The effective intensity modulation index for simulated scintillating stellar illumination patterns as a function of the theoretical modulation index. The blue dots show the effective modulation index values for different realisations of the phase screens. The red squares represent the mean value of the effective modulation indices with the same theoretical value. This plot shows the expected agreement for m < 0.15, a domain where the simulated screens are large enough not to suffer from statistical biases (see text).