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Digital holographic underwater camera.

The University of Aberdeen in Scotland, working with laser manufacturer Elforlight, offshore instrumentation company CDL and the FRS Marine Research Laboratory in Aberdeen, has built a digital holographic underwater camera to assist in the study of plankton and other small organisms and particles.

Professor John Watson of the University's School of Engineering developed one of the first underwater holography system in the early 1990s as an inspection tool for the city's offshore oil industry. This system, named HoloMar (Holographic Marine Recording) provided useful 3D images of underwater life on photographic plates.

The new camera, dubbed eHoloCam, uses a CMOS (Complementary Metal Oxide Semiconductor) visual chip to record the interference pattern created as an in-line hologram. The chip is both smaller than a plate-holder and much more sensitive than a holographic emulsion, so a lower power laser can be used. This means that the whole camera is more compact, at 90 cm long and 30 cm diameter, and weighing 75 kg. The laser and optics are housed in a waterproof unit, with the CMOS chip housed in a separate unit connected by a metal frame (see photograph). The laser beam is scattered by the intervening water and organisms in it, so that there is sufficient interference to create the pattern recorded by the chip.

The laser was custom-made by Elforlight, of Daventry, UK. It is NdYAG with the lasing cavity specially designed to deliver the 50 cm coherence length required for this holography. For underwater use Elforlight added a frequency doubling crystal to convert the IR output to green.

The original eHoloCam design called for a dual in-line and off-axis configuration, but the team had the opportunity to have the camera tested on the RV Scotia, the research vessel operated by the UK's Fisheries Research Council Aberdeen laboratory, so it was completed as an in-line system only, with the intention to build an off-axis system later. The units were assembled into the waterproof casing by CDL in Aberdeen, an offshore engineering company. These are waterproof to a depth of 1800m, but weather hampered the trials and restricted the depth to 500m, on a research frame towed behind the ship.

Understanding plankton and other small organisms is important to understand the migration and feeding patterns of fish stocks, knowledge which is increasingly important as edible fish stocks dwindle. Funding was provided through the Department of Trade & Industry's OSDA (Optical Systems for the Digital Age) fund. Watson told Holography News that there he has already received interest in using the camera to explore Loch Ness (now what would that be looking for?) and Lake Baikal.


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Title Annotation:TECHNOLOGY NEWS
Publication:Holography News
Geographic Code:4EUUK
Date:Aug 1, 2007
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