Exit signs stay continuously illuminated through either hardwired power with battery backup or photoluminescent glow, so the route stays visible when normal lights fail.
When the power cuts and the hallway goes dark, the exit sign above the door still points the way out. How do exit signs work so reliably? The answer comes down to two main designs — electrically powered units with emergency backup, and photoluminescent signs that charge during normal operation. Both exist to satisfy the same code requirement: marking the path of egress so occupants can find their way out in an emergency.
Electrically Powered Exit Signs
These are the classic red or green signs in every commercial building. Hardwired into the building’s electrical system, they stay lit whenever normal power flows. Most are equipped with a battery backup that automatically kicks in during an outage. NFPA 101, the Life Safety Code governing egress in most occupancies, requires backup-powered emergency lighting to last at least 90 minutes (1-1/2 hours) after loss of normal power. These signs illuminate the word “EXIT” from within using LEDs or incandescent bulbs so lettering remains visible without any outside light source. They are the most straightforward option: plug in, test, and they work whenever needed.
Photoluminescent Exit Signs
Newer buildings sometimes use photoluminescent signs — pale green or white panels during the day that glow in the dark when lights go out. They contain a strontium-aluminate pigment that absorbs ambient light during normal occupancy. When surrounding light disappears, the stored energy re-emits as a soft glow, making the “EXIT” legend visible in the dark. The catch? They only work if charged by continuous light during the day, and their visibility depends on the rated viewing distance of the specific product. Per guidance reviewed by the National Fire Protection Association, photoluminescent signs must be listed to UL 924 and placed so no point in an exit-access corridor is farther than the sign’s rated viewing distance, or 100 feet, whichever is less.
What the Code Actually Requires
Building codes and OSHA regulations spell out exactly how exit signs must perform. NFPA 101 requires that exits, other than obvious main exterior doors, be marked by an approved sign that is readily visible from any direction of exit access. For externally illuminated signs, the surface must receive at least 54 lux (5 foot-candles) of light with a contrast ratio of no less than 0.5. OSHA’s interpretation of 29 CFR 1910.37 similarly cites a 5 foot-candle value for the sign face. Lettering must be legible at a distance: new signs need letters at least 6 inches high with strokes at least 3/4 inch wide. Exit signs aren’t the same as emergency lights — signs mark the route, while separate emergency lighting illuminates the path itself at a minimum of 1 foot-candle average along the egress way.
| Requirement | Code Value | Notes |
|---|---|---|
| Emergency runtime | 90 minutes minimum | Per NFPA 101 after power loss |
| Sign face illumination | 5 foot-candles | Required by OSHA and NFPA for reliable light sources |
| Letter height | 6 inches minimum | For new signs per NFPA 101 |
| Viewing distance | 100 feet maximum | Or the sign’s rated viewing distance, whichever is less |
| Inspection interval | 30 days maximum | Visual check of illuminated exit markings |
Routine maintenance matters as much as the initial install. Visual inspections of illuminated exit markings should happen at intervals no longer than 30 days. For battery-backed units, confirm they still switch to backup and hold for the full 90 minutes when tested. For photoluminescent signs, check that nothing blocks the ambient light charging them. If you’re weighing options for a remodel or new build, our comparison of the best backlit exit signs lays out which models hold up best over years of code inspections.
One common mistake is assuming an obvious exterior door needs no sign at all — the code exception applies only to main exterior doors clearly identifiable as exits. Another is letting decorations or bright displays near the sign block the view or wash out its illumination. The most frequent error? Treating exit signs as a substitute for emergency lighting. You need both: one marks the way, the other lights the path.
FAQs
Are battery backup exit signs required in every building?
Most commercial buildings need exit signs with some form of backup power — batteries or an emergency generator — so the sign stays lit during an outage. The requirement appears in NFPA 101 and applies broadly across assembly, educational, hotel, mercantile, and business occupancies where egress paths must remain marked for at least 90 minutes after power loss.
Can exit signs use regular light bulbs instead of LEDs?
Yes, incandescent bulbs are still permitted, but LEDs dominate new installations because they draw far less power and last much longer. A battery backup lasts longer during an outage with LEDs, since the lower draw stretches the same battery over a longer runtime. Whatever bulb type you choose, the sign must still meet the 5 foot-candle illumination face requirement.
How often should exit signs be tested?
References & Sources
- National Fire Protection Association. “NFPA 101 Signage and Illumination Requirements.” NFPA Journal article covering illumination levels, letter sizing, and the 90-minute emergency duration baseline.
- OSHA. “Standard Interpretation: Exit Sign Illumination Requirements.” Official interpretation of 29 CFR 1910.37(q)(6) and (q)(7) citing 5 foot-candles on the sign face.
