Image forming apparatus

Abstract

To accurately detect a discharge defective nozzle at low cost.SOLUTION: A first test pattern has a first band including a thin line corresponding to a nozzle for each of a plurality of nozzle groups in which first nozzles are grouped into one nozzle group. A second test pattern has a second band including a white-outlined thin line corresponding to a nozzle, whose adjacent pixels are corrected, for each of a plurality of nozzle sub groups in which second nozzles in each nozzle group are grouped into one nozzle sub group. In a read-out image of the second test pattern with respect to the first band with density-reduction caused which is detected through a read-out image of the printed first test pattern, the nozzle corresponding to the white-outlined thin line in the second band with no density-reduction caused is detected as a discharge defective nozzle, on the basis of a density distribution in a reference range specified based on a distribution of sums or average values of a plurality of pixel values of the second band.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to an image forming apparatus.

Background Art

[0002] A certain image forming apparatus includes a recording head in which a plurality of nozzles are arranged, detects the landing position and landing area of each nozzle, measures the deflection value of a defective ejection nozzle, generates a nozzle profile, and executes correction processing based on the nozzle profile (see, for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, in the above-described image forming apparatus, in order to measure the above-described deflection value, a high-resolution image reading device such as 4800 dpi is required, resulting in high costs.

[0005] The present invention has been made in view of the above problems, and an object thereof is to obtain an image forming apparatus that accurately detects defective ejection nozzles at a relatively low cost.

Means for Solving the Problems

[0006] The image forming apparatus according to the present invention comprises a recording head that ejects ink corresponding to an image to be printed from an arranged nozzle, and a defective nozzle detection unit that detects a defective nozzle among the nozzles. The defective nozzle detection unit (a) sets up a plurality of nozzle groups by shifting by one nozzle each, with nozzles at predetermined intervals of a first number of nozzles as one nozzle group, (b) prints a first test pattern with the recording head, each having a first band containing a fine line corresponding to the nozzle in the nozzle group, (c) sets up a plurality of nozzle subgroups by shifting by one nozzle each, with nozzles at predetermined intervals of a second number of nozzles in the nozzle group as one nozzle subgroup, and (d) prints a second band in the nozzle subgroup that contains a white fine line corresponding to the nozzle and has correction processing applied to adjacent pixels of the white fine line. (e) print the second test pattern having a subgroup with the recording head, (f) acquire the read image of the printed first test pattern and the read image of the printed second test pattern, (g) detect the first band in which density loss has occurred in the read image of the first test pattern, and (g) in the read image of the second test pattern, detect the second band in which density loss has not occurred due to the correction process from among the plurality of second bands corresponding to the plurality of nozzle subgroups in the nozzle group of the detected first band, and detect the nozzle corresponding to the white thin line in the second band in which density loss has not occurred as the defective nozzle. Furthermore, when the defective nozzle detection unit detects the second band in which density loss has not occurred due to the correction process, it derives the sum or average value of the pixel values ​​of the plurality of second bands for each pixel position in the read image of the second test pattern, identifies a reference range based on the sum or average value, and detects the second band in which density loss has not occurred due to the correction process based on the density distribution of the reference range in the plurality of second bands. [Effects of the Invention]

[0007] According to the present invention, an image forming apparatus that can accurately detect defective nozzles at a relatively low cost can be obtained.

[0008] The above or other objects, features, and advantages of the present invention will become even more apparent from the following detailed description in conjunction with the accompanying drawings. [Brief explanation of the drawing]

