There are two types of mask.
First of all, masks with holes and round phosphorus
which are most traditional. The monitors with this type
of mask are found in Graphic arts, and this is called
Shadow Mask or Invar. It has several advantages, in
particular to that obtain a better restitution of the
colors.
The
second type of mask is a grid with slits and phosphorus
in bands. This technology is called Trinitron (invented
by Sony) and Diamondtron (alternative to Trinitron system
and invented by Mitsubishi). It is very less used in
the field of the Graphic arts, since contrast is generally
too high and the phosphorous used is very less to the
needs for the Graphic arts.
Between these two technologies of mask positionnercelle
setting by NEC (entitled CromaClear) can ahead, which
presents holes in the shape of ellipse, rather than
round.
Phosphorus
can show different characteristics according to the
manufacturer and chemical components selected by the
manufacturer. for ex. phosphorus EBU-P22 is used in
the applications of the Graphic arts and not for application
of office automation, for which phosphorus with higher
contrast is used.
As
we saw, the sweeping of the papered surface of luminophores
RGB is carried out line by line with an aim of obtaining
an image, and this in a split second. This sweeping
is characterized by a vertical and horizontal frequency
(expressed in unit Hertz or Hz). This frequency is associated
with the cooling of the screen. The image will be more
stable if this is more raised. It has a value criticizes
endessous of which (approximately
72 Hz) of the headaches can appear.
The
quality of an image on screen is dependent on the concept
of mask.
This represents the distance between two pixels, measured
in millimetres. Of Course it is necessary to know type
of screen (Trinitron or Invar for example), because
in the first type of mask, this distance is equivalent
to that which exists between two holes, and in the second,
with that between two slits.
LCD Screens
(Liquid Crystal Display)
LCD
Screens belongs to the family of the flat-faced screens,
to which, we can associate the screen plasma. LCD screens
takes very little space, and consume relatively less
energy, but they are rather expensive. Moreover, they
are adapted very less to graphic industry and the image
processing (lack of true system of calibration , and
the smaller gamut).
The
flat-faced screens called LCD (Liquid Crystal Display)
monitors are based on technology with liquid crystals.
This technology was initially used on the portable computers,
now we can also see this principle of operation in data-processing
screens. The most widespread technology is named TFT
(Thin-Film Transistor).
Technology
A
LCD monitor is made up of two walls of translucent glass,
having furrows whose orientation is very clearly defined
(angle of 90°). Between these two walls, a layer
of liquid crystals is inserted. The orientation of each
is same as that of the furrows constituting the walls.
Under the effect of an electric field (caused by a tension
of video electric-signal), the orientation of these
liquid crystals changes and is carried out according
to direction of this electric field (one of the properties
of these crystals is to propagate the light under the
effect of an electric field). This is similar to the
furrows of the walls and the crystals, the light (coming
from a fluorescent source) is propagated naturally under
the effect of the field. When the crystals change orientation,
the light is blocked. For the restitution of a color,
this system applies an identical way. Indeed, three
transistors are applied for each pixel (one by channel
color). The light intensity of each channel is modulated
according to the electric tension applied to each transistor.
Thus, the three luminous components move towards a network
of RGB filters placed in front of the wall before LCD
monitor. When the electric tension is stopped, the orientation
of the liquid crystals returns to its state of origin.
The
principal advantages and disadvantages of this LCD technology,
within the framework of graphic applications is described
below.
Advantages
:
LCD
Screens are less bulky than the CRT, and can thus saves
a considerable place in a manufacturing unit. From a
safety point of view, LCD screens are less dangerous
for the operator as CRT screens emits electromagnetic
radiations. (safety requirements for the observation
and work on screen CRT). Many administrations chose
this type of data-processing screens because of these
advantages.
Disadvantages
:
For
graphic industry, LCD Screen comprises of many disadvantages.
Indeed, the systems of calibration of these screens
are almost non-existent nowadays. However, the importance
of this operation of calibration is established perfectly
in a graphic flow of production and prevents the use
of this type of material. Currently, the color management
on this type of monitor is impossible.
