Exposure device and image forming device

Abstract

To improve the subjective image quality of images formed by using an exposure device.SOLUTION: An exposure device for exposing a photoreceptor includes multiple light-emitting chips arranged regularly in a first direction parallel to the axial direction of the photoreceptor. Each of the light-emitting chips includes K (K is an integer of 2 or more) current sources and multiple light-emitting elements aligned in the first direction and grouped into K light-emitting element groups. With k=1, 2, ..., K, the light-emitting elements that belong to the k-th light-emitting element group are supplied with the current from the k-th current source. For any one of the K light-emitting element groups, the width occupied by the one light-emitting element group in the first direction is greater than a lower limit value corresponding to the spatial frequency at which the contrast sensitivity of the visual sensation of a human represented by a visual transfer function peaks.SELECTED DRAWING: Figure 14

Description

[Technical Field]

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

[0002] A commonly known type of electrophotographic image forming apparatus is a solid-state exposure apparatus that forms a latent image by exposing a photosensitive drum to light emitted by an LED (e.g., an organic EL element) rather than laser light. The exposure head of this type of apparatus includes a light-emitting element array including a plurality of light-emitting elements arranged parallel to the axial direction of the photosensitive drum, and a rod lens array that focuses the light from the light-emitting element array onto the surface of the photosensitive drum. The length of the light-emitting element array in the axial direction corresponds to the maximum width of the image to be formed on the surface of the photosensitive drum. The pitch of the light-emitting elements in the light-emitting element array corresponds to the maximum resolution of the image. For example, the pitch of the light-emitting elements in the light-emitting element array of an apparatus capable of forming an image at 1200 dpi is approximately 21.16 micrometers (μm).

[0003] Generally, the light emitting element array of a solid-state exposure device has an elongated shape. For example, the light emitting element array of the exposure device disclosed in Patent Document 1 is made up of multiple light emitting chips arranged in a staggered pattern, and wiring from the power supply to each light emitting element extends along the longitudinal direction of the light emitting element array.

[0004] Furthermore, Non-Patent Document 1 introduces a visual transfer function (VTF) model that relates to the influence of uneven light intensity on image quality that is subjectively perceived through human vision. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent Publication No. 2021-37706 [Non-patent literature]

[0006] [Non-Patent Document 1] RP Dooley and R. Shaw, "Noise Perception in Electro-photography", Journal of Applied Photographic Engineering, Vol.5, No.4, pp.190-196, 1979 Summary of the Invention [Problem to be solved by the invention]

[0007] The resistance of the longitudinally extending wiring in the current supply circuit of an exposure device causes a significant voltage drop in the voltage applied to the light-emitting element, which can lead to uneven light intensity. To reduce this voltage drop, it is possible to provide multiple current sources on the light-emitting chip containing the light-emitting element array and shorten the wiring length from the current sources to the light-emitting element. However, manufacturing variations between different current sources also cause uneven light intensity and result in image quality degradation such as image streaks. The degree to which image streaks are noticeable to human vision depends on their spatial frequency. According to the Dooley-Shaw VTF model introduced in Non-Patent Document 1, assuming an observation distance of approximately 300 millimeters (mm), human visual contrast sensitivity peaks at a spatial frequency of approximately 1.0 cycle / mm.

[0008] In view of the above circumstances, an object of the present invention is to improve the subjective image quality of an image formed using an exposure device. [Means for solving the problem]

[0009] According to one aspect, an exposure device that exposes a photosensitive body includes a plurality of light-emitting chips that are regularly arranged in a first direction parallel to an axial direction of the photosensitive body, each of the plurality of light-emitting chips including K current sources (K is an integer of 2 or more) and a plurality of light-emitting elements that are aligned in the first direction and are grouped into K light-emitting element groups, where k=1, 2, ..., K, and a light-emitting element belonging to the k-th light-emitting element group is supplied with current from the k-th current source;the light emitting elements belonging to each of the K light emitting element groups are arranged consecutively in the first direction; An exposure device is provided in which, for any one of the K light emitting element groups, the width occupied by that one light emitting element group in the first direction is greater than a lower limit value corresponding to the spatial frequency at which the contrast sensitivity of human vision, represented by the visual transfer function, peaks. [Effects of the Invention]

[0010] According to the present invention, the subjective quality of an image formed using an exposure device can be improved. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a diagram illustrating a schematic configuration of an image forming apparatus according to an embodiment. [Figure 2] FIG. 2 is an explanatory diagram of the configuration of a photosensitive member and an exposure head according to an embodiment. [Figure 3] FIG. 2 is an explanatory diagram illustrating 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. [Figure 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. 2 is a circuit diagram showing the control configuration of an exposure apparatus according to an embodiment. [Figure 8] 10 is a signal chart related to access to a register of a light-emitting chip according to an embodiment; [Figure 9] 10 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. 10 is an explanatory diagram of multiple exposure using light-emitting elements arranged in a stepped pattern. [Figure 12A] FIG. 4 is an explanatory diagram of a procedure for light emission control based on input image data. [Figure 12B] FIG. 4 is an explanatory diagram of a procedure for light emission control based on input image data. [Figure 12C] FIG. 4 is an explanatory diagram of a procedure for light emission control based on input image data. [Figure 12D] FIG. 4 is an explanatory diagram of a procedure for light emission control based on input image data. [Figure 13] Contrast sensitivity graph for the Dooley-Shaw VTF model. [Figure 14] FIG. 2 is a block diagram showing a first embodiment of the configuration of a current supply circuit for a light-emitting chip. [Figure 15] 4 is an explanatory diagram of the width occupied in the first direction by the two-dimensional arrangement of light emitting elements in each light emitting element group. FIG. [Figure 16] FIG. 10 is a block diagram showing a second embodiment of the configuration of a current supply circuit for a light-emitting chip. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments will be described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the scope of the invention claimed. Although multiple features are described in the embodiments, not all of these multiple features are necessarily essential to the invention, and multiple features may be combined arbitrarily. Furthermore, in the accompanying drawings, the same reference numerals are used to designate the same or similar components, and redundant explanations will be omitted.

