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LED Application Series: Outdoor Area Lighting

Lighting of outdoor areas including streets, roadways, parking lots, and
pedestrian areas is currently dominated by metal halide (MH) and highpressure
sodium (HPS) sources. These relatively energy-efficient light sources
have been in use for many years and have well-understood performance
characteristics. Recent advances in LED technology have resulted in a new
option for outdoor area lighting, with several potential advantages over
MH and HPS sources. Well-designed LED outdoor luminaires can provide
the required surface illuminance using less energy and with improved
uniformity, compared to HID sources. LED luminaires may also have
significantly longer life (50,000 hours or more, compared to 15,000 to
35,000 hours) with better lumen maintenance. Other LED advantages
include: they contain no mercury, lead, or other known disposal hazards;
and they come on instantly without run-up time or restrike delay. Further,
while MH and HPS technologies continue to improve incrementally, LED
technology is improving very rapidly in terms of luminous efficacy, color
quality, optical design, thermal management, and cost.

Current LED product quality can vary significantly among manufacturers, so
due diligence is required in their proper selection and use. LED performance
is highly sensitive to thermal and electrical design weaknesses that can
lead to rapid lumen depreciation or premature failure. Further, long-term

performance data do not exist given the early stage of the technology’s development. Interested users should continue to monitor available information sources on product performance and lifetime, such as CALiPER test results and GATEWAY demonstration program reports, available on the DOE Solid State Lighting website (www.netl.doe.gov/ssl).
Design and Specification Considerations
Many issues enter into design and specification decisions for outdoor lighting. Energy efficiency is especially a priority in this application due to the long running hours and relatively high wattages typically involved. This section looks in detail at energy efficiency factors, as well as issues related to durability, color quality, life and lumen maintenance, light distribution, glare, and cost.
Energy efficiency
Energy effectiveness encompasses luminous efficacy of the light source and appropriate power supply in lumens per watt (lm/W), optical efficiency of the luminaire (light fixture), and how well the luminaire delivers light to the target area without casting light in unintended directions. The goal is to provide the necessary illuminance in the target area, with appropriate lighting quality, for the lowest power density. One step in comparing different light source and luminaire options is to examine luminaire photometric files. Look for photometry in standard IES file format from qualified independent or qualified manufacturer-based laboratories.1 The photometry should be based on an actual working product, not a prototype or computer model.

Luminaires differ in their optical precision. Photometric reports for outdoor area luminaires typically state downward fixture efficiency, and further differentiate downward lumens as “streetside” and “houseside.” These correspond to forward light (F) and backlight (B), respectively, referenced in the Luminaire Classification System (LCS). How does luminaire photometry translate to site performance? The next step is to analyze illuminance levels provided to the target areas, both horizontal and vertical. This is done through lighting design software and actual site measurements.
Table 2 compares measured illuminance data from the recent installation of LED outdoor luminaires referenced in Figure 1, in which existing 70W HPS luminaires were replaced with new LED luminaires.2 The LED luminaires installed used three arrays containing 20 LEDs each. An option using two arrays was also modeled in lighting software...View more information. Please download the article here

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