Holography Monitors Microscopic Marine Life

Richard Gaughan

Scientists estimate that 80 percent of the oxygen in the atmosphere is produced by phytoplankton, ocean-dwelling one-celled photosynthesizing organisms. It seems understandable, therefore, that we would want to investigate the distribution and interaction of plankton, but how can we make meaningful measurements of microscopic organisms in the vastness of the oceans? Researchers at the University of Aberdeen have coordinated a European consortium in the development of an undersea holographic camera to address this problem.

To better understand marine phytoplankton, researchers have developed a camera that generates in-line and off-axis holograms of the organisms.

Using 10-ns pulses of 532-nm radiation from a custom two-output, frequency-doubled Nd:YAG laser, the team, led by John Watson, a reader in engineering at the university, makes holographic exposures at depths of up to 100 m. The 2-m-long, 1-m-diameter submersible holographic camera, which the researchers have named the HoloCam, incorporates the laser, beam-forming optics and two holographic plate cassettes.

The camera generates in-line holograms using the interference of the backscattered light from the particles in an illuminated water volume with the directly transmitted beam. In-line holograms can resolve 10- to 250-µm particles in this 500-mm-long, 100-mm-diameter volume.

The HoloCam also generates off-axis holograms using the other laser output, which is conditioned to create a reasonably uniform diffuse illumination. The camera combines the reflected light from the particles in the 100-liter volume with a reference beam and can resolve particles down to 30 or 40 µm.

The holograms are reconstructed in the laboratory using collimated 442-nm illumination. A computer-controlled stage moves either a video camera or a microscope through the images, and the organisms can be measured as they were in their environment. The researchers now are working on automated object recognition and classification to speed the technique.

The tank and dockside trials of the HoloCam at Southampton Oceanography Centre in Empress Dock, UK, are encouraging. The group will conduct further trials in the inshore waters off Scotland, in the North Sea and off the Irish coast -- waters that are rich in plankton, Watson said.

If successful, they hope that the HoloCam will become an essential tool for marine biologists. They also note that a slightly modified version of the instrument could also find useful applications in inspection and measurement for the offshore oil and gas industries.