Many facets of light pollution

The article “Lighting and Astronomy” by Chris Luginbuhl, Connie Walker, and Richard Wainscoat (Physics Today, December 2009, page 32 ) is a welcome and important discussion of the impact of outdoor lighting on astronomical observation. It seems to fall short, however, with regard to actual practice and the effects outdoor lighting has on communities other than astronomers. Light pollution is wasted light and should be minimized for both environmental and financial reasons. Despite good intentions, many outdoor lighting applications waste light. The following important questions are not addressed in the article: How much light are we wasting now? How do we measure wasted light? And what can be done to reduce that waste?

People complain about three aspects of outdoor lighting: sky glow, light trespass, and glare. In terms of sky glow, the topic addressed by the authors, reflected light, rather than light directly leaving the fixture, typically contributes more than 80% of the light leaving the property. ¹ As the authors correctly point out, however, the direction of that escaping light matters for astronomical observations; low-angle light is relatively more important than highangle light, but proximity to and absolute levels of the light sources are even more important. That point is also made indirectly by the data the authors present from Flagstaff and Phoenix in their figure 4. More stray light comes to the US Naval Observatory Flagstaff Station from Flagstaff, a close, small city with very stringent fixture requirements in place, than from Phoenix, a distant, large city with less stringent requirements.

The impact of fully shielded fixtures is rather small. The authors use 10% as the amount of light directed upward for “typical” community lighting fixtures, but a sample of commonly installed fixtures tested by the National Lighting Product Information Program ² had uplight values averaging less than 0.3%. Counting only partially cut-off fixtures like the one illustrated in figure 3 of the article, the average uplight value was still less than 0.5%. Therefore, a value of 10% misrepresents what is actually being installed. Limiting the typical value of 0.3% uplight to 0% from fully shielded fixtures will have little incremental effect on reducing sky glow beyond current practice. Parenthetically, the statement attributed by the authors to members of the lighting profession that partially shielded fixtures permit wider spacing than fully shielded ones is incorrect; many fully shielded fixtures can be spaced farther apart than many unshielded ones. ³

The direction of light leaving the property matters most in terms of light trespass and glare. Both are also measures of wasted light and to some communities can be just as important as sky glow is to astronomers. The outdoor site-lighting performance (OSP) system ¹ has been used to develop methods to measure and limit light trespass onto adjacent windows from the lights illuminating, for example, a car dealer’s lot. Similarly, glare into the eyes of automobile drivers from a fixture on a golf driving range can be measured, and limits can be established.

Our systematic review of current outdoor lighting practice and of the reasons people complain about light pollution ¹ shows the state of outdoor lighting with regard to glow, trespass, and glare and suggests how to make improvements using the OSP system. Fully shielded fixtures are one way to limit wasted light, but they are not sufficient to reverse any of the three problems called light pollution. Luginbuhl and coauthors point out that unintentional use of vegetation and other structures reduces by 50-60% the impact of wasted light contributing to sky glow. Given that value, intentional use of vegetation and structures to prevent light from leaving a property could be more effective at controlling sky glow than the use of fully shielded fixtures advocated by the authors.

The OSP system is a practical computational tool to compare proposed and existing designs that limit wasted light with those used in current practice. It also gives owners and communities practical methods and effective criteria for minimizing glow, trespass, and glare not only to slow the growth of wasted light—including encroachment on the night sky—but to reverse it. We therefore applaud the authors for making the case to reduce wasted light as it affects astronomical observations, but many more issues associated with outdoor lighting need to be considered, including its benefits. With practical and effective tools, each community can best decide how to address the multifaceted issues of light pollution.