Light Pollution

Light pollution from excess articial light causes problems for people and wildlife. Sometimes we cannot even see the stars because of unnecessary artificial lights shining upward.

We can reduce the problem through community awareness, legislation (for example, requiring shielding on outdoor lights), and improved lighting technology.

Detrimental effects of poor outdoor lighting include glare, light trespass and sky glow.

What is Glare?

What is Light Trespass?

What is Sky Glow?

Effects of Artificial Light

Good Neighbor Outdoor Lighting


Bright Lights: A Survival Primer for Acton Residents

1984 Acton Outdoor Lighting Bylaw

Good Neighbor Outdoor Lighting

How, you might ask, could "outdoor lighting" be a subject on a site devoted to environmental concerns? Well, consider that half our environment is night time, and for millennia, humans have become well adapted to (and actually dependent on) darkness. In less than one hundred years, use of artificial lighting has become widespread. Although artificial lighting has been a benefit for moving about safely at night, it can also easily and inadvertently become obtrusive to our neighbors and others-light does not stop at property boundaries.

Good Neighbor Outdoor Lighting aims to control the direction of light and avoid detrimental effects of poor outdoor lighting (glare, light trespass and sky glow) while still letting us enjoy the benefits of seeing our way at night.
Earth at Night
Picture of the world at night from space.

Major population centers are easily recognizable from their light pollution. (Photo from NASA).

We can have safe and effective outdoor lighting without becoming a nuisance to our neighbors, by using shielded lighting and also using the right amount, which allows us to see where we are going without being excessive and wasting energy. Learn about these simple and effective shielding principles.

Sometimes we are on the unwelcome receiving end of obtrusive lighting. Here is some information on Acton's Outdoor Lighting bylaw and some tips on how to handle the situation.

What are Glare, Light Trespass and Sky Glow?

Detrimental effects of poor outdoor lighting include glare, light trespass and sky glow.


Glare is the direct view of light source against dark background. Glare overloads our eyes and actually reduces our ability to see in a generally dark environment, which can cause accidents. The cause of glare is poorly shielded lights that are aimed at least partially horizontally, not down onto the pavement. Glare Example

Glare can easily cause us not to see important objects such as the couple crossing in front of the car. (Photos from IDA).

The left picture illustrates how glare of a light fixture might affect our vision while driving. The right picture shows how the scene would look if the light was shielded, so the direct view of the lamp is hidden from the driver's view.

Light Trespass

Light Trespass is unwelcome light spilling off originating property. As with glare, the causes are, first, poorly shielded lights which are aimed partially horizontally, not down, and secondly, too much light power used for the application. Light Trespass Example

This picture shows an extreme example of light pollution. A bright unshielded fixture off-screen to the left of the picture is shining horizontally and lighting up the entire left wall of this building. The streetlight on the pole in the upper left of the picture is off, even though it is night. (Photo from IDA).

The light trespass has caused the light sensor in the streetlight to think it is daytime! Imagine what it would be like to have a bedroom in one of the building rooms.

Sky Glow

Sky Glow is a general brightening of the normally black night sky caused by artificial light pollution. Excessive sky glow can almost completely block out views of stars. The main causes are upward-directed light (again from poorly shielded fixtures) and too much light power used for the application. Sky Glow Example

These pictures show the sky in Toronto, Ontario in 2003.

The left picture was taken during a blackout before the general use of artificial lighting in this area.

The right picture shows the now-normal night sky, after the blackout. Light pollution has almost completely blocked out the any view of the stars.

These pictures were taken in August, 2003, during and after a blackout which spanned the eastern seaboard. (Photos courtesy of Todd Carlson).

Effects of Artificial Light

General use of artificial lighting, which began about 100 years ago, has been accelerating in the past 50 years, and if unchecked will completely block out any natural-sky views in the not-too-distant future.

