Image forming device

Abstract

To provide an image forming apparatus that can satisfactorily suppress density unevenness in a main scanning direction for all gradations.SOLUTION: When a user operates an operation panel, and density adjustment control in a main scanning direction is executed accordingly, an image forming apparatus forms a belt-like pattern with high gradation first, and obtains density unevenness in the main scanning direction of the belt-like pattern with high gradation by using a density sensor being detection means (S1, S2). Next, the image forming apparatus determines, on the basis of, the density unevenness in the main scanning direction, a shading correction amount as exposure value correction data to correct a laser light quantity being an exposure value of an optical writing unit being exposure means (S3). Subsequently, the image forming apparatus forms a plurality of belt-like patterns with intermediate or lower gradations at the laser light quantity corrected by using the shading correction amount, and obtains density unevenness in the main scanning direction of the belt-like patterns by using the density sensor (S4, S5). The image forming apparatus then acquires a gradation correction amount as image correction data on the basis of, the density unevenness in the main scanning direction (S6).SELECTED DRAWING: Figure 10

Description

[Technical Field]

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

[0002] Conventionally, there has been known an image forming apparatus that includes a latent image carrier, an exposure means that exposes the latent image carrier based on image data to form a latent image, and a detection means that detects density unevenness in the main scanning direction of a toner image obtained by developing the latent image, and that corrects the exposure amount of the exposure means based on the density unevenness in the main scanning direction detected by the detection means.

[0003] Patent document 1 describes an image forming device in which the toner image, in which density unevenness in the main scanning direction is detected by a color sensor as a detection means, is converted into a long, strip-shaped test pattern in the main scanning direction with an image density of 100%. Summary of the Invention [Problem to be solved by the invention]

[0004] However, depending on the gradation, density unevenness in the main scanning direction may remain. [Means for solving the problem]

[0005] In order to solve the above-mentioned problems, the present invention provides a method for detecting density unevenness in a main scanning direction of a toner image obtained by developing a latent image on a latent image carrier, the method comprising: a latent image carrier; an exposure unit that exposes the latent image carrier to light based on image data to form a latent image; and a detection unit that detects density unevenness in a main scanning direction of a toner image obtained by developing the latent image, and Only the exposure amount is corrected to form an image portion with a high gradation, and the exposure amount and the image data are corrected based on the density unevenness to form an image portion with a medium gradation or lower. It is characterized by the above. [Effects of the Invention]

[0006] According to the present invention, it is possible to effectively suppress density unevenness in the main scanning direction for all gradations. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a schematic configuration diagram showing a printer according to an embodiment. [Figure 2] FIG. [Figure 3] FIG. 2 is a schematic diagram illustrating the configuration of an image element included in a density sensor. [Figure 4] FIG. 3 is a cross-sectional view of the density sensor taken along a plane perpendicular to the main scanning direction. [Figure 5] (a) shows the image density unevenness in the main scanning direction of a low-gradation image, (b) shows the image density unevenness in the main scanning direction of a medium-gradation image, and (c) shows the image density unevenness in the main scanning direction of a high-gradation image. [Figure 6] 10A and 10B are diagrams for explaining conventional correction of density unevenness in the scanning direction. [Figure 7] FIG. 10 is a diagram showing an example of tone correction. [Figure 8] (a) is a diagram showing density unevenness in the main scanning direction after gradation correction for low gradations, (b) is a diagram showing density unevenness in the main scanning direction after gradation correction for medium gradations, and (c) is a diagram showing density unevenness in the main scanning direction after gradation correction for high gradations. [Figure 9] FIG. 4 is a control block diagram of density adjustment control in the main scanning direction according to the present embodiment. [Figure 10] FIG. 6 is a flowchart of density adjustment control in the main scanning direction according to the present embodiment. [Figure 11] FIG. 10 is a diagram showing an example of a multi-tone band-shaped pattern formed on recording paper. [Figure 12] 5A to 5C are diagrams illustrating shading correction according to the present embodiment. [Figure 13] FIG. 10 is a diagram showing an example of a band-shaped pattern for image data correction. [Figure 14] 10 is a graph showing the correlation between image density and gradation. [Figure 15] FIG. 10 is a diagram showing an example of a tone correction table. [Figure 16] 5A to 5C are diagrams for explaining tone correction according to the present embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0008] Hereinafter, an embodiment of a printer that forms images by electrophotography will be described as an image forming apparatus to which the present invention is applied. First, the basic configuration of the printer according to the embodiment will be described.

[0009] FIG. 1 is a schematic diagram showing the configuration of a printer according to an embodiment. The printer includes four process units 2Y, 2M, 2C, and 2K for forming toner images of yellow (Y), magenta (M), cyan (C), and black (K). The printer also includes a paper feed path 30, a pre-transfer conveyance path 31, a manual paper feed path 32, a manual feed tray 33, a pair of registration rollers 34, and a conveyor belt unit 35. The printer also includes a fixing device 40, a conveyance switching device 50, a pair of paper discharge rollers 52, a paper discharge tray 53, a first paper feed cassette 101, a second paper feed cassette 102, a re-feed device, and the like. The printer also includes two optical writing units 1YM and 1CK. The process units 2Y, 2M, 2C, and 2K each include drum-shaped photosensitive members 3Y, 3M, 3C, and 3K, which serve as latent image carriers.

