AGL-CS-090 — Sign Photometric Performance Failure
The AGL Knowledge Hub | Engineering Case Study Series. An illuminated sign photometric performance investigation where optical-system degradation and LED aging caused luminance reduction and non-uniform illumination — from electrical verification through photometric measurement, optical inspection, and performance restoration.
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 periodic airfield visual aid inspection, inspectors reported that a runway location sign appeared noticeably dim and unevenly illuminated despite being electrically operational.
Pilots also reported that the sign was difficult to identify during night operations, particularly from extended taxi distances.
The sign remained illuminated, the Constant Current Regulator (CCR) was operating normally, and no electrical faults were detected.
Because the sign appeared operational but visually inadequate, a comprehensive investigation was initiated covering luminance measurement, illumination uniformity, color compliance, optical transmission, LED aging, diffuser condition, reflector performance, and internal optical contamination.
The investigation identified optical-system degradation and LED aging, resulting in luminance reduction and non-uniform illumination that failed ICAO photometric performance requirements.
This case demonstrates that an illuminated sign can be electrically healthy while failing operational visual-performance requirements due to photometric deterioration.
2Learning Objectives
After completing this case study, the learner should be able to:
- Diagnose illuminated sign photometric failures.
- Measure sign luminance.
- Evaluate illumination uniformity.
- Verify color compliance.
- Assess optical-system degradation.
- Restore photometric performance.
- Conduct post-repair photometric verification.
3System Background
Illuminated sign performance depends on:
- LED output.
- Driver stability.
- Optical diffuser transmission.
- Reflector efficiency.
- Internal cleanliness.
- Color filters.
- Uniform light distribution.
- Optical alignment.
Photometric failures may result from:
- LED aging.
- Diffuser contamination.
- Reflector deterioration.
- Optical yellowing.
- Internal dust accumulation.
- Color-filter degradation.
- Driver output reduction.
- Uneven LED performance.
Visual performance depends on optical quality, not only electrical operation.
4Problem Statement
The reported condition was:
- Sign illuminated.
- Reduced brightness.
- Uneven illumination.
- Poor night conspicuity.
- Electrical system healthy.
Why is the sign visually inadequate despite remaining electrically operational?
5Initial Observations
| Sign illumination | ON |
| Brightness | Low |
| Uniformity | Poor |
| Color appearance | Slightly faded |
| Electrical operation | Normal |
The problem appeared optical rather than electrical.
6Initial Engineering Hypotheses
The investigation considered:
Low input voltage.
LED aging.
Optical diffuser contamination.
Reflector deterioration.
Color-filter degradation.
7Sign Photometry Principle
Photometric performance depends on optical efficiency.
8Test Equipment
The investigation used:
- Portable sign photometer.
- Luminance meter.
- Chromaticity meter.
- Uniformity measurement equipment.
- Optical inspection equipment.
- LED output analyzer.
- ICAO Annex 14 photometric reference data.
9Investigation
Step 1: Electrical Verification
Measurements showed:
| Input voltage | Normal |
| Driver output | Normal |
| Current consumption | Normal |
| Transformer output | Normal |
The electrical system was healthy.
Step 2: Luminance Measurement
Measured luminance:
72 cd/m²
Reference requirement:
≥ 120 cd/m²
The sign failed the luminance requirement.
Step 3: Uniformity Measurement
Measured luminance distribution:
| Zone | Luminance |
| Left | 92 cd/m² |
| Center | 74 cd/m² |
| Right | 51 cd/m² |
Uniformity ratio exceeded acceptable limits.
Step 4: Color Measurement
Chromaticity measurements showed:
- Red background shifted toward orange.
- White legend slightly yellowed.
Color degradation was measurable.
Step 5: Optical Inspection
Inspection revealed:
- Diffuser haze.
- Internal dust.
- Reflector discoloration.
- Surface contamination.
Optical transmission had deteriorated significantly.
Step 6: LED Aging Assessment
Measured LED output:
Approximately 65% of original luminous flux
Significant lumen depreciation had occurred.
Step 7: Controlled Optical Restoration
After:
- Replacing LED arrays.
- Replacing diffusers.
- Cleaning optical surfaces.
- Replacing degraded reflectors.
Measured luminance increased to:
138 cd/m²
Uniformity was restored.
10Root Cause
The sign photometric failure was caused by combined optical-system degradation and LED aging, including:
- LED lumen depreciation.
- Diffuser transmission loss.
