Average emissivity SED for the five I_ν(250 μm)/I_ν(500 μm) bins of Fig. 3, using (black lines). For I_ν(250 μm)/I_ν(500 μm)=4.5, the calibration errors are shown. Purple lines show the SED from the THEMIS model, averaged over the I_ν(250 μm)/I_ν(500 μm) values in each bin. Purple dots mark the emissivities after averaging over the Herschel filter response functions. The MW emissivities are also shown.