GE ERX1 Whitepapers

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 signicant

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 simplied 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 Reector

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: Simplied block diagram of outdoor LED system

Figure 2: Open view of roadway fixture

Reliability denitions

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 specic
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 denes 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 dierent 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.