In order to control the light or air permeability of an elastic extensible screen (1) disposed between an area to be protected and an external area, and having a permeability that varies with a pulling force exerted on the screen, a motorized mechanism (3) adjusts the pulling force on the screen in response to a setpoint signal, which can be determined on the basis of one or a plurality of signals from one or a plurality of sensors (12, 14, 15), in particular light sensors.