[0013] 1. General 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.

[0014] 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 drawing during image formation. The charger 107 charges the photoconductor 102. The exposure head 106 exposes the photoconductor 102 according to image data to form an electrostatic latent image on the surface of the photoconductor 102. The developer 108 develops the electrostatic latent image on the surface of the photoconductor 102 with toner to form a toner image. The toner image formed on the surface of the photoreceptor 102 is transferred onto a sheet transported on a transfer belt 111. By transferring the toner images of the four photoreceptors 102 onto the sheet in an overlapping manner, a color image containing four color components, namely black, yellow, magenta, and cyan, can be formed.

[0015] The conveying unit 105 controls the feeding and transport of sheets. Specifically, the conveying unit 105 feeds a 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 a transport path of the image forming apparatus 1. The fed sheet is transported to the registration rollers 110. The registration rollers 110 transport the sheet onto the transfer belt 111 at an appropriate timing so that the toner images on the photoconductors 102 are transferred to the sheet. As described above, the toner image is transferred to the sheet while the sheet is transported on the transfer belt 111. The fixing unit 104 heats and pressurizes the sheet to which the toner image has been transferred, thereby fixing the toner image to the sheet. After the toner image is fixed, the sheet is discharged to the outside of the image forming apparatus 1 by the discharge rollers 112. An optical sensor 113 is disposed downstream of the image forming unit 101a and facing the transfer belt 111. The optical sensor 113 is used to detect positional misalignment (color misalignment) between color components of a test image formed on the transfer belt 111 by the image forming unit 101. If a color misalignment is detected, the image formation positions of the image forming units 101a, 101b, 101c, and 101d are corrected under the control of an image controller 700 (described later) to compensate for the detected color misalignment.

[0016] Although an example has been described here in which a toner image is directly transferred from each photoconductor 102 to a sheet on the transfer belt 111, the toner image may also be indirectly transferred from each photoconductor 102 to a sheet via an intermediate transfer body. Also, although an example has been described here in which a color image is formed using toners of multiple colors, the technology according to the present disclosure is also applicable to an image forming apparatus that forms a monochrome image using toner of a single color.

[0017] 2. Exposure head configuration example 2(A) and 2(B) show the photoconductor 102 and the exposure head 106. As described above, the exposure head 106 is an exposure device that exposes the photoconductor 102 in accordance with image data. The exposure head 106 has a light-emitting element array 201, a printed circuit board 202 on which the light-emitting element array 201 is mounted, a rod lens array 203, and a housing 204 that supports the printed circuit board 202 and the rod lens array 203. The photoconductor 102 has a cylindrical shape. The exposure head 106 is disposed so that its longitudinal direction is parallel to the direction of the rotation axis of the photoconductor 102 (first direction D1) and the surface on which the rod lens array 203 is attached faces the surface of the photoconductor 102. While the photosensitive member 102 rotates in the circumferential direction (second direction D2) around the rotation axis, the light emitting element array 201 of the exposure head 106 emits light, and the rod lens array 203 focuses the light on the surface of the photosensitive member 102.

[0018] 3(A) and 3(B) show an example of the configuration of the printed circuit board 202. Note that FIG. 3(A) shows the surface on which the connector 305 is mounted, and FIG. 3(B) shows the surface on which the light-emitting element array 201 is mounted (the surface opposite to the surface on which the connector 305 is mounted). In this embodiment, the light-emitting element array 201 is formed by 20 light-emitting chips 400-1 to 400-20 regularly arranged in the longitudinal direction. In the example of FIG. 3(B), the light-emitting chips 400-1 to 400-20 are arranged in a staggered pattern along the longitudinal direction of the printed circuit board 202 of the exposure head 106. Specifically, when n is an odd number, 10 light-emitting chips 400-n form one line, and when n is an even number, another 10 light-emitting chips 400-n form another line. In this specification, the light emitting chips 400 in the former line are also referred to as odd-numbered light emitting chips 400, and the light emitting chips 400 in the latter line are also referred to as even-numbered light emitting chips 400. The light emitting chips 400-1 to 400-20 are also collectively referred to as light emitting chips 400. Each light emitting chip 400 on the printed circuit board 202 is connected to the image controller 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 arranged along the first direction D1 may be referred to as the "left" and the side with the larger branch number may be referred to as the "right" below. 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] FIG. 4 is an explanatory diagram illustrating an example of the arrangement of light-emitting elements 602 in a light-emitting chip 400. In this embodiment, each light-emitting chip 400 includes a two-dimensional array of light-emitting elements consisting of M rows (M is an integer of 2 or greater) in a second direction D2 perpendicular to the first direction D1 and N columns (N is an integer of 2 or greater) in the first direction D1 (however, FIG. 4 only shows a portion of the two-dimensional array). In one example, N=748 and M=4, in which case each light-emitting chip 400 has a total of 2992 (=748×4) light-emitting elements 602. In a light-emitting element array consisting of 20 light-emitting chips 400, 14960 light-emitting elements are aligned in the first direction D1. The pitch between adjacent light-emitting elements 602 in the first direction D1 may be approximately 21.16 μm, which corresponds to a resolution of 1200 dpi. In this case, the length of the entire light-emitting element array in the first direction D1 is approximately 316 mm (the maximum width of an image that can be formed), and the length of each light-emitting chip 400 in the first direction D1 is approximately 15.8 mm. The M light-emitting elements in each column are shifted in the first direction D1 by a displacement of approximately 5 μm, which corresponds to a resolution of 4800 dpi, and are arranged in a stepped pattern. While FIG. 4 shows 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. 4, the light-emitting elements in the rightmost column of the odd-numbered light-emitting chips 400 and the light-emitting elements in the leftmost column of the even-numbered light-emitting chips 400 may be arranged to overlap in the first direction D1. Similarly, the light-emitting elements in the leftmost column of the odd-numbered light-emitting chips 400 and the light-emitting elements in the rightmost column of the even-numbered light-emitting chips 400 may be arranged to overlap in the first direction D1. The interval Ly between the light-emitting element arrays of these light-emitting chips 400 may be, for example, approximately 105 μm. By overlapping the light-emitting element arrays in the first direction D1 in this way, it is possible to prevent gaps from occurring in the exposure range due to variations in implementation. When such an overlapping arrangement is adopted, typically, only one of the light-emitting elements in the overlapping rows is used as an effective light-emitting element that can emit light according to image data. The light-emitting elements in the other row do not need to be used regardless of the image data. Note that the number of rows or light-emitting elements overlapping in the first direction D1 is not limited to the above example and may be any number.

