Image forming device

Abstract

To provide an improved mechanism for correcting an uneven quantity of light of an image.SOLUTION: An image forming apparatus includes: an image forming unit including a photoreceptor and an exposure head; a generation unit that generates correction data for correcting an uneven quantity of light of the exposure head by using a read image of a correction chart; and a correction unit that, based on the correction data, corrects image data to be input to the image forming unit. The exposure head includes a plurality of light emitting chips, each having an array of light emitting elements arranged in the first direction. The correction chart includes: a plurality of regions arranged in a second direction orthogonal to the first direction; and first and second reference marks respectively arranged on both sides of the plurality of regions in the second direction. The generation unit determines a partial region formed by each of the plurality of light emitting chips based on respective positions of the first and second reference marks and generates the correction data, based on a measurement result of a density for each partial region.SELECTED DRAWING: Figure 11

Description

[Technical field]

[0001] The present disclosure relates to an image forming apparatus. Place Regarding. [Background technology]

[0002] As one type of electrophotographic image forming apparatus, a solid-state exposure type apparatus is generally known, which exposes a photosensitive drum with light emitted by an LED (e.g., an organic EL element) instead of laser light to form a latent image. The exposure head of this type of apparatus includes a light-emitting element group including a plurality of light-emitting elements arranged parallel to the axial direction of the photosensitive drum, and a rod lens array that focuses light from the light-emitting element group on the surface of the photosensitive drum. Patent Document 1 discloses a method for correcting image unevenness (uneven light amount) caused by differences in light amount between a plurality of light-emitting chips each having a light-emitting element group, based on the reading result of a test chart, in a solid-state exposure type image forming apparatus. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2018-1679 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, in the correction method disclosed in Patent Document 1, the recognition accuracy of the sections corresponding to each light-emitting chip included in the test chart is insufficient, and there are cases where the unevenness in the amount of light cannot be corrected well. The causes of unevenness in the amount of light can include not only errors in units of light-emitting chips, but also errors in lenses in a lens array and individual differences in circuits in one light-emitting chip. In order to capture the position of such local unevenness with high accuracy and perform correction, it is important to be able to accurately recognize which part of the chart corresponds to which part of each chip of the exposure head.

[0005] In view of the above problems, an object of the present invention is to provide an improved mechanism for correcting light intensity unevenness in an image. [Means for solving the problem]

[0006] According to one aspect, an image forming apparatus is provided, comprising: an image forming unit including a photoconductor and an exposure head that exposes the photoconductor according to image data; a generation unit that causes the image forming unit to form an image of a correction chart and generates correction data for correcting unevenness in the amount of light from the exposure head using a read image of the correction chart obtained by optically reading the formed image; and a correction unit that corrects the image data input to the image forming unit based on the correction data generated by the generation unit. The exposure head includes a plurality of light-emitting chips arranged at different positions in a first direction parallel to the axial direction of the photoconductor, and each of the plurality of light-emitting chips includes an array of a plurality of light-emitting elements aligned at least in the first direction. The correction chart includes a plurality of regions that are aligned in a second direction perpendicular to the first direction and have different gradations from each other, at least one first reference mark that is arranged on one side of the plurality of regions in the second direction and indicates a boundary between adjacent light-emitting chips among the plurality of light-emitting chips, and at least one second reference mark that is arranged on the other side of the plurality of regions in the second direction and indicates the boundary between adjacent light-emitting chips among the plurality of light-emitting chips. The generation unit determines partial areas formed by the multiple light-emitting chips within the multiple regions based on the positions of the first reference mark and the corresponding second reference mark in the read image of the correction chart, and generates the correction data based on the density measurement results for each determined partial area. Effect of the Invention

[0007] SUMMARY OF THE PRESENTLY PREFERRED EMBODIMENTS The present invention provides an improved mechanism for correcting light variations in an image. [Brief description of the drawings]

[0008] [Figure 1]1 is a diagram showing a schematic configuration of an image forming apparatus according to an embodiment; [Diagram 2] FIG. 2 is an explanatory diagram of a configuration of a photoconductor and an exposure head according to the embodiment. [Diagram 3] FIG. 4 is an explanatory diagram of the configuration of a printed circuit board of an exposure head according to an embodiment. [Figure 4] 3A and 3B are explanatory views of a light-emitting chip and a light-emitting element group in the light-emitting chip according to an embodiment. [Diagram 5] FIG. 1 is a plan view showing a schematic configuration of a light-emitting chip according to an embodiment. [Figure 6] FIG. 1 is a cross-sectional view showing a schematic configuration of a light-emitting chip according to an embodiment. [Figure 7] FIG. 13 is an explanatory diagram of multiple exposure using light-emitting elements arranged in a stepped pattern. [Figure 8] FIG. 4 is a configuration diagram of a control circuit for controlling light emission of a light-emitting chip on a printed circuit board. [Figure 9] FIG. 2 is a block diagram showing a configuration of a circuit related to current supply in the light-emitting chip. [Figure 10] 11A and 11B are explanatory diagrams of a method for correcting unevenness in the amount of light by changing local area gradation. [Figure 11] FIG. 4 is an explanatory diagram for explaining a configuration of a correction chart according to an embodiment. [Figure 12] FIG. 12 is an explanatory diagram showing an example of a gradation-density characteristic curve measured using the correction chart of FIG. [Figure 13] FIG. 4 is an explanatory diagram showing an example of a light amount distribution along the axial direction of one light-emitting chip. [Figure 14] 11A and 11B are diagrams illustrating the influence of an abnormal spot on the light amount distribution. [Figure 15] FIG. 11 is an explanatory diagram showing in comparison the gradation-density characteristic curves of a normal spot and an abnormal spot. [Figure 16] FIG. 4 is a block diagram showing a detailed configuration of a light amount correction unit according to an embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Hereinafter, the embodiments will be described in detail with reference to the attached drawings. Note that the following embodiments do not limit the invention according to the claims. Although the embodiments describe a number of features, not all of these features are essential to the invention, and the features may be combined in any manner. Furthermore, in the attached drawings, the same reference numbers are used for the same or similar configurations, and duplicated descriptions are omitted.

[0010] <1. Schematic configuration of image forming apparatus> 1 shows an example of a schematic configuration of an image forming apparatus 1 according to an embodiment. The image forming apparatus 1 includes a reading unit 100, an image creating unit 103, a fixing unit 104, and a conveying unit 105. The reading unit 100 optically reads an original placed on a platen and generates read image data. The image creating unit 103 forms an image on a sheet based on the read image data generated by the reading unit 100, or based on print 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 are collectively referred to as the image forming unit 101 below. The photoconductor 102 of the image forming unit 101 is rotated clockwise in the figure 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 surface of the photoconductor 102. The developer 108 develops the electrostatic latent image on the surface of the photoconductor 102 with toner to form a toner image. The toner image formed on the surface of the photoconductor 102 is transferred to a sheet transported on a transfer belt 111. By transferring the toner images of the four photoconductors 102 onto a sheet in an overlapping manner, a color image containing four color components, namely black, yellow, magenta, and cyan, can be formed.

[0012] The conveying unit 105 controls the feeding and conveying of the sheet. Specifically, the conveying unit 105 feeds the 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 the conveying path of the image forming apparatus 1. The fed sheet is conveyed to the registration roller 110. The registration roller 110 conveys the sheet onto the transfer belt 111 at an appropriate timing so that the toner image of each photoconductor 102 is transferred onto the sheet. As described above, the toner image is transferred onto the sheet while the sheet is being conveyed on the transfer belt 111. The fixing unit 104 fixes the toner image onto the sheet by heating and pressing the sheet onto 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 1 by the discharge roller 112. An optical sensor 113 is disposed at a position opposite the transfer belt 111. The optical sensor 113 is used to detect positional misalignment (color misalignment) between color components of a test image formed on the transfer belt 111 by the image forming unit 101. If a color misalignment is detected, the image forming positions of the image forming units 101a, 101b, 101c, and 101d are corrected under the control of an image controller 800 (described later) so as to compensate for the detected color misalignment.

