Number of photons (quanta of radiation, ) per time interval. SI unit is .
- Mathematical definition: . If the number of photons is constant over the time interval, .
- This quantity can be used on a chemical amount basis by dividing the photon flux, number basis, , by the Avogadro constant, the symbol then being , the name 'photon flux, amount basis', SI unit is ; common unit is .
- Although the symbol recommended by CEI is , the symbol is preferred since is reserved for quantum yield.
PAC, 2007, 79, 293 (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 395