An exit sign is a graphic device that indicates or identifies an escape route or the location of, or direction to, an exit or the closest emergency exit in the event of fire or other emergency that requires evacuation. Exit pathways and exit access doors must be clearly identified with exit signs that are designed to be absolutely unmistakable and understandable to anyone. Exit signs are increasingly in pictogram form and may be supplemented with the word “EXIT” and/or the equivalent in the local language. Exit signs generally have green lettering to imply a safe place of exit. In the United States, however, the signs are either red or green in color depending on enacted building codes of a state or city. The signage must be visible from any direction of egress travel at all times. Typically, a minimum luminance of greater than 2cd/m2 is required and the ratio of maximum to minimum luminance in any part of the pictogram should be less than 10:1. The ratio of luminance of the background color to the contrast color must be no less than 15:1 and no more than 5:1.
Types of exit signs
Exit signs fall into two general types: self-luminous exit signs and illuminated exit signs. Self-luminous exit signs are either radio-luminescent or phosphorescent signs that use no power and are commonly installed in locations where electrical power is not conveniently accessible. Radioluminescence uses low-energy beta particles emitted by the tritium to bombard the phosphor, causing it to glow. Phosphorescence uses light-emitting pigments which absorb energy from an electromagnetic radiation source and reemit it for a much longer time after the exciting radiation has been removed. The luminance of the exit sign is a crucial determining factor for visibility: the higher the sign luminance, the better the sign visibility. The luminance of self-luminous exit signs, however, is limited and may not meet building codes. Illuminated exit signs provide a far higher luminance level, which can ensure optimal perceptibility even in haze or smoke that may be involved in an emergency. Illuminated exit signs rely upon the use of electrical power. An externally illuminated sign has an externally mounted light source and makes use of reflected light for visibility. An internally illuminated sign contains its own light source and provides a more effective approach of emphasizing the way out. The majority of exit signs are therefore internally illuminated.
In the past, incandescent, halogen lamps, and fluorescent/compact fluorescent lamps had been in their turns to provide a source of light for illuminated exit signs. Conventional lighting technologies suffer from a number of limitations, including high operating voltages, high energy consumption, bulky source size, fragile construction, and short lifespan. These limitations lead to the high operating and maintenance costs of the installation. Excessive power consumption places high demand on the battery capacity which should maintain operation of the exit lights during a power failure for a period no less than 90 minutes. Today, exit signs are illuminated by light emitting diodes (LEDs) which use semiconductor electroluminescence to produce visible light for illumination. LEDs boast an unusually high conversion efficiency and have such long lifespans that they are virtually maintenance-free. Lighting attributes, such as color temperature, color rendering, optical distribution, and intensity level can be controlled and optimized for the application and the occupants in the space.
Exit signs must be mounted in a way that no point in the egress pathway is more than 100 feet in distance from the nearest visible exit sign, which means exit signs often need to visible from up to 100 feet. LED exit signs could be made even brighter to compensate for the limited visibility in a smoke-filled atmosphere without excessive power consumption. The high efficacy of LEDs ensures that the connected loads don’t over-discharge the battery and makes it possible to provide longer periods of emergency lighting or to significantly reduce the physical volume of the battery bank. Direct integration of the compact LEDs allows to design products for architectural applications where a pleasing aesthetic and a slim profile are desired.
Internally illuminated exit signs come in various forms: edge-lit, matrix, panel, and stencil. Both matrix exit signs and stencil exit signs have luminous lettering and pictogram while the background is opaque. However, the illuminated lettering and pictogram in matrix exit signs formed by points of light provided by LED matrices. A panel exit sign has a conventional configuration in which both the sign and the background are luminous. In an edge-lit sign, light from an enclosed, side-firing LED module is directed by total internal reflection (TIR) through a transparent light guide panel (LGP). The LGP has lettering and pictogram etched in or attached to its surface, which disturb the TIR and allow the light to escape through the exit surface. Line-powered exit signs are either equipped with a local rechargeable power source or connected to building’s emergency lighting circuits which provide the exit sign backup power from a UPS or a generator during power outages.