The international distinctive sign shall be as large as appropriate under the circumstances. The distinctive sign shall, whenever possible, be displayed on flat surfaces or on flags visible from as many directions and from as far away as possible. Subject to the instructions of the competent authority, civil defence personnel shall, as far as possible, wear headgear and clothing bearing the international distinctive sign. At night or when visibility is reduced, the sign may be lighted or illuminated; it may also be made of materials rendering it recognisable by technical means of detection.