Abstract:
A special test pattern and image quality analysis system and process for evaluating the test pattern are provided to accurately measure color-color registration in an image output device that prints in CMYK color. The process is robust even when using relatively low-resolution CCD-based scanners and also is unique in that the process automatically factors out errors originating from skew between the detector and the subject of measurement. Further, registration of each of C, M, and Y relative to K are independently determined. The test pattern includes an upper part and a lower part, with both the upper and lower parts having a black K component and the upper part also including color components for C, M and Y. The system and method have potential for being built in to color printers or copiers to allow measurement of color misregistration within the paper path and automatically correct for color registration errors.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The invention relates to an image quality analysis system and method that can measure color-color registration to perform evaluation of printer and copier image quality automatically. 
     2. Description of Related Art 
     It is well known that customer satisfaction can be improved and maintenance costs reduced if problems with copiers and printers can be fixed before they become serious enough to warrant a service call by the customer. While current technology exists to enable printers and copiers to call for service automatically when sensors detect certain operating parameters outside of permissible ranges, there is not a very comprehensive manner of detecting incipient system failure or automatically diagnosing when problems with image quality reach a level where human observers perceive a reduction in quality. This is caused not only by the large number of operating parameters that would need to be tracked, but also because these parameters are strongly coupled to one another. That is, a given parameter at a certain value may or may not be a problem depending on the values of other parameters. 
     It is well known that color-color registration is critical for obtaining the optimal print quality from color printers and copiers. Currently used techniques for color-color measurements typically rely on absolute position measurements. Absolute position measurements require expensive, very high-accuracy equipment (e.g., motion stages). Such measurements are also easily affected by skew of the measured print relative to the measurement device. 
     As such, there are problems with existing color-color measurement systems. 
     SUMMARY OF THE INVENTION 
     There is a need for image output devices, such as printers and copiers, to better self-diagnose problems relating to image quality. Applicants have found that to comprehensively and reliably measure the system performance of a printer or copier, the image quality of the output must be measured. 
     One exemplary embodiment of the systems and methods of the invention overcomes such problems by developing powerful diagnosing tools within a digital printer or copier for self-diagnosis and evaluation of image quality. Image quality analysis can be performed to monitor many aspects of the printed output of the printing system. Of particular importance to overall image quality is color-color registration. 
     In this embodiment, the system provides: one or more digital test patterns stored in memory (or stored in hard copy form in the case of copiers) for providing one or more hard copy output test images; an input scanner that can scan the hard copy test image to form a digital raster image; and an image quality analysis module that receives information about the position of the digital raster image and produces test results relevant to determination of image quality , particularly color-color registration. The input scanner and image quality analysis module may form part of the image output device or may be stand-alone components used to test the device. Optionally, a communication module may be provided that is capable of contacting a service department or a more sophisticated diagnostic module if further analysis or service is necessary, depending on the outcome of the image quality analysis. Alternatively, information relating to color-color misregistration may be used by a corrective procedure within the image output device being tested to calibrate the device to correct for detected misregistration. 
     The systems and methods of the invention allow highly accurate measurements that are robust against skew At the same time the technique relies only on relative measurements, which can be performed sufficiently accurate with standard input scanners. The technique therefore also allows an alternative embodiment, where a special sensor (for example a small RGB CCD array) is placed in the paper path of the output device to detect color registration errors, and allow subsequent correction without the need to use a full-page scanner. 
     A special test pattern and measurement technique is used to allow highly accurate measurements of color-color registration in an image output device that prints in CMYK color (Cyan, Magenta, Yellow, Black). The method has been demonstrated to be accurate and robust using relatively low-resolution CCD-based scanners. The technique is unique in that it automatically factors out errors originating from skew between the detector and the subject of measurement Moreover, this invention determines the registration of each of C, M, and Y relative to K such that each of the C-K, M-K, and Y-K measurements are independent, and can obtain higher accuracy by averaging results over two or more lines. 
     In the case of C-K measurements, two reflectance profiles, using the red channel of the scanner, are taken across upper and lower parts of the image. From these profiles the centroids of the K and C lines are calculated. The offset between the centroid of upper(C) line # 3  and lower(K) line # 3  is a measure of the C-K misregistration, but also includes a contribution due to skew of the image. The offset between the centroid of upper(K) line # 1  and lower(K) line# 1  is a measure of the skew. By subtracting the two offsets, a skew-independent measure of the C-K misregistration is obtained. The M-K and Y-K registration errors are determined in similar fashion, using green and blue scanner channels respectively. 
