A thermal imaging camera (also known as a thermography , thermal or infrared camera , or thermal imaging device in the military (often referred to as FLIR for Forward Looking Infrared )) is an imaging device similar to a conventional camera , but which receives infrared radiation . The infrared radiation is in the wavelength range from approx. 0.7 µm to 1000 µm. Thermal imaging cameras, however, use the spectral range from approx. 3.5 to 15 µm (medium and long-wave infrared ) due to the typical emission wavelengths in the vicinity of the ambient temperature . This range is also suitable for measuring and visualizing temperatures in the ambient temperature range when the emissivity is known. However , depending on the material, this varies between 0.012 and 0.98 - the temperature assignment can be correspondingly imprecise.
Since the normal atmosphere in this area is largely transparent, the lateral irradiation of the sun and artificial light sources hardly disturbs as long as the distance is only a few meters. At greater distances, the natural radiation of the air can falsify the result.
Thermography is a non-contact imaging process that makes visible heat radiation (mid- infrared ) from an object or body that is invisible to the human eye . With thermography, temperature distributions on surfaces and objects are recorded and displayed. In addition to the passive temperature measurement, active irradiation by infrared radiators can also take place. Methods for material testing, for example, are based on this.
The thermal imaging camera only evaluates differences in the received power , which is why objects with widely different emission factors can result in a large measurement error (apparent temperature difference). The assumed emission factor can be preselected on every thermal imaging camera. Radiation measurements should therefore be viewed with caution.
In principle, the camera is structured like a normal electronic camera for visible light, but the sensors differ in structure and function depending on the wavelength to be detected. It is not possible to record very long-wave radiation with conventional films , because the photosensitive emulsion would be “exposed” to the inherent thermal radiation even when packaged.
Images generated by infrared cameras are initially available as intensity information. Thermal imaging cameras usually display these in gray levels, common camera models are able to resolve up to 256 (8 bit) gray levels. However, it is not possible for the human observer to resolve such fine shades of gray; it is therefore useful to produce false-color images, which almost all thermal imaging cameras are capable of. The complete visible color space of the eye offers more differentiation than pure (gray) brightness differences. In the image colored in this way, the “brightness”, which indicates a thermal anomaly, is represented by a change in the displayed color instead of by different shades of gray. Different color palettes are usually available for coloring the gray-scale images. Often the lightest (warmest) part of the picture becomes white, the intermediate temperatures are shown in yellow and red tones and the dark (equally colder) parts of the picture in blue tones. In military applications, a false color display is normally not used, since this reduces the perceptibility of the image object for the human observer.
The geometric resolution of commercial thermographic cameras is considerably lower than that of cameras for the visible spectral range. It is typically 160 × 120, 320 × 240 or 384 × 288 image points ( pixels ). Detectors with 640 × 480 pixels have also recently been used. With micro scanning , the camera resolution can be improved up to 1280 × 960. The resolution determines the interaction with the inserted lenses or the field of view ( field of view ) of the camera the smallest definable measurement spot of thermography system.
An image is projected onto an electronic image sensor through a lens with lens (s). Cameras for the wavelength range from 8 to 14 µm use optics made from moisture-sensitive salts such as sodium chloride (table salt), silver salts or from monocrystalline semiconductor materials such as germanium or zinc selenide .
There are several methods by which infrared image sensors work.
- Silicon sensors are used for very short wavelengths around 800 nm . They convert the photons directly into a photocurrent via the photoelectric effect .
- Indium gallium arsenide sensors (InGaAs) or lead sulfide sensors are used for wavelengths of 1 to 2 µm (SWIR) .
- In the wavelength range 3–5 µm (MWIR), indium - antimony detectors (InSb) and cadmium - mercury - telluride detectors (MCT) are mainly used . A cold filter limits the wavelength downwards. Indium-antimony detectors with corresponding cold filters offer a sensitive spectral range from 1 to 5 µm.
- Gallium arsenide quantum well detectors (QWIP) and cadmium - mercury - telluride detectors are often used for the long wave range from 8 to 14 µm (LWIR) . Microbolometer arrays that detect the radiation by heating a sensor element are also well suited for this wavelength range. Common materials for microbolometer arrays are vanadium oxide (VOx) or amorphous silicon (a-Si).
Thermographic cameras can be divided into two types:
Cooled infrared detectors
Cooled infrared detectors work on the internal photo effect , that is, they consist of an array of photo receivers. The detectors are usually housed in a vacuum sealed housing and are cryogenically cooled. The working temperature of the detectors is typically between 4 K and 110 K, the usual value being around 80 K (slightly above the boiling temperature of nitrogen). This means that the detectors are usually much colder than the objects to be observed, which significantly increases the thermal sensitivity (temperature resolution) of the thermographic system compared to the uncooled systems. A disadvantage of this method: If the cooling of the detector fails, the thermographic system is blind.
Further disadvantages of cooled systems are the increased acquisition and operating costs as well as the sometimes long start-up times until the system has cooled the detector down to operating temperature. On the other hand, there is the outstanding image quality compared to uncooled systems.
