Absorptance is the ratio of the radiation absorbed by a surface to that incident upon it. Also known as absorbance.
Applicable units are those of quantitykind:Absorptance
https://en.wikipedia.org/wiki/Absorptance
https://www.researchgate.net/post/Absorptance_or_absorbance
http://en.wikipedia.org/wiki/Absorbance
Absorptance is the ratio of the radiation absorbed by a surface to that incident upon it. Also known as absorbance.
$\alpha = \frac{\Phi_a}{\Phi_m}$, where $\Phi_a$ is the absorbed radiant flux or the absorbed luminous flux, and $\Phi_m$ is the radiant flux or luminous flux of the incident radiation.