JP5908421B2 - Image forming apparatus and image forming method - Google Patents

Description

  The present invention relates to an image forming apparatus and an image forming method for forming dots of a plurality of colors including dark and light colors that are similar colors on a recording medium.

  In recent years, with the dramatic advancement of inkjet technology, color large format printing that achieves both high speed and high image quality using an inkjet image forming apparatus is becoming possible. This apparatus is used in a wide range of fields especially for sign / display applications, and can be applied to, for example, printing of storefront POP (Point Of Purchase), wall posters, outdoor advertisements / signboards, and the like.

  In the inkjet method, a printed matter can be obtained by discharging a plurality of types of ink liquids on a recording medium to form a large number of dots. That is, countless colors can be reproduced on a recording medium by combining various dots having different colors. In particular, in order to suppress deterioration of granularity in a highlight area (low density gradation area), various techniques for expressing continuous tone colors using inks of similar colors have been proposed.

  In Patent Document 1, only a light color is used in a low density gradation area, only a dark color is used in a high density gradation area, and a dark color and a light color are used in an intermediate density gradation area other than these. Specific examples have been proposed. Then, by adjusting each recording characteristic so as to satisfy a specific relationship between the light-colored peak point and the dark-colored start point, the color mixture at the joint of the low-to-medium-density gradation range may be smoothed. Have been described.

Japanese Patent Laid-Open No. 10-175318 (FIG. 14, paragraphs [0006] and [0007])

  By the way, according to the earnest study of the present inventor, for example, in the case of using an ink that is cured by irradiating an actinic ray including ultraviolet rays (so-called UV curable ink), the color tone of the ink before and after curing is determined by a combination of a light source and an ink. Found that could change. In other words, even when the hue is originally between dark and light colors with the same hue, hue shift occurs after the ink is cured, resulting in color irregularities and color jumps on the gradation image, and the appearance of the image (quality) ) Has a problem of lowering.

  However, the method disclosed in Patent Document 1 does not assume any case where a hue shift occurs between the dark color and the light color. Moreover, in Patent Document 1, attention is focused on the connection between the low to medium density gradation areas, and the transition of hue over the entire density gradation area (hereinafter also referred to as a mixed color range) in which dark and light dots coexist. No mention of smoothness.

  The present invention has been made to solve the above-described problem, and even when a hue shift occurs between a dark color and a light color, the entire density gradation region where dark and light dots coexist. An object of the present invention is to provide an image forming apparatus and an image forming method capable of suppressing the occurrence of uneven color and color jump.

  An image forming apparatus according to the present invention uses a recording head that forms dots of a plurality of colors including dark and light colors that are similar colors on a recording medium, and a recording characteristic that represents a dot recording rate for each density gradation level. An image processing unit that converts an input image signal into a control signal; and the recording head based on the control signal converted by the image processing unit under relative movement between the recording head and the recording medium. And a head driving circuit for controlling the color difference information relating to a hue difference between the color of the dark color flat image and the color of the light color flat image on the recording medium. A color difference range that is a density gradation range in which the dark color and the light color coexist as the hue difference obtained by referring to the hue difference information received by the hue difference reception unit increases. The recording characteristics are wide. It characterized by having a recording characteristic setting section for.

  Within a mixed color range, which is a density gradation region where dark and light colors coexist, a transition starting from one color on the hue transition line of the light color alone and ending on one color on the hue transition line of the dark color alone The hue transitions along the transition line. When a divergence occurs between the two transition lines, the gradient (the degree of change in hue) of the transition transition line connecting the two increases accordingly. As a result, color unevenness and color jump are likely to occur not only in the vicinity of the start point or end point but also in the entire color mixture range.

  Therefore, a hue difference receiving unit that receives hue difference information regarding a hue difference between the color of the dark flat halftone image and the color of the light flat halftone image on the recording medium, and the hue difference receiving unit that has received the hue difference receiving unit. Since the recording characteristic setting unit that sets the recording characteristic with a wider color mixing range as the hue difference obtained by referring to the hue difference information is larger is provided, the slope of the transition transition line connecting the two transition lines (the change in hue) Degree) can be reduced. Thereby, it is possible to suppress the occurrence of color unevenness and color jump over the entire density gradation region where dark and light dots coexist.

  The recording head ejects ink droplets that are cured by irradiation with actinic rays toward the recording medium, and the droplets ejected from the recording head adhere to the image formed on the recording medium. It is preferable to further comprise a curing light source for irradiating the actinic ray. When using ink that is cured by irradiation with actinic rays, the tendency of hue transition characteristics between dark and light colors may be different, which is particularly effective.

  An information storage unit that stores two or more of the recording characteristics with different color mixing ranges in association with the type of ink, and the hue difference receiving unit is configured to store the ink used for forming the image. The type is received as the hue difference information, and the recording characteristic setting unit corresponds to the type of ink received by the hue difference receiving unit from the two or more recording characteristics stored in the information storage unit. It is preferable to read and set the recording characteristics.

  The information storage unit further includes a recording characteristic determining unit that determines the recording characteristic according to the hue difference using colorimetric data relating to the color of the flat halftone image formed on the recording medium by the recording head. Preferably, the recording characteristic determined by the recording characteristic determination unit is stored in the storage.

  It is preferable that the recording characteristic setting unit sets the recording characteristic such that a lower limit value of the color mixture range is monotonously non-increased as the hue difference increases.

  An image forming method according to the present invention uses a recording head that forms dots of a plurality of colors including dark and light colors that are similar colors on a recording medium, and a recording characteristic that represents a dot recording rate for each density gradation level. An image processing unit that converts an input image signal into a control signal; and the recording head based on the control signal converted by the image processing unit under relative movement between the recording head and the recording medium. And a head drive circuit for controlling the color, the color relating to the hue difference between the color of the dark flat screen image and the color of the light flat screen image on the recording medium The step of receiving the phase difference information, and the larger the hue difference obtained by referring to the received hue difference information, the wider the mixed color range that is the density gradation region where the dark color and the light color coexist, and the recording characteristics are set. Steps to do Characterized in that it obtain.

  According to the image forming apparatus and the image forming method of the present invention, the hue difference information relating to the hue difference between the color of the dark flat screen image and the color of the light flat screen image on the recording medium is received, The larger the hue difference obtained by referring to the hue difference information is, the larger the color mixing range, which is the density gradation region where the dark color and the light color coexist, is set. It is possible to reduce the slope (degree of change in hue) of the transient transition line to be connected. Thereby, it is possible to suppress the occurrence of color unevenness and color jump over the entire density gradation region where dark and light dots coexist.

