AGL-CS-070 — Incorrect Photometer Alignment
The AGL Knowledge Hub | Engineering Case Study Series. A photometric measurement error case where incorrect detector alignment caused a false 15% intensity failure — from calibration verification through optical-axis alignment, sensitivity analysis, and procedural correction.
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 a laboratory goniophotometric acceptance test, a newly manufactured runway edge light failed the required peak-intensity specification by approximately 15%.
The manufacturer challenged the result because the same fixture had passed internal testing before shipment.
A repeat test performed by another laboratory produced fully compliant photometric results.
A detailed investigation was initiated covering photometer alignment, optical-axis verification, angular positioning, measurement distance, detector height, and reference-axis calibration.
The investigation identified incorrect photometer alignment with the fixture optical axis, causing the detector to measure the beam slightly off-center, resulting in artificially reduced measured intensity and distorted beam-distribution data.
This case demonstrates that incorrect photometer alignment can create false photometric failures even when the AGL fixture is fully compliant.
2Learning Objectives
After completing this case study, the learner should be able to:
- Diagnose photometric errors caused by detector misalignment.
- Verify photometer optical-axis alignment.
- Assess angular positioning accuracy.
- Evaluate distance and height alignment.
- Understand systematic measurement errors.
- Perform alignment verification before testing.
- Prevent false photometric failures.
3System Background
Photometric measurements require precise alignment of:
- Fixture optical axis.
- Photometer optical axis.
- Measurement distance.
- Detector height.
- Horizontal angle.
- Vertical angle.
- Reference coordinate system.
Alignment errors may occur due to:
- Detector displacement.
- Angular offset.
- Height error.
- Distance error.
- Fixture rotation.
- Incorrect reference-axis setup.
- Goniophotometer calibration error.
Photometric testing is fundamentally a geometric measurement process.
4Problem Statement
Laboratory results:
| Parameter | Laboratory A | Laboratory B |
| Peak intensity | 43,500 cd | 51,200 cd |
| Horizontal beam | Narrow | Normal |
| Vertical beam | Depressed | Normal |
| Chromaticity | Pass | Pass |
The same fixture produced significantly different photometric results.
5Initial Observations
| Same fixture | Yes |
| Different laboratories | Yes |
| Peak intensity difference | 17.7% |
| Beam shape difference | Yes |
| Chromaticity | Similar |
The large intensity difference required investigation.
6Initial Engineering Hypotheses
The investigation considered:
Fixture degradation.
LED instability.
Photometer calibration error.
Photometer alignment error.
Power-supply variation.
7Alignment Error Principle
Small angular errors can produce large intensity errors.
8Test Equipment
The investigation used:
- Precision goniophotometer.
- Calibrated photometer.
- Laser alignment system.
- Optical alignment telescope.
- Digital inclinometer.
- Distance measurement system.
- Alignment reference target.
- ICAO photometric reference data.
9Investigation
Step 1: Calibration Verification
Calibration records showed:
| Photometer calibration | Valid |
| Detector calibration | Valid |
| Goniophotometer encoder | Valid |
| Reference standard | Valid |
No calibration defect was identified.
Step 2: Alignment Verification
Laser alignment testing showed:
- Horizontal offset: 0.32°
- Vertical offset: 0.27°
- Detector centerline not coincident with fixture optical axis.
The photometer was misaligned.
Step 3: Optical-Axis Verification
When aligned correctly:
Peak intensity: 51,180 cd
When misaligned by 0.3°:
Peak intensity: 43,620 cd
Measured reduction:
14.8%
The alignment error reproduced the laboratory discrepancy.
Step 4: Beam Distribution Analysis
Misaligned measurements showed:
- Lower peak intensity.
- Narrower beam width.
- Vertical beam depression.
- Horizontal asymmetry.
The beam shape appeared defective even though the fixture was healthy.
Step 5: Distance Verification
Measurements showed:
Required distance: 25.00 m
Actual distance: 24.82 m
Distance error was small and not the primary cause.
Step 6: Height Verification
Detector height error:
18 mm
This contributed slightly to vertical measurement error.
Step 7: Alignment Sensitivity Study
Measured intensity versus angular offset:
| Offset | Measured Intensity |
| 0.0° | 51,200 cd |
| 0.1° | 49,800 cd |
| 0.2° | 47,600 cd |
| 0.3° | 43,600 cd |
| 0.4° | 39,900 cd |
A 0.3° alignment error caused a major photometric error.
10Root Cause
The photometric failure was caused by incorrect photometer alignment relative to the fixture optical axis, resulting in:
- Reduced measured peak intensity.
- Distorted beam-distribution data.
- Apparent beam asymmetry.
- False photometric non-compliance.
The fixture itself was fully compliant.
11Corrective Action
The laboratory:
- Realigned the photometer.
- Verified optical-axis coincidence.