[0020] 5 is a plan view showing a schematic configuration of the light-emitting chip 400. The plurality of 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 plurality of 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 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 portion of the cross section taken along line AA in FIG. 5. A plurality of lower electrodes 504 are formed on the light-emitting substrate 402. A light-emitting layer 506 is provided on the lower electrodes 504, and an upper electrode 508 is provided on the light-emitting layer 506. The upper electrode 508 is a common electrode for the plurality of lower electrodes 504. When a voltage is applied between the lower electrode 504 and the upper electrode 508, a current flows from the lower electrode 504 to the upper electrode 508, causing the light-emitting layer 506 to emit light. Therefore, one lower electrode 504 and the partial region of the light-emitting layer 506 and upper electrode 508 corresponding to that lower electrode 504 constitute one light-emitting element 602. In the figure, dx is the distance between two adjacent lower electrodes 504. dz is the distance between the lower electrode 504 and the upper electrode 508. By making dx larger than dz, leakage current between adjacent lower electrodes 504 can be suppressed, preventing light-emitting elements 602 that should not emit light from erroneously emitting light.

[0022] In this embodiment, each light-emitting element 602 is configured as an organic electroluminescence (EL) element. For example, an organic EL film can be used for the light-emitting layer 506. In other embodiments, each light-emitting element 602 may be configured as an inorganic EL element by using an inorganic EL film for the light-emitting layer 506. Generally, each light-emitting element 602 may be any type of LED (Light-Emitting Diode).

[0023] The upper electrode 508 is made of a transparent electrode such as indium tin oxide (ITO) so as to transmit the emission wavelength of the light-emitting layer 506. In the example of Fig. 6, the entire upper electrode 508 transmits the emission wavelength of the light-emitting layer 506, but the entire upper electrode 508 does not necessarily have to transmit the emission wavelength. Specifically, it is sufficient that a partial region through which light from each light-emitting element 602 passes transmits the emission wavelength.

[0024] By continuously depositing the light-emitting layer 506 and the upper electrode 508 without any seams as in the example of FIG. 6 , it is possible to reduce unevenness in the amount of light due to the structure of the light-emitting element. However, the structure of the light-emitting element is not limited to this example. For example, multiple light-emitting layers 506, each having a width equal to that of the lower electrode 504, may be formed on each lower electrode 504. Also, in FIG. 6 , the upper electrode 508 is formed as a single common electrode for the multiple lower electrodes 504, but multiple upper electrodes 508, each having a width equal to that of the lower electrode 504, may be formed corresponding to each lower electrode 504. Also, among the lower electrodes 504 of each light-emitting chip 400, a first plurality of lower electrodes 504 may be covered by a first light-emitting layer 506, and a second plurality of lower electrodes 504 may be covered by a second light-emitting layer 506. Similarly, a first upper electrode 508 may be commonly formed corresponding to a first plurality of lower electrodes 504 among the lower electrodes 504 of each light-emitting chip 400, and a second upper electrode 508 may be commonly formed corresponding to a second plurality of lower electrodes 504. Even 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.

[0025] 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 multiple signal lines (wires). The image controller 700 has a CPU 701, a clock generation unit 702, an image data processing unit 703, a register access unit 704, and a light-emitting control unit 705. 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 by the image controller 700 to write control data to a register of the light-emitting chip 400-n.

[0026] 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 generation unit 702 to the signal line CLK. The signals transmitted to the signal lines SYNC and EN will be described later.

[0027] 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 light emission of the light-emitting elements 602 of the light-emitting chips 400 on the printed circuit board 202. This image processing may include, for example, raster conversion, tone 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 writes and reads data to and from the registers of each light-emitting chip 400 via the light-emission control unit 705.

[0028] 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.

