ILLUMINANCE VS LUMINANCE
Light Falling On a Surface vs Brightness Seen by an Observer — illuminance describes how much light reaches a surface, while luminance describes how bright that surface or light source appears from a given viewing direction. For AGL work, the distinction separates surface illumination from what a pilot actually perceives.
At a Glance
| Feature | Illuminance | Luminance |
|---|---|---|
| Basic meaning | Light falling onto a surface | Light leaving toward the observer from a surface |
| What it describes | Illumination of a surface | Apparent brightness of a surface |
| SI unit | Lux (lx) | Candela per square metre (cd/m²) |
| Symbol | E | L |
| Depends on | Incident light | Direction, surface reflection/emission |
| Measured with | Lux meter | Luminance meter |
| Example | Runway surface receives 50 lx | Pavement appears at 10 cd/m² |
| Key question | How much light reaches the surface? | How bright does the surface appear? |
What Is Illuminance & Luminance?
Illuminance
The amount of luminous flux arriving at a surface per unit area. It describes light falling on a surface, regardless of how that surface then reflects or absorbs it.
Defining formula
E = Φ / A, where Φ is luminous flux in lumens and A is area in square metres. Measured directly at the surface with a lux meter.
Luminance
The light emitted, transmitted or reflected by a surface in a particular direction — what an observer actually sees when looking at that surface or source.
Defining relationship
Related to luminous intensity per unit projected area in the viewing direction. Measured with a luminance meter aimed at the surface from a specified direction.
Light IN vs Light OUT
Easy memory: Illuminance → IN (light arriving at the surface). Luminance → OUT (light coming from the surface toward the observer).
Lux vs Candela per Square Metre
This is the key unit distinction, and the two quantities cannot be substituted for one another.
| Quantity | Unit | Think |
|---|---|---|
| Illuminance | Lux (lx) | Light received |
| Luminance | Candela per m² (cd/m²) | Brightness seen |
Lux Meter
Measures illuminance at the sensor — the light falling directly onto its flat receptor from all directions above it.
Luminance Meter
Measures the luminance of a surface or source from a specified viewing direction, matching what an observer would perceive.
How Each Quantity Is Defined
Illuminance
- E = illuminance, lux
- Φ = luminous flux, lumens
- A = area, m²
Luminance
- L = luminance, cd/m²
- I = luminous intensity, cd
- Aproj = projected area in the viewing direction
Why This Matters on the Runway
Imagine a runway pavement illuminated by runway lighting. The light falling onto the pavement is measured as illuminance, in lux. The light reflected from that pavement toward a pilot is described as luminance, in cd/m².
A pilot does not directly perceive "lux." The pilot sees brightness and contrast from objects and surfaces — which is luminance, not illuminance. Luminance is therefore relevant when considering:
- Visual contrast
- Pavement appearance
- Light-emitting surfaces
- Signs and markings
- Visibility against background conditions
Surface Properties Matter
Two surfaces can receive exactly the same illuminance but appear different in brightness, because their reflection characteristics differ. A lighter surface generally reflects more of the incident light than a darker surface, so it will show a higher luminance under identical illumination.
Illuminance does not by itself determine luminance. The surface's reflectance and the observer's viewing direction both shape what is actually perceived as brightness.
Illuminance vs Luminance vs Luminous Intensity
These three photometric quantities are often confused with one another.
| Quantity | What It Describes | Unit |
|---|---|---|
| Luminous intensity | Light emitted in a particular direction by a source | Candela (cd) |
| Illuminance | Light arriving at a surface | Lux (lx) |
| Luminance | Light leaving a surface/source toward an observer | cd/m² |
Myths vs Facts
Incorrect. 100 lux describes the illuminance measured on the surface; 100 cd/m² would describe luminance measured for a particular viewing direction. They are different quantities and cannot be substituted for one another.
Luminance is directional. The same surface or source can present a different observed luminance depending on viewing direction and the surface's reflection characteristics.
Two surfaces receiving identical illuminance can appear very different in brightness, because luminance also depends on surface reflectance and geometry.
Full Engineering Parameters
| Parameter | Illuminance | Luminance |
|---|---|---|
| Symbol | E | L |
| Unit | lx | cd/m² |
| Light direction | Arriving at surface | Leaving toward observer |
| Surface required? | Measurement is at a surface | Surface/source being viewed |
| Instrument | Lux meter | Luminance meter |
| Visual brightness | Indirect | Directly related to perceived brightness |
| AGL relevance | Surface illumination | Visual appearance/contrast |
| Main question | How much light reaches it? | How bright does it look? |
Illuminance vs Luminance
Illuminance and luminance are not interchangeable quantities. The fundamental difference is the direction of the light being measured.
Illuminance measures light arriving at a surface, independent of how it is viewed. Luminance measures light leaving that surface toward an observer, which is what determines perceived brightness. Which measurement is appropriate depends on what the applicable lighting requirement is actually specifying.
For AGL: think of illuminance when discussing how much light reaches the runway/surface, and luminance when discussing how bright that surface or object appears to the pilot/observer.
Engineering & Expert Notes
Use the Quantity the Requirement Names
TerminologySigns use luminance; surfaces use illuminance. Match the measurement.
Control Geometry for Luminance Measurements
MethodViewing angle and distance affect results.
Use a Suitable Calibrated Instrument
EquipmentDifferent quantities need different meters.
State Units and Conditions
ReportingWrite units and setup in the report.
Frequently Asked Questions
Illuminance → IN (light arriving at a surface). Luminance → OUT (light leaving a surface toward an observer).
Not directly. Converting between the two requires knowing the surface's reflectance and the viewing geometry, since luminance also depends on how the surface reflects or emits the incident light.
A pilot perceives brightness and contrast, not incident light level. Luminance is what is directly related to that perceived brightness, while illuminance only describes what reaches the surface.
No. Luminous intensity (candela) describes light emitted in a direction by a source itself. Luminance (cd/m²) describes that light per unit projected area as seen from a surface or source.
Illuminance is measured with a lux meter placed at the surface. Luminance is measured with a luminance meter aimed at the surface from a specified viewing direction.
Yes. If their reflectance differs — for example a lighter versus darker pavement — they will show different luminance even under identical illuminance.
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 illuminance and luminance 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 illuminance, luminance and luminous intensity.
Final Engineering Note
Do not treat lux and cd/m² readings as interchangeable in a project specification. Confirm which photometric quantity the governing standard actually requires before selecting an instrument or acceptance criterion.
Illuminance is not luminance — one describes what reaches a surface, the other describes what an observer actually sees.
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