Abstract:
A laser is used for digitally imaging a printing plate. Conventional UV printing plates, as are used in mask film imaging, are generally not suited for computer-to-plate (CTP) imaging methods. Here, the imaging method and the corresponding imaging apparatus permit the use of the conventional and more beneficial UV plates in a CTP imaging method and apparatus, in that the laser used is a quasi continuous wave laser (QCW laser), which emits laser pulses of a wavelength in the UV range.

Description:
BACKGROUND OF THE INVENTION  
     Field of the Invention  
       [0001]     The present invention relates to an imaging method and an imaging apparatus for the digital imaging of a printing plate by using at least one laser.  
         [0002]     In particular, the invention relates to computer-to-plate offset imaging methods (CTP) and corresponding apparatuses.  
         [0003]     There are various alternative possible ways of imaging a printing form, in particular an offset printing plate. First of all, the printing plate can be imaged via an exposed film. In that case, depending on the type of plate, regions of the printing plate that are not to be exposed are covered by the film. This combination comprising printing plate and film is then acted upon with light of a specific wavelength by means of a specific exposure apparatus, so that the film is exposed in the regions which are not covered. For that purpose, the printing plate has a surface layer which is particularly sensitive to the wavelength or the wavelength range of the light used. In a further process step, the printing plate exposed in this way is developed further, possibly cleaned, and can then be used in a printing process. Frequently, in that type of exposure and imaging of the printing plate, UV light (ultra-violet) is used and the printing plates have a surface layer for the purpose which is particularly sensitive to UV radiation.  
         [0004]     Over the course of time, further imaging methods for printing plates have also been developed. In the so-called computer to plate method (CTP), the printing plate is exposed directly without the creation of a specific film or mask film being needed. For this purpose, the printing plate is exposed, for example by means of a laser, it being possible for a bitmap to be used as a digital original. The printing image on the printing plate is built up pixel by pixel by means of the laser. The printing plate itself in this method again has a surface layer which is specifically sensitive to the wavelengths of the laser used.  
         [0005]     The lasers that are employed are, for example, green lasers, infrared lasers, or violet lasers. For this purpose, there exist appropriate laser diodes with continuous laser signals, which are deflected onto the surface of the printing plate as a function of video signals which are obtained from the available bitmap. To this end, the laser beam is deflected pixel by pixel, that is to say point by point, by an optical modulator as a function of the video signals. A plurality of pixels or exposure points form a raster cell (screen cell), wherein they simulate a screen dot, the tonal value which is produced by the screen dot being determined by the number of exposure points exposed, the type of modulation used in the screening deciding on the precise arrangement of the pixels in a raster cell.  
         [0006]     Depending on the design of the CTP exposer that is used, the laser beam is deflected onto a rotatable optical element; this is, for example, a deflection prism. The laser signals are then deflected onto the printing form as a function of the video signals. The deflection prism scans the entire surface of the printing form by experiencing a forward movement. For this purpose, it is disposed within an imaging cylinder and axially displaced along the axis of the imaging cylinder. That system is referred to as an in-drum exposer.  
         [0007]     Other imaging methods can provide for the optical element for deflecting the laser to be located radially at a distance above an imaging cylinder and for the printing plate to rotate under this optical element. In this case, it is not necessary for a rotatable optical element to be used. The cylinder on which the printing plate is clamped rotates away under the optical element. That system is referred to as an ex-drum exposer. In particular, provision can also be made here for the laser or a large number of laser diodes to image the printing form. This can in particular be carried out directly.  
         [0008]     A further imaging method is used in the case of flatbed scanners. There, the surface of a printing form is exposed by a laser or by an optical element which deflects the laser signal onto the surface of the printing form. The printing form and the lasers or the optical deflection apparatus are displaced laterally with respect to each other. Only if the use of a laser is omitted, as described in German published patent application DE 195 45 821 A1 (cf. U.S. Pat. No. 6,074,065) and, instead, the printing form is exposed via micro mirrors by way of a UV lamp, for example a metal halide lamp, is it possible to use a conventional UV printing plate.  
