Propagation-based phase contrast imaging with a laboratory x-ray source is a valuable
tool for studying samples that show only low absorption contrast,
either because of low density, elemental composition, or small feature
size. If a propagation distance between sample and detector is
introduced and the illumination is sufficiently coherent, the phase
shift in the sample will cause additional contrast around interfaces,
known as edge enhancement fringes. The strength of this effect depends
not only on sample parameters and energy but also on the experimental
geometry, which can be optimized accordingly. Recently, x-ray lab
sources using transmission targets have become available, which
provide very small source sizes in the few hundred nanometer range.
This allows the use of a high-magnification geometry with a very short
source–sample distance, while still achieving sufficient spatial
coherence at the sample position. Moreover, the high geometrical
magnification makes it possible to use detectors with a larger pixel
size without reducing the image resolution. Here, we explore the
influence of magnification on the edge enhancement fringes in such a
geometry. We find experimentally and theoretically that the fringes
become maximal at a magnification that is independent of the total
source–detector distance. This optimal magnification only depends on
the source size, the steepness of the sample feature, and the detector
resolution. A stronger influence of the sample feature on the optimal
magnification compared to low-magnification geometries is
observed.