Exposure device and image formation apparatus

Abstract

To reduce local light amount unevenness while avoiding the image degradation.SOLUTION: An exposure device that exposes a photoreceptor according to image data, comprises: a light-emitting element array which is formed by multiple light-emitting chips arranged regularly in a first direction parallel to the axial direction of the photoreceptor; a light emission control unit which controls light emission of each light-emitting element in the light-emitting element array according to corresponding pixel values of the image data; a generation unit which generates correction data indicating a correction amount determined for each pixel position of the image data such that the light amount unevenness of the exposure device is suppressed; and a correction unit which corrects the pixel value of the image data according to the correction data generated by the generation unit. The generation unit determines the correction amount for each pixel position so that the correction amount does not exceed a predefined limit value.SELECTED DRAWING: Figure 16

Description

[Technical field]

[0001] The present disclosure relates to an exposure apparatus and an image forming apparatus. [Background technology]

[0002] As a type of electrophotographic image forming apparatus, a solid-state exposure type apparatus is generally known in which a photosensitive drum is exposed to 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 array 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 array on the surface of the photosensitive drum. Patent Document 1 discloses a correction method for correcting image unevenness (unevenness in light amount) caused by a difference in light amount between a plurality of light-emitting chips constituting the light-emitting element array in a solid-state exposure type image forming apparatus. In the correction method disclosed in Patent Document 1, a correction value of the light amount for each light-emitting chip is determined based on the result of reading a test chart, and each light-emitting chip is driven to emit light with the corrected light amount according to the determined correction value. [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, the unevenness in the amount of light does not necessarily occur on a light-emitting chip basis. For example, local errors in the rod lens array and differences in the amount of light for each light-emitting element in the light-emitting chip can also cause unevenness in the amount of light. Such local unevenness in the amount of light cannot be eliminated by the light-emitting chip-based correction method disclosed in Patent Document 1. In order to eliminate the local unevenness in the amount of light, it is conceivable to correct the pixel values ​​of the image data that determine whether each light-emitting element emits light or not, but excessive correction of the pixel values ​​may significantly change the shape of the dots formed by exposure, resulting in deterioration of the image.

[0005] In view of the above problems, an object of the present invention is to provide an improved mechanism capable of reducing localized unevenness in the amount of light while avoiding deterioration of an image. [Means for solving the problem]

[0006] According to one aspect, there is provided an exposure device that exposes a photosensitive member in accordance with image data, the exposure device comprising: a light-emitting element array formed by a plurality of light-emitting chips regularly arranged in a first direction parallel to the axial direction of the photosensitive member; a light-emitting control unit that controls the light emission of each light-emitting element of the light-emitting element array in accordance with a corresponding pixel value of the image data; a generation unit that generates correction data indicating a correction amount determined for each pixel position of the image data so that unevenness in the amount of light of the exposure device is suppressed; and a correction unit that corrects the pixel value of the image data in accordance with the correction data generated by the generation unit, wherein the generation unit determines the correction amount for each pixel position so that the correction amount does not exceed a predetermined limit value. Effect of the Invention

