The absorption rate of an object or surface is the ratio of the thermal radiation energy projected onto the object and absorbed to the total thermal radiation energy projected onto the object. Its value is always between 0 and 1, where 1 means that all the incident radiation is absorbed.
According to Gustav Kirchhoff's law of thermal radiation, the absorption rate of a medium is the same as its emissivity: the more electromagnetic radiation an object absorbs, the more radiation of the same kind it emits.
The absorption rate of a surface can actually depend on the direction of the incident rays. Thus, one can specify a directional absorption rate and a hemispherical absorption rate; The latter is an average value in different directions. In addition, the absorption rate generally depends on the wavelength of the light.
Absorbance should not be confused with absorbance.
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