photon exposure, Hp

Photon irradiance, $E p$ , integrated over the time of irradiation for a beam incident from all upward directions. SI unit is $m −2$ .

Notes:

Mathematical definition: $H p = ∫ t E p ⁢ d t$ . If the photon irradiance is constant over the time interval, $H p = E p t$ .
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.
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