JP2011000863A - Exposure head, and image forming apparatus - Google Patents

Abstract

There is provided a technique capable of realizing a good exposure by suppressing a variation in the size of convergent light regardless of a position variation of a surface (exposed surface) of an image carrier.
A first light emitting element that emits light having a wavelength λ11 and light having a wavelength λ12, and a light emitted from the first light emitting element in a first direction with the optical axis directed in a first direction. A first imaging optical system that emits and converges the image on the optical head, and an image carrier disposed in a first direction of the exposure head, wherein the first imaging optical system includes: The light of wavelength λ11 is imaged at the imaging position P11, and the light of wavelength λ12 is imaged at the imaging position P12 that is a distance Δ1 away from the imaging position P11 in the first direction. It is greater than or equal to the fluctuation width of the surface of the image carrier on the optical axis of one imaging optical system.
[Selection] Figure 1

Description

  The present invention relates to an exposure head that performs exposure by converging light from a light emitting element by an imaging optical system, and an image forming apparatus using the exposure head.

  A line head as such an exposure head is described in Patent Document 1. That is, the exposure head (line head) of the same document includes a light emitting element and an imaging optical system, and the light from the light emitting element is converged on the surface to be exposed by the imaging optical system. Thus, the surface to be exposed is exposed by the light converged on the surface to be exposed.

JP 2008-221790 A

  However, the position of the exposed surface is not always stable and may fluctuate. For example, as described in Patent Document 1, exposure is generally performed using the surface of a drum-shaped image carrier (photosensitive drum of the same Patent Document) rotating in a predetermined direction as an exposed surface. The drum shape of the carrier is not a perfect circle, and there is an error within the tolerance range. In such a case, with the rotation of the image carrier, the surface position of the image carrier changes on the optical axis of the imaging optical system. Alternatively, the center of the image carrier (center of the drum shape) is decentered from the center of rotation, and due to this decentering, the surface position of the image carrier is changed by the rotation of the image carrier. In some cases, it varies on the optical axis. When the position of the surface of the image carrier (exposed surface) fluctuates on the optical axis of the imaging optical system in this way, the size of the convergent light projected on the surface of the image carrier fluctuates, which is favorable. In some cases, exposure could not be performed.

  The present invention has been made in view of the above problems, and it is possible to realize good exposure by suppressing the fluctuation of the size of the convergent light regardless of the position fluctuation of the surface (exposed surface) of the image carrier. The purpose is to provide technology.

  In order to achieve the above object, an image forming apparatus according to the present invention includes a first light emitting element that emits light of wavelength λ11 and light of wavelength λ12, and an optical axis directed in a first direction. An exposure head having a first imaging optical system for emitting and converging light from the first light emitting element in a first direction; and an image carrier disposed in the first direction of the exposure head; The first imaging optical system forms an image of light having the wavelength λ11 at the imaging position P11 and has a wavelength at the imaging position P12 that is a distance Δ1 away from the imaging position P11 in the first direction. The light of λ12 is imaged, and the distance Δ1 is characterized by being not less than the fluctuation width of the surface of the image carrier on the optical axis of the first imaging optical system.

  In order to achieve the above object, an exposure head according to the present invention is provided with a first light emitting element that emits light of wavelength λ11 and light of wavelength λ12, and an optical axis directed in a first direction. A first image-forming optical system that emits light from one light-emitting element in a first direction and converges it on the surface to be exposed. The first image-forming optical system has a wavelength λ11 at the image-forming position P11. And the light of wavelength λ12 is imaged at the imaging position P12 that is separated by the distance Δ1 in the first direction with respect to the imaging position P11, and the distance Δ1 is the distance of the first imaging optical system. It is characterized by being not less than the fluctuation width of the exposed surface on the optical axis.

  The invention thus configured (image forming apparatus, exposure head) includes a first light emitting element that emits light of wavelength λ11 and light of wavelength λ12, and the first imaging optical system has an imaging position P11. In addition, the light of wavelength λ11 is imaged at the same time, and the light of wavelength λ12 is formed at the imaging position P12 which is a distance Δ1 away from the imaging position P11 in the optical axis direction (first direction) of the first imaging optical system Is imaged. In other words, the first imaging optical system forms an image of light from the first light emitting element at two imaging positions P11 and P12 that are separated by a distance Δ1 in the optical axis direction. An effect is obtained in which the depth of focus of the image optical system is apparently enlarged. Moreover, the distance Δ1 is not less than the fluctuation width of the image carrier surface (exposed surface) on the optical axis of the first imaging optical system. Therefore, it is possible to achieve good exposure by suppressing fluctuations in the size of the convergent light regardless of fluctuations in the position of the surface (exposed surface) of the image carrier.

  Further, the first light emitting element may be configured to have an emission spectrum having peaks at the wavelength λ11 and the wavelength λ12. This is because the above-described apparent depth-of-focus expansion effect is effectively achieved, and better exposure can be realized.

By the way, the present invention has an advantage that the variation in the size of the convergent light can be suppressed by setting the distance Δ1 to be equal to or larger than the variation width of the exposed surface on the optical axis of the first imaging optical system. Is. However, if the distance Δ1 is too large, the aberration of the convergent light is increased and the imaging performance is deteriorated, so that exposure unevenness and a reduction in resolution may be caused. Therefore, a first aperture stop provided in the first imaging optical system is provided, the diameter D of the first light emitting element, the magnification m of the first imaging optical system, and the first imaging optical system. And the image-side aperture angle u, which is a half angle extending from the image point to the diameter of the entrance pupil, is expressed by the relational expression Δ1 ≦ | m | × D / tan (u)
You may comprise so that it may satisfy | fill. Thereby, the influence on the imaging performance such as aberration can be suppressed, and better exposure can be performed.

  The present invention can also be applied to an exposure head having two imaging optical systems. That is, the exposure head is arranged with a second light emitting element that emits light of wavelength λ21 and light of wavelength λ22, and a light emitted from the second light emitting element in the first direction with the optical axis directed in the first direction. A second imaging optical system for emitting and converging in the direction of the optical axis. The second imaging optical system focuses the light of the wavelength λ21 at the imaging position P21 and at the imaging position P21. On the other hand, the light of the wavelength λ22 is imaged at the imaging position P22 which is separated by the distance Δ2 in the first direction, and the distance Δ2 is the fluctuation of the surface of the image carrier on the optical axis of the second imaging optical system. You may comprise so that it may be more than a width | variety. With this configuration, a good exposure can be realized by suppressing the change in the size of the convergent light by the second imaging optical system regardless of the position fluctuation of the surface (exposed surface) of the image carrier. It is possible.

  By the way, in such a configuration, the first imaging optical system converges the light in the vicinity of the intersection IS1 where the optical axis intersects the image carrier, and the second imaging optical system has the optical axis thereof. The light is converged in the vicinity of the intersection IS2 that intersects the image carrier. However, when the image carrier has a finite curvature, the positions of the intersections IS1 and IS2 may be shifted by a distance d in the first direction. In such a case, the first point formed near the intersection IS1. In some cases, the convergent light from the first imaging optical system differs from the convergent light from the second imaging optical system formed near the intersection IS2. Therefore, the distance d in the first direction between the intersection IS1 at which the optical axis of the first imaging optical system intersects the image carrier and the intersection IS2 at which the optical axis of the second imaging optical system intersects the image carrier. The imaging position P11 and the imaging position P21 may be configured to be separated from each other in the first direction. In other words, by shifting the imaging position of the first imaging optical system and the imaging position of the second imaging optical system by the distance d, it is possible to suppress the above-described difference in the size of the convergent light. It becomes.

