Abstract:
A full width array CCD sensor is incorporated in the media path to monitor fused pages by calculating area coverage from multiple engines in a Tightly Integrated Parallel Process (TIPP) architecture. With knowledge of the area coverage differences between print engines for a given pixel count to the ROS, a relative density difference of each engine is determined. Based on the determined relative density difference, an adjustment is calculated and applied to the engine with the largest error to match the area coverage(s) of the other engine(s).

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The following applications, the disclosures of each being totally incorporated herein by reference are mentioned: 
     U.S. Provisional Application Ser. No. 60/631,651, filed Nov. 30, 2004, entitled “TIGHTLY INTEGRATED PARALLEL PRINTING ARCHITECTURE MAKING USE OF COMBINED COLOR AND MONOCHROME ENGINES,” by David G. Anderson, et al.; 
     U.S. Provisional Application Ser. No. 60/631,656, filed Nov. 30, 2004, entitled “MULTI-PURPOSE MEDIA TRANSPORT HAVING INTEGRAL IMAGE QUALITY SENSING CAPABILITY,” by Steven R. Moore; 
     U.S. Provisional Patent Application Ser. No. 60/631,918, filed Nov. 30, 2004, entitled “PRINTING SYSTEM WITH MULTIPLE OPERATIONS FOR FINAL APPEARANCE AND PERMANENCE,” by David G. Anderson et al.; 
     U.S. Provisional Patent Application Ser. No. 60/631,921, filed Nov. 30, 2004, entitled “PRINTING SYSTEM WITH MULTIPLE OPERATIONS FOR FINAL APPEARANCE AND PERMANENCE,” by David G. Anderson et al.; 
     U.S. application Ser. No. 10/761,522, filed Jan. 21, 2004, entitled “HIGH RATE PRINT MERGING AND FINISHING SYSTEM FOR PARALLEL PRINTING,” by Barry P. Mandel, et al.; 
     U.S. application Ser. No. 10/785,211, filed Feb. 24, 2004, entitled “UNIVERSAL FLEXIBLE PLURAL PRINTER TO PLURAL FINISHER SHEET INTEGRATION SYSTEM,” by Robert M. Lofthus, et al.; 
     U.S. application Ser. No. 10/860,195, filed Aug. 23, 2004, entitled “UNIVERSAL FLEXIBLE PLURAL PRINTER TO PLURAL FINISHER SHEET INTEGRATION SYSTEM,” by Robert M. Lofthus, et al.; 
     U.S. application Ser. No. 10/881,619, filed Jun. 30, 2004, entitled “FLEXIBLE PAPER PATH USING MULTIDIRECTIONAL PATH MODULES,” by Daniel G. Bobrow; 
     U.S. application Ser. No. 10/917,676, filed Aug. 13, 2004, entitled “MULTIPLE OBJECT SOURCES CONTROLLED AND/OR SELECTED BASED ON A COMMON SENSOR,” by Robert M. Lofthus, et al.; 
     U.S. application Ser. No. 10/917,768, filed Aug. 13, 2004, entitled “PARALLEL PRINTING ARCHITECTURE CONSISTING OF CONTAINERIZED IMAGE MARKING ENGINES AND MEDIA FEEDER MODULES,” by Robert M. Lofthus, et al.; 
     U.S. application Ser. No. 10/924,106, filed Aug. 23, 2004, for PRINTING SYSTEM WITH HORIZONTAL HIGHWAY AND SINGLE PASS DUPLEX by Lofthus, et al.; 
     U.S. application Ser. No. 10/924,113, filed Aug. 23, 2004, entitled “PRINTING SYSTEM WITH INVERTER DISPOSED FOR MEDIA VELOCITY BUFFERING AND REGISTRATION,” by Joannes N. M. deJong, et al.; 
     U.S. application Ser. No. 10/924,458, filed Aug. 23, 2004 for PRINT SEQUENCE SCHEDULING FOR RELIABILITY by Robert M. Lofthus, et al.; 
     U.S. patent application Ser. No. 10/924,459, filed Aug. 23, 2004, entitled “PARALLEL PRINTING ARCHITECTURE USING IMAGE MARKING DEVICE MODULES,” by Barry P. Mandel, et al; 
     U.S. patent application Ser. No. 10/953,953, filed Sep. 29, 2004, entitled “CUSTOMIZED SET POINT CONTROL FOR OUTPUT STABILITY IN A TIPP ARCHITECTURE,” by Charles A. Radulski et al.; 
     U.S. application Ser. No. 10/999,326, filed Nov. 30, 2004, entitled “SEMI-AUTOMATIC IMAGE QUALITY ADJUSTMENT FOR MULTIPLE MARKING ENGINE SYSTEMS,” by Robert E. Grace, et al.; 
     U.S. patent application Ser. No. 10/999,450, filed Nov. 30, 2004, entitled “ADDRESSABLE FUSING FOR AN INTEGRATED PRINTING SYSTEM,” by Robert M. Lofthus, et al.; 
     U.S. patent application Ser. No. 11/000,158, filed Nov. 30, 2004, entitled “GLOSSING SYSTEM FOR USE IN A TIPP ARCHITECTURE,” by Bryan J. Roof; 
     U.S. patent application Ser. No. 11/000,168, filed Nov. 30, 2004, entitled “ADDRESSABLE FUSING AND HEATING METHODS AND APPARATUS,” by David K. Biegelsen, et al.; 
     U.S. patent application Ser. No. 11/000,258, filed Nov. 30, 2004, entitled “GLOSSING SYSTEM FOR USE IN A TIPP ARCHITECTURE,” by Bryan J. Roof; 
     U.S. application Ser. No. 11/001,890, filed Dec. 2, 2004, entitled “HIGH RATE PRINT MERGING AND FINISHING SYSTEM FOR PARALLEL PRINTING,” by Robert M. Lofthus, et al.; 
     U.S. application Ser. No. 11/002,528, filed Dec. 2, 2004, entitled “HIGH RATE PRINT MERGING AND FINISHING SYSTEM FOR PARALLEL PRINTING,” by Robert M. Lofthus, et al.; 
     U.S. application Ser. No. 11/051,817, filed Feb. 4, 2005, entitled “PRINTING SYSTEMS,” by Steven R. Moore, et al.; 
     U.S. application Ser. No. 11/109,996, filed Feb. 28, 2004, entitled “PRINTING SYSTEMS,” by Robert M. Lofthus, et al.; 
