What is the threshold-light beam?
How a green threshold light aims its beam toward approaching aircraft so the start of the runway is clear from the approach.
The threshold-light beam is the beam pattern of the runway threshold lights, the row of green lights across the runway that marks where the usable runway begins. Pilots see them from the approach, so each light is aimed back toward arriving aircraft. The beam is wide in azimuth, so the whole row can be seen even when the aircraft is not exactly on the centreline, and its intensity is kept in a band of low vertical angles.
The green function is unidirectional: it faces aircraft approaching the runway. A single fixture is often bidirectional, with green toward the approach and red toward the runway, so it also acts as the runway end light for aircraft travelling the other way. Threshold wing bars, where provided, stand on either side of the row and have their own isocandela diagram in Annex 14.
This follows the earlier beam entries: the beam direction (Entry 26) points toward the approach, the spread (Entry 27) is wide, and the complete distribution (Entry 28) is what the isocandela diagram for threshold lights describes. Compared with the edge-light beam (Entry 36), the threshold beam is aimed one way and shows a distinct colour. A tilted fixture or a faded green lens makes the threshold harder to recognise from the approach.
Frequently Asked Questions
Q1What is the threshold-light beam?
The beam pattern of a runway threshold light: how it is aimed and shaped so that the green threshold row is seen by pilots on the approach, marking where the usable runway begins.
Q2Are threshold lights unidirectional?
The green threshold function is unidirectional: it faces aircraft approaching the runway. The same fixture is often bidirectional, showing red toward the runway as the runway end light for aircraft travelling the other way.
Q3Why is the threshold-light beam wide in azimuth?
Because the row marks the full width of the runway and must be seen from the approach, including when the aircraft is off to one side during the final approach. The beam is shaped to cover a wide range of horizontal angles, with the exact pattern set by the standard.
Q4What can change the beam of a threshold light in service?
Tilt or rotation after impact or jet blast, a dirty or damaged lens, a faded or incorrect green filter, vegetation or snow blocking the view, and degraded light sources. A misaimed threshold light can look dim or off-colour from the approach.
Q5Where is the threshold-light beam defined?
In the isocandela diagrams of ICAO Annex 14, Appendix 2 for threshold, threshold wing bar and runway end lights, and in FAA AC 150/5345-46 and the fixture manufacturer's photometric data. Always confirm against the current edition.
See It. Understand It. Then Learn How to Engineer It.
Recognising an AGL system visually is the first step. The next step is knowing how to design, install, test, commission and maintain them correctly.
Build Practical AGL Engineering Skills
Go deeper with structured training from Precision AGL Academy, covering:
Learn how each light, sign and circuit works as part of a complete airport lighting system, not just as an isolated diagram.