Abstract:
A method for reducing artifacts in a color printed image, that are perceivable by the naked eye, includes using a different imaging beam to start imaging each printing plate that is used for a different color plane. Alternatively, a different set of imaging beams may be used to image each printing plate used for a different color plane. The method does not require manipulating image data. The method diffuses some errors throughout the printed image and prevents some errors from being imaged and/or printed which reduces or eliminates the visible perception of artifacts.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The invention herein generally relates to the field of imaging systems for use in the graphic arts industry. Further, the invention more specifically relates to reducing perceivable artifacts from being formed in printed images.  
           [0002]    Color images are often printed using four colors (more or less colors are also known), yellow, cyan, magenta, and black. For each color used on a printing press, a different printing plate is used. If a four color printing press is used to produce a color image, then as many as four printing plates are needed to produce the color image. Each printing plate designated for a different color, and to be used together to print a particular image (the same print job) has the same or a similar imaged placed upon each plate as is well known in the art.  
           [0003]    Platesetters are machines used to transfer an electronic image onto a printing plate, for subsequent use on a printing press. Platesetters often use laser based imaging systems to transfer an electronic image onto the plate, in a process called imaging. A typical laser imaging system employs many individual laser beams to image a printing plate. The plurality of laser beams used to image a plate, sometimes called writing beams, or just beams, often emanate from a moveable assembly referred to as an imaging head. An example of a multi-beam imaging head for an external drum platesetter is shown in FIG. 1. The optic energy produced by the laser (or lasers) is utilized to transfer an electronic image onto a printing plate that is photosensitive or thermally sensitive (including ablative) as is well know in the art.  
           [0004]    Color images printed using a printing press such as a lithographic printing press are verified for quality prior to, and during printing. A measure of the quality of a printed color image is the presence or absence of artifacts in the image. Artifacts are undesired variations in the printed image, such as the well known banding phenomenon shown in FIG. 3. Many causes contribute to generation of artifacts in a printed image. Periodic artifacts, such as banding may be caused by repetitive equipment errors in the printing press itself, or in the equipment used during the pre-printing press phase of production, called prepress. Platesetters are an example of prepress equipment.  
           [0005]    Image processing is often employed to compensate for equipment errors in an effort to remove artifacts from an image, or alternatively, to prevent artifacts from being formed in the image. U.S. Pat. No. 6,185,002 to Askeland et al is one example of where image data is manipulated to reduce artifacts such as banding. This intensive data manipulation carries a significant cost in terms of required computing power, memory, and additional software. This data manipulation step, or steps, serves to increase costs and reduce throughput in addition to changing the image via adding or deleting image pixels.  
           [0006]    What is needed is a means to reduce or eliminate artifacts in a printed image without having to resort to expensive image processing methods that increase cost.  
           [0007]    Further, it is also desirable to be able to reduce or eliminate artifacts without adding additional hardware to existing equipment.  
         SUMMARY OF THE INVENTION  
         [0008]    The invention herein solves the problems described supra and others, by using a different starting beam of a multi-beam imaging head when imaging each printing plate designated for a unique color plane, subsequently used to print a given image. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The following description may be further understood with reference to the accompanying drawings in which:  
         [0010]    [0010]FIG. 1 is a schematic of a multi-beam imaging machine for imaging printing plates.  
         [0011]    [0011]FIG. 2 is prior art showing how a printing plate is imaged.  
         [0012]    [0012]FIG. 3 is an example of an artifact referred to as banding.  
         [0013]    [0013]FIG. 4 demonstrates how multiple pixels (or dots) can be used to create a color image.  
         [0014]    [0014]FIG. 5 shows one mechanism causing banding.  
         [0015]    [0015]FIG. 6 shows how the invention herein reduces banding at the pixel level.  
         [0016]    [0016]FIG. 7 shows how the invention herein is implemented on a platesetter.  
         [0017]    [0017]FIG. 8 is the example of the banding artifact shown in FIG. 3 reproduced adjacent other figures to facilitate comparison with FIG. 9.  
         [0018]    [0018]FIG. 9 is the sample image of FIG. 8 showing the utility of the invention herein by removing (or preventing) the banding artifact (from being formed).  
