Conventional four-color printing processes use cyan, magenta, and yellow
subtractive primaries, and "ordered" halftone screening for color reproduction.
This confines the image detail and color gamut achievable to only a portion of
what the human visual system can resolve, of what a camera can capture on film,
or of what computer graphic or image software can model. High fidelity color
printing increases or changes colorants to expand the possible color gamut and
more accurately match printed colors with reality. It can also employ specific
screening techniques to enhance rendering and reproduction of color and detail,
and may employ multi-spectral separation whilst scanning for image acquisition
and data preparation.
Knowledge of the essential physical and mathematical principles requisite for
high fidelity printing and color separation have long existed, but the manual
and photomechanical processes for printing with a large palette of colorants
have proven too difficult, expensive, and overly dependent on crafts persons
for extensive application. Automation of many high fidelity process will occur
with the accessibility of potent computing tools, and the use of modern
appearance measurement and process control procedures. Most of the required
associated technologies are now available.
About seven years ago the concept of high fidelity color was introduced in the
graphic arts community as a reaction to digital color reproduction systems such
as video and the Internet. HiFi color is the tradename given to expanded gamut
process color printing by the Davis, Inc. HiFi Color Project. It refers to any
of several different methods for expansion of the printed process color tonal
range. As the phrase was originally used, it was very loosely defined as
"methodologies, which expand the color gamut, tone rendering, dynamic range,
spatial frequency modulation, and other appearance domains of the print
medium". What this means is that any method of expanding the color gamut beyond
what is available with conventional four-color process printing qualifies as
"high fidelity". Graphic arts manufacturers, suppliers and providers unified
their forces to develop new technologies for making print more vivid, more
convenient and economical. However, each provider has its own interpretation to
reach this goal and as a result there is general confusion over the difference
between conventional offset and HiFi color printing. Sense conventional offset
techniques can produce very pleasing images, the main criterion for HiFi color
printing is related to the quality of the printing process and hence HiFi color
printing corresponds to high quality printing.
Four-color prepress costs have declined by as much as 50-75% over the last six
years because of both over capacity and gains in technological efficiency. Very
powerful four-color image manipulation software now exists for standard desktop
computers. The result is that inexpensive off-the-shelf software can provide
all of the features that were available in the high-end integrated color
systems, with equivalent productivity. Four-color processes will become
commodity color. This force driving down the cost of four-color process
separations may also stimulate the development of new markets, where more
colorful and complex forms of printing can provide value added through visual
distinction.
Anticipating this trend and hoping to be one of the first to ride its wave, the
consultant firm of Davis Inc. organized The High Fidelity Color Research
Program. This was a multi-client investigation to identify emerging markets,
technology, and business opportunities for high fidelity color products and
services. For this research program, High Fidelity Color was defined as
"processes that can achieve more and better color than is possible with
conventional four-color printing, including process elements such as substrate,
colorant set, scanning, separation, and screening strategies, as well as
printing process measurement, control, and optimization techniques." Support
for the project has come from graphic arts manufactures such as Dupont, Fuji,
Kodak, Pantone and Scitex. Quite a lot of technical issues have to be solved
such as what is the gamut increase of processes with more than four inks, how
can we model such processes and make profiles, what about screening and is the
ICC profile specification flexible enough for such print processes.
Until the end of the nineteenth century, most reproductions featuring color
were made by the original lithographic method. Highly skilled craftsmen who had
many years of specialized training prepared the stones for the various color
"plates". Many of these old prints were not limited to our present standard
four process colors; printing in six, eight, or even ten different hues was not
at all unusual, and that fact accounts for the beautiful delicate shadings of
many old lithographs.
In the first half of this century, the use of additional colors was still a
common practice, especially in lithography. For example, a pink and a light
cyan have often been used in addition to a deep magenta and a deep cyan. This
improved the purity of light blues, pinks, and flesh tones, which are usually
degraded because of proportionality failure associated with the halftone
structure. Much lithographic printing prior to 1945 was done in six colors,
with two magentas and two cyans. A number of special sets of four-color or
five-color inks have been proposed. Zander (1908) patented a four-color system
based on the use of green, magenta, blue, and yellow inks. Jacobs (1994)
proposed the use of a fourth colored ink similar to Prussian blue. Murray
(1934) proposed the addition of purple or lavender ink. With these systems
there is considerable overlapping of the absorption bands. With the development
of better pigments and better plates after WW2, print quality improved to allow
for good four-color printing. During the 1970's several efforts were made to
improve the quality of color reproductions still further. Ernst Schumacher,
research director of Klimsch Camera Co. developed five and six-color printing
process using an orange color with CMYK to produce a 5-color process, and
orange and green plus CMYK to produce a six-color system. Lack of interest in
these systems derived mainly from the fact that printing required two passes
through the press. This created register problems because there were very few
sheet-fed presses with more than four printing units.
