https://doi.org/10.1351/goldbook.P04637

Number of photons (quanta of radiation, Np) per time interval (photon flux), qp, __incident__ from all __upward__ directions on a small __element of surface__ containing the point under consideration divided by the area of the element. SI unit is m-2 s-1. Equivalent definition: Integral, taken over the hemisphere visible from the given point, of the expression Lp.cos θ.dΩ the photon radiance at the given point in the various directions of the incident beam of solid angle Ω and θ the angle between any of these beams and the normal to the surface at the given point.**Notes: **

*Source: *

PAC, 2007,*79*, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 396 (https://doi.org/10.1351/pac200779030293)

- Mathematical definition: \(E_{\text{p}} = \text{d}q_{\text{p}}/\text{d}S\). If the photon flux is constant over the surface considered, \(E_{\text{p}} = q_{\text{p}}/S\). Equivalent definition: \(E_{\text{p}} = \int_{2\pi} L_{\text{p}}\, \text{cos}\:\theta\, \text{d}\varOmega\).
- This term refers to a beam
__not scattered or reflected__by the target or its surroundings. For a beam incident from__all directions__@P04635@ (\(E_{\text{p,o}}\)) is an equivalent term. - This quantity can be used on a @C01019@ basis by dividing \(E_{\text{p}}\) by the @A00543@, the symbol then being \(E_{n\text{,p}}\), the name 'photon irradiance, amount basis', SI unit is \(\text{mol m}^{-2}\ \text{s}^{-1}\); common unit is \(\text{einstein m}^{-2}\ \text{s}^{-1}\).

PAC, 2007,