# Underwater 2D Image Acquisition Using Sequential Striping Illumination
## 1 Introduction  
As one of the essential 3D measurement technologies, structured lighting is increasingly being applied in underwater imaging; those frequently required include, seabed mapping, pipeline or dock detection, target localization, and ROV (remotely operated vehicles)/AUV (autonomous underwater vehicles) navigation. A variety of structured lighting techniques have been developed for these applications, such as laser line scan systems [1], 3D laser line scan mapping systems [2], laser line sensors [3], and seabed-relative navigation by structured lighting techniques [4]. In general, structured lighting techniques modulate the appearance of a surface by projecting a particular pattern of light onto it [5]; subsequent correspondence of the image pattern and surface points, provided with calibration of the image sensor and projector, enables accurate computation of the range of surface points. Structured lighting techniques can be classified as multiple-shot or single-shot categories. The multiple-shot technique works with a light stripe scanning mode where the striping illumination is sequentially swept across an object (Figure 1), by which the 3D computation is completed frame by frame. The single-shot technique projects a complex coded illumination pattern onto the objects, and the 3D computation is completed from a single image by decoding the reflected surface light pattern. However, optical imaging in scattering media suffers from scattering problems [6]. The back scattered light, which is scattered back from the medium before projecting onto the object surface, veils the scene and degrades the image contrast. The forward scattered light, which comes from the illuminated object surface but deviates during the transmission to the camera, blurs the details of the scene and degrades the sharpness of the image. Thus, an optical image taken underwater usually loses meaningful image features of scenes. In practice, most single-shot techniques are inapplicable in turbid water medium,because the coded lighting pattern is hard to detect, with very low contrast caused by the back scattered light. Thus, in many applications, structured lighting techniques using sequential striping illumination is a reliable implementation of underwater 3D measurement, in terms of time and related cost [7]. Acquiring a 2D image while 3D measurement is implemented using the structured lighting illumination underwater, is highly desirable. Although additional flood lighting devices can induct extra deployment of hardware, the desire is mainly due to the fact that the sequential striping illumination modality is able to remove the back scattered light. For the removal of the back scattered light, a well-known approach is the physics-based gating technique, a typical example of which is the synchronous laser line scanning system (SLL) [8; 9]. The system works with the laser line (or stripe) illumination and the aperture-coupled opto-electric sensor, both of which scan synchronously. As the overlapped volume of the illuminated field of the laser and the view field of the aperture is very limited, the amount of the back scattered light received by the sensor is also very limited; most of the back scattered light generated outside the overlapped volume is spatially separated from the reflected light of the object. The image pixels are specified by stitching the sequentially scanned outputs of the opto-electric sensor. The SLL system has been deeply investigated, but only in terms of the cost of hardware complexes. Alternatively, a compact device consisting of a CCD camera and a structured light source can facilitate underwater image acquisition, albeit incorporated with 3D computation. In [10], a comprehensive analysis of the light stripe range scanning method is given. Based on the assumption of single scattering, an analytic image formation model is derived to describe the interactions of light with the medium and the scene.