https://doi.org/10.1351/goldbook.P04635
Rate of photon fluence. Total number of photons (\(N_{\rm{p}}\)) incident from all directions on a small sphere divided by the cross-sectional area of the sphere and per time interval. SI unit is \(\rm{m}^{-2}\ \rm{s}^{-1}\). Same as photon fluence rate.
Notes:
- Mathematical definition: \(E_{\rm{p,o}} = \rm{d}N_{\rm{p}}/\left ( \rm{d}t\, \rm{d}S \right ) = \rm{d}H_{\rm{p,o}}/\rm{d}t\). If \(E_{\rm{p,o}}\) is constant over the time interval and the surface, \(E_{\rm{p,o}} = N_{\rm{p}}/t\, S\). Equivalent definition: \(E_{\rm{p,o}} = \int _{4\pi}L_{\rm{p}}\rm{d}\varOmega\) with \(L_{\rm{p}}\) the photon radiance and \(\varOmega\) the solid angle of the beams passing through the given point on the surface.
- It reduces to photon irradiance \(E_{\rm{p}}\) 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 \(E_{\rm{p,o}}\) by the Avogadro constant, the symbol then being \(E_{n\rm{,p,o}}\), the name 'photon spherical irradiance, amount basis', SI unit is \(\pu{mol m-2 s-1}\); common unit is \(\pu{einstein m-2 s-1}\).