All objects that are not at absolute zero will emit electromagnetic radiation of different wavelengths. As the temperature of the object rises, the more vertical the thermal motion of molecules or atoms, the stronger the infrared radiation. The spectrum distribution or wavelength of radiation are related to the nature and temperature of the object. The quantity that measures the radiation capacity of an object is called the emissivity. Black objects or objects with darker colors surface have high emissivity and radiation transmission. Objects with bright colors or lighter colors surface have low emissivity and weaker radiation.
The human eye can only see electromagnetic radiation with a very narrow wavelength, called the visible spectrum. For radiation with a wavelength below 0.4um or above 0.7um, the human eye is powerless. The wavelength of the infrared region in the electromagnetic spectrum is between 0.7um and 1mm where the naked eye cannot see infrared radiation.
Modern thermal imaging equipment work in the mid-infrared region (wavelength of 3~5um) or far-infrared region (wavelength of 8~12um). By detecting the infrared radiation emitted by the object, the thermal imager produces a real-time image that provides a thermal image of the object and transforms the invisible radiation image into a clear image visible to the human eye. The thermal imager is very sensitive and can detect temperature differences of less than 0.1℃.
When working, the thermal imager uses optical instrument to focus the infrared energy emitted by objects in the scene on the infrared detector, and then the infrared data from each detector element is converted into a standard video format, which can be displayed on a standard video monitor on the screen, or recorded on a video tape. Because the thermal imaging system detects heat rather than light, it can be used around the clock. As it is a completely passive device, with no light radiation or radio frequency energy, it will not expose the user's location.
Infrared detectors are divided into two types: photon detectors and thermal detectors. After the photon detector absorbs the infrared energy, it directly produces an electrical effect. After the thermal detector absorbs the infrared energy, temperature changes, thereby generating an electrical effect. The electrical effects caused by temperature changes are related to material properties.
The photon detector is very sensitive, and its sensitivity depends on its own temperature. To maintain high sensitivity, the photon detector must be cooled to a lower temperature. The commonly used coolant is Stirling or liquid nitrogen.
Thermal detectors generally do not have as high sensitivity as photon detectors, but have good enough performance at room temperature, so cryogenic cooling is not required.
