Abstract: Interferometers always measure the relative phase distribution of a sample to a reference subject, and absolute measurement of the sample is difficult unless the exact shape of the reference phase distribution is known. A method of differential interferometer, in which the differences of two interferograms obtained before and after the slight change of parameters in the optical configuration is measured, can be used to measure the shape of the sample without the exact knowledge of the reference phase distribution. This method is applied for measuring the radius of curvature of a Newton gauge. !2
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