Fraser R. Dalgleish

A Focus on Recent Developments and Trends in Underwater Imaging

Underwater optical imaging advances from 2005 to the present are reviewed. Research and technical innovations are synopsized and organized much as the previous report (Kocak and Caimi, 2005) was. Examples of several recent novel system applications are given, as are brief summaries of emerging underwater imaging research and development trends.

Image enhancement for underwater pulsed laser line scan imaging system

Recent progress in system hardware such as laser, photon detectors and other electronic and optical components resulted in significant improvement for the underwater serial laser imaging system. Nevertheless, during normal system operation, system issues such as laser instability, electronic noise, and environmental conditions such as imaging in highly turbid water can still put constraint on the performance of imager. In this work, post-processing to take advantage of the improvement hardware to further reduce image noise and enhance the image quality as a critical aspect of the overall system design is studied. A novel realization of the bilateral principle based image/pulse noise reduction and image deconvolution using point spread function (PSF) predicted with EODES radiative transfer model is used to implement the processing chain. The concept is further extended to a multichannel deconvolution to exploit the benefit offered by the new multi element PMT configuration developed in HBOI. Two datasets were used to test the developed techniques respectively.

Improved LLS imaging performance in scattering-dominant waters

Experimental results from two alternate approaches to underwater imaging based around the well known Laser Line Scan (LLS) serial imaging technique are presented. Traditionally employing Continuous Wave (CW) laser excitation, LLS is known to improve achievable distance and image contrast in scattering-dominant waters by reducing both the backscatter and forward scatter levels reaching the optical receiver. This study involved designing and building prototype benchtop CW-LLS and pulsed-gated LLS imagers to perform a series of experiments in the Harbor Branch Oceanographic Institute (HBOI) full-scale laser imaging tank, under controlled scattering conditions using known particle suspensions. Employing fixed laser-receiver separation (24.3cm) in a bi-static optical geometry, the CW-LLS was capable of producing crisp, high contrast images at beyond 4 beam attenuation lengths at 7 meters stand-off distance. Beyond this stand-off distance or at greater turbidity, the imaging performance began to be limited mainly by multiple backscatter and shot noise generated in the receiver, eventually reaching a complete contrast limit at around 6 beam attenuation lengths. Using identical optical geometry as the CW-LLS, a pulsed-gated laser line scan (PG-LLS) system was configured and tested, demonstrating a significant reduction in the backscatter reaching the receiver. When compared with the CW-LLS at 7 meters stand-off distance, the PG-LLS did not become limited due to multiple backscatter, instead reaching a limit (believed to be primarily due to forward-scattered light overcoming the attenuated direct target signal) beyond 7 beam attenuation lengths. This result demonstrates the potential for a greater operational limit as compared to previous CW-LLS configuration.

Underwater imaging and optics: Recent advances

Obtaining satisfactory visibility of undersea objects has been historically difficult due to the absorptive and scattering properties of seawater. Mitigating these effects has been a long term research focus, but recent advancements in hardware, software, and algorithmic methods have led to noticeable improvement in system operational range. This paper is intended to provide a summary of recently reported research in the area of Underwater Optics and Vision and briefly covers advances in the following areas: 1) Image formation and image processing methods; 2) Extended range imaging techniques; 3) Imaging using spatial coherency (e.g. holography); and 4) Multipledimensional image acquisition and image processing.

Extended-Range Undersea Laser Imaging: Current Research Status and a Glimpse at Future Technologies

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.

Visualization and Image Enhancement for Multistatic Underwater Laser Line Scan System Using Image-Based Rendering

