THEORETICAL EVALUATION OF SIZING NONSPHERICAL PARTICLES USING DIFFRACTED LIGHT

The influence of non-sphericity on the precision of sizing of particles, using diffractively scattered light, is quantified by calculation. Laser light of 514.5 nm was collected by a lens with axis at 1.4 degrees, and with a mask at 0.9 degrees, from the forward-scattering direction. Ellipsoidal scatterers were considered with Gaussian probability distributions of aspect ratio with widths at the e(-2) point of 1.44 and 2.00. The largest uncertainty occurred for 20 mu m scatterers and was between 50% and 70% of the nominal diameter for the two PDF widths. The errors decreased asymptotically with increasing diameter up to about 75 mu m and fell to 10% and 20% for each of the two PDF widths. A validation criterion is proposed by which to reject measurement from particles with large aspect ratio, based on the use of two photodetectors collecting light from a common lens divided into two apertures with unequal area. When the validation tolerance was set at 10%, the precision for the 20 mu m nominal diameter particle improved from 50% to about 30% for the narrower PDF width of the aspect ratio. The corresponding improvement for a 75 mu m nominal diameter scatterer is from 20% to 10%. The better precision involves an approximate doubling of measurement time.