PART 4. ADDITIVE COLOUR MIXING

THE ADDITIVE PRIMARIES

We noted in Part 3 that the three cone types effectively divide the visible spectrum into three bands, a band of blue to violet colours, where S > (L + M) gives negative y/b values, a band of greenish colours, where M > L gives negative r/g values, and a band of reddish colours, where R > M gives positive r/g values. It should be evident from the way the opponent scores work that we will be able to produce a mixed light of any possible combination of positive and negative r/g and y/b values, and therefore a complete range of hues, from only three component lights, provided that one light is from each of these three bands.

This is how the colours on your screen are generated, from phosphors of just three colours, a red (R), a green (G), and a blue (B) (Figure 4.1). All of the colours that you have ever seen on a monitor or television, including white and grey, were created by mixtures of light of these three colours. This process of colour mixing by adding coloured lights is called additive mixing, and colours of lights used to create such mixtures are called additive primaries.

Figure 4.1. Spectra of red, green and blue CRT screen primaries. SOURCE: http://en.wikipedia.org/wiki/Image:CRT_phosphors.png

As you might expect, while such mixtures of three lights can produce colours in a full range of hues, these mixtures may not appear as saturated as a monochromatic light of the same hue. Any three coloured lights used as primaries will have a range or gamut of colours that they can produce by mixing, and will be unable to mix colours outside that gamut. The maximum range of colours produced by additive mixing of three lights is generated by monochromatic lights of wavelengths of 400 nm (bluish purple), 535 nm (yellowish green) and 700 nm (red). Monochromatic light would however be a very energy-inefficient way of creating colours on a computer screen. The colours used on monitors are chosen as a compromise between an acceptably broad gamut on the one hand and energy efficiency on the other. Figure 4.2 shows the maximum monochromatic gamut compared to a representative gamut of screen colours on a CIE chromaticity diagram. (The CIE system is one of the few aspects of the dimensions of colour that is already well-described in numerous books and websites, and need not be recounted here. For a good explanation of the system see here).

Figure 4.2. The maximum possible additive mixing gamut of three monochromatic lights (Hardy and Wurzburg, 1937), and the representative gamut of RGB phosphors shown on a CIE 1931 chromaticity diagram. Source (CIE diagram and phosphors): http://www.efg2.com/Lab/Graphics/Colors/CIE1931SMPTE.jpg

Some writers slip into referring to the R,G and B phosphor colours as the additive primary colours, which is not strictly correct, because light of any wavelength or wavelengths within each of the reddish, greenish and blue-violet spectral bands would make serviceable primaries for mixing a full range of hues at high saturation. On the other hand, the degree of arbitrariness of the additive primary colours is also frequently overstated. The exact wavelength or wavelengths used as primaries for mixing a full range of high-saturation hues is flexible, but their being located in one each of the three abovementioned spectral bands is not.

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