Best-fit sphere approximation to a general, aspheric surface.
In manufacturing an aspheric lens or mirror, it is often convenient to generate a spherical surface as a starting point. The radius of the sphere should be such that the volume of glass which must be removed to achieve the final aspheric is a minimum. Equations for this best-fit sphere are developed and arranged in a form well suited for computer programming.
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American Optical Company, SpaceDefense Division, Pittsburgh, Pennsylvania, USA.