JP2882646B2 - Color image recording device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a recording apparatus, and more particularly, to a color image recording apparatus that records a color image by a recording head having a plurality of recording elements.

2. Description of the Related Art Conventionally, as an example of this type of apparatus, there is an apparatus that converts read image data into a digital signal, performs predetermined data processing on the digital signal, and then records an image using a recording head. In such an apparatus, there is a case where there is non-uniformity of the recording density among the recording elements due to the characteristic variation due to the manufacturing process of the recording head and the characteristic variation of the material of the recording head, and the density unevenness occurs in the recorded image. there were.

For this reason, an apparatus has been proposed in which test printing is performed before recording, the output characteristics of each recording element are checked, the output characteristic data is stored, and input image data is corrected based on the stored data. The aforementioned uneven density was prevented.

The above-described correction will be described with reference to FIGS.

As shown in FIG. 8A, a plurality of recording elements 31 are arranged in the recording head 30. Even when the input signals to these recording elements 31 are uniform as shown in FIG. 3B, the density of the recorded image becomes non-uniform as shown in FIG. May occur.

In such a case, the input signal is corrected as shown in FIG. That is, a large input is given to a printing element in a low density portion, and a small input is given to a printing element in a high density portion.

In such a correction of the input signal, in a recording method capable of modulating the dot diameter or the dot density, the dot diameter recorded by each recording element can be modulated according to the input. For example, in a configuration in which the recording element ejects ink by the piezo method, the voltage or pulse width of the electric pulse applied to each piezo element is applied, and when the recording element is a heat transfer type heating element, the voltage or pulse width is applied to each heating element. The voltage or pulse width of the electrical pulse to be changed according to the input signal,
The dot diameter or dot density recorded by each recording element can be made uniform. As a result, the density distribution becomes uniform as shown in FIG.

Also, in a recording method in which it is impossible or difficult to modulate the dot diameter or the dot density, the number of dots is changed according to the input signal, and the recording element in the portion where the recording density is low prints many dots. The recording element in the portion where the density is high records a small number of dots, so that the density distribution can be similarly made uniform as shown in FIG. 8 (E).

The correction amount of the correction as shown in FIG. 8 (D) is obtained as follows, for example.

As an example, FIG. 9 to FIG. 9 show a case of correcting the density unevenness of a so-called multi-recording head having 4736 ejection ports.
This will be described with reference to FIG.

The density unevenness distribution when recording with a certain uniform image signal S is
Assume that the state is as shown in FIG. First, to determine the concentration 0D ₁ ~0D ₄₇₃₆ in the portion corresponding to each discharge port, the average concentration Then, OD / OD _k is calculated for each discharge port.

Here, for example, if the relationship between the value of the input image signal corresponding to the n-th ejection port and the recording density by this ejection port is as shown in FIG. 10, the density OD to be recorded is corrected by correcting the signal S. _In order to make the density uniform with _n being the average density OD, the signal S may be corrected by α × S = OD / OD _n × S.

For this purpose, the image signal may be subjected to table conversion as shown in FIG. In FIG. 11, a straight line A is a straight line having a slope of 1.0. In this relationship, the input is output without any conversion. On the other hand, the straight line B has a slope α = OD
/ OD _n is linear, the output signal when the input S in this relationship is the alpha · S.

Therefore, if the recording head is driven by performing a table conversion of the relationship shown by the straight line B in FIG. 11 on the input image signal corresponding to the n-th ejection port, the ink droplet ejected from this ejection port The density of the recorded part will be equal to the OD.

When the above-described processing is performed for all the ejection ports, the density unevenness is corrected, and the nonuniformity of the recording density between the ejection ports is eliminated. That is, the density unevenness can be corrected by previously obtaining data indicating what table conversion should be performed on an image signal corresponding to which ejection port.

Further, the correction of the density unevenness is not limited to the correction of the image signal (recording data) using the table as described above.
As described above, it can also be realized by correcting the voltage and pulse width of the drive pulse (drive signal), and further, the number of dots to be recorded.

However, it is possible to correct the density unevenness by the above-described method, but in this method, even if the unevenness can be corrected once, the recording density distribution may become non-uniform thereafter. Therefore, it is necessary to change the correction amount of the input signal.

