Submersible holographic microscope

A low-cost, 20-kg device has been developed that allows scientists to form three-dimensional images of the trajectories of tiny marine organisms in their natural environment. Scientists at Dalhousie University in Halifax, Canada, used a simple hologram arrangement: Laser light is focused onto a pinhole aperture in the laser’s watertight housing. The spherical waves that emanate from that point source then illuminate a sample of seawater, scatter from objects in the water, and recombine—at the photosensitive area of a CCD camera housed in a separate container—with the reference beam that went straight through the seawater without scattering. The digital holograms are then reconstructed into images of the objects. Shown here is a marine ciliate Favella, imaged with a 0.2-ms exposure at a depth of 10 m as the animal swam from right to left at 2.1 mm/s. Holograms with 1024 × 1024 pixels can be recorded at up to 10 frames per second, a rate at which the swimming and feeding characteristics of the organisms can be studied. Past generations of submersible holographic microscopes weighed several tons, had to be deployed from large ships, and used film as the recording medium. (S. K. Jericho et al., Rev. Sci. Instrum. 77 , 043706, 2006 http://dx.doi.org/10.1063/1.2193827 .)