         [0009]     None of the laser-driven CTP imaging methods described in the art allow the use of a conventional UV printing plates, as are used in the imaging methods with mask films described above. Instead, more expensive printing plates have to be used, which react sensitively to the corresponding wavelengths of the lasers. This also becomes clear, for example, from the article “Von Licht und Wärme” [Of Light and Heat], Druckmarkt Schweiz, Vol. 2-2001, pp. 24-26.  
         [0010]     UV plates cannot be used in a cost-saving manner in CTP exposers having lasers, since correspondingly necessary UV lasers are not available in a beneficial and inexpensive diode design.  
       SUMMARY OF THE INVENTION  
       [0011]     It is accordingly an object of the invention to provide a method and an apparatus for imaging a printing plate which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which makes it possible to use the conventional and more beneficial UV plates in a cost-effective manner in a CTP imaging method and a CTP apparatus.  
         [0012]     With the foregoing and other objects in view there is provided, in accordance with the invention, an imaging method for digitally imaging a printing plate, which comprises providing at least one quasi continuous wave (QCW) laser and emitting laser pulses of a wavelength in the UV (ultra-violet) range with the laser; and imaging the printing plate with the laser.  
         [0013]     In other words, the objects of the invention are achieved by an imaging method and an imaging apparatus for the digital imaging of a printing plate by means of at least one laser, wherein the laser used is a quasi continuous wave laser (QCW laser), which emits laser pulses of a wavelength in the UV range.  
         [0014]     A QCW laser emits laser pulses with a pulse duration in the neighborhood of 10 picoseconds and at a frequency in a range from 80 MHz to 120 MHz; higher frequencies are also possible. QCW lasers can be obtained relatively cheaply on the market, for example from the Light Wave Electronics company, Coherent or Spectra Physics. The wavelength of the laser light emitted can beneficially lie in the UV range around 366 nm; the conventional UV-sensitive printing plates are sensitive to light of this wavelength. They can therefore be imaged practically by means of this quasi CW laser. As emerges from the article “Quasi CW Solid-State Lasers Expand Their Reach (Photonics Spectra, December 2001, pp. 54-58)”, the direct exposure of printed circuit boards by means of such QCW lasers is already known. However, since this exposure is substantially less problematic than in the case of printing plates, since it is not necessary to pay attention to moirés or raster accuracy, this method cannot simply be transferred to UV-sensitive printing forms such as UV printing plates.  
         [0015]     Since a QCW laser does not emit a continuous laser signal but discharges a laser pulse, even if a high-frequency laser pulse, it may be necessary for various processes within the exposer to be driven as a function of the pulse frequency of the QCW laser. For this purpose, the invention beneficially provides for the pulse frequency of the QCW laser to be used to generate a master clock signal. In terms of apparatus, furthermore, a generating device for the generation of a master clock signal from laser pulses of the QCW laser is provided for this purpose.  
         [0016]     A master clock signal is to be understood to mean a regular clock signal in response to which various other regularly repeating method steps or drive signals can be triggered.  
         [0017]     In order to generate electric master clock signals from the laser pulses, the invention beneficially provides for the laser pulses to be deflected onto a photo-optical converter. According to the invention, this photo-optical converter is to be comprised by the generating device. The photo-optical converter can be, for example, a photodiode.  
         [0018]     A photo-optical converter has a specific time constant. The time resolution of the photo-optical converter corresponds to this time constant. Depending on the photo-optical converter used, these time constants can vary but there exists a technically possible minimum which currently lies at around 20 picoseconds. A laser pulse from the QCW laser lies in the range of about 10 picoseconds. The position of the laser pulse in time can therefore not be registered directly by the photo-optical converter.  
         [0019]     In order to be able to determine the position in time of the laser pulse more accurately, the invention provides for the generating device to have at least one optically active element which, on the basis of a laser pulse from the QCW laser, generates a light signal with a longer pulse duration for the purpose of registration by the photo-optical converter. By means of this optically active element according to the invention, a light signal having a longer pulse duration can be generated from at least one laser pulse. This light signal having the longer pulse duration can then be resolved and detected more accurately in time by the photo-optical converter. In this case, the pulse duration should lie at least in the region of the time constant of the photo-optical converter. This method step is not restricted to the pulse durations and time constants described but, in general, can be used when the pulse duration is shorter than the time constant, that is to say the measuring window of the photo-optical converter.  
