Image formation apparatus

Abstract

To provide an image formation apparatus which can perform multiple exposure with an exposure head without using a delay circuit.SOLUTION: An image formation apparatus comprises: an exposure head 6 for forming an electrostatic latent image on the surface of a photoreceptor drum 1, the exposure head 6 including a light emission part 50 that includes a lower electrode 54 having a plurality of electrodes two-dimensionally arrayed in the main-scanning direction and the sub-scanning direction, a light emitting layer 56 which is laminated on the lower electrode 54 and emits light by application with the voltage, and an upper electrode 58 that is laminated on the light emitting layer 56; and an image controller part 70 which can control application of the voltage to each of the plurality of electrodes included in the lower electrode 54 so as to perform multiple exposure with the plurality of light emission parts 50 arrayed in the sub-scanning direction. The plurality of electrodes are arranged such that a pitch in the sub-scanning direction of the plurality of electrodes included in the lower electrode 54 is the integral multiplication excluding the unity magnification of the resolution in the sub-scanning direction of the image formed by the image formation apparatus.SELECTED DRAWING: Figure 13

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus such as an electrophotographic copying machine or an electrophotographic printer that forms an image on a sheet using an electrophotographic image forming method. [Background technology]

[0002] When forming an image using an electrophotographic image forming apparatus, an electrostatic latent image is first formed on the surface of a photoconductor by irradiating the surface of the photoconductor with light according to image data, then a developing device attaches toner to the electrostatic latent image on the surface of the photoconductor to form a toner image, the toner image is transferred to a sheet, and the toner image transferred to the sheet is heated by a fixing device to fix it to the sheet, thereby forming an image.

[0003] Here, Patent Document 1 describes an image forming apparatus that forms an electrostatic latent image by irradiating a photosensitive member with light, and that includes an exposure head having a light-emitting unit using organic electroluminescence (EL) and a lens that focuses the light irradiated from the light-emitting unit on the surface of the photosensitive member. By using an exposure head in this way, it is possible to reduce the number of parts compared to a laser scanning type configuration in which an electrostatic latent image is formed by deflecting and scanning a laser beam using a rotating polygonal mirror, thereby enabling the image forming apparatus to be made smaller and the manufacturing costs to be reduced.

[0004] Furthermore, the light intensity of each light-emitting element using organic electroluminescence in the exposure head is not sufficiently high. Therefore, Patent Document 1 describes a configuration in which multiple light-emitting elements irradiate the same portion of the surface of a photoconductor with light to supplement the light intensity required to form an electrostatic latent image on the surface of the photoconductor. Specifically, in the exposure head, light-emitting elements are two-dimensionally arranged in the direction of the rotation axis of the photoconductor (main scanning direction) and the rotation direction (sub-scanning direction). Then, light-emitting elements adjacent to each other in the rotation direction of the photoconductor emit light at different timings depending on the rotation speed of the photoconductor, thereby irradiating the same portion of the surface of the photoconductor with light from multiple light-emitting elements. Hereinafter, irradiating the same portion of the surface of the photoconductor with light from multiple light-emitting elements in this manner will be referred to as multiple exposure. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2018-134820 Summary of the Invention [Problem to be solved by the invention]

[0006] Patent Document 1 does not mention the relationship between the resolution in the sub-scanning direction of an image formed by an image forming apparatus and the pitch in the sub-scanning direction of the light-emitting elements of the exposure head. Therefore, depending on the relationship between the two, it may be necessary to provide a delay circuit in the circuit that drives the light-emitting elements to shift the light emission timing of light-emitting elements adjacent in the sub-scanning direction during multiple exposure, which could result in an increase in the size of the circuit.

[0007] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an image forming apparatus that can perform multiple exposures using an exposure head without using a delay circuit. [Means for solving the problem]

[0008] A representative configuration of an image forming apparatus according to the present invention for achieving the above object is an image forming apparatus for forming an image by irradiating a surface of a photosensitive member with light to form an electrostatic latent image and attaching toner to the electrostatic latent image, the image forming apparatus including an exposure head for irradiating the surface of the photosensitive member with light to form the electrostatic latent image, the exposure head including a substrate, a first electrode layer including a plurality of electrodes arranged two-dimensionally in a rotation direction of the photosensitive member and in a rotation axis direction of the photosensitive member, the plurality of electrodes being separately arranged on the substrate, a light emitting layer laminated on the first electrode layer and emitting light when a voltage is applied thereto, and a second electrode layer arranged on the opposite side of the light emitting layer from the side on which the first electrode layer is arranged and through which light can pass. and a control unit that controls the application of voltage to each of the plurality of electrodes included in the first electrode layer based on image data so that the light-emitting layer emits light, the control unit being capable of controlling the voltage to each of the plurality of electrodes based on the image data so that one pixel is formed by controlling the voltage to a plurality of electrodes arranged at different positions in the rotation direction, wherein the plurality of electrodes are arranged so that the pitch in the rotation direction of the plurality of electrodes included in the first electrode layer is an integer multiple, excluding equal multiples, of the resolution in the rotation direction of an image formed by the image forming device. [Effects of the Invention]

[0009] According to the present invention, in an image forming apparatus, multiple exposure can be performed by the exposure head without using a delay circuit. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic cross-sectional view of an image forming apparatus. [Figure 2] 2A and 2B are a perspective view and a cross-sectional view of a photosensitive drum and an exposure head. [Figure 3] FIG. 2 is a diagram showing the mounting surface of a printed circuit board provided in the exposure head. [Figure 4] 10A and 10B are diagrams illustrating the positional relationship between a rod lens array and a light-emitting unit. [Figure 5] FIG. 2 is a schematic diagram of a light-emitting element array chip. [Figure 6] 1 is a cross-sectional view of a light-emitting element array chip; [Figure 7] FIG. 2 is a schematic diagram for explaining the arrangement of light-emitting units. [Figure 8] FIG. 2 is a block diagram showing a system configuration of an image controller unit and an exposure head. [Figure 9] FIG. 2 is a block diagram showing a system configuration of a light-emitting element array chip. [Figure 10] FIG. 4 is a circuit diagram of a data holding unit. [Figure 11] 10 is an operation timing chart of a data holding unit. [Figure 12] FIG. 2 is a circuit diagram of an analog section. [Figure 13] FIG. 4 is a diagram illustrating an exposure image of a photosensitive drum. [Figure 14] FIG. 4 is a diagram illustrating an exposure image of a photosensitive drum. [Figure 15] FIG. 4 is a diagram illustrating an exposure image of a photosensitive drum. [Figure 16] FIG. 2 is a schematic diagram for explaining the configuration of a light-emitting section. [Figure 17] FIG. 4 is a diagram illustrating an exposure image of a photosensitive drum. [Figure 18] FIG. 2 is a schematic diagram for explaining the configuration of a light-emitting section. [Figure 19] FIG. 4 is a diagram illustrating an exposure image of a photosensitive drum. DETAILED DESCRIPTION OF THE INVENTION

[0011] (First embodiment) <Image forming device> The overall configuration of the image forming apparatus A according to the first embodiment of the present invention will be described below with reference to the drawings, along with its operation during image formation. Note that the dimensions, materials, shapes, relative positions, etc. of the components described below are not intended to limit the scope of the present invention unless otherwise specified.

[0012] Image forming apparatus A is a full-color image forming apparatus that forms an image by transferring toner of four colors, yellow Y, magenta M, cyan C, and black K, onto a sheet. In the following description, components that use toner of each color are given the suffixes Y, M, C, and K, but the configuration and operation of each component are essentially the same except for the color of the toner used, so the suffixes will be omitted as appropriate unless a distinction is required.

