JP2011218560A - Density correction method and printer - Google Patents

Abstract

When printing is performed by ejecting white ink from a nozzle having a height difference at a position where ink is ejected, variation in ink ejection characteristics is corrected for each nozzle, and an image having a uniform density is printed.
A printing apparatus having a plurality of nozzles with different heights at an ink ejection position, wherein a first ink ejection amount, a second ink ejection amount, and a third ink ejection, which are standard ink ejection amounts, are output from the nozzles. A test pattern having a plurality of gradation patches is printed by ejecting ink on which the pigment settles in a quantity, and the lightness for each gradation value of the patch is measured for each ink ejection amount, and the lightness is When the same tone value is obtained for each ejection amount, the correction coefficient is calculated from the gradation value at each ejection amount with respect to the obtained gradation value of the first ink ejection amount. A correction table for each of the nozzles is created from the relationship between the ejection amount and the correction coefficient, and the correction coefficient for each nozzle is applied based on the correction table.
[Selection] Figure 9

Description

  The present invention relates to a density correction method and a printing apparatus.

  2. Description of the Related Art Printing apparatuses that perform recording by ejecting liquid from nozzles and landing ink droplets (dots) on a medium are known. In such a printing apparatus, variations in ink ejection characteristics (for example, ink ejection amount) may occur in each of the plurality of nozzles due to manufacturing errors or the like. When printing is performed using nozzles having different ejection characteristics, the size of the formed dots also varies, and a clean image cannot be printed.

  On the other hand, by using a plurality of types of ink, two or more types of ink droplets having different ink amounts are selectively ejected onto the medium to form a plurality of dots having different sizes in one pixel region. A method for correcting the variation for each nozzle is proposed. (For example, patent document 1).

JP 2006-88531 A

  In a printing apparatus that performs printing using white ink, a head for ejecting white ink may be installed obliquely in order to save space. On the other hand, the pigment component of the white ink generally has a specific gravity of 1 or more with respect to the solvent, and sedimentation due to gravity tends to occur. Therefore, when there is a height difference in the head, the density of the ink itself changes between a high position and a low position inside the head. For this reason, when the positions (heights) of the nozzles that eject ink are different even in the same head, the ink ejection characteristics (for example, the amount of ink ejection) may vary from nozzle to nozzle.

  On the other hand, the conventional method does not consider the case where the ink ejection characteristics change due to the difference in the ink ejection position (height) for each nozzle, and all nozzles with different heights are corrected under the same conditions. Was done. For this reason, in a printing apparatus having an oblique head as described above, variations in the ink ejection amount for each nozzle cannot be sufficiently corrected, and it has been difficult to print a uniform density image using white ink.

  In the present invention, when printing is performed by ejecting white ink from a nozzle having a height difference at a position where ink is ejected, the variation in the ink ejection characteristics is corrected for each nozzle, and an image having a uniform density is printed. It is aimed.

  The main invention for achieving the above object is (A) a printing apparatus having a plurality of nozzles with different heights at ink ejection positions, and a standard ink ejection amount from each of the nozzles at different heights. 1 ink ejection amount, a second ink ejection amount having a larger ink ejection amount than the first ink ejection amount, and a third ink ejection amount having a smaller ink ejection amount than the first ink ejection amount. The test pattern having a plurality of gradation patches is printed by ejecting the ink in which the pigment settles on the medium to form dots, and (B) each gradation value of the patch for each ink ejection amount. (C) gradation values of the patches when the lightness is the same, the first ink ejection amount, the second ink ejection amount, and the third ink ejection amount, respectively. A correction coefficient is calculated from the gradation value in the second ink ejection amount and the gradation value in the third ink ejection amount with respect to the obtained gradation value of the first ink ejection amount. D) A correction table for each of the nozzles at different heights is created from the relationship between the ink ejection amount and the correction coefficient, and (E) a printing apparatus including a plurality of nozzles having height differences in ink ejection positions For each of the nozzles at different heights, the amount of dots to be formed is corrected by applying a correction coefficient corresponding to the ink ejection amount for each nozzle based on the correction table. This is a density correction method.

  Other features of the present invention will become apparent from the description of the present specification and the accompanying drawings.

1 is a block diagram illustrating a configuration of a printer. FIG. 2A is a diagram illustrating the configuration of the printer according to the present embodiment. FIG. 2B is a side view illustrating the configuration of the printer according to the present embodiment. It is sectional drawing for demonstrating the structure of a head. It is a figure which shows arrangement | positioning of the head in a head unit. It is a figure which shows the example of the drive signal COM which defines operation | movement of the piezo element PZT. FIG. 6A is a diagram illustrating an example of the drive signal COM when the maximum potential difference Vh of the drive pulse is increased (Vhl). FIG. 6B is a diagram illustrating an example of the drive signal COM when the maximum potential difference Vh of the drive pulse is reduced (Vhs). FIG. 4 is a diagram illustrating a flow of print control performed by a printer driver. FIG. 3 is a diagram illustrating a state of sedimentation of white ink particles inside an oblique head of the printer. It is a figure showing the flow for creating a correction table. It is a figure which shows the example of the test pattern printed by this embodiment. It is a figure showing the combination of the amount of ink ejection at the time of test pattern printing, and the position of the nozzle which ejects ink. It is a figure which shows the relationship between the gradation value and the lightness (L *) of the test pattern printed using the MN nozzle. It is a figure explaining the method for calculating the correction coefficient for every ink ejection amount. It is a figure which shows the example of the correction table produced in this embodiment. It is a figure showing the flow for creating the correction table for back printing.

  At least the following matters will become clear from the description of the present specification and the accompanying drawings.

