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You are here:HomeLearn Lighting1.2-1 ----Characterising a light source
1.2-1 ----Characterising a light source

A light source used for general lighting is characterised by:
>>its color temperature or its position on the chromaticity diagram (see point 1.4)
>>its color rendering
>>its luminous flux

1.2.1Color temperature, hot or cold light
The light source color temperature is determined by comparing its spectrum with that of a black body  heated between 2000蝪 and 10000K.
For multi-chromatic fluorescent or electroluminescent (LED) sources, which have an irregular spectrum and thus resemble less than the spectrum of a black body, a Correlated Color Temperature (CCT) is calculated. In practice, we speak of color temperature and not of correlated temperature for all multi-chromatic light sources.
A light of low color temperature such as the standard incandescent lamp is called hot, its spectrum contains lots of yellow and red which gives the impression of a flame; it is perceived as sof and comfortable. From 5000K onwards, a light sources is called cold; it becomes blinding and is not used for general lighting.



Figure 4: Color temperature of different light sources (source: http://fr.wikipedia.org)

The technical sheets of lighting products give the color temperatures of sources. For example, for office lighting, we recommend using light having temperature ranging from 3000 to 4000K..

For LED
The white light of an LED generated from a blue source exciting a yellow phosphor gives a cold color ranging from 5000 to 10000K (see the different methods for producing white light from LEDs in point 6.1). When the color temperature is not specified by the manufacturer, it is generally more than 7000K.
Hot white LEDs have appeared recently with the development of phosphors that convert blue into a large spectrum or by adding a yellow phosphor and a red phosphor to the blue LED that reduces the color temperature between 2500 and 5000K. The reduction of the color temperature also results in the reduction of the LED yield associated with the use of thicker phosphors or with the addition of an additional phosphor to give the red color.

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