29 August 2008 High dynamic range scanning technique
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Measuring objects with high surface reflectivity variations (i.e., high dynamic range) is challenging for any optical method. This paper addresses a high dynamic range scanning (HDRS) technique that can measure this type of objects. It takes advantage of one merit of a phase-shifting algorithm: pixel-by-pixel phase retrieval. For each measurement, multiple shots of fringe images with different exposures are taken. And a sequence of fringe images with different overall brightness are captured: the brightest fringe images have good fringe quality for darker areas although the brighter areas may be saturated; while the darkest fringe images have good fringe quality in brighter areas although the fringes in the darker areas may be invisible. The sequence of fringe images is arranged from brighter to darker, i.e., from higher exposure to lower exposure. The final fringe images, used for phase retrieval, are produced pixel-by-pixel by choosing the brightest but the unsaturated corresponding pixel from one shot. A phase-shifting algorithm is employed to compute the phase, which can be further converted to coordinates. Our experiments demonstrate that the proposed technique can successfully measure objects with high dynamic range of surface properties.
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Song Zhang, Song Zhang, Shing-Tung Yau, Shing-Tung Yau, } "High dynamic range scanning technique", Proc. SPIE 7066, Two- and Three-Dimensional Methods for Inspection and Metrology VI, 70660A (29 August 2008); doi: 10.1117/12.791265; https://doi.org/10.1117/12.791265

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