00044 IUPAC Gold Book - absorptivity 10.1351/goldbook.A00044 A00044 current 1 Absorptance divided by the optical path length. For very low attenuance it approximates the absorption coefficient [within the approximation \((1 - e^{-A}) \sim A\)]. The use of this term is not recommended. Absorptance https://goldbook.iupac.org/terms/view/A00035 absorption coefficient https://goldbook.iupac.org/terms/view/A00037 attenuance https://goldbook.iupac.org/terms/view/A00513 PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2227 (https://doi.org/10.1351/pac199668122223) https://goldbook.iupac.org/terms/view/A00044/html https://goldbook.iupac.org/terms/view/A00044/json https://goldbook.iupac.org/terms/view/A00044/plain Citation: 'absorptivity' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. 10.1351/goldbook.A00044 The IUPAC Gold Book is licensed under Creative Commons Attribution-ShareAlike CC BY-SA 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) for individual terms. If you are interested in licensing the Gold Book for commercial use, please contact the IUPAC Executive Director at executivedirector@iupac.org . The International Union of Pure and Applied Chemistry (IUPAC) is continuously reviewing and, where needed, updating terms in the Compendium of Chemical Terminology (the IUPAC Gold Book). Users of these terms are encouraged to include the version of a term with its use and to check regularly for updates to term definitions that you are using. 2026-07-25T07:18:56+00:00