Investigations on the origin of the darkfield signal in X-Ray Talbot interferometry

Recently, darkfield imaging has been established as third imaging technique in X-Ray Talbot interferometry. The darkfield image shows the reduction of visibilities of the interferometer relative to the visibilities gained in reference measurements. The information perceivable from darkfield images can be different from what is observable in absorption and phase images. While the image formation for the absorption and phase image is widely understood, this is not the case for the darkfield signal. There are several attempts for an explanation. To pursue these explanations and, moreover, to get a comprehensive understanding of the darkfield image formation, an investigation based on simulations is carried out. These simulations include wave field propagation as well as Monte Carlo codes. The results suggest that small angle scattering and the micro structure of an object can not be the exclusive origins of the dark field signal. At least diffraction effects at steep edges and possibly visibility changes due to beam hardening effects also have to be accounted for.