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Investigation of error compensation in CGH-based form testing of aspheres

: Stuerwald, S.; Brill, N.; Schmitt, R.


Gorecki, C. ; Society of Photo-Optical Instrumentation Engineers -SPIE-, Bellingham/Wash.:
Optical Micro- and Nanometrology V : 15–17 April 2014, Brussels, Belgium
Bellingham, WA: SPIE, 2014 (Proceedings of SPIE 9132)
ISBN: 978-1-6284-1080-8
Paper 91320H, 10 pp.
Conference "Optical Micro- and Nanometrology" <5, 2014, Brussels>
Conference Paper
Fraunhofer IPT ()

Interferometric form testing using computer generated holograms is one of the main full-field measurement techniques. Till now, various modified measurement setups for optical form testing interferometry have been presented. Currently, typical form deviations in the region of several tens of nanometers occur in case of the widely used computer generated hologram (CGH) based interferometric form testing. Deviations occur due to a non-perfect alignment of the computer generated hologram (CGH) relative to the transmission sphere (Fizeau objective) and also of the asphere relative to the testing wavefront. Thus, measurement results are user and setup dependent which results in an unsatisfactory reproducibility of the form errors. In case of aligning a CGH, this usually requires a minimization of the spatial frequency of the fringe pattern by an operator. Finding the ideal position however often cannot be performed with sufficient accuracy by the operator as the position of minimum spatial fringe density is usually not unique. Therefore, the scientific and technical objectives of this paper comprise the development of a simulation based approach to explain and quantify the experimental errors due to misalignment of the specimen towards a computer generated hologram in an optical form testing measurement system. A further step is the programming of an iterative method to realize a virtual optimised realignment of the system on the basis of Zernike polynomial decomposition which should allow the calculation of the measured form for an ideal alignment and thus the subtraction of the alignment based form error. Different analysis approaches are investigated with regard to the final accuracy and reproducibility. To validate the theoretical models a series of systematic experiments is performed with hexapod-positioning systems in order to allow an exact and reproducible positioning of the optical CGH-based setup.