EP1176545B1 - Laser imaging with variable dot size - Google Patents

Abstract

A laser controller (426) varies the laser power or illumination intensity as a function of the spacing between laser source (40) and image point (410).

Description

The invention relates to a device and a method for pointwise imaging of printing surfaces by means of at least one laser beam which is moved relative to the printing surface.

When imaging plates using CtP (Computer-to-Plate) or Direct Imaging Printing machines must be the distance between the printing surface and the optical system the imaging device are maintained very accurately in order to achieve an optimal result receive. For example, by vibrations of the machine in operation, but it comes to Deviations from the nominal distance between printing surface and imaging laser. How strong the quality of the Bebilderungsergebnises depends on the deviation from the nominal distance is among other things, by the beam quality of the laser and the selected beam parameters founded. In general, results from a deviation from the desired distance deformed pressure point, which depending on the beam parameters either larger or smaller than the predetermined target size is. For very strong deviations even no pressure point generated more on the printing surface, since the laser beam is so much expanded that on no position of the printing area more the imaging threshold is reached.

US 5,764,272 describes an autofocus system for a laser imaging device disclosed. This system has a laser and corresponding optics for forming a Light beam, which is focused on an image plane on. By means of a photodiode if a signal characteristic of the light reflected from the image surface is produced, then that the focus of the laser beam on the image surface the characteristic signal can be adjusted accordingly. This creates a close connection between the picture surface and the image plane of the laser manufactured with its corresponding optics. to Shifting the focus of the imaging device may be the laser, the corresponding Optics or the image area are moved.

Such autofocus systems can work only limitedly fast. For example, the Laser optics procedure, a non-negligible mass must be accelerated quickly, exactly be positioned and quickly braked again. For high-frequency interference like her for example, by contamination under the pressure surface, dust grains or by Kinks occur in the printing surface, the regular times that such a system requires too long. It therefore often comes to Bebilderungsfehlern. In a multi-channel system, i. H., An imaging device with multiple parallel laser beams may typically not every single beam can be focused, because the entire imaging optics is moved. In other words, a compromise must be found so that the deviation from the nominal distance of all the simultaneous beams altogether becomes minimal. in the general is the construction of a mechanical, by movement of the Viewing optics autofocus system technically complex, needed corresponding space and causes relatively high costs.

From the document WO97 / 27065 is a device for pointwise imaging of Printing surfaces on a drum known, with a variety of infrared laser diodes whose Input currents are variable, allowing the light intensity during imaging is selectively variable to affect the size of pressure points to be generated. The Selective change is based on pre-calibration of the laser diode intensity by for this one correction function depending on the variations of the position of the Drum surface is defined relative to the device. The correction function becomes the Determining a correction of the laser diode intensity used during the imaging, so that the currently used input currents can be determined in real time.

Furthermore, it is disclosed in the document EP 0 922 573 A2 that by a Eccentricity of a drum, which receives a pressure surface to be imaged, conditional periodic variations in the distance of a device for pointwise Imaging of printing surfaces and compensated for the imaged printing surface be that an electronic compensation is made. The Output light intensity of the imaging device is varied by the Input power is adequately changed: one with the same period as the fluctuations changing correction factor is generated in a matching circuit.

Object of the present invention is therefore an apparatus for pointwise Imaging of printing surfaces by means of at least one laser beam, which relative to Pressure surface is moved to provide, with which a variable Imaging can be performed without removing parts of the device, such as For example, the imaging optics, must be mechanically moved to Compensate for variations in the distance between imaging optics and printing surface.

This object is achieved by a device having the features according to claim 1 and by a method according to claim 5 for imaging printing surfaces by means of at least one Laser beam solved.

Typically, the imaging optics of a imaging device are set in such a way that that at the desired distance the focus, i. H. the plane in which the laser beam its smallest diameter has, exactly on the surface of the printing surface comes to rest. A deviation from the nominal distance between laser and printing surface results in a Magnification of the beam diameter on the printing surface and thus depending on how the Laser parameters of power and focus diameter are set in one Increase or decrease the pressure point. By means of a detector is the Actual distance between pressure surface and laser detected, so that he with a setpoint can be compared. Depending on the deviation from the setpoint, the Light output, with which is imaged, increased or decreased. An increase in the Laser power is associated with an increase in the pressure point, since the spot size, on which energy is deposited on the printing surface which is the imaging threshold exceeds, increases. Accordingly, a reduction in laser power goes with a Reduction of the pressure point associated because the spot size, on which energy on the Pressure surface is deposited, which exceeds the Bebilderungsschwelle decreases.

