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Level I Course Manual – Section 2Publ No 1 560 009 C

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2. Introduction to ThermographyWhat will you learn

• Discovery and early history on infrared radiation

• Modern history and benchmarks

• An overview of infrared thermography and applications

The infrared radiation was discovered in the year 1800 by Friedrich WilhelmHerschel. Herschel originating from Hannover, Germany, came to England as amember of a military band because of the personal union of Hannover and theenglish crown. Because he was interested in astronomy he began to work withlenses to improve his telescopes. During these experiments he succeeded todetect the infrared radiation in the sunlight spectra with thermometers asdetectors.




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The existence of infrared radiation has been known since the year 1800.

However, only in 1964 AGA Infrared Systems (AGEMA Infrared Systems)presented the first commercially available infrared thermography system to themarket. Although heavy and bulky, this, the first system of its kind, soon foundits place as a tool for various applications, mainly electrical maintenance. Over the years our combined companies lead the development of thermographicequipment, adapting the products to the various needs of our many customers.Here are a few steps where ‘first’ means ‘first in the world’:

1965 First thermographic equipment for civilian purposes, Thermovision®6511971 First long-wave scanner, Thermovision®680 LW

1973 First really portable system, Thermovision® 7501988 First system with absolute temperature measurement,

Thermovision® 7801982 First computer connected thermographic system, Thermovision®782 with

DIPS (Digital Image Processing System)1986 First thermoelectrically cooled system, Thermovision® 8701988 First portable camera in one unit, Thermovision®4001991 First truly dual and digital 12-bit system for R&D, Thermovision® 9001993 First FPA camera1997 First uncooled camera, Thermovision® 570

Since the beginning of thermographic inspection in the mid 1960’s, the methodhas found an increasingly wider use in a variety of applications. The techniqueenjoys such popularity mainly because it makes remote inspection possible, i.e.the inspection can be carried out without disturbing the normal operation of theinspected component, which may be a high-voltage electrical component, ahouse, a microcircuit or virtually anything. This is called ‘remote inspection’ or ‘remote sensing’ or simply ‘radiometry’ – measurement by using the thermalradiation being emitted from the object.Below you can compare a thermogram from a Thermovision®680 camera from

the late 60’s with a thermogram from Thermovision® 570 from the late 1990’s,30 years later. Many of the old instruments are still in operation throughout theworld.




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What exactly is infrared thermography?

All objects emit infrared radiation. The intensity of the emitted radiation dependson two factors, the temperature of the object and the ability of the object toradiate. The latter is called emissivity or emittance.

There is a physical law which says that all matter with a temperature aboveabsolute zero, i.e. –273º C, radiates heat. Heat radiation is synonymous withinfrared radiation. The hotter the object, the more intense the radiation.However, for temperatures below about 500º C we cannot see the heat. Andthis is where thermography comes in. The camera can ‘see’ the heat at muchlower temperatures than the human eye. And the camera converts the infraredradiation into a state that is visible to us. That is how we can see and measurethe heat.

Normally we then need to convert the measured heat into temperature as theinfrared camera can only measure the heat intensity. To convert this value intotemperature we require two things: a well-known physical formula andknowledge about the radiative properties of the object. As mentioned above thisis called emissivity or emittance. It is essential that any user of infraredthermography, regardless of experience, has an understanding of the physicalproperties of the object.

However, emissivity is not the only important factor to consider whenconducting thermographic measurement. During the training course time will bedevoted to the acquisition of correct emissivity values and the consequences of having incorrect values. We will also cover other important parameters, which

must be considered in the measurement situation.

3.5°C

88.8°C

20

40

60

80




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Infrared Imaging and RadiometryGary L. OrloveInframetrics, Inc.16 Esquire Rd.

No. Billerica, MA 01862

ABSTRACTInfrared imaging and radiometry in the commercial sector have been availablesince the mid 1960's. Since that time, there has been an increase in thenumber and types of equipment available, and the applications for which

infrared is used. This paper presents an overview of the technology itself andmany of the current applications.

