PN B05577AC
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Theory
Theoretical Background
In characterizing particle size using light scattering, a sample concentration is optimized to a
proper range. This range allows enough scatter intensity for the measurement to be completed with
a desired signal-to-noise ratio, but not to the point of saturating the detecting system. The sample
concentration is also optimized for minimal particle-particle interaction and minimal multiple
scattering so that the measurement is performed based on elastic single particle scattering.
Also, in regards to light scattering measurement, it is assumed that the refractive index and density
of particles in the sample are uniform, which is true for most particulate systems. Thus, the
scattered intensity is only a function of scattering angle, particle shape, and particle size. If the
relations between scattering intensity, scattering angle, particle shape and particle size are known,
a size distribution for the particles can be resolved from the measured angular scattering intensity
pattern. Theories have been developed to aid in this particle size determination; some are discussed
next.
Figure 1.1
Scattering Patterns for Spheres
Mie Theory
The Mie theory describes the interaction of light with a particle of arbitrary size as a function of
angle, given that the wavelength and polarization of the light are known and that the particle is
smooth, spherical, homogeneous, and of known refractive index. This theory is more complex than
the theory set forth by Fraunhofer, in that it accounts for all possible interactions between particles
and light, yet it's only applicable to spheres.
Spheres produce light scattering patterns that are characterized by the presence of scattering
minima and maxima at different locations (
) depending on the properties of the particles.
At small angles (typically smaller than 10 degrees) the scattering pattern for spheres is centrally
symmetric instead of axially symmetric, i.e., it displays concentric rings in the direction of the
incident light. Therefore, large particles produce scattering intensities that are concentrated at
small angles and due mainly to diffraction effects from the edge of the particle.
3.1.2 Fraunhofer Theory
When the particle size is much larger than the wavelength of light or the materials are highly
absorptive, the edge effect of the particles contributes more to the total scattered intensity.
Interference effects are now due to the bending of light at the particle's boundary (diffraction). In
a light scattering measurement, because the light source is far away from scatterers and the optics
are usually designed so that the incident beam illuminating the scatterers is homogeneously
Содержание LS 13 320
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Страница 58: ...PN B05577AC 2 22 Installation Making Measurements Figure 2 19 Reference Background File Selection ...
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