A laser Doppler anemometer using photon correlation to measure the velocity of individual particles

A laser Doppler anemometer is described in which a photon correlator is used to measure the velocity of individual scattering particles. The mean velocity and the turbulence intensity found in this way are compared with the values obtained by fitting curves to the autocorrelation function averaged over many particles. The two methods agree to within 1% provided the turbulence intensity is not greater than 10%. At higher levels of turbulence agreement is less good due to various sources of systematic bias in the way the velocity is sampled when individual particle velocities are measured. The method is most likely to be useful when the density of scattering particles, and the intensity of the scattered light are low.