Extending the caustic test to general aspheric surfaces.
The caustic test is capable of a high degree of accuracy in the measurement of aspheric surfaces. The essence of the test is to measure the locations of the centers of curvature for off-axis zones of an aspheric surface with a modified Foucault test apparatus. We present equations that extend the usefulness of the caustic test to general aspheric surfaces. Equations are also given for calculating derivatives for several caustic test parameters that are useful for tolerancing optical systems or in derivative increment calculations for computer lens design applications. Guidelines are given for the correct use of the caustic test in actual testing setups.