Four color printing
|Helio real blue
|based on phthalocyanine blue
|based on a Rhodamine -B color lacquer , also dioxazine violet
|Pressure- resistant yellow pigments : Hansa Yellow , Permanent Yellow, Pigment Yellow 151
|Carbon black , refined with reflex blue
The three primary colors yellow, magenta and cyan would theoretically suffice. Because of the physically determined shape and position of the spectrum of technically available color pigments and graphic papers , deep black (depth of color) cannot be printed from CMY alone.
As a fourth printing color, black based on soot is also used for the “depth”. "K" stands for Key, as it is the key color for the color contrast. The word origin comes from the analog film separation. In German colloquial language, K is sometimes interpreted for contrast.
The color system is called the CMYK color model . The four standardized colors are defined as a color scale according to ISO 12647 and ISO 2846 . In addition to the ISO scale, the term Euroscale is also used in Europe .
Color separations must first be created from the original illustration . In the conventional prepress stage , the original is exposed one after the other through color filters on four separate films, so that only the cyan parts of the image appear on the first film, only the magenta parts on the second, then yellow and the dark level. These color separations still have to be rasterized (either in the same operation or afterwards).
The halftone image is exposed through a glass engraving or contact grid and broken down into small pixels. The shape of these grid points can vary from square to circular, elliptical to diamond-shaped, depending on the structure of the grid used. The center-to-center distance of the grid points is always identical within an image, but their filled area changes depending on the color intensity to be printed from “point not available” to “point fills the entire area”. This type of screening is also known as amplitude-modulated screening. The rasterization is necessary because either “color” or “no color” can be printed with four colors in the printing point. The printing process only allows gradations next to each other and not by applying a thick layer at the same place.
As an example, imagine a gray wedge that ranges from the purest white to the deepest black. All of these gray tones now have to be applied to paper with the one printing color black. Because the halftone dots are smaller in lighter shades of gray (or are printed less frequently) than in darker shades, the optical impression of a gray surface arises at a suitable viewing distance, although the dots are actually all printed from the same black. If you look at a photo in a newspaper or magazine with a magnifying glass , you can see the individual grid points very well.
In order to avoid a moiré effect in the print, the screen foils are rotated by a specified angle, the screen angle , before exposure . The angles in standard offset printing are: cyan 15 °, magenta 75 °, yellow 0 ° and black 45 °. In special cases (e.g. to reproduce skin colors) these angles can also be swapped. In modern digital prepress, these tasks are performed by a computer, the so-called RIP ( Raster Image Processor ). In the printing press , the paper passes through printing units with the four colors one after the other. The order of the colors depends on the order and printing process. In four-color offset printing, the sequence black, cyan, magenta, yellow is the rule.
In a real four-color printing machine, four complete single-color printing machines are connected in series. In smaller in-house print shops there is often only one printing unit, so that after the first color has been printed, the machine has to be completely cleaned and switched to the second color. To ensure that the prints of the individual colors also match perfectly, register marks are also printed outside the actual printing area . By adjusting the longitudinal and transverse register in the respective printing unit, the four colors can be printed precisely on top of each other. Furthermore, measuring fields are printed over the entire width of the printing sheet or the paper web , which help the printer to set the inking system of the machine correctly. Register marks and measurement fields are usually cut away after printing. They can often be left in place when printing the packaging, as they disappear under a cover flap after the packaging has been glued together (for example in the case of cigarette packets).
More color prints
Often there are further requirements for the print result. Due to the corporate design , it may be necessary that a specified “company color” is also printed as a fifth color. For example, this corporate color must not be screened in a company logo, but should be printed as a full tone.
Depending on the print job, it may be necessary to print a fifth color: printing varnish or special colors such as gold or silver tones and luminous colors.
For special printing requirements, the resolution is extended to more than the four process colors in order to improve the reproduction of the CMYK color space and additional chromatic colors are added. Primarily for art prints, six basic colors are used to achieve the intermediate tones better. The (limited) representable color space of four-color printing is enlarged, as offered by the Hexachrome process under the protected trademark Pantone Hexachrome . This works with the six printing colors: cyan, magenta, yellow, orange, green and black. This printing technique is associated with increased costs and technical effort, but leads to better printing results.
Another printing method with more than the three basic colors plus black are the photo printers offered by various manufacturers . In addition to the standard tones (yellow, cyan, magenta, black), these inkjet printers have the intermediate tones (red, green, blue) as an additional ink cartridge, which can be multi-color or single cartridges.
- Harald Küppers: The color theory of television. Photo and printing technology . Color theory of visual communication media. DuMont , Cologne 1985.
- Harald Küppers: DuMont color atlas . Over 5500 color nuances with digital color values, labeling and mixing instructions. Explanation of the standardization concept in offset printing. 10th edition, DuMont, Cologne 2007.