![]() 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. 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. Those elements are also largely variable in time and space, making it difficult to identify technological solutions that can be used in all conditions. 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. 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. 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. By summarizing and analyzing the existing problems that restrict the underwater optical imaging techniques, the future development trends are prospected. In each technique, we present their applicable environments and conditions according to the following indicators: operating distance (from 2 attenuation lengths (AL) to 13.5 AL), resolution (from centimeter to millimeter), and field of view (FOV). Moreover, we also present a detailed comparison of these techniques. We present the individual working mechanisms, latest representative advances, and suitable application conditions. In this work, we review the six most common methods based on signal light enhancement. The recent developments in image enhancement algorithms and strategies of signal light enhancement have brought improvement in some application areas. Recently, the research community has focused on mitigating these effects. The challenges associated with acquiring the clear images of objects in underwater environment are difficult to overcome due to the absorptive and scattering nature of seawater. Two more features, important for optimal imaging operation of the system, are the ability of the laser to operate in low temporal jitter when triggered by the video-camera signals in order to cope with the correct range-gating temporization and a refined optical beaming that allows a versatile control of the scene illumination over a wider range of distances and angular span. Other issues have been addressed and successfully managed with this new laser implementation: very compact packaging, low power consumption, and efficient cooling under a wide range of operating environmental conditions. ![]() The energy, pulse duration, and repetition rate goals sought for this item have been achieved: >2mJ <2ns, up to 1kHz. The new laser presents a novel MOPA (master oscillator-power amplifiers) architecture, chosen in order to render the output energy scalable in the present and future upgrades. The laser is based on a completely new conceptual design necessary to meet the tougher laser specifications as emerged after the successful results obtained with a preliminary off-the-shelf source (SystemZero implementation) and the prospected requirements for enhancing UTOFIA imaging system competitiveness. A novel, frequency doubled laser system emitting to be used as scene illuminator is the main result of this presentation. UTOFIA offers a new modus operandi for the main targeted domains of application: marine life monitoring, harbor and ocean litter detection, fisheries stock assessment, aquaculture monitoring, and seabed mapping. This will fill the current gap between short-range, high-resolution conventional video and long-range low-resolution sonar systems. Using range-gated imaging, the system will extend the imaging range by factor 2 to 3 over conventional imaging systems, while at the same time providing video-rate 3D information. The goal of UTOFIA (EU H2020 project) is to offer a compact and cost-effective underwater imaging system for turbid environments.
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