Tackling the Stepchild of Pollution – Light Pollution


Over the last number of years I have become increasingly aware of the silent enemy – light pollution.  Light pollution has not occupied the limelight like water and air pollution and of course the subject about which we have all become immensely aware – the urgent need to reduce energy consumption.  It would probably be more true to say that we are more motivated to save energy because of the negative impact has on our budgets whether it is our personal budget or that of a business.

Even people who religiously recycle their waste and try to minimize their carbon footprint are often unaware of light pollution – the silent enemy.  Yet the impact of this lesser known evil is no less than its more famous peers.

Many of you perhaps have heard of or read about light pollution but without any real understanding of what is really meant by the infamous light pollution monster!

Take a look at the photograph below.  It is really frightening when you look at the aerial photograph take from space of the light pollution in the British Isles.

Now let’s take a peek at a night view of Europe.  Of course the United Kingdom stands out as a significant light polluter.  You will easily be able to identify the major European cities as well.

When we either take off from or land at any of our South Africa airports, the twinkling lights below appear inviting.  If we were able to travel into near space, we would be shocked to see that the picture of South Africa cities at night are little different from those of the United Kingdom.

Now it is time to take a look at the big picture.  It is a night view of the world.

Do you see how our beautiful city, Cape Town, stands out at the foot of Africa?  Johannesburg of course is clearly visible as we would expect.

How is Outdoor Light Pollution Defined?

Light pollution, also known as photopollution or luminous pollution, is excessive or obtrusive artificial light.

The International Dark-Sky Association (IDA) http://www.darksky.org/ defines light pollution as – any adverse effect of artificial light including sky glow, glare, light trespass, light clutter, decreased visibility at night, and energy waste.

This time exposure photo of New York City at night shows skyglow, one form of light pollution.

Scientific definitions include the following:

  • Degradation of photic habitat by artificial light.
  • Alteration of natural light levels in the outdoor environment owing to artificial light sources.
  • Light pollution is the alteration of light levels in the outdoor environment (from those present naturally) due to man-made sources of light.  Indoor light pollution is such alteration of light levels in the indoor environment due to sources of light, which compromises human health.
  • Light pollution is the introduction by humans, directly or indirectly, of artificial light into the environment.

The first three of the above four scientific definitions describe the state of the environment.  The fourth (and newest) one describes the process of polluting by light.

Light pollution obscures the stars in the night sky for city dwellers, interferes with astronomical observatories, and, like any other form of pollution, disrupts ecosystems and has adverse health effects.  Light pollution can be divided into two main types:

  • Annoying light that intrudes on an otherwise natural or low-light setting.
  • Excessive light (generally indoors) that leads to discomfort and adverse health effects.


Since the early 1980s, a global dark-sky movement has emerged, with concerned people campaigning to reduce the amount of light pollution.

Light pollution is a side effect of industrial civilization.  Its sources include building exterior and interior lighting, advertising, commercial properties, offices, factories, streetlights, and illuminated sporting venues.  It is most severe in highly industrialized, densely populated areas of North America, Europe, and Japan and in major cities in the Middle East and North Africa like Tehran and Cairo, but even relatively small amounts of light can be noticed and create problems.  Like other forms of pollution (such as air, water, and noise pollution) light pollution causes damage to the environment.

A comparison of the view of the night sky from a small rural town (left) and a metropolitan area (right). Light pollution dramatically reduces the visibility of stars.

Example of useful light and light pollution from a typical pole-mounted outdoor luminaire.

The Impact on Energy Consumption

Energy conservation advocates maintain that light pollution can only be addressed by changing the habits of society, so that lighting is used more efficiently, with less waste and less creation of unwanted or unneeded illumination.  Several industry groups also recognize light pollution as an important issue.  For example, the Institution of Lighting Engineers in the United Kingdom provides its members with information about light pollution, the problems it causes, and how to reduce its impact.  The Institute of Lighting Professionals also take issue with light polluters including local governments: “Lighting experts have criticised Northamptonshire County Council for the way the authority has handled the county’s street light switch-off programme and for leaving its own headquarters floodlit.”

Since not everyone is irritated by the same lighting sources, it is common for one person’s light “pollution” to be light that is desirable for another.  One example of this is found in advertising, when an advertiser wishes for particular lights to be bright and visible, even though others find them annoying.

Other types of light pollution are more certain.  For instance, light that accidentally crosses a property boundary and annoys a neighbour is generally wasted and pollutive light.  Disputes are still common when deciding appropriate action, and differences in opinion over what light is considered reasonable, and who should be responsible, mean that negotiation must sometimes take place between parties.  Where objective measurement is desired, light levels can be quantified by field measurement or mathematical modelling, with results typically displayed as an isophote map or light contour map.

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 refers to multiple problems, all of which are caused by inefficient, unappealing, or (arguably) unnecessary use of artificial light.  Specific categories of light pollution include light trespass, over-illumination, glare, light clutter, and sky glow. A single offending light source often falls into more than one of these 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 the outside, causing problems such as sleep deprivation or the blocking of an evening view.

A number of cities in the USA have developed standards for outdoor lighting to protect the rights of their citizens against light trespass.  The International Dark Sky Association has developed a set of model lighting ordinances to assist the cities to curb light pollution.   The Dark-Sky Association was started to reduce the light going up into the sky which reduces visibility of stars, see sky glow below.  This is any light which is emitted more than 90° above nadir.  By limiting light at this 90° mark they have also reduced the light output in the 80–90° range which creates most of the light trespass issues.

US federal agencies may also enforce standards and process complaints within their areas of jurisdiction.  For instance, in the case of light trespass by white strobe lighting from communication towers in excess of FAA minimum lighting requirements the Federal Communications Commission maintains an Antenna Structure Registration database information which citizens may use to identify offending structures and provides a mechanism for processing consumer inquiries and complaints.  The US Green Building Council (USGBC) has also incorporated a credit for reducing the amount of light trespass and sky glow into their environmentally friendly building standard known as LEED.  The Green Building Council of South Africa has also started looking at the issue of light pollution.  In fact to qualify for Green Star building ratings, light pollution is one of the criteria that is considered.