Today many printing companies employ presses with six or more printing units in
order to enhance the visual appeal of printed products through the use of spot
colors, varnishes, and laminates. Such presses can be readily adapted to high
fidelity process reproduction methods. The existence of 7-, 8- and 10-color
sheet-fed presses has removed all register and press control constraints so
that seven and higher color printing systems are being developed that promise
to produce reproductions that can more closely match the color gamut, tone
range, and color purity of most originals.
Using terminology borrowed from the electronics field, alternative methods for
tone reproduction can improve print quality. AM (amplitude modulation)
describes the idea of varying the amount of color in a given area of print, by
changing the size of the dots. Each dot is given a value in terms of percent.
For example, a 50% dot area when viewed through a loop would have equal size
color and white dots. All dots would be evenly spaced, and in specific angled
rows. Because the dots are in even rows, traditionally, the numbers of dots per
inch are used to describe the fineness of the screen and are called "line
count." FM (frequency modulation) is sometimes refereed to as "stochastic dot"
or "random dot". What ever you call it, it is a radical departure from
traditional screening. Where both AM and FM screening are now computer
generated, and both are made up of dots, that is where any similarity ends. AM
relies on evenly spaced rows of dots of varying "percentages" to vary the
intensity of color, FM uses the approach of increasing the number of dots of
equal size in a given area to produce a similar effect. Since FM dots are not
in rows, and the process colors are not aligned in any pattern, angles do not
exist Conventional ordered or deterministic screening creates halftone dots
centered on a regular grid structure, and is susceptible to moiré.
Stochastic techniques incorporate a randomizing or probabilistic variable to
the screening process. This results in dot structure and placement that is
similar to that found in the antiquated collotype process, which allows for
more colors without moiré.
In a similar manner, waterless lithography offers the possibility of increasing
the fidelity of color printing. With waterless lithography there is minimal dot
gain. This means that much more ink can be laid down, with corresponding ink
density gains and no concerns about plugging. Since plugging is no longer a
problem, the printer can go to finer screens to enhance resolution and lessen
the effect of proportionality failure, while maintaining the opportunity to
print thicker ink films and more intense color.
Conventional color printing uses the tricolor principal. An original is scanned
and three channels of image information are created, once through a red filter,
once through a green filter, and once through a blue filter. The image
information obtained from these three scans is then printed with complementary
transparent colorants of cyan, magenta, and yellow to produce a subtractive
color print. It is possible to increase the gamut of the color printed by
creating a duotone for any or all of these information channels. A duotone is a
two-color printed image made from a single original information set with each
printed image emphasizing a different tonal range. Usually the image printed
with the darkest color emphasizes the shadow end of the illustration, while the
image printed with the lightest color emphasizes the highlight end. By result
of printing the image channel twice, a longer tonal range is possible than can
be achieved by only one impression. This means that more image information can
be printed, with smoother and subtler tonal transitions. Also, the use of two
different inks can increase the gamut of color in the image. Dupont's Hyper
Color is conventional four-color process printing with additional hits of each
of the colors only in the areas of 100% solid coverage.
In the greeting card market, Hallmark Cards introduced the addition of
fluorescent pink as a component of a duotone magenta channel to enhance skin
tones over what was possible with 4-color reproductions. The introduction of 5-
and 6-color sheetfed presses assured the success of this innovation. In the
1980's Hallmark added a sixth color, a second cyan, which was used to extend
the range of the conventional cyan printer. To ensure rigorous color
reproduction accuracy by this methodology, all three channels need to be
treated equally. The Swiss fine art publisher Lichtdruck uses a process that
splits the four standard process colors into two parts. They then print in
eight colors by making two four color passes - the first pass is used for light
colors, and the second for darker shades.
More than any other process, the Kuppers method probably produces the best
results. Demonstrated at DRUPA 90, this seven-color reproduction process was
developed by Harald Kuppers, director of the German repro house, Wittemann and
Kuppers. The seven colors are cyan, magenta, yellow, red, green, blue, and
black. The CMYK are essentially the same colors used in conventional 4-color
printing. The RGB are colors closely simulating the RGB additive primary
colors. The idea is, for example, rather than relying on the magenta and yellow
overprints to produce red, just print a red subtractive ink. The innovative
features of the method are the use of GCR (gray component replacement)
processing of the CMY colors, followed by selective CCR (color component
replacement) of the RGB constituents in an analogous manner, with a full range
black used to delineate all neutrals in the reproduction. Color image
processing for this technique is complex, but can be accomplished with software
now being developed for color electronic prepress systems. The results are
saturated, clean, and brilliant colors, with a visual impact well beyond
conventional 4-color printing. Interestingly, although employing more colors,
this process actually uses less ink when printing the image.
