JP3059451B2 - Image recording device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image recording apparatus, and more particularly to an image recording apparatus using a multi-head.

[Conventional technology]

With the spread of computers and communication devices, as an image recording apparatus, one that performs digital image recording using a recording head, such as an ink jet method or a thermal transfer method, is rapidly spreading. The recording head used is generally a multi-head in which a plurality of image recording elements are integrated in order to improve the recording speed.

For example, in an inkjet recording head, a multi-nozzle head in which a plurality of nozzles are integrated is generally used,
A thermal head for thermal transfer usually has a plurality of heaters integrated.

However, it is difficult to uniformly manufacture a multi-head image recording element, and the characteristics of the image recording element vary to some extent. For example, in an inkjet multi-head, variations occur in the shape and the like of nozzles, and in a thermal transfer multi-head, variations occur in the shape and resistance of a heater.

If the characteristics between the image recording elements are non-uniform, the size and density of dots recorded by each image recording element will be non-uniform, and density unevenness will occur in the recorded image. For example, when the input signals to each recording element are made uniform as shown in FIG. 13 (b) in the multi-head 1 in which the image recording elements 2 are arranged as shown in FIG. As shown in (c), there is a problem that density unevenness occurs.

As a method for solving such a problem, a method has been proposed in which a signal applied to each image recording element is corrected to make the recorded image density uniform. This is because, for example, when density unevenness occurs as shown in FIG. 13 (c), the input signal is corrected as shown in FIG. 13 (d), and a large input is applied to the image recording element in the low density portion. This is a method in which a small input is given to the image recording element in a high density portion.

According to such a method, in the case of a recording method capable of modulating the dot diameter or the dot density, the dot diameter recorded by each image recording element is modulated according to the input. For example, in a piezo-type inkjet method, a driving voltage or a pulse width applied to each piezo element is changed according to an input signal. In the thermal transfer method, the drive voltage or pulse width applied to each heater is changed in accordance with an input signal, so that the dot diameter or dot density of each recording element is made uniform, and the density distribution is as shown in FIG. Is homogenized.

Also, whether the dot diameter or dot density cannot be modulated,
Alternatively, when it is difficult, the number of dots is modulated according to the input signal, so that many dots are recorded in the image recording element in the low density portion, and few dots are recorded in the image recording element in the high density portion. . As a result, the concentration distribution
It is uniformed as shown in FIG.

Next, a procedure for correcting density unevenness in the case where the density distribution of a print image by a multi-head of 256 nozzles is as shown in FIG. 14 will be described.

(1) An average density OD of an image recorded by a multi-head of 256 nozzles is obtained.

(2) measuring the concentration OD ₁ ~OD ₂₅₆ in the portion corresponding to each nozzle.

(3) Obtain ΔOD _n = OD−OD _n (n = 1 to 256).

Here, when the relationship between the value of the image signal and the output density is as shown in FIG. 15, the image signal is _expressed by ΔS with respect to the density ΔODn.
It only needs to be corrected. For this purpose, the image signal may be converted into a table based on the characteristics shown in FIG.

The straight line A shown in FIG. 16 is a straight line having a slope of 1.0,
The input is output without any conversion. On the other hand, the straight line B is a straight line whose inclination is smaller than that of the straight line A, and the output when the input signal S is input is S-ΔS. Therefore, the image signal corresponding to the n-th nozzle is
If the head is driven after performing a table conversion such as the straight line B shown in the drawing, the density of the portion printed by this nozzle is
It is equal to OD.

If such processing is performed for all nozzles, density unevenness is corrected, and a uniform image is obtained. In other words, if data on what kind of table conversion should be performed on the image signal corresponding to which nozzle is obtained in advance,
Unevenness can be corrected.

With such a method, it is possible to correct the density unevenness. However, in this method, even if the unevenness can be corrected once, if the density unevenness subsequently changes, it is necessary to change the correction amount of the input signal.

In the case of the ink jet system, it is often observed that as the recording head is used, deposits from the ink adhere to the vicinity of the ink discharge ports or foreign matter adheres to the ink to change the density distribution.

