Laser Doppler Velocimetry (LDV) is a technique used to measure the instantaneous velocity of a flow field. This technique, like PIV is non-intrusive and can measure all the three velocity components. A laser light source is an essential part of the LDV. An argon ion or a Helium-Neon laser is used. Other equipments include a photo detector, optical arrangement for laser beam splitting. LDV crosses two beams of collimated, monochromatic, and coherent laser light in the flow of the fluid being measured. To ensure coherency, the two beams are obtained by splitting a beam. The two beams are made to intersect at the focal point of a laser beam, where they interfere and generate a set of straight fringes. The sensor is then aligned to the flow such that the fringes are perpendicular to the flow direction. As particles pass through the fringes, they reflect light (only from the regions of constructive interference) into a photo detector. The velocity can be calculated from the frequency of the signal received at the detector.
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