PHOTOMETRIC INTENSITY VS AVERAGE INTENSITY
Intensity at a Point vs Intensity Over a Range — photometric intensity describes the luminous intensity in one specific direction, while average intensity reduces several directional readings across a defined angular range into a single mean value. Both are expressed in candela, but they answer different engineering questions on an AGL fixture's beam.
At a Glance
| Feature | Photometric Intensity | Average Intensity |
|---|---|---|
| Meaning | Intensity at a specific direction/angle | Mean intensity over a specified range |
| Unit | Candela (cd) | Candela (cd) |
| Directional detail | High | Reduced into one value |
| Shows beam pattern | Yes | No, not by itself |
| Used for | Detailed photometric performance | Overall/representative intensity |
| AGL relevance | Very important | Useful for summarizing performance |
| Example | 8,000 cd at 3° elevation | 6,500 cd average from 2°–5° |
What Is Photometric Intensity & Average Intensity?
Photometric Intensity
The luminous intensity in a particular direction from a fixture. For an AGL light, each angle around the beam has its own photometric intensity value, and together these values describe the photometric intensity distribution.
Example distribution
| Direction | Intensity |
|---|---|
| 0° | 5,000 cd |
| 1° | 7,000 cd |
| 2° | 9,000 cd |
| 3° | 8,000 cd |
| 4° | 6,000 cd |
| 5° | 3,000 cd |
Average Intensity
The mean luminous intensity taken over a defined angular range or set of directions. It reduces several directional readings into one representative figure.
Worked example
Using the 2°–4° readings from the distribution at left:
Iavg = 7,667 cd for those three directions
Point/Direction vs Range/Mean
"What is the intensity HERE?"
Photometric intensity answers this at one specific angle — for example, 8,000 cd at 3°.
"What is the MEAN intensity over this range?"
Average intensity answers this across several directions — for example, (9,000 + 8,000 + 6,000) / 3 = 7,667 cd.
Easy memory: Photometric intensity → POINT/DIRECTION. Average intensity → RANGE/MEAN.
Why This Matters on the Beam
AGL lights are directional. An approach or runway light must produce appropriate intensity over a particular angular region as an aircraft moves through the beam. The individual directional intensities show how the light actually performs across that beam.
An average value can summarize part of that performance, but on its own it cannot show the detailed distribution an aircraft will actually encounter.
Same Average, Different Beams
Consider two fixtures with very different beam shapes that happen to share the same average intensity.
Fixture A (flat 10,000 cd throughout) and Fixture B (2,000 / 8,000 / 12,000 / 18,000 cd) both average to 10,000 cd — yet their beam distributions are clearly different. The same average intensity does not mean the same photometric performance.
Photometric vs Peak vs Average Intensity
Peak intensity is often confused with average intensity, but the two are distinct.
| Term | Meaning |
|---|---|
| Photometric intensity | Intensity at a specified direction |
| Peak intensity | Highest intensity found in the relevant distribution |
| Average intensity | Mean intensity over a defined range |
Peak intensity (at 2°) = 10,000 cd | Average over 0°–4° = 6,800 cd
Peak ≠ Average ≠ Individual directional intensity — each answers a different engineering question.
Horizontal-Direction Measurements
Imagine an AGL light with the following horizontal-direction photometric intensity readings:
| Angle | Photometric Intensity |
|---|---|
| −5° | 2,000 cd |
| −2.5° | 6,000 cd |
| 0° | 10,000 cd |
| +2.5° | 7,000 cd |
| +5° | 3,000 cd |
Iavg = 5,600 cd across those five points
- Directional intensity at 0° = 10,000 cd
- Average over those five points = 5,600 cd
- Peak intensity = 10,000 cd
"Average" Depends on the Governing Test
The exact meaning of average intensity depends on how the applicable test or specification defines the averaging. It could involve a defined angular interval, specific measurement directions, a specified beam sector, or multiple measurement points.
Do not assume "average intensity" always means a simple average of any arbitrary measurements. For an actual AGL compliance test, the applicable specification/test procedure controls the calculation.
Source-Side Intensity vs Surface Illuminance
Photometric and average intensity are both source-side directional quantities. They describe what the fixture emits, not what a surface downstream actually receives.
See also: Luminance vs Illuminance and Photometry vs Lux Measurement.
Myths vs Facts
Incorrect. A flat 10,000 cd distribution and a steeply varying 2,000–18,000 cd distribution can share the same 10,000 cd average while producing very different beam performance.
They are different quantities. Peak intensity is the single highest value in a distribution; average intensity is the mean across a defined range, and the two rarely coincide.
The applicable specification or test procedure defines exactly which points, intervals, or sectors go into the average — it is not left to arbitrary interpretation.
Full Engineering Parameters
| Parameter | Photometric Intensity | Average Intensity |
|---|---|---|
| Represents | Directional light intensity | Mean directional intensity |
| Measurement | Specific angle/direction | Defined range/set of directions |
| Unit | cd | cd |
| Beam detail | Preserved | Reduced |
| Peak identification | Possible | Not from average alone |
| Beam uniformity | Can be assessed from distribution | Limited |
| AGL fixture testing | Fundamental | Supplementary/summary measure |
| Same value for different beams? | Less likely | Possible |
Photometric Intensity vs Average Intensity
Photometric intensity and average intensity are not interchangeable quantities. Both are expressed in candela, but they answer different questions about the beam.
Photometric intensity preserves the detailed beam distribution at a specific direction; average intensity reduces a defined angular range to a single representative value. A fixture can share the same average with a very different fixture while performing very differently across the actual beam. Which value applies depends on what the governing specification is asking for.
For AGL: use photometric intensity to evaluate how the beam actually performs at each angle, and average intensity when a specification asks for a single representative figure over a defined range.
Engineering & Expert Notes
Do Not Let an Average Hide a Weak Point
PrincipleCheck individual points against requirements.
Report Point Values as Well as the Mean
ReportingShow the individual data.
Align Before Measuring
SetupOrientation affects readings.
Retest After Corrective Action
VerificationConfirm the fix with the same method.
Frequently Asked Questions
Photometric intensity → POINT/DIRECTION (intensity at one angle). Average intensity → RANGE/MEAN (intensity averaged over several angles).
Yes. Both are expressed in candela (cd), because both are based on luminous intensity. The difference is how the value is represented, not the unit.
Yes. A flat distribution and a widely varying distribution can produce the same average, even though their actual photometric performance across the beam is very different.
No. Peak intensity is the single highest value in a distribution, while average intensity is the mean over a defined range. The two are rarely equal.
No. The applicable test or specification defines which angular interval, directions, or beam sector are used in the averaging — it is not a fixed, universal calculation.
Photometric and average intensity are source-side quantities describing what the fixture emits in candela. Illuminance in lux describes what a surface downstream actually receives once that light travels and lands.
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 intensity requirements and averaging methods for runway and approach lighting.
ICAO Annex 14, Volume I
Aerodrome Design and Operations — photometric characteristics of aeronautical ground lights, including intensity distribution requirements.
CIE Photometry Standards
International Commission on Illumination definitions and measurement methods for luminous intensity and its averaging.
Final Engineering Note
Do not treat an average-intensity figure as equivalent to the full photometric distribution. Confirm which quantity, and which averaging method, the governing standard actually requires.
Photometric intensity is not average intensity — one preserves the beam's detailed distribution, the other summarizes a defined portion of it into a single figure.
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