Abstract:
A method of calibrating a media advance in a printer includes providing a mask to specify a printing configuration of a calibration target; forming a media feed calibration target on the print media by: i. printing the calibration target on a print media using an array of the marking elements, ii. advancing the print media by a media advance amount, iii. printing the calibration target on the print media using the array of the marking elements; iv. advancing the print media by the previous media advance amount plus an offset amount, and v. repeating steps iii. and iv. until the media feed calibration target is complete; measuring the optical reflectance of the media feed calibration target as a function of position along the media feed calibration target; identifying a position along the media feed calibration target corresponding to the location at which a maximum in the optical reflectance occurs; and comparing the location at which a maximum in the optical reflectance occurs to a predetermined location of the media feed calibration target to calibrate media advance in the printer.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a Divisional Application of U.S. patent application Ser. No. 12/241,112 filed Sep. 30, 2008. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates generally to the field of digitally controlled printing devices, and in particular to calibrating media advance through these devices. 
       BACKGROUND OF THE INVENTION 
       [0003]    Many types of printing systems include one or more printheads that have arrays of marking elements that are controlled to make marks of particular sizes, colors, etc. in particular locations on the print media in order to print the desired image. In some types of printing systems the array of marking elements extends across the width, and the image can be printed one line at a time. However, the cost of a printhead that includes a page-width array of marking elements is too high for some types of printing applications, so a carriage printing architecture is used. 
         [0004]    In a carriage printing system (whether for desktop printers, large area plotters, etc.) the printhead or printheads are mounted on a carriage that is moved past the recording medium in a carriage scan direction as the marking elements are actuated to make a swath of dots. At the end of the swath, the carriage is stopped, printing is temporarily halted and the recording medium is advanced. Then another swath is printed, so that the image is formed swath by swath. In a carriage printer, the marking element arrays are typically disposed along an array direction that is substantially parallel to the media advance direction, and substantially perpendicular to the carriage scan direction. 
         [0005]    Recording media, whether supplied as cut sheets or from continuous rolls of media, is typically advanced by a set of rollers driven by a motor. The amount of roller angular rotation is controlled by the printer controller. Angular rotation θ can be implemented by specifying a number of advance steps by a stepper motor, and/or θ can be monitored by use of a rotary encoder that is mounted coaxially with a feed roller, for example. The distance that media has been advanced is nominally Rθ, where R is the radius of the media advance roller that is coaxially mounted with the rotary encoder and where θ is measured in radians. However, there are a variety of sources of error in this nominal media advance distance. First of all, manufacturing variability or wear in rollers can result in a roller radius that is not exactly equal to R. Secondly, the distance of advance of the side of the paper on which marking will occur is actually (R+t)θ, where t is the thickness of the media being advanced. Media thickness can therefore affect the advance distance. Thirdly, there can be slippage between the media and the roller. 
         [0006]    For the case of under feeding the media, media advance errors can result in dark streaks in the image because adjacent swaths of printed data partially overlap. For the case of over feeding the media, media advance errors can result in white streaks in the image because there is a gap between adjacent swaths of printed data. In addition, overfeeding and underfeeding also results in the overall image length being too long or too short. Especially for long images, even relatively small systematic error in media feed distance can result in problems in framing or tiling of images. 
         [0007]    A variety of methods for correcting for media advance errors have previously been disclosed. U.S. Pat. No. 5,825,378 discloses printing a series of lines, where successive lines are separated by media advance steps. The line spacing can then be measured by rotating the sheet of media 90 degrees and measuring the distance between lines using an optical sensor that is mounted on the carriage. The actual distance between lines is compared to the nominal distance between lines and the error is used to correct the angular rotation that the roller is to be advanced for a given desired media advance. This method requires direct user intervention to rotate the media. 
         [0008]    U.S. Pat. No. 6,137,592 discloses printing a test pattern using successively increasing or decreasing values of media feed. The user then selects the region of the test pattern having a minimum amount of light or dark streaking. The media advance selected can then be stored in memory as the new nominal advance distance. This method requires user intervention to select the best looking portion of the test image, and is susceptible to user error. 
         [0009]    U.S. Pat. No. 7,210,758 discloses printing a test pattern including an on-off pattern such as a checkerboard and incrementing media feed values. At an optimal media feed, the dark patterns from a first pass will line up with the light patterns from a second pass, so that the pattern appears darkest (maximum optical density) for the optimal media feed. Examination of the printed pattern can be done automatically by measuring optical density of the pattern and identifying the optimal media feed value as corresponding to the region of the test pattern having the maximum optical density. 
         [0010]    While the aforementioned methods are satisfactory for some applications, as customer expectations for improved image quality continue to increase, there is a need for a media feed calibration method that is even more precise and less susceptible to measurement error. 
       SUMMARY OF THE INVENTION 
       [0011]    According to one aspect of the present invention, a method of calibrating a media advance in a printer, the printer including an array of marking elements that are disposed along a direction that is substantially parallel to a direction of media advance, includes: providing a mask to specify a printing configuration of a calibration target; forming a media feed calibration target on the print media by: i. printing the calibration target on a print media using the array of the marking elements, ii. advancing the print media by a media advance amount, iii. printing the calibration target on the print media using the array of the marking elements; iv. advancing the print media by the previous media advance amount plus an offset amount, and v. repeating steps iii. and iv. until the media feed calibration target is complete; measuring the optical reflectance of the media feed calibration target as a function of position along the media feed calibration target; identifying a position along the media feed calibration target corresponding to the location at which a maximum in the optical reflectance occurs; and comparing the location at which a maximum in the optical reflectance occurs to a predetermined location of the media feed calibration target to calibrate media advance in the printer. 
         [0012]    According to another aspect of the present invention, the method described above can also include forming a second media feed calibration target on the print media by: i. printing the calibration target on a print media using the array of the marking elements, ii. advancing the print media by a media advance amount, iii. printing the calibration target on the print media using the array of the marking elements; iv. advancing the print media by the previous media advance amount, and v. repeating steps iii. and iv. until the media feed calibration target is complete; measuring the optical reflectance of the second media feed calibration target as a function of position along the second media feed calibration target; identifying and storing a periodic variation in the optical reflectance as a function of angular rotation; and adjusting rotation of a media advance roller based on the stored periodic variation and the position of a marker that indicates the particular angular position of the roller. 