1. What is infrared radiation?1.1 How infrared radiation was discovered

In 1800, the German scientist Sir William Herschel is credited with the discoveryof the infrared spectrum. He was performing an experiment where he wasmeasuring the temperature of different colors light by placing the blackenedbulbs of thermometers in the rainbow of light from a prism. When he placed histhermometer in the dark area beyond the red band of light, he was surprised to

record the highest temperature1. This gave birth to the future investigations intothe infrared section of the spectrum.

1.2 IR - part of the electromagnetic spectrum

Infrared energy is part of the electromagnetic spectrum and behaves similarly tovisible light. It travels through space at the speed of light and can be reflected,refracted, absorbed, and emitted. The wavelength of IR energy is about anorder of magnitude longer than visible light, between 2 and 1000 µm. (Fig. 1.)

1.3 Facts about infrared radiation

Infrared radiation is emitted by all objects as a function of their temperature.This means all objects emit infrared radiation. Infrared is generated by thevibration and rotation of atoms and molecules. The higher the temperature, themore the motion and hence the more infrared energy is emitted. At absolutezero (-273ºC), material is at its lowest energy state so infrared radiation isminimized.




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0.1A 1A 10A 100A 0.1µ 1µ 10µ 100µ 0.1cm 10cm 1m

Gamma

RaysX Rays Ultra

Violet

Radio

MicrowavesVisible

InfraredUHF VHF

Visible Infrared

.4µ .7µ 2µ 5µ 8µ 12µ 50µ

Shortwave Longwave Fig. 1. The electromagnetic spectrum

1.4 Stefan - Boltzmann law

One of the most frequently seen formulas regarding infrared output from asurface is known as the Stefan -Boltzmann Law. This states that the totalinfrared power given off by an object is proportional to the 4th power of theobject's absolute temperature. From this, it can be seen that the total power output increases very rapidly with object temperature.1.5 Planck's radiation law

The German physicist Max Planck expressed what is the most usefulrelationships for infrared radiation, temperature, and wavelength in what isknown as Planck's Law. The formula itself is beyond the scope of thismanuscript but a discussion of what the law says can be most illuminating (pun

intended).Planck's Law tells how the radiative emissive power is distributed bywavelength for a specific temperature of an ideal radiating surface. It is mostoften represented by a family of curves that look like arches. As thetemperature of a surface increases, each arch is higher (meaning more infraredpower) and broader (covers more wavelengths) that its predecessor. (Fig. 2.)




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1.00E-05

1.00E-04

1.00E-03

1.00E-02

1.00E-01

1.00E+00

1.00E+01

1.00E+02

1.00E+03

1.00E+04

0.1 0.5 1 100

5727° C - Sun Temperature

25° C - Ambient Temperature

2 5 8 12

Fig. 2. Planck's Radiation Law

An interesting phenomenon is that the peaks or crests of these "arches" movetoward shorter wavelengths as the temperatures increase. If one draws a linethrough these a function known as Wien's Displacement Law is traced. Thistells us that the wavelength of maximum emission moves toward shorter wavelengths as the temperature of the radiating surface is increased.You have all seen the practical effects of this. For instance if a nail is heated bya propane torch, at first the nail does not radiate energy you can see; but slowlyas heating continues the nail turns a dark red to cherry red to yellow to white. At the same time the brightness keeps increasing dramatically. You have justseen the Planck's Law in action! (Fig.3.)

Heat Cool600°C 25°C

.4µ .7µ 2µ 5µ 8µ 12µ 50µ 100µ

Visible Infrared

Steel Bar Steel Bar

Fig. 3. Practical demonstration of Wien's displacement law.




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2. Infrared system design parametersInfrared systems are designed to "see" in one of two infrared wavelengthregions. Let's take a brief look at some of the criteria used to make this choice.

