Exposure device and image formation apparatus

Abstract

To provide a mechanism that can prevent image quality degradation by suppressing light amount unevenness in a solid exposure type exposure device employing an organic EL element.SOLUTION: An exposure device for exposing a photoreceptor includes: a light-emitting element array being an array of organic EL (Electro Luminescence) elements which is formed by multiple light-emitting chips arranged parallel to the axial direction of the photoreceptor; and control means which controls the drive current supplied to the organic EL elements of the multiple light-emitting chips on the basis of the temperature during operation predicted for each of the multiple light-emitting chips. The control means sets the drive current supplied to the organic EL elements of the light emitting chip with the higher predicted temperature to be lower than the drive current supplied to the organic EL elements of the light emitting chip with the lower predicted temperature.SELECTED DRAWING: Figure 17

Description

[Technical field]

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

[0002] In an electrophotographic image forming apparatus, an electrostatic latent image is formed on a photoconductor by exposing the photoconductor to light as it is rotated, and the electrostatic latent image is developed with toner to form an image. In particular, a solid-state exposure type exposure device that uses light from an array of light-emitting elements is attracting attention because it is easier to reduce the size, noise level, and cost compared to a laser scanning type exposure device.

[0003] Patent Document 1 discloses a solid-state exposure type exposure device that uses organic EL (Electro Luminescence) elements. The exposure device of Patent Document 1 adjusts the voltage to be applied to the light-emitting element array so as to suppress unevenness in the amount of light, based on the results of measurements of the variation in the amount of light obtained in advance at a factory.

[0004] Patent Document 2 discloses a solid-state exposure type exposure apparatus that employs light-emitting thyristors as light-emitting elements. The exposure apparatus of Patent Document 2 adjusts the drive current of each light-emitting element based on the estimated heat generation amount of the chip in order to compensate for the decrease in the light output of the light-emitting thyristors, whose output light amount decreases as the temperature increases. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent Publication No. 2021-35765 [Patent Document 2] JP 2020-59124 A Summary of the Invention [Problem to be solved by the invention]

[0006] Generally, organic EL elements have a lower light output compared to semiconductor lasers or inorganic EL elements, and therefore require a longer time for light emission when exposing a photoconductor. If there is unevenness in the light amount, deviation from the target light amount is accumulated over the exposure time, resulting in uneven density in the printed image, which becomes noticeable as degradation of image quality. Patent Document 2 discloses a technology for compensating for a decrease in the light amount of a light-emitting thyristor by increasing the drive current when the light-emitting chip generates a large amount of heat, but the temperature-light amount characteristics of an organic EL element are different from the temperature-light amount characteristics of a light-emitting thyristor.

[0007] An object of the present invention is to provide a mechanism capable of suppressing unevenness in the amount of light and avoiding deterioration of image quality in a solid-state exposure type exposure apparatus that employs organic EL elements. [Means for solving the problem]

[0008] According to one aspect, there is provided an exposure device for exposing a photoconductor, the exposure device comprising: a light-emitting element array, which is an array of organic EL (Electro Luminescence) elements formed by a plurality of light-emitting chips arranged parallel to an axial direction of the photoconductor; and a control means for controlling a drive current supplied to the organic EL elements of the plurality of light-emitting chips based on a predicted operating temperature for each of the plurality of light-emitting chips, the control means setting the drive current supplied to the organic EL elements of a light-emitting chip having a higher predicted temperature to be lower than the drive current supplied to the organic EL elements of a light-emitting chip having a lower predicted temperature. There is also provided an image forming apparatus including the exposure device and the photoconductor. Effect of the Invention

[0009] According to the present invention, it is possible to suppress unevenness in the amount of light in a solid-state exposure type exposure apparatus that employs organic EL elements, thereby avoiding deterioration in image quality. [Brief description of the drawings]

[0010] [Figure 1]1 is a diagram showing a schematic configuration of an image forming apparatus according to an embodiment; [Diagram 2] FIG. 2 is an explanatory diagram of a configuration of a photoconductor and an exposure head according to the embodiment. [Diagram 3] FIG. 4 is an explanatory diagram of the configuration of a printed circuit board of an exposure head according to an embodiment. [Figure 4] 1A and 1B are explanatory diagrams of a light-emitting chip and a light-emitting element array in the light-emitting chip according to an embodiment. [Diagram 5] FIG. 1 is a plan view showing a schematic configuration of a light-emitting chip according to an embodiment. [Figure 6] FIG. 1 is a cross-sectional view showing a schematic configuration of a light-emitting chip according to an embodiment. [Figure 7] FIG. 4 is a circuit diagram showing a control configuration of an exposure apparatus according to an embodiment. [Figure 8] 6 is a signal chart related to access to a register of a light emitting chip according to an embodiment; [Figure 9] 11 is a signal chart related to the transmission of image data to a light-emitting chip according to an embodiment; [Figure 10] FIG. 2 is a functional block diagram showing a detailed configuration of a light-emitting chip according to an embodiment. [Figure 11] FIG. 13 is an explanatory diagram of multiple exposure using light-emitting elements arranged in a stepped pattern. [Figure 12] 11 is a graph showing an example of a change in the amount of light of a light-emitting chip with respect to a change in driving current. [Figure 13] 11 is a graph showing an example of a change in potential of a photoconductor relative to a change in the amount of light from a light-emitting chip. [Figure 14] 4 is a graph showing an example of the relationship between the operating temperature and the output light amount of an organic EL element. [Figure 15] 11 is a graph showing an example of temporal changes in the operating temperatures of two light-emitting chips as an image formation operation progresses. [Figure 16] 11 is a graph showing an example of a change over time in the amount of output light from two light-emitting chips along with the progress of an image forming operation. [Figure 17] 10 is a flowchart showing an example of a flow of a current adjustment process according to an embodiment. [Figure 18]A graph showing an example of a change over time in the amount of output light from two light-emitting chips that has been equalized as a result of a current adjustment process. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, the embodiments will be described in detail with reference to the attached drawings. Note that the following embodiments do not limit the invention according to the claims. Although the embodiments describe a number of features, not all of these features are essential to the invention, and the features may be combined in any manner. Furthermore, in the attached drawings, the same reference numbers are used for the same or similar configurations, and duplicated descriptions are omitted.

