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 fixture1. 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 increases 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:
Background:
Light pollution can be simply defined as unnecessary or intrusive light that
humans generate. There are three main types of light pollution2:
Sky Glow: Light emitted upward into the atmosphere that is scattered by clouds or
particles in the atmosphere. In this way air pollution contributes to light pollution by
causing enhanced scattering.
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. A demonstration is outlined to show the effect of
sky glow, the efficacy of using shielded luminaires, and the effect of glare (i.e., for
security lighting). 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, brightly-lit buildings,3,4 and hatchling sea turtles are
distracted from returning to sea by beach-front lighting5. Night-lighting has been
observed to extend the natural feeding patterns of diurnal species6. 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 that production of the antioxidant melatonin is reduced when an
organism is illuminated7. 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
http://data.nextrionet.com/site/idsa/ida_wildlife_brochure.pdf 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 www.lightpollution.it/dmsp, 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 Data Center, 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. Most of the light that they must filter out comes from human activities on the
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
communities10.
How can light pollution affect astronomical observations? Artificial light that
makes its way into an astronomer’s telescope reduces the contrast of the image that is
formed and prevents measurements of faint objects. 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:
1. Badly installed lights 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 right used flash
photography). International Dark-Sky Association.
2. 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.
3. 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 to reduce sharp contrast and, even more importantly, adding shielding
to eliminate direct glare and direct all the light down on to the surface where it is needed.
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 atmosphere11. 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:
1. Light is emitted within the angle defined by the nadir (the line perpendicular to
the ground, seen in the figure below13) 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.
2. 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 lights14 (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
Incandescent
Mercury Vapor
Fluorescent
Metal Halide
High Pressure Sodium
Low Pressure Sodium
Lumens per watt
8 – 25
13 - 48
33 - 77
60 - 100
45 - 110
80 – 180
Average Lamp Life (Hours)
1000 – 2000
12000 - 24000+
10000 - 24000
10000 - 15000
12000 - 24000
10000 - 18000
3. 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.
4. The use of timers or motion detectors on street lights or any other outdoor
fixtures. This prevents the accidental usage of these lights during the daytime and
keeps the lights off when not needed. This is particularly useful for security
lighting, which can be turned on by a motion detector only when someone
approaches.
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 of all the common lamps, since about 95% of the
power used is converted into heat15. 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 lifetime16. Compared to incandescent bulbs, CFLs produce 70% less heat
and are thus safer to operate and may help reduce home cooling costs16. 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 working17.
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 mercury15. 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 settings15.
Demonstration
Equipment needed:
 0-30 V (or higher) variable voltage supply (if you don’t have this, the demo can
still work, but this makes it cooler)
 wire (i.e., for wiring the bulb to the variable voltage supply)
 cardboard (preferably black posterboard, available at craft stores, but cardboard
covered in black tape also works). You should have at least 20 cm x 60 cm.
 black plastic (i.e., trash bags)
 masking or duct tape
 posts, bases, and clamps (see Figure 2).
 2 portable light sockets (available at Home Depot)
 1 small lampshade (you could also find this at home depot, or make your own out
of cardstock and tinfoil)
 Two 60 watt bulbs
 1 desk lamp
Procedure
A. Build a light tight enclosure
1. Cut out a piece of cardboard, 20 cm x 60 cm.
2. Cut trash bags and tape them to the cardboard so that you can extend the enclosure ~50
cm forward from the cardboard backstop. Using the bases and posts, you can hold up the
plastic like a tent. Alternatively, you could build a rigid box with cardboard, leaving the
front part open for viewing, and taping plastic on the front to drape over the observer’s
head, preserving darkness.
3. If you have a voltage supply, wire its terminals to 2 wires attached to the prongs of the
portable light socket’s power cord. This way you can adjust the brightness of the bulb
(you probably won’t see any light until you have ~20 V across the filament). Put the
bulbs in the sockets, and place the bulbs inside the enclosure. If you don’t have a voltage
supply, just screw in both bulbs and place them inside the enclosure.
*** 60 W bulbs at full brightness are HOT (will melt the plastic if they come in
contact with it). Use caution. The bulb with 30 V across it will only be warm to the
touch.***
4. Poke some holes in the cardboard, simulating stars. We used a multimeter probe to
make ~ 1-2 mm sized holes. Make different size holes to simulate brighter or dimmer
stars. See Figure 2.
