Color separation

Color separation (literally color separation) refers to the calculation of individual (separated) printing inks , mostly in prepress . Before digitization , color separations with color filters were produced using a photographic method. Color separation is necessary because color information is usually not available in the form in which it is needed for printing. The four-color printing is usually carried out in the CMYK color model ( color mixing of the individual colors Cyan , Magenta , Yellow and Black ). The color data, on the other hand, can be available as composite color channels, in the RGB model , as special colors (such as Pantone , HKS , RAL ), other color definitions or as a non-digitized image.

Basics

overview

Color mixing in the RGB color space

The RGB model (red, green and blue) and the CMY model (cyan, magenta and yellow) form cube-shaped bodies. Millions of colors are represented within these bodies. Each of these millions of colors has to be mixed from just three basic colors (red, green and blue or cyan, magenta and yellow).

The different ways of mixing colors result in files with different color information.

application

In digital photography , RGB is almost always used. These are the basic colors for additive color mixing . When displaying photos on screens or outputting them as exposure , the highest possible photo quality is achieved .
The printing is based on subtractive color mixing . The colors CMY (cyan, magenta, yellow) are used.

Color separation

Sample images for color separation
RGB
CMYK

practice

The transfer of additive to subtractive is relatively simple. This color separation is only complicated by the use of black in the CMYK color model.

Black is used as an additional color in printing in order to save costs and - due to the associated loss of quality - to increase the impression of sharpness :

Small inaccuracies in the production of the cyan, magenta and yellow printing inks mean that pure black cannot be mixed. An exact production (to mix a pure black) is very complex and is therefore (almost) only used in professional technology (pictrography, thermal sublimation printer, ...) for photographers.
Printing technology is almost always used to print text and graphics. The need for black color is significantly higher than when outputting photos. Since black paint is cheaper to manufacture than other paints, it is always used.
The subtractive color mixture lacks the high contrast range that is characteristic of the additive mixture. The printer speaks of a lack of depth here. The addition of black improves the subjective contrast in the sharpness display .
Since printing processes are raster-oriented processes, there are strong subjective sharpness losses when displaying delicate colors (the raster width increases, so a delicate image detail contains less color, which is interpreted by the eye as a loss of sharpness). The addition of black creates a subjective compensation for this loss.

Problems

Inks used in four-color printing (subtractive color mixing): cyan, magenta, yellow

By using black, the inexpensive printing of a pure black tone should be achieved.

If parts of a photo are made of black, pure black can be used for printing.

If parts of a photo consist of very delicate colors, black can be omitted.

Image parts of a photo very rarely consist of only delicate colors or pure black. Color separation must therefore implement a gentle increase in the black component (from zero to 100%).
In the very dark parts of the image, there would be a four-fold application of color (100% cyan, 100% magenta, 100% yellow, 100% black). This would be uneconomical and also not feasible with most carrier materials (types of paper, ...) . Therefore, in addition to the gentle increase in black, the color tones cyan, magenta and yellow must also be reduced.
If the colors are reduced and replaced by black, "dirty" colors are created. They lack luminosity and saturation. This loss of quality is partially compensated for by a special reinforcement of the black component. The black portion is changed in such a way that a contrast increase and thus an increase in the subjective impression of sharpness is created.

The problems mentioned make color separation a very demanding process.

Black, colored, achromatic composition

Various methods have been developed in printing technology to improve all of the problems mentioned (costs, print quality ...):

Each of the methods described requires a special color separation.

Conversion programs

The color separation is carried out automatically by programs such as Adobe Photoshop or layout programs such as QuarkXPress or Adobe InDesign based on the color settings of the programs. This separation can also be done with special programs in the PDF . The existing standard settings of these programs meet average quality expectations. In the case of particularly demanding print files, only the experience of specialists can guarantee the necessary quality.

Conventional color separations

Cigar lid picture in spring point technique

Chromolithography: color extract brown I

Filter set for color separations

Before the invention of the glass engraving grid in 1881, color separations were made manually. In 1837 the Franco-German lithographer Godefroy Engelmann patented a colored variant of lithography under the name Chromolithography (color stone printing , color lithography), which was to be the most popular method for high-quality color illustrations until the 1930s. Chromolithographs consisting of up to 12, 16 and even 25 colors were not uncommon. The experienced chromolithographer started with the light colors and had them printed in the proof press. Depending on the progress of the work, he recognized in comparison with the original image what the next darker color plate, i.e. the color separation, had to look like. A 12-color chromolithography could, for example, consist of the following color extracts: blue I, red I, yellow, blue II, red II, brown I, brown II, blue III, red III, gray I, gray II, black. The lower digits indicate the lightest colors.

At the beginning of the 20th century, the Klimsch & Co company developed a reproduction camera with which two-dimensional templates could be reproduced exactly. This camera was significantly larger than the traditional studio or handheld cameras and could fill a large space. Maxwell had developed the filter technology for color separations as early as 1861 and Georg Meisenbach followed in 1881 with the invention of the glass engraved grid. This created the prerequisites for the use of photography in repro technology.

The repro photographer created color separations from a colored template using color filters . A four-color printing requires each one color separation for yellow, red, blue and black. The repro specialist calls these colors yellow or yellow, magenta , cyan and depth or black . During exposure, a color filter is placed upstream in the lens that corresponds to the complementary color of the color separation, i.e. a violet filter for yellow, a green filter for red and an orange filter for blue. Black, which is only used to increase the contrast, is recorded without a filter. In order to be able to print the color separations, a division into raster points is necessary. This rasterization also takes place in the reproduction camera by placing the glass engraving raster in front of the photographic plate or film to be exposed.

In four-color printing, in which several rasters are printed on top of each other to display a color image, an attempt is made to avoid the moiré effect by an angular distance of 30 ° per color separation. Yellow is excluded from this because the resulting moiré is barely visible. Common screen angles for four-color printing are according to DIN 16 547: yellow = 0 °, cyan = 75 °, black = 135 °, magenta = 15 ° or yellow = 0 °, cyan = 15 °, black = 45 °, magenta = 75 ° .