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The method of measuring the thickness of thin films by multiple-beam Fizeau fringes has been studied in order to assess the accuracy attainable. In this connection the small-scale irregularities on the surfaces of optical flats and other optically worked glass have been studied by multiple-beam methods. The effect of these irregularities on the accuracy of the thickness measurements is shown and the conditions for optimum accuracy are discussed.
O.S. Heavens (1951) studied this question.