spectral radiant exitance, M λ

The radiant exitance, $M$ , at wavelength $λ$ 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