A sensor has been developed and successfully implemented to automate robot calibration. Its simple design makes it ideal for production environments. It works in conjunction with precision spheres that are mounted in the workspace. The relative positions of the spheres are measured in advance, and the sensor is positioned over them automatically and precisely. The positioning of the sensor is governed by three light beams that define position with respect to the sensor. This eliminates the problems experienced by other short range devices in that it acts digitally and without contact. Design of the sensor and its autonomous control from a PC are presented along with proven measurement tactics and results of performance tests.