Phase Unwrapping of Metal Workpieces Based on Multifrequency Polarization Fringe Technology

Multifrequency fringe technology is widely used in fringe projection contour measurement methods, due to its stable and reliable measurement results, wide measurement range, and simple and fast algorithm. However, metal workpieces have a high reflectivity that often introduces errors to the wrapped phase of multifrequency fringes. These errors propagate layer by layer, ultimately leading to an increase in absolute phase error or the incapability to unwrap, which results in the difficult reconstruction of metal workpieces. To solve this problem, in this article, we propose a metal-workpiece phase-unwrapping strategy based on multifrequency polarization coding (MFPC). In this strategy, the traditional multifrequency gray code (MFGC) mode is replaced with an MFPC mode. Encoding polarization information into multifrequency fringe images is conducted to eliminate the influence of highlights on the wrapped phase of metal workpieces and prevent error propagation from the source. Unwrapping of high-frequency wrapped phases occurs with high-frequency polarized fringes being assisted by low-frequency polarized fringes, which improves the measurement accuracy and range. Experimental comparisons with traditional MFGC mode have shown that this method can reliably unwrap the phase of metal workpieces under the influence of highlight and obtain complete 3-D information on metal workpieces.