What is isocandela?
A curve that joins every direction in which a fixture gives the same intensity. Drawn together, the curves show how a light is distributed.
Isocandela means equal candela. An isocandela curve joins all the directions in which a fixture gives the same luminous intensity. It works like the contour lines on a map: a map joins points of equal height, and an isocandela curve joins directions of equal intensity. The fixture’s intensity is the hill, usually highest near the beam axis and falling away on every side.
An isocandela diagram draws several of these curves on a grid of horizontal angle across and vertical angle up, centred on the beam axis, with each curve marked with its candela value. To read it, find the direction by its two angles and see which curves it lies between: its intensity is between their values. Curves packed close together mean the intensity is changing quickly there.
This is how AGL light distribution is normally shown. For each light type the standard gives a diagram of the intensity required, and a fixture’s measured distribution is compared with it. The notes to each figure say how the values apply. The curves draw the beam pattern, and the diagram is one way of presenting the full photometric distribution.
Frequently Asked Questions
Q1What does isocandela mean?
Equal candela. An isocandela curve joins all the directions in which a fixture gives the same luminous intensity, measured in candela.
Q2What is an isocandela diagram?
A plot of several isocandela curves on a grid of horizontal and vertical angle around the beam axis, with each curve marked with its candela value. It shows a fixture's light distribution, or a requirement for it, in a single picture.
Q3How do I read an isocandela diagram?
Take the direction you are interested in, find it from its horizontal and vertical angles, and see which two curves it lies between. Its intensity is between their values. Where curves are close together, intensity is changing quickly.
Q4How is isocandela different from isolux?
Isocandela curves are drawn in angle around the fixture and show intensity in candela, which describes the light source. Isolux curves are drawn on a surface such as the ground and join points of equal illuminance in lux, which describes what the light does at a place.
Q5Where are isocandela diagrams used in AGL?
ICAO Annex 14 Vol. I, Appendix 2 gives one for each light type to show the intensity required, and other standards such as FAA AC 150/5345-46 do the same. Manufacturers publish them for their fixtures, and they are used in design and photometric acceptance. Always confirm against the current edition.
Q6Is an isocandela diagram the same as photometric distribution?
It is one way of showing it. The photometric distribution is the pattern of intensity in every direction, and it can be presented as an isocandela diagram, as polar or Cartesian curves, or as a table of candela values at defined angles.
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