An emergency light is a battery-backed lighting device that switches on automatically when a building experiences a power outage. In the United States, emergency lights are standard in new commercial and high occupancy residential buildings, such as college dormitories, apartments, and hotels. Most building codes in the US require that they be installed in older buildings as well. Incandescent light bulbs were originally used in emergency lights, before fluorescent lights and later light-emitting diodes (LEDs) superseded them in the 21st century.