What is a unidirectional fixture?
One beam, one direction: a light aimed only at the pilot who needs to see it, with no useful light behind it.
A unidirectional fixture has a single optical unit that sends its beam in one direction only. It is used where just one observer needs the light, so nothing is wasted behind it and nothing shows to the wrong aircraft.
Touchdown zone lights face the approaching aircraft. A stop bar shows red toward the aircraft approaching the holding position, and a runway guard light shows flashing yellow toward the same aircraft. Threshold lights show green toward the approach, and runway end lights show red toward the runway. Because the beam has one aim, orientation is everything: a fixture installed 180° round shows its light away from the aircraft it serves.
One beam, one direction. Aimed at a single approach or holding aircraft.
Two beams, 180° apart. Serves both ends of the runway from one body.
Frequently Asked Questions
Q1What is a unidirectional fixture in airfield ground lighting?
A light fixture with a single optical unit that sends its beam in one direction only. It is aimed at the observer who needs to see it, such as an aircraft on approach or a pilot waiting at a holding position.
Q2Which AGL lights are normally unidirectional?
Touchdown zone lights face the approaching aircraft, stop bars show red toward aircraft approaching the holding position, and runway guard lights show alternately flashing yellow toward the same aircraft. Threshold lights show green toward the approach and runway end lights show red toward the runway; where a threshold and an end light share one body, that fixture is bidirectional (green one way, red the other).
Q3What is the difference between unidirectional and bidirectional fixtures?
A unidirectional fixture has one beam. A bidirectional fixture has two optical units back to back, 180 degrees apart, and serves aircraft from both ends. Choose by who needs to see the light.
Q4Is the back of a unidirectional fixture dark?
Yes. Observers behind the fixture, or well off its beam, should see no useful light. This stops a stop bar or guard light from showing to aircraft it is not meant for.
Q5What happens if a unidirectional fixture is installed facing the wrong way?
The light goes to the wrong observer, or to nobody. A stop bar turned 180 degrees would show red away from the holding aircraft, so that aircraft would see no stop signal at all. Orientation is checked during installation and commissioning.
Q6Where are the aiming and photometric requirements defined?
In the applicable standard (ICAO Annex 14 Vol. I and the Aerodrome Design Manual Part 4, or FAA AC 150/5345-46 and AC 150/5340-30) together with the manufacturer's data. Always confirm against the current edition.
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