BEAM INTENSITY VS BEAM SPREAD
Strength vs Width — beam intensity describes how strong an AGL light is in a particular direction, measured in candela, while beam spread describes how wide that beam extends over angular directions, measured in degrees. They are closely related but not the same parameter, and an AGL fixture needs both specified correctly.
At a Glance
| Feature | Beam Intensity | Beam Spread |
|---|---|---|
| Main meaning | Strength of light in a direction | Angular width/coverage of the beam |
| Typical quantity | Luminous intensity | Angle |
| Unit | Candela (cd) | Degrees (°) |
| Main question | How strong is the beam here? | How wide is the beam? |
| Directional? | Yes | Yes |
| AGL importance | Required light level in viewing directions | Required angular coverage |
| Measurement | Photometric intensity | Angular distribution / beam width |
What Is Beam Intensity & Beam Spread?
Beam Intensity
Beam intensity describes the luminous intensity in a particular direction. Picture an AGL fixture pointing at an approaching aircraft: the intensity value tells you how strong the emitted light is along that specific line of sight.
Example
A fixture might read 10,000 cd at 0°, 8,000 cd at +2°, and 2,000 cd at +5° — the intensity changes with direction.
Beam Spread
Beam spread describes how far the beam extends angularly. Picture the same fixture's cone of light widening or narrowing: the spread value tells you the angular width between the beam's boundaries.
Example
A beam specified with a 20° spread extends its useful light output across a 20-degree angular sector, regardless of how strong that light is at the peak.
Strength vs Width
Easy memory: Intensity → STRENGTH, measured in candela. Spread → WIDTH, measured in degrees.
Same Beam, Two Different Numbers
Suppose an AGL light has a peak intensity of 10,000 cd, and its specified beam extends over 20°. These describe two different things about the same light:
The 10,000 cd figure describes intensity; the 20° figure describes angular spread. They are different parameters, and neither one can be inferred from the other.
Narrow vs Wide Beam
Consider two lights with the same peak intensity but different optical distributions:
Same peak intensity does not mean same beam spread. Light B distributes its 10,000 cd peak over a wider angular region than Light A.
A Wide Beam Doesn't Automatically Mean More Light
A wider beam does not automatically mean higher intensity. The optical system may simply be distributing the available light over a larger angular region rather than concentrating it. The actual intensity distribution across angle must be determined photometrically — it can't be assumed from the beam's apparent width alone.
Why AGL Fixtures Need Both Parameters
AGL fixtures are designed to produce specific directional distributions toward an approaching or taxiing aircraft. The designer needs both parameters satisfied at once, not just one:
Beam Intensity
Enough luminous intensity in the required viewing direction, so the light is visible at the necessary range and conditions.
Beam Spread
Enough angular coverage to serve the required viewing sector, so the light remains visible across the intended approach or taxi geometry.
Horizontal and Vertical Beam Spread
Beam spread is not a single number — an AGL fixture can have a different angular width in the horizontal (azimuth) plane than in the vertical (elevation) plane, which connects directly to horizontal vs vertical photometry.
| Plane | Describes |
|---|---|
| Horizontal spread | Angular width in the azimuth direction |
| Vertical spread | Angular width in the elevation direction |
An AGL fixture's horizontal and vertical beam spreads are typically specified separately, since a fixture may need a wide horizontal sector but a narrow vertical sector, or vice versa.
Beam Spread and Photometric Intensity
A photometric distribution typically shows intensity decreasing gradually away from the beam centre:
| Angle | Intensity |
|---|---|
| −10° | 500 cd |
| −5° | 3,000 cd |
| 0° | 10,000 cd |
| +5° | 3,000 cd |
| +10° | 500 cd |
A specified intensity threshold can then be used to define a particular beam width or spread, depending on the applicable photometric definition being applied.
How the threshold is usually set: ICAO Annex 14 and FAA AC 150/5340-30 typically define beam spread from the isocandela diagram, as the angle between the points where intensity has fallen to a specified fraction of the peak — commonly the 50% (half-intensity) points — not simply where the beam "looks like" it ends. In the table above, the true beam edges sit wherever the curve crosses that threshold, which will not necessarily line up with the ±5° or ±10° sample points shown. Many AGL specifications also work from main beam average intensity — the average intensity across the isocandela diagram — rather than the single peak figure used for simplicity in the examples on this page. Always confirm which definition the applicable standard and manufacturer's photometric report are using.
