spectral radiant intensity, I λ

https://dx.doi.org/10.1351/goldbook.S05827
The radiation intensity, I at wavelength λ per unit wavelength interval. The SI unit is W m −1 sr −1, but a commonly used unit is W nm −1 sr −1.
Source:
PAC, 1996, 68, 2223 (Glossary of terms used in photochemistry (IUPAC Recommendations 1996)) on page 2276