[0012] <1. Schematic configuration of image forming apparatus> 1 shows an example of a schematic configuration of an image forming apparatus 1 according to an embodiment. The image forming apparatus 1 includes a reading unit 100, an image creating unit 103, a fixing unit 104, and a conveying unit 105. The reading unit 100 optically reads an original placed on a platen and generates read image data. The image creating unit 103 forms an image on a sheet based on the read image data generated by the reading unit 100, or based on print image data received from an external device via a network, for example.

[0013] The image forming unit 103 includes image forming units 101a, 101b, 101c, and 101d. The image forming units 101a, 101b, 101c, and 101d form black, yellow, magenta, and cyan toner images, respectively. The image forming units 101a, 101b, 101c, and 101d have the same configuration, and are collectively referred to as the image forming unit 101 below. The photoconductor 102 of the image forming unit 101 is rotated clockwise in the figure during image formation. The charger 107 charges the photoconductor 102. The exposure head 106 exposes the photoconductor 102 to light to form an electrostatic latent image on the surface of the photoconductor 102. The developer 108 develops the electrostatic latent image on the photoconductor 102 with toner to form a toner image. The toner image formed on the surface of the photoconductor 102 is transferred to a sheet transported on a transfer belt 111. By transferring the toner images of the four photoconductors 102 onto a sheet in an overlapping manner, a color image containing four color components, namely black, yellow, magenta, and cyan, can be formed.

[0014] The conveying unit 105 controls the feeding and conveying of the sheet. Specifically, the conveying unit 105 feeds the sheet from a designated unit among the internal storage units 109a and 109b, the external storage unit 109c, and the manual feed unit 109d to the conveying path of the image forming apparatus 1. The fed sheet is conveyed to the registration roller 110. The registration roller 110 conveys the sheet onto the transfer belt 111 at an appropriate timing so that the toner image of each photoconductor 102 is transferred onto the sheet. As described above, the toner image is transferred onto the sheet while the sheet is being conveyed on the transfer belt 111. The fixing unit 104 fixes the toner image onto the sheet by heating and pressing the sheet onto which the toner image has been transferred. After the toner image is fixed, the sheet is discharged to the outside of the image forming apparatus 1 by the discharge roller 112. An optical sensor 113 is disposed at a position opposite the transfer belt 111. Optical sensor 113 optically reads the test chart formed on transfer belt 111 by image forming unit 101. When an error in the image formation range is detected for the test chart read by optical sensor 113, image controller 700 (described later) performs control to compensate for the error when a subsequent job is executed.

[0015] Although an example in which a toner image is directly transferred from each photoconductor 102 to a sheet on the transfer belt 111 has been described here, the toner image may be indirectly transferred from each photoconductor 102 to a sheet via an intermediate transfer body. Also, although an example in which a color image is formed using toners of multiple colors has been described here, the technology according to the present disclosure is also applicable to an image forming apparatus that forms a monochrome image using toners of a single color.

[0016] <2. Exposure head configuration example> 2(A) and 2(B) show the photoconductor 102 and the exposure head 106. The exposure head 106 has a light emitting element array 201, a printed circuit board 202 on which the light emitting element array 201 is mounted, a rod lens array 203, and a housing 204 that supports the printed circuit board 202 and the rod lens array 203. The photoconductor 102 has a cylindrical shape. The exposure head 106 is disposed such that its longitudinal direction is parallel to the axial direction D1 of the photoconductor 102, and the surface on which the rod lens array 203 is attached faces the surface of the photoconductor 102. While the photoconductor 102 rotates in the circumferential direction D2, the light emitting element array 201 of the exposure head 106 emits light, and the rod lens array 203 focuses the light on the surface of the photoconductor 102.

[0017] Fig. 3(A) and Fig. 3(B) show an example of the configuration of the printed circuit board 202. Fig. 3(A) shows the surface on which the connector 305 is mounted, and Fig. 3(B) shows the surface on which the light-emitting element array 201 is mounted (the surface opposite to the surface on which the connector 305 is mounted). Fig. 4 shows a schematic diagram of the light-emitting chip 400 and the array of the light-emitting elements 602 in the light-emitting chip 400.

[0018] In this embodiment, the light emitting element array 201 has a plurality of light emitting elements arranged two-dimensionally. The light emitting element array 201 as a whole includes N columns of light emitting elements in the axial direction D1 of the photoconductor and M rows of light emitting elements in the circumferential direction D2, where M and N are integers of 2 or more. In the example of FIG. 3(B), the light emitting element array 201 is divided into 20 light emitting chips 400-1 to 400-20, each including a subset of the entire plurality of light emitting elements, and the light emitting chips 400-1 to 400-20 are arranged in a staggered pattern along the axial direction D1. The light emitting chips 400-1 to 400-20 are also collectively referred to as light emitting chips 400. As shown in FIG. 3(B), the range occupied by all the light emitting elements of the 20 light emitting chips in the axial direction D1 is equal to or larger than the maximum width W of the input image data. 0is wider than the range occupied by the light emitting chip 400. Therefore, some light emitting elements located at both ends in the axial direction D1 may not be used to expose the photoconductor 102 unless an error in the image formation range is detected. Each light emitting chip 400 on the printed circuit board 202 is connected to the image controller 700 (FIG. 7) via a connector 305. For convenience of explanation, the side with the smaller branch number of the light emitting chips 400-1 to 400-20 aligned along the axial direction D1 may be referred to as the "left" and the side with the larger branch number as the "right". For example, the light emitting chip 400-1 is the leftmost light emitting chip 400, and the light emitting chip 400-20 is the rightmost light emitting chip.

