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.
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.
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
ink pigments, 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).
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.
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 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. Take a magnifier (or loupe) if you own an inkjet printer at home and take a peek at any image you've printed.
Stochastic screening or some hybrid of traditional linescreen and stochastic has become the standard screening method for many packaging applications.
