Developing a 3-D Imaging Systemto Trace the Path of a Rising Bubble
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A single CCD camera conventionally provides a 2-D image of a moving particle. Studying rising bubbles in fluids, however, necessitates a 3-D imaging system in order to extract information such as velocity and path of the bubble. Using a CCD camera, we develop a 3-D imaging system based on an approach introduced by C. Willert and M. Gharib . The shutter of a Pulnix-9701 camera is replaced by a metal piece containing three 1mm diameter holes arranged in an equilateral triangular pattern. Using such a modified system to image a defocused single point, we get a resulting picture consisting of 3 blurred images of the original point in an equilateral triangular pattern. The location (x0, y0) of the center of the image triangle is related to the X, Y coordinates of the point; the side B of the triangle is inversely related to the distance Z of the point from the camera. To determine the precise relations a series of calibrations is performed. Thus, the obtained images can be used to extract the 3-D coordinates of the original point. In addition, to tailor such a system to image rising bubbles, a controlling program is developed in CVI 5.0 to (1) take and save a sequence of images of the rising bubble, and (2) trigger a second camera which provides images of the bubble shape. Reference: C. Willert and M. Gharib, "Three-dimensional particle imaging with a single camera", Experiments in Fluids, Spring 1992.