The phase measuring deflectometry is a powerful technique for the in-situ measurement of of complex optics. Its measurement accuracy is comparable with conventional interferometry, but with higher flexibility, stability and efficiency. The three main challenges in the deflectometric measurement, namely the position-angle uncertainty in calculating the pixel correspondences, height-slope ambiguity in specifying the normal vectors, and rank deficiency in surface reconstruction are analyzed. Some significant error factors and effective solutions are introduced. The measuring accuracy of complex surfaces can achieve a level of 100 nm RMS.
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