[0009] [Figure 1] Figure 1 is a side view illustrating the mechanical internal configuration of an image forming apparatus according to an embodiment of the present invention. [Figure 2] Figure 2 is a plan view showing an example of recording heads 1a, 1b, 1c, and 1d in the image forming apparatus 10 shown in Figure 1. [Figure 3] Figure 3 is a block diagram showing the electrical configuration of an image forming apparatus 10 according to an embodiment of the present invention. [Figure 4] Figure 4 shows an example of the first test pattern. [Figure 5] Figure 5 shows an example of the second test pattern. [Figure 6] Figure 6 illustrates the detection of density defects in the reading image of the first test pattern. [Figure 7] Figure 7 illustrates the detection of a faulty nozzle based on the reading image of the second test pattern. [Figure 8] Figure 8 shows an example of the distribution of pixel values ​​(R, G, or B values) in the subscanning direction for bands 301A1 to 301A7 in Figure 7. [Figure 9] Figure 9 shows the distribution of the sum of pixel values ​​at each pixel position in the sub-scanning direction for bands 301A1 to 301A7 in Figure 8. [Modes for carrying out the invention]

[0010] Embodiments of the present invention will be described below with reference to the figures.

[0011] Figure 1 is a side view illustrating the mechanical internal configuration of an image forming apparatus according to an embodiment of the present invention. The image forming apparatus 10 according to this embodiment is a device such as a printer, copier, facsimile machine, or multifunction device.

[0012] The image forming apparatus 10 shown in Figure 1 comprises a print engine 10a and a sheet transport unit 10b. The print engine 10a physically forms the page image to be printed on a print sheet (such as a print sheet). In this embodiment, the print engine 10a is a line-type inkjet print engine.

[0013] In this embodiment, the print engine 10a is equipped with line-type recording heads 1a to 1d that correspond to four ink colors: cyan, magenta, yellow, and black.

[0014] Figure 2 is a plan view showing an example of recording heads 1a, 1b, 1c, and 1d in the image forming apparatus 10 shown in Figure 1. For example, as shown in Figure 2, in this embodiment, each recording head 1a, 1b, 1c, and 1d has multiple (in this case, three) head units 11. These head units 11 are arranged along the main scanning direction and are detachable from the apparatus body. Note that each recording head 1a, 1b, 1c, and 1d may have only one head unit 11. The head units 11 of the recording heads 1a, 1b, 1c, and 1d are equipped with nozzles arranged in a two-dimensional manner, and these nozzles eject ink corresponding to the image to be printed.

[0015] The sheet transport unit 10b transports the print sheet before printing to the print engine 10a along a predetermined transport path, and also transports the printed sheet after printing from the print engine 10a to a predetermined discharge destination (such as the discharge tray 10c).

[0016] The sheet conveyance unit 10b includes a main sheet conveyance unit 10b1 and a circulation sheet conveyance unit 10b2. In double-sided printing, the main sheet conveyance unit 10b1 conveys a print sheet used for printing the page image on the first side to the print engine 10a, and the circulation sheet conveyance unit 10b2 conveys the print sheet from the rear stage to the front stage of the print engine 10a while retaining a predetermined number of print sheets.

[0017] In this embodiment, the main sheet conveyance unit 10b1 includes an annular conveyance belt 2 disposed opposite to the print engine 10a for conveying the print sheet, a driving roller 3 and a driven roller 4 for suspending the conveyance belt 2, a suction roller 5 for nipping the print sheet together with the conveyance belt 2, and a discharge roller pair 6, 6a.

[0018] The driving roller 3 and the driven roller 4 circulate the conveyance belt 2. Then, the suction roller 5 nips the print sheet conveyed from the paper feed cassettes 20-1, 20-2 described later, and the nipped print sheet is sequentially conveyed to the print positions of the recording heads 1a to 1d by the conveyance belt 2, and the images of each color are printed by the recording heads 1a to 1d. After the color printing is completed, the print sheet is discharged to the discharge tray 10c or the like by the discharge roller pair 6, 6a.

[0019] Furthermore, the main sheet conveyance unit 10b1 includes a plurality of paper feed cassettes 20-1 and 20-2. The paper feed cassettes 20-1 and 20-2 accommodate print sheets SH1 and SH2, and push up the print sheets SH1 and SH2 upward with lift plates 21 and 24 to abut against pickup rollers 22 and 25. The print sheets SH1 and SH2 placed on the paper feed cassettes 20-1 and 20-2 are picked up one by one from above by the pickup rollers 22 and 25 and fed to paper feed rollers 23 and 26. The paper feed rollers 23 and 26 are rollers that convey the print sheets SH1 and SH2 fed from the paper feed cassettes 20-1 and 20-2 by the pickup rollers 22 and 25 one by one onto the conveyance path. The conveyance roller 27 is a conveyance roller on the common conveyance path for the print sheets SH1 and SH2 conveyed from the paper feed cassettes 20-1 and 20-2.