     U.S. application Ser. No. 11/070,681, filed Mar. 2, 2005, entitled “GRAY BALANCE FOR A PRINTING SYSTEM OF MULTIPLE MARKING ENGINES,” by R. Enrique Viturro, et al.; and, 
     U.S. application Ser. No. 11/081,473, filed Mar. 16, 2005, entitled “MULTI-PURPOSE MEDIA TRANSPORT HAVING INTEGRAL IMAGE QUALITY SENSING CAPABILITY,” by Steven R. Moore. 
     BACKGROUND 
     The present exemplary embodiment relates to printing systems. It finds particular application in conjunction with adjusting image quality in print or marking systems with multiple electrophotographic or xerographic print engines. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications. 
     Typically, in image rendering or printing systems, it is desirable that a rendered, or printed, image closely match, or have similar aspects or characteristics to a desired target or input image. However, many factors, such as temperature, humidity, ink or toner age, and/or component wear, tend to move the output of a printing system away from the ideal or target output. For example, in xerographic marking engines, system component tolerances and drifts, as well as environmental disturbances, may tend to move an engine response curve (ERC) away from an ideal, desired or target engine response and toward an engine response that yields images that are lighter or darker than desired. 
     In order to provide increased production speed, document processing systems that include a plurality of marking engines have been developed. In such systems, the importance of engine response control or stabilization is amplified. Subtle changes that may be unnoticed in the output of a single marking engine can be highlighted in the output of a multi-engine image rendering or marking system. For example, the facing pages of an opened booklet rendered or printed by a multi-engine printing system can be printed by different engines. For instance, the left-hand page in an open booklet may be printed by a first print engine while the right-hand page is printed by a second print engine. The first print engine may be printing images in a manner slightly darker than the ideal and well within a single engine tolerance; while the second print engine may be printing images in a manner slightly lighter than the ideal and also within the single engine tolerance. While an observer might not ever notice the subtle variations when reviewing the output of either engine alone, when the combined output is compiled and displayed adjacently, the variation in intensity from one print engine to another may become noticeable and be perceived as an issue of quality by a user. 
     One approach to improve consistency among multiple engines is for a user to periodically inspect the print quality. When inconsistency becomes noticeable, the user initiates printing of test patches on multiple engines and scans the test patches in. The scanner reads the test patches and adjusts the xerography of the engines to match. However, this approach requires a user intervention and the scanner to scan the test patches. Another approach to improve image consistency among multiple engines is to print test patches with the engines of the multiple engine system and compare the test patches against one another. However, such approach is complex as it involves substantial software development as well as elaborate scheduling of test patches to not interfere with the print job. 
     There is a need for methods and apparatuses that overcome the aforementioned problems and others. 
     REFERENCES 
     U.S. Pat. No. 4,710,785, which issued Dec. 1, 1987 to Mills, entitled PROCESS CONTROL FOR ELECTROSTATIC MACHINE, discusses an electrostatic machine having at least one adjustable process control parameter. 
     U.S. Pat. No. 5,510,896, which issued Apr. 23, 1996 to Wafler, entitled AUTOMATIC COPY QUALITY CORRECTION AND CALIBRATION, discloses a digital copier that includes an automatic copy quality correction and calibration method that corrects a first component of the copier using a known test original before attempting to correct other components that may be affected by the first component. 
     U.S. Pat. No. 5,884,118, which issued Mar. 16, 1999 to Mestha, entitled PRINTER HAVING PRINT OUTPUT LINKED TO SCANNER INPUT FOR AUTOMATIC IMAGE ADJUSTMENT, discloses an imaging machine having operating components including an input scanner for providing images on copy sheets and a copy sheet path connected to the input scanner. 
     U.S. Pat. No. 6,418,281, which issued Jul. 9, 2002 to Ohki, entitled IMAGE PROCESSING APPARATUS HAVING CALIBRATION FOR IMAGE EXPOSURE OUTPUT, discusses a method wherein a first calibration operation is preformed in which a predetermined grayscale pattern is formed on a recording paper and this pattern is read by a reading device to produce a LUT for controlling the laser output in accordance with the image signal (gamma correction). 
