A light guide has a first surface and a second surface with diffractive structures. A slanted facet is provided at one corner of the light guide. The diffractive structures have concentric fringes centered near the slanted facet. Fringes are arranged such that a light beam directed from the first surface at a location P toward the second surface is diffracted by the fringes only if the beam encounters the diffractive structures at a certain direction. The diffracted beam is guided and exits through the slanted surface at an exiting angle, which is correlated to the location P. If the light guide is illuminated in such a way that when an object is present at the first surface, it changes the light intensity of the exiting light beam. By detecting the exiting angle and the change of light intensity, the location of the object can be determined.