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Standard Laser Doppler Anemometer

The crossover area of the two beams shows a fringe pattern. At the bottem of the animation a cross section of this interference field is depicted. Scattering particles that float with the flow (white, typically up to 5 μm) scatter the laser light into all directions. Light received by a photodiode (not shown) is converted into an electrical signal whose time trace is written to the top of the animation. By knowing the fringe distance and the signal frequency the velocity component of the scattering particle perpendicular to the optical axis and in the plane of the beams can be calculated.