Theoretical and experimental investigations were carried out in respect to the influence of spurious spectra caused by specular reflection in laser Doppler velocimetry. In the measurement of the flow in a thin channel, the multiply reflected beams from channel walls work as incident beams to moving scatterers. The resultant scattered light, whose frequency is shifted by the Doppler effect, causes spurious spectra. The frequencies of the spurious spectra caused by specular reflection are closely related to the geometrical arrangement of the reflecting media such as channel walls. On the other hand, the frequencies are absolutely independent on the position of photodetector and the aperture size of receiving optics. These characteristics are entirely different from the spurious spectra caused by the scattered light from channel walls. The spurious spectra by specular reflection cause the spectral broadening and increase the ambiguity of the Doppler frequency, and its influence cannot be ignored when the receiving optical system is arranged in the backscattering region.
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