photon irradiance, Ep

Number of photons (quanta of radiation, $N p$ ) per time interval (photon flux), $q p$ , 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 $L p cos ⁡ θ d Ω$ 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:

Mathematical definition: $E p = d q p / d S$ . If the photon flux is constant over the surface considered, $E p = q p / S$ . Equivalent definition: $E p = ∫ 2 π L p cos ⁡ θ ⁢ d Ω$ .
This term refers to a beam not scattered or reflected by the target or its surroundings. For a beam incident from all directionsphoton fluence rate ( $E p , o$ ) is an equivalent term.
This quantity can be used on a chemical amount basis by dividing $E p$ by the Avogadro constant, the symbol then being $E n , p$ , the name 'photon irradiance, 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 396