[0019] The number J (J=N / 20) of the light-emitting elements 602 arranged in each row of one light-emitting chip 400 may be equal to, for example, 748 (J=748). Meanwhile, the number M of the light-emitting elements 602 arranged in each column of one light-emitting chip 400 may be equal to, for example, 4 (M=4). That is, in the exemplary embodiment, each light-emitting chip 400 has a total of 2992 (=748×4) light-emitting elements 602, with 748 in the axial direction D1 and 4 in the circumferential direction D2. The interval P between the center points of the light-emitting elements 602 adjacent to each other in the circumferential direction D2 is C may be, for example, about 21.16 μm, corresponding to a resolution of 1200 dpi. The distance between the center points of adjacent light emitting elements 602 in the axial direction D1 may also be about 21.16 μm, in which case 748 light emitting elements 602 occupy a length of about 15.8 mm in the axial direction D1. Note that, for convenience of explanation, FIG. 4 shows an example in which the light emitting elements 602 are completely arranged in a lattice in each light emitting chip 400, but the M (M=4) light emitting elements 602 in each row may be arranged in a stepped or partially stepped manner. The stepped arrangement of the light emitting elements 602 will be further described later.

[0020] 5 is a plan view showing a schematic configuration of the light emitting chip 400. The light emitting elements 602 of each light emitting chip 400 are formed on a light emitting substrate 402, which is, for example, a silicon substrate. A circuit unit 406 for driving the light emitting elements 602 is provided on the light emitting substrate 402. Signal lines for communicating with the image controller 700, power lines for connecting to a power source, and ground lines for connecting to a ground are connected to the pads 408-1 to 408-9. The signal lines, power lines, and ground lines may be wires made of, for example, gold.

[0021] FIG. 6 shows a part of the cross section taken along line AA in FIG. 5. A plurality of lower electrodes 504 are formed on the light emitting substrate 402. A gap of length d is provided between two adjacent lower electrodes 504. A light emitting layer 506 is provided on the lower electrode 504, and an upper electrode 508 is provided on the light emitting layer 506. The upper electrode 508 is a common electrode for the plurality of lower electrodes 504. When a voltage is applied between the lower electrode 504 and the upper electrode 508, a current flows from the lower electrode 504 to the upper electrode 508, causing the light emitting layer 506 to emit light. Therefore, one lower electrode 504 and a partial region of the light emitting layer 506 and the upper electrode 508 corresponding to the lower electrode 504 constitute one light emitting element 602. That is, in this embodiment, the light emitting substrate 402 includes a plurality of light emitting elements 602.

[0022] For example, an organic EL film can be used for the light-emitting layer 506. The upper electrode 508 is composed of a transparent electrode such as indium tin oxide (ITO) so as to transmit the emission wavelength of the light-emitting layer 506. Note that in this embodiment, the entire upper electrode 508 transmits the emission wavelength of the light-emitting layer 506, but it is not necessary for the entire upper electrode 508 to transmit the emission wavelength. Specifically, it is sufficient that a partial region through which light from each light-emitting element 602 passes transmits the emission wavelength.

[0023] In FIG. 6, one continuous light-emitting layer 506 is formed, but a plurality of light-emitting layers 506 each having a width equivalent to the width W of the lower electrode 504 may be formed on the lower electrode 504. In FIG. 6, the upper electrode 508 is formed as one common electrode for the plurality of lower electrodes 504, but a plurality of upper electrodes 508 each having a width equivalent to the width W of the lower electrode 504 may be formed corresponding to each of the lower electrodes 504. In addition, the first plurality of lower electrodes 504 of the lower electrodes 504 of each light-emitting chip 400 may be covered by the first light-emitting layer 506, and the second plurality of lower electrodes 504 may be covered by the second light-emitting layer 506. Similarly, the first upper electrode 508 may be commonly formed corresponding to the first plurality of lower electrodes 504 of the lower electrodes 504 of each light-emitting chip 400, and the second upper electrode 508 may be commonly formed corresponding to the second plurality of lower electrodes 504. In such a configuration, one lower electrode 504 and the region of the light emitting layer 506 and upper electrode 508 corresponding to the lower electrode 504 constitute one light emitting element 602 .

[0024] FIG. 7 is a circuit diagram related to a control configuration for controlling the light-emitting chip 400. The image controller 700 is a control circuit that communicates with the printed circuit board 202 via a plurality of signal lines (wires). The image controller 700 includes a CPU 701, a clock generating unit 702, an image data processing unit 703, a register access unit 704, a light-emitting control unit 705, a memory 706, a timer 707, and a temperature sensor 708. The light-emitting control unit 705 terminates the signal line between the image controller 700 and the printed circuit board 202. The n-th light-emitting chip 400-n (n is an integer from 1 to 20) on the printed circuit board 202 is connected to the light-emitting control unit 705 via a signal line DATAn and a signal line WRITEn. The signal line DATAn is used to transmit image data from the image controller 700 to the light-emitting chip 400-n. The signal line WRITEn is used for the image controller 700 to write control data to the register of the light-emitting chip 400-n.

[0025] One signal line CLK, one signal line SYNC, and one signal line EN are further provided between the light-emitting control unit 705 and each light-emitting chip 400. The signal line CLK is used to transmit a clock signal for transmitting data on the signal lines DATAn and WRITEn. The light-emitting control unit 705 outputs a clock signal generated based on a reference clock signal from the clock generating unit 702 to the signal line CLK. The signals transmitted to the signal lines SYNC and EN will be described later.

[0026] The CPU 701 controls the entire image forming apparatus 1. The image data processing unit 703 performs image processing on image data received from the reading unit 100 or an external device to generate binary bitmap image data for controlling the on / off of the light emission of the light emitting element 602 of the light emitting chip 400 on the printed circuit board 202. The image processing here may include, for example, raster conversion, gradation correction, color conversion, and halftone processing. The image data processing unit 703 transmits the generated image data to the light emission control unit 705 as input image data. The register access unit 704 receives control data to be written to the register in each light emitting chip 400 from the CPU 701 and transmits it to the light emission control unit 705. The light emission control unit 705 controls the light emission of the light emitting element 602 in each light emitting chip 400 based on the input image data input from the image data processing unit 703. The memory 706 is a non-volatile storage unit that stores computer programs and data required for light emission control. The timer 707 is a timing unit that measures the passage of time. The temperature sensor 708 is a temperature measuring means for measuring the environmental temperature.

