A Guide To Colour Temperature
LED Colour Temperatures – Are Your Whites Giving You The Blues?
“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 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”). This is illustrated in Figure 1 below.
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. The 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 (rather confusingly) reported in units of Kelvin (K).
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.
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