JP4844297B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus using a recording head that ejects ink droplets.

  An ink jet image forming apparatus that forms an image by ejecting ink liquid onto a recording sheet from a plurality of ejection nozzles provided in the recording head may cause uneven ink density in the image to be formed.

  In particular, a recording head in which the ejection nozzles are two-dimensionally arranged along a plurality of branch portions that supply ink liquid may cause ink density unevenness having periodic regularity. The ink density unevenness is caused by an influence of a decrease in acoustic capacity due to the narrow width of the tributary part, a fluid pressure loss of the ink liquid flowing through the tributary part, a manufacturing error of the recording head, and the like.

  An invention for reading a test pattern formed on a recording sheet in order to eliminate unevenness in ink density of an image formed on the recording sheet is disclosed (Patent Document 1). Further, an invention that combines a low resolution scan and a high resolution scan when reading a test pattern is disclosed (Patent Document 2).

In the invention described in Patent Document 1, a test pattern formed on a recording sheet is read by a scanner configured by, for example, a CCD, an ink density corresponding to the ejection nozzle of the recording head is obtained, and the ejection nozzle is based on the ink density. The amount of ink ejected from the ink is adjusted. Further, the invention described in Patent Document 2 creates image data by reading a test pattern formed on a recording sheet at a low resolution using a scanner, and from an allowable range where ink density unevenness is set from the image data. Ink density correction is performed by reading a large area with high resolution.
JP-A-5-69545 JP 2004-237725 A

  However, since the invention described in Patent Document 1 reads the entire test pattern with high resolution, it takes a long time to measure the ink density. The invention described in Patent Document 2 can shorten the time required for measuring the ink density compared to the invention described in Patent Document 1, but is used in a recording head in which ejection nozzles are two-dimensionally arranged. In some cases, the time required for measuring the ink density cannot be shortened. In the recording head in which the discharge nozzles are two-dimensionally arranged, the ink density unevenness has a periodic regularity corresponding to the tributary portion, and a portion exceeding the allowable range of the ink density unevenness frequently occurs at the end of the tributary portion. This is because the area of the test pattern image that requires high density and ink density measurement increases. Further, since the ink density unevenness within the allowable range is not a correction target, the ink density unevenness within the allowable range remains, and the highly accurate ink density unevenness correction cannot be performed.

  In consideration of the above-described facts, the present invention improves the speed required for ink density correction for eliminating ink density unevenness periodically generated by the image forming means in which the discharge nozzles are two-dimensionally arranged, and increases the accuracy of ink density correction. An object of the present invention is to obtain an image forming apparatus capable of performing the above.

The present invention, a plurality of branch portions each direction with respect to the arranged angularly inclined in Jo Tokoro with respect to the direction of movement record medium and relative, predetermined a plurality of ejection nozzles for ejecting ink droplets by providing in the pitch, the recording head in which the plurality of ejection nozzles are arranged with a two-dimensional regularity before type recording medium and the direction perpendicular to the direction and the recording medium and the relative moving direction of relative movement When the from the recording head and the recording medium and the length et previous SL plurality of discharge nozzles are moved relative to eject the ink droplets, the plurality of discharge nozzles in the direction perpendicular to the direction of the front SL relative movement An image forming unit that forms an image with a resolution corresponding to the pitch of the recording medium ; and a reference image that is arranged on the downstream side of the recording head in the moving direction of the recording medium and is formed on the recording medium by the image forming unit. Resolution An image reading unit obtaining a reference image data I read at a lower reading resolution, and the reference image data obtained by the image reading means, obtained for previously said recording head, said by the image forming means comprising said recording head the image formed on the recording medium, wherein on the basis of the periodic regularity of the ink density variation in a direction orthogonal to the direction of relative movement, the ink density distribution to create an ink concentration distribution of the plurality of ejection nozzles Based on the ink density distribution of the plurality of ejection nozzles obtained by the creation means and the ink density distribution creation means, ejection of the ink droplets ejected by the plurality of ejection nozzles so as to eliminate the ink density change Ink density correction table creating means for creating an ink density correction table for correcting the amount;
An ink density correction table storage unit that updates and stores the ink density correction table created by the ink density correction table creation unit every time the reference image data is obtained, and the ink density stored in the ink density correction table storage unit And a droplet discharge amount control means for controlling the discharge amount of the ink droplet for each of the plurality of discharge nozzles using a correction table.

