JP2008221570A - Image forming apparatus and image forming method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a technology of suppressing the occurrence of poor exposure regardless of a curvature shape of a surface of a latent image carrier, when the surface of the latent image carrier such as a photoreceptor is exposed to light by using a plurality of lens lines. <P>SOLUTION: A line head is arranged on the surface of the latent image carrier so that an image surface facing distance of a lens belonging to the lens line except the end lens line can become smaller than that of a lens belonging to the end lens line, when the lens line at each end in a width direction among the lens lines of N-lines (N is an integer not smaller than 3) is set as the end lens line. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a technique for forming an image by exposing a surface of a latent image carrier such as a photosensitive pair using a line head.

  A technique is known in which an electrostatic latent image is formed by exposing a light beam to the surface of the photoconductor while conveying the surface of the photoconductor in the sub-scanning direction. That is, in such a technique, a two-dimensional electrostatic latent image is formed on the surface of the photosensitive member by exposing the surface of the photosensitive member in the sub-scanning direction while aligning and exposing the light beam in the main scanning direction. Patent Document 1 describes a line head that forms an image with a light beam emitted from a light emitting element directed toward the surface of the photoreceptor, and a technique for exposing the surface of the photoreceptor using the line head. ing. More specifically, in such a line head, a plurality of light emitting element groups composed of a plurality of light emitting elements are arranged in the longitudinal direction corresponding to the main scanning direction. In addition, a plurality of lenses are arranged in one-to-one correspondence with the plurality of light emitting element groups. Each of the plurality of lenses forms an image of the light beam emitted from the light emitting element of the corresponding light emitting element group toward the surface of the photoreceptor. Then, the surface of the photoreceptor is exposed by the light beam imaged in this way.

Japanese Patent Laid-Open No. 2-4546

  By the way, in the technique described in Patent Document 1, in correspondence with the plurality of light emitting element groups arranged in the longitudinal direction, the plurality of lenses are arranged in a straight line in the longitudinal direction to form a lens row. Further, in the technique described in this document, there is only one lens row, but it has been conventionally proposed to use a plurality of lens rows for reasons such as increasing the resolution of the latent image formed on the surface of the photoreceptor. . However, the following problems may occur when performing exposure using a plurality of lens rows on a photoconductor having a curved surface shape such as a so-called photoconductor drum. It was.

  In a line head that uses a plurality of lens rows, the plurality of lens rows are arranged opposite to the surface of the photoconductor at positions different from each other in the width direction corresponding to the sub-scanning direction that is the conveyance direction of the photoconductor surface. . That is, the plurality of lens rows are opposed to different positions in the sub-scanning direction of the surface of the photosensitive member having a curvature shape. Accordingly, when the distance between the lens and the photosensitive member surface is defined as the image surface facing distance, a difference occurs in the image surface facing distance among a plurality of lens rows. If the arrangement relationship between the line head and the surface of the photoconductor is not appropriate, the difference in the image surface facing distance becomes large, which may cause the following problems. In other words, the distance between the imaging position of the light beam and the surface of the photoconductor may be greatly different among a plurality of lens rows due to the large difference in the image plane facing distance. As a result, there is a possibility that an image formed on the surface of the photoconductor is greatly different among a plurality of lens rows, and that a good exposure cannot be performed, that is, an exposure failure occurs. Therefore, from the viewpoint of suppressing the occurrence of such exposure failure, it is desired that the difference in the image plane facing distance between the plurality of lens rows is small.

  It is also conceivable to adjust the lens configuration of each lens in accordance with the image plane facing distance of the lens in order to cope with the above-described exposure failure caused by the difference in image plane facing distance between the lens rows. However, if the lens configuration is changed for each lens in a state where the difference in image plane facing distance between the plurality of lens rows is large, it may be necessary to change the lens configuration greatly between the lens rows. As a result, the design or manufacture of the lens may be complicated. Therefore, even when the lens configuration is adjusted according to the image plane facing distance, it is desired that the difference in the image plane facing distance between the plurality of lens rows is small.

  The present invention has been made in view of the above problems, and when exposing the surface of a latent image carrier such as a photoconductor using a plurality of lens rows, the lens rows between the lens rows due to the curvature shape of the surface of the latent image carrier are disclosed. An object of the present invention is to provide a technique capable of suppressing a difference in image plane facing distance and enabling good exposure.

  In order to achieve the above object, an image forming apparatus according to the present invention has a latent image carrier whose surface is conveyed in a sub-scanning direction substantially perpendicular to the main scanning direction, and a plurality of longitudinal image carriers in the longitudinal direction corresponding to the main scanning direction. A lens array in which lenses are arranged side by side in a width direction corresponding to N rows (N is an integer of 3 or more) in the sub-scanning direction and facing the surface of the latent image carrier, and a plurality of light emission Each of the plurality of lenses, and each of the plurality of lenses has a light beam emitted from the light emitting element of the light emitting element group corresponding to the lens, A line head that forms an image toward the surface of the latent image carrier facing the lens, and for each of the plurality of lenses, the apex of the lens surface on the latent image carrier side of the lens and the surface of the latent image carrier Len When the distance in the optical axis direction is defined as the image plane facing distance, each of the N lens rows is opposed to a different facing position on the surface of the latent image carrier in the sub-scanning direction. The surface area of the surface facing the lens array has a curvature in the sub-scanning section and is convex with respect to the lens array, and the line head is formed at each end in the width direction of the N lens rows. When the lens row is an end lens row, the latent image is set so that the image plane facing distance of lenses belonging to lens rows other than the end lens row is smaller than the image surface facing distance of lenses belonging to the end lens row. It is characterized by being arranged with respect to the surface of the carrier.

  In order to achieve the above object, the image forming method according to the present invention includes an exposure step in which the surface of the latent image carrier that is transported in the sub-scanning direction substantially perpendicular to the main scanning direction is exposed by a line head. The line head includes lens rows in which a plurality of lenses are arranged in the longitudinal direction corresponding to the main scanning direction and are arranged in different arrangement positions in the width direction corresponding to the N rows (N is an integer of 3 or more) and the sub-scanning direction. A lens array disposed opposite to the surface of the image bearing member; and a head substrate in which a light emitting element group including a plurality of light emitting elements is disposed for each of the plurality of lenses. The light beam emitted from the light emitting element of the corresponding light emitting element group is imaged toward the surface of the latent image carrier that the lens faces, and the latent image of the lens is carried on each of the plurality of lenses. When the distance between the apex of the side lens surface and the surface of the latent image carrier in the optical axis direction of the lens is defined as an image surface facing distance, each of the N lens rows is in the sub-scanning direction of the surface of the latent image carrier In the surface of the latent image carrier, the surface area facing the lens array has a curvature in the sub-scan section and is convex with respect to the lens array, When the head has a lens row at each end in the width direction among the N lens rows as an end lens row, an image surface facing distance of a lens belonging to a lens row other than the end lens row is the end lens row. It is characterized in that it is arranged with respect to the surface of the latent image carrier so as to be smaller than the image plane facing distance of the lens belonging to.

  In the invention thus configured (image forming apparatus, image forming method), the surface of the latent image carrier is exposed by the line head. The surface of the latent image carrier is conveyed in the sub-scanning direction substantially orthogonal to the main scanning direction. An electrostatic latent image can be formed by exposing the surface of the latent image carrier conveyed in the sub-scanning direction with a line head. At this time, the line head is arranged with respect to the latent image carrier so that the longitudinal direction thereof corresponds to the main scanning direction and the width direction thereof corresponds to the sub scanning direction.

  The line head includes a lens array and a head substrate. In the lens array, lens rows formed by arranging a plurality of lenses in the longitudinal direction are arranged opposite to the surface of the latent image carrier at different arrangement positions in N rows (N is an integer of 3 or more) in the width direction. The head substrate has a light emitting element group composed of a plurality of light emitting elements arranged for each of the plurality of lenses. Each of the plurality of lenses forms an image of the light beam emitted from the light emitting element of the light emitting element group corresponding to the lens toward the surface of the latent image carrier.

  In the line head, each of the N lens rows is opposed to a different facing position on the surface of the latent image carrier in the sub-scanning direction. On the other hand, the surface area facing the lens array in the surface of the latent image carrier has a curvature in the sub-scan section and is convex with respect to the lens array. Therefore, a difference occurs in the distance between the lens surface and the surface of the latent image carrier between the N lens rows. If the arrangement relationship between the line head and the surface of the latent image carrier is not appropriate, the difference in the image surface facing distance becomes large, which may cause the following problems. That is, due to the large difference in the image plane facing distance, there is a possibility that the distance between the imaging position of the light beam and the surface of the latent image carrier is greatly different among the N lens rows. As a result, the image formed on the surface of the latent image carrier is greatly different among the N lens rows, and there is a possibility that good exposure cannot be performed, that is, exposure failure occurs. Here, the image plane facing distance is defined as follows. That is, for each of the plurality of lenses, the distance in the optical axis direction of the lens between the apex of the lens surface on the latent image carrier side of the lens and the surface of the latent image carrier is the image surface facing distance.

  On the other hand, in the above-described invention, the line head has a lens line belonging to a lens row other than the end lens row when the lens row at each end in the width direction among the N lens rows is an end lens row. The image surface facing distance is arranged with respect to the surface of the latent image carrier so that the image surface facing distance of the lens belonging to the end lens row is smaller. Accordingly, it is possible to suppress the difference in the image plane facing distance between the N lens rows, and as a result, it is possible to suppress the occurrence of the above-described exposure failure and realize good exposure.

  Further, when the N lens rows are arranged at a predetermined lens row pitch in the width direction and are arranged substantially symmetrically in the width direction with respect to a symmetry axis substantially perpendicular to the width direction in the sub-scanning section, The line head may be arranged as follows. That is, the line head compares the image plane facing distance of the lens belonging to the central lens row closest to the symmetry axis among the N lens rows with the image plane facing distance of the lens belonging to each lens row other than the central lens row. It may be arranged with respect to the surface of the latent image carrier so as to be smaller. If this is the case, by disposing the line head with respect to the surface of the latent image carrier in this way, it becomes possible to suppress the difference in the image plane facing distance between the N lens rows, and good exposure can be achieved. This is because it is realized.

  When the curvature shape of the latent image carrier surface has the center of curvature, the line head may be arranged so that the axis of symmetry passes through the center of curvature. If the line head is arranged with respect to the surface of the latent image carrier, the difference in the image plane facing distance between the N lens rows can be further reduced, and good exposure can be realized. This is because that.

  Further, an aperture stop may be provided between each lens and the corresponding light emitting element group, and the image side of each lens may be configured to be telecentric. That is, the distance between the lens and the surface of the latent image carrier may vary due to the eccentricity of the latent image carrier. As will be described later, such fluctuations may cause fluctuations in the sub-scanning direction of the positions of spots formed on the surface of the latent image carrier. On the other hand, if the image side of each lens is configured telecentric, it is possible to suppress the fluctuation of the spot position in the sub-scanning direction, and it is preferable because good exposure is realized.

  Further, as described above, the line head is arranged with respect to the surface of the latent image carrier to suppress the difference in the image plane facing distance between the N lens rows, and each of the plurality of lenses has the lens. The lens shape of the lens is adjusted according to the image plane facing distance, and the imaging position of the light beam emitted from the light emitting element group corresponding to the lens is set as the position corresponding to the curvature shape of the surface of the latent image carrier. good. That is, in such a configuration, the line head is disposed on the surface of the latent image carrier so that the difference in the image plane facing distance between the N lens rows is suppressed. Therefore, it is possible to set the imaging position of the light beam to a position corresponding to the curvature shape of the surface of the latent image carrier without greatly changing the lens shape between the N lens rows. Therefore, it is possible to achieve very good exposure while suppressing the complexity of lens design or manufacture, which is preferable.

  FIG. 1 is a diagram showing an embodiment of an image forming apparatus according to the present invention. FIG. 2 is a diagram showing an electrical configuration of the image forming apparatus of FIG. This apparatus uses a color mode in which four color toners of black (K), cyan (C), magenta (M), and yellow (Y) are superimposed to form a color image, and only black (K) toner. Thus, the image forming apparatus can selectively execute a monochrome mode for forming a monochrome image. FIG. 1 is a diagram corresponding to the execution of the color mode. In this image forming apparatus, when an image formation command is given from an external device such as a host computer to a main controller MC having a CPU, a memory, etc., the main controller MC gives a control signal and the like to the engine controller EC as well as an image formation command. Corresponding video data VD is supplied to the head controller HC. The head controller HC controls the line head 29 for each color based on the video data VD from the main controller MC, the vertical synchronization signal Vsync from the engine controller EC, and parameter values. Thus, the engine unit EG executes a predetermined image forming operation, and forms an image corresponding to the image forming command on a sheet such as a copy sheet, a transfer sheet, a sheet, and an OHP transparent sheet.

  In the housing main body 3 of the image forming apparatus according to this embodiment, an electrical component box 5 is provided that incorporates a power circuit board, a main controller MC, an engine controller EC, and a head controller HC. An image forming unit 7, a transfer belt unit 8, and a paper feeding unit 11 are also disposed in the housing body 3. In FIG. 1, a secondary transfer unit 12, a fixing unit 13, and a sheet guide member 15 are disposed on the right side in the housing body 3. The paper feeding unit 11 is configured to be detachable from the apparatus main body 1. The paper feed unit 11 and the transfer belt unit 8 can be removed and repaired or exchanged.

  The image forming unit 7 includes four image forming stations Y (for yellow), M (for magenta), C (for cyan), and K (for black) that form a plurality of images of different colors. Each of the image forming stations Y, M, C, and K is provided with a cylindrical photosensitive drum 21 having a surface with a predetermined length in the main scanning direction MD. In this specification, the shape of the cylindrical peripheral surface is defined as “curvature shape”, and “surface has curvature” means that the shape of the surface is a curvature shape. And Further, in the present specification, when referred to as “curvature center of curvature”, the center of curvature means a point on the central axis of the cylinder. Each of the image forming stations Y, M, C, and K forms a corresponding color toner image on the surface of the photosensitive drum 21. The photosensitive drum is arranged so that the axial direction is substantially parallel to the main scanning direction MD. Each photosensitive drum 21 is connected to a dedicated drive motor and is driven to rotate at a predetermined speed in the direction of arrow D21 in the figure. As a result, the surface of the photosensitive drum 21 is conveyed in the sub-scanning direction SD substantially orthogonal to the main scanning direction MD. A charging unit 23, a line head 29, a developing unit 25, and a photoconductor cleaner 27 are disposed around the photoconductive drum 21 along the rotation direction. Then, a charging operation, a latent image forming operation, and a toner developing operation are executed by these functional units. Therefore, when the color mode is executed, the toner images formed at all the image forming stations Y, M, C, and K are superimposed on the transfer belt 81 of the transfer belt unit 8 to form a color image, and the monochrome mode is executed. In some cases, a monochrome image is formed using only the toner image formed at the image forming station K. In FIG. 1, the image forming stations of the image forming unit 7 have the same configuration, and therefore, for convenience of illustration, only some image forming stations are denoted by reference numerals, and the other image forming stations are omitted. .

