iPhoneOgraphy – 21 Sep 2016 (Day 265/366)
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.
While exit signs were more visible due to large letters, even a 60-watt bulb shown through a plastic or glass cover (see image), could appear somewhat dim under certain conditions. With the development of fluorescent lamp and light-emitting diode technology, exit signs could be made even brighter to show up in the limited visibility of a fire situation, or use less electricity. LED signs combine a large number of bright light-emitting diodes to illuminate the sign from inside. Fluorescent lamps work in the same way as incandescent bulbs, back-lighting both sides of an exit fixture from within. Because an exit sign is constantly lit, fluorescent bulbs need to be changed more often than LEDs, although the almost nonexistent on/off cycles extend the life of fluorescent lamps significantly. Generally, LEDs have a very long life, and may last for 10 years or more of continuous use, although the brightness may diminish. . Incandescent bulbs are still in use, because they are cheap and common, even though they use more electricity and require more or less frequent replacement. Bulbs lit 24/7 will have a greatly extended lifespan.
Decades ago, radioluminescent and phosphorescent signs that require no electricity have also been developed. These have been around since the 1970s. Radio Luminescence uses the radioactive decay of tritium to light the sign, while phosphorescence uses light-emitting phosphors to glow in the dark. While both of these signs meet California State Fire Marshall standards, where practical, electricity is used in the vast majority of signs.