Laser Doppler velocity measurements based on the reference beam technique are well known since the initial work by Yeh and Cummins (1964). Due to the specific signal generation of a reference beam arrangement, a location of the trajectory of the scattering particle inside the measurement volume is possible by applying at least two receivers Two alternative solutions to this problem with a spatial resolution of about 10 μm over a length of the measurement volume of more than 6 mm are described and experimentally verified in velocity profile measurements. The application of the phase Doppler principle to the reference beam technique allows simultaneous measurement of velocity, size and position of each particle inside the measurement volume. With this reference beam PDA, the correlation between velocity, size and position of the disperse phase in multiphase flows with high velocity gradients can be investigated without a relative displacement of the optical set-up. The paper presents the theory of the reference beam PDA and first results of simultaneous measurements of velocity and particle size profiles in boundary layers. According to the results of theoretical and experimental investigations, the reference beam PDA is especially suited for particle sizing in the diameter range below 3 μm, i.e. a size range in which traditional phase Doppler systems suffer from a poor diameter resolution, and allows in extension of conventional phase Doppler systems a general improvement of the spatial and diameter resolution of phase Doppler measurements.
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