Components of Optical Instruments Chapter 7

8/3/2019 Components of Optical Instruments Chapter 7

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Topics You Know By Now!Topics You Know By Now!

From Physics CoursesFrom Physics Courses

EM radiation and itsEM radiation and itspropertiesproperties

DiffractionDiffraction

RefractionRefraction

Coherent andCoherent and

incoherent radiationincoherent radiation

Polarization ofPolarization of

radiationradiation Scattering ofScattering of

radiationradiation

From ChemistryFrom Chemistry

CoursesCourses

Photoelectric effectPhotoelectric effect

ElectromagneticElectromagnetic

spectrumspectrum

BeerBeers Law, etc.s Law, etc.

Quantized states inQuantized states in

atomsatoms

lead to line spectralead to line spectra Quantized states inQuantized states in

moleculesmolecules

lead to broad orlead to broad orcontinuum spectracontinuum spectra


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Components of Optical Instruments:Components of Optical Instruments:The generic spectrometerThe generic spectrometer

General DesignsGeneral Designs

Sources and Sample HoldersSources and Sample Holders

Wavelength SeparatorsWavelength Separators

SlitsSlits

DetectorsDetectors


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LASERLASER

Light Amplification by Stimulated Emission ofLight Amplification by Stimulated Emission of

RadiationRadiation Emits very intense,Emits very intense, monochromaticmonochromatic light at highlight at high

power (intensity)power (intensity)

All wavesAll waves in phasein phase (unique), and parallel(unique), and parallel

All waves are polarized in one planeAll waves are polarized in one plane

Used to be expensiveUsed to be expensive

Not useful forNot useful for scanningscanning wavelengthswavelengths


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Laser SetupLaser Setup

Douglas A. Skoog, F. James Holler and Timothy A. Nieman, PrincipDouglas A. Skoog, F. James Holler and Timothy A. Nieman, Princip

les ofles of

Instrumental Analysis, Saunders College Publishing, PhiladelphiaInstrumental Analysis, Saunders College Publishing, Philadelphia, 1998., 1998.


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Population Inversion isPopulation Inversion is

Necessary for AmplificationNecessary for Amplification

Pumping: Usually Electrical or OpticalPumping: Usually Electrical or Optical

Douglas A. Skoog, F. James Holler and Timothy A. Nieman, PrincipDouglas A. Skoog, F. James Holler and Timothy A. Nieman, Princip

les ofles of



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Stimulated EmissionStimulated Emission

A photon incident on an excited stateA photon incident on an excited state

species causes emission of a second photonspecies causes emission of a second photonof the same frequency, returning the speciesof the same frequency, returning the species

to the lower stateto the lower state

M* + hM* + h > > M + 2hM + 2h


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OverallOverall

Douglas A. Skoog, F. James Holler and Timothy A. Nieman, PrincipDouglas A. Skoog, F. James Holler and Timothy A. Nieman, Principles ofles of



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Eugene Hecht,Eugene Hecht, OpticsOptics, Addison, Addison--Wesley, Reading, MA, 1998.Wesley, Reading, MA, 1998.

Light Amplification inLight Amplification in

the Resonance Cavity.the Resonance Cavity.


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13Chapter 7

Optical ResonanceOptical Resonance Form a resonant cavity, with mirrors on each endForm a resonant cavity, with mirrors on each end

Length of cavity is an integral multiple of theLength of cavity is an integral multiple of thedesired wavelengthdesired wavelength

Mirror Partially trans-

parent mirror

Active medium

Output beam


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Types of LasersTypes of Lasers Solid state lasersSolid state lasers

Nd:YAGNd:YAG neodymium yttrium aluminum garnetneodymium yttrium aluminum garnet

1064 nm1064 nm

Gas lasersGas lasers

lines w/ specificlines w/ specific s in UV/s in UV/visvis/IR/IR He/He/NeNe

ArAr++

, Kr, Kr++

COCO22 eximerseximers ((XeFXeF++,,.).)

Dye lasersDye lasers limitedlimited tunabilitytunabilityin the visiblein the visible

Semiconductor diode lasersSemiconductor diode lasers limitedlimited tunabilitytunabilityin the IR, redin the IR, red


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Absorbance: usually in a matched pair!

Fluorescence, Phosphorescence, Chemiluminescence

10 cm gas containing cell with

transparent windows at the ends.


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Wavelength SelectorsWavelength Selectors....

