Abstract: A method and a device for controlling a data processing system is provided. A light beam is emitted from a pointing device to a control surface which is equipped with one or more optical position detectors connected to the data processing system. The data processing system is influenced in accordance with the impact point of the light beam on the control surface. In order to be able to input different characters using the pointing device independently from where the light beam emitted by the pointing device is currently pointing, the light intensity emitted by the pointing device to the control surface oscillates in a pulse sequence characteristic for individual characters. The optical position detector is a flat luminescence optical wave guide with local photoelectric sensors enabling it to achieve the required high resolution. In a further embodiment, the identity of a pointing device is coded in the pulse sequence, enabling a clear distinction of several input devices.