LED technology lights the work-zone

By: David L. Evans

It is imperative that roadway work-zone light-emitting safety devices be dependable. To ensure the safety of both roadway work zone personnel and the motoring public, light-emitting safety devices must be capable of operating for long periods of time without the need for on-going short term maintenance.

The constant replacement of burned out incandescent lamps, the need for replacement of low batteries, and continual filling of fuel tanks associated with light-emitting devices in roadway work-zone areas adds a significant burden to the efforts of roadway engineers and contractors to provide for safety at all times. All of this extra burden dictates the need for a durable low power replacement light source. This new light source must equal or exceed the light emitting performance provided by incandescent lamps.

LED lamp devices have replaced incandescent lamps in roadway work zone light emitting safety devices. The long term reliability of a semiconductor light source has eliminated the need for constant lamp replacement. The light output performance has equaled that of incandescent lamps, and with the introduction of new generation LEDs in 1996 will exceed the light output performance possible with incandescent lamps. The low power consumption of LEDs allows the use of solar energy as the primary power source for driving LED light emitting work zone safety devices, reducing operating costs.

The use of solar power to drive LEDs makes light emitting safety devices compatible with the environment, and the low power consumption reduces the need for frequent battery replacement. Batteries are considered to be hazardous waste requiring special disposal procedures. Not having to frequently replace batteries translates into a reduced need for costly environmentally safe landfill disposal areas.

Similarly, eliminating the need for diesel-powered electrical generators reduces air pollution from noxious exhaust fumes in work zone areas, and personnel are not exposed to the dangers of handling a volatile fuel. This translates into a distinct financial saving to roadway work zone contractors in the form of reduced premiums for insurance and workman's compensation.

With the advent of aluminum indium gallium phosphide (AlInGaP) LED technology, LED lamp devices now challenge incandescent lamps in light output performance.

Introduced to the market in 1992, this technology has proven to be superior in overall performance to incandescent lamps and has become the light source of choice in the work zone safety lighting industry. Producing an amber color with a dominant wavelength of 590 nanometers (nm), and a light output efficiency equivalent to that of typical amber filtered incandescent lamps at ~10 to 15 lumens/watt, AlInGaP LEDs meet the light output requirements for variable message signs, arrowboards and barricade warning lights-both flashing and steady burn.

With continued development of the AlInGaP LED technology, it is anticipated that significant gains in light output efficiency will be achieved over the next few years. Efficiencies should exceed 20 lumens/watt in the amber color range.

With these improved LED light output efficiencies, roadway work zone light emitting safety devices will be able to operate at less power with improved performance and better visibility to motorists.

The saturated (pure) colors produced by AlInGaP LEDs do match ITE color requirements in the red, Portland Orange and amber (yellow) regions. InGaN has the capability of producing blue, matching that used on police-car lights, and blue-green, matching that required for traffic signals.

LED devices have many advantages. They provide a high degree of dependability and operate over long periods of time without the need for replacement. They can withstand high levels of mechanical shock and vibration and are not effected by wind, rain, snow, sand, or sunlight. The temperature range in which they can operate is from -40 C to 71 C.

When used in the T-13¦4 lamp package configuration LEDs may be grouped into a cluster to form a particular pixel size, providing versatility for design.

This package also increases an LED lamp's long term dependability, improving its resilience to moisture, shock and heat. With this package, it can operate at temperatures of 130 C, relieving concerns for dependable operation in high ambient temperature locations, such as the U.S. southwest.

A major feature, which allows the incorporation of LEDs into work zone light emitting devices is the significant reduction of electrical power consumption over that which is required with the use of incandescent lamps.

It has long been known that operating incandescent illuminated devices off solar power could not achieve either sufficient light output or long operation. Also, only limited operational on-time can be achieved from battery powered incandescent illuminated devices.

Just the opposite is true in both cases with the use of LED devices. This technology has the potential for many uses in work-zone safety devices, some of which are now on the market.

Two of the most popular uses of LED technology, which are already available, are changeable message signs (CMS) and arrowboards. Individual LED pixel elements in trailer-mounted portable CMSs, composed of between four and 12 LED lamps, consume between 120 mW and 480 mW, depending upon LED lamp count and drive current per pixel.

This low solar-power consumption is such that these large displays, and portable arrowboards, are now solar powered with battery back-up for nighttime operation. These CMSs and arrowboards can operate from only battery power, in the absence of sunlight, for approximately 30 days. The need for diesel generators is eliminated.

LEDs also can be used in battery, flasher roadway-marker-cones; however, LED-equipped cones are not yet available in the U.S. These cones would allow motorists to see the flashing amber LED lights on top of the cones at night, helping them to recognize the marked-off roadway section before their headlights illuminate the cones retroreflective stripping.

Portable all-LED traffic signals, as with other LED-illuminated work zone safety devices, will operate off solar and battery power. Not having to rely on local AC power, these signal units can be installed at remote roadway locations, and at critical disaster evacuation roadway locations. Multiple LED traffic signals at various locations can be controlled from a master unit via a microwave data link. Three-color LED traffic signals should come on the market by 1998.
Barricade warning lights and workman traffic-control flags also can be equipped with solar cells, batteries and LEDs to enhance their operating capabilities.

LED technology continues to improve. New precision-optical-performance AlInGaP LED lamp devices are coming on the market with enhancements specifically designed to meet the needs of the work-zone safety market. Some of these enhacemnts include new encapsulating epoxy and newly designed internal optics.

The new optical grade encapsulating epoxy has superior resilience to moisture absorption, contains uv-a and uv-b inhibitors, and has a high glass transition temperature (the temperature where the epoxy converts from a lattice structure to an amorphous structure resulting in high stresses on lamp internal parts).

The internal optics of the new precision-optical-performance LED lamps have been designed to ensure a precise optical-spatial radiation pattern. The optical axis of each LED lamp with the new optics, is closely aligned with the mechanical axis of the lamp package. The shape of its radiation pattern viewing angles are designed for specific application within the work-zone safety market.

The latest LED technology is indium gallium nitride (InGaN), grown on a sapphire substrate. This technology produces colors in the blue region and can produce the traffic signal blue-green color, 498 nm to 508 nm. The light output efficiency is 10 to 12 lumens/watt, thus offering the best possibility for replacing incandescent lamps in green traffic signals.

Some technical development is still necessary with InGaN LED technology to ensure anticipated dependability; however, InGaN in traffic- signal blue-green is currently available in low-volume, high-cost, engineering-sample quantities. It is anticipated to be available in high-volume quantities at reasonable cost in the foreseeable future.