The @[email protected], \(\varPhi_{\text{p}}\), at @[email protected] \(\lambda \) per unit @[email protected] interval. The SI unit is \(\text{s}^{-1}\ \text{m}^{-1}\), but a commonly used unit is \(\text{s}^{-1}\ \text{nm}^{-1}\). Alternatively, the term can be used with the amount of photons (mol or its equivalent @[email protected]), the SI unit then being \(\text{mol}\ \text{s}^{-1}\ \text{m}^{-1}\) and the common unit \(\text{mol}\ \text{s}^{-1}\ \text{nm}^{-1}\).