Image forming device

Abstract

To promote effective use of resources in relation to a rod lens array.SOLUTION: An exposure head has a plurality of light emitting devices that are arranged in an intersection direction intersecting the direction of rotation of a photoreceptor, and a rod lens array that forms rays of light output from the plurality of light emitting devices into an image on the photoreceptor based on image data, and the exposure head forms a latent image on the photoreceptor. Storage means stores correction data for correcting a streak image. Correction means corrects the image data based on the correction data read out from the storage means.SELECTED DRAWING: Figure 14

Description

[Technical Field]

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

[0002] Electrophotographic image forming devices are known that expose a photosensitive drum to light using an exposure head equipped with light-emitting diodes (LEDs) and organic electroluminescence (EL) elements to form a latent image. Each of the numerous light-emitting elements is equipped with a rod lens, and the light emitted from the light-emitting elements is focused by the rod lens to form a spot of a predetermined size on the photosensitive drum. The numerous rod lenses are arranged in a row to form a rod lens array. However, if any of the rod lenses in the rod lens array are positioned at an angle, uneven light intensity occurs. As a result, white or black streaks appear in the image formed on the sheet. Patent Document 1 describes a method for determining the quality of rod lenses based on the intensity distribution of light transmitted through the rod lens array. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-275692 Summary of the Invention [Problem to be solved by the invention]

[0004] In the past, rod lens arrays that were not of good quality were discarded. As a result, the yield of rod lens arrays decreased and the cost of image forming devices equipped with rod lens arrays increased. Therefore, an object of the present invention is to prevent the cost of image forming devices from increasing. [Means for solving the problem]

[0005] The present invention is, for example, an exposure head that includes a plurality of light-emitting elements arranged in a direction intersecting the rotation direction of a photosensitive member that is driven to rotate, and a rod lens array that forms an image on the photosensitive member using light output from each of the plurality of light-emitting elements based on image data; and a storage means for storing correction data for correcting streak images; a correction means for correcting the image data based on the correction data read from the storage means; With death, the exposure head includes a plurality of light-emitting chips regularly arranged in a first direction parallel to an axial direction of the photosensitive member; Each of the plurality of light emitting chips includes the plurality of light emitting elements regularly arranged at least in the first direction, the correction data includes frequency component data associated with one or more frequency components of the light intensity distribution measured along the first direction, the frequency component data being obtained by analyzing the light intensity distribution measured along the first direction; The correction means corrects the image data in accordance with a fluctuation in the amount of light in the light amount distribution, which is restored based on the frequency component data included in the correction data. The present invention provides an image forming apparatus. [Effects of the Invention]

[0006] According to the present invention, it is possible to prevent the cost of the image forming apparatus from increasing. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a diagram illustrating an image forming apparatus. [Figure 2] FIG. 2 is a diagram illustrating the arrangement of a photosensitive drum and an exposure head. [Figure 3] FIG. [Figure 4] FIG. 2 is a diagram illustrating a light-emitting element array. [Figure 5] FIG. 10 is a diagram illustrating an arrangement of light-emitting elements. [Figure 6] FIG. 1 is a block diagram showing an image controller and a printed circuit board. [Figure 7] Block diagram showing the digital section. [Figure 8] FIG. [Figure 9] FIG. 3 is a diagram illustrating a lighting control unit. [Figure 10] FIG. [Figure 11] FIG. [Figure 12] FIG. 10 is a diagram illustrating a streaky image. [Figure 13] FIG. [Figure 14] FIG. 3 is a block diagram of a streak correction unit. [Figure 15] FIG. [Figure 16] 6 is a timing chart showing the operation of the streak correction unit. [Figure 17] 10A and 10B are diagrams illustrating correction values ​​for combinations of streak rank and image data. [Figure 18] FIG. 10 is a diagram illustrating another example of a correction method. [Figure 19] 1A and 1B are explanatory diagrams of a light-emitting element array and a light-emitting element group in the light-emitting element array according to an embodiment. [Figure 20] FIG. 1 is a cross-sectional view showing a schematic configuration of a light-emitting element array according to an embodiment. [Figure 21] FIG. 10 is an explanatory diagram of multiple exposure using light-emitting elements arranged in a stepped pattern. [Figure 22] FIG. 2 is a diagram illustrating the configuration of a control circuit for controlling light emission of a light-emitting element array on a printed circuit board. [Figure 23] FIG. 2 is a block diagram showing a configuration of a circuit related to supplying current to a light-emitting element array. [Figure 24] FIG. 2 is an explanatory diagram illustrating the arrangement of a plurality of lenses in a rod lens array. [Figure 25] 1 is a diagram illustrating the periodic nature of light passing through a rod lens array. [Figure 26] FIG. 10 is a schematic cross-sectional view showing an example of the inclination of lenses in a rod lens array. [Figure 27] FIG. 10 is a schematic plan view showing an example of misalignment of a light-emitting element array on a printed circuit board. [Figure 28] An explanatory diagram of the components of uneven light intensity that are contributed by three different causes. [Figure 29] An explanatory diagram of the complex unevenness in light intensity caused by three different factors. [Figure 30] 10A and 10B are explanatory diagrams illustrating a method for correcting uneven light quantity by changing local area gradation. [Figure 31] 10 is a flowchart showing an example of a procedure for manufacturing an exposure apparatus according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0008] Hereinafter, embodiments will be described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the scope of the invention claimed. Although multiple features are described in the embodiments, not all of these multiple features are necessarily essential to the invention, and multiple features may be combined arbitrarily. Furthermore, in the accompanying drawings, the same reference numerals are used to designate the same or similar components, and redundant explanations will be omitted.

[0009] (First embodiment) <Image forming device> 1 shows an image forming apparatus 1 which is an electrophotographic copying machine. However, the image forming apparatus 1 may also be realized as a monochrome printer, a full-color printer, a facsimile communication device, or a multifunction device.

[0010] The scanner unit 100 is an original reading device that illuminates an original placed on a platen, optically reads the original image, converts the read result into an electrical signal, and creates image data. The printer engine 103 forms a toner image on a sheet P. The printer engine 103 rotates a photosensitive drum 102. A charger 107 charges the surface of the photosensitive drum 102 so that the surface potential of the photosensitive drum 102 becomes uniform. The exposure head 106 exposes the surface of the photosensitive drum 102 to light according to image data, forming an electrostatic latent image on the surface of the photosensitive drum 102. The developer 108 attaches toner to the electrostatic latent image formed on the photosensitive drum 102 to form a toner image. As the photosensitive drum 102 continues to rotate, the toner image reaches the transfer nip. At the transfer nip, the sheet P is conveyed while being sandwiched between the photosensitive drum 102 and a transfer belt 111. As a result, the toner image is transferred from the photosensitive drum 102 to the sheet P.

[0011] The printer engine 103 has four image creating units 101C, 101M, 101Y, and 101K corresponding to the toner colors cyan, magenta, yellow, and black (CMYK). The four image creating units 101C, 101M, 101Y, and 101K transfer toner images of different colors onto the sheet P, thereby forming a full-color image on the sheet P.

[0012] Feeding unit 105 feeds sheet P from a pre-instructed feeding device among feeding devices 109a and 109b provided in the main body of image forming apparatus 1, feeding device 109c provided outside the main body, and manual feeding device 109d. The fed sheet P is transported to registration rollers 110. Registration rollers 110 transport sheet P so that the timing of the arrival of the toner image at the transfer nip coincides with the timing of the arrival of sheet P. Transfer belt 111 transports sheet P, onto which the toner image has been transferred, to fixing unit 104.

[0013] The fixing device 104 applies pressure and heat to the toner image and the sheet P, thereby fixing the toner image onto the sheet P. The paper discharge rollers 112 discharge the sheet P to the outside of the image forming apparatus 1.

[0014] <Exposure head> 2(A) is a perspective view of exposure head 106 that exposes photosensitive drum 102. FIG. 2(B) is a schematic cross-sectional view of photosensitive drum 102 and exposure head 106. Exposure head 106 has a group of light-emitting elements 201, a printed circuit board 202, a rod lens array 203, and a housing 204. Light output from group of light-emitting elements 201 mounted on printed circuit board 202 is collected by rod lens array 203 and irradiated onto the surface of photosensitive drum 102. Printed circuit board 202 and rod lens array 203 are fixed to housing 204.

[0015] The exposure head 106 is disposed so that its longitudinal direction is parallel to the axial direction D1 of the photosensitive drum 102, and the surface to which the rod lens array 203 is attached faces the surface of the photosensitive drum 102. The light-emitting element group 201 includes a plurality of light-emitting elements regularly arranged parallel to at least the axial direction D1. While the photosensitive drum 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 forms an image of the light on the surface of the photosensitive drum 102. In this specification, the direction parallel to the axial direction D1 of the photosensitive drum 102 is also referred to as the first direction, and the direction on the surface of the printed circuit board 202 that is perpendicular to the first direction (substantially parallel to the circumferential direction D2) is also referred to as the second direction.

[0016] The exposure head 106 is assembled and adjusted individually. The adjustment includes spot size adjustment (focus adjustment) at the light condensing position and light intensity adjustment. In focus adjustment, the attachment position of the rod lens array 203 is adjusted so that the distance between the rod lens array 203 and the light emitting element group 201 is a predetermined value. In light intensity adjustment, the multiple light emitting elements included in the light emitting element group 201 are made to emit light one by one in sequence, and the drive current of each light emitting element is adjusted so that the light intensity of the light condensed via the rod lens array 203 becomes a predetermined light intensity.

[0017] <Configuration of light-emitting element group> 3A shows non-mounting surface 301 of printed circuit board 202. No light-emitting elements are mounted on non-mounting surface 301, but other electronic components such as connector 305 are mounted thereon. Connector 305 is connected to cables (power lines and signal lines) that transmit various signals such as clock signals to printed circuit board 202.

[0018] As shown in FIG. 3(B), the light emitting element group 201 is mounted on the mounting surface 302 of the printed circuit board 202. The mounting surface 302 is the surface opposite to the non-mounting surface 301. The light emitting element group 201 has m light emitting element arrays 300-1 to 300-m arranged in a staggered pattern. In the following, a case where m=20 will be described as an example. The light emitting element arrays may also be called light emitting chips. The light emitting element arrays 300-1 to 300-m may be collectively referred to as the light emitting element array 300.

[0019] As shown in FIG. 3C, each of the light emitting element arrays 300-1 to 300-m has a plurality of light emitting elements 350 arranged along the longitudinal direction of the light emitting element array 300.

[0020] 3(B), the light-emitting element arrays 300 are arranged in two rows. The first row includes light-emitting element array 300-1, light-emitting element array 300-3, ..., light-emitting element array 300-m-1. The second row includes light-emitting element array 300-2, light-emitting element array 300-4, ..., light-emitting element array 300-m.

