adjacent light detecting device such as x 39 2 is to sequentially activate light emitting devices and sequentially scan the arrays of phototransistors in other words when a given light emitting device is activated and emits light only the aligned phototransistor is activated although the adjacent phototransistors are illuminated thus activation of light emitting diode x1 would occur at the same time that phototransistor x 39 1 is activated to detect incident light the adjacent phototransistor x 39 2 upon which light from led x1 would also be incident would not be turned on during the interval in which led x1 is activated note that this sequential and synchronized activation of light emitting diodes and phototransistors permits accurate detection of the presence of an opaque element positioned between aligned leds and phototransistors the light from an adjacent led which is not blocked by an opaque element disposed in the field will not activate a 34 blocked 34 phototransistor simply because that led will not be turned on during the activation interval of the 34 blocked 34 phototransistor this sequential activation of light emitting devices and light detecting devices is achieved by conventional multiplexing techniques with scan rates of about twenty scans per second in the preferred embodiment of this invention both the light emitting elements and the light detecting elements are located in a matrix and are individually addressable although this invention is especially important in a touch input system using individually addressable light.