What is a polar diagram?
A plot of a fixture’s intensity in one plane: the angle round the centre is the direction, the distance from the centre is the candela, and the curve is the beam seen in section.
A polar diagram shows luminous intensity against direction in one plane. The fixture sits at the centre, the angle round the centre is the direction from the beam axis, and the distance from the centre is the intensity in candela. The curve is longest where the light is strongest, which is usually near the beam axis.
An AGL fixture is usually wider in one plane than the other, so a horizontal and a vertical section are drawn together: a broad lobe for light spread wide, a narrow lobe for light held near the axis. A polar diagram shows one plane at a time, where the isocandela diagram shows the whole pattern at once; both describe the same photometric distribution. It is a graph of intensity by angle, not a map of where the light falls on the airfield.
- Find the centre. The fixture is at the centre of the plot, with the beam axis pointing along the zero-degree line.
- Pick the plane. Each curve is one section through the beam, usually a horizontal section and a vertical section.
- Pick a direction. Choose it by its angle round from the beam axis.
- Follow it out. Run along that direction from the centre until it meets the curve.
- Read the distance. The distance from the centre, read against the rings, is the intensity in candela.
- Compare the lobes. A broad lobe means light spread widely in that plane; a narrow lobe means it is held near the axis.
Frequently Asked Questions
Q1What is a polar diagram?
A polar diagram plots a fixture's luminous intensity against direction in one plane. The fixture is at the centre, the angle round the centre is the direction measured from the beam axis, and the distance from the centre is the intensity in candela. The curve drawn is the intensity in every direction in that plane.
Q2How do you read a polar diagram?
Pick a direction by its angle from the beam axis, follow that direction out from the centre until it meets the curve, then read the distance against the rings. That distance is the intensity in candela in that direction. The curve is furthest from the centre where the intensity is highest.
Q3Why are there usually two curves?
An AGL fixture is usually wider in one plane than the other, so one curve is not enough. A horizontal section and a vertical section are shown together. A wide curve shows light spread widely in that plane, and a narrow curve shows it held tight.
Q4What is the difference between a polar diagram and an isocandela diagram?
A polar diagram shows one plane at a time, as intensity against angle round a centre. An isocandela diagram shows the whole pattern at once, as curves of equal candela on a grid of horizontal and vertical angle. Both describe the same photometric distribution.
Q5What does a narrow lobe mean?
A narrow lobe means the intensity falls away quickly as the direction moves off the beam axis in that plane, so the light is concentrated near the axis. A broad lobe means the intensity holds up over a wider range of angles.
Q6Is a polar diagram a picture of the beam on the ground?
No. It is a graph of intensity against angle, not a map of where the light lands. The shape of the curve says how strong the light is in each direction. It does not show distances on the airfield.
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