Position and function input system for a large area display

An input device for simultaneously entering position and function information into an electronic system having a viewing surface (20) upon which is displayed information generated by the electronic system. A light pen (22) is provided for projecting a light spot onto the viewing surface for indicating the position of a location marker thereon and for indicating a function to be performed. The optical output of the light pen is modified to represent the selected function. A sensor (28) receives the light spot and, in conjunction with discrimination electronics, determines the location of the centroid of the light spot relative to the viewing surface, and determines the function to be performed.