FR2467078A1 - Laser beam producing printing blocks - synchronously scans and modulates for line and half tone structures using sawtooth pulse generator and comparator - Google Patents

Abstract

A measure device for the preparation of printing blocks consists of three cylinders on a common drive shaft. The first carries the master plate which is scanned by an optoelectronic reader, the second fixes the boundaries for the line and half-tone with the of the block, and the third carries the printing plate on which a laser beam etches the raster grid or line block structure via a beam modulator. The electronic circuit includes a synchronised sawtooth pulse generator, a comparator, and a life-half toner commutator.. This system is suitable for combined line and half-tone structures. It is simplified by the absence of all photo reproductive and photochemical processes and speeds up the technological process for block making.

Description

 The present invention relates to the technique of manufacturing and reproducing printing forms, and more particularly to laser devices for manufacturing forms for all kinds of printing, typography, offset, gravure printing. , and screen printing and can be applied in the graphic arts industry, in publishing houses and printing houses, for printing images and inscriptions on various articles such as, for example, glass objects, and in applied art.

 Currently, for the majority of printed products, that is to say, books, daily and weekly newspapers, printing is used in letterpress and offset printing. The difference between letterpress and offset printing processes is the special characteristics of the printing forms.

 As printing forms for typography we use massive and heavy metal castings requiring for their use complicated processes of electroplating, photochemistry and photomechanical reproduction, as well as the use of complicated equipment. In addition, the shapes must be further machined. As a result, printing becomes an expensive and laborious industrial process requiring significant manpower and material resources.

 Offset printing forms are much simpler, lighter and less expensive. These are thin plates of metal, plastic or paper with a special coating on which an image is formed which is then reproduced.

In offset printing the most complicated, expensive and laborious operation is the manufacture of printing forms. For this purpose we carry out multiple photographic reports requiring the use of reproduction devices and photomechanical accessories of a large quantity of chemicals, and other materials, including precious metals (silver).

 In view of these considerations, intense research is being carried out worldwide to find simpler methods for the manufacture of offset forms. Laser radiation opens up great possibilities in this direction.

The essential characteristics of laser radiation are the unequal ability to concentrate energy in a small volume, as well as the easy and inertial control of laser radiation in time and space. This allows the efficient use of laser radiation for precision machining of practically all metallic and non-metallic materials.

 It is particularly effective to process non-metallic materials by this process, since fairly compact and inexpensive lasers can be used. It is for this reason that immediately after the invention of lasers, research work was started for their use in the graphic arts industry.

There is known an assembly called "Laser Graph" which is intended for the use of laser radiation for the manufacture of typographic forms - on a plastic base instead of a metal base.
As material for the printing forms, a Lavsan film with a thin metallic coating is used. The assembly already known, comprising two independent laser installations is an operating and engraving device. The material of the. form is fixed in the operating and engraving device in which a high-quality journal page, printed by a usual process, is used as the original. The reading of the original is done line by line using a brush with a helium-neon laser. The laser brush reflected by the original carrying the information on the image is received in an optical reader amplifies and employed to modulate the radiation of an argon laser carrying out the simultaneous and synchronized exploration of the material of the form.

 The brush of the argon laser of 1.5 W of power causes the vaporization of the metallic coating on the printing plate in the non-printing locations and leaves the metallic coating in the other places by forming metallic printing elements and in this way carrying out the facsimile transfer of the original to the surface of the printing form. This shape is then placed in an engraving block in which, by means of a 400 W CO 2 laser, the non-printing elements are deepened to the required value by evaporation of the Lavsan base. The typographic form produced in six minutes is used in a press for printing newspapers. The use of this already known assembly makes it possible to reduce the production cycle of the printing forms and to increase the speed of output of the products, but because of its high price its use is limited.

 We give an apparatus called "Laserite" employed for the exposure of the slides of publications by means of a laser radiation with argon. Its use is effective in an automatic control system for publishing work with transmission of images of publications by telecommunication channels between editions and printing works. After layout at the editorial office, the publication is transmitted by telecommunication to the printing press in the input block of the Laserite device with simultaneous exposure to a photographic film which is consecutively used for the manufacture of printing forms by a usual photomechanical process.

 We also know a laser device for the production of "Log E" printing forms. The offset printing form and the slide are simultaneously obtained in this apparatus using the radiation of a garnet laser. The form material is a base coated with a plastic layer.

Laser radiation joins the plastic by soldering to the base at the locations of the printing elements, but does not act on the plastic at the locations of the non-printing elements. After this treatment, the plastic is separated from the base and can be used as a slide, while the base with the brazed printing elements serves as a printing form.

This device was built in a single copy and its performance is low.

A laser device for manufacturing printing forms which is closest to the laser device which is the subject of the invention is described below.

