HORIZONTAL PHOTOMETRY VS VERTICAL PHOTOMETRY
Azimuth Distribution vs Elevation Distribution — horizontal photometry describes how a fixture's intensity changes left/right around the horizontal plane, while vertical photometry describes how it changes up/down through the vertical plane. An AGL fixture must provide the correct directional light distribution in both, not simply a high peak intensity.
At a Glance
| Feature | Horizontal Photometry | Vertical Photometry |
|---|---|---|
| Measurement plane | Horizontal plane | Vertical plane |
| Main variation examined | Left/right, azimuthal distribution | Up/down, elevation distribution |
| Angular parameter | Azimuth | Elevation |
| Shows | Horizontal beam spread | Vertical beam spread |
| Important for | Lateral coverage and alignment | Approach/viewing angles and vertical coverage |
| Unit of intensity | Candela (cd) | Candela (cd) |
| Typical output | Intensity vs azimuth | Intensity vs elevation |
What Is Horizontal & Vertical Photometry?
Horizontal Photometry
Examines the light distribution around the horizontal plane — imagine looking down at the fixture from directly above. Intensity is recorded at a series of azimuth angles around the fixture.
Question answered
How does the light intensity change as I move around the fixture horizontally?
Vertical Photometry
Examines the light distribution through a vertical plane — imagine viewing the fixture from the side. Intensity is recorded at a series of elevation angles above and below the horizontal.
Question answered
How does the light intensity change as I move above or below the horizontal direction?
Left/Right vs Up/Down
Easy memory: Horizontal → LEFT / RIGHT. Vertical → UP / DOWN.
The Key Terminology
This is the distinction that underpins both measurement types.
| Angle | Belongs To | Describes |
|---|---|---|
| Azimuth | Horizontal photometry | Horizontal angular direction — N/S/E/W around the fixture |
| Elevation | Vertical photometry | Vertical angular direction — above/below the horizontal reference |
Azimuth = horizontal angular direction. Elevation = vertical angular direction. Every photometric curve is plotted against one of these two angles.
Reading the Two Data Sets
The same fixture produces two separate data sets — one describing its horizontal distribution, one describing its vertical distribution.
| Azimuth | Intensity |
|---|---|
| −30° | 2,000 cd |
| −15° | 5,000 cd |
| 0° | 8,000 cd |
| +15° | 5,500 cd |
| +30° | 2,500 cd |
| Elevation | Intensity |
|---|---|
| −5° | 1,000 cd |
| −2° | 6,000 cd |
| 0° | 8,000 cd |
| +2° | 7,000 cd |
| +5° | 2,000 cd |
The horizontal set describes the fixture's horizontal intensity distribution; the vertical set describes its vertical intensity distribution. Both are needed to characterise the beam fully.
Two Curves, Two Planes
Each data set is typically represented as its own curve, plotted against its own angle.
Intensity plotted against azimuth angle — peak at 0°, tapering toward the sides.
Intensity plotted against elevation angle — the peak sits at the design elevation setting.
The exact shape of each curve depends entirely on the fixture's optical design and intended application.
Shaping the Beam in Two Directions
An AGL fixture is not simply a point source emitting equal light in every direction. Its beam is shaped, and the designer needs to know three things:
- Where the light goes horizontally
- Where the light goes vertically
- How much intensity is produced in each direction
Horizontal Photometry in AGL
Important when evaluating whether the light provides appropriate coverage across the required azimuthal sector — particularly relevant to directional fixtures that must cover a defined lateral viewing region.
Vertical Photometry in AGL
Important because the observer or aircraft can be at different elevations relative to the fixture — the fixture needs appropriate intensity across the required range of elevation angles.
