Underwater image enhancement by method combined the tail-gating and active polarization imaging technology

Underwater imaging always suffers strong absorption and scattering of water, leading to unsatisfactory imaging quality. Among these factors, scattering is considered as the main factor that makes underwater imaging fuzzy. Various methods have been introduced to eliminate the influence of scattering, and play a positive role in image enhancement, such as rangegated imaging, active polarization imaging and synchronous scanning technology. However, these methods appear limited effects in a higher level of turbidity conditions as it is generally believed that the multiple scattering photons are ascendant in a higher level of turbidity. In this paper, a new underwater image enhancing method was proposed. This method combined the tail-gating technology and the active polarization imaging technology. A range-gated imaging system was upgraded for verifying the novel method by adding polarizer in the light source unit and polarization analyzer in the light collecting unit. Experiments were conducted in a water tank size of 3m×0.6m×0.6m (L × W × H) that filled with running water is to simulate sea water condition in laboratory condition. The turbidly of sea water condition was simulated by adding Magnesium Hydroxide as scattered particles. Several groups of experiments under different range-gate setting and polarization strategy were conducted for comparison. The enhancement of the underwater imaging resolution was evaluated by computing the MTF (Modulation Transfer Function) values. The results said that the method combined the tail-gating and active polarization technology was capable of improving the imaging resolution in turbid underwater environment.