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15° C Air Temperature (59° F) 5.9mm Precipitable Water (46% Relative Humidity)

1013 MB Atmospheric Pressure

©1998 Putman Publishing Company and OMEGA Press LLC.

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TRANSACTIONS Volume 1 05

Non-Contact Temperature MeasurementA Technical Reference Series Brought to You by OMEGA

11

VOLUME

I N M E A S U R E M E N T A N D C O N T R O L

06 Volume 1 TRANSACTIONS

A Historical Perspective1

• IR Through the Ages

• From Newton to Einstein

• Today’s Applications

Figure 1-1: The First IR Thermometer

LensRetina Light Detector

To Brain

Eye

Theoretical Development2

10

I N M E A S U R E M E N T A N D C O N T R O L

TABLE OF CONTENTS

VOLUME 1—NON-CONTACT TEMPERATURE MEASUREMENTSection Topics Covered Page

• Radiation Basics

• Blackbody Concepts

• From Blackbodies to Real Surfaces

Figure 2-1: Radiation Energy Balance

Transmitted Energy

Radiant Energy

Absorbed Energy

Reflected Energy

17

IR Thermometers & Pyrometers3

• The N Factor

• Types of Radiation Thermometers

• Design & Engineering

Figure 3-6: Ratio Pyrometry Via a Filter Wheel

Viewing MicroscopeTemperature Controlled

Cavity

Objective Lens

Target Mirror And First Field Stop

Lens

Aperture Stop Second Field Stop

Lens

Sensor

Rotating Filter Wheel

Eye

24

Infrared Thermocouples4

• Thermocouple Basics

• Self-Powered Infrared Thermocouples

• Installation Guidelines

0 200 400 600 800

8

6

4

2

0

mV

°F

6.68 mV

4 mV

2.68 mV

Figure 4-1: Thermocouple Operation

1 3

2 38


Fiber Optic Extensions5

• Fiber Advantages

• Fiber Applications

• Component Options

Section Topics Covered Page

Figure 5-2: Typical IR Fiber Optic Probe

Lens

Low Temperature Optical FiberSingle Crystal

Sapphire (Al2O3)

Narrowband Filter

AnalyzerOptical

Detector

Coupler

Thin Film Metal Coating

Al2O3 Protective Film

Blackbody Cavity

Linescanning & Thermography6

43

Calibration of IR Thermometers7

Products & Applications8

• Infrared Linescanners

• 2-D Thermographic Analysis

• Enter the Microprocessor

Figure 6-3: 2-D Thermographic Camera

Two Dimensional Thermograph

False Color Image

Object

Area Being

Scanned

• Why Calibrate?

• Blackbody Cavities

• Tungsten Filament Lamps

Schematic of the Infrared Spectrum 02

Table of Contents 06

Editorial 08

About OMEGA 09

• Alternative Configurations

• Application Guidelines

• Accessories & Options

46

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ctiv

e Em

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, ε

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Figure 7-2: Effective Emissivity of Spherical Cavities

Cavity Surface Emissivity

φ

Figure 8-2: Sighting on a Specular Surface

Target

Hot Furnace Walls

Thermometer

53

56

REFERENCE SECTIONS

68 Information Resources

72 Emissivity of Common Materials

77 Glossary

80 Index

Editorial


Welcome to Transactions!S ince its founding in 1962, OMEGA has grown from a manufacturer of a single product…a fine

gauge thermocouple…into being an established global leader in the scientific and technical

marketplace, offering more than 68,000 state-of-the-art instrumentation and control

devices. And although OMEGA’s staff, facilities, and client services are the finest anywhere,

OMEGA’s legendary handbooks and encyclopedias have become its hallmark—engineers

throughout the world daily rely on these reference tools of unprecedented value not only for

OMEGA product data, but for the vital technical information necessary to effectively employ

today’s sophisticated instruments and process control devices.

But we’re not resting on our laurels. We realize that your need for basic and relevant scientific

data and information continues to grow, and that you require all the information you can get to

keep up with rapidly advancing and ever-more-complex instrumentation and control technology.

That’s why we’ve launched OMEGA’s Transactions in Measurement & Control, the first

issue of which you now hold in your hands. Conceived as a practical thesis, a

technical reference series for everyday users of instrumentation and

controls, rather than a series of erudite essays, each issue of

Transactions will be jam-packed with information on a different

measurement and control technology topic. This issue, for

instance, delves deeply into the issue of non-contact

temperature measurement, providing a historical and

theoretical context, engineering and design principles,

plus selection and application guidelines for devices

ranging from low-cost infrared thermocouples to

sophisticated linescanners. Neither advertisements or

promotionals will be present in the Transactions series.

Future issues of Transactions, to be published on a

quarterly basis, will systematically cover other aspects of

temperature, humidity, pressure, strain, flow, level, pH, and

conductivity instrumentation as well as other measurement,

data acquisition, and control topics.

We hope Transactions finds a permanent home on your reference

shelf, and that it proves itself of great value now and in the future.

S

About OMEGA


OMEGA’s Transactions in Measurement & Control series, as well as our legendary set of handbooks and

encyclopedias, are designed to provide at-your-fingertips access to the technical information you need

to help meet your measurement and control requirements. But when your needs exceed the printed

word—when technical assistance is required to select among alternative products, or when no “off-the-shelf”

product seems to fill the bill—we hope you’ll turn to OMEGA. There is no advertising or

promotional materials in the Transactions series. There will be none.

Our people, our facilities, and our commitment to customer service set the

standard for control and instrumentation. A sampler of our comprehensive resources

and capabilities:

• OMEGA’s commitment to leading-edge research and development and

state-of-the-art manufacturing keeps us firmly at the forefront of technology.

OMEGA’s Development and Engineering Center, located on our Stamford, Conn.,

campus, is home to OMEGA’s design and engineering laboratories. All product

designs are tested and perfected here prior to marketing. This facility houses

OMEGA’s metrology lab and other quality control facilities. The testing that takes place here assures

that you receive the best products for your applications.

• On the manufacturing side, our Bridgeport, N.J., vertically integrated manufacturing facility near

Philadelphia houses advanced thermocouple wire production equipment along with a host of other

computerized CNC milling machines, injection molding equipment, screw machines, braiders, extruders,

punch presses and much, much more.

• If our broad range of standard products don’t quite match your needs, OMEGA is proud to offer the most

sophisticated and extensive custom engineering capabilities in the process measurement and control industry.

Whether you need a simple modification of a standard product or complete customized system, OMEGA can

accommodate your special request. Free CAD drawings also are supplied with customized product orders or a

new design built to your specifications at no obligation.

• We believe in active versus reactive customer service. To complement our current business and

manufacturing operations, OMEGA continues to strive toward new levels of quality by pursuing ISO 9000

quality standards. This systematic approach to quality strengthens OMEGA’s competitive edge. Our

calibration services and quality control test center are trustworthy resources that help satisfy our customers’

needs for accuracy on an initial and ongoing basis.

• The company’s technical center welcomes many corporate groups of engineers and scientists who turn

to OMEGA for training. Our 140-seat auditorium, equipped with the latest in multimedia presentation

technologies, provides an ideal learning environment for training tailored to your company’s needs—from

basic refreshers to in-depth courses.

In short, it is our commitment to quality instrumentation and exceptional customer service that remains

the cornerstone of our success. OMEGA’s priority is clear: we exist to facilitate solutions to your needs.

For more information about Transactions or OMEGA Technologies, look us up on the Internet at

www.omega.com.

Exceeding Your ExpectationsO

Our eyes only see the tinyfraction of energy emittedby the sun in the form ofvisible light. However, if

we could see the infrared rays emit-ted by all bodies—organic and inor-ganic—we could effectively see inthe dark. Though invisible to thehuman eye, infrared radiation canbe detected as a feeling of warmthon the skin, and even objects thatare colder than ambient tempera-ture radiate infrared energy. Someanimals such as rattlesnakes, havesmall infrared temperature sensorslocated under each eye which cansense the amount of heat beinggiven off by a body. These sensorshelp them to locate prey and pro-tect themselves from predators.

Non-contact temperature sensorsuse the concept of infrared radiantenergy to measure the temperatureof objects from a distance. Afterdetermining the wavelength of the

energy being emitted by an object,the sensor can use integrated equa-tions that take into account thebody’s material and surface qualitiesto determine its temperature. In thischapter, we will focus on the historyof radiation thermometry and thedevelopment of non-contact tem-perature sensors.

IR Through the AgesAlthough not apparent, radiationthermometry has been practiced forthousands of years. The first practicalinfrared thermometer was the humaneye (Figure 1-1). The human eye con-tains a lens which focuses emittedradiation onto the retina. The retina isstimulated by the radiation and sendsa signal to the brain, which serves asthe indicator of the radiation. If prop-erly calibrated based on experience,the brain can convert this signal to ameasure of temperature.

People have been using infraredheat to practical advantage for thou-sands of years. There is proof fromclay tablets and pottery dating backthousands of years that the sun wasused to increase the temperature ofmaterials in order to produce moldsfor construction. Pyramids werebuilt from approximately 2700-2200B.C. of sun-dried bricks. TheEgyptians also made metal toolssuch as saws, cutting tools, andwedges, which were crafted by theexperienced craftsmen of their time.The craftsmen had to know how hotto make the metal before they couldform it. This was most likely per-formed based on experience of thecolor of the iron.

Because fuel for firing was scarce,builders of Biblical times had todepend on the sun’s infrared radiationto dry the bricks for their temples andpyramids. The Mesopotamian remainsof the Tower of Babel indicate that it


IR Through the Ages

From Newton to Einstein

Today's Applications

NON-CONTACT TEMPERATURE MEASUREMENTA Historical Perspective

1

OA Historical Perspective

Figure 1-1: The First IR Thermometer

LensRetina Light Detector

To Brain

Eye

was made of sun-dried brick, facedwith burnt brick and stone. In India,a sewer system dating back to 2500B.C. carried wastewater throughpottery pipes into covered brickdrains along the street and dis-charged from these into brick cul-verts leading into a stream.

In ancient Greece, as far back as2100 B.C., Minoan artisans producedthings such as vases, statues, textiles.By using sight, they could approximatewhen a piece of material could beshaped. Terra-cotta pipes were builtby heating them to a certain tempera-ture and casting them into a mold.

In more recent years, specialcraftsmen have relied on their ownsenses to visualize when a material isthe correct temperature for moldingor cutting. Sight has been used forsteel working, glass working, waxmolding, and pottery. From experi-ence, skilled craftsmen learned toestimate the degree of heat requiredin the kiln, smelter, or glass furnaceby the color of the interior of theheating chamber. Just as a classicalblacksmith, for example, might judgethe malleability of a horseshoe by itscherry-red color.

In countries around the world, thetechnique of sight is still being used.In Europe, glass molding craftsmenuse sight to determine when glass isready to be shaped (Figure 1-2). Theyput a large piece of glass in a heatingfurnace by use of a large metal rod.When the glass reaches the desiredcolor and brightness, they pull it outof the oven and immediately form itinto the shape they want. If the glasscools and loses the desired color orbrightness, they put it back in theoven or dispose of it. The glass mak-ers know when the glass is ready, bysight. If you have a chandelier madeof glass, or hand-made glasses from

Europe, most likely they wereformed in this way.

From Newton to EinsteinThe thermometer was invented inItaly by Galileo Galilei (1564-1642),about two hundred years before theinfrared light itself was discovered in1800, and about 100 years before thegreat English scientist Sir IsaacNewton (1642-1727) investigated thenature of light by experimentationwith prisms.As published in Opticks in 1704,Newton used glass prisms to showthat white light could be split up intoa range of colors (Figure 1-3). The leastbent portion of the light consisted ofred, and then following in order,orange, yellow, green, blue, indigo, andviolet, each merging gradually into thenext. Newton also show that the dif-ferent colors could be fed backthrough another prism to producewhite light again. Newton’s workmade it clear that color was an inher-ent property of light and that white

light was a mixture of different colors.Matter affected color only by absorb-ing some kinds of light and transmit-ting or reflecting others.

It was also Newton who, in 1675,proposed that light was made up ofsmall particles, or “corpuscles.” Withthis theory, Newton set out to mea-sure the relative sizes of these corpus-cles. From observations of theeclipses of the moons of Jupiter,Newton realized that all light traveledat the same speed. Based on thisobservation, Newton determined therelative sizes of the different colorlight particles by the refraction angles.

In 1678, Christiaan Huygens (1629-1695), a mathematician, astronomer,and natural scientist, challengedNewton’s “corpuscular” theory propos-ing that light could be better under-stood as consisting of waves. Throughthe 1800s, the theory was well accept-ed, and it eventually became importantin James Clerk Maxwell’s theory ofelectromagnetic radiation.

Ironically for the field of infrared



Figure 1-2: Glass Manufacture Using Visual IR Temperature Measurement

thermometry, infrared radiation wasfirst discovered by using a conven-tional thermometer. FriedrickWilliam Herschel (1738-1822), a scien-tist and astronomer, is known as thefather of sidereal astronomy. Hestudied the planets and was the firstscientist to fully describe the MilkyWay galaxy. He also contributed tothe study of the solar system and thenature of solar radiation. In 1800,England, he was experimenting withsunlight. While using colored glassesto look at the Sun, Herschel noticedthat the sensation of heat was notcorrelated to visible light (Figure 1-4).This led him to make experimentsusing mercury thermometers andglass prisms and to correctlyhypothesize the existence of theinvisible infrared heat waves. UntilHerschel, no one had thought toput a thermometer and a prismtogether to try to measure theamount of heat in each color.

In 1800, Herschel had formed a sun-light spectrum and tested differentparts of it with a thermometer to seeif some colors delivered more heatthan others. He found that the tem-

perature rose as he moved toward thered end of the spectrum, and itseemed sensible to move the ther-mometer just past the red end in orderto watch the heating effect disappear.It did not. Instead, the temperaturerose higher than ever at a spotbeyond the red end of the spectrum(Figure 1-4). The region was calledinfrared, which means “below the red.”

How to interpret the region wasnot readily apparent. The firstimpression was that the sun deliv-ered heat rays as well as light raysand that heat rays refracted to a less-er extent than light rays. A half-cen-tury passed before it was establishedthat infrared radiation had all theproperties of light waves except thatit didn’t affect the retina of the eyein such a way as to produce a sensa-tion of light.

The German physicist Joseph vonFraunhofer (1787-1826) investigatedthe solar spectrum in the early 1800s.His spectroscope introduced parallelrays of white light by passing sunlightthrough a slit. The light contacted aprism, where the prism broke thelight into its constituent rays. He pro-

duced an innumerable amount oflines, each an image of the slit andeach containing a very narrow bandof wavelengths. Some wavelengthswere missing however. The slitimages at those wavelengths weredark. The result was that the solarspectrum was crossed by dark lines.These lines would later becomeimportant to the study of emissionand radiation.

In 1864, James Clerk Maxwell (1831-1879) brought forth for the first timethe equations which comprise thebasic laws of electromagnetism. Theyshow how an electric charge radiateswaves through space at various defi-nite frequencies that determine thecharge’s place in the electromagneticspectrum—now understood toinclude radio waves, microwaves,infrared waves, ultraviolet waves,X-rays, and gamma rays.

In addition, Maxwell’s equations’most profound consequence was atheoretical derivation of the speedof electricity—300,000 km/sec.—extremely close to the experimen-tally derived speed of light. Maxwellobserved and wrote, “The velocity is

A Historical Perspective 1


Figure 1-3: Newton Splits, Recombines White Light

so nearly that of light, that it seemswe have strong reason to concludethat light itself…is an electromag-netic disturbance in the form ofwaves propagated through the elec-tromagnetic field according to elec-tromagnetic laws.” Maxwell wasable to predict the entire electro-magnetic spectrum.

Another German, physiologist andphysicist Hermann von Helmholtz(1821-1894), accepted Maxwell’s theo-ry of electromagnetism, recognizingthat the implication was a particletheory of electrical phenomena. “Ifwe accept the hypothesis that theelementary substances [elements]are composed of atoms,” statedHelmholtz in 1881, “we cannot avoidconcluding that electricity, also, pos-itive as well as negative, is dividedinto elementary portions whichbehave like atoms of electricity.”

Gustav Robert Kirchhoff (1824-1887), a physicist and mathematician,worked with Robert Bunsen (1811-1899), an inorganic chemist and aphysicist, in 1859 on a spectrometerthat contained more than one prism.The spectroscope permitted greaterseparation of the spectral lines than

could be obtained by Fraunhofer’sspectroscope. They were able toprove that each chemical elementemits a characteristic spectrum oflight that can be viewed, recorded,and measured. The realization thatbright lines in the emission spectraof the elements exactly coincidedin wavelength with the dark lines inthe solar spectrum indicated thatthe same elements that were emit-ting light on earth were absorbinglight in the sun. As a consequence ofthis work, in 1859, Kirchhoff devel-oped a general theory of emissionand radiation known as Kirchhoff’slaw. Simply put, it states that a sub-stance’s capacity to emit light isequivalent to its ability to absorb itat the same temperature.

The following year, Kirchhoff, setforth the concept of a blackbody.This was one of the results ofKirchhoff’s law of radiation. A black-body is defined as any object thatabsorbs all frequencies of radiationwhen heated and then gives off allfrequencies when cooled. This devel-opment was fundamental to thedevelopment of radiation thermom-etry. The blackbody problem arose

because of the observation thatwhen heating an iron rod, for exam-ple, it gives off heat and light. Itsradiation may be at first invisible, orinfrared, however it then becomesvisible and red-hot. Eventually itturns white hot, which indicates thatit is emitting all colors of the spec-trum. The spectral radiation, whichdepends only on the temperature towhich the body is heated and not onthe material of which it is made,could not be predicted by classicalphysics. Kirchhoff recognized that “itis a highly important task to find thisuniversal function.” Because of itsgeneral importance to the under-standing of energy, the blackbodyproblem eventually found a solution.

An Austrian physicist, Josef Stefan(1835-1893) first determined the rela-tion between the amount of energyradiated by a body and its tempera-ture. He was particularly interested inhow hot bodies cooled and howmuch radiation they emitted. Hestudied hot bodies over a consider-able range of temperatures, and in1879 determined from experimentalevidence that the total radiationemitted by a blackbody varies as the



Figure 1-4: Herschel Discovers Infrared Light

Infrared

Thermometers

Prism

RedOrangeYellowGreenBlue

Violet

fourth power of its absolute temper-ature (Stefan’s law). In 1884, one of hisformer students, Ludwig Boltzmann(1844-1906), determined a theoreticalderivation for Stefan’s experimentallyderived law of blackbody radiationbased on thermodynamic principlesand Maxwell’s electromagnetic theo-ry. The law, now known as the Stefan-Boltzmann fourth-power law, formsthe basis for radiation thermometry.It was with this equation that Stefanwas able to make the first accuratedetermination of the surface tem-perature of the sun, a value ofapproximately 11,000°F (6,000°C).

The next quandary faced by theseearly scientists was the nature ofthe thermal radiation emitted byblackbodies. The problem was chal-lenging because blackbodies did notgive off heat in the way the scien-tists had predicted. The theoreticalrelationship between the spectralradiance of a blackbody and itsthermodynamic temperature wasnot established until late in thenineteenth century.

Among the theories proposed toexplain this inconsistency was one bythe German physicist Wilhelm Wienand the English physicist JohnRayleigh. Wilhelm Wien (1864-1928)measured the wavelength distributionof blackbody radiation in 1893. A plotof the radiation versus the wavelengthresulted in a series of curves at differ-ent temperatures. With this plot, hewas able to show that the peak valueof wavelength varies proportionallywith the amount of energy, andinversely with absolute temperature.As the temperature increases, notonly does the total amount of radia-tion increase, in line with Stefan’s find-ings, but the peak wavelengthdecreases and the color of the emit-ted light changes from red to orange

to yellow to white. Wien attempted to formulate an

empirical equation to fit this rela-tionship. The complex equationworked well for high frequencyblackbody radiation (short wave-lengths), but not for low frequencyradiation (long wavelengths).Rayleigh’s theory was satisfactory forlow frequency radiation.

In the mid-1890s, Max Karl ErnstLudwig Planck (1858-1947), a Germanphysicist and a former student ofKirchhoff, and a group of Berlinphysicists were investigating the lightspectrum emitted by a blackbody.Because the spectrometer emitteddistinct lines of light, rather thanbroad bands, they hypothesized thatminute structures were emitting thelight and began to develop an atom-ic theory that could account forspectral lines.

This was of interest to Planckbecause in 1859 Kirchhoff had dis-covered that the quality of heatradiated and absorbed by a black-body at all frequencies reached anequilibrium that only depended on

temperature and not on the natureof the object itself. But at any giventemperature, light emitted from aheated cavity—a furnace, for exam-ple—runs the gamut of spectral col-ors. Classical physics could not pre-dict this spectrum.

After several false starts, beginningin 1897, Planck succeeded in finding aformula predicting blackbody radia-tion. Planck was able to arrive at a for-mula that represented the observedenergy of the radiation at any givenwavelength and temperature. He gavethe underlying notion that light andheat were not emitted in a steadystream. Rather, energy is radiated indiscrete units, or bundles. Planck dis-covered a universal constant, “Planck’sconstant,” which was founded onphysical theory and could be used tocompute the observed spectrum. Thisassumed that energy consisted of thesum of discrete units of energy hecalled quanta, and that the energyemitted, E, by each quantum is givenby the equation E = hυ = hc/λ, whereυ (sec-1) is the frequency of the radia-tion and h is Planck’s constant—now



Figure 1-5: The Sidewinder Missle's IR Guidance System

Reticle Location and Center of Rotation

Dome

Figure 1-5: The Sidewinder Missile’s IR Guidance System

known to be a fundamental constantof nature. By thus directly relating theenergy of radiation to its frequency, anexplanation was found for the obser-vation that higher energy radiation hasa higher frequency distribution.Planck’s finding marked a new era inphysics.

Before Planck’s studies, heat wasconsidered to be a fluid composedof repulsive particles capable ofcombining chemically with materialatoms. In this theory, the particles ofheat entered a system and movedbetween the particles. A mutualrepulsion of the particles of heat cre-ated a pressure. A thermometerdetected this pressure. Planck’s con-stant became known as a “fortunateguess.” It allowed for theoreticalequations which agreed with theobservable range of spectral phe-nomena, and was fundamental in thetheory of blackbody radiation.

Albert Einstein (1879-1955) studiedthe works of Maxwell and Helmholtz.In 1905, Einstein used the quantum as atheoretical tool to explain the photo-electric effect, showing how light cansometimes act as a stream of particles.He published three papers in volumeXVII of Annalen der Physik. In one, heset forth his now famous theory of rel-ativity, but another showed that a fun-damental process in nature is at workin the mathematical equation whichhad resolved the problem of black-body radiation.

Light, Einstein showed, is a streamof particles with a computableamount of energy using Planck’sconstant. Within a decade, this pre-diction confirmed experimentallyfor visible light.

Max Karl Ernst Ludwig Planck initi-ated quantum theory at the turn ofthe twentieth century and changedthe fundamental framework of

physics. Wrote Einstein, “He has givenone of the most powerful of allimpulses to the progress of science.”

Today’s ApplicationsThe first patent for a total radiationthermometer was granted in 1901.The instrument used a thermoelec-tric sensor; it had an electrical out-put signal and was capable of unat-tended operation. In 1931, the firstcommercially-available total radia-

tion thermometers were introduced.These devices were widely usedthroughout industry to record andcontrol industrial processes. They arestill used today, but mainly used forlow temperature applications.

The first modern radiation ther-mometers were not available untilafter the second World War. Originallydeveloped for military use, lead sulfidephotodetectors were the first infraredquantum detectors to be widely usedin industrial radiation thermometry.Other types of quantum detectorsalso have been developed for militaryapplications and are now widelyapplied in industrial radiation ther-mometry. Many infrared radiationthermometers use thermopile detec-tors sensitive to a broad radiationspectrum and are extensively used inprocess control instrumentation.

Infrared thermometers currently are

being used in a wide range of industri-al and laboratory temperature controlapplications. By using non-contacttemperature sensors, objects that aredifficult to reach due to extreme envi-ronmental conditions can be moni-tored. They can also be used for prod-ucts that cannot be contaminated by acontact sensor, such as in the glass,chemical, pharmaceutical, and foodindustries. Non-contact sensors can beused when materials are hot, moving,or inaccessible, or when materials can-

not be damaged, scratched, or torn bya contact thermometer.

Typical industries in which non-contact sensors are used includeutilities, chemical processing, phar-maceutical, automotive, food pro-cessing, plastics, medical, glass, pulpand paper, construction materials,and metals. Industrially, they areused in manufacturing, quality con-trol, and maintenance and havehelped companies increase produc-tivity, reduce energy consumption,and improve product quality.

Some applications of radiationthermometry include the heat treat-ing, forming, tempering, and anneal-ing of glass; the casting, rolling, forg-ing, and heat treating of metals; qual-ity control in the food and pulp andpaper industry; the extrusion, lamina-tion, and drying of plastics, paper,and rubber; and in the curing process



Figure 1-6: IR Optics for Missile Guidance

Reimaging Lens

Detector

Main Optics

Objects at Infinity

Reticle

of resins, adhesives, and paints. Non-contact temperature sensors

have been used and will continue tobe valuable for research in military,medical, industrial, meteorological,ecological, forestry, agriculture, andchemical applications.

Weather satellites use infraredimaging devices to map cloud pat-terns and provide the imagery seen inmany weather reports. Radiationthermometry can reveal the temper-ature of the earth’s surface eventhrough cloud cover.

Infrared imaging devices also areused for thermography, or thermalimaging. In the practice of medicine,for example, thermography has beenused for the early detection of breastcancer and for the location of thecause of circulatory deficiencies. Inmost of these applications, theunderlying principle is that patholo-gy produces local heating andinflammation which can be foundwith an infrared imager. Other diag-nostic applications of infrared ther-mography range from back problemsto sinus obstructions.

Edge burning forest fires have beenlocated using airborne infraredimagers. Typically, the longer wave-lengths of the emitted infrared radia-tion penetrate the smoke better thanthe visible wavelengths, so the edgesof the fire are better delineated.

On the research front, one sophis-ticated infrared thermometry appli-cation is in the study of faults in met-als, composites, and at coating inter-faces. This technique is known aspulsed video thermography. A com-posite material consisting of a car-bon-fiber skin bonded to an alu-minum honeycomb is subjected to

pulses of heat from a xenon flashtube. Infrared cameras record aframe-by-frame sequence of heatdiffusion through the object, whichis displayed on screen. Defects showup as deviations in the expected pat-terns for the material being tested.

Among the military applications ofradiation thermometry are night-visionand the “heat-seeking” missile. In thelatter case, the operator simply launch-es the missile in the general directionof the target. On-board detectorsenable the missile to locate the targetby tracking the heat back to the source.The most widely known militaryinfrared missile applications are theSidewinder air-to-air missile and asatellite-borne intercontinental ballis-tic missile (ICBM) detection system.

Both rely on detecting the infrared

signature of an emission plume or veryhot exhaust engine. The Sidewindermissile guidance system is shownschematically in Figure 1-5. A specialinfrared dome protects the opticalsystem inside. The optical system con-sists of a primary and secondary mirrorand a set of correction lenses to causean image to focus onto a special reti-cle. All the light from the reticle isfocused onto a detector (Figure 1-6).The reticle can modulate the radiationto distinguish between clouds andprovide directional information.

