The actual objects that exist in nature are almost 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 the type of material constituting the object, the preparation method, the thermal process, the surface state and the environmental conditions. Therefore, in order to make the law of black body radiation applicable to all practical objects, a proportional coefficient related to material properties and surface states must be introduced, that is, emissivity. This coefficient indicates how close the thermal radiation of the actual object is to the black body radiation, and its value is between zero and a value less than 1. According to the law of radiation, as long as the emissivity of the material is known, the infrared radiation characteristics of any object are known.

When using an infrared radiation thermometer to measure the temperature of a target, it is first necessary to measure the infrared radiation of the target within its band range, and then the temperature of the measured target is calculated by the thermometer. Monochromatic pyrometers are proportional to the amount of radiation within a band; dual-color pyrometers are proportional to the ratio of the amount of radiation in the two bands.