  Further, the present invention is also applicable to a configuration in which three or more imaging optical systems including the first imaging optical system and the second imaging optical system are arranged with the optical axis directed in the first direction. The invention can be applied. At this time, one of the first imaging optical system and the second imaging optical system has the optical axis closest to the center of curvature of the image carrier among the three or more imaging optical systems. The other optical axis different from one of the image optical system and the second image forming optical system may be configured to be farthest from the center of curvature of the image carrier among the three or more image forming optical systems. However, the distance d between the imaging positions is relatively large between the imaging optical system having the closest distance between the center of curvature of the image carrier and the optical axis and the imaging optical system having the farthest distance. However, if the distance d is too large, it is necessary to greatly change the optical configuration between the first imaging optical system and the second imaging optical system. Obtaining image performance is not easy.

  Therefore, (2N + 2) imaging optical systems (N is an integer of 1 or more) including the first imaging optical system and the second imaging optical system are arranged at intervals, and the first imaging optical system One of the system and the second imaging optical system may be configured to be located at the (N + 1) th or (N + 2) th from the end of the (2N + 2) imaging optical system. Thereby, since the distance d can be kept small, it is possible to easily obtain desired imaging characteristics.

  In addition, (2N + 1) imaging optical systems (N is an integer equal to or greater than 1) including the first imaging optical system and the second imaging optical system are disposed at intervals, and the first imaging optical system One of the system and the second imaging optical system may be configured to be located at the (N + 1) th position from the end of the (2N + 1) imaging optical system. Thereby, since the distance d can be kept small, it is possible to easily obtain desired imaging characteristics.

The figure for demonstrating the cause of a fluctuation | variation of the magnitude | size of a convergent light, and a countermeasure. 1 is a diagram illustrating an example of an image forming apparatus to which the present invention can be applied. FIG. 3 is a block diagram showing an electrical configuration provided in the image forming apparatus of FIG. 2. The perspective view which shows the outline of a line head. The partial top view which planarly viewed the head substrate from the thickness direction. FIG. 3 is a cross-sectional view taken along line AA of the line head in the first embodiment. The figure explaining the image formation operation | movement of the image formation optical system of this invention. The figure which showed the shake data of the photoconductor in the polar coordinate. Step sectional drawing in the AA line of the line head in 2nd Embodiment. The figure for demonstrating the optical structure with which 2nd Embodiment is provided. The figure which shows the structure of the line head in 3rd Embodiment. The figure which shows the structure of the line head in 4th Embodiment. The figure which shows the modification of the image forming apparatus concerning this invention. FIG. 10 is a diagram showing another modification of the image forming apparatus according to the present invention. The figure which shows the lens data of the upstream and downstream imaging optical system of an Example. The figure which shows the surface shape of S4 surface of an upstream and downstream imaging optical system. The figure which shows the surface shape of S7 surface of an upstream and downstream imaging optical system. The figure which shows the lens data of the center image formation optical system of an Example. The figure which shows the surface shape of S4 surface of a center imaging optical system. The figure which shows the surface shape of S7 surface of a center imaging optical system. The figure which shows the light ray figure of the upstream and downstream imaging optical system in the main scanning direction cross section. The figure which shows the light ray figure of the upstream and downstream imaging optical system in a subscanning direction cross section. The figure which shows the optical system item. The figure which shows the result of having calculated | required the imaging position of each light of 2 wavelengths by simulation. The figure which shows the result of having calculated | required the imaging position of each light of 2 wavelengths by simulation. The figure which shows the mode of the depth of focus expansion of an upstream and downstream imaging optical system. The figure which shows the mode of the depth of focus expansion of an upstream and downstream imaging optical system.

  As described above, when the position of the surface (exposed surface) of the image carrier fluctuates on the optical axis of the imaging optical system, the size of the convergent light may fluctuate, and good exposure may not be performed. Therefore, in the following, first, the cause of the variation in the size of the convergent light and its countermeasure will be described, and then a more specific embodiment will be described.

A. FIG. 1 is a diagram for explaining the cause and countermeasure of the fluctuation of the convergent light size, and the main scanning direction MD orthogonal to the sub-scanning direction SD. This corresponds to the case of The exposed surface ES has a finite curvature in the sub-scanning direction SD, in other words, has a finite radius of curvature in the cross section in the sub-scanning direction SD. Further, the imaging optical system OSα is arranged with the optical axis OAα directed in the direction Doa (optical axis direction). The imaging optical system OSα converges the light from the light emitting element Eα in the vicinity of the intersection ISα where the optical axis OAα of the imaging optical system OSα and the exposed surface ES intersect.

  However, the position of the exposed surface ES is not always stable and may fluctuate. In FIG. 1, the exposed surface ES varies between the position ESj and the position ESk. When the position of the exposure surface ES fluctuates on the optical axis of the imaging optical system OSα in this way, the size of the convergent light (spot) fluctuates, and there are cases where good exposure cannot be performed.

  Therefore, as a countermeasure against such a problem, the following configuration can be provided. In other words, the configuration shown in FIG. 1 includes a light emitting element Eα that emits light of wavelength λα1 and light of wavelength λα2, and the imaging optical system OSα forms an image of light of wavelength λα1 at the imaging position Pα1. The light of wavelength λα2 is imaged at an imaging position Pα2 that is a distance Δα away from the image position Pα1 in the optical axis direction Doa (first direction). That is, the imaging optical system OSα forms an image of light from the light emitting element Eα at two imaging positions Pα1 and Pα2 that are separated by a distance Δα in the direction of the optical axis OAα. The effect that the depth of focus is apparently enlarged is obtained. Moreover, the distance Δα is not less than the fluctuation width hα of the exposed surface ES on the optical axis OAα of the imaging optical system OSα. Therefore, good exposure can be realized by suppressing the fluctuation of the size of the convergent light regardless of the position fluctuation of the exposed surface ES.

  Hereinafter, specific embodiments will be described. Before that, the optical axis of the imaging optical system will be described. That is, the optical axis of the imaging optical system can be obtained as follows. Plane symmetry (mirror symmetry) with respect to a plane of symmetry perpendicular to the sub-scanning direction SD (second direction) and plane symmetry (mirror target) with respect to a plane of symmetry perpendicular to the main scanning direction MD (third direction) ), The imaging optical system includes a second symmetry plane perpendicular to the second direction and a third symmetry perpendicular to the third direction orthogonal to the second direction. And the intersection line between the second symmetry plane and the third symmetry plane can be obtained as the optical axis. In particular, when the imaging optical system is rotationally symmetric, the line of intersection between the second symmetric surface and the third symmetric surface coincides with the rotational symmetric axis, so that the rotational symmetric axis is obtained as the optical axis. Can do.

B-1. First Embodiment FIG. 2 is a diagram showing an example of an image forming apparatus to which the present invention is applicable. FIG. 3 is a block diagram showing an electrical configuration of the image forming apparatus shown in 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. 2 is a diagram corresponding to the execution of the color mode. In this image forming apparatus, when an image forming 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 to the engine controller EC and also outputs an image forming command. Corresponding video data VD is supplied to the head controller HC. At this time, every time the main controller MC receives the horizontal request signal HREQ from the head controller HC, the main controller MC supplies video data VD for one line to the head controller HC in the main scanning direction MD. 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. As a result, the engine unit ENG executes a predetermined image forming operation, and forms an image corresponding to the image forming command on a sheet such as copy paper, transfer paper, paper, and an OHP transparent sheet.