[0007] According to the present invention, it is possible to reduce local unevenness in the amount of light while avoiding deterioration of 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] FIG. 2 is an explanatory diagram illustrating an example of an arrangement of light-emitting elements in a 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] FIG. 4 is an explanatory diagram illustrating an example of the configuration of a correction chart used for measuring unevenness in light amount. [Figure 11] 11A and 11B are explanatory diagrams of a method for suppressing unevenness in the amount of light by locally changing the area gradation. [Figure 12] FIG. 4 is a block diagram showing an example of a detailed configuration of a light amount correction unit according to an embodiment. [Figure 13A] 6A and 6B are explanatory diagrams of current adjustment amounts and correction amounts in a first example of light amount distribution. [Figure 13B] FIG. 4 is an explanatory diagram showing how unevenness in the amount of light is suppressed in the first example of the light amount distribution. [Figure 14A] 13A and 13B are explanatory diagrams of the current adjustment amount and the correction amount in the second example of the light amount distribution. [Figure 14B] FIG. 11 is an explanatory diagram showing how unevenness in the amount of light is suppressed in a second example of the light amount distribution. [Figure 15] 11 is an explanatory diagram showing how unevenness in the amount of light is suppressed so that no significant difference in the amount of light occurs at chip boundaries; [Figure 16] 10 is a flowchart showing an example of the flow of a correction amount determination process according to an embodiment. [Figure 17]10 is a flowchart showing an example of the flow of a calibration process 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 pressurizing 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 facing the transfer belt 111 at the rear of the image forming unit 101a. 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. As described above, the exposure head 106 is an exposure device that exposes the photoconductor 102 according to image data. The exposure head 106 has a light emitting element array 201, a printed circuit board 202 on which the light emitting element array 201 is mounted, a rod lens array 203, and a housing 204 that supports the printed circuit board 202 and the rod lens array 203. The photoconductor 102 has a cylindrical shape. The exposure head 106 is disposed such that its longitudinal direction is parallel to the direction of the rotation axis of the photoconductor 102 (axial direction D1), and the surface on which the rod lens array 203 is attached faces the surface of the photoconductor 102. While the photoconductor 102 rotates in the circumferential direction D2 around the rotation axis, the light emitting element array 201 of the exposure head 106 emits light, and the rod lens array 203 forms an image of 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 array 201 is mounted (the surface opposite to the surface on which the connector 305 is mounted). In this embodiment, the light emitting element array 201 is formed by 20 light emitting chips 400-1 to 400-20 regularly arranged in the longitudinal direction. In the example of FIG. 3(B), the light emitting chips 400-1 to 400-20 are arranged in a staggered manner along the longitudinal direction of the printed circuit board 202 of the exposure head 106. Specifically, ten light emitting chips 400-n form one line when n is an odd number, and another ten light emitting chips 400-n form another line when n is an even number. 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] FIG. 4 is an explanatory diagram of an example of the arrangement of the light-emitting elements 602 in the light-emitting chip 400. In each light-emitting chip 400, the light-emitting elements 602 are two-dimensionally arranged to form 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 array consisting of 20 light-emitting chips 400, 14960 light-emitting elements are arranged in the axial direction D1. The pitch of 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 length of the entire light-emitting element array in the axial direction D1 is about 316 mm (the maximum width of the 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 corresponding to a resolution of 4800 dpi, and are 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 the axial direction D1 in this way, 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 may not be used regardless of the image data. Note that the number of rows or the number of light emitting elements overlapping in the axial direction D1 is not limited to the above example (one row, 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={1,2,3,4}) 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 in the axial direction of the light emitting element 602 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. Light Emission Control> <4-1. 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 acquisition unit 801 acquires image data from the reading unit 100 or an external device, and performs image processing on the acquired image data to generate binary bitmap image data for controlling the emission and non-emission of the light-emitting elements 602 of the light-emitting element array 201. 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 acquisition unit 801 outputs the generated image data to the light amount correction unit 802.

[0028] The light intensity correction unit 802 corrects pixel values ​​of the image data input from the image data acquisition unit 801 in accordance with correction data generated by a data generation unit 813 (described later) in order to suppress unevenness in the amount of light for each light emitting element of the exposure head 106. The light intensity correction unit 802 then outputs the corrected image data to the light emission control unit 803. The correction process performed by the light intensity correction unit 802 will be described in detail later.

[0029] The light emission control unit 803 controls the light emission of each light emitting element 602 of the light emitting element array 201 according to the corresponding pixel value of the corrected image data input from the light quantity correction unit 802. More specifically, the light emission control unit 803 reads out pixel values ​​within the corresponding readout range of the corrected image data in each line period identified by the line synchronization signal Lsync, and sends image data indicating the readout pixel values ​​to the 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 light emission control unit 803 specifies which light emitting chip 400 should receive each part of the image data DATA by the chip select signal CS. The light emission control 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, control data indicating the amount of current to be supplied to the light-emitting element 602 in each light-emitting chip 400, and correction data used for correcting image data by the light amount correction unit 802.

[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 light-emitting control 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 pixel values ​​included in the received image data.

[0033] <4-2. Supplying current to light-emitting elements> 9 is a block diagram showing a configuration of a circuit related to the supply of current in the light-emitting chip 400. A D / A 901, reference current sources 902-1 to 902-5, and a plurality of light-emitting elements 602 are disposed in each light-emitting chip 400. 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 position of the light-emitting element 602 in the axial direction.

[0034] The D / A 901 acquires control data for controlling the amount of current, which is generated by a data generating unit 813 described later, from the storage unit 810 of the printed circuit board 202. Then, the D / A 901 performs digital-to-analog conversion of the digital value of the amount of current to be supplied from each reference current source 902, and outputs an analog signal having a voltage corresponding to the amount of current to each reference current source 902. The amount of current here may be, for example, the sum of an adjustment amount determined by the data generating unit 813 described later and a reference amount determined in advance (common to a plurality of light-emitting chips 400). Each reference current source 902 supplies a current corresponding to the voltage of the analog signal input from the D / A 901 to the light-emitting element 602 connected to the reference current source 902. Note that the number of reference current sources 902 provided in each light-emitting chip 400 is not limited to the example shown in FIG. 9, and may be any number of one or more depending on the length of the wiring in the chip or the driving capability of the reference current source 902.

[0035] <4-3. Calibration> 8 again, 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 a 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 a request received via a user interface. Specifically, the CPU 811 can function as a measurement unit 812 and a data generation unit 813 in relation to the calibration.

