An Accurate Calibration Method for Multilayer Refractive Imaging in Underwater 3-D Scanning

Accurate calibration of imaging parameters is the key to achieve 3-D scanning in underwater environment. However, the methods based on perspective principle have insufficient precision because of the multilayer refraction caused by encapsulated window. In this article, an accurate calibration method for pose of camera, pose of window, and refractive index of water (RIOW) is proposed. A new derivation for closed solution based on the coplanar and flat refractive constraints is derived to solve the problem that unknown RIOW is coupled into calculation. In calibration, the problem of coplanar degeneration is solved and RIOW can be estimated by constructing high-order equation. The proposed method can analytically compute and simultaneously optimize all parameters including RIOW. Finally, simulations and practical experiments are carried out to validate the feasibility and superiority of the proposed method. It is observed that more parameters are estimated (RIOW included) and higher accuracy is achieved (improved by 30%) compared with existing methods.