2.1 Infrared "eyes"

Human eyes have been adapted to see those wavelengths where the sun emitsmaximum power. If we look at the Planck curves once more, we can find acurve representing 5727 C, the temperature of the sun. We can see that thepeak of the sun's energy falls at about 0.5 µm, just about in the middle of thevisible waveband! Now lets' find the curve for 25 C (this represents theapproximate temperature of the room). We will find that the peak radiation for objects at room temperature is about 10 µm. This represents the center of oneof the wavebands that infrared imagers use.

2.2 The atmospheric windows

Infrared imagers designed for terrestrial use have to look through a gaseousmedium, our atmosphere. Unfortunately, the atmosphere doesn't transmitinfrared energy equally well at all wavelengths, in fact at some particular wavelengths the transmission drops to a value near zero. The infrared designer must take this into account and designs the system spectral range in one of tworecognized "atmospheric windows". These are generally called the ShortwaveWindow (SW) that occupies about 3-5 µm; and the Longwave window (LW)occupying the 8-12 µm wavelength range. Infrared instruments using one of these two windows experience less degradation of the signal from a target. It isgenerally agreed that the LW window offers the least signal attenuation of thetwo. (Fig. 4.)

Wavelength (µm)2.0 4.0 6.0 8.0 10.0 12.0

1

0.9

0.8

0.7

0.6

0.50.4

0.3

0.2

0.1

0.0

Fig. 4. Atmospheric transmission at 25 C, 10 meters, and 50% relative humidity.




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2.3 Infrared detectors

An infrared detector converts infrared energy into a usable signal. An IRdetector can fall into two main categories: thermal, and photon or quantumdetectors. The thermal detector operates by producing a signal when its owntemperature changes. Examples of these detectors are thermocouples,bolometers, and pyroelectric materials.The photon detector is a semiconductor that produces a signal proportional tothe photon flux impinging on it. Photon detectors include photoconductive, andphotovoltaic varieties and usually require cooling to cryogenic temperatures for maximum performance.There are many materials used to manufacture infrared detectors. Examples

are HgCdTe which can be used for both SW and LW depending on doping;Indium Antimonide is a popular SW detector, and Platinum Silicide which is aSW detector most commonly used in focal plane arrays.Most infrared imagers utilize photon detectors due to their sensitivity and fastresponse time. Unfortunately, to achieve this performance these detectorsrequire cooling to cryogenic temperatures. Generally the performancecharacteristics of the detector are enhanced the colder it is. This used to bemost commonly accomplished by liquid nitrogen.In the past few years, other cooling technologies have become available toreplace liquid nitrogen. Peltier effect (thermoelectric @ 195 degrees K) andStirling Cycle (77 degrees K, -196ºC) (Fig. 5.) cooled systems are now

commonplace. Thermoelectric cooling has the advantage of no moving partsbut is limited to cooling to about -80 C in a room temperature environment. If the ambient temperature is raised, the detector temperature also goes upsacrificing performance. Peltier coolers are only available for 3-5 micrometer shortwave (SW) detectors.

Fig. 5. Inframetrics Stirling Cycle microcooler.




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Stirling Cycle coolers are generally more efficient and provide cooling to liquid

nitrogen temperature (77 K or -196ºC) or below if desired. The Stirling Cyclesystems in commercial use can maintain the 77 K temperature over a wide

range of ambient temperature conditions and are suitable for use with both SWand LW detectors.

2.4 Scanning mechanisms

Most infrared systems in commercial use today employ a mechanical scanningsystem to move the field of view of the detector (usually known as theInstantaneous Field of View or IFOV) to "paint" an image of the infrared energy

sensed from object space (known as the system Field of View (FOV). Thesescanning systems can comprise a series of reflecting or refracting prisms or mirrors to accomplish this (Fig. 6.). A scanner system may make use of asingle or multiple detector arrangement in order to produce an image. Usuallymultiple detector systems are limited to imaging applications due to thecomplexities in balancing the output from the detectors for accuratemeasurements. (Fig. 7.)

Fig. 6. Typical reflective galvanometer scan mechanism.