The infrared detectors in cooled systems usually consist of special semiconductor materials.
The detectors, which work according to the photoelectric effect, are cooled to temperatures in the region of 70 K so that the intrinsic radiation of the camera and the detector does not influence the measurement. In the past, liquid nitrogen or carbon dioxide was often used for cooling, modern cameras mostly work with Peltier elements , very precise models for scientific applications and many devices in the military sector, on the other hand, use Stirling coolers .
Uncooled infrared detectors
Uncooled thermographic cameras use infrared sensors that are kept at a constant temperature by thermoelectric coolers, Peltier elements , in order to reduce the signal drift of the receiver elements. They are usually thermostated close to the ambient temperature . All modern uncooled systems work on the principle of changes in resistance , voltage or current strength when the detector is heated by the infrared radiation. These changes are measured and compared with the values at operating temperature. This is used to determine the amount of radiation absorbed and - with the aid of a preset emission factor - a temperature is calculated.
Such systems do not require expensive, cumbersome cryogenic cooling devices. This means that these thermographic systems are significantly smaller and more cost-effective than cooled systems. However, they deliver a comparatively poorer result.
Uncooled detectors use pyroelectric or microbolometer arrays . Today they are available as compact modules (so-called thermal image sensors). The resolutions of these compact modules typically range from 320 × 240 to 1,024 × 768 pixels with measuring distances in the range of approx. 1 ... 10 m. Image readout frequencies go up to about 50 Hz. In addition to the classic analog image signals (PAL, NTSC), digital interfaces (USB, LAN, WLAN, Bluetooth) are also supported.
The detector cell of a microbolometer array consists of a radiation-sensitive disk only a few micrometers thick, which is held above the actual detector 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 incident infrared radiation is absorbed and leads to an increase in temperature of the disc, which in turn changes the 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 .
Both microbolometer arrays and pyrometric sensors require a mechanical shutter or at least periodic shading of the image sensor. The reason for pyrometric sensors is that they can only react to changes in temperature. In bolometer arrays, the 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).
Typical areas of application
Originally developed for military use during the Korean War , thermographic cameras can now be found in many areas of application. The development of new technologies and the associated drop in prices for thermographic systems has led to the spread of this technology. The improvement of the lenses used and the development of professional software for analysis and reporting are continuously expanding the possibilities of infrared thermography.
In the civil sector, uncooled infrared detectors are predominantly used. There are handheld devices that cover the temperature range from −20 ° C to 900 ° C and provide a temperature resolution of 0.025 K. Often, lenses with different opening angles can be used; the images can be saved or transferred to a PC .
- In building thermography, the procedure for testing the thermal insulation / detecting thermal bridges in houses , for building diagnostics / energy certification and checking flat roofs, for structural analysis of the masonry, for moisture detection in walls and roofs, to detect the course of underfloor heating and for localization used by cracks in pipelines .
- In photovoltaics , thermography is used for troubleshooting. You can z. B. find contact resistances on the cell contacts or in the module connection boxes. In addition, solar modules that are idle have a slightly higher temperature than those that are operated in MPP (Maximum Power Point). This allows you - z. B. with a thermography drone - find module strings that are not in operation.
- In industry and manufacturing, thermal imaging cameras are used, for example, to measure the distribution of power dissipation in electronic assemblies and to test electrical and mechanical systems.
- In the fire department , the thermal imaging cameras support the detection of embers in the event of a fire and the search for people in smoky buildings or extensive areas in the dark. In this area, grayscale images are used, which color the hottest point red from a certain temperature. Since the equipment used was initially very expensive (several thousand euros), it was not possible for many volunteer fire brigades to purchase a camera for a long time; sometimes several fire brigades shared a thermal imaging camera. In the meantime, thermal imaging cameras have become widespread.
- Scientists use thermal images to show the surface temperature of land or oceans . Due to the different thermal diffusivity, archaeologists can recognize structures hidden in the earth.
- The Federal Police use stationary and mobile thermal imaging cameras to monitor border sections. Illegal border crossings can be recognized: people (but also animals) and their recently abandoned camps are visible from a distance due to the increased temperature, even in the dark.
- In medicine, thermal imaging cameras are used for a wide variety of diagnostic purposes, such as to detect local sources of inflammation and for mass examinations of people for fever (see EN 80601-2-59 ), to examine blood flow and detect circulatory disorders and to detect breast cancer .
- In veterinary medicine, it is used to measure the heat and thus the blood circulation in horse's legs and thus to determine doping in equestrian sports .
- In recent times, the thermal imaging camera has also been gaining popularity among photo artists who use thermal images as a medium of expression. More examples can be found in the infrared photography article .
- Traffic technology: Detection of people and vehicles in traffic light control (particularly advantageous in comparison to conventional video and induction loop technology when detecting cyclists)
- Car night vision assistant : The latest development are driver assistance systems containing thermal imaging cameras , for example the system from Autoliv Inc. installed at BMW , which can recognize people and animals better due to their thermal radiation than conventional cameras in the near infrared, which can only penetrate fog better .
