Different optical configurations
Reflector, lens, prism or LED optic: four ways to turn a bare light source into a beam of the right direction, spread and colour.
Every fixture has the same optical job: gather the light, set the colour, and send it out as a beam of set direction and spread. The reflector, the Fresnel-type lens, the prism and the lens over each LED are four ways of doing it.
The configuration follows the fixture. Elevated fixtures typically use a stepped lens dome, inset fixtures use a prism that turns the beam to a shallow angle above the pavement, lamp-based optics may use a reflector, and LED fixtures put a small lens over each LED. Whatever the form, the optic also forms the sealed window, so it has to stay clean, sound and in place.
Curved surfaces gather and shape the light: a reflector behind the lamp, or a stepped dome around it.
Angled faces turn the light out at a shallow angle, or a small lens sits over each LED.
| Aspect | Reflector | Fresnel lens | Prism | LED + lens |
|---|---|---|---|---|
| Light source | Filament lamp at the reflector’s focus | Lamp inside the lens dome | Lamp or LED below the prism | LEDs on a board |
| Gathers light | Reflector turns light toward the front | Stepped rings bend the light | Flat faces refract and reflect the light | Small lens over each LED |
| Beam | Concentrated, directed forward | Thin beam near the horizon, round or two-way | Out to the side at a shallow angle | Built from the many LEDs, set by the front optic |
| Colour | Filter in the beam path | Coloured glass, clear one side and coloured the other if needed | Filter or coloured glass, or coloured LEDs | Coloured LEDs, no filter |
| Typical fixtures | Lamp-based fixtures | Elevated fixtures | Inset fixtures | LED fixtures |
| Watch points | Lamp must sit on the focus; dirt and moisture dim and misaim the beam | Cracks, crazing, O-ring and clamp band; correct type and colour, fitted square | Seal and gasket, rubber and dirt, wheel loads and heat; keyed against wrong refit | Lens over each LED intact and clean; check photometrics after any optical change |
What to check on each optic
Clean
Dirt and rubber deposits cut the light that gets out. Clean the lens or prism, and any channel in front of it, as the manufacturer instructs.
Inspect for damage
Look for cracks, chips, crazing, scratches and discolouration. A damaged optic changes the beam and can let water in.
Seals and moisture
Check gaskets, O-rings, clamp bands and retaining rings. Water inside the fixture dims and distorts the light.
Source position
The lamp or LED must sit correctly to the reflector, lens or prism. A lamp off the focus misaims the beam.
Right part and colour
Optics are matched to the fixture and normally keyed. Replace only with the correct type, colour and orientation.
Photometrics
After any optical change, confirm the output still meets the required intensity, colour and beam pattern.
Frequently Asked Questions
Q1What are the main optical configurations in airfield light fixtures?
Four common ones: a lamp at the focus of a reflector, a stepped Fresnel-type lens dome around the source (typical of elevated fixtures), a prism that turns the light out at a shallow angle (typical of inset fixtures), and a small lens over each LED in an LED fixture. Details vary by product.
Q2What is the difference between a lens and a prism?
Both shape the beam and form the sealed window of the fixture. A lens uses curved or stepped surfaces, as in the Fresnel-type dome of an elevated light. A prism uses flat angled faces that bend and reflect the light, as in an inset light where the beam leaves at a shallow angle above the pavement.
Q3Does an LED fixture need a reflector?
Usually not. An LED fixture typically puts a small lens over each LED to gather and shape the light, and the front lens or prism sets the final direction and spread. The exact arrangement depends on the manufacturer.
Q4How is colour produced in each configuration?
Lamp-based optics use a colour filter in the beam path, or coloured glass in the lens or prism, which can be clear on one side and coloured on the other. LED fixtures use coloured LEDs, so no filter is needed.
Q5Why does the optical configuration matter in maintenance?
Dirt, rubber deposits, cracks, moisture inside the fixture or a lamp that is not seated on the reflector's focus can make the light dim, uneven or wrongly aimed. Each configuration has its own parts to clean, inspect and seal.
Q6Can optical parts be swapped between fixture types?
No. Lenses, prisms and reflectors are matched to the fixture, its light source and its photometric approval, and are normally keyed so they cannot be refitted the wrong way. Replace them only with the correct manufacturer part and follow the manufacturer's instructions.
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