IOP PUBLISHING EUROPEAN JOURNAL OF PHYSICS

Eur. J. Phys. 28 (2007) S37–S50 doi:10.1088/0143-0807/28/3/S04

Infrared thermal imaging as a tool inuniversity physics education

Klaus-Peter Mollmann and Michael Vollmer

University of Applied Sciences Brandenburg, Magdeburgerstr. 50, 14770 Brandenburg, Germany

E-mail: vollmer@fh-brandenburg.de

Received 28 September 2006, in final form 21 November 2006Published 30 April 2007Online at stacks.iop.org/EJP/28/S37

AbstractInfrared thermal imaging is a valuable tool in physics education at the universitylevel. It can help to visualize and thereby enhance understanding of physicalphenomena from mechanics, thermal physics, electromagnetism, optics andradiation physics, qualitatively as well as quantitatively. We report on its useas lecture demonstrations, student projects and practical lab work.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Infrared (IR) radiation was discovered experimentally around 1800 by Herschel. Thequantitative explanation of incandescent radiation in 1900 by Max Planck started adevelopment which today has resulted in modern infrared technologies with so-calledthermocameras. These infrared imaging devices, which are also the result of scientificdevelopments in semiconductor physics and micro-system technologies, are ideal instrumentsto enhance the limited vision of man beyond the visible region. They are not only suitable fora great number of technological applications, but may also be used to qualitatively visualizeand/or quantitatively analyse a great number of physical phenomena. Such a visualizationis particularly important in physics education when dealing with phenomena involvingminute energy transfer, e.g., processes due to friction, where other methods for measuringor demonstrating the system changes usually fail. The present work briefly describes themethod of infrared imaging and then gives a selection of its numerous possible applications.Special emphasis is on its use at the university level in lectures, student projects and laboratorycourses. Focus will be, on one hand, on the visualization of phenomena discussed in highschool physics or introductory physics courses at college/university and, on the other hand,the lab courses and project work extend the use of IR thermal imaging towards practicalapplications in science and industry.

0143-0807/07/030037+14$30.00 c© 2007 IOP Publishing Ltd Printed in the UK S37

S38 K-P Mollmann and M Vollmer

(a) (b)

Figure 1. Spectra of blackbody radiation versus wavelength for different temperatures.

2. Basics of infrared imaging

Seeing requires light to enter the eye. Besides observing light sources, we usually see objectssince they scatter light from external sources into our eyes. In contrast, infrared imaging usesdirect sources, i.e. the fact that every object at a temperature T > 0 K emits thermal radiationwhich may enter our eyes (of course, optically smooth surfaces can also give rise to reflectionsof infrared radiation from the surroundings which then show up in the IR image of thesesurfaces; such thermal reflections must be avoided in quantitative analyses, see also figure 7 insection 3.4). Similar to visible light, thermal radiation is electromagnetic radiation, however,at longer wavelengths. Infrared imaging utilizes this thermal radiation with special infrareddetectors to generate an image similar as visible light generates an image in the light detectorsof a camcorder.

The spectra of thermal radiation can be calculated from Planck’s law for incandescentradiation, i.e. black bodies (see the usual textbooks or [1]). Typical temperatures lie in therange from 0 ◦C to 1000 ◦C; consequently, thermal radiation—if plotted versus wavelength—is located in the wavelength region from 1 to 20 µm (see figure 1). Note, however, that theinterpretation of peak positions of Planck’s law strongly depends on the chosen parameterregime. If spectra are plotted versus frequency, the peak positions shift [2]).

Black bodies are idealizations. For real bodies, the radiation of a black body of the sametemperature must be multiplied with the emissivity ε to yield the thermal radiation. Thisparameter ε which may depend on temperature, material and surface properties of the objectsas well as on wavelength is a measure of the fraction of thermal radiation with respect tothe thermal radiation emitted by a black body of the same temperature (figure 2). For mostpractical applications, ε is a constant which resembles a grey body. Exceptions are selectiveemitters, i.e. substances which have absorption and emission bands in the thermal infraredspectral range. On one hand, the resulting strong variations of emissivity with wavelengthcomplicate the analysis of thermal images; on the other hand, it enables special cameras todetect gaseous species [3].