         [0019]    [0019]FIG. 10 shows alternate embodiments of the invention. 
     
    
       [0020]    The drawings are shown for illustrative purposes only, and are not to scale.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    Though the following description of the invention herein is described in the context of an external drum platesetter, the application of the invention should not be limited to such. For example, the invention herein may also be employed on internal drum or flatbed platesetters, external or internal drum imagesetters and/or printing presses. Printing plate  34  may alternatively be a piece of film in lieu of a printing plate without deviating from the spirit of the invention. Further, the invention herein may be practiced with all types of printing plates, including but not limited to, aluminum, polyester, flexographic etc..  
         [0022]    Referring to FIG. 1, printing plate  23  is mounted on an external drum  21  of a platesetter  20 . Plate  23  has an imageable area  24  that an image (not shown) is transferred onto using moveable imaging apparatus  25 , often referred to as an imaging head. Drum  21  is rotatable as shown by arrow  22 , which in conjunction with moveable imaging head  25 , the movement thereof shown by arrow  27 , operates to move a plurality of N imaging beams  26  over the entire imageable area  24  of plate  23 . The number N, of imaging beams provided by multi-beam imaging head  25  is not restricted. Imaging heads having  96  beams, or as many as  1088  beams are known.  
         [0023]    Referring to FIG. 2, imageable area  24  can be viewed as consisting of many individual picture elements, or pixels, that must be imaged, or “turned on”. Though only a single row of pixels  28  are shown in FIG. 2, it is understood that the entire imageable area  24  consists of pixels. Imaging head  25  is shown having a plurality of imaging beams  26  aligned with the row of pixels  28 . The alignment is such that the first beam  32  of N beams is aligned with the first pixel  33  in the row of pixels  28 . An N beam imaging head can image a swath  29  having a width of N pixels. From this initial starting position, an image is transferred (imaged) onto plate  23 . The first beam used to image the first pixel  33  in this example is beam  32 , and is referred to as the starting beam. The starting beam as defined in this application, is the first beam, of a multi-beam imaging head, that is used to start imaging the first pixel of an image to be placed on image area  24 . In accordance with the invention herein, the starting beam may be a beam other than the first beam in a multi-beam array.  
         [0024]    The plurality of beams  26  are moved down plate  23  due to rotation of the drum  21  forming a swath  29  on imageable area  24 . Note that though the swaths herein are described as vertical, or parallel to an edge of plate  23 , the swaths may also be helical due to the combined rotational movement of drum  21  and lateral movement of imaging head  25 . After swath  29  has been imaged, imaging head  25  is positioned to image the next swath  30 , and imaging is continued in this manner until the entire electronic image is placed on imageable area  24 , the last swath being swath  31 . Applicant notes that though three swaths are described, the number of swaths may be any number, and is dependent upon the size of the plate and number of imaging beam among other variables. The total number of swaths is independent of the invention herein.  
         [0025]    Once plate  23  has been imaged, it may be further processed (e.g. developed) if required depending upon the type of plate before being used on a printing press. Plate  23  is designated for use with only one color of ink, called a color plane, and is used on a printing press. Consequently, an additional printing plate must also be imaged with the same or similar image for each primary color used in the subsequent printing process for a given print job as is well known in the art. Applicants note an image placed on a printing plate designated for a particular color, may not be identical to an image placed on a printing plate designated for a different color plane as is well known in the art. A printing plate designated for use with a particular color on a printing press is referred to as that particular color plate to avoid confusion with other plates. For example, a plate designated for use with the color yellow, is called a yellow plate even though the plate is not actually yellow in color.  
         [0026]    When a color image is printed using multiple plates for the different colors as is known in the art, artifacts may be created.  
         [0027]    Some types of artifacts may appear as a white line (or other color) in a color field or variations in intensity of a color field or fields. The color fields in which artifacts may appear are not limited to the primary color fields such as cyan, yellow, magenta (reddish) and black. Composite color fields may also be affected such as green. A green color field is created on a printing press by placing a yellow dot on a substrate with one printing plate, and then placing a cyan (a bluish color) dot on top of the yellow dot using a different printing plate. The two colors (composed of ink, wax etc.) mix yielding green.  
