Photon irradiance, , integrated over the time of irradiation for a beam incident from all upward directions. SI unit is .
- Mathematical definition: . If the photon irradiance is constant over the time interval, .
- 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 (, ) is an equivalent term.
- This quantity can be used on a chemical amount basis by dividing by the Avogadro constant, the symbol then being , the name 'photon exposure, amount basis', SI unit is ; common unit is .
PAC, 2007, 79, 293 (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 394