[0029] 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 peripheral speed of the photoconductor 102 is 200 mm / s and the peripheral resolution 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, each light-emitting chip 400 has 2992 light-emitting elements 602, and therefore, image data indicating the light-emitting or non-light-emitting status of each of the total 2992 light-emitting elements 602 must be transmitted 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.

[0030] FIG. 10 is a functional block diagram showing a detailed configuration of one light-emitting chip 400 (the 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 via 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 ground via a ground line. Each circuit of the circuit unit 406 and the upper electrode 508 are connected to 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 1004-748 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, for example, the amount of adjustment (or current amount) of a current, which will be described later.

[0031] The transfer unit 1003 receives input image data from signal line DATAn, including a series of pixel values ​​each indicating whether one light-emitting element 602 emits light or not, in synchronization with a clock signal from signal line CLK, starting from a line synchronization signal from signal line SYNC. The transfer unit 1003 performs serial-to-parallel conversion on the series of pixel values ​​serially received from signal line DATAn in units of M (e.g., M=4) pixel values. For example, the transfer unit 1003 has four cascade-connected D flip-flops, which parallelize pixel values ​​DATA-1, DATA-2, DATA-3, and DATA-4 input over four clocks and output the parallelized data to latch units 1004-0001 to 1004-748. The transfer unit 1003 also has four D flip-flops for delaying the line synchronization signal, and outputs a first latch signal to the latch unit 1004-001 via signal line LAT1 four clocks after the line synchronization signal is input.

[0032] The nth latch unit 1004-n (n is an integer from 1 to 748) holds, in a latch circuit, four pixel values ​​DATA-1, DATA-2, DATA-3, and DATA-4 input from the transfer unit 1003 simultaneously with the input of the nth latch signal. Except for the final latch unit 1004-748, the nth latch unit 1004-n delays the nth latch signal by four clocks and outputs the (n+1)th latch signal to the latch unit 1004-(n+1) via the signal line LAT(n+1). The nth latch unit 1004-n continues to output a drive signal based on the four pixel values ​​held in the latch circuit to the current drive unit 1104 during the signal period of the nth latch signal. For example, there is a four-clock delay between the timing at which the first latch signal is input to the latch unit 1004-1 and the timing at which the second latch signal is input to the latch unit 1004-2. Therefore, the latch unit 1004-1 outputs drive signals based on the first, second, third, and fourth pixel values ​​to the current driver 1104, while the latch unit 1004-2 outputs drive signals based on the fifth, sixth, seventh, and eighth pixel values ​​to the current driver 1104. Generally speaking, the latch unit 1004-n outputs drive signals based on the (4n-3), (4n-2), (4n-1), and (4n)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.

[0033] The current driver 1104 has 2992 light-emitting drive circuits corresponding to the 2992 light-emitting elements 602, each of which includes a partial region of the light-emitting layer 506. While the corresponding drive signal indicates a high level, which means that light is on, each light-emitting drive circuit applies a drive voltage corresponding to the amount of current indicated by the control data in the register 1102 to the light-emitting layer 506 of the corresponding light-emitting element 602. This causes a current to flow through the light-emitting layer 506, causing the light-emitting element 602 to emit light.

[0034] 3. Controlling multiple exposures As explained with reference to FIG. 4, in this embodiment, M light-emitting elements 602 in each row can 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 when M=4 is partially shown. R in the figure j_m (j={0, 2,..., N-1}, m={0, 1, 2, 3}) represents the light emitting element 602 in the j-th column from the left and the m-th row from the top. C As described above, the distance P between two adjacent light-emitting elements in the first direction D1 among the M light-emitting elements in each column may be about 21.16 μm. A may be about 5 μm, as mentioned above.

[0035] As shown in the example of Fig. 11, 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 D1. Then, the four light-emitting elements in the column corresponding to each pixel position of the input image data sequentially emit light while the photoconductor 102 is rotating, thereby forming spots corresponding to each pixel position on the surface of the photoconductor 102. In the example of Fig. 11, when the pixel value at the left end of the ith line of the input image data indicates that light is on, the light-emitting element R 0_0 , R 0_1 , R 0_2 , R 0_3 is a line L on the surface of the photoreceptor 102. i As a result, the line L i Similarly, when the j-th pixel value from the left of the ith line of the input image data indicates that light is emitted, the light-emitting element R j_0 , R j_1 , R j_2 , R j_3 is a line L on the surface of the photoreceptor 102. i As a result, the line L i The jth spot area from the left of is multiplexed and the corresponding spot SPj In this way, by sequentially emitting light at appropriate times from four light-emitting elements in each row of the twenty light-emitting chips 400, smooth lines of an electrostatic latent image can be formed on the surface of the photoreceptor 102, consisting of a series of spots that are partially overlapping each other and have a fixed spot spacing. As a result of these lines being continuously formed in the circumferential direction, a two-dimensional electrostatic latent image is created.

[0036] 12A to 12D are explanatory diagrams of the procedure for controlling light emission based on input image data. During image formation, the light emission control unit 705 receives input image data IM1 in a binary bitmap format from the image data processing unit 703. On the left side of FIG. 12A, the j-th pixel value from the left on the i-th line from the top of the input image data IM1, which is a two-dimensional pixel value array, is represented as (j, i) (j = {0, 1, 2, ...}, i = {0, 1, 2, ...}). The light emission control unit 705 adds dummy pixel values ​​for (M-1) lines to the beginning of the input image data IM1. When M = 4, including the added dummy pixel values, the range of pixel value index i is {-3, -2, -1, 0, 1, 2, ...}. The dummy pixel value may be, for example, zero, which indicates that light emission is off. The light emission control unit 705 may add dummy pixel values ​​to the left and right of the input image data IM1 so that the number of pixel values ​​in one line is equal to the number of light-emitting elements in the first direction D1, but for simplicity of explanation, only effective pixel values ​​are shown here with respect to the first direction D1.

