Abstract:
A method enables the positions of inoperable inkjets in printheads to be corrected for positional changes arising from shifts in media, shrinkage of the media, thermal expansion of optical sensors that detect the inoperable inkjets, and other measurement errors. The measurement errors are detected with reference to image data corresponding to a reference pattern printed within a portion of a print job. Thus, reference marks do not need to be printed in areas outside of a print job as previously required.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates to imaging devices that generate printed images on an image receiving member with pixels of colorant, and more particularly, to imaging devices that identify missing pixels during the operation of the imaging device. 
       BACKGROUND 
       [0002]    Imaging devices form images on image receiving members that include paper and other print media. Different imaging or printing techniques, which include laser printing, inkjet printing, offset printing, dye-sublimation printing, thermal printing, and the like, may be used to produce printed documents. In particular, inkjet imaging devices eject liquid ink from printheads to form images on an image receiving member. The printheads include a plurality of inkjets that are arranged in some type of array. Each inkjet has a thermal or piezoelectric actuator that is coupled to a printhead controller. The printhead controller generates firing signals that correspond to digital data for images. The printhead actuators respond to the firing signals by ejecting ink drops onto an image receiving member to form an ink image that corresponds to the digital image used to generate the firing signals. The size of the ink drops and the timing of the ejection of the ink drops are affected by the frequency and amplitude of the firing signals. 
         [0003]    Throughout the life cycle of imaging devices, the image generating ability of the device requires evaluation and, if the images contain detectable defects, correction. Various defects in the image generating process affect ink image quality. In an inkjet printing system, one such defect occurs when an individual inkjet becomes inoperable as either a “weak” or “missing” inkjet. A weak inkjet intermittently ejects ink drops or ejects ink drops having a mass that is different than expected for the firing signal used to operate the actuator for the inkjet. A missing inkjet fails to eject ink drops entirely. Inoperable inkjets, including both weak and missing inkjets, negatively impact the quality of printed images. 
         [0004]    Some existing printing systems are configured to detect and compensate for weak or missing inkjets. Identifying inoperable inkjets typically requires the printing of reference patterns, which are specially designed and arranged ink lines that are printed on the image receiving member. These reference patterns must be printed separately from the images printed as part of a print job. Consequently, the printing of reference patterns absorbs a portion of the resources to be used for productive printing. Because a printhead often includes hundreds or thousands of individual inkjets, correct identification of a single inoperable inkjet presents challenges. In some imaging devices, an optical sensor is used to generate image data of the reference pattern on an image receiving member and these data are analyzed and correlated to inkjet positions in a printhead to identify a weak or missing inkjet. Errors in the alignment of the photosensors in the optical sensor or in the calibration of the sensor along with distortions that arise from media shifting during operation of the printer affect the accuracy of the analysis of the image data. Consequently, improvements to the identification of weak or missing inkjets in an inkjet printer would be beneficial. 
       SUMMARY 
       [0005]    In one embodiment, a method for identifying weak or missing inkjets in a printhead has been developed. The method includes detecting at least one inoperable inkjet in the printhead of the inkjet printer with reference to image data of a portion of a print job printed onto an image receiving member, identifying an offset for a position of the at least one inoperable inkjet; adjusting a position for the detected at least one inoperable inkjet with reference to the identified offset; and compensating for the detected at least one inoperable inkjet with reference to the adjusted position for the at least one inoperable inkjet. 
         [0006]    In another embodiment, an inkjet imaging system that detects weak or missing inkjets has been developed. The system includes a plurality of printheads arranged in the inkjet imaging device to form a print zone in the inkjet imaging device, each printhead having a plurality of inkjets configured to eject ink, an optical sensor positioned between the print zone formed by the plurality of printheads and an exit for documents printed by the plurality of printheads, and a controller operatively connected to the plurality of printheads and to the optical sensor. The controller is configured to operate the plurality of printheads to eject ink onto an image receiving member as the image receiving member passes through the print zone in the inkjet imaging device, detect at least one inoperable inkjet in one of the printheads of the inkjet printer with reference to image data of a portion of a print job formed with ink ejected onto the image receiving member, identify an offset for a position of the at least one inoperable inkjet in the one printhead, adjust a position for the detected at least one inoperable inkjet with reference to the identified offset, and operate the printhead in which the detected at least one inoperable inkjet is located to compensate for the detected at least one inoperable inkjet with reference to the adjusted position for the at least one inoperable inkjet. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The foregoing aspects and other features of a system and method that identify weak or missing inkjets in a printhead are explained in the following description, taken in connection with the accompanying drawings. 
