JPH10315506A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an ink jet printer in which the image quality can be enhanced while suppressing the manufacturing cost. SOLUTION: The ink jet printer prints an image having density on a recording sheet by printing dots of a normal color ink or a photocolor ink depending on an image data. In such an ink jet printer, a matrix is set such that dots 503 having a single diameter are printed when a normal color ink is used and dots 501, 502 having a plurality of diameters are printed when a photocolor ink is used.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus using inks having different densities according to image data.

[0002]

2. Description of the Related Art An ink jet printer using a piezoelectric element (PZT) as a print head is conventionally known. In such a print head, a pulse voltage corresponding to the image data is applied to the piezoelectric element, and the ink in a predetermined container (ink channel) is pressurized by the distortion of the piezoelectric element caused by the application of the pulse voltage, and the ink is applied to the ink channel. The ink drop flies toward the recording sheet from the provided nozzle. An image based on image data transmitted from a personal computer or the like is formed on the recording sheet by these ink drops flying.

Further, a technique for printing an image having a color using the above-described ink jet printer is known. In such an ink jet printer, yellow, magenta, cyan, and black (hereinafter, referred to as Y,
M, C, and K) may be used to print a full-color image. further,
In addition to these normal colors with normal density,
There is known a technique for reproducing a smooth image having more gradations by using a photo color having a lower density (lighter color) than a normal color. As the photo color, magenta of normal color, photo magenta of lower density than cyan, and photo cyan (hereinafter sometimes referred to as Mp and Cp, respectively) are used.

In the above-described ink-jet printer, apart from the technique of printing a color image, an ink having a different size is generated in the piezoelectric element by changing the amplitude of a pulse voltage applied to the piezoelectric element, thereby causing the ink to fly. A technique for adjusting the amount of a drop is known. By adjusting the amount of the ink drop, ink dots having different diameters are printed on the recording sheet. In addition, a matrix in which dots of ink having different diameters are arranged on a plane is made to correspond to one pixel of an image to be printed, and a pattern of dots (dot matrix) in the matrix is adjusted according to the gradation of the image. By changing, more gradations are expressed.

[0005] The following is a dot matrix constituting an image printed by the first to third conventional ink jet printers using such techniques.
These dot matrices correspond to the density of the image to be printed and each of the dot matrices is specified by a numerical value starting from 0 following the tone. For example, in the case of a dot matrix having five gradations, each of the dot matrices has gradations 0 to 0.
It is specified by gradation 4. 21 to 24 showing dot matrices of the first to third conventional ink jet printers, the number of gradations for specifying these gradations is shown above each dot matrix.

FIG. 21 is a diagram showing a dot matrix of five gradations constituting an image printed by the first conventional ink jet printer.

In the ink jet printer of the first conventional example, only one type of dot constitutes a dot matrix, and five gradations can be expressed by using a matrix of two rows and two columns.

FIG. 22 is a diagram showing a dot matrix of 15 tones constituting an image printed by the second conventional ink jet printer.

In the ink jet printer of the second conventional example, the dots constituting the dot matrix are of two types, normal ink and photo ink, and the matrix can be expressed in 2 rows and 2 columns to express 15 gradations. it can.

FIG. 23 and FIG. 24 are diagrams showing a dot matrix of 28 gradations constituting an image printed by a third conventional ink jet printer.

In the third conventional ink jet printer, the dots constituting the dot matrix are of three types having large, medium and small diameters, and 27 gradations can be expressed by forming the matrix in two rows and two columns. .

[0012]

However, even if the above-described first to third conventional ink jet printers are used, a user cannot print a sufficiently smooth and high quality image.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and an object of the present invention is to provide an image forming apparatus capable of improving image quality while suppressing production costs.

[0014]

According to the first aspect of the present invention, a first ink having a certain density and a second ink having a lower density than the first ink are used in accordance with image data. This is an image forming apparatus that forms an image having a density on a sheet based on printing a dot on a sheet.

When the first ink is used, the image forming apparatus prints dots of a predetermined size.
When the second ink is used, a dot of one size is selected from a plurality of predetermined sizes and printed.

