ASNT NDT Handbook Volume 3 Infrared and Thermal Testing

PART 1. Thermal Detectors

In thermal detectors the incident temperature variations and thus need a
chopper modulating the incoming
radiation heats the surface. This heating radiation to maintain a signal output,
affects properties of the heated material unless the system is panned over the
scene. This added complexity is a severe
such as its electrical conductivity, which drawback.
in turn causes the signal output to vary.
One characteristic specific to thermal The figure of merit for pyroelectric
detectors is that the response is detectors is the pyroelectric coefficient p
independent of the wavelength. To get a defined as the slope of the material
wavelength band for a specific application polarization P versus temperature T, at the
an interference filter rejecting radiation of operating temperature:3
unwanted wavelengths will have to be
aP
placed in front of the detector.
The limit of sensitivity of thermal (3) aT

detectors is given by the effectit'e
conductivity GR:l

·where a= 5.67 x 10-12 V\'·cm-Z.K-4 is This effect can be improved with the

Stefan's constant, Tis the detector application of an electrical field bias. This
temperature and A is the detector is referred to as the field enhanced
sensitive area. D* is the detectivity limit:
pyroelectric effect or the ferroelectric
bolometer effect. In such a case, p comes
to the following:3

E aedp
aT dE
f(4)
~ ~(2) D • p ~ Po +

4kT G 0

·where k (\'V·m-1-K-1) is the thermal with Po being the pyroelectric coefficient

conductivity and G is the limit of GR. \Vithout bias, the dielectric permittivity
Theoretically a maximum of and E the applied electrical field.
Pyroelectric elements can be made as
0" = 1.8 x 1010 cm·\'(Hz)·VV-1 can be point or image detector.

obtained with a thermal detector; Photonic Detectors

practical factors limit the effective In photonic detectors the signal is
detector performance. Modern obtained by measuring directly the
technology has produced a detectivity excitation generated by the incident
limit of D* ~ 7 x 108 cm·'-'(Hz·\V-1). photons. Heating of the sensitive surface
is unnecessary. Photonic detectors are of
Pyroelectric Detectors two types: photoemissive and quantum
(photoelectric, photovoltaic or
Pyroelectricity is defined as the property photoconductor).
of certain crystals to produce·a state of
electric polarity in response to a change in Photoemissive Photonic Detectors
temperature. 2 A broad class of thermal
For photoemissive plwtonic detectors, the
detectors are p}'roelectric detectors, for signal observed expresses the measured
which electric charges are generated by electron flow (that is measurement of a
current i) pulled away from the
incident radiation absorption (heating) photocathode under the effect of both the
below the curie temperature. The curie incident photons and a static polarization
temperature is that below which there is a (rig. 1). The spectral sensitivity depends
spontaneous magnetization in absence of on the properties both of the material
an externally applied magnetic field. A used for the photocathode and of the
change in detector temperature generates outer envelope infrared transmittance.
a transient change in the surface charges Typical values span from ultraviolet to
thus causing a transient current available
for pickup by the readout unit.
Pyroelectric detectors are sensitive only to

186 Infrared and Thermal Testing

near infrared: 0.2 to 1 pm. Solid state made these detectors compact, reliable,
photoemissive detectors are also possible. robust and consequently popular.

In photomultiplier tubes electrons are For photoconductive detectors an
accelerated and multiplied by secondary external current is necessary to measure
emission from internal plates called conductivity change whereas
dynodes. Multiplication factors of 105 photoelectric (photovoltaic) detectors act
to 107 can be obtained for 10 stage tubes. as power generators supplying a signal
These detectors are point detectors without need for polarization. Because
although image converter tubes are made biasing current is needed for
as well, with such uses, for instance, as photoconductive detectors, high charge
night vision image intensification for capacity alkaline batteries can be used to
military and surveillance applications. induce minimum noise and ripple. Low
noise is essential to achieve stable results.
Quantum Detectors In this respect, because photovoltaic
detectors supply a signal by themselves,
Quantum detectors are solid state detectors they are much more attractive than
in ·which photon interactions either photoconductor detectors requiring less
change conductivity (photoconductive complex readout circuits. Common
detectors) or generate voltage materials used in photoelectric sensors
(photoelectric detectors, or photovoltaic (photodiodes and phototransistors)
detectors). Because no heating is needed include silicon, indium arsenide, indium
as for thermal detectors, their response antimonide and mercury cadmium
time is short. The solid state structure has telluride.

FIGURE 1. Schematic diagram of Figure 2 presents the spectral
photoemissive detector: (a) side view; detectivity curves for the most common
(b) axial view. detectors. On these curves, sharp cutoff
response is observed at longer
(a) Photocathode wavelengths. This can be explained
because of the presence of different
Glass envelope energy levels, within the atomic structure.
At low energy (low valence level)
"" electrons stay close to the nucleus. If
enough energy is supplied to electrons,
Incident they cross the forbidden baud, which frees
photons them from nucleus attraction and enables
them to participate in an electric current.
At Ions ·wavelengths the photons transmit
less energy to electrons:

(5)

Vacuum where lV is the radiated energy ~oule),
cis the velocity of light= 3 x 10 m·s-1,
Tube connections II is Planck's constant= 6.63 x l0-34 J·s-1

(b) and A is the radiation wavelength (meter).

Incident Because electrons cannot cross the
photons forbidden band and thus stay in the
valence band, there is a sharp cutoff in
the spectral detectivity curves.

At srnall wavelengths, photons have
greater energy and penetrate deeper in the

substrate, passing through sensitive areas

of the semiconductor without interacting.
This causes a gradual loss of detectivity as

observed on Fig. 2. Imaging detectors are
discussed below.

Static polarization Infrared Imaging Devices

A class of detectors lend themselves
without mechanical scanning operations
to imaging applications~ that is, to the
production of an array of points (also
called pictllre elements, or pixels) in either
one dimension, as in a line array, or in

Noncontact Sensors for Infrared and Thermal Testing 187

two dimensions, as in television image operation. Pyroelectric tubes are thermal
generation. Although infrared films such detectors where the signal is proportional
as liquid crystals made from specific dyes
sensitive in the 1 to 3 pm range are to the absorbed enerm•. For certain
available, they are impractical for real ferroelectric crystal materials such as
time imaging in nondestructive testing. triglydne sulfate that have a high
pyroelectric coefficient p, heating or
In imaging applications the output of
the detector device produces the image in cooling of the crystal slice creates an
one dimension or two dimensions. Two accumulation of charges. This is the
main image forming processes exist:
(1) direct image formation with a detector pyroelectric effect, which results in a
array, pyroelectric detector or infrared change of polarization.5
film; (2) electromechanical scanning of
Pyroelectric tubes are similar to
the scene with a single detector.
standard vidicon television cameras
Pyroelectric Detectors except for the face plate and target

Back in the 1960s1 the first successful material (Fig. 3). As the infrared image
infrared commercial products based on
pyroelectric detectors (vidicon tubes and impinges on the pyroelectric target1 a
single pyroelectric elements) were temperature distribution and in turn a
produced.4 The major advantage of this charge distribution appears on the
kind of thermal detector is the ambient or pyroelectric material. The electron beam
close to ambient temperature mode of
scans this material and two things may

happen. If no radiation has been absorbed
at the scanned spot there is no charge (no

polarization) and the electron beam is
redirected toward the mesh. On the

contrary, if radiation has been absorbed,

FIGURE 2. Spectral detectivity curves for infrared detectors. Atmospheric absorption is also

indicated. Unit conversions for temperature scale: K- 273 = 0 0 X 1.8) + 32 = 0 F. Unit
(; ((

conversion for measure of distance: em + 2.54 = in.

1011 n
Perfect
i7 photovo!taic
detector
I

""1: 1- Perfect
~n~~~~~~~~~~l'~t--.~~[':0eE photoconductor
~ 1010
" ~"' !"\. detectorth
*«
0 , / GeHg (28 K)

I' I r'"'" "m'' d'''"O'

0 '
i 1-1, (77 K)
JO'
Pyroe!ectric detector

Bolometer

I I '"f 'J'-Thermopil

·: (<

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

Wavelength /,, J-Im

legend

0 =atmospheric absorption (environment, T = 295 K f22 oc = 8 cfJ, 2 steradian)

AuGe =gold germanium
GeHg =germanium mercury

HgTeCd =mercury telluride cadmium
lnAs =indium arsenide
lnSb =indium antimonide
PbS =lead silicon
PbSe = lead selenide
PC = photoconductive
PV = photovoltaic

188 Infrared and Thermal Testing

charges accumulate and the electron Pyroelectric tubes have the following
beam reaches the signal electrode, thus drawbacks.
causing a current to appear. As the
elet·tron beam is scanned over the surface, 1. Their dynamic range is small (30 dB).
the video signal appears at the tube 2. Their linearity is poor, with less
output.
effective beam readout toward the
Because charges are released only when target edge.
the temperature of the pyroelectric 3. Their life span is restricted, typically
material changes, such tubes cannot 10 000 h, or 13 months at 24 h per
produce images of a static scene. If only day.
part of the scene is dynamic or moving, a 4. Because pyroelectric materials are
flying aircraft for instance, only the piezoelectric they require careful
moving part is observed. This effect can mechanical design to avoid
be useful in applications such as fire microphonic noise from excessive
detection where only the fast temperature target vibrations.
changes of flames are seen. Another use
may be nondestructive testing of parts Despite these drawbacks, pyroelectric
moving on a conveyor belt. tubes are attractive because of the
following advantages.
If the requirement for a scene to be
dynamic is undesired, the camera can be 1. They do not need cooling.
panned or the radiation can be modulated 2. They do not consume much power.
using a mechanical chopper. Camera 3. They are relatively inexpensive..
panning has the advantage of simplicity 4. They are not restricted to a fixed
and reliability because no part is moving
other than the panning mechanism. image format, as are detector arrays.
Changing the electron beam scanning
In the chopped (shuttered) mode, a rate suffices to obtain another video
format.
rotating chopper mounted in front of the
tube shuts off the scene during one field In the 1990s developments in
causing target cooling, a negative image is pyroelectric detectors made it possible to
then produced. 'A1hen the target is produce pyroelectric arrays.6•7 These
exposed again, the radiation distribution two-dimensional arrays (32 x 32 in 1987;
heats the pyroelectric material and the 100 x 100 in 1990) are made of
positive image is reformed. Although a ferroelectric ceramic thin film such as lead
signal is continuously produced it cannot scandium tantalate. They offer high
be displayed in this form because it is detectivity yet produce slow scan images
composed of alternating fields causing (1.0 Hz) without needing to cool.
heavy flickering of the useful positive Applications include low cost consumer
field. Special electronics make it possible products for detection of flames, detection
to overcome this problem by displaying of heat emitted by warm objects such as
the positive and reverted negative image. people (for intruder detection), medical
thermography and transport monitoring.
Pyroelectric material absorbs radiation Among advantages of these monolithic
evenly over a broadband wavelength chips of pyroelectric ceramic are the
spectrum, so the tube sensitivity is mainly following.
determined by the input window
material. Germanium (3 to 5 pm and 7 to 1. They are more rugged than
20 pm) and zinc selenide (0.6 to 20 pm) pyroelectric tubes with fragile glass
are the materials most commonly used for envelopes.
tube windows.
2. They are easy to operate, with small
FIGURE 3. Schematic diagram of pyroelectric vidicon tube. voltage requirements.

Cathode 3. They are compact.

~Video signal For these pyroelectric arrays, rcsponsivity
variations from clement to element are
Amplifier typically less than 10 percent. Absolute
temperature measurement thus requires
individual calibration of each element
which degrades considerably the image
acquisition time.

Ferroelectric materials can be used in
two modes: pyroelectric mode or dielectric
mode. The more conventional mode is
pyroelectric for which the signal origin is
the change of polarization because of the
heating of the detector by the incident
radiation, as described above. In the
dielectric mode, the change of
permittivity with temperature is used: this
change of permittivity is sensed as a
change in voltage across the detector after

Noncontact Sensors for Infrared and Thermal Testing 189

a proper bias has been applied. The Focal Plane Arrays: Schottky
drmvback of the dielectric mode of Barrier Detectors
operation is the need to stabilize the
detector temperature. This type of detector was first proposed in
1973 by Shepherd and Yang. 10 Since then
Improved spatial resolution can be large arrays (512 x 512) have been
obtained for pyroelectric imaging commercially introduced by many
detectors by reticulating the detector companies. Infrared cameras are also
surface to reduce the lateral heat spread. fabricated as large one~dimensional arrays
This reticulation can be done by ion beam such as 1024 x 1. 11 In this design,
machining a pattern of narrow groves at horizontal scanning is done electronically
30 to 40 pm depth all over the in the detector plane whereas the vertical
pyroelectric target.K scanning is achieved by optomechanical
means, by camera panning12 or by scene
In qualitative infrared nondestructive displacement with parts moving on a
testing, pyroelectric based infrared conveyor belt or air lifted.
cameras can be an excellent choice.
Quantitative measurement is also possible. The most common type is platinum
In some configurations the rotating silicide detectors operating in the
chopper, which has an high emissivity photoemissive mode and operating in the
surface, provides a knmvn temperature 3 to 5 J-Im band ·with cutoff wavelength of
reference from which scene temperature 5.6 pm. Gallium silicide is also available
can be determined. ~vfeasurement of the in the 8 to 14 and 8 to 16 ~m bands.
chopper temperature is carried out Platinum silicide detectors are often
indirectly by monitoring the signal from a fabricated with an aluminum mirror over
single~element pyroelectric detector signal the sensitive area and are illuminated
caused by a small blackbody target viewed from the back. The mirror enhances the
through the chopper. The blackbody platinum silicide responsivity but restricts
temperature is adjusted through a peltier the response to wavelengths greater than
device so that no temperature differential 1 J-Im because of the absorption by the
is observable between the chopper and silicon. 13,14 \-\'ith respect to intdnsic
the blackbody. The blackbody temperature photon detectors such as indium
is then measured using a thermistor.9 antimonide, quantum efficiency of
platinum silicide is rather small
Focal Plane Arrays (FPAs) (10 percent versus 85 percent).
for Infrared Testing
The design is generally similar to video
In the 1970s a new imaging device began charge coupled devices both for storage
revolutionizing the infrared community. and readout circuits. The radiation
Large infrared arrays have simplified induces charges that are stored in a
infrared camera construction. Because of capacitor like insulating layer. Charges are
thh technology, all that are needed to then transferred to the neighbor element
build an infrared camera are the optics, under the effect of an electric field. Stored
the focal plane array, the associated charges are next transferred in this
electronics and in some cases a cooling fashion element by element up to the
unit. Similar to conventional video charge array output where the signal is available.
coupled devices, these chips do not A multiphase (2 or 3 phases) clock
require any electromechanical scanning performs these step transfer processes. The
mechanism or other moving parts for video signal is generated by sequentially
image forming and are less cumbersome scanning all the rows in the array and by
and fragile than pyroelectric tubes. The multiplexing each row. In Jarge arrays
video signal is obtained directly by an (512 x 512), for compatible television rate
on~chip electronics drive. A variety of signal, clock rate greater than 8 MHz is
technOlogies have emerged. The present required to read all the detectors during
discussion reviews the schottky barrier, one frame. The fill factor is greatly
superlattice, intrlnslc and Z plane improved (up to 90 percent) if the readout
technologies. circuits are under the detector array.