           [0008]      FIG. 1  is a schematic diagram of a continuous feed printer. 
           [0009]      FIG. 2  is a block diagram of a process for identifying an inoperable inkjet in a printhead and for compensating for the inoperable inkjet. 
           [0010]      FIG. 3  is a block diagram of a process for identifying an offset of the inoperable inkjet identified in  FIG. 2 . 
           [0011]      FIG. 4  is a plan view of a media web with a reference pattern and printed images. 
           [0012]      FIG. 5  is a block diagram of another process for identifying the offset of the inoperable inkjet identified in  FIG. 2 . 
           [0013]      FIG. 6  is a plan view of a media web with printed images and registration marks printed concurrently with the printed images. 
           [0014]      FIG. 7  is a schematic diagram of inkjets in a printhead. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    For a general understanding of the environment for the system and method disclosed herein as well as the details for the system and method, reference is made to the drawings. In the drawings, like reference numerals have been used throughout to designate like elements. As used herein, the word “printer” encompasses any apparatus that forms a printed image on media. Examples of printers include, but are not limited to, digital copiers, bookmaking machines, facsimile machines, multi-function machines, or the like. The term “image receiving member” encompasses any print medium including paper, as well as indirect imaging members including imaging drums or belts. As used herein, the term “process direction” refers to a direction of travel of the image receiving member in the printer, and the term “cross-process direction” refers to a direction that is perpendicular to the process direction on the surface of the image receiving member. The terms “image” and “printed image” refer to any pattern of ink drops that a printer forms on the image receiving member. Examples of images include text and graphics in one or more colors that are printed on the image receiving member. 
         [0016]    The term “page” refers to an area of the surface of an image receiving member that receives a printed image that corresponds to one page of a document. The term “document zone” as used herein refers to a portion of the page that receives printed images. An “inter-document zone” refers to a margin that separates document zones in two adjoining pages where the printer does not print images of a print job portion during a print job, but some printers print images of reference patterns in the inter-document zones. A continuous media web may have a plurality of pages formed on its surface with a predetermined space left between adjacent printed images of each page to facilitate cutting the web into individual sheets. These predetermined spaces are an example of inter-document zones. A single sheet of paper may have printed images corresponding to different pages formed on each side of the sheet. 
         [0017]      FIG. 1  depicts an exemplary embodiment of a printer  100  that is configured to identify inoperable inkjets in one or more printheads. Printer  100  is a continuous web printer that includes six print modules  102 ,  104 ,  106 ,  108 ,  110 , and  112 ; a media path  124  configured to accept a print medium  114 , and a controller  128 . The print modules  102 - 112  are positioned sequentially along a media path  124  and form a print zone for forming images on a print medium  114  as the print medium  114  moves past the print modules. 
         [0018]    In printer  100 , each print module  102 ,  104 ,  106 ,  108 ,  110 , and  112  in this embodiment provides an ink of a different color. In all other respects, the print modules  102 - 112  are substantially identical. Print module  102  includes two print sub-modules  140  and  142 . Print sub-module  140  includes two print units  144  and  146 . The print units  144  and  146  each include an array of printheads that may be arranged in a staggered configuration across the width of both the first section of web media and second section of web media. Each of the printheads includes a plurality of inkjets in a configuration similar to the printhead  100  depicted in  FIG. 1 . In a typical embodiment, print unit  144  has four printheads and print unit  146  has three printheads. The printheads in print units  144  and  146  are positioned in a staggered arrangement to enable the printheads in both units to emit ink drops in a continuous line across the width of media path  124  at a predetermined resolution. 