FIG. 1A is a graph showing hue transition characteristics in a gradation image formed with magenta series dark and light colors. FIG. 1B is a graph showing hue transition characteristics of a gradation image formed with cyan series dark and light colors. It is a graph which shows the typical example of each recording characteristic in a dark color and a light color. FIG. 3A is a graph showing hue transition characteristics when the recording characteristics shown in FIG. 2 are applied to the colors having the transition characteristics shown in FIG. 1A. FIG. 3B is a graph showing hue transition characteristics when the recording characteristics shown in FIG. 2 are applied to the colors having the transition characteristics shown in FIG. 1B. It is a schematic block diagram which shows the main structures for implement | achieving the image forming method which concerns on this embodiment. FIG. 5 is a flowchart for explaining an operation of a recording characteristic determination unit shown in FIG. 4. FIG. It is a schematic front view of the monochromatic color chart shown in FIG. FIG. 7A is a graph showing an example of determining the lower limit value of the color mixture range from the hue difference between dark and light colors. FIG. 7B is a graph showing an example of determining the peak value of the output halftone from the hue difference between the dark color and the light color. FIG. 8A is a graph showing the determination result of recording characteristics in dark colors. FIG. 8B is a graph showing the determination result of the recording characteristics in light colors. 9B is a graph showing hue transition characteristics when the recording characteristics shown in FIGS. 8A and 8B are applied to the colors having the transition characteristics shown in FIG. 1B. 1 is an external perspective view of an image forming apparatus according to an embodiment. FIG. 11 is an explanatory diagram schematically illustrating a recording medium conveyance path in the image forming apparatus of FIG. 10. FIG. 11 is a perspective plan view illustrating an example of an arrangement form of a recording head, a temporary curing light source, and a main curing light source disposed on the carriage of FIG. 10. 2 is a block diagram illustrating a configuration of an ink supply system of the image forming apparatus. FIG. 1 is a block diagram illustrating a configuration of an image forming apparatus.

  Hereinafter, an image forming method according to the present invention will be described with reference to the accompanying drawings by giving preferred embodiments in relation to an image forming apparatus that performs the image forming method. In this specification, forming an image may be referred to as “printing” or “printing”.

[Explanation of terms]
In this image forming method, an image is formed by ejecting dark and light ink droplets, which are similar colors, using an image forming apparatus 100 (see FIG. 10 and the like). In the present specification, “similar colors” mean color groups having different densities and substantially the same hue. The “dark color” is the color with the higher density, and the “light color” is the color with the lower density.

“Hue” in the present specification represents an aspect of color and is an index taking any value within a range of 0 to 360 degrees. For example, on the CIELAB color system, the hue is calculated as H = tan ⁻¹ (b ^* / a ^* ) (0 ≦ H <360 [degrees]). Note that the hue is not limited to this, and may be variously defined in a color system such as CIELV, CIERGB.

  Further, “the hue is substantially the same” means that the absolute value of the difference in hue between the two colors (hereinafter referred to as “hue difference”) is in the range of 0 to 30 degrees. That is, the similar color means a color group having hues that are close enough to express a continuous tone color by combining the colors.

[Density gradation design combining dots of similar colors]
1A and 1B are graphs showing hue transition characteristics in gradation images formed with dark and light colors of each series. The horizontal axis of the graph in each figure is density (unit: D), and the vertical axis is hue (unit: degree).

  FIG. 1A is a graph showing hue transition characteristics in a gradation image formed with magenta series dark and light colors. The graph indicated by a solid line is a transition line 10d of magenta (hereinafter also referred to as “M”) as a dark color, and the graph indicated by a broken line is a transition line of light magenta (hereinafter also referred to as “LM”) as a light color. 10c. As understood from this figure, in the range of 0.2 to 0.7D, the hue difference between “M” and “LM” at the same density is about 5 degrees at the maximum, and the hue difference is relatively It can be said that it is small.

  FIG. 1B is a graph showing hue transition characteristics of a gradation image formed with cyan series dark and light colors. The graph indicated by a solid line is a transition line 12d of dark cyan (hereinafter also referred to as “C”), and the graph indicated by a broken line is a transition line 12c of light cyan (hereinafter also referred to as “LC”) as a light color. It is. As understood from this figure, in the range of 0.2 to 0.6D, the hue difference between “C” and “LC” at the same density is about 10 degrees at the maximum, and the hue difference is relatively It can be said that it is big.

  By the way, when an image is formed using ink that is cured by irradiating ultraviolet rays (so-called UV curable ink), the tendency of hue transition characteristics in dark and light colors may be different. This is partly because the color of a specific additive (initiator, sensitizer) contained in the ink changes before and after irradiation with ultraviolet rays depending on the combination of the light source and the type of ink. At this time, as shown in FIG. 1B, the lighter color having a relatively high contribution of the color of the specific additive is more susceptible to the change in color.

  FIG. 2 is a graph showing typical examples of recording characteristics in dark and light colors. Both the horizontal axis and the vertical axis of the graph are gradation levels expressed in percentage (unit:%). Here, in order to easily distinguish each value, the horizontal axis is referred to as “input network%” and the vertical axis is referred to as “output network%”. This output half-tone% corresponds to a “dot recording rate” which is a dot recording ratio (0 to 100%) with respect to the maximum number of dots capable of forming dots. That is, the recording characteristic represents the dot recording rate for each density gradation level. The graph indicated by the solid line is the recording characteristic in the dark color, and the graph indicated by the broken line is the recording characteristic in the light color.

  Regarding the dark color (solid line) recording characteristics, the value of the output net% is always 0 when the value of the input net% is in the range of 0 to 30%. In the range where the value of the input network% exceeds 30%, the value of the output network% increases in proportion to the increase of the input network%. When the value of the input network% is 100%, the value of the output network% is also 100%. In this example, the maximum value of the output mesh% is set to 100%. However, when the total amount of ink used is limited, the maximum value of the output mesh% is set to any value less than 100%. It may be set.

