AUTOHOLO: A novel, in situ, autonomous holographic imaging system for long-term particle and plankton characterization studies in diverse marine environments

Aditya R Nayak1,2, Malcolm McFarland1, James Michael Sullivan1, Fraser Dalgleish1 and Lysel Garavelli3, (1)Florida Atlantic University, Harbor Branch Oceanographic Institute, Fort Pierce, FL, United States, (2)Florida Atlantic University, Ocean and Mechanical Engineering, Boca Raton, FL, United States, (3)Pacific Northwest National Laboratory, Seattle, WA, United States
Digital holography has increasingly been embraced by the oceanographic community over the past few years as a valuable tool, due to its ability to facilitate characterization of 3-D spatial distributions of particles at high resolution. Several commercial and custom-built instruments have been used previously to characterize particle/phytoplankton community composition, harmful algal blooms, bubbles, marine snow and oil spills in situ in diverse marine environments. Here, we describe the development, fabrication and field testing of a new generation holographic imaging system (AUTOHOLO), which overcomes some of the issues associated with previous systems.

The AUTOHOLO is an untethered, battery powered system, with internal data logging capability. It uses a pulsed, 532 nm laser as the illumination source and a high resolution (4920 x 3280 pixels, 16 MP) camera as the recording medium. The optics and control electronics are packaged in two cylindrical housings, with the free stream sample volume located between them. The system is modular with deployment possible in a lens-less configuration as well as with a microscopic objective. This corresponds to a resolvable particle size range of 16.5 μm – 2.9 cm for the lens-less configuration or 8.25 μm - 1.45 cm with a 2X objective lens. A sampling length of 12 cm corresponds to a sampling volume of 71.4 mL per hologram for the lens-less configuration. This is an order of magnitude higher than the volume sampled per hologram in commercially available holographic imagers; at a sampling frequency of 3.5 Hz, this corresponds to sampling ~ 15 L of water every minute. The instrument is programmable to either acquire data continuously, or in burst mode, wherein it acquires data for a fixed period at regular intervals, thus allowing for deployments of several weeks. A copper shutter has been incorporated to minimize biofouling issues. Sample data obtained from controlled experiments in the laboratory as well as field tests/applications will be presented.