Nisa explains the “second-half syndrome.”
For many reasons, some scientific communities, as well as many ordinary people, believe LED lamps last a long time, or have such potential longevity. One reason is advertising claims; another is that LED indicator lamps on early electronic gear still produce light. Further, credible, laboratory tests (accelerated lifespan) of current LED lamps extrapolate lifetimes which should exceed 100,000 hours.
However, lighting professionals’ views differ from that of the public. They recognize, for example, that a manufacturer’s production run of two million, 100,000-hour lifespan, LED lamps produces roughly one million that will endure. Here, lifespan is akin to Mean Time Between Failure (MTBF). Some describe the long-life lamps as “first-half lamps,” because they prevail.
Therefore, assessing such a lamp-life characteristic shows that LED manufacturers shouldn’t claim a consistent lifespan for general-purpose, LED lamps, such as those sold for signage or illumination applications.
Preferably, manufacturers would test a production run of general-purpose LEDs for all relevant light characteristics and announce that half, at least, had endured as expected. Meaning, no light characteristics had dropped below published specifications.
This is rarely the case.
To justify advertised lifetimes, diligent manufacturers perform certain equivalent tests via some sampling, as well as accelerated temperature and aging tests. Such claims may be valid for smaller quantities, say for clusters of one hundred, rather than for tens of thousands, or millions.
Truth is, equivalency tests and samplings don’t always scale, because significant, temperature-related, performance variations exist among LEDs built from the same compound semiconductor epitaxial wafer, as well qas those from wafer batches. Other process variations may also cause performance variations within the entire lamp manufacturing line, but to lesser degrees.
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