  Regarding the light color (dashed line) recording characteristics, when the value of the input mesh% is in the range of 0 to 40%, the value of the output mesh% increases monotonously as the input network% increases. When the value of the input network% is in the range of 40 to 80%, the value of the output network% decreases monotonously as the input network% increases. In the range where the input network% exceeds 80%, the value of the output network% is always 0%. In other words, the input network% has a peak shape with a value of 40% as a peak, and has a mountain shape that becomes 0 at about 80%.

  Hereinafter, for the convenience of explanation, the density gradation range in which the input network% is 0 to 100% is classified into the following three ranges. In this example, the range where the input halftone is 0 to 30%, that is, the density gradation range where only light colors exist is defined as “light color range”. In this example, the range of the input halftone is 80 to 100%, that is, the density gradation range where only the dark color exists is defined as the “dark color range”. In this example, a range of 30 to 80%, that is, a density gradation range where light colors and dark colors coexist is defined as a “mixed color range”.

  The minimum value (0%) of the input network% is referred to as the lowest level Ll, and the maximum value (100%) of the input network% is referred to as the highest level Lh. The boundary between the “light color range” and the “color mixture range”, that is, the minimum value (30%) of the input network% belonging to the “color mixture range” is referred to as a lower limit level Lm1. The boundary between the “mixed color range” and the “dark color range”, that is, the maximum value (80%) of the input network% belonging to the “mixed color range” is referred to as an upper limit level Lm2.

  When the transition characteristics of FIG. 1A and the recording characteristics of FIG. 2 are considered together, the hue transitions along the transition line 10c in the “light color range” and the hue transitions along the transition line 10d in the “dark color range”. Is expected. Further, in the “mixed color range”, it is expected that the hue changes at an intermediate position between the transition lines 10c and 10d.

  FIG. 3A is a graph showing hue transition characteristics when the recording characteristics shown in FIG. 2 are applied to the colors having the transition characteristics shown in FIG. 1A.

  As shown in the graph, the magenta series hue transitions sequentially through the color C1, the color C2, the color C3, the color C4, and the color C5. Here, the colors C1, C2, C4, and C5 are colors corresponding to the lowest level L1, the lower limit level Lm1, the upper limit level Lm2, and the highest level Lh (see FIG. 2), respectively.

  A transitional transition line 14 illustrated by a one-dot chain line is a curve having a color C2 as a start point and a color C3 as an end point. Strictly speaking, the end of the transition transition line 14 is the color C4 (upper limit level Lm2). However, for convenience of illustration and explanation, the end of the color C3, which is a substantial merging position between the transition transition line 14 and the transition line 12d, is used. I will explain it.

  The shape of the transient transition line 14 is determined by the positional relationship between the transition lines 10c and 10d and the weighting of dark and light colors in the recording characteristics of FIG. Here, the transition transition line 14 is smoothly connected to a curve (a part of the transition line 10c) connecting the colors C1 and C2. Therefore, the color unevenness and color jump on the magenta series gradation image are not visually recognized by the observer.

  FIG. 3B is a graph showing hue transition characteristics when the recording characteristics shown in FIG. 2 are applied to the colors having the transition characteristics shown in FIG. 1B.

  As shown in this graph, the cyan hue shifts sequentially through color C1, color C2, color C3, color C4, and color C5. Here, the colors C1, C2, C4, and C5 are colors corresponding to the lowest level L1, the lower limit level Lm1, the upper limit level Lm2, and the highest level Lh (see FIG. 2), respectively.

  A transition transition line 16 illustrated by a one-dot chain line is a curve having a color C2 as a start point and a color C3 as an end point. The shape of the transitional transition line 16 is determined by the positional relationship between the transition lines 12c and 12d and the weighting of dark and light colors in the recording characteristics of FIG. Here, the transition transition line 16 is connected to the curve connecting the colors C1 and C2 (part of the transition line 12c). Accordingly, the observer may visually recognize color unevenness and color jump on a cyan-series gradation image.

  As described above, when an unexpected hue shift occurs between the dark color and the light color, color unevenness and color jump on the gradation image become apparent. Therefore, an image forming method for solving the above inconvenience is proposed.

[Configuration according to this embodiment]
FIG. 4 is a schematic block diagram showing a main configuration for realizing the image forming method according to the present embodiment. First, the recording characteristic determination unit 20 and its peripheral components will be described.

  The recording characteristic determination unit 20 determines a dark color recording characteristic and a light color recording characteristic based on the hue difference between the dark color and the light color. The recording characteristic determination unit 20 can exchange various data between the colorimeter 22 and the information storage unit 24.

  The recording characteristic determining unit 20 is a colorimetric data acquisition unit 28 that acquires colorimetric data of a single color chart 26 (see FIG. 6) to be described later, and a dark color / light color from the colorimetric data acquired from the colorimetric data acquisition unit 28. A hue difference calculating unit 30 for calculating a hue difference between the recording characteristics and a range changing unit 32 for changing the color mixing range (see FIG. 2) of the recording characteristics in accordance with the hue difference calculated by the hue difference calculating unit 30.

The colorimeter 22 acquires color measurement data from the measurement object in accordance with a predetermined operation. Here, the colorimetric data includes not only device-independent data such as tristimulus values XYZ and color values L ^* a ^* b ^{* in} a uniform color space, but also an optical physical quantity distribution with respect to wavelength (hereinafter referred to as “spectral data”). .) Is included. Examples of spectral data include spectral radiation distribution, spectral sensitivity distribution, spectral reflectance, or spectral transmittance.

  The information storage unit 24 stores various data necessary for executing this image forming method. In this example, a color separation table 34, a plurality of types of density tables 36 (recording characteristics), and a threshold matrix 38 are stored and stored, respectively.

[Operation of Recording Character Determining Unit 20]
Next, the operation of the recording characteristic determination unit 20 will be described in detail with reference mainly to the flowchart of FIG.

  In step S1, using the image forming apparatus 100 (see FIG. 10), the monochrome color chart 26 is printed by ejecting dark and light ink droplets (hereinafter, ink droplets) for each similar color. .

  FIG. 6 is a schematic front view of the monochrome color chart 26 of FIG. On the recording medium 40, patch strings 43, 44, 45, 46, an alphabetic character string 48, and a numerical character string 50 made up of a plurality of color patches 42 (flat mesh images and halftone solid images) are printed. Has been. The character string 48 represents the color attribute of the dots (ink liquid) used for forming the patch arrays 43 to 46. The character string 50 represents the dot recording rate (output network%) in each color patch 42 and reminds the operator as a user that the input network% changes in increments of 10% from the left side to the right side.

