photon fluence rate, Ep,o

Rate of photon fluence. Total number of photons ( $N p$ ) 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:

Mathematical definition: $E p , o = d N p / d t d S = d H p , o / d t$ . If $E p , o$ is constant over the time interval and the surface, $E p , o = N p / t S$ Equivalent definition: $E p , o = ∫ 4 π L p ⁢ d Ω$ with $L p$ the photon radiance and $Ω$ the solid angle of the beams passing through the given point on the surface.
It reduces to photon irradiance $E p$ 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 $E p , o$ by the Avogadro constant, the symbol then being $E n , p , o$ , the name 'photon fluence rate, amount basis', SI unit is $mol m −2 s −1$ ; 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