         [0013]    According to another aspect of the present invention, a method of calibrating a media advance in a printer, the printer including a media advance roller, a marker to indicate a particular angular position of the roller, and an array of marking elements that are disposed along a direction that is substantially parallel to a direction of media advance, includes: providing a mask to specify a printing configuration of a calibration target; forming a media feed calibration target on the print media by: i. printing the calibration target on a print media using the array of the marking elements, ii. advancing the print media by a media advance amount, iii. printing the calibration target on the print media using the array of the marking elements; iv. advancing the print media by the previous media advance amount, and v. repeating steps iii. and iv. until the media feed calibration target is complete; measuring the optical reflectance of the media feed calibration target as a function of position along the media feed calibration target; identifying and storing a periodic variation in the optical reflectance as a function of angular rotation; and adjusting rotation of the media advance roller based on the stored periodic variation and the position of the marker that indicates the particular angular position of the roller. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    In the detailed description of the preferred embodiments of the invention presented below, reference is made to the accompanying drawings, in which: 
           [0015]      FIG. 1  is a schematic representation of an inkjet printer system. 
           [0016]      FIG. 2  is a perspective view of a portion of a printhead chassis. 
           [0017]      FIG. 3  is a perspective view of a portion of a carriage printer. 
           [0018]      FIG. 4  is a schematic side view of a paper path in a carriage printer. 
           [0019]      FIG. 5  is an embodiment of a mask for a media feed calibration target. 
           [0020]      FIG. 6  shows an embodiment of a media feed calibration target. 
           [0021]      FIGS. 7-10  show magnified views of dot clusters of a media feed calibration target, with the amount of overlap varying in different regions of the target. 
           [0022]      FIG. 11A  shows a plot of the photosensor signal as a function of position corresponding to scanning the media feed calibration target of  FIG. 6 . 
           [0023]      FIG. 11B  shows the media feed calibration target of  FIG. 6  rotated clockwise by 90 degrees to clarify the relationship of features of the target and the plot in  FIG. 11A . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]    The present description will be directed in particular to elements forming part of, or cooperating more directly with, apparatus in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art. Referring to  FIG. 1 , a schematic representation of an inkjet printer system  10  is shown, as described in U.S. Pat. No. 7,350,902, and incorporated by reference herein in its entirety. Printer system  10  includes a source  12  of image data, which provides data signals that are interpreted by a controller  14  as being commands to eject drops. Controller  14  includes an image processing unit  15  for rendering images for printing, and outputs signals to a source  16  of electrical energy pulses that are inputted to an inkjet printhead  100 , which includes at least one printhead die  110 . 
         [0025]    In the example shown in  FIG. 1 , there are two nozzle arrays  120 ,  130  for inkjet printhead  100 . Nozzles  121  in the first nozzle array  120  have a larger opening area than nozzles  131  in the second nozzle array  130 . In this example, each of the two nozzle arrays  120 ,  130  has two staggered rows of nozzles, each row having a nozzle density of 600 per inch. The effective nozzle density then in each array  120 ,  130  is 1200 per inch. If pixels on the recording medium were sequentially numbered along the paper advance direction, the nozzles from one row of an array would print the odd numbered pixels, while the nozzles from the other row of the array would print the even numbered pixels. Even though neighboring pixels are printed by nozzles in different rows of the array in a staggered array geometry of marking elements, the marking elements that produce neighboring marking pixels are considered to be neighboring marking elements. Nozzle number one in the nozzle array is in the same row as nozzle number three in this example, while nozzle number two is in the other row. The physical distance between nozzle number one and nozzle number three is 1/600 inch. The center to center distance between nozzle number two and nozzle number one may be greater than 1/600 inch due to the spacing between rows of the array. However the distance along the array direction between nozzle number one and nozzle number two is d= 1/1200 inch in this example. The corresponding pixel spacing in the array direction is also 1/1200 inch in this example. 
         [0026]    In fluid communication with each nozzle array is a corresponding ink delivery pathway. Ink delivery pathway  122  is in fluid communication with nozzle array  120 , and ink delivery pathway  132  is in fluid communication with nozzle array  130 . Portions of fluid delivery pathways  122  and  132  are shown in  FIG. 1 , as openings through printhead die substrate  111 . One or more printhead die  110  can be included in inkjet printhead  100 , but only one printhead die  110  is exemplarily shown in  FIG. 1  for simplistic illustrative purposes. The printhead die is arranged on a support member as discussed below relative to  FIG. 2 . In  FIG. 1 , a first ink source  18  supplies ink to first nozzle array  120  via ink delivery pathway  122 , and a second ink source  19  supplies ink to second nozzle array  130  via ink delivery pathway  132 . Although distinct ink sources  18  and  19  are shown, in some applications it may be beneficial to have a single ink source supplying ink to both nozzle arrays  120  and  130  via ink delivery pathways  122  and  132  respectively. Also, in some embodiments, fewer than two nozzle arrays are included on printhead die  110 , in other embodiments more than two nozzle arrays are used. In some embodiments, all nozzles on a printhead die  110  may be the same size, rather than having multiple sized nozzles on a printhead die. 
         [0027]    Drop forming mechanisms (not shown in  FIG. 1 ) are associated with the nozzles. Drop forming mechanisms can be of a variety of types, some of which include a heating element to vaporize a portion of ink and thereby cause ejection of a droplet, or a piezoelectric transducer to constrict the volume of a fluid chamber and thereby cause ejection, or an actuator which is made to move (for example, by heating a bilayer element) and thereby cause ejection. In any case, electrical pulses from pulse source  16  are sent to the various drop ejectors according to the desired deposition pattern. In the example of  FIG. 1 , droplets  181  ejected from nozzle array  120  are larger than droplets  182  ejected from nozzle array  130 , due to the larger nozzle opening area. Typically other aspects of the drop forming mechanisms (not shown) associated respectively with nozzle arrays  120  and  130  are also sized differently in order to optimize the drop ejection process for the different sized drops. During operation, droplets of ink are deposited on a recording media  20 . 
         [0028]      FIG. 2  shows a perspective view of a portion of a printhead chassis  250 , which is an example of an inkjet printhead  100 . Printhead chassis  250  includes three printhead die  251  (similar to printhead die  110 ), each printhead die containing two nozzle arrays  253 , so that printhead chassis  250  contains six nozzle arrays  253  altogether. The six nozzle arrays  253  in this example may be each connected to separate ink sources (not shown in  FIG. 2 ), such as cyan, magenta, yellow, text black, photo black, and a colorless protective printing fluid. Each of the six nozzle arrays  253  is disposed along direction  254 , and the length L of each nozzle array along direction  254  is typically on the order of 1 inch or less. Typical lengths of recording media are 6 inches for photographic prints (4 inches by 6 inches), or 11 inches for 8.5 by 11 inch paper, while roll-fed printers can use media lengths as long as 100 feet or more. Thus, in order to print the full image, a number of swaths are successively printed while moving printhead chassis  250  across the recording media. Following the printing of a swath, the recording media is advanced along a media advance direction  304  that is substantially parallel to nozzle array direction  254 . 