     However, the aforementioned Patents are not concerned with methods for improving or achieving image consistency between or among a plurality of marking engines. 
     BRIEF DESCRIPTION 
     In accordance with one aspect, an automatic method for controlling image consistency in an image rendering system that includes a plurality of marking engines is disclosed. A first sheet of a print job is printed on a first print media with the first marking engine. A second sheet of the print job is printed on a second print media with the second marking engine. Concurrently with the printing of the first and second sheets of the print job, a density inconsistency is determined at least between the first and second sheets of the print job. Based on the inconsistency determination, at least one parameter of at least one of the first and the second marking engine is adjusted to improve image consistency between at least the first and second marking engines. 
     In accordance with another aspect, a document processing system is disclosed. The document processing system includes two or more xerographic print engines for printing sheets of a print job, each xerographic print engine having at least one xerographic actuator. A sensor measures light reflected from each sheet of the print job printed with the print engines. An analyzer analyzes light measurements of the sheet with the sheets printed with different engines and determines an amount of adjustment of at least one parameter of the document processing system for the at least one of xerographic actuators. A xerographic actuator adjuster adjusts the at least one xerographic actuator according to the adjustment amount determined by the analyzer to substantially match the xerography of the engines. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view of an image or document processing system including a plurality of print engines; 
         FIG. 2  is a block diagram of the document processing system including a plurality of print engines; 
         FIG. 3A  is an image representation of a first image printed with a first engine and measured by a sensor; 
         FIG. 3B  is an image representation of a second image printed with a second engine and measured with the sensor of  FIG. 3A ; 
         FIG. 4  is a flow chart outlining a method to control image consistency of the multiple marking engines; 
         FIG. 5  is a more detailed flow chart for a method of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1 , an image or document processing system  104 , that might incorporate embodiments of the methods and systems disclosed herein, includes a first image output terminal (IOT)  108 , a second image output terminal  110 , and an image input device  114 , such as a scanner, imaging camera or other device. Although only two output terminals are illustrated, it is contemplated that the document processing system can include a plurality of output terminals. Each image output terminal  108 ,  110  includes a plurality of input media trays  126  and an integrated marking engine as will be discussed in a greater detail below. The first image output terminal  108  may support the image input device  114  and includes a first portion  132  of a first output path. A second portion  134  of the first output path is provided by a bypass module  136 . The second image output terminal  110  includes a first portion  138  of a second output path. A third portion of the first path and a second portion of the second path begin at a final nip  142  of the second image output terminal  110  and include an input to a finisher  150 . 
     The finisher  150  includes, for example, first  160  and second  162  main job output trays. Depending on a document processing job description and on the capabilities of the finisher  150 , one or both of the main job output trays  160 ,  162  may collect loose pages or sheets, stapled or otherwise bound booklets, shrink wrapped assemblies or otherwise finished documents. The finisher  150  receives sheets or pages from one or both of the image output terminals  108 ,  110  via an input  152  and processes the pages according to a job description associated with the pages or sheets and according to the capabilities of the finisher  150 . 