  The charging unit 23 includes a charging roller whose surface is made of elastic rubber. The charging roller is configured to rotate in contact with the surface of the photosensitive drum 21 at the charging position, and at a peripheral speed in the driven direction with respect to the photosensitive drum 21 as the photosensitive drum 21 rotates. Followed rotation. The charging roller is connected to a charging bias generator (not shown). The charging roller is supplied with the charging bias from the charging bias generator and is charged at the charging position where the charging unit 23 and the photosensitive drum 21 come into contact with each other. The surface of 21 is charged.

  The line head 29 is arranged with respect to the photosensitive drum 21 such that the longitudinal direction thereof corresponds to the main scanning direction MD and the width direction thereof corresponds to the sub scanning direction SD. Accordingly, the longitudinal direction of the line head 29 is substantially parallel to the main scanning direction MD. The line head includes a plurality of light emitting elements arranged side by side in the longitudinal direction and is spaced from the photosensitive drum 21. From these light emitting elements, the surface of the photosensitive drum 21 charged by the charging unit 23 is irradiated with light (that is, exposed) to form a latent image on the surface (exposure process). In this embodiment, a head controller HC is provided to control the line heads 29 for the respective colors, and each line head 29 is controlled based on the video data VD from the main controller MC and a signal from the engine controller EC. ing. That is, in this embodiment, the image data included in the image formation command is input to the image processing unit 51 of the main controller MC. Various image processing is performed on the image data to create video data VD of each color, and the video data VD is given to the head controller HC via the main-side communication module 52. In the head controller HC, the video data VD is given to the head control module 54 via the head side communication module 53. As described above, the head controller module 54 is supplied with the signal indicating the parameter value related to the latent image formation and the vertical synchronization signal Vsync from the engine controller EC. Based on these signals, video data VD, and the like, the head controller HC creates a signal for controlling element driving for the line head 29 of each color, and outputs the signal to each line head 29. Thus, the operation of the light emitting elements is appropriately controlled in each line head 29, and a latent image corresponding to the image formation command is formed.

  In this embodiment, the photosensitive drum 21, the charging unit 23, the developing unit 25, and the photosensitive cleaner 27 of each of the image forming stations Y, M, C, and K are unitized as a photosensitive cartridge. Each photoconductor cartridge is provided with a nonvolatile memory for storing information related to the photoconductor cartridge. Then, wireless communication is performed between the engine controller EC and each photoconductor cartridge. In this way, information on each photoconductor cartridge is transmitted to the engine controller EC, and information in each memory is updated and stored.

  The developing unit 25 has a developing roller 251 on which toner is carried. The charged toner is developed at a developing position where the developing roller 251 and the photosensitive drum 21 come into contact with each other by a developing bias applied to the developing roller 251 from a developing bias generator (not shown) electrically connected to the developing roller 251. Is moved from the developing roller 251 to the photosensitive drum 21, and the electrostatic latent image formed by the line head 29 becomes obvious.

  The toner image that has been made visible at the developing position in this way is conveyed in the rotational direction D21 of the photosensitive drum 21, and then a primary transfer position TR1 at which each of the photosensitive drums 21 comes into contact with the transfer belt 81 described in detail later. 1 is primarily transferred to the transfer belt 81.

  In this embodiment, a photoreceptor cleaner 27 is provided in contact with the surface of the photoreceptor drum 21 on the downstream side of the primary transfer position TR1 in the rotation direction D21 of the photoreceptor drum 21 and on the upstream side of the charging unit 23. It has been. The photoconductor cleaner 27 abuts on the surface of the photoconductor drum to remove the toner remaining on the surface of the photoconductor drum 21 after the primary transfer.

  The transfer belt unit 8 includes a driving roller 82, a driven roller 83 (blade facing roller) disposed on the left side of the driving roller 82 in FIG. And a transfer belt 81 that is driven to circulate. Further, the transfer belt unit 8 is disposed on the inner side of the transfer belt 81 so as to be opposed to each of the photosensitive drums 21 included in the image forming stations Y, M, C, and K when the photosensitive cartridge is mounted. Four primary transfer rollers 85Y, 85M, 85C, and 85K are provided. Each of these primary transfer rollers 85 is electrically connected to a primary transfer bias generator (not shown). As will be described in detail later, when the color mode is executed, as shown in FIG. 1, all the primary transfer rollers 85Y, 85M, 85C, and 85K are positioned on the image forming stations Y, M, C, and K side. Then, the transfer belt 81 is pushed and brought into contact with the photosensitive drums 21 included in the image forming stations Y, M, C, and K, so that the primary transfer position TR1 is set between each photosensitive drum 21 and the transfer belt 81. Form. Then, by applying a primary transfer bias from the primary transfer bias generator to the primary transfer roller 85 at an appropriate timing, the toner images formed on the surfaces of the photosensitive drums 21 correspond respectively. A color image is formed by transferring to the surface of the transfer belt 81 at the primary transfer position TR1.

  On the other hand, when the monochrome mode is executed, among the four primary transfer rollers 85, the color primary transfer rollers 85Y, 85M, and 85C are separated from the image forming stations Y, M, and C facing each other, and the monochrome primary transfer is performed. By bringing only the roller 85K into contact with the image forming station K, only the monochrome image forming station K is brought into contact with the transfer belt 81. As a result, the primary transfer position TR1 is formed only between the monochrome primary transfer roller 85K and the image forming station K. Then, by applying a primary transfer bias from the primary transfer bias generator to the monochrome primary transfer roller 85K at an appropriate timing, the toner image formed on the surface of each photosensitive drum 21 is subjected to primary transfer. A monochrome image is formed by transferring to the surface of the transfer belt 81 at a position TR1.

  Further, the transfer belt unit 8 includes a downstream guide roller 86 disposed downstream of the monochrome primary transfer roller 85K and upstream of the driving roller 82. Further, the downstream guide roller 86 is formed between the primary transfer roller 85K and the photosensitive drum 21 at the primary transfer position TR1 formed by the monochrome primary transfer roller 85K contacting the photosensitive drum 21 of the image forming station K. It is configured to contact the transfer belt 81 on a common inscribed line.

  The drive roller 82 circulates and drives the transfer belt 81 in the direction of the arrow D81 in the figure, and also serves as a backup roller for the secondary transfer roller 121. A rubber layer having a thickness of about 3 mm and a volume resistivity of 1000 kΩ · cm or less is formed on the peripheral surface of the driving roller 82, and secondary transfer is omitted by grounding through a metal shaft. The conductive path of the secondary transfer bias supplied from the bias generation unit via the secondary transfer roller 121 is used. When the rubber layer having high friction and shock absorption is provided on the driving roller 82 in this way, the sheet enters the contact portion (secondary transfer position TR2) between the driving roller 82 and the secondary transfer roller 121. Is difficult to be transmitted to the transfer belt 81, and image quality deterioration can be prevented.

  The paper feed unit 11 includes a paper feed unit having a paper feed cassette 77 capable of stacking and holding sheets and a pickup roller 79 for feeding sheets one by one from the paper feed cassette 77. The sheet fed from the sheet feeding unit by the pickup roller 79 is fed to the secondary transfer position TR2 along the sheet guide member 15 after the sheet feeding timing is adjusted by the registration roller pair 80.

  The secondary transfer roller 121 is provided so as to be able to come into contact with and separate from the transfer belt 81 and is driven to come into contact with and separate from a secondary transfer roller drive mechanism (not shown). The fixing unit 13 includes a heating roller 131 that includes a heating element such as a halogen heater and is rotatable, and a pressure unit 132 that presses and biases the heating roller 131. The sheet on which the image has been secondarily transferred is guided to a nip formed by the heating roller 131 and the pressure belt 1323 of the pressure unit 132 by the sheet guide member 15, and in the nip, a predetermined value is formed. The image is heat-fixed at temperature. The pressure unit 132 includes two rollers 1321 and 1322 and a pressure belt 1323 stretched between them. A nip portion formed by the heating roller 131 and the pressure belt 1323 is formed by pressing the belt tension surface stretched by the two rollers 1321 and 1322 out of the surface of the pressure belt 1323 against the peripheral surface of the heating roller 131. Is configured to be widely taken. Further, the sheet thus subjected to the fixing process is conveyed to a paper discharge tray 4 provided on the upper surface portion of the housing body 3.

  Further, in this apparatus, a cleaner portion 71 is disposed to face the blade facing roller 83. The cleaner unit 71 includes a cleaner blade 711 and a waste toner box 713. The cleaner blade 711 removes foreign matters such as toner and paper dust remaining on the transfer belt after the secondary transfer by bringing the tip of the cleaner blade 711 into contact with the blade facing roller 83 via the transfer belt 81. The foreign matter removed in this way is collected in a waste toner box 713. Further, the cleaner blade 711 and the waste toner box 713 are integrally formed with the blade facing roller 83. Therefore, when the blade facing roller 83 moves as will be described below, the cleaner blade 711 and the waste toner box 713 also move together with the blade facing roller 83.

  FIG. 3 is a perspective view schematically showing an embodiment of the line head according to the present invention. FIG. 4 is a cross-sectional view in the width direction of one embodiment of the line head according to the present invention. As described above, the line head 29 is arranged with respect to the photosensitive drum 21 such that the longitudinal direction LGD corresponds to the main scanning direction MD and the width direction LTD corresponds to the sub-scanning direction SD. The longitudinal direction LGD and the width direction LTD are substantially orthogonal to each other. The line head 29 in the present embodiment includes a case 291, and positioning pins 2911 and screw insertion holes 2912 are provided at both ends of the case 291 in the longitudinal direction LGD. Then, the positioning pin 2911 covers the photosensitive drum 21 and is fitted into a positioning hole (not shown) formed in a photosensitive cover (not shown) positioned with respect to the photosensitive drum 21, thereby The head 29 is positioned with respect to the photosensitive drum 21. Further, the line head 29 is positioned and fixed with respect to the photosensitive drum 21 by screwing and fixing a fixing screw into a screw hole (not shown) of the photosensitive member cover through the screw insertion hole 2912.

  The case 291 holds the lens array 299 at a position facing the surface of the photosensitive drum 21, and includes a light shielding member 297 and a head substrate 293 in the order close to the lens array 299. A plurality of light emitting element groups 295 are provided on the back surface of the head substrate 293 (the surface opposite to the lens array 299 among the two surfaces of the head substrate 293). That is, the plurality of light emitting element groups 295 are two-dimensionally arranged on the back surface of the head substrate 293 so as to be separated from each other by a predetermined distance in the longitudinal direction LGD and the width direction LTD. Here, each of the plurality of light emitting element groups 295 is configured by two-dimensionally arranging a plurality of light emitting elements, which will be described later. In the present embodiment, a bottom emission type organic EL (Electro-Luminescence) element is used as the light emitting element. That is, in this embodiment, the organic EL element is disposed as a light emitting element on the back surface of the head substrate 293. Thus, all the light emitting elements 2951 are arranged on the same plane (the back surface of the head substrate 293). When each light emitting element is driven by the drive circuit formed on the head substrate 293, a light beam is emitted from the light emitting element toward the photosensitive drum 21. This light beam is directed to the light shielding member 297 via the head substrate 293.

  A plurality of light guide holes 2971 are formed in the light shielding member 297 on a one-to-one basis with respect to the plurality of light emitting element groups 295. Further, the light guide hole 2971 is formed as a substantially cylindrical hole penetrating the light shielding member 297 with a line parallel to the normal line of the head substrate 293 as a central axis. Therefore, all the light emitted from the light emitting elements belonging to one light emitting element group 295 is directed to the lens array 299 through the same light guide hole 2971, and interference between light beams from different light emitting element groups 295 is blocked by the light shielding member. 297 prevents it. Then, the light beam that has passed through the light guide hole 2971 formed in the light shielding member 297 is imaged as a spot on the surface of the photosensitive drum 21 by the lens array 299. Note that the specific configuration of the lens array 299 and the imaging state of the light beam by the lens array 299 will be described in detail later.

  As shown in FIG. 4, the back cover 2913 is pressed against the case 291 via the head substrate 293 by the fixing device 2914. That is, the fixing device 2914 has an elastic force that presses the back cover 2913 toward the case 291, and presses the back cover with the elastic force, thereby making the inside of the case 291 light-tight (that is, from inside the case 291. It is sealed so that light does not leak and so that light does not enter from the outside of the case 291. Note that a plurality of fixing devices 2914 are provided in the longitudinal direction of the case 291. The light emitting element group 295 is covered with a sealing member 294.

  FIG. 5 is a perspective view schematically showing the lens array. FIG. 6 is a cross-sectional view of the lens array in the longitudinal direction LGD. The lens array 299 has a lens substrate 2991. The first surface LSFf of the lens LS is formed on the back surface 2991B of the lens substrate 2991, and the second surface LSFs of the lens LS is formed on the surface 2991A of the lens substrate 2991. The first surface LSFf and the second surface LSFs of the lenses facing each other and the lens substrate 2991 sandwiched between these two surfaces function as one lens LS. The first surface LSFf and the second surface LSFs of the lens LS can be formed of, for example, a resin.

  In the lens array 299, the plurality of lenses LS are arranged such that the optical axes OA are substantially parallel to each other. The lens array 299 is arranged so that the optical axis OA of the lens LS is substantially orthogonal to the back surface of the head substrate 293 (the surface on which the light emitting element 2951 is disposed). At this time, the plurality of lenses LS are arranged one-on-one in the plurality of light emitting element groups 295. That is, the plurality of lenses LS are two-dimensionally arranged at a predetermined interval in the longitudinal direction LGD and the width direction LTD corresponding to the arrangement of the light emitting element groups 295. More specifically, a plurality of lens rows LSR in which a plurality of lenses LS are arranged in the longitudinal direction LGD are arranged in the width direction LTD. In the present embodiment, three lens rows LSR1, LSR2, and LSR3 are arranged in the width direction LTD. Further, the three lens rows LSR1 to LSR3 are arranged so as to be shifted from each other in the longitudinal direction by a predetermined lens pitch Pls.

  FIG. 7 is a diagram showing the arrangement of the light emitting element groups in the line head. FIG. 8 is a diagram showing the arrangement of light emitting elements in each light emitting element group. In the present embodiment, in each light emitting element group 295, eight light emitting elements 2951 are arranged at a predetermined element pitch Pel in the longitudinal direction LGD. Further, each light emitting element group 295 of the present embodiment includes a light emitting element row 2951R in which four light emitting elements 2951 are arranged at a predetermined interval (interval twice the element pitch Pel) in the longitudinal direction LGD, and an element in the width direction LTD. Two rows are arranged at an interval of the row pitch Perl. The plurality of light emitting element groups 295 are arranged as follows.

  That is, the plurality of light emitting element groups 295 are arranged such that three light emitting element group rows 295R configured by arranging a predetermined number of light emitting element groups 295 in the longitudinal direction LGD are arranged in the width direction LTD. Further, all the light emitting element groups 295 are arranged at different longitudinal positions. Further, the light emitting element groups 295 are arranged so that the light emitting element groups (for example, the light emitting element group 295_C1 and the light emitting element group 295_B1) whose longitudinal positions are adjacent to each other are different in the width direction. Note that in this specification, the geometric center of gravity of the light emitting element 2951 is defined as the position of the light emitting element 2951, and the geometric center of gravity of all light emitting element positions belonging to the same light emitting element group 295 is defined as the position of the light emitting element group 295. Further, the longitudinal direction position and the width direction position mean the longitudinal direction component and the width direction component of the position of interest, respectively.