(A) In a printing apparatus having a plurality of nozzles having different heights at ink ejection positions, a first ink ejection amount, which is a standard ink ejection amount, and the first ink ejection amount from each nozzle at different heights. The ink in which the pigment settles with the second ink ejection amount having a larger ink ejection amount than the first ink ejection amount and the third ink ejection amount having a smaller ink ejection amount than the first ink ejection amount is used as a medium. A test pattern having a plurality of gradation patches is printed by ejecting dots to form dots, (B) measuring the lightness for each gradation value of the patch for each ink ejection amount, and (C) the lightness Are determined for each of the first ink ejection amount, the second ink ejection amount, and the third ink ejection amount. A correction coefficient is calculated from the gradation value of the second ink ejection amount and the gradation value of the third ink ejection amount with respect to the gradation value of the ink ejection amount, and (D) the ink ejection amount and the correction coefficient. The correction table is created for each of the nozzles at different heights from each other, and (E) each of the nozzles at different heights of the printing apparatus having a plurality of nozzles having different heights at the ink ejection position A density correction method for correcting the amount of dots to be formed by applying a correction coefficient according to the ink ejection amount for each nozzle based on the correction table.
According to such a density correction method, when printing is performed by ejecting white ink from a nozzle having a height difference at a position where the ink is ejected, the variation in the ink ejection characteristics is corrected for each nozzle, and a uniform density is obtained. Images can be printed.

In this density correction method, the amount of dots formed by ejecting ink with the first ink ejection amount from the nozzle located in the center among the plurality of nozzles having a difference in height in the ink ejection position is calculated. It is desirable to create the correction table as a reference.
According to such a density correction method, since the standard density ink is used as a reference, variation in the correction amount can be reduced when density correction is performed for each nozzle.

In this density correction method, a straight line is obtained by approximating least squares of three points of data: a correction coefficient for the first ejection amount, a correction coefficient for the second ejection amount, and a correction coefficient for the third ejection amount. It is desirable to create the correction table by obtaining
According to such a density correction method, it is possible to create a table of correction coefficients for all ink ejection amounts simply by examining three ink ejection amount data (reference, large, and small).

In this density correction method, the test pattern is printed on a transparent medium, and the lightness is measured from the opposite side of the print surface on which the patch of the test pattern is printed, thereby correcting the correction table for back printing. It is desirable to create.
According to such a density correction method, it is possible to perform density correction in both cases of front printing and back printing by creating one type of test pattern.

In this density correction method, the brightness measurement is formed in the test pattern using a spectrocolorimeter that measures a colorimetric value of a predetermined range of an image and obtains a colorimetric value of the predetermined range. The patch is preferably performed by measuring the L * value of each color component value in the L * a * b * color space.
According to such a density correction method, the correction coefficient is calculated based on the L * value that is easily affected by the density change, so that an accurate correction coefficient corresponding to the density change can be obtained. it can.

  (A) a head unit having a plurality of nozzles with different heights in the ink ejection position; and (B) a control unit that controls the amount of ink ejected from the head unit, wherein the control unit is a standard ink. A first ink ejection amount that is an ejection amount, a second ink ejection amount that is greater than the first ink ejection amount, and a third ink ejection amount that is less than the first ink ejection amount, From the test pattern having a plurality of patches printed by ejecting the ink in which the pigment settles from the nozzles at each ink ejection amount, for each gradation value of the patch measured for each ink ejection amount Regarding the lightness, the gradation value of the patch when the lightness is the same is obtained for each of the first ink ejection amount, the second ink ejection amount, and the third ink ejection amount. Correction coefficients are calculated from the gradation value in the second ink ejection amount and the gradation value in the third ink ejection amount with respect to the obtained gradation value of the first ink ejection amount. Based on the relationship between the ejection amount and the correction coefficient, based on the correction table created for each nozzle at the different height, the ink ejection amount for each nozzle was determined for each nozzle at the different height. By applying the correction coefficient, a printing apparatus comprising: a control unit that corrects the amount of dots to be formed;

=== Basic Configuration of Recording System ===
As a form of a printing apparatus for carrying out the invention, an ink jet printer (printer 1) will be described as an example. The printer 1 is a color inkjet printer that forms ink dots by ejecting ink toward a medium such as paper, cloth, and the like and prints an image on the medium. Here, as the ink used for printing, it is possible to use an ultraviolet curable ink (hereinafter also referred to as UV ink) that is cured by irradiation with ultraviolet light (hereinafter also referred to as UV). In that case, the medium includes a transparent medium such as a film sheet. In addition, a general solvent-based ink (for example, water-based dye / pigment ink) that is fixed to a medium by drying or evaporation can be used.

  When printing on a transparent medium, there are an image for viewing the printed image directly from the printed side of the transparent medium (front printing) and an image for viewing through the transparent medium (back printing). Both can be printed. At this time, the image for directly viewing the image is a normal image of an original predetermined image, such as an image read by a scanner or an image taken by a digital camera, and the image for viewing through a transparent medium is In many cases, it is a mirror image of the original predetermined image. However, the present invention is not limited to this in the case where a left-right symmetrical image or an image obtained by inverting the left and right with respect to the original image is printed.

The UV ink is an ink containing an ultraviolet curable resin, and is cured by a photopolymerization reaction occurring in the ultraviolet curable resin when irradiated with UV. Note that the printer 1 of this embodiment performs printing using four colors of black (K), cyan (C), magenta (M), and yellow (Y) and white (W) UV ink.
Hereinafter, an example of printing using UV ink will be described.

<Printer configuration>
FIG. 1 is a block diagram illustrating the overall configuration of the printer 1.
The printer 1 is communicably connected to a computer 110 that is an external control device. A printer driver is installed in the computer 110. The printer driver is a program for displaying a user interface on the display device of the computer 110 and converting image data output from the application program into print data. This printer driver is recorded on a recording medium (computer-readable recording medium) such as a flexible disk FD or a CD-ROM. Also, the printer driver can be downloaded to the computer 110 via the Internet. In addition, this program is comprised from the code | cord | chord for implement | achieving various functions.

  In order for the computer 110 to print an image on the printer 1, the print data converted from the image data according to the image to be printed is transmitted to the printer 1. The print data is data in a format that can be interpreted by the printer 1 and includes various command data and pixel data. The command data is data for instructing the printer 1 to execute a specific operation. The command data includes, for example, command data for instructing paper feed, command data for indicating the carry amount, and command data for instructing paper discharge. The pixel data is data related to pixels of an image to be printed.

