The photon radiance, Lp, at wavelength per unit wavelength interval. The SI unit is s-1 m-3 sr-1, but a commonly used unit is s-1 m-2 sr-1 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 sr-1 and the common unit mol s-1 m-2 sr-1 nm-1.
Source:
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2276 (https://doi.org/10.1351/pac199668122223)