     A controller (not shown) controls the production of printed or rendered pages, the transportation over the path elements  132 ,  134 ,  138 ,  148  and  152 , and the collation and assembly as job output by the finisher  150 . The produced, printed or rendered pages may include images transferred to the document processing system via a telephone communications network, a computer network, computer media, and/or images entered through the image input device  114 . For example, rendered or printed pages or sheets may include images received via facsimile, transferred to the document processing system from a word processing, spreadsheet, presentation, photo editing or other image generating software, transferred to the document processor  104  over a computer network or on a computer media, such as a CD ROM, memory card or floppy disc, or may include images generated by the image input device  114  of scanned or photographed pages or objects. The images can be transferred, manually or automatically, to the image input device  114  to generate computer readable representations of the rendered images. 
     As discussed in a greater detail below, a full width CCD sensor  170 , which is disposed about the path  142 ,  152 , measures an amount of light reflected from each pixel of every line of images produced by the engines and converts the detected lightness information into digital numbers which can be read and analyzed by the controller, or some auxiliary device. For example, the information can contain a series of zeroes and ones, where “ones” represent the pixels for which LEDs were turned ON. The number of “ones” can be divided by total number of pixels in the image to measure output area coverage of each engine. The measured area coverages represent print densities of each printed page which are analyzed to measure relative engine density or lightness differences. The information about image lightness is accumulated overtime for each engine. By comparing the image lightness information of different engines, it can be determined whether an adjustment needs to be made. For example, relative lightness L*, as defined by the Commission Internationale de l&#39;Eclairages (CIE) can be analyzed and compensated for. Relative lightness L* is typically calculated by comparing a background lightness to the lightness of print. Thus, if necessary, some aspect of the engine with the largest error is adjusted in a manner predetermined or known to make an improvement in, or achieve, image consistency from one engine to another. For example, electrophotographic, xerographic, or other rendering technology actuators may be adjusted. Integration of the sensor  170  within the media path of the multiple engines provides relative measurements between the engines. Such relative measurements loosen calorimetric tolerances that are typically in place when the measurements are absolute. This results in a low cost set up of the focus, illuminant uniformity and sensor SNR. 
     With continuing reference to  FIG. 1  and further reference to  FIG. 2 , the image or document processing system  104  includes a plurality of print or marking engines, each of which is associated with a respective output terminal. For example, the plurality of marking or print engines includes first, second, . . . , n th  xerographic marking or print engines  214 ,  216 , . . . ,  218 . For simplicity, the xerographic marking engines  214 ,  216 , . . . ,  218  are illustrated as monochrome (e.g., black and white) marking engines. However, other embodiments including color marking engines are also contemplated. Furthermore, embodiments including marking engines of other technologies are also contemplated. 
     Each marking technology is associated with marking technology actuators. For example, the first xerographic marking engine  218  includes a charging element  222 , a writing element  224 , a developer  226  and a fuser  228 , which each can be associated with one or more xerographic actuators. 
     For instance, the charging element  222  may be a corotron, a scorotron, or a dicorotron. In each of these devices, a voltage is applied to a coronode (wire or pins)  230  to ionize surrounding air molecules, which in turn causes a charge to be applied to a photoconductive belt  232  or drum. Where the charging element  222  is a scorotron, the scorotron includes a grid  234 , to which a grid voltage is applied. The scorotron grid  234  is located between the coronode  230  and the photoconductor  232  and helps to control the charge strength and uniformity of the charge applied to the photoconductor  232 . The coronode voltage and the grid voltage are xerographic actuators. Changing either voltage may result in a change in the charge applied to the photoconductor  232 , which in turn may affect an amount of toner attracted to the photoconductor  232  and therefore the lightness or darkness of a printed or rendered image. Many xerographic marking engines include one or more electrostatic volt meters (ESV) for measuring the charge applied to the photoconductor  232 . In such systems, a control loop receives information from the ESV and adjusts one or both of the coronode voltage and the grid voltage in order to maintain a desired ESV measurement. 
