Herbig parameter space for the water line blend at 15.17 μm. In the plot the data points (all upper limits, in grey) are from Pontoppidan et al. (2010a). All the fluxes are scaled to 140 pc. The shaded yellow area contains the range of sensitivity we retrieved from the Spitzer spectra of Herbig stars HD 135344B and HD 100546. All the fluxes of the disks considered are scaled to 140 pc. The trend toward disks being flatter than β = 0.8 is shown with a black arrow, since models that are too flat will be self-shadowed, and the derived mid-IR colour cannot be compared in a trustworthy manner with observations, in which the shadowing can be partial owing to a more complex disk structure.