This device comprises an exploration device for the synchronized scanning of a mock-up serving as an original and of a printing plate, comprising an electrical control device rotating a scanning block made up of two cylinders arranged on a common axis. On one cylinder is fixed the original which can be, for example, a color slide, and on the other, a photosensitive film.

 This known device comprises an electro-optical reader block connected to the exploration device and a screening pulse generator.

In addition to the sets listed, this known laser device comprises an electronic circuit connected to the electro-optical reading unit and establishing the order in which the control signals for the parameters of the laser radiation are emitted. This circuit consists of a corrector, a comparator with a pulse shaper connected to the screening pulse generator; dt an analog-digital converter, a control computer and output amplifiers.

In addition, this laser device is equipped with a laser optical unit for forming the images, comprising a laser, optical modulators with optical amplifiers linked to the inputs of these modulators and optical units for directing and focusing the laser radiation on the surface. to explore.

 After laser and photochemical treatment, the photosensitive film becomes a color selection slide.

The selector block is designed for the formation of halftone images which are reproduced in the printing by separation into elements whose dimensions must be proportional to the optical density of the parts of the original to be reproduced.

This operation is called "screening" and the structure of the image obtained is called "a screened structure".

 A screen structure must have a strictly determined arrangement of the grid elements. When reproducing a monochrome image the screen elements are usually arranged at an angle of 45o, and in color printing, the screen elements of each color are arranged at different angles to the generated line of the cylinder, for example at angles of 15, 30 and 45o. For the screening of halftone images, in the known laser device is provided the development of a series of screening pulses located in a determined manner in time and space relative to the surface of the original and the print form.

 The framing pulses are generated by the framing pulse generator reading the succession of streaks (contrast marks) on the surface of one of the cylinders. The line-by-line reading of the image of the original is carried out by the electro-optical reading unit emitting an analog signal whose value is proportional to the optical density of the location of the original where the reading is made.

The series of framing pulses and the analog signal are sent in said electronic circuit. The bell-shaped screening pulses produced by the generator are transformed into rectangular pulses in the pulse shaper. The analog signal is received by the corrector where it is converted to correct the defects of the original and to ensure the reproduction of a combination of typographic colors which gives a natural resemblance to the reproduction of color images. After the corrector, the signal analog passes through the comparator in which the series of framing pulses sent from the pulse shaper is sent. The comparison of these two signals gives the comparator output a series of framing pulses whose duration is proportional to the density optics of the original when the pulse passes. These pulses, which are called information pulses on the frame, are sent first to the analog-to-digital converter and then to the address register of the computer.

Code information about the original image is passed from the address register to computer memory. In the computer are entered data determining the form of the screening elements, in the case considered rhombic form.

In accordance with these data, a train of pulses is formed in the memory of the computer, the number and duration of which depend on the optical density of the original in the part subjected to reading, the size of which is determined by the chosen line size. The developed pulse train is directed into a computer output register and then into a computer operation control block into which is also brought the synchronization framing pulse train determining the output rate of the pulse packets in the group of parallel output amplifiers. The amplified pulse packets are directed from the outputs of the amplifiers to the electronic inputs of the optical modulators, controlling the radiation intensity in the channels of the optical laser system. This system comprises an argon laser operating at a wavelength of approximately 0.5) 1 to which the photographic film has a high sensitivity.

To expose this film to light, with a linear speed of exploration equal to several meters per second, it is sufficient that the power of the laser radiation is of the order of a few tens of miîliwatts. For this reason, the normal operation of a multi-channel laser device is ensured by a laser with a power of several Watts. The laser brush is directed onto a series of semitransparent optical elements which branch out in five ways. Each channel comprises an optical modulator, an analyzer, a focusing objective and an optical wedge for the precise adjustment of the intensity of laser radiation in the channels, by means of which the power of the laser radiation is equalized in all the channels.

 The laser tracks at the predetermined times by the screening pulse generator simultaneously engrave on the photographic material a group of lines forming the screen elements of programmed shape. The surface of the screen elements depends on the optical density of the part of the original read, which makes it possible to reproduce on the phototechnical film good quality halftone images.

 The already known laser devices are characterized by the manufacture of a slide on a film, which is then used to obtain printing forms by the usual photomechanical process.

 As original in the known devices, high quality publications prints obtained by the usual offset or typographic processes are used.

 We will examine the manufacturing process of offset printing forms currently used. The print-ready copy is usually divided into typed text, illustration inscriptions, graphic illustrations and half-tone illustrations (photos). The dacrylographed text and the inscriptions on the illustrations are composed and then in the form of slides or page prints are sent for the assembly of the first model.

