Exposure head and image forming apparatus

Abstract

To achieve high output.SOLUTION: An exposure head comprises: a first light-emitting substrate group 41 in which a plurality of first light-emitting substrates 41-1, 41-2,..., 41-m having a plurality of first light-emitting elements arranged side by side in the X direction are alternately arranged along the X direction and Y direction; a first rod lens array 50 which condenses light emitted from the plurality of light-emitting elements provided on the first light-emitting substrate group 41 on the surface of a photoreceptor drum; a second light-emitting substrate group 141 in which a plurality of second light-emitting substrates 141-1, 141-2,..., 141-m having a plurality of second light-emitting elements arranged side by side in the X direction are alternately arranged along the X direction and Y direction and separated in the X direction from the first light-emitting substrate group 41; and a second rod lens array 150 which condenses light emitted from the plurality of light-emitting elements provided on the second light-emitting substrate group 141 on the surface of the photoreceptor drum.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to an exposure head for use in an image forming apparatus that forms an image using a developer by electrophotography or the like, and to an image forming apparatus. [Background technology]

[0002] Conventionally, in image forming devices such as electrophotographic printers, a method has been generally known in which a photosensitive drum is exposed to light using an exposure head that uses LEDs or organic electroluminescence (EL) to form a latent image. The exposure head generally includes a row of light-emitting elements arranged in the longitudinal direction of the photosensitive drum and a rod lens array that focuses light from the row of light-emitting elements onto the photosensitive drum. LEDs and organic ELs are known to have a surface-emitting configuration in which the direction of light emitted from the light-emitting surface is the same as the optical axis of the rod lens array.

[0003] The length of the light-emitting element array in the longitudinal direction of the photosensitive drum is determined by the width of the image area on the photosensitive drum, and the element spacing (pitch) is determined by the printer's resolution. For example, in a 1200 dpi printer, the pixel spacing (pitch) is approximately 21.16 μm (omitted after the third decimal point), so the element spacing is also 21.16 μm. Printers using such exposure heads use fewer components than laser scanning printers that deflect and scan a laser beam using a polygon motor, making it easier to reduce the size and cost of the device. One such exposure head proposed is one that uses a TFT circuit and an organic electroluminescent element (EL) on a transparent glass substrate (see Patent Document 1). This exposure head has a configuration in which a TFT circuit is formed on a transparent glass substrate, an organic electroluminescent layer is further formed on top of that, and light from the organic electroluminescent layer is extracted through the transparent glass substrate (see Figure 6 of Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-183436 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in the exposure head described in Patent Document 1, light from the organic EL light-emitting layer is extracted from the glass substrate side, so the light is blocked by wiring of the TFT circuit, etc., resulting in poor light utilization efficiency and low output. As such, because organic EL has low output, there is a problem that the amount of light reaching the surface of the photosensitive drum may be insufficient for high-speed applications such as POD (Print On Demand).

[0006] An object of the present invention is to provide an exposure head and an image forming apparatus that can achieve high output. [Means for solving the problem]

[0007] The exposure head of the present invention exposes a charged surface of an image carrier movable in a first direction to form an electrostatic latent image, and includes a first light-emitting substrate group in which a plurality of first light-emitting substrates having a plurality of first light-emitting elements arranged side by side in a second direction intersecting the first direction are arranged along the second direction and alternately in the first direction; The aforementioned Multiple No. 1 a first lens array that focuses light emitted from a light-emitting element onto a surface of the image carrier; a second light-emitting substrate group that is arranged along the second direction and alternately in the first direction, the second light-emitting substrates having a plurality of second light-emitting elements arranged side by side in the second direction, and that is spaced apart from the first light-emitting substrate group in the first direction; The aforementioned Multiple No. 2 a second lens array that focuses the light emitted from the light-emitting element onto the surface of the image carrier; The distance in the first direction between the first light emitting substrate group and the second lens array is set so that the amount of light incident on the second lens array out of the amount of light emitted from the first light emitting elements is 20% or less, and the distance in the first direction between the second light emitting substrate group and the first lens array is set so that the amount of light incident on the first lens array out of the amount of light emitted from the second light emitting elements is 20% or less. It is characterized by:

