Is chromaticity shift a fatal injury to LEDs?

LED lighting for general lighting was introduced about 20 years ago. As LEDs become more and more common, people begin to pay attention to the size and chromaticity shift of lumens, and at a more basic level, how should these properties be measured?

The Illuminating Engineering Society (IES) first released LM-80, "Measuring Lumen Maintenance of LED Light Sources" in 2008, and updated "IES Approved Method: Measuring Luminous Flux and Color Maintenance of LED Packages, Arrays" in 2015 and Modules". In addition to lumen maintenance, these specifications also provide functions for chromaticity shift measurement. All versions of the LM-80 standard require LED packages to operate for at least 6,000 hours and to record lumen and chromaticity measurements at least once every 1,000 hours.

Another concern of the lighting industry is the method of predicting long-term lumen maintenance and chromaticity shift based on the limited data set obtained in the LM-80 test. In response, IES released TM-21, "Technical Memorandum: Projecting Long Term Lumen Maintenance of LED Light Sources" in 2011, and updated it in 2019 to include projections of radiation and photon flux. In addition, TM-35, "Technical Memorandum: Projecting Long-term Chromaticity Coordinate Shift of LED Packages, Arrays, and Modules" was also released in 2019.

The data collected using the LM-80-08 and LM-80-15 methods form the basis of the DoE report issued by Lynn Davis and Monica Hansen in March 2020. The report characterizes the lumen and chromaticity maintenance of the LED package as an updated version of the report launched in 2015. The report only focuses on the lumen maintenance rate. The 2020 version evaluates four different LED package types used in general lighting applications: ceramic-based, polymer-based, chip-on-board (COB), and wafer-level. A total of 223 LM-80 data sets, representative products from leading LED packaging manufacturers, are used to predict lumen maintenance and chromaticity deviation according to the protocols introduced in TM-21-11 and TM-35-19, respectively.

Unsurprisingly, the results of the 2020 study show that since the start of the study in 2015, the life of all package types of LED packages has improved (except for wafer-level packages that were not included in the earlier study). In the 2015 study, about one-third of the LED packages are expected to have a service life of more than 60,000 hours (L70); in the 2020 study, 96% of the L70 packages are expected to have a service life of more than 60,000 hours. The report also pointed out that due to improvements in polymer materials, polymer-based packaging has achieved the greatest benefits.

However, the main point of interest in the industry, the area of ??projection chromaticity shift, was not covered by any standard until the release of TM-35-19. This study used the chromaticity shift mode (CSM) defined in the table below to characterize the chromaticity shift observed in the LM-80-15 protocol test.

In the steady state, the common CSM and chromaticity shift direction of LED devices.

Three of the four LED package types included in the 2020 study showed a chromaticity shift during the LM-80 test, as shown in Figure 1. Except for the COB package, the chromaticity shift mode seems to be related to the package type, so no conclusion can be drawn. The study recommends further research on the mechanism for COB package chromaticity shift.

Then apply the method of TM-35-19 to predict long-term chromaticity shift, compare these results with the collected data, and predict the direction of shift. The conclusion of the study is that the prediction of TM-35 is reasonable, and it is usually conservative in the offset range, but it is not enough in the offset direction.