What is a sign’s reflective / optical system?
The parts that decide how light reaches the viewer. Inside an illuminated sign they reflect and spread the LED light across the face. On a retroreflective sign the face sends an aircraft’s own light straight back.
A sign has to be read from a moving aircraft, at night and in poor visibility. Annex 14 asks for signs to be retroreflective and/or illuminated, and requires illumination where a sign serves runway visual range below 800 m, or night use at instrument runways and at non-instrument runways of code number 3 or 4. The parts that make that work are the sign’s reflective and optical system. There are two different ways of doing it, and they should not be confused.
1Illuminated: reflect, spread, show
In a back-lit sign the LEDs sit behind the face. White or reflective interior walls send stray light forward, and a diffuser between the LEDs and the face spreads it so there are no bright spots behind the legend. The translucent face carries the colour and the legend. In an edge-lit sign a light guide plate and a rear reflector do the same job.
2Retroreflective: return the light
A retroreflective face is covered in sheeting that sends light back toward where it came from. An aircraft’s own lights fall on the sign and return to the pilot, so the legend shows up without a lamp in the sign. It is passive: it needs no power, and it is only as bright as the light that reaches it.
3What to look after
Reflective surfaces, diffusers and sheeting all lose performance as they age or get dirty. Dull walls, a yellowed diffuser, moisture inside the sign or worn sheeting can make the face dim or uneven. Clean and replace these parts only as the manufacturer instructs.
| Element | Detail |
|---|---|
| Back-lit sign | LEDs behind the face, with reflective walls and a diffuser |
| Edge-lit sign | Light guide plate with a reflector behind it (see Entry 22) |
| Reflector | Returns stray light toward the face |
| Diffuser | Spreads LED light so the face looks even |
| Retroreflective face | Sheeting that returns light to its source |
| Power | Illuminated signs need it; a retroreflective face does not |
| Standard | Annex 14 asks for signs to be retroreflective and/or illuminated; illuminated for RVR below 800 m and for night use at instrument runways |
Frequently Asked Questions
Q1What is the reflective or optical system of a sign?
The parts that control how light reaches the viewer. In an illuminated sign they are the reflective interior, a diffuser or a light guide plate. On a retroreflective sign the face itself returns light that falls on it.
Q2What does the reflector do in a back-lit sign?
It sends light that would otherwise be lost back toward the face. More of the LED light reaches the legend, and the glow is more even.
Q3How is a retroreflective sign different from an illuminated one?
A retroreflective face only returns light that falls on it, such as an aircraft's lights, and uses no power. An illuminated sign has its own light source and can be read without outside light.
Q4Why does a sign need a diffuser?
LEDs are small and bright, so without a diffuser they would show as bright spots behind the legend. The diffuser spreads their light so the whole face looks even.
Q5When must an airfield sign be illuminated?
Annex 14 requires signs to be illuminated when they serve runway visual range conditions below 800 m, night operations at instrument runways, or night operations at non-instrument runways of code number 3 or 4. In other cases a retroreflective sign may be enough. Confirm against the edition in force and the national regulator’s rules.
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