Emergency Lighting Illumination Requirements
The NFPA (National Fire Protection Agency) defines the methods of illumination for exit signs, as follows below:
1. Externally illuminated—the light source is contained outside of the device or component that is going to be illuminated. The light source is typically a dedicated incandescent or fluorescent source.
2. Internally illuminated—the light source is contained inside the device or component that is illuminated. The light source is typically incandescent, fluorescent, electro luminescent, light-emitting diodes, or photoluminescent.
3. Photoluminescent—the emergency lighting sign is illuminated by self-contained power sources and operates independently of external power sources. NOTE: Batteries do not qualify as a self-contained power source.
The photoluminescent sign must be continually illuminated while the building is occupied. The exit sign shall be illuminated according to its listing. The charging light must be of the same type that is specified on the sign.
Photoluminescent signs need a specific type of light, as well as a minimum level of light to assure the sign will be able to provide adequate illumination in an emergency. During illumination, the sign should not be connected to an automatic timer.
The NFPA recognizes photoluminescent exit signs as an alternative to internally lighted electric powered signs. A photoluminescent sign absorbs light from an activation light source to emit light. The sign continues to emit light for a time after the activation light source has been removed.
Photoluminescent exit signs are in the category of an internally illuminated sign. The signs are permitted, provided they are listed in accordance with the UL924 standard for safety emergency lighting and power equipment, and if they meet all the requirements outlined in the criteria from its specified section in the Life Safety Code.
Every required exit sign must be suitably illuminated by a reliable light source and be visible in both normal and emergency lighting scenarios. The illuminated surface value of the exit sign should have a value of at least five feet-candles. There is a similar requirement for internally and externally illuminated signs, with exceptions made for approved self-luminous signs that provide evenly illuminated letters. These materials must have a minimum luminescence of 0.06 foot lambert as measured by a color-corrected photometer.