[0010] The first paper feed cassette 101 and the second paper feed cassette 102 each contain a stack of recording paper P therein. The rotation of paper feed rollers 101a and 102a sends the topmost recording paper P in the stack toward paper feed path 30. This paper feed path 30 is connected to a pre-transfer transport path 31 for transporting the recording paper just before the secondary transfer nip, which will be described later. The recording paper P, serving as a recording medium, sent from the paper feed cassettes (101, 102) passes through paper feed path 30 and enters pre-transfer transport path 31.

[0011] A manual feed tray 33 is disposed on the side of the printer housing so as to be able to open and close relative to the housing, and a stack of papers is manually fed onto the upper surface of the tray when it is open relative to the housing. The topmost recording paper P of the manually fed stack is sent out toward the pre-transfer conveyance path 31 by a feed roller of the manual feed tray 33.

[0012] The two optical writing units 1YM and 1CK, which act as exposure devices for exposing the photoconductor surfaces to light and forming electrostatic latent images on the photoconductor surfaces, each include a laser diode, a polygon mirror, and various lenses. Each optical writing unit 1YM and 1CK drives a laser diode as a light source based on image data read by an external scanner or image data sent from a personal computer. The laser diode then optically scans the photoconductors 3Y, 3M, 3C, and 3K of the process units 2Y, 2M, 2C, and 2K. Specifically, the photoconductors 3Y, 3M, 3C, and 3K of the process units 2Y, 2M, 2C, and 2K are rotated counterclockwise in the figure by driving devices. The optical writing unit 1YM performs optical scanning by irradiating the driven photoconductors 3Y and 3M with a laser beam deflected in the direction of their respective rotation axes. As a result, electrostatic latent images based on the Y and M image data are formed on the photoconductors 3Y and 3M. The optical writing unit 1CK performs optical scanning by irradiating the photoconductors 3C and 3K with laser light while deflecting it in the direction of their rotation axes, thereby forming electrostatic latent images based on the C and K image data on the photoconductors 3C and 3K.

[0013] Each of the process units 2Y, 2M, 2C, and 2K is a single unit that supports a photosensitive element (latent image carrier) and various peripheral devices on a common support, and is detachably attached to the printer body. They have similar configurations except for the toner colors they use. Taking the Y process unit 2Y as an example, it includes a photosensitive element 3Y and a developing device 4Y for developing the electrostatic latent image formed on the photosensitive element's surface into a Y toner image. It also includes a charging device 5Y that uniformly charges the surface of the rotationally driven photosensitive element 3Y, and a drum cleaning device 6Y that cleans residual toner adhering to the surface of the photosensitive element 3Y after it passes through the Y primary transfer nip (described later).

[0014] This printer has a so-called tandem configuration in which four process units 2Y, 2M, 2C, and 2K are arranged along the endless movement direction of an intermediate transfer belt 61, which will be described later.

[0015] The photoreceptor 3Y is a drum-shaped member made of a tube such as aluminum, on which a photosensitive layer is formed by applying an organic photosensitive material having photosensitivity, although an endless belt-shaped member may also be used.

[0016] The developing device 4Y develops a latent image using a two-component developer (hereinafter simply referred to as developer) containing magnetic carrier and non-magnetic Y toner. The developing device 4Y is appropriately replenished with Y toner from a Y toner bottle 103Y by a Y toner replenishing device. A toner concentration detecting means is provided within the developing device 4Y. The toner concentration detecting means detects the magnetic permeability caused by the carrier, which is a magnetic material, and calculates the toner concentration from the amount of carrier contained in a certain volume. This toner concentration detecting means detects the toner concentration within the developing device and controls the toner concentration within the developing device within a certain range (for example, 5 wt% to 9 wt%).

[0017] The drum cleaning device 6Y uses a polyurethane rubber cleaning blade that presses against the photoreceptor 3Y, but other types may also be used. To improve cleaning performance, this printer uses a rotatable fur brush that abuts against the photoreceptor 3Y. This fur brush also serves to scrape the lubricant from the solid lubricant, turning it into a fine powder and applying it to the surface of the photoreceptor 3Y.

[0018] A discharge lamp is disposed above the photoconductor 3Y, and this discharge lamp is also part of the process unit 2Y. The discharge lamp discharges the surface of the photoconductor 3Y by irradiating it with light after it has passed through the drum cleaning device 6Y. The discharged surface of the photoconductor 3Y is then uniformly charged by the charging device 5Y and optically scanned by the optical writing unit 1YM. The charging device 5Y is rotated while receiving a charging bias from a power source. Alternatively, a scorotron charger may be used, which charges the photoconductor 3Y without contact.

[0019] Although the process unit 2Y for Y has been described, the process units 2M, 2C, and 2K for M, C, and K also have the same configuration as that for 2Y.

[0020] A transfer unit 60 is disposed below the four process units 2Y, 2M, 2C, and 2K. This transfer unit 60 moves an intermediate transfer belt 61, an image carrier stretched by multiple rollers, endlessly in the clockwise direction in the drawing by rotating one of the rollers while bringing the intermediate transfer belt 61 into contact with the photosensitive elements 3Y, 3M, 3C, and 3K. This forms primary transfer nips for Y, M, C, and K where the photosensitive elements 3Y, 3M, 3C, and 3K come into contact with the intermediate transfer belt 61.

