Abstract:
A calibration system for a platesetter or imagesetter is applicable to systems that have a media drum and a carriage, including a light source and a spatial light modulator for selectively exposing the media that is held against the drum. The invention can be applied to internal or external drum systems. The calibration system comprises a calibration sensor that is scanned relative to the spatial light modulator. The controller then analyzes the response of the calibration sensor to generate calibration information that is used to configure the spatial light modulator. The use of this calibration sensor allows for job-to-job calibration of the spatial light modulator, in one example, that ensures the generation of a high quality images, without banding, for example, on the media. This calibration system is also used to detect a best focus position for projection optics by measuring a contrast ratio between exposure and OFF light levels for various focus settings. It selects the best focus position in response to the contrast ratio.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    Imagesetters and platesetters are used to expose the media that are used in many conventional offset printing systems. Imagesetters are typically used to expose film that is then used to make the plates for the printing system. Platesetters are used to directly expose the plates.  
           [0002]    In imagesetters and platesetters, throughput and uptime are critical metrics. These systems typically operate in commercial environments. Their throughput is often used as the criteria for selecting between the various commercially available systems.  
           [0003]    The cycle time, and consequently throughput, for a platesetter or imagesetter is largely dictated by the time that the imaging engine requires to expose the media. Most conventional systems expose the media by scanning. In a common implementation, the plate or film media is fixed to the outside or inside of a drum and then scanned with a laser source in a raster fashion. The laser&#39;s dot is moved longitudinally along the drum&#39;s axis, while the drum is rotated under the dot. As a result, by modulating the laser, the media is selectively exposed in is a continuous helical scan.  
           [0004]    In these drum-scanning systems, a number of criticalities can dictate the cycle time. One limitation can be the speed at which the laser is modulated. This is related to the resolution that is required on the media. Another limitation is laser power. As the scan rates increase, the power that the laser generates must also be increased since the time to expose each pixel on the media decreases.  
           [0005]    To overcome some of these inherent limitations, systems are being proposed that use a combination of a light source and a spatial light modulator (SLM). Such modulators are usually based on liquid crystal technology. In one example, the light source is pulsed with a fixed periodicity. The data determining the plate exposure is then used to drive the spatial light modulator. This results in the media being exposed in a series of separate sub-images in the fashion of a stepper. As a result, the speed of operation is no longer limited by the rate at which the laser can be modulated or the power that can be extracted from that single laser.  
         SUMMARY OF THE INVENTION  
         [0006]    Current designs for SLM-based systems, however, can be somewhat expensive to deploy. They rely on flashlamps, which have limited operational lifetimes and require complex drive electronics.  
           [0007]    A different system uses a combination of a continuous laser source and a spatial light modulator. It allows relatively inexpensive and energy efficient CW laser diodes to be used as the light source.  
           [0008]    However, to work properly and to yield high quality images on the media, the spatial light modulator must be calibrated. Artifacts can otherwise arise in the image on the media, such as banding, which is the result of the elements of the spatial light modulators exposing the media at different exposure levels.  
           [0009]    The present invention concerns a calibration process for an imaging system including a spatial light modulator. It ensures that the elements of the spatial light modulator expose the media at the same, uniform level across the extent of the modulator.  
           [0010]    In general, according to one aspect, the invention features a calibration system for a platesetter or imagesetter, for example. It is applicable to systems that have a media drum and a carriage, including a light source and a spatial light modulator for selectively exposing the media that is held against the drum. The invention can be applied to internal or external drum systems.  
           [0011]    The calibration system comprises a calibration sensor that is scanned relative to the spatial light modulator. The controller then analyzes the response of the calibration sensor to generate calibration information that is used to configure the spatial light modulator. The use of this calibration sensor allows for job-to-job calibration of the spatial light modulator, in one example, thereby ensuring the generation of a high quality images, without banding, for example.  
           [0012]    In specific embodiments, the calibration sensor comprises a photodiode and slit aperture. This configure enables the responses of individual elements of the spatial light modulator to be detected. In one example, this is achieved by loading a modulation pattern into the spatial light modulator that enables discrimination of exposure levels of the individual elements. In one example, the modulation pattern has on-state elements surrounded by off-state elements. The responses of all of the elements can then be acquired by scanning the spatial light modulator over the calibration sensor multiple times.  
           [0013]    According to the present configuration, the spatial light modulator comprises digital-to-analog (DAC) systems that include arrays of individual DAC&#39;s that control the on-state and the off-state of corresponding elements of the spatial light modulator. As a result, the calibration information corresponds to the control level data that is loaded into these DAC&#39;s dictating binary off and on states of the elements.  
