One may imagine a small homogeneous spherical material body labeled X at a temperature T X, lying in a radiation field within a large cavity with walls of material labeled Y at a temperature T Y. The body X emits its own thermal radiation. At a particular frequency ν, the radiation emitted from a particular cross-section through the centre of X in one sense in a direction normal to that cross-section may be denoted I ν, X (T X), characteristically for the material of X. At that frequency ν, the radiative power from the walls into that cross-section in the opposite sense in that direction may be denoted I ν, Y (T Y), for the wall temperature T Y. For the material of X, defining the absorptivity α ν, X, Y (T X, T Y) as the fraction of that incident radiation absorbed by X, that incident energy is absorbed at a rate α ν, X, Y (T X, T Y) I ν, Y (T Y).