Measured slope a against inverse brightness contrast W for efBm maps with spectral indexes ζ = 2.5,3,3.5 (full, dotted, and dashed lines, respectively). Top plot: result for S/N = ∞, bottom plot: result for S/N = 5. The data points for ζ = 3 and 3.5 (squares and triangles) are horizontally displaced by W = 0.02 and W = 0.04, respectively, from the ζ = 2.5 data points (diamonds) for better visibility. The error bars represent 1 σ variations in the statistical sample.