What is an isocandela diagram?
A chart of a fixture’s light on a grid of horizontal and vertical angle, with curves joining directions of equal intensity — read a direction by the two curves it lies between.
An isocandela diagram draws the curves of a fixture’s intensity on a grid of horizontal angle across and vertical angle up, centred on the beam axis. Each curve joins the directions of equal candela and is marked with its value, like the contour lines on a map.
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 changes quickly there. This is how AGL light distribution is normally shown. The standard gives a diagram of the required intensity for each light type, and a fixture’s measured distribution is compared with it. The whole pattern is what counts, not the peak alone, because two fixtures with the same peak can spread their light very differently.
- Find the axes. Horizontal angle runs across and vertical angle runs up, both measured from the beam axis at the centre.
- Pick a direction. Mark the direction you want by its horizontal and vertical angle.
- Find the curves. See which two curves the point lies between, or whether it sits on one.
- Read the band. Its intensity is between the two candela values marked on those curves.
- Check the spacing. Close curves mean the intensity is changing quickly; wide curves mean it changes gently.
- Compare. Set the measured diagram against the required diagram and notes for that light type.
Frequently Asked Questions
Q1What is an isocandela diagram?
An isocandela diagram is a chart of a fixture's light distribution. It is drawn on a grid of horizontal angle across and vertical angle up, centred on the beam axis, with curves that each join the directions of equal luminous intensity. Each curve is marked with its candela value.
Q2How do you read an isocandela diagram?
Find the direction you want by its two angles from the beam axis. Then see which two curves it lies between: its intensity is between their candela values. Directly on a curve, it has that curve's value.
Q3What is the difference between isocandela and an isocandela diagram?
Isocandela is the idea: equal candela, the contour of equal intensity. The isocandela diagram is the drawing that puts several of those curves on one angle grid so the whole beam can be seen and read at once.
Q4What do closely spaced curves mean?
Closely spaced curves mean the intensity changes quickly over a small change in angle, which is a steep fall. Widely spaced curves mean the intensity changes gently. The spacing shows how sharp or how soft the edge of the beam is.
Q5Why not just use the peak candela value?
Two fixtures can have the same peak and spread their light very differently. The diagram shows the whole pattern: how wide and tall the beam is, whether the peak is centred and how fast the intensity falls away. The pattern is what is measured and checked.
Q6How is the diagram used for AGL?
For each light type the standard gives a diagram of the required intensity, with notes on how its values apply. A fixture's measured distribution is compared with it to see whether the light meets the requirement in the directions that matter. The diagram is one way of presenting the full photometric distribution.
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