An automatic door installation with four traffic sensors mounted on the door jambs on opposite sides of the doorway opening and having radiant energy emitters and reflected energy receivers for sensing doorway traffic. Two sensors are employed to cover the exit or swing side of the doorway, a threshold...http://www.google.com/patents/US4565029?utm_source=gb-gplus-sharePatent US4565029 - Traffic responsive control system for automatic swinging door