[0013] Although an example in which a toner image is directly transferred from each photoconductor 102 to a sheet on the transfer belt 111 has been described here, the toner image may be indirectly transferred from each photoconductor 102 to a sheet via an intermediate transfer body. Also, although an example in which a color image is formed using toners of multiple colors has been described here, the technology according to the present disclosure is also applicable to an image forming apparatus that forms a monochrome image using toners of a single color.

[0014] <2. Exposure head configuration example> 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 mounting the rod lens array 203 to the printed circuit board 202. The photoconductor 102 has a cylindrical shape. The exposure head 106 is disposed such that its longitudinal direction is parallel to the axial direction D1 of the photoconductor 102, and the surface on which the rod lens array 203 is mounted faces the surface of the photoconductor 102. While the photoconductor 102 rotates in the circumferential direction D2, the light-emitting element group 201 of the exposure head 106 emits light, and the rod lens array 203 focuses the light on the surface of the photoconductor 102.

[0015] 3(A) and 3(B) show an example of the configuration of the printed circuit board 202. 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). FIG. 4 shows an arrangement of the light-emitting chip 400 and the light-emitting elements 602 in the light-emitting chip 400. In this embodiment, the light-emitting element group 201 is composed of light-emitting elements included in 20 light-emitting chips 400-1 to 400-20. The light-emitting chips 400-1 to 400-20 are arranged in a staggered manner along the axial direction D1. More specifically, when n is an odd number, 10 light-emitting chips 400-n form one line, and when n is an even number, another 10 light-emitting chips 400-n form another line, and the positions of the two lines in the circumferential direction D2 are different. In this specification, the light emitting chips 400 in the former line are also referred to as odd-numbered light emitting chips 400, and the light emitting chips 400 in the latter line are also referred to as even-numbered light emitting chips 400. The light emitting chips 400-1 to 400-20 are also collectively referred to as light emitting chips 400. Each light emitting chip 400 on the printed circuit board 202 is connected to the image controller 800 (FIG. 8) via a connector 305. In the following, for convenience of explanation, the side with the smaller branch number of the light emitting chips 400-1 to 400-20 arranged along the axial direction D1 may be referred to as the "left" and the side with the larger branch number as the "right". For example, the light emitting chip 400-1 is the leftmost light emitting chip 400, and the light emitting chip 400-20 is the rightmost light emitting chip.

[0016] The light emitting element array of each light emitting chip 400 is a two-dimensional array consisting of N columns (N is an integer of 2 or more) in the axial direction D1 and M rows (M is an integer of 2 or more) in the circumferential direction D2. In an exemplary embodiment, N=748 and M=4 may be used, in which case each light emitting chip 400 has a total of 2992 (=748×4) light emitting elements 602. In the entire light emitting element group 201 consisting of 20 light emitting chips 400, 14960 light emitting elements are arranged in the axial direction D1. The pitch between adjacent light emitting elements 602 in the axial direction D1 may be about 21.16 μm, which corresponds to a resolution of 1200 dpi. In this case, the entire length of the light emitting element group 201 in the axial direction D1 is about 316 mm (the maximum width of an image that can be formed), and the length of each light emitting chip 400 in the axial direction D1 is about 15.8 mm. The M light emitting elements in each row are shifted in the axial direction D1 at a pitch of about 5 μm, which corresponds to a resolution of 4800 dpi, and arranged in a stepped manner. FIG. 4 shows an example in which the lower light-emitting element of the two upper and lower light-emitting elements is shifted to the left, but the lower light-emitting element may be shifted to the right. Furthermore, as shown in FIG. 4, the light-emitting elements in the rightmost row of the odd-numbered light-emitting chips 400 and the light-emitting elements in the leftmost row of the even-numbered light-emitting chips 400 may be arranged so as to overlap in the axial direction D1. Similarly, the light-emitting elements in the leftmost row of the odd-numbered light-emitting chips 400 and the light-emitting elements in the rightmost row of the even-numbered light-emitting chips 400 may be arranged so as to overlap in the axial direction D1. The interval Ly between the light-emitting element arrays of these light-emitting chips 400 may be, for example, about 105 μm. By overlapping the arrangement of the light-emitting element arrays in this way in the axial direction D1, it is possible to avoid a situation in which a gap is generated in the range that can be exposed due to variations in mounting. When such an overlapping arrangement is adopted, usually, only one of the light-emitting elements in the overlapping rows is used as an effective light-emitting element that can emit light according to image data. The light-emitting elements in the other row do not need to be used regardless of the image data. The number of columns or the number of light emitting elements that are arranged overlappingly in the axial direction D1 is not limited to the above example (one column, four pixels) and may be any number.

[0017] 5 is a plan view showing a schematic configuration of the light-emitting chip 400. The light-emitting elements 602 of each light-emitting chip 400 are formed on a light-emitting substrate 402, which is, for example, a silicon substrate. A circuit section 406 for driving the light-emitting elements 602 is provided on the light-emitting substrate 402. The pads 408 are used to connect a signal line for communicating with the image controller 800, a power line for connecting to a power source, and a ground line for connecting to a ground to the circuit section 406. The signal line, the power line, and the ground line may be, for example, a wire made of gold.

[0018] FIG. 6 shows a part 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 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 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. Thus, one lower electrode 504 and a partial region of the light-emitting layer 506 and upper electrode 508 corresponding to the lower electrode 504 constitute one light-emitting element 602. dx in the figure is the distance between two adjacent lower electrodes 504. dz is the distance between the lower electrode 504 and the upper electrode 508. By making dx larger than dz, it is possible to suppress leakage current between adjacent lower electrodes 504 and prevent a light-emitting element 602 that should not emit light from erroneously emitting light.

[0019] In this embodiment, each light-emitting element 602 is configured as an organic EL (Electro-Luminescence) element. For example, an organic EL film can be used for the light-emitting layer 506. In other embodiments, each light-emitting element 602 may be configured as an inorganic EL element by using an inorganic EL film for the light-emitting layer 506. Generally, each light-emitting element 602 may be any type of LED (Light-Emitting Diode).

[0020] The upper electrode 508 is composed of a transparent electrode such as indium tin oxide (ITO) so as to transmit the emission wavelength of the light emitting layer 506. In the example of Fig. 6, the entire upper electrode 508 transmits the emission wavelength of the light emitting layer 506, but the entire upper electrode 508 does not necessarily have to transmit the emission wavelength. Specifically, it is sufficient that a partial region through which light from each light emitting element 602 passes transmits the emission wavelength.

[0021] In FIG. 6, one continuous light-emitting layer 506 is formed, but a plurality of light-emitting layers 506 each having a width equivalent to the width of the lower electrode 504 may be formed on the lower electrode 504. In FIG. 6, the upper electrode 508 is formed as one common electrode for the plurality of lower electrodes 504, but a plurality of upper electrodes 508 each having a width equivalent to the width of the lower electrode 504 may be formed corresponding to each of the lower electrodes 504. In addition, the first plurality of lower electrodes 504 of the lower electrodes 504 of each light-emitting chip 400 may be covered by the first light-emitting layer 506, and the second plurality of lower electrodes 504 may be covered by the second light-emitting layer 506. Similarly, the first upper electrode 508 may be commonly formed corresponding to the first plurality of lower electrodes 504 of the lower electrodes 504 of each light-emitting chip 400, and the second upper electrode 508 may be commonly formed corresponding to the second plurality of lower electrodes 504. In such a configuration, one lower electrode 504 and the region of the light emitting layer 506 and upper electrode 508 corresponding to the lower electrode 504 constitute one light emitting element 602 .

[0022] <3. Multiple Exposure> Although the use of organic EL elements as the light-emitting elements 602 facilitates miniaturization and cost reduction of the device, the amount of light that a single organic EL element can emit may be insufficient to form an image of a desired density. Therefore, in this embodiment, a multiple exposure technique is adopted in which a plurality of light-emitting elements 602 arranged in the circumferential direction of the photoconductor 102 are sequentially caused to emit light, thereby exposing each pixel region (spot region) on the photoconductor 102 in a multiple exposure manner.