[0020] The circulation sheet conveyance unit 10b2 returns the print sheet from a predetermined position on the downstream side to a predetermined position on the upstream side (here, a predetermined position on the upstream side of a line sensor 31 described later) of the print engine 10a during double-sided printing. The circulation sheet conveyance unit 10b2 includes a conveyance roller 41 and a switchback conveyance path 41a that reverses the traveling direction of the print sheet in order to switch the surface of the print sheet facing the print engine 10a from the first surface to the second surface.

[0021] Furthermore, the image forming apparatus 10 includes a line sensor 31 and a sheet detection sensor 32.

[0022] The line sensor 31 is an optical sensor that is arranged along the direction perpendicular to the conveyance direction of the print sheet and detects the positions of both end edges (both side edges) of the print sheet. For example, the line sensor 31 is a CIS (Contact Image Sensor). In this embodiment, the line sensor 31 is arranged between the registration roller 28 and the print engine 10a.

[0023] The sheet detection sensor 32 is an optical sensor that detects when the leading edge of the printed sheets SH1 and SH2 has passed a predetermined position on the transport path. The line sensor 31 detects the positions of both edges of the printed sheets when the leading edge of the printed sheets SH1 and SH2 is detected by the sheet detection sensor 32.

[0024] For example, as shown in Figure 1, the print engine 10a is positioned above or below the print sheet transport path (in this case, above), the line sensor 31 is positioned above or below the print sheet transport path (in this case, below), and the circulating sheet transport unit 10b2 transports the print sheet by switching back from downstream of the print engine 10a to upstream of the line sensor 31.

[0025] Figure 3 is a block diagram showing the electrical configuration of an image forming apparatus 10 according to an embodiment of the present invention. As shown in Figure 3, the image forming apparatus 10 includes an image output unit 71 having the mechanical configuration shown in Figures 1 and 2, as well as an operation panel 72, a storage device 73, an image reading device 74, and a controller 75.

[0026] The control panel 72 is located on the surface of the housing of the image forming apparatus 10 and includes a display device 72a such as a liquid crystal display, and input devices 72b such as hard keys and a touch panel. The display device 72a displays various messages to the user, and the input devices 72b accept user operations.

[0027] The memory device 73 is a non-volatile memory device (such as a flash memory or hard disk drive) that stores data, programs, etc., necessary for controlling the image forming apparatus 10.

[0028] The image reading device 74 includes a platen glass and an automatic document feeder. It optically reads images of documents placed on the platen glass or transported by the automatic document feeder, and generates image data of those images.

[0029] The controller 75 includes a computer that executes software processing according to a program, an ASIC (Application Specific Integrated Circuit) that executes predetermined hardware processing, and operates as various processing units. The computer includes a CPU (Central Processing Unit), ROM (Read Only Memory), RAM (Random Access Memory), etc., and loads programs stored in the ROM, storage device 73, etc., into the RAM and executes them with the CPU, thereby operating as various processing units (together with the ASIC as needed). In this case, the controller 75 operates as a control unit 81, an image processing unit 82, a nozzle defect detection unit 83, and a correction processing unit 84.

[0030] The control unit 81 controls the image output unit 71 (print engine 10a, sheet transport unit 10b, etc.) and executes the print job requested by the user. In this embodiment, the control unit 81 causes the image processing unit 82 to perform predetermined image processing and controls the print engine 10a (head unit 11) to eject ink and form a print image on the print sheet. The image processing unit 82 performs predetermined image processing such as RIP (Raster Image Processing), color conversion, and halftoning on the image data of the image to be printed on the print sheet.

