Customizing printmasks for printhead nozzle aberrations

An inkjet printer and printing method for improving print quality. The printer minimizes the visually perceptible effect of dot placement errors, dot size errors, and dot shape errors on a printed medium due to depositing drops of ink from lower print quality printhead nozzles. The printer provides a sensor which can test the ink drop output of the printhead nozzles to determine, for each particular printhead installed in the printer, which nozzles are of higher print quality and which are of lower print quality. A printmask is then defined based on the results of the testing for use in printing from that printhead. The printmask has a mask pattern which enables the deposition of more ink from higher quality nozzles and less ink from lower quality nozzles. Such a printer improves print quality without reducing throughput.

FIELD OFF THE INVENTION
 The present invention relates generally to modes of printing with
 swath-type printing systems. It relates more particularly to printmodes
 for improving the print quality of output produced by individual
 printheads used in an inkjet printer.
 BACKGROUND OF THE INVENTION
 Inkjet printers, and thermal inkjet printers in particular, have come into
 widespread use in businesses and homes because of their low cost, high
 print quality, and color printing capability. The operation of such
 printers is relatively straightforward. In this regard, drops of a colored
 ink are emitted onto the print media such as paper or transparency film
 during a printing operation, in response to commands electronically
 transmitted to the printhead. These drops of ink combine on the print
 media to form the text and images perceived by the human eye. Inkjet
 printers may use a number of different ink colors. One or more printheads
 may be contained in a print cartridge, which may either contain the supply
 of ink for each printhead or be connected to an ink supply located
 off-cartridge. An inkjet printer frequently can accommodate two to four
 print cartridges. The cartridges typically are mounted side-by-side in a
 carriage which scans the cartridges back and forth within the printer in a
 forward and a rearward direction above the media during printing such that
 the cartridges move sequentially over given locations, called pixels,
 arranged in a row and column format on the media which is to be printed.
 Each print cartridge typically has an arrangement of printhead nozzles
 through which the ink is controllably ejected onto the print media, and
 thus a certain width of the media corresponding to the layout of the
 nozzles on the print cartridge, can be printed during each scan, forming a
 printed swath. The printer also has a print medium advance mechanism which
 moves the media relative to the printheads in a direction generally
 perpendicular to the movement of the carriage so that, by combining scans
 of the print cartridges back and forth across the media with the advance
 of the media relative to the printheads, ink can be deposited on the
 entire printable area of the media.
 The quality of the printed output is a very important feature to purchasers
 of inkjet printers, and therefore manufacturers of inkjet printers pay a
 great deal of attention to providing a high level of print quality in
 their printers. Aberrations in the printhead nozzles can undesirably
 reduce print quality; such aberrations include, for example, not ejecting
 ink at all, ejecting an incorrect volume of ink in a drop, producing
 irregularly shaped drops with artifacts such as tails, or producing a
 spray of extraneous droplets in addition to the desired drop. Another
 common type of nozzle aberration is directionality error, also known as
 dot placement error, in which the drops of ink are not precisely printed
 in the intended locations on the print media. While sometimes printhead
 aberrations are due to the design of the printhead and thus are similar
 for all printheads of that particular type, other times the nozzle
 aberrations for a particular type of printhead differ from printhead to
 printhead. In addition, printhead aberrations can develop over time and
 with usage of the printhead; for instance, nozzles can become clogged or
 wear.
 Nozzle aberrations frequently result in banding, or streaks of unprinted
 areas, on the printed output. To minimize banding due to nozzle
 aberrations (and coincidentally to also reduce the effect of printing
 defects resulting from having too much ink on the print medium at one
 time, such as bleeding of one color area into another and warping or
 wrinkling of the print media), most printers do not print all the required
 drops of all ink colors in all pixel locations in the swath in one single
 scan, or "pass", of the printheads across the media. Rather, multiple
 scans are used to deposit the full amount of ink on the media, with the
 media being advanced after each pass by only a portion of the height of
 the printed swath. In this way, areas of the media can be printed in on
 more than one pass. In a printer which uses such a "multipass" printing
 mode, only a fraction of the total drops of ink needed to completely print
 each section of the image is laid down in each row of the printed medium
 by any single pass; areas left unprinted are filled in by one or more
 later passes. When printing of a page is complete, every area of the print
 medium has typically been printed on by the same multiple number of
 passes. Because each pass uses a different nozzle to print a particular
 row of the image, multipass printing can compensate for nozzle defects.
