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 einstein), the SI unit then being $\mathrm{mol}{\mathrm{s}}^{-1}$.