gives direct measurements of flow velocity for liquids
containing suspended particles flowing in a transparent pipe. Light from a laser is
focused by an optical system to a point in the flow, with fibre-optic cables being
commonly used to transmit the light. The movement of particles causes a Doppler
shift of the scattered light and produces a signal in a photodetector that is related to
the fluid velocity. A very wide range of flow velocities between 10 μm/s and 105 m/s
can be measured by this technique.