Patent Publication Number: US-2011069133-A1

Title: Image forming apparatus

Description:
BACKGROUND 
     1. Technical Field 
     The present invention relates to an image forming apparatus that exposes an image carrier by using an exposure head that forms an image of light from a light emitting device by an optical system. 
     2. Related Art 
     In an image forming apparatus described in JP-A-2008-221790, an exposure head (line head in JP-A-2008-221790) including a light emitting device and an image forming optical system is arranged to face an image carrier, and when the image forming optical system forms an image of light from the light emitting device, a spot is formed on (a surface of) the image carrier. The exposure head appropriately forms spots at positions corresponding to image data, so that a predetermined latent image can be formed on the surface of the image carrier. 
     In an image forming apparatus as described above, a distance between the image forming position of the light of the optical system and the image carrier is important. Because, if the distance is too large, the spot formed on the image carrier by the optical system blurs and a good latent image may not be formed. Therefore, a technique for positioning the exposure head and the image carrier so that the image forming position of the light of the optical system is located near (the surface of) the image carrier has been required. 
     SUMMARY 
     An advantage of some aspects of the invention is that it provides a technique capable of positioning the exposure head and the image carrier so that the image forming position of the light of the optical system is located near the image carrier and forming a good latent image by the exposure head. 
     To achieve the above object, the image forming apparatus according to an aspect of the invention includes: an exposure head that includes a light emitting device for emitting light having a first wavelength and a second wavelength and an optical system having a first lens surface through which the light emitted from the light emitting device enters and a second lens surface from which the light entered through the first lens surface is emitted, forms an image of the light having the first wavelength at a first image forming position, and forms an image of the light having the second wavelength at a second image forming position different from the first image forming position in an optical axis direction of the optical system; a latent image carrier onto which the light emitted from the second lens surface of the exposure head is irradiated and a latent image is formed; and a support member that supports the exposure head, wherein the support member supports the exposure head so that the exposure head and the latent image carrier have the following relationship: 
     d1&lt;di&lt;d2, 
     here, d1, di, and d2 are defined as follows: 
     d1: a distance between the second lens surface and the first image forming position in the optical axis direction, 
     d2: a distance between the second lens surface and the second image forming position in the optical axis direction, and 
     di: a distance between the second lens surface and the image carrier in the optical axis direction. 
     In the image forming apparatus constituted as described above, the optical system having the first lens surface and the second lens surface, and the light emitting device are provided, and the light that is emitted from the light emitting device and enters through the first lens surface is emitted from the second lens surface and irradiated onto the image carrier. The light emitting device according to an aspect of the invention emits the light having the first wavelength and the second wavelength, and the optical system forms images of the light of each wavelength at different positions (the first image forming position and the second image forming position) in the optical axis direction. The support member supports the exposure head having the optical system and the image carrier so that the relationship, d1&lt;di&lt;d2 is satisfied. Here, d1, di, and d2 are defined as follows: 
     d1: a distance between the second lens surface and the first image forming position in the optical axis direction, 
     d2: a distance between the second lens surface and the second image forming position in the optical axis direction, and 
     di: a distance between the second lens surface and the image carrier in the optical axis direction. 
     Specifically, in the image forming apparatus, the exposure head is positioned with respect to the image carrier so that (the surface of) the image carrier is located between the first image forming position and the second image forming position. By positioning the exposure head with respect to the image carrier in the manner described above, at least one of the two image forming positions (the first and the second image forming positions) can be located near (the surface of) the image carrier. Light formed into an image at the image forming position near the image carrier is used to form a spot on the image carrier, so that a good latent image with less blur can be formed. 
     The exposure head may include a second light emitting device that emits light and a second optical system that forms an image of the light emitted from the second light emitting device. In this case, the exposure head may be configured so that the image forming position at which the image of the light is formed by the second optical system may be located between the first image forming position and the second image forming position in the optical axis direction. In other words, according to an aspect of the invention, the exposure head is positioned with respect to the image carrier so that at least one of the two image forming positions (the first and the second image forming positions) of the optical system is located near (the surface of) the image carrier. Therefore, when the image forming position at which the image of the light is formed by the second optical system is located between the first image forming position and the second image forming position in the optical axis direction, the image forming position at which the image of the light is formed by the second optical system can be located near (the surface of) the image carrier, and as a result, a good latent image can be formed by the exposure head. 
     The range between the first image forming position and the second image forming position is a range between the first and the second image forming positions including the first image forming position and the second image forming position. 
     The support member may be configured to include a holding member that holds the exposure head and a contact member that is provided to the holding member and is in contact with the image carrier. By connecting the contact member coming into contact with the image carrier and the exposure head via the holding member in the manner described above, positioning accuracy of the exposure head with respect to the image carrier can be improved. 
    
    
     
       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 showing an example of an image forming apparatus to which the invention can be applied. 
         FIG. 2  is a block diagram showing an electrical configuration of the image forming apparatus of  FIG. 1 . 
         FIG. 3  is a partial perspective view showing an outline of a line head. 
         FIG. 4  is a partial plan view of a head substrate viewed from the thickness direction. 
         FIG. 5  is a partial cross-sectional view taken along V-V line of the line head. 
         FIG. 6  is a partial side view of the line head. 
         FIG. 7  is a longitudinal direction partial cross-sectional view showing a line head support mechanism. 
         FIG. 8  is a perspective view showing the line head support mechanism. 
         FIG. 9  is a diagram showing details of a support form of the line head. 
