JP7769655B2 - Image forming device - Google Patents

Description

The present invention relates to an image forming apparatus.

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 specified size on the photosensitive drum. A number of rod lenses are aligned 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.

Japanese Patent Application Laid-Open No. 2008-275692

In the past, non-defective rod lens arrays 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, the present invention aims to prevent increases in the cost of image forming devices.

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;
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 provides an image forming apparatus in which the correction means corrects the image data in accordance with fluctuations in the amount of light in the light amount distribution, which is restored based on the frequency component data included in the correction data .

This invention makes it possible to prevent increases in the cost of image forming devices.

FIG. 1 is a diagram illustrating an image forming apparatus. FIG. 2 is a diagram illustrating the arrangement of a photosensitive drum and an exposure head. FIG. FIG. 2 is a diagram illustrating a light-emitting element array. FIG. 10 is a diagram illustrating an arrangement of light-emitting elements. FIG. 1 is a block diagram showing an image controller and a printed circuit board. Block diagram showing the digital section. FIG. FIG. 3 is a diagram illustrating a lighting control unit. FIG. FIG. FIG. 10 is a diagram illustrating a streaky image. FIG. FIG. 3 is a block diagram of a streak correction unit. FIG. 6 is a timing chart showing the operation of the streak correction unit. 10A and 10B are diagrams illustrating correction values for combinations of streak ranks and image data. FIG. 10 is a diagram illustrating another example of a correction method. 1 is an explanatory diagram of a light-emitting element array and a light-emitting element group in the light-emitting element array according to an embodiment; FIG. 1 is a cross-sectional view showing a schematic configuration of a light-emitting element array according to an embodiment. FIG. 10 is an explanatory diagram of multiple exposure using light-emitting elements arranged in a stepped pattern. 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. FIG. 2 is a block diagram showing a configuration of a circuit related to supplying current to a light-emitting element array. FIG. 2 is an explanatory diagram illustrating the arrangement of a plurality of lenses in a rod lens array. 1 is a diagram illustrating the periodic nature of light passing through a rod lens array. FIG. 10 is a schematic cross-sectional view showing an example of the inclination of lenses in a rod lens array. FIG. 10 is a schematic plan view showing an example of misalignment of a light-emitting element array on a printed circuit board. 10 is an explanatory diagram of components of uneven light quantity contributed by three types of causes. 10 is an explanatory diagram of complex unevenness in light amount caused by three types of causes. 10A and 10B are explanatory diagrams illustrating a method for correcting uneven light quantity by changing local area gradation. 10 is a flowchart showing an example of a procedure for manufacturing an exposure apparatus according to an embodiment.

The following describes the embodiments in detail with reference to the attached drawings. Note that the following embodiments do not limit the scope of the claimed invention. While the embodiments describe multiple features, not all of these features are necessarily essential to the invention, and multiple features may be combined in any desired manner. Furthermore, in the attached drawings, the same reference numbers are used to designate identical or similar components, and redundant explanations will be omitted.

(First embodiment)
<Image forming apparatus>
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.

The scanner unit 100 is an original reading device that illuminates an original placed on a platen, optically reads the original image, and converts the read results into electrical signals to create image data. The printer engine 103 forms a toner image on the sheet P. The printer engine 103 rotates the photosensitive drum 102. The charger 107 charges the surface of the photosensitive drum 102 so that the surface potential of the photosensitive drum 102 is uniform. The exposure head 106 exposes the surface of the photosensitive drum 102 to light corresponding to the 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 the transfer belt 111. This causes the toner image to be transferred from the photosensitive drum 102 to the sheet P.

The printer engine 103 has four imaging units 101C, 101M, 101Y, and 101K, which correspond to the toner colors cyan, magenta, yellow, and black (CMYK). The four imaging 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.

The feeding unit 105 feeds the sheet P from a pre-specified feeding device among feeding devices 109a and 109b provided in the main body of the image forming apparatus 1, feeding device 109c provided outside the main body, and manual feeding device 109d. The fed sheet P is transported to the registration rollers 110. The registration rollers 110 transport the 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 the sheet P. The transfer belt 111 transports the sheet P with the transferred toner image to the fixing device 104.

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

<Exposure head>
Fig. 2A is a perspective view of the exposure head 106 that exposes the photosensitive drum 102. Fig. 2B is a schematic cross-sectional view of the photosensitive drum 102 and the exposure head 106. The 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 the group of light-emitting elements 201 mounted on the printed circuit board 202 is collected by the rod lens array 203 and irradiated onto the surface of the photosensitive drum 102. The printed circuit board 202 and the rod lens array 203 are fixed to the housing 204.

