Airfield Ground Lighting (AGL) photometry is often misunderstood as a simple brightness check. Many assume that if a light appears bright or meets a lux value on the ground, it is compliant. It is much more than that.
AGL photometry is not about how bright a light looks at close range. It is about how that light performs in three-dimensional space from the pilot's viewing position in the cockpit. Photometry evaluates the complete optical behaviour of a fitting, not just its surface illumination.
Photometry does not test how the light looks on the ground. It tests what the pilot actually sees — a light can look bright and still be unsafe, and a fitting can pass a lux test and still fail photometric compliance.
Light Intensity Distribution
Photometry measures luminous intensity in candelas across defined vertical and horizontal angles. Airfield lights are not designed to emit light uniformly in all directions. They are engineered to deliver precise intensity within specific angular sectors defined by standards such as ICAO Annex 14 and FAA specifications.
The intensity must:
- Meet minimum values within the primary beam
- Stay within maximum limits, where the standard specifies them, to prevent glare
- Drop off correctly outside defined zones
This directional intensity ensures pilots receive accurate visual guidance during approach, landing, and taxi operations.
Lux values alone cannot capture this behaviour.
Beam Spread and Orientation
Every AGL fitting has a designed beam shape. Photometry verifies:
- Vertical beam spread, which is critical for approach visibility
- Horizontal beam spread, which is critical for alignment cues
- Correct mechanical and optical aiming
A fitting may appear bright when viewed from directly above during inspection. However, if the beam is misaligned or improperly shaped, it may fall outside the required viewing window from the cockpit. Pilots do not see the light from the ground — they see it from a moving aircraft at defined approach angles.
Photometric testing confirms that the beam reaches them exactly where it should.
Uniformity Across Viewing Angles
Uniformity is a critical safety parameter. Photometry ensures:
- Smooth intensity transition across beam sectors
- No sudden dark patches within the primary beam
- No unexpected hot spots causing glare
Inconsistent intensity can distort visual perception, especially in low-visibility conditions. A runway should appear continuous and predictable, not uneven or patchy.
This consistency is defined in international standards and verified through controlled photometric measurement.
Compliance With ICAO-Defined Viewing Zones
Standards such as ICAO Annex 14 define exact angular windows (isocandela diagrams in Annex 14 Appendix 2, with equivalent FAA specifications) where intensity must meet specified values. Photometric testing checks:
- Minimum intensity within required vertical and horizontal angles
- Performance at critical cut-off points
- Intensity behaviour outside the main beam
A light can look bright during a ground check and still fail photometry if it does not meet intensity requirements within these defined cockpit viewing zones.
Compliance is not judged by appearance. It is judged by measured performance.
Directional Intensity vs. Lux Values
This distinction is fundamental. Lux measures illumination on a surface. AGL photometry measures luminous intensity in a direction. A light may show high lux directly beneath it yet fail to provide adequate intensity at a specific vertical approach angle where the pilot actually observes it.
| Aspect | Lux | Directional Intensity (Candela) |
|---|---|---|
| What it measures | Illumination on a surface | Luminous intensity in a specific direction |
| Reference point | Ground or fitting surface | Pilot's viewing angle in the cockpit |
| What it can miss | Performance at the actual approach angle | Single-point readings are not enough — measured at many angles across the beam |
| What defines safety | Does not define safety on its own | Directional intensity does |
Lux values do not define safety. Directional intensity does.
Testing What the Pilot Actually Sees
Photometry is not simply about testing lamps. It is about testing the visual signal delivered to the cockpit. It validates:
Photometry also covers colour. Chromaticity is checked against the colour boundaries in Annex 14 Appendix 1 (and the FAA equivalents), because a fitting can deliver correct intensity and still show a wrong or drifted colour.
It is carried out at two levels: laboratory (type) testing of the fitting on a goniophotometer, and in-service field measurement of installed fittings with a mobile photometric system. The field measurement is what verifies aiming and in-service degradation.
Two fittings may look physically identical. Only photometric analysis can reveal whether their optical output matches the required airfield category.
What AGL Photometry Actually Measures
Beam Geometry → Angular Intensity → Viewing-Zone Check → What the Pilot Sees
Conclusion
AGL photometry goes far beyond checking brightness. It measures:
- Intensity distribution
- Beam geometry
- Angular compliance
- Visual uniformity
- Colour (chromaticity)
A light can look bright and still be unsafe. A fitting can pass a lux test and still fail photometric compliance. Photometric compliance ensures correct visual cues from the cockpit, predictable appearance during approach, and no misleading brightness or dark zones.
Photometry does not test how the light looks on the ground. It tests what the pilot actually sees.
Frequently Asked Questions
What does AGL photometry measure?
AGL photometry measures luminous intensity in candelas across defined vertical and horizontal angles, along with beam geometry, uniformity and colour (chromaticity). It evaluates how a light performs in three-dimensional space from the pilot's viewing position in the cockpit, not how bright it looks at close range.
Why is a lux value not enough to prove an AGL fitting is compliant?
Lux measures illumination on a surface, whereas AGL photometry measures luminous intensity in a specific direction. A light may show high lux directly beneath it yet fail to provide adequate intensity at the vertical approach angle where the pilot actually observes it, so a fitting can pass a lux test and still fail photometric compliance.
What are the ICAO viewing zones in AGL photometry?
Standards such as ICAO Annex 14 define exact angular windows, through the isocandela diagrams in Appendix 2 with equivalent FAA specifications, where intensity must meet specified values. Photometric testing checks minimum intensity within the required vertical and horizontal angles, performance at critical cut-off points, and intensity behaviour outside the main beam.
Can a light look bright and still fail photometry?
Yes. Compliance is judged by measured performance, not appearance. If the beam is misaligned or improperly shaped, a fitting that looks bright from directly above during inspection may fall outside the required viewing window from the cockpit.
Why does uniformity across viewing angles matter?
Photometry checks for smooth intensity transition across beam sectors, no sudden dark patches within the primary beam and no unexpected hot spots that cause glare. Inconsistent intensity can distort visual perception, especially in low visibility, and a runway should appear continuous and predictable.
Does AGL photometry also check colour?
Yes. Chromaticity is checked against the colour boundaries in Annex 14 Appendix 1, and the FAA equivalents, because a fitting can deliver correct intensity and still show a wrong or drifted colour.
How is AGL photometry carried out?
It is carried out at two levels: laboratory (type) testing of the fitting on a goniophotometer, and in-service field measurement of installed fittings with a mobile photometric system. The field measurement verifies aiming and in-service degradation.