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Essay on Color, Para. 3, obtaining colors by mixing (continued)

3.2 Mixing Colors Subtractively
A bundle of light rays, containing various wavelengths, can be passed through a filter. This way, some wavelengths will be attenuated more than others; the color of the bundle is modified.

Important to note: These filters are "passive", they can only attenuate, the bundle power is diminished.

If we cascade two filters, making the bundle pass through one after the other, the second filter will diminish bundle power even further. And the second filter can do this at different wavelengths.

So, cascading two different filters will always narrow the resulting light bandwidth and -power. This has apparently led to naming it a "subtractive" color mixing. (Though, of course, attenuation is described mathematically by attenuation factors. And these, of course, multiply if you cascade filters.)

Knowing the bandwidth-narrowing effect of subtractive mixing you'll understand that for subtractive mixing, we prefer to take broadband primary colors. Especially well-established are the subtractive primary colors C(yan), M(agenta), and Y(ellow).

The next figure shows how they mix with one another.

    fig. 3.2-a: Subtractive mixing of CMY (30 kByte)

Important in fig. 3.2-a is the white background. It is intended for showing that outside the colored spots there is all light transmitted; the spots result from filters or imprinted inks on an otherwise totally white area.

In fig. 3.2-a, please note:

* The first-order (subtractive) mixtures of C, M, Y with 100% saturation each are exactly the primary colors R(ed), G(reen), B(lue) for additive mixing (para. 3.1).

* Mixing equal amounts of   C + M + Y   results in colorless black (or grey).

* White is obtained by setting   C = M = Y = 0 .

* In Fig.3.2-a, cyan and magenta and yellow are all presented at 100% saturation; mixtures with saturations varying over C and M and Y will accomplish all the rest of the gamut.

For understanding these mixing rules from the wavelength viewpoint, please refer again to fig. 2.3, presented in para.3.1.

How and where is subtractive color mixing used?

Probably the biggest applicational area is color printing.
No matter whether you use desk-top color printer, color copier, newspaper or offset or book printing machines or print on demand systems: They all use CMY primary colors.
Usually, the printed color saturation is set by choosing a screening factor, the latter being known since the times of grey half-tone printing.
For color mixing, two or more screened inks are printed onto the same area.
Because of imperfections that the inks and toners bear, C and M and Y are far from the ideal that is shown in fig. 2.3 (top) in para. 3.1. The most annoying consequence: Black areas will turn out to be brown, or bluish, or greenish. So people decided to introduce black as a fourth printing color. A decision which was not so easy to make since each and every new printing color needs its own new section in the printing machine.
And even more effort is put into art reproduction: here, 7 and more inks are printed in one run; see para. 3.3, gamuts.

An example of colored newspaper printing (promotion paper from famous swiss retailer "Migros") is shown in the next figure:

    fig. 3.2-b: Color print example (195 kByte)

The left side of fig. 3.2-b shows the printed image in normal size.
Besides a table with exotic fruits and besides text you see so-called registration marks.
With these marks, the craftsman at the printing machine controls synchronism of all 4 printing sections in the printing machine. (In a bit more demanding applications, these marks are cut off after printing.)

A portion of these registration marks is enlarged at the right side of fig. 3.2-b. From bottom to top you see a remainder of cyan registration, then magenta registration with sample screening factors of 2 - 3 - 5 - 8 % and the same repeated in yellow. Above this, the "grey" stripe turns out to consist of cyan-, magenta-, and yellow-colored dots ... a simple indicator for good color balance.

For printing purposes, the colors CMY are standardized, e.g. in DIN16539, Euro-Farbskala fuer den Offsetdruck. Though, with respect to para. 3.4 of this essay, I don't understand why spectral color definition is avoided and only tristimulus coordinates are given.

Other applications of subtractive color mixing include:
* lacquer and paint of all sorts,
* artist's paintings,
* ("Old fashioned") wet-chemical photographs and movies.

All subtractively mixing colorants (including inks, toners, paints) can come in one of two possible versions:
opaque or transparent.

Opaque colorants contain a considerable amount of pigments:
* Either white or black pigments (setting the saturation of color);
* or pigments that bear the color hue themselves.
Films of these colorants can be deposited on nearly arbitrarily colored substrates. Since they are opaque, the substrate shine-through is often neglegible. Substrate with film can even reflect more light than without.

Transparent colorants are clear dye solutions.
Films of these colorants optically work together with their substrates. They act like a clear glass filter upon the substrate. Substrate structure will shine through. Substrate with film is always darker than without (except for some rare colorants that fluoresce).

Link List and Literature

Subject used in source
mixing C M Y fig.3.2-a metacolor
printing C M Y K fig.3.2-b "Brueckenbauer, Wochenzeitung der MIGROS"
Dec.17th, 2002, p.27   brueckenbauer

  Continued: 3.3 gamuts     Contents of entire essay     Contents of entire web site

Last modified March 14th, 2003; 18:36