         [0019]    Print sub-module  142  is configured in a substantially identical manner to sub-module  140 , but the printheads in sub-module  142  are offset by one-half the distance between inkjet ejectors in the cross-process direction from the printheads in sub-module  140 . The arrangement of sub-modules  140  and  142  enables a doubling of linear resolution for images formed on the media web  114 . For example, if each of the sub-modules  140  and  142  emits ink drops at a resolution of 300 drops per inch (dpi), the combination of sub-modules  140  and  142  emits ink drops at a resolution of 600 dpi. 
         [0020]    During operation, the media web  114  moves through the media path  124  in the process direction. The media web  114  unrolls from a source roll  152  and passes through a brush cleaner  122  and a contact roller  126  prior to entering the print zone. The media web  114  moves through the print zone past the print modules  102 - 112  guided by a pre-heater roller  118 , backer rollers exemplified by backer roller  116 , apex roller  119 , and leveler roller  120 . The media web  114  then passes through a heater  130  and a spreader  132  after passing through the print zone. The media web passes an exit guide roller  134  and then winds onto a take-up roller  154 . The media path  124  depicted in  FIG. 1  is exemplary of one media path configuration in a web printing system, but various different configurations may lead the web past different rollers and other components. Alternative media path configurations include a duplexing unit that enables the printer  100  to form ink images on both sides of the media web  114 . 
         [0021]    In one embodiment, the source roll  152  is a roll of printing paper that winds through the media path  124  as the media web  114 . The paper provided at the source roll  152  includes a certain amount of moisture. The various heaters positioned in the media path  124  heat the media web, and the media web  114  shrinks due to evaporation of moisture from the media web  114 . Additionally, the media web  114  may experience oscillation in the cross-process direction as the media web  114  engages the rollers positioned along the media path  124 . In one embodiment, the media web  114  experiences oscillation with a magnitude of 30 μm at a frequency of 8 Hz. As described in more detail below, both shrinkage and oscillation of the media web  114  contribute to a cross-process offset between an apparent position of an inoperable inkjet identified from images formed on the media web and the actual position of the inoperable inkjet in the printer  100 . 
         [0022]    The printer  100  includes an optical sensor  138  that generates image data corresponding to light reflected from the media web  114  after the media web  114  has passed through the print zone. The optical sensor  138  is configured to detect, for example, the location, intensity, and/or location of ink drops jetted onto the receiving member by the inkjets of the printhead assembly. The optical sensor  138  also detects light reflected from unprinted portions of the media web  114 . The optical sensor  138  includes an array of optical detectors mounted to a bar or other longitudinal structure that extends across the width of the media web  114  in the cross-process direction. 
         [0023]    In one embodiment in which the media web  114  is approximately twenty inches wide in the cross process direction and the print modules  102 - 112  print at a resolution of 600 dpi in the cross process direction, over 12,000 optical detectors are arrayed in a single row along the bar in the optical sensor  138  to generate a single scanline across the image receiving member. The 12,000 optical detectors detect light corresponding to 12,000 pixels arranged on the surface of the image receiving member in the cross-process direction. The term “pixel” refers to one location in a grid-like pattern of potential locations where printed ink drops land on the image receiving member. In one embodiment, each optical detector generates a numeric value corresponding to the intensity of light reflected from one pixel on the image receiving member. The numeric value of the intensity corresponds to the amount of light reflected from the pixel, with a white image receiving member reflecting the most light to generate the highest numeric intensity value while a pixel filled with a black ink drop generates the lowest numeric intensity value. 
         [0024]    The optical detector  138  detects a single row of pixels in the cross-process direction at one time, and detects successive rows of pixels as the media web  114  moves through the media path  124 . The optical detectors are configured in association in one or more light sources that direct light towards the surface of the image receiving member. The optical detectors are arranged in the optical sensor  138  in a predetermined configuration in the cross-process direction. Consequently, the cross-process position of light reflected from the media web  114  can be identified with reference to the optical detector that detects the reflected light. 