The exposure head 106 is positioned 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 focuses the light onto the surface of the photosensitive drum 102. In this specification, the direction parallel to the axial direction D1 of the photosensitive drum 102 is referred to as the first direction, and the direction perpendicular to the first direction on the surface of the printed circuit board 202 (approximately parallel to the circumferential direction D2) is referred to as the second direction.

The exposure head 106 is assembled and adjusted individually. The adjustments include spot size adjustment (focus adjustment) at the light collection position and light intensity adjustment. In focus adjustment, the mounting 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 collected via the rod lens array 203 is a predetermined light intensity.

<Configuration of Light-Emitting Device Group>
3A shows a non-mounting surface 301 of the printed circuit board 202. No light-emitting elements are mounted on the non-mounting surface 301, but other electronic components such as a connector 305 are mounted thereon. The connector 305 is connected to cables (power lines and signal lines) that transmit various signals such as clock signals to the printed circuit board 202.

As shown in Figure 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. The following describes the case where m = 20 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 light-emitting element array 300.

As shown in Figure 3(C), each of the light-emitting element arrays 300-1 to 300-m has multiple light-emitting elements 350 arranged along the longitudinal direction of the light-emitting element array 300.

As shown in Figure 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.

As shown in Figure 3(C), the distance between two adjacent light-emitting elements 350 in a given light-emitting element array 300 is L. Distance L is the distance in the longitudinal direction of the light-emitting element array 300. At a resolution of 1200 dpi, L = approximately 21.16 μm. This distance corresponds to one pixel at 1200 dpi. Note that the distance between the right-most light-emitting element 350 of the i-th light-emitting element array 300-i and the left-most light-emitting element 350 of the i+1-th light-emitting element array 300-i+1 is also L, where i is any integer from 1 to m-1. As shown in Figure 3(C), in the transverse direction of the light-emitting element array 300, the distance S between the right-most light-emitting element 350 of the light-emitting element array 300-i and the left-most light-emitting element 350 of the light-emitting element array 300-i+1 is approximately 105 μm. This corresponds to the distance of five pixels at 1200 dpi. Note that distances L and S are just examples.

<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 contains a 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.

<Light-emitting part>
5 shows the light emitting element row that constitutes the light emitting unit 404. The light emitting unit 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.

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 account 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.

<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 the single color K of YMCK will be described. Similar processing and circuit configurations are also used for YMC.

The image controller 600 is a control circuit that generates and transmits signals to the printed circuit board 202 for controlling 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 the clock unit 608 and serves as a reference signal for the operation 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 image writing in the sub-scanning direction. The communication signal com is a communication signal sent 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, etc. via the 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 devices via 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 to the information storage unit 610 and the light-emitting element arrays 300-1 to 300-m via communication signal line 609.

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. 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.

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 mentioned 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 for 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.

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, synchronization unit 604, conversion unit 602, and printed circuit board 202. The CPU 603 executes the following processing 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.

The CPU 603 determines the generation cycle of the line synchronization signal lsync_x. The generation cycle is calculated, for example, based on the rotational speed of the photosensitive drum 102 (speed information at which the surface of the photosensitive drum 102 moves in the rotational 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.

The conversion unit 602 divides one line of image data output from the image data unit 601 via the streak correction unit 615 into m pieces of image data, data_1 to data_m. 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.

The synchronization unit 604 generates the 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.

On the printed circuit board 202, the light-emitting element array 300-i operates by receiving the line synchronization signal lsync_x, clock signal clk, image data data_i, and communication signal com. The information storage unit 610 is a memory circuit that stores head information. The head information includes information such as 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 the communication signal line 609 to read head information and write setting information. The information storage unit 610 may also store the setting value of the drive current adjusted during the exposure head 106 assembly process.

As shown in Figure 6, the clock signal line 605, communication signal line 609, and synchronization signal line 606 are connected to all light-emitting element arrays 300. The 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.

<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.

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

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 signal line 707-1. The lighting control unit 705-1 acquires 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 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 signal line 707-j+1. In this way, lighting control unit 705-1 receives a timing signal directly from timing unit 704, while lighting control units 705-2 to 705-n receive timing signals from the preceding lighting control units 705-1 to 705-n-1, respectively.

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

<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.

