- Exploring Optics Underwater with an iPad
STENNIS SPACE CENTER, Miss., April 11, 2012 — A new rigid frame device that securely holds a high-speed camera and an iPad can obtain field measurements of optical turbulence structures and quantify their impact on underwater imaging and beam propagation.
The structure, dubbed image Measurement Assembly for Subsurface Turbulence, or iMAST, was designed by Dr. Weilin Hou, an oceanographer in the Oceanography Div. at the US Naval Research Laboratory’s Stennis Space Center (NRL-SSC). His device may be able to explain just how far and well a diver and a vision system can see, especially under impacts from turbulence.
iMAST deployment configurations. The iMAST is designed to be deployed in horizontal and vertical orientations as shown. The horizontal orientation is designed to capture the maximal impacts from turbulence, while the vertical configurations allow close examination of the “shower curtain” effects of the turbulence and turbidity layer. (Images: US Naval Research Laboratory)
Understanding ocean optics is crucial to predicting environmental conditions, which help US Navy and US Marine Corps forces safely and effectively conduct operations involving signal processing, diver visibility, mine hunting and anti-submarine model performance prediction.
As part of the Bahamas Optical Turbulence Experiment, Hou and nine other researchers from NRL-SSC and Florida Atlantic University’s Harbor Branch Oceanographic Institute (HBOI) set sail on the coastal waters of Florida and the Bahamas to test the device.
Many factors influence visibility, one of which is the background, or path radiance. A self-illuminating active target will help reduce or completely remove such impacts when carried out at night. After exploring more than a dozen OEM suppliers to find a solution, Hou stumbled upon the iPad, which encompassed all of the characteristics he needed for the experiment: small size, lightweight, bright, self-contained and had low power consumption and low heat emissivity.
High Tech Secchi disk — These iPad images taken in field trial demonstrate how clarity of the target image would be degraded under turbulence conditions. Both are viewed with a range of 5 m, with similar turbidity of the water. The top was weak turbulence, while the bottom one is from regions of strong turbulence.
Aboard the University of Miami’s research vessel F.G. Walton Smith, Hou lowered the iPad, held in place in the iMAST, to display active targets such as resolution charts and image patterns. The iPad enabled control of the brightness of the patterns and charts, a high-tech Secchi disk of sorts.
The team measured the clarity of the target image in relation to optical turbulence structures in the water, first using a vertical microstructure profiler and 3-D velocimeter with a conductivity and temperature probe in close proximity in the field and, subsequently, with a velocimeter and CT probe mounted on the iMAST during moored deployments.
They calculated the turbulence kinetic energy dissipation rates and the temperature dissipation rates from both setups to compare with the derived imaging model, which estimated the limiting factors for underwater imaging components.
Dr. Weilin “Will” Hou, Capt. Shawn Lake (R/V Walton Smith), Dr. Sarah Woods, Mr. Steve Sofa, Dr. Ewa Jarosz, Mr. Ben Metzger (HBOI), Dr. Gero Nootz (HBOI), Dr. Alan Weidemann, Mr. Brian Ramos (HBOI), Dr. Fraser Dalgleish (HBOI), and Mr. Wesley Goode.
To investigate the impacts of optical turbulence on an active imaging system, such as laser-line scan (LLS), HBOI researchers designed Turbulence Research for Undersea Sensing Structure (TRUSS). TRUSS helped them determine the resolution limit of LLS systems resulting from beam wander at the target due to turbulence. Fourier transformed image patterns over turbulence were examined by placing a pinhole mask into the beam path. The same experimental setup also was used in the beam propagation experiment to study the effect of turbulence on the fringe pattern.
The team also tested the performance of pulsed LLS — where the receiver and the transmitter were mounted on the same side of the pole and the ground glass plate was replaced by a technical target and spectralon panel — to examine the impacts on lidar systems.
The HBOI team collected the data critical in understanding the impacts of optical turbulence on active electro-optical sensing from four stations for NRL, which cover various types of optical and physical conditions.
The data confirmed the team’s hypothesis that turbulence does play into optical visibility performance prediction and, at times, can greatly reduce the visibility range. However, more research is needed to better quantify and mitigate such effects, especially for the Navy’s next generation electro-optical systems, including active imaging, lidar and optical communications.
For more information, visit: www.nrl.navy.mil
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