As previous figures but using average surface brightness values in three τ(250 μm) intervals. The blue, green, and red symbols correspond to τ(250 μm) equal to (1 ± 0.5) × 10^-3, (2 ± 0.5) × 10^-3, and (3 ± 0.5) × 10^-3 respectively. The optical depths are derived from a fit to the SPIRE data using a fixed value of β = 1.8. The lines are least-squares fits to the points of the same colour and the slope values k are listed in the frames (unit K^-1). The median values are indicated with crosses (in the middle frame the red and green crosses overlap) and the median values of β are also given.