Delay circuit 801 is connected to synchronization signal line 606 and clock signal line 605, and delays the line synchronization signal lsync_x transmitted by synchronization signal line 606 by one cycle before outputting it to logic gate 802. Delay circuit 801 is implemented, for example, by a flip-flop circuit.

Logic gate 802 calculates the logical product (AND) of line synchronization signal lsync_x and the signal obtained by inverting the output signal of delay circuit 801 using inverter 803, to generate timing signal we[0]. Timing signal we[0] is output to signal line 707-1.

Figure 8 (B) 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 period equivalent to one cycle of the clock signal clk, and then returns to low.

<Details of 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. j is an integer between 1 and n. The signal line 707-j and the clock signal line 605 are connected to the delay circuit 901. 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.

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 it 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 the circuit that captures the image data data_i, but this is merely an example. Any circuit that can hold the image data data_i from the time the timing signal we[j] goes High until the next time the timing signal we[j] goes High may be used. For example, a flip-flop circuit may be used instead of the latch circuit 902.

Figure 9(B) is a timing chart of delay circuit 901. Timing signal we[j] is delayed by one cycle of clock signal clk to generate timing signal we[j+1].

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

<Details of the analog section>
10 is a block diagram of the analog unit 750. For the sake of 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.

Drive circuit 1001-j is a circuit that drives the jth light-emitting element 350-j. A lighting signal el[j] is supplied to drive circuit 1001-j via lighting signal line 708-j.

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.

The selection circuit 1007 generates a select signal that selects 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 causes only the signal line connected to one selected drive circuit 1001 out of the n drive circuits 1001-1 to 1001-n to go high. 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 for 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 goes 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.

<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.

MOSFET Q1 supplies a drive current to light-emitting element 350-j in accordance with the gate voltage applied to its gate. When the gate voltage is low, the drive current decreases, and light-emitting element 350-j turns off. An illumination signal line 708-j is connected to the gate of MOSFET Q2. MOSFET Q2 turns on when illumination signal el[j] is high, and transfers the voltage charged in capacitor C1 to MOSFET Q1. A signal line 1004-j is connected to the gate of MOSFET Q3. MOSFET Q3 turns on/off in accordance with the select signal from selection circuit 1007. In other words, MOSFET Q3 turns on when the select signal is high, and applies the analog voltage output from DAC 1002 to capacitor C1, charging it. In this embodiment, before image formation, DAC 1002 sets an analog voltage in capacitor C1. During the image formation period, MOSFET Q3 is turned off and capacitor C1 continuously maintains a voltage level. This causes MOSFET Q1 to supply or stop supplying a drive current corresponding to the set analog voltage to light-emitting element 350-1 in response to the lighting signal.

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

[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. Also, 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 use of resources. 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.

Figure 13 shows an example of streak information stored in the information storage unit 610. According to Figure 13, the streak information includes individual information for each streak that occurs at a specific main scanning position. In this example, the individual information includes the streak number, streak start position, and 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 Figure 13 indicate main scanning positions assuming a toner image is formed at 2400 dpi. For example, the streak start position of the streak assigned streak number "1" is 100 [pix]. Pix is an abbreviation for pixel. The streak start position of the 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 stripe information pieces may differ for each exposure head 106.

In some cases, the exposure head 106 supports 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 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.

The streak rank is information indicating the degree of streak-like density unevenness measured in advance. The streak rank may be expressed, for example, using 4 bits. In this case, the streak rank can indicate 16 levels of correction value. 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 individual pixels that make up this streak would require a large amount of storage space. Therefore, in this embodiment, the streak start position and streak rank can be used to identify multiple correction target pixels corresponding to the streak width. In the example of Figure 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 streak ranks, one streak rank may be held for every two, four, or eight pixels.

[Streak Correction]
As illustrated in FIG. 14 , the streak correction unit 615 includes a determination unit 1401, a correction calculation unit 1402, and a correction processing unit 1403. The determination 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 determination unit 1401 using a communication signal com. The determination unit 1401 counts the pixel data in the main scanning direction. For example, the determination unit 1401 may include a counter that counts clock signals clk. When the count value matches the streak start position, the determination unit 1401 outputs a determination signal indicating that the streak start position has been determined to the correction calculation unit 1402. For example, the determination 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.

The CPU 603 extracts a streak rank from the streak information and sets the streak rank in the correction calculation unit 1402 using the 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 the target for correction. 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 the pixel data that make up the image data one by one in sequence to the correction processing unit 1403 based on the clock signal clk.

