Blackbody is an idealized radiator that absorbs radiation energy of all wavelengths without any reflection or transmission of energy. Its surface emissivity is 1. However, almost all actual objects present in nature are not blackbodies. In order to understand and obtain the distribution law of infrared radiation, it is necessary to choose a suitable model in theoretical research, which is the quantized oscillator model of body cavity radiation proposed by Planck. This leads to the law of Planck blackbody radiation, which is the spectral radiance of blackbodies expressed in wavelength. This is the starting point of all infrared radiation theories, hence the blackbody radiation law. The radiation amount of all actual objects is not only dependent on the wavelength of radiation 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 that make up the object. Therefore, in order for the blackbody radiation law to apply to all practical objects, it is necessary to introduce a proportional coefficient related to material properties and surface states, namely emissivity. This coefficient represents the proximity between the thermal radiation of the actual object and the blackbody radiation, with a value between zero and a value less than 1. According to the law of radiation, as long as the emissivity of a material is known, the infrared radiation characteristics of any object are known. The main factors affecting emissivity include material type, surface roughness, physicochemical structure, and material thickness.