Portable surface-to-air missiles,SAMs, are effective defense unitsthat guide themselves to a target bydetecting and tracking the heat emit-ted by an aircraft, particularly theengine exhaust. T



References and Further Reading• Album of Science, The 19th Century, Pearce L. Williams, Charles Scribner’sSons, 1978.• Asimov’s Chronology of Science and Discovery, Isaac Asimov,HarperCollins Publishers, 1994.• The Biographical Dictionary of Scientists, 2nd ed., Oxford University Press,1994.• Dictionary of Scientific Biography, Vols. 9, 10, 11, Charles C. Gillispile,Charles Scribner’s Sons, 1973.• Engineering in History, Richard S. Kirby and Sidney Withington, Arthur B.Darling, Frederick G. Kilgour, McGraw-Hill, 1956.• The Invisible World of the Infrared, Jack R. White, Dodd, Mead &Company, 1984.• The McGraw-Hill Encyclopedia of Science and Technology, 8th ed., Vol. 9,McGraw-Hill, 1997.• Notable Twentieth-Century Scientists, Emily J. McMurray, Gale ResearchInc., 1995.• Pioneers of Modern Science, The World of Science, Bill MacKeith,Andromeda Oxford Limited, 1991.• The Scientific 100. A Ranking of the Most Influential Scientists, Past andPresent, John Simmons, Carol Publishing Group, 1996.• Theory and Practice of Radiation Thermometry, David P. DeWitt., and

All matter—animate or inani-mate, liquid, solid, or gas—constantly exchanges ther-mal energy in the form of

electromagnetic radiation with itssurroundings. If there is a tempera-ture difference between the objectin question and its surroundings,there will be a net energy transfer inthe form of heat; a colder object willbe warmed at the expense of its sur-roundings, a warmer object cooled.And if the object in question is at thesame temperature as its surrounding,the net radiation energy exchangewill be zero.

In either case, the characteristicspectrum of the radiation dependson the object and its surroundings’absolute temperatures. The topic ofthis volume, radiation thermometry,or more generally, non-contact tem-perature measurement, involves tak-ing advantage of this radiationdependence on temperature tomeasure the temperature of objectsand masses without the need fordirect contact.

. Radiation BasicsThe development of the mathemat-ical relationships to describe radia-tion were a major step in the devel-opment of modern radiation ther-mometry theory. The ability toquantify radiant energy comes,appropriately enough, from Planck’squantum theory. Planck assumedthat radiation was formed in dis-crete energy packages called pho-tons, or quanta, the magnitude ofwhich are dependent on the wave-

length of the radiation. The totalenergy of a quantum, E, is found bymultiplying Planck’s constant, h =6.6256 x 10-34, and, the radiation fre-quency, υ, in cycles per second.

In 1905, Albert Einstein postulat-ed that these quanta are particlesmoving at the speed of light, c =2.9979 x 108 m/s. If these photonstraveled at the speed of light, thenthey must obey the theory of rela-tivity, stating E2 = c2p2 , and eachphoton must have the momentum p= E/c = h/λ . The frequency can befound by dividing the speed of lightby its particle wavelength υ = c/λ .Substituting for momentum:

E = hυ = hc/λ

From this equation, it is apparentthat the amount of energy emitteddepends on the wavelength (or fre-quency). The shorter the wave-length, the higher the energy.

Emitted radiation consists of a

continuous, non-uniform distributionof monochromatic (single-wave-length) components, varying widelywith wavelength and direction. Theamount of radiation per unit wave-length interval, referred to as thespectral concentration, also varieswith wavelength. And the magnitudeof radiation at any wavelength aswell as the spectral distribution varywith the properties and temperatureof the emitting surface. Radiation isalso directional. A surface may prefera particular direction to radiate ener-gy. Both spectral and directional dis-tribution must be considered instudying radiation.

Wavelength can be thought of as atype of address to find where a ray’senergy is located. The map contain-ing all the wavelengths of electro-magnetic radiation is called the elec-tromagnetic spectrum (see the insidefront cover of this volume). The shortwavelengths are the gamma rays,X-rays, and ultraviolet (UV) radiation,


Radiation Basics

Blackbody Concepts

From Blackbodies to Real Surfaces

NON-CONTACT TEMPERATURE MEASUREMENTTheoretical Development

2

ATheoretical Development

Figure 2-1: Radiation Energy Balance

Transmitted Energy

Radiant Energy

Absorbed Energy

Reflected Energy

containing the highest amount ofenergy emitted. The intermediateportion of the spectrum, the heatregion, extends from approximately0.1 to 1000 µm (micrometers ormicrons: 1,000,000 microns = 1 meter),and includes a portion of the ultravio-let and all of the visible (VIS) andinfrared (IR) waves. This portion istermed thermal radiation, and isimportant in the study of heat trans-fer and radiation thermometry.

Non-contact temperature sensorswork in the infrared portion of thespectrum. The infrared range fallsbetween 0.78 microns and 1000microns in wavelength, and is invisi-ble to the naked eye. The infrared isregion can be divided into threeregions: near-infrared (0.78-3.0microns); middle infrared (3-30microns); and far infrared (30-300microns). The range between 0.7microns and 14 microns is normallyused in infrared temperature mea-

surement. The divisions have beenrelated to the transmission of theatmosphere for different types ofapplications.

Blackbody ConceptsIncident energy striking an objectfrom the surroundings, can beabsorbed by the object, reflected bythe object, or transmitted throughthe object (if it is not opaque) asseen in Figure 2-1. If the object is ata constant temperature, then therate at which it emits energy mustequal the rate at which it absorbsenergy, otherwise the object wouldcool (emittance greater thanabsorption), or warm (emittanceless than absorption). Therefore,for bodies at constant temperature,the emittance (absorption), thereflection and the transmittance ofenergy equals unity.

Central to radiation thermometry

is the concept of the blackbody. In1860, Kirchhoff defined a blackbodyas a surface that neither reflects ortransmits, but absorbs all incidentradiation, independent of directionand wavelength. The fraction of radi-ation absorbed by a real body iscalled absorptivity, α. For an idealblackbody, the absorptivity is 1.0 (αb

= 1). For non-blackbodies, the absorp-tion is a fraction of the radiation heattransfer incident on a surface, or 0 ≤α ≤ 1. Hence, in term of radiation heattransfer, q”:

q”absorbed = αq”incident

In addition to absorbing all inci-dent radiation, a blackbody is a per-fect radiating body. To describe theemitting capabilities of a surface incomparison to a blackbody,Kirchoff defined emissivity (ε) of areal surface as the ratio of the ther-mal radiation emitted by a surfaceat a given temperature to that of ablackbody at the same temperatureand for the same spectral anddirectional conditions.

This value also must be consideredby a non-contact temperature sensorwhen taking a temperature measure-ment. The total emissivity for a realsurface is the ratio of the total amountof radiation emitted by a surface incomparison to a blackbody at thesame temperature. The tables begin-ning on p. 72 give representative emis-sivity values for a wide range of mate-rials. If precise temperature measure-ments are required, however, the sur-face’s actual emittivity value should beobtained. (Although often used inter-changeably, the terms emissivity andemittivity have technically differentmeanings. Emissivity refers to a prop-erty of a material, such as cast iron,whereas emittivity refers to a property

Theoretical Development 2


Figure 2-2: Spectral Distributions

Rela

tive

Ene

rgy

Wavelength, Microns

ε=1.0 (Blackbody)

ε=0.9 (Graybody)

ε varies with

wavelength

(Non-graybody)

of a specific surface.)In 1879, Stefan concluded based

on experimental results that theradiation emitted from the surfaceof an object was proportional tothe fourth power of the absolutetemperature of the surface. Theunderlying theory was later devel-oped by Boltzmann, who showedthat the radiation given off by ablackbody at absolute temperatureTs (K) is equal to:

q” = σTs4

where ( is the Stefan-Boltzmann con-stant (σ = 5.67 x 10-8 W/m2 • K4 ). Theheat transfer rate by radiation for anon-blackbody, per unit area isdefined as:

q” = ασ(Ts4 - Tsur4)

where Ts is the surface temperatureand Tsur is the temperature of thesurroundings.

Although some surfaces comeclose to blackbody performance, allreal objects and surfaces have emis-sivities less than 1. Non-blackbodyobjects are either graybodies, whoseemissivity does vary with wave-length, or non-graybodies, whoseemissivities vary with wavelength.Most organic objects are graybodies,with an emissivity between 0.90 and0.95 (Figure 2-2).

The blackbody concept is impor-tant because it shows that radiantpower depends on temperature.When using non-contact tempera-ture sensors to measure the energyemitted from an object, dependingon the nature of the surface, theemissivity must be taken intoaccount and corrected. For example,an object with an emissivity of 0.6 isonly radiating 60% of the energy of a

blackbody. If it is not corrected for,the temperature will be lower thanthe actual temperature. For objectswith an emissivity less than 0.9, theheat transfer rate of a real surface isidentified as:

q” = εσTs4

The closest approximation to ablackbody is a cavity with an interi-or surface at a uniform temperatureTs, which communicates with thesurroundings by a small hole havinga diameter small in comparison tothe dimensions of the cavity (Figure2-3). Most of the radiation enteringthe opening is either absorbed orreflected within the cavity (to ulti-mately be absorbed), while negligi-ble radiation exits the aperture. Thebody approximates a perfectabsorber, independent of the cavi-ty’s surface properties.

The radiation trapped within theinterior of the cavity is absorbed andreflected so that the radiation withinthe cavity is equally distributed—some radiation is absorbed and somereflected. The incident radiant ener-gy falling per unit time on any sur-face per unit area within the cavity isdefined as the irradiance Gλ (W/m2 •µm). If the total irradiation G (W/m2)represents the rate at which radiationis incident per unit area from alldirections and at all wavelengths, itfollows that:

G = ∫0→∞Gλ (dλ)

If another blackbody is brought intothe cavity with an identical tempera-ture as the interior walls of the cavi-ty, the blackbody will maintain itscurrent temperature. Therefore, therate at which the energy absorbed bythe inner surface of the cavity will

equal the rate at which it is emitted.In many industrial applications,

transmission of radiation, such asthrough a layer of water or a glassplate, must be considered. For a spec-tral component of the irradiation,portions may be reflected, absorbed,and transmitted. It follows that:

Gλ = Gλ,ref + Gλ,abs + Gλ,tran

In many engineering applications,however, the medium is opaque tothe incident radiation. Therefore,Gλ,tran = 0, and the remaining absorp-tion and reflection can be treated assurface phenomenon. In otherwords, they are controlled byprocesses occurring within a frac-tion of a micrometer from the irra-diated surface. It is therefore appro-priate to say that the irradiation isabsorbed and reflected by the sur-face, with the relative magnitudesof Gλ,ref and Gλ,abs depending on thewavelength and the nature of thesurface.

Radiation transfer by a non-black-body encompasses a wide range ofwavelengths and directions. Thespectral hemispherical emissivepower, Eλ (W/m2 • µm) is defined asthe rate at which radiation is emit-ted per unit area at all possiblewavelengths and in all possibledirections from a surface, per unitwavelength dλ about λ and per unitsurface area.

Although the directional distribu-tion of surface emission variesdepends on the surface itself, manysurfaces approximate diffuse emit-ters. That is, the intensity of emittedradiation is independent of thedirection in which the energy is inci-dent or emitted. In this case, thetotal, hemispherical (spectral) emis-sive power, Eλ (W/m2) is defined as:



Eλ(λ) = πΙλ,e(λ)

or

E = πΙe

where Ιe is the total intensity of theemitted radiation, or the rate at whichradiant energy is emitted at a specificwavelength, per unit area of the emit-ting surface normal to the direction,per unit solid angle about this direc-tion, and per unit wavelength. Noticethat Eλ is a flux based on the actualsurface area, where Ιλe is based on theprojected area. In approximating ablackbody, the radiation is almostentirely absorbed by the cavity. Anyradiation that exits the cavity is due tothe surface temperature only.

The spectral characteristics ofblackbody radiation as a function oftemperature and wavelength weredetermined by Wilhelm Wien in1896. Wien derived his law for thedistribution of energy in the emissionspectrum as:

Eλ,b(λ,T) = 2h2/λ5 [exp(hco/λkT)]

where Eλ,b (b for blackbody) representsthe intensity of radiation emitted by ablackbody at temperature T, and wave-length λ per unit wavelength interval,per unit time, per unit solid angle, perunit area. Also, h = 6.626 x 10-24 J•s andk = 1.3807 x 10-23 J•K-1 are the universalPlanck and Boltzman constants,respectively; co = 2.9979 x 108 m/s isthe speed of light in a vacuum, and T isthe absolute temperature of the black-body in Kelvins (K).

Due to the fact that deviationsappeared between experimentalresults and the equation, Planck sug-gested in 1900 a refinement thatbetter fit reality:

Eλ,b(λ,T) = 2h2/λ5 [exp(hco/λkT) - 1]

It is from this equation that Planckpostulated his quantum theory. Amore convenient expression for thisequation, referred to as the Planckdistribution law (Figure 2-4), is:

Eλ,b(λ,T) = πIλ,b(λ,T)=C1/λ5[exp(C2/λT) - 1]

where the first and second radiationconstants are C1 = 2πhco

2 = 3.742 x 108

W • µm4/m2 and C2 = (hco/k) = 1.439x 104 µm • K.

Planck’s distribution shows that aswavelength varies, emitted radiationvaries continuously. As temperatureincreases, the total amount of energyemitted increases and the peak ofthe curve shifts to the left, or towardthe shorter wavelengths. In consider-ing the electromagnetic spectrum, itis apparent that bodies with very

high temperatures emit energy in thevisible spectrum as wavelengthdecreases. Figure 2-4 also shows thatthere is more energy difference perdegree at shorter wavelengths.

From Figure 2-4, the blackbodyspectral distribution has a maximumwavelength value, λmax, whichdepends on the temperature. By dif-ferentiating equation 2.12 withrespect to λ and setting the resultequal to zero:

λmaxT = C3

where the third radiation constant,C3 = 2897.7 µm • K. This is known asWien’s displacement law. Thedashed line in Figure 2-4 defines thisequation and locates the maximumradiation values for each tempera-ture, at a specific wavelength.Notice that maximum radiance isassociated with higher temperaturesand lower wavelengths.



Ι λ, ι

Figure 2-3: An Isothermal Blackbody Cavity

From Blackbodies to Real SurfacesAt first it would seem that a radiationthermometer would utilize the entirespectrum, capturing most of the radi-ant emission of a target in its particu-lar temperature range. There are sev-eral reasons why this is not practical.

In the equations for infrared radia-tion derived above, it was found thatat very low wavelengths, the radianceincreases rapidly with temperature, incomparison to the increase at higherwavelengths, as shown in Figure 2-4.Therefore, the rate of radiance changeis always greater at shorter wave-lengths. This could mean more precisetemperature measurement and tightertemperature control. However, at agiven short wavelength there is alower limit to the temperature thatcan be measured. As the process tem-perature decreases, the spectral rangefor an infrared thermometer shifts tolonger wavelengths and becomes lessaccurate.

The properties of the material atvarious temperatures must also beconsidered. Because no materialemits as efficiently as a blackbody ata given temperature, when measuringthe temperature of a real target,other considerations must be made.Changes in process material emissivi-ty, radiation from other sources, andlosses in radiation due to dirt, dust,smoke, or atmospheric absorptioncan introduce errors.

The absorptivity of a material isthe fraction of the irradiationabsorbed by a surface. Like emission,it can be characterized by both adirectional and spectral distribution.It is implicit that surfaces may exhib-it selective absorption with respectto wavelength and direction of theincident radiation. However, for mostengineering applications, it is desir-able to work with surface propertiesthat represent directional averages.The spectral, hemispherical absorp-

tivity for a real surface αλ(λ)isdefined as:

αλ(λ) ≅ Gλ,abs(λ)/Gλ(λ)

where Gλ,abs is the portion of irradia-tion absorbed by the surface. Hence,αλ depends on the directional distri-bution of the incident radiation, aswell as on the wavelength of theradiation and the nature of theabsorbing surface. The total, hemi-spherical absorptivity, α, representsan integrated average over bothdirectional and wavelength. It isdefined as the fraction of the totalirradiation absorbed by a surface, or:

α ≅ Gabs/G

The value of α depends on thespectral distribution of the incidentradiation, as well as on its direc-tional distribution and the natureof the absorbing surface. Although



Spec

tral

Em

issi

ve P

ower

, Eλ,

b, W

/m2 .

µm

Wavelength, λ, µm

0.1

109

0.2 0.4 0.6 1.0 2.0 4.0 6.0 2010 40 60 100

108

107 106 105 104 103 102 101

100 10-1 10-2 10-3 10-4

5800 K

100 K

300 K

800 K1000 K

2000 K

50 K

Visible Spectral Region

λmaxT=2898 µm . K

Solar Radiation

Figure 2-4: Planck Prediction of Blackbody Emissive Power

α is independent on the tempera-ture of the surface, the same maynot be said for the total, hemi-spherical emissivity. Emissivity isstrongly temperature dependent.

The reflectivity of a surfacedefines the fraction of incidentradiation reflected by a surface. Itsspecific definition may take severaldifferent forms. We will assume areflectivity that represents an inte-grated average over the hemisphereassociated with the reflected radia-tion to avoid the problems fromthe directional distribution of thisradiation. The spectral, hemispheri-cal reflectivity ρλ(λ), then, isdefined as the spectral irradiationthat is reflected by the surface.Therefore:

ρλ(λ) ≅ Gλ,ref(λ)/Gλ(λ)

where Gλ,ref is the portion of irradia-tion reflected by the surface. Thetotal, hemispherical reflectivity ρ isthen defined as:

ρ ≅ Gref/G

If the intensity of the reflectedradiation is independent of thedirection of the incident radiationand the direction of the reflectedradiation, the surface is said to bediffuse emitter. In contrast, if theincident angle is equivalent to thereflected angle, the surface is aspecular reflector. Although no sur-face is perfectly diffuse or specular,specular behavior can be approxi-mated by polished or mirror-likesurfaces. Diffuse behavior is closelyapproximated by rough surfacesand is likely to be encountered inindustrial applications.

Transmissivity is the amount ofradiation transmitted through a sur-face. Again, assume a transmissivitythat represents an integrated aver-age. Although difficult to obtain aresult for transparent media, hemi-spherical transmissivity is defined as:

τλ = Gλ,tr(λ)/Gλ(λ)

where Gλ,tr is the portion of irradia-tion reflected by the surface. Thetotal hemispherical transmissivity is:

τ = Gtr/G

The sum of the total fractions ofenergy absorbed (α), reflected (ρ),and transmitted (τ) must equal thetotal amount of radiation incidenton the surface. Therefore, for anywavelength:

ρλ + τλ + αλ = 1

This equation applies to a semitrans-parent medium. For properties thatare averaged over the entire spec-trum, it follows that:

ρ + τ + α = 1

For a medium that is opaque, thevalue of transmission is equal to zero.Absorption and reflection are sur-face properties for which:

ρλ + αλ = 1

and

ρ + α = 1


22 Volume 1 TRANSACTIONS26

Spec

tral

Tra

nsm

itta

nce,

τλ

Wavelength, λ, µm2.5 3 4 5 6 7 8

1.0

0.8

0.6

0.4

0.2

0109

3.43 Microns 4.8 to 5.3 Microns 7.9 Microns

0.009 In. Thick

0.027 In. Thick

0.061 In. Thick

0.124 In. Thick

0.231 In. Thick

Figure 2-5: Soda-Lime Glass Spectral Transmittance

For a blackbody, the transmitted andreflected fractions are zero and theemissivity is unity.

An example of a material whoseemissivity characteristics change rad-ically with wavelength is glass. Soda-lime glass is an example of a materialwhich drastically changes its emissiv-ity characteristics with wavelength(Figure 2-5). At wavelengths belowabout 2.6 microns, the glass is highlytransparent and the emissivity isnearly zero. Beyond 2.6 microns, theglass becomes increasingly moreopaque. Beyond 4 microns, the glassis completely opaque and the emis-sivity is above 0.97. T



References and Further Reading• Temperature Measurement in Engineering, H. Dean Baker, E.A. Ryder, andN.H. Baker, Omega Press, 1975.• Heat and Thermodynamics, 6th ed., Mark W. Zemansky, and Richard H.Dittman, McGraw-Hill, 1981.• Industrial Temperature Measurement, Thomas W. Kerlin and Robert L.Shepard, Publishers Creative Series, Inc., ISA.• Introduction to Heat Transfer, 2nd ed., Frank P. Incropera, and David P.DeWitt, John Wiley & Sons, 1990.• The Invisible World of the Infrared, Dodd, Jack R. White, Mead &Company, 1984.• Process/Industrial Instruments and Controls Handbook, 4th ed., DouglasM. Considine, McGraw-Hill, 1993. • Theory and Practice of Radiation Thermometry, David P. DeWitt andGene D. Nutter, John Wiley & Sons, 1988.• Thermodynamics, 5th ed., Virgil M. Faires, The Macmillan Company, 1971.

Pyrometer is derived from theGreek root pyro, meaning fire.The term pyrometer was orig-inally used to denote a device

capable of measuring temperaturesof objects above incandescence,objects bright to the human eye. Theoriginal pyrometers were non-con-tacting optical devices which inter-cepted and evaluated the visibleradiation emitted by glowing objects.A modern and more correct defini-tion would be any non-contactingdevice intercepting and measuringthermal radiation emitted from anobject to determine surface temper-ature. Thermometer, also from aGreek root thermos, signifying hot, isused to describe a wide assortmentof devices used to measure tempera-ture. Thus a pyrometer is a type ofthermometer. The designation radia-tion thermometer has evolved overthe past decade as an alternative topyrometer. Therefore the termspyrometer and radiation thermome-ter are used interchangeably by manyreferences.

A radiation thermometer, in verysimple terms, consists of an opticalsystem and detector. The optical sys-tem focuses the energy emitted byan object onto the detector, which issensitive to the radiation. The outputof the detector is proportional tothe amount of energy radiated by thetarget object (less the amountabsorbed by the optical system), andthe response of the detector to thespecific radiation wavelengths. Thisoutput can be used to infer theobjects temperature. The emittivity,or emittance, of the object is an

important variable in converting thedetector output into an accuratetemperature signal.

Infrared radiation thermometers/pyrometers, by specifically measur-ing the energy being radiated froman object in the 0.7 to 20 micronwavelength range, are a subset ofradiation thermometers. Thesedevices can measure this radiationfrom a distance. There is no needfor direct contact between the radi-ation thermometer and the object,as there is with thermocouples andresistance temperature detectors(RTDs). Radiation thermometers aresuited especially to the measure-ment of moving objects or any sur-faces that can not be reached or cannot be touched.

But the benefits of radiation ther-mometry have a price. Even the sim-plest of devices is more expensivethan a standard thermocouple or resis-tance temperature detector (RTD)assembly, and installation cost canexceed that of a standard thermowell.The devices are rugged, but do require

routine maintenance to keep thesighting path clear, and to keep theoptical elements clean. Radiation ther-mometers used for more difficultapplications may have more compli-cated optics, possibly rotating or mov-ing parts, and microprocessor-basedelectronics. There are no industryaccepted calibration curves for radia-tion thermometers, as there are forthermocouples and RTDs. In addition,the user may need to seriously investi-gate the application, to select theoptimum technology, method ofinstallation, and compensation need-ed for the measured signal, to achievethe performance desired.

Emittance, Emissivity, and the N FactorIn an earlier chapter, emittance wasidentified as a critical parameter inaccurately converting the output ofthe detector used in a radiation ther-mometer into a value representingobject temperature.

The terms emittance and emissiv-ity are often used interchangeably.


The N Factor

Types of Radiation Thermometers

Design & Engineering

NON-CONTACT TEMPERATURE MEASUREMENTIR Thermometers & Pyrometers

3

PIR Thermometers & Pyrometers

Lens

Optical Chopper

Temperature Controlled Cavity

To Recorder

Filter Detector

Preamplifier

Power Supply

Filter Rectifier Readout MeterSync. Motor

Figure 3-1: Traditional Infrared Thermometer

There is, however, a technical dis-tinction. Emissivity refers to theproperties of a material; emittanceto the properties of a particularobject. In this latter sense, emissivi-ty is only one component in deter-mining emittance. Other factors,including shape of the object, oxi-dation and surface finish must betaken into account.

The apparent emittance of amaterial also depends on the tem-perature at which it is determined,and the wavelength at which themeasurement is taken. Surface condi-tion affects the value of an object’semittance, with lower values forpolished surfaces, and higher valuesfor rough or matte surfaces. In addi-tion, as materials oxidize, emittancetends to increase, and the surfacecondition dependence decreases.Representative emissivity values for arange of common metals and non-metals at various temperatures aregiven in the tables starting on p. 72.

The basic equation used todescribe the output of a radiationthermometer is:

V (T) = ε K TN

Where: ε = emittivityV(T) = thermometer output with

temperatureK = constantT = object temperatureN = N factor ( = 14388/(λT))λ = equivalent wavelength

A radiation thermometer with thehighest value of N (shortest possibleequivalent wavelength) should beselected to obtain the least depen-dence on target emittance changes.The benefits of a device with a highvalue of N extends to any parameter

that effects the output V. A dirtyoptical system, or absorption ofenergy by gases in the sighting path,has less effect on an indicated tem-perature if N has a high value.

The values for the emissivities of

almost all substances are known andpublished in reference literature.However, the emissivity determinedunder laboratory conditions seldomagrees with actual emittance of anobject under real operating condi-tions. For this reason, one is likely touse published emissivity data whenthe values are high. As a rule of thumb,most opaque non-metallic materialshave a high and stable emissivity (0.85to 0.90). Most unoxidized, metallicmaterials have a low to medium emis-sivity value (0.2 to 0.5). Gold, silverand aluminum are exceptions, withemissivity values in the 0.02 to 0.04range. The temperature of these met-als is very difficult to measure with aradiation thermometer.

One way to determine emissivityexperimentally is by comparing theradiation thermometer measurementof a target with the simultaneousmeasurement obtained using a ther-mocouple or RTD. The difference in

readings is due to the emissivity,which is, of course, less than one. Fortemperatures up to 500°F (260°C)emissivity values can be determinedexperimentally by putting a piece ofblack masking tape on the target sur-

face. Using a radiation pyrometer setfor an emissivity of 0.95, measure thetemperature of the tape surface(allowing time for it to gain thermalequilibrium). Then measure the tem-perature of the target surface with-out the tape. The difference in read-ings determines the actual value forthe target emissivity.

Many instruments now have cali-brated emissivity adjustments. Theadjustment may be set to a value ofemissivity determined from tables,such as those starting on p. 72, orexperimentally, as described in thepreceding paragraph. For highestaccuracy, independent determina-tion of emissivity in a lab at thewavelength at which the thermome-ter measures, and possibly at theexpected temperature of the target,may be necessary.

Emissivity values in tables havebeen determined by a pyrometersighted perpendicular to the target.



Figure 3-2: Effect of Non-Blackbody Emissivity on IR Thermometer Error

1500 2000 2500 3000 3500 4000 4500 5000

800

700

600

500

400

300

200

100

Erro

r - °F

True Temperature, °F

E=0.3

E=0.5

E=0.7

E=0.9

If the actual sighting angle is morethan 30 or 40 degrees from the nor-mal to the target, lab measurementof emissivity may be required.

In addition, if the radiation pyrom-eter sights through a window, emissiv-ity correction must be provided forenergy lost by reflection from the twosurfaces of the window, as well asabsorption in the window. For exam-ple, about 4% of radiation is reflectedfrom glass surfaces in the infraredranges, so the effective transmittanceis 0.92. The loss through other materi-als can be determined from the indexof refraction of the material at thewavelength of measurement.

The uncertainties concerningemittance can be reduced usingshort wavelength or ratio radiationthermometers. Short wavelengths,around 0.7 microns, are useful

because the signal gain is high in thisregion. The higher response outputat short wavelengths tends toswamp the effects of emittance vari-ations. The high gain of the radiatedenergy also tends to swamp theabsorption effects of steam, dust orwater vapor in the sight path to thetarget. For example, setting thewavelength at such a band will causethe sensor to read within ±5 to ±10degrees of absolute temperaturewhen the material has an emissivityof 0.9 (±0.05). This represents about1% to 2% accuracy.

