Comparison of the R_FIR for different grain sizes and different opacities: silicate grains (blue), amorphous carbon grains (red), silicate grains with λ^–15 slope at λ > 100 µm (black-dashed), opacities that represent the opacities presented by (Fanciullo et al. 2020, black dotted), and models with silicate grains but with a modified stellar black body (black-solid). The models are calculated for the test model. See text for details).