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.
Click here to watch a tutorial on Sequoia’s hologram processing software.
- In-situ digital holography
- Particle Imagery
- Self-contained with internal datalogger
- Ethernet connection to PC for programmable data collection
- Power via external battery pack (accessory) or external power source
- Automated hologram processing
The LISST-HOLO differs from other LISST instruments in that it uses holography instead of laser diffraction to measure particle size. The other key difference is the LISST-HOLO is able to provide images of suspended particles. This allows for particle identification and recording of shape characteristics in addition to size. The LISST-HOLO is Sequoia’s only imaging particle size analyzer. Having the ability to view particles is especially important for those interested in biological particles or complex aggregates.
The optics end of the LISST-HOLO consists of a 50 mm optical path. A red (658 nm) laser beam traverses the optical path, overfilling a 7 × 4 mm CCD array. A hologram is formed when particles in the sample volume scatter the laser light which then creates an interference pattern on the CCD.
The holographic method used by the LISST-HOLO eliminates the typical depth of field issues common with submersible microscopes and camera systems. The LISST-HOLO has the ability to bring all the particles across the 50 mm spacing between the windows in to focus after the image is collected. The large open path of the LISST-HOLO allows particles to freely pass through the sample volume with minimal disturbance and therefore capture the true nature of the particles.
Sequoia provides state of the art software that can digitally reconstruct the hologram to provide crisp particle images. The software then automatically extracts particle characteristics and size from the reconstructed images.
The LISST-HOLO has onboard memory for storage of ~15,000 holograms. An additional external memory module can extend the data logging ability of the LISST-HOLO even further. The LISST-HOLO is configured through an Ethernet connection and a simple web browser interface.
- 2MB Hologram containing the interference pattern of all particles in the laser beam
Parameters derived upon hologram processing
- Particle Size Distribution (PSD)
- Standard Deviation of PSD
- Particle Area Concentration
- Mean and Median particle size
- 120 seconds processing time per hologram in MATLAB (typical, @ 2.2 GHz PC)
Sediment Size distribution and concentration range
- ~25-2500µm size range
- Beam attenuation values between 0 and 4 m-1
- Maximum concentration between 0 and 50 mg/l, depending on grain size.
- Solid state laser diode @ 658 nm, 8 mW
- 4.4µm pixel size digital camera, 1600 x 1200 pixels
- 2 MB hologram size
- Hologram capture rate programmable up to 0.2 Hz; 1 Hz in short bursts
- Optical path length 50 mm standard; Path reduction modules available for high-concentration environments.
- Sampling volume 1.86 cm 3
- Maximum current velocity/CTD speed during deployment: 0.4 m s -1
Mechanical and Electrical
- Dimensions: 13.3 cm × 76.7 cm (5.25″ × 30.21″) [Ø × L]
- Weight: 7.2 kg (15.8 lbs) in air; 1.0 kg (2.2 lbs) in water
- 300 m depth rating
- 32 GB internal solid state drive
- 32 GB external USB flash drive
- External Battery life: 10,000 holograms for standard 15V, 42Ah battery
- External power input: 15VDC nominal, 12-16VDC
- Power drain: 200µA / 500mA / 600mA / 880mA (sleeping / idling / laser on / laser + camera on)
- Weight of EXTERNAL battery case (sold separately): 8.2 kg (18 lbs) in air.
How does the LISST-HOLO work?
The LISST-HOLO contains a red (658nm) laser that emits collimated light into the sample volume. The light is scattered by suspended particles. The scattered light then interferes with the unscattered portion of the beam. The resulting interference pattern is captured by an onboard camera. The image captured by the camera is known as a hologram.
The hologram can be digitally reconstructed to produce an in-focus picture of all the particles in the sample volume. Information about the particles size, shape, and position can all be extracted from the hologram.
How is the LISST-HOLO different from an in-situ microscope?
The LISST-HOLO is similar to a microscope in that it can produce in-focus images of small suspended particles. However, the benefit of using a holographic system is that you can have a large sample volume while still maintaining proper focus. In traditional microscopes, increasing the magnification decreases the depth of field. Thus, you can only image a thin slab of water. In holography the depth of field problem is eliminated, because each particle can be focused individually after the hologram has been captured.
The larger depth of field allows for a larger sample volume than is possible with a traditional microscope. This has two benefits. (1) It allows for more particles to be imaged, which will statistically improve the size distribution measurement. (2) It allows for a larger gap between the send and receive optics. The larger spacing is less likely to influence fluid flow and/or break apart aggregated particles.
How are holograms reconstructed?
Sequoia provides two pieces of software for hologram reconstruction. One is for batch processing of holograms and is used to generate a size distribution and output automatically focused particle images. The second is for qualitative viewing of holograms and allows for manually stepping through the focused planes of the hologram.
Both programs use the Fresnel transformation to reconstruct in-focus images throughout the sample volume. Particles are then extracted and characterized from these images.
See the following paper and references therein for more information about hologram reconstruction:
Owen, R.B., Zozulya, A.A. 2000. In-line digital holographic sensor for monitoring and characterizing marine particulates. Opt. Eng. 39(8): 2187-2197.
What do the particle reconstructions look like?