Spectra with the logarithm of the spectral lines (red lines), photoionisation (blue lines), and reverse-bremsstrahlung (green lines) cross sections (left scale) vs. photon energy expressed in for two different temperature-electron density sets. The conditions for which they have been computed and the resulting Rosseland mean are detailed in each panel. The dashed lines denote the normalised temperature derivative of the Planck function in logarithm, log F(u) (right scale).