spectral radiance, Lλ 

https://doi.org/10.1351/goldbook.S05824
The radiance, L, at wavelength λ per unit wavelength interval. The SI unit is W m3 sr1, but a commonly used unit is W m2 sr1 nm1.
Source:
PAC, 1996, 68, 2223. (Glossary of terms used in photochemistry (IUPAC Recommendations 1996)) on page 2276 [Terms] [Paper]