Outdoor lighting is a critical component for communities because it can provide visibility, promote a sense of security, deter crime and attract economic activity. Its selection and implementation have to be carefully considered, however, to optimize energy efficiency while minimizing undesirable effects, such as glare. Because new light source technologies promising greater efficiency are now coming to market for outdoor lighting applications, including LEDs and induction lamps, municipalities and businesses are re-evaluating what type of lighting works best outdoors.
To help specifiers and decision makers evaluate their choices in outdoor lighting, the Alliance for Solid-State Illumination Systems and Technologies (ASSIST) has published two new volumes in its ASSIST recommends series. 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.
With these two new publications, ASSIST continues its goal of helping LED technology improve and gain ground in the lighting market, said Nadarajah Narendran, Ph.D., ASSIST organizer and director of research at the Lighting Research Center (LRC) at Rensselaer Polytechnic Institute. “Over the past 10 years, the LED and lighting industries’ leading stakeholders—through ASSIST—have been working with the LRC to identify barriers and overcome them,” said Dr. Narendran. “Through their active participation, these studies are being carried out to develop methods and recommendations for lighting decision makers.”
The two new ASSIST recommends are available for free download from the ASSIST web site here.
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 recommends extends the parking lot LSAE metric to street and roadway luminaires.
As with the parking lot lighting metric, the adaptation of LSAE to luminaires for street and roadway lighting allows for all light source technologies to be compared on the same basis. The requirements for illuminance, luminance, uniformity and glare correspond to those recommended in RP-08-00, the IES’s American National Standard Practice for Roadway Lighting; however, other preset criteria could be substituted for the RP-08-00 requirements, noted Jean Paul Freyssinier, LRC research scientist, lead author of the ASSIST volume.
The calculation involves defining the task plane (i.e., a segment of the roadway area), setting the pole spacing and luminaire mounting height, determining the illuminance criteria for the given application (based on the roadway type and pavement classification), and then counting the illuminance in each cell of a grid placed over the task plane. Cells with illuminance values outside the pre-determined criteria are penalized in the final efficacy calculation.
“In general, LSAE provides a good correlation to lighting power density and can be used to rank order individual luminaires all the way up to a complete installation,” said Freyssinier.
A detailed description of ASSIST’s street and roadway luminaire evaluation method can be found here.
Recommendations for Estimating Discomfort Glare
Because outdoor lighting is utilized at relatively low light levels and because outdoor lighting equipment (e.g., lamps and luminaires) tends to be relatively bright, there is a substantial potential for discomfort glare in outdoor lighting applications. This volume of ASSIST recommends describes 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, Ph.D., 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. With this finding, the LRC published a model of discomfort glare in 2008 as part of an outdoor site-lighting performance system for assessing the potential of an outdoor lighting installation to produce light pollution.
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.
A detailed description of ASSIST’s discomfort glare calculation method can be found here.
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