  Here, a pixel is a unit element constituting an image, and an image is formed by arranging these pixels two-dimensionally. Pixel data in the print data is data (for example, gradation values) related to dots formed on a medium (for example, paper S).

  The printer 1 includes a transport unit 10, a carriage unit 20, a head unit 30, an irradiation unit 40, a detector group 50, and a controller 60. The controller 60 controls each unit such as the transport unit 20 and the carriage unit 30 based on print data received from the computer 110 which is an external device, and prints an image on a medium. The situation in the printer 1 is monitored by the detector group 50, and the detector group 50 outputs the detection result to the controller 60. The controller 60 controls each unit based on the detection result output from the detector group 50.

<Transport unit 10>
FIG. 2A is a bird's-eye view showing the configuration of the printer 1 of this embodiment, and FIG. 2B is a side view showing the configuration of the printer 1. In the printer 1 of this embodiment, the medium is not transported horizontally, but is transported in an obliquely upward direction, thereby saving space and reducing the size of the entire apparatus.

  The transport unit 10 is for transporting the paper S or the like as a medium from an upstream side to a downstream side in a predetermined direction (in the present embodiment, an obliquely upward direction, hereinafter referred to as a transport direction). Here, the transport direction is a direction that intersects the moving direction of the carriage. The transport unit 10 includes an upstream transport roller 11, a downstream transport roller 13, a platen 15, and a transport motor (not shown) (FIGS. 2A and 2B).

  The paper feed roller 11 is a roller for feeding the paper S inserted into the paper insertion slot into the printer. The upstream transport roller 11 and the downstream transport roller 13 rotate while sandwiching the paper S from the front side and the back side, thereby transporting the paper S to a printable region, and then discharging the printed paper S from the print region. A roller, driven by a transport motor. The operation of the carry motor is controlled by the controller 60 on the printer side. The platen 15 is a member that supports the paper S being printed from the back side of the paper S. In the printing area during printing, the medium S is vacuum-sucked from below, and the medium S is held at a predetermined position.

<Carriage unit 20>
The carriage unit 20 is for moving (also referred to as “scanning”) the carriage 21 to which the head unit 30 is attached in a direction perpendicular to the transport direction (hereinafter referred to as a movement direction). The carriage unit 20 includes a carriage 21, a guide rail 22, a driving belt 23, and a carriage motor (not shown) (also referred to as a CR motor) (FIGS. 2A and 2B).

  The carriage 21 can reciprocate in the moving direction, and is driven by a driving belt 23 that is rotated by a carriage motor along the guide rail 22. The operation of the carriage motor is controlled by the controller 60 on the printer side. Further, the carriage 21 detachably holds an ink cartridge that stores ink.

<Head unit 30>
The head unit 30 is for ejecting ink onto the paper S to form an image. The head unit 30 includes a head 31 and a head controller HC. A plurality of nozzles that are ink ejecting portions are provided on the lower surface of the head 31, and an ink chamber containing ink is provided in each nozzle.

  The head 31 is installed obliquely according to the conveyance direction of the medium (see FIGS. 2A and 2B), and a plurality of heads 31 are provided on the carriage 21. When the carriage 21 moves in the movement direction, the head 31 also moves in the movement direction. Ink is ejected intermittently while the head 31 is moving in the moving direction, so that dot lines (raster lines) along the moving direction are formed on the paper. An operation for printing an image on the medium S supplied to the printing area (image forming operation), and an operation for conveying the medium S in the conveying direction by the conveying unit 20 and supplying a new medium S portion to the printing area (conveying operation). Are alternately repeated to print a large number of images on the medium S.

  FIG. 3 is a cross-sectional view showing the structure of the head 31. The head 31 includes a case 311, a flow path unit 312, and a piezo element group PZT. The case 311 houses the piezo element group PZT, and the flow path unit 312 is joined to the lower surface of the case 311. The flow path unit 312 includes a flow path forming plate 312a, an elastic plate 312b, and a nozzle plate 312c. The flow path forming plate 312a is formed with a groove portion serving as the pressure chamber 312d, a through hole serving as the nozzle communication port 312e, a through port serving as the common ink chamber 312f, and a groove portion serving as the ink supply path 312g. The elastic plate 312b has an island portion 312h to which the tip of the piezo element PZT is joined. An elastic region is formed by an elastic film 312i around the island portion 312h. The ink stored in the ink cartridge is supplied to the pressure chamber 312d corresponding to each nozzle Nz via the common ink chamber 312f. The nozzle plate 312c is a plate on which the nozzles Nz are formed.

  FIG. 4 is an explanatory diagram of the nozzle Nz provided in the head 31. As shown in FIG. 4, each nozzle row is configured by nozzles Nz serving as ejection ports for ejecting ink of each color being arranged at a predetermined interval D in the transport direction. Each nozzle row includes 360 nozzles Nz # 1 to # 360.

  On the nozzle surface, a yellow nozzle row Y that ejects yellow ink as a color ink ejection nozzle row that forms a main image, a magenta nozzle row M that ejects magenta ink, a cyan nozzle row C that ejects cyan ink, and black ink And a black nozzle row K for ejecting the gas. In addition, a white nozzle row W for ejecting white ink is provided in line with each color nozzle row of KCMY.

  The piezo element group includes a plurality of comb-like piezo elements PZT (drive elements), and is provided in a number corresponding to the nozzles Nz. When the drive signal COM is applied to the piezo element PZT, the piezo element expands and contracts in the vertical direction according to the potential of the drive signal COM. When the piezo element PZT expands and contracts, the island 312h shown in FIG. 3 is pushed toward the pressure chamber 312d or pulled in the opposite direction. At this time, the elastic film 312i around the island portion 312h is deformed, and the pressure in the pressure chamber 312d rises and falls, thereby ejecting ink droplets from the nozzles.

<Irradiation unit 40>
The irradiation unit 40 irradiates UV toward the dots (ink dots) of the UV ink landed on the medium. The dots formed on the medium by each head of the head unit 30 are cured by receiving UV irradiation from the irradiation unit 40. The irradiation unit 40 of this embodiment includes a provisional curing irradiation unit 41 and a main curing irradiation unit 42.