For example, in the case of the ink jet recording method, a deposit from the ink adheres to the vicinity of the ink discharge port or a foreign substance adheres from outside due to the use thereof, resulting in uneven recording density between the discharge ports. There is.

Further, even in the thermal transfer recording method, there is a case where the density distribution is changed due to deterioration and deterioration of each heating element.

In such a case, there is a problem that the density unevenness is insufficiently corrected with the initially set input correction amount, so that the density unevenness gradually becomes conspicuous with use.

In response to this, the present applicant has proposed a method of regenerating density unevenness correction data by providing a density unevenness reading unit in a printing apparatus and periodically reading the density unevenness distribution.
According to this method, even if the density unevenness distribution of the recording head changes, correction data is re-created in accordance with the change, so that a uniform image without density unevenness can be always maintained.

However, if non-uniformity correction data is created using the data obtained by reading as it is, the correction data becomes extremely noisy. It is for the following reasons.

As shown in FIG. 12, it is assumed that the density unevenness correction dot pattern is hit, for example, as shown by D in the figure.

As can be seen from the figure, there is a variation in dot size in the x direction in the figure, that is, at the timing when the dot pattern is printed. This is because the dot diameter may fluctuate depending on the time of recording in each recording element of the multi-head. (Note that this fluctuation is not enough to cause the density unevenness described above.) Therefore, the correction data greatly varies depending on at what point the density is measured.

Further, for example, the dots are not arranged at equal intervals in the y direction because the printing position accuracy of the dots varies. When measuring at such times, for example,
The density corresponding to the reading element 203b is high, and the reading element 203c
Is read lightly.

For this reason, the read data is averaged for a predetermined area in the x and y directions, and the result is used as data of a read pixel at the center of the area. The area to be averaged at this time is called an averaging area.

The size of the averaging region is an important factor for density unevenness correction. That is, if this value is reduced, a fine density unevenness pattern can be read. However, the difference between the minimum value and the maximum value of the density read by each read pixel increases, and a correction table and a pulse width necessary for correction for each recording element are required. Needs to have a wide modulation range.

Also, if the averaging area is enlarged, a gradual change in density unevenness is read, so that the difference between the minimum value and the maximum value of density becomes small, and the dynamic range of the correction amount, that is, the range of the correction table or the like may be small. On the contrary, it becomes impossible to read a subtle uneven pattern.

[Problems to be Solved by the Invention] However, when performing the density unevenness correction using the above-described averaging area, the color image recording apparatus prints a color in which the density unevenness is conspicuous, such as magenta, for example. It is necessary to read a subtle uneven pattern. For this reason, if the averaging area is set to be small, density unevenness is relatively inconspicuous, such as yellow, and the dynamic range of the correction amount is too large for a color in which a gradual change in density unevenness needs to be read. It is difficult to properly correct each color, for example, the modulation circuit for the pulse width or the pulse width is too large or too complicated.

The present invention has been made in order to solve the above-described conventional problems, and by setting an averaging area for each color in density unevenness correction of color image recording, it is possible to appropriately correct density unevenness for each color. It is an object of the present invention to provide a color image recording apparatus capable of performing the operation.

[Means for Solving the Problems] For this purpose, in the present invention, a plurality of recording heads each having a plurality of recording elements and capable of forming images in different colors, and different colors recorded by each of the plurality of recording heads Acquisition means for acquiring an average density of a region corresponding to the recording element of the test pattern for each of the plurality of recording heads, and density unevenness correction data of the plurality of recording elements based on the average density acquired by the acquisition means. A color image forming apparatus comprising: a calculating unit that calculates for each of the plurality of recording heads; and a correcting unit that corrects image formation by the plurality of recording heads based on density unevenness correction data calculated by the calculating unit. Acquiring means for acquiring an average density of an area of the test pattern corresponding to a predetermined number of print elements set for each of the plurality of print heads; It is characterized by that.

Further, in a density unevenness correction method of a color image forming apparatus for forming an image using a plurality of recording heads each having a plurality of recording elements and capable of forming images in different colors, recording is performed by each of the plurality of recording heads. The average density of test patterns of different colors is obtained, and the average density of the test pattern area corresponding to a predetermined number of printing elements set for each of the plurality of print heads is obtained, based on the obtained average density. The density unevenness correction data of the plurality of printing elements is calculated for each of the plurality of print heads, and image formation by the plurality of print heads is corrected based on the calculated density unevenness correction data.

