Research on the calibration technology of an underwater camera based on equivalent focal length

Calibration is one of the most crucial and inevitable processes in vision technology, whose performance can significantly affect the measurement accuracy of the vision system. To compensate for coefficients of water factors, the conventional calibration technology for the underwater camera generally introduces several higher-order distortion coefficients, which, unfortunately, suffer poor stability, low efficiency, as well as computational complexity. To overcome these drawbacks, a newish calibration method based on equivalent focal length is proposed in this paper. The method takes the refraction distortion of water factors as equivalent focal length changing to cause. Applying the invariance principle of linear perspective projection, distortion correction should first be carried out. Subsequently, the internal and external parameters of the camera are calculated by using Zhang's calibration method based on the camera linear model, which simplifies the process of underwater camera calibration. Finally, the experimental results show that the calibration method can effectively improve the accuracy of the underwater camera.