Abstract
Phase-measuring deflectometry is a full-field gradient technique that lends itself very well to testing reflective optical surfaces. In the past, the industry’s interest has been focussed mainly on the detection of defects and ripples, since it is easy to achieve sensitivity in the nm range. On the other hand, attempts to reconstruct the absolute surface shape from the gradient map have been plagued by systematic errors that accumulate to unacceptable uncertainties during data integration. Recently, thanks to improved measurement and evaluation techniques, the state of the art in absolute surface measurement has reached a level of maturity that allows its practical usage in precision optical manufacturing and qualification systems. We demonstrate the techniques, and the progress, by way of results from mirrors for telescopes, solar concentrators, and precision laboratory assemblies.
Highlights
Deflectometry utilises the deformation and displacement of a sample pattern after reflection from a test surface to infer the surface slopes [1, 2]
Particular difficulty is encountered in the shape acquisition of parabolic mirrors [9, 10], since a linear systematic error takes a parabolic form upon integration and is very hard to separate from the actual surface shape
This paper is organised as follows: in Section 2 we give a brief explanation for the most common origin of the parabolic shape error; in Section 3 we present several techniques to improve the evaluation and demonstrate them with practical measurement results; Section 4 summarises our view of what the state of the art currently is
Summary
Fraunhofer-Institut fur Solare Energiesysteme (ISE), Heidenhofstr. 2, 79110 Freiburg, Germany. Bremer Institut fur Angewandte Strahltechnik GmbH (BIAS), Klagenfurter Str. 2, 28359 Bremen, Germany. Phase-measuring deflectometry is a full-field gradient technique that lends itself very well to testing reflective optical surfaces. Attempts to reconstruct the absolute surface shape from the gradient map have been plagued by systematic errors that accumulate to unacceptable uncertainties during data integration. Thanks to improved measurement and evaluation techniques, the state of the art in absolute surface measurement has reached a level of maturity that allows its practical usage in precision optical manufacturing and qualification systems. The progress, by way of results from mirrors for telescopes, solar concentrators, and precision laboratory assemblies.
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