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A device is described which determines a zero reference position in terms of interference fringes and measures any shift to a small fraction of a fringe. It can be used with static or slowly moving fringes, and is based on the principle of superposition of two images of the fringe pattern in polarized light on the single slit of a photomultiplier tube. By means of a rotating Polaroid as optical shutter an alternating signal is produced of which the amplitude and phase are a function of the displacement.
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Peters et al. (1953) studied this question.
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