Examples for the two ways to produce a smooth differential phase signal: (1) a chromatic photocentre shift (blue) and (2) a purely spatial asymmetry of the brightness distribution (red). In case 1, the source is shifted to the bottom between λ ~ 10 μm and 12 μm; in case 2, the “binary” has a brightness ratio of 0.7:1.0. The resulting visibilities and phases are shown in the left panels, with the absolute Fourier phases φ_abs and the differential phases φ_diff (as would be measured with MIDI) plotted by dotted and continuous lines, respectively. In the right panels, the two brightness distributions are shown as false-colour images, with the brightness distribution at 13.0 μm, 11.0 μm and 8.0 μm mapped to the red, green and blue channels, respectively.