November's Topic: LED Lighting Life Prediction (2) By Jianzhong Jiao, Ph.D., Director of Regulations & Emerging Technologies • Osram Opto Semiconductors, Inc.
The manufacturers of LED lighting products often claim the long life of LEDs as one of the major benefits. Besides energy savings, long life of LED lighting products can also result in noticeable maintenance cost reductions. The term “long life” for general lighting applications refers to 25,000, 50,000 hours and sometimes longer, operation time. As a result, products properly installed in a given application may last as long as five to 10 years without needing replacement. How can the LED lighting manufactures claim such long life product without conducting years of reliability and durability tests? Very often they rely on the LED manufacturers’ claims for the life of the LED package.
As discussed in a previous column, the LED lighting standards committees in the industry have struggled to establish an acceptable standard method to be used by the LED integrators to project final product life. The three major challenges to developing a standard prediction method are: 1) variation of LED chip and packaging technologies that lead to significantly different lumen degradation behaviors especially within the first few thousands hours; 2) short testing duration based on IESNA LM-80; and 3) data collection errors due to repeatability and uncertainty. As challenging as it is, LED package life prediction is definitely needed. In practice, the major LED manufacturers have been diligently working in this area and have established some very reliable approaches to provide the LED lumen maintenance predictions, via data sheets, to the LED users including LED module, lamp, engine and luminaire manufacturers.
One approach used by some LED manufacturers is to first identify what causes the lumen output degradation for a given LED package. An LED package is defined, per ANSI/IESNA RP-16 as, “An assembly of one or more LED dies that includes wire bond or other type of electrical connection, possibly with an optical element and thermal, mechanical, and electrical interfaces.” In general, LED package degradation over time occurs for the die (or chip), phosphor, encapsulation material and lens used in the package. For simplicity it can be grouped into two parts – A) chip degradation and B) package degradation. Each part of the degradation or aging behaviors can be tested and analyzed separately. Based on years of semiconductor product development and testing experiences, experts in the industry have a well established approach to shorten the test duration, which is called accelerated life tests. In the accelerated life tests, much more severe operational and environmental conditions are applied to accelerate the degradations. This may include higher operating current, higher temperature and humidity. By conducting the accelerated life tests, the data collected can be used to compare with the degradation data under the normal conditions. Then the correlation between these two conditions can be found, and the behavior can be modeled mathematically.
For the chip part, depending on the technologies used by each LED manufacturer, the degradation behavior may be close to one of the mathematical models, which could be an exponential equation, an Arrhenius equation or others. For the package part, the degradation behavior may be found differently, in which a different equation such as Hill equation or others can be used with the data collected. Once each part of the degradation behavior is identified, one can superimpose these fitting curves and then a special mathematical model is developed to describe the combined degradation behavior for the LED package. Because of the variations in the LED packages, the models or the equations and the parameters used in the equations vary. The validity and accuracy of these models should also be confirmed by lumen maintenance test data under the non-accelerated life test condition.
Once the models for different LED packages are established, the LED manufacturers can also apply them to the family or group of products where the technologies or designs are similar. Using the prediction model associated with the LED package, the estimated lumen maintenance information over a long period of time, (25,000 to 50,000 hours or longer), can be provided.
Without an industry standard to reference, in the current Energy Star Program Requirement (page 17), it states: “The LED package(s) / module(s) / array(s) used in the fixture has/have been tested according to LM-80, and the package(s) / module(s) / array(s) demonstrated at least 91.8 percent lumen maintenance at 6,000 hours (residential indoor) or 94.1 percent lumen maintenance at 6,000 (residential outdoor and all nonresidential).” The thresholds of 91.8 percent and 94.1 percent are based on an exponential model that assumes the LED packages’ lumen decay over the time follows an exponential curve. For reasons stated previously, an exponential curve and 6,000 hour threshold can’t simply and reliably, in practice, describe the wide range of LED packages and their lumen maintenance. Also for the same reasons stated previously, the standards committees continue to struggle to establish standard lumen prediction models. Some alternative approaches for setting up a 6,000 hour threshold have been discussed in the standards committees. In the meantime, it is very important for the LED users to know the LED package lumen maintenance information before they commit to a luminaire design, production and installation.
So in the interim, between now and standardization there are a few suggested approaches to predict lumen maintenance. First, the LED users including luminaire manufacturers and lighting designers, need to extend the efforts to understand and qualify the LED package suppliers and their products. Two pieces of information may be requested from the LED package manufactures. One is the testing data such as IESNA LM-80, and another equally important part is how the lumen prediction curves associated with the LED package being tested are derived by the LED manufacturer. This effort may help the LED users to distinguish reliable and reputable LED package manufacturers.
Second, the LED users need to understand that LED package lumen maintenance prediction information should not be used to directly predict the luminaire product rated life or lumen maintenance. All LED luminaires contain many other components that impact the performance and decay over time.
There are two other major components or subsystems that have been identified, based on past experiences, which contribute to luminaire level lumen output decay over time. One contributor is the LED driver. Over the long period of time (25,000 hours or longer), without accounting for catastrophic failure, the driver efficiency reduces. In turn its output characteristics such as current may also change. LED is a current device, its lumen output is sensitively relating to the operation current. When the LED driver’s output current changes over a long period of time, even when the LED package maintains unchanged lumen output, the luminaire level light output may be reduced due to the drive degradation. The second contributor is the optical elements used in the luminaire. This includes secondary optics (lens, reflector, prism, diffuser and others) and the cover lens. If these optical elements are made of plastics or having metalized reflective surfaces, the optical characteristics will change over time. Transmittance will be reduced due to material hazing and coloring. The reflectance will be reduced due to surface blemish, cracking and out-gassing. The optical elements transmittance and reflectance losses over time significantly affect the overall luminaire lumen maintenance. For instance, a 30 percent transmission loss for plastic material over 25,000 hours of operational time is not uncommon.
Besides LED package lumen depreciation, other subsystems’ performance degradation must also be accounted for in long term luminaire lumen maintenance. Based on the knowledge from other lighting industries such as automotive and transportation lighting, the environmental impact of temperature, humidity, salt and other chemical corrosion to the luminaire light output reduction over time have been identified and corresponding testing and standards were established and are in use today. The general illumination industry standard committees are currently discussing the standardization for luminaire level reliability and durability. At the same time, it is encouraged that luminaire manufacturers make more efforts to identify other subsystems degradation behaviors beyond LED packages lumen maintenance.
The bottom line is that in order to get the full view of lumen maintenance degradation over time, one needs to look at not only the LED but also the luminaire and its components as a whole.
Dr. Jianzhong Jiao is an internationally recognized lighting expert and the director of Regulations and Emerging Technologies at Osram Opto Semiconductors, Inc. Dr. Jiao is responsible for representing Osram Opto Semiconductors in establishing and maintaining regulatory standards for solid-state lighting and LED products, and for researching and exploring emerging technologies for sensing, illumination and visualization. He can be reached at jianzhong.jiao@osram-os.com.
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Guest Column: LED Lighting Standardization Update