Image forming apparatus

Abstract

To provide a technique to suppress density unevenness of a formed image.SOLUTION: An image forming apparatus comprises: means to correct first image data based on correction information to generate second image data; and an exposure head that includes a plurality of light emission elements arranged at different positions in a main scanning direction, and which, on the basis of the second image data, forms an electrostatic latent image on a photoreceptor by repeatedly exposing one line in the main scanning direction of the photoreceptor driven to rotate. The first image data indicates whether each of a plurality of dots constituting an image is an exposed dot exposed by the exposure head or an unexposed dot not exposed by the exposure head, and the correction means, on the basis of the correction information, selects a first altered dot from among a plurality of first dots included in a partial image in the plurality of dots for each of a plurality of partial images obtained by dividing the image, and generates the second image data by altering the first altered dot to the unexposed dot when the first altered dot is an exposed dot.SELECTED DRAWING: Figure 11

Description

[Technical Field]

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

[0002] An electrophotographic image forming apparatus forms an image by exposing a rotating photoreceptor to light to form an electrostatic latent image on the photoreceptor, and then developing the electrostatic latent image with toner. The direction parallel to the rotation axis of the photoreceptor is referred to as the main scanning direction. Patent Document 1 discloses an image forming apparatus in which a plurality of chips each having a plurality of light-emitting elements are arranged in the main scanning direction, and exposure is performed for one line in the main scanning direction. Patent Document 1 also discloses a configuration for correcting density unevenness caused by the difference in light intensity between two adjacent chips in the main scanning direction. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2018-1679 Summary of the Invention [Problem to be solved by the invention]

[0004] However, differences in light intensity can occur not only between chips but also between multiple light-emitting elements within a chip. Therefore, even if only the differences in light intensity between chips are corrected, density unevenness can occur in the formed image. In other words, density unevenness occurs in the image formed by the configuration described in Patent Document 1.

[0005] In view of the above problems, an object of the present invention is to suppress uneven density in a formed image. [Means for solving the problem]

[0006] According to one aspect of the present invention, an image forming apparatus comprises a photosensitive element, a correction means for correcting first image data based on correction information to generate second image data, an exposure head including a plurality of light-emitting elements arranged at different positions in the main scanning direction, and for forming an electrostatic latent image on the photosensitive element by repeatedly exposing one line in the main scanning direction of the photosensitive element that is rotated based on the second image data using each of the plurality of light-emitting elements, and a development means for developing the electrostatic latent image on the photosensitive element to form an image on the photosensitive element, wherein the first image data indicates, for each of a plurality of dots that constitute the image, whether it is an exposure dot that is exposed by the exposure head or a non-exposure dot that is not exposed by the exposure head, and the correction means, based on the correction information, for each of a plurality of partial images obtained by dividing the image, selects a first change dot from a plurality of first dots included in the partial image among the plurality of dots, and if the first change dot is the exposure dot, generates the second image data by changing the first change dot to the non-exposure dot. [Effects of the Invention]

[0007] According to the present invention, density unevenness in the formed image can be suppressed. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a diagram illustrating the configuration of an image forming apparatus according to an embodiment. [Figure 2] FIG. 2 illustrates an exposure head and photoreceptor, according to one embodiment. [Figure 3] FIG. 2 illustrates a printed circuit board for an exposure head, according to one embodiment. [Figure 4] 1 is an illustration of the arrangement of light emitting elements within a light emitting chip according to one embodiment. [Figure 5] 1 is a plan view of a light-emitting chip according to one embodiment. [Figure 6] 1 is a cross-sectional view of a light-emitting chip according to one embodiment. [Figure 7] 4A and 4B are diagrams illustrating spots on a photoreceptor according to one embodiment. [Figure 8] FIG. 2 is a control configuration diagram of each light-emitting chip according to an embodiment. [Figure 9] 1 is a block diagram of a light-emitting chip according to one embodiment. [Figure 10] FIG. 4 is a block diagram of a light intensity correction unit according to one embodiment. [Figure 11] FIG. 6 is an explanatory diagram of processing in a light amount correction unit according to an embodiment. [Figure 12] FIG. 10 is a diagram illustrating a light intensity correction chart according to one embodiment. [Figure 13] FIG. 10 is an explanatory diagram of a process for generating correction information according to an embodiment. [Figure 14] FIG. 10 is an explanatory diagram of a process for generating correction information according to an embodiment. [Figure 15] FIG. 10 is an explanatory diagram of a process for generating correction information according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, embodiments will be described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the scope of the invention claimed. Although multiple features are described in the embodiments, not all of these multiple features are necessarily essential to the invention, and multiple features may be combined arbitrarily. Furthermore, in the accompanying drawings, the same reference numerals are used to designate the same or similar components, and redundant explanations will be omitted.

[0010] 1 is a schematic diagram of an image forming apparatus according to this embodiment. A reading unit 100 optically reads an original placed on a platen and generates image data representing the reading result. An image creating unit 103 forms an image on a sheet based on the image data generated by the reading unit 100 or based on image data received from an external device via a network, for example.

[0011] The image forming unit 103 includes image forming units 101a, 101b, 101c, and 101d. The image forming units 101a, 101b, 101c, and 101d form black, yellow, magenta, and cyan toner images, respectively. The image forming units 101a, 101b, 101c, and 101d have the same configuration and will hereinafter be collectively referred to as the image forming unit 101. The photoconductor 102 of the image forming unit 101 is rotated clockwise in the drawing during image formation. The charger 107 charges the photoconductor 102. The exposure head 106 exposes the photoconductor 102 according to image data to form an electrostatic latent image on the photoconductor 102. The developer 108 develops the electrostatic latent image on the photoconductor 102 with toner. The toner image on the photoreceptor 102 is transferred onto a sheet transported on a transfer belt 111. By transferring the toner images of the photoreceptors 102 in an overlapping manner, it is possible to reproduce colors different from black, yellow, magenta, and cyan.