[0023] 7 is an explanatory diagram of multiple exposure using light-emitting elements arranged in a stepped pattern. As described above, in this embodiment, M light-emitting elements 602 in each row of the light-emitting element array can be arranged in a stepped pattern at a constant pitch. Here, an example of the arrangement of light-emitting elements when M=4 is partially shown. j_m (j={1,2,...,N}, m={0,1,2,3}) represents the light emitting element 602 in the j-th column from the left in the axial direction and in the m-th row from the bottom in the circumferential direction. In the figure, W1 represents the width of the light emitting element 602 in the axial direction, and W2 represents the width of the light emitting element 602 in the circumferential direction. d1 represents the distance between adjacent light emitting elements 602 in the axial direction, and d2 represents the distance between adjacent light emitting elements 602 in the circumferential direction. d1 and d2 are the above-mentioned inter-electrode distance dx divided into two coordinate axes, and both are determined to be wider than the distance dz between the upper electrode and the lower electrode. As described above, the minimum pitch d3 of the light emitting elements 602 in the axial direction may be about 5 μm (equivalent to 4800 dpi).

[0024] As in the example of Fig. 7, four light-emitting elements in each row are arranged in a stepped manner, so that any two adjacent light-emitting elements among the four light-emitting elements occupy areas that partially overlap in the axial direction. Then, the four light-emitting elements in the row corresponding to each pixel position of the input image data sequentially emit light while the photoconductor 102 is rotating, so that a spot corresponding to each pixel position is formed on the surface of the photoconductor 102. In the example of Fig. 7, when the pixel value at the left end of the i-th line of the input image data indicates that light is on, the light-emitting element R 1_1 , R 1_2 , R 1_3 , R 1_4 is a line L on the surface of the photoconductor 102. i As a result, the line L i In the same manner, when the jth pixel value from the left of the i-th line of the input image data indicates that light is emitted, the light-emitting element R j_1 , R j_2 , R j_3 , R j_4 is a line L on the surface of the photoconductor 102.i As a result, the line L i The jth spot area from the left of is multiplexed and the corresponding spot SP j If the pitch of the light emitting elements in the circumferential direction is about 21.16 μm and the sheet conveying speed is 200 mm / s, each line is formed by one light emitting element R j_m The period (line period) of exposure by the 20 light-emitting chips 400 may be about 105.8 μs. In this manner, the four light-emitting elements in each row of the 20 light-emitting chips 400 sequentially emit light at appropriate times, so that smooth lines of an electrostatic latent image consisting of a series of spots that are partially overlapping each other and have a certain spot spacing can be formed on the surface of the photoconductor 102. As a result of such lines being continuously formed in the circumferential direction, a two-dimensional electrostatic latent image is created.

[0025] In the following description, a spot area in which a spot of an electrostatic latent image is formed as a result of multiple exposure due to the emission of M light-emitting elements 602 is also referred to as an "exposed dot," and a spot area in which a spot is not formed because the light-emitting elements 602 do not emit light is also referred to as a "non-exposed dot."

[0026] <4. Control circuit configuration> FIG. 8 shows an example of the configuration of a control circuit for controlling the light emission of the light emitting chip 400 on the printed circuit board 202. Here, for the sake of simplicity, a process for a single color component will be described, but in reality, a similar process is performed in parallel for four color components. The image controller 800 is connected to each of the light emitting chips 400 on the printed circuit board 202 via a plurality of signal lines 805 to 809. The chip select signal line 805 carries a chip select signal CS representing the effective range of image data. The clock signal line 806 carries a clock signal CLK. The data signal line 807 carries image data DATA. The synchronization signal line 808 carries a line synchronization signal Lsync for identifying the line period of the image data. The communication signal line 809 carries a control signal CTL.

[0027] The image data generating unit 801 performs image processing on image data received from the reading unit 100 or an external device to generate binary bitmap image data for controlling the on / off of the light emission of the light emitting element 602 of the light emitting chip 400 on the printed circuit board 202. The image processing here may include, for example, raster conversion and halftone processing (e.g., dithering). The image data after halftone processing is a set of bits indicating whether or not to emit light from the corresponding M light emitting elements 602 for each pixel position constituting the image to be formed. When a bit at a certain pixel position indicates "emission", the corresponding spot area on the surface of the photoconductor 102 becomes an exposed dot. When the bit indicates "non-emission", the corresponding spot area becomes a non-exposed dot. The image data generating unit 801 outputs the generated image data to the light quantity correcting unit 802.

[0028] The light quantity correction unit 802 performs a correction process for correcting unevenness in the amount of light of the exposure head 106 on the image data input from the image data generation unit 801, based on correction data generated by the CPU 811. The light quantity correction unit 802 then outputs the corrected image data to the chip data conversion unit 803. The correction of unevenness in the amount of light performed by the light quantity correction unit 802 will be described in detail later.

[0029] The chip data conversion unit 803 reads out pixel values ​​within a corresponding readout range of the image data input from the light quantity correction unit 802 in each line period identified by the line synchronization signal Lsync, and transmits image data indicating the readout pixel values ​​to a data signal line 807. The image data DATA is, for example, a sequence of pixel values ​​corresponding to the light emitting elements 602 of the 20 light emitting chips 400. The chip data conversion unit 803 specifies which light emitting chip 400 should receive each portion of the image data DATA by a chip select signal CS. The chip data conversion unit 803 generates a clock signal CLK and supplies it to each light emitting chip 400 in order to synchronize the timing of transmission and reception of individual signal values ​​with each light emitting chip 400.

[0030] The synchronization signal generation unit 804 determines the line boundaries of the image data, generates a line synchronization signal Lsync, and supplies the generated line synchronization signal Lsync to a synchronization signal line 808 .

[0031] The storage unit 810 of the printed circuit board 202 is a memory or a register that stores control data for controlling light emission by each light-emitting chip 400. As will be described later, the control data stored in the storage unit 810 may include, for example, a setting value related to the amount of supply current and correction data described later.

[0032] Each light-emitting chip 400 drives each of the light-emitting elements 602 in accordance with the image data after light intensity correction input from the chip data conversion unit 803 in each line period identified by the line synchronization signal Lsync. For example, when the chip select signal CS indicates the timing to receive data intended for the light-emitting chip, each light-emitting chip 400 receives a portion of the image data DATA intended for the chip itself via the data signal line 807. Then, each light-emitting chip 400 drives each of the light-emitting elements 602 in the light-emitting element array of M rows and N columns in accordance with the pixel values ​​included in the received image data.

[0033] The CPU 811 controls the entire image forming apparatus 1. For example, the CPU 811 controls the generation of the image data, the light amount correction, the generation of the line synchronization signal, and the transmission of the image data to the printed circuit board 202. In addition, in this embodiment, the CPU 811 executes calibration using a correction chart in response to an instruction via a user interface. Specifically, the CPU 811 causes the image forming unit 101 to form an image of the correction chart on a sheet. In addition, when the sheet on which the image of the correction chart is formed is set in the reading unit 100, the CPU 811 causes the reading unit 100 to optically read the image of the correction chart on the sheet. Then, the CPU 811 generates correction data for correcting the unevenness of the light amount of the exposure head 106 by using the read image of the correction chart obtained as a result of the reading. Therefore, in this embodiment, the CPU 811 functions as a generating unit that generates correction data. An example of correction data that can be generated by the CPU 811 will be described in detail later.

[0034] Fig. 9 is a block diagram showing a configuration of a circuit related to the supply of current in the light-emitting chip 400. Each light-emitting chip 400 includes a digital-to-analog converter (D / A) 901, a plurality of (five in the example of Fig. 9) reference current sources 902-1 to 902-5, and a plurality of light-emitting elements 602. Each light-emitting element 602 is supplied with a current from one of the reference current sources 902-1 to 902-5 depending on the axial position of the light-emitting element 602. Therefore, the light-emitting elements 602 of each light-emitting chip 400 can be grouped into five groups.

[0035] The D / A 901 performs digital-to-analog conversion of a digital value indicating a set value of the reference current set by the CPU 811, and outputs an analog signal having a voltage according to the set value to each reference current source 902. The set value of the reference current is stored in advance in the above-mentioned storage unit 810, read by the CPU 811, and output to the D / A 901. Each of the reference current sources 902-1 to 902-5 supplies a reference current according to the voltage of the analog signal input from the D / A 901 to the light-emitting element 602 of the corresponding group. The number of the reference current sources 902 provided in each light-emitting chip 400 is not limited to the above-mentioned example, and may be any number depending on the length of the wiring in the chip or the driving capability of the reference current source 902.

