Case of the DHGL grains (solid lines) and of the aggregates with 25% of porosity (dashed lines). Dust emissivity at 250 μm as a function of the central visual extinction A_V. The black and the blue lines show the emissivity measured in the Herschel and in the Planck-HFI bands respectively for the central pixels of the filaments described in Table 3, assuming the observer does know the true column densities of the clouds.