Because of the restricted applications Charge injection devices are also used
(military, law enforcement, in focal plane arrays. Charge injection
nondestructive testing), the price of such devices are similar to charge coupled
detectors, especially the silicon based devices but the generation of the video
ones, has been slow to approach those of signal is different. In charge injection
mass production consumer market device detectors, the video signal
technologies such as the video tape comprises the substrate current caused by
camcorder. charge injection, rather than charge
transfer as in charge coupled devices. This
current is proportional to the received
photons. Charge injection device
detectors are less prone to blooming.
Moreover, charges are Jess affected by

190 Infrared and Thermal Testing

radiation during device reading after the because they generate charges
accumulation interval. Because of the spontaneously under illumination by
addressing mode, cells can be read incident radiation. Thermal charge
individually. generations because of impurities in the
material degrade the signal, mainly
Platinum silicide is the more mature because of the shockley~read process and
schottky barrier detector. Its main the auger process (charges generated
drawback is its need for cooling to 77 K through impact ionization by carriers
(-196 "C ~ -321 "F). Other metal silicide whose energy exceeds the forbidden band
arrays have been made to operate at gap). Refinement in fabrication processes
longer wav('lengths; among them iridium should improve these aspects.
siliride, nk kel silicide and cobalt disilicide
are suitable for imaging applications. As mentioned in the case of
Iridium silicide has been reported to have superlattice detectors, hybrid technology
a cutoff \vavelength of 7.3 Jlnl at 62 K where detector layers are fused or glued
(-211 "C ~ -348 "F), 9.6 pm at 40 K on silicon readout circuits is attractive
(-233 "C ~ -388 "F) and 10.7 pm if the because silicon process is a well
detector is biasect.t5,l 6 Iridium silicide established fabricating process. Such
arrays are more difficult to fabricate hybrid arrays are fabricated with mercury
because impurities have a strong influence cadmium telluride detectors in other sizes,
on performance. Cooling below 60 K such as 128 x 128, 8 to 12 pm19 and
(-213 "C ~ -352 "I') is also a problem for 256 X 256, 3 tO 5 pm.ZO
silicon technology because charge storage
is difficult at low temperatures. 17 An alternative fabricating technique is
to grow mercury cadmium te11uride cells
Focal Plane Arrays: Superlattices on gallium arsenide buffers that are
themselves grown on silicon substrate
In superlattice detector arrays, alternating containing bipolar preamp1ifier transistors
layers of different semiconductors of and readout circuits. Advantages of this
different thickness makes it possible for monolithic configuration include
the wavelength of absorbed radiation to improved uniformity, reduced (-1 noise
be tuned: photoconduction occurs in a and higher operating temperature.1
narrow range of wavelengths. 1\•fonolithic focal plane arrays where
mercury cadmium telluride diodes are
A promising technology is gallium directly grown on silicon could also yield
arsenide-gallium aluminum arsenide, first to larger arrays.21
proposed in 1987.18 Typical cutoff
frequencies are between 6 and 11 pm PhotovoJtaic indium antimonide
with detectivities D* of about 1010 to hybrid focal plane arrays are fabricated as
IQ11 cm·~(Hz)·\·V-1 ·with proper cooling at wen.zz For instance, a widely used
SO to 70 K (-223 to -203 "C; -370 to commercial infrared camera of the 1990s
-334 °F). Other compositions have been is designed around a 256 x 256 indium
reported such as indium arsenic antimonide array with high frame rate:
antimonide-indium antimonide and 140 per second at 256 x 256 up to 1800 in
indium arsenide-gallium indium a 64 x 64 sub window. Other array
antimonide. formats are also available
(320 X 256, 512 X 512, 640 X 512).
The potential of this technology has
been evolving, especially with the Mercury cadmium telluride, indium
possibility of growing directly on the antimonide, lead tin telluride and indium
silicon the detecting superlattice layers gallium arsenide are common types of
that contain readout circuits. Indium intrinsic detectors. Compositions such as
gallium arsenide detectors operate in the mercury manganese telluride and mercury
0.8 to 2.6 pm band with important zinc telluride have been developed.
applications in spectroscopy for real time
determination of chemical composition, 1v{ercury cadmium telluride and its
surveillance and fiber optic extension, the SPRJTE (signal processing
telecommunications (particularly at 1.3 in the element) sensor is a widely used
and 1.55 pm). One of the advantages of detector. The SPRITE sensor was originally
indium gallium arsenide is the room developed by Elliot at the Royal Signals
temperature operation for intrinsic and Radar Establishment, United
detectors. Kingdom. 19

Focal Plane Arrays: Intrinsic A SPRITE sensor is made of a strip of
Photon Detectors mercury cadmium telluride mounted on a
substrate such as sapphire and is cooled at
Those detectors are in fact arrays of cryogenic temperatures (Fig. 4). The long
photoconductive or photoelectric axis is in the direction of scan and the
(photovoltaic) detectors that have been strip is biased so that the carrier drift
reviewed previously. Photoelectric velocity matches the speed at which the
(photovoltaic) detectors are more useful image is scanned. As a point in the image
moves along the strip, the charges it
induces move with it and the
accumulated charges are then read out at

Noncontact Sensors for Infrared and Thermal Testing 191

the end of the strip. Consequently, the importance, but if quantitative data must
main advantage of the SPRITE sensor is be extracted from the imaging array, some
that the signal is integrated on the focal sort of uniformity correction has to be
plane by the detector itself, thus reducing ccuried out.
the noise level and the timing
considerations. If a SPRITE sensor is used Although this calibration process can
and then read out serially, the scanning he done on chip in the case of the Z plane
rate js the same as ·with a single detector technology, it is generally executed off
element. Hm\'ever, if several SPRI'I'E chip. The array is exposed to a scene of
sensors are stacked together in a parallel uniform temperature and an image is
fashion,. the scan speed is reduced because recorded on computer memory. This
many rows cCin be integrated process is repeated for different scene
simultaneously.n temperatures. Next a calibration function
either linear or of higher order for
Z Plane Technology for Focal nonlinear response to the photon flux is
Plane Arrays computed pixel by pixeJ.25

Z plane technology has suggested ways to For linear functions, two coefficients
improve infrared camera systems by are obtained for every pixel. This is the
adding processing to the detection so-called flvo-point teclmique: gain and
function of the array. Silicon circuits are offset. Second order polynomials require
fabricated on a tiny ceramic board three parameters and more coefficients are
typically 100 pm (0.004 in.) thick with a needed for multiple points correction. In
detector attached on one edge. These single-point compensation average of several
boards are then stacked to form complete images recorded over a uniform
arrays. Among possible added processing background ·with a defocused lens is
functions are convolution operations, subtracted (in real time) from live video to
edge extraction and blobs dctection.24 remove nonuniformity. This· process is
however less effective than other
Silicon multiplexer technology makes techniques such as two and multiple
it possible for analog-to-digital converters points techniques. 13•14 During normal
to be incorporated on chip in each operation of the array these computed
column for higher data rates and low coefficients serve to correct images.
noise. Platinum silicide arrays exhibit better
uniformity; more sensitive, intrinsic
Uniformity Correction for Focal photon detectors have Jess uniformity,
Plane Arrays about 10 percent.

One problem with large arrays is the The nonuniform response is more
nonuniformity of response among easily handled in the case of a
detector cells caused by the fabrication single-detector based infrared camera. fn
process. For qualitative applications this this case a more complete correction
nonuniformity may be of little procedure can be carried .out to correct
vignetting and radiometric
FIGURE 4. Principle of operation of SPRITE (signal processing discrepancies.2S,l6 Periodic recalibration is
in the element) detector. needed because of system instabilities in
time, aging, unstable bias voltages, pixel
lncom·1ng radiation non linearities and (-1 noise. For static
scenes a quantitative measure of image
Charge carriers Readout region drift can be, for instance, the time
evolution of the spatial variance in the
Biasing ; - - - Drift region --~: image. This could help in deciding when
current to recalibrate. 'When applicable, noise
source reduction techniques such as running
averages contribute to reduce spatial noise
level although periodic recaJibration is
stili required for accurate quantitative
measurement. Z7,28

Summary

The different technology characteristics of
infrared detector arrays may be
summarized briefly.

Platinum silicide is the more mature
technology- large two-dimensional
arrays (512 x 512) are fabricated and
complete infrared camera systems are
commercially available. They operate in
the 3 to 5 pm hand and need cooling to

192 Infrared and Thermal Testing

77 K (-196 oc ~ -321 °F). Cooling may

not be a problem, for a compact stirling
engine cooling _u,njt can be integrated
with the detector array.

Thermal detectors operate at room
temperature but their detectivity is not as
high as for photonic detectors. Because
they respond to the rate of change in
scene temperature, a chopped mode of
operation or panning is needed for
viewing of static scenes.

Superlattices and Z plane technology
are more recent and have improved, with
operating temperatures close to 77 K
(~196 °C = -321 °F) possible in the long
wavelength band. Intrinsic photonic
detectors such as hybrid indium
antimonide and mercury cadmium
telluride are now common with operation
in both atmospheric bands (3 to 5 pm
and 8 to 12 pm) and 'With respectable size

although 77 K (-196 oc ~ -321 °!') cooling

is required.

Noncontact Sensors for Infrared and Thermal Testing 193

PART 2. Scanning Radiometric Imaging
Detectors

The difference between a scanning must be observed because thermal
radiometer and an· ordinary infrared information registered at different times is
camera is that, for a radiometer, the mixed together. On way to solve this
infrared signal is temperature calibrated problem is to split the video signal in its
thanks to the presence of internal basic fields and process them knmving the
temperature references seen by the time interval between each of them.
detector element durjng the image IIowever, this kind of analysis requires
formation process. This calibration signal careful manipulation of the signal.
permits recovery of the absolute
temperature after proper processing. In Cooling
such an instrument, the image is
electromechanically scanned over the As pointed above and shown on Fig. 2,
detector surface (single piece or SPRITE) superior detectivity can be achieved,
by means of the synchronous rotation of especially for photonic detectors, if
mirrors or prisms (Fig. 5). cooling is used. Such a cooling is needed
to reduce the noise to an acceptable level
'lb accommodate the standard video as seen above. There are different ways to
signal format of 30 frames per second (25 cool the detector: liquefied gas, cryogenic
in Europe), a very high scanning rate is engine, gas expansion or thermoelectric
required. This imposes a wide bandwidth effect. Cryogenic coo1ing uses a 1iquefied
from the associated electronics for the gas stored in a vacuum vessel called a
noise level to be kept small. To overcome dewar (from James Dewar, 1842-1923,
this problem, some manufacturers use Scottish physicist who was the first to
slower scanning rates and have frames liquefy hydrogen, in 1892). Dewars are
made of several fields with one field constituted of two envelope ·walls ·with
update at each scan (once every four evacuated space maintained between
scans, for example). As a result, the them; moreover, to prevent heat wall
output obtained, comprising several fields, losses the surfaces facing the vacuum are
is not updated in real time (that is at 30
or 25 Hz). This lack of updating may
cause problems when fast thermal events

FIGURE 5. Internal view of infrared radiometer. Filter
selection
liqu"1d n·1trogen
coolant at 77 K
(-196 "C = ~321 CF}

Dewar

Aperture control

Internal blackbody
temperature reference

Objective

Infrared detector

Collimation Horizontal Vertical scanning prism
Iem scanning prism

194 Infrared and Thermal Testing

heat reflective (Fig. 6). Metal dewars are -187 °C = -305 °1') at the tip of the
unbreakable, but relative porosity of metal expansion nozzle (on which the detector
welds restrict life span unless vessels me is fastened). As a gas, argon occurs freely
repumped regularly. Generally, in the atmosphere to the extent of
manufacturers provide a valve for this 0.935 percent. This mechanism permits
purpose ·whereas a cryopump or a greater autonomy than dewar operation
diffusion pump with vacuum rated in the but is noisy and the gas tank may be
cumbersome.
5 x 1o-6 torr range is recommended for
For applications where refilling is not
pumping. Dewar repumping is required practical, such as in remote areas or on
when sweating or condensed water vapor the production line, a closed stirling cycle
is observed on the outside of the dewar engine with cryostat can be used. This
and when rapid boiloff causes nitrogen machine cools through repetitive
gas to escape rapidly from the fill port.29 compression and expansion cycles of gas
Glass dewars do not have this problem, by a piston: it compresses gas at a low
but they are very fragile. temperature and allows it to expand at a
high temperature. Because of the cycling
In a popular configuration, the operation, strictly speaking the cooling is
detector is directly mounted on the cold not constant but temperature variations
surface \Vith a cold shield and an infrared can be made small (or large) depending of
transparent window. Because some the cycle characteristics. Although either
detectors (such as mercury cadmium rotary or linear motors can be used, a
telluride) tend to sublimate when exposed linear motor positioned at right angle to
to the vacuum, a protective coating such the detector plane causes less vibration in
as zinc sulfide can be applied on the integral stirling engines. Split cycle
sensitive surface to expand the life span. machines are also available for remote
The most commonly used and cheapest and low vibration operation. Typical
liquefied gas is liquid nitrogen at a input pm\'er is around 4 VV for 150 m\,V of
temperature of 77 K (-196 oc = -321 oF). cooling for a small engine (0.5 kg)
Some 70 percent of the Earth's whereas a large engine (2 kg) ·wm deliver
atmosphere is nitrogen. Liquid hydrogen 1 VV of cooling for 40 W of input power.
lvfany manufacturers offer infrared
(20 K =-253 oc =-424 oF) and liquid cameras with stirling cycle engines.
helium (4.2 K =-269 oc =-452 oF) are
Another mode of cooling infrared
more exotic because of their price and detectors is using thermoelectric elements
because a typical 1.0!. (1.0 dm3 = based on the peltier effect. This
0.26 gal) dewar will keep liquid gas for 3 phenomenon was discovered in 1834 by
to 4 h, thus requiring a regular refilling. ].C.A. Peltier: at the junction of two
dissimilar metals carrying a current,
Joule-thompson gas expansion is temperature rises or falls depending upon
another way to cool detectors. In this case the current direction. Vigure 7 shows that
the quick expansion of high pressure gas peltier effect is used for cooling,
(such as nitrogen or argon) produces, after thermoelectric effect is used for
a few minutes of operation, droplets of temperature measurement with
liquid nitrogen or liquid argon (86 K= thermocouple and thompson effect is
used for power generation. These three
FIGURE 6. Schematic diagram of dewar with
windowed aperture at angle 8.

FIGURE 7. Three processes of direct conversion of heat into
electricity or of electricity into heat.

