antimonide {lnSb) focal plane arrays are differences comparabfe to imaging
available in models designed to compete radiometers- 0.1 to 0.2 K (0.1 to 0.2 "C;
with top-of-the-line commercial thermal 0.18 to 0.36 oF) - and instantaneous
imagers. Some j11stJUJtll:~11ts in this fields of view considerably better than
category have the size and weight of a imaging radiometers (1 mrad or better
commercial video camera that fits in tlw with standard optics). Commercially
palm of the hand (Fig. lib); available quantitative infrared focal plane
array cameras use detector arrays made of
Quantitative platinum silicide or indium antimonide,
Thermographic Imagers either of which requires cooling.
Quantitative thermal imagers based on
Quantitative thermal imagers include uncooled focal plane arrays (using
mechanically scanned thermal imagers bolometric and ferroelectric detectors)
{imaging radiometers) and focal plane have also been developed. With
array radiometers. inherently faster response, no moving
parts and superior spatial resolution,
Mechanically Scanned Imagers infrared focal plane array cameras have
(Imaging Radiometers) been replacing infrared imaging
radiometers for most applications.
~v1echanically scanned thermal imagers
(imaging radiometers) provide a means for Thermal Imaging Display
measuring apparent target surface and Diagnostic Software
temperature with hjgh resolution image
quality and sometimes with extensive '"'hen the personal computer was
on-board diagnostic software. Most introduced as part of thermal imaging
commercially available imaging systems, the typical imager produced raw
radiometers use a single detector but some radiometric data whereas all of the
manufacturers offer dual detector or diagnostic software was contained in an
multidetector (linear array) instruments. ancillary, separately packaged computer
Most require detector cooling. that performed an of the diagnostics back
Imaging radiometers use refractive, 't\011 the bench. 1ith improved packaging
reflective or hybrid scanning systems and
operate in either the 3 to S pm or the 8 to technology in both computers and
14 pm atmospheric window. They thermal imaging equipment, there has
generally offer instantaneous fields of been a gradual trend toward providing
view on the order of 1 to 2 mrad with more and more on-board software so that
standard optics and minimum resolvable more diagnostics can be performed on
temperature differences of 0.05 to 0.10 K site.
(0.05 to 0.10 °C; 0.09 to 0.18 °1').
On-board capabilities often include Depending on manufacturer and
isotherm graphics features, spectral model, some software is incorporated into
filtering, interchangeable optics for instruments and some is available in
different total fields of view, color or computer driven software packages.
monochrome (black and white) displays, Although thermographi.c diagnostic
flexible video recording capabilities and software packages are usually proprietary
computer compatibility. Most feature to a particular manufacturer, there is a
compact, field portable, battery operable trend toward universality in image
sensing heads and control/display units. A storage. Common formats for storing
complete system, including battery and electronic images include the tagged
video recorder, can be handled by one image file format (TIFF) and other
person by mounting the components on a bitmapped formats. Retrieving images
cart or by assembling them on a harness. from· these formats is fast and easy.
Focal Plane Array Radiometers Quantitative Thermal
Measurements
Focal plane array radiometers are
adaptations of military and aerospace Some qualitative thermograms can be
forward looking infrared scanners but are converted to quantitative thermograms.
designed to measure the apparent Tile mw image produced by a quantitative
temperature at the target surface arHl to imager may be converted to a quantitative
produce quantitative thermograms. The thermogram; the raw image produced by
capabilities of early infrared focal plane a viewer may not. Quantitative thermal
array imagers were slow in developing. measurements provide the user with the
The quality of measurement capabilities true radiance or apparent temperature
has improved since 1990. value of any or all points on the target
surface. To present the thermogram in
Infrared focal plane array cameras offer true radiance measurements, the system
minimum resolvable temperature throughput attenuation must be
286 Infrared and Thermal Testing
considered as well as losses through the maximmn, minimum and average values~
measurement medium (atmosphere, in number of pixels or even a frequency
most cases). To present the thermogram histogram of the values within the area.
in true temperature values, the target Color scales can be created from 256
effective emissivity must also be colors stored in the computer. Electronic
considered. \'\1hen this capability is zoom features allow the operator to
provided, a menu instructs the user to expand a small area on the display for
enter system calibration constants on closer examination or to expand the
initial setup and a system of prompts colors for a small measurement range.
assures the operator that changes in Autoscale features provide the optimum
aperture settings, target distance, dhplay settings for any image if selected.
interchangeable lenses etc. will be fed into Three-dimensional features provide an
the keyboard each time a change in isometric thermal contour map or thermal
operating conditions occurs. Changes in profile map of the target for enhanc.ed
the corrections setting for target effective recognition of thermal anomalies.
emissivity are also monitored.
Image Recording, Storage and
In addition, digital cameras are Recovery
available to save visible images in
computer compatible format for archiving Images and data can be stored in and
with corresponding thermograms. retrieved from memory, hard disk, floppy
diskette, video tape, optical disks (writable
For most systems, the displayed compact disks and digital video disks) cmd
temperature readings are based on the Personal Computer h.,femory/Computer
assumption that the entire target surface Industry Association (PCMCIA) cards.
has the same effective emissivity. Some Commercial thermal imaging systems
syStems, however, aHow the assignment of incorporate some means, such as a floppy
several different emissivities to different disk drive or a PCMCIA card, to store
areas of the target selected by the operator images in the field. Usually, about forty
with the resulting temperature correction. images, with all accompanying data, can
A color scale or gray scale is provided be stored on a 3.5 in. diskette. Some
along one edge of the display with analysis usually can be done with
temperature shovm corresponding to each on-board software; more extensive
color or gray level in the selected range. diagnostics usually require a separate
The operator can place one or more spots c01nputer.
or crosshairs on the image and the
apparent temperature value of that pixel Options include IEEE or HS232 ports
will appear in an appropriate location on for access to additional storage and a
the display. The isotherm feature allows video recorder option so that an entire
the operator to select a temperature band measurement program can be recorded on
or interval and all areas on the target video tape. Video tapes can be played
within that band then appear enhanced hack into the system and images can be
in a predetermined gray shade or color saved to disk. Images can be stored from a
hue. frozen frame thermogram of a live target
on operator command or the operator can
Detailed processing and image set up an automatic sequence and a preset
diagnostics relies on software that allows number of images will be stored at preset
manipulation and analysis of each pixel time intervals. Stored images can be
in the thermogram presenting retrieved, displayed and further analyzed.
information in a wide variety of
qualitative and quantitative forms for the Image comparison (differential
convenience of the user. Some of these thermography) allows the automatic
capabilities are described in this chapter. comparison of thermograms taken at
different times. This includes time based
In addition to the spot measurement comparison of images taken of the same
capability discussed previously, line target, as well as the comparison of
profiles may be selected. The analog trace, images taken of different but similar
in X, Y or both, of the lines on the image targets. A special software program lets
intersecting at the selected spot will then the operator display two images,
appear at the edge of the display. Some side-by-side or in sequence; subtract one
systems allow the operator to display as image from another or one area from
many as seven sets of profiles another; and display a pixel-by-pixel
simultaneously. Profiles of skew lines can dif{ere11Ce thcnnosm111. Comparison
also be displayed on some systems. (subtraction) of images can be
Selected areas on the thermogram, in the accomplished between two images
form of circles, rectangles or retrieved from disk, between a live image
point-to-point free forms, can be shifted, and an image retrieved from disk and
expanded, shrunk or rotated or used to het\\'een a live image and an image stored
blank out or analyze portions of the in a computer's random access memory.
image. In this way, standard thermal images of
Detailed analysis of the entire image or
the pixels within the area can include
Equipment for Infrared and Thermal Testing 287
acceptable components, assemblies and (labels, d<1tes, conditions of measurement,
mechanisms can be archived and used as instrument settings etc.) as well as
models for comparison to subsequently thermograms have become rOutine. Soft
inspected items. It is also possible to copies can be made of real time images,
subtract a live image from a previous processed images, enhanced images and
baseline image for subsequent time based combined images on floppy disks, analog
thermal transient measurements. and digital magnetic tape, recordable
optical disks and Personal Computer
Database and Documentation Memory/Computer Industry Association
(PCMCIA) cards. Report preparation
Records, files, data and documents can be software allows images to be inserted into
saved in an orderly fashion. This ordinary word processing documents and
capability provides the thermographer printed by conventional laser or inkjet
with a filing system so that records of all printers.
measurement missions can be maintained
on magnetic media, including actual Making a hard copy directly from a
thermograms, time1 date, location/ stored or displayed image is done in a
equipment, equipment settings, variety of ways. A number of devices were
measurement conditions and other related introduced, before magnetic media werl'
observations. available, for directly photographing the
display screen with conventional or
}vfost manufacturers of thermal instant film. Using them generally
imaging equipment have developed report required considerable skill because
preparation software to facilitate timely ambient lighting and the screen curvature
and comprehensive reporting of the had to be considered. For this reason, it
findings of infrared surveys and other 1;vas difficult to achieve repeatable results.
measurement missions. These packages
provide templates that Jet the Online printers and plotters provide
thermographer prepare reports in reliable1 good quality copies when speed
standard word processing programs, into is not a consideration. Online printers
which tagged image file format (TIFF) and plotters are relatively slaw and
images, imported from various imaging usually tie up the computer and related
radiometers, can be directly incorporated. software during operation.
Additional diagnostic software is
customari1y provided in these packages so
that analysis and trending can be added
to reports.
Calibration Accessories
Infrared radiation reference sources are
used by manufacturers to calibrate
infrared sensing and imaging instruments
in the laboratory before they are shipped.
These same reference sources are used
later at periodic intervals to ensure
calibration stability. A radiation reference
source is designed to simulate a blackbody
radiatorj that is, a target surface with a
stable, adjustable known temperature and
a uniform emissivity approaching 1.0 at
all wavelengths. In addition to laboratory
reference sources, there are field portable
models suitable for periodic calibration
checks of fielded thermographic
equipment and for other tasks. The setup
and deployment of radiation reference
sources is discussed below.
Photorecording
Accessories for Hard
Copies
Since the advent of the personal computer
and its integration with thermal imagers,
magnetic storage and archiving of data
288 Infrared and Thermal Testing
-' ' _.'
PART 3. Interpretation of Infrared Test Results
Temperature Changes They are actually variations in
effective emissivity caused by changes
Distinguishing real temperature changes in surface configurations. An example
from apparent temperature changes is one of this is the apparent temperature
of the biggest challenges facing gradient in the far corner of an
thermographers. Thermal imaging enclosure that is at a uniform
instruments register temperature changes temperature. Geometric differences
in response to changes in radiosity at the diminish as target surface emissivity
target surface when, in many cases, there approaches unity.
is no change in real surface temperature.
To complicate matters further, external Causes of Real Temperature
mechanisms can exaggerate these Changes
misleading readings. To combat this
situation, thermographers should Real temperature changes may be caused
understand the ten basic causes of by differences in (1) mass transport (fluid
apparent temperature change- some of flow), (2) phase change (physical state),
which are only apparent and some of (3) thermal capacitance, {4) induced
which are the result of real temperature heating, (5) energy conversion (friction,
changes at the target surface. exothermic and endothermic reactions)
(6) direct heat transfer by conduction, '
Causes of Apparent Temperature convection and radiation (thermal
Changes resistance) or (7) a combination of two or
more of these causes.
Apparent temperature changes can be
caused by differences in (1) emissivity, 1. j\-Jass transport differences are real
(2) reflectivity, (3) transmissivity and temperature changes at the target
(4) target geometry. surface caused by various forms of
fluid flow. Free and forced convection
1. Emissivity differences at the target are two examples of mass transport
surface can change the target radiosity, differences. Cool air exiting an air
even on an isothermal target, and may conditioning register will cause the
give the appearance of temperature register to become cooler. Hot water
variations on the thermogram. flowing within a pipe will cause the
Frequently, these can be seen on inside surface of the pipe to become
painted metal surfaces where scratches warmer. (ThiS will result in the outside
expose bare metal that has a different of the pipe also becoming \Vanner.)
emissivity than the paint.
2. Phase change differences occur when
2. Reflectivity differences may become materials change physical state. An
apparent when heat sources external example of this is water evaporating
to the target surface reflect off low off a building surface. As the water
emissivity target surfaces into the evaporates, it has a cooling effect on
instrument. These can be point the entire surface. Thermal imaging
sources or extended sources and they equipment aimed at the building will
can add to or subtract from the register this cooling effect.
apparent temperature reading, as
discussed belov..•. 3. Thermal capacitance differences cause
temperature changes in transient
3. Trausmissivit)1 differences can be conditions when one part of a target
caused by heat sources behind the has a greater capacity to store heat
target if the target is partly transparent than another. In the thermogram of a
in the infrared range. These will only water tank (Hg. 12a) the water level
be seen if the target transmissivity is inside the tank is apparent because of
high enough and the heat source is the contrast in temperature, which h
different enough in temperature from caused by the difference in thermal
the target to contribute significantly to capacitance between water and air.
the total target radiosity. This real temperature change is also
evident in roof surveys.
4. Tm;get geometry differences are caused
by multiple reflections \Vithin recesses 4.luducetl healing differences occur when
or concavities on the target surface. ferrous metals are within a magnetic
field. Depending on the orientation of
Equipment for Infrared and Thermal Testing 289
the parts and the strength of the FIGURE 12. Thermograms: (a) indication of
magnetic field, induced currents water level in storage tank; (b) current
within the ferrous parts can cause carrying wire of twisted pair that generates
substantial heating. An example of heat to reveal insulation discontinuities;
this is when an aluminum bolt in a (c) catalytic cracker reformer vessel with
structure is mistakenly replaced with a insulation discontinuities; (d) motorcycle
ferrous bolt. If the structure is within a engine exhibits heat flow by combination of
magnetic field, the bolt may become mechanisms working simultaneously.
hot. This induction effect is exploited (a)
in the thermographic location of steel
reinforcing bars embedded in concrete (b)
structures. Here, a magnetic field is
introduced to the structure and the (c)
resultant warm spots on the
thermogram indicate the presence of (d)
the reinforcing bars.
5. Energy colll'ersiou differences occur
when energy is converted from one
form to another. Friction (mechanical
energy converted to thermal energy) is
a commonly observed example of
tenlperature changes because of energy
conversion. Another is electrical
energy converted to thermal energy
(Fig. 12b) where the current carrying
wire of a twisted pair generates heat,
revealing insulation discontinuities.
Exothermic or endothermic reactions
(chemical energy converted to thermal
energy) are further examples, typified
by the heating that accompanies the
curing of polymers.
6. Direct heat transfer differences are also
commonly observed in thermographic
survey programs. An example of this is
shown in the direct transfer of thermal
energy through the wall of a catalytic
cracker reformer vessel (Fig. 12c). The
differences in heat flow i1Justrate the
differences in thermal resistance
between good refractory material and
degraded material.
7. Thermal images of operating
equipment and systems will often
exhibit heat flow by a combination of
mechanisms ·working simultaneously.
Figure 12d illustrates the investigation
into the thermal design of a
motorcycle engine. The thermal
signature is a combination of fluid
flow (in the cooling fins), exothermic
reactions (within the cylinders)
friction (at the piston rings and within
the bearings) and thermal resistance
(in the exhaust system).
Image Interpretation
A clear understanding of pitfalls possible
in image interpretation helps the
thermographer to perform the required
tasks competently. As in the three modes
of heat transfer, these mechanisms
frequently occur in combinations.
Although the ability of the thermographer
to identify dearly the causes of
290 Infrared and Thermal Testing
temperature change in a particular target characteristics of the imaging radiometer
environment may be unnecessary when as well as the transmission spectra of glass
making measurements, it is absolutely envelopes of various thickncssc'>. Using a
essential for interpretation of results.
2.35 pm band pass filter with the
In situations ·where the thermographer instrument allows the instrument to see
is unfamiliar with the measurement tllrough the glass and monitor the
environment, a knowledgeable facility temperature of critical internal lamp
representative should accompany the
thermographer during the measurements components. Substituting a 4.8 pm high
or be available for consultation. By pass filter allows the instrument to
providing expert information concerning
the processes taking place and the likely monitor the glass envelope temperature.
sources of temperature differences, the Hgure 14 shows thermograms of the glass
thermographer will be able to anticipate
thermal behavior and better understand envelope and the internal lamp
and interpret the thermographic results. components respectively recorded in
Spectral Considerations in Product immediate sequence.
and Process Applications An important generic example of the
Many products, both simple and complex, need for spectral selectivity is in the
have complex spectral characteristics in measurement of plastics being formed
the infrared region. Spectral filtering of
the measuring instrument can exploit into films and other configurations. Thin
these complex spectral characteristics to films of many plastics are virtually
measure and control product temperature
without contact. transparent to most infrared wavelengths
For example, if it is necessary to but they do emit at certain wavelengths.
measure the temperature of objects from Polyethylene, polypropylene and other
473 to 1273 K (200 to 1000 oc; 392 to related materials have a very strong,
1832 oF) inside a heating chamber with a
glass port, or inside a thin walled glass though narrow, absorption hand at
bell jar, an instrument operating in the 2 3.45 pm. Polyethylene film is formed Jt
to 3 pm band 'Will see through the glass
and make the measurement easily. On the about 470 K (200 "C = 390 °F) in the
other hand, an instrument operating at
wavelengths longer than 4.8 pm will =presence of heaters that radiate at a
measure the surface temperature of the
glass. temperature near 970 K (700 oc
Spectral characteristics are exploited in 1290 °F).
the monitoring of incandescent lamp
temperatures during production (Figs. 13 Figure 15 shows the transmission
to 14). Hgure 13 shows the spectral
spectra of 40 rtm (1.6 x JQ-" in.) thick
polyethylene film and the narrow
absorption band at 3.45 pm. The
instrument selected for measuring the
surface of the film has a broadband
thermal detector and a 3.45 pm spike
band pass filter. The filter makes the
instrument blind to all energy outside of
3.45 pm and enables it to measure the
temperature of the surface of the plastic
film without being influenced by the hot
process environment.