           [0014]    In more detail, the controller determines ON control level data that provides for a uniform exposure level across the spatial light modulator.  
           [0015]    According to still further aspects of the preferred embodiment, the controller also determines dark levels provided by the elements by generating OFF control level data that provides for uniformity in the dark level across the spatial light modulator.  
           [0016]    In general, according to still another aspect, the invention features a method for calibrating an imaging engine of a platesetter or imagesetter. It comprises detecting exposure levels provided by elements of the spatial light modulator and determining control levels for the elements that yield improved uniformity of the exposure levels across the spatial light modulator. These control levels are then used during the exposure of media on the drum.  
           [0017]    The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    In the accompanying drawings, reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale; emphasis has instead been placed upon illustrating the principles of the invention. Of the drawings:  
         [0019]    [0019]FIG. 1 is a plan view of a platesetter imaging engine according to the present invention;  
         [0020]    [0020]FIG. 2 is a flow diagram illustrating a pre-plate exposure calibration sequence according to the present invention;  
         [0021]    [0021]FIG. 3 is a flow diagram of ON level calibration subsequence showing a process for generating a uniform exposure level across the spatial light modulator according to the present invention;  
         [0022]    [0022]FIG. 4 is a plot showing precalibration and post calibration exposure level data as a function of shutter position in the spatial light modulator used in the present invention;  
         [0023]    [0023]FIG. 5 is a flow diagram of the OFF level calibration subsequence showing the process for providing uniformity in the dark level across the spatial light modulator according to the present invention;  
         [0024]    [0024]FIG. 6 is a plot of precalibration and post calibration dark level data as a function of shutter position in the spatial light modulator used in the present invention;  
         [0025]    [0025]FIG. 7 is a plot of OFF level control data and ON level control data as a function of shutter position, these data being used used to control the exposure level and dark level for a calibrated spatial light modulator according to the present invention;  
         [0026]    [0026]FIG. 8 is a flow diagram showing a best focus calibration subsequence according to the present invention;  
         [0027]    [0027]FIG. 9 is a plot of exposure level and dark level data for different focus settings illustrating the change in the contrast ratio with changes in the focus setting; and  
         [0028]    [0028]FIG. 10 is a flow diagram showing an exposure level calibration subsequence according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0029]    [0029]FIG. 1 shows an imaging engine that has been constructed according to the principles of the present invention. This imaging engine  10  can be deployed in a platesetter in which the media  12  is a photosensitive plate. In another implementation, it is deployed in an imagesetter in which the media  12  is film.  
         [0030]    The imaging engine  10  comprises a media drum  110 . The drum  110  revolves around an axis-of-rotation  112  that is co-axial with the drum  110 . In the illustrated example, the media  12  is held against the outside of the drum  110 . This configuration is typically termed an external drum configuration.  
         [0031]    In an alternative implementation, the media  12  is held along an inner side of the drum  110  to provide an internal drum configuration.  
         [0032]    A carriage  120  is disposed adjacent to the drum  110 . It is controlled by a controller is  131  to move along track  140  that extends parallel to the rotational axis  112  of the drum  110 .  
         [0033]    In the internal drum configuration, the carriage  120  moves within the drum  110  and is typically supported on a cantilever-like track, generally extending down through the center of the drum  110 .  
         [0034]    In either case, the carriage  120  supports a light source  122 . In the present implementation, this light source  122  comprises an array of laser diodes. The beams from these laser diodes are combined into a single output and coupled into an integrator  124 .  
         [0035]    Generally, because of the multi-source nature and because individual laser diodes have spatial intensity profiles that are somewhat Gaussian, the integrator  124  is typically required to generate a beam  126  with a rectangular cross section and with a uniform spatial intensity profile.  
         [0036]    The spatially homogeneous beam  126  is coupled to projection optics  128 , which ensure that the beam has a rectangular cross-section and a planar phase front. This rectangular beam is then coupled through a spatial light modulator  130  to the media  12  held on the drum  110 . A Hall effect focus motor  129  is used to adjust the focus position provide by the projection optics under control of the controller  131 .  
         [0037]    In the present implementation, the spatial light modulator  130  comprises a linear array of grating light valves. The elements of the grating light valve array function as shutters that control the level of transmission to the media  12 . Generally, each grating light valve comprises an optical cavity that will propagate light through the grating light valve to the media in response to the optical size of the cavity and the wavelength of light generated by the light source  122 .  
