Same as Fig. 3, but for a wider range of values. The plot shows hints of non-linearity for high values of dust column densities or optical-depths, as shown by the curved fit (dashed line). We also report in this plot the linear fit obtained in the range of Fig. 3 (dotted line). A direct comparison between the results obtained fromshows how important it is to account for unresolved substructures and foreground stars in molecular clouds.