Correspondence: Glare in pedestrian-friendly outdoor lighting

Outdoor lighting contributes to the safety of people at night, on streets, in parking lots, at building entrances, in parks and on footpaths. However, conventional outdoor lighting design has focused primarily on the visual needs of the motorist. A US Department of Energy Gateway case study of two lighting projects at two pedestrian-focused sites suggests that lighting for pedestrians requires a different approach to that for drivers. The two projects studied yielded similar lessons: Daytime luminaire appearance matters to both residents and pedestrians. Both prefer a warm light appearance at night, possibly because both groups were accustomed to incandescent, 2700K CFL, or high pressure sodium luminaires. Illuminances on the ground at the low end of the Illuminating Engineering Society of North America’s recommendations are acceptable as long as the luminaire produces a soft-edged gradient of light. Glare from the luminaire matters as shown by the fact that all the light-emitting diode (LED) luminaires with visible LED arrays required a diffusing lens or sandetching of the prismatic refractor to improve glare acceptability. In one pedestrian-focused community, the only acceptable LED product used an LED module with large area remote phosphor. Glare is important because a bright pedestrian luminaire produces scattered light in the eye and this, in turn, reduces the luminance contrast of objects in the scene, making them less visible. A secondary effect may be the eye’s inability to see details in lower luminance areas when the glare source shifts its adaptation luminance higher. In other words, less glare means better visibility for the pedestrian. Luminaire appearance, light source colour, light distribution and the illuminances provided can be dealt with by any competent designer, but discomfort glare is not so easy to handle because there are several unanswered questions about the variables involved in its estimation. Some or all of the six factors listed below have been incorporated into glare metrics such as veiling luminance ratio and glare control mark for roadway lighting and unified glare rating for interiors. But, these factors are not easily quantified for outdoor lighting situations and especially for LED systems because they are difficult to measure or characterize. Consider the following: