ϵ_ν(250 μm) versus I_ν(250 μm)/I_ν(500 μm). Comparison between our results and the MW value for the cirrus. We also show the behavior of the THEMIS dust model under ISRFs of growing intensities (dots are for U = 0.2, 0.5, 1, 2, 5, 10, 20, and 50 from left to right, with ISRF = U × LISRF; see Appendix B). Grey contours and average emissivity as in Fig. 3. The errorbars for the average emissivity show the impact on ϵ_ν(250 μm) of the SPIRE calibration error.