Field Photometry vs Workshop Photometry
Two different ways of measuring AGL fixture light output — field photometry measures the photometric performance of AGL fixtures in their actual installed condition, while workshop photometry measures an individual fixture under controlled workshop/laboratory conditions. One answers "how is the installed light performing at the airport?" The other answers "how does the fixture perform under controlled test conditions?"
At a Glance
| Feature | Field Photometry | Workshop Photometry |
|---|---|---|
| Location | Installed at airport | Workshop / laboratory |
| Test object | Installed AGL system/fixture | Individual fixture |
| Environment | Actual field conditions | Controlled conditions |
| Fixture installation | Actual mounting and alignment | Test setup |
| Power supply | Actual CCR/circuit | Controlled test supply |
| Main focus | In-service performance | Fixture performance |
| Alignment effect | Included | Usually controlled separately |
| Circuit effects | Included | Generally isolated from field circuit |
| Environmental effects | Present | Minimized/controlled |
| Repeatability | Lower | Higher |
| Diagnostic value | Excellent for field condition | Excellent for product/fixture condition |
| Typical use | Maintenance, compliance verification | FAT, product testing, certification |
Terminology and scope: "Workshop photometry" here means laboratory photometry of an individual fixture (goniophotometer or photometric test bench); type-test and certification photometry is normally done by a qualified or accredited laboratory. Field photometry is normally done with a mobile measuring unit — Annex 14 calls for this on precision-approach and runway lighting — and mainly assesses the intensity, main-beam and orientation of installed lights. It does not reproduce a full goniophotometric distribution. See also Laboratory Photometry vs Field Photometry.
Field Photometry
Field photometry measures the light where it is actually installed and operating — as an in-service reading of the complete AGL installation, not just the fixture.
AGL FIXTURE
│
▼
INSTALLED POSITION
│
▼
ACTUAL AIRFIELD
│
▼
PHOTOMETRIC MEASUREMENT
It can reveal the combined effect of fixture optical performance, fixture alignment, mounting condition, CCR current, isolation transformer, circuit condition, lens contamination, physical deterioration, installation geometry, and environmental conditions — so it represents the actual operational condition of the AGL installation.
Workshop Photometry
Workshop photometry is normally performed with the fixture removed from service or before installation, using equipment such as a goniophotometer, photometric test bench, or other calibrated photometric instruments.
AGL FIXTURE
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▼
CONTROLLED POWER
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CONTROLLED SETUP
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PHOTOMETRIC TEST
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INTENSITY / BEAM /
COLOR ETC.
The purpose is to characterize the fixture itself under defined conditions, independent of any installation, alignment, or field circuit effects.
Installation Condition
This is the most important distinction between the two methods.
FIXTURE │ ▼ CONTROLLED SETUP │ ▼ MEASURE │ ▼ FIXTURE PERFORMANCE
FIXTURE │ ▼ ACTUAL INSTALLATION │ ▼ ALIGNMENT + CCR + CIRCUIT │ ▼ MEASURE │ ▼ INSTALLED PERFORMANCE
When a Good Fixture Still Fails in the Field
Suppose a runway edge light has excellent workshop photometry:
WORKSHOP │ ▼ INTENSITY ✓ BEAM ✓ COLOR ✓
After installation:
FIELD │ ▼ FIXTURE MISALIGNED │ ▼ PHOTOMETRIC PERFORMANCE ✕
The fixture itself may be perfectly manufactured — the problem is the installation/alignment. This is why workshop photometry cannot always replace field photometry.
Installation Effects Combined
FIXTURE + ALIGNMENT + MOUNTING + CCR CURRENT + CIRCUIT CONDITION + LENS CONDITION │ ▼ FIELD PHOTOMETRIC PERFORMANCE
Field measurement therefore provides a more realistic indication of what pilots actually receive from the installed lighting system.