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(problem related to the sensors
of the apparatuses, with the angle of measurement, adjustment
of gamma and smaller colorimetric
space) and thus limits this in administrative
and office automation tasks. It is not possible to carry
out final improvement of images and chromy with this
type of monitor.The current technology employed in LCD
screens does not allow an identical observation (and
a measurement) according to the angle under which one
looks the screen. However solutions are being studied
and will be able to make it possible to integrate LCD
screen in the digital chain.
Plasma display
screens or PDP (Plasma Display Panels)
Principle
of operation
In
its principle, it should be known that each pixel of
the screen receives a clean light source. There is
a certain number of cells containing the gas neon, behind
and in front of the two assembled glass plates. Those
have drivers on all their surface. Each cell can be
excited by an electric discharge, via these two networks
of drivers. The adjustment of the electrical current
must be controlled perfectly via various methods. The
plasma display screens (as CRT monitors) are thus emissive
screens, i.e. they are assimilated to sources of light.
Contrary
to these types of monitors, the liquid crystal displays
are not emissive since they will use external sources
of light.
Ergonomics-Standard-legislation
Recommendations
The
first recommendation goes back to 1987. A committee
of study formed by the Swedish government expresses
this request under the name of MPRI, at the series of
tests and research on the work of the pregnant women
in front of a computer screen. In 1990, recommendation
MPRI warns against the risks related to this type of
work and imposes a series of safety measures.In 1991
the Swedish Confederation of Professional Employees
(TCO) published recommendations (TCO91, TCO92, TCO95,
and TCO99) based on the principle of 6E (Ergonomics,
Emission, Energy, Ecology, Effectiveness, Economy).
Today, these labels are respected by the manufacturers
of monitors. Of Course, there are also other foreign
certifications such as TUV Rheinland, Energy Star and
Blue Angel. Fogra has its own standard screen display
in order to control color temperature, gamma, white
point and black point.
The
legislation
In
term of legislation, a decree (91-451 of May 14, 1991)
governs work on data-processing station. This decree
warns against the possible risks and diseases in connection
with work on screen.
The
points on standards
The
standards concerning the ergonomics of the interaction
between the man and the machine within the framework
of work were certified by the International Standard
Organization (ISO). A series of 17 ISO standards (9241)
is devoted to this effect. The general heading of these
standards is as follows: "ergonomic Requirements
for clerical work with display screen terminals (TEV)".
These standards of course are intended to the originators
of the product, with the employers and the users of
the data-processing stations.
Working
conditions in practice
The
ergonomic aspect is in close relationship to the practical
and technical aspect of the elements in the office,
which can influence the work of the employee. The variation
of lighting surrounding between space and the intensity
of the screen should not be of a too great amplitude.
A too strong contrast tires the eye much more quickly.
Moreover, it must always be placed at the perpendicular
of the window and a minimum distance of 1.5 m.
The
standard concerning the posting of the colors is not
approached here because it is in the framework of office
automation and not of image processing. The lighting
of the part in which we observe an image on a screen
or a luminous table must be controlled and is regulated
by standards. It is advised to use an ambient lighting
whose color temperature approaches 5000K. By this care
taken to ambient lighting, the critical observations
on screen and luminous table are facilitated. The use
of a cap for monitor is essential within the framework
of a professional work. Sources external of light such
as the windows, lamps of the ceiling light etc must
be controlled by the operator. Thus, a gray environment
would be the ideal (ground, wall, ceiling, curtain).
The distances from observation are also to be supervise,
just as the positioning of the monitor in the part,
and its orientation with respect to the openings. Never
put a screen face to face of the window (stray reflections,
contrast weakened etc.).
The temperature influences the electronic elements
of computer equipment and can in certain cases change
its performances.
Scanners
Fundamental
properties
Roles
of a CCD sensor
It
provides three different functions in data acquisition.
It must guarantee the activities of photoelectric conversion,
accumulation of electric charges from the signal and
finally transmission of loads .
light-electricity
Conversion
The
energy of a photon is inversely proportional to the
wavelength of the light, which is
ellemême responsible for the color. CCD
Sensor is a semiconductor material, i.e. its electrons
have specific characteristics allowing the conduction
of energy. A phenomenon thermal agitation acting on
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conduction as well as the
light. Already CCD Sensors underwent comparision with
very pure conducting semiconductors, a voluntary addition
of impurities. These sensors are thus doped semiconductors,
which increases the conduction of energy.
