Color 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:

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.

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 "black").

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

Printing inks cannot mix, or commingle. For this reason, only one color of ink can be allowed to be on a particular point of the paper at a time. The optical illusion created by the colors being so close together gives the viewer the impression of a continuous-tone image. Keeping the colors separate from each other is accomplished by using a screen, also known as a halftone.

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 obsoleted 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 ruling of 60 to 120 lines per inch (lpi) are used to reproduce color photographs in newspapers. This does not give high-quality reproduction, but this is the best halftoning possible on the cheap, highly absorbent paper used. A finer screen would lead to the ink spreading and the colors being muddied. A good way to see halftoning in action is to examine a color newspaper image with a magnifying glass. Screens with 133 to 175 lpi are used for magazines and commercial printing.

The measure of how much ink is absorbed and spread by the paper is called dot gain.

Stochastic screening

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 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. Take a magnifier (or loupe) if you own an inkjet printer at home and take a peek at any image you've printed.

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

References

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

Related articles

• Printing
• Letterpress printing
• Block printing
• Flexography
• Intaglio
• Lithography
• Offset printing
• Rotogravure
• Typography

See also: Color printing, Additive color, Block printing, CMYK, CMYK color model, Color space, Digital imaging, Flexography, Gamut, Grey component replacement