Almost all actual objects that exist in nature are not black bodies. 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 its value is between zero and a value less than 1. According to the radiation law, as long as you know the emissivity of the material, you can know the infrared radiation characteristics of any object.

When using an infrared radiation thermometer to measure the temperature of a target, the amount of infrared radiation of the target within its band range must first be measured, and then the temperature of the measured target is calculated by the thermometer. A single-color thermometer is proportional to the amount of radiation within the band: a two-color thermometer is proportional to the ratio of the amount of radiation in the two bands.