Accumulation
of the loads
Once
conversion from light-electricity is carried out, the
sensor has to accumulate the electric charges. The element
carrying out the transfer of load is a derivative of
MOS (Metal Oxide Semiconductor) technologies . Each
grid of CCD can be used like a volatile analogical report,
knowing that the time of memorizing depends on the temperature
of the substrate and thermal agitation presents at the
interior one.
A/D
Conversion (=CAN)
The
Phase A/D Conversion will start immediately after the
capture of the source light by the CCD or the PMT (amplified
signal). The A/D converters transform the analogical
tensions into digital data. One of its two phases
is sampling. Space sampling, carried out in a discrete
way by the CAN, provides a mosaic of square pixels to
some extent .
The quantification follows after the stage of sampling.
It consists of the transformation of an analogical signal
into a digital signal, to make correspond a number
to each level of the analogical signal, according to
a given scale. Each result of sampling is measured and
the number is allotted in a data-processing word of
N bits.
The value of N is defined as the depth of quantification,
which determines the échelledes
measurements. This scale depends on the dynamics of
the signal and the smoothness on which one wishes to
obtain. In this case, the image is coded in a finished
number of levels. We thus have a digital image defined
in the form of matrix of numbers. The scanner Pro 48
of Qubyx has a depth of 48 bits color (either 65,536
levels per color, that is to say 3x16 bits in mode RGB).
The tables are different for each scanner. That constitutes
a qualitative and quantitative stage in terms of interpretation
of the digitized image (to some extent the first level
of the color management takes place at this level).
The manufacturer himself has to decide the quality of
this table and results obtained by the output of CAN.
Operation
of a flat scanner
There
are two types of digitizers according to their technological
design: flat scanners and drum scanners. Drum scanners
are higher in quality of capture and a system of covering
than the flat scanners. Currently, the variations in
terms of quality tend to be reduced.
The
traditional composition of a scanner comprises of one
or two sources of light, a flat support (pane) to place
the original, allowing a system to capture and analyse
the light emitted by the source. These components also
determines the quality of the capture. The digitizing
software and the analysis of the signal constitutes
a basic element of the scanner.
The
principle of operation of a scanner is rather simple.
A source light, lits the document to be digitized. The
pencil of light passes then dansun
system of mirrors and convergent lenses which sends
it on a bar of CCD sensors, associated with a set of
RGB filters. The sensors receives the light signal and
convert this received energy into an electric tension
proportional to this one. Finally, an analog-to-digital
converter transforms into a digital data.
In general, the light source is mobile
and sweeps the glass plate on which the original is
posed. A mirror moves with it and returns the radiations
to a die cover often made up of another mirror and CCD
sensor.
The
PMT or photomultiplier
The drum scanners uses photomultipliers (PMT) system
of detection. The capture of the three RGB component
resulting from the light source of the scanner (for
example a lamp with Xenon) is carried out via three
PMT. The transparent documents are lit by a light
source located inside the drum, whereas the opaque
documents are to lit outside. The beam of light
is directed, to a set of mirrors towards PMT sensors,
after having crossed a set of RGB filters. The photomultiplier
is a photosensitive sensor in which electrons are
multiplied by the phenomenon known as secondary
emission. The beam of light arriving on the cathodic
element of PMT causes to release from the electrons
which move immediately and automatically towards
the dynodes. The secondary emission takes place
on the level of the dynode, which has a consequence
increase in release of electrons. This phenomenon
is called amplification. From this amplification,
the analogical signal is converted into digital
signal by the A/D converter. PMT Technology makes
it possible to obtain densities more important than
a CCD scanner, but its manufacturing cost confines
it only with the very high digitizers of range. |
|
From
an economic point of view, the scanner table has become
an essential tool for a printer. It is declined in version
intended to the amateurs but also for the professionals.
CCD Sensor integrated in the flat scanners is a technological
element in perpetual evolution. Nevertheless, it is
still different in terms of quality of results with
respect to those obtained by digitalization on drum
scanner. This has a system of analysis via the photomultiplier.
