摘要 :
Underwater laser-based imaging systems and data-processing techniques matured during the past decade. Active imaging systems can, nowadays, be integrated into platforms like remote-operated vehicles (ROV) or autonomous underwater ...
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Underwater laser-based imaging systems and data-processing techniques matured during the past decade. Active imaging systems can, nowadays, be integrated into platforms like remote-operated vehicles (ROV) or autonomous underwater vehicles (AUV). This article gives an overview of different civil and naval applications in underwater imaging with respect to underwater laser scanning (ULS) and laser gated viewing (LGV). Special emphasis has to be given to the environmental conditions, for example, the influence of the local and seasonal dependence of the turbidity with regard to the optical underwater channel. On the basis of tank and sea experiments, advanced techniques for 3D laser oblique scanning (LOS) and possibilities of contrast enhancements for gated viewing are presented.
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In images taken underwater, it is generally difficult to correctly extract distances and geometric information of objects. Different techniques, collectively referred to as photogrammetry, exist to measure features in images. One ...
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In images taken underwater, it is generally difficult to correctly extract distances and geometric information of objects. Different techniques, collectively referred to as photogrammetry, exist to measure features in images. One of these is to project a reference pattern onto an object in a scene viewed by a camera, and register the distortion of this pattern, to calculate the shape of, and distance to, that object. This method is implemented here on a miniaturized submersible explorer equipped with, among many other instruments, a camera. Diffractive optical elements (DOEs) have been designed and manufactured using microsystems technology, to, together with a laser diode, camera, and in-house developed software, provide a compact system for projecting reference patterns and analyzing their deformations. The system has been characterized by measuring the distances and angles of objects in a water tank, and attempting to reproduce their shapes. The range of operation of the system, verified to be at least one meter, is limited by the compact mounting in the small submersible and the cameras' performance. The system was found to work well under turbid conditions as well as in water containing larger particles. Together with a vehicle-mounted camera, the compact and low-power DOE laser projection system enables topographical measurement.
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Underwater target three-dimensional detection is crucial for effectively recognizing and
acquiring target information in complex water rings. The underwater robotic operating system
as a conventional underwater operating platfor...
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Underwater target three-dimensional detection is crucial for effectively recognizing and
acquiring target information in complex water rings. The underwater robotic operating system
as a conventional underwater operating platform, generally equipped with a binocular
or monocular camera, how to utilize the underwater monocular camera with high precision
and high efficiency to complete the target three-dimensional information acquisition is the
main research starting point of this paper. To this end, this paper proposes a laser-assisted
three-dimensional depth monocular detection method for underwater targets, which utilizes
three cross lasers to assist the monocular camera system in capturing the depth data at different
positions of the target plane at one time. The image correction by the four-point laser
calibration method in this paper solves the difficulties of image distortion caused by an
unstable underwater environment and lens effect, as well as laser angle deviation caused by
the tilting of the underwater robot. The instability of the underwater environment and the
lens can cause image distortion, and the tilt of the underwater robot causes the laser angle
to deviate. After correcting the image, the depth data between the target and the robot can
be calculated based on the geometric relationship that exists between the imaginary rectangle
formed by the laser dots and laser lines in the image and the imaginary rectangle
formed between the lasers on the device. This method uses a single image to obtain target
depth information and is capable of measuring not only horizontal planes but also multiplanes
and inclined planes. Experiments show that the algorithm improves the performance
accuracy in underwater environments and land environments compared to traditional methods,
and obtains depth information for the entire plane at once. The method provides a
theoretical and practical basis for underwater monocular 3D information acquisition.
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Recent advancements in obtaining visibility of undersea objects at extended ranges in coastal and oceanic waters are reviewed for the years 2009 to present. The paper focuses on the latest techniques that are utilized to reduce th...
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Recent advancements in obtaining visibility of undersea objects at extended ranges in coastal and oceanic waters are reviewed for the years 2009 to present. The paper focuses on the latest techniques that are utilized to reduce the undesirable effects of scattering, mainly due to suspended particulate within the imaging volume, leading to the loss of contrast and blurring characteristic of undersea optical images produced over long ranges. Several recent sets of experimental results obtained using both benchtop laboratory development systems as well as field-deployable prototypes of new system concepts are presented, with observed performance attributes being discussed. Simulation studies that make use of accurate radiative transfer physical models to enable design and operation of new system concepts within a turbid water environment are also presented. Finally, this paper includes a description and results from an extended-range laser system that has reached a level of packaging and automation necessary to be available as a commercial product.
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Some current operations with remotely operated vehicle (ROVs) require the vehicle to fix itself onto an underwater work site and carry out robotic tasks. For this to occur, a real-time computer model of the work site and its relat...
