Optical EngineeringTwyman-Green interferometer for testing microspheres
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Testing of spherical surfaces using the Twyman-Green interferometer with a laser source suffers from the presence of spurious interference fringes and dust diffraction. By reducing the spatial coherence while maintaining the temporal coherence a tremendous improvement of the fringe quality can be obtained. To conserve sufficient contrast, the reference mirror must be positioned to suitable locations relative to the test arm of the interferometer. These conditions are described and examples are presented.