A scatter removal technique to enhance underwater range-gated 3D and intensity images

In this paper we present how to exploit a combination of an underwater range-gated imaging system and an unsharp filtering method to effectively suppress the effects of forward- and backward-scatter. Typically, back-scattered light causes a contrast reduction in the image, while forward-scattered light causes a blurring of features. A range-gated imaging system can effectively suppress the majority of back-scatter by gating close to objects of interest, however, such systems do not have an inherent way of suppressing forward-scatter. The proposed range-gated system facilitates real-time 3D because it facilitates range-gated acquisition at 1kHz with an inter-range spacing of 18.8cm. A peak finding algorithm working on the range-gated sweep signal is used to estimate 3D. However, the scattering effects also cause unwanted peaks in the range-gated signal. The effect of forward-scatter (spatial blur) for the peak finding algorithm is that the extent of objects may be larger in the reported depth map than the real physical extent of the object. This can be detrimental to e.g., biomass estimation of fish. In this paper we describe an unsharp mask filtering approach to remove the combined effect of forward and backward-scatter on each individual range-gated image before the 3D peak finding algorithm is applied. The results of the proposed filtering are twofold: 1) removing the residual back-scatter which leads to a contrast increase in the gated images, and 2) removal of false-positive peaks in the range-gated signal due to forward-scatter. This leads to more accurate depth maps.