[0021] As shown in FIG. 3C, the distance between two adjacent light-emitting elements 350 in a given light-emitting element array 300 is L. The distance L is the distance in the longitudinal direction of the light-emitting element array 300. At a resolution of 1200 dpi, L is approximately 21.16 μm. This distance corresponds to one pixel at 1200 dpi. The distance between the rightmost light-emitting element 350 of the i-th light-emitting element array 300-i and the leftmost light-emitting element 350 of the i+1-th light-emitting element array 300-i+1 is also L, where i is an integer between 1 and m−1. As shown in FIG. 3C, the distance S between the rightmost light-emitting element 350 of the light-emitting element array 300-i and the leftmost light-emitting element 350 of the light-emitting element array 300-i+1 in the transverse direction of the light-emitting element array 300 is approximately 105 μm. This corresponds to a distance of five pixels at 1200 dpi. The distances L and S are merely examples.

[0022] <Configuration of light-emitting element array> FIG. 4 is a plan view of the light-emitting element array 300. The X direction indicates the longitudinal direction of the photosensitive drum 102. The Y direction indicates the rotational direction of the photosensitive drum 102. The light-emitting element array 300 has a light-emitting substrate 402, a light-emitting unit 404 including a plurality of light-emitting elements 350 mounted on the light-emitting substrate 402, and a WB pad 408 mounted on the light-emitting substrate 402. WB stands for wire bonding. The light-emitting substrate 402 has a built-in circuit unit 406 for controlling the light-emitting unit 404. The circuit unit 406 includes both an analog drive circuit (analog unit) and a digital control circuit (digital unit). Power is supplied to the circuit unit 406 and signals are input and output to and from the light-emitting element array 300 via the WB pad 408.

[0023] <Light-emitting part> 5 shows a row of light-emitting elements constituting the light-emitting section 404. The light-emitting section 404 has n light-emitting elements 350 arranged in a row. The multiple light-emitting elements 350 are arranged at a predetermined pitch (distance L = 21.16 μm) in the X direction.

[0024] In Figure 5, W1 is the length of the light-emitting element 350 in the X direction. d1 is the distance between two adjacent light-emitting elements 350 in the X direction. W2 is the length of the light-emitting element 350 in the Y direction. The length W2 is determined taking into consideration the scanning speed and resolution in the Y direction. As an example, the lengths W1 and W2 are 20.9 µm, and the distance d1 is 0.26 µm.

[0025] <control block> 6 shows a block diagram of the image controller 600 and the printed circuit board 202. In this embodiment, for the sake of simplicity, the circuit configuration and processing for K alone out of YMCK will be described. Similar processing and circuit configurations are also used for YMC.

[0026] The image controller 600 is a control circuit that generates signals for controlling the printed circuit board 202 and transmits them to the printed circuit board 202. These signals include a clock signal clk, image data data_1 to data_m, a line synchronization signal lsync_x, and a communication signal com. The clock signal clk is generated by a clock unit 608 and serves as a reference for the operations of various circuits. The image data data_1 to data_m are image data supplied to the light-emitting element arrays 300-1 to 300-m, respectively. The line synchronization signal lsync_x indicates the timing of writing an image in the sub-scanning direction. The communication signal com is a communication signal transmitted or received between the CPU 603 and the printed circuit board 202. The clock signal clk is transmitted from the clock unit 608 to the printed circuit board 202 and other components via a clock signal line 605. The line synchronization signal lsync_x is transmitted from the synchronization unit 604 to the printed circuit board 202 and other components via a synchronization signal line 606. The image data data_1 to data_m are transmitted to the light-emitting element arrays 300-1 to 300-m via image signal lines 607-1 to 607-m, respectively. The communication signal com is transmitted from the CPU 603 via a communication signal line 609 to the information storage unit 610 and the light-emitting element arrays 300-1 to 300-m.

[0027] The image data unit 601 applies image processing to image data received from the scanner unit 100 or a computer external to the image forming apparatus 1, and outputs the image data to the streak correction unit 615. The image processing includes, for example, dithering processing at a resolution instructed by the CPU 603. For example, dithering processing is performed at a resolution of 2400 dpi in the sub-scanning direction and 1200 dpi in the main scanning direction. The image data is 8-bit data and represents, for example, the amount of light being turned on.

[0028] The streak correction unit 615 corrects the image data input from the image data unit 601 based on the streak information stored in the information storage unit 610 during the assembly process of the exposure head 106, and outputs the corrected image data to the conversion unit 602. As described above, streak-like density unevenness (streak image) can occur due to tilt of the rod lens array 203. Streak images are reproducible. Therefore, during the assembly process of the exposure head 106, the main scanning position where density unevenness occurs is detected, the correction value of the image data at that position is measured, and the main scanning position to be corrected and the correction value are written to the information storage unit 610 as streak information. This makes it less likely that streak images will occur due to tilt of the rod lens array 203.

[0029] The clock unit 608 is an oscillator circuit that generates a clock signal clk with a constant period. The clock signal clk is supplied to the CPU 603, the synchronization unit 604, the conversion unit 602, and the printed circuit board 202. The CPU 603 executes the following processes in accordance with a control program stored in the ROM area of ​​the memory 650. The memory 650 also includes a RAM area for storing variables, etc.

[0030] The CPU 603 determines the generation cycle of the line synchronization signal lsync_x. The generation cycle is calculated, for example, based on the rotation speed of the photosensitive drum 102 (speed information at which the surface of the photosensitive drum 102 moves in the rotation direction) and the magnification ratio of the image in the sub-scanning direction. The CPU 603 sets the generation cycle of the line synchronization signal lsync_x in the synchronization unit 604. The CPU 603 also receives the line synchronization signal lsync_x from the synchronization unit 604 and recognizes the timing when generation of the line synchronization signal lsync_x is completed.

[0031] The conversion unit 602 generates image data data_1 to data_m by dividing one line of image data output from the image data unit 601 via the streak correction unit 615 into m pieces of image data. The conversion unit 602 transmits the image data data_1 to data_m to the printed circuit board 202 in synchronization with the line synchronization signal lsync_x and the clock signal clk.

[0032] The synchronization unit 604 generates a line synchronization signal lsync_x at a generation cycle instructed by the CPU 603. The line synchronization signal lsync_x is supplied to the printed circuit board 202, the conversion unit 602, and the CPU 603.

[0033] On the printed circuit board 202, the light-emitting element array 300-i is supplied with a line synchronization signal lsync_x, a clock signal clk, image data data_i, and a communication signal com to operate. The information storage unit 610 is a memory circuit that stores head information. The head information includes position information indicating the light emission amount and mounting position of each of the light-emitting element arrays 300-1 to 300-m. The CPU 603 accesses the information storage unit 610 via a communication signal line 609 to read out head information and write setting information. The information storage unit 610 may also store a setting value of the drive current adjusted during the assembly process of the exposure head 106.

[0034] 6, a clock signal line 605, a communication signal line 609, and a synchronization signal line 606 are connected to all of the light-emitting element arrays 300. Image signal lines 607 and light-emitting element arrays 300 are connected one-to-one. In other words, one image signal line 607 is connected to one light-emitting element array 300.

[0035] <Circuit configuration> 7 is a block diagram of the circuit unit 406 in the i-th light-emitting element array 300-i, where i is an integer between 1 and m. The circuit unit 406 has a digital unit 700 and an analog unit 750. The digital unit 700 synchronizes with a clock signal clk and generates a lighting signal for lighting the light-emitting elements 350 based on a preset value set by a communication signal com, a line synchronization signal lsync_x, and image data data. The digital unit 700 outputs the lighting signal to the analog unit 750 via a lighting signal line 708.

[0036] The communication IF 701 controls the writing and reading of setting values ​​to and from the register 702 based on the communication signal com from the CPU 603. The register 702 holds setting values ​​necessary for the operation of the light emitting element 350. The setting values ​​include a value indicating the drive current to be set in the analog unit 750.

[0037] The timing unit 704 generates a timing signal based on the line synchronization signal lsync_x and supplies the timing signal to the lighting control unit 705-1 via a signal line 707-1. The lighting control unit 705-1 retrieves image data data from the image signal line 607 in accordance with the timing signal. The number n of lighting control units 705 matches the number n of light-emitting elements 350. In other words, one lighting control unit 705 is provided for each light-emitting element 350. The lighting control unit 705-j outputs a lighting signal to the analog unit 750 via a lighting signal line 708-j, where j is an integer from 1 to n. The lighting control unit 705-j generates a timing signal for the lighting control unit 705-j+1 based on the input timing signal and supplies the timing signal to the lighting control unit 705-j+1 via a signal line 707-j+1. In this way, the lighting control unit 705-1 is supplied with a timing signal directly from the timing unit 704, but the lighting control units 705-2 to 705-n are supplied with timing signals from the lighting control units 705-1 to 705-n-1 at the preceding stage, respectively.

[0038] The analog section 750 drives the light emitting elements 350-1 to 350-n based on the pulsed lighting signal generated by the digital section 700.

[0039] <Details of the timing section> 8A is a circuit diagram of the timing unit 704. Here, the line synchronization signal lsync_x is assumed to be a negative logic signal, but it may also be a positive logic signal. we[0] is a timing signal. The timing unit 704 is a logic circuit that outputs the timing signal we[0] only when the line synchronization signal lsync_x changes from low to high.

[0040] The delay circuit 801 is connected to the synchronization signal line 606 and the clock signal line 605, and delays the line synchronization signal lsync_x transmitted by the synchronization signal line 606 by one cycle before outputting the signal to the logic gate 802. The delay circuit 801 is realized by, for example, a flip-flop circuit.

[0041] A logic gate 802 performs a logical AND operation on the line synchronization signal lsync_x and the signal obtained by inverting the output signal of the delay circuit 801 using an inverter 803, to generate a timing signal we[0]. The timing signal we[0] is output to a signal line 707-1.

[0042] 8B is a timing chart of the timing unit 704. When the line synchronization signal lsync_x changes from low to high, the timing signal we[0] goes high. The timing signal we[0] remains high for a time equivalent to one cycle of the clock signal clk, and then returns to low.

[0043] <Details of the lighting control unit> FIG. 9A is a circuit diagram of the jth lighting control unit 705-j in the i-th light-emitting element array 300-i, where j is an integer between 1 and n. The delay circuit 901 is connected to the signal line 707-j and the clock signal line 605. The delay circuit 901 delays the timing signal we[j] transmitted by the signal line 707-j by one cycle, generates a timing signal we[j+1] for the subsequent lighting control unit 705-j+1, and outputs it to the signal line 707-j+1. The delay circuit 901 may be any circuit that can delay an input signal by a time equivalent to one cycle of the clock signal clk. For example, a flip-flop circuit can be used as the delay circuit 901.