According to the first aspect of the present invention, a predetermined size dot using an ink having a certain density, and a plurality of predetermined size dots using an ink having a lower density than this ink are used. Is printed, an image having a density is formed on the sheet. This makes it possible to smoothly change the gradation of the density of a printed image without using a driving circuit for performing complicated processing, thereby improving the image quality while suppressing the production cost.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an ink jet printer which is one of embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a perspective view for explaining a schematic configuration of an ink jet printer 1 according to a first embodiment of the present invention.

The ink jet printer 1 is used for paper or OH
An inkjet head 3 which is an inkjet type print head for printing on a recording sheet 2 which is a recording medium such as a P sheet, a carriage 4 holding the inkjet head 3, and the carriage 4 is arranged in parallel with the recording surface of the recording sheet 2. Oscillating shafts 5 and 6 for reciprocating movement, a driving motor 7 for reciprocatingly driving the carriage 4 along the oscillating shafts 5 and 6, and an idle pulley for changing the rotation of the driving motor 7 to reciprocating movement of the carriage 4 8, a timing belt 9.

The ink jet printer 1 has a platen 10 also serving as a guide plate for guiding the recording sheet 2 along the transport path, and a paper holding plate 11 for holding the recording sheet 2 between the platen 10 and preventing floating. , Recording sheet 2
Roller 12, spur roller 13 for discharging
The ink jet head 3 includes a recovery system 14 that cleans the nozzle surface of the inkjet head 3 that discharges ink and recovers poor ink discharge, and a paper feed knob 15 for manually transporting the recording sheet 2.

The recording sheet 2 is fed to a recording section where the ink jet head 3 and the platen 10 face each other by a sheet feeding device such as a manual feed or a cut sheet feeder (not shown). At this time, the rotation amount of a paper feed roller (not shown) is controlled, and the conveyance to the recording unit is controlled.

The ink jet head 3 uses a piezoelectric element (PZT) as an energy source for flying ink. A voltage is applied to the piezoelectric element, causing distortion.
This distortion changes the volume of the channel filled with ink. Due to the change in the volume of the channel, ink is ejected from nozzles provided in the channel, and recording on the recording sheet 2 is performed.

The carriage 4 scans the recording sheet 2 in the horizontal direction by a drive motor 7, an idle pulley 8, and a timing belt 9, and the inkjet head 3 mounted on the carriage 4 records an image of one line. 1
Each time the recording for the line is completed, the recording sheet 2 is fed in the vertical direction and sub-scanned, and the next line is recorded.

The image is recorded on the recording sheet 2 in this manner, and the recording sheet 2 that has passed through the recording section has a discharge roller 12 disposed on the downstream side in the transport direction and a spur roller 13 contacting the discharge roller 12 at a constant pressure. Is discharged by.

Next, the carriage 4 will be described with reference to FIGS.
The peripheral configuration and the configuration of the inkjet head 3 will be described.

FIG. 2 is a perspective view for explaining the configuration around the carriage 4. As shown in FIG. An ink cartridge 40 containing ink and having a vent 404 is provided around the carriage 4.
3, a casing 401 for accommodating the ink cartridge 403, a casing lid 405, and an ink cartridge 4
An ink receiving pin 402 for receiving the ink to the inkjet head 3 while making the ink jet head 03 detachable;
05 when the casing lid 405 is closed.
Clutch 406 and urging clutch stop 407 for fixing the ink cartridge 403 and the ink cartridge 403 together with the casing lid 406 while pushing the ink cartridge 403 in the direction opposite to the direction in which the ink cartridge 403 is stored (the direction of the arrow D3). And a leaf spring 408 for holding. As the carriage 4 moves in the direction of arrow D1 shown in the figure, the recording sheet is scanned in the main direction, and ink drops are ejected in the direction of arrow D2.

The ink in the ink cartridge 403 includes seven colors of normal inks of yellow, magenta, cyan, and black and photo inks of magenta, cyan, and black. Hereinafter, these compositions will be described.

The normal yellow ink contains 74.5% of water as a solvent, 11.0% of polyhydric alcohol / diethylene glycol (DEG), 6.5% of polyhydric alcohol ether / triethylene glycol monobutyl ether (TGB), Thickener / polyethylene glycol (PEG)
4.5% of # 400. In addition, dye / Ba is used as a coloring material.
yer Y-CA51092 2.5%, surfactant / Olfin E1010 0.8% as additive, p
H regulator / NaHCO ₃ 0.2%.