A Ampere meter
(thermocouple,
r, thermoelectric effect)

A /o'
B • - Battery (power generation,
thompson effect)

~0>

Generator (cooling, pe!tier
effect)

Evacuated legend
5pace
A,B "" dissimilar metal!>

rh == high temperature

11 "" low temperature

Noncontact Sensors for Infrared and Thermal Testing 195

phenomena are in fact three applications important criteria for band selection are
of the same physical phenomenon of operating distance, indoor/outdoor
direct conversion of heat into electrical operation, temperature and emissivity of
t'tH.'1gy, or the reverse. Because of the low the bodies of interest. As Planck's law
efficiency of the conversion process stipulates high temperature bodies emit
peltier elements draw high current and more in the short wavelengths, so long
are generally stacked upon each other to wavelengths will be of more interest to
achieve sufficient heat removal and observe near room temperature objects.
temperature gradient (Fig. 8). Peltier Emitted radiation from ordinary objects at
elements are unattractive, however, for ambient temperature (300 K) peaks in this
temperatures below 200 K long wavelength range. Long wavelengths
are also preferred for outdoor operation
(-73 oc ~ -100 °!'). where signals are less affected by solar
radiation. For operating distances
Most infrared camera manufacturers restricted to a few meters in absence of
offer their products with an optional fog or water droplets, the atmosphere
choice among those various means of absorption has little effect.
cooling. For laboratory operation,
nitrogen cooling with metal dewar is Spectral emissivity is also very
perhaps the best choice because of important because it conditions the
reliability, quiet operation and the low emitted radiation. I'igure 9 shows spectral
emissivity cun•es for common materials.
temperature 77 K H 96 oc = -321 °!') Also plotted on these curves are the more
useful infrared bands of interest. Polisl1ed
achieved. For remote operations metals with emissivity smaller than 0.2
thermoelectric cooling is preferred can not be observed directiy1 for they
because it does not need any refilling and reflect more than they emit. A high
its lack of moving parts makes it reliable. emissivity coating (such as black paint) or
For the production environment, good a reflective cavity must be used.31 <B
choices are the stirling engine cooling,
joule-thompson gas expansion cooling or Although no specific rule can be
thermoelectrical cooling. formulated 1 generally the most u·seful
bands are 3 to 5 pm and 8 to 12 pm
Selection of Atmospheric because they match the atmospheric
transmission bands. Most of the infrared
Band commercial products fall in these
categories •whereas the near infrared (0.8
Because the atmosphere has not perfectly to 1.1 pm) part of the spectrum is easily
flat transmission properties (Fig. 2), the covered by standard ambient operation
selection of the operating wavelength temperature silicon detectors.
band will be conditioned by the final
application. For the majority of Another important point to consider is
nondestructive testing applications, the the detectivity D* of the detector used.
useful portion of the infrared spectrum From Hg. 2, for instance it is seen that a
lies in tile 0.8 to 20 pm range; beyond 77 K (-196 oc ~ -3Z1 ol') cooled indium
20 pm, applications are more exotic such
as high performance fourier transform antimonide detector operating in the 3 to
spectrometers operating near 25 pm.3o 5 pm range has a seven fold higher
The choice of an operating wavelength
band dictates the selection of the detector detectivity than a 77 K (-196 oc ~
type as Fig. 2 shows. Among the
-321 °F) cooled mercury cadmium
FIGURE 8. Multiple stage thermoelectric telluride detector operating in the 8 to
calling unit using peltier effect. 12 pm range. That means that even if, for
a specific application, the emitted
Absorbed heat radiation (temperature of interest or
spectral emissivity) is higher in the 8 to
Current 12 pm range, the contrast obtained may
be stronger in the 3 to 5 pm range
Rejected heat because of the superior D* of an indium
antimonide detector.

As a final notice we may point that
dct;!iled studies34·3f' have concluded that
for temperature in the 263 to 403 K
(-1 0 °C to +130 oc; 14 to 266 °F) interval,
measurements can be done without much
difference in both bands (3 to 5 pm and 8
to 12 pm). For some special applications
such as military, bispectral cameras
operating simultaneously in both bands
have been developed to characterize more
accurately target thermal signatures.

'Jb summarize, there are two main
categories of detectors: thermal and

196 Infrared and Thermal Testing

photonic. In thermal detectors the signal values r into thermal values G (gray level).
is proportional to the absorbed energy
whereas in photonic or quantum In recent infrared cameras, pixel coding is
detectors the response is a funrtion of the performed on 12 or even 14 bits yielding
number of absorbed photons. Figure 2 respectively to 4096 (~ 212) or 16384
shows different detector response curves (== 214) possible values. Thermograms from
as a function of wavelength. such systems do not require any further
conversion because the available span of
Radiometric Temperature values covers directly the whole dynamic
Measurement range of the infrared cameras. Older
systems digitizing infrared images on say
Different techniques can be used for 8 bits provide a limited 256 (== 28) possible
temperature measurement. Two of them values and require a particular conversion.
are considered here: single-color and In these cases, because the available span
bicolor. of values does not cover the whole
dynamic range of the infrared camera,
Before processing any thermal images, manufacturers introduce two settings, a
it is necessary to convert the raw image level and a range. These settings make it
possible to cover the whole dynamic span

FiGURE 9. Spectral emissivity curves of various materials:34 (a) crown glass; (b) red tile;
(c) foliage; (d) water; (e) concrete; (f) road asphalt; (g) black rubber; (h) metals. Unit
conversion: 1.0 f-Jffi == 4 x 1Q-5 in.

1.0(a) (e)
0.5.w£-~"
1 0 1 · · ·;. :- :::L:.t, . ..' .;;:''.·.~
·~ ~ ·· ; :
~.. :. -~ ~
00.5·~~ -
.
t~ ·•·.+.·I-...:{...,: . :, ., ::i--
05 • :
: ;. :::j:.t : ':;: :, .
: .. - ""i""i' : .. ' ' .~
0
0.5 2 3 4 5 6 8 10 12

wavelength ), (11m) wavelength }, (i.Jm)
(b)
(I)

1 . 0 1 . :. .'.:-;·;.-:: :
0 - T ":" :· '00
+ '' i•
l ''
0 5 . -~ :. ...}.~.:tt .;·--::.::;~.-...

' •'I"~ :· ,., " "

0.5-~:;:::

0
2 3 4 5 6 8 10 12

wavelength A (!Jm) wavelength ), (!Jm)
2 3 4 5 6 8 10 12
(c) 1.0 (g)
0.5pwG-"
1 0 1.. .....o5'. ' 1 ·.-·:Ti~-::-..';:! .: ....
··-~ 0~ .
.I '' _:'::.!;; :...,
.\:2·;:;
, _.
00.5E."!
' ., '''
t~
0.5 : : :l.:-~ - ~ ::: ::--

0
2 3 4 5 6 8 10 12

1.0(d) wavelength ), (j.Jm) 1.0(h) wavelength A (j.Jm)
0.5w-" 0.5w-"
2 3 4 5 6 8 10 12 .: ..:.:.:.:.:;.:
.£~ wavelength ), (tJm) .»~ ~~
10 12..., ' ' ''
-~ ~ -~ ~
2 3 45 6 8
0·- 0·~~ wavelength ), (pm)

00.5w·E!~- t~

Noncontact Sensors for Infrared and Thermal Testing 197

but have to be taken into account to Now considering a sequence of
compute the thermal values: thermograms taken at different
temperatures, it is noticed that this
(6) G r --7205C61C2:7:- Range + Level difference is a linear function of the
temperature in gray level:
'--
(8) di,j (c!rer,rref) a;,j G.~ref,pef
If no corrections are needed, as for
+ b;,j
instance in 12-bit or 14-bit systems,
then G = r. In the visible spectrum, a possible
hard\vare solution to this vignetting
Correction of Vignetting Effect in problem is to add an additional lens in
front of the objective.39 If the original
Single-Color Focal Plane Array optics of the camera introduces a
distortion P then a lens with a distortion
The next step before quantitative analysis function F-1 corrects the response of the
of thennograms is to convert the image global system. Of course such an
sequence into temperature images. In the approach would attenuate the signaL ·rhis
case of focal plane arrays, the image approach is possible if the function P does
restoration is generally limited to the not depend on the features of the scene
vignetting effect (if present) because the such as temperature due to self-emission
noise level is generally low. of the optical elements.40 Because in
infrared thermography F depends on the
Vignetting is explained as follows.:{? lf temperature, a software approach is
preferred. The idea is to create a
a cone of rays is formed from a point in (M x N x 2) matrix file (where A,/ x N is the
the object space and limited by the image format) containing the coefficients
diaphragm of the lens, and if this cone is ai,i and hi,; (for every location (i,j) in the
intercepted with the image plane image:
perpendicular to the lens axis, the
intercept is a circle if the object lies on the (9)
optical axis and more generally an ellipse
if the object is laterally displaced. wllere Gr~1rr is the gray level at the
Moreover, for many lenses, the front and reference point location corresponding to
rear apertures are too small to fully the actual ambient temperature (at the
transmit oblique rays and a part of the time the correction is computed) and
light cone may be cut off, causing an Gcc;;\;r is the temperature (in gray level) of
amplitude reduction at the edges of the the room when the correction matrix was
image. J~inally, this effect is also caused by created.
antireflection coatings of camera optics
optimized for normal incidence, thus Temperature Calibration
explaining reduced distortiOns for central
pixels. After assessment of vignetting, the next
step consists of converting gray level
Vignetting is more severe if expansion values into temperature. The common
rings are used to restrict the field of view procedure is as follow. A blackbody is set
because of the limited effective aperture at a given temperature, 283, 298, ... K (10,
obtained in this case.2s
25, ... oc; so, 77, ... 0 1") and positioned in
As predicted by theory, 38 experiments
carried out with the focal plane array front of the camera. For each blackbody
camera show this effect depends on both temperature, an image is recorded and
pixel location and temperature difference readings arc obtained in a restricted area
between the target and the ambient. at the center of the image where
Below ambient temperature, 31 S K radiometric distortion effects are less
(42 "C = 107 °F), vignetting has behavior important. Values in a central subwindow
opposite to that above ambient of say 10 x 10 pixels are averaged together
temperature: the curvature direction and plotted as function of blackbody
changes and at ambient temperature temperatures. A polynomial fitted to these
vignetting is not visible. values provides the calibration curve
(Fig. 10). For instance, in the case of a
Equation 7 expresses the difference focal plane array camera, the following
between the signal at the central reference relationship is obtained ·where G
point corresponding to the center of the represents gray level values (linear best fits
brightness area and three points placed at using a third order polynomial function):
different distances from it:

where Gf,; is the gray level of pixel (i,j) at
temperature t and G~rcf,)ref is the gray level

of the reference at temperature t.

198 Infrared and Thermal Testing

(10) T -13.4 + o.os sG that also uses emittance, emissivity is used
now.
1.6 x w-' G2
The main sourc~ of UQf~rtainty in
+ 2.2 x w-9 G 3 radiometric temperature measurement is
the unknown emissivity. The problem is
Obviously, such a calibration procedure worse because of emissivity variations
is valid only for a specific experimental with ·wavelength ).. and temperature T. It
setup. If the experimental conditions is, however, possible to assume a simple
change, it is necessary to repeat the relationship between emissivity and
process. wavelength to calculate the temperature
from measurements at different
If instead of the third order polynomial wave1engths. 41 -45 Pyrometers based on
of Eq. 10, a line fit is used, calibration is that principle are called multiwavelength
considered a two-step process. To speed pyrometers. The signal measured in
up calibration, conversion values might channel i of such a pyrometer is given by:
he precomputed and saved in a lookup
table for quick access: input of a gray where K; is a constant for this channel
value G directly provides the and C2 is the second radiation constant.
corresponding temperature T. If a Equation 11 is known as Planck's law
hardware lookup table is available, the under \1\,!ien's approximation.34
processing time is very fast. Digital frame
grabbers often include such a One simple assumption about the
variation of the emissivity and
functionality. wavelength is that a smooth curve exists
between these two variables over the
Two-Color Pyrometry for wavelength range of interest.41 •42 This
Temperature Evaluation assumption holds for graybodies (for which
emissivity is constant ·with wavelength)
The spectral surface emittance radiated by and to some extent to colored bodies (for
tm incandescent graybody depends only ·which emissivity varies with wavelength).
on its temperature 1~ its emissivity E and
the spectral bandpass of the observation. For bicolor pyrometers, an easy way to
The term is properly named the spectral compute the temperature is to form the
emittance referring to the property of a ratio R of the signals, because for a given
particular surface, rather than spectral set of wavelengths and temperature T the
emissivity, the intrinsic property of an spectral emittance ratio Iri2- 1 is unique:
uncontaminated, optically smooth
surface. However, to avoid the possibility
of confusion with the radiant energy flux

FIGURE 10. Infrared camera calibration curve and level and (12) R
range settings for temperature computations.

Calibration curve without radiometric
distortion for a given pixel (i,J)

which simplifies to:

Pixel of interest (13)

level K 1 regroups all the constant terms:

Blackbody temperature T (relative scale) (14) "Jj ~ P(R)

legend A calibration curve P(U) that relates the
G "" thermal value (arbitrary unit) measured ratio R to the calculated
temperature T 1 can be experimentally
i,f = coordinates that define pixel obtained. Although Planck's law is the
underlying physical basis, it docs not
r =- raw image value appear explicitly in Eq. 14.
T = temperature

Noncontact Sensors for Infrared and Thermal Testing 199

The error on the calculated
temperature 7'1 is given by the following
fonnula: 4S

;i)~(15) c2( -

Zero error is obtained with the
condition corresponding to a ratio of one
in Eq. 12. Equation 15 assumes no
measurement errors in either channel. If
any of such errors are present (random
errors from detector and photon noise,
detector nonlinearity, drift in channel
calibration, error in nominal channel
wavelengths), the equation becomes:44

(16) C2 t>.T
Tz

For instance, a 1 percent error in one
channel leads to an error in the
temperature of a!Jout ±15 K (±15 "C =

±27 "F) at 1273 K (1000 "C = 1832 "F) for

the \\'avelengths selected.