     The system and methods of the invention have potential for being built in to color printers to allow measurement and automatic correction of color registration within the image path. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described with reference to the following illustrative drawings, wherein like numerals refer to like elements and wherein: 
     FIG. 1 shows a typical digital copier machine having a user interface suitable for use with the invention; 
     FIG. 2 is a schematic diagram of a digital copier having a user interface for communicating with a remote diagnostic computer; 
     FIG. 3 is a flow chart showing an image analysis method according to the invention; 
     FIG. 4A is an exemplary digital test target used by the invention showing in dashed lines the virtual separation between upper and lower lines and zones within the upper and lower lines that are actually used for analysis purposes (the dimensions of this test target are on the order of 10 by 10 mm 2 ; 
     FIG. 4B is an exemplary full-page digital test pattern, incorporating the test target in FIG. 4A, five times in horizontal orientation and five times in vertical orientation, as well as containing different test targets which can be used for measurements of other image quality attributes; 
     FIG. 5 is an exemplary output from the digital copier based on the digital test pattern of FIG. 4A, showing both skew and mis-registration, and also showing in dashed lines the virtual separation between upper and lower lines and zones within the upper and lower lines that are actually used for analysis purposes; 
     FIG. 6 is a flow chart showing an exemplary color-color registration analysis according to the invention; 
     FIG. 7 is a detailed flow chart showing an exemplary centroid determination process according to the process of FIG. 6; and 
     FIG. 8 is an alternative image output device and image analysis system according to the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     An exemplary device to which automatic image quality analysis is to be performed will be described with reference to FIGS. 1-3. FIG. 1 shows an image output device, in particular a digital copier machine  10 , comprising a plurality of programmable components and subsystems which cooperate to carry out copying or printing jobs programmed through a touch dialog screen  42  of a user interface (UI)  11 . Internal operating systems of the digital copier  10  are disclosed in U.S. Pat. Nos. 5,038,319, 5,057,866, and 5,365,310, owned by the assignee of the present invention, the disclosures of which are incorporated herein by reference in their entirety. As such, no further detailed description thereof is necessary. Digital copier  10 , however, is merely representative of a preferred printing system to which the image quality determination is made. It should be understood that a loosely coupled printing or reproducing system is also applicable for use with the invention described herein, such as a printer or facsimile device. Moreover, while there may be benefits to use of the image quality analysis on a reproduction system, such as a digital copier having an integral scanner component, the invention also is applicable to a printer used in conjunction with a stand-alone scanner, such as a flatbed type scanner. 
     Referring to FIG. 2, operation of the various components of exemplary digital copier  10  is regulated by a control system which uses operating software stored in memory in the system controller  16  to operate the various machine components in an integrated fashion to produce copies and prints. The control system includes a plurality of printed wiring boards (PWBs), there being a user interface module (UIM) core PWB  18 , a scanner/imaging core PWB  20 , an input station core PWB  22 , a paper handling core PWB  24  and an output station core PWB  26 , together with various input/output (I/O) PWBs  28 . A shared line (SL)  30  couples the core PWBs  18 ,  20 ,  22 ,  24  and  26  with each other and with the electronic data node core  32 , while local buses  34  serve to couple the PWBs to the respective cores and to stepper and servo PWBs. Programming and operating control over digital copier  10  is accomplished through touch dialog screen  42  of UI  11 . The operating software includes application software for implementing and coordinating operation of system components. 
     Floppy disk port  38  provides program loading access to UIM core PWB  18  for the purpose of entering changes to the operating software, loading specific programs, such as diagnostic programs, and retrieving stored data, such as machine history data and fault data, using floppy disks. Hard disk  36  is used as a non-volatile memory (NVM) to store programs, machine physical data and specific machine identity information. One of the programs hard disk  36  may store is image quality analysis software that forms an image quality analysis module  70  used by the invention. Module  70  may also reside on a floppy disk used in floppy disk port  38 . 
     UIM core PWB  18  communicates with video engine  40  for driving a suitable visual display  42 , such as a CRT or flat screen of the user interface  11 . The UIM core  18  also has connected thereto a control panel I/O processor  44  and a generic accessories interface I/O processor  46 . The interface I/O processor  46  is in turn connected to a modem PWB  48 . The modem  48  provides communication between digital copier  10  and a communications channel, such as a public switched telephone network  50  to facilitate information transfer to and from a remote diagnostic computer  60 , which may also include image quality analysis module  70  as well as other diagnostic modules. 