         [0038]    In other implementations, different spatial light modulators are used. For example, in some examples, the spatial light modulator comprises a two-dimensional array of elements. Different types of spatial light modulators can also be used, such as spatial light modulators based on liquid crystal or tilt mirror technology.  
         [0039]    In the present implementation, the operation of the spatial light modulator elements is controlled by an ON DAC system  132  and an OFF DAC system  134 . These devices dictate the binary modulation level of the elements of the spatial light modulator  130 .  
         [0040]    The operation of the elements of the spatial light modulator  130  are controlled in a binary fashion such that, during operation, they are either in an ON or transmissive state to expose the corresponding pixel on the media  12 , or an OFF state or dark, non-transmissive state to leave the corresponding pixel on the media  12  unexposed. Whether the elements of the spatial light modulator  130  are in a transmissive or non-transmissive state depends on the size of their respective optical cavities. Each element of the spatial light modulator  130  has a corresponding ON digital-to-analog converter in the ON DAC system  132  and an OFF digital-to-analog converter in the OFF DAC system  134 . These DAC&#39;s are loaded with ON and OFF control level data that dictate the drive voltages used to control the elements during their on and off states. These ON and OFF control level data are loaded into the ON DACS  132  and the OFF DACS  134  by the controller  131 .  
         [0041]    According to the invention, a calibration sensor  150  is provided. In the present embodiment, this calibration sensor  150  comprises a photodiode  152  and a slit aperture  154 . The combination of the photodiode  152  and the slit aperture  154  enable the controller  131  to monitor the operation of individual elements of the spatial light modulator  130  when the carriage is moved to the calibration position  156 , such that it is opposite the calibration sensor  150 .  
         [0042]    [0042]FIG. 2 is a flow diagram illustrating a pre-plate exposure calibration sequence.  
         [0043]    Typically, this pre-plate exposure calibration sequence is run when the imagesetter or platesetter is first powered up. In an alternative implementation, this sequence is run before every exposure of the media  12  held on the drum  110 .  
         [0044]    Specifically, in step  210 , the controller  131  determines whether a focus set-up subsequence should be run. If the controller  131  determines that focus set up is required, then the focus set up subsequence  212  is performed. Generally, this focus set-up occurs on a periodic basis. Alternatively, it can be performed before every plate exposure cycle. Sometimes, it is only performed when the machine is initially powered-up.  
         [0045]    The laser power level is set in step  214 . Specifically, the controller  131  sets the drive current that is supplied to the light source  122  in the carriage  120 . Typically, the laser power level is read by the controller  131 . It can be the last laser power setting that was used, or it can be a laser power setting that is set in the machine during factory calibration.  
         [0046]    The ON DAC system  132  and the OFF DAC system  134  are next loaded with the ON/OFF control level data in step  216 . In this step, the controller  131  loads the DAC systems  132 ,  134  with the voltage level data that is used to drive the elements of the spatial light modulator  130 . Sometimes, the control level data for the elements are stored during a factory calibration step. In another implementation, this control level data is based upon the result of the last calibration sequence that was run on the imagesetter or platesetter.  
         [0047]    Next, in step  218 , the controller  131  determines whether the OFF level calibration is required. If it is, the OFF calibration subsequence is run in step  220 .  
         [0048]    Then, in step  222 , the controller  131  determines whether ON level calibration is required. If ON level calibration is required, the ON level calibration subsequence is performed in step  224 .  
         [0049]    Finally, the system determines whether the present job is related to a previous job in step  226 . The operator typically supplies this information. It is important, within the same job, that the average exposure levels are substantially the same. In this situation, the factory set exposure level may be too imprecise. As a result, in step  228 , if this present job is related to a previous job, an exposure level calibration subsequence is run in step  228 . Finally, in step  230 , the media  12  on the drum  110  is exposed based upon the image data provided to the spatial light modulator  130  by the controller  131 .  
         [0050]    [0050]FIG. 3 is a flow diagram showing an ON control level calibration subsequence  224  according to the present invention. Specifically, the laser power level is reset in step  250 . Then, the ON DAC system  132  and the OFF DAC system  134  are loaded with ON and OFF control level data for the elements of the spatial light modulator  130  in step  252 .  
         [0051]    The controller  131  then further loads the spatial light modulator with a 1-ON, 3-OFF image data modulation sequence in step  254 . This corresponds to an exposure pattern in which only every fourth element or shutter of the spatial light modulator  130  is in a transmissive state. Specifically, every fourth shutter is driven in response to the corresponding ON control level data held in its DAC of the ON DAC system  132 . The remaining shutters are driven in response to their corresponding OFF control level data held in the OFF DAC system  134 .  
