Light pollution

From Academic Kids

Light pollution (also known as photopollution, luminous pollution) is excess or obtrusive light created by humans. Among other effects, it can obscure all but a few stars to city dwellers, cause problems for astronomical observatories, and disrupt ecosystems. Since the early 1980s, a global dark-sky movement has emerged, with concerned people campaigning to reduce the amount of light pollution.

Missing image
This composite satellite photograph of the earth at night demonstrates how much of the world's light is projected into space, rather than its intended target. (Photo credit: NASA)

Light pollution is a side effect of industrial civilization. It comes from sources such as domestic lighting, advertising, commercial properties, offices, factories, streetlights, and lit sporting venues. It is most severe in the highly industrialised, densely populated areas of the United States, Europe, and Japan, but even relatively small amounts of light can be noticed and create problems.

With recent advances in private spaceflight, the prospect of space-based orbiting billboards appearing in the near future has provoked concern that such objects may become another form of light pollution. With this in mind, the United States Federal Aviation Administration sought permission, in May 2005, to enforce a law prohibiting "obtrusive" advertising in zero gravity [1] ( Similar intentions are yet to be expressed by authorities in most other countries, however.


Light pollution as a problem

"Light pollution" is a term that is used to refer to light that people find annoying. Some skeptics claim that light pollution should not be compared with "real" pollution, arguing that any instance of stray light has no long term effect. This is because unlike traditional forms of pollution, light pollution is not persistent. The amount of light pollution at any given time is directly related to the amount of artificial light that is being emitted at that time, meaning that if all artificial lights were immediately switched off, the visual effects would cease immediately. Critics who take this point of view contend that in some cases, light pollution should not even be worried about.

Campaigners wishing to reduce light pollution, however, argue that it is unrealistic to expect populations to ever switch off light en-masse, due to industrial society's economic reliance on artificial light. They argue, therefore, that it is a problem synonymous with traditional forms of pollution, where trends in society that are difficult to cease or reverse have long term negative effects. Campaigners contend that light pollution must be dealt with by changing the habits of society so that light is used more efficiently, with less light being wasted and directed towards places that are undesirable.

Because not everybody is irritated by the same sources of light, the term is subjective. It is common for one person's light "pollution" to be light that is desirable for another. This is often the case with advertising material, such as when an advertiser wishes for particular lights to be brightly visible, even though others find them annoying. With other types of light pollution, there is less ambiguity. For instance, light that accidentally crosses a property boundary and annoys a neighbour is generally wasted and pollutive light, as it is of no use to anyone. Disputes are still common, however, when deciding if it is necessary to do anything about it.

Differences in opinion over what light is considered reasonable, and who should be responsible, means that negotiation must sometimes take place between parties. Authorities have also taken a variety of measures for dealing with light pollution, depending on the interests, beliefs and understandings of the society involved. Measures range from doing nothing at all, to implementing strict laws and regulations about how lights may be installed and used.

Types of light pollution

"Light pollution" is a broad term that is generally accepted as referring to multiple problems, all of which are caused by inefficient, annoying or arguably unnecessary use of artificial light. Specific categories of light pollution include light trespass, glare, clutter, and sky glow, however it is common for instances of light pollution to fall into several categories.

Light trespass

Light trespass occurs when unwanted light enters one's property, for instance, by shining over a neighbour's fence. A common light trespass problem occurs when a strong light enters the window of one's home from outside, causing problems such as sleep deprivation or the blocking of an evening view.

Light trespass is particularly annoying for amateur astronomers, whose ability to observe the night sky from their property is likely to be inhibited by any stray light. Most major optical astronomical observatories are surrounded by zones, within which the light emissions are severely restricted.


Glare is the result of directly seeing the filament of an unshielded or badly shielded light. Light shining into the eyes of pedestrians and drivers can obscure night vision for up to an hour after exposure. Bright glare can also create a high contrast between light and dark areas, making it difficult for the human eye to adjust to the differences in brightness.

Glare is particularly an issue in road safety, as bright and badly shielded lights that surround roads may blind drivers unexpectedly, and potentially cause accidents.


Clutter refers to excessive groupings of lights. Groupings of lights may generate confusion, distract from obstacles, including those that they may be intended to illuminate, and potentially cause accidents. Clutter is particularly noticible on roads where the street lights are badly designed, or where brightly lit advertising surrounds the roadways. Depending on the motives of the person or organisation who installed the lights, their placement and design may even be intended to distract drivers, and can contribute to accidents.

