Quantities characterizing electromagnetic radiation (@R05046@, energy, energy density, intensity, @E02264@, @R05037@, irradiance, etc.) derived by differentiation with respect to @W06659@, frequency or wavenumber, e.g. spectral (concentration of) irradiance is the derivative of the irradiance with respect to @W06659@ (\(E_{\lambda }\)), frequency (\(E_{\nu }\)) or wavenumber (\(E_{\stackrel{˜}{\nu }}\)).