         [0052]    The carriage  120  is then moved on the track  140  to the calibration position  156  in which the spatial light modulator  130  is scanned opposite the aperture  154  of the calibration sensor  150  in step  256 . The controller  130  monitors the output of the photodiode  152  and compiles an array of precalibration exposure level data in step  258 . This exposure level data corresponds to the light that is transmitted through the spatial light modulator  130  and received at the image plane of the projection optics  128  for the media  12 .  
         [0053]    On the first pass through this process flow, however, the array of exposure level data is incomplete since data are gathered from 1 in 4 of the elements of the spatial light modulator  130 . As a result, in step  260 , it is determined whether data has been collected for all of the elements of the spatial light modulator  130 . If not, then the ON-1, 3-OFF spatial light modulator shutter pattern is incremented in step 262 and the process steps  256  and  260  repeated. This way, the system generates a complete array of precalibration exposure level data for all of the elements of the spatial light modulator  130 .  
         [0054]    The 1 -ON, 3-OFF shutter pattern, combined with successive scans is used to ensure that the controller  131  can discriminate the responses of the individual elements of the spatial light modulator  130 . For high-resolution systems, the corresponding size of the pixels at the image plane is small. Using the 1-ON, 3-OFF shutter pattern allows the calibration sensor to have a reasonably sized aperture, yet discriminate the responses of individual elements.  
         [0055]    In step  261 , the controller  131  compares the exposure level data across the spatial light modulator to a uniformity threshold. Generally, the controller  131  is determining whether there are large deviations in the level of exposure across the spatial light modulator  130 .  
         [0056]    If there is poor uniformity, as determined in step  264 , the controller  131  calculates new ON control level data in step  266 , which is then loaded in step  252 . The process repeats to ensure that this new control level data provides uniformity within the threshold.  
         [0057]    [0057]FIG. 4 is a plot of the exposure level data before and after calibration. Specifically, the level of exposure for exposure level data array  270  shows wide variations in exposure. Specifically, the data varies from approximately a count of 640 to approximately 540 for an analog-to-digital converter that monitors the output of the photodiode  152 .  
         [0058]    The exposure level data compiled after the recalculation of the ON DAC control level data (step  266 ) has been loaded in the ON DAC system  132  corresponds to data array  272 . Here, the exposure level generally is consistent, varying between 565 to 570 counts, showing good uniformity across the 700 shutters of the spatial light modulator  130 , in one implementation.  
         [0059]    [0059]FIG. 5 shows the OFF level calibration sequence  220 . Specifically, in step  310 , the laser power level is set. Then, in step  312 , the spatial light modulator  130  is loaded with a 2-ON, 724-OFF shutter pattern. This shutter pattern corresponds to a pattern in which most of the elements of the spatial light modulator  130  are in a non-transmissive state. Then, the OFF DAC system  134  is loaded so that each element is driven with the same OFF control level data in step  314 . Specifically, the digital-to-analog converters of the OFF DAC system  134  are loaded so that they all drive the elements of the spatial light modulator  130  to a level determined by a DAC count of 255. Then, in step  316 , the carriage  120  is moved to the calibration position  156  and scanned so that the spatial light modulator  130  passes in front of the aperture  154  of the calibration sensor  150 . The controller  131  monitors the response of the photodiode  152  during this scanning operation to generate an array of OFF or dark level data corresponding to this first DAC setting.  
         [0060]    In step  318 , the OFF DAC system  134  is loaded with a new OFF control level data. Specifically, in the specific implementation, it is loaded with a DAC count of 245, so that the elements of the spatial light modulator  130  are generally uniformly driven to this new off level. Then, in step  320 , the carriage is again moved to the calibration position  156  and scanned over the spatial light modulator  130 . This enables the controller  131  to generate a second array of OFF or dark level data corresponding to this second DAC setting.  
         [0061]    Finally, in step  322 , the OFF DAC system  134  is loaded with OFF control level data corresponding to a  235  DAC count. Then again, in step  324 , the carriage  120  is again scanned. This scanning allows the controller  131 , monitoring the output of the photodiode  152 , to generate a third array of OFF level data corresponding to this third DAC setting for the elements of the spatial light modulator  130 .  
         [0062]    In step  326 , the controller  131  evaluates the variation in the acquired OFF level data in the three data arrays. It then interpolates using the data of the three arrays to find an optimally uniform and optimally dark OFF control level setting for each of the elements of spatial light modulator in step  328 . The resulting, new corrected OFF control level data is then loaded into the OFF DAC  134  in step  330 .  
