An
exit sign is a device in a public facility (such as a building, airplane or boat) that displays where the
emergency exitAn emergency exit in a structure is a special exit for emergencies such as a fire: the combined use of regular and special exits allows for faster evacuation, while it also provides an alternative if the route to the regular exit is blocked by fire, etc....
is.
Most exit signs around the world are in
pictogramA pictograph is an ideogram that conveys its meaning through its pictorial resemblance to a physical object. Earliest examples of pictographs include ancient or prehistoric drawings or paintings found on rock walls...
form, with or without text supplement. There has been a shift towards the adoption of such exit signs in the recent decade. A small minority of nations adhered to the exit signs that show the word "EXIT" (or similar in another language). Its purpose is guiding people to the closest exit in case of fire or other emergency. The English word "exit" comes directly from the Latin word meaning "(he or she) goes out." Most fire codes require exit signs to be permanently lit.
Since visibility may be reduced in a fire, due to
smokeThis article is about the substance. For other uses, see Smoke .Smoke is the collection of airborne solid and liquid particulates and gases emitted when a material undergoes combustion or pyrolysis, together with the quantity of air that is entrained or otherwise mixed into the mass...
or failure of electric lighting, the sign is often permanently illuminated, usually by one of:
- Radioluminescence
Radioluminescence is the phenomenon by which luminescence is produced in a material by the bombardment of ionizing radiation such as beta particles...
, ('traser'), self-luminous, where a phosphorA phosphor is a substance that exhibits the phenomenon of phosphorescence ....
coating inside a glassIn general Glass refers to a solid, brittle, transparent material, commonly used for windows, bottles, or eyewear. Examples of glassy materials include, but are not limited to, soda-lime glass, borosilicate glass, acrylic glass, sugar glass, Muscovy-glass, or aluminium oxynitride. The term glass...
tube glows due to the beta decayIn nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted. In the case of electron emission, it is referred to as beta minus , while in the case of a positron emission as beta plus...
of radioactive tritiumTritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons...
in the tube. Radioluminescence exit signs are prohibited in US Department of Defense installations
- Phosphorescence
Phosphorescence is a specific type of photoluminescence related to fluorescence. Unlike fluorescence, a phosphorescent material does not immediately re-emit the radiation it absorbs. The slower time scales of the re-emission are associated with "forbidden" energy state transitions in quantum...
('glow in the dark'), Photoluminescent, where light is absorbed from the surroundings and slowly re-emitted.
- Electric light, with a local rechargeable power source
A rechargeable battery is a group of one or more secondary cells. Rechargeable batteries use electrochemical reactions that are electrically reversible. Rechargeable batteries come in many different sizes and use different combinations of chemicals...
.
- Electric light, with the building's emergency lighting circuits providing back-up power
Emergency power systems are a type of system, which may include lighting, generators, fuel cells and other apparatus, to provide backup power resources in a crisis or when regular systems fail. They find uses in a wide variety of settings from residential homes to hospitals, scientific...
from a UPSAn Uninterruptible power supply , also known as a battery backup, provides emergency power and, depending on the topology, line regulation as well to connected equipment by supplying power from a separate source when utility power is not available...
and/or a generatorA motor-generator is a device for converting electrical power to another form. In some contexts, the other form is mechanical energy; in other contexts, it is a different form of electricity. The two senses refer to different types of equipment.The Motor-generator set is used to convert...
in case normal power is lost.
History
Early exit signs were generally either made of metal and lit by a nearby
incandescent light bulbThe incandescent light bulb, incandescent lamp or incandescent light globe is a source of electric light that works by incandescence . An electric current passes through a thin filament, heating it until it produces light...
or were a glass cover that fit directly over a single-bulb
light fixtureA light fixture is an electrical device used to create artificial light or illumination. A luminaire is a lighting fixture complete with the light source or lamp, the reflector for directing the light, an aperture , the outer shell or housing for lamp alignment and protection, an electrical...
. 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.
As
battery-backupA rechargeable battery is a group of one or more secondary cells. Rechargeable batteries use electrochemical reactions that are electrically reversible. Rechargeable batteries come in many different sizes and use different combinations of chemicals...
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 main power and the
batteryAn electrical battery is a combination of one or more electrochemical cells, used to convert stored chemical energy into electrical energy. Since the invention of the first Voltaic pile in 1800 by Alessandro Volta, the battery has become a common power source for many household and industrial...
