Lighting lab setup predicts level of driver discomfort caused by LED streetlights

In many areas, nighttime drivers must repeatedly suffer the loss of night vision arising from the sudden stabs of brightness from streetlights. Of course, streetlights provide essential safety benefits. But the questions arise: Are certain types of streetlights easier on night vision than others? And, in particular, could the newly popular energy-efficient LED streetlamps being widely installed make things worse?

A team of researchers from China and the Netherlands has developed a way to evaluate the human impact of uncomfortable glare caused by LED road lighting.¹ They created a model that can predict the level of discomfort experienced by drivers under various lighting conditions.

"With the development of the LED industry, there is no doubt that more and more LED lights will be used for road lighting," says Yandan Lin, an associate professor and director of the Vision and Color Research Laboratory at Fudan University (Shanghai, China). "We believe that the lighting industry has an urgent need to update the ways to characterize discomfort glare caused by LED road lights."

LEDs smaller, bluer than traditional lamps
Traditional ways of predicting how much a person will want to look away from a bright light, characterized by so-called discomfort glare, are based on the nature of conventional light sources and may not be appropriate for LED streetlights, Lin notes. For example, LEDs are designed to emit the same amount of light as conventional bulbs from a smaller area, which could possibly increase the discomfort glare. In addition, white-light LEDs typically have more energy in the blue part of spectrum, which can also cause more glare because, based on past studies, blue light is perceived as brighter by the human eye under the same light levels.

To investigate, the team devised a laboratory setup to mimic visual conditions on the road. The researchers painted the floors and walls of a room completely black and positioned a collection of light sources around the room to simulate the illumination from an LED streetlamp as it would appear to a driver on a dark road. The researchers adjusted the light sources to create 72 different lighting conditions, and volunteer observers were asked to rate their level of discomfort glare under each condition on a standard deBoer 9-point rating scale, ranging from "unnoticeable" to "unbearable."

The team was guided in the selection of test-lighting conditions by previous work done on discomfort glare. "Although the characterization of discomfort glare for conventional light sources may not be applied to LEDs, the possible factors affecting discomfort glare may remain the same," Lin explains.

Lin and her colleagues systematically modified four lighting parameters in their experiments: the luminance (luminous intensity of light per given area) of the LED lights, the background luminance, the apparent size of the LED lights from the perspective of the observers in terms of solid angle, and the angle between the LEDs and the line of sight of the observers.

Lab work validated by on-the-road results
Based on the results, the team developed a model that related the four parameters mentioned above to the level of discomfort glare experienced by an observer. They validated the model with two additional experiments -- one carried out in the lab and one on the real roads.

The researchers found that the interaction between the LED luminance and the solid angle was the most significant factor affecting discomfort glare. To minimize discomfort, they recommend minimizing both the amount of light hitting at the eyes and the luminance contrast between the streetlights and the background, which can be accomplished through good optical design of the LED light unit and careful placement of each streetlight.

While more follow-up work would be needed before the team's results could guide industry standards, the researchers hope their model could still be put to immediate use by design and construction firms working on LED streetlight replacements and installations. Going forward, the team plans an interdisciplinary investigation of the physiological changes caused by glare, which could allow the researchers to ditch the 9-point rating scale in favor of more concrete measurements of discomfort such as eye movement and pupil size.

The researchers are from Fudan University, Philips Research Asia (Shanghai, China), and the Human Technology Interaction group, Eindhoven Technical University and Philips Research Laboratorie (Eindhoven, The Netherlands).

REFERENCE:

1. Yandan Lin et al., Optics Express (2014); http://dx.doi.org/10.1364/OE.22.018056