UnidirectionalVSBidirectional
Unidirectional runway lights produce their required light output in one specified direction. Bidirectional runway lights produce their required light output in two opposite directions. The main difference is the number of operational viewing directions served by the fixture — this is not a colour classification and is not the same as omnidirectional.
At a Glance
| Feature | Unidirectional Light | Bidirectional Light |
|---|---|---|
| Light output direction | One direction | Two opposite directions |
| Pilot visibility | From one intended direction | From both intended directions |
| Optical system | Single directional beam | Two directional beams/optics |
| Fixture complexity | Generally lower | Generally higher |
| Typical application | Direction-specific guidance | Two-direction runway operations |
| Installation | One optical orientation | Two opposite optical orientations |
| Colour output | One colour, set by the light's function | Same or different colour on each face |
| Examples | Runway threshold light (green); runway end light (red) | Runway edge light (e.g. white/white); threshold/end combination light (green/red) |
Unidirectional Runway Light
A unidirectional light is designed to be seen primarily from one specified direction. From the opposite direction, the fixture does not provide the same intended signal. Its purpose is to provide visual guidance for a particular direction of operation.
Light output is projected toward one required direction only.
- Single optical system aimed at one required direction
- Correct aiming is essential — incorrect orientation can leave the beam unseen by approaching aircraft
- Generally simpler fixture design
- Used where visual guidance applies only to aircraft moving or approaching from one direction, for example ICAO runway threshold lights (green) and runway end lights (red)
Bidirectional Runway Light
A bidirectional light provides light output in two opposite directions. Pilots approaching from either operational direction can receive the required indication from the same fixture location.
A single fixture produces compliant output toward two opposite directions.
- Two optical arrangements within one fixture, aimed at opposite directions
- Both directional outputs must be correctly aligned during installation
- Generally more complex than a unidirectional fixture
- Used where a runway or lighting position must provide information to aircraft operating in opposite directions, from one location, instead of installing two separate fixtures
The Core Distinction
Unidirectional = one viewing direction. Bidirectional = two opposite viewing directions. The distinction is purely about the number of intended operational directions the fixture serves — not brightness, not colour, and not fixture family.
A Clear Bidirectional Example
The runway threshold/end combination light is one of the clearest examples of a bidirectional runway light: green faces aircraft on approach and marks the threshold, while red faces the runway and marks the runway end for aircraft rolling toward that end. The same physical fixture therefore serves two directional functions.
Standards note: ICAO Annex 14 specifies threshold lights (green) and runway end lights (red) as separate unidirectional functions. A combined fitting is a bidirectional fixture that delivers both from one position, which is only possible where the threshold and the runway end coincide.
Bidirectional does not automatically mean two different colours. Depending on the application, both directions may show the same colour (e.g. runway edge lights: white ← light → white) — it simply means the fixture provides specified output in two opposite directions.
Where Direction Requirements Come From
The direction and colour required at each position are set by the light's function in the governing standard, not by the fixture family. The table shows how this plays out on a runway in ICAO Annex 14 terms.
| Runway light | Direction / colour requirement (ICAO Annex 14) | Typical fixture arrangement |
|---|---|---|
| Runway edge | White; visible at the angles in azimuth needed to guide a pilot landing or taking off in either direction, and at all angles where circling guidance is intended | Bidirectional (e.g. white/white) or omnidirectional, depending on intensity class and specification |
| Edge – caution zone | Last 600 m or one-third of the runway length, whichever is less, at the remote end from the take-off start, may show yellow | Bidirectional white/yellow: the yellow face is seen by aircraft rolling toward that end; the opposite face stays white |
| Edge – displaced threshold | Lights between the runway start and the displaced threshold show red in the approach direction | Bidirectional red/white |
| Threshold / wing bar | Fixed unidirectional, green, in the direction of approach | Unidirectional green, or the green face of a combined threshold/end fitting |
| Runway end | Fixed unidirectional, red, in the direction of the runway | Unidirectional red, or the red face of a combined threshold/end fitting |
| Runway centreline | White; alternating red and white from 900 m to 300 m before the runway end (on runways under 1,800 m, from the mid-point); red in the last 300 m | Typically bidirectional in the red/white sections near each end, where the two directions need different colours |
FAA equivalent: AC 150/5345-46 lists L-861 runway edge fixtures as omnidirectional or bidirectional, L-862 as bidirectional, and L-861E/L-862E threshold/end fixtures as bidirectional red/green or red/red (or unidirectional green). Check the current edition of each document before specifying.
Engineering Comparison
| Parameter | Unidirectional | Bidirectional |
|---|---|---|
| Number of intended directions | 1 | 2 |
| Optical outputs | One | Two |
| Fixture orientation | One direction | Two opposite directions |
| Optical complexity | Generally lower | Generally higher |
| Installation alignment | Single beam alignment | Both beams must comply |
| Reverse-direction signal | Not intended | Provided |
| Fixture functions | Usually one directional function | May perform two directional functions |
| Example | Threshold light or runway end light | Threshold/end combination fixture; runway edge light |
Where Each Is Used
Unidirectional
Used where lighting is intended for one operational direction, such as direction-specific visual guidance.
