3. Emissivity of real materials

If the emissivity of a material does not depend on wavelength, it is called a "gray body". However, this property is not encountered in reality, and a body whose emissivity does not vary in the spectral range under study or of interest is referred to as a "gray body". A gray body whose spectral properties do not vary with temperature has a total emissivity that also does not vary with temperature. Conversely, a surface whose spectral emissivity varies with wavelength will have a total emissivity that varies with temperature, as the spectral distribution of blackbody radiation varies with temperature.