Holograms give new insight into deep-sea relationships

17 Jun 2002

A group of European scientists will soon begin field trials of pulsed holographic techniques for studying plankton in their natural habitats. These techniques will allow biologists to non-invasively study how underwater organisms interact with others of the same species and with predators.

In the HOLOMAR project, plankton are imaged with a 'holocamera' based on a pulsed Nd:YAG (532nm) laser. This records up to 40 holograms on glass plates. The camera incorporates both the 'in-line' (single laser beam) and 'off-axis' (two reference beams) holographic geometries, so that both large and small organisms to be imaged. The camera is designed for operation down to depths of 100m.

The images are reconstructed by a dedicated scanning machine. The real image is projected from the holographic plate by a He-Cd (442nm) laser. This shorter wavelength minimises replay aberrations, which result from the difference in refractive index between the water, where the holograms were recorded, and the air, where they are replayed.

The system also includes sophisticated image analysis and identification software. The team includes researchers with image processing expertise. "While many workers have successfully used holography to record particle fields, almost none of them were able to analyse more than a couple of holograms, simply because of the amounts of data involved," explained one member of the team, Henry Nebrensky, of Brunel University, UK. Nebrensky presented a paper on this technique at the Sixth International Conference on Optics Within the Life Sciences (OWLS VI), which was held recently in Sydney, Australia.

The HOLOMAR project is funded by the EU MAST-3 programme and is led by John Watson at the University of Aberdeen, UK. It also involves researchers from Brunel University and Southampton Oceanography Centre, in the UK, University of Genoa and University of Udine, in Italy, as well as two French companies, HOLO 3 and Quantel, and NEMKO, of Norway.

The team is aiming for dockside trials and demonstration runs in July of this year.

SH