PROCESS PRINTING

Color printing is the reproduction of an image or text in color (as opposed to simpler black and white or monochrome printing). It may also commonly be called four-color process printing when only the colors cyan, magenta, yellow, and black (also known as CMYK) are used. Another emerging method of color printing is six-color process printing (for example, Pantone's Hexachrome system) which adds orange and green to the traditional CMYK for a larger and more vibrant gamut, or color range.

Color printing involves a series of steps, or transformations, in order to generate a quality color reproduction. Here are the main steps when reproducing a color image in CMYK print, along with some historical perspective.

Color separation process

The process of color separation occurs when the original artwork is digitally scanned and separated into red, green, and blue components. Before digital imaging was developed, the traditional method of doing this was to photograph the image three times, using a filter for each color. However this is achieved, the desired result is three grayscale images, which represent the red, green, and blue (RGB) components of the original image:

Image when separated into RGB components.

Image when separated into RGB components.

The next step is to invert each of these separations. When a negative image of the red component is produced, the resulting image represents the cyan component of the image. Likewise, negatives are produced of the green and blue components to produce magenta and yellow separations, respectively. This is done because cyan, magenta, and yellow are subtractive primaries which each represent two of the three additive primaries (RGB) after one additive primary has been subtracted from white light.

CMY separations of image derived from the RGB separations.

CMY separations of image derived from the RGB separations.

Cyan, magenta, and yellow are the three main pigments used for color reproduction. When these three colors are combined in printing, the result should be a reasonable reproduction of the original, but it is not. Due to limitations in the ink pigments, the darker colors are dirty and muddied. To resolve this, a black separation is also created, which improves the shadow and contrast of the image. Numerous techniques exist to derive this black separation from the original image; these include grey component replacement, under color removal, and under color addition. This printing technique is referred to as CMYK (the "K" being short for "key." In this case, the key color is black).

Cyan, magenta, yellow, and black (CMYK) inks when printed separately. During normal print production, these would be printed on top of one another.

Cyan, magenta, yellow, and black (CMYK) inks when printed separately. During normal print production, these would be printed on top of one another.

Today's digital printing methods do not have the restriction of a single color space that traditional CMYK processes do. Many presses can print from files that were ripped with images using either RGB or CMYK modes. The color reproduction abilities of a particular color space can vary; the process of obtaining accurate colors within a color model is called color matching.

SCREENING

Inks used in color printing presses are semi-transparent and can be printed on top of each other to produce different hues. For example, green results from printing yellow and cyan inks on top of each other. However, a printing press cannot vary the amount of ink applied except through "screening," a process that represents lighter shades as tiny dots, rather than solid areas, of ink. This is analogous to mixing white paint into a color to lighten it, except the white is the paper itself. In process color printing, the screened image, or halftone for each ink color is printed in succession. The screen grids are set at different angles, and the dots therefore create tiny rosettes, which, through a kind of optical illusion, appear to form a continuous-tone image. You can view the halftone screens that create printed images under magnification.

Cyan, magenta, yellow, and black (CMYK) separations with halftone exaggerated to show detail.
Final composite image.

Cyan, magenta, yellow, and black (CMYK) separations with halftone exaggerated to show detail.

Final composite image.

Traditionally, halftone screens were generated by inked lines on two sheets of glass that were cemented together at right angles. Each of the color separation films were then exposed through these screens. The resulting high-contrast image, once processed, had dots of varying diameter depending on the amount of exposure that area received, which was modulated by the grayscale separation film image.

The glass screens were made obsolete by high-contrast films where the halftone dots were exposed with the separation film. This in turn was replaced by a process where the halftones are electronically generated directly on the film with a laser. Most recently, computer to plate (CTP) technology has allowed printers to bypass the film portion of the process entirely. CTP images the dots directly on the printing plate with a laser, saving money, increasing quality (by reducing the repeated generations), reducing lead-times, and saving the environment from toxic film-processing chemicals.

Screens with a "frequency" of 60 to 120 lines per inch (lpi) are used to reproduce color photographs in newspapers. The coarser the screen (lower frequency), the lower the quality of the printed image. Highly absorbent newsprint requires a lower screen frequency than less-absorbent coated paper stock used in magazines and books, where screen frequencies of 133 to 200 lpi and higher are used.

The measure of how much an ink dot spreads and becomes larger on paper is called dot gain. This phenomenon must be accounted for in photographic or digital preparation of screened images. Dot gain is higher on more absorbent, uncoated paper stock such as newsprint.

STOCHASTIC SCREENING

CMYK image with stochastic screen enlarged to show detail.

CMYK image with stochastic screen enlarged to show detail.

Digital imaging technology has also given rise to new approaches to the screening process. The best-known is stochastic screening. Because the dots are the same size and randomly placed, the moiré effects that are generated by traditional half-tones are eliminated. A side benefit of stochastic screening is the ability to obtain a wider gamut of colors using additional inks such as orange or green (hexachrome). Due to the high resolution of the screen, using computer to plate imaging gives optimal results.

Almost all inkjet devices use stochastic screening. Viewing any image produced by a home inkjet printer with a magnifier (or loupe) will reveal the screening.

Stochastic screening or some hybrid of traditional linescreen and stochastic has become the standard screening method for many packaging applications.

REFERENCES

  • Bruno, Michael H. (Ed.) (1995). Pocket Pal: A Graphic Arts Production Handbook (16th ed.). Memphis: International Paper

  • Hunt, R.W.G., The Reproduction of Color (1957, 1961, 1967, 1975) ISBN 0-85242-356-X

  • Yule, John A.C., Principles of Color Reproduction (1967, 2000) ISBN 0-88362-222-X

 



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