Ratio of the absorbed to the incident radiant power. Also called absorption factor. When α ≤ 1, α ≈ Ae, where Ae is the Napierian absorbance.
Source:
Green Book, 2nd ed., p. 32 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
See also:
PAC, 1985, 57, 105. 'Names, symbols, definitions and units of quantities in optical spectroscopy (Recommendations 1984)' on page 114 (https://doi.org/10.1351/pac198557010105)
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2226 (https://doi.org/10.1351/pac199668122223)
PAC, 1996, 68, 957. 'Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)' on page 959 (https://doi.org/10.1351/pac199668040957)