[0027] FIG. 8 shows the transition of the signal level of each signal line when control data is written to the register of the light-emitting chip 400. An enable signal that is at high level during communication and indicates that communication is in progress is output to the signal line EN. The light-emitting control unit 705 transmits a start bit to the signal line WRITEn in synchronization with the rising edge of the enable signal. Next, the light-emitting control unit 705 transmits a write identification bit indicating a write operation, and then transmits the address of the register to which the control data is to be written (4 bits in this example) and the control data (8 bits in this example). When writing to the register, the light-emitting control unit 705 sets the frequency of the clock signal transmitted to the signal line CLK to, for example, 3 MHz.

[0028] FIG. 9 shows the transition of the signal level of each signal line when image data is transmitted to each light-emitting chip 400. A periodic line synchronization signal indicating the exposure timing of each line on the photoconductor 102 is output to the signal line SYNC. If the circumferential speed of the photoconductor 102 is 200 mm / s and the resolution in the circumferential direction is 1200 dpi (approximately 21.16 μm), the line synchronization signal is output at a period of approximately 105.8 μs. The light-emitting control unit 705 transmits image data to the signal lines DATA1 to DATA20 in synchronization with the rising edge of the line synchronization signal. In this embodiment, since each light-emitting chip 400 has 2992 light-emitting elements 602, it is necessary to transmit image data indicating the light emission / non-light emission of each of the total 2992 light-emitting elements 602 to each light-emitting chip 400 within a period of approximately 105.8 μs. Therefore, in this example, as shown in FIG. 9, when transmitting image data, the light-emitting control unit 705 sets the frequency of the clock signal transmitted to the signal line CLK to 30 MHz.

[0029] FIG. 10 is a functional block diagram showing a detailed configuration of one light-emitting chip 400 (n-th light-emitting chip 400-n). As also shown in FIG. 5, the light-emitting chip 400 has nine pads 408-1 to 408-9. The pads 408-1 and 408-2 are connected to a power supply voltage VCC by a power supply line. Power is supplied from this power supply voltage VCC to each circuit of the circuit unit 406 of the light-emitting chip 400. The pads 408-3 and 408-4 are connected to the ground by a ground line. Each circuit of the circuit unit 406 and the upper electrode 508 are connected to the ground via the pads 408-3 and 408-4. The signal line CLK is connected to the transfer unit 1003, the register 1102, and the latch units 1004-001 to 748 via the pad 408-5. The signal lines SYNC and DATAn are connected to the transfer unit 1003 via the pads 408-6 and 408-7. The signal lines EN and WRITEn are connected via pads 408-8 and 408-9 to a register 1102. The register 1102 stores control data indicating the magnitude of the drive current to be supplied to each light emitting element 602, for example.

[0030] The transfer unit 1003 receives input image data including a series of pixel values ​​each indicating the emission or non-emission of one light emitting element 602 from a signal line DATAn in synchronization with a clock signal from a signal line CLK, starting from a line synchronization signal from a signal line SYNC. The transfer unit 1003 performs serial-to-parallel conversion on the series of pixel values ​​serially received from the signal line DATAn in units of M (for example, M=4) pixel values. For example, the transfer unit 1003 has four cascaded D flip-flops, and parallelizes pixel values ​​DATA-1, DATA-2, DATA-3, and DATA-4 input over four clocks, and outputs the parallelized pixel values ​​to the latch units 1004-0001 to 1004-748. The transfer unit 1003 further has four D flip-flops for delaying the line synchronization signal, and outputs a first latch signal to the latch unit 1004-001 via a signal line LAT1 at a timing delayed by four clocks from the input of the line synchronization signal.

[0031] The kth latch unit 1004-k (k is an integer from 1 to 748) holds the four pixel values ​​DATA-1, DATA-2, DATA-3, and DATA-4 input from the transfer unit 1003 at the same time as the kth latch signal is input, by using a latch circuit. In addition, except for the last-stage latch unit 1004-748, the kth latch unit 1004-k delays the kth latch signal by four clocks and outputs the (k+1)th latch signal to the latch unit 1004-(k+1) via the signal line LAT(k+1). Then, the kth latch unit 1004-k continues to output a drive signal based on the four pixel values ​​held by the latch circuit to the current drive unit 1104 during the signal period of the kth latch signal. For example, there is a delay of four clocks between the timing when the first latch signal is input to the latch unit 1004-1 and the timing when the second latch signal is input to the latch unit 1004-2. Therefore, the latch unit 1004-1 outputs drive signals based on the 1st, 2nd, 3rd and 4th pixel values ​​to the current driver 1104, whereas the latch unit 1004-2 outputs drive signals based on the 5th, 6th, 7th and 8th pixel values ​​to the current driver 1104. Generally speaking, the latch unit 1004-k outputs drive signals based on the (4k-3), (4k-2), (4k-1) and (4k)th pixel values ​​to the current driver 1104. Therefore, in the embodiment shown in FIG. 10, 748 latch units 1004-001 to 1004-748 output 2992 drive signals for controlling the driving of 2992 (=748×4) light emitting elements 602 in approximately parallel to the current driver 1104. Each drive signal is a binary signal indicating a high level or a low level.

[0032] The current driver 1104 has 2992 light emission driver circuits corresponding to the 2992 light emitting elements 602 each including a partial region of the light emitting layer 506. Each light emission driver circuit passes a drive current of a magnitude indicated by the control data in the register 1102 to the light emitting layer 506 of the corresponding light emitting element 602 while the corresponding drive signal indicates a high level indicating that light emission is on. This causes the light emitting element 602 to emit light with a target amount of light. Note that the control data may indicate one individual current value for each light emitting element 602, may indicate one current value for each group of the light emitting elements 602, or may indicate one current value common to all the light emitting elements 602.

[0033] <3. Control of multiple exposure> 4 shows an example in which the light-emitting elements 602 in each light-emitting chip 400 are completely arranged in a lattice pattern, but the M light-emitting elements 602 in each row may be arranged in a stepped pattern at a constant pitch. FIG. 11 is an explanatory diagram of multiple exposure using light-emitting elements arranged in a stepped pattern. Here, an example of the arrangement of light-emitting elements in the light-emitting chip 400-1 when M=4 is partially shown. R in the figure j_m (j={0,1,...,J-1}, m={0,1,2,3}) represents the light emitting element 602 in the j-th column from the left in the axial direction and in the m-th row from the top in the circumferential direction. C As described above, the axial spacing between two adjacent light-emitting elements among the M light-emitting elements in each row, i.e., the axial pitch P A may be approximately 5 μm, which corresponds to a resolution of 4800 dpi.

