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Digital proofs are made directly from
the computer authored digital file without creating a set of films.
The digital file is the same file that will be used to create the printing
plates. The quality of digital proofs has improved to a level that is
acceptable for use as a contract proof, which is necessary as the industry
moves to a filmless workflow. Further improvements involve proofing
systems that create proofs containing halftone dots, which gives a more
accurate representation of the image to be reproduced.
When using digital systems for proofing,
be aware that the device(s) need to be color calibrated often to maintain
color accuracy, and color bars should be used on every proof.
A digital proofing process involves printing
each of the subtractive primary color components of the image (cyan,
magenta, yellow), and black on one pass through the output device to
create the full color digital proof as shown below.
Different types of digital color printers
are:
Digital Sublimation
A digital sublimation proof is the closest
of all digital proofers to the analog proofing method. Like the analog
method, the transparent colorant is transferred from a sheet or roll
(called ribbons) of clear polyester base, which has been coated with
the colorant. The ribbons are brought into contact with a receiver sheet
and varying temperatures of heat are used to transfer the colored material
in the image area to the receiver. The colorant begins as a solid and
then it is heated to become a gas. Once the colorant is on the receiver,
it cools and becomes a solid again. This procedure is completed for
each color. Most systems can transfer the colorant to the actual printing
stock either directly or as an additional step. The digital sublimation
proof system requires little maintenance and is reasonably consistent.
There are two different types of sublimation
proofers, dye sub and laser sub. The dye sub uses dyes to create the
color on the proof and the heat required in the proofing process is
supplied by a thin pointed metal stylus. The laser sub uses pigments
to create the color on the proof and a laser is used to provide the
heat that is required.
Dye Sublimation Proofers were the
first of the sublimation systems on the market. The proofs are called
Dye Subs. They utilize a series of thousands of styluses and dye based
colorants, which are relatively inexpensive and reasonably fast. The
disadvantages are: the dye colors on the proof will fade with time and
may not match the color at the press; the proofs must be viewed under
proper lighting conditions; the process has limited resolution and produces
a continuous tone effect.
Laser Sublimation Proofers are
the first of the dot oriented digital proofers. This system is much
more expensive and is slower than the dye sub system, but the quality
has been considered equal to the analog proofing system. The laser sublimation
proof system has been optimized for dot resolution and dye match to
the point where it is currently the standard. The disadvantages of this
system are that the proofs can also change color over time and they
must be viewed under proper lighting conditions.
Thermal Wax
Thermal wax printers work the same way
as the dye-sub printers except the printer ribbon is coated with colored
wax instead of using dyes. As the thermal head heats up, the wax fuses
to the paper. The thermal wax printer also prints only one process color
at a time so the paper must be fed into the printer four separate times
to produce the full color image. Thermal was printers produce a stronger
color than the ink jets, and they are much faster, but they are also
move expensive.
Inkjet
Inkjet printers range from inexpensive
desktop units suitable for letter size paper to floor units capable
of printing banners that are 36" wide and several feet long. The
ink comes in bottles containing the process colors and is sprayed from
nozzles onto the paper. Instead of printing a single row of pixels,
the ink jet printer prints a series of rows, vertically, each time it
travels across the sheet.
Some systems use six-colors to get 'photographic-quality'
output. The six-color inkjet printer add light cyan and light magenta
inks which produce more subtle flesh tones and finer color gradations
than the standard four colors.
There are several types of inkjet technology,
but the most common are drop on demand (DOD) and continuous flow. DOD
works by squirting small droplets of ink onto the paper. A continuous
flow system works with a continuous flow of very small droplets of ink
directed toward a receptor base mounted on a spinning drum. In locations
where color is not needed, the droplets are deflected and captured by
a waste collection system.
Since the proofing substrate is much
whiter and glossier than most papers, the proofs usually appear better
than the printed piece. To compensate for the paper difference, some
inkjet systems allow a simulation of the paper to be printed on the
proof, by printing the paper color with the rest of the document. Also,
some printers allow you to use the same paper that will be used for
printing the job on the press. This method of color management is called
absolute colorimetric rendering.
The inkjet printers require high-quality
coated or glossy paper for the production of photorealistic output.
The special paper that is required is composed of two layers. The base
layer is paper, and the second layer is a coating which accepts inkjet
ink. The surface must allow the ink to dry instantly during printing
to avoid smearing and ink spreading. The weight of the paper must be
heavy enough to avoid bleed through and curling, but of a weight that
can be folded and handled like regular printing stock.
The problems associated with inkjet printers
are:
- The ink has a tendency to smudge immediately after printing.
- The proofs are not waterproof.
- They are expensive to maintain. The special coated paper required
to produce high-quality output is very expensive, and the ink cartridges
which can also be expensive need be changed often. Most systems have
one cartridge for the black ink and one cartridge for the other three
colors (cyan, magenta, and yellow). If one of the three colors runs
out before the other two, the entire cartridge must be replaced.
Color
Laser Printer
Laser printers and copiers use a pre-charged
drum or belt that conducts a charge when exposed to light. Toner is
magnetically attracted to the appropriate areas of the image and repelled
from others. The printer transfers the image to the paper where it is
fused by heat and pressure.
Color laser copiers, interfaced with
fast and powerful RIPs, are now being used by service bureaus, quick
printers, and design shops as a way of producing quick proofs or short-run,
variable-data digital color printing.
Laser Thermal
Imaging Proofer
The thermal proofer is a high-end halftone
digital proofer that uses pigment based colors to produce exceptionally
high quality proofs. This is the fastest growing type of proofer because
it gives the closest match to the actual printing process. It is driven
by a single RIP that is also used to produce press plates.
The substrate is attached to a drum and
a donor sheet (ink sheet) is attached on the top of the substrate. An
infrared diode laser beam strikes the donor sheet and the material is
transferred from the colored donor sheet to the substrate. Since the
same RIP and laser exposure unit that creates the proof also creates
the printing plate, the proof reflects the same dot structure and angles
of the printed press sheet.
The following are factors to consider
when using digital proofs:
- Dots or no dots - many of the technologies cannot produce proofs
with halftone dots. The systems that are able to produce proofs with
halftone dots, may not produce dots that are the exact size and shape,
or at the same screen angle as those imaged on the printing plate.
With the differences in dot size and shape and screen angles, it would
be difficult to predict problems such as moiré patterns.
- Another factor is the ability to see the effects of overprinting.
- The key advantages of digital proofs over analog proofs are that
the costs can be less than half that of an analog color proof and
they are produced in much less time.
Tips
for checking a digital proof for accuracy
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