What is internal illumination?
A sign that makes its own light. The source sits inside the housing, so the legend is visible whether or not any other light falls on it.
A sign can be made visible in three ways. It can return other people's light, it can be lit from outside, or it can be lit from within. Annex 14 requires airfield signs to be retroreflective and/or illuminated, and calls for illumination where signs are used at night or in low visibility, which is why signs at a lit airfield are usually illuminated, most often from inside.
1Retroreflective and externally lit
A retroreflective sign only sends back light that falls on it, such as an aircraft's own lights, so it is dark when nothing shines on it. An externally lit sign needs a separate floodlight aimed at its face. Both depend on a source outside the sign.
2Lit from inside
An internally illuminated sign carries its source inside the housing, so the face is self-luminous. The pilot sees the legend because the sign itself is lit, and the sign is usually powered from the airfield lighting circuit, typically a series circuit through an isolating transformer, so its brightness follows the circuit's brightness step. Some designs run from a constant-voltage supply instead.
3Two ways to do it
Internal illumination is the principle, not a single design. A back-lit sign puts the sources behind the face, as in Entry 18. An edge-lit sign puts them at the edges of a light guide plate, as in Entry 17. The related entries on LED modules and drivers cover what sits inside either one.
| Element | Detail |
|---|---|
| Light source | Inside the sign housing |
| Legend | Self-luminous face; does not wait for outside light |
| Not reliant on | Aircraft lights or a separate floodlight |
| Designs | Back-lit (sources behind the face) and edge-lit (sources at the edges of a light guide plate) |
| Power | Usually the airfield series circuit via an isolating transformer (constant-voltage on some designs); brightness follows the circuit step |
| Requirement | Annex 14: signs retroreflective and/or illuminated; illuminated for the night and low-visibility uses it names |
Frequently Asked Questions
Q1What is internal illumination on an airfield sign?
Lighting the sign from inside its own housing, so the face is self-luminous and does not depend on other light falling on it.
Q2How is it different from a retroreflective sign?
A retroreflective sign only returns light that falls on it, such as an aircraft's lights. An internally illuminated sign produces its own light.
Q3How is it different from an externally lit sign?
An externally lit sign needs a separate floodlight aimed at its face. An internally illuminated sign carries its light source inside.
Q4Which designs use internal illumination?
Back-lit signs, with sources behind the face, and edge-lit signs, with sources at the edges of a light guide plate. Both are forms of internal illumination.
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