Spectral energy distributions based on surface brightness averaged over fields and over 10′ beams towards the centre of each field. Frame a) corresponds to the average over all fields, frame b) to averages over fields below (blue) and above (red) the median column density, and frame c) to averages over fields below and above the median temperature. The dotted lines are long-wavelength extrapolations using β_FIR. At 217 GHz the lower points (without error bars) correspond to data that have been corrected assuming a line ratio of (2–1)/(1–0) = 1.0 and thus with twice the default CO correction.