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Polarimetric imaging is an effective way for enhancing the image quality in underwater environments. Previous polarization-based methods for underwater imaging actually involve both optical and digital processing, while compared with digital processing, the importance of the optical method is not clear. In this paper, we propose a polarimetric method for underwater imaging based on the Mueller imager. In this method, in order to optimally filter out the backscattered light and enhance the image quality, the polarization states of illumination light together with the polarization filter in front of the camera are synergistically modulated in an optical way. In other words, the quality of images captured by the camera can be directly improved without any digital processing. A series of experimental results show the effectiveness of the proposed method, which indeed verifies its superiority in scattering media.

The scattering and absorption by the existing particles in turbid media is severely degrading the image quality and decreasing the visibility distance, especially in dense turbid media. In this paper, we investigate the polarization property of hazy images in dense turbid media, and we propose a simple pseudo-polarimetric dehazing method for dense haze removal, which only requires a single image captured by the detector. By our proposed method, two orthogonal polarization subimages are derived from the single original image, and then, based on these two subimages, one can obtain the recovered image by using the traditional polarimetric method. The real-word experimental results verify that, for the dense haze, our method based on a single image could have a comparable or better performance compared to the traditional polarimetric dehazing method with multiple images captured at different polarization states.

Contrast optimization is a key issue in polarimetric imaging for the purpose of target detection. In practice, the noise could induce the intensity fluctuation of the image and thus lead to the decrease of the image contrast. A joint noise reduction method is proposed for contrast enhancement in Stokes polarimetric imaging. The proposed method is based on the relation of the joint polarimetric image set, which includes four images taken to calculate Stokes vector and one image taken at the optimal state of a polarization state analyzer (PSA). By our method, the traditional contrast-enhanced image is modified to decrease the disturbance of noise, and the contrast of the image is further enhanced. Both the theoretical analysis and the real-world experimental results demonstrate that our method can effectively decrease the disturbance by the noise and thus increase the contrast of the image. In particular, it is found that the effect of the proposed method is independent of the optimal PSA state when the regular tetrahedron measurement matrix is implemented.

The underwater imaging could be severely degraded by the scattering media due the backscatter veiling and signal attenuation. Image recovery based on polarization information has shown its efficiency in improving the quality of image in scattering medium, but few of them considered the effect of nonuniform optical field on the image recovery. In this paper, we consider the condition of nonuniform optical field, and we propose a method of retrieving the objects radiance based on estimating the degree of polarization (DOP) and the intensity of backscatter at different positions of the image. The experimental results demonstrate that the quality of the underwater image can be effectively enhanced by this method. In addition, compared with the previous methods, the method proposed in this paper can maintain a better performance at different positions of the image, thanks to considering the DOP and intensity of backscatter as spatial variables instead of constants.

Underwater vision can be severely degraded by the scattering media due to backscatter veiling and signal attenuation. In this paper, we focus on the case that the polarization effect of the object could not be neglected, and we propose a method for recovering the underwater image based on the transmittance correction, which transforms the transmittance for the low depolarized objects from negative values to the positive values that optimize the image quality with a simple algorithm of polynomial fitting. We show with a real-world experiment that with the method proposed in this paper, the quality of the underwater image can be effectively enhanced either for the object with high depolarization degree or for that with low depolarization degree. In particular, without sacrificing the quality of image recovery for the objects with high depolarization degree, the method proposed in this paper can achieve a better performance than the previous methods.

We present a method for improving the spatial resolution and the amplitude by an optimized deskew filter in long-range optical frequency-domain reflectometry (OFDR). In a previous deskew-filter method, as the nonlinear phase estimated from an auxiliary interferometer is used to compensate for the nonlinearity effect in the beating signals generated from a main OFDR interferometer, the spatial resolution and amplitude of the reflection peak in a long range (i.e, 80 km) are deteriorated by a residual nonlinearity effect due to the estimation inaccuracy of the nonlinear phase. In the proposed optimized deskew-filter method, the estimation accuracy of the nonlinear phase is improved by the higher orders of Taylor expansion and the high accuracy of the estimation of the time delay in the auxiliary interferometer using a cepstrum. We experimentally demonstrate that the amplitude of a reflection peak at 80 km increases by 20.5 dB and that the spatial resolution is up to 80 cm, which is about 187 times enhancement when compared with that of the same OFDR trace without nonlinearity compensation.

The underwater imaging could be severely degraded by the scattering media due the backscatter veiling and signal attenuation. Image recovery based on polarization information has shown its efficiency in improving the quality of image in scattering medium, but few of them considered the effect of nonuniform optical field on the image recovery. In this paper, we consider the condition of nonuniform optical field, and we propose a method of retrieving the objects radiance based on estimating the degree of polarization (DOP) and the intensity of backscatter at different positions of the image. The experimental results demonstrate that the quality of the underwater image can be effectively enhanced by this method. In addition, compared with the previous methods, the method proposed in this paper can maintain a better performance at different positions of the image, thanks to considering the DOP and intensity of backscatter as spatial variables instead of constants. [ABSTRACT FROM AUTHOR]