Fast and reliable in-situ measurements of large and complex surfaces using a novel deflectometric device

Abstract

In-situ measurements of complex surfaces during the polishing process is a challenge for the production of aspheric surfaces or freeforms. We are providing a new attempt by using a scanning deflectometric device based on our recently published DaOS [1] principle, which allows in-situ measurements of large optical surfaces in realistic production environments and offers the conditions for direct intervention and correction in the polishing process. The results of insitu surface measurements after three polishing steps of a large glass substrate (320 mm in diameter) in a lever-polishing machine (NLP500 from Stock Konstruktion GmbH) are shown and critically compared with interferometric measurements on a SSI-A Interferometer. In this paper, the technical setup consisting of a highly precise scanning penta prism device and a Vignetting Field Stop (VFS) Sensor is explained. Secondly, we are discussing the mathematical algorithm to reconstruct the complete surface from angle measurements from a given number of cross-sectional cuts. The data of the surface reconstruction are transformed into a XYZ-file format to be analyzed with MetroPro®. The results are shown and discussed in terms of accuracy and reproducibility. Finally, a comparison with interferometric measurements on an SSI-A (QED) at TH Deggendorf (THD), Technology Campus Teisnach is shown to proof the degree of accuracy and applicability of our new, fast and reliable device for in-situ measurements of complex surfaces.