[0012] The conveying unit 105 controls the feeding and transport of sheets. Specifically, the conveying unit 105 feeds a sheet from a designated unit among the internal storage units 109a and 109b, the external storage unit 109c, and the manual feed unit 109d to a transport path of the image forming apparatus. The fed sheet is transported to the registration rollers 110. The registration rollers 110 transport the sheet onto the transfer belt 111 at a predetermined timing so that the toner images on the photoconductors 102 are transferred to the sheet. As described above, the toner image is transferred to the sheet while it is transported on the transfer belt 111. The fixing unit 104 fixes the toner image to the sheet by applying heat and pressure to the sheet to which the toner image has been transferred. After the toner image is fixed, the sheet is discharged to the outside of the image forming apparatus by the discharge rollers 112. An optical sensor 113 is disposed opposite the transfer belt 111. The optical sensor 113 detects a test chart for measuring the amount of color misregistration formed on the transfer belt 111 by the image forming unit 101. Based on the detection result of the test chart, a control unit (not shown) performs color misregistration correction control.

[0013] 2(A) and 2(B) show the photoconductor 102 and the exposure head 106. The exposure head 106 has a light-emitting element group 201, a printed circuit board 202 on which the light-emitting element group 201 is mounted, a rod lens array 203, and a housing 204 for attaching the rod lens array 203 to the printed circuit board 202. The rod lens array 203 focuses the light emitted by the light-emitting element group 201 onto the photoconductor 102, and forms an imaging spot (hereinafter simply referred to as a spot) of a predetermined size on the photoconductor 102.

[0014] 3(A) and 3(B) show the printed circuit board 202. Note that FIG. 3(A) shows the surface on which the connector 305 is mounted, and FIG. 3(B) shows the surface on which the light-emitting element group 201 is mounted (the surface opposite to the surface on which the connector 305 is mounted). In this embodiment, the light-emitting element group 201 has 20 light-emitting chips 400-1 to 400-20. The light-emitting chips 400-1 to 400-20 are arranged in two rows in a staggered pattern along the main scanning direction. More specifically, the light-emitting chips 400-(2k-1) (k is an integer from 1 to 10) are arranged in a row along the main scanning direction, and the light-emitting chips 400-2k are arranged in a row along the main scanning direction. The position of the row of the light-emitting chips 400-(2k-1) in the sub-scanning direction is different from the position of the row of the light-emitting chips 400-2k in the sub-scanning direction. The sub-scanning direction corresponds to the direction in which the photoconductor 102 rotates. The sub-scanning direction is perpendicular to the main scanning direction. In the following description, the light emitting chips 400-1 to 400-20 are also collectively referred to as light emitting chips 400. The light emitting chip 400-(2k-1) is referred to as the light emitting chip 400 in the odd-numbered row, and the light emitting chip 400-2k is referred to as the light emitting chip 400 in the even-numbered row. Each light emitting chip 400 has a plurality of light emitting elements. Each light emitting chip 400 on the printed circuit board 202 is connected to an image controller 800 (FIG. 8) which is a control unit via a connector 305.

[0015] FIG. 4 is an explanatory diagram of the arrangement of the light-emitting chips 400. In the light-emitting chip 400, 748 light-emitting elements 602 are arranged along the main scanning direction, and four sets are arranged in the sub-scanning direction. The pitch between adjacent light-emitting elements 602 in the main scanning direction is approximately 21.16 μm, which corresponds to a resolution of 1200 dpi. Therefore, the length of one set of 748 light-emitting elements in the main scanning direction is approximately 15.8 mm. Note that each set is arranged shifted in the main scanning direction by approximately 5 μm, which corresponds to a resolution of 4800 dpi. Furthermore, the light-emitting chips 400 in the even-numbered rows and the light-emitting chips 400 in the odd-numbered rows are arranged so as to overlap in the main scanning direction. The interval Ly between the light-emitting elements 602 of the light-emitting chips 400 in the even-numbered rows and the light-emitting elements 602 of the light-emitting chips 400 in the odd-numbered rows is, for example, approximately 105 μm.

[0016] 5 is a plan view of the light-emitting chip 400. The light-emitting chip 400 includes a light-emitting unit 404 including a plurality of light-emitting elements 602. The light-emitting unit 404 is formed on a light-emitting substrate 402. The light-emitting substrate 402 is also provided with a circuit unit 406 for controlling the light-emitting unit 404. A line for communicating with the image controller 800 is connected to a pad 408.

[0017] FIG. 6 shows a portion of the cross section taken along line AA in FIG. 5. A plurality of lower electrodes 504 are formed on the light-emitting substrate 402. A gap of length dx is provided between two adjacent lower electrodes 504. A light-emitting layer 506 is provided on the lower electrodes 504, and an upper electrode 508 is provided on the light-emitting layer 506. The upper electrode 508 is a common electrode for the plurality of lower electrodes 504. When a predetermined voltage is applied between the lower electrode 504 and the upper electrode 508, a current flows from the lower electrode 504 to the upper electrode 508, causing the light-emitting layer 506 to emit light. In other words, each lower electrode 504 is provided for one light-emitting element 602. By making the length dx longer than the length dz between the lower electrode 504 and the upper electrode 508, leakage current between adjacent lower electrodes 504 can be suppressed, thereby preventing erroneous emission of adjacent light-emitting elements 602.

