The number of photons (quanta, \(N\)) per unit time. (\(\frac{\text{d}N}{\text{d}t}\), simplified expression: \(\mathit{\Phi} _{\text{p}} = \frac{N}{t}\) when the number of photons is constant over the time considered). The SI unit is \(\text{s}^{-1}\). Alternatively, the term can be used with the amount of photons (mol or its equivalent @E01914@), the SI unit then being \(\text{mol s}^{-1}\).