[0021] Near the primary transfer nips for Y, M, C, and K, primary transfer rollers 62Y, 62M, 62C, and 62K disposed inside the belt loop press the intermediate transfer belt 61 toward the photoconductors 3Y, 3M, 3C, and 3K. A primary transfer bias is applied to each of these primary transfer rollers 62Y, 62M, 62C, and 62K by a power source. As a result, a primary transfer electric field is formed in the primary transfer nips for Y, M, C, and K that electrostatically moves the toner images on the photoconductors 3Y, 3M, 3C, and 3K toward the intermediate transfer belt 61.

[0022] As the intermediate transfer belt 61 moves endlessly clockwise in the figure, it passes through the primary transfer nips for Y, M, C, and K in sequence, and toner images are sequentially superimposed and primarily transferred onto the front surface of the intermediate transfer belt 61 at each primary transfer nip. As a result of this superimposed primary transfer, a four-color superimposed toner image (hereinafter referred to as a four-color toner image) is formed on the front surface of the intermediate transfer belt 61.

[0023] A secondary transfer roller 72 is disposed below the intermediate transfer belt 61 in the drawing, and this forms a secondary transfer nip by contacting the front surface of the intermediate transfer belt 61 with the secondary transfer backup roller 68 at a position where the intermediate transfer belt 61 is wound around the secondary transfer backup roller 68. This forms a secondary transfer nip where the front surface of the intermediate transfer belt 61 and the secondary transfer roller 72 come into contact with each other.

[0024] A secondary transfer bias is applied from a power source to the secondary transfer roller 72. Meanwhile, the secondary transfer backup roller 68 in the belt loop is grounded, thereby forming a secondary transfer electric field in the secondary transfer nip.

[0025] The above-mentioned registration roller pair 34 is disposed on the right side of the secondary transfer nip in the drawing, and the recording paper P sandwiched between the rollers is sent to the secondary transfer nip at a timing that allows it to be synchronized with the four-color toner image on the intermediate transfer belt 61. Within the secondary transfer nip, the four-color toner image on the intermediate transfer belt 61 is secondarily transferred all at once onto the recording paper due to the influence of the secondary transfer electric field and nip pressure, and combined with the white color of the recording paper, it becomes a full-color image.

[0026] A reflective optical sensor, the toner adhesion amount detection sensor 64, is disposed between the primary transfer nip K and the secondary transfer nip. The reflective optical sensor has a light-emitting element and a light-receiving element, and light emitted from the light-emitting element is reflected by the toner patch formed on the intermediate transfer belt 61, received by the light-receiving element, and converted into a signal. By reading the change in this signal, information on the test pattern is inferred and the amount of adhesion of the toner patch is detected.

[0027] Residual toner that was not transferred to the recording paper P at the secondary transfer nip adheres to the front surface of the intermediate transfer belt 61 that has passed through the secondary transfer nip. This residual toner is cleaned by a belt cleaning device 75 that comes into contact with the intermediate transfer belt 61.

[0028] The recording paper P that has passed through the secondary transfer nip is separated from the intermediate transfer belt 61 and delivered to the conveyor belt unit 35. This conveyor belt unit 35 has an endless conveyor belt 36 stretched between a drive roller 37 and a driven roller 38, and is moved endlessly counterclockwise in the drawing by the rotational drive of the drive roller 37. The recording paper delivered from the secondary transfer nip is then held on the upper stretching surface of the belt and conveyed as the belt moves endlessly, and delivered to the fixing device 40.

[0029] The recording paper P that has passed through the secondary transfer nip is sent into the fixing device 40 and sandwiched in the fixing nip, where the toner image is fixed by applying pressure, heat, and the like. The recording paper P, on whose first surface the toner image has been transferred in the secondary transfer nip and fixed by the fixing device 40 , is sent out toward the conveyance switching device 50 .

[0030] In this printer, the re-transport means is made up of the transport switching device 50, the re-feed path 54, the switchback path 55, the post-switchback transport path 56, etc. Specifically, the transport switching device 50 switches the subsequent transport destination of the recording sheet P received from the fixing device 40 between the discharge path 57 and the re-feed path 54. Specifically, when a print job in a single-sided mode in which an image is formed only on the first side of the recording sheet P is executed, the transport destination is set to the discharge path 57. As a result, the recording sheet P with an image formed only on the first side is sent to the pair of discharge rollers 52 via the discharge path 57 and discharged onto a discharge tray 53 outside the printer. Also, when a print job in a double-sided mode in which images are formed on both sides of the recording sheet P is executed, the transport destination is also set to the discharge path 57 when the recording sheet P with images fixed on both sides is received from the fixing device 40. As a result, the recording sheet P with images formed on both sides is discharged onto a discharge tray 53 outside the printer. On the other hand, when a print job in double-sided mode is executed, if the recording paper P with an image fixed only on the first side is received from the fixing device 40, the transport destination is set to the re-transport path .