This picture shows the view from space of upward directed light. The views of the mid-70's and 1997 are from satellite data; the late '50's and 2025 views are extrapolations from the satellite views, using known or expected amounts of outdoor lighting. All the light that is visible from space is obviously wasted energy which produces no useful effect (and lots of bad effects) for people on the ground. (Picture credit Fabio Falchi, The Light Pollution Science and Technology Institute [Istituto di Scienza e Tecnologia dell'Inquinamento Luminoso] www.istil.eu and CieloBuio, the organization that promotes the adoption of several of the best laws against light pollution: www.cielobuio.org)

Outdoor lighting is growing at greater than 5% per year. At the present time, artificial lighting obscures the Milky Way (our home galaxy) from view of more than 70% of the households in the United States. In a dark-sky site, about 2500 stars are visible; but in the sky over a large city like Boston, that number is reduced to less than 100.

What can we personally do about this? Use Fully Shielded light fixtures, use the proper amount of light for the job (resist the urge to over-light), and turn off light (either manually or with motion sensors) when there is no one around to use it. Advocate to others how these simple steps will result in both reduction of light pollution and conservation of energy. Demand that your cities and towns also follow these principles.

Principles of Good Neighbor Outdoor Lighting

We all like the freedom to do as we want on our own property. However, at the same time most of us realize it's also important not to intrude on our neighbor's rights by our actions. We would never think of either making very loud noises that would disturb our neighbors or of aiming our water sprinkler so it sprays on their house or of letting our dog exercise on their lawn. But often we don't stop to think that the outside lights we install on our house can be just as obtrusive to our neighbors.

The fact is that the light our fixtures produce doesn't stop at our property boundary. Light travels in straight lines from our outside fixture and can directly shine in our neighbor's windows or on his property, and then it becomes obtrusive and unwelcome. A bright, lighted, unshielded bulb on our house or yard will cause unpleasant glare in our neighbor's eyes. Glare is the uncomfortable situation in which the human eye is unable to respond both to the dark surround and also a very bright source. We instinctively squint our eyes to try to close out the offending light. If our light trespasses through the windows of our neighbors' house, and into, for example, their bedroom, then our light can even affect their ability (and right) to have a sound sleep.

The solution to prevent glare and the majority of light trespass is very simple and inexpensive. We can apply shielding to our bright outside lights, such as floodlights and other bare bulbs that are bright. "Bright" is defined In Acton's outdoor lighting bylaw as 120 watts for incandescent bulbs and 20 watts for fluorescent bulbs. Even though Acton's Outdoor Lighting Bylaw does not apply to single family dwellings we residents are on our honor to do the right thing in regard to our neighbor!


Shielding is used to block the lamp's rays from traveling upward (causing sky glow) or sideways (and off your property–potentially causing a nuisance to your neighbor). To direct the lamp's rays onto the ground–the place we are trying to illuminate after all–the shield should cover the top and sides of the lamp. The bottom–the only open side of the fixture–is parallel to the ground. If we view the fixture from a point horizontal to the lowest point of its shield, we will not see any direct light from the fixture. The only visible light should be that illuminating the ground–where of course it is the place where it is needed! A fixture like this -shielded on the top and sides with a flat horizontal opening on the bottom–is called Fully Shielded. An example of a Fully Shielded fixture for residential use (with the brand name Glare Buster) is shown here.

One light that is commonly used on many houses to light driveways and yards is the Parabolic Reflector (PAR) lamp. This is the familiar cone-shaped bulb about six inches in diameter. If unshielded, it can project light for long distances, because of the focusing reflector built into it.

On the left is a picture, taken from the neighbor's lot, of an unshielded PAR floodlight used over garage doors. The glare is obvious and unavoidable, and happens even if the floodlight is tipped downwards somewhat. (Picture credit Bob Crelin). It is easy to make a simple shield using aluminum flashing and to fix it to the PAR lamp with a clamp. Installing one on these garage lights (right picture), even without changing the direction in which they are aimed, dramatically reduces glare, without affecting the light that falls on the driveway.

The picture on the right shows a close-up of the light with shield attached. Quiz question—is this a Fully Shielded fixture? (Answer: Yes) Picture credit Bern Kosicki.

For those who would rather purchase a ready-made shield, these sites contain shades (or shields) that can be mounted to existing fixtures or bulbs:


You can learn much more about shielding and principles of good lighting from sites sponsored by the New England Light Pollution Advisory Group (NELPAG) and the International Dark Sky Association (IDA). We recommend all these sites to you.

