Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
2001-11-21
2004-10-26
Pham, Hai (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C347S258000
Reexamination Certificate
active
06809754
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains in general to an imaging device for illustrating a printing form, e.g., a printing plate mounted on a printing drum, which can be used, e.g., in web-fed rotary printing or newspaper printing. The present invention pertains, in particular, to an imaging optical system, especially a lens system, for an edge- or area-emitting laser, especially an edge- or area-emitting semiconductor laser diode, as well as to a process for illustrating a printing form.
BACKGROUND OF THE INVENTION
The use of lasers for illustrating printing plates has been known, and various processes are used to illustrate a printing plate with a laser beam. In the case of so-called inner drum exposure devices, the laser light beam is moved over the plate to be exposed, which is located in the drum, by using a multiple mirror rotating at a high speed. As a result of this, an image line is generated, and the rotating mirror is continuously moved along its axis of rotation in order to thus successively illustrate the entire plate surface by means of the laser. The laser light is modulated as a function of the image data such that the entire image can be generated on the printing plate. Furthermore, it has been known that an illustrating operation can be carried out in which the printing form is fastened outside on a cylinder or the cylinder surface itself forms the printing form, which is called the outer drum principle. This outer drum principle is used during the illustration of the printing form in the printing press. If a single laser were used, the printing form cylinder would have to be rotated at a high speed in order to carry out a complete illustration in an acceptable time, and the laser beam would have to be moved in parallel to the axis of rotation in order to illustrate the entire surface of the printing form. However, this process is technically extremely complicated because of the high speeds occurring. To make it possible to reduce the speed of rotation of the printing plate cylinder, a plurality of laser light sources are used, which are arranged as a laser array, in order to generate a plurality of columns of an image with such a laser array during each revolution of the printing form cylinder, and this array is moved along the printing form in order to illustrate the entire printing form.
For example, a plurality of laser diodes are used as the laser array, and the light emitted by the laser diodes is coupled into an optical fiber each and this light exits at the end of the fiber located opposite the printing plate as a light cone with an opening angle of, e.g., 16°, and is focused with lenses onto the printing form. Furthermore, edge-emitting laser diodes have been known, in which laser light exits in the form of a narrow strip, and one strip may have a width of, e.g., between 60 &mgr;m and 200 &mgr;m and a height of less than 1 &mgr;m. The laser light is emitted usually at a greater angle of radiation of, e.g., 35° to 40° in the vertical direction and at a smaller angle of, e.g., 8° to 10° in the horizontal direction. It is necessary to focus the laser light by a suitable imaging optical system onto the printing form to be illustrated such that the highest possible quality of the image generated by the laser light is achieved.
SUMMARY AND OBJECTS OF THE INVENTION
The object of the present invention is to propose an imaging optical system, a system and a process for illustrating a printing form with lasers, especially semiconductor lasers, which make it possible to generate images that meet the requirements in terms of quality and costs, e.g., in newspaper printing.
According to the invention, a laser lens system for illustrating a printing form is provided with at least one spherical lens which may be arranged between the laser and the printing form in order to bundle the laser light emitted by the laser. At least one aspherical lens may be arranged between the spherical lens and the printing form for focusing or converging the laser light onto a desired spot.
According to another aspect of the invention, a process for illustrating, a printing form is provided. A laser for exposing an area element of a predetermined width and height is operated continuously. The laser light generated by the laser is focused by the lens system such that a strip (L) is produced which has a width that approximately corresponds to the width of the area element to be exposed. The strip has a height (H) that is smaller than the height of the area element to be exposed. The laser light strip (L) generated is led over the area element to be exposed such that the entire height of the area element to be exposed is swept by the strip (L).
The lens system according to the present invention, which can be used as an imaging optical system for lasers, especially edge-emitting semiconductor laser diodes for illustrating a printing form, e.g., a printing plate on a printing drum, preferably has at least one spherical lens. This can be arranged between one or more laser light sources and a printing form to be illustrated for bundling the laser light emitted by one or more lasers. It is not absolutely necessary to focus the laser light. It is sufficient to reduce the angle of radiation of the laser light. Furthermore, the lens system has at least one aspherical lens, which is preferably arranged between the spherical lens and the printing form to be illustrated in order to converge the laser light focused by the at least one spherical lens and preferably to perform a correction of the spherical aberration, wherein the lens system is preferably directed such that the smallest possible irradiation angle is obtained at the printing form to be illustrated in order to obtain a great depth of field. High-quality illustration can be performed with the combination of spherical and aspherical lenses for focusing the light emitted preferably by an edge- or area-emitting laser on the printing form, because the spherical lens, which is usually ground, has a good surface finish and thus it can focus the light emitted by the laser at a certain angle of radiation without causing an imaging error due to surface inaccuracies. Such an imaging error directly after the emitting area of the laser would lead to a greater inaccuracy on the printing form. The at least one aspherical lens arranged behind the spherical lens in the beam direction of the laser light is usually prepared by pressing and cannot be ground like the spherical lens, so that the aspherical lens can be manufactured with a greater precision in terms of its desired curvature than the spherical lens, but it has a lower surface finish. Consequently, very good focusing of the laser light bundled by the spherical lens can be obtained with the at least one aspherical lens because its curvature can be manufactured with a very high precision, and errors and inaccuracies on the surface of the at least one aspherical lens have hardly any noticeable effect on the imaging, because only the radiation area of the laser located in the object plane of the lens system is imaged sharply, and surface inaccuracies, e.g., scratches, are imaged only diffusely with at extremely low power and thus lead qualitatively to a hardly noticeable deterioration. The at least one spherical lens used to bundle the laser light can consequently perform an accurate bundling of the laser light emitted by a laser because of the good surface finish, and essentially only spherical aberration occurs as an error. The downstream aspherical lens can bring about the most accurate focusing possible of the laser light because the desired curvature can be manufactured with precision, and the spherical aberration of the first lens is corrected. Surface errors of the downstream aspherical lens do not lead to a noticeable loss of quality of the imaging, as it would happen in the case of a lens that is arranged in the vicinity of the object. Thus, accurate focusing of the laser light emitted by a laser, e.g. an edge- or area-emitting laser, on a printing form can thus be obtained fo
Maschinenfabrik Wifag
Pham Hai
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