Patent Publication Number: US-2011050835-A1

Title: Exposure head and image forming apparatus

Description:
BACKGROUND 
     1. Technical Field 
     The present invention relates to an exposure head, which uses a plurality of lens arrays, and an image forming apparatus. 
     2. Related Art 
     An exposure head using lens arrays in which lenses are aligned in arrays has been known in the related art. An exposure head using two lens arrays has been proposed in JP-A-2009-098613. In this exposure head, the two lens arrays are supported so as to oppose each other, and a plurality of lenses aligned in one lens array faces a plurality of lenses aligned in the other lens array so as to make a one-to-one corresponding relationship. In addition, two lenses facing each other in this manner cooperate and thus function as one optical system. Moreover, in the exposure head, a light emitting element is provided so as to oppose the respective optical system, and the respective optical system forms an image of light from the opposing light emitting element, thereby forming a spot on an exposure target surface such as a surface of an image carrier or the like. 
     When it is attempted to allow an absolute value of a magnification of the optical system to be less than one (that is, when it is attempted to set the magnification of the optical system so as to form an reduction image) in the configuration in which two lenses cooperate and thus function as one optical system, there may occur the following problems. That is, when the absolute value of the magnification of the optical system is set to be less than one, the position and the surface precision of the lens, which is placed closer to the exposure target surface, from among the two lenses constituting the optical system tend to affect more greatly the optical performance of the optical system. Meanwhile, the light emitting element generates heat when emitting light. Therefore, if the heat from the light emitting element is transmitted to the lens array in which the lens closer to the exposure target surface is arranged, a thermal deformation occurs in the lens array. Therefore, the position of the lens arranged in that lens array (that is, the lens closer to the exposure target surface) is varied, and the surface precision of the lens is deteriorated in some cases. As a result, there is a concern that the optical system cannot appropriately exhibit its optical performance. 
     SUMMARY 
     An advantage of some aspects of the invention is to provide a technique for suppressing a thermal deformation of the lens array, in which lenses affecting more greatly the optical performance of the optical system are arranged, from among the lenses constituting the optical systems whose magnifications are less than one, and allowing the optical systems to appropriately exhibit their optical performances. 
     According to an aspect of the invention, there is provided an exposure head including: a light emitting element substrate in which light emitting elements are arranged in a first direction; first lens arrays with first lenses, to which the light from the light emitting elements is incident, arranged thereon; second lens arrays with second lenses, to which the light emitting from the first lenses is incident, and each of which constitutes with each of the first lenses an optical system whose absolute value of a lateral magnification is less than one, arranged thereon; first spacers which are arranged on the light emitting element substrate and support the first lens arrays; and second spacers which are arranged on the first lens arrays so as to be in different positions from those of the first spacers when seen from an optical axis direction of the optical system and support the second lens arrays. 
     The exposure head configured as described above is provided with the light emitting element substrate in which light emitting elements are arranged, the first lens arrays with first lenses, to which the light from the light emitting elements is incident, arranged thereon, and the second lens arrays with second lenses, to which the light emitting from the first lenses is incident, arranged thereon. The first lens arrays are supported by the first spacers, each of which is arranged between the first lens array and the light emitting element substrate. The second lens arrays are supported by the second spacers, each of which is arranged between the second lens array and the first lens array. Accordingly, when the light emitting elements in the light emitting element substrate generate heat along with the light emission, the heat is conducted to the first lens arrays via the first spacers in some cases. In such a case, if the heat is further conducted from the first lens arrays to the second lens arrays via the second spacers, there is a concern that the following problem may occur. 
     That is, in this exposure head, the light from the light emitting element is emitted from the first lens, then incident to the second lens, and subjected to the optical action by the optical system constituted by the first and second lenses. In addition, the absolute value of the lateral magnification of this optical system is less than one. In such a configuration, the position and the surface precision of the second lens greatly affect the optical performances of the optical system as in the same manner as described above. Accordingly, if the heat is conducted from the light emitting element substrate to the first lens array via the first spacer and then further conducted to the second lens array via the second spacer, and the thermal deformation occurs in the second lens array, the position of the second lens may be deviated, and the surface precision of the second lens may be deteriorated. As a result, there is a concern that the optical performances of the optical system may be degraded. 
     In order to solve this problem, according to this exposure head, the first spacers are arranged in the different positions from those of the second spacers when seen from the optical axis direction of the optical system. When the first and second spacers are arranged in different positions in this manner, it is possible to suppress the thermal conduction directing from the first spacers to the second spacers via the first lens arrays. Accordingly, it is possible to suppress the thermal conduction to the second lens arrays via the second spacers, and to thereby suppress the positional deviation of the second lenses along with the thermal deformations of the second lens arrays and the deterioration in the surface precision of the second lenses. As a result, it is possible to allow the optical systems constituted by the first and second lenses to exhibit their appropriate optical performances. 
     At this time, it is also applicable that first spacers and the second spacers are arranged in different positions in a second direction which is perpendicular to the first direction. 
     In addition, it is also applicable that the first spacers are arranged so as to be more distant from the optical axes of the optical systems in the second direction than the second spacers. The configuration in which the first spacers are further spaced from the optical axes of the second lenses than the second spacers in this manner is advantageous in suppressing the influence of the heat conducted to the first spacer on the optical performances of the optical systems (first and second lenses). 
     Moreover, it is also applicable that the width of the second spacer in the second direction is narrower than the width of the first spacer in the second direction. With this configuration, it is possible to further suppress the thermal conduction from the first spacers to the second lens arrays via the first lens arrays and the second spacers. As a result, it is possible to further suppress the positional deviation of the second lenses arranged in the second lens arrays, and to thereby allow the optical performances of the optical systems constituted by the first and second lenses to be more appropriate. 
     In addition, it is particularly preferable to apply the invention to the exposure head whose first spacers are made of a metal. That is, the first spacers made of a metal have a high thermal conductivity, thus the thermal conduction to the second lens array via the above-mentioned conduction path may easily occur. Accordingly, it is preferable to apply the invention to the exposure head configured as described above in order to suppress the thermal conduction to the second lens arrays, and thereby to secure the appropriate optical performances of the optical systems constituted by the first and second lenses. 
     In addition, it is particularly preferable to apply the invention to the exposure head provided with a driving element for driving the light emitting elements on the light emitting element substrate. That is, since the driving element generates heat along with the driving of the light emitting element, there is a concern that the heat from this driving element may be conducted to the second lens arrays via the above-mentioned conduction path. Accordingly, it is preferable to apply the invention to the exposure head with a driving element arranged on the light emitting element substrate in order to suppress the thermal conduction to the second lens array, and thereby to secure the appropriate optical performances of the optical systems constituted by the first and second lenses. 
     According to the invention, there is provided an image forming apparatus including: exposure heads, each of which includes a light emitting element substrate in which light emitting elements are arranged in a first direction, first lens arrays with first lenses, to which the light from the light emitting elements is incident, arranged thereon, second lens arrays with second lenses, to which the light emitted from the first lenses is incident, and each of which constitutes with each of the first lenses an optical system whose absolute value of a lateral magnification is less than one, arranged thereon, first spacers which are arranged on the light emitting element substrate and support the first lens arrays, and second spacers which are arranged on the first lens arrays so as to be in different positions from those of the first spacers when seen from an optical axis direction of the optical system and support the second lens arrays; and an image carrier which is irradiated with the light which is emitted from the light emitting elements and transmits through the optical systems constituted by the first lenses and the second lenses. 
