Proper lamp selection per any required color rendering and adherence to practical guidelines combine to create functional lighting installations.
The purpose of security lighting is to protect persons and property usually between dusk and dawn. Additionally, security lighting should provide enough light over an area so that anyone moving in or around it can be seen easily. For such an application, where lamps will be burning for at least 12 hrs continuously, economy of system operation is a major consideration.
Another point to remember is that although security lighting can double as safety lighting, the two do not serve the same purpose. For example, safety lighting can be considered as providing sufficient illumination on a stairwell so that a person can see steps and not trip and fall. Also, a person must be visually aware of possible hazards such as curbs, sloped walkways, or change of direction on a path.
An upgrade or retrofit security lighting project can be a factor in reducing the overall cost of security services, or it can be an important adjunct to increased security effectiveness.
After a decision to upgrade is made, the first task is to conduct an audit of the building's security lighting needs. Parking lots are the most commonly lighted area, then walkways and building perimeters. For parking lots and walkways, the key is to have uniform footcandle (fc) levels with maximum efficiency at ground levels. Often, the area surrounding a parking lot is neglected. A building perimeter can be illuminated for architectural enhancement, and in most cases, building accent or facade lighting can double as security lighting.
It's best to light as large an exposed area as possible, such as an expanse of wall, driveway, or paved ground surface; this will help to reflect and spread the light, creating a background or plane of low-level light against which an intruder can be seen.
The Illuminating Engineering Society lighting handbook offers some general guidelines for the illumination level in an area. One to 3 fc may be sufficient for a suburban facility, but 5 fc may be better for an urban area office building. Other factors, such as traffic patterns, may cause local or IFS guidelines to be exceeded.
You should select the most appropriate light source and wattage. Generally, the higher the wattage, the greater the efficacy (lumen per watt output) of the lamp. Additionally, high wattage lamps usually allow high spacing-to-mounting-height ratios, permitting the fixtures to be well elevated on a pole or structure, out of reach of vandals.
High-pressure-sodium (HPS) or metal-halide (M-H) sources are the two preferred lamp types. HPS lamps are available in ratings from 35W to 1000W, with efficacy ranges from 54 to 130 lumens per watt, including ballast losses. HPS lamps have an average rated life of 24,000 hrs, plus excellent lumen maintenance (90% mean lumens at end of the lamp life) in many wattage ratings.
M-H lamps are available in rating from 50W to 1500W. The single-ended lamp comes in three different outer bulb finishes: clear, phosphor-coated, and diffuse coated. The M-H lamp's lumen maintenance is not as good as an HPS lamp because of its construction. Also, the particular lamp design and burning position determines average rated life.
Exterior fluorescent fixtures may be used in certain applications, such as covered walkways or overhangs. Lamps and ballasts should be specified for cold weather application, if necessary.
Factors you should consider in selecting a light source are its luminous efficacy, color properties, resttike and runup times, and performance at temperatures likely to occur on the site. The color properties are important for accurate rendering of people and objects, though not necessarily for the detection of their presence. (Restrike is important in the event utility power is interrupted. Restrike refers to the ability of an HID lamp to regain full output after the arc has been extinguished.) Additionally, the lamp should start reliably in cold temperatures.
No single lamp type is the best for all applications. It's generally advisable not to mix lamp types, unless the two or more lamp types (and colors) are used as part of a specific lighting design. In addition, the color characteristics of a light source can be modified by inserting a filter in the lamp compartment.
With the availability of a variety of small envelope, clear M-H lamps, a variety of compact fixtures are now available. For example, using computerized design, one manufacturer has developed a forward-throw reflector system that, in the past, would require a fixture double in size. By using the ED28 reduced envelope, 400W M-H lamp and the BT37 reduced envelope, 1000W lamp, the manufacturer offers a fixture having a forward main beam at 60+ degrees with no fixture tilt and virtually no back light [ILLUSTRATION FOR FIGURE 1 OMITTED]. In a typical parking lot application, there is no spill light onto adjacent properties that may create problems.
Important points to consider
The following are important application parameters you must consider for security lighting.
* Excessive direct or reflected glare and harsh shadows can hamper seeing and cause visual confusion. Generally, the use of fixtures with cut-off optics that direct the light downward is recommended. Many municipalities in the United States have ordinances or laws requiring cut-off optics on exterior fixtures to avoid light trespass onto adjacent properties.
* Use equipment that can be shielded after installation if necessary. Many fixtures have an optional louver installed behind the lens so that the Effective Projected Area (EPA) (the wind loading of the fixture) is not increased.
* Use fixtures that are easy to mount and secure. The wiring (or ballast) compartment should have convenient openings or knockouts for conduit entry. Frequently, HID fixtures replace old incandescent PAR lamps in bullet-type fixtures installed on a building wall. Plan for easy access of conduit runs.
