Digital prepress technology usually uses laser recording equipment to output a full-page layout of color separation film. After the direct plate-making technology is put into practical application, the step of using the film as the data recording medium is no longer necessary, but the result of the digital prepress operation is directly recorded on the printing plate. In addition, small batch printing can also use digital printing machines to record digital prepress job results, digital proofing is similar to this.
Due to the operating characteristics of digital printing, the measurement and control strips used for analog printing cannot be output to a film and then printed on paper as a means of plate making, printing quality inspection and control. Any industrial production needs to be carried out in accordance with the prescribed quality standards. The quality standards for digital printed products can be implemented with reference to the standards established by traditional printing processes. But digital printing only has product quality inspection standards, and it also needs specific means to achieve product quality inspection and control. To this end, the Swiss Printing Research Promotion Association UGRA and the German Printing Research Association FOGRA launched a collaboration to develop laser imaging digital printing measurement and control strips, named UGRA / FOGRA Digital Control Strip, hereinafter referred to as digital printing measurement and control strips or simply measurement and control strips. UGRA / FOGRA digital printing measurement and control strip is defined in PostScript language and adopts modular structure, so it has great flexibility.
1. Composition of digital printing measurement and control strip
The digital printing measurement and control strip consists of three modules, of which module 1 and module 2 are used to monitor the printing and copying process, module 3 is used to monitor the exposure adjustment, and the size of each measurement control color patch is about 6mm × 6mm (sometimes may be smaller or larger This value). Since the control block included in the module 3 is mainly used to monitor the exposure recording process of digital printing, it is designed to correspond to the PostScript measurement and control strip used by UGRA / FOGRA to check and control the output of the film.
The advantage of using a modular structure is that users can freely choose any module in typesetting software or page formatting software, and can arrange the control color blocks in a row or column along the printing direction to form the required measurement and control strip. In addition to the measurement control block, the text in the measurement and control bar also contains details related to the exposure resolution in DPI. These texts can be used to measure the fineness of digital printing equipment screening and text scanning conversion. The main content of the text is the user name and the license number of the digital printing and control strip. The user name must be specified at the time of order, usually the company name.
1. Module 1
Contains the following 8 solid color patches: 1 for cyan, magenta, yellow and black solid patches, 3 for "cyan + magenta", "cyan + yellow", "magenta + yellow", "cyan One "+ magenta + yellow" solid color patch. These control patches are used to control the acceptable performance of digital printing inks and the superimposed printing effect of the three subtractive primary colors.
2. Module 2
(1) Color balance control color block
The color block is a specified gray tone value and is related to the output of the film. It actually contains two color blocks, of which the right color block is 80% black, used to control the mesh tone screening effect; the left color block is composed of 75% cyan, 62% magenta, and 60% yellow, the purpose is to 80% black color block comparison. If the gray balance is not well controlled during printing, the color block will exhibit color components.
(2) Field area
The solid area contains 4 solid color blocks, arranged in the order of black, cyan, magenta and yellow, with a color block placed every 4.8mm. The first solid color block (black block) is next to the color balance control color block. It is stamped with yellow on its four corners to check the printing color sequence, that is, yellow is printed before black or black is printed before yellow.
(3) D control block
D is Direction, so D control refers to the meaning of direction control, that is, to check the sensitivity of using specific copy technology, copy equipment and printing material combination to screen in different directions.
The D control blocks are divided into four groups, one for each of cyan, magenta, yellow and black. Each group contains 3 color blocks with a total size of 6mm × 4mm. When composing digital printing measurement and control strips, they are usually arranged in the order of black, cyan, magenta, and yellow, and the position is behind the solid color block. The three color blocks are screened with linear dots, and the screen angle is 0 ', 45' and 90 'from left to right. The number of screen lines used for each color block is 48 lines per cm, and the tone value is 60. % (60% black). The main reason why the 60% tone value is used instead of the intermediate tone value (50%) is that the color patch after output is slightly darker than the intermediate tone, which can more clearly identify the direction sensitivity of the screening process.