[0036] (1) Measurement of light intensity unevenness The measurement unit 812 measures the unevenness in the amount of light of the exposure head 106 using a read image based on optical reading of the image of the correction chart formed on the sheet using the exposure head 106. For example, when a request for calibration is received, the measurement unit 812 causes the image forming unit 101 to form the image of the correction chart on the sheet. At this time, it is assumed that a uniform amount of current is supplied to all the light emitting elements 602 of the light emitting element array 201 of the exposure head 106, and the light amount correction unit 802 does not perform correction processing. When the sheet on which the image of the correction chart is formed is set in the reading unit 100 by a user or an engineer, the measurement unit 812 instructs the reading unit 100 to read. The reading unit 100 optically reads the image of the correction chart on the set sheet to generate a read image, and outputs the generated read image to the measurement unit 812. The measurement unit 812 measures the unevenness in the amount of light using the read image of the correction chart acquired from the reading unit 100 in this manner.

[0037] Fig. 10 shows an example of the configuration of a correction chart used to measure light quantity unevenness. Referring to Fig. 10, a correction chart 1000 includes a plurality of strip regions 1001-1006, each of which is a rectangular region with an axial direction D1 as a longitudinal direction. The strip regions 1001-1006 have different gradations and are arranged side by side in a circumferential direction D2 perpendicular to the axial direction D1. The gradations of the strip regions 1001 and 1002 belong to a high gradation range. The gradations of the strip regions 1003 and 1004 belong to a medium gradation range. The gradations of the strip regions 1005 and 1006 belong to a low gradation range.

[0038] The correction chart 1000 includes at least one first reference mark 1011-1 to 1011-19 arranged on one side (upper side in the figure) of the strip-shaped regions 1001 to 1006 in the circumferential direction D2. In addition, the correction chart 1000 includes at least one second reference mark 1012-1 to 1012-19 arranged on the other side (lower side in the figure) of the strip-shaped regions 1001 to 1006 in the circumferential direction D2. The first reference mark 1011-1 to 1011-19 and the second reference mark 1012-1 to 1012-19 may be formed by light emission from an effective light-emitting element at least at one end in the axial direction D1 of each light-emitting chip 400. For example, the first reference mark 1011-1 and the second reference mark 1012-1 may be formed by light emission from four effective light-emitting elements 602 at the right end of the light-emitting chip 400-1. The first reference mark 1011-2 and the second reference mark 1012-2 can be formed by the emission of the four effective light emitting elements 602 at the right end of the light emitting chip 400-2. As a result, a line segment connecting a pair of representative positions of the first reference mark 1011-p and the second reference mark 1012-p approximately coincides with the boundary between the chips. For example, the dashed lines 1013-1 and 1013-2 in the figure correspond to such a boundary.

[0039] The measurement unit 812 determines the representative position of each reference mark in the read image of the correction chart 1000, and recognizes where the boundary between the light-emitting chips 400 is located in each of the band-shaped regions 1001 to 1006 based on the line segment connecting the determined representative positions. The representative position here may be, for example, the center of gravity of each reference mark, any vertex, or the position of the end point in the axial direction D1, depending on the mark shape. Then, the measurement unit 812 measures the density distribution of each band-shaped region that represents the change in density for each light-emitting chip and each light-emitting element, using the recognized boundary position as a reference. The relationship between density and light amount is represented by a density-light amount characteristic that is experimentally obtained in advance. Therefore, the measurement unit 812 can convert the measured density distribution into a light amount distribution using a conversion formula from density to light amount that is defined in advance. The light amount distribution derived by the measurement unit 812 in this manner indicates the light amount for each light-emitting element (or for each corresponding pixel position), and the spatial variation thereof indicates the light amount unevenness.

[0040] (2) Adjustment of the current The data generating unit 813 generates the above-mentioned control data and correction data so that the unevenness in the light amount appearing in the light amount distribution of the exposure head 106 measured by the measuring unit 812 is suppressed. Specifically, first, the data generating unit 813 determines the adjustment amount of the current to be supplied by each of one or more current sources of each light-emitting chip 400. For example, the data generating unit 813 may determine the adjustment amount of the current to be supplied by each current source based on the difference between the light amount measured for a specific light-emitting element selected from the multiple light-emitting elements 602 connected to the current source and the target light amount. The specific light-emitting element here may be, for example, the light-emitting element with the lowest output light amount among the light-emitting elements 602 to which the current is supplied. Which light-emitting element 602 has the lowest output light amount (output performance) can be determined in advance by an optical measurement test at the manufacturing stage of the device. The data generating unit 813 stores the control data indicating the adjustment amount of the current (or the current amount after adjustment) determined for each light-emitting chip 400 in the storage unit 810 of the printed circuit board 202. In this manner, by adjusting the amount of current supplied from the reference current source 902 to the light emitting element 602 in each light emitting chip 400, unevenness in the amount of light between different light emitting chips 400 is roughly eliminated.

