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The principle of an alternating PSM [18â22] is compared with a COG mask in Fig. 5.1. The Ä¥=0.4 line is bordered by transmitting regions with 180Â° phase difference on an alternating PSM and by clear areas of the same phase on a COG mask. Phase difference on the alternating PSM leads to destructive interference, resulting in a sharp dark image. The COG mask image is less robust because of the lack of phase interaction. The reliance on destructive interference between two bright regions to create dark images means that alternating PSM is a technique for imaging small dark areas.
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