After approval of the model by the editor it is dismantled. The line illustrations are mounted again and the halftone illustrations are subject to screening, after which a second model is made which is the final copy for publication. From this model, we obtain a slide at the scale required for publication. The material of the form is covered with a photosensitive layer on which the image of the slide is impressed, which is then tanned and retouched. The finished printing plate is used in a press for reproduction on paper or for printing a print.

 As follows from the description of the laser devices already known and from the offset printing process, all the laser devices used for the manufacture of the printing forms perform only one of the operations mentioned above. The use of known laser devices makes it possible to speed up the execution of certain operations, but does not bring improvements in the process for manufacturing printing forms.

 The argon or helium-neon lasers used in the known devices have a low efficiency. Great possibilities arise for the use of C02 lasers, whose efficiency is 100 times higher. However, pumped CO2 lasers are bulky and complicated to operate, and sealed CO2 lasers are unreliable due to the lack of good quality optical materials.

 The invention aims to provide a laser device for the manufacture of printing forms the assembly of a new design would improve the manufacturing process of printing forms and to obtain during a technological cycle images with lines and dots, to simplify the apparatus by eliminating all the photomechanical and photochemical reproduction operations, to reduce considerably the technological cycle and to obtain a greater reliability of the laser device thanks to the use of a laser with C 2 sealed with a polarizing element of new design.

 The subject of the invention is therefore a laser device for manufacturing printing forms comprising a system for synchronized exploration of a model and of the material of the form, an electro-optical reading unit, a pulse generator. of frame optically connected to the system for synchronized exploration of the model and the material of the form, an electronic circuit for producing a series of control signals for controlling the parameters of the laser radiation linked to the electro-optical reader unit and to the generator. framing pulses and comprising a corrector connected to the electro-optical reading unit, a comparator having its first input connected to the output of the corrector, a conformator having its input connected to the output of the framing pulse generator, --of which the input is electrically connected to the output of the shaper and the output of which is connected to the second input of the comparator, an output amplifier having its input connected electrically ment at the output of the comparator, as well as an optical laser system for forming the images on the surface of the form, said system comprising a laser, an optical modulator optically linked to the laser and having its input connected to an output amplifier , a rotary mirror placed between the laser and the optical modulator and an optical sub-assembly for directing and focusing the laser radiation on the surface to be explored of the form arranged between the optical modulator and the form, device characterized in that said electronic circuit includes a line image channel for controlling laser radiation during the formation of line images, which is paralleled with a dot image channel for controlling laser radiation at the formation of point images, a switch line-to-line picture-to-line channels, for automatically switching from the line-to-line image to the line-to-image channel, said switch having first and second inputs connected es at the outputs of the line image and dot image channels, and being kinematically connected to the synchronized exploration system of the model and of the shape, a block for fixing the limits of the halftone images to form the commands going to the switch image to dot-line image channels, the output of the block for fixing the limits of the halftone images being connected to a third input of this switch whose output is connected to the input of an output amplifier, a laser modulator being used as an optical modulator.

 It is advantageous that the laser device for the manufacture of the printing forms comprises put in series: a generator of the periods of movement of the system of synchronized exploration of the model and of the form optically connected to the exploration system, a counter periods of movement of the form and a staircase voltage generator having its output connected to the first input of the comparator, a phase switch, a phase shifting circuit whose input is associated with the first input of the phase switch and connected to the output of the shaper, the output of which is connected to a second input of the phase switch, the third input of which is connected to a second output of the shape displacement period counter, which is connected to a second input of the voltage generator staircase, tan say that the output of the phase switch is linked to the input of a synchronized sawtooth pulse generator.

 The phase shift circuit can be implemented in the form of an inverter.

It is preferable that the laser is provided with an optical resonator consisting of an output mirror, a fully reflecting spherical mirror and a polarizing element as a polarizing element. It is possible to use a totally reflecting plane mirror whose useful surface is arranged at an angle of 45 + 15 relative to the optical axis of the laser. The plane of incidence of the laser beam on the fully reflecting plane mirror of the optical resonator and the plane of incidence of the laser beam on the rotating mirror must coincide with the plane of incidence of the laser beam on the laser beam modulator, and the transmission factor of the output mirror of the laser optical resonator must exceed 50%.

To obtain in a single technological cycle a dot image and a line image without deformation on the shape, it is likewise preferable to direct the laser beam of the modulator within the limits of 3 to 6 compared to the normal line. on the surface of the form at the point of impact of the laser beam on the surface of the form.

 As a laser beam modulator, an electro-optical modulator or an acousto-optical modulator can be used.

Thanks to the use of the laser device for the manufacture of printing forms, according to the invention, it is possible to eliminate laborious and complicated operations from the technological process. The duration of the technological cycle is reduced by approximately one order. Use of this device for the production of printing forms in printing works makes it possible to reduce the number of machines, workers, occupied surfaces and service personnel.

