Color printing is the reproduction of an image or text
in color (as opposed to simpler black and white or monochrome printing).
It may also commonly be called four-color process printing when
only the colors cyan, magenta, yellow, and black (also known as CMYK)
are used. Another emerging method of color printing is six-color
process printing (for example, Pantone's Hexachrome system)
which adds orange and green to the traditional CMYK for a larger and more
vibrant gamut,
or color range.
Color printing involves a series of steps, or transformations, in order
to generate a quality color reproduction. Here are the main steps when
reproducing a color image in CMYK print, along with some historical perspective.
Color separation process
The process of color separation occurs when the original artwork is digitally
scanned and separated into red, green, and blue components. Before digital
imaging was developed, the traditional method of doing this was to photograph the
image three times, using a filter for each color. However this is achieved,
the desired result is three grayscale images, which represent the red,
green, and blue (RGB)
components of the original image:
Image when separated into RGB components.
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 are subtractive primaries
which each represent two of the three additive primaries
(RGB) after one additive primary has been subtracted from white light.
CMY separations of image derived from the RGB separations.
Cyan, magenta, and yellow are the three main pigments used for color reproduction.
When these three colors are combined in printing, the result should be
a reasonable reproduction of the original, but it is not. Due to limitations
in the inkpigments,
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 include grey
component replacement, under
color removal, and under
color addition. This printing technique is referred to as CMYK (the "K" being
short for "key." In this case, the key color is black).
Cyan, magenta, yellow, and black (CMYK) inks when
printed separately. During normal print production,
these would be printed on top of one another.
Today's digital printing methods do not have the restriction of a single color
space that traditional CMYK processes do. Many presses can print
from files that were ripped with images using either RGB or
CMYK modes. The color reproduction abilities of a particular color space
can vary; the process of obtaining accurate colors within a color model
is called color
matching.
SCREENING
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 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, or halftone for
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
of optical
illusion, appear to form a continuous-tone image. You can view the
halftone screens that create printed images under magnification.
Cyan, magenta, yellow, and black (CMYK) separations with
halftone exaggerated to show detail.
Final composite image.
Traditionally, halftone screens were generated by inked lines on two sheets
of glass that were cemented together at right
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, increasing quality (by reducing
the repeated generations), reducing lead-times, and saving the environment
from toxic film-processing chemicals.
Screens with a "frequency" of 60 to 120 lines per inch (lpi) are used
to reproduce color photographs in newspapers. The coarser the screen (lower
frequency), the lower the quality of the printed image. Highly absorbent
newsprint requires a lower screen frequency than less-absorbent coated
paper stock 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 called dot
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.
STOCHASTIC SCREENING
CMYK image with stochastic screen enlarged to show detail.
Digital imaging technology has also given rise to new approaches to the
screening process. The best-known is stochastic screening.
Because the dots are the same size and randomly placed, the moiré effects
that are generated by traditional half-tones are eliminated. A side benefit
of stochastic screening
is the ability to obtain a wider gamut of
colors using additional inks such as orange or green (hexachrome). Due
to the high resolution of the screen, using computer
to plate imaging gives optimal results.
Almost all inkjet devices
use stochastic screening. Viewing any image produced by a home inkjet printer
with a magnifier (or loupe) will reveal the screening.
Stochastic screening or some hybrid of traditional linescreen and stochastic
has become the standard screening method for many packaging applications.
REFERENCES
Bruno, Michael H. (Ed.) (1995). Pocket Pal: A Graphic
Arts Production Handbook (16th ed.). Memphis: International
Paper
Hunt, R.W.G., The Reproduction of Color (1957, 1961, 1967,
1975) ISBN
0-85242-356-X
Yule, John A.C., Principles of Color Reproduction (1967, 2000) ISBN
0-88362-222-X