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Some current operations with remotely operated vehicle (ROVs) require the vehicle to fix itself onto an underwater work site and carry out robotic tasks. For this to occur, a real-time computer model of the work site and its relationship to the vehicle must be determined, especially if the work site has not been designed for such interventions and no fixed docking structure is available. We described a laser-based system that can be used from an ROV to scan the underwater work site and translate the information into a CAD or virtual reality (VR) model. A single stripe is projected onto the work site and an image acquired by the ROV. An image-processing routine is used to optimize object range and shape information obtainable from this stripe and, when several such stripe positions have been determined, the information is combined. This paper concentrates on the image-processing techniques needed to isolate information from underwater images and the results obtained in trials of the system in a laboratory tank. The methodology is able to accommodate differing turbidities and the limitations of the system in terms of accuracy are discussed.
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We developed for underwater 3D imaging a scanning laser sensor that has a dome lens and coaxial optics to realize a wide-scanning-angle. Our sensor also has a feature in the sensitivity time control (STC) circuit that detects smal...
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We developed for underwater 3D imaging a scanning laser sensor that has a dome lens and coaxial optics to realize a wide-scanning-angle. Our sensor also has a feature in the sensitivity time control (STC) circuit that detects small signals by suppressing the unwanted signals backscattered by marine snows. We demonstrated the system performance in a pool and confirmed a 3D imaging a maximum distance of 20 m. We mounted our system on an autonomous underwater vehicle (AUV) and demonstrated seafloor mapping at the depth of 100 m in the ocean.
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Background and Aim Underwater endoscopic mucosal resection (UEMR) has become widespread for treating colorectal polyps. However, which observational mode is best suited for determining polyp margins underwater remains unclear. To ...
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Background and Aim Underwater endoscopic mucosal resection (UEMR) has become widespread for treating colorectal polyps. However, which observational mode is best suited for determining polyp margins underwater remains unclear. To determine the best mode, we analyzed three imaging modes: white light imaging (WLI), blue laser imaging (BLI) and linked color imaging (LCI). Methods Images of consecutive colorectal polyps previously examined by these three modes before UEMR were analyzed according to the degree of underwater turbidity (transparent or cloudy). Color differences between the polyps and their surroundings were calculated using the Commission Internationale d'Eclairage Lab color space in which 3-D color parameters were expressed. Eight evaluators, who were blinded to the histology, scored the visibility from one (undetectable) to four (easily detectable) in both underwater conditions. The color differences and visibility scores were compared. Results Seventy-three polyps were evaluated. Sixty-one polyps (44 adenomatous, 17 serrated) were observed under transparent conditions, and 12 polyps (seven adenomatous, five serrated) were observed under cloudy conditions. Under transparent conditions, color differences for the BLI (8.5) and LCI (7.9) were significantly higher than that of WLI (5.7; P < 0.001). Visibility scores for BLI (3.6) and LCI (3.4) were also higher than that of WLI (3.1; P < 0.0001). Under cloudy conditions, visibility scores for LCI (2.9) and WLI (2.7) were significantly higher than that of BLI (2.2; P P = 0.04, respectively). Conclusions BLI and LCI were better observational modes in transparent water; however, BLI was unsuitable for cloudy conditions.
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The imaging of underwater objects illuminated by artificial light has been of long-standing interest to investigators working in oceano-graphic environments. Pulsed lasers together with range-gated technology have been widely used...
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The imaging of underwater objects illuminated by artificial light has been of long-standing interest to investigators working in oceano-graphic environments. Pulsed lasers together with range-gated technology have been widely used for underwater optical imaging applications. In order to describe the formation of underwater range-gated images, a pulsed laser underwater imaging model based on pulse spatial and temporal broadening is proposed. Experiments based on a self-assembled laser range-gated imaging system were implemented in our laboratory. Results show good agreements between experiments and simulations. Both results also confirm higher image contrast toward the tail region of the target-reflected light. Furthermore, experiments on underwater image blur and restoration are also implemented and show good image recovery results. The modulation transfer function-based restoration mechanism also implies a way to eliminate the blur effect caused by light forward scattering.
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On laser shock peening and underwater laser micromachining, previous studies have shown that the immersion depth of the sample significantly affects laser ablation of submerged targets; however, the underlying mechanisms are still...
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On laser shock peening and underwater laser micromachining, previous studies have shown that the immersion depth of the sample significantly affects laser ablation of submerged targets; however, the underlying mechanisms are still obscure. In this work, we observe and study the formation, growth, and collapse of cavitation bubbles generated during nanosecond laser ablation of submerged titanium targets that are exposed at various immersion depths utilizing a stroboscopic shadowgraphy system. Our results show that the initial laser absorption and cavitation bubble formation after the laser incidence are not affected by the immersion depth. Nevertheless, when the immersion depth is less than the maximum radius of the generated cavitation bubble, the bubble shrinks asymmetrically during the collapse stage. Thus, the cavitation bubble is not fully compressed at the maximum implosion, and phenomena related to the violent implosive collapse, e.g., the second etching effect and the emission of strong shockwaves, are absent. We also propose a strategy to estimate the maximum radius of the laser-induced cavitation bubble, which helps to determine the optimal liquid depth for related engineering applications. Our results provide succinct explanations for the effects of immersion depth on pulsed laser ablation of submerged targets, which are important steps toward a deeper understanding of laser-materials' interactions in liquid environments.
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