The reflection plate interferometer, which is similar to a Schlieren system but with a small glass shearing plate replacing the knife edge, has been investigated and the necessary relations to understand its operating characteristics are presented. It was found that the optimum operating range is for incident angles between 40° and 65°, and that the fringe spacing is approximately inversely proportional to the shearing-plate thickness. The undisturbed fringe direction can be adjusted at any angle with respect to a test object. The interferograms are of very good quality and can be interpreted to give the density gradient of the test fluid.
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