Astronomical observatories are increasingly at risk due to the fast encroaching growth of communities and the commensurate output of light at night illuminating roadways, shopping centers, sports parks and neighborhoods.

In a report presented at the 228th meeting of the American Astronomical Society, held in San Diego, California, astrophysicist Dr. Eric R. Craine (STEM Laboratory, Tucson, AZ), and collaborators Dr. Brian L. Craine (Western Research Company, Tucson, AZ) and Dr. Roger B. Culver (Colorado State University, Fort Collins, CO) demonstrate how using a new satellite-based index offers insight into the understanding of light at night and how to better address these impacts.

Given that artificial light at night (ALAN) is fact of modern life, it is useful to consider not just the absolute brightness in a specific location, but rather, the range of ALAN use on a more global scale to determine "best practice" levels.

This allows for the identification of attainable levels, the recognition of areas where improvements could be expected and areas of unusual concern. This presentation describes the development of an indexing tool, Light at Night Index (LANI), to help in this type of analytical approach.

STEM Laboratory reports that it is annually producing the LANI value for each of more than 19,000 communities in the United States. This allows the ranking of communities in terms of their lighting efficiency, and indicates how new lighting installations, or community growth, impact dark night environments as a function of time.

Index developer, Dr. Brian L. Craine, commented that, "The LANI data can provide community planners with direction as to which communities would benefit from further light at night characterization, and more efficient light use."

The STEM Laboratory approach integrates data from five primary sources: a ground static survey, a ground mobile survey, an airborne survey, satellite data survey, and spectroscopic observation. Light sources are mapped and measured over time and from all directions.

Among the findings of the STEM Laboratory rankings is that thoughtful retrofit of older outdoor lighting can significantly move communities from poor to much more efficient use of light. This improves the community dark night ranking, saves significant community lighting costs, and provides maximum protection for dark skies.

"Light at night monitoring must be multi-dimensional and light sources must be properly characterized and ranked for brightness if we are to identify a strategy designed to save our urban observatories and address nighttime community light needs," Craine said.

STEM Laboratory has most recently used these methodologies to characterize dark night environments near the Mauna Kea observatories on the Big Island of Hawaii, where the community of Waikoloa has demonstrated significant improvement in lighting efficiency through novel filtered light emitting diode (FLED) streetlight installation.

"There is a demonstrated, viable program for monitoring light at night in all communities in a comprehensive and quantitative way that can lead to specific, effective mitigation proposals," Craine said. "This program can be implemented at a very low cost when compared to the enormous amounts of money wasted on inefficient lighting."

A summary of the report, "An Approach to Objectively Defining and Ranking Dark Night Communities" can be downloaded at the STEM Laboratory website http://www.stemlab.org/lan/events. In addition to Eric R. Craine, the study authors include Dr. Brian L. Craine, Western Research Co.; and Dr. Roger B. Culver, Colorado State University.

In addition to the research under way to evaluate light levels impacting astronomical observatories, STEM Laboratory staff are establishing baseline night light mapping of the San Pedro Riparian National Conservation Area. This significant bird and wildlife refuge contains nearly 57,000 acres of public land in Cochise County, Arizona, between the international border and St. David, Arizona.