Why is there still visible color difference between two lights, even at the same CCT?

Have you ever came across a situation like this...You brought two batches of LED strips one after another from the same company. When you installed them and you found the color difference but the labels shown that these are all 3000K. What are the reasons? Is it because of CCT tolerance 3000K±200K, so one is 3200K and the other one is 2800K? Or is it becuase these are from different LED batches? Well most of companies may explain like that, and indeed they could cause the color difference.BUT what if the test data shows that these two lights are at the same CCT? This time I am going to talk about a common phenomenon that the color difference caused by Iso-CCT lines and X-step SDCM.

From the picture above, it is obviously seen that one strip is reddish and the other one is greenish. The test data shows that the CCT is nearly same. This is because of Iso-CCT lines.

In CIE 1931 chromaticity diagram, the curve in the middle we explained last time is called Black Body Locus (BBL)which is "pure white". Also we found CCT is not a coordinate, it is a line across BBL. The coordinates of all light sources are same on the CCT lines. If the coordinate is above the BBL, the light color is greenish. If it is below the BBL, the light color is reddish. In the film lighting industry, there is a function in the luminares called "Green/Magenta" which is used to adjust the light coordinates on the CCT lines.

Therefore, in a photometic test report, please also be aware of the value of duv. This is another index to evaluate the color difference.

In next article, we are gonna talk about MacAdam Ellipsis and SDCM. Please remember LED BINs(in my previous article), Iso-CCT lines(in this article) and SDCM(in next article) are three key factors regarding visual sensitivities to color difference. Understanding these three concepts will be very helpful for lighting applications.