[Operation] According to the above configuration, an average density of a test pattern area corresponding to a predetermined number of recording elements set for each of a plurality of recording heads is acquired, and a plurality of average densities are acquired based on the acquired average density. Since the density unevenness correction data of the printing element is calculated for each of a plurality of print heads, the scale of a correction circuit such as a circuit for modulating a conversion table pulse width and the like is determined according to the size of the area where the average density is obtained. It can be determined for each recording head.

[Example] Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a schematic side view showing one embodiment of an ink jet recording apparatus according to the present invention.

In FIG. 1, 1Bk, 1y, 1m, 1c are black,
This is a bubble jet type recording head corresponding to yellow, magenta, and cyan ink colors, uses a heating resistor as a built-in ejection energy generator, and uses an air bubble generated in the ink when energized as a pressure source to eject ink droplets from an ejection port. And is fixed to the block 2. Each print head has 4736 ejection ports arranged at a density of 400 dpi. The density unevenness reading head 51 is also fixed to the block 2.

Reference numeral 3 denotes a capping unit, for example, for a standby.
At the time of non-recording, the block 2 is pulled up to the position shown by the one-dot chain line in the figure to cap the unit 3 so as to face the unit. Further, the capping unit 3 serves as a tray for waste ink fed from a recovery pump and an ink supply system (not shown) at the time of circulation recovery and extruded from the discharge port. The waste ink is guided to a waste ink tank (not shown).

Reference numeral 4 denotes an endless charging suction belt which is disposed opposite to each of the recording heads 1Bk, 1y, 1m, and 1c at a predetermined interval to convey a recording sheet, and 5 denotes each recording head via the charging suction belt 4. It is a back platen that is disposed to face the other side.

Reference numeral 6 denotes a paper feed cassette which contains recording sheets 7 such as plain paper and is detachably attached to the apparatus main body. Reference numeral 8 denotes a pickup roller which feeds out and feeds only one uppermost recording sheet 7. Reference numeral 9 denotes a transport roller for transporting the recording sheet 7 sent from the pickup roller 8 to the transport path 10, and reference numeral 11 denotes a transport roller disposed on the exit side of the transport path 10.

Reference numerals 13 and 14 denote heaters and fans for drying and fixing the ink droplets adhering to the recording sheet 7 by recording with hot air, 15 a discharge roller for discharging the fixed recording sheet 7 out of the mounting, and 16 a discharged recording sheet. 7 is a tray for sequentially stocking 7.

Next, the operation of the embodiment having the above configuration will be described.

First, the recording operation will be described. When a recording start operation is performed, a recording sheet 7 of a designated size is sent out of the paper feed cassette 6 by the pickup roller 8. The fed recording sheet 7 is transported by the transport rollers 9 and
By 11, it is rotated in a pre-charged state and is placed on the charging / absorbing belt 4 which has a planar shape by the back platen 5. The head of recording sheet 7 is head 1
In conjunction with the arrival at the lower portions of c, 1m, 1y, and 1Bk, the energy generator of each recording head is driven in accordance with image data via a drive circuit described later with reference to FIG. By this driving, ink droplets corresponding to the image information are ejected from the ejection ports onto the surface of the recording sheet 7 to perform recording.

When the recording sheet 7 has poor hygroscopicity, the droplets adhering to the surface do not dry and are rubbed to cause printing stains.
Forcible drying is performed by 13 and the fan 14 to fix. The recording sheet 7 on which the fixing has been completed is discharged to a tray 16 by a discharge roller 15.

As described above, a color image is formed by supplying a recording signal corresponding to each of the recording heads corresponding to the cyan, magenta, yellow, and black inks.

Hereinafter, the correction of uneven density according to the present invention will be described.

FIG. 2 is a block diagram showing a control configuration for driving a recording head of the ink jet recording apparatus shown in FIG.