2.5 Focal plane arrays

When many detectors are placed adjacent to each other in a matrix , theresulting sensor is usually referred to as a focal plane array. Imaging systemsbased on this technology are becoming increasingly more affordable. Thesesystems eliminate the need for a mechanical scanning system and, in theory atleast, should offer improvements in sensitivity due to the low duty cycle of eachindividual detector.




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2.6 IR Imaging Radiometry

To enable the system to measure temperatures, a thermal reference target isviewed at regular intervals inside the scanner. After this information isprocessed in the electronics, it is presented on a television monitor as a liveblack and white or color image representing thermal information in the scene.The images can also be sent to an image processing system for further enhancement and data extraction.

2.7 Electronics

The function of the system electronics is to convert the electrical signals from

the detector into a usable visible image. For radiometers, the electronics has tocalibrate the image and assign temperatures to gray levels. Usually theelectronics can also supply one of several synthetic color palettes to better illustrate temperature differences recorded in the image (Fig. 8.).

Fig. 7. The basic components of an infrared scanner system.

Several of the infrared detectors offer dynamic range (the maximumtemperature at signal saturation divided by the thermal sensitivity) in excess of 12 digital bits (4096 levels). The human eye can detect little more than 6 bits or

64 shades of gray. Electronics that digitize to 12 bits are useful where largetemperature ranges must be captured and later analyzed at smaller temperature spans where good thermal resolution is important. Mostapplications do not require this level of digitization and so 8 bits is commonlyused for these (256 levels).




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Fig. 8. Typical infrared scanner head and the image it produces;black is cold and white is hot.

Scanner systems also vary by the number of picture elements that are present.One can easily calculate the number of optical elements in a line by dividing thehorizontal FOV by the detector IFOV. Electronics should digitize at least asmany pixels as there are optical elements, oversampling is generally preferred.

3. Applications of IR imaging technologyInfrared technology has so many applications it would be impossible to list themall let alone describe them is this brief document. I will try to provide a brief smorgasbord of applications in several key areas. Some of the applications

have been around for a while, but are important because they are they form partof the backbone of the industry. Some others I will discuss are new leadingedge applications.

3.1 Process Control/Quality Assurance

3.1.1 Paper Machine Profiling

Paper is made from a slurry of wood pulp and other materials. This slurry iscompressed and dried in paper machines. A critical aspect for the final paper quality and strength lie in the uniformity of the drying process. Wet spots infinished paper rolls used to be very difficult to track before infrared

thermography was available. Now it is simply a matter of following the coolstreak (wet area) from the paper take-up spool back to the source, typically aclogged drying nozzle. (Fig. 9.)




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Fig. 9. Thermogram of paper machine problem; cool stripes indicate excessive moisture.

3.1.2 Testing turbine blades for blockages of cooling channels

General Electric Aircraft Engines was contracted in 1980 by the U.S. Air Forceto develop an infrared inspection program for non-destructive testing of jetengine turbine blades. Today, automated infrared inspection of turbine engineblades is cutting production costs and improving overall performance of GE'snewest military and commercial jet engines.

In high performance engines, many turbine blades contain internal passagesand surface cooling holes that allow them to operate at temperatures above the

service limits of the blade materials.2

The inspection technique operates on the principle that when hot air is injectedinto a blade's cooling passages and holes, an infrared imager can detecttemperature differences on the blade. Bright spots in the infrared imagecorrespond to warm (therefore open) surface holes; darker areas indicate cooler temperatures corresponding to blocked holes. (Fig. 10.)




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Hot Air

IR Camera

Fig. 10. Engine turbine blade inspection.

Since the original inception, GE has evolved the system to include a computer controlled part manipulator and image processing decision making software. Arobot loads each blade into the part manipulator which moves the blade thougha series of 6 to 20 positions for infrared imaging (the number of imagesdepends on the specific blade design). The software automatically detects anddisplays blocked hole locations.

To obtain each image, a 1 to 1.5 second burst of 100 degree F air is blownthough the blade and the infrared camera images the holes. The softwarecompares each image with preset parameters for hole size, position andthermal characteristics. If all holes are in their proper locations and havecorrect thermal characteristics, the blade is transferred by the robot to aconveyer for the next processing step.Blades with missing or blocked holes are sent to a reject conveyor for further testing and rework.

