Study On Underwater Polarization Imaging Based On Automatic Estimation Of Global Parameters

Underwater optical imaging is of great value in many fields,such as seabed resource exploration,marine environment monitoring,underwater archaeology and military reconnaissance,etc.However,affected by the absorption and scattering effects of the water body and its turbid media(i.e.suspended particles and soluble organic matter),underwater images are often lacking of details,with heavy noise and low contrast.Therefore,how to effectively enhance underwater vision has become a research hotspot.The main reason for the degradation of underwater image quality is the interference of backscattered light.Based on the fact that the backscattered light is partially polarized,underwater polarization imaging is proposed as an underwater optical imaging technology that can effectively suppress backscattered light.Benefiting from its simple structure,strong portability and low cost,underwater polarization imaging is considered as a promising underwater optical imaging technology,which has received wide attention.This dissertation makes improvements and innovations in view of shortcomings of some existing underwater polarization imaging methods,broadening the application scope of underwater polarization imaging technology.The specific research contents are as follows:(i)The underwater active polarization imaging method without prior knowledge may introduce a lot of noise into the restored images,for that only the contrast is taken into account.In order to solve the above problem,an underwater active polarization imaging method based on two-layer multi-index optimization is proposed.First,the mutual information and contrast are taken as the upper objective functions,and the Pareto optimal solution set is obtained by the multi-objective genetic optimization algorithm.Second,the information entropy is taken as the lower objective function to obtain the optimal parameters from this optimal solution set.Based on the optimal parameters,the restored images are obtained.According to the difference between the degrees of polarization(Do Ps)of target reflected and backscattered light,these restored images are further improved by the digital image processing method.The experimental results indicate that our method can not only enhance image details effectively but also balance various evaluation indexes of the imaging quality to obtain high-quality restored images.The proposed algorithm is appropriate for underwater targets with low and high Do Ps,with or without background regions.(ii)To address the difficulty of traditional methods in restoring underwater complex targets with non-uniform polarization characteristics and the limitation of image background regions,the basic physical model of underwater imaging is analyzed,and a new underwater polarization de-scattering method is proposed by establishing orthogonal polarization decomposition model of both the target reflected and backscattered light.The polarizeddifference intensity of the backscattered light can be directly calculated from the polarization orientation angle of the two horizontal decomposition components,avoiding the unreasonable assumption about the Do P of target reflected light.Meanwhile,a low-pass filter is used to suppress noise and globally evaluate the Do P of backscattered light.The experimental results demonstrate that this method can automatically realize image restoration of complex target with nonuniform polarization characteristics,and no human-machine interaction,background area or any prior information is required.It also shows good adaptability under water conditions with different turbidity levels.