[0044] The latch circuit 902 is connected to the signal line 707-j and the image signal line 607-i. The latch circuit 902 captures image data data_i during the period when the timing signal we[j] is High, and outputs the image data data_i to the light-up signal line 708-j as the light-up signal el[j]. In this embodiment, the latch circuit 902 is used as a circuit for capturing image data data_i, but this is merely an example. Any circuit can be used as long as it can hold the image data data_i from when the timing signal we[j] goes High until the next time the timing signal we[j] goes High. For example, a flip-flop circuit may be used instead of the latch circuit 902.

[0045] 9B is a timing chart of the delay circuit 901. The timing signal we[j] is delayed by one cycle of the clock signal clk to generate the timing signal we[j+1].

[0046] 9C is a timing chart of the latch circuit 902. While the timing signal we[j] is High, the image data data_i (in this example, "000") is captured and the light-up signal el[j] is generated.

[0047] <Details of the analog section> 10 is a block diagram of the analog unit 750. For simplicity, two light-emitting elements 350-1 and 350-n and two drive circuits 1001-1 and 1001-n are shown. In reality, there are n light-emitting elements 350-1 to 350-n and n drive circuits 1001-1 to 1001-n. For generalization, the j-th light-emitting element 350-j and the j-th drive circuit 1001-j will be described below. j is an integer from 1 to n.

[0048] The driving circuit 1001-j is a circuit that drives the j-th light emitting element 350-j. A light-up signal el[j] is supplied to the driving circuit 1001-j via a light-up signal line 708-j.

[0049] DAC 1002 converts the drive current data set in register 702 into an analog voltage and supplies the analog voltage to drive circuits 1001-1 to 1001-n via signal line 1003. DAC is an abbreviation for digital-to-analog converter. Here, the drive current data indicates the set value of the drive current supplied to light emitting elements 350-1 to 350-n.

[0050] The selection circuit 1007 generates a select signal for selecting the drive circuit 1001 based on the data set in the register 702. The selection circuit 1007 supplies the select signal to the drive circuits 1001-1 to 1001-n via signal lines 1004-1 to 1004-n. The select signal is a signal that sets high only on a signal line connected to one selected drive circuit 1001 out of the n drive circuits 1001-1 to 1001-n. When the drive circuit 1001-1 is selected, only the signal line 1004-1 is controlled to a high level. The signal lines 1004-2 (not shown) to 1001-n are controlled to a low level. An analog voltage is set in each of the drive circuits 1001-1 to 1001-n via the signal line 1003 at the timing selected by the selection circuit 1007 (the timing when the select signal sets high). The CPU 603 selects the drive circuits 1001-1 to 1001-n one by one in turn via the register 702, and sets an analog voltage corresponding to the selected drive circuit 1001. This makes it possible to set individual analog voltages for the n drive circuits 1001-1 to 1001-n using a single DAC 1002. In this way, the drive circuits 1001-1 to 1001-n each receive an analog voltage that determines the drive current and a lighting signal, causing the corresponding light-emitting elements 350-1 to 350-n to emit light.

[0051] <Drive circuit details> 11 is a circuit diagram of the j-th driving circuit 1001-j (j is an integer from 1 to n). The driving circuits 1001-1 to 1001-n all have the same circuit configuration.

[0052] The MOSFET Q1 supplies a drive current to the light-emitting element 350-j in response to a gate voltage applied to its gate. When the gate voltage is low, the drive current decreases, and the light-emitting element 350-j is turned off. The gate of the MOSFET Q2 is connected to an on-signal line 708-j. When the on-signal el[j] is high, the MOSFET Q2 is turned on and transfers the voltage charged in the capacitor C1 to the MOSFET Q1. The gate of the MOSFET Q3 is connected to a signal line 1004-j. The MOSFET Q3 is turned on / off in response to a select signal from the selection circuit 1007. That is, when the select signal is high, the MOSFET Q3 is turned on and applies an analog voltage output from the DAC 1002 to the capacitor C1, thereby charging the capacitor C1. In this embodiment, before image formation, the DAC 1002 sets an analog voltage in the capacitor C1. During the image formation period, the MOSFET Q3 is turned off, and the capacitor C1 continuously maintains a voltage level. As a result, the MOSFET Q1 supplies or stops supplying a drive current corresponding to the set analog voltage to the light emitting element 350-1 in response to the light-up signal.

[0053] If the input capacitance of the light-emitting element 350-j is too large, the response speed for switching the light-emitting element 350-j from on to off will be slow. Therefore, to improve the response speed, a MOSFET Q4 and an inverter 1101 may be added. A signal obtained by inverting the logic of the light-up signal el[j] by the inverter 1101 is input to the gate of the MOSFET Q4. When the light-up signal el[j] is at a low level, the gate of the MOSFET Q4 becomes high. Therefore, the MOSFET Q4 turns on, and it becomes possible to forcibly discharge the charge stored in the input capacitance of the light-emitting element 350-j.

[0054] [Story information] If some of the rod lenses constituting the rod lens array 203 are tilted, the spot size of light passing through that rod lens may widen. In this case, as shown in FIG. 12A, forming a low-density image 1201a may result in a streaky image 1202a that is lighter in density than the surrounding images. Similarly, as shown in FIG. 12B, forming a high-density image 1201b may result in a streaky image 1202b that is darker in density than the surrounding images. The streaky images 1202a and 1202b are generated parallel to the sub-scanning direction. In other words, the streaky images 1202a and 1202b appear at the same main-scanning position. Conventionally, such rod lens arrays 203 have been discarded. In this embodiment, the image data is corrected based on characteristic information of the rod lens array 203, thereby reducing the streaky density unevenness. This reduces the number of discarded rod lens arrays 203 and promotes efficient resource utilization. As a result, the yield of the rod lens array is improved, and an increase in the cost of the image forming apparatus can be suppressed.

[0055] FIG. 13 shows an example of streak information stored in the information storage unit 610. According to FIG. 13, the streak information includes individual information for each streak occurring at a specific main scanning position. In this example, the individual information includes a streak number, a streak start position, and a streak rank. The streak number is identification information for identifying multiple streaks. The streak start position indicates the start position of the streak image in the main scanning direction. In this example, pixel numbers indicating the main scanning position are defined so that the numbers increase from left to right in the main scanning direction. The streak start positions shown in FIG. 13 indicate main scanning positions assuming that a toner image is formed at 2400 dpi. For example, the streak start position of a streak assigned streak number "1" is 100 [pix]. "pix" is an abbreviation for pixel. The streak start position of a streak assigned streak number "2" is 1000 [pix]. In this example, streak information for five streaks is stored in the information storage unit 610, but this is merely an example. The number of pieces of streak information may differ for each exposure head 106 .

[0056] Incidentally, the exposure head 106 may support multiple resolutions, such as 2400 dpi and 1200 dpi. In this case, streak information for each resolution may be stored in the information storage unit 610. In this case, the CPU 603 reads out streak information corresponding to the resolution specified by the user from the information storage unit 610 and sets it in the streak correction unit 615.

[0057] The streak rank is information indicating the degree of streak-like density unevenness measured in advance. The streak rank may be expressed, for example, in 4 bits. In this case, the streak rank can indicate 16 levels of correction values. Furthermore, one piece of streak information may hold multiple streak ranks. Generally, the width of a streak in the main scanning direction spans multiple pixels. Holding all the numbers of the pixels that make up the streak would require a large amount of storage space. Therefore, in this embodiment, multiple correction target pixels corresponding to the streak width can be identified using the streak start position and streak rank. In the example of FIG. 13, 16 streak ranks are held for one streak. This is because the width (number of pixels) of one streak is generally 16 pixels or less. In this example, a streak rank is held for each pixel, but this is merely an example. To reduce the memory required to store the streak ranks, one streak rank may be held for every two, four, or eight pixels.

[0058] [Streak correction section] As illustrated in FIG. 14, the streak correction unit 615 includes an identification unit 1401, a correction calculation unit 1402, and a correction processing unit 1403. The identification unit 1401 determines whether input data (pixel data) is a pixel to be corrected for streak. The pixel data is, for example, an 8-bit illumination light amount (brightness value or density value). The CPU 603 extracts the streak start position from the streak information and sets the streak start position in the identification unit 1401 using a communication signal com. The identification unit 1401 counts the pixel data in the main scanning direction. For example, the identification unit 1401 may include a counter that counts a clock signal clk. When the count value matches the streak start position, the identification unit 1401 outputs an identification signal indicating that the streak start position has been identified to the correction calculation unit 1402. For example, the identification signal changes from Low to High. In parallel with this, the specifying unit 1401 outputs pixel data constituting the image data one by one in order to the correction calculation unit 1402 based on the clock signal clk.

[0059] The CPU 603 extracts a streak rank from the streak information and sets the streak rank in the correction calculation unit 1402 using a communication signal com. When the identification signal from the identification unit 1401 changes from low to high, the correction calculation unit 1402 recognizes the pixel data input from the identification unit 1401 as a correction target. The identification unit 1401 calculates a correction value based on the streak rank set by the CPU 603 and outputs a correction signal indicating the correction value to the correction processing unit 1403. In parallel with this, the identification unit 1401 outputs pixel data constituting the image data one by one in order to the correction processing unit 1403 based on the clock signal clk.

[0060] The correction signal and pixel data output from the correction calculation unit 1402 are input to the correction processing unit 1403. The correction processing unit 1403 corrects the pixel data in accordance with the correction signal. The image data made up of a series of pixel data output from the correction processing unit 1403 is output to the conversion unit 602. [Specific behavior to be corrected] FIG. 15 is a diagram showing an example of the configuration of the identification unit 1401. FIG. 16A is a timing chart showing the operation of the identification unit 1401. The CPU 603 writes the streak start position extracted from the streak information to the memory 1501. The memory 1501 sets the written streak start position in the comparison circuit 1503. As shown in FIG. 16A, the counter 1502 is a counter circuit that increments by 1 each time a clock signal clk is input. As shown in FIG. 16A, the count value of the counter 1502 indicates the main scanning position. Note that the counter 1502 is reset to 0 when a line synchronization signal lsync_x is input. The comparison circuit 1503 determines whether the streak start position matches the count value (current main scanning position). If the streak start position does not match the count value (current main scanning position), the comparison circuit 1503 maintains the identification signal det at a low level. On the other hand, if the streak start position and the count value match, the comparator circuit 1503 outputs a High-level identification signal det for a predetermined number of pixels. In the example shown in FIG. 16A, when the count value (main scanning position) matches 100, which is the streak start position, the identification signal det changes from Low to High. Also, in the example shown in FIG. 16A, the identification signal det is maintained at High for 16 pixels. This indicates that 16 pixels, including the pixel corresponding to the streak start position, are identified as correction targets. In this example, it is assumed that the number of pixels to be corrected matches the number of streak ranks. The delay circuit 1504 delays multiple pixel data constituting the image data by a time equivalent to one clock signal clk and outputs the delayed data. The delay circuit 1504 may be implemented, for example, by a flip-flop circuit. As shown in FIG. 16A, the output data is delayed by one clock signal clk relative to the input data.

