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Section 5: Glare and Sky Glow Issues

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Introduction

Roadway lighting systems are coming under greater scrutiny from various sectors of the public. Issues such as glare, sky glow, and aesthetic lighting have achieved widespread attention and are open to criticism. Lighting designers should become familiar with these issues and be prepared to design lighting systems that meet required illumination levels, while also considering the environmental and aesthetic effects.

Communities are adopting lighting ordinances meant to reduce sky glow (popularly termed “light pollution”). Lighting designers should be on notice that this is a very important issue. Light emitted above the horizontal does not benefit roadway lighting, but it can contribute to glare and may be considered visual clutter. Many people consider sky glow undesirable and even offensive. This is an immensely important issue with the astronomical community, professional and amateur, and is particularly annoying when equally effective lighting systems can be designed that reduce or eliminate direct-up lighting.

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Choice of Luminaires

Unless it is essential to have light aimed above the horizontal (as for building facades, landscapes, and central business districts, for example), non-cutoff luminaires should not be used for new roadway lighting. Improperly used non-cutoff luminaires may be considered a waste of energy.

Luminaires used for roadway lighting should at least be cutoff. Full cutoff should be considered first.

When emitting light above the horizontal is absolutely necessary and in accordance with Texas Health and Safety Code, Chapter 425, “ Regulation of Certain Outdoor Lighting,” the designer should strive to keep the above-horizontal light as low as practical to accomplish the intended effect. This can be achieved by using lower wattage luminaires, by shielding, or by luminaire design.

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Luminaire Modification

Adding glare shields to the existing roadway lighting luminaires of a continuous lighting system is generally not a good practice. Glare shields modify the photometric distribution of the luminaire and may cause an acceptable lighting system to no longer meet design standards. Luminaire photometrics are rarely measured with external shields installed, so the designer cannot know how a luminaire with glare shields will perform. A qualified lighting designer should investigate the effect of adding glare shields prior to their installation. It may be necessary to change the entire luminaire, rather than alter the photometric performance of existing luminaires.

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Resources

Knowledge of this subject and implementation of design techniques to reduce or eliminate these problems enhances the public’s perception of the professionalism of lighting designers and benefits citizens through a show of concern for neighbors and the complete aesthetic environment.

For further information on this subject, see:

  • ANSI RP-8-00
  • IESNA TM-10-99, “IESNA Technical Memorandum Addressing Obtrusive Light (Urban Sky Glow and Light Trespass) in Conjunction with Roadway Lighting”
  • IESNA RP-33-99, “Lighting for Exterior Environments”
  • CIE Report 126, 1997, “Guidelines for Minimizing Sky Glow.”
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