Gray body definition
If the monochromatic absorption of an object is independent of the wavelength of the radiant energy projected onto the object, that is, = α = constant, it is called a gray body.
The radiation spectrum of an object is continuous, and the ratio of the radiation intensity of all wavelengths of radiation at any temperature to the radiation intensity of the corresponding wavelength of blackbody at the same temperature is constant, then this object is called an ideal gray body, or simply Gray body. The relationship between the radiation intensity and wavelength of an actual object at a certain temperature is irregular, so it is not gray. But for the sake of convenience in engineering calculations, they are all regarded as gray bodies approximately.