Rate of photon fluence. Total number of photons () incident from all directions on a small sphere divided by the cross-sectional area of the sphere and per time interval. SI unit is . Same as photon spherical irradiance.
- Mathematical definition: . If is constant over the time interval and the surface, Equivalent definition: with the photon radiance and the solid angle of the beams passing through the given point on the surface.
- It reduces to photon irradiance for a parallel and normally incident beam not scattered or reflected by the target or its surroundings.
- This quantity can be used on a chemical amount basis by dividing by the Avogadro constant, the symbol then being , the name 'photon fluence rate, amount basis', SI unit is ; common unit is .
PAC, 2007, 79, 293 (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 395