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INFRACRVENA TERMOGRAFIJA INFRARED THERMOGRAPHY
STUDIJ: MEĐUNARODNI POSLIJEDIPLOMSKI STUDIJ
INTERNATIONAL MASTER OF SCIENCE PROGRAMME: SUSTAINABLE ENERGY
ENGINEERING
USTANOVA: FAKULTET STROJARSTVA I BRODOGRADNJE
SVEUČILIŠTA U ZAGREBU FACULTY OF MECHANICAL ENGINEERING AND
NAVAL ARCHITECTURE UNIVERSITY OF ZAGREB
Pripremili: Prof.dr.sc. Srećko Švaić, dipl.ing. Doc.dr.sc.
Ivanka Boras, dipl.ing. Fakultet strojarstva i brodogradnje
Sveučilište u Zagrebu
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1.0.0 INTRODUCTION Infrared (IR) Thermography IR thermography is
a contact less temperature and surface temperature distribution
measuring method. It is based on the measurement of IR radiation
intensity from the observed surface. The result of thermographic
measurement is a thermogram giving the temperature distribution at
the surfaces of the observed object in grey-scale or in a colored
code. The temperature distribution gives information of different
states of the surface itself or is the consequence of the structure
and internal state of the object. Electromagnetic Radiation All
bodies emit continuously electromagnetic radiation, traveling
through vacuum at speed of light – 3 ⋅ 108 m/s. Experiments have
proven that radiation is behaving like particles in interaction
with matter and like waves when traveling through space. Thus
electromagnetic waves are of dual nature: corpuscular and wave. The
radiation wavelength λ is connected to the wave frequency ν and the
wave propagation velocity c through the expression: λ⋅= vc (1)
Although bodies glow at high temperatures, visible light is not the
only radiation they emit. Emission spectra of solid bodies are
continuous and consist of all wavelengths. The energy distribution
at particular wavelengths depends on the temperature and physical
properties of the emitting surface. Fig. 1 represents the
electromagnetic spectrum. Thermal effects are bound to radiation in
the wavelength range from 0,1 to 100 µm. The visible part of the
spectrum is a very narrow band of the thermal radiation range, i.e.
the visible spectrum is only a part of the thermal radiation
spectrum that may be perceived by the human eye. It covers the
wavelength range from 0,4 to 0,7 µm. Following the increasing
wavelengths, the thermal radiation range may be divided into three
subdomains: ultraviolet, visible and IR range.
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Figure 1 The electromagnetic spectrum (gamma, Roentgen,
ultraviolet, visible, IR, microwaves, radio, visible, IR shortwave,
longwave, µm)
Figure 2 Photograph in visible spectrum and thermogram in IR
spectrum In most solids and liquids the neighboring molecules
absorb the radiation of a particular molecule. Therefore the
radiation of liquids and solids is emitted or absorbed only by the
molecules near to the surface: in metals this is a layer only a few
molecules thick and in non-metals a few micrometers. In such
materials emission and absorption may be regarded as surface
phenomena. On the other hand, mixtures of gases which include water
vapor particles, carbon dioxide or even solids partially
transmissive to radiation, the absorption is deep and the emitted
radiation may come from any part inside the observed gaseous
body.
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2.0.0 BLACKBODY A blackbody is an ideal body, which absorbs the
entire incoming radiation, regardless to its wavelength and
incidence angle, thus reflecting nothing. The evident consequence
of this definition is that the entire radiation coming from a
blackbody is emitted radiation and that at a given temperature and
wavelength the emission of a blackbody is the largest. A blackbody
has no preferred direction of radiation; the radiation is diffuse.
E*
1 ⋅ E*
Fig. 3 The blackbody absorbing the entire incoming radiation
Blackbodies emit in the entire range of the spectrum wavelength. In
the case of monochromatic emission of a blackbody, i.e. radiation
energy emitted from unit surface area at a certain wavelength
(W/m2µm), the spectral distribution of radiated energy is described
by Planck's law:
1/
51
2 −⋅
= ⋅−
TCb eCE λλ
λ (2)
where λ is the wavelength in µm, T the absolute temperature in
K, and the constants W⋅µm
81 10742,3 ⋅=C
4/m2 and µmK. The maximum of the spectral radiation density is
shifted to shorter wavelengths with the raise of temperature.
