Patent Publication Number: US-11644781-B2

Title: Reducing image burn-in artifacts using a compensation image

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 63/214,835, filed Jun. 25, 2021, which is incorporated herein by reference in its entirety. 
     Reference is made to commonly assigned, co-pending U.S. patent application Ser. No. 17/518,645, entitled: “Electrophotographic printing system including page rotations to reduce burn-in artifacts,” by T. Schwartz et al.; to commonly assigned, co-pending U.S. patent application Ser. No. 17/518,664, entitled: “Electrophotographic printing system including lateral translations to reduce burn-in artifacts,” by T. Schwartz et al.; and to commonly assigned, co-pending U.S. patent application Ser. No. 17/836,100, entitled: “Artifact reduction using a compensation image,” by C.-H. Kuo, each of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention pertains to the field of digital printing, and more particularly to electrophotographic printing systems having reduced artifacts. 
     BACKGROUND OF THE INVENTION 
     Electrophotography is a useful process for printing images on a receiver (or “imaging substrate”), such as a piece or sheet of paper or another planar medium (e.g., glass, fabric, metal, or other objects) as will be described below. In this process, an electrostatic latent image is formed on a photoreceptor by uniformly charging the photoreceptor and then discharging selected areas of the uniform charge to yield an electrostatic charge pattern corresponding to the desired image (i.e., a “latent image”). 
     After the latent image is formed, charged toner particles are brought into the vicinity of the photoreceptor and are attracted to the latent image to develop the latent image into a toner image. Note that the toner image may not be visible to the naked eye depending on the composition of the toner particles (e.g., clear toner). 
     After the latent image is developed into a toner image on the photoreceptor, a suitable receiver is brought into juxtaposition with the toner image. A suitable electric field is applied to transfer the toner particles of the toner image to the receiver to form the desired print image on the receiver. The imaging process is typically repeated many times with reusable photoreceptors. 
     The receiver is then removed from its operative association with the photoreceptor and subjected to heat or pressure to permanently fix (i.e., “fuse”) the print image to the receiver. Plural print images (e.g., separation images of different colors) can be overlaid on the receiver before fusing to form a multi-color print image on the receiver. 
     One problem that can occur in electrophotographic printing systems is known as “image burn-in” which occurs when a sequence of the same or similar pages having similar image data or patterns are printed. In this case, the performance of various system components can change as a function of position due to the repeated printing of the similar image data or patterns. This can cause subsequently printed images to have characteristics that vary in accordance with the repeated image data or pattern, thereby having a negative impact on image quality. Such artifacts are commonly referred to as “image burn-in artifacts.” This can require that the affected system components be more frequently serviced or even replaced to eliminate the artifacts which can add significant cost and system down-time. 
     Problems can also occur in electrophotographic printing systems if the rate that toner is supplied by the developing subsystem (sometimes referred to as the toner usage rate or the toner take-out rate) is too low or too high. If the toner take-out rate exceeds the system replenishment capability, then image nonuniformity artifacts can occur. On the other hand, if the toner take-out rate is low for an extended period, the toner electric charge in the development subsystem will reach an elevated level which can create imaging artifacts such as mottle at high density levels and disruption of halftone dots at low density levels. 
     There remains a need for an improved method to reduce image burn-in artifacts when printing an extended sequence of similar pages in an electrophotographic printing system, as well as artifacts associated with too low or too high toner usage rates. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a method for reducing artifacts in an electrophotographic printing system, including: 
     receiving a print job including image data for a set of pages to be printed with the electrophotographic printing system; 
     using the electrophotographic printing system to print a block of pages from the print job to provide corresponding printed pages; 
     analyzing the image data for the block of pages to determine a cross-track image profile for each page in the block of pages, wherein the cross-track image profile represents an average amount of toner in the printed pages as a function of cross-track position; 
     determining image data for a compensation image, wherein the compensation image has a cross-track image profile which has an inverted shape relative to an average of the cross-track image profiles for the block of pages; 
     determining a number of compensation images to be printed; and 
     using the electrophotographic printing system to print the determined number of compensation images; 
     wherein the printing of the compensation images reduces image burn-in artifacts which result from the printing of the block of pages. 
     This invention has the advantage that the compensation images reduce image burn-in artifacts by reducing surface roughness non-uniformities in a fuser roller. 
     It has the additional advantage that artifacts that result when toner usage rates in an electrophotographic printer are too high or too low are reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is an elevational cross-section of an electrophotographic printer suitable for use with various embodiments; 
         FIG.  2    is an elevational cross-section of one printing module of the electrophotographic printer of  FIG.  1   ; 
         FIG.  3 A  is an example of a page having image content that is susceptible to producing image burn-in artifacts; 
         FIG.  3 B  illustrates a fuser roller having a roughened surface in regions corresponding to a dark image content in the page of  FIG.  3 A ;  FIG.  3 C  illustrates image burn-in artifacts formed from the fuser roller of  FIG.  3 B ; 
         FIG.  4    is a flowchart of a method for printing with reduced artifacts in accordance with an exemplary embodiment; 
         FIG.  5    illustrates a cross-track image profile determined from image data for the exemplary page of  FIG.  3 A ; 
         FIG.  6    illustrates an exemplary cross-track image profile for a compensation image determined from the cross-track image profile of  FIG.  5   ; and 
         FIG.  7    illustrates an exemplary compensation image corresponding to the cross-track image profile of  FIG.  6   . 
     
