What is a fixture’s optical system?
The source only makes light. The optical system is everything after it: the reflector or lens, the colour filter and the front prism that turn that light into a beam with the right direction, spread and colour.
A runway light is not meant to shine in every direction. It has to be seen from the approach or the runway, at a set brightness and colour, and not wasted elsewhere. That is the job of the optical system, and it is the same job whether the source is a filament lamp or an LED.
1Gather and shape the light
A bare source sends light out in many directions. A reflector or a lens collects it and turns it toward the front. In a lamp-based fixture the lamp usually sits at the focus of a reflector or prism. In an LED fixture a small lens over each LED does the same work.
2Set the colour and the exit
Where the light is not white, a colour filter sits in the beam path, or the LEDs themselves are coloured. The front lens or prism then sets the beam’s direction and spread, and it also forms the sealed window of the fixture. In an inset fixture the prism turns the light to a shallow angle above the pavement.
3Keep it clean, clear and in place
The beam is only as good as the optics. Dirt, rubber deposits, scratches, 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. Clean and replace optical parts only as the manufacturer instructs.
| Element | Detail |
|---|---|
| Job | Turn source light into a beam of set direction, spread and colour |
| Source | Filament lamp or LEDs |
| Gathering | Reflector or prism (lamp-based); lens over each LED (LED) |
| Colour | Colour filter, or coloured LEDs |
| Front | Lens or prism, which also seals the fixture |
| Beam set by | The shape of the optics and how the fixture is aimed |
| Checked against | Photometric (isocandela) requirements |
| See also | Lamp-based runway fixture and LED runway fixture for the two source types; Beam spread for the beam |
Frequently Asked Questions
Q1What is the optical system of a fixture?
The parts that shape the light after the source makes it: a reflector or lens that gathers it, a colour filter or coloured LEDs that set the colour, and a front lens or prism that sets the beam's direction and spread.
Q2Why does a runway light need an optical system?
A bare lamp or LED sends light in many directions, and most of it would be wasted. The optics collect that light and send it into a controlled beam so the pilot sees the light at the right place and brightness.
Q3Is the optical system the same in LED and lamp-based fixtures?
The job is the same, but the parts differ. A lamp-based fixture usually has a reflector or prism around the filament, while an LED fixture usually has a small lens over each LED. Both must meet the same photometric requirements.
Q4What happens if the lens or prism is dirty or damaged?
The fixture can look dim, or its beam can be distorted or pointed wrongly. Dirt, rubber deposits on inset fixtures, scratches, cracks and moisture inside the fixture are the usual causes, so clean and replace parts as the manufacturer instructs.
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