     The information from the subsystem cores flows to and from the UIM core PWB  18 , which embodies software control systems including a user interface system manager and a user interface manager. The UI system manager includes a UI display manager subsystem for controlling the display of messages on the display  42 . A data manager subsystem provides data management to the UI system manager. 
     In a first embodiment of the invention, image quality analysis is performed by the process set forth in the flow chart of FIG.  3 . The process starts at step S 300  and advances to step S 310  where at least one specific digital test pattern, which can either be in hardcopy original form or a digital image stored in memory  36 , is provided. An exemplary test pattern is illustrated in FIG.  4  and will be described in more detail later. Preferably, multiple different test patterns are used to analyze various components relevant to a determination of image quality. Flow then proceeds to step S 320  where a corresponding hardcopy output of the test pattern is generated. This can be by outputting a printed hardcopy output from output station  26  using the digital test pattern as an input when the test pattern is stored in digital form, such as in hard disk  36  or floppy disk  38 . Alternatively, an accurate original hardcopy test pattern may be placed at scanner  20  and scanned into the digital copier  10  to form a digital test pattern, which can be used as an input to output station  26  to form the hardcopy output. Then, flow advances to step S 330  where the hardcopy output is scanned by scanner  20  to form a digital raster image for analysis purposes. 
     After step S 330 , flow advances to step S 340  where the digital image is preferably acted on by pattern recognition software, which can be located within hard disk  36  or floppy disk  38  and is associated with image quality analysis module  70 , to determine a precise location of various test elements within the scanned digital raster image. This software uses a Hough or similar transform to automatically detect locator marks on the image. A suitable pattern recognition system for use with the invention can be found in U.S. Pat. No. 5,642,202 to Williams et al., owned by the assignee of the present invention, the disclosure of which is incorporated herein by reference in its entirety. Alternatively, or in conjunction therewith, the test pattern may include a script that signifies a particular test pattern. The image quality analysis module  70  may use hardware/software to decipher the particular script embedded into the test pattern. The memory of the image quality analysis module  70  may be provided with a file corresponding to each possible script detailing the contents of the script and associated test pattern, as well as detailing the particular image quality analysis routine to be used to measure a particular part of overall image quality. A more detailed description of such a scripted test pattern can be found in co-pending U.S. Ser. No. 09/450,182 to Rasmussen et al., filed concurrently herewith, entitled “Method to Allow Automated Image Quality Analysis of Arbitrary Test Patterns”, the subject matter of which is incorporated by reference herein in its entirety. 
     After step S 340 , the process flows to step S 350  where image quality analysis is performed on the test image using image quality analysis module  70 . From step S 350 , flow advances to step S 360  where a determination is made by the image quality analysis module  70  whether the image quality for this particular test image is acceptable. If it is, flow advances to step S 380  where the process stops. However, if the image quality is not acceptable, flow advances from step S 360  to step S 370  where a call can be made to a diagnostic facility. This call may be an automatic service call made through modem  48  for scheduling an actual service visit by a service technician to correct the noted problems. Alternatively, it may be a call to a more sophisticated diagnostic module  80  located locally or at the remote facility that can further analyze the image quality problem along with values from various sensors and settings on the copier  10 . This would provide corrective feedback to the digital copier  10 , such as through modem  48  when module  80  is remotely located, allowing the digital copier  20  to adjust itself within acceptable parameters. 
     Alternatively, the image quality analysis module  70  may be remote from image output device  10 . An example of which is illustrated in FIG. 8 where image output devices are in the form of printers  10 A,  10 B which are associated with a personal computer  60  through appropriate data cables. A flat bed scanner  20  is also associated with personal computer  60  and image quality analysis module  70  is in the form of software provided in personal computer  60 . This embodiment operates as the previous embodiment in that the printers  10 A,  10 B (which ever is being tested) are given a test pattern to generate a hardcopy output from. This hardcopy output is then placed in scanner  20  to generate the digital test image. This digital test pattern is then analyzed to determine image quality of the printer. 
