A polar plot shows how a fixture distributes its light in different directions. The center of the plot is the source, the angle around the plot is the direction of the light, and the distance from the center shows the intensity in candela. A tight, long shape pointing straight down is a narrow spot, and a wide, rounded shape is a floodlight.

Photometric data, such as the IES and LDT files provided by manufacturers, lists candela values at many angles in two or more planes. The C0 and C90 planes show the distribution along and across the fixture, and for a symmetrical optic they look the same, while for a linear or an asymmetric fixture they differ. Plotting them together shows at a glance how the light is spread.

The beam angle is read from the plot as the angle where the intensity falls to half of the peak. Use the plot to compare optics, check that a distribution suits the space, and choose a fixture for wall washing, accent, or general lighting. The data can also be loaded into lighting design software, which calculates levels across a whole room.