IRDC model observations fitted with MCMC methods. Frame a shows the true τ(250 μm) optical depths, and frame d the colour temperature from the 1-MBB fit. Frames b and c show the τ maps from the 4-TMPL and 2-MBB fits (averages over the MCMC samples) divided by the true optical depth τ₀. Frames e and f show the posterior probability distributions for the four positions marked in the b and c frames, and the true values τ₀ are indicated with vertical lines of the same colour (black, blue, red, and cyan).