Estimates of ﹩{\zeta }_{p}({\rm{H}})﹩, derived from measurements of the H₂ and ﹩{{\rm{H}}}_{3}^{+}﹩ column densities, as a function of the measured ﹩N({{\rm{H}}}_{2})﹩ (top panel) and of the derived ﹩{A}_{{\rm{V}}}(\mathrm{tot})﹩ (bottom panel). Black diamonds: measurements obtained from direct observations of H₂. Magenta diamonds: measurements in which the H₂ column densities have been inferred indirectly from observations of CH or ﹩E(B-V)﹩. Dashed red lines: best fits to all the data.