The C.I.E. Color Space

The CIE system characterizes colors by a luminance parameter Y and two color coordinates x and y which specify the point on the chromaticity diagram. This system offers more precision in color measurement than do the Munsell and Ostwald systems because the parameters are based on the spectral power distribution (SPD) of the light emitted from a colored object and are factored by sensitivity curves which have been measured for the human eye.

Based on the fact that the human eye has three different types of color sensitive cones, the response of the eye is best described in terms of three "tristimulus values". However, once this is accomplished, it is found that any color can be expressed in terms of the two color coordinates x and y.

The colors which can be matched by combining a given set of three primary colors (such as the blue, green, and red of a color television screen) are represented on the chromaticity diagram by a triangle joining the coordinates for the three colors.

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The C.I.E. Chromaticity Diagram

The diagram at left represents the the mapping of human color perception in terms of two CIE parameters x and y. The spectral colors are distributed around the edge of the "color space" as shown, and that outline includes all of the perceived hues and provides a framework for investigating color.

The diagram given here is associated with the 1931 CIE standard. Revisions were made in 1960 and 1976, but the 1931 version remains the most widely used version. The diagram at lower left is a rough rendering of the 1931 CIE colors on the chromaticity diagram.

Diagram with annotation Associate with colors
The 1976 CIE Chromaticity Diagram
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The C.I.E. Chromaticity Diagram

On the CIE chromaticity diagram at left, some annotation is made about the significance of different parts of the diagram. The boundary represents maximum saturation for the spectral colors, and the diagram forms the boundary of all perceivable hues.

Calculation of coordinates
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Approximate colors can be assigned to areas on the CIE Chromaticity Diagram. These are rough categories, and not to be taken as precise statements of color. The boundaries and the color names are adapted from Brand Fortner, "Number by Color", Part 5, SciTech Journal 6, p32, May/June 1996.

Any attempt to depict the gamut of human color vision on a computer monitor must be accompanied by numerous qualifications and exceptions. In the first place, you cannot display the range of human color perception on an RGB monitor - the gamut of normal human vision covers the entire CIE diagram while the gamut of an RGB monitor can be displayed as a triangular region within the CIE diagram. Another qualification is that the hue and saturation associated with a given color name can vary over a considerable range. Add to that the variations with different kinds of display monitors, and you rightly conclude that an accurate rendition is impossible. With all those excuses, however, it still might be instructive to provide a rough idea of the regions of the CIE Diagram associated with common color names.

The display here was created by choosing representative RGB values for the color regions from a rendition of the 1976 CIE Chromaticity Diagram provided by Photo Research, Inc. Note that one representative value in about the middle of the hue and saturation ranges was chosen for each section of the diagram. The point chosen was just a visual judgment of a representative color in the range. The RGB values obtained are listed in the table at right. A different observer would likely have chosen different points to represent the color names, but at least these values might provide a starting point for preferred variations.

One characteristic of the commonly used 1931 CIE Chromaticity Diagram that is evident even from this crude portrayal is that the green takes up far too much of the landscape compared to the number of visually different colors in the region. That was one of the shortcomings that the 1960 and 1976 revisions sought to address.

Color name
Red
Green
Blue
Red
191
27
75
Pink
245
220
208
Reddish orange
216
119
51
Orange pink
240
204
162
Orange
228
184
29
Yellowish orange
231
224
0
Yellow
234
231
94
Greenish yellow
235
233
0
Yellow green
185
214
4
Yellowish green
170
209
60
Green
0
163
71
Bluish green
24
162
121
Bluegreen
95
164
190
Greenish blue
110
175
199
Blue
92
138
202
Purplish blue
88
121
191
Bluish purple
92
102
177
Purple
246
85
158
Reddish purple
196
64
143
Purplish pink
243
208
219
Red purple
175
35
132
Purplish red
209
65
136
White
255
255
255
Calculation of coordinates
Index

CIE concepts

Color measurement concepts
 
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