In cities around the world, telling a child, “I used to see stars from my backyard,” may soon sound as credible as, “I used to walk 10 miles to school in 6-foot-high snowdrifts uphill in both directions.” Sky glow, light pollution, and light trespass are the consequences of development and outgrowth in urban and rural landscapes. The damage isn’t merely aesthetic. Research suggests that excess night light can harm nearly everything living under the sun. The offender may be as simple as stadium lights fatally mistaken by fledgling birds for the moon, or the neighbor’s porch light that beams into your bedroom, resulting in fatigue and diminished productivity.

The amount of energy and money expended to illuminate what is essentially water vapor and floating particulates in our atmosphere is not trivial. According a 2009 document published by the nonprofit International Dark-Sky Association (IDA), the U.S. expends 22,000 gigawatt-hours of electricity—the equivalent of 3.6 million tons of coal—each year in light pollution. At the rate of 10 cents per kilowatt-hour, this energy translates to $2.2 billion annually.

Past Practices: The Bad and the Ugly

For decades, charts in outdoor-luminaire catalogs and standards such as ANSI/IESNA RP-8: Roadway Lighting dictated exterior lighting design. It seemed like nearly anyone could be a lighting designer—no calculations required. Manufacturers sold a bunch of luminaires and everything was overlit. Energy was cheap and sky glow was mostly ignored outside the astronomer community, which watched its observatories become decreasingly effective.

In the past half-century, the light source of choice for streets and parking lots was high-intensity discharge (HID) lamps, which included mercury vapor and low-pressure sodium lamps, but mostly the ubiquitous yellow-orange high-pressure sodium (HPS). The prevalence of HPS, possibly the greatest source of light pollution in the U.S., is evident in NASA’s aerial photographs, which are both beautiful and lamentable.

It is difficult to design optics that can control stray uplight and glare without significant tradeoffs in luminaire efficiency. Despite their notoriously poor color rendering, HPS lamps are inexpensive, can last up to 24,000 hours, and output a lot of light initially; the output declines as they age. In the last 30 years, the use of metal halide, an HID source that produces whiter light than HPS, has increased, but HPS still dominates.

Millions of HPS drop-lens or “cobra head” streetlight luminaires remain in use today. Because they produce large amounts of high-angle light, they can create a lot of glare. With routine maintenance such as lamp and ballast replacement, they can last for 30 or more years. Because they are still being installed, it will be decades before some are ready for replacement. In the U.S., most roadway lighting, which is owned by municipalities or utility companies, is not subject to energy codes.

Zoned In: New Codes and Standards

The California Energy Commission’s Title 24: Building Energy Efficiency Standards contains the most stringent lighting requirements in the nation. Though Title 24 first issued regulations regarding lighting for interior spaces in the early 1990s, it wasn’t until the 2005 edition that maximum exterior-lighting power densities were included. It also recognized that overlighting one area makes it difficult to see into surrounding areas, which, as a result, are also overilluminated.

Title 24 introduced the concept of lighting zones to regulate the power density of the exterior lighting allowed in each zone. U.S. Census Bureau maps determined the boundaries for three of the four zones: Zone 1 applies to parks, recreation areas, and wildlife preserves; Zone 2 encompasses rural areas, where low levels of ambient lighting are allowed; and Zone 3 is set aside for urban areas. Zone 4 is reserved for special-use districts, such as a sports complex, which is determined and adopted by local jurisdictions.

But Title 24 “is an energy standard,” says Nancy Clanton, president of Clanton & Associates, a lighting-design firm based in Boulder, Colo. “It can’t address issues like light pollution and glare.”

In 2004, Clanton co-chaired a task force to take on the problem of expanding the lighting zones Title 24 established into the Model Lighting Ordinance (MLO) that would address the issue of light pollution. The joint International Dark-Sky Association/Illuminating Engineering Society (IDA/IES) task force included dark-sky advocates, lighting manufacturers, lighting designers, and a city planner.

After years of research, intense negotiations among parties, and public reviews, the IDA and IES boards approved the MLO in June 2011. The writers intended it for use as a zoning overlay ordinance, meaning that its lighting zones could closely follow a municipality’s land-use maps. The MLO has five lighting zones: Lighting Zone 0 (LZ-0) is for environmentally sensitive areas that should have no light at all; LZ-1 is for one- and two-family residential neighborhoods and small rural communities where low ambient light is suitable; LZ-2 applies to areas for multifamily and institutional use where moderate ambient light is acceptable; LZ-3 is for commercial areas, where moderately high ambient light levels would be allowed; and LZ-4 is for special culture and entertainment districts, such as New York’s Times Square where extensive light is unavoidable.

In municipalities that choose to adopt the MLO, individuals seeking building permits or approvals from the local planning department would have to comply with its guidelines, subject to consequences determined by the particular city. By adopting the MLO, communities that want to be night-sky friendly do not need to hire engineers, planners, and lighting designers to develop regulations for them. The MLO provides regulatory consistency from town to town.

The MLO recommendations are similar to ANSI/ASHRAE/USGBC/IES 189.1P: Standard for the Design of High Performance Buildings, but they also cover residential lighting. However, they only cover lighting on private property; the MLO’s roadway-lighting section is optional.

The MLO uses “BUG ratings” developed by the IES in TM-15-07: Luminaire Classification System for Outdoor Luminaires. Manufacturers derive the Backlight-Uplight-Glare (BUG) ratings for luminaires from their photometric data. Backlight references the light that emanates from the back of the luminaire and often causes light trespass. Uplight—light 90 degrees or above nadir—is generally responsible for sky glow. Glare—visually disabling light that can cause discomfort or be a nuisance—originates from any part of the luminaire, though BUG ratings are primarily concerned with high-angle light projected from the luminaire’s front and back.