     The image forming apparatus configured as described above is provided with the above-mentioned exposure head according to the invention. Therefore, there was a concern that above-mentioned problem due to the thermal deformations of the second lens arrays along with the heat generation of the light emitting elements occurred. Thus, in this image forming apparatus, the first spacers are arranged in different positions from those of the second spacers when seen from the optical axis direction of the optical system. When the first and second spacers are arranged in different positions in this manner, it is possible to suppress the thermal conduction directing from the first spacers to the second spacers via the first lens arrays. Accordingly, it is possible to suppress the thermal conduction to the second lens arrays via the second spacers, and to thereby suppress the positional deviation of the second lenses along with the thermal deformations of the second lens arrays and the deterioration in the surface precision of the second lenses. As a result, it is possible to allow the optical systems constituted by the first and second lenses to exhibit their appropriate optical performances. 
     According to the above-mentioned exposure head in the image forming apparatus, the first lens and the second lens constitutes one optical system, and the optical system emits light that became incident from the first lens to the second lens. An image carrier is irradiated with the light emitted from the second lens. In such a configuration, the second lens arrays with the second lenses arranged thereon are arranged so as to be close to the image carrier. The first spacers are arranged so as to be more distant from the optical axes of the optical systems in the second direction which is perpendicular to the first direction than the second spacers. The width of the second lens array in the second direction may be narrower than the width of the first lens array in the second direction. It is possible to enhance the degree of freedom in the layout of the exposure head with respect to the image carrier by setting the width of the second lens array, which is arranged in the vicinity of the image carrier, to be narrower. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
         FIG. 1  is a diagram illustrating an example of an image forming apparatus to which the invention can be applied. 
         FIG. 2  is a block diagram illustrating an electronic configuration provided in the image forming apparatus shown in  FIG. 1 . 
         FIG. 3  is a partial perspective view illustrating a schematic configuration of a line head. 
         FIG. 4  is a partial plan view of a head substrate when seen from a thickness direction. 
         FIG. 5  is a partial sectional view of the line head taken along a line V-V. 
         FIG. 6  is a partial side view of the line head. 
         FIG. 7  is a detailed partial sectional view of the line head taken along the line VII-VII. 
         FIG. 8  is a diagram explaining a reason why a spacer SP 1  and a spacer SP 2  are arranged in different positions. 
         FIG. 9  is a diagram illustrating an arrangement relationship between the spacer SP 1  and the spacer SP 2 . 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       FIG. 1  is a diagram illustrating an example of an image forming apparatus to which the invention can be applied.  FIG. 2  is a block diagram illustrating an electronic configuration provided in the image forming apparatus shown in  FIG. 1 . This apparatus is an image forming apparatus capable of selectively executing a color mode for forming a color image by superimposing toners of four colors including black (K), cyan (C), magenta (M), and yellow (Y) colors and a monochrome mode for forming a monochrome image by using only a toner of black (K) color.  FIG. 1  is a drawing corresponding to the time when the color mode is executed. In this image forming apparatus, when an image forming command is supplied to a main controller MC provided with a CPU, a memory, and the like from an external apparatus such as a host computer, this main controller MC supplies a control signal or the like to an engine controller EC, and supplies video data VD corresponding to the image forming command to a head controller HC. At this time, the main controller MC supplies video data VD corresponding to one line in a main scanning direction MD to the head controller HC every time when receiving a horizontal request signal HREQ from the head controller HC. In addition, this head controller HC controls a line head  29  for each color based on the video data VD from the main controller MC and a parameter value and a vertical synchronization signal Vsync from the engine controller EC. With this configuration, an 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 transparent sheet for an OHP. 
     A housing main body  3  included in the image forming apparatus is provided therein with an electric component box  5  which embeds a power circuit substrate, the main controller MC, the engine controller EC, and the head controller HC. In addition, an image forming unit  7 , a transfer belt unit  8 , and a sheet feeding unit  11  are also arranged in the housing main body  3 . In  FIG. 1 , a secondary transfer unit  12 , a fixing unit  13 , and a sheet guiding member  15  are arranged on the right side in the housing main body  3 . The sheet feeding unit  11  is configured to be attachable and detachable with respect to an apparatus main body  1 . The sheet feeding unit  11  and the transfer belt unit  8  are respectively configured such that they can be detached for repair and replacement. 
     The image forming unit  7  includes four image forming stations Y (for the yellow color), M (for the magenta color), C (for the cyan color), and K (for the black color) for forming an image with a plurality 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 toner image of a corresponding color on a surface of the photosensitive drum  21 . The photosensitive drum  21  is arranged such that the axial direction thereof is parallel or substantially parallel to the main scanning direction MD. In addition, each of the photosensitive drums  21  is respectively connected to a dedicated driving motor, and rotated and driven at a predetermined speed in a direction of an arrow D 21  in the drawing. With this configuration, the surfaces of the photosensitive drums  21  are transported in a sub-scanning direction SD perpendicular or substantially perpendicular to the main scanning direction MD. A charging unit  23 , a line head  29 , a developing unit  25 , and a photosensitive cleaner  27  are arranged along a rotation direction in the circumference of the photosensitive drum  21 . These functional units execute a charging operation, a latent image forming operation, and a toner developing operation. Accordingly, a color image is formed by superimposing the toner images formed by all the image forming stations Y, M, C, and K onto a transfer belt  81  included in the transfer belt unit  8  at the time of executing the color mode, while the monochrome image is formed by using only the toner image formed by the image forming station K at the time of executing the monochrome mode. In  FIG. 1 , since each of the image forming stations in the image forming unit  7  has the same configuration, reference numerals are given only to a part of the image forming stations and omitted for the other image forming stations for the convenience of the illustration. 
     The charging unit  23  is provided with a charging roller with a surface constituted by a elastic rubber. This charging roller is configured to be driven and rotated while abutting on the surface of the photosensitive drum  21  at its charging position. The charging roller is driven and rotated at a rotation speed in a driven direction with respect to the photosensitive drum  21  along with the rotation operation of the photosensitive drum  21 . In addition, this charging roller is connected to a charging bias generating unit (not shown), receives a charging bias supplied from the charging bias generating unit, and charges the surface of the photosensitive drum  21  at the charging position where the charging unit  23  abuts on the photosensitive drum  21 . 
     The line head  29  is arranged so as to be spaced 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 parallel or substantially parallel to the sub-scanning direction SD. This line head  29  is provided with a plurality of light emitting elements, and each of the light emitting elements emits light in accordance with the video data VD from the head controller HC. An electrostatic latent image is formed on the surface of the photosensitive drum  21  by irradiating the charged surface of the photosensitive drum  21  with the light from the light emitting elements. 
