z Planar Irradiance method tests all relevant light specifications
z Easy to read reporting shows PAR, Photon Flux, Irradiance, Power
z Understand the energy consumption of your product
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The IES reliability standard development took
an approach similar to the IEC NP. The first
draft of IES LM-87 (to be changed to a TM document) focuses on the LED package level reliability or robustness tests that are under the
over-stress test conditions. These over-stress
test methods are provided in the JEDEC standards. The IES document defines failure, failure criteria, and LED package-specific test
conditions. It identifies six over-stress tests
that reflect general lighting applications:
HTOL, TMCL, resistance to soldering heat
(RSH), WHTOL, electrostatic discharge, and
continuous switching cycles until failure tests.
This group of tests is based on current industry best practices among LED manufacturers.
Although the results may not be directly
interpreted as a quantitative measure of
LED product reliability, they can indicate the
robustness of the LED package. In turn, they
help manufacturers to set and confirm product performance specifications. Parallel to
the LED package-level reliability test development, the IES also proposed developing a document for LED lamp and luminaire level tests.
LRC ASSIST project
Choosing the critical variables to test LED
lighting product reliability is essential for
establishing standardized test methods and
a possible projection method using test data.
Under an industry consortium a few years ago,
the Rensselaer Polytechnic Institute Lighting
Research Center ASSIST (Alliance for Solid-State Illumination and Technologies) program
initiated a project for an LED lamp reliability study. The ASSIST project’s objective was
to develop a test method that may help predict LED lamp life in any application by testing on/off power cycling with sufficient dwell
time, and considering both catastrophic and
parametric failure types.
Study results show for tested LED A-lamps
both catastrophic and parametric failure
types exist. On/off cycling encourages catastrophic failure, and maximum operating
temperature influences the lumen depreciation rate and parametric failure time. The
study indicates that LED lamp life is negatively affected by on/off switching; most of
the lamps failed catastrophically, considerably ahead of the LED light output reaching
the 70% mark or L70 level relative to the initial value.
The ASSIST project test method used setups based on on/off cycle and maximum operating temperature. Then the estimated average temperature difference between LED
lamp and room (ambient) temperatures can
be obtained. Using the established on/off cycle
and average temperature differential, the test
will continuously run to observe the failures of
LED lamps, both catastrophic and parametric.
So far, the ASSIST project test examples show
that the lifetime of LED lamps depends on the
application environment and use pattern.
Manufacturers have desired shorter test
times to determine product lifetimes. According to ASSIST, what will be needed to implement a test for LED lamps is to identify a suitable average temperature differential and
dwell time so the LED lamps fail due to failure modes usually present during applications. Overstressing will introduce additional
failure modes that may not be present in typical applications and could lead to underestimating LED lamp lifetimes.
For example, ASSIST proposed a test pro-
cedure such that the time-averaged tempera-
ture is in the range of 75–85°C, and the total
time for testing can be within 3000 hours.
This proposed hypothesis requires verifica-
tion by testing several LED lamps in the mar-
ketplace. When sufficient data (time to fails
with respect to the setup parameters) is col-
lected, the predictive model or projection can
be established. The results of this study show
a shorter timed test procedure can be devel-
oped to accurately predict LED lamp life in
any application by knowing the LED tempera-
ture and the switching cycle.
All these reliability standards development activities demonstrate the importance
of LED lighting reliability measurement. They
also reflect the challenges and degree of difficulty involved in writing them. And writing a
standard is not the goal. How the standard can
be effectively used and assist with LED lighting commercialization is the purpose; thus,
once developed, reliability standards must be
widely accepted and practiced by the industry.
Stay tuned for an update.