Coherence concepts in holographic particle image velocimetry
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A special holographic configuration has been introduced for the sheet-wise reconstruction of sample areas in a particle image field that is reconstructed for particle image velocimetry analysis. A distinct feature is the suppression of light from most of the out-of-focus region of the flow volume. This is realised by light-in-flight holography, where the illuminating laser light of short coherence length allows the discrimination of depth regions by selection of the position of the reconstruction aperture on the hologram. The basic features of the method are outlined, and results are given for flow visualisation in a free nozzle stream and for velocimetry in a wind tunnel vortex street and free nozzle jets. The method is supplemented by electro-optic switching for separate images and cross-correlation evaluation.