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 consisting of a series of pixel data output from the correction processing unit 1403 is output to the conversion unit 602.
[Specific motion 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 one 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 comparison circuit 1503 outputs a high-level specification 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 specification signal det changes from low to high. Also, in the example shown in FIG. 16A , the specification signal det is maintained high for 16 pixels. This indicates that 16 pixels, including the pixel corresponding to the streak start position, are specified 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.

[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 a correction value by referencing 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 a 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 each of the density values (brightness values) from 1 to 254.

Figure 16 (B) 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. Because 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. 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.

[Streak Correction Processing]
FIG. 16C is a timing chart showing the operation of the correction processing unit 1403. Since 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 merely an 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.

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.

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.

(1) Streak Correction Processing Using 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.

(2) Streak Correction Processing by Filtering Similar to pattern matching, filtering generates an output pixel using correction values of pixels before and after a pixel of interest. As an example, a pixel after a pixel before a pixel adjacent to the pixel of interest in the scanning direction is focused on. That is, the pixel value of the output pixel is determined from the correction values of three pixels, including the pixel of interest, and a filter coefficient. For example, a 3×1 filter is used as the filter, which is applied to a 3×1 pixel group consisting of three pixels in the main scanning direction and one pixel in the sub-scanning direction. As an 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 interest is 5 and the filter coefficient is 20%. In this case, the correction value of the pixel of interest is calculated as follows:

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

<Summary of First Embodiment>
As shown in FIG. 1 , the photosensitive drum 102 is an example of a rotationally driven photosensitive element. 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 element. 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 element. The exposure head 106 is an example of an exposure head that forms a latent image on the photosensitive element. The information storage unit 610 is an example of a memory unit that stores characteristic information (e.g., streak information) indicating 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 makes it possible to mount rod lens arrays 203, which would previously have been discarded, on the exposure head 106. In other words, it is possible to promote the effective use of resources related to the rod lens array 203.

The identification unit 1401 is an example of a 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.

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 the correction level to be applied to the pixel data of the pixel to be corrected (e.g., streak rank).

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

The characteristic information may include a correction level (e.g., 16 streak ranks) for each of the series of pixels to be corrected. As shown 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 greater than or equal to 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.

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 performs correction processing on image data for each pixel based on the correction value calculated for each pixel.

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.

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

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, second correction value, and 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, second correction value, and third correction value are not in a predetermined pattern.

The correction processing unit 1403 may calculate a first product by multiplying the 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 the 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 the 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, second product, and third product, and correct the pixel value of the pixel of interest with the total correction value.

The counter 1502 may function as a counting unit that counts the main scanning position based on a clock signal. The comparison circuit 1503 may function as a judgment unit that determines whether the main scanning position counted by the counting unit matches the position of the pixel to be corrected. The CPU 603 or the correction calculation unit 1402 begins 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 counting unit matches the position of the pixel to be corrected.

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

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 arranged regularly 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, with the two lines being positioned differently in the second direction. 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.

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 2,992 (= 748 x 4) light-emitting elements 350. A light-emitting element group 201 consisting of 20 light-emitting element arrays 300 has a total of 14,960 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 1,200 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 4,800 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 avoid 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. Note that the number of columns or 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.

FIG. 4 will be referenced again in the second embodiment. FIG. 4 is a plan view showing the schematic configuration of the light-emitting element array 300. The multiple 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. The light-emitting substrate 402 is also provided with a circuit section 406 for driving the multiple light-emitting elements 350. Multiple 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 section 406. The signal lines, power lines, and ground lines may be wires made of, for example, gold.

Figure 20 shows a portion of the cross section taken along line A-A in Figure 4. Multiple lower electrodes 2004 are formed on the 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 multiple 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 region 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 erroneously emitting light.

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 may 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 may be any type of LED (Light-Emitting Diode).

The upper electrode 2008 is composed 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 Figure 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 the partial area through which light from each light-emitting element 350 passes transmits the emission wavelength.

20, one continuous light-emitting layer 2006 is formed, but multiple light-emitting layers 2006, each having a width equal 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 multiple lower electrodes 2004, but multiple upper electrodes 2008, each having a width equal to that of the lower electrode 2004, may be formed corresponding to each lower electrode 2004. Also, of the lower electrodes 2004 of each light-emitting element array 300, a first plurality of lower electrodes 2004 may be covered by a first light-emitting layer 2006, and a second plurality of lower electrodes 2004 may be covered by a second light-emitting layer 2006. Similarly, a first upper electrode 2008 may be formed in common 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 formed in common 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.

