Abstract: A projection system includes an image projector, an invisible light transmitter and an invisible light sensor. The image projector is used for projecting a projection image on a physical plane. The invisible light transmitter is used for generating an invisible light plane, which is parallel with the physical plane. The invisible light sensor is in communication with the image projector. When a pointing object is placed on a touching point, an invisible light beam reflected from the pointing object is received by the invisible light sensor. According to the invisible light beam, a sensing signal indicative of a spatial coordinate position of the touching point is acquired and transmitted to the image projector. The image projector recognizes and calculates the spatial coordinate position of the touching point according to the sensing signal and performs a controlling action according to the spatial coordinate position.