According to the present invention, using a recording head in which ejection nozzles that eject ink droplets are two-dimensionally arranged, the image forming unit relatively moves the recording medium and the recording head. A reference image is formed on the recording medium with a resolution corresponding to the pitch of the ejection nozzles in a direction component orthogonal to the direction. The reference image is read at a reading resolution lower than the resolution, for example, by an image reading unit such as a CCD line sensor, and used as reference image data. Then, the ink density distribution creating unit calculates the reference image data and the ink in the direction orthogonal to the direction of relative movement of the image formed on the recording medium by the image forming unit including the recording head, which is obtained in advance. Based on the periodic regularity of the density change, the ink density distribution of the ejection nozzle is obtained. The ink density correction table creating means creates an ink density correction table for correcting the ejection amount of the ink droplets from the ejection nozzle so as to eliminate the ink density change from the ink density distribution of the ejection nozzle. The ink density correction table is stored in the ink density correction table storage unit, and the droplet discharge amount control unit controls the discharge amount of the ink droplets of the discharge nozzles using the ink density correction table.

  Accordingly, the speed of reading the reference image is increased, and the time required for correcting the ink density is shortened.

Further, according to the present invention, the ink density distribution creating unit calculates an approximate line of the ink density distribution for each of the plurality of tributaries, and obtains the ink density distribution of the plurality of ejection nozzles from the calculated approximate line. It is characterized by.

  Therefore, even if the reference image is read at a resolution lower than the resolution of the image forming apparatus, it is possible to perform highly accurate ink density correction by calculating an approximate line of the ink density distribution for each tributary part.

Furthermore, the present invention, the said image reading means reads an area of the reference image corresponding to the other end of the plurality of branch portions in the first reading resolution, corresponding to the end portion of the plurality of branch portions What reading an area of the reference image with the higher than first reading resolution second reading resolution, and wherein the Rukoto obtain the reference image data.

At the end of the branch portion, of the image formed on the recording medium, even if periodic regularity of the ink density variation in a direction orthogonal to the direction of the relative movement is changed, the area of the corresponding reference images first By reading at a second reading resolution higher than the reading resolution of 1, the ink density distribution in the region where the regularity has changed can be obtained with high precision, and thereby ink density correction with high precision can be performed.

  It is possible to improve the speed required for ink density correction for eliminating ink density unevenness that occurs periodically by the image forming means in which the discharge nozzles are two-dimensionally arranged, and to increase the accuracy of ink density correction.

  FIG. 1 is a schematic diagram showing the overall configuration of an ink jet image forming apparatus 10 according to an embodiment of the present invention.

  The image forming apparatus 10 includes a recording head array 12.

  The recording head array 12 includes a cyan ink liquid (C), a magenta ink liquid (M), a yellow ink liquid (Y), a black ink liquid (K), and a processing liquid (T). Correspondingly, five recording heads 14C, 14M, 14Y, 14K, and 14T are provided.

  The recording heads 14 </ b> C, 14 </ b> M, 14 </ b> Y, 14 </ b> K, and 14 </ b> T are long recording heads called FWA (Full Width Array) whose width is substantially equal to the width of the recording paper 16. The recording heads 14 </ b> C, 14 </ b> M, 14 </ b> Y, 14 </ b> K, and 14 </ b> T are fixed and discharge respective ink droplets and processing droplets onto the recording sheet 16 that is being conveyed, and based on image data input to the image forming apparatus 10. Form an image. The treatment liquid is colorless or light-colored, and the ink liquid of each color is dropped on the recording paper 16 and then dropped so as to overlap, thereby reducing ink bleeding and improving the image quality.

  The recording heads 14C, 14M, 14Y, 14K, and 14T are respectively connected to ink cartridges 18C, 18M, 18Y, 18K, and 18T that store CMYK ink liquids and processing liquids by pipes (not shown), respectively. Are supplied to the recording heads 14C, 14M, 14Y, 14K, and 14T. As the ink, various known inks such as water-based ink, oil-based ink, and solvent-based ink can be used.

