NEW FIXTURE PHOTOMETRY VS IN-SERVICE PHOTOMETRY
Baseline vs Actual Performance — new fixture photometry evaluates an AGL light when it is new and in its specified test condition, establishing the baseline/design performance. In-service photometry evaluates the photometric performance of a fixture that has been installed and operated over time, revealing its actual performance after operational exposure.
At a Glance
| Feature | New Fixture Photometry | In-Service Photometry |
|---|---|---|
| Fixture condition | New | Installed and used |
| Main purpose | Establish initial performance | Assess current performance |
| Ageing effects | Absent or minimal | Included |
| Dirt/contamination | Normally controlled/minimized | May be present |
| Optical degradation | Normally absent | May be present |
| Baseline comparison | Yes | Compared with baseline/specification |
| Installation effects | Usually excluded in fixture testing | May be included |
| Main question | "How does a new fixture perform?" | "How is the fixture performing now?" |
Establishing the Baseline
New Fixture Photometry
New fixture photometry establishes the initial optical performance of the AGL fixture. Typical characteristics assessed may include luminous intensity, peak intensity, beam distribution, horizontal coverage, vertical coverage, beam spread, and colour characteristics where applicable.
It essentially answers: "Does this new fixture provide the required photometric performance?"
Assessing Actual Performance
In-Service Photometry
In-service photometry assesses a fixture after it has been operating in the airfield environment. It answers: "Does the fixture still provide adequate photometric performance during service?"
What Started vs What It Delivers Now
Easy memory: New = what the fixture started with. In-service = what the fixture delivers now.
How Output Can Change Over Time
A new fixture may initially provide a specified peak intensity. After a period of operation, factors such as ageing may affect its output. The actual amount and cause of any change must be determined by measurement and should not simply be assumed.
What Can Affect a Fixture in Service
An AGL fixture in service may experience a number of factors that can potentially affect its actual photometric performance:
- Dirt accumulation
- Lens contamination
- Optical ageing
- LED ageing
- Lamp degradation
- Mechanical wear
- Vibration
- Environmental exposure
How the Comparison Is Structured
This comparison can help identify whether photometric performance has changed during service:
Comparing Measured Intensity
Suppose a new AGL fixture has a measured centre-beam intensity of 10,000 cd. After a period in service, this is measured at 8,000 cd.
Difference between the new and in-service measured intensity
Percentage change from the new measured value
Therefore, the measured intensity has decreased by 20% relative to that baseline. Whether the fixture remains acceptable depends on the applicable specification, maintenance criteria, and measurement method.
Clean Baseline vs Actual Condition
New Fixture
Testing generally seeks to characterize the fixture under its defined test condition — clean, unweathered, as-manufactured.
In-Service Fixture
Its measurement can reflect its actual operational condition — dust, wear — unless the test procedure requires cleaning, removal, or other preparation before testing.
The test condition must always be defined before comparing new and in-service photometric results.
Fixture-Only Testing vs As-Installed Testing
New Fixture
New fixture photometry often focuses on the fixture itself, tested independently of its final installation.
In-Service Fixture
In-service photometry may involve the fixture as installed, so results may potentially include effects from alignment, mounting condition, installation geometry, electrical operating condition, and actual optical condition, depending on the measurement method.
Where Each Applies
| Application | Better Fit |
|---|---|
| Product development | New fixture photometry |
| Type testing | New fixture photometry |
| Initial qualification | New fixture photometry |
| Establishing baseline performance | New fixture photometry |
| Maintenance assessment | In-service photometry |
| Detecting degradation | In-service photometry |
| Assessing operational condition | In-service photometry |
| Comparing current vs initial performance | Both |
A Conceptual Illustration
This is only a conceptual illustration. Real fixtures may maintain stable performance, degrade gradually, or change differently depending on their technology, maintenance, and operating environment.
Comparable Conditions Are Essential
A meaningful comparison requires comparable conditions. Otherwise, a difference in results may be caused by the test setup rather than actual fixture degradation.
Same measurement geometry? Confirm the test setup matches between new and in-service measurements.
Same operating condition? Confirm the fixture is powered and operated equivalently.
Same measurement method? Confirm the instrument and procedure are consistent.
Comparable results. Only then can a difference be reasonably attributed to the fixture itself.
Full Engineering Parameters
| Parameter | New Fixture Photometry | In-Service Photometry |
|---|---|---|
| Fixture age | New | Operational |
| Primary objective | Establish initial performance | Verify current performance |
| Baseline value | Provides it | Uses it for comparison |
| Ageing | Minimal | Potentially present |
| Contamination | Usually controlled | May reflect actual condition |
| Wear | Minimal | Potentially present |
| Installation influence | Often excluded | May be included |
| Maintenance assessment | Limited | Highly useful |
| Degradation detection | Reference point | Primary application |
Two Different Dimensions
These comparisons describe different dimensions. For example, you could have a new fixture tested in a laboratory, a new fixture verified in the field, an in-service fixture retested in a laboratory, or an in-service fixture tested in the field.
Test Location
Laboratory vs field describes where/how the measurement is performed.
Fixture Condition
New vs in-service describes the condition and service history of the fixture being measured.
New Fixture vs In-Service Photometry
New fixture photometry establishes the initial/baseline optical performance. In-service photometry measures the current performance after operational use.
For AGL maintenance and lifecycle assessment, new-fixture photometry provides the reference performance, while in-service photometry reveals the fixture's actual optical performance after exposure to real operating conditions.
Engineering & Expert Notes
Keep the New-Fixture Data as the Baseline
BaselineIt makes in-service results meaningful.
Record Condition Before and After Cleaning
PreparationThis shows how much loss is reversible.
Match Settings and Geometry
RepeatabilityUse the same step and positions.
Set Test Intervals From Trends
PlanningAdjust frequency by observed decline.
Frequently Asked Questions
New fixture photometry → BASELINE (how good the fixture started out). In-service photometry → CURRENT (how good the fixture is performing now).
Not necessarily. Whether a measured difference is acceptable depends on the applicable specification, maintenance criteria, and measurement method — the amount and cause of any change should be determined by measurement, not assumed.
That depends on the defined test procedure. The test condition must always be defined before comparing new and in-service photometric results, since an uncleaned fixture reflects actual operational condition while a cleaned one may not.
Yes. As-installed measurements may include effects from alignment, mounting condition, installation geometry, and electrical operating condition, not just the optical condition of the fixture.
No. Laboratory vs field describes where/how a measurement is performed. New vs in-service describes the condition and service history of the fixture being measured — the two dimensions can be combined in different ways.
Typical influences include dirt accumulation, lens contamination, optical ageing, LED ageing, lamp degradation, mechanical wear, vibration, and environmental exposure.
Governing Documents
For an actual project design, always verify the current edition of the applicable documents.
FAA AC 150/5345-46
Specification for Runway, Taxiway, Heliport, and Vertiport Light Fixtures — the actual fixture equipment specification, including the isocandela/photometric performance requirements a new fixture must meet at type approval.
FAA AC 150/5340-30
Design and Installation Details for Airport Visual Aids — photometric requirements relevant to interpreting new and in-service AGL performance.
ICAO Annex 14, Volume I
Aerodrome Design and Operations — photometric characteristics of aeronautical ground lights, including maintenance-related performance considerations.
CIE Photometry Standards
International Commission on Illumination definitions and measurement methods for luminous intensity and related photometric quantities.
Final Engineering Note
Do not compare new and in-service photometric results without first confirming the test condition, geometry, and method are comparable between the two measurements.
New fixture photometry is not in-service photometry — one describes the design baseline, the other describes what the fixture actually delivers in the field.
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