PART I - CHAPTER V
RECALLS ON THE PERIPHERALS

Monitors

          CRT Monitors (Cathodic Ray Tube)

                    Posting an image

          The traditional operation of a CRT screen is as follows: an electron beam resulting from three colors (red, green and blue) carry out a movement called sweeping on surface where a layer of phosphorus is on the internal wall of glass. The electrons which come into contact with these luminophores forms the coloured image when sweeping is carried out (line by line, in the left direction right-hand side, and upwards of the screen). All colors can be formed to the three luminophores red, green and blue. The reproduction of the colors is based on the additive synthesis.

                    The mask of the tube

          A metal plate called mask or grid (because it is perforated), is placed behind the pane of the monitor. The interaction between the pole + and the pole - creates a tension of 20,000 volt on the electrons which are projettés against the mask. Certain electrons miss the mask and pass through the holes to move on the phosphorus which is behind the pane. As the electrons are strongly accelerated, their energy shines phosphorus.



Diagram 11: Principle operation of a tube CRT

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.

          
profile of a CRT screen         Profile of a LCD screen

          

The spectrum of a CRT screen   The spectrum of a LCD screen

(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


Diagram 12: Principle of operation of a scanner flat.

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.

What is tramage?

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

What is flashage?

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.


Diagram 13: Example of Flow of work on the graphic chain

- PART I - CHAPTER V - RECALLS ON THE PERIPHERIQUES -
 
Publicity