Used to select the wavelength (or wavelength range) of lightUsed to select the wavelength (or wavelength range) of light

that eitherthat either

impinges on the sample (fluorescence and phosphorescence)impinges on the sample (fluorescence and phosphorescence)

is transmitted through the sample (absorption and emission)is transmitted through the sample (absorption and emission)

This selected wavelength then strikes the detectorThis selected wavelength then strikes the detector

the ability to select the wavelength helps you to discriminatedthe ability to select the wavelength helps you to discriminated

between phenomena caused by yourbetween phenomena caused by your analyteanalyte and that caused byand that caused byinterfering orinterfering or nonnon--relevantrelevant species.species.

Are often combined with a set ofAre often combined with a set of SLITSSLITS (discussed later)(discussed later)

Various typesVarious types

based on filters (CHEAP COLORED GLASS)based on filters (CHEAP COLORED GLASS)

based on prisms (LIMITED APPLICATIONS)based on prisms (LIMITED APPLICATIONS)

based on gratingsbased on gratings. (GREAT STUFF). (GREAT STUFF)


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FiltersFilters

Simple, rugged (no moving parts in general)Simple, rugged (no moving parts in general)

Relatively inexpensiveRelatively inexpensive

Can select some broad range of wavelengthsCan select some broad range of wavelengths

Most often used inMost often used in

field instrumentsfield instruments

simpler instrumentssimpler instruments

instruments dedicated to monitoring a single wavelengthinstruments dedicated to monitoring a single wavelength

range..range..

Two types of filters:Two types of filters:

InterferenceInterference filters depend on destructive interference offilters depend on destructive interference of

the impinging light to allow a limited range ofthe impinging light to allow a limited range of

wavelengths to pass through them (more expensive)wavelengths to pass through them (more expensive)

AbsorptionAbsorption filters absorb specific wavelength ranges offilters absorb specific wavelength ranges of

light (cheaper, more common)...light (cheaper, more common)...


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FiltersFilters

Ingle and Crouch,Ingle and Crouch, SpectrochemicalSpectrochemicalAnalysisAnalysis

BandpassBandpass

CutoffCutoff


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1-2% of at 80%T

0.1-0.2% at 10%T


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Absorption FiltersAbsorption Filters

Douglas A. Skoog and James J. Leary,Douglas A. Skoog and James J. Leary,Principles of Instrumental Analysis,Principles of Instrumental Analysis,

Saunders College Publishing, Fort Worth,Saunders College Publishing, Fort Worth,

1992.1992.

MellesMelles GriotGriot CatalogueCatalogue

Typical Effective Bandwidths = 30Typical Effective Bandwidths = 30250 nm250 nm

BAND or CUT-OFF


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Two basic functionsTwo basic functions..

cutoff filters absorb light in a specificcutoff filters absorb light in a specific

range of wavelengths. Theyrange of wavelengths. They cutoffcutoff thisthisrange from the detectors (e.g. cutoff forrange from the detectors (e.g. cutoff for

550 nm)550 nm)

bandpassbandpass filters absorb light outside of afilters absorb light outside of a

specific range (e.g. 350specific range (e.g. 350--550 nm)550 nm)

often made of a combination of twooften made of a combination of two

cutoff filters!cutoff filters!


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Absorption Filters vs. Interference FiltersAbsorption Filters vs. Interference Filters

Douglas A. Skoog and James J. Leary, Principles of InstrumentalDouglas A. Skoog and James J. Leary, Principles of InstrumentalAnalysis, Saunders College Publishing, Fort Worth, 1992.Analysis, Saunders College Publishing, Fort Worth, 1992.


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Wavelength SelectorsWavelength Selectors

FiltersFilters

PrismsPrisms

GratingsGratings

Michelson InterferometerMichelson Interferometer


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Wavelength SelectorsWavelength Selectors


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Why Wavelength Selectors ?Why Wavelength Selectors ?

MonochromatorsMonochromators

based on diffraction gratings or prismsbased on diffraction gratings or prisms

a slit is used to select a particulara slit is used to select a particularwavelength coming off the grating at awavelength coming off the grating at a

particular angleparticular angle


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CzernyCzernyTurnerTurnerMonochromatorMonochromator




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PrismsPrisms

First type of widely used,First type of widely used, scanningscanning

wavelength selection devices (TURN PRISM)wavelength selection devices (TURN PRISM)

Often made of salts such as sodium chloride,Often made of salts such as sodium chloride,

fluorites etc (Remember figure 7fluorites etc (Remember figure 7--2b).2b). VERY delicateVERY delicate. Often subject to damage in. Often subject to damage in

humidity and wide heat ranges.humidity and wide heat ranges.

Not widely used today in spectroscopyNot widely used today in spectroscopyequipment.equipment.

