Light Pollution
Authors: Luke Bissell, Josh Schoenly
Basic idea: More light is not necessarily better.
Jargon:
Lumen – Defined within the text.
Luminaire - A complete lighting unit including the lamp and the fixture[1]. A luminaire is anything from a table lamp to a street light.
Nadir – Defined within the text.
Spectroscopy - the study of the wavelength components of a light source.
Deeper explanation: Poorly designed outdoor lighting fixtures send unwanted/unnecessary light into adjacent properties, and can also increase the ambient light level of the night sky. Inefficient lighting consumes more power.
Importance in science:
Biology: Animal behavior is affected by light pollution.
Astronomy: Light pollution frustrates astronomers’ observations.
Engineering: designing efficient, non-obtrusive lighting.
Everyday examples:an orange-tinted sky on a cloudy night, unshielded street lamps emitting in all directions
Extended discussion and demonstration:
Background:
Light pollution can be simply defined as unnecessary or intrusive light that humans generate. There are three main types of light pollution[2]:
Sky Glow: Light emitted upward into the atmosphere that is scattered by clouds or particles in the atmosphere. In this way air pollution can contribute to light pollution.
Light Trespass: Light from a source on your property infringes on someone else’s property.
Glare: Lighting which is bright enough to cause viewing discomfort, or reduced night vision.
In this handout, we outline how light pollution affects several areas of science: biology, engineering, and astronomy. We conclude with a selected list of websites to consult for curriculum, and a cited reference list.
I. Biology
Since many plants and animals are nocturnal, it’s natural to suspect that artificial lighting may interfere with their behavior. Several studies have indeed confirmed this. For example, birds fly into tall, bright-lighted buildings,[3],[4] and hatchling sea turtles are distracted from returning to sea by beach-front lighting[5]. Night-lighting has been observed to extend the natural feeding patterns of diurnal species[6]. Some studies have even suggested that humans are adversely affected by artificial lighting, intimating links between artificial lighting and cancer, and heart disease. Much of this research can be summarized by the fact thatproduction of the antioxidant melatonin is reduced when an organism is illuminated[7]. A comprehensive review of research relating to the effects of light pollution on plants and animals can be found in Ref.6. For elementary to middle school-level audiences, you can go to and download this brochure for classroom use. It outlines the effects of light pollution on wildlife.
The introductory page to the Fatal Light Awareness Program confronts the viewer with a graphic depiction of how light pollution affects birds4.II. Astronomy
As the following map illustrates, there are few areas of the United States where the night sky can be viewed as it would have been 200 years ago. The same can be said for many other parts of the world—maps representing light pollution for the entire can be found at a website sponsored by the Light Pollution Science and Technology Institute in Thiene, Italy.
Map shows the ratios between the artificial sky brightness and the natural sky brightness of: <0.11 (black), 0.11-0.33 (blue), 0.33-1 (green), 1-3 (yellow), 3-9 (orange), >9 (red). Credit: P. Cinzano, F. Falchi (University of Padova), C. D. Elvidge (NOAA National Geophysical DataCenter, Boulder). Copyright Royal Astronomical Society. Reproduced from the Monthly Notices of the RAS by permission of Blackwell Science.This situation is particularly troublesome to astronomers, who face the challenge of collecting light from objects that are light years away, while rejecting light from other sources. Much of the light that they must filter out comes from earth itself—i.e., light pollution. This is more of a problem for amateur astronomers than for research astronomers, since most major observatories are either in remote locations or in areas where strict light pollution ordinances are in force.[8],[9] A notable exception to this, however, is the Mt.Palomar observatory in California. This installation’s research capabilities are threatened by light pollution from San Diego and surrounding communities[10].
How can light pollution effect astronomical observations? Artificial light that makes its way into an astronomer’s telescope reduces the contrast of the image that is formed. Astronomers who make spectroscopic measurements of stars (in order to deduce information about their chemical composition, temperature, and relative velocity) have to filter out wavelengths of light that come from common artificial lighting sources. For example, mercury vapor lamps are commonly used in street lights, but they emit light at many wavelengths that are relevant to astronomers. This hampers their ability to make meaningful measurements9.
III. Engineering
This section is divided into 3 specific applications: public security/safety, luminaire design, and environmentally-friendly lighting.
Security/Safety: The usual method of making people feel safer outside at night is to design luminaires (street lights, parking lot lights, flood lights in the backyard) which tend to over-illuminate a specified area. An example is lighting found at ATMs where there is an abrupt change of light to dark. The main security/safety problems associated with lights that are too bright are:
- Badly installed lights can create deep shadows (sharper contrast) making it easier for criminals to hide (see images below).
Street lights which are too bright create sharper contrast/ deeper shadows. The criminal lurking behind the truck is able to easily hide within the shadows (the image on the left used flash photography). International Dark-Sky Association.
- People can feel over-exposed in an over-lit area. Due to the deep contrast, anyone outside of the light looking in would clearly see a person within. Of course, some would regard the idea that over-illumination invites a criminal to attack, rather than warning the person in the light, as counter-intuitive.
- In terms of over-lit areas for motorists and other night-time travelers, poorly constructed luminaires (street lights, porch lights, commercial/industrial lights) can cause glare and light-scatter which inhibits night vision. An example would be opposing traffic with their high beams still on.
