An apparatus such as a touch screen display includes correlated emitter-detector pairs for determining the amount of electromagnetic radiation received at a detector from its paired emitter. The apparatus comprises an array of emitters operable to emit electromagnetic radiation, an array of detectors for detecting the electromagnetic radiation, and drive control circuitry configured to control the array of emitters so that they emit pulses of electromagnetic radiation. The amplitude of the pulses is modulated using mutually orthogonal binary vectors such as a Paley construction of the Hadamard matrix. Each emitter has a different vector associated with it. Detection circuitry is provided to detect the electromagnetic radiation reaching each particular detector in the array of detectors using the mutually orthogonal binary vector associated with the correlated emitter to produce a correlation between particular emitter-detector pairs. The arrangement can discriminate between near field and touching objects.