    
    
     It is to be understood that the attached drawings are for purposes of illustrating the concepts of the invention and may not be to scale. Identical reference numerals have been used, where possible, to designate identical features that are common to the figures. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is inclusive of combinations of the embodiments described herein. References to “a particular embodiment” and the like refer to features that are present in at least one embodiment of the invention. Separate references to “an embodiment” or “particular embodiments” or the like do not necessarily refer to the same embodiment or embodiments; however, such embodiments are not mutually exclusive, unless so indicated, or as are readily apparent to one of skill in the art. The use of singular or plural in referring to the “method” or “methods” and the like is not limiting. It should be noted that, unless otherwise explicitly noted or required by context, the word “or” is used in this disclosure in a non-exclusive sense. 
     As used herein, “toner particles” are particles of one or more material(s) that are transferred by an electrophotographic (EP) printer to a receiver to produce a desired effect or structure (e.g., a print image, texture, pattern, or coating) on the receiver. Toner particles can be ground from larger solids, or chemically prepared (e.g., precipitated from a solution of a pigment and a dispersant using an organic solvent), as is known in the art. Toner particles can have a range of diameters (e.g., less than 8 μm, on the order of 10-15 μm, up to approximately 30 μm, or larger), where “diameter” preferably refers to the volume-weighted median diameter, as determined by a device such as a Coulter Multisizer. When practicing this invention, it is preferable to use larger toner particles (i.e., those having diameters of at least 20 μm) in order to obtain the desirable toner stack heights that would enable macroscopic toner relief structures to be formed. 
     “Toner” refers to a material or mixture that contains toner particles, and that can be used to form an image, pattern, or coating when deposited on an imaging member including a photoreceptor, a photoconductor, or an electrostatically-charged or magnetic surface. Toner can be transferred from the imaging member to a receiver. Toner is also referred to in the art as marking particles, dry ink, or developer, but note that herein “developer” is used differently, as described below. Toner can be a dry mixture of particles or a suspension of particles in a liquid toner base. 
     As mentioned already, toner includes toner particles; it can also include other types of particles. The particles in toner can be of various types and have various properties. Such properties can include absorption of incident electromagnetic radiation (e.g., particles containing colorants such as dyes or pigments), absorption of moisture or gasses (e.g., desiccants or getters), suppression of bacterial growth (e.g., biocides, particularly useful in liquid-toner systems), adhesion to the receiver (e.g., binders), electrical conductivity or low magnetic reluctance (e.g., metal particles), electrical resistivity, texture, gloss, magnetic remanence, florescence, resistance to etchants, and other properties of additives known in the art. 
     In single-component or mono-component development systems, “developer” refers to toner alone. In these systems, none, some, or all of the particles in the toner can themselves be magnetic. However, developer in a mono-component system does not include magnetic carrier particles. In dual-component, two-component, or multi-component development systems, “developer” refers to a mixture including toner particles and magnetic carrier particles, which can be electrically-conductive or -non-conductive. Toner particles can be magnetic or non-magnetic. The carrier particles can be larger than the toner particles (e.g., 15-20 μm or 20-300 μm in diameter). A magnetic field is used to move the developer in these systems by exerting a force on the magnetic carrier particles. The developer is moved into proximity with an imaging member or transfer member by the magnetic field, and the toner or toner particles in the developer are transferred from the developer to the member by an electric field, as will be described further below. The magnetic carrier particles are not intentionally deposited on the member by action of the electric field; only the toner is intentionally deposited. However, magnetic carrier particles, and other particles in the toner or developer, can be unintentionally transferred to an imaging member. Developer can include other additives known in the art, such as those listed above for toner. Toner and carrier particles can be substantially spherical or non-spherical. 
     The electrophotographic process can be embodied in devices including printers, copiers, scanners, and facsimiles, and analog or digital devices, all of which are referred to herein as “printers.” Various embodiments described herein are useful with electrostatographic printers such as electrophotographic printers that employ toner developed on an electrophotographic receiver, and ionographic printers and copiers that do not rely upon an electrophotographic receiver. Electrophotography and ionography are types of electrostatography (printing using electrostatic fields), which is a subset of electrography (printing using electric fields). The present invention can be practiced using any type of electrographic printing system, including electrophotographic and ionographic printers. 
     A digital reproduction printing system (“printer”) typically includes a digital front-end processor (DFE), a print engine (also referred to in the art as a “printing module” or a “marking engine”) for applying toner to the receiver, and one or more post-printing finishing system(s) (e.g., a UV coating system, a glosser system, or a laminator system). A printer can reproduce pleasing black-and-white or color images onto a receiver. A printer can also produce selected patterns of toner on a receiver, which patterns (e.g., surface textures) do not correspond directly to a visible image. 
     The DFE receives input electronic files (such as Postscript command files) composed of images from other input devices (e.g., a scanner, a digital camera or a computer-generated image processor). Within the context of the present invention, images can include photographic renditions of scenes, as well as other types of visual content such as text or graphical elements. Images can also include invisible content such as specifications of texture, gloss or protective coating patterns. 
     The DFE can include various function processors, such as a raster image processor (RIP), image positioning processor, image manipulation processor, color processor, or image storage processor. The DFE rasterizes input electronic files into image bitmaps for the printing module to print. In some embodiments, the DFE permits a human operator to set up parameters such as layout, font, color, paper type, or post-finishing options. The printing module takes the rasterized image bitmap from the DFE and renders the bitmap into a form that can control the printing process from the exposure device to transferring the print image onto the receiver. The finishing system applies features such as protection, glossing, or binding to the prints. The finishing system can be implemented as an integral component of a printer, or as a separate machine through which prints are fed after they are printed. 
     The printer can also include a color management system that accounts for characteristics of the image printing process implemented in the printing module (e.g., the electrophotographic process) to provide known, consistent color reproduction characteristics. The color management system can also provide known color reproduction for different inputs (e.g., digital camera images or film images). Color management systems are well-known in the art, and any such system can be used to provide color corrections in accordance with the present invention. 
     In an embodiment of an electrophotographic modular printing machine useful with various embodiments (e.g., the NEXPRESS SX 3900 printer manufactured by Eastman Kodak Company of Rochester, NY) color-toner print images are made in a plurality of color imaging modules arranged in tandem, and the print images are successively electrostatically transferred to a receiver adhered to a transport web moving through the modules. Colored toners include colorants, (e.g., dyes or pigments) which absorb specific wavelengths of visible light. Commercial machines of this type typically employ intermediate transfer members in the respective modules for transferring visible images from the photoreceptor and transferring print images to the receiver. In other electrophotographic printers, each visible image is directly transferred to a receiver to form the corresponding print image. 
     Electrophotographic printers having the capability to also deposit clear toner using an additional imaging module are also known. The provision of a clear-toner overcoat to a color print is desirable for providing features such as protecting the print from fingerprints, reducing certain visual artifacts or providing desired texture or surface finish characteristics. Clear toner uses particles that are similar to the toner particles of the color development stations but without colored material (e.g., dye or pigment) incorporated into the toner particles. However, a clear-toner overcoat can add cost and reduce color gamut of the print; thus, it is desirable to provide for operator/user selection to determine whether or not a clear-toner overcoat will be applied to the entire print. A uniform layer of clear toner can be provided. A layer that varies inversely according to heights of the toner stacks can also be used to establish level toner stack heights. The respective color toners are deposited one upon the other at respective locations on the receiver and the height of a respective color toner stack is the sum of the toner heights of each respective color. Uniform stack height provides the print with a more even or uniform gloss. 
       FIGS.  1 - 2    are elevational cross-sections showing portions of a typical electrophotographic printer  100  useful with various embodiments. Printer  100  is adapted to produce images, such as single-color images (i.e., monochrome images), or multicolor images such as CMYK, or pentachrome (five-color) images, on a receiver. Multicolor images are also known as “multi-component” images. One embodiment involves printing using an electrophotographic print engine having five sets of single-color image-producing or image-printing stations or modules arranged in tandem, but more or less than five colors can be combined on a single receiver. Other electrophotographic writers or printer apparatuses can also be included. Various components of printer  100  are shown as rollers; other configurations are also possible, including belts. 
     Referring to  FIG.  1   , printer  100  is an electrophotographic printing apparatus having a number of tandemly-arranged electrophotographic image-forming printing modules  31 ,  32 ,  33 ,  34 ,  35 , also known as electrophotographic imaging subsystems. Each printing module  31 ,  32 ,  33 ,  34 ,  35  produces a single-color toner image for transfer using a respective transfer subsystem  50  (for clarity, only one is labeled) to a receiver  42  successively moved through the modules. In some embodiments one or more of the printing modules  31 ,  32 ,  33 ,  34 ,  35  can print a colorless toner image, which can be used to provide a protective overcoat or tactile image features. Receiver  42  is transported from supply unit  40 , which can include active feeding subsystems as known in the art, into printer  100  using a transport web  81 . In various embodiments, the visible image can be transferred directly from an imaging roller to a receiver, or from an imaging roller to one or more transfer roller(s) or belt(s) in sequence in transfer subsystem  50 , and then to receiver  42 . Receiver  42  is, for example, a selected section of a web or a cut sheet of a planar receiver media such as paper or transparency film. 
