Grey Body

In contrast to a black body, a gray body does not have a maximum absorption capacity, but otherwise has similar properties.

Electromagnetic radiation can interact with matter in three ways. It can be reflected or diffusely scattered back into the environment, transmitted by transparent objects, or absorbed in matter. In practical cases, one of these interactions often dominates. The law of conservation of energy requires that the sum of absorption (α), reflectivity (ρ), and transmissivity (τ) is equal to 100% or 1 = α + ρ + τ.

In general, all three parameters depend on the wavelength and object temperature. A Grey Body is a physical model for an object where α, ρ, and τ are constants and α is not equal to 1. It also follows that an emissivity determined for one wavelength is valid for all other wavelengths.

In practice, there are only approximations to this model. When using appropriate coatings, the parameters of the light-matter interaction can be assumed to be constants, at least within a limited temperature and spectral range.