Speckle noise suppression of range gated underwater imaging system
2009
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.
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