[0041] (3) Correction of input image data Furthermore, the data generating unit 813 determines a correction amount for each pixel position of the image data so as to suppress a residual component of the unevenness in the amount of light remaining after adjustment of the current supplied to each light-emitting chip 400, and generates correction data indicating the determined correction amount. The correction data generated by the data generating unit 813 is stored in the storage unit 810 of the printed circuit board 202, and is used by the light amount correcting unit 802 during subsequent image formation.

[0042] In this embodiment, the correction process executed by the light intensity correction unit 802 includes changing the area light intensity of an image region including each pixel of interest in the image data and surrounding pixels according to the correction amount indicated by the correction data. Therefore, the correction data generated by the data generation unit 813 indicates the correction amount determined from the viewpoint of the rate of change of the area light intensity for each pixel.

[0043] A method of suppressing unevenness in light quantity by locally changing the area light quantity will be described below with reference to FIG. 11. FIG. 11(A) shows, as an example, a small image IM1 around a certain reference pixel position represented by image data before correction. One 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.

[0044] FIG. 11B 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. 11B, 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.

[0045] 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. 11C shows an example of the result of correcting the small image IM1 using the correction matrix IM2. In the small image IM3 in FIG. 11C, 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. As a result, the local area light amount of the small image IM3 is reduced compared to the small image IM1. In the case of increasing the light amount at the reference pixel position, 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.

[0046] 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.

[0047] The data generating unit 813 calculates a residual component of the unevenness in the amount of light measured for each pixel position that remains after adjusting the supply current, and determines a correction amount corresponding to the calculated residual component (for example, capable of canceling or at least suppressing the residual component).The data generating unit 813 then generates correction data indicating the correction amount (change rate of area light amount) determined for each pixel position, and stores the generated correction data in the storage unit 810.

[0048] In one embodiment, the correction data may be a simple list of correction amounts indicating the correction amount for each pixel position in the main scanning direction. In another embodiment, the correction data may have, for example, two types of data items as follows: Light quantity correction value A: A correction value for each group of light emitting elements to which current is supplied from the same reference current source 902. Light intensity correction value B: Indicates the correction value for each light-emitting element. Generally, the variation in the light intensity of the light-emitting element 602 supplied with current from the same reference current source 902 is small, so by describing the correction amount by dividing it into two types of data items with different granularity in this way, the data size of the entire correction data can be reduced.

[0049] During image formation, the light quantity correction unit 802 repeatedly changes the pixel values ​​of the small images around each reference pixel position according to the correction amount (e.g., light quantity correction value A+B) indicated by the correction data while scanning the reference pixel position along the main scanning direction in the input image data. This suppresses the residual component of the light quantity unevenness (for each pixel or for each light emitting element) that remains after adjusting the supply current.

[0050] In another embodiment, the correction data may further include an additional data item called a spot correction value C. For example, when the spot size on the imaging surface of the light from the exposure head 106 becomes enlarged due to manufacturing variations, the density of the printed image for 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 for the same gradation value becomes higher in the high gradation range due to crushing between adjacent spots. In the following description, the imaged spot with an enlarged size is called an abnormal spot. The density of the intermediate gradation range is hardly affected by the abnormal spot. Therefore, the data generating unit 813 can detect the abnormal spot from the density distribution of each gradation range in the read image of the correction chart 1000 described using FIG. 10, and determine the position and spot size of each abnormal spot. The spot correction value C can indicate the position and spot shift amount (difference between the reference value and the spot size) of each abnormal spot detected in this way.

[0051] 12 shows an example of a detailed configuration of the light amount correction unit 802 when the correction data includes a light amount correction value A, a light amount correction value B, and a spot correction value C. Referring to FIG. 12, 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.

[0052] 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 areal gradation of the patch image (for example, an average of pixel values). For example, if the areal 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 areal 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 areal gradation of the patch image is equal to or less than the second threshold, the gradation range of the pixel of interest is classified as a low gradation range. The gradation determination unit 1105 notifies the gradation correction unit 1106 of the determination result of the gradation range for each pixel position.

[0053] 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.

[0054] 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.

[0055] 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. Furthermore, the image correction unit 1109 increases or decreases the area light amount at each pixel position by changing the pixel values ​​of small images around each pixel position, according to the method described in detail using FIG. 11. Then, the image correction unit 1109 outputs the corrected image data to the light emission control unit 803.

[0056] In this embodiment, the residual error of the unevenness in the amount of light for each light-emitting element is suppressed by changing the pixel value of the image data before the data is output from the light-emitting control unit 803 to each light-emitting chip 400. Therefore, the light-emitting control unit 803 can distribute the image data to each light-emitting chip 400 using a common control logic, without being affected by the unevenness in the amount of light.

