Absorptance
Author: the photonics expert Dr. Rüdiger Paschotta (RP)
Definition: the fraction of incoming radiant flux which is absorbed by an object
Alternative term: absorptivity
Related: absorptionabsorbanceemissivity
Units: (dimensionless)
Formula symbol: ($A$)
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DOI: 10.61835/sus Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn
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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.
Frequently Asked Questions
This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).
What is absorptance?
The absorptance of an object is the fraction of an incoming radiant flux which is absorbed. It is a dimensionless quantity with a value between 0 and 1.
How are absorptance and emissivity related?
According to Kirchhoff's law of thermal radiation, the absorptance of a medium is identical to its emissivity. This means an object that is good at absorbing radiation is also good at emitting it.
Does the absorptance of a surface depend on the direction of light?
Yes, absorptance can depend on the direction of the incoming light. One therefore distinguishes between directional absorptance for a specific direction and hemispherical absorptance, which is an average over all directions.
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