Mean gas opacity: comparison with Freedman et al. (2008). The Rosseland means (top panel) and the Planck means (bottom panel) as a function of gas temperature. The open squares are data from Freedman et al. (2008), the filled circles are from this work. Each curve corresponds to a constant gas density ρ [g cm^-3] = 10^-3, 10^-5, 10^-8 as labelled in the upper panel. In the lower panel the different lines are difficult to distinguish because the Planck mean is less sensitive to density.