Outdoor electric lighting at night which is unwanted, unneeded, or wasteful and which results in glare, light trespass and sky glow or other harmful effects on people and the environment.
Light Pollution Recognized
Electric lighting makes travel, working, playing, and other normal living activities possible at night. Indeed, the view of a lighted city from an airplane window or tall building is a hallmark of twenty-first-century civilization, yet little more than a century and a half ago, there were no such views and human activities outdoors at night were limited to those considered essential. Even then, they were carried out only with difficulty especially tasks involving work or travel. Limitations remained as candles and then gas flames and mantles began to generate more powerful and reliable night lighting, but the benefits were not widespread because of the relatively high cost of providing light especially over large outdoor areas at night.
When electric lighting was invented, one of its first uses was for outdoor illumination. Carbon arc lamps were used initially and then incandescent light sources, which were developed during the 1870s. These became the worldwide standard for all types of lighting until the commercial introduction of the fluorescent lamp in the late 1930s. Outdoor lighting, however, did not dramatically change until mercury lamps, the first of the so-called high-intensity discharge (HID) lamps became widely used for street, parking, and area lighting during the 1960s and then other types of HID lamps – high-pressure sodium (HPS) and metal halide (MH), due to their relatively higher efficacies, transformed the cost and use of outdoor lighting leading to rapid growth and the night time appearance of cities that we see today. Now, however, a new transformation is underway characterized by the growing use of solid state lighting in the form of Light Emitting Diodes (LEDs) which are increasingly being used for outdoor lighting.
Electric lighting has changed the night. And there are negative aspects that were recognized early on by critics who pointed out that bright lights frightened horses, annoyed pedestrians, and produced glare. There were those who also noticed that electric light changed the egg-laying habits of chickens, attracted insects, and modified the leaf and flowering characteristics of certain plants – early examples that hinted at more significant problems that later appeared due to unneeded or unwanted light. But the term “light pollution” did not appear until much later – in the 1980s. That’s when the large, powerful HID lamps began to be used in large quantities for major outdoor lighting systems such as street lighting, parking areas, signs, sports stadiums, and floodlighting. The stray, uncontrolled light from these new sources as typically used outdoors plus the light from the headlights of cars and light escaping through windows from the interior lighting systems of buildings at night has now transformed the night environment in conspicuous ways including making the night sky less visible. Instead, light into the sky blankets cities with “sky glow,” a fog, or haze of light.
Astronomers, both professional and amateur, were among the first to recognize and talk about sky glow as light pollution as they tried to observe the stars from urban locations. As a first response, professional astronomers moved telescopes and observatories from cities to new locations in remote areas. Then, it was recognized that such places were increasingly rare and costly to develop. Further, most amateur astronomers were not able to move or take advantage of such sites. Members of the public who simply wanted to catch a glimpse of the Milky Way from their backyard or to show their children a comet or star constellation were not able to do so. The result is a growing and widespread reaction to uncontrolled electric lighting outdoors at night.
Efforts began, usually via zoning, planning, or the so-called “nuisance” regulations to limit and control unwanted light especially around astronomical observatories. It was found that light from urban areas could significantly affect the sky brightness 100–200 km away and that minimizing light directed into the sky was a simple and effective way to control such light. The term “light pollution” became familiar and efforts began to understand the various associated problems and to find ways to limit the detrimental effects. A global organization, the International Dark Sky Association (IDA), founded in 1988, focused on the task and continues to work “to preserve and protect the nighttime environment … through environmentally responsible outdoor lighting” . Other regional and local organizations also appeared worldwide to raise awareness of the problems, write laws, provide educational materials, and organize local activities such as promoting the use of shielded luminaires. Lighting technical and professional organizations have also recognized light pollution problems and have sponsored research, issued reports, produced recommended lighting practices, and written model legislation [2, 3, 4] in order to control it.
Types of Light Pollution
Light trespass – Light generated in one location which, when poorly controlled, travels to another location where it is not wanted. An example would be a streetlight which is intended to light the street surface but also directs light into front yards or bedroom windows along the street.