[0008] The image forming apparatus of the present invention is characterized by comprising an image carrier, a charging means for charging the surface of the image carrier, the above-mentioned exposure head, a developing means for developing an electrostatic latent image formed on the surface of the image carrier by the exposure head with a developer, and a transfer means for transferring the developed image to a recording material. [Effects of the Invention]

[0009] According to the present invention, it is possible to achieve high output. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a cross-sectional view showing a schematic configuration of an image forming apparatus according to an embodiment. [Figure 2] 1A and 1B are a perspective view and a cross-sectional view, respectively, showing the peripheral configuration of an exposure head according to an embodiment. [Figure 3] 1A and 1B are diagrams showing the configuration of an exposure head according to an embodiment, in which FIG. 1A is a plan view and FIG. 1B is a cross-sectional view taken along line AA. [Figure 4] FIG. 2 is a plan view showing a first rod lens array according to the embodiment. [Figure 5] FIG. 2 is a plan view showing a first light emitter and a second light emitter according to the embodiment. [Figure 6] 3 is a cross-sectional view showing the relationship between a first light emitter and a first rod lens array according to the embodiment. FIG. [Figure 7] 10A and 10B are graphs showing the relationship between the distance from the light emitting element to the rod lens array according to the embodiment, where (a) shows the relationship with the imaging position shift and (b) shows the relationship with the amount of light. [Figure 8] FIG. 3 is a plan view showing the positions of a first light emitting element and a second light emitting element according to the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0011] An embodiment of the present invention will be described in detail below with reference to FIGS. 1 to 8. In this embodiment, a tandem-type full-color printer that forms an image on a recording medium using an electrophotographic system is described as an example of an image forming apparatus 1. Note that the image forming apparatus 1 is not limited to a tandem-type in which multiple image forming units are arranged side by side, but may be a rotary-type in which they are arranged in a cylindrical shape. Furthermore, it is not limited to a direct transfer system in which a toner image is directly transferred from a photoreceptor to a sheet S, which is a recording medium, but may be an intermediate transfer system in which an image is primarily transferred to an intermediate transfer body and then secondarily transferred to the sheet S. Furthermore, it is not limited to full color, but may also be monochrome or monocolor. The recording medium is the sheet S, and includes, in addition to plain paper, special paper such as coated paper, recording materials in special shapes such as envelopes and index paper, plastic film and cloth for overhead projectors, etc.

[0012] [Image forming equipment] 1 shows the configuration of image forming apparatus 1. Image forming apparatus 1 includes a scanner unit 2, image forming units PC, PM, PY, and PK, a fixing unit 3, a feeding unit 4, and a control unit (not shown) that controls these. Scanner unit 2 shines light onto a document placed on a document table to optically read the document image, and converts the image into an electrical signal to create image data.

[0013] Image forming unit PC forms a cyan image using cyan toner. Image forming unit PM forms a magenta image using magenta toner. Image forming unit PY forms a yellow image using yellow toner. Image forming unit PK forms a black image using black toner. The configurations of the image forming units PC, PM, PY, and PK are basically the same except for the color of the toner they contain. Therefore, the following describes the configuration and image formation process of the image forming unit PY using the yellow image forming unit PY as an example.

[0014] In the image forming units PC, PM, PY, and PK, a movable photosensitive drum 5, which is an example of an image carrier, is rotated, and a charger 6, which is an example of a charging means, charges the surface of the photosensitive drum 5. An exposure head 30 emits light in accordance with image data to form an electrostatic latent image. Details of the exposure head 30 will be described later. A developing unit 7, which is an example of a developing means, develops the electrostatic latent image formed on the surface of the photosensitive drum 5 by the exposure head 30 with a developer (toner). A transfer unit 8, which is an example of a transfer means, transfers the developed toner image onto a sheet S transported on a transfer belt 9.

[0015] Each image forming unit PC, PM, PY, and PK performs a series of electrophotographic processes (charging, exposure, development, and transfer) to form a full-color image by overlaying images in the order of cyan (C), magenta (M), yellow (Y), and black (K). Each image forming unit PC, PM, PY, and PK sequentially executes its own image forming operation after a predetermined time has elapsed since the image forming unit PC started forming images.

