FR2543395A1 - METHOD FOR RECORDING AN IMAGE - Google Patents

Abstract

The invention relates to a method for recording an image by scanning a photosensitive layer, by means of a beam of light, the intensity of which is modulated according to two predetermined levels and as a function of an image signal taken. 'A MEMORY. A PORTION OF THE DEFLECTED BEAM IS DERIVED BY MEANS OF A SEMI-TRANSPARENT MIRROR TO PRODUCE A POSITION SIGNAL. THE LIGHT BEAM IS MODULATED BY THE IMAGE SIGNAL IN SYNCHRONIZATION WITH THE POSITION SIGNAL, SO THAT THE MINIMUM INTENSITY OF THE MODULATED BEAM IS HELD AT A LEVEL TOO LOW TO IMPRESS THE PHOTOSENSITIVE MATERIAL. PREFERREDLY, THE POSITION SIGNALS ARE AMPLIFIED OR EXPECTED SO AS TO ALWAYS PRESENT THE SAME LEVEL TO FACILITATE THEIR PROCESSING. THE PROCESS CAN BE USED IN PARTICULAR FOR THE MANUFACTURE OF PRINTED CIRCUITS, AVOIDING ANY DIMENSIONAL ALTERATION.

Description

METHOD FOR RECORDING AN IMAGE

  The present invention relates to a method of recording an image, in which a light beam from a light source is deflected, and a recording surface coated with a layer is scanned with this deflected light beam. of photosensitive material. Such a method is primarily concerned with recording an image such as that of a wiring diagram or the like by printing the image on a support material coated with a film of photosensitive material, for example on a card Designed to be used in the manufacture of a printed circuit, the recording is made according to image input signals avoiding any dimensional alteration. PCBs can be formed on insulating boards, specifically designed for the manufacture of printed boards, either by contact exposure or by direct exposure. In the contact exposure method, a board carrying a sensitive film applied on its surface is exposed to light with a mask, in which a pattern is preformed in negative or positive, and which is maintained in contact with the sensitive film The card thus exposed is then developed to make the diagram of the circuits. However, the method of exposure by contact has many disadvantages. It takes several hours of work to prepare a mask from the desired image or pattern, for example by means of a coodinatograph or a similar device; on the other hand, dimensional changes are observed in the wiring diagrams formed on an insulating card, when such a mask undergoes a narrowing or an expansion and thus causes distortions by variation of the temperature and / or the humidity. When such a size error occurs, there are difficulties later, when piercing the printed circuit board, by the fact that the positions of some of the holes are offset with respect to the printed circuit. Unlike the conventional contact exposure method mentioned

  above, the direct exposure method forms an image or scheme in the following manner, without the use of a mask. In the present case, the two-dimensional image is stored as digital image signals in an image. The image data thus stored are then read and transmitted in the form of image signals. An insulating card, designed for the manufacture of a printed circuit, is then scanned by light which has been subjected to Prior to the effect of the image signals, by deflecting the light beam while advancing the insulating board according to a predetermined program. This method will be described later in more detail in reference to FIG. However, it makes it very difficult to obtain a good dimensional accuracy in the scanning direction because of the high accuracy required for the optical devices used. To overcome this disadvantage, the method has been perfected by superimposing on the light beam which impresses the sensitive layer a second beam of a different wavelength which has no effect on the sensitive layer. This second beam is used for detecting the position of the beam bent during scanning and for controlling, by a feedback circuit,

  The transmission of the image signals as a function of said position. Such a method 20 is described in more detail below with reference to FIG. 2, as well as certain disadvantages inherent to the apparatus it requires. the superposition of two beams of different wavelengths causes chromatic aberration phenomena that can only be overcome with difficulty and in a costly manner. Moreover, the perfect orientation of the two beams 25 along a common axis is very difficult to obtain. . Therefore, the object of the present invention is to provide a method of recording an image for improving the accuracy of each recorded image or pattern, as well as lowering the cost of manufacturing a printing device. recording images using this method, in comparison with the method according to the prior art illustrated in Figure 2, through the use of a single light beam. For this purpose, the method according to the invention is characterized in that a portion of the deflected light beam is derived by means of a semitransparent mirror and is sent to optical detection means of the position of deflection, and in that we modulate the light beam from the light source

  By means of an image signal delivered in synchronism with a position signal produced by said detecting means from said portion of the deflected light beam, so that the light intensity The minimum of the modulated light beam is maintained continuously at a level too low to impress sensibly the photosensitive material. According to a first form of the method, optical deflection position detection means are used which comprise an optical sensor and an elongated grid-like scale arranged in the conjugate position of an image point of the surface of the surface. relative to the semi-transparent mirror, this image point being the point of the recording surface which is exposed to the deflected light beam. On the other hand, it is possible to use optical detection means of the deflection position constituted by a row of photoelectric sensors, arranged in a linear form in a plane swept by said portion of the deflected light beam.