<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 a plurality of 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.

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 column of the light-emitting element array may be arranged in a stepped pattern at a constant pitch. An example of the arrangement of light-emitting elements when M=4 is partially shown here. R _{j_m} (j={1, 2, ..., N}, m={0, 1, 2, 3}) shown in FIG. 21 represents the light-emitting element 350 in the jth column from the left in the first direction and the mth 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. The minimum pitch d3 of the light emitting elements 350 in the first direction may be about 5 μm (equivalent to 4800 dpi), as described above.

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. 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 i-th line of the input image data indicates "on," light-emitting elements _{R1_1} , _{R1_2} , _{R1_3} , and _{R1_4} sequentially emit light at timings corresponding to line L _i on the surface of the photosensitive drum 102. As a result, the spot area at the left end of line L _i is multiplexedly exposed, forming a corresponding spot _SP1 . Similarly, if the jth pixel value from the left of the ith line of the input image data indicates light-on, the light-emitting elements _{Rj_1} , _{Rj_2} , _{Rj_3} , and _{Rj_4} sequentially emit light at timings corresponding to the line _Lj on the surface of the photosensitive drum 102. As a result, the jth spot area from the left of the line _Lj is multiplexedly exposed, forming the corresponding spot _SPj . If the pitch of the light-emitting elements in the second direction is approximately 21.16 μm and the sheet conveyance speed is 200 mm/s, the period (line period) at which each line is exposed by one light-emitting element _{Rj_m} can be approximately 105.8 μs. In this way, by the four light-emitting elements in each column of the 20-element light-emitting element array 300 sequentially emitting light at appropriate timings, smooth lines of an electrostatic latent image can be formed on the surface of the photosensitive drum 102, consisting of a series of spots that partially overlap each other at regular spot intervals. As a result of these lines being continuously formed in the circumferential direction, a two-dimensional electrostatic latent image is created.

In the following description, the spot areas where spots of the electrostatic latent image are formed as a result of multiple exposure due to the emission of M light-emitting elements 350 are also referred to as "exposed dots," and the spot areas where no spots are formed because the light-emitting elements 350 do not emit light are also referred to as "non-exposed dots."

<Configuration of control circuit>
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 the sake of simplicity, the processing for a single color component will be described here, but in reality, similar processing is performed in parallel for four color components.

The image controller 2200 is a component that, together with the exposure head 106, constitutes the exposure device. 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 2205-2209. 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 image data DATA. The synchronization signal line 2208 carries a line synchronization signal Lsync that identifies the line period of the image data. The communication signal line 2209 carries a control signal CTL.

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 emission of the light-emitting elements 350 of 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 that indicate whether or not to emit light to the corresponding M light-emitting elements 350 at each pixel position that constitutes the image to be formed. If the bit at a certain 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.

The light intensity correction unit 2202 performs a correction process on the image data input from the image data generation unit 2201 to correct uneven light intensity of the exposure head 106 using correction data stored in a memory (e.g., 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 uneven light intensity performed by the light intensity correction unit 2202 will be described in detail later. The conversion unit 2203 reads pixel values within the corresponding read range of the image data input from the light intensity correction unit 2202 in each line period identified by the line synchronization signal Lsync, and sends image data indicating the read pixel values to the 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 conversion unit 2203 uses a chip select signal CS to specify which light-emitting element array 300 should receive each portion of the image data DATA. The conversion unit 2203 generates a clock signal CLK and supplies it to each light-emitting element array 300 in order to synchronize the timing of transmission and reception of individual signal values with each light-emitting element array 300 .

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 the synchronization signal line 2208.

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

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 during each line period identified by the line synchronization signal Lsync. For example, when the chip select signal CS indicates the timing for receiving data intended for its own chip, each light-emitting element array 300 receives the 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 M-row, N-column light-emitting element array in accordance with the pixel values contained in the received image data.

The CPU 2211 controls the entire image forming apparatus 1. For example, the CPU 2211 controls the generation of the image data, light intensity correction, generation of a line synchronization signal, and transmission of image data to the printed circuit board 202. For example, prior to executing a job for image formation, the CPU 2211 reads out correction data that has been 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.

Figure 23 is a block diagram showing the configuration of circuits 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, multiple (five in the example of Figure 23) reference current sources 2302-1 to 2302-5, and multiple 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.