         [0028]    Referring to FIG. 4, the process can be visualized. A portion of a yellow printing plate  13  is shown consisting of nine pixels arranged in three columns  1 ,  2 , and  3 , of three pixels each. Applicants point out a printers “dot” is composed of a plurality of pixels. The number of pixels per dot is dependent upon the various resolutions required or used (e.g. dpi, number of line screens etc.) for a given type of printing technology and may or may not be in the range of ˜20-64 pixels per printers dot. Pixels are typically imaged on a printing plate using a platesetter, and dots are typically printed on a substrate (e.g. paper, cardboard, bumper sticker, cloth etc.) using a printing press. Again referring to FIG. 4, a portion of a cyan printing plate  14  is shown consisting of nine pixels arranged in three columns  1 ′,  2 ′, and  3 ′, of three pixels each. After plates  13  and  14  are imaged in the prepress phase of production, the plates are installed onto a printing press (not shown). Plate  13  is used to place yellow ink onto substrate  15  in the nine-pixel pattern shown. Plate  14  is then used to place cyan ink onto substrate  15 , on top of the yellow nine-pixel pattern, also in the same nine-pixel pattern shown. The resulting nine-pixel pattern on substrate  15  has a green color. Though the figures are black and white, the additive effect is similar in that the gray color of substrate  15  is darker than either plate  13  or  14  and represents the result of adding yellow and cyan for illustrative purposes only.  
         [0029]    Referring to FIG. 5, one mechanism that creates artifacts will be shown. The type of artifact to be shown is banding which can be a variation in color intensity, but well may be another type. Each column of pixels shown in FIGS.  4 - 6  corresponds to a unique, single imaging beam emanating from N beam imaging head  25 . For this example, in FIG. 5, the first three beams of multi-beam imaging head  25  are used to image pixel columns  1 ,  2 , and  3 . Also for this example we assume the second beam has very low optic power output such that pixels imaged by the second beam are not “fully imaged” resulting in a light color being produced when the plate is used for printing. This is shown on plate  13   a  by the columns of imaged pixels  4   a  and  6   a  being darker than column  5   a.  Since the same physical imaging beams, the first three beams in this example, are used to image all color plates, the same problem also exists on the cyan plate  14   a  as shown by imaged pixel columns  7   a  and  9   a  being darker than  8   a.  The net result is that when plates  13   a  and  14   a  are used to print the image on substrate  15   a,  column  11   a  of FIG. 5 is a much lighter green than columns  10   a  and  12   a.  This phenomena occurs at the pixel level on printing plates  13   a  and  14   a,  and appears as a series of alternating light and dark bands  37 ,  38  across the printed image  36  as shown in FIG. 3.  
         [0030]    An inventive step herein is demonstrated in FIGS. 6 and 7. When imaging plate  13   b  (designated for use with the color yellow on press) is imaged, the starting beam in this example is actually the second beam  39  of multi-beam imaging head  25 . This is seen in FIG. 6 because beam  39  is aligned with the first column of pixels  1  on plate  13   b.  FIG. 6 shows imaged pixel column  5   b  being lighter in color than imaged pixel columns  4   b  and  6   b  similar to the previous example. However, different from the previous example, the starting beam for cyan plate  14   b  is actually the fourth beam  48  of multi-beam imaging head  25 . Since, in this example, the fourth beam  48  has normal optic power output, imaged pixel column  8   b  has the same color intensity as adjacent imaged pixel columns  7   b  and  9   b.  Note that the starting beams for yellow plate  13   b  and cyan plate  14   b  are not the same, therefore a different set of imaging beams are used to image each color plate. This dilutes the effect of any imaging beam of multi-beam imaging head  25  that may be low in optic power or may be out of focus or have other spatial errors. The result of this inventive method is that the color field created on substrate  15   b  by a printing press using plates  13   b  and  14   b  is much more uniform as shown in FIG. 6. Printed color column  11   b  is much closer in color intensity to adjacent printed color columns  10   b  and  12   b,  than printed color column  11   a  is to columns  10   a  and  12   a  in FIG. 5.  
