|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.