Patent Application
20110316920
1. Field of the Invention
The present invention relates to an ejection characteristics measurement apparatus and ejection characteristics measurement method for inkjet printing apparatus and inkjet printing apparatus, in particular to evaluation of ejection characteristics of print heads required to prepare correction data used for head shading of print heads.
2. Description of the Related Art
Inkjet printing apparatuses to print color images using a plurality of ink colors are equipped with a plurality of print heads corresponding to each of the ink colors. Due to causes such as a manufacturing error, ejection characteristics are varied for each nozzle of the plurality of print heads. The variation of ejection characteristics causes color variations and density unevenness in images. Particularly in a printing apparatus with premises of 1-pass printing that completes an image by causing a line head longer than the width of the print medium to scan only once over the print medium, although the image can be printed at high-speed, density unevenness is more likely to occur compared to multi-pass printing. That is, in multi-pass printing, each raster can be printed using different nozzles so that density unevenness caused by the variation of ejection characteristics of nozzles can be reduced. However, in 1-pass printing in which each raster is printed by a single nozzle, ejection characteristics of the nozzle significantly affect the density of the image. Therefore in a printing apparatus of 1-pass printing, in accordance with ejection characteristics of a nozzle comprising a head, operation control of the nozzle to reduce density unevenness in an image, that is, head shading (HS) is required. In order to accurately perform such head shading, it is necessary to determine the aforementioned ejection characteristics of the nozzle as precisely as possible.
Japanese Patent Laid-Open No. H04-018364 (1992) of this applicant discloses the technology to perform such head shading. In the technology, density unevenness of patterns in a single color printed with a plurality of different ink ejection volumes (duties) are read by an optical reader using a complementary color of each of the single color. Subsequently, for a read value of a pattern printed with a duty with which density change appears sensitively, a weighted average is obtained by applying a large weight. Based on the weighted average value, the amount of signal compensation for each of the print heads is determined. According to the technology, head shading can be performed more precisely compared to a case in which the ejection feature of the nozzle are determined based on the read value of a pattern printed with a certain duty.
However, since the technology disclosed in said Japanese Patent Laid-Open No. H04-018364 (1992) determines the compensation value for head shading according to a read value obtained by reading the density unevenness in a single color pattern with a single complementary color, there exists the problem where adequate accuracy cannot be provided. On the other hand, enhancement of accuracy by increasing the pattern area of each duty and reading various positions in each pattern printed with the same duty has been proposed and implemented. In such a case, however, along with the increase of pattern areas formed for each duty, the problem of quality decline in total throughput of the printing operation occurs as the time to print patterns is increased, and at the same time, the problem of cost escalation occurs as ink consumption is increased.
It is an object of the present invention to provide an ejection characteristics evaluation apparatus for print heads that is capable of evaluating ejection characteristics of print heads precisely without increasing areas of detection patterns.
In order to achieve the above objective, the present invention has the following configuration.
The first aspect of the present invention is an ejection characteristics evaluation apparatus to evaluate ink ejection characteristics of a plurality of nozzles provided in print heads used for inkjet printers comprising: a print control unit configured to print a plurality of ejection characteristics detection patterns with different print duties, that are ratios between the number of all pixels within a unit area and the number of pixels in which a dot is formed on a print medium, by the print heads; a reading unit configured to divide each of the plurality of ejection characteristics detection patterns into areas having a predetermined number of pixels and to read each of the areas with a plurality of read colors; an evaluation value setting unit configured to specify, based on the read values by the read color obtained by the reading unit for each area, an evaluation value for each of the read colors that indicates an ejection variation volume that is the difference, in each area, between ink ejection volume in each area and standard ink ejection volume; and a comprehensive evaluation unit configured to obtain a weighted average value by applying weights determined based on reading accuracy of the reading unit to the evaluation value for each of the read colors of each area with different duties formed by the same nozzle and evaluate ejection characteristics of a nozzle that prints each of the areas using the weighted average.
