A plot of a quantity related to the absorption (@A00028@, cross section, etc.) multiplied by the @Q04991@ for the considered process against a suitable measure of photon energy, such as frequency, \(\nu \), wavenumber, \(\sigma \), or @W06659@, \(\lambda \), e.g. the conversion cross section, \(\sigma \varphi \), has the SI unit \(\mathrm{m}^{2}\).