642
23 Electricity-to-Light Conversion
(a)
(b)
Fig. 23.2 a Color-matching functions ¯
x, ¯
y, and ¯
z for the calculation of the CIE chromaticity, b color-matching functions
¯
r , ¯
g, and ¯
b for the calculation of RGB values
two-dimensional diagram, the CIE chromaticity diagram
6 as shown in Fig. 23.3a. White is represented
by x = y = z = 1/3. In order to relate the differences between colors as perceived by the human
eye more closely to the geometrical distance in the chart, a revision was made (Fig. 23.3b) with new
coordinates
u
= 4x/(−2x + 12y + 3)
(23.3a)
v
= 9y/(−2x + 12y + 3) .
(23.3b)
For CRTs the red-green-blue (RGB) color space is used.
7 The color matching functions for RGB values
are shown in Fig. 23.2b. The RGB values are related to the XYZ values according to
⎛
⎝
R
G
B
⎞
⎠ =
⎛
⎝
2.36461 –0.89654 –0.46807
–0.51517 1.42641 0.08876
0.00520 –0.01441 1.00920
⎞
⎠
⎛
⎝
X
Y
Z
⎞
⎠ .
(23.4)
The CIE RGB primaries from 1931 are at 700, 546.1, and 435.8 nm with the relative intensities 1.0,
4.5907, and 0.0601. A display device using three phosphors can only display colors in the triangular
area of the CIE chart between the three chromaticity coordinates. For sRGB,
8 the 1931 CIE primaries
and the NTSC
9 norm the coordinates are given in Table 23.2 and visualized in Fig. 23.3c. An optimal
coverage of the CIE chart involves monochromatic sources (for laser TV or LED displays) at about
680, 520 and 440 nm.
6 The coloring of the chart is provided for an understanding of color relationships. CRT monitors and printed materials
cannot reproduce the full gamut of the color spectrum as perceived in human vision. The color areas that are shown only
depict rough categories and are not precise statements of color.
7 RGB is an additive color system. However, printing devices use a subtractive color system. This means that the ink
absorbs a particular color, and the visible impression stems from what is reflected (not absorbed). When inks are combined,
they absorb a combination of colors, and hence the reflected colors are reduced, or subtracted. The subtractive primaries
are cyan, magenta and yellow (CMY) and are related to RGB via (C, M, Y ) = (1 − R, 1 − G, 1 − B).
8 Standard RGB color space as defined mainly by Hewlett-Packard and Microsoft, almost identical to PAL/SECAM
European television phosphors.
9 National television standard colors, US norm.
23 Electricity-to-Light Conversion
(a)
(b)
Fig. 23.2 a Color-matching functions ¯
x, ¯
y, and ¯
z for the calculation of the CIE chromaticity, b color-matching functions
¯
r , ¯
g, and ¯
b for the calculation of RGB values
two-dimensional diagram, the CIE chromaticity diagram
6 as shown in Fig. 23.3a. White is represented
by x = y = z = 1/3. In order to relate the differences between colors as perceived by the human
eye more closely to the geometrical distance in the chart, a revision was made (Fig. 23.3b) with new
coordinates
u
= 4x/(−2x + 12y + 3)
(23.3a)
v
= 9y/(−2x + 12y + 3) .
(23.3b)
For CRTs the red-green-blue (RGB) color space is used.
7 The color matching functions for RGB values
are shown in Fig. 23.2b. The RGB values are related to the XYZ values according to
⎛
⎝
R
G
B
⎞
⎠ =
⎛
⎝
2.36461 –0.89654 –0.46807
–0.51517 1.42641 0.08876
0.00520 –0.01441 1.00920
⎞
⎠
⎛
⎝
X
Y
Z
⎞
⎠ .
(23.4)
The CIE RGB primaries from 1931 are at 700, 546.1, and 435.8 nm with the relative intensities 1.0,
4.5907, and 0.0601. A display device using three phosphors can only display colors in the triangular
area of the CIE chart between the three chromaticity coordinates. For sRGB,
8 the 1931 CIE primaries
and the NTSC
9 norm the coordinates are given in Table 23.2 and visualized in Fig. 23.3c. An optimal
coverage of the CIE chart involves monochromatic sources (for laser TV or LED displays) at about
680, 520 and 440 nm.
6 The coloring of the chart is provided for an understanding of color relationships. CRT monitors and printed materials
cannot reproduce the full gamut of the color spectrum as perceived in human vision. The color areas that are shown only
depict rough categories and are not precise statements of color.
7 RGB is an additive color system. However, printing devices use a subtractive color system. This means that the ink
absorbs a particular color, and the visible impression stems from what is reflected (not absorbed). When inks are combined,
they absorb a combination of colors, and hence the reflected colors are reduced, or subtracted. The subtractive primaries
are cyan, magenta and yellow (CMY) and are related to RGB via (C, M, Y ) = (1 − R, 1 − G, 1 − B).
8 Standard RGB color space as defined mainly by Hewlett-Packard and Microsoft, almost identical to PAL/SECAM
European television phosphors.
9 National television standard colors, US norm.