Holography permits a photograph-like view of particles in water. Photography suffers from a small depth of focus, so that only a small volume of water can be studied with each photo. In contrast, holography has total depth-of-field and therefore, permits viewing particles in a larger volume with each image. Holography also achieves finer resolution, revealing features as small as a few microns in some cases. Thus, in addition to seeing what particles are present in water, one can also measure the particle size distribution (PSD) as equivalent spheres. Finally, because holography uses micro-second laser pulses (roughly 1000 times faster than photography), the images are frozen even at high relative water velocities (e.g. in profiling).
The see through figure shows generally how the LISST-Holo2 is put together. The housing includes laser, electronics, battery, data storage etc. The CCD camera is in the smaller housing. High-speed ethernet data offload is provided on the endcap. The open sample-volume enables holographic images of undisturbed particles.
In-line digital holography is different from the ‘Princess Leia’ type, where 3-D images can be seen. In line-holography permits only the view of silhouettes (profiles) of individual particles. It permits undisturbed views of the most fragile flocculated particles (flocs), or plankton and other material. The name ‘in-line’ derives from the fact that a laser beam passing through water is directly imaged on an imaging sensor (e.g. CCD or CMOS). Thus, the image is made of the overlap of the unscattered laser light and light scattered by particles in the beam. Because this overlap produces interference fringes, the images (‘holograms’) look like a set of concentric rings or other blurred shapes. See the demonstration below.
In this animation, we display objects that become focused at different distances from the CCD. Watch the slider at bottom, indicating distance from one window of the plane where the image is reconstructed. Different objects come into focus as the slider advances. Their location is indicated by heavy arrow from the figure title. Axes are in microns.
To see actual particles in the beam, a hologram must be processed – a Reconstruction is required. This is done via digital processing. The reconstruction essentially focuses particles at different distances from the imaging sensor. In the case of our LISST-Holo2, the laser beam between glass windows is 5 cm long, and particles are reconstructed at 50 planes (normal to the laser beam), each 0.5 mm apart. Thus from one hologram, 100 in-focus ‘photographic’ frames are produced, and particles appear in sharp focus at the plane where they existed. The reconstruction software is provided.
Holography is data intensive. Each hologram of the LISST-Holo2 is about 2MB. At 20Hz, that would mean a data rate of 40MB/sec. A user may slow down the frame rate, and also schedule the frame capture using provided software. A 10-minute full-speed capture would produce 12,000 holograms, i.e. 24GB of data!
The large number of holograms that might be collected in some experiment requires a method to sort them by importance. Beginning with LISST-Holo2, Sequoia is introducing a new algorithm that quickly ranks holograms by the richness of images in them, so that you may see the most interesting ones first, and so on.
The LISST-Holo2 is not sensitive to drifts in laser output. It is also not sensitive to changes in alignment. These are major advantages for field instruments. Got questions? Contact us.