Efficient underwater 3D shape measurement with robust dual-phase complementary unwrapping.

Area of Science:

• Optical Engineering
• Metrology
• Marine Technology

Background:

• Underwater 3D measurement using phase-shifting profilometry (PSP) is challenged by light scattering and absorption.
• Gray-code temporal phase unwrapping suffers from fringe order mismatches and phase-jump errors due to underwater scattering and phase instability.

Purpose of the Study:

• To develop a robust and efficient method for high-precision underwater 3D measurement.
• To address the limitations of existing phase unwrapping techniques in turbid water conditions.

Main Methods:

• Proposed a dual-phase complementary unwrapping method (DPC-Unwrap) for absolute phase reconstruction using two π-shifted wrapped phases.
• Implemented a hybrid ternary-binary Gray-code scheme with asymmetric gray-level mapping to enhance decoding reliability.
• Validated the method through water-tank experiments, turbidity-varying tests, and field measurements.

Main Results:

• The DPC-Unwrap method effectively suppresses phase-jump errors and phase unwrapping failures.
• Achieved millimeter-level measurement accuracy in diverse underwater environments.
• Demonstrated improved robustness and encoding efficiency under scattering interference.

Conclusions:

• The proposed DPC-Unwrap method significantly enhances the performance of underwater fringe projection profilometry (FPP).
• The approach is suitable for high-precision optical 3D measurement in complex natural waters, overcoming scattering and absorption issues.