# Thermography

The thermography , also thermography is an imaging technique for displaying the surface temperature of objects. The intensity of the infrared radiation emanating from a point is interpreted as a measure of its temperature .

Building thermography: Areas of increased temperature on the uninsulated outer wall are marked with the false color red .
Mobile thermal imaging camera

A thermal imaging camera converts infrared radiation, invisible to the human eye, into electrical signals . From this , the evaluation electronics generate an image in false colors , more rarely a grayscale image .

In contrast to near-infrared spectroscopy , no external light source is required for thermography.

## Historical

The astronomer and musician Wilhelm Herschel discovered thermal radiation in 1800 by directing sunlight through a prism and examining the area behind the red end of the visible spectrum with a thermometer. The temperature rose in this area, and Herschel concluded that an invisible form of energy must be operating there. Its designation “thermal radiation” is still common today and was replaced about 100 years later by “infrared” - in the German-speaking area the term “ ultrared ” was also used for some time .

Other researchers doubted his discovery at first because it was not yet known that the transparency for IR strongly depends on the type of glass of the prism. In search of a better material, the Italian physicist Macedonio Melloni discovered in 1830 that prisms made of crystalline rock salt hardly attenuate IR radiation and that thermal radiation can be bundled with lenses made of this material. A year earlier, Melloni was able to significantly increase the measurement accuracy by replacing the relatively imprecise mercury thermometer with the thermopile he had invented . Both - lenses made of rock salt and arrangements of thermopiles - were the essential components of the first thermal cameras.

The temperature distribution on surfaces (so-called “thermal images”) were made visible by Herschel in 1840 through different evaporation rates of a thin oil film. The temperature was later determined through direct contact with pressed thermal paper , which changes color when it comes into contact with sufficiently warm surfaces. All these methods have lost a lot of their importance because they only work in a narrowly limited temperature range, do not show changes over time or small temperature differences and are difficult to handle with curved surfaces. Compared to the contactless technology commonly used today, however, they were considerably cheaper.

Samuel Pierpont Langley made his breakthrough in the development of contactless temperature measurement in 1880 with the invention of the bolometer . Areas of application included tracking down icebergs and hidden people. Further developments, especially in the field of imaging, mostly took place in secret and research reports could not be published until after 1950 due to military confidentiality regulations. The devices have also been available for non-military purposes since around 1960.

The technology of imaging has changed fundamentally in its general use. Nowadays, a thermal imaging camera converts the thermal radiation ( infrared light ) from an object or body , which is invisible to the human eye , from a greater distance into electrical signals with the help of special sensors , which can be easily processed by computers. As a result, the temperature measurement range ( dynamic range ) has been significantly expanded, and tiny temperature differences can also be determined. Nowadays, thermography is mostly used as a synonym for infrared thermography.

## principle

Spectral distribution of the intensity of black body radiation. The temperature of the sun is marked in orange, the ambient temperature in red.

Every body with a temperature above absolute zero emits thermal radiation. In the ideal case ( emissivity ) is the spectrum of the emitted radiation corresponds to that of a black body radiator , in real surfaces, it differs (see Emissizität) . In the case of polished metal surfaces, the IR range drops to values ​​below 0.1. The following applies to common building materials . ${\ displaystyle \ varepsilon = 1}$${\ displaystyle \ varepsilon}$${\ displaystyle \ varepsilon \ approx 0 {,} 9}$

With increasing temperature, the transmitted spectrum shifts to shorter wavelengths ( Wien's law of displacement ).

Thermography is preferably used in the infrared range, i.e. at object temperatures around 300 K, which are in the range of normal ambient temperatures around 20 ° C. So that the measurements on objects located further away are only slightly falsified by the atmosphere between the object and the camera, the cameras usually work in restricted wavelength ranges in which the atmosphere hardly emits (and absorbs) its own radiation. Such a “window” is, for example, in the range from about 8 to 14 µm (see atmospheric counter-radiation / atmospheric window ).

