Light changes its direction of propagation before entering a camera enclosed in a waterproof housing owing to refraction, which means that perspective imaging models in the air cannot be directly used underwater. In this article, we propose an accurate binocular stereo measurement system in an underwater environment. First, based on the physical underwater imaging model without approximation and Tsai’s calibration method, the proposed system is calibrated to acquire the extrinsic parameters, as the internal parameters can be pre-calibrated in air. Then, based on the calibrated camera parameters, an image correction method is proposed to convert the underwater images to air images. Thus, the epipolar constraint can be used to search the matching point directly. The experimental results show that the proposed method in this article can effectively eliminate the effect of refraction in the binocular vision and the measurement accuracy can be compared with the measurement result in air.
In order to conduct measurement or 3d surface reconstruction in underwater environments, we have to place the camera in a waterproof house and take pictures. Due to the medium of water and air having different refractive indexes, the phenomenon of refraction occurs when light propagates in different medium, which led to the light is no longer a straight line before entering the camera, the incident light will not focus on one point. Traditional calibration model in air is no longer applicable in this situation. By analyzing the physical model of light propagation in the water, and according to the imaging model in air, we propose a nonlinear underwater imaging model and camera calibration method. Through the proposed approach, the physical parameters of our model that affecting the underwater imaging are calibrated. And the measurement experiments show that we can acquire the identical accuracy as in the air, or even better.
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