Temporal phase unwrapping with single frame spatial multi-level symmetric coding

A method for absolute phase unwrapping based on single frame spatial multi-level symmetric encoding is proposed. This method divides each fringe period into three or five equal parts and makes each codeword symmetrically distributed. The symmetry feature and the correlation between adjacent codewords are used to achieve highly robust fringe order correction. Combined with the three-step phase-shifting profilometry, only four frames of fringes need to be projected to achieve absolute phase unwrapping. Compared to traditional spatial encoding, the proposed encoding method can directly extract fringe order information using symmetry feature even when there are many defects in the fringes, and synchronously demodulate color textures from deformed patterns without additionally capturing specific color image, thereby achieving color fidelity 3D reconstruction of color objects. In order to improve the efficiency of real-time measurement reconstruction, a projection strategy of 5 phase-shifting fringes equipping with 1 code fringe is proposed. The reconstruction rate is increased from the traditional one reconstruction per four patterns to three reconstructions per six patterns. The experimental results have verified the effectiveness and practicality of the proposed absolute phase unwrapping method and projection strategy.