DAC 2301 performs digital-to-analog conversion on a digital value indicating the reference current setting value set by CPU 2211, and outputs an analog signal having a voltage corresponding to the setting value to each reference current source 2302. The reference current setting value is pre-stored in the storage unit 2210 described above, read by CPU 2211, and output to DAC 2301. Each of reference current sources 2302-1 to 2302-5 supplies a reference current corresponding to the voltage of the analog signal input from DAC 2301 to the light-emitting element 350. Note that 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 capacity of the reference current source 2302.

<Correction of uneven light intensity>
<Various causes>
There are various causes of uneven light intensity that occur in solid-state exposure devices. In this section, we will explain the following three causes:
24 and 25, the periodic nature of light passing through a rod lens array will be described. FIG. 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 FIG. 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 .

Curve 2500 shown in the upper part of Figure 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 the rod lens array 203. Because the rod lens array 203 is an array of multiple lenses 2410 arranged at a pitch _P1 , the light intensity distribution represented by curve 2500 has the property of repeatedly increasing and decreasing at a period _P1 in the longitudinal direction of the array. Meanwhile, the light emitting element array 300 arranged opposite the 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 varies depending on where each light emitting element 350 is located within the pitch _P1 of the lenses 2410.

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 these 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 these light-emitting elements 350 and passed through rod lens array 203 are light amounts _E2 and _E4 , respectively.

In this way, an exposure apparatus that uses a rod lens array to image light from a group of light-emitting elements onto the surface of a photosensitive member 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 parameters in the frequency domain. 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.

(2) Lens Tilting in the Rod Lens Array Ideally, the multiple lenses 2410 in the rod lens array 203 are all arranged so that their central axes are perpendicular to the light-emitting surface of the light-emitting element 350 (so that they are 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. Figure 26 is a schematic cross-sectional view of the rod lens array 203 taken along line B-B in Figure 24, showing an example of lens tilt due to manufacturing error. Direction D3 in the figure is a third direction perpendicular to the axial direction D1 (first direction) and circumferential direction D2 (second direction) of the photosensitive element. In the example of Figure 26, lens 2410-1 is positioned without error, and the central axis 2611-1 of lens 2410-1 is parallel to direction D3. On the other hand, lens 2410-2 is tilted due to manufacturing error, and the central axis 2611-2 of lens 2410-2 has a certain tilt angle with respect to direction D3. This phenomenon in which rod lenses in a rod lens array tilt 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 the pre-shipment inspection stage 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.

(3) Misalignment of Light-Emitting Element Arrays 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 with respect 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 the longitudinal direction of the light-emitting element array 300-2 is parallel to the first direction. On the other hand, the arrangement of the light-emitting element array 300-1 is misaligned due to a manufacturing error, and the longitudinal direction of the light-emitting element array 300-1 forms an angle _θ1 with respect to the first direction. On the other hand, the light-collection efficiency of the rod lens array is highest on the center line (shown by the dashed line in FIG. 27 ) of the rod lens array, and can decrease as the distance from the center line increases. Therefore, if the light-emitting element array 300 is misaligned as shown in the figure, linear unevenness in the amount of light will occur within the range of one light-emitting element array 300.

Note that individual differences between light-emitting element arrays 300 are also a cause of uneven light intensity. Similar to uneven light intensity caused by misalignment of the light-emitting element arrays 300, uneven light intensity caused by individual differences between the light-emitting element arrays 300 results in unevenness in the light intensity distribution per chip. Therefore, in this embodiment, correction of uneven light intensity caused by individual differences between 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.

(4) Complex Light Intensity Unevenness Caused by Multiple Causes The overall light intensity unevenness of the exposure head 106 is a complex unevenness caused by several of the causes exemplified above. Figure 28 shows the light intensity unevenness caused by three different causes, broken down into components by cause. Light intensity distribution 2801 in the figure is an example of a component of light intensity unevenness caused by the properties of the rod lens array. In light intensity distribution 2801, the light intensity periodically increases and decreases along the first direction. Light intensity distribution 2802 is an example of a component of light intensity unevenness caused by misalignment of the light-emitting element array 300. Light intensity distribution 2802 shows a linear change in light intensity in the section corresponding to each light-emitting element array 300. For example, the section from position _X11 to position _X12 corresponds to one light-emitting element array 300, and in this section, the light intensity increases linearly as the position moves. Light intensity distribution 2803 is an example of a component of light intensity unevenness caused by lens tilt in the rod lens array 203. In the example of FIG. 28, the light quantity distribution 2803 shows particularly low values at positions _X21 and _X22 .