[0061] [Calculation of correction value] FIG. 17 shows an example of a calculation table that receives streak rank and image data as input and outputs a correction value. The correction calculation unit 1402 determines the correction value by referring to the calculation table based on the input data value and the streak rank value. The correction calculation unit 1402 may be implemented by the CPU 603. The CPU 603 may calculate the correction value from the input data value and the streak rank value using a calculation table or a function. In the example shown in FIG. 17, only 0 and 255 are shown as image data (pixel data), but in reality, there are 16 streak ranks and 16 correction values ​​associated with them for each density value (brightness value) from 1 to 254.

[0062] 16B is a timing chart showing the operation of the correction calculation unit 1402. When the identification signal det input from the identification unit 1401 is High, the correction calculation unit 1402 calculates and outputs a correction value based on the streak rank set by the CPU 603. Since the correction calculation unit 1402 is composed of a logic circuit, it operates based on the rising edge of the clock signal clk. Therefore, the output data is delayed by one clock signal clk relative to the input data and output. Note that the correction of image data based on the correction value is performed by the correction processing unit 1403, so the correction calculation unit 1402 does not correct the image data.

[0063] [Streak Correction Processing] FIG. 16C is a timing chart showing the operation of the correction processing unit 1403. Because the correction processing unit 1403 is configured with a logic circuit, it operates based on the rising edge of the clock signal clk. The correction processing unit 1403 generates output data by adding a correction value indicated by the correction signal to input data. In this embodiment, the output data is generated by adding the correction value and the input data, but this is just one example. The output data may be determined by referring to a lookup table based on the input data and the correction value. Alternatively, pattern matching may be employed to generate output data based on the input data and the correction value. The output data may be generated by a filter operation based on the input data and the correction value.

[0064] In this way, the streak correction unit 615 performs streak correction on the image data based on the streak information stored in the information storage unit 610. This makes it possible to effectively utilize the rod lens array 203, which would have been discarded in the past. In other words, it is possible to promote the effective use of resources. Furthermore, it is possible to reduce density unevenness (streak images) caused by the tilt of the rod lens array 203 at a relatively low cost.

[0065] In the above embodiment, the streak correction unit 615 is described as being implemented by a logic circuit, but this is merely an example. The streak correction unit 615 may also be implemented by the CPU 603 and a program.

[0066] (1) Streak correction processing by pattern matching FIG. 18 shows an example of streak correction processing using pattern matching. Here, attention is focused on the correction values ​​of the preceding pixel and the succeeding pixel adjacent to the pixel of interest in the main scanning direction. If the combination of the correction values ​​of the preceding pixel, the target pixel, and the succeeding pixel matches a predetermined combination (pattern), the pixel value of the target pixel is corrected using the correction value of the target pixel. For example, if the correction value of the (i-1)th pixel is c, the correction value of the i-th pixel is c, and the correction value of the (i+1)th pixel is c+1, the correction value of the i-th pixel is determined to be +c. In other words, if the pixel value of the i-th pixel is r, the pixel value of the output pixel is r+c. As shown in FIG. 18, assume that the correction value of the preceding pixel is 4, the correction value of the target pixel is 4, and the correction value of the succeeding pixel is 5. In this case, the output pixel is corrected by +4 relative to the input pixel (127+4=131). These values ​​are merely examples.

[0067] (2) Streak correction processing using filter processing Similar to pattern matching, filtering generates an output pixel using the correction values ​​of the pixels before and after the pixel of interest. For example, the filter focuses on one pixel after a pixel before the pixel of interest, which is adjacent to the pixel of interest in the scanning direction. In other words, the pixel value of the output pixel is determined from the correction values ​​of three pixels, including the pixel of interest, and the filter coefficient. For example, a 3x1 filter is used, which is applied to a 3x1 pixel group consisting of three pixels in the main scanning direction and one pixel in the sub-scanning direction. For example, assume that the correction value of the previous pixel is 4 and the filter coefficient is 20%. Assume that the correction value of the pixel of interest is 4 and the filter coefficient is 80%. Assume also that the correction value of the pixel after the pixel is 5 and the filter coefficient is 20%. In this case, the correction value of the pixel of interest is calculated as follows:

[0068] Correction value = 4 × 0.2 + 4 × 0.8 + 5 × 0.2 = 1 + 3 + 2 = 5 (1) The figures are just an example.

[0069] <Summary of the First Embodiment> As shown in FIG. 1, the photosensitive drum 102 is an example of a photosensitive member that is rotated. The light-emitting elements 350-1 to 350-m are an example of a plurality of light-emitting elements arranged in a direction intersecting the rotational direction of the photosensitive member. The rod lens array 203 is an example of a rod lens array that focuses light output from each of the plurality of light-emitting elements on the photosensitive member. The exposure head 106 is an example of an exposure head that forms a latent image on the photosensitive member. The information storage unit 610 is an example of a memory unit that stores characteristic information (e.g., streak information) that indicates the imaging characteristics of the rod lens array. The CPU 603 or the streak correction unit 615 functions as a correction unit that reduces streak-like density unevenness that occurs in the latent image by correcting image data for driving the plurality of light-emitting elements based on the characteristic information read from the memory unit. This allows the rod lens array 203, which would previously have been discarded, to be installed in the exposure head 106. In other words, it is possible to promote the effective use of resources related to the rod lens array 203.

[0070] The identification unit 1401 is an example of an identification unit that identifies pixels to be corrected in image data based on characteristic information. The CPU 603 or the streak correction unit 615 corrects the pixel data of the pixels identified by the identification unit based on the characteristic information. This may reduce streak-like density unevenness.

[0071] The characteristic information may include position information (e.g., streak start position) indicating the position of the pixel to be corrected among the multiple pixels that make up the image data, and a correction level (e.g., streak rank) to be applied to the pixel data of the pixel to be corrected.

[0072] The position information (e.g., streak start position) may be information indicating the position of the first pixel in a series of pixels to be corrected among the multiple pixels that make up the image data, thereby reducing the storage capacity of the characteristic information.

[0073] The characteristic information may include a correction level (e.g., 16 streak ranks) for each of a series of pixels to be corrected. As illustrated in FIG. 13, the characteristic information may include one correction level for each pixel in the series of pixels to be corrected. Alternatively, the characteristic information may include one correction level common to N (N is an integer equal to or greater than 2) pixels in the series of pixels to be corrected. For example, one common streak rank may be provided for every two pixels, one common streak rank may be provided for every four pixels, or one common streak rank may be provided for every eight pixels.

[0074] The CPU 603 or the correction calculation unit 1402 may function as a calculation unit that calculates a correction value for each pixel based on the correction level. The CPU 603 or the correction processing unit 1403 may function as a correction processing unit that executes correction processing on image data for each pixel based on the correction value calculated for each pixel.

[0075] The CPU 603 or the correction processing unit 1403 may correct the pixel value of the pixel of interest by adding the correction value calculated for the pixel of interest by the CPU 603 or the correction calculation unit 1402 to the pixel value of the pixel of interest.

[0076] The CPU 603 or the correction processing unit 1403 may obtain the corrected pixel value for the pixel of interest by referring to a table based on the pixel value of the pixel of interest and the correction value calculated by the correction calculation unit 1402 for the pixel of interest.

[0077] The correction calculation unit 1402 calculates a first correction value for a previous pixel located before the pixel of interest in the main scanning direction. The correction calculation unit 1402 calculates a second correction value for the pixel of interest. The correction calculation unit 1402 calculates a third correction value for a subsequent pixel located after the pixel of interest in the main scanning direction. The CPU 603 or the correction processing unit 1403 may apply the second correction value to the pixel value of the pixel of interest if the first correction value, the second correction value, and the third correction value are in a predetermined pattern. Note that the CPU 603 or the correction processing unit 1403 may not correct the pixel value of the pixel of interest if the first correction value, the second correction value, and the third correction value are not in a predetermined pattern.

[0078] The correction processing unit 1403 may calculate a first product by multiplying a first correction value calculated for the previous pixel by a first filter coefficient. The correction processing unit 1403 may calculate a second product by multiplying a second correction value calculated for the pixel of interest by a second filter coefficient. The correction processing unit 1403 may calculate a third product by multiplying a third correction value calculated for the next pixel by a third filter coefficient. Furthermore, the correction processing unit 1403 may calculate a total correction value by summing the first product, the second product, and the third product, and correct the pixel value of the pixel of interest with the total correction value.

[0079] Counter 1502 may function as a count unit that counts the main scanning position based on a clock signal. Comparison circuit 1503 may function as a judgment unit that judges whether the main scanning position counted by the count unit matches the position of the pixel to be corrected. CPU 603 or correction calculation unit 1402 starts calculating the correction value when the judgment unit outputs a predetermined signal (e.g., specific signal det) indicating that the main scanning position counted by the count unit matches the position of the pixel to be corrected.

[0080] The storage unit (e.g., information storage unit 610) may be provided in the exposure head 106. [O11321] Stripe-like density unevenness caused by the manufacturing precision of the rod lens array 203 differs for each exposure head 106. Therefore, streak information may be acquired during the assembly process of the exposure head 106 and written to the information storage unit 610 of the exposure head 106. This will make it possible to correct stripe-like density unevenness using streak information appropriate for each exposure head 106.

[0081] Second Embodiment FIG. 19 schematically illustrates the arrangement of the light-emitting element array 300 and the light-emitting elements 350 within the light-emitting element array 300. In this embodiment, the light-emitting element group 201 is composed of light-emitting elements included in 20 light-emitting element arrays 300-1 to 300-20. The light-emitting element arrays 300-1 to 300-20 are regularly arranged in the first direction (here, in a staggered pattern of two lines). More specifically, when n is an odd number, 10 light-emitting element arrays 300-n form one line, and when n is an even number, another 10 light-emitting element arrays 300-n form another line, and the positions of the two lines in the second direction are different. In this specification, the light-emitting element arrays 300 in the former line are also referred to as odd-numbered light-emitting element arrays 300, and the light-emitting element arrays 300 in the latter line are also referred to as even-numbered light-emitting element arrays 300. The light-emitting element arrays 300-1 to 300-20 are also collectively referred to as light-emitting element arrays 300. Each light-emitting element array 300 on the printed circuit board 202 is connected to the image controller 2200 (FIG. 22) via a connector 305. For ease of explanation, the side with the smaller branch number of the light-emitting element arrays 300-1 to 300-20 aligned along the first direction may be referred to as the "left" and the side with the larger branch number as the "right." For example, the light-emitting element array 300-1 is the leftmost light-emitting element array 300, and the light-emitting element array 300-20 is the rightmost light-emitting element array.

