Ms. Hailan Li Profile

Hailan Li

at Beijing Institute of Technology

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Publications (3)

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Proceedings Article | 2 September 2009 Paper

Speckle noise suppression of range gated underwater imaging system

Hailan Li, Xia Wang, Tingzhu Bai, Weiqi Jin, Youwei Huang, Kun Ding

Proceedings Volume 7443, 74432A (2009) https://doi.org/10.1117/12.831994

KEYWORDS: Speckle, Imaging systems, Image filtering, Optical filters, Underwater imaging, Speckle pattern, Light sources, Digital filtering, Linear filtering, Electronic filtering

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Images associated with underwater imaging systems are normally degraded by the intervening water medium. The imaging instrument records not only the signal of interest, i.e., the radiance diffusely reflected from underwater target, but also the radiance scattered into the field of view by water molecule and particulates. In order to improve the system performance, range gated underwater imaging system is used to enhance image quality and visibility in turbid conditions. Range gated imaging utilizes time discrimination to improve signal-to-backscattering noise ratio by rejecting backscattered light from the medium. The range gated underwater imaging system basically consists of a pulsed laser system, a control and synchronous logics and a high-speed gated camera. Because a laser is a highly coherent light source, speckle noise results from the randomly constructive or destructive interference of the scattered light rays will appear in the images obtained from the range gated underwater imaging system. The random granular speckle noise brings great difficulty for the image processing. So the formation causes of speckle noise are discussed and several different material objects under standard light source and laser are chosen to carry out speckle noise comparative analysis. And a multidirectional morphological filtering algorithm for reducing speckle noise is proposed by using the characteristics of morphology's multi-resolution analysis and fast-computing. In order to evaluate the method objectively, equivalent number and speckle index are introduced. The experimental results demonstrate that the approach that is adopted not only can reduce the speckle noise of the image effectively but also can preserve the feature detail efficiently.

Proceedings Article | 2 September 2009 Paper

Image enhancement of range gated underwater imaging system based on least square error

Hailan Li, Xia Wang, Tingzhu Bai, Weiqi Jin, Xiao Zhang, Lin Zhao

Proceedings Volume 7443, 74432B (2009) https://doi.org/10.1117/12.832187

KEYWORDS: Image enhancement, Imaging systems, Image processing, Image quality, Image segmentation, Laser imaging, Signal to noise ratio, Underwater imaging, Laser systems engineering, Computer simulations

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Range gated underwater laser imaging technique can eliminate backscattering noise effectively. While the images associated with underwater imaging systems are normally degraded seriously by the intervening water medium. And the speckle noise is especially severe for the reason that we adopt the system based on intensified gate imaging technology. Well known causes of image degradation underwater include turbidity, particulate matters in the water column, and the interaction between light and medium as light travels through water. Consequently, using full image formation models to design restoration algorithms is more complex in water than in air because it's hard to get the values of the model parameters relating to water properties, e.g., attenuation and scattering coefficients. To improve the quality of the low signal-to-noise ratio images obtained through range gated laser imaging system, an enhancement algorithm is proposed. The main purpose of the algorithm proposed for processing underwater images is to filter out unwanted noises and remain desired signals. This algorithm is based on the principle of the least square error method, which fits discrete image data to continuous piecewise curves. To simply the fitting of image data, the interval of each row and column is subdivided into several subintervals. Then a curve is used to fit the image data within the subinterval. To merge two adjacent lines together, a weighting technique with a linear weighting factor is imposed. A series of experiments are carried out to study the effects of the algorithm. And the signal-to-noise ratio shows that the proposed algorithm can achieve high quality enhancement images.

Proceedings Article | 28 January 2009 Paper

Nonuniformly illuminated image enhancement based on range-gated underwater imaging system

Hailan Li, Xia Wang, Tingzhu Bai, Weiqi Jin, Youwei Huang, Kun Ding

Proceedings Volume 7156, 71563E (2009) https://doi.org/10.1117/12.812026

KEYWORDS: Imaging systems, Image processing, Image enhancement, Eye models, Underwater imaging, Visual process modeling, Image quality, Laser imaging, Pulsed laser operation, Visualization

Read Abstract +

Range-gated underwater laser imaging technique can eliminate most of the backscattering and absorption noise effectively. It has a range of from 4 to 6 times that of a conventional camera with floodlights in the strongly scattering waters, which becomes a useful technique in oceanic research, deep-sea exploration, underwater remote control and robotic works. While because of the laser pulse stretching, the image obtained through range gated underwater imaging system has obvious nonuniformly illuminated character, such as brighter center and darker edge. Low contrast and grayish white of the image also bring great difficulty for processing. In order to adjust the lightness of the nonuniformly illuminated image of range-gated underwater imaging system, the water degradation is assumed as illumination variation and retinal-cortex theory based on color constancy is introduced. Frame integral algorithm has to be applied first to eliminate system noise for the reason that we adopt the system based on intensified gate imaging technology. And gray stretch ensures that we can attain appropriate output. In retinal-cortex models, McCann model and McCann-Frankle model have obvious effect. So we choose the two models for comparison and improve the second one considering the exponential characteristics of eyes for illumination. In order to evaluate the methods objectively, strength uniformity of signals is applied. The experimental results demonstrate that the approaches we adopted are all effective and can enhance the image contrast. And the improved McCann-Frankle model gets more satisfying visual effect.

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