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LED外文文献精
LED LUMINAIRE LIFETIME:
Recommendations for Testing and Reporting
Solid State Lighting
Product Quality Initiative
FIRST EDITION
MAY 2010
Next Generation Lighting Industry Alliance
with the
U. S. Department of Energy
INTRODUCTION
UNDERSTANDING SSL LUMINAIRE LIFETIME
Surprisingly to many, the true reliability and lifetime of light‐emitting diode （LED lighting systems is currently unknown. Even worse, lumen maintenance values of LED devices are widely used as a proxy for the lifetime of an LED lighting system, which is misleading since light degradation or lumen maintenance is but one component of the reliability of a luminaire. For many manufacturers this approach cannot simply be ascribed to overly ambitious marketing efforts, but rather to dependence on anecdotal numbers in the absence of real data. In addition, we can impute simple ignorance in taking specifications at face value which may or may not live up to claims.
It isn't just about the LED. Good LEDs can be incorporated into poorly engineered products and turn the Methuselah of lighting into the exponent of “live fast, die young.” The promise of LED lifetime is often presented in terms of hours and years but with little background data to support anything beyond vacuous promises. The statement of 100,000 hours of LED luminaire lifetime has given way to the realization that there is little consistency, very little published data, and few hard facts around so‐called luminaire lifetime numbers. The situation is better at the LED package level, where reputable manufacturers have thousands of hours of data under varying conditions. But this is not enough.
To manufacturers and specifiers in the solid‐state lighting （SSL community, the dawning realization is that we need to work together towards understanding the issues surrounding true lifetime and reliability. We need to begin by cataloguing such failures and developing good models for underlying failure mechanisms. This process of understanding and explanation is very common in technological progress. Steam engines existed long before deep understanding of thermodynamic processes. With LEDs, we have a substantial head‐start on the underlying physics and many years of experience in both lighting and semi‐conductors as well as reliability of related products.
There is no reason not to begin this journey and every reason to start. We will figure this out, find reliability methods and metrics, and learn the underlying root causes of failure. But without data, experiments and models, it is all conjecture. We need a program to drive to reliability metrics. WHAT THIS GUIDE IS AND IS NOT
This guide is a set of recommendations for reporting and demonstrating reliability in terms of luminaire product lifetime. Initially, we sought to provide guidance for the Lighting FactsCM program, which gives users, retailers, and manufacturers a common short‐form reporting mechanism to improve the quality of solid‐state lighting products on the market. The Lighting Facts label provides a summary of key performance criteria but does not include lifetime, for which there have been numerous requests. Ideally, it would be the addition of a single number, e.g., “eight years.” But SSL product lifetime needs to be considered a combination of conventional failure mechanisms as well as a gradual decline of light output over time. Describing it is not simple; demonstrating it is difficult and expensive. So, we have come to appreciate the importance of gathering the full strength of the SSL community to address this critical issue. This guide is purposefully entitled “First Edition.” In developing these initial guidelines, the committee
LED Luminaire Lifetime Recommendations, May 2010 Page 2
recognizes that there remain many incomplete areas of knowledge, and we intend to pursue and improve these recommendations over time.
These recommendations have been developed by a working group under the U.S. Department of Energy Solid State Lighting program. This group is under the guidance of the SSL Quality Advocates oversight committee, a joint body of DOE and the Next Generation Lighting Industry Alliance
（NGLIA. The reliability and lifetime working group is composed of members of the NGLIA as well as other experts in reliability, lighting, and LED technology. As such, this guide is not an accepted international standard. Rather, it is meant to provide standards bodies with recommendations for their work in supporting the needs of the SSL community. These standards organizations will ultimately determine the details of the methods to measure and report the reliability of SSL luminaire products.