  Corresponding to the arrangement of the light emitting element groups 295 described above, a light guide hole 2971 is formed in the light shielding member 297 and a lens LS is arranged. That is, in the present embodiment, the center of gravity of the light emitting element group 295, the center axis of the light guide hole 2971, and the optical axis OA of the lens LS are configured to substantially coincide. Then, the light beam emitted from the light emitting element 2951 of the light emitting element group 295 enters the lens array 299 through the corresponding light guide hole 2971 and is connected as a spot on the surface of the photosensitive drum 21 by the lens array 299. Imaged.

  FIG. 9 is a diagram illustrating an imaging state of the lens in a cross section including the longitudinal direction and the optical axis. In addition, this figure shows the trajectory of the light beam from the virtual object points OM0, OM1, and OM2 on the back surface of the head substrate 293 in order to show the imaging state of the lens LS. Here, the virtual object point OM0 is on the optical axis OA. Further, the virtual object points OM1, OM2 are located at positions symmetrical to each other with respect to the optical axis OA. As shown by the locus, the light beam emitted from the virtual object point is incident on the back surface of the head substrate 293 and then emitted from the surface of the head substrate 293. The light beam emitted from the surface of the head substrate 293 reaches the image plane IP (the surface of the photosensitive drum 21) via the lens LS. Here, the head substrate 293 and the lens LS each have a predetermined refractive index.

  As shown in FIG. 9, the light beam emitted from the virtual object point OM0 is imaged at an intersection point IM0 between the image plane IP and the optical axis OA. The light beams emitted from the virtual object points OM1 and OM2 are imaged at the image plane positions IM1 and IM2, respectively. That is, the light beam emitted from the virtual object point OM1 is imaged at a position IM1 on the opposite side across the optical axis OA in the longitudinal direction LGD, and the light beam emitted from the virtual object point OM2 is In LGD, an image is formed at a position I2 on the opposite side across the optical axis OA. Thus, the lens LS in the present embodiment is a so-called reversal optical system having reversal characteristics. As shown in the figure, the distance between the positions IM1 and IM0 where the light beam is imaged is shorter than the distance between the virtual object points OM1 and OM0. That is, the absolute value of the magnification of the optical system including the head substrate 293 and the lens LS in this embodiment is less than 1. An aperture stop DIA is disposed at the front focal point between the head substrate 293 and the first surface LSFf of the lens LS (that is, the object space). As a result, the principal rays PRM0 to PRM2 of the light beam are all parallel to the optical axis OA in the image space. That is, the image side of the lens LS is configured to be telecentric.

  FIG. 10 is a diagram illustrating an imaging state of the lens in a cross section including the width direction and the optical axis. In addition, this figure shows the trajectory of the light beam from the virtual object points OS0, OS1, and OS2 on the back surface of the head substrate 293 in order to show the imaging state of the lens LS. Here, the virtual object point OS0 is on the optical axis OA. The virtual object points OS1 and OS2 are located at positions symmetrical to each other with respect to the optical axis OA. As shown by the locus, the light beam emitted from the virtual object point is incident on the back surface of the head substrate 293 and then emitted from the surface of the head substrate 293. The light beam emitted from the surface of the head substrate 293 reaches the image plane IP (the surface of the photosensitive drum 21) via the lens LS. As described above, the head substrate 293 and the lens LS each have a predetermined refractive index.

  As shown in FIG. 10, the light beam emitted from the virtual object point OS0 is imaged at the intersection IS0 between the image plane IP and the optical axis OA. The light beams emitted from the virtual object points OS1 and OS2 are imaged at the image plane positions IS1 and IS2, respectively. That is, the light beam emitted from the virtual object point OS1 is imaged at the opposite position IS1 across the optical axis OA in the width direction LTD, and the light beam emitted from the virtual object point OS2 is In the LTD, an image is formed at a position I2 on the opposite side across the optical axis OA. Thus, the lens LS in the present embodiment is a so-called reversal optical system having reversal characteristics. As shown in the figure, the distance between the positions IS1 and IS0 where the light beam is imaged is shorter than the distance between the virtual object points OS1 and OS0. That is, the absolute value of the magnification of the optical system including the head substrate 293 and the lens LS in this embodiment is less than 1. An aperture stop DIA is disposed at the front focal point between the head substrate 293 and the first surface LSFf of the lens LS (that is, the object space). As a result, the principal rays PRS0 to PRS2 of the light beam are all parallel to the optical axis OA in the image space. That is, the image side of the lens LS is configured to be telecentric.

  11 and 12 are explanatory diagrams of terms used in this specification. Here, the terms used in this specification will be organized using these drawings. In the present specification, as described above, the transport direction of the surface (image surface IP) of the photosensitive drum 21 is defined as the sub-scanning direction SD, and the direction substantially orthogonal to the sub-scanning direction SD is defined as the main scanning direction MD. ing. The line head 29 is arranged with respect to the surface (image surface IP) of the photosensitive drum 21 so that the longitudinal direction LGD corresponds to the main scanning direction MD and the width direction LTD corresponds to the sub-scanning direction SD. Has been.

  A set of a plurality of (eight in FIG. 11 and FIG. 12) light emitting elements 2951 arranged on the head substrate 293 in a one-to-one correspondence with the plurality of lenses LS included in the lens array 299 is defined as a light emitting element group 295. . That is, in the head substrate 293, the light emitting element group 295 including the plurality of light emitting elements 2951 is disposed for each of the plurality of lenses LS. Further, the light beam from the light emitting element group 295 is imaged toward the image plane IP by the lens LS corresponding to the light emitting element group 295, whereby a set of a plurality of spots SP formed on the image plane IP is obtained. It is defined as group SG. That is, the plurality of spot groups SG can be formed in one-to-one correspondence with the plurality of light emitting element groups 295. In each spot group SG, the most upstream spot in the main scanning direction MD and the sub-scanning direction SD is particularly defined as the first spot. The light emitting element 2951 corresponding to the first spot is particularly defined as the first light emitting element.

  11 and 12 show the case where the spot SP is formed in a state where the image plane is stationary so that the correspondence between the light emitting element group 295, the lens LS, and the spot group SG can be easily understood. Therefore, the formation position of the spot SP in the spot group SG is substantially similar to the arrangement position of the light emitting element 2951 in the light emitting element group 295. However, as will be described later, the actual spot forming operation is performed while conveying the image plane IP (the surface of the photosensitive drum 21) in the sub-scanning direction SD. As a result, the spots SP formed by the plurality of light emitting elements 2951 included in the head substrate 293 are formed on a straight line substantially parallel to the main scanning direction MD.

  Further, as shown in the column “on image plane” in FIG. 12, a spot group row SGR and a spot group column SGC are defined. That is, a plurality of spot groups SG arranged in the main scanning direction MD are defined as spot group rows SGR. The plurality of spot group rows SGR are arranged side by side in the sub-scanning direction SD at a predetermined spot group row pitch Psgr. A plurality (three in the figure) of spot groups SG arranged at the spot group row pitch Psgr in the sub scanning direction SD and at the spot group pitch Psg in the main scanning direction MD are defined as a spot group column SGC. The spot group row pitch Psgr is a distance in the sub-scanning direction SD between the geometric centroids of two spot group rows SGR arranged at the same pitch. The spot group pitch Psg is a distance in the main scanning direction MD between the geometric centroids of two spot groups SG arranged at the same pitch.

  Lens rows LSR and lens columns LSC are defined as shown in the “lens array” column of FIG. That is, a plurality of lenses LS arranged in the longitudinal direction LGD are defined as a lens row LSR. The plurality of lens rows LSR are arranged side by side in the width direction LTD at a predetermined lens row pitch Plsr. A plurality (three in the figure) of lenses LS arranged at the lens row pitch Plsr in the width direction LTD and at the lens pitch Pls in the longitudinal direction LGD are defined as a lens row LSC. The lens row pitch Plsr is a distance in the width direction LTD of the geometric centroids of the two lens rows LSR arranged at the same pitch. The lens pitch Pls is a distance in the longitudinal direction LGD between the geometric centroids of the two lenses LS arranged at the same pitch.

  As shown in the “head substrate” column of the same figure, a light emitting element group row 295R and a light emitting element group column 295C are defined. That is, a plurality of light emitting element groups 295 arranged in the longitudinal direction LGD is defined as a light emitting element group row 295R. The plurality of light emitting element group rows 295R are arranged side by side in the width direction LTD at a predetermined light emitting element group row pitch Pegr. Further, a plurality of (three in the figure) light emitting element groups 295 arranged at the light emitting element group row pitch Pegr in the width direction LTD and at the light emitting element group pitch Peg in the longitudinal direction LGD are defined as a light emitting element group column 295C. The light emitting element group row pitch Pegr is a distance in the width direction LTD between the geometric centroids of two light emitting element group rows 295R arranged at the same pitch. The light emitting element group pitch Peg is the distance in the longitudinal direction LGD of the geometric centroids of the two light emitting element groups 295 arranged at the same pitch.

  As shown in the column of “light emitting element group” in the drawing, a light emitting element row 2951R and a light emitting element column 2951C are defined. That is, in each light emitting element group 295, a plurality of light emitting elements 2951 arranged in the longitudinal direction LGD is defined as a light emitting element row 2951R. The plurality of light emitting element rows 2951R are arranged side by side in the width direction LTD at a predetermined light emitting element row pitch Pelr. In addition, a plurality of (two in the figure) light emitting elements 2951 arranged at the light emitting element row pitch Pelr in the width direction LTD and at the light emitting element pitch Pel in the longitudinal direction LGD are defined as a light emitting element column 2951C. The light emitting element row pitch Pelr is the distance in the width direction LTD of the geometric center of gravity of each of the two light emitting element rows 2951R arranged at the same pitch. The light emitting element pitch Pel is a distance in the longitudinal direction LGD between the geometric centroids of two light emitting elements 2951 arranged at the same pitch.

  As shown in the column “Spot Group” in the figure, a spot row SPR and a spot column SPC are defined. That is, in each spot group SG, a plurality of spots SP arranged in the longitudinal direction LGD are defined as spot rows SPR. The plurality of spot rows SPR are arranged side by side in the width direction LTD at a predetermined spot row pitch Pspr. Further, a plurality of (two in the figure) spots arranged at the spot pitch Pspr in the width direction LTD and at the spot pitch Psp in the longitudinal direction LGD are defined as a spot row SPC. The spot row pitch Pspr is the distance in the sub-scanning direction SD between the geometric centroids of two spot rows SPR arranged at the same pitch. The spot pitch Psp is the distance in the longitudinal direction LGD between the geometric centroids of two spots SP arranged at the same pitch.

  Here, the lens configuration and lens position of the lens LS used in this specification will be defined. First, the “lens configuration” is a concept including the lens shape of the lens LS, the lens thickness of the lens LS, the lens material of the lens LS, and the like. The lens position, lens thickness, and lens shape of the lens LS are as follows.

  FIG. 13 is an explanatory diagram of lens positions and the like. First, the lens position of the lens LS is the lens position when the origin is the intersection of the arrangement plane of the light emitting element group 295 corresponding to the lens LS (in this embodiment, the back surface of the head substrate 293) and the optical axis OA. This is the position of the vertex VTf of the first surface LSFf of LS. Here, the vertex VTf is an intersection of the first surface LSFf of the lens LS and the optical axis OA. The lens thickness THK of the lens LS is a distance between the first surface LSFf and the second surface LSFs of the lens LS. That is, as shown in the figure, the lens thickness THK is the distance between the vertex VTf of the first surface LSFf of the lens LS and the vertex VTs of the second surface LSFs of the lens LS. The vertex VTs is an intersection between the second surface LSFs of the lens LS and the optical axis OA. The lens shape of the lens LS is the shape of the first surface LSFf and the second surface LSFs of the lens LS. Therefore, lenses having different first surfaces LSFf and second surfaces LSFs have different lens shapes. Further, in this specification, the image plane facing distance ld is defined as follows. That is, the image surface facing distance ld is equal to the second surface LSFs of the lens LS, that is, the vertex VTs of the lens surface LSFs on the photosensitive drum side (lens surface on the latent image carrier side), and the photosensitive drum 21 facing the lens LS. Is the distance in the optical axis direction of the lens (that is, the direction in which the optical axis OA extends) to the surface (latent image carrier surface)

  FIG. 14 is a diagram showing a spot forming operation by the above-described line head. Hereinafter, the spot forming operation by the line head in this embodiment will be described with reference to FIGS. 2, 7, and 14. In order to facilitate understanding of the invention, here, a case where a plurality of spots are formed side by side on a straight line extending in the main scanning direction MD will be described. In the present embodiment, the head control module 54 causes a plurality of light emitting elements to emit light at a predetermined timing while transporting the surface of the photosensitive drum 21 (latent image carrier) in the sub scanning direction SD, thereby causing the main scanning direction MD to be emitted. A plurality of spots are formed side by side on a straight line extending in a straight line.

  That is, in the line head of this embodiment, six light emitting element rows 2951R are arranged side by side in the width direction LTD corresponding to the respective positions in the width direction LTD1 to LTD6 (FIG. 7). Accordingly, in the present embodiment, the light emitting element rows 2951R at the same width direction position emit light at substantially the same timing, and the light emitting element rows 2951R at different sub width direction positions emit light at different timings. More specifically, the light emitting element rows 2951R are caused to emit light in the order of the width direction positions LTD1 to LTD6. Then, while the surface of the photosensitive drum 21 is conveyed in the sub-scanning direction SD corresponding to the width direction LTD, the light-emitting element rows 2951R are caused to emit light in the above-described order, so that the surface extends on a straight line extending in the main scanning direction MD. A plurality of spots are formed side by side.

  Such an operation will be described with reference to FIGS. First, the light emitting elements 2951 of the light emitting element row 2951R in the width direction position LTD1 belonging to the most upstream light emitting element group 295_C1, 295_C2, 295_C3,. The plurality of light beams emitted by the light emission operation are reversed and imaged on the surface of the photosensitive member by the lens LS having the reversal characteristics described above. That is, a spot is formed at the position of the “first” hatching pattern in FIG. In the figure, white circles represent spots that have not yet been formed and are to be formed in the future. In the same figure, the spots labeled with reference numerals 295_C1, 295_B1, 295_A1, 295_C2 are spots formed by the light emitting element groups 295 corresponding to the reference numerals assigned thereto.

  Next, the light emitting elements 2951 in the light emitting element row 2951R at the width direction position LTD2 belonging to the same light emitting element group 295_C1, 295_C2, 295_C3. The plurality of light beams emitted by the light emission operation are reversed and imaged on the surface of the photosensitive member by the lens LS having the reversal characteristics described above. That is, a spot is formed at the position of the “second” hatching pattern in FIG. Here, while the conveyance direction of the surface of the photosensitive drum 21 is the sub scanning direction SD, the light emitting element rows 2951R on the downstream side in the width direction LTD corresponding to the sub scanning direction SD are sequentially (that is, the width direction). The reason why light is emitted in the order of the positions LTD1 and LTD2 is to correspond to the fact that the lens LS has a reversal characteristic.