Another way to vary the size of the pressure point is to use the Exposure time to extend or shorten specifically. A combination of Changes in performance and exposure time are also possible.

With the device according to the invention, the enlargement or reduction of the Pressure point due to a distance deviation are compensated: By the provided variable laser power can be an adjustment of the pressure point size done so that an acceptable Bebilderungsergebnis is achieved. In other words the pressure point size is variable. The value of the required light output or Exposure time can be calculated from the measured distance. This feature can z. B. in the raster generator, the pattern of pressure points to be imaged in a temporal Sequence of pulses for the laser imaging implements to be met. advantageously, is in advance by the functional context a table, a so-called Lookup-Table, created and saved, so that in situ the required value immediately Available.

In an advantageous embodiment of the invention, the device for pointwise Imaging of printing surfaces on multiple laser beams, with which simultaneously exposed becomes. Particular preference is given to individually controllable diode laser arrays. For each individual laser of the array, the power or imaging time can be varied be so that it is possible to achieve an acceptable Bebilderungsergebnis, as the Size of each printed by a laser pressure point variable and the size of the independent of other pressure points.

The present invention requires significantly fewer moving parts than the known ones Autofokussysteme and can react much faster to disturbances. she at the same time achieves a much better imaging result than a device without Autofocus. The realization of compact imaging device in integrated form is much easier. It is associated with lower costs.

Such a device can be inside or outside of a printing unit or a Printing machine can be used for point-by-point imaging.

Further advantages and advantageous embodiments of the invention will be described with reference to FIG following figures and their descriptions shown.

Fig. 1

Variation der Fleckgröße eines Laserstrahls

Fig. 2

Erzeugung eines Druckpunktes auf einer Druckfläche durch relative Bewegung eines Laserstrahls gegen die Druckfläche

Fig. 3

Beispiele für geschriebene Druckpunkte mit unterschiedlichen Laserparametern

Fig. 4

Schematische Ansicht der Bebilderung einer Druckfläche durch eine erfindungsgemäße Einrichtung

They show in detail:

Fig. 1

Variation of the spot size of a laser beam

Fig. 2

Generation of a pressure point on a pressure surface by relative movement of a laser beam against the pressure surface

Fig. 3

Examples of written pressure points with different laser parameters

Fig. 4

Schematic view of the illustration of a printing surface by a device according to the invention

In Fig. 1, the variation of the spot size of a laser for spotwise imaging is shown shown by pressure surfaces. The laser beam propagates along the optical axis 10, on which also its intensity maximum is. In focus 12, the Laser beam at its lowest waist. At this point, advantageously one Imaging done. In other words, the focus 12 defines the target distance of the laser to the printing surface. Both at a point 14 before the focus and at one Point 16 after the focus, the beam is widened. The lines 18 indicate the variation of the Limiting the light spot as a function of position along the propagation direction. In focus 12, in a region 110, a greater intensity than the threshold intensity reached for imaging. By the widening of the laser beam in front of and behind the focus 12, the area in which the intensity is above the threshold intensity becomes smaller, because the transported energy flows through a larger cross-sectional area. At detained Laser intensity thus results in the area 112, in which the imaging threshold is exceeded. At a reduced actual distance 114 from the laser to the printing surface is the Area to be imaged 116 greater than the area reached at a fixed intensity 112. Consequently, according to the invention, the intensity of the laser is increased so that the region, in which the threshold intensity for imaging is exceeded becomes larger. The Threshold intensity is then exceeded in the entire area 118. In actual distance 114 then the threshold intensity is reached in the entire area 116.

