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Subwave length-resolution optical image recording is demonstrated by moving an extremely narrow aperture along a test object equipped with fine-line structures. Details of 25-nm size can be recognized using 488-nm radiation. The result indicates a resolving power of at least λ/20 which is to be compared with the values of λ/2.3 obtainable in conventional optical microscopy.
Pohl et al. (1984) studied this question.