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 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), appeared somewhat dim. With the development of
fluorescent lampA fluorescent lamp or fluorescent tube is a gas-discharge lamp that uses electricity to excite mercury vapor. The excited mercury atoms produce short-wave ultraviolet light that then causes a phosphor to fluoresce, producing visible light....
and
light-emitting diodeA light-emitting diode , is an electronic light source. The first LED was built in the 1920s by Oleg Vladimirovich Losev, a radio technician who noticed that diodes used in radio receivers emitted light when current was passed through them...
technology, exit signs could be made even brighter to show up in the limited visibility of a fire situation, and/or use less electricity. LED signs work by combining a large number of bright light-emitting diodes to illuminate the sign from inside. Fluorescent bulbs work in the same way as incandescent bulbs, backlighting both sides of an exit fixture from the inside. Because an exit sign is constantly lit, fluorescent bulbs need to be changed more often than LEDs. As a rule, 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 frequent replacement. In recent years, radioluminescent and phosphorescent signs require no electricity have also been developed.
RadioluminescenceRadioluminescence is the phenomenon by which luminescence is produced in a material by the bombardment of ionizing radiation such as beta particles...
uses the radioactive decay of
tritiumTritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons...
to light the sign, while
phosphorescencePhosphorescence is a specific type of photoluminescence related to fluorescence. Unlike fluorescence, a phosphorescent material does not immediately re-emit the radiation it absorbs. The slower time scales of the re-emission are associated with "forbidden" energy state transitions in quantum...
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.
Modern installations
Most exit signs in the world, except in countries such as the United States, Australia, and Singapore are of pictogram type. Australia and Singapore have made changes to their respective life safety codes in the direction of pictogram use. In the United States, New York City has endorsed a new code requiring high rises to have glow in the dark exit signs with a pictogram (ISO).
Modern exit signs in the
United StatesThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
can be seen over every path to an exit in commercial and large residential buildings that comply with fire code. Certain circumstances, such as the year a building was built, may leave it exempt from some of these codes. However, in most situations, the owner of the building complies regarding exit signs. This is especially true in older buildings that serve as multiple residences, such as apartment buildings, hotels, and campus
dormitoriesA dormitory, often shortened to dorm, in the United States is a residence hall consisting of sleeping quarters or entire buildings primarily providing sleeping and residential quarters for large numbers of people, often boarding school, college or university students. In the U.K...
. Modern fixtures are usually in a rugged plastic or metal housing securely bolted to the wall or ceiling. The signs have the word EXIT, or a picture representing exit, on both sides. Single-sided signs are also available for wall-mount installations. The signs often have metal or plastic knock-outs which can be removed so that an arrow is also lit pointing left or right.
Many modern exit signs are combined with other safety devices, such as emergency floodlighting, as shown above. Exit signs draw a relatively small amount of power, and can generally be added onto any existing electrical circuit without adverse effects. Modern exit signs are also, to some degree, flame retardant.
Battery backup
BatteryAn electrical battery is a combination of one or more electrochemical cells, used to convert stored chemical energy into electrical energy. Since the invention of the first Voltaic pile in 1800 by Alessandro Volta, the battery has become a common power source for many household and industrial...
backup systems are the standard way of complying with new building code requiring that exit signs be permanently lit. Early systems were bulky and expensive, and generally unsuited for use with exit signs. Part of the reason for this was that they used lead acid batteries to supply the necessary 120 volts to light an incandescent bulb in the event of a power outage. For comparison, most automobiles use a single lead acid battery in the ignition system. This made early incandescent battery backup systems large, heavy, and costly.
Modern exit signs use much less power. LED, fluorescent, and new incandescent signs only require 3-12 volts to be fully lit, depending on the sign. All battery backed signs include a
transformerA transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core, and thus a varying magnetic field...
, which steps down the main current to charge the battery and operate the sign. In the case of a fluorescent light, the transformer only charges the battery; the bulb requires standard 120-volt current to supply the ballast.
