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To build a color image, we can selectively subtract some of the light. When we
print with red ink, we see it as red because it absorbs blue and green light
and reflects only red. In other words, red ink absorbs, or subtracts,
two-thirds of the visible spectrum, and reflects one-third. The same can be
said for blue and green. They each absorb two thirds of the visible spectrum
and reflect one third.
In the printing process, we use colored inks that each reflect not one-third,
but two-thirds of the visible spectrum. These special inks correspond to the
secondary colors of light; namely Cyan, Magenta, and Yellow.
Since each color absorbs one-third of the visible spectrum, it takes all three
inks to make black. Overprinting any two of these colors will produce red,
green, or blue. By overprinting selected amounts of cyan, magenta, and yellow,
the widest range of colors, within the limitations of paper and ink, can be
achieved.
Color Reproduction
In color reproduction, pigmented transparent inks (cyan, magenta, and yellow)
are used for printing. These inks are called process colors. A transparent
printing ink is made to absorb one component of white light and transmit the
other two.
The paper, or substrate, then reflects the transmitted colors back to the eye.
Cyan ink absorbs only red light, so appears blue-green. Magenta ink absorbs
only green light and appears bluish-red. Yellow ink absorbs blue light.
Color Absorption and Reflection
In theory, when each of the process colors - cyan, magenta, and yellow - are
overprinted, absorbing all the visible light reflected from the paper, the
result should be black.
In reality, solid layers of all three printing inks do not absorb all the
available light, and a brownish grey colour is produced. This is because of
impurities in process inks. Cyan ink not only absorbs red as it should, but
also absorbs some green and blue light.
The magenta ink should absorb only green light. It also absorbs some blue and
red. Yellow ink is nearly ideal. To overcome this problem, black ink is also
used. When black ink is added to the reproduction, it will add detail and
enhance contrast, making the dark areas appear darker and the light areas
appear lighter.
Cyan, magenta and yellow can therefore define the printed color space. This
representation of the color-space is referred to as the CMY color space -a
three-dimensional model, resembling the RGB model, yet distorted due to the
above-mentioned impurities.
Color separation
In color reproduction, the process of translating a color photograph or
transparency into its cyan, magenta, yellow and black components is called
color separation.
The principle of color separation is similar in both photographic and
electronic scanning processes. Each process utilizes the concepts of additive
and subtractive color for filtering the individual components of white light.
The photographic process consists of illuminating the original copy with white
light and then separating it into three images by placing red, green, blue
filters in between the copy and the film.
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A red filter is used to generate the cyan separation
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A green filter is used to generate the magenta separation
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A blue filter is used to generate the yellow separation
The black separation is made from a combination of the other three.
CMYK
The printing industry generally refers to the four process color separations
as CMYK where: C is for Cyan, M is for Magenta, Y is for Yellow and K is used
for Black, to distinguish it from Blue.
Different color languages
The devices comprising an open computer imaging system may all process colors
differently. Each type of device can be thought of as using its own 'language'
when interpreting and rendering color. This means that there can be no
automatic way of guaranteeing color fidelity throughout a system. Also, device
characteristics, such as age and wear, and operating conditions, such as
temperature, can affect color rendition. This can lead to disappointment when,
for instance, what was seen on the monitor looked fine, or when a screen
representation of a scanned artwork looks markedly different from the original.
Color spaces
Unfortunately, we are often not going to be able to faithfully reproduce an
object as seen by the human eye. Devices and systems available today are
simply not capable of doing so.
Monitors can represent more colors, operating in the RGB color space (red,
green and blue phosphors), than desktop printers and the offset lithographic
processes.
A scanner records, or samples, an image as a set of RGB values. The accuracy
of the color measurements taken from the image depends on the scanners
manufacture, maintenance and settings. A monitor will display that image to us
in a way that depends on viewing conditions, the screen phosphors used in its
manufacture, the state of its settings and on its condition. Further, when an
image is printed, the colors reproduced and perceived will depend on the inks,
as well as the paper used to print and the conditions under which the print is
viewed.
A system is required to compensate for the color handling differences between
the devices. This system is required to enable predictable reproduction of an
original image, in which the output is as close as possible to the original.
However, it must be acknowledged that the output may be comprised of a smaller
set of colors than that in the original scanned artwork. In other words, some
colors may be changed or lost.
Resources
The websites of most of the well-known imaging product manufacturers and
distributors now have pages dedicated to this issue. A small selection of
these and others providing information are listed below:
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