The second aspect of the present invention is an inkjet printing apparatus an inkjet printing apparatus to print an image on a print medium by using a plurality of print heads with a plurality of nozzles to eject ink and ejecting different ink colors from the plurality of print heads, the inkjet printing apparatus comprising: a print control unit configured to print a plurality of ejection characteristics detection patterns with different print duties, that are the ratios between the number of all pixels within a unit area and the number of pixels in which dots are formed on a print medium, by the print head; a reading unit configured to divide each of the plurality of ejection characteristics detection patterns into areas with a certain number of pixels and read each of the areas with a plurality of read colors; an evaluation value setting unit configured to specify, based on the read values by the read colors obtained by the read method for each area, an evaluation value for each of the read colors that indicates an ejection variation volume which are the difference, in each area, between ink ejection volume in each area and standard ink ejection volume; a comprehensive evaluation unit configured to obtain a weighted average value by applying weights determined based on reading accuracy of the reading unit to the evaluation value for each of the read colors of each area with different duties formed by the same nozzle and evaluate ejection characteristics of a nozzle that prints each of the areas using the weighted average; and a correction unit configured to correct ink ejection volumes from the print heads to the print medium so that the weighted average value is decreased.
According to the present invention having the configuration described above, highly-accurate evaluation of ejection characteristics of print heads is enabled without increasing areas of detection patterns and the total printing operation throughput can be improved and ink consumption can be reduced compared to before.
Further characteristics of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings).
In reference to the attached diagrams, embodiments of the present invention will be explained.
Embodiments of inkjet printing apparatus, ejection characteristics evaluation measurement apparatus and ejection characteristics evaluation method according to the present invention will be explained based on diagrams.
The inkjet printing apparatus also includes a pair of conveying rollers 104 that are rotated by the motion of a line feed motor 105 and conveys the print medium 106 in the direction of the arrow in the diagram. Further, the inkjet printing apparatus has an image scanner 107 (hereinafter referred to as a “scanner”) on the downstream side of the conveying direction of the print medium in relation to the print heads 101 to 103 and the conveying rollers 104. The scanner 107 is capable of reading an image printed on the print medium 106 by the print heads 101 to 103 as three-color outputs of R (red), G (green), and B (blue).
In such a printer, 1 raster of an output image is formed as ink is ejected from each head of C, M, and Y once respectively. Synchronizing with the line feed motor conveying the print medium, an image for one page is formed by repeating the ink ejection operation. As described above, the inkjet print apparatus using a long head according to the present embodiment is a full-line type printing apparatus that consecutively conveys the print medium 106 in the direction of the arrow in the diagram and ejects ink from the nozzles of each print head to print, that is, performs 1-pass printing. In other words, each nozzle of every print head prints each raster (line) consisting an image by scanning once.
The CCD 205 has a certain number of pixels (for instance, 600 dpi on a print medium) within the length obtained by reducing the reading width of the scanner corresponding to the width of the print medium 106 by the reduction ratio β of the image formation lens 204 and consists of three sensor arrays covered by R, G, and B color filter respectively. Therefore, a signal obtained by reading each color component of R, G, and B is outputted for each pixel. The output signals are amplified by an amplifier 206 first and then outputted as digital signals through the A/D converter 207. As described above, the scanner 107 outputs, as R, G, and B data of each pixel, one-dimensional digital signals obtained by reading the one-dimensional image on the print medium 106 that matches the longitudinal direction (direction perpendicular to the conveying direction of the print medium) of the print heads 101 to 103. Further, the print medium 106 is conveyed in the direction of the arrow in the diagram (sub-scan direction), and by repeating the one-dimensional reading described above at a certain timing (for instance, timing corresponds to 600 dpi on a print medium), two-dimensional digital scan signals of the print medium 106 can be obtained.