[0016] An optical sensor 10 is disposed opposite the transfer belt 9, and detects the position of a test chart printed on the transfer belt 9 to derive the amount of color misregistration between the image forming units PC, PM, PY, and PK. The amount of color misregistration derived here is notified to an image controller (not shown), which corrects the image position of each color. This control allows a full-color toner image to be transferred onto the sheet S without color misregistration.

[0017] The image forming apparatus 1 is provided with a built-in feeding unit 11, an external feeding unit 12, and a manual feeding unit 13. In the feeding section 4, a sheet S is fed from a previously designated one of these feeding units 11, 12, and 13, and is transported to a registration roller 14. The registration roller 14 transports the sheet S onto the transfer belt 9 at the timing when the toner image formed in the image forming units PC, PM, PY, and PK is transferred onto the sheet S.

[0018] The fixing unit 3 is made up of a combination of rollers and has a built-in heat source such as a halogen heater. Using heat and pressure, the toner on the sheet S onto which the toner image has been transferred from the transfer belt 9 is melted and fixed, and the sheet is then discharged to the outside of the image forming apparatus 1 by discharge rollers 15. The control unit communicates with the MFP control unit that controls the entire MFP, and executes control in accordance with its instructions. The control unit manages the status of each of the scanner unit 2, image forming units PC, PM, PY, PK, fixing unit 3, and feed unit 4, and issues instructions to ensure that the entire system operates smoothly and in harmony.

[0019] [Exposure head overview] Next, an overview of the exposure head 30 that exposes the photosensitive drum 5 will be described with reference to Figures 2(a) and (b). Figure 2(a) shows the arrangement of the exposure head 30 relative to the photosensitive drum 5. The photosensitive drum 5 and the exposure head 30 are arranged facing each other with their longitudinal directions parallel. Here, the rotation direction (movement direction) of the photosensitive drum 5 is defined as the Y direction as a first direction, and the direction intersecting (here, perpendicular to) the Y direction is defined as the X direction as a second direction. In other words, the X direction is the main scanning direction, and the Y direction is the sub-scanning direction.

[0020] 2(b) shows a cross-sectional view of the exposure head 30. The exposure head 30 has a printed circuit board 31, a first light-emitting element group 40 and a second light-emitting element group 140 provided on the printed circuit board 31, a first rod lens array 50 and a second rod lens array 150, and a housing 32 to which these are attached. In this embodiment, the light-emitting element groups 40, 140 and the rod lens arrays 50, 150 are each provided in two rows, and are arranged parallel to the Y direction with the X direction as the longitudinal direction.

[0021] Light emitted from the light-emitting element groups 40, 140 is focused onto the photosensitive drum 5 by the rod lens arrays 50, 150. The exposure head 30 and the photosensitive drum 5 are each attached to the image forming apparatus 1 by attachment members (not shown). Note that, although the present embodiment describes the case where the rod lens arrays 50, 150 are used as lenses, this is not limiting and lenses of other configurations may also be used.

[0022] The photosensitive drum 5 and the first rod lens array 50 are spaced a predetermined distance apart, and the first rod lens array 50 and the first light-emitting element group 40 are spaced a predetermined distance apart. Similarly, the photosensitive drum 5 and the second rod lens array 150 are spaced a predetermined distance apart, and the second rod lens array 150 and the second light-emitting element group 140 are spaced a predetermined distance apart. This allows light emitted from the first light-emitting element group 40 to be imaged on the photosensitive drum 5 via the first rod lens array 50, and light emitted from the second light-emitting element group 140 to be imaged on the photosensitive drum 5 via the second rod lens array 150.

[0023] When the exposure head 30 is assembled in a factory, the exposure head 30 is individually assembled and adjusted, and focus and light intensity adjustments are performed to adjust the spot size at the light-condensing position to a predetermined size. Therefore, during focus adjustment, the mounting position of the rod lens array 50, 150 is adjusted so that the distance between the rod lens array 50, 150 and the light-emitting element group 40, 140 is the desired value. During light intensity adjustment, each light-emitting element is individually and sequentially turned on, and the drive current of each light-emitting element is adjusted so that the light condensed through the rod lens array 50, 150 achieves a predetermined light intensity. In this way, the exposure head 30 emits light according to image data, and the light emitted from the chip surfaces of the arranged light-emitting element group 40, 140 is condensed onto the photosensitive drum 5 by the rod lens array 50, 150, forming an electrostatic latent image.

