Photometry VS Lux Measurement
Photometry is the complete science of measuring visible light as perceived by the human eye — intensity, distribution, uniformity and colour. A lux measurement is just one photometric quantity: illuminance at a single surface. In AGL, airport lighting performance is never judged on a lux reading alone.
At a Glance
| Parameter | Photometry | Lux Measurement |
|---|---|---|
| Meaning | Science of measuring visible light | Measurement of illuminance |
| Scope | Broad | Specific |
| Main quantities | Luminous intensity, illuminance, luminance, luminous flux | Illuminance only |
| Unit | Depends on quantity (lm, cd, lx, cd/m²) | Lux (lx) |
| What it tells you | Overall optical/light performance | How much light reaches a surface |
| AGL use | Complete lighting performance evaluation | Surface illumination measurement |
| Typical instrument | Photometer / goniophotometric system | Lux meter |
| Directional data | Yes — full beam distribution | No, not from a single reading |
| Example output | Light intensity distribution | 50 lx at a measurement point |
What Is Photometry & What Is Lux?
Photometry
Photometry is the measurement of visible optical radiation, weighted according to human visual sensitivity. It is the umbrella science behind every lighting performance figure — not a single reading but a family of related quantities that together describe how a light source behaves.
Key photometric quantities
- Luminous flux → lumen (lm)
- Luminous intensity → candela (cd)
- Illuminance → lux (lx)
- Luminance → candela per square metre (cd/m²)
Lux Measurement
Lux (lx) is the SI unit of illuminance — it describes how much luminous flux falls onto a surface. The basic relationship is 1 lux = 1 lumen per square metre. A lux meter placed at a point simply reports how much light is arriving there.
What a lux reading gives you
- Illuminance at one measurement point
- A single numeric value in lx
- Basis for area uniformity surveys
- No information on beam direction or shape
A Whole Science vs One Quantity Within It
Lux is not an alternative to photometry — it is part of photometry. Photometry covers luminous flux, luminous intensity, illuminance and luminance together; a lux measurement addresses only the illuminance branch, at a single surface.
Rule of thumb: if the question is "how bright does this fixture appear in every direction, and does its beam meet the design pattern?", that's photometry. If the question is "how much light is landing on this exact spot?", that's a lux measurement.
The Four Quantities Photometry Covers
Luminous Flux
Total quantity of visible light emitted by a source.
Luminous Intensity
Light emitted in a particular direction — the AGL beam figure.
Illuminance
Light arriving at a surface — what a lux meter reads.
Luminance
Light reflected or emitted from a surface toward the eye.
Why a Single Lux Reading Isn't Enough for AGL
Two runway lighting systems can both produce 100 lx at one measurement point without having the same photometric performance. They can still differ in beam distribution, directional intensity, coverage, uniformity, peak intensity and off-axis performance. A single point value cannot describe the complete light distribution — that is why AGL fixture performance is evaluated photometrically, over a range of angles, rather than from one illuminance reading.
In short: matching lux at one point is not proof of matching performance. The beam shape behind that number can be completely different.
Intensity at the Source vs Illuminance at the Surface
Candela
Luminous intensity describes how strongly a light source emits light in a particular direction. For an AGL fixture, intensity is directional — the photometric performance is represented as an intensity distribution rather than a single number, which is exactly why beam aiming matters for pilot guidance.
Lux
Illuminance describes how much light arrives at a surface, regardless of the source's directional behaviour. A lux meter gives the value at the sensor location only — it does not by itself tell you the intensity or beam shape of the fixture producing it.
This simplified relationship assumes an ideal point source at normal incidence — real AGL measurements can be more complex. Example: a 100 cd source at 10 m distance gives E = 100 / 10² = 1 lx under those idealised conditions. Candela and lux are related by geometry and distance, but they are never interchangeable units.
Photometric Testing vs Lux Survey
Photometric Testing
A proper photometric evaluation measures a fixture at many angles, both horizontal and vertical, to establish its full intensity distribution — a photodetector-based system characterises the light source itself, not just one location in front of it.
