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CIE Coding

Colours may be perceived differently depending on individuals and may be displayed differently depending on the peripheral display devices.

The International Lighting Commission (Commission Internationale de l'Eclairage, CIE) thus defined standards allowing a colour to be defined, independently from the peripheral devices used. For this purpose, the CIE defined criteria based on the perception of colour by the human eye, thanks to a triple stimulus.

In 1931, the CIE worked out the xyY colorimetric system which allows colours to be represented according to their chromaticity (X and Y axes) and their luminance (Y-axis). The chromaticity diagram (or chromatic diagram), resulting from a mathematical transformation, represents pure colours on the periphery, i.e. the monochromatic radiation corresponding to the colours of the spectrum (the colours of the rainbow), indicated by their wavelength. The line connecting the ends of the diagram (thus connecting the two ends of the visible spectrum) is known as the line of purples, because it corresponds to the colour purple, made up of the two monochromatic rays blue (420 nm) and red (680 nm):

The gamut of a display device is generally represented by tracing, in the chromatic diagram, a polygon containing all the colours which it is capable of producing.

However, this purely mathematical type of representation does not take into consideration physiological factors, i.e. colour perception by the human eye, which results in a chromaticity diagram that leaves too much space for greens.

In 1960 the CIE developed the Lu*v* model.

Finally in 1976, in order to overcome the problems with the xyY, model, the CIE developed the La*b* colorimetric model (also known as the CIELab), in which a colour is indicated by three values:

• L, luminance, expressed as a percentage (0 for black to 100 for white)
• a and b two colour ranges, from green to red and from blue to yellow respectively, with values ranging from -120 to +120.

The Lab mode thus covers the entire spectrum visible to the human eye and represents it in a uniform way. It thus makes it possible to describe all the visible colours in a way that is independent of any graphic technology.

It thus includes all the RGB and CMYK colours; this is why software such as PhotoShop use this mode to go from one representation model to another.

It is a mode that is very much used in industry, but not much favoured by most software since it is difficult to handle.

The CIE models are not intuitive, however the use of these guarantees that a colour created according to these models will be seen in the same way by everyone!
This document entitled « Video and digital imaging - CIE / Lab (L*a*b) coding » from Kioskea (en.kioskea.net) is made available under the Creative Commons license. You can copy, modify copies of this page, under the conditions stipulated by the licence, as this note appears clearly.