The normal magenta ink contains 74.5% of water, 11.0% of polyhydric alcohol / diethylene glycol (DEG), 6.5% of polyhydric alcohol ether / triethylene glycol monobutyl ether (TGB) as a solvent, Thickener / polyethylene glycol (PEG)
4.5% of # 400. In addition, dye / BA
2.5% SF Red FF-3282, 0.8% surfactant / Olfin E1010 as an additive,
Contains 0.2% of pH adjuster / NaHCO ₃ .

Normal cyan ink contains 74.0% of water as a solvent, 11.0% of polyhydric alcohol / diethylene glycol (DEG), 6.5% of polyhydric alcohol ether / triethylene glycol monobutyl ether (TGB), Thickener / polyethylene glycol (PEG)
4.5% of # 400. In addition, dye / Ba is used as a coloring material.
It contains 3.0% of yer CY-BG, 0.8% of surfactant / Olfin E1010 and 0.2% of pH adjuster / NaHCO ₃ as additives.

The normal black ink contains 77.9% of water as a solvent, 6.0% of polyhydric alcohol / diethylene glycol (DEG), 6.0% of polyhydric alcohol ether / triethylene glycol monobutyl ether (TGB), Thickener / polyethylene glycol (PEG)
4.5% of # 400. In addition, dye / Ba is used as a coloring material.
Yer BK-SP is contained 4.6%, and as an additive, 0.8% surfactant / Olfin E1010 and 0.2% pH adjuster / NaHCO ₃ are contained.

The photo magenta ink used as a solvent was 76.3% of water, 11.0% of polyhydric alcohol / diethylene glycol (DEG), 6.5% of polyhydric alcohol ether / triethylene glycol monobutyl ether (TGB), Thickener / polyethylene glycol (PEG)
4.5% of # 400. In addition, dye / BA
0.7% SF RED FF-3282, 0.8% surfactant / Olfin E1010 as an additive,
Contains 0.2% of pH adjuster / NaHCO ₃ .

The photocyan ink has a water content of 7 as a solvent.
6.2%, polyhydric alcohol / diethylene glycol (D
EG) and polyhydric alcohol ether / triethylene glycol monobutyl ether (TGB) at 11.0%.
5%, thickener / polyethylene glycol (PEG) # 4
00 of 4.5%. Dye / Baye
r CY-BG 0.8%, surfactant: Olfine E1010 0.8%, pH adjuster / N
Contains 0.2% of aHCO ₃ .

The photo black ink contains 81.3% of water as a solvent, 6.0% of polyhydric alcohol / diethylene glycol (DEG), 6.0% of polyhydric alcohol ether / triethylene glycol monobutyl ether (TGB), Thickener / polyethylene glycol (PEG)
4.5% of # 400. In addition, dye / Ba is used as a coloring material.
It contains 1.2% of yer BK-SP, 0.8% of surfactant / Olfin E1010 and 0.2% of pH adjuster / NaHCO ₃ as additives.

FIG. 3 to FIG.
FIG. 3 is a diagram for explaining a configuration (see FIG. 2). FIG. 3 is a perspective view showing the assembly of the inkjet head 3, FIG. 4 is a top view of the inkjet head 3, and FIG. 5 is a line XX of FIG. It is sectional drawing.

As shown in FIG. 4, the ink-jet head 3 includes a normal yellow ink head 31, a normal magenta ink head 32, and a normal magenta ink head 32 for discharging yellow, magenta, and cyan normal inks, respectively.
A normal cyan ink head 33, a photo magenta ink head 34, and a photo cyan ink head 35 for ejecting magenta and cyan photo inks.
And a normal black ink head 36 and a photo black ink head 37 for discharging black normal ink and black photo ink.

The heads 31 to 37 corresponding to each color in the ink jet head 3 include a nozzle plate 301 having nozzles for discharging ink drops, a common ink chamber plate 302 for forming an ink flow path, an inlet plate 303, Channel plate 304, partition 30
5, a piezoelectric element 306 that generates a distortion by applying a voltage to fly an ink drop, and a ceramic substrate 307. Also,
The side portion is formed by a head holder 308, and lead frames 315 and 316 are connected to the piezoelectric element 306.