         [0025]    The optical sensor  138  detects a cross-process position of ink drops formed on the image receiving member  114 , and the optical sensor  138  can also detect light streaks in printed images that correspond to an inoperable inkjet in one of the printheads that prints the image. The term “light streak” refers to a linear arrangement of pixels extending in the process direction on an image receiving member having an increased intensity level due to at least one inkjet corresponding to the pixels either failing to eject ink drops, or ejecting ink drops on an incorrect position of the image receiving member. The controller  128  identifies one printhead in the plurality of printheads that includes the inoperable inkjet with reference to printheads that formed the ink image that includes the light streak. In multi-color printing systems, each optical detector in the optical sensor  138  includes photodetectors that are selectively sensitive to red, green, and blue (RGB) light. Each optical detector records different amplitudes of reflected light detected by each of the RGB detectors, in addition to a sum of light received by all detectors to generate an RGB digital image of the ink image. The controller  128  can identify the color and corresponding printhead of an inoperable inkjet using the color image data. 
         [0026]    A light streak indicates that an inkjet is inoperable, and the cross-process location of each optical detector in the optical sensor  138  corresponds to an inkjet in at least one of the printheads. Due to spatial distortions of the media web, lateral movement of the media web, and thermal expansion of the optical sensor  138 , the inoperable inkjet may not correspond exactly to the optical detector that detects the light streak. In one example, web shrinkage near one edge of the media web  114  results in a two pixel wide offset in the cross-process direction between the apparent position of a light streak and the actual position of the inoperable inkjet. In another example, the entire media web oscillates in the cross-process direction and the magnitude and direction of the offset changes as the media web oscillates. 
         [0027]    Some distortions on the media web are non-uniform and are referred to as “local” distortions. The magnitude of offset due to media web shrinkage is a local distortion that is greatest near the edges of the media web and decreases near the center of the media web. Other sources of offset affect the entire media web in a uniform manner and are referred to as “global” distortions. A cross-process oscillation of the media web  114  as the media web  114  moves past the optical sensor  138  is a global distortion that affects the entire media web uniformly, at least in the region around the optical sensor  138 . In some configurations, multiple distortions including some or all of the distortions described above contribute to an offset between the apparent inoperable inkjet that is nominally aligned with the optical detector and an actual inoperable inkjet. 
         [0028]    The controller  128  is configured to control various subsystems, components and functions of printer  100 . The controller  128  is operatively connected to each of the printheads in the print modules  102 - 112  to control ejection of ink from each of the print modules  102 - 112 . The controller  128  is also connected to the optical sensor  138  to receive image data that the optical sensor  138  generates from light reflected from the media web  114 . 
         [0029]    The controller  128  stores and retrieves data, including stored program instructions, held in a memory  129 . Various embodiments of the memory  129  include volatile data storage devices, such as static and dynamic random access memory (RAM), as well as non-volatile data storage devices, which include magnetic and optical disks, solid-state storage devices including flash memory, and any other data storage device that is configured to store and retrieve data for the controller  128 . 
         [0030]    In various embodiments, controller  128  is implemented with general or specialized programmable processors that execute programmed instructions. These components may be provided on a printed circuit card or provided as a circuit in an application specific integrated circuit (ASIC). Each of the circuits may be implemented with a separate processor or multiple circuits may be implemented on the same processor. Alternatively, the circuits may be implemented with discrete components or circuits provided in VLSI circuits. Also, the circuits described herein may be implemented with a combination of processors, ASICs, discrete components, and VLSI circuits. 
         [0031]    Controller  128  is operatively coupled to the print modules  102 - 112  and controls the timing of ink drop ejection from the print modules  102 - 112  onto the media web  114 . The controller  128  generates a plurality of electrical firing signals for the inkjets in each of the print modules  102 - 112 . In one configuration, the controller  128  generates a predetermined sequence of firing signals for each of the printheads in the print modules  102 - 112  to generate reference pattern ink marks on the media web  114 . In another configuration, the controller  128  receives data corresponding to one or more images in a print job. The controller  128  generates firing signals for the print modules  102 - 112  corresponding to the print job data to print images on the image receiving member  114 . 