[0057] <4-4. Limits on correction amount> The method of locally changing the area gradation described with reference to FIG. 11 allows for precise adjustment of the amount of light of the exposure of the photoconductor 102. However, excessive correction of pixel values ​​of image data may result in a large change in the shape of dots formed on the photoconductor 102, resulting in image degradation. In this embodiment, the data generating unit 813 determines the amount of correction for each pixel position so that the amount of correction does not exceed a predetermined limit value. The limit value here may be expressed in terms of the rate of change of the area light amount in the correction process by the light amount correcting unit 802, similar to the amount of correction. The limit value may be set to any value taking into consideration the trade-off between suppressing unevenness in the amount of light and preserving the dot shape. As an example, the limit value may be ±30%. Different limit values ​​may be set for the reduction and increase of the area light amount.

[0058] Specifically, the data generating unit 813 determines whether or not a first correction amount corresponding to a residual component of the light amount unevenness remaining after the current adjustment for each pixel position exceeds a preset limit value. The first correction amount is typically a value that can cancel the residual component. The data generating unit 813 determines the correction amount for a pixel position where the first correction amount does not exceed the limit value as the first correction amount. On the other hand, the data generating unit 813 determines the correction amount for a pixel position where the first correction amount exceeds the limit value as a second correction amount that is equal to or less than the limit value. For example, the second correction amount may be equal to the limit value.

[0059] 13A and 13B are explanatory diagrams of the current adjustment amount and correction amount in a first example of the light amount distribution. A graph 1201 in FIG. 13A partially illustrates an example of the light amount distribution that is a measurement result of the unevenness in the amount of light. The horizontal axis of the graph represents the pixel position in the main scanning direction, and the vertical axis represents the measured light amount. T is the target light amount. Section B1 on the horizontal axis corresponds to the light emitting chip 400-1 located at one end in the longitudinal direction of the light emitting element array. Section B2 corresponds to the light emitting chip 400-2 adjacent to the light emitting chip 400-1 in the longitudinal direction of the light emitting element array. Section B3 corresponds to the light emitting chip 400-3 adjacent to the light emitting chip 400-2 in the longitudinal direction of the light emitting element array. The graphs for sections corresponding to the light emitting chips 400-4 to 400-20 are omitted.

[0060] For example, the data generation unit 813 determines the adjustment amount of the current to be supplied to the light-emitting chip 400-1 as ΔI1 based on the leftmost light-emitting element having the lowest light intensity in the section B1. Similarly, the data generation unit 813 determines the adjustment amounts of the current to be supplied to the light-emitting chips 400-2 and 400-3 as ΔI2 and ΔI3, respectively.

[0061] Also, for example, the data generating unit 813 may generate a pixel position P 10 The correction amount d to cancel the residual component of the light amount unevenness 10 is the preset limit value d TH Since the pixel position P 10 The correction amount is d10 On the other hand, the data generator 813 determines the pixel position P 20 The correction amount d to cancel the residual component of the light amount unevenness 20 is the limit value d TH Since the pixel position P 20 The correction amount is d 20 instead of d TH The data generator 813 determines the correction amounts for the other pixel positions in a similar manner.

[0062] 13B shows a light amount distribution in which the unevenness of the light amount is suppressed as a result of light emission control being performed according to the adjustment amount of the current for each light-emitting chip 400 and the correction amount for each pixel position. 10 At many pixel positions, including the target light amount I T , and a substantially uniform light distribution is achieved. 20 Light intensity in I 20 As a result of prioritizing the preservation of dot shape over the suppression of light intensity unevenness, the target light intensity I T As a result, image degradation caused by deformation of the dot shape is also prevented.

[0063] 14A and 14B are explanatory diagrams of the current adjustment amount and correction amount in the second example of the light amount distribution. The graph 1211 in FIG. 14A partially illustrates the light amount distribution as an example of the measurement result of the unevenness in the amount of light. As in the example of FIG. 13A, the data generating unit 813 determines the adjustment amounts of the current to be supplied to the light emitting chips 400-1, 400-2, and 400-3 as ΔI1, ΔI2, and ΔI3, respectively (these values ​​are different from the first example).

[0064] Furthermore, the data generator 813 calculates the pixel position P 11 The correction amount d to cancel the residual component of the light amount unevenness 11 is the limit value d TH Since the pixel position P 11 The correction amount is d 11 instead of dTH The pixel position P 11 is located at the right end of the section B1, that is, corresponds to the effective light-emitting element at the right end of the light-emitting chip 400-1 (the light-emitting element adjacent to the light-emitting chip 400-2). 21 Since the residual component of the light intensity unevenness is equal to zero, the pixel position P 21 The correction amount of the pixel position P is determined to be zero. 21 is located at the left end of the section B2, that is, corresponds to the leftmost effective light-emitting element of the light-emitting chip 400-2 (the light-emitting element adjacent to the light-emitting chip 400-1).

[0065] 14B shows a light amount distribution in which the unevenness in the amount of light is suppressed as a result of light emission control being performed in accordance with the adjustment amount of the current for each light-emitting chip 400 and the correction amount for each pixel position. In the graph 1212 as well, the amount of light at many pixel positions is equal to or smaller than the target light amount I T However, at pixel position P 11 In a part of section B1 including the target light amount I T Exceeds.

