An optical touch screen including a display surface, an optical illumination assembly arranged along at least part of a periphery of the display surface, at least one light detector arranged to detect light distribution produced by the optical illumination assembly and changes in the light distribution produced by the presence of an object in predetermined propinquity to the display surface, detection circuitry receiving at least one output from the at least one light detector and providing an output indication of a two-dimensional location of the object relative to the display surface, calibration functionality responsive to changes in light distribution other than changes produced by the presence of an object in predetermined propinquity to a region of the display surface for calibrating the detection circuitry, at least one auxiliary illumination source, and at least one partially transmissive, partially reflective optical element disposed along a light path to the at least one light detector, providing a light transmissive optical path for light to the at least one light detector and a light reflective optical path for light from the at least one auxiliary illumination source.