What is light distribution?
How an airfield light shares its intensity across every direction around it. It is the complete picture that beam direction and beam spread are read from.
Light distribution describes how a fixture's luminous intensity, measured in candela, is spread through space. Intensity is usually highest near the beam axis and falls away in the other directions, so a single number is never enough. The full description gives the intensity for each combination of horizontal and vertical angle.
The usual way to show it is an isocandela diagram. Horizontal angle runs along one axis, vertical angle along the other, and each contour joins directions of equal intensity, much like contour lines on a map. Standards use these diagrams to state what a light must provide in each region. Real diagrams are irregular shapes; the ellipses here are schematic.
This is why distribution matters in operation. As an aircraft moves, the pilot looks at the light from a continually changing angle, so the intensity seen depends on where that angle falls in the pattern. Beam direction and beam spread are two features of the same pattern: where its peak sits and how wide it is. The measurement view of the same idea is in Photometric Distribution and Isocandela Diagram.
Frequently Asked Questions
Q1What is light distribution in airfield ground lighting?
How a fixture's luminous intensity, measured in candela, is distributed in space. It describes the intensity in every direction around the fixture, in both horizontal and vertical angle, rather than in one direction only.
Q2How is light distribution shown?
Usually as an isocandela diagram: a plot with horizontal angle on one axis and vertical angle on the other, where each contour line joins the directions that have the same intensity. Standards use these diagrams to state the intensity a light must provide in each region.
Q3What is the difference between light distribution, beam direction and beam spread?
Beam direction is where the beam axis points. Beam spread is the width of the beam around that axis. Light distribution is the complete pattern, so direction and spread are both read from it.
Q4Why does light distribution matter in operation?
A pilot sees a light from a changing angle as the aircraft moves, so the intensity received depends on where that angle falls in the distribution. Misaligned, dirty or degraded fixtures can reduce or reshape the pattern, which is why photometric performance is checked and not judged by eye.
Q5Where are the required distributions defined?
In the isocandela diagrams of ICAO Annex 14, Appendix 2 for each light type, and in FAA AC 150/5345-46 and the fixture manufacturer's photometric data. Always confirm against the current edition.
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