[0018] The light-emitting layer 506 may be made of, for example, an organic EL film. Alternatively, the light-emitting layer 506 may be made of, for example, an inorganic EL film. The upper electrode 508 is made of a transparent electrode such as indium tin oxide (ITO) so as to transmit the emission wavelength of the light-emitting layer 506. In this embodiment, the entire upper electrode 508 transmits the emission wavelength of the light-emitting layer 506, but it is not necessary for the entire upper electrode 508 to transmit the emission wavelength. Specifically, it is sufficient that the region from which light is emitted from each light-emitting element 602 (corresponding to the lower electrode 504) transmits the emission wavelength.

[0019] As described with reference to FIG. 4 , each light-emitting chip 400 has four sets of light-emitting elements 602 arranged along the main scanning direction, and one of the sets adjacent in the sub-scanning direction is shifted 5 μm in the main scanning direction relative to the other set. When exposing one line of the photoconductor 102, the light-emitting timing of the four sets is controlled so as to expose that line of the photoconductor 102. Therefore, as shown in FIG. 7 , the four light-emitting elements 602 of the four sets, which are positioned approximately the same in the main scanning direction, expose the photoconductor 102 at positions shifted by 5 μm each. In this way, a smooth electrostatic latent image is formed by overlapping the spots generated by each light-emitting element 602. Note that although the number of sets is four in this embodiment, the number of sets can be two or more.

[0020] As described above, the exposure head 106 of this embodiment has 20 light-emitting chips 400 arranged in a staggered pattern in two rows along the main scanning direction. Each light-emitting chip 400 has four sets of multiple light-emitting elements 602 arranged along the main scanning direction. Each set is arranged along the sub-scanning direction, and the position of one adjacent set in the main scanning direction is shifted by 5 μm relative to the position of the other set in the main scanning direction, corresponding to a resolution of 4800 dpi. When viewed as a whole, the positions of the multiple light-emitting elements 602 in the main scanning direction are different. Note that although the positions of the multiple light-emitting elements 602 in the sub-scanning direction are not the same, the light emission timing of each light-emitting element 602 is adjusted so that they expose the same line on the photoconductor 102. Therefore, spots formed by each of the multiple light-emitting elements 602 can be formed on the photoconductor 102 at intervals of approximately 5 μm along one line in the main scanning direction. In the following description, the positions where each of the multiple light-emitting elements 602 forms a spot will be referred to as a "dot." Furthermore, when the light-emitting element 602 is made to emit light at the position of a dot, the dot is referred to as an "exposed dot," and when the light-emitting element 602 is not made to emit light at the position of a dot, the dot is referred to as a "non-exposed dot."

[0021] FIG. 8 shows the control configuration of each light-emitting chip 400 by the image controller 800. Image data indicating the gradation value of each pixel of the image to be formed is input to the image data generation unit 801. The image data generation unit 801 performs dithering (halftone processing) on ​​the image data at a resolution specified by the CPU 811 and outputs the processed image data to the light intensity correction unit 802. The halftone processed image data indicates whether each dot constituting the image is to be an exposed dot or a non-exposed dot. In other words, the halftone processed image data indicates whether the light-emitting element 602 corresponding to each dot is to emit light. The light intensity correction unit 802 corrects the light intensity of the image data based on the correction information and outputs the light intensity corrected image data to the chip data conversion unit 803. The synchronization signal generation unit 804 generates a line synchronization signal (Lsync) 808. The line synchronization signal 808 is used to determine the data portion of the image data corresponding to one line in the main scanning direction on the photoconductor 102. The chip data conversion unit 803 transmits one line of image data (DATA) 807 to each light-emitting chip 400 in synchronization with a line synchronization signal 808. The light-emitting chip 400 to which the image data 807 is addressed is indicated by a chip select signal (CS) 805. The chip data conversion unit 803 also transmits a clock signal (CLK) 806 to each light-emitting chip 400.

[0022] Correction information, which will be described later, is stored in a storage unit 810 of the printed circuit board 202. Each block shown in Fig. 8 is configured to be able to exchange various types of information by transmitting and receiving a control signal (CTL) 809. Upon receiving image data 807 from the chip data conversion unit 803, each light-emitting chip 400 performs a light-emitting operation according to the received image data 807 at the input timing of the next line synchronization signal 808.

[0023] FIG. 9 is a block diagram of the light-emitting chip 400. A digital-to-analog converter (D / A) 901 outputs an analog voltage corresponding to a digital value, which is a setting value set by the CPU 811. The digital value is indicated in correction information, and the CPU 811 determines the digital value to be set for the D / A 901 of each light-emitting chip 400 by reading the correction information stored in the storage unit 810. The light-emitting elements 602 of the light-emitting chip 400 are grouped into a plurality of blocks in the main scanning direction. Each group is provided with a corresponding reference current source 902. In FIG. 9, the light-emitting elements 602 are grouped into five groups, and therefore the light-emitting chip 400 has reference current sources 902-1 to 902-5 corresponding to each group. The reference current sources 902-1 to 902-5 output reference currents corresponding to the output values ​​output by the D / A 901, i.e., analog voltages, to each light-emitting element 602 of the corresponding group. In this way, the D / A 901 functions as a current control section that controls the reference current to the light emitting element 602. The amount of light emitted by the light emitting element 602 is controlled based on the reference current output by the corresponding reference current source 902.