 However, the typical multipass printmode in which all nozzles are enabled
 to deposit substantially the same amount of ink on each row of pixels is
 often insufficient to improve print quality to an acceptable level,
 particularly when some nozzles have worse errors than others, as in the
 case of nozzle aberrations as described above.
 One approach to overcoming the shortcomings of multipass printing for
 compensating for nozzle aberrations is disclosed in commonly-assigned U.S.
 Pat. No. 5,124,720 filed Aug. 1, 1990 and issued to Schantz on Jun. 23,
 1992 and titled "Fault-Tolerant Dot-Matrix Printing", which is hereby
 incorporated by reference in its entirety. This approach improves print
 quality by compensating for malfunctioning nozzles on a
 printhead-by-printhead basis. This method tests the printhead to identify
 inoperative printing elements, and then alters the scan path of the
 printhead so that properly functioning printing elements print where the
 inoperative printing elements normally would have. However, this method
 reduces the throughput (the number of pages that can be printed in a given
 unit of time, such as pages per minute) because it decreases the distance
 the paper is advanced after each pass of the printhead and thus increases
 the number of passes required to fully print a page.
 Throughput is often just as important or more important to an inkjet
 printer purchaser as is print quality. Accordingly, there is still a need
 for an inkjet printer that minimizes print quality defects due to nozzle
 aberrations but without significantly reducing the throughput of the
 printer.
 SUMMARY OF THE INVENTION
 In a preferred embodiment, the present invention provides a multipass
 inkjet printer that improves the quality of the printed output by
 compensating for dot placement error, dot shape error, and dot size error
 without compromising printing throughput. An embodiment of the printing
 system according to the present invention includes a printhead mounted in
 a carriage which is attached to a frame for relative motion with respect
 to a print medium. The printhead has an arrangement of nozzles, each
 having a print quality, through which ink is ejected onto a pixel grid of
 multiple rows on the print medium, each nozzle capable of depositing the
 drops of the ink onto a corresponding one of the rows during individual
 ones of the multiple printing passes. The printer also contains a print
 controller which activates the nozzles to deposit the ink onto the medium
 during each printing pass, as governed by a printmask. The printer further
 has the capability to test the nozzle print quality of individual
 printheads installed in the printer, and the capability to define the
 printmask such that it enables more printing from higher quality nozzles
 and less printing from lower quality nozzles in at least some of the rows.
 In some embodiments, the capability to test the printhead nozzle print
 quality may be implemented by a test pattern printed on the medium which
 is optically scanned by a sensor to detect nozzle quality. In alternate
 embodiments, the printhead is tested using either a pass-through detector
 inserted into the path through which ink drops are deposited onto the
 media, or an impact detector on which deposited ink drops impinge during a
 test operation. These detectors can be optical, piezoelectric,
 electrostatic, or other technology detector. In some embodiments, the
 capability to define the printmask may be implemented by a nozzle quality
 memory preferentially mounted on the printhead which stores indicia of
 nozzle quality, a processor which defines a printmask which allocates the
 ink deposition between higher and lower quality nozzles so as to improve
 the print output quality, and a printmask memory which stores the defined
 printmask. In some embodiments, the printmask has a hybrid mask pattern
 which uses a "hi-fipe" mode for lower quality nozzles and a "multidrop"
 mode for higher quality nozzles, while in other embodiments the printmask
 has a mask pattern which allows higher quality nozzles to print more
 possible times on a row than lower quality nozzles.