     In the illustrated embodiments, each receiver  42  can have up to five single-color toner images transferred in registration thereon during a single pass through the five printing modules  31 ,  32 ,  33 ,  34 ,  35  to form a pentachrome image. As used herein, the term “pentachrome” implies that in a print image, combinations of various of the five colors are combined to form other colors on the receiver  42  at various locations on the receiver  42 , and that all five colors participate to form process colors in at least some of the subsets. That is, each of the five colors of toner can be combined with toner of one or more of the other colors at a particular location on the receiver  42  to form a color different than the colors of the toners combined at that location. In an exemplary embodiment, printing module  31  forms black (K) print images, printing module  32  forms yellow (Y) print images, printing module  33  forms magenta (M) print images, and printing module  34  forms cyan (C) print images. 
     Printing module  35  can form a red, blue, green, or other fifth print image, including an image formed from a clear toner (e.g., one lacking pigment). The four subtractive primary colors, cyan, magenta, yellow, and black, can be combined in various combinations of subsets thereof to form a representative spectrum of colors. The color gamut of a printer (i.e., the range of colors that can be produced by the printer) is dependent upon the materials used and the process used for forming the colors. The fifth color can therefore be added to improve the color gamut. In addition to adding to the color gamut, the fifth color can also be a specialty color toner or spot color, such as for making proprietary logos or colors that cannot be produced with only CMYK colors (e.g., metallic, fluorescent, or pearlescent colors), or a clear toner or tinted toner. Tinted toners absorb less light than they transmit, but do contain pigments or dyes that move the hue of light passing through them towards the hue of the tint. For example, a blue-tinted toner coated on white paper will cause the white paper to appear light blue when viewed under white light, and will cause yellows printed under the blue-tinted toner to appear slightly greenish under white light. 
     Receiver  42   a  is shown after passing through printing module  31 . Print image  38  on receiver  42   a  includes unfused toner particles. Subsequent to transfer of the respective print images, overlaid in registration, one from each of the respective printing modules  31 ,  32 ,  33 ,  34 ,  35 , receiver  42   a  is advanced to a fuser module  60  (i.e., a fusing or fixing assembly) to fuse the print image  38  to the receiver  42   a.  Transport web  81  transports the print-image-carrying receivers to the fuser module  60 , which fixes the toner particles to the respective receivers, generally by the application of heat and pressure. The receivers are serially de-tacked from the transport web  81  to permit them to feed cleanly into the fuser module  60 . The transport web  81  is then reconditioned for reuse at cleaning station  86  by cleaning and neutralizing the charges on the opposed surfaces of the transport web  81 . A mechanical cleaning station (not shown) for scraping or vacuuming toner off transport web  81  can also be used independently or with cleaning station  86 . The mechanical cleaning station can be disposed along the transport web  81  before or after cleaning station  86  in the direction of rotation of transport web  81 . 
     In the illustrated embodiment, the fuser module  60  includes a heated fusing roller  62  and an opposing pressure roller  64  that form a fusing nip  66  therebetween. In an embodiment, fuser module  60  also includes a release fluid application substation  68  that applies release fluid, e.g., silicone oil, to fusing roller  62 . Alternatively, wax-containing toner can be used without applying release fluid to the fusing roller  62 . Other embodiments of fusers, both contact and non-contact, can be employed. For example, solvent fixing uses solvents to soften the toner particles so they bond with the receiver. Photoflash fusing uses short bursts of high-frequency electromagnetic radiation (e.g., ultraviolet light) to melt the toner. Radiant fixing uses lower-frequency electromagnetic radiation (e.g., infrared light) to more slowly melt the toner. Microwave fixing uses electromagnetic radiation in the microwave range to heat the receivers (primarily), thereby causing the toner particles to melt by heat conduction, so that the toner is fixed to the receiver. 
     The fused receivers (e.g., receiver  42   b  carrying fused image  39 ) are transported in series from the fuser module  60  along a path either to an output tray  69 , or back to printing modules  31 ,  32 ,  33 ,  34 ,  35  to form an image on the backside of the receiver (i.e., to form a duplex print). Receivers  42   b  can also be transported to any suitable output accessory. For example, an auxiliary fuser or glossing assembly can provide a clear-toner overcoat. Printer  100  can also include multiple fuser modules  60  to support applications such as overprinting, as known in the art. 
     In various embodiments, between the fuser module  60  and the output tray  69 , receiver  42   b  passes through a finishing system  70 . Finishing system  70  performs various paper-handling operations, such as folding, stapling, saddle-stitching, collating, and binding. 
     Printer  100  includes main printer apparatus logic and control unit (LCU)  99 , which receives input signals from various sensors associated with printer  100  and sends control signals to various components of printer  100 . LCU  99  can include a microprocessor incorporating suitable look-up tables and control software executable by the LCU  99 . It can also include a field-programmable gate array (FPGA), programmable logic device (PLD), programmable logic controller (PLC) (with a program in, e.g., ladder logic), microcontroller, or other digital control system. LCU  99  can include memory for storing control software and data. In some embodiments, sensors associated with the fuser module  60  provide appropriate signals to the LCU  99 . In response to the sensor signals, the LCU  99  issues command and control signals that adjust the heat or pressure within fusing nip  66  and other operating parameters of fuser module  60 . This permits printer  100  to print on receivers of various thicknesses and surface finishes, such as glossy or matte. 
     Image data for printing by printer  100  can be processed by a raster image processor (RIP; not shown), which can include a color separation screen generator or generators. The output of the RIP can be stored in frame or line buffers for transmission of the color separation print data to each of a set of respective LED writers associated with the printing modules  31 ,  32 ,  33 ,  34 ,  35  (e.g., for black (K), yellow (Y), magenta (M), cyan (C), and red (R) color channels, respectively). The RIP or color separation screen generator can be a part of printer  100  or remote therefrom. Image data processed by the RIP can be obtained from a color document scanner or a digital camera or produced by a computer or from a memory or network which typically includes image data representing a continuous image that needs to be reprocessed into halftone image data in order to be adequately represented by the printer. The RIP can perform image processing processes (e.g., color correction) in order to obtain the desired color print. Color image data is separated into the respective colors and converted by the RIP to halftone dot image data in the respective color (for example, using halftone matrices, which provide desired screen angles and screen rulings). The RIP can be a suitably-programmed computer or logic device and is adapted to employ stored or computed halftone matrices and templates for processing separated color image data into rendered image data in the form of halftone information suitable for printing. These halftone matrices can be stored in a screen pattern memory. 
       FIG.  2    shows additional details of printing module  31 , which is representative of printing modules  32 ,  33 ,  34 , and  35  ( FIG.  1   ). Photoreceptor  206  of imaging member  111  includes a photoconductive layer formed on an electrically conductive substrate. The photoconductive layer is an insulator in the substantial absence of light so that electric charges are retained on its surface. Upon exposure to light, the charge is dissipated. In various embodiments, photoreceptor  206  is part of, or disposed over, the surface of imaging member  111 , which can be a plate, drum, or belt. Photoreceptors can include a homogeneous layer of a single material such as vitreous selenium or a composite layer containing a photoconductor and another material. Photoreceptors  206  can also contain multiple layers. 
     Charging subsystem  210  applies a uniform electrostatic charge to photoreceptor  206  of imaging member  111 . In an exemplary embodiment, charging subsystem  210  includes a wire grid  213  having a selected voltage. Additional necessary components provided for control can be assembled about the various process elements of the respective printing modules. Meter  211  measures the uniform electrostatic charge provided by charging subsystem  210 . 
     An exposure subsystem  220  is provided for selectively modulating the uniform electrostatic charge on photoreceptor  206  in an image-wise fashion by exposing photoreceptor  206  to electromagnetic radiation to form a latent electrostatic image. The uniformly-charged photoreceptor  206  is typically exposed to actinic radiation provided by selectively activating particular light sources in an LED array or a laser device outputting light directed onto photoreceptor  206 . In embodiments using laser devices, a rotating polygon (not shown) is sometimes used to scan one or more laser beam(s) across the photoreceptor in the fast-scan direction. One pixel site is exposed at a time, and the intensity or duty cycle of the laser beam is varied at each dot site. In embodiments using an LED array, the array can include a plurality of LEDs arranged next to each other in a line, all dot sites in one row of dot sites on the photoreceptor  206  can be selectively exposed simultaneously, and the intensity or duty cycle of each LED can be varied within a line exposure time to expose each pixel site in the row during that line exposure time. 
     As used herein, an “engine pixel” is the smallest addressable unit on photoreceptor  206  which the exposure subsystem  220  (e.g., the laser or the LED) can expose with a selected exposure different from the exposure of another engine pixel. Engine pixels can overlap (e.g., to increase addressability in the slow-scan direction). Each engine pixel has a corresponding engine pixel location, and the exposure applied to the engine pixel location is described by an engine pixel level. 
     The exposure subsystem  220  can be a write-white or write-black system. In a write-white or “charged-area-development” system, the exposure dissipates charge on areas of photoreceptor  206  to which toner should not adhere. Toner particles are charged to be attracted to the charge remaining on photoreceptor  206 . The exposed areas therefore correspond to white areas of a printed page. In a write-black or “discharged-area development” system, the toner is charged to be attracted to a bias voltage applied to photoreceptor  206  and repelled from the charge on photoreceptor  206 . Therefore, toner adheres to areas where the charge on photoreceptor  206  has been dissipated by exposure. The exposed areas therefore correspond to black areas of a printed page. 
