Elimination of parasitic reflections for objects with high transparency in phase measuring deflectometry

In phase measuring deflectometry (PMD), the existence of parasitic reflections at the rear surface of transparent objects will lead to 'ghosted' fringe patterns, which results in phase error. Accurately extracting the phase from the 'ghosted' fringe patterns is considered as one of the main problems in PMD. Existing phase extraction methods still remain some drawbacks such as high cost of experiment equipment, the existence of ill-conditioned areas and difficulty in setting up a suitable threshold. In this paper, the envelope curve algorithm based on multifrequency approach is proposed to eliminate parasitic reflections. The parasitic reflections were eliminated successfully. The PV and the RMS of the front phase error are 0.1085 rad and 0.0072 rad, and those of the rear phase error are 0.1345 rad and 0.0081 rad respectively in the simulation. Experiments also proved the effectiveness of the proposed algorithm. What's more, the phase information of both the front and the rear surfaces can be attained simultaneously. The proposed algorithm could eliminate parasitic reflections successfully without destroying the tested object or special equipment. Factors which may influence the performance of this algorithm are discussed, and results showed that the proposed method is especially suitable for thick transparent object.