  Here, details of the recording heads 14C, 14M, 14Y, and 14K will be described with reference to FIG.

  Each of the recording heads 14C, 14M, 14Y, and 14K includes a main flow portion 800 through which ink liquid flows. The main flow unit 800 is supplied with ink liquid via the ink liquid supply unit 802 from 18C, 18M, 18Y, and 18K corresponding to the recording heads 14C, 14M, 14Y, and 14K. The ink liquid supplied to the main flow part 800 flows into the branch part 804.

  The branch portion 804 is provided with a plurality of discharge nozzles 812 at a predetermined pitch via the communication portion 806 and the pressure chamber 810. The plurality of branch portions 804 provided with the discharge nozzles 812 are arranged at a predetermined angle with respect to the direction of relative movement with respect to the recording paper 16 (the y direction which is the downward direction from the top in FIG. 9). As a result, the discharge nozzle 812 has two-dimensional regularity in the direction moving relative to the recording paper 16 and in the direction orthogonal to the direction moving relative to the recording paper 16 (the x direction which is the left-right direction in FIG. 9). Arranged. The resolution (nozzle resolution) of the images formed by the recording heads 14C, 14M, 14Y, and 14K is a directional component (in FIG. 9) orthogonal to the direction in which the recording heads 14C, 14M, 14Y, and 14K move relative to the recording paper 16. In the x-direction component), this corresponds to the pitch of the discharge nozzles 812 arranged two-dimensionally with regularity.

  Ink liquid is supplied to the pressure chamber 810 from the branch portion 804 via the communication portion 806. The volume of the pressure chamber 810 is changed by a piezoelectric element (not shown), and the discharge nozzle 812 provided for each pressure chamber 810 discharges ink droplets by the action.

  In addition, the method of ejecting ink droplets in each of the recording heads 14C, 14M, 14Y, and 14K is not limited to using a piezoelectric element. In addition, a heating resistor is provided to apply thermal energy to the ink so that the ink is discharged from the nozzle. A thermal method of discharging may be used.

  The recording head 14T has the same configuration as that shown in FIG. 9A, although there is a difference that the supplied liquid is a processing liquid.

  When the recording heads 14C, 14M, 14Y, and 14K in which the discharge nozzles 812 as shown in FIG. 9A are two-dimensionally arranged are regular, ink density unevenness having periodic regularity occurs. There is. FIG. 9B shows an example of periodic ink density unevenness. The ink density unevenness is a case where an image is formed while conveying the recording paper 16 from the top to the bottom in FIG. 9A, and has a periodicity corresponding to the branch portion 804. This periodicity has serrated regularity as shown in FIG. 9C or sinusoidal regularity.

  A conveyance belt 19 that is an endless belt is provided below the recording head array 12 that constitutes a part of the image forming apparatus 10 shown in FIG. The conveyor belt 19 is wound around the drive rolls 20A and 20B, and is driven to rotate in the A direction, which is the clockwise direction in FIG. 1, by the rotational force of the drive rolls 20A and 20B. The conveying belt 19 when facing the recording head array 12 is flat, and the recording paper 16 is conveyed to the flat area, and ink droplets are applied to the recording paper 16 from the recording heads 14C, 14M, 14Y, and 14K. As a result, an image is formed. At this time, the recording heads 14C, 14M, 14Y, and 14K each eject ink droplets from the ejection nozzle 812 to the recording paper 16 with a time difference. As a result, the ink droplets of the respective colors are superimposed on the recording paper 16, and an image is formed.

  A charging roll 22 is disposed on the upstream side in the driving direction from the area facing the recording head array 12 of the conveying belt 19 in FIG. A predetermined voltage is applied to the charging roll 22, and the charging roll 22 is driven while sandwiching the conveyance belt 19 and the recording paper 16 with the driving roll 20 </ b> A, thereby applying a charge to the recording paper 16. The recording paper 16 that has been charged by the charging roll 22 is electrostatically attracted to the transport belt 19 and is transported along with the circumferential drive of the transport belt 19.

