Light pollution is the intrusion of unwanted or unneeded artificial light into a man-made or natural environment. A variety of somewhat separate phenomena comprise the overall issue of pollution due to excess artificial light: Over-illumination, glare, light trespass and skyglow. Adverse impacts of light pollution include human annoyance, interference with ecosystems, human health effects, interference with astronomical observation and wasteful consumption of energy. Sources of interior light pollution consist chiefly of unnecessarily intense lighting; principal sources of outdoor light pollution include street lighting, poorly designed stadium and recreational lighting, gratuitous building uplighting and unnecessary use of office interior lighting intruding into the night sky.
Local governments have begun to regulate certain aspects of light pollution; in particular, numerous municipalities have established standards that control the amount of light traversing property boundaries. Regarding exterior lighting fixtures and skyglow issues, some governmental agencies and lighting manufacturers have established standards that limit the quantity of light that is wasted by emanating skyward and not illuminating the ground target.
Health effects of light pollution
See main article: Health effects of light pollution
 elevation of blood pressure and interference with circadian rhythm. In turn these basic metabolic disruptions can lead to derivative impacts such as medically defined stress, sexual dysfunction and immune system interference, which some research has associated with increased incidence of certain cancers.The root causes of health effects of excessive light relate not merely to artificial light intensity beyond natural exposure, but also to illumination spectra that depart from natural light and additionally to exposure to nighttime light disruptive to circadian rhythm. The most basic impact categories relate to headache triggers,
Over-illumination is the supply of light beyond amounts needed for safety or functionality. Within the USA the equivalent of two million barrels of oil per day is wasted due to excess lighting. Energy audits of existing buildings demonstrate that the lighting component of residential, commercial and industrial uses consumes about 20 to 40 percent of those land uses. Residential use lighting consumes only 10 to 30 percent of the energy, whereas lighting is the chief energy cost for commercial structures.  Over-illumination within the workplace is considered a chief cause of medical stress, headache, elevated blood pressure, anxiety and worker error rate.
Light trespass is defined as unwanted light crossing a property boundary; or in the case of astonomical observation or the greater public good, light trespass may include the emission of sufficient atmopheric uplighting to interfere with celestial observation or to cause unwanted skyglow that interferes with evening scenery. In the case of light crossing a porperty boundary, the issues may include invasion of privacy, sleep interference or simply the uninvited presence of over-illumination. The matter of light intrusion across property lines is subject to local government ordinances that can limit light transmission across property boundaries, and to provide enforcement via the police poweres of cities and counties. Numerous municipalities throughout the world have developed standards for outdoor lighting to protect the rights of their citizens against light trespass.
Massive amounts of energy is wasted from over-illumination aspects of light pollution. In interior environments this excess use arises from: (a) Illuminating work and living spaces with higher light intensities than required for functionality or aethestics; (b) failure to deploy occupancy sensors or other methods of excluding unoccupied areas from illumination; (c) use of aesthetically undesirable high levels of light, such as over-illumination of hallways and areas already illuminated by natural light; and (d) failure to utilize daylight savings time in certain world regions.
In the commercial world excessive energy use often arises when the one paying for energy use is not the same person who makes decisions about lighting. Building managers and not building owners typically structure such tasks as janitorial lighting use, setting of timers, and even choice of lighting fixtures. Another culprit is the structure of many commercial leases., where the tenant pays none of the electricity costs.
Architectural design can address many aspects of energy wastage by maximizing use of natural light, and carefully addressing specific light levels needed by work task and function. For example, modern office buildings often have large amounts of perimeter glazing, but usually fail to allow efficient use of this light by providing window tinting, adjustable shades or zonation of interior light banks that allow perimeter offices to have lights separately regulated from interior spaces, which lack abundant natural light. Advanced techniques can be employed such as designing windows with optimum angles of exposure to sun azimuth, and overall siting and orientation of a new structure to take advantage of use of natural light for interior occupants.
In outdoor venues, considerable light energy is wasted by failure to provide lighting fixtures that focus illumination on targeted needs. Too often street lighting as well as stadium, tennis court and other outdoor sports facilities allow considerable stray light to emanate at angles that do not add to surface illumination. Not only does this failure waste energy, but is a chief contributor to astronomical observation interference.
Virtually all world regions experience energy waste from light pollution or over-illumination. In Australia, for example, 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 carbon dioxide emissions; many of these fixtures are not targeted on delivering lighting to the ground level uses desired.
Light pollution causes significant adverse impacts to ecosystems by interrupting of certain animal migration and navigation; alteration of predator prey interactions; interference with plant and animal circadian rhythm; and fundamental metabolic interference with component species
Time management factors
The time management of lighting demand has two major components. The first is a very simple phenomenon of daylight savings time which is practiced in a large number of high energy demand countries. This trick simply takes advantage of a human behavior syndrome, whereby people generally postpone activities until a later time of day than purely rational. In a rational society, people would take advantage of the natural light cycle to conduct daily activities. A daylight savings program simply takes advantage of the institutional components of our society that locks in traditional work and school hours; clocks are simply advanced by one hour for a portion of the season, so that lighting demand is decreased by the greater length of natural light available during the time schedules that have been set. It is remarkable that few inquire why the practice isn’t extended for the full year. The true answer is that human behavior would eventually reset schedules such that the advantage would dissipate.
As an example of the energy savings realized by daylight savings programs Italy reports a monthly savings in excess of 100 million kilowatt hours for its daylight savings months.
A more subtle aspect of time management of lighting is the value of reducing lighting at the peak afternoon demand hours. This practice would not only save the over-illumination that is surplus to actual need, but the total grid demand could be shrunk to a point of requiring fewer total power plants, since this time of day governs the maximum demand and hence installed megawatt hour base. Thus the key leveraged component of over-illumination reduction is in the office sector, where enormous lighting demand coincides with peak grid demand in western countries.
- ^ Susan L.Burks. 1994. Managing your Migraine, Humana Press. ISBN 0-89603-2779
- ^ Craig DiLouie. 2006. Advanced Lighting Controls: Energy Savings, Productivity, Technology and Applications. The Fairmont Press, Inc. ISBN 0-88173-5108
- ^ World Health Organisation. IARC Monographs Programme finds cancer hazards associated with shiftwork, painting and firefighting. International Agency for Research on Cancer Monographs.
- ^ Doug Walo. 2005. Lighting audits. Stuart C. Irby Circuit Publications. vol. I, issue 2