 The present invention may also be implemented as a method of multipass
 printing. The method preferably includes providing a printhead for
 depositing ink onto a print medium, testing the printhead to identify
 lower print quality nozzles and higher print quality nozzles and
 allocating depositing of the ink between the lower quality nozzles and the
 higher quality nozzles based on the test results such that less than a
 given standard amount of ink from the lower print quality nozzles and more
 than a given standard amount of ink from the higher print quality nozzles
 is deposited in some rows. The testing can be performed during the
 printhead manufacturing process, during the printhead refilling process,
 after installation of the printhead in the inkjet printing system, and
 periodically during operation of the inkjet printing system. In some
 embodiments, the method also includes moving the printhead and the print
 medium relative to each other in a scan direction during each of the
 multiple passes, depositing the ink from certain nozzles onto pixel
 locations in certain rows as governed by the printmask while moving along
 the scan axis during each of the multiple passes, and moving the printhead
 and the print medium relative to each other in a medium advance direction
 in-between the multiple passes in order to position different nozzles over
 the certain rows. In some embodiments, allocating deposition of the ink
 includes defining a printmask which enables certain of the lower print
 quality nozzles identified by the testing to deposit the ink a relatively
 fewer total number of possible times during the multiple passes, and
 enables certain of the higher print quality nozzles to deposit the ink a
 relatively greater total number of possible times during the multiple
 passes. In other embodiments, allocating deposition of the ink includes
 defining a printmask which enables certain of the lower print quality
 nozzles identified by the testing to each deposit a small number of drops
 of the ink into specified pixel locations on at least two different rows
 during at least two corresponding passes, and enabling certain of the
 higher print quality nozzles to each deposit many drops of the ink rapidly
 into specified pixel locations on a given row during at least one of the
 multiple passes.
 Other aspects and advantages of the present invention will become apparent
 from the following detailed description, taken in conjunction with the
 accompanying drawings, illustrating by way of example the principles of
 the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 Referring now to the drawings, and more particularly to FIGS. 1, 2, and 3,
 there is illustrated a printer 10 constructed in accordance with the
 present invention which reduces visually objectionable print quality
 defects that occurs due to nozzle aberrations, and does so without
 reducing printer throughput. A preferred embodiment of the printer 10
 includes a frame indicated generally at 11 on which a carriage 20 is
 moveably mounted. The carriage 20 has stalls for holding at least one
 printhead 21 (FIG. 1 illustrates by way of example four printheads 21) and
 transporting them in a printing orientation adjacent the surface of a
 print medium 18 having a plurality of pixel locations, such as pixel
 location 19, organized in a rectangular array of rows and column. The
 carriage 20 is mounted in the frame 11 for relative motion with respect to
 the print medium 18 during a printing pass. Each printhead 21 has a
 plurality of nozzles 24 through which drops of ink are ejected onto the
 print medium 18 to form printed output, which may contain any combination
 of text, graphics, or photographs. As will be discussed hereinafter in
 further detail, the plurality of nozzles 24 is logically arranged as a
 linear array of nozzles substantially orthogonal to a scan axis 4, such
 that each nozzle is capable of depositing the drops of the ink onto a
 corresponding one of the rows of pixel locations during individual ones of
 the printing passes. The term "printing pass", as used herein, refers to
 those passes in which the printhead is enabled for printing as the nozzle
 arrangement 24 moves relative to the medium 18 in the scan axis direction
 4; in a bidirectional printer 10, each forward and rearward pass along the
 scan axis 4 can be a printing pass, while in a unidirectional printer
 printing passes can occur in only one of the directions of movement during
 each printing pass. Each printhead 21 contains a different color ink,
 typically the subtractive primary colors magenta, cyan, and yellow; other
 color shades are formed by depositing drops of these different colors in
 the same or nearby pixels (there is also usually a separate black ink
 printhead for producing a richer color black than is achieved by mixing
 the subtractive primary colors, and for producing some of the darker
 shades of other colors). The carriage 20 is moveable along the scan axis 4
 by a carriage advance mechanism, indicated generally at 12, mounted within
 the frame 11. The printer 10 also has a print medium advance mechanism,
 indicated generally at 17, mounted within the frame 11 which advances the
 print medium 18 along a medium advance axis 8 so as to change the row of
 pixel locations on which an individual nozzles prints. (The carriage
 advance mechanism 12 and the print medium advance mechanism 17 are well
 known to those skilled in the art, and will not be discussed further
 hereinafter.) A print controller 58 controls the carriage 20 and media 18
 movements and is electrically connected to the printhead so as to activate
 the nozzles 24 for ink drop deposition. By combining the relative movement
 of the carriage 20 along the scan axis 4 with the relative movement of the
 print medium 18 along the medium advance axis 8, each printhead 21 can
 deposit one or more drops of ink at each individual one of the pixel
 locations 19 in the rows on the print medium 18. A printmask 62 is used by
 the print controller 58 to govern the deposition of ink drops from each
 printhead 21 during each of the multiple passes. Typically a separate
 printmask 62 exists for each discrete intensity level (eg. light to dark)
 of each different color printhead. For each pixel position 19 in a row
 during an individual printing pass, the printmask 62 has a mask pattern
 which both (a) acts like a "gate" to enable the nozzle positioned adjacent
 the row to print, or disable that nozzle from printing, on that pixel
 location 19, and (b) defines the number of ink drops to be deposited from
 enabled nozzles. Whether or not the pixel location will actually be
 printed on by the corresponding enabled nozzle as it passes over depends
 on whether the image data 54 to be printed requires ink of that color in
 that pixel location.
