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An ultra-high-speed imaging system based on the background-oriented schlieren (BOS) technique has been built in order to capture a laser-induced underwater shock wave. This BOS technique is able to provide two-dimensional density-gradient field of fluid and requires a simple setup. The imaging system consists of an ultra-high-speed video camera, a laser stroboscope, and a patterned background. This system takes images every . Furthermore, since the density change of water disturbed by the shock is exceedingly small, the system has high spatial resolution . Using this BOS system, temporal position of a shock wave is examined. The position agrees well with that measured by conventional shadowgraph, which indicates that the high-speed imaging system can successfully capture the instantaneous position of the underwater shock wave that propagates with the speed of about 1500 …