Wien's law gives the relation between the temperature and
wavelength at maximum spectral radiation density:
42 104389,1 ⋅=C
2898max =⋅Tλ µmK (3) which explains the change of color of
surfaces from red to white at heating. The emission of a blackbody
is the energy emitted from its surfaces at all wavelengths. Its
amount is proportional to the fourth power of the body absolute
temperature, according to the Stefan-Boltzmann law: W/m4TEb ⋅=σ
2 (4) where σ = 5,6697 ⋅ 10-8 W/m2K4 is the Stefan-Boltzmann
constant. 3.0.0 REAL BODIES The radiation coming to the surfaces of
a real body is partially absorbed, partially reflected and
partially transmitted: (5) **** EdErEaE ⋅+⋅+⋅=
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E*
r ⋅ E*
d ⋅ E*
a ⋅ E*
Fig. 4 Absorbed, reflected and transmitted radiation The ratios
of the absorbed, reflected and transmitted radiation respectively
and the received radiation are called absorptivity (a),
reflectivity (r) and transmissivity (d). Equation (5) yields: dra
++=1 (6) The majority of surfaces interesting in engineering do not
transmit radiation (d = 0), except for some materials as glass and
plastic films. In that case the radiation is either absorbed or
reflected, so eq. (6) becomes: ra +=1 (7) The portion of the
incoming radiation, which will be absorbed or reflected, depends on
the material and state of the body surface, the radiation
wavelength and the incidence angle. It may also depend on the
temperature. For engineering practice it is suitable to use average
values of absortivity and reflectivity. The emission of real bodies
is essentially different from the emission of blackbodies and has a
different distribution of radiation intensity in the wavelength
spectrum. Emissivity ε is defined as the ratio of the real body
emission to the emission of a blackbody at equal temperatures:
)()(
TETE
b
=ε (8)
The emissivity of real bodies depends on the temperature and the
state of the surface, and significantly on the angle of the
radiation to the surface normal. The emissivity ε of the overall
radiation will differ from the emissivity of radiation
perpendicular to the surface εn. It may be calculated as:
2,1≅nεε for low emitting polished metal surfaces
98,0≅nεε for high emitting non-metal surfaces.
Accordingly, the Stefan-Boltzmann law for real bodies becomes:
(9) 4TE ⋅⋅= σεThe Kirchoff law defines the equality of emissivity
and absorptivity: )()( λλε a= (10) It becomes evident that the
emission spectra of real bodies, where the emissivity depends on
the wavelength, will not be equal to the radiation spectrum of the
blackbody.
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4.0.0 OPERATING PRINCIPLE OF THE THERMOGRAPHIC SYSTEM The
thermographic system consists of the IR camera and the thermogram
processing unit (PC). The camera includes the IR optics, IR sensor,
unit for conversion of electrical into video signals, display and
memory card. Thermograms are processed in the PC using special
software, and the PC stores data from the camera memory card.
Because the characteristics of electromagnetic radiation are the
same throughout the entire spectrum, the optics of IR cameras is
shaped as in usual photographic devices, but it is produced from
different materials, which must be transparent to IR radiation.
These are germanium, zinc-selenide and zinc-sulphide for longwave
IR, and silicon, sapphire, quartz or magnesium for mediumwave IR
radiation.
Fig. 5 Operating principle of a modern thermographic system The
IR sensor measures the amount of incident energy at its surface,
which corresponds to the radiation intensity of a defined IR
spectrum range. The energy radiated to the sensor of the camera
Ecam equals the sum of energies radiated from the observed body,
consisting of proper and reflected radiation (E+rE*), radiation
transmitted through the body dE** and radiation from the
environment Eenv: ( ) envcam EEdErEE +⋅+⋅+= *** (11)
E*
d ⋅ E*
a ⋅ E*
d ⋅ E**
r ⋅ E*
E**
E = Eb ⋅ ε
Eenv
a ⋅ E**
r ⋅ E**
Eenv
Fig. 6 Energy impinging the IR sensor at thermographic recording
of a body
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In order to calculate the correct temperature of the observed
body from the radiation received by the camera sensor, the
properties of the body surface, temperature of the surrounding
objects, camera to object distance, temperature and the humidity of
air must be known. All this parameters must be set as input data to
the camera software. The influence of the ambient radiation should
be minimized, especially if the observed object is at a temperature
similar to the ambient and/or has low emissivity. The basic purpose
of the camera software is to determine the temperature distribution
at the surface of a body of known emissivity. However it offers
other possibilities, e.g. it may be used to determine the
emissivity at the basis of all the mentioned parameters and known
temperature of the body. When it is necessary to eliminate the
transmitted radiation, various filters opaque to wavelengths to
which the observed object is transparent may be inserted in front
of the camera optics. 5.0.0 ACTIVE AND PASSIVE THERMOGRAPHY
According to the measurement approach and data processing,
thermography may be active or passive and qualitative or
quantitative. Active thermography is based upon observing the
dynamic behavior of the object surface exposed to thermal
stimulation. This is accomplished in various manners as impulse,
periodical, lock-in, vibration stimulation etc. The common aim to
all of them is to send a certain amount of energy to the observed
object and to analyse the object response to thermal stimulation in
form of the temporal development of the temperature distribution.