The two-dimensional digital signals obtained by the scanner 107 as described above are eventually sent to a host PC 300 described below.
The scanner 107 is configured to disperse the obtained scan light into RGB on the sensor side; however, a configuration in which LED to generate R, G, and B lights is provided as an optical source to read light by switching the optical source as the print medium 106 is conveyed by one pixel may be used. Also, the reduction/image formation lens 204 is used as the imaging optics of the present embodiment; however, Selfoc lens array that is an imaging optics of one-to-one magnification may be used.
In
In addition, the printing apparatus body 100 has the following configuration.
CPU 311 functions as a control unit, computing unit, and setting unit that perform various processing such as computing, detection, and control based on the program stored in ROM 313 and RAM 312. The RAM 312 is a volatile storage that temporarily stores programs and data. The ROM 313 is a non-volatile storage that stores programs and data. Data transfer I/F 314 is I/F to send/receive data to/from the PC 300.
A head controller 315 supplies print data to the print head that actually performs printing and controls printing. In particular, the head controller 315 is designed to read necessary parameters and data from a predetermined address on the RAM 312. The CPU 311 writes necessary parameters and data to the predetermined address on the RAM 312, activates the head controller 315, and starts actual printing operation.
An image processing accelerator 316 performs image processing at a higher speed than the CPU 311. In particular, the image processing accelerator 316 reads necessary parameters and data from a predetermined address on the RAM 312. When the CPU 311 writes necessary parameters and data in the predetermined address on the RAM 312, the image processing accelerator 316 is activated, and actual printing operation is performed. Note that this image processing accelerator 316 is not necessarily required, and image processing may be done by the CPU 311 only.
Next, an ejection characteristics measurement method of long print heads used for the inkjet printing apparatus in
First of all, as described above, an ejection characteristics detection pattern is printed on the print medium 106 by the conveying operation of the print medium 106 in the direction of the arrow and ejection operation from each head 101 to 103 (S401).
Next, the printed pattern 500 is read by the scanner 107 by each read color of R, G, and B (S403). As a result, R, G, and B values of each pixel are obtained for each print duty (S404). However, in the case of using dye ink, colors may be unstable after ejection characteristics detection patterns are printed. For this reason, as shown in S402, a certain amount of standby time may be inserted between the printing operation and reading operation, or time to promote stabilization of the ink using fixing means such as a dryer may be added.
Incidentally, in head shading, image data may be corrected for each pixel to be printed corresponding to each nozzle of the heads 101 to 103. However, since a large number of nozzles are used in a long head as in the present embodiment, in the case of correcting image data for each pixel, the load of performing data processing becomes larger. Therefore, by dividing printed patterns with each print duty into areas with a certain number of (plural in this case) pixels and reading patterns of each area unit, head shading may be performed for each nozzle group unit that printed each area. Based on the space frequency that is visible as density unevenness on the image, the size of each area is determined so that it becomes smaller than the space frequency. Here, an example of performing such head shading by area unit will be described. Thus the following sequence is controlled by the area unit.
The R, G, and B values of each pixel obtained in S404 are averaged for each area unit (S405). The respective average values of R, G, and B are converted into ejection volume variation values using the coefficient Knm obtained with reference to the coefficient table preliminarily stored in the ROM 313 by color (by RGB) and by print duty of the pattern (S406). The method of converting into ejection volume variation values will be described below.
Suppose that the color difference of R, G, and B is a variable cn (R: c1, G: c2, B: c3), the print duty of a pattern is a variable dm (d1: 20%, d2: 30% . . . , d9: 100%), and the coefficient for ΔR, G, B at variables cn and dm is Knm. In this case, the coefficient table stored in the ROM 313 is just as shown in
Here, the differences between read values of each read colors R, G, and B obtained by the scanner reading a pattern printed by a print head with a standard ink ejection volume and read values of each read colors obtained by the same scanner by reading a pattern printed by an actual print head are defined as dR, dG, and dB. Further, when the ejection volume variation is defined as ΔVnm, dR, dG, dB and ΔVnm have the following relationship:
ΔVnm=dR, dG, dB/Knm (formula 1)
Based on the relationship shown by this formula 1, in S406, variation volumes dR, dG, and dB in read values are converted into ejection variation volume ΔVnm for each R, G, and B and by print duty of a pattern. The obtained values are set as evaluation values.