3.2 Infrared Thermal Imaging in the Aerospace Industry

The diverse disciplines found in the aerospace industry offer a world of opportunities for application of infrared imaging systems. Thermographicanalysis has not found widespread use in the past because of high equipment

prices, insufficient performance, and/or lack of test procedures2. This ischanging as new research produces techniques which are practical and can beapplied in field situations.

In some cases, thermography is proving to be faster and more cost effectivethan traditional NDT techniques. More importantly, thermography is openingnew vistas in NDT which do not lend themselves to other forms of investigation.




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3.2.1 Inspection of cockpit windows

Aircraft cockpit windows are heated primarily to prevent icing and misting. Theyare laminated from a combination of materials to give the required strength andtoughness.

There are two main types of window elements; toughened glass with goldfilament heating elements for the front windows, and perspex with small filamentheating elements for the side windows.Traditional inspection methods can only check for discontinuity or satisfactorycurrent flow in the entire window. IR thermal inspection enables one to observethe thermal profile for heating element deterioration, hot spots, shorts infilaments, and overheating of connection points.

The inspection is usually performed from outside the aircraft. The scanner ispositioned so the window fills the field of view. The thermogram obtained can becompared to a "golden" thermogram of a known good window in the field usinga built-in floppy disk drive.

3.2.2 Fluid (water) ingress in control surfaces

Recently developed composite aircraft materials are extremely sturdy andlightweight. These materials are vital to aircraft performance and air worthiness. However, the honeycomb structure of this material presents apotentially dangerous problem: water ingress. British Airways was one of thefirst carriers to recognize this problem, and has pioneered inspection techniquesfor this malady.It has been discovered that certain control surfaces tend to absorb water in thehoneycomb structure, for reasons that are not fully understood. The problem isaggravated by the effects of lightning and hail which cause impact damagewhich is barely visible. The water enters the honeycomb and freezes when theaircraft is at high altitude. As the ice expands the cells in the structurebreakdown. This condition grows like a cancer and eventually jeopardizes theentire structural integrity of the component. (Fig. 11.)

Areas of water ingress

Fig. 11. Thermogram showing water ingress (dark areas)on illustrated section of aircraft.




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Until recently, the only effective method of diagnosing the problem was through

radiography. While this is still the most accurate way, it has severaldisadvantages; it is expensive in time, equipment, and man-power, and canexpose maintenance personnel to ionizing radiation.British Airways has found that thermography is an indispensable tool for inspecting planes for this problem. After the plane has landed, the ice remainsat 0 degrees C while it is melting. The rest of the plane has warmed to ambienttemperatures on the approach. This provides an ideal opportunity to search for the ice pockets with a thermal imaging system while the plane is being serviced.

British Airways uses an Inframetrics Model 600 to survey the entire aircraft in 20

minutes. Images are recorded onto videotape for later analysis by an imageprocessing workstation.

3.3 Target signature analysis

Target signature analysis is important for military aerospace applicationsbecause all powered aircraft use heat producing engines in their designs.Target signature analysis involves measuring the amount and pattern of infraredradiation emitted by military vehicles. Infrared cameras are utilized to evaluatethe effectiveness of methods used to reduce the intensity and shape of the

signature to avoid detection and classification by hostile armed forces. (Fig. 12.)

Fig. 12. Thermal signature of military aircraft model in the LW band.

Since there are two primary bands where infrared sensors are utilized, itbecomes important to evaluate the target in both bands. Specially designedinfrared systems, such as the Inframetrics 610, employ two detectors in onescan head to accomplish this. The advantage of the single optical path is that

the images are synchronized in time and object space.




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3.4 Night Vision

There are many areas where IR can be used for night vision or surveillance.These include navigation (not only on waterways but airplane landings, andeven automobile driving), security, and search and rescue. This last applicationis particularly appropriate for infrared because the heat from people penetratesfoliage well and rescue missions don't have to be terminated at sundown.

