Particle sizing by laser diffraction spectrometry in the anomalous regime.

The application of laser diffraction spectrometry to determine the size distributions of particles in the anomalous diffraction regime, i.e., particles with a refractive-index ratio close to one, has been examined. From a computer simulation, using the Mie theory and the geometrical optics approximation, it could be concluded that for suspensions with a refractive-index ratio near 1, the corresponding scattering matrix is required for calculation of the correct particle size distribution, even in the case of particles that are much larger than the wavelength of the incident light. In a system with the refractive-index ratio almost at unity, a suspension of ice crystals in a sucrose solution, the ice particles were sized by means of optical microscopy and laser diffraction spectrometry, and the results were compared.