Exit signs shall read only "exit" and shall be of the externally lighted, internally lighted, or electroluminescent type, except that they may be nonilluminated in buildings not provided with artificial lighting.
(a) The artificial light source on externally lighted signs shall provide a red light, either by the use of an incandescent colored bulb or other visible red light source, so as to provide at least twenty-five foot candles on the exposed face of the sign. Visibility of the sign shall not be obscured by the location of the light source.
(b) For internally lighted signs, the average initial brightness of the letters shall be at least twenty-five ft. lamberts, and where an illuminated background is used, its average initial brightness shall be at least two hundred fifty ft. lamberts. The light source shall not be modified or changed nor shall lamp life multipliers be used so as to reduce these brightness levels.
(c) The letters of exit signs shall be red. The background of externally lighted signs shall be white. The background of internally lighted signs shall be either stenciled metal with a light gray or white color, or translucent frosted, opal glass, slow-burning plastic, or the plastic edge-glow type with white plastic separators. The letters for internally lighted signs shall be translucent red.
(d) The letters shall be block lettering at least four and one-half inches high with nine-sixteenths inch strokes, except in buildings and spaces classified in occupancy group F and J-1, where they shall be at least eight inches high with the strokes at least three-quarters of an inch wide.
(e) In locations where breakage may occur, exit signs shall be of shock resistant materials, or shall otherwise be protected against breakage.
(f) Except for buildings not provided with artificial lighting and buildings which maintain one or more auxiliary systems for emergency exit lighting in the event of a public utility failure, there shall be either (1) an illuminated exit sign with the background thereon made of an approved phosphorescent material or (2) a supplemental exit sign made of an approved phosphorescent material with an opaque text and placed adjacent to or as close as possible to such illuminated sign. The phosphorescent material after exposure to normal lighting conditions shall be capable of remaining visible in total darkness for a period of at least eight hours. The signs shall be washable, non-toxic, nonradioactive and if subjected to fire must be self-extinguishing when the flame is removed.