Infrared night vision systems are routinely used in military and some civilianaircraft for flights at night or in poor visibility. Since the systems are sensitive toheat emissions and not light, visibility at night is extremely good. Typicallythese systems are non-radiometric and employ more than one detector for improved imaging resolution and sensitivity.

3.5 Non Destructive Testing

3.5.1 Defect and void detection in composite materials and structures

Composite materials are playing an increasing role in the manufacture of itemsas diverse as jet fighters to bicycles and tennis racquets. The extreme strengthto weight ratio of these materials are very attractive for these products.Composite materials include graphite epoxy, fiberglass, and/or sandwich panelshaving either Nomex or metallic honeycomb cores and graphite epoxy facesheets.

The original method for finding inclusions or delaminations in the panels withthermography was to induce a temperature differential by heating or cooling andlooking for differences in thermal conductivity which express themselves at thesurface by temperature patterns.




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Flash Lamp IR Camera

Specimen

Holes1 32 Reference

Time (sec)

0

50

100

150

200

250

0 20 40 60 80 100 120 14

Hole 1

Hole 2Hole 3Reference

Fig. 13. Flash pulse heating method and time-temperature curves(exaggerated for illustration purposes).

More recently, more sophisticated techniques have been developed. Theseinvolve the use of flash lamps3 that send a rapid pulse of heat to the specimen.The time vs. temperature information content from the IR imaging is recordedinto a computer. By selecting appropriate times to sample the data, an imagecan be formed displayed the defects at a particular depth in the material. (Fig.13 & 14.)




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Time (sec)

0

10

20

30

40

50

60

70

80

90

100

0 20 40 60 80 100 120 14

Hole 1

Hole 2Hole 3

Reference

Depth Squared

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14 16 18 20

X

X

X

Fig. 14. Flash pulse thermal contrast and peak time vs. hole depth squared

Using another image processing technique, a "thermal tomogram" can beproduced which shows the location of a defect and its depth as a function of itsassigned color.One of the techniques showing promise for certain types of defects is the use of an active laser scanned along with the image to produce heating that is inphase with the pixel being scanned. This is similar to the technique for gasimaging described later in this document.Structures that are amenable to these techniques include carbon fiber brakes,aerodynamic control surfaces and fuselage panels, and delamination andcorrosion of bonded metal honeycomb aerofoils.

3.6 Fugitive Gas Emission Location

If a gas were visible to the human eye, locating leaks or determining the extentand direction of a gas cloud from a leak or accidental spill would be easy. In thecase of hazardous gases, being at some distance is desirable. Since most of these gases are invisible to the naked eye, advanced technology must be reliedupon to obtain an image of the cloud. Active scanning is a technique whichaccomplishes these goals by integrating an infrared (CO2) laser and an infrared

imaging system.4




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Gas imaging occurs when a gas absorbs the laser light within the laser

illuminated field of view thereby attenuating the backscattered laser light andproducing a darkened region in the TV display wherever the gas is present.The higher the gas concentration, the greater is the absorption and the higher the contrast. In this manner, the usually invisible gas becomes visible on thevideo and its origin, size and direction of movement are easily determined. (Fig.15.)

In the absence of an absorbing gas, the energy reflected from the background(the backscattered laser component) produces an image of the terrain on theTV monitor in much the same manner as the thermal image produced on astandard TV monitor. However, when the absorbing gas is present, both the

incoming and backscattered laser radiation are reduced due to the strongmolecular absorption of the gas at the laser wavelength. The difference incontrast between the area where the gas is present produces an image or shadow of the gas cloud on the TV monitor.

Fig. 15. BAGI Gas imaging concept .

The IR detector instantaneous field of view (IFOV) senses about 1/350000 of the total system field of view (FOV) at a given instant. Therefore, the laser needs only a small amount of power to irradiate the same IFOV at the sametime. In contrast, to irradiate the entire FOV at one time would requiremegawatts of power. (Fig. 16.)