  The recording paper 16 is accumulated in a paper feed tray 24 provided on the lower side inside the image forming apparatus 10 of FIG. The recording paper 16 is taken out from the paper feed tray 24 one by one by the pick-up roll 26 and is conveyed by a recording paper conveyance unit 30 having a plurality of conveyance rollers 28.

  The recording paper transport unit 30 transports the recording paper 16 to the transport belt 19.

  A peeling plate 32 is disposed on the downstream side in the driving direction of the conveying belt 19 in FIG. 1 from the portion facing the recording head array 12. The peeling plate 32 peels the recording paper 16 from the transport belt 19. The recording paper 16 peeled from the transport belt 19 is transported by a plurality of discharge rollers 36 constituting a discharge transport section 34 and is discharged to a paper discharge tray 38 provided at the upper part of the image forming apparatus 10.

  A cleaning roll 40 capable of sandwiching the conveyance belt 19 with the drive roll 20B is disposed on the downstream side in the rotation direction of the conveyance belt 19 in FIG. The cleaning roll 40 cleans the surface of the conveyor belt 19.

  In addition, the recording paper 16 having an image formed on one side may be transported to the transport belt 19 again by the reverse transport unit 44 including a plurality of reversing rollers 42, and an image may be formed on the other surface. it can. The reverse conveyance unit 44 branches from the discharge conveyance unit 34 and is arranged so as to convey the recording paper 16 to the recording paper conveyance unit 30.

  An optical sensor 46 is disposed on the upstream side in the rotational direction from the position where the peeling plate 32 is disposed on the downstream side in the rotational direction of the conveyor belt 19 in FIG. 1 with respect to the portion facing the recording head array 12. The optical sensor 46 is composed of, for example, a CCD line sensor or a CCD area sensor, and reads, for example, a test pattern image for correcting the density of ink ejected by the ejection nozzle 812 with a predetermined reading resolution.

  FIG. 2 is a diagram illustrating a main part of a control system of the image forming apparatus 10.

  The image forming apparatus 10 includes a CPU 100 that controls the entire image forming apparatus 10. The CPU 100 includes a ROM 102, a RAM 104, a hard disk storage device 106, an image data input unit 108, an operation display unit 110, an image formation control unit 112, an image data processing unit 114, an optical sensor 46, and a bus 116 such as a control bus or a data bus. Connected.

  The ROM 102 stores a control program for controlling the image forming apparatus 10 and nozzle characteristic data (gamma table or the like) for each ejection nozzle 812. The RAM 104 is used as a work space for processing various data.

  The hard disk storage device 106 stores image data, test pattern data for forming a test pattern image, various data relating to image formation, and the like.

  The image data input unit 108 receives input of image data from a personal computer (not shown). The input image data is transmitted to the hard disk storage device 106.

  The operation display unit 110 includes a touch panel in which an operation function and a display function are integrated, and operation buttons for a user to perform various operations. The operation display unit 110 receives an operation such as the start of image formation on the recording paper 16 and notifies the user of the control state of the image forming apparatus 10.

  The image formation control unit 112 controls the recording heads 14C, 14M, 14Y, 14K, 14T, the conveyance belt 19 and the like in order to form an image on the recording paper 16 based on the image data.

  The image data processing unit 114 performs predetermined image processing such as ink density adjustment on the image data stored in the hard disk storage device 106. Further, the image data processing unit 114 processes the test pattern image data obtained by reading the test pattern image with the optical sensor 46 and corrects the ink density of the image formed on the recording paper 16. Create a table.

  FIG. 3 is a functional block diagram for creating an ink density correction table performed by the image data processing unit 114.

  The image data processing unit 114 includes an ink density distribution creation unit 200 and an ink density correction table creation unit 202.

  The ink density distribution creation unit 200 includes a scan data ink density distribution creation unit 204 and a complementary ink density distribution creation unit 206.

  The scan data ink density distribution creation unit 204 receives test pattern image data obtained by the optical sensor 46 reading the test pattern image, and creates an ink density distribution based on the test pattern image data.

  Here, the reading resolution when reading the test pattern image by the optical sensor 46 will be described with reference to FIG.

