Underwater 3D reconstruction based on double N-step orthogonal polarization state phase shift strategy

The nonlinear response of projection equipment and the uneven reflectance of HDR (high dynamic range) object surface pose challenges in underwater 3D reconstruction. As a result, the phase-shifting fringe pattern collected by the camera does not have good sinusoidality, leading to phase errors. Therefore, this paper proposes a double N-step orthogonal polarization state phase-shift strategy (DOPS). This technique uses the degree of linear polarization (DOLP) of polarized light to design a double N-step phase-shifting fringe pattern. The original phase shift fringes are encoded as horizontal polarization fringes, and the additional phase shift fringes are encoded as vertical polarization fringes. Then, the camera captures and extracts the phase information of the two groups of DOLP patterns, which are separately subjected to phase unwrapping. The resulting unwrapped phases are fused to reduce phase error. The experimental results indicate that this method reduces the measurement error by 57 % when dealing with 3D shape measurement of underwater HDR objects, compared to the polarization-encoded phase shift algorithm (PEPS). Additionally, this method is more accurate in measurement accuracy and improves measurement efficiency by 50 % compared to the double N-step phase shift method (DNPS).