Testing spherical surfaces: a fast, quasi-absolute technique

A technique for measuring the quality of spherical surfaces that provides a quasi-absolute result is presented. It requires only two measurement positions rather than the traditional method of absolute sphere measurement that requires three measurement positions. A measurement is taken with a mirror at the focus of the interferometer diverger lens and is subtracted from a measurement of the sphere tested at its center of curvature. This test assumes that the test sphere does not contain any aberrations with odd symmetry so that these aberrations can be subtracted to provide a fast, quasi-absolute measurement. We describe the new technique and compare measurement results from testing a lambda/12 peak-to-valley sphere (numerical aperture = 0.4) by using a phase-measuring Fizeau interferometer with results from the three-position absolute sphere measurement technique. The repeatability of this measurement technique is +/-0.01 waves peak to valley.