The radiant exitance, \(M\), at wavelength \(\lambda \) per unit wavelength interval. The SI unit is \(\rm{W m}^{-3}\), but a commonly used unit is \(\rm{W m}^{-2}\ \rm{nm}^{-1}\).
Source: PAC, 1996, 68, 2223. (Glossary of terms used in photochemistry (IUPAC Recommendations 1996)) on page 2276 [Terms] [Paper]