The “colour temperature” of a white light, whether LED or not, indicates the colour tint of the white light. Most white light is not pure white, it’s usually got a bit of a tint.

In other words, there are different shades of white.

“Warm white” generally means a yellow-ish type of white, and “cool white” means a blue-ish kind of white. Somewhere in between lies “daylight white”, which is pretty much the whitest kind of white. Cool, warm and daylight are pretty imprecise terms, though. It’s useful to have a more accurate system to indicates the tint of the light. That system is called the colour temperature, or Correlated Colour Temperature (CCT).

Why do we use the term “temperature” to describe the colour? Light doesn’t have a “temperature”, so why do we use this term, and what does it have to do with colour?

When a black object such as a piece of iron is heated up, it changes colour depending on the temperature that it’s heated to. Turns out that this range of colors is very useful for describing the colour tint of white light.

When iron gets hot enough, it begins to glow red. That’s where the term “red hot” comes from.

Heat up the iron some more and it starts to glow orange. Even after the iron melts, you could keep heating it up and the colour of its glow would continue to change, depending on its temperature. You can actually determine the temperature of hot iron by measuring its color.

When it reaches a temperature of 2,700º Kelvin, (about 4,400º F or 2,426º Celsius), its glow will roughly match the colour of light emitted by your typical incandescent lightbulb, a rather yellowish colour. Because of the yellowish colour produced, 2700K is generally referred to as “warm white”, because artists have traditionally called yellow a “warm” colour, and blue a “cool” colour.

Naturally, this causes some confusion, because in reality, the higher the colour temperature, the more blue the light becomes! So a high colour temperature actually indicates a more “cool” colour, not a “warm” colour.

In other words the term “cool” does not mean “low colour temperature”, it means “cool looking, like blue water or ice”.

Artists were never very good at physics….

The chart on the left is just to illustrate the point – the colour saturation on the chart is exaggerated and indicates the tint of the white light, rather than actual red, yellow or blue light.

As you can see, a standard incandescent lightbulb is rather yellowish at about 2,700K and a normal halogen is slightly “cooler/whiter” in color, about 3,000K.

For interior lighting in homes, especially in the Western world we have grown accustomed to incandescent and halogen
lighting, so we have a comfort with the “warm” colours they produce.

In more industrial settings, and for higher intensity lighting ie. “high bay” lighting, higher color temperatures of 4,500 to 5,000K are accepted. Metal Halide lights (HID) have traditionally been employed for this application, but of course LEDs are now available to do this job more efficiently.

Some specialty applications such as jewelery case lighting often use “cool” bluish lights of 6,500K or higher, in order to accentuate the sparkle and clarity of diamonds, silver and jewels.

Interestingly, as the intensity of light increases, the perception of the color temperature changes. For example, on a very bright day, the color temperature of the light may be as high as 8,000 or 10,000K, but we do not perceive that we are bathing in blue light, although the light from the blue sky is the predominant source.

So, you may ask… is the sky blue because the atmosphere is heated to 10,000 degrees Kelvin? No, the sky is blue because of the light scattering in the atmosphere; the shorter blue wavelengths are absorbed and re-radiated by gas molecules, resulting in a blue looking sky.

Now that you’re an expert on color temperature, please consider Lumicrest Apturi LED bulbs, they come in 2700K warm white, 3000K clean white and 4000K natural white. 2700K is similar to the color of a regular incandescent or halogen light, which looks a little yellow or “warm”. 3000K is a little bit more white, but is still in the “warm” range. 4000K is approaching sunlight colour temperatures, but still a bit on the warm side.