[0013] Fig. 1 is a schematic cross-sectional view of an image forming apparatus A. As shown in Fig. 1, the image forming apparatus A has an image forming unit that forms an image. The image forming unit has photosensitive drums 1 (1Y, 1M, 1C, 10K) as photosensitive members, charging devices 2 (2Y, 2M, 2C, 2K), exposure heads 6 (6Y, 6M, 6C, 6K), developing devices 4 (4Y, 4M, 4C, 4K), and transfer devices 5 (5Y, 5M, 5C, 5K).

[0014] Next, a description will be given of the image forming operation by the image forming apparatus A. When forming an image, first, a sheet S stored in a sheet cassette 99a or a sheet cassette 99b is sent to a registration roller 96 by pickup rollers 91a and 91b, feeding rollers 92a and 92b, and conveying rollers 93a to 93c. Thereafter, the sheet S is sent to the conveyor belt 11 by the registration roller 96 at a predetermined timing.

[0015] Meanwhile, in the image forming unit, the surface of the photosensitive drum 1Y is first charged by the charging device 2Y. Next, the exposure head 6Y irradiates the surface of the photosensitive drum 10Y with light in accordance with image data read by the image reading unit 90 or image data transmitted from an external device (not shown), thereby forming an electrostatic latent image on the surface of the photosensitive drum 10Y. Thereafter, the developing device 4Y causes yellow toner to adhere to the electrostatic latent image formed on the surface of the photosensitive drum 1Y, thereby forming a yellow toner image on the surface of the photosensitive drum 1Y. The toner image formed on the surface of the photosensitive drum 1Y is transferred to the sheet S being transported by the transport belt 11 by applying a transfer bias to the transfer device 5Y.

[0016] Using a similar process, photosensitive drums 1M, 1C, and 1K are also irradiated with light from exposure heads 6M, 6C, and 6K to form electrostatic latent images, and magenta, cyan, and black toner images are then formed by developing devices 4M, 4C, and 4K. Then, by applying a transfer bias to transfer devices 5M, 5C, and 5K, these toner images are transferred and superimposed on the yellow toner image on sheet S. As a result, a full-color toner image corresponding to the image data is formed on the surface of sheet S.

[0017] Thereafter, the sheet S carrying the toner image is conveyed by a conveyor belt 97 to a fixing device 94, where it is subjected to a heat and pressure treatment. This fixes the toner image on the sheet S to the sheet S. Thereafter, the sheet S with the fixed toner image is discharged onto a discharge tray 95 by a discharge roller 98.

[0018] <Exposure head> Next, the configuration of the exposure head 6 will be described.

[0019] Fig. 2(a) is a perspective view of the photosensitive drum 1 and the exposure head 6. Fig. 2(b) is a cross-sectional view of the photosensitive drum 1 and the exposure head 6. Figs. 3(a) and 3(b) are diagrams showing the mounting surfaces on one side and the other side of the printed circuit board 22 provided in the exposure head 6. Fig. 3(c) is an enlarged view of area V shown in Fig. 3(b).

[0020] 2, the exposure head 6 is fixed by a fixing member (not shown) at a position facing the surface of the photosensitive drum 1. The exposure head 6 has a light-emitting element array chip 40 that emits light, and a printed circuit board 22 on which the light-emitting element array chip 40 is mounted. The exposure head 6 also has a rod lens array 23 that focuses (focuses) the light emitted from the light-emitting element array chip 40 on the photosensitive drum 1, and a housing 24 to which the rod lens array 23 and the printed circuit board 22 are fixed.

[0021] A connector 21 is mounted on the surface of the printed circuit board 22 opposite to the surface on which the light-emitting element array chip 40 is mounted. The connector 21 is provided for transmitting control signals for the light-emitting element array chip 40 sent from the image controller unit 70 (FIG. 8) and for connecting a power supply line. The light-emitting element array chip 40 is driven via the connector 21.

[0022] As shown in Fig. 3, 20 light-emitting element array chips 40 are mounted on the printed circuit board 22 in a staggered arrangement in two rows. Each light-emitting element array chip 40 has 748 light-emitting units 50 arranged at a predetermined resolution pitch in its longitudinal direction (arrow X direction). Each light-emitting element array chip 40 has four light-emitting units 50 arranged at a predetermined pitch in its lateral direction (arrow Y direction). That is, in each light-emitting element array chip 40, the light-emitting units 50 are two-dimensionally arranged in the directions of arrows X and Y.

[0023] In this embodiment, the resolution pitch of the light-emitting element array chip 40 is 1200 dpi (approximately 21.16 μm). The distance from one end to the other end of the light-emitting element 50 in the longitudinal direction of each light-emitting element array chip 40 is approximately 15.828 mm. In other words, the exposure head 6 has a total of 14,960 light-emitting elements 50 in the direction of arrow X, which enables exposure processing corresponding to an image width in the longitudinal direction of approximately 316 mm (≒ approximately 15.8 mm × 20 chips).

[0024] In the longitudinal direction of the light-emitting element array chip 40, the interval L1 between the light-emitting sections 50 of adjacent light-emitting element array chips 40 is approximately 21.16 μm. In other words, the longitudinal pitch of the light-emitting sections 50 at the boundary between each light-emitting element array chip 40 is a pitch corresponding to a resolution of 1200 dpi. In addition, in the lateral direction of the light-emitting element array chip 40 (direction of arrow Y), the interval L2 between the light-emitting sections 50 of adjacent light-emitting element array chips 40 is approximately 127 μm (equivalent to six pixels at 1200 dpi, or four pixels at 800 dpi).

[0025] In this embodiment, the arrow X direction, which is the longitudinal direction of the light-emitting element array chip 40, is the direction of the rotation axis of the photosensitive drum 1 and is also the main scanning direction. The arrow Y direction, which is the lateral direction of the light-emitting element array chip 40, is the rotation direction of the photosensitive drum 1 and is also the sub-scanning direction. The rotation direction of the photosensitive drum 1 is the tangential direction of the photosensitive drum 1 at the exposure position on the photosensitive drum 1 where light is collected by the exposure head 6. The arrow Z direction is the stacking direction in which each layer of the light-emitting unit 50 having a layered structure described below overlaps. The longitudinal direction of the light-emitting element array chip 40 may be inclined by approximately ±1° with respect to the rotation axis direction of the photosensitive drum 1. The lateral direction of the light-emitting element array chip 40 may also be inclined by approximately ±1° with respect to the rotation direction of the photosensitive drum 1.

[0026] Fig. 4 is a diagram showing the positional relationship between the rod lens array 23 and the light-emitting units 50 of the light-emitting element array chip 40. As shown in Fig. 4, a predetermined number of rod lens arrays 23 are arranged in a staggered pattern in the direction of arrow X and in two rows in the direction of arrow Y so as to cover the light-emitting units 50 of the light-emitting element array chip 40. The diameter of the rod lens array 23 is set to 290 µm, and one rod lens array 23 is configured to collect light emitted from a plurality of light-emitting units 50.

[0027] <Light-emitting element array chip> Next, the configuration of the light-emitting element array chip 40 will be described.

[0028] Fig. 5 is a schematic diagram of the light-emitting element array chip 40. Fig. 6 is a cross-sectional view of the light-emitting element array chip 40 taken along the MM cross section shown in Fig. 5. Fig. 7 is a schematic diagram for explaining the arrangement of the light-emitting units 50 of the light-emitting element array chip 40.

