By Brian Liebel & Rick Utting, DarkSky International, Tucson, AZ.
Executive Summary
Upward-emitted outdoor light has long been identified as a source of atmospheric light pollution, a.k.a. sky glow. This form of light pollution is an obvious contributor to the brightening of the nighttime sky and can be observed as glowing domes of light above urban population centers and highly industrialized areas (see image above).
In fact, for the vast majority of urban populations on earth, light pollution is so prevalent it has significantly decreased the visibility of celestial objects in the night sky.1 For this reason, DarkSky International (“DarkSky”), a nonprofit organization representing thousands of advocates in more than 70 countries, has consistently recommended luminaire and lighting installation restrictions on upward light emissions.
However, recent concerns regarding the continued proliferation of skyward light pollution, growing by 10% annually,2 and its adverse effects upon nocturnal ecosystems1 obligate a broader evaluation of possible causes of light pollution and the development of mitigation strategies. Specifically, this white paper evaluates horizontal and near horizontal light emissions from the most prevalent types of outdoor applications, and their human usefulness versus known contributions to light pollution.
Note: The scope of this white paper is limited to the outdoor applications of highways, roadways, streets, sidewalks, parking lots, pathways, pedestrian areas, and large-area outdoor lighting applications (e.g., sports, ports, and industrial areas) utilizing pole or building mounted lighting with a mounting height of 3 meters (10 feet) or more above finished grade (AFG). DarkSky recognizes that there are other outdoor applications where horizontal and near horizontal light emission may be useful for purposes beyond the scope of this review.
As discussed further in Section 7.0 Conclusion, outdoor applications often have very specific, sometimes unique, goals and requirements for artificial light at night (ALAN). These application needs are typically designed with both objective requirements, such as electrical and energy codes, and subjective outcomes, such as visual success or emotional responses, in mind. The best way to discuss, communicate, and validate these application needs is by using a vocabulary that includes both horizontal and vertical illuminance and/or luminance values. As one might expect, the contributions and value of ALAN within these two differing planes will vary. This paper evaluates the usefulness and targeting success of commonly used luminaire emission types.
For many tasks and applications, it was found that the contribution and value of light emitted between 80 and 90 degrees above nadir is limited – and sometimes nonexistent – when compared to the visual and ecological benefits of reducing near-horizontal light pollution. As a result of this analysis, DarkSky International programs and policies now include limits on very high-angle light emitted between 80 and 90 degrees from nadir, ranging from 0% to a maximum of 5% of the total luminaire lumens.
Read the full white paper here.
Special thanks to the All Things Lighting Association (ATLA). Excerpt republished with permission.
Image above: A panoramic night sky image taken from Carlsbad Caverns National Park. Image credit: U.S. National Park Service.
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