The light intensity distribution 2900 in Figure 29 shows a complex light intensity variation caused by these three factors. The light intensity distribution measured for an actual exposure head 106 traces such a complex trajectory. Therefore, if an attempt is made to generate correction data that can accurately reproduce the light intensity distribution using conventional simple sampling of light intensity measurement results, it is necessary to set an extremely large number of data points and store data for each of those data points. 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, data points must be set 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 data from light intensity measurement results for such a large number of data points not only increases memory resource requirements but also causes delays associated with transferring large amounts of data read from memory.

In contrast, in this embodiment, the fluctuation in light quantity from the reference value (value _E0 in FIG. 29) in the light quantity 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 makes it possible to reduce the total number of data points in the correction data and suppress the data size, making it possible to save memory resources and shorten data transfer delays.

<Basic correction method>
Possible methods for correcting uneven light intensity 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 light intensity of the light-emitting element array 300, and then precisely corrects the remaining uneven light intensity using the latter correction method.

(1) Adjusting the Amount of Current Supplied to Light-Emitting Elements The former method can be performed by rewriting the setting value for the amount of current supplied among 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 varying its 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 current supplied to 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.

(2) Changing the Area Tone in Image Data The latter method, which enables precise correction of uneven light intensity, 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, the pixel position x-th from the left and y-th from the top of the small image is represented as (x, y), with the top left pixel as the reference.

Figure 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 based on the reduction (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% of its maximum value, four pixels out of a total of 100 (= 10 x 10) pixels are selected as pixels to be changed. In the example of Figure 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 well-known blue noise masking method.

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 that pixel to a non-exposure dot (i.e., inverts the pixel value). Figure 30(C) shows an example of the results of correcting the small image IM1 using the correction matrix IM2. In the small image IM3 in 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 cases where the light intensity at a reference pixel position is to be increased, the light intensity correction unit 2202 changes that pixel to an exposure dot if the 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 scans the reference pixel positions in the input image data and 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. As a result, precise correction of uneven light intensity in the first direction can be achieved.

(3) Increasing or Decreasing the Light Amount at 1/M Resolution In this embodiment, as described with reference 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, it is possible to reduce the light amount at 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, it is possible to increase the light amount at a resolution of 1/M of the pixel pitch. Therefore, the light amount correction unit 2202 may increase or decrease the light amount at this resolution (changing the emitting or non-emitting state of each of the M light-emitting elements) at 1/M resolution, in addition to changing the area gradation described above, so as to absorb fluctuations in the light amount of the light amount distribution restored based on the correction data.

<Example of correction data configuration>
In this embodiment, the light intensity distribution of the exposure head 106 is measured during the inspection stage before the product is shipped. Then, based on the measurement results, correction data is generated that enables subsequent restoration of fluctuations 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 is described below.

In this embodiment, the correction data may include the following three types of component data:

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

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 multiple 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. The frequency values can be derived from the known pitch, but the frequency component data may also include frequency values.

When the periodic light intensity unevenness of the rod lens array 203 traces an ideal sinusoidal wave trajectory, the fluctuations in the light intensity can be expressed by the amplitude and phase values at one data point, and the fluctuations 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.

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 light intensity. Typically, the linear components are mainly caused by misalignment of each light-emitting element array 300. At least two data points for the linear component data are set in the interval corresponding to each light-emitting element array 300. By retaining the offset values (intercepts) at these data points as linear component data, it is possible to reproduce the linear components of the fluctuation in light intensity at all pixel positions within that interval by linear interpolation. Alternatively, the linear component data may include the offset value and the slope of the light intensity at one data point set for each light-emitting element array 300.

The irregular component data is data related to the residual component remaining after removing the above-mentioned frequency components and linear components from the fluctuations in light intensity. In particular, the irregular component data in this embodiment represents the irregular component resulting from 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.

For example, suppose there are two or three data points for frequency component data per light-emitting element array 300, two data points for linear component data per light-emitting element array 300, and ten data points for 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 in conventional methods based on simple sampling and interpolation. Note that it is possible to share data points between frequency component data and linear component data, in which case the number of data points can be further reduced.

<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 in accordance with 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-light-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.