[0024] 10 is a block diagram of the light intensity correction unit 802. The light intensity correction value A, the light intensity correction value B, and the spot correction value C are indicated in the correction information. The CPU 811 reads out the correction information stored in the storage unit 810 and notifies the light intensity correction value A, the light intensity correction value B, and the spot correction value C to the light intensity correction unit 802.

[0025] The light intensity correction value A is a correction value for correcting the difference in light intensity between groups of the light-emitting chips 400. In this embodiment, the light-emitting elements 602 in one light-emitting chip 400 are grouped into five groups, and therefore five light intensity correction values ​​A are set for one light-emitting chip 400. One group, that is, one reference current source 902, is associated with one light intensity correction value A.

[0026] The light intensity correction value B is a correction value for correcting the difference in light intensity between the light-emitting elements 602 in a group. Details will be described later, but in this embodiment, four light intensity correction values ​​B are associated with one group of one light-emitting chip 400. For example, a plurality of light-emitting elements 602 that emit light with a reference current from one reference current source 902 are subgrouped into four subgroups according to their positions in the main scanning direction. The light-emitting elements 602 included in one subgroup form spots continuously in the main scanning direction. One light intensity correction value B is associated with one subgroup.

[0027] The spot correction value C indicates the amount of deviation of the spot from a reference value (hereinafter referred to as the spot deviation) to correct for a difference in light intensity caused by the spot created by the light-emitting element 602 becoming enlarged in the main scanning direction. The spot correction value C is set for each light-emitting element 602 whose spot becomes enlarged. Note that, for a light-emitting element 602 for which the spot correction value C is not set, the spot correction value C is interpreted as being 0. As will be described in detail later, the impact of the spot created by the light-emitting element 602 becoming enlarged in the main scanning direction varies depending on the gradation. Specifically, when the gradation is high, the density increases as the spot becomes enlarged in the main scanning direction. Therefore, when a spot for forming a portion with a high gradation value becomes enlarged in the main scanning direction, the light intensity is reduced. On the other hand, when the gradation is low, the density decreases as the spot becomes enlarged in the main scanning direction. Therefore, when a spot for forming a portion with a low gradation value becomes enlarged in the main scanning direction, the light intensity is increased. Note that, the absolute value of the increase or decrease in the light intensity increases as the spot deviation increases.

[0028] Correction information including the light intensity correction value A, the light intensity correction value B, and the spot correction value C is stored in the storage unit 810 before shipping. Furthermore, the CPU 811 can obtain the light intensity correction value A, the light intensity correction value B, and the spot correction value C using a method described below, and update the correction information stored in the storage unit 810.

[0029] The image data that has been dithered by the image data generation unit 801 is input to the gradation determination unit 1105 and the image correction unit 1109. As described above, the image data indicates whether or not each light-emitting element 602 is to emit light when exposing each line of the photosensitive member 102 in the main scanning direction.

[0030] The gradation determination unit 1105 determines the gradation value of a pixel based on the input image data and notifies the gradation-specific correction unit 1106. The gradation-specific correction unit 1106 has a correction table for each gradation. The correction table is included in the correction information. The correction table is a table that indicates a reference light intensity correction value for each gradation. A positive reference light intensity correction value indicates an increase in the light intensity, and a negative reference light intensity correction value indicates a decrease in the light intensity. As described above, the effect of a spot enlargement in the main scanning direction varies depending on the gradation. For example, gradations are classified into three categories: low gradation, medium gradation, and high gradation, based on a first threshold and a second threshold. The first threshold is greater than the second threshold, and gradation values ​​greater than the first threshold are high gradation, gradation values ​​less than the second threshold are low gradation, and gradation values ​​equal to or greater than the second threshold and equal to or less than the first threshold are medium gradation. The reference light intensity correction values ​​indicated by the correction table are positive values ​​at low gradations, negative values ​​at high gradations, and 0 at medium gradations. In other words, a negative reference light intensity correction value is 0 at low and medium gradations, and a positive reference light intensity correction value is 0 at high and medium gradations.

[0031] The gradation-by-gradation correction unit 1106 corrects the reference light intensity correction value for the gradation of a pixel notified by the gradation determination unit 1105 based on the spot shift amount indicated by the spot correction value C of the light-emitting element 602 that forms the dot that constitutes the pixel, to determine the light intensity correction value D for that dot. As an example, the gradation-by-gradation correction unit 1106 holds coefficient information that indicates the correspondence between the spot shift amount and a coefficient, and determines the light intensity correction value D by multiplying the reference light intensity correction value for the gradation notified by the gradation determination unit 1105 by the coefficient corresponding to the spot shift amount. Note that the light intensity correction value D of dots formed by light-emitting elements 602 for which no spot correction value C is set, i.e., light-emitting elements 602 for which the spot correction value C is 0, is always 0. The gradation-by-gradation correction unit 1106 outputs data indicating the light intensity correction value D of each dot that constitutes the image to the image correction unit 1109.

[0032] The calculation unit 1107 calculates the light intensity correction value E for each light-emitting element 602 arranged at different positions in the main scanning direction based on the light intensity correction value A and the light intensity correction value B. The light intensity correction value E for each light-emitting element 602 is the sum of the light intensity correction value A for the group to which the light-emitting element 602 belongs and the light intensity correction value B for the subgroup to which the light-emitting element 602 belongs. As will be described in detail later, the sum of the light intensity correction value A for the group to which the light-emitting element 602 belongs and the light intensity correction value B for the subgroup to which the light-emitting element 602 belongs is 0 or a negative value, not a positive value. In other words, the sum indicates that the light intensity should be maintained or decreased, but does not indicate that the light intensity should be increased. The light intensity correction value E is also the correction value for the light intensity of each dot on one line in the main scanning direction formed by the light-emitting elements 602 arranged at different positions in the main scanning direction. The calculation unit 1107 outputs the light intensity correction value E for each dot on one line in the main scanning direction to the image correction unit 1109.