[0037] In the first line period t0 of image formation, the light-emitting control unit 705 reads out pixel values ​​of the top four lines of the input image data IM1 and outputs a subset of 2992 (=748×4) of the read pixel values ​​to the light-emitting chip 400-n via the signal line DATAn. Focusing on the light-emitting chip 400-1 shown on the right in FIG. 12A, image data within a read range RD including pixel values ​​from (0,-3) to (748,0) is input via the signal line DATA1 during the line period t0. The light-emitting chip 400-1 serial-to-parallel converts the input image data and supplies drive signals based on these pixel values ​​to each of the 2992 light-emitting elements. For example, drive signals based on pixel values ​​(0,-3), (0,-2), (0,-1), (0,0), and (1,-3) are output to the light-emitting element R. 0_0 , R 0_1 , R 0_2 , R 0_3 and R 1_0 In particular, as shown by the broken line in the figure, the light emitting element R 0_3 A drive signal based on the effective pixel value of line DL0 of index i=0 in the input image data IM1 is supplied to the light-emitting elements in the fourth row, including line L0. As a result, line L0 on the surface of photoconductor 102 is exposed in accordance with the set of pixel values ​​of line DL0 in the input image data IM1. However, at this point, multiple exposure is still in progress, and the formation of the electrostatic latent image of line L0 is not complete.

[0038] FIG. 12B shows how the light-emitting chip 400-1 is driven during the next line period t0+1. In the line period t0+1, the light-emitting control unit 705 moves the readout range RD of the input image data IM1 down by one line, reads out pixel values ​​from (0,-2) to (748,1), and outputs them to the light-emitting chip 400-1 via the signal line DATA1. The light-emitting chip 400-1 supplies drive signals based on the input pixel values ​​to the 2992 light-emitting elements. For example, drive signals based on pixel values ​​(0,-2), (0,-1), (0,0), (0,1), and (1,-2) are supplied to the light-emitting elements R. 0_0 , R 0_1 , R 0_2 , R 0_3 and R 1_0In the line period t0+1, the light emitting element R 0_2 A drive signal based on the effective pixel values ​​of line DL0 of the input image data IM1 is supplied to the light-emitting elements in the third row, including line L0. At this time, since the photoconductor 102 is rotating in the circumferential direction, line L0 on the surface of the photoconductor 102 faces the light-emitting elements in the third row of the light-emitting chip 400-1. As a result, line L0 on the surface of the photoconductor 102 is again exposed in accordance with the set of pixel values ​​of line DL0 of the input image data IM1.

[0039] FIG. 12C shows how the light-emitting chip 400-1 is driven during the next line period t0+2. In the line period t0+2, the light-emitting control unit 705 moves the readout range RD of the input image data IM1 further down by one line, reads out pixel values ​​from (0, -1) to (748, 2), and outputs them to the light-emitting chip 400-1 via the signal line DATA1. The light-emitting chip 400-1 supplies drive signals based on the input pixel values ​​to the 2992 light-emitting elements. In the line period t0+2, the light-emitting elements R 0_1 A drive signal based on the effective pixel value of line DL0 of the input image data IM1 is supplied to the light-emitting elements of the second row including line L0 on the surface of the photoconductor 102. At this time, line L0 on the surface of the photoconductor 102 faces the light-emitting elements of the second row of light-emitting chip 400-1. As a result, line L0 on the surface of the photoconductor 102 is exposed a third time in accordance with the pixel value set of line DL0 of the input image data IM1.

[0040] FIG. 12D shows how the light-emitting chip 400-1 is driven during the next line period t0+3. In the line period t0+3, the light-emitting control unit 705 moves the read range RD of the input image data IM1 further down by one line, reads out pixel values ​​from (0,0) to (748,3), and outputs them to the light-emitting chip 400-1 via the signal line DATA1. The light-emitting chip 400-1 supplies drive signals based on the input pixel values ​​to the 2992 light-emitting elements. In the line period t0+3, the light-emitting elements R 0_0A drive signal based on the effective pixel value of line DL0 of the input image data IM1 is supplied to the light-emitting elements of the first row, including the light-emitting elements of the light-emitting chip 400-1. At this time, line L0 on the surface of the photoconductor 102 faces the light-emitting elements of the first row of the light-emitting chip 400-1. As a result, line L0 on the surface of the photoconductor 102 is exposed a fourth time according to the set of pixel values ​​of line DL0 of the input image data IM1. At this point, multiple exposure by the four light-emitting elements of each column of the light-emitting chip 400 has been performed, and the formation of line L0 of the electrostatic latent image is completed. Lines subsequent to line L0 of the electrostatic latent image can also be formed on the surface of the photoconductor 102 in a similar manner through repetition of this line cycle.

