Absolute sphericity measurement.
Several conventional methods for absolute sphericity testing of optical surfaces are known. One method has been used previously for real time interferometry, but a detailed investigation has not been carried out so far. In this work the known conventional methods are reviewed and assessed for suitability in real time interferometry. One method has been applied to phase stepping interferometry and examined. Adjustment peculiarities, experimental results of deviation measurements on normal surfaces, and application of normals are reported. Measuring errors, especially coherent noise, are analyzed and a special subsequent digital spatial filtering technique for diminution of noise is described.