[0034] By arranging the four light-emitting elements in each row in a stepped manner in this manner, any two adjacent light-emitting elements among the four light-emitting elements occupy areas that partially overlap in the axial direction. Then, the four light-emitting elements in the row corresponding to each pixel position of the input image data sequentially emit light while the photoconductor 102 is rotating, forming spots corresponding to each pixel position on the surface of the photoconductor 102. In the example of Figure 11, when the pixel value at the left end of the i-th line of the input image data indicates that light is on, the light-emitting element R 0_0 , R 0_1 , R 0_2 , R 0_3 is a line L on the surface of the photoconductor 102. i As a result, the line L i The leftmost spot area of ​​the 0 Similarly, when the jth pixel value from the left of the i-th line of the input image data indicates that light is on, the light-emitting element R j_0 , R j_1 , R j_2 , R j_3 is a line L on the surface of the photoconductor 102. i As a result, the line L i The jth spot area from the left of is multiplexed and the corresponding spot SP j is formed.

[0035] 11, just as two adjacent rows of light-emitting elements occupy a partially overlapping area, two rows of light-emitting elements located at the boundary between two adjacent light-emitting chips 400 also occupy a partially overlapping area in the axial direction. That is, of the two adjacent light-emitting chips 400, the light-emitting elements in the rightmost row of the left light-emitting chip 400 and the light-emitting elements in the leftmost row of the right light-emitting chip 400 also occupy a partially overlapping area in the axial direction. The axial pitch P of the light-emitting elements across all 20 light-emitting chips 400 is Ais constant at about 5 μm. By sequentially emitting light at appropriate times from the four light-emitting elements in each row of these light-emitting chips 400, a smooth line of an electrostatic latent image can be formed on the surface of the photoconductor 102, which is made up of a series of spots that are partially overlapping each other with a constant spot spacing. As a result of such lines being continuously formed in the circumferential direction, a two-dimensional electrostatic latent image is created.

[0036] <4. Uniformity of light intensity> <4-1. Adjustment of drive current before product shipment> The amount of light from the light-emitting element array depends on the magnitude of the drive current supplied from the current driver 1104. FIG. 12 shows the result of measuring the amount of light from one light-emitting chip 400 while changing the drive current supplied to the light-emitting element 602 of the light-emitting chip 400 from 0 mA to 46 mA. The measurement was performed using an optical power meter 3664 and an optical sensor 9742-10 manufactured by HIOKI EE Corporation. FIG. 13 shows the change in potential on the surface of the photoconductor 102 after the photoconductor 102 is exposed to the light amount shown in FIG. 12. If this potential is uniform across the multiple light-emitting chips 400 arranged parallel to the axial direction of the photoconductor 102, no density unevenness occurs in the printed image, but in reality, the unevenness in the amount of light from the light-emitting element array can cause uneven density in the printed image.

[0037] Examples of causes of uneven light intensity include manufacturing variations in the light emitting elements 602, misalignment of the light emitting chips 400, and variations in the lenses in the rod lens array 203. Uneven light intensity caused by these causes is measured at the factory before the product is shipped, and a setting value of the drive current for compensating for the measurement result and equalizing the light intensity is written into the memory or register of the product. The adjustment of the drive current may include, for example, rough adjustment for each light emitting chip and fine adjustment for each light emitting element.

[0038] <4-2. Influence of temperature during operation> However, in order to make the light quantity uniform, it is not enough to simply make adjustments based on the results of measuring the unevenness of the light quantity before the product is shipped, because the output light quantity of the light-emitting element has characteristics that depend on temperature, and if the operating temperature differs, the output light quantity also differs.

[0039] In general, inorganic EL elements including light-emitting thyristors have the characteristic that the amount of light decreases as the temperature increases due to an increase in the recombination of holes and electrons that do not contribute to light emission. In contrast, organic EL elements have the characteristic that the amount of light increases as the temperature increases due to an increase in the luminous efficiency of the organic film. Figure 14 shows an example of the relationship between the operating temperature and the output light amount of an organic EL element. According to the graph in Figure 14, when the operating temperature is 15°C, the output light amount is 0.27 μJ / cm 2 The organic EL element that outputs a light intensity of 0.29 μJ / cm at an operating temperature of 45°C 2 It outputs an amount of light.

[0040] The temperature of the operating light-emitting chips 400 or light-emitting elements 602 is not uniform even within the exposure head 106. If the ratio of pixels that are turned on to emit light in a certain light-emitting chip 400 (hereinafter also simply referred to as the image ratio) is high, the temperature of the light-emitting chip will be higher than that of other light-emitting chips 400 with lower image ratios. In addition, the influence of other heat sources (e.g., the fixing unit 104) in the device also differs depending on the chip position in the light-emitting element array.

[0041] FIG. 15 is a graph showing an example of the change over time in the operating temperature of two light-emitting chips of the exposure head 106 as the image forming operation progresses under certain conditions. In this example, a print job for printing 1800 pages at a print speed of 70 ppm is repeated for six hours with an interval (operation stop) of six minutes between jobs. The environmental temperature is about 10° C. The round marker in the figure is a plot of the temperature of the light-emitting chip 400-1, and the rectangular marker is a plot of the temperature of the light-emitting chip 400-10. The light-emitting chip 400-1 is located at the left end of the light-emitting element array, and the image ratio in the image forming range of the light-emitting chip 400-1 is 50%. On the other hand, the light-emitting chip 400-10 is located almost in the center of the light-emitting element array, and the image ratio in the image forming range of the light-emitting chip 400-10 is 25%.