[0029] 5, the light-emitting element array chip 40 has a light-emitting substrate 42 (substrate) incorporating a circuit section 46 for controlling the light-emitting sections 50, a light-emitting region 44 in which a plurality of light-emitting sections 50 are regularly arranged on the light-emitting substrate 42, and wire-bonding pads 48. Signals are input and output between the outside of the light-emitting element array chip 40 and the circuit section 46, and power is supplied to the circuit section 46 through the wire-bonding pads 48. The circuit section 46 can be an analog drive circuit, a digital control circuit, or a circuit including both.

[0030] As shown in FIG. 6, the light-emitting section 50 is composed of a light-emitting substrate 42, a plurality of lower electrodes 54 two-dimensionally arranged on the light-emitting substrate 42 at regular intervals (intervals d1 and d2 shown in FIG. 7) in the directions of arrows X and Y, a light-emitting layer 56, and an upper electrode 58.

[0031] The lower electrodes 54 (first electrode layer having a plurality of electrodes) are a plurality of electrodes formed in a layered and separated manner on the light emitting substrate 42, and are electrodes provided corresponding to each pixel. In other words, each lower electrode 54 is provided to form one pixel.

[0032] The upper electrode 58 (second electrode layer) is laminated on the light-emitting layer 56 at a position opposite to the side on which the lower electrode 54 is disposed with respect to the light-emitting layer 56. The upper electrode 58 is an electrode that can transmit (be transparent to) light of the emission wavelength of the light-emitting layer 56.

[0033] The circuit unit 46 controls the potential of the selected lower electrode 54 based on a control signal generated in response to image data, thereby generating a potential difference between the selected lower electrode 54 and the upper electrode 58. When a potential difference is generated between the upper electrode 58, which is an anode, and the lower electrode 54, which is a cathode, electrons flow from the cathode into the light-emitting layer 56, and holes flow from the anode into the light-emitting layer 56. The recombination of the electrons and holes in the light-emitting layer 56 causes the light-emitting layer 56 to emit light.

[0034] When the light-emitting layer 56 emits light, the light that travels toward the upper electrode 58 passes through the upper electrode 58 and is emitted. Light that travels from the light-emitting layer 56 toward the lower electrode 54 is reflected by the lower electrode 54 toward the upper electrode 58, and this reflected light also passes through the upper electrode 58 and is emitted. In this manner, the light-emitting section 50 emits light. Note that although there is a time difference in the emission timing between the light that is emitted from the light-emitting layer 56 directly toward the upper electrode 58 and the light that is reflected by the lower electrode 54 and emitted from the upper electrode 58, the emission can be considered to be nearly simultaneous because the layer thickness of the light-emitting section 50 is extremely thin.

[0035] In this embodiment, the light-emitting substrate 42 is a silicon substrate. The upper electrode 58 is preferably transparent to the emission wavelength of the light-emitting layer 56. For example, by using a transparent electrode such as indium tin oxide (ITO), the aperture ratio is substantially 100%, and light emitted by the light-emitting layer 56 passes through the upper electrode 58 and is emitted as is. In this embodiment, the upper electrode 58 is an anode provided in common to each of the lower electrodes 54. However, the upper electrode 58 may be provided individually for each of the lower electrodes 54, or one upper electrode 58 may be provided for each of the plurality of lower electrodes 54. In addition, when a transparent electrode is used as the upper electrode 58, the entire electrode does not necessarily have to be a transparent electrode; only the openings through which light is emitted may be transparent, and the area other than the openings may be wired with an electrode other than a transparent electrode, such as a metal wire.

[0036] The light-emitting layer 56 may be an organic EL film or an inorganic EL layer. When an organic EL film is used as the light-emitting layer 56, the light-emitting layer 56 may be a laminated structure including functional layers such as an electron transport layer, a hole transport layer, an electron injection layer, a hole injection layer, an electron blocking layer, and a hole blocking layer as needed. The light-emitting layer 56 may be formed continuously in the direction of arrow X, or may be divided into segments of the same size as the lower electrodes 54. Alternatively, each lower electrode 54 may be divided into multiple groups, and one light-emitting layer 56 may be laminated on top of the lower electrodes 54 belonging to each group.

[0037] When using a moisture-sensitive light-emitting material such as an organic EL layer (organic light-emitting layer) or an inorganic EL layer as the light-emitting layer 56, it is desirable to seal the light-emitting region 44 to prevent moisture from entering. For example, a sealing film is formed by forming a thin film, either alone or in layers, of silicon oxide, silicon nitride, aluminum oxide, or the like. A method that excels in covering structures such as steps is preferred as a method for forming the sealing film, and atomic layer deposition (ALD) can be used, for example. The materials, configurations, and formation methods of the sealing film are merely examples, and are not limited to the above examples; any suitable method may be selected as appropriate.

[0038] The lower electrode 54 is preferably made of a metal having a high reflectivity at the wavelength of light emitted by the light-emitting layer 56. Examples of suitable materials include Ag, Al, or an alloy of Ag and Al. The lower electrode 54 is formed using Si integrated circuit processing technology together with the circuit section 46 and is directly connected to the drive section of the circuit section 46. Forming the lower electrode 54 using Si integrated circuit processing technology allows for high precision with a process rule of approximately 0.2 μm, allowing the lower electrodes 54 to be arranged with high precision and high density. Furthermore, because the lower electrodes 54 can be arranged with high density, most of the light-emitting region 44 can be made to emit light, thereby improving the utilization efficiency of the light-emitting region 44. The spaces between each lower electrode 54 are filled with the organic material of the light-emitting layer 56, and each lower electrode 54 is separated by the organic material.

[0039] Furthermore, when the voltage across both ends of the light-emitting unit 50 exceeds a predetermined value, a current begins to flow, and the value of the current then increases approximately in proportion to the voltage value. The voltage at which a current begins to flow varies among the light-emitting units 50. Therefore, before the product is shipped from the factory, the light-emitting units 50 of the light-emitting element array chip 40 are individually and sequentially made to emit light, and the current flowing through the light-emitting units 50 is adjusted so that the light focused through the rod lens array 23 has a predetermined light intensity. Note that, before the product is shipped from the factory, the exposure head 6 is not only subjected to the above-mentioned light intensity adjustment, but also to focus adjustment, which adjusts the distance between the light-emitting element array chip 40 and the rod lens array 23.

[0040] As shown in FIG. 7 , the light-emitting sections 50 are arranged in a matrix in the light-emitting region 44 at predetermined intervals in the directions indicated by the arrows X and Y. In this embodiment, the width W1 of each light-emitting section 50 in the direction indicated by the arrow X is 19.80 μm, and the distance d1 between adjacent light-emitting sections 50 in the direction indicated by the arrow X is 0.68 μm. That is, the light-emitting sections 50 are arranged at a pitch of 21.16 μm (1200 dpi) in the direction indicated by the arrow X. Note that the pitch of the light-emitting sections 50 in the direction indicated by the arrow X may deviate within the tolerance range. The pitch tolerance of the light-emitting sections 50 in the direction indicated by the arrow X is ±1% of the nominal design pitch of the light-emitting sections 50 in the direction indicated by the arrow X. That is, the pitch tolerance of the light-emitting sections 50 in the direction indicated by the arrow X is ±0.21 μm in this embodiment. Furthermore, the width, shape, and arrangement of the light-emitting sections 50 are substantially determined by the width, shape, and arrangement of the lower electrodes 54 in this embodiment, and therefore can also be rephrased as the width, shape, and arrangement of the lower electrodes 54.

