Revisiting grating interferometry for spatial coherence characterization of diffraction-limited storage rings

Diffraction-limited storage rings (DLSRs) provide an excellent spatially coherent photon beam, enabling the rapid development of advanced experimental techniques. As a result, it is important to characterize the spatial coherence of the beam. However, the photon beam from DLSRs presents a challenge for many existing methods to measure the coherence accurately. Here, we investigate the limitations in grating interferometry for coherence measurement of DLSRs using numerical simulation and theoretical analysis. The simulation results of the widely accepted method for third-generation rings using grating interferometry show notable errors for DLSRs. Detailed theoretical analysis revealed that the grating-based method inherently assumes the spatial invariance of the beam, which is incompatible with the properties of DLSRs. The theoretical analysis can also be extended to assess the validity of other coherence measurement techniques. Our findings emphasize the necessity for revisiting the existing methods and developing new approaches for coherence characterization of DLSRs.