More and more communities are tackling light pollution—nuisance light that brightens the nighttime sky and washes out the stars, or light that leaves the boundaries of a property and trespasses into a neighbor’s home or causes glare for passing motorists. Light pollution laws often restrict the type of luminaires (light fixtures) allowed for outdoor lighting. But are these restrictions actually reducing light pollution? To answer that, we have to first understand what light pollution is and how we can evaluate it. We also need to know that we’re making a difference when we try to alleviate it.
Intuitively, restricting outdoor luminaires to full-cutoff over semi-cutoff types helps to ensure that we don’t emit light into the sky. While this seems to make sense, quantitative analyses of these measures have actually shown that they have little or no effect on minimizing light pollution. So, what do we need to do?
“We need a system for characterizing light pollution that allows specifiers to predict in advance the impact of their designs and change practice before installation,” says Jennifer Brons, a lead research specialist at the LRC. “They need to be able to compare several alternate designs, and they need to gauge their designs against standard industry practice.”
OSP and the ‘shoebox’
Working with a group of North American and European outdoor lighting manufacturers, the LRC developed Outdoor Site (Lighting) Performance, or OSP, a framework for quantifying light pollution from an installation. The manufacturers working with the LRC on OSP are Acuity Brands, Lumec, R-Tech/Schréder, and Philips.
“OSP is the first comprehensive, quantitative system of assessing light pollution based upon the concept that property owners should be able to meet their lighting design objectives while adhering to light pollution regulations set by their communities,” said Mark Rea, the director of the LRC.
OSP uses a calculation ‘shoe box’ that follows the boundaries of a property and a top plane. “Using property boundaries is a logical system because they form a natural dividing line between private property and public interests,” said Rea.
The OSP calculation box uses calculation planes that can be generated by any commercial lighting software that can calculate inter-reflections. OSP calculates the light crossing the planes in every direction and provides practical insight into the characteristics of the light that leaves a property.
OSP considers three metrics: glow—the average illuminance on the entire calculation box, indicative of potential to cause sky glow; trespass—the maximum illuminance on any of the vertical calculation planes, indicative of the potential to disturb neighbors; and glare—the degree to which illumination from luminaires will cause discomfort to observers. Using actual lighting installations, OSP provides a realistic and accurate basis for developing limits on glow and trespass. The LRC used indoor and outdoor experiments to develop sensible criteria for limiting discomfort glare.
Specific limits for glow, trespass, and glare are now being discussed by many interested stakeholders, from practicing engineers to dark sky advocates. LRC researchers have presented information about OSP at several industry meetings and conferences. Brons says many lighting practitioners in the U.S. and in Europe have indicated that OSP shows promise as a practical tool in the mitigation of light pollution and its negative consequences. The LRC is inviting engineers to participate in building a database on which to form a statistical foundation for OSP.
A paper showing the results of recent OSP work has been published in the October 2007 edition of “The Lighting Journal” from the U.K.’s Institution of Lighting Engineers (ILE) and is available online.
More information about the LRC’s OSP . . . can be found in the Outdoor Lighting section of the LRC Web site.
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