PART II - CHAPTER III
THE EPREUVAGE

What is epreuvage? Its goal?

          In the broad sense, the term epreuvage is a paramount phase in the graphic chain operation. This phase is very important for the company, since it saves time and money. Its goal is to simulate and validate before the final stage of the impression on press. This stage also considers the page-setting and the original colors. It must be put in conformity with the characteristics of the offset press (to approach nearest possible to its colorimetric space), and thus avoids the important divergences with the original parameters.
The epreuvage or proofing can be carried out in different manners. The analogical epreuvage, digital epreuvage or screen epreuvage. In this chapter we do not approach the system of press epreuvage, because it is less and less adapted to the technical and economic requirements in the market.


The analogical epreuvage

          The two most widespread systems of analogical epreuvage are Cromalin (Dupont of Nemours) and Matchprint (3M/Imation).In this systems of epreuvage, the dyes are fixed on the film, while as well as possible simulating the final test. In this case simulation is made starting from dyes in comparison with inks of impression. This principle is very well and gives reliable results, but unfortunately very expensive compared to the systems of digital and the screen epreuvage, but also ensures simulation close to the final impression, and guarantees the customer an insurance (contractual test) with respect to the printer .


The Cromalin Test


          Cromalin is a trade mark and comparable with a means of color epreuvage, i.e. method of quality control of an image before its impression on press. The output obtained is used as reference for the printer. From a technical point of view, Cromalin is obtained by a photomechanical process. In brief, we can say that a white support is rolled, exposed, then plasticized product is removed by leaving an adhesive coat to the surface of the white support. Then, each film CMYK is applied to this surface, and a final plasticization comes to recover the trial. With final simulation, We obtain impression in quadrichromy of the offset press. Matchprint d' Imation and PressMatch d' Agfa are also processes of analogical epreuvage, whose principle of operation varies. First of all, there is a phase of transfer of the primary sheet of color by the phenomenon of lamination. Then, the phase of exposure of these layers is carried out by using sources of UV light, and of selected films. The separation of the layer of color followed by the display, and thus a reproduction produces a selected film image.

          From an economic point of view, this process is more profitable than the epreuvage which was done directly on press. Apart from two large companies in the market (Dupont for Cromalin and 3M for Matchprint), companies like Fuji, Agfa or Hoechst also proposes similar solutions, with some technological differences.
          Cromalin has to meet certain conditions for the validity on a file called Bar of control. Operating conditions such as the ambient temperature in which the machine is, qualitative characteristics of the lamp being used for the insolation of films, the type of pigment employed, as well as the sequence of repoduction of these pigments (cyan, magenta, yellow and black) must be controlled. These operating conditions has to be controlled by the specialized companies such as Brunner, Fogra, etc. This allows a precise quality control of the system of epreuvage. The analogical test has technical imperfections, since the repetatibility of the colors is often unequal and this is because of the fact that the Cromalin process is a dry technology.

          The system of epreuvage must be based on a standard called Eurostandard, which is managing by the FEDEC. It is a European federation gathering six national associations belonging to the trades of the edition, publicity, communication and Art-Graphs. In France, it is the association ATEP (Association of the Technicians of the Edition and Publicity) which is associated within this federation, which regularly organizes meetings of exchanges between the various protagonists of the trades of Art-Graphs (meetings centered more particularly on the level marketing and techniques).


What is Eurostandard?

          Eurostandard is a standard adopted on the European market, contrary to American standard called SWOP. This standard makes it possible to obtain a qualitative management CMYK for the majority of work in printing offset works. On a control test (for example Cromalin), a range known as of Brunner is obligatorily present. This complete range makes it possible to control several parameters such as the fattening of the point, densites of inking, the balance of the gray, etc.

It is in responsibility of develop standards, mainly with regard to digital technologies. Thus Eurostandard is managed by this federation, and the company which wishes to put a technology relative to this standard, must have the agreement from the FEDEC and it should obtain Eurostandard Certification.

The digital epreuvage

          General

          This digital epreuvage has been under operation only a few years back and has both advantages and disadvantages compared to the analogical epreuvage. Certain parameters, are essential for a good simulation. for example, the epreuvage via a thermal system of sublimation erases the watered effect ( impression with continuous tone). The woven epreuvage systems do not have the same types of screens as those of a press, and this incompatibility results in nonoptimal quality ( the solution already exist for this, for example, the Aurelon company released the RIP, which uses the files, already woven by the RIP of flascheuse). Another problem is with same ink, which should have the tendence to change with time. If the customer gave its BAT on the digital test, and 2 week later it reclaims the quality of the impression on the printer, the printer cannot present any more BAT, because the color already changed.
The digital epreuvage is very faster compared to anological epreuvage. The results can be optimized if it is associated with a color management system. The companies Dupont Nemours, Epson, Agfa, Minolta QMS, Imation, etc, propose this type of epreuvage on various types of machines (laser, thermal sublimation, jet of ink, woven). These systems of epreuvage are proposed with RIP, and this generates an essential screen with the simulation of an output on press.

