Evolution of the critical ratio n_IC/n_C separating the low-contrast regime from the high-contrast regime of the Lyα radiative transfer in clumpy shell geometries. For highest ratio n_IC/n_C, all clumpy media belong to the low-contrast regime, whereas for lower n_IC/n_C the high-contrast regime is observed. Three curves are represented in this figure corresponding to the three different physical conditions applied to our clumpy media (see text).