Case of the DHGL populations. a) True hydrogen column density N_H at the centre of the modelled clouds as a function of the central visual extinction A_V (red line). The dashed line shows the N_H − A_V relation measured by Bohlin et al. (1978) for the diffuse ISM. The blue line represents the column density estimated from the SED measured in the Planck-HFI and IRIS 100 μm bands, whereas the black line comes from the SED-fits made in the Herschel bands. b) Ratio of the true column density of the modelled clouds to the column densities measured in Herschel and Planck bands (black and blue, respectively).