https://doi.org/10.1351/goldbook.ST07479
Derivative of fluence rate, \(E_{\rm{o}}\), with respect to wavelength, \(\lambda\). SI unit is \(\rm{W m}^{-3}\); common unit is \(\rm{W m}^{-2}\ \rm{nm}^{-1}\).
Note: All spectral terms may also be defined as derivatives with respect to frequency, \(\nu\), or wavenumber, \(\tilde{\nu}\), and are referred to, when necessary, as in terms of wavelength, or frequency or wavenumber, respectively.