What is a lens?
The clear or coloured dome in front of the light source. Its stepped surface shapes the light into a beam, its glass sets the colour, and it seals the head of the fixture.
A bare lamp lights the sky as much as the pavement. A pilot needs the light concentrated near the horizon and in the right colour. In an elevated light the lens does both, and it also keeps the weather out.
1It shapes the light
The lens is usually a glass dome with a surface made of rings or steps, called a Fresnel-type lens. The rings bend the light like a thick curved lens would, but in a thin shape. They squeeze it into a thin beam near the horizon, spread all round (omnidirectional) or in two directions (bidirectional). The inset equivalent is the prism, in Entry 6.
2It colours and protects
In many elevated fixtures the glass itself is coloured, so the lens sets the signal colour (in some LED fixtures the colour comes from the LEDs and the lens is clear), and one fixture can be clear on one side and coloured on the other. The lens is sealed to the body with an O-ring and a clamp band or similar. It is usually glass, and some fixtures offer a tough plastic version that is less likely to break if the fixture is knocked over.
3Keep it sound, and keep it right
Clean the lens, and look for cracks, crazing, discolouration, a damaged O-ring, a loose clamp band and water inside. The lens and the light source must stay in the right position to each other, and the lens must be the correct type and colour, so replace it only with the correct part, refitted square. Opening the fixture head on a series circuit is maintenance work: isolate and lock out first (see Safe Maintenance Sequence), and let a lamp cool before handling it.
| Element | Detail |
|---|---|
| Lens | Clear or coloured dome in front of the light source |
| Material | Usually glass; some fixtures offer tough polycarbonate |
| Surface | Stepped rings (Fresnel-type) that bend light into a beam |
| Colour | Often the glass itself; may be split, such as clear and yellow |
| Seal | O-ring and clamp band, or similar, keep water out |
| Direction | All round, or two beams; set by the lens and the fixture alignment |
| Light source | Lamp or LEDs inside the dome |
Frequently Asked Questions
Q1What is the lens of an airfield light?
The clear or coloured part in front of the light source. In an elevated light it is usually a glass dome with a stepped, Fresnel-type surface that shapes the light into a beam and also seals the head of the fixture.
Q2Why does a lens have rings or steps?
A Fresnel-type lens uses a series of rings or steps to bend light like a thick curved lens, but in a thin, light shape. The rings gather the light and send it out near the horizon, where pilots see it.
Q3How does the lens set the colour of a light?
In many elevated fixtures the glass itself is coloured, for example blue, green, red or yellow, and a lens can be clear on one side and coloured on the other. Fitting a lens of the wrong colour gives the wrong signal, so always use the correct part.
Q4What can go wrong with a lens?
Dirt cuts the output, and a crack, craze, discolouration, damaged O-ring or loose clamp band can let in water or change the beam. Clean and inspect the lens as the manufacturer instructs, and replace it with the correct type and colour.
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