Glare – Excessive light directed into the eye where it causes visual discomfort or reduces the ability to see. Overly bright unshielded or misaimed luminaires are usually the main sources of glare.
Sky glow – Light directed or reflected into the sky where it is scattered by air molecules, water, and other particles in the atmosphere and reflected diffusely to produce a fog or haze of brightness that reduces the visibility of the night sky.
Light pollution has energy implications since stray or misdirected light is luminous energy which has been converted from electric energy by the light source. That electric energy, the fuel, and other resources which produce it have therefore been wasted, delivering no value to those who pay for those resources.
The IDA estimates that some 30 % of the electric energy used for outdoor lighting goes into the sky. Considering the hours of operation and an average electrical energy rate of $0.10/kWh (USA), the cost of that wasted energy is well over $2 billion/year.
Electric lighting is the cause of light pollution. However, all light sources are not equally effective at generating light pollution of the types described. The scattering of light in the atmosphere that affects sky glow, for example, is a function of the wavelength of the light (Rayleigh scattering). Such scattering affects how far light pollution will travel. Similarly, the intensity (candlepower) of the luminaire as well as the way the light is aimed and directed matters too.
Other Light Pollution Effects
Research reported during the 1990s has shown that light pollution seriously affects the health and behavior of animals, including mammals, birds, amphibians, and insects. Changes in migration patterns, food gathering and feeding habits, mating, and social behavior have all been documented [7, 8]. However, bird and sea turtle problems have received the most attention.
Birds may become disorientated by lighting to the extent that they die from exhaustion while continuously circling lighted buildings or, confused, they collide with the buildings. According to a Chicago Audubon Society study  documented by actual counts of dead birds found at the base of tall buildings, bird deaths due to building collisions were reduced significantly when just the decorative floodlighting of buildings was switched off for most of the night hours during migration seasons.
Baby sea turtles hatch on beaches and have a short time to dig out of the sand and to find their way to the sea before they die by dehydration or are eaten by predators. The turtles appear to use the brightness of the horizon as a visual clue to find the water. Area and streetlighting along beachfront areas confuses the turtles, so efforts have been made to remove, turn off, or modify such systems including changing to amber-colored light sources which, because of their spectral sensitivity, appear less bright to turtle eyes .
Human Health Effects
A controversial issue involving light pollution is whether or not outdoor lighting negatively affects human health. Research beginning in the 1970s has indicated that certain uses and kinds of electric light and, particularly, light at night (LAN) when the human body expects and needs darkness for sleep, can be detrimental to health because it interferes with the body’s normal circadian rhythms.
These rhythms mark the periods of alertness and sleep due to the ebb and flow of hormones, such as melatonin, in the body. According to the research, strong circadian rhythms are essential for overall health and mental functioning and particularly for cell repair which helps protect against major diseases including cancer. The general rule is that people need bright days and dark nights so current research is exploring questions about light intensity, spectrum, timing, and exposure duration to determine the “dose” of light that might result in the interruption of the melatonin cycle and, further, what kind of interruptions lead to an increased risk of disease. The research literature on the subject is already extensive – one compilation now lists over 1,000 citations , and how outdoor lighting impacts human health will remain an important subject of research and discussion.
The American Medical Association adopted a resolution in 2009 entitled “Advocating and Support for Light Pollution Control Efforts and Glare Reduction for Both Public Safety and Energy Savings” . Significantly that AMA view was broadened in 2012 when they said, “The (AMA) policy also supports the need for developing lighting technologies that minimize circadian disruption and encourages further research on the risks and benefits of occupational and environmental exposure to light at night” .
Outdoor lighting, because such lighting impacts the natural environment and so many people see it, use it, and experience it, has become complicated. It is now a social, technical, medical, and environmental problem that has to be carefully analyzed rather than a traditional exercise in illuminating engineering so that people can see. Light pollution issues contribute to outdoor lighting’s complexity, but light pollution also calls attention to important and growing outdoor lighting problems that have to be addressed and resolved.
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