A method and apparatus for generating a control signal which corresponds to movement of an object in contact with a first surface are described. The apparatus includes a platen having the first surface. The platen is characterized by a critical angle beyond which light incident upon the first surface is not transmitted. A source of electromagnetic radiation emits electromagnetic radiation through the platen thereby illuminating the object. A detector having a plurality of sectors receives a portion of the electromagnetic radiation diffused by the object and transmitted through the platen beyond the critical angle. The sectors of the detector accumulate charge in response to the incident electromagnetic radiation. Conversion circuitry then converts the charge accumulated in the detector to the control signal, which in a specific embodiment is for controlling the movement of a pointer on a display.