Top: two different phase screen power laws. Bottom: the corresponding modulation indices as a function of the expected modulation index for the Kolmogorov turbulence. The 3 indices plotted at a given abscissa correspond to screens with β = 3.1 (red), 3.67 (Kolmogorov, blue, along the diagonal), and 3.9 (black) with the same R_diff. As the power law gets steeper, a larger modulation is expected. Blue dots show the Kolmogorov turbulence case.