[0031] A switchback path 55 is connected to the re-feed path 54, and the recording sheet P sent to the re-feed path 54 enters this switchback path 55. Then, when the entire area of ​​the recording sheet P in the conveying direction enters the switchback path 55, the conveying direction of the recording sheet P is reversed, and the recording sheet P switches back. In addition to the re-feed path 54, a post-switchback conveying path 56 is connected to the switchback path 55, and the switched-back recording sheet P enters this post-switchback conveying path 56. At this time, the recording sheet P is turned upside down. Then, the upside-down turned recording sheet P is re-fed to the secondary transfer nip via the post-switchback conveying path 56 and the above-mentioned paper feed path 30. The recording sheet P, on whose second side a toner image has also been transferred at the secondary transfer nip, passes through the fixing device 40, where the toner image is fixed to the second side, and is then discharged onto the paper discharge tray 53 via the conveying switching device 50 and the paper discharge roller pair 52. A density sensor 51 is disposed in front of the pair of paper discharge rollers 52 as a detection means for detecting the density of an image on the recording paper P, and detects the density of an image on the recording paper P during an adjustment operation, which will be described later.

[0032] Here, the configuration of the concentration sensor 51 will be described. FIG. 2 is a perspective view of the density sensor 51. The density sensor 51 is elongated in the main scanning direction. It has an internal imaging element that is elongated in the main scanning direction, and the density sensor 51 is sometimes called a line sensor. The detection width of the density sensor 51 in the main scanning direction is the width indicated by the dotted line in FIG. 2. Because this detection width is longer than the width of the recording paper P in the main scanning direction, it is possible to detect the image density over the entire area of ​​the recording paper P by transporting the recording paper P so that it passes through the width indicated by the dotted line in the main scanning direction.

[0033] FIG. 3 is a schematic diagram of the image element 111 included in the density sensor 51. As shown in FIG. 3, the image element 111 has a shape extending in the main scanning direction, and small light receiving elements 112-0 to 112-n (hereinafter referred to as light receiving elements 112 when there is no need to distinguish one from another) are arranged in the main scanning direction. The range in which the light receiving elements 112 are arranged is the detection width in the main scanning direction of the density sensor 51 described above.

[0034] FIG. 4 is a cross-sectional view of the density sensor 51 taken along a plane perpendicular to the main scanning direction. 4, the density sensor 51 includes, in addition to the image element 111, a light source 113, a lens array 114, and an output circuit 115. The dotted line represents the light emitted from the light source 113.

[0035] A light emitting element provided at the end of a light guide, an LED array, or the like can be used as the light source 113. The light source 113 emits RGB light. A Selfoc (registered trademark) lens or the like can be used as the lens array 114.

[0036] Light emitted from the light source 113 is reflected on the recording paper P and formed into an image by the lens array 114. The image element 111 receives the light formed into an image by the lens array 114 with each light receiving element 112 shown in Fig. 3 and outputs a signal corresponding to the received light. The image element 111 may be a CMOS sensor, a CCD sensor, or the like.

[0037] The output circuit 115 uses, for example, an ASIC (Application Specific Integrated Circuit), and converts the signals from each light receiving element 112 on the image element 111 into data indicating image density corresponding to the position of the toner pattern on the recording paper P in the main scanning direction, and outputs the data. For example, it outputs 0 to 255 gradations expressed in 8 bits. A state with no image is 0 gradation, and a solid image is 255 gradations.

[0038] In the electrophotographic image forming apparatus of this embodiment, an image is formed on recording paper P through multiple processes, including a developing process, a transfer process, and a fixing process. The developing process is a process in which the photosensitive member 3 is uniformly charged, and a latent image is formed by optical scanning using the optical writing unit 1, and toner supplied from the developing device 4 adheres to the latent image to develop it. The transfer process includes a first transfer process in which the toner image on the photosensitive member is transferred to the intermediate transfer belt, and a second transfer process in which the toner image is transferred from the intermediate transfer belt to the recording paper. The fixing process is a process in which the toner image on the recording paper is fixed to the recording paper P by the fixing device 40.

[0039] In each of these processes, variations in mechanical precision and supply characteristics in the main scanning direction, which is the axial direction of the photosensitive member, can cause variations in the charge of the photosensitive member in the main scanning direction, variations in the gap between the photosensitive member and the developing roller of the developing device, and variations in transfer pressure. These variations cause variations in image density in the main scanning direction (hereinafter referred to as density unevenness).

[0040] Figure 5(a) shows image density unevenness in the main scanning direction of a low-gradation image, Figure 5(b) shows image density unevenness in the main scanning direction of a medium-gradation image, and Figure 5(c) shows image density unevenness in the main scanning direction of a high-gradation image. The horizontal axis represents position in the main scanning direction, and the vertical axis represents density. As described above, density unevenness in the main scanning direction occurs due to a combination of several factors, and therefore, as shown in FIG. 5, the tendency of density unevenness may differ from low to high gradations.

[0041] FIG. 6 is a diagram illustrating conventional correction of density unevenness in the scanning direction. Conventionally, a long strip-shaped test pattern in the main scanning direction of a medium gradation is formed on recording paper P, and the test pattern on the recording paper P is detected by a density sensor 51 to obtain the density unevenness in the main scanning direction of the medium gradation. Then, based on the obtained density unevenness in the main scanning direction of the medium gradation, a shading correction amount is calculated to correct the light amount (LD power) of the laser light of the optical writing units 1YM and 1CK, which is the exposure amount, as shown in FIG. 6(b).