Further, even in the case of the thermal transfer method, the density distribution may change due to deterioration or deterioration of each heater. In such a case, there is a problem that the density correction becomes insufficient as the recording head is used since the density correction is insufficient for the initially set input correction amount.

As a method for solving such a problem, the present applicant has already proposed a method in which the density unevenness distribution is periodically read to generate density unevenness correction data. 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 unevenness can always be obtained.

[Problems to be Solved by the Invention] In this method, since the correction data is created by a machine actually operating in the market, the time required for the creation of the correction data is extremely short, and the downtime of the machine is reduced. Is necessary to shorten

However, in practice, it is not sufficient to perform the reading of the density unevenness and the creation of the correction data only once, and the correction effect is not sufficient, and a uniform image cannot be obtained unless it is repeated several times, sometimes ten or more times. There was a problem that often.

The present applicant has found that the cause of this problem is an individual difference in gradation characteristics of each head.

When the gradation characteristic of the recording head is a straight line C shown in FIG.
As described above, if there is a density unevenness of ΔOD in the image signal S, the unevenness ΔOD can be corrected by correcting the image signal by ΔS. However, when the gradation characteristic of the recording head is curve D shown in FIG. 17, when the density unevenness of ΔOD is detected and ΔS is corrected, the density unevenness is corrected only by ΔOD ′, and one operation is performed. Alone does not provide a uniform image.

As described above, when the gradation characteristics of the recording head are different, the necessary correction amount is different, so that the density nonuniformity is not sufficiently corrected in one and two readings and corrections, and the reading and the correction are repeated many times. There was a problem that had to be.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-described problems, to absorb individual differences in tone characteristics of a print head, and to reduce uneven density regardless of individual differences in tone characteristics of a print head used. An object of the present invention is to provide an image recording apparatus that can sufficiently correct the image.

[Means for Solving the Problems] In order to achieve such an object, the present invention provides an image recording apparatus that records an image using a recording head in which a plurality of image recording elements are arranged. Density unevenness correction means for correcting the density unevenness of the print head to a predetermined tone characteristic based on the unevenness characteristic; and, corresponding to the used print head, changing the tone characteristic of the print head to the predetermined tone characteristic. Acquisition means for acquiring gradation correction information for correcting to gradation characteristics, and gradation correction means for correcting gradation characteristics of the recording head to the predetermined gradation characteristics based on the acquired gradation correction information And characterized in that: The present invention further includes reading means for reading the test pattern recorded by the recording head.

[Operation] In the present invention, based on the density unevenness characteristic of the used print head, the density unevenness of the print head is corrected by a density unevenness correcting means for a predetermined gradation characteristic, and the density unevenness of the used print head is corrected. Then, the tone correction information for correcting the tone characteristics of the recording head to the predetermined tone characteristics is acquired by the acquisition unit, and based on the acquired tone correction information, The gradation characteristic of the recording head is corrected to the predetermined gradation characteristic.

〔Example〕

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

 FIG. 1 shows a first embodiment of the present invention.

In the figure, reference numerals 29a, 29b, and 29c denote unevenness correction RAMs for respective colors.
A selection signal of a correction straight line required for correcting each head unevenness is stored. That is, the non-uniformity correction signals having 61 kinds of values of 060 are stored for 256 nozzles, and the non-uniformity correction signals 30a, 30b, in synchronization with the input image signal.
30c is output.

Reference numerals 22a, 22b, and 22c denote unevenness correction tables serving as uneven density correction means, which convert the image signals 21a, 21b, and 21c so as to correct the unevenness of the heads 24a, 24b, and 24c. As shown in FIG. 2, there are 61 correction straight lines having slopes from Y = 0.70X to Y = 1.30X, each having a different inclination of 0.01, and the unevenness correction signal 30a,
The correction straight line is switched according to 30b and 30c. For example, when a signal of a pixel to be printed by a nozzle having a large dot diameter is input, a correction straight line having a small inclination is selected, and when a nozzle having a small dot diameter is selected, an image signal is corrected by selecting a correction straight line having a large inclination. .

