Measurement of inner centration of an asphere with computer generated holograms compared to an optical profiler

Centration measurements of lenses usually require two measurements of the center of curvature, one for each side of a lens. For aspheres, the situation is more complex, because an aspherical surface defines a set of centers of curvature for each diameter of the aspherical surface. This makes the measurement and its interpretation more complex. We present three different methods to measure the inner centration of an asphere and discuss accuracies and practical aspects. One approach is the use of a Computer-Generated Hologram in combination with a Fizeau-type interferometer. The CGH is split into a ring area for testing the first side of the asphere and a central circular CGH which is optimized for testing the spherical backside of the lens at the same time. The Zernike coefficients of the two interferometric measurements can be used to back calculate the inner centration. The second approach is based on an optical profiler using multi-wavelength interferometry. It collects surface data from the first and the second surface after another, by turning the lens surfaces using an additional tooling where common fiducials for both sides enable a precise mechanical relation. A third option is also based on this optical profiler, enabling the measurement of the back-surface through the lens. We have applied all three methods for testing one specific asphere and compare and discuss the results we have obtained.