  An electrical component box 5 containing a power circuit board, a main controller MC, an engine controller EC, and a head controller HC is provided in the housing main body 3 of the image forming apparatus. 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. 2, 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. 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 21 is arranged so that the axial direction is parallel or 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 that is orthogonal or 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. 2, since the image forming stations of the image forming unit 7 have the same configuration, only a part of the image forming stations is given a sign for convenience of illustration, and the sign is omitted for the other image forming stations. .

  The charging unit 23 includes a charging roller whose surface is made of elastic rubber. The charging roller is configured to be driven to rotate in contact with the surface of the photosensitive drum 21 at a charging position, and is rotated 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 a 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 spaced apart from the photosensitive drum 21, the longitudinal direction of the line head 29 is parallel or substantially parallel to the main scanning direction MD, and the width direction of the line head 29 is the sub-scanning direction SD. Parallel or substantially parallel to The line head 29 includes a plurality of light emitting elements, and each light emitting element emits light according to video data VD from the head controller HC. The surface of the photosensitive drum 21 is irradiated with light from the light emitting element, whereby an electrostatic latent image is formed on the surface of the photosensitive drum 21.

  The developing unit 25 has a developing roller 251 that carries toner on the surface thereof. 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. Moves from the developing roller 251 to the photosensitive drum 21, and the electrostatic latent image formed by the line head 29 is made visible.

  The toner image that has been made visible at the development position in this way is conveyed in the rotation direction D21 of the photosensitive drum 21, and then transferred to the transfer belt 81 at the primary transfer position TR1 where the transfer belt 81 and each photosensitive drum 21 abut. Primary transfer.

  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 clean and remove 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. 2, and the direction of the arrow D81 (conveying direction) stretched around these rollers. 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 the respective 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). When the color mode is executed, all the primary transfer rollers 85Y, 85M, 85C, 85K are positioned on the image forming stations Y, M, C, K side as shown in FIG. A primary transfer position TR 1 is formed between each photosensitive drum 21 and the transfer belt 81 by being pushed and brought into contact with the photosensitive drum 21 included in each of the forming stations Y, M, C, and K. Then, by applying a primary transfer bias from the primary transfer bias generating unit to the primary transfer roller 85 at an appropriate timing, the toner images formed on the surface of each photosensitive drum 21 correspond to each. 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, which face 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 on the downstream side of the monochrome primary transfer roller 85K and on the upstream side 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 being in contact with 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 drive 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 manner, 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 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. Then, the sheet on which the image is secondarily transferred is guided to a nip portion formed by the heating roller 131 and the pressure belt 1323 of the pressure portion 132 by the sheet guide member 15, and in the nip portion, a predetermined value is provided. 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 by pressing the belt tension surface stretched by the two rollers 1321 and 1322 against the peripheral surface of the heating roller 131 out of the surface of the pressure belt 1323. 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 unit 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 contacting the tip of the cleaner blade 711 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.

  FIG. 4 is a perspective view showing an outline of the line head. In the drawing, a part of the line head 29 is shown in a cross section in order to facilitate understanding of the configuration of the line head 29 in the thickness direction TKD. Here, the thickness direction TKD is a direction perpendicular or substantially perpendicular to the longitudinal direction LGD and the width direction LTD, and a light emitting element E to be described later emits light (that is, a side from the line head 29 toward the photosensitive drum 21). ). The line head 29 includes a head frame 291 that is long in the longitudinal direction LGD, and the first lens array LA1 and the second lens array LA2 are held on one side in the thickness direction TKD of the head frame 291. The head substrate 293 is held on the other side of the head frame 291 in the thickness direction TKD. A light shielding member 297 is disposed inside the head frame 291. Thus, the line head 29 has a schematic configuration in which the head substrate 293, the light shielding member 297, the first lens array LA1 and the second lens array LA2 are arranged in this order in the thickness direction TKD. Next, a detailed configuration of each member will be described with reference to FIGS. 4, 5, and 6. In the description of the embodiment, the downstream side (upper side in FIG. 4) in the thickness direction TKD is referred to as “one side (in the thickness direction TKD)” and the upstream side in the thickness direction TKD (lower side in FIG. 4). Is referred to as “the other side (in the thickness direction TKD)”. In addition, one surface of the substrate or the flat plate is referred to as a front surface, and the other surface of the substrate or the flat plate is referred to as a back surface.

  FIG. 5 is a partial plan view of the head substrate 293 viewed from the thickness direction TKD, and corresponds to a case where the back surface 293-t of the head substrate 293 is seen through from the downstream side (upper side in FIG. 4) of the thickness direction TKD. To do. FIG. 6 is a step sectional view taken along line AA of the line head in the first embodiment, and corresponds to a case where the section is viewed from the longitudinal direction LGD (main scanning direction MD).

  In FIG. 5, the light emitting element group EG formed on the head substrate 293, the first lenses LS1a, LS1b, LS1c (indicated by reference numeral LS1 in FIG. 4) and the second lens array formed on the first lens array LA1. In order to show the positional relationship of the second lenses LS2a, LS2b, LS2c (indicated by reference numeral LS2 in FIG. 4) formed on LA2, the first lenses LS1a, LS1b, LS1c and the second lenses LS2a, LS2b, LS2c Are also indicated by alternate long and short dash lines. Incidentally, the descriptions of the first lens LS1a, LS1b, LS1c and the second lens LS2a, LS2b, LS2c are for showing their positional relationship, and the first lens LS1a, LS1b, LS1c and the second lens It does not indicate that LS2a, LS2b, and LS2c are formed on the head substrate back surface 293-t (FIG. 6).

  The head substrate 293 is made of a glass substrate that transmits light. On the back surface 293-t of the head substrate, a plurality of light emitting elements E that are bottom emission type organic EL (Electro-Luminescence) elements are formed, and a sealing member It is sealed with 294 (FIG. 6). The plurality of light emitting elements E have the same emission spectrum, and emit a light beam toward the surface of the photosensitive drum 21. As shown in FIG. 5, the arrangement of the plurality of light emitting elements E formed on the back surface 293-t of the head substrate has a group structure. That is, 15 light emitting elements E are arranged in two rows and staggered in the longitudinal direction LGD to form one light emitting element group EG, and a plurality of light emitting element groups EG are dispersed in three rows and staggered in the longitudinal direction LGD. Are arranged.

  In more detail, this arrangement | positioning aspect can be demonstrated as follows. That is, in each light emitting element group EG, 15 light emitting elements E are arranged at different positions in the longitudinal direction LGD, and the longitudinal direction LGD of two light emitting elements E and E whose positions in the longitudinal direction LGD are adjacent to each other. Is the inter-element distance Pel (in other words, in each light emitting element group EG, 15 light emitting elements E are arranged in the longitudinal direction LGD with a pitch Pel). A plurality of light emitting element groups EG are discretely arranged along the longitudinal direction LGD with a group distance Peg longer than the element distance Pel to form one light emitting element group row GRa and the like. Further, three light emitting element group rows GRa, GRb, GRc are discretely arranged at different positions in the width direction LTD with a distance Dt therebetween, and each of the light emitting element group rows GRa, GRb, GRc is: They are mutually shifted by a distance Dg in the longitudinal direction LGD.