[0042] As shown in the graph of FIG. 15, the temperature of the light-emitting chip 400-1 rises at a high rate during operation because the amount of integrated current increases due to the high image ratio. Heat received from the main body of the image forming apparatus 1 also contributes to the temperature rise of the light-emitting chip 400-1. The cooling of the light-emitting chip 400-1 during the interval between jobs is not sufficient, and the temperature of the light-emitting chip 400-1 reaches 50° C. two hours after the start of measurement. On the other hand, the temperature of the light-emitting chip 400-10 rises at a relatively low rate during operation due to the relatively low image ratio. The influence of heat from the main body of the image forming apparatus 1 may also be lower in the light-emitting chip 400-10 compared to the light-emitting chip 400-1. The temperature of the light-emitting chip 400-10 is about 20° C. two hours after the start of measurement, and the temperature of the light-emitting chip 400-10 gradually rises thereafter.

[0043] The temperature of the light-emitting chip 400-1 and the temperature of the light-emitting chip 400-10 are different from each other throughout most of the measurement period shown in Fig. 15. As a result, if the driving currents supplied to the light-emitting chips 400-1 and 400-10 are the same, the output light quantities of the two chips will differ due to the temperature-light quantity characteristics of the organic EL elements, that is, unevenness in the light quantity that causes uneven density will occur. Fig. 16 is a graph showing an example of the change over time in the output light quantity of the two light-emitting chips 400-1 and 400-10 corresponding to the change over time in the operating temperature of Fig. 15.

[0044] <4-3. Dynamic drive current adjustment based on predicted temperature> It is conceivable to prevent unevenness in the amount of light by providing a temperature sensor in each light-emitting chip 400 and adjusting the drive current of each light-emitting chip 400 based on the measured temperature. However, additional mounting of a temperature sensor has the disadvantages of increasing manufacturing costs and hindering miniaturization. Therefore, in this embodiment, instead of directly measuring the temperature of each light-emitting chip 400, a mechanism for predicting or estimating the temperature of each light-emitting chip 400 is adopted.

[0045] Specifically, the CPU 701 of the image controller 700 functions as a control means that predicts the temperature during operation of each light-emitting chip 400 and controls the drive current supplied to the light-emitting element 602 of each light-emitting chip 400 based on the predicted temperature. Taking into account the temperature-light amount characteristics of the organic EL element, the CPU 701 sets the drive current supplied to the light-emitting element 602 of the light-emitting chip 400 having a higher predicted temperature to be lower than the drive current supplied to the light-emitting element 602 of the light-emitting chip 400 having a lower predicted temperature.

[0046] The CPU 701 holds, for example, a variable Tn (n=1,...,20) indicating the temperature of the light-emitting chip 400-n in the memory 706. Then, the CPU 701 dynamically (for example, periodically) updates the temperature variable Tn according to a certain temperature prediction model so as to track the temperature change of the light-emitting chip 400-n during the operation of the exposure head 106.

[0047] Typically, the temperature prediction model may include a temperature rise when the exposure head 106 is in operation, and a temperature drop when the exposure head 106 is not in operation.

[0048] The temperature rise can be calculated by multiplying a parameter related to the operation amount of the light-emitting chip 400 by a temperature rise coefficient α. The parameter related to the operation amount of the light-emitting chip 400 may be, for example, the time during which the light-emitting chip 400 is used for an image formation operation (hereinafter, referred to as the operation time) or the number of times the light-emitting chip 400 is used for an image formation operation (hereinafter, referred to as the operation number).

[0049] However, even if the light-emitting chips 400 operate for the same time (or the same number of times), the amount of heat generated differs if the ratio of the light-emitting elements 602 that emit light differs. The temperature rise coefficient α is intended to reflect this difference. For example, the CPU 701 calculates an image ratio Rn for each light-emitting chip 400 based on the input image data. The image ratio Rn represents the ratio of light-emitting elements that emit light for each light-emitting chip, and can be calculated, for example, as the ratio of pixels whose pixel values ​​indicate light emission on among all pixels in the same chip. Then, the CPU 701 can determine a temperature rise coefficient (first coefficient) αn to be used in calculating the temperature rise of the light-emitting chip 400-n based on the image ratio Rn calculated for the light-emitting chip 400-n. The relationship between the image ratio and the temperature rise coefficient is determined, for example, in a test before product shipment, and is stored in the memory 706 in the form of a relational expression or a look-up table (LUT). Table 1 below shows an example of the relationship between the image ratio [%] of the light-emitting chip and the temperature rise coefficient multiplied by the operation time [hours] of the light-emitting chip.

[0050] [Table 1]

[0051] The relationship between the image ratio and the temperature rise coefficient may be defined differently for each position of the light-emitting chip 400 in the light-emitting element array. For example, when the image ratio is the same, a relational expression or LUT may be defined for each chip position so that the temperature rise coefficient αn of the chip located at the end of the array, which is easily affected by other heat sources in the device, is larger than that of the chip located in the center of the array. Table 2 below shows an example of the relationship between the image ratio and the temperature rise coefficient of the light-emitting chip depending on the chip position. For an intermediate chip position, any of the temperature rise coefficients shown in the table may be selected, or an intermediate temperature rise coefficient may be calculated by interpolation.

[0052] [Table 2]

[0053] The temperature drop amount can be calculated by multiplying the elapsed time from the last operation of the light-emitting chip 400 (hereinafter, also referred to as the left-alone time) by the temperature drop coefficient β. As an example, the temperature drop coefficient β may be 9 [° C. / hour]. Here, during the period when the exposure head 106 is not operating, the temperature of each light-emitting chip 400 may drop faster as the environmental temperature is lower. Therefore, the CPU 701 may determine the temperature drop coefficient β based on the environmental temperature measured by the temperature sensor 708. In addition, when the left-alone time of each light-emitting chip 400 exceeds a predetermined threshold, the CPU 701 may determine that a sufficient time has passed for the chip temperature to become equal to the environmental temperature, and reset the predicted temperature of each light-emitting chip 400 to the environmental temperature measured by the temperature sensor 708. The memory 706 may store in advance the relationship between the environmental temperature and the temperature drop coefficient (second coefficient).

[0054] When the exposure head 106 operates for the image forming operation, the CPU 701 dynamically updates the temperature variable Tn of each light-emitting chip 400-n according to the above-mentioned temperature prediction model. For example, the CPU 701 subtracts the temperature drop during the unused time, which can be calculated using the temperature drop coefficient β, from the temperature variable Tn updated during the previous operation. In addition, the CPU 701 periodically adds the temperature rise calculated using the temperature rise coefficient αn to the temperature variable Tn during the progress of the image forming operation. The CPU 701 determines the value of the drive current to be supplied to each light-emitting chip 400-n based on the predicted temperature tracked in this way, and writes the determined drive current value to the register of each light-emitting chip 400-n.

