The number of photons (quanta, ) per unit time. (, simplified expression: when the number of photons is constant over the time considered). The SI unit is . Alternatively, the term can be used with the amount of photons (mol or its equivalent einstein), the SI unit then being .
See: spectral photon flow
See also: radiant power
PAC, 1996, 68, 2223 (Glossary of terms used in photochemistry (IUPAC Recommendations 1996)) on page 2263