Term: photon exitance, Mp
https://doi.org/10.1351/goldbook.P04631

Definition:
Photon flux, number basis, qp, emitted by an element of the surface containing the source point under consideration divided by the area S of that element. SI unit is s-1 m-2.

Notes:
0) Mathematical definition: dq(p)/dS. If qp is constant over the surface area considered, Mp = qp/S.
1) Formerly called photon emittance.
2) This quantity can be expressed on a @C01019@ basis by dividing Mp by the @A00543@. In this case the symbol is Mn,p, the name 'photon @E02264@, amount basis', SI unit is mol s-1 m-2; common unit is einstein s-1 m-2.

Related Terms:
1) chemical amount (http://doi.org/10.1351/goldbook.C01019).
2) avogadro constant (http://doi.org/10.1351/goldbook.A00543).
3) exitance (http://doi.org/10.1351/goldbook.E02264).

Source: PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 394 (https://doi.org/10.1351/pac200779030293)

Citation: 'photon exitance, Mp' 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.P04631

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.