[0042] In calculating the shading correction amount, a predetermined area in the main scanning direction is used as a reference area, and the difference between the density of the reference area and the image density of each area in the main scanning direction other than the reference area is calculated. The shading correction amount is then calculated to cancel this difference. Alternatively, the average image density in the main scanning direction may be used as a reference, and the difference between the average image density and the image density of each area in the main scanning direction may be calculated to cancel this difference. As a result of shading correction, density unevenness in the main scanning direction for medium gradations can be eliminated, as shown in Figure 6(c). However, in the past, latent images for low-gradation and high-gradation images were also formed using this shading-corrected laser light amount. Therefore, when the density unevenness trends differ among low-gradation, medium-gradation, and high-gradation images, density unevenness in the main scanning direction for low-gradation and high-gradation images could not be sufficiently improved, as shown in Figure 6(c).

[0043] It is also conceivable to calculate a shading correction amount corresponding to low gradation, a shading correction amount corresponding to medium gradation, and a shading correction amount corresponding to high gradation, and change the amount of shading correction to be applied depending on the gradation of the image to be printed. However, when printing an image in which high gradation image portions and low gradation image portions are mixed, it is difficult to implement control such that the amount of shading correction to be applied depending on the gradation of the image portion to be printed during the printing operation.

[0044] One method for suppressing density unevenness in the main scanning direction is to correct the gradation of image data. Printers represent images using a collection of small dots, and the 0 to 255 gradations expressed in 8 bits are expressed by the image density, which is expressed by the density of the color dots. The printer's image processing unit 204 (see Figure 9) converts image data read by a scanner or image data sent from a personal computer into a binary pseudo-gradation image using a predetermined method such as dithering, density patterning, or error diffusion. Then, based on this pseudo-gradation image, the laser diode of the optical writing unit is controlled to turn on and off to form an electrostatic latent image. Gradation correction of image data corrects the gradation (density of the color dots) of this converted pseudo-gradation image.

[0045] FIG. 7 is a diagram showing an example of tone correction as density correction by correcting image data. Figure 7(a) shows a pseudo-gradation image (dot pattern) with N gradations. To lighten the image density, a gradation correction is performed using a predetermined algorithm to change color dots to white dots and reduce the number of color dots to lower the gradation, as shown in Figure 7(b1). On the other hand, to darken the image density, a gradation correction is performed using a predetermined algorithm to change white dots to color dots and increase the number of color dots to raise the gradation, as shown in Figure 7(b2).

[0046] Fig. 8(a) shows density unevenness in the main scanning direction after tone correction for low gradations, Fig. 8(b) shows density unevenness in the main scanning direction after tone correction for medium gradations, and Fig. 8(c) shows density unevenness in the main scanning direction after tone correction for high gradations.

[0047] As can be seen from Figure 8, for low and medium gradations, gradation correction as a correction of image data was able to eliminate density unevenness in the main scanning direction. However, for high gradations, as shown in Figure 8(c), gradation correction could not be performed to bring light image density areas back to the standard density, and density unevenness in the main scanning direction could not be sufficiently improved. This is because, with gradation correction as shown in Figure 7, the number of white dots decreases as the gradation increases, and the number of dots that can be converted to color dots decreases. Therefore, the correction range in the direction of increasing image density (high gradation) becomes narrower as the gradation increases. As a result, light image density areas could not be corrected to the standard density, and gradation correction could not sufficiently improve density unevenness in the main scanning direction for high gradations.

[0048] On the other hand, in low gradations, the number of color dots is small, and the number of dots that can be converted to white dots is also small, so the correction width in the direction of reducing image density (low gradation) becomes narrower as the gradation becomes lower. However, in the case of low gradations, the amount of adhered toner is small, so the density fluctuation width α1 (the difference between maximum and minimum density) in the main scanning direction is narrower than the fluctuation width α2 in medium gradations and the fluctuation width α3 in high gradations. Therefore, even if the correction width in the direction of reducing image density is narrower in low gradations, it is possible to eliminate density unevenness in the main scanning direction through gradation correction.

[0049] As described above, shading correction alone or tone correction alone cannot effectively suppress density unevenness in the main scanning direction for all tones. Therefore, in this embodiment, shading correction and tone correction are combined to effectively suppress density unevenness in the main scanning direction for all tones. The following describes the characteristics of the present invention.

[0050] FIG. 9 is a control block diagram of density adjustment control in the main scanning direction according to this embodiment. The control unit 200, which functions as a control unit and a density correction unit, has a CPU 201, a ROM 202, a RAM 203, an image processing unit 204, a shading correction amount calculation unit 205, an image data correction amount calculation unit, etc. The control unit 200 is connected to the optical writing units 1YM and 1CK, the density sensor 51, an operation panel 220, a storage unit 210, an external communication I / F 230, etc.

[0051] The CPU 201 controls the operation of the printer. Specifically, the CPU 201 uses the RAM 203 as a work area (working region) and executes programs stored in the ROM 202, etc., to control the operation of the entire printer and realize various functions such as the printer function.

[0052] The ROM 202 is a non-volatile semiconductor memory that can retain data even when the power is turned off. The RAM 203 is a volatile semiconductor memory that temporarily stores programs and data.

