Light pollution/lighting regulation
Population and economic growth have been accompanied by an increase in outdoor lighting in both urban areas and smaller communities in proximity to major observatories. Observatory management realized that community engagement and lighting regulation would be required to preserve dark sky access. The earliest example is the ordinance from the City of Flagstaff in 1958 prohibiting the use of searchlights (Lockwood 2002). Kitt Peak, Steward, and Smithsonian Observatories undertook a campaign of community engagement with the City of Tucson and surrounding Pima County to develop outdoor lighting ordinances in the early 1970s, while the Kitt Peak National Observatory 4-m and other telescopes were under construction (e.g. Walker 1973).
Today, light pollution control and the specific protection of major observatory sites are enshrined in local and national regulations. The current status is summarized in Table 5. The general approach in the United States is to make these regulations part of the local Zoning Code, which regulates the usage of property. The majority of other codes are established for protection of a specific astronomical site and/or are part of environmental or sustainability regulation.
There are many common elements to these regulations. Almost all are prospective only, applying to new development or major redevelopment of properties. An exception is the Andalucia statute for protection of Calar Alto, which did require that all fixtures with greater than 25% up-light be removed within three years of enactment in 2010. Most require fully shielded fixturing, those that allow no light to be directly radiated above horizontal. As discussed above, radiation into directions immediately above horizontal has the greatest impact on observatory sites outside of the urban area from which it emanates (Luginbuhl et al. 2009a). The IAU Commission on Site Protection made full cutoff its highest priority for protection of dark skies (Green 2012). Exceptions are granted for low-intensity and decorative fixtures. Searchlights and bottom-lit signs tend to be specifically prohibited.
Given the ~ 1/r2 impact of light sources, many codes create special zones of protection immediately around observatory sites. Many restrict the lumens per fixture depending on the radial zone around the observatory. In Arizona, Flagstaff, Coconino County, Pima County and City of Tucson codes restrict the total number of lumens per acre (hectare) for new development, depending on distance to major observatories and land use (commercial vs. residential). Many codes offer incentives to deploy fully shielded fixtures by increasing the allowed limit on lumens if the installation is 100% full cutoff. All grant exceptions to the full cutoff requirement for emergency installations, and many exempt roadway lighting.
As discussed above, with the new generation of solid-state LED lamps, there is a growing issue of spectral management, which typically requires an update of existing codes. The Chilean national code (the Norma Lumínica) specifies the fraction of total energy output allowed in three specific wavelength ranges:
300–379 nm: luminous intensity < 0.15 × luminous intensity for 380–780 nm.
380–499 nm: luminous intensity < 0.15 × luminous intensity for 380–780 nm.
780–1000 nm: luminous intensity < 0.5 × luminous intensity for 380–780 nm.
The new proposed Norma Lumínica sets much tighter limits on the fraction of the radiation outside of the 500–780 nm band (MMA 2021).
The code for protection of Calar Alto in southern Spain allows 0% of the energy below 440 nm, and mandates a monochromatic (or narrow-band) source when color rendition is not critical to the use. Others create incentives by allowing more lumens of low-pressure sodium than for other spectral sources. U.S. codes have started placing limits on the correlated color temperature rating of lamps (typically applied to LED), from San Diego County requiring < 2500 K near the observatory, and the Southern Arizona codes setting CCT < 3500 K, while the City of Phoenix just purchased roadway lighting with CCT = 2700 K.
Time management of artificial illumination is also critical for astronomical site protection. Most codes contain a curfew, typically 23:00 or midnight, at which commercial illumination must be turned off or greatly reduced in intensity. La Palma also requires that lighting of public spaces be dimmed after midnight. Warrumbungle Shire took a contemporary approach to protection of Siding Spring Observatory by requiring that brighter fixtures be activated only by motion detectors.
The following sections provide examples of locations that have had some measure of success in successful light pollution mitigation. There is one location (Flagstaff, Arizona) where all three aspects of the problem—technical standards; identification of a threshold; successful implementation—have to a degree been successfully addressed, with measurable results. The degree of success for most is uncertain, due to the lack of ongoing measurement or monitoring as noted previously. Nonetheless, they may serve as illustrative examples of aspects of effective regulatory approaches and overall strategies.
