Technologies Behind Thermal Imaging Cameras

What is infrared radiation?
The Latin prefix "infra" means "under" or "below". Thus, "infrared" indicates a spectrum outside the red end of the visible light spectrum. The infrared zone is located between the visible light zones and the microwave zone of the electromagnetic spectrum. Because heated objects emit energy in the infrared spectrum, it is often referred to as the thermal zone of the spectrum. All objects emit energy in the infrared spectrum to some extent, even objects at room temperature and ice.
The higher the temperature of an object, the higher the energy it emits and the shorter the dominant or peak wavelength. Peak radiation from an object occurs at room temperature at a wavelength of 10 microns. The sun has an equivalent temperature of 5900 K and a peak wavelength of 0.53 microns (green light). It emits a large amount of energy in spectra ranging from ultraviolet to far infrared.
Most of the infrared radiation spectrum cannot be used by detection systems, as this radiation is absorbed by water or carbon dioxide in the atmosphere. However, there are several wavelength ranges with good transmission. Long-wave infrared radiation (LWIR) is presented in the range of 8-14 microns, while in the range of 9-12 microns, almost 100% transmission coefficient is provided. The LWIR range provides excellent visibility to most terrestrial objects. The MWIR mid-wave infrared range (3.3 - 5.0 µm) also provides nearly 100% transmission, with the advantage of less background noise. Shortwave and visible infrared radiation (SWIR) or near infrared radiation (NIR - 0.35-2.5 μm) matches the transmission range in the upper atmosphere and peak solar radiation, providing the possibility of using sensors with the highest visibility and resolution of all three ranges. However, the quality of SWIR images drops significantly without lunar or artificial lighting, and they are unable to provide visibility of objects with a temperature of 300K.
 Short Wave Infrared Technology (SWIR)
Shortwave infrared sensors have unique capabilities that often complement LWIR and MWIR sensors. The SWIR sensor is the same optical sensor as the cooled LWIR or MWIR sensors. But unlike them, SWIR sensors use (mainly) reflected light. For this indicator, SWIR sensors do not differ from cameras in the visible range or the human eye. Therefore, SWIR images are comparable to images of the visible range (black and white) in terms of resolution and detail.
The introduction of InGaAs material marked a breakthrough in the development of SWIR sensors without cryogenic cooling. InGaAs SWIR sensors typically use Peltier chillers. They have a sensitivity ranging from 900 to 1700 nm.
SWIR cameras can be used for shooting day and night (under the light of the stars), which makes them stand out in comparison with other technologies. Other advantages include small size (compared to cooled detectors), the ability to see hidden lasers, and the ability to see through glass.
Medium wave infrared technology
Mid-wave infrared is also called "thermal infrared" because it radiates from the object itself and does not require an external light source to display it. How bright an object appears in an image is influenced by two factors: the temperature of the object and emissivity (a physical property of a material that indicates how efficiently a material emits).
As the object heats up, it emits more energy and becomes brighter for thermal imaging systems. Phenomena that usually result in reduced visibility have less effect on scattering in the mid-wave infrared range than in the short-wave infrared wavelength range, so these cameras are less sensitive to phenomena such as smoke, dust or fog.
MWIR captures light in the 3 to 5 micron range. MWIR cameras are used when the main goal is to obtain high quality images, rather than measuring temperature and movement.
Long wave infrared technology (LWIR)
For decades, infrared cameras using long-wave infrared and medium-wave infrared sensors have been used by the military to detect human movement. Such thermal imaging cameras detect heat radiation from people, vehicles, animals.
There are two types of long-wave infrared detectors - uncooled LWIR microbolometers and the much rarer cooled LWIR detectors.
An uncooled LWIR microbolometer is a thermal sensor with essentially a resistor at each pixel. Microbolometers are usually made from amorphous silicon (a-Si) or vanadium oxide (Vox). The resistance value changes depending on