[0082] In this embodiment, the light-emitting element array 300 has a two-dimensional array of N columns (N is an integer of 2 or greater) in the first direction and M rows (M is an integer of 2 or greater) in the second direction. In an exemplary embodiment, N=748 and M=4, in which case each light-emitting element array 300 has a total of 2992 (=748×4) light-emitting elements 350. A light-emitting element group 201 consisting of 20 light-emitting element arrays 300 has 14960 light-emitting elements aligned in the first direction. The pitch between adjacent light-emitting elements 350 in the first direction may be approximately 21.16 μm, corresponding to a resolution of 1200 dpi. In this case, the overall length of the light-emitting element group 201 in the first direction is approximately 316 mm (the maximum width of an image that can be formed), and the length of each light-emitting element array 300 in the first direction is approximately 15.8 mm. The M light-emitting elements in each column are shifted in the first direction at a pitch of approximately 5 μm, corresponding to a resolution of 4800 dpi, and arranged in a stepped pattern. While FIG. 19 illustrates an example in which the lower light-emitting element of two upper and lower light-emitting elements is shifted to the left, the lower light-emitting element may also be shifted to the right. Furthermore, as shown in FIG. 19, the light-emitting elements in the rightmost column of the odd-numbered light-emitting element arrays 300 and the light-emitting elements in the leftmost column of the even-numbered light-emitting element arrays 300 may be arranged to overlap in the first direction. Similarly, the light-emitting elements in the leftmost column of the odd-numbered light-emitting element arrays 300 and the light-emitting elements in the rightmost column of the even-numbered light-emitting element arrays 300 may be arranged to overlap in the first direction. The spacing Ly between the light-emitting element arrays of these light-emitting element arrays 300 may be, for example, approximately 105 μm. By overlapping the light-emitting element arrays in this manner in the first direction, it is possible to prevent gaps in the exposure range due to variations in implementation. When such an overlapping arrangement is adopted, typically, only one of the overlapping columns is used as an effective light-emitting element capable of emitting light according to image data. The light-emitting elements in the other column may be unused regardless of the image data. The number of columns or the number of light-emitting elements that are arranged overlappingly in the first direction is not limited to the above example (four pixels per column), and may be any number.

[0083] FIG. 4 will be referred to again in the second embodiment. FIG. 4 is a plan view showing a schematic configuration of the light-emitting element array 300. The plurality of light-emitting elements 350 of each light-emitting element array 300 are formed on a light-emitting substrate 402, which is, for example, a silicon substrate. A circuit unit 406 for driving the plurality of light-emitting elements 350 is also provided on the light-emitting substrate 402. A plurality of pads 408 are used to connect signal lines for communicating with the image controller 2200, power lines for connecting to a power source, and ground lines for connecting to ground to the circuit unit 406. The signal lines, power lines, and ground lines may be wires made of, for example, gold.

[0084] FIG. 20 shows a portion of the cross section taken along line AA in FIG. 4. A plurality of lower electrodes 2004 are formed on a light-emitting substrate 402. A light-emitting layer 2006 is provided on the lower electrodes 2004, and an upper electrode 2008 is provided on the light-emitting layer 2006. The upper electrode 2008 is a common electrode for the plurality of lower electrodes 2004. When a voltage is applied between the lower electrode 2004 and the upper electrode 2008, a current flows from the lower electrode 2004 to the upper electrode 2008, causing the light-emitting layer 2006 to emit light. Therefore, one lower electrode 2004 and the partial regions of the light-emitting layer 2006 and upper electrode 2008 corresponding to that lower electrode 2004 constitute one light-emitting element 350. In the figure, dx is the distance between two adjacent lower electrodes 2004. dz is the distance between the lower electrode 2004 and the upper electrode 2008. By making dx larger than dz, leakage current between adjacent lower electrodes 2004 can be suppressed, and light emitting elements 350 that should not emit light can be prevented from emitting light by mistake.

[0085] In this embodiment, each light-emitting element 350 is configured as an organic EL (Electro-Luminescence) element. For example, an organic EL film can be used for the light-emitting layer 2006. In other embodiments, each light-emitting element 350 can be configured as an inorganic EL element by using an inorganic EL film for the light-emitting layer 2006. Generally, each light-emitting element 350 can be any type of LED (Light-Emitting Diode).

[0086] The upper electrode 2008 is made of a transparent electrode such as indium tin oxide (ITO) so as to transmit the emission wavelength of the light emitting layer 2006. In the example of Fig. 20, the entire upper electrode 2008 transmits the emission wavelength of the light emitting layer 2006, but the entire upper electrode 2008 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 350 passes transmits the emission wavelength.

[0087] 20 shows that one continuous light-emitting layer 2006 is formed, but a plurality of light-emitting layers 2006 each having a width equivalent to that of the lower electrode 2004 may be formed on the lower electrode 2004. Also, in FIG. 20, the upper electrode 2008 is formed as one common electrode for the plurality of lower electrodes 2004, but a plurality of upper electrodes 2008 each having a width equivalent to that of the lower electrode 2004 may be formed corresponding to each of the lower electrodes 2004. Also, of the lower electrodes 2004 of each light-emitting element array 300, a first plurality of lower electrodes 2004 may be covered with a first light-emitting layer 2006, and a second plurality of lower electrodes 2004 may be covered with a second light-emitting layer 2006. Similarly, a first upper electrode 2008 may be commonly formed corresponding to a first plurality of lower electrodes 2004 among the lower electrodes 2004 of each light-emitting element array 300, and a second upper electrode 2008 may be commonly formed corresponding to a second plurality of lower electrodes 2004. Even in such a configuration, one lower electrode 2004 and the region of the light-emitting layer 2006 and upper electrode 2008 corresponding to that lower electrode 2004 constitute one light-emitting element 350.

[0088] <Multiple exposure> While using organic EL elements as the light-emitting elements 350 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 the desired density. Therefore, in this embodiment, a multiple exposure technique is adopted in which multiple light-emitting elements 350 arranged in the circumferential direction of the photosensitive drum 102 are sequentially caused to emit light, thereby exposing each pixel area (spot area) on the photosensitive drum 102 in multiple exposures.

[0089] FIG. 21 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 350 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. R j_m (j={1,2,...,N}, m={0,1,2,3}) represents the light-emitting element 350 in the j-th column from the left in the first direction and the m-th row from the bottom in the second direction. In FIG. 21, W1 represents the width of the light-emitting element 350 in the first direction, and W2 represents the width of the light-emitting element 350 in the second direction. d1 represents the distance between adjacent light-emitting elements 350 in the first direction, and d2 represents the distance between adjacent light-emitting elements 350 in the second direction. d1 and d2 represent 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 350 in the first direction may be approximately 5 μm (equivalent to 4800 dpi).

[0090] As shown in the example of Fig. 21, four light-emitting elements in each column are arranged in a stepped pattern, so that any two adjacent light-emitting elements among the four light-emitting elements occupy areas that partially overlap in the first direction. Then, the four light-emitting elements in the column corresponding to each pixel position of the input image data sequentially emit light while the photosensitive drum 102 is rotating, thereby forming spots corresponding to each pixel position on the surface of the photosensitive drum 102. In the example of Fig. 21, when the pixel value at the left end of the ith 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 photosensitive drum 102. i As a result, the line L i Similarly, when the j-th pixel value from the left of the ith line of the input image data indicates that light is emitted, the light-emitting element R j_1 , Rj_2 , R j_3 , R j_4 is a line L on the surface of the photosensitive drum 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 When the pitch of the light emitting elements in the second 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 exposure period (line period) by the 20 light-emitting element arrays 300 may be approximately 105.8 μs. In this way, by sequentially emitting light at appropriate times from four light-emitting elements in each column of the 20 light-emitting element arrays 300, smooth lines of an electrostatic latent image can be formed on the surface of the photoconductor drum 102, consisting of a series of spots that partially overlap each other and have a constant spot spacing. As a result of these lines being continuously formed in the circumferential direction, a two-dimensional electrostatic latent image is created.

[0091] In the following description, the spot area where a spot of an electrostatic latent image is formed as a result of multiple exposure due to the emission of M light-emitting elements 350 is also referred to as an "exposed dot," and the spot area where a spot is not formed because the light-emitting elements 350 do not emit light is also referred to as an "unexposed dot."

[0092] <Control circuit configuration> 22 shows an example of the configuration of a control circuit for controlling the light emission of the light-emitting element array 300 on the printed circuit board 202. For simplicity of explanation, the processing for a single color component will be described here, but in reality, similar processing is performed in parallel for four color components.

[0093] The image controller 2200 is a component that constitutes the exposure device together with the exposure head 106. The image controller 2200 is connected to each of the light-emitting element arrays 300 on the printed circuit board 202 via multiple signal lines 2205 to 2209. The chip select signal line 2205 carries a chip select signal CS that indicates the effective range of the image data. The clock signal line 2206 carries a clock signal CLK. The data signal line 2207 carries the image data DATA. The synchronization signal line 2208 carries a line synchronization signal Lsync for identifying the line period of the image data. The communication signal line 2209 carries a control signal CTL.

[0094] The image data generation unit 2201 performs image processing on image data received from the scanner unit 100 or an external device to generate binary bitmap image data for controlling the on / off of light-emitting elements 350 in the light-emitting element array 300 on the printed circuit board 202. This image processing may include, for example, raster conversion and halftone processing (e.g., dithering). The image data after halftone processing is a collection of bits indicating whether to emit light for each of the M corresponding light-emitting elements 350 at each pixel position constituting the image to be formed. If a bit at a pixel position indicates "emission," the corresponding spot area on the surface of the photosensitive drum 102 becomes an exposed dot. If the bit indicates "non-emission," the corresponding spot area becomes a non-exposed dot. The image data generation unit 2201 outputs the generated image data to the light intensity correction unit 2202.

[0095] The light intensity correction unit 2202 performs correction processing on the image data input from the image data generation unit 2201 to correct unevenness in the amount of light from the exposure head 106 using correction data stored in a memory (for example, a storage unit 2210) described later. The light intensity correction unit 2202 then outputs the corrected image data to the conversion unit 2203. The correction of unevenness in the amount of light performed by the light intensity correction unit 2202 will be described in detail later. The converter 2203 reads pixel values ​​within a corresponding read range of the image data input from the light intensity corrector 2202 in each line period identified by the line synchronization signal Lsync, and sends image data indicating the read pixel values ​​to a data signal line 2207. The image data DATA is, for example, a sequence of pixel values ​​corresponding to each of the 20 light-emitting elements 350 of the light-emitting element array 300. The converter 2203 specifies which light-emitting element array 300 should receive each portion of the image data DATA using a chip select signal CS. The converter 2203 generates a clock signal CLK and supplies it to each light-emitting element array 300 to synchronize the timing of transmission and reception of individual signal values ​​with each light-emitting element array 300.