  For example, the color patch 42 in the upper right corner represents a color whose cyan (C) dot recording rate is 100%. The color patch 42 in the lower left corner represents a color having a light magenta (LM) dot recording rate of 0%. That is, each color patch 42 is formed by dots of any one color (single color) among a plurality of colors.

  In step S2, the colorimetric data acquisition unit 28 acquires the colorimetric data of the monochrome color chart 26 printed in step S1. Specifically, the user uses the colorimeter 22 to measure the colors of the color patches 42 in the patch arrays 43 to 46. Then, the color measurement data acquisition unit 28 acquires the color measurement data of each color patch 42 in association with the identification information (character strings 48 and 50).

  In step S3, the hue difference calculation unit 30 calculates the hue difference between the dark color and the light color in each type of similar color from the colorimetric data acquired in step S2. As described above, various hues and / or hue differences may be defined as long as the difference between dark and light colors can be quantified. Here, the first color of the color patch 42 (belonging to the light-colored patch rows 44 and 46) having a dot recording rate of a predetermined value (for example, 100%) and the color patch 42 (dark color) equal to the density of the first color. The hue difference between the second color of the patch rows 43 and 45) is calculated. The hue difference calculation unit 30 may calculate the hue difference using various calculation methods after creating the graphs illustrated in FIGS. 1A and 1B from the colorimetric data.

  In step S4, the recording characteristic determining unit 20 determines the recording characteristics of dark and light colors for each similar color. Prior to this determination, the range changing unit 32 changes the color mixture range (FIG. 2) in the recording characteristics according to the hue difference calculated in step S3.

  FIG. 7A is a graph showing an example of determining the lower limit value of the color mixture range from the hue difference between dark and light colors. The horizontal axis of the graph is the hue difference (unit: none), and the vertical axis is the lower limit (unit:%) of the color mixture range. As can be seen from the figure, the lower limit value is constant (30%) if the hue difference is in the range of 0 to 5, and the lower limit value decreases linearly if the hue difference is in the range of 5 to 12. If the hue difference is 12 or more, the lower limit value is constant (15%).

  FIG. 7B is a graph showing an example of determining the peak value of the output halftone from the hue difference between the dark color and the light color. The horizontal axis of the graph is the hue difference (unit: none), and the vertical axis is the peak value (unit:%) of the output net% in the light color. As can be understood from this figure, the peak value is constant (60%) if the hue difference is in the range of 0 to 5, and if the hue difference is in the range of 5 to 15, the peak value is monotonously and nonlinearly. If the hue difference is 15 or more, the peak value is constant (50%).

  By the way, when the hue difference is in the range of 0 to 5, the lower limit value (30%) in FIG. 7A and the peak value (60%) in FIG. 7B are constant, so the recording characteristics shown in FIG. It is done. When the hue difference in the magenta series (see FIG. 1A) is 3, the range changing unit 32 does not change the lower limit value (30%) and peak value (60%) in the recording characteristics.

  On the other hand, when the hue difference is 5 or more, the lower limit value in FIG. 7A and the peak value in FIG. 7B are changed with respect to the recording characteristics shown in FIG. When the hue difference in the cyan series (see FIG. 1B) is 10, the range changing unit 32 changes the lower limit value in the recording characteristics to 20% and changes the peak value to 55%.

  Then, the recording characteristic determination unit 20 determines the recording characteristics of dark and light colors in the magenta series and cyan series, respectively. In the magenta series, the dark color (solid line) recording characteristics shown in FIG. 2 and the light color (dashed line) recording characteristics shown in FIG. 2 are determined. Similarly, in the cyan series, a dark color recording characteristic indicated by a solid line in FIG. 8A and a light color recording characteristic indicated by a solid line in FIG. 8B are determined. Here, the broken line graphs in FIGS. 8A and 8B show the recording characteristics before the change by the range changing unit 32, and the solid line graphs show the recording characteristics after the change by the range changing unit 32.

  As described above, the recording characteristic determination unit 20 determines the recording characteristic so that the color mixture range becomes wider as the hue difference increases. For example, the lower limit value (lower limit level Lm1) of the color mixture range may satisfy the relationship of monotonically non-increasing, that is, decreasing or maintaining as the hue difference increases. Alternatively, the upper limit value (upper limit level Lm2) of the color mixture range may satisfy a relationship of monotonic non-decrease, that is, increase or maintenance as the hue difference increases.

  In step S5, the recording characteristics determined in step S4 are set / saved. Specifically, the recording characteristic determination unit 20 sends data relating to the recording characteristic (for example, a table format) to the outside, and stores it in the information storage unit 24 as a density table 36. Here, the shading table 36 is stored in association with various conditions (hereinafter, image forming conditions) on which the monochrome color chart 26 is printed. Examples of the image forming conditions include the type of ink, the recording medium 40, or the image forming apparatus 100. In particular, when UV curable ink is used, it is more preferable that two or more density tables 36 with different color mixing ranges are stored in association with the ink type in consideration of, for example, a difference in hue.

  In this way, the operation of the recording characteristic determination unit 20 is completed through steps S1 to S5 in FIG.

[Configuration and Operation of Image Processing Unit 60]
Second, the configuration and operation of the image processing unit 60 in FIG. 4 will be described.

  The image processing unit 60 generates a control signal used for image formation (more specifically, control of the recording head 106 in FIG. 14 and the like) from the input image signal (hereinafter, input image signal). The image processing unit 60 includes a hue difference receiving unit 61, a resolution conversion unit 62, a first color conversion processing unit 64, a second color conversion processing unit 66, a halftone processing unit 68, and a recording characteristic setting unit 70. Is basically provided.

  The input image signal is multi-gradation data composed of a plurality of color channels. For example, 8-bit (256 gradations per pixel) RGB TIFF format data may be used.

  Prior to the processing operation of the input image signal, the hue difference receiving unit 61 receives information on the hue difference between the dark color and the light color on the recording medium 40 (hereinafter referred to as hue difference information). For example, the numerical value of the hue difference may be directly received through a manual operation by the operator. Alternatively, an image forming condition (for example, ink type) associated with the input image signal may be received as hue difference information.

  The resolution conversion unit 62 converts the resolution of the input image signal to a resolution corresponding to the image forming apparatus 100 (see FIG. 10 and the like) using an image enlargement / reduction process that enlarges or reduces the image size. The first intermediate image signal obtained here has the same data definition as the input image signal, but the data size is different. Various known algorithms including interpolation calculation may be applied to the image enlargement / reduction processing.

