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.
The imaging of underwater objects which have been illuminated by natural or artificial light has been of long-standing
interest to investigators working in oceanographic environments. Analysis of the propagation of imaging light through
water is complicated by extreme attenuating and scattering properties of water, especially multiple scattering. A
comprehensive computer model to simulate the formation of underwater images has been presented in this article. The
model incorporates the temporal and spatial broadening properties of the propagation of light in water. A degraded image
is approximated as a convolution of an initial image and point spread functions (PSFs). The spatial PSF plays a
significant role in describing the spatial propagation of light and image blurring due to forward scattering. The temporal
PSF can describe the pulse broadening effect, which is of great significance to evaluate the performance of range-gated
technology. Computed and measured images compare favorably. It is concluded that the use of spatial and temporal
broadening models are convenient and viable techniques for analytic computation of underwater images.
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.
KEYWORDS: Imaging systems, Laser imaging, Pulsed laser operation, Laser systems engineering, CCD cameras, Cameras, Control systems, Power supplies, High power lasers, Backscatter
The range-gated laser imaging technology can eliminate the backscattering noise and 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. The characteristics of range-gated
underwater laser imaging were analyzed in this paper, and the basic requirements for gated ICCD were presented. As a
result, Gated Gen II+ Image Intensifier and progressive scanning CCD were assembled together to meet the requirements,
which was called the Gate Intensified CCD camera. Combined with the small-sized programmable high voltage power
supply, video image acquisition/control system and high power lamp-Pumped blue-green Nd:YAG pulse laser, the
experimental system was developed. Experiments were carried out in pipes full of saturated salt water and large pools.
When tested by USAF 1951 resolution target, the detecting range can extend to about 30 meters.
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