Method of determining the area coverage of printing plates

#The invention pertains to the field of electronic reproduction technology and relates to a method of determining the area coverage of printing plates. The area coverage is the proportion of the area of a printing plate that is covered with printing ink during printing, which ink is, then, transferred to the printing material (e.g., paper). For an image, the area coverage is given by the proportion of the area that is covered by the halftone dots, in relation to the total area of the image. For a text, the area coverage is given by the proportion of the area that is covered by the lines and curves of the letters, in relation to the total area of the text block. Global and local area coverages, determined strip by strip, are needed for presetting the inking zones in the inking unit of an offset printing press. For the inking zones divided up over the width of the printing plate, the quantity of printing ink fed in must be set such that it corresponds to the quantity of printing ink taken off the corresponding strip on the printing plate, which, in turn, depends on the area coverage of the printing plate in the relevant strip, that is to say, on the size of the halftone dots, the thickness of the lines of the text and so on.

#In reproduction technology, printing originals for printed pages are produced that contain all the elements to be printed, such as texts, graphics, and images. For colored printing, a separate printing original is produced for each printing ink, which contains all the elements that are printed in the respective color. For four-color printing, these are the printing inks cyan, magenta, yellow, and black (CMYK). The printing originals, separated in accordance with printing inks, are also referred to as color separations. The printing originals are generally scanned and exposed by an exposer onto films, which are, then, processed further in order to produce printing plates for printing large editions. Alternatively, the printing originals may also be exposed directly onto printing plates in special recorders, or they are output directly to a digital printing press as digital data.

#According to the prior art, the printing originals are reproduced electronically. Here, images are scanned in a color scanner and stored in the form of digital data. Texts are produced by text processing programs and graphics by symbol programs. Using a layout program, the image, text, and graphics elements are assembled to form a printed page. Following separation into the printing colors, the printing originals are, then, present in digital form. Nowadays, the page description languages Postscript and Portable Document Format (PDF) are largely used as data formats for describing the printing originals. Before the recording of the printing originals, such a description is converted into a description in which the image points are each assigned a gray value if the printed page is printed in black and white or, respectively, the image points are each assigned color values if the printed page is a colored print. If the printed page is printed in the four printed colors CMYK, then the amounts of ink to be printed in each image point are described by four color values. For example, four bytes are produced for each image point, corresponding to the four color values. The color values, then, have one of 256 possible steps between 0 and 255. The printing-original data for a printing color can, therefore, also be understood as a “black and white page,” whose “gray values” specify the amount of the associated printing color in each image point.

#FIG. 1 is a block circuit diagram of a prior art working sequence in the production of printing-original data;

#FIG. 2 is a graph illustrating dot gain curves for various screen systems;

#FIG. 3 is a graph illustrating print transfer curves for various screen systems;

#Referring now to the figures of the drawings in detail and first, particularly to FIG. 1 thereof, there is shown a working sequence predominantly used in the prior art during the production and recording of printing-original data. The printed-page data 1, which is described in the page description language PostScript or PDF, for example, is fed to a feed or sheet mounting system 2, which can be a computer specifically optimized for such a task or a program on a standard computer. The printed-page data 1 describes the content of a single printed page. It is produced in a prior process by an application program for assembling a printed page (layout program). The sheet mounting system 2 assembles a plurality of printed pages in accordance with an imposition scheme to form a printed sheet and produces the printed-sheet data 3, which describes the content of a printed sheet, for example, likewise in PDF data format. In addition to the printed pages, the printed-sheet data 3 also contains register crosses, cutting and folding marks, and a print control strip at the edge of the printed sheet. The imposition scheme depends on how the printed sheet is folded and cut after being printed, in order, for example, to produce a multi-page brochure or a folded sheet.

#In an interpretation system 4, the printed-sheet data 3 present in a page description language is analyzed and interpreted and, in the process, converted into color-value printing data 5, that is to say, into a data format that, for each image point, specifies the color values of the color separations in accordance with the number of printing inks used. For the four standard printing inks CMYK, the color-value printing data 5 contain four bytes for each image point, for example, which each specify the quantity of the color to be printed. The color-value printing data 5 is, therefore, also referred to as “byte map.” The resolution of the color-value printing data 5 generally corresponds to the resolution with which the printing originals are to be recorded on film material or directly on a printing plate, for example, the resolution is 100 image points/mm. However, the data from the color-value printing data 5 can also be produced in another, for example, lower, resolution and interpolated to the resolution of the film or plate recorder only before recording. Furthermore, the data from the color-value printing data 5 can be compressed by a data compression system to save storage space, for example, using run-length coding or using the Joint Photographic Experts Group (JPEG) method.

#The data from the color-value printing data 5 are fed, in a further step, to a raster generator 6, which converts the color values into areas filled with raster points and transfers them as rastered printing data 7 to the exposer 8, for example, a plate exposer, which records the rastered printing data 7 directly onto a printing plate. The production of the raster points is carried out, for example, in a conventional way by comparing the color separation values with a threshold value matrix that contains the configuration and size of the raster points. The raster point size and/or the number of raster points per unit area, depending on the raster system used, is varied in this case in accordance with the color separation value in the color-value printing data 5. The rastered printing data 7 has a high resolution, for example, 100 points/mm (=2540 dpi) and are described with one bit for each image point and color separation. The bit indicates whether the corresponding point on the printing plate is exposed or not. The rastered printing data 7 is, therefore, also referred to as a “bitmap.”