CMYK Color Model

The CMYK color model (process color, four color) is a subtractive color model, used in color printing, and is also used to describe the printing process itself. CMYK refers to the four inks used in some color printing: cyan, magenta, yellow, and key (black). Although it varies by print house, press operator, press manufacturer, and press run, ink is typically applied in the order of the abbreviation.

The "K" in CMYK stands for key because in four-color printing, cyan, magenta, and yellow printing plates are carefully keyed, or aligned, with the key of the black key plate. Some sources suggest that the "K" in CMYK comes from the last letter in "black" and was chosen because B already means blue. However, some people disagree with this because C for Cyan is classed as the blue when printing in CMYK format. Some sources claim this explanation, although useful as a mnemonic, is incorrect, that K comes only from "Key" because black is often used as outline and printed first.

In additive color models, such as RGB, white is the "additive" combination of all primary colored lights, while black is the absence of light. In the CMYK model, it is the opposite: white is the natural color of the paper or other background, while black results from a full combination of colored inks. To save cost on ink, and to produce deeper black tones, unsaturated and dark colors are produced by using black ink instead of the combination of cyan, magenta, and yellow.

The CMYK model works by partially or entirely masking colors on a lighter, usually white, background. The ink reduces the light that would otherwise be reflected. Such a model is called subtractive because inks "subtract" brightness from white.

With CMYK printing, halftoning (also called screening) allows for less than full saturation of the primary colors; tiny dots of each primary color are printed in a pattern small enough that humans perceive a solid color. Magenta printed with a 20% halftone, for example, produces a pink color, because the eye perceives the tiny magenta dots on the large white paper as lighter and less saturated than the color of pure magenta ink.

Halftoning

Without halftoning, the three primary process colors could be printed only as solid blocks of color, and therefore could produce only seven colors: the three primaries themselves, plus three secondary colors produced by layering two of the primaries: cyan and yellow produce green, cyan and magenta produce blue, yellow and magenta produce red (these subtractive secondary colors correspond roughly to the additive primary colors), plus layering all three of them resulting in black. With halftoning, a full continuous range of colors can be produced.

Screen Angle

To improve print quality and reduce moiré patterns, the screen for each color is set at a different angle. While the angles depend on how many colors are used and the preference of the press operator, typical CMYK process printing uses any of the following screen angles:

C	15°	15°	105°	165°
M	75°	45°	75°	45°
Y	0°	0°	90°	90°
K	45°	75°	15°	105°

Benefits Of Using Black Ink

The "black" generated by mixing commercially practical cyan, magenta, and yellow inks is unsatisfactory, so four-color printing uses black ink in addition to the subtractive primaries. Common reasons for using black ink include:

• In traditional preparation of color separations, a red keyline on the black line art marked the outline of solid or tint color areas. In some cases a black keyline was used when it served as both a color indicator and an outline to be printed in black. Because usually the black plate contained the keyline, the K in CMYK represents the keyline or black plate, also sometimes called the key plate.
• Text is typically printed in black and includes fine detail (such as serifs), so to reproduce text or other finely detailed outlines, without slight blurring, using three inks would require impractically accurate registration.
• A combination of 100% cyan, magenta, and yellow inks soaks the paper with ink, making it slower to dry, causing bleeding, or (especially on cheap paper such as newsprint) weakening the paper so much that it tears.
• Although a combination of 100% cyan, magenta, and yellow inks should, in theory, completely absorb the entire visible spectrum of light and produce a perfect black, practical inks fall short of their ideal characteristics and the result is actually a dark muddy color that does not quite appear black. Adding black ink absorbs more light and yields much better blacks.
• Using black ink is less expensive than using the corresponding amounts of colored inks.

When a very dark area is desirable, a colored or gray CMY "bedding" is applied first, then a full black layer is applied on top, making a rich, deep black; this is called rich black. A black made with just CMY inks is sometimes called a composite black.

