An elevator system has a pair of horizontally opposed vertical surfaces defining therebetween a doorway or opening to an elevator cage and a door moving horizontally to open and close the opening. In particular, the system has a first optical device having a light emitter for emitting light and a second optical device having a light receiver for receiving the light emitted from the light emitter. The first and second optical devices are positioned in a vertical plane crossing the opening and adjacent to the opening. One of the first and second optical devices is positioned below the other of the first and second optical devices and mounted in the vertical surface.