  Here, the inter-element distance Pel can be obtained as a distance in the longitudinal direction LGD between the geometric centroids of the two light emitting elements E to be processed. Further, the inter-group distance Peg is the geometric center of gravity of the light emitting element E at the other end of the light emitting element group EG on one side in the longitudinal direction LGD, and the longitudinal direction LGD of the two target light emitting element groups EG. The distance in the longitudinal direction LGD with the geometric center of gravity of the light emitting element E at the one end of the other light emitting element group EG can be obtained. The distance Dg can be obtained as a distance in the longitudinal direction LGD between the geometric centroids of two light emitting element groups EG whose positions in the longitudinal direction LGD are adjacent to each other. The distance Dt can be obtained as the distance in the width direction LTD between the geometric centroids of the two light emitting element groups EG whose positions in the width direction LGD are adjacent to each other.

  Thus, the plurality of light emitting element groups EG are discretely arranged on the back surface 293-t of the head substrate 293. On the other hand, the surface 293-h of the head substrate 293 is fixed to the other side in the thickness direction TKD of the head frame 291 by an adhesive while being in contact with the light shielding member 297 disposed inside the head frame 291. Abut. Note that the other side of the light shielding member 297 in the thickness direction TKD is fixed to the head substrate surface 293-h with an adhesive. The light shielding member 297 is formed with a light guide hole 2971 that penetrates in the thickness direction TKD. The light guide hole 2971 has a circular shape in plan view from the thickness direction TKD, and the inner wall has a black color. It is plated. One light guide hole 2971 is formed for each light emitting element group EG. That is, one light guide hole 2971 is opened for one light emitting element group EG. Thus, the light shielding member 297 is fixed in contact with the head substrate surface 293-h in a state where the light guide hole 2971 is opened in the light emitting element group EG.

  The purpose of providing such a light shielding member 297 is to prevent so-called stray light from entering the lenses LS1 and LS2. That is, each light emitting element group EG is provided with a dedicated imaging optical system composed of a pair of lenses LS1 and LS2. In such a configuration, it is desirable that the light beam is incident only on the imaging optical systems LS1 and LS2 provided corresponding to the light emitting element group EG which is its own emission source. However, some of the light beams may become stray light without being directed to the imaging optical systems LS1 and LS2 provided in the light emitting element group EG that is the emission source. If such stray light enters the imaging optical systems LS1 and LS2 provided corresponding to the light emitting element group EG that is not its own emission source, a so-called ghost may occur. On the other hand, in this embodiment, a light shielding member 297 is provided between the light emitting element group EG and the imaging optical systems LS1 and LS2. The light shielding member 297 is provided with a light guide hole 2971 whose inner wall is black-plated so as to open to the light emitting element group EG. Therefore, most of the stray light is absorbed by the inner wall of the light guide hole 2971. As a result, the above-described ghost can be suppressed and a good exposure operation can be realized.

  On one side of the light shielding member 297 in the thickness direction TKD, a first lens array LA1 having a substantially flat plate shape is provided on both sides 291A and 291B in the width direction LTD of the head frame 291. On the back surface of the first lens array LA1, one first lens LS1 (LS1a, LS1b, LS1c) is formed for each light emitting element group EG, that is, for one light emitting element group EG. One first lens LS1 faces each other. Thus, in the first lens array LA1, the plurality of first lenses LS1 are arranged in a three-row zigzag pattern. In other words, three first lenses LS1 (positions adjacent to each other in the main scanning direction MD (longitudinal direction LGD)) are arranged. LS1a, LS1b, and LS1c) have different positions in the sub-scanning direction SD (width direction LTD). Therefore, in FIG. 5 and FIG. 6, the first lens LS1 is shown separately according to the position in the sub-scanning direction SD. In other words, the first lens LS1 that is in the uppermost stream in the sub-scanning direction SD is labeled LS1a, the first lens LS1 that is in the center in the sub-scanning direction SD is labeled LS1b, and the sub-scanning direction Reference sign LS1c is assigned to the first lens LS1 located on the most downstream side in SD.

  Further, on one side of the first lens array LA1 in the thickness direction TKD, a second lens array LA2 having a substantially flat plate shape is provided on both sides 291A and 291B in the width direction LTD of the head frame 291. On the back surface of the second lens array LA2, one second lens LS2 (LS2a, LS2b, LS2c) is formed for each light emitting element group EG, that is, for one light emitting element group EG. One second lens LS2 faces each other. Thus, in the second lens array LA2, the plurality of second lenses LS2 are arranged in a three-row zigzag pattern, in other words, three second lenses LS2 (positions adjacent to each other in the main scanning direction MD (longitudinal direction LGD)). LS2a, LS2b, and LS2c) are different in position in the sub-scanning direction SD (width direction LTD). Therefore, in FIG. 5 and FIG. 6, the second lens LS2 is shown separately according to the position in the sub-scanning direction SD. That is, the second lens LS2 that is in the uppermost stream in the sub-scanning direction SD is labeled LS2a, the second lens LS2 that is in the center in the sub-scanning direction SD is labeled LS2b, and the sub-scanning direction Reference sign LS2c is assigned to the second lens LS2 which is the most downstream in SD.

  Each of the lens arrays LA1 and LA2 includes a glass light-transmitting lens array substrate SB, and resin lenses LS1 and LS2 are formed on the back surface SB-t of the lens array substrate SB. That is, the resin-made first lenses LS1 (LS1a, LS1b, LS1c) are all formed on the back surface (same plane) of the substrate SB of the first lens array LA1. Further, the second lenses LS2 (LS2a, LS2b, LS2c) made of resin are all formed on the back surface (same plane) of the substrate SB of the second lens array LA2. Such lens arrays LA1 and LA2 can be produced by various methods conventionally proposed. For example, the lens arrays LA1 and LA2 can be produced by a method using a mold. In this method, a photocurable resin is interposed between the mold and the lens array substrate SB in a state in which the mold having a concave portion corresponding to the shape of the lenses LS1 and LS2 is in contact with the back surface SB-t of the lens array substrate SB. Is filled. When the photocurable resin is irradiated with light, the resin is cured and lenses LS1 and LS2 are formed on the lens array substrate SB.

  Thus, the three imaging optical systems, that is, the upstream imaging optical systems LS1a and LS2a, the central imaging optical systems LS1b and LS2b, and the downstream imaging optical systems LS1c and LS2c are at different positions in the sub-scanning direction SD. Arranged. The optical axes OAa, OAb, OAc of the imaging optical systems LS1a, LS2a, etc. are parallel to each other and face the optical axis direction Doa shown in FIG. Here, the optical axis direction Doa is a direction parallel to the respective optical axes OAa, OAb, and OAc and facing the direction in which light from the light emitting element E travels, and is parallel to the above-described thickness direction TKD. Further, the distances between the upstream imaging optical systems LS1a and LS2a, the central imaging optical systems LS1b and LS2b, and the downstream imaging optical systems LS1c and LS2c in the sub-scanning direction SD are equal to each other at a distance Lls. Here, the interval between the imaging optical systems LS1a, LS2a, etc. can be obtained as the distance between these optical axes OAa, OAb, OAc.

  Then, the upstream imaging optical systems LS1a and LS2a, the central imaging optical systems LS1b and LS2b, and the downstream imaging optical systems LS1c and LS2c arranged in this way sensitize light from the light emitting element E. It converges on the peripheral surface of the body drum 21. These imaging optical systems converge light in the vicinity of the intersections ISa, ISb, ISc of the respective optical axes OAa, OAb, OAc and the peripheral surface of the photosensitive drum 21 (FIG. 6), thereby the sub-scanning direction. Convergent light (spot SP) is formed at different positions on the SD. Note that the imaging optical system of the present embodiment forms an inverted reduced image, and its magnification is negative and has an absolute value of less than 1.

