Color printing:

Definition:Color separation is the process by which original artwork is separated into individual color components for printing. The components are cyan, magenta, yellow and black, known asCMYK. By combining these colors, a wide spectrum of colors can be produced on the printed page. In this four color printing process, each color is applied to a printing plate. When the colors are combined on paper (they are actually printed as small dots), the human eye combines the colors to see the final image. The use of plates for printing is part of the process known aslithography.Also Known As:Four-color Process

The act of decomposing a color graphic or photo into single-color layers. For example, to print full-color photos with anoffset printingpress, one must first separate the photo into the four basic ink colors: cyan, magenta, yellow, and black (CMYK). Each single-color layer is then printed separately, one on top of the other, to give the impression of infinite colors.This type of color separation, mixing three or four colors to produce an infinite variety of colors, is calledprocess colorseparation. Another type of color separation, calledspot colorseparation, is used to separate colors that are not to be mixed. In this case, each spot color is represented by its own ink, which is specially mixed. Spot colors are effective for highlightingtextbut they cannot be used to reproduce full-color images.Traditionally, process color separation has been performed photographically with different coloredfilters. However, many moderndesktop publishingsystemsare now capable of producing color separations forgraphicsstoredelectronically. This capability is essential if you want to create full-colordocumentson yourcomputerand then print them using an offset printer. You don't need to perform color separation if you are printing directly to acolor printerbecause in this case theprinteritself performs the color separation internally.

Color printingorcolour printingis the reproduction of an image or text in color (as opposed to simpler black and white ormonochromeprinting). Any natural scene or color photograph can be optically and physiologically dissected into threeprimary colors, red, green and blue, roughly equal amounts of which give rise to the perception of white, and different proportions of which give rise to the visual sensations of all other colors. The additive combination of any two primary colors in roughly equal proportion gives rise to the perception of asecondary color. For example, red and green yields yellow, red and blue yieldsmagenta(a purple hue), and green and blue yieldcyan(a turquoise hue). Only yellow is counter-intuitive. Yellow, cyan and magenta are merely the "basic" secondary colors: unequal mixtures of the primaries give rise to perception of many other colors all of which may be considered "tertiary."

Modern techniques[edit]While there are many techniques for reproducing images in color, specific graphic processes and industrial equipment are used for mass reproduction of color images on paper. In this sense, "color printing" involves reproduction techniques suited forprinting pressescapable of thousands or millions of impressions for publishing newspapers and magazines, brochures, cards, posters and similar mass-market items. In this type of industrial or commercialprinting, the technique used to print full-color images, such as color photographs, is referred to asfour-color-processor merelyprocessprinting. Four inks are used: three secondary colors plus black. These ink colors arecyan,magentaandyellow; abbreviated asCMYK. Cyan can be thought of as minus-red, magenta as minus-green, and yellow as minus-blue. These inks are semi-transparent or translucent. Where two such inks overlap on the paper due to sequential printing impressions, a primary color is perceived. For example, yellow (minus-blue) overprinted by magenta (minus green) yields red. Where all three inks may overlap, almost all incident light is absorbed or subtracted, yielding near black, but in practical terms it is better and cheaper to use a separate black ink instead of combining three colored inks. The secondary orsubtractive colorscyan, magenta and yellow may be considered "primary" by printers and watercolorists (whose basic inks and paints are transparent).Two graphic techniques are required to prepare images for four-color printing. In the "pre-press" stage, original images are translated into forms that can be used on a printing press, through "color separation," and "screening" or "halftoning." These steps make possible the creation ofprinting platesthat can transfer color impressions to paper on printing presses based on the principles oflithography.An emerging method of full-color printing issix-color process printing(for example,Pantone'sHexachromesystem) which adds orange and green to the traditional CMYK inks for a larger and more vibrantgamut, or color range. However, such alternate color systems still rely on color separation, halftoning and lithography to produce printed images.Color printing can also involve as few as one color ink, or multiple color inks which are not the primary colors. Using a limited number of color inks, or specific color inks in addition to the primary colors, is referred to as "spot color" printing. Generally, spot-color inks are specific formulations that are designed to print alone, rather than to blend with other inks on the paper to produce various hues and shades. The range of available spot color inks, much like paint, is nearly unlimited, and much more varied than the colors that can be produced by four-color-process printing. Spot-color inks range from subtle pastels to intense fluorescents to reflective metallics.Color printing involves a series of steps, or transformations, to generate a quality color reproduction. The following sections focus on the steps used when reproducing a color image in CMYK printing, along with some historical perspective.

Color separation process[edit]

The first widely reproduced image printed using the three-color process, byWilliam Kurtz(January 1893)Typically color separation is the responsibility of the Color Separator. This includes cleaning up the file to make it print ready and creating a proof for the prepress approval process. The process of color separation starts by separating the original artwork into red, green, and blue components (for example by a digital scanner). Beforedigital imagingwas developed, the traditional method of doing this was tophotographthe image three times, using a filter for each color. However this is achieved, the desired result is threegrayscaleimages, 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 aresubtractiveprimaries which each represent two of the threeadditiveprimaries (RGB) after one additive primary has been subtracted from white light.Cyan, magenta, and yellow are the three basic colors used for color reproduction. When these three colors are variously used in printing, the result should be a reasonable reproduction of the original, but in practice this is not the case. Due to limitations in theinks, 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 includegrey component replacement,under color removal, andunder color addition. This printing technique is referred to as CMYK (the "K" stands for Key, a traditional word for the black printing plate).Today's digital printing methods do not have the restriction of a singlecolor spacethat traditional CMYK processes do. Many presses can print from files that were ripped with images using eitherRGBor CMYK modes. The color reproduction abilities of a particular color space can vary; the process of obtaining accurate colors within a color model is calledcolor matching.

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 to particular picture areas 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, orhalftonefor 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 ofoptical illusion, appear to form a continuous-tone image. You can view the halftoning, which enables printed images, by examining a printed picture under magnification.

Traditionally, halftone screens were generated by inked lines on two sheets of glass that were cemented together atright 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, and eliminating the film step. The amount of generation loss in printing a lithographic negative onto a lithographic plate, unless the processing procedures are completely ignored, is almost completely negligible, as there are no losses of dynamic range, no density gradations, nor are there any colored dyes, or large silver grains to contend with in an ultra-slow rapid access negative.Screens with a "frequency" of 60 to 120 lines per inch (lpi) reproduce color photographs in newspapers. The coarser the screen (lower frequency), the lower the quality of the printed image. Highly absorbentnewsprintrequires a lower screen frequency than less-absorbentcoated paperstock 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 calleddot 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.