     The developing unit  25  includes a developing roller  251  which carries a toner on its surface. The charged toner is moved from the developing roller  251  to the photosensitive drum  21  at a developing position where the developing roller  251  abuts on the photosensitive drum  21 , by a developing bias applied to the developing roller  251  from a developing bias generating unit (not shown) electrically connected to the developing roller  251 , and the electrostatic latent image formed by the line head  29  is visualized. 
     The toner image visualized at the developing position in this manner is transported in the rotation direction D 21  of the photosensitive drum  21 , and then primarily transferred onto the transfer belt  81  at primary transfer positions TR 1  at each of which the transfer belt  81  abuts on the respective photosensitive drums  21 . 
     In this embodiment, a photosensitive cleaner  27  is provided on the downstream side of the primary transfer position TR 1  in the rotation direction D 21  of the photosensitive drum  21  and on the upstream side of the charging unit  23  so as to abut on the surface of the photosensitive drum  21 . This photosensitive cleaner  27  abuts on the surface of the photosensitive drum and removes the toner remaining on the surface of the photosensitive drum  21  after the primary transfer. 
     The transfer belt unit  8  includes a driving roller  82 , a driven roller  83  (blade opposing roller) arranged on the left side of the driving roller  82  in  FIG. 1 , and a transfer belt  81  which is stretched over these rollers and circularly driven in a direction of an arrow D 81  in the drawing (transport direction). The transfer belt unit  8  is provided in the inner side of the transfer belt  81 , and four primary transfer rollers  85 Y,  85 M,  85 C, and  85 K are respectively arranged so as to oppose the respective photosensitive drums  21  included in the respective image forming stations Y, M, C, and K at the time of mounting a photosensitive cartridge while making a one-to-one relationship. These primary transfer rollers  85  are electrically connected to primary transfer bias generating units (not shown), respectively. At the time of executing the color mode, all the primary transfer rollers  85 Y,  85 M,  85 C, and  85 K are positioned on the sides of the image forming stations Y, M, C, and K as shown in  FIG. 1 , the transfer belt  81  is pushed toward the photosensitive drums  21  included in the image forming stations Y, M, C, and K so as to abut on them, and primary transfer positions TR 1  are formed between the respective photosensitive drums  21  and the transfer belt  81 . A primary transfer bias is applied from the primary transfer bias generating unit to the primary transfer rollers  85  at an appropriate timing, and the toner image formed on the surface of the respective photosensitive drums  21  is transferred onto the surface of the transfer belt  81  at the corresponding primary transfer positions TR 1  to thereby form a color image. 
     On the other hand, at the time of executing the monochrome mode, the primary color transfer rollers  85 Y,  85 M, and  85 c are allowed to be spaced from the respectively opposing image forming stations Y, M, and C from among the four primary transfer rollers  85 , and only the primary monochrome transfer roller  85 K is allowed to abut on the image forming station K. Thus, only the monochrome image forming station K abuts on the transfer belt  81 . As a result, the primary transfer position TR 1  is formed only between the primary monochrome transfer roller  85 K and the image forming station K. The primary transfer bias is applied from the primary transfer bias generating unit to the primary monochrome transfer roller  85 K at an appropriate timing, and the toner image formed on the surface of the respective photosensitive drums  21  is transferred onto the surface of the transfer belt  81  at the primary transfer position TR 1  to thereby form a monochrome image. 
     Moreover, the transfer belt unit  8  is provided with a downstream guide roller  86  arranged on the downstream side of the primary monochrome transfer roller  85 K and the upstream side of the driving roller  82 . This downstream guide roller  86  is configured to abut on the transfer belt  81  on an internal common tangent of the primary transfer roller  85 K and the photosensitive drum  21  at the primary transfer position TR 1  formed when the primary monochrome transfer roller  85 K abuts on the photosensitive drum  21  of the image forming station K. 
     The driving roller  82  circularly drives the transfer belt  81  in the direction of the arrow D 81  in the drawing, and also functions as a backup roller for a secondary transfer roller  121 . A circumferential surface of the driving roller  82  is provided with a rubber layer formed thereon, which has a thickness of about  3  mm and a volume resistivity of not more than 1000 kΩ·cm and functions as a conduction path of the secondary transfer bias supplied from a secondary transfer bias generating unit (not shown) via the second transfer roller  121  by grounding it via a metal shaft. When the driving roller  82  is provided with a rubber layer which has a high frictional property and an impact absorbing property in the above manner, the impact generated when a sheet enters to the abutting portion (secondary transfer position TR 2 ) between the driving roller  82  and the secondary transfer roller  121  is hardly transmitted to the transfer belt  81 . Accordingly, it is possible to prevent the image quality from being deteriorated. 
     The sheet feeding unit  11  includes a sheet feeding section having a sheet feeding cassette  77  capable of holding sheets in a laminated manner and a pick-up roller  79  for feeding sheets one by one from the sheet feeding cassette  77 . The sheet fed from the sheet feeding section by the pick-up roller  79  is fed to the secondary transfer position TR 2  along the sheet guide member  15  after the adjustment of the sheet feeding timing by a resist roller pair  80 . 
     The secondary transfer roller  121  is provided so as to be freely separated from and abutted on the transfer belt  81 , and driven to be separated and abutted by a secondary transfer roller driving mechanism (not shown). The fixing unit  13  includes a freely rotatable heating roller  131  which installs a heat generating body such as a halogen heater or the like and a pressurizing section  132  for pressing and biasing this heating roller  131 . The sheet with a surface on which an image was secondarily transferred is guided by the sheet guiding member  15  to a nip section formed by the heating roller  131  and a pressurizing belt  1323  of the pressurizing section  132 , and the image is thermally fixed at a predetermined temperature at the nip section. The pressurizing section  132  includes two rollers  1321  and  1322  and a pressurizing belt  1323  stretched over these rollers. The nip portion formed by the heating roller  131  and the pressurizing belt  1323  is configured to be as large as possible by pressing the stretched belt surface, which is stretched by the two rollers  1321  and  1322  from among the surface of the pressurizing belt  1323 , toward the circumferential surface of the heating roller  131 . The sheet after this fixing process is transported to a paper discharge tray  4  provided on the upper surface portion of the housing main body  3 . 
     In this apparatus, a cleaner unit  71  is arranged so as to oppose the blade opposing 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 powder from the papers, toner remaining on the transfer belt after the secondary transfer, and the like by abutting its leading end portion on the blade opposing roller  83  via the transfer belt  81 . The foreign matters removed in this manner are recovered in the waste toner box  713 . 
       FIG. 3  is a partial perspective view illustrating the schematic configuration of the line head.  FIG. 3  shows a section of an end portion of the line head  29  in the longitudinal direction LGD (lower left end portion in  FIG. 3 ) for allowing the configuration of the line head  29  in the thickness direction TKD to be easily understood. Here, it is assumed that the thickness direction TKD is a direction perpendicular or substantially perpendicular to the longitudinal direction LGD and the width direction LTD, and a direction in which light emitting elements E, which will be described later, emit light (that is, a direction directing from the line head  29  toward the photosensitive drum  21 ). In the following description of the embodiment, the downstream side in the thickness direction TKD (upper side in  FIG. 3 ) will be referred to as “one side (in the thickness direction TKD)”, and the upstream side in the thickness direction TKD (lower side in  FIG. 3 ) will be referred to as “the other side (in the thickness direction TKD)”. In addition, the surface on one side of the substrate or a plate will be referred to as a front surface, and the surface on the other side of the substrate or the plate will be referred to as a rear surface. 