Reference numerals 101a, 101b, and 101c denote gradation correction tables as gradation correction means. As shown in FIG. 3, the ROM includes 20 types of gradation correction curves having different shapes. The optimum gradation correction table for each head is obtained by a characteristic test after manufacturing the head, and a label is displayed or marked on each head.

103a, 103b and 103c are tone correction table selection sections as input means for selecting the optimum tone correction curve for each head from the operation section of the apparatus by software or by using a switch on an electric board. .

31a, 31b and 31c are binarization circuits for binarizing the signal corrected by the gradation correction table 101 by a dither method, an error diffusion method or the like. Reference numerals 24a, 24b, and 24c denote inkjet heads each having 256 nozzles for each color.

Reference numeral 25 denotes a reading unit as a reading unit, which has a CCD having filters of three colors of red (R), green (G), and blue (B). The CCD has a reading density of 400 dpi, which is the same as the recording density of the head. The number of pixels of the CCD is at least larger than the number of nozzles of the head, that is, 256. 32 is RAM
And stores the read signals 26a, 26b, 26c from the reading unit 25. Reference numeral 27 denotes a CPU that calculates unevenness correction data based on the R, G, and B signals from the RAM 32.

21a, 21b, and 21c are image signals of three colors of cyan, magenta, and yellow, respectively, 23a, 23b, and 23c are image signals after unevenness correction for each color, and 26a, 26b, and 26c are R and R output from the reading unit 25. G and B read signals, 28a, 28b, and 28c respectively represent unevenness correction data for cyan, magenta, and yellow, 30a, 30b,
30c is an unevenness correction signal for each color.

 Next, the operation will be described.

The image signals 21a, 21b, 21c correspond to the unevenness correction tables 22a, 22b, 2
2c is converted so as to correct the unevenness of the heads 24a, 24b, 24c, and the unevenness correction signal 30 is synchronized with the input image signal.
a, 30b, and 30c are output. The signals 23a, in which the unevenness has been corrected by the γ straight line selected by the unevenness correction signals 30a, 30b, and 30c,
23b and 23c are input to the tone correction tables 101a, 101b and 101c, and each of the heads 24 is output from the tone correction tables 101a, 101b and 101c.
The gradation characteristics of a, 24b, and 24c are corrected and output.

In this embodiment, the optimum gradation correction table for each head is
It is required by a characteristic test after head manufacture, and each head is labeled or marked. When installing the head, the operator looks at this and sees the gradation correction table setting unit 10
A gradation correction selection signal is input to the address of the gradation correction table by 3a, 103b, and 103c, and an optimum table is selected.

For example, as shown in FIG. 4, when the gradation characteristic of the head is curve B, a gradation correction table like curve A is selected, and the relationship between the input signal and the image density is a straight line like line C. Correct so that When the gradation characteristic of the head is the curve E shown in FIG. 5, a table like the curve D is selected, and correction is performed so that a linear gradation like the straight line C is obtained. That is, by selecting the optimum gradation correction table for each head, the relationship between the input signal and the image density is always the same straight line.

The signal corrected in this way is binarized by binarization circuits 31a, 31b, 31c using a dither method, an error diffusion method, or the like, and based on signals from the binarization circuits 31a, 31b, 31c, The multi-nozzle inkjet heads 24a, 24b, 24c are driven. As a result, the number of dots of the nozzles having a large dot diameter is corrected to be small, and the number of dots of the nozzles having a small dot diameter is corrected to be large.

FIG. 6 is a flowchart showing a procedure for correcting uneven density by the first illustrated CPU 27.

In step S72, all of the non-uniformity correction tables 22a, 22b, and 22c are set to straight lines with a slope of 1.0 by a control signal (not shown), and no non-uniformity correction is performed. Then, step S7
At 3, an unevenness correction pattern is output from a signal source (not shown), and the unevenness correction pattern is printed out by the heads 24a, 24b, and 24c. The unevenness correction pattern may be a uniform pattern with an arbitrary print duty, but a pattern with a duty of about 30% to 75% is appropriate. Here, a 50% duty uniform halftone is printed in cyan, magenta, and yellow.