  A reading pattern A shown in FIG. 4A shows a case where the optical sensor 46 reads a test pattern image with the same resolution as the nozzle resolution that is the resolution of the image formed by the recording heads 14C, 14M, 14Y, and 14K. . That is, high resolution reading. In this case, the ink density distribution of the discharge nozzle 812 can be obtained directly. However, it takes a long time to read the test pattern image.

  On the other hand, a reading pattern B shown in FIG. 4A shows a case where the optical sensor 46 reads a test pattern image at a resolution lower than the nozzle resolution. That is, low resolution reading. In this case, the ink density distribution of the discharge nozzles 812 cannot be obtained directly, and a plurality of discharge nozzles within a predetermined range, for example, individual regions surrounded by a broken line of the read pattern B in FIG. An average value of the ink density of 812 is obtained. However, the time for reading the test pattern image is shorter than that for reading with the read pattern A.

  In the present embodiment, the optical sensor 46 reads the test pattern image at a low resolution that is the reading pattern B. For this reason, the reading speed of the test pattern image is faster than that when reading at a high resolution as the reading pattern A. For example, when a test pattern image is read at 150 dpi with respect to the recording heads 14C, 14M, 14Y, and 14K having a nozzle resolution of 600 dpi, a 16-fold speed improvement is expected compared to reading at the same reading resolution as the nozzle resolution. it can.

  The scan data ink density distribution creating unit 204 aligns the test pattern image data read by the optical sensor 46 with the ejection nozzles 812 to create an ink density distribution. In FIG. 4B, the vertical axis is the ink density, the horizontal axis is the identification number of the discharge nozzle 812, the optical sensor 46 reads the test pattern image, and the scan data ink density distribution is based on the obtained test pattern image data. An example of the ink density distribution created by the creation unit 204 is shown. The discharge nozzle 812 having a younger identification number is located closer to the main flow portion 800. As shown in FIG. 4B, the ink density distribution is a sparse ink density distribution that does not correspond to each of all the ejection nozzles 812 because it is read at a low resolution. However, it can be seen that the ink density distribution shown in FIG. 4B has a linear tendency.

  The complementary ink density distribution creation unit 206 receives data relating to the ink density distribution created by the scan data ink density distribution creation unit 204. The complementary ink density distribution creating unit 206 is connected to the ink density unevenness regularity storage unit 208 provided in the hard disk storage device 106.

  The ink density unevenness regularity storage unit 208 stores a function representing a periodic regularity tendency peculiar to the recording heads 14C, 14M, 14Y, and 14K in which the discharge nozzles 812 are arranged in two dimensions. To do. The function representing the tendency of regularity is different for each of the recording heads 14C, 14M, 14Y, and 14K. For example, an ink density distribution having a sawtooth periodic regularity as shown in FIG. , B is used to repeat the regularity represented by a linear function as in equation (1).

y = ax + b (1)
In addition, for example, an ink density distribution having a periodic regularity of a half cycle of a sine wave is represented by a sine function. The periodic regularity for each of the recording heads 14C, 14M, 14Y, and 14K is obtained in advance, and a function indicating the periodic regularity corresponding to each of the recording heads 14C, 14M, 14Y, and 14K is stored.

  The complementary ink density distribution creation unit 206 uses the function stored in the ink density unevenness regularity storage unit 208 to calculate, for example, the equation (1) from the sparse ink density distribution created by the scan data ink density distribution creation unit 204. Are obtained, and an approximate expression indicating the ink density distribution is obtained. Then, a complementary ink density distribution is created by complementing the ink density corresponding to each of the ejection nozzles 812 based on the obtained approximate expression of the ink density distribution.

  FIG. 4C shows a complementary ink density distribution obtained by approximating the sparse ink density distribution of FIG. 4B with a linear function and complementing the ink density corresponding to each of the ejection nozzles 812.

  A line A indicated by a solid line in FIG. 4C is an ink density distribution that is complemented based on an approximate expression that approximates the ink density distribution in FIG. A line B indicated by a broken line in FIG. 4C is an ink density distribution when a test pattern image is read at a low resolution. As described above, the complementary ink density distribution (line A in FIG. 4C) substantially matches the ink density distribution (line B in FIG. 4C) obtained by reading the test pattern with high resolution.