Light trespass can be reduced by selecting light fixtures which limit the amount of light emitted more than 80° above the nadir.  The IESNA definitions include full cutoff (0%), cutoff (10%), and semi-cutoff (20%). (These definitions also include limits on light emitted above 90° to reduce sky glow.)


An office building is illuminated by high pressure sodium (HPS) lamps shining upward, of which much light goes into the sky and neighbouring apartment blocks and causes light pollution, in Nijmegen, the Netherlands.

Over-illumination is simply the excessive use of light.  Specifically within the United States, over-illumination is responsible for approximately two million barrels of oil per day in energy wasted.  This is based upon US consumption of equivalent of 50 million barrels per day (7,900,000 m3/d) of petroleum.

It is further noted in the same US Department of Energy source that over 30% of all energy is consumed by commercial, industrial and residential sectors.  Energy audits of existing buildings demonstrate that the lighting component of residential, commercial and industrial uses consumes about 20–40% of those land uses, variable with region and land use. (Residential use lighting consumes only 10–30% of the energy bill while commercial buildings major use is lighting).  Thus lighting energy accounts for about four or five million barrels of oil (equivalent) per day.  Again energy audit data demonstrates that about 30–60% of energy consumed in lighting is unneeded or gratuitous.

The experience that BHA Lighting Design & Consulting have gained having conducted many lighting energy audits in the Western Cape is that lighting accounts for between 5% and 70% depending on the category of business which has been audited.

An alternative US DOE calculation starts with the fact that commercial building lighting consumes in excess of 81.68 terawatts (1999 data) of electricity, according to the US DOE.  Thus commercial lighting alone consumes about four to five million barrels per day (equivalent) of petroleum, in line with the alternate rationale above to estimate US lighting energy consumption.

Over-illumination stems from several factors:

  • Not using timers, occupancy sensors or other controls to extinguish lighting when not needed;
  • Improper design, especially of workplace spaces, by specifying higher levels of light than needed for a given task;
  • Incorrect choice of fixtures or light bulbs, which do not direct light into areas as needed;
  • Improper selection of hardware to utilize more energy than needed to accomplish the lighting task
  • Incomplete training of building managers and occupants to use lighting systems efficiently;
  • Inadequate lighting maintenance resulting in increased stray light and energy costs;
  • “Daylight lighting” demanded by citizens to reduce crime or by shop owners to attract customers;
  • Substitution of old mercury lamps with more efficient sodium or metal halide lamps using the same electrical power;
  • Indirect lighting techniques, such as lighting a vertical wall to bounce photons on the ground.

Most of these issues can be readily corrected with available, inexpensive technology, and with resolution of landlord/tenant practices that create barriers to rapid correction of these matters.  Most importantly public awareness would need to improve for industrialized countries to realize the large payoff in reducing over-illumination.


Example of lighting that can result in disability and discomfort glare.

Glare can be categorized into different types.  One such classification is described by Bob Mizon, coordinator for the British Astronomical Association’s Campaign for Dark Skies in his book “Light Pollution: Remedies and Responses”. according to this classification:

  • Blinding glare describes effects such as that caused by staring into the Sun.  It is completely blinding and leaves temporary or permanent vision deficiencies.
  • Disability glare describes effects such as being blinded by oncoming car lights, or light scattering in fog or in the eye, reducing contrast, as well as reflections from print and other dark areas that render them bright, with significant reduction in sight capabilities.
  • Discomfort glare does not typically cause a dangerous situation in itself, though it is annoying and irritating at best.  It can potentially cause fatigue if experienced over extended periods.

According to Mario Motta, president of the Massachusetts Medical Society, “… glare from bad lighting is a public-health hazard-especially the older you become.  Glare light scattering in the eye causes loss of contrast and leads to unsafe driving conditions, much like the glare on a dirty windshield from low-angle sunlight or the high beams from an oncoming car.”   In essence bright and/or badly shielded lights around roads can partially blind drivers or pedestrians and contribute to accidents.

The blinding effect is caused in large part by reduced contrast due to light scattering in the eye by excessive brightness, or to reflection of light from dark areas in the field of vision, with luminance similar to the background luminance.  This kind of glare is a particular instance of disability glare, called veiling glare. (This is not the same as loss of night vision which is caused by the direct effect of the light itself on the eye.)

To determine the mounting height of luminaires, the CIE suggests the following considerations:

  • Higher mounting heights can often be more effective in controlling spill light, because floodlights with a more controlled light distribution (i.e., narrower beam) may be used, and the floodlights may be aimed in a more downward direction, making it easier to confine the light to the design area.
  • Lower mounting heights increase the spill light beyond the property boundaries.  To illuminate the space satisfactorily, it is often necessary to use floodlights with a broader beam and to aim the floodlights in directions closer to the horizontal than would occur when using higher mounting heights.
  • Lower mounting heights make bright parts of the floodlights more visible from positions outside the property boundary, which can increase glare.

The diagrams below show how a higher mounting height compares to a lower mounting height for providing a given amount of light.

Floodlight at a higher mounting height with narrow beam angle, resulting in less spill light.

Floodlight at a lower mounting height with wider beam angle, resulting in more spill light.

Light clutter

Las Vegas displays excessive groupings of colourful lights. This is a classic example of light clutter.

Light 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 noticeable 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 organization that installed the lights, their placement and design can even be intended to distract drivers, and can contribute to accidents.

Clutter may also present a hazard in the aviation environment if aviation safety lighting must compete for pilot attention with non-relevant lighting.  For instance, runway lighting may be confused with an array of suburban commercial lighting and aircraft collision avoidance lights may be confused with ground lights.

