beams at different levels and a corresponding array of photo sensitive detectors arranged on the other door for sensing those light beams thus creating a system which can sense objects of different heights between the door for maximum sensitivity the light beams are generally collimated by a lens to ensure that each sensor is responsive to one light source this arrangement although generally satisfactory has a disadvantage because the light beams are spaced apart from each other objects between the beams are not sensed for instance an arm extended horizontally may fail to be sensed the sensitivity could of course be improved for instance by spacing the parallel beams more closely together in effect providing substantially contiguous light beams but that would be costly to implement and to maintain sensitivity the light beams would need to be highly collimated to prevent stray light from one sensor from reaching an adjacent the wrong sensor as that could blind the sensor the large number of photodetectors and their optical systems for beam collimation that would be needed would of course add significantly to cost mirrors may be used to extend the path length of a light beam from its source so that the beam traverses the door opening a number of times prior to striking a photodetector but while that reduces the number of detectors required it also reduces the intensity of the light beam.