  By the way, in such a configuration, the position of the spot by the upstream imaging optical systems LS1a and LS2a (near the intersection ISa) and the position of the spot by the central imaging optical systems LS1b and LS2b (near the intersection ISb) are: They are shifted from each other by a distance di in the optical axis direction Doa. Similarly, the spot position by the downstream imaging optical systems LS1c and LS2c (near the intersection ISc) and the spot position by the central imaging optical systems LS1b and LS2b (near the intersection ISb) are in the optical axis direction Doa. Are shifted from each other by a distance di. In such a case, the size may differ between the spots formed by the upstream imaging optical systems LS1a and LS2a and the spots formed by the central imaging optical systems LS1b and LS2b. Similarly, spot size differences may occur between the downstream imaging optical systems LS1c and LS2c and the central imaging optical systems LS1b and LS2b. In order to cope with this, the imaging positions of the upstream imaging optical systems LS1a and LS2a and the imaging positions of the central imaging optical systems LS1b and LS2b correspond to the distance d (distance di) in the optical axis direction Doa. Are shifted from each other by the distance) (FIG. 7). The downstream imaging optical systems LS1c and LS2c and the central imaging optical systems LS1b and LS2b are configured in the same manner. That is, the above-described difference in spot size is suppressed by shifting the imaging position by the distance d.

  FIG. 7 is a diagram for explaining the image forming operation of the image forming optical system of the present invention, and corresponds to the case when viewed from the main scanning direction MD. Incidentally, since the imaging operation of the downstream imaging optical system is the same as the imaging operation of the upstream imaging optical system, the description of the imaging operation of the downstream imaging optical system is omitted in FIG. . Further, in the drawing, in order to enlarge and show the vicinity of the imaging position, the description of the imaging optical system is omitted, and only the optical axis of the imaging optical system is shown.

  Further, as described above, when the circumferential surface position of the photosensitive drum 21 fluctuates because the photosensitive drum 21 is not a perfect circle or because the photosensitive drum 21 is eccentric with respect to the rotation center. There is. Therefore, in FIG. 7, a broken line indicates that the circumferential surface of the photosensitive drum 21 reciprocates between the position 21 j and the position 21 k with the rotation cycle of the photosensitive drum 21.

  As shown in the figure, the upstream-side imaging optical systems LS1a and LS2a image light of wavelength λ1 at the imaging position Pa1, and image formation separated from the imaging position Pa1 by the distance Δ1 in the optical axis direction Doa. The light of wavelength λ2 is imaged at position Pa2. As a result, an effect is obtained in which the depth of focus of the upstream imaging optical systems LS1a and LS2a is apparently enlarged. The distance Δ1 is equal to or greater than the fluctuation width h1 of the circumferential surface of the photosensitive drum 21 on the optical axis OAa of the upstream imaging optical systems LS1a and LS2a. Thus, the circumferential surface of the photosensitive drum 21 always exists between the image forming position Pa1 and the image forming position Pa2 regardless of the position variation. Therefore, it is possible to realize good exposure by suppressing the fluctuation of the spot size by the upstream imaging optical systems LS1a and LS2a regardless of the position fluctuation of the circumferential surface of the photosensitive drum 21.

  The central imaging optical systems LS1b and LS2b image light having the wavelength λ1 at the imaging position Pb1, and light having the wavelength λ2 at the imaging position Pb2 that is separated from the imaging position Pb1 by the distance Δ2 in the optical axis direction. Is imaged. As a result, an effect is obtained in which the focal depth of the central imaging optical systems LS1b and LS2b is apparently enlarged. The distance Δ2 is equal to or greater than the fluctuation width h2 of the circumferential surface of the photosensitive drum 21 on the optical axis OAb of the central imaging optical systems LS1b and LS2b. Thus, the circumferential surface of the photosensitive drum 21 always exists between the image forming position Pb1 and the image forming position Pb2, regardless of the position variation. Therefore, it is possible to realize good exposure by suppressing the fluctuation of the spot size caused by the central imaging optical systems LS1b and LS2b regardless of the position fluctuation of the circumferential surface of the photosensitive drum 21.

  Incidentally, the position fluctuation of the circumferential surface of the photosensitive drum 21 can be obtained as shake data of the photosensitive drum. FIG. 8 is a diagram showing the shake data of the photoreceptor in polar coordinates. This shake data can be obtained as follows. First, the photosensitive drum 21 is rotated with the distance sensor facing the circumferential surface of the photosensitive drum 21. A well-known distance sensor can be used. Thus, the distance between the circumferential surface of the photosensitive drum 21 and the distance sensor (the distance between the drum and sensor) is acquired for one revolution of the photosensitive drum 21 and stored in the memory. Then, the fluctuation amount of the distance between the drum and the sensor (that is, the difference between the distance between the drum and the sensor at each angle and the minimum value of the distance between the drum and the sensor) is plotted, so that the photosensitive member shake data shown in FIG. Can be requested. The maximum value of the data in FIG. 8 is the fluctuation range.

  In the first embodiment, the light emitting element E has an emission spectrum having peaks at the wavelengths λ1 and λ2. As a result, the above-described apparent depth-of-focus expansion effect is effectively achieved, and better exposure can be realized.

B-2. Second Embodiment By the way, in the first embodiment, the distances Δ1 and Δ2 in the optical axis direction Doa between the imaging position of the light of wavelength λ1 and the imaging position of the light of wavelength λ2 are set on the circumferential surface of the photosensitive drum 21. By making the fluctuation widths h1 and h2 or more, it is possible to suppress the fluctuation of the spot SP size. However, if the distances Δ1 and Δ2 are too large, the spot aberration increases and the imaging performance deteriorates, which may result in uneven exposure and a decrease in resolution. Therefore, in the second embodiment, the same configuration as that of the first embodiment is provided, and the following configuration is further provided. In addition, it cannot be overemphasized that 2nd Embodiment can have the effect similar to 1st Embodiment by providing the structure which is common in 1st Embodiment.

  FIG. 9 is a step sectional view taken along line AA of the line head in the second embodiment, and corresponds to a case where the section is viewed from the longitudinal direction LGD (main scanning direction MD). As shown in FIG. 9, in the line head of the second embodiment, a diaphragm plate 295 is provided between the first lens array LA1 and the light shielding member 297, and each imaging optical system is provided on the diaphragm plate 295. On the other hand, an aperture stop Aa (Ab, Ac) is formed. Accordingly, the amount of light incident on the imaging optical systems LS1a, LS2a, etc. is limited by the aperture stop Aa. And in 2nd Embodiment, after providing this aperture stop Aa etc., it has the following optical structures.

FIG. 10 is a diagram for explaining an optical configuration provided in the second embodiment. When the influence of the aberration of the light of the second wavelength λ2 on the imaging plane IP1 of the light of the wavelength λ1 (first wavelength) becomes about the size of the light emitting element image on the image plane, the resolution is significantly reduced. Become. Therefore, when it is intended to realize fine image formation, it is desirable to suppress such a decrease in resolution. Therefore, in each imaging optical system of the second embodiment, the diameter D of the light-emitting element E in the main scanning direction MD, the lateral magnification m of the imaging optical system in the main scanning direction MD, and from the image point to the entrance pupil The image side aperture angle u, which is a half of the angle spanning the diameter, is a relational expression Δ ≦ | m | × D / tan (u) Equation 1
Meet. In Expression 1, the distances Δ1 and Δ2 are expressed as the distance Δ. With this configuration, it is possible to perform better exposure while suppressing the influence on the imaging performance such as aberration.