In FIG. 2, reference numeral 101 denotes image data read by an image reading unit (not shown) in FIG. 1, reference numeral 102 denotes an image processing unit which performs processing such as logarithmic conversion, masking, UCR, color balance adjustment, and reference numeral 103 denotes image processing. Image signal after 104, ROM in which an unevenness correction table is stored, 105 is an image signal after unevenness correction,
106 is a binarization circuit, 107 is a binarized image signal, 108 is a head drive circuit, 109 is a head drive signal, 1Bk, 1y, 1m, and 1c are the multiple recording heads shown in FIG. The uneven density reading head shown in FIG.
Is a RAM, 115 is a CPU, 116 and 118 are unevenness correction signals, 117 is an unevenness correction RAM, and 130 is a color selection signal.

The signal 103 subjected to the image processing is stored in the unevenness correction table ROM 104.
Is converted to correct the unevenness of the recording head.

For example, when the conversion input is X and the conversion output is Y, the unevenness correction table 104 for cyan has a Y = Y
There is a conversion relationship represented by 61 correction straight lines whose slopes differ by 0.01 from 0.70X to Y = 1.30X, and the correction straight line for conversion is switched according to the unevenness correction signal 118.
For example, when a signal to be recorded at an outlet having a tendency to increase the dot diameter to be recorded is input, a correction straight line having a small inclination is selected, and when the ejection port has a small dot diameter, a correction straight line having a large inclination is selected. The image signal is corrected by the selection.

The unevenness correction RAM 117 stores a signal for selecting a correction straight line required for correcting unevenness of each head. For example, in the case of the cyan color, an unevenness correction signal having 61 types of values from 0 to 60 is stored for 256 ejection ports for each color, and the unevenness correction signal 118 is synchronized with the image signal 103 input to the ROM 104. Output. The corrected image signal 105 corrected by the relationship indicated by the selected correction straight line is binarized by a binarization circuit 106 using a dither method, an error diffusion method, or the like, and then input to a head drive circuit 108. . A head drive pulse is output, and recording is performed by discharging ink droplets from the recording heads 1Bk, 1y, 1m, and 1c.

By performing the above-described unevenness correction processing, the printing duty is reduced for the dots by the ejection openings that tend to have a high density, and the printing duty is high for the dots by the ejection openings that tend to be light. As a result, density unevenness of the recording head is corrected, and a uniform image is obtained.

However, as described above, the density unevenness pattern of the recording head changes with use, so that the unevenness correction signal becomes inappropriate and the image becomes uneven. Even in such a case,
In the unevenness correction signal rewriting mode, the unevenness correction data is rewritten.

In the unevenness correction data rewriting mode, the following operation is performed. In the case of cyan, first, all the conversion relations of 61 in the unevenness correction table 104 are inclined by a control signal (not shown).
A straight line of 1.0, and no unevenness correction is performed.
Subsequently, an unevenness correction pattern is output from a signal source (not shown), and is recorded by the head 1c. Here, a uniform halftone with a 50% duty is used as the unevenness correction pattern.

As shown in FIG. 1, uneven density reading heads 51 are provided beside the four recording heads. The density unevenness read head is a CCD having a read pixel density equal to the print element density of the print head, and reads the density unevenness of a pattern printed by the method described with reference to FIG.

Before entering the unevenness correction data rewriting mode, an ink color for which correction data is to be rewritten, that is, a recording head of the ink color is selected in advance from a recording color designation unit (not shown).
Thereby, the color selection signal 130 is input to the CPU 115.

The density data 112 read by the read head 51 is stored in the RAM 11
Once stored in 3. At this time, the density data to be stored is 100 in the x direction and 4736, the same as the number of pixels in the y direction, and a total of 100 × 4736 = 473600 density data is stored.

The average value of the density data of the averaged density corresponding to the color selection signal 130 among the density data is set as the density data recorded by the ejection port corresponding to the central pixel of the area.

For example, in the case of cyan color selection, as shown in FIG. 4, 250 pixel dots of 5 dots × 50 dots are set as an averaging area, and this average value is determined by the ejection port corresponding to the target pixel shown in FIG. The density data is used. The two pixels at both ends are 3
The same as the density data of the pixel.

The CPU 115 performs the following calculation based on the 4736 pieces of density data obtained by averaging the density data of the averaging area and corresponding to each of the ejection ports, and rewrites the unevenness correction data.

