The Problem
The increasing desires to light exterior areas for safety, security, amenities, commercial activities, and sports events makes the problem of light trespass more frequent and complex. The multitude of lighting products, very often misapplied, badly aimed, or poorly designed, results in many complaints. Glare may be visible from nearby or distant areas. Frequently the lighting is not
confined to the originating property. Spill light falling over property lines can illuminate adjacent grounds or buildings in an objectionable manner. These are probably the most frequent light trespass complaints.
The gradual increase of ordinances indicate that communities feel that light trespass is a problem that must be addressed. This can and has resulted in the creation of laws and penalties which are often subjective and even overly restrictive. This fact may well hinder the process of getting affordable quality lighting installations in the regulated areas due to fear of liability and/or the time and cost to research the design of an installation.
The Illuminating Engineering Society of North America (IESNA, or IES) has established a committee to research the problems of light trespass, glare, and sky glow, and, hopefully, will set guidelines that the lighting industry and communities can follow. As of 1996, there is not yet a published report. Several other IES committees are also addressing the issues.
Solutions
Because light trespass and glare are quite subjective, they are difficult to eliminate, but they can be minimized through good design practices. In many cases, all that is required is the proper placement of poles, selection of luminaire optics, and shielding accessories. The fact that light trespass is a concern can be reasonably stated in municipal codes. However, overly rigid regulations are just as undesirable as light trespass itself. Codes must allow for design flexibility through a reasonable review process.
Requirements should be written using available measurable parameters. Terms such as “glare”, “eye discomfort”, and “excessive brightness” are subjective terms without scientific backup or guidelines. Luminaire optic restrictions should be defined through mathematical criteria, not the generalized term “cutoff”. Mounting heights and lighting levels should follow recognized industry practices, but they should have provisions to allow a review body to impose restrictions or allow exceptions for special circumstances.
Recommended Criteria for Exterior
Lighting Ordinances Direct Glare: Direct glare is defined as the visual discomfort resulting from insufficiently shielded light sources in the field of view. One should “see the effect, not the light source”. Use of the term “direct glare” is recommended in lieu of the word “glare” alone. The direct glare definition means if you can see an unshielded lamp, or the luminaire maximum candlepower zone, you may well have glare. However, usually when you are near a luminaire, you will see these parameters, of course. So a reasonable definition limiting the field of view is required.
Limitation of Observation of Direct Glare: Direct glare shall not be observable (outside the originating property limits) at an angle greater than 85 degrees from the nadir of the vertical axis of the light source.
This definition is a reasonable one. In practice, it will result in limiting the distance from the light source that you see direct glare to less than ten times the mounting height of the light source. In many cases, it would be approximately six times the mounting height. There are many cutoff luminaire types that can meet this requirement easily. Also many floodlight luminaires and other area luminaire types with proper installation and shielding can also meet this requirement.
Light Trespass: Limit the exterior lighting originating on a property to a maximum of 0.5 horizontal foot candles (HFC) at a distance of 25 feet beyond the property lines. This specification will allow the controlled placement of lighting poles and luminaires adjacent to the property lines. With many outdoor luminaires, it is difficult to comply with low level foot candle requirements at the property line. An example: A typical 250 watt high pressure sodium (HPS) luminaire at the property line would have about a 5 HFC below the light fixture, but it could be shielded so that there is no more than
0.5 HFC at 25 feet from the property line.
Luminaire Mounting Height: A mounting height of 30 feet allows the use of a variety of luminaires in an energy efficient manner. There should be provisions in any code for needed exceptions, such as for sport lighting installations or other high mast designs.
Illumination Levels: Use the terms horizontal foot candle (HFC) or vertical foot candle (VFC) to define the type of illumination levels and measurements. Recommended illumination levels are published in various IES publications and by many luminaire manufacturers. These levels should be followed unless there are adequate design reasons not to do so.
Summary
These recommendations are far from perfect. Indeed, it is probably impossible to produce an outdoor lighting ordinance that would be perfect for all applications and locales. However, using the combination of these lighting design factors will greatly alleviate many complaints and will certainly improve the quality and effectiveness of most all outdoor area lighting applications. Any lighting control code should contain provisions for a ruling body to grant essential exceptions to the code provisions if it is clear that the intent of the code will still be met. One wants to maximize the creativity of the designer to produce quality lighting, while at the same time limiting greatly the amount of poor lighting so commonly seen today.