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