Thermal imagers detect infrared radiation, radiation that's emitted by every object.
Every object emits electromagnetic radiation. The wavelength of the radiation depends upon the temperature. The sun, for example, has a temperature of 5,000 Kelvin and puts out radiation with a peak wavelength of about 550 nanometers (nm), smack dab in the middle of the visible spectrum. Cooler objects, such as airplanes, cars, and human bodies also emit radiation, but at much longer wavelengths, peaking around 8 or 9 micrometers (um). Thermal imagers let us see these invisible wavelengths.
The Infrared Spectrum
The infrared spectrum, just like the optical, covers a range of wavelengths.
Electromagnetic waves cover an energy spectrum stretching from high-energy gamma rays through low-energy radio waves. The energy carried by an electromagnetic wave depends upon its wavelength. Gamma rays have a wavelength of about one hundredth of a nanometer, or one hundredth of a billionth of a meter. Radio waves can have wavelengths many meters long. In between those extremes are the near, mid, and far infrared. The mid and far infrared cover the spectrum from about 2 um (2 millionths of a meter) to about 100 um.
MCT
Mercury-Cadmium-Telluride is a semiconductor sensitive to radiation from 5 um to 12 um. This is the heart of the region called the "thermal infrared," specifically because these are the wavelengths emitted by objects such as people, buildings, and cars. Most MCT detectors need to be cooled to cryogenic temperatures, either with liquid nitrogen or a mechanical cooler. MCT is very sensitive, but also expensive and relatively short-lived.
InSb
Indium Antimonide is another semiconductor sensitive to mid- and far-infrared wavelengths. InSb is most commonly used in the 3 to 5 µm band. In this mid-infrared band, the difference between the signal from a car exhaust and a human body, for example, is fairly large, which means the dynamic range of the detector must be adjusted to match the expected target. For optimum performance the user needs to select between a high gain sensitive to cool targets or a lower gain where the signal from hot targets won't overload the detector. Still, InSb detectors are often operated "uncooled," but this doesn't mean they aren't cooled at all --- just that they don't have to be cooled way down to cryogenic temperatures. This makes them generally cheaper and more reliable than MCT.
Microbolometers
Silicon microbolometers are devices that measure the temperature change due to absorbed radiation. A thin square of silicon is thermally isolated by being suspended on tiny silicon posts above a silicon readout microcircuit. Any change in temperature is due to radiation striking the suspended square, rather than conduction through the silicon chip. The microbolometer changes its resistance with temperature. Theoretically, these detectors are sensitive to all radiation that strikes them, but they are engineered with some kind of filter mechanism to limit their response to a given spectral region, say from 7 to 14 um. They can operate uncooled as InSb, but they can also be operated without any cooling at all. These detectors are less expensive than the photodiodes and can image the same scenes with only slightly less sensitivity.
QWIP
Quantum-well infrared photodetectors are engineered to have a specific, very narrow window of energy sensitivity. These detectors are fabricated out of many thin layers of a semiconductor material, most often gallium arsenide. The thinness of the layers themselves creates a confined structure, a quantum well, that will only absorb a narrow range of energy. By placing many slightly different layers on top of each other, the detectors automatically provide detailed spectral information from each pixel. Because objects at a given temperature have a specific ratio of one wavelength with respect to another, more detailed information about scene temperature can be obtained from QWIPs than from other technologies, although it comes at a higher price. QWIPs are a relatively new technology, and must be operated at cryogenic temperatures.
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