Effect of the grain size distribution on I_λ/ ℛ, as predicted by the C11 model. The SEDs shown are the coldest (blue dot-dashed line), the median (green solid line) and the warmest (red dotted line); normalisation is the same as per Fig. 4. The corresponding observations are plotted in grey behind the models. Larger symbols indicate larger average grain size, and smaller symbols indicate smaller average grain size (see text for details).