13 has guide end pieces 17 that can engage in slides with which the uprights 18 of the door are fitted when reference is made to fig2 it can be seen that the curtain is fitted with a stay 20 that is bolted into the strut connecting the two stiffeners it should be noted that the curtain 2 has a symmetrical structure and there is the same construction at the other end of the stiffener bars 7 8 the absolutely characteristic point of this curtain is that the stay supports a cell 21 that emits an optical beam 23 that is parallel to and upstream of the ballast and sealing element 13 facing this emitting cell 21 the second supporting stay is fitted for its part with a receiving cell in another possibility one of the stays 20 is fitted with an emitting receiving cell while the other stay simply receives a passive cell for sending back the beam the receiving cell is connected via a wire connection 22 to the control electronic or electromechanical elements of the curtain 2 note furthermore that the ballast and sealing element 13 receives on its face that is opposite the supporting stays 20 at each of its ends a detection flag 24 that consists of an l shaped section the operation of the detection device is therefore as follows when the ballast and sealing element 13 encounters an obstacle during its descent it deforms since it consists for its essential part of a spring and a foam sleeve the deformation of the ballast and sealing element 13 changes.