As shown in FIG. 5, each head 31-37 is provided with a common ink chamber 31 by each plate 301-305.
2, an ink flow path including an ink chamber 313 and a nozzle 314 is formed.
8, an ink introduction path 311 and a receiving pin 402
2 (see FIG. 2).
Is connected and ink is supplied.

The operation of the ink jet head 3 having such a configuration is controlled by the control unit of the ink jet printer 1. Head discharge drive unit 1 of control unit
05 (see FIG. 6), the lead frames 315, 31
A predetermined pulse voltage based on the image data is applied during 6, and the piezoelectric element 306 is deformed in a direction to push the partition 305. The deformation of the piezoelectric element 306 is transmitted to the partition 305, which pressurizes the ink in the ink chamber 313,
The ink drop 320 flies toward the recording sheet 2 (see FIG. 1) via.

Next, the control section of the ink jet printer 1 will be described. FIG. 6 is a block diagram for explaining the configuration of the control unit of the inkjet printer 1.

The control unit of the ink jet printer 1 is C
PU 101, RAM 102, ROM 103, data reception unit 104, head ejection drive unit 105, head movement drive unit 106, paper feed motor drive unit 107, recovery system motor drive unit 108, various sensor units 109 Contains.

The CPU 101 for controlling the whole executes a program stored in the ROM 103 by using the RAM 102 as necessary. The program includes a head ejection drive unit 105, a head movement drive unit 106, a paper feed motor drive unit based on image data read from the data reception unit 104, which is connected to a host computer or the like and receives image data to be recorded. 107, various sensor units 1
09 to control an image recording on the recording sheet 2 and, if necessary, the recovery system motor drive unit 108 and various sensor units 109 to recover the nozzle surface of the inkjet head 3 to a good state. And a part for.

Under the control of the CPU 101, the head ejection drive unit 105 applies a pulse voltage corresponding to image data to
06, the head movement drive unit 106 drives the drive motor 7 that moves the carriage 4 holding the inkjet head 3, and the paper feed motor drive unit 107 drives the paper feed roller. Further, based on the control of the CPU 101, the recovery system motor drive unit 108 drives a motor or the like necessary to recover the nozzle surface of the inkjet head 3 to a good state.

As described above, the pulse voltage corresponding to the image data is applied to the piezoelectric element 306 from the head ejection drive unit 105, and the image data includes a dot stored in the ROM 103 in advance as described later. Processing is performed so that the pattern is printed in accordance with the density gradation.

Next, a procedure for processing the above image data will be described. FIG. 7 shows the CPU 1 for image data.
FIG. 2 is a block diagram for explaining a procedure of a process performed by a first embodiment.

The image data of 256 gradations for each color corresponding to red, green, and blue (hereinafter sometimes referred to as R, G, and B, respectively) from the data receiving unit 104 (see FIG. 6)
The gradation is corrected by the gradation correction unit 1101. The R, G, and B image data subjected to the gradation correction is supplied to a color conversion unit 1012.
Is converted into image data corresponding to C, M, and Y. Next, in the black generation + UCR unit 1013, the converted C,
A gray component is separated from the M and Y image data (under color removal, also referred to as UCR), K image data is generated, and image data corresponding to Cp, Mp, and Kp is generated.

Further, these image data are subjected to dither processing in a dither processing unit 1014, and image data of 256 gradations for each color is supplied from the head ejection drive unit 105 to the piezoelectric element 30.
The data is converted into data corresponding to the pulse voltage applied to 6.

Hereinafter, a pattern of a dot printed as one pixel corresponding to one piece of the image data described above and an effect of using the dot in this manner will be described with reference to FIGS.

FIGS. 8 and 9 show the ink jet printer 1.
FIG. 4 is a diagram showing a dot pattern printed by the dot pattern.

A dot matrix corresponding to one pixel which forms an image printed by the ink jet printer 1 is composed of two rows and two columns. The dots 501 in the matrix indicating the gradation 1 indicate small-diameter dots formed by photo ink, the dots 502 in the matrix indicating the gradation 2 indicate medium-sized dots formed by photo ink, and the dots 503 in the matrix indicating the gradation 3 indicate Shows large diameter dots with normal ink.
Actually, dots 501 and 502 have Cp, Mp,
Kp is used, and C, M,
Either Y or K is used.