[0024] [Exposure head configuration] Next, the detailed configuration of the exposure head 30 will be described with reference to Figures 3(a) and (b). Figure 3(a) is a schematic plan view of the exposure head 30. In this embodiment, the printed circuit board 31 is, for example, a general glass epoxy printed circuit board. The printed circuit board 31 is provided with a first light-emitting board group 41 and a second light-emitting board group 141 that is spaced apart from the first light-emitting board group 41 in the Y direction.

[0025] The first light-emitting substrate group 41 is composed of a collection of first light-emitting substrates 41-1, 41-2, ..., 41-m, which are transparent glass substrates on which TFTs (Thin Film Transistors) are formed, for example. In this embodiment, there are m collections, where m can be set appropriately. The first light-emitting element group 40 is composed of a collection of first light emitters 40-1, 40-2, ..., 40-m. The first light-emitting substrates 41-1, 41-2, ..., 41-m are provided with first light emitters 40-1, 40-2, ..., 40-m, respectively. The first light-emitting substrates 41-1, 41-2, ..., 41-m are provided with first driver ICs 42-1, 42-2, ..., 42-m for driving the first light emitters 40-1, 40-2, ..., 40-m. In this embodiment, the first light emitting substrate group 41 is formed by arranging a plurality of first light emitting substrates 41-1, 41-2, . . . , 41-m in a staggered pattern along the X direction and alternately in the Y direction.

[0026] Similarly, the second light-emitting substrate group 141 is made up of a collection of second light-emitting substrates 141-1, 141-2, ..., 141-m, which are, for example, transparent glass substrates on which TFTs are formed. The second light-emitting element group 140 is made up of a collection of second light emitters 140-1, 140-2, ..., 140-m. The second light-emitting substrates 141-1, 141-2, ..., 141-m are provided with second light emitters 140-1, 140-2, ..., 140-m, respectively. The second light-emitting substrates 141-1, 141-2, ..., 141-m are provided with second driver ICs 142-1, 142-2, ..., 142-m for driving the second light emitters 140-1, 140-2, ..., 140-m. In this embodiment, the second light emitting substrate group 141 is formed by arranging a plurality of second light emitting substrates 141-1, 141-2, . . . , 141-m in a staggered pattern along the X direction and alternately in the Y direction.

[0027] 3(b) is a cross-sectional view taken along line AA in FIG. 3(a), and the Z direction in the drawing indicates the optical axis direction. For example, a first light emitter 40-1 is provided on the first light-emitting substrate 41-1 facing the printed circuit board 31. A first sealing substrate 43-1 for sealing the first light emitter 40-1 is interposed between the first light-emitting substrate 41-1 and the printed circuit board 31. In this embodiment, an organic EL is used as the first light emitter 40-1. Because organic EL is vulnerable to moisture, a sealing space 44-1 is formed in the first sealing substrate 43-1 so that the first light emitter 40-1 is contained within the first sealing substrate 43-1, and the first light emitter 40-1 is isolated from the outside air. Similarly, in other locations, a first sealing substrate 43-2 and second sealing substrates 143-1 and 143-2 are provided between the light emitting substrate and the printed circuit board 31, forming sealed spaces 44-2, 144-1 and 144-2 to isolate the light emitters from the outside air.

[0028] The first rod lens array 50 is arranged such that a first side portion 50a and a second side portion 50b, each having two rows of rod lenses aligned in the Y direction, face the first light emitters 40-1 and 40-2, respectively. The first rod lens array 50 is preferably arranged so that the Y-direction centers of the first light emitters 40-1 and 40-2 and the Y-direction center of the first rod lens array 50 coincide with each other. Similarly, the second rod lens array 150 is arranged such that a first side portion 150a and a second side portion 150b, each having two rows of rod lenses aligned in the Y direction, face the second light emitters 140-1 and 140-2, respectively. The Y-direction centers of the second light emitters 140-1 and 140-2 and the Y-direction center of the second rod lens array 150 are preferably arranged so that they coincide with each other. Furthermore, each rod lens array 50, 150 is fixed, for example, with an adhesive to a bracket not shown, and the distance between each rod lens array 50, 150 and each light-emitting element group 40, 140 is adjusted so that an appropriate spot diameter is obtained on the photosensitive drum 5.