The amount of black to use to replace amounts of the other ink is variable, and the choice depends on the technology, paper and ink in use. Processes called under color removal, under color addition, and gray component replacement are used to decide on the final mix; different CMYK recipes will be used depending on the printing task.

CMYK Color Model

Other Printer Color Models

CMYK or process color printing is contrasted with spot color printing, in which specific colored inks are used to generate the colors appearing on paper. Some printing presses are capable of printing with both four-color process inks and additional spot color inks at the same time. High-quality printed materials, such as marketing brochures and books, often include photographs requiring process-color printing, other graphic effects requiring spot colors (such as metallic inks), and finishes such as varnish, which enhances the glossy appearance of the printed piece.

CMYK are the process printers which often have a relatively small color gamut. Processes such as Pantone's proprietary six-color (CMYKOG) Hexachrome considerably expand the gamut. Light, saturated colors often cannot be created with CMYK, and light colors in general may make visible the halftone pattern. Using a CcMmYK process, with the addition of light cyan and magenta inks to CMYK, can solve these problems, and such a process is used by many inkjet printers, including desktop models.

Comparison With RGB Displays

Comparisons between RGB displays and CMYK prints can be difficult, since the color reproduction technologies and properties are very different. A computer monitor mixes shades of red, green, and blue light to create color pictures. A CMYK printer instead uses light-absorbing cyan, magenta, and yellow inks, whose colors are mixed using dithering, halftoning, or some other optical technique.

Similar to monitors, the inks used in printing produce a color gamut that is "only a subset of the visible spectrum" although both color modes have their own specific ranges. As a result of this items which are displayed on a computer monitor may not completely match the look of items which are printed if opposite color modes are being combined in both mediums. When designing items to be printed, designers view the colors which they are choosing on an RGB color mode (their computer screen), and it is often difficult to visualize the way in which the color will turn out post printing because of this.

CMYK Color Model

Conversion

Since RGB and CMYK spaces are both device-dependent spaces, there is no simple or general conversion formula that converts between them. Conversions are generally done through color management systems, using color profiles that describe the spaces being converted. Nevertheless, the conversions cannot be exact, particularly where these spaces have different gamuts.

The problem of computing a colorimetric estimate of the color that results from printing various combinations of ink has been addressed by many scientists. A general method that has emerged for the case of halftone printing is to treat each tiny overlap of color dots as one of 8 (combinations of CMY) or of 16 (combinations of CMYK) colors, which in this context are known as Neugebauer primaries. The resultant color would be an area-weighted colorimetric combination of these primary colors, except that the Yule–Nielsen effect ("dot gain") of scattered light between and within the areas complicates the physics and the analysis; empirical formulas for such analysis have been developed, in terms of detailed dye combination absorption spectra and empirical parameters.

What is Imposition ?

Imposition is one of the fundamental steps in the prepress printing process. It consists of the arrangement of the printed product’s pages on the printer’s sheet, in order to obtain faster printing, simplify binding and reduce paper waste.

Correct imposition minimizes printing time by maximizing the number of pages per impression, reducing cost of press time and materials. To achieve this, the printed sheet must be filled as fully as possible.

Task Description :

The arrangement of pages on the printer’s sheet is affected by five different parameters:

• Format of the product: The size of the finished page determines how many pages can be printed on a single sheet.
• Number of pages of the printed product: The compositor must determine how many sheets are to be printed to create a finished book.
• Stitching/binding method: The compositor must understand how the sheets are placed to form the signatures that compose the finished book.
• Paper fiber direction: Many papers have a "grain," reflecting the alignment of the paper fibers. These fibers must run lengthwise along the fold, which influences the alignment, hence the position, of the pages on the printed sheet.
• Finishing and binding.

To understand how the pages are related to each other, an imposition dummy may be used. This is made by folding several sheets of paper in the way the press will print and fold the product. A little copy is then created, and this can help paginate the product.