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The technique is generically termed "Backscatter/Absorption Gas Imaging"

(BAGI) and the first system was built at the Lawrence Livermore NationalLaboratory. The BAGI technique is different from other laser-based remotedetection techniques in that it is designed for the sole purpose of locating leaksor tracking as clouds. The image of the escaping gas allows the operator toquickly identify the location of the leak. The technique is capable of detectingextremely small leaks and displaying them in real-time in a standard TV format.Inframetrics is the exclusive worldwide licensee for the BAGI technology andmarkets a line of gas imaging equipment under the GasVue name.

Detector

Horizontal Mirror

Vertical Mirror

Lens

Laser Beam

Fig. 16. BAGI scanner concept.

A single tracer gas BAGI system can be used to test the integrity of gas andliquid tight components on the assembly line such as heat exchangers, fueltanks, pumps, exhaust systems, compressors, hydraulic lines and vessels. Thisoffers a rapid and clean alternative to conventional leak location methods suchas dip tanks, soaping and pressure decay. Furthermore, due to the real-timevideo aspect of this new technology, automation of production line leakdetection and location is now possible.

A multigas BAGI system can be used to locate fugitive gas emissions fromtanks, pipes, valves and containment vessels. Over 80 different gases can bedetected and identified from distances up to 30 meters. In many cases theleaking gases are hazardous or are at locations which are very difficult to reach.However, the BAGI system allows the operator to stand off up to 30 metersfrom the structure being leak inspected and to examine a large area in a shortperiod of time.




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For the tracer gas system a CO2 laser is tuned to the wavelength of the tracer

gas being used while the multigas system incorporates a tunable CO2 laser for multigas detection and can be varied from wavelength of 9.05 µm to beyond11.81 µm.

A proposed gas imaging application employs a single gas BAGI system for thelocation of tracer gas leaks from automobile gasoline tanks on an assembly line. As the tanks are positioned in the leak checking station they are pressurizedwith a tracer gas. If the unit under observation has a leak, an alarm is soundednotifying the operator who quickly identifies the point of leak by observing thereal-time TV image. A proprietary alarm system alleviates the need for a fulltime operator and an unattended operation will account for further cost savings.

Production rates are estimated to increase from 117 to 200 tanks per hour.Similarly, automotive wheels need to be leak checked for porosity duringmanufacturing. Typically 75 to 90 wheels per hour can be leak checked usingthe standard method of water testing. With a GasVue system it is expected thatleak checking rates will increase production.

3.7 Building Diagnostics

3.7.1 Roof Moisture Surveys

Roof moisture surveys are probably the most common type of buildingdiagnostic work performed in the U.S. today.The inspection works on the premise that wet areas of roof have a much higher thermal capacitance than dry areas. Hence when the sun shines on the roof,more energy is absorbed by the wet area than the dry. At night, when theenergy is released back to the environment, the dry roof cools rapidly but thewet roof remains warmer. This makes it quite easy to spot the wet areas withan infrared camera and mark them with spray paint. Utilizing this technique notonly locates the leak area, but can save a tremendous amount of money to thebuilding owner because only the faulty section of roof need be replaced.(Fig.17.)

Fig. 17. Thermograms of wet roofs (warm blocky looking areas).




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3.7.2 Building Heat Loss Inspections

In the mid seventies to early eighties, energy conservation was an importantbuzzword in residential buildings. Many homes were reinsulated to help saveheating/cooling fuel and hence operating costs. This was the era thatthermography use in the building market was most visible in the U.S. Since thattime this market has cooled somewhat but still remains strong in Europe andCanada where energy costs are much higher.

Fig. 18. Building heat loss from an uninsulated wall (outdoor thermogram).Note: the cool appearance of the wall studs indicating the lack of any insulating material.

This type of inspection relies on the fact that wall surfaces that are poorlyinsulated appear cold from the inside and warm from the outside to an infraredsystem. As can be seen from the image below, the wall studs appear cold fromthe outside of an uninsulated building while the uninsulated area between thestuds appears warm. (Fig.18.)

