Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
2003-03-07
2004-12-28
Tran, Huan (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C347S251000, C347S240000
Reexamination Certificate
active
06836283
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to the field of electronic reproduction technology and pertains to a method of setting the line width of recorded image lines in an exposer (output scanner, output recorder) for recording printing originals, such as printing plates.
In electronic reproduction technology, recording devices are used to expose printing originals and printing forms. For this purpose, a laser beam is, for example, produced by a laser diode, shaped by an optical device, and focused onto the recording material. The laser beam is deflected over the recording material, point by point, and line by line, by a deflection system. There are also recording devices which, in order to increase the exposure speed, produce a bundle of laser beams, for example, using a separate laser diode for each laser beam, and expose a plurality of image lines of the printing form simultaneously each time they sweep over the recording material. The printing forms can be exposed on film material to produce color separation films that can subsequently be used for producing printing plates with a photographic recopying method. Instead, the printing plates themselves can also be exposed in a plate exposer or directly in a digital press. The recording material can be located on a drum (external drum exposer), in a cylindrical hollow (internal drum exposer) or on a flat surface (flat bed exposer).
In the case of an external drum exposer, the material to be exposed, in the form of films or printing plates, is mounted on a rotatably mounted drum. While the drum rotates, an exposure head is moved axially along the drum at a relatively short distance. The exposure head focuses one or more laser beams onto the drum surface. The beams sweep over the drum surface in the form of a narrow helical line. In this way, during each revolution of the drum, one or more image lines are exposed on the recording material.
In the case of an internal drum exposer, the material to be exposed is mounted on the internal surface of a partly open hollow cylinder and is exposed with a laser beam that is aimed along the cylinder axis onto a deflection device that reflects the laser beam at right angles onto the material. The deflection device, a prism or a mirror, is rotated during operation and is additionally moved slowly in the direction of the cylinder axis, so that the deflected laser beam makes circular or helical image lines on the material, in order to scan the entire surface of the material little by little.
Flat bed exposers operate predominantly with a rapidly rotating polygonal mirror, whose mirror surfaces deflect the laser beam transversely over the recording material, while at the same time the recording material is moved at right angles to the deflection direction of the laser beam. In this way, exposure is carried out image line by image line. Since the length of the light path changes during the movement of the laser beam over the recording material, complicated imaging optics are required, which compensate for size changes thereby induced in the exposure point.
As compared with flat bed exposers, internal drum exposers have the advantage that the optical paths are shorter, so that less expenditure for vibration damping is required. In addition, the length of the light path is constant, so that it is possible to manage with simpler imaging optics. Because of the few and rapidly moving parts, high exposure speeds are achieved with internal drum exposers. Because of the construction, when using an external drum exposer, the achieved exposure speeds are not so high, since a large mass has to be moved and the rotational speed of the drum is limited by the centrifugal forces which, at too high a rotational speed, would separate the recording material from the drum. The advantage with the external drum exposer is that the light path from the laser to exposure point is very short. This permits a configuration of the imaging optics with which a very sharp beam profile is produced. Because of the longer light path, this is not possible in an internal drum exposer, so that the beam profile has a distribution of the power density from the center to the edge of the laser beam in the form of a Gauss bell curve, for example.
During the exposure of printing originals, the laser beam is not modulated with a continuously varying signal, but is switched on and off on the basis of a binary image signal which can assume only two values. The reason for this is that the printing originals are screened, that is to say different tonal values in the color separations are represented by halftone dots whose size is varied in accordance with the tonal values. During the exposure of the printing originals, the half tone dots are composed of many small exposure points, for example, with a resolution of 1,000 exposure points/cm. In order to achieve this resolution, the image lines must have a spacing of a=10 &mgr;m, for example. Whether the diameter d of the exposure points in this example must likewise be set to d=10 &mgr;m depends on the beam profile of the laser beam and on the exposure characteristics of the recording material. For the following explanations, the diameter d will be defined as 50% diameter that results from the radius at which the power density of the laser beam has fallen to 50% of its maximum value at the center of the beam.
If there is a sharp beam profile, such as in the case of an external drum exposer, that is to say there is a uniform distribution of the power density over the area of the exposure point, the 100% diameter of the exposure point is set to the dimension of the line spacing a or somewhat larger. This is because adjacent image lines adjoin one another exactly or overlap somewhat, which ensures that no unexposed areas are produced between adjacent image lines, which would be disruptively visible in the finished recording of the printing original. If, as in an internal drum exposer for example, the beam profile follows a gaussian curve, the 50% diameter d of the exposure point is preferably set such that it corresponds to the image line spacing a, that is to say, in the example of a resolution of 1,000 exposure points/cm, the line spacing a=10 &mgr;m and the 50% diameter is likewise d=10 &mgr;m. This basic setting of the diameter of the exposure point has been tried and tested in practice for films and printing plates with a photosensitive layer of silver halide, which generally have a light sensitivity curve with a pronounced exposure threshold. If the incident energy density exceeds the exposure threshold, the material is exposed, and at an energy density lying below this, it remains unexposed. The basic setting of the line width in an exposer is optimized during daily operation by being readjusted in fine steps by changing the laser power. In this way, varying characteristics of the recording material and of the developing scheme are compensated for. For this purpose, test exposures are carried out with various line widths and are measured. The parameters that result in the best line connection between adjacent image lines are then set in the exposer.
For some time, offset printing plates have been available which are built up from a carrier material and a photopolymer layer applied thereto and which behave differently than a typical photosensitive layer based on silver halide.
FIG. 1
shows in qualitative terms a typical exposure curve for a photopolymer printing plate. Plotted on the horizontal axis is the energy density H with which the printing plate is exposed, and the effect W of the exposure is plotted on the vertical axis. When a first exposure threshold M
1
is exceeded, polymerization is triggered in the photopolymer layer, and crosslinks the molecules. As a result, the exposed areas become resistant to the developer solution, with which the unexposed areas are washed out in a subsequent process. However, the exposed areas are not yet resistant enough against the wear in the press if they have been exposed only with an en
Greenberg Laurence A.
Heidelberger Druckmaschinen AG
Mayback Gregory L.
Stemer Werner H.
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