[0066] 14B, although the unevenness in the amount of light is suppressed overall, a step in the amount of light occurs at the chip boundary between the light-emitting chip 400-1 and the light-emitting chip 400-2. Such a step in the amount of light appears in the printed image as a noticeable density variation that is easily visually recognized by the user.

[0067] Therefore, in one modified example, the data generating unit 813 determines the adjustment amount of the current to be supplied by the current source of the light emitting chip 400-k based on the correction amount determined for the pixel position corresponding to the right end light emitting element of the light emitting chip 400-(k-1). Note that, here, the upstream side in the main scanning direction is the left side. Therefore, the light emitting chip 400-(k-1) is the light emitting chip adjacent to the left side of the light emitting chip 400-k. Hereinafter, the effective light emitting element at the right end of the light emitting chip 400-(k-1) that contacts the boundary between the light emitting chip 400-(k-1) and the light emitting chip 400-k is called the first light emitting element, and the effective light emitting element at the left end of the light emitting chip 400-k that contacts the same boundary is called the second light emitting element. In addition, the pixel position corresponding to the first light emitting element is called the first pixel position, and the pixel position corresponding to the second light emitting element is called the second pixel position. In the example of FIG. 14A and FIG. 14B, the pixel position P 11 is the first pixel position, and pixel position P 21 is the second pixel position. When the data generating unit 813 determines the correction amount for the first pixel position to be the second correction amount that does not substantially cancel out the residual component of the unevenness in the amount of light, it determines the adjustment amount of the current in the light-emitting chip 400-k so that a significant difference in the amount of light is not generated between the first light-emitting element and the second light-emitting element. For example, in the example of FIG. 14A, the adjustment amount of the current to be supplied by the current source of the light-emitting chip 400-2 is determined by subtracting ΔI2 from the gap d 11 -d TH As a result, the second pixel position P 21 The light intensity at is the target light intensity I T Instead, the amount of light I 11 When the light-emitting element array 201 is composed of three or more light-emitting chips 400, the data generating unit 813 first determines the adjustment amount of the current to be supplied by the current source of the light-emitting chip 400-1 without depending on the correction amount determined for the light-emitting elements of the other light-emitting chips 400. Then, the data generating unit 813 determines the adjustment amount of the current to be supplied by each current source of the light-emitting chips 400 after the light-emitting chip 400-2 in order from the upstream to the downstream in the main scanning direction, depending on the correction amount determined for the light-emitting elements of the other (upstream) light-emitting chips 400.

[0068] 15 is related to the second example of the light amount distribution, and shows how the light amount unevenness is suppressed so that no significant light amount difference occurs at the chip boundary in the above-mentioned modified example. 11 and the second pixel position P 12 The step in the amount of light that occurred at the chip boundary between is eliminated in graph 1213. In graph 1213, the amount of light (or image density) is not completely uniform, but only shows gentle fluctuations. Therefore, the possibility of the user perceiving degradation in the image is low.

[0069] <5. Processing flow> 16 and 17, examples of the flow of processes that can be executed by the image forming apparatus 1 will be described. In the following description, a processing step will be abbreviated as 'S'.

[0070] <5-1. Correction amount determination process> 16 is a flowchart showing an example of a flow of a correction amount determination process according to an embodiment. The correction amount determination process in FIG. 16 is executed when the data generating unit 813 generates correction data for light amount correction after determining the adjustment amount of the current of each light-emitting chip 400.

[0071] First, in S100, the data generating unit 813 focuses on one pixel position and calculates the residual component of the light intensity unevenness at the focused pixel position by subtracting an offset based on the current adjustment from the light intensity indicated by the measurement result of the light intensity unevenness. Next, in S102, the data generating unit 813 calculates a first correction amount d1 (for example, a rate of change in area gradation to cancel the residual component) corresponding to the calculated residual component.

[0072] Next, in S104, the data generating unit 813 calculates the first correction amount d1 in accordance with a preset limit value d TH It is determined whether the first correction amount d1 exceeds the limit value d TH If it does not exceed (d1≦d TH), in S106, the data generating unit 813 determines the correction amount for the pixel of interest to be the first correction amount d1. On the other hand, when the first correction amount d1 is smaller than the limit value d TH If d1>d TH ), and in S108, the data generating unit 813 sets the correction amount of the pixel of interest position as a second correction amount d2 (for example, d2=d TH ) will be decided.

[0073] Next, in S110, the data generating unit 813 judges whether the correction amount has been determined for all pixel positions. If there are still pixel positions for which the correction amount has not been determined, the above-mentioned S100 to S108 are repeated with attention focused on the new pixel positions. When the correction amount has been determined for all pixel positions, the correction amount determination process of FIG. 16 ends.