Drum
scanners
The
drum scanners are considered to be a very top-of-the-range
material, and are reserved to professional customers.
The constraints related to this type of digitizer (especially
in term of handling) are with the detriments of a high
productivity but have the advantage of generating images
of optimal quality. The ray of light crosses the document
(example of a transparent document) which itself is
assembled on a cylinder of rotary glass. A rather simple
set of mirrors makes it possible to reach the three
photosensitive sensors (a filter by component of the
light).
Systems of impression
The
systems of impression also known as restitution (the
screen is also a peripheral of restitution) are declined
in two great families (two types of technologies): modes
of impression to screen and continuous tone.
Technology with
tramage
The
ink jet
The
ink jet was first used in office automation, before
tackling the market of photography, prepress (for example
the printer Epson Proofer 5000 for the digital épreuvage),
and the large size impression. The ink jet market is
divided between three large manufacturers Epson, Hewlett-Packard
and Canon. They employ different technologies with a
varying number of colors (for ex. 4 colors CMYK
or 7 colors black, cyan, clear cyan, magenta, clear
magenta, yellow, clear yellow.)
In all, these technologies are explained technically
by the projection of fine drops of ink on surface of
paper (paper is an essential parameter with made up
of final impression and each company develops its own
range with its own inks).
Piezo-electric
technology is used in the small average formats of Epson
printers (technology Microphone-piezo) and in the Xerox
large sizes. The print heads used in the Epson ink jet
printers have a variable number of tubes which deposits
ink drops on the surface of paper. Precision of the
size of these drops, as well as the way in which they
are deposited on paper are very important in this technology.
It applies a tension to the piezo crystals, which themselves
operate the pumps containing ink (the pumps are located
in the print head). The size of the drop (technology
called UltraMicrodot at Epson) is in the center of a
commercial battle between the protagonists of the ink
jet.
The
Thermal ink Jet technology (Thermal Ink Jet or TIJ)
is second largest in the ink jet market. Hewlett-packard
and Canon use this technology as a competitor to electric
one. At present, It is applied to the small and large
size printers. From a technological point of view, the
print head of the machine has a cavity inside a microphone-resistance
. A negligible part of the ink penetrates inside the
cavity is heated at very high temperature (phase known
as of nucleation) and thus causes a thermal shock which
results in to expel the remainder ink which is not heated,
but present in the cavity. This proportion of expelled
ink arrives on the surface of paper while the cavity
absorbs a new quantity of ink. The number of holes in
a print head is variable (several hundreds), and expelled
ink are several thousands a second. Of course, as in
the case of piezo-electric ink jet, the size of the
globule is extremely fine (some picolitres). Some differences
exists between this technology and piezo-electric, it
should be announced that the change of toners for a
printer to thermal transfer, obligatorily induced a
renewal of the print heads (present in each cartridge
CMYK). A print head of a printer piezo has less tubes,
which induces a less smoothness, but a speed of execution
is more important .
Xerography
Xerography
is a system of impression to tramage
which consists of a transfer of solid ink by a phenomenon
called electrostatism. It is a dry process which calls
upon the intrinsic properties of electrically charged
materials, which were sensitive to the light. This electrophotographic
process was invented in 1938 by C.Carlson, and put on
the market eleven years later by the Xerox company,
which was called xerography. It uses the projection
of the image document on a photoconductive plate (electrically
charged). Each variation of intensity of luminous projection
on the photoconductive plate corresponds to the creation
of electric charges. By the electrostatic phenomenon,
the electrically charged dyes are attracted on the plate
before being transferred on surface from paper. The
fixation of the dyes is carried out by fusion. It is
this technology of impression which equips a great number
of electrostatic copieursdits,
and which thus uses, a paper surface untreated chemically
and intrinsic characteristics of Selenium (photoconductive
substance). This technology is based on the principle
of the static electricity.