The economical and technical reasons for adapting digital epreuvage is described below. The economic reason is that, the investment on the digital epreuvage machine is much more profitable than analogical. The cost of the digital display makes difference towards its direct competitors (Matchprint or Cromalin). Another is purely technical, Systems CTP (Computer To Plate), whose principle is to engrave directly the plates from the computer files, and thus jumps to the stage of creation of CMYK films (Flashage), so their market is developing at high speed in the medium of the Art-Graphs. This is why the digital epreuvage is inevitably proposed by the companies having systems CTP. A contractual test of digital epreuvage (woven or in continuous tone) avoids them to produce films, which will be used for engraving the offset plates. The saving of time and the money seems to be the principal advantages of the digital epreuvage. As for the quality of the output, there are two types of digital epreuvage. The first one is digital epreuvage with continuous tone and the second woven digital epreuvage.


          The digital epreuvage with continuous tone

          The people receiving benefits use peripherals of impression without considering the screen. Range of control, measured system of epreuvage using a densitometer, in order to highlight the possible drifts between a contractual test and the final print. The system in which screen not being simulated is adapted to work which doesnot require woven quality. The companies which uses the continuous tone are Epson, Hewlett Packard, Gun, Imation, CreoScitex, Dupont, Drytone, Agfa and Xerox. Technologies of the systems of impression are the continuous ink jet (Iris), thermal sublimation (Approval Kodak), the ink jet on demand (or DOD - Epson Proofer 5000), and laser copiers. Currently, the system of digital epreuvage with continuous tone is more appreciated in the prepress medium, because it gives the best results.

 

Woven digital epreuvage

It is the most qualitative system of digital epreuvage. The screen must be simulated by the same RIP being used to test and to engrave the plates with the pledge of precision and quality. It is the system of epreuvage approved by kodak and is most recognized in market. This system is however much more expensive than the first. Other types present in the market are Polaroid, CreoScitex, Aurelon or Imation.

          Properties of the digital epreuvage

        The systems of digital epreuvage must manage and shape ICC. it is applied to all the systems described previously. The range of control is essential on each digital epreuvage. The densitometer is used for the measurement of this range to know if the tests are in the standards compared to the councils of the manufacturer. The essential parameter of fattening of the point checked by comparing with the press offset.

Proof Impression and FOGRA Standard

For a standardization of the Proof impression and pulling impression, the Institute of Research FOGRA, in collaboration with the Bundesverband Druck Federal of Impression (BVD) grouping set up of the directives for impression. These directives primarily includes the following :

 -   the division of different kinds of papers into four categories
 - determination of increases in minimum and maximum     impression for each different types of papers
-  an audit roll for the pulling impression and the proof impression
-  a method for determination of optimal intensity for each kind of     papers in the pulling impression and the epreuvage.

For example, for the art paper, FOGRA gives the following value 40% for the covering of surface on film, the FOGRA indicates, that on art paper the covering of surface must be around 56%, and on a not-art paper this value must be around 62%.

The use of a bar of control is recommended for all the systems of
epreuvage.



Criteria for the selection of digital system of epreuvage.

          The system must be able to to simulate different types of impression, between art papers and newsprints. The system should have a system of calibration called linearization (but different from the characterization) to be able to control the inking and the balance of gray. This system works with the bar of control, which is measured using a densitometer. The measured values returned directly to the impression device, or RIP software, which creates a new curve of inking. This system is simple, rapid and makes it possible to have a constant quality of epreuvage.

          In practice, the majority of printing works do not function according to the principles of the FOGRA and the BVD. The group of FOGRA has a recourse in order to control the intensities of tone, to increase in tone and the balance of the gray in the pulling impression. However, instead of identifying the optimal tone according to the directives of the FOGRA and the BVD, printing works with experimental values and establishes a precise adjustment using an analogical or digital epruvage.


          The screen epreuvage

          Currently, this system of epreuvage is very economic and easily integrable to the color management system. In the screen epreuvage, Simulation does not take place physically as in the two preceding cases, but on the screen via the CMS.The application of ICC profiles implements the screen epreuvage logically. This can be carried out in two ways. By the software of final improvement of image like Photoshop, or by a software which is specifically dedicated for this purpose such as PressProofEdit of Qubyx or ColorBlind Edict etc.



Software QUBYX PressProofEdit allows a previsualisation / Softproof of the Cromalin output, or by taking of account profile of screen and profile of the scanner. The file can be in LAB, RGB or CMYK.




In Photoshop we can visualize the Softproof by applying the profile of the printer even to a RGB file, and it can simulate the white paper.





