Abstract: A light barrier is restricted to detecting blockage in a limited, constant region before the closing edges of a lift cabin-twin door. Two light emitters or transmitters are displaceably arranged in the door sill and are operatively associated with entrainment elements fastened to the lift cabin doors. The light emitters are synchronously displaced with light receivers mounted at the upper edges of the cabin doors. To increase reliability of operation, the lower edges of the lift cabin doors are provided with cleaning devices which clean the light exit locations of infrared luminescent or light-emitting diodes forming the light emitters during every opening and closing operation. Thus, the cleaning device of one door is arranged to clean the infrared luminescent diode of the light emitter of the other door and vice versa.