Is Your Tower a Good Neighbor?

With the rapid expansion of the communications and broadcast industries, towers are springing up like new neighbors, ever closer to the back door. Although this is a positive by-product for those in the tower and obstruction lighting industries, residents who are located in the vicinity of towers have been less enthusiastic. Concern stems from the possibility of light pollution caused by the aviation warning beacons. Now, thanks to equipment specification changes by the Federal Aviation Administration (FAA), as well as response from several manufacturers, this 1-time nuisance doesn't have to be an issue.

In October 1995, the FAA published new standards in Aviation Circular 150/5345-43E -- Specification for Obstruction Lighting Equipment. This revised specification, which became effective during the first half of 1996, restricts light output from beacons at certain vertical angles below the horizon, which results in a tightly controlled lower beam edge. By restricting the amount of stray light measured at 10 degrees below the horizon, the FAA ensures that neighborhood residents will detect little, if any, light from the tower. Limited to 3% of the peak intensity at the same radial, the maximum allowable nighttime light intensity at -10 degrees below horizontal is only 75 candelas, approximately that of a 60W light bulb. This standard is stricter than the previous FAA standard, which did not specify the amount of stray light that could be directed toward the ground.

Think of the new specification as the beam of light visible from a flashlight. To someone looking directly into the beam of light, such as a pilot flying toward a tower, the light is extremely bright. However, to someone standing to the side or below the flashlight's beam, such as nearby residents, the light is barely visible. This linear property of light ensures aviation safety without dis-turbing residents near the tower.

THE FRESNEL LENS Companies that manufacture medium-intensity strobe lights have found that the Fresnel lens design is ideal for meeting and exceeding the FAA's stray light restrictions. First used in 1822 in marine lighthouses, the time-proven design of this lens captures light and forms it into a tight beam. Based on the laws of refraction, the Fresnel lens gathers light rays emitted from a source located at the primary focus of the lens. These light rays then are refracted (or bent) by the lens and emerge as essentially parallel or slightly divergent rays.

Improvements in design and materials have capitalized on the superior optical capabilities of the Fresnel lens, while virtually eliminating its once inherent limitations. Traditional Fresnel lenses, made of molded glass, cannot be polished as ordinary glass refractive lenses can. The result is a surface area riddled with tiny imperfections. These optical shortcomings emerge as scattered light, or light that falls outside the directed beam. Today, many Fresnel lenses are made of molded plastic, creating an almost flawless surface with no scatter light.

As with any other lens, the total beam spread from a Fresnel lens depends on the size of the source in relation to the focal length of the lens. Smaller sources and longer focal lengths produce more compact beams. There are practical limitations to minimizing the geometry and dimensions of a flashtube that is suitable for obstruction light applications. A totally engineered optical system, where the light source (flashtube) and Fresnel lens have been designed together toperform a specific lighting function, will offer the best results. Beacons now are available with focal lengths extended to 400mm (300 mm was the former limit). While producing 2,000 candelas at the beam peak, these beacons only direct about 25 candelas at -10 degrees, which is about one-third of the amount the FAA allows.

BEING NEIGHBORLY Several years ago, an alternate medium-intensity beacon was in-troduced as a possible replacement to the Fresnel lens. Originally touted as the solution to ground scatter, this parabolic beacon has proved ineffective in controlling the light directed toward the ground. It is composed of three identical linear parabolic sections confined to a diameter comparable to that of a 400mm Fresnel lens. In order to fit within the compact size of the flashhead, the parabolic sections must be shallow.

Unfortunately, with shallow optics, a large percentage of the total light is not collected, and therefore does not become part of the desired beam. A large portion of the uncollected light from the medium-intensity parabolic is directed downward, where it impinges unhampered on the ground near the tower and on anyone in the vicinity. In other words, there goes the neighborhood.

There are other drawbacks linked to the inefficient design, including shortened tube life and increased energy consumption. Because most of the total light is wasted (uncollected) by shallow optics, the tube must work harder to compensate.

RESIDENTIAL PREFERENCE Today's Fresnel beacons offer a dual-lighting function. These all-in-one beacons are designed to provide white light during the day and red light at night. Although the FAA approves of white light for both day and night visibility, nearby residents generally prefer red light. For years the red incandescent beacon was the only option for nighttime obstruction warning, and people became accustomed to seeing red lights in the night sky. Because early white beacons produced a substantial amount of light directed at the ground, white light usually is perceived as more intrusive than red light. Even though today's white Fresnel beacons are precise and actually appear dim from the ground, there is still a bias for red lighting.

The medium-intensity dual-strobe light is the ideal solution for residential or more densely populated areas. This beacon contains a single 400mm lens that houses a white flashtube. An automated red filter surrounds the tube for nighttime operation, then lowers for daytime white lighting. Tested and certified to meet -43E standards, this beacon produces almost no discernible ground-directed light. As a replacement for the traditional red incandescent beacon, which is two large filament bulbs inside a red glass lens, the dual strobe offers many advantages including reduced ground illumination.

Tower owners also benefit from the characteristics of this option. Most importantly, it is more energy efficient. Compared to the dual strobe's average 110W of power consumption, the incandescent beacons use approximately 1,240W. The incande-scents also are notorious for consuming bulbs and require replacements approximately every six to nine months. Manufacturers usually recommend that you replace flashtubes every three years as part of a routine maintenance program.

Because the dual beacon provides white lighting for daytime visibility, you won't have to paint the tower with glaringly bright bands of aviation orange and white. This eliminates several problems. Residents usually consider these color bands unattractive and complain that they distract from the surrounding landscapes. In addition, you have to reapply this paint on a regular basis to maintain its conspicuousness. According to tower owners, this multiplies costs and leaves paint splatters on everything near the tower.

ENVIRONMENTALLY FRIENDLY Generating electricity produces pollutants such as carbon dioxide, sulfur dioxide and nitrogen oxides. As a result, the U.S. Environmental Protection Agency (EPA) urges businesses to upgrade to more energy-efficient services, including lighting (the Green Lights Program). Through overall electricity reduction and upgraded lighting, you will save money, help curb acid rain and smog, and slow the greenhouse effect. The EPA estimates that if green lights are fully implemented in the United States, the reduction of environmental pollutants would be equivalent to 5% of U.S. utility emissions.

The Fresnel dual-strobe beacon operates with less than one-fifth the power required for red incandescent operation. This equals annual savings of approximately 4,467kWh of power per beacon. On a typical 400-foot tower, which normally requires three beacons, you save 13,401kWh per year. Dual-strobe beacons that use parabolic reflectors rather than Fresnel optics also are less energy efficient, requiring approximately 2,234kWh more per year on a typical triple- beacon tower. Multiplied by the tens of thousands of towers that require obstruction lighting, there is significant potential to save energy.

LESSONS Rest assured the FAA is concerned with the environmental impact of tower lighting, as well as aviation safety. Combined with technological advancements in obstruction lighting, today's towers have come a long way in aesthetic and environmental acceptability.

The restriction of nuisance light has resulted in lighting that is less intrusive than typical street lights. And even as the number of towers proliferates, improvements in energy efficiency allow the opportunity to decrease environmental pollutants through vast reductions in energy consumption.

In the long run, careful selection of obstruction lighting will help towers move in more peacefully and unobtrusively, so that you can be the best neighbor possible.