Sky Glow

Skyglow refers to the glow effect that can be seen over populated areas.  It is the combination of all light reflected from what it has illuminated escaping up into the sky and from all of the badly directed light in that area that also escapes into the sky, being scattered (redirected) by the atmosphere back toward the ground.  This scattering is very strongly related to the wavelength of the light when the air is very clear (with very little aerosols). Aerosol is a colloid suspension of fine solid particles or liquid droplets in a gas. Examples are clouds, and air pollution such as smog and smoke. Rayleigh scattering dominates in such clear air, making the sky appear blue in the daytime.  When there is significant aerosol (typical of most modern polluted conditions), the scattered light has less dependence on wavelength, making a whiter daytime sky.  Because of this Rayleigh effect, and because of the eye’s increased sensitivity to white or blue-rich light sources when adapted to very low light levels (Remember the Purkinje effect), white or blue-rich light contributes significantly more to sky-glow than an equal amount of yellow light.  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 any but the brightest stars.

The Bortle Dark-Sky Scale, originally published in Sky & Telescope magazine, is sometimes used (by groups like the US National Park Service) to quantify skyglow and general sky clarity.  The nine-class scale rates the darkness of the night sky and the visibility of its phenomena, such as the gegenschein and the zodiacal light (easily masked by skyglow), providing a detailed description of each level on the scale (with Class 1 being the best).

Light is particularly problematic for amateur astronomers, because it inhibits their ability to observe the night sky from their property because of stray light from nearby.  Most major optical astronomical observatories are surrounded by zones of strictly enforced restrictions on light emissions.  However, The Sutherland Observatory find that the light pollution which originates from Century City, Cape Town, inhibits their ability to see the full glory of the heavens when observing in that direction.

Direct skyglow is reduced by selecting lighting fixtures which limit the amount of light emitted more than 90° above the nadir.  The IESNA definitions include full cutoff (0%), cutoff (2.5%), and semi-cutoff (5%).  Indirect skyglow produced by reflections from vertical and horizontal surfaces is harder to manage; the only effective method for preventing it is by minimizing over-illumination.  But it has to be taken into account that, according to late 2010 publications, Italian regions using full cut off lighting only does not increase skyglow.   Anyway light reflected upwards by dark surfaces such as roads or building can be considered as minor, so debate about contribution of indirect skyglow will last long.

In pristine areas clouds appear black and blot out the stars.  In urban areas skyglow is strongly enhanced by clouds

Sky glow is made considerably worse when clouds are present.  While this has no effect on astronomical observations (which are not possible at visible wavelengths under cloud cover), it is very important in the context of ecological light pollution.  Since cloudy nights can be up to ten times brighter than clear nights, any organisms that are affected by sky glow (e.g. zooplankton and fish that visually prey on them) are much more likely to have their ordinary behavior disturbed on cloudy nights.

Measurement and global effects

False colors show various intensities of radiation, both direct and indirect, from artificial light sources that reach space.

Measuring the effect of sky glow on a global scale is a complex procedure.  The natural atmosphere is not completely dark, even in the absence of terrestrial sources of light and illumination from the Moon.  This is caused by two main sources: airglow and scattered light.

At high altitudes, primarily above the mesosphere, there is enough UV radiation from the sun of very short wavelength that ionization occurs.  When these ions collide with electrically neutral particles they recombine and emit photons in the process, causing airglow.  The degree of ionization is sufficiently large to allow a constant emission of radiation even during the night when the upper atmosphere is in the Earth’s shadow.  Lower in the atmosphere all of the solar photons with energies above the ionization potential of N2 and O2 have already been absorbed by the higher layers and thus no appreciable ionization occurs.

Apart from emitting light, the sky also scatters incoming light, primarily from distant stars and the Milky Way, but also the zodiacal light, sunlight that is reflected and backscattered from interplanetary dust particles.

The amount of airglow and zodiacal light is quite variable (depending, amongst other things on sunspot activity and the Solar cycle) but given optimal conditions the darkest possible sky has a brightness of about 22 magnitude/square arcsecond.  If a full moon is present, the sky brightness increases to 18 magnitude/sq. arcsecond, 40 times brighter than the darkest sky.  In densely populated areas a sky brightness of 17 magnitude/sq. arcsecond is not uncommon, or as much as 100 times brighter than is natural.

To precisely measure how bright the sky gets, night time satellite imagery of the earth is used as raw input for the number and intensity of light sources.  These are put into a physical model of scattering due to air molecules and aerosoles to calculate cumulative sky brightness.  Maps that show the enhanced sky brightness have been prepared for the entire world.

Inspection of the area surrounding Madrid reveals that the effects of light pollution caused by a single large conglomeration can be felt up to 100 km (62 mi) away from the centre.  Global effects of light pollution are also made obvious.  The entire area consisting of southern England, Netherlands, Belgium, west Germany, and northern France have a sky brightness of at least 2 to 4 times above normal.  The only places in continental Europe where the sky can attain its natural darkness is in northern Scandinavia and in islands far from the continent.

In North America the situation is similar.  From the east coast to west Texas up to the Canadian border there is very significant global light pollution.

What are the consequences?

Energy waste

Lighting is responsible for about 20% of all electricity consumption worldwide, and case studies have shown that several forms of over-illumination constitute energy wastage, including non-beneficial upward direction of night-time lighting.  In 2007, Terna, the company responsible for managing electricity flow in Italy, reported a saving of 645.2 million kWh in electricity consumption during the daylight saving period from April to October.  It attributes this saving to the delayed need for artificial lighting during the evenings.

In Australia, public lighting is the single largest source of local government’s greenhouse gas emissions, typically accounting for 30 to 50% of their emissions.  There are 1.94 million public lights — one for every 10 Australians — that annually cost A$210 million, use 1,035 GWh of electricity and are responsible for 1.15 million tonnes of CO2 emissions.

