PARTITIVE MIXING

Figure 4.8 and 4.9 show examples of a kind of additive mixing known as partitive mixing. Depending on the viewing distance, as the stripes become narrower, the light from them is eventually seen as a single colour whose hue and saturation follow the laws of additive mixing, but whose brightness is less than would result from simple additive mixing. Thus in Figure 4.8, where each pair of colours are additive complementaries, the resulting mixtures are neutral, because the total numbers of R,G and B phosphors glowing are equal in each case. However, as only half the number of phosphors are glowing as in an area of white screen, the mixture is seen as grey, not white. (The nonlinear response of our visual system to brightness explains why the grey looks more than half as bright as white). Similarly in Figure 4.9, the mixtures have the hue and saturation but not the brightness (or chroma) of the colours that would be expected from simple additive mixing. Partitive mixing may be thought of as a kind of averaging process as opposed to simple addition.

Figure 4.8. Partitive mixing of additive complementaries.

Figure 4.9. Partitive mixing of additive primaries.

An alternative term for partitive mixing is "optical mixing", a term which refers to the idea that the coloured light mixes"in the eye", and which sometimes creates the impression that the process is more subjective than it really is. Partitive mixing occurs when two visual stimuli for whatever reason can not be separately distinguished, and most or all such effects can be photographed as well as seen. Another example of partitive mixing is the mixing of colours with spinning discs (Figure 4.10).

Figure 4.10. Spinning disc displaying partitive mixing of additive complementaries, Ultramarine and Cadmium Yellow Light. Note that these pigments would mix physically to produce a green rather than a grey mixture.

Perhaps the first example of partitive mixing in art to come to mind is the pointillist technique of the Neo-Impressionist painters. This technique is sometimes erroneously said to produce brighter colours than can be obtained by the physical mixing of pigments. Optical mixing may or may not produce a brighter colour than a physical mixture of the same pigments, but pointillist technique on the whole produces colours that are more neutral but livelier than uniform physical mixtures of colours. Partitive mixing is also an important component (combined with subtractive mixing) in halftone printing and in the physical mixing of opaque paints (see Part 6).

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Next: Part 5: Subtractive Colour Mixing