Abstract:
An imaging system is disclosed for imaging recording media. The imaging system includes an input opening through which recording media may be input to the imaging system, and an imaging unit for imaging the recording media that is received through the input opening. The imaging unit provides a calibration image on the recording media. The imaging system also includes a calibration image receiving unit for receiving image data representative of the calibration image. The calibration image receiving unit produces imaging unit calibration adjustment data. The imaging system also includes an imaging unit adjustment controller, and an output opening. The imaging unit adjustment controller adjusts the imaging unit responsive to the imaging unit calibration adjustment data. The recording media exits the imaging system through the output opening.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The invention generally relates to the field of imaging systems, and specifically relates to systems and methods for providing plates (e.g., lithographic or flexographic plates) in a plate making system.  
           [0002]    In many pre-press imaging systems, such as imagesetters or platesetters, a plurality of sheets or plates (hereafter referred to as plates) of recording media are separately exposed by an imaging source. Each plate may provide a pattern for a different color (e.g., yellow, magenta, cyan and possibly black), and these plates must be registered with one another during printing of the final multi-color image. The recording media to be imaged by a pre-press imaging system is commonly supplied in web form or in discrete plates. The recording media may include photosensitive material, radiation sensitive material, thermally sensitive material, or any other type of imageable material.  
           [0003]    During imaging, a movable optical carriage is typically used to displace a laser system or other imaging source in a slow scan direction along a stationary or moving, curved or planar, media support surface (e.g., an external drum, and internal drum, a flatbed, or other support surface). The imaging source exposes a supply or recording media supported on, and held against, the media support surface. Generally, the imaging source includes an optical system for scanning one or more lasers or other radiation beams, each modulated by a digital information signal, over the recording media to record an image onto the recording media. Generally, the information signal is recorded onto a supply of recording media mounted about the external drum by displacing the imaging source relative to the media support surface, e.g., an external drum. This may be accomplished in a number of ways, including rotation of the external drum in combination with a lateral translation of the imaging source, etc. In certain systems, the external drum is rotated while the imaging source is displaced in discrete steps or continuously along the length of the external drum to record data onto the recording media.  
           [0004]    Calibration of the imaging system is typically performed during a set-up procedure that occurs either at the manufacturer or at a user&#39;s site. Calibration typically involves analyzing an imaged plate after the plate has been processed by a chemical processor that is used to develop the image on the plate.  
           [0005]    There is a need, therefore, for an improved system and method for calibrating imaging systems, and in particular, for calibrating pre-press imaging systems responsive to variations in imaging media.  
         SUMMARY OF THE INVENTION  
         [0006]    The invention provides an imaging system for imaging recording media. In an embodiment, the imaging system includes an input opening through which recording media may be input to the imaging system, and an imaging unit for imaging the recording media that is received through the input opening. The imaging unit provides a calibration image on the recording media. The imaging system also includes a calibration image receiving unit for receiving image data representative of the calibration image. The calibration image receiving unit produces imaging unit calibration adjustment data. The imaging system also includes an imaging unit adjustment controller, and an output opening. The imaging unit adjustment controller adjusts the imaging unit responsive to the imaging unit calibration adjustment data. The recording media exits the imaging system through the output opening. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    The following description may be further understood with reference to the accompanying drawings in which:  
         [0008]    [0008]FIG. 1 shows an illustrative isometric view of an external drum imaging system of the invention together with a processing station and an output table;  
         [0009]    [0009]FIG. 2 shows an illustrative isometric view of an external drum imaging system in accordance with the invention with the housing removed;  
         [0010]    [0010]FIG. 3 shows an illustrative side view of the imaging system of FIG. 1 with the housing removed;  
         [0011]    [0011]FIG. 4 shows an illustrative top view of the writing and calibration system of FIG. 2 taken along line  4 - 4  thereof during writing;  
         [0012]    [0012]FIG. 5 shows an illustrative top view of the writing and calibration system of FIG. 4 during calibration; and  
         [0013]    FIGS.  6 A- 6 C show illustrative diagrammatic views a calibration image in accordance with an embodiment of the invention.  
         [0014]    The drawings are shown for illustrative purposes only, and are not to scale.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    An embodiment of a system of the invention is illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout the drawings. Although the drawings are intended to illustrate an embodiment of the present invention, the drawings are not necessarily drawn to scale.  
         [0016]    A calibration system of the invention may be used with an external drum imaging system that is configured to record digital data onto imaging media. Although described below with regard to an external drum platesetter, many aspects of the present invention may be used in conjunction with a wide variety of other types of external drum, internal drum, or flatbed imaging systems, including imagesetters and the like, without departing from the intended scope of the present invention.  
         [0017]    As shown in FIG. 1, in accordance with an embodiment of the invention, imaging media may be placed onto an input tray  30  of an image recorder, such as a platesetter  10  having a housing  2 . The imaging media is then imaged, and a portion of the imaged media may then be scanned by a calibration unit. The imaging system may then be adjusted responsive to the output of the calibration unit, and imaging of the media may continue. The media is then output from the platesetter  10  via output port  4 , passed to a processing station  6 , and then deposited onto a table  8  as shown in FIG. 1.  