Current public lighting in Australia, particularly for minor roads and streets, uses large amounts of energy and financial resources, while often failing to provide high quality lighting.  There are many ways to improve lighting quality while reducing energy use and greenhouse gas emissions as well as lowering costs.

Effects on animal and human health and psychology

Medical research on the effects of excessive light on the human body suggests that a variety of adverse health effects may be caused by light pollution or excessive light exposure, and some lighting design textbooks use human health as an explicit criterion for proper interior lighting.  Health effects of over-illumination or improper spectral composition of light may include:

  • Increased headache incidence
  • Worker fatigue
  • Medically defined stress
  • Decrease in sexual function
  • Increase in anxiety.

Likewise, animal models have been studied demonstrating unavoidable light to produce adverse effect on mood and anxiety.  For those who need to be awake at night, light at night also has an acute effect on alertness and mood.

In 2007, “shift work that involves circadian disruption” was listed as a probable carcinogen by the World Health Organization’s International Agency for Research on Cancer.  (IARC Press release No. 180).  Multiple studies have documented a correlation between night shift work and the increased incidence of breast cancer.

A more recent discussion (2009), written by Professor Steven Lockley, Harvard Medical School, can be found in the CfDS handbook “Blinded by the Light?”. Chapter 4, “Human health implications of light pollution” states that “… light intrusion, even if dim, is likely to have measurable effects on sleep disruption and melatonin suppression.  Even if these effects are relatively small from night to night, continuous chronic circadian, sleep and hormonal disruption may have longer-term health risks”.  The New York Academy of Sciences hosted a meeting in 2009 on Circadian Disruption and Cancer.  Forty Danish female shift workers in 2009 were awarded compensation for breast cancer “caused” by shift work made possible by light at night – the most common cause of light pollution.

In June 2009, the American Medical Association developed a policy in support of control of light pollution.  News about the decision emphasized glare as a public health hazard leading to unsafe driving conditions.  Especially in the elderly, glare produces loss of contrast, obscuring night vision.

Disruption of ecosystems

When artificial light affects organisms and ecosystems it is called ecological light pollution.  While light at night can be beneficial, neutral, or damaging for individual species, its presence invariably disturbs ecosystems.  For example, some species of spiders avoid lit areas, while other species are happy to build their spider web directly on a lamp post.  Since lamp posts attract many flying insects, the spiders that don’t mind light gain an advantage over the spiders that avoid it.  This is a simple example of the way in which species frequencies and food webs can be disturbed by the introduction of light at night.

Light pollution poses a serious threat in particular to nocturnal wildlife, having negative impacts on plant and animal physiology.  It can confuse animal navigation, alter competitive interactions, change predator-prey relations, and cause physiological harm.  The rhythm of life is orchestrated by the natural diurnal patterns of light and dark, so disruption to these patterns impacts the ecological dynamics.

Studies suggest that light pollution around lakes prevents zooplankton, such as Daphnia, from eating surface algae, helping cause algal blooms that can kill off the lakes’ plants and lower water quality.  Light pollution may also affect ecosystems in other ways.  For example, lepidopterists and entomologists have documented that night time light may interfere with the ability of moths and other nocturnal insects to navigate.  Night-blooming flowers that depend on moths for pollination may be affected by night lighting, as there is no replacement pollinator that would not be affected by the artificial light.  This can lead to species decline of plants that are unable to reproduce, and change an area’s long term ecology.

A 2009 study also suggests deleterious impacts on animals and ecosystems because of perturbation of polarized light or artificial polarisation of light (even during the day, because direction of natural polarization of sun light and its reflection is a source of information for a lot of animals).  This form of pollution is named polarized light pollution (PLP).  Unnatural polarized light sources can trigger maladaptive behaviours in polarization-sensitive taxa (one or a group of organisms) and alter ecological interactions.

Lights on tall structures can disorient migrating birds.  Estimates by the US Fish and Wildlife Service of the number of birds killed after being attracted to tall towers range from 4 to 5 million per year to an order of magnitude higher.  The Fatal Light Awareness Program (FLAP) works with building owners in Toronto,Canada and other cities to reduce mortality of birds by turning out lights during migration periods.

Similar disorientation has also been noted for bird species migrating close to offshore production and drilling facilities. Studies carried out by Nederlandse Aardolie Maatschappij b.v. (NAM) and Shell have led to development and trial of new lighting technologies in the North Sea.  In early 2007, the lights were installed on the Shell production platform L15.  The experiment proved a great success since the number of birds circling the platform declined by 50 to 90%.

Sea turtle hatchlings emerging from nests on beaches are another casualty of light pollution.  It is a common misconception that hatchling sea turtles are attracted to the moon.  Rather, they find the ocean by moving away from the dark silhouette of dunes and their vegetation, a behavior with which artificial lights interfere.  The breeding activity and reproductive phenology of toads, however, are cued by moonlight.  Juvenile seabirds may also be disoriented by lights as they leave their nests and fly out to sea.  Amphibians and reptiles are also affected by light pollution. Introduced light sources during normally dark periods can disrupt levels of melatonin production.  Melatonin is a hormone that regulates photoperiodic physiology and behaviour.  Some species of frogs and salamanders utilize a light-dependent “compass” to orient their migratory behaviour to breeding sites.  Introduced light can also cause developmental irregularities, such as retinal damage, reduced sperm production, and genetic mutation.