         FIG. 10  is an enlarged diagram of an area near image forming positions P 1  and P 2 . 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       FIG. 1  is a diagram showing an example of an image forming apparatus to which the invention can be applied.  FIG. 2  is a block diagram showing an electrical configuration of the image forming apparatus of  FIG. 1 . The image forming apparatus selectively performs a color mode in which a color image is formed by superimposing four color toners of black (K), cyan (C), magenta (M), and yellow (Y), and a monochrome color mode in which a monochrome image is formed by only black toner (K).  FIG. 1  is a diagram when performing the color mode. In the image forming apparatus, when an image forming instruction is provided from an external apparatus such as a host computer to a main controller MC including a CPU, a memory, and the like, the main controller MC provides a control signal or the like to an engine controller EC and provides video data VD corresponding to the image forming instruction to a head controller HC. In this case, every time the main controller MC receives a horizontal request signal HREQ from the head controller HC, the main controller MC provides the video data VD for one line in the main scanning direction MD to the head controller HC. The head controller HC controls line heads  29  of each color on the basis of the video data VD from the main controller MC and a vertical synchronous signal Vsync and a parameter value from the engine controller EC. Based on this, an engine section ENG performs a predetermined image forming operation and forms an image corresponding to the image forming instruction on a sheet such as copying paper, transfer paper, plain paper, and OHP transparent sheet. 
     A housing main body  3  of the image forming apparatus includes an electrical component box  5  in which a power supply circuit board, the main controller MC, the engine controller EC, and the head controller HC are mounted. An image forming unit  7 , a transfer belt unit  8 , and a paper feed unit  11  are also arranged in the housing main body  3 . In  FIG. 1 , a secondary transfer unit  12 , a fixing unit  13 , a sheet guide member  15  are arranged at the right side in the housing main body  3 . The paper feed unit  11  is configured to be attachable/detachable to/from an apparatus main body  1 . The paper feed unit  11  and the transfer belt unit  8  are configured so that they can be detached and repaired or replaced respectively. 
     An image forming unit  7  includes four image forming stations C (for cyan), M (for magenta), Y (for yellow), and K (for black) that form images of a plurality of different colors. Each image forming station Y, M, C, and K includes a photoconductor drum  21  having a cylindrical shape and a surface with a predetermined length in the main scanning direction MD. Each image forming station Y, M, C, and K forms a toner image of corresponding color on the surface of the photoconductor drum  21 . The photoconductor drum  21  is arranged so that the axis direction is in parallel or approximately in parallel with the main scanning direction MD. Each photoconductor drum  21  is connected to a dedicated driving motor thereof, and driven to rotate at a predetermined speed in a direction of an arrow D 21  in  FIG. 1 . Accordingly, the surface of the photoconductor drum  21  is transported in a sub-scanning direction SD perpendicular to or approximately perpendicular to the main scanning direction MD. A charging section  23 , the line head  29 , a developing section  25 , and a photoconductor cleaner  27  are arranged around the photoconductor drum  21  along the rotation direction. A charging operation, a latent image forming operation, and a toner developing operation are performed by these functional sections. Accordingly, a color image is formed by superimposing toner images formed by all the image forming stations Y, M, C, and K on a transfer belt  81  of the transfer belt unit  8  when performing the color mode, and a monochrome image is formed using only a toner image formed by the image forming station K when performing the monochrome mode. Since the respective image forming stations in the image forming unit  7  have the same configuration, reference characters are given to only some of the image forming stations while being not given to the other image forming stations in order to facilitate the diagrammatic representation in  FIG. 1 . 
     The charging section  23  includes a charging roller, the surface of which is made of an elastic rubber. This charging roller is configured to be rotated by being held in contact with the surface of the photoconductor drum  21  at a charging position. As the photoconductor drum  21  rotates, the charging roller is rotated at the same circumferential speed in a direction driven by the photoconductor drum  21 . This charging roller is connected to a charging bias generator (not shown in  FIG. 1 ) and charges the surface of the photoconductor drum  21  at the charging position where the charging section  23  and the photoconductor drum  21  are in contact with each other upon receiving a supply of a charging bias from the charging bias generator. 
     The line head  29  is arranged separately from the photoconductor drum  21 . The longitudinal direction of the line head  29  is in parallel with or approximately in parallel with the main scanning direction MD and the width direction of the line head  29  is in parallel with or approximately in parallel with the sub-scanning direction SD. The line head  29  includes a plurality of light emitting devices, and each light emitting device emits light in accordance with the video data VD from the head controller HC. The surface of the charged photoconductor drum  21  is irradiated by light beams emitted from the light emitting devices, and thereby a latent image is formed on the surface of the photoconductor drum  21 . 
     The developing section  25  includes a developing roller  251  whose surface carries toner. Charged toner moves from the developing roller  251  to the photoconductor drum  21  at a developing position where the developing roller  251  and the photoconductor drum  21  are in contact with each other by a developing bias applied to the developing roller  251  from a developing bias generator (not shown  FIG. 1 ) electrically connected to the developing roller  251 , and the latent image formed by the line head  29  is visualized. 
     The toner image visualized at the developing position is transported in the rotation direction D 21  of the photoconductor drum  21 , and thereafter primary-transferred to the transfer belt  81  at primary transfer positions TR 1  where the transfer belt  81  and each photoconductor drum  21  are in contact with each other. 
     In this embodiment, a photoconductor cleaner  27  is provided in contact with the surface of the photoconductor drum  21  on the downstream side of the primary transfer position TR 1  and on the upstream side of the charging section  23  in the rotation direction D 21  of the photoconductor drum  21 . The photoconductor cleaner  27  cleans and eliminates the toner remaining on the surface of the photoconductor drum  21  after the primary transfer by coming in contact with the surface of the photoconductor drum. 