     In one embodiment, the writing element  224  is a raster output scanner (ROS). Typically, a raster output scanner includes a laser, and a polygonal arrangement of mirrors, which is driven by a motor to rotate. A beam of light from the laser is aimed at the mirrors. As the arrangement of mirrors rotates, a reflected beam scans across a surface of the photoconductor  232 . The beam is modulated on and off. As a result, portions of the photoconductor  232  are discharged. Alternatively, the ROS includes one or more light emitting diodes (LEDs). For instance, an array of LEDs may be positioned over respective portions of the photoconductor  232 . Lighting an LED tends to discharge the photoconductor at positions associated with the lit LED. ROS exposure is a xerographic actuator. For example, the exposure, or amount of light that reaches the photoconductor  232 , is a function of ROS power and/or ROS exposure time. The higher the laser or LED power, the more discharged associated portions of the photoconductor  232  become. Alternatively, the longer a particular portion of the photoconductor  232  is exposed to laser or LED light, the more discharged the portion becomes. The degree, to which the portions of the photoconductor  232  are charged or discharged, affects the amount of toner that is attracted to the photoconductor  232 . Adjusting ROS exposure adjusts the lightness of a rendered or printed image. 
     The developer  226  includes a reservoir of toner. The concentration of toner in the reservoir has an effect on the amount of toner attracted to charge portions of the photoconductor  232 . For instance, the higher the concentration of toner in the reservoir, the more toner is attracted to portions of the photoconductor  232 . E.g., toner concentration in the reservoir is a xerographic actuator. Toner concentration can be controlled by controlling the rate at which toner from a toner supply is delivered to the developer toner reservoir. 
     With continuing reference to  FIG. 2 , print media, such as sheets of paper or velum, is transported on a media transport  236 , while toner on the photoconductor  232  is transferred to the media at a transfer point  238 . The print media is transported to the fuser  228  where elevated temperatures and pressures operate to fuse the toner to the print media. Pressures and temperatures of the fuser  228  are xerographic actuators. 
     Other xerographic actuators are also known. Additionally, other printing technologies include actuators that can be adjusted to control the lightness or darkness of the printed or rendered image. For example, in ink jet based marking engines, a drop ejection voltage controls an amount of ink propelled toward print media with each writing pulse. Therefore, drop ejection voltage is a factor in an ink jet actuator. 
     The second and n th  xerographic print engines  216 , . . . ,  218  include elements similar to the first xerographic marking engine  214  such as a charging element  242 ,  262 , a writing element  244 ,  264 , a developer  246 ,  266 , a fuser  248 ,  268 , a coronode  250 ,  270  and a photoconductor  252 ,  272 . The charging element may include a charging grid  254 ,  274 . A media transport  256 ,  276  carries print media to a transfer point  258 ,  278  and to the fuser  248 ,  268 . 
     The document or image processing system  104  also includes an analyzer  284  and an actuator adjuster  288 . The system  104  may also include one or more of printing, copying, faxing and scanning services  292 . In one embodiment, the analyzer  284  and actuator adjuster  288  are embodied in software which is run by a controller (not shown). Alternatively, one or more of the analyzer  284 , and actuator adjuster  288  are implemented in hardware, which is supervised by the controller (not shown). 
     The analyzer  284 , actuator adjuster  288 , and two or more of the plurality of print or marking engines  214 ,  216 , . . . ,  218 , cooperate to perform one or more methods which control image consistency. 
     With continuing reference to  FIG. 2  and further reference to  FIGS. 3A and 3B , the printed sheets or pages or images  300 ,  302  from each of the plurality of print engines  214 ,  216 , . . . ,  218  are delivered automatically to the sensor  170  which operates to generate a computer readable representation of the printed image which is analyzed by the analyzer  284 . The analyzer  284  determines an amount by which at least one xerographic actuator should be adjusted based on the analysis. The actuator adjuster  288  adjusts the at least one xerographic actuator according to the amount determined by the analyzer  284 . The analyzer  284  and actuator adjuster  288  are included as a means for controlling or adjusting image quality in the print job production so that the density of portions of the print job printed with the first engine  214  substantially matches the density of portions of the print job printed with other engines  216 , . . . ,  218 . 