  Next, the light emitting elements 2951 in the light emitting element row 2951R at the width direction position LTD3 belonging to the second light emitting element group 295_B1, 295_B2, 295_B3. The plurality of light beams emitted by the light emission operation are reversed and imaged on the surface of the photosensitive member by the lens LS having the reversal characteristics described above. That is, a spot is formed at the position of the “third” hatching pattern in FIG.

  Next, the light emitting elements 2951 in the light emitting element row 2951R in the width direction position LTD4 belonging to the light emitting element groups 295_B1, 295_B2, 295_B3. The plurality of light beams emitted by the light emission operation are reversed and imaged on the surface of the photosensitive member by the lens LS having the reversal characteristics described above. That is, a spot is formed at the position of the “fourth” hatching pattern in FIG.

  Next, the light emitting elements 2951 in the light emitting element row 2951R at the width direction position LTD5 belonging to the light emitting element groups 295_A1, 295_A2, 295_A3. The plurality of light beams emitted by the light emission operation are reversed and imaged on the surface of the photosensitive member by the lens LS having the reversal characteristics described above. That is, a spot is formed at the position of the “fifth” hatching pattern in FIG.

  Finally, the light emitting elements 2951 in the light emitting element row 2951R at the width direction position LTD6 belonging to the light emitting element groups 295_A1, 295_A2, 295_A3. The plurality of light beams emitted by the light emission operation are reversed and imaged on the surface of the photosensitive member by the lens LS having the reversal characteristics described above. That is, a spot is formed at the position of the “sixth” hatching pattern in FIG. In this way, by performing the first to sixth light emitting operations, a plurality of spots are formed side by side on a straight line extending in the main scanning direction MD.

  As described above, in the present embodiment, in the light emitting element group, a plurality of light emitting elements 2951 are arranged at different positions in the longitudinal direction LGD, and two light emitting elements that emit light to form adjacent spots are provided. They are arranged at different positions in the width direction LTD. Then, the light emitting elements 2951 are caused to emit light at timings corresponding to the movement of the photosensitive drum 21 in the sub scanning direction SD, and the light beams emitted from the light emitting elements 2951 are connected to the surface of the photosensitive member at different positions in the main scanning direction MD. The spot SP is formed side by side in the main scanning direction MD.

  Thus, the line head 29 in the present embodiment arranges the three lens rows LSR1 to LSR3 side by side in the width direction LTD. The line head 29 is arranged with respect to the photosensitive drum 21 such that the longitudinal direction LGD corresponds to the main scanning direction MD and the width direction LTD corresponds to the sub scanning direction SD. On the other hand, the surface of the photosensitive drum 21 has a curvature shape. Therefore, as will be described in detail later, the lens rows LSR1 to LSR3 face different positions in the sub-scanning direction SD on the surface of the photosensitive drum having a curvature shape. Therefore, if the arrangement relationship between the line head 29 and the photosensitive drum 21 is not appropriate, there is a problem that the exposure of the surface of the photosensitive drum by the line head 29 cannot be performed satisfactorily, that is, an exposure failure occurs. The contents of such exposure failure will be described next. Further, following the description of the exposure failure, details of this embodiment for suppressing the occurrence of the exposure failure will be described.

  FIGS. 15 and 16 are sub-scan sectional views showing the positional relationship between the line head and the photosensitive drum when the line head is not properly positioned. Note that the upper part of FIG. 15 is an enlarged view of the broken-line square part in the lower part of the figure. The cause of the exposure failure will be described with reference to these drawings.

  First, the configuration of the line head 29 will be described. In the line head 29, three lens rows LSR1 to LSR3 are arranged at different arrangement positions AP1 to AP3 in the width direction LTD. More specifically, the three lens rows LSR1 to LSR3 are arranged at the lens row pitch Plsr in the width direction LTD, and are arranged substantially symmetrically in the width direction LTD with respect to the symmetry axis SA. The symmetry axis SA is substantially orthogonal to the width direction LTD. In addition, the lens configuration and the lens position of each of the plurality of lenses LS included in the lens array 299 are the same. Therefore, the vertices VTs1 to VTs3 of the second surfaces LSFs1 to LSFs3 (that is, the lens surfaces LSFs1 to LSFs3 on the photosensitive drum side) of the lenses LS1 to LS3 are substantially in the same plane SPL_vts. Further, the lenses LS1 to LS3 are arranged so that their optical axes OA1 to OA3 are parallel to each other. In the figure, the optical axis OA of the lens LS2 coincides with the symmetry axis SA.

  The lens rows LSR1 to LSR3 are all disposed so as to face the surface of the photosensitive drum 21. At this time, each of the lens rows LSR1 to LSR3 faces opposite positions FCP1 to FCP3 that are different from each other in the sub-scanning direction SD on the surface of the photosensitive drum (latent image carrier surface). Accordingly, the lens LS1 belonging to the lens row LSR1 forms an image of the light beam LB1 emitted from the light emitting element group 295 opposed to the lens LS1 toward the facing position FCP1. As a result, the light beam LB1 is imaged at the imaging position FP1. The lens LS2 belonging to the lens row LSR2 forms an image of the light beam LB2 emitted from the light emitting element group 295 opposed to the lens LS2 toward the facing position FCP2. As a result, the light beam LB2 is imaged at the imaging position FP2. Further, the lens LS3 belonging to the lens row LSR3 forms an image of the light beam LB3 emitted from the light emitting element group 295 facing the lens LS3 toward the facing position FCP3. As a result, the light beam LB3 is imaged at the imaging position FP3.

  As described above, the imaging positions FP of the light beams formed by the lenses LS belonging to different lens rows LSR are different from each other in the sub-scanning direction SD. Here, the imaging position FP is a position where the light beam LB that has passed through the lens LS forms an image with the smallest spot diameter and its vicinity. As described above, the lens configurations and lens positions of the lenses LS1 to LS3 are equal to each other. Therefore, the imaging positions FP1 to FP3 are on the same plane SPL_fp.

  By the way, the surface of the photosensitive drum 21 facing the lens array 299 is convex with respect to the lens array 299 with a curvature in the sub-scan section. Therefore, there is a difference in the image plane facing distances ld1 to ld3 between the lens LS and the photosensitive drum surface among the three lens rows LSR1 to LSR3. 15 and 16, it can be seen that the difference Δld_max in the image plane facing distance ld is large because the positional relationship between the line head 29 and the photosensitive drum 21 is not appropriate. Here, the difference Δld_max is a difference between the maximum value and the minimum value of the image plane facing distance ld of each lens LS.

  More specifically, in FIGS. 15 and 16, the image surface facing distance ld1 among the image surface facing distances ld1 to ld3 of the lenses LS1 to LS3 is made smaller than the other image surface facing distances ld2 and ld3. The line head 29 is disposed with respect to the photosensitive drum 21. In other words, the line head 29 is arranged so that the image plane facing distance ld1 of the lens LS1 belonging to the end lens row LSR1 is minimized. Here, the end lens rows are the lens rows LSR1 and LSR3 at each end in the width direction LTD among the three lens rows LSR1 to LSR3.

  When the line head 29 is thus arranged with respect to the surface of the photosensitive drum, the difference Δld_max between the image plane facing distances ld1 to ld3 increases. In other words, when the line head 29 is arranged so that the image plane facing distance ld1 of the lens LS1 belonging to the end lens row LSR1 is minimized, the distance from the end lens row LSR1 to the width direction LTD (lens row LSR2,. As it goes to LSR3, the image plane facing distance ld monotonously increases (in the example shown in FIGS. 15 and 16, the image plane facing distance ld1 to ld3 increases monotonously in the order). That is, the change in the width direction LTD of the image plane facing distance ld is monotonously increased. Therefore, the difference between the image surface facing distance ld1 and the image surface facing distance ld3 corresponding to each of the end lens rows LSR1 and LSR3 is increased, and as a result, the image surface facing distance between the three lens rows LSR1 to LSR3. The difference Δld_max of ld becomes large.

  Such a difference in the image surface facing distance ld causes a difference in the distance between the imaging positions FP1 to FP3 of the light beam and the surface of the photosensitive drum (image photosensitive member distances fd1 to fd3). Here, the image photosensitive member distance fd is a distance between the imaging position FP and the surface of the photosensitive drum in the direction of the optical axis OA of the lens LS corresponding to the imaging position FP. That is, the image photosensitive member distances fd1 to fd3 are distances between the imaging positions FP1 to FP3 and the surface of the photosensitive drum in the direction of the optical axis OA of the lenses LS1 to LS3 corresponding to the imaging positions FP1 to FP3. That is, as shown in the upper part of FIG. 15, the image photosensitive member distances fd1 to fd3 monotonously increase toward the width direction LTD. As a result, the difference between the minimum value fd1 and the maximum value fd3 of the image photoreceptor distance is large. When the difference in the image photosensitive member distance fd becomes large, the image formed on the surface of the photosensitive drum is greatly different among the three lens rows LSR1 to LSR3, and a satisfactory exposure cannot be performed, that is, exposure failure. Could occur.

  Next, in order to facilitate understanding of the invention, the above-described exposure failure will be described using a more specific comparative example 1. That is, the specific contents of the exposure failure will be described through simulation results on the arrangement relationship between the photosensitive drum 21 and the line head 29 as shown in FIGS.

Comparative Example 1
In the simulation in the comparative example 1, the positional relationship between the photosensitive drum 21 and the line head is the positional relationship shown in FIGS. That is, the image plane facing distance ld1 (that is, the image plane facing distance ld1 of the lens LS1 belonging to the end lens row LSR1) among the image plane facing distances ld1 to ld3 of the lenses LS1 to LS3 is the other image plane facing distance. The line head 29 is arranged so as to be smaller than ld2 and ld3. In the simulation, the lens row pitch Plsr and the light emitting element group row pitch Pegr were both 1.65 mm. In addition, a simulation was performed for each case where the diameter of the photosensitive drum 21 was 25 mm, 40 mm, and 80 mm.

  Table 1 shows lens data of the lens LS used in the simulation of Comparative Example 1. The surface numbers S1 to S6 will be described with reference to FIGS. The surface number S1 corresponds to the object surface, that is, the back surface of the head substrate 293 on which the light emitting element 2951 is disposed. The surface number S2 corresponds to the surface of the head substrate 293. The surface number S3 corresponds to the surface (aperture surface) on which the aperture stop DIA is arranged. As described above, the aperture stop DIA is disposed at the front focal point of the lens LS, and image-side telecentricity is realized. The surface number S4 corresponds to the first surface LSFf of the lens LS. The surface number S5 corresponds to the second surface LSFs of the lens LS. The surface number S6 corresponds to the image surface IP, that is, the surface of the photosensitive drum (latent image carrier). Here, the sum of the surface intervals from surface numbers S1 to S3 gives the lens position. Further, the surface distance of the surface number S4 gives the lens thickness. In addition, although several lens data are suitably shown below, the surface corresponding to a surface number is the same also in any lens data.

  Table 2 shows the aspheric coefficients of the aspheric surfaces S4 and S5. Equation 1 is an expression that gives an aspherical shape. That is, the shapes of the aspheric surfaces S4 and S5 (in other words, the lens shape of the lens LS) are determined by Table 2 and Equation 1.

Table 3 shows the optical system specifications used in the simulation in Comparative Example 1. Here, the wavelength is the wavelength of the light beam emitted from the light emitting element. The lens diameter is the diameter of the exit surface of the lens LS, that is, the second surface LSFs. The light source diameter is the diameter of the light emitting element 2951. Further, the object height of 0.6 mm in the same specification means that the simulation was performed under the condition that the light beam was emitted from the virtual light emitting element having the object height of 0.6 mm. At this time, since the magnification is −0.5, the image height is −0.3 mm.

  Table 4 shows the simulation results executed under the above conditions. The value Δld shown in the table is a value obtained by subtracting the minimum image plane facing distance ld1 from each of the image plane facing distances ld1 to ld3 corresponding to the lenses LS1 to LS3. Therefore, the maximum value Δld is the difference Δld_max between the image plane facing distances ld1 to ld3 described above. In the following, simulation results will be described for each case where the diameter of the photosensitive drum 21 is 25 mm, 40 mm, and 80 mm.

  First, the case where the photosensitive drum diameter is 25 mm will be described. As shown in the column “photosensitive member diameter φ25 mm” in Table 4, when the diameter of the photosensitive drum 21 is 25 mm, the difference Δld_max between the image plane facing distances ld1 to ld3 is 0.443 mm. As described above, the reason why the large difference Δld_max in the image plane facing distance is generated is that the position of the line head 29 is not appropriate as described above. As indicated by the spot diameter in the same column, the spot diameter formed on the surface of the photosensitive drum 21 due to the difference between the image plane facing distances ld1 to ld3 is between the lenses LS1 to LS3. Different. Specifically, the spot diameter of the spot formed by the lens LS1 has a minimum value of 29.1 μm, whereas the spot diameter of the spot formed by the lens LS3 has a maximum value of 224.7 μm. That is, in the comparative example, the maximum spot diameter difference is 195.6 μm (= 224.7 μm−29.1 μm). As described above, in Comparative Example 1, when the diameter of the photosensitive drum 21 is 25 mm, an exposure failure occurs in which the diameter of the spot to be formed differs by a maximum of 195.6 μm between the lens rows LSR1 to LSR3. .

  Next, a case where the photosensitive drum diameter is 40 mm will be described. As shown in the column “photosensitive member diameter φ40 mm” in Table 4, when the diameter of the photosensitive drum 21 is 40 mm, the difference Δld_max between the image plane facing distances ld1 to ld3 is 0.274 mm. As described above, the reason why the large difference Δld_max in the image plane facing distance is generated is that the position of the line head 29 is not appropriate as described above. As indicated by the spot diameter in the same column, the spot diameter formed on the surface of the photosensitive drum 21 due to the difference between the image plane facing distances ld1 to ld3 is between the lenses LS1 to LS3. Different. Specifically, the spot diameter of the spot formed by the lens LS1 has a minimum value of 29.1 μm, whereas the spot diameter of the spot formed by the lens LS3 has a maximum value of 133.0 μm. That is, in the comparative example, the maximum spot diameter difference is 103.9 μm (= 133.0 μm−29.1 μm). As described above, in Comparative Example 1, when the diameter of the photosensitive drum 21 is 40 mm, an exposure failure occurs in which the diameters of spots to be formed differ by a maximum of 103.9 μm between the lens rows LSR1 to LSR3. .

