Abstract: A method and apparatus for measuring the velocity of moving particles such as red blood cells in a tissue sample is disclosed, characterized by digital processing techniques and autocorrelation. The moving particles are illuminated to produce a spread spectrum optical signal resulting from the Doppler shift occurring when photons are scattered by the moving particles. A spread spectrum electrical signal corresponding with the optical signal and containing spectral and noise components is generated from the optical signal. The electrical signal is filtered to produce the bandpass and DC signals which are subsequently converted to digital form. A first autocorrelation function is calculated from the bandpass signal and a noise autocorrelation function is determined in accordance with the DC signal level. The first and noise autocorrelation functions are compared to produce an autocorrelation function free of a noise component.