To give a clearer idea of the invention, an embodiment of the invention is described below, the description being given with reference to the appended drawings in which
- Fig. 1 is the technological diagram of the offset printing process carried out by means of a laser device for the manufacture of printing forms, according to the invention
- Fig. 2 is the block diagram of the laser device for manufacturing the printing forms according to the invention
- Fig. 3 is a diagrammatic illustration of the laser which in the case considered is a laser sealed at C02, according to the invention
- Figs. 4A to 4C represent diagrams explaining the formation of a dot image, according to the invention
- Figs. 5L and 5T respectively give fragmentary views of a real dot image: L, in the lights, T in the blacks, according to the invention.

 By means of the laser device which is the subject of the invention, printing forms are produced, for example, offset forms during a single technological cycle, from an absolutely simple model comprising the text, the line images and halftone halftone images.

The technological cycle includes all of the operations for manufacturing the printing forms, that is to say, reading the model, forming the definite image and obtaining this image on the surface of the printing form does not require any treatment before printing printing.

The diagram of the operations consecutively carried out in the laser device for the production of offset printing forms is illustrated in FIG. 1.

Copy 1 and inscriptions 2 of illustrations are directed from composers 3 and illustrations 4 are scaled by means of a scaling block 5. The composition obtained from text 1 and inscriptions 2 illustrations, as well as illustrations-4 to the necessary scale are placed on an assembly table 6 where the model is made, which in the finished state includes the composition pages, the titles, the thumbnails, as well as the illustrations line and halftone halftone. They are glued to a paper or film support, or fixed to a laser device 7 for the manufacture of the printing forms. From this simple model, in the laser device 7, a printing form is produced with automatic screening of images, which without requiring additional processing can be used in 8 presses to print the print.

The laser device for manufacturing print forms comprises a system for synchronized exploration of the model and the form, said system comprising an electrical control device 9 (FIG. 2), the shaft 10 of which carries cylinders of work 11 and 12. The cylinder 11 on which the model 13 is fixed carries a series of lines or contrast mark 14 and the material for the printing form 15 is fixed on the cylinder 12.

 The laser device likewise comprises an electro-optical reader block 16 which is optically connected to a system for synchronized exploration of the model and the shape, a generator 17 of screening pulses, an electronic circuit for producing a series of control signals for controlling the parameters of the laser radiation, comprising a corrector 18, an input 19 of which is connected to the output of the electro-optical reading unit 16, a synchronous sawtooth pulse generator 20, a comparator 21 of which an input 22 is connected to the output of the corrector 18 and an input 23 of which is connected to the output of the synchronous sawtooth pulse generator 20. The corrector 18 and the comparator 21 form a weft path ensuring radiation control laser when forming halftone images.

The electronic circuit further comprises a comparator 24, the input 25 of which is connected to the output of the electro-optical reading unit 16. The comparator 24 is the line channel ensuring the control of the laser radiation at the formation of the line images.

 The electronic circuit likewise comprises a shaper 26, an input of which -27 is connected to the output of the framing pulse generator 17, a switch 28 of the dot image - line image channels for automatically switching the line image channel to the line dot image and obtain on the material of form 15, in the course of a single technological cycle, line and dot images, and having its inputs 29, 30 connected to the outputs of the dot and d image channels line image.

The electronic circuit likewise comprises a block 31 for fixing the limits of the halftone images which forms the commands applied to the switch of the dot image and line image channels and which consists of a programmer cylinder 32 mounted on the axis 10 and an electro-optical sensor 33 which is optically connected to contrast plates 34 fixed on the cylinder 32. The output of the electro-optical sensor 33 is connected to the input 35 of the switch 28 of the channels dot image and line image.

 The electronic circuit also includes an output amplifier 36, for example, a transistor 37. In the collector circuit of transistor 37 is inserted a resistor 38 having a terminal connected to a power supply 39. The base of transistor 37 is connected at the switch output of the dot image and line image channels.

 The laser device for manufacturing print forms comprises a laser optical system for forming the image on the surface of the form 15 comprising a laser 40 connected to a power supply unit 41 which is a stabilized current source, a modulator 42 of the laser beam optically connected, by means of a rotating mirror 43 to the laser 40 and an optical unit 44 for directing and focusing the laser radiation on the surface to be explored in the form 15. At the entrance 45 of the modulator 42 of the laser beam is connected to the output of the amplifier 36.

 As modulator 42 of the laser beam, an electro-optical modulator or an acousto-optical modulator can be used.

