CANDELA VS LUX
Light Emitted vs Light Received — candela describes the luminous intensity a fixture sends out in a particular direction, while lux describes the illuminance actually falling on a surface. For AGL work, fixture and beam performance is expressed in candela, while surface illumination is expressed in lux.
At a Glance
| Feature | Candela (cd) | Lux (lx) |
|---|---|---|
| Quantity | Luminous intensity | Illuminance |
| Measures | Light emitted in a direction | Light arriving at a surface |
| Unit definition | lm/sr | lm/m² |
| Depends on direction | Yes | Depends on geometry/angle |
| Depends on distance | Intensity itself does not | Yes, at the receiving surface |
| Typical instrument | Goniophotometer / photometric system | Lux meter |
| AGL relevance | Light/beam intensity | Surface illumination |
| Key question | How strong is the light in this direction? | How much light reaches this surface? |
What Is Candela & Lux?
Candela
Candela measures luminous intensity — the strength of light emitted by a source in a particular direction. Picture an AGL fixture pointing its beam toward an approaching aircraft: the candela value describes how strong that emitted light is in that specific direction.
Example
10,000 cd in a specified direction means the fixture has a luminous intensity of 10,000 candela in that direction. It says nothing about what happens further along the beam.
Lux
Lux measures illuminance — how much light actually arrives at a surface. Picture light falling down onto a runway pavement: the lux meter reads the illuminance at that measurement location.
Example
50 lx means the measured surface receives an illuminance of 50 lux at that specific point.
Source vs Surface
Easy memory: Candela → SOURCE (how strongly the source sends light in a direction). Lux → SURFACE (how much light arrives at a surface).
How Candela and Lux Connect
Candela and lux are related, but they are not interchangeable. For an ideal point source under appropriate conditions, illuminance falls off with the square of the distance:
Inverse-square relationship between intensity and illuminance
- E = illuminance, in lux
- I = luminous intensity, in candela
- r = distance, in metres
This form holds at normal incidence (surface perpendicular to the light direction). For any other incidence angle, the more general form applies: E = I · cos θ / r², where θ is the angle between the light direction and the surface normal.
So if I = 1,000 cd and r = 10 m at normal incidence, then E = 1,000 / 10² = 10 lx — the illuminance would be 10 lux under this simplified geometry.
Same Source, Different Illuminance
A light with a fixed intensity of 1,000 cd produces a different illuminance depending on how far away the receiving surface is:
The same source intensity can therefore produce very different illuminance at different distances — the candela rating alone never tells you what's on the ground.
AGL Lights Are Highly Directional
Approach lights, runway edge lights, runway centreline lights, taxiway centreline lights, PAPI units, and stop bars are all designed to provide light in particular directions and viewing angles. A single "brightness" number is therefore not enough — the intensity can vary substantially with angle, so the beam distribution is often as important as the peak intensity.
Candela is directional. A light could have 10,000 cd in one direction, 5,000 cd in another, and only 500 cd in a third. Stating "this light is 10,000 cd" without specifying the relevant direction and photometric condition may be incomplete.
Lux Is a Surface Measurement
A lux measurement is normally taken at a defined point or across a grid of points on the runway or taxiway surface. Readings from several points can then be used to assess the illumination of the area and, where applicable, its uniformity.
| Grid Point | Illuminance |
|---|---|
| Point 1 | 42 lx |
| Point 2 | 47 lx |
| Point 3 | 51 lx |
| Point 4 | 45 lx |
Candela vs Lux in AGL
| Application | More Relevant Quantity |
|---|---|
| AGL fixture beam intensity | Candela |
| Directional light output | Candela |
| Photometric intensity distribution | Candela |
| Runway surface illumination | Lux |
| Taxiway surface illumination | Lux |
| Area illuminance survey | Lux |
| Light seen from a particular direction | Candela |
| Surface illumination uniformity | Lux |
Candela vs Lux vs Luminance
These three photometric quantities are frequently confused with one another.
| Quantity | What It Describes | Unit |
|---|---|---|
| Candela | How strongly the source sends light in a direction | cd |
| Lux | How much light arrives at a surface | lx |
| Luminance | How bright the surface appears from a direction | cd/m² |
Myths vs Facts
No. They are different physical quantities — candela describes intensity at the source, lux describes illuminance at a surface. They are not directly equal or interchangeable.
No. Illuminance depends on distance and geometry. The same 10,000 cd source produces very different lux readings depending on how far away, and at what angle, the surface is.
Not directly. Lux tells you the illuminance at the measurement location, not the intensity rating of the fixture that produced it.
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.
Full Engineering Parameters
| Parameter | Candela | Lux |
|---|---|---|
| Symbol | I | E |
| Unit | cd | lx |
| Light direction | Leaving the source | Arriving at the surface |
| Distance dependence | Not intrinsic to the value | Follows inverse-square law |
| Instrument | Goniophotometer / photometric system | Lux meter |
| Direction-sensitive? | Yes, by definition | Depends on incidence angle |
| AGL relevance | Fixture/beam performance | Surface illumination |
| Main question | How strong in this direction? | How much reaches the surface? |
Candela vs Lux
Candela and lux are not interchangeable quantities. The fundamental difference is where the light is being measured — at the source, or at the receiving surface.
Candela describes how strongly an AGL fixture emits light in a given direction, independent of distance. Lux describes how much of that light actually reaches a defined surface, which depends on distance and geometry. Which measurement is appropriate depends on whether the applicable requirement concerns fixture performance or surface illumination.
For AGL: think of candela when discussing fixture or beam performance, and lux when discussing how much illumination reaches the runway or taxiway surface.
Engineering & Expert Notes
Know Which Quantity Each Requirement Uses
TerminologyFixture requirements are in candela; surface lighting is in lux. Mixing them leads to wrong conclusions.
Geometry Matters for Both
MeasurementDistance and angle change lux readings. Record the setup with each result.
Use the Right Instrument for the Quantity
EquipmentA lux meter does not measure intensity of an aeronautical light. Select instruments for the requirement.
State Units in Every Report
ReportingAmbiguous units invite disputes. Always write the unit and conditions.
Frequently Asked Questions
Candela → SOURCE (how strongly light is emitted in a direction). Lux → SURFACE (how much light arrives at a surface).
Only with additional information. For an ideal point source, E = I / r² relates the two, but this requires knowing the distance and, in general, the angle of incidence at the surface.
AGL lights are highly directional — approach lights, edge lights, centreline lights, PAPI units and stop bars are all designed to project light along specific directions and viewing angles, so their performance is expressed as intensity in candela.
Because surface illumination is about how much light actually lands on the pavement, which depends on the distance and angle from every contributing fixture — that is exactly what illuminance in lux captures.
No. Candela describes the intensity of the source in a given direction and does not itself change with distance. It is the resulting illuminance at a surface, in lux, that changes with distance.
No. A lux meter only reads the illuminance at its own location. Characterizing a directional AGL fixture requires measuring its intensity/distribution in candela across the relevant directions, typically with a goniophotometric system.
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 for runway and approach lighting, relevant to interpreting candela and lux in AGL specifications.
ICAO Annex 14, Volume I
Aerodrome Design and Operations — photometric characteristics of aeronautical ground lights, intensity and colour requirements.
CIE Photometry Standards
International Commission on Illumination definitions and measurement methods for luminous intensity, illuminance and luminance.
Final Engineering Note
Do not treat candela and lux readings as interchangeable in a project specification. Confirm which photometric quantity the governing standard actually requires before selecting an instrument or acceptance criterion.
Candela is not lux — one describes what a fixture emits, the other describes what a surface receives.
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