The velocity measurement of phase objects

Conventional laser velocimeters rely upon small contaminant dust particles to scatter the laser light. A technique is described which requires no contaminant particles but relies upon minute refractive index variations in the fluid. By placing the detector far from the measuring region a set of shadow bands is produced which is convected with the fluid. The velocity is measured by either correlating the light transmitted by two slits or by measuring the frequency of the light transmitted by a grating.