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

First, in step S11, an assembly process performed in a factory, various circuit elements including multiple light-emitting element arrays 300 are attached to the printed circuit board 202 of the exposure head 106. The printed circuit board 202 and rod lens array 203 are then attached to the housing 204. The storage section (memory) 810 may be attached to the printed circuit board 202 in a form mounted on the light-emitting element array 300, or may be attached separately to the printed circuit board 202. After the exposure head 106 has been 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 installation position of the rod lens array 203 may be adjusted.

Next, in step S12, the light intensity of each light-emitting element array 300 in the exposure head 106 is measured, and the average light intensity for each light-emitting element array 300 is calculated. Next, in step S13, a setting value for the amount of supply current required 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.

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.

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 amplitude and phase values for at least one data point for each of one or more significant frequency components. Next, in step S16, the frequency components reconstructed from the frequency component data (e.g., through an inverse Fourier transform) are removed from the fluctuations in light intensity indicated by the light intensity distribution to obtain residual components.

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 for at least two data points set in the section corresponding to each light-emitting element array 300. In addition, 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 no lens tilt is detected in the rod lens array 203, step S18 may be omitted.

Next, in step S19, correction data, which 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.

Although not shown in Figure 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 device 1 may be manufactured by incorporating an exposure device manufactured in this manner into an image forming unit 101, which may include a photosensitive drum 102 and other components.

<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 lens arrangement direction of 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 and reduces 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.

In the above-described embodiment, the frequency components may include a frequency component corresponding to the pitch of the arrangement of the multiple 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 accurately represent the periodic unevenness in light intensity caused by the properties of the rod lens array without requiring large data sizes.

In the above-described embodiment, the frequency components may also include frequency components corresponding to the pitch of the arrangement of the multiple light-emitting element arrays in the exposure head. By including the amplitude and phase values of this frequency component at data points set in the intervals corresponding to each light-emitting element array in the correction data, it is possible to accurately represent periodic light intensity variations that may be affected by individual differences between the individual light-emitting element arrays.

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 in the correction data the offset value of this linear component at the data point set in the section corresponding to each light-emitting element array, it is possible to accurately represent the partially linear unevenness in light intensity caused by misalignment of individual light-emitting element arrays. Therefore, it is possible to accurately correct unevenness in light intensity by also taking into account the residual component obtained by excluding the frequency component from the fluctuations in light intensity in the light intensity distribution of the exposure head.

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

In addition, 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, by correcting the unevenness in light intensity in the digital domain at a stage prior to the distribution of data to multiple light-emitting element arrays, it is possible to avoid complicating the configuration of the circuit that drives the light-emitting element array and reduce manufacturing costs. Furthermore, by 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 a single imaging spot, it is possible to correct the unevenness in light intensity with high resolution, further improving image quality.

In the above 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 greater than or equal to one. 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.

<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., an ASIC) that realizes one or more functions.

The invention is not limited to the above-described embodiments, and various modifications and variations are possible without departing from the spirit and scope of the invention. Therefore, the following claims are appended to clarify the scope of the invention.

102: Photosensitive drum, 106: Exposure head, 610: Information storage unit, 615: Streak correction unit

Claims (18)

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 . 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. 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. 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: The image forming apparatus of claim 4, wherein the position information includes information indicating the position of the first pixel in a series of pixels to be corrected among the plurality of pixels constituting the image data. The image forming apparatus of claim 5, wherein the characteristic information includes the correction level for each of the series of pixels to be corrected. The image forming apparatus of claim 6, wherein the characteristic information includes one correction level for each pixel in the series of pixels to be corrected. The image forming apparatus of claim 6, wherein the characteristic information includes one common correction level for N (N is an integer greater than or equal to 2) pixels in the series of pixels to be corrected. 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. The image forming apparatus of claim 9, wherein the correction means 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. The image forming apparatus of claim 9, wherein the correction means obtains the corrected pixel value for the pixel of interest by referencing a table based on the pixel value of the pixel of interest and the correction value calculated for the pixel of interest. The image forming apparatus of claim 9, wherein the correction means applies the second correction value to the pixel value of the pixel of interest when a predetermined combination of a first correction value calculated for a previous pixel located before the pixel of interest in the main scanning direction, a second correction value calculated for the pixel of interest, and a third correction value calculated for a subsequent pixel located after the pixel of interest in the main scanning direction is obtained. 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. 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. The image forming apparatus of claim 1, wherein the storage means is provided in the exposure head. 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. 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. 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 .