  The imaging plane is a direction orthogonal to the optical axis OA. In FIG. 10, the imaging plane IP2 for light of wavelength λ2 is shown together with the imaging plane IP1 for light of wavelength λ1.

B-3. Third Embodiment FIG. 11 is a diagram showing a configuration of a line head in the third embodiment, and corresponds to a case when viewed from the main scanning direction MD. The main difference between the third embodiment and the first embodiment is that the optical axes OAb of the central imaging optical systems LS1b and LS2b are deviated from the center of curvature CT21 of the photosensitive drum 21. As a result, the relationship between the distances Ba, Bb, and Bc is Ba>Bc> Bb (the distance Ba is the maximum and the distance Bb is the minimum). Here, the distance Ba is the distance between the optical axis OAa of the upstream imaging optical system and the curvature center CT21, and the distance Bb is the distance between the optical axis OAb of the central imaging optical system and the curvature center CT21, and the distance Bc. Is the distance between the optical axis OAc of the downstream imaging optical system and the center of curvature CT21.

  In such a configuration, the distance d between the imaging positions (reference numeral dmx in FIG. 11) is compared between the imaging optical system having the shortest distance between the curvature center CT21 and the optical axis and the imaging optical system having the furthest distance. Become bigger. However, if the distance dmx is too large, it is necessary to greatly change the optical configuration between the imaging optical system having the closest distance between the curvature center CT21 and the optical axis and the imaging optical system having the farthest distance. Obtaining the desired imaging performance with an optical configuration is not easy. In particular, in order to use the lens arrays LA1 and LA2 in which the upstream, central, and downstream lenses LS are integrally formed as described above, it is difficult to change the position of the lens surface and the distance dmx is large. It becomes difficult to obtain good imaging performance by optical design.

  In contrast, in the line head 29 of the third embodiment, (2N + 1) imaging optical systems (N is an integer equal to or greater than 1 and N = 1 in the third embodiment) are equally spaced in the sub-scanning direction SD. The imaging optical systems LS1b and LS2b which are arranged at Lls and have the shortest distance from the curvature center CT21 are located at the (N + 1) th position from the end of the (2N + 1) imaging optical system. Thereby, since the distance dmx can be kept small, it is possible to easily obtain desired imaging characteristics.

B-4. Fourth Embodiment FIG. 12 is a diagram showing the configuration of a line head in the fourth embodiment, and corresponds to the case seen from the main scanning direction MD. The main difference between the fourth embodiment and the first embodiment is that the lenses are arranged in a four-row zigzag manner in the lens arrays LA1 and LA2, and as a result of such an arrangement, as shown in FIG. Four imaging optical systems (that is, imaging optical systems LS1a and LS2a, imaging optical systems LS1b and LS2b, imaging optical systems LS1c and LS2c, and imaging optical systems LS1d and LS2d) are arranged in the sub-scanning direction SD. They are arranged at equal intervals Lls.

  As shown in the figure, in the fourth embodiment, the relationship between the distances Ba, Bb, Bc, and Bd is Bd> Ba> Bc> Bb (the distance Bd is the maximum and the distance Bb is the minimum). Here, the distance Ba is the distance between the optical axis OAa of the imaging optical systems LS1a and LS2a and the curvature center CT21, and the distance Bb is the distance between the optical axis OAb of the imaging optical systems LS1b and LS2b and the curvature center CT21. The distance Bc is the distance between the optical axis OAc of the imaging optical systems LS1c and LS2c and the curvature center CT21, and the distance Bd is the distance between the optical axis OAd of the imaging optical systems LS1d and LS2d and the curvature center CT21.

  As described in the third embodiment, in such a configuration, the distance between the imaging positions between the imaging optical system with the closest distance between the curvature center CT21 and the optical axis and the imaging optical system with the farthest distance is between. The distance d (reference numeral dmx in FIG. 11) becomes relatively large. However, if the distance dmx is too large, it is necessary to greatly change the optical configuration between the imaging optical system having the closest distance between the curvature center CT21 and the optical axis and the imaging optical system having the farthest distance. It is not easy to obtain a desired imaging performance with a simple optical configuration.

  On the other hand, in the line head 29 of the fourth embodiment, (2N + 2) imaging optical systems (N is an integer equal to or greater than 1, N = 1 in the fourth embodiment) are equally spaced in the sub-scanning direction SD. The imaging optical systems LS1b and LS2b, which are arranged at Lls and have the smallest distance from the center of curvature CT21, are located at the (N + 1) th or (N + 2) th position from the end of the (2N + 2) imaging optical system. Thereby, since the distance dmx can be kept small, it is possible to easily obtain desired imaging characteristics.

C. Others As described above, in the above embodiment, the line head 29 corresponds to the “exposure head” of the present invention, the photosensitive drum 21 corresponds to the “image carrier” of the present invention, and the optical axis direction Doa corresponds to the present invention. Corresponds to the “first direction”. Further, in the description in the column “Causes and countermeasures for the variation in the size of the convergent light”, the light emitting element Eα corresponds to the “first light emitting element” of the present invention, and the wavelength λα1 is the “wavelength λ11” of the present invention. The wavelength λα2 corresponds to the “wavelength λ12” of the present invention, the imaging optical system OSα corresponds to the “first imaging optical system” of the present invention, and the imaging position Pα1 corresponds to “ The imaging position Pα2 corresponds to the “imaging position P11”, and the imaging position Pα2 corresponds to the “imaging position P12” of the present invention. In the first embodiment, if the upstream imaging optical systems LSa1 and LSa2 are the “first imaging optical system” of the present invention, the central determination optical systems LSb1 and LSb2 are the “second imaging optical system” of the present invention. This corresponds to an “imaging optical system”.

  The present invention is not limited to the above-described embodiment, and various modifications can be made to the above-described one without departing from the spirit of the present invention. FIG. 13 is a diagram showing a modification of the image forming apparatus according to the present invention. The point that this modification is greatly different from the first embodiment is the mode of the photoconductor. In other words, in this modified example, the photosensitive belt 21 </ b> B 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). Further, 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 finite curvature. The reason why the line head 29 is arranged opposite to the winding portion in this way is as follows. That is, the tension surface of the photoreceptor belt 21 </ b> B has a large fluttering as compared with the 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 face the winding portion of the surface of the photosensitive belt 21B around the roller 28 with relatively little flapping. ing. However, the surface position of the photoreceptor belt 21b may fluctuate because the roller 28 is not a perfect circle or because the roller 28 is eccentric with respect to the center of rotation. In such a case, it is preferable to apply the present invention to the line head 29.

  FIG. 14 is a diagram showing another modification of the image forming apparatus according to the present invention. The point that this another modification is greatly different from the first embodiment is that the intermediate transfer belt 81 is not used. That is, in this embodiment, the toner image formed on the photosensitive drum 21 is directly transferred to the sheet by the transfer roller 85 and then fixed by the fixing device 13. However, the circumferential surface position of the photosensitive drum 21 may fluctuate because the photosensitive drum 21 is not a perfect circle or because the photosensitive drum 21 is eccentric with respect to the rotation center. In such a case, it is preferable to apply the present invention to the line head 29.

  Further, in the above embodiment, the peak intensity of the light emitting element at the wavelengths λ1 and λ2 is not particularly mentioned. However, the peak intensity at the wavelengths λ1 and λ2 may be configured to be stronger than half of the maximum value of the emission spectrum. Thereby, the effect | action of depth of focus expansion is show | played more effectively.

