ASSIST recommendations provide new methods for evaluating street and roadway lighting

The Alliance for Solid-State Illumination Systems and Technologies (ASSIST), part of Lighting Research Center (LRC), has published recommendations to help specifiers and decision makers evaluate their choices in outdoor lighting. The two issues - ‘Recommendations for evaluating street and roadway luminaires’ and ‘A method for estimating discomfort glare from exterior lighting systems’ - offer new calculation methods for estimating efficacy and discomfort glare, respectively, from luminaires used outdoors.

Recommendations for evaluating street and roadway luminaires

In 2009, ASSIST published an alternative method for evaluating outdoor luminaires designed for parking lot lighting. The ASSIST metric, called luminaire system application efficacy (LSAE), is based on the concept of application efficacy in which efficacy is measured by the amount of luminous flux reaching the task plane that meets the application’s photometric requirements rather than all the lumens exiting the luminaire. For a parking lot luminaire, this meant counting the lumens reaching the parking lot ground that conformed to recommended illuminance and uniformity guidelines, and discounting everything falling outside the application area or not conforming to photometric requirements. This new volume of ASSIST recommendations extends the parking lot LSAE metric to street and roadway luminaires.

Recommendations for estimating discomfort glare

Because outdoor lighting is utilised at relatively low light levels and because outdoor lighting equipment tends to be relatively bright, there is a substantial potential for discomfort glare in outdoor lighting applications. This volume of ASSIST recommends a calculation method for predicting discomfort glare from outdoor lighting systems, based on an existing rating scale and a published discomfort glare model.

Discomfort glare is defined as the annoying or even painful sensation from viewing a bright light, whereas disability glare is the reduction in visibility that a bright light might cause. “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,” said LRC senior research scientist John Bullough, PhD, lead author of the ASSIST volume. Current assessments typically use a subjective rating scale developed in the 1960s, called the De Boer rating scale. Recent LRC research has shown that De Boer ratings of discomfort glare are much more strongly related to the glare source’s illuminance than to its luminance.  The ASSIST calculation method is an extension of this model that incorporates the source luminance, resulting in improved predictions of the De Boer rating for a given lighting system.

The two new ASSIST recommendations are available for free download from the ASSIST website: www.lrc.rpi.edu/assist.