In these figures, for example, the gradation 22
(Refer to FIG. 8) indicates that, when the image data corresponding to a certain pixel has a gradation of 22, the first row and the first column of a matrix partitioned into a 2 × 2 grid corresponding to the pixel And a small dot 501 made of photo ink is printed on the section of the first row and the second column with the center of the section aligned with the center of the section.
A large-diameter dot 503 of normal ink is printed on the section of the first row in the row, with the center of the section aligned with the center,
It is assumed that the medium dot 502 is printed by photo ink with the center of the section aligned with the section of the row.

FIG. 10 shows the reflection optical density (Op Op) using a dot pattern printed by the ink jet printer 1.
FIG. 9 is a diagram for comparing the optical density of a third conventional example with a reflection optical density using a dot pattern printed by an inkjet printer according to a third conventional example.

In FIG. 10, the horizontal axis represents FIGS. 8 and 9 (FIG.
FIG. 24) shows a graph in which gradations having a dot pattern as shown in FIG. 24 are taken, and the vertical axis represents reflection optical densities corresponding to these gradations. The white circle is the inkjet printer 1
, And the solid circles correspond to the third conventional inkjet printer.

The graph shown in FIG. 10 was obtained by measuring the reflection optical density of an image printed for each gradation, and used as a recording sheet "IJ Printer / Word Processor Paper High" manufactured by Kao Corporation. Grade color K
JHA4100 ”, an ink having the composition shown in FIG. 2 and a reflection optical density measuring device Sakura (now Konica Corporation) Sakura Densitometer.
r (PDA65) is used.

Using the above-described ink, a dot pattern having a gradation corresponding to the horizontal axis of the graph (the dot pattern printed by the ink jet printer 1 shown in FIGS. 8 and 9 and the dot pattern shown in FIGS. 23 and 24). 3) is printed on the recording sheet so as to be an area of 5 mm × 5 mm or more on the recording sheet.
From these, the reflection optical density was measured by the measuring device described above.

In any of the printed images of the ink jet printer 1 and the third conventional ink jet printer, the reflection optical density of the recording sheet itself corresponding to gradation 0 is about 0.1, The reflection optical density when the corresponding recording sheet was solid-coated was about 1.5.

Referring to the results, in the image printed by the ink jet printer of the third conventional example, the gradient γ of the reflection optical density with respect to the gradation is slightly larger than that of the ink jet printer 1 in gradation 0 to gradation 9. Taken, gradation 1
A value equivalent to that of the ink jet printer 1 is taken at 0 to 18. Further, in the third conventional ink jet printer, γ gradually decreases as the gradation increases, and γ takes a value close to 0 near gradation 27.

Referring to the results, 9 gradations are assigned to the reflection optical density range of 0.1 to 0.8 in the third conventional ink jet printer, and 15 gradations are assigned to the ink jet printer 1 in the range. In the range of 1.2 to 1.5 of the reflection optical density, 10 gradations are allocated in the third conventional inkjet printer, and 6 gradations are allocated in the inkjet printer 1. I understand.

This means that, particularly in an image printed by the ink jet printer 1, a fine difference in density in a low density portion (highlight portion) can be reproduced as compared with the ink jet printer of the third conventional example. Is shown.

As described above, multi-value printing is performed when photo ink is used (dots having a plurality of diameters are printed), and binary printing is performed when normal ink is used (having a single diameter). By setting a matrix for printing dots, it is possible to print an image having a smooth gradation particularly in a highlight portion. By thus smoothly reproducing the image of the highlight portion, which is an area sensitive to human vision, the quality of the entire image is improved. In addition, an ink jet printer that prints such an image does not require a driving circuit for performing complicated processing, and thus does not increase the production cost.

Next, the dot pattern printed by the ink jet printer according to the second embodiment will be described. The overall configuration of the inkjet printer according to the second embodiment, the configuration of the print head, the configuration of the control unit, and the like are as follows.
This is the same as the inkjet printer according to the first embodiment.

FIGS. 11 to 18 are diagrams showing dot patterns printed by the ink jet printer according to the second embodiment.