In September 2009, the 9th European Dark-Sky Symposium in Armagh, Northern Ireland had a session on the environmental effects of light at night (LAN).  It dealt with bats, turtles, the “hidden” harms of LAN, and many other topics.  The environmental effects of LAN were mentioned as early as 1897, in a Los Angeles Times article—the text of which can be obtained from Dr. Travis Longcore of the Urban Wildlands Trust, California.  The following is an excerpt from that article, called “Electricity and English songbirds”:

“An English journal has become alarmed at the relation of electricity to songbirds, which it maintains is closer than that of cats and fodder crops.  How many of us, it asks, foresee that electricity may extirpate the songbird?…With the exception of the finches, all the English songbirds may be said to be insectivorous, and their diet consists chiefly of vast numbers of very small insects which they collect from the grass and herbs before the dew is dry.  As the electric light is finding its way for street illumination into the country parts of England, these poor winged atoms are slain by thousands at each light every warm summer evening….The fear is expressed, that when England is lighted from one end to the other with electricity the song birds will die out from the failure of their food supply.”

Effect on astronomy

Astronomy, both amateur and professional, is very sensitive to light pollution. The night sky viewed from a city bears no resemblance to what can be seen from dark skies.  Skyglow (the scattering of light in the atmosphere) reduces the contrast between stars and galaxies and the sky itself, making it very much harder see fainter objects.  This is one factor that has caused newer telescopes to be built in increasingly remote areas.  Some astronomers use narrow-band “nebula filters” which only allow specific wavelengths of light commonly seen in nebulae, or broad-band “light pollution filters” which are designed to reduce (but not eliminate) the effects of light pollution by filtering out spectral lines commonly emitted by sodium- and mercury-vapor lamps, thus enhancing contrast and improving the view of dim objects such as galaxies and nebulae.  Unfortunately these light pollution reduction (LPR) filters are not a cure for light pollution.  LPR filters reduce the brightness of the object under study and this limits the use of higher magnifications.  LPR filters work by blocking light of certain wavelengths, which alters the color of the object, often creating a pronounced green cast.  Furthermore, LPR filters only work on certain object types (mainly emission nebulae) and are of little use on galaxies and stars.  No filter can match the effectiveness of a dark sky for visual or photographic purposes.  Due to their low surface brightness, the visibility of diffuse sky objects such as nebulae and galaxies is affected by light pollution more than are stars.  Most such objects are rendered invisible in heavily light polluted skies around major cities.  A simple method for estimating the darkness of a location is to look for the Milky Way, which from truly dark skies appears bright enough to cast a shadow.

In addition to sky glow, light trespass can impact observations when artificial light directly enters the tube of the telescope and is reflected from non-optical surfaces until it eventually reaches the eyepiece.  This direct form of light pollution causes a glow across the field of view which reduces contrast.  Light trespass also makes it hard for a visual observer to become sufficiently dark adapted.  The usual measures to reduce this glare, if reducing the light directly is not an option, include flocking the telescope tube and accessories to reduce reflection, and putting a light shield (also usable as a dew shield) on the telescope to reduce light entering from angles other than those near the target.  Under these conditions, some astronomers prefer to observe under a black cloth to ensure maximum dark adaptation.  In one Italian regional lighting code this effect of stray light is defined as “optical pollution”, due to the fact that there is a direct path from the light source to the “optic” – the observer’s eye or telescope.

Increase in atmospheric pollution

A study presented at the American Geophysical Union meeting in San Francisco found that light pollution destroys nitrate radicals thus preventing the normal night time reduction of atmospheric smog produced by fumes emitted from cars and factories.  The study was presented by Harald Stark from the National Oceanic and Atmospheric Administration.

How do we tackle light pollution?


This kind of LED droplight could reduce unnecessary light pollution in building interiors.

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:

  • Utilizing light sources of minimum intensity necessary to accomplish the light’s purpose.
  • Turning lights off using a timer or occupancy sensor or manually when not needed.
  • 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. Mercury, metal halide and above all first generation of blue-light LED road luminaries are much more pollutant than sodium lamps: Earth atmosphere scatters and transmits blue light better than yellow or red light.  It is a common experience observing “glare” and “fog” around and below LED road luminaries as soon as air humidity increases, while orange sodium lamp luminaries are less prone to show this phenomenon.
  • Evaluating existing lighting plans, and re-designing some or all of the plans depending on whether existing light is actually needed.

Improving lighting fixtures

The four IESNA classifications are defined as follows (IESNA 2000):

  • Full cutoff—The luminous intensity (in candelas) at or above an angle of 90° above nadir is zero, and the luminous intensity (in candelas) at or above a vertical angle of 80° above nadir does not numerically exceed 10% of the luminous flux (in lumens) of the lamp or lamps in the luminaire.
  • Cutoff—The luminous intensity (in candelas) at or above an angle of 90° above nadir does not numerically exceed 2.5% of the luminous flux (in lumens) of the lamp or lamps in the luminaire, and the luminous intensity (in candelas) at or above a vertical angle of 80° above nadir does not numerically exceed 10% of the luminous flux (in lumens) of the lamp or lamps in the luminaire.
  • Semi-cutoff—The luminous intensity (in candelas) at or above an angle of 90° above nadir does not numerically exceed 5% of the luminous flux (in lumens) of the lamp or lamps in the luminaire, and the luminous intensity (in candelas) at or above a vertical angle of 80° above nadir does not numerically exceed 20% of the luminous flux (in lumens) of the lamp or lamps in the luminaire.
  • Noncutoff—There is no candela limitation in the zone above maximum candela.

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

Full cutoff fixtures first became available in 1959 with the introduction of General Electric’s M100 fixture.

A full cutoff fixture, when correctly installed, reduces the chance for light to escape above the plane of the horizontal. Light released above the horizontal may sometimes be lighting an intended target, but often serves no purpose.

When it enters into the atmosphere, light contributes to sky glow.  Some governments and organizations are now considering, or have already implemented, full cutoff fixtures in street lamps and stadium lighting.