         [0029]    A single pass print mode is one in which each marking element in the array is assigned to print all of the pixel locations within a given row of pixels in a single swath. Single pass print modes are relatively fast and are typically used in draft modes. However, if a jet is misdirected or malfunctioning in other ways, single pass print modes can result in objectionable white or dark streaks in the printed image. Therefore, higher quality print modes use multiple overlapping passes to print the image. In multi-pass printing, print masks are used to assign responsibility to different marking elements for printing the various printing locations within a row of pixels. In addition, the media advance distance between printing passes is less than the length of the nozzle array in multi-pass printing. Instead, if the length of the nozzle array is L and the number of passes in the multi-pass print is m, then the desired media advance distance will be approximately L/m. 
         [0030]    A flex circuit  257  to which the printhead die  251  are electrically interconnected, for example by wire bonding or TAB bonding, is also shown in  FIG. 2 . The interconnections are covered by an encapsulant  256  to protect them. Flex circuit  257  bends around the side of printhead chassis  250  and connects to connector board  258 . When printhead chassis  250  is mounted into the carriage  200  (see  FIG. 3 ), connector board  258  is electrically connected to a connector (not shown) on the carriage  200 , so that electrical signals may be transmitted to the printhead die  251 . 
         [0031]      FIG. 3  shows a portion of a desktop carriage printer. Some of the parts of the printer have been hidden in the view shown in  FIG. 3  so that other parts may be more clearly seen. Printer chassis  300  has a print region  303  across which carriage  200  is moved back and forth in carriage scan direction  305  along the X axis, between the right side  306  and the left side  307  of printer chassis  300 , while drops are ejected from printhead die  251  on printhead chassis  250  that is mounted on carriage  250 . Carriage motor  380  moves belt  384  to move carriage  200  along carriage guide rail  382 . An encoder sensor (not shown) is mounted on carriage  200  and indicates carriage location relative to an encoder fence  383 . 
         [0032]    Also mounted on carriage  200  is an optical sensor (also called a carriage sensor)  210 , as shown in  FIG. 4 . Carriage sensor  210  includes a light emitter such as an LED that shines light onto the recording medium. Light reflected from the recording medium is received by a photosensor that is also included in carriage sensor  210 . A common use of a carriage sensor  210  in prior art is to evaluate printhead alignment by providing an electronic signal that that may be analyzed to determine the positions of printed marks. 
         [0033]    Printhead chassis  250  is mounted in carriage  200 , and ink supplies  262  and  264  are mounted in the printhead chassis  250 . The mounting orientation of printhead chassis  250  is rotated relative to the view in  FIG. 2 , so that the printhead die  251  (shown in  FIG. 2 ) are located at the bottom side of printhead chassis  250 , the droplets of ink being ejected downward onto the recording media in print region  303  in the view of  FIG. 3 . Ink supply  262 , in this example, contains five ink sources cyan, magenta, yellow, photo black, and colorless protective fluid, while ink supply  264  contains the ink source for text black. Paper, or other recording media (sometimes generically referred to as paper, media or print media herein) is loaded, in this example, along paper load entry direction  302  at the front portion  308  of printer chassis  300 . 
         [0034]    A variety of rollers are used to advance the recording media through the printer, as shown schematically in the side view of  FIG. 4 . In this example, a pickup roller  320  moves the top sheet  371  of a stack  370  of paper or other recording media in the direction of arrow  302 . A turn roller  322 , toward the rear portion  309  of the printer chassis  300  shown in  FIG. 3 , acts to move the paper around a C-shaped path (in cooperation with a curved rear wall surface) so that the paper continues to advance along direction arrow  304  from the rear direction  309  of the printer shown in  FIG. 3 . The paper is then moved by feed roller  312  and idler roller(s)  323  to advance along the Y axis  9  in  FIG. 3 ) and across print region  303 , and from there to a discharge roller  324  and star wheel(s)  325  so that a paper, printed with an image, exits along direction  304 . In the vicinity of the print region  303 , the paper is fed along direction  304 , so that with regard to printing, direction  304  is referred to as the media feed direction. Feed roller  312  includes a feed roller shaft  319  along its axis, and feed roller gear  311  is mounted on the feed roller shaft  319 . Feed roller  312  can include of a separate roller mounted on feed roller shaft  319 , or a thin high friction coating on feed roller shaft  319 . A rotary encoder (not shown) can also be coaxially mounted with feed roller  312  on shaft  319 , so that it can monitor the angular rotation θ of feed roller  312 . 
         [0035]    The motor (e.g. a DC servo motor or a stepper motor) that powers the paper advance rollers is not shown in  FIG. 1 , but the hole  310  at the right side  306  of the printer chassis  300  (shown in  FIG. 3 ) is where the motor gear (not shown) protrudes through in order to engage feed roller gear  311 , as well as the gear for the discharge roller (not shown). For normal paper pick-up and feeding, it is desired that all rollers rotate in forward direction  313 . Toward the left side  307  in the example chassis  300  shown in  FIG. 3  is the maintenance station  330 . 
         [0036]    Toward the rear portion  309  of the printer in chassis  300  is located electronics board  390 , which includes cable connectors  392  for communicating via cables (not shown) to the printhead carriage  200  and from there to the printhead chassis  250 . Also mounted on the electronics board  390  are motor controllers for the carriage motor  380  and for the paper advance motor, a processor and/or other control electronics (shown schematically as  14  and  15  in  FIG. 1 ) for controlling the printing process, including image processing, and an optional connector for a cable to a host computer. 
         [0037]    Embodiments of the present invention make use of the fact that, particularly for a dark color ink such as black, the optical density of a region having multiple layers of ink dots in a particular location on a white recording medium is not much greater than the optical density of a single layer of dots. However, if the various layers of dots are displaced from each other so that the group of dots covers a greater area of the white recording medium, then the optical density of the region does increase significantly. (Correspondingly the optical reflectance of a region having dots covering a greater area of the white recording medium decreases significantly). A special type of print mask is used to print a media feed calibration target, such that corresponding marking elements in different sections of the printhead are directed by the mask to print dots that will land on top of each other if the actual media feed distance is equal to the nominal media feed distance, but which will be displaced from each other if the actual media feed distance is either greater than or less than the nominal media feed distance. Between printing successive swaths of the media feed calibration target, the media advance distance is successively incremented or decremented. After printing the target, the optical density (or optical reflectance) of the target is measured as a function of position within the target, for example by scanning carriage sensor  210  across the target and analyzing the signal from the photosensor due to light reflected from the target. 