- In non-destructive testing of materials and components, the test part is heated in a targeted manner by means of an excitation source, so that hidden defects can be measured through different thermal behavior. The NDT methods include pulse thermography, lock-in thermography and thermoelastic stress analysis. To carry out these methods, particularly fast and high-resolution infrared cameras are required.
- The airtightness of buildings and conditional gas leaks from containers can also be checked by means of thermographic examination .
- Security technology: In modern perimeter protection, thermal imaging cameras reduce the number of sensor components required. The high level of contrast (heat difference between people and the environment) forms a reliable basis for video analysis for detection.
In the military sector, thermal imaging devices (WBG) are used for observation and reconnaissance in the dark or in poor visibility. The WBG of the battle tank Leopard 2 , for example, based on a detector of mercury cadmium telluride (engl .: mercury cadmium telluride MCT), which is cooled to approximately -190 ° C, which requires a lead time of about 15 minutes. The display is green-monochrome with a selectable polarity of black or white, so that heat sources appear particularly bright or dark. If there is a sufficient temperature difference between individual objects, an observed section of the terrain can be recognized very well.
Thermal imaging devices have the advantage over night vision devices that there is no residual light , and there is no need to use an infrared headlight , which in turn can be very easily detected and switched off. Furthermore, objects that are optically well camouflaged during the day can in many cases be easily recognized due to the heat signature. Hiding heat sources - especially when the outside temperature is low - is only possible with great effort.
However, there are limits to the use of WBG in heavy rain , fog or blowing snow .
Target search systems of self-guiding missile weapons can partially differentiate the heat sources of an aircraft engine from those dropped decoys on the basis of the thermal signature .
Advantages of taking pictures with a thermal imaging camera
- The temperature distribution over a large area can be monitored at the same time - a time advantage compared to point-by-point registration with thermometers.
- In technical systems, points of increased temperature can be detected before further damage occurs.
- The measurement is carried out contactless even over long distances, for example in high voltage systems or with rotating components.
- You can discover objects of different temperatures in a dark environment (see also pit organ ).
Disadvantages and limitations of the procedure
- Thermal imaging cameras with good resolution (more than 320 × 240 pixels) are very expensive.
- The images are difficult to interpret if the emission factor is unknown.
- Reflections (such as from sunlight) on bare metal surfaces can be very annoying.
- The accuracy is usually worse than ± 2% and therefore significantly lower than with contact measurement with a thermometer.
- You can only measure surface temperatures.
- In strong winds, solar radiation or a damp surface, the measurement accuracy drops considerably.
- Snowfall or rain reduce the transmission factor of the air, which is why the temperature displayed hardly relates to the surfaces "behind".
- The detection of fast moving movements is limited by the often low frame rate (<50 Hz). In the meantime, however, there are already high-speed thermography systems in the high-end sector that can record over 1000 images per second.
- Werner Brügel: Physics and technology of ultrared radiation . Vincentz, Hanover 1961, 448 pp.
- Helmut Israel: Measuring and Locating with Infrared . Franzis, Munich 1988, 127 pp.
- Thomas Zimmermann: Textbook of Infrared Thermography . Fraunhofer IRB, Stuttgart 2012, 170 pp.
- Keller, Maass, Reichard, Witte: WBK training manual for the fire brigade . Cologne 2012
- Picture gallery with thermographic images of animals, houses, power plants, electrical devices, etc.
- VATh - Federal Association for Applied Thermography
- Report on the use of a thermal imaging camera Bochumschau
- www.wbk- Einsatz.de - Information site about thermal imaging cameras in fire service
- Measuring with the thermal imaging camera - explained for schoolchildren on the KIT physics school laboratory initiative
- ↑ Jörg Böttcher: Compendium of measurement technology and sensor technology: characteristics of thermal image sensors. Retrieved September 19, 2019 .
- ↑ BB Lahiri, S. Bagavathiappan, T. Jayakumar, John Philip: Medical applications of infrared thermography: A review. Infrared Physics & Technology , Vol. 55, No. 4, July 2012, pp. 221-235
- ↑ Hsin Wang; Dwight R. Wade, Jr .; Jack Kam: IR imaging of blood circulation of patients with vascular disease. Proc. SPIE 5405, Thermosense XXVI, 115 (April 12, 2004); doi : 10.1117 / 12.545899
- ↑ Hairong Qi; Kuruganti, PT; Zhongqi Liu: Early detection of breast cancer using thermal texture maps, Biomedical Imaging , 2002, Proceedings of the 2002 IEEE International Symposium on Biomedical Imaging, pp. 309-312 doi : 10.1109 / ISBI.2002.1029255
- ^ EY-K. Ng: A review of thermography as promising non-invasive detection modality for breast tumor , International Journal of Thermal Sciences, Vol. 48, No. 5, May 2009, pp. 849-859 doi : 10.1016 / j.ijthermalsci.2008.06.015
- ↑ Mobile thermal imaging camera in anti-doping use. Badische Zeitung, September 22, 2008, accessed on September 13, 2014 .