Page:Colorimetry104nime.djvu/33

Fig. 19.Dimensions of the surface-color solid: (a) opaque surf aces (b) transparent volumes.

c. Munsell System

The basis of the Munsell system is description of colors perceived to belong to surfaces in terms of hue, lightness, and saturation. Each such tridimensional description can be represented by a point plotted in a space diagram known as the surface-color solid, as shown in figure 19. In the surface-color solid the central axis represents the grays extending from black at the bottom to white at the top. Lightness of a chromatic (nongray) color determines the gray to which it is equivalent on this scale. Lightness is represented in the color solid by distance above the base plane. Hue determines whether a color is perceived as red, yellow, green, blue, purple, or some intermediate; it is represented in the color solid by angle about the central axis. Saturation indicates the degree of departure of a surface-color perception from the gray of the same lightness; it is represented by distance from the central gray axis.

The Munsell color system specifies a surface color by giving for usual viewing conditions its position on more or less arbitrary hue, lightness, and saturation scales having perceptually nearly uniform steps [107]. The Munsell term corresponding to lightness is Munsell value, that for saturation is Munsell chroma, and that for hue is Munsell hue. Munsell value is zero for the ideal black surface I having luminous reflectance equal to zero, and it is for the ideal white diffusing surface having luminous reflectance equal to 1. Munsell chroma is expressed in arbitrary units intended to be perceptually of the same size regardless of value and hue. The strongest known pigment colors have chromas of about 16; neutral grays have zero chroma as do black and white. Munsell hue is expressed on a scale intended to divide the hue circuit (red, yellow, green, blue, purple, back to red) into 100 perceptually equal steps. According to one convention the 100 Munsell hues are identified simply by a number from 1 to 100, and on this scale hues that differ by 50 are nearly complementary. Fig. 20.Diagram of hue circle with Munsell hue notation. Fig. 21.Alternate ways of expressing Munsell hue notation. The most common convention, however, is to divide these 100 hues into 10 groups of 10 hues each, and identify each group by initials indicating the central member of the group, thus: red R, yellow red YR, yellow Y, green yellow GY, green G, blue green BG, blue B, purple blue PB, purple P, and red purple RP. The hues in each group are identified by the numbers 1 to 10. Thus, the most purplish of the red hues (1 on the scale of 100) is designated as IR, the most yellowish as 10R, and the central hue as 5R, or often simply as R; see figures 20 and 21. The transition points (10R, 10 YR, 10Y, and so on) between the groups of hues are also sometimes designated by means of the initials of the two adjacent hue groups, thus: R-YR = 10R, YR-Y = 10YR, Y-GY = 10Y, and so on; but this convention is little used. The Munsell notation is commonly written: Hue value/chroma, this is, the hue notation, such as 6R, then the value, such as 7, and finally the chroma, such as 4, the latter two being separated by a solidus: 6R 7/4. More precise designations are given in tenths of the arbitrary steps of the scales, thus: 6.2R 7.3/4.4. The grays are indicated by the symbol N for neutral followed by the value notation, thus: N 7/ or N 7.3/; the chroma being zero for neutrals is not specifically noted.

Three representations of the Munsell system