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A dual-angle streaming ultramicroscope for submicrometre particle size measurement

Streaming ultramicroscopy is a potentially valuable technique for size analysis of submicrometre particles but suffers from a number of limitations. In particular, the need to observe light pulses scattered from the particles at small angles, in order to obtain a monotonic particle size-intensity relationship, leads to high background count rates from non-streaming particles in the incident beam. The author describes the use of a novel dual-angle coincidence technique, in which light pulses scattered at 90 degrees to the incident beam are used to gate the small-angle signal, thus rejecting pulses from particles not in the central particle stream. This arrangement has the advantages of high sensitivity and background rejection due to the perpendicular viewing arrangement, while retaining the monotonic size-intensity relationship up to a particle diameter of 2.3 mu m at a relative refractive index of 1.1. The author believes that this is a significant improvement in the design of single-particle optical size measurement instruments.