IRDC observations fitted in Fourier space. Frame a shows the true optical depth and frame e the colour temperature from the 1-MBB fit. Frames b–d show the optical depths estimated with the 1-MBB, 4-TMPL, and 12-TMPL fits, respectively. The corresponding ratios of estimated and true optical depths are shown in frames f-h and the χ² values in frames j-l. Frame i shows the standard deviation of the temperature values from the 12-TMPL fit. The values quoted in the frames are the 10%, 50%, and 90% percentiles of the plotted quantity. The maps cover the same 256 × 256 pixel area as in Fig. 8.