Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
1996-08-21
2001-04-03
Metjahic, Safet (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S040000
Reexamination Certificate
active
06211948
ABSTRACT:
The invention relates to a process for photomechanical production of structured surfaces on a pattern carrier as a copy of an electronically stored model, especially for irradiation of offset press plates, in that the model is broken apart electronically into fragmentary images, and the fragmentary images are represented in succession on a liquid crystal screen and copied on the pattern carrier in such a way that the copies combine again on the pattern carrier to form a total copy of the model.
The invention also relates to an apparatus for photomechanical production of structured surfaces, especially for irradiating offset press plates, having a light source, a digitally stored model, and a pattern carrier, in particular of an offset press plate, in which the model can be displayed on a liquid crystal screen and can be copied onto the pattern carrier by means of an optical component provided in the beam path, wherein the copy and the pattern carrier are embodied as movable relative to one another in at least one axis.
Still today, up to 90% of offset press plates are irradiated with film models using contact copier technology, or individually with production systems as well. This means that before an offset press plate can be irradiated, a film model is prepared. The process step for preparing the film model disadvantageously increases the time and expense of putting pictures on offset press plates.
To solve this problem, German Patent Disclosure DE-A 37 08 147 has disclosed a process of the generic type in question. The photoplotter described there holds a sheet of light-sensitive material for the sake of irradiation from an associated light source. A light valve arrangement inserted between the light source and the sheet has a main surface parallel to the sheet that is subdivided into many subregions or copies, whose light transmissivities are controlled individually by an associated computer to assure that the irradiation is in fact that of a desired graphic illustration or work.
German Patent Disclosure DE-A 38 13 398 also proposes a process and an apparatus according to which and with which the process for producing a light mask for irradiation of offset press plates is simplified and shortened. To generate a latent image, for one of the photosensitive offset press plates, by means of a beam transmitted by a light source and modulated by means of a light mask, the light mask is embodied as a light valve matrix and triggered with signals that correlate with the image. A display of the desired image is thus produced in the form of light spots on an liquid crystal layer, and the display is copied onto the photosensitive coating of an offset press plate.
Finally, from German Patent Disclosure DE-A 28 12 206, an optical printer with a light source and a template, disposed between the light source and the photosensitive recording carrier, for forming the characters to be printed; the template is embodied as a light switching mask by integrated thin-film technology, and the light switching elements of the mask are electrically controllable by a character generator and as a result are transparent as needed at predetermined character raster points to the light transmitted constantly by the light source.
The known processes have not become established thus far, because in practice they prove to be too slow and to lack quality. Especially in the preparation of offset press plates for printing newspapers, the known processes are unsuitable, since the sequential irradiation leads to intolerable timing bottlenecks and poorer-quality printing plates.
The object of the invention is to disclose a process and an apparatus of the type referred to at the outset that furnish better-quality offset press plates with reduced irradiation times.
In a process of this type, this object is attained in that the information content of a fragmentary image is investigated, and as a function of this investigation copying of the fragmentary image onto the pattern carrier is effected. To that end, the apparatus has an appropriate image information evaluation circuit or software.
It can happen that not all the fragmentary images actually contain image information. According to the invention, therefore, only fragmentary images that have picture contents are transmitted to the pattern carrier. Because of the reduced number of fragmentary images, the total irradiation time is shortened approximately in proportion to the sum of fragmentary images without picture content. The irradiation head approaches every position of a fragmentary image over a meandering path. The total irradiation time still required for the remainder can be further reduced if each fragmentary image is assigned a first data set for its image content and a second data set for its position, and the image content of each fragmentary image is investigated for whether picture information is contained in it, and only coherent data sets with image content are used to generate a succession of control data sets for transfer to a numerical controller. As a result, the times required between the individual irradiation times for changing positions between two irradiation locations can also be reduced.
The time for offset press plate preparation can be shortened by a further travel reduction, in that the quantity of the second data sets is sorted such that the smallest possible sum of individual spacings between the positions of the fragmentary images results, and the data sets are transferred in this order to the numerical controller. In this way, the total image is prepared over the shortest possible process path.
High motion speeds of the irradiation head are attained if at least one electric linear drive is used as the drive to generate a relative motion between the pattern carrier and the liquid crystal screen. The thus-shortened positioning times can be followed, without a waiting period, by the irradiation times, because the effects of elasticity, play and friction, on the one hand, and natural oscillation on the other are largely averted by the electric linear drive.
Since offset press plates may have a waviness that is sometimes greater than 2 mm, and since these irregularities are also variously distributed, adequate sharpness or depth of focus in the copy must be attained by choosing suitably small apertures. The low light yield, however, lengthens the irradiation times for the fragmentary image. To shorten these irradiation times, it is contemplated that before each copying of a fragmentary image, a measurement of the spacing from the pattern carrier is made, and deviations from a previously inlet value are automatically corrected. In this way, with a substantially shallow depth of field, high copy quality can be attained in short irradiation times even in wavy plates. The correction can preferably be done by moving the entire irradiation head. In this way, even variously thick offset press plates can be irradiated. The focal length may be corrected accordingly as well, however, although this can cause copying errors.
Uniform copying quality over the entire surface of the model is attained if for the copying, a light source is used whose luminous flux is measured, and deviations from the predetermined luminous flux are automatically corrected.
This correction can either be made by making the correction by varying the irradiation time, or by varying an electrical supply to the light source.
Even the most high-quality printing models can be produced by the process, if the liquid crystal screen is copied on an altered, preferably reduced scale. By suitable reduction of the liquid crystal screen, resolutions of 2540 dpi, for instance, can be attained. However, the model must then also be broken up into correspondingly many fragmentary images, which are irradiation in succession and joined together to form the total model. Because of the thus-increased number of individual irradiations, the total irradiation time for the model rises considerably.
If positioning of the fragmentary images is effected with an accuracy of better than 5 &mgr;m and in
Lüllau Friedrich
Lüllau Rolf
Mayer Claus
Chizmar John
Luellau Friedrich
Metjahic Safet
Striker Michael J.
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