Evolution of the CO-dark to total H₂ column-density ratio with the 2MASS visual extinction. The molecular fraction in the DNM varies from 50% (lower grey bound) to 100% (upper bound). The gas column densities in the CO and DNM phases have been obtained with the X_COγ factor and the γ-ray N_HDNM map from the γ + A_VQ analysis. The other analyses yield equivalent curves. The thick and dotted lines respectively give the standard error of the mean and the standard deviation of the ratios for each A_V bin, in the case of a fully molecular DNM.