  The pre-curing irradiation unit 41 irradiates UV for pre-curing the formed dots. Temporary curing refers to curing performed to prevent the formed dots from coming into contact with each other and bleeding. In the present embodiment, the provisional curing irradiation unit 41 includes a light emitting diode (LED) as a light source for UV irradiation. The LED can easily change the irradiation energy by controlling the magnitude of the input current.

  The provisional curing irradiation unit 41 is provided on the downstream side in the conveyance direction of the head unit 30 (a position obliquely above the head unit 30) (see FIG. 2). The length in the medium width direction of the pre-curing irradiation unit 41 is equal to or greater than the medium width, and the dots corresponding to the medium width can be pre-cured by one irradiation.

  The main curing irradiation unit 42 irradiates UV for main curing dots formed on the medium. In the present embodiment, the main curing is curing performed to completely solidify the dots. The main curing irradiation unit 42 includes a lamp (a metal halide lamp, a mercury lamp, or the like) as a light source for UV irradiation.

  The main curing irradiation section 42 is provided on the downstream side in the transport direction of the temporary curing irradiation section 41 (a position obliquely above the temporary curing irradiation section 41) (see FIG. 2). This is because the color ink dots temporarily cured by the pre-curing irradiation unit 41 are completely solidified. The length of the main curing irradiation unit 42 in the medium width direction is also equal to or greater than the medium width.

<Detector group 50>
The detector group 50 includes a rotary encoder, a linear encoder, an optical sensor (all not shown), and the like. The rotary encoder detects the rotation amounts of the upstream conveyance roller 11 and the downstream conveyance roller 12 and detects the conveyance amount of the medium based on the detection result. The linear encoder detects the position in the moving direction of the carriage 21 that moves while being guided by the guide rail 22. The optical sensor detects the presence or absence of the paper S at the opposing position by the light emitting part and the light receiving part attached to the carriage 21, for example, detects the position of the edge of the paper while moving, and detects the width of the paper can do.

<Controller 60>
The controller 60 is a control unit (control unit) for controlling the printer. The controller 60 includes an interface unit 61, a CPU 62, a memory 63, and a unit control circuit 64.

  The interface unit 61 transmits and receives data between the computer 110 that is an external device and the printer 1. The CPU 62 is an arithmetic processing device for performing overall control of the printer 1. The memory 63 is for securing an area for storing a program of the CPU 62, a work area, and the like, and is configured by a storage element such as a RAM or an EEPROM. Then, the CPU 62 controls each unit such as the transport unit 10 via the unit control circuit 64 in accordance with a program stored in the memory 63. Further, the CPU 62 outputs a head control signal for controlling the operation of the head 31 to the head unit 30.

<About the drive signal COM>
FIG. 5 shows an example of the drive signal COM that determines the operation (deformed shape) of the piezo element PZT. The drive signal COM is composed of drive pulses as shown in FIG. The drive pulse corresponds to a waveform portion for operating the piezo element PZT, and the waveform is determined based on the operation performed by the piezo element PZT. In the repetition period T, the expansion and contraction of the piezo element PZT can be controlled by changing the maximum potential difference Vh of the drive pulses or combining drive pulses having different shapes.

  FIG. 6A shows an example of the drive signal COM when the maximum potential difference Vh of the drive pulse in FIG. 5 is increased (Vhl). FIG. 6B shows an example of the drive signal COM when the maximum potential difference Vh of the drive pulse in FIG. 5 is reduced (Vhs). A drive pulse indicated by a broken line in the figure has a waveform (corresponding to FIG. 5) in the case of the maximum potential difference Vh.

  For example, when the drive pulse shown in FIG. 5 is applied to the piezo element PZT within the repetition period T, the piezo element PZT vibrates to the extent that it ejects an amount of ink that forms a medium dot. On the other hand, when a driving pulse having a maximum potential difference as shown in FIG. 6A within the repetition period T is applied to the piezo element PZT, the piezo element PZT ejects an amount of ink that forms a large dot. Vibrate. In addition, when a driving pulse having a large maximum potential difference as shown in FIG. 6B is applied to the piezo element PZT within the repetition period T, the piezo element PZT ejects an amount of ink that forms a small dot. Vibrate.

  As described above, the drive signal COM is generated for each piezo element PZT provided in each nozzle Nz, and defines the amount of ink ejected from each nozzle Nz.

<About printing operation>
When the printer 1 receives print data from the computer 110, the controller 60 first rotates the upstream side conveyance roller 11 and the downstream side conveyance roller 13 of the conveyance unit 10 to send the medium to be printed in the conveyance direction. The medium is conveyed without stopping at a constant speed, and passes through the head unit 30 and the irradiation unit 40. During this time, UV ink is intermittently ejected from the nozzles of each head of the head unit 30 to form dots on the medium. Then, UV is irradiated from the provisional irradiation unit 41 of the irradiation unit 40 to temporarily cure the dots, and then UV is irradiated from the main curing irradiation unit 42 to completely cure each color ink dot. Thus, an image is printed on the medium.
Finally, the controller 60 discharges the medium on which image printing has been completed.

=== About print control processing by the printer driver ===
FIG. 7 shows a flowchart of print control performed by the printer driver in this embodiment.
The printer driver receives image data from the application program, converts it into print data in a format that can be interpreted by the printer 1, and outputs the print data to the printer. When converting image data from an application program into print data, the printer driver performs resolution conversion processing (S101), color conversion processing (S102), halftone processing (S103), rasterization processing (S104), and command addition processing (S105). )I do.

  Hereinafter, various processes performed by the printer driver for printing characters and the like from image data will be described.

<S101: Resolution Conversion Process>
The resolution conversion process is a process for converting image data (text data, image data, etc.) output from an application program into a resolution (print resolution) for printing on a medium. For example, when the print resolution is specified as 720 × 720 dpi, the vector format image data received from the application program is converted into bitmap format image data with a resolution of 720 × 720 dpi.
Each pixel data of the image data after the resolution conversion process is RGB data of each gradation (for example, 256 gradations) represented by the RGB color space.

