4.4 ADDITIVE-AVERAGING MIXING

When two or more light stimuli can not be distinguished separately, the light from those stimuli is seen as a single colour that follows the rules of additive mixing. This type of colour mixing has long been known as optical mixing, a term which refers to the idea that the coloured light mixes "in the eye". This should not be taken to imply that the process is more subjective than it really is; most or all such effects can be photographed as well as seen first hand. When the stimuli can not be distinguished because they are too small, the process is called spatial mixing, as for example of the RGB lights 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 called temporal mixing, for example of the colours on a spinning disc.

Although the colours of the light mixtures follow the rules of additive mixing, all of these examples of spatial and temporal mixing differ from the additive mixing of light beams in that the mixture is seen as an object colour. In the example of the RGB lights on a monitor, where the light is emitted from the object, we judge the colour mixtures in relation to the white composed of all three RGB lights at their maximum brightness. This means (1) that mixtures seen as white and bright colours can be produced if the lights are at full intensity, and (2) that colours such as grey, brown, olive etc, that are not seen as colours of lights can be produced if the lights are at low intensity.

However in the examples where light is reflected from objects, i.e. printed dots and spinning discs, the additive mixing is of a particular kind known as averaging or additive-averaging mixing (or more precisely, averaging colour-stimulus synthesis [Burnham et al., 1967]). Additive-averaging mixtures (viewed normally) can not appear as white or bright colours, because the remitted light is in effect averaged over the area being viewed instead of simply being added. Additive-averaging mixing (combined with subtractive mixing) plays an important role in physical mixtures of opaque paints (see Part 6). The newer term partitive mixing is sometimes used in the broad sense of optical mixing, and sometimes in the narrower sense of additive-averaging mixing.

Additive-averaging can also be demonstrated on a light-emitting monitor, as long as the mixing involves averaging of discrete areas of colour that are interspersed rather than overlapping (Figs 4.4.1, 4.4.2). 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 4.4.1, 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 4.4.2, 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 4.4.1. Additive-averaging mixing of additive complementaries.

 

Figure 4.4.2. Additive-averaging mixing of additive primaries.

Similarly, in this example of a spinning disc, 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, so the colour of the spinning disc is seen as grey.

Figure 4.4.3. Spinning disc displaying partitive mixing of additive complementaries, Ultramarine and Cadmium Yellow Light. These pigments would mix physically to produce a green rather than a grey mixture.

Optical mixing has long been mistakenly said to produce brighter or stronger colours than can be produced by the physical mixing of paints, and to have been used with this intention by the Impressionist painters. Optical mixtures are typically lighter than physical mixtures of the same paints, but as a whole are neither brighter nor more chromatic than physical mixtures of paints. Indeed, optical mixtures of varied colours inevitably tend towards medium lightness and low chroma. The pointillist technique of the Neo-Impressionist painters utilized partial optical mixing at the intended viewing distance, to produce a "soft" yet lively effect not seen in physical paint mixtures.

 

 


Page modified August 5, 2012. Original text here.

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