  Further, the imaging optical system of the above embodiment forms an inverted reduced image, and its magnification is negative and has an absolute value of less than 1. However, the magnification of the imaging optical system is However, it may be positive and may have an absolute value of 1 or more.

  In the above embodiment, the lenses are arranged in a three-row zigzag or a four-row zigzag in each of the lens arrays LA1 and LA2, but the lens arrangement is not limited to this.

  In the third and fourth embodiments, the case where the integer N is 1 has been described. However, the integer N is not limited to 1, and may be 2 or more.

  In the above embodiment, the imaging optical systems are arranged at equal intervals Lls in the sub-scanning direction SD. However, the arrangement intervals of the imaging optical systems may not be equal.

  In the above embodiment, the lenses LS1 and LS2 are formed on the back surfaces of the lens arrays LA1 and LA2. However, for example, the lenses LS1 and LS2 may be formed on the surfaces of the lens arrays LA1 and LA2.

  In the above embodiment, resin lenses LSa1, LSa2 and the like are formed on the light transmissive substrates SB1, SB2 made of the lens arrays LA1, LA2. However, the lens arrays LA1 and LA2 can be integrally formed of one material.

  Moreover, in the said 1st Embodiment, although the several light emitting element group EG was arrange | positioned by 3 rows zigzag, the arrangement | positioning aspect of the some light emitting element group EG is not restricted to this.

  In the above embodiment, the light emitting element group EG is composed of 15 light emitting elements E. However, the number of light emitting elements E constituting the light emitting element group EG is not limited to this.

  Moreover, in the said embodiment, in the light emitting element group EG, although the some light emitting element E was arrange | positioned by 2 rows zigzag, the arrangement | positioning aspect of the some light emitting element E in the light emitting element group EG is restricted to this. Absent.

  In the above embodiment, a bottom emission type organic EL element is used as the light emitting element E. However, a top emission type organic EL element may be used as the light emitting element E, or an LED (Light Emitting Diode) other than the organic EL element may be used as the light emitting element E.

  In the above embodiment, the light emitting element E having an emission spectrum having peaks at the wavelengths λ1 and λ2 is used. However, it is not essential to have peaks at the wavelengths λ1 and λ2. In short, the use of the light emitting element E that emits the light with the wavelength λ1 and the light with the wavelength λ2 makes it possible to increase the depth of focus.

  Next, examples of the present invention will be shown. However, the present invention is not limited by the following examples as a matter of course, and it is of course possible to implement the present invention with appropriate modifications within a range that can meet the gist of the preceding and following descriptions. They are all included in the technical scope of the present invention.

  Hereinafter, a more specific implementation applied to a line head in which three imaging optical systems are arranged at different positions in the sub-scanning direction SD as shown in FIG. 9 and the like in the second embodiment. An example will be described. FIG. 15 is a table showing lens data of the upstream and downstream imaging optical systems of the example. FIG. 16 is a table summarizing data indicating the surface shape of the S4 surface of the upstream and downstream imaging optical systems. FIG. 17 is a table summarizing data indicating the surface shape of the S7 surface of the upstream and downstream imaging optical systems. FIG. 18 is a table showing lens data of the central imaging optical system of the example. FIG. 19 is a table summarizing data indicating the surface shape of the S4 surface of the central imaging optical system. FIG. 20 is a table summarizing data indicating the surface shape of the S7 surface of the central imaging optical system.

  In this embodiment, the optical axes OAb of the central imaging optical systems LS2b and LS1b pass through the center of curvature CT21 of the photosensitive drum 21 (FIG. 9), and the radius of curvature R of the photosensitive drum 21 is 39 [mm] (photosensitive member diameter). φ78 [mm]), and the fluctuation range of the circumferential surface of the photosensitive drum 21 is about 25 [μm] (FIG. 8). The distance Lls between the imaging optical shapes is 1.77 [mm] (FIG. 9). Further, the distance between the lens exit surface S9 and the image plane S10 is different by 40 [μm] between the upstream and downstream imaging optical systems and the central imaging optical system, and the distance d≈40 [μm]. Yes.

  FIG. 21 is a diagram showing a ray diagram of the upstream and downstream imaging optical systems in the cross section in the main scanning direction. FIG. 22 is a diagram showing a ray diagram of the upstream / downstream imaging optical system in the cross section in the sub-scanning direction. FIG. 23 is a table showing the specifications of the optical system used when obtaining the ray diagrams of FIGS. 21 and 22. As shown in FIG. 23, the object side pixel group main direction full width (width Wm in FIG. 21) is 0.885 [mm], and the object side pixel group sub direction full width (Ws in FIG. 22) is 0.150 [mm]. The diameter D of the light emitting element is 28.6 [μm], the object side aperture angle (half angle) is 12.6 [°], the image side aperture angle u (half angle) is 17.6 [°], The ray diagrams of FIGS. 21 and 22 were obtained with the magnification m set to −0.7056.

  As shown in the lens data of FIGS. 15 to 17 and the ray diagrams of FIGS. 21 and 22, the upstream and downstream imaging optical systems are composed of two lenses. These two lenses are made of a lens material (resin) having a small Abbe number (νd = 30). As a result, the upstream and downstream imaging optical systems have a relatively large chromatic aberration. Then, light from a light emitting element having an emission spectrum having peaks at two wavelengths (wavelengths λ1, λ2) is imaged by the imaging optical system having a large chromatic aberration. As a result, as shown in the enlarged view in FIG. 21, light of each wavelength is imaged at two imaging positions P1 and P2 that are separated by a distance Δ in the optical axis direction.

  FIG. 24 and FIG. 25 are diagrams showing the results of obtaining the imaging positions of the two wavelengths (wavelengths λ1, λ2) by simulation. Specifically, in FIG. 24, the spot diameter (dashed curve in the same graph) when the light of wavelength λ 1 = 610 [nm] is converged as a spot by the imaging optical system according to the example, The spot diameter (solid curve in the graph) when the light of wavelength λ2 = 670 [nm] is converged as a spot by such an imaging optical system is also shown. In FIG. 25, the spot diameter when the light of wavelength λ1 = 565 [nm] is converged as a spot by the imaging optical system according to the example (the dashed curve in the same graph) and the result according to the example. The spot diameter (solid curve in the graph) when the light of wavelength λ2 = 715 [nm] is converged as a spot by the image optical system is also shown. In this figure, the horizontal axis indicates the focus shift [μm] and the vertical axis indicates the spot diameter [μm]. That is, these figures show the variation of the diameter of the spot SP (diameter in the main scanning direction) with respect to the displacement amount (focus shift) of the formation position of the spot SP in the optical axis direction. The minimum position of each curve corresponds to the imaging position of light having a wavelength corresponding to the curve.

  As shown in FIG. 24, the imaging position of wavelength λ1 (= 610 [nm]) and the imaging position of wavelength λ2 (= 670 [nm]) are shifted by a distance of 30 [μm] in the optical axis direction Doa. ing. That is, by adopting a light source that emits light of two wavelength components of wavelength 610 [nm] and wavelength 670 [nm], the distance Δ between these imaging positions becomes 30 [μm]. The effect that the depth of focus of the imaging optical system is apparently increased can be obtained. Further, as shown in FIG. 25, the imaging position of the wavelength λ1 (= 565 [nm]) and the imaging position of the wavelength λ2 (= 715 [nm]) are only a distance 60 [μm] in the optical axis direction Doa. There is a shift. That is, by adopting a light source that emits light of two wavelength components of wavelength 565 [nm] and wavelength 715 [nm], the distance Δ between these imaging positions becomes 60 [μm]. The effect that the depth of focus of the imaging optical system is apparently increased can be obtained.

