# Polarization Imaging Method for Underwater Low-Visibility Metal Target Using Focus Dividing Plane
## 1 Introduction  
During the imaging of targets in underwater low-visibility environments, the images are affected by the absorption and scattering of light by water and lighting equipment. There are problems such as image brightness attenuation and contrast reduction. The problems in these images will bring some difficulties to underwater environment monitoring, military reconnaissance and other target detection activities [1; 2]. Polarization imaging is a new optical imaging technology. The polarization characteristic distribution can be obtained based on the target light intensity information, including the degree of polarization, the polarization angle and other information [3]. According to the polarization information difference of objects with different shapes, materials and surface roughness, it can enhance the edge contour, texture details and other information of underwater targets [4]. It can effectively detect targets in complex underwater environments [5]. [PERSON] used the method of rotating the polarizer. He conducted polarization imaging on targets with six different materials. It is proved that the polarization image containsenough brightness information and rich detail information that is easy for human eyes to recognize [6]. [PERSON] studied the influence of different materials on polarization imaging through an underwater polarization experiment. The conclusion was that the degree of polarization of the target with a smooth surface is higher than that of the target with a rough surface [7]. [PERSON] designed a polarization imaging experimental system based on LED auxiliary lighting. For targets with different polarization characteristics, different processing methods are used to obtain high-resolution images [8]. [PERSON] proposed a material classification method based on polarized light imaging. In the study, it was observed that there are different reflectivity ratios between metals and nonmetals within a certain observation angle range, and a Fresnel reflectivity ratio method was proposed to distinguish metal and nonmetal targets [9]. [PERSON] studied the polarization degree information of metal aluminum plates, cloth, rubber, ceramics, sediment and paper through a linear polarization imaging experiment. It was concluded that the polarization degree of a metal target is obviously higher than that of other objects. It can be used to quickly identify metal chips [10]. In a haze environment, [PERSON] conducted polarization imaging on plastic objects and metal objects under low background illumination. It was concluded that polarization imaging can be used as a complementary approach to obtain target details [11]. In the analysis of underwater target imaging technology, underwater polarization imaging technology can be studied by detectors with polarizers [12; 13]. [PERSON] studied the spatial distribution of the degree of polarization of the reflected light on the target surface, and obtained that the metal and nonmetal material targets can be classified by using the degree of the polarization image [14]. [PERSON] used multibands as factors; the benefit of polarization imaging in a sea fog environment was assessed objectively using contrast, information entropy, degree of polarization, and other evaluation indices [15]. Through theoretical analysis, [PERSON] conducted polarization imaging experiments on water bodies with different turbidity and different materials. The mechanism of active linear polarization imaging was demonstrated [16]. [PERSON] proposed an underwater polarization imaging method based on a Mueller imager. In this method, optical comodulation of the polarization state of the light source and the direction of the polarizer in front of the camera is established. It can not only filter the underwater scattered light, but also improve the image quality [17].