System and method for calibrating an imaging system during imaging

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.

BACKGROUND OF THE INVENTION

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.

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.

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.

Calibration of the imaging system is typically performed during a set-up procedure that occurs either at the manufacturer or at a user'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.

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

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.

The drawings are shown for illustrative purposes only, and are not to scale.

DETAILED DESCRIPTION OF THE INVENTION

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.

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.

As shown inFIG. 1, in accordance with an embodiment of the invention, imaging media may be placed onto an input tray30of an image recorder, such as a platesetter10having a housing2. 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 platesetter10via output port4, passed to a processing station6, and then deposited onto a table8as shown inFIG. 1.

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 platesetter10, for recording the rasterized digital files onto a printing plate or other recording media. The external drum platesetter10records 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 platesetter10by an operator. Alternately, or in addition to manual loading, the printing plate may be provided and loaded onto the external drum platesetter10by a media supply or autoloading system, which may accept a plurality of the same size or different size printing plates.

As shown inFIG. 2, the external drum platesetter10includes an external drum12having a cylindrical media support surface14for supporting the printing plate16(shown inFIG. 3) during imaging. The external drum platesetter10further includes a writing and calibration system18, coupled to a movable carriage20, for recording digital data onto the imaging surface22of the printing plate16using a single or multiple imaging beams24(shown inFIG. 2). The carriage20rides along a stable base26, and the drum12rotates about a drum drive system28. The base26may be formed of heavy material, such as a polymer-concrete mixture, granite, or the like, to vibrationally isolate the external drum12and writing and calibration unit18from external vibrations.

Generally, during use a plate is positioned on an input tray30, 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 belts32that are driven by pulleys34about pulley shafts36. From the output tray, the plate may be exited from the platesetter10in either of a direction as indicated at C or a direction as indicated at D by rollers40that contact the plate after the pulleys34and belts32are lowered with respect to the rollers40. In further embodiments, the plate may be exited from the platesetter10in a direction as indicated at E.

As shown inFIG. 2, the scanning system18is displaced by the movable carriage20in a slow scan (axial) direction along the length of the rotating external drum12to expose the printing plate16in 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. The external drum12is rotated by a drive system28in a clockwise or counterclockwise fast scan direction, typically at a rate of about 100-1000 rpm. In an embodiment, the printing plate16is loaded onto the external drum12while rotating the drum in a first clockwise direction. The printing plate16is then imaged while the drum is rotated in the first, or in an opposite second, direction. Finally, the printing plate16is unloaded from the external drum12while rotating the drum in the second direction.

As shown inFIG. 3, a plate16is positioned on the input tray30above a pair of resilient input nip rollers50, one of which may be driven by a drive assembly52. The leading edge38of the plate16is positioned by the input tray30to rest substantially between the input nip rollers50. The rollers50are positioned above the external drum12, and are oriented such that the common tangent of the rollers50is tangent to the media support surface14. The input tray30is oriented such that the loading path of the plate16extends along a line that is tangent to the external drum12at a leading edge clamping mechanism40.

A curved input/output guide platen54, mounted to a frame member (not shown) of the external drum platesetter10may be provided to direct the leading edge38of the printing plate16toward the leading edge clamping mechanism40during the loading of the printing plate16onto the external drum12. In addition, the curved input/output guide platen54is configured to direct the printing plate16off of the external drum12toward the plate output area after imaging is complete.

During loading of a plate16, the drum12is rotated until the leading edge clamping mechanism40is positioned to receive the leading edge38of the plate16. A clamping portion54of the clamping mechanism40is held in an open position by an actuator56, exposing registration pins58. A trailing edge clamping mechanism44is rotated by the drive system28, if necessary, to position a clamping bar60out of the way of the loading path of the plate16. An actuation system62for the trailing edge clamping mechanism44, and an ironing roller system64, may also be retracted away from the media support surface14of the external drum12out of the way of the loading path. After the leading edge38of the plate16is properly positioned against the registration pins58, the leading edge clamping mechanism is closed, thereby pinching the plate16against the external drum12while the leading edge38remains in contact with the registration pins58.

After the leading edge clamping operation, the external drum12is rotated a few degrees by the drive system28. The ironing roller assembly of the stationary ironing roller system64is then extended and positioned against the plate16by an actuating system. The plate is drawn around the drum12until the trailing edge42of the plate16is positioned adjacent the trailing edge clamping mechanism44. The clamping bar60is then positioned over the trailing edge42of the plate16. If the size of the plate is not known and pre-programmed into the system, a sensor68may be used to detect the trailing edge of the plate16. The drum and clamping bar60are then rotated together, and the clamping bar60, which is normally biased away from the drum12, is then forced against the drum12by the actuation system62. Vacuum may also be used to facilitate securing the plate to the drum.

During imaging, the leading edge38of the plate16is held in position against the media support surface14by the leading edge clamping mechanism40. Similarly, the trailing edge42of the printing plate16is held in position against the media support surface14by the trailing edge clamping mechanism44. Both the trailing edge clamping mechanism44and the leading edge clamping mechanism40provide a tangential friction force between the printing plate16and the external drum12sufficient to resist the tendency of the edges of the printing plate16to pull out of the clamping mechanisms40,44, 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 edges38,42, is held against the external drum12by the leading and trailing edge clamping mechanisms40,44, thereby preserving as much of the available imaging area of the printing plate16as possible.

As shown inFIG. 4, a calibration image80is recorded on a portion of the surface14of the imaging media12by a writing illumination source82of the writing and calibration unit18. The calibration image80may then be viewed by a calibration detection unit84within the unit18as shown inFIG. 5. The calibration detection unit84may include, in particular, a dispersing filter86and a charge couple device (CCD) camera88.

As shown inFIGS. 6A and 6B, the calibration image80may 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 source82is improperly calibrated, then the individual boxes90may be slightly undersized as shown at90A inFIG. 6A, or slightly oversized as shown at90B inFIG. 6B. Because the calibration detection unit84includes a dispersing filter86, the CCD camera88receives a blurred homogenous image of the calibration image80, 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 at90A inFIG. 6A, then the shade value of the image received by the CCD camera84will be lower than a desired target shade value. If the calibration image includes oversized boxes as shown at90B inFIG. 6B, then the shade value of the image received by the CCD camera84will be higher than the desired target shade value. Finally, if the calibration image includes correctly-sized boxes as shown at90C inFIG. 6C, then the shade value of the image received by the CCD camera84will be equal to the desired target shade value. The position of the writing unit82may then be adjusted as indicated at92inFIG. 5responsive 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.

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.

With reference again toFIG. 3, during output of the plate16from the drum12, the drive system28rotates the drum12in a counterclockwise direction, the trailing edge clamping mechanism44is released, and the leading edge clamping mechanism40is released. The trailing edge42of the plate16is guided by the input/output platen54toward resilient output nip rollers70, one of which may include a drive system72. The plate16is then received in the output area47by the belts32that are rotated about pulleys34in a direction that causes the top surface of the belts32to 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 pulleys34is turned off. In other embodiments, plate advancement may cease responsive to the output of a position sensor.