A newly published study from the Lighting Research Center (LRC) at Rensselaer Polytechnic Institute shows that improving the uniformity of lighting in parking lots can increase the sense of safety and security, as well as provide up to an additional 75% energy savings by lowering total light levels.
The study, “Energy and user acceptability benefits from improved illuminance uniformity of parking lot illumination,” now in press withLighting Research and Technology journal, describes a field demonstration, human factors study, and systems analysis that tested the relationship between illuminance, uniformity, and user acceptance in parking lots, and calculated the advantage that LED luminaires offer at producing more uniform beam patterns compared with high-intensity discharge (HID) luminaires.
Outdoor lighting is a rapidly growing market for LED-based lighting solutions. Municipalities, commercial property owners, and utilities are considering replacing existing HID lighting with LEDs. However, the main question they face is how to meaningfully compare LED-based solutions with traditional technologies, not only on the basis of light source efficacy but as a system and, more importantly, as an application. In 2009, the LRC’s Alliance for Solid-State Illumination Systems and Technologies (ASSIST) program developed a metric called luminaire system application efficacy (LSAE), which allows designers and specifiers to select luminaires that will provide the industry-recommended light levels and uniformity levels on the parking lot surface while using the least possible electricity. However, current recommended practice does not favor highly uniform designs. With more uniformity, light is distributed evenly across the parking lot surface, while less uniformity creates alternating brighter and darker areas between light poles.
“With the LSAE metric, we wondered if there was a possible tradeoff between illuminance and uniformity. In other words, if the lighting is evenly distributed across the whole parking lot, could we lower the total light level to save energy and still have people feel safe and secure?” said Nadarajah Narendran, Ph.D., the LRC’s director of research and principal investigator of the study. Additionally, LRC researchers wanted to know if it was possible for LEDs to provide better uniformity than traditional light sources, which would lower the power demand.
To test the first hypothesis, the LRC created an “improved uniformity” (3:1 ratio) and “standard uniformity” (10:1 ratio) LED lighting demonstration in its Troy, New York, parking lot and invited regular users, neighbors and visitors to answer survey questions about the two uniformity conditions at six different light levels. Overall, those surveyed perceived the lighting as more “safe” and “good” at lower light levels when presented with the improved uniformity compared with the standard uniformity. Even when presented with the highest light levels, the subjects gave lower ratings to the standard uniformity condition compared to the improved uniformity condition. From the survey results and light levels tested, LRC researchers determined that a parking lot with improved uniformity can be lighted to about 9 lx (0.9 fc), and there is no need to illuminate at higher levels if a uniform distribution is implemented.
Following the human factors survey, LRC researchers used modeling software to conduct a performance analysis of commercial parking lot luminaires and compare them with a custom LED prototype designed for improved uniformity. The modeling showed that LEDs can not only provide better uniformity but can do so at twice the application efficacy, thus lowering power demand, because LEDs are more effective at creating uniform distributions than larger HID sources traditionally used for parking lot lighting.
“In general, we now know that improved uniformity allows for equal or greater acceptance at much lower light levels. This is expected to translate to energy savings of up to 75%,” Dr. Narendran said.