     In the illustrated embodiment, meter  212  is provided to measure the post-exposure surface potential within a patch area of a latent image formed from time to time in a non-image area on photoreceptor  206 . Other meters and components can also be included (not shown). 
     A development station  225  includes toning shell  226 , which can be rotating or stationary, for applying toner of a selected color to the latent image on photoreceptor  206  to produce a developed image on photoreceptor  206  corresponding to the color of toner deposited at this printing module  31 . Development station  225  is electrically biased by a suitable respective voltage to develop the respective latent image, which voltage can be supplied by a power supply (not shown). Developer is provided to toning shell  226  by a supply system (not shown) such as a supply roller, auger, or belt. Toner is transferred by electrostatic forces from development station  225  to photoreceptor  206 . These forces can include Coulombic forces between charged toner particles and the charged electrostatic latent image, and Lorentz forces on the charged toner particles due to the electric field produced by the bias voltages. 
     In some embodiments, the development station  225  employs a two-component developer that includes toner particles and magnetic carrier particles. The exemplary development station  225  includes a magnetic core  227  to cause the magnetic carrier particles near toning shell  226  to form a “magnetic brush,” as known in the electrophotographic art. Magnetic core  227  can be stationary or rotating, and can rotate with a speed and direction the same as or different than the speed and direction of toning shell  226 . Magnetic core  227  can be cylindrical or non-cylindrical, and can include a single magnet or a plurality of magnets or magnetic poles disposed around the circumference of magnetic core  227 . Alternatively, magnetic core  227  can include an array of solenoids driven to provide a magnetic field of alternating direction. Magnetic core  227  preferably provides a magnetic field of varying magnitude and direction around the outer circumference of toning shell  226 . Development station  225  can also employ a mono-component developer comprising toner, either magnetic or non-magnetic, without separate magnetic carrier particles. 
     Transfer subsystem  50  includes transfer backup member  113 , and intermediate transfer member  112  for transferring the respective print image from photoreceptor  206  of imaging member  111  through a first transfer nip  201  to surface  216  of intermediate transfer member  112 , and then to a receiver  42  which receives respective toned print images  38  from each printing module in superposition to form a composite image thereon. The print image  38  is, for example, a separation of one color, such as cyan. Receiver  42  is transported by transport web  81 . Transfer to a receiver is affected by an electrical field provided to transfer backup member  113  by power source  240 , which is controlled by LCU  99 . Receiver  42  can be any object or surface onto which toner can be transferred from imaging member  111  by application of the electric field. In this example, receiver  42  is shown prior to entry into a second transfer nip  202 , and receiver  42   a  is shown subsequent to transfer of the print image  38  onto receiver  42   a.    
     In the illustrated embodiment, the toner image is transferred from the photoreceptor  206  to the intermediate transfer member  112 , and from there to the receiver  42 . Registration of the separate toner images is achieved by registering the separate toner images on the receiver  42 , as is done with the NEXPRESS SX 3900. In some embodiments, a single transfer member is used to sequentially transfer toner images from each color channel to the receiver  42 . In other embodiments, the separate toner images can be transferred in register directly from the photoreceptor  206  in the respective printing module  31 ,  32 ,  33 ,  34 ,  25  to the receiver  42  without using a transfer member. Either transfer process is suitable when practicing this invention. An alternative method of transferring toner images involves transferring the separate toner images, in register, to a transfer member and then transferring the registered image to a receiver. 
     LCU  99  sends control signals to the charging subsystem  210 , the exposure subsystem  220 , and the respective development station  225  of each printing module  31 ,  32 ,  33 ,  34 ,  35  ( FIG.  1   ), among other components. Each printing module can also have its own respective controller (not shown) coupled to LCU  99 . 
     As discussed in the background section, one problem that can occur in electrophotographic printing systems is known as “image burn-in” which can occur when a sequence of the same or similar pages having similar image data or patterns are printed. In this case, the performance of various system components can change as a function of position due to the repeated printing of the similar image data or patterns. This can cause subsequently printed images to have characteristics that vary in accordance with the repeated image data or pattern (i.e., “image burn-in artifacts”). This can require that the affected system components be frequently serviced, or even replaced, to eliminate the artifacts which can add significant cost and system down-time. 
     One system component that has been found to be particularly susceptible to the formation of image burn-in artifacts is the fuser module  60  (see  FIG.  1   ). The toner stacks that form the print image  38  on the receiver  42  generally have a higher roughness than the receiver  42 . As a result, when the receiver  42  passes through the fuser module  60 , the roughness of the toner can affect the surface of the fusing roller  62 . If a large number of pages are printed that have high-density image content at the same cross-track position, the surface characteristics of the corresponding portion of the fusing roller  62  can be changed (e.g., roughened) relative to the portions of the fusing roller  62  that correspond to low-density image content. The spatially-dependent surface characteristics of the fusing roller  62  can then affect the image quality of subsequently printed pages. For example, the gloss of the printed image can be lower in the image regions corresponding to the roughened surface of the fusing roller  62 . 
     Other system components besides the fuser module  60  can also contribute to the formation of burn-in artifacts. For example, the response of the photoreceptor  206  ( FIG.  2   ) can become position dependent when the same image content is repeated imaged. 
       FIG.  3 A  shows an example of a page  250  that is susceptible to producing image burn-in artifacts when printed repeatedly. The page  250  includes dark image content  252  which occurs in a particular cross-track position together with light image content  254 . Over time, the dark image content  252  can modify the surface of the fusing roller  62 , producing a roughened surface  256  as shown in  FIG.  3 B , when many copies of the page  250  are printed. The roughened surface  256  can in turn produce image burn-in artifacts  260  in a subsequently printed page  251  (in this example a uniform gray field) as shown in  FIG.  3 C . In this case, the image burn-in artifacts  260  show up as a lower gloss level in the cross-track positions that correspond to the dark image content  252  in page  250 . 
     Other types of artifacts can occur in electrophotographic printers  100  when the rate at which toner is supplied by the development subsystem  225  is too high or too low. If the toner usage rate is too high, there is insufficient time for the newly added toner to reach the target electric charge evenly across the toning station. (The toner usage rate is sometimes referred to as the toner take-out rate.) This can result in low frequency nonuniformity artifacts in high coverage image area. On the other hand, if the toner usage rate is low for an extended period, the toner electric charge in the development subsystem will reach an elevated level which can create imaging artifacts such as mottle at high density levels and disruption of halftone dots at low density levels. 
     The present invention represents a method or system for reducing image burn-in artifacts and artifacts resulting from low or high toner usage rates by printing “compensation images” at specified intervals. The compensation images are not part of the print job and are intended to be discarded after printing. In an exemplary embodiment, the compensation images have cross-track image profiles that are inverted relative to the average cross-track image profiles for the pages in the print job in order to mitigate the image burn-in artifacts, and have a controlled average density level that mitigates the charge buildup/depletion in the development subsystem  225 . 
       FIG.  4    shows a flowchart of a method that can be used to implement the present invention in accordance with an exemplary embodiment. A print job  305  including image data for a set of pages  310  is received using a receive print job step  300 . 
     In some cases, the print job  305  may contain a sequence of pages  310  that include image data or patterns (for example, image content) that are identical or substantially similar. For example, the print job  305  may contain multiple copies of the same page  310 , or may include a series of pages  310  that include similar image content (e.g., a form letter where the name and address of the recipient vary but the rest of the page content is identical). The similar pages may correspond to all of the pages  310  in the print job  305 , or may correspond to a subset of the pages  310  in the print job  305 . In cases where there are a significant number of similar pages, image burn-in artifacts can be formed as discussed relative to  FIGS.  3 A- 3 C   
     In accordance with the present invention, the pages  310  in the print job  305  are processed in a block-wise fashion, where a print block of pages step  320  is used to print a block of pages  315  using the printer  100  to provide corresponding printed pages  325 . In an exemplary embodiment the block of pages  315  has a block size of S=50 pages. In other embodiments, other block sizes can be used such as block sizes in the range of 10≤S≤200 pages  310 . 
     An analyze image data step  330  is then used to analyze the image data for the block of pages  315 . In an exemplary embodiment, the analyze image data step  330  determines an average toner usage rate  335  and cross-track image profiles  340  for the block of pages  315 . 
     Consider a block of images  315  where the image data for the i th  page  310  is given by I i   h (x, y) , where his the color channel (e.g., cyan, magenta, yellow, black or other spot colors) and x and y are the cross-track and in-track pixel positions, respectively. The image data will typically be represented by 8-bit integers where a value of 0 corresponds to printing no toner, and a value of 255 corresponds to printing a maximum amount of toner. 
     Cross-track image profiles  340 , P h   i (x), for each page can be determined by averaging all of the pixels at a particular cross-track position: 
                       P   h   i     (   x   )     =       1     N   y       ⁢       ∑     y   =   1       N   y               I   h   i     (     x   ,   y     )                 (   1   )               
where N is the number of rows in the image data (i.e., the number of pixels in the in-track dimension).
 