FIGURE 13. Spectral selectivity for measuring surface and internal temperatures of incandescent
lamps. Transmission of various
glass envelopes 4.8 1-1m high pass filter
100 . t- r/J.2n;!,;@1@i,l;) ., / . --
90
1.5 h:!_m (b.o6 n.) ··- - f-· -
80 e.- f- -
. j_
-...~, i\i
70 --6. mm( ..
60 2.35 !Jm \II
Ij~ L_ =i I1-- -··- ·- ··-- ·
band __l ___ Spectra! response of _
50 pass thermographic imager
filter
J'40
I -lc- I- 1- -
30 l- ,1.-
20 I- ,I_. - ·- ....
10
..... I !\. . . -j
-~
2 3 4 5 6 7 8 9 10 11
Wavelength }, (!-1m)
Equipment for Infrared and Thermal Testing 291
Figure 16 shm\;s a similar solution for site line is required, additional line
13 pm (5 x JQ-4 in.) thick polyester scanners may be deployed.
(polyethylene terephthalate) film under
about the same temperature conditions. FIGURE 15. Measuring temperature of polyethylene.
Here the strong polyester absorption band
from 7.7 to 8.2 pm dictates placement of 3.45 prn 38 !Jrn (1.5 x lQ-3 in.)
a 7.9 pm spike filter in front of the same spike lilter thick polyethylene
broad band detector as that used in the
polyethylene application. 100
Using Line Scanners for 90
Monitoring Continuous Processes
"'.•~•cs 80
Continuous processes are most often 70
processes in constant and uniform 60
motion. ''Vhen this happens, an imaging
system may not be required to cover the c 50 -[
full process image. To monitor and
control processes in motion, an infrared 0 t=tl
Hne scanner can be used, scanning :~ 40
normal to the process flow, to generate a li 20
thermal strip map of the product as it 30
passes the measurement site line as shown c
in Fig. 17. If more than one measurement ~ 20
FIGURE 14. Thermograms of plasma lamp: (a) surface 10
temperature; (b) internal temperature.
5 10 15
(a) Wavelength ),, pm
FIGURE 16. Measuring temperature of polyester.
7.9 prn 13 prn (5 x 10 4 in.)
spike filter thick polyester
100
.••c~s"' 90 80
70
60
c 50
0
-~ 40
·~ 30
c
20
~ 10
5 10 15 20
Wavelength ), (!Jm)
11:56:02.92 FIGURE 17. line scanner for continuous
l'falerial research labora process monitoring.
(b) UnQ scanner
Scan tine
width
Scan sector_,..-~ Sequential
sc.:~n !ines
generate
l11ermogram
1m''"ot'm'"n
norma! to
scan!~
I
292 Infrared and Thermal Testing
PART 4. Infrared Thermographic Equipment
Operation
Because of product performance advances function of working distance and for
and meticulous human engineering on emissivity correction. Default ~ettings for
the part of manufacturers, infrared these values are normally in effect unless
thermographic equipment is far easier to the operator chooses to alter them.
operate in the twenty-first ~entury than it
was in the 1990s. It IS relatively sunple for Checking cJiibration of a thermal
the novice thermographer to turn on the imaging system in detail requires placing
equipment, aim at a target and acquire an a blackbody reference source in front of
image. Consequently, it is easy to the instrument so that it subtends a
misinterpret findings. substantial area in the center of the
displayed image (much greater than the
Preparation of Equipment instJntaneous field of view). The correct
for Operation measurement conditions must be set into
the computer where applicable (for
Even when using point sensing example, \Vorking distance:::::: 10m (33ft),
instruments, preparation for making
measurements requires an instrument ambient temperature = 298 K
operation check, a battery status check
and a simple calibration check. This (25 oc = 77 °F), emissivity= 1 etc.) and
preparation follows a simple checklist,
which is a critical element in the the temperature reading must be
successful field operation of thermal compared to the reference source setting.
imaging equipment. Equipment The spot measurement soft\vare diagnostic
preparation is crucial in field should be used if available. The detailed
measurements because of time calibration should include the widest
consumption, measurement scheduling range of temperatures possible.
and the availability of on-site personnel.
A seemingly small oversight in equipment If the instrument is out of calibration,
preparation can waste time and money. it may be possible to recalibrate it under
certain conditions. (Refer to the operator's
Calibration against a known handbook.) Othenvise, it may be
temperature reference is required for all necessary to return it to the factory for
infrared measuring instruments and is recalibration. A detailed calibration check
normally accomplished through radiation should be made at least every six months.
reference sources, also known as blackbody Periodic calibration spot checks should
simulators. These temperature controlled also be performed. Ideally, calibration
cavities or high emissivity surfaces are checks should be done before and after
designed to simulate a blackbody target at each field measurement mission and can
a specific temperature or over a specific be accomplished by means of a high
temperature range, with traceability to the quality radiation thermometer and high
National Institute for Standards and emlssivity sample targets.
Technology (NIST). Factory calibration
and traceability is provided by the To perform a spot check, place the
manufacturer. Because most quantitative target in front of the instrument. Set
thermographic instruments measure emissivity the same for both instruments
radiant enert,T)' values converted to and measure the apparent temperature
temperature readings by a computer, simultaneously with the imager and the
calibration information is usually stored ra'diation thermometer. Spot checks
in the computer software and is identified should be run at a few temperatures
with a specific instrument serial number. covering the r"'nge of temperatures
If a specific instmment calibration is not anticipated for the specific measurement
available in the software, the computer mission. Because the fields of view and
will usually default to a generic spectral ranges of the two instruments
calibration for that class of instrument. In may not match, exact correlation may not
addition to a blackbody calibration, the be possible. The errors should be
software is usually provided with repeatable from day to day, however, and
correction functions for amhlent effects the procedure will provide a high degree
such as atmospheric attenuation as a of confidence in the results of the
measurement mission.
Transfer calibration using a radiation
reference source in the field is effective
where extremely accurate measurements
are required within a narrow range of
temperatures. Typically, instrument
Equipment for Infrared and Thermal Testing 293
calibrations are performed over a broad 1. Set up the test pattern such that 8T
range of temperatures, with certain exceeds the manufacturer's
maximum allowable errors occurring at specification for minimum resolvable
temperatures within this broad range. The temperature difference.
transfer calibration can optimize accuracy
over a limited range. The procedure 2. Determine the spatial frequency It of
requires introducing a radiation reference the target in cycles per milliradian as
source into the total field of view along follows: the number of radians equals
with the target of interest with the the bar width 11' divided by the
reference set very close to the temperature distanced to the target. For example,
range of interest. Using the diagnostic 2 mm at 1 m = 2 mrad; and the spatial
software to measure the apparent frequency It= 1 cycle/(1 bar+ 1 space)
temperature differences between the = 1/(W + S). If W = 2 mrad and
reference and various points of the target S = 2 mrad, for example, then
of interest should provide improved I,~ 1/(2 + 2) ~ 0.25 cycles per
accuracy. milliradian.
The equipment checklist used in 3. Reduce the 8T until the image is just
preparation for a day of field lost (note 8T11). Raise 8T until the
measurements helps ensure that there will image is just reacquired (note 8Tc). See
be no surprises on site. A standard Eq. 4.
checklis~ should be prepared to include all
items in the thermographic equipment 4. Then change distances or use different
inventory. These should include size bar targets to plot minimum
instruments, spare lenses, tripods, resolvable temperature difference for
harnesses, transport cases, carts, batteries, other spatial frequencies.
chargers, liquid or gaseous cryogenic
coolant, safety gear, special accessories, FIGURE 18. Standard test configuration for
film, diskettes, spare fuses, tool kits, data measurement of minimum resolvable
sheets, operator manuals, calibration data, temperature difference: four bars at
radiation reference sources, seven~to~one ratio of bar height H to bar
interconnecting cables, accessory cables width W. The same configuration is used for
and special fixtures. measurement of modulation transfer
function, where 8 T = T2 - T1.
The batteries mentioned on the mission
checklist should be fully charged batteries. IV r,
It is the thermographer's responsibHity to
ensure that there is a comfortable surplus r r,
of battery power available for each field ,lH
measurement session. The fact that s
batteries become discharged more rapidly ''
in cold weather also must be considered '' '
when preparing for field measurements.
' ' '' ' ' ' ' ,,' '
Procedures for Checking
Critical Instrument ;~~;r~~n
Performance Parameters >emo•U
There are established procedures for Legend
checking the critical performance
parameters discussed above. The H"' bar height
parameters that are most important to 5""' space between two adiacent bars
most measurement programs me T1 = temperature of ambient grid
(1) thermal resolution, or minimum T2 = temperature of four slots
resolvable temperature difference (MRTD), W =o bar width
(2) imaging spatial resolution, or
instantaneous field of view (IFOV), and
(3) measurement spatial resolution
(!FOVmeas).
Thermal Resolution
Thermal resolution can be measured using
a procedure developed for military
evaluation of night vision systems. This
procedure uses standard resolution targets
as illustrated in Fig. 18 and is described as
follows.
294 Infrared and Thermal Testing
(4) liT ABS(t.7jj) + ABS(ii'I~) 2. Select distance to simulate the
manufacturer's specified imaging
z spatial resolution. The bar width H'
represents one resolution element. For
Imaging Spatial Resolution example, instantaneous field of view
can be calculated where bar width H' =
Imaging spatial resolution of scanning 2 mm and distanced= 1 m. See Eq. 6.
imagers can he measured using another
procedure that stems from military night 3. Display imager's horizontal line scan
vision evaluation protocol and uses the through the center of the bar target.
same standard bar target. The procedure
measures the modulation transfer function 4. Calculate the modulation transfer
(MTF), a measure of imaging spatial function as shown in Eq. 7.
resolution. Modulation is a measure of
radiance contrast: 5. If the modulation transfer function
(tvfTF) = 0.35 or greater, the imager
(5) Modulation \~mx Fmln meets the imaging spatial resolution
specification. (If tl1e signal
l~nax + \~11in representing the horizontal scan line is
not accessible, consult the
where Vis the voltage analog of the manufacturer for an alternate means
instantaneous radiance measured. hy which the modulation transfer
function can be verified. (In a digital
Modulation transfer is the ratio of the image, the gray level may replace the
modulation in the observed image to that voltage value.) There are
in the actual object. For any system, the disagreements among users and
modulation transfer function will vary manufacturers regarding the
with scan angle and background and will acceptable minimum value of the
almost always be different when measured modulation transfer function to verify
along the high speed scanning direction imaging spatial resolution, with values
than it is when measured normal to it. For varying between 0.35 and 0.5,
this reason, a methodology was depending on the manufacturer and
established and accepted by the purpose of the instrument.}
manufacturers and users alike to measure
the modulation transfer function of a Equation 6 shmNs calculation of
scanning imager and, thereby, to verify instantaneous field of view (IFOV) as in
the spatial resolution for imaging (night step 2, above.
vision) purposes. A sample setup is
illustrated in Fig. 19 for a system where (6) !FOV w 2 111111
the instantaneous field of view is specified 1m
at 2.0 mrad using the same setup as d
illustrated in Fig. 18. The procedure is as 2 mrad
follows.
Equation 7 shows relationship~ for
1. Set nT (where fiT~ T2 - T 1) to at least calculation of modulation transfer
1Ox the manufacturer's specified function as in step 4, above.
minimum resolvable temperature
difference. (7) MTF \~nax Fmin
\~nax + \~nin
fiGURE. 19. Measurement of modulation where dis distance to target (m), MTI~ is
transfer function, using test configuration in modulation transfer function (a ratio),
fig. 18. Fmax is maximum measured voltage (V),
\'min is minimum measured voltage (V),
H' is bar ·width (mm). The measurement
units are meter (m), millimeter (mm),
milliradian (mrad) and volt (V).
legend Measurement Spatial Resolution
VmJ• = ffi<lximum signal voltage (V) ·Measurement spatial resolution
Vmn == minimum signal voltage (V) (lFOVmeas) can be measured using a
procedure that measures the slit response
V, = baseline signal voltage (II) function (SRF) of the imaging system. This
procedure was developed by instrument
manufacturers and is generally accepted
throughout the industry. In this
technique, a single variable slit is placed
in front of a blackbody source and the slit
width is varied until the resultant signal
approaches the signal of the blackbody
Equipment for Infrared and Thermal Testing 295
reference. Because there are other errors in S. Close slit until Vweas = 90 percent of
the optics and the 100 percent level of slit Vmax and measure slit width (lY).
response function is approached rather
slowly, tl1e slit width at which the slit 6. Compute: JFOVmeas = Y\'·d-1. This
response function reaches 0.9 is usually should be equal to or smaller than the
accepted as the measurement spatial manufacturer's imaging spatial
resolution. resolution specification.
Again, there are disagreements about Again1 if the signal representing the
whether 0.9 or 0.95 should be considered horizontal scan line is not accessible,
acceptable. The test can establish whether consult the manufacturer for an alternate
the imager meets the manufacturer's means by which measurement spatial
specifications for measurement spatial resolution can be verified.
resolution.
Common Mistakes in
The test configuration for slit response Instrument Operation
function determination is illustrated in
Fig. 20. The procedure is as foJlows: Remembering a few key cautions
regarding proper equipment application
1. Set liT (T2 - T1) to at least !Ox the can help the thermographer to avoid
manufacturer's specified minimum some common mistakes. The following
resolvable temperature difference. guidelines should be observed.
2. Select distance and slit width to 1. Select an instrument appropriate to
simulate the manufacturer's specified the measurement application.
measurement spatial resolution. The
bar width W (mm) represents one 2. Learn and memorize the startup
resolution element. For example, for a procedure.
3 mrad measurement spatial
resolution, if d = 1 m, 3. Learn and memorize the default
W = (1.0 x 0.003) = 3 mm. values.
3. Display imager's horizontal line scan 4. Set or use the correct emissivity and be
through the center of the bar target. particularly cautious with emissivity
settings below O.S.
4. Open slit until Fmeas = Fmax·
S. Make sure the target to be measured is
FIGURE 20. Test configuration for larger than the measurement spatial
measurement of slit response function, resolution of the instrument.
where tJ.T = T2- T,.
6. Aim the instrument as close to normal
~ W I-+-- Adjustable s!it width (perpendicular) with the target surface
as possible.
r, . Extended surface
7. Check for reflections off the target
~I ~) I blackbody surface and elther avoid or
reference source compensate for them.
I
II 8. Keep sensors or sensing heads as far
II away as possible from very hot targets.
I
Learning the Startup Procedure
j: ,I
Learning the startup procedure
legend thoroughly is essential, particularly for
thermographers who operate several
T1 "' temperature or ambient spacer different models of thermographic and
thermal sensing equipment. Efficient
T1 "' temperature of reference source startup lets the data gathering process
Vma, =maximum signal voltage (V) begin without unnecessary delays; it saves
valuable on~site time and inspires
Vm<o> = measured signal voltage (II) confidence of facility personnel.· A quick
review of the operator's manual and a
111= slit width short dry run before leaving home base is
usualJy alJ that is required.
Memorizing of Default Values
Memorizing the default values provided
in the operator's manual is another
important contribution to time efficiency
and cost effectiveness. These include
default values for several important
variables in the measurement such as
emissivity1 ambient (background)
temperature, distance from sensor to
296 Infrared and Thermal Testing
target, temperature scale (degrees 4. Make certain that the value for
fahrenheit or celsius), lens selection and background temperature has been
relative humidity. It is important to properly entered. Then set the
remember that the instrument's data instrument emissivity control to the
processing software automatically uses knmvn emissivity of the coating and
these values to compute target measure the temperature of the coated
temperature unless the thermographer area with the instrument. ~ccord the
changes these values to match actual reading.
measurement conditions. Typical default
values are 1 m (3 ft) distance to target, 5. Immediately point to the uncoated
emissivity of 1.0 and background area and adjust the emissivity set until
temperature of 298 K (25 'C = 77 'F). the reading obtained in step 4 is
Failure to correct for these can result in repeated. This is the emissivity value
substantially erroneous results if, for that should be selected in measuring
example, the target is known to be l 0 m the temperature of this material with
{33 ft) away, is known to have an effective this instrument.
emissivity of about 0.7 and is reflecting an
ambient background temperature of 283 K Measuring and Reporting
Temperature Accurately- Filling
(10 oc =so °F). By memorizing the default of Instantaneous Field of View
values, the thermographer wm know
If true temperature measurement of a spot
when it is necessary to change them and on a target is required, the spot must
when time can be saved by using them completely fill the instrument's
unchanged without referring to a menu. measurement spatial resolution
(IFOVmeas). If it does not, some useful
Setting the Correct Effective information about the target can still be
learned but an accurate reading of target
Emissivity temperature cannot be obtained. The
simple expression, D =ad, can be med to
Setting the correct effective emissivity is compute measurement spot size D at the
critical in making temperature target plane from a working distance d
measurements. Refer to a table of where a is taken to be the manufacturer's
emissivity values when obtaining precise
temperature values is not critical. ·when FIGURE 21. Steps for determination of effective emissivity
measurement accuracy is important, it is using reference emitter technique. (See text.)
always better to directly determine the
effective emissivity of the surface to be Infrared Imager
measured using the actual instrument to or sensor
be used in the measurement and under
similar operating conditions. This is 5~---4
because emissivity may vary with
temperature, surface characteristics and '\' '\ ' '·.---------_\c'T-'~----------7''
measurement spectral band and may even
vary among samples of the same material. \
There are several techniques that may be
used to quickly estimate target effective ' '' '~'
emissivity. One, knmvn as the reference
emitter technique, can be used to Sample material
determine the emissivity setting needed
for a particular target material. The Heat
determination uses the same instrument
that will be used for the actual legend
measurement. The procedure is illustrated 1. Prepare sample.
in fig. 21 and is described as follows. 2. Apply highly emissive (oating.