Over the last several decades, developments in underwater laser line scan (LLS) serial imaging sensors have resulted in significant improvements in turbid water imaging performance. In the last few years, there has been renewed interest in distributed, truly multistatic time-varying intensity (TVI) (i.e., multiple transmitter nonsynchronous LLS) sensor configurations. In addition to being capable of high-quality image acquisition through tens of beam attenuation lengths, while simultaneously establishing a non-line-of-sight free-space communications link, these system architectures also have the potential to provide a more synoptic image coverage of larger regions of seabed and the flexibility to simultaneously examine a target from different perspectives. A related issue worth investigation is how to utilize these capabilities to improve rendering of the underwater scenes. In this regard, light field rendering (LFR)-a type of image-based rendering (IBR) technique-offers several advantages. Compared to other IBR techniques, LFR can provide signal-to-noise ratio (SNR) improvements and the ability to image through obscuring objects in front of the target. On the other hand, multistatic nonsynchronous LLS can be readily configured to acquire image sequences needed to generate LFR. This paper investigates the application of LFR to images taken from a distributed bistatic nonsynchronous LLS imager using both line-ofsight and non-line-of-sight imaging geometries to create multiperspective rendering of an unknown underwater scene. The issues related to effectively applying this technique to underwater LLS imagery are analyzed and an image postprocessing flow to address these issues is proposed. The results from a series of experiments at the Harbor Branch Oceanographic Institute at the Florida Atlantic University (HBOI-FAU, Fort Pierce, FL, USA) optical imaging test tank demonstrated the capability of using bistatic/multistatic nonsynchronous LLS system to generated LFR and, therefore, verify the proposed image processing flow. The benefits of LFR to underwater imaging in challenging environments were further demonstrated via imaging against a variety of obstacles such as mesh screens, bubbles, and water at different turbidity. Image quality metrics based on mutual information and texture features were used in the analysis of the experimental results.

Efficient laser pulse dispersion codes for turbid undersea imaging and communications applications

The objective of this work was to develop and validate approaches to accurately and efficiently model channel characteristics in a range of environmental and operational conditions for underwater laser communications systems that use high frequency amplitude modulation (AM) or coded pulse trains. Two approaches were investigated: 1) a Monte Carlo model to calculate impulse responses for a particular system hardware design over a large range of environmental and operational conditions, and 2) a semi-analytic model which has the potential to be more computationally efficient than the Monte Carlo model. The formulation of the Monte Carlo code is presented in this paper, together with test results used to evaluate the range of accuracy of the model against 500ps laser-pulse propagation measurements from well-controlled and characterized particle suspensions in a 12.5m test tank. A multiple scattering study using the Monte Carlo simulation code was also performed and some results are presented. Results from the semi-analytic model will be compared with these test tank measurements and the Monte Carlo model in a later paper.

Quantification of optical turbulence in the ocean and its effects on beam propagation.

The influence of optically active turbulence on the propagation of laser beams is investigated in clear ocean water over a path length of 8.75 m. The measurement apparatus is described and the effects of optical turbulence on the laser beam are presented. The index of refraction structure constant is extracted from the beam deflection and the results are compared to independently made measures of the turbulence strength (Cn2) by a vertical microstructure profiler. Here we present values of Cn2 taken from aboard the R/V Walton Smith during the Bahamas optical turbulence exercise (BOTEX) in the Tongue of the Ocean between June 30 and July 12, 2011, spanning a range from 10-14 to 10-10  m-2/3. To the best of our knowledge, this is the first time such measurements are reported for the ocean.

An AUV-deployable pulsed laser line scan (PLLS) imaging sensor

Laser line scan (LLS) underwater imaging is a serial imaging technique that involves the optical scanning of a narrow instantaneous field of view (IFOV) receiver in a synchronous fashion with a highly collimated laser source over a wide swath. It is widely regarded as the optimal technology for extended range underwater optical imaging, with up to 6 attenuation lengths achievable in turbid seawater. These imagers, which utilize high power green CW lasers, require an adequate source- receiver separation to reduce near field backscattering image degradation. They have been successfully deployed onboard towed bodies, and have potential as AUV-deployed imagers except their large size and high sensitivity to changes in operating conditions and environment makes them difficult to use successfully with these platforms. Harnessing recent developments in high power, high repetition rate green pulsed lasers, high speed gate-able photomultiplier tubes, rapid parallel digital signal processors and compact underwater scanning systems, this paper describes the latest approach to demonstrating a prototype pulsed laser line scan (PLLS) imager which has the required compactness and ease of operability to make it more compatible for implementation onboard the common classes of AUV.

Extended Range Underwater Optical Imaging Architecture

Synchronous scan imagers have demonstrated improved ability to see underwater allowing operation at up to 6 optical attenuation lengths. Recent developments in laser and scanning systems show promise for further miniaturization of these systems to allow operation from small AUVs. This paper describes an approach that exhibits several advantages compared to larger systems having a similar receive aperture and Field of View (FOV)