Term: intensity https://doi.org/10.1351/goldbook.I03072 Definition: The traditional term for photon flux, fluence rate, irradiance or radiant power (radiant flux). In terms of an object exposed to radiation, the term should now be used only for qualitative descriptions. Related Terms: 1) photon flux (http://doi.org/10.1351/goldbook.P04636). 2) fluence rate (http://doi.org/10.1351/goldbook.P04635). 3) irradiance (http://doi.org/10.1351/goldbook.P04637). 4) radiant power (http://doi.org/10.1351/goldbook.R05046). 5) radiant flux (http://doi.org/10.1351/goldbook.R05044). Source: PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2248 (https://doi.org/10.1351/pac199668122223) Citation: 'intensity' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.I03072 License: 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. Disclaimer: 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.