spectral radiant intensity, \(I_{\lambda}\)

https://doi.org/10.1351/goldbook.S05827
The radiation intensity, \(I\) at wavelength \(\lambda \) per unit wavelength interval. The SI unit is \(\text{W m}^{-1}\ \text{sr}^{-1}\), but a commonly used unit is \(\text{W nm}^{-1}\ \text{sr}^{-1}\).
Source:
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2276 (https://doi.org/10.1351/pac199668122223)