Title: photon exitance
Long Title: IUPAC Gold Book - photon exitance
DOI: 10.1351/goldbook.P04631
Status: current
Index: quantity

Definition
Photon flux, number basis, \(q_{\rm{p}}\), emitted by an element of the surface containing the source point under consideration divided by the area \(S\) of that element. SI unit is \(\rm{s}^{-1}\ \rm{m}^{-2}\).

Notes
1) Mathematical definition: \(\rm{d}q_{p}/\rm{d}S\). If \(q_{\rm{p}}\) is constant over the surface area considered, \(M_{\rm{p}} = q_{\rm{p}}/S\).
2) Formerly called photon emittance.
3) This quantity can be expressed on a chemical amount basis by dividing \(M_{\rm{p}}\) by the Avogadro constant. In this case the symbol is \(M_{n\rm{,p}}\), the name 'photon exitance, amount basis', SI unit is \(\pu{mol s-1 m-2}\); common unit is \(\pu{einstein s-1 m-2}\).

Related Terms
- Avogadro constant: https://dev.goldbook.iupac.org/terms/view/A00543
- chemical amount: https://dev.goldbook.iupac.org/terms/view/C01019
- exitance: https://dev.goldbook.iupac.org/terms/view/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)

Other Outputs
- html: https://dev.goldbook.iupac.org/terms/view/P04631/html
- json: https://dev.goldbook.iupac.org/terms/view/P04631/json
- xml: https://dev.goldbook.iupac.org/terms/view/P04631/xml

Citation: Citation: 'photon exitance' 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.P04631
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Accessed: 2026-06-13T02:54:33+00:00