An optical control system is described, which employs a series of optical emitters and detectors arranged about a touch area. The emitters are driven by a series of orthogonal functions, and the system is operable to correlate the signals received at the detectors with the modulation functions to determine the amount of energy received by the detectors from each of the emitters. This system means that all or many emitters can be modulated simultaneously, so the scanning process is accelerated. Also, the energy estimates arrived at by correlation with the modulating functions are largely unaffected by external interference signals as well by each other. An additional benefit of the present invention is that the correlation processing makes use of many detector samples and tends to yield results which have higher resolution than the sample values themselves, due to the averaging of noise over the sample set.