Visibilities versus baseline length for HD 100546 at 10 micron, along a position angle of 30 degree (the same as the 41 meter baselines). The colored lines show a very round wall in blue, a steeper wall in dotted red, a vertical wall in solid red, and a model without a gap in dotted gray. Note that in the model without a gap the emission is coming from much closer to the star, and visibilities are much higher. Indicated in green are the spatial frequency ranges probed by the MIDI spectra used in this paper (λ = 8....13 micron), displayed as effective baseline at ten micron (Baseline/λ × 10  μm).