[0036] <5. Correcting uneven light intensity> <5-1. Causes of uneven light intensity> Examples of factors that cause light quantity unevenness in a solid-state exposure type image forming apparatus may include the following: Optical errors caused by manufacturing variations in lenses in rod lens arrays Errors in the amount of current supplied to the light-emitting element due to individual differences in the current source and circuit -Concentration errors due to spot size variations Optical errors caused by manufacturing variations in the lens may occur at any position in the longitudinal direction of the light-emitting chip 400 (the axial direction of the photoconductor 102), causing localized unevenness in the amount of light. Errors in the amount of supplied current caused by individual differences in the current source may occur in groups of light-emitting elements 602 to which current is supplied from each reference current source 902. However, since the current loss in the circuit may differ depending on the position of the light-emitting element 602, errors in the amount of supplied current are not necessarily uniform even within one group. Variations in spot size affect the characteristics of density with respect to the gradation value of the image to be reproduced. Specifically, when the spot size on the imaging surface of the light from the exposure head 106 becomes enlarged, the density of the printed image with respect to the same gradation value becomes lower in the low gradation range due to insufficient light. On the other hand, when the spot size becomes enlarged, the density of the printed image with respect to the same gradation value becomes higher in the high gradation range due to crushing between adjacent spots. The main cause of the variation in spot size is the variation in manufacturing the exposure head 106. In the following description, an image spot with an enlarged size is referred to as an abnormal spot. An abnormal spot causes local density unevenness in the longitudinal direction of the exposure head 106. Note that, since the density of each spot in the formed image correlates with the amount of light of the exposure light by the corresponding light-emitting element, the terms light amount unevenness and density unevenness can be used interchangeably in this specification.

[0037] <5-2. Basic correction methods> Methods for correcting the unevenness in the amount of light may include a method for adjusting the amount of current supplied from the reference current source 902 of the light-emitting chip 400, and a method for correcting the input image data so as to change the local area gradation. This embodiment incorporates both methods. The two correction methods may be combined in various ways, and for example, it may be possible to correct the rough unevenness in the amount of light between the light-emitting chips 400 by the former, and to correct the fine unevenness in the amount of light within each light-emitting chip 400 by the latter.

[0038] The former method can be performed by rewriting a setting value related to the amount of current supply among the control data stored in the storage unit 810. The unevenness in the amount of light between the light-emitting chips 400 may be measured at an inspection stage after assembly of the product, for example, without using the correction chart 200 described later. For example, in a state where all the light-emitting elements 602 are made to emit light, the amount of light from each light-emitting element 602 is measured, and the light-emitting element 602 with the lowest measured value is selected for each light-emitting chip 400. Then, a setting value related to the amount of current supply for each light-emitting chip 400 (adjustment target value T described later) is determined so that the amount of light from each selected light-emitting element 602 becomes a constant target amount of light across the light-emitting chips 400. The determined setting value is written in the storage unit 810 of each light-emitting chip 400. Then, during subsequent image formation in the image forming apparatus 1, the difference in the amount of light between the light-emitting chips 400 is almost eliminated, and the unevenness in the amount of light in each light-emitting chip 400 may be corrected by the latter method.

[0039] The latter method, which enables fine correction of uneven light quantity, will be described below with reference to FIG. 10. FIG. 10(A) shows, as an example, a small image IM1 around a certain reference pixel position, which is represented by image data before correction. Each square in the figure represents one pixel. A shaded pixel indicates that the corresponding spot area is an exposed dot, i.e., the corresponding M light-emitting elements emit light. A blank pixel indicates that the corresponding spot area is a non-exposed dot, i.e., the corresponding M light-emitting elements do not emit light. Here, the pixel position that is x-th from the left and y-th from the top is represented as (x, y) with the top left pixel of the small image as the reference.

[0040] FIG. 10B shows a correction matrix IM2 as an example. The correction matrix IM2 is a matrix (bitmap) of the same size as the small image IM1, and one square in the figure is one element. The shaded elements represent pixels to be changed that are selected based on the result of calibration. There may be two types of correction matrices, one for reducing the amount of light and one for increasing the amount of light, but the correction matrix IM2 is assumed to be a matrix for reducing the amount of light. As an example, when the amount of light at the reference pixel position is to be reduced by 4% from the maximum value, four pixels out of a total of 100 (=10×10) pixels are selected as pixels to be changed. In the example of FIG. 10B, pixels at pixel positions (4,2), (7,5), (2,8), and (8,10) are selected as pixels to be changed. The pixel positions of the pixels to be changed may be selected according to, for example, a known blue noise mask method.

[0041] When a pixel at the same position as the selected change pixel in the correction matrix IM2 indicates an exposure dot in the small image IM1, the light amount correction unit 802 changes the pixel to a non-exposure dot (i.e., inverts the pixel value). FIG. 10C shows an example of a result of correcting the small image IM1 using the correction matrix IM2. In the small image IM3 in FIG. 10C, the pixels at pixel positions (7,5), (2,8), and (8,10) that were exposure dots in the small image IM1 are changed to non-exposure dots. In a case where the light amount at the reference pixel position is to be increased, when a pixel at the same position as the selected change pixel indicates a non-exposure dot in the small image IM1, the light amount correction unit 802 changes the pixel to an exposure dot. The light amount correction unit 802 repeats correction of the small images around each reference pixel position based on the correction data generated as a result of the calibration while scanning the reference pixel positions in the input image data in the axial direction. As a result, a fine correction of the unevenness in the light amount in the axial direction can be realized.

[0042] Here, if the rate of increase or decrease in the amount of light is large, it is necessary to select a large number of change pixels and change the pixel values. However, such a change may destroy the dot shape of the image to be reproduced and degrade the image quality. In this embodiment, as described above, the rough unevenness in the amount of light between the light-emitting chips 400 is corrected by adjusting the amount of supply current, and only the remaining unevenness in the amount of light is corrected by changing the area gradation, so that the rate of increase or decrease in the amount of light due to the latter is suppressed to be relatively small. Therefore, it is possible to prevent the image quality from deteriorating due to the destruction of the dot shape.

[0043] Although the example in which the size of the small image is 10×10 pixels has been described here, the present embodiment is not limited to such an example. For example, when selecting change pixels according to the blue noise mask method, the size of the small image can be set to a larger size of 128×128 pixels or 256×256 pixels to diffuse spatial frequencies more randomly and significantly suppress interference moire.

[0044] <5-3. Example of correction chart configuration> In order to effectively correct the above-mentioned unevenness in the amount of light, it is important to accurately capture the local position where the unevenness occurs and to accurately determine at which position in the axial direction the amount of light should be increased or decreased by how much. Therefore, in this embodiment, the image forming apparatus 1 uses a correction chart 200 as shown in FIG.

[0045] 11, the correction chart 200 includes a plurality of strip regions 2101-2106, each of which is a rectangular region with the longitudinal direction being the axial direction D1. The strip regions 2101-2106 are arranged side by side in a circumferential direction D2 perpendicular to the axial direction D1. The strip regions 2101-2106 are image regions having different gradations. The gradations of the strip regions 2101 and 2102 belong to a high gradation region. The gradations of the strip regions 2103 and 2104 belong to a medium gradation region. The gradations of the strip regions 2105 and 2106 belong to a low gradation region. As will be described in detail later, the strip regions 2103 and 2104 can be mainly used to measure an error in the amount of light of each light emitting element or group of light emitting elements in each light emitting chip 400. The swaths 2101 , 2102 , 2105 and 2106 may be used primarily to measure the variation in spot size at the imaging plane of the light from the exposure head 106 .