  Finally, a case where the photosensitive drum diameter is 80 mm will be described. As shown in the column of “photosensitive member diameter φ80 mm” in Table 4, when the diameter of the photosensitive drum 21 is 80 mm, the difference Δld_max between the image plane facing distances ld1 to ld3 is 0.136 mm. As described above, the reason why the large difference Δld_max in the image plane facing distance is generated is that the position of the line head 29 is not appropriate as described above. As indicated by the spot diameter in the same column, the spot diameter formed on the surface of the photosensitive drum 21 due to the difference between the image plane facing distances ld1 to ld3 is between the lenses LS1 to LS3. Different. Specifically, the spot diameter of the spot formed by the lens LS1 has a minimum value of 29.1 μm, whereas the spot diameter of the spot formed by the lens LS3 has a maximum value of 69.7 μm. That is, in the comparative example, the maximum spot diameter difference is 40.6 μm (= 69.7 μm−29.1 μm). As described above, in Comparative Example 1, when the diameter of the photosensitive drum 21 is 80 mm, the exposure defect that the diameter of the spot to be formed differs by a maximum of 40.6 μm between the lens rows LSR1 to LSR3 occurs. .

  As described with reference to the first comparative example, a difference occurs between the image plane facing distances ld1 to ld3 due to an inappropriate arrangement position of the line head 29. As a result, the surface of the photosensitive drum 21 is changed. An exposure failure occurs when a difference occurs in the spot diameters of the formed spots. Therefore, an embodiment of the present invention that suppresses the occurrence of such exposure failure will be described in detail through Example 1 below.

Example 1
FIGS. 17 and 18 are sub-scan sectional views showing the positional relationship between the line head and the photosensitive drum in the first embodiment. Note that the upper part of FIG. 17 is an enlarged view of the broken-line square part in the lower part of the figure. That is, both FIG. 17 and FIG. 18 represent the case where the arrangement relationship between the line head and the photosensitive drum is viewed from the longitudinal direction LGD.

  In the line head 29, three lens rows LSR1 to LSR3 are arranged at different arrangement positions AP1 to AP3 in the width direction LTD. More specifically, the three lens rows LSR1 to LSR3 are arranged at the lens row pitch Plsr in the width direction LTD, and are arranged substantially symmetrically in the width direction LTD with respect to the symmetry axis SA. The symmetry axis SA is substantially orthogonal to the width direction LTD. In addition, the lens configuration and the lens position of each of the plurality of lenses LS included in the lens array 299 are the same. Therefore, the vertices VTs1 to VTs3 of the second surfaces LSFs1 to LSFs3 (that is, the lens surfaces LSFs1 to LSFs3 on the photosensitive drum side) of the lenses LS1 to LS3 are substantially in the same plane SPL_vts. Further, the lenses LS1 to LS3 are arranged so that their optical axes OA1 to OA3 are parallel to each other. In the first embodiment, the optical axis OA of the lens LS2 coincides with the symmetry axis SA. The lens array 299 is arranged so that the symmetry axis SA passes through the center of curvature CC21 of the surface shape of the photosensitive drum 21. Therefore, the symmetry axis SA passes through the rotation axis of the photosensitive drum 21.

  The lens rows LSR1 to LSR3 are all disposed so as to face the surface of the photosensitive drum 21. At this time, each of the lens rows LSR1 to LSR3 faces opposite positions FCP1 to FCP3 that are different from each other in the sub-scanning direction SD on the surface of the photosensitive drum (latent image carrier surface). Accordingly, the lens LS1 belonging to the lens row LSR1 forms an image of the light beam LB1 emitted from the light emitting element group 295 opposed to the lens LS1 toward the facing position FCP1. As a result, the light beam LB1 is imaged at the imaging position FP1. The lens LS2 belonging to the lens row LSR2 forms an image of the light beam LB2 emitted from the light emitting element group 295 opposed to the lens LS2 toward the facing position FCP2. As a result, the light beam LB2 is imaged at the imaging position FP2. Further, the lens LS3 belonging to the lens row LSR3 forms an image of the light beam LB3 emitted from the light emitting element group 295 facing the lens LS3 toward the facing position FCP3. As a result, the light beam LB3 is imaged at the imaging position FP3.

  As described above, the imaging positions FP of the light beams formed by the lenses LS belonging to different lens rows LSR are different from each other in the sub-scanning direction SD. As described above, the lens configurations and lens positions of the lenses LS1 to LS3 are equal to each other. Therefore, the imaging positions FP1 to FP3 are on the same plane SPL_fp. The surface of the photosensitive drum 21 facing the lens array 299 is convex with respect to the lens array 299 with a curvature in the sub-scan section. Accordingly, if the arrangement relationship between the line head 29 and the surface of the photosensitive drum is not appropriate, the difference between the image surface facing distances ld1 to ld3 becomes large, and the above-described exposure failure may occur.

In order to deal with this problem, in Example 1 of the present embodiment, the line head 29 is arranged as follows. That is, in the line head 29, the end surface lens row LS2 has an image plane facing distance ld2 of the lens LS2 belonging to the lens rows LSR2 other than the end lens rows LSR1 and LSR3 at each end in the width direction LTD among the three lens rows. The lenses LS1 and LS3 belonging to LSR1 and LSR3 are arranged with respect to the surface of the photosensitive drum so as to be smaller than the image plane facing distances ld1 and ld3. In other words, the following magnitude relationship ld2 <ld1
ld2 <ld3
The line head 29 is arranged so that Therefore, it is possible to suppress the difference Δld_max in the image plane facing distance ld between the three lens rows LSR1 to LSR3, and as a result, it is possible to suppress the occurrence of the above-described exposure failure and realize good exposure.

  That is, when the line head 29 is arranged in this way, the occurrence of a situation in which the image plane facing distance ld monotonously increases from the end lens row LSR1 toward the width direction LTD as shown in Comparative Example 1 is suppressed. Has been. More specifically, in FIGS. 17 and 18, the image plane facing distance ld of the lens LS1 belonging to the end lens row LSR1 is the distance ld1. Then, the image surface facing distance ld decreases from the end lens row LSR1 in the width direction LTD, and the image surface facing distance ld of the lens LS2 belonging to the lens row LSR2 is the distance ld2. Further, when going in the width direction LTD, the image surface facing distance ld increases, and the image surface facing distance ld of the lens LS3 belonging to the end lens row LSR3 becomes the distance ld3. At this time, the difference Δld_max in the image plane facing distance ld is determined by the image surface facing distance ld1 (or image surface facing distance ld3) corresponding to the end lens row LSR1 (or end lens row LSR3) and the lens row LSR2. This is the difference from the corresponding image plane facing distance ld2. As shown in FIGS. 17 and 18, the difference Δld_max in the image plane facing distance ld is smaller in the first embodiment than in the first comparative example.

  As described above, when the line head 29 is arranged as shown in FIGS. 17 and 18, the change in the width direction LTD of the image plane facing distance ld turns to an increasing tendency after showing a decreasing tendency. Therefore, it is possible to suppress the difference Δld_max in the image plane facing distance as compared with the case where the change in the width direction LTD of the image plane facing distance ld is monotonously increased. As a result, it is possible to achieve good exposure while suppressing the occurrence of the exposure failure.

  In particular, in the present embodiment, the lens rows LSR1 to LSR3 are arranged at the lens row pitch Plsr in the width direction LTD, and are substantially symmetrical in the width direction LTD with respect to the symmetry axis SA substantially perpendicular to the width direction LTD in the sub-scanning section. Is arranged. The image plane facing distance ld2 of the lens LS2 belonging to the lens row LSR2 (center lens row) closest to the symmetry axis SA is the image plane of the lenses LS1 and LS3 belonging to the lens rows LSR1 and LSR3 other than the center lens row LSR2. The line head 29 is arranged so as to be smaller than the facing distances ld1 and ld3. Therefore, the difference Δld_max in the image plane facing distance can be more effectively suppressed.

  That is, the difference Δld_max is a difference between the maximum value and the minimum value of the image plane facing distance ld. Here, the maximum value of the image plane facing distance ld is the image plane facing distance ld of the lens LS belonging to one of the end lens rows LSR1 and LSR3 at both ends in the width direction LTD. That is, the maximum value of the image surface facing distance ld is the larger of the image surface facing distance ld1 and the image surface facing distance ld3. Therefore, from the viewpoint of suppressing the difference Δld_max, it is preferable that both the image plane facing distances ld1 and ld3 of the lenses LS belonging to the end lens rows LSR1 and LSR3 are made as small as possible. On the other hand, the image plane facing distances ld1 and ld3 corresponding to the end lens rows LSR1 and LSR3 have a relationship in which, when one is reduced, the other is increased. That is, for example, even if the position of the line head 29 is adjusted in the width direction LTD (that is, moved in the width direction LTD) to reduce both the image plane facing distances ld1 and ld3, if one is reduced, the other is increased. There is a relationship. Under such a relationship, in order to suppress the difference Δld_max, it is preferable that the image plane facing distances ld1 and ld3 are approximately the same.

  Therefore, in the image forming apparatus according to the present embodiment, the lens rows LSR1 to LSR3 are arranged substantially symmetrically with respect to the symmetry axis SA, and the image plane facing distance ld2 of the lens LS2 belonging to the central lens row LSR2 closest to the symmetry axis SA. However, the line head 29 is arranged to be smaller than the image plane facing distances ld1 and ld3 of the lenses LS1 and LS3 belonging to the lens rows LSR1 and LSR3 other than the central lens row LSR2. This is because with this configuration, the image plane facing distances ld1 and ld3 corresponding to the end lens rows LSR1 and LSR3 can be made comparable. As a result, in the image forming apparatus according to the present embodiment, the difference Δld_max in the image plane facing distance is suppressed, and good exposure is realized.

  Further, when the line head 29 is disposed on a latent image carrier having a surface center of curvature CC21 such as the photosensitive drum 21, it is preferable to dispose the line head as follows. That is, in Example 1, the line head 29 is arranged so that the symmetry axis SA passes through the center of curvature CC21 of the surface shape of the photosensitive drum 21 (that is, the rotational axis of the photosensitive drum 21). Accordingly, the image plane facing distances ld1 and ld3 corresponding to the end lenses LSR1 and LSR3 are equal to each other. As a result, it is possible to more effectively suppress the difference Δld_max in the image plane facing distance, and it is possible to achieve better exposure, which is preferable. Therefore, for easy understanding of the invention, the result of the simulation executed under the condition that the line head 29 is arranged as shown in FIGS. 17 and 18 will be described below.

  In the simulation in the first embodiment, the lens rows LSR1 to LSR3 are arranged in the width direction LTD at a predetermined lens row pitch Plsr. The line head 29 is arranged such that the image plane facing distance ld2 corresponding to the central lens row LSR2 is smaller than the image plane facing distances ld1 and ld3 corresponding to the other lens rows LSR1 and LSR3. The symmetry axis SA of the lens rows LSR1 to LSR3 passes through the center of curvature CC21 of the surface shape of the photosensitive drum. In the simulation, the lens row pitch Plsr and the light emitting element group row pitch Pegr were 1.65 mm. In addition, a simulation was performed for each case where the diameter of the photosensitive drum 21 was 25 mm, 40 mm, and 80 mm. Note that Example 1 and Comparative Example 1 are the same in terms of the lens configuration and lens position of the lens LS. That is, Example 1 and Comparative Example 1 are the same in terms of lens data, aspherical coefficients, and optical system specifications.

  Table 5 shows the simulation results in Example 1 executed under the above conditions. The simulation results will be described below for the case where the diameter of the photosensitive drum 21 is 25 mm, 40 mm, and 80 mm.

  First, the case where the photosensitive drum diameter is 25 mm will be described. As shown in the column “photosensitive member diameter φ25 mm” in Table 5, when the diameter of the photosensitive drum 21 is 25 mm, the difference Δld_max between the image plane facing distances ld1 to ld3 is 0.109 mm. It can be seen that this is an improvement over 0.443 mm. As indicated by the spot diameter in the same column, the spot diameter formed by the lenses LS1 to LS3 has a minimum value of 29.1 μm (spot diameter of the spot by the lens LS2), while the maximum value is 59. 1 μm (the spot diameter of the spots formed by the lenses LS1 and LS3). That is, in Example 1, the maximum spot diameter difference is 29.9 μm (= 59.0 μm−29.1 μm), which is a significant improvement compared to the spot diameter difference 165.7 μm in Comparative Example 1. I understand that.

  Next, a case where the photosensitive drum diameter is 40 mm will be described. As shown in the column of “photosensitive member diameter φ40 mm” in Table 5, when the diameter of the photosensitive drum 21 is 40 mm, the difference Δld_max between the image plane facing distances ld1 to ld3 is 0.068 mm. It can be seen that this is an improvement over 0.274 mm. Further, as shown by the spot diameter in the column, the spot diameter formed by the lenses LS1 to LS3 has a minimum value of 29.1 μm (spot diameter of the spot by the lens LS2), while the maximum value is 40. 2 μm (the spot diameter of the spots formed by the lenses LS1 and LS3). That is, in Example 1, the maximum spot diameter difference is 11.1 μm (= 40.2 μm−29.1 μm), which is greatly improved compared to the spot diameter difference 103.9 μm in Comparative Example 1. I understand that.

  Next, the case where the photosensitive drum diameter is 80 mm will be described. As shown in the column “photosensitive member diameter φ80 mm” in Table 5, when the diameter of the photosensitive drum 21 is 80 mm, the difference Δld_max between the image plane facing distances ld1 to ld3 is 0.034 mm. It can be seen that this is an improvement over 0.136 mm. As indicated by the spot diameter in the same column, the spot diameter formed by the lenses LS1 to LS3 has a minimum value of 29.1 μm (spot diameter of the spot by the lens LS2), while the maximum value is 32. 3 μm (the spot diameter of the spots formed by the lenses LS1 and LS3). That is, in Example 1, the maximum spot diameter difference is 3.2 μm (= 32.3 μm−29.1 μm), which is a significant improvement compared to the spot diameter difference of 40.6 μm in Comparative Example 1. I understand that.

  As described above, in the first embodiment, the difference Δld_max between the image plane facing distances ld1 to ld3 is suppressed by arranging the line head 29 as shown in FIGS. In addition, by suppressing the difference Δld_max in the image plane facing distance in this way, the difference in the spot diameters of the spots formed on the surface of the photosensitive drum 21 is suppressed as compared with the first comparative example. That is, in Example 1, better exposure is realized as compared with Comparative Example 1.

  By the way, in the first embodiment, the line head 29 is arranged with respect to the surface of the photosensitive drum as shown in FIGS. 17 and 18 to suppress the difference in the image plane facing distance ld between the lens rows LSR1 to LSR3. is doing. Therefore, in each of the plurality of lenses LS, the lens shape of the lens 1S is adjusted according to the image plane facing distance 1d of the lens LS, and the light emitted from the light emitting element group 295 corresponding to the lens LS. The beam imaging position FP may be a position corresponding to the curvature shape of the surface of the photosensitive drum.

  That is, by disposing the line head 29 as shown in FIGS. 17 and 18, the difference in the image plane facing distance ld between the lens rows LSR1 to LSR3 is suppressed. Therefore, as shown in the following second embodiment, the image formation position FP of the light beam can be set to a position corresponding to the curvature shape of the surface of the photosensitive drum without changing the lens shape greatly between the lens rows LSR1 to LSR3. Is possible. Therefore, very good exposure can be realized while suppressing the complexity of lens design or manufacture.

