Beam forming and sensing apparatus for aerodynamic particle sizing system

Abstract

Apparatus, intended primarily for use in an aerodynamic particle sizing system, for generating a pair of closely-spaced, substantially parallel light beams and for determining particle sizes by measuring the times of flight of particles between the two light beams. The apparatus for generating the light beams includes a laser source, an aperture slit for establishing the shape of the beams, a cylindrical lens for focusing the laser beam on the aperture slit, a diffraction grating for converting the beam from the aperture slit into diffracted beams in the zero order lobe and one first order lobe and a transfer lens for directing the diffracted beams as substantially parallel beams. The particle measuring apparatus includes a prism, a lens arrangement for imaging light scattered by particles passing through one beam on a first face of the prism and for imaging light scattered by particles passing through the other beam on a second face of the prism that is perpendicular to the first face, and a pair of photomultiplier tubes for detecting light reflected by the faces of the prism. Start and stop pulses from the photomultiplier tubes are processed to determine particle time-of-flight which is then converted to particle size.