# A Survey of Target Detection and Recognition Methods in Underwater Turbid Areas
## 4 Research on Target Detection and Recognition in Turbid Waters
### Underwater Image Processing Based on Polarization Imaging and Scattering  
The most difficult problem encountered during optical detection in turbid areas is the scattering effect of water on the light wave, which mainly results in low image contrast, a reduction in resolution, and image blurring. Scattering includes forward and backward scattering processes. When forward scattering occurs, light deviates from the original transmission path, resulting in a reduced image resolution and blurred image. Backscattered light, which carries suspended particulate information, produces a 'curtain effect' on the target image and reduces the image contrast. Therefore, it is necessary to overcome the scattering and reflection problems in underwater imaging to improve the imaging distance and quality. Polarized imaging technology has obvious advantages in removing background scattered light and achieving clear underwater images by deeply mining the uniqueness and differences in polarization information in a scattered light field. Currently, accurate estimation of the polarization characteristics and relationship between target information light and background scattered light, inverting the intensity distribution of target information light and background scattered light, are key research areas of underwater imaging technology. Research has shown that the polarization characteristics of incident polarized light can be used to separate these two kinds of light in a scene, effectively restoring a clear scene, improving the contrast and clarity of imaging results, and aiding underwater target detection and recognition. Because underwater search and rescue operations often face target detection problems in high-turbidity water, exploration of polarization imaging technology that is suitable for turbid water is necessary. An overview of this method is depicted in Figure 5.  
With a long research history and significant basic experience, polarization imaging technology is suitable for more in-depth research in this direction. Currently, polarization imaging methods are being developed for the unique environment of turbid waters. Underwater models for studying turbidity and illumination have been established, which is helpful for optical research of target detection and recognition in turbid waters. [PERSON] et al. [44] proposed a model based on the spatial variation in underwater environments and coherent light and used it for low-contrast target detection in turbid water. This model was used to theoretically study the effects of turbidity, projection space-frequency variation, and three-dimensional target shapes on unstructured scattered light components and the target structured return signal. The results showed that the model's accuracy is adequate for the modeling of noise reduction technology. This result indicates that the received three-dimensional target image can be modeled with backscattering and structured illumination, and noise reduction and target identification can be achieved in the model environment. Based on the image degradation model, [PERSON] [45] considered image degradation due to the joint effects of forward and backward scattered light, estimated the degradation function of forward scattered light using the edge method, and further restored clear scene images. He constructed a turbid water polarization imaging model, and then obtained the polarization degree of target information light and background scattered light using the optical correlation principle to restore the clear scene. On the premise of establishing an optical model, [PERSON] [45] proposed an active underwater polarization imaging method, which is based on the imaging noise analysis model, and studied the effect of noise that is introduced during the polarization imaging process on the final imaging quality. This method resulted in the best polarization azimuth image for active underwater polarization imaging, and the relationship between different polarizer images and the final imaging quality was established.