The wavefronts are rotated or reversed with respect to each other when a plane mirror is used in one beam and a cube-corner prism or a right-angle prism is used in the second beam of a Twyman–Green interferometer. The purpose of this paper is to explore the conditions under which fringes of equal thickness could be obtained. This is related to the spatial coherence of the collimated beam incident on the beam splitter. Use of a gas laser source is discussed.
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