[0053] The shading correction amount calculation unit 205 calculates the amount of shading correction based on the density unevenness data in the main scanning direction detected by the density sensor 51. The calculated amount of shading correction is stored in the memory unit 210. The gradation correction amount calculation unit 206 calculates the amount of correction for correcting gradation based on the density unevenness data in the main scanning direction detected by the density sensor 51. The gradation correction amount calculation unit 206 calculates the amount of gradation correction for each gradation equal to or lower than the specified gradation, and creates a gradation correction table as shown in FIG. 15 based on the amount of gradation correction. The created gradation correction table is stored in the memory unit 210. In this embodiment, the specified gradation is 230 gradation. In this embodiment, for gradations above the specified gradation (230 gradations), only shading correction is performed, and for gradations below the specified gradation, both shading correction and gradation correction are performed. In this embodiment, gradations above 230 gradations are considered high gradations, and gradations below 230 gradations are considered medium gradations or lower. The specified gradations, which are divided into "high gradations" where only shading correction is performed and "medium gradations" or lower where both shading correction and gradation correction are performed, can be set appropriately depending on the characteristics of the device, etc.

[0054] The image processing unit 204 performs image processing such as converting the image data received via the external communication I / F 230 from a scanner or personal computer external to the printer into a pseudo-gradation image, thereby converting the image into an image that can be written by an optical writing unit. Furthermore, the image processing unit 204 performs gradation correction for each area in the main scanning direction for image portions with gradations below the specified gradation, based on a gradation correction table stored in the storage unit 210.

[0055] The storage unit 210 is configured with a flash memory such as an HDD or SSD, and stores shading correction amounts and tone correction tables. The external communication I / F 230 is an interface for connecting to a network such as the Internet or a LAN (Local Area Network), etc. The external communication I / F 230 can receive print instructions, image data, etc. from external devices such as a scanner or a personal computer.

[0056] The operation panel 220 accepts various inputs in response to user operations and displays various information (for example, information indicating the accepted operations, information indicating the printer's operating status, information indicating the device's setting status, etc.). The operation panel 220 is configured, for example, by a liquid crystal display (LCD) equipped with a touch panel function, but is not limited to this. For example, the operation panel 220 may be configured by an organic electroluminescence (EL) display equipped with a touch panel function. Furthermore, in addition to or instead of this, an operation unit such as hardware keys and a display unit such as a lamp may also be provided.

[0057] FIG. 10 is a flowchart of density adjustment control in the main scanning direction according to this embodiment. The density adjustment control in the main scanning direction is executed by the user operating the operation panel 220. The density adjustment control in the main scanning direction may also be executed automatically when the device is turned on or after each specified number of prints.

[0058] When the density adjustment control in the main scanning direction is executed, the control unit 200 first performs shading correction. Specifically, first, a band-shaped pattern with high gradation of Y, M, C, and K is printed on the recording paper P (S1). FIG. 11 shows an example of a high-gradation band-shaped pattern of Y, M, C, and K formed on recording paper P. The gradation of the high-gradation band-shaped pattern is a gradation that exceeds the above-mentioned specified gradation (230 gradation), and is set to an image density of, for example, approximately the center gradation (243 gradation) of 230 gradation to 255 gradation. "R" in FIG. 11 indicates the back side of the device, "C" in the figure indicates the center, and "F" in the figure indicates the front side of the device. Here, the front side of the device is the side from which the user operates operation panel 220.

[0059] The density sensor 51 detects the image density at each position in the main scanning direction of the high-gradation band-shaped pattern of Y, M, C, and K formed on the recording paper P, and obtains the density unevenness in the main scanning direction (S2). Next, the shading correction amount calculation unit 205 of the control unit 200 calculates a shading correction amount for correcting the laser light intensity of each optical writing unit 1YM, 1CK based on the acquired density unevenness in the main scanning direction (S3). Specifically, based on the density unevenness in the main scanning direction of the high-gradation Y band-shaped pattern, the shading correction amount for correcting the laser light intensity of the Y laser diode of the optical writing unit 1YM that irradiates the photoconductor 3Y with laser light is calculated. Similarly, based on the density unevenness in the main scanning direction of the high-gradation M band-shaped pattern, the shading correction amount for correcting the laser light intensity of the M laser diode of the optical writing unit 1YM. Furthermore, based on the density unevenness in the main scanning direction of the high-gradation C band-shaped pattern, the shading correction amount for correcting the laser light intensity of the C laser diode of the optical writing unit 1CK. Furthermore, based on the density unevenness in the main scanning direction of the high gradation belt-like pattern of K, a shading correction amount for correcting the laser light amount of the laser diode corresponding to K of the optical writing unit 1CK is calculated.

[0060] The calculated amount of shading correction for each color is stored in the storage unit 210. When printing, the amount of shading correction for each color is read out from the storage unit 210, and a latent image is formed on the photosensitive member using the amount of laser light corrected by the amount of shading correction.

[0061] FIG. 12 is a diagram illustrating shading correction according to this embodiment. In this embodiment, shading correction is performed by creating a high-gradation band-shaped pattern. Therefore, the shading correction amount calculated by the shading correction amount calculation unit 205, shown in Fig. 12(b), corresponds to the density unevenness in the main scanning direction of high gradations. Therefore, as shown in Fig. 12(c), when the shading correction amount is applied and image portions of low, medium, and high gradations are printed, density unevenness in the main scanning direction is eliminated only in the image portions of high gradations.