 As will be discussed in greater detail subsequently, a printer 10 according
 to the present invention also has the capability to test each of the
 nozzles 24 of each printhead 21 to determine whether or not they are
 functioning properly, and consequently to assign indicia of print quality
 for each nozzle. Such a printer 10 also has the capability to construct
 for each individual printhead 21 a printmask 62 based on knowledge of the
 print quality of each nozzle which will improve the quality of the printed
 output without reducing throughput; this will also be subsequently
 discussed in further detail. The printmask 62 is constructed to have a
 printmask pattern such that less ink from lower print quality nozzles and
 more ink from higher print quality nozzles is enabled to be deposited in
 at least some rows of the pixel locations 19 on the print medium 18. The
 less ink and the more ink are relative to a given standard amount of ink.
 Typically this standard amount is a substantially equal amount of ink from
 each nozzle.
 Before discussing the nozzle testing and printmask construction in further
 detail, however, it is beneficial to consider with reference to FIGS. 4A
 through 4C several types of nozzle aberrations known to those skilled in
 the art and for which the present invention can compensate. FIG. 4A
 illustrates by way of example directionality error (also known as dot
 placement error). A nozzle 24 exhibiting directionality error does not
 deposit ink drops precisely in the intended location 41, but rather places
 them in an actual location 42 different from the intended location 41 by
 some amount of directionality error. This directionality or dot placement
 error may have a component in the direction of the scan axis 4 (known as
 scan axis directionality, or SAD, error), and a component in the direction
 of the media or paper advance axis 8 (known as paper axis directionality,
 or PAD, error). Embodiments of the present invention can improve the print
 quality produced from printheads which exhibit either SAD, PAD, or both
 SAD and PAD. FIG. 4B illustrates by way of example dot size error (dot
 volume error). A nozzle 24 exhibiting dot size error deposits an actual
 amount of ink 44 different from the intended amount of ink 43 (in the
 illustration, the actual amount of ink 44 is less than the intended amount
 43, as might occur using a weak or clogged nozzle). FIG. 4C illustrates by
 way of example dot shape error. A nozzle 24 exhibiting dot shape error
 deposits ink in an actual pattern 46 which is not substantially circular
 as intended 45. The actual pattern 46 can include non-circular shapes,
 tails, and spray.
 As is well known to those skilled in the art, printheads are typically
 formed on silicon substrates. One or more printheads, each for a different
 ink, may be formed on a single substrate. Considering now the plurality of
 nozzles 24 of a printhead 21 in greater detail with reference to FIG. 3, a
 preferred embodiment of a printhead 21 has two vertical columns 70a-b of
 nozzles 24 which, when the printhead 21 is installed in the printer 10,
 are perpendicular to the scan axis 4. The columnar vertical spacing 74
 between adjacent nozzles in a column is typically 1/300th inch in
 present-day printheads. However, by using two columns instead of one and
 logically treating the nozzles as a single column, the effective vertical
 spacing 72 between logical nozzles is reduced to 1/600th inch, thus
 achieving improved printing resolution in the direction of the media
 advance axis 8. As an illustration, the print controller 58 would print a
 vertical column of 1/600th inch pixel locations on the print medium 18 by
 depositing ink from the nozzles in column 70a, then moving the printhead
 21 in the scan axis direction 4 an amount equal to the inter-column
 distance 76 before depositing ink from the nozzles in column 70b.
