MIDI visibilities of AC Her at the three baselines (one in each panel) as a function of wavelength. Overplotted are predictions for four double-power-law radiative transfer models: the standard best-fit model (full blue lines, Model 2 in Table 2); the standard model added with 1% of 0.1 μm forsterite grains (dashed green lines); the standard model added with 1% of forsterite grains with a size distribution between 0.1 and 2.0 μm (dotted red lines); and the New Forst model from Sect. 7.2.2 which has 1% of 0.1 μm forsterite grains, but with a different spatial distribution (thin black lines).