Objective Underwater optical imaging technology is of great significance for applications such as seabed resource exploration, marine ecological monitoring, underwater search and rescue, and underwater archaeology. Compared to traditional underwater cameras, underwater range-gated imaging (RGI) technology can filter out backscattered noise and environmental background noise outside the gated slice, achieving high-quality underwater imaging. However, in turbid water bodies, it is still affected by backscattered noise inside the slice, resulting in a shorter imaging distance.

Methods In view of the problem of short RGI distance in highly turbid water bodies, underwater polarization gating imaging technology combining optical polarization and RGI was studied. By utilizing the good polarization preservation of backscattered light, the backscattered noise within the gating slice range was removed, and the target recognition distance was improved (Fig.1). Firstly, a physical model for polarized-range-gated imaging (PRGI) is established, a formula for calculating the signal-to-noise ratio of PRGI is derived, a normalized simulation curve for signal-to-noise ratio is drawn. Subsequently, RGI and PRGI are performed on underwater targets such as fishing nets and corals, and signal-to-noise ratio normalization experimental curves are drawn. The simulation curves and experimental curves are compared and analyzed.

Results and Discussions  When the water attenuation coefficient is 0.21 m⁻¹, the PRGI recognition distance is about 15 m, and the RGI recognition distance is about 17 m (Fig.2). The reason why the recognition distance of PRGI is smaller than RGI is that under the low water attenuation coefficient, the backscattering of the water body is small, and the absorption effect of the water body plays a major role in limiting the recognition distance. When the water attenuation coefficient is 0.42 m⁻¹, the recognition distance of PRGI is about 8 m, and RGI recognition distance is about 9 m (Fig.3). The gap between the two has narrowed. The reason is that as water attenuation coefficient increases, the backscattering of water increases, and the scattering effect of water will reduce the target recognition distance. When the water attenuation coefficient is 0.63 m⁻¹, the recognition distance of PRGI is between 5.5 m and 6 m, and the recognition distance of RGI is between 5 m and 5.5 m (Fig.4). When the water attenuation coefficient is relatively high, the backscattering effect of water is severe, PRGI can improve the recognition distance compared with RGI. The experimental results of fishing net imaging under the water attenuation coefficient of 0.21 m⁻¹ show that the signal-to-noise ratio of PRGI at 16 m is lower than RGI (Fig.7). The experimental results of fishing net imaging under the water attenuation coefficient of 0.42 m⁻¹ show that the signal-to-noise ratio of PRGI at 9 m is lower than RGI (Fig.8). The experimental results of fishing net imaging under the water attenuation coefficient of 0.63 m⁻¹ show that the signal-to-noise ratio of PRGI at 5.5 m is better than RGI (Fig.9). The experimental results of coral imaging under the water attenuation coefficient of 0.21 m⁻¹ show that PRGI have severe device noise at 19 m, and the signal-to-noise ratio of the image is worse than RGI (Fig.10). The experimental results of coral imaging under the water attenuation coefficient of 0.54 m⁻¹ show that PRGI at 9.5 m is slightly worse than RGI (Fig.11). The coral experiment results under the water attenuation coefficient of 0.89 m⁻¹ show that the signal-to-noise ratio of PRGI at 5 m is better than RGI (Fig.12).

Conclusions According to the comparison experiment between fishing nets and coral, there should be a critical attenuation coefficient c₀₁ between 0.42 m⁻¹ and 0.63 m⁻¹. When the water attenuation coefficient is higher than c₀₁, the maximum recognition distance of PRGI of fishing nets is greater than RGI; There is a critical attenuation coefficient c₀₂ between 0.54 m⁻¹ and 0.89 m⁻¹. When the water attenuation coefficient is higher than c₀₂, the maximum recognition distance of PRGI of coral is greater than RGI. 　　Based on the comprehensive simulation and experimental results, the following conclusions can be drawn. 　　1) There exist a critical attenuation coefficient c₀, which determines the applicable water quality for RGI and PRGI. 　　2) The critical attenuation coefficient c₀ is related to the target reflectivity.