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The description of colour
The characterisation of the colour of an object perceived by a standard observer under standardised lighting can be performed mathematically by means of colorimetric coordinates that enable the colour to be positioned within a colorimetric space. It is by positioning different samples within this space that colorimetric differences can be calculated.

In 1976, the CIE defined a colour space known as CIE L*a*b*, which is still widely used to measure colour. Any colour may be characterised in this space as a function of two types of coordinates: rectangular (L* a* b*) or cylindrical (L* C* h).

• Rectangular coordinates
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2 axes define the chromatic plane:
The [-a; +a] axis, which represents the red - green variations
The [-b; +b] axis, which represents the yellow - blue variations
The L* axis, perpendicular to the chromatic plane, which describes the lightness (grey values axis) from 0 (black) to 100 (white).
• Cylindrical coordinates
 In this system, 3 polar coordinates define the colour:
C*, which represents the saturation (chroma): this value corresponds to the “purity” of the colour. If the colour is close to the centre of the colorimetric circle, it is considered “dirty”. On the other hand, the further away it is from the centre of the circle, the more it is saturated and therefore “vivid”.
h which represents the Hue angle, expressed in degrees.
0° corresponds to +a* (red)
90° corresponds to +b* (yellow)
180° corresponds to –a* (green)
270° corresponds to –b* (blue)
C* and h describe the chromatic plane.
C L* represents the Lightness, identical to that used in the rectangular coordinates.

These coordinates are essential to mathematically characterise colours, but also have a qualitative equivalent:
The tint (or tonality) defines the visual sensation (denomination of colours: blue, red, green, etc.)
The saturation (or chroma) enables the sensation of purity of a colour to be expressed. It describes the more or less coloured or vivid character of the surface of an object.
The lightness (or luminosity) represents the quantity of light reflected or transmitted by the object (relative to a white surface lit in an identical manner).
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