         [0038]    The special mask used for the media feed calibration target differs from a typical mask used in multipass printing of images, in that while a mask used in multipass printing of images has complementary patterns of 1&#39;s and 0&#39;s in the various zones of the mask corresponding to successive passes, the special mask for the media feed calibration target has a substantially identical pattern of  1 &#39;s and  0 &#39;s in each zone of the mask. Therefore, if the actual media feed distance were the nominal media feed distance, the special mask would cause multiple layers of dots to land on top of each other. 
         [0039]      FIG. 5  shows an example of a mask  400  for a media feed calibration target for 4 pass printing of the target. Printing the entire media feed calibration target requires more than 4 swaths of printing. What is meant by 4 pass printing in this case is that for the nominal media feed distance, corresponding marking elements from each quarter of the printhead deposit four layers of dots (corresponding to the 1&#39;s in the mask) in particular regions of the paper. 
         [0040]    Exemplary mask  400  is used to control the firing of dots from marking element array  420 , where in this example, the marking element array  420  has 40 inkjet nozzles of a single color, such as black or cyan, disposed along nozzle array direction  254 , which is substantially parallel to the media feed direction  304 . Mask  400  includes 40 rows of entries along the media feed direction  304 , corresponding to the 40 inkjet nozzles. The 40 rows are separated into 4 zones of 10 rows each for four pass printing of a media feed calibration target. The patterns of 1&#39;s and 0&#39;s are substantially identical in each of the four zones. In the example of mask  400 , the 1&#39;s are arranged as two dimensional clusters, in particular as 2×2 clusters  406 . The clusters are separated from one another along the media feed direction  304  by isolation regions  407 , which are 2×8 groups of 0&#39;s in this example. 
         [0041]    Each row of mask  400  has 10 entries along the carriage scan direction  305 . When a corresponding nozzle is near a pixel position on the recording medium, the mask directs the printhead either to make a dot at that pixel location corresponding to mask values of 1, or not to make a dot at that pixel location corresponding to mask values of 0. Even though the rows in mask  400  only have 10 entries, the mask is typically tiled across a larger area so that the printed target is significantly larger than 40 pixels by 10 pixels. 
         [0042]    For four pass printing (i.e. m=4) of the media feed calibration target, nozzles  1 - 10  are first used to print dots in the locations corresponding to the first zone  401  of the mask during the first pass as the carriage moves along carriage scan direction  305 . In particular, if the carriage  200  is moving left to right, mask  400  would direct nozzles  1  and  2  to fire at the first and second pixel locations along the carriage scan direction  305 , nozzles  7  and  8  to fire at the third and fourth pixel locations along the carriage scan direction  305 , nozzles  3  and  4  to fire at the fifth and sixth pixel locations along the carriage scan direction  305 , etc. Then the media is advanced by a distance that is approximately equal to L/m, i.e. by about 10 pixel spacings, plus an offset amount to be described below. During the second pass of printing, nozzles  1 - 10  print the pattern corresponding to the first zone  401  of the mask on a previously unmarked region of paper, while nozzles  11 - 20  print the pattern corresponding to the second zone  402  of the mask  400  in the same region that nozzles  1 - 10  just printed. As a result, nozzles  11  and  12  print a cluster of  4  dots substantially on top of the cluster of dots just printed by nozzles  1  and  2 , etc. Then the media is advanced by a distance that is approximately equal to L/m, i.e. by about 10 pixel spacings, incremented again by the offset amount. During the third pass of printing, nozzles  1 - 10  print the pattern corresponding to the first zone  401  of the mask on a previously unmarked region of paper, while nozzles  11 - 20  print the pattern corresponding to the second zone  402  of the mask  400  in the same region that nozzles  1 - 10  just printed, and nozzles  21 - 30  print the pattern corresponding to the third zone  403  of the mask  400  in the same region that nozzles  11 - 20  just printed. Then the media is advanced by a distance that is approximately equal to L/m, i.e. by about 10 pixel spacings, incremented yet again by the offset amount. During the fourth pass of printing, nozzles  1 - 10  print the pattern corresponding to the first zone  401  of the mask on a previously unmarked region of paper, while nozzles  11 - 20  print the pattern corresponding to the second zone  402  of the mask  400  in the same region that nozzles  1 - 10  just printed, nozzles  21 - 30  print the pattern corresponding to the third zone  403  of the mask  400  in the same region that nozzles  11 - 20  just printed, and nozzles  31 - 40  print the pattern corresponding to the fourth zone  404  of the mask  400  in the same region that nozzles  21 - 30  just printed. 
         [0043]    As the media feed calibration target is printed, the amount of media feed is incrementally swept to well outside the range of media feed errors normally anticipated for the printing system and/or media type. In one embodiment, the media feed distance is swept from L/m−4 pixels to L/m+4 pixels at 0.25 pixel increments per feed event to provide a media feed calibration target with a very distinctive reflectance signal that can be scanned to determine the correct media feed calibration setting automatically. In other words, the media advance amounts are increased or decreased in successive passes such that the minimum media advance distance is less than Lim and the maximum media advance distance is greater than L/m. In this embodiment the media feed distance of L/m is substantially at the center of the media feed calibration target. The media feed increments to greater than L/m and to less than L/m are disposed symmetrically about the center of the target. This produces a target such that if the actual media feed is equal to the nominal media feed, there will be near perfect overlap of the dots printed by successive passes at the center of the media feed calibration target. The distance between the actual point of near perfect overlap (as measured by a maximum in optical reflectance of the target) and the physical center of the target thus provides a measure of the error in the actual media feed. The nominal media feed distance L/m does not need to be located at the center, and the target does not need to be symmetric in sweep increments about the center, but that is perhaps the simplest version of the target design. Also, the calibration sweep range that would work is not limited to ±4 pixels or feed increments of 0.25 pixel, and systems using other embodiments having wider or narrower sweep ranges could be made to work with appropriate print masking and target dimensions. 
         [0044]    The mask dot pattern for exemplary mask  400  shows clusters of 4 dots, i.e. 2×2 clusters of mask value 1 that are isolated from other clusters by regions of mask value 0. It has been found that such clusters of dots reduce the signal loss that occurs from dot placement errors due to misdirected jets, carriage velocity errors, printhead to media spacing variation and nozzle array tilt. However, the method will work with single pixel dots or with more than 4 dots arranged as 2×2 clusters. As described below, clusters which are further elongated along the media feed direction (e.g. a 2×3 cluster) will extend the portion of the media feed calibration target in which overlap occurs between printing of dot clusters in successive passes, in spite of fairly large overfeeding or underfeeding of the media. 