  Note that the human visual sensitivity to an image is sensitive to changes in ink density that change at low frequencies, but is insensitive to changes in ink density that change at high frequencies. Specifically, in the modulation transfer function (MTF) of the visual system at an observation distance of 30 cm as shown in FIG. 8, a low frequency region having a spatial frequency of 8 cycles / mm or less is a visually sensitive region, and 8 cycles. A high-frequency region exceeding / mm is an insensitive region.

  Here, for example, the ink density of the line B in FIG. 4C corresponding to the discharge nozzle number 7 in FIG. 4C is the ink density obtained from the approximate expression of the line A in FIG. 4C. Is larger than the ink density corresponding to the other ejection nozzle numbers. That is, the ink density change corresponding to the discharge nozzle number 7 changes at a high frequency. However, the line A in FIG. 4C, which is an approximate expression, substantially coincides with the line B in FIG. 4C, and the low frequency change of the ink density change can be captured. Therefore, it is considered that the high-frequency ink density change at the discharge nozzle number 7 has little influence on human eyes.

  The ink density correction table creation unit 202 corrects the ejection amount of the ink droplets ejected by the ejection nozzle 812 based on the complementary ink density distribution created by the complementary ink density distribution creation unit 206 so that the ink density unevenness does not occur. An ink density correction table is created for this purpose. The created ink density correction table is stored in the ink density correction table storage unit 210 of the hard disk storage device 106. At this time, if the previously created ink density correction table is already stored in the ink density correction table storage unit 210, the ink density correction table is rewritten to the newly created ink density correction table.

  The droplet discharge amount control unit 212 of the image formation control unit 112 includes discharge nozzles 812 constituting the recording heads 14C, 14M, 14Y, and 14K based on the ink density correction table stored in the ink density correction table storage unit 210. The ejection amount of the ink droplet to be ejected is controlled.

  The operation of the first embodiment will be described below.

  With reference to the flowchart of FIG. 5, the control related to the ink density correction table creation process will be described.

  First, in step 500, a test pattern image is formed on the recording paper 16.

  Next, in step 502, the optical sensor 46 reads the test pattern image formed on the recording paper 16 with low resolution.

  Next, in step 504, the scan data ink density distribution creation unit 204 creates an ink density distribution based on the test pattern image data of the test pattern image read by the optical sensor 46.

  Next, in step 506, the complementary ink density distribution creation unit 206 obtains an approximate expression of the ink density distribution created by the scan data ink density distribution creation part 204, and uses the ink density distribution for each ejection nozzle 812 from the approximate expression. Create a complementary ink density distribution.

  In step 508, the ink density correction table creation unit 202 creates an ink density correction table based on the complementary ink density distribution.

  Next, in step 510, the ink density correction table storage unit 210 stores the ink density correction table, and the ink density correction table creation process ends.

  As described above, the optical sensor 46 reads the test pattern image for correcting the density of the ink ejected by the ejection nozzle 812 at a resolution lower than the nozzle resolution, obtains the test pattern image data, and creates the ink density distribution. . Since the ink density distribution is created based on test pattern image data read at a low resolution, it is not an ink density distribution for each ejection nozzle 812 but sparse. Therefore, an approximate expression is obtained from the known periodic regularity of the ink density unevenness, thereby creating a complementary ink density distribution that complements the ink density for each ejection nozzle 812 and creating an ink density correction table.

  As a result, the speed required for ink density correction that eliminates ink density unevenness periodically generated by the recording heads 14C, 14M, 14Y, and 14K in which the ejection nozzles 812 are two-dimensionally arranged is improved, and an approximate expression of the ink density distribution is obtained. Thus, the accuracy of ink density correction can be increased by obtaining the ink density for each ejection nozzle 812.

Note that the image forming apparatus 10 described in the present embodiment forms an image (including characters) on the recording paper 16, but the image forming apparatus 10 of the present invention is not limited to this. Absent. In other words, the recording medium is not limited to the recording paper 16, and the liquid to be discharged is not limited to the ink liquid. For example, the present invention can also be applied to other droplet discharge recording apparatuses such as a pattern forming apparatus that discharges droplets onto a sheet-like substrate for pattern formation of a semiconductor or a liquid crystal display.
(Second Embodiment)
The second embodiment of the present invention will be described below. Note that, in the second embodiment of the present invention, the same components as those in the first embodiment are denoted by the same reference numerals, and description of the configuration is omitted.