[0046] The correction chart 200 includes at least one first reference mark 2111-1 to 2111-19 arranged on one side (upper side in the figure) of the band-shaped regions 2101 to 2106 in the circumferential direction D2. In addition, the correction chart 200 includes at least one second reference mark 2112-1 to 2112-19 arranged on the other side (lower side in the figure) of the band-shaped regions 2101 to 2106 in the circumferential direction D2. The first reference marks 2111-1 to 2111-19 indicate boundaries between adjacent light-emitting chips of the multiple light-emitting chips 400. The second reference marks 2112-1 to 2112-19 also indicate these boundaries. When generating correction data based on the read image of the correction chart 200, the CPU 811 determines the representative positions of the first reference marks 2111-p and the corresponding representative positions of the second reference marks 2112-p (p=1, 2, ..., 19) in the read image. The representative positions here may be, for example, the center of gravity of each reference mark, any vertex, or the position of an end point in the axial direction D1, depending on the mark shape. Then, the CPU 811 determines the partial areas formed by the light-emitting chips 400-1 to 400-20 in each of the strip-shaped areas 2101 to 2106, using the line segments (shown by dashed lines in the figure) connecting the determined representative positions as boundaries. The CPU 811 measures the density of each partial area determined in this manner, and generates correction data (described later) based on the results of the measurement.

[0047] The first reference marks 2111-1 to 2111-19 and the second reference marks 2112-1 to 2112-19 may be formed by the effective light-emitting elements at at least one end in the axial direction D1 of each light-emitting chip 400 emitting light. For example, the first reference mark 2111-1 and the second reference mark 2112-1 may be formed by the four effective light-emitting elements 602 at the right end of the light-emitting chip 400-1 emitting light. The first reference mark 2111-2 and the second reference mark 2112-2 may be formed by the four effective light-emitting elements 602 at the right end of the light-emitting chip 400-2 emitting light. In this way, when a pair of the first reference mark 2111-p and the second reference mark 2112-p is formed by driving a specific light-emitting element 602 adjacent to each boundary, the line segment connecting the representative positions of these reference marks approximately coincides with the boundary between the chips. For example, the six partial regions between the dashed lines 2113-1 and 2113-2 in the figure are image regions formed by the light emitting chip 400-2.

[0048] For example, in response to an instruction to perform calibration via a user interface, the CPU 811 causes the image forming unit 101 to form an image of the correction chart 200 on a sheet. A user or an engineer sets the sheet on which the image of the correction chart 200 is formed in the reading unit 100 and instructs the reading of the image. The reading unit 100 optically reads the image of the correction chart 200 and outputs the read image to the CPU 811. This read image is an input for generating correction data for correcting unevenness in the amount of light. During such a reading operation, a tilted read image may be obtained due to the skew of the sheet. In this embodiment, since the correction chart 200 has six strip-shaped areas 2101 to 2106, the influence of the skew of the sheet is more magnified compared to the case where only a single or a small number of strip-shaped areas exist. If the correction chart 200 has only one of the first reference mark 2111 and the second reference mark 2112, it is difficult to accurately determine the boundary between the light-emitting chips in the tilted read image. However, in this embodiment, the correction chart 200 has both the first reference mark 2111 and the second reference mark 2112. Therefore, the CPU 811 can accurately determine the boundaries between the light-emitting chips based on the positions of these reference marks, and appropriately extract each partial area from the read image.

[0049] When the reading resolution of the reading unit 100 differs from the resolution of the image formed by the exposure head 106, the CPU 811 extracts a partial area from the read image of the correction chart 200 while converting the pixel positions according to the ratio of the two resolutions. For example, when the reading resolution is one-fourth of the resolution of the image formed by the exposure head 106, the difference in resolution can be compensated for by multiplying the index of the pixel position in the read image by 4 (or by performing 4-fold upsampling).

[0050] <5-4. Derivation of light quantity distribution> The CPU 811 extracts partial areas formed by each light-emitting chip 400 based on the position of the reference mark from the read image of the correction chart 200, and measures the density of the image for each extracted partial area. FIG. 12 shows the average density values ​​measured for each of the partial areas of the band-shaped areas 2101 to 2106 formed by one light-emitting chip 400, plotted on a plane with the horizontal axis representing the gradation value and the vertical axis representing the density. For example, the point 221 represents the average density value measured for the gradation value g1 of the band-shaped area 2101, the point 222 represents the average density value measured for the gradation value g2 of the band-shaped area 2102, and the same is true for the points 223 to 226. The density value can be derived by substituting the three-dimensional signal value of the read image into a predetermined conversion formula, but the luminance value output from the image sensor may be used as the density value. 12 represents the grayscale-density characteristic of the light-emitting chip 400 estimated from the six average density values, and the gradient of the density may vary depending on the grayscale value. For example, assuming that the average density values ​​of the points 221 and 222 are d1 and d2, respectively, the conversion coefficient k for converting the density difference in the high grayscale range into the difference in grayscale value (i.e., the light amount difference) can be calculated according to the following formula: k = (d1-d2) / (g1-g2) Similarly, a conversion coefficient for converting the density difference in the mid-tone range to a difference in tone value can be calculated by dividing the difference in tone value between points 223 and 224 by the density difference. Also, a conversion coefficient for converting the density difference in the low-tone range to a difference in tone value can be calculated by dividing the difference in tone value between points 225 and 226 by the density difference.

[0051] Furthermore, by multiplying the density at each pixel position measured along the axial direction for the partial region of the band-shaped region 2101 by the conversion coefficient k calculated according to the above formula, it is possible to derive the light quantity distribution in the axial direction of the light-emitting chip 400 for the high gradation region. In this embodiment, the CPU 811 derives such light quantity distributions for the high gradation region, the intermediate gradation region, and the low gradation region. Then, the CPU 811 generates a light quantity correction value A and a light quantity correction value B, which will be described later, using the light quantity distribution in the intermediate gradation region. Moreover, the CPU 811 generates a light quantity correction value C, which will be described later, using the light quantity distribution in the high gradation region and the low gradation region.

[0052] In addition, instead of making the gradation values ​​of the image data of the correction chart different between pairs of band-shaped regions used to calculate the conversion coefficients described above, the difference in gradation of the images of the band-shaped regions formed on the sheet may be realized by changing the amount of current supplied to the light-emitting element 602.

[0053] <5-5. Generation of correction data> (1) Fine correction of light intensity unevenness (light intensity correction value A and light intensity correction value B) FIG. 13 shows an example of a light amount distribution in the intermediate gradation range that can be derived according to the method described in the previous section. The horizontal axis of FIG. 13 represents the position (pixel position) of a light-emitting element in the axial direction based on, for example, the leftmost light-emitting element in one light-emitting chip 400. The vertical axis represents the amount of light. The light amount distribution graph 230 is a graph that traces the amount of light (for example, the product of the measured concentration and the conversion coefficient) calculated for each position. The light amount T shown by the dashed line in the figure is the target light amount value of this light-emitting chip 400. For each pixel position, the ratio of the difference between the light amount value shown by the light amount distribution graph 230 and the target light amount value T represents a correction value for correcting the unevenness of the light amount.

[0054] FIG. 13 shows five points 231 to 235 on the light intensity distribution graph 230. These five points correspond to the representative coordinates of the five reference current sources 902-1 to 902-5 of one light-emitting chip 400, respectively. For example, the point 231 corresponds to the center position of the group of light-emitting elements 602 to which a current is supplied from the reference current source 902-1, and the point 232 corresponds to the center position of the group of light-emitting elements 602 to which a current is supplied from the reference current source 902-2. The CPU 811 generates the correction data by dividing it into a correction value for each group (light intensity correction value A) and a correction value for the residual component for each element in the group (light intensity correction value B). Generally, the variation in the light intensity of the light-emitting elements 602 to which a current is supplied from the same reference current source 902 is small, so by dividing and generating the correction value in this way, the data size of the entire correction data can be reduced. For example, if the correction resolution is 1%, the average correction value for a group is 15%, and the residual component of the correction value is a maximum of 3%, the correction value per group can be represented in 4 bits, and the correction value for each element can be represented in 2 bits per element.

[0055] The CPU 811 may update the target light amount value T so that the light amount correction values ​​A and B become minimum based on the light amount distribution graph 230 derived using the read image of the correction chart 200. For example, the CPU 811 can update the target light amount value T by additionally writing, into the storage unit 810, an offset value to be applied to the set value of the supply current amount determined at the product inspection stage.

