Measuring deformations with deflectometry
2014
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.
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