DOE publishes research on LED lumen and chromaticity maintenance or shift
The US Department of Energy (DOE) has published new research that is based on analysis of LM-80 testing datasets of 223 separate packaged LEDs to discern component behavior over time relative to lumen maintenance and to chromaticity shift. The analysis considers four different package types that generally correspond to what we would call mid-power, high-power, chip-on-board (COB), and chip-scale package (CSP) LEDs. The analysis reveals relatively predictable and acceptable lumen maintenance performance for all LED types but vastly different chromaticity shift characteristics — data that can be vital to solid-state lighting (SSL) manufacturers, lighting designers/specifiers, and others.
The authors of the report are Lynn Davis, a fellow in engineering research at RTI International, and Monica Hansen, principal at consultancy LED Lighting Advisors. That same duo was behind a similar report back in 2015 that was solely focused on lumen maintenance. We did not publish a news story on that earlier report, but we did publish a contributed article from the authors of the report leading up to a presentation the duo made at Strategies in Light 2015.
Since the earlier report, it’s become increasingly evident that chromaticity shift, especially in applications such as retail or perhaps museum lighting, can essentially render an LED to be at the end of useful life just as easily as can lumen depreciation. Thus the new report examines both failure modes and also adds coverage of CSP LEDs. The report recognizes the long understood fact that LEDs don’t often fail catastrophically yet degrade to a point where they no longer produce illumination that’s useful for the application at hand.
The report is completely focused on phosphor-converted white LEDs (pc-LEDs). The four package types studied include polymer-based (sometimes referred to as plastics and common in mid-power), ceramic-based (high-power), COB-array, and two types of CSP LEDs. We published our first detailed article on CSP technology back in 2015 and many other articles since. Some CSP LEDs emit from five surfaces — the flat top and the four sides — while others have reflective sidewalls that limit emission to the top. The DOE report refers to the top emitters as having polymer sidewalls.
The new research found that lumen maintenance performance is generally greatly improved across all package types so long as SSL system designs are accomplished cognizant of LED manufacturer guidelines. The data reveals that 96% of the components deliver L70 lumen maintenance (70% or more of initial lumen output) for more than 60,000 hours. There are of course caveats. Generally, mid-power LEDs must be driven at less than 200 mA to achieve such lifetime. Both high-power and COB LEDs can be driven reliably to 1.5A. And for CSP LEDs the key is temperature. The system design must keep junction temperature to 120°C or less.
Generalizing, the authors said mid-power LEDs have essential matched high-power LEDs in reliability. It was only a few years ago when we ran a technical feature indicating just the opposite. The report said materials improvement has bolstered mid-power reliability. We began to see that happen with what Lumileds twice called mid-power March announcements.
Turning to chromaticity shift, the story is quite different. Product developers can choose from LEDs in any single package type and achieve SSL systems with relatively good chromaticity uniformity compared to what has been common in the past. Yet the different LED types vary widely in how chromaticity shifts once it begins.
The authors of the DOE research refer to the phenomena as “chromaticity shift mode” (CSM). Mid-power LEDs tend to shift toward the blue region. High-power LEDs shift toward the yellow. COB devices from different manufacturers shift in a variety of directions. The authors did not offer this conclusion, but we would suggest that greatly varied COB architectures are the culprit. COB LEDs are built on aluminum and ceramic substrates. Some manufacturers use relatively large numbers of mid-power LEDs in such an array while others use far fewer high-power LEDs in an array.
CSP LEDs are also inconsistent, and the report breaks the situation down based on whether the device has the polymer sidewalls. Five-sided emitters exhibited either green or blue shift. CSPs with reflective sidewalls exhibited yellow or blue shift. Why does it matter? Well, a space with a mixture of luminaire types would generally also have a mixture of LED types in those luminaires. The mismatch could become very noticeable and significant over time.
The is a lot of great detail in the new report, far more than we can go into here. You can download the entire report in PDF form from the DOE website.
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Maury Wright | Editor in Chief
Maury Wright is an electronics engineer turned technology journalist, who has focused specifically on the LED & Lighting industry for the past decade. Wright first wrote for LEDs Magazine as a contractor in 2010, and took over as Editor-in-Chief in 2012. He has broad experience in technology areas ranging from microprocessors to digital media to wireless networks that he gained over 30 years in the trade press. Wright has experience running global editorial operations, such as during his tenure as worldwide editorial director of EDN Magazine, and has been instrumental in launching publication websites going back to the earliest days of the Internet. Wright has won numerous industry awards, including multiple ASBPE national awards for B2B journalism excellence, and has received finalist recognition for LEDs Magazine in the FOLIO Eddie Awards. He received a BS in electrical engineering from Auburn University.