  The first color conversion processing unit 64 converts the first intermediate image signal acquired from the resolution conversion unit 62 into a device color signal handled by the image forming apparatus 100 (see FIG. 10 and the like). Specifically, the first color conversion processing unit 64 converts the RGB color signal into the CMYK color signal by referring to the color separation table 34 read from the information storage unit 24. The second intermediate image signal obtained here corresponds to a multi-tone device color signal. For example, color separation is performed into device color signals for each of four color channels of yellow (Y), magenta (M), cyan (C), and black (K).

  The second color conversion processing unit 66 further converts the second intermediate image signal acquired from the first color conversion processing unit 64. Prior to this conversion, the recording characteristic setting unit 70 refers to the hue difference information acquired from the hue difference receiving unit 61 and reads out one type of the plurality of types of density tables 36 stored in the information storage unit 24. The density table 36 is set in the second color conversion processing unit 66. The second color conversion processing unit 66 refers to the set shading table 36 to convert the device color signal of a specific color channel (for example, magenta, cyan) into a signal for each color channel of the similar color. Separate (separate).

  In this example, the first color conversion processing unit 64 and the second color conversion processing unit 66 are provided separately, and the color separation processing is sequentially executed in two stages, but is configured to be executed at a time. May be.

  The halftone processing unit 68 converts the third intermediate image signal acquired from the second color conversion processing unit 66 into a dot on / off signal. This halftone process is a process for converting a continuous tone image signal into a binary or multilevel control signal. As means for halftone processing, various known methods including an error diffusion method, a dither method, a threshold matrix method, a density pattern method, and a random dot method can be applied.

  In this way, the image processing unit 60 generates a control signal used for ejecting ink droplets. This control signal corresponds to a signal for controlling driving / non-driving (dot on / off) of ink droplet ejection from each nozzle row 140 (see FIG. 12) and a droplet amount (dot size).

[Effects of this image forming method]
Next, effects obtained by this image forming method will be described. FIG. 9 is a graph showing hue transition characteristics when the recording characteristics shown in FIGS. 8A and 8B are applied to the colors having the transition characteristics shown in FIG. 1B.

  8A and 8B, by changing the lower limit level Lm1 to 20% while fixing the upper limit level Lm2, the light color range becomes narrower and the color mixture range becomes wider. Then, as shown in FIG. 9, the color C2 (circle not filled) moves in the direction of the arrow D1 on the low density side along the transition line 12c, and the color C3 (circle not filled) transitions. It moves in the direction of arrow D2 on the low concentration side along 12d. As a result, another transient transition line 18 starting from the filled circle color C2 and ending at color C3 is formed.

  As understood from the graph of FIG. 9, the colors C1, C2, and C3 are smoothly connected as compared to FIG. 3B. Therefore, even if it is a magenta series gradation image, color unevenness and color jump are not visually recognized by the observer.

[Overall Configuration of Image Forming Apparatus 100]
FIG. 10 is an external perspective view of the image forming apparatus 100 according to the present embodiment. The image forming apparatus 100 is a wide format printer that forms a color image on a recording medium 40 using ultraviolet curable ink (UV curable ink). Here, the “wide format” printer means a device that supports A3 Nobi or higher.

  The image forming apparatus 100 includes an apparatus main body 102 and support legs 104 that support the apparatus main body 102. The apparatus main body 102 includes a drop-on-demand type recording head 106 that discharges ink toward the recording medium 40, a platen 108 that supports the recording medium 40, a guide mechanism 110 as a head moving unit (scanning unit), and a carriage. 112 is provided.

  The guide mechanism 110 is above the platen 108 along a scanning direction (Y direction) orthogonal to the conveyance direction (X direction) of the recording medium 40 and parallel to the medium support surface of the platen 108 (supporting surface of the recording medium 40). It is arranged to extend. The carriage 112 is supported so as to be able to reciprocate in the Y direction along the guide mechanism 110. The carriage 112 is mounted with the recording head 106, and temporary curing light sources 114 a and 114 b for irradiating the ink on the recording medium 40 with ultraviolet rays, and main curing light sources 116 a and 116 b.

  The temporary curing light sources 114a and 114b are light sources that irradiate ultraviolet rays for temporarily curing the ink so that the adjacent ink droplets do not coalesce after the ink droplets ejected from the recording head 106 have landed on the recording medium 40. It is. The main curing light sources 116a and 116b are light sources that irradiate ultraviolet rays for performing additional exposure after temporary curing and finally completely curing (main curing) the ink.

  The recording head 106, the temporary curing light sources 114a and 114b, and the main curing light sources 116a and 116b disposed on the carriage 112 move integrally with the carriage 112 along the guide mechanism 110. Hereinafter, the reciprocating direction of the carriage 112 may be referred to as “main scanning direction”, and the conveyance direction of the recording medium 40 (also simply referred to as “conveyance direction”) may be referred to as “sub-scanning direction”.

  For the recording medium 40, various media such as paper, non-woven fabric, vinyl chloride, synthetic chemical fiber, polyethylene, polyester, tarpaulin, etc., regardless of material, permeable medium or non-permeable medium may be used. it can. The recording medium 40 is fed from the back side of the apparatus in a roll paper state (see FIG. 11), and after printing is wound up by a winding roller 128 (see the same figure) on the front side of the apparatus. Ink droplets are ejected from the recording head 106 to the recording medium 40 conveyed on the platen 108, and the temporary curing light sources 114a and 114b and the main curing light sources 116a and 116b are applied to the ink droplets attached on the recording medium 40. Each is irradiated with ultraviolet rays.

  A mounting portion 120 for the ink cartridge 118 is provided on the left front surface facing the front of the apparatus main body 102. The ink cartridge 118 is a replaceable ink supply source that stores ultraviolet curable ink. The ink cartridge 118 is provided corresponding to each color ink used in the image forming apparatus 100 of this example. Each color ink cartridge 118 is connected to the recording head 106 by an ink supply path (not shown) formed independently.

[Conveying path of recording medium 40]
FIG. 11 is an explanatory diagram schematically showing a conveyance path of the recording medium 40 in the image forming apparatus 100. As shown in the figure, the platen 108 is formed in an inverted bowl shape, and the upper surface thereof serves as a support surface of the recording medium 40. In the vicinity of the platen 108 and upstream in the X direction, a pair of nip rollers 122 that are conveying means for intermittently conveying the recording medium 40 are disposed. The nip roller 122 moves the recording medium 40 on the platen 108 in the X direction.

