13 July 2008

Appreciating LED useful life time

With Light emitting diodes (LEDs), outright failure is very unlikely. Contrary to conventional lighting sources which typically fail suddenly or burnout, LEDs are solid state electronic components and as such gradually degrade. But because of their long expected lifetimes, conventional light sources’ life testing is impractical to estimate the useful life of LEDs.

Predicted Life Time
Useful life of conventional lighting sources is commonly expressed as the time to failure. This primary metric is based on the time it takes for 50% of lamps to fail. LEDs, being replaceable semiconductors, are using “mean time to failure” (MTTF) to express their failure rate. This is a well-defined statistical reliability metric commonly used in the electronics industry.
Power LED manufacturers typically predict high brightness LED MTTF to be on the order of 50.000 - 100,000 hours, provided LEDs “are properly packaged and used in accordance with manufacturers’ recommendations”.

Translating these durations in plain English, a LED light source would have an average life time between 5 and 12 years if left on all day. For “normal” general lighting usage, even when considering a 12 hours average daily usage, this would translate into an average life time comprised between 12 and 24 years. A very long life times indeed.

Average Lumen Maintenance
Even when operated within the manufacturers’ specifications, both conventional and LED light sources experience loss of light over time. This is known as lumen depreciation, and is typically expressed as lumen maintenance, i.e. the percentage of initial lumens remaining after a specified period of time.
If you have ever changed a light bulb, you have certainly noticed how bright the new bulb is compared to the older bulb, then you have seen the effects of lumen depreciation. Lumen depreciation in incandescent lamps mainly occurs by depletion of the filament over time and accumulation of evaporated tungsten particles on the bulb wall. In fluorescent lamps, it occurs by photochemical degradation of the phosphor coating and the glass tube, and by accumulation of light-absorbing deposits within the lamp over time.

LEDs also experience lumen depreciation, but many factors can influence light degradation, such as ambient temperature and humidity, drive current and thermal management. That said, the primary cause is heat generated at the LED junction. High junction temperatures accelerate degradation in lumen maintenance, but also result in a temporary reduction in luminous flux. Contrary to other light sources, LEDs do not emit heat as infrared radiation, so the heat must be removed from the component by conduction or convection. If a LED application has inadequate means of removing the heat, such as heat sinking, the temperature will rise and light output will decrease.

Let’s translate this into plain English. For general ambient lighting applications, the commonly admitted light output decline is set at 70% of initial lumens. If for example we are considering LEDs that deliver 70% lumen maintenance at a 50,000 hours rated life, it means we should expect to receive 70% of the initial lumens after 50,000 hours.

Lumen depreciation has to be taken into account when appreciating the “useful” life of very long life components such as LEDs. In short, the “useful” life time of a LED results from a combination of the MTTF and the lumen degradation. From an application perspective, a catastrophic failure of the semiconductor or the lumen performance falling below 70% always boils down to a degraded service. Because of the LEDs very high MTTF, LED applications are more likely to falter because of lumen degradation, which in turn is highly dependent on the appropriate thermal management.

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