Two detailed examples
Flagstaff, Arizona USA
In 1989 Flagstaff adopted the first known outdoor lighting code with standards based on limiting sky brightness at an observatory site. This code was matched by a nearly identical code in the surrounding Coconino County adopted in the same year. These remain to our knowledge the only lighting codes with standards designed to achieve a sky brightness management target. Besides the innovative limits on overall lumen amounts (see below), these codes also included strict standards for the use of low-pressure sodium for “Class 2” lighting (defined as “lighting for general illumination” such as for parking areas, security, and roadways), and strict shielding standards.
The lumen limits were devised to limit the increase in total zenith sky brightness over the U.S. Naval Observatory Flagstaff Station (NOFS) to 30% over the then current condition, which meant, based on then current sky brightness measurement and sky brightness modeling, an increase from V ~ 21.65 to V ~ 21.37 mag/arcsec2. Modeling of the expected sky brightness increase from development of vacant commercial/industrial properties, performed by R. Garstang using an early version of his model (Garstang 1986) were used to establish a limit of 25,000 (lamp) lumens per acre (10,100 lm per hectare) within a zone of 2.5 mile radius centered at NOFS. Combined with the available area for development within this zone, this lumen per acre translated to approximately 5,000,000 lm.
The lumen caps in further zones were increased in steps by factors of two, to 50,000 and 100,000 lumens per acre, as sky brightening impacts of development in these outer areas were decreased by distance. It was anticipated in a rough sense that lighting added by additional development of vacant land in these outer zones would be offset by redevelopment and replacement of old lighting under the tighter standards of the new code.
It is important to note that the conceptual basis for the lumen cap in the innermost lighting zone was not lighting needs or lighting industry recommendations. It was understood that the limit would have implicit impacts on possible land uses or typical development practices. In the more distant zones, with limits of 50,000 lm per acre and above, arguments can be made that industry-recommended illumination needs for most uses can be met with minimal adaptation (IDA 2002), and experience since the adoption has borne this out. This is one instance showing the result of a policy balancing the competing purposes of development and dark sky protection.
The lighting code has been successful in achieving its goals of limiting sky brightness increase at NOFS. Measurements made in June 2015 indicate an actual decrease of approximately 10% compared to the measures made in the late 1980s. This decrease is thought to arise from the further shift of Flagstaff outdoor lighting based on the 1989 lighting code standards and most roadway lighting to low-pressure sodium, which has decreased impact in the Johnson V band, the beneficial effects of improved shielding, not fully understood in the modeling effort in the late 1980s, and the effects of near-ground blocking, not understood or included in the 1980s modeling.
Following the June 2015 sky brightness measures, and using improved light pollution modeling, NOFS has recommended, and the community planning bodies have supported, the revision of the Flagstaff and Coconino County lighting codes to achieve a new management target of only a 10% increase over current sky brightness conditions. These amendments involve conversion of the former lamp lumen standards to fixture lumens (amounting to a 0.7× decrease); stricter shielding requirements in the closest Lighting Zone; reduction of the lumen limit in the outermost zone (formerly 100,000 lamp lumens or 70,000 fixture lumens per acre) to the values of the next innermost zone (35,000 fixture lumens per acre); and the elimination of the requirement for roadway lighting in residential areas. These amendments are in process as of spring 2021.
This result demonstrates not only the effectiveness of the technical aspects of the lighting code standards, but also the successful implementation and maintenance of the standards through the governing process. The most critical aspect of this successful implementation is the ongoing engagement of observatory staff at both NOFS and Lowell observatories with the local planning process, as well as the efficacy of a citizen-based group advocating for dark skies based on community values (and not just economic benefits of having professional astronomical observatories). This engagement involves attendance at hearings and both public and where possible internal planning staff meetings where lighting plans or rezoning are considered, consultation with planning staff to assure understanding of and effective implementation of the codes, and assistance in developing code amendments where misunderstandings develop or technology changes necessitate. This engagement is a crucial aspect to successful light pollution mitigation, one rarely achieved.
