photon exposure, H p

https://dx.doi.org/10.1351/goldbook.P04632
Photon irradiance, E p, integrated over the time of irradiation for a beam incident from all upward directions. SI unit is m −2.
Notes:
  1. Mathematical definition: H p = ∫ t E p ⁢ d t. If the photon irradiance is constant over the time interval, H p = E p t.
  2. This term refers to a beam not scattered or reflected by the target or its surroundings. For a beam incident from all directions photon fluence (H p , o, F p , o) is an equivalent term.
  3. This quantity can be used on a chemical amount basis by dividing H p by the Avogadro constant, the symbol then being H n , p, the name 'photon exposure, amount basis', SI unit is mol m −2; common unit is einstein m −2.
Source:
PAC, 2007, 79, 293 (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 394