3. Heat sample to uniform temper<Jture.
1. Prepare a sample of the material large 4. Set emissivity while viewing (O<Jted area.
enough to contain several spot sizes or 5. Readjust emissivity while viewing uncoated sample.
instantaneous fields of t•ielv of the 6. Apply result 5etting in further measurements.
instrument. A 100 x 100 mm
(4.0 x 4.0 in.) sample may be big
enough.
2. Spray half of the target sample with
flat black (light absorbing) paint, cover
it with black masking tape or use some
other substance of known high
emissivity.
3. Heat the sample to a uniform
temperature as close as possible to the
temperature at which actual
measurements will be made.
Equipment for Infrared and Thermal Testing 297
published value for measurement spatial Errors due to the reflection of an
resolution. For example, if the target spot extended source, however, cannot be
to be measured is SO mm (2.0 in.) and the eliminated this way. The ambient
calculated spot size, Dis 100 mm {4.0 in.), instrument hackground (what the
move the instrument closer to the target instrument sees reflected off the target
or use a higher magnification lens if surface) is the most common extended
either is possible. If not, expect the source reflection. Errors due to extended
reading to be affected by the temperature source reflections are more likely when
of the scene behind the target. Also, be the target emissivity is low or when the
sure to allow for aiming errors and target is cooler than its surroundings.
instrument imperfections. An extra 30
percent should be enough. t\·fost instrument menus include a
provision for entering the ambient
Aiming Normal to the Target background temperature if it is different
from the default setting. The system will
Aiming normal (perpendicular) to t11e automatically correct the temperature
target surface, or as close as possible to reading. This will also work if the ambient
normal, is important because the effective background is an extended source such tiS
emissivity of a target surface is partly a large boiler. In this situation,
dependent on the surface texture. It substituting the boiler's surface
stands to reason, then, that if the surface temperature for the background ambient
is viewed at a skimming migle, the setting will correct the temper<tture
apparent texture will change, the effective reading.
emissivity ·will change greatly and the
meJsurement 'iVill be affected hy Measuring of Appropriate
misleading reflections. These can result in Background Temperature Using
cold errors as well as hot errors. A safe rule the Instrument
is to view the target at an angle within 30
degrees of normal (perpendicular). If the A technique commonly used by
target emiSsivity is very high this can be thermograpllers to determine an
increased to as high as a 60 degree angle if appropriate setting for "ambient
necessary. background temperature11 requires a piece
of aluminum foil large enough to fill the
Recognizing and Avoiding total field of view of the instrument. First,
Reflections from External Sources crush the foil into a ball and then flatten
it so that it simulates a diffuse reflecting
Recognizing and avoiding reflections from surface. Next, place the foil so that it fills
external sources is an important acquired the instrument's total field of view and
skm for the thermographer. If there is a reflects the ambient background into the
concentrated source of radiant energy instrument. Allow the foil to come to
(point source) in a position to reflect off thermal equilibrium. VVith the
the target surface and into the instrument's emissivity set to 1.00,
instrument, steps should be taken to measure the apparent temperature of the
avoid misleading results. There is the foil. Use this apparent temperature
greatest likelihood of errors due to point reading as the ambient background
source reflections when the target temperature setting.
emissivity is low, the target is cooler than
its surroundings or the target surface is Avoiding Radiant Heat Damage to
curved or irregularly shaped. the Instrument
It should be noted that, although most Avoiding radiant heat damage to the
errors due to reflections are from sources instrument is always important. Unless an
hotter than the target, reflective errors infrared sensing or imaging instrument is
from cold sources can also occur and specifically selected or equipped for
should not be discounted. A common continuous operation in close proximity
source of reflective error is the reflection to a very hot target, it may be damaged
of the cold sky off glass or other reflective hy extensive thermal radiation from the
surfaces. target. A good rule for the thermographer
to follow is "don't leave the instrument
If a temperature anomaly is caused by a sensing head in a location where you
point source reflection, it can be could not keep your hand without
identified hy moving the instrument and suffering discomfort." Accessories such as
pointing it at the target from several heat shields and environmental
different directions. If the anomaly enclosures are available from
appears to move with the instrument, it is manufacturers for use when exposure to
a point source reflection. Once identified, direct radiant heat is unavoidable. These
the effect can be eliminated by changing accessories should be used to protect the
the viewing angle, by blocking the line of instrument when appropriate.
sight to the source or by doing both.
298 Infrared and Thermal Testing
Temperature Differences between 4. It is advisable to transfer some of the
Similar Materials liquid from the storage dewar to a
smaller vessel (that is, a vacuum jug)
Particularly in electrical applications, it is for more convenient filling and to
critical to measure and report temperature minimize spillage. Slowly pour a small
differences between similar components amount into the instrument's liquid
with similar surface materials, such as the nitrogen chamber and wait until
fuses on different phases of the same boiling ceases. This ensures that the
supply. Strict observance of the chamber is at the same temperature as
procedures regarding the correct effective the liquid and minimizes splashing
emissivity value, filling the measurement and spillage. Fill the chamber
spatial resolution, using the correct completely and replace the filler cap.
background temperature setting and using
the correct viewing angles, will ensure Batteries
that these differences are measured and
reported correctly. Procedures for the handling of batteries
and their safe disposal must also be
Safety and Health followed by the thermographer. In
general, these procedures are included in
Safety an.d health considerations are the safet}' regulations for each facility.
critical to successful thermography They can also often be found in the
programs as \Veil as to the welfare of the instrument operator's manuals.
thermographer and client personnel. Strict
adherence to applicable codes is the Generally, instructions for the safe
responsibility of the thennographer. It is disposal of batteries are provided in the
essential that the basics of these literature accompanying the batteries. In
regulations be understood. the absence of such instructions,
exhausted batteries should be considered
Liquid and Compressed Gases as hazardous waste and handled
accordingly.
Some instruments in the field use liquid
or compressed gases for detector cooling. Electrical Safety
The handling of these materials can be
hazardous and it is the thennographer's Failure to recognize and observe electrical
responsibility to learn safe practices and safety regulations can result in electrical
to adhere to them. In general, these shock and irreparable damage to the
procedures are included in the safety human body. Electrical current flowing
regulations for each facility. They can also through the heart, even as small as a few
be found in the operator's manuals for milliampere, can disrupt normal heart
these instruments. functions and cause severe trauma and
sometimes death. In addition, body tissue
Some instruments use liquid nitrogen can be severely and permanently
as a detector coolant. Liquid nitrogen is damaged. Shock hazards are proportional
not very hazardous but some safety to equipment operating voltage levels and
precautions should be observed. The distance from the hazard. Voltage levels as
following four guidelines for using and low as 60 \~ causing current to flow
storing liquid nitrogen are adapted from through the chest area with low skin
an operator's handbook.4 resistance can be lethal. Examples of
electric shock current thresholds and
1. Never store the liquid in sealed typical electrical contact resistances are
containers. Liquid nitrogen and given in Tables 1 and 2.
similar cryogenic liquids are always
stored in dewar flasks or the Safety practices are important as ·well.
equivalent insulated containers, with One good safety rule to follow is never to
loosely fitting covers, which allow the
gas to vent without building up TABLE 1. Electric shock current thresholds.
dangerous pressures.
Threshold Current
2. Never come into direct contact with (kA)
liquid nitrogen. Serious frostbite injury
(similar to a burn) can result if the Sensation 0.001
liquid is allowed to splash into the Pain 0.005
eyes or onto the skin. Muscle paralysis 0.010
Stoppage of breathing 0.030
3. Always replace the filler cap after Ventricular fibrillation 0.075
filling to avoid the risk of spillage and Tissue burning 5.0
condensation. Household electrical current 15.0
Equipment for Infrared and Thermal Testing 299
touch electrical contacts unless qualified Record Keeping
to do so.
Keeping thorough and detailed recC"":-d~ >~,
Arcing can also be lethal- low voltage very important to the thermographer,
equipment may produce killing arcs. It is particularly when performing a
important that only trained personnel comprehensive program of thermographic
wearing arc protective gear be permitted facility surveys. Most equipment
to approach energized equipment. manufacturers sell software that provides
Spectators should not approach at all. the thermographer with a filing system to
maintain records of all images and
Safety codes have been developed that accompanying data and comprehensive
specify the minimum distances to he report preparation software for timely and
maintained from live equipment and, in comprehensive reporting of the findings
addition, protective clothing and devices of infrared surveys and other
(face shield, protective clothing and measurement missions. Although
insulated gloves) are required in all recording the actual findings is the basic
facilities. Although the codes may vary reason for record keeping, support records
from facility to facility, they all spell out are also important. These records include
the safety rules to which thermographers equipment status history as well as
are expected to adhere. Examples of personnel qualification documentation.
National Electrical Safety (NES) codes
currently being observed in facilities in Records of surveys should be
the United States and.Canada that specify documented to include the follo·wing:
the minimum clearance zone from (1) day, date, location, identification of
operating high voltage equipment in inspection site and equipment or
terms of voltage and distance are components inspected;
described in Table 3. (2) thermographer's identification and
qualifications; (3) equipment used and
Thermographers must be aware of the calibration history (when last calibrated,
safety regulations in force and know the when last spot check was made etc.);
recommended protective clothing. It is (4) what was inspected, what was not
recommended that the applicable safety inspected and why; (S) visual test reports
guidelines be reviewed. S,6 of cracking etc. ·with photographs if
appropriate; (6) other observations noted
TABlE 2. Skin contact resistances. by the inspector, such as noise and aroma;
(7) backup video tapes of the entire
Contact Resistance measurement survey; and (8) specific
(kn) mention of any critical findings.
Finger touch, dry 50.0 All images should be maintained as
Finger touch, wet 5.0 files for future reference and trending.
Holding pliers, dry 5.0 Reports may be tailored to include only
Holding pliers, wet 2.0 those items considered significant but
Foot to wet ground, wet shoe 5.0 records should be maintained for all
Hand in water 0.3 measurements. 1vfaintenance and repair
records of all equipment and accessories
TABLE 3. Examples of specified clearance distances from should also be kept.
high voltage.
Easily accessible and easily understood
United States Canada notes and records are a measure of the
competence and professionalism of the
thermographer and lead to credibility in
the eyes of management, whatever the
industry or discipline.
Distance to Distance to
Voltage Emelo~ee Voltage EmEio~ee
(kV) m (It) (kV) m (It)
1 to 34 0.6 (2.0) 0.75to15 0.6 (2.0)
46 0.8 (2.5) 15 to 35 0.9 (3.0)
69 0.9 (3.0) 35 to 50 1.2 (4.0)
138 1.0 (3.5) 50 to 150 1.5 (5.0)
230 1.5 (5.0) 150 to 350 2.0 (7.0)
350 to 550 3.7 (12.0)
300 Infrared and Thermal Testing
PART 5. Infrared Borescopy7
An infrared borescopic imaging system range of 2 to 15 pm with aspect ratios
has been developed, combining infrared specific to application requirements. A
technology and borescopic systems preferred version of the imaging system
engineering for applications in industrial incorporates coaxial infrared and visible
nondestructive testing. A thermal image is spectrum channels within a 10 to 12 mm
produced in an objective section and is (0.4 to O.S in.) diameter tube, with an
relayed over a length of 0.30 m (12 in.) insertion depth of about 0.30 m (12 in.).
within a 10 mm (0.4 in.) diameter tube. The infrared channel is about 6 mm
(0.25 in.) in diameter.
System Design
The optimal system design wavclt'ngth
The system level design of an optimized range can he determined theoretically. 7- w
duaJ channel infrared borescope requires Given an understanding of the theory, the
integration of infrared systems system designer must assess the
engineering and borescope design. The commercial availability of camera detl:'ctor
conventional wisdom for the design of a technologies and the relative departure of
borescope is1 in some instances, opposed the camera systems from theoretical
to that of the design of more detector performance. In general, the
conventional infrared lens systems. most sensitive detectors will be quantum,
as opposed to thermal, detectors.
Like cameras, many borescopes are Therefore, Planck's law describing the
described in terms of the (ocalmunber, spectral radiant photon emittance should
which is the ratio of focal distance to he interpreted in terms of photons rather
aperture diameter. This quantity is often than watt.
referred to as F~Jmmber or f/#. Many
cameras that record visible light images An understanding of the expected
have adjustable focal numbers or offer performance for a theoretically perfect
several selectable focal number settings. staring focal plane array must include a
For a given exposure, the focal number discussion of signal~to~noise ratio. The
varies inversely with the shutter speed: present analysis assumes background
the higher the focal number is, the slower limited performance (BLIP) detectors. That
the shutter speed must be to achieve an is, the noise is fundamentally limited by
equivalent exposure. Borescopic design the mean square fluctuation in the
entails a high ratio of length to diameter number of background photons. The
and so requires an imaging system with a conventional formalism for describing
relatively high focal number, on the order signal-to-noise ratio is to introduce the
of 14 or higher. concept of noise equivalent power (NFP),
'''hich is the radiant flux necessary to give
The optimal infrared borescopic system an output signal equal to the detector
design is application specific. For noise. A more intuitive figure of merit is
example, borescopic dimensions require called the detectivity, which increases
sufficient depth of focus to allow for the with increasing detector performance. It is
generally nonplanar object field to be in defined with respect to noise equivalent
focus over the full field, thereby limiting power. 8
the focal number to the order of a focal
number of 4 or greater. Additionally, more Photoconductive detectors experience
noise from both the generation and
=demanding target temperatures of about recombimttion processes whereas
photovoltaic detectors do not undergo
300 K (27 oc 80 °F) must be considered recombination processes.9
in the optimization process. The nominal The ratio of signal to noise for a
specifications for insertion depth of quantum detector is proportional to the
approximately 0.30 m (12 in.) and total number of signal photons and
maximum diameter of between 10 and inversely proportional to the noise
12 mm (0.4 to O.S in.) place further equivalent power normalized by the
constraints on the system. energy per photon. Thus1 the
signal~to-noise ratio is proportional to the
Detection Wavelength product of the number of signal photons
and the normalized detectivity times the
Borescopic infrared imaging systems can energy per photon. The sensitivity of tht
be configured to work in the spectral mercury cadmium telluride band would
Equipment for Infrared and Thermal Testing 301
be about five tirnes that of the indium widely available and did not offer
antimonide band under ideal conditions. performance as close to ideal as offered by
indium antimonide focal plane arrays. A
A focusable camera objective transmits representative, developed infrared
images onto a high sensitivity focal plane borescopic system uses an indium
array such as indium antimonide. High antimonide focal plane array camera
Sensitivity focal plane arrays such as chosen for reasons of performance,
indium antimonide and mercury compactness and commercial availability.
cadmium telluride, coupled with efficient Focal plane array technologies continue to
closed cycle coolers, have made infrared be developed for possible application in
borescopes a practical reality. Focal plane the spectral region centered around
arrays using platinum silicide have low 9.4 pm. There are many industrial
relative quantum efficiencies and are not applications for which the target is much
sufficiently sensitive for the most wanner than room temperature -
demanding room temperature applications tor \Vhich platinum silicide
applications. Low mass indium . radiometric cameras would be effective.
antimonide focal plane array cameras of The present discussion concerns
small size have been developed. applications with target temperatures near
room temperature.
Camera Selection
A promising semiconductor based
A wavelength band centered around the infrared detector technology is that of the
temperature peak of the speCtral radiant quantum well infrared photodetector
photon emittance derivative may provide (Q,.YIP), based on absorption by confined
a benefit for resolving small temperature carriers in multiple quantum wells. 11 The
changes. For a target temperature on the devices can be tailored to match any
transition in the 3 to 20 pm wavelength
order of 310 K (37 oc = 98 °F), this range. They can be made using III~V
semiconductors based on gallium arsenide
corresponds \\'ell to the 7.S to 12 pm or indium phosphide, for which a mature
wavelength band. The stoichiometry of production technology exists. The
mercury cadmium telluride can be detectors are expected to have excellent
designed to have a peak wavelength on uniformity and thermal stability.
the order of 12 pm. Thus, photovoltaic Additionally, quantum well infrared
mercury cadmium telluride detectors may photodetectors may be configurable as
play an important role in the system multispectral imagers, which would open
optimization of an infrared borescope. up a host of new applications in
Given an understanding of the theory, the spectroscopic imaging. These detectors
system designer must assess the can offer significant improvement in
commercial availability of detectors and system performance for infrared
the relative departure of those detectors borescopy.
from theoretical performance.
