What is beam spread?
How wide an airfield light spreads its beam around its axis. It decides how far off-axis a pilot can be and still see the light.
Beam spread is the angular width of a fixture's beam. Every light has a peak intensity on its axis, and intensity falls away on either side. Spread is usually taken as the angle between the two points where intensity has dropped to half of the peak, quoted separately in the horizontal and vertical planes. Some documents use a different fraction, so check the definition in the one you are working from.
Spread is a trade-off. A narrow beam concentrates the light into a small angle, giving a higher peak intensity on the axis but little off it. A wide beam gives lower intensity on the axis and keeps the light visible to observers further off the axis. Each type of light is seen from a different position relative to the fixture, so the required coverage differs between light types. The values are set by the isocandela diagrams for that light.
Beam direction (Entry 32) is where the axis points; spread is how wide the beam is around it. The two work together: the narrower the beam, the less tolerance there is for a fixture that is misaligned, and the more it matters that orientation is checked at installation and commissioning.
Frequently Asked Questions
Q1What is beam spread in airfield ground lighting?
The angular width of a fixture's beam: how far either side of the beam axis the light stays useful. It is described separately in the horizontal and vertical planes.
Q2How is beam spread measured?
As the angle between the points on either side of the beam axis where intensity falls to a stated fraction of the peak, commonly 50 percent. Horizontal and vertical spread are quoted separately, and the fraction used should be checked in whichever document you are working from.
Q3Why do different runway lights have different beam spreads?
Each type of light is seen from a different place relative to the fixture. A wider spread gives coverage to observers well off the axis, while a narrower spread concentrates more intensity along the axis. The required coverage for each light type is set by the standard.
Q4What is the difference between beam direction and beam spread?
Beam direction is where the beam axis points. Beam spread is how wide the beam is around that axis. A narrow beam that is misaimed can miss the observer, which is why alignment is checked at commissioning.
Q5Where are the required beam values defined?
In the isocandela diagrams of ICAO Annex 14, Appendix 2 for each light type, and in FAA AC 150/5345-46 and the fixture manufacturer's photometric data. Always confirm against the current edition.
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