         [0018]    The imaging system generally includes a front end computer or workstation (not shown) for the design, layout, editing, and/or processing of digital files representing pages to be printed, a raster image processor (RIP) for further processing the digital pages to provide rasterized page data (e.g., rasterized digital files) for driving an image recorder, and an image recorder, such as an external drum platesetter  10 , for recording the rasterized digital files onto a printing plate or other recording media. The external drum platesetter  10  records the digital data provided by the RIP onto a supply of photosensitive, radiation sensitive, thermally sensitive, flexographic or other type of suitable printing plate. In the present embodiment, the printing plate is manually loaded onto a staging area of the external drum platesetter  10  by an operator. Alternately, or in addition to manual loading, the printing plate may be provided and loaded onto the external drum platesetter  10  by a media supply or autoloading system, which may accept a plurality of the same size or different size printing plates.  
         [0019]    As shown in FIG. 2, the external drum platesetter  10  includes an external drum  12  having a cylindrical media support surface  14  for supporting the printing plate  16  (shown in FIG. 3) during imaging. The external drum platesetter  10  further includes a writing and calibration system  18 , coupled to a movable carriage  20 , for recording digital data onto the imaging surface  22  of the printing plate  16  using a single or multiple imaging beams  24  (shown in FIG. 2). The carriage  20  rides along a stable base  26 , and the drum  12  rotates about a drum drive system  28 . The base  26  may be formed of heavy material, such as a polymer-concrete mixture, granite, or the like, to vibrationally isolate the external drum  12  and writing and calibration unit  18  from external vibrations.  
         [0020]    Generally, during use a plate is positioned on an input tray  30 , and transferred to an imaging station in a direction as indicated at A. After imaging, the plate is transferred from the imaging station to an output tray in a direction as indicated at B using transfer belts  32  that are driven by pulleys  34  about pulley shafts  36 . From the output tray, the plate may be exited from the platesetter  10  in either of a direction as indicated at C or a direction as indicated at D by rollers  40  that contact the plate after the pulleys  34  and belts  32  are lowered with respect to the rollers  40 . In further embodiments, the plate may be exited from the platesetter  10  in a direction as indicated at E.  
         [0021]    As shown in FIG. 2, the scanning system  18  is displaced by the movable carriage  20  in a slow scan (axial) direction along the length of the rotating external drum  12  to expose the printing plate  16  in a line-wise manner when a single beam is used or in a section-wise manner for multiple beams. Other types of imaging systems may also be used in the present invention.  
         [0022]    The external drum  12  is rotated by a drive system  28  in a clockwise or counterclockwise fast scan direction, typically at a rate of about 100-1000 rpm. In an embodiment, the printing plate  16  is loaded onto the external drum  12  while rotating the drum in a first clockwise direction. The printing plate  16  is then imaged while the drum is rotated in the first, or in an opposite second, direction. Finally, the printing plate  16  is unloaded from the external drum  12  while rotating the drum in the second direction.  
         [0023]    As shown in FIG. 3, a plate  16  is positioned on the input tray  30  above a pair of resilient input nip rollers  50 , one of which may be driven by a drive assembly  52 . The leading edge  38  of the plate  16  is positioned by the input tray  30  to rest substantially between the input nip rollers  50 . The rollers  50  are positioned above the external drum  12 , and are oriented such that the common tangent of the rollers  50  is tangent to the media support surface  14 . The input tray  30  is oriented such that the loading path of the plate  16  extends along a line that is tangent to the external drum  12  at a leading edge clamping mechanism  40 .  
         [0024]    A curved input/output guide platen  54 , mounted to a frame member (not shown) of the external drum platesetter  10  may be provided to direct the leading edge  38  of the printing plate  16  toward the leading edge clamping mechanism  40  during the loading of the printing plate  16  onto the external drum  12 . In addition, the curved input/output guide platen  54  is configured to direct the printing plate  16  off of the external drum  12  toward the plate output area after imaging is complete.  
         [0025]    During loading of a plate  16 , the drum  12  is rotated until the leading edge clamping mechanism  40  is positioned to receive the leading edge  38  of the plate  16 . A clamping portion  54  of the clamping mechanism  40  is held in an open position by an actuator  56 , exposing registration pins  58 . A trailing edge clamping mechanism  44  is rotated by the drive system  28 , if necessary, to position a clamping bar  60  out of the way of the loading path of the plate  16 . An actuation system  62  for the trailing edge clamping mechanism  44 , and an ironing roller system  64 , may also be retracted away from the media support surface  14  of the external drum  12  out of the way of the loading path. After the leading edge  38  of the plate  16  is properly positioned against the registration pins  58 , the leading edge clamping mechanism is closed, thereby pinching the plate  16  against the external drum  12  while the leading edge  38  remains in contact with the registration pins  58 .  
