How to Choose Streetlights & Avoid Zebra Effect
Oct 14, 2026
Introduction
When driving on city roads at night, you often encounter a phenomenon where the road surface alternates between bright and dark areas. This visual discontinuity of alternating light and dark is known as the Zebra Effect .
The core reason for the zebra effect is not insufficient street light power or total luminous flux, but rather the unreasonable spatial distribution of light . The light distribution curve is the physical expression that determines the direction of the light beam.
What Are Light Distribution Curves and the C-Gamma Coordinate System?
A light distribution curve is a geometric trajectory of the luminous intensity (Candela, cd) of a luminaire in all directions in space. To eliminate the influence of power and facilitate comparison between different models, the drawings usually normalize the luminous intensity to cd / 1000 lm (luminous intensity per kilolumen).
When evaluating road lighting, the internationally accepted C-Gamma ( C-γ ) spatial rectangular polar coordinate system is used :
γ (Gamma) angle : The angle that opens outwards from directly below the light fixture ( γ = 0°). γ = 90° represents the horizontal direction; beams exceeding 90° are considered waste light directed towards the sky.
C plane : Angle of rotation around the vertical axis of the luminaire:
- C0 - C180 Plane : Longitudinal section parallel to the direction of road extension.
- C90 - C270 Plane : A transverse section perpendicular to the road (C90 points to the center of the lane, C270 points to the sidewalk or green belt behind the streetlights).
Graphical Explanation of Beam Distribution Curves: From Polar Coordinates to Beam Distribution
When interpreting photometric curves, focus on analyzing the following three characteristics:
Light Distribution Shape: Batwing vs. Lambertian
- Lambert beam distribution (circular/elliptical) : The maximum light intensity is located directly below ( γ = 0°). The light accumulates near the base of the lamp post, and the light attenuates rapidly in the far-end area between the two posts, directly causing alternating light and dark on the road surface (zebra crossing effect).
- Batwing beam pattern : The light intensity is appropriately reduced directly below the poles, forming peaks on both sides around γ = 60° ~ 68°, resembling the shape of an outstretched bat wing. By projecting a portion of the light intensity directly below onto the far-end area between the two poles, the overall brightness of the road surface is leveled.
Peak Angle and Glare Control
- Optimal projection angle : The peak value of the main beam γ should be maintained between 60° and 65°.
- Glare suppression (TI value) : In the region where γ ≥ 75°, the light intensity must decrease sharply. High-angle light rays directly entering the driver's retina will cause disabling glare. According to national standards, the light intensity values in the γ = 80° and 90° directions are strictly limited (requirements for cutoff/semi-cutoff luminaires).
Asymmetric Light Distribution
On the C90-C270 cross section, the light intensity distribution of excellent LED streetlights is asymmetrical:
- C90 direction (lane side) : The light spreads over a wide area, covering multiple lanes.
- C270 Direction (Rear) : Light is quickly cut off to reduce useless light directed onto roadside green belts or windows of buildings along the street (avoiding light pollution).
The Impact of Light Distribution Curves on Actual Engineering Performance
|
Road Indicators |
Physical Meaning |
Factors Determining the Photometric Curve |
|
Overall Uniformity (U0) |
Emin / Eavg (the ratio of minimum illuminance to average illuminance) |
Batwing spread angle and half-power angular width at C0-C180 cross sections |
|
Utilization Factor (UF) |
The proportion of effective luminous flux falling on the road surface |
Lateral light output control of C90-C270 polarizing lenses |
|
Glare Limiting (TI) |
Percentage of visual sensory loss caused by glare |
Cutoff slope for light intensity at high angles of γ = 75° ~ 90° |
In the relationship between pole height H and spacing S, the span-to-height ratio (S/H) is typically between 3.0 and 4.5. The larger the angle of the light distribution curve, the longer the allowable pole spacing S, thereby reducing the number of poles and luminaires per kilometer.
Product Recommendations
The SL12 intelligent LED street light integrates IoT control and efficient optical design, specifically designed for modern smart cities. The product supports NEMA/ZHAGA standard interfaces and integrates long-distance microwave and photosensitive sensors, enabling remote dimming, timed switching, and fault self-diagnosis. It employs high-efficiency LED chips batwing polarizing lens design (Type II/III) to output a uniform light spot and effectively prevent glare and reduce light pollution. Combined with a die-cast aluminum integrated heat dissipation shell and IP66/IK09 high protection rating, it fully meets the high energy efficiency and long lifespan maintenance requirements of urban main roads and industrial parks.
Frequently Asked Questions (FAQ)
Q1: Why do two LED streetlights with the same total lumen output have such different effects on illuminating the road?
A: Because lumens only represent the total luminous flux, while the light distribution curve is what truly determines where the light falls. If the light is concentrated directly under the light pole, it will cause the area under the pole to be bright and the area between the two poles to be dark; light fixtures that use polarized batwing light distribution can refract the light to the far end and the center of the lane, greatly improving the uniformity of illuminance and eliminating the zebra crossing effect at the same lumen output.
Q2: What are the differences between Type I, Type II, and Type III street light light distribution types in the ANSI/IES standards?
A: These are the lateral beam projection types categorized based on road width and installation location. Type I is suitable for narrow alleys or median strips; Type II is suitable for narrow roads with 1-2 lanes; Type III is the most common beam distribution for urban arterial roads, covering 3-4 lanes; Type IV is an extremely forward-projecting type, often used for edge installations; Type V is a 360° symmetrical beam distribution, mostly used in plazas and intersections.
Q3: What do the C0-C180 and C90-C270 profiles on the photometric curve represent?
A: C0-C180 is a longitudinal section parallel to the road's extension direction, used to observe the batwing shape and peak angle of the stretched pole spacing; C90-C270 is a transverse section perpendicular to the road (C90 points towards the lane, C270 points behind the lamppost), used to observe the polarization capability, i.e. whether the light is accurately projected onto the road surface and quickly cut off behind it to prevent light pollution.
Q4: What is the relationship between BUG Rating and photometric curve?
A: The BUG rating is an index of light pollution and glare prevention calculated from light distribution curve data (values from 0 to 5, the lower the better). Among them, B (Backlight) represents the waste light projected behind the light pole, U (Uplight) represents the upward light projected into the sky (should be close to U0), and G (Glare) represents the large-angle glare that causes discomfort to the driver.
In Conclusion
Understanding light distribution curves essentially shifts the focus from "how bright a light to buy" to "where the light should be directed." In road engineering projects, the proper use of batwing and polarized light designs can not only completely eliminate the zebra crossing effect and ensure driving safety, but also improve road surface uniformity while reducing light pole density, achieving dual control over construction and maintenance costs.
