The @[email protected], \(L_{\text{p}}\), at @[email protected] per unit @[email protected] interval. The SI unit is \(\text{s}^{-1}\ \text{m}^{-3}\ \text{sr}^{-1}\), but a commonly used unit is \(\text{s}^{-1}\ \text{m}^{-2}\ \text{sr}^{-1}\ \text{nm}^{-1}\). Alternatively, the term can be used with the amount of photons (mol or its equivalent @[email protected]) the SI unit then being \(\text{mol s}^{-1}\ \text{m}^{-3}\ \text{sr}^{-1}\) and the common unit \(\text{mol s}^{-1}\ \text{m}^{-2}\ \text{sr}^{-1}\ \text{nm}^{-1}\).