[0096] The synchronization signal generation unit 2204 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 2208 .

[0097] The storage unit 2210 of the printed circuit board 202 is a memory (e.g., a non-volatile memory) that stores control data for controlling light emission by each light-emitting element array 300. As will be described later, the control data stored in the storage unit 2210 may include, for example, a setting value related to the amount of supply current and correction data used to correct input image data.

[0098] Each light-emitting element array 300 drives each of the light-emitting elements 350 in accordance with the light-intensity-corrected image data input from the conversion unit 2203 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 its own chip, each light-emitting element array 300 receives a portion of the image data DATA intended for its own chip via the data signal line 2207. Then, each light-emitting element array 300 drives each of the light-emitting elements 350 in the light-emitting element array of M rows and N columns in accordance with pixel values ​​included in the received image data.

[0099] The CPU 2211 controls the entire image forming apparatus 1. For example, the CPU 2211 controls the generation of the image data, the light intensity correction, the generation of a line synchronization signal, and the transmission of the image data to the printed circuit board 202. For example, prior to the execution of a job for image formation, the CPU 2211 reads out correction data stored in advance in the storage unit 2210 of each light-emitting element array 300, and provides the read correction data to the light intensity correction unit 2202.

[0100] Fig. 23 is a block diagram showing the configuration of a circuit related to the supply of current in the light-emitting element array 300. Each light-emitting element array 300 includes a digital-to-analog (D / A) converter 2301, a plurality of (five in the example of Fig. 23) reference current sources 2302-1 to 2302-5, and a plurality of light-emitting elements 350. Each light-emitting element 350 is supplied with current from one of the reference current sources 2302-1 to 2302-5 depending on the position of the light-emitting element 350 in the first direction.

[0101] The DAC 2301 performs digital-to-analog conversion on a digital value indicating the setting value of the reference current set by the CPU 2211, and outputs an analog signal having a voltage corresponding to the setting value to each reference current source 2302. The setting value of the reference current is stored in advance in the storage unit 2210, read out by the CPU 2211, and output to the DAC 2301. Each of the reference current sources 2302-1 to 2302-5 supplies a reference current corresponding to the voltage of the analog signal input from the DAC 2301 to the light-emitting element 350. The number of reference current sources 2302 provided in each light-emitting element array 300 is not limited to the example described above, and may be any number depending on the length of the wiring within the chip or the driving capability of the reference current sources 2302.

[0102] <Correction of uneven light intensity> <Various causes> There are various causes of uneven light intensity in solid-state exposure equipment. This section explains the following three causes: Properties of rod lens arrays Lens tilt in rod lens arrays - Misalignment of light-emitting element array (1) Properties of rod lens arrays First, the periodic nature of light passing through a rod lens array will be described using Figures 24 and 25. Figure 24 shows an example of the arrangement of multiple lenses in a rod lens array 203. The rod lens array 203 includes multiple lenses 2410 arranged in at least a first direction. In the example of Figure 24, the multiple lenses 2410 in the rod lens array 203 form two lines parallel to the first direction, and the lenses 2410 are regularly arranged in each line at a pitch P1.

[0103] Curve 2500 shown in the upper part of Fig. 25 schematically represents the light intensity distribution that can be measured along the first direction after completely uniform parallel light emitted from an ideal light source passes through rod lens array 203. Because rod lens array 203 is an arrangement of multiple lenses 2410 arranged at pitch P1, the light intensity distribution represented by curve 2500 has the property of repeatedly increasing and decreasing at period P1 in the longitudinal direction of the arrangement. Meanwhile, light emitting element array 300 arranged opposite rod lens array 203 also has multiple light emitting elements 350 regularly arranged parallel to the first direction. Therefore, the actual light intensity distribution of transmitted light differs depending on where each light emitting element 350 is located within pitch P1 of the lenses 2410.

[0104] In the enlarged view of curve 2500 shown in the lower part of Fig. 25, positions X1, X2, X3, and X4 are candidate positions for light-emitting elements 350 that can be arranged at a pitch smaller than pitch P1 of lenses 2410. When light-emitting elements 350 are located at positions X1 and X3, for example, the amounts of light emitted from those light-emitting elements 350 and passed through rod lens array 203 are light amounts E1 and E3, respectively. On the other hand, when light-emitting elements 350 are located at positions X2 and X4, the amounts of light emitted from those light-emitting elements 350 and passed through rod lens array 203 are light amounts E2 and E4, respectively.

[0105] In this way, an exposure apparatus that focuses light from a group of light-emitting elements onto the surface of a photosensitive member using a rod lens array will exhibit a light intensity distribution that includes some frequency components, even if there is no manufacturing variation. The correction data, which will be described in detail later, expresses the frequency components of this light intensity distribution using frequency domain parameters. Typically, the light intensity distribution of light that has passed through the rod lens array 203 includes a frequency component corresponding to the pitch P1 of the lenses 2410. Alternatively, the light intensity distribution of light that has passed through the rod lens array 203 also includes a frequency component corresponding to the pitch of the light-emitting elements 350 in each light-emitting element array 300.

[0106] (2) Lens tilt in rod lens array Ideally, the multiple lenses 2410 of the rod lens array 203 are arranged so that their central axes are all perpendicular to the light-emitting surface of the light-emitting element 350 (parallel to the optical axis). However, in reality, the rod lens array 203 may be manufactured with a small number of lenses 2410 having tilted central axes. FIG. 26 is a schematic cross-sectional view of the rod lens array 203 taken along line BB in FIG. 24, illustrating an example of lens tilt due to manufacturing errors. The direction D3 in the figure is a third direction perpendicular to the axial direction D1 (first direction) and the circumferential direction D2 (second direction) of the photosensitive element. In the example of FIG. 26, the lens 2410-1 is positioned without error, and the central axis 2611-1 of the lens 2410-1 is parallel to the direction D3. On the other hand, the lens 2410-2 is tilted due to a manufacturing error, and the central axis 2611-2 of the lens 2410-2 has a certain tilt angle with respect to the direction D3. This phenomenon of tilting the rod lenses in a rod lens array is also called lens tilt. This lens tilt occurs irregularly, causing blurring of the image formed on the surface of the photosensitive drum 102 and non-periodic unevenness in the amount of light. While it is possible to exclude rod lens arrays in which lens tilt is detected during pre-shipment inspection from shipment, if the effects of lens tilt can be eliminated by correcting the amount of light, this would be beneficial from the perspective of manufacturing costs, as it would improve yield.

[0107] (3) Misalignment of the light-emitting element array Ideally, each light-emitting element array 300 is mounted on the printed circuit board 202 so that its longitudinal direction is parallel to the longitudinal direction of the rod lens array 203 (i.e., the first direction). However, in reality, the light-emitting element array 300 may be mounted on the printed circuit board 202 with its longitudinal direction tilted relative to the first direction. FIG. 27 shows an example in which two light-emitting element arrays 300-1 and 300-2 are arranged on the printed circuit board 202, similar to FIG. 19. In the example of FIG. 27, the light-emitting element array 300-2 is arranged without error, and its longitudinal direction is parallel to the first direction. On the other hand, the arrangement of the light-emitting element array 300-1 is misaligned due to manufacturing errors, and its longitudinal direction forms an angle θ1 with the first direction. On the other hand, the light-collection efficiency of the rod lens array is highest along the center line of the rod lens array (shown by the dashed line in FIG. 27) and may decrease with increasing distance from the center line. Therefore, when the light emitting element array 300 is misaligned as shown in the figure, linear unevenness in the amount of light is caused within the range of one light emitting element array 300.

[0108] Note that individual differences among the light-emitting element arrays 300 are also one of the causes of uneven light intensity. The uneven light intensity caused by the individual differences among the light-emitting element arrays 300 causes unevenness in the light intensity distribution per chip, similar to the uneven light intensity caused by misalignment of the light-emitting element arrays 300. Therefore, in this embodiment, correction of uneven light intensity caused by individual differences among the light-emitting element arrays 300 is treated together with correction of uneven light intensity caused by misalignment of the light-emitting element arrays 300.

[0109] (4) Complex uneven light intensity caused by multiple factors The unevenness in the amount of light of the exposure head 106 as a whole is a composite unevenness that is contributed by several of the causes exemplified above. Figure 28 shows the unevenness in the amount of light that is contributed by three types of causes, broken down into components according to the cause. Light amount distribution 2801 in the figure is an example of a component of the unevenness in the amount of light that is caused by the properties of the rod lens array. In the light amount distribution 2801, the increase and decrease in the amount of light are repeated periodically along the first direction. Light amount distribution 2802 is an example of a component of the unevenness in the amount of light that is caused by misalignment of the light emitting element array 300. Light amount distribution 2802 shows a linear change in the amount of light in the section corresponding to each light emitting element array 300. For example, at position X 11 From position X 12 The section from position X to position X corresponds to one light emitting element array 300, and in this section the light intensity increases linearly as the position moves. A light intensity distribution 2803 is an example of a component of uneven light intensity caused by lens tilt in the rod lens array 203. In the example of FIG. 28, the light intensity distribution 2803 is 21 and position X 22 It shows a particularly low value in

[0110] The light intensity distribution 2900 in Figure 29 shows a complex light intensity variation resulting from these three factors. The light intensity distribution measured for an actual exposure head 106 follows this complex trajectory. Therefore, if we were to generate correction data that accurately reproduces the light intensity distribution using conventional simple sampling of the light intensity measurement results, we would have to set an extremely large number of data points and store the data for each data point. For example, if the light intensity distribution of an exposure head with an image formation width of 316 mm repeatedly increases and decreases in light intensity at intervals of approximately 0.30 mm, we would need to set data points at intervals of 0.15 mm, which is at least half of that interval, resulting in a total of more than 2,100 data points. Storing the light intensity measurement data for such a large number of data points not only increases memory resources but also causes delays associated with transferring large amounts of data read from memory.

[0111] In contrast, in this embodiment, the fluctuation in light intensity from the reference value (value E0 in FIG. 29) in the light intensity measurement result is decomposed into multiple components, and correction data is generated in advance as a set of parameters representing the characteristics of each component and stored in memory. This reduces the total number of data points in the correction data and suppresses the data size, making it possible to save memory resources and shorten data transfer delays.

[0112] <Basic correction method> Possible methods for correcting unevenness in the amount of light include adjusting the amount of current supplied from the reference current source 2302 of the light-emitting element array 300, and correcting input image data so as to change the local area gradation. This embodiment incorporates both methods. Specifically, the image forming apparatus 1 uses the former correction method to equalize the average amount of light of the light-emitting element array 300, and then precisely corrects the remaining unevenness in the amount of light using the latter correction method.