[0041] The width W2 of the light-emitting section 50 in the direction of the arrow Y is also 19.80 μm, the same as the width W1. That is, the light-emitting section 50 of this embodiment is square-shaped with one side measuring 19.80 μm. Although the light-emitting section 50 is square-shaped because the widths W1 and W2 are equal, the dimensions of these widths W1 and W2 may vary within the tolerance range. In this embodiment, the tolerance of both widths W1 and W2 is ±0.2 μm.

[0042] The distance d2 between adjacent light-emitting sections 50 in the direction of arrow Y is also 0.68 μm, similar to the distance d1. The light-emitting sections 50 are also arranged at a pitch of 21.16 μm (1200 dpi) in the direction of arrow Y. The pitch of the light-emitting sections 50 in the direction of arrow Y may deviate within a tolerance range. The tolerance of the pitch of the light-emitting sections 50 in the direction of arrow Y is ±1% of the nominal design pitch of the light-emitting sections 50 in the direction of arrow Y. In other words, the tolerance of the pitch of the light-emitting sections 50 in the direction of arrow Y in this embodiment is ±0.21 μm. The distances d1 and d2 between the light-emitting sections 50 are set wider than the distance dz ( FIG. 6 ) between the upper electrode 58 and the lower electrode 54. This configuration can suppress leakage current between adjacent lower electrodes 54 in the directions of arrow X and arrow Y, thereby suppressing erroneous light emission by the light-emitting sections 50.

[0043] In the present invention, the shape of the light-emitting unit 50 is not limited to a square, and may be a polygon with more sides than a square, a circle, an ellipse, or the like, as long as it emits light in an exposure area size corresponding to the output resolution of the image forming apparatus A and the image quality of the output image satisfies the design specifications of the image forming apparatus A. However, because organic light-emitting materials emit less light than LEDs, it is preferable to form the light-emitting unit 50 in a square shape and reduce the distance between adjacent light-emitting units 50, thereby ensuring a light-emitting area sufficient to obtain a light amount sufficient to change the potential of the photosensitive drum 1. Furthermore, the number of light-emitting units 50 arranged in parallel in the direction of arrow Y is not limited to four, as long as two or more are provided, and is determined based on the light amount and resolution required for the exposure process of the exposure head 6.

[0044] Furthermore, the distance between the light-emitting sections 50, i.e., the distance between the lower electrodes 54, is defined based on the center of gravity of the nominal design lower electrodes 54. That is, if the shape of the lower electrodes 54 is a regular polygon, the distance between the lower electrodes 54 is set based on the intersection of the diagonals, if the shape is a perfect circle, the center of the circle, or if the shape is an ellipse, the intersection of the major and minor axes. Note that if the shape of the lower electrodes 54 is a regular polygon, the corners do not have to be perfect corners and may be rounded.

[0045] <Exposure head system configuration> Next, we will explain the configuration of the exposure head 6 and the image controller unit 70 (control unit) that controls the exposure head 6. The image controller unit 70 is provided on the main body side of the image forming apparatus A. Note that although the following explains the control performed when processing one image data (monochromatic), when performing image forming operations, similar processing is performed in parallel for four image data corresponding to yellow, magenta, cyan, and black.

[0046] Fig. 8 is a block diagram showing the system configuration of the image controller unit 70 and the exposure head 6. As shown in Fig. 8, the image controller unit 70 includes an image data generation unit 71, a chip data conversion unit 72, a CPU 73, and a synchronization signal generation unit 74. Using these components, the image controller unit 70 processes image data and image formation timing, and sends control signals to the printed circuit board 22 of the exposure head 6 to control the exposure head 6.

[0047] Image data of a document read by the image reading unit 90 and image data transferred from an external device via a network are input to the image data generation unit 71. The image data generation unit 71 performs dithering processing on the input image data at a resolution specified by the CPU 73, and generates image data for outputting an image. In this embodiment, dithering processing is performed at a resolution of 2400 dpi in both the main scanning direction and the sub-scanning direction.

[0048] The synchronization signal generating unit 74 periodically generates a line synchronization signal (control signal) indicating the start of image data capture and transmits it to the chip data converting unit 72. The CPU 73 determines, for a preset rotation speed of the photosensitive drum 1, the period during which the surface of the photosensitive drum 1 moves in the rotation direction by a pixel size corresponding to the resolution in the sub-scanning direction of the image formed by the image forming apparatus A, as one line period, and instructs the synchronization signal generating unit 74 on the time interval of the signal period.

[0049] In this embodiment, the resolution of the image formed by the image forming apparatus A in the sub-scanning direction is 2400 dpi, and the photosensitive drum 1 rotates at 200 mm / s. Therefore, the time it takes for the photosensitive drum 1 to move the distance of a pixel size of 2400 dpi (approximately 10.58 μm) is 52.92 μm, and the period of the line synchronization signal is 52.92 μm. The rotational speed of the photosensitive drum 1 is calculated by the CPU 73 based on a setting value stored in a memory unit (not shown).

[0050] The chip data conversion unit 72 divides image data of one line x four columns (the number of light-emitting units 50 in the direction of the arrow Y) into each light-emitting element array chip 40 in synchronization with the line synchronization signal generated and input by the synchronization signal generation unit 74. The chip data conversion unit 72 then transmits the image data together with the clock signal and line synchronization signal to each light-emitting element array chip 40 via a line synchronization signal line 75, a clock signal line 76, and an image data signal line 77. The number of image data signal lines 77 provided is four, which is the same as the number of light-emitting units 50 in the direction of the arrow Y.

[0051] The head information storage unit 171 provided in the exposure head 6 is connected to the CPU 73 via a communication signal line 79. The head information storage unit 171 stores, as head information, the light emission amount and mounting position information of each light-emitting element array chip 40. The light-emitting element array chip 40 causes the light-emitting unit 50 to emit light based on the setting values ​​of each of the above signals input from the image controller unit 70. The light-emitting element array chip 40 also generates a line synchronization signal used by other light-emitting element array chips 40 connected via a line synchronization signal line 75.

[0052] <System configuration of light-emitting element array chip> Next, the system configuration of the light-emitting element array chip 40 will be described.

[0053] Fig. 9 is a block diagram showing the system configuration of the light-emitting element array chip 40. In Fig. 9, wiring is omitted because clock signals are input to all blocks of the digital section 80. As shown in Fig. 9, the circuit section 46 of the light-emitting element array chip 40 is made up of the digital section 80 and an analog section 86.

[0054] The digital section 80 includes a communication IF section 81, a register section 82, a capture signal generation section 83, a line synchronization signal generation section 84, and a data holding section 85. Using these sections, the digital section 80 generates pulse signals for causing the light-emitting sections 50 to emit light based on preset settings, image data signals, and line synchronization signals that are synchronized with a clock signal by a communication signal, and transmits the pulse signals to the analog section 86. There are 748 data holding sections 85 (85-001 to 85-748), which is the number of light-emitting sections 50 in one light-emitting element array chip 40 in the direction of the arrow X.

[0055] The line synchronization signal generation unit 84 delays the input line synchronization signal by a predetermined time to generate a line synchronization signal to be used by other light-emitting element array chips 40 connected via the line synchronization signal line 75. The capture signal generation unit 83 outputs a data latch signal we001 to the data holding unit 85-001 at a timing delayed by a predetermined set time input from the register unit 82 from the input line synchronization signal.

[0056] The register unit 82 stores information on the delay time of the capture signal generation unit 83 described above, setting information on the drive current set by the analog unit 86, etc. The communication IF unit 81 controls writing and reading of setting values ​​to and from the register unit 82 based on a communication signal input from the CPU 73.