     The transfer belt unit  8  includes a driving roller  82 , a driven roller  83  (blade-facing roller) arranged to the left of the driving roller  82  in  FIG. 1 , and the transfer belt  81  mounted on these rollers and driven to circulate in a direction of an arrow D 81  (transportation direction) shown in  FIG. 1 . The transfer belt unit  8  includes, on an inner side of the transfer belt  81 , four primary transfer rollers  85 Y,  85 M,  85 C, and  85 K that are arranged to face the respective photoconductor drums  21  included in the image forming stations Y, M, C, and K on one-on-one basis when a photoconductor cartridge is mounted. These primary transfer rollers  85  are electrically connected to a primary transfer bias generator (not shown in  FIG. 1 ). When performing the color mode, as shown in  FIG. 1 , by positioning all the primary transfer rollers  85 Y,  85 M,  85 C, and  85 K on the image forming stations Y, M, C, and K, the transfer belt  81  is pressed to be in contact with each photoconductor drum  21  included in the image forming stations Y, M, C, and K, and the primary transfer positions TR 1  are formed between each photoconductor drum  21  and the transfer belt  81 . By applying a primary transfer bias from the primary transfer bias generator to the primary transfer rollers  85  at an appropriate timing, toner images formed on each photoconductor drum  21  are transferred to the surface of the transfer belt  81  at the primary transfer positions TR 1  on each photoconductor drum  21 , and a color image is formed. 
     On the other hand, when performing the monochrome mode, the color primary transfer rollers  85 Y,  85 M, and  85 C among the four primary transfer rollers  85  are separated from the image forming stations Y, M, and C respectively facing the color primary rollers, and only the monochrome primary transfer roller  85 K is brought into contact with the image forming station K, so that only the monochrome image forming station K is brought into contact with the transfer belt  81 . As a result, the primary transfer position TR 1  is formed only between the monochrome primary transfer roller  85 K and the image forming station K. By applying a primary transfer bias from the primary transfer bias generator to the monochrome primary transfer roller  85 K at an appropriate timing, a toner image formed on the photoconductor drum  21  is transferred to the surface of the transfer belt  81  at the primary transfer position TR 1 , and a monochrome image is formed. 
     Further, the transfer belt unit  8  includes a downstream guide roller  86  arranged on the downstream side of the monochrome primary transfer roller  85 K and on the upstream side of the driving roller  82 . The downstream guide roller  86  is configured to be in contact with the transfer belt  81  on an internal common tangent of the primary transfer roller  85 K and the photoconductor drum  21  at the primary transfer position TR 1  formed by the monochrome primary transfer roller  85 K coming in contact with the photoconductor drum  21  of the image forming station K. 
     The driving roller  82  drives and circulates the transfer belt  81  in the direction of the arrow D 81  shown in  FIG. 1 , and also serves as a backup roller of a secondary transfer roller  121 . A rubber layer having the thickness of about 3 mm and volume resistivity equal to or lower than 1000 kΩ·cm is formed on a circumferential surface of the driving roller  82 . The driving roller  82  is grounded via a metal shaft to thereby form a conductive path for a secondary transfer bias supplied from a secondary transfer bias generator not shown in  FIG. 1  via the secondary transfer roller  121 . The rubber layer having high friction and impact absorption is provided to the driving roller  82  in this way. Consequently, impact caused when a sheet enters a contact portion between the driving roller  82  and the secondary transfer roller  121  (a secondary transfer position TR 2 ) is less easily transmitted to the transfer belt  81 , and thus, it is possible to prevent deterioration in an image quality. 
     The paper feed unit  11  includes a paper feed cassette  77  capable of holding stacked sheets and a paper feed section having a pickup roller  79  that feeds the sheets one by one from the paper feed cassette  77 . A paper feed timing of the sheet fed from the paper feed section by the pickup roller  79  is adjusted at a pair of registration rollers  80 , and then fed to the secondary transfer position TR 2  along the sheet guide member  15 . 
     The secondary transfer roller  121  is provided to freely separate from and come into contact with the transfer belt  81  and is driven to separate from and come into contact with the transfer belt  81  by a secondary transfer roller driving mechanism (not shown in  FIG. 1 ). The fixing unit  13  includes a heating roller  131  that incorporates a heating member such as a halogen heater and freely rotates and a pressing section  132  that presses and urges the heating roller  131 . The sheet on the surface of which an image is secondarily transferred is guided by the sheet guide member  15  to a nip portion formed by the heating roller  131  and a pressing belt  1323  of the pressing unit  132 . The image is thermally fixed at predetermined temperature in the nip portion. The pressing unit  132  includes two rollers  1321  and  1322  and the pressing belt  1323  mounted on these rollers. A belt stretched surface stretched by the two rollers  1321  and  1322  of the surface of the pressing belt  1323  is pressed against a circumferential surface of the heating roller  131 , so that the nip portion formed by the heating roller  131  and the pressing belt  1323  is largely secured. The sheet on which fixing processing is performed in this way is transported to a paper discharge tray  4  provided in an upper surface section of the housing main body  3 . 
     In this apparatus, a cleaner section  71  is disposed to face the blade-facing roller  83 . The cleaner section  71  includes a cleaner blade  711  and a waste toner box  713 . The cleaner blade  711  brings a distal end thereof into contact with the blade-facing roller  83  via the transfer belt  81  to remove foreign matters such as a toner and paper powder remaining on the transfer belt after the secondary transfer. The foreign matters removed in this way are collected in a waster toner box  713 . 
       FIG. 3  is a partial perspective view showing an outline of the line head. In  FIG. 3 , to easily understand the configuration of the line head  29  in the thickness direction TKD, a cross-section of an edge portion (lower left edge portion in  FIG. 3 ) in the longitudinal direction LGD of the line head  29  is shown. Here, the thickness direction is a direction perpendicular or approximately perpendicular to the longitudinal direction LGD and the width direction LTD, and the thickness direction is a direction in which light is emitted by the light emitting device E described below (in other word, a direction from the line head  29  to the photoconductor drum  21 ). In the description below of the embodiment, the downstream side of the thickness direction TKD (upper side of  FIG. 3 ) is referred to as “one side (of the thickness direction)” and the upstream side of the thickness direction TKD (lower side of  FIG. 3 ) is referred to as “the other side (of the thickness direction)”. One surface of a substrate or a 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. 
