Incomplete coverage of a surface with patches of another material with a lateral size smaller than the beam footprint on the surface (Figure ) results in an equivalent XRR curve of a film with a reduced electron density due to averaging over the illuminated area. The coverage may vary with time as in the case of single-atom-thick graphene crystals during their growth on liquid copper. Based on such a system, Figure provides a simulation where the XRR curves of a patchy surface (graphene flakes) with a variable coverage ratio X are normalized to the XRR curve of the substrate (liquid Cu). The drop in the intensity around the qz = 0.8 Å -1 is due to the Kiessig minimum, related to the height separation between the liquid surface and the graphene sheet. Typically, the analysis of such patches with XRR is quite problematic unless supported by additional information such as diffuse off-specular scattering measurements or obtained by grazing incidence small angle X-ray scattering (GISAXS). However, these methods are applicable mainly for islands of nanometric size (up to a few microns in the best cases) and are not sensitive to larger objects at the μm to mm scale. Imaging of a flat surface in XRR geometry using synchrotron X-rays cannot be applied directly because of the small beam size, usually below a few tens of μm, which has the same size on the detector plane after specular reflection (Figure ). This size is smaller than the pixel size of recent 2D photon-counting area detectors (Maxipix-ESRF: 55 μm, Eiger-Dectris: 75 μm, Pilatus-Dectris: 172 μm). To visualize the surface under the beam footprint, an optical element, i.e. a 2D X-ray lens, is needed after the sample to transform a bunch of parallel beams into a fan of diverging beams. The larger the obtained angular spread of the beam fan, the higher the magnification of the surface image.