How to Interpret the Light Distribution Curve of LED Streetlights?

A light distribution curve is a graphical representation of how a luminaire’s luminous intensity is distributed in space; it is particularly critical for LED streetlights. When interpreting such a curve, focus on the following aspects:

1. Lateral Light Distribution (perpendicular to the road): The horizontal axis typically represents the angle (-90° to +90°), while the vertical axis represents luminous intensity (in candelas, cd). An ideal streetlighting distribution forms a "batwing" shape—with higher intensity between ±60° and ±80° to illuminate the road surface on either side of the pole—while maintaining lower intensity directly beneath the luminaire (at 0°) to minimize glare and "hot spots."

2. Longitudinal Light Distribution (along the road): The light should be spread as uniformly as possible to ensure a smooth transition in illuminance levels between street poles.

3. Angle of Maximum Intensity: This is usually tilted downward (e.g., 15° to 30°) to ensure that the majority of the light is projected onto the road surface.

4. Cut-off Angle: This refers to the angle at which the luminous intensity drops to 10% of its maximum value. "Full cut-off" luminaires have a cut-off angle of ≥65° (meaning virtually no light is emitted above 65°), which effectively reduces light pollution and glare.

5. Utilization Factor: This indicates the proportion of the total light emitted by the luminaire that actually falls onto the road surface (a factor dependent on the light distribution).

Light distribution data is typically provided in an IES file format, which can be imported into software such as DIALux for simulation. When purchasing streetlights, you should request the IES file to verify whether the light distribution curve is appropriate for the specific road width and pole spacing. If a manufacturer cannot provide a reliable IES file, it may indicate that their optical design is not sufficiently mature.