Maunakea, Hawaii USA
Approaches to local regulation
Maunakea has been protected by a strong lighting ordinance since 1990. The ordinance has been recently adapted to allow use of LEDs. The core component of the original lighting ordinance was widespread use of low-pressure sodium (LPS) lighting for all applications where color rendition was not important. LPS lighting has several important advantages for astronomy. The first is that it is nearly monochromatic, with nearly all of the energy emitted near 589 nm wavelength (amber color). This means that for spectroscopic observations with a telescope, only one wavelength is affected, and that same wavelength is already compromised naturally by sodium emission in the upper atmosphere from sodium deposited by micrometeorites. The second reason relates to Rayleigh scattering. At most major observatory sites (particularly the island sites—Hawaii and La Palma), the air is clean with very little aerosol. The dominant scattering mechanism for artificial light is therefore Rayleigh scattering by air molecules, which is very strongly wavelength dependent. Blue light at 450 nm scatters three times as much as LPS light. LPS light is nevertheless close in wavelength to the photopic peak in the eye’s sensitivity at 555 nm, so it is effective for human vision.
The disadvantage of LPS lighting is that it is difficult to control. Early versions of the lighting ordinance on the Island of Hawaii did not require full shielding of lights because of the limited availability and efficiency of fully shielded LPS fixtures. Later updates to the lighting ordinance did require full shielding of all lights. Luginbuhl et al. (2009a, b, c) showed that full shielding of light sources is one of the most important tools for protecting the night sky. Light directed at small angles above the horizontal is particularly damaging. The lighting ordinance on the Island of Hawaii now requires that all artificial light sources are fully shielded, and emit no light above the horizontal plane.
Another important aspect is the amount of light used (cf. Sect. 3.2). On the Island Hawaii, streetlights are commonly mounted on utility poles, which have spacing that makes it difficult to achieve uniformity and lighting levels that are recommended by lighting organizations such as CIE and the Illumination Engineering Society of North America (IESNA). The lighting is nevertheless adequate and safe. Electricity is very expensive on the Island of Hawaii, with prices sometimes as high as US$ 0.50/kWh. This provides strong additional motivation for energy efficiency (one of the arguments supporting the use of LPS), and to use only the amount of light needed, and to not over-light.
Dimming of light sources is another important tool to protect the dark night sky. Lighting levels are often set by counting the number of pedestrians in the early evening. There are many fewer pedestrians (and cars) later at night, so following the pedestrian count rationale, lights can be dimmed. Dimming is expected to be implemented on some state highways, and in other counties in the state of Hawaii. Dimming of LEDs is relatively simple; it is much more difficult to dim arc discharge lamps such as LPS and high-pressure sodium (HPS). On the Island of Hawaii, street lighting levels on county roadways are already close to minimum levels recommended by the lighting industry, and it is unlikely that these lights can be further dimmed. Haleakala observatory on the Island of Maui is a smaller observatory that has been protected by a much weaker lighting ordinance. The core requirement of the Maui County lighting ordinance is for full shielding of lights. However, metal halide lamps, which are very damaging, are not required to be shielded (because of their use at recreational facilities, and the cost associated with replacing them with shielded fixtures). Maui’s lighting ordinance has very weak spectral power distribution limitations. It urgently needs to be updated, particularly to adapt to the use of LED lighting.
Two techniques are possible. The blue LED light (from a white LED) can be filtered out. Or LEDs that intrinsically emit little or no blue light can be used (e.g., amber LEDs). Due to energy efficiency considerations, the approach that has been adopted on the Island of Hawaii for streetlights is use of filtered LEDs. A filter is used that absorbs essentially all light shortward of 500 nm. The resulting light has a greenish-yellow hue if a 5000 K white LED is used as the base illumination, or a yellow hue if a 3000 K LED is used as base illumination (the spectra of these filtered LEDs are shown in Fig. 4 as FLEDcw and FLEDww, respectively). The similarity of the color of amber LED lighting and the amber (yellow) traffic signal was raised as a concern on the Island of Hawaii. This results from many years of use of partially shielded LPS lights allowed under the early code standards. The color of an LPS light is indistinguishable from that of an amber traffic signal. Likewise, the color of an amber LED is indistinguishable from the amber traffic signal, because the amber LEDs are used in traffic signals. Fortunately, full shielding of streetlights reduces the likelihood that a streetlight could be mistaken for a traffic signal. Phosphor converted amber LED and filtered white LEDs both have color rendering indices of approximately 60. Both sources are deficient in blue light. Filtered LEDs are now being used for street lighting on the Island of Hawaii.