     This thickness direction TKD is parallel with optical axes (optical axes OAa, OAb, and OAc in  FIG. 7 ) of an image forming optical system constituted by a lens LS 1  in a lens array LA 1  and a lens LS 2  in a lens array LA 2 . Here, the optical axis is defined as follows. In many cases, the image forming optical system is plane-symmetrical (reflective symmetry) with respect to a symmetry plane which is perpendicular to the main scanning direction MD, and also plane-symmetrical (reflective symmetry) with respect to a symmetry plane which is perpendicular to the sub-scanning direction SD. As described above, the image forming optical system includes a first symmetry plane which is perpendicular to the main scanning direction and a second symmetry plane which is perpendicular to the sub-scanning direction SD intersecting the main scanning direction MD at a right angle, and a line of intersection between the first symmetry plane and the second symmetry plane is defined. When the image forming optical system is rotationally symmetrical, the line of intersection between the first symmetry plane and the second symmetry plane coincides with the optical axis. When the image forming optical system is not rotationally symmetrical, the optical axis of the image forming optical system is not defined in some cases when strictly speaking. However, the aforementioned line of intersection may be regarded as the optical axis in such cases. 
     The line head  29  has a schematic configuration in which a head substrate  293 , a light shielding member  297 , a lens array LA 1 , and a lens array LA 2  are arranged in this order in the thickness direction TKD. A predetermined plural number of light emitting elements E are made into a light emitting element group EG, and some light emitting element groups EG are two-dimensionally and discretely arranged on the rear surface of the head substrate  293 . A sealing member  294  for sealing the plurality of light emitting elements E is attached to the rear surface of the head substrate  293 . Moreover, a rigid member  299  for supporting the aforementioned respective members constituting the line head  29  is attached to the rear surface of this sealing member  294 . 
     A spacer SP 1  is provided between the head substrate  293  and the lens array LA 1 . This spacer SP 1  defines the space between the head substrate  293  and the lens array LA 1 . In addition, the light shielding member  297  is arranged between the head substrate  293  and the lens array LA 1 , and the spacer SP 1  supports the lens array LA 1  with a small space between the lens array LA 1  and the light shielding member  297  on one side in the thickness direction TKD. A spacer SP 2  is provided between the lens array LA 1  and the lens array LA 2 , and this spacer  2  supports the lens array LA 2  while defining the space between the lens array LA 1  and the lens array LA 2 . 
     In the line head  29 , the head substrate  293 , the light shielding member  297 , and the lens arrays LA 1  and LA 2  are arranged in this order as described above. The light from the light emitting elements E on the head substrate  293  transmits through light guiding holes  2971  of the light shielding member  297 , and an image is formed by the lenses LS 1  and LS 2  in the lens arrays LA 1  and LA 2 . Next, detailed configuration of the respective members will be described with reference to  FIGS. 3 ,  4 , and  5 . 
       FIG. 4  is a partial plan view of the head substrate  293  when seen from the thickness direction TKD, and corresponds to the case of seeing the rear surface  293 - t  of the head substrate  293  through other components from one side (upper side in  FIG. 3 ) in the thickness direction TKD.  FIG. 5  is a partial sectional view of the line head taken along the line V-V, and corresponds to the case of seeing the section in the longitudinal direction LGD (main scanning direction MD). This sectional view taken along the line V-V passes through respective geometric gravity centers (or respective lens centers) of three light emitting element groups EG (or three lenses LS 1 , and the like) which are arranged in one column while being spaced with each other by a distance Dg in the longitudinal direction LGD and by a distance Dt in the width direction LTD. The direction Dlsc shown in  FIGS. 4 and 5  is a direction parallel to the line V-V. Moreover, one-dotted dashed lines in  FIG. 4  represent both the lenses LS 1  and the lenses LS 2  in order to show the positional relationship of the light emitting element groups EG formed on the head substrate  293 , the lenses LS 1  formed on the lens array LA 1 , and the lenses LS 2  formed on the lens array LA 2 . In addition, the lenses LS 1  and LS 2  are shown in the same drawing in order to show the positional relationship therebetween, which does not mean that the lenses LS 1  and LS 2  are formed on the rear surface  293 - t  of the head substrate ( FIG. 5 ). In  FIG. 5 , light permeable members (that is, transparent members) are shown by being hatched with plural dots. 
     The head substrate  293  is made of a glass substrate through which the light is permeable (light permeable substrate). A plurality of light emitting elements E, which are bottom emission type organic EL (Electro-Luminescence) elements, are formed on the rear surface  293 - t  of the head substrate, and sealed by the sealing member  294  ( FIGS. 3 and 5 ). Each of the plurality of light emitting elements E has the same light emitting spectrum, and emits a light beam toward the surface of the photosensitive drum  21 . As shown in  FIG. 4 , the plurality of the light emitting elements E formed on the rear surface  293 - t  of the head substrate is arranged so as to have a group structure. That is, fifteen light emitting elements E are arranged in a two-row zigzag manner in the longitudinal direction LGD to constitute one light emitting element group EG, and a plurality of light emitting element groups EG is discretely arranged in a three-row zigzag manner in the longitudinal direction LGD. 
     More specifically, this arrangement can be described as follows. That is, in the respective light emitting element groups EG, fifteen light emitting elements E are arranged in the positions which are different from each other in the longitudinal direction LGD, and the distance in the longitudinal direction LGD between two light emitting elements E and E, whose positions are adjacent to each other in the longitudinal direction LGD, corresponds to a distance between the elements Pel (in other words, fifteen light emitting elements E are arranged at the pitch Pel in the longitudinal direction LGD in the respective light emitting element groups EG). Moreover, a plurality of light emitting element group EG is discretely arranged along the longitudinal direction LGD while being spaced to each other by a distance between the groups Peg, which is longer than the distance between the elements Pel, to constitute one row of the light emitting element group GRa, or the like. Furthermore, three rows of light emitting element groups GRa, GRb, and GRc are discretely arranged in different positions in the width direction LTD while being spaced by a distance Dt, and mutually shifted by a distance Dg in the longitudinal direction LGD. As described above, three light emitting element groups EG are arranged in one column in the direction Dlsc while being spaced by the distance Dg in the longitudinal direction LGD and by the distance Dt in the width direction LTD. 
     Here, the distance between the elements Pel can be obtained as a distance between the geometric gravity centers of the two target light emitting elements E in the longitudinal direction LGD. In addition, the distance between the groups Peg can be obtained as a distance between the geometric gravity center of the light emitting element E, which is positioned in the end portion on the other side of the light emitting element group EG positioned on one side in the longitudinal direction LGD, and the geometric gravity center of the light emitting element E, which is positioned in the end portion on one side of the light emitting element group EG positioned on the other side in the longitudinal direction LGD, from among the two target light emitting element group EG. In addition, the distance Dg can be obtained as a distance in the longitudinal direction between the respective geometric gravity centers of the 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 a distance in the width direction LTD between the respective geometric gravity centers of the two light emitting element groups EG whose positions in the width direction LTD are adjacent to each other. 
