spectral radiant power, \(P_{\lambda}\)

https://doi.org/10.1351/goldbook.S05828
The radiant power at wavelength λ per unit wavelength interval. The SI unit is W m-1, but a commonly used unit is W nm-1.
Source:
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2276 (https://doi.org/10.1351/pac199668122223)