American standard SWOP


          The development of this standard was started at the end of the Sixties and the beginning of seventies, when the publications was using the traditional techniques for the offset. The SWOP (Specifications for Web Offset Publications) develops technology with an aim of improving the production. Its specification became a trade mark in 1994. Its current goal is to raise the quality level of the systems of impression by applying a certain number of constraints and tolerances. The recent developments in data-processing made it possible to replace films with digital files. SWOP Standard has to modify according to the demand of the industrial market, and it should set up a series of new recommendations. The prepress, advertising agencies, edition and impression uses this standard, and it has direct competition with the Eurostandard of Europe. Additional information can be consulted on the official site: http://www.swop.org



          Edition of ICC profiles

          There is a dedicated softwares for the edition of ICC profiles. For example, GretagMac-Beth proposes a software called Profile Editor, just like Colorsynergy of CandelaColor or ColorBlind Edict, Profile Editor from ColorVision. The purpose of all softwares is to be able to publish a profile in a specific manner for each one of them.
Thus, the edition of profile makes it possible to post and publish the characteristics of a profile, with an aim of being able to control the quality and, if necessary, to carry out corrections on the profile itself. Once the corrections ane carried out, we can visualize them on screen and also can compare the results obtained from the corrections on the profile and the initial image. In this precise case, we have to employ a logic of software proofing, method which is described in the preceding chapter. This is a complementary stage for the creation of ICC profiles for input (scanners, monitors) and output (system of impression) is very essential to get the optimized quality of a color management.


          publishing a profile

          Adjustments are necessary, If the profile requires corrections or profile is erroneous, in order to modify and to improve the profile.
The edition of ICC profiles is necessary in several cases. As we specified previously, if the original has defects such as under-exposure, then in this precise case, it is essential for the reestablishment of the image. Thus, by choosing the Source Profile and Destination Profile, we can display the chromatic representation of these profiles (gamuts in colorimetric spaces Co L*a*b *, Yxy, or Co L*u*v *), and compares them visually and graphically. The color balance, black point, white point, contrast, luminosity and saturation are the parameters on which the edition of profiles can allow modifications.



Example of edition of a profile (software Editor Profile)




The ColorQuatet software makes it possible to record colorimetric corrections directly in the profile of the scanner, or in the profile of output.


The ColorSynergy software also offers many possibilitiessuch as modification of the profile, and also the white point, black point, gray point, contrast, luminosity, selective correction of the colors.

QUBYX Software PressProof Edit makes it possible to publish profiles of input and output with curves or using variants with adjustments of +-Magenta, +-cyan, +-yellow, +-density.




Figure 29: Example of range of a profile (software Colorsync Profiles Inspector)




The software ColorBlind Edit offers very vast posibilities of adjustments to publish a profile.



The PostScript language

          PostScript, created by the Adobe company, is a programming language optimized for the impression of graphs and texts. Since 1997, the PostScript language using is version 3 (1st version was created in 1982 and came to market in 1985). It is compatible with many operating systems (Windows 3.1, 95, NT, MacOs, DOS, UNIX, etc.), and it is supported by a great number of applications. It is considered as standard of impression. The format known as EPS (Encapsulated PostScript), is the format based on the language PostScript (file image). The R.I.P. must accept this type of format, and it should transform these data into bitmap data. Thus, this language is independent of the system of impression, since the format used is vectorial, and each printer uses a specific PostScript file provided by the manufacturer of the peripheral.

 


CRD or Color Rendering Dictionary

          This dictionary belongs to the PostScript language (level 2 and 3), and its function is to be able to define the characteristics of conversion between two colorimetric spaces. While selecting such as CRD in the options of the printer driver, the PostScript language has the possibility of conversion variations by using these Color Rendering Dictionaries. These dictionaries are used in particular by the RIP.


Raster Processor Image or R.I.P.

The R.I.P. is the principal control unit before the system of impression. Its function is to translate the PostScript data, coming from the driver controlling the impression, in data bitmap usable by the system of impression. The color management can be carried out on the level of the R.I.P. itself, while selecting ICC profiles or CRDs to make the conversion of a space to another. The principle of RIP is to use two ICC profiles, that of the proofer (printing, for example Epson or HP) and profile of the offset printing, who must be simulated.
The majority of the RIP creates the screen with their own system, with the result that a printer is able to print another color with this new screen, that with the original driver. After having rippe the image, the color management of the RIP will apply to the printer profile to simulate the final impression.
Manufacturing RIP in general founissent their systems with ICC profiles for a certain number of printer and their own paper proof.
To be able to use these RIP like a system of epreuvage, it is necessary to use the manufacturing paper, or to create its own profile.
The RIP uses the programming language PostScript. The current RIP is associated with server, the principle goal is to improve the productivity by controlling each parameter of the process (checking of page-setting of the document, integrated images, queue of impression, etc.). The individual working stations do not carry out any more all these tasks, which leaves them time and workspace for the production server(final improvement of images, page-setting, etc.). This is why the majority of the people receiving benefits chooses a RIP Server, because it is able to control post-production of daily work. However, it should not be believed that today all is automated in a WorkFlow system. There are still certain tasks which must be carried out manually, what causes direct to slow down and minimize the output of flow. These RIP servers tend to use as platform of Windows NT.
- PART II - CHAPTER III - THE EPREUVAGE -
 
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