Types of Radiation ThermometersHistorically, as shown in Figure 3-1, aradiation thermometer consisted ofan optical system to collect theenergy emitted by the target; a

detector to convert this energy to anelectrical signal; an emittivity adjust-ment to match the thermometer cal-ibration to the specific emittingcharacteristics of the target, and anambient temperature compensationcircuit, to ensure that temperaturevariations inside the thermometerdue to ambient conditions did notaffect accuracy.

The modern radiation thermome-ter is still based on this concept.However the technology hasbecome more sophisticated towiden the scope of the applicationsthat can be handled. For example,the number of available detectorshas greatly increased, and, thanks toselective filtering capabilities, thesedetectors can more efficiently bematched to specific applications,improving measurement perfor-mance. Microprocessor-based elec-tronics can use complex algorithmsto provide real time linearization andcompensation of the detector out-put for higher precision of measuredtarget temperature. Microprocessorscan display instantaneous measure-ments of several variables (such ascurrent temperature, minimum tem-perature measured, maximum tem-perature measured, average tempera-ture or temperature differences) onintegral LCD display screens.

A convenient classification of radi-ation thermometers is as follows:• Broadband radiation ther-mometers/pyrometers;• Narrow band radiation ther-mometers/pyrometers;• Ratio radiation thermometers/pyrometers;• Optical pyrometers; and• Fiber optic radiation ther-mometers/pyrometers.

These classifications are not rigid.For example, optical pyrometers can

IR Thermometers & Pyrometers 3


Inte

nsit

y

Wavelength

Figure 3-3: Blackbody Radiation in the Infrared

5µm

10µm

200F

400F

600F

800F

1200F

Ultra violet

Visible Light

Infrared Radio Waves

1nm .77µ 1mm 1m 1km

Blue

RedX-

Rays

Wavelength

be considered a subset of narrowband devices. Fiber optic radiationthermometers, to be discussed indetail in another section, can beclassified as wide band, narrowband, or ratio devices. Likewise,infrared radiation thermometers canbe considered subsets of several ofthese classes.

• Broadband RadiationBroadband radiation thermometerstypically are the simplest devices,cost the least, and can have aresponse from 0.3 microns wave-length to an upper limit of 2.5 to 20microns. The low and high cut-offsof the broadband thermometer are afunction of the specific optical sys-tem being used. They are termedbroadband because they measure asignificant fraction of the thermalradiation emitted by the object, in thetemperature ranges of normal use.

Broadband thermometers aredependent on the total emittance ofthe surface being measured. Figure 3-2 shows the error in reading for vari-ous emissivities and temperatureswhen a broadband device is calibrat-ed for a blackbody. An emissivitycontrol allows the user to compen-sate for these errors, so long as theemittance does not change.

The path to the target must beunobstructed. Water vapor, dust,smoke, steam and radiation absorp-tive gases present in the atmospherecan attenuate emitted radiation fromthe target and cause the thermome-ter to read low.

The optical system must be keptclean, and the sighting window pro-tected against any corrosives in theenvironment.

Standard ranges include 32 to1832°F (0 to 1000°C), and 932 to 1652°F

(500 to 900°C). Typical accuracy is0.5 to 1% full scale.

• Narrow Band RadiationAs the name indicates, narrow bandradiation thermometers operate overa narrow range of wavelengths.Narrow band devices can also bereferred to as single color ther-

mometers/pyrometers (see OpticalPyrometers). The specific detectorused determines the spectralresponse of the particular device.For example, a thermometer using asilicon cell detector will have aresponse that peaks at approximate-ly 0.9 microns, with the upper limitof usefulness being about 1.1microns. Such a device is useful formeasuring temperatures above1102°F (600°C). Narrow band ther-mometers routinely have a spectralresponse of less than 1 micron.

Narrow band thermometers usefilters to restrict response to aselected wavelength. Probably themost important advance in radiationthermometry has been the introduc-tion of selective filtering of theincoming radiation, which allows aninstrument to be matched to a par-ticular application to achieve highermeasurement accuracy. This was

made possible by the availability ofmore sensitive detectors andadvances in signal amplifiers.

Common examples of selectivespectral responses are 8 to 14microns, which avoids interferencefrom atmospheric moisture over longpaths; 7.9 microns, used for the mea-surement of some thin film plastics; 5microns, used for the measurementof glass surfaces; and 3.86 microns,which avoids interference from car-bon dioxide and water vapor inflames and combustion gases.



Figure 3-4: The 'Two-Color' IR Thermometer

5

4

3

2

1

Rela

tive

Ener

gy R

adia

ted,

H

Wavelength, λ.4 .8 1.2 1.6 Microns

λ 1 λ2

T1

T2

T3

T4

T5

Ratio H1/H2 changes as a function of temperature (T1...T5)

The choice of shorter or longerwavelength response is also dictatedby the temperature range. The peaksof radiation intensity curves move

towards shorter wavelengths as tem-perature increases, as shown in Figure3-3. Applications that don’t involvesuch considerations may still bene-fit from a narrow spectral responsearound 0.7 microns. While emissivi-ty doesn’t vary as much as youdecrease the wavelength, the ther-mometer will lose sensitivitybecause of the reduced energyavailable.

Narrow band thermometers withshort wavelengths are used to measurehigh temperatures, greater than 932°F(500°C), because radiation energy con-tent increases as wavelengths getshorter. Long wavelengths are used forlow temperatures -50°F (-45.5°C).

Narrow band thermometers rangefrom simple hand-held devices, tosophisticated portables with simul-taneous viewing of target and tem-perature, memory and printout capa-bility, to on-line, fixed mounted sen-sors with remote electronics havingPID control.

Standard temperature ranges varyfrom one manufacturer to the next,but some examples include: -36 to1112°F (-37.78 to 600°C), 32 to 1832°F (0

to 1000°C), 1112 to 5432°F (600 to3000°C) and 932 to 3632°F (500 to2000°C). Typical accuracy is 0.25% to2% of full scale.

• Ratio RadiationAlso called two-color radiation ther-mometers, these devices measurethe radiated energy of an objectbetween two narrow wavelengthbands, and calculates the ratio of thetwo energies, which is a function ofthe temperature of the object.Originally, these were called twocolor pyrometers, because the twowavelengths corresponded to differ-ent colors in the visible spectrum(for example, red and green). Manypeople still use the term two-colorpyrometers today, broadening the

term to include wavelengths in theinfrared. The temperature measure-ment is dependent only on the ratioof the two energies measured, andnot their absolute values as shown inFigure 3-4. Any parameter, such astarget size, which affects the amountof energy in each band by an equalpercentage, has no effect on thetemperature indication. This makes aratio thermometer inherently moreaccurate. (However, some accuracy is lost when you’re measuring smalldifferences in large signals). The ratiotechnique may eliminate, or reduce,errors in temperature measurementcaused by changes in emissivity, sur-face finish, and energy absorbingmaterials, such as water vapor,between the thermometer and thetarget. These dynamic changes mustbe seen identically by the detector atthe two wavelengths being used.

Emissivity of all materials doesnot change equally at differentwavelengths. Materials for whichemissivity does change equally atdifferent wavelengths are calledgray bodies. Materials for which thisis not true are called non-gray bod-ies. In addition, not all forms ofsight path obstruction attenuate theratio wavelengths equally. For exam-ple, if there are particles in the sightpath that have the same size as one



Figure 3-5: Beam-Splitting in the Ratio IR Thermometer

OutputRatio

Beam Splitter

Colimator

Target

λ1

λ2

Figure 3-6: Ratio Pyrometry Via a Filter Wheel

Viewing MicroscopeTemperature Controlled

Cavity

Objective Lens

Target Mirror And First Field Stop

Lens

Aperture Stop Second Field Stop

Lens

Sensor

Rotating Filter Wheel

Eye

of the wavelengths, the ratio canbecome unbalanced.

Phenomena which are non-dynamic in nature, such as the non-gray bodiness of materials, can bedealt with by biasing the ratio of thewavelengths accordingly. This adjust-ment is called slope. The appropriateslope setting must be determinedexperimentally.

Figure 3-5 shows a schematic dia-gram of a simple ratio radiation ther-mometer. Figure 3-6 shows a ratiothermometer where the wavelengthsare alternately selected by a rotatingfilter wheel.

Some ratio thermometers usemore than two wavelengths. A multi-wavelength device is schematicallyrepresented in Figure 3-7. Thesedevices employ a detailed analysis ofthe target’s surface characteristicsregarding emissivity with regard towavelength, temperature, and sur-face chemistry. With such data, acomputer can use complex algo-rithms to relate and compensate foremissivity changes at various condi-tions. The system described in Figure3-7 makes parallel measurement pos-sible in four spectral channels in therange from 1 to 25 microns. Thedetector in this device consists of anoptical system with a beam splitter,and interference filters for the spec-

tral dispersion of the incident radia-tion. This uncooled thermometerwas developed for gas analysis.Another experimental system, usingseven different wavelengths demon-strated a resolution of ±1°C measur-ing a blackbody source in the rangefrom 600 to 900°C. The same systemdemonstrated a resolution of ±4° Cmeasuring an object with varyingemittance over the temperaturerange from 500 to 950°C.

Two color or multi-wavelengththermometers should be seriouslyconsidered for applications whereaccuracy, and not just repeatability, is

critical, or if the target object is under-going a physical or chemical change.

Ratio thermometers cover widetemperature ranges. Typical commer-cially available ranges are 1652 to 5432° F (900 to 3000°C) and 120 to6692°F (50 to 3700°C). Typical accura-cy is 0.5% of reading on narrowspans, to 2% of full scale.

• Optical PyrometersOptical pyrometers measure theradiation from the target in a narrowband of wavelengths of the thermalspectrum. The oldest devices use theprinciple of optical brightness in thevisible red spectrum around 0.65microns. These instruments are alsocalled single color pyrometers.Optical pyrometers are now avail-able for measuring energy wave-lengths that extend into the infraredregion. The term single color pyrom-eters has been broadened by someauthors to include narrow band radi-ation thermometers as well.

Some optical designs are manual-ly operated as shown in Figure 3-8.



Aperture

Incident Radiation

Infrared Window

Beam Splitter

Filters

Responsive Elements

Preamplifiers

Out

puts

Figure 3-7: Schematic of a Multispectral IR Thermometer

Typical configuration of an industrial infrared temperature probe.

The operator sights the pyrometeron target. At the same time he/shecan see the image of an internallamp filament in the eyepiece. Inone design, the operator adjusts thepower to the filament, changing itscolor, until it matches the color ofthe target. The temperature of thetarget is measured based uponpower being used by the internal fil-ament. Another design maintains aconstant current to the filamentand changes the brightness of thetarget by means of a rotatable ener-gy-absorbing optical wedge. Theobject temperature is related to theamount of energy absorbed by thewedge, which is a function of itsannular position.

Automatic optical pyrometers,sensitized to measure in the infraredregion, also are available. Theseinstruments use an electrical radia-tion detector, rather than the

human eye. This device operates bycomparing the amount of radiationemitted by the target with thatemitted by an internally controlledreference source. The instrumentoutput is proportional to the differ-ence in radiation between the targetand the reference. A chopper, drivenby a motor, is used to alternatelyexpose the detector to incomingradiation and reference radiation. Insome models, the human eye is usedto adjust the focus. Figure 3-9 is aschematic of an automatic opticalpyrometer with a dichroic mirror.Radiant energy passes through thelens into the mirror, which reflectsinfrared radiation to the detector,but allows visible light to passthrough to an adjustable eyepiece.The calibrate flap is solenoid-oper-ated from the amplifier, and whenactuated, cuts off the radiationcoming through the lens, and focus-

es the calibrate lamp on to thedetector. The instrument may have awide or narrow field of view. All thecomponents can be packaged into agun-shaped, hand-held instrument.Activating the trigger energizes thereference standard and read-outindicator.

Optical pyrometers have typicalaccuracy in the 1% to 2% of fullscale range.

• Fiber Optic RadiationAlthough not strictly a class untothemselves, these devices use a lightguide, such as a flexible transparentfiber, to direct radiation to thedetector, and are covered in moredetail in the chapter beginning on p.43. The spectral response of thesefibers extends to about 2 microns,and can be useful in measuringobject temperatures to as low as210°F (100°C). Obviously, thesedevices are particularly useful whenit is difficult or impossible to obtaina clear sighting path to the target, asin a pressure chamber.

Design and ConstructionThe manufacturer of the radiationthermometer selects the detectorand optical elements to yield theoptimum compromise based uponthe conflicting parameters of cost,accuracy, speed of response, andusable temperature range. The usershould be cognizant of how the dif-ferent detectors and optical ele-ments affect the range of wave-lengths over which a thermometerresponds. The spectral response of apyrometer will determine whether ausable measurement is possible,given the presence of atmosphericabsorption, or reflections from



Figure 3-8: Optical Pyrometry By Visual Comparison

EyepieceRed Filter

Calibrated Tungsten Lamp

Lens

Lens

Target

Target

Battery

Battery

Ammeter Is Calibrated In Units Of Temperature

Slide Wire

Constant Current

Flow

Eyepiece

Red Filter Standard Lamp

Sliding Gate

Sliding Gate Opening Is

Calibrated in Unit Of Temperature

other objects, or trying to measurethe temperature of materials likeglass or plastics.

• Detectors Thermal, photon, and pyroelectricdetectors are typically used in radi-ation pyrometers. Radiation detec-tors are strongly affected by ambi-ent temperature changes. Highaccuracy requires compensation forthis ambient drift.

The responsivity of a radiationdetector may be specified in termsof either the intensity of radiation, orthe total radiant power incidentupon the detector.

When the image formed by thetarget surface area is larger than theexposed area of the detector, theentire detector surface is subjectedto a radiation intensity proportionalto the brightness of the target. Thetotal radiant power absorbed by thedetector then depends on the areaof its sensitive surface. The actualsize of the effective target area isdetermined by the magnification of

the optical system. Sensitivity typi-cally is not uniform over the surfaceof a detector, but this has no effectif the target brightness is uniform. Ifsubstantial temperature differencesoccur on the target surface withinthe patch imaged on the detector,an ambiguously weighted averagewill result.

In the case of total radiant power,the area of the target surfaceimaged on the detector is limited bya stop optically conjugate to thedetector. This area can be madearbitrarily small. As a result, localtemperatures can be measured onthe target body surface. The respon-sivity of the detector may dependon the location of this target sourceimage on the detector surface.Constancy of calibration willdepend on maintaining the elementin a fixed position with respect tothe optical system.

Thermal detectors are the mostcommonly used radiation ther-mometer detectors. Thermal detec-tors generate an output because

they are heated by the energy theyabsorb. These detectors have lowersensitivity compared to otherdetector types, and their outputsare less affected by changes in theradiated wavelengths. The speed ofresponse of thermal detectors islimited by their mass.

Thermal detectors are blackenedso that they will respond to radiationover a wide spectrum (broadbanddetectors). They are relatively slow,because they must reach thermalequilibrium whenever the targettemperature changes. They can havetime constants of a second or more,although deposited detectorsrespond much faster.

A thermopile consists of one ormore thermocouples in series, usual-ly arranged in a radial pattern so thehot junctions form a small circle,and the cold junctions are main-tained at the local ambient temper-ature. Advanced thin film ther-mopiles achieve response times inthe 10 to 15 millisecond range.Thermopiles also increase the out-put signal strength and are the bestchoice for broadband thermome-ters. Ambient temperature compen-sation is required when thermopiledetectors are used. A thermostati-cally controlled thermometer hous-ing is used to avoid ambient temper-ature fluctuations for low tempera-ture work. Self-powered infraredthermocouples are covered in thechapter beginning on p. 38.

Bolometers are essentially resis-tance thermometers arranged forresponse to radiation. A sensing ele-ment with a thermistor, metal film,or metal wire transducer is oftencalled a bolometer.

Photon detectors release electriccharges in response to incident radi-ation. In lead sulfide and lead



Figure 3-9: An Automatic Optical Pyrometer

Adjustable EyepieceCircle ReticleDichroic

MirrorLens

Calibration Lamp

Optical Chopper

Filter

Detector

Amplifier

Calibrate Flag

Eye

Sync. Motor

selenide detectors, the release ofcharge is measured as a change inresistance. In silicon, germanium, andindium antimonide, the release ofcharge is measured as a voltage out-put. Photon detectors have a maxi-mum wavelength beyond which theywill not respond. The peak responseis usually at a wavelength a littleshorter than the cutoff wavelength.Many radiation thermometers usephoton detectors rather than ther-mal detectors, even though theymeasure over a narrower band ofwavelength. This is because withinthe range of useful wavelengths, thephoton detectors have a sensitivity1000 to 100,000 times that of thethermal detector. Response time ofthese detectors is in microseconds.They are instable at longer wave-lengths and higher temperatures.They are often used in narrow bandthermometers, or broadband ther-mometers at medium temperatures(200 to 800°F/93 to 427°C), and oftenprovided with cooling.

Pyroelectric detectors change sur-face charge in response to receivedradiation. The detector need notreach thermal equilibrium when thetarget temperature changes, since itresponds to changes in incomingradiation. The incoming radiationmust be chopped, and the detectoroutput cannot be used directly. Achopper is a rotating or oscillatingshutter employed to provide ACrather than DC output from the sen-sor. Relatively weak AC signals aremore conveniently handled by con-ditioning circuitry. The detectorchange can be likened to a changein charge of a capacitor, which mustbe read with a high impedance cir-cuit. Pyroelectric detectors haveradiation absorbent coatings so theycan be broadband detectors.

Response can be restricted byselecting the coating material withappropriate characteristics.

Photon and pyroelectric detectorshave thermal drift that can be over-come by temperature compensation(thermistor) circuitry, temperatureregulation, auto null circuitry, chop-

ping, and isothermal protection. Figure 3-10 shows the different

sensitivity for various radiationdetectors. PbS has the greatest sensi-tivity, and the thermopile the least.

• Optical SystemsAs shown in Figure 3-11, the opticalsystem of a radiation pyrometer maybe composed of lenses, mirrors, orcombinations of both. Mirror sys-tems do not generally determine thespectral response of the instrument,as the reflectivity is not dependenton wavelength over the range usedfor industrial temperature measure-

ment. A mirror system must be pro-tected from dirt and damage by awindow. Copper, silver and gold arethe best materials for mirrors in theinfrared range. Silver and copper sur-faces should be protected againsttarnish by a protective film.

The characteristics of the window

material will affect the band ofwavelengths over which the ther-mometer will respond. Glass doesnot transmit well beyond 2.5microns, and is suited only for highertemperatures. Quartz (fused silica)transmits to 4 microns, crystallinecalcium fluoride to 10 microns, ger-manium and zinc sulfide can transmitinto the 8 to 14 micron range. Moreexpensive materials will increase thetransmission capability even more, asshown in Figure 3-12.

Windows and filters, placed infront of or behind the optical sys-tem, and which are opaque outside agiven wavelength range, can alter the



Figure 3-10: Relative Sensitivity of IR Detectors

106

105

104

103

102

101

1

Rela

tive

Sen

siti

vity

Wavelength, µm

.1 .2 .3 .5 .7 1 2 3 5 7 10 20

Chopped Unchopped

Metal Thermopile

Thin Film Thermopile

Thermistor Bolometer

Pyro Electric Detector

InSb

InAsSi

PbS

Ge

transmission properties greatly, andprevent unwanted wavelengths fromreaching the detector.

Mirror systems are generally usedin fixed focus optical instruments.Varying the focus of the instrumentrequires moving parts, which is lesscomplicated in a lens system. Theselection of lens and window mate-rial is a compromise between theoptical and physical properties ofthe material, and the desired wave-length response of the instrument.The essential design characteristicsof materials suitable for lenses,prisms, and windows includeapproximate reflection loss, andshort and long wavelength cut-offs.

Figure 3-13 shows the transmittanceof some common materials as afunction of wavelength. Chemicaland physical properties may dictatechoice of material to meet givenoperating conditions.

The aberrations present in a singlelens system may not permit preciseimage formation on the detector. Acorrected lens, comprised of two ormore elements of different material,may be required.

The physical shape of the opticalsystem, and its mounting in the hous-ing, controls the sighting path. Formany designs, the optical system isaligned to surface and measures sur-face temperature. This is satisfactory

for sizable targets. Visual aimingaccessories may be required forsighting very small targets, or forsighting distant targets. A variety ofaiming techniques are availablewhich include: simple bead andgroove gun sights, integrated ordetachable optical viewing finders,through-lens sighting, and integratedor detachable light beam markers.

• Field of ViewThe field of view of a radiation ther-mometer essentially defines the sizeof the target at a specified distancefrom the instrument. Field of viewcan be stated in the form of a dia-gram (Figure 3-14), a table of targetsizes versus distance, as the targetsize at the focal distance, or as anangular field of view.

Figure 3-15 shows typical wideangle and narrow angle fields of view.With a wide angle field of view, tar-get size requirements neck down to aminimum at the focal distance. Thenarrow angle field of view flares outmore slowly. In either case, cross sec-tional area can vary from circular, torectangular, to slit shaped, depend-ing on the apertures used in the ther-mometer optics system.

Telescopic eyepieces on somedesigns can magnify the radiantenergy so smaller targets can beviewed at greater distances. Targetsas small as 1/16 inch in diameter aremeasurable using the correct ther-mometer design. A common opticssystem will produce a 1-inch diame-ter target size at a 15-inch workingdistance. Other optical systems varyfrom small spot (0.030 inch) forclose up, pinpoint measurement, todistant optics that create a 3-inchdiameter target size at 30 feet. Theangle of viewing also affects the tar-



Figure 3-11: Typical Optical Systems

Objective Mirror

Detector

Detector

Eyepiece LensObjective Lens

Eyepiece Lens

Beam Splitting Prism

Movable Objective Lens For Variable Focus

Prism or Mirror

Eyepiece Lens

Detector

Reticule, Usually In Fixed Position

Eyepiece Lens

Eyepiece Lens

Half Silvered MIrror Reflects Infrared, Passes Visible Energy

Mirror

ReticuleSmall Opaque Mirror

Detector

Detector

Reticule

get size and shape.In calibrating a radiation ther-

mometer, the radiation source mustcompletely fill the field of view inorder to check the calibration out-put. If the field of view is not filled,the thermometer will read low. If athermometer does not have a welldefined field of view, the output ofthe instrument will increase if theobject of measurement is larger thanthe minimum size.

The image of the field stop at thefocal distance for most thermometersis larger than the diameter of the fieldstop. Between the focal distance, thefield of view is determined by the lensdiameter and the image diameter.Lines drawn from the image, at thefocal distance, to the lens diameterenclose the field of view. Beyond thefocal distance, the field of view isdetermined by rays extending fromthe extremities of the lens diameterthrough the extremities of the imageat the focal distance.

In practice, any statement of fieldof view is only an approximationbecause of spherical and chromaticaberration. Spherical aberration iscaused by the fact that rays hitting thelens remote from its axis are bentmore than rays passing the lens near

its axis. A circular field stop is imagedas a circle with a halo around it.Mirrors also have spherical aberration.

Chromatic aberration occursbecause the refractive index of opti-cal materials changes with wave-length, with the refractive indexlower at shorter wavelengths. Thismeans rays of shorter wavelength arebent more and focus nearer the lens,while rays of longer wavelength arefocused farther from the lens. Theimage of a field stop over a band ofwavelengths is hence a fuzzy image.

Fuzziness of the field of view canalso be caused by imperfections inthe optical material, and reflectionsfrom internal parts of the ther-mometer. Quality materials, andblackening of inside surfaces reducethese latter effects.

Some manufacturers state a fieldof view that includes effects of aber-rations, and some do not. If the tar-get size and stated field of view arenearly the same, it may be wise todetermine the field of view experi-mentally. Sight the thermometer ona target that gives a steady, uniformsource of radiation. At the focal dis-tance, interpose a series of aperturesof different diameter. Plot the ther-mometer output versus the aperture

area. The output of the thermometershould increase proportional to theaperture area for aperture areas lessthan the nominal target area. Theoutput should increase only mini-mally for increasing areas, above thenominal target area. Increases of afew tenths of a percent in output foreach doubling of the aperture areaindicates the nominal field of viewtakes into account the effects ofaberrations. If these are not takeninto account, the thermometer out-put may show significant increases inoutput as the viewable target area isincreased above the nominal value.

• ElectronicsThe calibration curves of detectoroutput versus temperature of alldetectors is non-linear because theequations relating the amount ofradiation emitted by an object arepower functions. The radiation ther-mometer electronics must amplify,regulate, linearize and convert thissignal to an mV or mA output pro-portional to temperature.

Before microprocessors, theadvantage of high N values was off-set by the fact that the useful rangeof temperature measurement with an



Figure 3-12: IR Transmission of Optical Materials

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

100

90

80

70

60

50

40

30

20

10

0

Tran

smit

tanc

e, %

Wavelength, µm18 19 20

Quartz

All can be AR coated except BaF2, MgF2, KBr

Zinc Sulfide Potassium Bromide

Thickness 1mm 300K

77K

Magnesium

Fluoride

Barium Fluoride

Sapphire

instrument of fixed span was verylow. For example, for N=15, an instru-ment reading 100% of scale at 1000°C would read approximately 820°C at10% of full scale. If the target tem-perature were expected to fall out-side this narrow band, linearizationor range switching was necessary.Today, microprocessors easily permitsuch signals to be linearized verycost effectively.

Microprocessor-based electronics(Figure 3-16) are superior to conven-tional analog electronics because insitu computing can be used to cor-rect detector imperfections, provideemissivity compensation, and pro-vide digital outputs for two waycommunications between the ther-mometer and a PC or a control sys-tem workstation.

Many of the shortcomings of ther-mal type detectors can be handledby sophisticated data processingtechniques available in digital com-puters. The target temperature is anexponential function of the detectortemperature. The output signal fromthe detector is a small voltage pro-portional to the difference in tem-perature between the target and thedetector itself. To get the target tem-

perature, it is necessary to accuratelymeasure the detector temperature.Detector body temperatures spanthe range of the environment, from-50 to 100°C. Over this range, themost precise and accurate tempera-ture transducer is the thermistor.However, thermistor outputs arehighly non-linear and vary widelyfrom unit to unit. Analog devices mustabandon use of the thermistor for aless accurate and easier to use ele-ment, such as an integrated circuit,which has a linear output. But highlynon-linear responses are no problemfor a computer, and units with micro-processors can employ thermistors.

Detector responsivity is also anon-linear function of the detectorbody temperature. It is typicallygrossly corrected in analog deviceswith a simple linear gain correctionproduced by a temperature sensitiveresistor in the preamplifier feedbacknetwork. A microprocessor can use acomplex algorithm for the detectionbody temperature to correct forchanges in detector responsivity.

The net radiant target signalpower impinging on the detector ishighly non-linear with the targettemperature, and for temperatures

under 1000°F, it is also dependent onthe detector temperature itself.Again, a microprocessor can makeaccurate compensation for boththese effects.

There is a fourth power relation-ship between the detector outputvoltage and target temperature.Analog devices typically use linearapproximation techniques to charac-terize this relationship. A computercan solve, in real time, a complexalgorithm, with as many as seventerms, instead of linear approxima-tion, for higher accuracy.

Detector zero drift due to ambienttemperature conditions can also becorrected using a microprocessor.This avoids errors of several degrees



Figure 3-13: IR Transmission Characteristics

2 4 6 8 10 12 14 16 18 20 22 24

100

90

80

70

60

50

40

30

20

10

0

Tran

smit

tanc

e, %

Wavelength, µm

Thickness 1mm 300K

77K

KRS-5

Silicon

Zinc Selenide Germanium

Figure 3-14: Field of View

Focal Distance

Lens Diameter

Field Stop

Detector

Minimum Target Diameters

Angular Field of View

when you move an instrument fromone room to another having a differ-ent temperature.

