What is a Cartesian photometric diagram?
A plot of a fixture’s intensity in one plane on straight axes: the angle from the beam axis runs along the bottom, the candela runs up the side, and the curve is the beam cut open into a line.
A Cartesian photometric diagram shows luminous intensity against direction in one plane on ordinary graph axes. The angle from the beam axis runs along the bottom, the candela runs up the side, and the curve is the intensity at every angle. It is the polar diagram laid out flat: the same section of the beam, opened out from a circle into a line.
Straight axes make exact values easy to read and curves easy to compare. The peak is usually on the beam axis at zero degrees, the beam spread is the width between the half-peak points, and the angle axis can be scaled to just the range that matters, which a fixed polar circle cannot do. An AGL fixture is usually wider in one plane than the other, so a horizontal and a vertical curve are drawn together. It is a graph of intensity by angle, not a map of where the light falls on the airfield.
Angle along a line, height is candela. Best for reading exact values and comparing curves.
Angle round a centre, distance is candela. Best for seeing the shape of the lobe at a glance.
- Find the axes. The angle from the beam axis runs along the bottom, and the candela runs up the side.
- Pick the plane. Each curve is one section through the beam, usually a horizontal section and a vertical section.
- Pick an angle. Find it along the bottom, to the left or right of the beam axis.
- Run up to the curve. Go straight up from that angle until you meet the curve.
- Read across. The height there, read against the side scale, is the intensity in candela at that angle.
- Find the half-peak points. Where the curve crosses half of its peak. The width between the two points is the beam spread in that plane.
Frequently Asked Questions
Q1What is a Cartesian photometric diagram?
A Cartesian photometric diagram plots a fixture's luminous intensity against direction in one plane on ordinary graph axes. The angle from the beam axis runs along the bottom, the intensity in candela runs up the side, and the curve drawn is the intensity at every angle in that plane.
Q2How do you read a Cartesian photometric diagram?
Find the angle you want along the bottom, run straight up until you meet the curve, then read across to the side scale. That height is the intensity in candela at that angle. The curve is highest where the intensity is highest, which is usually on the beam axis at zero degrees.
Q3What is the difference between a Cartesian and a polar diagram?
They show the same section of the beam. A polar diagram wraps the angle round a centre and uses the distance from the centre as the candela. A Cartesian diagram opens that circle out into a straight axis and uses the height as the candela. Both describe the same photometric distribution.
Q4Why use a Cartesian diagram instead of a polar one?
Straight axes make exact values easier to read and curves easier to compare. The angle axis can also be scaled to just the range that matters, so a narrow beam can be shown in detail, which a fixed polar circle cannot do.
Q5What are the half-peak points on a Cartesian diagram?
They are the two places where the curve falls to half of its peak value, one on each side of the beam axis. The angle between them is the beam spread in that plane. Some documents use a different fraction, so check the definition in the one you are working from.
Q6Is the horizontal axis a distance on the airfield?
No. The horizontal axis is an angle in degrees measured from the beam axis, not a distance on the ground. The diagram says how strong the light is in each direction. It does not show where the light lands on the airfield.
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