     The thickness direction TKD is in parallel with an optical axis (optical axis OA in  FIG. 9 ) of an image forming optical system constituted by a lens LS 1  of a lens array LA 1  and a lens LS 2  of a lens array LA 2 . While a plurality of image forming optical systems of LS 1  and LS 2  are arranged in the line head  29  according to the embodiment, the optical axes OA of each image forming optical system are in parallel with each other. As described below, between the light emitting device E and the surface of the photoconductor drum  21 , the lens LS 1 , a glass substrate SB, the lens LS 2 , the glass substrate SB, and a support glass SS are arranged in this order, and these components cooperate to function as one image forming optical system. In this embodiment, this image forming optical system is abbreviated as an image forming optical system LS 1 , LS 2  (or LS 1   s , LS 2   s ). 
     Here, the optical axis is defined as described below. Many image forming optical systems are plane symmetry (mirror symmetry) with respect to a symmetry plane perpendicular to the main scanning direction MD and plane symmetry (mirror symmetry) with respect to a symmetry plane perpendicular to the sub-scanning direction SD. In this way, the image forming optical system has a first symmetry plane perpendicular to the main scanning direction MD and a second symmetry plane perpendicular to the sub-scanning direction SD perpendicular to the main scanning direction MD, and a line of intersection of the first symmetry plane and the second symmetry plane is defined. When the image forming optical system is rotation symmetry, the line of intersection of the first symmetry plane and the second symmetry plane corresponds to the optical axis. When the image forming optical system is not rotation symmetry, the optical axis of the image forming optical system may not be technically defined. In such a case, the line of intersection described above may be used as the optical axis. 
     The line head  29  has a schematic configuration in which a head substrate  293 , a light shielding member  297 , a diaphragm plate  295 , the lens array LA 1 , and the lens array LA 2  are arranged in the thickness direction TKD in this order. On the back surface of the head substrate  293 , a plurality of light emitting devices E are divided into groups as light emitting device groups EG, each of which includes a predetermined number of light emitting devices E, and arranged two-dimensionally and discretely. To the back surface of the head substrate  293 , a sealing member  294  to seal the plurality of light emitting devices E is attached. Furthermore, to the back surface of the sealing member  294 , a rigid member  299  to support the above components constituting the line head  29  is attached. 
     A spacer SP 1  is provided between the head substrate  293  and the lens array LA 1 , and the spacer SP 1  defines the distance between the head substrate  293  and the lens array LA 1 . The light shielding member  297  and the diaphragm plate  295  mounted on the light shielding member  297  are arranged between the head substrate  293  and the lens array LA 1 , and the spacer SP 1  supports the lens array LA 1  while maintaining a certain distance between the diaphragm plate  295  and the lens array LA 1  in one side of the thickness direction TKD. A spacer SP 2  is provided between the lens array LA 1  and the lens array LA 2 , and the spacer SP 2  defines the distance between the lens array LA 1  and the lens array LA 2  and supports the lens array LA 2 . 
     In this way, in the line head  29 , the head substrate  293 , the light shielding member  297 , the diaphragm plate  295 , and the lens arrays LA 1  and LA 2  are arranged in this order. Light from the light emitting device E on the head substrate  293  passes through a light guide hole  2971  in the light shielding member  297  and an aperture stop  2951  of the diaphragm plate  295 , and the light is formed into an image by the lenses LS 1  and LS 2  of the lens arrays LA 1  and LA 2 . Next, detailed configuration of each component will be described with reference to  FIGS. 3 ,  4 , and  5 . 
       FIG. 4  is a partial plan view of the head substrate  293  viewed from the thickness direction TKD.  FIG. 4  corresponds to a perspective view of the back surface  293 - t  of the head substrate  293  viewed from one side in the thickness direction TKD (viewed from the upper side of  FIG. 3 )  FIG. 5  is a partial cross-sectional view taken along V-V line of the line head, and corresponds to a view of the cross-section viewed from the longitudinal direction LGD (the main scanning direction MD). This V-V line cross-section passes through geometric centers of gravity (or centers of lenses) of three light emitting device groups EG (or three lenses LS 1  or the like) aligning in a line at an interval of Dg in the longitudinal direction LGD and at an interval of Dt in the width direction LTD. The direction Dlsc shown in  FIGS. 4 and 5  is in parallel with the V-V line. Further, in  FIG. 4 , to show positional relationship among the light emitting device group EG formed on the head substrate  293 , the lens LS 1  formed in the lens array LA 1 , and the lens LS 2  formed in the lens array LA 2 , the lens LS 1  and the lens LS 2  are indicated by a dashed-dotted line. The description regarding the lens LS 1  and the lens LS 2  in  FIG. 4  is to show the positional relationship among them, but not to show that the lens LS 1  and the lens LS 2  are formed on the back surface  293 - t  of the head substrate ( FIG. 5 ). In  FIG. 5 , shading using dots is given to light transmissive components (or transparent components). 
     The head substrate  293  is constituted by a glass substrate (light transmissive substrate) through which light passes. On the back surface  293 - t  of the head substrate, a plurality of light emitting devices E, which are bottom emission type organic EL (Electro-Luminescence) devices, are formed and sealed by the sealing member  294  ( FIGS. 3 and 4 ). The plurality of light emitting devices E have the same emission spectrum and emit light beams to the surface of the photoconductor drum  21 . As shown in  FIG. 4 , an arrangement pattern of the plurality of light emitting devices E formed on the back surface  293 - t  of the head substrate has a group structure. Specifically, 15 light emitting devices E are arranged in a two-row zigzag pattern in the longitudinal direction LGD to form one light emitting device group EG, and further a plurality of light emitting device groups EG are discretely arranged in a three-row zigzag pattern in the longitudinal direction LGD. 
