spectral radiant power, P λ

https://dx.doi.org/10.1351/goldbook.S05828
The radiant power at wavelengthλ 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
See also:
PAC, 1985, 57, 105 (Names, symbols, definitions and units of quantities in optical spectroscopy (Recommendations 1984)) on page 112