         [0045]      FIG. 6  shows an embodiment of a media feed calibration target  430  (enlarged slightly in the figure) that has been printed using a mask similar to mask  400  (but with 640 rows rather than 40 rows, and optionally a different dot cluster pattern than shown in mask  400 ) using 4 pass printing by a inkjet nozzle array of 640 nozzles having a nozzle spacing of 1/1200 inch. The nominal media feed advance distance for a 640 nozzle printhead for 4 pass printing is 640/4=160 nozzle spacings, i.e. 160/1200 inch ˜0.133″. However during the printing of the pattern the media feed distance is incremented from an intended 156 pixel spacings (the nominal media feed advance minus 4 pixel spacings) at the top of media feed calibration target  430 , to an intended 156.25 pixel spacings, to 156.5, 156.75, 157, . . . 159.75, 160, 160.25, . . . 163.5, 163.75, and finally to an intended 164 pixel spacings at the bottom of media feed calibration target  430 . In other words, toward the top of media feed calibration target  430 , the media is being underfed, which results in some overlap between adjacent swaths, and toward the bottom of media feed calibration target  430  the media is being overfed, which results in some gaps between adjacent swaths. The gaps are particularly noticeable as the show up as white streaks  432  on a relatively dark background. The white streaks  432  indicate the boundaries between two adjacent swaths in this region of the target. 
         [0046]    In the above description, the different media feed distances were designated as an “intended” number of pixel spacings. In other words, knowing the approximate radius R o  of feed roller  312 , the feed roller  312  is rotated by a DC servo motor, for example, through an angle θ (as monitored by rotary encoder mounted on feed roller shaft  319 ) such that R o θ equals the intended media feed amount such as 156 pixel spacings, etc. However, due to manufacturing variation or wear of the feed roller  312 , the actual radius R of feed roller  312  may not be precisely equal to R o . In addition there can be other sources of feed error such as media slippage. An intent of the present invention is to relate the actual distance that media is fed for a variety of different intended media feed amounts, and then indicate with high precision the correct amount that media should be fed so that underfeeding and overfeeding can be avoided when printing images. 
         [0047]    In boxes  440   FIG. 6  also schematically shows using magnified views a, b, c, . . . i, the approximate appearance of the multi-pass printed dot clusters within the swaths as a function of media feed error. These magnified views are only intended to show schematically the effect of more white paper showing when the dot clusters overlap each other toward the center of media feed calibration target  430 , so that the target appears light gray near its center and dark gray toward the ends. Actual configurations of dot clusters as a function of media feed error will be described below. 
         [0048]    Also shown schematically in  FIG. 6  is the field of view  212  of the photosensor in carriage sensor  210 . After printing media feed calibration target  430 , carriage  200  is moved along carriage scan direction  305  until field of view  212  of the carriage sensor  210  is aligned with target  430 , for example in location  212   a.  Then the DC servo motor turns feed roller  312  at a constant rate so that the paper is moved at a substantially constant speed along media feed direction  305 . As the media feed calibration target  430  is moved relative to the field of view  212 , the photosensor signal is monitored as a function of nominal position within the target  430 , where the nominal position is provided by the rotary encoder that is mounted on feed roller shaft  319 . The photosensor signal is larger when more white paper is within field of view  212 , and the photosensor signal is smaller when the paper within field of view  212  is covered to a greater extend by printed dots. Thus the photosensor signal will be larger near central position  434  than near end regions  436  or  438 . Optionally the photosensor signal is amplified and converted to digital data by an analog to digital converter and stored in printer controller  14  as a function of the nominal position provided by the rotary encoder. The photosensor signal will be at its highest level for unmarked paper, such as when the field of view is at position  212   a.  In the example shown in  FIG. 6 , field of view  212  is about 3 mm in diameter (0.12″) and can fit entirely within a swath, as shown by field of view position  212   b  relative to the distance between white streaks  432 . 
         [0049]      FIGS. 7-10  show magnified views of configurations of multi-pass printed dot clusters that will be formed in various regions of media feed calibration target  430 , assuming four pass printing of 2×2 dot clusters where the media feed increment is one quarter pixel.  FIG. 7A  shows the vertical position of the individual 2×2 dot clusters printed during each of the four passes in a region of target  430  near position  434  where the media feed error is close to zero. The dot clusters in  FIG. 7A  are displaced from each other horizontally so that they can be seen individually. The dot cluster composite patterns in  FIG. 7B  represent the appearance of the overlying dot clusters after each pass. The dotted lines represent spacings of a quarter pixel spacing. Dot cluster  441  is printed, for example by nozzles  1  and  2  in two adjacent positions along the carriage scan direction  305 . Each dot in the dot cluster has a diameter of about 1.5 pixel spacings, so that diagonally adjacent dots overlap. Dot cluster  446  (the composite pattern after 1 pass) is the same as dot cluster  441 . Dot cluster  442  is printed by nozzles  161  and  162  after a media feed of 159.75 pixel spacings (i.e. a media feed error of minus one quarter pixel spacing) so it is vertically displaced from dot cluster  441  by a quarter pixel. Dot cluster composite  447  shows dot cluster  442  overlying dot cluster  441 . Dot cluster composite  447  is idealized from the standpoint that there is no jet misdirectionality between dot clusters  441  and  442 , and there is no spreading of the ink as the dot clusters overlap. Because the media feed is incremented by one quarter pixel for each swath of media feed calibration target  430 , the media feed amount before dot cluster  443  is 160 pixel spacings, i.e. the nominally correct media feed for 4 pass printing of a 640 jet nozzle array. Thus, dot cluster  443  (printed by nozzles  321  and  322 ) is at the same vertical position as dot cluster  442 , and dot cluster composite  448  looks just like dot cluster composite  447  because dot cluster  443  lands precisely on dot cluster  442 . Prior to the printing of dot cluster  444  by nozzles  481  and  482 , the media is advanced 160.25 pixel spacings (i.e. a media feed error of plus one quarter pixel spacing). Dot cluster composite  449  looks just like dot cluster composites  447  and  448  because dot cluster  444  lands precisely on dot cluster  441 . Dot cluster composite is the actual configuration of dot cluster composites in box e of  FIG. 6 , assuming no jet misdirectionality and actual media feeds approximately equal to the nominal media feed of 160 pixel spacings. 
         [0050]      FIGS. 8A and 8B  are similar to  FIGS. 7A and 7B , but for media feed errors near +1 pixel spacing. Dot cluster  451  is printed by nozzles  1  and  2  and is the same as dot cluster composite  456  after one pass. Dot cluster  452  is offset vertically from dot cluster  451  by 0.75 pixel spacing, so that dot cluster composite  457  covers significantly more paper than dot cluster composite  456 . Dot cluster  453  is offset vertically from dot cluster  452  by one pixel spacing and from dot cluster  451  by 1.75 pixel spacings, so that dot cluster composite  458  is even larger. Finally dot cluster  454  is offset vertically from dot cluster  452  by 1.25 pixel spacings, and from dot cluster  451  by 3.0 pixel spacings. The final appearance of dot cluster composite  459  after 4 passes is what the dot cluster composites would nominally look like in boxes d and fin  FIG. 6 . 