  The recording medium 40 sent from the supply-side roll (feeding supply roller 124) constituting the roll-to-roll type conveying means is intermittently moved in the X direction by a pair of nip rollers 122 provided at the entrance of the printing unit 126. Be transported. The recording medium 40 that has reached the printing unit 126 immediately below the recording head 106 is printed by the recording head 106, and is taken up by the take-up roller 128 after printing. A guide member 130 for guiding the recording medium 40 is provided on the downstream side in the X direction with respect to the printing unit 126.

  A temperature adjustment unit for adjusting the temperature of the recording medium 40 during printing is provided on the back surface of the platen 108 (the surface opposite to the surface supporting the recording medium 40) at the position facing the recording head 106 in the printing unit 126. 132 is provided. When the recording medium 40 at the time of printing is adjusted to have a predetermined temperature, the physical properties such as the viscosity of the ink droplets landed on the recording medium 40 and the surface tension become desired values, and a desired dot diameter is obtained. It becomes possible. If necessary, the pre-temperature control unit 134 may be provided on the upstream side in the X direction with respect to the temperature control unit 132, or the after-temperature control unit 136 may be provided on the downstream side in the X direction with respect to the temperature control unit 132. In the first place, when it is not necessary to adjust the temperature of the recording medium 40, the configuration of the temperature adjustment unit 132 may be omitted.

[Description of Recording Head 106]
FIG. 12 is a plan perspective view showing an example of the arrangement of the recording head 106, the temporary curing light sources 114a and 114b, and the main curing light sources 116a and 116b arranged on the carriage 112. FIG.

  The recording head 106 includes yellow (Y), magenta (M), cyan (C), black (K), light cyan (LC), light magenta (LM), transparent ink (CL), and white (W). For each ink, nozzle rows 140Y, 140M, 140C, 140K, 140Lc, 140Lm, 140CL, and 140W for ejecting the respective color inks are provided. In this figure, the nozzle rows are indicated by dotted lines, and the individual illustration of the nozzles is omitted. In the following description, the nozzle rows 140Y, 140M, 140C, 140K, 140Lc, 140Lm, 140CL, and 140W may be collectively referred to as the nozzle row 140.

  The ink color type (number of colors) and the color combination are not limited to the present embodiment. For example, a form in which the LC and LM nozzle arrays are omitted, a form in which the CL and W nozzle arrays are omitted, and a form in which a nozzle array for ejecting special color ink is added are possible. Further, the arrangement order of the nozzle rows for each color is not particularly limited.

  By configuring a head module for each color nozzle row 140 and arranging them, the recording head 106 capable of forming a color image or a monochrome image can be configured. For example, the head modules 106Y and 106M having nozzle arrays 140Y, 140M, 140C, 140K, 140Lc, 140Lm, 140CL, and 140W, respectively, that discharge ink of each color of Y, M, C, K, LC, LM, CL, and W. , 106C, 106K, 106Lc, 106Lm, 106CL, and 106W may be arranged at equal intervals so as to be aligned along the reciprocating movement direction (X direction) of the carriage 112. It is also possible to interpret each color-specific head module (for example, the head module 106Y) as a “recording head”. Alternatively, a configuration in which an ink flow path is formed separately for each color within one recording head 106 and a nozzle row that discharges a plurality of colors of ink with one head is also possible.

  Each nozzle row 140 includes a plurality of nozzles arranged in a row along the X direction at regular intervals. In the recording head 106 in this example, the arrangement pitch of the nozzles constituting each nozzle row 140 is 254 μm, the number of nozzles constituting one nozzle row 140 is 256 nozzles, and the total length Lw of the nozzle row 140 is about 65 mm (254 μm). × 255 = 64.8 mm). The discharge frequency is 15 kHz, and three types of discharge amounts of 10 pl, 20 pl, and 30 pl can be determined by changing the drive waveform.

  As an ink ejection method of the recording head 106, a method (piezo jet method) in which ink droplets are ejected by deformation of a piezoelectric element (piezo actuator) is employed. In addition to a configuration using an electrostatic actuator (electrostatic actuator method) as a discharge energy generating element, a heating element (heating element) such as a heater is used to heat ink to generate bubbles and to eject ink droplets with that pressure. It is also possible to adopt a form (thermal jet system).

[Explanation of UV irradiation equipment]
As shown in FIG. 12, provisional curing light sources 114a and 114b are arranged on both the left and right sides of the recording head 106 in the scanning direction (Y direction). Further, main curing light sources 116 a and 116 b are arranged on the downstream side in the transport direction (X direction) of the recording head 106.

  The ink droplets ejected from the nozzles of the recording head 106 and landed on the recording medium 40 are immediately irradiated with ultraviolet rays for temporary curing by the temporary curing light sources 114a and 114b passing thereover. Ink droplets on the recording medium 40 that have passed through the printing unit 126 (see FIG. 11) along with the intermittent conveyance of the recording medium 40 are irradiated with ultraviolet rays for main curing by the main curing light sources 116a and 116b.

  Each of the temporary curing light sources 114a and 114b has a structure in which a plurality of UV-LED elements 142 are arranged. The two temporary curing light sources 114a and 114b have a common configuration. The six UV-LED elements 142 are arranged so that UV irradiation can be performed at once on a region having the same width as the entire length Lw of the nozzle row 140 of the recording head 106.

  Each of the main curing light sources 116a and 116b has a structure in which a plurality of UV-LED elements 144 are arranged. The two main curing light sources 116a and 116b have a common configuration. In the main curing light sources 116a and 116b, six UV-LED elements 144 in the Y direction and two in the X direction are arranged in a matrix.

  In addition, the number of LED elements in the temporary curing light sources 114a and 114b (or the main curing light sources 116a and 116b) and the arrangement form thereof are not limited to this example. If necessary, the temporary curing light sources 114a and 114b may be omitted and only the main curing light sources 116a and 116b may be provided.

[Description of ink supply system]
FIG. 13 is a block diagram illustrating a configuration of an ink supply system of the image forming apparatus 100. As shown in the drawing, the ink stored in the ink cartridge 118 is sucked by the supply pump 146 and sent to the recording head 106 via the sub tank 148. The sub tank 148 is provided with a pressure adjusting unit 150 for adjusting the pressure of the ink inside.