Infrared imaging only uses a small portion of the spectrum between λ = 1 µm and20 µm. The restriction is due to several factors, the most important being the transmission ofthe atmosphere (usually objects are observed through the atmosphere). The absorption bandsof the greenhouse gases H2O and CO2 lead to the so-called atmospheric windows. Figure 3

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(a) (b)

Figure 2. Emissivities (left) and resulting spectra (right) of thermal radiation for objects of T =100 ◦C. The emissivity may be unity (black body), constant (grey body) or vary as a function ofwavelength (selective emitter).

(a) (b)

Figure 3. Atmospheric transmission (low resolution) for the US 1976 standard atmosphere at T =296 K and a path length of 10 m (left) and 1000 m (right).

depicts the transmission of the atmosphere for horizontal paths of 10 m and 1000 m. In thepast few decades, thermal imaging used the second and third windows in the spectral regions3.5 to 5.5 µm and 8 to 14 µm, respectively. Recently, the very short bands between 0.9 and1.7 µm and 1.5 to 5.5 µm have been used for shorter path lengths, too.

The used portion of the thermal radiation is also influenced by the transmission of the usedIR optics and the spectral response of the detectors. Most commercial cameras nowadays usearrays of 240 × 320 pixels with cooled semiconductor detectors or uncooled micro-bolometers.The most recent development is quantum well infrared photon detectors (QWIPs), operating ina very narrow spectral region between 8 and 9 µm. Very recently, detector arrays with 480 ×640 pixels have become available commercially. For educational purposes and several specialapplications, cheaper systems with, e.g., 120 × 160 pixels are available. More informationcan be easily found via, e.g., Google in the internet.

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If all parameters are known, the analysis of the camera signal allows us to extract theobject temperature, provided the emissivity is known (or vice versa). More information onprinciples of IR technology and exemplary applications can be found in [1, 4, 5].

Infrared cameras are too expensive for schools, but they can be found more frequently inuniversities. Our aim is to motivate the use of such systems at universities by giving examplesof their potential uses in lectures, lab work and student projects. Here, we restrict ourselvesmainly to qualitative results.

3. Examples of the use of IR imaging in physics lecture demonstrations

The following examples of thermal infrared images intend to visualize the respective physicalphenomena, as can be done in physics lectures. Some of these phenomena were investigated indetail in student projects. Although all phenomena are of course due to a temperature increaseof objects, they are classified according to the respective field of physics to which they belong.

Usually, thermal images are presented for a constant value of ε; this means objects ofthe same temperature may show differences due to differing ε values. The images presentedhere are usually still the raw data, containing both temperature and ε variations. For mostcases of qualitative visualization of certain phenomena, this is sufficient. A word of caution:it is very easy to generate nice false colour IR images, but the interpretation of these imagesis nontrivial. It usually requires a substantial knowledge of all aspects of IR physics andtechnologies.

All data were taken with a LW camera (8–14 µm) of FLIR systems or a SW camera(3.5–5.5 µm) by Agema (now also FLIR systems), both cameras having 240 × 320 pixels.

3.1. Mechanics

A very important field for IR imaging in physics education concerns the visualization ofmechanical phenomena involving friction. For example, consider a bicycle which is brakingwith blocked tyres. Due to friction, the kinetic energy will be transferred into heat, finallyresulting in a temperature increase of the floor as well as the tyre [6, 7]. As a matter offact, walking or riding a vehicle with wheels can only take place due to frictional forces.Hence, one can easily observe footsteps on the floor or detect the elevated temperatures ofa bicycle, motorcycle (this was part of a student project) or car wheel due to normal drivingwithout braking. Thermography is, e.g., also used to check the tyres of racing cars, since hotspots during normal operation give hints regarding possible future failure. More generally,thermography can be used to visualize the variation of frictional forces due to an increasednormal force while keeping the contact surfaces between an object sliding on top of anotherthe same.

