We developed a technique for measuring instantaneous, three-dimensional velocity components in liquid flows by using a digital image processing system, and here we describe a particle tracking velocimery technique improving the tracking of the initial and terminal positions of tracer particles. By using color cameras and a RGB flashlight source three images of the tracer particles are obtained: one at their initial positions; their ter-minal positions, and along their pathline. The images of the initial and the terminal posi-tions are paired by using the pathline image, and the velocity vectors are calculated. We studied image processing algorithms for obtaining three-dimensional-flow vectors from the images produced by the two cameras and for distinguishing duplicate images of the tracer particles. We confirmed this technique is useful for three-dimensional flow observa-tion.
