Underwater 3D Imaging Research Articles

A Study on Refraction Error Compensation Method for Underwater Spinning Laser Scanning Three-Dimensional Imaging.

Laser scanning 3D imaging technology, because it can obtain accurate three-dimensional surface data, has been widely used in the search for wrecks and rescue operations, underwater resource development, and other fields. At present, the conventional underwater spinning laser scanning imaging system maintains a relatively fixed light window. However, in low-light situations underwater, the rotation of the scanning device causes some degree of water fluctuation, which warps the light strip data that the system sensor receives about the object's surface. To solve this problem, this research studies an underwater 3D scanning and imaging system that makes use of a fixed light window and a spinning laser (FWLS). A refraction error compensation algorithm is investigated that is based on the fundamentals of linear laser scanning imaging, and a dynamic refraction mathematical model is established based on the motion of the imaging device. The results of the experiment on error analysis in an optimal underwater environment indicate that the error in reconstructing the radius is decreased by 60% (from 2.5 mm to around 1 mm) when compensating for the measurement data of a standard sphere with a radius of 20 mm. Moreover, the compensated point cloud data exhibit a higher degree of correspondence with the model of the standard spherical point cloud. Furthermore, we examine the impact of physical noise, measurement distance, and partial occlusion of the object on the imaging system inside an authentic underwater setting. This study is a good starting point for looking at the refractive error of an underwater laser scanning imaging system. It also provides to us some ideas for future research on the refractive error of other scanning imaging methods.

The influence of active projection speckle patterns on underwater binocular stereo vision 3D imaging

Due to the phenomena of refraction, absorption and scattering, underwater stereo vision faces more difficulties than in air. These phenomena will bring various problems such as blurred details, reduced contrast and brightness to underwater images, which will affect the accuracy and reliability of stereo matching and increase the difficulty of stereo matching. The method based on active speckle pattern is a more simple and reliable method to enhance the surface texture of underwater objects. A good-quality speckle pattern can provide more robust and accurate results for matching the corresponding points of the left and right images. This is a more important research problem in water than in air. In this paper, the row entropy of speckle pattern is adopted to quantify the quality of the projection speckle pattern suitable for underwater stereo vision, and the relevant experiment and analysis are carried out to prove the correctness and effectiveness of new parameters. Further, the influence of different projection speckle patterns on underwater 3D imaging is researched. The actual underwater experiment indicates that the speckle pattern exerts a great influence on the accuracy of underwater 3D imaging based on the binocular stereo vision, and the row entropy of higher-quality projection speckle pattern should be as small as possible. This work can give a reference for the selection of projection speckle patterns in underwater binocular stereo vision. An appropriate speckle pattern can effectively improve the accuracy of underwater 3D measurement.

High-Resolution Underwater Single-Photon Imaging With Bessel Beam Illumination

The ability to achieve precise underwater 3D imaging is an urgent need in marine research and development. Yet, due to the strong scattering by the water, the laser beam is easily spread out in the underwater propagation, which hampers the imaging quality with low resolution. In this paper, we demonstrate a single-photon LiDAR with Bessel beam illumination, which central spot shows a stable diameter of about 0.73 mm after propagating 18 m in water. Together with a single-photon detector to sense the weak returning laser echoes, the transverse resolution reaches about a few millimeters at 17.5 m in water. In the experiment, an underwater optical image of 103.1 mm × 192.1 mm with 516 × 97 pixels is reconstructed. The result shows that our single-photon LiDAR with Bessel beam illumination greatly improves the imaging resolution in the long distance underwater sensing.

High precision underwater 3D imaging of non-cooperative target with frequency comb

High-precision underwater imaging technology has an important role in underwater navigation and identification, underwater pipeline inspection, sinker docking, and underwater vibration measurement. Due to the different mechanisms of underwater measurement sources, such as 3D gated imaging and streak tube imaging, the accuracy of underwater target imaging can reach the millimeter level. In this paper, we propose a high-precision underwater imaging method based on frequency combs. When measuring the non-cooperative target underwater, the spectrogram is modulated due to the strong dispersion property of water. After that, for the gained spectrogram, we use two algorithms, i.e., center offset frequency and phase slope, to obtain the distance and imaging, respectively. By comparing with reference values, the underwater experimental results combining two algorithms show an agreement within 8 μm in the range of 2.5 m, corresponding to 3.2 × 10−6 relative precision. This work can provide a strong guarantee and support for high-precision underwater applications.