[0055] Table 3 below shows the relationship between the temperature [°C] and the drive current [mA] of the light emitting chip 400 and the output light amount [μJ / cm 2 ] is an example of the relationship between

[0056] [Table 3]

[0057] A relational expression or LUT expressing the correspondence as shown in the above table is determined in a test before product shipment and stored in the memory 706. For example, the CPU 701 can search an LUT such as Table 3 to determine a drive current value indicating a predetermined target light amount at the predicted temperature Tn of each light-emitting chip 400. As an example, when the predicted temperature of the light-emitting chip 400-1 is 30° C. and the target light amount is 0.56 μJ / cm 2 , the value of the driving current to be set for the light-emitting chip 400-1 is 40 mA. For an intermediate chip temperature or output light amount that does not match any entry in the table, the CPU 701 may determine the value of the driving current by interpolation (for example, linear interpolation) based on the values ​​indicated by multiple entries.

[0058] 17 is a flowchart showing an example of the flow of a current adjustment process that can be executed by the CPU 701. Here, focusing on the n-th light-emitting chip 400, an example of the flow of a process for dynamically adjusting the driving current supplied to the light-emitting chip 400 will be described. In practice, similar temperature prediction and driving current adjustment can be performed in parallel for the 20 light-emitting chips 400-1 to 400-20.

[0059] First, in S101, the CPU 701 acquires the predicted temperature value (temperature variable) Tn of the n-th light-emitting chip 400 from the memory 706 when execution of a print job is started. Next, in S102, the CPU 701 determines whether or not the idle time Δt from the previous operation exceeds a predetermined threshold value (e.g., 7 hours). If the idle time Δt does not exceed the threshold value, the process proceeds to S103. On the other hand, if the idle time Δt exceeds the threshold value, the process proceeds to S104.

[0060] In S103, the CPU 701 subtracts a temperature drop equal to the product of the standing time Δt and the temperature drop coefficient β from the predicted temperature Tn. Meanwhile, in S104, the CPU 701 resets the predicted temperature Tn to the environmental temperature measured by the temperature sensor 708. Note that, in S103 as well, the CPU 701 may reset the predicted temperature Tn to the environmental temperature if the predicted temperature Tn after subtraction is lower than the environmental temperature.

[0061] Next, in S105, the CPU 701 determines a temperature rise coefficient αn to be used for predicting the temperature of the n-th light-emitting chip 400. For example, the CPU 701 may calculate an image ratio in an image formation range of the light-emitting chip 400 based on the input image data, and determine the temperature rise coefficient αn from the calculated image ratio.

[0062] Next, in S106, the CPU 701 determines whether or not to update the drive current value of the n-th light-emitting chip 400 based on the latest predicted temperature Tn. For example, the CPU 701 may determine to update the drive current value when starting execution of a print job. The CPU 701 may also determine to update the drive current value of the n-th light-emitting chip 400 when the fluctuation of the predicted temperature Tn from the time when the drive current value was last updated exceeds a predetermined threshold value. Since frequent updates of the drive current value increase the delay of the image forming operation, it is possible to prevent a decrease in productivity due to the delay of the operation by omitting the update of the drive current value when the fluctuation of the predicted temperature Tn is small. When the drive current value is updated, the process proceeds to S107. On the other hand, when the drive current value is not updated, the process proceeds to S109.

[0063] In S107, the CPU 701 determines a driving current value for outputting a target amount of light corresponding to the latest predicted temperature Tn of the n-th light-emitting chip 400 by referring to the relational expression or LUT stored in the memory 706. Next, in S108, the CPU 701 sets the determined driving current value in the n-th light-emitting chip 400 (writes control data indicating the driving current value into the register of the n-th light-emitting chip 400).

[0064] When the execution of a print job is started, the CPU 701 repeats monitoring of factors contributing to a temperature rise of the light-emitting chip 400 and updating of the predicted temperature Tn in a constant control cycle. For example, in S109, the CPU 701 monitors the operation time X of the light-emitting chip 400 using the timer 707. Next, in S110, the CPU 701 adds a temperature rise amount equal to the product of the temperature rise coefficient αn and the operation time X to the predicted temperature Tn. Next, in S111, the CPU 701 determines whether the operation of the exposure head 106 has ended. If the operation of the exposure head 106 has not ended, the process returns to S105, and the temperature rise coefficient αn may be redetermined and the driving current value may be updated as necessary.

[0065] If it is determined in S111 that the operation of the exposure head 106 has ended, the CPU 701 records the latest value of the predicted temperature Tn of the n-th light-emitting chip 400 together with the last update time in the memory 706, and the current adjustment process in FIG. 17 ends.

[0066] FIG. 18 shows an example of the change over time in the output light amount of the two light-emitting chips 400-1 and 400-10 when the dynamic current adjustment process described above is applied while performing the image forming operation under the same conditions as in FIG. 15 and FIG. 16. Compared with FIG. 16, in FIG. 18, the output light amount of the light-emitting chip 400-1 with a relatively high image ratio and the light-emitting chip 400-10 with a relatively low image ratio are uniform throughout the operation time. This is because the driving current supplied to each of the light-emitting chips 400-1 and 400-10 is reduced so as to suppress the increase in the light amount caused by the individual temperature rise of the light-emitting chips 400-1 and 400-10. As a result, the unevenness in the amount of light in the surface of the light-emitting element array 201 is suppressed, and it is possible to print a high-quality image with little unevenness in density.

[0067] The temperature prediction model described in this section is merely an example. In a modification, the predicted temperature may be further refined by incorporating feedback of the temperature measured by the temperature sensor 708 into the current adjustment process described with reference to FIG. 17. In another modification, instead of calculating the temperature rise by multiplying the operation time or the number of operations of each light-emitting chip 400 by a temperature rise coefficient, the temperature rise may be calculated based on an integrated value of the driving current amount of each light-emitting chip 400.

