A new optical theft deterrent device wherein a ray from a light source is divided by a beam splitter mounted on a key into a plurality of light beams which are detected by photosensors to determine authorized use of the key only when predetermined one or ones of the photosensors produce electrical outputs. The beam splitter serves to uniformly divide the ray from the light source into the light beams and also to accurately guide these beams toward the photosensors. The key may be utilized to mechanically and electrically unlock a locking device. The new system may preferably be applied to a steering column shaft locking device to prevent theft of an automotive vehicle on which the system is equipped.