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See Illustration
An Essential Process for Color Printing.
When you need something to be printed in color and mass-produced,
you need to avail of color separation services. Examples of these
are full-color brochures and magazine or souvenir program covers.
Mass-production means a quantity of several hundred to thousands
and the most cost-effective means of doing this is through the
use of an offset printing machine. Although your color or inkjet
printer will allow you to print several color copies, it becomes
very expensive and time-consuming when you need hundreds or thousands.
How Colors are Printed.
Offset printing machines are basically single-color devices
that make use of printing plates that hold the image that is
going to be printed. The color that is printed on paper depends
on the ink color that is loaded on the machine. If you want to
print the Philippine flag, for example, you will need a printing
plate that holds the image of the blue field, another one for
the red field, and a third one with the image of the sun and
the stars for the yellow plate. All these images on the plates
are in black. To print the flag, the plate for the blue field
is attached and blue ink is loaded; the plate for the red field
together with the red ink; and the same is done with the yellow
part. Therefore, for a flag with three colors, paper is fed through
and printed three times with three color-separated plates together
with their corresponding ink colors.
The RGB (Red,
Green, Blue) System
How Colors are Created.
By understanding how color images are projected on you television
set will help you understand color separation. Your TV has three
light sources: the red, green and blue (RGB) light guns that
combine in certain ways to produce the entire color spectrum
that you see. When your set is off, it is almost black and when
the R, G, and B guns add up, they produce a white image, the
colors having an additive effect.
While your TV set is black when there is no light, paper, on
the other hand is white when there is no ink printed on it. For
printing, the reverse of the RGB system is implemented. The CMYK
system is used: for Cyan (combination of green and blue), Magenta
(red and blue), Yellow (red and green), and Black. This system
is subtractive. Theoretically, the combination of C, M, and Y
should produce black (or no colors), but due to imperfections
of inks a muddy brown results. This is why a fourth ink, black,
was introduced.
The CMY (Cyan,
Magenta, Yellow) System
With the CMYK system, full-color photographs and images can
be printed with certain combinations of cyan, magenta, yellow
and black inks. The problem is how to determine and separate
these components.
Basics of Color Separation.
The art of color separation involves taking the entire visual
range of light reflected or transmitted by an original image
and describing that range with individual colors. When these
individual colors are combined on a device such as a printing
press, a full-color reproduction of the original image is produced.
A color separation is a classic example of the whole being greater
than the sum of its parts.
The number of individual colors that you can see is immense.
The ideal situation is to identify every color in an image and
then print that color. Unfortunately, the logistics of printing
20,000 or more colors proves to be a bit unreasonable. Fortunately,
in the early 1860’s, British physicist James Maswell established
the basics of trichromatic color photography and laid the foundation
for most modern color reproduction. He reduced the variables
of light into three primary colors. Maxwell divided the spectrum
of colors by projecting light through separate red, green, and
blue filters. Using the lowest number of primary colors possible,
he literally established how to reproduce most of the visual
spectrum discovered by Newton. Although the tools have evolved
over time, modern printers and color separators use the same
basic techniques that Maxwell established over a century ago.
For over 50 years, color separation for commercial printing relied
on analog photographic methods. From the mid-80s to the present,
the printing industry has shifted to mainly digital methods of
color reproduction. The printing devices used to produce color
have become more computerized and sophisticated, but the actual
printing process of putting ink or dyes on a substrate has not
changed. Because the end process of printing is the same, the
techniques involved with color separation remain the same. Computerization
and digital file handling have refined parts of the procedure,
but mastering the theories and techniques must still be accomplished
to achieve good reproduction results.
Separating Tones and Color
Every image that you see with your eyes reflects or transmit
tones and colors from a range of light wavelengths (400-700nm).
The separation of color involves a simplified description of
an image’s visual range of color. To simplify the visible
spectrum of light which brings the image to your eye, the spectral
information is broken into three main bandwidths of Red, Green,
and Blue (RGB), called trichromatic color system.
The combination of red, green, and blue in additive light theory
makes white light--which represents nearly all the visible wavelengths
of light.
After the initial separation of the spectrum is accomplished,
the total light data is easier to describe and manipulate. After
the colors of an image are assigned trichromatic RGB values by
a scanner, the color data is digitized. At this point you can
say that the total color description of the original image has
been reduced to three groups of binary on-off commands. The digital
data is more manageable that the complex series of light wavelengths
that enable the eye to comprehend tones and color.
When color separation were made in the predigital age, light
from the original was passed through RGB filters and focused
on photographic film. The colored filters separated the wavelengths
of light. The result was an individual halftone film for each
separate color. Halftone film is composed of groups of black;
metallic-silver dots of various sizes. A halftone firm is a black
or white (solid or clear) record of a single color. The black
or white composition of the film emulates the digital function
of on or off (one or zero). The physical form of the halftone
film acts just like a digital file version of an image.
Analog-produced, halftone separation films and digital color
data files may be different physically, but they are similar
in function. Both are black-and-white records of a visual set
of tones and colors. The complex sensation of color has been
reduced to a set of simple instructions.
After the visual color experience is separated into the digital
components of RGB color, you must convert the RGB file to the
complementary color space of CMY in order to physically print
the colors. CMYK inks are used on printing devices. But when
other uses for the image data required, color spaces such as
YCC (for photo CD), CIE L*a*b*, and HSV are available to manipulate
the data while it is still in digital form.
Digital files are available for one of two possible types of
printing: continuous tone or halftone printing. Continuous tone
printing is slower and very expensive compared to halftone printing.
Cheaper and quicker, halftone printing is geared more for producing
mass quantities of prints. |
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