         [0032]    In the printer  100 , the controller  128  is configured to identify one or more inoperable inkjets by detecting light streaks in the image data of a portion of print job generated by the optical sensor  138 . The controller  128  is further configured to identify a magnitude and direction of a cross-process direction offset between the apparent position of an inoperable inkjet in the image data and the actual position of the inoperable inkjet in the printhead. The controller  128  executes instructions stored in the memory  129  to implement one or more of the processes described herein to identify and compensate for distortion in the image data that affects the identification of the location of inoperable inkjets. 
         [0033]      FIG. 2  depicts a process  200  for identification and compensation of an inoperable inkjet in a printhead. Process  200  is described in conjunction with the printer  100  for illustrative purposes. In process  200 , the printer  100  prints an image on the media web  114  (block  204 ). In one configuration, the printed image corresponds to a single page of a print job that includes various text and graphical elements. 
         [0034]    The image receiving member with the printed image moves past an optical sensor that generates image data corresponding to the printed image on the image receiving member (block  208 ). In the printer  100 , the optical sensor  138  generates image data for one or more rows of pixels arranged in the cross-process direction on the media web  114 . Process  200  detects an inoperable inkjet at first cross-process position from the image data (block  212 ). In the embodiment of the printer  100 , the controller  128  detects a light streak in the image data that indicates an inoperable inkjet. The cross-process position of the optical detector in the sensor  138  that generates the image data corresponding to the light streak indicates a first position of an inoperable inkjet in one of the printheads in the printer  100 . The first identified position is referred to as an apparent position because various components in the printer including either or both of the media web  114  and the optical sensor  138  produce a cross-process direction offset between the apparent position of the inoperable inkjet in the image data and the actual position of the inoperable inkjet in a printhead. 
         [0035]    Process  200  continues by identifying a magnitude and direction of the offset between the apparent position of the inoperable inkjet and the actual position of the inoperable inkjet (block  216 ). In one embodiment described in more detail in  FIG. 3 , the controller  128  identifies spatial distortions of the media web  114  and optical sensor  138  during initialization of the printer  100 . During a print job, the controller  128  identifies the cross-process position of one edge of the media web  114  to serve as a reference location, and then identifies the offset of the inoperable ejector with reference to the measured distortions and the identified position of the edge of the media web  114 . In another embodiment described in more detail in  FIG. 5 , the printer  100  prints a series of marks on the image receiving member while printing images in the print job. The marks are generated with a low-coverage pattern that is difficult for the naked eye to detect. The optical sensor  138  detects the marks and the controller  128  identifies a cross-process direction offset from the predetermined cross-process positions of the inkjets that form the pattern and the identified cross-process positions of the marks from the image data. 
         [0036]    Process  200  adjusts the position of the identified inoperable inkjet from the first cross-process position of the inoperable inkjet in the image data with reference to the identified offset (block  220 ). In some embodiments, the first position and the adjusted position of the inoperable inkjet are indexed as a number of pixels extending across the image receiving member. 
         [0037]    As depicted in  FIG. 7 , an exemplary printhead  700  includes a plurality of inkjets arranged in the cross-process direction  720 . In printhead  700 , inkjet  704  is inoperable, but the image data generated from printed images produced by the printhead  700  include a light streak at the pixel location corresponding to the operating inkjet  708 . Process  200  identifies the correct inoperable inkjet  704  using the identified position corresponding to the ejector  708  adjusted by an offset  712 . The offset  712  is two pixels to the right as viewed on the face of the printhead  700 . The adjusted pixel position of the inoperable inkjet corresponds directly to the inoperable inkjet  704 . 
         [0038]    After identifying the inoperable inkjet, process  200  operates one or more printheads to compensate for the inoperable inkjet (block  224 ). In one configuration, the controller  128  compensates for the inoperable inkjet by operating adjacent inkjets  716  and  718  to reduce or eliminate the light streak and does not generate signals to operate the inoperable inkjet. In another configuration, inkjets in one or more different printheads in the printer that are aligned with the inoperable inkjet  704  in the cross-process direction eject ink drops to reduce or eliminate light streaks and other image artifacts caused by the inoperable inkjet  704 . In some alternative embodiments, a printhead maintenance process revives the inoperable inkjet, or the printer generates an alert to request serviced or replacement of the printhead with the inoperable inkjet. 
