Measuring the velocity of particles in a fluid, or in a moving liquid or solid surface

摘要

Measuring the velocity of particles in a fluid flow or in a moving liquid or solid surface, comprises directing at the moving item (1) a pair of light beams (2) spaced apart by a predetermined distance, the beams carrying respectively first and second modulations and being directed in a direction generally transverse to the direction of flow; detecting at a common detector means (3) light reflected from or scattered by the particles, liquid or surface, passing through the two beams to produce a detector signal; separating said detector signal into a first signal carrying the first modulation and a second signal carrying the second modulation; comparing the pattern of variations in amplitude in the first and second signals to establish a correlation between the patterns and measuring the temporal displacement in the correlation; and calculating from the temporal displacement and the known distance between the beams the velocity of the particles, liquid or surface.