Example 2
FIG. 19 is a sub-scan sectional view showing a lens array in Embodiment 2 of the present invention. The upper part of the figure is an enlarged view of the broken-line square part in the lower part of the figure. The arrangement relationship between the lens array 299 and the photosensitive drum 21 is the same as the arrangement relationship described with reference to FIGS. 17 and 18 (that is, the arrangement relationship in the first embodiment). Therefore, in the second embodiment, the difference Δld_max between the image plane facing distances ld1 to ld3 of the lenses LS1 to LS3 is suppressed. In Example 2, the diameter of the photosensitive drum 21 was 80 μm. As described below, in the second embodiment, the image plane facing distance ld is suppressed as described above, and the lens configuration of the lens LS is further adjusted to form the light beam imaging positions FP1 to FP1 to LS3. FP3 is set to a position corresponding to the curvature shape of the photosensitive drum 21. Specifically, the lens shapes of the lenses LS1 to LS3 are adjusted so that the imaging positions FP1 to FP3 correspond to the curvature shape.

  Table 6 shows lens data of the lens LS2, and Table 7 shows aspherical coefficients of the lens LS2. On the other hand, Table 8 shows lens data of the lenses LS1 and LS3, and Table 9 shows aspheric coefficients of the lenses LS1 and LS3. As can be seen from these tables, in Example 2, the aspherical coefficient is changed between the lens LS2 and the lenses LS1 and LS3 (that is, the lens shape is changed). In all of the lenses LS1 to LS3, the lens thickness and the lens position are the same. Table 10 shows the specifications of the optical system used in the simulation in Example 2. Thus, in Example 2 of the present invention, the lens LS1 and the lens LS3 are the same. On the other hand, the lenses LS1, LS3 and the lens LS2 are different.

  Table 11 shows simulation results when the lenses LS1 to LS3 are configured based on the data given in Tables 6 to 10 described above. In the simulation, other conditions such as the lens row pitch Plsr and the light emitting element group row pitch Pegr are the same as those in the first embodiment. The difference Δfd shown in Table 11 is the difference between the image photoreceptor distances fd1 to fd3 corresponding to the lenses LS1 to LS3 and the image photoreceptor distance fd2. That is, the difference Δfd between the image photosensitive member distances fd1 to fd3 is obtained on the basis of the image photosensitive member distance fd2. As shown in the table, the difference Δfd is 0 in the first embodiment. That is, the image photosensitive member distances fd1 to fd3 are equal to each other. As shown in FIG. 19, the image forming positions FP1 to FP3 of the light beams by the lenses LS1 to LS3 are positions corresponding to the curvature shape of the photosensitive drum 21 (in the second embodiment, as shown in FIG. This is because it is adjusted to the substantially surface) of the photosensitive drum 21. As shown in the “Spot Diameter” column of Table 11, the difference in the spot diameters of the spots formed on the surface of the photosensitive drum 21 is suppressed by suppressing the difference in the image photosensitive member distances fd1 to fd3. You can see that Specifically, the minimum value is 29.1 μm (spot spot diameter by the lens LS2), while the maximum value is 29.5 μm (spot spot diameter by the lenses LS1 and LS3). Therefore, the difference in spot diameter in Example 2 is 0.4 μm (= 29.5 μm−29.1 μm), which is an improvement compared to the difference in spot diameter in Example 1 of 3.2 μm. . That is, in the second embodiment, as compared with the first embodiment, the difference in the diameters of the spots formed between the plurality of lens rows LSR1 to LSR3 is further suppressed, and better exposure is realized.

  As described above, in the second embodiment, the image surface facing distance ld is suppressed, and the lens configuration of the lens LS is further adjusted, so that the imaging positions FP1 to FP3 of the light beams by the lenses LS1 to LS3 are changed to the photosensitive drum. The position is in accordance with the 21 curvature shape. Accordingly, it is possible to set the image forming position FP of the light beam at a position corresponding to the curvature shape of the surface of the photosensitive drum without greatly changing the lens shape between the lens rows LSR1 to LSR3. Therefore, it is possible to realize very good exposure while suppressing the complicated design or manufacture of the lens, which is preferable.

  By the way, in the above-described comparative example 1 and examples 1 and 2, the present invention has been described using the line head 29 in which three lens rows LSR are arranged in the width direction LTD. However, the number of lens rows is not limited to this, and may be four or more. Therefore, a case where the line head 29 having four lens rows is used will be described. In the following description, first, it will be described with reference to simulation results that the above-described exposure failure occurs when there are four lens rows (Comparative Example 2). Following the description of Comparative Example 2, specific examples of the present invention will be described with showing simulation results (Examples 3 and 4).

Comparative Example 2
20 and 21 are sub-scanning cross-sectional views corresponding to the case where the arrangement of the line head is not appropriate. The upper part of FIG. 20 is an enlarged view of the broken-line square part in the lower part of the figure. The cause of the exposure failure will be described with reference to these drawings.

  The line head 29 arranges four lens rows LSR1 to LSR4 at different arrangement positions AP1 to AP4 in the width direction LTD. More specifically, the four lens rows LSR1 to LSR4 are arranged at the lens row pitch Plsr in the width direction LTD, and are arranged substantially symmetrically in the width direction LTD with respect to the symmetry axis SA. The symmetry axis SA is substantially orthogonal to the width direction LTD. The vertices VTs1 to VTs4 of the second surfaces LSFs1 to LSFs4 of the lenses LS1 to LS4 (that is, the lens surfaces LSFs1 to LSFs4 on the photosensitive drum side) are substantially on the same plane SPL_vts. In addition, the lens configuration and the lens position of each of the plurality of lenses LS included in the lens array 299 are the same. Further, the lens rows LSR1 to LSR4 are arranged such that the optical axes OA1 to OA4 of the lenses LS1 to LS4 belonging to each of them are parallel to each other.

  The line head 29 is arranged with respect to the photosensitive drum 21 as follows. That is, the line head 29 is arranged so that the image surface facing distance ld1 of the image surface facing distances ld1 to ld4 of the lenses LS1 to LS4 is smaller than the other image surface facing distances ld2 to ld4. . In other words, the line head 29 is arranged so that the image plane facing distance ld1 of the lens LS1 belonging to the end lens row LSR1 is minimized. Here, the end lens rows are the lens rows LSR1 and LSR4 at the respective ends in the width direction LTD among the four lens rows LSR1 to LSR4. The symmetry axis SA does not pass through the center of curvature CC21 of the surface shape of the photosensitive drum.

  In this way, when the line head 29 is arranged so that the image plane facing distance ld1 of the lens LS1 belonging to the end lens row LSR1 is minimized, the lens head is moved from the end lens row LSR1 to the width direction LTD (lens row). As the distance from LSR2 to LSR4 increases, the image plane facing distance ld monotonously increases. Therefore, the difference between the image surface facing distance ld1 and the image surface facing distance ld4 corresponding to each of the end lens rows LSR1 and LSR4 increases. Therefore, the difference Δld_max in the image plane facing distance ld between the four lens rows LSR1 to LSR4 becomes large. Such a difference in the image surface facing distance ld causes a difference in the image photosensitive member distances fd1 to fd4. That is, as shown in the upper part of FIG. 20, the difference between the minimum value fd1 and the maximum value fd4 of the image photoreceptor distance is large. When the difference in the image photosensitive member distance fd becomes large, the image formed on the surface of the photosensitive drum is greatly different among the four lens rows LSR1 to LSR4, and a satisfactory exposure cannot be performed. Could occur. Therefore, in order to facilitate understanding of the invention, the above-described exposure failure will be described using more specific simulation results. That is, the specific contents of the exposure failure will be described through simulation results on the arrangement relationship between the photosensitive drum 21 and the line head 29 as shown in FIGS.

  In the simulation in Comparative Example 2, the positional relationship between the photosensitive drum 21 and the line head is the positional relationship shown in FIGS. That is, the image plane facing distance ld1 (that is, the image plane facing distance ld1 of the lens LS1 belonging to the end lens row LSR1) among the image plane facing distances ld1 to ld4 of the lenses LS1 to LS4 is the other image plane facing distance. The line head 29 is arranged so as to be smaller than ld2 to ld4. In the simulation, the lens row pitch Plsr and the light emitting element group row pitch Pegr were 1.65 mm. In addition, a simulation was performed for each case where the diameter of the photosensitive drum 21 was 25 mm, 40 mm, and 80 mm. The lens data, aspheric coefficient, and optical system specifications in the simulation of Comparative Example 2 are the same as those of Comparative Example 1. That is, the simulation in Comparative Example 2 was executed based on the data in Tables 1 to 3 and Equation 1.

  Table 12 shows the simulation results executed under the above conditions. The value Δld shown in the table is a value obtained by subtracting the minimum image plane facing distance ld1 from the image plane facing distances ld1 to ld4 corresponding to the lenses LS1 to LS4. Therefore, the maximum value Δld is the difference Δld_max between the image plane facing distances ld1 to ld4 described above. The simulation results will be described below for the case where the diameter of the photosensitive drum 21 is 25 mm, 40 mm, and 80 mm.

  First, the case where the photosensitive drum diameter is 25 mm will be described. As shown in the column “photosensitive member diameter φ25 mm” in Table 12, when the diameter of the photosensitive drum 21 is 25 mm, the difference Δld_max between the image plane facing distances ld1 to ld4 is 1.022 mm. As described above, the reason why the large difference Δld_max in the image plane facing distance is generated is that the position of the line head 29 is not appropriate as described above. As indicated by the spot diameter in the same column, the spot diameter formed on the surface of the photosensitive drum 21 due to the difference between the image plane facing distances ld1 to ld4 is between the lenses LS1 to LS4. Different. Specifically, the spot diameter of the spot formed by the lens LS1 has a minimum value of 29.1 μm, whereas the spot diameter of the spot formed by the lens LS4 has a maximum value of 567.3 μm. That is, in the comparative example, the difference in spot diameter is 538.2 μm (= 567.3 μm−29.1 μm) at the maximum. As described above, in Comparative Example 2, when the diameter of the photosensitive drum 21 is 25 mm, an exposure failure occurs in which the diameters of spots to be formed differ by a maximum of 538.2 μm between the lens rows LSR1 to LSR4.

  Next, a case where the photosensitive drum diameter is 40 mm will be described. As shown in the column of “photosensitive member diameter φ40 mm” in Table 12, when the diameter of the photosensitive drum 21 is 40 mm, the difference Δld_max between the image plane facing distances ld1 to ld4 is 0.622 mm. As described above, the reason why the large difference Δld_max in the image plane facing distance is generated is that the position of the line head 29 is not appropriate as described above. As indicated by the spot diameter in the same column, the spot diameter formed on the surface of the photosensitive drum 21 due to the difference between the image plane facing distances ld1 to ld4 is between the lenses LS1 to LS4. Different. Specifically, the spot diameter of the spot formed by the lens LS1 has a minimum value of 29.1 μm, whereas the spot diameter of the spot formed by the lens LS4 has a maximum value of 328.3 μm. That is, in the comparative example, the difference in spot diameter is a maximum of 299.2 μm (= 328.3 μm−29.1 μm). As described above, in Comparative Example 2, when the diameter of the photosensitive drum 21 is 40 mm, an exposure failure occurs in which the diameter of the spot to be formed differs by a maximum of 299.2 μm between the lens rows LSR1 to LSR4.

  Finally, a case where the photosensitive drum diameter is 80 mm will be described. As shown in the column of “photosensitive member diameter φ80 mm” in Table 12, when the diameter of the photosensitive drum 21 is 80 mm, the difference Δld_max between the image plane facing distances ld1 to ld4 is 0.307 mm. As described above, the reason why the large difference Δld_max in the image plane facing distance is generated is that the position of the line head 29 is not appropriate as described above. As indicated by the spot diameter in the same column, the spot diameter formed on the surface of the photosensitive drum 21 due to the difference between the image plane facing distances ld1 to ld4 is between the lenses LS1 to LS4. Different. Specifically, the spot diameter of the spot formed by the lens LS1 has a minimum value of 29.1 μm, whereas the spot diameter of the spot formed by the lens LS4 has a maximum value of 151.4 μm. That is, in the comparative example, the maximum spot diameter difference is 122.3 μm (= 151.4 μm−29.1 μm). As described above, in Comparative Example 2, when the diameter of the photosensitive drum 21 is 80 mm, an exposure failure occurs in which the diameters of spots to be formed differ by a maximum of 122.3 μm between the lens rows LSR1 to LSR4.

  As described with reference to Comparative Example 2, a difference occurs between the image plane facing distances ld1 to ld4 due to an inappropriate arrangement position of the line head 29. As a result, the surface of the photosensitive drum 21 is changed. An exposure failure occurs when a difference occurs in the spot diameters of the formed spots. However, the occurrence of such exposure failure can be suppressed by making the arrangement relationship between the line head 29 and the photosensitive drum 21 appropriate. Therefore, in the third embodiment, it will be described that the occurrence of exposure failure is suppressed while showing a simulation result when the arrangement relationship between the photosensitive drum 21 and the line head 29 is appropriate.

Example 3
22 and 23 are sub-scan sectional views showing the positional relationship between the line head and the photosensitive drum. In the upper part of the figure, the broken-line square part in the lower part of the figure is enlarged and displayed. That is, both FIG. 22 and FIG. 23 show a case where the arrangement relationship between the line head and the photosensitive drum is viewed from the longitudinal direction LGD.

  The line head 29 arranges four lens rows LSR1 to LSR4 at different arrangement positions AP1 to AP4 in the width direction LTD. More specifically, the four lens rows LSR1 to LSR4 are arranged at the lens row pitch Plsr in the width direction LTD, and are arranged substantially symmetrically in the width direction LTD with respect to the symmetry axis SA. The symmetry axis SA is substantially orthogonal to the width direction LTD. The vertices VTs1 to VTs4 of the second surfaces LSFs1 to LSFs4 of the lenses LS1 to LS4 (that is, the lens surfaces LSFs1 to LSFs4 on the photosensitive drum side) are substantially on the same plane SPL_vts. In addition, the lens configuration and the lens position of each of the plurality of lenses LS included in the lens array 299 are the same. Further, the lens rows LSR1 to LSR4 are arranged such that the optical axes OA1 to OA4 of the lenses LS1 to LS4 belonging to each of them are parallel to each other. The lens array 299 is arranged so that the symmetry axis SA passes through the center of curvature CC21 of the surface shape of the photosensitive drum 21. Therefore, the symmetry axis SA passes through the rotation axis of the photosensitive drum 21.

