Optical design of an underwater wide working distance robot vision lens

The continuous exploration of the ocean boosts an increasing use of optical means to guide underwater object grasping or underwater docking. In this paper, an underwater wide working distance lens is designed, which is suitable when accurate guidance by machine vision at close range is needed. The lens is adaptable for underwater manipulator grabbing the target, docking of unmanned underwater vehicles and so on. The commonly used sonar guidance methods for underwater long-distance work are difficult to meet the accuracy of close-range work. The method of optical guidance is also affected by factors such as water's absorption and scattering of light, and can only work at close distances. Combining the limitations of both, the designed working distance of this system is within the range of 0.1-10 m, and the use of polarization can further improve the effectiveness of underwater imaging. In view of the effects of seawater refractive index, water pressure, total length limit, and water temperature in the seawater use environment, this paper designs an underwater lens with the maximum working depth of 1500 meters and the working water temperature of 0-30 degrees. The lens is designed with flat underwater optical window and 10 spherical elements in 8 groups. The diagonal field of view (FoV) in water is 35 degrees, F-number (F/#) of 1.8, and the total length is less than 70mm, which is used with the polarization image sensors of 2/3inch optical format. The good imaging quality of the lens satisfies the MTF greater than 0.4@145lp/mm in the whole working distance.