[0029] [Rod lens array] The rod lens arrays 50 and 150 are formed by arranging two rows of SELFOC (registered trademark) lens arrays in the Y direction, each of which is made up of two rows of SELFOC (registered trademark) lenses. The first rod lens array 50 is an example of a first lens array, and the second rod lens array 150 is an example of a second lens array. Since the first rod lens array 150 and the second rod lens array 250 have the same configuration, the first rod lens array 150 will be described below as a representative.

[0030] The first rod lens array 50 will be described with reference to FIG. 4. FIG. 4 is a schematic plan view of the first rod lens array 50. As shown in FIG. 4, the first rod lens array 50 has a configuration in which rod lenses 51, each having a diameter D, are two-dimensionally arranged in the Y and X directions. A larger diameter D of the rod lenses 51 has the advantage of higher light utilization efficiency and a smaller amount of light required for the light-emitting element, but on the other hand, it has the disadvantage of a shallower depth of focus and greater likelihood of defocusing. Furthermore, it also has the problem of greater lens aberration, which tends to deteriorate the shape of the focused spot and result in reduced image quality. On the other hand, a smaller diameter D has the disadvantage of a deeper depth of focus, less likelihood of defocusing, and a better spot shape, resulting in a sharp image, but it has the disadvantage of lower light utilization efficiency and a greater likelihood of insufficient light intensity.

[0031] Therefore, in this embodiment, the number of rod lenses is increased in the Y direction without increasing the diameter D, thereby ensuring a sufficient amount of light while suppressing degradation of image quality. In this embodiment, the first rod lens array 50 is configured in such a way that two rows of rod lenses 51 (first side portion 50a and second side portion 50b) with a diameter D of 270 μm are arranged in the Y direction. The same applies to the second rod lens array 250.

[0032] [Light-emitting element group configuration] FIG. 5 shows an enlarged view of the first light emitter 40-1 and the second light emitter 140-1. As shown in FIG. 5, the first light emitter 40-1 and the second light emitter 140-1 are configured by arranging a plurality of light emitting elements 45, 145 in the X direction and the Y direction, respectively. That is, the plurality of first light emitting elements 45 and the plurality of second light emitting elements 145 are arranged side by side in the X direction and the Y direction, respectively. In this embodiment, the size and shape of the light emitting elements 45, 145 are circular with a diameter of 60 μm. The light emitting elements 45, 145 are not limited to circular shapes, and may be oval or polygonal, such as rectangular or hexagonal. In this embodiment, four light emitting elements 45, 145 are arranged in the Y direction, and the width W in the Y direction is 240 μm. The light emitting elements 45, 145 may be arranged in a single row in the Y direction instead of in multiple rows. In this case, the shape of the light emitting elements 45, 145 may be rectangular or oval, for example, with the dimension in the Y direction larger than the dimension in the X direction.

[0033] In this embodiment, the light-emitting elements 45, 145 adjacent to each other in the Y direction are arranged so as to be offset in the X direction by a distance corresponding to the main scanning resolution. For example, for the light-emitting elements 45-1 and 45-2, the distance X0 in the X direction between the centers of gravity of the light-emitting elements 45-1 and 45-2 is 21.16 μm, which corresponds to 1200 dpi. The same applies to the other light-emitting elements 45, 145.

[0034] Here, it is assumed that the light emitting element 145-1 is upstream and the light emitting element 45-1 is downstream in the Y direction, which is also the rotation direction of the photosensitive drum 5. In this case, after the light emitting element 145-1 emits light to expose a predetermined area of ​​the photosensitive drum 5, if the light emitting element 45-1 is made to emit light after a predetermined time Δt, the same area as that exposed by the light emitting element 145-1 can be exposed. In other words, exposure can be achieved with twice the amount of light that the light emitting elements 145-1 and 45-1 individually emit.