Three thermal outputs contribute to the result:

• The main part P object is emitted by the measuring object itself, the surface of which should have the highest possible emissivity.
• The objects in the environment, but also the sun, radiate energy P environment , the proportion is scattered on the measuring object and adds up to the result. This annoying addition is particularly pronounced on smooth metal surfaces.${\ displaystyle (1- \ varepsilon)}$
• The air in between supplies P air .

All three parts are weakened when passing through the air; for distances of around two meters, a degree of transmission of can be expected. ${\ displaystyle \ tau \ approx 0 {,} 9}$

The entire received service is calculated

${\ displaystyle P _ {\ text {Total}} = \ varepsilon \ tau P _ {\ text {Object}} + (1- \ varepsilon) \ tau P _ {\ text {Environment}} + (1- \ tau) P_ { \ text {air}}}$

Scattered radiation from sunlight and hot, side emitters are easiest to avoid with careful measurement. The problem, however, is the radiation power of the air mass between the object and the sensor when the distance increases. For this reason, ground-based infrared telescopes can only be used for observing the relatively nearby sun. Objects farther away can only be seen if the thickness of the air layer (as in the Stratospheric Observatory for Infrared Astronomy ) is greatly reduced or (as in the Wide-Field Infrared Survey Explorer and Spitzer Space Telescope ) is switched off completely.

## Possible measurement errors

Black polyethylene film is transparent in the MIR range (see next picture)
The arm can be seen in the thermogram through the black film (see picture above), but the lens is not transparent. The unit of the temperature scale is degrees Fahrenheit .

Real surfaces emit less radiation than a black body . The ratio is always between zero and one and is called the emissivity . It depends on the material and the surface properties, but hardly on the temperature, and is particularly small for polished metal surfaces. An example illustrates the associated problem: A heavily rusted iron plate with a uniform temperature of 30 ° C = 303 K is polished in strips, which results in a “picket fence effect” of strong and weak IR radiation due to the widely differing emissivities. From the Stefan Boltzmann law

${\ displaystyle P = \ varepsilon (T) \ cdot \ sigma \ cdot A \ cdot T ^ {4}}$

follows for the radiated power per unit area

${\ displaystyle {\ frac {P _ {\ text {polished}}} {P _ {\ text {rusted}}}} = {\ frac {0 {,} 05} {0 {,} 85}} = 0 {, } 059}$

The thermal imaging camera only evaluates the power received from the different areas, resulting in a ratio of the absolute temperatures of

${\ displaystyle {\ frac {T _ {\ text {polished}}} {T _ {\ text {rusted}}}} = {\ sqrt [{4}] {0 {,} 059}} = 0 {,} 49 }$

calculated. If the thermal imaging camera is set so that the rusted surface is assigned 303 K, i.e. approximately 30 ° C, it should assign the absolute temperature of 149 K, which corresponds to −124 ° C, to the polished strips. In fact, a significantly higher temperature will probably be displayed because unwanted IR radiation from the environment is "faded in" on the reflective surface.

The assumed emission factor can be preselected on every thermal imaging camera. If you were to set this so that the temperature of the polished surfaces corresponds to reality, this measuring device would register so much more radiation power from the rusted areas that it would calculate a temperature of 342 ° C = 615 K. Radiation measurements should therefore be viewed with caution. If the temperature of bare metal surfaces has to be determined, manufacturers of measuring devices recommend painting a sufficiently large area or covering it with adhesive tape.

The influence of the temperature on the emissivity can in most cases be neglected for measurements in the temperature range from 0 ° C to 100 ° C.

Many non-metals have an emissivity close to one in the mid-infrared. Examples are glass, mineral substances, paints and varnishes of any color, anodized layers of any color, plastic materials (except for polyethylene; see adjacent pictures), wood and other building materials, water and ice. This makes the temperature measurement less erroneous.

The temperature of surfaces with a low emissivity, such as that of metals, can often not be reliably determined with thermography.

## Process variants

### Passive thermography

With passive thermography, the temperature distribution of the surface caused by the environment or the process is recorded. This is used, for example, in structural engineering to find thermal bridges or on technical devices in operation to identify heat loss sources and defects. Another application is e.g. B. indirect process monitoring in injection molding , by observing the outflow of the heat introduced by the melt on the demolded component and using it to check and readjust process parameters. Due to the different cooling speeds near and far from the surface, there are heat flows within the component. Internal structures such as unintentional flaws can act like a thermal barrier, so that this is expressed by a changed temperature distribution on the surface.

