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Proceedings Paper

Measuring deformations with deflectometry
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Paper Abstract

Phase-measuring deflectometry is a powerful method to measure reflective surfaces. It is relatively easy to extract slope and curvature information from the measured phase maps; however, retrieving shape information depends very sensitively on the calibration of the camera and the geometry of the measurement system. Whereas we have previously demonstrated shape uncertainties below 1 μm, the range below 100 nm is currently inaccessible to deflectometric shape measurement. On the other hand, the astounding sensitivity of deflectometry can be put to good use for deformation measurements. The evaluation of corresponding shape differences rather than absolute shapes is much less susceptible to system calibration errors and its resolution is given mostly by the measurement system’s sensitivity. We give an overview of recent progress in difference deflectometry. Firstly we show results from solar mirror substrates under load to detect flaws with high sensitivity. Secondly we present a preliminary simulation study of achievable deformation-measurement uncertainties to assess the feasibility of deflectometric characterisation of actuator performance and gravity sag for the mirror segments of the European Extremely Large Telescope (E-ELT). Results for the relevant Zernike terms show reliable detection of Zernike coefficients at the 25 nm level. Random artefacts related to noise in the phase measurements are seen to translate into bogus Zernike terms, and we discuss possible mitigation techniques to enhance the sensitivity and accuracy further.

Paper Details

Date Published: 18 August 2014
PDF: 12 pages
Proc. SPIE 9203, Interferometry XVII: Techniques and Analysis, 92030F (18 August 2014); doi: 10.1117/12.2063446
Show Author Affiliations
Wansong Li, VEW Vereinigte Elektronik Werkstätten GmbH (Germany)
Philipp Huke, Bremer Institut für angewandte Strahltechnik GmbH (Germany)
Jan Burke, Bremer Institut für angewandte Strahltechnik GmbH (Germany)
Christoph von Kopylow, VEW Vereinigte Elektronik-Werkstätten GmbH (Germany)
Ralf B. Bergmann, Bremer Institut für angewandte Strahltechnik GmbH (Germany)


Published in SPIE Proceedings Vol. 9203:
Interferometry XVII: Techniques and Analysis
Katherine Creath; Jan Burke; Joanna Schmit, Editor(s)

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