Particle image velocimetry: image labeling by use of adaptive optics to modify the point-spread function

A simple method of labeling the first- and second-exposure images of a particle-image-velocimetry recording is discussed. One achieves this method by changing the transfer characteristics of the recording optics between exposures by inserting a rotating optic into the iris plane of the camera objective. Spatial filtering can then be used to address the images recorded by each exposure independently, permitting cross-correlation analysis to be implemented. Two practical systems using either an aperture plate or a phase plate are demonstrated, and we show that the phase plate is significantly more light efficient. Finally, the feasibility of image labeling high-speed flows using electro-optic devices is discussed.