Particle sizing by laser diffraction is currently the most widely prevalent
method in research and industry. It is a multi-parameter measurement.
Just as a set of 3 equations can be solved for 3 unknowns, a
measurement of light scattering at multiple angles can be solved for
equally as many concentrations in different sizes. For example, the
LISST-200X measures scattering into 36 angles; consequently, one
obtains concentrations in 36 size classes of particles. This is called the
particle size distribution (PSD). That is the essence of this method. In
order that you appreciate the importance of some steps in operation of
the LISST-200X, we recommend reading the rest of this chapter.
The measurement of scattering at multiple angles is done with a
collimated beam illuminating particles in water. Laser light scattering at
an angle from the beam arrives, through a daylight rejection filter, at the
focal plane of a receive lens at the same angle from lens axis (see
figure). Thus, distance from lens axis in the lens focal plane
corresponds to scattering angle. The photodetectors in the LISST
series instruments are a series of silicon rings spanning 60-degree
arcs. Each ring covers a small range of scattering angles. Rings are
used to achieve stability of inversion – conversion of set of multi-angle
scattering measurements to the PSD.
Notice that the laser beam itself is focused by the receive lens and
passes through a small aperture in the ring detector, centered on the
rings. This beam is sensed by a photodiode placed behind the ring
detector. This is the transmission sensor. As light is removed from the
laser beam by scattering, the beam is attenuated, i.e. the light
transmitted through water is reduced in intensity. A similar attenuation
also affects the light that is scattered by particles and sensed by ring
detectors. Thus, the transmission sensor provides a vital measurement
to de-attenuate the measured scattered light.
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