[0041] In this manner, in this embodiment, a drive signal based on the pixel value of each pixel position is input to four light-emitting elements in a corresponding column of the light-emitting element array. Specifically, for example, a drive signal based on a pixel value (0,0) is input to four light-emitting elements R 0_3 , R 0_2 , R 0_1 and R 0_0 When these four light-emitting elements emit light in accordance with the drive signals, a spot corresponding to the pixel value (0,0) is formed on the surface of the photoconductor 102. Similarly, a drive signal based on the pixel value (1,0) is input to the four light-emitting elements R 1_3 , R 1_2 , R 1_1 and R 1_0 When these four light emitting elements emit light in accordance with the drive signals, a spot corresponding to the pixel value (1,0) is formed on the surface of the photoconductor 102.

[0042] As can be understood from the above description, the light emission control unit 705 causes the plurality of light emitting elements 602 to emit light based on pixel values ​​read from a read range spanning M lines of the input image data IM1. The read range moves by one line per line period.

[0043] <4. Improvement of subjective image quality> <4-1. Basic principles> As described above, the light-emitting element array of each light-emitting chip 400 has an elongated shape along the first direction D1. If each light-emitting chip 400 were provided with only one current source for supplying power to the light-emitting elements, the wiring resistance between the current source and the light-emitting elements would cause a non-negligible voltage drop in the voltage applied to the light-emitting elements that are far from the current source.

[0044] Therefore, in this embodiment, K current sources (K is an integer equal to or greater than 2) are provided in each light-emitting chip 400. Furthermore, the light-emitting elements in the light-emitting element array of each light-emitting chip 400 are grouped into K light-emitting element groups. Then, a current supply circuit is designed to supply current from the k-th current source to the light-emitting elements belonging to the k-th light-emitting element group, where k=1, 2, ..., K. By setting the number of current sources (equal to the number of light-emitting element groups) K to a sufficiently large value, the wiring length from the current sources to the light-emitting elements can be shortened and voltage drops due to wiring resistance can be suppressed.

[0045] However, to prevent manufacturing variations in the current source and related circuitry from causing significant degradation of image quality as perceived by human vision, a limit is placed on the width of one light emitting element group in the first direction D1. Typically, manufacturing variations in the current source and related circuitry cause periodic variations in light intensity with a period equal to the width of the light emitting element group, which manifests as image streaks in printed images. The degree to which image streaks are noticeable to human vision depends on their spatial frequency. The Dooley-Shaw VTF model expresses the spatial frequency-dependent contrast sensitivity of human vision using a VTF function as follows:

number

[0046] FIG. 13 shows a graph of contrast sensitivity according to the Dooley-Shaw VTF model when the viewing distance L is 300 mm. The horizontal axis of the graph represents spatial frequency λ [cycle / mm], and the vertical axis represents the output value (sensitivity) of the VTF function. As can be seen from the graph in FIG. 13, assuming a viewing distance L of 300 mm, the contrast sensitivity of human vision peaks at a spatial frequency of approximately 1.0 cycle / mm. This spatial frequency corresponds to a period of approximately 1 mm for image streaks. In other words, when the width of the image streaks is equal to 1 mm, the image streaks are most strongly perceived as a subjective degradation of image quality.

[0047] Increasing the number K of current sources (increasing the density of current sources) makes the width of the image stripes narrower than 1 mm (increasing the spatial frequency), and the above-mentioned peak in contrast sensitivity can be avoided. However, the inventors recognized that high-frequency image stripes cause other types of image degradation, such as moire caused by interference with a halftone screen. Therefore, in order to avoid the peak in contrast sensitivity and improve image quality, it is meaningful to design the arrangement of current sources so that the spatial frequency of the image stripes is on the left side (low frequency side) of the contrast sensitivity peak rather than the right side (high frequency side).

[0048] Based on the above-mentioned concept, the arrangement of the current sources in each light-emitting chip 400 is designed to satisfy the following condition 1. Condition 1: For any one of the K light emitting element groups, the width W that the one light emitting element group occupies in the first direction D1 is G is greater than the lower limit corresponding to the spatial frequency at which the contrast sensitivity of human vision reaches its peak.

[0049] The lower limit here may vary depending on the assumed observation distance L, but may be, for example, 1 mm assuming an observation distance L of 300 mm based on the general use of printed images. Alternatively, by setting the lower limit to 1.5 mm, the contrast sensitivity can be significantly reduced from its peak, further suppressing degradation of subjective image quality.

[0050] Furthermore, the arrangement of the current sources in each light-emitting chip 400 can be designed to also satisfy the following condition 2. Condition 2: For any one of the K light emitting element groups, the width W that the one light emitting element group occupies in the first direction D1 is G is smaller than the upper limit corresponding to the allowable voltage drop in the current supply from the current source to the light emitting element.

[0051] The upper limit here may vary depending on the characteristics of the current supply circuit and the target value of the current amount, but may be, for example, 4 mm. Alternatively, by setting the upper limit to 2 mm, the voltage drop can be further reduced.

[0052] <4-2. First Example> Below, several examples of designs for current supply circuits that satisfy conditions 1 and 2 will be described.

[0053] In the first embodiment, as described above, the total number of rows of light-emitting elements in the light-emitting element array 201 in the first direction D1 is 14,960, and the pitch of the light-emitting elements is approximately 21.16 μm, which corresponds to a resolution of 1,200 dpi. This allows for the formation of an image with a maximum width of approximately 316 mm in the first direction D1. When the light-emitting element array 201 is configured with 20 light-emitting chips 400-1 to 400-20, the width of the light-emitting element array in each light-emitting chip 400 is approximately 15.8 mm, and the number of rows of light-emitting elements is 748. Therefore, for example, by grouping the two-dimensional light-emitting element array of each light-emitting chip 400 into 10 light-emitting element groups (i.e., K=10), the width W per group can be increased by approximately 15.8 mm. G can be set to about 1.58 mm on average.