This document covers only luminaire lifetime—i.e., change over time in normal operation—and not initial performance criteria or product consistency. Initial performance criteria for LED luminaires have been separately discussed in the December 2008 publication Reporting LED Luminaire Product Performance, found on the DOE SSL Web site at . TYPES OF FAILURES ENCOUNTERED
An LED luminaire is in many ways more
complex than a traditional lighting fixture. It
is an electromechanical system that includes,
in addition to the essential light‐emitting
source, provisions for heat transfer, electrical
control, optical conditioning, mechanical
support, and protection, as well as aesthetic
design elements. Because the LEDs
themselves are expected to have long life, all
of these other components, adhesives, and
other materials must be equally long‐lived,
or, to the extent they are not, they will limit
LED Luminaire Lifetime Recommendations, May 2010Page 3 Figure 1:
Total system or luminaire reliability is the product of all of the individual reliability considerations as follows:
RLuminaire = RLEDs * ROptical * RPCB * RFinish * RMechanical * RThermal RHousing * RGaskets * RSealants * RElectrical * RDriver * RManufacturing. SOURCE:
PHILIPS HADCO the system lifetime.
While LEDs do not radiate heat, with current
products half or more of the input energy
may be converted to heat that must be
conducted away from the diodes. This
situation requires a reliable, heat‐conducting
assembly, be it mechanical or adhesive, in
addition to a heat sink component or means
for further conduction. For proper operation,
the power supply and electronics must
provide a well‐controlled DC drive current
and possibly other control features, and must
not fail for the life of the product. Any optical
components must be able to withstand years of exposure to intense light and possibly heat without yellowing, cracking, or other significant degradation. Reflecting materials need to stay in place and maintain their optical efficiencies.
Even if the design itself has addressed all of these issues, questions of proper manufacturing remain. Was the epoxy properly mixed?
Was an essential heat‐transmitting paste omitted?
Were the wire bonds properly made?
Any of the failure mechanisms inherent in electronic assemblies, and many others, may apply to an LED luminaire. Figure 1, above, is an attempt to visualize this larger scope of SSL reliability.
For the best lifetime in a well‐designed and properly assembled luminaire, the principal failure mechanism should be lumen depreciation—a gradual reduction of light output from the packaged LED over time. But other mechanisms could come into play. One LED vendor has reported that the principal causes for customer complaints involve either the use of chemicals in the luminaire that are incompatible with the LED or the driver overstressing the LED. These are design issues, but equally important are early failures of other components or subsystems or manufacturing defects. These additional failure mechanisms will always persist to some degree and will usually lead to catastrophic failure, but they may sometimes simply accelerate lumen depreciation. An important message of this guide is that these mechanisms must be accounted for in describing product life. THE ROLE OF WARRANTIES
Given the early stage of LED lighting technology, it is difficult to accurately predict product lifetime. Warranties provide a means for the potential buyer to reduce risk. While this document provides some guidance on what might be included in a warranty, it is ultimately the manufacturer’s decision on how best to provide this protection and how much risk to take in doing so. The warranty may logically be shorter than the claimed lifetime as defined below. As there will be a distribution of times of failures for any product, the degree of risk on the part of the manufacturer depends on the level of confidence in the projected lifetime and the amount of variation in that distribution.
LUMEN LIFETIME
RECOMMENDED DEFINITION OF LIFETIME
For conventional technologies, the “rated average lamp life” is the point at which half the lamps cease to emit light. All sources lose light output over time, but generally not more that 15–20 percent over the rated lamp life as defined by complete, lights‐out failure. A well‐designed LED package, however, would typically have a very long‐rated life, as conventionally defined, but, because it is so long, would also have more lumen depreciation over that life than conventional technologies. Accordingly, we need a new approach.
Whatever the stated lifetime of any lighting product, it is a statistical measure of the performance of a given design. For an individual LED package, lifetime has typically been considered to be the hours of operation at which the light output has fallen to 70 percent of its original value （“L70”.
LED Luminaire Lifetime Recommendations, May 2010 Page 4
Lifetime is typically reported as the median time to failure of a population of diodes under normal operating conditions, called “B50.” In other words, after this per
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