         [0039]      FIG. 3  depicts a process  300  for identifying the offset between an apparent cross-process position of an inoperable inkjet as identified in image data generated from the image receiving member and the cross-process position of the inoperable inkjet in a printhead. Process  300  is described in conjunction with the printer  100  for illustrative purposes. In process  300 , the printer  100  prints a reference pattern on an image receiving member during either or both of initialization of the printer  100  and prior to commencing a print job (block  304 ). 
         [0040]      FIG. 4  depicts an example reference pattern  404  printed on the media web  114  and images printed on the media web  114  with an inoperable inkjet as described in process  300 . The reference pattern  404  includes marks, which are depicted as a plurality of dashes in  FIG. 4 , generated by a selected set of inkjets in some or all of the printheads in the printer  100 . A printhead  450  that prints a portion of the reference pattern  404  includes an ejector  454 . The printhead  450  and ejector  454  have a predetermined position in the cross-process direction. The ejector  454  ejects ink drops to form a series of dashes  424  on the media web  114  as part of the reference pattern  404 . The printer  100  prints the reference pattern  404  or another suitable reference pattern on the media web  114  with the images of a print job. 
         [0041]    In one embodiment, the reference pattern  404  is printed as a “top of form” (TOF) reference pattern in an inter-document zone that precedes each copy of a document in a print job. As used in this document, the TOF reference pattern refers to a reference pattern printed on the media web at a location that is proximate to one edge of a printed sheet after the sheet is cut from the media web. Examples of TOF reference patterns include patches or lines formed with black ink near one edge of the page after the page is cut from the media web. In one embodiment, the printer forms a TOF reference pattern as small rectangular black patches or bar codes that are located just outside the top-left corner of a printed page form, which is referred to as the top of the form. The TOF is formed outside of the area of the media sheet containing the printed image, and the portion of the cut sheet that contains the TOF is trimmed during the final finishing process. In addition to use with process  300 , the printer  100  uses the TOF to identify pages and to match first and second side images for printed pages on the media web  114 . 
         [0042]    Process  300  generates image data corresponding to the printed reference pattern on the image receiving member (block  308 ). In the printer  100 , the optical sensor  138  generates image data as the media web  114  and reference pattern  404  move past the optical sensor. In  FIG. 4 , an optical detector  139 C is aligned with the inkjet  454  in the cross-process direction, but due to distortions in the media web  114  and optical detector  138 , a different optical detector  139 D detects the light reflected from the dashes  424 . Another optical detector  139 A identifies the cross-process location of an edge  412  of the media web  114  as a reference to identify an expected cross-process distance between the edge  412  of the media web  114  and the dashes  424 . 
         [0043]    Process  300  identifies cross-process offsets between the marks in the reference pattern and the predetermined cross-process locations of the inkjets that print the reference pattern marks on the image receiving member (block  312 ). In the printer  100 , thermal expansion of the optical sensor  138  and distortions in the media web  114 , such as web shrinkage, generate an offset between the cross-process position of dashes identified in the reference pattern  404 . An offset  426  separates the inkjet  454  and the identified cross-process position of the dashes  424  that the inkjet  454  prints on the media web  114 . In one embodiment, the offset  426  is measured as a number of pixels in the cross-process direction. For example, the number of pixels between optical detector  139 D and the optical detector  139 C that is aligned with the predetermined position of the inkjet  454  represent a measurement of the offset  426 . The offset between various inkjets and the corresponding positions of dashes in the image data varies for different inkjets due to local distortions of the media web  114  and the optical sensor  138 . In the printer  100 , the controller  128  is configured to store each of the identified offset values in the memory  129  in association with the cross-process position of each associated inkjet. The stored offset values are calibrated at the time the printer prints the reference pattern  404  to identify the offset of inoperable inkjets that are detected during a subsequent print job. 