<S102: Color Conversion Process>
The color conversion process is a process for converting RGB data into data in the CMYK color space. The image data in the CMYK color space is data corresponding to the ink color of the printer. This color conversion processing is performed based on a table (color conversion lookup table LUT) in which gradation values of RGB data and gradation values of CMYK data are associated with each other.
The pixel data after the color conversion processing is 8-bit CMYK data with 256 gradations represented by the CMYK color space.

<S103: Halftone processing>
The halftone process is a process for converting high gradation number data into gradation number data that can be formed by a printer. For example, data representing 256 gradations is converted into 1-bit data representing 2 gradations or 2-bit data representing 4 gradations by halftone processing. In the halftone process, a dither method, a γ correction, an error diffusion method, or the like is used. The data subjected to the halftone process has a resolution equivalent to the print resolution (for example, 720 × 720 dpi). The image data after halftone processing corresponds to 1-bit or 2-bit pixel data for each pixel, and this pixel data is data indicating the dot formation status (presence / absence of dot, dot size) in each pixel. Become.

<S104: Rasterization processing>
The rasterizing process rearranges the pixel data arranged in a matrix for each pixel data in the order of data to be transferred to the printer 1. For example, the pixel data is rearranged according to the arrangement order of the nozzles in each nozzle row.

<S105: Command addition processing>
The command addition process is a process for adding command data corresponding to the printing method to the rasterized data. The command data includes, for example, conveyance data indicating the medium conveyance speed.
The print data generated through these processes is transmitted to the printer 1 by the printer driver.

=== Regarding Variation in Print Density Generated for Each Nozzle ===
<Properties of white ink>
In the printer 1 of this embodiment, printing is performed using white (W) ink. White ink contains titanium oxide and hollow resin as pigment particles. Since these particles have a specific gravity of 1 or more with respect to the solvent, they have the property of precipitating in the solvent if left for a long period of time. Therefore, when the period in which printing is not performed continues, the pigment component settles in the ink chamber inside the head.

  Even if the sedimentation of the pigment component occurs inside the head, if the head is horizontal and the position in the height direction of each nozzle is the same, that is, if the ink ejection position is the same height, the ink is ejected from any nozzle. The density of the ink to be used is also equal. Therefore, apart from problems such as nozzle clogging, the sedimentation of the pigment component does not affect the ink density ejected from each nozzle, and the printed image does not become uneven. On the other hand, when the head is installed obliquely as in the present embodiment, a problem arises.

  FIG. 8 is a diagram for explaining how white ink particles settle inside the head according to the present embodiment. FIG. 8 schematically shows a cross section of the nozzle rows (# 1 to # 360) in the head 31 and the common ink chamber. Since the head 31 is installed obliquely, the nozzle # 1 on the most upstream side in the transport direction is at the lowest position (height direction), and the nozzle # 360 on the most downstream side in the transport direction is at the highest position (height direction). )become. As described above, since the pigment component of the white ink settles below the ink chamber, the pigment component is condensed and the ink density is high at the nozzle (for example, the nozzle NL in FIG. 8) on the upstream side (low position) in the transport direction. Become. On the other hand, in the nozzle (for example, the nozzle NH in FIG. 8) on the downstream side (high position) in the transport direction, the pigment component decreases and the ink density decreases. Note that the nozzle NM at the center of the head has many or less pigment components of the ink, and therefore it is assumed that standard density ink is ejected.

  If printing is performed in this state, the density of ink ejected differs depending on the difference in the installation height of each nozzle, resulting in variations in printing density. On the other hand, it is difficult to return the pigment component to a uniform state over the entire white ink by re-stirring the ink already retained in the ink chamber. Therefore, in order to print an image having a uniform density using white ink, it is necessary to correct the ink ejection amount for each nozzle having a different installation height.

=== This Embodiment ===
In this embodiment, a correction table is created in advance for each nozzle having a different height, and the ink ejection amount is corrected by applying a correction value obtained from the correction table according to the ink ejection amount of each nozzle. Thus, an image with little density unevenness is printed.

<How to create a correction table>
First, a correction table for calculating a correction value for each nozzle is created. The correction table is used by using a printer as a reference for creating a correction table in the development stage of the printer. Then, based on the created correction table, the density is corrected for each nozzle of the manufactured printer.

  FIG. 9 shows a flowchart for creating the correction table. The correction table is created through the five steps S201 to S205, and one type of correction table is created for each nozzle having a different height. Details of each step will be described below.

<S201: Test pattern printing>
First, a test pattern is printed using white ink.
FIG. 10 shows an example of a test pattern printed in the present embodiment. The test pattern has a plurality of rectangular patches formed of white ink on a transparent medium. Each patch is formed with a plurality of gradations of gradation value 0 to gradation value 255. The numbers in the patches in FIG. 10 indicate the gradation values. The medium on which the test pattern is printed may be black or the like, but a white medium cannot be used. This is because the brightness cannot be measured even when white ink dots are formed on the white medium. Further, in the modification described later, since the color measurement is performed from the back side of the test pattern, the medium needs to be transparent.

  As the test pattern, a plurality of types of patterns are printed while changing the ink ejection amount and the location (nozzle position) where the ink is ejected. FIG. 11 shows a table of combinations of ink ejection amounts and nozzle positions during test pattern printing.

  The ink ejection amount includes a reference ejection amount, a large ejection amount in which the amount of ink ejected per unit time is larger than the reference ejection amount, a small ejection amount in which the ink amount ejected per unit time is less than the reference ejection amount, There is. The reference ejection amount is an ink amount ejected by the drive signal COM shown in FIG. 5 during normal printing. Further, the large ejection amount is the drive signal COM represented in FIG. 6A, and the small ejection amount is the ink amount ejected by the drive signal COM represented in FIG. 6B.

  The nozzle position for ejecting ink is printed using a nozzle NM located at the center of the head, a nozzle NH located downstream in the head transport direction, and a nozzle NL located upstream in the head transport direction. (See FIG. 8).

