CMYK
http://en.wikipedia.org/wiki/CMYK_color_model
http://desktoppub.about.com/cs/basic/g/cmyk.htm
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). Though it varies by print house, pressoperator, press manufacturer, and press run, ink is typically applied in the order of the abbreviation.
The "K" in CMYK stands for key since 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, this explanation, although useful as a mnemonic, is incorrect.
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 subtractivebecause inks "subtract" brightness from white.
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 money 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.
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 human beings 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.
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 a purplish 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.
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:
Comparison with RGB displays
Comparisons between RGB displays and CMYK prints can be difficult, since the color reproduction technologies and properties are so different. A computer monitor mixes shades of red, green, and blue to create color pictures. A CMYK printer instead uses light-absorbing cyan, magenta and yellow inks, whose colors are mixed usingdithering, 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.
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 profilesthat 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.
Definition:
To reproduce full-color photographic images, typical printing presses(and some inkjet printers) use 4 colors of ink. The four inks are placed on the paper in layers of dots that combine to create the illusion of many more colors. CMYK refers to the 4 ink colors used by the printing press — the the subtractive primaries plus black.
C is cyan (a blue-green color)
M is magenta (a reddish pink color)
Y is yellow
K is a pure black ink, the key plate or keyline color
CMYK is not the only full color printing process but is the most common. Other full color methods include 6C Hexachrome and 8C Dark/Light (6 colors and 8 colors instead of just 4).
There are differences between images in RGB (how most digital images start out) and CMYK so that when converting from RGB to CMYK there may be some shifts in the color. Companies that have precise corporate or logo colors may find that when converting their print logo for use on the Web (or the other way around), the logo colors may not look exactly the same. When CMYK colors are displayed on screen, such as on the Web or in your graphics software, they are just approximations of what the color will look like when printed. There will be differences.
A mistake often made when submitting artwork for 4-color printing is not converting the images to the CMYK color space. This is needed so that the file can be separated into the four colors (see illustration) so that a separate printing plate can be made for each of the colors.
Also Known As: 4-color | 4C | process colors
Alternate Spellings: CYMK
Examples:The illustration on this page shows a color photograph (center) separated into its CMYK components. A separate plate for the printing press would be made from each one. Those areas on the C plate, for example, that are black and shades of gray would print in varying shades of Cyan. The white areas get no Cyan. Each ink is added in turn to create the final full color image on paper.
http://graphicdesign.about.com/od/colorbasics/a/cmyk.htm
The CMYK color model is used in the printing process. To understand it, it is best to start with RGB color. The RGB color model (made up of red, green and blue) is used in your computer monitor, and is what you will view your projects in while still on screen. These colors, however, can only be viewed with natural or produced light, such as in the computer monitor, and not on a printed page. This is where CMYK comes in.
When two RGB colors are mixed equally they produce the colors of the CMYK model, known as subtractive primaries. Green and blue creates cyan (C), red and blue creates magenta (M), and red and green creates yellow (Y). Black is added to the model because it cannot be created with the 3 subtractive primaries (when combined they create a dark brown). The K, or “key,” stands for black.
CMYK in the Printing Process
The four-color printing process uses four printing plates; one for cyan, one for magenta, one for yellow and one for black. When the colors are combined on paper (they are actually printed as small dots), the human eye sees the final image.
CMYK in Graphic Design
Graphic designers have to deal with the issue of seeing their work on screen in RGB, although their final printed piece will be in CMYK. Digital files should be converted to CMYK before sending to printers, unless otherwise specified. Because of this issue, it is important to use “swatches” when designing if exact color matching is important. Swatches provide a designer and client with a printed example of what a color will look like on paper. A selected swatch color can then be chosen in Photoshop (or a similar program) to insure the desired results. Even though the on-screen color won’t exactly match the swatch, you know what your final color will look like. You can also get a “proof” from a printer, which is an example of your printed piece provided before the entire job is run.