200 Infrared and Thermal Testing

PART 3. Schemes for line Scanning

In thermal nondestructive testing the constants arc combined into a single
surface of an object is usually heated with parameter called thermal diffusivity a:
a periodic or pulsed heat source
generating time dependent heat flow (18) " k
inside the object. The inner structure and pc
possible anomalies of the object affect the
heat flow. Information about the effects In the case of a scanned line source,
can be obtained by monitoring the the excitation is directed on the surface of
surface temperature of the object in time.
the solid and as such the case does not
In line scanning techniques the time represent volume heating. Therefore, it is
dependent heating is obtained by moving more convenient to put q = 0 and to give
a line shaped source across the object the excitation as a boundary condition to
surface or by translating the object itself
in relation to a stationary Jine source. Eq. 17. For example, if a semiinfinite
Thus each surface point is subjected to a
varying amount of heat in time. Because solid, whose surface is the z = 0 plane, is
of heat diffusion in the object, the
temperature of a particular surface point heated with a moving laser beam, the
reaches its maximum after the source has heat flow in the direction normal to the
passed the point. In this sense, the surface is caused by a scanned line source:
moving Hne source is followed by a
thermal wake. The behavior of the wake I(19) k iJT(r,t)
depends on the object material and the
possible anomalies in the object. ()z z=O

As the surface area of the object is +
heated line by line, the monitoring of the
surface temperature is usually done the where a is the 1·rl radius of the gaussian
same way. The detection line is located a line source in the X direction and bin the
distance behind the heat source to allow Y direction. The source moves to the
time for the heat diffusion. The correct positive Y direction ·with a velocity\' and
distance has considerable importance in has an amplitude of Qo. In Eq. 19. it is
subsurface discontinuity detection. assumed that the surface absorbs the
excitation completely.
The monitoring is performed in a
noncontact way by using infrared photon Equation 17 can now be solved with
detectors with suitable optics. \-\'ith standard Green's function techniques:46,47
postprocessing the monitored lines are
composed into a two-dimensional surface t
temperature map, the usual form of Q~~/Z Jdt0
information used for evaluating the (20) T(.t,)•,z,t)
characteristics of the object.
pc1r --e-o

Scanned Line Excitation x exp[ z -2x2

Time dependent heat transfer phenomena a + 8o:(t- to)
in solids are described by the heat -Z(y- vt )2
diffusion equation:
' b2+Ba(t- to)
iJT(r,t)
(17) pc--- + 4u(tz~ t )]
0
ilt
7 [ v~s-,-(t---~o-)
where cis specific heat U·kg-1-K-1), k is
thermal conductivity (VV-m-I.kl), q is x Jb2+8a(t-t0)
heat input per unit volume (\-\1·111-:{) at a
x ~4a(t -t0 )]
given time (second) and point F in the

solid, f is position, Tis temperature
(kelvin) and pis mass density (kg·m-3).
Usually, the last three material dependent

Noncontact Sensors for Infrared and Thermal Testing 201

Equation 3 cannot be solved models. Already in a two-layer case the
analytically but can be integrated analysis is quite complex, 48 although
numerically. By putting a== 15 mm, analytical expressions can be obtained
b ~ 0.3 mm, p ~ 1400 kg·nr3, c ~ 935 even for general multilayer structures.-t9,~0
}kg-1-K-1 and K ~ 0.87 W·m-1-K- 1 and by However, the introduction of true
three-dimensional object geometry or
plotting the isotherms at the Z = 0 plane discontinuities ln the object renders
(Fig. 11) it is revealed that the surface analytical solutions in most cases
temperature profiles are distorted in the)' impossible. Therefore, these kinds of
direction as the result of the movement of models have to be solved numerically,
the line source. As a consequence of the which can be accomplished witll a variety
asymmetry of the isotherms in the of techniques.zs,SI-53
y direction, a net heat flow can be
expected in this direction. The finite difference technique, for
example, has been successfully applied to
r:igure ] 2 shows surface temperature several line scan problems by
profiles computed with tbe same Hartikainen, 54 Lehtiniemi55 and Varis.56 In
parameters at various scanning velocities. this technique, the object is described
The nonsymmetrical behavior due to the with a discrete three-dimensional grid.
motion of the source is evident here also. The diffusion values in Eq. 17 with
As the line source moves across the object suitable boundcuy conditions are replaced
sltrface, every surface point in turn \Vith their discrete equivalents. The
experiences a heating pulse, whose resulting large set of equations is usually
duration tis the source width b divided by solved both in the case of the object's
the scanning velocity 1'. Therefore, the having a discontinuity and the object's
higher the scanning velocity, the less time not having one. To determine the effect
there is to deposit heat into the object. As caused by the discontinuity, the surface
a result, the peak surface temperature temperature profiles are plotted and
drops as the scanning velocity is compared as shown in Fig. 13. The
increased. Because of heat diffusion, lt materials parameters are the same as used
takes a certain amount of time for the for plotting Figs. 11 and 12. Because of
surface temperature to reach the peak the diffusion again, it takes a certain
value, during which time the source has amount of time for the heat pulse to
moved a distance vl. Because of this, the reach the discontinuity and reflect back to
maxima of the temperature profiles occur the object surface. For this reason, the
behind the line source. surface temperature profiles are practically
identical on the leading edge; the
For practical purposes, the simple differences emerge on the trailing edge.
semiinfinite object is seldom adequate.
Many advanced materials have The surface temperature difference
complicated layered structures that must between the object with the discontinuity
be taken into account by theoretical and the Object without one can be
determined by simply subtracting the

FIGURE 11. Isotherms on surface of solid heated with moving FIGURE 12. Surface temperature profiles at various scanning
gaussian line source. velocities generated by moving gaussian line source.

+1.0 (+0.04) ~~ -~1 ~-· ~·----.------~,~---------r'-~~~

l0 1.25

-1.0 (-0.04) 1.00
0vc
E ·3c"
E -2.0 (-0.08) c~ 0.75
g
.!::2~
X :e 0.50

~ ~
h
>- -3.0 (-0.12)
0.25
-4.0(-0.16)
1 0 CL----~~-L~~j_~~~~~od
-5.0 (--0.20) 1
-20 -~•~~.~ . ~ ~ ~-~J -5.0 --4.0 -3.0 -2.0 -1.0 0 1.0
-0.8) (-0.20) (-0.16) (-0.12) (-0.08) (-0.04) (0.04)

Distance from heat source center, mrn (in.)

-10 0 +10 +20 Legend

(-0.4) (0.4) (+0.8) T =- temperature
vl-= scanning velocity of 6 mrn-s-1 (14.2 in.·rnin-1)
X axis, mm (in.) v2 0; scanning velocity of 8 mm·s-1 (18.9 in.·min-l)
v3 = scanning velocity of 10 mm-~-1 (23.6 in.·min-1)

202 Infrared and Thermal Testing

curves from each other. Figure 13b shows maximum gives an estimate of the
the surface temperature difference caused detectability of the discontinuity. If the
by the discontinuity obtained from surface temperature difference caused by
Fig. 13a and similar curves computed ·with the discontinuity is smaller than the
different line source velocities. As can be temperature difference that can be
seen, the temperature difference maxima resolved with the detecting system, there
and their locations depend on the is obviously little hope of success. The
scanning velocities. Naturally, they location of the maximum is the optimum
depend on the depth of the discontinuity point for detection and therefore
as welJ. 57 determines how far away from the
excitation the detection point should be
Both the magnitudes and locations of placed. Unfortunately in practice, there is
the maxima have great practical seldom prior knowledge about the depth
importance. The magnitude of a of the discontinuity, which makes the
determination of the optimum point
FIGURE 13. Comparison of objects with and without merely an approximate technique in real
discontinuity: (a) numerically computed surface temperature experiments.
increases for object having defect (solid line) and for object
without defect (dashed line) generated by moving gaussian Line Heating Methods
line source at 10 mm-s-1 (23.6 in.·min-1); (b) surface
temperature differences between object without A typical infrared line scanning setup is
discontinuity and object with discontinuity at various shown in Fig. 14, ·where a heating line
scanning velocities. and a line scanner is focused on the
surface of a moving object. By combining
(a) the horizontal translation of the object
and the infrared detection alongside a
18 ,---,~--,--·-,- vertical line, a two-dimensional surface
temperature map (an infrared image) can
· - -16 - · - f - - ··- f---·+~--+~. be constructed.

14-· ·-- The detection line has to he some
distance away from the heating line to
12-· -- allow for heat diffusion in the object as
discussed previously. Otherwise, the
g 10 . - I··--\--- -+-~--+-11_-11-----1 heating and the monitoring of the object
surface happen simultaneously.
~ 8 -- Discontinuity---+·---+/-"Cf-~1\--·-
: "-, -==~r--- The line detection can be obtained
.-~--t"---+1 with a single-element infrared detector, an
infrared lens and a suitable scanning
2 "-No discontinuity"- \-~- optical element like a deflection mirror
(Fig. 14a), a rotating polygon mirror or a
0 -~--L--~-- ~.. rotating prism. Also, it is possible to
-5.0 forego the scanning optical elements
(-0.20} -4.0 -3.0 -2.0 -1.0 0 1.0 completely by using modern
(-0.16} one-dimensional focal plane array
(-0.12) (-0.08) (-0.04) (0.04) detectors (Fig. 14b).

Distance from heat source center, mm (in.) There are various ways to generate line
heating: (1) lasers, (2) hot air jets and
(b) (3) induction currents. Basically, any
source that can excite heat in the sample
0.25 and be formed into a remotely line
shaped form is applicable.
0.20
Heating by lasers
0.15
Perhaps, the most often used heating
g source is a laser beam. The laser beam can
be formed to the line shape by first
h 0.10 expanding it and then focusing it with a
<I cylindrical lens (Fig. 1Sa).58 Another
variation is to use a focussed laser spot
0.05 and to move it rapidly with a deflection
mirror (Fig. 1Sb).59 \o\'ith two mirrors this
0 --4.0 -3.0 -2.0 -1.0 0 LO technique has also been used for
-5.0 (-0.16) (-0.12} (-0.08) (-0.04) generating a two-dimensional raster
(0.04) SCaiL 61 l-r,z The size of the heated area is
(-0.20) easily controlled with optics and a highly
localized source can be generated.
Distance from heat source center, mm (in.)

legend
T = temperature

vl =velocity of 6 mm·s 1(14.2 ln.·min·l)
v2 =velocity of 8 mm·s 1 (18.9 in.·min-1)
v3 =velocity of 10 mrn·s-1 (23.6 in.·min-1)

Noncontact Sensors for Infrared and Thermal Testing 203

The power of the source can be semiconductor lasers are available. The
controlled fairly accurately, too. Possible laser beam can be transferred via an
heating lasers are, for example, argon ion optical fiber, 63 if the wavelength of the
lasers,:;,!'\ neodymium-yttrium aluminum light is in the visible or near infrared
garnet Jasers63 and carbon dioxide lasers.64 region. This makes it possible to place the
Argon lasers yield output power in the often fragiJe laser device farther from the
order of few watts, whereas both the test area, which plays an important role in
neodymium-yttrium aluminum garnet the development of a transportable
and carbon dioxide lasers can provide testing systems.6s
output power in excess of 20 \~'if
necessary. Even high power Disadvantages of laser heating are the
price of high power lasers, the fragility of
'.·· the lasers and the ·water cooling
requirements of the lasers. Because of the
fiGURE 14. Operational principle of infrared line scanning optical fiber the fragility and the cooling
measurement system: (a) single-element detector with requirements do not pose a problem. The
scanning optics; (b) multiple-element line detector. length of the fiber can be several meters
without any considerable loss of output
(a) power. Therefore, the laser itself can be

Germanium tens
Infrared detector

Deflection fiGURE 15. Arrangements: (a) line focused laser heating;
mirror (b) flying spot laser heating; (c) infrared lamp heating;
(d) hot air jet heating with fish tail nozzle; (e) radio
( frequency induction heating.

Detection tine (a) (c) Lamp

Object Heating Beam r-,
line !'Xpander
(b)
Object ~
Focal plane translation {\
array
K-I laser Reflector
infrared beam
detector -I

~ Object

Cylindrical
lens

Object

(b) (d)

Def lection t==~H~o=.,je:t r~r

mir~~ las~

Heating line lens ----------- ~
Nozzle
ObjeCt Object

(e)

Induction
coil

Detection tine

Object Object

204 Infrared and Thermal Testing

placed near a water source and in a place Otherwise, the coil is mechanically very
of safety. An alternative technique for rugged, which makes it a good choice for
generating heat with light is to use a high industrial applications. Furthermore, the
power infrared lamp with a reflector necessary equipment is relatively
(Fig. 1Sc).66 For example, quartz lamps inexpensive.
can have radiative power exceeding
1000 W. The lamps are typically 200 to Imaging Methods
300 mm (8 to 12 in.) long and thus are
capable of heating much larger objects In the line scan schemes the infrared
compared to laser heating situations. images are formed by combining in the
Naturally, the generated heat source is not computer the object translation and the
as localized as in the case of a laser beam. image line scanning perpendicular to the
translation in the computer (Fig. 14).
Heating by Hot Air jets Usually, this is achieved by placing the
object in a translation stage (Fig. 16a).5t>
Another excitation technique that can be Alternatively, the object can be kept still
used is hot air jet heating (Fig. 1Sd).s6,67.69 and the scanner moved (Fig. 16b),65
Bringing a narrow fisll tail nozzle near the although this configuration is rare. In
surface of the sample directing the hot air both these cases, the sample )las a planar
flow through it can generate an geometry. However, other possibilities
approximately line shaped heat source on exist as well. The sample can have a ring
the sample surface. Unfortunately, the hot shaped geometry (Fig. 16c), in which case
air jet generates a wide source, which does the ring is rotated around its axis and the
not provide a good spatial resolution. heating and the detection line is arranged
However, there is a lot of heating power alongside the radius of the ring. 55 Also
available with the hot air jet heating, possible are cylindrical shapes like tubes
which can be used for looking for and shafts (l;ig. 16d), which can he
discontinuities that have large dimensions rotated around their longitudinal axis. 56
and are located relatively deep.56 This
technique is also suited for materials with FIGURE 16. Imaging techniques for (a) moving planar object;
low diffusivities. 69 In addition to the good (b) stationary planar object; (c) ring shaped object;
pmver reserve (0.1 to 10 kW), hot air guns (d) cylindrical object.
are much less expensive than lasers. An
interesting variation of the technique is to (a) (c)
use cold air with already hot objects.
Plasma jets instead of air jets are also Sample translation Ring rotation
possible.67
Df'tector Lens 'l\ O~f!ection
Heating by Induction Currents
(b) )l mirror
Radio frequency induction is a heating
technique that can be used for heating (d)
electrically conducting materials
conveniently. Heat is excited by resistive Scanner
losses of the eddy currents induced in the translation
material (comprehensive treatments have
been published 70,71), although it is also
possible to make a conductive filament
radiate enough heat.72

In thermal nondestructive testing,
induction heating can be used for
generating harmonic source73 as well as a
line source. 74 The source is generated with
an induction coil that is driven with an
amplified radio frequency signal
(Fig. !Se). The coil has to be matched to
the object material and to the cable
carrying the signal with an impedance
matching circuit. The resonance
frequency of the matching circuit depends
on the size of the coil and the material of
the object. Typically, the frequency is in
the MHz region. Amp1ifier powers range
from few hundreds of watt to few tens of
kilowatt.

Downsides of induction heating are the
large source size and the incapability of
heating nonconductive materials.