[0035] As shown in FIG. 5, when the distance in the Y direction between the centers of gravity of the light emitting elements 145-1 and 45-1 is Y0 and the process speed is v, the difference Δt in the light emission timing of the light emitting elements 145-1 and 45-1 can be calculated by Equation 1. △t=Y0 / v (Equation 1)

[0036] In this embodiment, if the distance Y0 is, for example, 480 μm and the process speed v is 600 mm / s, Δt is 0.8 ms according to Equation 1. Therefore, by causing the light-emitting element 145-1 to emit light and then causing the light-emitting element 45-1 to emit light 0.8 ms later, the same location on the photosensitive drum 5 can be exposed using the light-emitting elements 145-1 and 45-1. In this way, by increasing the number of light-emitting elements 45 in the Y direction, a sufficient amount of light can be obtained by multiple exposure.

[0037] [Width of light emitter in sub-scanning direction] On the other hand, if the width W of the light emitters 40-1 and 140-1 in the Y direction becomes larger, adverse effects may occur. FIG. 6 shows a schematic diagram of an out-of-focus state. FIG. 6 compares the first light emitter 40-1 positioned as shown by the solid line with the first light emitter 40-1 positioned as shown by the two-dot chain line. The first light emitter 40-1 positioned as shown by the solid line has its center line aligned with the first rod lens array 50, so no misalignment occurs on the surface of the photosensitive drum 5. In contrast, the first light emitter 40-1 positioned as shown by the two-dot chain line has its center line deviated from the first rod lens array 50 by a distance d1, resulting in a misalignment of a distance d2 on the surface of the photosensitive drum 5.

[0038] When the focus is off as shown in Fig. 6, as the distance between the center of the first light emitter 40-1 and the center of the first rod lens array 50 increases, the image position on the photosensitive drum 5 shifts significantly. Fig. 7(a) is a graph showing the relationship between the distance between the first rod lens array 50 and the light source and the amount of deviation of the image position on the drum from the nominal when focus is off, and shows values ​​when the focus is off by 100 µm. As shown in Fig. 7(a), as the width W increases, the light emitting element moves away from the center of the first rod lens array 50, which increases the deviation of the image position on the photosensitive drum 5 from the nominal.

[0039] Furthermore, if the width W in the Y direction shown in FIG. 5 becomes larger, other problems may occur. FIG. 7B shows the relationship between the distance between the first rod lens array 50 and the light-emitting element and the amount of light focused on the photosensitive drum 5. The amount of light focused on the photosensitive drum 5 is normalized by the value when the distance between the light-emitting element and the center of the first rod lens array 50 is zero. As shown in FIG. 7B, the amount of light focused on the photosensitive drum 5 decreases as the light-emitting element moves away from the center of the first rod lens array 50. Therefore, in a configuration having optical characteristics such as those of this embodiment, it is desirable to position the first rod lens array 50 within 200 μm of the center of the first light-emitting element group 40 in the Y direction. Similarly, it is desirable to position the second rod lens array 150 within 200 μm of the center of the second light-emitting element group 140 in the Y direction.

[0040] 7(a) and 7(b), increasing the width W in the Y direction of the light-emitting element groups 40 and 140 may cause problems such as misalignment of the imaging position and reduction in the amount of light on the photosensitive drum 5. For this reason, it is desirable to obtain the amount of light while keeping the width W in the Y direction of the light-emitting element groups 40 and 140 to a certain extent.

[0041] [Sub-scanning direction spacing] Next, the distance Y0 (pitch) in the Y direction between adjacent light-emitting elements 40-1, 140-1 in the Y direction will be described with reference to FIG. 5. If the distance Y0 between the multiple light-emitting elements 40-1, 140-1 in the Y direction is too close, light may be incident on the adjacent rod lens array 50, 150, and an image may be formed on the photosensitive drum 5, resulting in an image density that differs from that at other positions in the X direction. This is the case, for example, in FIG. 3(b), when light emitted from the first light-emitting element group 40 is imaged on the photosensitive drum 5 by the second rod lens array 150. For this reason, the distance Y0 in the Y direction between the light-emitting element groups 40, 140 needs to be set far enough apart to prevent such problems from occurring.

