Is Chroma Shift the Achilles heel of LED

Ordinary LED lighting came into use about 20 years ago. As LEDs become more commonplace, there is concern about lumen size and chromaticity shift. A more fundamental 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 to measure lumen maintenance and chromaticity shift. In its latest revision, LM-80-20, in addition to luminous flux, guidelines are provided for measuring radiant flux and photon flux to meet the evolving LED horticultural lighting market. All versions of the LM-80 standard require the LED to operate for a minimum of 6,000 hours, with lumen and color measurements recorded at least every 1,000 hours.

The lighting industry is also concerned about what method to use to predict long-term lumen maintenance and chromaticity shift based on the limited data obtained from the LM-80 test. IES released TM-21, Technical Memorandum: Projecting Long Term Lumen Maintenance of LED Light Sources in 2011, and updated it in 2019 to include radiation and photon flux forecasts. Also released in 2019 is the TM-35, Technical Memorandum: Projecting Long-term Chromaticity Coordinate Shift of LED Packages, Arrays, and Modules.

Data collected using the LM-80-08 and LM-80-15 methods formed the basis for a March 2020 DoE report by Lynn Davis and Monica Hansen, which builds on a 2015 report published on lumen maintenance only Updated to add content related to LED lumen and chromaticity maintenance. The new 2020 edition evaluates four different LED package types used in general lighting: ceramic-based packages, polymer-based packages, chip-on-board (COB) and chip-scale packages. There are a total of 223 LM-80 datasets from leading LED packaging manufacturers to predict lumen maintenance and chromaticity shift according to the protocols presented in TM-21-11 and TM-35-19, respectively.

Unsurprisingly, the 2020 study results show improvements in LED lifetime across all package types since the study began in 2015 (except for chip-scale packages not included in the earlier study). In the 2015 study, approximately one-third of the LEDs were expected to have a lifespan of more than 60,000 hours (L70); in the 2020 study, 96 percent of the LEDs were expected to have a lifespan of more than 60,000 hours (L70). The report states that encapsulation with polymers benefits the most due to improvements in polymer materials.

However, the industry's main point of interest, predicting chrominance shift, was not mentioned in any standard until the release of TM-35-19. This study used the Chrominance Shift Mode (CSM) defined in Table 1 to characterize the chrominance shift measured according to the LM-80-15 protocol.

The 2020 study included four LED package types, three of which showed a chromaticity shift during LM-80 testing, as shown in Figure 1. Chroma shift patterns seem to be related to package type, with the exception of COB packages (since no conclusions can be drawn), the study recommends further understanding of the mechanisms of COB package chroma shift.

The TM-35-19 method can be applied to predict longer-term chrominance shifts, and these results are compared with collected data on the magnitude and direction of predicted shifts. The study concluded that the TM-35 predictions are reasonable, generally conservative in the magnitude of the offset and insufficient in the direction of the offset. These findings have important implications for higher-end environments, such as museums, restaurants, retail stores and even grocery stores, where the color of light affects the appearance of illuminated objects.

As can be seen from the discussion above, more work is needed in the LED industry to develop better methods for evaluating long-term chromaticity shifts. It is also important to remember that the research results are based on LED package data collected in a controlled environment. After LEDs are installed in lamps, a number of conditions in the actual use of these lamps, including losses due to improper thermal management, operating in harsh environments, and light loss due to UV and dirt build-up, can affect lumen maintenance and chromaticity shift, and no evaluation studies have been conducted to date.