The described invention makes use of at least two screens which have complementary transparent and opaque patterns and which are relatively movable towards and away from each other so that when adjacent they obturate radiation and when separated they pass it. Thus radiation, for example upon a plant-cultivation area or into a green house, is controllable. Selective filtration may be used to control selected parts of the spectrum. The operation is preferably by inflating and deflating spaces between the screens at least one of which is pliable.