  The pressure adjusting unit 150 includes a valve 152, a pressure increasing / decreasing pump 154 communicated with the sub tank 148 via the valve 152, and a pressure gauge 156 provided between the valve 152 and the pressure increasing / decreasing pump 154.

  During normal printing, the pressure increasing / decreasing pump 154 operates in a direction to suck ink in the sub tank 148. Thereby, the internal pressure of the sub tank 148 and the internal pressure of the recording head 106 are maintained at a negative pressure.

  On the other hand, during maintenance of the recording head 106, the pressure increasing / decreasing pump 154 operates in a direction to pressurize the ink in the sub tank 148. As a result, the inside of the sub tank 148 and the inside of the recording head 106 are forcibly pressurized, and the ink in the recording head 106 is discharged through each nozzle row 140 (see FIG. 12).

[Description of Control System of Image Forming Apparatus 100]
FIG. 14 is a block diagram illustrating a configuration of the image forming apparatus 100. As shown in the figure, the image forming apparatus 100 includes a control device 160 as control means. As the control device 160, for example, a computer equipped with a central processing unit (CPU) can be used. The control device 160 functions as a control device that controls the entire image forming apparatus 100 according to the read program, and also functions as a calculation device that performs various calculations.

  The control device 160 includes a recording medium conveyance control unit 162, a carriage drive control unit 164, a light source control unit 166, a recording characteristic determination unit 20 (see FIG. 4), an image processing unit 60 (see FIG. 4), and an ejection control unit 168. Have. Each of these units is realized by a hardware circuit or software, or a combination thereof.

  The recording medium conveyance control unit 162 controls a conveyance driving unit 170 (conveying means) for conveying the recording medium 40 (see FIG. 11). The conveyance drive unit 170 includes a drive motor that drives a pair of nip rollers 122 (see the same figure) and a drive circuit thereof. The recording medium 40 conveyed on the platen 108 (see the same figure) is intermittently fed in the sub-scanning direction in units of swath widths in accordance with the reciprocal scanning (movement of the printing pass) in the main scanning direction by the recording head 106.

  The carriage drive control unit 164 controls a main scanning drive unit 172 (conveying means) for moving the carriage 112 (see FIG. 10) in the main scanning direction. The main scanning drive unit 172 includes a drive motor connected to a moving mechanism of the carriage 112 and a control circuit thereof.

  The encoder 174 is attached to the drive motor of the main scanning drive unit 172 and the drive motor of the transport drive unit 170, and outputs a pulse signal corresponding to the rotation amount and rotation speed of the drive motor. The pulse signal is sent to the control device 160. Based on the pulse signal output from the encoder 174, the position of the carriage 112 and the position of the recording medium 40 are grasped.

  The light source control unit 166 adjusts the amount of light emitted from the temporary curing light sources 114a and 114b (UV-LED element 142) via the LED drive circuit 176, and the main curing light sources 116a and 116b (UV−) via the LED drive circuit 178. It is a control means for adjusting the light emission amount of the LED element 144).

  The LED drive circuits 176 and 178 output a voltage having a voltage value corresponding to a command from the light source control unit 166 to adjust the light emission amount of the UV-LED elements 142 and 144. The adjustment of the light emission amount may be performed by changing the duty ratio and frequency of the drive waveform instead of changing the voltage.

  An input device 180 such as an operation panel and a display device 182 are connected to the control device 160. The input device 180 is means for inputting a manual external operation signal to the control device 160. For example, various forms such as a keyboard, a mouse, a touch panel, and operation buttons can be adopted. Various forms such as a liquid crystal display, an organic EL display, and a CRT can be adopted for the display device 182. The operator can input printing conditions and input / edit attached information by operating the input device 180. Further, the operator can confirm various information such as input contents and search results through the display on the display device 182. Alternatively, the colorimeter 22 (FIG. 4) may be connected so that the colorimetric data of the monochrome color chart 26 can be acquired.

  In addition, the image forming apparatus 100 is provided with an information storage unit 24 (see FIG. 4) for storing various types of information and an image input I / F 186 for capturing an input image signal. As the image input I / F 186, a serial interface or a parallel interface may be applied. In this part, a buffer memory (not shown) for speeding up communication may be mounted.

  The information storage unit 24 stores a program executed by a CPU (not shown) included in the control device 160 and various data necessary for control. Examples of data stored in the information storage unit 24 include color conversion conditions (a color separation table 34 and a plurality of types of shade tables 36) shown in FIG.

  The recording characteristic determination unit 20 determines a dark color recording characteristic and a light color recording characteristic based on the hue difference between the dark color and the light color. The recording characteristic determination unit 20 illustrated in the example of FIG. 4 includes a colorimetric data acquisition unit 28, a hue difference calculation unit 30, and a range change unit 32. Since the function and operation of each part are as described above, the description thereof is omitted.

  The image processing unit 60 performs desired image processing on the input image signal acquired via the image input I / F 186 to generate a control signal used for ink droplet ejection control. The image processing unit 60 illustrated in FIG. 4 includes a resolution conversion unit 62, a first color conversion processing unit 64, a second color conversion processing unit 66, a halftone processing unit 68, and a recording characteristic setting unit 70. ing. Since the function and operation of each part are as described above, the description thereof is omitted.

  The discharge control unit 168 generates a discharge control signal for the head drive circuit 188 based on the control signal acquired from the image processing unit 60. A common drive voltage signal is applied to each ejection energy generating element of the recording head 106 via the head driving circuit 188, and is connected to the individual electrode of each energy generating element according to the ejection timing of each nozzle (not shown). By switching on and off the switch elements, ink droplets are ejected from the corresponding nozzles in the nozzle row 140 (see FIG. 12).

[Effect of the present invention]
As described above, the image forming apparatus 100 represents the recording head 106 that forms a plurality of dots including dark and light colors that are similar colors on the recording medium 40, and the dot recording rate for each density gradation level. An image processing unit 60 that converts an input image signal into a control signal using a recording characteristic (light / dark table 36), and a converted control signal under relative movement between the recording head 106 and the recording medium 40. And a head driving circuit 188 for controlling the recording head 106 on the basis thereof.