     With continuing reference to  FIGS. 3A and 3B  and further reference to  FIG. 4 , a method  310  controls image consistency in the image processing system  104  that includes the marking engines  214 ,  216 . Although illustrated with reference to only two print engines, it is contemplated that the control method  310  is applicable to printing systems which include more than two print engines. Integration of the sensor  170  within the media path  136 ,  142  allows utilizing the method  310  with any configuration of the multiple engine document system  104 . A first sheet or image  300  is printed  314  with the first marking engine  214  to generate a first rendered version of the first sheet. In one embodiment, the control method  310  is restricted to a certain area coverage to minimize interference of measurement noise. More specifically, from available input data  316 , the analyzer  284  determines  318  whether the area coverage of the image  300  is greater than a predetermined area coverage threshold value. For example, if the input area coverage is less than about 5% -10%, no measurements are taken and no adjustments to the engines are to be made. Such restriction of the area coverage ensures a robust estimate of the differences between the engines in terms of detecting lightness, darkness, and the like. If it is determined that the area coverage is greater than the predetermined area coverage threshold, the sensor  170  measures  320  light reflected by each pixel in a cross section direction  322  of each line of the image  300  in a process direction  324 . Since the measurement takes place on the fly, a clocking mechanism  326  is implemented. For example, the clocking mechanism can be a timer which signals the sensor  170  to take measurements, for example, every 0.1 seconds. Of course, it is contemplated the setting of such timer can be varied to accommodate various printer or production needs. As another example, a timing wheel can be implemented which provides a hardwired clock to the sensor  170  to take measurements. Of course, it is contemplated that other timing mechanisms can be implemented. The sensor  170  is the full width array sensor, n×M, where n is a number of sensing elements  328  in the process direction  324 , and M is a number of sensing elements  328  in the cross process direction  322 . For example, the number of elements in the process direction  324  can be 1, while the number of sensing elements in the cross process direction  322  can be equal to a number of cross process direction pixels of the given printer. Such sensor is a line sensor, or 1×M sensor. However, other arrangements of the sensing elements  328  of the sensor  170  are also contemplated. A second sheet or image  302  is printed  330  with the second marking engine  216  to generate the second rendered version of the second sheet, which is measured  332  by the sensor  170 . The data of the first and second sheets  300 ,  302  are analyzed  334 . Based on the analysis, if it is determined that an adjustment is needed, at least one aspect associated with at least one of the marking engines  214 ,  216  is adjusted  338  in a manner predetermined to improve engine to engine consistency. 
     In this manner, each image is measured as the sheets, which are printed by different engines, are transferred through the sensor  170 . As one example, the method  310  can be effectuated when the print engines  214 ,  216  print the same image. As another example, the method  310  can be effectuated when the input area coverage of the engines  214 ,  216  is the same, but the output area coverage does not match. Because the control method  310  automatically adjusts the printers based on the measured printed images, no special scheduling of test patches is needed, which minimizes production downtime, customer intervention and waste of sheets. The calculations of adjustments are made after the measurements are averaged over many prints, thus filtering noise and providing robust estimates before adjustments. 
     With continuing reference to  FIGS. 3A ,  3 B and  4  and further reference to  FIG. 5 , analyzing  334  the first and second images  300 ,  302  can include any analysis appropriate to measure image and the aspect or aspects of marking engine processes that are being studied, analyzed, adjusted or compensated for. 
     In one embodiment, average gray level values over the respective first and second sheets  300 ,  302  are determined  400 ,  402  according to equation: 
                       GL   meas     =         ∑     j   =   0       N   -   1       ⁢       ∑     i   =   0       M   -   1       ⁢     GL   ij           M   *   N         ,           (   1   )               
where GL meas  is the average gray level value over the respective image  300 ,  302 ;
 