There have been many inquiries as to
Indium antimonide staring focal plane how platinum silicide focal plane arrays
array technology compares favorably with or the class of uncooled thermal detectors,
ideal theoretical performance. Departures such as microbolometer detector arrays,
from theoretical performance are a ferroelectric arrays or thermopile arrays,
consequence of excess noise sources may perform with infrared borescopic lens
systems. None of these technologies
including multiplexer readout noise, f 1 ·would be expected to perform adequately
relative to indium antimonide or mercury
noise, pixel~to~pixel responsivity cadmium telluride focal plane arrays for
non uniformity, gain drift, responsivity ncar room temperature targets.. For
drift and microphonics. Several camera example, the quantum efficiency of
manufacturers offer indium antimonide p1atinum silicide is extremely poor
focal plane array cameras coupled to relative to that of indium antimonide.
closed cyCle coolers, resulting in a high The high relative quantum efficiency of
performance package tl1at is portable and indium antimonide requires that the
very reliable. One disadvantage of some integration time, for a typie<ll standard
indium antimonide cameras is that they lens focal number on the order of a focal
are not stable enough for accurate number of 1.0 to 2.5, be limited to much
radiometry. Indium antimonide offers less than a typical video frame rate. This is
excellent sensitivity within the restrictions not the case for platinum silicide focal
of its spectral response, but calibration plane arrays, which are generally
drift with time causes inaccuracies in integrated for times approaching the
radiometric measurements. Some video frame rate bec(luse of poor quantum
industrial applications do not require efficiency. An infrared borescope, with a
absolute accuracy; many industrial focal number of about 5, would have
applications, however, would welcome a between four and 25 times Jess signal
more radiometrically precise camera with incident on the focal plane, at an
sensitivity equivalent to or improved over
that of the indium antimonide camera.
In the 1990s, mercury cadmium
telluride focal plm1e arrays were not
302 Infrared and Thermal Testing
equivalent integration time, relative to about 0.30 m (12 in.}. The system can be
the standard lens. The indium antimonide configured as an infrared only lens system
focal plane array can partially make up for or include both infrared and visible
the lost signal by integrating the signal up coaxial lens systems. Figure 22b suggests
to a level approaching saturation, or the the geometry of the combined visible and
video frame rate. The signal~to-noise ratio infrared borescope system. Fiber optic
will increase as the square of the illumination fibers fill the area between
integration time. Thus, indium the infrared and visible lens tubes and the
antimonide has a clear advantage for luw outer 10 to 12 mm (0.4 to 0.5 in.)
signal flux levels relative to platinum diameter tube. The illumination bundle is
silicide. The performance of the uncooled normally connected to a high output
thermal detectors is expected to be even xenon arc lamp. The thermal and visible
worse than that of the platinum silicide. images can be viewed simultaneously on
These lower sensitivity devices may be separate monitors or combined in a fused
suitable only for high temperatme target or picture-in-picture format on a single
applications. monitor.
As mentioned above, pixel to pixel A typical horescope system is depicted
responsivity nonuniformity represents an in Fig. 22. The borescopic lens system
important consideration with respect to comprises infrared transmitting optical
temperature sensitivity. Higher levels of materials with high refractive indices,
responsivity nonuniformity, ~r such as germanium and silicon. A group
nonlinearity of nonuniformity, result in a of lenses comprising the objective lens
decrease in a camera's ability to resolve acts to form the first intermediate image
small changes in temperature while of the object. The image formed by the
maintaining good overall image contrast. Objective lens is telecentric, allowing all
This type of noise is not treated by field angles to pass through the system
theoretical analysis but could prove to be with minimal vignetting. The real image
a substantial noise mechanism for a formed by the objective lens group is then
practical imaging system. reimaged by the first group of relay lenses.
The relay groups include field lenses in
Optical Design the vicinity of the internal images. Field
lenses act to increase the numerical
A representative infrared borescope is aperture that can pass through the system
configured ·with a tube diameter of with low attenuation.
10 mm (0.4 in.) and an insertion depth of
FtGURE 22. Representative infrared borescopic system: (a) lens groups; (b) radial section
showing visible and infrared optical bundles.
(a) Field lens and
relay lens pairs
last
intermediate
image
Objective Intermediate
!ens images
group
(b)
Illumination
optical fibers
Equipment for Infrared and Thermal Testing 303
A typical high performance borcscope limits the minimum focal number to the
uses three sets of relay and field lenses to order of a focal number of 4 to 6. Thus
relay the image to the camera or eye. the signal flux on the focal plane array is
1)'pically, the aperture stop or a pupH ol reduced significantly from that which
the system is coincident with the would be expected for standard infrared
midpoint between the relay lens pair of a camera lenses. This forces the requirement
given group of relay and field lenses. for high sensitivity camera technology.
In general, camera lens systems The number of lens surfaces that must
transmit flux to the focal plane in be antireflection coated for an infrared
proportion to the square of the numerical borcscope is on the order of three to ten
aperture of the system. The numerical times greater than for a standard infrared
aperture, in the case of a borescope, is lens system. This places extremely
severely limited by the small diameter of stringent requirements on the coatings.
the lens tube. The lens system depicted in Visible borescopic lens systems can use
Fig. 22 is representative of a Hopkins rod low absorption index matching cements
lens system. H.H. Hopkins shows that the to reduce the requirements imposed on
brightness of an endoscope or borescope the coatings. Such cements can not be
scales as the square of the optical invaricml, readily applied, however, on high
which is proportional to the numerical refractive index infrared materials such as
aperture.12 The optical invariant is germanium and silicon.
consistent with the brightness theorem,
which states that the product of the iridex Stray light rejection is an important
of refraction, the ray height and the sine design issue for any infrared lens system.
of the ray angle at a given position is For a borescope comprised of twenty to
constant throughout an optical system. thirty lens elements, the problem is
The ray height and sine of the ray angle exacerbated. Integrated stray light effects
as linear functions of the lens diameter. can reduce contrast substantially.
The brightness can be shown to increase
as the square of the number of relay As in the design of any visible
groups for a flxed diameter and tube borcscope system, the bulk absorption of
length. For a given lens tube diameter and the lens materials must be low enough to
length, the system brightness can be have minimal effect on the system's
significantly improved by the use of transmissivity. This is parameter is
optical materials of index greater than 1.0 generally not important for a typical
or by increasing the number of relay infrared lens systems. l;or a borescope,
groups. For a fixed tube diameter on the however, with a total optical thickness
order of 10 to 12 mm (0.4 to 0.5 in.) for a approaching 0.30 m (12 in.) or more, bulk
typical rigid horescope, there will be a absorption effects are critical. Bulk
tradeoff between the brightness of the absorption and index of refraction
visible and infrared channels for dual changes with temperature can also be
spectrum systems. pro!Jiematic.
The design must adequately address The infrared system designer is familiar
the needs of the device's intended use: the with the concept of narcissus, which is
optimal balance of lens diameters will he manifest as a circular ring or ring~
application specific. Borescopes coupled superimposed on the image, reducing
to cameras generally use a group of lenses effective image contrast. It results when
to reimage the last intermediate image the cooled detector is imaged back onto
onto the focal plane of the detector and itself by retroreflection off of one of the
to allow for focus adjustment. The lens elements or stops. It is particularly
focusing group of lenses is not limited in problematic for systems that will undergo
diameter to the tube depth, allowing for focus adjustment, as this will perturb the
more degrees of freedom in the optical imaging of the unwanted retroreflections.
design of that group. Careful attention must be paid to the
design to minimize these effects. This is a
Borescopic Constraints on Infrared difficult task for a borescoplc system
System Design comprised of twenty or more lens
elements.
Certain design issues must be addressed to
optimize the performance of an infrared Industrial Applications for
borescopic system. The design of an Infrared Borescopy
infrared borescope is constrained by tlw
demands of both the borescope and the Infrared horescopy can find numerous
infrared camera system. applications in the industrial market. The
system will allow for high spatial and
The brighllless of a lens system varies temperature resolved thermal imagery in
as the square of the system's numerical locations that were previously
aperture or inversely as the square of the inaccessible. The system is expected to
system's focal number. The depth of focus play an important role in the future of
requirement for near object distances
304 Infrared and Thermal Testing
industrial nondestructive testing. The
more commercial applications that can be
developed, the greater the economy of
scale will drive suppliers of infrared
components and camera systems toward
cost effective volume manufacturing
processes.
There are several key industrial
applications for the infrared borescope.
Tl1e infrared borescope represents a tool
in predictive maintenance, nondestructive
testing, quality assurance, diagnostic
assessment, process monitoring and
process control. Specific applications
include aircraft engine and wing
inspection, furnace inspection, storage
tank inspection, pollution monitoring,
electrical circuit fault detection and
process monitoring, and thermal process
control, such as in the manufacture of
semiconductor materials.JJ
The discussed desigri measures 10 mm
(0.4 in.) in diameter over a length of 0.30
m (12 in.). Specific applications may
require longer insertion depths or
different diameters. Such requirements
can be- accommodated by design. The
practical limitation on length depends on
the sensitivity required for the specific
application and is limited primarily by the
antireflection coatings. Longer insertion
depths require more relay pairs for a fixed
diameter, thereby increasing the reflective
losses due to imperfect antireflection
coatings.
The capability of this thermal imaging
tool is best illustrated by example. There
is significant precedent for the thermal
inspection of electrical personal computer
boards. Thermal imagery can be used to
predict the premature failure of discrete
electrical components during operation.
Current imaging systems do not allow for
the boards to be tested in situ. The hoards
are placed in a test fixture and powered
up according to their typical operating
voltages or currents. The performance of
the board may be much different,
hmvever, inside its enclosure. The thermal
properties of the electronic board will be
impacted by surrounding boards, power
supplies, baffles, cooling air flow etc.
Borescopic imaging systems can be fitted
with a 90 degree direction-of-view
attachment to allmv the system to image
the board's components in situ and with a
wide field of view.
Equipment for Infrared and Thermal Testing 305
References
1. Kaplan, H. Practical Applications uf 12. Hopkins, H.ll. Chapter 1. E11eiosropy.
Ne\v York, Appleton-Century-Crofts
l11(rared Semi11g and Imaging Equipment, (1976): p 12-18.
second edition. SPIE Tutorial
13. Lodge, G. "IR Borescope Introduced."
Text IT34. Bellingham, WA: Infrared Imaging News. Vol. 3, No. 1.
International Society for Optical Fairfield, CT: Maxtech International
Ganuary 1997): p I.
Engineering (1999).
2. ASTM E 1256. Test Methods for
Radiation Thermometers (Sh1gle
Waveband Type). VVcst Conshohocken,
PA: American Society for Testing and
Materials (1995).
3. Gibson, C.E., B.K. Tsai and A. C. Parr.
NIST Measurement Sen'ices: Radiance
Temperature CalibraUollS. NIST Special
Publication 250-43. Gaithersburg, MD:
National Institute of Standards and
Technology {1997).
4. AGEMA Model 782 Operator~' Handbook.
FLJR Systems AB (formerly Agema),
Danderyd, Sweden.
5. NFPA 70B, Recornmended Practice for
Electrical Equipment Maintenance.
Quincy, MA: National Fire Protection
Association {1994).
6. NFPA 70E, Standard for Electrical Safetr
Requirements for Employee 1'\'orkplace.
Quincy, MA: National Fire Protection
Association (1995)
7. Brukilacchio, T.]., LJ. Bonnell and
D.C. Leiner. ''A Novel Infrared
Borescope System for Industrial
Nondestructive Testing. 11 ASNTs
Infrared Thermography Topical
Conference [Cleveland, OH, june 1997].
Columbus, OH: American Society for
Nondestructive Testing (1997):
p 79-87.
8. Hudson, R.D., Jr. Infrared Systems
Engineering. New York, NY: John Wiley
& Sons (1969): p 34-39, 348-354,
421-423.
9. Brukilacchio, 1:]., M.D. Skeldon
and R. W. Boyd.
"Generation~Recombination Noise in
Extrinsic Photoconductive Detectors."
journal of the Optical Society ofA1nerica
B. Vol. I. Washington, DC: Optical
Society of America Gune 1994): p 354.
10. Boyd, R. W. Radiometry and the
Detection of Optical Radiation. Chapters
10 and 11. New York, NY: John \'Viley
and Sons (1983).
11. Razeghi, M. 11 IR Imaging Arrays Turn
to Quantum \Veils." Photonics Spectra.
Vol. 31. Pittsfield, MA: !.aurin
Publishing Company Uanuary 1997):
p 108.
306 Infrared and Thermal Testing
CHAPTER
Techniques of Infrared
Thermography
Xavier P.V. Maldague, University Laval, Quebec,
Quebec, Canada (Parts 2 and 3)
jean Louis Beaudoin, Universite de Reims
Champagne-Ardennes, France (Part 7)
Christian Bissieux, Universite de Reims
Champagne-Ardennes, France (Part 7)
Gerd Busse, Universitat Stuttgart, Stuttgart, Germany
(Part 4)
Fran<;ois R. Galmiche, University Laval, Quebec,
Canada (Part 3)
E.G. Henneke, II, Virginia Polytechnic and State
University, Blacksburg, Virginia (Part 6)
Minh Phong Luong, Ecole Polytechnique, Paris, France
(Part 8)
Stephan Offermann, Universite de Reims
Champagne-Ardennes, France (Part 7)
Robert Osiander, johns Hopkins University, Applied
Physics Laboratory, Laurel, Maryland (Part 5)
Yuri A. Plotnikov, GE Research and Development,
Niskayuna, New York (Part 1)
Samuel S. Russell, National Aeronautics and Space
Administration, Marshall Space Flight Center,
Huntsville, Alabama (Part 6)
jane M. Spicer, johns Hopkins University, Applied
Physics Laboratory, Laurel, Maryland (Part 5)
PART 1. Passive Techniques
Thermal nondestructive evaluation is equipment or suboptimal operation.
conducted using two different approaches: Often, it is helpful to record such scans
passive and active. In active thermography, for subsequent image processing or for
heat flows are produced by way of comparison with thennograms obtained
external heating or cooling of the during earlier tests.
structure. lly perturbing such heat flows,
subsurface flaws may produce measurable Passive thermography is a simple and
surface temperature patterns that exist cost effective way of ensuring optimal
under both transient and persistent ope-ration of engineering equipment.
excitation regimes. lvfost of the techniques Faults may be quickly found and rectified,
discussed in this chapter are active and inefficient operation averted through
techniques. appropriate condition based scheduling of
preventive maintenance. The technique
In the passive approach, no external can be applied again to establish the
heating or cooling is applied, instead success or failure of any repair or
existing differences in temperature within maintenance operation.
the evaluated structure or between it and
its surroundings establish the necessary Although thermographic equipment is
temperature patterns. generally easy to use, proper
interpretation of the results requires a
Passive thermography is commonly great deal of experience. An operator
applied to assess or monitor the state of should have good knowledge about the
an industrial process or manufacturing tested structure and be appropriately
stage on the basis of temperature patterns. trained in thermal image processing.
Aspects of the measured temperature
distribution useful in assessing the Process and Product
condition of an object are its absolute Monitoring
value, its time rate of change, and its
difference with respect to a reference Passive thermographic testing is used in
value. An appealing and rather unique industry during the fabrication stage for
feature of infrared thermography is its production monitoring. It is used in
ability to provide information about a metal, paper and glass production.
target's condition during normal Numerous applications also include
operation and to provide this information welding process monitoring, failure
in real time and from a safe working analysis of printed circuit boards and
distance. testing of refrigeration and heating
installations. The results of testing are
A common configuration for passive used for real time control of the
thermal nondestructive evaluation is technological processes and for
shown in l~ig. 1. An operator is scanning classification of the fabricated
the tested structure for telltale hot or cold components.
spots that may indicate malfunctioning
The absolute value of the temperature
FiGURE 1. Observation of electrical is often of great importance for
installations using infrared imager. thermography in production monitoring.
Change of the temperature from its
normal value often indicates an
abnormality of the technological pron:'~s.
Detailed descriptions of tile most
important applications of the passive
thermographic technique are given in
another chapter of this volume.
Preventive Maintenance
In preventive maintenance the technique
is used to find components that, even if
designed and built correctly, have
degraded because of poor material quality,
308 Infrared and Thermal Testing
age or envlronmental coi1ditions. Passive diseases in dermatology, neurolob')',
thermal testing complies with the major oncology, orthopedics and
requirements of predictive engineering. It rheumatology. 6·8 Because accurate
is robust, inexpensive, user friendly, absolute temperature readings are of
effective for wide area coverage and suited concern, a dual+\\'avelcngth camera could
for repeated tests with registration of their offer some advantage over conventional
results. The applications include tests of single wavelength systems.''
electrical installations/ electronic
components, buildings and infrastructure Military
and buried piping systems.