The subsequent analysis yields conclusions of the inside structure
of the material, possible inhomogeneities, cracks or processes
occurring below the surface.
24,6°C
48,1°C
25
30
35
40
45
LI01
LI02
LI03
Fig. 7 Active thermography: Measurement of phenol resin sample,
t = 300 s At passive thermography objects are observed in a
stationary state. The recorded IR radiation differences coming from
the object surface are consequence of temperature and/or property
differences.
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Fig. 8 Passive thermography: Photograph and thermogram of a wall
at the Croatian National and
University Library at Zagreb The processing of thermograms
stored in the PC may be qualitative, which that only differences in
the shading (grey scale or colour code) are analysed (Fig. 9), or
quantitative, which includes the estimation of temperature values,
temperature differences or emissivities at distinct locations of
the thermogram (Figs. 10, 11 and 12)
Fig. 9 Areas of higher and lower temperatures are easily
spotted
Fig. 10 Thermal load of machine parts, an analysis with the
"Isotherm dual above" tool
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Fig. 11 Thermal image of a vessel with a vertical temperature
profile line
Fig 12 Evaluation of the state of a building using the histogram
analysis of two thermogram areas 6.0.0 THERMAL IMAGING SYSTEM
ThermaCAM 2000 Camera basic data Measurement accuracy +/- 2 %
Thermal sensitivity < 0,08 oC at 30 oC Field of view(FxV) /
min.focus distance 24 o x 18 o / 0,5m Detector type FPA 320 x 240
pixel (uncooled bolometer) Spectral range 7,5 – 13 µm Video output
VH Display colour LCD PC card drive type II or type III Image
storing real time, 14 bit digital Battery system ACU Nickel-metal
hydride Size 209 x 122 x 130 mm Weight 2,43 kg Visual camera 640 x
480 pixels
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) 6 1 6 8 2 2 2 , F a x . : ( 0 1 ) 6 1 5 6 9 4 0
Object temperature measurement range - 40 oC – 120 oC 0 oC – 500
oC 350 oC – 1500 oC
Menus and choice possibilities The menu FILE with its submenus
enables the opening of thermograms formerly saved to the disc,
individual or periodical thermogram saving to chosen or new
directories, erasing of thermograms and input of various notes to
individual thermograms, as sound or text data. The ANALYSIS menu
offers with series of submenus the definition of important
characteristics of the observed object and its surrounding:
emissivity, ambient temperature and air humidity. The submenus
Spot, Area, Isotherm and Profile enable an immediate analysis of
the recorded object through spot temperature metering, line
temperature profile and temperature analysis of particular areas.
The IMAGE menu contains submenus enabling the choice among IR and
video recording, selection of temperature range, adjusting the
temperature level and temperature range of thermograms, freezing
the displayed image, automatic focus and colour adjustment, and
setting of markers in video mode, which helps at the analysis of
thermograms. The SETUP menu and its submenus enable setting of
options at spot, area or isotherm metering (colour, size etc.),
changing of picture parameters, manual or automatic adjustment of
thermograms, choice of colours, correction of noise and indication
of temperature saturation. A series of options to define the
thermogram organization is offered, along with the choice of
measurement units, language, date, saving mode, text, sound, save
format and general selection of information appearing at the
thermogram. 6.1.0 RUNNING THE ThermaCAM-Researcher 2002 SOFTWARE
The basic purpose of the ThermaCAM-Researcher 2002 software is
processing of IR recordings (thermograms) coming from the camera in
real time. However, the software may receive and process
thermograms from other media as PC hard disc or memory card. The
program handles fast/medium/slow thermal processes and, depending
on the set configuration, it may display thermograms or save them
on disc and analyse them later. The thermograms as measurement
results may be processed using the following tools: isotherm, spot,
area or line. The results obtained using these tools are displayed
at the monitor along with the thermogram as windows showing the
temperature profile, histogram, basic result data table or drawing.
The measurement results can also be linked and processed using
various subprograms. The standard application in this program is
the adjustment of the image marked with "lock". This facilitates
locking of the temperature scale, object parameters or the zoom
factor. This means that a previously defined specific temperature
scale, adjusted to the user's wishes may be used. The present and
the following displays will be shown using this specific
temperature scale, although they are saved with another. After
unlocking, each thermogram will be displayed with the original
temperature scale.