         [0020]     Advantageously, the invention can provide for the optically active element to comprise at least one optical medium that is excited to fluoresce by a laser pulse. This can be excited by the laser pulse to emit a light signal. Beneficially, optically active elements and fluorescent media can be used for this purpose which emit light signals which are longer than the pulse duration of the laser and, particularly advantageously, are longer than the time constant of the photo-optical converter. The photo-optical converter can use the fluorescence signal, which has the same frequency as the laser signal, to generate a master clock signal which corresponds to the frequency of the laser pulse. This is possible in particular as a result of the fact that the time interval between two laser pulses is considerably more than the pulse duration of a laser pulse or the duration of a fluorescence signal.  
         [0021]     In an alternative embodiment, provision is made for the optically active element to comprise at least one optical medium that can be excited by the laser pulse to emit scattered light. This optical medium can be, for example, an optical fiber. This optical fiber can be positioned in such a way that the laser pulse passes through it axially and excites it to emit scattered light over its length. This scattered light is then emitted radially from the optical medium. The length of this optical medium determines the duration of the resultant light signal; it can first be collected and passed on or deflected directly onto the photo-optical converter. For example, an optical element or optical medium having a length of 6 mm should generate a light signal with a duration of about 20 picoseconds.  
         [0022]     In a further alternative embodiment, provision is made for the optically active element to comprise at least one dispersive optical medium, which broadens the laser pulse in time. Within a dispersive optical medium, components of the laser pulse with a different frequency exhibit a different speed. They therefore traverse this medium at different times, which means that the resultant laser pulse is broadened in time. It can then be detected better by the photo-optical converter, in particular if its width in time is longer than the time constant of the photo-optical converter.  
         [0023]     In a further development according to the invention and a corresponding alternative apparatus, provision is made for the optically active element to comprise at least one dispersive optical medium and an optical filter for high-frequency constituents of the laser pulse. It is then possible for high-frequency components to be filtered out of the broadened laser pulse and for the remaining parts of the laser pulse to be superimposed again. In this way, too, a light signal with a longer time is generated from the laser pulse. This can accordingly be detected by the photo-optical converter. In this way, the photo-optical converter is able to generate electronic signals which are available as a master clock signal in order to synchronize the sequences within the imaging apparatus with the pulse frequency of the QCW laser.  
         [0024]     When using a QCW laser for imaging a printing form, in particular a printing plate, it is a problem that the pulse frequency of the QCW laser and the frequency of the video signals are substantially of the same order of magnitude, that is to say lie around 100 megahertz. In this way, it is possible for moirés, floating effects to occur as a result of the slightly different frequencies of the video signals and the laser pulses. According to the invention, provision is therefore made for the laser pulses to be deflected by an optical modulator as a function of video signals for imaging the printing form, the modulation frequency of the optical modulator being synchronized with the master clock signal. If the frequencies of the video signals and of the laser pulses are coordinated with one another in this way, it is ensured that one pixel corresponds to one laser pulse. Moirés then beneficially no longer occur.  
         [0025]     The optical modulator is driven by the video signals. When driving the optical modulator, a drive delay occurs. In order to avoid exposure defects arising from the drive delay of the optical modulator, the invention provides for the drive delay to be taken into account when synchronizing the frequency of the optical modulator and the video signals with the pulse frequency of the quasi CW laser.  
         [0026]     By driving the optical modulator by means of a video signal, a time window, a modulation window of the optical modulator, is opened. Within this time, the laser pulse corresponding to a video signal can be deflected or remain unaffected. It is necessary here to take account of the fact that, as a result of the scanning, time-dependent flanks of the drive of the modulator within the modulation window occur. Ideally, therefore, a laser pulse should traverse the modulator in the middle of a modulation window. In order to generate correct deflection of the laser pulse, the invention therefore provides for the width in time of the modulation window of the optical modulator to be taken into account for the synchronization.  
         [0027]     In order to take into account the drive delay and/or the time width of the modulation window of the optical modulator, the invention beneficially provides that either the drive delay of the optical modulator or both the setting delay and the time interval between a laser pulse and the middle of the modulation window of the optical modulator are detected. If the sum of these two time intervals is known, the control of the video signals can be adapted appropriately, so that pixel-accurate driving of the optical modulator can be carried out.  
         [0028]     According to the invention, provision is made for the time difference between the driving of the optical modulator and of a deflected laser pulse to be detected for this purpose.  
