Individuals, environmental groups, and governing organizations at all levels have become increasingly concerned about unwanted or intrusive light in the nighttime. This light is referred to as uncontrolled light, an environmental problem that includes glare, overlighting, light trespass, and skyglow among its manifestations.

What are the issues?

When the level of the ambient light causes visual discomfort or loss of visibility, it is called glare. Disability glare is a condition in which a bright light source appears in the eyes’ field of view, reducing visibility. Overlighting, on the other hand, occurs when the brightness exceeds what is needed or recommended.

Skyglow results from light that is unnecessarily directed upward and is scattered by the atmosphere, rather than being focused directly on the target area.

Obtrusive light, or light trespass, is light that is distributed where it is not wanted or needed. It is also commonly referred to as spill light, which is emitted light that falls outside the boundaries of the property on which the lighting installation is located. It usually results from luminaires being misapplied, badly aimed, or poorly designed.

Nearby street lamps, sports field lighting, and/or residential, parking lot, or shopping area lighting, might be the culprit.

Warning to electrical contractors

Light trespass is not only visually disabling and aesthetically unappealing, but it also wastes costly energy. The complaints arising from it are wide ranging, from issues of aesthetics to concerns about safety, health, and conserving energy resources.

As a result, many communities have passed ordinances in an effort to legislate a solution. These discussions and their implications for outdoor lighting directly affect the lighting designer, particularly when legislation is enacted to control lighting equipment selection and applications.
A number of organizations are devoted to publicizing the problems related to light trespass, which has led to a growing body of ordinances that are either in effect or being proposed. There is little agreement about which aspects of light trespass need to be controlled and how.

Light trespass ordinances differ by municipality, which creates a nightmare for lighting designers planning outdoor installations. Lighting professionals need to be alert to this growing body of legislation when creating lighting designs. They need to take the initiative in developing standards and making recommendations to lawmakers. A proactive stance will ensure that logical, technically achievable, and well-coordinated ordinances are enacted.

Brightness and proximity

Defining the problem objectively is complicated because light trespass and glare are subjective, and there is always an emotional or personality component involved. A light source that is highly objectionable and intrusive to one person may not be so to another.

Research studies with human subjects have attempted to pinpoint more precisely and objectively the characteristics of light leading to light trespass. Study results strongly indicate that brightness of the source is the main characteristic people consider objectionable.

In one experiment aimed at defining light trespass characteristics, participants were asked to observe a test fixture with a light source placed inside of it. Under one of the test conditions, while the subjects were watching television inside, the fixture was placed outside the window to simulate a street light or floodlight. The interior lighting was low, and the light fixture was 32 feet from the subject.

The brightness of the light was incrementally varied at 12 different levels, and the wideness of the fixture opening that exposed the light to the subject was adjusted to three different sizes. The subjects were asked to judge the lighting from the fixture based on its location on residential property. The ratings ranged from 1 to 5, with "1" being "Not Objectionable" and "5" being "Highly Objectionable."

In general, the test results indicate that increasing brightness, whatever the fixture opening’s size, raised the objectionable rating. In addition, a wider fixture opening, even at constant light levels, produced more objectionable ratings.

The level of the interior or ambient lighting had no effect on the rating. When the subjects were placed farther from the light source, their rating of it as objectionable decreased. Not surprisingly, greater distance from the source reduces objections.

These results indicate that light trespass is caused by overly bright light that is too close for comfort. They also indicate that the fixture holding the light source is a significant factor in whether or not light trespass occurs. One significant aim of light trespass ordinances, which this study’s results validate, is to specify the type of fixture allowed in lighting installations. The results of this study appear to validate this legislation.

What do the laws say about fixtures?

The ordinances in question require a fixture designed to focus light output where it is needed. In essence, this fixture, referred to as a "full cut-off fixture," directs light down and sideways, thereby eliminating light directed toward the eye or the sky. For retrofits, several manufacturers offer full cut-off screw-on shields.

Fixture makers increasingly offer these full cut-off fixtures and shields because of their technical advantages, including their ability to reduce light trespass, glare, and skyglow.

The ordinances requiring the use of full cut-off fixtures are highly specific regarding the definition of such fixtures, the cut-off angle to be used, and the permissible rated light output in lumens. Typically, they specify that a full cut-off fixture, or fully shielded fixture, is one that emits zero light at 90 degrees (and less than 10 percent of lamp lumens at 80 degrees.

The cut-off angle is simply the point at which you can no longer see the light source directly. In general, these ordinances use the "maintained horizontal illuminance recommendations" set by the Illuminating Engineering Society of North America (IESNA).

Highly specific requirements

Some laws are specific about which light source types and wattages must be shielded and which are exempt. High-pressure sodium, incandescent, fluorescent, and metal halide lamp types are in general listed as those that must be fully shielded.

Low-pressure sodium must be partially shielded, meaning that only a specified percentage of the light rays are emitted by the fixture above the horizontal plane. Incandescent lamps below 160 watts and all other light sources of 50 watts or less may not require shields. A number of ordinances simply ban all forms of uplighting.

In many instances, these laws are referred to as lighting control ordinances, because they specify the fixture type that must be used and the light pole height, distance, and location within the site, dimming requirements, and shut-off times. Lighting control ordinances also often specify municipal area and/or lighting type (i.e. display lighting) to which each restriction applies.

The drive to eliminate light trespass is the force behind the growth of restrictive lighting control ordinances. Lighting designers need to be aware of the problems and the solutions. The outdoor lighting designs they create should reflect their knowledge and expertise in eliminating light trespass, before this growing body of legislated restrictions takes the initiative from them.

Dr. Lewin is the founder and president of Lighting Sciences Inc. of Scottsdale, Ariz., which is part of Advanced Lighting Technologies, Inc. Dr. Lewin is president and a fellow of the Illuminating Engineering Society of North America (IESNA) and was the recipient of the 1997 IES Medal, the society’s highest honor. This article is adapted from his presentation at the IESNA Annual Technical Conference, August 1999.