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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).

One interpretation of the differential Doppler laser technique for measuring flow velocity is that real fringes are formed in the flow region. Particles crossing alternate bright and dark interference fringes produce a modulation of the scattered radiation. Measurement of the modulation frequency determines the flow velocity. The fringe image technique is similar in principle except that it uses localized fringes formed by the optical image of a grating (e.g., a Ronchi grating) illuminated with an incandescent source.

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