Sensors in Engineering and Science 53
Further modifications can be made by adding other beams of different
frequencies in different Cartesian coordinate directions to yield all three
components of the velocity.
Phase Doppler Anemometer
For the phase Doppler anemometer, two additional photodetectors
are added next to the existing photodetector of a dual-beam laser Doppler
velocimeter. The three photodetectors are spaced apart equally by a small
distance (several mm). When a microparticle passes through the measure-
ment volume, this results in a Doppler burst that lags in time between one
photodetector to the next. This phase lag can be related to the diameter
of the microparticle using light-scattering theory. Thus, a three-component
system can measure