  Planet formation has been known for many years to be tied to the spatial
distribution of gas and dust in disks around young stars. To constrain planet
formation models, imaging observations of protoplanetary disks are required. In
this framework, we have undertaken a mid-infrared imaging survey of Herbig Ae
stars, which are pre-main sequence stars of intermediate mass still surrounded
by a large amount of circumstellar material. The observations were made at a
wavelength of 20.5 $\mu$m with the CAMIRAS camera mounted at the Cassegrain
focus of the Canada France Hawaii Telescope.
  We report the observations of three stars, HD135344, CQTau and HD163296. The
circumstellar material around the three objects is spatially resolved. The
extensions feature a disk like shape. The images provide direct information on
two key parameters of the disk : its inclination and its outer radius. The
outer radius is found to be quite different from the one deduced from disk
models only constrained by fitting the Spectral Energy Distribution of the
object. Other parameters of the disk, such as flaring, dust mass have been
deduced from fitting both the observed extension and the spectral energy
distribution with sophisticated disk models.
  Our results show how important imaging data are to tighten constraints on the
disk model parameters.