Noncontact Sensors for Infrared and Thermal Testing 205

In this case, the heating and the detection of a problem. For most cases, the
are placed alongside the longitudinal axis. difficulties with surface reflection can be
The rotation can be achieved, for removed by using hot air jet heating or
example, with direct current motors and induction heating. The emissivity
can be controlled ·with the computer by problem cannot be solved with a proper
using optical encoder disks. In principle, choice of heat source. The effect of
other slightly curved objects could be emissivity variations can be eliminated
imaged as well but the control of the with a combination of controlled
translation system lJecomes much more temperature measurements and image
involved. post processing,80 but the technique is
rather involved to be applied for line scan
The infrared image is formed in the schemes.
computer. Usually, the computer controls
the translation of the object in such a way One of the advantages of the modern
that a preset number of line scans is infrared testing method is that it produces
performed on the object. The number of images, where the locations of
the line scans and the ri1agnification of discontinuities are readily recognizable by
the optics define the surface area that can comparing to the object geometry.
be imaged. Each line scan consists of a Usually, the discontinuities are easy to
preset number of pixels, in which the recognize without any special training of
signal of the infrared detector is sampled. the operator. Hmvever, sometimes it is
The signal is amplified and filtered before necessary to process the image further and
the sampling. If a deflection mirror or the computer program controlling the
rotating polygon mirror is used, the imaging system should be able to. Normal
computer controls the mirror rotation so operations include correction for
that the image point at the sample surface heterogeneous heating profile, noise
moves correctly. elimination by median filtering, contrast
enhancement to correct for dynamic
Most often, the thermal image is range problems and edge tracing for shape
formed after one heating round. llowever recognition. Beyond simple
in the case of low diffusivity materials, it postprocessing are problems in
is possible that good enough contrast determining the sizes and depths of
cannot be obtained with one frame. This discontinuities or the severity of
is especially true for deep discontinuities, discontinuities. In these cases, rather
In situations like these, it is possible to complicated and specialized algorithms
use several heating rounds to enhance the are needed.
contrast as long as proper care is taken to
avoid damaging the object.69 To improve Applications
signal-to-noise ratio, the consecutive
frames are averaged and embedded into a Detection of Adhesion
single image. Another possibility is to Discontinuities in Ceramic Coated
average pixels during the sampling. Steel Rings

The pixel size on the surface of the Thermal nondestructive testing methods
object is defined by the magnification of have proven to be beneficial in several
the optics. In principle, the smallest detail applications where traditional methods
possible to resolve is the size of the like radiographic testing, electromagnetic
detector multiplied by the magnification. testing and ultrasonic testing have been
However in practice, various experimental problematic. The applications in question
factors limit this. The spatial resolution in usually relate to new materials or
thermal imaging systems is complicated advanced materials. These include
and no analytical theory in the 1900s has ceramics, composite materials, plastics,
described it. Experimental studies show semiconductors etc.
that in addition to the size of the detected
area, the spatial resolution is affected by A typical example is plasma sprayed
the size of the heated area and the coatings on steel substrates like the ones
diffusion in the sample.75 The used on hydrostatic seal rings in a
discontinuity size and geometry affect the circulatory water pump in a nuclear
thermal image as well. 76 Several authors power plant. ss,sH The ring material is an
have also discussed the problem of austenitic stainless steel, American Iron
defining the point spread function of and Steel Institute (AISI) 316 (18 percent
thermal imaging to reconstruct the size, chromium, I0 percent nickel, :~ percent
the shape and the depth of the molylJdenum). The actual seal consists of
discontinuity accurately. 77-79 two such rings that rub against each
other. To prevent excessive wear, the
Besides spatial resolution, other factors surfaces of the rings are coated with
that influence the detectability of dichromium trioxide layers. The bonding
discontinuities are object surface between the coating and the substrate has
reflectivity and emissivity problems.
Surface reflectivity is difficult especially
\Vith metal objects heated with lasers or
lamps. \\1ith nonmetals reflectivity is less

206 Infrared and Thermal Testing

to be strong enough to keep the coating Crack, Inclusion and Impact
from breaking loose from the substrate. It Damage Detection in Carbon
is not even necessary to have an air gap Fiber Reinforced Composites
between the coating and the steel.
Weakened adhesion is enough. The infrared line scanning technique can
be used for finding discontinuities like
Unfortunately, ultrasound is strongly cracks,81 inclusions,56 and impact
attenuated in the coating material and damage65,74 in carbon fiber reinforced
electromagnetic testing is not sensitive to composites. One of the advantages of the
bonding strength variations. However, infrared line scanning technique is the
thermal nondestructive testing has proven generation of lateral heat flow in the
to be suitable for the task. Figure 17 shows direction of the movement of the line
an infrared image of a 2 x 2 mm source. In most of the pulsed thermal
(0.08 x 0.08 in.) adhesion discontinuity nondestructive techniques the heat flow is
between a coating and a seal ring. ss The in the direction normal to the surface of
discontinuity can be seen clearly as the the composite, which makes it difficult to
hotter surface area (white) against the detect vertical cracks or broken fibers.
otherwise cooler surroundings. In this
case, the weaker adhesion hinders heat Hgure 18 shows that the detection of
flo'N from the coating to the substrate fiber cracks is feasible with the infrared
causing the hot spot. The size of the line scanning technique. In this case, the
imaged area is 100 x 14 mm sample in question is a unidirectional
(4.0 x 0.6 in.). carbon fiber composite that has protective
skins on both sides. The composite is
The infrared image in Fig. 17 was heated with the induction technique. In
obtained by using the line scanning the infrared image an area of 50 x 30 rum
measurement setup depicted in Fig. 16c. (2.0 x 1.2 in.) of the composite having a
The heat source was an argon ion laser 20 nun (0.8 in.) long and 160 ~m
beam focused on a 20 mm (0.8 in.) long (0.006 in.) wide cut in it is shown. The
and 300 ~m (0.012 in.) wide line on the direction of the fibers is from left to right
surface of the ring. The radial direction of in the image.
the ring in Fig. 17 was from top to
bottom. The output power of the laser In addition to the cut in the middle of
was 4 W. The diameter of the ring was the image, a lot of finer details of the
270 mm (11 in.) and the \Vidth 37 mm structure of the protective skin are
(1.5 in.). The thickness of the coating was resolved with high contrast. These details
300 ~m (0.012 in.). For the detection, a are typical for infrared images obtained
single-element mercury cadmium telluride from carbon fiber composites heated with
detector measuring 25 x 25 pm the induction technique.
(0.001 x 0.001 in.} ·with a germanium lens
and a deflection mirror 'i\'as used (see The technique is especially attractive
Fig. 14a). The detector operates in the 8 to for carbon fiber reinforced composites
12 pm wavelength region and is cooled because they contain both conductive
with liquid nitrogen to a temperature of materials (carbon fibers) and
77 K (-196 'C = -321 'F). The line rate of nonconductivc materials (epoxy and glass
the deflection mirror was about 100 Hz. fibers). Discontinuities affecting the
The ring rotation and the data acquisition carbon fibers alter the fibers' electrical
were controlled ·with a microcomputer.
FIGURE 18. Infrared image of 20 mm (0.8 in.) long x 160 ~m
FiGURE 17. Thermal image showing 2 x 2 mm (6.3 x 10-3 in.) wide cut in unidirectional carbon fiber
(0.08 x 0.08 in.) delamination in ceramic coated steel ring. composite plate.

Noncontact Sensors for Infrared and Thermal Testing 207

properties, which in turn affect the eddy stationary to
currents and therefore the heat generation
in the sample. On the other hand, the transported t '1lposite panels and e
nonconductive elements contribute to the shaemcpolvee,rtehdebS0taat-dtCeCstasreitae.oIfntshteeasdaomfrtJle1 can
heat generation hardly at all. As a result (Fig. 16h). 'fl~~ moving the line scanne!
the thermal image will show only details
corresponding to the elements responsible applied man s movement can even b 1.9~
for the structural integrity of the sample. In the il1} ltaiJy by hand115 as in Hg.
with a vicke age, a carbon fiber compostt
A good spatial resolution is vital in the r~1,hine
detection of closed cracks. J:or such representeq dentation on it is s of a
purposes, the wide source generated by glass fiber Q. composit e consist
the induction coil is not the best possible. with carbo 11P~xy matrix strengthened .
However, the induced eddy currents, nets. The btl lber bundles forming loose
which flow in the carbon fibers, are verr
sensitive to the fiber breakage. In this {0.3 in.) ap<tr~~les are about 7 mm
sense, the induction heating again induction teq IIl. each net. Again, th~ ·til ,
provides a selective feature. \-\'hether this conductive ll1 1n1que hea ts the eiectnC• 11)e
factor provides enough heat to be carbon fibe aterials. As a result, onlY t
detected by infrared testing is r
questionable. More likely, the detection is {-eimage. The spets are resolved in th~
possible because of the stopped heat flow SO x 30 m 111 of the i.maged area IS
in the broken fibers. The heat input in the indentation 2·0 x 1.2 m.). The f
composite is sensitive to the distance 1the image a.sC'an be seen in the middle ~)t
between the induction coil and the spot. a srnall diamond shaped
sample. Therefore, any curvature or
notches in the composite surface affect Figure 19 h- t
the surface temperature. These effects can quality typic <IS the same high cont:as
also be detected in Fig. 18.
tgthe induco0 a} of images obtained wtth
Figure 18 was obtained by using the l~eating. However
measurement setup described in Fig. 16a. comparing
The radio frequency signal was obtained dmeataniulsalblscuarr11e~~lnttgnpagroevwidieths aFonifg.itmh1e8a,gsectl~1'ne"11.,111e~
from a standard signal generator and
and variation ?Y the jitter
amplified \Vith a 200 '"' radio frequency \Vith the det s 1 ~ the scanning veloCJ~}·
amplifier. The high power signal was then Fig. 14a, the ~C'hon system described 111
fed through an impedance matching cmoamnpuaacl tsc'Hal1c1i 0~IIalgn.neInr
circuit to a four turn rectangular coil used is too heavy for
for coupling the power to the composite future, more
(Fig. J5e). The size of the coil was about become poss·~ensitive scanners \'\'ill
70 x 6 mm (3 x 0.25 in.) and the 1
resonance frequency 10 MHz. The plane array t , le with the use of focal
direction of the coil movement was from 0t~hchhtn~ohlforgeyqu(eFnigc,ie1s4ab)n-d
left to right in the image. Again for the Because all
detection, a single-element cooled skin depth,
mercury cadmium telluride detector SD1'
(Fig. 14a) was used. The measurement
setup was controlled with a computer. aradtdhiteior np,o_otrh~1eeptIhnd~uecnetitornathioena.tiInng haS a

Another example of the power of the induction co· couphng between the
induction he<Hing is impact damage
detection in carbon fiber composites. become diffi Il and the sample will
Impact damages are an important
category of discontinuities. This kind of a differs from cult if the sample geometrY
discontinuity can result, for example, example, Wif~anar geometry- as, ~or. 56
from a bird striking an aircraft or from a Figure 2f0ibseh, 0 tube isnhfarpareeddciommapgoesoifte~s~.-
tool dropped during maintenance of an carbon \Vs an Ill5
aircraft. Characteristically, there is only a 1. s 2C o.mposJ. te e r,(1(
minor mark on the impact side of the 8 mm (0.~--{ tn.) and tube, wh os
composite, whereas severe delaminations thickn ess 2 J11 111
and fiber breaking can occur inside and
on the other side of the composite. FIGURE19,fiea , . Q12X0.12in,)
Unfortunately, the other side of the
composite is often inaccessible. vickers hard 11estsStiegsntaitnudreenotfat3ioxn 3 mm (· glass fiber
composite. in car bon
Lehtiniemi and llartikainen7·1 have
discussed the detection of impact damage
with the same kind of equipment as
represented above in the case of the crack
detection. However, it is also possible to
design a line scanning measurement
system that can be used for testing

208 Infrared and Thermal Testing

(0.08 in.). Instead of induction heating, Other Applications
the composite is now heated with a hot.
air jet. The tube consists of six layers Line scanning systems have been used for
embedded in epoxy matrix. In the top discontinuity detection in glass fiber tubes
layer the carbon fiber bundles, which can and aluminum honeycomb compositesJ2
be seen as wide vertical stripes in Fig. 20, It is also possible to detect moisture in
are parallel to the longitudinal axis of the resin matrix composites and honeycomb
tube. Between the bundles are glass fibers. structures. A variety of applications use a
The thickness of the first layer is 0.5 mm line heat source for heating an objecti an
(0.02 in.). The deeper layers are wound infrared canlera is applied instead of a line
helically around the tube axis. Signs of scanner for imaging. Such applications
the second layer can be seen as hot stripes are, for example, the study of adhesive
at an angle of about 45 degrees. The bonds in aluminum laminates, where
almost horizontal stripes correspond to infrared lamps, cool and hot air jets are
fine grooves on the epoxy. These grooves used as heat sources. 6b,s2 A related
steer the hot air jet along the tube surface.
As a result a distracting heat signature is application studies corrosion in aircraft
constantly present during the testing. fuselages.83 Line scanning can be used
However, the discontinuities in carbon both as a reflection and a transmission
fiber composites can be quite large. measur~ment technique, although
Sometimes they can be detected despite detection from a single side of an object is
all the complications. In this case, the usually preferable.
composite has an inclusion that can be
seen between the third and fourth carbon The applications discussed previously
fiber bundle from the left. apply active heating for detecting
discontinuities. Naturally, line scan
l:igure 20 was obtained by using a techniques can be used for passive
measurement setup described in Fig. 16d. monitoring as well. Line scanners are easy
The hot air jet was directed through a to place above conveyor belts, where they
narrow nozzle as shown in I:ig. lSd. The can be used for monitoring various
nozzle was aligned parallel to the industrial products. Commercial line
longitudinal axis of the composite tube. scanners are available for such
The narrow end of the nozzle has the applications, although infrared cameras
dimensions of SO x 3 mm (2.0 by are becoming more and more popular in
0.12 in.). The heating was generated with industry. Other passive monitoring
a standard 1400 W hot air gun. The applications are airborne or satellite
angular velocity of the tube rotation was infrared imaging applications. Line
adjusted to equal the scanning velocity, scanners have been used for night aerial
that is, one full rotation of the tube surveys, mapping of land resources,
matches one frame exactly. The infrared meteorological observations and various
detection system was the same as military purposes.s4
described earlier.

FIGURE 20. Infrared image of inclusion in carbon fiber
composite tube.

Noncontact Sensors for Infrared and Thermal Testing 209

PART 4. Multicolor Radiometry near Ambient
Temperatures

Infrared radiometric imaging has been A technique of determining the target
widely used to estimate the temperature in the environment is
two-dimensional temperature distribution proposed by radiation intensities of the
on the surface concerned. As a means of infrared radiometer with different wave
nondestructive remote sensing, infrared hands. The respective infrared radiometers
thermography is applicable to various and those sensors have a selected wave
engineering problems such as band in the infrared range. Described next
is the methodology of determination of
discontinuity detection, condition the temperature by multicolor radiometry
monitoring and characterization of heat in consideration of the reflection intensity
and mass transfer. It is important to in a simple model near ambient
analyze the fundamental characteristics of temperature.
thermography to establish a practical
technique to distinguish the temperature The applicability of the numerical
from the measured radiation intensity. simulation to the temperature
The technology described below has been determination is confirmed through a
documented in the published parametric study on how different values
literature.H7-93 of emissivity correspond to output of the
individual infrared radiometer. The
A multicolor radiometer, often called a experimental study is also performed to
radiation difference pyrometer, measures the estimate the measurement error. The
radiation intensities at different tricolor technique radiometer permits
wavelengths and deduces the temperature quantitative temperature measurement
of the surface from the balance of these without presuming temperature,
spectral radiation intensities. Generally, emissivity and reflectivity of the surface
the multicolor radiometer is mainly used wall. The temperature difference
to calculate the surface temperature \\'hen calculated with the tricolor pyrometer is
the surface temperature of the objective compared with that calculated with the
material is higher than that of the hicolor pyrometer.
surroundings. The reflection intensity of
the surface injected from the Test Setup
surroundings is smaller than the emission
intensity of the surface. If the radiance Jv1easurement of the temperature by
intensity is measured by infrared means of bicolor and tricolor infrared
thermography near ambient temperature, radiometers is carried out using a specially
the reflected ratio of the surface from designed test apparatus to stabilize the
surrounding surfaces cannot be neglected. background radiation. Two groups of the
Therefore, it is necessary to exclude the
reflection rate of total incident energy.

FIGURE 21. Schematic illustration of apparatus for testing of infrared bicolor radiometer.

