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Correlated colour temperature (CCT) is one measure of the quality of white light. White light consists of a mix of different wavelengths and CCT describes the relative proportions of the low frequency reds and oranges and the higher frequency blues and violets in any particular white light.

The variation in the pro­portions of different wavelengths gives us different types of white light, from “warm” feeling light, rich in red and yellow wavelengths (as we would expect to find in our living rooms or a cosy restaurant) to “cool” feeling white light, rich in the blue wavelengths, that we might find in an office or the gym.

Electromagnetic Spectrum Diagram

CCT is measured in kelvin (K) and gives us a precise and quantifiable means of expressing the hue of a white light.

White light of a specific corelated colour temperature is one where the colour of the light emitted correlates with (or is equivalent to) the colour of the light emitted by a black-body at the same temperature.

Background: White light consists of a mix of different colours, each of which has its own wavelength.

Any object above absolute zero (0K) emits some electromagnetic radiation, but at lower levels (below about 500ºC) this is not visible to the human eye.

As the temperature of the object rises, the dominant wavelength of light emitted changes, so the object gradually shifts from being “red-hot” to “white-hot”. You can visualise this by imagining a piece of iron being heated by a blacksmith or you yourself putting a poker in an open fire. At about 798K (525ºC) the metal starts to glow a dull red. At 1,000K (727ºC) it is bright red and at 6,000K (5,727ºC) it appears white.

White light of a specific correlated colour temperature, say 4,000K, is one where the colour of the light emitted correlates to the light emitted by a black-body at 4,000K.

Black-body radiation: For an accurate correlation of temperature with the colour of emitted light, physicists will refer to “black-body radiation”. In this context a black-body has some specific characteristics (eg it should be non-reflective and opaque) and must be in thermodynamic equilibrium with its environment – but the principle explained above (that as a body gets hotter the quality of light that it emits changes) is very consistent throughout nature.

Metal workers judge the temperature of the piece of iron, steel (or anything else) they are working on with great accuracy just by observing its colour, even though it does not fulfil all the exact requirements of a perfect black-body.

CCT in everyday use: Any lamp or luminaire that is sold should have its CCT written on it. Commonly available CCT values are as follows:

Colours Temperature 2700k to 6000K diagram

CCT choices in different countries: Selecting the appropriate CCT for a particular space or activity is to a large extent a matter of taste. While there is objective evidence that different wavelengths of light have differing effects on our metabolism (see https://www.nvcuk. com/technical/human-centric-v-circadian-lighting/1040.htm ) it is also true that geography and culture effect our choices of CCT.

For many years it has been observed that for commercial applications the Scandinavian countries buy more lighting in the range of 3000K – 3500K, while southern European countries are more likely to purchase lighting in the range of 4000K – 6500K.

Colour selectable lighting: This is lighting where the installer or user can select the CCT that a fitting will emit, typically by activating a switch on the product itself.

Tuneable white lighting: This is lighting where the CCT emitted can be dynamically adjusted, usually according to some pre-set parame­ters by a control system. 

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