  FIG. 26 and FIG. 27 are diagrams showing the results of the simulation of how the focal depth of the imaging optical system is expanded. In both figures, the horizontal axis indicates the focus shift [μm] and the vertical axis indicates the spot diameter [μm]. That is, these figures show the variation of the diameter of the spot SP (diameter in the main scanning direction) with respect to the displacement amount (focus shift) of the formation position of the spot SP in the optical axis direction. In FIG. 26, a spot formed by imaging light having two wavelength components of 610 [nm] and 670 [nm] and a light having a single wavelength of 640 [nm] are formed. In FIG. 27, a spot formed by imaging light having two wavelength components of 565 [nm] and 715 [nm] and a single spot of 640 [nm] are shown in FIG. A comparison is made with a spot formed by imaging light of a wavelength.

  Since the imaging position of the light of wavelength 610 [nm] and the imaging position of the light of wavelength 670 [nm] are shifted by 30 [μm] (= distance Δ) in the optical axis direction, as shown in FIG. It can be seen that the variation in the spot SP is smaller and the focal depth is apparently enlarged when the light having two wavelength components is imaged as compared with the case where the light having the wavelength of 640 [nm] is imaged. .

  Similarly, the imaging position of the light with the wavelength of 565 [nm] and the imaging position of the light with the wavelength of 715 [nm] are shifted by 60 [μm] (= distance Δ) in the optical axis direction. Thus, compared to the case where the light having a single wavelength of 640 [nm] is formed, the variation in the spot SP is smaller and the depth of focus is apparently enlarged when the light having two wavelength components is formed. I understand that.

  Moreover, in both FIG. 26 and FIG. 27 (FIGS. 24 and 25), the distance Δ (= 30 [μm], 60 μ [μm]) between the imaging positions is both the fluctuation range of the circumferential surface of the photosensitive drum 21. (= 25 [μm]) or more. Therefore, it is possible to perform favorable exposure while suppressing the fluctuation of the spot size regardless of the position fluctuation of the circumferential surface of the photosensitive drum 21.

Further, the distance Δ also satisfies the above-described expression 1, and it is possible to perform better exposure while suppressing the influence on the imaging performance such as aberration. That is, the right side of Equation 1 is
| −0.7056 | × 28.6 μm / tan (17.6 °) = 63.6 μm
The distance Δ (= 30 [μm], 60 [μm]) between the imaging positions shown in FIGS. 26 and 27 (FIGS. 24 and 25) is shorter than 63.6 [μm].

  Note that, similarly to the upstream and downstream imaging optical systems, the central imaging optical system is configured so that the apparent depth of focus is expanded and the expression 1 is satisfied. There is an effect equivalent to the effect of the downstream imaging optical system.

  DESCRIPTION OF SYMBOLS 21 ... Photosensitive drum, 29 ... Line head, E, E (alpha), E (beta) ... Light emitting element, OS (alpha), OS (beta) ... Imaging optical system, OA (alpha), OA (beta) ... Optical axis, Doa ... Optical axis direction, Doa ... Optical axis direction (1st Direction), SD ... sub-scanning direction, MD ... main scanning direction, CT, CT21 ... center of curvature, Pα1, Pα2, Pβ1, Pβ2 ... imaging position, ISα, ISβ ... intersection, Δ, Δα, Δβ ... distance, d , Dmx ... distance, LS1a, LS2a ... lens (imaging optical system), LS1b, LS2b ... lens (imaging optical system), LS1c, LS2c ... lens (imaging optical system), LS1d, LS2d ... lens (imaging optics) System), Aa, Ab, Ac ... Aperture stop

Claims (9)

A first light-emitting element that emits light of wavelength λ11 and light of wavelength λ12, and an optical axis that is disposed in a first direction and emits light from the first light-emitting element in the first direction An exposure head having a first imaging optical system for convergence
An image carrier disposed in the first direction of the exposure head;
With
The first imaging optical system forms an image of the light having the wavelength λ11 at the imaging position P11 and the imaging position P12 that is a distance Δ1 away from the imaging position P11 in the first direction. Imaging light of wavelength λ12,
The image forming apparatus according to claim 1, wherein the distance Δ1 is equal to or greater than a fluctuation width of the surface of the image carrier on the optical axis of the first imaging optical system.   The image forming apparatus according to claim 1, wherein the first light emitting element has an emission spectrum having peaks at the wavelength λ11 and the wavelength λ12. A first aperture stop provided in the first imaging optical system; a diameter D of the first light emitting element; a magnification m of the first imaging optical system; and the first imaging. For the optical system, the image side aperture angle u, which is a half angle extending from the image point to the diameter of the entrance pupil, is a relational expression Δ1 ≦ | m | × D / tan (u)
The image forming apparatus according to claim 1, wherein: The exposure head includes a second light emitting element that emits light having a wavelength λ21 and light having a wavelength λ22, and an optical axis directed in the first direction so that light from the second light emitting element is emitted from the second light emitting element. A second imaging optical system that emits and converges in a first direction;
The second imaging optical system forms an image of the light of the wavelength λ21 at the imaging position P21, and at the imaging position P22 that is a distance Δ2 away from the imaging position P21 in the first direction. Imaging light of wavelength λ22,
4. The image forming apparatus according to claim 1, wherein the distance Δ <b> 2 is equal to or greater than a fluctuation width of the surface of the image carrier on the optical axis of the second imaging optical system. 5.   The intersection IS1 where the optical axis of the first imaging optical system intersects the image carrier and the intersection IS2 where the optical axis of the second imaging optical system intersects the image carrier in the first direction. The image forming apparatus according to claim 4, wherein the imaging position P11 and the imaging position P21 are separated from each other in the first direction by a distance d. Three or more imaging optical systems including the first imaging optical system and the second imaging optical system are arranged with the optical axis directed in the first direction,
One optical axis of the first imaging optical system and the second imaging optical system is closest to the center of curvature of the image carrier in the three or more imaging optical systems, and The optical axis of the other of the image forming optical system and the second image forming optical system different from the one is farthest from the center of curvature of the image carrier in the three or more image forming optical systems. The image forming apparatus according to 5. (2N + 2) imaging optical systems (N is an integer equal to or greater than 1) including the first imaging optical system and the second imaging optical system are disposed at intervals,
The one of the first imaging optical system and the second imaging optical system is located at the (N + 1) th or (N + 2) th from the end of the (2N + 2) imaging optical system. 6. The image forming apparatus according to 6. (2N + 1) imaging optical systems (N is an integer equal to or greater than 1) including the first imaging optical system and the second imaging optical system are arranged at intervals,
The image according to claim 6, wherein the one of the first imaging optical system and the second imaging optical system is positioned at the (N + 1) th position from the end of the (2N + 1) imaging optical system. Forming equipment. A first light emitting element that emits light of wavelength λ11 and light of wavelength λ12;
A first imaging optical system that is disposed with the optical axis directed in a first direction and that emits light from the first light emitting element in the first direction and converges it on an exposed surface;
With
The first imaging optical system forms an image of the light having the wavelength λ11 at the imaging position P11 and the imaging position P12 that is a distance Δ1 away from the imaging position P11 in the first direction. Imaging light of wavelength λ12,
The exposure head according to claim 1, wherein the distance Δ1 is equal to or greater than a fluctuation width of the exposed surface on the optical axis of the first imaging optical system.

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