     As described above, a plurality of light emitting element groups EG is two-dimensionally and discretely arranged on the rear surface  293 - t  of the head substrate  293 . Meanwhile, a light shielding member  297  is arranged on the front surface  293 - h  of the head substrate  293 . A plurality of light guiding holes  2971  is formed in the shielding member  297  so as to pass therethrough in the thickness direction TKD. The respective light guiding holes  2971  have a circular shape in a plan view from the thickness direction TKD, and a black coating was made on its inner wall. One light guiding hole  2971  is formed for each of the light emitting element groups EG. That is, one light guiding hole  2971  is opened for one light emitting element group EG. The light shielding member  297  is abutted on and fixed to the front surface  293 - h  of the head substrate in a state in which the light guiding holes  2971  are opened to the light emitting element groups EG. 
     Such a light shielding member  297  is provided in order to prevent so-called stray light from being incident to the lenses LS 1  and LS 2 . That is, a dedicated image forming optical system constituted by a pair of the lens LS 1  and the lens LS 2  is provided for each of the light emitting element groups EG. In such a configuration, it is preferable that the light beam is incident only to the image forming optical system LS 1 , LS 2  provided in the light emitting element group EG, which is the light emitting source of the light beam, to form an image. However, a part of the light beam does not direct to the image forming optical system LS 1 , LS 2  provided in the light emitting element group EG, which is the light emitting source of the light beam, and becomes stray light. If the stray light is incident to the image forming optical system LS 1 , LS 2  provided in the light emitting element group EG, which is not the light emitting source of the stray light, there is a fear that a so-called ghost may occur. In order to solve this problem, a light shielding member  297  is provided between the light emitting element group EG and the image forming optical system LS 1 , LS 2  in this embodiment. Since this light shielding member  297  is provided with the light guiding holes  2971  with inner walls, for each of which a black coating was made, so as to open to the light emitting element groups EG, most of the stray light is absorbed by the inner walls of the light guiding holes  2971 . As a result, it is possible to prevent the aforementioned ghost, and thereby to achieve a satisfactory exposure operation. 
     As described above, the lens arrays LA 1  and LA 2  are provided on one side in the thickness direction TKD of the head substrate  293  and the light shielding member  297 , and supported by the spacers SP 1  and SP 2 , respectively. Hereinafter, the detailed description will be made of the supporting structures for the lens arrays LA 1  and LA 2  with reference to  FIG. 6  in addition to  FIGS. 3 to 5 . 
       FIG. 6  is a partial side view of the line head, and corresponds to the case of seeing the line head  29  in a plan view from the width direction LTD. A plurality of spacers SP 1  with the same shape and size is arranged in a column while being spaced to each other at an interval CL 1  in the longitudinal direction LGD on the front surface of the head substrate  293 . This column of the spacers SP 1  is provided on each side of the width direction LTD ( FIGS. 3 and 5 ). As described above, two columns of the spacer SP 1  are arranged so as to interpose in the width direction an area, in which the light emitting elements E are formed, on the rear surface  293 - t  of the head substrate when seen in a plan view from the thickness direction TKD (in other words, two columns are arranged so as to interpose the shielding member  297  in the width direction LTD). These spacers SP 1  are fixed to the front surface  293 - h  of the head substrate  293  by an adhesive, or the like. 
     The lens array LA 1  is bridged over the spacers SP 1 , which are arranged in two columns, in the width direction LTD in this manner. With this configuration, the lens array LA 1  is positioned on one side in the thickness direction TKD of the head substrate  293 . At this time, the lens array LA 1  is arranged such that the area in the lens array LA 1 , in which the lenses LS 1  are formed, is positioned between the two columns of the spacers SP 1  arranged in the width direction LTD. This lens array LA 1  includes a glass substrate SB with a parallelogram shape whose opposite ends in the longitudinal direction LGD were obliquely cut (so as to be parallel to the direction Dlsc). A plurality of lenses LS 1  formed by photo-curable resin is arranged in arrays on the rear surface of this glass substrate SB. The plurality of lenses LS 1  is arranged in a three-row zigzag manner so as to correspond to the arrangement of the opposing light emitting element groups EG ( FIG. 4 ). 
     As shown in  FIGS. 3 and 6 , the plurality of lens arrays LA 1  is arranged in the longitudinal direction LGD. That is, the spacers SP 1  support the plurality of lens arrays LA 1  arranged in the longitudinal direction LGD to constitute one lens array L-LA 1  with a long length in this embodiment. In addition, a length of a spacer SP 1  with a hexahedron shape is shorter than a length of an end side in the width direction LTD of the lens array LA 1  in the longitudinal direction LGD, and one lens array LA 1  is supported by a plurality of spacers SP 1  arranged in the longitudinal direction LGD. Specifically, a center spacer SP 1 - b  from among these spacers SP 1  supports a substantially central portion of the lens array LA 1  in the longitudinal direction LGD, and an end portion spacer SP 1 - a  supports two lens arrays LA 1  and LA 1  which are adjacent to each other in the longitudinal direction LGD so as to cross over a gap BD 1  between these two lens arrays LA 1  and LA 1 . Moreover, the spacers SP 1  and the lens arrays LA 1  are fixed by an adhesive or the like. 
     A plurality of spacers SP 2  with the same shape and size are arranged in a column while being spaced to each other by an interval CL 2  in the longitudinal direction LGD, on one side surface of the lens array L-LA 1  with a long length, which is configured as described above, in the thickness direction TKD. This column of the spacer SP 2  is provided on each side of the width direction LTD ( FIGS. 3 and 5 ). With this configuration, two columns of the spacers SP 2  are arranged so as to interpose an area in the lens arrays LA 1 , in which the lenses LS 1  are formed, in the width direction LTD when seen in a plan view from the thickness direction TKD. These spacers SP 2  are fixed on the front surface of the glass substrates SB of the lens arrays LA 1  by an adhesive or the like. 
     The lens array LA 2  is bridged over the spacers SP 2 , which are arranged in two columns, in the width direction LTD in this manner. With this configuration, the lens array LA 2  is positioned on one side in the thickness direction TKD of the lens array LA 1 . At this time, the lens array LA 2  is arranged such that the area in the lens array LA 2 , in which the lenses LS 2  are formed, is positioned between the two columns of the spacers SP 2  arranged in the width direction LTD. This lens array LA 2  includes a glass substrate SB with a parallelogram shape whose opposite ends in the longitudinal direction LGD were obliquely cut (so as to be parallel to the direction Dlsc). A plurality of lenses LS 2  formed by photo-curable resin is arranged in arrays on the rear surface of this glass substrate SB. The plurality of lenses LS 2  is arranged in a three-row zigzag manner so as to correspond to the arrangement of the opposing light emitting element groups EG ( FIG. 4 ). 
