Multiple wavelength instrument for measurement of particle size

Abstract

A method for measurement of the size distribution of particles suspended in a gas or in a liquid. The particle suspension is illuminated by a collimated beam of colored light, which is produced by passing a collimated beam of substantially white light through a spectral filter which has spatial sections of narrow spectral passband in the blue, green and red. Part of the light scattered by the particles is collected by a lens and is passed through a spatial filter placed in the focal plane of the lens. The light transmitted by the filter is measured by a photodetector. The photodetector output is measured as different spatial filters are switched in place. A computer, microprocessor, or analog device acts on the measured values and produces the particle size distribution as an output. The data reduction algorithm consists of a linear transformation of the measured data vector, followed by the construction of a linear combination of basis functions for the size distribution. The spatial filters consist of color transparencies with non-uniform spatial transmittance functions in the blue, green, and red. The average scattering angle is not restricted, but for broad polydisperse particle distributions near back scatter is preferred, in order to avoid an ill-conditioned data reduction problem. One of the spatial filters has uniform transmittance for the purpose of back ground light subtraction, and to provide a bias in order to allow effectively indefinite filter transmittance functions.