A spatially partially coherent two-dimensional beamlike wave field with arbitrary distributions of the optical intensity and complex degree of spatial coherence is assumed incident on a diffraction grating with an arbitrary periodic permittivity modulation. Rigorous electromagnetic diffraction theory of gratings is used, together with the coherent-mode representation of partially coherent fields, to calculate near-field and far-zone distributions of the zeroth-order beam and the Bragg-diffracted beam. A volume grating with significant Bragg selectivity is shown to increase the spatial coherence of the diffracted beam, but the diffraction efficiency is simultaneously reduced. #1997 Optical Society of America. [S0740:3232(97)02801-9]
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