LED Color Stability

LED color shifts over time are far harder to predict than lumen depreciation. Here's what we do know.

In the last ten years, a lot has been published on lumen depreciation. As I discussed in a previous article, the Illuminating Engineering Society provides guidance on predicting lumen depreciation in their document TM-21 "Projecting Long Term Maintenance of LED Light Sources." While some might dispute the method, it at least provides a foundation for projecting the long-term effects of time, temperature, and operation to the light output of an LED. The model is straightforward to apply to predict what will happen to lumen output over time for an LED. Color stability, however, is a different matter.

Chromaticity over time of some of the lamps tested by CALiPER in 2008-2011. Not only are the starting and ending points in color space seemingly random and unpredictable for any given lamp, but the paths each lamp's output takes through that space is even more so.

IES document LM-80-08 "Approved Method: Measuring Lumen Maintenance of LED Light Sources," standardizes the test conditions, set-up, and measurement of life data for LEDs. The document calls for lumen measurements for a minimum of 6000 hours, at prescribed ambient conditions, forward currents, and monitoring.

The corollary document to LM-80, TM-21-11 -- "Projecting Long Term Maintenance of LED Light Sources published by the Illuminating Engineer Society" -- prescribes a standard method for extrapolating the LM-80 data. The DOE offers a free, Excel-based calculator using the model. Many LED manufacturers even offer TM-21 predictions together with the LM-80 data. Alas, there is no similar guidance to predict long-term color shift. LM-80 does at least require corresponding chromaticity measurements in addition to the life data, so there is hope that the data can serve as a basis to develop projection methods.

Perhaps there is no corresponding guidance on chromaticity shift because it will be significantly more challenging. Lumen maintenance is two-dimensional, i.e. a function with one variable. Color shift, on the other hand, is much more complex. Color shift can occur in any direction on the chromaticity diagram. It could migrate along the blackbody curve, or more likely, vary in any other conceivable direction.

DOE's minimal guidance

The US Department of Energy, as part of their Energy Star program, is driving industry to standardize measurement and extrapolation in LEDs and LED fixtures of both lumen depreciation and color shift. In September, the Department of Energy released a report entitled "Color Maintenance of LEDs in Laboratory and Field Applications." It is perhaps one of the most comprehensive studies to date on LED chromaticity shift. It notes the issue of projecting long term color performance of LEDs and LED fixtures:

There are no specified acceptable limits for color shift, and there is no method for projecting future color shift as there is for lumen depreciation... The ENERGY STAR program does mandate that Δu'v' at 6,000 hours of operation not exceed 0.007 [EPA 2012, 2013]. This is perhaps the only industry-wide criterion for color shift. It is a reasonable starting point, but may not be strict enough to ensure very high-quality lighting, especially since the lifetimes of LED products routinely far exceed 6,000 hours.

The document studied long-term chromaticity performance of LED luminaires in several different applications, including a Gateway museum lighting project at the Smithsonian American Art Museum, the CALiPER program, and the L Prize winner. Many of the luminaires studied exceeded a Δu'v' of 0.007 in less than 6,600 hours. The exception was the L Prize winner, which showed remarkable color stability. However, it uses a remote phosphor technology that is not representative of most of the available LED products on the market.

The DOE report gives some insight into the mechanisms that cause color shift in LEDs. It concludes by identifying the need and desirability of improving the long-term color stability of LED products, but it stops short of offering any guidance in predicting color change. So, for the time being at least, we're left to our own devices.

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