Water tank Cathode ray
tube
~Hc--------=-H
Central
Test plate processing
unit

Personal
computer

210 Infrared and Thermal Testing

infrared radiometer with filters are used infrared radiometer near ambient
for the experiment. temperature. Three infrared radiometers
are installed in an experimental cubic
The group A consists of a mercury space. The sidewall of the space consists
cadmium telluride infrared radiometer of the insulation layer inside covered with
with three calcium fluoride filters cooled a black velvet texture. Temperature of the
by liquid nitrogen. The group B consists cubic space is measured using several
of an indium antimonide and two thermocouples. It is kept constant using a
mercury cadmium telluride radiometers suction blower connected to the wall.
cooled by a stirling engine.
A test piece with the heater is placed
Figure 21 shows a schematic 700 mm (28 in.) from the three infrared
illustration of the experimental apparatus radiometers. The test piece consists of
for testing the hicolor infrared radiometer polished copper, stainless steel and black
near ambient temperature. The apparatus painted copper plates. The size of test
consists of the test piece with the electric plates measures 300 x 100 mm (12 x 4 in.)
heater, conical water hood regulated by and is 10 mm (0.4 in.) thick. A silicon
the heat exchanger, infrared radiometers, rubber heater is stuck on the backside of
cathode ray tube monitor and personal the test piece. The surface temperature is
computer. measured by the thermocouple of 0.1 mm
(0.004 in.) diameter embedded in the
The water hood is installed between plate of 1 mm (0.04 in.) deep. The
the test piece and infrared radiometer. The experiment is performed at steady state
temperature of the inner blackbody wall condition in the temperature range from
of the water hood regulated by the heat 322 to 435 K (49 to 162 "C; 120 to
exchanger can be constant. A velvet cloth 323 "F). A blackbody furnace is used to
the emissivity of which is nearly unity calibrate each sensor. Thermography is
covers the surface of the inner wall of the performed at a steady state condition.
water hood at a uniform temperature. The
covered black wall avoids the Multicolor Technique
multireflection effect from the
surroundings, people and light. The Radiative Characteristics of
experiment is performed at steady state Infrared Radiometer
condition in the ambient temperature
range- from 293 to 373 K (+20 to Figure 23 shows a detection sensitivity of
100 "C; 68 to 212 "1'). the infrared radiometer with three
filters A1, A2 and A3. The central
The test piece is made of stainless steel, wavelength of the filters are 8.37, 10.86
graphite, epoxy and polycarbonate resins and 11.46 pm. The waveband width of
heated by the electric heater. The size of each filter is about 1 pm.
test plates measures 150 x 150 mm
(6 x 6 in.) and is 15 mm (0.6 in.) thick. FIGURE 23. Detection sensitivity of bicolor infrared
The temperature of the test piece is
measured by chromel-to-alumel radiometer with three filters. . . -,,-""'l
thermocouples embedded in the test
plate.

Figure 22 shows a schematic
illustration of the experimental apparatus
for testing the bicolor and tricolor

FIGURE 22. Schematic illustration of apparatus for testing of
infrared bicolor and tricolor radiometers.

1 + - - - - - 1 . 9 m (75 in.)_-:----~1
Test piece
t Heater Thermographic
cameras I
I
50 8 10 12
14
.cg Wavelength i, (~rn)

u

*0
0
6

legend

}.1 =filter 1 wavelength= 8.383 pm
),2 = filter 2 wavelength= 10.8821-Jm
/,3 "'" filter 3 wavelength~ 11.482 IJm

Noncontact Sensors for Infrared and Thermal Testing 211

Figure 24 shows a detectable energy of 3. Sensor A.{ has a wavelength
the radiometer with three sensors 'A= 11.462 pm, temperature exponent
calculated using the Maxwell-Boltzmann 11.-... == 3.6064 and detected blackbody
equation and the detection :..e11:-.ilivity, as etH::'rgy Qh == 3.8526 X J0-2 G·'fthOM.
already shown in Fig. 23. The upper
straight line shows the detected blackbody Figure 25 shows tile detection
energy denoted by the Stefan-Boltzmann sensitivity of three infr<Hed radiometers
Ja-w: B1, B2 and B:t. Three infrared radiometers
are an indium antimonide sensor at 2 to
·where Qh is energy (\'V·m-2) absorbed by 5 pm wavelength, a mercury cadmium
the blackbody, Tis temperature {kelvin) telluride sensor at 6 to 9 pm and a
and Stefan-Boltzmann's constant o- = mercury cadmium tt'lluride sensor at 8 to
5.67 x 10--X \V·m-Z.K-4. 13 pm.

The detected energy equations for the Figure 26 shows a detectable energy of
three mercury cadmium telluride sensors the three radiometers B 1, B2 and BJ
are simplified in the first ordered equation calculated by a Maxwell-Boltzmann
in the temperature range from 293 to equation applied to the detection
373 K (20 to 100 oc; 68 to 212 °F): sensitivity. TJ1e upper straight line shovvs
the detectable blackbody energy, as
1. Sensor A 1 has a ·wavelength /, = 8.363 defined by Eq. 21. Detection eneq,')'
pm, temperature exponent 111 = 4.8050 equations of indium antimonide and
and detected blackbody energy mercury cadmium telluride radiometers
are simplified in the first ordered equation
Qb ~ 2.2451 x I()-' <>· T 4·8°5° in the temperature range from 322 to
2. Sensor A2 has a ·wavelength 435 K (49 to 162 oc; 120 to 323 °F):

), = 10.86 pm, temperature exponent 1. Sensor B1 has an indium antimonide
liz~ 3.7916) and detected blackbody element, a wavelength I~= 2 to 5 pm,
a temperature exponent 114 = 9.2493
energy Qh = 1.1818 x 10-2 cr·Y:~.7YI6 and detected blackbody energy
Qh = 2.083 x 1o-16 o· y9.2-19].
fiGURE 24. Detectable energy of bicolor radiometer with
three filters. 2. Sensor B2 has a mercury cadmium
telluride element, a wavelength 1, = 6
to 9 pm, a temperature exponent
115 ~ 6.5377 and detected blackbody

energy Qb = 6.5284 x 1o-9 o· 'f6.SJ77.

3. Sensor B3 has a mercury cadmium
telluride element, a wavelength ), == 8
to 13 pm, a temperature exponent
116 = 4.3067 and detected blackbody
energy Qh = 4.3476 x H>-] 0;ruom.

Bicolor and tricolor thermography tests
using radiometers A and B are carried out
with combination of two radiometers
selected from three radiometers.

FIGURE 25. Detection sensitivity of bicolor and tricolor
infrared radiometers.

200 f 300 400 500
(-73) (127) (227)
f-100] (+27) [2601 [440)
[+81]

Temperature T, K(OC) [GF}

23 4 5 6 7 8 9 10 11 12 13
Wavelength A (!Jm)
legend
legend
A Blackbody emission, Q/1 ,.-- GT,4.
B. Qb=1.9818x1Q·lxo-f,4.Ct~l<' ------ "'short wave, indium antimonide
- =medium wave, mercury cadmium lellwide
(mercury cadmium telluride, /,at 8 to 13 j.Jrn). -·-·- =long wave, mereury c.1dmium telluride
C. Ql, = 3.8526 >·. J0-1 x oT,3LC6l ().1 at 11.462 pm).
D. Qb = 1.1818 x 1Q-2 x oT?-79!6 (>, 2 a! 10.862 jJm).
L Ov = 2.2451 x 10-s x or,uCtso (J,3 at 8.363 J.lm).

212 Infrared and Thermal Testing

Graybody and Pseudo Graybody The hypothesis that T~ and Ta are
Approximation much larger than Tv. is usually true when
the sensor is cooled by liquid nitrogen.
In case when lhe h'~l piece is enclosed Equation 21 is therefore rt>written in the
with the blackbody surroundings, an following form:
enclosure model is shown in Fig. 27. \•Ve
consider that tested surface S is (23) ''"
surrounded by blackbody surfaces A
maintained at ambient temperature '(11 to where a is the radiosity coefficient and T5
simplify the measurement field as a is the radiation temperature.
standard measurement system. The
radiosity heat flux /i is a quantity leaving In general, the grayhody
the surfaceS and is the sum of emitted approximation of nonmetal diffused
and reflected energies. The reflected surfaces nm be applied:
enert,•y is incident from the surroundings
at '/~ and is transferred to the sensor R. (24) c + p ~
The sensor is cooled at temperature of
liquid nitrogen TR. The measurement fields for metal
surfaces generally become complicated
Because the test piece is surrounded by owing to the influence of specular
the blackbody surfaces1 the effect of reflection of the surface. The sample
multiple reflections between A and S is surface is estimated in a different way.
negligible. Transferred heat flux /i from S Under a constant surface temperature,
toR using emissivity and reflectivity is two radiosity coefficients a1 and a2
expressed in the following form: corresponding to two ambient
temperatures ~~~ and T112, are shown in
where c is emissivity and p is reflectivity. the following form using Eq. 23.

FIGURE 26. Detectable energy of bicolor and tricolor (T'I~:I]"
radiometers.

10' (26) (/2

1O'

FIGURE 27. Enclosure model.

L_ I Enclosed surface

200 I /
(-73)
[-100] I Sensor
I
/
I

____l_l_

300 400 500 Radiosity --+- R
Ct27) (127) (227)
{+80] [260] [440]

legend Refl('ction

A Blackbody cmis5ion, Ot. =crT/. /
B. Q =- 4.3476 ;<. 10-l x of,4 J01.9
A
(mercury cadmium telluride, ), at 8 to 13 11m).
C. Q= 6.5284 x lQ-9 xof,6Hll legend
A=- blackbody surf<Jce
(mercury cadmium telluride, }, at 6 to 91-fm). R =sensor
D. Q = 0.2083 x 10· 15 x af,9.249l (indium antimonide, /,at 2 to 5 1-1m). 5 ""test surfilce

Noncontact Sensors for Infrared and Thermal Testing 213

Using Eqs. 25 and 26, the summation where ni is temperature index for three
of both emissivity E and reflectivity pis filters, T5 is temperature of test piece, 'fa is
represented by the pseudo graybody temperature of surrounding wall, £ is
appwximation b: emissivity, p is reflectivity, subscript s
represents the test piece and subscript a
(27) b = £ + r the surrounding wall. The radiosity
coefficient a includes the emission and
a,crt- 7;~1) - az(Tsn -1~q reflection of the incident irradiation.

r;1 -1~~ The radiosity coefficient a and
emissivity e for three filters are expressed
A glossy metal surface has a directional
property if its temperature is measured by as follows:
a narrow band infrared radiometer. The
summation of both emissivity E and b (32) (/
does not become unity. The summation b
is the pseudo graybody approximation in
a way similar to the graybody
approximation; b is the characteristic
value for various metal surfaces; a is also
given by the following form by

e)(~ rtransforming Eq. 23 using b.

(28) (/ = £ + (b-

(33) E 1 - (7_,~)"'
7~
a-{~)"
i: r(29) £ T.,Iili _ b 1~ni
1 - /J( Tti - Tani

The radiative properties for metal and TABlE 1. Radiative properties on various material surfaces
nonmetal surfaces are shown in Table 1. measured by three sensors. Ta == 293 to 373 K (20 to
The data are measured by the indium 100 °(; 68 to 212 °F),
antimonide (2 to 5 pm), mercury
cadmium telluride (6 to 9 pm) and Material Indium Mercury
mercury cadmium telluride (8 to 13 pm) Properties Antimonide Cadmium Telluride
sensors in the range of 293 to 373 K (20
to 100 oc; 68 to 212 °F) where the mean (2 to 5 ~m) (6 to9~m) (8 to 13 ~m)

temperature is T, = 293 K (20 oc = 68 °F). Stainless Steel 0.30 0.39 0.22
Emissivity £ 0.60 0.59 0.71
Those data arc applied to calculation of Reflectivity p 0.90 0.98 0.93
the multicolor technique for metal b =E+p
surfaces. 0.22 0.25 0.22
Brass 0.61 0.65 0.71
Temperature Calculation Emissivity £ 0.83 0.90 0.91
Reflectivity p
It is assumed that emissivity£, reflectivity b =£+p 0.15 0.14 0.15
p and term b for each filter are constant 0.75 0.76 0.83
and that graybody and pseudo graybody Aluminum 0.90 0.90 0.98
Emissivity r
approximations can apply. The heat flux li Reflectivity p 0.40 0.40 0.35
b =o£+p 0.50 0.47 0.63
from the surface of the test piece is 0.90 0.90 0.98
expressed as: Steel
Emissivity E 0.96 0.95 0.94
(31) I; cr(cTti + PT.lni) Reflectivity p 0.04 0.05 0.06
1.00 1.00 1.00
a( cT~ni + (b - c)7~111 1 b=~·+p

Concrete
Emissivity £
Reflectivity p
b = £+r

214 Infrared and Thermal Testing

where i = 1,2,3 ... and T~ is the radiation (37) F, (7;,) . [b'J~li~('J;iti]'J~lk
temperature for three filters used. The
ambient temperature Ta is already known TJll + (7~'"'rk- bTtk
for bicolor calculation.
Ti1k('l~l)m· - Tsni (T;k) nk
These approximations are applied in
the range where two wavelengths of + (T;k)nk ~ bTilk
bicolor filters are located close1y. The
radiation temperatures measured by the =0
bicolor radiometer 7~1 and T~1 are
expressed in following equations:

(34) (T;i)ni aT~ni i:;(where i = 1,2,3; j = 1,2,3; k = 1,2,3; i

€J~ni + pl~:Ii *for Eq. 36; and k j for Eq. 37.
cTti + (b- c)~~~i Because T~i• T;2 and Ta are measured by
thermography using the tricolor
(35) (T;jfi a'/~11 i radiometer and thermocouple, the
€J:t1+ p7~lj temperature of the test surface T~ is
numerically and simultaneously solved by
tT~nj + (b- c)~111i
simultaneous Eqs. 36 and 37. Three
Eliminating the emissivity, 7~ is given calculation temperatures using three
by solving the following equation Fi(T5).
filters 1'12, T2:{ and Tu are solved as
follows:

1. T 12, F1(T,), i = 1 and j = 2; and F.1(T,),
i = 3 and k = 1;

i2. Tn, F2(1~), i = 2 and = 3; and F1(T,1),

i = 1 and k = 2;
3. Tu, F1(T,), i = 3 and j = 1; and F2(T,),

i = 2 and k = 3.

[ ( )";].bT;1m. - '}~i T;lJ.
Tsni + .L--~-~'---- Experimental Results and
Numerical Analysis
(TS.'J·)nj - bTnj

a

nj Result and Analysis of Bicolor Test

~1nj (T~i')ni - J:ti (T;i)

+

Results of the birolor test were applied to

0 calculate test surface temperatures, using

the test apparatus shown in Pig. 21.

1-'igure 28 shows the relation between the

where i = 1,2,3 and j = 1,2,3 and i ":/: ;. calculated temperature 7 13 and function
FJCJ~) of the epoxy resin using sensor A1
As the radiation temperatures T511 T\ 2 (8.37 pm) and sensor A3 (11.46 pm) of the
and T53 are measured by thermography
and the ambient temperature by the mercury cadmium telluride radiometer.

thermocouple Ta (the temperature of the In this experiment, the temperature of

test surface 7~ is solved numerically. Three the ambient Ta is 293.2 K
calculation temperatures '1'121 '123 and T13
using three filters are solved in the (20.1 °C = 68.1 oF) and the temperature Ts

following conditions: of the test piece is 363.8 K (90.7 "C =

1. T 12, F1('!;), i = 1 and j = 2; 195.2 °F). The calcul ated temperature T 13
358.8
at F3(T,) =0 is K (85.7 oc = 186.2 °F)

2. T 23 , Fz(T~), i =2 and j = 3; and isS K (Soc= 9 oF) less than the
3. and j
T 13 , F., ('I;), i =3 = 1. temperature measured using the

The cycle of numerical calculation is thermocouple 'J~.
iterated 20 to 30 times until the values
Figure 28b shows the relation between

converge. the calculated temperature T12 and
The tricolor technique does not need function F 1(Ts) of the polycarhonate resin

to consider the ambient temperature Ta of using the mercury cadmium telluride
the enclosed wall in numerical calculation
sensor A1 (8.37 pm) and sensor A2
process. Eliminating emissivity €, the (10.86 pm). The ambient temperature'/~

values of T5 and Ta are deterrnined by is :l03.9 K (30.8 "C = 87.4 °F) and that of
solving Eqs. 36 and 37:
the test piece T, is 375.2 K (102.1 'C =

215.7 °F). The calculated temperature T 12

at F1 u·,) =0 is 378.0 K (104.9 oc =

220.7 °F) and is 2.8 K (2.8 oc = 5.4 "F)

gre<~ter than the temperature measured

using the thermocouple Ts.

