Showing papers by "Weilin Hou published in 2012"

Optical turbulence on underwater image degradation in natural environments

Abstract: It is a well-known fact that the major degradation source on electro-optical imaging underwater is from scattering by particles of various origins and sizes. Recent research indicates that, under certain conditions, the apparent degradation could also be caused by the variations of index of refraction associated with temperature and salinity microstructures in the ocean and lakes. The combined impact has been modeled previously through the simple underwater imaging model. The current study presents the first attempts in quantifying the level of image degradation due to optical turbulence in natural waters in terms of modulation transfer functions using measured turbulence dissipation rates. Image data collected from natural environments during the Skaneateles Optical Turbulence Exercise are presented. Accurate assessments of the turbulence conditions are critical to the model validation and were measured by two instruments to ensure consistency and accuracy. Optical properties of the water column in the field were also measured in coordination with temperature, conductivity, and depth. The results show that optical turbulence degrades the image quality as predicted and on a level comparable to that caused by the particle scattering just above the thermocline. Other contributing elements involving model closure, including temporal and spatial measurement scale differences among sensors and mitigation efforts, are discussed.

Restoration of turbulence degraded underwater images

Abstract: The ability to image underwater is highly desired for scientific and military applications, including optical communications, submarine awareness, diver visibility, and mine detection. Underwater imaging is severely impaired by scattering and optical turbulence associated with refractive index fluctuations. This work introduces a novel approach to restoration of degraded underwater imagery based on a multi-frame correction technique developed for atmospheric distortions. The method represents synthesis of "lucky-region" fusion with nonlinear gain and optical flow-based image warping. The developed multiframe image restoration algorithm is tested on underwater imagery collected in a laboratory tank and in a field exercise. Reliance of image restoration on accuracy of the optical flow algorithm is revealed. The developed algorithm demonstrates significant resolution improvement of the restored image in comparison to any single frame or the mean of the underwater image sequence.

Automated underwater image restoration via denoised deconvolution

Abstract: A method for calculating an image quality metric for evaluating the quality of a digital image including the steps of denoising the data of the image, identifying edges in the denoised data, determining an edge profile of the edges, determining a grayscale angle for each identified edge in the edge profile that is associated with the edge, and calculating the image quality metric based on a weighted average of the grayscale angles for all the edges.

Bahamas Optical Turbulence Exercise (BOTEX): preliminary results

Abstract: The Bahamas Optical Turbulence Exercise (BOTEX) was conducted in the coastal waters of Florida and the Bahamas from June 30 to July 12 2011, onboard the R/V FG Walton Smith. The primary objective of the BOTEX was to obtain field measurements of optical turbulence structures, in order to investigate the impacts of the naturally occurring turbulence on underwater imaging and optical beam propagation. In order to successfully image through optical turbulence structures in the water and examine their impacts on optical transmission, a high speed camera and targets (both active and passive) were mounted on a rigid frame to form the Image Measurement Assembly for Subsurface Turbulence (IMAST). To investigate the impacts on active imaging systems such as the laser line scan (LLS), the Telescoping Rigid Underwater Sensor Structure (TRUSS) was designed and implemented by Harbor Branch Oceanographic Institute. The experiments were designed to determine the resolution limits of LLS systems as a function of turbulence induced beam wander at the target. The impact of natural turbulence structures on lidar backscatter waveforms was also examined, by means of a telescopic receiver and a short pulse transmitter, co-located, on a vertical profiling frame. To include a wide range of water types in terms of optical and physical conditions, data was collected from four different locations. . Impacts from optical turbulence were observed under both strong and weak physical structures. Turbulence measurements were made by two instruments, the Vertical Microstructure Profiler (VMP) and a 3D acoustical Doppler velocimeter with fast conductivity and temperature probes, in close proximity in the field. Subsequently these were mounted on the IMAST during moored deployments. The turbulence kinetic energy dissipation rate and the temperature dissipation rates were calculated from both setups in order to characterize the physical environments and their impacts. Beam deflection by multiple point patterns are examined, using high speed camera recordings (300 to 1200 fps), in association with measured turbulence structures. Initial results confirmed our hypothesis that turbulence impacted optical transmissions. They also showed that more research will be needed to better quantify and mitigate such effects, especially for the U.S. Navy's next generation EO systems, including active imaging, lidar and optical communications.

Underwater image quality degradation by scattering

Abstract: Image quality assessment is a challenging task. Numerous metrics and algorithms have been developed, with a fair share of these efforts focused on referenced targets. For applications associated with most civilian and military needs in the field of intelligence, surveillance, reconnaissance such that automatic target recognition is required, an objective, no reference metric is needed. The task becomes even more challenging when the signals obtained from the imaging sensor are time-varying, including those introduced by the turbulence induced index of refraction fluctuations along the imaging path. This study is aimed to develop an objective metric, tuned to be used for underwater imaging applications where the scattering degradations from particulates, as well as optical turbulence are present. Following recent research outcome, that spatial coherence length is a direct proxy to optical turbulence strength, we developed a metric based on this principle, while including the static scattering contributions with previous developed underwater image quality metric. The results show very good agreement with referenced metric such as the structure similarity image metric, as well as visual, subjective validation, using images obtained from both lab and field experiments.

Restoring Turbulence-Degraded Underwater Images

Abstract: : Underwater imaging is essential for a variety of scientific and military applications, such as optical communications, situational awareness, diver visibility, and mine detection. Unfortunately underwater imaging is severely impaired by scattering and optical turbulence associated with refraction index fluctuations. Because of the similarities between underwater and atmospheric image degradation, approaches taken to solve the underwater image restoration problem originate in atmospheric turbulence compensation algorithms. However, applying unmodified atmospheric techniques to underwater images fails because of the differences in the scale of the refraction index fluctuations.