PHOTOMETRY VS VISUAL INSPECTION
Measure the Light vs Look at the Light — photometry is the objective, instrument-based measurement of a light's optical performance, while visual inspection is the subjective, qualitative examination of the installation by an observer. Visual inspection can identify obvious defects; photometry verifies whether the light actually meets its required optical performance.
At a Glance
| Feature | Photometry | Visual Inspection |
|---|---|---|
| Method | Instrument measurement | Human observation |
| Nature | Quantitative | Mainly qualitative |
| Main result | Numerical data | Observations / findings |
| Measures intensity | Yes | No precise measurement |
| Measures beam distribution | Yes | Only visually / qualitatively |
| Typical units | cd, lx, cd/m², etc. | No physical unit |
| Repeatability | High when test conditions are controlled | Depends on observer/conditions |
| Detects visible damage | Sometimes indirectly | Yes |
| Verifies photometric compliance | Yes | Not by itself |
| Typical use | Performance testing | Routine condition checks |
What Is Photometry & Visual Inspection?
Photometry
Photometry uses instruments to measure the actual optical characteristics of an AGL light. It is a chain: the light is fed into a photometric measurement instrument, which produces numerical data.
Typical measurements
- Luminous intensity
- Beam distribution
- Horizontal and vertical coverage
- Illuminance / luminance
- Colour characteristics
Visual Inspection
Visual inspection means examining the light or installation for observable problems. An inspector looks at the fixture and forms a condition assessment.
What an inspector checks
- Broken or missing components, loose fittings
- Dirty or cracked lenses, water ingress
- Misalignment, physical damage
- Incorrect visible colour, obvious light failure
Measure the Light vs Look at the Light
Easy memory: Photometry → MEASURE the light. Visual inspection → LOOK at the light.
A Light Can Pass Visual Inspection but Fail Photometry
A fixture may illuminate normally, appear bright, and show no visible physical damage — but still have insufficient intensity in a required direction.
"It looks bright" is not a photometric measurement. A fixture can appear entirely correct to the eye while its measured intensity in a required direction is out of tolerance.
A Light Can Fail Visual Inspection but Still Emit Light
The reverse is also true. A fixture with a cracked lens, dirt accumulation, damaged housing, loose mounting, or water ingress may still operate electrically and emit light — but visual inspection identifies a maintenance problem before it develops into a complete optical or electrical failure.
Photometry Provides Pass/Fail Data
Photometry compares a measured value against a required value and returns a clear result:
Photometry provides measurable data rather than relying on visual judgement.
What Visual Inspection Provides
A typical visual inspection identifies condition, not performance:
- Lens — Clean?
- Housing — Damaged?
- Mounting — Secure?
- Seal — Intact?
- Alignment — Correct?
- Light — Operating?
These observations can be extremely important for maintenance but do not provide a complete photometric characterization.
Photometry vs Visual Inspection in AGL
| Requirement | Best Method |
|---|---|
| Verify luminous intensity | Photometry |
| Verify beam distribution | Photometry |
| Verify angular coverage | Photometry |
| Check for damaged lens | Visual inspection |
| Check cleanliness | Visual inspection |
| Check physical mounting | Visual inspection |
| Verify precise photometric performance | Photometry |
| Identify obvious physical defects | Visual inspection |
A Dirty AGL Fixture
A visual inspection can identify lens contamination immediately. However, photometry can determine whether that contamination has reduced the light output enough to affect required performance. The two methods complement each other:
Instrument vs Observer
Photometry
Instrument → measured value → cd / lx / cd/m². Results can be compared directly against specified requirements.
Visual Inspection
Observer → visual judgement → "condition appears normal / abnormal." Visual inspection can be influenced by observer position, ambient lighting, weather, visibility, experience, and individual judgement.
Where Each Method Fits
A simplified AGL maintenance process may run:
The exact sequence depends on the airport's maintenance procedures and applicable requirements.