- Reflector deterioration.
- Internal contamination.
- Color degradation.
The electrical system remained healthy.
The primary failure mechanism was loss of optical efficiency.
11Corrective Action
The maintenance team:
- Replaced aged LED arrays.
- Replaced optical diffusers.
- Cleaned internal optical surfaces.
- Replaced degraded reflectors.
- Verified driver performance.
- Measured luminance.
- Verified uniformity.
- Measured chromaticity.
- Conducted night legibility assessment.
- Updated photometric maintenance records.
12Post-Repair Verification
Optical restoration recovered full photometric performance.
13Root-Cause Analysis
Sign appeared dim
Healthy
Low
Poor
Degradation found
Lumen depreciation
Optical degradation and LED aging
Restore optical efficiency
14Why This Case Was Important
Photometric failures can cause:
- Poor sign legibility.
- Reduced taxiway guidance.
- Increased pilot workload.
- Night-operation degradation.
- ICAO compliance failures.
- Reduced operational safety margins.
Electrical operation alone does not ensure visual performance.
15Engineering Lessons
An illuminated sign can fail photometric requirements while remaining operational.
An illuminated sign can fail photometric requirements while remaining operational.
Luminance and uniformity must be measured objectively.
Luminance and uniformity must be measured objectively.
LED aging is a significant long-term degradation mechanism.
LED aging is a significant long-term degradation mechanism.
Optical contamination can reduce sign performance substantially.
Optical contamination can reduce sign performance substantially.
Photometric verification should be part of preventive maintenance.
Photometric verification should be part of preventive maintenance.
16Engineer's Decision Point
You observe: Sign illuminated. Noticeably dim. Uneven brightness. Electrical system healthy. What should you investigate first?
17Knowledge Check
18Interview Questions
19Frequently Asked Questions
Quick answers about sign photometric performance, luminance and uniformity.
Q1What is AGL-CS-090?
AGL-CS-090 is an engineering training case study. It covers a runway location sign that stayed lit but was dim and unevenly illuminated, so it failed photometric requirements. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2Can an illuminated sign be electrically healthy but still fail?
Yes. Input voltage, driver output, current and transformer output were all normal, yet the sign failed luminance and uniformity requirements.
Q3What did measurements show in AGL-CS-090?
Luminance was 72 cd/m² against a requirement of at least 120 cd/m². It varied across the sign: left 92, centre 74 and right 51 cd/m², so uniformity exceeded acceptable limits.
Q4What colour problems were found?
The red background had shifted toward orange and the white legend had yellowed slightly.
Q5What did optical and LED assessment show?
Inspection found diffuser haze, internal dust, reflector discoloration and surface contamination. The LEDs were producing about 65% of their original luminous flux.
Q6What was the root cause of AGL-CS-090?
Combined optical-system degradation and LED aging: lumen depreciation, diffuser transmission loss, reflector deterioration, internal contamination and colour degradation.
Q7How was performance restored?
Replacing the LED arrays, diffusers and degraded reflectors and cleaning the optical surfaces raised luminance to 138 cd/m² and restored uniformity.
Q8What other checks were made after repair?
The team verified driver performance, measured luminance, uniformity and chromaticity, conducted a night legibility assessment and updated photometric maintenance records.
Q9What is the key lesson from this case?
Illumination alone does not prove compliance. Measure sign luminance, uniformity and colour, not just electrical operation.
Engineering & Expert Notes
Measure Luminance and Uniformity With Calibrated Instruments
PhotometryVisual inspection cannot assess compliance.
Plan Replacement as LEDs Age
LifecycleGradual output loss is predictable. Schedule refurbishment.
Clean Diffusers and Check Reflectors
MaintenanceCleaning may recover part of the loss.
Check Colour of the Sign Face
ColourColour shift reduces legibility and compliance.
20Case Study Conclusion
Illuminated sign photometric failures are optical-performance failures caused by LED aging, diffuser contamination, reflector deterioration, and loss of optical transmission, and objective measurement of luminance, uniformity, chromaticity, and legibility is essential for determining ICAO compliance and restoring effective visual guidance.
Systematic photometric measurement, optical inspection, LED-performance assessment, diffuser evaluation, reflector restoration, and post-repair compliance testing provide a complete engineering methodology for restoring ICAO-compliant illuminated sign performance.
In AGL engineering, a sign that is illuminated is not necessarily a sign that is visible, and photometry is the engineering discipline that measures the difference.
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