[0062] On the other hand, even after applying the shading correction amount to the low-tone and medium-tone image portions, density unevenness in the main scanning direction remains, as shown by the solid line in Figure 12(c). Therefore, in order to improve the density unevenness in the main scanning direction for low to medium gradations, gradation correction is performed as a correction of the image data. Specifically, as shown in Figure 10, after shading correction is performed (S1 to S3), multiple belt-shaped patterns with different numbers of gradations for each of Y, M, C, and K are printed on recording paper P (S4) at or below the specified gradation (230 gradations). The multiple belt-shaped patterns for each color are imaged by correcting the laser light intensity using the shading correction amount calculated by the shading correction.

[0063] FIG. 13 is a diagram showing an example of a belt-shaped pattern for correcting image data. In this embodiment, a single color band pattern is printed on one sheet of recording paper P. In this embodiment, 11 band patterns with different numbers of gradations are formed on one sheet of recording paper P. Multiple band patterns are formed so that the number of gradations increases by 20 from the downstream side in the recording paper travel direction. In this embodiment, band patterns of 20, 40, 60, 80, 100, 120, 140, 160, 180, 200, and 220 gradations are printed for one color. The number of band patterns formed on one sheet of recording paper and the number of gradations of each band pattern may be set as appropriate.

[0064] The image density at each position in the main scanning direction of multiple band-shaped patterns of Y, M, C, and K colors formed on four sheets of recording paper P is detected by a density sensor 51, and multiple density unevenness in the main scanning direction for each color is obtained (S4). In this embodiment, a total of 11 density unevennesses in the main scanning direction are acquired: 20 gradations, 40 gradations, 60 gradations, 80 gradations, 100 gradations, 120 gradations, 140 gradations, 160 gradations, 180 gradations, 200 gradations, and 220 gradations.

[0065] Next, the tone correction amount calculation unit 206 of the control unit 200 calculates the tone correction amount for each tone equal to or less than the specified tone (tone 230) based on the acquired 11 density unevennesses in the main scanning direction (S6). As an example of calculating the amount of gradation correction, the gradation correction amount calculation unit 206 first calculates the average image density for each gradation from the 11 density variations in the main scanning direction. By calculating the average image density for each gradation, the correlation between gradation and image density as shown in FIG. 14 is obtained. Specifically, the correlation between gradation and image density is expressed by a cubic approximation formula. By differentiating this cubic approximation formula, the number of gradations per image density at a predetermined density can be obtained. The cubic approximation formula showing the correlation between gradation and image density may be obtained in advance by experiment or the like and stored in the storage unit 210.

[0066] Next, the image density corresponding to the gradation of each band-shaped pattern is calculated from the correlation shown in Figure 14. Using this image density as a reference, the density difference value for each area in the main scanning direction is calculated from the density unevenness in each band-shaped pattern in the main scanning direction. The required amount of gradation correction is obtained from these density difference values ​​and the number of gradations per image density in the image density corresponding to this gradation, which is calculated by differentiating a cubic approximation formula that shows the correlation between gradation and image density. In other words, the amount of gradation correction is calculated to cancel the difference between the target gradation and the actual gradation of each area in the main scanning direction. This calculation is performed because gradation and density are not correlated by a linear approximation, and the amount of gradation correction required per unit density varies depending on the image density.

[0067] Next, after determining the amount of tone correction for each area in the main scanning direction for each tone of the band-shaped patterns, the amount of tone correction for each area in the main scanning direction for the tone between the band-shaped patterns is determined by interpolation. Note that the amount of tone correction for each area in the main scanning direction for the tone between the band-shaped patterns may also be determined as follows. That is, based on the density unevenness in the main scanning direction of multiple band-shaped patterns, the density unevenness in the main scanning direction of the tone between the band-shaped patterns is calculated by interpolation. Then, based on the calculated density unevenness in the main scanning direction of the tone between the band-shaped patterns, the amount of tone correction for each area in the main scanning direction for each tone between the band-shaped patterns is determined.

[0068] After determining the amount of tone correction for each area in the main scanning direction for 0 to 230 tones in this way, the target number of tones for each area in the main scanning direction at each tone is determined from the amount of tone correction for each area in the main scanning direction, and a tone correction table such as that shown in Fig. 15 is created. The created tone correction table is stored in the storage unit 210. Note that instead of the target number of tones for each area in the main scanning direction at each tone, the amount of tone correction for each area in the main scanning direction at each tone may also be used. In this way, once the tone correction table is stored in the storage unit 210, the density adjustment control in the main scanning direction is completed.

[0069] The image processing unit 204 performs gradation correction based on the gradation correction table shown in FIG. 15 stored in the storage unit 210. For example, the image processing unit 204 obtains the target number of gradations for a certain area in the main scanning direction of the image data from the gradation number of that area and the gradation correction table. The image processing unit 204 then corrects the image data so that the target number of gradations is achieved. For example, when the target number of gradations is higher than the number of gradations of the image data, the image processing unit 204 converts a predetermined number of white dots from the dot pattern shown in FIG. 7 representing the gradation (image shading) into color dots to achieve the target number of gradations, and corrects the image data to perform gradation correction. On the other hand, when the target number of gradations is lower than the number of gradations of the image data, the image processing unit 204 converts a predetermined number of color dots from the dot pattern shown in FIG. 7 representing the gradation (image shading) into white dots to achieve the target number of gradations, and corrects the image data to perform gradation correction.