 Returning now to the means for testing the printhead nozzles 24 to
 determine the print quality of each, the present invention contemplates
 the use of a wide variety of different detectors, also known as sensors,
 for measuring the quality of the ink drops deposited from the nozzles. One
 preferred embodiment includes an in-flight passthrough sensor 30 which
 detects and characterizes ink drops in flight as the drops pass through
 the sensor 30. The sensor 30 can be mounted in the frame 11 such that the
 carriage 20 positions the nozzles 24 of the printhead 21 in a test
 position, or alternatively can be mounted on the carriage 20 between the
 printhead 21 and the media 18 in the path of deposited ink drops such that
 the ink drops pass through the sensor 30 during normal printing operation
 (not shown). Nozzle quality is determined based on the detection and
 characterization of ink drops from the selected nozzle. The in-flight
 detector 30 may be implemented using a number of technologies known to
 those skilled in the art, including optical and electrostatic
 technologies. Optical in-flight detectors usable with the present
 invention are described in greater detail in U.S. Pat. No. 4,922,270,
 filed Jan. 31, 1989 and issued May 1, 1990 to Cobbs et al., and U.S. Pat.
 No. 5,434,430, filed Apr. 30, 1993 and issued Jul. 18, 1995 to Stewart,
 both of which are assigned to the assignee of the present invention and
 are hereby incorporated by reference in their entirety. Examples of
 electrostatic in-flight detectors usable with the present invention are
 described in U.S. Pat. No. 3,953,860 issued Apr. 27, 1976 to Fujimoto et
 al., titled "Charge Amplitude Detection for Ink Jet System Printer".
 Another preferred embodiment includes an impact sensor 31 which detects and
 characterizes ink drops on impact as the drops strike the sensor 31. The
 sensor 31 can be mounted in the frame 11 such that the carriage 20
 positions the nozzles 24 of the printhead 21 in a test position. Nozzle
 quality is determined based on the detection and characterization of ink
 drops from the selected nozzle. The impact detector 31 may be implemented
 using a number of technologies known to those skilled in the art,
 including piezoelectric and electrostatic technologies. Use of a
 piezoelectric membrane impact detector suitable for use with the present
 invention is described in greater detail in U.S. Pat. No. 5,124,720, filed
 Aug. 1, 1990 and issued Jun. 23, 1992 to Schantz, titled "Fault-Tolerant
 Dot-Matrix Printing", which is assigned to the assignee of the present
 invention and hereby incorporated by reference in its entirety. Examples
 of electrostatic impact detectors usable with the present invention are
 described in U.S. Pat. No. 4,323,905 issued Apr. 6, 1982 to Reitberger et
 al., titled "Ink Droplet Sensing Means".
 Yet another preferred embodiment for testing the printhead nozzles 24 uses
 an ink drop test pattern 33 printed on the print medium 18 from the
 nozzles 24. This embodiment includes a print sensor 32 mounted on the
 carriage 20 for relative motion with respect to the print medium 18. After
 the test pattern 33 is printed, the carriage 20 moves the sensor 32 over
 the print medium 18 in one or more sensing passes in order to scan and
 analyze the test pattern 33 so as to determine the print quality of the
 nozzles 24. One type of print sensor 32 that is usable with the present
 invention is an optically reflective sensor, such as is described in
 greater detail in the above-referenced copending U.S. application Ser. No.
 08/811,412, by Armijo et al., filed Mar. 4, 1997, titled "Detection of
 Printhead Nozzle Functionality by Optical Scanning of a Test Pattern".
 Further details of how the various types of sensors described above are
 used to test the printhead will be discussed subsequently.
 Considering now a method of printing with an inkjet printer 10 according to
 the present invention, and with reference to FIG. 5, the method includes
 both (a) a configuration portion 64 that configures a printmask 62 for
 each printhead 21 of the printer 10 in order to maximize the quality of
 the printed output, by minimizing print quality defects that occur due to
 nozzle aberrations but without significantly reducing the throughput of
 the printer 10, and (b) a printing portion 65 which uses the configured
 printmask to print an image on the printer 10.
 The configuration portion 64 begins with a step S51 which tests the
 printhead 21 to determine the print quality of the nozzles 24. This
 testing uses the above-mentioned sensors to determined the nozzle quality;
 the testing method will be described subsequently in greater detail.
 In step S52, the test results generated in step S51 are stored in a nozzle
 quality memory 35. The memory 35 is readable and writeable by a processor
 59 operatively connected to the print controller 58. The test results
 represent indicia of nozzle quality. The preferred indicia include
 identifying individual nozzles 24 as capable of generating output of
 either higher or lower print quality, or assigning nozzles a value of one
 of N levels of print quality. Alternatively, sections of the printhead 21
 containing groups of nozzles may be identified as capable of generating
 output of either higher or lower print quality. In a preferred embodiment,
 a separate memory 35 is used for storing the indicia of nozzle quality for
 each printhead 21, and this memory 35 is preferentially incorporated in
 each printhead 21. A memory incorporated in the printhead that is usable
 with the present invention is described in greater detail in U.S. Pat. No.