Or write to us at outdoorlighting@greenacton.org for advice on how to solve your lighting needs with Good Neighbor Outdoor Lighting principles. Here is a short list of sites that provide fully shielded complete fixtures for home use:


The Glare Buster, www.theglarebuster.com, is also available from some large hardware stores. Again, remember that you are looking for Fully Shielded lights, which are easy to recognize since their only open side is horizontal to the ground and points straight down.

Bright Lights in Your Eyes? A Survival Primer for Acton Residents

Acton has an Outdoor Lighting Bylaw that affects primarily commercial developments. Any commercial development after 2004 that requires a Site Plan Special Permit (developments that are above a certain size need this permit) must comply with the 2004 Acton Outdoor Lighting Bylaw. (See Section 10.6 of the Zoning Bylaws http://www.acton-ma.gov/index.aspx?nid=167.) Site Plan Special Permit developments that were permitted between 1984 and 2004 must follow the previous 1984 Acton Outdoor Lighting Bylaw.

If you believe that the offending commercial development falls into one of these categories, then you should call the Acton Zoning Enforcement Officer, Scott Mutch (978) 264-9612 (Planning Department) and state your case.

Various Acton Boards of Selectmen have agreed that the Town should also follow the outdoor lighting bylaw (but there is no binding agreement that requires them to do so).

For issues with lights that are owned by the municipal government (library, streetlights, South Acton Train Station parking lot, transfer station, police and fire departments), call the Municipal Properties Director, Dean Charter (978) 264-9629.

For problems associated with any school-owned property, call the Coordinator of Facilities and Transportation, John D. Head; (978) 264-4700.

Acton's bylaws don't cover residential lighting. However, often times a simple discussion with the owner can solve the problem. Here are some tips on how to approach that discussion, abstracted from an excellent International Dark Sky information sheet (http://docs.darksky.org/PG/PG3-residential-lighting.pdf):

  1. Make friends, not enemies. Your neighbors probably don't realize the light is bothersome. Always approach people in a friendly, non-threatening way, and don't argue. Be tactful and understanding about their right to light their property. Suggest alternatives to their current fixture. (Ask them to move the light, add a shield to it or add a motion sensor).
  2. Be informative. There are many reasons to use dark sky friendly lighting. Safety is important, but brighter does not mean safer. A starry sky is a natural resource.
  3. Do your homework and be prepared to address the real issues. It is useful to know the local costs of electricity (cents per KWH), and the local lighting control ordinances. You may also want to compile a list of local businesses with good quality lighting as examples. A list of shielded light fixtures to provide as alternatives to your neighbor's current light is also recommended.
  4. Stay positive. Don't let bad lighting create a feud in your residential area.

We encourage you to read the entire IDA document that these tips were borrowed from. If you need for more help, write to outdoorlighting@greenacton.org and we'll try to provide support to you.

1984 Acton Outdoor Lighting Bylaw

The purpose of Acton's 1984 Outdoor Lighting Bylaw is to control glare by requiring bright lights to be aimed down, and also to control light trespass from the originating property onto an adjacent property by additional shielding. Outdoor Lighting–In the area of new construction all outdoor lighting, with the exception of pedestrian lighting with a height of less than eight feet, shall be designed and located so that:

Less than 76º

  1. The luminaire has an angle of cutoff less than 76 degrees;
  2. A line drawn from the height of the luminaire along the angle of cutoff intersects the ground at a point within the development site, and
  3. The bare light bulb, lamp or light source is completely shielded from direct view at any point five feet above the ground on neighboring properties or STREETS. Pedestrian lighting with a height of less than eight feet shall be regulated through the Building Commissioner's Office.

This bylaw is strict in terms of requiring effective shielding. However, OLEC found the following shortcomings in this bylaw:

Technical requirements for shielded lights; the current bylaw requires a strongly shielded light, but doesn't use modern terminology.
There is no limit to the amount of light that may be used. There are now guidelines established that suggest effective and considerate lighting levels.
Hours of operation are not addressed. Lights that are kept on after close of business, and are not used specifically for safety or security, should be turned off.
Special provisions; there are special cases that are not well handled by the current bylaw, and consequently, there is in practice no control over these situations.