* Use fixtures with tamper-resistant hardware and vandal-resistant polycarbonate refractors. Fixtures with electrical components mounted to a diecast aluminum housing provide proper transfer of heat generated by the ballast.
Fixtures can be selected that serve for both landscape and security lighting. Light only those objects in the landscape that are pleasant to look at, such as certain trees, shrubs, statuary, fences, etc.
A useful design technique is to provide layering of light in an urban setting. General illumination can be provided by standard roadway/street lighting luminaires with poles ranging in height from 20 ft to 30 ft. A second layer can be provided by fixtures on poles ranging in heights from 8 ft to 15 ft, with the spacing planned to provide a design feature. This fixture can have a decorative element such as edge lit acrylic lens added at the top, perhaps. A third layer of light can be provided by small scale fixtures, such as bollards, to provide physical as well as psychological cues to a specific locale, which could be a walkway, for example. Multifunction bollards are also available, with one side having a controlled downlight component while the other side may have one or more accent or adjustable assemblies for highlighting plantings or buildings. Some bollards are made of heavy gauge cast aluminum and use 18W to 150W HID lamps.
RELATED ARTICLE: OUTDOOR SECURITY LIGHTING STUDY
A Manhattan security lighting installation, part of a utility program, works for building tenants and pedestrians.
A two-building site located in an industrial area on the West Side of Manhattan uses 60 1000W metal-halide (M-H) fixtures to provide security lighting. The architect for the project, Jeff J. Vandeberg, developed this lighting design in response to the needs of the tenants (among them, a restaurant and bar) and in conjunction with the local electric utility, Consolidated Edison of New York. The utility offers a rebate for increased night and security lighting as long as energy-efficient fixtures are used. This part of the project, which is called the "Around the Clock" marketing program, increases lighting loads at off-peak times and thereby improves the utility's load factor. The goal of the project is to provide adequate visibility in order for pedestrians to see and participate in a range of activities, including reading signage and locating businesses. Two major concerns at the outset were the prevention of vandalism and unnecessary light trespass. The installing contractor was Lightech, Inc., Stamford, Conn.
What's it about?
Effectively, the project overlights the area by about six times, compared with what would be considered proper security lighting design. But there is a particular need: The building tenants wanted to "take back the neighborhood" and to have undesirable nighttime activity migrate away from the area. A specific need of the architect was to provide true color rendition; thus, the high-pressure-sodium (HPS) light source was eliminated in preference to the color output of the M-H lamp.
The fixture selected [ILLUSTRATION FOR FIGURE 1 OMITTED] answered the engineering needs of the project: It's a parking/roadway luminaire constructed of seamless die cast aluminum. Located within a totally enclosed, 22-in.-long lamp housing, the reflector provides an IES (Illuminating Engineering Society) Type II candlepower distribution. An optional .19-in (5mm) polycarbonate vandal shield is specified to deter objects that might break the fixture lens. The open-end design allows self-cleaning as well as ventilation for cooling both lens and fixture. Rated at 240V, the high power factor, constant wattage autotransformer ballast provides excellent voltage regulation for lamp operation.
A bi-metal type photocell control provides automatic dusk-to-dawn operation. It is mounted through a knockout provided on the wiring compartment cover of the adjustable filter. An advantage of this decentralized control is that if a single photocell fails, it will not affect operation of any of the other fixtures.
Special bracket mounting hardware secures the fixture to the structural wall of the building.
Site details
The larger of the two structures is actually a grouping of interconnected buildings occupying an entire city block; the second structure is an almost square-shaped building abutting the riverfront. On the smaller structure, 20 fixtures are mounted 25 ft above the sidewalk on the second floor spandrel centerlines. Two fixtures are mounted on an enclosed elevated pedestrian bridge connecting the two sites.
At the full city block site, 38 fixtures are mounted on pilaster centerlines. They start 25 ft above the sidewalk at the 9th Ave. entry and continue around the building, level with this elevation. Nine of the fixtures located on the northeast side have an external glare shield that attaches to the floodlight lens frame with four stainless steel crews, as seen in Fig. 1. Use of this accessory eliminates any spill light.
Candlepower distribution from the rear of the fixture is reflected off the building wall and redirected down to street level, thus illuminating the surrounding area and minimizing the possibility of deep down shadows in doorways, etc.
Lighting Research Center study
This lighting installation was included in a lighting research project conducted by the Lighting Research Center (LRC) in cooperation with the local utility, Con Edison. Fifty observers made a subjective evaluation of the security lighting at this location, along with 26 other sites in New York City and Albany, N.Y. The LRC, which is headquartered at Rensselaer Polytechnic Institute in Troy, N.Y., included this research in a book it is publishing on security lighting.
The LRC is a recognized information resource, publishing a variety of materials, such as the National Lighting Product Information Program.