Theoretically, when the same number of screen lines and dot shapes are used, the three color patches should have the same density value. If the actual measured three density values ​​are significantly different, it indicates that the copy technology, copy equipment and printing material combination used by the user is too sensitive in a certain screen direction. [next]
(4) Mesh control block 40% and 80%
The control block also has four groups of cyan, magenta, yellow and black, each group of control blocks is composed of two color blocks of 40% and 80%, and is screened by 150LPI. This number is consistent with the recording accuracy used in most commercial prints. The two mesh tone control color patches are asymmetrically distributed with the midtone dot percentage, which represents a percentage of dots that are slightly lighter (closer to midtone) and closer to the ground than the midtone. Different digital printing processes adopt different screen copying technologies, and different output effects will be obtained. Therefore, these two control blocks can be used to evaluate the performance and behavior characteristics of specific digital printing screening technology, and to measure whether the screening technology can obtain the required recording effect. When forming the measurement and control strip combination, they are arranged in the order of black, cyan, magenta and yellow, and the position is behind the D control block.
3. Module 3
The module includes 15 gray blocks of different degrees, each color block has the same size (6mm × 10mm), and is printed with black ink. 15 color blocks make up 5 columns, each column contains 3 color blocks, but different dot structure is adopted. The ink coverage of the color blocks is 25%, 50%, and 75%, respectively, where the leftmost column is 25%, the second, third, and fourth columns are 50%, and the fifth column is 75%. Figure 4 is the 15 gray blocks that make up the control block. In order to let the reader see the difference, the color blocks are enlarged.
The reason for printing these color blocks with black ink is very simple, that is to save the space occupied by the measurement and control strip. The first line of the control block is always copied with the highest recording resolution achievable by the output device, the second line of color block recording resolution is one-half of the first line, and the third line is one-third of the first line One. Thus, the large dot structure can be seen from the second and third row of color patches. The second, third, and fourth columns of the control block are all 50% black. The second column is named 50cb (CheckerBoard). They are all lattice patterns; the third column contains horizontal lines; the fourth column contains vertical lines.
In theory, after module 3 is printed, the tone value of each column of color patches should be the same, the only difference is the recording resolution; in the row direction, the 3 color patches in the middle of each row should be copied to the paper. The same tone value. Therefore, if there is a difference in the tone of the three color blocks in each row, this difference must be related to the copy method. The cause of the difference can be found from the angle of the network line. When outputting, the recording equipment should be adjusted to minimize the difference in tone in the line direction. When the tone values ​​of the color blocks in the column direction are different, it reflects the influence of the number of screen lines on the replication effect.
4. Module connection
The module 3 of the digital printing and control strip is used alone, and the assembly of module 1 and module 2 is in principle free, but in order to arrange more regularly, the following order can be adopted: first arrange the balance control block; then black, green, Magenta and yellow solid blocks, plus black D control blocks and black mesh tone screening control blocks; followed by black, cyan, magenta, and yellow solid block, plus cyan D control blocks and cyan mesh tone adjustments Net control block; followed by black, cyan, magenta and yellow solid blocks, followed by magenta D control block and magenta mesh tone screening control block; finally black, cyan, magenta and yellow solid blocks , And then add the yellow D control block and the mesh tone control block. [next]
2. Screening and RIP output of digital printing measurement and control strip
1. Output digital printing measurement and control strips with different recording equipment and RIP
(1) Color reproduction equipment and offline proofing system
The question resolution function of the driver interface of the color copying device or the offline proofing system can specify the output resolution of the digital printing measurement and control strip. The color patches contained in 2 will get the correct output. This means that for color reproduction devices or devices that use RIP interpretation and output, their recording resolution must be higher than 600 DPI, otherwise the correct color block output cannot be obtained.
(2) Digital printing equipment
Digital printing equipment has become one of the important devices for outputting page content to paper. Such equipment should be able to accept PostScript files, such as direct imaging waterless offset presses produced by Heidelberg, and digital printing systems produced by XEIKON and INDIGO. In the digital printing system, UGRA / FOGRA digital printing measurement and control strip can be output to the paper without difficulty, used to control the quality of digital printing.
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