### Active thermography

Active thermography is used to discover hidden structures or structural defects that are shown by a locally changed heat flow due to a different thermal conductivity. For this purpose, the component to be tested must be thermally excited in order to generate a heat flow in the object (DIN 54190-1). There is periodic excitation, e.g. B. in lock-in thermography, and one-time stimulation applied (pulse thermography). distinguished.

Inhomogeneities influence the heat flow into the inside of the component (excitation and camera on the same side, so-called reflection arrangement) or through the component (excitation from the rear, i.e. transmissive, e.g. applicable to walls, housings, body parts accessible from both sides) and thereby lead to local temperature differences on the surface.

The thermal excitation can take place as follows:

• Optical by means of flash lamps or laser radiation, in which the radiation is absorbed on the surface.
• In the case of ultrasonic excitation, ultrasound is coupled into the component, which is preferably attenuated at defects or converted into heat by friction at loose contact points and consequently leads to locally detectable heating.
• Inductive heating is used for metals, preferably ferrous materials. Also, carbon fiber composite material can be inductively excite. Here cause z. B. breaks in the conductive fibers reduce heat generation.

Experience has shown that u a. in the case of plastics, that only defects can be recognized whose depth in the component corresponds at most to their extent projected onto the surface.

With lock-in thermography, the excitation is intensity-modulated and periodic. Lock-in thermography is frequency selective, i. that is, it only responds to temperature changes at the specific excitation frequency. The phase image obtained by a pixel-by-pixel discrete Fourier analysis therefore shows, in contrast to the amplitude image, the thermal structures below the surface regardless of the illumination quality and the emissivity. The penetration depth depends primarily on the modulation frequency and the thermal diffusivity. The lower the excitation frequency, the higher the penetration depth and the required measuring time.

Active thermography is particularly suitable for the non-contact examination of homogeneous, large-area and thin-walled components with a simple geometry. In the case of plastics, use is usually limited to thin walls in the millimeter range. Thermography can primarily show three-dimensional defects close to the surface, but it can also detect flat defects such as delamination , missing connections in weld seams or the lack of fiber layers. Even the lack of individual rovings in fiber composite components such as the rotor blades of wind turbines can be recorded.

Active thermography in the automotive sector is a non-contact and non-destructive process with which the result is displayed in the form of a reproducible image. In contrast to conventional paint layer thickness measurements , with which only punctual measured values ​​are determined in the form of numerical values, active thermography not only captures punctual parts of the vehicle, but also the entire vehicle flank. With the vehicle scanners can vehicle bodies from steel , aluminum , fiberglass , carbon fiber and even foiled vehicles are inspected.

### Thermography for detecting sports injuries

Thermography has also been used in sports since 2010. First of all, this was used to search for injuries / disorders in racehorses that you couldn't ask where it hurts. It is now used systematically in athletes. In football it is used for the early detection of bruises after training and competition and has proven itself. This involves taking thermographic images of both legs. Temperature differences of more than 0.4 degrees at the same point on the right / left are considered noticeable and therefore require a sports medicine check. In the meantime, uniform standards for sport have also been agreed internationally in order to be able to create thermographic recordings according to the same principles and thus be able to compare them.

The costs and dangers of the source of excitation are also disadvantageous. The light sources used for optical excitation are potentially dangerous to the eyes. Magnetic fields during inductive excitation are sometimes higher than the precautionary limit values. The resolution, which decreases rapidly with depth, is a disadvantage compared to other imaging methods.

The advantages of material testing using active thermography result from the special applications. It is thus possible - in contrast to X-ray testing  - to work without ionizing radiation . Areas accessible from one side can be checked. It can e.g. B. large areas can also be checked in one step using image evaluation.

## Image generation

Calibrated thermal imaging cameras are used to generate images in the mid-infrared.

