Laser Doppler velocimetry uses a laser as its light source to measure the velocity of particles, fluids, or moving surfaces.
In a common arrangement, one laser beam is split into two coherent beams that cross inside the flow. Their intersection forms a pattern of bright and dark interference fringes. Small particles carried by the fluid scatter the light as they pass through this measurement volume, and the scattered signal fluctuates at a frequency related to their velocity through the fringes.
The method became practical with the development of reliable lasers, especially helium-neon lasers. A 1962 helium-neon laser produced concentrated continuous-wave light at 632.8 nanometres, supporting early optical flow measurements. Early applications included rocket-exhaust studies and measurements of blood flow; modern systems also examine wind-tunnel flows, machinery, conveyor belts, and microscopic vibrations.
The laser is not merely a bright lamp. Its monochromatic, coherent, and well-collimated light makes the interference geometry and frequency measurement precise. A related technique, laser Doppler vibrometry, measures surface motion rather than fluid velocity, but both rely on laser light and Doppler frequency changes.