Airfield Ground Lighting (AGL) systems are designed to provide precise visual guidance to pilots during taxiing, landing, and takeoff. However, the effectiveness of these lights is not determined simply by whether they turn on or off. What truly matters is whether the light performs exactly as required for safe aircraft operations.
This is where the concept of the 4Cs of AGL Photometry becomes important. These four parameters — Colour, Candela, Coverage, and Configuration — evaluate whether an AGL fitting delivers the correct visual signal to pilots, and together they determine whether a lighting system is compliant with aviation standards and effective in real operational conditions.
A note on terminology: the “4Cs” is a practical teaching framework used in this article, not a term defined by ICAO or the FAA. The underlying requirements come from ICAO Annex 14 Volume I (Chapter 5, with Appendix 1 for colours and Appendix 2 for light characteristics), the FAA AC 150/5345 equipment specifications, and the national regulator’s rules (in India, the DGCA aerodrome requirements). Always check the current official edition.
The 4Cs transform airfield lighting from simple illumination into precise visual guidance for pilots — Colour, Candela, Coverage, and Configuration together, not any single parameter alone.
Colour – Correct Visual Identification
Colour is one of the most important visual signals in airfield lighting. Different colours are used to convey specific information to pilots:
Photometric testing verifies that each light produces chromaticity coordinates inside the colour boundaries defined in aviation standards (ICAO Annex 14 Appendix 1; FAA AC 150/5345-46 for fixtures). Colour is tested at every brightness step, because the filtered light from an incandescent or halogen lamp shifts towards red/yellow as the lamp is dimmed, whereas LED fittings are generally more stable but must still be verified.
If the colour shifts outside the allowed limits — through filter ageing, lamp or LED degradation, or the wrong filter being fitted — pilots may misinterpret the signal, especially during night or low-visibility operations.
Candela (Intensity) – Adequate Light Output
Candela measures luminous intensity: the light emitted in a particular direction. It is not the same as total light output (lumens) or the illuminance on a surface (lux). In AGL systems, the intensity in each direction must be carefully controlled.
- Too low: the light may appear dim or difficult to see
- Too high: it may cause glare and reduce pilot visibility
International standards such as ICAO Annex 14 (Appendix 2) and the FAA specifications define the required intensity for each type of airfield light, mainly as isocandela diagrams giving minimum intensities at defined angles and limits on how intensity varies across the main beam. Photometric measurements, taken with the fitting at its rated current and at each brightness step, confirm that it produces the required intensity at those angles.
Compliance at the time of testing is not the end of the story: lamp or LED ageing, dirt, water ingress and lens damage all reduce in-service intensity, which is why routine measurement and maintenance limits matter.
This ensures that pilots receive a clear and comfortable visual signal, even during challenging environmental conditions.
Coverage – Correct Beam Orientation
Coverage refers to the direction and spread of the light beam emitted by the fitting. Airfield lights are designed to direct light toward specific viewing angles that correspond to a pilot's line of sight during aircraft operations:
| Fitting Type | Coverage Requirement |
|---|---|
| Runway edge lights | Visible from approach and runway roll angles |
| Taxiway lights | Guidance along the taxi route, with beam spread suited to the type (edge, centreline, curves) |
| Approach lights | Aimed (vertical angle and alignment) so the beam is seen along the aircraft descent path |
| In-pavement centreline and touchdown zone lights | Beam aligned along the runway direction, visible from the approach and roll-out |
Photometric testing verifies the horizontal and vertical beam distribution to ensure that the light reaches the pilot's viewing position without unnecessary light spill or loss of intensity.
Two checks are involved. The laboratory test confirms that the fitting’s beam pattern is correct; the field check confirms that the installed fitting is aimed correctly. A fully compliant fitting that is rotated or tilted out of position after installation or maintenance will still send the wrong signal to the pilot.
Proper beam coverage ensures that the lighting system performs exactly as intended in real operational scenarios.
Configuration – Correct Placement and System Layout
Configuration refers to how individual lights are arranged and installed across the airfield. Even if each fitting performs correctly on its own, improper placement or spacing can reduce the effectiveness of the entire lighting system. Configuration checks ensure:
Unlike colour, candela and beam coverage, configuration is not measured in a photometric laboratory. It is verified by survey, inspection against the approved layout, and flight inspection.
When the configuration is correct, the lighting system appears uniform and predictable to pilots.
The 4Cs Framework at a Glance
Colour → Candela → Coverage → Configuration
Conclusion
The performance of an Airfield Ground Lighting system cannot be evaluated by brightness alone. The true effectiveness of the system depends on the combined performance of the 4Cs: Colour, Candela, Coverage, and Configuration.
These parameters ensure that every AGL fitting delivers the correct visual signal, in the correct direction, with the correct intensity, and at the correct location. The 4Cs framework therefore provides a practical way to evaluate AGL photometric performance and understand why these parameters are essential for aviation safety and regulatory compliance.
The 4Cs transform airfield lighting from simple illumination into precise visual guidance for pilots.
Frequently Asked Questions
What are the 4Cs of AGL photometry?
The 4Cs are Colour, Candela, Coverage and Configuration. They are a practical teaching framework, not a term defined by ICAO or the FAA, and together they evaluate whether an AGL fitting delivers the correct visual signal to pilots. The underlying requirements come from ICAO Annex 14 Volume I (Chapter 5, Appendix 1 for colours and Appendix 2 for light characteristics), the FAA AC 150/5345 equipment specifications and the national regulator's rules.
Why does colour matter in airfield lighting?
Different colours convey specific information to pilots, such as white for runway edge and centreline lights, green for threshold, red for runway end and stop bars, blue for taxiway edge and yellow for caution zones. Photometric testing verifies that chromaticity falls inside the Annex 14 Appendix 1 colour boundaries, at every brightness step. If colour shifts through filter ageing, lamp or LED degradation or the wrong filter, pilots may misinterpret the signal.
What does candela measure in AGL?
Candela measures luminous intensity, the light emitted in a particular direction. It is not the same as total light output (lumens) or illuminance on a surface (lux). If intensity is too low the light may appear dim, and if it is too high it may cause glare, so standards define it mainly as isocandela diagrams with minimum intensities at defined angles.
What does coverage mean in AGL photometry?
Coverage is the direction and spread of the light beam, which must reach the pilot's viewing position during aircraft operations. A laboratory test confirms the fitting's beam pattern is correct, and a field check confirms the installed fitting is aimed correctly. A compliant fitting that is rotated or tilted after installation or maintenance will still send the wrong signal.
What does configuration cover and how is it verified?
Configuration is how individual lights are arranged and installed across the airfield, including spacing, alignment, installation height and layout compliance. For example, runway edge lights are spaced at not more than 60 m on instrument runways. Unlike colour, candela and coverage, it is not measured in a photometric laboratory but verified by survey, inspection against the approved layout and flight inspection.
Why can't AGL performance be judged by brightness alone?
Effectiveness depends on whether the light performs exactly as required, not just whether it turns on. Each fitting must deliver the correct signal, in the correct direction, with the correct intensity and at the correct location, which is why the 4Cs must be satisfied together and not one at a time.
Does a fitting stay compliant after it passes testing?
Not necessarily. Compliance at the time of testing is not the end of the story, because lamp or LED ageing, dirt, water ingress and lens damage all reduce in-service intensity. This is why routine measurement and maintenance limits matter.