     While shown in FIG. 8 to be loosely associated, the invention can also be practiced with completely discrete components, such as a separate printer, scanner and computer or other source for containing image quality analysis module  70 . In this case, the hardcopy output from the printer can be provided to a non-associated scanner for scanning. Then, the digital test image from the scanner can be stored or converted onto a portable recording medium, such as a floppy disk and provided to a non-associated computer having the image quality analysis module. 
     The test pattern used can be one of several test patterns designed to provide evaluation of a particular parameter relevant to image quality analysis of the output of the printing system, such as color registration, motion quality, micro and macro uniformity, color correction, and font quality. 
     This particular invention relates specifically to determination of color-color registration, which forms a part of the overall image quality analysis. For a more detailed description of the overall image quality analysis system, see co-pending U.S. Ser. No. 09/450,185 to Rasmussen et al., filed concurrently herewith, entitled “Virtual Tech Rep By Remote Image Quality Analysis”, the disclosure of which is incorporated herein by reference in its entirety. 
     Another exemplary test pattern and analysis suitable for determining pixel placement accuracy to a high precision can be found in co-pending U.S. Ser. No. 09/450,184 to Dalal et al., filed concurrently herewith, entitled “Quantification of Motion Quality Effect on Image Quality”, the subject matter of which is incorporated by reference herein in its entirety. Furthermore, a test pattern and analysis may be used to distinguish and categorize various non-uniformities. Such an exemplary analysis can be found in co-pending U.S. Ser. No. 09/450,180 to Rasmussen et al., filed concurrently herewith, entitled “Image Processing Method for Characterization of Uniformity of Printed Images”, the subject matter of which is incorporated by reference herein in its entirety. Other test patterns can be used to determine quality of font reproduction. An example of such can be found in co-pending U.S. Ser. No. 09/450,177 to Rasmussen et al., filed concurrently herewith, entitled “Outline Font For Analytical Assessment of Printed Text Quality”, the subject matter of which is incorporated by reference herein in its entirety. 
     The image quality analysis according to this invention is preferably performed with as much automation as possible to reduce the amount of human involvement with the process. In the case of a digital copier or reprographic machine, such as machine  10  having both an output station and a scanner station, analysis can be initiated automatically by the image quality analysis module  70 , which can be stored within hard disk  36 , for example. That is, the image quality analysis module  70  may operate as a self-executing program either initialized at start-up or after a predetermined period of use or number of copies made, such that module  70  causes the test pattern to be printed by output section  26  and then causes the hardcopy output therefrom scanned by scanner section  20 . This can be achieved in an automated fashion, for example, by having the print output of the station  26  linked to the scanner station  20  input, as in U.S. Pat. No. 5,884,118 to Mestha et al., owned by the assignee of the present invention, the disclosure of which is incorporated herein by reference in its entirety. 
     An exemplary full-page test pattern useful to determine color-color registration as well as other image quality metrics, in a color image output device is illustrated in FIG.  4 B. The specific region of the full-page test pattern that is used for color registration measurements is shown in enlargement in FIG.  4 . This particular test pattern is useful in a device that outputs using a CMYK color space, with C being Cyan, Y being Yellow, M being Magenta and K being Black. Similar to that disclosed in co-pending U.S. Ser. No. 09/450,182, this test pattern (FIG. 4B) can be provided with a script to identify the particular test, as well as identify the analysis procedure to be implemented to analyze the test pattern. In this example, the full-page test pattern is provided with a human-readable script name (IQAF-TP2-v2) which could also be represented as bar code or other machine-readable form. Suitable bar code readers or optical character recognition equipment can be used to decipher the script, which will be recreated when the input pattern is output as a hardcopy by the image output device  10 . The script is preferably provided to specify the location of subregions of interest from the various sub-regions provided on the test pattern, as well as indicating what type of analysis is to be performed on that particular sub-region. 
     The test pattern has 10 upper line positions (labeled U 1  through U 10 ) and 10 lower line positions (labeled L 1  through L 10 ), which merge at the imaginary dashed line I, which does not form part of the test pattern but is provided for illustrative purposes. The 10 lines in the lower part (L 1 -L 10 ) are black (K) only. The upper part has lines in each of K, C, M and Y. In particular, U 1  and U 2  are black, U 3  and U 4  are cyan, U 5  and U 6  are magenta, U 7  and U 8  are yellow, and U 9  and U 10  are also black. 
     A method of using the test pattern to determine color-color registration will now be described with reference to FIG.  6 . 
