DOE releases research on pedestrian-centric outdoor lighting and glare

Jan. 13, 2014
Tests of LED and legacy lighting at two pedestrian-dominant communities reveal that prevailing wisdom on light levels may be inappropriate and that improving SSL technology with warmer CCTs, especially with diffusers in place, can deliver lighting that's preferred by residents.

The US Department of Energy (DOE) has released a report via its Gateway Demonstration Program on pedestrian-friendly outdoor lighting. The report is based on projects at Stanford University near Palo Alto, California and at Chautauqua Institution located adjacent to Chautauqua Lake in southwestern New York State. Both of the demonstration areas are pedestrian-centric and tested multiple light fixtures based on LED and legacy sources in an attempt to identify luminaires that were both safe and pleasant for the residents with minimal glare. The results of the demonstrations show that LED luminaires can serve the communities' needs with warmer CCTs and glare reduction techniques, and also may change the thinking of lighting professionals on appropriate light levels.

DOE solid-state lighting (SSL) Gateway reports are intended to share lessons learned from real installations of LED-based lighting in a variety of scenarios. For example, we covered a Gateway report on LED street lighting back in August 2013.

The new report notes that outdoor lighting installations are most often guided by metrics that apply more to drivers of vehicles than to pedestrians, and therefore may not be appropriate in cases where "cars are subordinate to bicycles and users on foot." For example, glare ratings are based on the Illuminating Engineering Society (IES) Luminaire Classification System BUG (backlight, uplight, glare) rating in which high-angle glare is the concern for driver discomfort. The researchers found that pedestrians are much more bothered by glare from lower angles measured from 0–75° from the pole toward the 90° level at the height of the luminaire.

As we covered in a recent interview with Mark Rea with the Lighting Research Center, there is a lack of good metrics to characterize discomfort glare. And as Rea pointed out, the problem is more acute when the subject is close to the light sources. The DOE demonstration would reveal that tests of different products and installation scenarios were needed to identify a low-glare design.

Indeed, the demonstration programs tested different lighting options through multiple rounds of mockups before settling on LED-based luminaires that would be considered for campus-wide deployment. The mockups revealed unacceptable options based on both LED and legacy sources. Moreover, some legacy products could have been suitable, although when short product lifetimes were factored in LED options won out.

The key to LED success was in part the evolution of the technology, especially in color temperature. Surveys revealed that test subjects preferred lights at 4000K and warmer, and products in the 2700K were most preferred, especially given the character of the neighborhood and the incumbent warmer light sources.

The work verified that glare is a significant factor in pedestrian ratings of lighting. Indeed, the LED products that proved most preferred were augmented with diffusers to further soften the appearance of the lights. The test subjects also preferred a soft-edge pattern of light on the ground, whereas streetlight projects often seek sharp boundaries on beam patterns.

The research also revealed that area lighting in pedestrian-centric areas could essentially offer better nighttime visibility at the low end of IES guidelines on luminance and illuminance. Elimination of glare, enabled in part by lower lighting levels, allows the human visual system to better perceive objects and possible danger in areas of lower light away from the nearest light source.

You can peruse the full report on the DOE website. Moreover, the report includes the detailed surveys used in the two projects that could help other communities compose an appropriate survey for their own use.