         [0051]    In  FIGS. 9 and 10 , only the composite dot clusters are shown after each of the four passes. Dot cluster composite  462  consists of a dot cluster like  461  plus another dot cluster that is vertically offset from  461  by 1.75 pixel spacings. Dot cluster composite  463  consists of dot cluster composite  462  plus another dot cluster that is vertically offset from  461  by 1.75+2.0=3.75 pixel spacings. Dot cluster composite  464  consists of dot cluster composite  463  plus another dot cluster that is vertically offset from  461  by 6.0 pixel spacings. The final appearance of dot cluster composite  464  is what the dot cluster composites would nominally look like in boxes c and g in  FIG. 6 . 
         [0052]    In  FIG. 10 , dot cluster composite  472  consists of a dot cluster like  471  plus another dot cluster that is vertically offset from  471  by 2.75 pixel spacings. Note that the two dot clusters making up dot cluster composite  472  are completely separate from each other. They will cover no additional paper by spreading even further apart. (By contrast, a 2×3 dot cluster geometry would still provide overlap in the dot cluster composite at this amount of vertical offset between clusters.) Dot cluster composite  473  consists of dot cluster composite  472  plus another dot cluster that is vertically offset from  471  by 2.75+3.0=5.75 pixel spacings. Dot cluster composite  474  consists of dot cluster composite  473  plus another dot cluster that is vertically offset from  471  by 9.0 pixel spacings. The final appearance of dot cluster composite  474  is what the dot cluster composites would nominally look like in boxes b and h in  FIG. 6 . 
         [0053]    As described above relative to  FIG. 6 , after the media feed calibration target  430  is printed, the carriage  200  is moved into position so that carriage sensor  210  is aligned with target  430 . Photosensor data is stored as a function of nominal position provided by the rotary encoder.  FIG. 11A  shows a plot  530  of photosensor data versus position corresponding to target  430 . Target  430  has been rotated clockwise 90 degrees in  FIG. 11B  relative to  FIG. 6  and lined up approximately with plot  530  for a clearer understanding of how various parts of the plot relate to various parts of the target  530 . 
         [0054]    Regions  431  outside of the media feed calibration target  430  consist of white paper, corresponding to the highest values  531  of the photosensor signal (approximately a value of 620 on the vertical axis in the plot  530  shown in the example of  FIG. 11A ). If the target  430  is scanned from the end  439  toward the end  437 , then as more and more of the target  430  enters the field of view  212  of the photosensor, the photosensor signal decreases. At location  539  of plot  530 , the edge  439  is in the center of the field of view  212  of the photosensor, and the signal level has dropped to about midway between its maximum level and its minimum level. Similarly at location  537  of plot  530 , the edge  437  is in the center of the field of view of the photosensor, and the signal level rises again to about midway between its maximum level and its minimum level. 
         [0055]    The region of the media feed calibration target near left edge  439  is the region where overfeeding has occurred (i.e. feeding greater than 160 pixel spacings), which results in white streaks  432 . As a white streak  432  enters the field of view  212 , the photosensor signal increases correspondingly until the white streak exits the field of view. The white streaks  432  entering and exiting the field of view  212  of the photosensor is what gives rise to the jagged bumps  532  in the data curve  530 . 
         [0056]    Point  433  of the media feed calibration target  430  is midway between end  439  and end  437 . This midway point corresponds to point  533  on data plot  530 . Point  533  is found by finding the midway position (given by the position on the horizontal axis) between points  539  and  537 . 
         [0057]    The peak  535  in photosensor signal for the reflectance data occurs when the field of view  212  is centered on the lightest region  435  of the target  430 . This is the portion where there is maximum overlap of the dot clusters. If the target  430  is designed to be symmetrical, and if the media feed is nominally accurate, then the peak  535  will occur approximately at midway point  533 , rather than being displaced from midway point  533  as it is in  FIG. 11A . 
         [0058]    The plot  530  is smoother in appearance between the peak  535  and end  537  than it is between the peak  535  and end  539 . This is because the reflectance change at the swath boundaries for this target is not as apparent for underfeeding as it is for the white streaks  432  on the dark gray background. 
         [0059]    The separation distance between the peak point  535  and the midway point  533  can be used to calculate the media feed error. The horizontal scale in  FIG. 11A  is in units of 1/600 inch. In the example of  FIG. 11A , the peak  535  occurs at a position of  2156  and the midway point  533  occurs at a position  2000 . Thus the distance between the peak  535  and the midway point  533  is 156/600 inch, which is the same as 312/1200 inch. Since the media feed was incremented by 0.25 pixel for every swath of about 160/1200 inch, and since the peak  535  is on the underfed side of the midway point  533 , the desired media feed as measured relative to peak  535  is given by: 
         [0060]    desired media feed= 160/1200″−(312/160) (0.25/1200″) ˜ 160/1200″− 0.4875/1200″. 
         [0061]    In actuality, a small error is made when the expression 312/160 is used. A slightly better calculation would be to use the average number of pixel spacings of media advance on the underfed side of the target  430  between the midway point  430  and the peak position  435  rather than the nominal distance of 160 pixel spacings in the ratio 312/160. In this case that is the average of 160 and 159.75, i.e. 159.875 pixel spacings. However, the error is less than one part in 1000 (i.e. the result of the better calculated value is 160/1200″− 0.4879/1200″, which is almost the same as the result provided above for the desired media feed). 
         [0062]    In the printing system in which this method was tested, the resolution of the rotary encoder corresponded to one twelfth of a pixel spacing, i.e. to 0.083/1200″, so errors that are less than half that value (i.e. 0.041/1200″) are negligible in this example. Even if the peak value  535  were as far to the right as an underfeed of about 3 pixel spacings, corresponding to coordinate  3000  on the horizontal axis of the plot in  FIG. 11A  (which is about as far out as the peak could be and still be recognized as a peak) the use of the nominal value 160 pixel spacings in the calculation still provides a negligible error to the nearest 1/12 pixel spacing, which is the resolution of the rotary encoder in our example. Even though the media feed has not previously been calibrated, the nominal value of 160 pixel spacings can be used in the calculation, without making a substantial error in the determination of the desired correction for the rotary encoder. 
         [0063]    Thus, in the above example, in order to provide the correct media feed, the intended value at the rotary encoder should not correspond to 160 pixel spacings, but 159.49 pixel spacings, which is rounded off to 159.5 to the nearest 1/12 pixel spacing. This is a correction of half a pixel spacing or 6 counts on the rotary encoder since the rotary encoder resolution is one twelfth of a pixel spacing in this example. Thus, the proper rotary encoder rotation can be adjusted by minus 6 encoder counts to provide the half pixel correction value in order to provide the proper amount of media feed. In other words, rather than the rotary encoder count for media feed being 160×12=1920, the corrected rotary encoder count for a proper media feed in this example would be 1914. 
