Spectral irradiance ($E_{\lambda },E_{\nu }$)

Incident radiant flux per unit area and unit wavelength or frequency. Integrating $E_{\lambda }$ over a wavelength interval gives the contribution to irradiance in that interval. Conversion between $E_{\lambda }$ and $E_{\nu }$ requires the absolute Jacobian determinant associated with $\nu =c/\lambda$, as for spectral radiance.