Coherent Detection of Scattered Light from Submicron Aerosols

Abstract

The van Cittert—Zernike theorem states that light scattered from small particles can be made coherent if gathered at the focal point of a lens. Coherent-detection techniques can then be used to measure the extent of the phase shift due to the particles. Scattering mechanisms can relate the phase shift to the particle diameter, so particle size can be determined. Theoretical estimates of the ideal minimum detectable particle diameter are of the order of 5 × 10⁻³ μm; estimates of the minimum detectable diameter for a practical system are of the order of 5 × 10⁻² μum. Particles are counted one at a time as they travel through an optically defined view volume. Electronic signal processing allows particle size distributions to be deduced from the individual signals. The sampling of particle concentrations up to 10⁹/cm³ is possible with this technique. The coherent optical particle spectrometer can be used to measure aerosol dynamics in high-temperature environments and to determine the properties of single submicron particles.