  According to another embodiment of the method, the level of the position signal produced by the detection means is changed to a predetermined level, depending on the level of the image signal which controls the intensity of the light beam from In the output of the optical detection means of the deflection position, it is possible, in particular, to have means arranged for modifying an amplification factor of the position signal depending on whether the level of the image signal is low. Alternatively, at the output of the optical detection means of the deflection position, there may be provided means arranged to amplify the position signal when the level of the image signal is low and to attenuate the signal. position when the image signal level is high. The various aspects of the present invention, as well as its features and advantages, will be better understood from the following detailed description of the methods according to the prior art and the method according to the invention with reference to FIG. 1 is a block diagram illustrating an example of a traditional direct exposure device for the manufacture of printed circuits. FIG. 2 is a block diagram of a conventional direct exposure device.

  4. FIG. 3 is a front view of an example of the grid-shaped ladder 5 shown in FIG. 2, FIG. 4 is a block diagram of an image recording device using the first embodiment of the method according to the invention, and FIG. 5 is a block diagram of a direct exposure device using another form of the image recording method according to the invention. Referring to FIG. 1, a conventional direct exposure device for the manufacture of printed circuits will first be described. In this device, image signals are stored in binary form in memory 1 and are classified. in two dimensions of the image The image signals are controlled by a central processing unit 2, so that they are converted into sequential scan signals for recording, by the technique scanning, an image or a diagram corresponding to the image binary signals stored in the memory 1.

  The resulting scanning signals are transmitted to a light modulator 3 of the acousto-optical type. A light beam originating from an exposure light source, for example an argon ion laser tube 4, is modulated in all or nothing by the acousto-optical light modulator 3, then it is guided towards a ref lec The various reflecting surfaces of the polyhedron reflector 7 rotate under the action of a motor 8 and deflect the beam of light. exposure by scanning, by the deflected beam, a predetermined angle (angle of deflection). The exposure light beam then passes through a focusing lens 35 disposed close to the polyhedral reflector 7, and is then reflected by a fixed mirror 11 disposed near an insulating card 12 suitable for the manufacture of a printed circuit. Thus, the light beam sweeps and im-

  -5 - press the insulating card 12 by forming image points on its surface. The insulating board 12 is fixed on a board 13 which is movable at a constant speed in a direction perpendicular to the scanning direction of the bundle (i.e. in the direction indicated by the arrow O in the figure ) with a motor 14 driving an auxiliary mechanism of scanning. When the polyhedral reflector 7 rotates and the platen 13 advances, the insulating card 12 is then scanned successively over its entire surface at the image point 10 of the exposure light beam.

  The precision of the movement of the plate 13 in the auxiliary scanning direction, under the effect of the mechanical drive, can be maintained without too much difficulty to a sufficient level in the direct exposure device, provided that the motor 14 and the transmission mechanism are adequately designed However, the accuracy of the deflection speed of the beam and the width of the scanning field produced by the rotation of the polyhedral reflector 7, in other words the accuracy of the scanning in the main direction, is determined by the design and fabrication accuracy of the optical system formed by the polyhedral reflector 7, the lens 10, etc. It is not easy to minimize the errors inherent in the design. and / or the manufacture and to achieve a satisfactory level of precision In addition, it is extremely difficult technically to maintain the linearity with sufficient accuracy. the angle of rotation of the polyhedral reflector 7 and that of the position of the image points along the line swept by the light beam. FIG. 2 is a block diagram of a conventional exposure device which tends to remedy the drawbacks described above. In this figure, all the elements or device parts identified by the same reference numerals as in FIG. 1 perform the same functions as the elements described in FIG 1 Their detailed description is superfluous. In comparison with the device illustrated in FIG. 1, the device of FIG. 2 comprises additional equipment constituted by an elongate scale 15 configured as a gate, a photoelectric sensor 16 and an auxiliary laser tube 17. the fixed mirrors 5, they are replaced by semi-mirrors

