The invention relates to a light pen input system with a light pen which is adapted to generate at least one scanning light line sweep for scanning a surface such as for example a display screen. The time from starting a scan until a light sensing element placed at a known position detects a scanning light line is measured and processed for determining the coordinates of the pointing position of the light pen. Thus, an “inverse” light pen is provided which may be implemented at low costs and which may be used with non-CRT displays. Furthermore, the system is independent of the display screen size.