Irradiance, E, at wavelength λ per unit wavelength interval. The SI unit is W m-3, but a commonly used unit is W m-2 nm-1.
Source:
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2275 (https://doi.org/10.1351/pac199668122223)
See also:
PAC, 1990, 62, 2167. 'Glossary of atmospheric chemistry terms (Recommendations 1990)' on page 2198 (https://doi.org/10.1351/pac199062112167)