  When calculating a correction value for each nozzle at a different height, a test pattern should be printed for every nozzle from # 1 to # 360. However, in the present embodiment, for the sake of explanation, attention is paid to three nozzles (NM, NH, NL) at different heights, and seven conditions in FIG. 11 are tested as representative test patterns. Create a pattern.

  In condition 1 in FIG. 11, ink is ejected from the nozzle NM at the center of the head with a reference ejection amount. In this embodiment, this condition is used as the reference condition. Since the density of the white ink is considered to be neither light nor dark at the center of the head (FIG. 8), the ink density ejected from the central nozzle NM is used as a reference density and compared with other conditions. Do. Thereby, the variation in the density from the reference density can be reduced, and the deviation of the correction amount can be reduced when the density correction is performed for each nozzle.

<S202: Brightness measurement>
For the patch of each gradation value of the printed test pattern, the brightness is measured using a colorimeter.
The colorimeter is a spectrocolorimeter for measuring a colorimetric value of a predetermined range of an image and acquiring the colorimetric value of the predetermined range, and in this embodiment, a gradation value formed in a test pattern The colorimetric value of each patch is measured and used to obtain the colorimetric value of each patch.

  In the present embodiment, the colorimetric values of each patch and the colorimetric values acquired by the colorimeter are values represented as the respective color component values in the L * a * b * color space. In particular, in this embodiment, the lightness (L * value) of the color component values in the L * a * b * color space is used as the colorimetric value and the colorimetric value. This is because lightness is most susceptible to changes in density among colorimetric values, and is a suitable colorimetric value used for obtaining a correction coefficient and creating a correction table. However, the present invention is not limited to this, and other color component values (a * value, b * value) may be used. Further, it may be a set of values including an L * value, an a * value, and a b * value, and a predetermined ratio (for example, 2) with respect to the L * value, the a * value, and the b * value. : 1: 1 etc.) may be used. Further, it may be a value expressed as each color component of another color space (for example, XYZ color space or L * u * v * color space).

  When performing color measurement, it is preferable to apply a black paper or the like from the back of the transparent medium on which the test pattern is printed so that the color values of the white ink dots can be easily measured.

<S203: Tone value calculation>
The relationship between the calculated lightness (L *) and the gradation value is checked for each ink ejection amount, and the gradation value at the same lightness (L *) is obtained for each case where the ink ejection amount is large and small. .

  FIG. 12 shows the relationship between the gradation value and the lightness (L *) of the test pattern printed using the MN nozzle. The horizontal axis in FIG. 12 represents the gradation value, and the vertical axis represents the lightness (L *). In the figure, a curve representing the relationship between the gradation value and the brightness measured for the test pattern under Condition 1 in FIG. 11 is displayed as a reference curve. A curve representing the relationship between the gradation value and the lightness for each of condition 2 and condition 3 in FIG. 11 is displayed.

  Even when printing is performed with the same gradation value using nozzles having the same height (nozzle NM in FIG. 12), the size of the dots formed varies depending on the amount of ink ejection, so the lightness (L *) is also the ink. Different values are measured depending on the amount of ejection. For example, assume that 100 dots are formed with the reference ink ejection amount (condition 1) in order to form a patch having a gradation value of 150 in FIG. When printing is performed using ink of the same density and with an increased amount of ink ejection (Condition 2), the number of dots for forming a patch having the same gradation value 150 is larger than 100 because the number of dots formed is large. Less. Also, when printing is performed using ink of the same density and with less ink ejection (condition 3), the number of dots to form a patch with the same gradation value 150 is less than 100 because the number of dots formed is small. Will also increase. Thereby, the lightness (L *) measured becomes a different value.

  In other words, in order to obtain the same lightness (L *), it is necessary to decrease the gradation value when the ink ejection amount is large, and it is necessary to increase the gradation value when the ink ejection amount is small. is there. For example, in FIG. 12, when the lightness (L *) value with respect to the gradation value a of the reference condition (condition 1) is A, the gradation value when the lightness is A in condition 2 where the ink ejection amount is large is b is lower than a, and the tone value is b when the lightness is A under Condition 3 where the ink ejection amount is small.

  Similarly, the relationship when the white ink density is low (when the nozzle NH at a high position is used) is examined. That is, a graph similar to that in FIG. 12 is created for condition 1, condition 4, and condition 5 in FIG. Note that condition 1 is always used as a reference. In order to correct the ink ejection amount at the nozzle (NH) by examining the relationship when printing is performed by changing the ink ejection amount using the nozzle (NH) located at a high position with respect to this reference condition. The correction value is calculated. The correction value calculation method will be described later.

  Further, the relationship when the white ink density is high (when the nozzle NL at a low position is used) is examined in the same manner. That is, a graph similar to that in FIG. 12 is created for conditions 1, 6 and 7 in FIG.

  As a result, a graph representing the relationship between the gradation value and the lightness (L *) when the ink ejection amount is changed for each of the nozzles (NM, NH, NL) at different heights is obtained.

<S204: Correction coefficient calculation>
A graph plotting gradation values for each ink ejection amount when the lightness (L *) becomes the same value is created, a straight line is obtained for each ink ejection amount, and a correction coefficient for each ink ejection amount from the inclination of the straight line Is calculated.

  FIG. 13 is a diagram illustrating a method for calculating a correction coefficient for each ink ejection amount. The horizontal axis represents each gradation value under the reference condition (condition 1), and the vertical axis represents the gradation value at each ejection amount when the lightness (L *) is the same as the lightness (L *) in each gradation value on the horizontal axis. Is shown.

  In FIG. 13, what is indicated by ● is a plot of gradation values for condition 1. Under condition 1, since the gradation value on the horizontal axis is equal to the gradation value on the vertical axis, ● are arranged on a straight line with an inclination of 1 as shown in FIG.

  In FIG. 13, Δ indicates a plot of gradation values when the ink ejection amount is large (condition 2). As described in S203, when the ink ejection amount is large, the same lightness (L *) as the reference condition is displayed when the gradation value is low. For example, in FIG. 12, when the ink ejection amount is large (condition 2) with respect to the brightness A at the gradation value a in the reference condition (condition 1), the gradation value at which the brightness is A is b (condition 2). a> b). Therefore, as shown in FIG. 13, Δ in Condition 2 indicates a gradation value relatively lower than ● in Condition 1 with respect to the reference gradation value on the horizontal axis.

