LED technology lights the work-zone

David L. Evans / December 28, 2000

The need for a cost effective, power efficient, and dependable light source
has prompted the implementation of light-emitting diode (LED) technology
in roadway work-zone light-emitting safety devices. In years preceding 1992,
most work-zone light-emitting safety devices used incandescent lamps. Then
during the years 1992 through 1994, the work zone safety industry made a
conversion from incandescent lighting to LED technology as the light source
of choice.

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

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.

About the Author

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