For accurate radius of curvature measurements of spherical specimens on an interferometric radius bench, the axial displacement between the cat’s eye position and the confocal position needs to be measured at the height of the optical axis of the form measuring interferometer. We propose an alignment technique which allows for the alignment of the Abbe-offset well below 100 μm within a few seconds based on fringe nulling. The alignment technique bases on a special designed optical element and a pre-adjustment procedure. The implementation of the alignment technique in a radius measurement setup with a repeatability of 30 nm will be shown.
Two techniques for the measurement of aspheres as well as freeforms are presented. The first method, the tilted wave interferometer, is a full aperture interferometric measurement method without any moving parts during the measurement. The second method applies an optical single point sensor in conjunction with two translational and one rotational axes. Both techniques are compared by measuring a selected asphere and a special freeform surface. Differences between both measurement principles are discussed.