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Partially coherent periodic stop filtering for inspection of periodic objects.

A partially coherent system is introduced for defect inspection of periodic objects such as integrated circuit photomasks. An experimental investigation demonstrates that the new system is more robust in the presence of optical setup noise than the corresponding coherent spatial filtering system. The system illumination is provided by an array of mutually incoherent point sources that image onto a grid of opaque spots at the pupil plane. Nonperiodic defects are displayed at the image plane as bright structures against a dark background.