photon irradiance, E p
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 angleΩ and θ the angle between any of these beams and the normal to the surface at the given point.
  1. 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.
  2. 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.
  3. 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.
PAC, 2007, 79, 293 (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 396