654 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS of color and is called chrorna on the Munsell map. Chroma is also divided into a series of steps, with the neutral axis or gray being 0, whereas a fully saturated sample has a chroma of 12. Color, or hue as it is called in the Munsell system (14), is presented by different planes around the vertical axis of the Munsell solid. In this system, the whole solid is divided into ten equal vertical segments with five principal hues (red, yellow, green, blue, and purple) occupying the central planes of alternate segments while intermediate hues (yellow-red, green-yellow, blue-green, purple-blue and red-purple) occupy the remaining planes. Each hue segment is further divided into ten sections numbered 1 to 10, with the main hue segment always numbering 5. The hue of a sample is designated by a number, indicating the section of the segment, followed by symbols which show the color of the segment involved. For example, 10P indicates the hue section 10 of the purple segment. To complete the specification of the sample, the value quan- tity follows the hue which is then followed by a stroke and then the chroma. Here the color defined at 10 P •/• has the hue 10 P, the value 5, and the chroma of 8. COLOR DIFFERENCE AND TOLERANCE It is of interest to point out that color differences are not entirely linear throughout the entire CIE chromaticity diagram since, if one cal- culates two points on this diagram, another set of points twice as far apart do not necessarily have a totally linear difference in color (9, 15). In other words, equal distances on the diagram do not always indicate the same degree of color change. Judd (16) devised the Uniform Chromaticity Scale (UCS) adopted by the National Bureau of Standards (NBS). Adams (17) followed with his chromatic-value diagram which was accepted by the American Society for Testing Materials (ASTM). MacAdam (18) investigated color dif- ferences where the observer made a large number of matches on a series of test colors located at various points in the chromaticity diagram. The spread in the color settings necessary to make a match (Fig. 5) was used by MacAdam as the criterion of sensitivity to color differences. The distances from points on the boundary of each ellipse to the central point within the ellipse approximately correspond to the chromaticity difference just perceptible with certainty under specified viewing condi- tions this just perceptible color difference is defined as one MacAdam unit. The orientation of the major axis of individual ellipses is a func- tion of the dominant wavelength and purity changes, but does not

COLOR AND ITS MEASUREMENT 655 Y 1.s 52.• 530 •0 •'X •4• o.4- 499 • • •oo F•gure 5. MacAdam's perceptible chromaticity differences in the CIE diagram. The axis of each ellipse has been multiplied by 10 for illustrative purposes SOURCE MONOCHROMATOR PHOTO-SENSO MPLE Figure 6. Schematic diagram representing the major components of a spectrophotometer or colorimeter. Spectrophotometers use prismatic or grating monochrometers where colori- meters would contain filters change with lightness i.e., the shape and orientation remain constant at different luminosities. The ellipses do vary in size with a change of lightness, becoming smaller as lightness increases, since errors in making color decisions decrease under improved light-viewing conditions. COLOR MEASURING INSTRUMENTS Reflectance Spectrophotometers This type of instrument, diagrammed in Fig. 6, is based on a beam of monochromatic light penetrating into a sample. This light is scattered in many directions, is partially absorbed, and finally re-emerges to the