[0113] (1) Adjusting the amount of current supplied to the light-emitting element The former method can be performed by rewriting the setting value for the amount of supply current in the control data stored in the storage unit 2210. Each light-emitting element array 300 in the exposure head 106 is driven to emit light while changing the drive current, and the light amount is measured using an image sensor with a sufficiently large light-receiving surface. This allows the light amount versus drive current characteristics (IL characteristics) of each light-emitting element array 300 to be obtained. Then, based on the obtained IL characteristics, a setting value for the amount of supply current of each light-emitting element array 300 is determined so that the average light amount is equal across all light-emitting element arrays 300. The determined setting value is written to the storage unit 2210 of each light-emitting element array 300. Then, when the image forming apparatus 1 subsequently attempts to form an image, the difference in average light amount between the light-emitting element arrays 300 is approximately zero. The reference value E0 shown in FIG. 29 may be equal to the value of the average light amount of the light-emitting element array 300 adjusted in this manner.

[0114] (2) Changing the area gradation of image data The latter method, which enables precise correction of light intensity unevenness, will be described below with reference to FIG. 30. FIG. 30(A) shows an example of a small image IM1 around a certain reference pixel position, 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 white 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, with the top left pixel of the small image as the reference, the xth pixel position from the left and the yth pixel position from the top will be represented as (x, y).

[0115] FIG. 30(B) shows an example of a correction matrix IM2. The correction matrix IM2 is a matrix (bitmap) of the same size as the small image IM1, and each square in the figure represents one element. The shaded elements represent pixels to be changed that are selected according to the reduction rate (or increase rate) of the amount of light. There are two types of correction matrices: one for reducing the amount of light and one for increasing the amount of light. The correction matrix IM2 is assumed to be a matrix for reducing the amount of light. As an example, if the amount of light at the reference pixel position is to be reduced by 4% relative to 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. 30(B), the 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, for example, according to a known blue noise mask method.

[0116] If 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 intensity correction unit 2202 changes the pixel to a non-exposure dot (i.e., inverts the pixel value). Figure 30(C) shows an example of the result of correcting the small image IM1 using the correction matrix IM2. In the small image IM3 of Figure 30(C), the pixels at pixel positions (7,5), (2,8), and (8,10), which were exposure dots in the small image IM1, have been changed to non-exposure dots. In a case where the light intensity at a reference pixel position is to be increased, if a pixel at the same position as the selected change pixel indicates a non-exposure dot in the small image IM1, the light intensity correction unit 2202 changes the pixel to an exposure dot. While scanning the reference pixel positions in the input image data, the light intensity correction unit 2202 repeatedly corrects the small image set at each reference pixel position according to the fluctuations in light intensity restored based on the correction data read from the storage unit 2210. Specifically, the light intensity correction unit 2202 changes the area light intensity of the small image at each reference pixel position so as to absorb fluctuations in the restored light intensity, thereby achieving precise correction of uneven light intensity in the first direction.

[0117] (3) Increase or decrease the light intensity with a resolution of 1 / M In this embodiment, as described in relation to FIG. 21 , a spot corresponding to one pixel position is formed by sequentially emitting light from M light-emitting elements 350 in each column of the light-emitting element array. Therefore, by changing one or more of the M light-emitting elements to non-emitting light, it is possible to reduce the amount of light with a resolution of 1 / M of the pixel pitch. Similarly, by changing one or more of the M non-emitting light-emitting elements to emitting light, it is possible to increase the amount of light with a resolution of 1 / M of the pixel pitch. Therefore, the light amount correction unit 2202 may increase or decrease the amount of light (change the emitting or non-emitting state of each of the M light-emitting elements) with this 1 / M resolution, in addition to changing the area gradation described above, so as to absorb fluctuations in the amount of light in the light amount distribution restored based on the correction data.

[0118] <Example of correction data configuration> In this embodiment, the light intensity distribution of the exposure head 106 is measured during an inspection stage before product shipment. Then, based on the measurement results, correction data is generated that enables subsequent restoration of variations in light intensity along the first direction in the obtained light intensity distribution. The generated correction data is stored in the storage unit 2210. It is assumed that the average light intensity of the light-emitting element array 300 of the exposure head 106 is equalized through adjustment of the amount of supply current prior to measurement of the light intensity distribution. An example of the configuration of correction data generated based on the measurement results of the light intensity distribution will be described below.

[0119] In this embodiment, the correction data may include the following three types of component data.

[0120] Frequency component data Linear component data Irregular component data However, if the residual component obtained by subtracting the frequency component from the fluctuation in the amount of light is zero or very small, the linear component data and the irregular component data do not need to be included in the correction data.

[0121] The frequency component data is data related to one or more frequency components of the light intensity distribution. The frequency component data can be obtained by analyzing the spatial frequency of the light intensity distribution. For example, one or more frequency components contained in the light intensity distribution can be extracted by performing a Fourier transform on the light intensity distribution. Typically, the first frequency component resulting from the properties of the rod lens array 203 including the plurality of lenses 2410 described above is the main frequency component, and its frequency may correspond to the pitch of the lenses 2410. Alternatively, the one or more extracted frequency components may also include a second frequency component having a frequency corresponding to the pitch of the light-emitting elements 350. The frequency component data includes amplitude and phase values ​​of each frequency component at at least one data point. Although the frequency value can be derived from the known pitch, the frequency component data may also include the frequency value.

[0122] When the periodic light intensity unevenness of the rod lens array 203 traces an ideal sinusoidal wave, the fluctuation in the light intensity can be expressed by the amplitude and phase values ​​at one data point, and the fluctuation in the light intensity at all pixel positions can be reproduced from these amplitude and phase values. However, considering the discontinuity of the light source for each light-emitting element array, it is desirable to set at least one data point for each light-emitting element array 300 and retain the amplitude and phase values ​​at that data point. Alternatively, data points may be set at two locations, for example, at both ends of each light-emitting element array 300, or at three locations, for example, at both ends and the center of each light-emitting element array 300.

[0123] The linear component data is data related to the linear components of the interval corresponding to each light-emitting element array 300 among the residual components obtained by subtracting the frequency components represented by the frequency component data from the fluctuation in the amount of light. Typically, the linear components are mainly caused by misalignment of each light-emitting element array 300. At least two data points of the linear component data are set in the interval corresponding to each light-emitting element array 300. By retaining offset values ​​(intercepts) at these data points as the linear component data, it is possible to reproduce the linear components of the fluctuation in the amount of light at all pixel positions within the interval by linear interpolation. Alternatively, the linear component data may include the offset value and the slope of the amount of light at one data point set for each light-emitting element array 300.

[0124] The irregular component data is data related to residual components obtained by removing the above-described frequency components and linear components from fluctuations in light intensity. In particular, the irregular component data in this embodiment represents irregular components caused by lens tilt detected during inspection of the rod lens array 203. The irregular component data includes, for example, at least one irregular component position in the first direction and an offset value corresponding to each irregular component position.

[0125] For example, suppose there are two or three data points for the frequency component data per light-emitting element array 300, two data points for the linear component data per light-emitting element array 300, and ten data points for the irregular component data across the entire exposure head 106. In this embodiment, 20 light-emitting element arrays 300 are attached to the exposure head 106, so the total number of data points is 90 or 110. Therefore, in this case, the number of data points is reduced to 6% or less compared to the number of correction points exceeding 2,100 required in a conventional method based on simple sampling and interpolation. Note that the data points for the frequency component data and the data points for the linear component data can be shared, in which case the number of data points can be further reduced.

[0126] <Restoring fluctuations in light intensity and correcting image data> When an image is formed in the image forming apparatus 1, the light intensity correction unit 2202 restores one or more frequency components of the fluctuations in light intensity along the first direction from the frequency component data included in the correction data read from the storage unit 2210. The light intensity correction unit 2202 also restores linear components of sections corresponding to each light-emitting element array 300 from the linear component data included in the correction data. The light intensity correction unit 2202 also restores irregular components from the irregular component data included in the correction data. The light intensity correction unit 2202 calculates a light intensity correction coefficient by combining the restored frequency components, linear components, and irregular components for each pixel position scanned along the first direction. The calculated correction coefficient represents the rate of increase or decrease in light intensity for correcting uneven light intensity. The light intensity correction unit 2202 corrects the input image data by changing the areal light intensity of the small image at each pixel position according to the calculated correction coefficient. At this time, the light intensity correction unit 2202 may change the area light intensity with fine resolution by changing the light emitting or non-emitting state of each of the M light emitting elements 350 corresponding to one pixel. Then, the light intensity correction unit 2202 outputs the corrected image data to the conversion unit 2203.

[0127] <Manufacturing of exposure equipment> FIG. 31 is a flowchart showing an example of a procedure for manufacturing an exposure apparatus according to this embodiment.

[0128] First, in step S11, which is an assembly process performed in a factory, various circuit elements including a plurality of light-emitting element arrays 300 are attached to the printed circuit board 202 of the exposure head 106. In addition, the printed circuit board 202 and the rod lens array 203 are attached to the housing 204. The storage section (memory) 810 may be attached to the printed circuit board 202 in a state where it is mounted on the light-emitting element array 300, or may be attached to the printed circuit board 202 separately. After the exposure head 106 is assembled in this manner, the spot size on the imaging plane of the rod lens array 203 may be measured, the focus may be adjusted, and the attachment position of the rod lens array 203 may be adjusted.

[0129] Next, in step S12, the light intensity of each light-emitting element array 300 of the exposure head 106 is measured, and the average light intensity is calculated for each light-emitting element array 300. Next, in step S13, a setting value for the amount of supply current to equalize the average light intensity among the light-emitting element arrays 300 is determined, and the determined setting value is stored in the storage unit 2210 of each light-emitting element array 300.

[0130] Next, in step S14, with the average light intensity uniformed among the light-emitting element array 300, the light intensity from the exposure head 106 is measured along a first direction parallel to the axial direction D1 of the photosensitive drum 102, thereby obtaining the light intensity distribution of the entire exposure head 106. For example, the light intensity distribution may be obtained by scanning an image sensor having a slit with a certain opening width in the longitudinal direction of the exposure head 106. Alternatively, the light intensity distribution may be obtained by sequentially emitting light from the light-emitting elements 350 of the light-emitting element array 300 while keeping the image sensor fixed. It is desirable that the light intensity measurement here be performed with a spatial resolution of at least 1 / 10 of the pitch P1 of the lenses 2410, for example.

