H₂ and CO column densities from UV and radio absorption spectra. Shown are UV absorption line measurements from Sonnentrucker et al. (2007), Burgh et al. (2007) and Sheffer et al. (2008) (downward-pointing triangles), mm-wave results from Liszt & Lucas (1998) (small orange circles), and results from this work (green filled squares). Some informative upper limits to UV measurements are shown in red. The mm-wave results assume N(H₂) = N(HCO⁺)/3 × 10⁻⁹ and the present results use Eq. (2) to derive N(CO). UV absorption sightlines in Chamaeleon are marked by blue asterisks, see Table 4. Grey dashed lines show the locii of relative abundances N(CO)/N(H₂) as indicated. Only sighlines with N(CO) > 10¹⁵   cm⁻² are observable in present wide-field CO emission surveys.