In order to analyse the effects of frictional energy transfer in more detail, two differentweights of 1 kg and 5 kg, respectively, were placed on small wooden plates and drawnsimultaneously across the floor (student project). The heavier weight led to a much largerwarming of the floor as expected [7]. Again, as was the case in the bicycle example, thesurface of the wooden plate was also getting warmer. In general, the distribution of theavailable energy onto the two surfaces sliding with respect to each other depends on theirsurface structures and their thermal properties.

Another frequently encountered example of energy transfer into heat occurs for inelasticcollisions, e.g., in sports. In tennis, the question arises whether a service was in or out. Incritical cases, even slow motion might be insufficient, disregarding the fact that decisionshave to be made immediately. In late 1996, the method of IR imaging was tested for the

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(a) (b)

Figure 4. Left: a tennis ball hits a carpet, resembling the court. The image was taken justafter touching the ground. The ball was also heated up during the collision, but—due to its fastmovement—only left the vertical trace. Right: analysis of the temperature of the contact point onthe floor as a function of time.

first time at the world championships in Hannover (Germany). It utilizes the fact that theball suffers an inelastic collision upon hitting the floor, transforming a substantial amount ofenergy into the floor (the lines were made visible by emissivity contrast). For services at about200 km h−1, temperature increases of more than 10 K occur, decaying over a period of morethan 10 s. Even an amateur hitting a ball in a lecture hall can do an easily detectable job (seefigure 4).

Similar observations can be made for other ball games such as squash (student project,see [7]). Another example of an inelastic collision is the jumping of two persons of differentweights from a given height to the floor (and quickly stepping aside). The resulting temperatureincrease of the floor may be used to estimate the mass difference of the jumpers.

3.2. Thermal physics

Obviously, nearly every thermal phenomenon can be visualized using IR imaging. Fourexamples will be briefly discussed (see [7–9]). Consider first three geometrically identicalrods made of glass, steel and aluminium, respectively. Upon heating with a Bunsen burnerat one side, the temperature increase as a function of distance from the burner depends onthe coefficient of heat conductivity. In real time, one may readily observe the transientbehaviour. The temperature increases nicely correspond to the expected behaviour due to theheat conductivities of 0.8 W (m K)−1, 45 W (m K)−1 and 220 W (m K)−1, respectively.

Second, the temperature-driven motion of fluids yields many fascinating pictures, be itthe images of ice cubes in a drink on a hot summer day or Benard convections, i.e. the regularstationary cell structures due to convection. The latter can be observed, e.g., in a pan filledwith 3 mm of oil and heated to about 120 ◦C to 150 ◦C.

Another well-known fact in thermodynamics is thirdly the cooling of a gas upon adiabaticexpansion. Since gases are optically thin, one may observe this effect indirectly either bydirecting the cold gas stream onto a solid or by detecting the temperature decrease of the valvethrough which the gas expands. In practice, the valve of an automobile tyre cools down withinseconds. A fourth visualization is easily realized for the heat of condensation: the alcohol inafter-shave quickly evaporates and leads to a cooling of the skin. The effect can be enhancedby using a warm (!) air fan.

S42 K-P Mollmann and M Vollmer

(a) (b)

Figure 5. Visualization of the Peltier effect: two copper wires are connected to a constantan wire(horizontal). A current through the wires leads to cooling at one contact and heating at the other.Changing the direction of the current reverses the situation for the two contacts.

3.3. Electromagnetism

All applications of infrared imaging in electricity and magnetism are based on temperatureincreases due to transfer of electrical into thermal energy. This can, e.g., be due to eddycurrents. An impressive example illustrates the Peltier effect. An U-shaped swing is madeof two copper wires connected on the bottom of the U with a constantan wire. If a current isdirected through the wires, one contact between copper and constantan cools while the otherone gets warmer (see figure 5). Changing the direction of the current leads to the expectedchange: the formerly warmer contact will cool down and vice versa. The effect can only beseen if the wires are quite thick, otherwise, resistant heating within the wires leads to a moreor less homogeneous temperature profile along the wires.

A very important application concerns the design of electronic boards for computers.Every board contains heat-producing elements. Regarding the progress in miniaturization,it is obvious that the heat which is produced on a small area leads to temperature increaseswhich must not exceed the given limits for the electronic components. Consequently, the heat-producing elements must be well separated on the boards. On our website [7], an example ofa PC board is shown. The Pentium processor is cooled by a fan and not visible. However,other components warm up by about 30 K (this application is also a standard investigation inthe student lab course).

