Internal illumination means the light is built into the sign. External illumination means a separate fixture shines onto the sign. Halo lighting puts the light behind raised letters so it glows on the wall around them. These words describe how a sign is lit; the local code decides which ones are allowed. The ISA glossary has the industry definitions.
How one real code handles it
Richmond Heights, Ohio makes external lighting the default in section 1179.08(a) of its sign chapter. For signs that are allowed to be lit, the external lighting must meet these conditions:
“A. Only direct lighting from an external source shall be used to illuminate the sign.
B. The source of light shall not be visible from the street or adjacent property.”
The next clause says “No variances to this Section 1179.08(a)(1) shall be sought or granted,” so there is no asking your way around it. Internal lighting is allowed only in nonresidential districts and certain residential parcels, and only as reverse pan channel or halo-lit letters, or as a cabinet with a solid opaque background and knock-out copy. Electronic message signs have their own section. In practice, a standard translucent-face box sign is off the table there, and so is any fixture whose light source can be seen from the street. That shapes the fabrication order before anyone draws a letter. (Ord. 3-2006, passed February 28, 2006. We read it on September 28, 2026.)
Read the unit with the number
Luminance is light coming off a surface; illuminance is light landing on a surface. NIST explains the difference. A limit on sign-face brightness and a light reading at the property line are two different tests, so keep the unit, the measuring spot and the method together in the spec.
Use the illumination checklist and the brightness measurement guide before choosing fixtures or programming a display.