[0042] As shown in Fig. 5, the distance between the end of the light-emitting element of the second light emitter 140-1 closest to the adjacent first light emitter 40-1 and the center in the Y direction of the adjacent first light emitter 40-1 is defined as L. As the light source moves away from the center of the rod lens arrays 50 and 150, the amount of light focused on the photosensitive drum 5 decreases, so the distance L must be set so that this amount of light is sufficiently small. As shown in Fig. 7(b), the amount of light decreases to about 20% when the light source is approximately 500 µm away from the first rod lens array 50, and becomes almost zero when the light source is approximately 700 µm or more away. Therefore, the distance L is preferably 500 µm or more, and more preferably 700 µm or more.

[0043] That is, the distance in the Y direction between the first light-emitting substrate group 41 and the second rod lens array 150 is set so that the amount of light emitted from the first light-emitting element group 40 that enters the second rod lens array 150 is 20% or less. Also, the distance in the Y direction between the second light-emitting substrate group 141 and the first rod lens array 50 is set so that the amount of light emitted from the second light-emitting element group 140 that enters the first rod lens array 50 is 20% or less.

[0044] However, if the distance L is too large, another problem occurs. It is desirable that the light-emitting element groups 40, 140 and the photosensitive drum 5 are parallel in the X direction, but it is difficult to always eliminate assembly errors, and an angle θ may occur due to assembly errors in the X direction of the light-emitting element groups 40, 140 and the photosensitive drum 5. If the assembly error is ±0.1 mm and the length of the light-emitting element groups 40, 140 in the X direction is 340 mm, then Equation 2 can be obtained. tanθ=0.1×2 / 340 (Equation 2)

[0045] Here, we calculate how much the light-emitting elements that are located at the same X-direction position by design but are farthest apart in the Y-direction are shifted in the X-direction when the angle is θ. For example, in FIG. 5, the light-emitting elements that are located at the same X-direction position by design but are farthest apart in the Y-direction are first light-emitting element 45-1 and second light-emitting element 145-1, so only these light-emitting elements 45-1 and 145-1 are extracted, and the positional relationship when the angle is θ is shown in FIG. 8. If the distance between light-emitting elements 45-1 and 145-1 in the Y-direction is distance dY and the distance in the X-direction is distance dX, then the relationship in Equation 3 is satisfied when the angle is θ. dX=dY×tanθ (Equation 3)

[0046] Here, since the image positional deviation becomes easily visible if it exceeds 5 μm, the distance dX needs to be kept to 5 μm or less. Therefore, Equation 4 can be obtained from Equation 2 and Equation 3. dY≦(340 / (0.1×2))×0.005=8.5(mm) (Formula 4)

[0047] 5, in this embodiment, the designed distance between light-emitting elements 45-1 and 145-1 is Y0, as shown in Equation 5. Therefore, the distance Y0 (pitch) between the first light-emitting element group 40 and the second light-emitting element group 140 is preferably 8.5 mm or less. Y0≦8.5 (mm) (Formula 5)

[0048] Furthermore, L=Y0-W / 2, and since W=240 μm as described above, L≦8.38 mm. Therefore, a preferable range for the distance L is 0.7 mm≦L≦8.38 mm. That is, the first light-emitting element 45-1 and the second light-emitting element 145-1, which is arranged to expose the same position on the surface of the photosensitive drum 5 as the first light-emitting element 45-1, are arranged so that the distance in the X direction is 5 μm or less.

