Effective extinction curves for clumpy circumstellar shells as a function of N_cl using the aCSi model. The line colours correspond to the models indicated in the top left. As the number of clumps decreases, the clumps become more optically thick and the effective extinction curve flattens. The white solid line shows the input dust cross-sections normalised to the V-band, and the black dotted line the same after a reduction in the scattering efficiencies by (1 − g). The two panels refer to different dust masses and hence homogeneous-shell optical depths as indicated on the panel.