 5,812,156, filed Jan. 21, 1997 and issued Sep. 22, 1998 to Bullock et al.,
 titled "Apparatus Controlled by Data from Consumable Parts with
 Incorporated Memory Devices", which is assigned to the assignee of the
 present invention and hereby incorporated by reference in its entirety.
 Alternatively, the memory 35 may be located in the printer 10, or in a
 computer (not shown) which is connectable to the printer 10.
 In step S53, a printmask 62 to govern ink deposition is provided. The
 printmask 62 allocates the amount of ink which can be deposited from each
 nozzle 24 during each of the multiple printing passes of the printhead 21
 relative to the print medium 18. The printmask 62 is defined, as described
 in step S54, so as to enable the depositing, in at least some rows of
 pixel locations on the print medium 18, of relatively less ink from lower
 print quality nozzles, and relatively more ink from higher print quality
 nozzles. The processor 59 performs the computation and control operations
 required to define the printmask. Details of a method according to the
 present invention to perform this allocation will be described
 subsequently in further detail.
 In step S55, once the printmask 62 is constructed, it is stored in a
 printmask memory 64 which is operatively connected to the processor 59 and
 the print controller 58. In a preferred embodiment, the printmask memory
 64 is mounted within the frame of the printer 10. Alternatively, the
 printmask memory 64 may be stored external to the printer 10, for example
 in a computer (not shown) which is attachable to the printer 10.
 The printing portion 65 begins with a step S56 in which all or part of the
 image to be printed on the printer 10 is obtained. It is to be understood
 that the term "image" refers not only to pictures or photographs, but to
 any information to be output to the print medium 18, including graphics or
 text.
 In step S57, the printhead 21 and the print medium 18 move in relative
 motion in the scan direction 4 during each printing pass. For each section
 of the image which corresponds to the position of the printhead 21 over
 the print medium 18, nozzles 24 deposit ink, as governed by the printmask
 62 for the printhead 21, onto corresponding rows of pixel locations on the
 print medium 18 during the scanning operation, as indicated in step S58.
 In step S59, if the image has been completely printed, the printing
 operation ends. If some of the image remains to be printed, then step S60
 is performed, in which the the printhead 21 and the print medium 18 move
 in relative motion in the medium advance direction 8 between passes so as
 to position a different swath of the medium 18 under the printhead 21.
 Following step S60, the method then continues at step S57.
 The testing (S51) and storing (S52) steps of the abovementioned method as
 illustrated in FIG. 5 can be performed at different times, including
 outside the printer (for example, using a test system designed for testing
 printheads) during the manufacturing process for the printhead 21, during
 a process of refilling a previously manufactured printhead 21 with ink,
 after installation of the printhead 21 in the printer 10 of an inkjet
 printing system, and periodically in the printer during operation of the
 inkjet printing system. In a similar fashion, the steps of providing a
 printmask (S53) and defining the printmask (S54) can be performed
 following the completion of steps S51 and S52, or can be deferred to a
 later time prior to printing.
 Considering now in further detail the method of testing the printhead of
 step S51, and as best understood with reference to FIG. 6, the steps of
 this method depend on the type of detection operation to be performed. A
 preferred method which determines nozzle quality by assessing ink drops as
 they are deposited begins with a step S61 which deposits one or more drops
 from a nozzle which is operationally positioned adjacent the sensor. For
 an impact sensor 31, the nozzle is positioned such that ink drops from the
 nozzle will strike the sensor 31 to create its output on impact, as in
 step S62. For an in-flight pass-through sensor 30, the nozzle is
 positioned such that the ink drops from the nozzle will pass through an
 opening in the sensor 30 to break a light path and create its output, as
 in step S63. A number of alternatives for positioning the sensor are
 contemplated by the present invention. A single sensor can be repositioned
 to detect and analyze a number of different nozzles, a single sensor can
 be provided with a detecting area of sufficient size to assess multiple
 nozzles without movement, or a single sensor may have multiple detecting
 elements for measuring multiple nozzles. In step S64, the output of the
 sensor is used to determine the print quality of the corresponding nozzle.