[0057] <Data storage section> Next, the configuration of the data holding unit 85 will be described.

[0058] Fig. 10 is a circuit diagram of the data holding unit 85. As shown in Fig. 10, four lines of image data (image data 1 to 4), a clock signal, and a data latch signal wen (n = 1 to 748) are input to the data holding unit 85. Each data holding unit 85 has four flip-flop circuits and four gate circuits for latching the four lines of image data that are simultaneously input at the timing when the data latch signal is input. Each data holding unit 85 also has one flip-flop circuit for delaying the data latch signal by one clock and outputting it.

[0059] 11 is an operation timing chart of the data holding unit 85. As shown in FIG. 11, four lines of image data (D1[1] to D1[4]) are simultaneously input to the data holding unit 85-001. The data holding unit 85-001 latches this image data at the timing when the data latch signal we001 is input from the capture signal generating unit 83, and generates drive signals (P001[1] to P001[4]). The data holding unit 85-001 also delays the input data latch signal we001 by one clock and transmits it to the next data holding unit 85-002 as data latch signal we002.

[0060] Four lines of image data (D2[1] to D2[4]) are also input simultaneously to the data holding unit 85-002. The data holding unit 85-002 latches this image data at the timing when the data latch signal we002 is input from the data holding unit 85-001, and generates drive signals (P002[1] to P002[4]). The data holding unit 85-002 also delays the data latch signal we002 by one clock and sends it to the data holding unit 85-003 as data latch signal we003.

[0061] In this way, the data holding units 85 (-001 to 748) sequentially latch the image data while transmitting the data latch signal up to the 748th data holding unit 85. Then, when the data holding units 85 (-001 to 748) latch the image data, they transmit the latched signal as a drive signal to the analog unit 86. In this embodiment, four lines of image data are latched with one data latch signal, so drive signals for four lines (four pixels) are output simultaneously.

[0062] <Analog section> Next, we will explain the configuration of the analog unit 86. The analog unit 86 is made up of a drive circuit that is connected one-to-one to each light-emitting unit 50. For ease of explanation, the following will explain one drive circuit, but it is assumed that there are the same number of similar drive circuits as the number of light-emitting units 50, that is, 748 units x 4 columns = 2992 units.

[0063] Fig. 12 is a circuit diagram of the analog unit 86. As shown in Fig. 12, the analog unit 86 is composed of a DAC 61 for setting a current, a MOSFET 62 for controlling a current, and a MOSFET 63 for switching. The DAC 61 receives a current setting value to be flowed to the light-emitting unit 50 from the register unit 82 of the digital unit 80 as a digital value, converts it into an analog voltage, and outputs it.

[0064] The current control MOSFET 62 is a Pch MOSFET, with its source terminal connected to the power supply voltage VDD and its gate terminal connected to the output of the DAC 61. The current flowing from the source to the drain increases as the analog voltage input from the DAC 61 increases.

[0065] The switching MOSFET 63 is a Pch MOSFET, and its source terminal is connected to the drain terminal of the current control MOSFET 62, and its gate terminal receives a drive signal output from the data holding unit 85. The drive signal is a binary signal of Hi level and Low level, and when a Hi level signal is input, the MOSFET 63 turns ON and a current controlled by the current control MOSFET 62 flows from the source to the drain. The drain terminal is connected to the anode terminal of the light-emitting unit 50, and this current serves as a drive current for the light-emitting unit 50. In this embodiment, drive current for four lines (four pixels) is output simultaneously, and therefore four lines (four pixels) of the light-emitting units 50 emit light simultaneously.

[0066] <Lighting control of light-emitting units during image formation> Next, the lighting control of the light-emitting unit 50 during image formation will be described. In the following description, light emission by the light-emitting unit 50 refers to the light emitted by the light-emitting unit 50 being sufficient to change the charge potential of the photosensitive drum 1 to an extent that a toner image is not developed as a visible image. In other words, light emission does not include the light-emitting unit 50 emitting light having an amount sufficient to change the charge potential of the photosensitive drum 1 to an extent that a toner image is not developed as a visible image.

[0067] Fig. 13 is a diagram showing an exposure image of the photosensitive drum 1. In Fig. 13, squares on the photosensitive drum 1 indicate pixels on the photosensitive drum 1, and the numbers (1-1 to 16-4) within the pixels indicate the type of image data written to each pixel. Although the number of pixels in the direction of the arrow X is 748 pixels x 20 chips = 14,960 pixels, for ease of explanation, Fig. 13 shows only four pixels in the direction of the arrow X.

[0068] 13, first, at time T1, image data for four lines is sent from the image controller unit 70 to the exposure head 6. As a result, the four light-emitting units 50 arranged in parallel in the direction of the arrow Y emit light simultaneously, and four lines of pixels (1-1, 3-1, 5-1, 7-1) on the photosensitive drum 1 are exposed simultaneously.

[0069] One clock later, the image controller unit 70 transmits the next four lines of image data to the exposure head 6. As a result, the four light-emitting units 50 adjacent to the four light-emitting units 50 that initially emitted light in the direction of the arrow X simultaneously emit light, and four lines of pixels (1-2, 3-2, 5-2, 7-2) adjacent to the direction of the arrow X on the photosensitive drum 1 are simultaneously exposed.

[0070] By repeating this operation every clock cycle, four lines of pixels at 2400 dpi are exposed on the photosensitive drum 1 during 52.92 μs, which is the period of the line synchronization signal. Here, at time T1, image data (1-1 to 1-4) for exposing the first line of the photosensitive drum 1 is transmitted to the light-emitting unit 50 located furthest downstream in the rotational direction of the photosensitive drum 1. Meanwhile, in this embodiment, the image resolution in the sub-scanning direction is 2400 dpi, and the resolution pitch of the light-emitting element array chip 40 is 1200 dpi (approximately 21.16 μm). Therefore, for the light-emitting unit 50 to which the image data for exposing the first line is transmitted, image data spaced one line at a time at 2400 dpi is transmitted to the light-emitting unit 50 adjacent to the light-emitting unit 50 located upstream in the rotational direction of the photosensitive drum 1. For example, image data (3-1 to 3-4) for exposing the third line of the photosensitive drum 1 is transmitted to the light-emitting unit 50 located one line upstream of the light-emitting unit 50 for exposing the first line. Similarly, image data (5-1 to 5-4) and image data (7-1 to 7-4) are transmitted to the further upstream light-emitting unit 50. That is, the image formed on the photosensitive drum 1 at time T1 is spaced one line apart at 2400 dpi as shown in FIG.

[0071] Next, at time T2, when the photosensitive drum 1 has rotated in the sub-scanning direction (arrow Y direction) by one line (10.58 μm) of 2400 dpi relative to time T1, image data for four lines is transmitted from the image controller unit 70 to the exposure head 6 in the same manner as at time T1. Here, the image data transmitted to each line of the light-emitting unit 50 at time T2 is transmitted shifted by one line relative to time T1.

[0072] That is, at time T2, image data (2-1 to 2-4) for exposing the second line of the photosensitive drum 1 is transmitted to the light-emitting unit 50 located furthest downstream in the rotation direction of the photosensitive drum 1. Furthermore, image data (4-1 to 4-4) for exposing the fourth line of the photosensitive drum 1, spaced one line at a time at 2400 dpi, is transmitted to the light-emitting unit 50 adjacent to the light-emitting unit 50 on the upstream side in the rotation direction of the photosensitive drum 1 to which the image data for exposing the second line has been transmitted. Similarly, image data (6-1 to 6-4) and image data (8-1 to 8-4) are transmitted one line at a time to the light-emitting unit 50 adjacent to the light-emitting unit 50 in the rotation direction of the photosensitive drum 1.