         [0044]    Process  300  continues during a print job as the printer forms ink images on the image receiving member. The printer detects an inoperable inkjet in image data generated from the printed image as described above in process  200 . Process  300  identifies a cross-process location of an edge of the media web as the inoperable inkjet is identified in the image data (block  316 ). In  FIG. 4 , an inoperable inkjet  474  in a second printhead  470  results in a light streak  432  formed through printed images on the media web  114  during the print job. In the example of  FIG. 4 , the second printhead  470  did not produce the reference pattern  404 , but is aligned with printhead  450  in the cross-process direction. A controller in the printer  100  identifies the printhead  470  that includes an inoperable inkjet from the image data generated by the optical detector  139 E. A second optical detector  139 B detects the edge  412  of the media web  114  at substantially the same time as the detection of the light streak  432 . The identified cross-process position of the edge  412  media web  114  serves as a reference that reduces or eliminates the effects of media web oscillations that occur between the printing of the reference pattern  404  and the detection of an inoperable inkjet. 
         [0045]    Process  300  identifies the offset in the image data in the cross-process direction with respect to lateral shifts in the media web and the offsets stored for inkjets that are proximate to the inoperable inkjet (block  320 ). The lateral movement of the media web  114  is a global distortion that changes the reference used to identify the inoperable inkjet. For example, in  FIG. 4 , the media web shifts right in direction  416  by a distance of five pixels in the time between the printing of the reference pattern  404  and the detection of the light streak  432 . The stored offsets for one or more inkjets that are located near the inoperable inkjet provide offset data corresponding to local distortions in the media web  114  and the sensor  138 . In the example of  FIG. 4 , dashes in the reference pattern  404 , which includes the dashes  424 , are positioned proximate to the cross-process position of the light streak  432 . 
         [0046]    In the printer  100 , the controller  128  generates an average local offset using the stored offset data for one or more inkjets that are proximate to the light streak, such as inkjet  454 . The controller  128  may generate an average offset value for several inkjets located near the light streak  432 . The controller identifies the offset  476  as the net sum of the average offset of the inkjets proximate to the light streak  432  with the identified lateral offset of the entire media web  114 . The offset data also specify a positive or negative value, corresponding to either a left or right offset in the cross-process direction as depicted in  FIG. 4 . Using the example offsets in  FIG. 4 , the average offset of surrounding inkjets is eight pixels left in the cross-process direction, while the lateral movement of the media web  114  is five pixels right in the cross-process direction. The resulting net offset  476  is three pixels to the left between the cross-process position of the optical detector  139 E and the cross-process position of the inoperable inkjet  474 . Process  200  uses the generated offset value to identify the offset  476  in the image data to determine the inkjet  474  that failed to operate and hence for the light streak. Once the inoperable inkjet is accurately identified, the controller compensates for the inoperable inkjet  474  by operating other inkjets that eject ink drops in the vicinity of the light streak in the image data. 
         [0047]      FIG. 5  depicts another process  500  for identifying the offset between a cross-process position of an inoperable inkjet identified with reference to the image data of the image receiving member and the cross-process position of the inoperable inkjet in the printhead. Process  500  is described in conjunction with the printer  100  for illustrative purposes. In process  500 , the printer  100  prints a low-coverage pattern on a region of the media web that is proximate to the inoperable inkjet (block  504 ). 
         [0048]    The low-coverage patterns are formed by printing ink drops in a low density pattern on the image receiving member to form marks that the optical sensor  138  detects but are difficult to perceive with the naked eye. In one example, low-coverage patterns include a plurality of dashes that are formed with an ink drop density of less than 20%. For example, a dash that is forty pixels in length in the process direction formed from two ink drops printed at intervals over the length of the dash has a density of 5%. The low coverage patterns are printed on the image receiving member with portions of the print job without producing a noticeable degradation in the quality of the printed print job images. In a CMYK color printer embodiment that prints on white paper, the yellow printheads print the low-coverage patterns since yellow ink is more difficult for the human eye to perceive. The identified offsets between the yellow dashes printed in the reference pattern and the known positions in the yellow printhead of the inkjets that ejected the yellow drops in the dashes are used to identify the offsets for inoperable inkjets in printheads of any color in the printer. In situations where an inkjet in a yellow printhead is inoperable, the printer ejects additional yellow ink drops from operating yellow ejectors, or can select printheads ejecting a different color, such as cyan, to print the low-coverage patterns. 