The use of full cutoff fixtures help to reduce sky glow by preventing light from escaping above the horizontal.  Full cutoff typically reduces the visibility of the lamp and reflector within a luminaire, so the effects of glare are also reduced.  Campaigners also commonly argue that full cutoff fixtures are more efficient than other fixtures, since light that would otherwise have escaped into the atmosphere may instead be directed towards the ground.  However, full cutoff fixtures may also trap more light in the fixture than other types of luminaires, corresponding to lower luminaire efficiency, suggesting a re-design of some luminaires may be necessary.

The use of full cutoff fixtures can allow for lower wattage lamps to be used in the fixtures, producing the same or sometimes a better effect, due to being more carefully controlled. In every lighting system, some sky glow also results from light reflected from the ground.  This reflection can be reduced, however, by being careful to use only the lowest wattage necessary for the lamp, and setting spacing between lights appropriately.  Assuring luminaire setback is greater than 90° from highly reflective surfaces also diminishes reflectance.

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, and because people typically like to see the source of illumination.  Due to the specificity with their direction of light, full cutoff fixtures sometimes also require expertise to install for maximum effect.

The effectiveness of using full cutoff roadway lights to combat light pollution has also been called into question. According to design investigations, luminaires with full cutoff distributions (as opposed to cutoff or semi cutoff, compared here) have to be closer together to meet the same light level, uniformity and glare requirements specified by the IESNA.  These simulations optimized the height and spacing of the lights while constraining the overall design to meet the IESNA requirements, and then compared total uplight and energy consumption of different luminaire designs and powers.  Cutoff designs performed better than full cutoff designs, and semi-cutoff performed better than either cutoff or full cutoff.  This indicates that, in roadway installations, over-illumination or poor uniformity produced by full cutoff fixtures may be more detrimental than direct uplight created by fewer cutoff or semi-cutoff fixtures. Therefore, the overall performance of existing systems could be improved more by reducing the number of luminaires than by switching to full cutoff designs.

Are the IESNA cutoff classifications a good indicator of direct uplight?

Except for the full cutoff designation, the Illuminating Engineering Society of North America (IESNA) cutoff classifications are not a good indicator of direct uplight, because glare control was the original reason they were developed.  A luminaire that has the IESNA full cutoff classification does not have any light going directly upward from the luminaire and will not, if mounted correctly, emit light directly into the sky.  The direct uplight from a cutoff luminaire can vary from 0% to 16% of the light output of the lamp(s) in the luminaire, and the uplight from a semi-cutoff luminaire can vary from 0% to 31% of the lamp light output.

However, careful consideration of these classification definitions can be very important when evaluating outdoor luminaires for their potential to cause light trespass or glare.  Please read the following sentence carefully:

The definitions are given in terms of luminous intensity (in candelas), but the values are made in reference to luminous flux of the light source (inlumens).

Casually skimming these definitions could lead to the assumption that for a cutoff luminaire, no more than 10% of the lamp luminous flux is emitted between 80° and 90° from nadir, or that no more than 2.5% of the lamp luminous flux is emitted above 90° from nadir.  In fact, neither of these assumptions is correct.  The following is a technical discussion of these discrepancies.

Bullough states the following:

Consider the hypothetical luminous intensity distribution [shown below] for a luminaire equipped with a 1000-lumen lamp.  The luminous intensity at any angle between and including 80° and 90° from nadir is 100 candelas, which is 10% of the numerical value of the lamp lumens.  The luminous intensity 90° from nadir and at any angle above is 25 candelas, which is 2.5% of the numerical value of the lamp lumens.  [The distribution of the remaining 73% of lamp lumens at angles below 80° from nadir does not affect the cutoff classification of the luminaire and, thus, is not shown below]  This hypothetical luminaire can be classified as a cutoff luminaire. Interestingly, a luminaire with this distribution would emit 11% of the lamp lumens between 80° and 90° from nadir, and nearly 16% of the lamp lumens above 90° from nadir. (Bullough 2002, reprinted with permission from IESNA).

Source: Bullough 2002, © IESNA, used with permission

Hypothetical luminous intensity distribution at 80°–180° for a cutoff luminaire containing a 1000-lumen lamp (units are in candelas)

The quantity of lumens emitted above 90° from nadir is calculated by multiplying the maximum allowable intensity value, 25 candelas, by the solid angle over which they are emitted, 2π steradians, totaling 157 lumens or 16% of the 1000 lumens emitted by the lamp.  For the percentage of lamp lumens in the glare zone, the solid angle from 80° to 90° is approximately 1 steradian.  Multiplying the solid angle, 1 steradian, by the maximum allowable intensity value, 100 candelas, the total allowable lumens in the glare zone is 109 lumens or 11% of the 1000 lumens emitted by the lamp.

Bullough further explains:

Conversely, consider another hypothetical luminous intensity distribution, also for a luminaire equipped with a 1000-lumen lamp.  The luminous intensity at one angle between 80° and 90° from nadir is 125 candelas, exceeding 10% (100 candelas) of the numerical value of the lamp lumens.  The luminous intensity at one angle above 90° from nadir is 40 candelas, exceeding 2.5% (25 candelas) of the numerical value of the lamp lumens. Such a luminaire would emit only 3% of the lamp lumens between 80° and 90° from nadir, and about 1% of the lamp lumens above 90° from nadir. [As in the example above, the distribution of the remaining 96% of lamp lumens at angles below 80° from nadir does not impact the luminaire’s cutoff classification].  Because of its luminous intensity values, this luminaire cannot be classified as a cutoff luminaire even though it emits significantly less light upward than the [previously shown] hypothetical luminaire.  Indeed, even if this same luminaire were fully shielded so that it emitted no light above 90°, it could not be considered a full cutoff or even a cutoff luminaire, because of its higher-than permitted intensity at one angle between 80° and 90°.(Bullough 2002)

Source: Bullough 2002, © IESNA, used with permission

Hypothetical luminous intensity distribution at 80°–180° for a luminaire containing a 1000-lumen lamp failing to meet the cutoff classification (units are in candelas)

Certainly, these luminous intensity distribution examples represent extreme cases.  They do, however, serve to emphasize the caution that is required when interpreting the various cutoff classifications.  If, for example, one is concerned about minimizing direct uplight from a luminaire, it is not necessarily true that a cutoff luminaire will emit a smaller proportion of its luminous flux upward than a semicutoff (or even a noncutoff) luminaire, even if the luminaires are equipped with the same lamp.  A full cutoff luminaire, on the other hand, will never emit direct uplight.

