emitting receiving cell cooperating with a passive beam return cell can also be considered several emitting cell 8 receiving cell 12 couples can also be considered in order to increase the width of the door 1 the detecting means also comprise at least one detecting flag 14 for each emitting cell 8 receiving cell 12 couple the detecting flag 14 can be constituted by an l profile it is supported here by the ballasting and sealing element 6 the operation of the detecting device is as follows when the ballasting and sealing element 6 encounters an obstacle during its descent it deforms as it is constituted for its essential part by a spring 16 and a foam sheath 18 the deformation of the ballasting and sealing element 6 modifies the base of the detecting flag 14 which cuts off the beam 10 which is normally parallel to the ballasting and sealing element 6 the beam 10 being cutoff a signal is sent to the control electronics or electromechanics of the door 1 which then gives the order on the one hand to stop the closing of the curtain 2 and on the other hand to re open it as is visible in fig1 to 4 the emitting cell 8 and the receiving cell 12 are each supported by a prop 20 each prop 20 is.