In practice, a proof is usually understood to be a representation on an at least paper-like carrier material. In the broader sense of the word, however, intangible simulation processes can also be meant (e.g. soft proof , ie display on the computer screen).
With a proof, you want to simulate what the subsequent print result will look like as early as possible within the production chain. The background to this is that in classic printing processes such as offset printing , gravure printing or flexographic printing, errors become more costly the later they are discovered. If a master copy manufacturer recognizes a typesetting error on the screen, it takes a few seconds and a few keystrokes to correct it. If, on the other hand, the error is only discovered when the completely engraved gravure cylinder is hanging in the machine, the order must be removed from the machine, corrected and later put back into the machine. This creates considerable costs and problems in production planning.
While a proof is certainly not required to detect simple typesetting errors - after all, they can already be clearly assessed on the screen or after being output on simple office printers - there is one area in which the special qualities of proofing systems are particularly important: the color.
In principle, when creating a print template - which is usually done on the computer today - it is not easily possible to exactly predict the future appearance of the print result. Due to the fact that it is a self-luminous object using additive color mixing , the colors of the screen already have a different appearance than those of the later print. This fundamental difference can not be completely compensated for by calibration techniques or color management .
The usual color printing systems for the office area are also not suitable for generating color-accurate print simulations. Although in principle they often work with the same color mixing technique as the classic printing process ( subtractive color mixing with the CMYK color model with the primary colors cyan , magenta , yellow and black ) B. the color locations of the primary colors cyan, magenta, yellow and black, the halftone process for halftone simulation , the change in tone value between the database and the print result and much more. Of course, it is possible to achieve output with such printers that “roughly” looks like the later print.
However, this is completely insufficient for professional applications. Here it is important to be able to make a precise statement about the later colors before printing. Changes to the template can then be discussed on the basis of a color-binding proof, and if the proof is satisfactory, it can be used as a contract proof , i.e. represent the legally binding template for a print order. On this basis, z. For example, complaints can also be made if the result of the print run deviates too much from the proof ( Delta E color difference ).
It is precisely here that it becomes clear that a proof must very precisely anticipate the flow of color information from the file or film to the print, and this in certain circumstances for several machines and several printing processes. The independent discipline of color management developed from this requirement .
Proof processes can be differentiated according to whether they work analog or digital.
Analog proof processes
Analog (or "conventional") proofing processes require the presence of print films and usually work with photographic means. The basic principle is usually the application of a UV radiation-sensitive color layer to a carrier material (laminating a film or dusting with a color toner). The printing film is placed on this color layer and exposed. Through a photochemical process, the non-printing or non-ink-bearing areas are changed so that they can be washed out during the later development process. The color-bearing areas, on the other hand, remain on the proof and thus form the respective color separation. The whole thing has to be repeated for each color separation.
These analog proofing methods, such as B. Matchprint from Kodak or Cromalin from DuPont , require a certain amount of manual work. It has to be laminated, powdered, mounted, exposed and developed, so that the time that a skilled user needs to produce a four-color A2 proof is around an hour for both processes. In addition, the analog proofing processes with their standard color foils and powders are usually only applicable to a single printing process on common substrates, such as B. the Euroscale offset printing, designed. At best, they can be adapted to in-house standards by varying the exposure times within tight tolerances. Textile and screen printing and other special printing processes usually cannot be reproduced. One advantage of the analog methods, however, is that they basically reproduce the screen of the original printing process and thus make screen-related problems such as moirés etc. visible. In addition, they are characterized by high consistency and reproducibility of the results.
Digital proofing process
Since the mid-1990s, digital processes have gained more and more importance in prepress . Today almost all print templates are produced digitally. Consequently, digital proofing systems are the method of choice today. After all, it would be senseless and expensive to produce a print film just for the proof, if one - e.g. B. when working with a CtP system - is not required for printing. In addition, digital proofing systems work faster, less complicated and cheaper than their analog counterparts. The hardware consists of an electronic printer that works in one of the non-impact processes , e.g. an inkjet or thermal sublimation device. The associated software (usually a combination of raster image processor , color management module and workflow solution) is responsible for processing the incoming data and converting it into the printer-specific format. In addition, the color management is done here.
In digital proofing systems, color matching is usually controlled using color profiles (a distinction is made here between ICC- based and proprietary color profiles). ICC-based systems have the upper hand today, because they are much more flexible than analog systems: By simply exchanging the profiles, a wide range of printing processes, in-house and industry standards can be reproduced. It is also possible to achieve consistent results on inkjet printers from various manufacturers and designs.
However, digital printing processes are still said to have slight deficits in reproduction quality. Depth markings, gradients, difficult special colors, gray balances and similarly demanding parts of the image are usually reproduced somewhat better by the analog proofing process, ie more similar to the edition print. However, due to the price, speed and versatility of digital systems, they no longer play a role in the market today.
With the introduction of the new ISO 12647-7 since 2013, which also takes into account and requires optical brighteners, so-called OBA (Optical Brightning Agents) in print and proof, serious changes in the proof are still in progress: Current proofs according to PSO, SWOP and GRACol must be printed and measured in the so-called M1 standard. To do this, the proof service providers need the latest software, new measuring technology (measuring mode M1) and new proof papers that contain optical brighteners. The changeover to the new standards took place in 2018.
Classification of proofs according to their binding nature
Digitally produced prints can be roughly classified according to their intention or their commitment. The type of creation of these prints is not taken into account.
Should show the correctness of the content and the placement of the elements used. Little or no value is placed on color accuracy. Often a reduced output is made. Most often, electrophotographic printing systems are used .
Shows the position (stand) of the pages on the press sheet . It should be generated from the data prepared by the Raster Image Processor (RIP) for the exposure of the printing form . Color accuracy is not mandatory. Common expression today: Formproof.
Color-binding and legally binding proof (contract proof)
A color-binding proof should almost predict the print result in terms of color. The printing media standard and ISO 12647-7 regulate when such a proof is color- accurate. In addition to a halftone proof, which uses halftone processes such as those provided by the printer driver manufacturer, halftone proofs can also be generated that simulate the screen used later in printing (offset rosette). Today inkjet printers are used almost exclusively to create proofs , which are controlled via RIPs with a built-in color management system. In order for a contract proof is as binding and legally binding as soon as the basis for continuous printing, the printing of a is UGRA / FOGRA - media wedge required. The standardized values of the media wedge are measured on the proof. If the deviations are within the specified limits of the ISO standard, a test report is printed or stuck on that documents the accuracy of the proof within the tolerances. With this test report, a contract proof becomes color and legally binding.
- eci.org : European Color Initiative: The most important European group of experts that deals with color-accurate processing and designs proof profile standards.
- Fogra.de : Fogra Forschungsgesellschaft Druck eV One of the most important standardization and research institutes in the field of printing worldwide.
- Proof profiles : An overview of all current proof profiles (as of 2018) and some older but common proof profiles