An Optical Flow technique based o n t h e u s e o f D y n a m i c Programming has been applied to Particle Image Velocimetry yielding a signi cant increase in the accuracy and spatial resolution of the velocity eld. Results are presented for calibrated synthetic sequences of images and for sequences of real images taken for a thermally driven ow of water with a freezing front. The accuracy remains better than 0.5 pixel/frame for tested twoimage sequences and 0.2 pixel/frame for four-image sequences, even with a 10% added noise level and allowing 10% of particles to appear or disappear. A velocity vector is obtained for every pixel of the image.

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