Research On The Polarization Optics Properties Analysis And Target Detection Method In Underwater Bubble Environment

Underwater optical imaging is an important technical means of ocean exploration and target detection.When taking optical imaging observations in seawater,due to the sea wave,ship wake,Marine life,and Seabed resource exploration,a large number of the bubble will appear in the water,and severely affect the quality of underwater target imaging.Light scattering and the veiling effect of a bubble tend to lower the contrast and definition of the underwater image,and seriously decline the image quality,it is difficult to identify the target because the ability of underwater optical imaging and target detection efficiency is limited by bubble.Preliminary studies have shown that the polarization imaging technology has the characteristics of suppressing the scattering and improving the contrast of the target in a strong scattering medium compared with the traditional optical radiation imaging technology.To improve the optical imaging efficiency in the bubble environment and effectively suppress interference from bubble on imaging,this thesis has carried out polarized optical characteristics analysis and research on target detection method in an underwater bubble environment,and studies the radiation transmission mechanism of polarized light and target polarization reflection characteristics in a bubble environment,proposes the solutions to the interference of imaging under the condition of different water environment,and carries out the corresponding verification experiment.The thesis has mainly carried the thorough research from the following four aspects.1.Studying the mechanism of polarization radiation transmission in a bubble environment.First of all,establishing a quantitative model of the intensity and polarization characteristics of light in a single bubble with observation geometry through the Fresnel reflection law,and getting the transmission energy function in all parts of the water-gas interface.Then,establishing a quantitative model of the intensity and polarization characteristics and the observation geometry when the incident light has transmitted through a bubble group by increasing the number of bubbles,and obtaining the relationship between the light transmission efficiency and the distance in the bubble group medium.Finally,constructing a polarized radiation transmission model when light transmitting in a complex bubble medium based on geometric optics and the Monte Carlo random process,simulating the influence on the light transmission in the bubble groups with different radii of bubble size and light transmission distance and bubble density.Simulation results show that in the transmission process of light waves in a bubble group consisting of a large-scale bubble,the changing state of polarization is strongly associated with incident angle.When the incident angle is less than 30°,radiation intensity decreases slowly,and the bubble number that penetrated is more after light has passed through the bubble group.With the increase in transmission distance,the degree of polarization to backward transmission light has a downward trend,contrary to forwarding transmission.When linear polarized light is transmitted to a bubble environment,the degree of polarization to backward transmission light gradually reduces in the linear trend with gradually increasing transmission distance.2.Studying the polarized reflection characteristic of typical materials commonly used in the water.Firstly,constructing the polarized reflection characteristic model of the target based on the Stokes formula.On this basis,simulating the changing trend of the intensity and polarization characteristic on the surface of the target with various materials under different transmission medium environments.Finally,using a portable pBRDF measurement system to verify the model and the variation trend,and obtaining the observation geometry conditions suitable for underwater environment imaging.The results show that the degree of polarization to aluminum,iron and copper materials with similar surface smoothness decreases with the increase of wavelength to incident light in the waveband from 450nm to 700nm.When the air medium is changed into the water medium in the transmission process,it will lead to a decrease in the Brewster angle of the coated target.The following results are obtained through simulation and sample experiment on coated target,the degree of polarization to red coated target and grey coated target are slightly lower in the underwater environment than in the air environment,and black coated target have the opposite results.When the observation geometry is less than Brewster angle in an underwater environment,the degree of polarization to the coated target increases with the increase of the zenith angle.As each coated targets contain different composition,and the degree of polarization to coated target in the specular reflection observation direction is black,red and gray in descending order.3.Studying a polarization imaging device adapted to target detection in a bubble environment and experimental verification method.First of all,the performance evaluation and test of the imaging detection device have been carried out to ensure that the measurement accuracy of the device meets the polarization measurement requirements of the target in the bubble environment.Then,setting up a polarized optical imaging experiment platform in an underwater bubble environment to carry out verification experiments.Finally,the effects of incident angle and observation angle,imaging band and polarization state of incident light on target detection in a bubble environment are verified by changing the experimental conditions.The experiment results show that in a low-concentration bubble scene,the imaging device is fixed and set at 45°from the target normal direction,the angle between incident and observation is greater than 90°,and the natural light or linearly polarized light is used for illumination,the polarization characteristics of the target are obvious.In the high concentration to bubble scenario,the polarization imaging effect using a linear polarized light source for lighting is better than the natural incident light.Based on the depolarization difference between the target and the bubble,the illumination source with a specific polarization direction by rotating the polarization device can weaken the bubble interference and improve the target contrast in the polarization images.4.Studying the detection method of the target in an underwater bubble environment based on polarization imaging.Firstly,obtaining the continuous frame image data in the bubble scene by using a polarization imaging device based on the optimal observation geometry and bubble motion characteristics in the bubble environment.Secondly,establishing the assumption that the Stokes-Q parameters of the bubble in water have constant significance based on the difference of polarized reflection characteristic between the target and the bubble,and the polarization parameter difference algorithm is proposed to effectively suppress the bubble interference and restore the image under the environment of high concentration of bubble.Finally,according to the polarization scattering characteristic of a complex underwater environment,optimizing the definition of underwater transmission rate,establishing the polarization reconstruction model of the underwater target,and automatically estimating the polarization information of the target region by using the covariance parameter,to realize the image sharpening in a complex scene.The verification experiment is carried out simultaneously and the experimental results show that the proposed method can effectively suppress the interference of bubbles on optical imaging in both high-concentration and low-concentration bubble environments.Compared with the original image,the structural similarity(SSIM)and peak signal-to-noise ratio(PSNR)are significantly improved.When the underwater environment contains both bubbles and suspended particles,this method can suppress the scattering of suspended particles and interference of bubbles.Compared with the traditional underwater imaging algorithm,the image contrast and target identification are significantly improved.The research content of this thesis has a certain reference value and provide scientific and technological support to improve the target imaging effect in a complex aquatic environment,and enhance the information extraction ability of underwater target.