[0031] Specifically, the control unit 81 instructs the print engine 10a to print user-specified image data based on print image data specified by the user, as well as test patterns described later.

[0032] Furthermore, in this embodiment, the control unit 81 has an automatic centering function that (a) identifies the center position of the print sheet as the actual sheet center position based on the positions of both edges of the print sheet detected by the line sensor 31, and (b) adjusts the center position of the image to be printed based on the actual sheet center position, and executes the automatic centering function as hardware processing.

[0033] Specifically, in the automatic centering function, the control unit 81 changes the drawing position of the image to be printed along the main scanning direction by the difference between the reference center position of the print engine 10a and the actual sheet center position. In this embodiment, since the nozzles in the recording heads 1a to 1d do not move, the nozzle corresponding to each pixel in the image to be printed is changed according to the drawing position of the image to be printed.

[0034] In this way, the control unit 81 determines the nozzle corresponding to the image to be printed (a nozzle corresponding to each pixel) according to the position of the print sheet, and causes the recording heads 1a to 1d to eject ink from the nozzle.

[0035] The nozzle failure detection unit 83 detects a nozzle failure among the nozzles of the recording heads 1a to 1d.

[0036] The nozzle ejection defect detection unit 83 (a) sets up multiple nozzle groups by shifting by one nozzle each, with nozzles at predetermined intervals of a certain number of nozzles in each recording head 1a to 1d as one nozzle group, and (b) prints a first test pattern with the recording heads 1a to 1d, each having a first band containing a fine line corresponding to the nozzle in the nozzle group.

[0037] Figure 4 shows an example of the first test pattern. In the first test pattern shown in Figure 4, the nozzles in each recording head 1a to 1d are classified into four nozzle groups A, B, C, and D, such that every three nozzles (the predetermined first number mentioned above) form one nozzle group. Note that the first test pattern is not limited to the one shown in Figure 4.

[0038] Ink is ejected by nozzle Ai(j) of nozzle group A (i=1,...,N1,j=1,...,N2, where N1 and N2 are constants, N1 is the number of nozzles included in the nozzle subgroup described later, and N2 is a value corresponding to the total number of nozzles), and band 101A (i.e., the fine line 111 in band 101A) is drawn. This fine line 111 is an image with density, having a width of 1 dot in the main scanning direction and a length of L1 (6 dots in this case) in the sub-scanning direction. In other words, ink is not ejected in areas other than this fine line.

[0039] Similarly, ink is ejected by nozzle Bi(j) of nozzle group B, drawing band 101B; ink is ejected by nozzle Ci(j) of nozzle group C, drawing band 101C; and ink is ejected by nozzle Di(j) of nozzle group D, drawing band 101D.

[0040] Furthermore, the nozzle defect detection unit 83 sets up multiple nozzle subgroups by shifting each nozzle by one nozzle, with (c) nozzles at predetermined intervals of two from among the nozzles in the nozzle group, and (d) printing a second test pattern with the recording heads 1a to 1d, which has a second band for each nozzle subgroup that includes a white thin line corresponding to the nozzle in the nozzle subgroup and has correction processing applied to the adjacent pixels of the white thin line.

[0041] Figure 5 shows an example of a second test pattern. In the second test pattern shown in Figure 5, the nozzles Ai(j) in nozzle group A are classified into seven nozzle subgroups A1, ..., A7, such that every six nozzles (the predetermined second number mentioned above) form one nozzle subgroup. Note that the second test pattern is not limited to the one shown in Figure 5.

[0042] Here, nozzle subgroup A1 includes nozzle A1(j) from nozzle Ai(j), nozzle subgroup A2 includes nozzle A2(j) from nozzle Ai(j), nozzle subgroup A3 includes nozzle A3(j) from nozzle Ai(j), nozzle subgroup A4 includes nozzle A4(j) from nozzle Ai(j), nozzle subgroup A5 includes nozzle A5(j) from nozzle Ai(j), nozzle subgroup A6 includes nozzle A6(j) from nozzle Ai(j), and nozzle subgroup A7 includes nozzle A7(j) from nozzle Ai(j).

