The @P04631@, \(M_{\text{p}}\), at @W06659@ per unit @W06659@ interval. The SI unit is \(\text{s}^{-1}\ \text{m}^{-3}\), but a commonly used unit is \(\text{s}^{-1}\ \text{m}^{-2}\ \text{nm}^{-1}\). Alternatively, the term can be used with the amount of photons (mol or its equivalent @E01914@) the SI unit then being \(\text{mol s}^{-1}\ \text{m}^{-3}\) and the common unit \(\text{mol s}^{-1}\ \text{m}^{-2}\ \text{nm}^{-1}\).