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Color Addition

Additive mixing of color takes place when two or more light beams with different colors are superimposed on a screen or directly on the retina of the observing eye. One example is theater stage illumination where several colored light projectors illuminate the same region. Another example is the well-known rotating top with a disc of colors. If all colors of the spectrum are in the disc, a neutral gray color is observed when the top is rotating. A third example is in color television screens, or in computer screens. Extremely small red, green and, blue dots are produced on the screen. The relative intensities of these colors produce a wide range colors for the eye.

The CIE chromaticity diagram is a useful tool to predict the color produced by different color additive mixtures. Assume that two light fields with spectral powers P₁(λ) and P₂(λ) are mixed. If the tristimulus values are (X₁, Y₁, Z₁) and (X₂, Y₂, Z₂), the total tristimulus values are expressed as

The chromaticity coordinates for the addition of the two fields are (x_T, y_T, z_T) given by

Now, when we define the weight w_i of the colored light beam i, plotted in Fig. 6.1 as

Thus, with these definitions, the chromaticity coordinates for the combination of two colored light beams become

This result can be interpreted as a lever in equilibrium, where each of the two colors with weights w₁ and w₂ are on each end of the resultant color at the equilibrium point (Fig. 6.2). The conclusion is that any color in the line joining the two added colors can be reproduced with the proper weight of these components. Generalizing this result, we conclude that with three colors—A, B, and C in the chromaticity diagram (Fig. 6.3)—any color inside the triangle can be faithfully reproduced. To find the resultant color a first step is to find the color resulting from the combination of A with B, and then combine that color with color C. Figure 6.4 shows how three colors combine by color addition. Unfortunately, these rules do not apply for the CIELAB uniform color system.