[0056] (2) Removal of the effects of abnormal spots (spot correction value C) FIG. 14 is an explanatory diagram for roughly explaining the influence of an abnormal spot on the light amount distribution caused by the manufacturing variation of the exposure head 106. In FIG. 14, a solid light amount distribution graph 240 shows an example of a light amount distribution in the middle gradation region, a dashed light amount distribution graph 241 shows an example of a light amount distribution in the high gradation region, and a dashed light amount distribution graph 242 shows an example of a light amount distribution in the low gradation region. Here, it is assumed that an abnormal spot exists at pixel position z1. The light amount distribution graphs 240, 241, and 242 can be derived from the measurement results of the density of partial regions of the strip regions 2101, 2103, and 2105, respectively. When a spot becomes enlarged, the crushing of the spot (exposure dot) between adjacent spots causes an increase in density in the high gradation region, so that a local increase in density appears in the light amount distribution graph 241. On the other hand, in the low gradation region, the insufficient light amount of the enlarged spot causes a decrease in density, and a local decrease in density appears in the light amount distribution graph 242.

[0057] FIG. 15 shows the gradation-density characteristic curve 220 of a normal spot (same as that shown in FIG. 12) in a solid line, and the gradation-density characteristic curve 250 of an abnormal spot in a dashed line. The density corresponding to the gradation value g1 belonging to the high gradation range is higher on the characteristic curve 250 than on the characteristic curve 220, while the density corresponding to the gradation value g5 belonging to the low gradation range is lower on the characteristic curve 250 than on the characteristic curve 220. The difference in density for the gradation value g3 belonging to the intermediate gradation range between the characteristic curve 220 and the characteristic curve 250 is approximately zero. Within the range of gradation values, the gradation value at which the density difference of the characteristic curve 250 with respect to the characteristic curve 220 is maximum in the positive direction can be determined through experiments. The gradation value of the band-shaped region 2101 of the correction chart 200 can be determined in this way. The gradation value at which the density difference of the characteristic curve 250 with respect to the characteristic curve 220 is zero can also be determined through experiments. The gradation value of the strip region 2103 of the correction chart 200 can be determined in this manner. The gradation value at which the density difference of the characteristic curve 250 relative to the characteristic curve 220 is maximum in the negative direction can also be determined through experimentation. The gradation value of the strip region 2105 of the correction chart 200 can be determined in this manner. The gradation values ​​of the strip region 2102, the strip region 2104, and the strip region 2106 can be determined by subtracting a constant offset from the gradation values ​​of the strip region 2101, the strip region 2103, and the strip region 2105, respectively.

[0058] The density of the partial regions of the band-shaped regions 2103 and 2104 having gradations in the intermediate gradation range is hardly affected by the presence or absence of abnormal spots. Therefore, as described above, the CPU 811 measures the error in the amount of light for each light-emitting element or for each group of light-emitting elements using the light amount distribution obtained from the measurement results of the densities of the partial regions of the band-shaped regions 2103 and 2104, and generates the light amount correction values ​​A and B. On the other hand, the light amount distribution in the high gradation range obtained from the measurement results of the densities of the partial regions of the band-shaped regions 2101 and 2102, and the light amount distribution in the low gradation range obtained from the measurement results of the densities of the partial regions of the band-shaped regions 2105 and 2106 can be used to detect abnormal spots. For example, the CPU 811 detects abnormal spots by examining the maximum value in the light amount distribution in the high gradation range and the minimum value in the light amount distribution in the low gradation range. Then, the CPU 811 generates the position of the detected abnormal spot and the amount of change in the spot size at each position (the difference from a predetermined reference value, also called the spot shift amount) as the spot correction value C.

[0059] (3) Storing and updating correction data The initial values ​​of the correction data including the light intensity correction value A, the light intensity correction value B, and the spot correction value C described above may be generated in an inspection stage before the shipment of the product and stored in the storage unit 810 of each light-emitting chip 400. When the CPU 811 executes calibration after the shipment of the product, it updates the data stored in the storage unit 810 of each light-emitting chip 400 with the correction data generated as a result of the calibration.

[0060] It should be noted that the generation of correction data at the inspection stage does not necessarily have to be performed using the correction chart 200. For example, the size of the imaging spot of each of the light-emitting elements 602 may be measured by causing the light-emitting elements 602 to emit light discretely one by one and receiving the light with a CCD (Charged Coupled Device) camera arranged on the imaging plane.

[0061] <5-6. Details of the light intensity correction section> In this section, a detailed configuration example of the light amount correction unit 802 that corrects image data using the above-mentioned correction data (light amount correction value A, light amount correction value B, and spot correction value C) will be described. When the image forming apparatus 1 executes a job for image formation, the CPU 811 reads out the correction data from the storage unit 810 and provides it to the light amount correction unit 802. Together with this correction data, image data in a bitmap format generated by the image data generation unit 801 is input to the light amount correction unit 802. FIG. 16 shows an example of a detailed configuration of the light amount correction unit 802. Referring to FIG. 16, the light amount correction unit 802 includes a gradation determination unit 1105, a gradation correction unit 1106, a calculation unit 1107, and an image correction unit 1109.

[0062] The light intensity correction value A is a correction value for correcting the unevenness of the light intensity in units of groups of the light emitting elements 602 of each light emitting chip 400. The light intensity correction value A can correct the unevenness of the light intensity caused by the error of the amount of current supplied to the light emitting elements due to the individual difference of the reference current source 902. The light intensity correction value B is a correction value for correcting the residual component of the unevenness of the light intensity in units of the light emitting elements in the group. The spot correction value C is a correction value for correcting the unevenness of the light intensity caused by the abnormal spot. The spot correction value C can indicate the position of the detected abnormal spot and the spot shift amount at each position. As described above, the influence of the abnormal spot differs depending on the gradation value. Therefore, in this embodiment, it is determined whether to decrease or increase the light intensity depending on the gradation value in the image data. The absolute value of the increase or decrease amount is determined from the spot shift amount.

[0063] The gradation determination unit 1105 determines the gradation range of each pixel position based on the input image data. For example, the gradation determination unit 1105 focuses on a patch image including a pixel (a pixel of interest) at each pixel position and surrounding pixels, and determines whether the gradation of the pixel of interest belongs to a low gradation range, a middle gradation range, or a high gradation range based on the area gradation of the patch image (for example, an average of pixel values). The size of the patch image may be, for example, approximately the same as the dither matrix, or may be about 3×3 pixels. In order to avoid excessive correction, the gradation determination unit 1105 may determine the gradation range of each pixel position by receiving feedback of the image data after correction instead of the image data before correction. For example, if the area gradation of the patch image is greater than a first threshold, the gradation range of the pixel of interest is classified as a high gradation range. If the area gradation of the patch image is equal to or less than the first threshold and greater than a second threshold (first threshold>second threshold), the gradation range of the pixel of interest is classified as a middle gradation range. If the area gradation of the patch image is equal to or smaller than the second threshold, the gradation range of the pixel of interest is classified as a low gradation range. A gradation determination unit 1105 notifies a gradation correction unit 1106 of the determination result of the gradation range for each pixel position.

[0064] The gradation-by-gradation correction unit 1106 has a correction table that holds a reference light amount correction value that is determined in advance for each of the high gradation range, the middle gradation range, and the low gradation range. Generally, the reference light amount correction value of the high gradation range indicates a negative light amount correction value, and the sign being negative means that the light amount is decreased. The reference light amount correction value of the low gradation range indicates a positive light amount correction value, and the sign being positive means that the light amount is increased. The reference light amount correction value of the middle gradation range may be zero. The gradation-by-gradation correction unit 1106 acquires a reference light amount correction value corresponding to the gradation range notified by the gradation judgment unit 1105 for each pixel position of the abnormal spot indicated by the spot correction value C by referring to the correction table. Next, the gradation-by-gradation correction unit 1106 calculates a light amount correction value D based on the acquired reference light amount correction value and the spot shift amount indicated by the spot correction value C. For example, the light amount correction value D may be a product of the reference light amount correction value and a coefficient corresponding to the spot shift amount. To calculate the light amount correction value D, the gradation correction unit 1106 may have a coefficient table or a relational expression that defines the relationship between the spot shift amount and a coefficient. Then, the gradation correction unit 1106 outputs the calculated light amount correction value D to the image correction unit 1109. Note that for pixel positions where no abnormal spot is detected, the light amount correction value D may be zero.