  Similarly, the circles in FIG. 13 indicate the gradation values when the ink ejection amount is small (condition 3). When the ink ejection amount is small (condition 3), the gradation value at which the lightness is A is c (c> a) with respect to the lightness A at the gradation value a in the reference condition (condition 1). Therefore, with respect to the reference gradation value on the horizontal axis, ○ in condition 3 indicates a relatively higher gradation value than ● in condition 1.

  Next, for the plot Δ of condition 2 and the plot ◯ of condition 3, straight lines are obtained by least square approximation, respectively. As shown in FIG. 13, when the ink ejection amount is large (condition 2), the straight line has a smaller slope than the reference condition (condition 1) (inclination β), and when the ink ejection amount is small (condition 3). A straight line (inclination γ) having a larger slope than the reference condition (condition 1) is obtained.

  The inclination of these straight lines is used as a correction coefficient for each ink ejection amount. That is, the correction coefficient for the reference ink ejection amount is 1, the correction coefficient when the ink ejection amount is large is β, and the correction coefficient when the ink ejection amount is small is γ.

  Depending on the ink ejection characteristics of each nozzle, the nozzle with a large ink ejection amount is multiplied by a correction coefficient β (1> β) to reduce the amount of dots formed on the medium and print in the reference state. The print density is corrected so as to be the same as that in the case where it is applied. Conversely, for nozzles with a small ink ejection amount, the same print density as when printing in the standard state with the amount of dots formed on the medium increased by multiplying the correction coefficient γ (γ> 1). It is corrected to become.

  This operation is performed for each nozzle located at a different height. That is, in addition to the graph of FIG. 13, the correction coefficient (the slope of the straight line in the condition 4 and the condition 5) for the nozzle (NH) at the high position is calculated by creating the graph for the condition 1, the condition 4, and the condition 5. Is done. Similarly, by creating graphs for condition 1, condition 6, and condition 7, the correction coefficient (the slope of the straight line in condition 6 and condition 7) for the low-position nozzle (NL) is calculated.

<S205: Creation of Correction Table>
A correction table is created from the correction coefficients for the three types of ink ejection amounts calculated in S204.
FIG. 14 shows an example of a correction table created in this embodiment. The horizontal axis in FIG. 14 indicates the ink ejection amount, and the vertical axis indicates the correction coefficient. A straight line obtained by least square approximation is obtained for three points obtained by plotting the correction coefficients (1, α, β) obtained by the above-described method with respect to each ink ejection amount. As a result, a correction table representing the relationship between the ink ejection amount and the correction coefficient is created.
The correction table is created for each nozzle having a different height. In this embodiment, three types of correction tables for the nozzles NM, NH, and NL are created.

<Density correction>
The correction table created for each nozzle is stored in the memory 63 of each printer being manufactured in the manufacturing process of the printer 1.
At the time of printing, the printer driver requests the printer 1 to transmit the correction table stored in the memory 63 to the computer 110. The printer driver stores the correction table transmitted from the printer 1 in a memory in the computer 110. A correction coefficient is calculated from the ink ejection amount information for each nozzle of the printer using a corresponding correction table. By applying the calculated correction coefficient to the correction target nozzle, the amount of ink ejected from the nozzle is adjusted.

  The correction at the time of printing is performed between the color conversion process (S102) and the halftone process (S103) of the print control by the printer driver. By applying the correction coefficient to 256-gradation pixel data after the color conversion process, the amount of dots formed is adjusted by increasing or decreasing the ink ejection amount or changing the gradation value. As a result, variations in ink ejection characteristics due to nozzle manufacturing errors and the like can be reduced, and printing with a uniform density can be performed.

  In the present embodiment, correction tables are created for nozzles having different heights. As a result, even when printing is performed using white ink or other ink that has a property that particles are likely to settle due to the influence of gravity, an appropriate correction coefficient is calculated for each nozzle having a different height so that there is no density unevenness. Printing can be performed.

=== Modification ===
In the printer 1 of the present embodiment, reverse printing for printing an image for viewing through a transparent medium is also performed. Here, when the printing surface on the medium is viewed from the front side (at the time of front printing) and when viewed from the back side (at the time of back printing) due to the difference in light transmittance of the medium used at the time of back printing The color of the white ink dots may differ. Therefore, even when correction is performed using the correction table obtained by the above-described method, when the printed material on which reverse printing is performed is viewed from the back side of the medium, the variation in the density of white ink dots is not necessarily eliminated. Not exclusively.
In such a case, it is necessary to prepare a correction table for reverse printing separately from the correction table for front printing. Therefore, in a modification of the present embodiment, a method for acquiring a correction table for back printing will be described.

FIG. 15 is a flowchart for creating a back printing correction table.
The basic flow is the same as the correction table creation method for surface printing (S201 to S205), and only the brightness measurement step of S302 is different. Only different points will be described below.

<S302>
The lightness measurement of the modification is performed from the back side of the medium on which the test pattern is printed, and the lightness is measured using a colorimeter for each tone value patch of the test pattern, as in S202. As the test pattern, the test pattern for surface printing created in S201 can be used as it is.
Also in the modification, the color value of each patch and the colorimetric value acquired by the colorimeter use the lightness (L * value) of each color component value in the L * a * b * color space. . When performing colorimetry, it is preferable to apply black paper or the like from the surface of the transparent medium on which the test pattern is printed so that the color values of the white ink dots can be easily measured.
By obtaining graphs corresponding to FIGS. 12 to 14 from the relationship between the measured lightness (L *) and the gradation value, it is possible to create a correction table for reverse printing for each nozzle having a different height. .
That is, it is possible to create both a correction table for front printing and a correction table for back printing from one test pattern.

