CMYK color model

Color printing typically uses ink of four colors: cyan, magenta, yellow, and key (black).
When CMY “primaries” are combined at full strength, the resulting “secondary” mixtures are red, green, and blue. Mixing all three gives an imperfect black.

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.[1][2] 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.[3]

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.

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.

Halftoning

This diagram shows three examples of color halftoning with CMYK separations, as well as the combined halftone pattern and how the human eye would observe the combined halftone pattern from a sufficient distance.

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 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

Typical halftone screen angles.

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:[4][5]

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

A color photograph of the Teton Range.
The image above, separated for printing with process cyan, magenta, and yellow inks.
The same image, this time separated with maximum black, to minimize colored-inks use.

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:[6]

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.[7] 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.

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.[8]

Comparison with RGB displays

Comparison of some RGB and CMYK color gamut on a CIE 1931 xy chromaticity diagram.

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.[9] 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.

Spectrum of Printed Paper

To reproduce color, the CMYK color model codes for absorbing light rather than emitting it (as is assumed by RGB). The 'K' component absorbs all wavelengths and is therefore achromatic. The Cyan, Magenta, and Yellow components are used for color reproduction and they may be viewed as the inverse of RGB. Cyan absorbs Red, Magenta absorbs Green, and Yellow absorbs Blue (-R,-G,-B).

Spectrum of the visible wavelengths on printed paper (SCA Graphosilk). Shown is the transition from Red to Yellow. White, red, blue, and green are shown for reference. Readings from a white orchid flower, a rose (red and yellow petals), and a red cyclamen flower are shown for comparison. The units of spectral power are simply raw sensor values (with a linear response at specific wavelengths).

Conversion

Early representation of the three color process (1902).

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.[10] 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.[10]

See also

References

  1. ↑ Galer, Mark; Horvat, Les (2003). Digital Imaging: Essential Skills. Focal Press. p. 74. ISBN 978-0-240-51913-5.
  2. ↑ Jennings, Simon (2003). Artist's Color Manual: The Complete Guide to Working with Color. Chronicle Books LLC. p. 21. ISBN 978-0-8118-4143-6.
  3. ↑ Gatter, Mark (2004). Getting It Right in Print: Digital Pre-press for Graphic Designers. Laurence King Publishing. p. 31. ISBN 978-1-85669-421-6.
  4. ↑ Campbell, Alastair. The Designer's Lexicon. 2000 Chronicle, San Francisco. p 192
  5. ↑ McCue, Claudia. Real World Print Production. 2007 Peachpit, Berkeley. p 31.
  6. ↑ Roger Pring (2000). WWW.Color. Watson–Guptill. ISBN 0-8230-5857-3.
  7. ↑ R. S. Hodges (2003). The Guild Handbook of Scientific Illustration. John Wiley and Sons. ISBN 0-471-36011-2.
  8. ↑ Carla Rose (2003). Sams Teach Yourself Adobe Photoshop Elements 2 in 24 Hours. Sams Publishing. ISBN 0-672-32430-X.
  9. ↑ Damien van Holten, print international.org, "RGB Vs CMYK" http://www.printernational.org/rgb-versus-cmyk.php
  10. 1 2 Gaurav Sharma (2003). Digital Color Imaging Handbook. CRC Press. ISBN 0-8493-0900-X.
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