Title: photon fluence rate
Long Title: IUPAC Gold Book - photon fluence rate
DOI: 10.1351/goldbook.P04635
Status: current
Index: quantity

Definition
Rate of photon fluence. Total number of photons (\(N_{\rm{p}}\)) incident from all directions on a small sphere divided by the cross-sectional area of the sphere and per time interval. SI unit is \(\rm{m}^{-2}\ \rm{s}^{-1}\). Same as photon fluence rate.

Notes
1) Mathematical definition: \(E_{\rm{p,o}} = \rm{d}N_{\rm{p}}/\left ( \rm{d}t\, \rm{d}S \right ) = \rm{d}H_{\rm{p,o}}/\rm{d}t\). If \(E_{\rm{p,o}}\) is constant over the time interval and the surface, \(E_{\rm{p,o}} = N_{\rm{p}}/t\, S\). Equivalent definition: \(E_{\rm{p,o}} = \int _{4\pi}L_{\rm{p}}\rm{d}\varOmega\) with \(L_{\rm{p}}\) the photon radiance and \(\varOmega\) the solid angle of the beams passing through the given point on the surface.
2) It reduces to photon irradiance \(E_{\rm{p}}\) for a parallel and normally incident beam not scattered or reflected by the target or its surroundings.
3) This quantity can be used on a chemical amount basis by dividing \(E_{\rm{p,o}}\) by the Avogadro constant, the symbol then being \(E_{n\rm{,p,o}}\), the name 'photon spherical irradiance, amount basis', SI unit is \(\pu{mol m-2 s-1}\); common unit is \(\pu{einstein m-2 s-1}\).

Related Terms
- Avogadro constant: https://goldbook.iupac.org//terms/view/A00543
- angle: https://goldbook.iupac.org//terms/view/A00346
- chemical amount: https://goldbook.iupac.org//terms/view/C01019
- fluence rate: https://goldbook.iupac.org//terms/view/FT07376
- photon irradiance: https://goldbook.iupac.org//terms/view/P04637
- photon radiance: https://goldbook.iupac.org//terms/view/P04639

Source
- PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 395 (https://doi.org/10.1351/pac200779030293)

Other Outputs
- html: https://goldbook.iupac.org/terms/view/P04635/html
- json: https://goldbook.iupac.org/terms/view/P04635/json
- xml: https://goldbook.iupac.org/terms/view/P04635/xml

Citation: Citation: 'photon fluence rate' 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.P04635
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.
Collection: If you are interested in licensing the Gold Book for commercial use, please contact the IUPAC Executive Director at executivedirector@iupac.org .
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.
Accessed: 2026-06-06T22:39:00+00:00