Enhanced Laser Diffraction
The Laser Diffraction method of measuring particle size takes advantage of an optical principle which dictates that small particles in the path of a light beam scatter the light in characteristic, symmetrical pattern which can be viewed on a screen. Given a certain pattern of scattered light intensity as a function of angle to the axis of the incident beam (‘flux pattern’), the distribution of particle sizes can be deduced.
The simplest flux pattern , that from a monomodal dispersion of spheres, consists of a central bright spot (known as the Airy disk), surrounded by concentric dark and bright rings whose intensity diminishes further from the center of the pattern, that is at higher scattering angles. The scattering angle at which the first dark ring, or diffraction minimum, occurs, depends on the size of the particles; the smaller the particle, the higher the angle of the first dark ring (or, alternatively, the larger the size of the Airy disk).
These flux patterns obey the rule of linear superposition. In other words, the pattern from a mixture of two (or more) monomodal dispersions of particles can be constructed by adding the intensity functions of the constituent particles in the mixture. The goal of a Laser Diffraction particle size measurement of course is to measure the flux pattern accurately enough to determine the distribution of particles.
The Coulter® LS 230 Enhanced Laser Diffraction Analyzer uses a patented technique for the characterization of sub-micron particles. This technology is needed as conventional diffraction theory loses its applicability in the sub-micron particle size region. PIDS or Polarization Intensity Differential Scattering is unique to the COULTER® LS 230 and provides exceptional performance extending the size measurement down to 0.04 µm.
For further information of the theory of Enhanced Laser Diffraction several technical monographs / reprints are available. Please e-Mail or call for your copies.
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