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.
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2276 (https://doi.org/10.1351/pac199668122223)
PAC, 1985, 57, 105. 'Names, symbols, definitions and units of quantities in optical spectroscopy (Recommendations 1984)' on page 112 (https://doi.org/10.1351/pac198557010105)