Do exit signs need to be self-illuminating?

Employers may use non-illuminated type exit signs as long as they are suitably illuminated by a reliable light source provided at or above five foot candles on the illuminated surface. Electrical power supply from a public utility company is considered to be reliable. The exit sign surface may be passive, electrically internally illuminated, luminescent, fluorescent, reflective, or radioactive isotope self-energized.

Although a glow-in-the-dark sign might be sufficiently illuminated when fully charged, the brightness of these signs may fade in the absence of direct light. Exit signs must be sufficiently lit from within or sufficiently illuminated by external lights at all times, including when the overhead lights have been off for some time. A glow-in-the-dark sign might not sustain the required visibility in the absence of area lighting to keep it charged.