Abstract:
A laser imager comprises a modulator with an array of electrodes to control picture elements individually or by groups to produce images on a recording plane. The light intensity of each picture element or group of elements is determined by electrical potential applied between sets of controlling electrodes. These levels can be manually set or automatically exercised during a balancing routine through a feed-back loop using a photodetector to monitor and detect elements producing a light intensity higher than a set value. Control means assign to these elements a “correction voltage” to lower the density of elements they control. These correction values obtained after one or repeated balancing routines are stored in a memory and are used during the imaging cycle to produce images of substantially equal intensity on the recording medium. Different electrode arrangements to control the intensity of picture elements can be applied to different kinds of modulators and for different power levels of radiated energy.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is converted from and claims priority from provisional patent application Serial No. 60/136,035 filed May 26, 1999, which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to intensity control of modulators used to form images on radiation sensitive media from a constant wave radiation beam. Although the invention can be applied to any system, it is particularly aimed at the imaging of printing plates sensitive to infrared radiation. 
     BACKGROUND OF THE INVENTION 
     The use of lasers associated with optical modulators for the exposure of printing plates is well known. For example a laser beam is directed to an array of electrodes located on a substrate, allowing emerging sub-beams to selectively reach a radiation sensitive media such as a printing plate. The transmission of each beam to the image receptor surface is selectively inhibited in accordance with a predetermined pattern or program. In a machine developed by applicant the laser beam is divided into 256 individually controllable “gates” allowing 256 pixels or light dots to reach the radiation sensitive surface or be prevented from doing so by stops or deflection means downstream of the modulator. However, the intensity of different pixels, may not be uniform along the line of pixels. Corrections should be introduced as found necessary following initial tests. Also, the intensity of various pixels constituting a light bar may vary during the operation of the imager. These variations may be caused by various factors for different reasons. Although they may be so small as to be acceptable for certain applications, they may not be acceptable for the production of high quality printing plates. 
     U.S. Pat. No. 4,780,731 describes a system for intensity control applied to a strip of light emitting diodes in which the regulation is obtained by affecting the duration of activated diodes. This system is not applicable to the production of bands or swaths as explained above because the exposure of a pixel is very short, of the order of 10 microseconds. The high frequencies necessary for such a mode are not possible in the present state of the art to control spatial electrooptic modulators. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a method for the manual or automatic intensity control of each pixel or group of pixels by initial and periodic adjustment of the potential of gating electrodes. This adjustment is preferably accomplished by biasing the counter-electrodes individually or by groups without interfering with the control leads. This adjustment is independent of the control of selected marginal electrodes acting as an “electronic mask” as described in co-pending European Patent Application No. 99104942.0, which is incorporated herein by reference. The invention is not limited to a specific modulator. For example its scope covers PLZT as well as TIR modulators as either can accomplish the same function by similar means. 
     The present invention is applicable to the imaging of high quality thermal printing plates, for example, but without limitation, to machines and methods such as described in co-pending European Patent application No. 99112797.8, which is incorporated herein by reference. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 represents light intensity variation of a line of pixels on the y axis, I along a line of pixels shown in the x axis d. 
     FIG. 2 is a curve representing the light transmission T as a function of the applied voltage to control electrodes of an electrooptic modulator, either for a PLZT modulator between two parallel polarizers or a TIR modulator in bright field mode. 
     FIGS. 3- a  and  3   b  are partial representations of a PLZT electroded modulator. 
     FIGS. 3- c  and  3   d  are schematic partial representations of an electroded TIR modulator. 
     FIG. 4 shows how electric fields may be applied to a modulator to control the light reaching the imaging plane for correction purposes. 
     FIG. 5 schematically represents a circuit with a common counter-electrode. In this figure, as an example, the same electrical potential is applied every two electrodes. 
     FIG. 6 shows an arrangement in which the controlled counter-electrodes are arranged by groups of four. 
     FIG. 7 represents a circuit for the generation of controlling electrical potentials in any modulator. 
     FIG. 8 is a schematic representation of a circuit for the automatic adjustment of pixel intensity in a system based on a PLZT modulator. 
     FIG. 9 is a schematic representation of a TIR modulator with mechanical scanning of the image of projected spots in view of correction. 
