DOE recommends new approach to LED luminaire lifetime ratings
Recognizing that catastrophic failure of electronics in an LED luminaire is a concern, the DOE recommends a new way to account for such failures in addition to the widely used L and B specifications.
LED luminaires are extremely complex relative to legacy lighting technologies and the industry has struggled with an accurate way to rate the lifetime of such solid-state-lighting (SSL) products. The DOE has published a new recommendation for testing and reporting LED luminaire lifetime that accounts both for failure attributable to declining light output or lumen maintenance, and to catastrophic failure of the luminaire.
The new "LED Luminaire Lifetime: Recommendations for Testing and Reporting" guide recommends that luminaire makers specify an F rating for catastrophic failures – for example F10 at 30,000 hours would imply that 10% of the luminaires in a given population would fail catastrophically by 30,000 hours of usage.
DOE Lighting Program Manager Jim Brodrick mentioned the report in his weekly Postings newsletter writing, "Longevity is considered one of SSL's major advantages over traditional lighting technologies, and manufacturers are quite naturally touting it as a big selling point. But the topic is extremely complicated." Indeed both LED manufacturers and luminaire makers need a way to accurately define lifetime. Moreover, buyers need accurate information to justify SSL purchases and calculate accurate payback windows.
The new recommendation builds on the L and B ratings that originated with Philips Lumileds. First lumen maintenance was defined with a figure such as L70 at 50,000 hours implying that an SSL product would decline to 70% of its initial light output after 50,000 hours of usage – essentially reaching the end of its useful life. The B figure was added such that L70/B50 at 50,000 hours implies that 50% of a population of lights reach the L70 point in 50,000 hours.
The L and B specifications were first targeted at lumen maintenance at the LED component level. LED brightness can decline based on age, drive current, and operating temperature. Indeed, LED makers such as Philips provide L and B specifications relative to current and temperature.
Shortest of L/B and F specifications is figure of merit for lifetime rating
Luminaire makers must provide fixture level specifications based on the entire system. The L and B luminaire ratings must account for lumen maintenance and for catastrophic failure of one or more individual LEDs. Luminaire designs based on arrays of LEDs can suffer individual LED failures without falling below an L70 spec.
L and B ratings at the luminaire level have not accounted for catastrophic failures. Such failures would not likely be due to LED failure but rather to failed drive electronics, failed solder joint or other problem. A simple component such as an electrolytic capacitor can cause a catastrophic failure.
The DOE recommendation actually identifies 12 reliability considerations that could contribute to a catastrophic failure lifetime rating including electrical connections, printed circuit boards, thermal elements, the drive electronics and more. The F rating should account for all of the considerations.
The DOE further recommends that luminaire makers choose the shortest life rating between L/B and F specifications as the one used in a luminaire lifetime rating. For example, if a luminaire L70/B50 point is 50,000 hours and the F10 point is 40,000 hours then the luminaire maker should report a lifetime of 40,000 hours.
The new report was developed by a working group under the guidance of the DOE SSL Quality Advocates program. The group was formed jointly by the DOE and the Next Generation Lighting Industry Alliance. This work follows the earlier "Reporting LED Luminaire Product Performance" report that led to the Lighting Facts label.
Of course defining a way to specify lifetime still doesn't solve the inexact science of determining actual numbers. As Brodrick noted in his newsletter, products rated for 50,000 hours can't be actually tested in real time. And the fast-paced SSL industry will result in new components, drive electronics, and other changes long before a single product makes it through a six-year, 50,000-hour deployment. So luminaire and LED makers must extrapolate the lifetime ratings.