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Talking Photometry: A Guide to Colour Temperature

CCT

“Colour temperature” is that parameter which is used to describe the colour of light emitted by a white light source. It simplifies the communication of the colour of a light source which would otherwise have to be described more confusingly in either 2 or 3 numbers (chromaticity coordinates xy, uv or u’v’ or tristimulus values XYZ). The problem with the colour temperature metric is that it is possible for two flourescent lamps, LEDs or LED modules to have the same colour temperature specification but to appear distinctly different. This presents quite a problem if you wish (for example) to second-source an LED module for your product that needs to look identical to the other vendor’s unit.
 
To understand how two LEDs with the same colour temperature can look markedly different, we need to look at the definition of colour temperature more carefully. True colour temperature is the colour of radiation emitted from a perfect blackbody radiator held at a particular temperature. Colour temperature is reported in units of Kelvin (K). In a CIE colour space diagram, the plot of the chromaticity coordinates of a blackbody radiator with temperatures from 1,000 to 20,000 Kelvin is called the Planckian locus. Colours on this locus in the range from about 2,000 to 20,000 K are considered to be “white”, with 2,000 K being reddish white (“warm white”) and 20,000 K being bluish white (“cool white”).
 
Some further definitions may be helpful. A blackbody radiator is a source that emits blackbody radiation. This is radiation that is full or complete, containing all wavelengths. The spectral power distribution (“spectrum”) of light emitted from a blackbody is a function of its temperature only and is described by Planck’s radiation law. It may help to consider a real life Planckian radiator – a hot metal. Pass an electric current through a piece of tungsten wire (as Edisson once did) and the current experiences a resistance that is in proportion to the current passed. The consequence of this resistive heating is that the tungsten wire gets hot and starts to glow – a process called incandescence. With increasing current and higher temperatures, the light emitted transitions through red to white. Most incandescent light bulbs (remember those?) emit light with a colour temperature of about 2,800 to 3,100 Kelvin. Light of this colour temperature is called warm white light as there is still a red (warm) hue to the light.

Other, more energy efficient light sources – such as fluorescent or discharge lamps, or LEDs – are not blackbody or incandescent sources. Taking one example, LEDs emit light by a process called electroluminescence.vThe chromaticity coordinates of the white light emitted by an LED will not necessarily fall directly on the Planckian locus in the colour space diagram. For those light sources, we should refer to them as having a correlated colour temperature (CCT) . CCT describes the colour temperature of those white light sources (non blackbody emitters) whose colours don’t fall exactly on the Planckian locus. The CCT of a non-Planckian light source is the blackbody colour temperature that the source resembles most closely. Correlated colour temperature is also reported in units of Kelvin (K).
 
It is in the exact definition of CCT that we can be badly tripped up. While the chromaticity coordinates of a true blackbody source must (by definition) fall exactly on the Planckian locus, the chromaticity coordinates for an LED of a certain correlated colour temperature can fall anywhere along a so-called “ISO-CCT” line that intersects the blackbody locus at the equivalent (true) colour temperature. In other words, a CIE standard illuminant A incandescent lamp with a true colour temperature of 2856 K will have chromaticity coordinates of exactly x = 0. 4476 and y = 0.4075. In the CIE 1960 colour space, these coordinates become u = 0.2560  and v = 0.3495. A light source with a correlated colour temperature of 2,856 K can have actual chromaticity coordinates which deviate from the blackbody source by up to duv = ± 0.02. Given that in the 1960 uv colour space a difference of just ± 0.001 in u or v is generally considered to be noticeable, the definition of CCT permits the colour of white light sources to deviate more than 20 times beyond the point where an observer would start to notice the difference.

What this means in practice is that if you only define the colour of your white LEDs by means of their CCT, you will potentially end up with a whole variety of shades of white. Fine if you wish to decorate your Christmas tree, not so good if you wish to produce a high quality lighting product with a consistent colour. There is of course an obvious solution.  While it is convenient to resort to CCT as a simplified colour metric, you can uniquely define the colour of the white LED that you require in terms of the CIE chromaticity coordinates. That way, your whites won’t end up giving you the blues.
 
Photometric Testing provides a service for almost any application in measuring the colour and brightness of LEDs, luminaires, lamps and displays - including the measurement of correlated colour temperature. We can measure the colour temperature of your lamp, LED, LED module or luminaire spectroradiometrically (for the highest accuracy) and also the variation of colour with angle (as required by IES LM-79-08).