In the United States, county and state laws do not affect federal installations. County laws for the County of Maui do not affect lighting on the Island of Oahu (where Honolulu is located); Oahu makes the night sky bright in the northwest seen from Haleakala. State facilities, such as airports and harbors, may try to exempt themselves from County lighting requirements. And federal facilities, such as the Pohakuloa Training Area, are not required to follow the County lighting ordinance.
Photographs of Hawaii obtained by astronauts on the International Space Station have shown that the airports and harbors are among the brightest source of light on the Island of Hawaii, and careful lighting there is vitally important. The army training area at Pohakuloa is located only 10 km from the observatory. Because of its close proximity, it is critically important that their lighting is state-of-the-art in terms of reducing impact to the observatory.
In Hawaii, state laws have been established that require the state Department of Transportation to follow county lighting ordinances for highway, airport and harbor lighting. State coastline lighting laws have also been enacted. A state night sky protection advisory committee has been established, and a state law requiring all state lighting to be fully shielded and to have correlated color temperature ≤ 4000 K is now in place. The state law will begin to address the impact that the bright lights from Honolulu have on Haleakala observatory and on the more distant Maunakea observatory. Although the Pohakuloa Training Area is not required to follow the county lighting ordinance, they are voluntarily complying. The army minimizes impact on the night sky by using careful shielding and selecting light sources that minimize emission at blue and green wavelengths.
Hawaii has numerous endangered birds and turtles, and many of these animals are profoundly affected by light at night. The endangered Newell’s Shearwater, found mostly on the Island of Kauai, circles around an unshielded light until it is exhausted, at which time it collapses to the ground near the light, where it is vulnerable to predators. The US endangered species act produces strong motivation to use proper shielding of lights. As a result, almost all lighting on the Island of Kauai is fully shielded, despite there being no observatories on that island. The lighting requirements for protecting endangered species and for protecting the observatories are closely aligned, both in terms of shielding and spectral energy distribution. Endangered turtles are much more affected by white light than by amber light.
It is important to update lighting ordinances regularly to adapt them to changes in lighting technology and other changes. The lighting ordinance on the Island of Hawaii is presently being revised. One change that has become necessary is to limit nighttime light emission from greenhouses. Grow lights are sometimes used in greenhouses at night to increase crop production. These can have a strong impact on the night sky, and so should be appropriately shuttered at night to prevent escape of light. Architectural lighting is often not properly shielded, and is often damaging to the dark night sky. The revised lighting ordinance will explicitly prohibit illumination of rooftops.
Results in night sky protection
The Island of Hawaii has a population of nearly 200,000. As a result of the strong lighting ordinance on the Island of Hawaii, the night sky over Maunakea observatory remains very dark, and is well suited to the demanding requirements of the deep sky astronomy performed by the large telescopes located there (Fig. 12). In fact, the night sky over Maunakea is among the darkest night skies in the world. Astronomical observatories benefit from nearby communities, because these communities provide infrastructure such as schools, shops, and medical facilities for the staff that work at the observatories, as well as providing other logistical benefits such as transportation. However, very careful and strong lighting regulations are required for observatories to successfully coexist with urban development. In contrast, the Island of Maui has a much weaker lighting ordinance, and as a consequence, the night sky over Haleakala observatory is less dark (Fig. 13) Haleakala is also closer to the city of Honolulu and the Island of Oahu (population approximately 1 million), where the lighting is less well regulated. At a distance of approximately 180 km, Honolulu and Oahu have a significant impact on Haleakala’s night sky, and make the northwestern sky brighter.
Flagstaff provides a second example at a moderately polluted site, and provides a quantitative insight into what is achievable through effective sky glow mitigation. Measurements also made by the U.S. NPS (see Fig. 14) from 27 km outside of Flagstaff (population 70,320) show that its integrated V band artificial sky brightness is less than 1/10 (1/11.9) the integrated sky brightness over Cheyenne Wyoming (population 63,335) as observed from 31 km (Pipkin et al. 2017). Though this is a single point comparison, the long history of aggressive light pollution control efforts in Flagstaff contrasted to a similarly-sized community with no particular historical concern for light pollution shows that dramatic reduction may be more widely achievable.