[0049] As described above, the image forming apparatus 1 of this embodiment has two rows of light-emitting substrate groups 41, 141 and corresponding rod lens arrays 50, 150. By arranging multiple light-emitting element groups in the Y direction, each of which has multiple light-emitting elements arranged in the X direction, multiple exposures can be performed using the multiple light-emitting elements in the Y direction, thereby reducing the amount of light required for exposure for the light-emitting elements. This allows for a higher level of productivity in the image forming apparatus 1. Furthermore, by providing a rod lens array facing each light-emitting element group and arranging the Y-direction center of each light-emitting element group opposite the Y-direction center of the rod lens array, light from the light-emitting element group can be efficiently guided to the lens array. This allows for a higher level of productivity in the image forming apparatus 1. Thus, the image forming apparatus 1 of this embodiment performs multiple exposures using each light-emitting element group 40, 140, thereby achieving an exposure head 30 that can ensure a sufficient amount of light for high productivity without compromising image quality. This allows for an increased amount of light and higher output compared to when only one row of light-emitting element groups is provided.

[0050] In the above-described embodiment, organic EL elements are used as the light sources of the light-emitting element groups 40 and 140, but the present invention is not limited to this and gallium arsenide LEDs may also be used. [Explanation of symbols]

[0051] 1...image forming apparatus, 5...photosensitive drum (image carrier), 6...charger (charging means), 7...developer (developing means), 8...transfer section (transfer means), 30...exposure head, 41...first light-emitting substrate group, 41-1, 41-2, 41-m...first light-emitting substrate, 45, 45-1, 45-2...first light-emitting element, 50...first rod lens array (first lens array), 141...second light-emitting substrate group, 141-1, 141-2, 141-m...second light-emitting substrate, 145, 145-1...second light-emitting element, 150...second rod lens array (second lens array)

Claims

1. an exposure head that exposes a charged surface of an image carrier that is movable in a first direction to form an electrostatic latent image; a first light emitting substrate group in which a plurality of first light emitting substrates, each having a plurality of first light emitting elements arranged side by side in a second direction intersecting the first direction, are arranged along the second direction and alternately in the first direction; a first lens array that focuses light emitted from the plurality of first light-emitting elements provided on the first light-emitting substrate group onto a surface of the image carrier; a second light emitting substrate group, in which a plurality of second light emitting substrates each having a plurality of second light emitting elements arranged side by side in the second direction are arranged along the second direction and alternately in the first direction, and are spaced apart from the first light emitting substrate group in the first direction; a second lens array that condenses light emitted from the plurality of second light-emitting elements provided on the second light-emitting substrate group onto a surface of the image carrier, a distance in the first direction between the first light emitting substrate group and the second lens array is set so that the amount of light incident on the second lens array out of the amount of light emitted from the first light emitting elements is 20% or less; a distance in the first direction between the second light-emitting substrate group and the first lens array is set so that the amount of light incident on the first lens array out of the amount of light emitted from the second light-emitting elements is 20% or less; An exposure head characterized by:

2. The plurality of first light-emitting elements and the plurality of second light-emitting elements are arranged side by side in the first direction and the second direction, respectively.

2. The exposure head according to claim 1.

3. An exposure head that exposes a charged surface of an image carrier that is movable in a first direction to form an electrostatic latent image, a first light emitting substrate group in which a plurality of first light emitting substrates, each having a plurality of first light emitting elements arranged side by side in a second direction intersecting the first direction, are arranged along the second direction and alternately in the first direction; a first lens array that focuses light emitted from the plurality of first light-emitting elements provided on the first light-emitting substrate group onto a surface of the image carrier; a second light emitting substrate group, in which a plurality of second light emitting substrates each having a plurality of second light emitting elements arranged side by side in the second direction are arranged along the second direction and alternately in the first direction, and are spaced apart from the first light emitting substrate group in the first direction; a second lens array that condenses light emitted from the plurality of second light-emitting elements provided on the second light-emitting substrate group onto a surface of the image carrier, the first light-emitting element and the second light-emitting element, which are arranged so as to expose the same position on the surface of the image carrier as the first light-emitting element, are arranged so that the distance in the second direction is 5 μm or less; An exposure head characterized by:

4. The plurality of first light-emitting elements and the plurality of second light-emitting elements are arranged side by side in the first direction and the second direction, respectively.

4. The exposure head according to claim 3.

5. an image carrier; a charging means for charging the surface of the image carrier; An exposure head according to any one of claims 1 to 4; a developing means for developing the electrostatic latent image formed on the surface of the image carrier by the exposure head with a developer; and a transfer means for transferring the developed image onto a recording material. An image forming apparatus characterized by:

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