  6 - transparencies 5 ', 11'. 3 represents a part of the elongated grid-shaped scale 15 shown in FIG. 2. In the device of FIG. 2, the elongated grid-shaped scale 15 is arranged in the conjugate position of the point of FIG. exposure of the insulating card 12, ie the image point of the recording surface of this card relative to the semi-transparent mirror 11 '. As in the device of FIG. 1, a laser beam, coming from the argon laser tube 4, scans the insulating card 12 so as to impress it. On the other hand, the auxiliary laser tube 17 is arranged in such a way that any The laser beam that it produces follows the same optical axis as the laser beam from the argon laser tube 4. The laser beam from the auxiliary tube 17 has a wavelength that is outside the range of color sensitivity of the laser beam. light-sensitive material whose insulating card 12 is coated The laser beam from the auxiliary tube 17 scans the insulating card 12 and, at the same time, it scans the front surface of the grid-shaped scale 15, which reaches it in a 20 direction substantially parallel to the length of the plate 15. The laser beam which scans the scale 15 passes through the openings of this grid-shaped scale (see FIG. 3), thus being converted into a laser beam interrupted periodically at a frequency of This photoelectric sensor 16 converts this laser beam into electrical pulses as a function of the deflection rate. The pulsed signals that result are introduced. in the central unit 2, which then reads the image signals in the memory in synchronization with the pulsed signals. An image or a diagram is thus obtained without dimensional alterations on the insulating board 12. As shown in FIG. 2, two types of laser beams having different frequencies are used. Even if a lens whose chromatic aberration is minimized is used for the objective of focusing 10,

  There is still some residual chromatic aberration (magnification function). In addition, an aberration may also occur in the semi-transparent mirror 11 ', depending on the angle of incidence of each laser beam. Therefore, the grid-like scale shown in FIG. 3 must be given a non-linear configuration in order to correct such errors. Since there is a relatively long path between the polyhedral reflector 7 and the insulating card 12 for the laser beam which sweeps the insulating card 12, another disadvantage may arise if the optical axes of the two laser beams do not coincide perfectly with one another. can then observe a difference between the image points of the two laser beams on the insulating board 12, which gap depends on the deflection angle of the beam.

  To overcome these drawbacks, the method according to the present invention can be implemented for example by means of the devices described below with reference to FIGS. 4 and 5. It implies the use of a particular characteristic of the exposure, in particular other terms of the relationship between the amount of exposure light and the resulting image density or pattern, after development in the photosensitive material coating the insulating board used for manufacturing a printed circuit board D Generally, in the process according to the invention, a photosensitive material suitable for the production of printed circuits and having a high characteristic e is used, so that it is not impressed at all with a light. whose intensity is equal to or less than a certain threshold, but it is clearly impressed by a light whose intensity exceeds this threshold. A first embodiment of the invention is illustrated by the recording device shown in FIG. 4. This device differs from that of FIG. 2 in that the auxiliary laser tube 17 has been removed and the semi-transparent mirror 5 'has been replaced by a fixed mirror 5. As described with reference to FIG. 2, the input laser beam is modulated in the acousto-optic light modulator 3 as a function of the binary signals. image transmitted by the central unit 2 and it leaves the light modulator 3 in the form of a laser beam having alternately two different intensity levels, namely an intensity to impress

