# 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
This method can effectively realize the imaging distance in a high-turbidity water body, improve the imaging quality and detection effect, and providing support for underwater search and rescue work in rivers and offshore areas. [PERSON] [46] proposed a polarization image restoration algorithm and a new curve fitting-based method to estimate the target signal of polarization difference images. Based on the polarization effect of reflected light in underwater imaging, the former was used to restore underwater blurred images with polarization imaging, and study the imaging model of underwater active illumination imaging systems and the transmission behavior of polarization information in an underwater turbid medium. The latter considers the polarization effect of the reflected light from the object in the scene to derive the true transmission coefficient image and underwater restored image. Both can overcome the invalid detection problem in the area corresponding to objects with a low degree of deflection and effectively enhance the underwater imaging quality. As backscattered light occurs due to the presence of high concentrations of impurities in turbid water, the reflected light from an object is easily confused, which makes it difficult to distinguish the object from the environment. Therefore, the division of reflected light from interfering light, such as backscattered light, which reflects the characteristic of the object, is a core issue for underwater image processing using polarized imaging. At present, several methods, such as optical sensing technology, the polarization filter method, and the backscattering interference suppression method, that can separate coherent light from incoherent light exist. [PERSON] et al. [47] proposed new optical sensing technology  
Figure 5: An overview of underwater data processing methods based on polarization imaging. based on the orbital angular momentum (OAM). The target is illuminated by a Gauss beam. By setting a diffraction spiral phase plate at the receiving end, the reflected and backscattered light of the object passes through the phase plate to form vortex light, thus spatial separation of coherent and incoherent light is achieved. Experiments have shown that the echo of a ballistic target can achieve detection that is two to three orders of magnitude the level of backscattered clutter. The detection of this coherent element is realized using a complex optical heterodyne scheme. In addition, the detection of this small coherent signal is completed without the use of any complex optical heterodyne scheme, which indicates that the unique characteristics of OAM can be used to distinguish between objects and the environment. [PERSON] et al. [48] used a polarized imaging optical system to reduce the influence of underwater beam diffusion on image acquisition and optimized the DCP method. They used a low-pass polarization Gauss filter to calculate the illumination from the input image and enhance underwater optical imaging, which reduced the long running time and ameliorated the efficiency reduction of traditional algorithms with the increase in turbidity. Moreover, the visibility of this method was significantly higher than that of the traditional DCP method and the processing time was reduced by nearly 50-fold. [PERSON] et al. [49] proposed an underwater image restoration method based on transmission correction for when the object polarization effect cannot be ignored. Without sacrificing the quality of image restoration, this method showed a better performance, has a simpler algorithm, and used less computational time than previous methods. This approach converts the transmittance of a low depolarized object from a negative value to a positive value and uses a simple polynomial fitting algorithm to optimize the image quality. The results showed that it can effectively improve the quality of underwater images regardless of the degree of depolarization of the target.