[0043] Similarly, nozzles Bi(j) in nozzle group B are classified into seven nozzle subgroups B1, ..., B7, such that every six nozzles form one nozzle subgroup. Similarly, nozzles Ci(j) in nozzle group C are classified into seven nozzle subgroups C1, ..., C7, such that every six nozzles form one nozzle subgroup. Similarly, nozzles Di(j) in nozzle group D are classified into seven nozzle subgroups D1, ..., D7, such that every six nozzles form one nozzle subgroup.

[0044] The second test pattern includes bands 201A1 to 201A7 corresponding to nozzle subgroups A1, ..., A7; bands 201B1 to 201B7 corresponding to nozzle subgroups B1, ..., B7; bands 201C1 to 201C7 corresponding to nozzle subgroups C1, ..., C7; and bands 201D1 to 201D7 corresponding to nozzle subgroups D1, ..., D7.

[0045] The nozzles of nozzle subgroups A1,...,A7 correspond to the white thin lines 311 in bands 201A1~201A7, the nozzles of nozzle subgroups B1,...,B7 correspond to the white thin lines 311 in bands 201B1~201B7, the nozzles of nozzle subgroups C1,...,C7 correspond to the white thin lines 311 in bands 201C1~201C7, and the nozzles of nozzle subgroups D1,...,D7 correspond to the white thin lines 311 in bands 201D1~201D7. Note that the white thin lines 311 are images with a width of 1 dot in the main scanning direction and a length of L2 (4 dots in this case) in the sub-scanning direction, and have no density.

[0046] Ink is not ejected by nozzle A1(j) of nozzle subgroup A1, but a predetermined amount of ink (midtone density) is ejected by nozzles other than nozzle A1(j), and band 201A is drawn. At that time, the amount of ink ejected by pixel 312 adjacent to the white thin line 311 in the main scanning direction (i.e., by the nozzle adjacent to nozzle A1(j)) is increased by a correction process so that the white thin line 311 becomes invisible after printing. The remaining bands 201A2~201A7, 201B1~201B7, 201C1~201C7, and 201D1~201D7 are drawn in the same manner.

[0047] Furthermore, the nozzle defect detection unit 83 (e) acquires a reading image of the first test pattern printed on a print sheet or the like and a reading image of the printed second test pattern, (f) detects a first band in which density loss occurs in the reading image of the first test pattern, and (g) detects a second band in the reading image of the second test pattern in which no density loss occurs due to correction processing, among the second bands corresponding to multiple nozzle subgroups in the nozzle group of the detected first band, and detects the nozzle corresponding to the white thin line in the second band in which no density loss occurs as a nozzle defect.

[0048] The images of these test patterns are acquired using the line sensor 31 or the image reader 74. When the line sensor 31 is used to detect the ejection defect position as described above, the printed sheet with the test pattern is automatically transported to the line sensor 31, scanned, and the image of the test pattern (image data) is supplied to the controller 75. The printed sheet with the test pattern is then ejected. Alternatively, instead of the line sensor 31, the printed sheet with the test pattern is immediately ejected, and the image of the printed sheet (read image) set by the user in the image reader 74 is scanned by the image reader 74, and the image of the test pattern (image data) is supplied to the controller 75.

[0049] Figure 6 illustrates the detection of density defects in the reading image of the first test pattern. In this embodiment, the discharge defect nozzle detection unit 83 smooths the reading image of the first test pattern and detects the first band in which density defects occur in the smoothed reading image of the first test pattern.

[0050] If a nozzle group corresponding to a first band includes a defective nozzle Ax (such as one with distortion, i.e., a nozzle where the impact point is misaligned in the main scanning direction), density loss will occur in the band's read image, as shown in Figure 6, for example. On the other hand, if a nozzle group does not include a defective nozzle Ax, no density loss will occur in the read image of the band corresponding to that noise group.