     As shown in  FIGS. 3 and 6 , the plurality of lens arrays LA 2  is arranged in the longitudinal direction LGD. That is, the spacers SP 2  support the plurality of lens arrays LA 2  arranged in the longitudinal direction LGD to constitute one lens array L-LA 2  with a long length in this embodiment. In addition, a length of a spacer SP 2  with a hexahedron shape is shorter than a length of an end side in the width direction LTD of the lens array LA 2  in the longitudinal direction LGD, and one lens array LA 2  is supported by a plurality of spacers SP 2  arranged in the longitudinal direction LGD. Specifically, a center spacer SP 2 - b  from among these spacers SP 2  supports a substantially central portion of the lens array LA 2  in the longitudinal direction LGD, and an end portion spacer SP 2 - a  supports two lens arrays LA 2  and LA 2  which are adjacent to each other in the longitudinal direction LGD so as to cross over a gap BD 2  between these two lens arrays LA 2  and LA 2 . Moreover, the spacers SP 2  and the lens arrays LA 2  are fixed by an adhesive or the like. 
     As described above, the two lens arrays LA 1  and LA 2  are arranged so as to oppose each other in the thickness direction TKD. As a result, the plurality of lenses LS 1  in the lens array LA 1  and the plurality of lenses LS 2  in the lens array LA 2  are opposed to each other while making a one-to-one relationship, and the positions of the lens arrays LA 1  and LA 2  are adjusted such that the opposing lenses LS 1  and LS 2  are interposed with each other in a plan view from the thickness direction TKD. 
     In this embodiment, a supporting glass SS with a long length in the longitudinal direction LGD is also provided. Specifically, this supporting glass SS is formed so as to be longer than the length of the lens array LA 2  in the longitudinal direction LGD, and has substantially the same length as that of the lens array L-LA 2  with a long length. This supporting glass SS is attached to one side surface of the lens array L-LA 2  with a long length, and supports the plurality of lens arrays LA 2  from the opposite side of the spacers SP 2 . A surface SS- h  (one side plane) of the supporting glass SS opposes the surface of the photosensitive drum  21  with a clearance. 
     In this embodiment, the lenses LS 1  and LS 2 , which oppose each other in the thickness direction TKD, constitute one image forming optical system. This image forming optical system is for forming an inversed reduction image, and the lateral magnification is a negative value and has an absolute value of less than one. Accordingly, the light beams emitted from the light emitting elements E transmit through the lenses LS 1  and LS 2 , are then emitted from the front surface SS- h  of the supporting glass SS, and are irradiated onto the surface of the photosensitive drum  21  as spots ST ( FIG. 5 ). As disclosed in  FIG. 11  of JP-A-2008-036937, it is possible to form a line latent image extending in the main scanning direction MD by controlling the light emitting of the respective light emitting elements E in accordance with the movement of the surface of the photosensitive drum  21  in the sub-scanning direction SD. 
       FIG. 7  is a detailed partial sectional view of the line head taken along the line VII-VII, and corresponds to the case of seeing the sectional view from the longitudinal direction LGD (main scanning direction MD).  FIG. 7  does not show the light shielding member  297 . The detailed configuration of the line head  29  will be described with reference to the same drawing. As described above, two lens arrays LA 1  and LA 2  are arranged so as to oppose each other in the thickness direction TKD, and the lenses LS 1  and LS 2  are arranged in the respective lens arrays LA 1  and LA 2 . The two lenses LS 1  and LS 2  opposing each other in the thickness direction TKD constitute one image forming optical system. In the same drawing, reference numerals OAa, OAb, and OAc are respectively given to the optical axes of the image forming optical systems in this order from the other side to one side of the width direction LTD, and a reference numeral Rls is given to a lens forming area in which the lenses LS 1  and LS 2  are formed in the plan view from the thickness direction TKD. 
     As shown in  FIG. 7 , the spacers SP 1  are arranged on both sides of the lens forming area Rls in the width direction LTD on the front surface  293 - h  of the head substrate, and the lens array LA 1  is bridged over these spacers SP 1  and SP 1 . The spacers SP 2  are arranged on both sides of the lens forming area Rls in the width direction LTD on the front surface of the lens array LA 1 , and the lens array LA 2  is bridged over these spacers SP 2  and SP 2 . The spacer SP 1  has a hexahedron shape with a width Wsp 1  in the width direction LTD, and is formed by a metal such as iron, or the like. The spacer SP 2  has a hexahedron shape with a width Wsp 2  in the width direction LTD, and is formed by a material with a thermal conductivity which is lower than that of the spacer SP 1 . In addition, the width Wsp 1  of the spacer SP 1  is equal to the width Wsp 2  of the spacer SP 2 . 
     As described above, the spacers SP 1  and SP 2  are arranged so as to be laminated in the thickness direction TKD via the lens array LA 1  in each of one side and the other side of the lens forming area Rls. The spacers SP 1  and SP 2 , which are arranged so as to be laminated as described above, are shifted with each other in the width direction LTD, and arranged in the different positions when seen from the thickness direction TKD (optical axis direction). The expression that “the spacer SP 1  is shifted with respect to the spacer SP 2  toward one side (the other side) in the width direction LTD” used in this specification means the state in which in the width direction LTD, an inner wall IW 1  of the spacer SP 1  is shifted with respect to an inner wall IW 2  of the spacer SP 2  toward one side (the other side), and an outer wall OW 1  of the spacer SP 1  is also shifted with respect to an outer wall OW 2  of the spacer SP 2  toward one direction (the other side). Here, the inner walls IW 1  and IW 2  of the spacers SP 1  and SP 2  are the wall surfaces of the spacers SP 1  and SP 2  on the side of the lens forming area Rls, and the outer walls OW 1  and OW 2  of the spacers SP 1  and SP 2  are the wall surfaces of the spacers SP 1  and SP 2  on the other side of the lens forming area Rls. As will be described later with reference to  FIG. 9 , the expression that “the spacers SP 1  and SP 2  are arranged in the different positions when seen from the thickness direction TKD (optical axis direction)” means the case in which there is at least a part where the spacers SP 1  and SP 2  are not overlapped with each other when seen through other components in a plan view from the thickness direction TKD (optical axis direction). On the other hand, the expression that “the spacers SP 1  and SP 2  are arranged in the same position when seen from the thickness direction TKD (optical axis direction)” means the case in which the entire part of the spacer SP 2  is positioned completely within the spacer SP 1  when seen through other components in a plan view from the thickness direction TKD (optical axis direction). 
     Hereinafter, the arrangement of the spacers SP 1  and SP 2  will be described while exemplifying the spacers SP 1  and SP 2  arranged on the other side in the width direction LTD as their representative. As shown in  FIG. 7 , the spacer SP 1  is arranged so as to be shifted with respect to the spacer SP 2  toward the other side in the width direction LTD. That is, the inner wall IW 1  of the spacer SP 1  is shifted with respect to the inner wall IW 2  of the spacer SP 2  toward the other side in the width direction LTD by a shift amount sfi, and the outer wall OW 1  of the spacer SP 1  is shifted with respect to the outer wall OW 2  of the spacer SP 2  toward the other side in the width direction LTD by a shift amount sfo. Since the width Wsp 1  of the spacer SP 1  is equal to the width Wsp 2  of the spacer SP 2 , the shift amount sfi of the inner wall surface is equal to the shift amount sfo of the outer wall surface. As described above, the spacer SP 1  is arranged so as to be shifted with respect to the spacer SP 2  toward the outer side in the width direction LTD. As a result of this arrangement, the distances da 1 , db 1 , and dc 1  between the spacer SP 1  and the respective optical axes OAa, OAb, and OAc of the image forming optical systems are longer than the distance da 2 , db 2 , and dc 2  between the spacer SP 2  and the respective optical axes OAa, OAb, and OAc of the image forming optical systems (that is, da 1 &gt;da 2 , db 1 &gt;db 2 , dc 1 &gt;dc 2 ). 