[0131] Next, in step S15, the light intensity distribution obtained as the light intensity measurement result is analyzed (e.g., through a Fourier transform) to generate frequency component data related to one or more frequency components of the light intensity distribution. The generated frequency component data may include an amplitude value and a phase value at at least one data point for each of one or more significant frequency components. Next, in step S16, a residual component is obtained by removing the frequency component reconstructed from the frequency component data (e.g., through an inverse Fourier transform) from the fluctuations in light intensity indicated by the light intensity distribution.

[0132] Next, in step S17, linear component data for each light-emitting element array is generated from the residual component of the fluctuation in light intensity. The linear component data generated here may include offset values ​​at at least two data points set in the section corresponding to each light-emitting element array 300. Furthermore, in step S18, irregular component data related to the irregular component measured for each of the multiple lenses 2410 of the rod lens array 203 is generated. If lens tilt is not detected in the rod lens array 203, step S18 may be omitted.

[0133] Next, in step S19, correction data that may include the frequency component data generated in step S15, the linear component data generated in step S17, and the irregular component data generated in step S18 is stored in the storage unit 2210 of each light-emitting element array 300.

[0134] 31, the printed circuit board 202 of the exposure head 106 is connected to the image controller 2200 via multiple signal lines. The image forming apparatus 1 may be manufactured by incorporating the exposure device manufactured in this manner into the image forming unit 101, which may include the photosensitive drum 102 and other components.

[0135] <Summary of the second embodiment> Various embodiments have been described in detail above using FIGS. 1 to 31. According to the above-described embodiments, the light intensity distribution of an exposure head having a rod lens array is measured, and frequency component data related to the frequency components of the measured light intensity distribution is generated by analyzing the measured light intensity distribution. Correction data including the generated frequency component data is then stored in memory. Image data for driving the light-emitting element array of the exposure head is then corrected according to the fluctuations in light intensity restored based on the frequency component data in the correction data read from memory. Therefore, when the light intensity distribution of the exposure head traces a trajectory that repeatedly increases and decreases along the direction of lens arrangement in the rod lens array, the fluctuations in light intensity can be represented as a set of parameters at a relatively small number of data points. This saves memory resources required for storing correction data, thereby reducing the manufacturing cost of the device. Furthermore, the delay in data transfer associated with reading the correction data is reduced, thereby shortening the time required to start up the device or execute a job.

[0136] In the above-described embodiment, the frequency components may include a frequency component corresponding to the pitch of the arrangement of the lenses in the rod lens array. By including the amplitude and phase values ​​of this frequency component at at least one data point in the correction data, it is possible to satisfactorily represent periodic unevenness in the amount of light caused by the properties of the rod lens array without requiring large data sizes.

[0137] In the above-described embodiment, the frequency components may also include frequency components corresponding to the pitch of the arrangement of the plurality of light-emitting element arrays in the exposure head. By including in the correction data the amplitude and phase values ​​of these frequency components at data points set in sections corresponding to each light-emitting element array, it is possible to accurately represent periodic unevenness in the amount of light that may be affected by individual differences between the individual light-emitting element arrays.

[0138] In the above-described embodiment, the correction data may further include linear component data related to the linear component measured for each light-emitting element array. By including an offset value of this linear component at a data point set in a section corresponding to each light-emitting element array in the correction data, it is possible to accurately represent the partially linear unevenness in light intensity caused by misalignment of each light-emitting element array. Therefore, it is possible to accurately correct the unevenness in light intensity by taking into account the residual component remaining after removing the frequency component from the fluctuation in light intensity in the light intensity distribution of the exposure head.

[0139] In the above-described embodiment, the correction data may further include irregular component data related to irregular components measured for each of the plurality of lenses. For example, by expressing residual components of irregular light intensity unevenness, which occurs infrequently but has no regularity, such as irregularity caused by lens tilt that may occur accidentally during the manufacturing process of the rod lens array, as irregular component data, the accuracy of the correction of the light intensity unevenness can be further improved.

[0140] In the above-described embodiment, the image data can be corrected by changing the area light intensity of the small image set at each pixel position of the image data so as to absorb fluctuations in the light intensity distribution. In this way, correcting the unevenness in the light intensity in the digital domain at a stage prior to the distribution of data to the light-emitting element arrays can avoid complicating the circuit configuration for driving the light-emitting element arrays and reduce manufacturing costs. Furthermore, adjusting the area light intensity by changing the light-emitting or non-light-emitting state of each of the M light-emitting elements involved in forming one imaging spot enables correction of the unevenness in the light intensity with high resolution, further improving image quality.

[0141] In the above-described embodiment, specific numerical values ​​are used for the purpose of 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 element arrays provided on one printed circuit board is not limited to 20 and can be any number equal to or greater than 1. Furthermore, the size of the light-emitting element arrangement of each light-emitting element array 300 is not limited to 4 rows and 748 columns and may be any other size. Furthermore, the circumferential pitch and axial pitch of the light-emitting elements are not limited to approximately 21.16 μm and approximately 5 μm and may be any other value.

[0142] <Other embodiments> The above-described 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, or by a circuit (e.g., ASIC) that realizes one or more functions.

[0143] The invention is not limited to the above-described embodiments, and various changes and modifications can be made without departing from the spirit and scope of the invention. Accordingly, the following claims are appended to apprise the public of the scope of the invention. [Explanation of symbols]

[0144] 102: photosensitive drum, 106: exposure head, 610: information storage unit, 615: streak correction unit

Claims

1. an exposure head that includes a plurality of light-emitting elements arranged in a direction intersecting the rotation direction of a photosensitive member that is driven to rotate, and a rod lens array that forms an image on the photosensitive member using light output from each of the plurality of light-emitting elements based on image data; and a storage means for storing correction data for correcting streak images; a correction means for correcting the image data based on the correction data read from the storage means; and the exposure head includes a plurality of light-emitting chips regularly arranged in a first direction parallel to an axial direction of the photosensitive member; Each of the plurality of light-emitting chips includes the plurality of light-emitting elements regularly arranged in at least the first direction, the correction data includes frequency component data associated with one or more frequency components of the light intensity distribution measured along the first direction, the frequency component data being obtained by analyzing the light intensity distribution measured along the first direction; The image forming apparatus includes: an image corrector that corrects the image data in accordance with a variation in the amount of light in the light amount distribution, the variation being restored based on the frequency component data included in the correction data.

2. the correction data includes characteristic information indicating an imaging characteristic of the rod lens array, 2. The image forming apparatus according to claim 1, wherein the correction unit corrects the image data for driving the plurality of light-emitting elements based on the characteristic information read from the storage unit, thereby reducing streaky density unevenness that occurs in the latent image.

3. The correction means Identifying pixels to be corrected in the image data based on the characteristic information; The image forming apparatus according to claim 2 , wherein the pixel data of the identified pixel is corrected based on the characteristic information.

4. The characteristic information is position information indicating the position of the pixel to be corrected among a plurality of pixels constituting the image data; a correction level to be applied to pixel data of the pixel to be corrected; The image forming apparatus according to claim 3 , comprising:

5. 5. The image forming apparatus according to claim 4, wherein the position information includes information indicating the position of a first pixel in a series of pixels to be corrected, among the plurality of pixels constituting the image data.

6. The image forming apparatus according to claim 5 , wherein the characteristic information includes the correction level for each of the series of pixels to be corrected.

7. The image forming apparatus according to claim 6 , wherein the characteristic information includes one correction level for each pixel in the series of pixels to be corrected.

8. The image forming apparatus according to claim 6 , wherein the characteristic information includes one common correction level for N (N is an integer of 2 or more) pixels among the series of pixels to be corrected.

9. The correction means calculating a correction value for each pixel based on the correction level; The image forming apparatus according to claim 6 , wherein correction processing is performed on the image data for each pixel based on the correction value calculated for each pixel.

10. 10. The image forming apparatus according to claim 9, wherein the correction unit corrects the pixel value of the pixel of interest by adding the correction value determined for the pixel of interest to the pixel value of the pixel of interest.

11. 10. The image forming apparatus according to claim 9, wherein the correction unit acquires the corrected pixel value of the pixel of interest by referring to a table based on the pixel value of the pixel of interest and the correction value calculated for the pixel of interest.

12. 10. The image forming apparatus according to claim 9, wherein the correction means applies the second correction value to the pixel value of the target pixel when a predetermined combination of a first correction value calculated for a previous pixel located before the target pixel in the main scanning direction, a second correction value calculated for the target pixel, and a third correction value calculated for a subsequent pixel located after the target pixel in the main scanning direction is obtained.

13. The correction means multiplying a first correction value obtained for a previous pixel located before the pixel of interest in the main scanning direction by a first filter coefficient to calculate a first product; multiplying the second correction value calculated for the pixel of interest by a second filter coefficient to calculate a second product; multiplying a third correction value obtained for a subsequent pixel located subsequent to the pixel of interest in the main scanning direction by a third filter coefficient to calculate a third product; summing the first product, the second product, and the third product to determine a total correction value; The image forming apparatus according to claim 9 , wherein the pixel value of the pixel of interest is corrected by the total correction value.

14. The correction means Counting the main scanning position based on the clock signal, determining whether the counted main scanning position and the position of the pixel to be corrected coincide with each other; 10. The image forming apparatus according to claim 9, wherein calculation of the correction value is started when a predetermined signal indicating that the counted main scanning position and the position of the pixel to be corrected match is output.

15. 2. The image forming apparatus according to claim 1, wherein said storage means is provided in said exposure head.

16. the correction data further includes linear component data associated with a linear component measured for each of the plurality of light-emitting chips; 2. The image forming apparatus according to claim 1, wherein the correcting means restores the fluctuation in the light amount in the light amount distribution further based on the linear component data included in the correction data.

17. 2. The image forming apparatus according to claim 1, wherein the correction means corrects the image data by changing the area light quantity of the small image set at each pixel position so as to absorb the fluctuation in the light quantity in the light quantity distribution.

18. a photosensitive member that is driven to rotate; an exposure means for exposing the photosensitive member, The exposure means an exposure head that includes a plurality of light-emitting elements arranged in a direction intersecting the rotation direction of the photosensitive member, and a rod lens array that forms an image on the photosensitive member using light output from each of the plurality of light-emitting elements based on image data; and a storage means for storing correction data for correcting streak images; a correction unit that corrects the image data based on the correction data read from the storage unit, the exposure head includes a plurality of light-emitting chips regularly arranged in a first direction parallel to an axial direction of the photosensitive member; Each of the plurality of light-emitting chips includes the plurality of light-emitting elements regularly arranged in at least the first direction, the correction data includes frequency component data associated with one or more frequency components of the light intensity distribution measured along the first direction, the frequency component data being obtained by analyzing the light intensity distribution measured along the first direction; The image forming apparatus includes: an image corrector that corrects the image data in accordance with a variation in the amount of light in the light amount distribution, the variation being restored based on the frequency component data included in the correction data.

Images