[0051] As shown in Figure 6, specifically, in the brightness distribution of the band reading image, if there is a location where the brightness exceeds a predetermined threshold (or where the density in the density distribution is below a predetermined threshold), that location is detected as a density defect location. When the reading image of the first test pattern is smoothed, the brightness difference (density difference) between the density defect location and the other locations becomes larger, making it easier to accurately detect the density defect.

[0052] Figure 7 illustrates the detection of a faulty nozzle based on the reading image of the second test pattern.

[0053] For example, if a density defect is detected in band 101A in the reading image of the first test pattern, as shown in Figure 7, the density distribution (luminance distribution) of bands 301A1 to 301A7 in a predetermined range near the density defect location in the main scanning direction is referenced for the corresponding bands 301A1 to 301A7 in the reading image of the second test pattern, and a band 301Ak in which no density defect is detected (near the density defect location) is identified (band 301A2 in Figure 7). Among the nozzles of the nozzle subgroup corresponding to the identified band 301Ak, the nozzle within that predetermined range (nozzle A2(2) in Figure 7) is detected as a defective nozzle. The width of this predetermined range is set to be less than or equal to the period of the white thin line 311.

[0054] In other words, if the nozzle of the white outline 311 is a faulty nozzle, the correction process will eliminate the white streaks caused by the faulty nozzle, and the density defect will not be detected. On the other hand, if the nozzle of the white outline 311 is not a faulty nozzle, the white outline 311 is located in a different position from the white streaks caused by the faulty nozzle, so the white streaks will not be eliminated by the correction process and will remain, and the density defect will be detected.

[0055] Figure 8 shows an example of the distribution of pixel values ​​(R, G, or B values) in the subscanning direction for bands 301A1 to 301A7 in Figure 7. Figure 9 shows the distribution of the sum of pixel values ​​at each pixel position in the subscanning direction for bands 301A1 to 301A7 in Figure 8.

[0056] Furthermore, when detecting bands where no density loss has occurred due to the correction process, the ejection defect nozzle detection unit 83 derives the sum or average value of the pixel values ​​of multiple bands 301A1 to 301A7 for each pixel position in the read image of the second test pattern, identifies a reference range based on that sum or average value, and detects bands where no density loss has occurred due to the correction process based on the density distribution of the identified reference range in bands 301A1 to 301A. Note that this pixel value is the value of the color (R value, G value, or B value) with the highest sensitivity according to the ink color among RGB. For example, if the ink color is cyan, this pixel value is the R value.

[0057] Furthermore, the discharge defect nozzle detection unit 83 (a) identifies the pixel location of density loss based on the sum or average value described above within its reference range, and (b) identifies the second band with the highest density at the identified pixel location as a band where no density loss has occurred through correction processing.

[0058] If the seven bands 301A1 to 301A7 have a pixel value distribution as shown in Figure 8, for example, the sum described above will have a distribution as shown in Figure 9. In this way, the noise contained in the pixel values ​​of bands 301A1 to 301A7 is relatively suppressed by the sum or average, making it easier to accurately identify pixel locations with density defects. Specifically, among all pixel locations in bands 301A1 to 301A7, pixel locations with density defects are identified at pixel locations where the sum or average value exceeds a predetermined threshold, and the band with the highest density (e.g., the lowest R value) at the identified pixel locations is identified as a band without density defects through the correction process. Therefore, pixel locations where the sum or average value is below the predetermined threshold are excluded from the list of pixel locations with density defects. For example, in the case of the distribution shown by the dashed line in Figure 9, the sum or average value is below the predetermined threshold at all pixel locations, so no density defect locations are detected. In this way, the erroneous detection of pixel locations with density defects due to noise, etc., is suppressed.

[0059] Returning to Figure 1, the correction processing unit 84 performs a correction process as a hardware process for each detected defective nozzle in the image to be printed. In this correction process, for example, the image data (pixel value) of a pixel adjacent to the pixel from which ink is ejected by a defective nozzle is corrected so that the density of that pixel becomes higher.

