The Lighting Research Center (LRC), via its Alliance for Solid-State Illumination Systems and Technologies (ASSIST) program, has published two new reports in the ASSIST Recommends Series that apply to LED-based, and other types, of outdoor lighting. Recommendations for Evaluating Street and Roadway Luminaires describes a methodology for evaluating street and roadway luminaires with a metric called luminaire system application efficacy (LSAE). Recommendations for Estimating Discomfort Glare uses a published discomfort glare model and describes a method for calculating a prediction of discomfort glare for outdoor-lighting systems including solid-state lighting (SSL).
LSAE is based on measuring outdoor lighting using a 2x2-ft grid that covers what the LRC calls a task plane – a defined segment of a roadway for example. The methodology relies on luminous-flux levels at the task plane rather than measurement of light leaving the luminaire.
ASSIST had previously described LSAE in relationship to parking-lot lighting. The new publication extends the concept to street and roadway lighting.
The concept behind the LSAE metric awards credit to grid squares where light meets the photometric requirements while essentially penalizing a luminaire for under-performing areas of the grid and for grid areas where the light level is higher than the recommended value since it’s considered wasted light. And obviously light wasted outside of the task plane won’t help an LSAE score.
LSAE is ultimately defined as the ratio of task-conforming light (Φtask-conforming) to input power. The report details a methodology for calculating the Φtask-conforming element of the equation.
The ASSIST publication utilizes photometric requirements defined by the IES RP-08-00 American National Standard Practice for Roadway Lighting as a standard for minimum and average illuminance measurements in each grid square. The metric provides a measure of both appropriate light levels and uniformity. While the LRC utilizes the AES standard, you could easily apply other criteria using the same methodology.
Predicting discomfort glare
The glare publication is focused on predicting discomfort glare in outdoor lighting. The LRC defines discomfort glare as “the annoying or even painful sensation from viewing a bright light.” In contrast, disability glare is the actual temporary vision loss attributed to looking at a bright light.
The LRC asserts that a study of discomfort glare has been needed by the lighting industry. LRC Senior Research Scientist, and lead author of the ASSIST volume, John Bullough, said “Disability glare and its mechanisms have been well understood for a long time, but discomfort glare is something less well known and no accurate method of measurement or prediction has existed.”
LRC’s work in discomfort glare extends prior research. It’s based on the De Boer rating scale developed on the 1960s. But recent LRC work has revealed that illuminance has more impact on discomfort glare than does luminance. The new model incorporates the latest research presumably providing more accurate predictions.
ASSIST is a collaboration between the LRC and researchers, manufacturers and government organizations. The LRC is part of the Rensselaer Polytechnic Institute.