[0033] The image correction unit 1109 divides data indicating the light intensity correction value D of each dot constituting the image into first data indicating the light intensity correction value D of dots for which the light intensity is increased and second data indicating the light intensity correction value D of dots for which the light intensity is decreased. The image correction unit 1109 also generates third data indicating the light intensity correction value E of each dot constituting the image based on the light intensity correction value E of each dot on one line in the main scanning direction. The image correction unit 1109 then adds the absolute values ​​of the light intensity correction values ​​D and E of the same dot in the second and third data to generate fourth data indicating the total light intensity correction value of each dot constituting the image. The total light intensity correction value of each dot indicated by the fourth data indicates that the amount of light intensity reduction is equal to or greater than 0, and is hereinafter referred to as subtraction data. On the other hand, the light intensity correction value D of each dot indicated by the first data indicates that the amount of light intensity increase is equal to or greater than 0, and is hereinafter referred to as addition data. The image correction unit 1109 corrects the image data based on the subtraction data and addition data. In this embodiment, the image correction unit 1109 performs image correction on a partial image of a predetermined size, which is a part of the image to be formed. In this example, the size of the partial image is 10 x 10 pixels (100 pixels in total). FIG. 11A shows an example of a partial image of 10 x 10 pixels of an image formed using image data before correction. In FIG. 11A, one square represents one pixel. Note that shaded pixels indicate pixels to which toner adheres, and white pixels indicate pixels to which toner does not adhere.

[0034] In this embodiment, one pixel is formed by 10 consecutive dots in each of the main scanning direction and the sub-scanning direction. FIG. 11B shows an example of image data for forming one pixel in FIG. 11A. Hereinafter, the ten light-emitting elements 602 in the main scanning direction that form one pixel in FIG. 11B will be referred to as light-emitting elements #1 to #10. The Kth (K is an integer from 1 to 10) square from the left in FIG. 11B indicates a dot (spot) formed by light-emitting element #K. Specifically, a shaded square indicates an exposed dot, i.e., light-emitting element #K is made to emit light, and a white square indicates a non-exposed dot, i.e., light-emitting element #K is not made to emit light. Note that the vertical direction in FIG. 11B corresponds to the position in the sub-scanning direction. For example, light-emitting element #1 is shown to emit light at the first to fourth, seventh, and eighth of the ten dot formation positions in the sub-scanning direction. FIG. 11(B) can be regarded as indicating whether each of the 10×10 dots that form one pixel is an exposed dot or a non-exposed dot.

[0035] In the following, the dots (spots) shown in Figures 11(B) to 11(D) will be identified by numbers in the main scanning direction and sub-scanning direction. In the main scanning direction, the leftmost dot is numbered 1 and the rightmost dot is numbered 10. In the sub-scanning direction, the topmost dot is numbered 1 and the bottommost dot is numbered 10. For example, the second dot in the main scanning direction and the third dot in the sub-scanning direction will be represented as (2, 3).

[0036] The image correction unit 1109 has a threshold matrix table for subtraction and a threshold matrix table for addition. The threshold matrix table is a table showing thresholds for 10 x 10 pixels, i.e., 100 x 100 dots, for which image correction is performed. The image correction unit 1109 compares the absolute value of the total light intensity correction value of a dot corresponding to a partial image among the dots in the image indicated by the subtraction data with the threshold value of the corresponding dot indicated in the threshold matrix table for subtraction. The image correction unit 1109 then determines as a first change dot a dot whose absolute value of the total light intensity correction value exceeds the threshold in the threshold matrix table for subtraction.

[0037] FIG. 11C shows an example of the determination result using the threshold matrix table for subtraction of one pixel portion corresponding to FIG. 11B. In FIG. 11C, the dots at positions (4,2), (7,5), (2,8), and (8,10) are determined to be first change dots. If the first change dot is an exposed dot, the image corrector 1109 changes the first change dot to a non-exposed dot. On the other hand, if the first change dot is a non-exposed dot, the image corrector 1109 leaves the first change dot as a non-exposed dot. Therefore, the image corrector 1109 corrects the image data of FIG. 11B as shown in FIG. 11D. The dot at position (4,2) in FIG. 11B was originally a non-exposed dot, and therefore remains a non-exposed dot after correction. On the other hand, the dots at positions (7,5), (2,8), and (8,10) have been corrected from exposed dots to non-exposed dots.

[0038] Similarly, the image correction unit 1109 determines the second change dot using the addition data and the threshold matrix table for addition. If the second change dot is a non-exposed dot, the image correction unit 1109 changes the second change dot to an exposed dot. On the other hand, if the second change dot is an exposed dot, the image correction unit 1109 leaves the second change dot as an exposed dot. If the same dot is selected as both the first change dot and the second change dot, the image correction unit 1109 does not change the exposure / non-exposure of the dot, and leaves it in the state indicated by the original image data. Note that the threshold matrix table can be one with high-frequency spatial frequency characteristics used in the commonly known blue noise mask method.

[0039] The same threshold matrix table (100 x 100 dots in this example) is repeatedly used in both the main scanning direction and the sub-scanning direction. However, the subtraction data and addition data that are compared are for the entire image and are not repeated periodically, which helps prevent image defects from occurring at processing boundaries. The thresholds in the threshold matrix table are set so that the spacing between the first change dots and the second change dots are not uniform. Making the spacing between the first change dots and the second change dots uneven helps prevent moire from occurring.