[0060] Next, the operation of the image forming apparatus 10 will be described.

[0061] (a) Determination of the location of the discharge defect that needs to be corrected

[0062] The nozzle defect detection unit 83, via the control unit 81, causes the image output unit 71 to print the first test pattern and the second test pattern described above onto the print sheet.

[0063] As described above, the nozzle ejection defect detection unit 83 uses the line sensor 31 and the image reading device 74 to acquire read images (image data of each ink color) of the first test pattern and the second test pattern.

[0064] The discharge defect nozzle detection unit 83 then determines whether a concentration deficiency, as shown in Figure 6, has occurred in each band of the first test pattern, and if a band with a concentration deficiency has occurred, it detects that band.

[0065] If there are no bands with concentration gaps, the dispensing defect nozzle detection unit 83 determines that there are no dispensing defect nozzles.

[0066] On the other hand, if there is a band with a density defect, the dispensing defective nozzle detection unit 83 (a) identifies a nozzle subgroup corresponding to the nozzle group corresponding to that band, (b) identifies a band corresponding to the identified nozzle subgroup in the reading image of the second test pattern, and (c) identifies the density distribution within a predetermined range from the density defect location in the identified band, identifies a band in that predetermined range where no density defect has occurred, and identifies the nozzle of the nozzle subgroup corresponding to the identified band as a dispensing defective nozzle.

[0067] Then, nozzle information (such as nozzle number) of the defective nozzle is stored as data in the storage device 73.

[0068] In this way, defective nozzles that are subject to correction processing are detected and set.

[0069] (b) Operation during printing

[0070] When the control unit 81 receives a print request, the image processing unit 82 performs image processing on the image specified by the print request to obtain image data of the image to be printed, and the image output unit 71 transports the print sheet and prints the image to be printed onto the print sheet based on the image data.

[0071] At that time, the correction processing unit 84 reads data on defective nozzles from the storage device 73 before printing starts to identify the defective nozzles, and when the position of the print sheet is detected by the line sensor 31, it (a) identifies the nozzle corresponding to each pixel in the above image, (b) identifies the defective nozzle in the above image, and (c) performs correction processing on the defective nozzle. Then, the control unit 81 performs the above print based on the image data after the correction processing.

[0072] As described above, according to the above embodiment, the ejection defect nozzle detection unit 83 (a) sets up multiple nozzle groups by shifting by one nozzle each, with nozzles at predetermined first intervals from the nozzles of the recording heads 1a to 1d as one nozzle group, (b) prints a first test pattern with the recording heads 1a to 1d, each having a first band containing a fine line corresponding to the nozzle in the nozzle group, (c) sets up multiple nozzle subgroups by shifting by one nozzle each, with nozzles at predetermined second intervals from the nozzles in the nozzle group as one nozzle subgroup, and (d) includes a white thin line corresponding to the nozzle in the nozzle subgroup and applies to adjacent pixels of the white thin line. A second test pattern having a second band that has been corrected for each nozzle subgroup is printed by recording heads 1a to 1d, (e) a read image of the printed first test pattern and a read image of the printed second test pattern are acquired, (f) the first band with density loss is detected in the read image of the first test pattern, and (g) the second band that does not have density loss due to the correction process is detected in the read image of the second test pattern from among the multiple second bands corresponding to multiple nozzle subgroups in the nozzle group of the detected first band, and the nozzle corresponding to the white thin line in the second band without density loss is detected as a defective nozzle. Furthermore, when detecting bands where no density loss has occurred due to the correction process, the ejection defect nozzle detection unit 83 derives the sum or average value of the pixel values ​​of multiple second bands 301A1 to 301A7 for each pixel position in the read image of the second test pattern, identifies a reference range based on that sum or average value, and detects bands where no density loss has occurred due to the correction process based on the density distribution of the identified reference range in the second bands 301A1 to 301A.