[0074] <5-2. Calibration process> Fig. 17 is a flowchart showing an example of the flow of a calibration process according to an embodiment. The calibration process in Fig. 17 can be executed, for example, when a sheet on which an image of a correction chart for calibration is formed is set in the reading unit 100 by a user or an engineer.

[0075] First, in S200, the measurement unit 812 acquires the light quantity distribution of the exposure head 106. For example, the measurement unit 812 acquires a read image of the correction chart 1000 based on the reading of a sheet by the reading unit 100, detects band-like regions 1001 to 1006 in the read image, and measures the light quantity distribution. The measurement unit 812 outputs the acquired light quantity distribution to the data generation unit 813.

[0076] Next, in S202, the data generating unit 813 determines an adjustment amount of a current to be supplied to K (e.g., K=20) light-emitting chips 400 so as to suppress unevenness in the amount of light per light-emitting chip that appears in the light amount distribution acquired by the measuring unit 812. The data generating unit 813 generates control data indicating the adjustment amount (or the amount of current after adjustment) determined for each light-emitting chip 400, and stores the generated control data in the storage unit 810.

[0077] 16, thereby determining the correction amount for each pixel position of the K light-emitting chips 400. The data generation unit 813 generates correction data indicating the correction amount for each pixel position, and stores the generated correction data in the storage unit 810.

[0078] The subsequent process is repeated for each of the second to K-th light-emitting chips 400-k (k=2, . . . , K) (S206). In S208, the data generating unit 813 determines whether the first correction amount d1 is greater than or equal to the limit value d at the pixel position corresponding to the rightmost light-emitting element of the k-1-th light-emitting chip 400-(k-1). TH It is determined whether the first correction amount d1 exceeds the limit value d TH If the kth light-emitting chip 400-K is exceeded, in S210, the data generating unit 813 offsets the adjustment amount of the current of the kth and subsequent light-emitting chips so that a significant difference in light amount does not occur at the chip boundary adjacent to the pixel position, and updates the control data stored in the storage unit 810 in S202. When the repetition up to the Kth light-emitting chip 400-K is completed, the calibration process in FIG. 17 ends.

[0079] <6. Summary> Various embodiments, examples, and modifications of the technology according to the present disclosure have been described in detail above with reference to Figs. 1 to 17. According to the above-described embodiment, in an exposure device that exposes a photoconductor, when image data for controlling the emission of light-emitting elements of a light-emitting element array is corrected so as to suppress unevenness in the amount of light, the amount of correction for each pixel position is determined so as not to exceed a limit value. This makes it possible to reduce local unevenness in the amount of light while avoiding image degradation caused by significant deformation of the shape of dots formed as a result of exposure.

[0080] In the above-described embodiment, the adjustment amount of the current to be supplied from the current source disposed in each light-emitting chip to the light-emitting element is determined so as to suppress the unevenness in the amount of light per light-emitting chip, and the correction amount for each pixel position is determined so as to suppress the residual component of the unevenness in the amount of light remaining after the adjustment. According to this configuration, the proportion of pixel positions requiring a correction amount exceeding a limit value to cancel out the unevenness in the amount of light is reduced, so that even if the unevenness in the amount of light cannot be completely canceled out at those pixel positions, a sufficiently good light amount distribution can be achieved.

[0081] In the above-described embodiment, the adjustment amount of the current to be supplied by each current source is determined based on the difference between the light amount measured in advance for a specific light-emitting element selected from the multiple light-emitting elements connected to the current source (for example, to compensate for the difference). In general, the variation in the light amount between multiple light-emitting elements supplied with current from the same current source is small. Therefore, by using a method of adjusting the light amount of a specific light-emitting element to the target light amount while correcting image data for pixel positions corresponding to other light-emitting elements supplied with current from the same current source, each correction amount can be suppressed to a relatively small value.

[0082] In the above-described embodiment, the adjustment amount of the current of the second light-emitting chip adjacent to the first light-emitting chip is determined based not only on the difference between the light amount of a specific light-emitting element and the target light amount, but also on the correction amount determined for the pixel position at the boundary of the first light-emitting chip. Therefore, it is possible to prevent the occurrence of a light amount difference that causes a significant density fluctuation in the printed image at the boundary between the first light-emitting chip and the second light-emitting chip. In addition, when the light-emitting element array is composed of three or more light-emitting chips, the adjustment amount of the current and the correction amount for each pixel position are determined in order starting from the first light-emitting chip at one end, thereby realizing a properly calibrated light amount distribution of the exposure device.

[0083] In the above-described embodiment, the correction of the image data includes, for each pixel of interest, changing the area light quantity of an image region including the pixel of interest and its surrounding pixels according to the correction amount determined for the pixel of interest. The rate of change in the area light quantity can be treated as a quantitative index that correlates with the degree of deformation of the dot shape. Therefore, by setting the limit value in terms of the rate of change in the area light quantity, it is possible to effectively restrict the correction amount determined for each pixel position so that the dot shape is not excessively deformed.

