spectral radiant exitance, M λ

https://dx.doi.org/10.1351/goldbook.S05825
The radiant exitance, M, at wavelengthλ per unit wavelength interval. The SI unit is W m −3, but a commonly used unit is W m −2 nm −1.
Source:
PAC, 1996, 68, 2223 (Glossary of terms used in photochemistry (IUPAC Recommendations 1996)) on page 2276