Fig. 2 shows the generation of a pressure point by relative movement of a laser beam against a printing surface. A laser beam with a spot 22 falls onto a printing surface 20. The laser is scanned over the printing surface 20 in such a way that the threshold intensity for imaging is exceeded in the entire region 24. In a preferred embodiment, it is an elliptical Gaussian laser beam which has two different semi-axes. Typically, the longer spot diameter w _x 26 is perpendicular to the direction of movement. The shorter spot diameter w _y 28 lies in the direction of movement. With such a device, both lines and points can be written, since the pressure point width d _x 210 and the pressure point height d _y 212 can be preset accordingly.

Examples of boundary lines of written pressure points of different laser parameters are shown in FIGS. 3a, 3b and 3c. In other words, that area is shown on which the threshold intensity for imaging is exceeded. 3a shows the boundary line f of a pressure point with widths d _x 9.3 microns and d _y 10.6 microns. The pressure point with boundary line f shown by an elliptical laser beam in focus with the spot diameters w _x = 8.0 Microns and w _y = 6.0 microns produced. It is also to see the boundary line u of a pressure point, as it arises at deflection of 100 microns from the nominal distance at the same held laser power. Its width d, is 8.5 microns, and its height d _y is 9.8 microns. The laser wavelength is 830 nanometers, and the diffraction factor M ² is 1.1. At this distance from the focus, the spots w _x and w _{y are} 8.8 microns and 7.7 microns, respectively. It is shown in Fig. 3a the boundary line a of a pressure point, as it can be achieved with the aid of the device according to the invention. To produce a pressure point of width d, 9.4 microns and height d _y of 10.7 microns at the given effective distance, 100 microns from the focus, the power of the laser is increased by 10%. The laser wavelength 830 nanometers and the diffraction index M ² = 1.1 are selected as in the other two cases.

With the device according to the invention, the pressure point size can be made variable. FIG. 3 b shows by way of example how a power adjustment can lead to a reduced pressure point. With reduced power, which is optimized for writing a line, the boundary line 1 of a pressure point of width d _x of 8.1 microns and the height d _y of 9.5 microns is generated. The actual distance deviates again by 100 micrometers from the nominal distance in the focus of the laser. There, the spot diameter w _x 8.8 microns and w _y 7.7 microns.

FIG. 3 c shows by way of example how an extension of the exposure time, in other words of the duration of the load beam, leads to an increase in the pressure point both in the x direction and in the y direction. In addition to the boundary lines f and u (exposure in focus and 100 microns out of focus respective), a boundary line v of a pressure point is seen, which is generated at a time line extension of the exposure from 10 microseconds to 11 microseconds. The point thus created has the widths d _x 9.5 microns and d _y 10.8 microns. The parameters of the generating beam are the same as for the beam, which produces a pressure point with the boundary line u, as also shown in Fig. 3a.

The shown series of images in FIGS. 3 a, 3 b and 3 c exemplifies how a pointwise imaging of printing surfaces with the help of at least one laser beam Variable pressure point size achieved by variable laser power or exposure time becomes. Changes in distance between the printing surface and laser focus are instead of a Movement of the imaging optics, the laser itself or the printing surface, as in Autofocus systems is common, balanced by adjusting the laser power.

In Fig. 4, a preferred embodiment of the invention for imaging a Pressure surface, which is located on a rotatable cylinder shown. Such Embodiment may in particular in a printing unit or a printing press be realized. The laser light source 40 generates a laser beam 42 which mediates an imaging optics 44 on the located on a cylinder 46 pressure surface 48 in Item 410 is mapped. The cylinder 46 is rotatable about its axis of symmetry. These Rotation is indicated by the double arrow B. The laser light source 40 may be parallel to Symmetryeachse of the cylinder 46 are moved in a linear manner, which by the Double arrow A is marked. To illustrate the cylinder 46 rotates with the Pressure surface 48 according to the rotational movement B, and the laser light source 40 moves along the cylinder according to the translation direction A. The result is an image, which rotates in helical ways the axis of symmetry of the cylinder 46. Of the Path of the pixel 410 is indicated by line 412. The rangefinder 414 emits a light beam 416 which reaches the printing surface 48 at the pixel 418. As a result, the necessary information about the distance of the laser light source 40 with the pixel 410, which is used for imaging, are obtained to the printing surface 48. By means of a connection for the exchange of data and / or control signals 420 is the rangefinder 414 with a device for calculating the necessary Laser power 422 linked. Via connection 424 is the means for calculation the necessary laser power or exposure time 422 with the laser control 426 linked, which in particular can determine the laser power. Data and / or Control signals between laser control 426 and laser light source 40 are on the Transfer connection 428.

