Photorefractive Particle Image Velocimetry
Optical signal processing techniques are attractive when the input is a two-dimensional transparency. With the help of the photorefractive effect, this processing is achieved quite easily in real-time using holographic schemes. We have recently demonstrated particle image velocimetry (or PIV) over a large area of a transparency in parallel using a photorefractive correlator.1 In such an optical system, the input is a double exposure photograph of a particle flow. The autocorrelation function of this flow is carried out in a two-step procedure. First, the Fourier transform of an area of the transparency and a plane reference wave induce a phase grating in a photorefractive BSO crystal. The reference wave is then switched off and the hologram is read out with the Fourier transformed object beam itself to yield autocorrelation on the output screen. The result consists of two side-peaks and a central spot in the case of a uniform particle velocity. The distance between the peaks and their relative orientation determine the velocity and direction of the flow.
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