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Essay on Color (continued)

3. TECHNOLOGY: Obtaining Colors by Mixing
Hues and shades of color surround us by the thousands. If we want to reproduce them adequately in image-forming devices, do we really need to prepare thousands of colored inks, of pigments, of lamps, of filters?

Basic principles of human visual system (para. 1.2, neurological facts) suggest that it is possible to describe any color in a tristimulus system (para. 1.3, measuring color sensation) ... provided that you choose three appropriate primary colors.

And indeed some artist began very early to use only three primary colors (plus white) for obtaining all the other necessary hues by mixing.

Their habit is employed in modern color-image forming devices, too.
Admitted: sometimes the gamut of producible colors may turn out to be limited (para.3.3, gamuts). But in most cases these technically produced images really show good and pleasing color rendition.

Which colors are apt for primary colors? -- Well, in principle you could choose any triple of colors, provided that
* all colors are sufficiently saturated   AND
* the entire triple is evenly distributed across the visible spectrum.

But the actual choice of primary colors is governed by one question: do we mix colored lights (additively) or surface colors (subtractively)?

3.1 Mixing Additively
The basic idea behind additive mixing:
Use three narrow-band colored lights, each of which stimulates just one of the three cone types (fig. 1.2-c in para.1.2, neurological facts). Then add weighted amounts of these three stimuli (from three adjustable light sources) within every image point to which you want to assign its own color.

Now the three cone spectral characterstics show considerable overlap. So, the basic idea will not perform ideally. But we get quite sufficient color rendering performance if we use for example the
R(ed), G(reen), B(lue)
color triple shown in fig. 3.1-a and in fig 2.3 (bottom) .

These three colors are called "primary colors for additive color mixing".

    fig.3.1-a: additive mixing of R G B (19 kByte)

Important in fig. 3.1-a is the black background. It is intended for showing that there is no light outside the colored spots; these spots result from light beams on an otherwise totally dark screen.

One additional hint:
Fig. 3.1-a shows many number triples, for example 0,255,255. These are the R,G,B-intensities or -lightnesses in 8-bit (decimal) representation. So, "255" simply means "100%". And the black background would be represented by "0,0,0".

In fig. 3.1-a, please note:

* The first-order (additive) mixtures of R, G, B with 100% lightness each are exactly the primary colors C(yan), M(agenta), Y(ellow) for subtractive mixing (para. 3.2).

* Mixing equal amounts of   R + G + B   results in colorless white (or grey).

* Black is obtained by setting   R = G = B = 0 .

* In Fig.3.1-a, red and green and blue are all presented at 100% intensity; mixtures with intensities varying over R and G and B will accomplish all the rest of the gamut.

For better explaining the facts from physics, I'll just this once repeat a figure which you already know from para.2.3:

    fig.2.3: colorful spectral characteristics (188 kByte)

Now please assume that behind each of these 6 filters there is a white lamp adjusted to the same intensity of 100%. Then, from the filter curves you can clearly see that

+ light from the B filter together with light from the G filter is the same as light from the C filter;

+ light from the R filter together with light from the B filter is the same as light from the M filter;

+ light from the R filter together with light from the G filter is the same as light from the Y filter;

+ lights from all three filters   R, G, B   add up to white light;

+ the colors   R, G, B   are darker than   C, M, Y   because they are narrow-band (when compared to   C, M, Y  ).

But who in the world might mix three colored lights for making up a color?

Well, there's quite a lot of applications, including:

    fig.3.1-b: color television CRT (33 kByte)

* In color television sets and computer monitors, every picture element is made up of 3 sub-elements (Fig. 3.1-b). Each sub-element displays one of the colors R,G,B. For the eye, the sub-elements are usually too small; they remain invisible. Via overlapping blur circles, their color stimuli add to one another for our eyes.

* Highly efficient three-band fluorescent lamps (fig. 2.2-b in para.2.2) dominate the lighting technique in schools, department stores and the like. Their color temperature (para. 1.3.4) is set by the appropriate mixture of red, green, and blue phosphor components.

* White LEDs seem to be light sources of our future. They either use three chips for R,G, and B in one housing -- or they add a broad-band yellow phosphor emission to the blue emission peak of a single LED chip.

This last application example leads us to the idea of complementary colors:
Pairs of colors that result in white light upon additive mixing are called "complementary". From the spectral characteristics in fig. 2.3, we see that complementary color pairs are:
* red   and cyan,
* green and magenta,
* blue  and yellow.

Complementary colors are opposed to one another in Goethe's color circle, which you find in fig. 1.1-c at the end of para. 1.1.

Complementary colors are not only defined by the "adding results in white"-statement. You can also find complementary pairs by simple experiments (which already Goethe performed):
Prepare two similar sheets of paper. Leave one blank. Put a big spot of saturated red color on the other one. Gaze at the well-illuminated red spot for some minutes. Then swiftly replace the sheet with the red spot by the blank sheet and look at the latter. What do you see? - You're going to see the disappeared spot, and it will have the complementary color.
(Reason is an exaggerated bleaching process in the corresponding cone dye, see fig. 1.2-c in para. 1.2.)

Please be cautious when calling colors "complementary". This is not the same as "maximum color contrast"; the latter is described in para. 1.3.2.

Having seen several application examples of additive mixing with R,G,B primary colors you won't be astonished that these colors are standardized. But in my opinion the standards are not worth while.
First, because there are different standards for different applications.
Second and most important: They all define these colors with just their tristimulus values ... leaving their applications prone to metamerism (see para. 3.4).

Link List and Literature

Subject used in source
mixing R G B fig.3.1-a teleseminar-2000
filter set curves fig.2.3 Edmund Industrie Optik GmbH, "2002 Optics and Optical Instruments Catalog" p.76 (modified)
color CRT fig.3.1-b Microsoft Encarta Enzyklopaedie 2002 (modified)

Cont'd: 3.2 mixing subtractively   Contents of entire essay   Contents of entire web site

Last modified March 13th, 2003; 11:11