Range-Gated Imaging System for Underwater Monitoring in Ocean Environment

Abstract

High-quality video observations are very much needed in underwater environments for the monitoring of several ecosystem indicators and to support the sustainable development and management of almost all activities in the ocean. Reliable video observations are however challenging to collect, because of the generally poor visibility conditions and the difficulties to deploy cost-effective sensors and platforms in the marine environment. Visibility in water is regulated by natural light availability at different depths, and by the presence of suspended particles, scattering incident light in all directions. Those elements are also largely variable in time and space, making it difficult to identify technological solutions that can be used in all conditions. By combining state-of-the-art "time of flight" (ToF) image sensors and innovative pulsed laser illumination, we have developed a range-gated camera system (UTOFIA) that enables affordable and enhanced 3D underwater imaging at high resolution. This range-gated solution allows users to eliminate close-range backscattering, improving quality of the images and providing information on the distance of each illuminated object, hence giving access to real-time 3D measurements. Furthermore, as the system is based on pulsed laser light, it is almost independent of natural light conditions and can achieve similar performances at an extended depth range. We use this system to collect observations in different oceanographic conditions and for different applications, including aquaculture monitoring, seafloor mapping, litter identifications and structure inspection. Performances are evaluated by comparing images to regular cameras and by using standard targets to assess accuracy and precision of distance measurements. We suggest that this type of technology can become a standard in underwater 3D imaging to support the future development of the ocean economy.Sustainability 2019, 11, 162 2 of 13 create a turbid environment that strongly increases light scattering and enhances the absorption probability of photons [3]. When the light source is the sun, this process effectively decreases the amount of ambient light present at any depth and limits the range of visual observations. With artificial illumination, the range for underwater vision can be extended (for example we can move deeper) but at the cost of degrading image contrast due to (forward-and back-) scattering generated by suspended particles. The situation is similar to driving a car in foggy conditions with the headlights on: increasing the power of illumination does not improve the visibility, as the backscattering increases proportionally. Image contrast is also lowered with shorter visual range as the light attenuation will reduce the illumination of distant targets. These factors remain the outstanding challenges in underwater imaging and limit the application of visual observations in many sectors [1,2,4].Various optical and acoustic imaging systems for mitigating or solving these problems are under constant develo...

State of the Art of Underwater Active Optical 3D Scanners

“…One-shot illumination consists of illuminating and acquiring the whole scene at once. Risholm et al [76,134] and Chua et al [75] used a doe to flash diffuse light in every direction with no spatial structure. Other authors employ spatially-structured light: Massot-Campos and Oliver-Codina [60] used a special doe to project 25 lines onto the surface of the scanned object, whereas Sarafraz and Haus [135] and Risholm et al [112,136] used commercial projectors to illuminate the scene with specifically-designed, more complex patterns.…”

“…The devices presented by Risholm et al [76], Mariani et al [134] and Chua et al [75] are range-gated ToF sensors. This means that their light receiver opens with a delay after the light has been sent out.…”

“…This is done to make them more robust against backscatter (see Figure 1 and Figure 3). Risholm et al [76], Mariani et al [134] developed a peak-finding algorithm that can deal with forward scatter and at the same time can perceive distant peaks that are barely higher than peaks caused by noise. This way, they can increase the theoretical depth resolution of 18.8 cm by a factor of 20.…”

“…Scanners that are able to provide more frames per second are more suitable for scenes with faster dynamics. The reported refresh rates are 0.1 Hz (Risholm et al [136]), 0.1 Hz to 6 Hz, depending on lateral resolution (Palomer et al [48]), 0.2 Hz (Imaki et al [82]), 1 Hz (Digumarti et al [97] with Intel RealSense) and 10 Hz (Risholm et al [76,134] and Anwer et al [147] with Kinect v2). Typical underwater manipulation tasks require a refresh rate higher than a few hertz.…”

Research on the Visualization of Ocean Big Data Based on the Cite-Space Software

“…The work in [19] notes that monitoring the characteristics of the ice in the Bohai Sea can be used for ice disaster prevention in marine transportation prevention, oil field operations, and regional climate change. The study in [20] developed a range gate camera system (UTOFIA) that enables enhanced high-resolution 3D imaging underwater. The use of this solution can eliminate close backscattering, improve image quality, and provide each illumination distance Information object, as well as access real-time 3D measurement data.…”