GONIOPHOTOMETER VS LUX METER
Beam vs Surface — a goniophotometer characterizes the directional photometric distribution of a fixture, rotating or scanning it through angular positions to build a full intensity profile. A lux meter simply reads the illuminance arriving at one measurement point. For AGL fixture testing, they answer two different questions and are not interchangeable.
At a Glance
| Feature | Goniophotometer | Lux Meter |
|---|---|---|
| Main purpose | Measures directional light distribution | Measures illuminance |
| Main question | How much light is emitted in each direction? | How much light reaches this surface? |
| Typical output | Photometric intensity distribution | Illuminance value |
| Main unit | Candela (cd) | Lux (lx) |
| Angular measurement | Yes | No, not by itself |
| Beam pattern measurement | Yes | No |
| Typical setup | Fixture rotated/measured at multiple angles | Handheld or fixed sensor at a point |
| AGL use | Fixture photometric testing | Surface/area illuminance measurement |
What Is a Goniophotometer & a Lux Meter?
Goniophotometer
A goniophotometer measures how the luminous intensity of a light fixture changes with direction. During measurement, the fixture and/or detector is moved through defined angular positions to obtain luminous intensity versus direction.
Example
0° → 10,000 cd, +5° → 3,500 cd, +10° → 600 cd — a set of readings across angles that together form a photometric distribution.
Lux Meter
A lux meter measures illuminance at the position of its sensor — how much light is falling on a surface or reaching a specific measurement point.
Example
50 lx — a single illuminance reading at the location where the sensor was placed.
Angular Beam vs Fixed Point
A goniophotometer is designed to investigate different angular directions, with the fixture or detector stepped through positions around the source. A lux meter is generally placed at one specified measurement point — moving it to another location gives another illuminance reading, but this alone does not build a complete, standardized photometric distribution.
Easy memory: Goniophotometer → measures the BEAM. Lux meter → measures the ILLUMINATION received.
What Each Instrument Reports
Goniophotometer Output
Typically represented as:
- Intensity vs horizontal angle
- Intensity vs vertical angle
- Polar distribution
- Photometric data tables
Lux Meter Output
Typically a single value, or a small set of point readings:
- Measurement Point → 35 lx
- Point 1 → 32 lx
- Point 2 → 38 lx
- Point 3 → 35 lx
What a Goniophotometric Test Evaluates
AGL lights are often designed to provide specific light output within specified angular sectors, so a goniophotometric test on an AGL fixture can evaluate:
- Peak intensity
- Intensity distribution
- Horizontal spread
- Vertical spread
- Angular coverage
Area Illuminance Measurement
A lux meter is useful for measuring illuminance across an area, taking readings at a grid of points to help evaluate surface illumination, illuminance distribution, and uniformity where required.
| Grid Point | Illuminance |
|---|---|
| P1 | 45 lx |
| P2 | 50 lx |
| P3 | 42 lx |
| P4 | 48 lx |
Can a Lux Meter Replace a Goniophotometer?
Generally, no. A lux meter can measure illuminance at specified locations, but a goniophotometric system is designed to accurately characterize the directional photometric performance of a fixture — "how much light arrives here?" is a different question from "how does the fixture perform over many directions?"
How the Two Readings Connect
Under simplified point-source and normal-incidence conditions, the intensity a goniophotometer measures and the illuminance a lux meter measures are related by the inverse-square law:
Inverse-square relationship between intensity and illuminance
- E = illuminance, in lux
- I = luminous intensity, in candela
- r = distance, in metres
This illustrates the relationship, although real photometric measurements must account for measurement geometry and applicable test procedures.
Same AGL Light, Two Different Readings
With a Goniophotometer
−5° → 3,000 cd
0° → 8,000 cd
+5° → 4,000 cd
You learn about
The beam pattern.
With a Lux Meter
One sensor, one location → 25 lx
You learn about
The illuminance at that point.
