4 November 2003 Absolute high-accuracy deflectometric measurement of topography
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The absolute measurement of the surface figure of large, slightly curved specimens with nanometer accuracy is a demanding task and has not yet been satisfactorily solved. Deflectometric methods offer the possibility of measuring specimens of arbitrary large size and arbitrary figure without a reference surface being required. Here the slope is measured and from this the topography is obtained by integration. The method needs a calibrated (multi-) angle measuring tool (e.g. a commercial autocollimator) and well-defined lateral positioning. Recently, two systems have been described that eliminate the influences of scanning guide and other errors by the use of angle differences only (Extended Shear Angle Difference - ESAD). With the first one of these systems, sub-nm uncertainty has already been achieved for optical flats 150 mm in diameter. In this paper we present recent experimental results obtained with the second system and scan lengths up to 88 cm. The contribution of experimental uncertainties to the uncertainty of topography will be discussed. The behavior of ESAD systems with respect to their spatial frequency response will be discussed. It is shown that the uncertainty of the parabolic part (i.e. curvature) of the figure in particular has to be treated separately and dominates the topography. Applications might reach from wafer inspection to straightness stamdards to flat glass production to free-form synchrotron mirror correction.
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Jens Illemann, Jens Illemann, "Absolute high-accuracy deflectometric measurement of topography", Proc. SPIE 5188, Advanced Characterization Techniques for Optics, Semiconductors, and Nanotechnologies, (4 November 2003); doi: 10.1117/12.507867; https://doi.org/10.1117/12.507867


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