ADDITIVE MIXING

Let's now look at the process of additive mixing. The richest screen colours are all obtained by combining the red (R), green (G) and blue (B) primaries two at a time. Mixing R and G without B generates a series of colours with positive y/b signals. When R and G are both at their full intensity, we see the result as bright yellow. This result is particularly surprising for artists used to the mixing of paints. Reducing one or other component shifts the yellow towards red and green respectively. Mixing the B and G phosphors together in various proportions creates a continuous range of hues with generally negative y/b and negative r/g, passing through a blue-green colour called cyan. R and B in various proportions generate a range of intermediate hues of negative y/b and positive r/g, passing through the red-violet colour we call magenta (Figure 4.3). Together these various combinations of two primaries at a time span the entire range of the hue circle.

Figure 4.3. Additive mixing of RGB colours.

Less saturated (i.e. more whitish) colours are produced by adding the third primary. The brightnesses of the RGB phosphors are configured so that when all three are at their maximum brightness, the mixture of light produced is seen as being neutral (white). Light is seen as being white whenever the mix of light wavelengths generates r/g and y/b signals that are both zero. You can explore all of these possible combinations using the interactive sliders in Figure 4.4.

Figure 4.4. Interactive demonstration of additive mixing of RGB screen colours. Sliders control the brightnesses of the R (top), G(middle) and B (bottom) screen phosphors. Copyright David Briggs and Ray Kristanto, 2007.

Mixing of projected light beams is another example of additive mixing, and produces the same results as those we saw with on-screen additive mixing (Figure 4.5).

Figure 4.5: Interactive demonstration of additive mixing of spotlights. Top, middle and bottom sliders control the brightnesses of the red, green and blue spotlights respectively. Copyright David Briggs and Ray Kristanto, 2007.

We've just seen that we experience light as being white if it produces r/g and y/b signals that are both zero. One way that this can occur is if all wavelengths of the spectrum are represented in equal energy. This is almost the case with direct sunlight, which is a little deficient in violet-blue wavelengths, and so looks slightly yellowish. But it can also be the case with much spikier spectral distributions, such as the white on your computer screen, or fluorescent lighting, as long as the inputs from the three bands of the spectrum balance out. This phenomenon of different spectral distributions looking identical in colour, i.e. being indistinguishable to our colour sensing system, is called metamerism. Metameric differences in the spectra of different light sources can cause a potentially serious annoyance for painters, in that it can result in substantial shifts in the tonal and colour relationships within a painting under different kinds of apparently similar lighting.

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