Arguably, the biggest gap in LED metrics is a product lifetime metric and test procedure. Currently, the industry is rife with fraudulent emitter life claims used as LED system product life claims (lamps, luminaires, and engines). It is common to see LED product spec sheets with exaggerated L70 for the emitters (only), both violating of the TM-21 6X extrapolation rule, and being used as product lifetime claims.
Early last year, the Lighting Research Center (LRC) was approached by the International Energy Agency’s 4E Solid State Lighting Annex and asked to explore and summarize the literature on LED system lifetime. Over the course of a year, LRC conducted an international literature search that included definitions of LED life, failure mechanisms of LED components and systems, parameters that accelerate failure, and available test methods for estimating LED system lifetime. The results were accumulated into a report published in June by the IEA 4E SSL Annex, Literature Summary of Lifetime Testing of Light Emitting Diodes and LED Products, which is available online.
The major outcome of this report is the LRC’s recommendation of two test methods as the most promising for accurate life prediction of LED lighting products. The selection of these two methods was based on an understanding—from both the literature and from LRC’s own laboratory testing and research for the past two decades—of the ways in which LED systems fail and the operating conditions that lead to their failure. Here are the two proposed methods:
- The first method, adopted recently by the European Union, considers both environmental condition and use pattern. This is an important advancement in the industry because it recognizes the effect of the operating conditions of LED products in different applications on lifetime. However, in its present form, this method only considers one temperature and use pattern condition and was implemented to report lumen depreciation and percentage of surviving products at the end of the test. Before this method can be used for predicting lifetime in different applications, other test conditions representative of those applications need to be added.
- The second method was proposed by the Lighting Research Center and formalized by the Alliance for Solid-State Illumination Systems and Technologies (ASSIST). This method allows for any combination of environment temperature and use pattern to be specified, and thus provides a means to predict LED product lifetime within the boundary conditions of the test. One thing to note is that humidity is not considered in either of these test methods. Humidity was conveyed as a concerning factor in a number of studies discovered during our literature search. Considering the dominance of LED products for outdoor applications, such as for lighting parking lots, roadways, parks, airports and more, the effects of humidity should be weighed, especially where safety is concerned.
Read the full article in LD+A Online here.