Exit sign

An exit sign is a device in a public facility (such as a building, aircraft or boat) denoting the location of the closest emergency exit in case of fire or other emergency.

Most relevant codes (fire, building, health or safety) require exit signs to be permanently lit.

Exit signs are designed to be absolutely unmistakable and understandable to anyone. In the past this generally meant exit signs that show the word "EXIT" or the equivalent in the local language, but increasingly exit signs around the world are in pictogram form, with or without text supplement.

Early exit signs were generally either made of metal and lit by a nearby incandescent light bulb or were a white glass cover with EXIT written in red 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 could be difficult to see in a fire where smoke often reduced visibility, despite being relatively bright. The biggest problem was the exit sign being hardly distinguishable from an ordinary safety lighting fixture commonly installed above doors in the past. The problem was partially solved by using red-tinted globes instead.

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 one or 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.

As battery-backup systems became smaller and more efficient, some exit signs began to use a dual-power system. Under normal conditions, the exit sign was lit by mains 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 mains 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 batteries improved, so did the amount of time that a fixture could remain lit on batteries.

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Wikipedia