AGL-CS-069 — Photometry Result Changes Between Tests
The AGL Knowledge Hub | Engineering Case Study Series. A laboratory photometric investigation where repeated tests of the same fixture produced significantly different results — from calibration verification through thermal stabilization analysis, alignment assessment, environmental monitoring, and procedure standardization.
Important: This is an engineering simulation created for training. It is not presented as an actual airport incident. All readings, values, dates and outcomes are illustrative, and no airport, organisation, person or manufacturer is identified or implied.
1Case Study Overview
During routine laboratory photometric testing of a runway edge LED light, the same fixture was tested twice within a 48-hour period using the same goniophotometer.
The first test produced compliant photometric results, while the second test showed a 12% reduction in peak intensity and measurable changes in beam distribution.
The manufacturer questioned the validity of the laboratory results, arguing that the fixture had not been modified between tests.
A detailed investigation was initiated covering measurement repeatability, equipment calibration, environmental conditions, fixture stabilization, optical alignment, power-supply stability, and measurement uncertainty.
The investigation identified combined effects of fixture thermal stabilization, optical positioning variation, and laboratory environmental conditions, which produced measurable differences between repeated photometric tests.
This case demonstrates that photometric result variation can occur even when testing the same fixture, and the investigation must be based on measurement science rather than assumptions.
2Learning Objectives
After completing this case study, the learner should be able to:
- Investigate changing photometric results between tests.
- Evaluate measurement repeatability and reproducibility.
- Assess goniophotometer calibration.
- Analyze fixture stabilization effects.
- Understand measurement uncertainty.
- Identify laboratory environmental influences.
- Develop reliable photometric testing procedures.
3System Background
Photometric testing depends on:
- Equipment calibration.
- Angular positioning accuracy.
- Detector calibration.
- Power-supply stability.
- Fixture stabilization.
- Ambient temperature.
- Optical cleanliness.
- Alignment repeatability.
- Measurement methodology.
Common causes of changing results include:
- Thermal stabilization differences.
- Alignment variation.
- Power variation.
- Detector calibration drift.
- Ambient temperature changes.
- Fixture aging.
- Optical contamination.
- Measurement uncertainty.
Photometric testing is a precision metrology process.
4Problem Statement
Test results:
| Parameter | Test 1 | Test 2 |
| Peak intensity | 52,400 cd | 46,100 cd |
| Beam center | Normal | Slight shift |
| Vertical spread | Normal | Reduced |
| Horizontal spread | Normal | Slightly reduced |
| Chromaticity | Pass | Pass |
The same fixture produced significantly different photometric results.
5Initial Observations
| Same fixture | Yes |
| Same laboratory | Yes |
| Same goniophotometer | Yes |
| Same operator | Yes |
| Different result | Yes |
The variation required systematic investigation.
6Initial Engineering Hypotheses
The laboratory team considered:
Equipment calibration error.
Fixture degradation.
Alignment variation.
Thermal stabilization difference.
Measurement uncertainty.
7Measurement Repeatability Principle
Photometric testing requires controlled measurement conditions.
8Test Equipment
The investigation used:
- Precision goniophotometer.
- Reference standard lamp.
- Calibrated photometer.
- Temperature logger.
- Humidity logger.
- Regulated power supply.
- Alignment verification tools.
- Measurement uncertainty analysis software.
- Laboratory calibration records.
9Investigation
Step 1: Equipment Calibration Verification
Calibration records showed:
| Detector calibration | Valid |
| Angular encoder | Valid |
| Rotation accuracy | Valid |
| Reference standard | Valid |
No calibration defect was identified.
Step 2: Repeatability Testing
Five repeated measurements were performed.
Results:
| Test | Peak Intensity |
| 1 | 52,200 cd |
| 2 | 52,000 cd |
| 3 | 52,350 cd |
| 4 | 52,180 cd |
| 5 | 52,260 cd |
The system demonstrated good repeatability under controlled conditions.
Step 3: Thermal Stabilization Analysis
Temperature measurements showed:
| Condition | Fixture Temperature |
| Test 1 | 41°C |
| Test 2 | 27°C |
The fixture had not reached thermal stabilization before Test 2.
LED optical output varied with temperature.
Step 4: Alignment Verification
Laser alignment measurements showed:
- 0.18° vertical deviation.
- 0.11° horizontal deviation.
The fixture had been repositioned between tests.
Small angular errors affected measured intensity.
Step 5: Environmental Analysis
Laboratory conditions:
| Parameter | Test 1 | Test 2 |
| Temperature | 23°C | 19°C |
| Humidity | 46% | 62% |
Environmental conditions had changed between tests.
Step 6: Power-Supply Verification
Supply measurements showed:
| Test | Current | Voltage |
| Test 1 | Stable | Stable |
| Test 2 | Stable | Stable |
Power supply was not the cause.
Step 7: Uncertainty Analysis
Combined uncertainty contributors:
- Alignment.
- Detector uncertainty.
- Angular positioning.
- Thermal stabilization.
- Environmental conditions.
Expanded uncertainty:
±4.8%
The observed 12% variation exceeded expected uncertainty.
