What is an optical filter?
A piece of coloured glass or coating that lets the wanted colour through and absorbs the rest.
An optical filter is a piece of coloured glass or a coloured coating in the path of the light. It lets the wanted colour through and absorbs the rest, so the white light from a lamp comes out as aviation red, green, blue or yellow.
1The filter picks the colour
Every ray that reaches the filter is treated the same way: it passes only a narrow band of the light and absorbs everything else. The colour that results must stay within the colour limits set in the standards. Some fixtures colour the lens or prism glass instead, and many LED fixtures make their colour directly with coloured LEDs, so not every fixture has a separate filter.
2The cost is lost light and heat
The light a filter absorbs does not vanish: it warms the filter. It also means a coloured beam is weaker than a white beam from the same lamp, and the fixture design allows for this. A filter has to withstand that heat, day after day, without fading or cracking.
3Keep it the right colour, intact and clean
The usual faults are fading, cracks, dirt, moisture and the wrong filter. A wrong colour is a safety fault, because the colour is the message. Replace a filter only with the type the manufacturer specifies, and check the colour and the light output afterwards.
| Part | What it does | Main risk |
|---|---|---|
| Colour | Passes only the wanted colour | Fading shifts the colour |
| Absorption | Removes the unwanted light as heat | Heat can crack the filter |
| Fit | Sits in the path of all the light | A chip or gap leaks off-colour light |
| Surface | Lets the coloured light through | Dirt or moisture dims the beam |
Frequently Asked Questions
Q1What is an optical filter in an airfield light fixture?
An optical filter is a piece of coloured glass, or a coloured coating, placed in the path of the light. It lets the wanted colour through and absorbs the rest, so white light from a lamp comes out as aviation red, green, blue or yellow.
Q2Why do airfield lights need a filter?
A lamp gives white light, but airfield lighting uses colour to tell the pilot what each light means. A filter gives the lamp's light its colour. Because it absorbs part of the light, a coloured beam from a filtered lamp is weaker than a white beam from the same lamp, and the fixture design allows for this.
Q3Do all airfield light fixtures use a filter?
No. Some fixtures colour the lens or prism glass itself, and many LED fixtures make their colour directly with coloured LEDs, so they may have no separate filter. Check the product design to see where the colour comes from.
Q4What happens when a filter fades, cracks or is the wrong one?
A faded or discoloured filter shifts the colour of the light, and a cracked or chipped one can let off-colour light leak out. Either way the light may fall outside the colour limits in the standards. A filter of the wrong colour sends the pilot the wrong message, so it is a safety fault, not just a cosmetic one.
Q5How should a filter be checked or replaced?
Only as the manufacturer's instructions allow, and only with the filter type the manufacturer specifies. Check its colour, its condition and its fit, and clean it gently without abrasives or harsh cleaners. Do not open a sealed optical chamber outside a controlled procedure, and test the colour and output of the light afterwards.
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