We always look forward to bringing our audience webcasts that educate them on industry standards, design challenges, and product and application development guidance. But this week’s webcast was one I particularly anticipated because it has the potential to change how the solid-state lighting (SSL) industry utilizes test and measurement guidance, and effectively end the notion that the lumen is the only light characteristic that matters with regard to LED lifetime performance.
I’m talking about the presentation by Dr. Eric Bretschneider, an established consultant in the LED and lighting space, where he addressed a new analytical model to predict chromaticity shift in LEDs and SSL products.
Why is chromaticity shift important?
“This is something the industry has been interested in. With the long life of LED products and the focus on lumen maintenance, the products are lasting long enough that people are starting to see these chromaticity shifts, and it’s been a real problem,” said Bretschneider. And with good reason.
As Bretschneider presented an example of current lifetime test methodology on a sample SSL product (see below), he said, “Here’s why it’s important… If you look at this product in the first 6000 hours, it looks really great. If you just look at this, you say, ‘This is fantastic; I’ve got no color shift. This is going to last forever.’ The real problem is what happens after that.” The initial period during which Δu’v’ was constant was referred to as “incubation.”
Showing the abrupt upward curve representing change in Δu’v’ with a shift of ~0.007 over the time period, Bretschneider explained, “[W]e’ve got no change… all of a sudden you see it takes off and it never looks back” — what he referred to as “emergence,” the period during which Δu’v’ increases as approximately a linear function of time.
Image credit: DOE Gateway Demonstrations: Color Maintenance of LEDs in Laboratory and Field Applications.
So you may ask, why can’t we rely solely on lumen maintenance to determine failure rate going forward? Bretschneider said that the color shift emerges after the period over which lumen maintenance would typically already be measured. However, critical chromaticity shift (which is considered a failure) may happen long before the actual lumen maintenance failure of a product.
So essentially, you are either stuck with performing a much more extensive test (much longer than the LM-80 test of 6000 hours, for instance)…or you can learn how to use a predictive model to be proactive in determining color shift. This could reduce a lot of customer dissatisfaction by setting realistic expectations on the light quality and maintenance timeline for installed fixtures.
A self-proclaimed math nerd, Bretschneider goes into considerably more detail about the presently available methods, and provides insight on how a model he called “differential chromaticity analysis” (DCA) can be used to build a table of predictive data with time steps, that helps those who are more math averse. There is so much more to glean from his presentation than I can convey here.
You can register and view this webcast on demand to get the full details on predicting chromaticity shift in SSL products.