photon fluence rate, Ep,o 

https://doi.org/10.1351/goldbook.P04635
Rate of photon fluence. Total number of photons (Np) incident from all directions on a small sphere divided by the cross-sectional area of the sphere and per time interval. SI unit is m−2 s−1. Same as photon spherical irradiance.
Notes:
  1. Mathematical definition: Ep,o=dNp/(dtdS)=dHp,o/dt. If Ep,o is constant over the time interval and the surface, Ep,o=Np/tS. Equivalent definition: Ep,o=∫4πLpdΩ with Lp the photon radiance and Ω the solid angle of the beams passing through the given point on the surface.
  2. It reduces to photon irradiance Ep 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 Ep,o by the Avogadro constant, the symbol then being En,p,o, the name 'photon fluence rate, amount basis', SI unit is molm−2s−1; common unit is einstein m−2 s−1.
Source:
PAC, 2007, 79, 293. (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 395 [Terms] [Paper]
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. https://doi.org/10.1351/goldbook.P04635 RIS BibTex EndNote
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Last revised: February 24, 2014