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| LISST Instruments > LISST-HOLO |
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LISST-HOLO |
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Click here to read an article by Dr. John Ryan, MBARI, on integrating the LISST-HOLO on an Autonomous Underwater Vehicle (AUV).
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The LISST-HOLO from Sequoia is the world’s first commercially available submersible Digital Holographic Particle Imaging System. The system was developed by Dr. Alex Nimmo Smith at the Marine Institute of the University of Plymouth (UK) and Sequoia holds a world-wide license for manufacture.
The LISST-HOLO is particularly well suited to the measurement of large, complex flocs and biological particles. However, existing users are also using the unit to investigate frazil ice formation. Others are considering use of the LISST-HOLO to measure the size distribution and concentration of falling snow flakes in situ.
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The optics end of the LISST-HOLO consists of a 5 cm optical path. A red (658 nm) laser beam traverses the optical path, overfilling a 7 × 4 mm CCD array, thereby creating a hologram.
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The LISST-HOLO creates in-situ images of particles, using in-line digital holography to overcome the depth of field limitation of traditional particle imaging systems. Numerical reconstruction of the digitally recorded holograms gives in-focus, high resolution images of all particles from 25 µm to 2.5 mm equivalent spherical diameter throughout the entire sample volume (~2 ml). |
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This series of 3 pictures show a reconstruction of a hologram with sand grains. The grid size is 100 µm. In the first image a sand grain is seen to be in focus at a distance of 5 mm from the window. Two other grains are out of focus.
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The second image shows a sand grain coming into focus at 7 mm depth, while the sand grain that was in focus at 5 mm is now out of focus.
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The last image shows a sand grain coming into focus at 9 mm depth, while the 2 other sand grains are now both out of focus.
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The LISST-HOLO is particularly well suited to the measurement of large, complex flocs and biological particles. The processing software, available in MATLAB as well as compiled format for non-MATLAB users, allows the automatic focusing of particle images and the extraction of size distributions. From the images, shape parameters can be computed and particles may be manually identified, e.g. as plankton or flocculated sediment grains |
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The LISST-HOLO is equipped with an external memory module on the connector endcap (red circle), capable of holding 15,000 holograms. The external memory makes for fast and easy offloading of holograms, simply by swapping out the memory module with an empty one.
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When the external memory module has been removed, the holograms are offloaded by connecting it to a PC using the USB offload cable. One 32GB memory module and one offload cable is included with the LISST-HOLO.
The sampling frequency is adjustable, with a maximum sampling frequency of 0.2 Hz (one hologram stored every 5 seconds). Power is provided externally, either from an optional external battery pack (sold separately) or from another instrument, e.g. a CTD.
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Related Links
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Updated 10/28/2011
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|  | | Sequoia visit to Colombia and Ecuador! |  | | Sequoia's Yogi Agrawal and Ole Mikkelsen visited C'Particulas, Sequoia's Colombian distributor 28-30 November 2011. See the pictures on our facebook page! | |  | |  | |
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