In the example above, a 16-page book is prepared for printing. There are eight pages on the front of the sheet, and the corresponding eight pages on the back. After printing, the paper is folded in half vertically (page two falls against page three). Then it is folded again horizontally (page four meets page five). A third fold completes this process (page nine meets page eight). The example below shows the final result prior to binding and trimming.

Non-Digital Techniques :

Imposition has been a requirement since the earliest days of printing. When pages were set using movable type, pages were assembled in a metal frame called a chase, and locked into place using wedges called quoins.

By the late twentieth century, most typesetting was onto photographic film. These sheets were combined manually on a light table, in a process called stripping. Skilled workers would spend many hours stripping pieces of film together in the correct sequence and orientation. The term stripping was also used for other changes to a prepared page, such as a spelling correction, or a stop press story in a newspaper. Digital techniques rendered stripping less necessary, but what has forced increasing numbers to abandon it completely is the introduction of "platesetters", which put pages directly onto printing plates; these plates cannot be adjusted with a sharp knife. In addition, an extremely high precision would be needed for stripping of colour work, as each ink colour is on a separate piece of film.

Digital Techniques :

Manual imposition processes tend to cause bottlenecks of the whole printing production. The first digital imposition software, Impostrip, was released in 1989.[2] The advent of digital imposition has not only helped a lot in making sure layout and sheet arrangement are correct with higher register precision, but it significantly reduces the usual imposition errors (e.g., slight movements of register due to parallax). An entire book may be imposed and many complex functions applied in an instant. Binding options may be changed on the fly and impositions produced to multiple output devices at once, often with no user intervention. In turn, digital techniques help to reduce material costs, time and resolves production bottlenecks. There are several different approaches to digital imposition.

• Imposition in the design application. Software packages that can be used to design single pages can often be used to design entire printed sheets, sometimes by a process as simple as copy/paste onto a larger sheet. This is still in use, especially for low volumes of work, but a popular alternative is an imposition function built in, or added in, to the design tool. This typically takes a document prepared as single pages and creates a new document with full-sheet layouts. This larger layout is then printed to film or a plate.
• Post-design imposition. A post-design application might take a PostScript or PDF file in single pages and produce a new PostScript or PDF file with imposed sheet layouts for printing. A variation of this is to take a large number of single-page source files as input. This is especially suitable for a magazine or newspaper, where pages may be worked on by different groups simultaneously.
• Print driver imposition. Some printer drivers enable the source application's single-page printed output to be sent to the printer as full sheets. This is not often found in professional production, but is popular for such things as booklet printing on office laser printers. A variation of this offers the ability to print layouts as an option in the application.
• Output device imposition. This is sometimes called "in-RIP imposition". This allows regular pages to be printed by any suitable means, and the output device handles imposition. While this offers the advantage of enabling specific tuning of the imposition for an output device, the cost is that there is no preview until the output is produced. This may mean a costly printing plate that takes some time to produce, or even (with a digital press) errors in finished copies: expensive mistakes are possible.

Where an imposition layout is viewed on screen, it may be referred to as a printer's spread. This is used to contrast with reader's spread, which shows a finished printed piece on screen as it will appear to the reader, rather than the printer; specifically, in a reader's spread for a typical book, pairs of facing pages are shown side-by-side (that is, pages 2 and 3 together).

Montax Imposition Software

Imposition Proof :

The imposition proof is the last test that is performed before beginning the print run.

This test is performed to verify, through the formation of a prototype, that the imposition was successful. Typical checks are that the pages are on the correct spot and the crossover bleeds work. It cannot be used as a check proof for images or colors or layout because it is printed on a large, low-resolution inkjet printer.

Since the inkjet printer can print on only one side of the paper, the full proof (the front and rear sides) is printed on two separate sheets. They are first cut along the crossover bleeds, checking to see if they are in the correct position. The two sheets are then attached together to form a single sheet printed on both sides, and then this sheet is folded to form a prototype of the signature.

This proof is still called blue copy, digital blue copy to prototype, or blues plotter.