[0084] In the above-described embodiment, an example has been described in which the correction amount for each pixel position of the image data is determined so that the residual component of the unevenness in the amount of light remaining after the adjustment of the current supplied from the reference current source is suppressed, but the technology according to the present disclosure is not limited to such an example. For example, the data generating unit 813 may determine the correction amount for each pixel position (within a range not exceeding a limit value) so that the unevenness in the amount of light itself appearing in the measured light amount distribution is suppressed, instead of the residual component of the unevenness in the amount of light.

[0085] <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.

[0086] 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]

[0087] 1: image forming apparatus, 100: reading unit, 101 (101-a to 101-d): image forming unit, 102: photoconductor, 106: exposure head (exposure device), 201: light emitting element array, 400 (400-1 to 400-20): light emitting chip, 602: light emitting element, 802: light quantity correction unit, 803: light emission control unit, 812: measurement unit, 813: data generation unit, 902 (902-1 to 902-5): reference current source, D1: axial direction (first direction), D2: circumferential direction (second direction), R j_m : Light emitting element, d1: First correction amount, d2: Second correction amount, d TH :Limit value

Claims

1. An exposure device that exposes a photoconductor according to image data, a light emitting element array formed by a plurality of light emitting chips regularly arranged in a first direction parallel to an axial direction of the photoconductor; a light emission control unit that controls light emission of each light emitting element of the light emitting element array in accordance with a corresponding pixel value of the image data; a generation unit that generates correction data indicating a correction amount determined for each pixel position of the image data so as to suppress unevenness in the amount of light of the exposure device; a correction unit that corrects pixel values ​​of the image data in accordance with the correction data generated by the generation unit; Equipped with the generation unit determines the correction amount for each pixel position such that the correction amount does not exceed a predetermined limit value. Exposure equipment.

2. Each of the plurality of light emitting chips includes one or more current sources for supplying current to a light emitting element of the light emitting chip; The generation unit is determining an adjustment amount of the current to be supplied by each of the one or more current sources so that the unevenness in the amount of light is suppressed; determining the correction amount for each pixel position so that a residual component of the light amount unevenness remaining after the adjustment of the current is suppressed; 2. The exposure apparatus according to claim 1.

3. The generation unit is determining a correction amount for a pixel position where a first correction amount corresponding to the residual component does not exceed the limit value as the first correction amount; determining a second correction amount equal to or less than the limit value as the correction amount for a pixel position where the first correction amount corresponding to the residual component exceeds the limit value; The exposure apparatus according to claim 2 .

4. 3. The exposure apparatus according to claim 2, wherein the generation unit determines the adjustment amount of the current to be supplied by each current source based on a difference between a light amount measured in advance for a specific light-emitting element selected from a plurality of light-emitting elements connected to the current source and a target light amount.

5. 5. The exposure apparatus according to claim 4, wherein the specific light-emitting element is a light-emitting element having the lowest light output amount among the plurality of light-emitting elements.

6. The plurality of light emitting chips include at least a first light emitting chip and a second light emitting chip adjacent to the first light emitting chip; The first light emitting chip has a first light emitting element adjacent to the second light emitting chip, The generation unit determines the adjustment amount of the current to be supplied by the current source of the second light-emitting chip based on a correction amount determined for a pixel position corresponding to the first light-emitting element.

4. The exposure apparatus according to claim 3.

7. The exposure apparatus of claim 6, wherein the generation unit determines the adjustment amount of the current to be supplied by the current source of the second light-emitting chip so that, when the correction amount determined for a pixel position corresponding to the first light-emitting element is the second correction amount, no significant difference in light intensity occurs between the first light-emitting element and a second light-emitting element of the second light-emitting chip adjacent to the first light-emitting element.

8. The plurality of light emitting chips includes three or more light emitting chips, the first light emitting chip is located at one end of the light emitting element array in the first direction; The generation unit is determining the adjustment amount of the current to be supplied by the current source of the first light-emitting chip without depending on a correction amount determined for a light-emitting element of another light-emitting chip; determining the adjustment amount of the current to be supplied by each current source of the light-emitting chips other than the first light-emitting chip depending on a correction amount determined for a light-emitting element of the other light-emitting chips; 7. The exposure apparatus according to claim 6.

9. 2. The exposure apparatus according to claim 1, wherein the correction of the pixel values ​​of the image data by the correction unit includes changing, for each target pixel of the image data, the area light amount of an image region including the target pixel and surrounding pixels in accordance with the correction amount indicated by the correction data.

10. The exposure apparatus according to claim 9 , wherein the limit value is set in advance in terms of a rate of change of the area light quantity.

11. The photoreceptor; An exposure apparatus according to any one of claims 1 to 10, An image forming apparatus comprising:

Images