Thermographic recordings of musicians with a thermal imaging camera . The warm areas show particularly stressed muscle areas, which often become health burdens due to excessive demands.
Hot connection point and cable end on a contactor that can be identified by the heating (white-red-yellow) , usually due to increased contact resistance at the connection

In principle, a thermal imaging camera is structured like a normal electronic camera for visible light, but the sensors differ in structure and functionality depending on the wavelength to be detected. It is not possible to record such long-wave radiation with conventional films.

An image is projected through a lens onto an electronic image sensor. Cameras for the wavelength range from 8 to 14  µm use lenses made of monocrystalline germanium or zinc selenide . Monocrystalline sodium chloride would also be suitable, but is sensitive to moisture.

Deeply cooled photo semiconductors are often used as electronic image sensors, whereas microbolometer arrays , thermopile arrays or pyroelectric sensors do not necessarily have to be cooled.

The photoelectrically working detectors are often cooled to temperatures around 77 K (liquid nitrogen) so that the sensors can even work as photo receivers . The thermal sensitivity (temperature resolution) of the thermographic system can be significantly increased compared to uncooled systems. Uncooled infrared sensors are also often thermoelectrically thermostated in order to reduce signal drift in the receiver elements. Such devices are significantly smaller and more cost-effective than deep-cooled systems. However, they deliver a comparatively poorer result.

The detector cell of a microbolometer array consists of an absorbing disk only a few micrometers thick, which is held by two curved contacts (so-called microbridges). The disks are made of a material with a strongly temperature-dependent resistance (for example vanadium oxide). The absorbed infrared radiation increases the temperature of the disc, which in turn changes its resistance. The measured voltage drop is output as a measurement signal.

Pyroelectric sensors , on the other hand, only deliver a voltage with a very high source impedance when the temperature changes .

Pyrometric sensors require a mechanical chopper, microbolometer arrays at least periodic shading of the image sensor. The reason for pyrometric sensors is that they can only react to changes in temperature. With bolometer arrays, the chopper or shutter is used to obtain a dark image, which is deducted from the recorded image pixel by pixel as a sensor-specific reference (each pixel has an individually different resistance).

## Thermographic testing standards

German Institute for Standardization (DIN)
• DIN 54162, Non-Destructive Testing - Qualification and certification of personnel for thermographic testing - General and special principles for levels 1, 2 and 3
• DIN 54190-1, Non-destructive testing - Thermographic testing - Part 1: General principles
• DIN 54190-2, Non-destructive testing - Thermographic testing - Part 2: Devices
• DIN 54190-3, Non-destructive testing - Thermographic testing - Part 3: Terms
• DIN 54191, Non-Destructive Testing - Thermographic testing of electrical systems
• E DIN 54192, Non-Destructive Testing - Active Thermography
• DIN EN 13187, Thermal behavior of buildings - Detection of thermal bridges in building envelopes - Infrared method
• DIN EN ISO 9712, personnel for non-destructive testing according to DIN EN ISO 9712: 2012 - Thermography (TT) method
International Organization for Standardization (ISO)
• ISO 6781, Thermal insulation - Qualitative detection of thermal irregularities in building envelopes - Infrared method
• ISO 18434-1, Condition monitoring and diagnostics of machines - Thermography - Part 1: General procedures
• ISO 18436-7, Condition monitoring and diagnostics of machines - Requirements for qualification and assessment of personnel - Part 7: Thermography

## literature

• Dietrich Schneider: Introduction to practical infrared thermography , 2nd corrected edition, Shaker Verlag, Aachen 2019.
• Norbert Schuster, G. Valentin Kolobrodov: Infrared thermography. Wiley-VCH, Weinheim 2004.
• Helmut Budzier, Gerald Gerlach: Thermal infrared sensors. Wiley-VCH, Weinheim 2010.
• Nabil A. Fouad, Torsten Richter: Guide to thermography in construction. Fraunhofer IRB, Stuttgart 2005.
• Thomas Zimmermann, Martina Zimmermann: Textbook of infrared thermography. Fraunhofer IRB, Stuttgart 2012.
• G. Schwalme: A process for the production of plastic moldings. Patent, No. DE 102010042759 B4, October 21, 2010.
• W. Roth, G. Schwalme, M. Bastian: Thermal fingerprint - process control and regulation based on inline thermography. In: Plastverarbeiter. 04, 2012, p. 36.
• G. Schober, T. Hochrein, P. Heidemeyer, M. Bastian and others: Safe enjoyment - detection of non-metallic foreign substances in food. In: LVT food industry. 1/2, 2014, p. 20.
• S. Neuhäusler, G. Zenzinger, T. Krell, V. Carl: Optimization of the impulse thermography test technology by laser scans and flash sequences. DGZfP report volume 86, Thermography Colloquium, Stuttgart, September 25, 2003.
• T. Hochrein, G. Schober, E. Kraus, P. Heidemeyer, M. Bastian: I see something that you don't see. In: plastics. 10, 2013, p. 70.