     The arrangement pattern will be described below in more detail. Specifically, in each light emitting device group EG, 15 light emitting devices E are arranged at positions different from each other in the longitudinal direction LGD, and further, the distance between two light emitting devices E and E adjacent to each other in the longitudinal direction LGD is a device-to-device distance Pel (in other words, in each light emitting device group EG, 15 light emitting devices E are arranged in the longitudinal direction LGD at a pitch Pel). The plurality of light emitting device groups EG are discretely arranged along the longitudinal direction LGD with a group-to-group distance Peg larger than the device-to-device distance Pel in between, and one light emitting device group row GRa or the like is formed. Further, three light emitting device group rows GRa, GRb, and GRc are discretely arranged at positions different from each other with a distance Dt in between in the width direction LTD, and the light emitting device group rows GRa, GRb, and GRc are shifted from each other by a distance Dg in the longitudinal direction LGD. In this way, three light emitting device groups EG are aligned in a direction Dlsc with the distance Dg in between in the longitudinal direction LGD and with the distance Dt in between in the width direction LTD. 
     Here, the device-to-device distance Pel can be obtained as a distance between geometric centers of gravity of two target light emitting devices E in the longitudinal direction LGD. The group-to-group distance Peg can be obtained as a distance between two target light emitting device groups EG in the longitudinal direction LGD, specifically as a distance between a geometric center of gravity of a light emitting device E at an end of one light emitting device group EG near the other light emitting device group EG in the longitudinal direction LGD and a geometric center of gravity of a light emitting device E at an end of the other light emitting device group EG near the one light emitting device group EG in the longitudinal direction LGD. The distance Dg can be obtained as a distance in the longitudinal direction LGD between geometric centers of gravity of two light emitting device groups EG located adjacent to each other in the longitudinal direction LGD. The distance Dt can be obtained as a distance in the width direction LTD between geometric centers of gravity of two light emitting device groups EG located adjacent to each other in the width direction LTD. 
     As described above, on a back surface  293 - t  of the head substrate  293 , a plurality of light emitting device groups EG are arranged two-dimensionally and discretely. On the other hand, on a front surface  293 - h  of the head substrate  293 , the light shielding member  297  is disposed. In the light shielding member  297 , a plurality of light guide holes  2971  penetrating in the thickness direction TKD are formed. Each light guide hole  2971  has a circular shape in a plan view seen from the thickness direction TKD, and black plating is performed on an internal wall thereof. The light guide hole  2971  is formed for each light emitting device group EG, in other words, one light guide hole  2971  is opened for one light emitting device group EG. In this way, the light shielding member  297  is in contact with the front surface  293 - h  of the head substrate  293  and fixed to the front surface  293 - h  of the head substrate  293  while the light guide holes  2971  are opened for the light emitting device groups EG. 
     The purpose of providing the light shielding member  297  is to prevent so-called stray light from entering the lens LS 1  or LS 2 . Specifically, the image forming optical system constituted by a pair of lenses LS 1  and LS 2  is dedicatedly provided to each light emitting device group EG. In such a configuration, a light beam is desired to enter only the image forming optical system of LS 1  and LS 2  provided to the light emitting device group EG from which the light beam is emitted and be formed into an image. However, a part of the light beam is not directed to the image forming optical system of LS 1  and LS 2  provided to the light emitting device group EG from which the light beam is emitted and becomes the stray light. When such stray light enters an image forming optical system of LS 1  and LS 2  that is not provided to the light emitting device group EG from which the light beam is emitted, a so-called ghost may appear. Considering the above, in this embodiment, the light shielding member  297  is provided between the light emitting device group EG and the image forming optical system of LS 1  and LS 2 . Since the light guide holes  2971  in which black plating is performed on the inner wall thereof are provided and opened to the light emitting device groups EG, a large part of the stray light is absorbed by the inner walls of the light guide holes  2971 . As a result, the ghost mentioned above is suppressed and a good exposure operation is realized. 
     Further, the diaphragm plate  295  is mounted on one end of the light shielding member  297  in the thickness direction TKD. In the diaphragm plate  295 , the aperture stop  2951  penetrating in the thickness direction TKD is formed for each light emitting device group EG, in other words, one aperture stop  2951  is opened for one light emitting device group EG. The aperture stop  2951  is provided for the image forming optical system constituted by the lens LS 1  and the lens LS 2  to perform a desired image forming operation. Specifically, the aperture stop  2951  controls an amount of the light beam entering the lens LS 1  and adjusts an amount of light that is used to form a spot and affects the size and the shape of the spot formed finally. 
     As described above, on one side of the head substrate  293 , the light shielding member  297 , and the diaphragm plate  295  in the thickness direction TKD, the lens arrays LA 1  and LA 2  are provided, and the lens arrays LA 1  and LA 2  are supported by the spacers SP 1  and SP 2 . Details of the support structure of the lens arrays LA 1  and LA 2  will be described with reference to  FIGS. 3 ,  5 , and  6 . 
       FIG. 6  is a partial cross-sectional view of the line head, and corresponds to a plan view of the line head  29  viewed from the width direction LTD. On the front surface of the head substrate  293 , a plurality of spacers SP 1  having the same shape and size are arranged in a row with an interval CL 1  in the longitudinal direction LGD. The row of the spacers SP 1  is provided at both sides in the width direction LTD ( FIGS. 3 and 5 ). In this way, in a plan view viewed from the thickness direction TKD, two rows of spacers SP 1  are arranged sandwiching the area where the light emitting devices E are formed on the back surface  293 - t  of the head substrate from the width direction LTD (in other word, two rows of spacers SP 1  are arranged sandwiching the light shielding members  297  from the width direction LTD). The spacers SP 1  are fixed to the front surface  293 - h  of the head substrate  293  by adhesive or the like. 