         [0049]      FIG. 6  depicts low coverage patterns printed on the media web  114  and images printed on the media web  114  with an inoperable inkjet as described in process  500 .  FIG. 6  depicts two alternative configurations of the low-coverage patterns. In one configuration, the printer  100  prints a low-coverage reference pattern  604  at regular intervals such in the document zone for each page of a print job. Printer  100  prints the marks in the low-coverage reference pattern  604  across the media web  114  without regard to inoperable inkjets. When an inoperable inkjet is identified, the marks in the low-coverage reference pattern  604  that are proximate to the inoperable inkjet are used to identify the offset of the inoperable inkjet. In the example of  FIG. 6 , some of the marks in the low-coverage reference pattern are printed over the printed images on the media web  114 , while other marks are printed between images within the page. The process-direction position of each mark in the low-coverage reference pattern is selected to enable the optical detector  138  to detect the mark while the mark remains difficult to detect with the naked eye. 
         [0050]    In another configuration, the low-coverage patterns are printed in response to the identification of an inoperable inkjet. In  FIG. 6 , the optical sensor  138  generates image data corresponding to the light streak  432  of an inoperable inkjet  474 . The printer  100  operates inkjets  654  and  658  in printhead  650  and inkjets  665  and  668  in printhead  660  to form low-coverage patterns  606  on the media web  114 . The low-coverage patterns  606  are printed proximate to the light streak  432  in the cross-process direction in the document zone to enable the optical detector  138  to identify local distortions of the media web  114  around the light streak  432 . The printer  100  prints the low coverage patterns  606  in portions of the document zone on the media web  114  where the contrast between the ink in the pattern and the ink corresponding to the content in the document is low. In some embodiments, the low-coverage patterns  606  are printed on subsequent pages of a print job after a light streak corresponding to an inoperable inkjet is detected in the image data. The printer  100  detects the inoperable inkjet, prints the low-coverage reference pattern, and identifies the position of the inoperable inkjet over the course of one or more pages in a print job. 
         [0051]    Process  500  generates image data of the low-coverage patterns printed on the media web (block  508 ) and identifies offsets of the inkjets that print the low-coverage patterns (block  512 ). In the printer  100 , the optical detector  138  detects the low-coverage patterns  604  or  606 . Process  500  identifies an offset between each mark in the low-coverage pattern and the corresponding inkjet that printed the mark. For example, inkjet  654  in printhead  650  is located at a predetermined position in the cross-process direction. An optical detector  139 F in the optical sensor  138  detects the low-coverage pattern that the inkjet  654  prints on the media web  114 . The offset  656  is the separation between the optical detector  139 F and the inkjet  654  in the cross-process direction. As described above, the offset can be measured in terms of pixels. The controller  128  in the printer  100  identifies offsets for two or more inkjets that are proximate to the light streak  432 , such as offsets for inkjets  658 ,  664 , and  668 . 
         [0052]    Process  500  identifies an offset of the inoperable inkjet using the identified offsets for the inkjets that printed the low-coverage pattern on the image receiving member (block  516 ). The inkjets that print the low-coverage pattern are proximate to the inoperable inkjet to account for offset due to local distortions in the media web  114  and the optical sensor  138 . Additionally, the printer  100  prints the low-coverage patterns close in time to detection of the light streak in the image data. Consequently, lateral oscillations of the media web  114  have minimal effect on the measured offsets since the low-coverage patterns are printed within a short time before or after detection of the light streak  432 . In the printer  100 , the controller  128  generates an average value of the offset for the inkjets that print the low-coverage pattern on the media web  114 , such as inkjets  654 ,  658 ,  664 , and  668 . Process  200  uses the generated offset value to identify the offset  476  of the inoperable inkjet  474  and compensates for the inoperable inkjet  474 . 
         [0053]    It will be appreciated that variants of the above-disclosed and other features, and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art, which are also intended to be encompassed by the following claims.