The laser device for manufacturing printing forms likewise comprises in series: a sensor 46 of the periods of movement of the system for synchronized exploration of the model and of the form, which is for example optically connected to a contrast mark 47 on the working cylinder 12, a counter 48 for the periods of movement of the form 15 and a staircase voltage generator 49, the output of which is connected to the input 22 of the comparator 21.

In addition, the laser device comprises a phase switch 50, a phase shift circuit 51, an input 52 of which is connected to an input 53 of the phase switch 50 and is connected to the output of the pulse trainer 26. The output of the phase shift 51 is connected to an input 54 of the phase switch 50, an input 55 of which is connected to a second output of the counter 48 connected to input 56 of the staircase voltage generator 49. The output of the phase switch 50 is connected to input 57 of the synchronous sawtooth pulse generator.

 The phase shift circuit 51 can be produced in the form of an inverter.

In FIG. 3 is given the diagram of the laser 40.

 Inside an gas discharge tube 58 are mounted an anode 59, a cathode 60 and a polarizing element which is a totally reflecting plane mirror 61, the working surface of which is arranged at an angle of 45 + 15 relative to the optical axis 52 of the laser. The laser likewise comprises a fully reflecting spherical mirror 63 and an exit mirror 64 forming together with the fully reflecting plane mirror 61 the optical resonator of the laser 40.

 The laser device for manufacturing the print forms operates as follows.

The electrical control device 9 (FIG. 2) rotates the axis 10 on which the working cylinders 11 and 12 are rigidly wedged. The information contained in the model 13 containing the text and the illustrations in line and in half - non-screened shades (photos) is read by lines by means of the electro-optical reading unit 16 which is moved along the working cylinder 11 by means of an autonomous stepping motor (not shown in the drawing). At the output of the electro-optical reading unit 16, an analog electric signal is formed, the value of which is proportional to the optical density at the location of the model 13 where the reading is made. The analog signal passes from the output of the electro-optical reader block 16 in two channels: the line image channel and the dot image channel.

 The signal received by the line image channel passes through comparator 24 with an adjustable threshold voltage. By choosing the level of the threshold voltage we can eliminate the reading of different erasures on. the line part of the model 13: the traces of glue, the contours of the parts of the image, the difference in tones in these parts.

 The useful analog signal is transmitted from the output of the comparator 24 to the output 30 of the switch from the dot image to the line image.

The signal received in the dot image channel passes through the corrector 18 by which the quality of the print form is made independent of the imperfections of the halftone images of the model 13, and it is then directed to the input 22 of comparator 21, and finally at input 29 of the dots image switch - line image.

 Simultaneously the succession of contrast reference lines 14 is read by the screening pulse sensor 17 which establishes the succession in which the screening pulses are emitted in time and in space in a manner determined with respect to the surface of the working rolls 11 and 12. The number of screening pulses is equal to the number of weft elements on the material of the shape 15 determining the line size of the weft of the halftone image.

 The screening pulse train is directed to the input 27 of the pulse shaper 26 and then to the phase switch 50 and to the phase shift circuit 51.

The phase switch 50 and the phase shift circuit 51 are intended to form the screened structure of the halftone images.

 Then, the series framing pulses coming from the output of the phase switch 50 pass through the synchronous sawtooth pulse generator 20 and into the comparator 21.

 As a result of the comparison of the sawtooth pulses 65 (FIG. 4A to 4C) with an analog signal 66 serving as the threshold level, a series of information pulses is formed in the comparator 21 (FIG. 2) frame whose duration is proportional to the optical density at the point read on the model 13. The information transmitted by the line image and dot image channels is received by the switch 28 from the dot image to the line image and by means of this switch, one of these channels is automatically connected to the output amplifier 36. The command for switching the channels is received from block 31 for fixing the limits of the halftone images .On the programmer cylinder 32 of this block 31 are fixed contrast plates 34 whose arrangement and shape correspond to those of the halftone images on the model 13.

 The limits of the contrast plates 34 are read by means of the electro-optical sensor 33. The pulse generated by the sensor 33 corresponding to the crossing of the border of the halftone image is directed to the switch 28 in control pulse quality for switching from the line image line to the dot image channel. By means of block 31 it is likewise possible to transmit the parts of the model 13 at different speeds, such as for example, negative-positive, in color. The block 31 can also be used for the selection of the scale of different parts of the model 13.

 The image is impressed on the material of the form 15 by means of the laser beam which is focused on the surface of the form 15 by means of the optical unit 44. The displacement of the optical unit 44 is likewise carried out by means of a motor step by step (not shown in the drawing).

