Absolute position information to reduce uncertainty in flexible asphere and freeform metrology

Optical surface metrology is a vital part of precision optics fabrication, especially for asphere and freeform surfaces. The fabrication process requires areal measurement data as feedback for the iterative correction of the surface form. Flexible interferometric measurement techniques such as tilted wave interferometry (TWI) can measure surface forms with a departure of several hundred micrometers from its best fitting sphere without wavefront-shaping elements such as computer-generated holograms (CGHs). The flexibility and accuracy of TWI is based on a volume calibration that takes the retrace errors of the interferometer optics into account. A major part in the uncertainty budget of areal interferometric form measurements in general and of TWI measurements in particular stems from positioning uncertainties. In this contribution we present two approaches to reduce these uncertainties and their integration into the TWI system. The first approach is based on a sophisticated specimen alignment procedure together with a direct measurement of the vertex distance between surface under test (SUT) and interferometer lens (e.g. using a white light interferometer). The second method is a multiple wavelength interferometry approach applied to a TWI Fizeau system to increase the unambiguous range using synthetic wavelengths.