Abstract: An input device which can be employed to input commands to a computer directly through a CRT display is disclosed. The input device employs a plurality of infrared light emitting diodes and phototransistors aligned in individual emitter-detector pairs. Infrared beams from the emitter cross the display field before striking the corresponding detector. Emitters and detectors are sequentially activated and each emitter is pulsed or modulated at a frequency different from the variation of ambient conditions during each emitter activation interval. The detection circuitry thus can distinguish the varying emitter signal from ambient or spurious variations. A number of pulses at the pulsed or modulated emitter frequency would be detected unless an opaque element were blocking the particular beam.