Laser Doppler Velocimetry (LDV) is a technique which allows the measurement of velocity at a point in a flow field with a high temporal resolution. Whenever a micron-sized liquid or solid particle entrained in a fluid passes through the intersection of two laser beams, the scattered light received from the particles fluctuates in intensity. LDV makes use of the fact that the frequency of this fluctuation is equivalent to the Doppler shift between the incident and scattered light, and is thus proportional to the component of particle velocity which lies in the plane of two laser beams.
Phase Doppler Interferometry (PDI) measures the diameter and velocity of small droplets. The measurement of drop size distribution and mean values can be done without the requirement of assuming a distribution function. It is based on the laser light wavelength which is known to high accuracy. The particle sizing is independent of light intensity so attenuation and window contamination have a minimal effect on drop sizing performance.