 If all nozzles have been tested, then the "yes" branch of step S65 is
 taken and testing of the printhead is concluded. If all nozzles have not
 been tested, then the "no" branch of step S65 is taken; the printhead 21
 or sensor is repositioned if necessary in step S66, and the method then
 continues at step S61.
 An alternate method of testing the printhead determines nozzle quality by
 assessing the output on the medium 18 of a printed test pattern 33. This
 method begins with a step S67 in which a nozzle test pattern is printed on
 the medium 18. In a preferred embodiment, a single test pattern is printed
 to test all nozzles; one such test pattern usable with the present
 invention is described in further detail in the above-referenced
 co-pending U.S. application Ser. No. 08/811,412, by Armijo et al., filed
 Mar. 4, 1997, titled "Detection of Printhead Nozzle Functionality by
 Optical Scanning of a Test Pattern". If a print sensor is used to automate
 the assessment of nozzle quality, then the printed test pattern is scanned
 in step S68 in order to determine the print quality of the nozzles.
 Alternatively, if the determination of nozzle quality is to be visually
 made by the operator of the printer 10, then in step S69 the operator
 visually analyzes the test pattern to determine the print quality of the
 nozzles, and in step S70 enters the nozzle quality information into the
 inkjet printing system using, for example, the keyboard of a computer (not
 shown) which is connected to the printer 10, or a keypad mounted on the
 printer 10.
 Considering now in further detail step S54 of FIG. 5, FIG. 7 illustrates by
 way of example a method according to the present invention for defining
 the printmask 62 to enable the printing of less ink from lower quality
 nozzles and more ink from higher quality nozzles so as to improve print
 quality without reducing throughput. By way of introduction, different
 types of printers may provide different capabilities for printing a pixel
 location with a given intensity of a color ink. Some printers,
 particularly those which have relatively large drop volumes, or a
 relatively low repetition rate at which multiple drops can be deposited
 from a given nozzle, print a given pixel location with only a single drop
 in a given printing pass, and that single drop will provide the full
 amount of ink required to completely print that pixel location. Such a
 printer is described in the abovereferenced co-pending U.S. application by
 Askeland titled "Banding Reduction in Multipass Printing" (U.S.
 application Ser. No. 09/399473).
 Alternatively, other printers, particularly those which have relatively
 small drop volumes such that several drops are required to provide the
 full amount of ink required to completely print a pixel location, and a
 relatively high repetition rate at which multiple drops can be deposited
 from a given nozzle, can print in different modes during the printing of a
 single image. In a first printing mode, known as "hi-fipe" mode, a nozzle
 deposits a small number of drops (typically one drop) into pixel locations
 on different rows during each of several passes. On a given pixel location
 on a given row, the nozzle provides only a fraction of the total amount of
 ink required to completely print the pixel location, and so additional
 drops must be deposited from other nozzles into the pixel location during
 other passes. In a second printing mode, known as "multidrop" mode, a
 nozzle prints a given location by depositing several drops (typically at
 least two drops) rapidly into a given pixel location in a small number of
 passes (typically one pass). Typically, the several drops completely print
 the pixel location during a single pass. Advantageously, some nozzles can
 operate in a hi-fipe mode while other nozzles operate in a multidrop mode.
 Such a printer is described in the abovereferenced co-pending U.S.
 application by Bland et al. titled "Hybrid Printmask for Multidrop Inkjet
 Printer" (U.S. application Ser. No. 09/399534).
 Returning to the discussion of step S54 of FIG. 5 with reference to FIG. 7,
 different steps are performed depending on whether or not the printer is
 operated in a single drop per pass printing mode, or in a hybrid
 hi-fipe/multipass printing mode. If a single drop per pass printing mode
 is used, the printmask specifies the total number of possible times each
 of the nozzles can be activated during the multiple printing passes. The
 first step S71 in defining the printmask is to enable lower print quality
 nozzles identified by the testing to print relatively fewer possible total
 times during the multiple passes. Next, in step S72, higher print quality
 nozzles identified by the testing are enabled to print relatively more
 possible total times during the multiple passes, to compensate for the
 reduced amount of printing performed using the lower print quality
 nozzles. The relatively more possible total times and the relatively fewer
 possible total times are relative to a substantially equal number of
 possible times for all nozzles.