LEDs are
electronicElectronics is a branch of science and technology that deals with the controlled flow of electrons. The ability to control electron flow is usually applied to information handling or device control. Electronics is distinct from electrical science and technology, which deals with the generation,...
diodes, and as such the entire sign can be run directly off of the low voltage power supplied by the battery. Modern incandescent bulbs resemble
flashlightA flashlight is a portable electric spotlight that emits light from a small incandescent lightbulb or from one or more light-emitting diodes...
bulbs, and are extremely bright while only using a few
wattThe watt is a derived unit of power in the International System of Units . It measures rate of energy conversion. One watt is equivalent to 1 joule of energy per second....
s of power. They can also be lit directly from the battery. The primary bulbs in fluorescent signs are high voltage, but the signs include a second set of incandescent bulbs. In the event of a power outage, the sign automatically switches to the lower voltage bulbs. Most battery backup systems use NiCd batteries, which last a few years before needing replacement.
CadmiumCadmium is a chemical element with the symbol Cd and atomic number 48. The soft, bluish-white transition metal is chemically similar to the two other metals in group 12, zinc and mercury. Similar to zinc it prefers oxidation state +2 in most of its compounds and similar to mercury it shows a low...
is a heavy metal and requires proper disposal, so other battery types such as NiMH might be environmentally preferable.
Some manufacturers have replaced batteries with capacitors. This makes the product more environmentally friendly, as it contains only conventional electronics. Capacitor's lifetime is equal to LED's lifetime, eliminating the need for frequent battery change. Typically these types of exit signs use the latest super- or ultracapacitors together with high power LEDs.
Self-luminous exit signs
Self-luminous exit signs do not require an external power source or battery backup. They use a mechanism similar to fluorescent lighting, but powered by
tritiumTritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons...
instead of an electrical current, and are available in models that are rated to last up to 20 years. The
half-lifeHalf-life is the period of time, for a substance undergoing decay, to decrease by half. The name originally was used to describe a characteristic of unstable atoms , but may apply to any quantity which follows a set-rate decay....
of tritium is 12.5 years and is hermetically sealed in a glass tube, and poses little risk of radiation exposure. (One manufacturer claims that if a person were enclosed in a 10' X 10' room and all the tubes on the sign were to break, they would be exposed to an amount of radiation similar to that used in a dental x-ray.) They are prohibited in some areas. Proper disposal of the radioactive material is required, and may be expensive.
Red or green
- In most regions, including the European Union
The European Union is an economic and political union of 27 Member States, located primarily in Europe. Committed to regional integration, the EU was established by the Treaty of Maastricht on 1 November 1993 upon the foundations of the pre-existing European Economic Community...
, AustraliaAustralia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the continental mainland , the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans...
, New ZealandNew Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses , and numerous smaller islands, most notably Stewart Island/Rakiura and the Chatham Islands. The indigenous Māori named New Zealand Aotearoa, commonly translated as The Land of the Long White Cloud...
, Japanis an island country in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, People's Republic of China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
, South KoreaSouth Korea, officially the Republic of Korea and often simply referred to as Korea, is a country in East Asia, located on the southern half of the Korean Peninsula. It is neighbored by China to the west, Japan to the east, and North Korea to the north. Its capital is Seoul, the second largest...
and ChinaChina is a cultural region, an ancient civilization, and, depending on perspective, a national or multinational entity extending over a large area in East Asia....
, exit signs are greenGreen is a color, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 520–570 nanometres. In the subtractive color system, it is not a primary color, but is created out of a mixture of yellow and blue, or yellow and cyan; it is considered...
, because they give the direction to go in an emergency. (In this color scheme, red is used to show prohibited activities).
- In the United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
exit signs can be either red or green. This is due to states or cities enacting building codes which specify the sign color. For example, in Maryland, green is required. New York CityNew York is the most populous city in the United States, and the center of the New York metropolitan area, which is among the most populous urban areas in the world. A leading global city, New York exerts a powerful influence over worldwide commerce, finance, culture, fashion and entertainment...
, Los AngelesLos Ángeles is the capital of the province of Biobío, in the municipality of the same name, in Region VIII , in the center-south of Chile. It is located between the Laja and Biobío rivers. The population is 123.445 inhabitants...
and ChicagoChicago is the largest city in the U.S. state of Illinois, and with more than 2.8 million people, the 3rd largest city in the United States...
require that exit signs have red lettering.
In the Senate Chamber of New Parliament House in Australia the exit signs are red with white writing. This is to match the colour scheme, and is legal because of a special law passed for that purpose.
However, the general
rule of thumbA rule of thumb is a principle with broad application that is not intended to be strictly accurate or reliable for every situation. It is an easily learned and easily applied procedure for approximately calculating or recalling some value, or for making some determination...
is that most newly installed exit signs in Australia are green with white "running man" figure. (AS2293)