Then, in step S74, the pattern printed out in step S73 is read by the reading unit 25, and the read signals 26a, 26b, 26c of the three colors at that time are temporarily stored in the RAM 32. Among the red, green, and blue signals obtained by the reading, the uneven distribution of the cyan head is obtained from the red signal, and the uneven distribution of the magenta head is obtained from the green signal.
The uneven distribution of the yellow head is obtained from the blue signal.
Here, for the sake of simplicity, a case will be described in which unevenness distribution of cyan heads is obtained and unevenness correction is performed.

In step S75, the head signal R _n (n = 1 to 256) obtained for each nozzle of the cyan head is compared with C _n =
An operation of −log (R _n / R _o ) (R _o is a constant satisfying R _o ≧ R _n ) is performed to convert the signal into a cyan density signal, thereby obtaining an uneven density distribution.
Then, in step S76, the average density is calculated by the following equation (1).
From In step S77, how much the density corresponding to each nozzle deviates from the obtained average density is determined from the following equation (2).

ΔC _n = C _n − (2) In step S78, the signal correction amount ΔS _n corresponding to ΔC _n is calculated by ΔS
_n = K × ΔC _n

Here, K is a coefficient determined by the gradation characteristic of the head. In this embodiment, the optimum coefficient is set when the gradation characteristic is a straight line.

Then, in step S79, a selection signal of a correction straight line to be selected according to ΔS _n is obtained.
An unevenness correction signal having one type of value is stored in the unevenness correction RAM 29a for 256 nozzles.

A different γ straight line is selected for each nozzle based on the unevenness correction data created in this way, and the unevenness in density is corrected. Variations in gradation characteristics for each head are corrected by selecting an optimum gradation correction table, so that an optimum correction value can be obtained in a short time and density unevenness can be corrected.

In the present embodiment, the example of the cyan head has been described. However, in the case of the magenta head and the yellow head, the density unevenness correction can be performed in a short time by the same procedure, and the downtime of the machine can be minimized. be able to.

In the present embodiment, the operation of reading the printed correction pattern may be performed by a user or a serviceman operating the output sample in the reading unit. Alternatively, the printed sample may be automatically read by a machine.

Second Embodiment FIG. 7 shows a second embodiment of the present invention.

This embodiment is different from the first embodiment in the means for inputting gradation correction information.

That is, in the first embodiment, when the head is mounted on the head, the optimum gradation correction table labeled or marked on each head is viewed by the operator, and the gradation correction table setting units 103a, 103b, 103c determine the gradation. Although the correction selection signal is input to the address of the gradation correction table to select the optimum table, in the present embodiment, the optimum gradation correction curve information indicating sections 60a, 60b, and 60c attached to the respective heads are used. The value corresponding to the optimum gradation correction curve of each head is set as the ROM data or the resistance value of the variable resistor, and the optimum gradation correction curve information is read by the optimum gradation correction curve information reading units 61a, 61b, and 61c. And the control signals 62a, 62b, and 62c according to the read information of the optimal curve.
b, 101c to automatically set the optimum gradation correction curve for each head.

With such a configuration, the operation and effect of this embodiment is the first.
It is not essentially different from that of the embodiment. However, there is an advantage that it is not necessary to set an optimum gradation correction curve for each head mounted.

In the present embodiment, an example is described in which a ROM or a variable resistor is used as the optimum gradation correction curve information indicating section, but a switch having several stages may be used.

Further, a configuration may be adopted in which the shape of a part of the head is changed according to the optimum curve, and a control signal is generated according to the shape when the head is engaged with the recording apparatus.

Further, the optimum various correction curve information indicating section does not necessarily have to be integrated with the head. For example, a ROM storing information may be prepared separately from the head, and when the head is mounted, the ROM may be mounted at the same time.