Precise emissivity corrections canbe called up, either from as many as 10values stored in EEPROM, or from acomplex real-time algorithm depen-dent on target time-temperature rela-tionships. An example is a program tocompensate for the emissivity of apiece of steel, which oxidizes as itheats to higher temperatures.

Preprocessing by an onboardmicroprocessor may allow extrac-tion of only the pertinent dataneeded by control systems. Forexample, only out of range data,determined by setpoints pro-grammed into the microprocessor,may be desired for data transmis-sion. This data can be transmitteddigitally, on a priority interruptbasis. This is more efficient thanhaving the user transmit all mea-sured data to the host system, onlyto have the pertinent informationsorted there.

An intelligent radiation ther-

mometer can be programmed to runpreprogrammed internal calibrationprocedures during gaps, or windowsin measurement activity. This pre-vents internal calibration checksfrom taking the device off-line at acritical moment in the process. Athermometer reading the tempera-ture of cans on a conveyor belt canrun an internal calibration programwhenever a gap between successivecans is sensed.

An internal microprocessor canalso perform external control func-tions on external loop elements, using

contact closure or relay outputs pro-vided as options, and based on theincoming temperature data. In addi-tion, intelligent devices can acceptauxiliary inputs from thermocouples,RTDs or other radiation thermome-ters, and then use this data to supportinternal functions. For example, a hightemperature setpoint could be con-tinuously, and automatically reset bythe microprocessor in response toinput variable history.

A sample-and-hold function isuseful when a selected event servesto trigger the temperature measure-ment of an object. The thermometermeasures temperature at that instant,disregarding earlier or later measure-ments. Analog circuitry exhibited aslow drift of the measurement duringthe hold period, but modern digitalinstruments hold the value withoutdegradation for indefinite periods.

Sometimes, the highest tempera-ture within the field of view is ofinterest during a given period.Intelligent electronics can be pro-grammed to store into memory thehighest temperature it saw in a sam-pling period. This is called peak pick-ing. Valley picking, when the lowesttemperature measured over a givenperiod is of interest, also is possible.

Averaging is used to prevent rapidexcursions of the object temperature



Figure 3-15: Typical Narrow and Wide Angle Sighting Paths

2436

412 Dia.

Target

1810

6

234 Dia.

Target13

4 Dia. Target

12 Dia.

Target

78 Dia.

Target13

8 Dia. Lens

Sighting Path of the Wide-Angle Total Radiation Pyrometer

Sighting Path of the Narrow-Angle Total Radiation Pyrometer

2436

138 17

8134Dia. Lens Dia. TargetDia. Target

Optical Assembly

Optical Chopper

IR Sensor

ADCAnalog MUX

+9 VDC

Micro- computer

Digital Output

Reference Thermistor

Low Noise Reamp

Figure 3-16: Microprocessor-Based IR Thermometer

from the average value from causingnoise in the control system. A com-mon way to accomplish this is toslow down the response of theinstrument via software in the micro-processor-based electronics.

• ConstructionFigure 3-11, p. 33, illustrates the com-mon types of construction found inindustrial radiation thermometers.The constructions in (a) and (b) aretypical of instruments using detec-tors that give a stable DC millivoltoutput without preamplificationsuch as thermopiles and silicon cells.The construction in (a) has also beenused for detectors whose DC driftdemands that they be used in an ACmode. A spinning disk or vibratingreed is interposed between the lensand the detector to cyclically inter-rupt the radiation. Thus the detectorsees pulses of radiation. The outputof the detector is AC. The detectorpackage must be small enough sothat it doesn’t interfere with opticalsighting to the target.

The constructions in (c), (d), and (e)are useful when the detector pack-age is too large to permit sightingaround it. Optical chopping betweenthe lens and the detector is commonin these constructions. The back sur-face of the chopping disk, or blade,may serve as a local ambient tem-perature reference. The detectoralternately sees the target and themodulating device, which is at localambient temperature.

In some designs, a local hotsource, or hot surface, may be main-tained at a known reference temper-ature. The detector alternately sees

the target and the reference source.The resulting AC signal can then becalibrated in terms of the unknowntarget temperature.

In ratio thermometers, the filtersthat define the pass band of the tworadiation signals that are ratioed maybe on the chopping disc.

Figure 3-17 illustrates a portableradiation thermometer for spot mea-

surement of the temperature of asurface. Radiation from the target ismultiply reflected from the hemi-spherical mirror shown. A detectorreceives this radiation through asmall opening in the reflector. Theradiation multiply reflectedbetween the mirror and targetappears to the detector to be from ablackbody. A commercial pyrometerusing this technique can read thetemperature of targets with emissivi-ty as low as 0.6 without correction.The reflector must be placed closeto the surface being measured toeliminate extraneous radiation andprevent losses. It can only be usedfor short time durations becauseheating of the reflector will affectthe measurement accuracy. In addi-tion, the energy reflected back to thetarget surface may cause its temper-ature to change. T



Figure 3-17: Surface Temperature Pyrometer

Detector

Gold-Plated Hemispherical Reflector Target

References and Further Reading• Handbook of Temperature Measurement & Control, Omega Press, 1997.• New Horizons in Temperature Measurement & Control, Omega Press,1996.• “Evolution in the Application of Optical Fiber Thermometry”, F.G. BearTinsley, Bruce Adams, Proceedings of the International Conference andExhibitions, Instrument Society of America, 1991.• Infrared Temperature Measurement, MIT Video Series, R. John Hansman,Jr., Massachusetts Institute of Technology.• “Progress on the Development of Multi-wavelength Imaging Pyrometer,”Michael B, Kaplinsky, Jun Li, Nathaniel J. McCaffrey, Edwin S. H. Hou andWalter F. Kosonecky, SPIE Proceedings, 1996.• Temperature Measurement in Industry, E. C. Magison, Instrument Societyof America, 1990.• “Uncooled Multispectral Detectors and their Applications”, VolkmanNorkus, Gunter Hofman and Christine Schiewe, SPIE Proceedings, 1966.

As described in chapter 3 on “IRThermometers & Pyrometers”,thermocouples have beenused as detectors in radia-

tion thermometry for many decades.Often, a series of thermocouples, orthermopile, was the thermal detec-tor of choice. But in more recentyears a new class of low-cost, self-powered “infrared thermocouples”has been developed, and hasopened up a broad market for non-contact temperature measurementin such industries as food, electron-ics, paper, pharmaceutical, plastics,rubber, and textiles.

All infrared thermocouple sensorswork in a fashion similar to a stan-dard thermocouple: a small millivolt-age or electromotive force (emf)relates to the temperature beingmeasured. To correctly apply anysuch instrument, the user or designermust be aware of certain basic char-acteristics of all thermocouples andthe circuitry involved. Just how doesthe thermocouple function in pro-viding a usable emf measuring signal?And what is important to observe sofar as metering that signal to accu-rately indicate the measured temper-ature? What is the effect of changesin ambient temperature—at the ther-mocouple and at the meter? A dis-cussion with reference to Figure 4-1will help make such points clear.

Thermocouple BasicsLet’s start with T. J. Seebeck, who in1821 discovered what is now termedthe thermoelectric effect. He notedthat when two lengths of dissimilar

metal wires (such as iron andConstantan) are connected at bothends to form a complete electriccircuit, an emf is developed whenone junction of the two wires is at adifferent temperature than theother junction.

Basically, the developed emf (actu-ally a small millivoltage) is dependentupon two conditions: (1) the differ-ence in temperature between the hotjunction and the cold junction. Notethat any change in either junctiontemperature can affect the emf valueand (2) the metallurgical compositionof the two wires.

Although a “thermocouple” isoften pictured as two wires joinedat one end, with the other ends notconnected, it is important toremember that it is not a true ther-mocouple unless the other end isalso connected! It is well for theuser to remember this axiom:

“Where there is a hot junction thereis always a cold or reference junc-tion” (even though it may seem hid-den inside an instrument 1,000 feetaway from the hot junction).

Still in Seebeck’s century, twoother scientists delved deeper intohow the emf is developed in a ther-moelectric circuit. Attached to theirnames are two phenomena theyobserved—the Peltier effect (for JeanPeltier in 1834) and the Thompsoneffect (for Sir William Thompson a.k.aLord Kelvin in 1851). Without gettinginto the theories involved, we canstate that the Peltier effect is the emfresulting solely from the contact ofthe two dissimilar wires. Its magni-tude varies with the temperature atthe juncture. Similarly, the Thompsoneffect can be summarized as havingto do with emf’s produced by a tem-perature gradient along a metal con-ductor. Since there are two points of


Thermocouple Basics

Self-Powered Infrared Thermocouples

Installation Guidelines

NON-CONTACT TEMPERATURE MEASUREMENTInfrared Thermocouples

4

AInfrared Thermocouples

0 200 400 600 800

8

6

4

2

0

mV

°F

6.68 mV

4 mV

2.68 mV

Figure 4-1: Thermocouple Operation

1 3

2

contact and two different metals oralloys in any thermocouple, there aretwo Peltier and two Thompson emfs.The net emf acting in the circuit is theresult of all the above named effects.

Polarity of the net emf is deter-mined by (a) the particular metal oralloy pair that is used (such as iron-constantan) and (b) the relationship ofthe temperatures at the two junctions.The value of the emf can be measuredby a potentiometer, connected intothe circuit at any point.

In summary, the net emf is a func-tion primarily of the temperature dif-ference between the two junctionsand the kinds of materials used. If thetemperature of the cold junction ismaintained constant, or variations inthat temperature are compensated for,then the net emf is a function of thehot junction temperature.

In most installations, it is not prac-tical to maintain the cold junction ata constant temperature. The usualstandard temperature for the junc-tion (referred to as the “referencejunction”) is 32°F (0°C). This is thebasis for published tables of emf ver-sus temperature for the various typesof thermocouples.

The Law of IntermediateTemperatures provides a means ofrelating the emf generated underordinary conditions to what it shouldbe for the standardized constanttemperature (e.g., 32°F). Referring toFigure 4-1, which shows thermocou-ples 1 and 2 made of the same twodissimilar metals; this diagram willprovide an example of how the lawworks. Thermocouple 1 has its coldjunction at the standard referencetemperature of 32°F and its hot junc-tion at some arbitrary intermediatereference temperature (in this case,300°F). It generates 2.68 mv.Thermocouple 2 has its cold junction

at the intermediate reference pointof 300°F and its hot junction at thetemperature being measured (700°F).It generates 4.00 mv. The Law ofIntermediate Temperatures statesthe sum of the emfs generated bythermocouples 1 and 2 will equal theemf that would be generated by asingle thermocouple (3, shown dot-ted) with its cold junction at 32°F andits hot junction at 700°F, the mea-sured temperature. That is, it wouldhypothetically read 6.68 mv and rep-resent the “true” emf according tothe thermocouple’s emf vs. tempera-ture calibration curve.

Based upon this law, the manufac-turer of an infrared thermocoupleneed only provide some means ofsubstituting for the function of ther-mocouple 1 to provide readings ref-erenced to the standard 32°F coldjunction. Many instruments accom-plish this with a temperature-sensi-

tive resistor which measures the vari-ations in temperature at the coldjunction (usually caused by ambientconditions) and automatically devel-ops the proper voltage correction.

Another use of this law shows thatextension wires having the samethermoelectric characteristics asthose of the thermocouple can beintroduced into the thermocouplecircuit without affecting the net emfof the thermocouple.

In practice, additional metals areusually introduced into the thermo-couple circuit. The measuring instru-ment, for example, may have junc-tures that are soldered or welded.Such metals as copper, manganin, lead,tin, and nickel may be introduced.

Would not additional metals likethis modify the thermocouple’s emf?Not so, according to the Law ofIntermediate Metals. It states thatthe introduction of additional metals



Figure 4-2: Equivalent Thermocouple Circuits

All three circuits

generate same EMF

A

A

A

B

B

B

C

CD

E

F

T1 T2

will have no effect upon the emfgenerated so long as the junctions ofthese metals with the two thermo-couple wires are at the same temper-ature. This effect is illustrated inFigure 4-2, with A and B representingthe thermocouple wires.

A practical example of this law isfound in the basic thermoelectricsystem shown in Figure 4-3. Theinstrument can be located at somedistance from the point of measure-ment where the thermocouple islocated. Several very basic and prac-tical points are illustrated in this ele-mentary circuit diagram:

Quite often the most convenientplace to provide the cold junctioncompensation is in the instrument,remote from the process.

With the compensation meanslocated in the instrument, in effect,the thermoelectric circuit is extend-ed from the thermocouple hot junc-tion to the reference (cold) junctionin the instrument.

The actual thermocouple wiresnormally terminate relatively nearthe hot junction. Conventional cou-ples have what is called a “terminalhead” at which point interconnectingwires, known as “extension wires” arerequired as shown. Since these wiresare in the thermoelectric circuit, theymust essentially match the emf vs.temperature characteristics of thethermocouple.

With the cold junction locatedinside the instrument, internal exten-sion wires of the proper materialsmust be used between the instru-ment terminals and the cold junction.

With this set-up, there are ineffect three added thermocouplesin the circuit: one in the thermocou-ple assembly, one in the externalextension wire, and in the internalextension wire. However, accordingto the Law of IntermediateTemperatures, the actual tempera-tures at the terminal head and at theinstrument terminals is of no conse-

quence: the net effect of the threethermocouples is as if one thermo-couple ran from the hot junction tothe cold junction.

The Infrared ThermocoupleOver the past decade or two, therehas been a mushroom growth in thesmall, application-specific designs ofinfrared thermocouples. These con-tain a sophisticated optical systemand electronic circuitry that belie thesimplicity of their external, tube-likeappearance. They use a special propri-etary design of thermopile whichdevelops enough emf to be connect-ed directly to a conventional thermo-couple type potentiometer or trans-mitter for all types of indication,recording, and control.

A wide variety of these devices arecommercially available, covering tem-perature ranges from -50 to 5000°F (-45 to 2760°C) with up to 0.01°C pre-cision. The range of models includes:

Infrared Thermocouples 4


Hot Junction Terminal Head

Cold Junction

Instrument

Figure 4-3: Typical Thermocouple Installation

Thermocouple

Extension Wire (External)

Extension Wire

(Internal)

• Standard units, simulating the ther-mocouple outputs J, K, T, E, R, and Sand offering 12 different fields ofview from 1:2 to 100:1. Minimum spotsize is 1 mm and focused spot sizesavailable range from 4 mm to 12 mm. • Handheld scanning models forsuch applications as accurate scan-ning of electrical equipment andNIST traceable surface temperaturecalibration—a must for ISO 9001,9002, 9003 programs.• Thermal switches that act like pho-tocells for use in production linequality inspection of thermalprocesses with line speeds of up to1000 feet per minute.

All infrared thermocouples areself-powered, using only the incom-ing infrared radiation to produce anmv output signal through thermo-electric effects. The signal thus fol-lows the rules of radiation thermalphysics and produces a curve asshown in Figure 4-4.

Over a specific, relatively narrowtemperature range, the output is suffi-ciently linear to produce an mv out-put that can be closely matched tothe mv vs. temperature curve of agiven thermocouple type (Figure 4-4).

What’s more, the designer can matchthe two curves to be within a degreeof tolerance such as ±2%, as speci-fied by the buyer.

Each model is specificallydesigned for optimum performancein the region of best linear fit withthe thermocouple’s mv vs. tempera-ture curve. The sensor can be usedoutside that range, however, by sim-ply calibrating the readout deviceappropriately. Once so calibrated,the output signal is smooth andcontinuous over the entire range ofthe thermocouple, and will main-tain a 1% repeatability over theentire range.

The user can select a model toprovide, say, a 2% accuracy, by refer-ring in the supplier’s literature to aRange Chart which provides a verti-cal list of “Range Codes” with a cor-responding Temperature Range overwhich 2% accuracy is to be expected.The user also specifies the type ofthermocouple (J, K, etc).

A typical infrared thermocouplecomprises a solid, hermetically-sealed,fully-potted system. As such, evenduring severe service, it does notchange either mechanically or metal-

lurgically. It contains no active elec-tronic components and no powersource other than the thermocoupleitself. Thus, suppliers rate its long-termrepeatability, conservatively, at 1%.

Long term accuracy is influencedby the same factors that affect relia-bility. In comparison to the applica-tion of conventional thermocouples,the infrared thermocouple is wellprotected inside a rigid, stainlesssteel housing. Along with the solid,fully-potted construction, this designessentially eliminates the classicaldrift problems of conventional ther-mocouples. Double annealing at tem-peratures above 212°F (100°C) helpsensure long term stability.

Installation GuidelinesLike all radiation-based sensing sys-tems, the infrared thermocouplemust be calibrated for specific sur-face properties of the object beingmeasured, including amount of heatradiated from the target surface andenvironmental heat reflections.

The calibration is performed bymeasuring the target surface temper-ature with a reliable independent sur-



Actual IR Thermo- couple Signal

Mill

ivo

lt O

utp

ut

Target Temperature

Conventional Thermocouple

Mill

ivo

lt O

utp

ut

Target Temperature

Linear Region

Figure 4-4: IR Thermocouple Output

face-temperature probe. One suchdevice is a handheld infrared ther-mometer with a built-in automaticemissivity compensation system.

The following procedure is recom-mended:

(1) Install the infrared thermocou-ple as close as practical to view thetarget to be measured.

(2) Connect the infrared thermo-couple to the supervisory controlleror data acquisition system in stan-dard fashion (including shield). Aswith conventional thermocouples,the red wire is always negative.

(3) Bring the process up to normaloperating temperature and use thehand-held radiation thermometer tomeasure the actual target temperature.

(4) Make the proper adjustmentson the readout instrument so that its

calibration matches the reading ofthe handheld device. T

Infrared Thermocouples 4


References and Further Reading• Handbook of Temperature Measurement & Control, Omega Press, 1997.• New Horizons in Temperature Measurement & Control, Omega Press,1996.• The Infrared Temperature Handbook, Omega Engineering, 1994.• Handbook of Non-Contact Temperature Sensors, The IRt/c Book, ThirdEdition, Exergen Corp., 1996.• Instrument Engineers’ Handbook, Third Edition, B. Liptak, Chilton BookCo. (CRC Press), 1995.• Process/Industrial Instruments and Controls Handbook, Fourth Edition,D. M. Considine, ed., McGraw-Hill, 1993.• “Some Basic Concepts of Thermoelectric Pyrometry,” C.C. Roberts andC.A. Vogelsang, Instrumentation, Vol. 4, No.1, 1949.• Temperature Measurement in Engineering, H. Dean Baker, E. A. Ryder, andN. H. Baker, Omega Press, 1975.

Fiber optics are essentially lightpipes, and their basic opera-tion may be traced back morethan a century when British

Physicist John Tyndall demonstratedthat light could be carried within astream of water spouting out andcurving downward from a tank. Athin glass rod for optical transmis-sion was the basis of a 1934 patentawarded to Bell Labs for a “LightPipe.” American Optical demonstrat-ed light transmission through shortlengths of flexible glass fibers in the1950s. However, most modernadvances in fiber optics grew out ofCorning Glass developments in glasstechnology and production methodsdisclosed in the early 1970s.

Like many technical developmentssince WWII, fiber optics programswere largely government funded fortheir potential military advantages.Projects primarily supported telecom-munications applications and laserfiber ring gyroscopes for aircraft/mis-sile navigation. Some sensor develop-ments were included in manufactur-ing technology (Mantech) programs aswell as for aircraft, missile and ship-board robust sensor developments.More recently the Dept. of Energy andNIST have also supported variousfiber optic developments.

Commercial telecommunications

has evolved as the fiber optics tech-nology driving force since the mid-’80s. Increased use of fiber opticswell correlates with fiber materialsdevelopments and lower componentcosts. Advances in glass fibers haveled to transmission improvementsamounting to over three orders ofmagnitude since the early CorningGlass efforts. For example, ordinaryplate glass has a visible light attenua-tion coefficient of several thousanddBs per km. Current fiber optic glass-es a kilometer thick would transmitas much light as say a /” plate glasspane. Table 5-1 indicates relative dig-ital data transmission losses for cop-per and fiber.

Fiber AdvantagesImproved glass transmissions haveresulted in undersea cables withrepeaters required about every 40miles—ten times the distancerequired by copper. Bandwidth androbustness have led to cable serviceproviders selecting fiber optics asthe backbone media for regionalmultimedia consumer services. Theworld market for fiber optic compo-nents was in the $4 billion range in1994 and is projected to reach $8 bil-lion in 1998.

Whether used for communications

or infrared temperature measurement,fiber optics offer some inherentadvantages for measurements inindustrial and/or harsh environments:• Unaffected by electromagneticinterference (EMI) from large motors,transformers, welders and the like;• Unaffected by radio frequencyinterference (RFI) from wireless com-munications and lightning activity;

• Can be positioned in hard-to-reachor view places;• Can be focused to measure smallor precise locations;• Does not or will not carry electri-cal current (ideal for explosive haz-ard locations);• Fiber cables can be run in existingconduit, cable trays or be strapped ontobeams, pipes or conduit (easily installedfor expansions or retrofits); and,• Certain cables can handle ambienttemperatures to over 300°C—higherwith air or water purging.

Any sensing via fiber optic linksrequires that the variable cause a


Fiber Advantages

Fiber Applications

Component Options

NON-CONTACT TEMPERATURE MEASUREMENTFiber Optic Extensions

5

FFiber Optic Extensions

Figure 5-1: Fiber Optic Probe Construction

Optical Fiber Core

Jacket

Cladding

Clear Elastometer

Phosphor in Elastomer

26 gage twisted wire pair

19 gage twisted wire pair

RG 217/u coaxial cable

Optical fiber 0.82 µm wavelength carrier

1.5Mb/s 6.3Mb/s 45Mb/s

24 48 128

10.8 21 56

2.1 4.5 11

3.5 3.5 3.5

Table 5-1: Relative Transmission Losses for Digital Data Losses in dB/km

14

modulation of some type to an opti-cal signal—either to a signal pro-duced by the variable or to a signaloriginating in the sensing device.Basically, the modulation takes theform of changes in radiation intensi-ty, phase, wavelength or polarization.For temperature measurements,intensity modulation is by far themost prevalent method used.

The group of sensors known as fiberoptic thermometers generally refer tothose devices measuring higher tem-peratures wherein blackbody radiationphysics are utilized. Lower tempera-ture targets—say from -100°C to400°C—can be measured by activatingvarious sensing materials such asphosphors, semiconductors or liquidcrystals with fiber optic links offeringthe environmental and remotenessadvantages listed previously.

Fiber ApplicationsFiber optic thermometers haveproven invaluable in measuring tem-peratures in basic metals and glassproductions as well as in the initialhot forming processes for suchmaterials. Boiler burner flames andtube temperatures as well as critical

turbine areas are typical applica-tions in power generation opera-tions. Rolling lines in steel and otherfabricated metal plants also poseharsh conditions which are wellhandled by fiber optics.

Typical applications include fur-naces of all sorts, sintering opera-tions, ovens and kilns. Automatedwelding, brazing and annealingequipment often generate large elec-trical fields which can disturb con-ventional sensors.

High temperature processingoperations in cement, refractory andchemical industries often use fiberoptic temperature sensing. At some-what lesser temperatures, plasticsprocessing, paper making and foodprocessing operations are makingmore use of the technology. Fiberoptics are also used in fusion, sput-tering, and crystal growth processesin the semiconductor industry.

Beyond direct radiant energy col-lection or two-color methods, fiberoptic glasses can be doped to servedirectly as radiation emitters at hotspots so that the fiber optics serveas both the sensor and the media.Westinghouse has developed suchan approach for distributed temper-

ature monitoring in nuclear reactors.A similar approach can be used forfire detection around turbines or jetengines. Internal “hot spot” reflect-ing circuitry has been incorporatedto determine the location of thehot area.

An activated temperature measur-ing system involves a sensing headcontaining a luminescing phosphorattached at the tip of an optical fiber(Figure 5-1). A pulsed light sourcefrom the instrument package excitesthe phosphor to luminescence andthe decay rate of the luminescence isdependent on the temperature.These methods work well for non-glowing, but hot surfaces belowabout 400°C.

A sapphire probe developed byAccufiber has the sensing end coatedby a refractory metal forming a black-body cavity. The thin, sapphire rodthermally insulates and connects toan optical fiber as is shown in Figure 5-2. An optical interference filter andphotodetector determines the wave-length and hence temperature.

Babcock & Wilcox has developed aquite useful moving web or rollertemperature monitoring system

Fiber Optic ExtensionsΩ 5


Figure 5-2: Typical IR Fiber Optic Probe

Lens

Low Temperature Optical FiberSingle Crystal

Sapphire (Al2O3)

Narrowband Filter

AnalyzerOptical

Detector

Coupler

Thin Film Metal Coating

Al2O3 Protective Film

Blackbody Cavity

Figure 5-3: Multipoint Pick-up Assembly

Surface of Target Web

Single Strand Optical FiberGlass Focusing Sphere

Fiber/Lens Holder

Upper Air Purge Plate

Lower Air Purge Plate

Trans- parent

Retainer Plate

Purge Air In

which will measure temperaturesfrom 120°C to 180°C across webs upto 4 meters (13 ft.) wide (Figure 5-3).The system combines optical andelectronic multiplexing and can haveas many as 160 individual pickupfibers arranged in up to 10 rows. Thefibers transfer the radiation througha lens onto a photodiode array.

Component OptionsFiber optics for temperature mea-surements as well as for communi-cations depends on minimizing loss-es in the light or infrared radiationbeing transmitted. Basics of lightconduction (Figure 5-4) is a centralglass fiber which has been carefullyproduced to have nearly zeroabsorption losses at the wave-lengths of interest. A cladding mate-rial with a much lower index ofrefraction reflects all non-axial light

rays back into the central fiber coreso that most of the conducted radi-

ation actually bounces down thelength of the cable. Various metal,Teflon or plastics are used for outerprotective jackets.

The difference in refractive indicesof the core and cladding also identi-fy an acceptance cone angle for radi-ation to enter the fiber and be trans-mitted. However, lenses are oftenused to better couple the fiber witha target surface.

For relatively short run temperaturesensing, losses in the fiber optic linkare generally negligible. Losses in con-nectors, splices and couplers predom-inate and deserve appropriate engi-neering attention. Along with thefiber optic cable, a temperature mea-suring system will include an array ofcomponents such as probes, sensorsor receivers, terminals, lenses, cou-plers, connectors, etc. Supplementalitems like blackbody calibrators andbacklighter units which illuminateactual field of view are often neededto ensure reliable operation. T

Fiber Optic Extensions5


Figure 5-4: Fiber Optic Cable Construction

CladdingCore

Cladding

Core

Very Small Θ0

Plastic TubingCore and Cladding

Jacket (PVC)

Tape or Sleeve Separator

Braided (Strength) MemberLaquer Coated

Optical Fiber

Silicon (RTV) Inner Jacket

Plastic Outer Jacket

Jacket (PVC)

Coated Optical Fibers

Plastic Tube Containers

Tape Separator

Jacketed Strength Member

Jacket (PVC)

Laquered Optical Fibers

Fabric Strength Members

Plastic Tube Containers

Polyurethane Sheath

SINGLE-FIBER CABLES

MULTIFIBER CABLES

References and Further Reading• Handbook of Temperature Measurement & Control, Omega Press, 1997.• “Fiber Optic PLC Links,” Kenneth Ball, Programmable Controls, Nov/Dec 1988.• Fiber Optic Sensors, Eric Udd, John Wiley & Sons, 1991.• Handbook of Intelligent Sensors for Industrial Automation, Nello Zuech,Addison-Wesley Publishing Company, 1992.• “Infrared Optical Fibers”, M.G. Drexhage and C.T. Moynihan, ScientificAmerican, November 1988.• Measurements for Competitiveness in Electronics, NIST Electronics andElectrical Engineering Laboratory, 1993.• “Multichannel Fiber-Optic Temperature Monitor,” L. Jeffers, Babcock &Wilcox Report; B&W R&D Division; Alliance, Ohio.• Optical Fiber Sensors: Systems and Applications, Vol 2, B. Culshaw and J. Dakin, Artech House; 1989.• Process Measurement and Analysis; Instrument Engineers’ Handbook,Third Edition, B. Liptak, Chilton Book Company, 1995.• “Radiation Thermometers/Pyrometers,” C. Warren, Measurements & Control, February, 1995.• Sensors and Control Systems in Manufacturing, S. Soloman, McGraw-Hill, 1994.