 The beam is generated by the laser 40 which is a sealed CO 2 laser, working at a wavelength of 10.6} and giving a beam with a power of about 30 W in continuous regime. This beam is directed by means of a rotating mirror 43 onto the laser beam modulator 42, controlling the intensity of the beam, and then onto the optical unit 44. The intensity of the laser beam is modified by changing the parameters of the electric signal received by the laser beam modulator 42.

The image impressed during a technological cycle on the material of the form 15 is the reproduction of the model 13 containing, for example, text and illustrations in line and in halftone non-halftone (photos).

 To reproduce, for example, the text of the model 13, in the block 31, a control signal is generated which is transmitted to the input 35 of the switch 28. The line image channel is connected to the amplifier 36 which receives on its input an analog signal whose value is proportional to the optical density at the point of the model 13 where the reading takes place. At the output of the amplifier 36, a signal is formed which, via the modulator 42, controls the intensity of the laser radiation.

The modulated laser beam is focused by means of the optical unit 44 into a spot with a diameter of 40 microns on the surface of the 1S form. The diameter of the laser beam and the displacement pitch of the optical unit 44 are chosen so as to reproduce without deformation a line with a minimum width of 0.1 mm. To reproduce such a trait without deformation it must be impressed for at least three revolutions of the working cylinders. Taking this fact into account, the weft linearity for the shape is chosen equal to 300 lines per cm.

In this way, the text and line illustrations are reproduced in facsimile.

 Under the action of a corresponding control signal emitted from block 31, the dot image channel is connected to amplifier 36 and a train of information pulses on the frame is received at the input of this amplifier. At its output there appears a signal which by means of the laser beam modulator 42 controls the intensity of the laser radiation.

The half-tone raster images are likewise formed by the combination of lines impressed by the laser beam on the material of the form 15. However, these lines are not a facsimile reproduction of the images of the model 13.

The grid elements are artificially formed by impressing the lines sequentially. The width of the frame elements is determined by the number of lines, and the height, by the duration of the information pulses on the frame generated by the output amplifier 36 and controlling the laser beam modulator 42. The dimensions raster elements determine the linearity of the halftone halftone image. For this reason, in the laser device for the manufacture of printing forms the lineature is modified by changing the number of lines of the registered line elements and by modifying the pitch of the contrast lines 14 on the cylinder 11 which are read by the sensor 17.

The pattern for forming the grid elements, for example, with a line size of -40 lines / cm, is shown in
Fig. 4A to 4C. With reference to FIG. 4A is described the formation of a halftone structure when reproducing a gray tone. The required shape of the frame control pulses is acquired at the output of the comparator 21 (FIG. 2). At its input 23 is received a sawtooth pulse train 65 (Fig.4A), the frequency of which is predetermined by the frame pulse sensor 17 (fige2), in accordance with the coordinates and the transmission time of these pulses from the surface of the cylinders 11 and 12 which are rigidly linked together. The phase of these pulses is kept constant while the elements of the frame over their entire length are impressed line by line. In accordance with the chosen linearity, that is to say, 300 1 / cm, the total width of the elements of weft with a linearity of 40 lines / cm measured along a diagonal of the outline of the image is impressed during five revolutions of the working cylinders 11 and 12.

The time diagrams corresponding to five revolutions of working cylinders 11 and 12 are designated by a II III Iv goes, a, a and a or I, II, III, IV and V are the number of revolutions of cylinders ll and 12 (Fig .2).

"a" designates the group of revolutions of the cylinders 11 and 12 during which 11 weft element is formed. The groups of consecutive revolutions to form the following grid elements are designated by the letters b and c.

The combination of the sawtooth signal with the stepped signal designated by the corresponding number I, II,
III, IV and or V is maintained during a complete revolution of the cylinder 12 (Fig. 2).

The count of revolutions is carried out by the counter 48 of the displacement periods which receives an impulse at each revolution of the cylinder 12 coming from the sensor 46. After five revolutions, at the output of the counter 48 is generated an impulse sent to the input 55 of the phase switch 50
In the presence of this signal, from the output of the phase inverter 50 to the input 57 of the generator 20 is applied a train of screening pulses which are offset by half a period in comparison with the frame pulses of the previous group of revolutions of the cylinders 11, 12, said pulse train being applied to the input 54 of the phase switch 50 from the output of the phase shift circuit 510 As a result of the action of the phase switch 50, the phase of the information pulses on the frame is offset by a half-period of repetition of these pulses and the laser beam begins to impress the series of weft elements in the next line, starting with the Sixth revolution of the cylinder work 12.

 In FIG. 4A, the time diagrams of the weft elements which correspond to these revolutions of the working cylinder 12 are designated by b1, bII, b111, bIV, bV. Cleared from revolutions of cylinder 12 (Fig. 2) the return of the phases of the frame pulses to the initial state is carried out in the same way and starting with the eleventh revolution of working cylinder 12, the laser beam begins to impress the I succession of elements Weft I III IV V third succession of weft elements c, c, c, c V, c after which the process is repeated.