  The lens rows LSR1 to LSR4 are all arranged to face the surface of the photosensitive drum 21. At this time, each of the lens rows LSR1 to LSR4 faces different facing positions FCP1 to FCP4 in the sub scanning direction SD on the surface of the photosensitive drum (latent image carrier surface). Accordingly, the lens LS1 belonging to the lens row LSR1 forms an image of the light beam LB1 emitted from the light emitting element group 295 opposed to the lens LS1 toward the facing position FCP1. As a result, the light beam LB1 is imaged at the imaging position FP1. The lens LS2 belonging to the lens row LSR2 forms an image of the light beam LB2 emitted from the light emitting element group 295 opposed to the lens LS2 toward the facing position FCP2. As a result, the light beam LB2 is imaged at the imaging position FP2. Further, the lens LS3 belonging to the lens row LSR3 forms an image of the light beam LB3 emitted from the light emitting element group 295 facing the lens LS3 toward the facing position FCP3. The lens LS4 belonging to the lens row LSR4 forms an image of the light beam LB4 emitted from the light emitting element group 295 opposed to the lens LS4 toward the facing position FCP4. As a result, the light beam LB4 is imaged at the imaging position FP4.

  As described above, the imaging positions FP of the light beams formed by the lenses LS belonging to different lens rows LSR are different from each other in the sub-scanning direction SD. Here, the imaging position FP is a position where the light beam LB that has passed through the lens LS forms an image with the smallest spot diameter and its vicinity. As described above, the lens configurations and lens positions of the lenses LS1 to LS4 are equal to each other. Therefore, the imaging positions FP1 to FP4 are on the same plane SPL_fp.

  By the way, the surface of the photosensitive drum 21 facing the lens array 299 is convex with respect to the lens array 299 with a curvature in the sub-scan section. Therefore, a difference occurs in the image surface facing distances ld1 to ld4 between the lens LS and the photosensitive drum surface among the four lens rows LSR1 to LSR4. If the arrangement relationship between the line head 29 and the surface of the photosensitive drum is not appropriate, the difference between the image surface facing distances ld1 to ld4 becomes large, and the exposure failure as shown in Comparative Example 2 may occur. It was.

In order to cope with such a problem, in the present embodiment, the line head 29 is arranged as follows. That is, the line head 29 includes the image plane facing distances ld2 and ld3 of the lenses LS2 and LS3 belonging to the lens rows LSR2 and LSR3 other than the end lens rows LSR1 and LSR4 at the respective ends in the width direction LTD among the four lens rows. Is arranged with respect to the surface of the photosensitive drum so as to be smaller than the image plane facing distances ld1 and ld4 of the lenses LS1 and LS4 belonging to the end lens rows LSR1 and LSR4.
In other words, the following magnitude relationship ld2 <ld1
ld2 <ld4
ld3 <ld1
ld3 <ld4
The line head 29 is arranged so that Accordingly, it is possible to suppress the difference Δld_max in the image plane facing distance ld between the four lens rows LSR1 to LSR4, and as a result, it is possible to suppress the occurrence of the above-described exposure failure and realize good exposure.

  That is, when the line head 29 is arranged as shown in FIGS. 22 and 23, the image plane facing distance ld is monotonous from the end lens row LSR1 as shown in FIGS. 20 and 21 toward the width direction LTD. Occurrence of an increasing situation is suppressed. More specifically, in FIGS. 22 and 23, the image plane facing distance ld of the lens LS1 belonging to the end lens row LSR1 is the distance ld1. The image plane facing distance ld decreases from the end lens row LSR1 in the width direction LTD, and the image plane facing distances ld of the lenses LS2 and LS3 belonging to the lens rows LSR2 and LSR3 are distances ld2 and ld3. Furthermore, the image plane facing distance ld increases toward the width direction LTD, and the image plane facing distance ld of the lens LS4 belonging to the end lens row LSR4 becomes the distance ld4. At this time, the difference Δld_max in the image plane facing distance ld is determined by the image surface facing distance ld1 (or image facing distance ld4) corresponding to the end lens row LSR1 (or the end portion LSR4) and the lens row LSR2 (or lens). This is the difference from the image surface facing distance ld2 (or image surface facing distance ld3) corresponding to the row LSR3). As a result, the difference Δld_max in the image plane facing distance ld in FIGS. 22 and 23 is smaller than the difference Δld_max in the image plane facing distance ld in FIGS.

  As described above, when the line head 29 is disposed as shown in FIGS. 22 and 23, the change in the width direction LTD of the image plane facing distance ld starts to decrease and then increases. Therefore, it is possible to suppress the difference Δld_max in the image plane facing distance as compared with the case where the change in the width direction LTD of the image plane facing distance ld is monotonously increased. As a result, it is possible to achieve good exposure while suppressing the occurrence of the exposure failure.

  In particular, in the present embodiment, the lens rows LSR1 to LSR4 are arranged at the lens row pitch Plsr in the width direction LTD, and are substantially symmetrical in the width direction LTD with respect to the symmetry axis SA substantially perpendicular to the width direction LTD in the sub-scanning section. Is arranged. Then, the image plane facing distances ld2 and ld3 of the lenses LS2 and LS3 belonging to the lens rows LSR2 and LSR3 (center lens row) closest to the symmetry axis SA are in the lens rows LSR1 and LSR4 other than the center lens rows LSR2 and LSR3. The line head 29 is arranged so as to be smaller than the image plane facing distances ld1 and ld4 of the lenses LS1 and LS4 to which it belongs. Therefore, the difference Δld_max in the image plane facing distance can be more effectively suppressed.

  That is, the difference Δld_max is the difference between the maximum value and the minimum value of the image plane facing distance ld, and the maximum value of the image plane facing distance ld is any of the end lens rows LSR1 and LSR4 at both ends in the width direction LTD. This is the image surface facing distance ld of the lens LS belonging to the lens. That is, the maximum value of the image surface facing distance ld is the larger of the image surface facing distance ld1 and the image surface facing distance ld4. Therefore, from the viewpoint of suppressing the difference Δld_max, it is preferable to make both the image plane facing distances ld1 and ld4 of the lenses LS belonging to the end lens rows LSR1 and LSR4 as small as possible. On the other hand, the image plane facing distances ld1 and ld4 corresponding to the end lens rows LSR1 and LSR4 are in a relationship in which when one is reduced, the other is increased. That is, for example, even if the position of the line head 29 is adjusted in the width direction LTD (that is, moved in the width direction LTD) to reduce both of the image plane facing distances ld1 and ld4, if one is reduced, the other is increased. There is a relationship. Under such a relationship, in order to suppress the difference Δld_max, it is preferable that the image plane facing distances ld1 and ld4 are approximately the same.

  Therefore, the image forming apparatus according to the present embodiment arranges the lens rows LSR1 to LSR4 substantially symmetrically with respect to the symmetry axis SA, and images of the lenses LS2 and LS3 belonging to the central lens rows LSR2 and LSR3 closest to the symmetry axis SA. The line head 29 is arranged so that the surface facing distances ld2, ld3 are smaller than those of the lens rows LSR1, LSR4 other than the central lens rows LSR2, LSR3. This is because with this configuration, the image plane facing distances ld1 and ld4 corresponding to the end lens rows LSR1 and LSR4 can be made comparable. As a result, in the image forming apparatus according to the present embodiment, the difference Δld_max in the image plane facing distance is suppressed, and good exposure is realized.

  Further, when the line head 29 is disposed on a latent image carrier having a surface center of curvature CC21 such as the photosensitive drum 21, it is preferable to dispose the line head as follows. That is, in Example 3, the line head 29 is arranged so that the symmetry axis SA passes through the center of curvature CC21 of the surface shape of the photosensitive drum 21 (that is, the rotational axis of the photosensitive drum 21). Accordingly, the image plane facing distances ld1 and ld4 corresponding to the end lenses LSR1 and LSR4 are equal to each other. As a result, it is possible to more effectively suppress the difference Δld_max in the image plane facing distance, and it is possible to achieve better exposure, which is preferable. Therefore, for easy understanding of the invention, the result of the simulation executed under the condition that the line head 29 is arranged as shown in FIGS. 22 and 23 will be described below.

  In the simulation in Example 3, the positional relationship between the photosensitive drum 21 and the line head is the positional relationship shown in FIGS. That is, the lens rows LSR1 to LSR4 are arranged in the width direction LTD at a predetermined lens row pitch Plsr. The line head 29 is arranged so that the image plane facing distances ld2 and ld3 corresponding to the central lens rows LSR2 and LSR3 are smaller than the image plane facing distances ld1 and ld4 corresponding to the other lens rows LSR1 and LSR4. ing. The symmetry axis SA of the lens rows LSR1 to LSR4 passes through the center of curvature CC21 of the surface shape of the photosensitive drum. In the simulation, the lens row pitch Plsr and the light emitting element group row pitch Pegr were 1.65 mm. In addition, a simulation was performed for each case where the diameter of the photosensitive drum 21 was 25 mm, 40 mm, and 80 mm. Note that Example 3 and Comparative Example 2 are the same in terms of the lens configuration and lens position of the lens LS. That is, Example 3 and Comparative Example 2 are the same in terms of lens data, aspheric coefficient, and optical system specifications.

  Table 13 shows simulation results in Example 3 executed under the above conditions. The simulation results will be described below for the case where the diameter of the photosensitive drum 21 is 25 mm, 40 mm, and 80 mm.

  First, the case where the photosensitive drum diameter is 25 mm will be described. As shown in the column “photosensitive member diameter φ25 mm” in Table 13, when the diameter of the photosensitive drum 21 is 25 mm, the difference Δld_max between the image plane facing distances ld1 to ld4 is 0.22 mm. It turns out that it is improving compared with 1.022 mm. As indicated by the spot diameter in the same column, the spot diameter formed by the lenses LS1 to LS3 has a minimum value of 29.1 μm (the spot diameter of the spots by the lenses LS2 and LS3), while the maximum value is 107.6 μm (the spot diameter of the spots by the lenses LS1 and LS4). That is, in Example 3, the maximum spot diameter difference is 78.5 μm (= 107.6 μm−29.1 μm), which is greatly improved compared to the spot diameter difference 538.2 μm in Comparative Example 2. I understand that.

  Next, a case where the photosensitive drum diameter is 40 mm will be described. As shown in the column of “photosensitive member diameter φ40 mm” in Table 13, when the diameter of the photosensitive drum 21 is 40 mm, the difference Δld_max between the image plane facing distances ld1 to ld4 is 0.137 mm. It can be seen that this is an improvement compared to 0.622 mm. As indicated by the spot diameter in the same column, the spot diameter formed by the lenses LS1 to LS4 has a minimum value of 29.1 μm (the spot diameter of the spots by the lenses LS2 and LS3), while the maximum value is 70.0 μm (the spot diameter of the spots by the lenses LS1 and LS4). That is, in Example 3, the maximum spot diameter difference is 40.9 μm (= 70.0 μm−29.1 μm), which is a significant improvement compared to the spot diameter difference 299.2 μm in Comparative Example 2. I understand that.

  Next, the case where the photosensitive drum diameter is 80 mm will be described. As shown in the column of “photosensitive member diameter φ80 mm” in Table 13, when the diameter of the photosensitive drum 21 is 80 mm, the difference Δld_max between the image plane facing distances ld1 to ld4 is 0.068 mm. It can be seen that this is an improvement compared to 0.307 mm. As indicated by the spot diameter in the same column, the spot diameter formed by the lenses LS1 to LS4 has a minimum value of 29.1 μm (the spot diameter of the spots by the lenses LS2 and LS3), while the maximum value is 40.2 μm (the spot diameter of the spots by the lenses LS1 and LS4). That is, in Example 3, the maximum spot diameter difference is 11.1 μm (= 40.2 μm−29.1 μm), which is greatly improved compared to the spot diameter difference 122.3 μm in Comparative Example 2. I understand that.

  As described above, in the third embodiment, the difference Δld_max between the image plane facing distances ld1 to ld4 is suppressed by arranging the line head 29 as shown in FIGS. In addition, by suppressing the difference Δld_max in the image plane facing distance in this way, the difference in the spot diameter of the spots formed on the surface of the photosensitive drum 21 is suppressed as compared with the comparative example 2. That is, in Example 3, better exposure is realized compared to Comparative Example 2.

  In the third embodiment, the line head 29 is arranged on the surface of the photosensitive drum as shown in FIGS. 22 and 23, so that the difference in the image plane facing distance ld between the lens rows LSR1 to LSR4 is reduced. Suppressed. Therefore, in each of the plurality of lenses LS, the lens shape of the lens 1S is adjusted according to the image plane facing distance 1d of the lens LS, and the light emitted from the light emitting element group 295 corresponding to the lens LS. The beam imaging position FP may be a position corresponding to the curvature shape of the surface of the photosensitive drum.

  That is, by disposing the line head 29 as shown in FIGS. 22 and 23, the difference in the image plane facing distance ld between the lens rows LSR1 to LSR4 is suppressed. Therefore, as shown in Example 4 below, in the lens row intervals LSR1 to LSR4, the image formation position FP of the light beam can be set to a position corresponding to the curvature shape of the surface of the photosensitive drum without changing the lens shape greatly. Is possible. Therefore, it is possible to achieve very good exposure while suppressing the complexity of lens design or manufacture.

Example 4
FIG. 24 is a sub-scan sectional view showing a lens array in Embodiment 4 of the present invention. The upper part of the figure is an enlarged view of the broken-line square part in the lower part of the figure. The arrangement relationship between the lens array 299 and the photosensitive drum 21 is the same as the arrangement relationship described with reference to FIGS. 20 and 21 (that is, the arrangement relationship in the third embodiment). Therefore, in the fourth embodiment, the difference Δld_max between the image plane facing distances ld1 to ld4 of the lenses LS1 to LS4 is suppressed. In the simulation in Example 4, the diameter of the photosensitive drum 21 was 80 μm. As described below, in the fourth embodiment, the image plane facing distance ld is suppressed as described above, and the lens configuration of the lens LS is further adjusted to form the light beam imaging positions FP1 to FP1 to LS4. FP4 is set to a position corresponding to the curvature shape of the photosensitive drum 21. Specifically, the lens shapes of the lenses LS1 to LS4 are adjusted so that the imaging positions FP1 to FP4 correspond to the curvature shape.

  Table 14 shows lens data of the lenses LS2 and LS3, and Table 15 shows aspheric coefficients of the lenses LS2 and LS3. On the other hand, Table 16 shows lens data of the lenses LS1 and LS4, and Table 17 shows aspheric coefficients of the lenses LS1 and LS4. As can be seen from these tables, in Example 4, the aspheric coefficient is changed between the lenses LS2 and LS3 and the lenses LS1 and LS4. The lens thickness and the lens position are the same between the lenses LS2 and LS3 and the lenses LS1 and LS4. Table 18 shows the optical system specifications used in the simulation in Example 4. Thus, in Example 4 of the present invention, the lens LS2 and the lens LS3 are the same, and the lens LS1 and the lens LS4 are the same. Further, the lenses LS2 and LS3 are different from the lenses LS1 and LS4.

