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Advancements in LED Technology
Advancements in LED (Light Emitting Diode) technology have made this source an attractive alternative to traditional light
sources in a variety of applications. One such application is outdoor xtures. This segment includes roadway and decorative
street lighting, as well as general area lighting traditionally occupied by discharge light sources. In comparison to historic
technologies (such as high pressure sodium and metal halide), today’s outdoor LED lighting xtures can provide signicant
energy savings over their useful life.
Another advantage is the long lifetime of today’s LED fixtures.
Overall system reliability is comprised of several key subsystems
and their components: the electrical subsystem, the optical
subsystem and the outer enclosure. A simplied block diagram
is shown below as an example to illustrate the relationship
between these subsystems and their corresponding components.
Mechanical Enclosure & Finish Subsystem
Electrical Subsystem
Driver
TVSS Reector
Wiring/Interconnects
Seals/Gaskets
Fuses Lens
Control System
Optical Subsystem
PCB LEDs
This paper addresses the reliability of GE’s outdoor LED lighting
systems through examples of rigorous testing and reliability
modeling, resulting from GE’s deep technical experience as one
of the world’s largest LED systems companies.
Mounting Subsystem
Figure 1: Simplied block diagram of outdoor LED system
Figure 2: Open view of roadway fixture
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Reliability denitions
The reliability bathtub curve is often used to depict the expected failure rate of a family
of products over time. This model is comprised of three segments: infant mortality, useful
life and wearout, as illustrated in Figure 3.
Product “useful life”
Approximate constant
failure rate
Product “wearout”
Increasing failure rate
“infant mortality”
Failure Rate
Figure 3: Example of reliability bathtub curve
Decreasing failure rate
Time
Infant mortality is an initial period of failures usually resulting from manufacturing
defects or quality excursions, and has a decreasing failure rate over a relatively short
time frame. Product useful life is shown as the bottom portion of the bathtub curve. It is a
period of random failures with a nearly constant failure rate. The weakest component
in any system will determine the duration of this portion of the curve. At the end of
useful life, wearout failure modes, such as fatigue and material depletion, will cause
the failure rate to increase with time. This nal segment of the curve is called wearout.
Reliability during useful life is often a focus when considering products for a specic
application. In the case of GE outdoor LED systems, an exponential distribution may
be applied to model system reliability. Reliability values are often requested in the form
of an MTBF (Mean Time Between Failures) value. MTBF is often misunderstood since it
is expressed as a time value, but more accurately denes the failure rate during the
useful life of the product. The relationship is illustrated below, where λ is the failure
rate with units of hrs-1.
It is important to note that this failure rate is valid only during the useful life portion of the
bathtub curve where the failure rate is relatively constant. When the failure rate begins to
increase, a product has entered wearout and a dierent mathematical model is needed
to represent this behavior. For this reason, it is important to understand when wearout
failure modes begin. Both component-level and full-system testing are utilized by GE Lighting
engineers to understand and accurately model the reliability of outdoor LED systems.