The best way to estimate the luminous flux emitted directly upward by a particular luminaire is to consult a zonal luminous flux summary prepared by the luminaire’s manufacturer.

Continuing with the logic above and generalizing for each cutoff classification, ranges of lamp lumen percentage are shown in below in terms of uplight and lamp lumens in the glare zone. The direct uplight of a luminaire that has the IESNA semicutoff classification theoretically can vary from 0% to 31% of the total lamp lumens and the lamp lumens in the glare zone can vary from 0% to 22%.

What is the difference between full cutoff and fully shielded?

The term full cutoff has and is being used to describe luminaires that have no direct uplight (no light emitted above horizontal).  However, in addition to that limitation, the Illuminating Engineering Society of North America (IESNA) definition also requires luminaires to comply with the glare requirement limiting intensity of light from the luminaire in the region between 80° and 90°.

The term full cutoff is often substituted for the term fully shielded.  The terms are not equivalent.  Fully shielded luminaires emit no direct uplight, but have no limitation on the intensity in the region between 80° and 90°.  Luminaires that fall under the IESNA full cutoff, cutoff, semi-cutoff, and non-cutoff definitions, may also qualify as fully shielded.  It may be obvious that a luminaire that is characterized as an IESNA full cutoff luminaire is fully shielded, but not as obvious when luminaires with other IESNA classifications may also qualify.  Consider a semi-cutoff luminaire containing a 1000 lumen lamp that has no direct uplight but a candela value of 150 between 80° and 90°. This luminaire is considered to be fully shielded.  However, if it were mistakenly labelled a full cutoff luminaire, this can become quite confusing. In 2002, the IESNA chartered a new committee to address the inconsistencies and confusion.

There is also a confusing assumption that a luminaire with a flat lens qualifies as a full cutoff luminaire.  While this may be true sometimes, it is not always the case.  Depending on the structure of the luminaire, reflections off the housing may result in some amount of direct uplight from the luminaire.  Consider the hypothetical luminaire below. Reflections from below the lens may result in some direct uplight from the luminaire.  The IESNA full cutoff classification also has a limitation on light in the glare zone between 80° and 90°.  A flat lens on a luminaire does not guarantee that this requirement is met.

Example of a flat lens luminaire that may have some uplight reflected from the mechanical structures below the lens.

The majority of Italian regions require “zero upward light”, which usually implies use of overall full cut-off lamps for new luminaries, but violations are common.

However, using the definition of “light pollution” from some Italian regional bills (i.e., “every irradiance of artificial light outside competence areas and particularly upward the sky”) only full cutoff design prevents light pollution.  The Italian Lombardy region, where only full cutoff design is allowed (Lombardy act no. 17/2000, promoted by Cielobuio-coordination for the protection of the night sky), in 2007 had the lowest per capita energy consumption for public lighting in Italy.  The same legislation also imposes a minimum distance between street lamps of about four times their height, so full cut off street lamps are the best solution to reduce both light pollution and electrical power usage.

Adjusting types of light sources

Several different types of light sources exist, each having different properties that affect their appropriateness for certain tasks, particularly efficiency and spectral power distribution.  It is often the case that inappropriate light sources have been selected for a task, either due to ignorance or because more sophisticated light sources were unavailable at the time of installation.  Therefore, badly chosen light sources often contribute unnecessarily to light pollution and energy waste.  By re-assessing and changing the light sources used, it is often possible to reduce energy use and pollutive effects while simultaneously greatly improving efficiency and visibility.

Some types of light sources are listed in order of energy efficiency in the table below.

Many astronomers request that nearby communities use low pressure sodium lights as much as possible, because the principal wavelength emitted is comparably easy to work around or in rare cases filter out.  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 color cannot be distinguished, due to its emitting principally a single wavelength of light (see security lighting). Due to the substantial size of the lamp, particularly in higher wattages such as 135 W and 180 W, control of light emissions from low pressure sodium luminaires is more difficult.  For applications requiring more precise direction of light (such as narrow roadways) the raw lamp efficacy advantage of this lamp type is decreased and may be entirely lost compared to high pressure sodium lamps.  Allegations that this also leads to higher amounts of light pollution from luminaires running these lamps arise principally because of older luminaires with poor shielding, still widely in use in the UK and in some other locations.  Modern low-pressure sodium fixtures with better optics and full shielding, and the decreased skyglow impacts of yellow light preserve the luminous efficacy advantage of low-pressure sodium and result in most cases is less energy consumption and less visible light pollution.  Unfortunately, due to continued lack of accurate information, many lighting professionals continue to disparage low-pressure sodium, contributing to its decreased acceptance and specification in lighting standards and therefore its use.  Another disadvantage of low-pressure sodium lamps is that some people find the characteristic yellow light very displeasing aesthetically.

Because the light is scattered by the atmosphere, different sources produce dramatically different amounts of skyglow from the same amount of light sent into the atmosphere.

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.  Timers are especially valuable for this purpose.  One of the world’s first coordinated  legislative efforts to reduce the adverse effect of this pollution on the environment began in Flagstaff, Arizona, in the US.  There  it has taken over  three decades to develop the legislation  with the full support of the population and  often with government support, the support of community advocates, and with the help of major local observatories, including the United States Naval Observatory Flagstaff Station.  Each component helps to educate, protect and enforce the imperatives to intelligently reduce detrimental light pollution.

