What is photometric distribution?
The full pattern of how a fixture’s light intensity is spread in every direction around it.
Photometric distribution is the pattern of a fixture’s luminous intensity in every direction around it, in candela. It is shown as curves in a plane, as an isocandela diagram, or as a table of candela values at defined angles, and all of these describe the same pattern. Intensity in candela is a property of the fixture and does not change with distance; the illuminance it produces on a surface falls with the square of the distance.
An AGL fixture usually spreads its light differently in the horizontal and vertical planes (an elevated taxiway edge light, for example, is nearly all-round horizontally but narrow vertically), so a horizontal and a vertical section are shown together. The distribution, not a single peak value, is what is measured and checked against the requirement, because two fixtures with the same peak candela can spread their light very differently.
Frequently Asked Questions
Q1What is photometric distribution?
The pattern of a fixture’s luminous intensity in every direction around it. It records how many candela the fixture sends at each angle, not just in one direction.
Q2How is photometric distribution shown?
As curves in a plane, either polar or Cartesian, as an isocandela diagram, or as a table of candela values at defined angles. All of them describe the same pattern in different forms.
Q3Why are a horizontal and a vertical plane shown?
An AGL fixture usually spreads its light differently in the two planes. The two sections show how the pattern spreads sideways and up and down, and together they give a fuller picture than either one alone.
Q4How is photometric distribution different from beam intensity?
The distribution is the whole pattern in all directions. Beam intensity refers to the values inside the main beam, which is one region of that pattern.
Q5Why does photometric distribution matter in AGL?
Requirements are written as intensity at defined angles, and a pilot sees a fixture from many directions. Two fixtures with the same peak candela can have very different distributions, so the pattern is what is checked.
Q6Where does photometric distribution data come from?
It is measured during photometric testing and reported by the manufacturer or the test laboratory. It is then used for design and for checking acceptance against the applicable standard.
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