         [0064]    In the example described above, the maximum amount of overlap of the dot clusters in the media feed calibration target  430  was identified as the peak  535  in the reflectance value. An alternative is to use the centroid  546  of the peak region rather than the peak  535  itself. Using the centroid can have an advantage of averaging to reduce the impact of dot misplacement due to mechanical vibration, for example. In the example shown in  FIG. 11A , centroid  546  was calculated as follows. First, select the region near the peak  535 . This can be done by first truncating the data on the left at position  543  and at the right at position  544 . Truncation positions  543  and  544  are selected to be a known distance (such as  200  units on the horizontal axis) inside the positions of the ends  537  and  539 . Then a threshold value  542  is identified. This threshold value  542  can be selected on the basis of the maximum and minimum values between truncation positions  543  and  544 . In this example, threshold value  542  was selected as: 
         [0065]    Threshold value=minimum+0.25 (maximum−minimum)˜200. 
         [0066]    The region for centroid calculation is then chosen as the values between truncation positions  543  and  544  that exceed the threshold  542 . The threshold value ( 200 ) is then subtracted from the data in this region of curve  530  to produce peak region curve  540 . The centroid position  546  is the position such that the area under the curve  540  to the left of centroid  546  is equal to the area under the curve  540  to the right of centroid  546 . 
         [0067]    In the example of  FIG. 11A , the centroid  546  is located at horizontal position  2166  compared to the horizontal position  2156  of peak  535  that was used in the calculation above. Thus the distance between the centroid  546  and the midway point  533  is 166/600 inch, which is the same as 332/1200 inch. Since the media feed was incremented by 0.25 pixel for every swath of about 160/1200 inch, and since the centroid is on the underfed side of the midway point  533 , the desired media feed as measured relative to centroid is given by: 
         [0068]    Desired media feed= 160/1200″−(332/160) (0.25/1200″) ˜ 160/1200″− 0.5188/1200″. 
         [0069]    To the nearest one twelfth of a pixel spacing (the resolution of the rotary encoder), the correction as calculated by the reflectance centroid method is thus minus one half a pixel spacing (i.e. minus 6 encoder counts), just as it was for the calculation by the reflectance peak method in this example. 
         [0070]    Referring back to exemplary mask  400  in  FIG. 5  that controls the printing of the dots in the media feed calibration target, note that the mask entries in zone  401  are identical to the mask entries in zones  402 ,  403  and  404 . Such a mask configuration is advantageous, in that for the region of greatest overlap of the dots printed by the multipass printing, the greatest amount of white paper will be exposed, so that the peak in optical reflectance is pronounced at the correct media feed. However, the method will still work even if the mask entries are not identical from zone to zone, but are predominantly the same from zone to zone. In the example of mask  400 , each zone of the mask has 100 entries, consisting of twenty 1&#39;s and eighty 0&#39;s. For all 100 entries, if there is a 0 in a particular location for zone  401 , there will be a 0 in the corresponding location for zone  402 , etc. and similarly for locations of 1&#39;s. While it is not required that the mask entries in one zone be exactly the same as the entries in a second zone, the method works better if more than 75% of the mask entries are the same in the second zone as they are in the first zone. 
         [0071]    As described above, each zone of mask  400  includes twenty 1&#39;s and eighty 0&#39;s. In other words, the number of pixels designated by each zone of the mask as to be marked is 20% of the total number of pixels in the zone. A mask that is relatively sparsely populated by 1&#39;s is advantageous such that as the media feed is progressively incrementally overfed or underfed, marked regions do not start overlapping other marked regions. However, depending upon factors such as how far the underfeeding or overfeeding is incremented, the method will still work for greater than 20% of the pixels in a zone being marked. The 2×3 cluster described above would correspond to 30% of the pixels in a zone being marked. The method works better if the number of pixels designated by each zone of the mask as marked is less than 50% of the pixels in the zone. 
         [0072]    Again referring to the example  FIGS. 5-10 , the 2×2 marking clusters printed in one pass have a dimension S along the media feed direction such that S is 2 spacings. (The printed dot diameter is generally a bit larger than 1.414 times the vertical or horizontal pixel spacing so that the dots will overlap along the diagonal, but the corresponding dimension S in the mask is 2 pixel spacings.) Adjacent the 2×2 clusters of  1 &#39;s in mask  400  along the media feed direction  305  (or marking element array direction  254 ) is an isolation region of 2×8 0&#39;s. In other words, the isolation region has a dimension along the media feed direction that is 8 spacings, i.e. 4 times S. Although it is not required that the isolation region be as large as 4 times S, the method works better if the dimension of the isolation region along the media feed direction is greater than S. In addition, in mask  400 , the isolation region is made up only of 0&#39;s. Although it is not required that there be no 1&#39;s in the isolation region, the method works better if fewer than 25% of the pixels in the isolation region are designated for marking. Also, it is not required that the marked pixels be in dot clusters, nor that each cluster have all of the adjacent pixels in the media advance direction being designated for marking, but the dot clusters in  FIGS. 5-10  have that property. 
         [0073]    The amount of media feed increment (i.e. the offset amount between successive passes) in the examples described above was 0.25 pixel spacing, i.e. 0.25d, where d is the distance between neighboring marking elements. The choice of 0.25d as the offset amount (i.e. the amount of increase or decrease of the media advance distance in successive passes) works well, but other choices are possible. The method works better if the offset amount is less than 2d. 
         [0074]    Note also that the nominal media advance corresponding to the rotary encoder resolution in the examples described above is 1/12 d, i.e. 0.083d, which is less than the 0.25d amount of increase or decrease of the media advance distance in successive passes. It has been demonstrated that the inventive method of calibrating a media advance in a printer is capable of identifying a correction value that is smaller than the amount of increase or decrease of the media advance distance in successive passes. In some embodiments, it is advantageous to use an offset amount (e.g. 0.25d) in the media advance sweep that is greater than the nominal media advance distance (e.g. 0.083d) corresponding to the rotary encoder resolution. This is because fewer swaths are required to print the pattern, making the pattern more compact. In addition such a media feed calibration target can be less susceptible to noise from pass to pass, due for example to dot misplacement resulting from mechanical vibration in the printing system. 
         [0075]    Although the need for precision and convenience have driven the development of the embodiments described above in which calibration of media feed is done automatically, the method could also be adapted for manual calibration embodiments. 