A dot matrix corresponding to one pixel, which constitutes an image printed by the ink jet printer according to the second embodiment, has two rows and three columns. A dot 506 in the matrix indicating the gradation 1 indicates a small diameter dot formed by photo ink, and a dot 5 in the matrix indicating the gradation 2
Reference numeral 07 denotes a medium-sized dot formed by photo ink, dots 508 in a matrix indicating gradation 4 indicate large-sized dots formed by photo ink, and dots 509 in a matrix indicating gradation 9 indicate large-sized dots formed by normal ink. ing. Actually, dots 506 to 508 have Cp, M
Either p or Kp is used, and C,
One of M, Y, and K is used.

In these figures, for example, the gradation 47
(Refer to FIG. 12) indicates that, when the image data corresponding to a certain pixel has a gradation of 47, the first row and first column of the matrix partitioned into a 2 × 3 grid for that pixel A small-diameter dot 506 made of photo ink is printed on the section, the section of the second row and the second column, with the center of the section aligned with the center of the section.
Medium diameter dots 507 made of photo ink are printed on the section of the third row and the center of the section, and large diameter dots 509 of normal ink are printed on the section of the first row and the second column. Is printed.

With such a dot matrix, the same effects as those of the ink jet printer of the first embodiment can be obtained, and an image having a smooth gradation can be printed particularly in a highlight portion. By thus smoothly reproducing the image of the highlight portion, which is an area sensitive to human vision, the quality of the entire image is improved. In addition, an ink jet printer that prints such an image does not require a driving circuit for performing complicated processing, and thus does not increase the production cost.

Subsequently, the image quality indexes of the ink jet printers of the first and second embodiments and the first to third conventional ink jet printers are calculated.

The image quality index Q is a number used as a guide when evaluating the smoothness of an image, and M represents the pixel density (p
pixels / mm), where N is the number of gradations, and Q = M × √
It is a number represented by (N-1). This is illustrated in FIG.
It looks like 9.

In the ink jet printer of this embodiment, the dot density is about 400 (360 dpi), and the dot pitch is about 63.5 μm.

In the ink jet printer of the first embodiment, the dot matrix has two rows and two columns, the pixel pitch is 63.5 μm × 2 = 127 μm, and the pixel density M is M = 1/127 μm ≒ 8 pixels / mm. Becomes Accordingly, the image quality index Q1 is calculated as follows: Q1 = 8 × √ (28−
1) It becomes $ 42.

In the ink jet printer according to the second embodiment, the image quality index Q2 in the main scanning direction (the direction in which three dots are arranged in one pixel) is calculated. Pixel density M
Is M = 1 / (63.5 μm × 3) ≒ 5.3 pixels
s / mm, and the image quality index Q2 is Q2 = 5.3 × √.
(190-1) ≒ 73.

In the same manner, the image quality index is calculated for the first to third conventional ink jet printers. The first to third conventional inkjet printers include:
As in the ink jet printer of the first embodiment, the pixel density is 8 pixels / mm in each case.

In the first conventional ink jet printer, the number of gradations N is 5, and the image quality index Q3 is Q3 = 8 × √.
(5-1) = 16. In the ink jet printer of the second conventional example, the number of gradations N is 15, and the image quality index Q4
Is Q4 = 8 × {(15-1)} 30. Also, the third
In the conventional ink jet printer of the above, the number of gradations N is 2
8, and the image quality index Q5 is Q5 = 8 × {(28-1)}.
42.

Generally, as an OA device, the image quality index Q is 3
It is used in the range of 2 <Q <64. According to the calculation of the image quality indices Q1 to Q5 as described above, those having Q in this range are the ink jet printer of the first embodiment and the ink jet printer of the third conventional example. The printer having the above Q is the ink jet printer of the second embodiment. Images printed with these inkjet printers are sufficiently smooth as full-color images printed for OA.

The ink jet printer of the first embodiment and the third conventional ink jet printer have the same value of Q. However, for the reason described with reference to FIG. It can be said that the inkjet printer of the embodiment can print a smoother image. In the ink jet printer according to the second embodiment, the image quality index Q4 exceeds 64, and a smoother full-color image is printed.

In the above embodiment, an ink jet printer having a dot matrix of 2 rows and 2 columns and an ink jet printer having a dot matrix of 2 rows and 3 columns have been described.
The dot matrix can be formed in three rows and three columns or more as in the following modified example of the ink jet printer of the present invention.

FIG. 20 is a diagram for explaining a dot matrix constituting an image printed by an ink jet printer according to a modification of the present invention.

