spectral photon exitance, M p λ

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.
PAC, 1996, 68, 2223 (Glossary of terms used in photochemistry (IUPAC Recommendations 1996)) on page 2275