The photon exitance, M p, at wavelength per unit wavelength interval. The SI unit is s-1 m-3, but a commonly used unit is s-1 m-2 nm-1. Alternatively, the term can be used with the amount of photons (mol or its equivalent einstein) the SI unit then being mol s-1 m-3 and the common unit mol s-1 m-2 nm-1.