LEDs Change the Balance of Post-Top Lighting
The US Department of Energy (DOE) has published the evaluation report from a GATEWAY demonstration in New York City’s Central Park, conducted in collaboration with The Climate Group and the New York City Department of Transportation (NYCDOT). LED post-top luminaires from five different manufacturers were installed along paved walking trails and evaluated against the metal halide (MH) luminaires that were in the park at the time. The report includes product comparisons across the performance criteria of energy savings, improved horizontal and vertical illuminance, life-cycle cost, and simple payback, as well as considerations regarding orientation, spacing, and color quality.
Key findings include:
- All of the LED products evaluated provided significant energy reductions relative to the MH baseline, ranging from 50% to 83% energy savings.
- Four of the LED products offered lower life-cycle costs than the incumbent luminaire based on an analysis period of 75,000 operating hours.
- The LED products provided a larger percentage of their output as downlight – because of the directional nature of LEDs and better optical control offered by the smaller emitters – and produced considerably less uplight than the baseline. This difference partially explains how some of the LED products produced higher measured illuminance values even with lower overall output.
- Proper orientation of directional luminaires can be a challenge. With an objective to focus illuminance on the pathway versus the surrounding grassy areas, this evaluation considered both symmetric and asymmetric distributions, but a strictly asymmetric approach would not necessarily work for all sites.
The park’s meandering walkway lighting – mounted 9 feet above grade – is spaced at irregular distances that vary with the vegetation and undulating landscape. So in order to facilitate the comparison of luminaire performance regardless of the luminaire’s location, DOE employed a nonstandard polar measurement system that accounted for both horizontal and vertical illuminance.
Horizontal and vertical illuminance measurements are both important for walkways in general. Horizontal illuminance describes how much light is available for navigating the path, as well as how much is distributed onto the adjacent areas; while vertical illuminance describes whether there’s enough light to identify the faces of people who approach, for example,
The LED products evaluated in this demonstration were purchased in late 2009 and thus reflect the prices and performances available at that time, which have improved since then. This past summer, after the Central Park demonstration was concluded, NYCDOT replaced the park’s incumbent MH lighting with an updated version of one of the evaluated LED products, which, like the earlier version, has an asymmetric distribution but is roughly 15 percent more efficacious.
The full report is available for download at www.ssl.energy.gov/gatewaydemos_results.html, along with other DOE GATEWAY demonstrations that showcase high-performance LED products for general illumination in a variety of applications.