One example of a lighting plan assessment can be seen in a report originally commissioned by the Office of the Deputy Prime Minister in the United Kingdom, and now available through the Department for Communities and Local Government.  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.

In another example, the city of Calgary has recently replaced most residential street lights with models that are more  energy efficient.  The motivation is primarily operation cost and environmental conservation.  The costs of installation are expected to be regained through energy savings within six to seven years.

The Swiss Agency for Energy Efficiency (SAFE) uses a concept that promises to be of great use in the diagnosis and design of road lighting, “consommation électrique spécifique (CES)”, which can be translated into English as “specific electric power consumption (SEC)”.  It is based on observed lighting levels in a wide range of Swiss towns, SAFE has defined target values for electric power consumption per metre for roads of various categories. SAFE currently recommends an SEC of 2 to 3 watts per meter for roads of less than 10 metre width (4 to 6 watts per metre for wider roads).  Such a measure provides an easily applicable environmental protection constraint on conventional “norms”, which usually are based on the recommendations of lighting manufacturing interests, who may not take into account environmental criteria.  In view of ongoing progress in lighting technology, target SEC values will need to be periodically revised downwards.

A newer method for predicting and measuring various aspects of light pollution was described in the journal Lighting Research Technology (September 2008).  Scientists at Rensselaer Polytechnic Institute’s Lighting Research Center have developed a comprehensive method called Outdoor Site-Lighting Performance (OSP), which allows users to quantify, and thus optimize, the performance of existing and planned lighting designs and applications to minimize excessive or obtrusive light leaving the boundaries of a property.  OSP can be used by lighting engineers immediately, particularly for the investigation of glow and trespass (glare analyses are more complex to perform and current commercial software does not readily allow them), and can help users compare several lighting design alternatives for the same site.

Crossroad in Alessandria, Italy: luminaries with mercury lamps are in the background, LED street lights in the middle, luminaries with high pressure sodium lamps are in the foreground. Supposed superiority of blue-white lamps for common road lighting intensities seems to be questionable.

In the effort to reduce light pollution, researchers have developed a “Unified System of Photometry,” which is a way to measure how much or what kind of street lighting is needed.  The Unified System of Photometry allows light fixtures to be designed to reduce energy use while maintaining or improving perceptions of visibility, safety, and security.  There was a need to create a new system of light measurement at night because the biological way in which the eye’s rods and cones process light is different in night time conditions versus daytime conditions.  Using this new system of photometry, results from recent studies have indicated that replacing traditional, yellowish, high-pressure sodium (HPS) lights with “cool” white light sources, such as induction, fluorescent, ceramic metal halide, or LEDs can actually reduce the amount of electric power used for lighting while maintaining or improving visibility in nighttime conditions.

On the other hand remember that human night vision is better in blue or white light only if light intensity is low, while lighting and power industries are pushing to increase more and more ground illuminance: to avoid a conflict of interests, ground illuminance or luminances required by standard have to be lowered if blue or white light sources are used.  Blue and white light scatters more than yellow or red light.  Overall light pollution would be increased using metal halide lamps, whereas directional LED lamps would help to decrease light pollution whilst also providing better visibility for users.

The International Commission on Illumination, also known as the CIE from its French title, la Commission Internationale de l’Eclairage, will soon be releasing its own form of unified photometry for outdoor lighting.

Concluding Observations

The situation has been acknowledged as being so serious that in 2009, The Royal Commission on Environmental Pollution issued their report titled Artificial Light in the Environment.  The report is available to purchase from the Royal Commission at £14.35.

The report deals with the following subjects:

  • A Growing Sense of Loss.
  • Social Benefits and Drawbacks of Outdoor Lighting.
  • Impact of Light Pollution on Organisms and Ecosystems.
  • Road Lighting Technology: An Opportunity and a Challenge.
  • Conclusions and Recommendations.

There were some startling revelations.  The United Kingdom’s skies are so saturated with light pollution.  Campaigners warn that half the population cannot see many stars.

The latest annual star count survey by the Campaign to Protect Rural England (CPRE) and the Campaign for Dark Skies (CFDS) showed 53% of those taking part could see 10 stars or fewer within the major constellation of Orion.  Only 9% could see between 21 and 30 stars within the constellation and just 2% had really dark skies above them and were able to see 31 or more stars on a clear night earlier this year.  The number of people living with severe light pollution had decreased only very slightly, a total of 1% in 2011, the online survey found.

Peter Dean, who has been stargazing for more than 25 years said “Over time the skies have been becoming increasingly lighter.  It has become increasingly difficult to see the stars in their full glory.  You have to go a long way to far flung areas to see them.”

Tim Murphy, chairman of CPRE Surrey, said there are many reasons why we should change the situation.

“It is important to understand the impact of inappropriate lighting on our wildlife.  It fundamentally changes their patterns of life and from a human point of view too much lighting can affect people’s sleep patterns and other aspects of their health.”

Artificial light can:

  • Confuse migrating birds, usually with fatal outcomes
  • Cause birds to fly into tall buildings as they are drawn to powerful lights
  • Birds not killed in such collisions still waste plenty of energy flying around and around buildings, limiting their likelihood of survival
  • Cause birds to start breeding early by simulating shorter nights
  • Disturb feeding patterns of bats due to insects clustering around outside lights
  • Disrupt the normal routines of many plants, such as flowering
  • Cause health problems in humans, including inhibiting melatonin production
  • Certain breast cancers have been linked to melatonin production
  • Disrupt people’s sleep due to light from road lamps glaring into homes


My personal research and study about this multi-faceted problem is by no means exhaustive or for that matter conclusive.  I have no illusion about this documented study.  I am sure that there are many far more qualified and competent engineers and scientists who will eventually find the solution and wise committed governments will legislate against poor lighting practice which results in light pollution.



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