  The photosensitive material, and an intensity not permitting to impress the photosensitive material, but allowing the photoelectric sensor 16 to detect the laser beam in the form of pulses. This technique of controlling and modulating a two-level laser beam by means of the acousto-optic light modulator 3 and the technique of detecting the laser beam by means of the photoelectric sensor 16 are not in themselves objects. Of the present invention Any suitable known technique can be used for this purpose. Therefore, these techniques are not detailed in this specification. The present invention makes it possible to dispense with one of the two laser tubes and to further use a lens which is not specifically corrected to reduce chromatic aberration. Thus, the invention has advantageous effects in this respect. meaning that it avoids certain assembly and adjustment work such as the alignment of the optical axes of the beams, it makes it possible to reduce the cost of manufacturing an image recording device and it makes it possible to avoid aggravation of the errors produced by the variations of the scanning angle due to misalignment of the optical axes of the two laser beams. In the first embodiment described above, the grid scale 15 and a photoelectric sensor 16 are used. Instead of these elements, a row of linearly arranged photoelectric sensors could also be used. In the first embodiment described above, a light beam having an intensity too weak to sensitively impress the photosensitive material is sent to the photosensitive layer as long as the beam is scanning an empty portion of the image. In contrast to the prior art, it is a portion of the image-bearing deflected light beam that is moved to detect a position exactly corresponding to the image point on the photosensitive layer. On the basis of the thus obtained position signal, the device produces a feedback signal which makes it possible to control the optical modulation as a function of time. In this first embodiment, a portion of the recording light beam is derived by means of a semi-transparent mirror or the like

  9 - and the recording light beam is assigned two different intensity levels. Because a photoelectric sensor having a single level of sensitivity to the deflected light beam is used, both when the light layer, which is too weak to impress the photosensitive material, is sent to the sensitive layer only when an image portion is recorded on the sensitive layer, the beam level may be reduced. A deflected light of low intensity, corresponding to an empty portion of the image, is too different from the level of the high intensity deflected light beam corresponding to a positive portion of the image. The resulting pulses can thus be have different levels, which is likely to make unstable the processing of the light beam delivered by the light source, this processing being supposed to be synchronized with the image signals. Fig. 5 illustrates a direct exposure device suitable for carrying out the image recording method according to the second form of the present invention, which method makes it possible to stably perform the processing mentioned herein. -above. In essence, the image-recording method according to the second form of the present invention consists, for example, in arranging an amplifier at the output of the photoelectric sensor, in modifying the amplification factor of the amplifier. function of the image signals and to output reaction pulses which are suitable for the time correction of the optical modulation by synchronization of the image signals, while having levels which allow correct digital processing, so maintaining at a predetermined constant level the pulsed detection signals produced during the scanning of an empty portion of the image, that is, while the light intensity of the deflected beam is at its lower level. In the device of FIG. 5, an amplifier 18 has been added in the pulse reaction circuit P, which circuit extends between the photoelectric sensor 16 and the central unit 2 of FIG. parts or elements are identical to those of f ig 4 and we will therefore refrain from describing them. In this device, the laser beam which has passed through the gate 15 is converted into pulses P in the photoelectric sensor 16 in relation to the deflection speed of the laser beam. The pulses P serve to transmit a

  Position information indicating the corresponding image point of the light beam deflected on the insulating board 12, for example by counting the pulses or by a similar method The pulses P are then amplified by the amplifier 18 whose amplification factor is variable, then they are transmitted in reaction to the central unit 2. At this stage, a binary image signal read by the central unit 2 has already been introduced into the amplifier 18 in the form of a signal. a signal G for controlling the amplification factor The amplifier 18 changes amplification factor according to whether the image signal is of a high level or of a low level Thus, the amplification factor takes its lowest value when the pulse levels P are high It takes its higher value when the pulse levels P are low With the amplifier 18, it is possible to obtain pulses having a predetermined constant level, poorly It is appreciated that the photoelectric sensor 16 delivers pulses with varying levels. Therefore, in the case of a laser beam modified according to an image signal corresponding to an empty portion of the image and having a low light intensity, the amplification factor of the pulses corresponding to the light having passed through the grid-shaped scale 15 is increased, so that the signal-to-noise ratio of each position information element, transmitted in response to the central unit 2, is improved and that the control as a function of time can in the case of a laser beam modulated according to an image signal corresponding to a positive portion of the image and therefore having a high intensity. When illuminated, the amplification factor of the corresponding pulses is decreased so that the level of these pulses becomes substantially equal to those corresponding to an empty portion of the image. The pulses from the photoelectric sensor 16 are cut to a suitable threshold by a technique known per se, so as to form reaction pulses Pa of rectangular shape. They are then converted into high frequency pulses by a circuit. phase or similar A count of these pulses is introduced in the central unit 2 to control the speed of reading of the image signal in the memory 1 Thus, it is possible to