         [0031]    In a preferred embodiment, a different start beam is used for each printing plate that is designated for use with a unique color plane when used on a printing press for a particular print job. In other words, if a print job (e.g. a magazine cover) requires four colors on press, then the four required printing plates (corresponding to the four required colors) are imaged on a imaging machine using a different starting beam for each plate. The sequence that the starting beams are selected from plate to plate, may be random, pseudo-random, fixed offset, or even sequential. For example, if four plates are required (e.g. designated for yellow, cyan, magenta, and black) the starting beams could be 1, 13, 21, and 4 respectively. In a preferred embodiment, a random beam selection process is utilized  
         [0032]    Referring to FIG. 7, the invention herein is shown compared to the previous technique of using the first beam  32  as the starting beam and using all available beams for all plates. The starting beam  40  is actually the third imaging beam of N beams in multi-beam imaging head  25 . This means that the first two beams  32 ,  39  are not used to image this particular plate. In a preferred embodiment, the first two beams  32 ,  39  are not used to image only the first swath  44 , and all N beams are used to image the remaining swaths (except for residual swath  47 ). Since a subset of the full number of N beams is used to image at least some of the plates, some pixels  42  remain that would otherwise have been imaged if the full number of N beams were utilized. Consequently, at least one additional swath  47  may be required to fully image the imageable area  24  of plate  23 . Using the inventive technique herein, swaths  44 - 47  are required verses swaths  29 - 31  to image the same imageable area  24 . Each color printing plate, each of which is imaged using a different starting beam (or different set of beams) may each require a different number of swaths in order to fully image each plate.  
         [0033]    [0033]FIG. 9 shows the same image as FIG. 3 (reproduced as FIG. 8 to facilitate comparison) with the exception that the image in FIG. 9 shows the effect of implementing the invention herein. FIG. 8 clearly shows the banding phenomenon on what is otherwise a uniform color image. Alternating bands of light  37  and dark  38  color intensity are seen in the image  36  of FIG. 8 and are absent in the image  36   a  of FIG. 9. FIGS. 8 and 9 are macro (whole image) representations of the micro representations (pixel level of image) shown in FIGS.  4 - 6 . Even though some banding may still remain after implementing the inventive method herein, the residual banding is virtually undetectable to the human eye. The banding has essentially been obscured or hidden and is simply not perceivable to the unaided eye.  
         [0034]    In a first alternate embodiment, the ending beam may be varied and the starting beam may be the same for each color plate. For example, referring to FIG. 10, starting beam  32  is used to image the first pixel  33  on each color plate (the imageable area  24  for only one plate is shown for clarity). However, the ending beam will be different for each color plate. One implementation of this alternate embodiment could be beams  1  to N are used to image a yellow plate, beams  1  to (N−1) used to image a cyan plate, beams  1  to (N−2) used to image a magenta plate, and beams  1  to (N−3) used to image a black plate. This is illustrated in FIG. 10 by beams  34 ,  35 , and  41  shown as dotted lines indicating that one or more of beams  34 ,  35 , or  41  is not used to image some of the plates.  
         [0035]    A result of the first alternate embodiment is the first swath of the printed image will still have banding since the same beams are used to image equivalent pixels on all the plates. However, the remaining portion of the printed image will have the banding removed (or obscured). Since a single swath is actually a very small portion of the total printed image, the banding in the first swath may not be prominent.  
         [0036]    A second alternate embodiment uses different starting and ending beams for each color plate. However, the printed image will still have banding which may be acceptable in some print runs.  
         [0037]    The previously described embodiments are variable swath techniques on a plate to plate basis. A third alternate embodiment is to use any of the previously described embodiments, wherein a given color plate (e.g. the yellow plate) has the starting beam, ending beam, or both starting and ending beams varied on a swath to swath basis within the same plate. The result would be some residual banding may remain, which may be acceptable depending on the particular print run.  
         [0038]    Though the invention herein has been described for use with printing systems that use printing plates, the invention is not limited to such. The invention herein may also be adapted for use with color printers that do not use printing plates, but use separate color cartridges, such as inkjet printers, laser printers or any multi-beam scanning system.  
         [0039]    Further, the invention is also suitable for use with on-press plate making systems (alternatively referred to as plateless printing systems) where temporary printing plates are actually created on a cylinder (or other support surface).  
         [0040]    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.