  The feature of the second embodiment is that the regularity of the ink density unevenness in the vicinity of the end portion of the tributary portion 804 is changed when noise occurs near the end portion of the tributary portion 804 and the regularity of the ink density unevenness changes. For the sake of clarity, the region corresponding to the end portion of the tributary portion 804 of the test pattern image data is read with a higher reading resolution than in the first embodiment.

  FIG. 6A shows the reading resolution when reading the test pattern image with the optical sensor 46 when the regularity of the ink density unevenness changes in the vicinity of the end of the branch portion 804.

  The optical sensor 46 reads a region corresponding to the end of the branch portion 804 of the test pattern image with the reading pattern C. The read pattern C is a case where the test pattern image is read at a higher reading resolution than the read pattern B, and matches the nozzle resolution (read pattern A), for example. The area other than the region corresponding to the end of the tributary part 804, that is, the central part of the tributary part 804 is read with the reading pattern B having a low resolution. By reading the region corresponding to the end of the tributary unit 804 with the high-resolution reading pattern C, the complementary ink density distribution creating unit 206 increases the accuracy of complementing the ink density distribution and creates a highly accurate ink density correction table. can do.

  FIG. 6B shows the ink density distribution obtained by the optical sensor 46 reading the test pattern image at low resolution and high resolution. Area α is the ink density distribution at the end of the tributary section 804 that is read at high resolution, and the ink density distribution is at the center of the tributary section 804 that is read at low resolution. The region γ is the end opposite to the side where the ink liquid flows in the tributary portion 804 read at high resolution. Since the ink density distribution in the area α and the area γ has a smaller slope than the ink density distribution in the area β, it is necessary to create a different approximate expression for each area.

  6C approximates the ink density distribution of FIG. 6B with different linear functions in the areas α, β, and γ, and the complementary ink density distribution (206) created by the complementary ink density distribution creating unit 206. FIG. 6C shows a line C). A line D indicated by a broken line in FIG. 6C is an ink density distribution when a test pattern image is read at a high resolution. Thus, even when the regularity of the ink density distribution changes, the complementary ink density distribution (line C in FIG. 6C) is an ink density distribution obtained by reading the test pattern with the same resolution as the nozzle resolution. This substantially coincides with (line D in FIG. 6C).

  FIG. 7A shows the ink density distribution when the area of the test pattern image corresponding to the end of the tributary portion 804 is read by the reading pattern C, as in FIG. The ink density distribution in the region α and the region γ in FIG. 7A shows a sinusoidal tendency.

  7B approximates the ink density distribution in the region α and the region γ with a sine function from the ink density distribution in FIG. 7A, and approximates the ink density distribution in the region β with a linear function to obtain a complementary ink density distribution. The complementary ink density distribution (line E in FIG. 7B) created by the creation unit 206 is shown. A line F indicated by a broken line in FIG. 7B is an ink density distribution when a test pattern image is read at a high resolution. Thus, even when the regularity of the ink density distribution changes greatly from a straight line to a curved line, the complementary ink density distribution (line E in FIG. 7B) shows the ink density obtained by reading the test pattern with high resolution. It almost coincides with the distribution (line F in FIG. 7B).

  As described above, when the regularity of the ink density distribution changes at the end portion of the tributary portion 804, the test pattern image corresponding to the end portion of the tributary portion 804 is read at a higher reading resolution than the central portion. Complementary ink density distributions are created using different approximation formulas for the central part and the central part. Thereby, even if the regularity of the ink density distribution changes, it is possible to create an ink density complement table with high accuracy.