[0040] Furthermore, by setting the correction resolution to be sufficiently fine with respect to the pixel size of the original image data, it becomes possible to perform light intensity correction with high precision. Furthermore, by performing light intensity correction before the chip data conversion unit 803 divides the image data into image data for each light-emitting chip 400, it is possible to suppress the occurrence of image defects at the boundaries of the light-emitting chips 400, compared to a configuration in which light intensity correction is performed after division.

[0041] In this manner, in this embodiment, the exposure / non-exposure of dots obtained by dividing one pixel indicated by image data is corrected in units of partial images of a predetermined area (10 x 10 pixels in this example). This configuration allows for simpler and more accurate correction compared to light intensity correction using complex analog circuits. Furthermore, by performing light intensity correction based on the light intensity correction value of each light-emitting element 602 that forms spots consecutively in the main scanning direction, i.e., the light intensity correction value at each consecutive position in the main scanning direction, it is possible to prevent image defects from occurring. Furthermore, by performing light intensity correction in units of dots obtained by dividing one pixel, it is possible to perform light intensity correction with high accuracy.

[0042] In this embodiment, the light intensity fluctuation of each light-emitting element 602 in the main scanning direction is corrected in two stages. First, in the first stage of correction, the digital value set in the D / A 901 of each light-emitting chip 400 is adjusted to correct the light intensity fluctuation between the light-emitting chips 400. To determine the digital value to set in the D / A 901 of each light-emitting chip 400, all light-emitting elements 602 in the light-emitting chip 400 are turned on and the light intensity values ​​of each light-emitting element 602 in the light-emitting chip 400 are measured. Then, the digital value to set in the D / A 901 is determined so that the light intensity of the light-emitting element 602 with the lowest light intensity in the light-emitting chip 400 becomes equal to or exceeds the target light intensity. Therefore, as described above, the light intensity correction value E of each light-emitting element is a value indicating whether to maintain or reduce the light intensity. By performing the first stage of light intensity correction using the digital value set in the D / A 901, the correction amount in the light intensity correction unit 802 can be reduced, thereby suppressing degradation of image quality due to image data correction.

[0043] The second stage of correction is correction of light intensity fluctuations within the light-emitting chip 400, and is performed by the light intensity correction unit 802 correcting the image data as described above. The light intensity correction value A, light intensity correction value B, and spot correction value C used by the light intensity correction unit 802 and the digital values ​​input to the D / A 901 of each light-emitting chip 400 described above are generated based on measurement results in the assembly and adjustment process of the exposure head 106, and are stored as correction information in the storage unit 810. The CPU 811 reads out the correction information, sets the light intensity correction value A, light intensity correction value B, and spot correction value C in the light intensity correction unit 802, and sets the digital values ​​in the D / A 901 of each light-emitting chip 400.

[0044] The spot correction value C indicates the amount of deviation (spot deviation) of the spot from a reference value. To measure the spot correction value C, each light-emitting element 602 is individually made to emit light. Then, the spot size is measured by reading the spot with a CCD camera, and the amount of change from the reference value is measured. The position of occurrence, that is, the relationship with the light-emitting element 602, is taken as the spot correction value C.

[0045] The correction information may be generated within the image forming apparatus. Fig. 12 shows a light intensity correction chart, which is a measurement image formed on a sheet in order to generate the correction information. The light intensity correction chart is formed by the image forming apparatus in response to a user instruction to perform light intensity unevenness adjustment. The user causes the reading unit 100 to read the sheet on which the light intensity correction chart has been formed. As a result, the CPU 811 obtains chart data, which is the reading result by the reading unit 100. The chart data is data indicating the density distribution in the main scanning direction for each of the multiple gradation images 2101 to 2106.

[0046] The light intensity correction chart has a plurality of gradation images 2101 to 2106 arranged in a band shape in the main scanning direction, and reference marks 2111-1 to 2119-19 and 2112-1 to 2112-19 arranged above and below the gradation images 2101 to 2106. Each reference mark is a marker image for identifying the position of each light-emitting chip 400, and is formed by light emission from the light-emitting elements 602 at the end of each light-emitting chip 400 in the main scanning direction. For example, the reference mark 2111-2 is formed by light emission from the four light-emitting elements 602 at the right end of the light-emitting chip 400-2 and the four light-emitting elements 602 at the left end of the light-emitting chip 400-3. The CPU 811 determines each reference mark based on the chart data. Then, the CPU 811 determines the areas formed by the light-emitting chips 400-1 to 400-20 for each of the gradation images 2101 to 2106 using a line connecting the reference mark 2111-p (p is an integer from 1 to 19) and the reference mark 2112-p. For example, the area B1 in FIG. 12 is determined to be formed by the light-emitting chip 400-2. Note that the left end in FIG. 12 is determined to be formed by the light-emitting chip 400-1, and the right end in FIG. 12 is determined to be formed by the light-emitting chip 400-20.

[0047] The set of gradation images 2101 and 2102 is formed using image data of the same gradation value. However, when forming the gradation image 2102, the CPU 811 sets the digital value set in the D / A 901 of each light-emitting chip 400 lower by a predetermined ratio than the digital value set in the D / A 901 of each light-emitting chip 400 when forming the gradation image 2101. Therefore, the density of the gradation image 2102 is lower than the density of the gradation image 2101. The same applies to the set of gradation images 2103 and 2104 and the set of gradation images 2105 and 2106. Note that the gradation values ​​indicated by the image data used to form the set of gradation images 2101 and 2102, the set of gradation images 2103 and 2104, and the set of gradation images 2105 and 2106 are different. Specifically, the gradation value indicated by the image data when forming the set of gradation images 2101 and 2102 is set to the highest, and the gradation value indicated by the image data when forming the set of gradation images 2105 and 2106 is set to the lowest. Note that the density of gradation image 2102 is higher than the density of gradation image 2103, and the density of gradation image 2104 is higher than the density of gradation image 2105.