Which Instrument for Which Task
| Application | Better Instrument |
|---|---|
| Measure luminous intensity | Goniophotometer |
| Measure beam distribution | Goniophotometer |
| Measure horizontal photometry | Goniophotometer |
| Measure vertical photometry | Goniophotometer |
| Determine beam spread | Goniophotometer |
| Measure surface illuminance | Lux meter |
| Perform illuminance survey | Lux meter |
| Check area illumination uniformity | Lux meter |
Full Engineering Parameters
| Parameter | Goniophotometer | Lux Meter |
|---|---|---|
| Measures | Directional photometric performance | Illuminance |
| Primary unit | cd | lx |
| Angular scanning | Yes | No |
| Beam distribution | Yes | No |
| Surface illuminance | Not its primary purpose | Yes |
| Fixture characterization | Excellent | Limited |
| Area illumination survey | Not typical | Excellent |
| Test complexity | Higher | Lower |
| Typical result | Photometric curve/table | Point/grid readings |
Myths vs Facts
Not by itself. A lux meter only measures illuminance at its own location. A complete photometric characterization of a directional AGL fixture requires intensity/distribution measurements across the relevant directions.
No. Repositioning a lux meter gives more point readings, but not a standardized angular intensity distribution referenced to the fixture itself.
No. Candela describes intensity emitted by the source in a direction; lux describes illuminance received at a surface. They are related by geometry, not identical.
Goniophotometric testing is central to type/photometric approval, but distribution checks may also be relevant whenever fixture design, optics, or in-service condition changes are being assessed.
Goniophotometer vs Lux Meter
A goniophotometer and a lux meter answer different questions. The fundamental difference is what is being characterized — the fixture's directional beam, or the illuminance at a receiving location.
A goniophotometer characterizes an AGL fixture's photometric performance across angles, producing the intensity distribution that fixture and beam specifications are built around. A lux meter measures how much of that light actually lands on a surface at a given point, which is what surface illuminance and uniformity requirements are built around. Neither instrument substitutes for the other.
For AGL: reach for a goniophotometer to test and characterize a fixture's photometric performance, and a lux meter to check illuminance on the runway or taxiway surface.
Engineering & Expert Notes
Use the Instrument for the Question Being Asked
PrincipleOne characterises a fixture; the other measures illumination. Do not substitute.
Control Position and Orientation With a Lux Meter
SetupSmall changes alter readings. Mark positions and record them.
State Calibration Status in Reports
CalibrationInclude calibration dates and traceability for both.
Interpret Results Against the Right Requirement
InterpretationCandela data and lux data answer different requirements.
Frequently Asked Questions
Goniophotometer → BEAM (how the fixture's intensity is distributed across directions). Lux meter → SURFACE (how much light reaches a given point).
Generally no. A lux meter reads illuminance at its own location; it does not by itself produce a standardized angular photometric distribution of the fixture.
Intensity values across angles, typically presented as intensity vs horizontal angle, intensity vs vertical angle, a polar distribution, or a photometric data table.
Because AGL lights are highly directional — approach lights, edge lights, centreline lights and PAPI units are designed to project specific intensity within defined angular sectors, so performance has to be verified across those angles, not at a single point.
Under simplified point-source, normal-incidence conditions, E = I / r² relates illuminance (lux) to intensity (candela) and distance, though real measurements must account for geometry and applicable test procedures.
When the question is about surface illumination — how much light reaches the pavement across an area, and how uniform it is — rather than about the fixture's own directional performance.
Governing Documents
For an actual project design, 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 interpreting goniophotometric intensity data and lux-based illuminance in AGL specifications.
ICAO Annex 14, Volume I
Aerodrome Design and Operations — photometric characteristics of aeronautical ground lights, intensity and colour requirements over specified angular sectors.
CIE Photometry Standards
International Commission on Illumination definitions and measurement methods for luminous intensity, illuminance, and goniophotometric test geometry.
Final Engineering Note
Do not treat a lux reading as a substitute for a goniophotometric intensity distribution in a project specification. Confirm which photometric quantity the governing standard actually requires before selecting an instrument or acceptance criterion.
A goniophotometer characterizes the light source and its beam distribution; a lux meter measures the resulting illuminance at the receiving location.
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