Experiment Explores Optics with iPad

As any other dutiful scientist, Dr. Weilin "Will" Hou, an oceanographer in the Oceanography Division at NRL Stennis Space Center (NRL-SSC), did his research. Earlier last summer, he decided the hottest tablet computer available on the market-the iPad-was the best option for an upcoming experiment.

Upon receipt of the No. 1 wished-for item on everyone's Christmas list, he promptly threw it in the water.

iCan See

Understanding ocean optics is crucial to predicting environmental conditions, which help Navy and Marine Corps forces safely and effectively conduct operations involving signal processing, diver visibility, mine hunting and anti-submarine model performance prediction.

While in the office, Hou heads the NRL Ocean Hydro Optics Sensors and Systems Section. As such, he develops and manages new programs to improve understanding of adaptive optics, turbulence quantification, optical flow and signal transmissions over turbulence.

Hou's optics research furthers the general understanding of ocean optics through the combined use of in situ observations, remotely sensed data and physical models. How far and how well a diver and a vision system can see, especially under impacts from turbulence, is one of the focus areas Hou is working to explore.

iMAST Keeps It on Target

As part of the Bahamas Optical Turbulence Experiment (BOTEX), Hou and nine other researchers from NRL-SSC and Florida Atlantic University's Harbor Branch Oceanographic Institute (HBOI), set sail in the coastal waters of Florida and Bahamas aboard R/V F.G. Walton Smith on June 30, 2011.

The team set out to obtain field measurements of optical turbulence structures and quantify their impact on underwater imaging and beam propagation.

To measure these phenomena, Hou designed a rigid frame to securely hold a high-speed camera and a target (active or passive), which he called the image Measurement Assembly for Subsurface Turbulence, or iMAST. The name was fitting, as the active target that was mounted to it was the iPad.

Why the iPad?

Many factors influence visibility, one of which is the background, or path radiance. An active target (self illuminating) will help to reduce or completely remove such impacts (if carried out at night), which helps to isolate the main factor under examination.

After exploring more than a dozen OEM suppliers to find a solution, Hou found the best solution required a 5-month delivery schedule and cost more than $5,000.

Although he had thought of using a tablet computer before, Hou had always dismissed the idea due to the refresh rate limit (60 hz) of most LCD screens, which would not work for his project, since he needed to film at high frame rates (more than 100 frames per second).

When all the other options seemed remote, out of frustration, he set up an experiment to determine the performance limit of the LCD screen. To his surprise, there wasn't any! Hou discovered the common conception of refresh rate is defined differently for the LCD.

The buzz of the newest gizmo iPad caught his eye as the perfect candidate: small, light, bright, self contained, low power consumption, and low heat emissivity. What's more, it cost an order of magnitude less!

He quickly placed the order, and fellow NRL employee Wesley Goode began constructing a waterproof case to attach to iMAST.

Getting it Wet

At night from aboard R/V F.G. Walton Smith, Hou used the iPad, which was safely enclosed in a waterproof casing, to display active targets, such as resolution charts and image patterns. The iPad, securely held in place in the iMAST, was then lowered into the water.

The iPad allowed control of the brightness of the patterns and charts, a high-tech Secchi disk of sorts. (Incidentally, Hou is also responsible for an improved Secchi disk theory based on an imaging approach, compared to the traditional radiance-based version for the last 100 years.)

The team then measured the clarity of the target image in relation to optical turbulence structures in the water, first using a Vertical Microstructure Profiler and 3D velocimeter with a conductivity and temperature (CT) probe in close proximity in the field, and subsequently with a velocimeter and CT probe mounted on the iMAST during moored deployments.

Hou and the rest of the NRL-SSC team then calculated turbulence kinetic energy dissipation rate and temperature dissipation rates from both setups to compare to the derived imaging model, which estimates the limiting factors for underwater imaging components.

Collaboration with HBOI

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 assisted researchers in determining the resolution limit of LLS systems due to 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 was also used in the beam propagation experiment to study the effect of turbulence on the fringe pattern.

The team 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.

HBOI collected the data, which are critical in understanding the impacts of optical turbulence on active electro-optical sensing. They collected data from four stations for NRL, covering different types of optical and physical conditions. Because HBOI was funded by ONR, there is no added cost to Hou's core project and this leverage nested Hou's project a complete set of data needed.

BOTEX Results

Initial results 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 Navy's next generation electro-optical systems including active imaging, lidar and optical communications.

Hou arrived at NRL-SSC in 2006 after working as a research professor at the University of South Florida. He earned his doctoral degree in oceanography from the University of South Florida.

He is the editor of four books, 15 peer-reviewed papers and 40 proceedings papers and is credited as a co-inventor for three patents (two filed, one pending).

They can all be viewed on the iPad.

Source: U.S. Naval Research Laboratory