What Each Method Uses
Photometry
AGL light → photodetector / photometer → measurement system → intensity data.
Visual Inspection
Inspector → light fixture, potentially supported by cameras, inspection equipment, or maintenance checklists.
What Each Method Cannot Do Alone
Visual Inspection Cannot Reliably Determine
- Exact candela output
- Precise beam distribution
- Compliance with numerical photometric requirements
- Exact intensity reduction
Photometry Alone May Not Efficiently Identify
- Mechanical defects
- Corrosion issues
- Loose components
- Housing damage or installation problems
One method should not automatically replace the other. Photometry and visual inspection cover different, complementary failure modes.
Full Engineering Parameters
| Parameter | Photometry | Visual Inspection |
|---|---|---|
| Type | Instrumental | Observational |
| Result | Numerical | Qualitative |
| Units | cd, lx, cd/m², etc. | None |
| Accuracy | Measurement-dependent | Observer-dependent |
| Beam analysis | Yes | Limited |
| Condition assessment | Limited | Excellent |
| Compliance verification | Yes, where measurements match the specified method | No, by itself |
| Routine defect detection | Limited | Very useful |
| Main purpose | Optical performance | Physical/visible condition |
Photometry vs Visual Inspection
Photometry and visual inspection are not substitutes for each other. One measures optical performance instrumentally; the other observes physical condition.
In AGL maintenance, visual inspection tells you whether the system appears physically sound; photometry tells you whether its optical performance meets the required characteristics. A light can pass one and fail the other, so a complete assessment uses both.
For AGL: use visual inspection for routine condition checks, and photometry to verify that a fixture actually meets its required optical performance.
Engineering & Expert Notes
Do Both
ComplementVisual checks find physical defects; photometry measures output.
Do Visual Checks at Night
InspectionDim or misaligned lights are easier to see in darkness.
Use Photometric Trends to Plan Cleaning and Replacement
PlanningData shows what visual checks cannot.
Record Both Sets of Findings
RecordsKeep a joint history.
Frequently Asked Questions
Photometry → MEASURE the light (instrument, numbers). Visual inspection → LOOK at the light (eyes, condition).
Yes. A fixture can illuminate normally with no visible physical damage and still have insufficient intensity in a required direction. "It looks bright" is not a photometric measurement.
Yes. A cracked lens, dirty lens, damaged housing, or loose mounting can all be identified visually while the fixture still operates electrically and emits light.
No. Visual inspection cannot reliably determine exact candela output, precise beam distribution, or compliance with numerical photometric requirements.
Not efficiently. Photometry alone may not readily catch mechanical defects, corrosion, loose components, housing damage, or installation problems that a visual check identifies quickly.
A typical sequence is: visual inspection → identify obvious defects → clean/repair/align → functional test → photometric test → verify performance. The exact sequence depends on the airport's maintenance procedures.
Governing Documents
For an actual project or maintenance programme, always verify the current edition of the applicable documents.
FAA AC 150/5340-30
Design and Installation Details for Airport Visual Aids — photometric requirements referenced when verifying AGL fixture performance.
FAA AC 150/5340-26
Maintenance of Airport Visual Aid Facilities — the governing guidance for the routine visual inspection and preventive maintenance side of this comparison, as distinct from the photometric testing procedures above.
ICAO Annex 14, Volume I
Aerodrome Design and Operations — photometric characteristics of aeronautical ground lights, plus maintenance and serviceability provisions.
CIE Photometry Standards
International Commission on Illumination definitions and measurement methods for luminous intensity, illuminance and luminance.
Final Engineering Note
Do not treat a passed visual inspection as evidence of photometric compliance, or vice versa. Confirm which check the applicable maintenance procedure actually requires.
Visual inspection tells you whether the system looks physically sound; photometry tells you whether it actually performs to requirement.
Compare the Systems. Then Learn How to Engineer Them.
Understanding the difference between AGL systems is the first step. The next step is knowing how to design, install, test, commission and maintain them correctly.
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