[0054] Fig. 14 shows a first embodiment of the configuration of a current supply circuit of the light-emitting chip 400. Referring to Fig. 14, the light-emitting chip 400 includes a digital-to-analog converter (D / A) 901, ten reference current sources 902-1 to 902-10, and a plurality of light-emitting elements 602 grouped into ten light-emitting element groups 903-1 to 903-10. Each light-emitting element 602 belongs to one of the light-emitting element groups 903-1 to 903-10 depending on the position of the light-emitting element 602 in the first direction D1.

[0055] The D / A 901 performs digital-to-analog conversion on the digital value of the current amount indicated by the control data read from the register and outputs an analog signal having a voltage corresponding to the current amount to each reference current source 902. The current amount here may be, for example, the sum of a predetermined reference amount (common to the plurality of light-emitting chips 400) and an adjustment amount predetermined for each light-emitting chip 400. In this case, the D / A 901 functions as a current adjustment circuit. By transmitting the current amount from the D / A 901 to each reference current source 902 as an analog signal, which is a voltage signal, it is possible to prevent a voltage drop in this portion of the wiring. Each reference current source 902 applies a drive voltage to each light-emitting element 602 in accordance with the analog signal input from the D / A 901 when the drive signal described with reference to FIG. 10 indicates a high level, thereby supplying each light-emitting element 602 with a current for emitting light at a target light intensity.

[0056] In other words, the number of rows of light emitting elements in the first direction D1 of the light emitting chip 400 is 748. In the first embodiment, since the number of light emitting element groups K=10, for example, the number of rows of light emitting elements belonging to each light emitting element group 903 may be as follows: Light-emitting element groups 903-1 to 903-9 ... 75 rows each Light-emitting element group 903-10 ... 73 rows

[0057] FIG. 15 shows the width W of the two-dimensional array of light emitting elements in each light emitting element group 903 in the first direction D1. GIn the first embodiment, the light emitting elements belonging to each of the K light emitting element groups 903 are arranged consecutively in the first direction D1. This optimizes the length of the wiring from each reference current source 902 to the corresponding light emitting element 602. The width W that the k-th light emitting element group 903-k occupies in the first direction D1 is Gk corresponds to the distance between the two light-emitting elements that are spaced the widest apart in the first direction D1 among the light-emitting elements belonging to the light-emitting element group 903-k, i.e., the light-emitting elements located at both ends. In Fig. 15, the light-emitting elements located at both ends of each light-emitting element group 903 are indicated by hatching.

[0058] Width W of the first light emitting element group 903-1 G1 is equal to the product of the number of rows (K=75) and the pitch (approximately 21.16 μm), which is approximately 1.587 mm. G2 and the width W of the third light emitting element group 903-3 G3 The same is true for the width W of the tenth light emitting element group 903-10. G10 is equal to approximately 1.545 mm, which is the product of the number of rows (K=73) and the pitch (approximately 21.16 μm). G1 ~W G10 Both of these satisfy the above-mentioned conditions 1 and 2. Therefore, it is possible to suppress voltage drops and reduce unevenness in the amount of light, while avoiding peaks in the contrast sensitivity of human vision without causing interference moiré, thereby improving subjective image quality.

[0059] Of course, the number K of reference current sources 902 (the number of light-emitting element groups) provided in each light-emitting chip 400 is not limited to the above example. For example, more (or fewer) light-emitting chips 400 may be mounted on the exposure head 106, and each light-emitting chip 400 may accordingly be provided with fewer (or more) reference current sources 902.

[0060] <4-3. Second Example> In the second embodiment, the total number of rows of light-emitting elements in the light-emitting element array 201 is 14,960, the pitch of the light-emitting elements is approximately 21.16 μm, the number of light-emitting chips 400 mounted on the exposure head 106 is 20, and the number of light-emitting element groups 903 is 10 (K=10). Therefore, the W occupied by one light-emitting element group in the first direction D1 is G is approximately 1.58 mm on average.

[0061] Fig. 16 shows a second embodiment of the configuration of the current supply circuit of the light-emitting chip 400. Referring to Fig. 16, the light-emitting chip 400 includes ten D / As 901-1 to 901-10, ten reference current sources 902-1 to 902-10, and a plurality of light-emitting elements 602 grouped into ten light-emitting element groups 903-1 to 903-10. Each light-emitting element 602 belongs to one of the light-emitting element groups 903-1 to 903-10 depending on the position of the light-emitting element 602 in the first direction D1.

[0062] The kth (k=1,...,10) D / A 901-k is connected to the kth reference current source 902-k. The D / A 901-k performs digital-to-analog conversion on the digital value of the current amount for the kth reference current source 902-k indicated by the control data read from the register, and outputs an analog signal having a voltage corresponding to the current amount to the reference current source 902-k. The current amount here is, for example, the sum of a predetermined reference amount and an adjustment amount predetermined for each reference current source 902. In other words, each D / A 901 functions as a current adjustment circuit. The kth reference current source 902-k applies a drive voltage to the light-emitting elements 602 belonging to the kth light-emitting element group 903-k when the drive signal described with reference to FIG. 10 indicates a high level, thereby supplying each light-emitting element 602 with a current for emitting light at a target light-emitting intensity.