Table 19 shows simulation results when the lenses LS1 to LS4 are configured based on the data given in Tables 14 to 18 described above. In the simulation, other conditions such as the lens row pitch Plsr and the light emitting element group row pitch Pegr are the same as those in the third embodiment. The difference Δfd shown in Table 19 is the difference between the image photoreceptor distances fd1 to fd4 corresponding to the lenses LS1 to LS4 and the image photoreceptor distance fd2. That is, the difference Δfd between the image photosensitive member distances fd1 to fd4 is obtained on the basis of the image photosensitive member distance fd2. As shown in the table, the difference Δfd is 0 in the fourth embodiment. That is, the image photosensitive member distances fd1 to fd4 are equal to each other. As shown in FIG. 24, this is because the positions FP1 to FP4 of the light beams by the lenses LS1 to LS4 correspond to the curvature shape of the photosensitive drum 21 (in the fourth embodiment, as shown in FIG. This is because the surface of the photosensitive drum 21 is adjusted. As shown in the “Spot Diameter” column of Table 19, the difference in the spot diameters of the spots formed on the surface of the photosensitive drum 21 is suppressed by suppressing the difference in the image photosensitive member distances fd1 to fd4. You can see that Specifically, the minimum value is 28.7 μm (spot spot diameter by the lenses LS1 and LS4), while the maximum value is 29.1 μm (spot spot diameter by the lenses LS2 and LS3). Therefore, the difference in spot diameter in Example 4 is 0.4 μm (= 29.1 μm−28.7 μm), which is an improvement compared to the spot diameter difference in Example 3 of 11.1 μm. . That is, in the fourth embodiment, compared to the third embodiment, the difference in the diameter of the spots formed between the plurality of lens rows LSR1 to LSR4 is further suppressed, and better exposure is realized.

Others The present invention is not limited to the above-described embodiment, and various modifications other than those described above can be made without departing from the spirit of the present invention. For example, in the above embodiment, the lens rows LSR are arranged at the lens row pitch Plsr in the width direction LTD, and are arranged substantially symmetrically in the width direction LTD with respect to the symmetry axis SA substantially perpendicular to the width direction LTD in the sub-scan section. ing. The line head 29 is arranged so that the symmetry axis SA passes through the center of curvature CC21 of the surface shape of the photosensitive drum 21. However, it is not an essential requirement for the present invention that the symmetry axis SA passes through the center of curvature CC21 of the photosensitive drum 21. In short, the image plane facing distance ld of the lens LS belonging to the lens row LSR other than the end lens row LSR at each end in the width direction LTD is smaller than the image plane facing distance ld of the lens LS belonging to the end lens row LSR. By arranging the line head 29 in such a manner, the effect of the present invention can be achieved in which the difference ld_max in the image plane facing distance ld is suppressed. However, by arranging the line head 29 so that the symmetry axis SA passes through the center of curvature CC21 of the photosensitive drum 21, the image plane facing distance ld can be more effectively suppressed, which is preferable as described above. It is.

  In the above embodiment, the lens rows LSR are arranged at the lens row pitch Plsr in the width direction LTD, and are arranged substantially symmetrically in the width direction LTD with respect to the symmetry axis SA substantially perpendicular to the width direction LTD in the sub-scanning section. ing. The image plane facing distance ld of the lens LS belonging to the lens row LSR (center lens row) closest to the symmetry axis SA is equal to the image surface facing distance ld of the lens LS belonging to each lens row LSR other than the center lens row LSR. The line head 29 is arranged so as to be smaller in comparison. However, such a configuration is not an essential requirement for the present invention. In short, the image plane facing distance ld of the lens LS belonging to the lens row LSR other than the end lens row LSR at each end in the width direction LTD is smaller than the image plane facing distance ld of the lens LS belonging to the end lens row LSR. By arranging the line head 29 in such a manner, the effect of the present invention can be achieved in which the difference ld_max in the image plane facing distance ld is suppressed. However, the line head 29 so that the image plane facing distance ld of the lens LS belonging to the central lens row LSR is smaller than the image plane facing distance ld of the lens LS belonging to each lens row LSR other than the central lens row LSR. As described above, it is possible to more effectively suppress the image plane facing distance ld by arranging.

  In the above embodiment, the case where the number of lens rows LSR is 3 or 4 has been described. However, the number of lens rows LSR is not limited to this, and the present invention is applicable to the line head 29 having N rows (N is an integer of 3 or more) lens rows LSR.

  As described with reference to FIGS. 9 and 10, in the above-described embodiment, the aperture stop DIA is disposed at the front focal point of the lens LS, and the image side of the lens LS is configured to be telecentric. However, it is not an essential requirement for the present invention that the image side of the lens LS is configured to be telecentric. However, the distance between the lens LS and the surface of the photosensitive drum may vary due to the eccentricity of the photosensitive drum 21 or the like. Such fluctuations may cause fluctuations in the sub-scanning direction SD of the positions of spots formed on the surface of the photosensitive drum. On the other hand, when the lens LS is configured to be telecentric, the effect of suppressing the fluctuation of the spot position in the sub-scanning direction SD can be obtained, and favorable exposure is realized, which is preferable. This will be described in detail.

  FIG. 25 is a sub-scan sectional view showing an effect when the image side of the lens is telecentric. The surface DSF represents the surface of the photosensitive drum 21 when there is no eccentricity. The surface DSFe is a surface of the photosensitive drum 21 that is decentered in the photosensitive drum 21 and is shifted by a distance CHoa in the direction of the optical axis OA of the lens LS. The principal ray PRMt is a principal ray of a light beam that forms a spot at the position IM on the surface of the photosensitive drum when image-side telecentricity is realized. On the other hand, the principal ray PRMnt is a principal ray of a light beam that forms a spot at the position IM on the surface of the photosensitive drum when image-side telecentricity is not realized. That is, when the photosensitive drum 21 is not decentered, the positions of the principal ray PRMt and the principal ray PRMnt on the surface of the photosensitive drum 21 are the same.

  Here, consider a case where the photosensitive drum 21 is decentered and the surface of the photosensitive drum 21 is shifted by the distance CHoa in the direction of the optical axis OA of the lens LS. At this time, the spot position by the light beam of the principal ray PRMt is the position IMe. On the other hand, the spot position by the light beam of the principal ray PRMnt is the position IMech. As shown in the figure, when image-side telecentricity is realized, the principal ray PRMt of the light beam is parallel to the optical axis OA of the lens LS. Therefore, even when the surface of the photosensitive drum 21 changes in the direction of the optical axis OA, the position of the formed spot only changes in the direction of the optical axis OA, and is substantially in the sub-scanning direction SD. Does not fluctuate. On the other hand, when image-side telecentricity is not realized, the principal ray PRMnt of the light beam is not parallel to the optical axis OA of the lens LS. Therefore, when the surface of the photosensitive drum 21 varies in the direction of the optical axis OA, the position of the formed spot varies by the distance CHs in the sub-scanning direction SD. As described above, when the lens LS is configured to be telecentric, it is possible to achieve an effect of suppressing the fluctuation of the spot position in the sub-scanning direction SD, and it is preferable that favorable exposure is realized.

  In the above-described embodiment, the photosensitive drum 21 is used as the latent image carrier. However, the latent image carrier to which the present invention is applicable is not limited to the photosensitive drum 21. In short, the present invention can be applied to all latent image carriers in which the surface area facing the lens array 299 has a curvature in the sub-scan section.

  FIG. 26 is a sub-scan sectional view showing an image forming apparatus equipped with a line head according to the present invention. The point that this embodiment differs greatly from the embodiment of FIG. 1 is the mode of the photoreceptor. That is, in this embodiment, a photosensitive belt 21B is used instead of the photosensitive drum 21. In addition, since the other structure is the same as that of the said embodiment, about the same structure, the same or equivalent code | symbol is attached | subjected and description of a structure is abbreviate | omitted.

  In this embodiment, the photosensitive belt 21B is stretched between two rollers 28 extending in the main scanning direction MD. The photosensitive belt 21B is rotationally moved in a predetermined rotational direction D21 by a drive motor (not shown). A charging unit 23, a line head 29, a developing unit 25, and a photoconductor cleaner 27 are disposed around the photoconductor belt 21B along the rotation direction D21. Then, a charging operation, a latent image forming operation, and a toner developing operation are executed by these functional units.

  In this embodiment, the line head 29 is disposed so as to face the winding portion of the photosensitive belt 21B around the roller 28. The roller 28 is cylindrical. Therefore, the winding portion of the photoreceptor belt 21B has a curvature shape. The reason why the line head 29 is arranged so as to face the winding portion is as follows. In other words, the tension surface of the photoreceptor belt 21B has a large fluttering compared to the winding portion around the roller 28. Therefore, the distance between the line head 29 and the surface of the photosensitive belt 21B is stabilized by disposing the line head 29 so as to oppose the winding portion of the surface of the photosensitive belt 21B around the roller 28 with relatively little flutter. ing.

  However, the surface shape of the photoreceptor belt 21B at the portion around the roller 28 has a curvature in the sub-scanning section. Therefore, the exposure failure as described above may occur. Therefore, in the image forming apparatus having the configuration as shown in FIG. 26, the present invention is applied, and the imaging position of the light beam is set to a position corresponding to the curvature shape of the surface of the photosensitive belt 21B. Therefore, it is possible to realize a proper exposure.

1 is a diagram showing an embodiment of an image forming apparatus according to the present invention. FIG. 2 is a diagram illustrating an electrical configuration of the image forming apparatus in FIG. 1. 1 is a perspective view schematically showing an embodiment of a line head according to the present invention. Sectional drawing of the width direction of one Embodiment of the line head concerning this invention. The perspective view which shows the outline of a lens array. Sectional drawing of the longitudinal direction of a lens array. The figure which shows arrangement | positioning of the light emitting element group in a line head. The figure which shows arrangement | positioning of the light emitting element in each light emitting element group. The figure which shows the image formation state of the lens in the cross section containing a longitudinal direction and an optical axis. The figure which shows the image formation state of the lens in the cross section containing the width direction and an optical axis. Explanatory drawing of the term used by this specification. Explanatory drawing of the term used by this specification. Explanatory drawing about the position etc. of a lens. The figure which shows the spot formation operation | movement by a line head. FIG. 6 is a sub-scan sectional view corresponding to a case where the line head is not properly arranged. FIG. 6 is a sub-scan sectional view corresponding to a case where the line head is not properly arranged. FIG. 3 is a sub-scanning cross-sectional view illustrating a positional relationship between a line head and a photosensitive drum. FIG. 3 is a sub-scanning cross-sectional view illustrating a positional relationship between a line head and a photosensitive drum. FIG. 4 is a sub-scanning cross-sectional view illustrating a configuration of a lens array. FIG. 6 is a sub-scan sectional view corresponding to a case where the line head is not properly arranged. FIG. 6 is a sub-scan sectional view corresponding to a case where the line head is not properly arranged. FIG. 3 is a sub-scanning cross-sectional view illustrating a positional relationship between a line head and a photosensitive drum. FIG. 3 is a sub-scanning cross-sectional view illustrating a positional relationship between a line head and a photosensitive drum. FIG. 4 is a sub-scanning cross-sectional view illustrating a configuration of a lens array. FIG. 6 is a sub-scanning sectional view showing an effect when the image side is configured telecentric. 1 is a diagram showing an image forming apparatus equipped with a line head according to the present invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 21 ... Photosensitive drum (latent image carrier), MD ... Main scanning direction, SD ... Sub scanning direction, 29 ... Line head, 293 ... Head substrate, 295 ... Light emitting element group, 2951 ... Light emitting element, 299 ... Lens array, LS, LS1, LS2, LS3, LS4 ... Lens, LSR ... Lens row, LGD ... Longitudinal direction, LTD ... Width direction, ld1 to ld4 ... Image surface facing distance

Claims (6)

A latent image carrier whose surface is conveyed in a sub-scanning direction substantially perpendicular to the main scanning direction;
The latent image carrier has lens rows formed by arranging a plurality of lenses in a longitudinal direction corresponding to the main scanning direction, and N rows (N is an integer of 3 or more) at different arrangement positions in the width direction corresponding to the sub-scanning direction. A lens array disposed opposite to the surface; and a head substrate in which a light emitting element group including a plurality of light emitting elements is disposed for each of the plurality of lenses, and each of the plurality of lenses includes: A line head that forms an image of the light beam emitted from the light emitting element of the corresponding light emitting element group toward the surface of the latent image carrier facing the lens,
For each of the plurality of lenses, when the distance in the optical axis direction of the lens between the apex of the lens surface on the latent image carrier side of the lens and the surface of the latent image carrier is defined as an image plane facing distance,
Each of the N lens rows faces opposite positions on the surface of the latent image carrier in the sub-scanning direction,
Of the surface of the latent image carrier, the surface region facing the lens array has a curvature in the sub-scan section and is convex with respect to the lens array,
The line head faces the image plane of the lens belonging to a lens row other than the end lens row when the lens row at each end in the width direction among the N lens rows is an end lens row. The image forming apparatus, wherein the distance is arranged with respect to the surface of the latent image carrier so that the distance is smaller than an image surface facing distance of the lens belonging to the end lens row. The N lens rows are arranged at a predetermined lens row pitch in the width direction, and are arranged substantially symmetrically in the width direction with respect to an axis of symmetry substantially perpendicular to the width direction in the sub-scan section.
The line head is configured such that an image plane facing distance of the lens belonging to the central lens row closest to the symmetry axis among the N lens rows is the lens belonging to each lens row other than the central lens row. The image forming apparatus according to claim 1, wherein the image forming apparatus is disposed on the surface of the latent image carrier so as to be smaller than an image surface facing distance.   The image forming apparatus according to claim 2, wherein the curvature shape of the surface of the latent image carrier has a center of curvature, and the line head is disposed so that the axis of symmetry passes through the center of curvature.   4. The image forming apparatus according to claim 1, wherein an aperture stop is provided between each lens and the corresponding light emitting element group, and an image side of each lens is configured to be telecentric. 5. .   In each of the plurality of lenses, the lens shape of the lens is adjusted according to the image plane facing distance of the lens, and the imaging position of the light beam emitted from the light emitting element group to which the lens corresponds corresponds to the latent image. The image forming apparatus according to claim 1, wherein the position is set according to a curvature shape of the surface of the image carrier. An exposure step of exposing the surface of the latent image carrier carried by the surface in a sub-scanning direction substantially perpendicular to the main scanning direction by a line head;
The line head has N rows (N is an integer of 3 or more) arranged in a longitudinal direction corresponding to the main scanning direction at different arrangement positions in the width direction corresponding to the sub-scanning direction. A lens array disposed facing the surface of the latent image carrier, and a head substrate in which a light emitting element group composed of a plurality of light emitting elements is disposed for each of the plurality of lenses, and each of the plurality of lenses Is formed by focusing the light beam emitted from the light emitting element of the light emitting element group corresponding to the lens toward the surface of the latent image carrier facing the lens,
For each of the plurality of lenses, when the distance in the optical axis direction of the lens between the apex of the lens surface on the latent image carrier side of the lens and the surface of the latent image carrier is defined as an image plane facing distance,
Each of the N lens rows faces opposite positions on the surface of the latent image carrier in the sub-scanning direction,
Of the surface of the latent image carrier, the surface region facing the lens array has a curvature in the sub-scan section and is convex with respect to the lens array,
The line head faces the image plane of the lens belonging to a lens row other than the end lens row when the lens row at each end in the width direction among the N lens rows is an end lens row. An image forming method, wherein the distance is arranged with respect to the surface of the latent image carrier so that the distance is smaller than an image surface facing distance of the lens belonging to the end lens row.

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