Is chromaticity shift the fatal injury of LED?
Ordinary LED lighting began to be used about 20 years ago. As LEDs become more and more popular, people begin to pay attention to the size and chromaticity shift of lumens. A more basic question is, 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 and Modules in 2015. These specifications are used for the measurement of lumen maintenance and chromaticity shift. In its latest revision LM-80-20, in addition to luminous flux, it also provides guidelines for measuring radiant flux and photon flux to meet the growing LED garden lighting market. All versions of the LM-80 standard require LEDs to work for at least 6,000 hours, and to record lumen and chromaticity measurements at least once every 1,000 hours.
The lighting industry is also concerned about the question of what method is used to predict long-term lumen maintenance and chromaticity shift based on the limited data obtained from the LM-80 test. For this reason, IES released TM-21, Technical Memorandum: Projecting Long Term Lumen Maintenance of LED Light Sources in 2011, and included radiation and photon flux predictions after the 2019 update. Also released in 2019 is TM-35, Technical Memorandum: Projecting Long-term Chromaticity Coordinate Shift of LED Packages, Arrays, and Modules.
The data collected using the LM-80-08 and LM-80-15 methods became the basis of the DoE report released by Lynn Davis and Monica Hansen in March 2020. This report is based on the report released only for lumen maintenance in 2015. Update, added the related content of LED lumens and chromaticity maintenance rate. The 2020 version evaluates four different LED package types used in general lighting: ceramic-based packaging, polymer-based packaging, chip-on-board packaging (COB), and chip-scale packaging. There are a total of 223 LM-80 data sets, products from leading LED packaging manufacturers, to predict lumen maintenance and color shift 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 lifetime of LEDs in all package types has improved (except for chip-scale packages that were not included in the earlier studies). In the 2015 study, it is estimated that about one-third of the LEDs have a service life of more than 60,000 hours (L70); in the 2020 study, it is estimated that 96% of the LEDs have a service life of more than 60,000 hours (L70). The report pointed out that due to the improvement of polymer materials, the use of polymer encapsulation benefits the most.
However, the main point of interest in the industry-predicting chromaticity shift, was not mentioned in any standard until the release of TM-35-19. This study used the chromaticity shift mode (CSM) defined in Table 1 to characterize the chromaticity shift measured according to the LM-80-15 protocol.
The 2020 study included four LED package types, three of which showed chromaticity shifts during the LM-80 test, as shown in Figure 1. The chromaticity shift mode seems to be related to the package type, except for the COB package (because no conclusion can be drawn). The study recommends further understanding of the COB package chromaticity shift mechanism.
The TM-35-19 method can be applied to predict longer-term chromaticity shifts, and compare these results with the collected data on the magnitude and direction of the predicted shifts. The conclusion of the study is that the prediction of TM-35 is reasonable, and it is usually conservative in the offset range and not sufficient in the offset direction. These findings are of great significance to higher-end environments, such as museums, restaurants, retail stores, and even grocery stores, because in these environments, the color of light affects the appearance of illuminated objects.
As can be seen from the above discussion, the LED industry still needs to make more efforts to develop better methods to assess long-term chromaticity shift. It is also important to remember that the research results are based on LED package data collected in a controlled environment. After the LEDs are installed in the lamps, some of the actual use of these lamps, including the loss caused by improper thermal management, working in harsh environments, and the light loss caused by the accumulation of ultraviolet rays and dirt, will affect the lumen maintenance and Chromaticity shift, and no evaluation research has been conducted so far.