Various military applications represent a
Detecting the presence of a failure large class of nonindustrial infrared
(subsurface leak in buried pipelines devices/ A large number of advanced
carrying water, gas, petroleum or sewage) infrared systems are used on military
can be obtained by surveying the entire platforms such as man portable weaponry,
inspected Clrea with the infrared system. 1 personnel helmets, ground vehicles, ships,
Segments identified as having an air vehicles and satellite systems. It is
abnormal temperature pattern are marked \Yorth noting that infrared systems are
as suspicious and are then subject to more classified into two categories according to
testing. Often a threshold level or a operating wavelengths: near infrared and
temperature limit is set in the infrared long infrared or thermal imagers.
camera to distinguish an exceedingly hot
or cold element from the backgrounct.2 Devices in the first category use the
image intensifier tube technology in the
Components with a normal near infrared band (750 nm to 2.5 ~m) for
operational temperature different from night vision. Relatively little ambient light
the background, are compared with a is needed for these systems to function
qualified component. properly in passive mode. lf ambient light
is inadequate an external infrared light
Sometimes/ rate of temperature change source is necess<Jry. The variety of these
is used as an indicator of health in passive devices includes several generations of
themography. Integral electronic memory image intensifiers. 10, 11 The night vision
circuits for example are examined using devices of this kind are often used for
electronic stimulation during a functional security purposes at night or in poor
test. -~ A faulty chip is one whose heating visibility conditions,
rate is outside of some specified range.
The long \Yave or thermal band (3 to
Nonindustrial Applications 1S pm) devices usc the heat illumination
for targeting and night vision. Tank,
ivfilitary and medical infrared sensitive automobile, aircraft and missile engines,
systems are the most important and well human presence and other kinds of
developed nonindustrial applications of enemy activities produce heat patterns
the passive approach. that can be detected and recognized by
the thermal scanning systems having
Medicine working principals similar to infrared
imagers for industrial applications. 12.u
Medical applications are numerous Night can be more suitable for these
because the absolute value of the human applications because the absence of
body temperature is a well known health sunlight reduces the chan<..:es of spurious
indicator. Conventional contact solar reflections. 14 This technology is also
thermometers (mercury filled or liquid used for human body surveillance during
filled glass hottles) and contemporary fast rescue operations.
pyrometers are used for temperature
measurements.
Infrared scanning imagers have
introduced a new approach for
monitoring the heat radiation from the
patients. They provide instant
noninvasive measurement of differences
in the surface temperature of the skin
caused by a variety of internal disorders.
The thermograms can be invaluable in
some cases in health monitoring and
diagnostics. During the past two dect~des
thermal imaging has been used as a
highly effective form of screening for
breast diseases and vascular disorders.4,5
Infrared thermography is hecoming a
widely accepted tool in the medical and
veterinary sector for identification of
Techniques of Infrared Thermography 309
PART 2. Pulse Thermography15
Pulse thermography is one of the most shallow and contrasts are weak. An
popular thermal stimulation methods in empirical rule of thumb says that the
infrared thermography.16 One reason for radius of the smallest delectable discontinuity
this popularity is the quickness of the test should be at least one to two times larger
relying on a short thermal stimulation than its depth under the surfilce. This rule is
pulse, with duration going from about valid for homogeneous isotropic
3 ms for high conductivity material materials. In case of anisotropic materials,
testing (such as metal parts) to about 4 s that rule is more constrained (i.e., higher
for low conductivity specimens (such as values of radius-to-depth ratio are
plastics and graphite epoxy laminates). required, for instance a value of 10 is
Such quick thermal stimulation allows reported in the case of an
direct deployment on the plant floor with aluminum-to-aluminum epoxy bonded
convenient heating sources. ivforeover, the laminate.7
brief heating (generally a few degrees
above initial component temperature) In the case of active thermography,
prevents damage to the component and various configurations are possible
pulse duration varies from about 3 ms (Fig. 2).
to 2 s).
1. Point testing is heating with a laser or
BasicaHy, pulse thermography consists a focused light beam. Its advantages
of briefly heating the specimen and then are uniform and repeatable heating. A
recording the temperature decay curve. drawback is that the need to move the
Qualitatively, the phenomenon is as test head to fully inspect a surface
follows. The temperature of the material slows down the test process.
changes rapidly after the initial thermal
pulse because the thermal front 2. Line testing involves heating hy line
propagates, by diffusion, under the lamps, heated wire, scanning laser,
surface. The presence of a discontinuity line of air jets (cool or hot).
reduces the diffusion rate so that when Advantages include fast testing rate
observing the surface temperature, (up to 1 m2.s~1 is reported) and good
discontinuities appear as areas of different uniformity thanks to the lateral
temperatures with respect to surrounding motion. A drawback is that only part
sound area once the thermal front has of the temperature history curve is
reached them. Consequently, deeper available because of the lateral motion
discontinuities \Viii be observed later and of the specimen and the fixed distance
\vith a reduced contrast. In fact, the between thermal stimulation and
observation time t is a function (in a first temperature signal pickup. Projection
approximation) of the square of the of a series of line heating strips is also
reported to detect cracks.JR
depth z and the loss of contrast C is
3. Surface inspection uses heating by
proportional to the cube of the depth. 17 lamps, flash lamps or scanning laser.
Advantages include the complete
(1) t - analysis of the phenomenon because
the whole temperature history curve is
and recorded. A drawback is the anisotropy
of heating by lamps, flashes, heat gun,
(2) c - l 3 laser or microwave. 19
where o: is the thermal diffusivity of the If the temperature of the part to
material. inspect is already higher than ambient
temperature, it can be of interest to use a
Tn infrared thermography, as seen from cold thermal source such as a line of air
Eqs. 1 and 2, because of the spreading jets (or ·water jets, sudden contact with ice
effect of the thermal front (i.e., the heat or snow etc.). Obviously, following the
deposited on the surface propagates into fourier equation of conduction, a thermal
the material in all directions with a front propagates the same way whether
dilution effect as depth increases), hot or cold: ·what is important is the
observable discontinuities are generally temperature differential between the
thermal source and the specimen.
Another advantage of a rold thermal
source is that it does not induce spurious
thermal refJcctions into the infrared
310 Infrared and Thermal Testing
FIGURE 2. Various configurations for pulse infrared camera as in the case of a hot thermal
thermography inspection: (a) by point; (b) by line; (c) by source. The main limitations of cold
surface. stimulation sources are related to practical
(a) considerations for instance, it is easier,
following the law of entropy, to heal
I rather than to cool. Moreover, thermal
coupling with the test surface is not as
L good for cold sources leading to limited
thermal contrasts.
Possible part
There are two basic arrangements for
(b) observation: (1) in reflection, the thermal
source and detector are located on the
same side of the inspected component;
(2) in transmission, the heating source
and the detector are located one on each
side of the component to inspect (Hg. 3).
Generally, the reflection approach is
used for detection of discontinuities
located close to the heated surface
whereas the transmission approach allows
detection of discontinuities close to the
rear surface because of the spreading
effect of the thermal front. Moreover if
the rear surface is not accessible, the
transmission approach is not possible.
FIGURE 3. Observation techniques: (a) in reflection; (b) in
transmission.
-(a) Applied heat
Part moving Reflected heat
(c) (b)
Applied heat
@-- 5
5
Transmitted heat
legend legend
1. Sample. 1. Sample.
2. Subsurface discontinuHy. 2. SubstJrface discontinuity.
3. Thermal stimulation source. 3. Thermal stimulation source.
4. Infrared detection and processing system. 4. Infrared detection ilnd processing system.
5. Surface temperature profile. 5. Surface temperature proftle.
6. Obse1valion area. 6. Observation area.
Techniques of Infrared Thermography 311
FIGURE 4. Pulse infrared thermography of carbon fiber Finally, in the transmission approach, the
reinforced plastic plate containing simulated fluorocarbon discontinuity depth cannot lJe estimated
resin implants: (a) two high power flashes heat surface because of the same travel distance
observed by infrared camera; (b) discontinuities 0.3 mm whatever is the discontinuity depth
(0.01 in.) under front surface with sizes (from left to right) (because the transit time through the total
20 X 20, 10 X 10, 3 X 3 mm (0.8 X 0.8, 0.4 X 0.4, 0.1 X 0.1 in.); material thickness is the smne).
(c) discontinuities size 20 x 20 mm (0.8 x 0.8 in.) inserted
(from left to right) 0.3, 1.12, 2.25 mm (0.01, 0.04, 0.09 in.) As an illustration of pulse
under front surface. thermography, l:ig. 4 shows a typical
(a) experimental setup as well as infrared
images, or themwgtams, recorded on a
(b) carbon fiber reinforced plastic plate in
which fluorocarbon resin implants of
various sizes were inserted at different
depths between plies before curing.
Heating was performed in reflection using
high power flashes with back reflector (fOJ
a total of 12.8 kj of electric energy, pulse
duration was sf't to 200 ms). Figure 4h
shows the effect of discontinuity size
whereas Fig. 4c depicts the effect of depth
from 0.3 to 2.25 mm (0.01 to 0.089 in.)
under the front surface (implant size
constant at 20 x 20 mm, 0.8 x 0.8 in.): the
three dimensional spreading of the
thermal front is clearly seen from left to
right as discontinuity edges are getting
blurred for deeper implants.
To summarize, one of the main
advantages of pulse thermography is it.s
quick deployment (results available right
after the thermal pulse). A drawback is the
limited resolution both in depth and
spatially.
(c)
;.. "-",",' .!''•''
•, .·..•.,.::·.-·<~,·,··~i'li~t:'~d
T,, T,,
legend
Tb1 = 293.8 to 304.0 K (20.6 to 30.8 oc; 69.1 to 87.4 cf)
Tb2 = 300.9 to 302.2 K(27.7 to 29.0 cc; 81.9 to 84.2 "F)
fb1"' 293.8 to 304.0 K(20.6 to 30.8 "C; 69.1 to 87.4 "F)
Tct = 293.8 to 304.0 K(20.6 to 30.8 'C; 69.1 to 87.4 °f)
T~2 = 298.2 to 307.5 K(25.0 to 34.3 °(; 77.0 to 93.7 °f)
Td = 305.0 to 307.1 K(31.8 to 33.9 ~c; 89.2 to 93.0 "F)
312 Infrared and Thermal Testing
PART 3. Pulsed Phase Thermography
Pulsed phase thermography was Longer pulses are deployed depending
introduced in 1996.20 This technique was on the specimens' thermal conductivity,
described as a link between pulsed which provides a measure of the speed of
thermography and lockin thermography, the propagating thermal waves. IIowever,
which are classical techniques for frequency distribution of longer pulses
nondestructive testing using infrared limit possibilities. The main objective of
thermography. pulsed phase thermography is to launch
thermal \\'aves of different frequencies
In pulsed thermography, a pulse of inside the sample using only one
energy is applied to the surface of the experiment. \'\7ith an ideal pulse, thermal
specimen. The heating pulse launches ·waves of all frequencies are available.
thermal waves of different frequencies in Such a situation cannot occur in practict',
the specimen. The measurement is made for a real pulse modeled by a rectangular
in the transient state and the temperature pulse has a corresponding fourier
evolution of the surface is recorded using transform (FoTr) being a sine function:
an infrared camera.
(3) Rect(l)
In lockin thermography, the specimen
is subjected to a sinusoidal thermal (4) FoTr[Rect(t)] sin (rru)
stimulation. The temperature evolution of nu
the surface is also measured using an
infrared camera but in the steady state. sine (u)
This technique launches in the specimen
a thermal wave of unique frequency For a 10 ms pulse, the highest excited
corresponding to a specific depths of frequency having more than 90 percent of
probed material. Large areas can be the maximum amplitude is 25 Hz; for a
inspected at once because heating 0.1 s pulse the maximum is 2.5 Hz.
constraints such as uniformity are not so
severe. Such an approach is efficient but Practically, just after the thermal pulse,
can be slow if different frequencies have a series of infrared images of the specimen
to be tested one at a time. In that sense, surface is recorded. The acquisition rate is
pulsed thermography testing is faster important because of the sampling
because many frequencies are analyzed theorem and the possible aliasing.21 To
simultaneously. reduce aliasing distortions it is necessary
to have a sampling frequency at least
Pulsed phase thermography was equal to twice the maximum frequetKy
introduced to make a frequency per within the specimen. Thh translate:::. in a
frequency analysis similar to the corresponding acquisition rate.
frequency analysis made in lockin
thermography from measurements The fourier transform of the temporal
obtained in pulsed thermography. This evolution of the temperature T(l) of each
frequency analysis is performed with the pixel is calculated (Fig. 5):
fourier transform.
(5) FoTr[T(t)] ~ FTT(f)
Theory
J'M
Data acquisition is the same for pulsed T(t)exp(-i2rrtt)t11
phase thermography as for pulsed
thermography: pulsed phased A(f) exp[i¢(1)]
thermography is only a piggyback signal
processing from pulse thermography where FfT(f) is the discrete fourier
measurements. Generally, one or more
lamps are used to apply a brief and transform of T(t), A(f) is the amplitude of
powerful energy pulse on the surface of the result, t is time (s) and$(/) its phase.
the specimen. For short pulses,
photographic flashes arc preferred with a
pulse in the 10 ms range and with a
power that depends on the nature of the
specimen and the distance from the
flashes.
Techniques of Infrared Thermography 313
Figure 6 is obtained from different advantages of the phase over the
simulations of a pulsed thermography amplitude are as follows.
experinwnt on aluminum. The temporal
evolution of the temperature for different Phase is less affected by heating
depth discontinuities and for a free anisotropy because all information related
discontinuity zone can be observed. to thermal pulse amplitude is included in
Figure 7 shows the phase of the fourier the amplitude of the fourier spectrum.
spectrum of the graphs of Fig. 6; Fig. 8 Because of anisotropic illumination only
shows their amplitude. It is noticed that transverse heat transfers could affect the
the maximum visibility of the phase but are generally not significant.
discontinuities is obtained for a low This is an important advantage because
frequency. This makes it possible to anisotropy of heating distribution is a
increase the length of the thermal pulse. common problem with the classical
In nondestructive testing, the main analysis of pulsed thermography.
FIGURE 5. Data acquisition and analysis for pulsed phased therrmography: (a) sequence of
thermal images; (b) temperature evolution for pixel m,n; (c) from fourier transform, phase
evolution for pixel m,n; (d) from fourier transform, amplitude evolution for pixel m,n.
(a) (c)
;
Q(m,n)
XX
XX
XX
'-::: XX ~
" " ""'""'~ (m,n)
""'·
~
F,e
9it('/)
<:v
(b) (d)
T(m,n) A(m,n)
Legend
A= amptitude (relative scale)
i,j = coordinates of display axes
m,n = indices of pixel
T = temperature
4l =phase
314 Infrared and Thermal Testing
1\·faximum depth penetration of the fluorocarbon resin inserts, each 10 mm
phase is about twice the depth (0.4 in.) in diameter, one 1 mm (0.04 in.)
penetration of the amplitude.22
deep and the other 2 mm (0.08 in.) deep.
'fo illustrate this last concept, this
Hgure 9 shows the phase for four different
technique is applied on a carbon fiber
reinforced plastic sample with two circular frequencies. The discontinuity edges are
well defined. It is also noticed that a
FIGURE 6. Temporal evolution of temperature for different depth discontinuities.
296.0 (22.8) [73.0)
295.8 (22.6) [72.7]
(22.4) [72.3]
E 295.6 (22.2) [72.0]
(22.0) [71.6)
t 295.4 (21.8) (71.2]
"1'.' 295.2 (21.6) (70.9)
il (21.4) [70.5)
295.0 (21.2) [70.2)
~ (21.0) (69.8)
~
Q.
294.8
E
2l
u~ 294.6
"~ 294.4
~
294.2
294.0 (20.8) (69.4) 0.10 1.00 10.0
O.ol Time after beginning of flash (s)
legend
;; 0.5 mm (0.02 in.)
• ;; 1.5 mm (0.06 in.)
0 = 2.5 mm (0.10in.}
" = 3.5 mm (0.14 in.)
·· -- = no discontinuity
FIGURE 7. Phase evolution for different depth discontinuities. FIGURE 8. Amplitude evolution for different depth
discontinuities.
0.6
0.4 308 (35) [95)
0.2 303 (30) (86)
0 ~ 298 (25) (77]
.'0; -0.2 "E"' 293 (20) (68)
"" -0.4 "~'-
" 288 (15) [59)
:3(
.-2
"'m -0.6
Q_
~
E 283 (10) (50)
-0.8
"
-1 278 (5) [41)
-1.2 5 10 15 20 25 273 (0) (32] 1.0 10 100
0.1 Frequency (Hz)
-1.4
0
Frequency (Hz)
legend legend
= no discontinuity "= no discontinuily
+ = 0.5 mm (0.02 in.)
o = 1.5 mm (0.06 in.) * = 0.5 mm (0.02 in.)
t>. = 2.5 mm (0.10 in.)
-- - ;; 3.5 mm (0.14 in.) o = 1.5 mm (0.06 in.)
t>. = 2.5mm(O.l0in.)
"' 3.5 mm (0.14 in.)
Techniques of Infrared Thermography 315
discontinuity's appearance is related to (7) t ~
frequency. This can be understood with
respect to the thermal diffusion length: where a is the thermal diffusivity of the
material. The fourier transform suppresses
(6) ~I ~ ~· this time information. 25 For this reason,
depth extraction is challenging in pulsed
where a is thermal diffusivity and (!) is
angular frequency. Equation 6 links phase thermography based on the fourier
detection depth and frequency of interest. transform.
