A method for tracking the three-dimensional position and computing the three velocity components of a micro-particle using information encoded in the ring structure of the particle image is introduced. The proposed technique can be employed in most existing micro-particle image velocimetry systems without any additional hardware and details of the optical train are not required. The technique utilizes a set of calibration images of a particle at various known distances from the focal plane (in the out-of-plane direction) in order to create a calibration curve. The calibration curve, which relates the radius of the outermost ring to the distance of the particle from focus is used to deduce the out-of-plane location of particles in experimental images. From successive images the particle's velocity is computed. The in-plane particle velocity is deduced from traditional particle tracking methods.
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