5Color

Figure 5.1 A three-dimensional color solid in the form of a double cone. The hue circle forms the common base of the two cones, and the neutral grays are situated on the central axis. Object colors find their natural position within this double cone. Bl ¼ black; W ¼ white. (See also color plate section.)

the paper and textile industries, in interior design and painting, in lighting engineering etc., have given rise to different practical solutions of the three-dimensional ordering of color stimuli. Many of the color systems are hybrid in that they have been constructed on compromises between perceptive arrangements and technical requirements. Equal perceptual scaling of visual differences is one important requirement for a system that is to be used for product control. This has been attempted in the widely used Munsell System.

The best known color-ordering systems today, where color atlases are available, are the Munsell System from the USA, the Swedish NCS (Natural Color System), and until recently the German DIN system (Deutsche Industrienormen). The structure of the Munsell system is shown in Figure 5.4(a). A vertical cut through the color solid is displayed in Figure 5.4(b), with the coordinates chroma (color strength) and value (lightness). Ideally, in this system all chroma steps and all value steps should have the same perceptual difference (which is, however, only approximately fulfilled). These differences were determined empirically by over 70 subjects participating in scaling experiments. Because of the established use of the Munsell system in determining color tolerances in industrial processes, i.e. in the dying of ceramic tiles and textiles, this system has served as a reference standard for quantitative color scaling and discrimination, and also for color vision models.

The NCS is an attempt to realize a system based on Leonardo da Vinci’s simple colors and Hering’s unique colors by using them as reference points under welldefined viewing conditions (Johansson, 1952). The hue circle has been scaled in terms of relative contributions of elementary hues (Figures 5.1–5.3).

B Y

Bl

W

(c)

G R

Bl

Figure 5.2 (a) A hue circle with the elementary hues Y (yellow), R (red), B (blue) and G (green) on the axes. The hues in between contain proportions of the two nearest elementary hues. (b, c) Examples of opposing hue triangles from vertical cuts through the color solid of Figure 5.1. The horizontal and vertical dimensions are chromatic strength and lightness, respectively. (See also color plate section.)

Unique yellow is characterized by it being ‘neither reddish nor greenish’. It is thus determined purely subjectively by means of the other, neighboring unique hues on the hue circle. Unique blue satisfies the same criterion. The yellow–blue pair is opponent in that the two color percepts mutually exclude one another. No object color is seen as

Figure 5.3 A hue circle with 24 steps (after Miescher et al., 1961), here reproduced from the original (the printing process will have reduced chromatic strength and has introduced hue distortions as well). Opposing unique hues on the axes. (See also color plate section.)

both yellowish and bluish at the same time, in the way that purple can be said to be perceptually composed of blue and red. The same reasoning applies to the ‘neither yellowish nor bluish’ unique red or unique green.

All other hues are experienced as being a transition between two unique, or elementary hues (e.g. orange is a transition between yellow and red). This leads to an arrangement of surface colors in a three-dimensional color solid, e.g. like the double cone in Figure 5.1. The vertical central axis represents all neutral, achromatic colors between black and white, whereas the chromatic colors of maximum color strength are situated on a hue circle, in a common plane of the double cone [Figures 5.2(a) and 5.3]. Figure 5.3 reproduces a 24-step hue circle made by the Swiss chemist and color scientist Karl Miescher (1892–1974). In an experiment with 28 subjects, Miescher used Hering’s principle to construct a hue circle for daylight illumination (Miescher, 1948; Miescher et al., 1961). A vertical cut through the solid of Figure 5.1 displays schematically the dimensions of color strength and relative lightness (Figure 5.2(b) and (c)]. A similar schematic order of colors was described in 1611 by the Swede A.S. Forsius in his book Physica.

Before we present other, psychophysical color systems, we need to take a closer look at the physical properties of light and matter that are of importance for color perception and for color technology.