Great demonstration tools for kidsGreat demonstration tools for kids

Nice on the cover of a Pink Floyd albumNice on the cover of a Pink Floyd album


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Reflection Gratings... (Transmission?)Reflection Gratings... (Transmission?)

Widely used in instruments today.Widely used in instruments today.

Light reflected off a surface, and notLight reflected off a surface, and not

cancelled out by destructive interference, iscancelled out by destructive interference, isused for selection of wavelengthsused for selection of wavelengths

Constructed of various materialsConstructed of various materials.. Polished glass, silica or polymer substratePolished glass, silica or polymer substrate

Grooves milled or laser etched into the surfaceGrooves milled or laser etched into the surface

Coated with a reflective material (silvered) such asCoated with a reflective material (silvered) such asa shiny metala shiny metal

VERY FRAGILE!!VERY FRAGILE!! Sealed inside the instrument. DO NOT TOUCH!Sealed inside the instrument. DO NOT TOUCH!


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Grating EquationGrating Equation

1 travels AB further than 21 travels AB further than 2

after the reflection.after the reflection.

2 travels CD further than 12 travels CD further than 1

to reach the grating.to reach the grating.

For constructiveFor constructiveinterference:interference:

mm = (CD= (CDAB)AB)

m = 0,m = 0, 1,1, 2,2, 3,3,


CD =CD = ddsinsin iiAB =AB = --ddsinsin rr

dd(sin i + sin r) =(sin i + sin r) = mm


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There are multiple orders ofThere are multiple orders of at a single r.at a single r.


d(sind(sin ++ sinsin) =) = mm

For Example:For Example:

11stst Order = 400 nmOrder = 400 nm

22ndnd Order = 200 nmOrder = 200 nm

33rdrd Order = 133 nmOrder = 133 nm

First order = 90% TFirst order = 90% T

Higher orders can beHigher orders can be

eliminated by filterseliminated by filters


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Example 7-1

Grating with 1450 blazes/mmPolychromatic light at i=48 deg

L of the monochromatic reflected light atR=+20,+10 and 0 deg?

dd(sin i + sin r) =(sin i + sin r) = mm 1) Calculate1) Calculate dd

d= 1 mm/1450 blazes convert to nm x106 689.7 nm per groove!

dd(sin i + sin r) =(sin i + sin r) = mm 2) Calculate2) Calculate for n=1 at +20 degfor n=1 at +20 deg

= 689.7 nm ( sin 48 + sin 20)/1 = 748.4 nm!

Grating will give a monochromatic beam of light of 748.4 nm at

20 deg, 632 nm at 10 deg and 513 nm at 0 deg. For n=1!


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Eugene Hecht,Eugene Hecht, OpticsOptics, Addison, Addison--Wesley, Reading, MA, 1998.Wesley, Reading, MA, 1998.

Diffraction GratingsDiffraction Gratings

Plane or convex plate ruledPlane or convex plate ruled

with closely spaced grooveswith closely spaced grooves

(300(300--2000 grooves/mm).2000 grooves/mm).

New holographic gratings canNew holographic gratings can

have up to 64K grooves!have up to 64K grooves!

More grooves = better resolving powerMore grooves = better resolving power

R =R = // (1K to 10K)(1K to 10K)

R =R = nnNN (N = grooves!)(N = grooves!)How well can you focus on twoHow well can you focus on two

adjacent wavelengths!adjacent wavelengths!


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`

Conventional=Ech

ellette

M th ti f G ti I t tM th ti f G ti I t t


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Mathematics of Gratings: ImportantMathematics of Gratings: Important

ConsiderationsConsiderations..

Dispersion:Dispersion: Angular dispersion is the change in the angle of reflection withAngular dispersion is the change in the angle of reflection with aa

change in the wavelength of light (change in the wavelength of light (dr/ddr/d)) Linear dispersion is the change in the wavelength of light alongLinear dispersion is the change in the wavelength of light along

some distance on the focal plane of the grating (some distance on the focal plane of the grating (dy/ddy/d)) Resolving Power:Resolving Power:

The ability of a grating to resolve separate wavelengths of lighThe ability of a grating to resolve separate wavelengths of lightt

dilluminateblazesofnumbertheN

orderndiffractiothen

nN

R

=

=

==

R

Slits hole in the wallSlits = hole in the wall


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Slits = hole in the wallSlits = hole in the wall

Control the entrance of light into and out from theControl the entrance of light into and out from the

monochromatormonochromator. They control quality!. They control quality!