 Following this step, the method ends. The resulting printmask 62, which has
 a mask pattern allowing some of the nozzles to deposit drops in fewer
 possible pixel locations on a row and allowing others of the nozzles to
 deposit drops in more possible pixel locations on the row, provides for an
 unequal printing load between higher and lower quality nozzles, with
 higher quality nozzles being enabled more total times on a row than lower
 quality nozzles. However, the proper number of total drops are enabled for
 printing on the row because a compensating higher quality nozzle will
 print during one pass on rows printed by lower print quality nozzles
 during a different pass. The structure and method of operation of a
 printmask 62 resulting from the execution of this method is described in
 the abovereferenced co-pending U.S. application by Askeland titled
 "Banding Reduction in Multipass Printing" (U.S. application Ser. No.
 09/399473).
 If a hybrid hi-fipe/multipass printing mode is used, the printmask 62
 specifies the number of drops of the ink that each of the nozzles can
 deposit into the pixel locations during each of the multiple passes, where
 at least two drops of the ink are required to fully print a pixel location
 62 with a given intensity level of color. The first step S73 in defining
 the printmask is to enable lower print quality nozzles identified by the
 testing to deposit a small number of drops into individual pixel locations
 on at least two different rows during at least two corresponding passes.
 Next, in step S74, higher print quality nozzles identified by the testing
 are enabled to deposit many drops rapidly into a specific pixel location
 on a single row during at least one pass. Optionally, nozzles defined to
 be of intermediate print quality can be enabled to deposit both a small
 number of drops into individual pixel locations on at least two rows
 during at least two corresponding passes, and many drops rapidly into a
 specific pixel location on a single row during at least one other pass.
 Following this step, the method ends. The resulting printmask 62, which
 has a hi-fipe mask subpattern for a some nozzles and a multidrop mask
 subpattern for other nozzles, provides for an equal printing load from
 higher and lower print quality nozzles; after completion of all passes,
 the higher and lower print quality nozzles will have been enabled to
 deposit substantially the same number of drops. However, because the lower
 print quality nozzles operate in a hi-fipe mode which limits their ink
 contribution to any specific pixel to a fraction of the total ink required
 to fully print that pixel, with other higher print quality nozzles
 contributing the remainder of the ink to that specific pixel by printing
 on it in different passes, the effect of erroneous printing from the lower
 print quality nozzles is more evenly distributed throughout the printed
 output, and consequently less visually perceptible. The structure and
 method of operation of a printmask 62 resulting from the execution of this
 method is described in the abovereferenced co-pending U.S. application by
 Bland et al. titled "Hybrid Printmask for Multidrop Inkjet Printer" (U.S.
 application Ser. No. 09/399534).
 From the foregoing it will be appreciated that the printer and method
 provided by the present invention represents a significant advance in the
 art. A printer can be constructed according to the present invention so as
 to reduce visually objectionable banding that occurs due to nozzle
 aberrations occurring on individual printheads without significantly
 reducing printing throughput. Although several specific embodiments of the
 invention have been described and illustrated, the invention is not to be
 limited to the specific methods, forms, or arrangements of parts so
 described and illustrated. In particular, the invention may be used with
 bidirectional printing where printing passes occur in both directions of
 movement along the scan axis 4, or unidirectional printing where printing
 passes occur only in one direction along the scan axis 4; with
 even-advance printmodes where the medium 18 is advanced the same distance
 between passes, or with uneven-advance printmodes in which the medium 18
 is advanced different distances between passes; with multipass printers
 requiring two or more passes to fully print rows on the print medium; with
 printmasks having any number of cells in width; with all types of swath
 printers including band printers and drum printers; with all types of
 inkjet printers including thermal and piezo printing technologies; and
 with printing systems in which all the components of the printer may not
 be located in the same physical enclosure. According to the present
 invention, a single sensor can be positioned to detect and analyze a
 number of individual nozzles; a single sensor can be designed to have a
 detecting area of sufficient size to detect multiple nozzles without
 movement; or a sensor may have multiple detecting elements for detecting
 the output of multiple nozzles. Also, the invention is usable with
 printheads having lower and higher print quality nozzles regardless of
 where on the printhead those nozzles are located. The invention is limited
 only by the claims.