Continuous Tone
technology
Thermal
sublimation
The
technology of a printer with thermal sublimation is
based on the passage of a plasticized film (which comprises
a layer of coloured wax) in front of a heating print
head. This print head has several hundreds of elements
being able to be carried at a very high temperature
(between 220 and 370°C). In the principle, sublimation
is the instantaneous passage of a solid body to a gas
state, without passing through the liquid state. The
heat emitted by a barette
of electrodes is directly in contact with the paper
covered with a chemical substance, and of the transparent
film to which a layer of coloured wax was applied (Cyan,
Magenta, Yellow, Black). The coloured wax vaporizes
and penetrates into fibres of the paper sheet. The temperature
of the heat produced by the barette of electrode is
directly proportional to the quantity of gas produced
(vaporization), thus controls the density of wax in
coloured paper. This process occurs in three passes,
sometimes four additional pass in the case of of black
ink. Each element of the system must have characteristics
suitable for thermal sublimation. The transparent film
covered with coloured wax, the heating barette
of diodes, paper and its specific surface, rollers of
pressure of paper on film, as well as the debtor and
receiving reels of film must be coordinated perfectly
to obtain a good quality of thermal sublimation process.
Printers on silver
paper
This
technology is used on printers such as Fuji Pictrography.
The principle of operation is as follows: The impression
takes place starting from a digital file. The exposure
is carried out via three laser diodes on the surface
of paper line by line, with a system of sweeping allowing
to traverse the surface of impression. In fact, significant
surface is made up of specific photographic paper. In
the case of Pictrography, there is a coupled transmitter
and a receiver, developed using a distilled water. The
dyes are contained in photographic paper.
The imagor (films
and papers)
A
digital imagor allows to obtain images and supports
opaque or transparent computer files. The impression
or the insolation of photosensitive surface can be carried
out either by the projection of a cathode ray tube (CRT)
or by a laser RGB. According to the format of photosensitive
surface and the type of support (transparent or opaque),
there are particular systems deriving from these two
principle types of insolation.
digital impression - General
The
digital impression can be defined as a process of traditional
reproduction which is associated with data-processing
from which the digital files are transmitted to the
system of impression. The digital impression is a simple
reproduction of color printer, with the concept of productivity
which delimits its properties clearly. This process
rises directly from xerography. A dozen companies divide
the current market (Gun, Oce, Xerox, Xeikon, Oki, Agfa,
IBM, Indigo, Screen, Heidelberg, Scitex etc.). The digital
system of impression is in full expansion at the present
time, since it combines advantages that does not have
the offset process (proceeded major of the traditional
impression). Indeed, the flexibility of work, the speed
of execution of the various stages and the great productivity
which in result from this the systems of impression
is best adapted to a production work. Its type of originals
can vary in terms of formats, contents (texts, images,
graphs, etc.), and sources (network, Internet). This
process is based on the principle of the quadrichromy,
but cannot employ the colors of accompaniment as in
the case of the offset.
There
are three great technological families in the field
of the digital impression. First is called electrophotography
and was developed by Xeikon. Its principle is to use
a system of impression via a dry toner. The principle
systems present in the market are ChromaPress (Agfa),
DigiPress (Barco) and InfoColor (IBM) systems.
To understand the role of a screen in the system
of impression, it is always necessary to know that
the ink deposited on the surface of the image must
always have the same thickness. The solution to
obtain half-tones is to use a system of opaque surfaces,
but invisible with the naked eye under the normal
conditions. (distance and lighting of observation).
It is a system which modulates the printed image.
According to the surface of the points it gives
an illusion of more or less dense. The result obtained
is called imitation. The current systems of impression
in quadrichromy or hexachromy use the technique
continuous half-tone to simulate the reproduction
of the tone. This technique simulates a whole nuances
colors while varying the size of the points (variation
of the percentage of these points). The screen must
give the quality of smoothness of impression (smoothness
of screen). Indeed, the outline of screen must be
selected according to the type of work. On standard
newsprint paper, the outline of screen is sufficient
about 65 to 110 lines per inch (lpi), whereas on
art paper, it is about 133 to 200 lpi. There are
two types of screens: the traditional and random
screen. The traditional screen is a regular alignment
with a well defined spacing enters the various points
of impression. The moire is a characteristic defect
that can be defined as a visually awkward optical
effect whose two principle reasons are as follows.
