CCT and Chromaticity

CCT and Chromaticity

Correlated Color Temperature is actually the color that a "black body" object would appear, when heated to that specific Kelvin temperature. This is why we call it a "temperature."

We can represent this on the chromaticity chart as the path that the color a "black body" takes, as its temperature changes. This is called the Planckian Locus.

Here is a zoomed-in view of the chromaticity chart. Note the X and Y coordinates on the side. The Planckian Locus is the curve running through the middle. There are several lines drawn perpendicular to this curve, where the CCT is a specific value, or level of total energy. 

If you look along the curve, you'll see that 6000K is a blue tone, while 3500K is a warm white tone - just like you'd expect. But, if you look more closely along each temperature line, you'll see that 5500K could be a greenish tone towards the top, or a blue tone towards the bottom, based on the Y coordinate... and both colors are still considered 5500K!

So, CCT is not precise. We could have more green or violet energy, but end up with the same exact CCT. This is why we use color quadrants to describe specific colors in the automotive world, rather than CCT. We need to make sure the color is correct in every direction, not just along the blackbody curve. 

Although consumers describe white colors by CCT, technically this is not precise at all - unless they're within established boundaries around that "white line," like an SAE Quadrant. If they're not near the white line, a "6000K" white could be greenish-white or blue-white, rather than a correct 6000K pure white color.