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LED Streetlight Design Curbs Light Pollution

Some of the shortcomings of a traditional streetlight — light pollution, glare, wasted energy, and uneven illumination — could be eliminated under a new LED-based system that shines only where needed.

Existing solutions are notoriously inefficient, wasting as much as 20 percent of their light by projecting it onto nearby objects or washing out the night sky.

The new LED streetlights developed by researchers at National Central University and the Autonomous University of Zacatecas could reduce that number to just 2 percent by shining light only in areas that need illumination. Additionally, LED lamps could lower power consumption by 40 to 60 percent, according to Ching-Cherng Sun of National Central University.

The proposed lamp is based on a novel three-part lighting fixture. The first part contains a cluster of LEDs, each fitted with a special lens called a total internal reflection (TIR) lens that focuses the light so that the rays are parallel to one another instead of intersecting — a process called collimation. The lens-covered LEDs are mounted inside a reflecting cavity that “recycles” the light, ensuring that as much of it as possible is used to illuminate the target. As the light leaves the lamp, it passes through a diffuser that eliminates unwanted glare. The collimation and filtering combination also enables researchers to control the beam’s shape: The current design yields a rectangular pattern suitable for street lighting, the researchers say.

This photograph (left) illustrates some of the shortcomings of a traditional streetlight: light pollution, glare, wasted energy and an uneven illumination of the ground. The streetlight design (far right) developed by researchers in Taiwan and Mexico would eliminate these problems. Images courtesy of Optics Express.

The team tested the design’s performance by analyzing how little the beam would spread as it hit its target — a road or sidewalk 10 m or more away from the light source. The lamp’s performance was quantified using the optical utilization factor (OUF), a number that describes the relationship between the light’s flow rate at the target compared with that coming directly out of the LEDs. Higher OUFs indicate better performance. Simulations of the design achieved an OUF of 51 to 81 percent, outperforming a recent design that reached 45 percent.

An example of the new design’s adaptability: If an LED streetlight projected its light from the center of the street, the light pattern on the road would be a rectangle. If the streetlight is at the side of road, it projects its light at an angle, producing a trapezoidal light pattern. The new design includes alternative configurations that would maintain the light beam's rectangular shape for a lamp that must project its beam at an angle, ensuring an even illumination of the road.

The module is simple to fabricate, the researchers say, because it comprises just four parts, including a type of LED bulb commonly used in the lighting industry.

A prototype of the design is expected to be completed in the next three to six months, with practical installations set to begin as early as next year.

The research was published in Optics Express (doi: 10.1364/OE.21.010612). 

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