Colorimetry

Systems CIELAB and CIELUV

The eye's ability to separate two distinct colors depends of the hue, saturation and brightness of the observed color. As a consequence, the sensitivity to color differences will vary from one region to another within the CIExy diagram. Many studies have tried to study these variations: Mac Adam [] defined 25 points, spread out inside the (x,y) diagram, around which he drew ellipses with different sizes and orientations: all the colors inside an ellipse appear to be matched with the central color of the ellipse for a standard observer (see figure 19).

Figure 19 : Minimum discriminable color differences (Mac Adam's discrimination ellipses, magnified 10x)
Figure 19 : Minimum discriminable color differences (Mac Adam's discrimination ellipses, magnified 10x) [zoom...]Info

In the diagram, colors which appear to be different “by the same amount” are represented in the diagram as dots separated by very different distances depending on the color hue, which means that the (xy) space is not uniform. Transformations of the color space CIE XYZ 1931 have been proposed in order to obtain a better correlation between the “color difference” (as perceived by an observer) and the length separating two representative points.

Chromatic space L*u*v* 1976 (CIELUV)

n 1976 the Commission Internationale de l'Eclairage (CIE) has defined a nearly uniform chromatic space by defining two coordinates and such as :

with

is the CIE 1976 lightness [] defined by if or if .

, and stand for the values of , and for a given illuminant (see examples in table1):

Table 1 : Values of u', v' and Y for the most widely used normalized illuminants.
Table 1 : Values of u', v' and Y for the most widely used normalized illuminants. [zoom...]Info

The CIE recommends using the CIELUV color space for the characterization of color displays.

Chromatic space L*a*b* 1976 (CIELAB)

The CIELAB space is obtained by representing in a Cartesian coordinates system the parameters and defined by :

with

and  has the same definition as in the CIELUV color space.

, and are the values of , and corresponding to the illuminant, as tabulated in table 2:

Table 2 : Values of X, v' and Y for the most widely used normalized illuminants.
Table 2 : Values of X, v' and Y for the most widely used normalized illuminants. [zoom...]Info

The CIE recommends using the CIELAB color space for the characterization of colored surfaces and dyes.

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