Experimental treating of hydrodynamic instabilities in laser-accelerated planar targets have pointed out the creation of a mix between a heavy material and a light one. The experiment described here is intended to answer the question: “Is this mix homogeneous or is it an interpenetration of both materials in the form of bubbles and spikes?” The method chosen to study the mix is self-backlighting, that is, backlighting of the target by its own X-ray emission. Comparison of the front and rear X-ray pinhole images of the target allows us to obtain information about the local areal density. Indeed, if 20-μm diameter Au dots are deposited on the target rear face some structures corresponding to these dots are observed on the rear image. The resolution imposed by the pinholes is about 10 μm. By using stable targets Au/CH, we obtained a good coincidence between front and rear pinhole images of the target. By doing the same comparison for unstable targets Au/CH/Au, the coincidence of the images is not significantly different from that obtained with stable targets. Therefore, in this experiment hydrodynamic instabilities do not create structures with dimension larger than 10μm.