     FIG. 9 a  represents intensity curves obtained by the dynamic measure of the intensity of pixels arranged by groups of four obtained by the scanning arrangement of FIG.  9 . 
     FIGS. 10 to  11   b  represent schematically parts of a laser platesetter embodying the invention. 
    
    
     DESCRIPTION OF THE INVENTION 
     In the following description of the invention the terms “dots” or “spots” or “pixels” are used interchangeably. They all refer to elementary areas assembled in order to produce images. Also “radiation” or “light” is used to designate radiation of various wave lengths and “optical components” generally referred to as lenses (or other optical items) may differ in material composition or surface quality according to the wave lengths of the beams on which they act. “Electrodes” mean metal deposit on or close to an electrooptic media to control impinging light rays. “Imaging electrodes” refer to electrodes usually connected to a positive voltage source while “counter-electrodes” refer to adjacent electrodes usually connected to a ground potential. The OFF state of the modulator means that no or very little radiation can go through. The ON state means that most impinging radiation can go through. Finally, it is understood that the term “laser” encompasses any kind of system producing coherent light beams, such as laser diodes with or without scattered emitters. 
     In the present description, the “OFF” state means that no radiation can reach the image plane. The “ON” state means that controlled radiation can reach the image plane. This control is preferably obtained by applying a selected bias potential to each electrode or group of electrodes. The method described in the present invention makes it possible to equalize pixel intensity to ±1%. This result, close to the ideal zero density variation is illustrated in FIG. 1 by a dotted line. The solid line represents a typical intensity variation of pixels not subjected to correction or calibration as per the present invention. Near uniform illumination between pixels within the limits shown is obtained by measuring means, by computation and by specific configuration of electronic circuits and of electrodes. 
     The graph of FIG. 2 represents the light transmission T as a function of the applied voltage V to control electrodes of an electrooptic modulator that could be either a PLZT wafer associated with two parallel polarizers or a TIR modulator in the bright field mode. 
     PLZT modulators for high radiant energy are schematically represented in FIGS. 3 a  and  3   b  with two different wiring arrangements. In the example shown in these figures, electrodes  48  and  50  are located on each side of the PLZT substrate  62 . The size of each electrode is 1.5 mm by 160 microns. The gap  49  between electrodes is  80  microns. A sheet-like bundle of rays illuminates the inter-electrode corridor  56 . It may be longer than the length of the corridor and slightly narrower than gap  49 . The width of the PLZT substrate is of the order of 6 mm. 
     A typical TIR modulator with interleaved electrodes located on the surface of a crystal as described in U.S. Pat. No. 4,281,904, is represented in FIGS. 3 c  and  3   d . The line of light  82  enters the crystal  84  to be submitted to the influence of electrodes  64 . The operation of the TIR modulator is similar to the operation of the PLZT modulator described above. 
     For correction purposes, in order to obtain a density curve similar to the one illustrated in FIG. 1, light equalization is obtained by selectively decreasing the amount of light received by pixels at the image plane whose intensity is above others. As illustrated in FIG. 4, this result is obtained by changing the potential between electrodes corresponding to pixels of higher intensity. In the top part of this figure, a “blocking potential” VH, through switch  70  is applied to the controlling electrode  44  to prevent the light from reaching the image plane. This state is referred as the (light) “Off” state as opposed to the “ON” state in which, no potential (except as may be necessary for correction purposes) appearing between electrodes, the maximum amount of radiation is allowed to reach the recording media located at the image plane. Correction is obtained by placing electrodes  44  and  46  under a selected potential to decrease the amount of light allowed to reach the recording media. Thus, in the lower part of the figure, switch  70  has been opened, cutting off the connection to potential VH and switch  71  has been closed connecting electrode  44  to a selected potential Vi to place it on its dependence for the purpose of lowering the intensity of pixels corresponding to points located above the ideal line of FIG.  1 . 
     In one embodiment of the invention, commercially available circuit boards are used, such as boards available from Supertex as illustrated in FIG. 5 and 6. In these figures, the switches used for correction purposes are arranged by groups of two, as represented at  80 . Each group is submitted to the same correcting potential. The electrodes are located alternatively on each side of the substrate, for example electrodes  72 ,  72 ′ are on the front face and electrodes  73 ,  73 ′ are located on the back side. A common counter-electrode is shown at  74 . The “OFF” potential or blocking voltage is represented as VH. Different potential values used for correction are shown at Vi−1, Vi, Vi+1 etc. 
