Section 27-385. Exit sign design

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,
  non-radioactive and if subjected to fire must be self-extinguishing when
  the flame is removed.