10Root Cause
The changing photometric results were caused by combined thermal stabilization and alignment variation, including:
- Incomplete thermal stabilization before Test 2.
- Small fixture repositioning errors.
- Minor environmental differences.
The fixture itself remained electrically healthy.
The measurement process had not been fully controlled between tests.
11Corrective Action
The laboratory:
- Established a mandatory thermal stabilization period.
- Standardized fixture mounting procedures.
- Implemented laser alignment verification.
- Controlled laboratory temperature.
- Recorded environmental conditions.
- Verified repeatability before certification.
- Updated testing procedures.
- Revised uncertainty documentation.
- Trained laboratory personnel.
- Updated quality-assurance records.
12Post-Correction Verification
Controlled testing produced:
| Test | Peak Intensity |
| 1 | 52,240 cd |
| 2 | 52,180 cd |
| 3 | 52,210 cd |
| 4 | 52,260 cd |
| 5 | 52,190 cd |
Variation was less than 0.2%.
13Root-Cause Analysis
Different photometric results
Acceptable
Good
Incomplete
Variation found
Different
Thermal and alignment variation
Standardize stabilization and alignment procedures
14Why This Case Was Important
Changing photometric results can affect:
- Product certification.
- Factory acceptance testing.
- Third-party testing.
- Customer disputes.
- Compliance decisions.
- Quality assurance.
- Laboratory credibility.
Repeatability is a fundamental requirement of photometric metrology.
15Engineering Lessons
Photometric testing requires controlled thermal conditions.
Photometric testing requires controlled thermal conditions.
Small alignment errors can significantly affect measured intensity.
Small alignment errors can significantly affect measured intensity.
Measurement uncertainty must be quantified.
Measurement uncertainty must be quantified.
Repeatability testing is essential before drawing conclusions.
Repeatability testing is essential before drawing conclusions.
Laboratory procedures must be standardized and documented.
Laboratory procedures must be standardized and documented.
16Engineer's Decision Point
You observe: Same fixture. Same laboratory. 12% difference between tests. Calibration records valid. What should you investigate first?
17Knowledge Check
18Interview Questions
19Frequently Asked Questions
Quick answers about photometric repeatability, thermal stabilization and measurement uncertainty.
Q1What is AGL-CS-069?
AGL-CS-069 is an engineering training case study. It covers a laboratory test where the same runway edge LED light gave different photometric results within 48 hours. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2Why did the same fixture give different results in AGL-CS-069?
Peak intensity was 52,400 cd in Test 1 and 46,100 cd in Test 2, a 12% reduction. The investigation found incomplete thermal stabilization, small fixture repositioning errors and minor environmental differences, not a fixture defect.
Q3Does a photometric difference between two tests mean the fixture changed?
Not necessarily. The fixture stayed electrically healthy. The investigation must rest on measurement science: calibration, repeatability, environment, stabilization, alignment and uncertainty.
Q4How did thermal stabilization affect the result?
The fixture was at 41°C for Test 1 but only 27°C in Test 2, so it had not reached thermal stabilization. LED optical output varies with temperature.
Q5What role did alignment and environment play?
Laser measurements showed 0.18° vertical and 0.11° horizontal deviation after repositioning. Laboratory conditions also changed from 23°C and 46% humidity to 19°C and 62%.
Q6Was the equipment at fault?
No. Detector, angular encoder, rotation accuracy and reference standard calibrations were all valid. Five repeat measurements under controlled conditions ranged from 52,000 to 52,350 cd, and the power supply was stable.
Q7Was the 12% variation within measurement uncertainty?
No. The combined expanded uncertainty was ±4.8%, so the observed 12% variation exceeded what uncertainty alone would explain.
Q8How did the laboratory correct the process?
It set a mandatory thermal stabilization period, standardized mounting, added laser alignment verification, controlled laboratory temperature and recorded environmental conditions. It also verified repeatability before certification and updated procedures, uncertainty documentation and training.
Q9What is the key lesson from this case?
Photometric testing needs controlled, documented conditions. Verify stabilization, alignment and repeatability before treating a result difference as a product failure.
Engineering & Expert Notes
Allow Time for Thermal Stabilisation
MethodOutput changes as the fixture warms. Define and record the stabilisation period.
Mark the Fixture Position for Repeat Tests
RepeatabilitySmall shifts change the result. Use reference marks or jigs.
Quantify Measurement Uncertainty
UncertaintyKnow how much variation to expect before deciding a change is real.
State Test Conditions in the Report
ReportingTemperature, settings and positions let others interpret the result.
20Case Study Conclusion
Photometry result changes between tests are often measurement-process problems involving thermal stabilization, alignment repeatability, environmental control, and measurement uncertainty, and reliable photometric evaluation requires controlled laboratory procedures, quantified uncertainty, and demonstrated repeatability.
Systematic calibration verification, repeatability testing, thermal stabilization control, alignment verification, environmental monitoring, and uncertainty analysis can identify the true causes of changing photometric results and ensure reliable ICAO photometric certification.
In AGL engineering, a changing photometric result is not automatically a changing fixture; it is often a changing measurement condition.
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