The watered effect appears at the time of the interaction
between several screens or a screen and an element
of the image. The solution used to avoid the watered
effect is to incline the screens by modifying the
angle of screen at the time of the impression. The
second solution is to use a random screen or with
frequency modulation, whose principle is to utilize
points of identical size whose spacing between them
is variable. Currently, the random tramage is defined
digitally and It is employed more and more by its
effectiveness. |
Recall on the
process of traditional offset
The traditional offset is a photomechanical process
of impression (together with the processes photography
and the chemical treatment allows the preparation
of the forms of impression), indirect by planographic
printing form (plane surface). Its principle is
based on the repulsion which exists between a greasy
substance (ink) and water. The printing form consists
of an absorbent part, and of another hydrophobic
subject. The system of offset printing is composed
of three principal elements which are: the cylinder
Carries plate or printing Plate, the cylinder Carries
strainer, and pressure rolls it. In its operation,
the originals (texts, drawings, images, graphs)
are deferred photographically to the surface of
a thin metal plate. This plate is having a form
of impression, it is rolled up around the cylinder
plate. After having undergone a damping and an inking,
it turns in contact with another cylinder covered
with rubber (cylinder strainer) and the printing
elements inks are transferred there. With each turn
of cylinder, the printing form is fed out of water
and ink. On this one, certain zones are inks whereas
others push back ink by the repulsive effect. As
a preliminary, the plates of offset printing were
treated chemically so that inks are retained only
on the printing zones. This cylinder comes in its
turn in contact with paper (roller or sheet), carried
by a third cylinder. The printing elements are transferred
there to carry out the impression. This process
has advantages such as the possibility of using
paper of good quality, thus to have a final quality
of better impression, but also a speed of important
execution. Currently, the offset is able to carry
out any type of printed color (large size with the
label). |
The second technological process is the liquid system
of inking developed by Indigo, the principle uses the
press with fluid ink. The last process was developed
by Heidelberg (digital system of press). This last is
besides directly at the origin of CTP technology (Computer
To Plate).
All
in all, a digital system of impression has two characteristic
units which are the engine of impression and the DIGITAL
Front End (DFE) or print server. The role of this is
to provide the data-processing data to the engine which
directs the impression. these two elements can be added
to a RIP (Raster Imaging Processor), a software of preparation
of the data (documents to be gauged, to impose), the
whole integrated in a flow of production (Workflow).
CTP Technology
(Computer To Plate)
Computer
To Plate (CTP) technology or "Direct Exposure of
the Plates" has an entirely digital principle of
operation until the system of impression. The stage
of creation of films for the press offset (four films
CMYK) is removed. The direct reproduction of the plates
of impression makes it place. The reference here is
the plate instead of film. Among the advantages of this
technique of impression, we can propose the more precise
point, the facilitated location, the fattening of the
points controlled better, and a productivity more adaptable
for the traditional process of impression. The market
of the CTP is currently in full effervescence, what
explains why more and more printers adopt this technology
in parallel of their system of offset impression. The
quality of the printed image is due to the fact that
the plates are especially designed for impression CTP,
but also with the precision of the system of exposure
of the plates by laser. The restitution high and low
lights is better via impression CTP.
The principle of operation is as follows.
The
insolation of the plates directly starting from the
digital files is dependent on the two elements which
we described previously, i.e. the digital type of plate
and laser heads. The computer file is transmitted to
the RIP (Raster Imaging Processor), who transcribes
this one in a bitmap
file and
sends it to system CTP. A bitmap file consists
of black and white spots which indicates each elementary
point of the plate if the laser must be or not to be
activated. The insolation of the plate is carried out
line by line, and it is is the optical system, that
is to say the plate which carries out displacement in
order to carry out the complete writing of the digital
plate. This writing must be extremely reliable and fast.
This is why this system is equipped with a significant
and variable number of lasers by
The flashage is a stage of the graphic chain being
located between that of the graphic creation (put
on page, improve, chromy, etc.) and that of the
impression. A flasheuse be to impress a photosensitive
surface using a calculating unit (computer) associated
a RIP. On film resulting from the flashage, indications
such as the features of cut, and other systems of
control, are present with an aim of bringing to
the photoengraver the most information necessary.