     The lens array LA 1  is mounted in the width direction LTD on the spacers SP 1  arranged in two rows in this way, so that the lens array LA 1  is positioned on one side of the thickness direction TKD of the head substrate  293 . At this time, the lens array LA 1  is disposed so that an area where the lenses LS 1  are formed in the lens array LA 1  is located between the two rows of spacers SP 1  arranged in the width direction LTD. The lens array LA 1  includes a rhomboid-shaped glass substrate SB whose both edges in the longitudinal direction LGD are cut obliquely (in a direction in parallel with the direction Dlsc). On the back surface of the glass substrate SB, a plurality of lenses LS 1  formed of a light curing resin are arranged in arrays. The plurality of lenses LS 1  are arranged in a three-row zigzag pattern corresponding to the arrangement of the opposing light emitting device groups EG ( FIG. 4 ). 
     As shown in  FIGS. 3 and 6 , a plurality of lens arrays LA 1  are arranged along the longitudinal direction LGD. Specifically, in this embodiment, the plurality of lens arrays LA 1  arranged along the longitudinal direction LGD are supported by the spacers SP 1 , and a long lens array L-LA 1  is formed. The length of the spacer SP 1  having a rectangular solid shape is smaller than the longitudinal direction LGD length of the width direction LTD side of the lens array LA 1 , and one lens array LA 1  is supported by the plurality of spacers SP 1  arranged along the longitudinal direction LGD. Specifically, regarding the spacers SP 1 , a center spacer SP 1 - b  supports the approximate center of the lens array LA 1  in the longitudinal direction LGD, and an end spacer SP 1 - a  bridges a gap BD 1  between two lens arrays LA 1  and LA 1  adjacent to each other in the longitudinal direction LGD and supports the lens arrays LA 1  and LA 1 . The spacers SP 1  and the lens array LA 1  are fixed to each other by adhesive or the like. 
     On one surface in the depth direction TKD of the long lens array L-LA 1  formed in this way, a plurality of spacers SP 2  having the same shape and size are arranged in a row with an interval CL 2  in the longitudinal direction LGD. The row of the spacers SP 2  is provided at both sides in the width direction LTD ( FIGS. 3 and 5 ). In this way, in a plan view viewed from the thickness direction TKD, two rows of spacers SP 2  are arranged sandwiching the area where the lenses LS 1  of the lens array LA 1  are formed from the width direction LTD. The spacers SP 2  are fixed to the front surface of the glass substrate SB of the lens array LA 1  by adhesive or the like. 
     The lens array LA 2  is mounted in the width direction LTD on the spacers SP 2  arranged in two rows in this way, so that the lens array LA 2  is positioned on one side of the thickness direction TKD of the lens array LA 1 . At this time, the lens array LA 2  is disposed so that an area where the lenses LS 2  are formed in the lens array LA 2  is located between the two rows of spacers SP 2  arranged in the width direction LTD. The lens array LA 2  includes a rhomboid-shaped glass substrate SB whose both edges in the longitudinal direction LGD are cut obliquely (in a direction in parallel with the direction Dlsc). On the back surface of the glass substrate SB, a plurality of lenses LS 2  formed of a light curing resin are arranged in arrays. The plurality of lenses LS 2  are arranged in a three-row zigzag pattern corresponding to the arrangement of the opposing light emitting device groups EG ( FIG. 4 ). 
     As shown in  FIGS. 3 and 6 , a plurality of lens arrays LA 2  are arranged along the longitudinal direction LGD. Specifically, in this embodiment, the plurality of lens arrays LA 2  arranged along the longitudinal direction LGD are supported by the spacers SP 2 , and a long lens array L-LA 2  is formed. The length of the spacer SP 2  having a rectangular solid shape is smaller than the longitudinal direction LGD length of the width direction LTD side of the lens array LA 2 , and one lens array LA 2  is supported by the plurality of spacers SP 2  arranged along the longitudinal direction LGD. Specifically, regarding the spacers SP 2 , a center spacer SP 2 - b  supports the approximate center of the lens array LA 2  in the longitudinal direction LGD, and an end spacer SP 2 - a  bridges a gap BD 2  between two lens arrays LA 2  and LA 2  adjacent to each other in the longitudinal direction LGD and supports the lens arrays LA 2  and LA 2 . The spacers SP 2  and the lens array LA 2  are fixed to each other by adhesive or the like. 
     In this way, the lens array LA 1  and the lens array LA 2  are arranged so that they face each other in the thickness direction TKD. As a result, a plurality of lenses LS 1  of the lens array LA 1  and a plurality of lenses LS 2  of the lens array LA 2  face each other in a one-to-one relation, and positions of the lens array LA 1  and the lens array LA 2  are adjusted so that the lenses LS 1  and the lenses LS 2  facing each other overlap each other in a plan view viewed from the thickness direction TKD. 
     Further, in this embodiment, a long support glass SS is provided in the longitudinal direction LGD direction. Specifically, in the longitudinal direction LGD, the support glass SS is formed longer than the lens array LA 2 , and has approximately the same length as that of the long lens array L-LA 2 . The support glass SS is attached to one surface of the long lens array L-LA 2 , so that the support glass SS supports the plurality of lens arrays LA 2  from the opposite side of the spacers SP 2 . A front surface SS- h  (one flat surface) of the support glass SS faces a surface  21   s  (drum circumferential surface) of the photoconductor drum  21  with a clearance (distance di) between them ( FIG. 7 ). 