     FIG. 6 shows another circuit board, similar to the board of FIG. 5 but in which electrodes are arranged by groups of four in order to correct four pixels at a time. Correcting potential values are obtained by projecting the imaging pixels to a light detector as explained later in connection with FIGS. 8 and 9. These values are memorized by the arrangement of FIG. 7. A section of a commercially available electronic board is represented at  100 . It contains analog multiplexers and decoders-driver. The data necessary for correction, as determined by light detector circuitry, is entered by connecting link  98 . Different voltages Vi+1, Vi, Vi−1, Vi−2, memorized in condensers or other memory devices  94  to  97  will be used during the operation of the imager to equalize the light intensities of the pixels. 
     The generation of correction values will now be described in connection with FIGS. 8 and 9 respectively representing the embodiment of the invention in a PLZT and a TIR modulator. 
     FIG.8 represents the embodiment of the invention in an imager comprising a PLZT modulator. A high power polarized light emitting assembly including laser and optical means is shown at  108 . A sheet-like beam of polarized light  106  emerging from this assembly is directed to the inter-electrode gap of the modulator assembly  104 . Light beams  107  that have been allowed to cross the polarizer  126  are shown at  129 . A relatively small part of these beams of the order of 4%, by example, is deflected by a beam-splitting mirror  130  located between the polarizer and the imaging objective  132 . This deflected light reaches detector  128  located at the focal point of a field lens positioned between the modulator and the beam splitter. The function of this field lens is to concentrate the beams emerging from the modulator to the objective lens to decrease its numeric aperture. The advantage of arrangement just described is that each pixel from the modulator arrives exactly at the same location on the surface of the detector. The values detected by the light detector are transferred over connection  127  to processor  116 . The D/A converter shown at  117  is connected as shown to power supply  114 . Its purpose is to control the laser intensity so that it corresponds to an assigned minimum. A cooling circuit helpful to absorb the heat generated by the high-energy laser unit is show at  110 . Processor  116  transfers to the control unit via connector  124  through wiring  118 ,  120  and  122  data necessary for intensity correction. To obtain these data the modulator is commanded to serially project onto the light detector selected groups of pixels. The identity of each group is, of course, known by processor  116  in which the signals from detector  128  are entered. The intensity of each group of pixel is compared to a reference value entered into block  116  by wire  119 , which may correspond to the desired minimum light intensity necessary for equalization. Correction voltages are sent by wire  118  via A/D circuit  111  to control block  124  where they are memorized as commanded by processor  116  which will also control via wiring  122  the voltages applied to certain pixels to bring their intensity to the desired value as explained above. On/Off switching circuits are shown at  112  and storage circuits at  56 . 
     FIG. 9 represents a system similar to the previous one where the PLZT modulator has been replaced by a TIR modulator whose operation is well known in the art. In the present description it is operated in the so-called “bright field” mode. A high-power light-emitting assembly  109  produces a sheetlike beam of light  136  directed to electro-optic material  134 , of modulator assembly  160  through lens  137 . The diffracted output beams, through lens  135  are directed to stop plate  138  having an aperture allowing only the zero order rays to go through, reach imaging optics  140  and finally the recording plane impinged by image-carrying pixels rays  139 . A light measuring assembly is shown at  151 . It comprises a carriage  150  that can slide in a direction transverse to the direction of rays  139  in a plane parallel to the recording media. The carriage is provided with a coder cooperating with a grating  133  in order to define its position. Attached to the sliding carriage is a block  158  provided with masks  143  and  144  slightly spaced apart to leave a narrow slit  145  between them. The masks are located on the image recording plane and are made to slide parallel to said plane. Beyond the slit is a partially reflecting mirror  146  directing beams that have passed the slit to light detector  148 . The light intensity values produced by the detector are transferred by connection  154  to processor  152  via A/D circuit  141 . The A/D converter shown at  147  is connected to power supply  115 . Its purpose is to control the laser intensity so that it corresponds to an assigned minimum. 