3D surface measurement of static and dynamic objects in underwater environments based on the propagation and projection of CW sinusoidal signal at 532 nm

This work carries out both theoretical and experimental studies on three dimensional surface measurement of objects in underwater environments based on the use of the projection of continuous-wave laser-beam-based monochromatic structured light. The mathematical analysis and description of the generation and propagation of sinusoidal fringe patterns in air and underwater environments are presented, which provides the physical basis for the proposed method of measuring the underwater objects. In the experimental investigation we completed the in-door measurements of both static and dynamic objects located in a tank filled with real turbid seawater or lake water. Experimental results have indicated that the proposed approach is capable of yielding high-accuracy reconstructions for underwater 3D imaging of both static and dynamic objects, which shows the possibility of building a portable instrument based on the optical design and measuring technique presented in this work for carrying out high-accuracy in-situ underwater measurement.

Correction of range walk error for underwater photon-counting imaging.

Due to the characteristics of photon-counting LIDAR, there exists range walk error (RWE) when the intensity of the signal fluctuates. In this paper, an effective method to rectify underwater RWE was proposed. The method allows the separation of signal detections from noise detections, and based on a prior model, the method can compensate for RWE. An underwater experiment verified its feasibility and results showed RWE of three parts in a plane was reduced from 75mm to 7mm, from 45mm to 3mm and from 5mm to 0mm, respectively, even when the rate of backscatter photons reached 4.8MHz. The proposed correction method is suitable for high precision underwater photon-counting 3D imaging application, especially when the signal intensity varies sharply.

Axially shifted pattern illumination for macroscale turbidity suppression and virtual volumetric confocal imaging without axial scanning

Structured illumination has been widely used for optical sectioning and 3D surface recovery. In a typical implementation, multiple images under non-uniform pattern illumination are used to recover a single object section. Axial scanning of the sample or the objective lens is needed for acquiring the 3D volumetric data. Here we demonstrate the use of axially shifted pattern illumination for virtual volumetric confocal imaging without axial scanning. In the reported approach, we project illumination patterns at a tilted angle with respect to the detection optics. As such, the illumination patterns shift laterally at different z sections, and the 3D sample information can be recovered based on the captured 2D images. We demonstrate the reported approach for virtual confocal imaging through a diffusing layer and underwater 3D imaging through diluted milk. We show that we can acquire the entire confocal volume in ∼1 s with a throughput of 420 megapixels per second. Our approach may provide new insights for developing confocal light ranging and detection systems in degraded visual environments.

Range-Gated Imaging System for Underwater Monitoring in Ocean 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. 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.

Underwater 3D Imaging Using a Fiber-Based Endoscopic System for Arthroscopic Surgery

Arthroscopic surgery is a minimally invasive surgical procedure that is widely used on joints. However, conventional endoscope-based arthroscopic surgery does not provide stereoscopic information to the surgeon. To overcome this limitation, we have developed a modified endoscopic system that uses an optical fiber (1 mm diameter) for three-dimensional imaging of knee joints. Our endoscopic system is able to operate underwater in real time. It consists of a laser beam that is projected on the surface of the object to be imaged via an optical fiber. A prism is used to decrease the length and diameter of baseline and endoscope tube, respectively. The small diameter (8 mm) of our endoscope makes it extremely convenient for application in arthroscopic surgery. The feasibility of the proposed system has been demonstrated via experiments on synthetic knee joints.

PILOT APPLICATION OF 3D UNDERWATER IMAGING TECHNIQUES FOR MAPPING POSIDONIA OCEANICA (L.) DELILE MEADOWS

Abstract. Seagrass communities are considered one of the most productive and complex marine ecosystems. Seagrasses belong to a small group of 66 species that can form extensive meadows in all coastal areas of our planet. Posidonia oceanica beds are the most characteristic ecosystem of the Mediterranean Sea, and should be constantly monitored, preserved and maintained, as specified by EU Habitats Directive for priority habitats. Underwater 3D imaging by means of still or video cameras can allow a detailed analysis of the temporal evolution of these meadows, but also of the seafloor morphology and integrity. Video-photographic devices and open source software for acquiring and managing 3D optical data rapidly became more and more effective and economically viable, making underwater 3D mapping an easier task to carry out. 3D reconstruction of the underwater scene can be obtained with photogrammetric techniques that require just one or more digital cameras, also in stereo configuration. In this work we present the preliminary results of a pilot 3D mapping project applied to the P. oceanica meadow in the Marine Protected Area of Capo Rizzuto (KR, Calabria Region – Italy).

3D color reconstruction based on underwater RGB laser line scanning system

Laser line scanning (LLS) as an optical 3D survey method is recently used for underwater 3D imaging. This paper presents an approach for both 3D reconstruction and color restoration based on the LLS system. The system is set up with RGB laser scanning components for acquiring 3D data and color data at the same time. Direct calibration algorithms and RGB additive color model are involved for processing 3D color data. The experiment result shows that underwater targets can be feasibly 3D reconstructed with color texture simultaneously, at millimeter scales.

