Lighting Design Glossary

Absorption
Absorption factor
Absorptance
Absorption Coefficient

(Terms of radiometry/photometry)

The dissipation of light (radiation) within a surface or medium, caused by the conversion of radiant (luminous) energy to a different form of energy, usually heat, by interaction with matter. The Absorption is the "missing piece", when comparing the total reflected and transmitted energy with the incident energy.

The ratio of the total absorbed radiant or luminous flux to the incident flux is called absorptance (formerly also absorption factor).

Standard unit of absorptance is percent (%) or a factor between 0 an 1.

The fraction of light absorbed per unit distance in a participating medium (eg. mist in the Radiance software) is called absorption coefficient.

Standard unit of the absorption coefficient is fraction per meter (/m)

In the Radiance software, absorptance and absorption coefficient are specified seperately for red, green and blue.