Amount of photons (quanta of radiation) incident on a small sphere from all directions, divided by the cross-sectional area of that sphere and integrated over time. Photons per surface area (). SI unit is .
- Mathematical definition: with the photon fluence rate, integrated over the duration of the irradiation, . If is constant over the time interval,
- This quantity can be used on a chemical amount basis by dividing by the Avogadro constant, the symbol then being , the name 'photon fluence, 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 395