# Deepwater 3D Measurements with a Novel Sensor System
The main reason for this is the short range and the comparatively small measuring volume compared to laser-based systems. Systems with structured lighting have been described in recent years by, for example, [PERSON] et al. [22] and [PERSON] et al. [23]. Structured light-supported systems have significant potential in terms of the high measurement accuracy of this reconstruction technique. The absolute accuracy of photogrammetric 3D measuring systems for applications in the air can be up to 1:10,000 (see [24]), i.e., for measured variables such as lengths of 1 m, the absolute error can be less than 0.1 mm. In underwater applications, the absolute measurement accuracy is limited due to unfavorable environmental conditions such as beam refraction, water turbidity, suspended particles in the water, etc., with up to 1:1000 but are still high compared to ToF and sonar systems. When using structured illumination, the measurement accuracy can be increased even further for objects with a limited natural surface structure compared to passive photogrammetry. However, for the illumination of distant objects with a large field of view, the illumination output of the sample projectors is typically too weak. Additionally, short scanning times, fast data processing, and the creation of 3D surface models in real-time are required. Hence, advanced camera technology and powerful computing equipment to process large amounts of data are necessary. It is expected that new hardware developments will be able to meet these requirements soon. The quality of a 3D measurement system depends heavily on its ability to compensate for the influence of the sensor or the measurement of object movements on captured 3D data. This is one of the most demanding tasks in 3D stereo scanning systems based on structured lighting because of the long exposure times required for a single scan. As a rule, ten subsequent images are required to realize a point of correspondence. The distance traveled by a moving sensor in the time required to capture ten images results in the displacement (or blurring) of the projected pattern observed in a particular pixel. Motion correction can reduce or correct the resulting image distortion. Various methods are available for movement compensation. A new method for pattern projection-based 3D scanner systems involving a projection unit in the 3D point calculation is presented by [PERSON] et al. [25]. [PERSON] et al. [26] describe the use of motion blur for motion compensation. Recent applications have targeted the use of low-cost components in 3D reconstruction in coastal shallow waters using video streams from an action sports camera [27]. Structure-from-motion technology (SfM) was used to create an overall 3D model from the measurements [28]. With our novel underwater 3D sensor system, we attempted to obtain the precise, continuous 3D capturing and modeling of underwater structures at medium-range measurement distances and speeds of up to 1 m/s while processing it into a complete 3D model of the observed scene with a high level of detail. The novelty of the presented system compared to commercially available underwater 3D measurement systems is the combination of the SLAM technology based on visual odometry (vSLAM) with highly precise, detailed 3D measurements of objects and the generation of dense, high-resolution 3D models, including color capture. This work describes a novel 3D sensor system, which is suitable for underwater inspection tasks at sea depths of up to 1000 m by connecting it to a work-class Remotely Operating Vehicle (ROV). First, the hardware of the system is described, and the measurement properties are presented. Experimental measurement examples for practical applications are given, as well as measurements for the metrological characterization of the sensor system.