This is a technique to determine the local velocity of a flow by fine particles in the air with high temporal resolution and in a non-intrusive way. The particles within the flow pass through interference fringes created by interaction between two coherent laser beams. The wavelengths used are usually in the visible. ln industrial or research applications, it is necessary to have two wavelengths to analyze the particle velocity in two dimensions and even three wavelengths for three dimensions.

The principle of LDV (Laser Doppler Velocimetry) is to create interference between two laser beams with the same properties. For this, a technique proposes to separate a single beam into two coherent beams using a beam splitter. Thanks to transmission optics, the two beams cross at their waists. Their intersection creates interference fringes locally, it is the measurement volume. Through a flow, the particles scroll in this volume and diffuse the light when they are on constructive interferences. The fluctuation of the light scattered by the particles corresponds to the Doppler shift.