Quantitative determination of the spatial coherence from the visibility of equal-thickness fringes

Spatial coherence of the undulator radiation in the horizontal direction is measured by a wedge-shaped crystal interferometer. The Borrmann effect, the effect of the angular divergence of the incidence and the wavefront matching of the collimating asymmetric diffractions with the interferometer are discussed to estimate the coherence factor. The spatial coherence length of 145 µm is experimentally determined, which is enhanced by the collimators about 100 times more than expected from the source size.