In first approximation, this equation
makes it possible to estimate discontinuity Nevertheless, neural networks or
depth.
statistical techniques can be used in some
Depth Discontinuity cases to bypass this problem.26 It is
Measurement in Pulsed reported 27 that neural networks give more
Phased Thermography
than 77 percent of good depth detection
Figure 10 is obtained from a simulation of in aluminum. A statistical techniquezr.
a pulse thermography experiment in
aluminum with discontinuities of modeling the temperature, the phase and
different depths.2~ The evolution of the the amplitude as gaussian random
phase for different frequencies and processes2H to make depth retrieval allmvs
different depths is presented. This figure to reach up to 94 percent of good depth
demonstrates that the phase depth
relationship is not one to one, which retrieval, still in aluminum.
means that it is not possible to easily Another technique for the depth
retrieve the depth from the phase. This
problem is due to the fact that retrieval consists of another transform
discontinuity depth appearance depends
on travelling time of the thermal waves preserving tlle time information. As said,
up to the discontinuity and back on the the main problem for depth extraction in
surface.24 The relation between time and
depth is in a first approximation given by: pulsed phased thermography is the Joss of
the time information ·with the fourier
tr~nsform. Th: ·wavelet transform keeps
th1s temporat mformation.29 The \vavelet
transform of a temperature function T{t)
is defined as:
Js l [(8) w,(s,TrF)+~
T(il)
x fl•(t-_:rrf)11]
FIGURE 9. Phase obtained for different frequencies: ·where S is the scaling factor that refers to
(a) 0.020 Hz; (b) 0.063 Hz; (c) 0.188 Hz; (d) 3.125 Hz. the frequency, TrF is the translation factor
(a) (c) that refers to the time information needed
(b) (d) FIGURE 10. Frequency fat different values of phase versus
discontinuity depth.
0
-0.2
-0.4
U' -0.6
6.,. -0.8
•~
L -1
~
-1.2
-1.4
-1.6
0 1.0 2.0 3.0 4.0 5.0 6.0 ' 7.0 8.0 9.0 10
(0.04)(0.08) (0.12)(0.16)(0.20){0.24)(0.28)(0.32}(0.36) (0.4)
Material thickness, mm (in.)
316 Infrared and Thermal Testing
to retrieve discontinuity depth, and ll(t) is
the mother wavelet. For discontinuity
depth retrieval, the mother wavelet
selected is the morlet wavelet30 defined as:
(9)
Such a wavelet processing in pulsed
phase thermography keeps the advantages
of the fourier transform as well as the
time information necessary to retrieve the
discontinuity depth.
Conclusion
Pulsed phase thermography processing is
an interesting addition to classical pulsed
thermography processing. It combines
some advantages of both pulsed
thermography and locking thermography.
Techniques of Infrared Thermography 317
PART 4. lockin Thermography
Harmonic heat flow was first described by raster images obtained pixel after pixel
Fourier:H and used later on by AngstrOm using photothermal radiometry <lfe too
to determine the thermal diffusivity of slow for many industrial applications
solids from phase angle measurements.32 where it is necessary to monitor thermal
The wave phenomena correlated with
periodical heat injection (thermal waws) features in about 1 mm (0.04 in.) depth of
have been described elsewhere in this
volume. Under this aspect thermal waves material. On this background it was very
can be used for local probing of solids to attractive to think about techniques
determine their thermal properties and to alluwing for a reduction of testing time.
locate subsurface boundaries.
Principle of lockin
Photothermal radiometry is a Thermography
technique for remote nondestructive
testing where a small surface spot is One approach is lockin thermography
periodically illuminated by an intensity where the laser is replaced by a modulated
modulated laser beam to launch thermal lamp, which generates a low frequency
waves into the test object while the thermal wave simultaneously on the
resulting local thermal response is whole surface of the inspected
monitored by an infrared detector and component. Also the infrared detector
subsequently analyzed by a lockin monitoring just one spot is replaced a
amplifier with respect to amplitude and continuously recording thermography
phase of temperature modulation.33 If camera that monitors many spots in the
such measurements are performed in a illurninated area.:W·43 (See Fig. 11.) After
raster fashion a thermal wave image is about 1000 images have been recorded
obtained where either amplitude or phase over several modulation cycles, a fourier
of the local response can be used. It has analysis performed at each pixel results in
been found that the phase angle image is local amplitude and phase of modulation
more important because it is independent attributed to this pixel. This information
of local changes of illumination, surface derived from the set of thermographic
absorption or local thermal emission images is then presented as one pair of
coefficienfH whereas amplitude images images where the amplitude image is the
contain all this nonrelevant information.
Another advantage of phase angle images FIGURE 11. Principle of lockin thermography using optical
is that the depth range for probing with generation of thermal waves: camera monitors temperature
thermal waves is almost twice the thermal field while lamp intensity is modulated.4 3
diffusion length p:
(1 0) fl
with thermal diffusivity a and Sample
modulation frequency (whereas depth Control unit
range is only about Jl if signal amplitude
is used. 35<1H The consequence is that an Thermographic
increase of depth range by a factor of two processor
requires a reduction of modulation
frequency by a factor of four. Therefore a
phase angle image has a depth range
achieved by amplitude images only after a
factor of four more times. The acquisition
time of an image obtained pixel by pixel
depends on the number of pixels and the
inverse modulation frequency. For
materials with low diffusivity (for
example, polymers and ceramics) the
modulation cycle for application relevant
depth ranges may be about 60 s. Therefore
318 Infrared and Thermal Testing
superposition of illumination intensity1 emission coefficient because these values
optical surface absorption, thermal result in local factors that cancel in the
emission coefficient and thermal features ratio of Eq. 1 l.
while the phase image of temperature
modulation displays the thermal features. The difference between the two kinds
In this case the testing time is not given of images is visualized by the result
by the product of the number of pixels obtained on a polymer sample with a
and the modulation period time but only structured surface illuminated at an angle
a by few modulation cycles. This way (Fig. 12): whereas the amplitude image
within 3 min an image can be obtained at displays the effects of topography and of
a modulation frequency in the 10 ml-lz illumination, the phase image has just
range giving several millimeters (0.1 or one color where the noise is larger in
0.2 in.) depth range in polymer areas with less intensity. HAS
composites or ceramic materials.
The well known facts that thermal
Compared to conventional wave range depends on frequency46 and
photothermal radiometry this technique that phase has more depth range35-3S is
may be considered as spatially multiplexed confirmed in Fig. 13 where an inclined
photothermal radiometry because the whole slot milled into a carbon fiber reinforced
sample surface is monitored during one polymer plate is imaged at two different
modulation cycle and not just one point. frequencies. 47
Compared to conventional thermography,
this technique responds selectively to a FIGURE 12. Effect of test piece shape and
temperature modulation and analyzes it heterogeneities on Jockin thermograms of
only at the frequency of the optical optically generated thermal waves: (a) test
intensity modulation whose phase is the piece; (b) effect on amplitude; (c) effect on
reference. In these terms it is phase phase image of thermal waves.44.45
sensitive modulation thermography. The
term locki11 tlzermograplly relates to the (a)
device that usually performs the same
analysis of signals with respect to a (b)
reference frequency~ the device whose
coupling to the time dependent signal of
a pixel would provide the same local data
amplitude and phase. In the present case,
however, there is no hardware lockin
amplifier, its function is simulated by the
fourier transform performed at each pixel
by a computer.
If the intensity of the modulation is
sinusoidal and the resulting surface
temperature modulation as well, then the
fourier analysis means that the 1000
images mentioned above may be
compressed to only four, equidistantly
distributed over one modulation cycle. If
these images are symbolized by S1(x) to
S4(x) \Vhere x denotes the pixel address,
then the amplitude image A(x) and phase
image q'l(X) are given:42
Sr(x) - S3(x)]
arctan [ S2 (x) ~S4 (x)
{[s1(x) + s_,(x)]2 (c)
(1 2) A(x1) [Sz(x) + S4(x)lz}Yz
From the structure of these equations it
is obvious (1) that the technique responds
only to signal modulations and (2) that
that the phase image (j)(X) is independent
of local variations of illumination
intensity, surface absorption or thermal
Techniques of Infrared Thermography 319
FIGURE 13. Inclined slot milled into rear After these fundamental experiments
surface of carbon fiber reinforced polymer have confirmed that lockin thermography
plate. Images obtained on front surface with allows for thermal wave imaging of a
lockin thermography of optically generated sample at low frequencies within a few
thermal waves: (a) test piece; (b) phase minutes, it is obvious that the technique
image at 0.47 Hz; (c) phase image at is an answer to many application relevant
0.12 Hz; (d) amplitude image at 0.47 Hz; questions such as the following.
(e) amplitude image at 0.12 Hz.43,44
(a) lmm Applications for lockin
Thermography with
(0.04 in.) Optical Excitation
(c) Thermal waves are sensitive to boundaries
from which they are reflected according
(d) to the mismatch of thermal impedance.
Therefore lockin thermography with
(e) optical excitation ("OLT"= opticallockin
thermography) is applicable to
nondestructive evaluation of components
where boundaries or their integrity need
to be monitored, e.g., layered materials.
Coatings
For coatings the topics of interest are
thickness and the detection of local
disbands. Areas ·where such questions
arise include paints, veneered wood and
ceramic coatings on metal.
Measurement of paint thickness is no
problem if the substrate is metal so that
capacitive or inductive techniques art'
applicable. However, such techniques fail
if the substrate is a polymer. Though in
such a case the substrate and the coating
are very similar, the difference in thermal
wave impedance is usually still strong
enough for thermal wave reflection and
thickness effects. Figure 14 shov..'s the
phase angle image obtained with lockin
thermography on a sample where paint
thickness varied stepwise in the range of
IS to 74 pm.47 The steps are clearly
revealed and the plot of observed phase
versus known thickness provides a
calibration curve that allmvs for thickness
determinations with an accuracy of about
±5 pm (2 x 1Q-4 in.). However, it should
he mentioned that such a calibration is
specific for the kind of paint and the kind
of polymer substrate. So it must be
established for each combination. As the
technique does not require physical
contact with the inspected area, it allows
also for inspection of paint while wet or
sticky. Of course the mentioned
calibration curve is affected by the drying
process. Because the thermal wave
reflection depends strongly on the
boundary situation an image of surface
contaminations (for example, a
fingerprinC grease or corrosion) can be
obtained even after the paint has been
applied.
Paint not only is decoration hut also is
supposed to protect the substrate
320 Infrared and Thermal Testing
underneath. This is particularly true for relative to each other while there is still a
hard coatings, such as ceramics, on metal. force acting between them. Quality
In this case the interest is in the detection control is interested to detect
of disbands, where damage will occur. An deterioratiOn at such an early state. Also
example is discussed below together with this topic will be discussed again later.
results obtained '~Nith internal thermal
\\'ave generation. Laminates
Loss of adhesion is also important in Compared to metals, laminates have a
the wood industry where veneer or a high specific strength hut do not suffer by
polymer foil is applied to fiber boards corrosion. That is why they are used for
more homogeneous than wood and aerospace components where safety is
cheaper. Lockin thermography is suited to critical. So any material changes due to
identify such discontinuities by their production or use must be detected early
boundaries.48 An example for the enough to prevent failure.
detection of a disbanded area is shown in
Fig. 15. An intermediate situation during Laminates consist of fibers (typica1ly
the development of disband may allow glass or carbon) embedded in a polymer
for motion of a substrate and coating matrix. The fibers provide the strength;
the matrix keeps them together and
FIGURE 14. Painted polymer surface: remote determination of determines the shape of the component.
Therefore fiber orientation is one topic
paint thickness with phase angle image of OLT: (a) thermal that needs to be monitored even if the
image; (b) measurements.44.45 laminate is covered by paint. If heat is
injected into a point, the temperature
(a) field spreads out according to the thermal
anisotropy49 correlated with fiber
15 pm 37 ~m 60 pm 74 pm orientation. For periodical injection the
thermal wave propagation is described by
{5.9 X 10-4) (1.5 X lo-l) (2.4 X Jo-l) (2.9 X 1Q·3) an angular dependence of thermal
diffusion length that results in an elliptic
_____.,.__...,...llllll...,..,llllllllllllllllll Paint shape of the equaphase field. The ratio of
the axis lengths is proportional to the
Polymer square roots of thermal conductivities
along these directions:t9 However, this is
(b) IL true only for very thick samples, as is
' shown in I;ig. 16 where a modulated laser
72 I beam was split by a two-dimensional grid
l' and focused on a carbon fiber reinforced
71 plastic sample. The two-dimensional array
of modulated focal points resulted in
v~ 70 thermal wave point sources on the sample
that consisted of two mutually
~"~' perpendicular carbon fiber directions in
carbon fiber reinforced plastic laminate
&ro 69 0/90 with varying thickness ratio
(Fig. 16b). The eccentricity in the
~ observed array of elliptical phase fields
~ depends on the local thickness of the
upper laminate. Therefore not only fiber
68 orientation can he determined but also
the thickness of the layer with this
67 orientation.50-52
0
FiGURE 15. Delamination of 0.8 mm (0.03 in.) thickness
polymer layer on fiber board.
Delamination
20 "-~~~- 60 -I
(0"8) (2A)
40 80
(L6) (3" 1)
Thickness, pm (1 o-3 in.)
Techniques of Infrared Thermography 321
Also the thickness of material is filled areas would result in bright spots.s4
important. As an example the nose Lockin thermography makes it possible to
section of a small airplane is shown monitor the intev.rity of structures
(Fig. 17). The phase angle image ignores remotely and quickly. It should be
the optical structure and shows only the mentioned that the intensity of optical
areas where the glass fiber reinforced illumination corresponds to the sunshine
layers overlap.53 at noon in summer.
Besides fiber orientation and laminate For metal airplanes, integrity
structure, another topic of interest is monitoring relates also to corrosion and
disbanding that may be initiated by the tightness of screws and bolts. It is
excessive load or impact. It is necessary to possible to monitor both with lockin
detect such discontinuities during thermography because of the loss of
maintenance inspections to prevent thickness or to the change of thermal
failure. contact between the joint metal plates. As
an example, Fig. 18 shows the phase angle
Loss of stringer adhesion (which means image of two metal plates pressed
dangerous loss of stiffness) would be together by screws fastened at different
identified by an interruption of the lines torque levels..ss The phase angle profiles
vd1ereas impact, delamination or water along lines AA and BB display maxima
that can be correlated to the torque. Such
FIGURE 16. Point array for determination of local fiber a calibration qlfvc (valid only for the
orientation in 0/90 carbon fiber reinforced plastic laminate: specific metal-to-screw system) obviously
(a) setup; (b) sample geometry; (c) lockin thermographic allows for the determination of previously
phase image of optically generated thermal waves.43,44 applied torque with an accuracy of
±10 percent. Similar investigatiOns ·were
(a) performed for bolts with a similar result. 55
In this way the local loss of compressive
force in rows of screws or bolts can be
identified remotely.
laser 0 FIGURE 17. Lockin thermography of optically
generated thermal waves to evaluate
(b) Fiber structural integrity: (a) photograph of
orientation graphite fiber reinforced plastic airplane with
90 degrees dashed lines in nose area; (b) phase angle
image of subsurface features.54
(a)
(b)
Fiber orientation 0 degrees
(c)
322 Infrared and Thermal Testing
FIGURE 18. Remote lockin thermography of torque in array of Lockin Thermo~raphy with
screws using optically generated thermal waves: (a) phase Internal Excitatron
image of 2 mm (0.08 in.) thick steel plates pressed together
by screws fastened at different torque levels, with In all these cases the test object is
illumination intensity modulated at 0.06 Hz; (b) phase irradiated by an intensity modulated
profiles taken along lines A-A and B-B; (c) phase dependent lamp. On the surface of the illuminated
on applied torque level.55 component, absorption generates a
temperature modulation that propagates
(a) as a thermal wave into the component
where it is reflected at subsurfarC'
A boundaries. Superposition with the
original wave causes a phase change that
(b) indicates the hidden thermal feature (see
Fig. 19a). Therefore both discontinuities
237 and intact stmctures contribute to the
image, discontinuities can be revealed
236 50 N·m only by comparing the observed features
with expected features provided by theory
40N·m or by a reference sample. Discontinuity
detection would be much easier if a
235 mechanism were involved where the
feature of interest responds selectively so
-;;- 234 l~ u that the image ·would not contain the
confusing background of the nonrelevant
~ intact features. This advantage knmvn
from dark field techniques in optics
e~n 233 makes testing easier and more reliable.
Besides this, the internally generated
~ thermal wave travels only half the
distance involved in surface heating, so
~ 232
FIGURE 19. Comparison of two means of generating thermal
~ 231 waves for lockin thermography: (a) thermal wave generated
.rco optically on whole surface; (b) thermal waves generated
~ selectively in discontinuity (with enhanced loss angle) by
230 absorption of amplitude modulated ultrasound.