     In this embodiment, the lens LS 1  and the lens LS 2  facing each other in the thickness direction TKD constitute one image forming optical system. This image forming optical system forms an upside-down reduced-size image, and the magnification thereof is negative and has an absolute value smaller than 1. Therefore, after the light beam emitted from the light emitting device E passes through the lenses LS 1  and LS 2 , the light beam is emitted from the front surface SS- h  of the support glass SS and irradiated onto the surface of the photoconductor drum  21  as a spot ST ( FIG. 5 ). A line latent image extending in the main scanning direction MD can be formed by controlling light emitted from each light emitting device E in accordance with the movement of the surface of the photoconductor drum  21  in the sub-scanning direction SD as shown in  FIG. 11  or the like in JP-A-2008-036937. 
     By the way, each spot ST used to form the line latent image needs to be formed in a size corresponding to resolution of the latent image. However, as described above, when the line head  29  (exposure head) and the photoconductor drum  21  (image carrier) are not appropriately positioned, the spots ST blur, and spots ST having a desired size may not be formed. Therefore, in the image forming apparatus according to this embodiment, a line head support mechanism  6  is provided to correctly position the line head  29  with respect to the photoconductor drum  21 . 
       FIG. 7  is a longitudinal direction partial cross-sectional view showing the line head support mechanism.  FIG. 8  is a perspective view showing the line head support mechanism. Although the line head support mechanism  6  is disposed at each of both ends of the line head  29  in the longitudinal direction LGD, the configurations of both line head support mechanisms  6  and  6  are the same. Therefore, the configuration of the line head support mechanism  6  disposed at one end in the longitudinal direction LGD will be described in detail below. As described above, the thickness direction TKD is in parallel with the optical axis of the image forming optical system constituted by the lenses LS 1  and LS 2 . Therefore, in  FIGS. 7 and 8  (and  FIGS. 9 and 10  described below), the optical axis direction OA is indicated along with the thickness direction TKD. 
     As described in  FIGS. 7 and 8 , the line head support mechanism  6  includes a holding member  61  that holds the line head  29 , a gap roller  63  that comes in contact with the surface  21   s  (drum circumferential surface  21   s ) of the photoconductor drum  21 , a roller support member  65  that is fixed to the holding member  61  and supports the gap roller  63 , a slider  67  that fits into the holding member  61 , and a pressing spring  69  that urges the holding member  61  in the thickness direction TKD. 
     The holding member  61  supports the line head  29  from the direction opposite to the optical axis direction OA of the photoconductor drum  21 . More specifically, one side of the holding member  61  in the optical axis direction OA is a holding surface  61   s  formed into a flat surface shape perpendicular to the optical axis direction OA, and the holding surface  61   s  is brought in contact with the rigid member  299  of the line head  29  and fixed. Further, the roller support member  65  is attached to the holding surface  61   s  of the holding member  61 . The roller support member  65  includes a roller support shaft  651  in parallel with the rotation axis of the photoconductor drum  21 , and the gap roller  63  is rotatably supported by the roller support shaft  651 . The gap roller  63  has a disk shape with the roller support shaft  651  at its center. The gap roller  63  is rotated in accordance with the rotation of the photoconductor drum surface  21   s  while the circumferential surface of the gap roller  63  is in contact with the photoconductor drum surface  21   s.    
     The pressing spring  69  is provided between the holding member  61  and a main body frame  32  of the housing main body  3  in the optical axis direction OA. One end of the pressing spring  69  is attached to the holding member  61  in the opposite side of the optical axis direction (opposite side of the holding surface  61   s ), and the other end of the pressing spring  69  is attached to the main body frame  32 . Therefore, the holding member  61  is urged in the optical axis direction OA by the pressing spring  69 , so that the holding member  61  freely moves in the optical axis direction OA. A cylindrical shaped fitting hole  61   h  is formed in the holding member  61  in the opposite side of the optical axis direction OA, and a cylindrical shaped slider  67  fits into the fitting hole  61   h.    
     Although the holding member  61  receives an urging force from the pressing spring  69 , the holding member  61  is controlled to move in the optical axis direction OA by the fitting hole  61   h  and the slider  67 . Therefore, the holding member  61  is urged in the optical axis direction OA. On the other hand, the gap roller  63  is provided on one side of the holding member  61 . Therefore, the holding member  61  is in a stable state at a position a predetermined distance from the photoconductor drum surface  21   s  while the gap roller  63  is pressed against the photoconductor drum surface  21   s  by the urging force from the pressing spring  69 . By holding the line head  29  by the holding member  61 , the line head  29  can be supported with a predetermined clearance from the photoconductor drum surface  21   s . More specifically, in this embodiment, the line head  29  is supported so that the relationship described below is satisfied. 
       FIG. 9  is a diagram showing details of a support form of the line head, and corresponds to a plan view of the V-V line cross-section viewed from the width direction LTD. Like the light emitting device Es shown in  FIG. 9 , the line head  29  according to this embodiment includes a light emitting device emitting light having a wavelength λ1 and a wavelength λ2 different from the wavelength λ1. The image forming optical system of LS 1   s  and LS 2   s  faces the light emitting device Es, forms an image of the light having the wavelength λ1 at an image forming position P 1 , and forms an image of the light having the wavelength λ2 at an image forming position P 2  a distance A apart from the image forming position P 1  in the optical axis direction OA. FIG.  10  is an enlarged diagram of an area near the image forming positions P 1  and P 2 . As shown in  FIG. 10 , the light from the light emitting device Es is formed into an image at the image forming position P 1  and the image forming position P 2  a distance A apart from the image forming position P 1  in the optical axis direction OA. 