From the multitude of other applications, we mention the direct visualization of theprocesses within a microwave oven during operation [10, 11]. This requires the microwaveoven to be modified by cutting a hole into the front door and replacing it by an infraredtransparent NaCl window. Figure 6 depicts an example. A glass plate with a thin cold waterfilm was placed in a microwave oven in a height of 8 cm and heated for about 15 s with amicrowave power of 800 W without using the turntable of the oven. Obviously, the horizontalmode structure of the standing electromagnetic waves within the cavity is easily detectable,since the water film heats up most at the nodes of the microwave field.

3.4. Optics

Some fascinating experiments are possible with glasses made of various materials. Figure 3in [6] (see also [7]) depicts a person who is wearing special glasses: one side is made ofNaCl; the other side contains normal glass. Since normal glass is IR absorbing, the glass side

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Figure 6. Modes of a microwave oven.

(a) (b)

Figure 7. Transition from regular to diffuse reflection from oxidized brass plate (for details, seethe text).

is opaque and the thermal radiation of the warmer eye behind it is not transmitted. Rather,the colder surface of the glass—due to the rather poor thermal contact of the glasses at theears and the nose—is shown in the picture. In contrast, the NaCl side is IR transmitting inthe wavelength range of the camera. Similarly, we also investigated opaque materials in thevisible which are transparent in the IR like Si wafers [7].

In optics classes, there is a clear distinction between reflecting surfaces (mirrors) anddiffusely scattering surfaces (e.g. blackboard, walls, etc). However, the transition from one tothe other is usually performed theoretically. Diffuse scattering takes place if the wavelengthof the electromagnetic radiation is comparable to the dimensions of the surface roughness.If the latter dimensions are small compared to the wavelength, regular reflection takes place(analogously, a soccer ball will bounce back from a mesh wire according to the law ofreflection, whereas a table tennis ball with similar dimensions to the mesh will behave like adiffuse scatterer). Using visible and IR electromagnetic radiation, the transition from diffuseto specular reflection can be directly demonstrated. Consider, e.g., a person in front of abrass plate which is oxidized and a diffuse scatterer in the visible: no mirror image can be seen(figure 7). However, the wavelength of the IR radiation detected in λ = 8–14 µm IR cameras isabout a factor of 10 larger. Therefore, the IR image can already demonstrate regular reflection[7]. This phenomenon led to a project dealing with the identification and suppression ofthermal reflections in the wavelength range of IR cameras [12].

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(a) (b)

Figure 8. Observing a Leslie cube plate of constant temperature T = 68 ◦C through a cuvette filledwith air (left) and pure CO2 (right) at atmospheric pressure. The difference is due to absorption ofIR radiation in CO2, leading to an apparent temperature of only 64 ◦C.

3.5. Quantum physics

Since infrared cameras are based on the laws of radiation, they can also be used to visualizethe underlying principles. The most important parameter in thermography is the emissivity ε.Bodies of equal temperature but different emissivities emit different amounts of thermalradiation (figure 2). This is most easily demonstrated with a Leslie cube, filled with hot water.A Leslie cube is a copper metal container with four differently treated surfaces, one black,one white, one of oxidized metal and the last one of polished metal. Due to the differencesin emissivity, one can easily detect different amounts of thermal radiation already with athermopile detector. More easily, IR imaging helps to visualize the differing emissivities [7].If the camera is operated at constant ε, the polished metal with the lowest ε emits the least,which is interpreted as lower temperature. It therefore looks almost dark whereas the blackand white colour surfaces have higher emissivities and emit more IR radiation.

Max Planck discovered his famous formula while studying the radiation from black bodiesin the form of cavities held at fixed temperature with small holes, through which radiationis emitted. These cavities can be characterized by emissivities close to unity, the exactvalue depending mostly on geometry (for details, see [1, 9, 13, 14]). We performed severalexperiments [9] to measure cavity emissivities by using three identical cylindrical enclosures.On top of these three enclosures, a plate with three different opening diameters was mounted,giving theoretical emissivities of 0.99, 0.98 and 0.96, respectively. Such differences are easyto detect experimentally: the metal block with the enclosures was put on a regulated hot plateto achieve thermal equilibrium conditions. The IR signals from the holes varied according tothe different emissivities [9].