         [0029]     In an alternative embodiment, the invention provides that, in order to detect this time difference, a second laser, which emits an auxiliary beam, is used. This auxiliary beam can be in particular a continuous laser beam which has a wavelength which differs from the wavelength of the laser pulse. In this case, the auxiliary beam and the laser pulse should be deflected substantially parallel to each other by the same optical modulators. In the further course, a filter can be incorporated which, for example, allows only laser signals above or below a predefined wavelength to pass, so that only laser pulses from the QCW laser can pass. This second laser can be used on its own for the purpose of detecting the drive delay and the relative position of a drive signal from the AOM in relation to the width of the modulation window.  
         [0030]     In a further development of the invention, provision is advantageously made for a rotatable optical element to be used to deflect the laser pulses onto the printing form, the rotational frequency of the optical element being synchronized with the master clock signal. The rotational frequency of this optical element should in this case be an integer divisor of the frequency of the master clock signal. In this way, floating effects (moirés) are avoided as a result.  
         [0031]     According to the invention, in a particular embodiment of the invention, provision is made for the rotatable element used to be a rotational prism. By means of the combination of the use of a QCW laser in the UV range and a rotational prism, it is possible to increase the rotational speed of the optical element and therefore to reduce the imaging duration. The reason for this is that the spot size of a UV laser is smaller than the spot size of the previously used lasers in the violet range or in other previously used wavelength ranges. As a result of this reduced spot size, it is possible to use a rotational prism with a smaller extent, which has a lower air resistance, which makes a higher rotational speed possible. A further advantage in the use of a rotational prism lies in the fact that the resolution of the printed image can alternatively be increased by the reduced spot size.  
         [0032]     Other features which are considered as characteristic for the invention are set forth in the appended claims.  
         [0033]     Although the invention is illustrated and described herein as embodied in an imaging method and apparatus for imaging a printing plate, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.  
         [0034]     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0035]      FIG. 1  is a diagram showing the structure of a conventional drum exposer;  
         [0036]      FIG. 2  is a raster cell having printing dots;  
         [0037]      FIG. 3  shows a video signal belonging to the raster cell;  
         [0038]      FIG. 4  shows a sequence of laser pulses from a quasi continuous wave (QCW) laser;  
         [0039]      FIG. 5  is a diagram showing the structure of an internal drum exposer using a quasi CW laser;  
         [0040]      FIG. 6A  shows a possible structure of a master clock generating device;  
         [0041]      FIG. 6B  shows an alternative structure of a master clock generating device;  
         [0042]      FIG. 6C  shows a further alternative structure of a master clock generating device;  
         [0043]      FIG. 6D  shows a further alternative structure of a master clock generating device;  
         [0044]      FIG. 7  is a diagrammatic view of a detail from a plate exposer having elements for the synchronization of the optical modulator; and  
         [0045]      FIG. 8  is a diagrammatic view of an alternative structure of an apparatus for the synchronization of the optical modulator. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0046]     Referring now to the figures of the drawing in detail and first, particularly, to  FIG. 1  thereof, there is seen the structure of an internal drum plate exposer according to the prior art. This is a conventional structure. Use is made of a laser source  1  which, in the case illustrated here, comprises a continuous wave laser. The laser source  1  emits a laser signal  2 , which is transmitted through an acousto-optical modulator (AOM)  3 , which modulates the laser signal  2  as a function of a video signal  4 . A modulated laser signal  5  emerges from the acousto-optical modulator. By means of a lens  6 , the modulated laser signal  5  is focused onto a specific location of a printing plate  11  via a rotational prism  7  and as a function of the video signal  4  and the lateral position of the rotational prism  7 . The printing plate  11  is in this case clamped in the internal drum  10  of the printing plate exposer. The rotational prism  7  is driven in rotation about an axis of rotation  9  by a prism drive  8 . The printing plate  11  is exposed line by line by way of the modulated laser signal  5  as a result of the rotation of the rotational prism  7 . A non-illustrated forward movement device moves the rotational prism  7  in a forward movement direction  13 . In this way—i.e., the relative movement between the plate  11  and the prism  7 —the entire region of the printing plate  1  to be exposed can be exposed.  
