The characterisation of the coherence properties of neutron beams in double perfect crystal arrangements has still some open questions. Owing to the narrow angular width of perfect crystal reflections, neutron beams with large coherence lengths can be produced. Coherence properties may be observed by the interaction of the neutrons with various samples containing scatterers of different length scale, respectively. Associated phenomena are full coherent scattering, partial coherent scattering with incoherent superposition and geometrical optics refraction. These phenomena have been identified previously but not investigated systematically up to now. With this proposal we aim at such an investigation at the instrument S18, applying its USANS option. There, the monochromator perfect crystal will serve for beam preparation with large coherence length while the searched for phenomena will be resolved by the analyser perfect crystal. This should allow us to define the range of wave optics and geometrical optics more clearly and to study the transition region between these ranges in more detail and to link it to the coherence properties of the specifically prepared neutron beam.