Caveat Emptor, and Carry a Flashlight?
Photoluminescent (PL) exit signs can be an excellent addition to the life safety outfitting of many facilities. Their ease of application and use lend them to be favored by many building operators and sometimes even required by life safety codes. However, PL signage must be applied and maintained properly and has its limitations. Too often advertising claims gloss over or ignore some very critical points that anyone considering PL signage should take into account as they consider implementing it.
Photoluminescent exit signage marketers often claim the signage requires no electricity to operate because it can charge off ambient light in the space. It is often claimed that this feature further makes PL exit Signage "Green" and a great solution for anyone trying to save energy. This claim is in itself contradictory, as it establishes a need for light in a space to charge the signs and thus some electricity usage. Whether or not something is "Green" is subjective, but claims of zero electricity usage appear rooted in the logic that the lights in the building will be turned on for the occupants, and thus light provided for the PL signage. This claim also ignores the fact that a PL exit sign needs a specific minimum amount of light (lux) hitting it to sufficiently charge the photoluminescent material. If one accepts that many facilities turn out most or all of the lighting at certain times when the building is dormant, the PL signage would be discharging some of the time and not fully charged during and just following this period.
The accepted charge time to reach full performance for most PL signage is one hour. This complicates the safety of having PL signage as the primary exit marking if someone enters the building during normal off time when lights are routinely dimmed or turned off. If an emergency occurred before the PL signage was charged, the safety of those inside would be lessened by some factor. This of course does not take into account that there may not have been sufficient ambient light in the space around the PL exit sign to begin with. If the lights are always on, because of the needs of the PL exit signage, the claims of energy savings may not be realized.
Having established that PL signage needs light (electricity and dollars spent) to charge, the next issue becomes how much. All the various life safety codes require a certain amount of light (lux) hit the PL exit sign to fully charge it, and in some cases require dedicated emergency circuit lighting, thus negating all ambient light only, zero electricity claims. An uneducated purchaser/installer also may not be well versed in determining light levels nor capable of evaluating proper code compliance. PL exit signage installed in a normally lit passageway may not be receiving sufficient light to fully charge and thus not meet code.
Additionally, some life safety codes allow for lower illuminance for PL signs as compared to internally lit signs. For instance, UL 924 requires less luminance by a factor of ten for PL exit signage as compared to internally (battery/electric) lit exit signage. The performance characteristics of the photoluminescent material also contribute to a steep decay in luminance over time. A properly functioning battery operated LED exit sign has little if any charge over the first critical hour of discharge time following a power loss. While properly installed and properly functioning PL exit signage may meet its pertinent safety code standards, it will often be dimmer than a comparable electric (battery) sign. At this time there is a dual standard for exit signage, divided between PL and electric.
These are just some of the concerns regarding photoluminescent signage. As with any matter dealing with life safety, anyone considering using PL exit signage to replace existing internally lit signage would do well to research the practicality and feasibility themselves, and not rely too heavily on manufacturers claims. Further discussion of this subject may be found in the soon to be released NEMA Lighting Systems Division white paper LSD-46.