         [0047]     The video signal  4  is transferred by an AOM controller  22  to the acousto-optical modulator (AOM) and in this case consists of a pulse train as illustrated in  FIG. 3 . The AOM controller receives the video data belonging to the video signals from a video data source  12 .  
         [0048]     In order to image the printing plate  11 , the rotational prism  7  is set rotating and, at the same time, moved in the forward movement direction  13 . The laser beam  2  is modulated by the AOM  3  as a function of the exact position of the rotational prism  7  and the associated video data from the video data source  12 . In this way, printing points  15  on the surface of the printing plate  11  are exposed. Depending on the printing plate used, the exposed printing points  15  are points which, following subsequent development of the printing plate  11 , can accept ink or act in an ink-repellent manner. The individual printing points  15  are generated in this case from a printing original by way of a raster image processor (RIP). The latter is not shown here, for reasons of simplicity.  
         [0049]     The rotational prism  7  has a rotational frequency of approximately 1 kHz. This corresponds approximately to a beam speed of 1000 m per second on the surface of the printing plate  11 . A printing point  15 , or image dot  15 , should have a resolution of approximately 10 μm. This corresponds to a time length of 10 ns for the appropriately time-modulated laser signal  5 . From this modulation window of the acousto-optical modulator AOM, this results in a modulation frequency of the AOM of 100 MHz. If the laser source  1  used is a CW laser, it is sufficient to synchronize the modulation frequency of the AOM with the rotational frequency of the rotational prism  7 . In this case, the laser beam  2  has a constant intensity.  
         [0050]     If the laser source  2  used is a quasi CW laser (QCW), however, then both the AOM  3  and the rotational prism  7  must be synchronized with the repetition rate, that is to say the pulse rate, of the laser source  1 . This is not possible with the structure shown here. It proves to be particularly problematic that the modulation frequency of the AOM  3  at its 100 MHz lies in the region which corresponds to the pulse frequency of the laser. Slight timing differences between the driving of the AOM  3  and the pulse frequency of the laser source  1  then lead to moirés or artifacts in the resultant printed image. The same is also true of time differences between the repetition rate of the laser  1  and the rotational frequency of the rotational prism  7 .  
         [0051]      FIG. 2  shows a raster cell  14  comprising individual printing points  15 , as are imaged on the printing plate  11  as a function of the video signals  4 . Individual printing points  15  image pixels, or image dots, not illustrated here, on the printing plate  11 . The width of a printing point  15  is in this case 10 μm.  
         [0052]      FIG. 3  illustrates the variation over time of the pulses of a video signal  4 , which modulate the acousto-optical modulator  3  in such a way that the printing points  15  illustrated in  FIG. 2  are imaged on the printing plate  11 . In the example shown here, the frequency of the video signal  4  is 100 MHz. This corresponds to a modulation window of the AOM  3  of 10 ns.  
         [0053]      FIG. 4  shows the variation over time of the laser pulses  16  from a QCW laser of the laser source  1 . The variation over time is shown here in such a way that it can be assigned to the pulses of the video signal  4  from  FIG. 3 . The phase angle of the laser pulses  16  is chosen such that the maximum values of the laser pulses  16  in each case fall in the middle of the modulation windows of the video signals  4 .  
         [0054]     The frequency of the laser pulses  16  is 100 MHz here, corresponding to the frequency of the video signals  4 . The time interval  18  between two laser pulses  16  is therefore 10 ns. The width  17  of a laser pulse  16  is 10 μs.  
         [0055]      FIGS. 2, 3 , and  4  are arranged in vertical alignment so that it is possible to see the way wherein the laser pulses  16  interact with the video signals  4  in order ultimately to produce printing points  15  of a raster cell on a printing plate  11 . If the frequency of the video signals differs from the frequency of the laser pulses  16 , then the position of the pulses of the video signal  4  is displaced relative to the maximum values  43  of the laser pulses  16 . In this way, moirés can arise in a printing image on the printing plate  11 .  
         [0056]     The structure of an in-drum exposer, wherein a QCW laser is used, is sketched in  FIG. 5 . Identical reference numbers here designate the same elements as in the preceding figures.  
         [0057]     As distinct from  FIG. 1 , the laser source  1  used here is a QCW laser. The structure of the internal drum exposer is therefore wherein by additional elements and apparatuses which, in an inventive way, make the QCW laser usable for imaging the printing plate  11 .  
