Modules - Beriled

Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | SubscribeqMqM | Next PageqqM qMMQmagsTHE WORLD’S NEWSSTAND ®standards | TM-21The elusive “life” of LEDs:How TM-21 contributes to the solutionThe newly-introduced TM-21 document presents a method for lumen-depreciation projection basedon LM-80 data, enabling LED lifetime extrapolations beyond 6000 hours, says ERIC RICHMAN.In August 2011, the IlluminatingEngineering Society (IES) published theTM-21 document entitled “Lumen degradationlifetime estimation method for LED lightsources.” TM-21 is the IES-recommendedmethod for projecting lumen degradation ofan LED package, array or module based ondata collected according to LM-80.The lighting community expects TM-21 tobecome the standard method for projectinguseful LED lighting product life at realisticoperating temperature. This article presentsthe development process behind TM-21, andclarifies how and when to apply the lifetimeextrapolation method to arrive at reasonableand useful estimations.Lumen output(%)Typical "life" of standard lighting technologies10510095908580LED75INC FL HID706560551,000 11,000 21,000 31,000 41,000 51,000 61,000Hours of operationRepresentative only - not to scaleFIG. 1. Lumen degradation and failure behavior for incandescent (INC), fl uorescent(FL), high-intensity discharge (HID), and LED lamps.Why TM-21 and why now?We are all familiar with the very real butsometimes exaggerated long-life attributesof LED technology. Not the least of theseis the potential for very long life that helpsmake it an attractive design choice. Thetrick has been and continues to be how tomeasure or estimate this longevity to provideassurance to users of this technology’sreliability (life) compared to other options.We also understand that the overall reliabilityof a complete LED lighting fixturecan be affected by the reliability of individualproduct components (driver, lens, etc.)and should be accounted for in lifetimeestimations.The useful life of standard lighting technologiesis defined as the time to filamentor cathode failure. For most of these lamps,the time period prior to failure exhibitsacceptable levels of light output, as shownERIC RICHMAN (eric.richman@pnnl.gov) is a Senior Research Engineer with Pacific NorthwestNational Laboratories and follows standards development for the DOE SSL program (www.ssl.energy.gov). PNNL (www.pnl.gov) is a research laboratory of the US Department of Energy.______with the solid lines in Fig. 1. This makes iteasy to determine when to replace the lamp.However, LEDs do not have filament burnoutthat conveniently announces the endof life (dashed line in Fig. 1). Further, therapid development of the technology andthe desire to bring products to market ina timely manner does not allow for actualtesting verification of the long lives claimed(100,000 or even 35,000 hours). As a result,the industry has come to accept a definitionof the end of the useful life of an LED as thepoint when it no longer provides a specifiedlevel of light output.And finally, the life and performance ofLED lighting products depends greatly onexcess heat retained at the diode. This is whyLEDs require testing at multiple temperaturessuch that when a source is installed ina luminaire, its actual operating temperaturecan be measured and lumen depreciationof the product can be derived.Therefore, to serve the solid-state lightingindustry, the Technical Procedures Committee(TPC) of the IES proceeded to developappropriate tests for use in rating LED productlongevity. The initial need was a measureof the basic lumen degradation of LEDsource components identified by a module,package, or array of diodes and this came inthe form of LM-80. Importantly, LM-80 onlyspecifies how to measure lumen depreciationto a minimum of 6000 hours (but recommendstesting to 10,000 hours or longer).LM-80 stops short of using that data to estimateany depreciation after that, which iswhere TM-21 comes in.The TM-21 working group (WG) as partof the IES TPC was formed to develop thelumen depreciation projection methodand spent over three years exploring manyoptions. The WG evaluated various projectionoptions starting with an analysis of variousmathematical, engineering-based modelsto provide effective depreciation fit and auseful projection method.LEDsmagazine.com NOVEMBER/DECEMBER 2011 37Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | SubscribeqMqM qMM MQmags| Next Page q qTHE WORLD’S NEWSSTAND ®