Woods Hole Oceanographic Institution

Underwater digital holography for plankton imaging and automated identification

Cabell Davis (WHOI), George Barbastathis (MIT), Jerome Milgram (MIT), Jose A. Dominguez-Caballero (MIT)

Traditional tools for monitoring plankton populations, such as nets and Niskin bottles, suffer from two serious drawbacks: they destroy fragile plankton, and mobile zooplankton are able to avoid capture.  These tools are also limited in their ability to spatially locate plankton, either taking large trawls over an extended area or sucking in a limited amount of water from a single location.  Various in-situ imaging tools have been developed over the years to analyze plankton.  Many of these, however, are limited to a small in-focus volume determined by the optics.

Digital holography (DH) is an interferometric non-imaging technique for recording 3D information about a sample.  It has the benefit of extremely high imaging resolution across a wide viewing field without the traditional depth of field limits.  DH achieves this by recording the diffracted optical field directly, which contains the complete information about sample volume.  This allows a user to chose an arbitrary focal depth after the hologram has been recorded -- and gives the ability to scan through an entire 3D volume.  An image of the optical field is reconstructed computationally, making it immediately available for additional processing.

Our project focuses on creating a low-power, compact DH unit which can be used for underwater imaging of plankton.  A prototype has been built and deployed, returning high quality images with imaging resolutions down to 9-12 microns over a 300 ml sample volume (per hologram).  Plankton between 200 microns and 3 mm can be imaged, depending on the system parameters.  Work to increase the processing speed and perform automated identification of the plankton is underway.

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