A body that completely absorbs all radiation incident upon it, irrespectively of wavelength, is called a perfect black body. A black body when heated emits radiation of all wavelengths at that temperature. The wavelength of the emitted radiation is independent of the material of the body and depends only on the temperature of the body.

The absorptance is defined as the ratio of the radiation (power) absorbed within a given waveband λ to λ + d λ to the radiation (power) incident on the body in the same waveband. Mathematically, a λ = P λ a P λ i = P λ a d t P λ i d t a λ = E λ a E λ i

The reflectance of the body is defined as the ratio of the part of the heat radiation reflected Q r by the body to the total heat radiation incident Q i on it. Mathematically, t = Q r Q i