Linescannning and thermogra-phy essentially extend the con-cept of point radiation ther-mometry to one-dimensional

profiles or two-dimensional picturesof non-contact temperature data.

One-dimensional linescanners havea wide range of application. They areused in the production of fiberboard,carpets, vinyl flooring, paper, packag-ing material, pressure sensitive tapes,laminates, float glass, safety glass,and nearly any other web-type prod-uct for which temperature control iscritical. Linescanners also monitorhot rolling mills, cement and limekilns, and other rotary thermal pro-cessing equipment.

Thermographic “cameras” find usein the maintenance function in man-ufacturing plants, especially in theasset-driven capital intensive indus-tries in which temperature is anactive concern and diagnostic tool.Typical targets for infrared inspec-tion include electrical equipment,frictional effects of power transmis-sion equipment, and thermal pro-cessing steps in a production line.

Infrared LinescannersThe typical sensor unit for linescanthermography uses a single detectorthat, by itself, is limited to measuringthe temperature at only one point.However, a rotating mirror assemblyfocuses a single, constantly changing,narrow slice of real-world view onthe detector surface (Figure 6-1).

Although adequate thermal mea-surement resolution may requireonly a few dozen scans per second,

contemporary units offer up to 500scans per second. The electronic cir-cuitry behind the detector elementchops the thermal data for each lin-ear pass into several hundred to sever-al thousand individual measurementpoints. High speed data collection cir-cuitry then quantifies, digitizes, andcaptures the temperature of theobject at the measurement pointsalong each scan. Additional circuitrythen analyzes and manipulates thedigital data to produce a real-time dis-play of the temperature of the viewpresented to the detector.

By itself, a linear temperature scanconstitutes a rather myopic view of astationary object. However, an objectmoving past a stationary linescannerprovides a data-rich measuring envi-

ronment. Mounted several feet aboveand focused on the moving web ofproduct in a production line makesthe linescanner an element of a real-time process feedback and closed-loop control scheme.

• Linescanner OperationThe resolution of a linescanner is afunction of the speed of the movingweb, the scan rate, the number ofmeasurements per scan, and thewidth of the scanned line. The accu-racy and response of the linescannerdepends on a clean optical pathbetween the target and the overheaddetector, which can be problematicin a typical manufacturing environ-ment. Linescanners are available with


Infrared Linescanners

2-D Thermographic Analysis

Enter the Microprocessor

NON-CONTACT TEMPERATURE MEASUREMENTLinescanning & Thermography

6

LLinescanning & Thermography

Figure 6-1: Linescanner Operation

Stationary Thermograph

Point Being Measured

Object

Line Being Measured

False Color

Isometric Representation

Linescanning Thermograph

Moving Web of MaterialPoint A

Point B

ZERO-DIMENSIONAL THERMOGRAPHY

ONE-DIMENSIONAL THERMOGRAPHY

A

B

air purge system to keep the opticsclean. Water cooling of the sensorassembly may be necessary to main-tain reliable operation in hot envi-ronments. Sealed housings and bear-ings protect delicate componentsagainst moisture and dust.

The digital electronic output fromthe linescanner typically feeds a per-sonal computer running softwarethat converts the stream of data intoa real-time moving image of the webpassing the detector assembly.Because the human retina is not sen-sitive to infrared radiation, the screenimage is necessarily rendered in falsecolor the hue of which correspondsto the temperature of the specificlocation.

The linescan output converts thetraditional plot of temperature ver-sus time into a three-axis measure-ment of temperature as a function oftime and location across the web ofmaterial (Figure 6-2). The scanner“maps” the “thermal terrain” movingbelow the detector assembly. As istypical of false color thermographicoutput, the electronics assigns thecolor of a given screen pixel on thebasis of the temperature implied bythe measured infrared radiation.

• Linescanner ApplicationsLinescanning technology offersindustry an opportunity to optimizethermally-based processes. Forexample, plastic thermoforming fab-rication requires heating a plasticsheet using an array of heaters. Thetemperature is critical if the vacuummolding process is to successfullyform pleats, deeply drawn recesses,and sharp corners in the completedpiece part. A high-resolution lines-canner, used in a closed-loop config-uration to control the output of the

individual heaters, helps to maximizethe productivity of the press.

Another application for linescan-ning in the plastic industry is in blow-molding plastic film. The processinvolves extruding a polymer meltthrough a circular die and drawingthe formed tube upward. At some

point as the plastic rises it solidifiesat what is called the frost line.Additional cooling must occur sothat the plastic sheeting achieves theproper temperature as it enters theoverhead takeup rolls.

Maintaining a proper temperatureprofile of the moving plastic is criti-cal to eliminating functional and aes-thetic defects in the sheeting.Mounting a scanner to monitor therising plastic gives the operatorsinstant information about the preciselocation of the frost line and othertemperature-related information inreal-time.

A similar approach is used in theglass industry. Glass sheets are heattreated to give them the requiredstrength. As the glass moves on aconveyor belt, electric heaters raiseits temperature as uniformly as possi-ble. After a suitable holding time inthe oven, the glass sheet is cooleduniformly with compressed air. In thisprocess are all the elements

of a linescanner application for a moving web of temperaturesensitive material.

2-D Thermographic AnalysisAdding another simultaneous geo-metric dimension to the linescanningconcept leads to the practices of

two-dimensional thermographicanalysis. Two-dimensional figures areplanes exhibiting only length andwidth but no height. The two-dimen-sional plane in question is the viewyou see with your own eyes. The ther-mographic equipment in question isan infrared camera comparable in sizeto a video camera (Figure 6-3).

Devices used for thermographicanalysis generally fall into two broadclasses. Radiometry devices are usedfor precise temperature measure-ment. The second, called viewingdevices, are not designed for quantita-tive measurements but rather for qual-itative comparisons. A viewer onlytells you that one object is warmerthan another, whereas radiometry tellsyou that one object is, for instance,25.4 degrees warmer than the otherwith an accuracy in the neighborhoodof 2 degrees or 2 percent.

Whereas a standard video cameraresponds to visible light radiatingfrom the object in view, thermo-



Figure 6-2: 1-D Scans Composited Into a 2-D Image

Width of Web

Travel Distance of Web

in One Can

Tem

pera

ture

Distance Along Web

graphic units responds to the object’sinfrared radiation. The scene throughthe camera’s viewfinder is presentedin false colors designed to conveytemperature information.

Specular surfaces, especiallymetallic ones, reflect infrared radia-tion. The image of a shiny metal sur-face viewed through an infrared cam-era contains thermal informationinherent to and radiated by the sur-face as well as thermal informationabout the surroundings reflected bythe surface. When monitoring thetemperature of a transparent object,the optical system may pick up athird source of radiation—that trans-mitted through from objects on theother side. Modern imagers haveemissivity controls that adjust theresponse of the unit so that it readsaccurately. (See p. 72 for emissivitytables of common materials.)

The compact size of thermograph-ic imagers eliminates the need fortripods and other factors that limitmobility. In fact, in an industrial set-ting, the technician using the hand-held imager may well be readingtemperatures or capturing imageswhile walking around. Doing so canbe dangerous since the user’s atten-tion is on data collection and not onenvironmental trip hazards. For thatreason, imagers do not have the typ-ical “eyepiece” found on a videocameras. Imagers use, instead, a 4-inch flat panel display, typically acolor liquid crystal display.

• Detectors OptionsThe earliest thermal imaging systemsfeatured a single detector and a spin-ning mirror that scanned the imagecoming through the lens of the cam-era and focused the pixels of thetwo-dimensional image on the

detector in sequence. The electron-ics that captures data is synchronizedwith the mirror so that no thermalinformation is lost or garbled. One ofthe problems with the single detec-tor approach is dwell time. Scanninga 120 x 120 pixel image with a spin-ning mirror does not give any singlepixel very much time to register areading on the detector.

The newest thermal imaging sys-tems eliminate the need for the spin-ning mirror by replacing the singlepoint detector with a solid-statedetector that continuously “stares”at the entire image coming throughthe lens. Dwell time is no longer anissue because the scene comingthrough the optics maps directly onthe active surface of the focal planearray detector. Using the focal planarray technology brings several bene-fits to the user.

The most obvious benefit is fewermoving parts in the camera. Fewerparts leads to higher reliability andalmost certainly higher durabilityagainst physical abuse and otherhazards of the workplace. Thenewer thermal imagers are smallerand lighter than their predecessors.In fact, the latest infrared imagersare of a size not much larger thanthe smallest of the modern hand-held video camera.

The resolution of the focal planarray—a minimum of 320 x 244 pix-els—is much greater than thatoffered by the single detector mod-els. A finer resolution leads directlyto being able to discern smaller “hotspots” in the field of view.

• Detector CoolingThe detector in either type of imagermust be cooled if it is to work prop-erly. This is analogous to looking out

of a window at night. If the roomlights are on, it is difficult to seeclearly because there is too muchvisible light coming from the roomitself. Turning the lights off makes itmuch easier to see outside. Similarly,accurately measuring the tempera-ture can be difficult if the cameraparts surrounding the detector aregiving off too much infrared radia-tion. A cooler detector is equivalentto turning off the lights in the room.Infrared imaging technology relies ona refrigerated detector. The earliestthermography cameras used lique-fied gases to cool the detector.Certainly the technology was newand the operating refinements werecrude. As you can imagine, the earlyunits were not all that portable. Thekey element of contemporaryradiometers is a sufficiently small,battery-operated Stirling cycleengine to keep the detectors cold.

There are two common methodsfor cooling the detector chip. AStirling cycle engine provides thecryogenic cooling required by preciseradiometry devices. Thermoelectriccooling provides the temperaturestabilization required by a viewer. Incryogenic cooling, the detector ischilled to around -200°C, whereastemperature stabilization requirescooling the detector to somewherenear room temperature.

Depending on the design of thedetector, the stabilization tempera-ture may be in the range of 20 to 30°C or it may be at the Curie temper-ature in the range of 45-60°C.Operating at the Curie temperatureoffers better sensitivity to theincoming infrared radiation. Ineither case, the temperatures mustbe constant from reading to readingif the imager is to provide consis-tent and reproducible results. As an

Linescanning & Thermography 6


aside, in common industry parlance,units that rely on thermoelectriccooling are referred to as“uncooled” units relative to cryo-genically cooled devices.

• The Stirling EngineIn 1816, Robert Stirling developedwhat is called in thermodynamicterms a closed-cycle regenerableexternal combustion engine. Thismachine produces no waste exhaustgas, uses a diversity of heat sourcesto power it, remains quiet duringoperation, and has a high theoreticalthermal efficiency. The Stirling cycleconsists of four steps: heating at con-stant volume, isothermal expansion,cooling at constant volume, andisothermal compression (Figure 6-4).The device converts heat into itsequivalent amount of mechanicalwork. In effect, one obtains shaftwork by heating the engine.Fortunately, the Stirling cycle is areversible thermodynamic process,and mechanical work can be used toproduce a cooling effect.

This need for cooling is a limita-tion only in the sense that one can-not achieve true “instant on” with aninfrared imager. Cryogenic coolingrequires five to nine minutes toachieve the very low temperaturesrequired for the detector to respond.Thermoelectric cooling, on the otherhand takes about one minute or less.

• Other Detector DevelopmentsSoon, researchers are expected toproduce less expensive uncooledradiometric detectors with sensitivi-ty and resolution at least as good astoday’s cryogenic units. One of theproblems to be surmounted is therelatively low yields in the detector

manufacturing process. Until theproduction problems find a solution,the cost of the high-performanceuncooled detectors will remain on apar with cryogenic units and theirStirling engines.

There is one true “instant on” unitthat depends on pyroelectric arraysfor the detector elements. Theseimagers require absolutely no cool-ing but they require a constantlychanging image signal. If the scenepresented to the lens does notchange, the camera ceases to resolveany image at all. Imagers based onthe pyroelectric principle are suit-able for viewer use only, not radiom-etry. However, it is possible to use apoint detector aligned with the cen-terline of the image to record onetemperature to represent the entireframe. Because the pyroelectric ele-ment is piezoelectric, it generates anextraneous signal in response tovibration of the camera housing.Such sensitivity requires that theunits be shock mounted to damp outthe vibrations.

• Spatial, Temperature ResolutionThere are two types of thermograph-ic detector resolution to be distin-guished. First is spatial resolution.The detector assemblies in focal

plane arrays have multiple detectorelements on a single detector chipthat map directly to the aperture ofthe optical system. High spatial reso-lution means the camera can distin-guish between two closely spaceditems. Temperature resolution refersto the ability of the camera to distin-guish temperature differencesbetween two items. Temperature res-olution is a function of the type ofdetector element; spatial resolutionis a function of the number of detec-tor elements.

Specification sheets for infraredimagers give the spatial resolution interms of milliradians of solid angle(Figure 6-5). The milliradian value isrelated to the theoretical object areacovered by one pixel in the instanta-neous field of view. Obviously, atgreater distances more of the objectarea maps to a single pixel and a larg-er area means less precise thermalinformation about any single ele-ment in that larger area.

• Applications of ThermographyAn effective predictive maintenanceprogram implies the need to collectsometimes rather sophisticated datafrom productive assets locatedaround the plant floor. A plant main-tenance technician normally follows



Figure 6-3: 2-D Thermographic Camera

Two Dimensional Thermograph

False Color Image

Object

Area Being

Scanned

a predetermined route and visitingplant assets in a specific sequence.This makes data collection as effi-cient as possible. At each asset, thetechnician collects data from dis-crete sensors while working from acheck list so as not to miss a reading.In the case of thermography, the dataconsists of one or more images ofthe relevant machine parts. A barebones infrared camera is nothingmore than a data collector. Raw ther-mal data is of little value; it is whatone does with the data that makesthe difference.

After the thermographic data col-lection is complete, the technicianor the data analyst evaluates theimages for evidence of thermalanomalies that indicates a need foreither scheduled or immediaterepair. If maintenance work is war-ranted, the analyst prepares a reportboth to justify to others the need tospend money for repairs and toretain as part of the permanentrecords for the given plant asset.

• Framing the ImageA certain level of skill and experienceaided by common sense is a primerequisite in gathering and analyzingthermographic data. The technicianusing the imager in the field must beaware of reflections of irrelevantheat sources that appear to be com-ing from the object being scanned.Physically moving the imager to theleft or right could make a dramaticdifference in the apparent tempera-ture of the object. The difference iscaused by the reflections of shopfloor lighting fixtures, sunlightthrough windows, and other extrane-ous sources.

As with taking snapshots with asingle lens reflex camera, framing the

scene is somewhat of an art. If theobject is in the path of the heated orcooled air issuing from an HVAC sys-tem, the readings will be skewed. Itmay make more sense to return aftersundown or to shut down the HVACsystem to gather meaningful data.Analysts need common sense, aswell. Modern cameras offer resolu-tions of a fraction of a degree.

• Data Analysis ToolsSome thermal imagers work in con-junction with on-board microproces-sors and specialized software thatgive the user the ability to quicklyprepare diagnostic reports. In fact,downloading the field data to adesktop PC frees the technician tocontinue gathering data while theanalyst prepares a report. But, reportpreparation is not the only enhance-ment to standard infrared picture.

Some thermal imagers simplifywork for the user by making it possi-ble to annotate thermal images withvoice messages stored digitally withthe digital image itself. Some unitsautomate the process of setting thecontrols to permit the camera tocapture the best, most information-rich thermal image as long as theview is in clear focus.

Watching a part “age” may be ofgreat value to the plant maintenancedepartment in predicting the expect-ed failure date for a machine compo-nent. Being able to trend thermaldata as a function of time is one wayto watch the aging process. The soft-ware package that processes thethermal data from the camera canproduce a graph of the time-seriestemperature data corresponding tothe same point in the infrared imageof the plant asset. The analyst simplymoves a spot meter to the part ofthe image to be trended and thetemperature data for that spot linksto a spreadsheet cell.

These enhancements to the basicthermographic technology continueto make the IR imagers easier to use.With a few hours of training, even anovice can generate excellent thermalscans that capture all the thermalinformation present in a given scene.

• Industrial ApplicationsElectrical wiring involves many dis-crete physical connections betweencables and various connectors, andbetween connectors and mountingstuds on equipment. The hallmarkof a high-quality electrical connec-tion is very low electrical resistance



Figure 6-4: The Stirling Cycle

V

P3

24

1

S

T

Th3

2

4

1TL

between the items joined by theconnection. Continued electricalefficiency depends on this low con-tact resistance.

Passing a current through an electri-cal resistor of any sort dissipatessome of the electrical power. The dis-sipated power manifests itself as heat.If the quality of the connectiondegrades, it becomes, in effect, anenergy dissipating device as its electri-cal resistance increases. Withincreased resistance, the connector orjoint exhibits a phenomenon calledohmic heating. Electricians and main-tenance technicians use the thermo-graphic camera to locate these hotspots in electrical panels and wiring.The heated electrical componentsappear as bright spots on a thermo-gram of the electrical panel.

Three-phase electrical equipmentconnects to the power supplythrough three wires. The currentthrough each wire of the circuitshould be equal in magnitude.However, it is possible to have anunbalance in the phases. In this case,the current in one of the phases dif-fers significantly from the others.Consequently, there exists a tempera-

ture difference among the three con-nections. Thermographic cameras canillustrate this imbalance quite easilyand dramatically. Consider, for amoment, the ease with which a ther-mographer can inspect overheadelectrical connections or pole-mounted transformers from a remote,safe place on the ground.

Thermography also finds use ininspecting the building envelope. Itcan locate sections of wall that haveinsufficient insulation. It can also spotdifferences in temperatures that indi-cate air leaks around window anddoor frames. Thermal imaging is use-ful for inspecting roofs as well.

If a defect in the outermost roofmembrane admits rain water thatgets trapped between the layersmaking up the roof, the thermal con-ductivity of the waterlogged sectionof roof is greater than that of the sur-rounding areas. Because the thermalconductivity differs, so does thetemperature of the outer roof mem-brane. An infrared camera can easilydetect such roof problems. A ther-mal scan of the roof and a can ofspray paint is all that is needed toidentify possible roof defects for a

roofing contractor to repair.Because thermography is a non-

contact measurement method, itmakes possible the inspection ofmechanical systems and componentsin real time without shutting downthe underlying production line.

Energy constitutes a major cost inmost manufacturing plants. Everywasted BTU represents a drain onplant profitability. Thermographylends itself to eliminating the energyloss related to excessive steam con-sumption and defective steam traps.If steam is leaking through a steamtrap, it heats the downstream con-densate return piping. The heatedsection of piping is clearly visible toan infrared imager.

Heat loss to the surrounding envi-ronment is a function of temperatureof the inside temperature. The heatloss increases nonlinearly withincreased temperature because radi-ant losses can easily exceed convec-tive and conductive losses at highertemperatures. For example, therefractory block installed inside of akiln, boiler, or furnace is intended tominimize heat loss to the environ-ment. Thermography can quickly



S

Figure 6-5: Spatial Resolution of a Thermographic Camera

R

Θ

S = RΘk S = Spot Diameter R = Distance to Target Θ = Imaging Spot Resolution [=] Milliradians k = Constant Based on Units of Measure for S & R

k Values Distance UnitsFeet Meters

Spot Size Units

Milli- meters

0.305 1.000

0.012 0.039Inches

locate any refractory defects.Another application for the technol-ogy is a blast furnace, with its mas-sive amount of refractory.

The location of the blockage in aplugged or frozen product transferline can sometimes be detected withthermography. If the level indicatoron a storage tank fails, thermographycan reveal the level of the inventoryin the tank.

Thermography finds further usein the inspection of concrete bridgedecks and other paved surfaces. Thedefects in question are voids anddelamination in and among the var-ious layers of paving materials. Theair or water contained within theinterlaminar spaces of the pavementslab affects its overall thermal con-ductivity. The IR imager can detectthese defects.

Painted surfaces become multilay-er composites when a bridge or stor-age tank has been repainted numer-ous times during its service life. Here,too, the possibility of hidden rust,blistering, cracking, and other delami-nation defects between adjacentpaint layers make objective visualinspection difficult. A techniquecalled transient thermography returnsobjectivity to the evaluation of apotentially costly repainting project.

Transient thermography entailsusing a pulse of thermal energy, sup-plied by heat lamps, hot air blowers,engine exhaust, or some similarsource of energy, to heat the surfacefrom behind for a short period oftime. Because the imager detectstemperature differentials of less thana degree, voids and delaminationsbecome readily apparent.

Forestry departments use ther-mography to monitor the scope andrange of forest fires to most effi-ciently deploy the valuable, limited,

urgently needed resources of man-power and fire-retardant chemicals.Corporate research and develop-ment rely on radiometric thermogra-phy, as well. Auto makers use thetechnology to optimize the perfor-mance of windshield defrosting sys-tems and rear window defoggers.Semiconductor manufacturers use itto analyze operational failures incomputer chips.

Enter the MicroprocessorMicroprocessors and the softwarebehind thermography units areimportant to the versatility of thetechnology. Digital control and high-speed communication links givethermographic devices the intercon-nectivity and signal processingexpected in a digital manufacturingenvironment. For example, the lines-canner output can be subdividedinto several segments or zones, eachcorresponding to a portion of thewidth of the moving web. Each ofthese zones can provide individual 4-20 mA control and alarm signals tothe process machinery.

Because the thermal data is digi-tized, it is easy to store the optimumthermograph for use as a standard ofcomparison. This standard thermalimage—the golden image— is used

to simplify process setup, a featureespecially valuable when changingproducts in the processing line.

Contemporary thermographicunits use 12-bit dynamic range archi-tecture. This is the practical mini-mum if radiometry is to capture allthe thermal information that thescene contains. It allows the analystto position a set of crosshairs on asingle pixel and determine the pre-cise temperature that it represents.

The microprocessor also makesinterpreting the thermogram easier.The analyst can spread the colorpalette across the full range of tem-peratures represented by the ther-mogram. For instance, wheninspecting a roof in July, the temper-ature of every point in the scene ishigh and the difference in tempera-ture between sound areas anddefects is relative small, perhaps 20degrees. On the other hand, pro-duction process may well mean thatparts of the scene are 250 degrees(or more) warmer than the back-ground objects. In either case, theanalyst can spread 256 colors acrossthe 20-degree and the 250-degreerange in the scene to generate ausable picture. T



References and Further Reading• Applications of Thermal Imaging, S.G. Burnay, T.L. Williams, and C.H. Jones(editor), Adam Higler, 1988.• Infrared Thermography, (Microwave Technology, Vol. 5), G. Gaussorguesand S. Chomet (translator), Chapman & Hall, 1994.• An International Conference on Thermal Infrared Sensing for Diagnosticsand Control, (Thermosene Vii), Andronicos G. Kantsios (editor), SPIE, 1985.• Nondestructive Evaluation of Materials by Infrared Thermography, XavierP.V. Maldague, Springer Verlag, 1993.• Practical Applications of Infrared Thermal Sensing and Imaging Equipment(Tutorial Texts in Optical Engineering, Vol. 13), Herbert Kaplan, SPIE, 1993

Because of normal variationsin the properties of materialsused to construct radiationtemperature sensors, new

instruments must be individually cal-ibrated in order to achieve evenmoderate levels of accuracy. Initialcalibration is likely to be performedby the sensor manufacturer, but peri-

odic recalibration—in-house or by athird-party laboratory or the originalmanufacturer—is necessary if any butthe most qualitative measurementsare expected.

The ongoing accuracy of a non-contact temperature sensor willdepend on the means by which thecalibration is performed, how fre-quently it is recalibrated, as well asthe drift rate of the overall system.Ensuring the absolute accuracy ofnon-contact temperature measure-ment devices is more difficult thanwith most direct contactingdevices, such as thermocouples andresistance temperature detectors(RTDs). Limiting the absolute accu-racy to 1% is difficult; even in themost sophisticated set-ups, better

than 0.1% accuracy is seldomachieved. This arises, in part, fromthe difficulty in accurately deter-mining the emissivity of real bodies.Repeatability or reproducibility is,however, more readily achievablethan absolute accuracy, so don’tpay more if consistency will do.

If absolute accuracy is a concern,then traceability to standards such asthose maintained by the NationalInstitute of Standards & Technology(NIST) will also be important.Traceability, through working to sec-ondary to primary standards is cen-tral to the quality standards compli-ance such as those defined by theISO 9000 quality standard.

Why Calibrate?There are generally three methodsof calibrating industrial radiationthermometers. One method is to

use a commercial blackbody simula-tor, an isothermally heated cavitywith a relatively small aperturethrough which the radiation ther-mometer is sighted (Figure 7-1). Asexplained in the earlier chapter on“Theoretical Development,” thistype of configuration approachesblackbody performance and itsemissivity approaches unity. A stan-dard thermocouple or resistancetemperature detector (RTD) insidethe cavity is used as the tempera-ture reference. At higher tempera-tures, calibrated tungsten filamentlamps are commonly used as refer-ences. A final alternative is to used areference pyrometer whose calibra-tion is known to be accurate, adjust-ing the output of the instrumentbeing calibrated until it matches.

In any case, the radiation sourcemust completely fill the instrument’sfield of view in order to check the


Why Calibrate?

Blackbody Cavities

Tungsten Filament Lamps

NON-CONTACT TEMPERATURE MEASUREMENTIR Thermometer Calibration

7

BIR Thermometer Calibration

Back Thermocouple Front Thermocouple

Refractory Sphere

Figure 7-1: A Spherical Blackbody Cavity

Control ThermocoupleController

Working much like a hot plate, this infrared cali-bration source uses a high emissivity, specially tex-tured surface to provide a convenient temperaturereference.

IR Thermometer Calibration 7


calibration output. If the field ofview is not filled, the thermometerwill read low. In some instruments,calibration against a blackbody refer-ence standard may be internal—achopper is used to alternatebetween exposing the detector tothe blackbody source and the sur-face of interest. Effectively, this pro-vides continuous recalibration andhelps to eliminate errors due to drift.

Blackbody CavitiesBecause calibration of a non-contacttemperature sensor requires a sourceof blackbody radiation with a precisemeans of controlling and measuringthe temperature of the source, theinterior surface of a heated cavityconstitutes a convenient form, sincethe intensity of radiation from it isessentially independent of the mate-rial and its surface condition.

In order for a blackbody cavity towork appropriately, the cavity mustbe isothermal; its emissivity must beknown or sufficiently close to unity;and the standard reference thermo-

couple must be the same tempera-ture as the cavity. Essentially, theblackbody calibration reference con-sists of a heated enclosure with asmall aperture through which theinterior surface can be viewed (Figure7-1). In general, the larger the enclo-sure relative to the aperture, themore nearly unity emissivity is

approached (Figure 7-2). Althoughthe spherical cavity is the most com-monly referenced shape, carefullyproportioned cone- or wedge-shaped cavities also can approachunity emissivity.

In order to provide isothermal sur-roundings for the cavity, the follow-ing materials commonly are used:• Stirred water bath for 30-100°C (86-212°F) temperature ranges;• Aluminum core for 50-400°C (122-752°F) temperature ranges; and • Stainless steel core for 350-1000°C(662-1832°F) temperature ranges.