     The spacers SP 1  and SP 2  on one side in the width direction LTD are also arranged in the same arrangement, and the spacers SP 1  are also arranged on one side in the width direction LTD so as to be shifted with respect to the spacer SP 2  toward the outer side in the width direction LTD. As a result, the interval between the spacers SP 2  and SP 2  arranged on the opposite sides in the width direction LTD is narrower than the interval between the spacers SP 1  and SP 1  arranged on the opposite sides in the width direction LTD. In this embodiment, the widths Wla 1  and Wla 2  of the lens arrays LA 1  and LA 2  in the width direction LTD are allowed to be changed in accordance with the difference in the intervals of the spacers SP 1  and the spacers SP 2 , which support the lens arrays LA 1  and LA 2 , respectively, and the width Wla 2  of the lens array LA 2  is set to be narrower than the width Wla 1  of the lens array LA 1  (width Wla 2 &lt;width Wla 1 ). 
     As described above, according to this embodiment, the spacer SP 1  is arranged so as to be shifted with respect to the spacer SP 2 , and the spacers SP 1  and SP 2  are arranged in the different positions when seen from the thickness direction TKD (optical axis direction). Next, the description will be made of the reason why such arrangements are employed for the spacers SP 1  and SP 2  with reference to  FIG. 8  in addition to  FIG. 7 .  FIG. 8  is a diagram explaining a reason why the spacers SP 1  and SP 2  are arranged in different positions when seen from the thickness direction TKD (optical axis direction), and a reference example (the left half of the same drawing) is also shown in addition to the structure of this embodiment (the right half of the same drawing). The arrows with the reference numerals Q 1  to Q 3  in  FIG. 8  show the heat conducted in the arrow directions, and the width of the respective arrows schematically represents the heat amount of the heat Q 1  to Q 3 , respectively. 
     In the line head shown in  FIGS. 7 and 8 , when the light emitting elements E (light emitting element groups EG) formed on the head substrate  293  generate heat along with the emission of the light, the heat Q 1  from the light emitting elements E (light emitting element groups EG) is conducted to the lens arrays LA 1  via the spacers SP 1  in some cases. In such cases, if the heat is further conducted from the lens arrays LA 1  to the lens arrays LA 2  via the spacers SP 2 , the following problem may occur. 
     That is, in this line head  29 , the light from the light emitting element E is emitted from the lens LS 1 , then incident to the lens LS 2 , and thereby subjected to the optical action by the image forming optical system constituted by these lenses LS 1  and LS 2 . Moreover, the absolute value of the lateral magnification of this image forming optical system is less than one. With such a configuration, the position and the surface precision of the lens LS 2  (the lens on the side of the image surface from among the lenses constituting the image forming optical system) greatly affect the optical performances such as the image forming performance of the optical system, and the like. For this reason, if the heat is conducted from the head substrate  293  to the lens array LA 1  via the spacer SP 1 , and further to the lens array LA 2  via the spacer SP 2 , and the thermal deformation occurs in the lens array LA 2 , the position of the lens LS 2  may be deviated, or the surface precision of the lens may be deteriorated. As a result, there is a fear that the optical performances of the image forming optical system may be degraded. 
     In order to solve this problem, the spacers SP 1  and SP 2  are arranged in the different positions when seen from the thickness direction TKD (optical axis direction) in this line head  29 . When the spacers SP 1  and SP 2  are arranged in different positions in the thickness direction TKD (optical axis direction) in this manner, it is possible to suppress the thermal conduction directing from the spacer SP 1  to the spacer SP 2  via the lens array LA 1 . Accordingly, it is possible to suppress the thermal conduction to the lens array LA 2  via the spacer SP 2 . The description will be made while comparing the reference example and the embodiment with reference to  FIG. 8 . As shown in  FIG. 8 , the heat amount of the heat Q 2  conducted to the lens array LA 2  is relatively large in the reference example in which the spacer SP 1  is not shifted with respect to the spacer SP 2 . On the other hand, the heat amount of the heat Q 3  conducted to the lens array LA 2  is suppressed to be a relatively small amount in the embodiment in which the spacer SP 1  is shifted with respect to the spacer SP 2 . Accordingly, it is possible to suppress the thermal deformation of the lens array LA 2 , and to thereby suppress the positional deviation of the lens LS 2 . As a result, it is possible to allow the image forming optical system constituted by the lenses LS 1  and LS 2  to exhibit its appropriate optical performances. 
     In this embodiment, the spacers SP 1  are arranged in the positions more distant from (the axis of) the image forming optical system than the spacers SP 2  in the width direction LTD. Such a configuration is advantageous in suppressing the influence of the heat conducted to the spacer SP 1  on the optical performances of the image forming optical system. 
     It is preferable that the invention is applied to the line head  29  whose spacer SP 1  is made of metal as in this embodiment. That is, since the spacer SP 1  made of metal has a high thermal conductivity, the thermal conduction to the lens array  2  via the above-mentioned conduction path (the light emitting element E→the head substrate  293 →the spacer SP 1 →the lens array LA 1 →the spacer SP 2 ) easily occurs. Accordingly, it is preferable to secure the appropriate optical performances of the optical system constituted by the lenses LS 1  and LS 2  by applying the invention to such a line head  29  to suppress the thermal conduction to the lens array LA 2 . 
     In addition, the line head  29  is arranged so as to be close to the photosensitive drum  21  inside the image forming apparatus in order to irradiate the surface of the photosensitive drum  21  with the spots ST. Accordingly, the lens array LA 2  is arranged in an opposing manner so as to be close to the photosensitive drum  21 . In this embodiment, the width Wla 2  of the lens array LA 2  is narrower than the width Wla 1  of the lens array LA 1 . With this configuration, it is possible to suppress the width of the lens array LA 2 , which opposes the photosensitive drum  21  so as to be close thereto, in the rotation direction of the photosensitive drum  21  (sub-scanning direction SD). Therefore it is possible to sufficiently secure the space for arranging other functional units (charging unit  23 ) and the like in the circumference of the line head  29 , and to thereby enhance the degree of freedom in the layout of the line head  29  with respect to the photosensitive drum  21 . 
     Others 
     As described above, the line head  29  in the embodiment corresponds to the “exposure head” in the invention. The head substrate  293  corresponds to the “light emitting element substrate” in the invention, the lens array LA 1  corresponds to the “first lens array” in the invention, the lens array LA 2  corresponds to the “second lens array” in the invention, the lens LS 1  corresponds to the “first lens” in the invention, the lens LS 2  corresponds to the “second lens” in the invention, the spacer SP 1  corresponds to the “first spacer” in the invention, the spacer SP 2  corresponds to the “second spacer” in the invention, and the image forming optical system constituted by the lenses LS 1  and LS 2  corresponds to the “optical system” in the invention. In addition, the longitudinal direction LGD and the main scanning direction MD correspond to the “first direction” in the invention, and the width direction LTD and the sub-scanning direction SD correspond to the “second direction” in the invention. 