Sky glow

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Los Angeles at night, with a brightly illuminated sky.

Sky glow refers to the "glow" effect that can be seen over populated areas. It is the combination of all of the badly directed light in that area, being refracted in the surrounding atmosphere. Sky glow is of particular irritation to astronomers, because it reduces contrast in the night sky to the extent where it may even become impossible to see the brightest stars.

Astronomers have begun to use the Bortle Dark-Sky Scale, to quantify sky glow, since it was published in Sky & Telescope magazine (Bortle, 2001). The Bortle Scale rates the darkness of the sky, inhibited by sky glow, on a scale of 1 to 9, providing a detailed description of each position on the scale.

Consequences of light pollution

Several negative consequences may result from light pollution, whether as a direct result of the light, or otherwise. Consequences that are commonly publicised include energy waste and reduced security. There is also some evidence that light pollution may have negative effects on people's health, and for animals in the surrounding environment.

Energy waste

Energy waste results from light being directed to places where it is not intended to go, and is not useful. For instance, lighting fixtures that allow light to escape above the horizontal, rather than directing it towards the ground, are allowing light to be wasted. Energy waste also results from using lights that are overpowered, or by installing more lights than are necessary for the task at hand.

Energy wastage due to inefficient light has become a significant issue for some national governments, particularly after the Kyoto Protocol has obligated them to search for ways to reduce energy consumption. On a smaller scale, individuals and local authorities sometimes look to improve lighting efficiency in order to reduce their electricity bill. A notable example has been the city of Calgary, in which most residential street lights have been replaced with models that are comparably energy efficient [2] (

Reduced security

Although some people claim that increased lighting improves security and safety, and there is little doubt that more light increases the feeling of safety for many people, campaigners for the reduction of light pollution claim that badly installed lighting may actually reduce safety. Deep contrasting shadows created by poorly directed home security lighting, or by lights that emit a lot of glare, for example, may provide intruders with more places to hide. It is sometimes also argued that glare from badly installed lighting fixtures can prevent people from adequately seeing and assessing their surroundings.

Loss of the night sky

Large numbers of people are unable to see the night sky, with the exception of astronomical objects that are exceptionally bright, such as the Moon and bright planets ("The Problem with Light Pollution", 1996). Light pollution, and sky glow in particular, tends to occur in places where there are concentrations of population, blotting out the sky's features. Although light pollution decreases whilst leaving populated areas ("Estimating the Level of Sky Glow Due to Cities", 1996), one might still have to travel many tens of kilometers to reach a reasonably dark sky. It may be necessary to travel even further if the population is large, if the population is spread out, or if a completely unaffected sky is wanted. According to a study performed by Backpacker magazine, Natural Bridges National Monument, Utah has the least amount of light pollution, and thererefore the darkest skies in the continental United States.

The gradual loss of the night sky is a major cause of concern for back-yard amateur astronomers, for most of whom extensive travel is unrealistic. Some also believe that there are negative effects for the overall population, citing anecdotal cases suggesting that city-dwellers may be less informed and appreciative of space, astronomy, and perhaps science in general, than those who live in rural areas and are more familiar with the night sky.

Health problems

Medical research relating to the effects of excessive light on the human body is slim, but there is at least some argument that it may cause health problems. Of note, two studies have been published that suggest a link between exposure to light at night and risk of breast cancer, due to suppression of the normal nocturnal production of melatonin (Davis et al., 2001; Schernhammer et al., 2001). As always in new fields of scientific study, more research is needed.

Interruption of the eco-system

Some researchers believe that light pollution affects insect and other animal behaviors, disrupting ecosystems. In particular, light pollution has been shown to confuse the navigation systems of many animals, particularly those that assume the Moon as the brightest source of light, in order to find their way.

Studies have shown that light pollution around lakes prevents fish from eating surface algae, helping cause algal blooms that can kill off the lakes' plants and lower water quality (Moore et al., 2000). Light pollution may also affect ecosystems in other ways. For example, Lepidopterists and entomologists have documented that night-time light may interfere with moths' ability to navigate (Frank 1988), sometimes preventing the pollination of night-blooming flowers. Michael Mesure, founder of the Fatal Light Awareness Program, believes that excessive light in cities causes the deaths of millions of birds every year, through confusing their sense of navigation.

So far there is no certainty of whether light pollution will, or will not, result in long term negative effects for the ecosystem, but many people argue there is reason for caution. A book that collects together research on the subject is currently in production, and is scheduled to be released in November 2005 (Rich and Longcore, 2005).