=== Other Embodiments ===
Although a printer or the like as one embodiment has been described, the above embodiment is for facilitating the understanding of the present invention, and is not intended to limit the present invention. The present invention can be changed and improved without departing from the gist thereof, and it is needless to say that the present invention includes equivalents thereof. In particular, the embodiments described below are also included in the present invention.

<About printing devices>
In the above-described embodiment, the type of printer 1 that moves the head 31 together with the carriage has been described as an example. However, the printer may be a so-called line printer in which the head is fixed.

<About ink>
In the above-described embodiment, ink (UV ink) that is cured by being irradiated with ultraviolet rays (UV) is ejected from the nozzle. However, the liquid to be ejected is not limited to such an ink, and may be cured by receiving irradiation of electromagnetic waves other than UV. In this case, an electromagnetic wave for curing the liquid may be irradiated from the temporary curing irradiation unit and the main curing irradiation unit.

  Further, instead of ink that is cured by electromagnetic waves, general solvent-based ink (for example, aqueous dye / pigment ink) that is fixed to a medium by drying or evaporation may be used.

<About ink colors to be used>
In the above-described embodiment, an example in which printing is performed using five colors of YMCKW inks has been described. However, the present invention is not limited to this. For example, recording may be performed using ink of a color other than YMCKW, such as light cyan, light magenta, and clear.

  In the above-described embodiment, the arrangement order of the nozzle rows that eject ink of each color is YMCKW. However, the present invention is not limited to this. For example, the positions of the W nozzle row and the K nozzle row may be switched, or there may be two W nozzle rows.

<About test pattern printing>
In the present embodiment, the test pattern is printed using white (W) ink, which is likely to cause sedimentation, but the present invention is not limited to this. For example, it is also possible to create a correction table by printing a test pattern using multicolor inks such as black (K) ink and measuring the color values of the formed patches.

<About piezo elements>
In each of the above-described embodiments, the piezo element PZT is exemplified as the element that performs the operation for ejecting the liquid. However, other elements may be used. For example, a heating element or an electrostatic actuator may be used.

<About the printer driver>
The printer driver processing may be performed on the printer side. In that case, a recording apparatus is configured by the printer and the PC on which the driver is installed.

1 printer, 10 transport unit, 11 upstream transport roller,
13 downstream transport roller, 15 platen, 20 carriage unit,
21 Carriage, 22 Guide rail, 23 Drive belt,
30 head units, 31 heads, 311 case, 312 flow path unit,
312a flow path forming plate, 312b elastic plate, 312c nozzle plate,
312d pressure chamber, 312e nozzle communication port, 312f common ink chamber,
312g Ink supply path, 312h island part, 312i elastic film,
40 irradiation unit, 41 provisional irradiation unit, 42 main curing unit,
50 detector groups, 60 controllers, 61 interface units,
62 CPU, 63 memory, 64 unit control circuit,
110 computer

Claims (6)

(A) A printing apparatus having a plurality of nozzles with different heights at the ink ejection position,
From each said nozzle at a different height,
A first ink ejection amount that is a standard ink ejection amount, a second ink ejection amount that is larger than the first ink ejection amount, and a third ink that is smaller than the first ink ejection amount. With each ink ejection amount,
A test pattern having a multi-tone patch is printed by ejecting ink on which the pigment is precipitated onto the medium to form dots,
(B) For each ink ejection amount, measure the lightness for each gradation value of the patch,
(C) A gradation value of the patch when the brightness is the same is obtained for each of the first ink ejection amount, the second ink ejection amount, and the third ink ejection amount,
A correction coefficient is calculated from the gradation value in the second ink ejection amount and the gradation value in the third ink ejection amount with respect to the obtained gradation value of the first ink ejection amount,
(D) creating a correction table for each of the nozzles at different heights from the relationship between the ink ejection amount and the correction coefficient;
(E) A correction coefficient corresponding to the ink ejection amount for each nozzle is applied to each of the nozzles at different heights of a printing apparatus including a plurality of nozzles having different heights at the ink ejection position based on the correction table. To correct the amount of dots formed,
A density correction method characterized by the above. The density correction method according to claim 1,
The correction table is created based on the amount of dots formed by ejecting ink at the first ink ejection amount from the nozzle located at the center among a plurality of nozzles having different heights in the ink ejection position. A density correction method characterized by: The density correction method according to claim 1, wherein:
A correction coefficient for the first ejection amount, a correction coefficient for the second ejection amount, a correction coefficient for the third ejection amount,
A density correction method characterized in that the correction table is created by obtaining a straight line by least-square approximation of the three data points. The density correction method according to any one of claims 1 to 3,
The test pattern is printed on a transparent medium,
By measuring the brightness from the opposite side of the printed surface on which the patch of the test pattern is printed,
A density correction method, characterized by creating a correction table for reverse printing. The density correction method according to any one of claims 1 to 4,
The lightness measurement is performed using a spectrocolorimeter that measures a colorimetric value of a predetermined range of an image and obtains a colorimetric value of the predetermined range,
A density correction method comprising: measuring the L * value of each color component value in the L * a * b * color space for the patch formed in the test pattern. (A) a head portion having a plurality of nozzles with different heights at the ink ejection position;
(B) a control unit for controlling the amount of ink ejected from the head unit,
The controller is
A first ink ejection amount that is a standard ink ejection amount, a second ink ejection amount that is larger than the first ink ejection amount, and a third ink that is smaller than the first ink ejection amount. From a test pattern having a plurality of patches printed by ejecting ink from which the pigment settles at each ink ejection amount from the nozzle,
About the lightness for each gradation value of the patch measured for each ink ejection amount,
The tone value of the patch when the lightness is the same is obtained for each of the first ink ejection amount, the second ink ejection amount, and the third ink ejection amount,
A correction coefficient is calculated from the gradation value of the second ink ejection amount and the gradation value of the third ink ejection amount with respect to the obtained gradation value of the first ink ejection amount,
Based on the relationship between the ink ejection amount and the correction coefficient, based on a correction table created for each of the nozzles at different heights,
For each of the nozzles at different heights, the amount of dots formed is corrected by applying a correction coefficient according to the ink ejection amount for each nozzle.
A control unit characterized by:
A printing apparatus comprising:

Images