spectral radiant exitance, \(M_{\lambda}\)

https://doi.org/10.1351/goldbook.S05825
The radiant exitance, \(M\), at wavelength \(\lambda \) per unit wavelength interval. The SI unit is \(\text{W m}^{-3}\), but a commonly used unit is \(\text{W m}^{-2}\ \text{nm}^{-1}\).
Source:
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2276 (https://doi.org/10.1351/pac199668122223)