Letter to the Editor: Lifetime limitations of LED products #2

Jan. 20, 2009
The performance and reliability of an LED system are only as good as the integrity of its parts and how they work together, writes Kevin Dowling.
To the Editor:

We thank Lindsay Pelsar (Letter, January 19) for raising a critical issue which, if left unaddressed, will hurt the adoption of LED lighting in its entirety.

While LEDs do outlast their conventional counterparts, in some cases by many years, they are but one component of a complete system that's required for illumination. To extract the most useful life from LED sources, the system must be designed with close attention to many different factors; particularly component selection, circuit design and thermal management.

Even the highest quality LEDs will fall short of their expected lifetime if the system is not properly engineered in these respects. Additionally, the system will be affected by environmental conditions, ambient operating temperatures and other environmental factors. As a result, true system lifetime can vary.

The lifetime of LED sources -- not systems or luminaires -- is defined in terms of “lumen maintenance,” meaning the useful life of the source before a certain amount of output diminishes. There is now a standard in place (IESNA LM-80) to govern the proper measurement of lumen maintenance and further work is underway to provide valid prediction beyond the actual measurement time.

LED system manufacturers rely on the lumen maintenance data provided by their LED suppliers, and must then take into account the factors above to best project system lifetime. For example, Philips designs and rigorously tests white LED fixtures to sustain at least 70% of their initial output at 50,000 hours based on LED source data, which equates to nearly 6 years of continuous 24/7 use.

True lifetime is best represented in terms of reliability of the entire system. Lumen maintenance is but one factor. The points raised regarding specific electrical components are germane to system and driver reliability, but resistors and capacitors are passive components whose reliability, assuming good system engineering, is quite high. Certain types of capacitors can be problematic if they are not selected properly. In this case, mil-spec devices or metal-film devices provide lifetimes that can exceed that of LED lumen maintenance.

When specifying LED systems, pay close attention to the warranty, the support, the quality of the product and how its lifetime was calculated, and the viability of the manufacturer. Warranties based on hours are unusual and I am not familiar with this, but multi-year warranties in the LED lighting business are not unusual. Philips warranties are 2 years.

In short, the performance and reliability of an LED system are only as good as the integrity of its parts and how they work together. Overstated claims and underachieving LED products will put a major roadblock in the otherwise incredibly exciting future of lighting.

Kevin Dowling,
VP Innovation, Philips Color Kinetics
Burlington, MA, USA