Beckman Coulter LS 13 320, Руководство пользователя

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PN B05577AC xxv
LS 13 320 Introduction
Determining PSD
An important component of making this measurement in an LS 13 320 instrument is the Fourier
lens (
Figure 3 ). A Fourier lens serves two functions: it focuses the incident beam so it will not
interfere with the scattered light, and it also transforms the angularly-scattered light into a
function of location on the detection plane. The most important feature of Fourier optics is that the
scattered light of any particle at a specific angle will be refracted by the lens so as to fall onto a
particular detector, regardless of the particle's position in the beam.
Figure 3 Fourier Optics
NOTE
Large particles scatter at small angles, and vice versa.
The result is that the Fourier lens forms an image of the composite scattering pattern of all particles.
This pattern is centered at the same fixed point in the Fourier plane regardless of the position or
velocity of the particle in the sensing zone.
The individual scattering patterns are superimposed, creating a single composite scattering pattern
that represents the contributions from all the particles in the sample cell. Detectors the Fourier
plane record this composite scattering pattern. Over the course of a measurement, a running
average is created from the changing flux patterns. When the duration of the measurement is long
enough that the flux pattern accurately represents the contributions from all particles, an analysis
of the resulting pattern yields the true particle size distribution of the sample.
Determining PSD
The composite scattering pattern is measured by 126 detectors placed at angles up to approximately
35 degrees from the optical axis. When you view intensity in flux units (light intensity per unit area),
you are looking at the scattering pattern.
In order to compute the size distribution, the composite scattering pattern is deconvolved into a set
of individual number, one for each size classification, and the relative amplitude of each number is
a measure of the relative volume of equivalent spherical particles of that size. This deconvolution is
based on either the Fraunhofer or Mie theories of light scattering.
1.
Fourier Lense
2.
Particles
3.
Detector