That is, assuming that 4736 density data are R ₁ , R ₂ ,..., R ₄₇₃₆ , these data are (R ₀ is a constant that satisfies R ₀ ≧ R _n , n = 1, 2,..., 4736) and is converted into a density signal C _n .

Next, average density Is calculated.

Subsequently, the concentration C _n corresponding to respective discharge ports, a measure of how far to the average concentration C is calculated as follows.

ΔC _n = C / C _n (n = 1,2,... 4736) Next, the signal correction amount ΔS _n according to ΔC _n is obtained by ΔS _n = K × ΔC _n (n = 1, 2,... 4736) .

Here, K is a coefficient determined by the gradation characteristics of the recording head.

Subsequently, a signal for selecting a correction straight line is obtained according to the signal correction amount ΔS _n obtained above, and in the case of cyan,
The non-uniformity correction signal having the value of the type is stored in the non-uniformity correction RAM 117 so as to correspond to 4736 ejection openings.

A correction straight line is selected according to each ejection port based on the unevenness correction data created in this way, and uneven density is corrected.

As is apparent from the above description, the unevenness correction table ROM 104 and the unevenness correction RAM 117 shown in FIG.
Are provided independently for each color, and the circuit scale is varied depending on the ink color.

For example, the unevenness correction required for the yellow color may be a gentle unevenness correction as described above. Therefore, according to the color selection signal 130 shown in FIG. 9 × 80 = 720 dots shown, and the average value can be used as the data of the ejection port corresponding to the target pixel.

As a result, the unevenness correction signal of cyan is fine in the arrangement direction of the ejection openings and the swing width of the signal is large as shown in FIG. 6, but the unevenness correction signal of yellow is smaller than that of cyan as shown in FIG. Becomes smaller.

Therefore, the unevenness correction table ROM 104 for yellow
As shown in FIG. 5, it is only necessary to have 21 correction lines each having a slope from Y = 0.9X to Y = 1.1X, each of which has a difference of 0.01, and the scale of the unevenness correction RAM 117 for storing the selection number of the correction line. Can be reduced.

In the above embodiment, as a means for calculating the average density of the averaged area, the density data obtained in this area is arithmetically averaged, but the present invention is not limited to this. The optical image may be read by an optical sensor, and the output may be set as the average density in a predetermined area. In this case, as a method of changing the size of the averaging region, there are a method of changing the aperture size of the optical sensor, a method of changing the illumination condition of the illumination system, and the like.

Further, in the above embodiment, the method of changing the digital signal by the correction table is adopted as the density unevenness correction means. However, the correction means of the present invention is not limited to this. May be used to change the dot diameter.

Further, although an example in which the density unevenness read head has a plurality of read elements has been described, the present invention can be similarly carried out even when reading is performed by a single read element.

In this case, the reading element scans in the x direction in FIG. 12, reads the data, performs a sub-scan in the y direction by one pixel, and scans again in the x direction, and stores the read data in the RAM once. . After that, a process of averaging data in a predetermined area may be performed as in the above embodiment.

Further, the data to be averaged does not necessarily have to be the density data itself, and may be light amount data before logarithmic conversion, or may be data subjected to another conversion. These may be determined according to the characteristics of the recording paper and the characteristics of the reading head.

Further, in the above-described embodiment, the density correction of the recording element according to the ink jet recording method has been described. However, the present invention can be applied to, for example, the correction of the recording element according to the thermal recording method.

[Effects of the Invention] As is clear from the above description, according to the present invention, the average density of the test pattern area corresponding to a predetermined number of print elements set for each of a plurality of print heads is acquired, and this acquisition is performed. The correction data for the density unevenness of a plurality of printing elements is calculated for each of a plurality of print heads based on the average density thus obtained. Can be determined for each recording head according to the size of the area in which is obtained.

