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Large projection screens are increasingly present for everyday use at work. This paper introduces a new reliable system that utilizes spotlight emitted from a laser pointer device, a camera and a projection screen for interaction between human and computer. A camera is placed in a presentation room at a fixed but unknown distance and directed to the projected screen displayed on a wall. Although the locations, orientations and optical parameters of the camera and projected display are unknown, the system self-calibrate by inferring the mapping between the pixels in the camera image to the pixels on the projected screen. The camera is subsequently used to recognize and detect the location of the spotlight on the screen, allowing the laser pointer for substituting the pointing actions of a mouse. The proposed method reliably detects the spotlight in real time and over extremely dynamic backgrounds under varying illumination conditions.