Simulation and Test of Polarization Reflection Characteristics of Marine Oil Film
Oil spills are a common form of marine pollution.In the field of satellite remote sensing,utilizing polarization techniques for oil spill detection holds significant potential.Researchers conducted tests,simulations,and analyses on the polarization characteristics of oil spills.These studies focused on the optical feature model of the oil film,neglecting the geometric feature modeling,resulting in an incomplete exploration of polarization characteristics in marine oil spills.Based on these findings,this paper encompasses polarization sky radiation,dynamic oil film reflection,detector performance parameters,and more,establishing a comprehensive oil spill model.Firstly,a model for polarization distribution in the sky was created,with sunlight as the incident light and scattered light as linearly polarized light.Through the calculation method of Rayleigh particles,the polarization degree and polarization angle of sky light were obtained,further deriving the Stokes vector of polarized sky light.Secondly,a submodule for wind-wave and swell-wave spectrum was constructed,utilizing the Jonswap spectrum model for wind-waves and the Gaussian model for swell-waves.These were combined into the wave submodule.Thirdly,a submodule for polarization radiation conversion of the sea surface was generated.Through the transformation of the'global-to-local'incident Stokes rotation matrix,the transformation matrix was multiplied by the incident vector,then by the local pBRDF matrix,and finally by the'local-to-global'transformation matrix.The calculated results were the global reflected luminance.Finally,a submodule for the polarization distribution of the oil film was built,deriving the polarization degree and polarization angle of quasi-monochromatic light from the boundary conditions of electric and magnetic vectors based on the refractive index,incidence angle,and emergence angle.The simulation framework in this paper comprises two parts full digital simulation and semi-physical simulation.Results of full digital simulations are as follows:Firstly,the azimuth and elevation of the sun are determined by time,latitude,and longitude.The sun is non-polarized light,while the sky is polarized light.Both constitute the light source.Based on the elevation and azimuth angle between the sun and the point in the sky,the polarization degree and polarization angle of a point light source were calculated,obtaining the Stokes value.Secondly,wave height was obtained through Fourier transform of wind-wave and swell-wave spectrum.The inclination of the wave surface was calculated by subtracting the wave height in the neighborhood.Thirdly,incident and reflection rotation matrices were constructed based on the wave surface inclination angle,while the Mueller matrix of pBRDF was constructed based on the refractive index.Through the'global-local-global'transformation,polarization degree and polarization angle were obtained.Semi-physical simulation involves capturing real water surfaces with a measurement camera and replacing parts of water with oil to simulate the oil film.The research process in this paper involved four steps Firstly,simulating sea surfaces,including the superposition effect of wind-waves,swell-waves,and wind-wave fluctuations based on wind speed changes.Secondly,simulating optical radiations,where the intensity of the sea surface becomes brighter as the angle between some points in the sky and the sun decreases,reaching saturation in the camera.Additionally,the phase angle gradually increases,and the polarization degree changes from'small-large-small'.Thirdly,simulating rotations of the observed axis,where observed axis rotation has no effect on intensity and linear polarization degree but affects the polarization angle,redistributing the polarized value.Fourthly,semi-physical simulations,where in water tests,approaching Brewster Angle maximizes the polarization degree of water and oil.The ocean,unlike most solids,is a relatively smooth liquid.The distribution of pBRDF in the ocean is concentrated.Even slight waves on the ocean surface cause significant changes when sunlight is reflected,leading to camera saturation,especially during repeated alternations between the sky and the sun.Secondly,intensity is preferred over polarization degree in sea surface detection,and polarization reaches its maximum at the Brewster angle.The polarization angle mainly reflects the rotation of the light source in the sky around the sun and the axis rotation of the camera.Finally,to better distinguish oil spills,understanding the inclination angle of each micro-surface,irradiation environment,and 3D imaging algorithm is essential.This approach helps mitigate the influence of waves and improves the probability of oil spill identification.
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