The photon irradiance \(E_{\rm{p}}\), at wavelength \(\lambda \) per unit wavelength interval. The SI unit is \(\rm{s}^{-1}\ \rm{m}^{-3}\), but a commonly used unit is \(\rm{s}^{-1}\ \rm{m}^{-2}\ \rm{nm}^{-1}\). Alternatively, the term can be used with the amount of photons (mol or its equivalent einstein), the SI unit then being \(\pu{mol s-1 m-3}\) and the common unit \(\pu{mol s-1 m-2 nm-1}\).