 When reproducing the black and white illustrations, the screen elements must be arranged at an angle of 450 relative to the generator of cylinder 12 (Fig. 2).

To this end, the phase of the framing pulses determined by the phase shift circuit 51 is shifted to 1800; as a phase shift circuit 51, an inverter can be used.

The series of sawtooth pulses applied to the input of comparator 21 is compared with a reference voltage; as a reference voltage, an analog signal from the corrector 18 can be used. When reproducing the "gray" tone, the value of the analog signal 66 '(Fig.

4A) is chosen equal to half the amplitude of the sawtooth pulses 65.

 To form the weft elements of ~ triangular-trapezoidal shape enabling a better quality halftone image to be obtained, the step voltage is added to the analog signal at the input of the comparator 21. The step voltage is formed by the generator 49 by adding the value of the voltage fixed in the counter 48 to the previous voltage value. After five revolutions (for a line size of 40 lines / cm) under the control of the counter 48 the total voltage is reset to zero and the process is repeated. As a result of such an addition, the analog signal 66 obtained at the input 22 (Fig. 2) of the comparator 21 also has a periodically stepped shape.

 The addition of the sawtooth pulses 65 to the analog step signal 66 makes it possible to obtain at the output of the comparator 21 (FIG. 2) a series of pulses, the duration of which determined by the points of intersection of the compared signals. changes with each revolution of cylinders 11 and 12, the tonality of the halftone image being maintained without variations.

During the first revolution there is an impression of the longest lines and during the fifth revolution there is an impression of the shortest lines.

 Thanks to the use of the laser device according to the invention, when the tone of the original image changes, not only the length bes drawn in the succession of the frame elements, but also their number is subject to a change. For example, in the case of recording a light gray tone (see Fig. 4B) only two lines are impressed during revolutions I and II of cylinders 11, 12 within the limits of the screen element. the reproduction of the lightest tone (see Fig.4C) only a short line is impressed during revolution I and on the darkest tone (Fig.4D) on the last line impressed during revolution V only ruptures remain minimum length.

 An example of a halftone structure obtained on the printing form is illustrated in FIGS. 5L and 5T with an enlargement of 16 times. The grid elements have a triangular-trapezoidal shape. In this case, the minimum dimension of the screen element “in the lights” is of the order of 20 microns (FIG. 5L) and in the “blacks” of 50 microns (FIG.

ST).

 In the case where the laser radiation is linearly polarized, it is controlled by means of a modulator and for this reason the laser 40s used in the given device, ensures the linear polarization of the radiation.

The laser device for manufacturing the printing forms makes it possible to obtain them directly and to eliminate the intermediate operations. For this purpose, the laser beam used to impress the form must be of sufficient power.

 The process for manufacturing an offset printing form with an engraving speed of around 200 m / min, guaranteeing high technical and economic efficiency of the device forming the subject of the invention, is carried out in practice by means of a sealed CO2 laser, giving a linearly polarized beam with a power of 30 W. This laser is the most economical, simple to use and to manufacture.

However, the optical element used in a laser
Modern C02 to ensure linear polarization of the radiation is unreliable. As an optical element, an optical polished plate of NaCl or KC1 is used, which is arranged at a Brewster angle relative to the optical axis of the laser. This plate is glued to the end face of the gas discharge tube and is part of the vacuum-tight envelope of this tube. Given the properties of these materials, this joint cannot be made by welding or brazing, and the known adhesives give off a complex gas mixture causing the rapid degradation of the mixture of C02 gases from the laser so that the laser can be put out. service in a time impossible to control.

 In order to increase the reliability of the laser at C02, a totally reflecting plane metallic mirror 61 -Fig.3) is used as the polarizing element forming part of the optical resonator of the laser 40. The working surface of the mirror 61 is at an angle by 45 + 15 with respect to the optical axis 62 of the laser 40. The design of the optical system ensures the propagation of the laser beam in a single plane, that is to say that the plane of incidence of the laser beam on the plane mirror totally reflecting 61 of the optical resonator and the plane of incidence of the laser beam on the rotating mirror 43 (FIG. 2) are in coincidence with the plane of incidence of the laser beam on the. modulator 42.

 The degree of polarization of the radiation depends not only on the properties of the polarizing element, but also on the number of reflections of the laser beam in the optical resonator of the laser 40 (FIG. 3) which in turn depends on the reflection coefficient of the output mirror. 64.

To increase the degree of polarization of the radiation, the number of reflections of the laser beam must be increased.