- Calibrated detector position.
- Verified detector height.
- Verified measurement distance.
- Repeated the photometric test.
- Confirmed beam-distribution accuracy.
- Updated alignment procedures.
- Implemented mandatory pre-test alignment verification.
- Updated laboratory quality procedures.
12Post-Correction Verification
| Parameter | Before | After |
| Peak intensity | 43,500 cd | 51,200 cd |
| Horizontal beam | Narrow | Compliant |
| Vertical beam | Depressed | Compliant |
| Certification result | Fail | Pass |
Correct alignment restored the true photometric result.
13Root-Cause Analysis
Photometric failure reported
Acceptable
Passing result
Photometer offset detected
Misalignment confirmed
Incorrect photometer alignment
Realign detector and repeat testing
14Why This Case Was Important
Alignment errors can cause:
- False certification failures.
- Incorrect acceptance decisions.
- Manufacturer disputes.
- Unnecessary product rejection.
- Incorrect beam-distribution analysis.
- Laboratory credibility problems.
Alignment verification is a fundamental requirement of photometric metrology.
15Engineering Lessons
Photometer alignment is as important as photometer calibration.
Photometer alignment is as important as photometer calibration.
Small angular errors can create large intensity errors.
Small angular errors can create large intensity errors.
Beam-distribution distortion may be a measurement error.
Beam-distribution distortion may be a measurement error.
Optical-axis verification should precede every photometric test.
Optical-axis verification should precede every photometric test.
Geometric alignment must be documented in laboratory procedures.
Geometric alignment must be documented in laboratory procedures.
16Engineer's Decision Point
You observe: A fixture fails peak intensity by 15%. Calibration records are valid. Another laboratory reports a passing result. What should you investigate first?
17Knowledge Check
18Interview Questions
19Frequently Asked Questions
Quick answers about photometer alignment errors and false photometric failures.
Q1What is AGL-CS-070?
AGL-CS-070 is an engineering training case study. It covers a laboratory acceptance test where a compliant runway edge light appeared to fail because the photometer was misaligned. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2Can incorrect photometer alignment cause a false photometric failure?
Yes. In AGL-CS-070 a fully compliant fixture failed by about 15% because the detector measured the beam slightly off-centre.
Q3Why did two laboratories get different results?
Laboratory A measured 43,500 cd with a narrow, depressed beam, while Laboratory B measured 51,200 cd with a normal beam. Laboratory A's photometer was not aligned with the fixture optical axis.
Q4How large was the alignment error?
Laser testing showed a 0.32° horizontal and 0.27° vertical offset. A 0.3° misalignment reproduced the discrepancy: 51,180 cd aligned versus 43,620 cd misaligned, a 14.8% reduction.
Q5How sensitive is measured intensity to alignment?
In the sensitivity study, intensity fell from 51,200 cd at 0.0° to 49,800 cd at 0.1°, 47,600 cd at 0.2°, 43,600 cd at 0.3° and 39,900 cd at 0.4°. Small angular errors cause large intensity errors.
Q6Did calibration, distance or height contribute?
Calibration records were valid, so calibration was not the cause. The distance error (24.82 m against 25.00 m) was small and not the primary cause. An 18 mm detector height error contributed slightly to vertical error.
Q7Why did the beam look defective when the fixture was healthy?
Misaligned measurements showed lower peak intensity, narrower beam width, vertical depression and horizontal asymmetry, all of which came from measuring off-axis.
Q8How did the laboratory correct the problem?
It realigned the photometer, verified optical-axis coincidence, detector position, height and distance, and repeated the test. It also made pre-test alignment verification mandatory and updated its quality procedures.
Q9What is the key lesson from this case?
Before declaring a fixture non-compliant, verify the measurement setup, especially alignment with the optical axis. A discrepancy between laboratories is a reason to check the method first.
Engineering & Expert Notes
Check the Optical Axis Before Every Measurement
SetupA quick alignment check prevents false failures.
Use Fixtures or Jigs for Repeatable Positioning
RepeatabilityMarked positions reduce operator variation.
Verify Photometer Calibration and Orientation
EquipmentCalibration alone is not enough. Orientation affects what the sensor sees.
Repeat a Failed Measurement Before Condemning the Fixture
VerificationA second, independent setup confirms whether the failure is real.
20Case Study Conclusion
Incorrect photometer alignment is a geometric measurement error that can significantly reduce measured intensity, distort beam distribution, and create false ICAO photometric failures, even when the AGL fixture is fully compliant.
Systematic optical-axis verification, detector alignment, height verification, distance verification, angular positioning control, and repeat measurement validation can eliminate alignment-related photometric errors and ensure reliable AGL photometric certification.
In AGL engineering, a photometer measures exactly where it is pointed, not necessarily where the light is designed to shine, and alignment accuracy is therefore a fundamental requirement of every photometric measurement.
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