Calibration-free single camera stereo-digital image correlation for small-scale underwater deformation measurement.

Stereo-digital image correlation (stereo-DIC) has been routinely used as a practical and powerful optical technique for surface 3D full-field shape and deformation measurements in various scenarios. However, it is challenging to perform accurate stereo-DIC measurements for submerged objects due to the significant refraction presented at the interfaces of air and water. In this paper, a novel underwater full-field 3D profile and deformation measurements method using the single camera stereo-DIC technique that combines single bilateral telecentric lens imaging and bi-prism-assisted pseudo stereovision is proposed. In using this technique, an immersed surface projects through the (semi-) submerged bi-prism and the bilateral telecentric lens, forming two virtual images on left and right parts of the camera sensor. Matching the virtual left and right images using DIC and substituting the matched image points into a set of newly derived linear equations, accurate 3D profiles and further 3D deformation fields can be readily obtained. The effectiveness and accuracy of the proposed method are successfully validated by a set of real experiments including underwater 3D shape reconstruction, in-plane and out-of-plane translation, and membrane inflation experiments. Because of the distinctive advantages of simple and compact optical configuration, without the need of stereo calibration, and strong robustness against water fluctuation and ambient light variation, the proposed method is expected to be a simple yet effective method for many underwater applications like in vitro biological tissues deformation measurements and submerged materials characterization.