Beam Spread vs Light Coverage
Beam spread is an angular characteristic. Coverage, by contrast, considers the actual physical area or region illuminated at a particular distance and geometry. The same angular spread covers a wider physical width as distance increases, because the angle stays fixed while the geometry scales.
Angle Stays Fixed, Physical Width Grows
Suppose a beam has a fixed angular spread. Close to the fixture, that spread covers a narrow physical width; further away, the same angular spread covers a wider physical width:
Beam Intensity vs Illuminance
This also connects to candela vs lux. Beam intensity is a source/directional property; once that beam reaches a surface at some distance, the resulting light level there is illuminance:
Full Engineering Parameters
| Parameter | Beam Intensity | Beam Spread |
|---|---|---|
| Describes | Strength | Angular width |
| Main unit | cd | ° |
| Question | How strong? | How wide? |
| Related to | Luminous intensity | Angular distribution |
| Depends on direction | Yes | Yes |
| Can be plotted | Intensity vs angle | Beam width vs angle |
| AGL use | Ensure sufficient directional light | Ensure required angular coverage |
| Same for all fixtures? | No | No |
Two Runway Lights Compared
| Parameter | Light A | Light B |
|---|---|---|
| Peak intensity | 10,000 cd | 10,000 cd |
| Horizontal spread | 10° | 20° |
| Vertical spread | 5° | 8° |
Both lights have the same peak intensity, but Light B covers a wider angular region.
| Parameter | Light A | Light B |
|---|---|---|
| Peak intensity | 5,000 cd | 10,000 cd |
| Beam spread | 20° | 20° |
Here, angular coverage is similar, but Light B is much more intense at its peak — showing that intensity and spread vary independently of each other.
Myths vs Facts
No. A wider beam spreads the available light over a larger angular region — it does not automatically increase peak intensity, and can even reduce it if the same light output is distributed more broadly.
No. Peak intensity alone says nothing about angular width. Two fixtures can share the same peak candela figure while having very different horizontal and vertical beam spreads.
Not quite. Beam spread is an angular characteristic of the fixture. Physical coverage on a surface depends on that angle combined with distance and geometry.
Beam Intensity vs Beam Spread
Beam intensity and beam spread are not interchangeable quantities. The fundamental difference is what each one describes — how strong the light is versus how wide it reaches.
Beam intensity describes the strength of light an AGL fixture emits in a given direction, expressed in candela. Beam spread describes how far that beam extends angularly, expressed in degrees. A complete beam specification always needs both, since neither one can be inferred from the other.
For AGL: specify both — sufficient intensity in the required viewing directions, and the correct horizontal/vertical angular coverage to serve the intended sector.
Engineering & Expert Notes
Both Must Be Right for the Light to Work
PrincipleA bright narrow beam seen off-axis, or a wide dim one, can fail the purpose. Verify intensity and distribution together.
Alignment Changes What You Measure
AlignmentSmall aiming errors alter measured intensity at a point. Align before drawing conclusions.
Look at the Distribution, Not One Number
MethodReview the measured points across the envelope rather than a single peak value.
Report the Measurement Geometry
ReportingRecord angles and positions with each result so others can interpret it.
Frequently Asked Questions
Intensity → STRENGTH (how strong the light is in a direction). Spread → WIDTH (how wide the beam extends angularly).
Yes. Two fixtures can share an identical peak intensity while distributing that light over very different angular widths — one narrow, one wide.
Not necessarily by definition, but in practice, distributing a given light output over a wider angular region often reduces the peak intensity compared to concentrating it narrowly. The actual figures must be confirmed photometrically.
Because the required angular coverage in the azimuth plane and the elevation plane usually serve different operational purposes, so an AGL fixture's beam can be wide in one plane and narrow in the other.
No. Beam spread is the fixture's angular characteristic. Coverage on a surface also depends on distance and geometry, since the same angular spread covers a larger physical width further from the fixture.
Beam intensity is expressed in candela, the same source-side quantity used in the candela vs lux comparison. Once that beam reaches a surface at some distance, the resulting light level there is illuminance, in lux.
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 and beam distribution requirements relevant to runway and approach lighting.
ICAO Annex 14, Volume I
Aerodrome Design and Operations — photometric characteristics of aeronautical ground lights, including intensity and angular distribution requirements.
CIE Photometry Standards
International Commission on Illumination definitions and measurement methods for luminous intensity and angular beam characteristics.
Final Engineering Note
Do not treat a candela figure and a beam-spread figure as substitutes for each other in a project specification. Confirm both are stated, and for the correct viewing directions and planes.
Beam intensity is not beam spread — one describes how strong the light is, the other describes how far it reaches angularly.
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