Distinction between color mixtures

Addition of colored light beams is performed onto the retina. It can be obtained by three different ways:

1. By superposition over the same retinal area of colored light beams, scattered onto the observed object towards the eye of the observer (think for instance of lighting spots in a theater stage)

2. By juxtaposition of colored zones on very close retinal areas, separated by less than the spatial resolution of the eye (visual acuity ≈ 1.5 arcminute). This is the case of TV or computer screens pixels, made up of three sub-pixels (Red, Green, Blue) which are not distinguishable at normal viewing distance.

3. By successive display of colored beams with a frequency higher than the characteristic cutoff frequency of the human “sensor” (eye+brain). It is for instance the case of the spinning color wheel, composed of different colored segments: it appears white when the spinning speed is high enough

An additive mixture of (see figure 6) :

• Green and Red gives Yellow (Green + Blue = Cyan ; Red + Blue = Magenta)

• Yellow and Blue gives White (as well as Cyan + Red or Green + Magenta)

• Red, Green and Blue gives White.

In this case the spectrum of the light is modified by its propagation in a colored medium before entering the eye. One can distinguish two types of subtractive processes:

1. Absorption and selective transmission of light. This is the case of a colored filter which absorbs selectively one part of the incident spectrum. This principle is used in color CCD sensors, for instance. .

2. Absorption et selective scattering of light. It is the case of painting pigments, which absorb selectively one part of the incident spectrum and reflect the other part (a yellow pigment will absorb blue, for instance).

A subtractive mixture of (see figure 7):

• Green pigments (reflecting only intermediate wavelengths, absorbing blue and red) and red pigments (reflecting only long wavelengths) will give black (or in practice, dark brown).

• Yellow pigments (absorbing only short wavelengths, i.e. blue) and cyan pigments (absorbing only long wavelengths, i.e. red) will give green. Yellow and Magenta will give Red, Cyan and Magenta will give Blue.

• Magenta, Cyan and Yellow pigments will give black.

In the printing industry, color images are produced using a four-color process with cyan, magenta, yellow and black inks: halftone dots are sized depending on the desired color tone. When overprinted, some of the dots corresponding to the individual colors are adjacent to one another, while others partially or entirely overlap. In this case the resulting color is the result of both subtractive mixing and additive mixing, since when looking at an offset-printed item at a normal viewing distance, our eyes are unable to distinguish the individual dots. The combination of additive and subtractive mixing is called autotypical mixing (figure 8).