Another example of radiation physics is the visualization of the direct absorption of IRradiation by CO2, which is a very important greenhouse gas [3, 9]. This example shall alsoserve as an example for a more quantitative analysis. A short wave IR camera (operatingbetween 3.5 and 5.5 µm) observed a homogeneous hot surface (T = 68 ◦C) through a cylinderof length 10 cm with IR transparent NaCl windows. In one case, the cylinder was filled withnormal air, then the cylinder was evacuated and filled with CO2 at atmospheric pressure. Thedifferences between the two IR images directly visualize the drastic effect of the absorptionband of CO2 at 4.2 µm wavelength (figure 8).

In normal air, the attenuation of the IR radiation due to 10 cm of air with about 360 ppmof CO2 is negligible; hence the transmitted radiation, corrected for the transmission of thewindows, led to the real surface temperature of 68 ◦C. In contrast, 10 cm of CO2 at atmospheric

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Figure 9. FTIR spectrum of the CO2 sample.

pressure have a very strong absorption. This means that the camera receives a lower radiationinput signal. Since the camera software calculates the temperature according to the radiationlaws from the received signal, a lower signal leads to a lower temperature reading. In theexperiment, an apparent temperature of 64 ◦C was recorded.

In order to quantitatively understand this behaviour, the absorption spectrum of CO2

was measured quantitatively with a Fourier transform IR spectrometer. Figure 9 depicts themeasured transmission of the sample in the wavelength region 4.0 to 4.5 µm. The vibrational–rotational absorption band is so strong that, to first order, the transmission may be modelled aszero in the wavelength range from 4.19 µm to about 4.44 µm and unity elsewhere. No otherabsorption features are present in the range from 3.5 to 5.5 µm, i.e. in the wavelength rangeof our SW camera.

For the simplest analysis, the Leslie cube, i.e. the radiation source, is regarded as a blackbody. For the experiment with air, the camera signal is, hence, given by the detected portionof the blackbody radiation with temperature T = 68 ◦C. The camera optics and detector onlyselect a portion of this radiation. For simplicity, we neglect any wavelength dependence ofoptics, filters and detector and just assume that all of the radiation between 3.5 µm and 5.5 µmcontributes to the signal. Figure 10 illustrates the idea for the analysis. The camera receivesradiation, corresponding to the area below the radiance curve for T = 341 K (solid line) from3.5 µm to 5.5 µm. CO2 removes the area in the region from 4.19 to 4.44 µm. Hence, thecamera only receives a reduced signal (shaded area), which is however, interpreted as beingdue to a blackbody source of lower temperature (here: area from 3.5 µm to 5.5 µm below theradiance curve of 337 K).

For a quantitative test, blackbody radiances were integrated from 3.5 µm to 5.5 µm for atemperature range from 50 ◦C to 80 ◦C. If the integration limits were from zero to infinity, theintegral would give the Stefan–Boltzmann law, the total radiation varying like T 4. Using thegiven limits, this behaviour changes appreciably. As expected, the total radiance varies nearlyexponentially with T since we are dealing with the short wavelength region of the spectrum.Figure 11 depicts the result which students may derive by numerically integrating Planck’slaw. The idea behind a quantitative prediction for the measured temperature is to estimate thearea below the 68 ◦C curve, i.e. start at T = 341 K to find the corresponding integratedsignal (here about 11.5). The second step is to estimate the area from the CO2

absorption between 4.19 and 4.44 µm (from a similar graph as in figure 11, computed

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Figure 10. Spectrum of blackbody radiation for T = 68 ◦C. The camera detects radiation between3.5 µm and 5.5 µm (area below the solid line), whereas CO2 absorbs all the radiation between 4.19and 4.44 µm. The thus reduced (shaded) area equals the total radiation in the same wavelengthrange 3.5 µm to 5.5 µm for a black body of lower temperature (area below the broken line from3.5 to 5.5 µm).

