Exposure device for printing plates including digitally...

Photocopying – Projection printing and copying cameras – Image transferred from individual documents to film strip

Reexamination Certificate

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Details

C355S040000, C355S071000

Reexamination Certificate

active

06411366

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method for triggering an exposure device for the photomechanical production of structured surfaces as a copy of an electronically memorized model, in particular for exposing printing plates, in which the exposure device includes at least one light source, one picture generating unit comprising movable, electronically individually triggerable micro mirrors arranged in a grid, and one copying optical element.
The invention also relates to an exposure device suitable for performing the method, having a movably embodied exposure head, which includes a light source, one picture generating unit comprising movable, electronically individually triggerable micro mirrors arranged in a grid, and one copying optical element.
2. Prior Art
Even today, up to 90% of offset printing plates are exposed via fill models with the aid of contact copy technology or in isolated cases with projection systems. This means that before an offset printing plate can be exposed to light, a film model is made. This is done with film exposers and film developing machines developed especially for this purpose.
The method step for preparing the film model disadvantageously increases the time and expense involved in putting images on offset printing plates.
From German Patent Disclosure DE 41 21 509 A1, a device suitable for exposing printing plates is known, which has an elongated light source in the form of a linear arc lamp, a collimator lens, an elongated light modulator comprising electronically triggerable movable micromirrors, and a copying optical element, all these elements being disposed in stationary fashion. The elongated linear form of the light source is used here to attain a higher light yield than in point-type light sources. Consequently, however, the stationary exposure arrangement can expose only a very narrow strip of a printing plate. To expose the complete printing plate, the plate is therefore displaced continuously relative to the stationary exposure arrangement. To assure that the copied data remains stationary relative to the exposed material, the transmission of the data within the mirror array must also be displaced in synchronism with the motion of the printing plate.
The known apparatus has the disadvantage that because of the continuous motion of the printing plate, it can expose only in strips and can therefore utilize only a narrow region of the micromirror array. This leads to excessively long total exposure times. Also, because of the limited length of the micromirror array, as a rule it is not possible to expose the entire length of the printing plate simultaneously. Instead, the printing plate must be exposed column by column and moved back and forth for the purpose. This in turn, however, requires that the entire heavy table on which the printing plate is spread out be displaced with accuracy in the micrometer range. Because of the mass to be moved, this cannot be done arbitrarily quickly with the requisite precision. The result is a further lengthening of the total exposure time.
From International Patent Disclosure WO 95/22787, an apparatus for photomechanically making structured surfaces, in particular for exposing printing plates, is known that has a movable exposure head with a light source, a picture generating unit, and a copying optical element. The movable exposure head is compact in design and relatively light in weight. As a result, it can be positioned with micrometer accuracy. The picture generating unit comprises a liquid crystal screen that is disposed between two polarizers. From this reference an exposure method is also known in which the electronically memorized models are broken down into partial images, and the partial images are shown in succession on the liquid crystal screen and copied onto the printing plate in such a way that they combine into a total copy of the model. To that end, the exposure head is moved with extreme precision from one exposure position to the next between each two partial exposures. For exposing the entire printing plate, however, a great number of partial images have to be exposed. This can result in a very long exposure time.
In principle, the exposure time can be shortened by using higher light intensities for the exposure. Given the necessarily narrow design of the exposure head, however, higher light intensities lead to a no longer tolerable heat burden, especially since for the requisite polarization of the light entering the liquid crystal screen, a polarization foil is used. Such polarization filters admit the portion of the incident light that has the “correct” direction of polarization, while the remainder with the “wrong” direction of polarization is absorbed. A considerable amount of heat is thus created in the polarization filter at high light intensities and this heat must be dissipated if destruction of the polarization filter is to be averted. In the liquid crystal screen itself and in the second polarization foil that follows as well, heat from absorbed light occurs.
Particularly in producing offset printing plates for newspaper printing, the sequential exposure in known methods and apparatuses leads to disadvantageous bottlenecks in terms of time.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an improved exposure device for printing plates of the above-described type, which produces exposure results of satisfactory quality with reduced total exposure times.
It is also an object of the present invention to provide an improved method for controlling an exposure device of the above-described type for exposing printing plates according to an electronically stored model, whereby exposure times may be reduced while still producing exposure results of satisfactory quality.
In a method of this generic type, this object is attained in that the model is broken down electronically into two-dimensional partial images; that the partial images are shown in succession, but each partial image is shown with its entire area at the same time, by means of the picture generating unit, and successive partial exposures of the surface to be structured are made, in that a exposure head, provided with the light source, the picture generating unit and the copying optical element, moves between each two partial exposures from one exposure position to the next and stops there for the exposure, so that the individual partial copies are combined again into a total copy of the electronically memorized model.
The invention adopts the known method, in conjunction with a precision-controlled movable exposure head, which is based on the production of flat partial images using a liquid crystal screen, to a stationary exposure arrangement that is based on linear picture generation by means of micromirrors. For that purpose, in a first step of the invention, the stationary exposure device is embodied as a movable exposure head, and in a second step of the invention the quasi-linear micromirror array is replaced by a genuinely two-dimensional array, and in a third step of the invention the electronic process control for the motion of the exposure head and the exposure of the partial images is transferred. In this respect, the teaching provided in the first paragraph of the background section of German Patent Disclosure DE 41 21 509 A1 actually leads one skilled in the art away from using two-dimensional light modulators. If conversely one skilled in the art takes as his point of departure the apparatus and the associated method known from WO 95/22787, then by simply adopting the stationary exposure arrangement with micromirrors known from DE 41 21 509 A1, he certainly does not arrive at the subject of the invention. Nor does simply replacing the liquid crystal screen with the linear micromirror array lead directly to the invention. That would require a genuinely flat-area, two-dimensional micromirror array, which furthermore must be triggered quite differently from the linear array. Furthermore, the micromirror array cannot simply be used

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