         [0058]     As already outlined in relation to  FIG. 1 , the laser source  1  emits a laser beam or a laser signal  2 , which is modulated by an AOM  3  as a function of the video signal  4  and then is deflected accordingly by a rotational prism onto the printing plate  11  within the internal drum exposer. In order to make the QCW laser usable, provision is made here for the rotational prism  7  to be controlled as a function of the frequency of the laser source  1 . For this purpose, a prism controller  23 , which drives the rotational prism  7 , is provided. To this end, the prism controller  23  firstly has a rotation controller  23   a , which controls the rotational frequency of the rotational prism  7 . Secondly, the prism controller  23  has a forward movement controller  23   b , which drives the forward movement speed of the rotational prism in a forward movement direction  13 . The rotation controller  23   a  is in turn matched to the clock rate of the laser source  1  by a rotation synchronizer  24 .  
         [0059]     In order to synchronize the video signal  4  with the laser pulses  16  of the laser source  1 , provision is made for the AOM controller  22  to be matched appropriately to the frequency of the laser source  1 .  
         [0060]     The forward movement controller  23   b  of the rotation synchronizer  24  and the AOM controller  22  are in each case matched to the frequency of the laser source  1  via a master clock signal  21 . The master clock signal  21  is generated from a laser beam  19  from the laser source  1  via a master clock generating device  20 . The AOM controller  22  is therefore synchronized with the master clock signals, by their phase angle being adapted appropriately. The frequency of the video signal  4  can therefore in particular coincide with the frequency of the laser pulses  16 . In these ways, by means of synchronization or triggering of the video signal  4  as a function of the master clock signal  21 , exact superimposition of the driving of the acousto-optical modulator  3  as a function of the laser pulses  16  can be achieved. In this way, a moiré can successfully be avoided.  
         [0061]     Within the rotation synchronizer  24 , the frequency of the master clock signal  21  is divided by a divisor, which is not illustrated here. The rotational frequency of the rotational prism  7  multiplied with the divisor is intended to result exactly in the frequency of the master clock signal  21 . The divided frequency of the master clock signal should therefore correspond to the rotational frequency of the rotational prism  7 . To this extent, the rotational frequency which results from the rotation controller  23   a  is compared with the divided master clock frequency in the rotation synchronizer  24 . The rotation controller  23  will then automatically adapt the rotational frequency of the rotational prism  7  to such an extent that the rotational frequency corresponds to the divided master clock frequency and has an identical phase angle. In this way, it is possible to ensure that each position of the rotational prism  7  is assigned to a specific printing point on the printing plate  11 . As a result of the matching or triggering of the rotational frequency of the rotational prism  7  to the master clock frequency  21 , it is ensured that it is not possible for moirés to occur on account of slight mismatching of the corresponding frequencies.  
         [0062]     Furthermore, provision is advantageously made for the forward movement speed in the forward movement direction  13  of the rotational prism  7  to be regulated as a function of the master clock frequency. All the constituent parts of the imaging of the printing plate  11  are therefore synchronized with the frequency of the master clock signal  21 . In this way, exact imaging of the printing plate  11  can be carried out.  
         [0063]      FIGS. 6A  to  6 D show various alternative embodiments of the structure of a master clock generating device.  
         [0064]     As already outlined, the width  17  of a laser pulse  16  from the QCW laser is too small to generate a master clock signal  21  directly therefrom. The structures illustrated in  FIGS. 6A  to  6 D are therefore used for the purpose of spreading out the time duration of the laser pulse  16  of the laser signal  19  to such an extent that the spread signal can be detected by a photodiode  26  and a master clock signal  21  can be generated directly by this photodiode  26 .  
         [0065]     The laser signal  19  can in this case be derived from the laser source  1 . It is possible that the laser signal  19  is led out at a different point than the laser signal  2  in relation to the laser source  1 . The two points can, however, also coincide in one point. In particular, optical elements can be present which split an emitted laser signal  2  into a laser signal  2  leading onward, which is used to image the printing plate  11 , and a laser signal  19  which is used to generate the master clock signal  21 .  
