The CMYK Process for Printing Images.

Source Information:
the following information is from http://en.wikipedia.org/wiki/CMYK_color_model.

THE CMYK PROCESS FOR PRINTING IMAGES.

CMYK: (short for cyan, magenta, yellow, and key (black), and often referred to as process color or four color) is a subtractive color model, used in color printing, also used to describe the printing process itself. Though it varies by print house, press operator, press manufacturer and press run, ink is typically applied in the order of the abbreviation.

The CMYK model works by partially or entirely masking certain colors on the typically white background (that is, absorbing particular wavelengths of light). Such a model is called subtractive because inks “subtract” brightness from white.

HALFTONE

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 single color. Magenta printed with a 20% halftone, for example, produces a pink color, because the eye perceives the tiny magenta dots and the white paper between the dots 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 six colors: the three primaries themselves, plus three complementary 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 complementary colors correspond roughly to the additive primary colors). With halftoning, a full continuous range of colors can be produced.

SCREEN ANGLE

To improve print quality and reduce moiré patterns, the screens for individual colors are set at unique angles. While the specific 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 75˚/15˚/105˚

M 15˚/45˚/75˚

Y 0˚/0˚/90˚

K 45˚/75˚/15˚

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