       FIG.  5    shows a cross-track image profile  340  determined using Eq. (1) for the exemplary page  250  shown in  FIG.  3 A . It can be seen that toward the left edge of the cross-track image profile  340  there is a large peak in the cross-track image profile  340  corresponding to the black bar in the image content which extends down the length of the page  250 . There is also a broad peak with a smaller magnitude corresponding to the horizontal lines of text in the page  250 . 
     Returning to a discussion of  FIG.  4   , the analyze image data step  330  also determines an average toner usage rate  335  for the block of pages  315 . In an exemplary embodiment, the average toner usage rate  335  for color channel h can be determined using the following equation: 
                       V   _     h     =       1       N   x     ⁢   S       ⁢       ∑     i   =   1     S             ∑     x   =   1       N   x               P   h   i     (   x   )                   (   2   )               
where S is the number of pages in the block of pages  315  and N x  is the number of columns in the image data (i.e., the number of pixels in the cross-track dimension). Note that this average toner usage rate value will be in terms of the average pixel value in the block of images. Conventionally, toner usage rates are measured in terms of physical values such as average toner mass per unit area per page. The determined value of  V   h  will be approximately proportional to the physical toner usage rate. Within the context of the present invention, the term “toner usage rate” is used to refer to any quantity which is used as a measure of the amount of toner applied per page, whether or not the quantity is tied directly to physical quantities.
 
