The long life spans of solid-state lighting systems have been among the strongest selling points of the technology and one of the main reasons, along with energy efficiency, that LEDs are now well on their way to replacing legacy lighting technologies in most applications. The need to standardize and verify the life-span estimates of lighting manufacturers has focused primarily on lumen maintenance of the LED light source, but a new study from the Next Generation Lighting Industry Alliance (NGLIA) and the industry consortium it formed with the U.S. Department of Energy, called the LED Systems Reliability Consortium (LSRC), takes a close look at one of the many other factors that can limit the useful life of an LED lighting system: chromaticity stability.Chip-on-board LED module. Image by Binarysequence [CC BY-SA 4.0], via Wikimedia Commons
The Illuminating Engineering Society’s standards LM-80 (for measuring lumen maintenance) and TM-21 (for projecting long-term lumen maintenance) have been the go-to resource for designers and specifiers wanting a reliable benchmark for the expected life of a fixture and they are useful for comparing products. In a real-world application, though, the stability of the color of light produced by a luminaire can have a significant impact on the luminaire’s useful life.
“While lumen maintenance has dominated discussions about LED lifetime, the color stability (also known as chromaticity stability) is another important performance attribute that can be a barrier to purchase or lead to unmet expectations of LED lighting,” the LSRC study says.
The report goes to some lengths to point out that color shift isn’t a problem that’s unique to LED lighting. It happens as much or more in legacy lighting technologies but because they need to be replaced more frequently for other reasons (i.e., failure) has kept color shift from becoming a central concern.
The report, titled LED Luminaire Reliability: Impact of Color Shift and available from the DOE website, characterizes how color shift differs depending on the LED technology in use – in rough terms high power LEDs tend to shift toward yellow, mid-power LEDs tend to shift blue, chip-on-board (COB) LED packages predominantly shift green (all of which vary depending on the specific materials used) – and aging lens and diffuser materials contribute their own yellow shift.
The report charts a path forward for better understanding chromaticity shift and the need for a model to characterize the magnitude of shifts. A TM-31 working group at IES is developing a standard for projecting long-term chromaticity maintenance of LED packages and the DOE SSL program has funded research on the topic.