Lyα (blue lines) and UV (red line) escape fractions as a function of the dust optical depth in the high-contrast regime, for various values of expansion velocity v_exp and hydrogen column density . The extremely clumpy media have FF = 0.23, CF = 0.997 (R_min = 49 and R_max = 64 cells), n_ic/n_c = 0.00 and b = 40 km s^-1 in common. We adopt as well the following expansion velocities v_exp and H i column densities : v_exp = 0, 100, and 250 km s^-1 and = 10¹⁹ cm^-2 and 2 × 10²⁰ cm^-2. We indicate the critical dust optical depth τ_c where the curve of f_esc(Lyα) for v_exp = 0 km s^-1 cross the curve of f_esc(UV).