     An acceptable toner usage rate test  345  is used to compare the determined average toner usage rate  335  to a predefined acceptable toner usage rate range  380 . In an exemplary embodiment, the acceptable toner usage rate range  380  is defined by a minimum acceptable toner usage rate V L  and a maximum acceptable toner usage rate range V H  such that the acceptable toner usage rate test  345  can be performed by determining whether or not the average toner usage rate  335  satisfies the inequality: V L ≤ V   h ≤V H . In an exemplary embodiment, the minimum acceptable toner usage rate V L  and the maximum acceptable toner usage rate range V H  are determined by experimentation. The minimum acceptable toner usage rate V L  corresponds to the point where imaging artifacts are formed due to toner electric charge in the development subsystem ( FIG.  2   ) reaching an elevated level. The maximum acceptable toner usage rate range V H  corresponds to the point where image nonuniformity artifacts are formed when the toner usage rate exceeds the system replenishment capability. In some embodiments, the same acceptable toner usage rate range can be used for each color channel. In other embodiments, different acceptable toner usage rate ranges can be defined for each of the color channels. 
     The acceptable toner usage rate range  380  can be determined using any appropriate means. For example, V L  can be determined experimentally as the lowest average coverage where a 1000 page print job can be printed with a negligible increase in the charging voltage, and V H  can be defined as the highest average coverage where a 100 page print job with monochrome flat field images with 100% coverage remain within a uniformity specification. 
     Preferably, the average toner usage rates  335  for each of the color channels are evaluated against the acceptable toner usage rate range  380 . In an exemplary embodiment, if the average toner usage rate  335  for any of the color channels do not satisfy the inequality then the acceptable toner usage rate test  345  returns a value of “No” and control is passed to the form compensation image step  350 , and if the average toner usage rate for all of the color channels satisfy the inequality then the acceptable toner usage rate test  345  returns a value of “Yes” and the method proceeds to print the next block of pages  315 . 
     A form compensation image step  350  is used to form a compensation image  355  and determine a number of compensation images  360  that should be printed. For embodiments where the compensation image  355  is used to compensate for average toner usage rates  335  that are outside of the acceptable toner usage rate range  380 , the compensation image  355  and the number of compensation images  360  are determined such that the average toner usage rate will be brought back into the acceptable toner usage rate range  380  when the determined number of compensation images  360  are printed using the electrophotographic printing system (i.e., printer  100 ). 
     The new average toner usage rate  V ′ h  can be computed by computing a weighted average of the average toner usage rates  335  for the block of pages  315  and an average toner usage rate  V   c  for the compensation images  355 : 
                       V   _     h   ′     =         S   ⁢       V   _     h       +     K   ⁢       V   _     c           S   +   K               (   3   )               
where K is the number of compensation images  360 .
 
