A thermal imaging camera (also known as a thermographic, thermal, or infrared camera, or in the military, a thermal imaging device) is a device similar to a conventional camera, but it receives infrared radiation and, unlike a pyrometer, reproduces the IR radiation as an image of the object.
The infrared radiation is in the wavelength range from approx. 0.7 μm to 1000 μm. However, thermal imaging cameras use the spectral range of approx. 3.5 to 15 μm (medium and long-wave infrared) due to the typical emission wavelengths close to 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 scatters 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 is hardly disturbing as long as the distance is only a few meters. At greater distances, the intrinsic radiation of the air can falsify the result.
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Thermography is a non-contact imaging technique that visualizes the thermal radiation (mid-infrared) of an object or body that is invisible to the human eye. In thermography, temperature distributions on surfaces and objects are recorded and displayed. In addition to passive temperature measurement, active irradiation can also be carried out by infrared emitters. This is the basis for materials testing procedures, for example.
The thermal imaging camera only evaluates differences in the received power, which is why objects with very different emission factors can result in a large measurement error (apparent temperature difference). The presumed emission factor can be preselected on each thermal imaging camera. Radiation measurements should therefore be viewed with caution.
In principle, the camera is designed like a normal electronic camera for visible light, but the sensors differ in design and functionality 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” by the thermal radiation even in its packaged state.
Images generated by infrared cameras are initially available as intensity information. Thermal imaging cameras usually display these in grayscale, while common camera models are capable of resolving up to 256 (8 bit) grayscales. However, it is not possible for the human observer to dissolve such fine shades of grey; it is therefore useful to produce images in false color, 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” that indicates a thermal anomaly is represented by a change in the color displayed, rather than by different shades of gray. There are usually different color palettes available for coloring the grayscale images. Often the brightest (warmest) part of the image is white, the intermediate temperatures are shown in yellow and red tones, and the dark (equally colder) parts of the image in shades of blue. In military applications, a false-color representation is not normally used, as it reduces the recognizability of the image object to the human viewer.
Technology Behind Thermal Imaging Camera
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 pixels. Recently, detectors with 640 × 480 pixels have also been used. Micro scanning can be used to improve the camera resolution up to 1280 × 960. The resolution, in combination with the lenses used or the field of view of the camera, determines the smallest definable measuring spot of the thermography system. Thermal imaging cameras are also available for smartphones.
Through a lens consisting of lenses, an image is projected onto an electronic image sensor. Conventional cameras operate passively (i.e. without their own light source) in the wavelength range of 8 to 14 μm and use optics made of germanium, which is permeable to these wavelengths but costs about 100 times more compared to glass lenses. In addition, single-crystalline semiconductor materials such as silicon or zinc selenide are also suitable. On the other hand, salts such as sodium chloride (table salt), silver salts or chalcogenides are usable in principle, but unsuitable for practical applications due to moisture sensitivity.