[0065] The calculation unit 1107 calculates a light intensity correction value E for each pixel position based on the light intensity correction value A and the light intensity correction value B. The light intensity correction value E may be the sum of the light intensity correction value A determined for a group to which the light emitting element 602 corresponding to each pixel position belongs and the light intensity correction value B determined for the light emitting element 602. Typically, the light intensity correction value E may be zero or a negative value. The calculation unit 1107 outputs the calculated light intensity correction value E to the image correction unit 1109.

[0066] The image correction unit 1109 determines the rate of increase or decrease in the amount of light at each pixel position based on the light amount correction value D input from the gradation correction unit 1106 and the light amount correction value E input from the calculation unit 1107. The image correction unit 1109 also increases or decreases the area light amount at each pixel position by correcting the image data of the small image around each pixel position according to the method described in detail with reference to FIG. 10. Then, the image correction unit 1109 outputs the corrected image data to the chip data conversion unit 803.

[0067] In this embodiment, the pixel values ​​of the image data are changed before the data is output from the chip data conversion unit 803 to each light-emitting chip 400 to correct the unevenness in the amount of light. Therefore, the chip data conversion unit 803 can distribute the image data to each light-emitting chip 400 using a common control logic, regardless of the correction of the unevenness in the amount of light.

[0068] <6. Summary> Various embodiments of the technology according to the present disclosure have been described in detail above with reference to FIGS. 1 to 16. According to the above-described embodiments, the exposure head of the image forming apparatus includes a plurality of light-emitting chips arranged at different positions in a first direction parallel to the axial direction of the photoconductor, and each of the plurality of light-emitting chips includes an array of a plurality of light-emitting elements aligned at least in the first direction. The correction chart used for correcting the unevenness in the amount of light of the exposure head includes a plurality of areas aligned in a second direction perpendicular to the first direction, and a first reference mark and a second reference mark arranged on one side and the other side of the plurality of areas in the second direction, respectively. These first reference mark and second reference mark indicate the boundary between adjacent light-emitting chips among the plurality of light-emitting chips of the exposure head. Therefore, when generating correction data for correcting the unevenness in the amount of light based on the read image of the correction chart, the image forming apparatus can accurately determine the boundary between the light-emitting chips in the read image based on the positions of the first reference mark and the corresponding second reference mark. This makes it possible to capture with high accuracy at which position of the light-emitting element in each light-emitting chip the local unevenness in the amount of light occurs, thereby improving the performance of the correction of the unevenness in the amount of light. In the above-described embodiment, the boundary of the partial region can be determined by a simple geometric calculation based on a line segment connecting the representative position of the first fiducial mark and the representative position of the corresponding second fiducial mark, so that the boundary of the partial region can be accurately determined with low calculation cost without requiring a complex algorithm.

[0069] In the above-described embodiment, the first and second reference marks can be formed by emitting light from an effective light-emitting element at at least one end in the first direction of each light-emitting chip. With this configuration, the end points of the exposure range of each light-emitting chip (i.e., the boundary positions of the partial areas to be extracted from the read image) can be directly indicated by each reference mark.

[0070] In the above-mentioned embodiment, the plurality of regions may include at least a region for measuring the error of the light quantity of each light-emitting element or each group of light-emitting elements in each light-emitting chip, and a region for measuring the variation of the spot size on the light imaging surface. These regions may be arranged side by side in the circumferential direction of the photoconductor, that is, in the conveying direction of the sheet on which the image is formed. In this way, when many regions are arranged side by side in the conveying direction of the sheet in the correction chart, there is a risk that the influence of a slight skew of the sheet when reading the image of the correction chart will cause a relatively large shift in the boundary position. However, in the above-mentioned embodiment, since the reference marks are printed on both sides of the plurality of regions, even if the sheet is skewed, the boundary between the light-emitting chips in the read image of the correction chart can be accurately determined.

[0071] For example, two of the multiple regions have different gradations that belong to the middle gradation range, and the light amount distribution of each light-emitting chip in the first direction can be derived based on the measurement results of the densities of the partial regions of the two regions. Since the density of the middle gradation range is hardly affected by the presence or absence of an enlarged imaging spot, the light amount distribution of the middle gradation range can be used to measure the error in the light amount to generate correction data that enables fine correction of the unevenness in the amount of light. The other two of the multiple regions can have different gradations that belong to the high gradation range, and the other two regions can have different gradations that belong to the low gradation range. Then, the light amount distribution of the high gradation range and the light amount distribution of the low gradation range can be derived based on the measurement results of the densities of the partial regions of these regions. The light amount distribution of the high gradation range and the light amount distribution of the low gradation range in the first direction can be derived. The influence of the enlarged imaging spot is easily manifested in the light amount distribution of the high gradation range and the low gradation range. Therefore, by detecting an abnormal spot using these light amount distributions and measuring the fluctuation in the spot size, correction data that can also correct the unevenness in the amount of light caused by the abnormal spot can be generated.

[0072] In the above embodiment, specific numerical values ​​are used for explanation, but these specific numerical values ​​are merely examples, and the present invention is not limited to the specific numerical values ​​used in the embodiment. Specifically, the number of light-emitting chips provided on one printed circuit board is not limited to 20, and may be any number equal to or greater than 1. In addition, the size of the light-emitting element array of each light-emitting chip 400 is not limited to 4 rows and 748 columns, and may be any other size. In addition, the circumferential pitch and axial pitch of the light-emitting elements are not limited to about 21.16 μm and about 5 μm, and may be any other value.

[0073] <7. Other embodiments> The above-mentioned embodiment can also be realized in the form of a process in which a program for realizing one or more functions is supplied to a system or device via a network or a storage medium, and one or more processors in a computer of the system or device read and execute the program. It can also be realized by a circuit (e.g., ASIC) for realizing one or more functions.

