Infrared light is a kind of light invisible to the human eye, but in fact it is an objective substance like any other light. As long as the temperature of any object is above thermodynamic zero, infrared rays will radiate around it. Infrared light is located in the visible light other than red light, so it is called infrared. Its wavelength range is roughly within the spectrum of 0.75-100μm.

The physical nature of infrared radiation is thermal radiation. The higher the temperature of the object, the more infrared radiation, the stronger the energy of infrared radiation. It has been found that the thermal effect of various monochromatic light in the solar spectrum increases gradually from violet light to red light, and the maximum thermal effect occurs in the frequency range of infrared radiation, so people also call infrared radiation thermal radiation or heat rays.

Thermal sensors make use of the thermal effect of radiation, so that the detector device receives the radiation energy caused by the temperature rise, and then make the sensor in a column with the temperature of the performance changes. By detecting a change in one of these properties, radiation can be detected. In most cases is through the Seebeck effect to detect radiation, when the device receives radiation, causing a non-electrical physical changes, but also through the appropriate changes into the amount of electricity after the measurement.