- Runway threshold and wing bar lights (green)
- Runway end lights (red)
Bidirectional
Used where aircraft operating in opposite directions require visual guidance from the same fixture location.
- Threshold/end combination lights (green/red)
- Runway edge lights (white/white, white/yellow, red/white)
- Runway centreline lights near the runway ends (different colour each way)
The exact application depends on the lighting system design and governing standard.
Bidirectional Does Not Mean Omnidirectional
These terms should not be confused with each other.
Unidirectional
One direction only.
Bidirectional
Two opposite directions.
Omnidirectional
All horizontal directions (360° in azimuth).
Omnidirectional is a real fixture category, not just a term to avoid: some medium-intensity runway edge lights and blue taxiway edge lights are omnidirectional. The three terms simply describe how many directions a fixture is designed to serve.
Bidirectional ≠ omnidirectional. A bidirectional fixture serves exactly two opposite directions, not all directions.
Myths vs Facts
No. Bidirectional refers specifically to two opposite intended directions, not all-around (omnidirectional) visibility.
Not necessarily. Both directions may show the same colour depending on the application — the defining feature is the two directions, not the colours used.
The specified beam and colour apply only in the intended direction. Some stray light outside that beam can exist, but it is not part of the specified signal and is not what pilots should rely on for guidance.
Not always. Depending on intensity class and specification, edge fixtures may be bidirectional or omnidirectional. The requirement is guidance for pilots landing or taking off in either direction.
Unidirectional vs Bidirectional
The choice depends on whether the runway lighting system must provide guidance in one operational direction or in two opposite directions. In practice the governing standard fixes the direction and colour required at each position; the designer then specifies a certified fixture type that delivers it.
Easy memory: UNI = ONE. BI = TWO. Unidirectional → one way. Bidirectional → two ways (not all ways).
Engineering & Expert Notes
Orientation Is Part of the Installation Check
OrientationA unidirectional light aimed the wrong way fails its purpose. Verify direction during installation and after replacement.
Check Both Outputs on Bidirectional Fixtures
VerificationEach side must meet its requirement. Test and record both.
Keep Spares Matched to the Fixture Type
SparesMixing types in stores risks fitting the wrong one. Label clearly.
Record Direction and Colour at Each Position
RecordsRegisters should show which way each light faces and what it displays.
Frequently Asked Questions
The number of intended viewing directions. A unidirectional light provides its required output in one direction; a bidirectional light provides required output in two opposite directions.
No. That would be omnidirectional. Bidirectional means two opposite intended directions only.
Yes. A combined threshold/end fitting shows green toward aircraft on approach (threshold) and red toward the runway (runway end). ICAO Annex 14 specifies threshold lights and runway end lights as separate unidirectional functions; the combined fitting delivers both from one position.
Neither by definition. ICAO Annex 14 requires edge lights to be visible at the angles needed to guide a pilot landing or taking off in either direction, and at all angles in azimuth where circling guidance is intended. In practice the fixtures are bidirectional (for example white/white, or white/yellow in the caution zone) or omnidirectional, depending on the intensity class and specification.
No. Many show the same colour both ways, such as white/white runway edge lights. Others show different colours, such as green/red threshold/end lights or white/yellow caution-zone edge lights. The defining feature is the two opposite directions, not the colours.
One fixture can deliver both required outputs from a single installed position, so fewer fixtures and installation points are needed than with two back-to-back unidirectional fixtures. The trade-off is a more complex fixture whose two optical outputs must both be correctly aligned.
The required beam may not reach approaching aircraft from the intended direction, so correct aiming during installation, and re-checking after maintenance, is essential.
Governing Documents
For an actual project design, always verify the current edition of the applicable documents. This page is an educational reference and does not replace the governing standard or the project specification.
FAA AC 150/5345-46 and AC 150/5340-30
AC 150/5345-46 (Specification for Runway, Taxiway, Heliport and Vertiport Light Fixtures) defines fixture types, light direction (omni-, uni- or bidirectional), colours and photometric requirements. AC 150/5340-30 (Design and Installation Details for Airport Visual Aids) covers layout and installation.
ICAO Annex 14, Volume I
Aerodrome Design and Operations. Chapter 5 (Visual Aids for Navigation) sets direction, colour and intensity requirements for edge, threshold, runway end and centreline lights; Appendices 1 and 2 give colours and light characteristics. Guidance: Doc 9157, Aerodrome Design Manual, Part 4 (Visual Aids).
DGCA / AAI Requirements
For Indian airport projects, also verify DGCA CAR Section 4, Series B, Part I (Aerodrome Design and Operations), AAI requirements, airport-specific technical specifications, project drawings and contract specifications.
Final Engineering Note
Do not assume directionality from colour or fixture family alone. Verify the specified photometric output, intended direction(s), colour requirements and installation orientation against the applicable current governing standard.
Unidirectional → light output in one required direction. Bidirectional → light output in two opposite required directions, not all directions.
Page last updated: 20 September 2026.
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