[0073] Furthermore, at time T3, when the photosensitive drum 1 has rotated in the sub-scanning direction (arrow Y direction) by one line (10.58 μm) of 2400 dpi relative to time T2, image data for four lines is transmitted from the image controller unit 70 to the exposure head 6 in the same manner as at times T1 and T2. Here, the image data transmitted to each line of the light-emitting unit 50 at time T3 is transmitted shifted by one line relative to time T2.

[0074] That is, at time T3, image data (3-1 to 3-4) for exposing the third line of the photosensitive drum 1 is transmitted to the light-emitting unit 50 located furthest downstream in the rotation direction of the photosensitive drum 1. Furthermore, image data (5-1 to 5-4) for exposing the fifth line of the photosensitive drum 1, spaced one line at a time at 2400 dpi, is transmitted to the light-emitting unit 50 adjacent to the light-emitting unit 50 to which the image data for exposing the third line is transmitted. Image data (7-1 to 7-4) and image data (9-1 to 9-4) are also transmitted line by line to the light-emitting unit 50 adjacent to the light-emitting unit 50 in the rotation direction of the photosensitive drum 1.

[0075] As a result, the third, fifth, and seventh lines on the photosensitive drum 1 are subjected to two multiple exposures by the light-emitting units 50 at times T1 and T3. In other words, one pixel is formed by the multiple light-emitting units 50 performing multiple exposure. Thereafter, from time T4 onwards, the same processes as those performed at times T1, T2, and T3 are performed. As a result, at time T7, the seventh line on the photosensitive drum 1 is subjected to exposure processes at times T1 to T7, for a total of four multiple exposures. This operation is repeated for one page of image, and an electrostatic latent image that has been subjected to four multiple exposures is formed over the entire area of ​​the photosensitive drum 1 except for lines 1 to 6.

[0076] As described above, in this embodiment, the pitch of the light-emitting units 50 of the exposure head 6 in the sub-scanning direction is an integer multiple of the resolution pitch in the sub-scanning direction (the rotation direction of the photosensitive drum 1, the direction of the arrow Y) of the image formed by the image forming apparatus A. With this configuration, the photosensitive drum 1 can be multiplexed exposed simply by shifting the image data exposed by the light-emitting units 50 arranged in parallel in the direction of the arrow Y, without providing a delay circuit in the exposure head 6 to shift the light emission timing of the light-emitting units 50 arranged in parallel in the direction of the arrow Y. This prevents the circuit scale of the exposure head 6 from becoming too large, and reduces manufacturing costs.

[0077] In this embodiment, the photosensitive drum 1 is driven at a rotational speed of 200 mm / s, but the present invention is not limited to this. Optimal image formation conditions vary depending on the type of sheet S, etc. For example, when fixing a toner image on cardboard or coated paper in the fixing device 94, a larger amount of heat is required than when fixing a toner image on plain paper, so it is preferable to slow down the conveyance speed of the sheet S and lengthen the fixing time. Therefore, in the following, we will consider a case where the photosensitive drum 1 is driven at a rotational speed of 100 mm / s in order to slow down the conveyance speed of the sheet S.

[0078] When the photosensitive drum 1 is driven at a rotational speed of 100 mm / s, the time required to expose a distance of 2400 dpi (10.58 μm) is 211.66 μs. Therefore, as shown in Figure 14, the photosensitive drum 1 is driven at a rotational speed of 100 mm / s, and the period of the line synchronization signal is set to 105.83 μs. The order in which the light-emitting element 50 of the exposure head 6 emits light and the image data written for each line is controlled in the same way as the control described above using Figure 13.

[0079] In this configuration, the rotation speed of the photosensitive drum 1 is 100 mm / s, half that of the configuration described using FIG. 13, so the time required for exposure at 2400 dpi (10.58 μm) is doubled. Therefore, if the light-emitting unit 50 is driven with the same drive current as in the configuration in which the rotation speed of the photosensitive drum 1 is 200 mm / s, the photosensitive drum 1 will be exposed with twice the intensity. Therefore, it is preferable to adjust the exposure intensity by changing the setting value of the current-setting DAC 61 in accordance with the rotation speed of the photosensitive drum 1. For example, in a configuration in which the rotation speed of the photosensitive drum 1 is 100 mm / s, it is preferable to set the current setting value of the DAC 61 to half that of the configuration in which it is 200 mm / s, so that the exposure intensity is the same.

[0080] Furthermore, in a configuration in which the photosensitive drum 1 is driven at a rotational speed of 100 mm / s, the following configuration can be considered as a configuration in which the exposure intensity is equivalent to a configuration in which the photosensitive drum 1 is driven at a rotational speed of 200 mm / s without changing the current setting value of the DAC 61. As shown in Figure 15, first, the photosensitive drum 1 is driven at a rotational speed of 100 mm / s, and the period of the line synchronization signal is set to 52.92 us without changing from the configuration in which the photosensitive drum 1 is driven at a rotational speed of 200 mm / s.

[0081] 13, the light-emitting unit 50 is controlled to expose 2400 dpi×4 lines of pixels spaced one line apart every 52.92 μs, which is the period of the line synchronization signal, on the photosensitive drum 1. Here, although the photosensitive drum 1 rotates at 100 mm / s, the period of the line synchronization signal is 52.92 μs, which is equivalent to 200 mm / s, and therefore the length of the exposed area on the photosensitive drum 1 in the direction of the arrow Y at time T1 is 5.29 μm, half of 10.58 μm.

[0082] Next, at time T2 when the photosensitive drum 1 has rotated 5.29 μm relative to time T1, image data is not sent from the image controller unit 70 to the exposure head 6, and no exposure is performed on the photosensitive drum 1. Next, at time T3 when the photosensitive drum 1 has rotated 10.58 μm (one line at 2400 dpi) relative to time T1, the light emitting unit 50 is caused to emit light in the same manner as the control of the light emitting unit 50 at time T2 described using FIG.

[0083] As a result, the seventh line on the photosensitive drum 1 is subjected to two multiple exposures by the light-emitting unit 50 at times T1 and T5. Thereafter, similar control is performed until time T13. In other words, the line synchronization signal is output twice, and one of these times image data is not transmitted (image data is thinned out), and the light-emitting unit 50 is turned off (non-emitting). As a result, at time T13, the seventh line on the photosensitive drum 1 is subjected to exposure processing at times T1, T5, T9, and T13, respectively, resulting in a total of four multiple exposures. By repeating this operation, an electrostatic latent image that has been subjected to four multiple exposures is formed over the entire area of ​​the photosensitive drum 1 except for lines 1 to 3.

[0084] By performing such control, the exposure time for each line can be made the same in a configuration in which the photosensitive drum 1 is driven at a rotational speed of 100 mm / s as in a configuration in which the photosensitive drum 1 is driven at a rotational speed of 200 mm / s. Therefore, the exposure intensity can be made the same without changing the current setting value of the DAC 61. Note that with this configuration, the length of the exposure area for each line on the photosensitive drum 1 in the sub-scanning direction (direction of arrow Y) is halved. However, this does not halve the image resolution; it merely reduces the spot diameter in the sub-scanning direction, so there is no adverse effect on the image and the sharpness of the image is improved.

[0085] (Second embodiment) Next, a second embodiment of the image forming apparatus A according to the present invention will be described. Portions that overlap with the first embodiment will be denoted by the same reference numerals and drawings, and descriptions thereof will be omitted.

