The ratio of absorption-corrected X-ray (0.1–2.4 keV) to extinction-corrected optical (V band) luminosity as a function of visual extinction, with the insert showing the range up to A_V = 12 mag. The ordinate corresponds to the correction which has to be applied to the X-ray to optical luminosity ratio if the observed values without extinction correction are used. Shown are two blackbody (bb) models with 50 and 150 eV, two thermal bremsstrahlung (tb) models with 100 eV and 900 eV, and two power law (po) models with photon index 1.5 and 2.0, respectively. Except for very soft X-ray spectra (50 eV blackbody and 100 eV thermal bremsstrahlung) this ratio is within a factor of two with respect to the uncorrected luminosity values. This justifies the decision to use observed, not extinction-corrected values to determine the X-ray to optical luminosity ratio (see text).