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Battling outdoor light

Photonics Spectra
Jul 2009
Anne L. Fischer, Senior Editor,

The battle against light pollution recently had a mathematical model added to its arsenal. Developed by Chris Baddiley, the scientific adviser to the British Astronomical Association’s Campaign for Dark Skies, the algorithm calculates how much illumination from streetlights is reflected by surrounding surfaces.

Called the skyglow model, it determines how light scattering is affected by the atmosphere, taking into account the wavelength, angle and viewing distance of light. In constructing the model, Baddiley found that the white light typically used in contemporary light sources, which often use blue and green wavelengths, produces greater reflectivity and scattering than previous light source models, which used more yellow and orange; switching to sources with longer wavelengths, he noted, could significantly cut light pollution.

Skyglow over Worcester, UK, is caused by lights shining above the horizon. Photo courtesy of C.J. Baddiley.

The skyglow model, which took seven years to develop, could be used to improve the design of streetlights, cutting glare by a factor of three to five, Baddiley said. These findings have been published by the Institution of Lighting Engineers and incorporated into the standards of the UK Highways Agency.

The movement toward reducing light pollution is effecting change throughout the world. Slovenia, for example, enacted a decree against outdoor light pollution in 2007.

The association Dark Sky Slovenia recently released a study of streetlighting on two roads, comparing conditions in 2007 before lighting reform with conditions in 2008. Using fully shielded luminaires and switching 250-W bulbs for 150-W bulbs with improved optics reduced glare and illuminated the road more evenly. In addition, energy consumption was decreased by 40 percent.


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