[0086] 16 is a schematic diagram for explaining the configuration of the light-emitting unit 50 of the light-emitting element array chip 40 according to this embodiment. As shown in FIG. 16, in this embodiment, the light-emitting units 50 adjacent to each other in the direction of the arrow Y are shifted in the direction of the arrow X by a distance d3. In this embodiment, the distance d3 is set to 5.29 μm (4800 dpi).

[0087] The widths W1 and W2 and the intervals d1 and d2 are W1=W2=19.8 μm and d1=d2=0.68 μm, respectively, as in the first embodiment. That is, the pitch of the light-emitting units 50 in the direction of the arrow Y is set to 21.16 μm (1200 dpi), as in the first embodiment. The image forming apparatus A according to this embodiment forms an image with a resolution of 2400 dpi in the sub-scanning direction, and the rotation speed of the photosensitive drum 1 is 200 mm / s. Therefore, the exposure time for a distance of 2400 dpi (10.58 μm) is 52.92 μs, and the period of the line synchronization signal is also 52.92 μs. The remaining configuration of the image forming apparatus A according to this embodiment is the same as that of the first embodiment, except for the control described below.

[0088] Fig. 17 is a diagram showing an exposure image of the photosensitive drum 1. In Fig. 17, squares on the photosensitive drum 1 indicate pixels on the photosensitive drum 1, and the numbers (1-1 to 16-4) within the pixels indicate the type of image data written to each pixel. Although the number of pixels in the direction of the arrow X is 748 pixels x 20 chips = 14,960 pixels, for ease of explanation, Fig. 17 shows only four pixels in the direction of the arrow X.

[0089] As shown in Figure 17, first, at time T1, the light-emitting unit 50 is controlled in the same manner as the control of the light-emitting unit 50 at time T1 described using Figure 13, and pixels of 2400 dpi x 4 lines on the photosensitive drum 1 are exposed, with one line spaced between each line, for 52.92 us, which is the period of the line synchronization signal.

[0090] Next, at time T2, when the photosensitive drum 1 has rotated in the sub-scanning direction (arrow Y direction) by one line (10.58 μm) of 2400 dpi relative to time T1, image data for four lines is transmitted from the image controller unit 70 to the exposure head 6 in the same manner as at time T1. Here, the image data transmitted to each line of the light-emitting unit 50 at time T2 is transmitted shifted by one line relative to time T1.

[0091] That is, at time T2, image data (2-1 to 2-4) for exposing the second line of the photosensitive drum 1 is transmitted to the light-emitting unit 50 located furthest downstream in the rotation direction of the photosensitive drum 1. Furthermore, image data (4-1 to 4-4) for exposing the fourth line, spaced one line apart at 2400 dpi, on the photosensitive drum 1 is transmitted to the light-emitting unit 50 adjacent thereto on the upstream side in the rotation direction of the photosensitive drum 1 relative to the light-emitting unit 50 to which the image data for exposing the second line has been transmitted. Similarly, image data (5-1 to 5-4) and image data (7-1 to 7-4) are transmitted to the light-emitting unit 50 further upstream. That is, the image formed on the photosensitive drum 1 at time T1 is spaced one line apart at 2400 dpi, as shown in FIG. 1.

[0092] Furthermore, at time T3, when the photosensitive drum 1 has rotated in the sub-scanning direction (arrow Y direction) by one line (10.58 μm) of 2400 dpi relative to time T2, image data for four lines is transmitted from the image controller unit 70 to the exposure head 6 in the same manner as at times T1 and T2. Here, the image data transmitted to each line of the light-emitting unit 50 at time T3 is transmitted shifted by one line relative to time T2.

[0093] That is, at time T3, image data (3-1 to 3-4) for exposing the third line of the photosensitive drum 1 is transmitted to the light-emitting unit 50 located furthest downstream in the rotation direction of the photosensitive drum 1. Furthermore, image data (5-1 to 5-4) for exposing the fifth line of the photosensitive drum 1, spaced one line at a time at 2400 dpi, is transmitted to the light-emitting unit 50 adjacent to the light-emitting unit 50 to which the image data for exposing the third line is transmitted. Image data (7-1 to 7-4) and image data (9-1 to 9-4) are also transmitted line by line to the light-emitting unit 50 adjacent to the light-emitting unit 50 in the rotation direction of the photosensitive drum 1.

[0094] As a result, the third, fifth, and seventh lines on the photosensitive drum 1 are subjected to two multiple exposures by the light-emitting unit 50 at times T1 and T3. After that, the same processes as those at times T1, T2, and T3 are performed from time T4 onwards. As a result, at time T7, the seventh line on the photosensitive drum 1 is subjected to exposure processes at times T1 to T7, for a total of four multiple exposures. This operation is repeated for one page of image, and an electrostatic latent image that has been subjected to four multiple exposures is formed over the entire area of ​​the photosensitive drum 1 except for lines 1 to 6.

[0095] In this embodiment as well, the pitch of the light-emitting units 50 of the exposure head 6 in the sub-scanning direction is an integer multiple of the resolution pitch in the sub-scanning direction (the direction of rotation of the photosensitive drum 1, the direction of the arrow Y) of the image formed by the image forming apparatus A. Therefore, as in the first embodiment, multiple exposure of the photosensitive drum 1 can be performed without providing a delay circuit in the exposure head 6 to shift the light emission timing of the light-emitting units 50 arranged in parallel in the direction of the arrow Y. This prevents the circuit scale of the exposure head 6 from becoming too large, and reduces manufacturing costs.

[0096] Furthermore, in this embodiment, the light-emitting units 50 adjacent to each other in the direction of the arrow Y are arranged with a positional shift of 5.29 μm (4800 dpi) in the direction of the arrow X. As a result, the exposure positions on the photosensitive drum 1 of the light-emitting units 50 adjacent to each other in the direction of the arrow Y are shifted by 5.29 μm in the main scanning direction (direction of the arrow X), and the exposure resolution in the main scanning direction is 4800 dpi. Therefore, according to the configuration of this embodiment, the exposure resolution can be improved compared to the configuration of the first embodiment, and image quality can be improved.

[0097] (Third embodiment) Next, a third embodiment of the image forming apparatus A according to the present invention will be described. Portions that overlap with those of the first and second embodiments will be denoted by the same reference numerals and drawings, and descriptions thereof will be omitted.

[0098] 18 is a schematic diagram illustrating the configuration of the light-emitting section 50 of the light-emitting element array chip 40 according to this embodiment. As shown in FIG. 18, in this embodiment, in order to increase the light intensity of the light-emitting section 50, the width W2 of the light-emitting section 50 is set to W2=31.07 μm, which is larger than the width W2 of the first embodiment. Furthermore, the width W1 and the spacing d1 and d2 are W1=19.8 μm and d1=d2=0.68 μm, as in the first embodiment. That is, in this embodiment, the pitch of the light-emitting sections 50 in the direction of the arrow Y is set to 31.75 μm (800 dpi).

[0099] Furthermore, the image forming apparatus A according to this embodiment forms an image with a resolution of 2400 dpi in the sub-scanning direction, and the rotation speed of the photosensitive drum 1 is 200 mm / s, the same as in the first embodiment. Therefore, the time required to expose a distance of 2400 dpi (10.58 μm) is 52.92 μs, and the period of the line synchronization signal is also 52.92 μs. The other configurations of the image forming apparatus A according to this embodiment are the same as those of the first embodiment, except for the control described below.

