Theory

XYZ tristimulus values
the Yxy colour space
the CIE L*a*b* system
the L*, C*, h color space
The Reflection of light on one object
           The direction of light : specular reflection and diffuse reflection
           The different reflection of light
           Example of reflection on different material
           The reflection of the surface and reflection of volume
           Le facteur de réflexion lumineuse d'un matériau
RAL Color
          RAL Classic
          RAL Design
NCS Color
Homogeneity
Full scale material aspect
 

The XYZ tristimulus values

The concept for the XYZ tristimulus values is based on the three-component theory of color vision, which state that the eye possesses receptors for the three primary colors (red,green and blue) and that all colors are seen as mixtures of these three primary colors. The CIE in 1931 defined the standard Observer to have the color matching functions shown in the figure here below. The XYZ tristimulus value are calculated using the Standard Observer color-matching functions.

Figure 1 : Spectral sensitivity corresponding to the human eye (source : Minolta, "Precise color communication : color control from feeling to instrumentation", 1994.)

The Yxy color space

The tristimulus values XYZ are useful for defining a color, but the results are not easily visualized. Because of this, the CIE also defined a color space in 1931 for graphing color in two dimensions independent of lightness; this is the Yxy color space. Y is the lightness or the reflexion coefficient while x and y are the chromaticity coordinates calculated from the tristimulus values X, Y and Z.

The figure 1 shows the CIE x, y chromaticity diagram, for a Y fixed value. The achromatic colors are toward the center of the diagram, and the chromaticity increases toward the edge.

For example, an apple having the following chromaticity coordinates : Y = 13.37, x = 0.4832 et z = 0.3045 is at point A on the diagram.

Figure 1 : representation of the apple's color in the chromaticity diagram (source : Minolta, "Precise color communication : color control from feeling to instrumentation")

The CIE L*a*b* color system

Figure 2 : Representation of the apple's color in the L*a*b* color space
(source : Minolta, "Precise color communication : color control from feeling to instrumentation", 1994.)

The L*a*b* color space (also referred to as CIELAB) is presently one of the most popular color space measuring object color and is widely used in virtually all fields. It reduces one of the major problems of the original Yxy color space : that equal distances on the x, y chromaticity diagram did not correspond to equal perceived color differences.
In this color space, L* indicates lightness, a* is the red-green chromaticity value and b* is the yellow-blue chromaticity value.
The center of the diagram is achromatic ( of white color); as the a* and b* values increase and the point moves out from the center, the saturation of the color increases. Figure 1 shows the CIE L*a*b* color space and figure 2 shows an horizontal section in this space, for a constant L* value. For example, the apple previously represented in the Yxy color space, can also be representeed by point A, marked at figure 2.

The L* C* h color space

The L* C* h color space uses the same diagram as the L*a*b* color space, but uses cylindrical coordinates instead of rectangular coordinates. In this color space, L* indicates lightness and is the same as the L* of the L*a*b* color space;C* is chroma and h is the hue angle. The value of chroma C* is 0 at the center and increases according to he distance from the center. Hue angle h is defined as starting at the +a* axis and is expressed in degrees; 0° would be +a* (red), 90° would be +b* (yellow), 180° would be -a* (green), and 270° would be -b* (blue). The relationship between the L*a*b* color space and the L*C* h color space are the following :

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