Light & Color
Process printing is an attempt to reproduce all the colors of the spectrum with just three colors plus black. To understand why we use the specific colors we do in four-color process printing, it is important to understand the nature of light and color.
Light and Color.
Light contains all the colors of the spectrum. This can be demonstrated by directing a beam of light through a prism. Although the spectrum contains all possible colors, it can be broken down into three color regions-red, green, blue-each representing a third of the visible spectrum.
As it is possible to break light down into three colors, conversely these same three colors when projected on top of one another create white light. Whereas all three colors overlapping produce white, if one is removed, leaving only two overlapping colors, a totally different color is created: red and green, with the blue removed, produce yellow; red and blue, with the green removed, produce magenta; green and blue, with red removed, produce cyan.
Because the red, green, and blue combine to produce white light, they are called additive primaries. Because the yellow, magenta, and cyan are formed by taking away one of the three additive primaries, they are called subtractive primaries.
How We See Light and Color.
Because light contains all colors, it is light that gives color to everything we see. When we see color, we are really seeing light.
There are three ways we see light: as direct, reflected, or transmitted. Direct light, as the name suggests, is from a light source, such as the sun, a candle, or a lightbulb. Reflected light is reflected off some opaque surface, such as that of a photograph, painting, or drawing. Transmitted light is passed through a transparent material, such as a photographic transparency, piece of colored glass, or filter.
As designers, we are mainly concerned with the last two kinds of light: reflected and transmitted. Let's look at a simple example of both:
First, let's examine what happens when light strikes a red apple. We have already established that light is made up of red, green, and blue. When this light strikes the apple, every color is absorbed by the apple except the color it is -red. The red is reflected off its surface, so the color that we see is the reflected color red.
Now let's take a piece of red glass. Once again, all the colors of light, except the color of the object, are absorbed. But in this case the red passes through the object rather than reflecting off its surface. You might say that the red glass has filtered out the green and blue, allowing only the red to pass through. As with the apple, the color we see is red, but it is a transmitted color red, not a reflected color.
Now let's apply this principle to separating color for process printing.
Separating Color. All colors are made up of varying amounts of the three additive primaries: red, green, and blue. Therefore it should follow that if we can separate a full-color image into these three colors, we should be able to recreate it by using the same three colors in printing inks. Unfortunately, we cannot. The problem is that there is a limit to the number of colors that can be created using these three particular colors. For example, although red and green light produce yellow light, red and green inks produce a brown/black color. Therefore, not only would it be impossible to create yellow, but any color brighter than the original colors. The solution to the problem lies in the subtractive primaries: yellow, magenta and cyan. Using inks in these three colors it is possible to re-create all the colors of the spectrum.
To separate the full-color image into yellow, magenta, and cyan, it is necessary to photograph the copy three times, through filters which are the same color as the additive primaries: red, green, and blue. When the copy is Photographed through the red filter, the green and blue is absorbed and the red passes through, producing a negative with a record of the red. By making a positive of this negative we will obtain a record of everything that is not red, or more specifically, a record of the green and blue. The green and blue, as we have seen earlier, combine to produce cyan; therefore, we have a record of the cyan. The same process is repeated for the magenta, using a green filter, and for the yellow, using a blue filter. As each filter covers one-third of the spectrum, we now have a record of all the colors found in the original copy. When combined and printed with the correct colors, yellow, magenta, and cyan we should be able to reproduce all the colors of the original.
In theory, this is correct. Unfortunately, printing inks are not "pure"; they absorb colors that they would not absorb if they were pure. For this reason, the printed image will appear dirty or muddied unless color corrections are made on the separations to compensate for these ink deficiencies. Another problem is a lack of density in the shadow areas. To overcome this, a black separation is made by using a yellow filter or a combination of all three filters. The addition of black improves shadow density and overall contrast.
Reproducing the Image. The four separations are screened at different angles, and from these screened separations the printing plates are made. When printed, the image is reproduced as thousands of tiny dots laid down in thin layers of color. The particular color is dictated by the size of the dots, the manner in which they overlap, and their relation to one another. Therefore the colors are produced not in the physical mixing of the inks, but in the optical mixing of individual colors by the viewer's eye.
Ink and Paper. The paper used has a major influence on the quality of the color printed on it. Because process inks are transparent, it is the light reflected from the paper's surface that supplies the light to the ink. For example, let's look at some cyan printed on a sheet of paper. The light passes through the transparent cyan ink as through a glass filter. The cyan absorbs the color it is (not red) -and allows the color it is (blue and green) -to pass through. These two colors reflect off the paper and back up through the ink. What we see is a blue and green color, or cyan. It is the quality and quantity of the reflected light that dictates the quality of the color. For this reason, the paper must be bright if it is to reflect maximum light. Also, because a rough surface will scatter the light and distort the color, the paper should be smooth so the ink will lie flat and filter properly.