         [0063]    [0063]FIG. 6 is a plot of dark level data as a function of the shutter in the spatial light modulator  130 . It shows that for the data arrays corresponding to the DAC setting of  255 , see data  340 , the DAC setting  245 , see data array  342 , and the DAC setting  235 , see data array  344 .  
         [0064]    There is generally poor uniformity across the shutters of the spatial light modulator  130 , illustrating that simply selecting a uniform DAC level for every element of the spatial light modulator  130  will generally yield poor performance. However, in step  328  of FIG. 5, the controller  131  uses the information from the three data arrays  340 ,  342 ,  344  to generate corrected OFF control level data by selecting counts between  235  and  255  for the various DACs of the OFF DAC system  134  by an interpolation process. The selection yields the corrected OFF light level data  346 . This shows that a generally uniform level is achieved across the shutters of the spatial light modulator  130  using the data from the three arrays of dark level data collected in steps  314 - 322  of FIG. 5.  
         [0065]    [0065]FIG. 7 is a plot of OFF control level data and ON control level data for the shutters of the spatial light modulator, across shutters  200 - 900 . These control level data are generated during the calibration subsequences of FIGS. 3 and 5. Generally, the OFF level data  710  exhibits a trend across the spatial light modulator. This is typically due to wafer-level process variation during fabrication. The ON level data  712  tend to be less spatially correlated.  
         [0066]    [0066]FIG. 8 is a flow diagram illustrating the focus subsequence  212 . Specifically, the laser power level is set in step  350 . Then, the elements of the spatial light modulator  130  are loaded with a 1-ON, 3-OFF shutter pattern in  352 . To review, in this shutter pattern, only every fourth shutter is in a transmissive state.  
         [0067]    In step  354 , the ON DAC system  132  and the OFF DAC system  134  are loaded with the control level data. Further, in step  356 , the carriage  120  is moved to the calibration position  156  in front of the calibration sensor  150  such that the spatial light modulator  130  is scanned opposite the aperture  154 . This scanning occurs in step  358  while the focus setting for the projection optics  128  is changed.  
         [0068]    The controller  131  then monitors the response of the photodiode  152  to generate a contrast ratio map in step  360 . A contrast ratio map plots the on-light levels and the off-light levels for various shutters of the spatial light modulator and for various focus settings. Specifically, the focus setting of the projection optics  128  is changed in a continuous fashion across the scan of the spatial light modulator  130 . As a result, the exposure level data and the dark level data exhibit variation across the spatial light modulator that corresponds to the changes in the focus setting during the scan.  
         [0069]    In step  362 , the controller  131  selects the focus setting from the contrast map generated in step  360  to maximize the contrast ratio between the OFF light level data and the exposure light level data.  
         [0070]    [0070]FIG. 9 is a plot of the contrast ratio map that is generated during the scan of step  358 .  
         [0071]    Specifically, the exposure level data  912  and the dark level data at different shutter positions corresponds to different focus settings for the projection optics  129  under control of the Hall motor  129 . The maximum contrast ratio focus setting corresponds to the focus setting applied when elements approximately 190 to 200 were scanned over the calibration sensor  150 . The corresponding Hall motor position is stored as the best focus position by controller  131 . In this way, the present invention sets the best focus setting to maximize the contrast ratio. In the spatial light modulator systems, this contrast ratio is a figure of merit determining their performance.  
         [0072]    [0072]FIG. 10 is a flow diagram illustrating an exposure level calibration sub sequence  228 . Many times, especially within the same job, it is important for the platesetter or imagesetter to expose successive plates within the same job at the same exposure setting. The process of FIG. 10 accomplishes this.  
         [0073]    Specifically, in the first step  410 , the laser power level of the light source  122  is set. Then, the ON DAC system  132  and the OFF DAC system  134  are loaded with the control level data in step  412 . Then, in step  414 , the carriage  120  in moved to the calibration position  156  and the spatial light modulator  130  scanned in front of the aperture  154  of the calibration sensor  150  in step  416 .  
         [0074]    The controller  132  then monitors the output of the photodiode  152  and determines an average exposure level across the entire scan of the spatial light modulator  130  in front of the calibration sensor  150  in step  418 . This detected average light level is then compared to the light level for a previous exposure of a plate for the same job or a similar pre-exposure calibration step. If it is determined to be outside an acceptable tolerance level, in step  420 , the laser power level is adjusted by the controller  131  in step  422  and then, the sequence repeated to ensure that the average exposure level is the same for the two media exposures in the same job.  
         [0075]    While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.