     The process starts at step S 600  and advances to step S 610  where the digital test pattern of FIG. 4 is used as an input to obtain a hardcopy output from an image output device to be tested, which for example can be copier  10  in FIG.  2 . Then, at step S 620 , the hardcopy output is placed into a suitable image capture device, such as a scanner or CCD device  20 , and scanned to provide a digital image corresponding to the hardcopy output. A particularly suitable image capture device is a flatbed scanner, which may be a stand-alone scanner (as in FIG. 8) when testing is being performed on a printer or other image output device with or without an integral scanner, or could be the integral scanner (as in FIG. 2) of the image output device in the case of a copier. Thus, for example, the scanner section  20  of copier  10  may be used to scan the hardcopy output to obtain the digital image. Best results are obtained with a color scanner, for example an RGB scanner, but in principle a monochrome scanner could be used, provided it gave sufficiently high contrast and signal-to-noise ratio for cyan, magenta, and yellow lines. 
     Once the digital test image is obtained from step S 620 , flow advances to step S 630  where image quality analysis module  70  selects a Red channel (the channel complementary to the color of the lines being analyzed) of the image for processing. Flow then advances to step S 640  where a horizontal intensity profile is obtained from the digital test image. In particular, this intensity profile is an upper red intensity profile determined from the upper lines U 1 -U 10 . However, to avoid erroneous results, the entirety of lines U 1 -U 10  and L 1 -L 10  are not analyzed. Instead, only the central region CR 1  shown in dashed outline in FIG. 4 is used (similarly, when the lower intensity profile is subsequently determined, only the central region CR 2  is used). This prevents extraneous information or artifacts found at the extremities of the lines, particularly at the merge area of the upper and lower lines, from affecting the analysis. 
     In particular, step S 640  involves averaging the pixel values of the image over the vertical direction within the central region CR 1  for each horizontal pixel position. The profile is an array of intensities, with one intensity for each horizontal pixel position corresponding to the average intensity in the vertical direction. This horizontal intensity profile is referred to as the upper red profile. 
     From Step S 640 , flow advances to step S 645  where a background intensity (BI) is determined from a maximum value of the lower red profile (the area bounded by the dashed box CR 2 ). After step S 645 , flow advances to Step S 650  where centroid positions of lines U 1 , U 2 , U 3 , U 4 , U 9  and U 10  are determined from the upper red profile. This can be achieved rather easily as both the black and cyan lines provide high contrast when viewed through the red channel, and as the approximate line locations are known from the digital test pattern. 
     In particular, step S 650  can be broken up into the following subroutine described with reference to FIG.  7 . The process starts at step S 700  and advances to step S 710  where edge positions of a first line are determined (U 1 ). This can be achieved in several ways. One exemplary way is to first determine approximate edge positions. Then, the average intensity of the line (U 1 ) within those approximate edge locations is determined. Then, exact edge locations are set as the points corresponding to intensity levels midway between the background intensity and the average line intensity. 
     From step S 710 , flow advances to step S 720  where the line surround is defined. In a first iteration, we are working with line U 1 . This surround is defined such that it is certain to contain not only the region defined by the line edges of U 1  (from step S 710 ), but sufficient white surround around this region as well. In practice, this is usually satisfied when about 200 microns are included that extend outside of the line edges. Alternatively, the surround can be set by 10%-90% line edge widths of the U 1  line as calculated by the lower red profile, with the line surround for U 1  being set to extend three times the edge width beyond the edge locations. This line surround is used later to calculate the centroid location. It is important in this calculation that the region covers image area outside of the actual edge of the line. 
     From step S 720 , flow advances to step S 730  where a weight profile is calculated from the lower red profile by assigning each pixel location X a weight W(X) that is equal to the following equation: 
     
       
           W ( X )= IB−R ( X ), 
       
     
     where R(X) is the lower red profile at pixel location X and IB is the background intensity. 
     Then, the process advances to step S 740  where the centroid position of the particular line (U 1  in the first iteration) is calculated as the centroid of the weight profile over the line surround (U 1 -surround). 
     Then, at step S 750 , the process proceeds to S 760  and stops if no further lines need calculations. Otherwise, flow returns to step S 710  to complete calculations for remaining ones of lines U 1 , U 2 , U 3 , U 4 , U 9  and U 10 . At the end of the process shown in FIG. 7, centroids XU 1 , XU 2 , XU 3 , XU 4 , XU 9  and XU 10  are determined. 
