Comparison between the differential phase signal on the shortest MIDI baseline and the ISO spectrum in the same wavelength range. The red squares show the MIDI differential phases after subtraction of a linear trend (fit to the blue points), inversion, and normalization. The ISO spectrum was first smoothed with a Gaussian kernel (σ = 0.025 μm, comparable to the resolution of the MIDI grism at 13 μm). The full black line shows this smoothed ISO spectrum after subtraction of a linear trend (fit to the green points) and normalization. The differential phase of the intermediate baseline can be scaled similarly, but without sign inversion and with a smaller normalization factor, because these spatial frequencies fall largely in the second lobe of the radial visibility curve along this direction. The ISO spectrum of the base model added with 1% of 0.1 μm forsterite grains (see Sect. 7) is shown as the dashed line. We note that this model is normalized independently.