Speckle noise suppression of range gated underwater imaging system

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.