And while blackbody cavitieshave their appeal, they also havesome disadvantages. Some portable,battery-operated units can be usedat low temperatures (less than100°C), but blackbody cavities are,for the most part, relatively cum-bersome and expensive. They alsocan take a long time to reach ther-mal equilibrium (30 minutes ormore), slowing the calibration pro-cedure significantly if a series ofmeasurements is to be made.

0 20 40 60 80 100 120 140 160 180

1.0

0.8

0.6

0.4

0.2

0.0

Effe

ctiv

e Em

issi

vity

, ε

Aperture Angle, φ (Deg)

0.950.9

0.8

0.7

0.5

0.3

0.1

Figure 7-2: Effective Emissivity of Spherical Cavities

Cavity Surface Emissivity

φ

A handheld IR thermometer is calibrated against a commercial blackbody source—the internal cavityis designed to closely approach a blackbody’s unity emissivity.

IR Thermometer Calibration 7


Tungsten FilamentsAs a working alternative to black-body cavities, tungsten ribbon lamps,or tungsten strip lamps, are com-monly used as standard sources(Figure 7-3). Tungsten strip lamps arehighly reproducible sources of radi-ant energy and can be accurately cal-ibrated in the 800°C to 2300°C range.They yield instantaneous and accu-rate adjustment and can be used athigher temperatures than those read-ily obtainable with most cavities.

Lamps, however, must be calibrat-ed in turn against a blackbody stan-dard; the user typically is providedwith the relationship between elec-tric current to the filament and itstemperature. Emissivity varies withtemperature and with wavelength,but material is well understoodenough to convert apparent tem-peratures to actual.

Just as a blackbody cavity includesa NIST-traceable reference thermo-couple, instrument calibration againsta ribbon lamp also can be traced toNIST standards. In a primary calibra-tion, done mostly by NIST itself, fila-

ment current is used to balance stan-dard lamp brightness against thegoldpoint temperature in a black-body furnace, in accordance with theITS-90. Typical uncertainties rangefrom ±4°C at the gold point to ± 40°Cat 4000°C.

In secondary standard calibration,the output of a primary pyrometer,i.e., one calibrated at NIST, is com-pared with the output of a sec-ondary pyrometer when sightedalternately on a tungsten strip lamp.Many systematic errors cancel out inthis procedure and make it morepractical for routine calibration. T

Pointer

Filament

Nickel Support

Glass/ Ceramic

Base

Figure 7-3: Typical Tungsten Lamp Filament

References and Further Reading• Handbook of Temperature Measurement & Control, Omega Press, 1997.•New Horizons in Temperature Measurement & Control, Omega Press, 1996.• Temperature Measurement in Engineering, H. Dean Baker, E. A. Ryder, andN. H. Baker, Omega Press, 1975.• The Detection and Measurement of Infrared Radiation, R.A. Smith, F. E.Jones, and R. P. Chasmar, Oxford at Clarendon Press, 1968.• Handbook of Temperature Measurement & Control, Omega EngineeringCo., 1997.• Infrared Thermography (Microwave Technology, Vol 5), G. Gaussorguesand S. Chomet (translator), Chapman & Hall, 1994.• Instrument Engineers’ Handbook, Third Edition, B. Liptak, Chilton BookCo. (CRC Press), 1995.• Process/Industrial Instruments and Controls Handbook, 4th ed., DouglasM. Considine, McGraw-Hill, 1993. • Theory and Practice of Radiation Thermometry, David P. DeWitt andGene D. Nutter, John Wiley & Sons, 1988.

While many of the earlierchapters of this volumehave explored thephysics and technology

behind non-contact temperaturemeasurement, now it’s time to delveinto the wide array of products thatare available to take advantage ofradiation phenomena—and howthey’re applied to industrial use.

Non-contact temperature sensorsallow engineers to obtain accuratetemperature measurements in appli-cations where it is impossible or verydifficult to use any other kind of sen-sor. In some cases, this is because theapplication itself literally destroys acontact-type sensor, such as whenusing a thermocouple or resistancetemperature detector to measuremolten metal. If the electrical inter-ference is intense, such as in induc-tion heating, the electromagneticfield surrounding the object willcause inaccurate results in conven-tional sensors. A remote infrared sen-sor is immune to both problems.

For maintenance, no other sensoris able to provide long-distance, non-contact temperature measurementsneeded to find hot spots or troubleareas in distillation columns, vessels,insulation, pipes, motors or trans-formers. As a maintenance and trou-bleshooting tool, it’s difficult to beata hand-held radiation thermometer.

Although non-contact temperaturesensors vary widely in price, theyinclude the same basic components:collecting optics, lens, spectral filterand detector. For more detailed tech-nical information on each sensor type,see the previous chapters.

Alternative ConfigurationsThe user can select among non-con-tact temperature sensors that operateover just about any desired wave-length range, both wide and narrow.Radiation thermometer sensitivityvaries inversely proportionally withwavelength. Therefore, an instrumentoperating at 5 microns only has one-fifth the sensitivity of an instrumentoperating at 1 micron. This also meansthat optical noise and uncertainties inemissivity will result in measurementerrors five times greater in the long

wavelength instrument. Radiation thermometer optics are

usually the fixed focus type,although designs with through-the-lens focusing are available for mea-suring over longer distances. Fixedfocus devices can also be used tomeasure at long distances if the tar-get area is smaller than the lensdiameter in the optical system.

Non-contact temperature sensorsrange from relatively inexpensiveinfrared thermocouples, priced fromabout $99, to sophisticated, comput-


Alternative Configurations

Application Guidelines

Accessories & Options

NON-CONTACT TEMPERATURE MEASUREMENTProducts & Applications

8

WProducts & Applications

Low-End IR Pyrometer/ Thermometer

Portable and convenient

Inexpensive (from $235)

Excellent maintenance tool

Maximum probe cable length of 1 m limits use

High-End IR Thermometer

Can focus on any target at almost any distance

Portable or fixed-place operation

Camera-like operation (point and shoot)

Low to medium cost (from $350)

Measures only a fixed spot on target

Accuracy affected by smoke, dust, etc. in line of sight

Affected by EMI

Fiber Optic Works in hostile, high-temperature, vacuum or inaccessible locations

Can bypass opaque barriers to reach target

Unaffected by EMI

Fairly expensive ($1600-$2600)

Fixed Focus

Two-Color Sees through smoke, dust and other contaminants in line of sight

Independent of target emissivity

Fairly expensive (from $3600 for sensor, and $5000 for display/controller)

Linescanner Only sensor that makes full-width temperature measurements across product

Measures continuously as product passes by

Computer can produce thermographic images of entire product and its temperature profile

Very expensive (from $10,000 for sensor alone, $50,000 for complete system)

INSTRUMENT TYPE STRENGTHS WEAKNESSES

IR Thermocouple

Inexpensive (from $99)

Self-powered

No measurement drift

Plugs into standard thermocouple display and control devices

Reaches into inaccessible areas

Intrinsically safe

Nonlinear output

Susceptible to EMI

Table 8-1: Strengths and Weaknesses of Non-Contact Temperature Sensors

er-based $50,000 linescanners. Inbetween is a wide variety of hand-heldand permanently mounted measuringsystems that meet just about any tem-perature monitoring need imaginable.

• Infrared ThermocouplesAn infrared thermocouple is anunpowered, low-cost sensor thatmeasures surface temperature ofmaterials without contact. It can bedirectly installed on conventionalthermocouple controllers, trans-mitters and digital readout devicesas if it were a replacement thermo-couple. An infrared thermocouplecan be installed in a fixed, perma-nent location, or used with a hand-held probe.

Because it is self-powered, itrelies on the incoming infrared radi-ation to produce a signal via ther-moelectric effects. Therefore, itsoutput follows the rules of radia-tion thermal physics, and is subjectto nonlinearities. But over a givenrange of temperatures, the output issufficiently linear that the signal canbe interchanged with a convention-al thermocouple.

Although each infrared thermo-couple is designed to operate in aspecific region, it can be used out-side that region by calibrating thereadout device accordingly.

• Radiation Thermometers/PyrometersRadiation thermometers, or pyrome-ters, as they are sometimes calledcome in a variety of configurations.One option is a handhelddisplay/control unit, plus anattached probe. The operator pointsthe probe at the object being mea-sured—sometimes getting within afraction of an inch of the surface—

and reads the temperature on thedigital display. These devices areideal for making point temperaturemeasurements on circuit boards,bearings, motors, steam traps or anyother device that can be reachedwith the probe. The inexpensivedevices are self-contained and runoff battery power.

Other radiation thermometers arehand-held or mounted devices thatinclude a lens similar to a 35mm cam-era. They can be focused on anyclose or distant object, and will takean average temperature measure-ment of the “spot” on the target thatfits into its field of view.

Handheld radiation thermometersare widely used for maintenance andtroubleshooting, because a techni-cian can carry one around easily,focus it on any object in the plant,and take instant temperature read-ings of anything from molten metalsto frozen foods.

When mounted in a fixed position,radiation thermometers are oftenused to monitor the manufacturingof glass, textiles, thin-film plastic andsimilar products, or processes such astempering, annealing, sealing, bend-ing and laminating.

• Fiber Optics ExtensionsWhen the object to be measured isnot in the line of sight of a radiationthermometer, a fiber optic sensorcan be used. The sensor includes atip, lens, fiber optic cable, and aremote monitor unit mounted up to30 ft away. The sensor can be placedin high energy fields, ambient tem-peratures up to 800°F, vacuum, or inotherwise inaccessible locationsinside closed areas.

• Two-Color SystemsFor use in applications where the tar-get may be obscured by dust, smokeor similar contaminants, or changingemissions as in “pouring metals,” atwo-color or ratio radiation ther-mometer is ideal. It measures tem-perature independently of emissivity.Systems are available with fiber opticsensors, or can be based on a fixed orhand-held configurations.

• LinescannersA linescanner provides a “picture” ofthe surface temperatures across amoving product, such as metal slabs,glass, textiles, coiled metal or plas-tics. It includes a lens, a rotating mir-



Typical fiber optic probe, transmitter, and bench top display.

ror that scans across the lens’ field ofview, a detector that takes readingsas the mirror rotates, and a comput-er system to process the data.

As the mirror rotates, the linescanner takes multiple measure-ments across the entire surface,obtaining a full-width temperatureprofile of the product. As the prod-uct moves forward under the sensor,successive scans provide a profile ofthe entire product, from edge toedge and from beginning to end.

The computer converts the profileinto a thermographic image of theproduct, using various colors to rep-resent temperatures, or it can pro-duce a “map” of the product. The 50or so measurement points across thewidth can be arranged in zones, aver-aged, and used to control upstreamdevices, such as webs, cooling sys-tems, injectors or coating systems.

Linescanners can be extremelyexpensive, but they offer one of theonly solutions for obtaining a com-plete temperature profile or imageof a moving product.

• Portable vs. MountedNon-contact temperature measure-ment devices also can be classified asportable or permanently mounted.Fixed mount thermometers are gen-erally installed in a location to con-tinuously monitor a process. Theyoften operate on AC line power, andare aimed at a single point. Measureddata can be viewed on a local orremote display, and an output signal(analog or digital) can be providedfor use elsewhere in the controlloop. Fixed mount systems generallyconsist of a housing containing theoptics system and detector, connect-ed by cable to a remote mountedelectronics/display unit. In someloop-powered designs, all the ther-mometer components and electron-ics are contained in a single housing;the same two wires used to powerthe thermometer also carry the 4 to20 mA output signal.

Battery powered, hand-held “pis-tol” radiation thermometers typical-ly have the same features as perma-nently mounted devices, but withoutthe output signal capability. Portableunits are typically used in mainte-nance, diagnostics, quality control,

and spot measurements of criticalprocesses.

Portable devices include pyrome-ters, thermometers and two-colorsystems. Their only practical applica-tion limit is the same as a humanoperator; i.e., the sensors will functionin any ambient temperature or envi-ronmental condition where a humancan work, typically 32-120°F (0-50°C).

At temperature extremes, wherean operator wears protective cloth-ing, it may be wise to similarly pro-tect the instrument. In shirt-sleevemanufacturing or process controlapplications, hand-held instrumentscan be used without worrying aboutthe temperature and humidity, butcare should be taken to avoidsources of high electrical noise.Induction furnaces, motor starters,large relays and similar devices thatgenerate EMI can affect the readingsof a portable sensor.

Portable non-contact sensors arewidely used for maintenance andtroubleshooting. Applications varyfrom up-close testing of printed cir-cuit boards, motors, bearings, steamtraps and injection moldingmachines, to checking temperatures

Products & Applications 8


Figure 8-1: Ambient Effects on IR Thermometer Accuracy

Atmosphere Emission and Absorption

Radiation Thermometer

Surroundings

Target

Hand-held IR thermometers include such

options as laser sights.

remotely in building insulation, pip-ing, electrical panels, transformers,furnace tubes and manufacturing andprocess control plants.

Because an infrared device mea-sures temperature in a “spot” definedby its field of view, proper aiming canbecome critical. Low-end pyrome-ters have optional LED aiming beams,and higher end thermometers haveoptional laser pointing devices tohelp properly position the sensor.

Permanently mounted devices aregenerally installed on a manufactur-ing or process control line, and out-put their temperature signals to acontrol or data acquisition system.Radiation thermometers, two-colorsensors, fiber optics, infrared ther-mocouples, and linescanners can allbe permanently mounted.

In a permanent installation, aninstrument can be very carefullyaimed at the target, adjusted for theexact emissivity, tuned for responsetime and span, connected to aremote device such as an indicator,controller, recorder or computer, andprotected from the environment.Once installed and checked out,such an instrument can run indefi-nitely, requiring only periodic main-tenance to clean its lenses.

Instruments designed for perma-nent installation are generally morerugged than lab or portable instru-ments, and have completely differ-ent outputs. In general, systems thatoperate near a process are ruggedi-zed, have NEMA and ISO industrial-rated enclosures, and output stan-dard process control signals such as4-20 mA dc, thermocouple mV sig-nals, 0-5 Vdc, or serial RS232C.

For very hot or dirty environ-ments, instruments can be equippedwith water or thermoelectric coolingto keep the electronics cool, and

nitrogen or shop air purging systemsto keep lenses clean.

Application GuidelinesFor first level sorting, consider speedof response, target size (field ofview), and target temperature. Oncethe list of possible candidates for theapplication has been narrowed, con-sider things like band pass and sensi-tivity of the detector, transmission

quality of the optical system andtransmission quality of any windowsor atmosphere in the sighting path,emissivity of the target, ambientconditions, and the process dynam-ics (steady state variations or stepchanges). These are shown graphical-ly in Figure 8-1.

If 90% response to a step changein temperature is required in lessthan a few seconds, pyrometers withthermal detectors may not be suit-able, unless you use thermopiles. Apyrometer with a photon detectormay be a better choice.

Thermometers with targets of 0.3to 1 inch diameter with a focal dis-tance of 1.5 to 3 feet from the lensare common. If a target size in this

range is needed to sight on a largetarget through a small opening in afurnace, a pyrometer in which targetsize increases rapidly with distancebeyond the focal plane may be fitthe bill. Otherwise, a thermometerwith more sophisticated optics andsignal conditioning may be required.

If the temperature to be measuredis below 750°F (400°C) a more sophis-ticated pyrometer with opticalchopping can improve performance.

If the surroundings between thethermometer and the target are notuniform, or if a hot object is present,it is desirable to shield the field ofview of the instrument so that thesephenomenon have minimal effect onthe measurement.

Any radiation absorbed or generat-ed by gases or particles in the sightingpath will affect measured target tem-perature. The influence of absorbingmedia (such as water vapor) can beminimized by proper selection of thewavelengths at which the thermome-ter will respond. For example, apyrometer with a silicon detectoroperates outside the absorptionbands of water vapor and the error isnil. The influence of hot particles can



Figure 8-2: Sighting on a Specular Surface

Target

Hot Furnace Walls

Thermometer

be eliminated by ensuring they do notenter the sighting path, or by peak orvalley picking, if they are transientlypresent. A open ended sighting tube,purged with a low temperature gascan provide a sighting path free ofinterfering particles.

Thermometers selected to mea-sure transparent targets, such asglass or plastic films, must operateat a wavelength where the transmis-sion of these materials is low so hotobjects behind the target do notinterfere with the measurement. Forexample, most glass is opaque atwavelengths above 5 microns if it is3 mm or thicker. The emittance ofglass decreases at higher wave-lengths above 8 microns because ofits high reflection, so measurementat higher wavelengths is not asdesirable. If the incorrect band ispicked, the thermometer will sightthrough the glass and not read thesurface temperature.

Imagine, for example, two ther-mometers measuring the surfacetemperature of a lightbulb. One ther-mometer operates in the 8 to 14micron range, and the other operatesat 2 microns. The 8 to 14 microndevice reads the surface temperatureof the bulb as 90°C. The 2-microndevice, sees through the surface ofthe glass, to the filament behind, andreads 494°C.

Other parameters to considerwhen selecting a non-contact tem-perature sensor include:• Target material—The compositionof a target determines its emissivity,or the amount of thermal energy itemits. A blackbody is a perfect emit-ter, rated 1.0 or 100%. Other materialsare somewhat lower; their emissivitycan be anywhere from 0.01 to 0.99, or0-99%. Organic materials are very effi-cient, with emissivities of 0.95, while

polished metals are inefficient, withemissivities of 20% or less. Tables onlygive the emissivity of an ideal surface,and cannot deal with corrosion, oxi-dation or surface roughness. In thereal world, emissivity variations rangefrom 2 to 100% per 100°F temperaturechange. When in doubt, obtain anappropriate instrument and measurethe emissivity exactly.• Temperature range—The emissivi-ty and the range of expected tem-peratures of the target determine thewavelengths at which the target willemit efficiently. Choose a sensor thatis sensitive at those wavelengths.Accuracy is listed as a percent of fullscale or span, so the closer the tem-perature range to be measured canbe specified, the closer sensormatch, and the more accurate thefinal measurements.• Wavelength choice—Manufacturers

typically lists their products with agiven temperature range and wave-length, with wavelengths listed inmicrons. Note that more than onewavelength can apply in any givenapplication. For example, to measureglass, a wavelength of 3.43, 5.0 or 7.92microns can be used, depending onthe depth you want to measure, thepresence of tungsten lamps, or toavoid reflections. Measuring plasticfilms presents the same problems.You may want to use a broad spec-trum to capture most of the radiantemissions of the target, or a limitedregion to narrow the temperaturerange and increase accuracy. In manyapplications, various conditions andchoices may exist. You may want toconsult with your supplier. • Atmospheric interference—What ispresent in the atmosphere betweenthe sensor and the target? Most non-



DH

Figure 8-3: Use of Shielding and Cooling

Hot SourceHot Source

Open End Sighting Tube

Target

Cooled Shield

contact temperature sensors requirean environment that has no dust,smoke, flames, mist or other contam-inants in the sensor’s line of sight. Ifcontaminants exist, it may be neces-sary to use a two-color sensor. If thereis an obstructed line of sight, it may benecessary to use a fiber optic probe togo around the obstacles.• Operating Environment—Intowhat kind of environment will thesensor itself be installed? If it is haz-ardous, hot, humid, corrosive orotherwise unfriendly, it will be nec-essary to protect the instrument.Lenses and cases are available towithstand corrosives; air purge sys-tems can protect lenses fromprocess materials; and various cool-ing systems are available to cool thelenses, optics and electronics.

If the surrounding temperature isthe same as the target temperature,the indicated temperature from aradiation thermometer will be accu-rate. But if the target is hotter thanthe surroundings, it may be desirableto use a device with a high N* valueto minimize the emissivity error andminimize radiation from the sur-roundings reflected into the ther-mometer. Two approaches can beused when the target is at a lowertemperature than the surroundings.The first method, Figure 8-2, is pos-sible if the target is fixed, flat, andreflects like a mirror. The ther-mometer is arranged so that it sightsperpendicular to the target.

To measure the temperature of atarget with a matte surface, youmust shield the field of view of thethermometer so that energy fromhot objects does not enter. Oneapproach, shown in Figure 8-3,involves sighting the thermometerthrough an open ended sightingtube. The other approach is to use a

cooling shield. The shield must belarge enough so that D/H ratio is 2to 4. This method can not be usedfor slowly moving or stationary tar-gets. An uncooled shield can beused to block out radiation from asmall, high temperature source thatwill not heat it significantly.

A closed end sight tube is an acces-sory that can be used to protectoptics and provide a clear sight pathfor broadband thermometers. Theone end of the tube reads the sametemperature as the target (it may betouching the target or very close to it),while cooling can be used to protectthe thermometer itself, at the otherend of the tube, from high tempera-tures. A closed or open end sight tubecan prevent attenuation of emittedradiation by water vapor, dust, smoke,steam and radiation absorptive gasesin the environment.

Industrial applications invokeeither surface temperature of

objects in the open, or tempera-tures inside vessels, pipes and fur-naces. The target may need to sightthrough a window in the lattercase. The thermometer, if perma-nently installed, can be mounted toan adjacent pedestal, or attachedto the vessel. Hardware is availablefrom manufacturers to accomplishthis. The thermometer housing mayneed to be protected from exces-sive heat via a cooling mechanism,and/or may require a continuousclean gas purge to prevent dirtaccumulation. Hardware is option-ally available for both needs.

The accessories needed for diffi-cult applications, for example, topermanently install a radiation ther-mometer on the wall of a furnace,can easily escalate the cost of aninfrared thermometer into the thou-sands of dollars, doubling the priceof the standard instrument. In Figure8-4, for example, the thermocouple



Figure 8-4: Accessories for Furnace-Wall Installation

Refactory Target Tube

Silicon Carbide Sight Tube

Sight Tube

Pipe Mount Flange

Safety Shutter

Air Purge Assembly

Sensor Head in Protective Cooling

Jacket

End Cover

*Refer to page 25

sensor head and its aiming tube aremounted inside a cooling jacket. Thecoolant flow required depends onthe actual ambient conditions whichexist. Also shown are an air purgeassembly, and a safety shutter. Thelatter allows the furnace to be sealedwhenever the radiation thermometermust be removed.

If the target and the surroundingsare not at the same temperature,additional sensors, as shown inFigure 8-5 need to be supplied. Thisconfiguration allows automaticcompensation in the radiation ther-mometer electronics for the effectsof the surroundings on the targettemperature reading.

There is a lot to consider whenselecting and installing a non-con-tact sensor to measure a criticalprocess temperature. And to the

unfamiliar, the task can seem mindboggling. How do I get emissivitydata? Which wavelength(s) is best formy application? What options do Ireally need? .... and a thousand otherquestions easily come to mind. Buthelp is available. For example, manymanufacturers have open Internetsights that contain an abundance of

helpful information to assist the firsttime user in getting started. (See listof resources, p. 68.) In addition, thereare consultants, as well as the manu-facturers themselves, who can sup-ply all the assistance needed to getup and running quickly.

• Industrial ApplicationsIn most cases, at least one of the sen-sors we’ve discussed can be used tomeasure temperature in any kind of

application, from -50 to 6,500°F . Thekey is to identify the sensor that willdo the best job. This can be a verysimple or an extremely difficultchoice. Perhaps some of the applica-tions listed below will give you a fewideas on how to use a non-contacttemperature sensor in your plant. • Airplane Checkout—The sheer sizeand height of a widebody 747 aircraftmakes it very difficult for techniciansto check the operation of variousdevices, such as pitot tubes andheating tapes used to warm pipes,water and waste tanks in variousparts of the aircraft. Before, a techni-cian had to climb a 25-ft ladder andtouch the surfaces to see if thedevices were working properly.

Now, a radiation thermometer isused during final assembly to checkthe operation of various heating ele-ments. The technician stands on theground, and aims the thermometer ateach pitot tube or heating element.Boeing reports saving 4-5 construc-tion hours on each jet.• Asphalt—Asphalt is very sensitiveto temperature during preparationand application. Thermocouples nor-mally used to measure asphalt tem-perature usually have severe breakageproblems because of the abrasivenessof the material. Infrared thermocou-ples are an ideal replacement.

The sensor can be mounted sothat it views the asphalt through asmall window in the chute, or slight-ly above for viewing at a distance. Ineither case, the sensor should havean air purge to keep the lens cleanfrom vapor or splashes. Plus, it can beconnected to the control system as ifit was a thermocouple.• Electrical System Maintenance—Infrared scanning services arebecoming widely available. Typically,a scanning service brings in a



Figure 8-5: Compensation for Elevated Ambient Temperatures

Sum of Emitted And Reflected Signals

Background TemperatureSignal Processor

Corrected Object Temperature

Emissivity Reflectivity Correction

portable imaging processor andscanner twice a year to check abuilding’s switchgear, circuit breakers,and other electrical systems. The ser-vice looks for hot spots and temper-ature differences.

Between visits, maintenance per-sonnel can perform spot checks andverify repairs with an inexpensive radi-ation thermometer. Attaching a datalogger lets a technician determineheating trends of switchgear duringpeak periods, and identify the parts ofsystem that suffer the most whenelectrical consumption goes up.• Flame Cutting—In flame cutting,before a computer cuts variousshapes from steel plate, the steelsurface has to be heated by a natur-al gas or propane flame. When a“puddle” of molten metal is detect-ed by the operator, oxygen is inject-ed into the gas stream. This blowsthe molten metal through the plateand the cutting cycle begins. If oxy-gen is injected prematurely, it makesa defective cut, leaving an objec-tionable rough and wide pit-likedepression in the plate.

A fiber optic sensor can be mount-ed on the torch and aimed to lookthrough the gas stream at the platesurface. It will detect the proper platetemperature for puddling, and informthe operator.• Glass—An infrared thermometer isideal for measuring the temperatureof soda-lime-silica glass, predomi-nantly used in making sheet, plate,and bottles. The biggest problem isthat glass has relatively poor thermalconductivity, so temperature gradi-ents exist at various depths. Thethree most commonly used wave-lengths for measuring glass—3.43, 5.0,and 7.92 microns—each see a differ-ent distance into the glass. A sensorwith 7.92 microns sees only the sur-

face, while a 3.43 micron sensor cansee up to 0.3 in. into the glass.

The trick is to select a thermome-ter which is not adversely influencedby thickness variations. Your best betmay be to send samples of glassproducts to the thermometer manu-facturer, and let them advise you onwhat device to use.

During installation, select the aim-ing point so that the instrumentdoesn’t see any hot objects behind

the transparent glass, or any reflectedradiation from hot objects in front ofthe glass. Aim the sensor at an anglethat avoids reflections, or install anopaque shield to block the reflec-tions at the source. If neither is possi-ble, use either of the higher wave-length sensors, because they are notaffected as much by reflections.

Be careful of applications wherethe glass is heated with high inten-sity, tungsten filament quartz



Cement Kilns Burning zones, preheaters

Energy Conservation Insulation and heat flow studies, thermal mapping

Filaments Annealing, drawing, heat treating

Food Baking, candy-chocolate processing, canning freezing, frying, mixing, packing, roasting

Furnaces flames, boiler tubes, catalytic crackers

Glass Drawing, manufacturing/processing bulbs, containers, TV tubes, fibers

Maintenance Appliances, bearings, currentoverloads, drive shafts, insulation, power lines, thermal leakage detection

Metals (ferrous and nonferrous) annealing, billet extrusion, brazing, carbonizing, casting, forging, heat treating, inductive heating, rolling/strip mills, sintering, smelting Metals, Pouring

Quality Control printed circuit boards, soldering, universal joints, welding, metrology

Paint Coating, ink drying, printing, photographic emulsions, web profiles

Paper Blow-molding, RIM, film extrusion, sheet thermoforming, casting

Plastic Blow-molding, RIM, film extrusion, sheet thermoforming, casting

Remote Sensing Clouds, earth surfaces, lakes, rivers, roads, volcanic surveys

Rubber Calendaring, casting, molding, profile extrusion, tires, latex gloves

Silicon Crystal growing, strand/fiber, wafer annealing, epitaxial deposition

Textile Curing, drying, fibers, spinning

Vacuum Chambers Refining, processing, deposition

2=2-color sensor H=High Temperature L=Low Temperature

2 H L 2 H L

• • • •

• •

• •

• •

• • • •

• • • • • •

• •

• • • •

• • • • • •

•

• •

• •

• • •

• •

• • • •

• •

• •

Table 8-2: Successful Radiation Thermometer ApplicationsMOUNTED PORTABLES

•

lamps. These generate radiationlevels that interfere with ther-mometers operating below 4.7microns. In this case, use a 7.92micron sensor.• Glass Molds—The temperature ofthe mold or plunger used to makeglass containers is critical: if too hot,the container may exit the mold andnot retain its shape; if too cool, itmay not mold properly. Molds mustbe measured constantly to ensurethat cooling is proceeding correctly.

