Laser anemometry and particle sizing

The use of LDA signals to measure the size of scattering particles is examined. The principal methods of particle sizing use a signal intensity and the pedestal or Doppler component combination, a visibility factor or modulation depth, phase shifts of the Doppler signals as measured by two or more detectors, and the angular distribution of scattered light. The techniques include the use of: (1) diffracted light, (2) Mie scattering, (3) off-axis aperture, (4) phase technique, and (5) backscatter signals. The applicability of the methods depends on the direction of scattering, the types of particles, and the size range. The depth of modulation of the signal or visibility, and the relation between visibility and the particle diameter are studied. Techniques for countering the problems of particle sizing by LDA methods due to nonuniform distribution of intensity through the probe volume are described. Analogue and digital processings of signals for determining visibility phase are discussed.