spectral irradiance, \(E_{\lambda}\)

Irradiance, \(E\), at wavelength \(\lambda \) per unit wavelength interval. The SI unit is \(\text{W m}^{-3}\), but a commonly used unit is \(\text{W m}^{-2}\ \text{nm}^{-1}\).
PAC, 1996, 68, 2223. (Glossary of terms used in photochemistry (IUPAC Recommendations 1996)) on page 2275 [Terms] [Paper]
See also:
PAC, 1990, 62, 2167. (Glossary of atmospheric chemistry terms (Recommendations 1990)) on page 2198 [Terms] [Paper]