As a result, it is possible to obtain a color image recording apparatus in which the storage means such as the ROM and the RAM and the arithmetic circuit for correction have an appropriate scale.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view of a color image recording apparatus according to an embodiment of the present invention, FIG. 2 is a block diagram showing a configuration for performing density unevenness correction in the apparatus shown in FIG. 1, and FIG. Is a diagram conceptually showing the contents of the unevenness correction table shown in FIG. 2, FIG. 4 is a conceptual diagram for explaining an averaging area according to an embodiment of the present invention, and FIG. 5 is a diagram in FIG. FIGS. 6 and 7 are diagrams conceptually showing the contents of the irregularity correction table shown, FIGS. 6 and 7 are diagrams showing the signals subjected to the irregularity correction in correspondence with the respective ejection ports of the recording head, and FIG. 8 (A). 8A and 8B are schematic front views showing a recording head and recording elements arranged on the recording head. FIGS. 8B and 8C show an uncorrected input signal for driving the recording head shown in FIG. FIG. 8 (D) is a diagram showing the density recorded by this. FIG. 9E is a diagram showing a corrected input signal for driving the print head shown in FIG. 9A, and a diagram showing the density recorded by the same. FIG. 9 is a diagram showing the density of the image recorded for each ejection port. FIG. 10 is a diagram showing different states, FIG. 10 is a diagram showing a difference between an average density and a recording density according to an image signal, and FIG. 11 is a diagram showing a relationship of a conversion table for converting and outputting an input image signal. FIG. 12 is a schematic diagram for explaining a method for reading uneven density. 1Bk, 1y, 1m, 1c: recording head, 51: density unevenness reading head, 102: image processing unit, 104: unevenness correction table ROM, 106: binarization circuit, 108: drive circuit, 113 … RAM, 115… CPU, 117… Mura correction RAM.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-1-160182 (JP, A) JP-A-1-180682 (JP, A) JP-A-1-181168 (JP, A) JP-A-1-181682 180681 (JP, A) JP-A-1-151376 (JP, A) JP-A-1-129667 (JP, A) JP-A-3-73665 (JP, A) (58) Fields investigated (Int. ⁶ , DB name) B41J 29/46 B41J 2/525 B41J 2/21

Claims (7)

(57) [Claims] A plurality of print heads each having a plurality of print elements and capable of forming images in different colors, and areas corresponding to print elements of test patterns of different colors printed by each of the plurality of print heads. Acquiring means for acquiring the average density of each of the plurality of recording heads, and calculating the density unevenness correction data of the plurality of recording elements for each of the plurality of recording heads based on the average density acquired by the acquiring means. A color image forming apparatus comprising: a correction unit configured to correct image formation by the plurality of recording heads based on density unevenness correction data calculated by the calculation unit; A color image forming apparatus for acquiring an average density of a region of the test pattern corresponding to a predetermined number of printing elements set in (1). 2. The method according to claim 1, wherein the acquiring unit detects a density corresponding to each recording element of the test pattern, and averages the detected densities of the predetermined number of recording elements, thereby obtaining the predetermined number of recording elements. 2. The color image forming apparatus according to claim 1, wherein an average density of an area of the test pattern corresponding to an element is obtained. 3. An average density of an area of the test pattern corresponding to the predetermined number of recording elements by detecting a density of a range of the test pattern corresponding to the predetermined number of recording elements. The color image forming apparatus according to claim 1, wherein the color image is obtained. 4. A printing apparatus according to claim 1, wherein said correction means has a conversion table for converting print data to said plurality of print elements based on said density unevenness correction data, for each of said plurality of print heads. 2. The color image forming apparatus according to claim 1, wherein the capacity corresponds to the set number. 5. The image forming apparatus according to claim 1, wherein the correction unit corrects image formation by the plurality of recording heads by changing drive signals to the plurality of recording elements based on the density unevenness correction data. 2. The color image forming apparatus according to 1. 6. A recording element of each of the plurality of recording heads,
The color image forming apparatus according to any one of claims 1 to 5, wherein each of the color image forming apparatuses has an ink discharge port, and discharges ink from the ink discharge port using thermal energy. 7. A density unevenness correction method for a color image forming apparatus for forming an image using a plurality of recording heads each having a plurality of recording elements and capable of forming an image in different colors, wherein each of the plurality of recording heads Average densities of test patterns of different colors recorded by the method, and the average densities of the test pattern areas corresponding to a predetermined number of recording elements set for each of the plurality of recording heads. Calculating density unevenness correction data of the plurality of printing elements for each of the plurality of printheads based on density; and correcting image formation by the plurality of printheads based on the calculated density unevenness correction data. Color image forming apparatus.