Abstract

Mie theory angular scattering distributions for spherical particles of refractive index m = 1.2 (0.2) to 2.4 and 2 − i, and of size 0.1 < πd/λ < 100, have been integrated both over a range of illuminating wavelengths and over scattering angles with ranges and weighting appropriate to the geometries of various designs of aerosol counters and photometers. The use of white light alone smooths the curves of collected flux vs particle size which are relatively insensitive to the precise spectral distribution, and not much further smoothing is introduced by the use of large-aperture illuminating and collecting lenses. The smoothing by no means eliminates the effect of the optical properties of the particles on the responses. In all designs the collected flux per particle increases initially as the fifth or sixth power of diameter reaching a first maximum when d ∼ ⅓(m − 1) μ. Instruments using 40° to 50° scattering are the best for transparent particles since for all refractive indices their response varies roughly as (diam)2 when d > 1μ then varies little with m for 1.4 < m < 2.4; but their response is weaker for absorbing particles, e.g., m = 2 − i. In 90° scattering the variation with transparent refractive index is considerable and the response does not vary as (diam)2 until d > 10 μ at least, but as much flux may be received from absorbing as from transparent particles. In instruments measuring the forward lobe of the scattering pattern the differences in response for transparent and absorbing particles are least, but not negligible, and the collection of this lobe is difficult for particles larger than a few microns. Axially symmetric instruments do not necessarily measure forward scattering. No single instrument design can meet all requirements of aerosol measurement.

© 1965 Optical Society of America

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