In a preferred embodiment of the invention, the laser controller 426 may be above Be linked via a connection 430 with the machine control 432.

In an advantageous embodiment of the invention, the laser light source 40 is made a laser diode array whose individual lasers can be controlled separately. It can then take place a simultaneous imaging of multiple pressure points whose Size is variable. For each individual pressure point, the deviation of the actual position of the desired position of the printing surface to the laser focus by the variable laser power or Exposure time be compensated.

LIST OF REFERENCE NUMBERS

10

Optical axis

12

beam focus

14

Exploded beam in front of focus

16

Exploded beam after focus

18

Variable limitation of laser spot as a function of position

110

imaging area

112

Intensity above threshold at nominal distance

114

actual distance

116

Desired imaging area

118

Intensity above threshold at actual distance

20

print area

22

Stain of the imaging laser

24

to be written pressure point

26

Focus diameter in x-direction w _x

28

Focus diameter in y-direction w _y

210

Width of the pressure point d _x

212

Height of the pressure point d _y

A

translational motion

B

rotational motion

f

Boundary line of the pressure point during exposure in focus

u

Limit line of the pressure point at exposure 100 microns outside the focus

a

Boundary line of the pressure point during exposure to adjusted power

l

Limit line of the pressure point at exposure 100 microns outside the focus

u

Limit line of the pressure point with extended exposure time

40

Laser light source

42

laser beam

44

imaging optics

46

cylinder

48

print area

410

pixel

412

Path of the pixels

414

rangefinder

416

Beam for distance measurement

418

Pixel of the beam for distance measurement

420

Connection for exchanging data and / or control signals

422

Device for calculating the necessary laser power or exposure time

424

Connection for exchanging data and / or control signals

426

Laser control, in particular control of the laser power or exposure time

428

Connection for exchanging data and / or control signals

430

Connection to the machine control

432

machine control

Claims (8)

1. Device for the spot-wise imaging of printing surfaces with the aid of at least one laser beam (42) that is moved relative to a printing surface (48), the device including a laser light source (40) and a laser control (426) for varying the laser power or the exposure time as a function of the distance between the laser light source (40) and an image spot (410),
   characterized in that the device includes a distance meter (414) for determining the distance between the laser light source (40) and the image spot (410), the distance meter (414) being connected to the laser control.
2. Device for the spot-wise imaging of printing surfaces according to claim 1,
   characterized in that the laser light source (40) is a laser diode.
3. Device for the spot-wise imaging of printing surfaces according to claim 1 or 2,
   characterized in that the laser light source (40) includes a plurality of light beams (42) to simultaneously image a plurality of printing spots, the light beams (42) being separate from each other in space.
4. Device according to one of the preceding claims,
   characterized in that the laser light source (40) is an individually addressable laser diode array.
5. Method of imaging printing surfaces in a spot-wise manner with the aid of at least one laser beam (42), the method including the steps of

   providing a laser light source (40) for generating a laser beam (42) with location-dependent intensity distribution in the two spatial directions perpendicular to the axis of expansion and a given divergence,

   providing a printing surface (48) at a distance to the laser light source (40),

   exposing the printing surface (48) located at the given distance to the laser light source (48),

   varying the laser power or exposure time by means of a laser control (426) as a function of the distance between the laser light source (40) and the image spot (410) on the printing surface (48) to modify the spot size of the image spot (410) on the printing surface (48),

   characterized by

   determining the distance between the laser light source (40) and the image spot (410) by means of a distance meter (414) connected to the laser control.
6. Method according to claim 5,
   characterized by
   an adjustment of the spot size on the printing surface (48) to a predetermined value by varying the laser power or the exposure time.
7. Printing unit,
   characterized in that the printing unit includes at least one device according to one of claims 1 to 4.
8. Printing machine,
   characterized in that the printing machine includes at least one printing unit according to claim 7.

Images