The dot matrix of the ink jet printer of this modified example is composed of four rows and four columns. In this dot matrix, small diameter dots 511 made of photo ink, medium diameter dots 512 made of photo ink, and large diameter dots 513 made of normal ink are used. . When the image quality index Q6 is calculated in the same manner as in the above-described inkjet printer, Q
6 = 4 × √ (16-1) ≒ 16, and Q6 is an image quality index of the same level as that of Conventional Example 1. These facts indicate that the pixel index is the product of the primary pixel density and the 1 / 2-order gradation number, and that the pixel index is more susceptible to the pixel density than the gradation number. Therefore, when increasing the dot matrix, it is necessary to sufficiently increase the resolution.

As described above, in the ink jet printer according to the present embodiment, it is possible to smoothly change the gradation of the density of a printed image without using a driving circuit for performing complicated processing. Image quality can be improved while suppressing production costs.

[Brief description of the drawings]

FIG. 1 is a perspective view for explaining a schematic configuration of an inkjet printer 1 according to a first embodiment of the invention.

FIG. 2 is a perspective view for explaining a configuration around a carriage 4. FIG.

FIG. 3 is a perspective view showing the assembly of the inkjet head 3.

4 is a top view of the inkjet head 3. FIG.

FIG. 5 is a sectional view taken along line XX of FIG. 4 for explaining the flow of ink in the inkjet head 3.

FIG. 6 is a block diagram for explaining a configuration of a control unit of the inkjet printer 1.

FIG. 7 is a block diagram for explaining a procedure of a process performed by a CPU 101 on image data.

FIG. 8 is a first diagram illustrating a pattern of dots printed by the inkjet printer 1.

FIG. 9 is a second diagram showing a pattern of dots printed by the inkjet printer 1.

FIG. 10 is a diagram for comparing a reflection optical density using a dot pattern printed by the ink jet printer 1 with a reflection optical density using a dot pattern printed by the third conventional ink jet printer. is there.

FIG. 11 is a first diagram illustrating a dot pattern printed by the ink jet printer according to the second embodiment.

FIG. 12 is a second diagram illustrating a dot pattern printed by the ink jet printer according to the second embodiment.

FIG. 13 is a third diagram showing a dot pattern printed by the ink jet printer according to the second embodiment.

FIG. 14 is a fourth diagram illustrating a dot pattern printed by the ink jet printer according to the second embodiment.

FIG. 15 is a fifth diagram illustrating a dot pattern printed by the ink jet printer according to the second embodiment.

FIG. 16 is a sixth diagram illustrating a dot pattern printed by the inkjet printer according to the second embodiment.

FIG. 17 is a seventh diagram illustrating a dot pattern printed by the ink jet printer according to the second embodiment.

FIG. 18 is an eighth diagram illustrating a dot pattern printed by the ink jet printer according to the second embodiment.

FIG. 19 is a diagram illustrating an image quality index with respect to the number of gradations and the pixel density.

FIG. 20 is a diagram for explaining a dot matrix forming an image printed by an ink jet printer according to a modified example of the invention.

FIG. 21 is a diagram showing a dot matrix of five gradations that constitutes an image printed by the first conventional inkjet printer.

FIG. 22 is a diagram showing a dot matrix of 15 tones constituting an image printed by the second conventional inkjet printer.

FIG. 23 is a first diagram showing a dot matrix of 28 gradations that forms an image printed by a third conventional inkjet printer.

FIG. 24 is a second diagram showing a 28-gradation dot matrix constituting an image printed by the third conventional inkjet printer.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Ink jet printer 2 Recording sheet 3 Ink jet head 101 CPU 103 ROM 105 Head ejection drive part 306 Piezoelectric element 307 Ceramic substrate 314 Nozzle 320 Ink drop 501 Small dot with photo color ink 502 Medium dot with photo color ink 503 Normal color Large dots with ink

Claims (1)

[Claims] 1. A method for printing dots on a sheet based on printing dots using a first ink having a certain density and a second ink having a lower density than the first ink according to image data. An image forming apparatus for forming an image having a density of: when using the first ink, prints a dot of a predetermined size, and when using the second ink,
An image forming apparatus, wherein a dot of one size is selected from a plurality of predetermined sizes and printed.