[0063] The current adjustment amount for each reference current source 902 can be determined in advance based on measurement results of the light intensity emitted from the light-emitting element array during manufacturing or inspection of the exposure head 106. For example, a constant current is supplied from the reference current sources 902-1 to 902-10 to the corresponding light-emitting elements 602, and the resulting distribution of the light intensity emitted from the light-emitting element array indicates unevenness in the light intensity due to manufacturing errors. Based on the measurement results of the light intensity, the current adjustment amount for each reference current source 902 (e.g., an offset for compensating for the difference between the target light intensity and the measured light intensity) can be determined so that the light intensity emitted from the K light-emitting element groups is uniform. The control data written to the register of each light-emitting chip 400 can indicate the adjustment amount determined in this manner. When the exposure head 106 is powered on, the CPU 701 reads the control data from the register of each light-emitting chip 400 and sets the adjustment amount (or current amount) in each D / A 901. The D / As 901-1 to 901-10 in the second embodiment adjust the amount of current supplied to the corresponding light emitting elements from the reference current sources 902-1 to 902-10, respectively, according to the settings.

[0064] In this embodiment as well, the current supply circuit is designed to satisfy the above-mentioned conditions 1 and 2, and as a result, it is possible to achieve both a reduction in unevenness in the amount of light by suppressing voltage drops and an improvement in subjective image quality by avoiding peaks in the contrast sensitivity of human vision.

[0065] Note that the distribution of light quantity may be measured by a scanner reading a test pattern formed on a sheet by the image forming apparatus 1, instead of using a light quantity meter during the manufacturing or inspection stage of the exposure head 106. In a modified example, a scanner (reading device) that optically reads the test pattern formed on a sheet by the image forming apparatus 1 may be connected downstream of the image forming apparatus 1. Also, the amount of adjustment of the current for each reference current source 902 may be automatically determined (for example, by the image controller 700) by analyzing a read image of the test pattern. Alternatively, the amount of adjustment of the current for each reference current source 902 may be specified via a user interface by a user who visually checks the test pattern on the sheet.

[0066] Although some specific values ​​related to the light-emitting element array have been described in this specification, the present invention is not limited to those specific values. The number of light-emitting chips provided on one printed circuit board is not limited to 20 and can be any number equal to or greater than 1. Furthermore, the size of the light-emitting element array of each light-emitting chip 400 is not limited to 4 rows and 748 columns and can be any other size. Furthermore, the pitch of the light-emitting elements is not limited to approximately 21.16 μm and can be any other value.

[0067] <5. Other embodiments> The above-described embodiment can also be realized in the form of a process in which a program for realizing one or more functions is supplied to a system or device via a network or a storage medium, and one or more processors in a computer of the system or device read and execute the program, or by a circuit (e.g., ASIC) that realizes one or more functions.

[0068] The invention is not limited to the above-described embodiments, and various changes and modifications can be made without departing from the spirit and scope of the invention. Accordingly, the following claims are appended to apprise the public of the scope of the invention. [Explanation of symbols]

[0069] 1: image forming apparatus, 102: photosensitive element, 106: exposure head (exposure device), 201: light emitting element array, 400: light emitting chip, 602: light emitting element, 901: D / A (adjustment circuit), 902: reference current source, 903: light emitting element group, D1: first direction (axial direction), D2: second direction (circumferential direction), R j_m : Light-emitting element, W G1 ~W G10 :Width of the light-emitting element group

Claims

1. An exposure device that exposes a photosensitive member, a plurality of light-emitting chips regularly arranged in a first direction parallel to an axial direction of the photosensitive member; Equipped with Each of the plurality of light emitting chips includes: K current sources (K is an integer equal to or greater than 2); a plurality of light emitting elements arranged in the first direction, the plurality of light emitting elements being grouped into K light emitting element groups; Including, where k=1, 2, . . . , K, a light emitting element belonging to the k-th light emitting element group is supplied with current from the k-th current source, the light emitting elements belonging to each of the K light emitting element groups are arranged consecutively in the first direction; For any one of the K light emitting element groups, the width occupied by the one light emitting element group in the first direction is greater than a lower limit value corresponding to a spatial frequency at which the contrast sensitivity of human vision represented by a visual transfer function reaches a peak. Exposure device.

2. The visual transfer function is a Dooley-Shaw VTF function expressed by the following equation: [Equation 2] where λ represents the spatial frequency.

2. The exposure apparatus according to claim 1.

3. The exposure apparatus according to claim 2 , wherein the lower limit is 1 millimeter.

4. 2. The exposure apparatus of claim 1, wherein for any one of the K light emitting element groups, the width occupied by that one light emitting element group in the first direction is smaller than an upper limit value corresponding to the allowable amount of voltage drop in the current supply from the current source to the light emitting element.

5. 5. The exposure apparatus according to claim 4, wherein the upper limit is 4 millimeters.

6. Each of the plurality of light-emitting chips includes a two-dimensional array of light-emitting elements having M rows (M is an integer of 2 or more) in a second direction perpendicular to the first direction and N columns (N is an integer of 2 or more) in the first direction, the width occupied by the one light emitting element group in the first direction corresponds to the distance between two light emitting elements having the widest distance in the first direction among the light emitting elements belonging to the one light emitting element group; 2. The exposure apparatus according to claim 1.

7. Each of the plurality of light emitting chips includes: K adjustment circuits that adjust the amounts of current supplied from the K current sources so that the amounts of light emitted from the K light emitting element groups are uniform; The exposure apparatus of claim 1 further comprising:

8. the photoreceptor; an exposure apparatus according to any one of claims 1 to 7; An image forming apparatus comprising:

Images