Peak strength of the effective extinction curve without (left) and with additional destruction (right) of the carriers as function of the two parameters  ⟨ A_V ⟩  and σ_{lnN/ ⟨ N ⟩}  of the turbulent screen. The black curves are lines of constant peak strengths. They are labelled with the percentage of the peak strength relative to the intrinsic value. In the right figure the carriers are assumed to be destroyed below a critical column density of  [N_H] _crit = 10²¹   cm^-2. The corresponding R_V-values are shown as grey solid lines.