Further, the correction of the density unevenness does not necessarily have to be performed for each image recording element, but may be performed for each block by using a plurality of image recording elements arranged as one block.

Third Embodiment FIG. 8 shows a third embodiment of the present invention.

This embodiment is different from the first embodiment in the means for inputting gradation correction information.

That is, in the first embodiment, an optimum gradation correction table for each head is obtained by a characteristic test after head manufacture, and a label is displayed or marked on each head. Correction table setting unit 103a, 1
According to 03b and 103c, the gradation correction selection signal is input to the address of the gradation correction table to select the optimum table.In this embodiment, the gradation characteristics of the mounting head are measured in the actual machine, and the The CPU 27 determines the optimum curve based on the result, and selects the optimum tone correction curve selection signals 90a, 90 according to the determination result.
b and 90c are input to the gradation correction table to select the optimum gradation correction table.

FIG. 9 is a flowchart showing a procedure for correcting uneven density by the CPU 27 in this embodiment.

In step S110, the gradation correction curve is set to a straight line with a slope of 1.0, and in step S111, a gradation characteristic measurement pattern is printed out according to a signal from a gradation characteristic measurement pattern generator (not shown). As shown in Fig. 10, the pattern for gradation characteristic measurement printed out has a print duty of 10% or less.
Change 10 steps in 10% increments up to 100%.

In step S112, the output pattern printed and output in step S110 is read, and the read signal of each gradation is stored in the RAM. Subsequently, in step S113, after converting the read signal into a density, an averaging process is performed to obtain an average density for each gradation.
▼ (m = 1 to 10) is obtained. Then, step S114
At an average concentration C _m and table conversion by the gradation correction table, at step S115, it computes the square error between the gradation characteristics which are table conversion and the ideal straight line. That is, the average density of each gradation after table conversion is defined as ▲ (m = 1 to 1).
0), and when the value on the ideal straight line is _Com (m = 1 to 10), Is calculated.

In step S116, it is determined whether the calculation of the square error has been performed for all tables, that is, for all 20 types of tone correction curves. As a result of the determination, if not performed, the process returns to step S114.
Move to S117.

In step S117, a gradation correction curve with the least square error is determined, and a selection signal corresponding to the optimum gradation correction curve is output. After that, in step S118, density unevenness correction data is created as in the first embodiment.

With this configuration, even when the gradation characteristic changes due to a temporal change of the head or the like, the unevenness correction data can always be created in the shortest time, and the downtime of the machine can be minimized.

Fourth Embodiment FIG. 11 shows a fourth embodiment of the present invention.

This embodiment is different from the third embodiment in the means for inputting gradation correction information.

That is, in the third embodiment, the gradation characteristic of the mounting head is measured in the actual machine, the CPU 27 determines the optimum curve based on the result, and selects the optimum gradation correction curve selection signals 90a, 90b according to the determination result. , 90c is the R of the gradation correction tables 101a, 101b, 101c.
Although the optimum gradation correction table is selected by inputting it to the OM, in this embodiment, after measuring the gradation characteristics of the head in an actual machine, an appropriate gradation correction table is obtained from the measured density of each gradation. Inversely calculates the curve and calculates the correct corrected curve data 150
a, 150b, and 150c are stored in the RAM of the gradation correction tables 102a, 102b, and 102c.

FIG. 12 is a flowchart showing a procedure for correcting uneven density by the CPU 27 in the present embodiment.

Steps S110 to S113 show the same steps as the steps shown in FIG. After obtaining the average density for each tone in step S113, an ideal tone correction curve is inversely calculated from the measured density of each tone in step 124, and the ideal correction is performed in step 125. The curve is stored in the RAM of the gradation correction unit. After that, density unevenness correction data is created in the same manner as in the first embodiment.

With this configuration, no matter what the gradation characteristics of the head are, irregularity correction data can always be created in the shortest time, and downtime of the machine can be minimized.

In the first to fourth embodiments, an example of ink jet has been described, but a thermal head for thermal transfer may be used.

In addition, multi-head is not only semi-multi-head,
A full multi-head having the same width as the image width may be used.