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Program screen layout There are several layout options
available. These are controlled by tabs in the bottom part of the
ThermaCAM Researcher window. You can see combinations of the IR
image, the profile, the histogram, the plot and result table
windows. All tabs have an IR image with a temperature scale in the
top left corner.
Fig. 13 One of the possible interfaces of the ThermaCAM
Researcher Tools enabling the thermogram processing are located at
the following tool bars: ■ standard tool bar (creating, opening,
saving etc.), ■ play images tool bar, ■ recording tool bar, ■ image
directory tool bar, ■ analysis tool bar ■ scaling tool bar. In
order to get a good image from the camera, you should establish a
connection, select an appropriate measurement range, auto adjust it
and focus it. No matter if you have a live image, a frozen image or
a disk image you should now consider the object parameters
(emissivity, ambient temperature, atmospheric temperature, relative
humidity of the air, the distance and the external optics
transmission and temperature). They describe the physical
properties of the body of interest and its environment and the
atmosphere between the object and the camera. You can reach them
via Settings in the Image menu or this button:
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Figure 14 Settings in the Image menu It is important that these
parameter values become correct. Otherwise the scale temperatures
and displayed colours will be wrong. The image parts for which the
object parameters are wrong will get incorrect temperatures and
colours. (The measurement functions have object parameters of their
own which are used to handle the case when there are two different
targets in the same image.) If the colours of the image are
inappropriate, you can change them. The selection Palette tool
button will bring up a dialogue window with the palettes available.
How to use the analysis tools to get numerical temperatures and
statistical information out of a single image The analysis tools
will show their results in the result table, plot, profile or
histogram window or directly inside the IR image. Results are also
available through the OLE functions, such as Copy Value. Both
absolute measurements (i.e. the result is a real temperature) and
relative measurements (i.e. the result is a difference temperature)
can be made. The relative measurements are made relative to the
reference temperature that you can enter in the dialogue window
Image Settings (in the Image menu), the Object Parameters tab. The
analysis tools work both with live images and recorded images. The
isotherm tool An isotherm is a marker in an infrared image that
highlights areas where the radiation from the object is equal. The
name isotherm can be misleading, since it implies that equal
temperatures are highlighted. This is only true if the emissivity
of the object is the same all over the image. If you bring up the
menu on this button, you will see that there are five types of
isotherms in ThermaCAM Researcher. The most commonly used one is
the interval isotherm. It will highlight a temperature interval
with a certain (selectable) width. The spot meter tool This tool
measures the temperature in one spot on the image and shows the
result in the result table or beside its symbol in the IR image.
The results are also available through OLE. You can obtain the
following values: Temperature, Temperature relative to the
reference temperature, Emissivity, Object distance and the image
co-ordinates of the spot meter.
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The flying spot meter This tool only measures the temperature at
the mouse cursor and displays it beside the cursor in a tool tip
window. There is just one single flying spotmeter. The area tool
This tool measures the maximum, minimum, average and standard
deviation temperature within a chosen part of the image and
presents these values in the result table window or beside its
symbol in the image. Results can also be displayed graphically in
the histogram window. The line tool This tool measures the minimum,
maximum, average and standard deviation temperature along a
straight or bendable line within the image. The temperature in one
spot, the line cursor, can also be measured. These values are
presented in the result table or beside the line symbol in the
image. The line temperatures can also be graphically presented in
the profile window. The Formula tool This tool is used for adding
and editing formulas. A formula can contain all common mathematical
operators and functions, such as +, -, *, / square root, etc. Also,
numeric constants such as 3.14 can be used. Most importantly,
references to measurement results, formulas and other numerical
data can be inserted into formulas. Object's parameters Frequently,
the object emissivity or distance is varying between different
parts of the IR image. All analysis tools (except the isotherm) can
be forced to use their own values on these object parameters. 7.0.0
CONCLUSION Every experimental method has the capabilities and
limitations. For thermography we could say that the advantages
are:
■ Contact less technique: no physical contact, no interaction
with specimen ■ Fast, surface inspection ■ Ease of interpretations
of thermograms ■ Great versatility of application ■ Ease of
numerical thermal modeling
And the limitations are: ■ Variable emissivity ■ Cooling losses
(convection/radiation causing perturbing contrast) ■ Absorption of
infrared signals by the atmosphere ■ Difficult to get uniform
heating (for active procedure) ■ Limited contrasts and limited
signal/noise ratio, causing false alarms ■ Observable defects
generally shallow ■ Works only if thermal contrast naturally
present
For correct quantitative and qualitative analyses of thermograms
it is necessary that in measurements are included trained person
knowing the problem.
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