A well-designed exterior lighting scheme is a crucial part of any facility's security system. In addition to helping people see better at night, the right kinds of fixtures, properly placed, can discourage criminal activity. The differences between systems that work well and those that don't are quite subtle. Some lighting systems that produce industry-recommended light levels will not work well when put into use. It is important that building designers as well as those who maintain existing structures understand where to place luminaires and what kinds to specify.
The conventional wisdom about outdoor security lighting is simple: Make an area bright and criminal activity will be discouraged. Unfortunately, things are rarely this simple. In some situations an area may be made less secure if it is lit. Other factors to consider include how well a light source renders color, what kind of fixtures are available, and how it will be turned on and off.
The reasons why people feel safer in well-lit spaces than in dark ones are largely psychological. People feel more comfortable when they can see well enough to avoid hazards. Intelligently located and aimed light fixtures also eliminate shadows where a perpetrator can hide. Light that renders color accurately allows people to feel more comfortable because they can see more clearly. If a crime is committed, color rendering may enable a witness to identify a subject.
What makes lighting a deterrent is fairly obvious: Those who are intent on committing a crime generally don't want to be seen, identified or caught. A light strong enough to allow a witness or a surveillance camera to view and describe a criminal is a deterrent.
In communities where crime is accepted as a way of life, the amount of deterrence that a security lighting system can offer is limited. For lighting to work, criminals must perceive that their activities are being monitored by someone who will report them to the authorities, who in turn will act promptly. If criminals know they are likely to be ignored, good lighting actually may help, rather than hinder, their actions.
If video surveillance is contemplated as part of a new security system, consultations between the lighting designer and the video consultant are crucial. Selection of the wrong light source, inappropriate lighting levels and poor fixture placement can render an expensive surveillance system useless.
|BAD COLOR Security lighting that renders hues inaccurately makes it difficult to describe suspects. (Photo © Elliot Johnson)|
LIGHT SOURCES AND FIXTURES
Generally, there are three types of lamps that can be used in outdoor lighting fixtures: incandescent, fluorescent and high-intensity discharge types. Each has its own positive characteristics and drawbacks.
Incandescent lamps are commonly used for outdoor lighting because they are widely available, cheap, easy to install, and have excellent color-rendering properties.
Heavy glass incandescent lamps known as PAR lamps are the most popular. These have their own built-in reflectors, which give them well-defined beam-spreads. They can be installed in simple lamp-holders and aimed in almost any direction.
Double-ended quartz are the runners-up in popularity. These lamps are about the size and shape of a pencil and are most frequently mounted in die-cast housings whose parabolic shape is used to focus the lamp's light output.
PAR and quartz lamps share some serious shortcomings. First, their life-expectancy is short. Quartz lasts about 3,000 hours, and a PAR will last about 1,000. These lamps require frequent maintenance, and they are the least energy-efficient of all light sources. A PAR lamp may only produce about 11 lumens per watt (lm/W), making it difficult to provide bright coverage over a large area with a few fixtures.
Quartz produces about 18 lm/W, better than PAR, but these lamps typically consume 1,000 watts. They produce far more light than is needed and waste energy.
Compact fluorescent lamps are up to five times more energy-efficient than incandescents and at 100 lm/W, double-ended tubular fluorescents are even better. Both have good color rendering, but have drawbacks when they're used outdoors. A fluorescent lamp's output is directly proportional to its size. This does not matter so much for lighting store awnings, but it is impractical when lighting large areas like parking lots. Many lamps, and large fixtures, would be required. Also, the efficiency of a fluorescent lamp is related to outside temperature. If the weather is either too hot or too cold, a lamp's output will decline. If exterior fluorescent must be used, special lamps made for cold environments can be specified.
|DANGEROUS GLARE Light aimed into the eyes creates unsafe conditions. (Photo © Elliot Johnson)|
High-intensity discharge (HID) lamps are the best choice for security lighting for several reasons. They are compact in size, and can be up to 10 times more energy-efficient than incandescent PARs. The initial cost is higher than other types, but energy savings and low maintenance offset that drawback. Manufacturers make an almost endless variety of fixtures.
Metal halide's superior color rendering makes it the best HID source for security lighting. Still, high-pressure sodium (HPS) is more popular despite its orange-yellow color rendering that is inferior to metal halide's. That's because HPS can last up to 24,000 hours, a few thousand hours longer than a typical metal-halide lamp, and at around 110 lm/W, is slightly more efficient.
High-pressure sodium lamps also are a little cheaper than metal-halide lamps. HPS is typically found lighting streets and parking lots, and can be used for security lighting applications where color rendering is not critical.
Low-pressure sodium lamps produce 200 lumens per watt and are the most efficient sources available. However, the monochromatic, yellowish light they produce is completely incapable of rendering color, making them useless except for industrial applications.
Sodium lamps are sometimes selected for difficult-to-service locations because they can last up to six years. But their color rendering is so poor that they are not recommended for security purposes.
Mercury-vapor lamps also have poor efficiency. They render neutral colors as greens, have high mercury content and are not recommended for security lighting.
DISTRIBUTION IS CRITICAL
Almost all lamps are housed inside some kind of enclosure, and all but the very worst of these incorporate some kind of optical system comprising a reflector and lens. These components help distribute light where it is needed.
Light that goes where it is not wanted, such as into the night sky or onto a neighbor's property, is considered "light pollution." This is a sure sign that energy is being wasted. High-quality HID luminaires that are engineered to eliminate excessive spill light provide "sharp-cutoff" optics.
The light output characteristics of an optical assembly are summarized in charts called photometric reports. Architects and engineers use these when designing a lighting system to predict how each fixture will distribute its light in both horizontal and vertical directions. Photometrics allow designers to calculate how many fixtures will be needed, and how to optimally locate them.
A lighting designer will also consider whether exterior lighting should operate from dusk until dawn. Energy management systems can be programmed to shut off or lower lighting levels after a building has closed for the evening. These typically feature astronomical time clocks that enable a facilities manager to preprogram lights to turn on and off at the appropriate time each day. This is commonly based upon when the sun rises and sets. Systems use photocontrols and motion detectors. The main drawback is that the controls are often prone to false triggering, which wastes energy.
Emergency Lighting Systems
One of the most important lessons learned when the World Trade Center was bombed in 1993 was that emergency lighting systems do fail. Dark stairwells caused confusion and slowed the evacuation. In the aftermath, the Port Authority of New York and New Jersey installed redundant power supplies for its emergency lighting systems. On Sept. 11, 2001, the emergency lights in the towers' exit stairs had two sources of power. Survivors say that emergency lighting systems did operate and helped people escape.
The effectiveness of an emergency lighting system is dependent upon two factors: its power source and what kinds of fixtures are chosen for it. Systems that are powered by emergency generators are usually wired to power a few selected fixtures at full light output along an egress route. These stay lit as long as the generators' fuel supplies last, although the dangers of storing gasoline and diesel fuel are problematic. Central inverter systems, which comprise large banks of batteries that power the emergency lighting throughout a building, are used in a similar fashion. But generators and central inverters are useless if their power lines are severed. They can be backed up with small emergency lighting units with their own individual battery packs. To conserve power, these put out a minimal amount of light for just a few hours.