Uniform Color Scale

Color diagram according to CIE 1976 UCS

The CIE-UCS color table is a color table which is intended to improve the CIE standard color table .

The CIE standard color table is not perceptual, as MacAdam was able to prove in an experiment in 1940. I.e. The CIE standard color table is correct for the mathematical description of colors, but not tailored to the color perception of the human eye.

If you measure two colors of the same brightness , one speaks of a perceptible color difference - the color difference threshold.

description

MacAdam ellipses

MacAdam was able to show that the color difference threshold in the standard color table has the shape of ellipses ( MacAdam ellipses , shown enlarged by a factor of 10 in the figure). These show how strongly two color stimuli have to differ so that the color difference is visible to the eye. The green colors take up a very large space in the color space, the red and blue areas are smaller.

The UCS color table was created by distorting the standard color table. The ellipses are distorted into circles by mathematical transformation in order to approximate the Delta E equidistance. x and y are transformed to u and v :

${\displaystyle u={\frac {4x}{-2x+12y+3}}}$

${\displaystyle v={\frac {6y}{-2x+12y+3}}}$

This system was adopted by the CIE in 1960 based on MacAdam.

The u'v 'system

In 1976, the uv color table was converted to the u ' v ' system in order to improve equality. Here v is increased by 50%:

${\displaystyle u'={\frac {4x}{-2x+12y+3}}=u}$

${\displaystyle v'={\frac {9y}{-2x+12y+3}}=1,5v}$

application

The CIE-UCS color table should be used when colors are perceived as being represented. It should be noted, however, that this color table is only approximately equally spaced.

The color distance is determined as the Euclidean distance :

${\displaystyle \Delta c={\sqrt {(\Delta u^{})^{2}+(\Delta v^{})^{2}}}}$

With

• ${\displaystyle \Delta u=u_{2}-u_{1}}$
• ${\displaystyle \Delta v=v_{2}-v_{1}}$.

It indicates how strongly two colors differ from one another according to their perception.

literature

• Bergmann, Schaefer: Textbook of Experimental Physics , Volume 3, Optics, Berlin 2004, deGruyter
• E. Dörsam: Color rendering in the media , TU Darmstadt, lecture WS 08-09
• R. Greule: Script for the lecture colorimetry , HAW-Hamburg