The primary function of the notification appliance is to alert persons at risk. Several methods are used and documented in industry specifications. Alerting methods generally include sound, light and textual. These methods are intended to trigger a response other than evacuation, such as clean agent warning annunciation, or shelter in place tones.
In 1996, the ANSI and the NFPA recommended a standard evacuation pattern to eliminate confusion. The pattern is uniform without regard to the sound used. This pattern for fire alarm notification is named the Temporal-Three alarm signal, often referred to as "T-3" . (ISO 8201 and ANSI/ASA S3.41 Temporal Pattern) The Temporal Pattern produces an interrupted four count, three half second pulses, followed by a one and one half second pause, repeated for a minimum of 180 seconds.
In the 1980s, most new installations began to include visual signals, and more strobes started to appear. In the United States, the 1990 Americans with Disabilities Act (ADA) 
triggered changes in evacuation signaling methods to include the hearing impaired. Audible notification appliances would now have to include strobe lights to alert the hearing impaired. The 1996 ADA also required that the strobe be at least 15 candelas and have a flash rate of at least 60 flashes a minute. Today, strobe synchronization is often used to synchronize all strobes in a uniform flash pattern. This is to prevent individuals with photosensitive epilepsy from potentially experiencing seizures due to unsynchronized strobes.