With a reflection coefficient of the output mirror 64 equal to 35 MB, the degree of linear polarization of the laser radiation provided by the totally reflecting plane mirror 61 arranged obliquely is equal to 95%, which is insufficient to obtain good engraving of the image by the laser beam. When the reflection coefficient of the output mirror 64 is greater than 50%, the degree of polarization becomes greater than 98 60, a value sufficient to ensure correct operation of the laser beam modulator 42.

 Usually, more or less of the laser radiation is reflected by the material of the grave form by the laser beam (Fig. 2). The reflected radiation returns to the optical resonator of the laser 40 causing a deformation of the characteristics of the radiation generated by the laser 40, an instability of the power level and variations in the character of the polarization.

To eliminate this uncontrollable effect, it is necessary to direct the laser beam leaving the laser modulator 42 at an angle of 3 to 6 relative to the line normally drawn on the surface of the form 15 at the point of impact of the laser beam on the surface. material of the form 15.

Thus, the laser device according to the invention ensures the possibility of manufacturing printing forms (mainly, for offset) containing halftone and line illustrations and eliminating the complicated and laborious operations of the printing process. manufacture of printing forms.

Provided an appropriate choice of materials is used for the printing forms, the laser engraver described can also be used for the manufacture of the forms for other kinds of printing, that is to say, for typography, rotogravure and serigraphy. It is possible to use this laser device as an output device in automated assemblies for editing work with transmission of information via communication channels.

Claims (7)

1 - Laser device for manufacturing printing forms comprising a system for synchronized exploration of a model and of the material of the form, an electro-optical reading block, a screening pulse generator optically connected to the synchronized exploration system of the model and the material of the form, an electronic circuit connected to the electro-optical reader unit and to the screening pulse generator and intended to produce a series of control signals to control the parameters of the laser radiation, said circuit being consisting of a corrector connected to the electro-optical reading unit, a comparator, a first input of which is connected to the output of the screening pulse generator, a synchronized sawtooth pulse generator, the input is electrically connected to the output of a pulse shaper while its output is connected to the second input of the comparator, an output amplifier whose input is connected electrically at the output of the comparator and likewise comprising an optical laser system for forming the images on the surface of the material of the form, said laser system consisting of a laser, of an optical modulator, which is optically connected laser, with an output amplifier coupled to its between, a rotating mirror arranged between the laser and the optical modulator and an optical unit for directing and focusing the laser radiation on the surface to be explored in the form, arranged between the optical modulator and the shape, characterized in that said electronic circuit comprises a line image channel for controlling the laser radiation during the formation of line images, which is put in parallel with a point image channel intended to control laser radiation during dot image formation, a dot image and line image channel switch to automatically switch the line image channel to the dot image channel and don t the first and second inputs are linked to the outputs of the line image and dot image channels, a block for fixing the limits of the halftone images, to form the control signals seen in the channel switch dot image and line image, the output of the block for fixing the limits of the halftone images being connected to a third input of this switch whose output is connected to the input of the output amplifier, a laser modulator being used as an optical modulator.  2 - Laser device according to claim 1, characterized in that it comprises in series, a sensor of the periods of movement of the synchronized exploration system of the model and of the shape, optically connected to the exploration system, a counter periods of movement of the form and a staircase voltage generator whose output is connected to the first input of the comparator, a phase switch, a phase shifting circuit whose input is joined to a first between the phase switch and linked at the output of the impulse shaper and whose output is connected to a second input of the phase switch, of which a third input is connected to a second output of the displacement period counter of the shape which is connected to a second input of the generator of staircase voltage, the output of the phase switch being connected to the input of the synchronized sawtooth pulse generator.  3 - Laser device according to claim 2, ca characterized in that the phase shifting circuit can be realized in the form of an inverter.  4 - Laser device according to claims 1 to 3, characterized in that the laser comprises an optical resonator consisting of an output mirror, a fully reflecting spherical mirror and a polarizing element produced in the form of a totally reflecting plane mirror whose working surface is arranged at an angle of 45 + 15u with respect to the optical axis of the laser, the plane of incidence of the laser beam on the totally reflecting plane mirror of the optical resonator and the plane of incidence of the laser beam on the rotating mirror coinciding with the plane of incidence of the laser beam on the laser beam modulator and the reflection factor of the output mirror of the optical resonator of the laser being greater than 50 S.  5 - Laser device according to claim 4, characterized in that the laser beam is directed from the output of the modulator at an angle between 3 and 6 relative to the line normal to the surface of the material of the shape passing through the point contact of the laser beam with the surface of the form.  6 - Laser device according to claim 5, characterized in that the laser beam modulator is produced in the form of an electro-optical modulator.  7 - Laser device according to claim 5, characterized in that the laser beam modulator is produced in the form of an acousto-optical modulator.