         [0066]     In the alternative structures, the laser signal  19  passes through an optical element in each case, by which means a longer light signal is generated which has a time constant which lies at least in the range of the time constant of the photodiode  26 . The photodiode  26  can in each case detect the light signal generated and convert it into a master clock signal  21 . In this way, the master clock signal  21  has the same frequency as the laser signal  19  and has an identical, possibly displaced, phase angle.  
         [0067]     In  FIG. 6A , the optical element is a fluorescent optical medium  25 . This medium  25  is excited by the laser signal  19  to emit light signals  44 . In this case, the medium  25  is chosen such that the duration of the light signals  44  in each case exceeds the time constant of the photodiode. Light signals  44  then strike the photodiode  26  and generate the master clock signal  21  there.  
         [0068]     In  FIG. 6B , the laser signal  19  passes through a fiber  45 . During this passage through the fiber  45 , scattered light  27  is generated along the optical path of the laser signal  19  and is emitted at right angles to the direction of the laser signal  19 . In this case, the length of the fiber  45  is somewhat more than 6 mm, so that the scattered light  27  falls onto a sufficiently large photodiode  26  over a sufficiently large time period. In this way, too, a corresponding master clock signal  21  is generated.  
         [0069]     In  FIG. 6C , the optical element is a stepped index fiber  29 . Within the stepped index fiber  29 , the laser signal  19  experiences dispersion. The laser pulse  16  is dispersed as a result, so that its width in time  17  changes accordingly. On the other side of the stepped index fiber  29 , a correspondingly lengthened light pulse  30  then exits. The length of the stepped index fiber  29  is chosen such that the width in time of the light pulse  30  is greater than the time constant of the photodiode  26 . In this way, a master clock signal  21  can be generated by the light pulses  30  striking the photodiode  26 .  
         [0070]     In  FIG. 6D , the laser signal  19  is achieved by means of broadening the laser pulses  16  over time by means of dispersion and spectral filtering. The laser signal  19  first passes through a first prism  31 , which splits the laser signal  19  into corresponding light signals  32 . These light signals  32  then pass through an optical filter  33 . This optical filter  33  then filters individual spectral components of the laser pulse  16  out of the laser signal  19 . In this way, laser signals  32 ′ which emerge from the filter  33  are obtained. The light signals  32 ′ have a smaller spectral distribution than the light signals  32 . They are then focused by a second prism  31  and result in a light pulse  30 . Because of the smaller spectral width, this light pulse has a greater width in time than the signal  19 . By means of suitable selection of the filters  33  or of the filter  33 , it is possible to achieve the situation wherein the width of the light signals  30  is sufficient to generate a master clock signal  21  by means of the photodiode  26 .  
         [0071]      FIGS. 7 and 8  show different alternatives of structures having elements for the synchronization of the AOM  3 . In each case, these are extracts from a corresponding plate exposer with QCW laser. These structures can in particular be provided in addition to the structures described for the synchronization of the video signal  4  and the rotational frequency and forward movement speed of the rotational prism  7 , as have been described in  FIG. 5 .  
         [0072]     In the structure sketched in  FIG. 5 , the laser signal  2  from the laser source  1  is modulated by a modulator AOM  3  in accordance with the video signal  4 , so that a modulated laser signal  5  accordingly leaves the AOM  3 . Here, the same reference numbers also signify the same elements as in the preceding figures.  
         [0073]     The laser signal  2  is deflected by the modulator (AOM)  3 . Depending on the video signal  4  present, the result is a modulated laser signal  5  or a second, deflected laser signal  39 . In terms of its structure, this laser signal  39  corresponds to a complementary laser signal to the modulated laser signal  5 . This laser signal  39  then enters a delay detection element  34 . This delay detection element generates signals  35  from the laser signal  39 . In terms of their frequency, these signals  35  correspond to the laser signals  39 , and their phase angle is caused by the delay, that is to say the “inertia” of the AOM with respect to the video signal  4 . The “inertia” of the AOM  3  is brought about here by propagation times of the signal in relation to the AOM  3 , by the limited speed of sound within the AOM  3 , by the spot size of the laser signal  33  and possibly further factors. Within the AOM controller  22 , the phase angle of the video signal  4  for driving the AOM  3  can then be compared with the signals  35 . In this way, the time offset between the driving of the AOM  3  via the video signal  4  and the deflection of the laser signal  2  into laser signals  39  actually carried out can be determined. This time offset, which has become known from the latter, can be used to drive the AOM  3  via the AOM controller  22 , so that, in particular in conjunction with the comparison with the master clock signal  21 , the pulses  16  from the laser signal  2  lie in the middle of the modulation window of the AOM  3 . Corresponding correct driving of the AOM  3  was illustrated in FIGS.  2  to  4 . Here, the maximum values  43  of the laser pulses  26  lie in the middle of the modulation window of the AOM  3 . In this way, the entire laser pulse  16  can be used for imaging the printing plate  11  without the laser signal being distorted by rising or falling flanks of the modulation window of the AOM  3 .  
