From the world's tallest skyscrapers to underground subways facilities, safety features and directional aids that many of us take for granted are exit signs. Every exit and every staircase entrance has such a sign near or above a doorway. In fact, in a modern 100 storey skyscraper there can be as many as ten to twenty signs per floor, or one thousand to two thousand throughout an entire building!
In the United States and Canada the common design of an exit sign is backlit red text encased by a white border, although colour variations include green text. Exit sign designs more common throughout Europe and Asia portray an illustration of a person walking through a door. The colour green is used in order to differentiate warning or prohibited activity signs coloured red. This pictorial design is beginning to appear more often in Canada and has been used alongside traditional white and red text-based signs in New York City.
Positioned near or above building exits and emergency exits, the exit sign is designed to appear unmistakable to all occupants of public buildings and direct individuals towards the exact location of an exit. Even in low light conditions or during electrical failures, modern exit signs are designed to shine brightly in low visibility conditions (such as areas with heavy smoke) and remain fully lit through the use of batteries or diesel powered generator backup power supplies. However, the most high-tech energy source for exit signs that does away with having to wire exit signs to backup power generator circuits or ensure backup batteries are in working order, relies on tritium, a substance that essentially allows exit signs to remain self-luminous. Tritium is a radioactive isotope of hydrogen and has a half-life of nearly twelve-and-a-half years, meaning an exit sign powered by tritium can remain fully lit for approximately twenty years. These signs are restricted in some cases due to their radioactive power source (although the radioactivity taking place is extremely low and well within accepted levels for use in public spaces).
Regardless of the energy source, to conserve energy and in particular to provide extra battery life during periods of electrical failure resulting in battery backups, LED bulbs in exit signs have grown in popularity due to their extremely efficient power consumption relative to light output. LED bulbs allow exit signs to remain on for significantly longer periods than would be possible through the use of other types of bulbs during electrical failures relying on battery backups.