     Returning now to the flow chart of FIG.  6  and the completion of step S 650 , flow advances to step S 655  where a lower red profile is calculated similar to that of the upper red profile in step S 640 . Then, at step S 660 , lower centroids of lines L 1 , L 2 , L 3 , L 4 , L 9  and L 10  are calculated similar to that of the upper centroids, only using the central region CR 2 . This results in determination of centroids XL 1 , XL 2 , XL 3 , XL 4 , XL 9  and XL 10 . 
     From step S 660 , flow advances to step S 670  where misregistration is calculated. In a first iteration, misregistration MC of Cyan (C) relative to Black (K) is calculated using the following equations: 
     
       
           UMC= 0.5*(( XU   3 + XU   4 )−( XL   3 + XL   4 )); 
       
     
     
       
           SM= 0.25*(( XU   1 + XU   2 + XU   9 + XU   10 )−( XL   1 + XL   2 + XL   9 + XL   10 )); 
       
     
     
       
         
           MC=UMC−SM, 
         
       
     
     where UMC is misregistration not corrected for skew and SM is the estimated contribution of skew to UMC. 
     After cyan misregistration MC is calculated, flow advances to step S 675  where it is determined whether additional printed colors need to be tested. As magenta and yellow registrations have not yet been measured, the process advances to step S 680  where the channel that provides the highest contrast for Magenta (Green) is selected and the process returns to step S 640  for a second iteration of steps S 640  to S 670  using the green channel to ultimately determine misregistration of Magenta relative to Black using three equations similar to those above, but substituting use of the magenta lines U 5 , U 6 , L 5  and L 6  for the cyan lines U 3 , U 4 , L 3  and L 4 . Then, at step S 675  it is determined that the Yellow registration has not yet been measured and flow advances to step S 680  where the channel that provides the highest contrast for Yellow (Blue) is selected and flow returns to step S 6640  for a third and final iteration of steps S 640  to S 670  substituting use of the yellow lines U 7 , U 8 , L 7  and L 8  for the cyan lines U 3 , U 4 , L 3  and L 4 . This time, as all three colors have been analyzed, the determination in step S 675  is no and process advances to step S 690  where the analysis is stopped. 
     The algorithm described above can be varied in several regards without changing its substantial features. For example, the test pattern (FIG. 4) could contain geometrical shapes other than lines from which centroids are determined; the number of lines of a given color could be changed; or the specific method to determine centroid location could be modified. Additionally, first and second sections could be horizontally aligned (rather than vertically aligned as in FIG. 4A) to determine lateral mis-registration. However, the key principles of the algorithm are: (1) a single channel of an image input device is used for measurement of a given color relative to another color; (2) the same algorithm as is used to measure the displacement of centroids of one color relative to another color, is applied to measure the displacement of two centroids for one and the same color, as caused by skew, and this result is further used to correct the color registration measurement for skew; (3) more than one line (or other geometrical shape) can be used to increase the accuracy of the measurement (in FIG. 4A two are used for C, M, and Y, and four are used for black); (4) the use of color lines in the digital test pattern at the exact same horizontal position as the (black) reference lines implies that the results will not be affected by quantization errors when the digital test pattern is rasterized; (5) the lines (or other geometrical shapes) are placed such that the measurements of relative displacements involve only very small distances, and therefore do not require high accuracy over larger distances. 
     Also, by using multiple channels of data (e.g., RGB data), the registrations of each of C, M, and Y relative to K are done independently using the channels R, G, and B, respectively. This means that the results are not affected by the registration of the R, G, B channels of the measurement device. This method would work equally well on any multiple color system, such as a copier with highlight color, where a channel is used that is complementary to the color being tested. 
     The technique has already been implemented in an image quality analysis system, tested with off-line scanners, and has demonstrated accuracy around 5 microns using standard 600 dpi CCD-based scanners. 
     Once this analysis is determined, a service call may be made using the communication module of FIG. 2 if any mis-registration is detected. Alternatively, information relating to color-color misregistration may be used by a corrective procedure within the image output device being tested to calibrate the device to correct for detected misregistration. 
     The present invention has been described with reference to specific embodiments, which are intended to be illustrative and non-limiting. Various modifications can be made to the invention without departing from the spirit and scope of the invention as defined by the appended claims.