Despite the security risks listed above, this doesn’t necessarily advocate not using street lights (you still need some light to see!). A properly constructed luminaire adequately illuminates a specific area without creating deep shadows. This is typically done by dimming the light.
Luminaire Design: Extensive design is necessary to make a luminaire energy efficient that provides the necessary illumination. Many times efficiency is compromised for beauty. A common aesthetically pleasing, environmentally unfriendly street light design is the “acorn” fixture (seen below). The “acorn” street light fixture is an example of a poorly designed luminaire since a great deal of the light is emitted upward into the sky and outward into a person’s property. A Salem Cutoff (made by GE lighting) is similar to the “acorn” fixture but is energy efficient since it directs the upward emitted light downwards using a reflector at the top.
An example of an “Acorn” (left) and Salem Cutoff (right) fixture. International Dark-sky Association.A properly designed street light (or any other outdoor light, for that matter) emits light in an energy efficient manner. It is estimated that 30% of outdoor lighting is wasted either due to over-illumination or photons directed upwards into the atmosphere[11]. This costs the U.S. nearly $10.4 billion per year, corresponding to 38 million tons of unnecessary CO2 produced11. In 2005, light waste from street lamps in Brighton, NY contributed to 485 tons of CO2 produced, costing the town $402,000.[12] An energy efficient, low light pollution outdoor luminaire could have one of the following characteristics:
- Light is emitted within the angle defined by the nadir (the line perpendicular to the ground, seen in the figure below[13]) and the astronomical horizon.
Any light emitted or reflected above the astronomical horizon is considered light pollution. Special design must be considered for street lamps since light emitted at too large of an angle from nadir may land on someone else’s property (light trespass). Properly designed fixtures reflect light emitted upward from the lamp downwards, thus inhibiting light pollution and providing more light to the necessary area.
- The fixture contains an energy efficient light source (measured in lumens per watt). A lumen is a measure of light perceived by the human eye, and a watt is a measure of electrical power used by the lamp. Below is a table of the six common lamps used in street lights (obtained from the International Dark-sky Association). The most common lamp used in street lights is the 175 watt mercury vapor lamp, the second most energy inefficient lamp next to the incandescent lamp (the common light bulb). By replacing the mercury vapor lamp with a low pressure sodium lamp, a street light could produce the same light at about ¼ of the power usage.
Type of Lamp / Lumens per watt / Average Lamp Life (Hours)
Incandescent
Mercury Vapor
Fluorescent
Metal Halide
High Pressure Sodium
Low Pressure Sodium / 8 – 25
13 - 48
33 - 77
60 - 100
45 - 110
80 – 180 / 1000 – 2000
12000 - 24000+
10000 - 24000
10000 - 15000
12000 - 24000
10000 - 18000
- Multiple luminaires are appropriately spaced from each other. Luminaires located too close too each other, or too many luminaires lighting a particular area in general, can lead to over illumination, and thus wasted light and energy.
- The use of timers on street lights or any other outdoor fixtures. This prevents the accidental usage of these lights during the daytime.
Environmentally Friendly Light Bulbs: The most popular home lighting lamp in the U.S. is the incandescent light bulb. Invented in 1879 by Thomas Edison, these lamps are the most inefficient and short-lived out of all the common lamps, since about 95% of the power used is converted into heat[14]. In order to save money and cut down on CO2 emissions, many people and companies are switching to “greener” light bulbs. One such green light bulb is the compact fluorescent light bulb (CFL) which uses 1/3 less energy than the incandescent bulb and lasts up to 10 times longer, saving around $30 in energy costs over its lifetime[15]. Compared to incandescent bulbs, CFLs produce 70% less heat and are thus safer to operate and may help reduce home cooling costs15. Some of the drawbacks of CFLs are flickering, shortened life if the CFL is too frequently turned on and off, delayed light levels, and they contain a trace amount of Mercury (5 mg compared to the 500 mg in old thermometers), which needs to be appropriately disposed of when the CFL stops working[16].
Another green light source finding popularity is the Light Emitting Diode (LED) which consumes less energy than CFLs, lasts for about 10,000 hrs (11 years), and contains no mercury14. LEDs are semiconductor chips that emit light of different colors when electricity passes through them. They are being used in more than half of the traffic lights in the U.S. and are finding popularity in office and industrial settings14.
Useful Websites
International Dark Sky Association homepage
Links to curriculum on light pollution, designed for children ages 5-13. A little simplistic.
Maps representing light pollution in different parts of the world.
National Geographic Nov. 2008 cover story on light pollution
References
[1]
[2]
[3]D. Malakoff (2001). "Faulty towers". Audubon103(5): 78–83
[4]
[5]M. Salmon (2003). "Artificial night lighting and sea turtles". Biologist50: 163–168
[6]Catherine Rich and Travis Longcore (2006). Ecological consequences of artificial night lighting. Island Press. See ch. 13.
[7]Navara KJ, Nelson RJ (2007) The dark side of light light at night: physiological, epidemiological, and ecological consequences. J. Pineal Res. 2007; 43:215–224
[8]
[9]
[10]
[11]Ben Harder, "Turning Out the Lights,"U.S. News & World Report, March 14, 2008.
[12]Final Report of the Green Brighton Task Force, BrightonNY, August 2008. available online:
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[14]
[15]
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