The absorptance of an object or surface is the fraction of an incoming radiant flux which is absorbed. It always has a value between 0 and 1, where 1 means that all incoming radiation is absorbed.
According to Gustav Kirchhoff's law of thermal radiation, the absorptance of a medium is identical to its emissivity: the more an object can absorb electromagnetic radiation, the more it can also emit the same kind of radiation.
The absorptance of a surface can actually depend on the direction of incoming light. Therefore, one can specify a directional absorptance and a hemispherical absorptance; the latter is an average over different directions. Also, the absorptance is generally dependent on the optical wavelength.
Absorptance should not be confused with absorbance.
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