Translated Abstract
Underwater optical imaging plays an important role in the fields of marine resources development, marine military applications, and marine environmental protection. The work on searching for the wreckage of Malaysia Airlines MH370 passenger plane makes underwater target detection and recognition based on optical methods become the hot spot in the optical sensing field once again. When light passes through water, scattering and absorption degrades the image quality severely, and reduces the image contrast and the visibility. Therefore, general optical imaging systems are difficult to achieve clear image. Underwater target detection and recognition using optical methods has become a very challenging problem.
To solve the problems mentioned above, the developing stages of underwater polarization imaging are disscussed, modified laser polarization imaging methods are developed. The scattering lights from the underwater objects and background are separated from each other by exploiting the difference in polarization responses. As a result, degraded image quality caused by light scattering can be improved, and underwater optical image contrast and detection distance can be enhanced. This makes underwater target detection and recognition more effective.
In order to reduce the influence of background scattering on the underwater image from the physical mechanism, the transmission characteristics of polarized light that propagating in turbid water is investigated in the case of active detection. The experimental results demonstrate that when polarized light interacts with turbid water, both forward and backward scattered light can preserve their original polarization state. Also, there exists a coupling effect between the linear polarization elements when polarized light propagate in turbid water, which makes a difference in the polarization angle between the incident light and the scattered light.
According to the law of interaction between polarized light and turbid media, the physical model for underwater polarization imaging is established. On the basis of difference in polarization responses between the background scattering and the target signal, polarization filtering techniques are designed to remove the influence of background scattering on the underwater imaging. For underwater targets with rough surface, orthogonal polarization imaging is used to remove the background significantly, and polarization subtraction method is further employed to eliminate the background completely. In order to obtain the original characteristics of the target, improved Schechner’s descattering model based on the polarization properties of light with different scattering process is proposed. For underwater camouflage targets, polarizarion-based laser range-gated underwater imaging is proposed based on the depolarization difference in the target and the background, uniform illumination and further filtering of the background is achieved too. For underwater targets with smooth surface, Stokes-vector-based polarization difference imaging based on the difference in vector direction between the target and the background is proposed to achieve rapid polarization imaging, which improves efficiency of underwater imaging and detection. Experiments show the validity of these polarization imaging methods.
Although the modified methods mentioned above could enhance the signal-to-noise ratio between target and background, the flux of imaging light is reduced, which affects the visual image quality. A Matlab program is used to process the polarization image to achive a better detection performance based on the physical mechanism of denoising.
This thesis demonstrated that modified polarization technique could eliminate the influence of attenuation on the imaging light caused by scattering and absorption in the turbid water, and remove the background scattering imposed on the target image, which results in the improvement of underwater image quality. Futher developed ideas is proposed to improve the image quality better.
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