Digital proofing and its main points
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At present, there are at least two cases where digital proofing technology must be used: First, the final result of prepress work is directly recorded on the printing plate, which is known as computer direct plate making (CTP); the second case is that the prepress preparation results are directly Output on a digital printing system. In both cases, the step of printing the film after the output film is skipped, and there is no reason to use the proofing process of the simulated offset printing.
First, digital proofing and digital printing
There is a principle difference between computer and proofing and digital printing, the only difference is the output speed and output purpose. The digital graphic output targeted for proofing uses a technology similar to digital printing. The focus is on checking how the results of the digital workflow will behave in actual printing, as a technical basis for mass printing. Due to this feature, the speed requirement for digital proofing equipment is not the main factor. The key is the difference between the result of digital proofing and the actual printed matter. Through years of efforts, with the participation of color management systems, digital proofing results have become closer to traditional offset printing and accepted by more and more printers.
Digital proofing is a key technology that supports the overall digital workflow of CTP workflow printing. It involves not only customers but also the print shop. Both the customer and the press operator are required to receive the most basic training in various digital proofing techniques. They should be made aware of the replication technology features used in different digital proofing processes, as well as the screening techniques determined by the replication characteristics and the color reproduction requirements. The extent and limitations of the number of screened lines can be achieved. Printer operators must abandon the analog halftone proofing process, telling them that even with the traditional printing process to achieve the same effect as digital proofing, it is not difficult.
Second, the characteristics of the CTP workflow
The CTP workflow omits the process of outputting to the film, and the phenomenon that the image-recording of the color image is enlarged by the imagesetter will no longer exist. Therefore, an important feature of the CTP workflow is that the dot gain is much smaller than that of film recording and reprinting, so the output curve (transfer function) when the page is recorded on the imagesetter cannot be used; the press operator must also understand the CTP print. The difference between the version and the PS version requires different ink and water balance adjustment parameters. When using a metal CTP plate in a workflow, the printing conditions also change due to the decrease in dot gain, which affects the ink density setting and the gradation of the image. Compared to traditional workflows, binders need to be instructed so that digitally discharged large versions meet all printing and binding requirements.
The key issue in implementing the CTP process is to solve the digital proofing. Because once the film is removed from the workflow, digital proofing is imperative. In theory, the digital proofing results should reflect the same halftone dot shape, number of screen lines, and gradation changes as the printing process. However, most digital proofing methods usually cannot simulate the halftone dot properties of actual printing, because some digital proofing devices do not use halftone screening technology to simulate continuous tone originals (such as continuous digital proofing equipment), even if The halftone technology-based digital proofing system may not necessarily produce the same dot structure as the CTP plate. The proofing and plate generation in the CTP process are two separate processes, so the dots on the proof and the dots on the printed matter. It is often different. Therefore, the user must be aware that digital proofing and recording to the plate are two RIP processes, which is the main reason for the different levels of change and resolution of the proof and plate. In fact, the printing company should understand that even if the analog proofing or offset proofing process is used, whether the proofing result reflects the actual production effect of the printing process is also uncertain.
Third, digital proofing points
Printing companies and their customers have not yet reached the stage of digital proofing in the transition from analog to digital workflows, because such changes are not easy or even difficult and require time. The press operator needs time to learn how to match the print results to the proof results, rather than simply using the dot analog approach as before. Note that it is important to match the printed color to the proofing result, rather than using a magnifying glass to check the proximity of the printed dot to the proofing dot. This conceptual change is a prerequisite for accepting the digital proofing process. In order to check whether the color of the print matches the color of the proof, you first need to observe under the standard light source. For prints with low quality requirements, visual inspection may be used; however, if the quality of the printed matter is high, instrument inspection is required. A typical densitometer can only measure optical density. When it is required to be high, consider using a spectrodensitometer or similar device to measure the actual color value of the proof and print.
Fourth, digital proofing technology
There are two types of digital color proofing methods, one for halftone proofing and the other for continuous proofing. Each of these two proofs has many technical implementations, such as Laser Dye Removal technology and color spray. Ink replication technology, dye sublimation technology, laser color printing technology and thermal transfer replication technology. Each digital proofing technique has its advantages and disadvantages, and the price of equipment and consumables can vary greatly. But with proper management and calibration, each digital proofing technique produces very accurate colors. When discussing digital proofing techniques, you should not only consider the advantages and disadvantages of various color proofing techniques, but also judge each type of proofing technology from an economic perspective. For example, both inkjet replication technology and dye sublimation technology can produce very accurate color proofing, but the cost of the two is different. Therefore, finding the right digital proofing method for each customer is not an easy task, because the customer's requirements and preferences are varied.
Fifth, page proofing and spelling large version proofing
Color proofing is of course important, and page proofing and spelling proofing should have an equally important position. The purpose of page proofing is to check fonts, line drafts, and layouts. This proofing should begin early in the production cycle. If the user device has the ability to combine page proofing and color proofing, the page proofing can be combined with color proofing, but the two should be independent of each other and clearly distinguishable from each other.
The transfer of pre-press preparation results to large-scale output also requires some type of layout proofing system, as well as a system for performing calibration on color proofing equipment. The operator must confirm that the pages of each publication have been positioned according to the requirements of the book, and that the page numbers have been properly imprinted according to the folding and saddle stitching. The equipment used to proof the layout results is usually a large-format inkjet printer, such as the Atlas produced by Agfa, a continuous-adjusting drum proofing device. If you purchase the Apogee workflow software developed by Agfa, its RIP and output management module Print Drive support special preview functions, which can carefully check the jobs to be output before output.
The large-scale proofing is to check the correctness of the operation as a whole. If the combination of the layout is unreasonable, the printing and binding workshop will be at a loss. Therefore, the prepress and platemaking departments should have regular communication with the printing and binding workshops, and mutual understanding is the fundamental way to prevent rework.