Next, the weighted average of the calculated corresponding value dVnm is obtained to find an average ejection volume variation (weighted average value) ΔVave (S407). The value ΔVave becomes a comprehensive evaluation value for ink ejection characteristics of print heads. The average ejection volume variation ΔVave can be obtained by the following formula when the weighted ratio by R, G, and B and by print duty of the pattern is Wnm.
A method to determine the weighted ratio will be described below.
In an ejection characteristics detection pattern printed by an inkjet printing apparatus, as shown in
Considering such characteristics shown in
1. ΔRGB sensitivity as a detecting system to the brightness difference recognized by a human vision is high.
2. Measurement error is low.
As described above, it is desirable to increase the weight of ΔVnm obtained based on the read values obtained with a print duty and read color that satisfies the condition in which S/N ratio (signal-to-noise ratio) becomes high at a high sensitivity, that is, the condition in which the scan accuracy becomes high. Taking this point into account, characteristics as shown in
Using the Wnm determined in this way, the ejection volume variation value ΔVave expressed by the above formula 2 is determined eventually (S408), and correction of head shading is performed (S409) so that the ejection volume variation value ΔVave is decreased (to zero ideally). The correction of head shading is performed by a known method such as controlling the control value (pulse width, pulse voltage etc.) for the head ejection volume of driving pulse etc. or controlling the number of dots in image processing.
As described above, in the present embodiment, the same ejection characteristics detection pattern is read with a plurality of colors (R, G, B) and G, B, and the weighted average is obtained as applying a larger weight to the evaluation value (Vnm) based on the read value read under the condition in which the scan accuracy becomes higher. Therefore, the ejection characteristics evaluation of print heads can be performed highly precisely without increasing the area of ejection characteristics detection pattern as is conventionally done. As a result, the running cost can be reduced by lowering ink consumption and the print operation throughput can be improved.
The above paragraph explained the case in which the ejection characteristics detection pattern of C is read with G and B in addition to the complementary color (other than R) normally used. However, the ejection characteristics detection pattern of M can be read with read color B other than the complementary colors (other than G). In that case, just as in the case of ejection characteristics detection pattern of C, highly accurate evaluation is enabled without increasing the pattern area to be formed. However, since sensitivity of R to the detection characteristics detection pattern of M is very low (the value detected by the scanner 107 barely changes in relation to the change of ejection volume), reading with R is not conducted.
Therefore, based on the ejection characteristics detection pattern of M, the following formula 3 is applied to obtain the above described ΔVave.
Further, since detection sensitivities of R and G to the ejection characteristics detection pattern of Y are very low, the pattern is read only by B.
In the above described embodiment, as shown in
Further, in the step S407 of the above embodiment, ΔR, G, and B are independently weighted based on Knm to obtain the corresponding value ΔVnm. However, for the purpose of increasing S/N ratio as a measurement system, weight may be limited to 0% and 100% depending on the print duty. In this way, the calculation process can be simplified, and it is possible to respond to higher-speed printing.
In the above embodiment, an example of a case in which the ejection characteristics detection apparatus and ejection characteristics detection method of the present invention are applied to a full-line type inkjet printing apparatus is described. However, the present invention can be applied to so-called serial type printing apparatuses that perform printing as moving print heads in the direction crossing the conveying direction of the print medium.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of Japanese Patent Application No. 2010-143095, filed Jun. 23, 2010, which is hereby incorporated by reference herein in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2010-143095 | Jun 2010 | JP | national |