[0048] Next, a method for converting chart data read by the reading unit 100 into light intensity data will be described with reference to FIG. 13. The CPU 811 calculates average densities 2101_D to 2106_D for each of the gradation images 2101 to 2106 by averaging the read densities at each position in the main scanning direction. FIG. 13 shows the relationship between the digital values ​​set in the D / A 901 when forming each of the gradation images 2101 to 2106 and the average densities 2101_D to 2106_D. The CPU 811 calculates a change amount k1 in the digital values ​​corresponding to a change in density for the set of gradation images 2101 and 2102. Specifically, the CPU 811 calculates the change amount k1 by dividing the difference between the digital values ​​set in the D / A 901 when forming the gradation images 2101 and 2102 by the difference between the average densities 2101_D and 2102_D. Similarly, the CPU 811 calculates the amount of change k2 between the set of gradation images 2103 and 2104, and the amount of change k3 between the set of gradation images 2104 and 21054.

[0049] The CPU 811 multiplies the density at each position in the main scanning direction of the gradation image 2101, determined based on the chart data, by the gradient k1 to determine the light intensity distribution in the main scanning direction of the gradation image 2101. Similarly, the CPU 811 determines the light intensity distribution in the main scanning direction for the gradation images 2103 and 2105. Note that the light intensity distribution can also be determined by changing the gradation of the image data used to form the gradation images 2101 and 2102, rather than changing the digital value set in the D / A 901.

[0050] In this embodiment, the CPU 811 determines the digital value to be set in the D / A 901 and the light intensity correction value A and the light intensity correction value B based on the light intensity distribution of the gradation image 2103, which is an image in the intermediate density region. The CPU 811 also determines the spot correction value C based on the light intensity distribution of the gradation image 2101 in the high density region and the gradation image 2105 in the low density region. A method for determining the digital value to be set in the D / A 901 and the light intensity correction value A and the light intensity correction value B will be described below with reference to FIG. 14.

[0051] FIG. 14 shows the light intensity distribution of the gradation image 2103. Note that FIG. 14 shows only a portion corresponding to one light-emitting chip 400. As described with reference to FIG. 12, which portion of the gradation image 2103 is formed by which light-emitting chip 400 can be determined by the reference mark. The CPU 811 determines a digital value to be set in the D / A 901 so that the lowest light intensity in FIG. 14 becomes the target light intensity T (target value). Specifically, the CPU 811 determines the digital value to be set in the D / A 901 by increasing the digital value set in the D / A 901 when forming the gradation image 2103 by a digital value corresponding to the value obtained by subtracting the lowest light intensity in FIG. 14 from the target light intensity T.

[0052] The light intensity 2301 in FIG. 14 is the light intensity at a predetermined position within the range where dots are formed by the plurality of light-emitting elements 602 in the group corresponding to the reference current source 902-1. Similarly, the light intensities 2302 to 2305 are the light intensities at predetermined positions within the range where dots are formed by the plurality of light-emitting elements 602 in the groups corresponding to the reference current sources 902-2 to 902-5, respectively. The CPU 811 determines, for example, the difference between the light intensity 2301 and the lowest light intensity in FIG. 14 as the light intensity correction value A associated with the reference current source 902-1. As described above, the lowest light intensity in FIG. 14 becomes the target light intensity T by the digital value set in the D / A 901. The CPU 811 similarly determines the light intensity correction values ​​A associated with the reference current sources 902-2 to 902-5.

[0053] Furthermore, the CPU 811 determines the light intensity at, for example, four positions within the range where dots are formed by the plurality of light-emitting elements 602 in the group corresponding to the reference current source 902-1. The positions for determining the light intensity are each selected from the range where dots are formed by the plurality of light-emitting elements 602 in one subgroup. The CPU 811 determines the difference between each of the four determined light intensities and the light intensity 2301 as the light intensity correction value B associated with each subgroup of the reference current source 902-1. The CPU 811 similarly determines the light intensity correction value B associated with the reference current sources 902-2 to 902-5.

[0054] In this way, all of the light emitting elements 602 in the group are corrected using light intensity correction value A, which is based on the reference current source 902, and fluctuations in the light intensity of the light emitting elements 602 in the group are corrected using light intensity correction value B. With this configuration, light intensity correction value B can be expressed using a smaller number of bits, thereby reducing the data amount of correction information. As an example, light intensity correction value A can be expressed using 4 bits, and light intensity correction value B, which indicates the fluctuations in the light intensity within the group, i.e., the residual component, can be expressed using 2 bits.

[0055] Next, a method for calculating the spot correction value C will be described. Fig. 15(A) shows the light intensity distribution of gradation images 2101, 2103, and 2105. Note that Fig. 15(A) displays the normalized light intensity of each of the gradation images 2101, 2103, and 2105. In other words, for gradation image 2101, the value on the vertical axis of Fig. 15 is the light intensity at each position in the main scanning direction of gradation image 2101 divided by the average light intensity of gradation image 2101. The same is true for gradation images 2103 and 2105. Because of the normalization, the light intensities of gradation images 2101, 2103, and 2105 have similar values ​​at most positions in the main scanning direction.

