spectral photon radiance, Lpλ

https://doi.org/10.1351/goldbook.S05822
The photon radiance, Lp, at wavelength per unit wavelength interval. The SI unit is s1 m3 sr1, but a commonly used unit is s1 m2 sr1 nm1. Alternatively, the term can be used with the amount of photons (mol or its equivalent einstein) the SI unit then being mols1m3sr1 and the common unit mols1m2sr1nm1.
Source:
PAC, 1996, 68, 2223. (Glossary of terms used in photochemistry (IUPAC Recommendations 1996)) on page 2276 [Terms] [Paper]