Exit signs were generally either made of metal and lit by a nearby incandescent light bulb or were a glass cover that fit directly over a single-bulb light fixture. The inherent flaws with these designs were that, in a fire, the power to the light often failed. In addition, the fixtures were small and dim, making them hard to spot in a fire where smoke often reduced visibility. Better signs were soon developed that more resembled today’s modern exit sign, with an incandescent bulb inside a rectangular-shaped box that backlit the word “EXIT” on both sides. Being larger than its predecessors, this version of the exit sign solved some of the visibility problem. The sign was only useful as long as main power remained on.

Photoluminescent Exit Signs became smaller and more efficient, some exit signs began to use a dual-power system. Under normal conditions, the exit sign was lit by main power and the battery was in a charge state. In the event of a power outage, the battery would supply power to light the sign. It continued to discharge until main power returned to the unit or the battery was no longer able to provide sufficient power to light the sign. Early battery-backup systems were big, heavy, and costly. Modern systems are lightweight, can be installed virtually anywhere, and are integrated into the fixture, rather than requiring a separate box. As Self Luminous Exit Signs improved, so did the amount of time that a fixture could remain lit on batteries.