# Why and how do the lamps with the same reported colour temperatures look different?

We see more and more light sources that have a reported/indicated colour temperature that when compared to another light source actually appear different to human eye. So, how and why does this happen?

When discussing colour temperature (CT), people are usually referring to correlated color temperature (CCT). There is a key difference between these two measures.

Colour temperature (CT)

This measure defines the exact spot of the light source on the planckian locus line. This line, pictured in the image below, is the black arc. For CT,  two light sources that have a color temperature of 4000K, will look exactly the same as they both occupy the same spot on the arc.

Correlated color temperature (CCT)

This measure is used when the light source is not on the planckian locus line. If CT defines the exact point on locus, then CCT defines the perpendicular line which runs directly through that exact point. So if a light source is off from the locus, then CCT is the CT point which is closest on the locus. For example if a light source has a CCT of 4000K, that means that it can be on any point on the line that runs through the 4000K point on the locus. You can see these lines for 10000K to 1000K on the image below.

Typically light sources indicate/measure colour temperature as CCT. So, this is what two light sources that have a CCT of 4000K, can look very different, as they are on the same line that runs through the 4000K point on the CT locus. Chromaticity coordinates

A light source’s chromaticity can be defined in a diagram using chromaticity coordinates. The chromaticity diagram below can be used to determine how close a particular light source’s colour temperature is to the locus. A quick way of determining roughly where a luminaire’s CCT lies in relation to the locus:

• If the light has a greenish hue, that will mean that the chromaticity coordinate is above the planckian locus.
• If the light has a purple hue, then chromaticity coordinates are below the locus.
• If the light is white, then the chromaticity coordinates are on the locus or at least very close to it.

So the key takeaway from this blog post is…if you are using two different light sources in the same space (or building for that matter) with the same CCT, you must confirm the chromaticity coordinates to determine that they are actually the same colour.

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