         [0076]    In the embodiments described above, the intent of the calibration is to quantify and correct for the component of media feed error that is independent of angular position of the feed roller. In some printing system applications, the feed roller is sufficiently round and the mechanical mounting is sufficiently precise that no further correction is needed. However, in some printing systems, feed rollers may be not precisely circular in cross-section, or they may be eccentrically mounted, or there may be wobble or noise or other deviation from a constant media feed amount for a constant angular rotation of the feed roller. 
         [0077]    The methods described above can be used to quantify and correct for periodic variation or noise in the media feed. In addition to the angular scale of the rotary encoder, a marker (which can be part of the rotary encoder, for example) is provided to indicate a particular angular position of the roller. The first step then is to calibrate the media feed so that the angular-position-independent component of the error is quantified, for example, by measuring optical reflectance due to the overlap of dot clusters for a range of media feed amounts as described in embodiments above. Then use the same mask (e.g. exemplary mask  400 ), but in this case set the media feed amount between swaths to a corrected amount based on the measurement of the angular-position-independent component of the error (i.e. not sweeping the media feed incrementally, but keeping it constant at a corrected value). After printing the target, align the carriage  200  with the target so that it can be scanned with the carriage sensor  210  as the media is advanced past it. Measure the optical reflectance versus position, where position is correlated relative to the marker that indicates a particular angular position of the roller. A printing system with significant run-out or eccentricity of the roller will show a periodic variation (e.g. sinusoidal) in reflectance. The optical reflectance data as a function of position then can be used to characterize the proper number of encoder counts to rotate the roller as a function of roller position. Suppose the nominal media feed distance corresponds to 160/1200″ or 1920 encoder counts, but the corrected media feed distance corresponds to 1914 encoder counts (as in the above example). In measuring the angular dependence it might be found that at 0 degrees relative to the roller marker, the proper media feed distance corresponds to 1916 encoder counts, while at 90 degrees it is 1914, at 180 degrees it is 1912, and at 270 degrees it is 1914 (with the overall average being 1914 for a full rotation of the roller). These angular variations can be stored in a table in printer controller  14  and used to correct for the media feed advance as a function of roller angular position. 
         [0078]    Such calibration can be done at the factory or by the user. If there is excessive noise or variation found at the factory (detected as excessive variation in the reflectance of the target), that assembly can be reworked or rejected before leaving the factory. 
         [0079]    The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention. 
       PARTS LIST 
       [0080]      10  Inkjet printer system 
         [0081]      12  Image data source 
         [0082]      14  Controller 
         [0083]      15  Image processing unit 
         [0084]      16  Electrical pulse source 
         [0085]      18  First fluid source 
         [0086]      19  Second fluid source 
         [0087]      20  Recording medium 
         [0088]      100  Ink jet printhead 
         [0089]      110  Ink jet printhead die 
         [0090]      111  Substrate 
         [0091]      120  First nozzle array 
         [0092]      121  Nozzle in first nozzle array 
         [0093]      122  Ink delivery pathway for first nozzle array 
         [0094]      130  Second nozzle array 
         [0095]      131  Nozzle in second nozzle array 
         [0096]      132  Ink delivery pathway for second nozzle array 
         [0097]      181  Droplet ejected from first nozzle array 
         [0098]      182  Droplet ejected from second nozzle array 
         [0099]      200  Carriage 
         [0100]      210  Carriage sensor 
         [0101]      212  Field of view of carriage sensor 
         [0102]      250  Printhead chassis 
         [0103]      251  Printhead die 
         [0104]      253  Nozzle array 
         [0105]      254  Nozzle array direction 
         [0106]      256  Encapsulant 
         [0107]      257  Flex circuit 
         [0108]      258  Connector board 
         [0109]      262  Multichamber ink supply 
         [0110]      264  Single chamber ink supply 
         [0111]      300  Printer chassis 
         [0112]      302  Paper load entry 
         [0113]      303  Print region 
         [0114]      304  Paper exit 
         [0115]      305  Carriage scan direction 
         [0116]      306  Right side of printer chassis 
         [0117]      307  Left side of printer chassis 
         [0118]      308  Front portion of printer chassis 
         [0119]      309  Rear portion of printer chassis 
         [0120]      310  Hole for paper advance motor drive gear 
         [0121]      311  Feed roller gear 
         [0122]      312  Feed roller 
         [0123]      313  Forward rotation of feed roller 
         [0124]      319  Feed roller shaft 
         [0125]      320  Pickup roller 
         [0126]      322  Turn roller 
         [0127]      323  Idler roller 
         [0128]      324  Discharge roller 
         [0129]      325  Star wheel 
         [0130]      330  Maintenance station 
         [0131]      370  Stack of media 
         [0132]      371  Top sheet 
         [0133]      372  Main paper tray 
         [0134]      373  Photo paper stack 
         [0135]      374  Photo paper tray 
         [0136]      376  Recess in paper tray 
         [0137]      380  Carriage motor 
         [0138]      382  Carriage rail 
         [0139]      383  Encoder fence 
         [0140]      384  Belt 
         [0141]      390  Printer electronics board 
         [0142]      392  Cable connectors 
         [0143]      400  Mask for media feed calibration target 
         [0144]      401  First zone of mask 
         [0145]      402  Second zone of mask 
         [0146]      403  Third zone of mask 
         [0147]      404  Fourth zone of mask 
         [0148]      406  Cluster 
         [0149]      407  Isolation region 
         [0150]      420  Marking element array 
         [0151]      430  Media feed calibration target 
         [0152]      431  Region outside target 
         [0153]      432  White streaks due to overfeeding 
         [0154]      433  Point midway between ends of target 
         [0155]      434  Central position of target 
         [0156]      435  Lightest region of target 
         [0157]      436  End region of target 
         [0158]      437  End of target 
         [0159]      438  End region of target 
         [0160]      439  End of target 
         [0161]      440   a - i  Magnified view boxes 
         [0162]      441 - 444  Dot clusters 
         [0163]      446 - 449  Dot cluster composites 
         [0164]      451 - 454  Dot clusters 
         [0165]      456 - 459  Dot cluster composites 
         [0166]      461 - 464  Dot cluster composites 
         [0167]      471 - 474  Dot cluster composites 
         [0168]      530  Plot of photosensor data 
         [0169]      531  Photosensor data for region  431   
         [0170]      531  Bump in data corresponding to white streak  432   
         [0171]      533  Photosensor data corresponding to target center  433   
         [0172]      535  Peak of reflectance data 
         [0173]      537  Photosensor data for end  437  in center of field of view 
         [0174]      539  Photosensor data for end  439  in center of field of view 
         [0175]      540  Peak region curve 
         [0176]      542  Threshold value 
         [0177]      543 ,  544  Truncation positions 
         [0178]      546  Centroid of peak region