Airborne particle shape and size classification from spatial light scattering profiles

A new instrument for the study of airborne particles is described. The instrument incorporates a laminar flow sample delivery system which constrains airborne particles to traverse a focussed laser beam in single file. Particle transits through the beam are of 2–3 μs duration. The transient spatial intensity distributions of light scattered from individual particles are recorded by an intensified charge-coupled-device (CCD) camera as two-dimensional photon distributions. These distributions, or scattering profiles, represent the light scattered from the particles over approximately 84% of the total sphere of scattering. Preliminary results using standard latex spheres in the range 1–4.45 μm diameter have shown good agreement with Mie theory for both particle singlets and doublets. Preferential orientations of the doublets towards alignment with the sample delivery airflow have been observed. Further data recorded from samples of precision micromachined fibre particles of 12 μm length are also shown, and these further illustrate the orientational behaviour of the particles in the sample delivery airflow. The potential of the instrument for the classification of airborne particles on the basis of shape and size is briefly discussed.