When two or more light stimuli can not be separately distinguished, the light from those stimuli is seen as a single colour that follows the same rules of additive mixing as established for physically mixed lights. This type of colour mixing has long been known as optical mixing, a term that refers to the idea that the coloured light mixes "in the eye" of the observer. This should not be taken to imply that the process is more subjective than other kinds of stimulus mixing; such effects can be photographed with a camera as well as seen first hand. When the stimuli can not be distinguished because they are too small, the process is spatial optical mixing, as for example of the RGB subpixels on a computer screen, or finely interspersed dots of ink in a printed image.When the stimuli can not be distinguished because they are moving too fast, the process is temporal optical mixing, for example of the colours of a spinning disc.

Although the colours of the light stimulus follow the rules of additive mixing, most examples of optical mixing differ from the physical mixing of lights in that the light stimulus is seen as a property of an object rather than an independent light, and is therefore perceived as an object colour, judged relative to white object (see Fig. 1.1.4).

In the example of the RGB subpixels on a screen, where the light is emitted from the object, we judge the colour mixtures in relation to a white composed of all three RGB lights at their maximum brightness. This means that colours such as grey, brown, olive etc, that are not seen as colours of independent lights, are seen if the RGB lights are at low intensity, while white and bright colours can be seen if the lights are at full intensity. However in the examples where light is reflected from objects, i.e. printed or painted dots and spinning discs, the additive mixing is of a particular kind known as averaging colour-stimulus synthesis (Burnham et al., 1967), or more simply averaging or additive-averaging mixing. Additive-averaging mixtures, viewed normally, can not appear as white or bright colours, because the reflected light is in effect averaged over the area of the object instead of simply being added. For example in Figure 4.4.1, very little violet-blue light is reflected from the area of the yellow disc, and little red, yellow or green light is reflected friom the area of the ultramarine disc, so while the light reflected from the spinning discs is white, the amount of white light is less than a white disc would reflect, and the colour of the disc is therefore seen as grey.

Figure 4.4.1. Spinning disc displaying additive-averaging mixing of additive complementaries, Ultramarine and Cadmium Yellow Light. These pigments would mix physically to produce a green rather than a grey mixture.

Additive-averaging mixing can also be demonstrated on a light-emitting monitor if the mixing involves averaging of discrete areas of colour that are interspersed rather than superimposed (Figs 4.4.2, 4.4.3). 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 4.4.2, 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, because 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 4.4.3, the mixtures have the hue and saturation but not the brightness (and thus chroma) of the colours that would be expected from simple additive mixing.

Figure 4.4.2. Additive-averaging mixing of additive complementaries.


Figure 4.4.3. Additive-averaging mixing of additive primaries.

The term partitive mixing is sometimes used in the broad sense of optical mixing, and sometimes in the narrower sense of additive-averaging mixing, as used here. Additive-averaging mixing, combined with subtractive mixing, plays an important role in physical mixtures of opaque paints (see Part 6).

There is a widespread myth in art teaching that optical mixing of paints can produce brighter and/or stronger colours than can be produced by the physical mixing, and or at least that it was used with this misguided intention by the Neoimpressionist painters beginning with Seurat. An optical mixture of two or more paints is certainly higher in value than a physical mixture of the same paints, but optical mixtures as a whole are not lighter or more chromatic than physical mixtures. Indeed, optical mixtures of varied paint colours tend, due to the averaging principle, to lie towards the middle of colour space, i.e. medium value and low to medium chroma. The pointillist technique of the Neo-Impressionist painters utilized optical mixing that, because of the size of the dots, was only partial at the intended viewing distance, to produce a "soft" yet lively effect not obtainable using physical paint mixtures, based on the recommendation of Ogden Rood in his 1879 book Modern Chromatics. The myths mentioned above seem to derive from a misreading of Rood's book by contemporary and later critics.



Page modified August 5, 2012. Original text here.

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