The measured velocity profile across a diameter of the pipe can be integrated with respect to cross sectional area to obtain the volumetric flow rate. Our LDV system operates on the Differential Doppler principle, also called the Dual Beam arrangement. The key components are a pair of crossed laser beams produced by splitting the beam from a single laser.
Coherent beams have a sharp phase difference that is uniform over time.
This distinct and continuing phase relationship allows the interaction between the beams to be persistent enough to be observed as interference effects. In the picture above, the crossed beams are clearly visible in the water filled pipe. The region where the beams cross is called the measurement volume. The velocity of the particles in the water is only measured inside this measurement volume. This arrangement allows for precise positioning of the measurement volume anywhere inside the transparent water pipe.