[0070] FIG. 16 is a diagram for explaining tone correction according to this embodiment. As shown in Fig. 16(a), in this embodiment, by performing shading correction, it is possible to effectively suppress density unevenness in the main scanning direction in image areas with high gradations (greater than 230 gradations), but density unevenness in the main scanning direction remains in low and medium gradations. However, by performing the above-described gradation correction for 0 to 230 gradations, it is possible to suppress density unevenness in the main scanning direction in all gradations, low, medium, and high gradations, as shown in Fig. 16(b).

[0071] In this embodiment, shading correction is performed for all gradations, which eliminates the need to determine whether or not to perform shading correction depending on the number of gradations in the image area, thereby simplifying the control of the optical writing unit.

[0072] Furthermore, although tone correction was not performed on images with high gradations exceeding 230 in the above description, tone correction may be performed on all gradations. This allows for density unevenness in the main scanning direction that could not be fully corrected by shading correction for high gradations to be corrected by tone correction. Note that for high gradations, the range of variation in density unevenness is suppressed by shading correction. Therefore, even if the number of white dots is small and the number of dots that can be converted to color dots is small, tone correction can be performed on areas in the main scanning direction that have a density lighter than the reference density so that they reach the reference image density.

[0073] The above description is merely an example, and each of the following aspects provides unique effects. (Aspect 1) The device includes a latent image carrier such as a photosensitive member 3, an exposure means such as an optical writing unit 1 that exposes the latent image carrier based on image data to form a latent image, and a detection means such as a density sensor 51 that detects density unevenness in the main scanning direction of a toner image obtained by developing the latent image, and corrects the exposure amount of the exposure means, such as the amount of laser light, and the image data based on the density unevenness in the main scanning direction detected by the detection means. According to this, for gradations where density unevenness in the main scanning direction remains when the exposure amount of the exposure means is corrected, the density unevenness in the main scanning direction can be suppressed by correcting the image data, and therefore density unevenness in the main scanning direction can be effectively suppressed for all gradations.

[0074] (Aspect 2) In aspect 1, only the exposure amount is corrected based on the density unevenness to form an image area with a high gradation (in this embodiment, a gradation exceeding 230 gradations), and the exposure amount and image data are corrected based on the density unevenness to form an image area with a gradation other than the high gradation. This makes it possible to obtain a good image in which density unevenness in the main scanning direction is suppressed, as explained in the embodiment.

[0075] (Aspect 3) In aspect 2, exposure amount correction data such as a shading correction amount that corrects the exposure amount of an exposure means such as a laser light amount is obtained based on density unevenness in the main scanning direction of a high-gradation toner image such as a high-gradation band-like pattern detected by a detection means such as a density sensor 51, and image correction data such as a gradation correction amount that corrects image data based on density unevenness in the main scanning direction of a toner image of a gradation lower than the high gradation obtained using the exposure amount corrected by the exposure amount correction data. As described in the embodiment, this allows for effective suppression of density unevenness in the main scanning direction in high-gradation image portions by correcting the exposure amount, such as the laser light amount, based on the acquired exposure amount correction data, such as the shading correction amount. Furthermore, by correcting the image portions of the image data with high or lower gradations based on the acquired image correction data, such as the gradation correction amount, it is also possible to suppress density unevenness in the main scanning direction in the image portions with high or lower gradations. This allows for the production of a good image in which density unevenness in the main scanning direction is suppressed across all gradations.

[0076] (Aspect 4) In any of the first to third aspects, the dot pattern of a pixel made up of a plurality of dots is set according to the gradation. This allows the dot pattern to express the shading of pixel image density and to express gradation.

[0077] (Aspect 5) In the fourth aspect, the correction of image data based on density unevenness is a correction of the dot pattern. According to this, as explained in aspect 4, the dot pattern can express the shade of pixel image density, and therefore, by correcting the dot pattern based on the density unevenness, it is possible to correct the density unevenness in the main scanning direction. [Explanation of symbols]

[0078] 1: Optical writing unit 3: Photoreceptor 51: Concentration sensor 111: Image element 112: Light receiving element 113 :Light source 114: Lens array 115: Output circuit 200: Control section 204: Image processing unit 205: Shading correction amount calculation unit 206: Gradation correction amount calculation unit 210: Storage section 220: Operation panel 230: External communication I / F P: Recording paper [Prior art documents] [Patent documents]

[0079] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-170195

Claims

1. a latent image carrier; an exposure unit that exposes the latent image carrier to light based on image data to form a latent image; a detection means for detecting density unevenness in the main scanning direction of a toner image obtained by developing the latent image, correcting only the exposure amount of the exposure means based on the density unevenness in the main scanning direction detected by the detection means to form a high-gradation image portion; an image forming apparatus for forming an image portion having a gradation of a medium gradation or less by correcting the exposure amount and the image data based on the density unevenness;

2. 2. The image forming apparatus according to claim 1, acquiring exposure amount correction data for correcting the exposure amount of the exposure means based on the density unevenness in the main scanning direction of the high gradation toner image detected by the detection means; An image forming apparatus characterized by acquiring image correction data that corrects image data based on density unevenness in the main scanning direction of a toner image of a medium gradation or lower obtained using an exposure amount corrected by the exposure amount correction data.

3. 3. The image forming apparatus according to claim 1, An image forming apparatus characterized in that a dot pattern of pixels made up of a plurality of dots is set according to gradation.

4. 4. The image forming apparatus according to claim 3, The image forming apparatus is characterized in that the correction of the image data based on the density unevenness is a correction of the dot pattern.

Images