Examples of Kuppers Method
Image Information | With GCR | With CCR | |||
Cyan | 20% | Black | 20% | Black | 20% |
Magenta | 40% | Magenta | 20% | Red | 20% |
Yellow | 70% | Yellow | 50% | Yellow | 30% |
Image Information | With GCR | With CCR | |||
Cyan | 90% | Cyan | 60% | Cyan | 50% |
Magenta | 40% | Magenta | 10% | Blue | 10% |
Yellow | 30% | Black | 30% | Black | 30% |
Image Information | With GCR | With CCR | |||
Cyan | 50% | Cyan | 40% | Cyan | 20% |
Magenta | 10% | Black | 10% | Black | 10% |
Yellow | 30% | Yellow | 20% | Green | 20% |
Pantone Inc. introduced a six-color process at the DRUPA '95 Graphic arts show in Germany. Hexachrome is a fully integrated six-color process printing system, which includes new ink set separations, proofing and the PANTONE Hexachrome Color Selector. The proprietary ink set, developed by Pantone, consists of enhanced versions of the subtractive primaries yellow, magenta and cyan, along with black, vivid orange and intensified green. In a single pass on a six-color press, the company claims that Hexachrome can simulate over 90% of the PANTONE MATCHING SYSTEM Colors, more than twice the number than can be obtained using conventional four-color process printing. Hexachrome can be achieved with conventional screening with only two screen angles (same screen angle is assigned to colors which don't print together) or stochastic screening techniques. Heavy marketing and the availability of inexpensive separation processing software have made this the most well known HIFI process. Pantone also claims that Hexachrome offers several other advantages including:
VISU Technologies, a division of the Van Ginneken & Mostaard Group, one of
Western Europe's leading and most innovative prepress companies, developed an
innovative color processing system. The Interactive Color Independent Software
System (ICISS) allows a user to take an existing digital image, and separate it
into as many as sixteen channels based on targeted output inks or colorants.
The ICISS system comprises software called ICISS scanner and ICISS Separator,
which streamlines the separation workflow for CMYK output, duotones, tritones,
spot colors, bump plates and Hexachrome jobs. ICISS Scanner features high-end
system functionality, such as global and selective retouching without masks,
gradation, color balance and unsharp masking controls. Scripts are created that
contain the file changes. These scripts are then used to separate the high-res
file - automatically, in the background, and in batch mode, if desired, via
ICISS Separator. All of this can be done while reviewing the image with
"Digital/Ink" technology, which simulates ink and substrates on the computer
monitor.
All the above systems involve the same basic conventional tricolor principle,
with all manipulation of image information occurring as a function of output.
An original is scanned and three channels of image information are created,
once through a red filter, once through a green filter, and once through a blue
filter. To advance the concepts of High Fidelity printing to even greater
fundamental potential, different input scanning could be employed.
At the most rudimentary, the use of photographic narrow-band filters, such as
the green (61), blue (47B), and red (29), instead of the wide-band filters
green (58), blue (47), and red (25), can produce separations which make some
colors stronger than in normal separations. Of even more interest, it is
possible to separate the additive primaries of red, green and blue from an
original by using complementary colored filters. Photographically a cyan (44A)
filter can be used to separate red, a magenta (32) filter for green, and a
yellow (12) filter for blue. The graphic arts manufacturer Isomet says its 455
scanner features a new color computer that can scan for RGB, CMYK, brown and
violet.
Friedman (1944) suggested splitting the spectrum into four instead of three
equal bands, and Ball (1950) adjusted these bands to take advantage of what is
known about color vision and color mixture. Ball suggested several sets of inks
associated with these bands, each set consisting of a yellow, a pink, a purple
and a cyan. Some overlapping of absorption bands is of course unavoidable with
available pigments. His compromise set is intended to split the spectrum at
485, 545, and 600 nanometers. The purple ink absorbs what he called the "chlor"
band, a greenish yellow band extending from 545 to 600 nm. He also suggested a
five-color set that covered an even larger gamut of colors.
Greater sophistication of image information content will assist the print
medium to remain competitive in an increasingly visual world. High fidelity
color is a strategy that some innovators in the industry are currently
exploring to establish image sophistication. In addition to the main methods
described above, other more elaborate and exotic methods such as colorimetric
or spectro-radiometric scanning, 3D scene synthesis, advanced coating and
finishing, micro-lenticular lamination, and illumination/display techniques
have all been proposed. It is also probable that combinations of different high
fidelity process can be employed in the same print to gain further refinements.
In this regard, stochastic screening, waterless lithography and the Kuppers
Seven Color Process could all concurrently exist, with the whole being greater
than the sum of the parts. In all these scenarios, process control remains
paramount to success.