Reliability & Lumen
Maintenance
Lumen maintenance is simply the amount of light emitted from a source at any given time relative to the light output when the source was first measured. This is usually expressed as a percentage. If you've ever changed a light bulb and noticed how bright a new bulb is when compared with the older bulb, you're seeing the effects of lumen depreciation. This steady decline over time is known as lumen maintenance (L). The materials inside the bulb will continue to deteriorate until finally the bulb will no longer emit any light. This is known as the bulb's mortality (B). With conventional light sources, the bulb usually fails before our eyes notice the change in lumen maintenance.
LEDs also experience lumen depreciation but it happens over a much longer period of time, usually tens of thousands of hours. Compared to conventional light sources, you notice very little degradation of light with LUXEON LEDs.
Not all LEDs deliver the same lumen maintenance. An LED is a complex package of materials that must all work together to deliver long lifetimes. Everything from the design of the chip, thermal management, optics material, phosphors and even the assembly of the entire package will affect lumen maintenance. Some LEDs demonstrate very rapid depreciation (Fig. 1) but Philips Lumileds technical advances in all of these areas has led to the industry’s longest lived LEDs — LUXEON.
Figure 1.
LUXEON LEDs vs. conventional light sources.
To truly understand the life of an LED light source, the user needs to
understand both mortality (B) and lumen maintenance (L). Let's say for
example you are working with LEDs that on average deliver 70% lumen
maintenance at 20,000 hours (L70). This tells you to expect to receive 70%
of the relative lumens after 20,000 hours. What this doesn't tell us is
what percent of LEDs are performing below this level which can affect
overall system performance. To get a complete understanding of
performance, the user needs to know the rated life or mortality (B) and
the lumen maintenance (L) of the light source.
Figure 2.
Lifetime for InGaN LUXEON K2 Power LEDs
Providing this critical information to the lighting community
requires that thousands of hours of testing be completed
on statistically relevant sample sizes. The more hours
of testing that are completed, the more accurate the
resulting data will be.
Philips Lumileds has compiled data over a long-enough time period to accurately describe lumen maintenance and mortality expectations and is the first manufacturer to make this information publicly available (Fig. 2).
Learn more about the tools and information available by reading our
white paper—Understanding
Power LED Lifetime Analysis.
Further information regarding lifetime and reliability of specific products
is available in our reliability datasheets.
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