“Laser Doppler velocimetry” is an optical method of measuring local fluid velocity (velocity at-a-point). In this method velocity is inferred from the Doppler shift of laser light scattered by small (~ 1 micron) particles moving with the fluid. Highly sensitive measurement of the Doppler shift is accomplished by the process of optical heterodyne mixing in which the frequency-shifted light is combined on a photodetector with unshifted light to produce a photo signal that oscillates at the difference of the two frequencies. The special advantages of the method are that it does not require calibration, it responds uniquely to a single component of the velocity vector, it can distinguish between positive and negative velocities, it has a good accuracy (~ 0.1%), and it can be used in a variety of transparent fluids. One, two or three components of the velocity can be measured at a common point.

F. Durst, A. Melling and J. H. Whitelaw, Principles and Practice of Laser-Doppler Anemometry, Academic, New York, 1976.

R. J. Adrian, editor, Selected Papers on Laser Doppler Velocimetry, SPIE Milestone Series,  MS78, SPIE Optical Engineering Press, Bellingham, Washington, 1993