Absorptivity

(Sometimes called absorptance, or absorptive power) When radiation falls onto a surface, some is reflected and some absorbed; the proportion absorbed is the absorptivity of the surface. It may vary according to the wavelength of the radiation, and so may be different between visible light and radiant heat. Absorptivity may also change somewhat with temperature.

The absorptivities of materials are valuable data for the design of buildings and rooms for energy efficiency and comfort. It can be shown that the absorptivity of a surface is equal to its emissivity, i.e. surfaces which are good at absorbing heat are also good at emitting it by radiation. For tabulated figures, see emissivity.