What is a prism?
The shaped glass block in front of the light source. Its angled faces bend the light into a beam, and in an inset light it is also the window that takes the wheel loads.
A lamp or LED sends light in many directions. A pilot needs it in one: a beam of set direction and spread, low over the pavement. The prism is the part that does that shaping.
1It bends the light
A prism has flat, angled faces. Light is bent, or refracted, as it enters and leaves the glass, and some is turned inside the glass by reflection. Together these turn the source’s light into a beam. In an inset fixture the prism sends the light out to the side at a shallow angle above the pavement. A lens does a similar job with curved surfaces.
2It is also the window
In an inset light the prism sits at the top of the body, flush or nearly flush with the pavement. It lets the beam out and closes the top, seated on a gasket or O-ring and held by a retaining ring and bolts, so it must stay sealed and carry wheel loads and heat. Colour comes from a filter (often dichroic), coloured glass or coloured LEDs. Direction comes from the prism shape and the fixture’s alignment, and a bidirectional light has a prism at each end while a unidirectional one usually blanks the other aperture.
3Keep it clean and sound
Dirt and rubber deposits cut the light that gets out, and a crack, chip or failed seal lets in water and changes the beam. Clean the prism, and any channel in front of it, and inspect it as the manufacturer instructs. Optical parts are normally keyed so they cannot be refitted the wrong way, so replace a damaged prism only with the correct part.
| Element | Detail |
|---|---|
| Prism | Shaped glass block that bends light into a beam |
| Material | Usually glass; the maker specifies the type |
| Light source | Lamp or LEDs below the prism |
| Gasket or O-ring | Seals the prism to the body |
| Retaining ring and bolts | Hold the prism and seal in place |
| Colour | Set by a filter (often dichroic), coloured glass or coloured LEDs |
| Direction | Set by the prism shape and alignment; an unused aperture is blanked |
Frequently Asked Questions
Q1What is the prism in an airfield light?
A shaped block of clear material, usually glass, in front of the light source. Its angled faces bend the light so it leaves the fixture as a beam of the right direction and spread.
Q2Is a prism the same as a lens?
Both bend light by refraction. A lens has curved surfaces, while a prism has flat, angled ones. Many fixtures use one or the other, and some use both, and the shape is chosen for the beam the fixture needs.
Q3Why is the prism also the window of an inset light?
In an inset light the prism sits at the top, flush or nearly flush with the pavement. It lets the beam out and closes the top of the fixture, so it must be sealed against water and strong enough for wheel loads.
Q4What problems can affect a prism?
Dirt and rubber deposits cut the output, and a crack, chip or failed seal lets in water and changes the beam. Clean it and inspect it as the manufacturer instructs, and replace a damaged one with the correct part.
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