absorptance, α

https://dx.doi.org/10.1351/goldbook.A00035
Ratio of the absorbed to the incident radiant power. Also called absorption factor. When α ≤ 1, α ≈ A e, where A e is the Napierian absorbance.
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See also:
PAC, 1985, 57, 105 (Names, symbols, definitions and units of quantities in optical spectroscopy (Recommendations 1984)) on page 114
PAC, 1996, 68, 957 (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 959
PAC, 1996, 68, 2223 (Glossary of terms used in photochemistry (IUPAC Recommendations 1996)) on page 2226