Figure 11. Scheme showing the quantitative analysis of detected temperatures upon absorption byCO2 (for details, see the text).

for the changed integration limits). In the present case, ≈10% of the radiation at68 ◦C was blocked by the CO2 sample. Hence, we calculate in the third step 0.90 × 11.5,giving the value for the shaded area in figure 10, here about 10.4. This new signal corresponds(step 4) to a temperature of 337 K, i.e. 64 ◦C.

This simple analysis demonstrates that the measured decrease of the IR signal can bequantitatively understood with basic radiation laws and knowledge of the detection rangeand optics of the camera as well as the absorption spectrum of the gas. In a more thoroughanalysis, the detected camera signal is proportional to

(T 4 − T 4

amb

)rather than just to T 4,

with Tamb being the ambient temperature. For the present example, resulting temperaturedeviations—compared to the simple model—are below 1 K.

Similar to CO2, many molecular gases have absorption bands in the thermal IR region andconsequently, all of those gases can in principle be detected. If the absorption/emission lines

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(a) (b)

Figure 12. Absorption of cold SF6 (T ≈ −20 ◦C, left) and emission of warm SF6 (T ≈ 80 ◦C,right) in front of a room temperature wall (detected in the range 8–14 µm).

are very narrow, broadband detection with a camera is usually not sensitive enough; however,narrowband filters may help. Luckily for practitioners and students, there is a multitude ofgases, which do, however, have broad absorption/emission features in the wavelength rangeof conventional IR cameras. As an example, figure 12 depicts the absorption of cold SF6

in front of a warmer background as well as the emission of hot SF6 in front of a colderbackground (for other examples and details, see [3]). This example visualizes the transitionfrom absorption to emission of IR radiation by molecular species.

4. Student projects and labs

Many of the examples discussed above are shown during regular lectures as demonstrations;however, most of them have also been studied during student projects and sometimes they arethe content of student lab work. The student projects as well as the lab work take place in thethird or fourth year, i.e. after the students have a sound knowledge of fundamental physics.They also participated in a special course on infrared technologies (30 h total on radiationlaws, radiation quantities, infrared emitters and detectors, optical materials and elements,IR spectrometry, devices, pyrometers and thermography, system parameters applications;this course shall soon be part of a common master programme in Applied Physics with theUniversity of Paisley, Scotland). The project work is very flexible and students can work for,say, 10 to 14 days on a specific topic, first discussing a project, sometimes building somemodels, etc, then taking IR images and finally analysing the images. An example for a modelwill be given below. The lab courses are regular courses with thermography being an importantpart. The following topics are treated in the IR lab course: first, camera and object parameterssuch as emissivity, geometrical resolution of cameras and instantaneous field of view; second,measurements such as building thermography, transient phenomena of electronic circuits, truetemperature of electronic boards, etc.

Here, we briefly discuss two special student projects in detail whose results were lateralso included in the lab course.

4.1. Hidden structures

One of the most well-known applications of thermal imaging is building thermography, whichis based on differences in thermal properties of different materials. Obviously, it only works

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(a) (b)

Figure 13. Half-timbered structures of a house which are hidden behind plaster. The visible imageshows no structure at all, whereas IR imaging immediately reveals the hidden structure. The whitespot in the centre corresponds to a small top window which was just opened after recording thevisible image. The warm air from the inside heated up the parts adjacent to the window.

(a) (b) (c)

Figure 14. Model of hidden structure with front (left) and back (middle) sides as well as IR image(right) when heated from the back.

if large temperature gradients between inside and outside exist and it is usually performedonly in winter time. Whenever the thermal insulation of a building is poor, there will be anenergy flow from the warm inside of a house to the cold outside. Depending on the thermalconductivities and heat capacities of the wall materials, certain parts will show variations ofthe surface temperature distribution. The best reproducible results are obtained with indoorthermography. Sometimes, outdoor thermography proves useful, too. It allows us to detecthidden structures behind walls or floors. Examples are half-timbered structures, visibly hiddenbehind plaster, but readily detectable using infrared imaging due to the differing thermalproperties of the wood and the rest of the wall. Figure 13 shows an example with a visible andan IR image.