This preliminary stage with that of the impression
on press is essential if one wishes to carry out
a pulling in many specimens. This reproduction on
four different films (CMJN) is associated a test
of control. This stage is it-also necessary and
avoids the drifts at the time of the impression
on final press, in particular with regard to the
control of the colors (cf chapter devoted to the
épreuvage).
The films produced at the time of the flashage are
also used to create the plates of offset printing.
The traditional format to use at the time of the
flashage is EPS (Encapsulated PostScript), which
is a specific format for the safeguard of text and
images in a model, whose principle is based on the
programming language PostScript. |
CTP. The time of insolation is called time of maintenance
or dwell time and must thus be controlled perfectly.
The choice of the lasers is thus directly dependent
on this property and they are selected according to
their power and their wavelength of emission. Synchronization
must be perfect between the data resulting from the
RIP and activations and deactivations of the lasers.
Moreover, the screen is supported by this technology,
since a point of screen is in fact made up of a number
of elementary
points. The plates used can be several natures, polyester
or aluminium. The principal manufacturers of systems
CTP are Heidelberg CPS, Scitex, Screen, PurupEskofot,
Agfa, and Cymbolic Sciences.
digital presses
This
process results in to radically change the stages of
the graphic chain of production, or more simply to remove
some of them. Indeed, stages of creation of films by
flashage, the imposition and the assembly of these films,
are as many phases not taking place more to exist with
the digital system of press. The chain of production
adapted to such a system of impression can be as follows:
the digital file is sent to the RIP which converts
the data into a file with the PostScript format, to
be then stored (file d`attente), before the impression
on digital press. The stage of completion is always
present. This process differs from a system CTP (Computer
To Plate) for which the characteristics of the plates
of impression and the system of exposure are particular.
In the case of presses digital, there are several
technological alternatives, adapted to types of particular
work. digital presses DI (DIGITAL Impressionde Heidelberg),
Karat (KBA-Scitex), DICOweb (MAN Roland) are intended
for the impression of a series starting from a single
original. With each new original, the plate offset is
changed.
The
digital presses can also carry out impressions of
which the number of originals is higher than the unit.
Based on known technologies of electrophotography with
solid inks or liquids, and on the ink jet, the printing
form does not exist physically. By the system of electrophotography,
the drum plays the part of printing form and undergoes
a preparation with each new impression.
These
systems of digital presses are opened at the markets
of the impression to the request and pullings to weak
quantity. Adaptability, the versatility, the personalization
and speed of execution are the principal advantages
of this system of impression. The digital presses
do not compete with the traditional presses offset,
but are an interesting complement as well as a system
CTP.
The specific case
of the hexachromy
The
traditional impression uses the principle of quadrichromy.
With the four colors the green and the orange can be
associated, to form the process called hexachromy. The
range of the colors obtained in hexachromic impression
is wider than in the case of the quadrichromy. Colors
difficult to obtain, such as the brightnesses and fluorescences,
can be obtained thanks to this technology. In this precise
case, we can do without the colors of accompaniment
(synthesized artificially according to customer requirements').
In other words, the combination of hexachromic inks
makes it possible to exempt use of personalized inks,
to obtain nuances of colors more diversified much. The
separation of a file is carried out by specific algorithms
which make it possible to obtain six colors CMJNOV.
The Pantone company provides plug-ins Photoshop, Illustrator
(respectively HexVector and HexImage) which makes it
possible to apply to the files colors of the libraries
Pantone Hexachromes. The files separated in six layers
can be recorded in other graphic applications (X-Press
Quark, for example).
From
a practical point of view, the tramage
must be selected advisedly at the time of the impression
on press. It is advised to use a random screen in the
case of reproduction in HifiColor. The printing must
obligatorily be carried out on a press 6 colors. The
qualitative management of this process is more complex
than for the quadrichromy, in particular in colorimetric
term (management of the profiles colors). Its economic
cost is much higher than the traditional CMJN. They
is d`ailleurs for this reason which the HifiColor process
did not know yet to be established in the medium of
the impression. It is currently confined with the printing
of luxury.
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