Obtrusive Lighting: When Too Much Light Is Annoying
Donald R. Monahan, PE
Increased awareness of the importance of safety and security in parking facilities has resulted in more and brighter lighting. However, when does an excessively bright lighting system become a nuisance to neighbors surrounding the site or impact other sensitive environmental issues? This article is intended to assist the reader in understanding these issues and how to design a system to prevent obtrusive lighting.
Obtrusive lighting may consist of unwanted light falling onto an adjacent property, often called light trespass. It may also consist of excessive glare from a light source or group of light sources viewed from outside the boundaries of the parking facility site. Another form is "sky glow" caused by direct or reflected light emitted into the atmosphere. Sky glow, often called light pollution, is objectionable to astronomers and aircraft pilots. It also wastes energy and consumes nonrenewable energy resources.
Inappropriate Lighting Regulations
Municipalities and other government entities are increasingly developing regulations regarding outdoor lighting practice to control obtrusive lighting. The next revision to California Title 24, for example, will have regulations to control obtrusive lighting. ASHRAE 90.1 has limitations on the power consumption of parking facility lighting systems of 0.3 watts per square foot. This energy limitation effectively caps the maximum illumination in a parking structure at 10 to 12 average maintained foot-candles, depending on the efficiency of the lighting system. The energy limitations of the ASHRAE 90.1 publication have been adopted by the federal government and many states.
Many municipalities adopt ordinances to minimize obtrusive lighting, often without expert advice from lighting designers, that have unintended consequences as follows:
Mandating the use of full cutoff luminaires (i.e., no direct light emitted above a horizontal line through the center of the luminaire) will reduce light emitted directly into the night sky, but can increase sky glow from light reflected off ground surfaces. Full cutoff luminaires usually require a horizontal orientation of the lamp, which creates a hot spot below the luminaire that often results in poor lighting uniformity versus a lamp with a vertical orientation.
Pole height limitations often result in poor lighting uniformity, increased costs, greater sky glow and possibly higher energy consumption. On a recent parking structure roof lighting project, the requirement for a 12-foot pole height resulted in 24 light poles with two dozen 200-watt light fixtures at an installed cost of $60,000 versus six light poles with one dozen 400-watt light fixtures at a 25-foot mounting height at an installed cost of $27,000. The energy consumption of both designs was approximately equivalent. The average maintained illuminance of the 12-foot design was 3.4 foot-candles and 3.2 foot-candles for the 25-foot design. However, the maximum/minimum uniformity ratio for the 12-foot design was 9.7 versus 6.7 for the 25-foot design. Therefore, the higher pole height resulted in a better design at less than half the cost.
Some ordinances specify that the light source cannot be visible from anywhere off the property. The fixture housing will typically shield the light source at an angle of approximately 16 degrees below a horizontal line through the fixture. (The center of the light source is approximately 3.5 inches above the bottom of the housing, and the opening at the bottom of the housing is approximately 24 inches in diameter.) A 12-foot mounting height then requires that the light fixture must be 42 feet from the property line in order to completely shield the light source from view. A 25-foot mounting height requires that the light fixture must be 87 feet from the property line in order not to see the light source.
A rule-of-thumb indicates that the light fixture will provide adequate minimum illuminance at a distance equivalent to approximately two times the mounting height. Therefore, the interior light poles at 42 feet from the perimeter for a 12-foot mounting height -- or 87 feet from the perimeter for a 25-foot mounting height -- will not achieve adequate minimum illuminance at the perimeter of the parking area. Perimeter light poles are required. A pole-mounted light fixture located on or near the property line then must have a supplemental shield that hangs down below the fixture approximately 12 to 24 inches to shield the light source. Shielded light fixtures are not recommended for interior locations as the shield will block the light distribution toward the perimeter and have an adverse impact on the minimum illuminance.
Low-pressure sodium lighting is often specified in areas surrounding astronomic observatories, because the monochromatic characteristics of that light source can be easily filtered. However, it has poor color rendering characteristics and reduces contrast. This type of light source may cause color identification problems and reduce nighttime visibility.
A curfew is often established late at night (e.g., 10 to 11 p.m.), after which the lighting must be reduced or turned off. This requirement is problematic for hospitals, casinos and other 24-hour operations, because it compromises safety and security in the parking facility.
Appropriate Lighting Regulations
The Illuminating Engineering Society of North America (IESNA) and the International Commission on Illumination (Commission Internationale de L'Eclairage, or CIE) have conducted extensive research regarding the thresholds that result in obtrusive lighting. These studies have found that the background lighting conditions (ambient lighting) affect the brightness level of the light source that is objectionable. Therefore, a series of lighting classifications, or lighting zones, were defined based on ambient lighting conditions, and the limitations on light source brightness were determined for each lighting zone.
Establishment of curfews is a logical method to provide partial control of obtrusive light. For instance, it may not be possible to design sports lighting that provides adequate lighting for the sporting activity and is not objectionable to the neighbors. Specifying a time at night when the lighting must be reduced or extinguished is then a compromise between the desire to provide for that outdoor activity balanced with the neighbors' desire to minimize obtrusive light late at night. Where a curfew is established, the local ordinance for pre-curfew hours can allow higher limitations for light trespass. During post-curfew hours, lighting that is nonessential, such as sports lighting, building floodlighting and outdoor advertising, could be extinguished and lower limitations on light trespass specified. However, the IESNA-recommended practice for safety and security should be met even when the curfew is in effect.
(Note: The position of the observer is generally taken at the property line of the site on which the outdoor lighting is installed. However, where the site is surrounded by a street right of way, the writer recommends that the position of the observer be taken at the property line across the public right of way or at the closest adjoining private property line.)
There are right and wrong ways to control obtrusive lighting. Governing authorities should engage lighting professionals to assist with developing ordinance requirements that are reasonable and not unduly restrictive. The research and recommendations of the IESNA and CIE represent the best available information to date.
Don Monahan is a Vice President of Walker Parking Consultants in Denver. He is Chairman of the Off-Roadway Lighting Committee of the Illuminating Engineering Society of North America, and is a member of the obtrusive lighting task force responsible for the TM-11 publication. Contact him for specific tables on light trespass. He can be reached via email at firstname.lastname@example.org.
Lewin, I. Light Trespass Research, Lighting Research Office of the Electric Power Research Institute, Report Number:
TR-114914, Palo Alto, CA, March 2000.
IESNA publication TM-11-2000, Light Trespass: Research, Results, and Recommendations, prepared by the Obtrusive Light Subcommittee of the IESNA Roadway Lighting Committee, December 2000.
CIE publication 150: 2003, Guide on the Limitation of the Effects of Obtrusive Light from Outdoor Lighting Installations, International Commission on Illumination, Vienna,
Article Abstract from May, 2005