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Section 5: Glare and Sky Glow Issues

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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 important issue with the astronomical community, and is particularly annoying when equally effective lighting systems can be designed that reduce or eliminate up-lighting.

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Cutoff and BUG Rating

Prior to 2007, the Illumination Engineering Society (IES) used the cutoff classification system to rate the amount of uplight produced by a luminaire. In 2007, cutoff classifications were replaced with BUG (Backlight, Uplight, Glare) ratings. BUG ratings are specified in IES document TM-15-11.

The center letter U of the BUG rating may range from U0 with no uplight, to U5 with over 1000 lumens of uplight. TxDOT's standard LED luminaires are specified to have an uplight rating of U0. If the luminaires are mounted level they will not emit any uplight.

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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), luminaires with minimal uplight should be used for lighting projects.

When emitting light above the horizontal is absolutely necessary and in accordance with Texas Health and Safety Code, Chapter 425 (see appendix), 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

Glare shields may sometimes be added to existing luminaires to reduce unwanted light straying onto private property. However, glare shields change the light distribution of the luminaire, so proposed luminaire modifications should be carefully analyzed to be sure the roadway lighting will remain acceptable.

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