### General

• T. Hochrein and others: NDT: I see what you don't see. In: plastics. 11/2013, pp. 70-74.
• G. Busse: Thermal wave generator for imaging thermal structures. Patent, No. DE 3217906 A1, November 17, 1983.
• G. Busse, D. Wu, W. Karpen: Thermal wave imaging with phase sensitive modulated thermography. In: Journal of Applied Physics. 71/1992, p. 3962.
• B. Köhler: Method of non-destructive testing and material characterization for plastics. SKZ seminar "Quality assurance in the processing of fiber composite materials", Hall 2010.
• D. Wu: Lock-in thermography for non-destructive material testing and material characterization. Dissertation . University of Stuttgart, 1996.
• J. Aderhold, G. Dobman, M. Goldammer, W. Pia, T. Hierl: Guide to heat flow thermography - non -destructive testing with image processing. Fraunhofer Vision Erlangen Alliance, 2005.

### Ultrasonic excitation

• A. Dillenz, T. Zweschper, G. Busse: Elastic wave burst thermography for NDE of subsurface features. In: Insight. 42/2000, p. 815.
• J. Rantala et al .: Amplitude modulated lockin vibrothermography for NDE of polymers and composites. In: Research in Nondestructive Evaluation. 7/1996, p. 215.

### Inductive excitation

• J. Vrana: Basics and applications of active thermography with electromagnetic excitation. Dissertation. Saarland University, 2008.
• G. Riegert: Induction lock-in thermography - a new method for non-destructive testing. Dissertation. Faculty of aerospace engineering and geodesy at the University of Stuttgart, 2007.

### Pulse phase thermography

• T. Krell, J. Wolfrum, B. Deus: Pulse-phase thermography on defined damaged and repaired fiber composite components. DGZfP Thermography Colloquium, Stuttgart, 27. – 28. September 2007.

Commons : Thermography  - collection of images, videos and audio files

## Individual evidence

1. ^ G. Schwalme: A process for the production of plastic moldings. Patent, No. DE 102010042759 B4, October 21, 2010.
2. W. Roth, G. Schwalme, M. Bastian: Thermal fingerprint - process control and regulation based on inline thermography. In: Plastverarbeiter. 04, 2012, p. 36.
3. ^ T. Hochrein, G. Schober, E. Kraus, P. Heidemeyer, M. Bastian: I see what you do not see. In: plastics. 10, 2013, p. 70.
4. G. Schober, T. Hochrein, P. Heidemeyer, M. Bastian et al.: Safe enjoyment - detection of non-metallic foreign substances in food. In: LVT food industry. 1/2, 2014, p. 20.
5. S. Neuhäusler, G. Zenzinger, T. Krell, V. Carl: Optimization of the impulse thermography test technology by laser scans and flash sequences. DGZfP report volume 86, Thermography Colloquium, Stuttgart, September 25, 2003.
6. Vehicle thermography - recognizing hidden damage caused by accidents (explanation with video). svs-gutachten.de, December 29, 2019, accessed on December 29, 2019 .
7. Soroko, M. & Howel, K. (2018), Infrared thermography: current applications in equine medicine, J. Equ. Vet. Sci. , 60 (1), 9096
8. Hildebrandt, C., Raschner, C. & Ammer, K. (2010), An overview of recent application of medical infrared thermography in sports medicine in Austria, Sensors , 10 (5), 4700–4715
9. ^ Arnd Krüger : Thermography for prophylaxis. Performance sports 49 (2019), 3, 32-33