     The invention is not limited to the above embodiment, and various modification can be added to the above embodiment without departing from the gist. For example, although the width Wsp 2  of the spacer SP 2  is equal to the width Wsp 1  of the spacer SP 1  in the above embodiment, the relationship between the widths of the spacers SP 1  and SP 2  are not limited thereto. The width Wsp 2  of the spacer SP 2  may be narrower than the width Wsp 1  of the spacer SP 1 . With such a configuration, it is possible to further suppress the thermal conduction from the spacer SP 1  to the lens array LA 2  via the lens array LA 1  and the spacer SP 2 . As a result, it is possible to further suppress the positional deviation of the lens LS 2  arranged in the lens array LA 2 , and to thereby allow the optical performances of the optical system constituted by the lenses LS 1  and LS 2  to be more appropriate. 
     In the above embodiment, the spacers SP 1  and SP 2  are arranged in different positions when seen from the thickness direction TKD (optical axis direction) by allowing the spacers SP 1  and SP 2  to be shifted with each other in the width direction LTD. However, various modifications can be made for the arrangement relationship between the spacers SP 1  and SP 2 . In short, it is possible to achieve the above effect by arranging the spacers SP 1  and SP 2  in the different positions when seen from the thickness direction TKD (optical axis direction), that is, by arranging the spacers SP 1  and SP 2  in a manner which will be described below with reference to  FIG. 9 . 
       FIG. 9  is a diagram illustrating an arrangement relationship between the spacer SP 1  and the spacer SP 2  when seen through other components in a plan view from the thickness direction TKD (optical axis direction), and shows both the case (upper portion of the same drawing) in which the spacers SP 1  and SP 2  are arranged in the different positions when seen from the thickness direction TKD (optical axis direction) and the case (lower portion of the same drawing) in which the spacers SP 1  and SP 2  are arranged in the same position when seen from the thickness direction TKD (optical axis direction).  FIG. 9  shows the joining portion between the spacer SP 1  and the lens array LA 1  as the representative of the spacer SP 1 , and shows the joining portion between the spacer SP 2  and the lens array LA 2  as the representative of the spacer SP 2 . In addition, the joining portion between the spacer SP 1  and the lens array LA 1  (first joining portion) is shown by being hatched with a plurality of diagonal lines from the upper right side to the lower left side, and the joining portion between the spacer SP 2  and the lens array LA 2  (second joining portion) is shown by being hatched with a plurality of diagonal lines from the upper left side to the lower right side. The overlapping portion between the first joining portion and the second joining portion (in other words, the overlapping portion between the spacers SP 1  and SP 2 ) is shown by being hatched with a plurality of diagonal lines intersecting with each other. 
     In any of the four examples showing the arrangement relationships in the upper portion of the same drawing, the spacers SP 1  and SP 2  are partially overlapped with each other while the other portions are not overlapped with each other, and are arranged in different positions when seen through other components in a plan view from the thickness direction TKD (optical axis direction). As a result, the area of the overlapping portion between the first joining portion and the second joining portion is smaller than the area of any one of the areas of the first joining portion and the second joining portion, which is smaller than the other. On the other hand, in any of the four examples showing the arrangement relationships in the lower portion of the same drawing, the entire part of the spacer SP 2  is completely within the spacers SP 1 , and the spacers SP 1  and SP 2  are arranged in the same position when seen through other components in a plan view from the thickness direction TKD (optical axis direction). It is possible to achieve the above effects by arranging the spacers SP 1  and SP 2  in the different positions when seen from the thickness direction TKD (optical axis direction) as shown in the upper portion of the same drawing. 
     In addition, it is also applicable that a driving element such as a TFT (Thin Film Transistor) is provided on the rear surface  293 - t  of the head substrate  293  so as to cause the driving element to drive the light emitting element E. It is particularly preferable to apply the invention to such a configuration. That is, since the driving element generates heat along with the driving of the light emitting element, the heat from this driving element may be conducted to the lens array LA 2  via the above-mentioned conduction path. Accordingly, it is preferable to apply the invention to the line head  29  with a driving element arranged on the head substrate  293  in order to suppress the thermal conduction to the lens array LA 2 , and thereby to secure the optical performances of the image forming optical system constituted by the lenses LS 1  and LS 2 . 
     Although the above embodiment was described such that the supporting glass SS was provided, it is also applicable that the supporting glass SS is not provided. 
     In addition, various modifications can be made for the dimensional relationships of the respective members such as the lens array LA 1 , the lens array LA 2 , and the like, and the dimensional relationships other than the ones described above are also applicable. 
     Although the above embodiment was described such that the plurality of lens arrays LA 1  had the same shape and size, various modifications can be made regarding this configuration. In addition, similar modifications can be made for the plurality of lens arrays LA 2 . 
     Although the above embodiment was described such that the plurality of spacers SP 1  had the same shape and size, various modifications can be made regarding this configuration. In addition, similar modifications can be made for the plurality of spacers SP 2 . 
     Although the above embodiment was described such that the image forming optical system is for forming the inversed image, it is also applicable that the image forming optical system is for forming a normal image (that is, a non-inverted image). 
     Although the above embodiment was described such that the lenses LS 1  are formed on the rear surface (the other side surface in the thickness direction TKD) of the lens array LA 1 , the position for forming the lenses LS 1  is not limited thereto. The same is true for the lenses LS 2  in the lens array LA 2 . 
     Although the above embodiment was described such that the lenses are arranged in a three-row zigzag manner in each of the lens arrays LA 1  and LA 2 , the arrangement of the lenses is not limited thereto. 
     The lens arrays LA 1  and LA 2  in the above embodiment were described such that the lenses LS 1  and LS 2  made of resin are formed in the light permeable substrate SB made of glass. However, it is also applicable that each of the lens arrays LA 1  and LA 2  is integrally formed by one material. 
     Although the above embodiment was described such that the plurality of the light emitting element groups EG is arranged in a three-row zigzag manner, the arrangement of the plurality of light emitting element groups EG is not limited thereto. 
     The above embodiment was described such that the light emitting element group EG is constituted by fifteen light emitting elements E. However, the numbers of the light emitting elements E constituting the light emitting element group EG is not limited thereto. 
     Although the above embodiment was described such that the plurality of the light emitting elements E is arranged in a two-row zigzag manner in a light emitting element group EG, the arrangement of the plurality of light emitting elements E in the light emitting element group EG is not limited thereto. 
     The above embodiment was described such that the bottom emission type organic EL elements were used as the light emitting elements. However, the top emission type organic EL (Electro-Luminescence) elements may be used as the light emitting elements E. Alternatively, LEDs (Light Emitting Diodes) or the like other than the organic EL elements may be used as the light emitting elements E. 
     The entire disclosure of Japanese Patent Applications No. 2009-202447, filed on Sep. 2, 2009 is expressly incorporated by reference herein.