Noncontact Sensors for Infrared and Thermal Testing 215

Figure 29 shows the experiment<tl and However, the temperature difference
theoretical result on the epoxy resin to
summarize the relation between the !J.T23 = Tn-T~ (combining sensors A2 and
calculated temperature using the bicolor A3) is 40 to 70 K (40 to 70 "C; 72 to
theory and the temperature measured 126 "F).
using the thermocouple. The result shows
that the temperature difference between FIGURE 29. Experimental results on epoxy resin.
the experimental temperature and
calculated temperature is 5 to 20 K (5 to t-.-:- -+--=t·500 (227) [440]~..
·t-(187)[368]~-t-···460 -J ..:
t•:r20 "C ~ 9 to 36 "1'): 12 ~ T12 - T,
.l_
(combining sensors A1 and A2) and 8T1.1 =
Tu- T~ (combining sensors A1 and A3). i

FIGURE 28. Relation between temperature Ts and function E_"_ -{ t-1--~=-~G -JI-ll

F3Ts of epoxy resin: (a) experiment using sensor 1 and "-'
sensor 2; (b) experiment with slightly different parameters 420
(147) [296]!
and using sensor 3 instead of sensor 2. "-tl ' .""1-;;, 380 (107) [224]1- - -
:_!}-340 (67) [152], __
(a) H-- c~

6 t= 300 (27) [80) 1 _l
300
4 . r- (27) __l___ 420 460 500
[80) (147) (187) (227)
0 340 380 [296] [368] [440]
(67) (107)
0 [152] [224]

X2 Temperature T5, K (0 C) [°F)
;:?
B
J: 0
l legend
A
-~ + = filters 1 and 2
I-2
+ 370 390 o = filters 2 and 3
(97) (117)
[206] [242] A. =filters 3 and 1

~ = theoretical line

-4 ...L _.L.L ·: -

250 270 290 310 330 350 fiGURE 30. Measured emissivity 8£ and
(-23) (-3) (17) (37) (57) (77) temperature difference 8 T;i of epoxy and
[-10] [+23] [62] [98] [134] [170] polycarbonate resins.

Temperature Til K ec) [°F]

(b)

2

0 0 D.

0 290 310 330 350 370 390
(17) (37) (57) (77) (97) (117)
"'X [62] [98] [134] [170] [206] [242]
Temperature T1, K (0 C) [°F]
;:?
~ -2

-3

-4

-5 270
(-3)
250 [+23]
(-23)
[-10]

legend Drift of emissivity ~~£

A. Temperature Ts = 293.2 K(20 oc = 68 <F). legend

B. Temperature Ts = 358.2 K(85 °C = 185 "F). + = epoxy re~in results using filters 1 and 2

C. Temperature fs = 303.9 K{31 oc = 87 oF). • =epoxy resin results using filters 2 and 3
J,. = epoxy resin results using filters 3 and 1
D. Temperature fs = 378.0 K(105 '>c = 221 °F}. (> "'" polycarbonate results using filters 1 and 2
D = po!ycarbonate results using filters 2 and 3
1:, = polycarbonate results using filters 3 ands 1

216 Infrared and Thermal Testing

Temperature measurement results on a Experimental result of the bicolor test
is applied to solve the test surface, using
polycarbonate resin test piece using the the test apparatus of Fig. 22. Three
radiometers of indium antimonide and
bicolor technique show that the mercury cadmium teJiuride sensors B are
used to measure the radiation temperature
calculated temperature difference i'l.T12 of of the polished copper surface. The
sensors A1 and A2 combined and graybody approximation is applied for
this calculation. The test result shows that
temperature difference i'l.Tu o f sensors oAc1; the temperature difference Tii is from 0.3
and A3 combined is S t o 20 K (S to 20 to 8.0 K (0.3 to 8.0 'C; 0.5 to 14 ol') and is
4.0 K (4.0 'C = 7.2 'F) in mean.
9 to 36 °F). However, the calculated
Result and Analysis of Tricolor Test
temperature difference i'l.Tn in the second
Table 4 shows the numerical results of
combination of sensors A2 and A3 is over calculated surface temperature Tst with the
100 K (1 00 'C = 180 'F). The result on emissivity as a parameter in the case that

polycarbonate resin is quite similar. The the true temperature is 340 K (67 °C =

temperature difference T23 that the central 152 ol;). Calculated temperature Ts and its
detection wavelength approaches with emissivity are shown in the table. The
graybody approximation is applied for
each other becomes inferior to that of 1'12 this calculation.
and T13 •
Table S shows the calculated results of
Figure 30 shows the calculated the surface temperature of blackbody and
copper surface. Proposed equations Fi(T5)
emissivity curve versus measured in Eq. 36 and Fi(7~) expressed in Eq. 37
are used to obtain the calculated
temperatures Tii for epoxy and temperature. ?vieasured and calculated
temperature is shown in the table. Result
polycarbonate resins. The temperature

differences 1'12 and 1'13 become smaller
than 10 K (10 'C = 18 'F) where

emissivity E = 0.02. However, 1'23 is 65 K
(-208 'C = -343 'F) where £ = 0.02. Filters

A2 and A3 cannot be combined for bicolor
measurement. An interval of the center to

center wavelength of two filters is needed

to be greater than the waveband ·width of

each filter.

The pseudo graybody approximation is

applied to calculate for the stajnless steel

surface. Table 2 shows the true

temperature 1~; calculated temperatures TABLE 3. Temperatures T9 calculated temperature T13, T23,
T31; and temperature difference i'J. T13, t1 T23, tl T31 of
1'12, Tn, Tz3; temperature difference graphite at T, ~ 293 K (20 'C = 70 °f).
i'l.Ttz = 1'12- 1~; temperature difference
r, t).Tt2 Tn nTu T23 ~T23 T,
tlT13 = 1'13 - Ts; and temperature
difference L'J.72.-.. = T23 - 1~. The mean value

of the temperature differences of i'l.T12 and 299.9 -2.4 299.9 -2.4 300.3 -2.0 302.3
8T13 are 1.3 and 1.5 K (1.5 'C = 2. 7 'F) 321.9 -1.9 319.3 -4.5 320.8 -3.0 323.8
respectively. However, AT23 are 2.6 K 330.8 -1.5 332.4 332.4 332.3
°F) greater than values of 7'12 340.3 -1.8 344.0 0.1 343.5 0.1 342.1
(2.6 oc = 4.7 350.1 -2.5 353.2 1.9 353.5 1.4 352.6
361.2 -1.4 364.5 0.6 365.8 0.9 362.6
and T13• 370.7 -1.8 375.2 1.9 377.5 3.2 372.5
!Ts12! !Tsnl 2.7 5.0
Table 3 shows the calculated result of 1.9 2.0 1Ts231 2.2

the graphite surface using radiometer A.

The temperature differences of i'J.7'12, tlT13
and i'J.T23 are 1.9 and 2.0 and 2.2 K (1.9
and 2.0 and 2.2 'C; 3.4 and 3.6 and

4.0 °r), respectively.

TABLE 4. Numerical results of surface temperature by
= 340
TABLE 2. Temperatures Ty calculated temperature T13, T23, tricolor technique: f 12(T,), F23 (T,), T, K

T31i and tempera tu re diff eKre(n2c0e'Ci'l.T=136, L\T23, i'J.T31 of (67 'C = 152 'F).
stainless steel at T, 293
= 8 'F). Calculated Ts Emissivity
K ('C) ['F]
r, o.T12 Tn flTn T23 nT23 T, c,'J E2

302.7 0.3 304.9 2.5 302.0 -0.4 302.4 333.43 (60.28) [140.50] 0.90 0.91 0.92
315.7 0.4 315.4 0.1 313.3 -2.0 315.3 347.91 (74.76) [166.57] 0.52 0.50 0.51
326.5 0.8 326.6 0.9 323.3 -2.4 325.7 331.96 (58.81) [137.86] 0.22 0.21 0.20
332.8 0.8 330.0 -2.0 329.3 -2.7 332.0 333.45 (60.30) [140.54] 0.12 0.11 0.10
343.6 -1.3 343.1 -1.8 340.0 -4.9 344.9 329.05 (55.90) [132.62] 0.62 0.61 0.60
350.5 -1.3 349.4 -2.4 347.6 -4.2 351.8 348.84 (75.69) [168.24] 0.60 0.20 0.40
364.5 1.7 362.1 0.7 359.8 -3.0 362.8
376.4 3.9 373.9 1.4 371.5 -1.0 372.5
ITs12! 1.3 lT5nl 1.5 lT523 l
2.6

Noncontact Sensors for Infrared and Thermal Testing 217

TABlE 5. Calculated results of surface temperature by tricolor method. F1(T5) by n1 and
n2, F3(T,) by n2 and n3, T, = 340 K.

True Tem~erature T5 Calculated Tem~erature Tem~erature Difference
K ('C) ['F]
K ('C) ['F] K ('C) roFJ

Blackbody [133.52] 343.97 (70.82) [159.48] -14.42 (-14.42) [-25.96]
329.55 (56.40) [161.33] 344.73 (71.58) [160.84] -0.26 (-0.26) [-0.47]
345.00 (71.85) [177.44] 348.86 (75.71) [168.28] -5.09 (-5.09) [-9.16]
353.95 (80.80)
[144.32] 346.49 (73.34) [164.01] 10.94 (10.94) [19.69]
Polished Copper [160.70] 352.68 (79.53) [175.15] -8.23 (-8.23) [-14.81]
335.55 (62.40)
344.65 (71.50)

of the proposed calculation shows that
estimated error isS to 10 K (S to 10 oc; 9
to 18 oF) at nearly ambient temperature.

The tricolor radiometer can measure
the surface temperature of reflective and
emissive materials without considering
the surrounding wall at nearly ambient
temperatures. ?vfeasured temperature error
of the tricolor radiometer isS to 10 K (S
to 10 oc; 9 to 18 °F) greater than that of
the bicolor one.

218 Infrared and Thermal Testing

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Noncontact Sensors for Infrared and Thermal Testing 225



CHAPTER

Contact Sensors for
Thermal Testing and

Monitoring

E. john Dickinson, University Laval, Quebec, Quebec,
Canada (Part 6)
Erik E. Muller, Agilent Technologies, Everett,
Washington (Parts 1 to 5)
Dennis P. Redline, Tempi!, Incorporated, South
Plainfield, New jersey (Part 7)

Parts 1 to 5 adapted with permission from Hewlett-Packard Application Note 290, Practical
Temperature Measurements. © 1997 Agilent Technologies, Everett, Washington.

PART 1. Temperature Measurement1

This chapter discusses contact sensors for Development of
temperature measurement. The four most Thermometric Scales
common temperature transducers provide
electrical signals: the thermocouple, the Galileo is credited with inventing the
resistance temperature detector (RTD), the thermometer circa 1592,2<~ Jn an open
therrnistor and the integrated circuit container filled with colored akohol, he
sensor (Hgure 1 and Table 1). '1\vo types suspended a long narrow throated glass
of material applied to the surface of the tube at the upper end of which was a
test object are also discussed: liquid ho!Jow sphere. \'Vhen he<1ted, the <1ir in
crystals and temperature sensitive lacquer tht' sphere expanded <md bubbled
or crayon. Both have chemistries that through the liquid. Cooling the ~phere
change color when temperature changes.

FIGURE 1. Four kinds of temperature sensors: (a) thermocouple; (b) resistance temperature
detector; (c) thermistor; (d) integrated circuit.

(a) (c)

Temperature T Temperature T

(b) (d)

Temperature T Temperature T
228 Infrared and Thermal Testing

caused the liquid to move up the tube.2 48 and eventually 96 parts. The
Fluctuations in the temperature of the fahrenheit scale gained popularity
sphere could then be observed hy noting primarily hecause of the repeatability and
the position of the liquid inside the tube. quality of tht' thermometers that
This upside down_ thermometer was a poor Fahrenheit built.
indicator because the level changed with
barometric pressure and the tube !lad no Around 1742, Anders Celsius proposed
scale. Vast improvements were made in that the melting point of ice and the
temperature measurement accuracy with boiling point of water be used for the two
the development of the Horentine benchmarks. Celsius selected zero degrees
thermometer, which incorporated sealed as the boiling point and 100 degrees as
construction and a graduated :scale. the melting point. l.ater, the end points
were reversed and the centigrade scale \Vas
In the ensuing decades, many born. In 1948 the name was officially
thermometric scales were conceived, all changed to the celsius scale.
based on two or more fixed points. One In the early 1800s \-\'illiam Thomson
scale, however, was not universally (Lord Kelvin), developed a universal
recognized until the early 1700s when thermodynamic scale based on the
Gabriel Fahrenheit, a Dutch instrument coefficient of expansion of an ideal gas.
maker, produced accurate and repeatable Kelvin established the concept of absolute
meKury thermometers. For the fixed zero and his scale remains the standard
point on the low end of his temperature for modern thermometry.
scale, Fahrenheit used a mixture of ire
water and salt (or ammonium chloride). As an increment, one kelvin (K) equals
This was the lowest temperature he could one degree celsius ("C). One degree
reproduce and he labeled it zero degrees. fahrenheit ("1:) equals one degree rankine
For the high end of his scale, he chose ("H). One kelvin equals 1.8 degree
human blood temperature and called it fahrenheit. So I K = 1 oc ~ 1.8 op = 1.8 °1(,
96 degrees. The rankine scale ("R) is simply the
fahrenheit equivalent of the kelvin scale
\'\'hy 96 and not 100 degrees? Earlier and was named after an early pioneer in
scales had been divided into twelve parts. the field of thermodynamics, W.].1'f.
1n an apparent quest for more resolution, Rankine. Notice that the official kelvin
Fahrenheit divided his scale into 24, then scale does not carry a degree sign. The
units arc expressed in kelvin, not degrees
TABLE 1. Advantages and disadvantages of temperature kelvin.

sensors. On the absolute scale, the freezing

Advantages Disadvantages point of water is 273.1 S K = 0 "C =32 "F.

