Standard deviation σ₂₅₀ of the 250 μm surface brightness within an annulus vs. 250 μm surface brightness Σ₂₅₀ in the annulus (left panel), and the mean σ₂₅₀ over all the profiles in a galaxy vs. the ratio of maximum and minimum Σ₂₅₀ in the galaxy (right panel). Only radial points with S/N ≥ 3 are shown. In the left panel, the dashed lines correspond to 10% variation (upper) and 1% variation (lower) relative to Σ₂₅₀. The error bars in the right panel reflect the standard deviation within a given profile. In both panels, points are plotted by Hubble type with (red) circles corresponding to early types (T ≤ 0), (green) triangles to spirals (0 <T ≤ 6), and (blue) squares to late types (T> 6); in the left panel filled symbols correspond to R/R_opt≤ 0.8 and open ones to larger radii.