3.8 Electrical distribution

Perhaps the oldest application of commercial infrared imaging radiometryinvolves the inspection of electrical distribution equipment. This is still thelargest single use of equipment in the industry. Utilities, manufacturing plants,and office facilities routinely use IR to inspect distribution equipment,transformers, switchgear, and motor control centers. (Fig. 19.)




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Fig. 19. IR and visual image of an oil circuit breaker in need of repair.

The underlying principle upon which this works is that all conductors carryingcurrent produce heat. If an area of localized higher resistance is formedbecause of corrosion, or a loose connection, a localized hot spot is formed thatcan be easily found with a thermal imager and quantified as to its severity.Repairing these on a timely basis will prevent unexpected failures in equipmentand help keep productivity high.

3.9 Medical

Medical applications have also been around for a while. The first serious

application of IR was in the early screening of breast cancer. For variousreasons, the breast cancer screening lost favor with physicians in the U.S., butis still routinely used in other countries. There are several other areas whereinfrared is used today: soft tissue injury screening (pain syndromes of varioustypes), burn injury diagnosis, detection of deep vein thrombosis, and diagnosingthe extent and location of injuries to racehorse legs.

3.9.1 Thermal Coronary Angiography

A very promising new technique for thermal imaging is called Thermal Coronary Angiography (TCA). In the U.S., over 250,000 bypass procedures areperformed annually. Sometimes, the operations do not meet the goal of restoring full blood perfusion to the cardiac tissue. This means that the patientmust undergo another surgical procedure or be restricted in his activities.

Thermal Coronary Angiography5 offers the cardiac surgeon a new technique toensure that blood is flowing properly to all affected areas of the heart while he isperforming the procedure. He can then take corrective action right away.




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Known obstruction area

Obstruction is removed and

venous graft inserted

Cold saline is injected into

graft prior to attachment

before suturing the open

end into the blood supply

Additional occlusions are instantly

visible to the surgeon from the

thermal image

Fig. 20. The three basic steps in the TCA procedure.

A bypass operation entails taking a vein from some other part of the body andgrafting it to bypass a section of the cardiac venous structure where blood flowhas been severely restricted or stopped by atheroma buildup (athersclerosis or arterioscelrosis). Thermal Coronary Angiography involves using a real-timeinfrared imager to view the new grafted bypass vein while a solution of coldsaline is injected. The areas of the heart that are open to flow instantly cool off,indicating where the flow channels are. Those areas still occluded will show notemperature decrease, indicating further work that is necessary by the surgeon.(Fig. 20.)

The majority of the initial research on TCA is currently being conducted inGermany.




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4. Conclusion

As we have seen, infrared thermography has an extremely diversifiedapplication base. Continuing research is opening new opportunities for applyingthe technology. More powerful IR imaging equipment, coupled with advancedimage processing, are taking these new techniques from the laboratory to thefield.

5. references1. Burton Bernard, abc's of INFRARED, Howard W. Sams & Co. Inc.,

Indianapolis, Indiana, 1970.1. Gary L. Orlove, "Infrared Thermal Imaging in the Aerospace Industry",

Sensors, February 1992.2. L.D. Favro, T. Ahmed, D. Crowther, H.J. Jin, P.K. Kuo, R.L. Thomas, and

X. Wang, "Infrared thermal-wave studies of coatings and composites",Thermosense XIII, George S. Baird, Editor, Proc. SPIE 1467, pp. 290-294, 1991.

3. Robert P. Bruno, "Tracking Gas Leaks with Active IR Scanning",Photonics Spectra, February, 1992.

4. Friedrich W. Mohr, MD, Jack Matloff, MD, Warren Grundfest, MD, AurelioChaux, MD, Robert Kass, MD, Carlos Blanche, MD, Po Tsai, MD, FrankLitvack, MD, and James Forrester, MD, "Thermal Coronary Angiography: A Method for Assessing Graft Patency and Coronary Anatomy in

Coronary Bypass Surgery", Ann Thorac Surg, 1989, 47: pp. 441-9.



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