Experimental study of the influence of refraction on underwater three-dimensional reconstruction using the SVP camera model

In an underwater imaging system, a perspective camera is often placed outside a tank or in waterproof housing with a flat glass window. The refraction of light occurs when a light ray passes through the water-glass and air-glass interface, rendering the conventional multiple view geometry based on the single viewpoint (SVP) camera model invalid. While most recent underwater vision studies mainly focus on the challenging topic of calibrating such systems, no previous work has systematically studied the influence of refraction on underwater three-dimensional (3D) reconstruction. This paper demonstrates the possibility of using the SVP camera model in underwater 3D reconstruction through theoretical analysis of refractive distortion and simulations. Then, the performance of the SVP camera model in multiview underwater 3D reconstruction is quantitatively evaluated. The experimental results reveal a rather surprising and useful yet overlooked fact that the SVP camera model with radial distortion correction and focal length adjustment can compensate for refraction and achieve high accuracy in multiview underwater 3D reconstruction (within 0.7 mm for an object of dimension 200 mm) compared with the results of land-based systems. Such an observation justifies the use of the SVP camera model in underwater application for reconstructing reliable 3D scenes. Our results can be used to guide the selection of system parameters in the design of an underwater 3D imaging setup.

Experimentation of structured light and stereo vision for underwater 3D reconstruction

Current research on underwater 3D imaging methods is mainly addressing long range applications like seafloor mapping or surveys of archeological sites and shipwrecks. Recently, there is an increasing need for more accessible and precise close-range 3D acquisition technologies in some application fields like, for example, monitoring the growth of coral reefs or reconstructing underwater archaeological pieces that in most cases cannot be recovered from the seabed. This paper presents the first results of a research project that aims to investigate the possibility of using active optical techniques for the whole-field 3D reconstructions in an underwater environment. In this work we have tested an optical technique, frequently used for in air acquisition, based on the projection of structured lighting patterns acquired by a stereo vision system. We describe the experimental setup used for the underwater tests, which were conducted in a water tank with different turbidity conditions. The tests have evidenced that the quality of 3D reconstruction is acceptable even with high turbidity values, despite the heavy presence of scattering and absorption effects.

Polarimetry as a valid means to reduce optical noise in underwater 3D imaging by means of amplitude-modulated laser optical radar systems

We report the results of a series of underwater imaging experiments in the visible, carried out at ENEA (Frascati, Rome) by using a bistatic, amplitude-modulated laser optical radar system. In these experiments, polarimetry is used for minimizing the water backscattering signal and improving the accuracy of phase measurements directly related to distance. The presented technique enables one to obtain 3D images of underwater real scenes characterized by high quality, space resolution, and contrast. The results are of remarkable importance for applications in the 3D imaging of submerged objects, such as submarine archaeological sites.

Improving underwater imaging in an amplitude modulated laser system with radio frequency control technique

We present the results of an experiment aimed to demonstrate the low-pass filter dependence of water backscattered power in an amplitude-modulated laser scanner for underwater 3D imaging. We also demonstrate that improvements in target imaging are obtained by allowing the device to operate in the stop-band region. A simple model is described to account for the physics underlying the effect and suggesting future experimental schemes based on demodulated detection techniques.

Underwater Multi-View Three-Dimensional Imaging

In this paper, is proposed the use of multi-view three-dimensional (3D) imaging techniques, such as synthetic aperture integral imaging (SAII) for underwater applications. We analyze SAII systems for reconstructing 3D objects submerged in water. The effects of beam propagation in shallow water on the traditional SAII system are studied and computational reconstructions of 3D scenes are presented. We present experiments with SAII to reconstruct underwater 3D objects placed behind heavy occlusion. These systems could benefit deployments in unmanned underwater vehicles. To the best of our knowledge, this is the first report on underwater multi-view 3D imaging.

Experimental evidence of signal-optical noise interferencelike effect in underwater amplitude-modulated laser optical radar systems

We report experimental evidence that in an amplitude-modulated laser optical radar system for underwater 3D imaging the observed contrast oscillations as a function of the modulation frequency originate from an interference-like effect between target signal VT and water backscattered radiation VW. The demonstration relies on the ability to perform a direct measurement of VW in a 25 m long test tank. The proposed data processing method enables one to remove the contribution of water backscattering from the detected signal and drastically reduce signal fluctuations due to the medium. Experiments also confirm the possibility to improve the signal to optical noise ratio and contrast by increasing the modulation frequency.