Controlled, Repeatable Results
CONTROLLED POWER
+
CONTROLLED POSITION
+
CONTROLLED ENVIRONMENT
+
CALIBRATED EQUIPMENT
│
▼
REPEATABLE PHOTOMETRIC RESULT
Workshop testing removes many field variables, which makes it useful for:
- Product development
- Design verification
- Type testing
- Production testing
- FAT
- Certification
- Comparison between fixtures
What Each Method Measures
| Parameter | Field | Workshop |
|---|---|---|
| Light intensity | Yes | Yes |
| Beam distribution | Limited (main-beam region, equipment-dependent) | Yes (full distribution) |
| Beam spread | Limited (equipment-dependent) | Yes |
| Color | Usually checked separately | Yes |
| Alignment | Yes | Controlled separately |
| Installed geometry | Yes | No |
| Fixture optical performance | Indirect (combined with installation effects) | Yes |
| Actual CCR condition | Yes | Usually controlled |
| Circuit effects | Yes | Usually minimized |
| Lens contamination | Yes | Controlled/clean |
| Environmental effects | Yes | Minimized |
The exact measurement parameters and acceptance criteria depend on the fixture type and applicable ICAO/FAA/project requirements. Field measurement with a mobile unit is not normally used for chromaticity, flashing lights or PAPI units, which are verified by other methods.
Identifying Degradation in Service
NEW FIXTURE
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100% PERFORMANCE
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IN SERVICE
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95%
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90%
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85%
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INVESTIGATE / MAINTAIN
Illustrative percentages only. Maintenance thresholds are set by the applicable standard and the airport maintenance manual.
A declining field result may indicate:
- LED degradation
- Lamp aging
- Dirty lens
- Optical deterioration
- Misalignment
- Electrical problems
- Mechanical movement
The root cause should be investigated rather than assuming the fixture itself has failed.
Reference point: Annex 14 treats a light as unserviceable when its main-beam average intensity falls below 50% of the value specified for a new light. Airport maintenance programmes for precision-approach and runway lighting therefore include periodic field measurement of intensity, beam spread and orientation. Verify the exact provisions and frequency in the current edition.
Combine both methods: When a field reading is low, removing the fixture for workshop photometry and comparing it with the type-test data separates optical degradation (lens, reflector, LED or lamp) from installation, alignment or circuit causes.
Verifying Before Installation
MANUFACTURING
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WORKSHOP PHOTOMETRY
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VERIFY PERFORMANCE
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FAT / ACCEPTANCE
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INSTALLATION
This establishes the fixture's controlled photometric performance before it is installed at the airport.
Two Different Questions
FAT / Workshop
Does the manufactured fixture meet its specified photometric requirements?
Field
Does the installed fixture meet the required operational performance?
WORKSHOP │ ▼ PRODUCT COMPLIANCE │ ▼ INSTALLATION │ ▼ FIELD │ ▼ INSTALLED PERFORMANCE
Checking a PAPI Both Ways
Workshop Photometry
The unit can be tested under controlled conditions for light intensity, beam characteristics, transition characteristics, color, and optical performance.
Field Verification
After installation, the system must also be correctly positioned, aligned, elevated, oriented, and set to the required visual/approach geometry.
A PAPI can pass workshop photometry but still require field adjustment and verification after installation.
On site, PAPI unit setting angles and alignment are verified by direct angular measurement and confirmed by flight inspection. This is a geometry and alignment check rather than a mobile-unit intensity measurement.
Controlled vs Variable Conditions
CONTROLLED
CONDITIONS
│
▼
REPEATABLE
RESULTS
WEATHER
+
INSTALLATION
+
ALIGNMENT
+
CIRCUIT
+
CONTAMINATION
+
ACTUAL OPERATING
CONDITIONS
│
▼
FIELD RESULT
Workshop conditions can be controlled, so results are generally more repeatable. Field results have more variables and can therefore vary more than controlled laboratory measurements.
Full Engineering Parameters
| Parameter | Field Photometry | Workshop Photometry |
|---|---|---|
| Environment | Actual airport | Controlled |
| Installation | Actual | Test arrangement |
| Alignment | Actual | Controlled |
| Power source | Actual system | Controlled |
| Circuit condition | Included | Usually excluded |
| Fixture condition | In-service | New/test fixture |
| Repeatability | Lower | Higher |
| Product characterization | Limited | Excellent |
| Installed performance | Excellent | Not representative |
| Maintenance | Very useful | Limited |
| FAT | Limited | Very useful |
| Type testing | Not normally primary | Very useful |
Verify the Fixture, Verify the Installation
│
▼
ACTUAL
INSTALLATION
│
▼
"HOW IS THE LIGHT
PERFORMING IN
SERVICE?"