     In some embodiments, the compensation images are only used to compensate for average toner usage rates  335  that are outside the acceptable toner usage rate range  380 , and are not used to compensate for image burn-in artifacts. In this case, the simplest case is to define a uniform compensation image  355 . For example, if the average toner usage rate  335  is too low (i.e.,  V   h &lt;V L ), the fastest way to bring the average toner usage rate back into the acceptable toner usage rate range  380  is to use a uniform compensation image  355  having a code value of V H  such that  V   c =V H  . The minimum number of compensation images  350  can then be determined by solving Eq. (3) for K using  V ′ h =V L : 
                     K     m   ,   h       =     ⌈       S   ⁡   (       V   L     -       V   _     h       )       (       V   H     -     V   L       )       ⌉             (     4   ⁢   a     )               
where the notation ┌g┐ denotes a rounding up to the next integer operation.
 
     Similarly, if the average toner usage rate  335  is too high (i.e.,  V   h &gt;V H ), the fastest way to bring the average toner usage rate back into the acceptable toner usage rate range  380  is to use a uniform compensation image  355  having a code value of 0 (i.e., a blank page) such that  V   c =0. The minimum number of compensation images  350  can then be determined by solving Eq. (3) for K using  V ′ h =V H : 
     
       
         
           
             
               
                 
                   
                     K 
                     
                       m 
                       , 
                       h 
                     
                   
                   = 
                   
                     ⌈ 
                     
                       
                         S 
                         ⁡ 
                         ( 
                         
                           
                             
                               V 
                               _ 
                             
                             h 
                           
                           - 
                           
                             V 
                             H 
                           
                         
                         ) 
                       
                       
                         V 
                         H 
                       
                     
                     ⌉ 
                   
                 
               
               
                 
                   ( 
                   
                     4 
                     ⁢ 
                     b 
                   
                   ) 
                 
               
             
           
         
       
     
     Generally, different minimum numbers of compensation images  350  may be determined for each color channel. Then the number of compensation images  350  can then be set to the maximum of the values computed for each color channel:
 
 K   m =max[ K   m,h ]  (5)
 
where the max[⋅] operation is performed across the color channels, h.
 
     In some cases, it may be desirable to use a number of compensation images (K)  360  that is larger than the minimum number K m . For example, it may be desirable to use a number of compensation images  360  that is a multiple of the number of pages that fit on the transport web  81  ( FIG.  1   ). 
     The code values used for the compensation image  355  can be different in each color channel. For example, if the average toner usage rate  335  for one color channel is too low, then the code value for that color channel of the compensation image  355  can be set to VH, while the code value for another color channel can be set to 0 if the average toner usage rate  335  is too high. For color channels where the average toner usage rate  335  is within the acceptable toner usage range  380 , the code value can be set to V L  to insure that the average toner usage rate remains within the acceptable range while minimizing the amount of toner that is used. 
     In some embodiments, the compensation images  355  are used to reduce image burn-in artifacts. As discussed earlier with respect to  FIG.  3 B , image burn-in artifacts can result when the image content of the pages  310  causes position-dependent roughening of the fusing roller  62 . The roughened surface  256  can in turn produce artifacts such as a differential gloss on subsequently printed page  251  as shown in  FIG.  3 C . To mitigate the formation of such artifacts, the compensation images  355  of the present invention can be used to reduce the position-dependent roughening of the fusing roller  62  by arranging the image content of the compensation images  355  such that it is inverted relative to that of the block of pages  315 . In this way, printing the compensation images  355  will roughen the surface of the fusing roller  62  in a way that is complementary to the roughening caused by the block of pages  315 , therefore keeping the surface roughness approximately uniform. 
     In an exemplary embodiment, the form compensation image step  350  can determine compensation images  355  for reducing image burn-in artifacts using the following process. First, a low-pass filter is applied to the cross-track image profiles  340 , P h   i (x), for each page to blur them out in the cross-track direction forming corresponding filtered cross-track image profiles,  (x) :
 
 ( x )= P   i   h ( x )* L ( x )   (6)
 
where “*” indicates a convolution operation, and L(x) is the low-pass filter (for example, a 32 pixel wide box-car filter or a Gaussian filter).
 
     Next a total compensation image profile for each color channel, C h (x) , is determined by inverting and summing the filtered cross-track image profiles: 
                       C   h     (   x   )     =         ∑     i   -   1     S             C   h   i     (   x   )       =       ∑     i   -   1     S       (       P   max     -       (   x   )         )                 (   7   )               
where P max  is a maximum possible value of the filtered cross-track image profiles  (x) . For example, in some embodiments, P max =255. It can be seen that Eq. (7) has the effect of inverting the cross-track image profiles so that the total compensation image profile will have an inverted shape relative to the cross-track image profiles. That is, where the average of the cross-track image profiles is high, the total compensation image profile will be low, and vice versa.
 
     To minimize the toner usage when printing the compensation images  355 , the minimum value of the total compensation image profile can be subtracted without any loss in the effectiveness at reducing the image burn-in artifacts:
 
 C′   h ( x )= C   h ( x )−min[ C   h ( x )]  (8)
 
where the min[⋅] operation finds the minimum value of the total compensation image profile across all cross-track positions.
 
     The shifted total compensation image profile C′ h (x) represents the total amount of toner that should be applied in the compensation images  355  as a function of cross-track position. Generally, the maximum value of C′ h (x) will exceed  255 , indicating that the toner to be printed will need to be divided between multiple compensation images  355 . The minimum number of compensation images K m  can be determined by dividing the maximum value of the shifted total compensation image profile C′ h (x) by the maximum printable code value (e.g., P max =255) and rounding up: 
                     K   m     =     ⌈       max   [       C   h   ′     (   x   )     ]       P   max       ⌉             (   9   )               
where the max[⋅] operation is performed across all cross-track positions. As discussed earlier, in some cases, it may be desirable to use a number of compensation images (K)  360  that is larger than the minimum number K m . The number of compensation images  360  can be defined using any appropriate selection strategy as long as K≥K m .
 