1 is a schematic diagram illustrating an overall configuration of an ink jet image forming apparatus. FIG. 2 is a diagram illustrating a main part of a control system of the image forming apparatus. It is a functional block diagram regarding creation of an ink density correction table performed in an image data processing unit. (A) is a figure which shows the reading resolution by the optical sensor based on 1st Embodiment, (B) is the ink density distribution obtained by reading the test pattern image concerning 1st Embodiment. FIG. 6C is a diagram illustrating a complementary ink density distribution according to the first embodiment. 6 is a flowchart relating to control relating to creation processing of an ink density correction table. (A) is a diagram showing the reading resolution by the optical sensor according to the second embodiment, and (B) is a tributary gradient of the ink density distribution at the end of the tributary portion according to the second embodiment. It is a figure which shows the ink density distribution obtained by reading a test pattern image in case it is smaller than the inclination of the ink density distribution in the center part of a part. (C) is a figure which shows the complementary ink density distribution obtained based on FIG. 6 (B) concerning 2nd Embodiment. (A) is a test according to the second embodiment when the ink density distribution at the end of the tributary part is approximated by a sine function and the ink density distribution at the central part of the tributary part is approximated by a linear function. The figure which shows the ink density distribution obtained by reading a pattern image, (B) is a figure which shows the complementary ink density distribution obtained based on FIG. 7 (A) concerning 2nd Embodiment. It is a figure which shows the sensitivity of human vision in the modulation transfer function of a visual system. (A) is a diagram showing a structure of a recording head in which ejection nozzles are two-dimensionally arranged, and (B) is a diagram showing a change in periodic ink density distribution peculiar to a recording head in which ejection nozzles are two-dimensionally arranged. (C) is a diagram showing the regularity of a periodic change in the ink density distribution based on the ink density and the discharge nozzle position.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Image forming apparatus 14 Recording head 16 Recording paper (recording medium)
46 Optical sensor (image reading means)
112 Image formation control unit (image forming means)
200 Ink density distribution creation unit (ink density distribution creation means)
202 Ink density correction table creation unit (ink density correction table creation means)
210 Ink density correction table storage unit (ink density correction table storage means)
212 Droplet discharge amount control unit (droplet discharge amount control means)
804 Branch 812 Discharge nozzle

Claims (3)

As a record medium and relative movement relative to the direction in which the plurality of branch portions each arranged angularly inclined in Jo Tokoro with respect to the direction, providing a plurality of ejection nozzles for ejecting ink droplets at a predetermined pitch it is a recording head to which the plurality of ejection nozzles are arranged with a two-dimensional regularity before type recording medium and the direction perpendicular to the direction and the recording medium and the relative moving direction of the relative movement,
Wherein from the recording head and the recording medium and the length et previous SL plurality of discharge nozzles are moved relative to ejecting the ink droplet, before Symbol pitch of the plurality of discharge nozzles in the direction perpendicular to the direction of relative movement An image forming means for forming an image with a resolution equivalent to
Than said recording head is disposed in the downstream side in the movement direction of the recording medium, wherein the reference image formed on the recording medium by the image forming unit, an image reading obtain the reference image data I read in the lower resolution reading resolution Means,
A direction perpendicular to the direction of the relative movement of the reference image data obtained by the image reading unit and an image formed on the recording medium by the image forming unit including the recording head, which is obtained in advance for the recording head. ink based on the periodic regularity of density change, and the ink concentration distribution creation means to create an ink concentration distribution of the plurality of ejection nozzles in,
Based on the ink density distribution of the plurality of ejection nozzles obtained by the ink density distribution creating means, the ejection amount of the ink droplets ejected by the plurality of ejection nozzles is corrected so as to eliminate the ink density change. Ink density correction table creating means for creating an ink density correction table for
An ink density correction table storage unit that updates and stores the ink density correction table created by the ink density correction table creation unit every time the reference image data is obtained;
Droplet ejection amount control means for controlling the ejection amount of the ink droplets for each of the plurality of ejection nozzles using the ink density correction table stored in the ink density correction table storage means;
An image forming apparatus. The ink density distribution creating unit calculates an approximate line of the ink density distribution for each of the plurality of tributaries, and obtains the ink density distribution of the plurality of ejection nozzles from the calculated approximate line. The image forming apparatus according to 1. The image reading means reads a region of the reference image corresponding to a portion other than the ends of the plurality of branch portions at a first reading resolution, and reads the region of the reference image corresponding to the ends of the plurality of branch portions. first I read at a higher than reading resolution second reading resolution, the image forming apparatus according to claim 1 or claim 2, wherein Rukoto obtain the reference image data.