M is a number of pixels in the cross-process direction  322  and is equal to R x *h;
 
N is a number of scan lines in the process direction  324  and is equal to R y *w;
 
R x  is a resolution of the printbar in the cross-process direction  322 ;
 
R y  is a resolution of the printbar in the process direction  324 ;
 
h is a height of the sheet  300 ,  302 ; and
 
w is a width of the sheet  300 ,  302 .
 
     First and second area coverages AC meas (A), AC meas (B) of the first and second sheets  300 ,  302  are determined  404 ,  406  according to equation:
 
AC meas =GL meas /GL white   (2)
 
where AC meas  is the measured output area coverage of each marking engine  214   216 ; and
 
GL white  is a gray level of print media.
 
     The gray level value of the print media is preferably a known value which can be determined in advance. Optionally, as the gray level value might change from one stack of print media to another, the gray level value of the print media is determined  408 ,  410  by measuring the average gray level of the blank sheet in accordance with the Equation (1). Such measurement, for example, can take place by internally scheduling to process a blank sheet periodically. 
     An error between the first and second area coverages ERR(AB) is determined  412  according to equation:
 
ERR( AB )=AC meas ( A )−AC meas ( B ),  (3)
 
where ERR(AB) is the error between the first and second area coverages AC meas (A), AC meas (B).
 
     The determined error ERR(AB) between the first and second engines  214 ,  216  is compared  420  to a prespecified threshold E TH . If the determined error ERR(AB) is greater  422  than the threshold E TH , errors of the first and second engines ERR(A), ERR(B) are determined  424 ,  426 . 
     The error of the first engine ERR(A) is determined  424  according to equation:
 
ERR( A )=AC meas ( A )−(AC IN(A)   +C ),  (4)
 
where AC(IMAGE) is the input area coverage; and
 
C is a constant representative of the system growth.
 
     The error of the second engine ERR(B) is determined  426  according to equation:
 
ERR( B )=AC meas ( B )−(AC IN(B)   +C ),  (5)
 
where AC(IMAGE) is the input area coverage; and
 
C is a constant representative of the system growth.
 
     In a perfectly calibrated system, the area coverage of the output perfectly matches the area coverage of the input. As in reality the systems are not perfectly calibrated, the area coverage of the output rarely perfectly matches the area coverage of the input. The constant C is added to the area coverage of the input in the Equation (5) to prevent the system shrinkage and possible loss of information. 
     An absolute value of the error of the first engine ERR(A) is compared  430  to the absolute value of the error of the second engine ERR(B). If the absolute value of the error of the first engine ERR(A) is greater  432  than the absolute value of the error of the second engine ERR(B), then an adjustment of an exposure EXP(A) of the first engine  214  is calculated  440  as a function of the determined error ERR(A) according to equation:
 
EXP( A )=EXP 0 ( A )+ERR( A )*sE,  (6)
 
where EXP(A) is the adjustment of the exposure of the first engine  214 ;
 
EXP 0 (A) is the current setting of the exposure of the first engine  214 ;
 
ERR(A) is the relative error of the first engine  214 ; and
 
sE is the exposure sensitivity.
 
     The first engine  214  is adjusted  442  by the calculated adjustment EXP(A). 
     If the absolute value of the error of the second engine ERR(B) is greater than the absolute value of the error of the first engine ERR(A), then an exposure EXP(B) of the second engine  216  is calculated  444  as a function of the determined error ERR(B) according to equation:
 
EXP( B )=EXP 0 ( B )+ERR( B )*sE,  (7)
 
where EXP(B) is the adjustment of the exposure of the second engine  216 ;
 
EXP 0 (B) is the current setting of the exposure of the second engine  216 ;
 
ERR(B) is the relative error of the second engine  216 ; and
 
sE is the exposure sensitivity.
 
     The second engine  216  is adjusted  446  by the calculated adjustment EXP(B). 
     In this manner, the determined new exposure drives the first marking engine and the second marking engine toward each other. As a result, image consistency is substantially improved between prints rendered or printed with different marking engines  214 ,  216 . 
     Although the discussion above concerns only two print engines, the control method  310  is applicable to the printing system which includes more than two print engines. In such system, the engine having the largest error is adjusted to closer match the xerography of other engines. 
     In one embodiment, the statistical performance of each engine is gathered overtime. For example, the engine errors ERR(A), ERR(B) are accumulated for the respective engines  214 ,  216  and averaged in a predetermined manner. The error between the engines is determined as
 
ERR′( AB )=ERR′( A )−ERR′( B ), where
 
ERR′(AB) is an averaged error between the averaged first and second engines errors ERR′(A), ERR′(B).
 
     The averaged error ERR′(AB) is compared to the predetermined threshold T TH . If the averaged error ERR′(AB) is greater than the threshold T TH.  the appropriate adjustments are calculated and applied in the manner discussed above. When the engine&#39;s change is slow, with variation occurring in several 10,000 prints per L* unit, such control mechanism provides a robust statistical estimate prior to actuation of a given xerographic parameter such as exposure. The engines are kept consistent over time. 
     It will be appreciated that various of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that 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.