[0073] This allows for the accurate and low-cost detection of defective nozzles without the need for high-resolution image readers, by using a test pattern in which fine lines and white lines corresponding to the nozzles are intermittently arranged. Furthermore, since the reference range is determined based on the distribution of the sum or mean of pixel values ​​along the pixel positions of multiple second bands, detection errors caused by noise are suppressed.

[0074] Furthermore, various changes and modifications to the embodiments described above will be obvious to those skilled in the art. Such changes and modifications may be made without deviating from the spirit and scope of the subject matter and without diminishing the intended advantages. In other words, such changes and modifications are intended to be included in the claims.

[0075] For example, in the above embodiment, the ejection defect nozzle detection unit 83 may determine the above-mentioned reference range based on the sum or average value mentioned above, and the background color value of the print sheet on which the second test pattern is printed in the read image of the second test pattern. In that case, for example, the threshold value mentioned above is set according to the background color value of the print sheet (pixel value of the background color portion), and the reference range is determined based on that threshold value. In this case, the higher the background color value of the print sheet, the higher the threshold value is set. [Industrial applicability]

[0076] The present invention is applicable, for example, to an inkjet-type image forming apparatus. [Explanation of symbols]

[0077] 1a~1d Recording head 10 Image forming apparatus 101A, 101B, 101C, 101D bands (an example of the first band) 201A1~201A7, 201B1~201B7, 201C1~201C7, 201D1~201D7 bands (an example of the second band) 111 Thin line 311 White outline with thin lines 312 pixels (an example of adjacent pixels)

Claims

1. A recording head that ejects ink corresponding to the image to be printed using a set of nozzles, The system includes a nozzle failure detection unit that detects a nozzle failure, The nozzle defect detection unit (a) sets up multiple nozzle groups by shifting each nozzle by one nozzle, with nozzles at predetermined first intervals from the nozzles as one nozzle group; (b) prints a first test pattern with the recording head, each having a first band containing a fine line corresponding to the nozzle in the nozzle group; (c) sets up multiple nozzle subgroups by shifting each nozzle by one nozzle, with nozzles at predetermined second intervals from the nozzles in the nozzle group as one nozzle subgroup; and (d) prints a second band containing a white fine line corresponding to the nozzle in the nozzle subgroup, with correction processing applied to adjacent pixels of the white fine line, to the nozzle (e) print the second test pattern having each subgroup with the recording head, (f) acquire the read image of the printed first test pattern and the read image of the printed second test pattern, (g) in the read image of the first test pattern detect the first band in which density loss has occurred, and in the read image of the second test pattern detect the second band in which density loss has not occurred due to the correction process, among the plurality of second bands corresponding to the plurality of nozzle subgroups in the nozzle group of the detected first band, and detect the nozzle corresponding to the white thin line in the second band in which density loss has not occurred as the defective nozzle. The nozzle failure detection unit, when detecting a second band in which no density loss has occurred due to the correction process, derives the sum or average value of the pixel values ​​of the plurality of second bands for each pixel position in the read image of the second test pattern, identifies a reference range based on the sum or average value, and detects a second band in which no density loss has occurred due to the correction process based on the density distribution of the reference range in the plurality of second bands. An image forming apparatus characterized by the following.

2. The image forming apparatus according to claim 1, characterized in that the ejection defect nozzle detection unit (a) identifies the pixel location of the density defect based on the sum or the average value in the reference range, and (b) identifies the second band with the highest density at the identified pixel location as the second band in which no density defect occurs by the correction process.

3. The image of the second test pattern is read from the print sheet on which the second test pattern is printed. The nozzle failure detection unit determines the reference range based on the sum or average value and the background color value of the print sheet in the reading image of the second test pattern. The image forming apparatus according to claim 1, characterized by the following:

4. The image forming apparatus according to claim 1, characterized in that the nozzle defect detection unit smooths the read image of the first test pattern and detects the first band in which density defects occur in the smoothed read image of the first test pattern.

5. The image forming apparatus according to any one of claims 1 to 4, further comprising a correction processing unit that performs a correction process corresponding to the defective nozzle in the image.

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