5. Place the desk lamp behind the cardboard back stop. This will be the source for
lighting up the “stars” in the backboard. Note that in Figure 1, we placed the desk lamp
so that the bulb was above and on the back right corner of the enclosure. The closer you
have the desk lamp to the backboard, the brighter you will have to turn up the variable
brightness bulb (i.e., “street lamp”) to see the effect of light pollution. See step B 1.
B. Simulating sky glow, efficacy of using cutoffs.
1. Adjust the desk lamp position so that as you turn up the voltage to the variable
brightness bulb, some of the stars “wash out”. It may be helpful to place a baffle
(cardboard taped to a rigid object works fine) between your eyes and the variable
brightness bulb, so that you don’t get glare from the bulb. You still want to be able to see
the stars in the background, obviously. Why does sky glow obscure our vision of the night
sky?
2. If you put the lampshade over the bulb, you should be able to see the stars better.
So why should you use cutoffs on light fixtures? (Note that because enclosure is not very
tall, we didn’t opt to mount the bulb upright, but to the side. We simply slid the
lampshade on its side so that it was baffling the bulb. You could take some time to
engineer it so that the bulb was upright and place the lampshade on top of the bulb, but
the effect is the same.)
3. You can place an “eye chart” on one side of the box- say, a white mailing label with
different sized letters on it. (See Figure 2, left side). If you experiment with placing the
lampshade on or off the bulb, you should notice that you can see the letters better when
with the lampshade is on, because more light is being directed toward the object (again
we are assuming a bulb on its side orientation, as shown in Figure 2).
C. Simulating glare
1. Keep the variable bulb on. Use a baffle to cast a shadow with the bright bulb. Choose
some object within the shadowy area (a black dot that you place on the table, say, or even
a base or post) to observe while you turn the bright bulb on and off. You should notice
that your ability to see the object in the shadows (i.e., the “burglar) is actually enhanced
when the bright bulb is off and only the variable bulb is on. How can brighter light
actually be worse for security?
Figure 1. Light tight enclosure, 20 cm x 60 cm x 50 cm. Power supply is on
the left. The desk lamp can be seen on the back right hand side.
Figure 2. Inside the enclosure. The “stars” can be seen poked in the
cardboard. The variable brightness bulb is on the left. The lampshade is in
the middle. The bright bulb is seen on the right. Bases with posts hold up
the plastic.
Useful Websites
http://www.darksky.org International Dark Sky Association homepage
http://www.darksky.org/mc/page.do?sitePageId=59533 Links to curriculum on light
pollution, designed for children ages 5-13. A little simplistic.
www.lightpollution.it/dmsp Maps representing light pollution in different parts of the
world.
http://ngm.nationalgeographic.com/2008/11/light-pollution/klinkenborg-text National
Geographic Nov. 2008 cover story on light pollution
References
1
http://www.darksky.org/mc/page.do?sitePageId=59746
2
http://www.laserfocusworld.com/display_article/339339/12/none/none/TECHN/Software
-optimizes-illumination-design-while-minimizing-light-pollutio
3
D. Malakoff (2001). "Faulty towers". Audubon 103(5): 78–83
4
http://www.flap.org
5
M. Salmon (2003). "Artificial night lighting and sea turtles". Biologist 50: 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
http://en.wikipedia.org/wiki/Light_pollution
9
http://curious.astro.cornell.edu/question.php?number=194
10
http://www.astro.caltech.edu/palomar/lp.html
11
Ben Harder, "Turning Out the Lights," U.S. News & World Report, March 14, 2008.
12
Final Report of the Green Brighton Task Force, Brighton NY, August 2008. available
online: http://www.colorbrightongreen.org/site/uploads/GBTF_FinalReport.pdf
13
http://en.wikipedia.org/wiki/Nadir
14
http://data.nextrionet.com/site/idsa/is052.pdf
15
http://money.cnn.com/2007/02/13/magazines/fortune/gunther_pluggedin_lightbulb.fortun
e/index.htm
16
www.18seconds.org
17
http://www.spiked-online.com/index.php?/site/article/4281/