A black body is an ideal radiator that absorbs radiant energy of all wavelengths without energy reflection or transmission, and its surface emissivity is 1. However, almost all actual objects existing in nature are not black bodies. In order to clarify and obtain the distribution rules of infrared radiation, an appropriate model must be selected in theoretical research. This is the quantized oscillator model of body cavity radiation proposed by Planck. Planck's law of black body radiation was derived, that is, the black body spectral radiance expressed in wavelength. This is the starting point of all infrared radiation theories, so it is called the black body radiation law. The radiation amount of all actual objects depends not only on the radiation wavelength and the temperature of the object, but also on factors such as the type of material, preparation method, thermal process, surface state and environmental conditions of the object. Therefore, in order to make the black body radiation law applicable to all real objects, a proportional coefficient related to the material properties and surface state must be introduced, that is, the emissivity. This coefficient represents how close the thermal radiation of an actual object is to blackbody radiation and has a value between zero and a value less than 1. According to the radiation law, as long as the emissivity of the material is known, the infrared radiation characteristics of any object can be known. The main factors that affect emissivity are: material type, surface roughness, physical and chemical structure, and material thickness.