Color differences

The parameters L*u*v* (or L*a*b*) of the CIELUV (CIELAB) color spaces may be represented on 3 orthogonal axes. The case of the CIELAB space is shown in figure 21.

In these color spaces, a colored stimulus may be specified either by its cartesian coordinates L*u*v* (or L*a*b*) or by its cylindrical coordinates (see figure 22) :

• The hue angle h (h=0° for the red hue and +a* (or +u*) direction ; h=90° pour the yellow hue and +b* (or +v*) direction; h=180° for the green hue and -a* (or -u*) direction; h=270° for the blue hue and -b* (or -v*) direction.)

• The chroma or saturation s (iso-chroma lines (or ) are concentric circles centered at the origin a*=b*=0 (or u*=v*=0) corresponding to the white reference stimulus).

• The lightness L* (défined in § I.C.3.a).

From figure 22, and given two stimuli M1 and M2 having coordinates in the CIELUV (CIELAB) system L*u*v* (L*a*b*, respectively), the differences , , , and are defined by the following relations (see table 3):

The color difference between the two stimuli can be thought of as being composed of a difference of lightness , a difference in chroma and a difference in hue .

In the graphic arts and printing industry, the maximum acceptable color difference in the CIELAB space is:

• =1,5 for most demanding works (reproduction of works of art...),

• =2,0 for magazines,

• =2,5 for newspapers.