# Underwater 2D Image Acquisition Using Sequential Striping Illumination
## 3 Removal of Back Scattered Light  
Eliminating back scattered light is a critical requirement in underwater imaging systems, for which a few techniques have been well developed [11; 12; 13]. The synchronous scanning system works with a collimated laser beam (or stripe) and an opto-electric sensor with a narrow field of view; both are synchronously scanned to limit back scattered light within the small intervening volume during image scanning. Alternatively, as described in [10], the back scattered light generated by stripe lighting can be computed, if the 3D reconstruction of the scene is obtained and the attenuation coefficient is evaluated. Thus, the scene appearance without scattered light can be evaluated for each lighted point of the object. Both approaches, however, require either complex system constructions or prior measurements of the optical properties of the medium.  
As illustrated in Figure 1, for stripe lighting in a scattering medium, the back scattered background appears as a light sheet from which brightness falls off exponentially along the projection direction, ending at the intersection of the surface. If the reflected light by the surface could yield to a discontinuity of brightness at the end of the light sheet, we may separate the flat brightness of the light sheet from the surface reflected light by detection of such discontinuity. However, the brightness of the back scattered light plane is usually mixed with surface light, so detection of such a discontinuity is not reliable. A straightforward approach is to employ a window mask on the image to cut off the back scattered light sheet from the surface light, which is the same principle as the synchronous scanning system. However, the field of view is limited by a virtual \"aperture\", rather than the physics-based setup of the opto-electric sensor in the synchronous scanning system [14].  
The size of the window mask is application dependent, which is needed to cover the reflected surface light and to remove back scattered light as much as possible. As the coverage of the projected  
Figure 3: (**a**) Scheme of sequential illumination process. (**b**) Sequential frame images of the real object during scanning illumination.  
striping illumination region is known, we can preset a threshold to first determine the starting point of the window at the empty side of the light stripe (far end of the back scattered light sheet), then determine the ending point (near end of the back scattered light sheet) according to the coverage of the striping illumination region. Considering that noise and texture of the surface albedo may induce uncertainties in the reflected surface light, we apply the smoothing filter before masking the support of the surface light by thresholding. Figure 4 shows an example of the determination of a window mask for a single frame image. Figure 5 shows results for synthesized images before and after the removal of back scattered light.  
Figure 4: _Cont._  
Although the frame image with removal of back scattered light is not the computed exact scene image, as described previously in the method of [10], the narrow field of view by the imposed virtual aperture can significantly eliminate back scattered light in the case of striping illumination. It should be noted that the masking operation inevitably involves loss of the scene on each frame image. However, such a loss of the scene can be recognized as a result of the change of illumination function _I(x)_ in the model of Equation (5). If we can properly estimate the changed illumination function through blind deconvolution, we can fully recover the albedo of the object.