Reducing losses in optical coatings implies a search for optimal realization conditions of multidielectric coatings to obtain minimal absorption and scattering. Numerous works have been devoted to the absorption problem. Here we deal with scattering minimization. Scattering from a coated substrate is a function of numerous factors that can be successively measured: substrate roughness, interfaces roughness, cross-correlation state between interfaces. First, the substrate roughness is determined by scattering measurement before coating. Then, the interfaces roughness depends on the grain size of the material in thin film form. For a given material, we find that the grain is, in a large extent, function of the preparation conditions of the layer. And mainly, when a whole layer stack is concerned, the spatial distribution of the scattered light depends on the cross-correlation state between successive interfaces. With a scattering vector theory and an apparatus for measuring scattering curves at our disposal, we can show how it is possible to determine the interface roughness and the cross-correlation state, by a systematic comparison between theory and experiment. In order to eliminate any ambiguity in interpreting the experimental results, we must work with multilayer stacks the design of which is chosen so that the scattering curves have an aspect very different according to the cross-correlation state.