Find the coverage width of a fixture beam at a given range, and how much neighbouring beams overlap.
Enter the beam spread from the fixture data sheet, the range you care about and the fixture spacing. The tool gives the width the beam covers, the overlap or gap between adjacent fixtures, the range at which beams first meet, and the spread or spacing needed to reach a target overlap.
Applies to every length. The geometry is the same in either unit.
From the data sheet. Note the intensity level it is defined at, such as 50 % of peak.
Gives the beam height at range. The usable percentage is applied to it as well.
Distance from the fixture along the beam axis.
Fixtures are taken to point the same way, side by side.
Trims the coverage to the part of the beam you consider effective. This is your own rule.
The overlap you want between adjacent beams. This is your own rule, not a standard value.
| Range | Effective width | Overlap or gap | Overlap (%) | Status |
|---|
Every step is shown so the result can be checked by hand.
The beam widens with range. At range d the full width across the axis follows from the full spread angle θ, then the usable percentage is applied.
Adjacent fixtures overlap when the width is more than the spacing, and leave a gap when it is less.
Beams from neighbours first touch where the width equals the spacing.
For a target overlap fraction t, the width must be the spacing divided by (1 − t). That gives the spread needed, or the largest spacing for a given spread.
| Item | Value |
|---|---|
| 10° spread at 300 m | W = 52.5 m |
| Spacing 60 m | Gap of 7.5 m |
| Beams first meet at | 342.9 m |
| 20 % overlap needs | W = 75 m, 14.25° spread |
| Largest spacing for 20 % | 42.0 m at 10° |
It does not model intensity across the beam, aiming tilt, or the visibility needed by a pilot. Beam spread depends on the intensity level it is defined at, so two fixtures with the same quoted spread can differ in usable width. Use the photometric data for intensity. For fixtures certified to ICAO Annex 14 or FAA specifications, read the angle from the isocandela diagram at the intensity level you need, because a lower intensity level gives a wider beam.
Quick answers on the coverage width of a fixture beam and how neighbouring beams overlap.
It finds the coverage width of a fixture beam at a given range, and how much neighbouring beams overlap. It also gives the range at which beams first meet, and the spread or spacing needed to reach a target overlap.
Enter the horizontal beam spread as a full angle from the fixture data sheet, the range to the point of interest, and the spacing between adjacent fixtures. The vertical spread is optional and gives the height of the beam at range. Note the intensity level at which the data sheet defines the spread, such as 50 percent of peak. Lengths can be in metres or feet.
The full width across the axis at range d is 2 times d times the tangent of half the spread angle, times the usable width percentage. The usable width trims the coverage to the part of the beam you consider effective, which is your own rule.
Adjacent fixtures overlap when the width is more than the spacing, and leave a gap when it is less. The overlap is the width minus the spacing, and the overlap percentage is that divided by the width, times 100. Fixtures are taken to point the same way, side by side.
Beams from neighbours first touch where the width equals the spacing. That range is the spacing divided by 2 times the tangent of half the spread angle, times the usable percentage. The tool also shows how many beams cover the point, which is the width divided by the spacing.
For a target overlap fraction t, the width must be the spacing divided by 1 minus t. That gives the spread needed at your range and spacing, and the largest spacing for your spread and range, which is the width times 1 minus t. The target overlap is your own rule, not a standard value.
A 10 degree spread at 300 m gives a width of 52.5 m. With a spacing of 60 m there is a gap of 7.5 m, and the beams first meet at 342.9 m. A 20 percent overlap needs a width of 75 m, which means a 14.25 degree spread, or a largest spacing of 42.0 m at 10 degrees.
It uses flat-plane geometry with the beam axis square to the coverage plane. It does not model intensity across the beam, aiming tilt, or the visibility needed by a pilot. The edge of a real beam is not sharp, so treat the width as the stated spread and not a cut-off. Two fixtures with the same quoted spread can differ in usable width, so take the beam spread and its intensity level from the manufacturer's photometric data.
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