NFPA 101
7.10.1.2.1* Exits, other than main exterior exit doors that obviously and clearly are identifiable as exits, shall be marked by an approved sign that is readily visible from any direction of exit access.
7.10.5 Illumination of Signs.
7.10.5.1* General. Every sign required by 7.10.1.2, 7.10.1.5, or 7.10.8.1, other than where operations or processes require low lighting levels, shall be suitably illuminated by a reliable light source. Externally and internally illuminated signs shall be legible in both the normal and emergency lighting mode.
7.10.5.2* Continuous Illumination.
7.10.5.2.1 Every sign required to be illuminated by 7.10.6.3, 7.10.7, and 7.10.8.1 shall be continuously illuminated as required under the provisions of Section 7.8, unless otherwise provided in 7.10.5.2.2.
7.10.5.2.2* Illumination for signs shall be permitted to flash on and off upon activation of the fire alarm system.
It goes on further about internally/externally illuminated signs, but it's all in NFPA 101, Chpt 7.