Difference in filling factor, ΔA_{0° −90°}, as a function of the geometrical parameters 2r/d and r/z. The white dotted curve indicates the combinations of parameters that reproduce the ΔA_{0° −90°} inferred from the successful model, marked by the white dot. The black dotted line shows an upper limit on 2r/d, imposed by the absence of optical-depth saturation in the observed azimuthal signature.