Thermocouple nonlinear Reference Temperatures
self-powered low voltage
simple reference required A temperature divider cannot he built as
rugged least stable can a voltage divider, nor can
inexpensive least sensitive temperatures be added as lengths would
wide temperature range be added to measure distance. It is
wide \'ariety of physical forms expensive
slow TABLE 2. Fixed temperature points in International
Resistance temperature detector current source required
most stable small resistance change Temperature Scale.4
most accurate four-wire measurement
more linear than thermocouple Element Type Temperature "F
nonlinear K oc
Thermistor limited temewture range
high output fragile Hydrogen triple point 13.8033 ~259.347 -434.824
fast current source required 24.5561 -415.469
two·wire ohm measurement self-heating Neon triple point 54.3584 --248.594
83.8058 ~361.825
lntergrated Circuit Sensor T < 523 K (250 "C ~ 482 'F) Oxygen triple point 234.315 ~218.792 ~308.820
most linear (temperature T) power supply required 273.16 ~189.344
highest output slow Argon triple point 302,9146 ~37.903
inexpensive self-heating 429.7485 ~38.8350
limited configurations Mercury triple point 505.078 32.018
692.677 0.0100 85.5763
Water triple point 933.473 29.7646 313.8773
1234.93 156.5985 449.470
Gallium melting point 1 337.33 231.928 787.149
419.527 1220.58
Indium freezing point 660.323 1763.20
961.780 1947.52
Tin freezing point 1064.18

Zinc freezing point

Aluminum freezing point

Silver freezing point

Gold freezing point

Contact Sensors for Thermal Testing and Monitoring 229

necessary to rely on temperatures
established by physical phenomena that
are easily observed and consistent in
nature.

The International Temperature Scale
(ITS) is based on such phenomena.
Revised in 1990, it establishes seventeen
fixed points and corresponding
temperatures. A sampling is given in
Table 2. 4

Because only these fixed temperatures
are available instruments must interpolate
between them. But_ accurately
interpolating between these temperatures
can require fairly exotic transducers,
many of \\'hicll are too complicated or
expensive for practical situations.

The discussion immediately below
emphasizes the practical considerations of
transducer placement, signal conditloning
and instwmentation. Despite the
widespread popularity of the
thermocouple, it is frequently misused.
For this reason, the discussion
concentrates on thermocouple
measurement techniques. A more
thorough discussion of thermocouple
theory has been published elsewhere.5

230 Infrared and Thermal Testing

PART 2. Thermocouples1

\'\'hen two wires composed of dissimilar FIGURE 2. Thermoelectric effect:
metals are joined at both ends and one of (a) continuous current in thermoelectric
the ends is heated, a continuous current circuit; (b) net open circuit voltage, function
flows in the thermoelectric circuit (Fig. 2). of junction temperature and composition of
Thomas Seebeck made this discovery in two metals.
1821.
(a)
If this circuit is broken at the center,
the net open circuit voltage (the @-<~---Me-ta-iA--1---;-Mre-tal-A--~1,
thermoelectric voltage) is a function of
the junction temperature and the MetaiB 0
composition of the two metals (Fig. 2h).
(b)
All dissimilar metals exhibit this effect.
Table 3 lists common combinations of Metal A
two metals along with important
characteristics. For small changes in
temperature the thermoelectric voltage is
linearly proportional to temperature:

(1) aT

wl1ere thermoelectric coefficient a is the legend
constant of proportionality. For real ·world
thermocouples, a. is not constant but eAll = thermoelectric voltage
varies with temperature. Conversion from i = electric current (A)

voltage to temperature is discussed below.

TABLE 3. Thermocouple properties.

Type Metal Color Thennoelectric Wire Error NIST
Proportion Codea Coefficient Ranged
Range Standard Special
Resistance S (ttV·K-1) (K)< (percent) (percent) (K)<
(see noteb) at T (K)'

B (+) 0.7 platinum ,0.3 rhodium gray 0.22 5.96 at 273 1143 to 1973 ±0.50 ±0.25 0 to 1820
0.71 58.67 at 873 273 to 1173 ±0.50
B (-) 0.94 platinum, 0.06 rhodium red 0.36 50.38 at 273 273 to 1023 ±0.75
0.59 39.45 at 273 273 to 1523 ±0.75
E (+) 0.9 nickel, 0.1 chromium violet 0.78 25.93 at 273 273 to 1523 ±0.75 ±0.40 -270 to 1000
0.19 11.36 at 873 273 to 1723 ±0.25
E (-)constantan red 0.19 10.21 at 873 273 to 1723 ±0.25
0.30 38.75 at 273 273 to 623 ±0.75
I (+)iron white ±0.40 -210 to 1200
I (-) constantan red

K (+) 0.9 nickel, 0.1 chromium yellow ±0.40 -270 to 1372

K (-)nickel red

N (+) nicrosit orange ±0.40 -270 to 1300

N (-) nisil red

R (+) 0.87 platinum, 0.13 rhodium black ±0.10 -50 to 1768

R (-)·platinum red ±0.10 -50 to 1768
black
s (+) 0,9 platinum, 0.1 rhodium

s (-)platinum red

T (+)copper blue ±0.40 -270 to 400

T (-·)constantan red

a. Standard United States color code.
b. Ohm per 305 mm (per double foot) at 293 K (at 20 ~c = 68 "F) for 20 American wire gage (AWG) wire.
c. 273 K= 0 "C = 32 <F.
d. Specified material range is for 8 AWG wire and decreases with decreasing wire size.

Contact Sensors for Thermal Testing and Monitoring 231

Measuring Thermocouple only \'1 but connecting the volt meter in
Voltage an attempt to measure the output of
junction j 1 has created two more metallic
The thermoelectric voltage cannot be
measured directly because a volt meter junctions: 12 and J.t- Because h is a
must first be connected to the
thermocouple. The volt meter leads copper~to·copper junction, it creates no
themselves create a new thermoelectric
circuit. thermal electromotive force (v~ = 0) but Jz

A volt meter may be connected across a is a copper~to-constantan junction, \Nhich
copper~to-constantan (copper nickel alloy)
thermocouple (type T) to produce a will add an electromotive force 0'2) in
voltage output (Fig. 3). It might he
expected that the volt meter would read opposition to \11• The resultant volt meter
reading \f will be proportional to the
fiGURE 3. Measuring junction voltage with
digital volt meter. Diagrams of equivalent temperature difference between J1 and Jz,
thermoelectric circuits: (a) volt meter is
connected to copper constantan (type T) The temperature at j 1 cannot be
thermocouple; (b) thermoelectric circuit is
created by connecting volt meter to determined unless the temperature at h is
thermocouple (V3 = 0 V); (c) thermoelectric
circuit is equivalent to that in Fig. 3b, measured first.
because copper-to-copper connection
junction creates no electromotive force. Reference Junction
(a) One way to determine the temperature Jz

---1 is to physically put the junction into an
ice bath, forcing its temperature to be
lh
273 K (0 oc ~ 32 °F) and establishing], as
+ ~rH.. igh1"1'...-------._-----"c''--'~+ h
v~ the reference junction. Because both volt
Cn - ~ meter terminal junctions are now
copper-to-copper, they create no thermal
(b) electromotive force and the reading 1' on
the volt meter is proportional to the

temperature difference between j 1 and Jz.

Now the volt meter reading is (see Fig. 4):

If 1J 1 is specified in degrees celsius:

FtGURE 4. External reference junction for copper constantan
thermocouple: (a) thermocouple connected to volt meter;
(b) equivalent thermoelectric circuit.

(a)

-vrylow Cu + h

+ V2- 1',
Cn -

(c) Vo~m~!~~TJ J4

legend +~(b)
Cn = constantan resistance alloy (50 to 65 percent v - vl r 11
copper, 35 to 50 percent nickel} - +v, -
Cu =copper
b
I= electrical junction Legend

\ 1 = voltage (V) Cn =constantan resistance alloy (SO to 6S percent copper, 3S to
SO percent nickel)

Cu =copper
I= electrical junction
T"" temperature (K)
V = voltage (V)

232 Infrared and Thermal Testing

(3) T11 ('C) + 273.15 This circuit still provides moderately
accurate measurements as long as the volt
then the volt meter reading F becomes: meter high and low terminals (h and j 4)
act in opposition (Fig. 6).
(4) v1 - \;z
If both front panel terminals are not at
o:[(T11 + 273.15) the same temperature, there will be an
error. For a more precise measurement,
(T12 + 2n 1s)] the copper volt meter leads should be
extended so the copper-to-iron junctions
o:(7j1 - T12 ) are made on an isothermal (uniform
(T11 - 0) temperature) block (J'ig. 7).
aT11
The isothermal block is an electrkal
This protracted derivation is used to insulator but a good heat conductor and
emphasize that the ice bath junction
output V2 is not 0 V. It is a function of it serves to hold 1:~ and J4 at the same
absolute temperature.
temperature. The absolute block
By adding the voltage of the ice point temperature is unimportant because the
reference junction, the reading F has now two copper iron junctions act in
been referenced to 273 K (0 'C = 32'F). opposition:
This technique is very accurate because
the ice point temperature can be precisely (5) v
controlled. The ice point is used by the
National Institute of Standards and FIGURE 6. Junction voltage cancellation:
Technology (NIST) as the fundamental
reference point for their thermocouple v, = vif v, = V4.
tables, so thermocouple tables can be
consulted to convert directly from legend
voltage \1 to temperature 7)1• Cu = copper

The copper constantan thermocouple I = electrical junction
shown in Fig. 4 is a unique example
because the copper wire is the same metal V = voltage (V)
as the volt meter terminals. If an iron
constantan (type J) thermocouple is used FIGURE 7. Removing junctions from digital volt meter
instead of a copper constantan terminals.
thermocouple, the iron ·wire (Fig. 5)
increases the number of dissimilar metal Cu h
junctions in the circuit as both volt meter +
terminals become copper-to-iron
thermocouple junctions. legend
Cn = constantan resistance alloy (50 to 65 percent copper, 15 to
FIGURE 5. Iron constantan (copper nickel 50 percent nickel)
alloy) couple. Cu =copper
Fe = iron
Cu + ,,
~Cn -v, I = efectrie<ll !unction
+
v T = temperature (K)

Vo!t~e~ J 14 \'=voltage M

Legend

Cn ==constantan resistance alloy (50 to 65 percent
copper, 35 to 50 percent nlckel)

Cu =copper
Fe= iron

J =electrical junction
V =voltage M

Contact Sensors for Thermal Testing and Monitoring 233

Reference Circuit junction will have no effect on the output
voltage as long as the two junctions
The circuit in Fig. 7 will gh•e accurate formed by the additional metal are at the
readings but it would be expedient to same temperature (Fig. 9). This conclusion
eliminate the ice bath if possible.
is useful because it completely obviates
The ice bath can be replaced with the iron wire in the low lead (Fig. 10).
another isothermal block (Fig. 8). The new
block is at reference temperature Trer and Again F = o:(T1 - 'J~l'f) l\'here o: is the
because hand 14 are still at the same thermoelectric coefficient for an
temperature it can again be shown that:
iron-to-constantan thermocouple.
This is still a rather inconvenient Junctions J~ and J-1 take the place of the
circuit because two thermocouples must ice bath. These two junctions now
be connected. The extra iron wire in the become the reference junction.
negative (low) lead can be eliminated by
combining the copper~to-iron junction J.J- The next logical step is directly to

and the iron-to-constantan junction .Iref· measure the temperature of the

This connection can be accomplished isothermal block (the reference junction)
and use that information to compute the
by first joining the two isothermal blocks
(Fig. 8b). The output voltage V remains unknown temperature 7J 1 (Fig. 11).
unchanged (Eq. 5): A thermistor, whose resistance R-r is a

Now the law of intermediate metals function of temperature, provides a way

eliminates the extra junction. This FIGURE 9. law of intermediate metals:
empirical law states that a third metal (in (a) intermediate metal with isothermal
junctions; (b) lead equivalent to Fig. 9a;
this case, iron) inserted between the two (c) lower lead in Fig. Sb; (d) lead equivalent
dissimilar metals of a thermocouple to Fig. 9c.

(a)

Metal A Metal B Meta! C

\) \)

fiGURE 8. Elimination of ice bath: (a) replacing of ice block
with isothermal block; (b) joining of isothermal blocks.

(a) Isothermal block Isothermal connection
---,
,----,/ (b)

+ ,, Metal A Metal C

Cn

Volt meter I. (c)

Cu Fe Cn

r..:f isothermal block \) \

(b) [

High Cu.---------~

,,+ --1>---, II''+~
IV
vI h I _v'_j ''
(d)
Cu: Fe +~
low

L -''----- -'-"' J

\

Isothermal block at T" 1

legend legend

Cn =constantan resistance atroy (50 to 65 percent copper, 35 to Cn =constantan resistance a!toy (50 to 65 percent
50 percent nickel) copper, 35 to 50 percent nickel)

cu =copper Cu =copper
Fe= iron
Fe =iron T" 1= reference temperature (K)

I = electrical junction

V =voltage (V)

234 Infrared and Thermal Testing

to measure the absoh1te temperature of any device that has a characteristic
the reference junction. junctions hand ;.1 proportional to absolute temperature: a
and the thermistor are all assumed to be resistance temperature detector, a
at the same temperature, because of the thermistor or an integrated circuit sensor.
design of the isothermal block.
Advantages of Thermocouples
1. Use a digital multimeter to measure Rr
and find 'J~cf· H seems logical to ask why, if a device
that will measure absolute temperature
2. Convert Tree to its equivalent reference (like a resistance temperature detector or
junction voltage Free· thermistor) is available, a thermocouple
that requires reference junction
3. 1\,feasure V. compensation is needed. The single most
4. Add Vrcf and F to find \'1• important answer to this question is that
5. Convert \11 to temperature TJJ. the thermistor, the resistance temperature
detector and the integrated circuit
This procedure is known as softwme transducer are only useful over a certain
compensation because it relies on temperature range. Thermocouples, on
software in the instrument or a computer the other hand, can be used over a range
to compensate for the effect of the of temperatures and optimized for various
reference junction. The isothermal atmospheres.
terminal block temperature sensor can be
Thermocouples are much more rugged
FIGURE 10. Equivalent circuit. than thermistors, as evidenced by the fact
that thermocouples are often welded to a
Cu metal part or damped under a scre\\'. They
can be manufactured on the spot, either
+ by soldering or welding. In short,
thermocouples are the most versatile
v temperature transducers available and
because the measurement system
Cu performs the entire task of reference
compensation and software
II voltage-to-temperature conversion, using
: ,, 1"- a thermocouple becomes as easy as
____ j T,d connecting a pair of wires.

Legend Thermocouple measurement becomes
especially convenient when it is necessary
Cn = constantan resistance alloy (50 to 65 percent to monitor many data points. This is
copper, 35 to 50 percent nickd) accomplished by using the isothermal
reference junction for more than one
Cu = copper thermocouple element (Fig. 12). A relay
Fe = iron scanner connects the volt meter to the

J = electrical junction
T,.1 = reference temperature (K)

V = voltage (V}

FIGURE 11. External reference junction FIGURE 12. Switching multiple thermocouple types.
without ice bath.

Block +~
temperature v I High

IT"' -~Low
__i ~--
Cu
Fe Vo!t meter
All copper wires
v+, '· \ Pt/10Rh

I Cu Cn ~ Isothermal
Volt~et~ J block
R, (zonE' box)

Legend Legend

Cn =- constantan resistance alloy (50 to 65 percent Cn = constantan resistance alloy (50 to 65 percent copper, 35 to
coppN, 35 to 50 percent nickel) 50 percent nickel)

Cu = coppN Fe = iron
Fe=- iron Pt = platinum
Pt/lORh = 90 percent pl"tinum; 10 percent rhodium
J = electrical junction Rr = resistance measurement
Rr = resistance measurement V =voltage 0/)
V = voltage (V)

Contact Sensors for Thermal Testing and Monitoring 235