  it is possible to record an image or a diagram with an exact synchronization of the revolution speed of the polyhedral reflector 7 and the reading speed of the image signal. In the second embodiment described above, the laser beam corresponding to an empty portion of the image and the one corresponding to a positive portion of the image are both converted into pulses P for detecting the deflection position - by the sensor 16 using the semitransparent mirror 11 'and the gate 15 The amplification factors of the two types of pulses 10 are respectively modified by the image signals corresponding to an empty portion and in a positive portion of the image or pattern to be recorded In some cases, it may be preferable to perform only attenuation of the detection pulses of a laser beam corresponding to positive portions of the image In some other cases it may be preferable to perform only an amplification of the corresponding laser beam detection pulses of the empty portions of the image. The amplification factor can be changed by changing the ratio between a feedback resistor and an input resistor of an operational amplifier by means of a switching element when the amplifier 18 is constituted by the amplifier. amplifier, or by installing two different types of amplifiers, attenuators or the like and switching their output by means of a switch, so as to obtain at the output of the amplifier 18 deflection whose levels are the same. The image signals read in the memory 1 are converted into sequential scan signals which are accurately synchronized with the beam deflection speed, thereby recording an image on the insulating board 12. or a diagram having no dimensional alteration. The second embodiment of the present invention is described above assuming that an insulating board is impressed to make a printed circuit. It goes without saying that the present invention can also be applied to recording on other carriers. image, for example graphic photo films, heat-sensitive materials designed for infrared laser beam exposure, etc.

  In the embodiment described with reference to FIG. 5, the amplification or attenuation factor in the reaction circuit is modified as a function of the intensity level of the light beam modulated by each image signal. In such a way that the position pulses produced by the deflected beam are amplified or attenuated to a prescribed pulse level Therefore, it is still possible to obtain position detection pulses having substantially the same level when scanning an empty portion or a positive portion of the image, which makes it possible to produce the image or the prescribed diagram without dimensional changes.

The method according to the invention, as described above, furthermore makes it possible to reduce the overall manufacturing cost of image recording devices using this method, since it uses only one tube. Unlike the known method described above, this laser requires an expensive device because of the dependence of two laser tubes. On the other hand, the present invention does not require consideration of chromatic aberration, which makes it possible to use an inexpensive lens as a focusing objective. It is sufficient to take into account the aberration of a single lens. color with respect to the grid-shaped scale. The most advantageous feature of the invention is the use of a single laser tube. This renders unnecessary the task of aligning the optical axes which is necessary when using two laser tubes. On the other hand, the methods according to the invention completely solve the problem of errors caused by variations in the scanning angle, variations which were themselves increased by a difference between the two optical axes. The processes described above may be subject to many variations and modifications obvious to those skilled in the art without departing from the scope of the present invention.

Claims (6)

claims   A method of recording an image, in which a light beam from a light source is deflected, and a recording surface coated with a layer of material is scanned with this deflected light beam. photosensitive material, characterized in that a portion of the deflected light beam is drifted by means of a semi-transparent mirror and is sent to optical detection means of the deflection position, and in that the light beam from the light source is modulated by means of an image signal delivered in synchronism with a position signal produced by said detecting means from said portion of the deflected light beam, in such a way that the minimum luminous intensity of the modulated light beam is continuously maintained at a level too low to substantially impress the photosensitive material.   Method according to claim 1, characterized in that means 15 for optical detection of the deflection position are used which comprise an optical sensor and an elongated scale shaped like a grid, arranged in a conjugated position of a point. image of the recording surface with respect to the semitransparent mirror, this image point being the point of the recording surface which is exposed to the deflected light beam.   3 Process according to claim 1, characterized in that optical deflection position detection means constituted by a row of photoelectric sensors arranged in a linear form in a plane swept by said portion of the light beam are used. deflected.   Method according to Claim 1, characterized in that the level of the position signal produced by the detecting means is changed to a predetermined level, depending on the level of the image signal which controls the signal. intensity of the light beam from the light source.   5. Method according to claim 4, characterized in that, at the output of the optical detection means of the deflection position, means are provided for modifying an amplification factor of the position signal according to the signal level. image is low or high.   6 Process according to claim 4, characterized in that, at the output of the optical detection means of the deflection position, means are provided for amplifying the position signal when the level of the image signal is low and to attenuate the position signal when the level of the image signal 5 is high.