An infrared thermometer can beused to take mold measurements. Afew suggestions: Don’t measure newmolds. They are usually shiny andclean, so they are reflective and havelow emissivity. As they get older,

they get dull and non-reflective, andthe emissivity becomes higher andmore repeatable. Use a radiationthermometer with a short wave-length, such as 0.9 microns, or a two-color instrument. • Humidity—An infrared thermocou-ple can be used to measure relativehumidity in any situation wherethere is a convenient source of waterand flowing air. Aim the device at a

wet porous surface with ambient airblowing across. When air movesacross a wet surface, water cools byevaporation until it reaches the wet-bulb temperature, and cooling stops.The sensor can be connected to adisplay that records the lowest tem-perature, which is the wet-bulb tem-perature, and can be used to calcu-late the relative humidity.• Immersion Thermowells—Thermowells protrude into a high-pressure vessel, stack, pipe or reac-tor, allowing a temperature sensorto get “inside” while maintainingprocess integrity. An infrared ther-mocouple or fiber optic sensor canbe positioned outside the ther-mowell looking in, rather than being

mounted inside the thermowell.Conventional sensors subjected toconstant high temperatures suffermetallurgical changes that affectstability and drift. But the non-con-tact sensors, because they are out-side, do not suffer such problems.They also respond more quickly;essentially, the response time of aradiation sensor is the same as thethermowell. Also, since the sensor

is outside, it will survive muchlonger in a very high temperatureenvironment than a conventionalsensor will.

To install a radiation thermometerin a thermowell, mount it so it isaimed directly into a hollow ther-mowell, and adjust its distance sothat its “spot size” is the same diam-eter as the thermowell. This way, thesensor will monitor temperature atthe thermowell tip. If the thermowellhas a sight glass, select a sensor thatcan see through it. • Induction Heating—Measuring thetemperature of an induction heatingprocess can be accomplished withinfrared thermocouples, thermome-ters or fiber optic sensors.

An infrared thermocouple willoperate in the very strong electricalfield surrounding induction heaters.Make sure the sensor’s shield wire isattached to a proper signal ground.The preferred method is to view thepart between the coil turns or fromthe end. If there is excessive heatingon the sensor, use a water coolingjacket (you can use the same watersource used to cool the inductioncoil).

Fiber optic sensors should bemounted so the viewing end isplaced close to the target. The tip ofthe fiber can be positioned betweenthe induction coils. Replaceableceramic tips can be used to minimizedamage and adverse effects from theradio frequency field. If the fiberwon’t fit, use a lens system to moni-tor the surface from a distance. Fiberoptic sensors are not normallyaffected by induction energy fields,but if the noise is excessively high,use a synchronous demodulationsystem. The demodulator convertsthe 400 Hz ac signal from the detec-tor head to dc, which is more



Table 8-3: Typical Application Temperature RangesAPPLICATION TEMP. RANGES

General purpose for textile, printing, food, rubber, thick plastics, paints, laminating, maintenance

Life sciences, biology, zoology, botany, veterinary medicine, heat loss and research

Thin film plastic, polyester, fluorocarbons, low temperature glass

Glass and ceramic surfaces, tempering,annealing, sealing, bending and laminating

See-through clean combustion flames and hot gases. Furnace tubes

Medium to high temperature ferrous and non-ferrous metals. See-through glass

Hot and molten metals, foundries, hardening, forging, annealing, induction heating

-50 to 1000°C -58 to 1832°F

0 to 500°C 32 to 932°F

50 to 600°C 122 to 1112°F

300 to 1500°C 572 to 2732°F 500 to 1500°C 932 to 2732°F

250 to 2000°C 482 to 3632°F

600 to 3000°C 1112 to 5432°F

immune to noise.• Plastic Film—A film of plastic orpolymer emits thermal radiation likeany other material, but it presentsunique measuring problems for anysensor, including a radiation ther-mometer. As with glass, when mea-suring film temperature, it’s impor-tant to install it so the instrumentdoesn’t see any hot objects behindthe transparent film, or any reflect-ed radiation from hot objects infront of the film.

For films of 1, 10 or 100 mil thick-

nesses, a wavelength of 3.43 or 7.92microns will work for celluloseacetate, polyester (polyethyleneterephthalate), fluoroplastic (FEP),polymide, polyurethane, polyvinylchloride, acrylic, polycarbonate,polymide (nylon), polypropylene,polyethylene and polystyrene.

As with glass, be careful of appli-cations where the film is heated withhigh intensity, tungsten filamentquartz lamps. These generate radia-tion levels that interfere with ther-mometers operating below 4.7

microns. In this case, use a 7.92micron sensor.• Web Rollers—Infrared sensors canbe used to measure the temperatureof rollers used in various web process-es, even if they are chrome plated.Uncoated metal or chrome rollers aredifficult for an IR sensor to see,because they have low emissivity andthe sensor sees too many environ-mental reflections. In such a case,paint a black stripe on an unused por-tion of the roller and aim the devicedirectly at the stripe.



Aluminum

Asphalt

Automotive

Appliances

Ammunition

Batteries

Cement

Construction Materials

Fiberglass

Food Processing

Foundry

Glass-Melting

Glass-Flat

Glass Bottles

Heat Treating

Induction Heating

Kilns

Metalworking

Mining

Non-ferrous Metals

Ovens

Paper

Pharmaceutical

Plastics

Plastic Films

Rubber

Semiconductors

Steel

Textiles

Utilities

• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •

TYPICAL 0.65 0.9 1.0 0.7-1.08 1.55 1.65 2.0 3.43 3.9 5.0 7.9 8-14 APPLICATIONS and 1.68 and 1.68 2-color 2-color

Table 8-4: Application Wavelengths (Microns)

SOUR

CE: IR

CON

Dull metal rollers often providereliable signals. Emissivity can shift ifthe rollers get covered with dirt,moisture or oil. If in doubt, simplypaint a stripe. Non-metallic surfacedrollers provide a reliable signal nomatter where the device is pointed.

Accessories, Features & OptionsRadiation thermometers and ther-mocouples are available with a hostof features to solve a wide range ofapplication conditions. All infraredsensors are available in a wide rangeof wavelengths, temperature rangesand optical systems. Portable unitsalmost always are available with car-rying kits, and permanently mount-ed units are ruggedized. Listedbelow are other options, featuresand accessories that make thesesensors more useful for certaintypes of applications.

Backlit LCD displays, integrallyattached or remotely mountedfrom the thermometer, are avail-able. Multiple variables can beviewed simultaneously on thesedisplays. These data can includecurrent temperature, minimummeasured temperature (time based),maximum measured temperature(time based), average temperaturemeasured (time based), and differ-ential temperature (for example,between the target and the sur-roundings).

Microprocessor-based radiationthermometers have input options toallow data to be integrated into themeasurement from other sensors orthermometers in the loop. For exam-ple, a separate thermocouple or RTDinput to the thermometer can beused to compensate the measuredtarget temperature for changingambient temperature conditions.

Protection from high electromag-netic and radio frequency interfer-ence (EMI/RFI) is available if thethermometer must be installed in adifficult environments.

Most infrared thermometers canbe supplied with an emissivityadjustment. In addition, somedevices can be supplied with anadjustable field of view. This isaccomplished by installing an iris inthe optical system that can beopened or closed to provide wide ornarrow angle field of views.

• Handheld IR ThermometersHandheld instruments are generallycompletely self-contained, battery-powered units, with manual controlsand adjustments and some form ofdigital readout. Units can be mountedon tripods. Other accessories include:• Laser sights, which paint a visiblespot on the target, making it easier todetermine where the instrument ispointed. This option is available bothintegrally attached or detachablefrom the thermometer. Hand-helddevices used for up-close spot tem-perature measurement (for example,to measure component temperatureon printed circuit boards) can haveaudible focusing guides instead oflight markers.• Dataloggers, for acquiring datafrom thermometers and recording itfor future use;• Digital printers• Electrical system scanners, designedspecifically for finding hot spots inelectrical panels, switchgear, fusepanels, transformers, etc. • Handheld, shirt-pocket-size scan-ner for general surface temperaturemeasurement.• Outputs: RS232C serial and/or 1mV/degree.

• Infrared ThermocouplesThese self-powered devices generate athermocouple signal output usingradiated energy, but usually have nosignal processing or display systems.An infrared thermocouple is a sensoronly, but it does have a few optionsand accessories.• Cooling jacket kits for air or watercooling;• Handheld version for precise spotmeasurements;• Close-focus model with up to 60:1field of view;• Periscope kit for right-angle mea-surements;• Low-cost ($99) model with ABSplastic housing;• Adjustable emissivity;• Two-color pyrometry unit thatuses short-wave and long-waveinfrared thermocouples.

• Fiber Optic SensorsProbes are available in lens cells ofvarious sizes, with replaceableglass or quartz tips. Optionsinclude a ceramic/metal tip forhigh temperatures, a polymer boltfor extrusion applications, ejectorpin probe for injection molding,and right angle prisms. Sensorprobes also are available as opticalrods up to 60 cm long.

Cables can be supplied in single,bifurcated or trifurcated fiber opticbundles, and enclosed in jacketsmade of flexible stainless steel (stan-dard), ceramic, heavy duty wire braidfor abrasion resistance, or Teflon forhigh radio frequency fields. Cablestypically are up to 30 ft long.

• Indicators and Controllers Display units and controllers areavailable in models ranging from a



simple digital panel meter that dis-plays the signal as a temperature in °For °C, to complex multi-channelprocessors that perform signal con-ditioning, linearization, peak-picking,alarm monitoring, saving min/maxvalues, signal averaging, data loggingand a host of other signal processingand manipulation functions.

• Mounted IR ThermometersThe same basic features, optionsand accessories are available forradiation thermometers, two-colorsystems, and line scanners.Ruggedized for use on the plantfloor, all these devices have severalaccessories to help them survive inhostile environments.• Air purge—Attaches to front endof sensor housing and provides posi-tive air pressure in front of the lens,preventing dust, smoke, moisture andother contamination from reachinglens. In two-color systems, it canattach to front of re-imaging lens. • Air or water cooling jackets—Available for warm (35°F above ambi-ent) and hot (up to 400 °F) environ-ments, cooling jackets keep sensor

temperature at normal levels insidethe enclosure.• Peltier effect cooling—Some linescanners have electronic cooling sys-tems, using Peltier effect devices. • Sighting accessories, includingsight tubes, laser pointing devices,and scopes.

• Onboard data logging functionsare available, as well as options forthermal printers to retrieve storeddata. Data can also be remotelytransmitted digitally.• Transmitters—Ruggedized NEMA 4housing with 4-20 mAdc and/orRS232C/RS485 outputs. T



References and Further Reading• Handbook of Temperature Measurement & Control, Omega Press, 1997.• New Horizons in Temperature Measurement & Control, Omega Press,1996.• Product Previews in Temperature Measurement & Control, 21st Century™Preview Edition, Omega Press, 1997.• Temperature Measurement in Engineering, H. Dean Baker, E. A. Ryder, andN. H. Baker, Omega Press, 1975.• “Glass Temperature Measurement,” Technical Note 101, Ircon Inc., Niles, Ill.• Handbook of Non-Contact Temperature Sensors, Exergen Corp.,Watertown, Mass., 1996.• “How Do You Take Its Temperature?, ” Aviation Equipment Maintenance,February 1992.• “How Infrared Thermometers are Gaining Acceptance,” Paul Studebaker,Control, July 1993.• IR Answers and Solutions Handbook, Ircon Inc., Niles, Ill.• “On-Line Industrial Thermal Imaging Systems Evolve Expanding InfraredMeasuring Capabilities,” George Bartosiak, Industrial Heating, December 1992.• “Plastic Film Measurement,” Technical Note 100, Ircon Inc., Niles, Ill.• “Preventive Maintenance Program Averts Crashes with IRThermometer/Thermal Scanning,” Engineer’s Digest, September 1989.

Academy of Infrared Thermography2955 Westsyde Road, Kamloops, BC, Canada, V2B 7E7 250/579-7677 www.netshop.net/~academy/

American Ceramic Society65 Ceramic Drive, Columbus, OH 43214 614/268-8645 www.acers.org

American Institute of Chemical Engineers (AIChE) 345 East 47 Street, New York, NY 10017-2395 212/705-7338 www.aiche.org

American Society of Mechanical Engineers (ASME)345 East 47th Street, New York, NY 10017 212/705-7722 www.asme.org

Electric Power Research Institute (EPRI)3412 Hillview Avenue, Palo Alto, CA 94303 415/855-2000 www.epri.com

Fiber Optics Sensor System Facilities & Optical Fiber Drawing & Measuring Facilities, Dept. of the Navy 202/767-3744

4555 Overlook Avenue, Washington, DC 20375

Infrared Information and Analysis Center (IRIA)Dept. of the Navy 313/994-1200 www.erim.org/IRIA

PO Box 8618, Ann Arbor, MI

Infraspection InstituteShelburne, VT 802/985-2500 www.together.net/~werir

Institute of Electrical & Electronics Engineers (IEEE)445 Hoes Lane, Piscataway, NJ 08855-1331 732/981-0060 www.ieee.org

ISA—The International Society for Measurement & Control67 Alexander Drive, Research Triangle Park, NC 27709 919/549-8411 www.isa.org

International Society for Optical Engineers (SPIE)PO Box 10, Bellingham, WA 98277 206/676-3290 www.spie.org

Lawrence Berkeley National Laboratory, Infrared Thermography Laboratory 510/486-6844 http://ucaccess.uirt.uci.edu/

Berkeley, CA 94720 (Dariush Arasteh)

National Institute of Standards & TechnologyGaithersburg, MD 20899-0001. 301/975-3058 www.nist.gov

Information Resources

Information ResourcesORGANIZATIONSNAME/ADDRESS PHONE WEB ADDRESS


For the Latest Informationon Non-Contact TemperatureInstrumentation Products:

Omega Engineering, Inc.One Omega DriveP.O. Box 4047Stamford, CT 06907-0047Phone: 800-82-66342 (800-TC-OMEGASM)Email: [email protected]: www.omega.com



Book of Books: Scientific & Technical Books, Software & Videos, Omega Press, 1998.

Handbook of Temperature Measurement & Control, Omega Press, 1997.

New Horizons in Temperature Measurement & Control,Omega Press, 1996.

Omega Handbook & Encyclopedia, Purchasing Agent Edition,Omega Press, 1995.

Product Previews in Temperature Measurement & Control, Omega Press, 1997.

Product Previews in Temperature Measurement & Control, 21st Century Preview Edition, Omega Press, 1997.

Temperature Measurement in Engineering, H. Dean Baker, E. A. Ryder, and N. H. Baker, Omega Press, 1975.

Album of Science, The 19th CenturyPearce L. Williams, Charles Scribner’s Sons, 1978.

Applications of Infrared Technology (SPIE Proceedings, Vol. 918) T.L. Williams (editor), SPIE, 1989.

Applications of Thermal Imaging S.G. Burnay, T.L. Williams, and C.H. Jones (editor), Adam Hilger, 1988.

Asimov’s Chronology of Science and Discovery Isaac Asimov, HarperCollins Publishers, 1994.

The Biographical Dictionary of Scientists2nd ed., Oxford University Press, 1994.

Dictionary of Scientific Biography, Vols. 9, 10, 11Charles C. Gillispile, Charles Scribner’s Sons, 1973.

Detecting Delaminations in Bridge Decks Using Infrared Thermography (ASTM Standard D4488-88).

The Detection and Measurement of Infrared RadiationR.A. Smith, F. E. Jones, and R. P. Chasmar, Oxford at Clarendon Press, 1968.

Engineering in HistoryRichard S. Kirby and Sidney Withington, Arthur B. Darling, Frederick G. Kilgour, McGraw-Hill, 1956.

Fiber Optic SensorsEric Udd, John Wiley & Sons, 1991.

Fundamentals of Infrared Detector Operation and Testing (Wiley Series in Pure and Applied Optics)John David Vincent, John Wiley & Sons, 1990.

Glass Temperature Measurement, Technical Note 101Ircon Inc., Niles, Ill.

Handbook of Infrared Optical Materials (Optical Engineering Series, Vol. 30)Paul Klocek (editor), Marcel Dekker, 1991.

OMEGA PRESS REFERENCES

OTHER REFERENCE BOOKS



Handbook of Intelligent Sensors for Industrial AutomationNello Zuech, Addison-Wesley Publishing Company, 1992.

Handbook of Non-Contact Temperature SensorsExergen Corp., Watertown, Mass., 1996.

Handbook of Temperature Measurement & ControlOmega Engineering Co., 1997.

Heat and Thermodynamics, 6th ed.Mark W. Zemansky, and Richard H. Dittman, McGraw-Hill, 1981.

Industrial Temperature MeasurementThomas W. Kerlin and Robert L. Shepard, Publishers Creative Series, Inc., ISA.

Infrared DetectorsR. D. Hudson and J. W. Hudson (editor), Van Nostrand Reinhold, 1975.

Infrared Detectors : State of the Art (SPIE Proceedings,Vol. 1735)Wagih H. Makky (editor), SPIE, 1992.

Infrared Detectors : State of the Art II (SPIE Proceedings, Vol. 2274)Randolph E. Longshore (editor), SPIE, 1994.

Infrared Detectors and Systems (Wiley Series in Pure and Applied Optics)Eustace L. Dereniak and G. D. Boreman, John Wiley & Sons, 1996.

The Infrared and Electro-Optical Systems HandbookJoseph S. Accetta, David L. Shumaker (editor), SPIE, 1993.

Infrared and Optoelectronic Materials and Devices (SPIE Proceedings, Vol. 1512)Ahmed Naumann, SPIE, 1991.

Infrared Fiber Optics III (SPIE Proceedings Series, Vol. 1591)James A. Harrington and Abraham Katzir (editor), SPIE, 1992.

Infrared RadiationHenry L. Hackford, McGraw-Hill, 1960.

Infrared System Engineering (Pure and Applied Optics)Richard D. Hudson, John Wiley & Sons, 1969.

Infrared Technology Fundamentals (Optical Engineering Series 22)Irving J. Spiro and Monroe Schlessinger, Marcel Dekker, 1989.

The Infrared Temperature HandbookOmega Engineering, 1994.

Infrared Thermography (Microwave Technology, Vol 5)G. Gaussorgues and S. Chomet (translator), Chapman & Hall, 1994.

Instrument Engineers’ Handbook, Third EditionB. Liptak, Chilton Book Co. (CRC Press), 1995.

An International Conference on Thermal Infrared Sensing for Diagnostics and Control (Thermosene Vii)Andronicos G. Kantsios (editor), SPIE,1985.

Introduction to Heat Transfer, 2nd ed.Frank P. Incropera, and David P. DeWitt, John Wiley & Sons, 1990.

An Introduction to the Principles of Infrared PhysicsHayes Aircraft Corp., Infrared Radiation Staff, , Birmingham, Alabama, 1956.



The Instrument Engineer’s HandbookB. Liptak, ed., Chilton, 1996.

IR Answers and Solutions HandbookIrcon Inc., Niles, Ill.

The Invisible World of the InfraredJack R. White, Dodd, Mead & Company, 1984.

The McGraw-Hill Encyclopedia of Science and Technology, 8th ed., Vol. 9McGraw-Hill, 1997.

Measurements for Competitiveness in ElectronicsNIST Electronics and Electrical Engineering Laboratory,1993.

Nondestructive Evaluation of Materials by Infrared ThermographyXavier P.V. Maldague, Springer Verlag, 1993.

Notable Twentieth-Century ScientistsEmily J. McMurray, Gale Research Inc., 1995.

Optical Fiber Sensors: Systems and Applications, Vol 2B. Culshaw and J. Dakin, Artech House; 1989.

Pioneers of Modern Science, The World of ScienceBill MacKeith, Andromeda Oxford Limited, 1991.

Plastic Film Measurement, Technical Note 100Ircon Inc., Niles, Ill.

Practical Applications of Infrared Thermal Sensing and Imaging Equipment (Tutorial Texts in Optical Engineering, Vol. 13), Herbert Kaplan, SPIE, 1993.

Process/Industrial Instruments and Controls Handbook, 4th ed.Douglas M. Considine, McGraw-Hill, 1993.

The Scientific 100. A Ranking of the Most Influential Scientists, Past and PresentJohn Simmons, Carol Publishing Group, 1996.

Sensors and Control Systems in ManufacturingS. Soloman, McGraw-Hill, 1994.

Theory and Practice of Radiation ThermometryDavid P. DeWitt and Gene D. Nutter, John Wiley & Sons, 1988.

Thermodynamics, 5th ed.Virgil M. Faires, The Macmillan Company, 1971.

Emissivity Tables

Alloys20-Ni, 24-CR, 55-FE, Oxidized 392 (200) .9020-Ni, 24-CR, 55-FE, Oxidized 932 (500) .9760-Ni, 12-CR, 28-FE, Oxidized 518 (270) .8960-Ni, 12-CR, 28-FE, Oxidized 1040 (560) .8280-Ni, 20-CR, Oxidized 212 (100) .8780-Ni, 20-CR, Oxidized 1112 (600) .8780-Ni, 20-CR, Oxidized 2372 (1300) .89

AluminumUnoxidized 77 (25) .02Unoxidized 212 (100) .03Unoxidized 932 (500) .06Oxidized 390 (199) .11Oxidized 1110 (599) .19Oxidized at 1110°F (599°C) 390 (199) .11Oxidized at 1110°F (599°C) 1110 (599) .19Heavily Oxidized 200 (93) .20Heavily Oxidized 940 (504) .31Highly Polished 212 (100) .09Roughly Polished 212 (100) .18Commercial Sheet 212 (100) .09Highly Polished Plate 440 (227) .04Highly Polished Plate 1070 (577) .06Bright Rolled Plate 338 (170) .04Bright Rolled Plate 932 (500) .05Alloy A3003, Oxidized 600 (316) .40Alloy A3003, Oxidized 900 (482) .40Alloy 1100-0 200-800 (93-427) .05Alloy 24ST 75 (24) .09Alloy 24ST, Polished 75 (24) .09Alloy 75ST 75 (24) .11Alloy 75ST, Polished 75 (24) .08

BismuthBright 176 (80) .34Unoxidized 77 (25) .05Unoxidized 212 (100) .06

Brass73% Cu, 27% Zn, Polished 476 (247) .03

73% Cu, 27% Zn, Polished 674 (357) .0362% Cu, 37% Zn, Polished 494 (257) .0362% Cu, 37% Zn, Polished 710 (377) .0483% Cu, 17% Zn, Polished 530 (277) .03Matte 68 (20) .07Burnished to Brown Color 68 (20) .40Cu-Zn, Brass Oxidized 392 (200) .61Cu-Zn, Brass Oxidized 752 (400) .60Cu-Zn, Brass Oxidized 1112 (600) .61Unoxidized 77 (25) .04Unoxidized 212 (100) .04

Cadmium 77 (25) .02Carbon

Lampblack 77 (25) .95Unoxidized 77 (25) .81Unoxidized 212 (100) .81Unoxidized 932 (500) .79Candle Soot 250 (121) .95Filament 500 (260) .95Graphitized 212 (100) .76Graphitized 572 (300) .75Graphitized 932 (500) .71

Chromium 100 (38) .08Chromium 1000 (538) .26Chromium, Polished 302 (150) .06Cobalt, Unoxidized 932 (500) .13Cobalt, Unoxidized 1832 (1000) .23

ColumbiumUnoxidized 1500 (816) .19Unoxidized 2000 (1093) .24

CopperCuprous Oxide 100 (38) .87Cuprous Oxide 500 (260) .83Cuprous Oxide 1000 (538) .77Black, Oxidized 100 (38) .78Etched 100 (38) .09Matte 100 (38) .22Roughly Polished 100 (38) .07

Emissivity of Common Materials


Note: Because the emissivity of a given material will vary with temperature and surface finish, the value in these tables should be used only as a guidefor relative or differential temperature measurements. The exact emissivity of a material should be determined when high accuracy is required.

