Due to the absorption and scattering effects of light in water bodies and the non-uniformity and insufficiency of artificial illumination, underwater images often present various degradation problems, impacting their utility in underwater applications. In this paper, we propose a model-based underwater image simulation and learning-based underwater image enhancement method for coping with various degradation problems in underwater images. We first derive a simplified model for describing various degradation problems in underwater images, then propose a model-based image simulation method that can generate images with a wide range of parameter values. The proposed image simulation method also comes with an image-selection part, which helps to prune the simulation dataset so that it can serve as a training set for learning to enhance the targeted underwater images. Afterwards, we propose a convolutional neural network based on the encoder-decoder backbone to learn to enhance various underwater images from the simulated images. Experiments on simulated and real underwater images with different degradation problems demonstrate the effectiveness of the proposed underwater image simulation and enhancement method, and reveal the advantages of the proposed method in comparison with many state-of-the-art methods.
By inserting a microlens array (MLA) between the main lens and imaging sensor, plenoptic cameras can capture 3D information of objects via single-shot imaging. However, for an underwater plenoptic camera, a waterproof spherical shell is needed to isolate the inner camera from the water, thus the performance of the overall imaging system will change due to the refractive effects of the waterproof and water medium. Accordingly, imaging properties like image clarity and field of view (FOV) will change. To address this issue, this paper proposes an optimized underwater plenoptic camera that compensates for the changes in image clarity and FOV. Based on the geometry simplification and the ray propagation analysis, the equivalent imaging process of each portion of an underwater plenoptic camera is modeled. To mitigate the impact of the FOV of the spherical shell and the water medium on image clarity, as well as to ensure successful assembly, an optimization model for physical parameters is derived after calibrating the minimum distance between the spherical shell and the main lens. The simulation results before and after underwater optimization are compared, which confirm the correctness of the proposed method. Additionally, a practical underwater focused plenoptic camera is designed, further demonstrating the effectiveness of the proposed model in real underwater scenarios.
In practical application of underwater wireless optical communication (UWOC), the transmitter should have a larger divergence angle to make it easier to establish a communication link, besides high modulated rate and high optical power. Laser diodes (LD) are suitable to design such transmitter, thanks to their simpler structure and much faster switching speed. However, it is difficult to implement for widespread use in ocean engineering because of its quite small divergence angle. For this, we present a simple way to enlarge the divergence angle for an LD transmitter based on an engineered diffuser in this paper. First, we design a blue LD transmitter that has 476 mW output power, 50 Mbps rate, and 50° divergence angle. Then, using such transmitter, we establish a UWOC system in a large experimental tank with 13.3 m communication distance and about 0.26 m − 1 attenuation coefficient of water. The results show that if the deviation of the transmitting direction is up to ± 25 ∘ , the communication system is workable. Emission light from the transmitter could cover a 42.5% solid angle of the hemisphere space. The combination performances of speed, angular coverage, and optical power are suitable for ocean engineering. Also, it implies that a light field could be designed by using a suitable engineered diffuser for UWOC. The method presented in this paper is simple and pragmatic, which is useful to reduce the difficulty in establishing communication links and is easy to popularize.
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