     Processor  152  transfers to control unit through connector  124  and chip  159  through wiring  121 ,  123  and  125  data necessary for intensity correction. To obtain this data the position of imaging head  140  relative to carriage  151  is moved so that the edge of bundle of rays  139  is adjacent to slit  145 . The carriage  151  is then moved to serially project pixels onto the light detector one at a time or by selected groups. The identity of each group is, of course known by processor  152  in which the signals from detector  148  are entered. The intensity of each pixel or group of pixels is compared to a reference value entered into block  152  in which a value representing the desired minimum light intensity necessary for equalization has been stored. Correction voltages are sent by wire  118  via A/D circuit  111  to control block  124  where they are memorized as commanded by processor  152  which will also control via wiring  121  the voltages applied to certain pixels to bring their intensity to the desired value as explained above. 
     FIG. 9 a  graphically illustrates how the light detecting arrangement can be utilized. Signals received from photodetector  148  are represented on coordinate “I” and the scanning motion of slit  145  on coordinate “d”. Curves, as shown, represent measurement made alternatively by groups of four pixels at a time. All odd number groups are placed in the ON state and all even number groups in the OFF state as follows: 4 ON-4 OFF-4ON-4OFF and so on. The first passage of the signal to a maximum is detected, memorized and affected to the first group of pixels. The second passage is affected to the third group and so on. Then the routine is started again with 4 OFF-4 ON-4 OFF-4 ON and so on. Correction voltages are derived from measured values based on the transmission curve of the modulator in function of the tension (FIG.  2 ). After these voltages have been applied, new measures are made again to correct, if necessary, residual errors by an interactive process. 
     The application of the invention to a flat-bed platesetter will now be described with reference to FIGS. 9,  10  to  11   b  and co-pending European patent application No. 99112797.8 (incorporated herein by reference). The imaging head assembly shown at  260  comprises a pre-modulation optical assembly  202  that could be as described in co-pending European patent application No. 99104943.8 (incorporated herein by reference), a modulator assembly  204  and associated projection optics  206  such as described in co-pending European patent application No. 99104942.0 (incorporated herein by reference). Emerging from the objective lens located at the output of assembly  260  are a multiplicity of individually controllable rays  208 . For the imaging of a printing plate such as  214 , assembly  260  moves to and fro over the plate along path  230  in order to create successive swaths of pixels. At the completion of each swath, the plate, secured by vacuum grippers attached to plate feeding carriage  218  is moved in the direction of arrow  232  by a distance substantially equal to the width of a swath. The complete imaging of the plate is obtained by a series of adjacent swaths. 
     When a plate has been completely imaged (plate  214  in the figure), in order to start a calibration cycle, projection head  260  is positioned mid-course, at the center of imaging area  216 , for example at location  260 ′ shown in dotted lines over a light detector assembly attached to carriage  218 . 
     Referring to FIGS. 10 and 10 a , attached to the plate feeding carriage  218  is an extension shown more in detail in FIG. 11 b . It comprises the same components as in FIG. 9 also represented by the same reference numbers. 
     Mounted on extension  228  is an assembly comprising plates  143  and  144  leaving between them a slit  143  to allow rays  139  emerging from objective lens  140  to impinge a light detector  148  via deflection mirror  146 . These components are located in box  286  attached to extension  228 . The light produced by rays  139  is measured in the same manner as described above in connection with FIG. 9, except that the function of separate sliding carriage  151  is replaced by the extension  228 . The reading of the light value of pixels is sequentially made by stepping the plate holding carriage by small pre-determined steps under the control of the command of a linear motor as explained in the above-referenced copending European patent application No. 99112797.8. The detecting and storing operation of pixel intensity values is the same as described in conjunction with FIG.  9  and will not be repeated here. 
     The intensity calibration just described takes advantage of the time elapsed between the completion of the imaging of a plate and the beginning of the imaging of the following plate, as shown in FIG.  10 . The arrangement just described does not necessitate a separate light detector carriage but it is limited to one re-calibration per plate. However this frequency has been found sufficient in practice. 
     The embodiments described herein are included only by way of example and are not meant to limit the scope of the invention. For example, the intensity calibration according to the present invention can be achieved without the specific automated calibration means previously noted by making manual or automated adjustments to the potential of the gating electrodes after observing or measuring banding effects that may appear due to intensity variations on imaged printing plates or other media. The intensity can be recalibrated with a frequency (minutes, hours, days, months, etc.) necessary to correct any unwanted banding effects that appear on the imaged media.