What is a back-lit sign?
The light comes from behind. Sources inside the housing shine through a diffuser and a translucent face, so the legend glows straight at the viewer.
The back-lit sign is the direct approach to lighting an airfield sign: put the light sources behind the face and let the light pass through. Annex 14 sets the luminance and colour of an illuminated face, and a back-lit sign meets it by making sure the light arrives at the face evenly, rather than as a row of bright spots.
1Sources behind the face
LED modules, or lamps in older designs, are mounted at the back of the housing and shine forward. They sit behind the face, not at the edges, so the light travels directly toward the pilot.
2Cavity and diffuser
Light crosses the cavity behind the face and spreads as it goes. A diffuser in front scatters it so that no individual source shows through, and the translucent face carries the legend and the background colour.
3What it costs the design
The cavity needs depth, so a back-lit housing is thicker than an edge-lit one. In exchange, the light reaches the face directly and with high output. The edge-lit alternative, covered in Entry 11, trades that depth for a thin sign.
| Element | Detail |
|---|---|
| Light source | LED modules (or lamps) behind the face |
| Cavity | Housing depth behind the face, so the light can spread |
| Diffuser | Spreads the light and hides individual sources |
| Face | Translucent panel carrying the legend |
| Result | Bright, direct illumination; deeper housing |
| Compared with | Edge-lit sign, with sources at the edges and a light guide plate |
| Power | From the series circuit through an isolating transformer and an LED driver |
Frequently Asked Questions
Q1What is a back-lit sign?
An airfield sign whose light sources sit inside the housing behind a translucent face, shining straight through it to light the legend.
Q2Why does a back-lit sign need a diffuser?
Without one, each light source would show as a bright spot on the face. The diffuser spreads the light so the whole face glows evenly.
Q3Why is a back-lit housing deeper than an edge-lit one?
The light sources need space behind the face so their light can spread before it reaches the diffuser. An edge-lit sign puts the sources at the edges and needs no such cavity.
Q4How is a back-lit sign different from an edge-lit sign?
In a back-lit sign the light comes from behind the face, directly. In an edge-lit sign it enters at the edges and a light guide plate spreads it across the face.
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