     The cross-track image profile Ĉ h (x) for the compensation images  355  can be determined by dividing the shifted total compensation image profile C′ h (x) by the number of compensation images:
 
 Ĉ   h ( x )= C′   h ( x )/ K    (10)
 
     In some embodiments, the equation for cross-track image profile can be modified to include a sensitivity threshold δ and a tunable gain factor λ which can be used to account for the damping effect from the printed image data to the overall printing system:
 
 Ĉ   h ( x )=min[λ(max[ C′   h ( x )/ K−δ,  0]),  P   max  ]  (11)
 
where the max[⋅] and min[⋅] operations have the effect of limiting the values to be between 0≤Ĉ h (x)≤P max .
 
     The compensation image  355  can then be formed by repeating the cross-track image profile Ĉ h (x) for each row of the compensation image  355 . 
       FIG.  6    illustrates an exemplary cross-track image profile  510  for a compensation image  355  determined for a block of pages  315  having an average cross-track image profile  340  of  FIG.  5   . It can be seen that the cross-track image profile  510  for a compensation image  355  has an inverted shape relative to the average cross-track image  340  determined for the block of pages  315 . 
       FIG.  7    shows an exemplary compensation image  355  formed from the cross-track image profile  510  of  FIG.  6   . The cross-track image profile  510  is used to define the pixel values for a row of the compensation image  355 , and the row is then repeated down the page. 
     A toner usage rate for the resulting compensation image  355  can be determined using: 
                       V   _     c     =       1     N   x       ⁢       ∑     x   =   1       N   x                 C   )     h     (   x   )                 (   12   )               
An average toner usage rate  335  including the block of pages  315  and the compensation images  355  can then be computed using Eq. (3) to confirm that it falls within the acceptable toner usage rate range  380 . If the average toner usage rate  335  falls outside of the acceptable toner usage rate range  380 , adjustments can be made in a number of different ways. In an exemplary embodiment, after printing the compensation images  355  with the determined cross-track image profile Ĉ h (x) , additional uniform compensation images can be printed (e.g., blank pages to reduce the ink usage rate or solid pages with code value V H  to increase the ink usage rate as was discussed earlier). The number of additional uniform compensation images can be calculated using Eqs. ( 4   a )-( 4   b ) where the average toner usage rate used in the equations is that computed from the printed block of pages  315  together with the printed compensation images  355 . In other embodiments, the cross-track image profile for the compensation image can be modified to increase or decrease the toner usage rate as necessary. In this case, it will generally be necessary to increase the number of compensation images  360  accordingly in order to keep the same differential toner laydown.
 
     Once the compensation image  355  and the number of compensation images  360  have been determined, a print compensation images step  365  is used to form printed compensation images  370 . Since the printed compensation images  370  do not belong to the print job  305 , they should not be directed into the same locations as the printed pages  325  (e.g., output tray  69 ) and can be discarded using a discard compensation images step  375 . In an exemplary embodiment, the discard compensation images step  375  directs the printed compensation images  370  into an appropriate waste receptacle. For example, the waste receptacle could be an alternate output tray, or a trash bin, or some other designated output location which is periodically emptied by the operator. 
     After the compensation images are printed, the method proceeds to print the next block of pages  315 . This process is repeated until all of the pages  310  in the print job  305  have been printed. 
     In some embodiments of the present invention, the compensation images  355  can be used to compensate for the image burn-in artifacts without addressing any artifacts that may result from the toner usage rate. In this case, the method of  FIG.  4    can be modified by skipping the acceptable toner usage rate test  345 . 
     In some embodiments, the method of the present invention can be combined with the methods described in commonly assigned, co-pending U.S. patent application Ser. No. 17/518,645, entitled: “Electrophotographic printing system including page rotations to reduce burn-in artifacts”, by T. Schwartz et al. and commonly assigned, co-pending U.S. patent application Ser. No. 17/518,664, entitled: “Electrophotographic printing system including lateral translations to reduce burn-in artifacts”, by T. Schwartz et al. to further reduce the magnitude of the burn-in artifacts. 
     In embodiments of the present inventions, a controller (such as the logic and control unit  99  in  FIG.  1   ) is used to implement the various operations such as some or all of the steps in the method of  FIG.  4   . In some embodiments the controller is a single processing unit operated by appropriate software. In other embodiments, the controller can include a plurality of different processing units each of which implement a portion of the operations. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations, combinations, and modifications can be effected by a person of ordinary skill in the art within the spirit and scope of the invention. 
     Parts List 
       31  printing module 
       32  printing module 
       33  printing module 
       34  printing module 
       35  printing module 
       38  print image 
       39  fused image 
       40  supply unit 
       42  receiver 
       42   a  receiver 
       42   b  receiver 
       50  transfer subsystem 
       60  fuser module 
       62  fusing roller 
       64  pressure roller 
       66  fusing nip 
       68  release fluid application substation 
       69  output tray 
       70  finishing system 
       81  transport web 
       86  cleaning station 
       99  logic and control unit 
       100  printer 
       111  imaging member 
       112  intermediate transfer member 
       113  transfer backup member 
       201  first transfer nip 
       202  second transfer nip 
       206  photoreceptor 
       210  charging subsystem 
       211  meter 
       212  meter 
       213  grid 
       216  surface 
       220  exposure subsystem 
       225  development subsystem 
       226  toning shell 
       227  magnetic core 
       240  power source 
       250  page 
       251  printed page 
       252  dark image content 
       254  light image content 
       256  roughened surface 
       260  image burn-in artifacts 
       300  receive print job step 
       305  print job 
       310  page 
       315  block of pages 
       320  print block of pages step 
       325  printed pages 
       330  analyze image data step 
       335  average toner usage rate 
       340  cross-track image profiles 
       345  acceptable toner usage rate test 
       350  form compensation image step 
       355  compensation image 
       360  number of compensation images 
       365  print compensation images step 
       370  printed compensation images 
       375  discard compensation images step 
       380  acceptable toner usage rate range 
       510  cross-track image profile