Method for measuring the size and velocity of spheres by dual-beam light-scatter interferometry

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Abstract
A method is described for obtaining real-time in situ size and velocity measurements of spherical particles or droplets using crossed-beam interferometry. The optical arrangement, which is similar to a dual-scatter laser Doppler velocimeter (LDV), consists of two laser beams focused to a crossover region. Droplets passing through the focal volume scatter light to the collecting lens situated at some off-axis angle. The dual-beam light scatter is analyzed by the geometric optics theory to relate the scattered fringe pattern to the droplet diameter. Because the droplet size measurement is based on the relative phase shift between the two light waves passing through it, the method is independent of the incident intensity, droplet absorption, or absolute scattering intensity. Experimental measurements of monodisperse droplet streams show good agreement with the theory. The technique can be applied to spray-droplet measurements over the size range of 3 μm to 5 mm. By using large off-axis scatter detection angles, the measurement of the droplet size and velocity distributions in relatively dense spray environments is made possible.