EntranceEntrance slits control theslits control the intensityintensity of light entering theof light entering the

monochromatormonochromator and help control the range ofand help control the range of

wavelengths of light that strike the gratingwavelengths of light that strike the grating

Less important than exit slitsLess important than exit slits

ExitExit slights help select theslights help select the range of wavelengthsrange of wavelengths that exitthat exitthethe monochromatormonochromator and strike the detectorand strike the detector

More important than entrance slitsMore important than entrance slits

Can be:Can be: Fixed (just a slot)Fixed (just a slot)

Adjustable inAdjustable in widthwidth (effective bandwidth and intensity)(effective bandwidth and intensity)

Adjustable inAdjustable in heightheight (intensity of light)(intensity of light)


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Wider slits = greater intensity,

Poorer resolution

Narrower slits = lower intensity,

Better resolution


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Michelson InterferometerMichelson Interferometer



I t fInterferograms


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InterferogramsInterferograms

ff= 2r/= 2r/



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Fourier Transform of theFourier Transform of the

InterferogramInterferogramgives the Spectrumgives the Spectrum

Ingle and Crouch,Ingle and Crouch, SpectrochemicalSpectrochemicalAnalysisAnalysis

Optical FibersOptical Fibers


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Optical FibersOptical Fibers

Used to transmit light waves over nonUsed to transmit light waves over non-- linear paths.linear paths.

Often used in remote sensing, solution samplingOften used in remote sensing, solution sampling

(dipping probes) and field instruments(dipping probes) and field instruments

Based on the fact that light inside a fiber can beBased on the fact that light inside a fiber can be

continuously (totally internally reflected) if the angle itcontinuously (totally internally reflected) if the angle itstrikes the fiber surface at is correct (determinesstrikes the fiber surface at is correct (determines

radius of bends, etc.).radius of bends, etc.).

Used in construction ofUsed in construction ofoptodesoptodes (optical fiber based(optical fiber basedchemical sensor)chemical sensor)


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1sinsince

claddintocorefromgoeslightno

1sinn

nif

claddingtheis2core,theis1

sinnsinn

LawSnells

r

i

2

1

r2i1

=


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DETECTORSDETECTORS

Just photon transducers!Just photon transducers!


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Respond to the intensity of EMR striking them by changing aRespond to the intensity of EMR striking them by changing avoltage or current emitted or required by themselvesvoltage or current emitted or required by themselves

Do NOT respond selectively to specific wavelengthsDo NOT respond selectively to specific wavelengths (that is(that iswhat the wavelength selector is for) but work over a range ofwhat the wavelength selector is for) but work over a range ofwavelengths (DUMB COUNTERS OF PHOTONS!)wavelengths (DUMB COUNTERS OF PHOTONS!)

Various typesVarious types

Photographic films (not widely in use any more)Photographic films (not widely in use any more)

Phototubes (used in simpler instruments)Phototubes (used in simpler instruments)

Photomultiplier tubes (used in more complex instruments)Photomultiplier tubes (used in more complex instruments)

MultichannelMultichannel transducerstransducers

Diode arraysDiode arrays

Charged coupled devices (Charged coupled devices (CCDCCDss, like in many camcorders), like in many camcorders)


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Different wavelengths require different detectors!!

Most UV-VIS instruments have two photomultiplier tubes.Why? Different work functions for the cathode materials.Remember what the work function is?

Phototubes (found in SPEC 20Phototubes (found in SPEC 20s for example)s, for example)


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Phototubes (found in SPEC 20Phototubes (found in SPEC 20 s, for example)s, for example)

Function based on the photoelectric effectFunction based on the photoelectric effect

Photomultiplier tubes (found in more advanced,Photomultiplier tubes (found in more advanced,

i UVi UV VIS d t i i t t )VIS d t i i t t )


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scanning UVscanning UV--VIS and spectroscopic instruments)VIS and spectroscopic instruments)

Also function based on the photoelectric effectAlso function based on the photoelectric effect

AdditionalAdditional signal is gained by multiplying the number ofsignal is gained by multiplying the number of

electrons producedelectrons produced

by the initial reaction in the detector.by the initial reaction in the detector.

Each electron produces as series of photoEach electron produces as series of photo--electrons, multiplyingelectrons, multiplyingits signal. Thus the name PMT!its signal. Thus the name PMT!

Very sensitive to incoming light.Very sensitive to incoming light. Most sensitive light detector in the UVMost sensitive light detector in the UV--VIS range.VIS range.

VERY rugged. They last a long time.VERY rugged. They last a long time.