229 20Nm J -iONm__ '~ (a)
228 f _"_,JQ m
51 101 151 201
Pixel
(c)
237 SON"!
236 40 N-m
30N·m
23S
30N·m
~ 234 i
10000 15000 20000 25000 30000
~ 233
en 232 Discontinuity
~ Sample
~ 231
~ 230 (b)
.rco 229
~
228
227 I ON·m Ultrasonic
226
wave source
0 5000 ltM
Sample
Force (N) ~ ···········"~Discontinuity
Legend ) . ····..... ...···>:-"
A,B,A',B' = Points \. :-::······· .···
-fr--- = Phase profile B-B'
........Q--.- = Phase profile A-A' Legend
.....-{]-- = Plate one
- - = optical wave
-+- = Plate two ·-----=elastic wave
············· =thermal wave
Techniques of Infrared Thermography 323
there is less attt>nuation and a better thermal wave generation by periodical
resolution (Fig. 19b). As there is no loss angle heating with clastic waves at
superposition involved, l.Nhich is the ultrasound frequencies. These high
reason for depth range limitation if the frequencies are very efficient in heating
surface is illuminated, the detection of the because many hysteresis cycles are
thermal wave on the surface is only noise performed per second (the area of
limited. The phase angle found at the hysteresis is heat energy per volume, so
surface is proportional to the distance area per time is heating power per
traveled, which is the depth of the source volume).
underneath the surface. In addition, such
a technique would be more economical The technique, called ultrasonic lorkin
because not the whole component is t/lermosrapll;~ is suited to reveal the
heated but mainly the discontinuities in following, for example: (1) cracks in
it. metals and ceramics, (2) impact damage
and disband in laminates, (3) corrosion in
Activation of internal thermal waves metal and (4) deterioration of smart
can be achieved using various techniques. structures. Therefore it is of considerable
If the structure of interest is electrically interest in the field of nondestructive
conductive, modulated electrical heating testing for fast discontinuity detection.
is possible. This is convenient to monitor
the structure of electrical heaters (or heat It should be mentioned that the images
generating electronic components), their provided by the two techniques (optically
depth or their thermal coupling to other generated thermal waves and ultrasonic
components. 47 As compared to lotkin thermography) are based on
conventional thermography of heated substantially different mechanisms.
electronic systems, it should be \•Vhereas optical lockin thermography
mentioned that spatial resolution is better provides images of modulated heat flow
because an image is obtained not of hot and its perturbation, ultmsonic lockin
areas but only of areas where the thermography gives an image of local loss
temperature is modulated. This is only angle or hysteresis. The magnitude image
near the thermal wave source because is a mixture of hysteresis area and its
thermal waves are highly attenuated. depth underneath the surface whereas the
Internal generation of thermal waves can phase image displays only depth. So there
also be achieved by the modulated flow of is some relation to ultrasound imaging
gas or liquids that have different based on the real part of elastic properties
temperatures. This phenomenon is of whereas ultrasonic lockin thermography
interest for the testing of subsurface uses the imaginary part. As the speed of
tubing (such as heaters in the floor) or elastic waves is much higher than the
channels. The technique is also applicable speed of low frequency thermal waves it
to image selectively areas of blood flow can be assumed that thermal waves are
underneath the surface of the body.56 generated simultaneously everywhere in
the sample according to the amplitude
Principle of Lockin Thermography modulation of the elastic wave.
~vfechanical resonances of the inspected
Using Elastic Waves
FIGURE 20. Setup for lockin thermography with ultrasonically
Mechanical discontinuities differ from modulated internal excitation.57
their surroundings by their mechanical
weakness. They may cause stress Sample
concentrations and under periodical load
there may be friction in cracks and Ultrasonic
delaminations. Therefore a discontinuity transducer
may be an area where mechanical
damping is enhanced so that ultrasound Thermographic
is attenuated because it is converted into processor
heat more efficiently. If the amplitude of
the injected ultrasound is modulated at a
low frequency, the discontinuity acts as a
local thermal wave source thus revealing
itself like in dark field optical imaging.
Though the technique is similar to
ultrasonic testing it should be pointed out
that, in the thermal technique, the
ultras.onic transducer does not scan across
the sample but is attached at a spot from
wllich the acoustic waves are launched
into the whole volume where they are
reflected until they disappear preferably in
a discontinuity and generate heat
(Fig. 19b). So this is low frequency
324 Infrared and Thermal Testing
component are avoided because they because the injected ultrasound wave may
result in a lwterogeneous response arross be multiply reflected inside the inspected
the <:.urface. component so that all directions are
equivalent, resulting in excitation of
The experimental arrangement used for cracks with any directions.
ultrasonic lockin thermography is shown
in Fig. 20.57 The arrangement is similar to Thermal wave emission occurs only if
the one used previously for pulsed or the crack is not open. This is important
transient sonic heating thermography.ss for the interpretation of observed features
The main difference from the in ultrasonic lockin thermography images.
arrangement in Fig. 11 is that the acoustic Figure 21 displays images obtained on
transducer is attached to the sample vertical cracks in metal (Fig. 2la) and
\Vhereas the synchronization output of ceramic (Fig. 21b).57 Though in both cases
the lockin thermography system controls the discontinuities are selectively imaged
the acoustic amplitude and not the (similar to dark field microscopy), in the
intensity of the lamp. However, the power metal tensile sample the strongest signal
may be similar in both cases. is found at the crack tips (see arrnws)
whereas in the ceramic cup the whole
To avoid confusion, it should be crack is displayed. llecause this material is
mentioned how ultrasonic lockin not ductile the crack does not open. The
thermography differs from two other
techniques where thermal effects of FIGURE 21. Crack detection with lockin
periodical mechanical loading are thermography with ultrasonically generated
monitored. thermal waves: (a) in metal; (b) in ceramic
cup. 57 Only crack tips are displayed in
1. Vibrothermography is a technique ductile material (see arrows). Bright spot
where the average temperature field of between them is hole where crack started.
an oscillating component is imaged
using thermography.59 As the (a)
amplitude of oscil1ation is not
modulated, there is no thermal ·wave (b)
emission involved, so the resolution is
different from ultrasonic lockin
thermography.
2. The stress pattern analysis by thermal
emission (SPATE)60 uses low frequency
mechanical loading at constant
amplitude to generate an oscillating
temperature field \Vhose modulated
part is analyzed. However, in this case
the involved s~ress fields are
monitored by using the thermoelastic
effect01 that is reversible and linear in
the trace of the stress tensor.
Applications of Ultrasonic
Lockin Thermography
Cracks in Ceramics and Metal FIGURE 22. Lockin thermography using
optically generated thermal waves for
In ceramic components cracks may be detection of fatigue crack in riveted
caused by wrong sintering, surface aluminum structure measuring 300 x
treatment or excessive local load. The
early detection of cracks is important 200 mm (12.0 x 8.0 in.)''
because their propagation may result in
sudden failure of the whole component.
This is also true for metals. Cracks differ
from their surroundings by the presence
of two surfaces that can rub against each
other under oscillating load. The heat
generation depends on the product of
stress perpendicular to the boundary and
the relative velocity. The orientation of
cracks is relevant for lockin thermography
using optically generated thermal waves,
where the injection of heat flow has only
one direction. Crack orientation is less
critical for ultrasonic Jockin thermography
Techniques of Infrared Thermography 325
open crack would be found ·with optically part is the small area where the boundary
generated thermal waves, so the change is strong enough to he detectable
combination of optically generated using optically generated thermal waves.
thermal waves with ultrasonic lockin However, the area around it does not yet
thermography makes it possible to result in a boundary modification but
distinguish between open and closed rather in a locally enhanced mechanical
crack areas. llmvever, any crack whose tip loss angle thereby revealing the whole
is closed can be revealed by ultrasonic area of damage.S7
lockin thermography. A realistic example
is the riveted structure in Fig. 22 where Laminates are multilayered materials
the length of a fatigue crack along the ·where interlaminar strength is important.
rivets was determined using eddy current Impact damage causes a local damage
testing. However, with ultrasonic lockin from which subsequent delamination may
thermography it was found that the crack expand to critical sizes. Therefore early
was much longer.62 detection of impact damage in safety
relevant laminates is important for safe
Delamination and Impact in operation of structures. Figure 24 shows a
Layers or Laminates carbon fiber reinforced polymer plate with
seven impact damage locations selectively
The decorative or protecting effect of detected using ultrasonic lockin
coatings is acl)ieved only if the coating is thermography.63
firmly attached to the substrate. Relative
motion with respect to each other will The ultrasonic lockin thermography
end up in fatigue and finally disband. So technique is applicable also to smart
the relative motion in the presence of structures that contain embedded
partial adhesion (which presents buckling) actuators for shape contro!J•2,6J Early
may be considered as an early stage whose detection of disband is essential for the
detection is a maintenance task. safe operation also of such structures
increasingly being used in aerospace
As an example, Fig. 23a shows the applications.
ultrasonic lockin thermographic image of
veneered ·wood with bonding Corrosion
discontinuities in a rectangular area.63
For metals, another problem besides
Another example for monitoring cracking is corrosion, difficult to detect
disband at an early stage is a ceramic between riveted plates. As corrosion
coating on metal (Fig. 23b). The central results in the reduction of cross section
and in corrosion products, the local
FIGURE 23. lockin thermographic images of mechanical behavior under the oscillating
bonding discontinuities, using ultrasonically load of an acoustic "\Vave is modified so
generated thermal waves: (a) amplitude that lossy relative motion is possible. This
image of wood panels;63 (b) phase angle
image of zirconia coated metal (carrier fiGURE 24. lockin thermographic techniques
frequency 20 kHz, amplitude modulation at on carbon fiber reinforced plastic sample
0.06 Hz)Y with seven impact points: (a) phase image,
optically generated thermal waves;
(a) (b) amplitude image, ultrasonically
generated thermal waves; (c) phase image,
ultrasonically generated thermal waves.63
(a) (b) (c)
(b)
326 Infrared and Thermal Testing
interpretation is consistent with the result eliminates sensitivity to artifacts.
shown in Fig. 25 where the images Therefore the technique is applicable for
obtained with optically generated thermal industrial tests even under rough
waves and ultrasonic 1ockin conditions.
thermography are compared on a riveted
and corroded aluminum structure Generation of thermal waves can be
(airplane component) of 4 mm (0.16 in.) performed optically from the outside by
thickness. Lockin thermography using periodical illumination (optim/Jockin
optically genen1ted thermal waves reveals thermography) or selectively at
the differences of thermal diffusivity discontinuity enhanced loss angles inside
caused by the rivets and to some extent the component by injecting amplitude
by corrosion.62•6J Ultrasonic lockin modulated acoustic waves (ultrasonic
thermography is remote loss angle lockin thermography). The resulting
imaging using the correlated thermal images are substantially different
effects1 so the image is dominated by concerning the mechanism of contrast
corroded areas and not by rivets. generation.
Conclusion The examples show that thermal waves
with optical excitation are applicable for
Lockin thermography combines the nondestructive maintenance testing of a
advantages of photothermal radiometry wide variety of structures ranging from
(phase information and adjustable depth the human body to painted surfaces,
range) and thermography (speed of veneered wood, turbine blades1 ceramics
imaging and ease of operation) but avoids and airplanes where impact damage,
their specific disadvantages. Phase angle hidden corrosion and loss of adhesion can
images with application relevant depth be detected. As compared to coJwentional
range are obtained within typically 3 min ultrasonic testing/ the advantage is that
where the modulation technique no coupling medium is required for
thermal wave detection. The power
FIGURE 25, Detection of corrosion in riveted aluminum density caused by optical illumination is
stringers using lockin thermography at 0.03 Hz: (a) sample the same as the one on a sunny summer
geometry; (b) ultrasonically generated thermal waves; day.
(c) light optical excitation of thermal waves.62,6l
'·Vith acoustic excitation of the sample,
(a) the advantage of selective discontinuity
heating is obvious: automatic
Rivets discontinuity detection is possible with a
much higher reliability and the energy is
(b) used in a very efficient way only for
discontinuity heating.
Qff!)·,f!::,:,~~·.['':;~·~::;:'\····:~"::~:':
Though the applicability of pulsed
L_>H :J~:!:\i.'• .;~~;;:- acoustic fields58 instead of sine shaped
modulation is obvious, the preliminary
Ultrasonic comparative measurements indicate that
excitation the load on the test object must be
substantially higher to achieve the same
(c) signal-to-noise ratio.M So the criteria to be
applied are the same as for the choice
between pulse thermography and lockin
thermography with optical excitation.
The problem that still needs to be
solved is the injection of high power
ultrasound. VVhile optical excitation of
thermal waves allows for testing of 3 to
5 m2 (30 to 50 ft 2), testing using
ultrasonic lockin thermography is
presently limited to areas of about 0.3 m 2
(3 ft2). Future activities will concentrate
on this problem and on the inversion of
the obtained results for discontinuity
characterization.
Techniques of Infrared Thermography 327
PART 5. Step Heating
In most infrared radiometric techniques current image processing equipment allow
of nondestructive testing the sample cools time resolved algorithms previously used
after pulsed heating. In contrast, the only on single-point measurements to be
technique of time resolved infrared applied to full images. It should be noted
radiometry with step heatiug65,66 follows the that time resolved infrared radiometry
surface temperature rise as a function of with step heating requires wavelength
time during the heating pulse. This separation between the heating source
approach allows identification of and the detector. This wavelength
subsurface features and determination of separation usually requires a laser source
thermal properties with the same speed as that provides enough power to
other thermal techniques but keeps the homogeneously heat an area of typically
required heating power and resulting about a square foot. Laser diode arrays are
surface temperature small. This permits now viable for this as their power levels
heat sources such as microwaves and have increased in the last few years. The
radio frequency induction heating where requirement for wavelength separation is
high peak power often is not available. not a problem for nonoptical heating
One of the most attractive features of time sources such as micra·waves and inductive
resolved infrared radiometry is the ability heating.
to calibrate the temperature response at
early times, when the sample is thermally A significant challenge associated with
thick. This allows correction for fast infrared cameras is the enormous
heterogeneous heat source distributions amount of data to be dealt with, typically
and differentiation between backing on the order of 16384 bytes per image for
materials. A fast algorithm has been a 128 x 128 focal plane array. \-\1hile most
developed to calculate thermal transit scientists working in the field have
times and therefore generate quantitative developed expertise for viewing time
depth images of subsurface features. sequences of infrared images and can
Below are des.cribed infrared radiometry discern features by looking at successive
with step heating and the (malysis of its images, a typic3I aircraft inspector, for
time response, including the calibration at example, will not have this same
early times. extensive background. Therefore, an effort
must he made to make time resolved
In time resolved infrared radiometry infrared techniques more accessible by
with step heating the surface temperature reducing the infrared time sequences to
of a sample is monitored via infrared single images showing the spatial
emission during application of a heating distribution of properties and
pulse from an optical, microwave or discontinuities. Such images can reduce
induction source. While thermal the variability in interpretation during a
nondestructive testing techniques test procedure in the field.
generally concentrate on discontinuity
detection and imaging, time resolved Data Analysis for Step
infrared radiometry with step heating has Heating
also been used successfully to determine
material parameters such as thermal To derive thermal properties from time
diffusivity and thickness. This has allowed resolved, step heating measurement, the
information about material structure such temporal dependence of the surface
as presence of corrosion, porosity or voids temperature during heating is compared
to be obtained. Time resolved infrared to an analytical model. The thermal
radiometry with step heating results in properties of interest are the thermal
smaller heating intensities compared to conductivity k or thermal diffusivity o.:
the more common short pulse
techniques.6N>9 Furthermore, the prompt (13) u ~ -k
response of temperature versus time
ano·ws self-calibration to be performed for cp
each pixel in the image, thus correcting
for emissivity and heating beam intensity where c is the specific heat and p is the
variations. The development of infrared density. For a layered system as shown in
focal plane arrays with their full field Fig. 26, with a surface layer of thickness L
imaging capabilities at high speeds and
328 Infrared and Thermal Testing
and properties ao. Co and Po and a backing Therefore it is advantageous to plot the
temperature as a function of the square
layer ·with properties a 1, c1 and p11 the root of time to sec deviations from the
surface temperature for step heating is bulk behavior, a straight line when
plotted versus square root time. Figure 27
given as a function of time t.71 shows the temperature rise for step
heating as a function of the square root of
(14) T5(0,t) JFP0 [ 1- R(i.) normalized time HLZ·a-1)-1. LZ·a-1 is a
characteristic time scale for a layered
~oPon system) for a range of thermal mismatch
factor 1.
x { 1 + 21~(-r)"
To demonstrate the effect of a
x[exp(- ~~~:) subsurface discontinuity -ln this case a
layer with different thermal properties
;£fd~1 collected in the thermal mismatch factor
ErFc( 1)]} - the ratio between the temperature rises
for a two-layer sample and a bulk sample
where J~~~- -~ are also shown in the group of curves at
(15) 1 the top of the graph. The temperature rbe
~!PI - JKoCoPo initially increases as the square root of
time and then deviates after a
where Po is the intensity of the light characteristic time, (.{L2·a-1)-1 = 0.5. Note
modulated with frequencr, R(i,) is the that this thermal transit time is not
wavelength dependent reflectivity at the affected by the value of the thermal
mismatch factor and only depends on the
surface, r is thermal mismatch factor or surface layer thickness and thermal
diffusivity. For early times diffusion occurs
thermal reflection coefficient, r = -Y(Kcp) is only in the top layer and the response is
that of a bulk sample.
thermal effusivity, and ErFc(x) = 1 -
ErF(x), where ErF(.x) is the error function. As time increases, tile diffusion extends
into the substrate and the temperature is
For a bulk sample of infinite thickness, for reflected at the interface. This is described
very short times or for very thin layer by the summation in Eq. 14, which
thicknesses, the terms in the sum vanish FIGURE 27. Temperature rise for step heating as function of
square root of normalized time.
and the temperature increases as .Yt.
32
FIGURE 26. Schematic for photothermal 2.5 n
experiment with two layers in
one-dimensional geometry. ·E "'0 0
OJ
Po .,c" 2 ~
cOrJ.
