The Challenges of Exterior Lighting
By John L. Fetters - April 2003 - Lighting
A growing number of education, medical and other institutional and commercial organizations are using their facilities at night, so effective nighttime lighting is essential for illuminating walkways, automobiles and obstructions. Proper lighting also provides security for visitors and occupants who are walking, driving or biking. It helps accentuate building entrances and signage, and it provides an attractive outdoor environment with no glare.
Facilities with quality outdoor lighting are safer when pedestrians and drivers can see better, and quality lighting produces an attractive nighttime environment. Quality lighting fixtures provide well-controlled light distribution, and they do not produce glare.
But managers responsible for providing and maintaining good outdoor lighting for parking lots, parking garages, storage areas and walkways face a number of challenges in specifying products that produce effective outdoor lighting.
Good Lighting vs. Bad Lighting
Most outdoor lighting installations provide more bad lighting than good. Bad outdoor lighting is counterproductive, reducing safety and security and creating higher-than-normal operating costs. Poorly designed, misapplied, defective, improperly located or badly aimed fixtures can cause glare, harsh shadows, wasted light and light trespass.
Such fixtures are easy to spot at night because they produce obtrusive light or glare. Obtrusive light is the generalized category of unwanted light that includes sky glow and light trespass, which causes annoyance, discomfort, distraction or reduction in the ability to see. Light trespass is obtrusive light that crosses a property line.
Many people mistakenly believe that a great deal of glare means there is a great deal of light. Actually, glare is an indicator of inefficient and ineffective light. Glare never aids visibility. While glare is rather subjective, in most cases all that is needed to prevent it is to select the correct luminaire optics, use the correct pole height, put the pole in the correct place, and use shielding accessories where required.
The best way to minimize glare and prevent obtrusive light is to replace offending fixtures with full-cut-off fixtures, which have no up-light or glare and, when they use energy-efficient lamps, are energy efficient. Cut-off fixtures are designed for streetlights, parking lots, garages and pedestrian walkways.
Well-designed, full-cut-off fixtures prevent light above the horizontal plane that causes “sky glow” and usually are energy-efficient. The Illuminating Engineering Society of North America (IESNA) recommendations for such fixtures can be found in RP-33, “Lighting for Exterior Environments.”
One new streetlight or roadway luminaire designed to replace the glary cobra-head design is the mongoose, which is designed to provide superior optical performance and reduce maintenance costs.
The primary purpose of lighting parking lots is to benefit pedestrians, according to IESNA. The design intent requires that a driver or pedestrian who looks at the brightest spot in the field of view also can see an object in the dark areas within the field of view. Light levels must be balanced to provide the uniformity necessary to produce this result. White-light sources also help drivers find their cars by enabling them to identify colors more accurately.
Wall-mounted wall-packs are available with cut-off optics and can be used to light narrow parking areas between or adjacent to buildings. They are available in a variety of architectural styles and should be mounted no higher than 25 feet.
Managers also can specify bollards — which look like short, thick posts — to effectively light walkways and building entrances. Low-wattage metal halide or compact fluorescent lamps (CFL) also are options for primary light sources. Managers should ensure that bollards and ground-mounted floods are protected from lawn mowers and snow-removal equipment.
Floodlights often put light in people’s eyes and are another common source of direct glare. But even well-designed floods must be installed properly, with beams aimed so they do not cause glare. Wall packs also can be a source of glare unless they are designed with good light control.
Globe fixtures look good in the daytime, but because they tend to deliver a high amount of light above the horizontal centerline, they are a source of light pollution. Another source of light pollution is sign lights that are aimed to light from below. Especially on foggy mornings, the beams of these floodlights can project over signs, lighting the sky.
Too much light is a common cause of bad outdoor lighting and can impair visual adaptation. When people move from areas that are too bright to those that are too dark — and vice versa — poor visibility results. When lighting designs do not minimize the effect called transient adaptation, the steep transitions of light to dark inhibit visibility.
The trend in outdoor lighting is toward white light with a good color rendering index (CRI). The use of yellow or orange low-pressure — where the CRI is 0 — and high-pressure — where the CRI is 25 — sodium lamps has been declining for some time. There is also an increase in visual acuity when lamps that are scotopically enhanced, such as metal halide lamps with higher color temperature — more than 4,000 K — that are the most popular white-light source for outdoor lighting.
Also, sales of pulse-start metal-halide lamps are starting to outpace those of standard metal halides because of improvements in efficiency, restrike time and color consistency. They also have good CRI values of 70–80.
Some higher-wattage CFLs are being used in walkway and entrance fixtures. They are energy-efficient, deliver long life and have good CRI values, in the low 80s. For outdoor applications in northern climates, managers might consider amalgam lamps — most triple tubes are amalgam — to maintain lumen output at low- and high-temperature extremes. Managers should ensure that they are used in enclosed fixtures and close to a building to minimize the effects of low temperatures and wind chill.
Long-life, white, electrodeless lamps are finding applications in roadway and walkway lighting, and they offer managers an option that reduces maintenance labor costs. Electrodeless lamps also have good color rendering with values in the mid-80s, similar to fluorescent lamps.
Managers should avoid using obsolete mercury-vapor lamps for outdoor lighting. They have severe lumen depreciation, they seem to burn forever, even with very little light output, and they have poor color rendering, with values of 22–52.
Managers need to ensure that outdoor lighting systems are controlled so they can be turned off when they are not required. The best control is a photocell, an electrical switching device that operates when light falls on it. Photocells are used to turn on lighting at dusk and turn it off at dawn. They have a built-in time delay, so they will not turn off a lighting system when lightning flashes at night.
When photocells fail, they fail on, or fail-safe. So, when an outdoor lighting system controlled by a photocell is on during daylight hours, the photocell usually has failed.
In the northern hemisphere, managers should make sure photocells are aimed north to “see” the reflected light of the northern sky so they are not influenced by the directionality of east/west exposure or degraded by intense southern exposure.
The least expensive type of photocell uses cadmium-sulfide cells that degrade from exposure to sunlight and lose sensitivity after being in service for a few years. This loss of sensitivity decreases savings by keeping exterior lighting on longer than necessary.
To avoid this loss of energy savings, managers should replace cadmium-sulfide cells with electronic cells that use solid-state, silicon phototransistors or photodiodes, which do not lose sensitivity over time. Also, solid-state photocells last longer, as evidenced by their longer warranties — up to six years — and pay back before that time in energy and labor savings.
Finally, managers can use time clocks or energy-management systems to limit outdoor lighting within certain hours during the night when some systems can be turned off.
Improvements in lighting equipment — especially luminaires with excellent cut-off optics and new white-light sources — are available to help managers relight their facilities’ exterior areas. Relighting with quality equipment can produce better visibility for facility visitors and occupants and make outdoor environments both more attractive and safer.
Light Pollution: A Closer Look
Light pollution is defined as the glow of the atmosphere resulting from outdoor lighting that obstructs the beauty of the universe, according to the International Dark Sky Association (IDSA). IDSA is a non-profit, tax-exempt, membership-based organization organized to help preserve and restore dark skies while maximizing the quality and efficiency of nighttime outdoor lighting.
The IDSA has emerged as an advocate for restricting light pollution. Avoiding skyward illumination is one hallmark of sustainable lighting design, and many exterior designs can be revisited and redesigned to avoid creating sky glow.
— John L. Fetters