LISST-200X User’s ManualAppendix G: Spherical and Random Particle Shape Models Page 112
Appendix G: Spherical and Random
Particle Shape Models
We briefly comment on shape effects here. Particles of different shapes scatter light
differently. Two models are offered by Sequoia – the spherical model, and a random shape
model. The resulting inversion of data will differ slightly for the two models.
The spherical particle model performs the mathematical inversion of scattering data under
the assumption that the particles that scattered light are all spheres. Light scattering by
spheres of any size and refractive index is modeled by Mie theory. According to Mie theory,
the angular scattering depends on the size of the spherical particle, and its refractive index
relative to water. Sequoia employs the full Mie scattering model, i.e. no simplifications, for
inversion of LISST measurement as a distribution of spheres. The Mie solution is a
generalized solution to the scattering of light from spheres and is commonly used as the
standard model by all laboratory laser diffraction instrument manufacturers.
Sequoia provides an alternate model to Mie theory to invert the measured scattering pattern
with the assumption that the scattering particles are randomly (or irregularly) shaped. This is
an empirically determined model, since theoretical development of such models is not
complete. The exact details of how this scattering model was established are described in a
paper by Agrawal et al. [Light scattering by random shaped particles and consequences on
measuring suspended sediments by laser diffraction. Journal of Geophysical Research, Vol.
113, C04023, doi:10.10-29/2007JC004403.], which can be downloaded from the library
section on Sequoia’s website (www.SequoiaSci.com). The direct URL is
http://www.sequoiasci.com/library/technical-papers/. The paper is also included on your ship
disk. A brief version of the method is described in this Article on Sequoia’s website:
http://www.sequoiasci.com/article/random-shaped-particles-lissts/. It is noteworthy that no
other instrument manufacturer has a scattering model for irregular particles. Instead, they
often use a Mie model with large imaginary refractive index, in effect, assuming the particles
to be highly absorbing (i.e. black). This is an obvious fudge with consequences!
Key differences in results of the spherical vs random shape model are: the random shaped
model interprets particles to be smaller than the spherical model, and when scattering from
irregular particles is inverted with the spherical model, the resulting particle size distribution
invents fine spherical particles.
So a LISST user is likely to ask which model she/he should use. If the plan is to compare
LISST results with those from another laser particle sizer, one should choose the spherical
particle model. If comparing with sieved particle size data, the irregular shape model is
appropriate. When using the spherical particle model, the PSD measured by LISST
instruments should generally match those measured by other instruments. Subtle
differences may arise due to different refractive indices built into the instrument software.
Generally, it is recommended that you keep both boxes checked during inversion (this is
also the default option), so that you won’t have to reprocess your data because you need to
see what the data look like when processed as randomly shaped particles.
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