In Phase Doppler Velocimetry, particles are radiated by two laser beams and the phase shift of the scattered light signals is measured. Two laser beams are split into four beams of equal intensity. The beams are focused and made to intersect. The scattered light from particles passing through the beams at their intersection is mixed at the photo detector surface and results in a ‘difference’ signal.

Since the rays enter the particle at different angles, the optical paths to a common arbitrary point on the detector differ; the light waves are shifted relatively to each other. Two detectors allow determination of particle size and droplet size.

The technique is used for characterization of droplet size and velocity distribution in sprays and nozzles from 0.5 to 90 µm. The measurements are performed on a TSI Phase Doppler Particle Analyzer and the report consists of size and velocity distribution results.