SNR in optical velocimeters: effect of particle number density.

Measurements of SNR as a function of number density (n) were carried out employing a Differential Heterodyne Laser Doppler Velocimeter (DLDV) and a White Light Fringe Image Velocimeter (WFIV). These two techniques made it possible to investigate separately the coherent and incoherent scattering modes, respectively. It was observed that with the WFIV the SNR is proportional to n for n values corresponding to less than one particle in the probe volume. The SNR remains constant for large n values. In the case of the DLDV the SNR was always found to be proportional to n when the coherence condition was satisfied. Consequently, it is concluded that the DLDV signal results entirely from coherent scattering, even when the number of particles in the probe volume is as low as 0.02 (the lower limit of the measurements).