A Runway Light Viewed on Approach
Consider a runway light viewed by an approaching aircraft.
| Photometry Type | Question It Asks |
|---|---|
| Vertical | How much intensity does the fixture provide at different elevation angles? |
| Horizontal | How much intensity does it provide as the aircraft or viewing direction moves laterally? |
Horizontal vs Vertical — What Each Covers
| Question | Horizontal | Vertical |
|---|---|---|
| Does the beam cover left/right? | Yes | No |
| Does the beam cover above/below? | No | Yes |
| Main angle | Azimuth | Elevation |
| Plane | Horizontal | Vertical |
| Typical view | Top/plan view | Side/elevation view |
| Intensity unit | cd | cd |
Myths vs Facts
They are not the same. Horizontal and vertical photometry describe the fixture's own directional intensity distribution in candela, plotted against azimuth or elevation. Illuminance describes light landing on a surface, measured in lux with a lux meter.
A single candela figure only describes intensity at one angle. Without the horizontal and vertical curves, it says nothing about how that intensity falls off with azimuth or elevation.
Both are needed together. Horizontal photometry describes lateral coverage, while vertical photometry describes coverage across the elevation angles an approaching aircraft will actually occupy.
Full Engineering Parameters
| Parameter | Horizontal Photometry | Vertical Photometry |
|---|---|---|
| Plane | Horizontal | Vertical |
| Angle | Azimuth | Elevation |
| Direction | Around the fixture | Above/below reference plane |
| Main purpose | Lateral beam distribution | Vertical beam distribution |
| Output | Intensity vs azimuth | Intensity vs elevation |
| Unit | cd | cd |
| AGL importance | Lateral coverage | Vertical viewing/coverage |
| Typical representation | Plan/polar diagram | Elevation/vertical curve |
Horizontal vs Vertical Photometry
Horizontal and vertical photometry are not alternative ways of describing the same thing. They describe intensity distribution in two different planes, using two different angular parameters.
Horizontal photometry answers how the beam spreads left and right. Vertical photometry answers how it spreads up and down. Together they define the fixture's full directional photometric performance — neither one alone is sufficient for AGL design or acceptance.
For AGL: use horizontal photometry to judge lateral alignment and coverage, and vertical photometry to judge whether the beam reaches the aircraft at its actual approach elevation.
Engineering & Expert Notes
Read Both Planes Together
PrincipleA beam can be right in one plane and wrong in the other.
Align Before Measuring
SetupOrientation errors move the pattern. Verify alignment.
Record Angles and Positions
RecordsInclude measurement geometry with each result.
Compare With the Specification Envelope
ComplianceCheck points at the specified angles, not only the peak.
Frequently Asked Questions
Horizontal → left/right, measured by azimuth. Vertical → up/down, measured by elevation.
No. Both express intensity in candela (cd). What differs is the angle each is plotted against — azimuth for horizontal, elevation for vertical — not the unit itself.
Yes. The two planes are shaped independently by the optical design, so a fixture can be wide horizontally while remaining tightly focused vertically, or vice versa.
Horizontal and vertical photometry are the two planar slices most commonly used to characterise a fixture's overall luminous intensity distribution, which in full form is a three-dimensional surface.
Neither matters "more" in general — but on final approach, the aircraft's elevation relative to the fixture changes continuously, making vertical coverage critical for the light to remain visible at the correct glide angle.
Horizontal and vertical photometry describe the fixture's own directional output in candela. Illuminance describes light that has already arrived at a surface, measured in lux — a different quantity entirely.
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 — specifies the horizontal and vertical photometric envelopes required for runway and approach light fixtures.
ICAO Annex 14, Volume I
Aerodrome Design and Operations — photometric characteristics of aeronautical ground lights, including required beam spread in azimuth and elevation.
CIE Photometry Standards
International Commission on Illumination definitions and measurement methods for luminous intensity distribution in horizontal and vertical planes.
Final Engineering Note
Do not accept a fixture based on a single peak-intensity value. Confirm both the horizontal (azimuth) and vertical (elevation) photometric curves against the governing standard.
Horizontal photometry is not vertical photometry — one describes where the light goes left/right, the other describes where it goes up/down. For AGL, both together define the fixture's directional photometric performance.
Compare the Systems. Then Learn How to Engineer Them.
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