  Within a mixed color range where dark and light colors coexist, one color (C2) on hue transition lines 10c and 12c in light color alone starts from one color (C2) on hue transition lines 10d and 12d in dark color alone The hue transitions along the transitional transition lines 14 and 16 with (C3) as the end point. When a divergence occurs between the transition lines 12c and 12d, the gradient (the degree of change in hue) of the transient transition line 16 connecting the two increases. As a result, color unevenness and color jumping easily occur not only in the vicinity of the start point (color C2) or end point (color C3) but also in the entire color mixture range.

  Therefore, the color difference information between the color of the dark color patch 42 and the color of the light color patch 42 on the recording medium 40 is received, and the color mixture range becomes larger as the hue difference obtained from the hue difference information is larger. Since the recording characteristic setting unit 70 for setting a wide recording characteristic (light / dark table 36) is provided, it is possible to reduce the inclination (degree of change in hue) of the transition transition line 18 connecting the transition lines 12c and 12d. It is. Thereby, generation | occurrence | production of the color nonuniformity and color jump over the whole color mixing range can be suppressed.

  In addition, this invention is not limited to embodiment mentioned above, Of course, it can change freely in the range which does not deviate from the main point of this invention.

  In the above embodiment, the recording head 106 is reciprocally scanned along the width direction of the recording medium 40, and the recording medium 40 is relatively moved along the conveyance direction of the recording medium 40 to generate an image. Although the shuttle method (multi-pass method) has been illustrated, the single-pass method may be applied without being limited to this recording method. The single pass method uses a line head including a plurality of nozzles arranged along the width direction of the recording medium 40 to relatively move the recording medium 40 only once along a predetermined linear path to generate an image. Is a recording method for generating.

  In the above-described embodiment, the image is formed using the ultraviolet curable ink, but an embodiment using an ink that is cured by an actinic ray other than the ultraviolet ray may be employed. In this case, as the temporary curing light source 114a (b) and the main curing light source 116a (b), a light source capable of irradiating the active light may be used.

  In the above-described embodiment, the wide format printing apparatus is exemplified, but the scope of application of the present invention is not limited to this. Application to image forming apparatuses other than the wide format is also possible. In addition, the present invention is not limited to graphic art (printing) applications, but also includes a wiring drawing device for electronic circuit boards, a device for manufacturing various devices, and resist printing using a resin liquid as a functional liquid for ejection (equivalent to “ink”). The present invention can be variously applied to an image forming apparatus capable of forming an image pattern, such as an apparatus or a fine structure forming apparatus.

  In the above-described embodiment, an ink jet printer is exemplified as the image forming apparatus 100. However, the present invention is not limited to this, and it is needless to say that the present invention can be applied to any system that forms dots on the recording medium 40.

10c, 10d, 12c, 12d ... transition lines 14, 16, 18 ... transition transition lines 20 ... recording characteristic determination unit 24 ... information storage unit 26 ... monochromatic color chart 36 ... darkness table 40 ... recording medium 42 ... color patch 61 ... color Phase difference receiving unit 70 ... Recording characteristic setting unit 100 ... Image forming apparatus 106 ... Recording head 160 ... Control device 170 ... Conveyance driving unit 172 ... Main scanning driving unit 186 ... Image input I / F
188 ... Head drive circuit

Claims (6)

A recording head for forming dots of a plurality of colors including dark and light colors that are similar colors on a recording medium;
An image processing unit that converts an input image signal into a control signal using a recording characteristic that represents a dot recording rate for each density gradation level; and
A head drive circuit that controls the recording head based on the control signal converted by the image processing unit under relative movement between the recording head and the recording medium;
The image processing unit
A hue difference receiving unit that receives hue difference information relating to a hue difference between the color of the dark flat mesh image and the color of the light flat mesh image on the recording medium;
The larger the hue difference obtained by referring to the hue difference information received by the hue difference receiving unit, the wider the mixed color range that is the density gradation region where the dark color and the light color coexist, and the recording characteristics are set. have a record characteristic setting unit,
The recording characteristic setting unit starts from one color on a hue transition line when the light color is alone within the color mixture range, and ends as one color on a hue transition line when the dark color is alone. When the hue transitions along the transition transition line, the recording characteristics having a wide color mixing range are set so that the slope of the transition transition line connecting the light transition line and the dark transition line becomes small. An image forming apparatus. The image forming apparatus according to claim 1.
The recording head ejects ink droplets that are cured by irradiation with actinic rays toward the recording medium,
An image forming apparatus, further comprising: a curing light source that irradiates the actinic ray to an image formed on the recording medium by adhering the droplets ejected from the recording head. The image forming apparatus according to claim 2.
An information storage unit that stores the two or more recording characteristics having different color mixing ranges in association with the type of ink;
The hue difference receiving unit receives the type of the ink used for forming the image as the hue difference information,
The recording characteristic setting unit reads and sets the recording characteristic corresponding to the ink type received by the hue difference receiving unit from the two or more recording characteristics stored in the information storage unit. An image forming apparatus. The image forming apparatus according to claim 3.
Using a colorimetric data relating to the color of the flat halftone image formed on the recording medium by the recording head, further comprising a recording characteristic determining unit that determines the recording characteristic according to the hue difference;
The image forming apparatus, wherein the information storage unit stores the recording characteristics determined by the recording characteristic determination unit. The image forming apparatus according to claim 1, wherein:
The image forming apparatus, wherein the recording characteristic setting unit sets the recording characteristic such that a lower limit value of the color mixture range is monotonously non-increasing as the hue difference increases. Using the recording head that forms dots of multiple colors including dark and light colors that are similar colors on the recording medium, and the recording characteristics that indicate the dot recording rate for each density gradation level, the input image signal is controlled as a control signal. An image processing unit that converts the recording head, and a head drive circuit that controls the recording head based on the control signal converted by the image processing unit under relative movement between the recording head and the recording medium. An image forming method using an apparatus,
Receiving hue difference information relating to a hue difference between the color of the dark flat mesh image and the color of the light flat mesh image on the recording medium;
Setting the recording characteristics with a wider mixed color range, which is a density gradation region in which the dark color and the light color coexist, as the hue difference obtained by referring to the received hue difference information increases .
In the step of setting the recording characteristics, in the mixed color range, the light color starts from one color on the hue transition line alone and the dark color ends on one color on the hue transition line alone The recording characteristics having a wide color mixing range so that the transition of the transition transition line connecting the light transition line and the dark transition line becomes small when the hue transitions along the transition transition line an image-forming method, comprising that you set the.

Images