PROJECTION OF COLOR SHIFT
Unlike many of the other parameters outlined in this document, color shift is not very well-understood, well-studied, or even commonly used as a metric, even for incumbent technologies. That said, we believe it is worth studying and characterizing, because SSL products may remain in place for a long time, and color shift may well be an important reliability consideration for certain applications. Although LM-80 requires LED manufacturers to collect data on color shift over 6,000 hours of operation, there is no accepted, standard way to use this data to extrapolate color shift. While the IES TM-21 committee is working to define a method to project long-term lumen maintenance of LEDs from LM-80 test data, developing a method to extrapolate color shift is outside the scope of this working group’s task. At present there does not appear to be any standardized color shift projection under consideration. It is also important to note that often the actual measurements of color shift for LM-80 testing are not done in situ or at steady state operation but rather in short-duration, relative photometry measurements at room temperature. Additionally, color changes in luminaires with multiple types of LEDs may not be easy to characterize using single-LED LM-80 testing.
Factors that will make color shift so difficult to extrapolate include differences in LED design, materials, manufacturing process, optics applied to the LED, and the temperature and time the LED operates. Many experts indicate that it will be a long while before there is general agreement on how to project color shift for an LED over an extended period of time.
Finally, there are practical limitations to consider. While color and color shift can be measured relatively easily in an integrating sphere, testing and measurement of multiple samples under various operating conditions can become very time consuming and hence expensive. It may also require specialized equipment, such as a large thermal chamber to test a roadway fixture at different temperatures. While such expenses may be justified in limited professional applications sensitive to color, that is the exception rather than the rule.