The invention relates to an optical-signal based control surface for a data processing system, the surface being designed as a planar optical fiber, on which photoelectric sensors are provided, the measured signals of which are used to deduce the position of a light signal striking the control surface and the position being associated with a processing mark in the data processing system. A layer having photoluminescent properties is in contact with at least one layer of the planar optical fiber. The light of the light beam shining on the control surface is converted into longer-wave light by way of photoluminescence, the light being coupled into the waveguide, diffusing therein, and being measured by the photoelectric sensors.