Effect of wall scattering on SNR in laser Doppler anemometry

A method, based on geometrical optics and diffraction theory of Gaussian beams, is presented and describes the image formation of scattered light in laser Doppler anemometry. The intensity distributions of the images of scattered light from both the control volume and a nearby transparent wall are given as a function of distance between the control volume and the wall, collecting lens aperture radius, magnification, incident beam radius, and beam angle. The effect of the scattered light from the wall surface on the Doppler SNR is discussed. The minimum distance close to the wall for which a good signal quality can be obtained is defined and quantitatively determined. The results show that this distance is strongly affected by the aperture size of the collecting lens and beam angle.