│
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CONTROLLED
CONDITION
│
▼
"HOW DOES THE
FIXTURE PERFORM
BY ITSELF?"
One-line distinction: Workshop photometry verifies the fixture. Field photometry verifies the installed lighting performance.
Field Photometry vs Workshop Photometry
Field photometry is the photometric measurement of an AGL fixture/lighting system in its actual installed and operating condition, including the effects of alignment, mounting, circuit conditions, and in-service degradation.
Workshop photometry is the photometric measurement of an individual AGL fixture under controlled test conditions to verify and characterize its optical performance. Neither replaces the other — the practical sequence runs new fixture → workshop photometry → FAT/product verification → installation → field alignment → field photometry → commissioning/in-service verification.
Passing workshop photometry does not by itself prove that the installed AGL system will meet field photometric requirements.
Engineering & Expert Notes
Do Not Compare the Two Without Context
InterpretationField conditions add alignment, current and contamination. Differences are expected.
Clean and Align Before Field Measurement
PreparationNote any cleaning and the alignment method used.
Use Workshop Data as the Reference
BaselineIt shows what the fixture can do. Field values show what it is doing.
Document the Setup for Both
ReportingState instruments, positions, current and temperature.
Frequently Asked Questions
Field photometry measures an AGL fixture's photometric performance in its actual installed condition at the airport, while workshop photometry measures an individual fixture under controlled workshop or laboratory conditions before or after removal from service.
Yes. A fixture may be perfectly manufactured and pass every workshop test, but still show poor field photometric performance due to misalignment, mounting condition, CCR current, circuit effects, or lens contamination after installation.
Workshop photometry is generally more repeatable because power, position, and environment can be controlled. Field photometry has more variables — weather, installation, alignment, circuit condition, and contamination — so results can vary more.
Field photometry is used for maintenance and compliance verification of installed AGL systems, since it captures the combined effect of the fixture, alignment, mounting, CCR current, circuit condition, and environmental factors together.
Workshop photometry is used for product development, design verification, type testing, production testing, FAT, certification, and comparison between fixtures — anywhere a controlled, repeatable measurement of the fixture itself is required.
No. A PAPI unit can pass workshop photometry for intensity, beam characteristics, and color, but still require field adjustment and verification for positioning, alignment, elevation, orientation, and the required visual/approach geometry after installation.
Governing Documents
For an actual project or maintenance program, always verify the current edition of the applicable documents and the approved test procedure.
ICAO Annex 14, Volume I — Appendices & Chapter 10
Aerodrome Design and Operations: Appendix 1 (colours) and Appendix 2 (characteristics of aeronautical ground lights, including intensity and isocandela requirements), and Chapter 10 (maintenance, including field measurement of intensity, beam spread and orientation, and serviceability levels). Verify clause numbers in the current edition.
ICAO Aerodrome Design Manual, Doc 9157 Part 4
Visual Aids: design guidance supporting the Annex 14 light characteristics. Guidance material, not a Standard.
ICAO Airport Services Manual, Doc 9137 Part 9
Airport Maintenance Practices: guidance on visual aid maintenance, including light serviceability.
FAA AC 150/5345-46
Specification for Runway and Taxiway Light Fixtures: photometric requirements for fixtures, the basis for workshop and type-test photometry.
FAA AC 150/5345-53
Airport Lighting Equipment Certification Program: third-party certification of airport lighting equipment.
FAA AC 150/5340-26
Maintenance of Airport Visual Aid Facilities: recommended maintenance practice for airport lighting.
FAA AC 150/5345-28
Specification for PAPI systems: requirements for PAPI units, relevant to the PAPI example above.
DGCA / AAI Requirements (India)
For Indian airport projects, verify applicable DGCA and AAI requirements, airport-specific maintenance manuals and approved photometric test procedures.
Final Engineering Note
Do not substitute one method for the other. The applicable specification and approved test procedure should always govern the scope, sequence, and pass/fail criteria of workshop and field photometry.
Workshop photometry is not a substitute for field photometry, and field photometry is not a substitute for workshop photometry — each answers a different question about fixture and installation performance.
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