[0068] 17, it may be determined whether or not to update the driving current value based on the difference in image ratio between the light-emitting chips 400. For example, when the difference in image ratio between the light-emitting chips 400 exceeds a predetermined threshold, the CPU 701 may set the frequency of updating the driving current value higher than when the difference in image ratio between the light-emitting chips 400 exceeds a predetermined threshold, since this is a situation in which unevenness in the amount of light is likely to appear. This makes it possible to achieve an appropriate balance between image quality and productivity.

[0069] <5. Summary> Various embodiments of the technology according to the present disclosure have been described in detail above with reference to FIGS. 1 to 18. In the above-described embodiment, in an exposure device that exposes a photoconductor using an array of organic EL elements formed by a plurality of light-emitting chips, the drive current supplied to the organic EL element of each light-emitting chip is controlled based on the temperature during operation predicted for each light-emitting chip. Then, the drive current supplied to the organic EL element of the light-emitting chip having a higher predicted temperature is set lower than the drive current supplied to the organic EL element of the light-emitting chip having a lower predicted temperature. According to this configuration, the temperature-light amount characteristic inherent to the organic EL element, that is, the higher the temperature, the greater the output light amount, is appropriately taken into consideration, thereby making it possible to suppress the unevenness in the light amount between the light-emitting chips caused by temperature changes during operation and to avoid deterioration of image quality.

[0070] In the above-described embodiment, a variable indicating a predicted temperature is held for each light-emitting chip, and a temperature rise derived by multiplying the operation time or the number of operations of each light-emitting chip by a first coefficient can be added to the variable. By tracking the temperature rise of each light-emitting chip in this manner, the temperature of each light-emitting chip can be easily predicted without providing a temperature sensor for each of the light-emitting chips. Furthermore, by determining the first coefficient based on the ratio of light-emitting elements that emit light for each light-emitting chip (i.e., image ratio), it is possible to accurately track temperature changes that may differ for each light-emitting chip. Furthermore, by determining the first coefficient based on the chip position in the light-emitting element array, it is possible to track temperature changes that may differ for each light-emitting chip with even better accuracy, taking into account the influence of other heat sources in the device.

[0071] In the above embodiment, the temperature drop derived by multiplying the elapsed time from the last operation of each light-emitting chip by a second coefficient can be subtracted from the above variable. When the elapsed time from the last operation of each light-emitting chip exceeds a predetermined threshold, the predicted temperature of each light-emitting chip can be reset to the environmental temperature. With this configuration, the temperature drop during the interval between the operations of the device can be appropriately reflected in the predicted temperature of each light-emitting chip.

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

[0073] <6. Other embodiments> The above-mentioned embodiment can also be realized in the form of a process in which a program for realizing one or more functions is supplied to a system or device via a network or a storage medium, and one or more processors in a computer of the system or device read and execute the program. It can also be realized by a circuit (e.g., ASIC) for realizing one or more functions.

[0074] The invention is not limited to the above-described embodiments, and various modifications and variations are possible without departing from the spirit and scope of the invention. Accordingly, the following claims are appended to apprise the public of the scope of the invention. [Explanation of symbols]

[0075] 1: image forming apparatus, 102: photoconductor, 106: exposure head, 201: light emitting element array, 400-1 to 400-20: light emitting chips, 602: light emitting element, 701: CPU (control means), 705: light emission control unit, 706: memory, 707: timer, 708: temperature sensor

Claims

1. An exposure device for exposing a photoconductor, comprising: a light emitting element array, which is an array of organic EL (Electro Luminescence) elements, formed by a plurality of light emitting chips arranged parallel to an axial direction of the photoconductor; a control unit that controls a drive current supplied to the organic EL element of each of the light-emitting chips based on a temperature during operation predicted for each of the light-emitting chips; Equipped with the control means sets the driving current supplied to the organic EL element of a light-emitting chip having a higher predicted temperature to be lower than the driving current supplied to the organic EL element of a light-emitting chip having a lower predicted temperature. Exposure equipment.

2. The control means A variable indicating the predicted temperature for each light-emitting chip is stored. During operation of the exposure device, a temperature rise is derived by multiplying an operation time or an operation count of each light-emitting chip by a first coefficient, and the temperature of each light-emitting chip is predicted by adding the temperature rise to the variable.

2. The exposure apparatus according to claim 1.

3. 3. The exposure apparatus according to claim 2, wherein the control means determines the first coefficient used for predicting the temperature of each light-emitting chip based on a ratio of light-emitting elements that emit light for each light-emitting chip.

4. 3. The exposure apparatus according to claim 2, wherein the control means determines the first coefficient used for predicting the temperature of each light-emitting chip based on a chip position in the light-emitting chip array.

5. 3. The exposure apparatus according to claim 2, wherein the control means predicts the temperature of each light-emitting chip by subtracting from the variable a temperature drop derived by multiplying an elapsed time since the last operation of each light-emitting chip by a second coefficient.

6. The exposure apparatus further includes a measuring unit for measuring an environmental temperature, the control means resets the predicted temperature of each light-emitting chip to the environmental temperature measured by the measurement means when the elapsed time from the last operation of each light-emitting chip exceeds a threshold value; 6. The exposure apparatus according to claim 5.

7. 2. The exposure apparatus according to claim 1, wherein the control means updates the value of the driving current supplied to the organic EL element of a light-emitting chip for which the predicted temperature fluctuation from the previous update of the driving current exceeds a threshold value based on the predicted temperature of the light-emitting chip.

8. 2. The exposure apparatus according to claim 1, wherein the control means sets a frequency for updating a value of the drive current supplied to the organic EL element of each of the light-emitting chips based on a difference in a ratio of light-emitting elements that emit light among the plurality of light-emitting chips.

9. 2. The exposure apparatus according to claim 1, wherein the control means determines the driving current to be supplied to each light-emitting chip from the temperature predicted for each light-emitting chip using a table or a relational equation that defines the relationship between the temperature of the light-emitting element, the supplied driving current, and the light amount of the organic EL element to which the driving current is supplied at the temperature.

10. An exposure apparatus according to any one of claims 1 to 9, The photoreceptor; An image forming apparatus comprising:

Images