Color Systems

Color systems combine data from three elements:
• light source
• observer
• object
They are the tools to communicate and document color and color differences.

The system which is recommended by the CIE and widely used today, is the CIELab system.

It consists of two axes a* and b* which are at right angles and represent the hue dimension or color. The third axis is the lightness L*. It is perpendicular to the a*b* plane.
Within this system, any color can be specified with the coordinates L*, a*, b*. Alternatively L*, C*, h° are commonly used. C* (= Chroma) represents the intensity or saturation of the color, whereas the angle h° is another term to express
the actual hue.

To keep a color on target a standard needs to be established and the production run is compared to that standard; a typical customer / supplier situation. Therefore, color communication is done in terms of differences rather than absolute values.

The total change of color, ∆E*, is commonly used to represent a color difference.

 ∆E* =  √ (∆L*)² + (∆a*)² + (∆b*)²

The same ∆E* value can be obtained for two sample sets, and yet look completely different:

	Sample Set 1	Sample Set 2
∆L*	0.57	0.0
∆a*	0.57	0.0
∆b*	0.57	1.0
∆E*	1.0	1.0

To determine the actual change in color, the individual colorimetric components ∆L*, ∆a*, ∆b* or ∆L*, ∆C*, ∆H* need to be used.

The calculation and interpretation of the differences are done as follows:

The color differences that can be accepted must be agreed upon between customer and supplier. These tolerances are dependent both on demands and technical capabilities.