"Up Periscope" soon a thing of the past

Crew members of the Starship Enterprise watching an object
on the ship's tele-visor screen.  Is this a reality in our near future?
It's always a tense moment when the captain of a submarine comes shallow  and raises the ship's periscope.  What's he going to see?  An anti-submarine destroyer bearing down on his ship?  A freighter or other target of opportunity?

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It appears that the periscope is soon to be replaced by a screen, perhaps similar to the large visor screen on the Starship Enterprise or from the movie and television show, "Voyage to the Bottom of the Sea."

A team of Technion-Israel Institute of Technology researchers has developed an underwater imaging system that allows submariners to view objects above the water's surface - without a periscope. The unique technology gets around the inevitable distortion caused by the water-surface waves when using a submerged camera because of the sharp refractive differences between water and air, random waves at the interface present distortions that are worse than the distortion atmospheric turbulence creates for astronomers peering into space.

The technology behind a submerged, "virtual periscope" is called "Stella Maris" (Stellar Marine Refractive Imaging Sensor). The heart of the underwater imaging system is a camera, a pinhole array to admit light (a thin metal sheet with precise, laser-cut holes), a glass diffuser, and mirrors. Sunrays are projected through the pinholes to the diffuser, which is imaged by the camera, beside the distorted object of interest. The latter is then corrected for distortion.

Researchers explain that raw images taken by a submerged camera are degraded by water-surface waves similarly to degradation of astronomical images by our atmosphere. Researchers borrowed the concept from astronomers who use the Shack-Hartmann astronomical sensor on telescopes to counter blurring and distortion caused by layers of atmosphere, Stella Maris is a novel approach to a virtual periscope as it passively measures water and waves by imaging the refracted sun.

"When the water surface is wavy, sun-rays refract according to the waves and project onto the solar image plane," explains one researcher. "With the pinhole array, we obtain an array of tiny solar images on the diffuser." When all of the components work together, the Stella Maris system acts as both a wave sensor to estimate the water surface, and a viewing system to see the above surface image of interest through a computerized, "reconstructed" surface.

The Stella Maris virtual periscope is just the latest technology developed by the researchers, who have also found ways to exploit "underwater flicker," i.e., random change of underwater lighting, caused by the water surface wave motion. Members in the Schechner Hybrid Imaging Lab turned the tables on underwater flicker and used the natural rapid and random motion of the light beams to obtain three-dimensional mapping of the sea floor.

According to the developers, the virtual periscope may have potential uses in addition to submarines, where they could reduce the use of traditional periscopes that have been in use for more than a century. Submerged on the sea floor, Stella Maris could be useful for marine biology research where and when viewing and imaging both beneath and above the waves simultaneously is important. Stella Maris could, for example, monitor the habits of seabirds as they fly, then plunge into water and capture prey.
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Story Source:  Materials provided by American Technion Society, written by Kevin Hattori. "Submarine: 'Virtual periscope' sees above-surface/airborne objects from underwater view." ScienceDaily


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