Reducing light pollution

Reducing light pollution implies many things, such as reducing sky glow, reducing glare, reducing light trespass, and reducing clutter. The method for best reducing light pollution, therefore, depends on exactly what the problem is in any given instance. Possible solutions include:

  • Improving lighting fixtures, so that they direct their light more accurately towards where it is needed, and with less side effects.
  • Adjusting the type of lights used, so that the light waves emitted are those that are less likely to cause severe light pollution problems.
  • Evaluating existing lighting plans, and re-designing some or all of the plans depending on whether existing light is actually needed.

Improving lighting fixtures

Missing image
This drop-lens cobra luminaire allows light to escape sideways and upwards, where it is unlikely to be useful and may cause problems.
Missing image
An alternative flat-lens cobra luminaire is preferable for reducing light pollution, as it ensures that light is only directed below the horizontal.

Most campaigners for the reduction of light pollution advocate the use of full cutoff lighting fixtures as much as possible. It is also commonly recommended that lights be spaced appropriately for maximum efficiency, and that lamps within the fixtures not be overpowered.

A full cutoff fixture, when correctly installed, ensures that no light escapes above the plane of the horizontal. Light released above the horizontal normally serves no purpose, but when released into the atmosphere it contributes to sky glow in populated areas. Some governments and organisations are now considering, or have already implemented, full cutoff fixtures in street lamps and stadium lighting.

By preventing light from escaping unnecessarily, the use of full cutoff fixtures helps to reduce sky glow. Effects of glare are also reduced, since by their nature, full cutoff fixtures usually prevent luminaries from being directly visible. They are also more efficient than other fixtures, since light that would otherwise have escaped is instead directed towards the ground.

The improved efficiency of full cutoff fixtures allows for lower powered bulbs to be used in the fixture with the same, or sometimes better effect due to being more carefully controlled. In badly designed systems, some sky glow also results from light reflected from the ground. This reflection can be minimized, however, by being careful to avoid overpowering the lamp wattage within the fixtures, and setting spacing between lights appropriately ([NYSERDA-Planners], 2002).

A common criticism of full cutoff lighting fixtures is that they are sometimes not as aesthetically pleasing to look at. This is most likely because historically there has not been a large market specifically for full cutoff fixtures. Due to the specificity with their direction of light, full cutoff fixtures sometimes also require expertise to install for maximum effect.

Adjusting types of lighting

Several different types of luminaire exist, each consisting of different properties that determine their appropriateness for certain tasks, as well as their efficiency. It is often the case that inappropriate luminaire have been selected for a task, either due to ignorance or because more sophisticated luminaries were unavailable at the time of installation. Therefore, badly chosen luminaries often contribute to light pollution unnecessarily. By re-assessing and changing the luminaries used, it is often possible to reduce the effects of light pollution.

Some types of luminaire, in order of energy efficiency, are:

Type of luminaire Colour Efficiency
(lumens per watt)
Low pressure sodium yellow 80 - 200
High pressure sodium orange/pink 45 - 130
Metal Halide white 60 -120
Mercury Vapour blue/white 13 - 48
Incandescent yellow/white 8 - 25

Many astronomers prefer their neighbouring societies to use low pressure sodium lights as much as possible, because the single wavelength involved is comparably easy to filter. The low cost of operating sodium lights is another feature. In 1980, for example, San Jose, California, replaced all street lamps with low pressure sodium lamps, whose light is easier for nearby Lick Observatory to filter out. Similar programs are now in place in Arizona and Hawaii.

Disadvantages of low pressure sodium lighting are that fixtures must usually be larger than competing fixtures, and that colour cannot be distinguished due to its emitting only a single wavelength of light. This has led many authorities to instead adopt high pressure sodium lighting for their street lights.

Re-designing lighting plans

In some cases, evaluation of existing plans has determined that more efficient lighting plans are possible. For instance, light pollution can be reduced by turning off unneeded outdoor lights, and only lighting stadiums when there are people inside.

A working example of a lighting plan assessment can be seen in a report commissioned by the Office of the Deputy Prime Minister in the United Kingdom ("Towards better practice", 2005). The report details a plan to be implemented throughout the UK, for designing lighting schemes in the countryside, with a particular focus on preserving the environment.


See also


External links

Campaign groups

Research about light pollution

Collections of links related to light pollution


da:Lysforurening de:Lichtverschmutzung eo:luma poluo fi:Valosaaste fr:Pollution lumineuse ja:光害 nl:Lichtvervuiling sv:Ljusförorening


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