Influence of the size of scatterers on the recorded frequency in laser Doppler spectroscopy systems

A theoretical analysis is reported of the dependence of the frequency of a signal obtained from a laser Doppler anemometer on the size of scattering particles. This analysis is made for a differential optical system and a noncoherent model is employed. It is shown that in general the Doppler frequency shift is a function of two parameters: the ratio of the size of the scatterers to the spatial period of the interference pattern and the number of interference fringes in the investigated region. It depends also on the nature of the intensity distribution function in the scattering region and on the particle scattering function. The results apply to large scatterers in a small scattering region. Although an increase in the Doppler frequency shift caused by the influence of the finite size of the scatterers is accompanied by a reduction in the depth of modulation of the signal, under normal conditions the measurement error may reach 1–2%.