# A New Sensor System for Accurate 3D Surface Measurements and Modeling of Underwater Objects
## 1 Introduction  
Three-dimensional acquisition of objects underwater is gaining importance in the field of inspection. Industrial facilities, such as offshore wind turbine foundations, oil and gas pipeline systems, underwater structures, and other objects, including anchor chains, are regularly required to be measured underwater [1, 2]. Other objects of interest for underwater 3D measurements include archaeological sites on the seabed [3, 4, 5, 6, 7], sunken shipwecks [8], plant and coral growth [9], and the biomass or size of fish populations [10, 11, 12]. Several contactless methods have been applied to perform 3D reconstruction underwater. Examples includes techniques using sonar systems, laser scanning, time-of-flight (ToF) cameras, and photogrammetry. Whereas sonar systems [13, 14, 15, 16] provide fast measurements of large areas over long distances, their measurement accuracy is low compared to that of optical sensors. [PERSON] et al. introduced a new ToF based system called UTOFIA [17], providing an accuracy of approximately 10 cm at a 14 m distance. A commercially available ToF system is provided by 3D at Depth [18]. Laser scanning systems [1, 2, 19, 20] provide fast acquisition, long ranges, high measurement accuracy, and robustness against water turbidity. Additionally, measurements can beperformed in motion. Because line laser scanning systems typically capture only a single or a few 2D profiles at a time, 3D registration and merging of individual consecutive 3D scans is not feasible. Therefore, for 3D mapping purposes, the trajectory is often estimated using visual odometry [21] or vehicle navigation and motion data [22]. However, the resulting measurement accuracy is not comparable to the potential accuracy of photogrammetric measurements. Commercially available laser scanning systems for underwater use are produced by, e.g., CathXOCean [23] and Voyis [24]. However, high-accuracy photogrammetry requires either preparation effort comprising placement of markers, textured objects, or support of structured illumination. Photogrammetric systems have been introduced by [PERSON] [25], [PERSON] and [PERSON] [26], [PERSON] and [PERSON] [27], and others [3; 28; 29; 3]. [PERSON] [30] provided a photogrammetric solution for reconstruction of large underwater structures with low accuracy using video streams of a stereo rig. An interesting new approach to underwater 3D measurements was provided by [PERSON] [31] by using a plenoptic camera. Structured illumination has been applied for precise underwater measurements in several research projects. It has been shown to be suitable for close distances and small objects. Examples of such systems have been presented by [PERSON] et al. [32], [PERSON] et al. [33], and [PERSON] et al. [34]. These systems have shown potential regarding high measurement accuracy of this reconstruction technique. However, illumination power of the pattern projectors in these systems are typically too low to satisfy the needs of challenging industrial inspection tasks. Namely, they should have a large field of view, be able to measure along large distances, be able to accommodate the very small amount of time available for scanning and data processing, and, finally, be able to generate 3D surface models of the observed object in real time. In addition to an accurate camera technique, this requires a high illumination power for structured light patterns, a cooling concept for the dissipation of produced heat, and high-power computational equipment for processing the large quantity of data. Modern developments in hardware components, which are necessary to build structured light-based optical 3D scanners, offer hope of overcoming the current restrictions regarding insufficient measurement volume size and measurement distance of such devices for underwater use. One of the most challenging tasks regarding the use of 3D stereo scanning systems based on structured illumination is the capture of fast-moving objects or the motion of the sensor itself.