         [0074]     The delay detection element  34  is illustrated dashed here, since, in terms of structure, it is constructed in such a way that a measurable synchronized signal  35  can be generated from the short laser pulses  16 . Its structure corresponds to that of the master clock generating device  20 ; in this regard, reference is made in particular to the possible alternative embodiments which have been described in  FIGS. 6   a  to  6   d.    
         [0075]     A similar structure to that in  FIG. 7  is shown in  FIG. 8  but here, in order to determine the time offset, that is to say the delay of the AOM  3  as a function of the video signals  4 , a second laser source  36  is used. This laser source  36  emits a continuous laser signal  37 . In this case, this laser signal  37  can in particular have a wavelength which is different from the wavelength of the laser signal  2 . For example, the laser signals  2  can have a wavelength of 355 nm while the wavelength of the laser signal  37  is 370 nm. The laser signal  37  is modulated in accordance with the video signal  4 , so that a modulated laser signal  38  and a correspondingly complementary deflected laser signal  40  are generated. Since the deflected laser signal  40  is a laser signal which is modulated but still continuous, the duration of the individual signal pulses of the modulated deflected laser signal  40  corresponds to the length in time of the modulation windows of the AOM  3  and thus substantially to the duration of the video signals  4 . The frequency of the deflected laser signal  40  is therefore at most 100 MHz in the example illustrated here. This corresponds to a shortest duration of a signal section of 10 ns of the deflected laser signal  40 . Light pulses of this order of magnitude can be detected without difficulty by a photodiode  42 . In this way, via the deflected laser signal  40 , which falls onto a photodiode  42 , a signal  35  can then be generated without difficulty. As described in relation to  FIG. 7 , this signal  35  can then be compared with the time waveforms of the video signal  4  within the AOM controller  22 . A corresponding time offset can be detected and taken into account appropriately when driving the AOM  3 . In order to avoid the laser signal  38  modulated by the AOM  3  being able to act in any way on the rotational prism  7  or on the 3-D imaging of the printing plate  11 , provision can be made for a filter  41  to be provided in the beam path of the modulated laser beams  38  and  5 . This optical filter is then designed in such a way that it filters out light of the wavelength of the laser signal  37  while allowing laser signals of the wavelength of the laser signal  2  through. In this way, the imaging of the printing plate  11  can readily be carried out.  
         [0076]     Via the generation of the master clock signal  21 , all the elements involved in the imaging of the printing plate  11  can be synchronized appropriately with the frequency of the laser signal  2 . This includes, in particular, the rotational frequency of the rotational prism  7 , the forward movement speed of the rotational prism  7  and the driving of the video signal  4  via the AOM controller  22 . Via the additional detection of the delay of the AOM  3 , as has been described in  FIGS. 7 and 8 , the time offset of the modulation window of the AOM  3  in relation to its actual driving can be detected and taken into account. In this way, moirés can be prevented in an ideal manner and the video signal  4  can drive the AOM  3  in such a way that the laser pulses  16  lie with their maximum values  23  in the middle of a modulation window of the AOM  3  in each case.  
         [0077]     In this way, it can beneficially be made possible for quasi continuous wave lasers with a wavelength of 355 nm to be available for the imaging of printing plates  11 . These quasi continuous wave lasers are substantially less expensive than conventional continuous UV lasers and can advantageously image the conventional UV-sensitive printing plates  11 .  
         [0078]     This application claims the priority, under 35 U.S.C. § 119, of German patent application No. 10 2005 019 308.0, filed Apr. 26, 2005; the entire disclosure of the prior application is herewith incorporated by reference.