Mixing Processes
Of all types of mixing, we are probably most familiar with mixing paints. This becomes our point of reference for how colour behaves, instead of recognizing that the behaviour of coloured paints in mixtures is indicative of a specific type of process. Many misconceptions about the nature of colour arise when we only consider colour’s behaviour within the context of paints, such as all colours can be mixed from a limited set of three primaries. We are surprised when the mixtures of sources of light are not the same as what we find in mixing paints. To understand mixing, we need understand that there are different types of mixing processes, and different media can be mixed using each type of process. The process, the specific media used and even the brand of media used all impact the final result.
It is important to recognize that we do not mix colour, rather we mix coloured media. We perceive various colours, as our visual system absorbs and interprets light distributions which enter our eyes. In each type of mixing process, the media interact with light in slightly different ways.
1) Subtractive mixing process
The subtractive mixing process can be thought of as a physical or external mixing process (compared to the additive mixing process, which can be thought of as a perceptual or internal mixing process). In this process, different colorants are combined, resulting in a mixture with a new colour.
Subtractively mixed colorants (or filters) remove light, each with a different range of wavelengths from the same light beam. The remaining light is reflected to our eyes, and our visual system interprets the light as the mixed colour.
For example, if we physically mix blue and yellow-coloured paints, each paint removes light with its own set of wavelengths from a light beam. The light reflected by the mixture enters our eye, and our visual system interprets the mixed paint colour as green.
Examples of various greens mixed from different blue and yellow-coloured paints. Image courtesy of Robin Kingsburgh.
The exact mixed colour will vary, depending on the type of paint used (e.g. oil vs acrylic), the specific paints used (e.g. phthalo blue vs ultramarine blue) and also the brand of paints used. Each of these impacts the way a particular paint absorbs and reflects light.
The mixed colour also depends on the relative proportions of paint.
Mixtures from varying proportions of Ultramarine and Diarylide Yellow. Image courtesy of Maggie Maggio.
Another example of the subtractive mixing process occurs when a single beam of white light is sent through stacked coloured filters. In this instance, instead of a coloured paint reflecting light, a coloured filter will transmit light with a particular range of wavelengths. The Figure below demonstrates what happens when a single beam of light is sent through blue and yellow-coloured filters. In this instance, light with middle wavelengths can pass through both filters most easily, resulting in the perception of green.
A single beam of white light passing through both blue and yellow-coloured filters produces a subtractively mixed green. Image courtesy of Maggie Maggio.
2) Additive Mixing Process
Additive mixing processes can be thought of as perceptual or internal mixing processes (compared to the subtractive mixing process, which can be thought of as a physical or external a mixing process). There are several variations of the additive mixing process.
2.1) Simple Additive Mixing Process
Two separate beams of white light passing through blue and yellow-coloured filters produce an additively mixed white, where the coloured light beams overlap. Image courtesy of Maggie Maggio.
Simple Additive Mixing occurs when lights of different colours from two or more sources combine or add together before the combined light beam reaches your eye. The lights from different sources add together, and your visual system interprets all the light entering your eye as one colour perception.
The Figure demonstrates what happens when a beam of white light is sent through a blue coloured filter, a second beam of white light is sent through a yellow- coloured filter, and the two coloured light beams overlap. In this case the overlapped areas result in a mixed colour close to white.
2.2) Optical Mixing Process
In Optical Mixing, similar to Simple Additive Mixing, lights of different colours from two or more sources combine or add together before the combined light beam reaches your eye. The lights from different sources add together, and your visual system interprets all the light entering your eye as one colour.
2.2.1 Example: Spatial optical mixing on screens
One example of optical mixing occurs as we view colours on our phone, computer or television screen. These types of displays contain pixels, each containing multiple sources which emit light over narrow wavelength ranges, typically centred on short, middle and long wavelengths, corresponding to blue, green and red coloured light beams. To generate different colours on a screen, each source within each pixel emits varying amounts of light, which is then interpreted by our visual system.
Representation for magnified blue, yellow and white areas of a screen. Magnified areas depict which sources emit light within each pixel, for each additive spatial optical mixture. Image source: David Briggs’ Figure 4.4.2 D from Additive-averaging and pointillist mixing.
2.2.2 Example: Averaging/Partitive Mixing
Some optical mixing is also known as averaging/partitive mixing. It is a form of additive mixing where there is a dilution or averaging of the intensity of light that enters our eyes compared to simple additive mixing. The mixed colours appear more muted or diluted compared to the potential vibrant mixtures found in simple additive mixing or spatial optical mixing on a screen. The averaging can occur either spatially, or temporally.
The figure below shows examples of spatial optical mixing. In A-C, stripes of alternating colours become thinner and thinner, until they become too thin to be distinguished as separate sources. Their combined light enters the eye to generate the sensation of a single colour. In this case, the achromatic colour of grey is perceived.
Examples of spatial optical mixing. Image source: David Briggs’ Figure 4.4.2 A-C from Additive-averaging and pointillist mixing.
2.2.3 Example: Temporal additive mixing with spinning disks
In temporal additive mixing, light from sources become averaged in time when they are moving too rapidly to be distinguished. This effect is seen when mixing coloured areas using a rapidly spinning disk.
Preparation of a disk to demonstrate comparative mixing of blue and yellow-coloured paints. Central portion of disk contains subtractively mixed blue and yellow paints. Outer portion of disk contains separate areas of blue and yellow, which will optically mix when disk is spun rapidly. Image courtesy of Maggie Maggio.
Demonstration of temporal optical mixing with a spinning disk. GIF courtesy of David Briggs.