In order to simulate this situation, students were given the task of building a model ofsuch a hidden structure. The differing thermal properties were realized by using Styrofoam,air, metal and wood. Figure 14 (left) shows the front and figure 14 (middle) shows the backside of the final model with dimensions of 62 cm × 42 cm and a thickness of 2 cm. TheIR imaging test was done by placing it in front of a planar electrically heated plate of 0.6 ×0.9 m2 (which within minutes reached surface temperatures of about 130 ◦C. Figure 14 (right)depicts the result. The hidden structures already became visible a minute after starting to heatthe plate.

Similarly, the hidden structure of a floor heating system may be made visible [15].

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(a) (b) (c)

Figure 15. Visible and IR images of the electronic board of a speaker, which is connected to acomputer. First, the speaker was turned on, but operated without emitting any sound (middle),then the speaker was used to play music which was clearly audible (right). Obviously, the fourdiodes on the board (within circle at end of arrow in VIS image) were heated up to about 75 ◦Cduring operation of the speaker.

4.2. Electronic boards and switches

A different aspect of IR thermal imaging in projects and the lab deals with electrical switchesand electronic boards. The first topic is related to typical industrial applications of engineers,since the regular testing and surveying of electrical switches is essential for reducing serviceshut-down times, and hence for guaranteeing a high productivity of machines in factories.Therefore, another student project dealt with an electrical switch assembly to simulatemalfunctioning and subsequently identify the relevant parts.

The second topic has to do with an electronic board design. Due to the ongoingminiaturization of all electrical components, in particular in the microelectronic boards ofcomputers, the problem of malfunctioning of whole boards becomes important. Althoughthe voltage and also the currents are very low, they still produce heat and many componentsmalfunction above a critical temperature. The old 486 processors still operated uncooled andsometimes at working temperatures above 80 ◦C. Starting with the Pentium series, processorswere air cooled by fans. Still, it is necessary to separate all heat-producing elements on aboard from each other. In order to test new boards, it is essential to operate the boards underworking conditions and analyse, e.g., IR images.

In the lab, students regularly study the temperature distributions of electronic boards ofcomputers (see [7]) as well as other related equipment. As an example, figure 15 depicts theelectronic board of a small speaker of a type which is usually connected to computers. Thevisible image shows the board which is mounted on the back cover of the speaker, which wasopened for the experiment. The playing of music obviously led to a heating up of four diodeson the board, whereas the power transistors were sufficiently cooled by the metallic coolingfins.

5. Discussion and conclusions

Selected examples from all fields of physics were discussed where infrared imaging canhelp to visualize complex physical phenomena which otherwise often have to be believedby students. Infrared imaging provides a slightly unusual perspective of viewing physicalphenomena. Our qualitative experiences with students show that this different physical way ofviewing the outside world is quickly accepted and proves to be very successful in enhancingan understanding of the respective phenomena.

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The present work only gave a small selection of phenomena and applications. Its potentialuses for a later professional career of students are also convincing. IR imaging is a commontool in industry, e.g., for condition monitoring of large factories in the chemical industry orto perform contactless surveys of electrical appliances in all voltage and current ranges, inparticular, the detection of leaks in high voltage lines.

Another important field for infrared imaging is medical applications. It is possible to detectchanges in the blood circulation due to smoking, drinking alcohol or doing sports via changesof the surface temperature of a body. Recent developments include, e.g., thermocoronaryangiography.

Another recent—though obvious—application of infrared imaging is the visualization andquantitative analysis of chemical reactions. It is easily possible to detect temperature changesof exothermic or endothermic reactions for very small quantities of substances. This leads toa new field of application: the study of chemical reactions in micro-reactors. A final example:the most recent high technology use was the testing of on-orbit IR camera inspections duringspace shuttle flights in order to ensure safe returns during re-entry. There are many moreapplications and contents of the project, and lab work will accordingly change in the future tomake these courses even more attractive.

Acknowledgment

We wish to thank D Karstadt and F Pinno for helpful discussions in part of this project.

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