[0100] Fig. 19 is a diagram showing an exposure image of the photosensitive drum 1. In Fig. 17, squares on the photosensitive drum 1 indicate pixels on the photosensitive drum 1, and the numbers (1-1 to 7-4) within the pixels indicate the type of image data written to each pixel. Although the number of pixels in the direction of the arrow X is 748 pixels x 20 chips = 14,960 pixels, for ease of explanation, Fig. 19 shows only four pixels in the direction of the arrow X.

[0101] 19, first, at time T1, the light-emitting unit 50 is controlled in the same manner as the control of the light-emitting unit 50 at time T1 described using Fig. 13, and 1200 dpi x 4 lines of pixels are exposed on the photosensitive drum 1 during 52.92 us, which is the period of the line synchronization signal. In this embodiment, an image with a resolution of 2400 dpi in the sub-scanning direction (direction of arrow Y) is formed by the image forming apparatus A, and the spacing between the light-emitting units 50 in the sub-scanning direction is 800 dpi, so at time T1, an electrostatic latent image with a spacing of 2 lines at 2400 dpi is formed on the photosensitive drum 1.

[0102] Next, at time T2, when the photosensitive drum 1 has rotated in the sub-scanning direction (arrow Y direction) by one line (10.58 μm) of 2400 dpi relative to time T1, image data for four lines is transmitted from the image controller unit 70 to the exposure head 6 in the same manner as at time T1. Here, the image data transmitted to each line of the light-emitting unit 50 at time T2 is transmitted shifted by one line relative to time T1.

[0103] That is, at time T2, image data (2-1 to 2-4) for exposing the second line of the photosensitive drum 1 is transmitted to the light-emitting unit 50 located furthest downstream in the rotation direction of the photosensitive drum 1. Furthermore, image data (5-1 to 5-4) for exposing the fifth line of the photosensitive drum 1 is transmitted to the light-emitting unit 50 adjacent to the light-emitting unit 50 on the upstream side in the rotation direction of the photosensitive drum 1 relative to the light-emitting unit 50 to which the image data for exposing the second line has been transmitted. Image data (8-1 to 8-4) and image data (11-1 to 11-4) are also transmitted in the same manner to the light-emitting units 50 adjacent to the light-emitting unit 50 in the rotation direction of the photosensitive drum 1. In this way, at time T2, each line on the photosensitive drum 1 is not multiple-exposed.

[0104] Next, at times T3 and T4, the same control as at times T1 and T2 is performed. As a result, at time T4, the fourth, seventh, and tenth lines on the photosensitive drum 1 are multiple-exposed twice by the light-emitting unit 50 at times T1 and T4. At times T5 and T6, the same control as at times T3 and T4 is performed. As a result, at time T5, the fifth, eighth, and eleventh lines on the photosensitive drum 1 are multiple-exposed twice by the light-emitting unit 50 at times T2 and T5. At time T6, the sixth, ninth, and twelfth lines on the photosensitive drum 1 are multiple-exposed twice by the light-emitting unit 50 at times T3 and T6.

[0105] Thereafter, at time T7, four lines of image data are transmitted from the image controller unit 70 to the exposure head 6 in the same manner as at time T1. Here, the image data transmitted to each line of the light-emitting unit 50 at time T7 is transmitted one line later than at time T6. As a result, at time T7, the tenth line on the photosensitive drum 1 is exposed at times T1, T4, T7, and T10, respectively, resulting in a total of four multiple exposures. This operation is repeated for one page of image, forming an electrostatic latent image that has been subjected to four multiple exposures on the entire area of ​​the photosensitive drum 1 except for lines 1 to 9. Note that, although lines 1 to 3 on the photosensitive drum 1 are not subjected to multiple exposure, lines 4 to 9 are subjected to at least two or more multiple exposures.

[0106] As described above, in this embodiment, the pitch of the light-emitting elements 50 of the exposure head 6 in the sub-scanning direction is set to an integer multiple of the resolution in the sub-scanning direction (the rotation direction of the photosensitive drum 1, the direction of the arrow Y) of the image formed by the image forming apparatus A. Even with this configuration, the photosensitive drum 1 can be multiplexed exposed simply by shifting the image data exposed by the light-emitting elements 50 arranged in parallel in the direction of the arrow Y, without providing a delay circuit in the exposure head 6 to shift the light emission timing of the light-emitting elements 50 arranged in parallel in the direction of the arrow Y. This makes it possible to prevent the circuit scale of the exposure head 6 from becoming too large, and to reduce manufacturing costs.

[0107] In the first and second embodiments, the resolution of the image formed by the image forming apparatus A in the sub-scanning direction (arrow Y direction) is 2400 dpi, and the pitch of the light-emitting units 50 in the sub-scanning direction (arrow Y direction) is 1200 dpi. In the third embodiment, the resolution of the image formed by the image forming apparatus A in the sub-scanning direction (arrow Y direction) is 2400 dpi, and the pitch of the light-emitting units 50 in the sub-scanning direction (arrow Y direction) is 800 dpi. However, the present invention is not limited to this. That is, as long as the pitch of the light-emitting units 50 of the exposure head 6 in the sub-scanning direction is an integer multiple, excluding unity, of the resolution of the image in the sub-scanning direction (direction of rotation of the photosensitive drum 1, arrow Y direction) of the image formed by the image forming apparatus A, the resolution of the image in the sub-scanning direction and the pitch of the light-emitting units 50 in the sub-scanning direction may be freely set. [Explanation of symbols]

[0108] 1...Photosensitive drum (photoconductor) 6...Exposure head 42...Light-emitting substrate (substrate) 50...Light emitting part 54... Lower electrode (first electrode layer including multiple electrodes) 56...Emitting layer 58...Top electrode (second electrode layer) 70...Image controller unit (control unit) A...Image forming device

Claims

1. An image forming apparatus that forms an image by irradiating a surface of a photosensitive member with light to form an electrostatic latent image and then attaching toner to the electrostatic latent image, an exposure head that irradiates light onto a surface of the photosensitive member to form the electrostatic latent image, the exposure head comprising: a substrate; a first electrode layer including a plurality of electrodes that are two-dimensionally arranged in a rotation direction of the photosensitive member and in a rotation axis direction of the photosensitive member and are separately arranged on the substrate; a light-emitting layer that is stacked on the first electrode layer and emits light when a voltage is applied; and a second electrode layer that is arranged on the opposite side of the light-emitting layer from the side on which the first electrode layer is arranged and is light-transmittable; a control unit that controls application of a voltage to each of the plurality of electrodes included in the first electrode layer based on image data so that the light-emitting layer emits light, and that is capable of controlling the voltage to each of the plurality of electrodes based on the image data so that one pixel is formed by controlling voltages to a plurality of electrodes that are arranged at different positions in the rotation direction; Equipped with An image forming apparatus characterized in that the plurality of electrodes included in the first electrode layer are arranged so that the rotational pitch of the plurality of electrodes is an integer multiple, excluding equal multiples, of the resolution in the rotational direction of an image formed by the image forming apparatus.

2. The image forming apparatus according to claim 1, characterized in that the control unit controls the application of voltage to the electrodes of the plurality of electrodes included in the first electrode layer that are arranged in parallel in the rotational direction so that the light emitting units arranged in parallel in the rotational direction simultaneously emit or do not emit light according to image data.

3. 3. The image forming apparatus according to claim 1, wherein the length of the electrodes included in the first electrode layer in the rotational direction is equal to the length in the rotational axis direction.

4. 3. The image forming apparatus according to claim 1, wherein the length of the electrodes included in the first electrode layer in the rotational direction is longer than the length in the rotational axis direction.

5. 5. The image forming apparatus according to claim 1, wherein the light emitting layer is an organic light emitting layer.

Images