[0056] However, if the spot formed by the light-emitting element 602 changes locally due to manufacturing variations in the exposure head 106, the light intensities of the grayscale images 2101, 2103, and 2105 will differ. Specifically, in the case of the low-gradation image 2105, if the spot becomes locally large, sufficient light emission intensity cannot be obtained, and the density becomes low. In other words, when converted into light intensity, the light intensity decreases as indicated by reference numeral 2307 in FIG. 15(A). On the other hand, in the case of the high-gradation image 2101, the density increases due to the gap between adjacent pixels being collapsed. In other words, when converted into light intensity, the light intensity increases as indicated by reference numeral 2306 in FIG. 15(A). Note that reference numeral 2308 in FIG. 15(A) indicates the light intensity of the medium-gradation image 2103.

[0057] The solid line in Figure 15(B) shows the density characteristics when there is no spot fluctuation, and the dotted line shows the density characteristics when the spot becomes larger than the reference. As shown in Figure 15(B), when the spot becomes larger than the reference, the density increases in the high gradation region and decreases in the low gradation region. Note that the effect in the medium gradation region is small.

[0058] The CPU 811 calculates a peak value difference, which is the difference between the peak value of the normalized light intensity of the gradation image 2101 (reference numeral 2306 in FIG. 15A) and the peak value of the normalized light intensity of the gradation image 2105 (reference numeral 2307 in FIG. 15A). The image forming apparatus stores in advance determination information indicating the relationship between the peak value difference experimentally calculated and the amount of spot misalignment. The CPU 811 uses the determination information based on the calculated peak value difference to calculate a spot correction value C associated with the light-emitting element 602 corresponding to the position in the main scanning direction where the normalized light intensity is fluctuating.

[0059] Note that the specific values ​​used in the description of this embodiment are merely examples, and the present invention is not limited to the use of these specific values.

[0060] As described above, in this embodiment, the light intensity fluctuation of each light-emitting element 602 in the main scanning direction is corrected in two stages. First, the image controller 800 corrects the light intensity difference between the light-emitting chips 400 using a digital value set in the D / A 901 in each light-emitting chip 400. Then, the image controller 800 corrects the light intensity fluctuation of each light-emitting element 602 in each light-emitting chip 400 by correcting image data. By correcting the light intensity difference between the light-emitting chips 400 using a digital value set in the D / A 901 in each light-emitting chip 400, the amount of image data correction can be reduced, thereby suppressing degradation of image quality due to the image data correction. Furthermore, correcting the light intensity difference between the light-emitting elements 602 in each light-emitting chip 400 by correcting the image data allows for simpler and more accurate correction compared to a configuration in which a correction circuit for individually correcting the current flowing through each light-emitting element 602 is provided. In other words, the configuration of this embodiment allows for suppression of density unevenness without increasing the circuit size compared to a configuration in which a correction circuit for correcting the current flowing through each light-emitting element 602 in each light-emitting chip 400 is provided on the chip.

[0061] Furthermore, image data correction is performed by changing the exposed dots and non-exposed dots for each partial image using a threshold matrix of the same size as that partial image. The same threshold matrix is ​​used repeatedly for each partial image that makes up the image. However, the light intensity correction value compared with the threshold matrix is ​​for the entire image and is unrelated to the size of the threshold matrix, which prevents image defects from occurring at the boundaries of partial images. Furthermore, because the exposed dots / non-exposed dots are changed for each of the multiple dots that make up one pixel, light intensity correction can be performed with high precision.

[0062] [Other embodiments] The present invention can also be realized by supplying a program that realizes one or more functions of the above-described embodiments to a system or device via a network or a storage medium, and having one or more processors in the computer of the system or device read and execute the program.The present invention can also be realized by a circuit (e.g., ASIC) that realizes one or more functions.

[0063] The invention is not limited to the above-described embodiments, and various changes and modifications can be made without departing from the spirit and scope of the invention. Accordingly, the following claims are appended to apprise the public of the scope of the invention. [Explanation of symbols]

[0064] 106: exposure head, 400-1 to 400-20: light emitting chips, 602: light emitting element, 801: image data correction unit, 802: light quantity correction unit, 803: data conversion unit, 811: CPU

Claims

1. A rotating photoreceptor; a light-emitting chip including a plurality of light-emitting units that expose the photosensitive member, the plurality of light-emitting units being arranged along a rotation axis of the photosensitive member; a generating unit that generates image data; a driver that drives the plurality of light-emitting units based on the image data generated by the generator; Equipped with the image data is a set of bit data indicating whether the plurality of light-emitting units are lit or not, the image data includes first image data and second image data; the generation unit generates the second image data from the first image data in which a number of bit data indicating light emission of the plurality of light-emitting units is changed, based on the correction information; the drive unit drives the plurality of light-emitting units based on the second image data. An image forming apparatus characterized by:

2. The image forming device described in Claim 1, characterized in that the generation unit generates the second image data by changing selected bit data among the bit data indicating light emission contained in the first image data to bit data indicating light extinction based on the correction information.

3. The image forming device described in Claim 1, characterized in that the generation unit generates the second image data by changing selected bit data from the bit data indicating off contained in the first image data to bit data indicating on, based on the correction information.

4. The image forming device described in Claim 1, characterized in that the generation unit divides the first image data into multiple partial images and changes the number of bit data indicating light emission for each of the multiple partial images.

5. An image forming apparatus as described in Claim 4, characterized in that each of the multiple partial images is a rectangular area containing a predetermined number of pixels in the direction of the rotation axis of the photosensitive body and the rotation direction of the photosensitive body.

6. An image forming device as described in claim 1, characterized in that each of the multiple light-emitting units is an organic EL element.

Images