METALSMATERIAL TEMP °F (°C) ε-EMISSIVITY MATERIAL TEMP °F (°C) ε-EMISSIVITY

A

B

C

Emissivity Tables


Polished 100 (38) .03Highly Polished 100 (38) .02Rolled 100 (38) .64Rough 100 (38) .74Molten 1000 (538) .15Molten 1970 (1077) .16Molten 2230 (1221) .13Nickel Plated 100-500 (38-260) .37

Dow Metal 0.4-600 (-18-316) .15Gold

Enamel 212 (100) .37Plate (.0001)Plate on .0005 Silver 200-750 (93-399) .11-.14Plate on .0005 Nickel 200-750 (93-399) .07-.09Polished 100-500 (38-260) .02Polished 1000-2000 (538-1093) .03

Haynes Alloy COxidized 600 2000 (316-1093) .90-.96

Haynes Alloy 25Oxidized 600-2000 (316-1093) .86-.89

Haynes Alloy XOxidized 600-2000 (316-1093) .85-.88

InconelSheet 1000 (538) .28Sheet 1200 (649) .42Sheet 1400 (760) .58X, Polished 75 (24) .19B, Polished 75 (24) .21

IronOxidized 212 (100) .74Oxidized 930 (499) .84Oxidized 2190 (1199) .89Unoxidized 212 (100) .05Red Rust 77 (25) .70Rusted 77 (25) .65Liquid 2700-3220 (1516-1771) .42-.45

Iron, CastOxidized 390 (199) .64Oxidized 1110 (599) .78Unoxidized 212 (100) .21Strong Oxidation 40 (104) .95Strong Oxidation 482 (250) .95Liquid 2795 (1535) .29

Iron, Wrought

Dull 77 (25) .94Dull 660 (349) .94Smooth 100 (38) .35Polished 100 (38) .28LeadPolished 100-500 (38-260) .06-.08Rough 100 (38) .43Oxidized 100 (38) .43Oxidized at 1100°F 100 (38) .63Gray Oxidized 100 (38) .28

Magnesium 100-500 (38-260) .07-.13Magnesium Oxide 1880-3140 (1027-1727) .16-.20Mercury 32 (0) .09

Mercury 77 (25) .10Mercury 100 (38) .10Mercury 212 (100) .12

Molybdenum 100 (38) .06Molybdenum 500 (260) .08Molybdenum 1000 (538) .11Molybdenum 2000 (1093 .18Oxidized at 1000°F 600 (316) .80Oxidized at 1000°F 700 (371) .84Oxidized at 1000°F 800 (427) .84Oxidized at 1000°F 900 (482) .83Oxidized at 1000°F 1000 (538) .82

MonelMonel, Ni-Cu 392 (200 .41Monel, Ni-Cu 752 (400) .44Monel, Ni-Cu 1112 (600) .46Oxidized 68 (20) .43Oxidized at 1110°F 1110 (599) .46

NickelPolished 100 (38) .05 Oxidized 100-500 (38-260) .31-.46Unoxidized 77 (25) .05Unoxidized 212 (100) .06Unoxidized 932 (500) .12Unoxidized 1832 (1000) .19Electrolytic 100 (38) .04Electrolytic 500 (260) .06Electrolytic 1000 (538) .10Electrolytic 2000 (1093) .16

Nickel Oxide 1000-2000 (538-1093) .59-.86Palladium Plate (.00005 on

MATERIAL TEMP °F (°C) ε-EMISSIVITY MATERIAL TEMP °F (°C) ε-EMISSIVITY

D

G

H

I

M

N

P

Emissivity Tables


.0005 silver) 200-750 (93-399) .16-.17Platinum 100 (38) .05

Platinum 500 (260) .05Platinum 1000 (538) .10

Platinum, Black 100 (38) .93Platinum, Black 500 (260) .96Platinum, Black 2000 (1093) .97Oxidized at 1100°F (593°C) 500 (260) .07Oxidized at 1100°F (593°C) 1000 (538) .11

Rhodium Flash (0.0002 on 0.0005Ni) 200-700 (93-371) .10-.18

SilverPlate (0.0005 on Ni) 200-700 (93-371) .06-.07Polished 100 (38) .01Polished 500 (260) .02Polished 1000 (538) .03Polished 2000 (1093) .03

SteelCold Rolled 200 (93) .75-.85Ground Sheet 1720-2010 (938-1099) .55-.61Polished Sheet 100 (38) .07Polished Sheet 500 (260) .10Polished Sheet 1000 (538) .14Mild Steel, Polished 75 (24) .10Mild Steel, Smooth 75 (24) .12Mild Steel, Liquid 2910-3270 (1599-1793) .28Steel, Unoxidized 212 (100) .08Steel Oxidized 77 (25) .80

Steel AlloysType 301, Polished 75 (24) .27Type 301, Polished 450 (232) .57Type 301, Polished 1740 (949) .55Type 303, Oxidized 600-2000 (316-1093) .74-.87Type 310, Rolled 1500-2100 ((816-1149) .56-.81Type 316, Polished 75 (24) .28Type 316, Polished 450 (232) .57Type 316, Polished 1740 (949) .66Type 321 200-800 (93-427) .27-.32Type 321, Polished 300-1500 (149-815) .18-.49Type 321 w/BK Oxide 200-800 (93-427) .66-.76Type 347, Oxidized 600-2000 (316-1093) .87-.91Type 350 200-800 (93-427) .18-.27Type 350 Polished 300-1800 (149-982) .11-.35

Type 446, Polished 300-1500 (149-815) .15-.37Type 17-7 PH 200-600 (93-316) .44-.51Type 17-7 PH Polished 300-1500 (149-815) .09-.16Type C1020, Oxidized600-2000 (316-1093) .87-.91Type PH-15-7 MO 300-1200 (149-649) .07-.19Stellite, Polished 68 (20) .18

TantalumUnoxidized 1340 (727) .14Unoxidized 2000 (1093) .19Unoxidized 3600 (1982) .26Unoxidized 5306 (2930) .30

TinUnoxidized 77 (25) .04Unoxidized 212 (100) .05Tinned Iron, Bright 76 (24) .05Tinned Iron, Bright 212 (100) .08

TitaniumAlloy C110M, Polished 300-1200 (149-649) .08-.19Alloy C110M, Oxidized

at 1000°F (538°C) 200-800 (93-427) .51-.61Alloy Ti-95A, Oxidized

at 1000°F (538°C) 200-800 (93-427) .35-.48Anodized onto SS 200-600 (93-316) .96-.82

TungstenUnoxidized 77 (25) .02Unoxidized 212 (100) .03Unoxidized 932 (500) .07Unoxidized 1832 (1000) .15Unoxidized 2732 (1500) .23Unoxidized 3632 (2000) .28Filament (Aged) 100 (38) .03Filament (Aged) 1000 (538) .11Filament (Aged) 5000 (2760) .35

Uranium Oxide 1880 (1027) .79Zinc

Bright, Galvanized 100 (38) .23Commercial 99.1% 500 (260) .05Galvanized 100 (38) .28Oxidized 500-1000 (260-538) .11Polished 100 (38) .02Polished 500 (260) .03Polished 1000 (538) .04Polished 2000 (1093) .06


R

S

T

U

Z

Emissivity Tables


Adobe 68 (20) .90Asbestos

Board 100 (38) .96Cement 32-392 (0-200) .96Cement, Red 2500 (1371) .67Cement, White 2500 (1371) .65Cloth 199 (93) .90Paper 100-700 (38-371) .93Slate 68 (20) .97Asphalt, pavement 100 (38) .93Asphalt, tar paper 68 (20) .93

Basalt 68 (20) .72Brick

Red, rough 70 (21) .93Gault Cream 2500-5000 (1371-2760) .26-.30Fire Clay 2500 (1371) .75Light Buff 1000 (538) .80Lime Clay 2500 (1371 .43Fire Brick 1832 (1000) .75-.80Magnesite, Refractory 1832 (1000) .38Grey Brick 2012 (1100) .75Silica, Glazed 2000 (1093) .88Silica, Unglazed 2000 (1093) .80Sandlime 2500-5000 (1371-2760) .59-.63

Carborundum 1850 (1010) .92Ceramic

Alumina on Inconel 800-2000 (427-1093) .69-.45Earthenware, Glazed 70 (21) .90Earthenware, Matte 70 (21) .93Greens No. 5210-2C 200-750 (93-399) .89-.82Coating No. C20A 200-750 (93-399) .73-.67Porcelain 72 (22) .92White Al2O3 200 (93) .90Zirconia on Inconel 800-2000 (427-1093) .62-.45

Clay 68 (20) .39Fired 158 (70) .91Shale 68 (20) .69Tiles, Light Red 2500-5000 (1371-2760) .32-.34Tiles, Red 2500-5000 (1371-2760) .40-.51Tiles, Dark Purple 2500-5000 (1371-2760) .78

ConcreteRough 32-2000 (0-1093) .94Tiles, Natural 2500-5000 (1371-2760) .63-.62

Brown 2500-5000 (1371-2760) .87-.83Black 2500-5000 (1371-2760) .94-.91

Cotton Cloth 68 (20) .77Dolomite Lime 69 (20) .41Emery Corundum 176 (80) .86Glass

Convex D 212 (100) .80Convex D 600 (316) .80Convex D 932 (500) .76Nonex 212 (100) .82Nonex 600 (316) .82Nonex 932 (500) .78Smooth 32-200 (0-93) .92-.94

Granite 70 (21) .45Gravel 100 (38) .28Gypsum 68 (20) .80-.90Ice

Smooth 32 (0) .97Rough 32 (0) .98

LacquerBlack 200 (93) .96Blue, on Al Foil 100 (38) .78Clear, on Al Foil (2 coats) 200 (93) .08 (.09)Clear, on Bright Cu 200 (93) .66Clear, on Tarnished Cu 200 (93) .64Red, on Al Foil (2 coats) 100 (38) .61 (.74)White 200 (93) .95White, on Al Foil (2 coats) 100 (38) .69 (.88)Yellow, on Al Foil (2 coats) 100 (38) .57 (.79)

Lime Mortar 100-500 (38-260) .90-.92Limestone 100 (38) .95Marble

White 100 (38) .95Smooth, White 100 (38) .56Polished Gray 100 (38) .75

Mica 100 (38) .75Oil on Nickel

0.001 Film 72 (22) .270.002 Film 72 (22) .460.005 Film 72 (22) .72Thick Film 72 (22) .82

Oil, LinseedOn Al Foil, uncoated 250 (121) .09

NON-METALSMATERIAL TEMP °F (°C) ε-EMISSIVITY MATERIAL TEMP °F (°C) ε-EMISSIVITY

A

B

C

E

G

D

I

L

M

O

Emissivity Tables

On Al Foil, 1 coat 250 (121) .56On Al Foil, 2 coats 250 (121) .51On Polished Iron, .001 Film 100 (38) .22On Polished Iron, .002 Film 100 (38) .45On Polished Iron, .004 Film 100 (38) .65On Polished Iron, Thick Film 100 (38) .83

PaintsBlue, Cu2O3 75 (24) .94Black, CuO 75 (24) .96Green, Cu2O3 75 (24) .92Red, Fe2O3 75 (24) .91White, Al2O3 75 (24) .94White, Y2O3 75 (24) .90White, ZnO 75 (24) .95White, MgCO3 75 (24) .91White ZrO2 75 (24) .95White, ThO2 75 (24) .90White, MgO 75 (24) .91White PbCO3 75 (24) .93Yellow, PbO 75 (24) .90Yellow, PbCrO4 75 (24) .93

Paints, Aluminum 100 (38) .27-.6710% Al 100 (38) .5226% Al 100 (38) .30Dow XP-310 200 (93) .22

Paints, Bronze Low .34-80Gum Varnish (2 coats) 70 (21) .53Gum Varnish (3 coats) 70 (21) .50Cellulose Binder (2 coats) 70 (21) .34

Paints, OilAll colors 200 (93) .92-.96Black 200 (93) .92Black Gloss 70 (21) .90Camouflage Green 125 (52) .85Flat Black 80 (27) .88Flat White 80 (27) .91Grey-Green 70 (21) .95Green 200 (93) .95Lamp Black 209 (98) .96Red 200 (93) .95White 200 (93) .94

Quartz, Rough, Fused 70 (21) .93Glass, 1.98 mm 540 (282) .90Glass, 1.98 mm 1540 (838) .41Glass, 6.88 mm 540 (282) .93Glass, 6.88 mm 1540 (838) .47Opaque 570 (299) .92Opaque 1540 (838) .68

Red Lead 212 (100) .93Rubber

Hard 74 (23) .94Soft, Gray 76 (24) .86

Sand 68 (20) .76Sandstone 100 (38) .67Sandstone, Red 100 (38) .60-.83Sawdust 68 (20) .75Shale 68 (20) .69Silica

Glazed 1832 (1000) .85Unglazed 2012 (1100) .75

Silicon Carbide 300-1200 (149-649) .83-.96Silk Cloth 68 (20) .78Slate 100 (38) .67-.80Snow

Fine Particles 20 (-7) .82Granular 18 (-8) .89

SoilSurface 100 (38) .38Black Loam 68 (20) .66Plowed Field 68 (20) .38

SootAcetylene 75 (24) .97Camphor 75 (24) .94Candle 250 (121) .95Coal 68 (20) .95

Stonework 100 (38) .93Water 100 (38) .67Waterglass 68 (20) .96Wood Low .80-.90

Beech, Planed 158 (70) .94Oak, Planed 100 (38) .91Spruce, Sanded 100 (38) .89


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Glossary


Absolute zero: Temperature at which thermal energy is ata minimum. Defined as 0 Kelvin or 0 Rankine (-273.15 °C or-459.67°F). Absorptivity: The fraction of incident radiationabsorbed by a surface, α.Accuracy: Closeness of a reading or indication of a mea-surement device to the actual value of the quantitybeing measured.Ambient compensation: The design of an instrumentsuch that changes in ambient temperature do not affectthe readings of the instrument. Ambient temperature: The average or mean tempera-ture of the surrounding air which comes in contact withthe equipment and instruments under test.Ampere (amp): A unit used to define the rate of flowof electricity (current) in an electrical circuit; units areone coulomb (6.25x1018 electrons) per second.Symbolized by A.American National Standards Institute (ANSI): TheUnited States standards body responsible for designatingstandards developed by other organizations as nationalstandards.

Blackbody: A theoretical object that radiates the maxi-mum amount of energy at a given temperature, andabsorbs all the energy incident upon it. A blackbody isnot necessarily black. (The name blackbody was chosenbecause the color black is defined as the total absorp-tion of light energy.)Boiling point: The temperature at which a substance in theliquid phase transforms to the gaseous phase. Commonlyrefers to the boiling point of water, 100°C (212°F).Bolometer: Infrared thermometer detector consistingof a resistance thermometer arranged for response toradiation.BTU: British thermal unit, the amount of energy requiredto raise one pound of water one degree Fahrenheit.

Calibration: The process of adjusting an instrument orcompiling a deviation chart so that its reading can becorrelated to the actual value being measured.Calorie: Measure of thermal energy, defined as theamount of heat required to raise one gram of water one

degree Celsius at 15°C.Celsius (Centigrade): A temperature scale defined by 0 °Cat the ice point and 100°C at the boiling point of water. Color code: The ANSI established color code for ther-mocouple (and infrared thermocouples) wires in whichthe negative lead is always red. Color code for basemetal thermocouples is yellow for Type K, black for TypeJ, purple for Type E and blue for Type T.Common-mode rejection ratio: The ability of an instru-ment to reject interference from a common voltage at itsinput terminals with relation to ground, usuallyexpressed in decibels (dB).Compensating alloys: Alloys used to connect thermo-couples and IR thermocouples to instrumentation. Thesealloys are selected to have similar thermal electric prop-erties as the thermocouple alloys over a limited temper-ature range. Compensated connector: A connector made of ther-mocouple alloys used to connect thermocouple and IRthermocouple probes and wires.CPS: Cycles per second, also Hertz (Hz).Cryogenics: The measurement of very low temperatures,i.e., below -200°C.Current: The rate of flow of electricity. The unit is theampere (A), which equals one coulomb per second.

Degree: An incremental value in a temperature scale.Diffuse emitter: A surface that emits radiation equally inall directions.DIN: Deutsche Industrial Norms, a German agency thatsets engineering and dimensional standards that nowhave worldwide acceptance.Drift: A change in an instrument’s reading or setpointvalue over extended periods due to factors such as time,line voltage, or ambient temperature effects.Dual element sensor: A sensor assembly with two inde-pendent sensing elements.

Electromotive force (EMF): A measure of voltage in anelectrical circuit.Electromagnetic interference (EMI): electrical noiseinduced upon signal wires with the possible effect ofobscuring the instrument signal. Emissive power: Rate at which radiation is emitted from

GlossaryA

B

C

D

E

a surface, per unit surface area per unit wavelength.Emissivity/emittivity: The ratio of energy emitted by asurface to the energy emitted by a blackbody at thesame temperature, symbolized by ε. Emissivity refers toan overall property of a substance, whereas emittvityrefers to a particular surface’s characteristics.Error: The difference between the correct of desiredvalue and the actual read or value taken.

Fahrenheit: A temperature scale define by 32°F at the icepoint and 212°F at the boiling point of water at sea level. Fiber optic radiation thermometer: Radiation ther-mometer that uses a fiber optic probe to separate thedetector, housing, and electronics from the radiationgathering point itself. Used to measure temperature inhard-to-reach places or in hostile conditions.Field of view: A volume in space defined by an angularcone extending from the focal plane of an instrument. Freezing point: The temperature at which a substancegoes from the liquid phase to the solid phase. Frequency: The number of cycles over a specified timeperiod over which an event occurs. For electromagneticradiation, normally symbolized by υ.

Gain: The amount of amplification used in an electricalcircuit.Ground: The electrical neutral line having the samepotential as the surrounding earth; the negative side of adirect current power system; the reference point for anelectrical system.

Heat: Thermal energy, typically expressed in calories orBTUs. Heat transfer: The process of thermal energy flowingfrom a body of high energy to a body of lower energy viaconduction, convection, and/or radiation.Hertz (Hz): Unit of frequency, defined in cycles persecond.

Ice point: The temperature at which pure water freezes,0°C, 32°F, 273.15°K.Impedance: The total opposition to electrical flow.Infrared (IR): A range of the electromagnetic spectrumextending beyond red visible light from 760 nanometersto 1000 microns.Infrared thermocouple: Radiation thermometer whoseoutput simulates that of a standard type thermocouple,typically over a more limited temperature range.

Interchangeability error: A measurement error thatcan occur if two or more sensors are used to make thesame measurement. Caused by slight variations fromsensor to sensor.Intrinsically safe: An instrument in which electricalenergy is limited such that it will not spark or otherwiseignite a flammable mixture. ISA: Formerly the Instrument Society of America, now referredto as the International Society for Measurement & Control.

Joule: Basic unit of thermal energy.Junction: The point in a thermocouple where the twodissimilar metals are joined.

Kelvin: Absolute temperature scale based on the Celsiusscale, but with zero K defined at absolute zero. 0°C cor-responds to 273.15°K.

Linearity: The deviation of an instrument’s responsefrom a straight line.Linescanner: Device that uses a series of moving mirrorsto measure temperature or other properties at variouspoints across a moving web or surface.Loop resistance: The total resistance of a completeelectrical circuit.

Measuring junction: The thermocouple junctionreferred to as the hot junction that is used to measure anunknown temperature.Melting point: The temperature at which a substancetransforms from a solid phase to a liquid phase.Micron (µm): One millionth of a meter.Milliamp (mA): One thousandth of an ampere.Millivolt (mV): One thousandth of a volt.

N = N factor (= 14388/(lT))Narrow-band radiation thermometer: Radiation ther-mometer that measures radiation in a tightly controlledrange of wavelengths, typically determined by the opti-cal filter used.Noise: Any unwanted electrical interference on a signal wire.Normal-mode rejection ratio: The ability of an instru-ment to reject electrical interference across its input ter-minals, normally of line frequency (50-60 Hz).

Ohmeter: A device used to measure electrical resistance.Optical isolation: Two networks or circuits in which an

K

L

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N

O

Glossary


F

G

H

I

J

Glossary


LED transmitter and receiver are used to maintain elec-trical discontinuity between the circuits.Optical pyrometer: Infrared thermometer that measuresthe temperature of very hot objects by the visible wave-length radiation given off.

Phase: A time-based relationship between a periodicfunction and a reference. Photon detector: Radiation thermometer detector thatreleases electric charges in response to incident radiation. Polarity: In electricity, the quality of having two oppo-sitely charged poles, one positive and one negative.Power supply: A separate unit or part of a circuit thatprovides power to the rest of a circuit.Primary standard: The standard reference units andphysical constants maintained by the National Instituteof Standards & Technology (NIST) upon which all mea-surement units in the United States are based.Pyroelectric detector: Radiation thermometer detec-tor that changes surface charge in response to receivedradiation. Pyrometer: Device used to measure the infrared radia-tion (hence temperature) given off by a body or surface.

Radiation: The movement of energy in the form of elec-tromagnetic waves.Range: An area between two limits within which a quanti-ty is measured, stated in terms of a lower and upper limit.Rankine: Absolute temperature scale based on theFahrenheit scale, but with zero R defined at absolutezero. 0°F corresponds to 459.67°R.Reference junction: The cold junction in a thermocou-ple circuit that is held constant at a known or measuredtemperature.Reflectivity/reflectance: The fraction of incident radia-tion reflected by an object or surface.Radio frequency interference (RFI): Noise inducedupon signal wires by ambient radio-frequency electro-magnetic radiation with the effect of obscuring theinstrument signal. Repeatability: The ability of an instrument to give the sameoutput or reading under repeated, identical conditions.Resistance: The resistance to the flow of electric cur-rent, measured in ohms, Ω.

Secondary standard: A standard of unit measurementderived from a primary standard.Sensitivity: The minimum change in a physical variable

to which an instrument can respond.Span: The difference between the upper and lower lim-its of a range, expressed in the same units as the range.Spectral filter: A filter that allows only a specific band-width of the electromagnetic spectrum to pass, i.e., 4-8micron infrared radiation.Spot size: The diameter of the circle formed by the crosssection of the field of view of an optical instrument at agiven distance.Stability: The ability of an instrument or sensor to maintaina consistent output when a constant input is applied.Sterling cycle: Thermodynamic cycle commonly used tocool thermographic detectors.

Thermal detector: Radiation thermometer detector thatgenerates a signal based on the heat energy absorbed.Thermocouple: The junction of two dissimilar metalsthrough which a measurable current flows depending onthe temperature difference between the two junctions.Thermography: The presentation and interpretation oftwo-dimensional temperature pictures.Thermometry: The science of temperature measurement.Thermopile: an arrange of multiple thermocouples inseries such that the thermoelectric output is amplified.Thermowell: A closed-end tube designed to protect atemperature sensor from harsh process conditions. Transmittance/transmissivity: The fraction of incidentradiation passed through an object.Two-color pyrometer: A radiation thermometer thatmeasures the radiation output of a surface at two wave-lengths, thus reducing any effects of emissivity variationwith wavelength.

Volt (V): The electrical potential difference between twopoints in a circuit. One volt is the potential needed tomove one coulomb of charge between two points whileusing one joule of energy.

Wavelength: Distance, from peak to peak, of any wave-form. For electromagnetic radiation in the infrared region,typically measured in microns and symbolized by λ.Working standard: A standard of unit measurement cal-ibrated from either a primary or secondary standardwhich is used to calibrate other devices or make com-parison measurements.

Zero offset: The non-zero output of an instrument,expressed in units of measure, under conditions of true zero.

P

R

S

T

V

W

Z

Index


A

Index

B

C

D

E

F

G

H

I

K

Absorptivity 18, 21Application guidelines, 59-65

atmospheric interference 60operating environment 61target material 60temperature range 60wavelength choice 60

Blackbody,behavior 14use in calibration 54-55definition 13, 18real approximation 19

Bibliography 68Bolometer 31Boltzmann, Ludwig 14Bunsen, Robert 13

Calibration,blackbody sources 54-55importance 53isothermal options 54internal 36traceability 55tungsten filament refence 55

Camera, thermographic 46Chopper 30, 37Cooling,

sensor assembly 60detector 36

Detectorerror compensation 35photon 31-32pyroelectric 32sensitivity 32responsivity 35thermal 31-32

Electronics,control functions 36detector compensation 36filtering 37

linescanning 52signal analysis 36thermography 51

Einstein, Albert 15Electromagnetism, basic laws 12Emissivity,

definition 18, 25experimental determination 25rules of thumb 25values for common materials 72

Emittivity 18, 25Emitttance 18Error 58, 60

Fiber optics,applications 30, 43, 57, 66cable construction 45historical development 43noise immunity 43probe construction 43transmission efficiency 43

Field of view (see Optical systems)

Filters(see also Optical systems),narrow band 27wheel configuration 28

Franhofer, Joseph von 12

Galilei, Galileo 11Glossary 77-79Gray body 18

Heat balance, radiation 17-18Helmholtz, Hermann von 13Herschel, Frederick William 12Huygens, Christian 11

Information resources 68International temperature scale 55Intermediate temperatures, law of 39

Kelvin, Lord 38Kirchhoff, Gustav Robert 13

Index


S

T

L

M

N

O

P

Q

R

Kirchhoff’s law 13

Linescanner, infraredapplications 47, 58electronics 51principles of operation 46two-dimensional imaging 47

Maxwell, James Clerk 11Maxwell-Boltzmann equationMirrors

(see Optical systems)Multicolor pyrometer

(see Radiation thermometer, ratio)

Newton, Sir Isaac 11N-Factor 25

Omega Engineering,about 9contact information 68

Optical pyrometer(see Radiation thermometer, optical)

Optical systems,configuration 32field of view 33, 35sighting path 33transmission characteristics 32, 34

Peltier, Jean 38Peltier effect 38Photoelectric effect 15Planck, Max Karl Ernst Ludwig 14Planck’s constant 14Planck’s distribution law 20Planck’s equation 14, 17Purging 61Pyrometer

(see Radiation thermometer)

Quantum theory 14

Radiation, infrareddefinition 12, 18directional dependence 17, 21discovery 12energy balance 19historical uses 11

Radiation thermometer,accessories 66advantages 24alternative configurations 56application guidelines 59broadband 27definition of 24design considerations 30, 37detector options 31electronics 35features 66handheld 57, 66industrial applications 15-16, 57mounted 57, 67limitations 24multi-wavelength 29narrow band 27operation 26optical 29-30N factor equation 25ratio 25, 28, 57single-color (see narrow band)two-color (see ratio)

Rayleigh, John 14Reference texts 69Reflectance 18Reflectivity 22Ribbon-filament lamp

(see Tungsten filament lamp)

Seebeck, T.J. 38Shielding 60Sighting path

(see Optical systems)Sighting tube 61Spectrum, electromagnetic 2-3, 12, 17Stefan, Josef 13Stefan-Boltzmann constant 19Stefan-Boltzmann equation 14, 19Sterling cycle 50Surfaces,

diffuse 22non-ideal 21specular 22

Thermocouplecompensation 38-39operating principles 38

Index


W

intermediate temperatures, law of 39Thermocouple, infrared

accessories 66application guidelines 57calibration 42configuration options 41installation guidelines 41operating principles 40

Thermography,applications 49-52detector cooling 48detector options 47electronics 51image analysis 50operating principles 47radiometric devices 47resolution 49viewing devices 47

Thermopile 31

Thermometer,definition 24invention 11

Thermowell 64Thompson, William

(see Kelvin, Lord)Thompson effect 38Transmissivity 22Transmittance 18Tungsten filament lamp

(see Calibration)Two-color pyrometer

(see Radiation thermometer, ratio)

Wavelength 14Website resources 68Wien, Wilhelm 14Wien’s displacement law 20Wien’s law 20

Index

1-1. The First IR Thermometer 10

1-2. Glass Manufacture Using Visual IR

Temperature Measurement 11

1-3. Newton Splits, Recombines White Light 12

1-4. Herschel Discovers Infrared Radiation 13

1-5. The Sidewinder Missile’s IR Guidance System 14

1-6. IR Optics for Missile Guidance 15

2-1. Radiation Energy Balance 17

2-2. Spectral Distributions 18

2-3. An Isothermal Blackbody Cavity 20

2-4. Planck Prediction of Blackbody Emissive Power 21

2-5. Soda-Lime Glass Spectral Transmittance 22

3-1. Traditional Infrared Thermometer 24

3-2. Effect of Non-Blackbody Emissivity

on IR Thermometer Error 25

3-3. Blackbody Radiation in the Infrared 26

3-4. The ‘Two-Color’ IR Thermometer 27

3-5. Beam-Splitting in the Ratio IR Thermometer 28

3-6. Ratio Pyrometry Via a Filter Wheel 28

3-7. Schematic of a Multispectral IR Thermometer 29

3-8. Optical Pyrometry By Visual Comparison 30

3-9. An Automatic Optical Pyrometer 31

3-10. Relative Sensitivity of IR Detectors 32

3-11. Typical Optical Systems 33

3-12. IR Transmission of Optical Materials 34

3-13. IR Transmission Characteristics 35

3-14. Field of View 35

3-15. Typical Narrow and Wide Angle Sighting Paths 36

3-16. Microprocessor-Based IR Thermometer 36

3-17. Surface Temperature Pyrometer 37

4-1. Thermocouple Operation 38

4-2. Equivalent Thermocouple Circuits 39

4-3. Typical Thermocouple Installation 40

4-4. IR Thermocouple Output 41

5-1. Fiber Optic Probe Construction 43

5-2. Typical IR Fiber Optic Probe 44

5-3. Multipoint Pick-up Assembly 44

5-4. Fiber Optic Cable Construction 45

6-1. Linescanner Operation 46

6-2. 1-D Scans Composited Into a 2-D Image 47

6-3. 2-D Thermographic Camera 49

6-4. The Stirling Cycle 50

6-5. Spatial Resolution of a Thermographic Camera 51

7-1. A Spherical Blackbody Cavity 53

7-2. Effective Emissivity of Spherical Cavities 54

7-3. Typical Tungsten Lamp Filament 55

8-1. Ambient Effects on IR Thermometer Accuracy 58

8-2. Sighting on a Specular Surface 59

8-3. Use of Shielding and Cooling 60

8-4. Accessories for Furnace-Wall Installation 61

8-5. Compensation for Elevated

Ambient Temperatures 62


List of FiguresSection 1A Historical Perspective

Section 2Theoretical Development

Section 3IR Thermometers & Pyrometers

Section 4Infrared Thermocouples

Section 5Fiber Optic Extensions

Section 6Linescanning & Thermography

Section 7IR Thermometer Calibration

Section 8Products & Applications

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