Legend ~ -1 0
c "' specific heat
L "' surface layer thickness (m) ·" 2
a oo thermal difft.isivity
p =density •~ 1.5
·~
~
1•"0'1.
E
!"- 0.5
0 0.5 1.5
0
Square rool of normalized time (\Is)
legend
A. -1.0 thermal mismatch factor.
B. ·-0.8.
c. -0.5.
D. 0.0.
E. 0.5.
F. 0.8.
G. 1.0.
Techniques of Infrared Thermography 329
represents a number of successive determining~ ::: .J((Lui)·L-1• A faster
temperature reflections at z = L. The algorithm can be generated when looking
magnitude of this reflections is governed at the inverse function to determine
by the thermal mismatch factor r. r is ~(T,r). For example, because T{r == -1,
~ = 1) = 0.18, J}-a-1 = tat the time when
negative when the backing layer has a the normalized temperature reaches 0.18.
This algorithm allows a fast determination
much lower thermal effusivity t = Y(kcp) of L2-a-1 at each pixel position. The
algorithm can be extended to calculate
(for example, for a disband consisting of the thermal mismatch factor I for known
air behind a metal layer, r = ~1); r is 0 if top layer properties. Examples of the
application of this algorithm will be given
both layers have the same effusivity; and in the next section.
r is positive when the second layer has a Applications of Step
Heating Technique
much higher effusivity (for example, a
The data analysis algorithm described
ceramic coating on a metal). above can be used to generate images of
Equation 14 is the exact solution for subsurface discontinuity depths in
samples with known (air) backing or to
the case of an extended1 homogeneous characterize the backing materials for
optical heating source absorbed at the samples with knmvn top layer thickness
surface and referred to as one*dimensional and thermal diffusivity. To demonstrate
the viability of the approach outlined
heat flow. Under the assumption that above for measuring thermal transit times,
spatial variations of the light source and measurements were performed on a test
sample consisting of flat bottomed holes
discontinuity distribution are sma111 it can milled in a 12 mm (0.47 in.) thick
be applied to each single point of the graphite epoxy composite panel. The
sample monitored by the pixels on the diameters of the holes were 12.4 mm
(0.49 in.) and 18.3 mm (0.72 in.) for a
camera. A data analysis algorithm has series of different depths, 0.81 1.4 and
been developed that allows application of 2.4 mm (0.03, 0.06 and 0.09 in.). An
expanded carbon dioxide laser beam was
Eq. 14 pixeHo-pixel for calibration of used as the heating source. Figure 28a
variations in the optical properties of the shows the mw image data obtained after
sample surface as well as a technique for 15.6 s ofheating for a region of the panel
rapid calculation of interface position. containing groupings of 12.4 mm
(0.49 in.) diameter holes at three different
Equations 16 and 17 can be substituted in depths. The three brightest holes toward
Eq. 14 to yield Eq. 18: the right side of the image are at 0.8 mm
(16) c Po[I-R(ic)]
~ kcprr
(17) s
(18) T,(O,t) c z'f~(~ + T,,(s))T,,(s)
\ 11=1 FIGURE 28. Region of graphite epoxy
-v-(r)"[exp( 112) composite panel containing groups of
t ErFc(f)] 12.7 mm (0.50 in.) diameter holes (three
each) at depths of 0.8 mm (0.03 in.),
For times short enough that the sample 1.4 mm (0.055 in.) and 2.4 mm (0.094 in.)
appears to be a bulk sample (the
below surface: (a) raw infrared image
thermally thick case)1 the temperature
increases as a linear function of the square obtained after 15.6 s of carbon dioxide laser
root of time as seen in Fig. 27. This early heating; (b) image of calibration constant
time can be used to calculate the prefactor
calculated from early dependence of time
C from a linear fit to the early data at
each pixel position. After division by resolved infrared thermographic data set;
c..,J(t) and subtraction of 1, only the sum (c) image of normalized temperature
on 1~1 remains and is a function
depending only on the thermal mismatch .;showing source intensity distribution
rand the variable~. Applying this process
to an experimental data set at each pixel removed from data.
yields the normalized temperature T11' The (a) (b) (c)
normalized temperature can be fit to this
~i'l•
function just by stretching the time axis
for the best fit and from there
330 Infrared and Thermal Testing
(0.03 in.) below the surface; the three \!\'hen two different materials are
holes to the left of these are at 1.4 mm bonded together, the thermal transport
will depend on the th~rmal transit time
(0.06 in.) below the surface; and there are for the top layer and also on the thermal
a further three holes to the left, 2.4 mm mismatch factor between the top layer
and the hacking material. When the
(0.09 in.) below the surface. The image backing material has a lower thermal
shown in Fig. 28b is an image of the effusivity than the top layer, the
calibration constant (C in Eq. 16) normalized temperature wi11 be positive,
as seen in the extreme case of flat bottom
calculated from the swift initial data set. holes in \Vhich air is the backing material.
J!or a sample with uniform optical If the backing material has a greater
properties, this image represents the thermal effusivity than the upper layer,
the normalized temperature will be
intensity distribution of the heating negative. For a homogeneous upper layer
source. Figure 28c shows an image of the of constant thickness, the transit time is
the same everywhere and the normalized
normalized temperature, with the temperature images can be used to
intensity distribution removed from the determine the thermal mismatch factor,
characteristic of the backing material.
data.
Time resolved measurements were
The time dependence of the performed on a test sample consisting of
normalized temperature at a position at small plates of different materials bonded
onto a 1 mm (0.04 in.) thick layer of
the center of the flat bottom hole is fiberglass epoxy composite. The resulting
plotted in Fig. 29 for three different temperature images were nonnalized
depths. These curves each slww the same using the technique described above.
behavior as seen in Fig. 26 for the Figure 31 shows the normalized
temperature image after a 2 s heating
response when thermal mismatch factor r pulse. The different plates· are easily
detected and the different materials show
= -1. Note that the horizontal axis in different and distinct behaviors.
Fig. 29 is square root time, --It and not the The range of behaviors is shown more
clearly in Fig. 32, where the normalized
normalized square root time, -l(at)·L-1, temperature is plotted as a function of
used in Fig. 27 for the general case. The square root time for pixel locations on
time for V(at)·L-1 = 1 is determined at the each of the different materials. As
time where the normalized temperature expected, the sign of the thermal
intersects with the line at y = 0.18. Then mismatch factors for the thermally
the depth of the discontinuity can be conductive metals (steel, copper, brass,
determined if the thermal diffusivity a of aluminum) and the thermally insulating
the material is knnwn. This analysis can materials (acrylic and fiberglass) are
be done for each pixel in Fig. 28c, thus
creating an image of the thermal transit
times. This was done in Fig. 30, which
shmvs a transit time image for the
graphite epoxy composite panel with flat
bottomed holes. These images summarize
the information contained in the stack of
infrared images collected as a function of
time during heating.
fiGURE 29. Time dependence of normalized temperature at fiGURE 30. Transit time image of graphite
center of flat bottom hole in graphite epoxy composite epoxy composite sample using algorithm
panel plotted for three different depths of holes. described in text.
1.5
~ ., ""® -~- ,- - ,-
il ' - - - - - -I - ________L__ ) _
I"
~
"-
E
~ 0.5
"~
.~
-;;;
§0
z0
-0.5 ---
0 0.5
1.5 2 2.5 3 3.5 4
Square root time {\Is)
legend 1.S 2 2.5
0 "'0.8 mrn (0.03 in.) Transit square root time Os)
0"' 1.4 mm (0.06 in.)
0 "'2.8 mm (0.11 in.)
Techniques of Infrared Thermography 331
different. The thermal transit time to the Furthermore, the spatial resolution in
interface for all locations on the sample is these measurements is determined by the
the same because the fiberglass upper infrared wavelength and not by the
layer thickness was constant at 1 mm microwave wavelength as occurs iil
conventional microwave imaging
(0.04 in.). techniques. As a result, image resolutions
A full thermal analysis at the location of better than 30 pm (1.2 x 10-3 in.) can
be obtained.
of the metal backing layers is more
complicated, because the metal backing The measurements described here use
layer thickness is too thin for a two-layer microwaves at a frequency of 9 GHz and a
model to be valid. The reflection at the maximum power of 2.3 W fed into a
rear metal interface causes the normalized single-flare horn antenna through a
temperature to be higher than predicted rectangular wave guide. The antenna has
by the two-layer model. However, a beam width of about SO degrees and is
aluminum has the lowest value of thermal
effusivity among the metals in the test placed about 0.15 m (6 in.) from the
specimen and exhibits the least negative sample. A 128 x 128 indium antimonide
normalized temperature as predicted by focal plane array with 12-bit
the theory. These measurements have analog-to-digital converter operating in
demonstrated the feasibility of using time the 3 to S }lm band is used for detection
resolved infrared radiometry with step of infrared radiation. The benefits of
heating for characterizing the thermal microwave heating in specific applications
properties of thick backing materials. is demonstrated by the foUr images in
Fig. 33. The specimen is a section of steel
Microwave heating techniques ·with the pipe ·with an epoxy coating that has
time resolved step heating technique experienced some disbanding. This is a
provide unique capabilities compared to coating system widely used for corrosion
heating with conventional optical protection of buried gas pipelines. Time
sources. 19•71 ivficrowave heating sources resolved infrared radiometry with step
have distinct advantages for optically heating by a conventional laser source is
opaque but microwave transparent illustrated in Figs. 33a and 33b for a
materials containing localized absorbing disband that was first dry and then filled
regions, such as entrapped water in with water. The disbanded region is much
composites. For particular specimen more clearly delineated when dry because
geometries and material properties, the of the high thermal contrast between the
presence of the discontinuity region can epoxy coating and the underlying air.
be imaged at higher contrast and better
spatial resolution than obtainable with '"'hen the disband is filled with water, as
the surface heating technique. Because the is often encountered in a field situation,
heat has only to diffuse to the top surface, the time resolved infrared radiometry
the characteristic thermal transit times for with step heating image is predominately
the measurement are shorter. determined by the spatial distribution of
FIGURE 31. Normalized temperature image FIGURE 32. Normalized temperature plotted as function of
after 2 s heating pulse on fiber glass epoxy square root time for pixel locations on six materials.
composite specimen with different backing
materials. 0.8
Acrylic ~ 0.6
3
~
0.4
~
Q.
E 0.2
2
"0
~
0.0
.~
"'E
z6 -0.2
-0.4
Fiberglass 0 0.2 0.4 0.6 0.8 1.2 1.4
Square root time (\Is)
02 4 legend
Normalized temperature
0 ~acrylic
..t. =fiberglass
• =aluminum
D. = steel
m =copper
0 =brass
332 Infrared and Thermal Testing
the laser heating source and does not FIGURE 33. Time resolved infrared images of partially
clearly show the disbanded region. The disbanded epoxy coated steel sample after 15 s of laser
time resolved infrared radiometry with heating: (a) for empty void; (b) for void filled with water;
step heating image in Fig. 33c shows (c) for empty void after 30 s of microwave heating; (d) for
micro-wave heating of the dry disband void filled with water after 30 s of microwave heating.
and there is not any appreciable heat
deposition in the specimen because the (a) (c)
epoxy coating is microwave transparent.
The image in Fig. 33d was taken after the
disband region was filled with water. Here
the water is readily heated by the
microwaves and the infrared image of the
surface of the coating provides an outline
of the disband region.
Summary (d)
The application of step heating and
observation of the surface temperature
during heating has been applied to time
resolved infrared radiometry and has been
described for one-dimensional systems.
Further a data analysis algorithm for
generating images showing interface
depth and thermal properties of backing
materials has been demonstrated. A major
advantage of step heating technique over
pulsed techniques are that the power
requirements of the heating source are
lower, making it applicable to many
nonoptical or single-wavelength heating
sources. Further it allows pixel-to-pixel
calibration without a reference sample.
Images of disband depth or thermal
properties can be generated with a simple
and fast algorithm. It also has been shuwn
that the blurring of small or deeply buried
discontinuities by lateral diffusion is
reduced for step heating. 72 Nevertheless,
the choice of temporal heating pattern,
for example step heating or pulsed
heating, in infrared radiometry usually
will he based on availability of equipment
such as heating sources rather than based
on the materials system under
investigation.
Techniques of Infrared Thermography 333
PART 6. Vibrothermography59
'J'he vibrothermographic nondestructive vibrothermography, ultrasonic testing,
testing technique entails the mapping of a X~radiography and visual testing. The
particular rotor examined (Fig. 34) was
structure's surface temperature '''hile the damaged by failure of the single screw
structure is subjected to forced mechanical while pumping. Because the gate rotor
oscillations. Regions of imperfection was composed of semitransparent epoxy
convert energy to heat through glass and the part was operated in
viscoelastic dissipation, collisions of hydraulic fluid, the damaged regions in
internal free surfaces in cracks or other the part can be seen visually as stained,
mechanisms. Discontinuities may appear making the part a onique .subject in that
hot when the surface temperature is results of other techniques can easily be
mapped. The thermal map is generally confirmed. Comparing the results of the
presented as isothermal lines on the vibrothermography, X~radiography and
surface and is obtained either by a ultrasonic testing to the stained regions
scanning infrared camera or by coating suggests the abilities of these techniques
the surface with temperature sensitive to identify damage in this machined
liquid crystals. composite part.
Liquid crystals have been usecF3·74 as A pair of gate rotors is used in a
temperature sensors for nondestructive balanced rotor single-screw hydraulic
testing of adhesive bonds and pump for sealing. This part (Fig. 35) is
interlaminar flaws and for locating machined from a (0, 90) cross ply epoxy
fractures in composite materials. This glass panel and each tooth is numbered
technique requires preparation of the for later reference. The gate rotor
specimen before a temperature sensitive investigated had been in service for about
coating is applied. Only a few isotherms 70 h when the companion bronze worn
can be identified and the temperature screw in the center of Hg. 34 faited.
differences vary among isotherms, so each Because hydraulic oil surrounds the seal
coating must be calibrated individually. during operation, regions of delamination
Real time scanning infrared cameras are damage have been stained in the
more flexible, requiring little surface semitransparent epoxy glass panel. A
preparation and alluwing the range of photograph (Fig. 36) of the region around
isotherms to be changed during a test. tooth 7 illustrates this staining. These
Infrared camera systems like the one regions have been mapped as irregular
described below have performed reliably
in various investigations.ls.sz FIGURE 34. Single~screw gate rotor pump, top case removed,
with gate rotor seal on left marked with R.
Many nondestructive testing
techniques- including most ultrasonic
and eddy current techniques- are point
techniques and require the operator to
evaluate many locations during a test.
Vibrothermography is useful for
identifying discontinuities because it is a
field technique- that is, because it
provides an image of an area of the
surface of the test piece in a single sensing
operation. Once discontinuities are
identified, other techniques can be used
to get additional information on regions
of interest for the characterization of
service life, failure load and part stiffness.
Single-Screw Gate Rotor
Pump
The case of service produced damage in a
glass fiber epoxy hydmulic pump seal is
considered here as an example of
discontinuity detection by
334 Infrared and Thermal Testing
curves on the teeth of the gate rotor in Methods Applied
Fig. 35. It should be noted that each tooth
has two sides cut on a 45-degree bevel, so Vibrothermography
only half the area of each tooth has a
constcmt thickness. Because of its complex In a vibrothermographic test of the gate
geometry, the gate rotor is difficult to rotor seal (Fig. 37) two aluminum disks
examine with most nondestructive test and the cap screw served as clamps to
techniques. However, because hold the gate rotor to the shaker. The
discontinuities are stained, the rotor rotor was excited by the piezoelectric
provides a unique test of the effectiveness shaker, with the displacements normal to
of any nondestructive test technique. the plane of the gate rotor disk at
Results of vibrothermographic frequencies between 9 and 30 kHz.
investigation \\rere compared to those of Stopcock grease was placed on the
ultrasonic C-scans and penetrant surfaces of the disk that contact the gate
enhanced radiographs. Discussion of these rotor or the shaker to aid in transmission
techniques follows. of mechanical energy bet\vcen the shaker
and the rotor. During an examination of
fiGURE 35. Gate rotor seal with damaged regions (stained) the rotor, the sweep oscillator used to
indicated. drive the power amplifier for the shaker
was allowed to sweep frequencies from 9
Sectmn A A to 30 kHz at a rate of about 1 kHz per
') minute. The images of thermograms in
"-l28 mm Fig. 38 were produced by photographing
the color display with a 35 mm camera.
(1. t in.)
Ultrasonic C-Scan and Penetrant
\ 109 mm (4.3 ;n.) d;ometN Enhanced X-Radiography
86 mm (3.4 in.) diameter Zinc oxide, tetra bromoethane and gold
chloride were used as X-ray opaque
102 mm (4.0 in.) radius penetrants to enhance discontinuity
detection. These penetrants were effective
FIGURE 36. Photograph of tooth 7 showing only on discontinuities open to the
stained delamination regions. outside of the structure because a
penetration path to the discontinuity was
needed.
A pulse echo instrument was used to
provide an ultrasonic C-scan. Water
immersion provided coupling of the
sound between the 10 MHZ transducer
and the specimen. The technique
FiGURE 37. Vibrothermographic inspection of gate rotor seal.
Co!or display unit Detector (liquid nitrogen, cooled
indium antimony crystal)
9.5 mm (0.375 in.) cap screw
control unit Aluminum disk
Scanning !ens system
Vibration
direction
Techniques of Infrared Thermography 335
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