         [0026]    After the leading edge clamping operation, the external drum  12  is rotated a few degrees by the drive system  28 . The ironing roller assembly of the stationary ironing roller system  64  is then extended and positioned against the plate  16  by an actuating system. The plate is drawn around the drum  12  until the trailing edge  42  of the plate  16  is positioned adjacent the trailing edge clamping mechanism  44 . The clamping bar  60  is then positioned over the trailing edge  42  of the plate  16 . If the size of the plate is not known and pre-programmed into the system, a sensor  68  may be used to detect the trailing edge of the plate  16 . The drum and clamping bar  60  are then rotated together, and the clamping bar  60 , which is normally biased away from the drum  12 , is then forced against the drum  12  by the actuation system  62 . Vacuum may also be used to facilitate securing the plate to the drum.  
         [0027]    During imaging, the leading edge  38  of the plate  16  is held in position against the media support surface  14  by the leading edge clamping mechanism  40 . Similarly, the trailing edge  42  of the printing plate  16  is held in position against the media support surface  14  by the trailing edge clamping mechanism  44 . Both the trailing edge clamping mechanism  44  and the leading edge clamping mechanism  40  provide a tangential friction force between the printing plate  16  and the external drum  12  sufficient to resist the tendency of the edges of the printing plate  16  to pull out of the clamping mechanisms  40 ,  44 , at a high drum rotational speed. In accordance with the present invention, only a small section (e.g., 6 mm) of the leading and trailing edges  38 ,  42 , is held against the external drum  12  by the leading and trailing edge clamping mechanisms  40 ,  44 , thereby preserving as much of the available imaging area of the printing plate  16  as possible.  
         [0028]    As shown in FIG. 4, a calibration image  80  is recorded on a portion of the surface  14  of the imaging media  12  by a writing illumination source  82  of the writing and calibration unit  18 . The calibration image  80  may then be viewed by a calibration detection unit  84  within the unit  18  as shown in FIG. 5. The calibration detection unit  84  may include, in particular, a dispersing filter  86  and a charge couple device (CCD) camera  88 .  
         [0029]    As shown in FIGS. 6A and 6B, the calibration image  80  may include an checkerboard-type array of black and white boxes, and may, for example be about 1 inch by 1 inch in size. If the writing source  82  is improperly calibrated, then the individual boxes  90  may be slightly undersized as shown at  90 A in FIG. 6A, or slightly oversized as shown at  90 B in FIG. 6B. Because the calibration detection unit  84  includes a dispersing filter  86 , the CCD camera  88  receives a blurred homogenous image of the calibration image  80 , rather than distinguishing between individual boxes. The blurred homogenous image will be recorded by the CCD camera and accompanying computer system as a shade having a particular shade value. If the calibration image includes undersized boxes as shown at  90 A in FIG. 6A, then the shade value of the image received by the CCD camera  84  will be lower than a desired target shade value. If the calibration image includes oversized boxes as shown at  90 B in FIG. 6B, then the shade value of the image received by the CCD camera  84  will be higher than the desired target shade value. Finally, if the calibration image includes correctly-sized boxes as shown at  90 C in FIG. 6C, then the shade value of the image received by the CCD camera  84  will be equal to the desired target shade value. The position of the writing unit  82  may then be adjusted as indicated at  92  in FIG. 5 responsive to the shade value of the received image to achieve an optimal calibration for that particular imaging media. In various embodiments, the system may adjust to any shade value, e.g., 50% as described above, or any value between 0% and 100%. Further, the system may record and analyze either the brightness (the white areas) and/or the dark areas, and may record the brightness/darkness for a variety of shades (e.g., 25%, 50% and 75%) and provide the appropriate (linear or non-linear) correction as needed. In other embodiments, the system may not blur the image, but rather may record the actual shapes (e.g., each square shape) in the calibration image. Moreover, the calibration image may form a part of the overall image being recorded on the plate.  
         [0030]    The calibration image may be written on a small portion of the media near an edge in order to preserve as much of the media as possible for imaging of the desired image during pre-press imaging. In further embodiments, the calibration image may be written over a large portion of imaging media that is used only for the calibration process.  
         [0031]    With reference again to FIG. 3, during output of the plate  16  from the drum  12 , the drive system  28  rotates the drum  12  in a counterclockwise direction, the trailing edge clamping mechanism  44  is released, and the leading edge clamping mechanism  40  is released. The trailing edge  42  of the plate  16  is guided by the input/output platen  54  toward resilient output nip rollers  70 , one of which may include a drive system  72 . The plate  16  is then received in the output area  47  by the belts  32  that are rotated about pulleys  34  in a direction that causes the top surface of the belts  32  to travel with the plate as it emerges from the imaging area, and thereby carry the plate away from the imaging area. Once the plate reaches a stop surface, the plate stops moving and the drive system for the pulleys  34  is turned off. In other embodiments, plate advancement may cease responsive to the output of a position sensor.  
         [0032]    Those skilled in the art will appreciate that numerous modifications and variations may be made to the above disclosed embodiments without departing from the spirit and scope of the present invention.