Sensitive to excessive stray light (room light + powered PMTSensitive to excessive stray light (room light + powered PMT= DEAD PMT)= DEAD PMT)

Always used with a scanning or moveable wavelength selectorAlways used with a scanning or moveable wavelength selector(grating) in a(grating) in a monochromatormonochromator


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http://elchem.kaist.ac.kr/vt/chem-ed/optics/detector/pmt.htm

Photomultiplier TubePhotomultiplier Tube


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pp

Douglas A. Skoog and James J. Leary, Principles of InstrumentalDouglas A. Skoog and James J. Leary, Principles of Instrumental

Analysis, Saunders College Publishing, Fort Worth, 1992.Analysis, Saunders College Publishing, Fort Worth, 1992.

8819 dynodes (919 dynodes (9--10 is10 ismost common).most common).

Gain (m) is # eGain (m) is # e-- emittedemitted

per incident eper incident e-- (() to the) to thepower of the # ofpower of the # of

dynodes (k).dynodes (k).

m =m = kke.g. 5 ee.g. 5 e-- emitted / incident eemitted / incident e--

10 dynodes.10 dynodes.

m =m = kk = 5= 51010 1 x 101 x 1077

Typical Gain = 10Typical Gain = 1044 -- 101077

Spectral ResponseSpectral Response


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HamamatsuHamamatsu CatalogueCatalogue

Silicon DiodesSilicon Diodes


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Silicon DiodesSilicon Diodes

Constructed of charge depleted and charge rich regions of silicoConstructed of charge depleted and charge rich regions of siliconn(silicon doped with other ions)(silicon doped with other ions)

Light striking the detector causes charge to be created in eachLight striking the detector causes charge to be created in each

region.region.

The charge collected is then measured and the array isThe charge collected is then measured and the array is resetreset forforthe next collectionthe next collection

Used in instruments where the grating is fixed in one position aUsed in instruments where the grating is fixed in one position andndlight strikes an array of silicon diodes (light strikes an array of silicon diodes (akaaka the diode array)the diode array)

Can have thousands of diodes on an arrayCan have thousands of diodes on an array

Each diode collects light from a specific wavelength rangeEach diode collects light from a specific wavelength range

The resolution is generally poorer than with a PMTThe resolution is generally poorer than with a PMT

However, you can scan literally thousands of times a minute sincHowever, you can scan literally thousands of times a minute sinceethere arethere are NO moving partsNO moving parts!!

Charge coupled devices work differently, but are arranged inCharge coupled devices work differently, but are arranged insimilar arrays.similar arrays.

SemiconductorsSemiconductors


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Semiconductors

Silicon or Germanium are common.Silicon or Germanium are common.

nn--type:type: Si (orSi (or GeGe) doped with group V element (As,) doped with group V element (As, SbSb))

to add electrons.to add electrons.

pp--type:type: Doped with group III element (In,Doped with group III element (In, GaGa) to added) to added

holes.holes.

J. MichaelJ. Michael HollasHollas,, Modern SpectroscopyModern Spectroscopy, John Wiley & Sons, New York, 1992., John Wiley & Sons, New York, 1992.


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PhotodiodePhotodiodeA photon promotes an electron from theA photon promotes an electron from the

valence band (filledvalence band (filled orbitalsorbitals) to the) to the

conduction band (unfilledconduction band (unfilled orbitalsorbitals) creating an) creating anelectron(electron(--)) -- hole(+) pairhole(+) pair

The concentration of these electronThe concentration of these electron--hole pairshole pairs

is dependent on the amount of light strikingis dependent on the amount of light strikingthe semiconductorthe semiconductor


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PhotodiodesPhotodiodes


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Photodiode Arrays (PDA or DAD)Photodiode Arrays (PDA or DAD)


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y ( )y ( )




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DiodeDiode--ArrayArray


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yy

SpectrophotometerSpectrophotometer

Schematic of a diode-array spectrophotometer



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Optical diagram of the HP 8453 diode-array spectrophotometer



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Optical system of the HP 8450A diode-array

spectrophotometer

Thermal DetectorsThermal Detectors


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(infrared detectors)(infrared detectors) ThermocouplesThermocouples

potential difference developed at a junction of twopotential difference developed at a junction of twodifferent metaldifferent metal--metal junctions at two differentmetal junctions at two different

temperaturestemperatures junction is black to absorb lightjunction is black to absorb light

BolometersBolometers

resistance thermometerresistance thermometer PyroelectricPyroelectric transducerstransducers

changes in temperature change the polarizationchanges in temperature change the polarizationbehavior of specific materialsbehavior of specific materials

triglycinetriglycine sulfate (TGS)sulfate (TGS)

Components of Optical Instruments Chapter 7

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