What Is the Planckian Locus? Understanding Why Two 3000K Luminaires Can Look Different

When selecting LED lighting, one of the first specifications people look at is the Correlated Colour Temperature (CCT). Whether a luminaire is 2700K, 3000K or 4000K gives a good indication of whether the light will appear warm, neutral or cool.
However, colour temperature tells only part of the story.
Have you ever compared two LED luminaires from different manufacturers that were both specified as 3000K, yet one appeared slightly greener while the other looked cleaner or even had a subtle pink tint?
If they have the same colour temperature, shouldn’t they look identical?

The answer lies in the Planckian Locus.

THE PLANCKIAN LOCUS: THE REFERENCE FOR WHITE LIGHT

The Planckian Locus is a reference curve on the CIE 1931 chromaticity diagram. It represents the colour of an ideal light source, known as a black-body radiator, as its temperature increases.
As the temperature rises, the emitted light gradually changes from warm orange and yellow tones to neutral white and eventually cool bluish white. This continuous transition forms the Planckian Locus.Every colour temperature, such as 2700K, 3000K, 4000K or 6500K, has its own position somewhere along this curve.Traditional light sources, such as incandescent and halogen lamps, closely follow the Planckian Locus. LEDs, however, generate light in a completely different way, which means they rarely sit exactly on the curve.

WHY TWO 3000K LEDS DON’T ALWAYS LOOK THE SAME

When an LED luminaire is specified as 3000K, it does not mean the LED is positioned exactly on the 3000K point of the Planckian Locus.
Instead, it means its chromaticity is closest to that point.
A simple way to understand this is to imagine the Planckian Locus as a road.
Moving along the road changes the colour temperature from warm white to cool white.
However, LEDs can also be positioned slightly above or below that road while still being classified as 3000K.
Although these LEDs share the same correlated colour temperature, they do not necessarily produce identical white light.

ΔUV: THE TINT OF WHITE LIGHT

The distance between an LED’s chromaticity and the Planckian Locus is expressed as Δuv.
Unlike CCT, which describes whether the light is warm or cool, Δuv describes the subtle tint of the white light.

• On or very close to the Planckian Locus – the light appears neutral.
• Above the Planckian Locus (positive Δuv) – the white light shifts slightly towards green or yellow-green.
• Below the Planckian Locus (negative Δuv) – the white light shifts slightly towards pink or magenta.

These differences are usually very small when measured, but the human eye is remarkably sensitive to them, especially when luminaires are installed next to each other.

READING THE CIE CHROMATICITY DIAGRAM

Once you understand the Planckian Locus, reading a CIE chromaticity diagram becomes much easier.
The position along the Planckian Locus determines the colour temperature.
The position above or below the Planckian Locus determines the tint of the white light.
This means that several LEDs can all be specified as 3000K, while each occupies a slightly different position around the Planckian Locus. Their colour temperature is the same, but their appearance is not.
This is exactly why two luminaires with identical specifications on paper can still produce noticeably different white light in a real installation.

WHY COLOUR CONSISTENCY MATTERS

For many applications, these differences are hardly noticeable. In professional lighting projects, however, colour consistency can make a significant difference.

Retail stores rely on accurate colour presentation to make products look their best. Museums and galleries require uniform illumination to preserve the appearance of artwork. Hospitality projects depend on comfortable white light to create the right atmosphere, while architectural projects often contain long rows of luminaires where even small colour differences become immediately visible.

For these applications, achieving consistent chromaticity is just as important as selecting the correct colour temperature.

MIXING DIFFERENT LEDS: WHAT CAN HAPPEN?

This is why it is generally recommended not to mix different LED or COB types within the same installation.
Even when they have the same specified colour temperature, their position relative to the Planckian Locus may differ. As a result, the white light they produce may not appear identical.
The same applies when combining luminaires from different manufacturers.
Two luminaires may both be specified as 3000K with CRI >90, yet if their Δuv values differ, one may appear slightly greener while the other appears slightly pinker. Installed side by side, these differences can become clearly visible and reduce the visual uniformity of the installation.
For this reason, professional lighting manufacturers carefully control LED selection, colour binning and chromaticity consistency to ensure every luminaire within a project delivers the same visual appearance.

COLOUR TEMPERATURE IS ONLY PART OF THE STORY

Colour temperature remains one of the most important specifications when selecting LED lighting, but it should never be the only one.
Professional lighting design also considers Δuv, colour consistency, MacAdam ellipses, CRI, optics, glare control and beam distribution.
Together, these parameters determine whether a lighting installation appears comfortable, natural and visually uniform.
The next time you compare two luminaires with the same Kelvin rating, remember that CCT tells you where the light is on the Planckian Locus, while Δuv tells you how far it is from it.
Understanding both helps explain why two luminaires with the same colour temperature can still look noticeably different, and why colour consistency is so important in professional lighting.