Product Selection Guide Page 1114 Product Selection Guide-11th Edition

1114 PRODUCT INFORMATION Life & Depreciation LED TECHNICAL INFORMATION PSG11 TECHNICAL INFORMATION While there has been great excitement about the potential for LED lighting to save energy, certainly at least as much anticipation has surrounded the promise of LEDs to deliver extremely long life. The practical service life of LEDs for lighting applications is a complex issue and it is VERY important that facts are separated from hype. As LED lighting technology evolves, what is seemingly solid knowledge today will change as R&D efforts (and associated rigorous testing programs) combine with field experience to frame up a more complete picture of the capabilities of this source. With todays conventional light sources (HID, fluorescent, incandescent), the industry-set definition of service life is the point at which 50% of lamps have failed. Each of these source types has a distinct failure mode (e.g. electrode wear, cathode burnout, burned filaments, among others) that is known and anticipated before a new lamp is even launched. The rich history of these lamp development efforts over the decades has enabled manufacturers to build a comprehensive data set to use in creating statistical projections of lamp life. All this has defined the current industry- accepted normal life functions and have shaped customers expectations of source mortality. All of this changes with LEDs. To date, no consistent failure modes across the array of currently available LEDs have been uncovered. So, in the absence of catastrophic (i.e. non-operational) failures at the LED- package level, the Department of Energy (DOE) and Illuminating Engineering Society (IES) have agreed to define life for an LED as the time at which it degrades in light output to a given percent of its initial value. The nomenclature is given as L50 (50% of initial), L70 (70% of initial), and so on. It is important to realize however, the rate of this depreciation is highly dependent on the operating conditions of the LEDnamely junction temperature, operating current, and in some cases local ambient temperature (defined as the temperature immediately surrounding the LED). Since life is integrally tied to the LEDs operating conditions, we can no longer separate the life of the LED from the characteristics of the luminaire. So, given the same LED, a luminaire with a poorly designed thermal system (leading to higher junction temperature) will have a far shorter life than one with a solid thermal management approach. Bottom line: LED service life and lumen depreciation metrics are fundamentally interconnected and must be provided in the context of a specific luminaire design and intended application. For example, the figure at right on the next page shows three curves for luminaires with varying degrees of thermal management. LIFE & DEPRECIATION How is service life measured for LED systems?

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