Lux Survey
A lux survey establishes a grid of measurement points across a runway or apron area and records the illuminance at each point (for example 45 lx, 52 lx, 48 lx, 55 lx). These values are then assessed against the required illuminance and uniformity criteria for that surface.
Where Lux Measurement Is Especially Useful
Lux measurements come into their own when evaluating uniformity over an illuminated surface. A grid of readings that stay close together (for example, all in the high-70s to low-80s) indicates consistent illumination. A grid with wide swings between very high and very low values indicates poor uniformity even if the average looks acceptable — an average alone can hide a non-uniform installation.
Photometry vs Lux by AGL Requirement
| AGL Requirement | More Relevant Measurement |
|---|---|
| Light output distribution | Photometry |
| Directional intensity | Candela / photometry |
| Surface illumination | Lux |
| Area uniformity | Lux measurements |
| Fixture beam pattern | Photometry |
| Light colour | Photometric / colour measurement |
| Complete fixture performance | Photometric testing |
Photometric System vs Lux Meter
Photometric System
A professional photometric setup pairs a photodetector with a measurement system to characterise the light source across a range of directions, producing intensity/distribution data rather than a single value.
Lux Meter
A lux meter is a simpler handheld instrument: a sensor reads the light arriving at its position and displays that illuminance directly on screen — useful in the field, limited on its own.
Common Misunderstandings
A lux meter measures illuminance at the sensor position, not the source's luminous intensity directly. The two are related only through geometry and distance.
They can still differ in beam distribution, peak intensity, coverage and off-axis performance. A single lux reading cannot describe the complete light distribution.
They measure different quantities — luminous intensity at the source versus illuminance at a surface — and are only related through the inverse-square relationship and the measurement geometry.
Which Measurement Applies Where?
Photometry is the complete science of measuring visible light performance — luminous flux, intensity, illuminance and luminance together. Lux is one photometric quantity within that science: illuminance at a surface.
Use photometric testing to characterise an AGL fixture's beam distribution, direction and colour; use lux measurement and lux surveys to assess surface illumination and uniformity at specified points. Neither replaces the other in a full compliance evaluation.
Engineering & Expert Notes
Use Lux for What It Measures
PrincipleLux shows illuminance at a point, not fixture intensity.
Record Geometry
MethodDistance and angle affect readings.
Use Proper Photometry for Compliance
ComplianceUse the specified method.
State Units
ReportingWrite units and conditions.
Frequently Asked Questions
Yes. Lux measures illuminance, which is one of the quantities covered under the broader science of photometry — it is part of photometry, not an alternative to it.
Yes. They can have different beam distributions, directional intensities, coverage and off-axis performance even while reading the same illuminance at one point.
Candela measures luminous intensity — how strongly a source emits light in a given direction. Lux measures illuminance — how much light arrives at a surface. They are related by distance and geometry, not directly interchangeable.
A lux survey is well suited to assessing surface illumination and uniformity at defined points. Full photometric testing is needed to characterise a fixture's beam distribution, direction and colour performance.
Not necessarily. A higher point reading says nothing about uniformity, beam shape or directional performance — it must be assessed alongside the applicable photometric and uniformity requirements.
Governing Documents
For an actual airport project, verify the current applicable editions of the relevant standards before using this comparison for project design.
ICAO Annex 14, Volume I
Aerodrome Design and Operations — light characteristics, photometric requirements and intensity distributions for AGL fixtures.
FAA AC 150/5340-30
Design and Installation Details for Airport Visual Aids — the specific photometric performance requirements (intensity distribution by light type) that photometric testing verifies fixtures against.
ICAO Aerodrome Design Manual
Applicable material covering photometric testing methods and lighting performance criteria.
National CAA / DGCA / AAI Requirements
For Indian airport projects, also verify applicable DGCA requirements, AAI requirements, airport-specific technical specifications and project drawings.
Final Engineering Note
Photometric acceptance criteria and lux/uniformity requirements can vary with the governing standard and project specification. Use the applicable current standard for final compliance, not a generic comparison page.
Photometry tells you about a light's overall optical performance; a lux measurement tells you how much illuminance reaches a particular surface or location.
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