[0074] The above embodiment includes at least the following image forming apparatus, correction chart, and method. (Item 1) an image forming unit including a photoconductor and an exposure head that exposes the photoconductor in accordance with image data; a generation unit that generates correction data for correcting unevenness in the amount of light from the exposure head, using a read image of the correction chart obtained by causing the image forming unit to form an image of the correction chart and optically reading the formed image; and a correction unit that corrects the image data to be input to the image forming unit based on the correction data generated by the generation unit; Equipped with the exposure head includes a plurality of light-emitting chips arranged at different positions in a first direction parallel to an axial direction of the photoconductor, each of the plurality of light-emitting chips including an array of a plurality of light-emitting elements aligned at least in the first direction; The correction chart is A plurality of regions arranged in a second direction perpendicular to the first direction and having different gradations; At least one first reference mark is arranged on one side of the plurality of regions in the second direction and indicates a boundary between adjacent light emitting chips among the plurality of light emitting chips; at least one second reference mark arranged on the other side of the plurality of regions in the second direction and indicating the boundary between adjacent light emitting chips among the plurality of light emitting chips; Including, The generation unit is determining partial areas formed by the plurality of light-emitting chips in the plurality of areas based on positions of the first reference mark and the corresponding second reference mark in the read image of the correction chart; generating the correction data based on the measurement results of the densities of the determined partial regions; Image forming device. (Item 2) 2. The image forming apparatus according to item 1, wherein the generation unit determines a boundary of the partial region based on a line segment connecting a representative position of the first reference mark and a corresponding representative position of the second reference mark. (Item 3) 3. The image forming apparatus according to item 2, wherein the first reference mark and the second reference mark are formed by light emission from an effective light-emitting element at at least one end in the first direction of each of the plurality of light-emitting chips. (Item 4) The plurality of regions include: At least one region for measuring an error in the amount of light of each light-emitting element or each group of light-emitting elements in each light-emitting chip; and at least one area for measuring a variation in spot size at an imaging plane of light from the exposure head; 4. The image forming apparatus according to any one of items 1 to 3. (Item 5) 5. The image forming device according to item 4, wherein the at least one region for measuring the error in the amount of light per light-emitting element or per group of light-emitting elements in each light-emitting chip includes two regions having different gradations belonging to a mid-tone range. (Item 6) The image forming apparatus described in item 5, wherein the generation unit measures the error in light intensity per light-emitting element or per group of light-emitting elements using a light intensity distribution in the mid-tone region in the first direction of each light-emitting chip, which is derived based on the measurement results of the densities of partial regions of the two regions in the mid-tone region. (Item 7) 7. The image forming apparatus according to any one of items 4 to 6, wherein the at least one area for measuring the variation in spot size includes two areas having different gradations belonging to a high gradation range, and two areas having different gradations belonging to a low gradation range. (Item 8) 8. The image forming apparatus according to item 7, wherein the generation unit measures the variation in spot size using a light intensity distribution in the high gradation region and the low gradation region in the first direction of each light-emitting chip, the light intensity distribution being derived based on the measurement results of the densities of partial regions of the two regions in the high gradation region and the two regions in the low gradation region. (Item 9) 9. The image forming apparatus according to any one of items 1 to 8, further comprising a reading unit that optically reads the image of the correction chart formed by the image forming unit and generates the read image of the correction chart. (Item 10) A correction chart used to correct unevenness in the amount of light of an exposure head of an image forming apparatus, The image forming apparatus includes: an image forming unit including a photoconductor and the exposure head that exposes the photoconductor in accordance with image data; a generation unit that generates correction data for correcting the unevenness in the amount of light by using a read image of the correction chart obtained by causing the image forming unit to form an image of the correction chart and optically reading the formed image; and a correction unit that corrects the image data to be input to the image forming unit based on the correction data generated by the generation unit; having the exposure head includes a plurality of light-emitting chips arranged at different positions in a first direction parallel to an axial direction of the photoconductor, each of the plurality of light-emitting chips including an array of a plurality of light-emitting elements aligned at least in the first direction; The correction chart is A plurality of regions arranged in a second direction perpendicular to the first direction and having different gradations; At least one first reference mark is arranged on one side of the plurality of regions in the second direction and indicates a boundary between adjacent light emitting chips among the plurality of light emitting chips; at least one second reference mark arranged on the other side of the plurality of regions in the second direction and indicating the boundary between adjacent light emitting chips among the plurality of light emitting chips; Including, correction chart. (Item 11) 1. A method for correcting unevenness in the amount of light of an exposure head in an image forming apparatus including an image forming unit including a photoconductor and an exposure head that exposes the photoconductor in accordance with image data, comprising: the exposure head includes a plurality of light-emitting chips arranged at different positions in a first direction parallel to an axial direction of the photoconductor, each of the plurality of light-emitting chips including an array of a plurality of light-emitting elements aligned at least in the first direction; The method comprises: causing the image forming unit to form an image of a correction chart; generating correction data for correcting unevenness in the amount of light of the exposure head using a read image of the correction chart obtained by optically reading the formed image; correcting the image data to be input to the image forming unit based on the generated correction data; Including, The correction chart is A plurality of regions arranged in a second direction perpendicular to the first direction and having different gradations; At least one first reference mark is arranged on one side of the plurality of regions in the second direction and indicates a boundary between adjacent light emitting chips among the plurality of light emitting chips; at least one second reference mark arranged on the other side of the plurality of regions in the second direction and indicating the boundary between adjacent light emitting chips among the plurality of light emitting chips; Including, Generating the correction data includes: determining partial areas formed by the plurality of light-emitting chips in the plurality of areas based on positions of the first reference mark and the corresponding second reference mark in the read image of the correction chart; generating the correction data based on a measurement result of the density of each determined partial region; A method comprising:

[0075] The invention is not limited to the above-described embodiments, and various modifications and variations are possible 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]

[0076] 1: image forming apparatus, 100: reading unit, 101 (101-a to 101-d): image forming unit, 102: photoconductor, 106: exposure head, 400 (400-1 to 400-20): light emitting chip, 602: light emitting element, 802: light quantity correction unit, 811: CPU (generation unit), D1: axial direction (first direction), D2: circumferential direction (second direction), R j_m : Light emitting element,

Claims

1. an image forming unit including a photoconductor and an exposure head that exposes the photoconductor in accordance with image data; a generation unit that generates correction data for correcting unevenness in the amount of light from the exposure head, using a read image of the correction chart obtained by causing the image forming unit to form an image of the correction chart and optically reading the formed image; and a correction unit that corrects the image data to be input to the image forming unit based on the correction data generated by the generation unit; Equipped with the exposure head includes a plurality of light-emitting chips arranged at different positions in a first direction parallel to an axial direction of the photoconductor, each of the plurality of light-emitting chips including an array of a plurality of light-emitting elements aligned at least in the first direction; The correction chart is A plurality of regions arranged in a second direction perpendicular to the first direction and having different gradations; at least one first reference mark arranged on one side of the plurality of regions in the second direction and indicating a boundary between adjacent light emitting chips among the plurality of light emitting chips; at least one second reference mark arranged on the other side of the plurality of regions in the second direction and indicating the boundary between adjacent light emitting chips among the plurality of light emitting chips; Including, The generation unit is determining partial areas formed by the plurality of light-emitting chips in the plurality of areas based on positions of the first reference mark and the corresponding second reference mark in the read image of the correction chart; generating the correction data based on the measurement results of the densities of the determined partial regions; The plurality of regions include: At least one area for measuring an error in the amount of light of each light-emitting element or each group of light-emitting elements in each light-emitting chip; and two regions having different gradations belonging to a high gradation range and two regions having different gradations belonging to a low gradation range for measuring a variation in spot size on an imaging plane of light from the exposure head; 13. An image forming apparatus comprising:

2. The image forming apparatus according to claim 1 , wherein the generation section determines the boundary of the partial region based on a line segment connecting the representative position of the first reference mark and the corresponding representative position of the second reference mark.

3. The image forming apparatus according to claim 2 , wherein the first reference mark and the second reference mark are formed by emitting light from an effective light emitting element at least at one end in the first direction of each of the plurality of light emitting chips.

4. 2. The image forming apparatus according to claim 1, wherein the at least one region for measuring the error in the amount of light per light-emitting element or per group of light-emitting elements in each light-emitting chip includes two regions having different gradations belonging to a mid-tone range.

5. The image forming apparatus of claim 4, wherein the generation unit measures the error in light intensity per light-emitting element or per group of light-emitting elements using a light intensity distribution in the mid-tone region in the first direction of each light-emitting chip, derived based on the measurement results of the densities of partial regions of the two regions in the mid-tone region.

6. 2. The image forming apparatus of claim 1, wherein the generation unit measures the variation in spot size using a light intensity distribution in the high gradation region and the low gradation region in the first direction of each light-emitting chip, the light intensity distribution being derived based on the measurement results of the densities of partial regions of the two regions in the high gradation region and the two regions in the low gradation region.

7. The image forming apparatus according to claim 1 , further comprising a reading section that optically reads the image of the correction chart formed by the image forming section, and generates the read image of the correction chart.

8. An image forming apparatus for forming a correction chart image, comprising: A photoconductor that rotates around a rotation axis; an exposure head having a plurality of light emitting units arranged along a first direction which is a direction of the rotation axis; a lens array that condenses light emitted from the plurality of light emitting units onto a surface of the photoconductor; a control unit that controls light emission by the plurality of light emitting units so that the correction chart image is formed; a reading unit that reads the formed correction chart image; Equipped with The correction chart image is a first region having a first gray level; a second region having a second gradation higher than the first gradation and disposed at a position different from that of the first region in a second direction perpendicular to the first direction; a third region having a third gradation higher than the second gradation and disposed at a position different from the first region and the second region in the second direction; Including, The image forming apparatus includes: a measurement unit that measures a variation in a spot size of light on the surface of the photoconductor based on a result of reading the first area and the third area, and measures an error in the amount of light emitted from the plurality of light-emitting units based on a result of reading the second area; The image forming apparatus further comprises:

9. The image forming device described in Claim 8, wherein the exposure head includes a plurality of light-emitting chips arranged along the first direction, each of the light-emitting chips having a plurality of light-emitting portions.

10. The correction chart image is At least one first reference mark is arranged on one side of a plurality of regions including the first region, the second region, and the third region in the second direction, and indicates a boundary between adjacent light-emitting chips in the first direction among the plurality of light-emitting chips; at least one second reference mark arranged on the other side of the plurality of regions in the second direction and indicating the boundary between adjacent light emitting chips in the first direction among the plurality of light emitting chips; The image forming apparatus of claim 9 .

11. An image forming device as described in Claim 10, wherein the first reference mark and the second reference mark are formed by emitting light from an effective light-emitting portion at at least one end in the first direction of each of the plurality of light-emitting chips.

Images