https://doi.org/10.1351/goldbook.S05827
The radiation intensity, \(I\) at wavelength \(\lambda \) per unit wavelength interval. The SI unit is \(\rm{W m}^{-1}\ \rm{sr}^{-1}\), but a commonly used unit is \(\rm{W nm}^{-1}\ \rm{sr}^{-1}\).