Term: photon flux, qp, Φp
https://doi.org/10.1351/goldbook.P04636

Definition:
Number of photons (quanta of radiation, Np) per time interval. SI unit is s-1.

Notes:
0) Mathematical definition: qp = dNp/dt. If the number of photons is constant over the time interval, qp = Np/t.
1) This quantity can be used on a @C01019@ basis by dividing the photon flux, number basis, qp, by the @A00543@, the symbol then being qn,p, the name 'photon flux, amount basis', SI unit is mol s-1; common unit is einstein s-1.
2) Although the symbol recommended by CEI is Φp, the symbol qp is preferred since Φ is reserved for @Q04991@.

Related Terms:
1) chemical amount (http://doi.org/10.1351/goldbook.C01019).
2) avogadro constant (http://doi.org/10.1351/goldbook.A00543).
3) quantum yield (http://doi.org/10.1351/goldbook.Q04991).

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)

Citation: 'photon flux, qp, Φp' 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.P04636

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.