Лако-красочные материалы — производство

The principles of colorimetry are based on the fact that all color stimuli can be sim­ulated by additively mixing only three selected color stimuli (trichromatic principle). A color stimulus can, however, also be produced by mixing the spectral colors. Thus, it has a spectral distribution, which in the case of nonluminous, perceived colors is

Fig. 1.3 Figure 1.3 Spectral reflectance curves of some inorganic pigments in paints. (a) Manganese blue; (b) CdS; (c) a-FeOOH; (d) a-Cr2O3; (e) a-Fe2O3.

called the spectral reflectance p(X). After defining three reference stimuli, the trichro­matic principle allows a three-dimensional color space to be built up in which the color coordinates (tristimulus values) can be interpreted as components of a vector (CIE system; for standards, see Table 1.1, “Colorimetry”; CIE = Commission Inter­nationale de l’Eclairage). The three CIE tristimulus values depend on the spectral
reflectance р(ё) and the spectrum of the illuminant S^) as follows:

400

X = [х(ё)р(ё )S(X]dё

700

400

Y = [у(ё)р(ё )S^)dё

700

400

г(ё)р(ё )S(ё )dё

700

where x (ё), y (ё), and х(ё) are the CIE tristimulus values of the spectral colors and are called the CIE spectral tristimulus values (color matching function). The CIE chromaticity coordinates (x, y, and z) are given by

x

У

z = 1 — x — y

They are represented as coordinates in a color plane. The chromaticity coordinates x and y are used to specify the saturation and hue of any color in the CIE chromaticity diagram. See Figure 1.5 for illumination D 65. The CIE spectral tristimulus value у(ё) corresponds to the lightness sensitivity curve of the human eye. Therefore, a third color variable is specified in addition to x and y, namely the CIE tristimulus value Y, which is a measure of lightness.

This system allows exact measurement of color with worldwide agreement. For pigment testing, however, this is not sufficient because small color differences usually have to be determined and evaluated (e. g., between test and reference pigment). Using the CIE system, it is certainly possible to say which spectral distributions are visually identical, but this is not suitable for determining color differences. To establish color differences an “absolute color space” must be used. Here, colors are arranged three-dimensionally such that the distance between two colors in any direction in space corresponds to the perceived difference. Such a type of color space can be based on the color qualities lightness, hue, and saturation. Several such systems exist. The most widespread color system is probably the Munsell system, which is available in the form of an atlas.

For the quantitative determination of color differences, the transformation rela­tionships between the CIE system (which has to be used for color measurement) and the physiologically equidistant color system must be established. Color differ­ences can then be calculated in the latter system. A large number of color difference systems have been developed, mainly as needed for industrial color testing. The

Fig. 1.6 Representation of the CIELAB system.

Adams-Nickerson (AN) system, well known for many decades and derived from the Munsell system, was recommended for pigment testing by DIN (German Standards

Institute) and later worldwide by the CIE (“CIELAB”; for standards, see Table 1.1, “Color differences”). The three coordinates are denoted by a* (the red-green axis), b* (the yellow-blue axis), and L* (the lightness axis). See Figure 1.6 for a simple representation of the CIELAB system. To calculate the CIELAB coordinates, X, Y, and Z are first converted into the functions X*, Y*, and Z * by using a relationship that approximately takes account of the physiologically equidistant lightness steps:

X * = X/X n;

Y * = Y/Y n;

Z * = 3 Z/Zn

where Xn, Yn, and Zn are the CIE tristimulus values of the illuminant, especially a standard illuminant. For radicands g0.008856, these equations become:

X* = (7.787X/Xn ) + 0.138

Y* = (7.787Y/Yn ) + 0.138

Z* = (7.787Z/Zn ) + 0.138

Values of a*, b*, and L* are obtained from the values of X *, Y*, and Z *: a* =500 (X*- Y*) b*= 200 (Y* — Z*)

L*= 116 Y* — 16

The components of the color difference are obtained as differences between the test sample (T) and the reference pigment (R):

Aa = aT — aR ; Ab = bx — bR ; AL = Lt —Lr

The color difference is finally calculated as the geometrical distance between the two positions in the CIELAB color space:

AEab* = yjAa * 2 + Ab * 2 + AL *2

An important advantage of the CIELAB system is that the resulting color difference can be split into component contributions, namely lightness, saturation, and hue, corresponding to the arrangement of the color space:

Lightness difference

AL = Lt — Lr

Chroma difference (saturation difference)

ACab * = J aT * 2 + bx * 2 — J aR* 2 + bR * 2 Hue difference

AHb = xjAEab * 2 — AL *2 — ACab * 2

In 1994 the CIE proposed a modified formula called “CIE94”. This formula is to be tested by interested laboratories [1.26].

1.3.1.2