Furthermore, although the example of the unevenness correction has been described in which the number of dots is changed, the area of the dots may be changed by changing the width or voltage of the drive pulse.

Furthermore, an example of an image recording apparatus that obtains a color image using three colors of cyan, magenta, and yellow has been described.
It may be a single-color image recording device.

[Effects of the Invention] As described above, according to the present invention, the density unevenness correcting means corrects the density unevenness of the recording head to a predetermined gradation characteristic, and also adjusts the gradation characteristic of the used recording head. Since the tone correction means corrects the tone characteristics to the predetermined tone characteristics, it is possible to absorb individual differences in tone characteristics of the print head, and to sufficiently reduce uneven density regardless of individual differences in tone characteristics of the print head used. Can be corrected.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a first embodiment of the present invention, FIG. 2 is a diagram showing an example of a density unevenness correction straight line, FIG. 3 is a diagram showing an example of a gradation correction curve, FIG. 4 and FIG. FIG. 6 is an explanatory diagram for explaining the gradation correction, FIG. 6 is a flowchart showing a density unevenness correction procedure by the CPU 27 shown in FIG. 1, FIG. 7 is a block diagram showing a second embodiment of the present invention, and FIG. FIG. 9 is a block diagram showing a third embodiment of the invention, FIG. 9 is a flowchart showing a density unevenness correction procedure by the CPU 27 shown in FIG. 8, FIG. 10 is a diagram showing an example of a gradation characteristic measuring pattern, and FIG. FIG. 12 is a block diagram showing a fourth embodiment of the invention, FIG. 12 is a flowchart showing a procedure for correcting uneven density by the CPU 27 shown in FIG. 11, FIG. 13 is an explanatory diagram for explaining conventional tone correction, and FIG. FIG. 15 is a diagram showing an example of a relationship between an image signal and an image density, FIG. Explanatory diagram for explaining correction of density unevenness, FIG. 17 is an explanatory diagram for explaining the correction of density unevenness when the density unevenness distribution is changed. 22a, 22b, 22c ... Unevenness correction table, 24a, 24b, 24c ... Head, 25 ... Reading unit, 27 ... CPU, 29a, 29b, 29c ... Unevenness correction RAM, 101a, 101b, 101c ... Floor Tone correction table, 103a, 103b, 103c ... gradation correction table setting unit.

 ──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-63-267559 (JP, A) JP-A-62-227767 (JP, A) JP-A-62-256575 (JP, A) JP-A-62-267575 145266 (JP, A) JP-A-1-129667 (JP, A)

Claims (6)

(57) [Claims] An image recording apparatus for recording an image using a recording head in which a plurality of image recording elements are arranged, wherein the density irregularity of the recording head is reduced to a predetermined gradation based on the density irregularity characteristic of the recording head used. Density unevenness correction means for correcting characteristics, and acquisition means for acquiring gradation correction information for correcting the gradation characteristics of the recording head to the predetermined gradation characteristics corresponding to the recording head to be used. An image recording apparatus, comprising: tone correction means for correcting tone characteristics of the recording head to the predetermined tone characteristics based on the acquired tone correction information. 2. An image recording apparatus according to claim 1, wherein said acquisition means selects gradation correction information corresponding to the recording head to be used from a plurality of gradation correction information stored in advance. 3. The image recording apparatus according to claim 1, further comprising reading means for reading a test pattern recorded by said recording head. 4. The image forming apparatus according to claim 3, wherein said acquiring means comprises: a measuring means for measuring a gradation characteristic of the recording head used by said reading means; and a plurality of information stored in advance based on a measurement result by said measuring means. Selecting means for selecting tone correction information corresponding to the print head to be used from the tone correction information. 5. The image forming apparatus according to claim 3, wherein the acquiring unit measures a gradation characteristic of the recording head used by the reading unit, and calculates gradation correction information based on the measurement result by the measuring unit. An image recording apparatus comprising: 6. An image recording apparatus according to claim 1, wherein said recording head discharges ink by heat of said plurality of image recording elements.