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Sources' Spectra, Roadway Surfaces and the Potential for Uplight Scattering

by Jefferey F. Knox* & David M. Keith, FIES+

Authors' Affiliations:
* Luminous Design, Inc., Longmont, CO
+ Marshall Design, Inc., Boulder, CO


The growing concerns about light pollution raise questions about what steps lighting designers and society can take to control and reduce it. The first step is understanding that skyglow is the result of redirection of light by the atmosphere. Evaluating the potential for atmospheric redirection of light is the subject of this paper.

The dominant phenomenon in atmospheric scattering is Rayleigh scattering. The Rayleigh Scatter Index is a simplified model of the energy-and-wavelength interaction in Rayleigh scattering that provides relative information about the potential for scattering of radiant energy by the atmosphere under clear sky conditions. To calculate RSI, the radiant energy at each wavelength is multiplied by the associated wavelength's value raised to the power of negative four, and by a constant, and then summed over the visible spectrum.

RSI is applied to eleven source spectra and two roadway surfaces. Sources considered include an equal energy source, CIE D65, CIE Illuminant A and conventional high pressure sodium and metal halide sources representing commercially available lamps. Concrete and asphalt surfaces are represented by spectral reflectivity data from the ASTER Spectral Library.

The results show that the potential for scattering varies significantly between the sources considered. Most significantly for outdoor lighting, the RSI values indicate that for the same amount of radiant energy, high pressure sodium sources produce around one-half to three-quarters of the skyglow produced by metal halide sources. When these sources are compared on a lumen-by-lumen basis, this difference increases.

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