Research Of Phase Unwrapping In Specular Objects’3D Measurement Based On Phase Deflectometry

In optical measuring, the surface structured light technology has wide application indiffuse objects because of the advantage of large range, fast speed and high precision,but the relevant technology is not ripe for specular objects. There is not evaluationstandard of specular surfaces now, and its measurement is behind of the industrialneed. This paper adopted the Phase Measurement Deflectometry (PMD) technology tomeasure the specular surface, which used LCD as the displaying device and the CCDas the image acquisition device. The3D shape can be gotted using the phase map ofthe measured surface and the reference mirrors. But only the wrapped phase can beacquired directly, while the unwrapped phase was needed in the PMD technology.This paper mainly researched how to get the coded grating efficiently and how toget the unwrapped phase accurately and quickly when measuring the3D shape of thespecular objects with PMD technology. The content of the paper were as following:1. A mathematical measurement model of measuring the highly reflectedspecular surfaces had been proposed, and the measuring system had been set up basedon the model. Theory and experiment demonstrated that the method was feasible.2. Analyzed the mathematical expression of the phase unwrapping, discussed theadvantage and disadvantage of linear increment method, index increment method, theleast squared method, the center alteration method and the cut branch method, andproved the conclusion in simulation and experiment.3. Proposed a new method of phase unwrapping, in which an auxiliary alterationgrating combined with the modulation method. The alteration grating determined thestarting point’s real phase, and the modulation decided the path which the phaseunwrapping direction is following. This method realized automatic measurement.4. An improved composite grating was proposed, in which the alteration gratingis adding to the traditional composite grating. The measuring speed was improvedbecause of high efficiency of the composite grating and the measuring was automatic.5. Completed3D measurement of specular step objects and the plane mirror. Theprecision can reach50μm.