spectral photon exitance, M pλ

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 $mol s −1 m −2 nm −1$ .

Source:

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