     In this embodiment, the line head  29  is supported so that the distance d1, the distance d2, and the distance di satisfy the following relation equation A: 
     d1&lt;di&lt;d2 . . . Relation equation A 
     Here, d1, di, and d2 are defined as follows:
     d1: a distance between an optical surface (front surface SS-h of the support glass) nearest to the photoconductor drum surface  21   s  in the optical surfaces of the image forming optical system of LS 1   s  and LS 2   s  and the image forming position P 1  in the optical axis direction OA,   d2: a distance between the optical surface (front surface SS-h of the support glass) nearest to the photoconductor drum surface  21   s  in the optical surfaces of the image forming optical system of LS 1   s  and LS 2   s  and the image forming position P 2  in the optical axis direction OA,   di: a distance between the optical surface (front surface SS-h of the support glass) nearest to the photoconductor drum surface  21   s  in the optical surfaces of the image forming optical system of LS 1   s  and LS 2   s  and the photoconductor drum surface  21   s  in the optical axis direction OA.   

     In this embodiment, the light emitting device Es and the image forming optical system of LS 1  and LS 2  as described above are arranged in approximately the center of the latent image forming area (approximately the center of the main scanning direction MD) of the line head  29 . In this case, image forming positions where other image forming optical systems of LS 1  and LS 2  form images of the light from the light emitting devices E are not largely apart from the photoconductor drum surface  21   s , so that a good latent image can be formed by the spots ST with less blurs. 
     As described above, in this embodiment, the image forming optical system in which a lens surface of a first lens LS 1  (a first lens surface) is used as an entrance surface and the front surface SS-h of the support glass is used as an emission surface, and the light emitting device Es are provided. The light emitting device Es emits the light having the wavelength λ1 (first wavelength) and the wavelength λ2 (second wavelength), and the image forming optical system of LS 1   s  and LS 2   s  forms images of the light of the wavelengths λ1 and λ2 at different positions (the image forming position P 1  and the image forming position P 2 ) in the optical axis direction OA. The line head support mechanism  6  (support member) supports the line head  29  including such an optical system and the photoconductor drum  21  so that the above relation equation A is satisfied. Specifically, in this image forming apparatus, the line head  29  is positioned with respect to the photoconductor drum  21  so that the surface  21   s  of the photoconductor drum  21  is positioned between the image forming position P 1  (first image forming position) and the image forming position P 2  (the second image forming position). By positioning the line head  29  with respect to the photoconductor drum  21  in this way, the surface  21   s  of the photoconductor drum  21  can be positioned in a range between the two image forming positions P 1  and P 2  of the image forming optical system of LS 1   s  and LS 2   s  (in other words, in a range indicated by the reference character Δ in  FIG. 10 ), and the other image forming optical systems of LS 1  and LS 2  can form an image of the light from the light emitting device E at a position near the photoconductor drum  21 . Thus, the line head  29  can form a good latent image by the spots ST with less blur. The range between the mage forming position P 1  and the image forming position P 2  is a range between the two image forming positions P 1  and P 2  including the image forming position P 1  and the image forming position P 2 . 
     The line head  29  includes the light emitting devices E in addition to the light emitting device Es, and the image forming optical systems of LS 1  and LS 2  that form images of the light from the light emitting devices E in addition to the image forming optical system of LS 1   s  and LS 2   s . The image forming positions of the light by the image forming optical systems of LS 1  and LS 2  other than the image forming optical system of LS 1   s  and LS 2   s  are desired to be in a range between the image forming position P 1  and the image forming position P 2  in the optical axis direction OA, and in such a case, the image forming optical systems of LS 1  and LS 2  other than the image forming optical system of LS 1   s  and LS 2   s  can form spots ST with further less blur. Specifically, the line head  29  is positioned with respect to the photoconductor drum  21  so that at least one of the two image forming positions P 1  and P 2  of the image forming optical system of LS 1   s  and LS 2   s  is positioned near the surface of the photoconductor drum  21 . Therefore, when the image forming positions of the light by the image forming optical systems of LS 1  and LS 2  other than the image forming optical system of LS 1   s  and LS 2   s  are in the range between the image forming position P 1  and the image forming position P 2  in the optical axis direction OA, the image forming positions of the light by the image forming optical systems of LS 1  and LS 2  other than the image forming optical system of LS 1   s  and LS 2   s  can be positioned within a distance Δ from the surface  21   s  of the photoconductor drum  21 , and as a result, a good latent image can be formed by the line head  29 . 
     In the above embodiment, the line head support mechanism  6  includes the holding member  61  that holds the line head on the opposite side of the optical axis direction OA of the photoconductor drum  21 , and the gap roller  63  (contact member) that is disposed to the holding member  61  and brought into contact with the photoconductor drum  21 . By connecting the gap roller  63  coming into contact with the photoconductor drum  21  and the line head  29  via the holding member  61  in the manner described above, positioning accuracy of the line head  29  with respect to the photoconductor drum  21  can be improved. 
     Others 
     As described above, in the above embodiment, the line head  29  corresponds to the “exposure head” of the invention, the photoconductor drum  21  corresponds to the “image carrier” of the invention, and the line head support mechanism corresponds to the “support member” of the invention. The light emitting device Es corresponds to the “light emitting device” of the invention, the light having the wavelength λ1 corresponds to the “light having the first wavelength” of the invention, the light having the wavelength λ2 corresponds to the “light having the second wavelength” of the invention, the image forming optical system of LS 1   s  and LS 2   s  corresponds to the “optical system” of the invention, the image forming position P 1  corresponds to the “first image forming position” of the invention, and the image forming position P 2  corresponds to the “second image forming position” of the invention. The light emitting device E other than the light emitting device Es corresponds to the “second light emitting device” of the invention, and the image forming optical system other than the image forming optical system of LS 1   s  and LS 2   s  corresponds to the “second optical system” of the invention. The gap roller  63  corresponds to the “contact member” of the invention. The lens surface of the lens LS 1  corresponds to the “first lens surface” of the invention, and the front surface SS-h of the support glass corresponds to the “second lens surface” of the invention. 
     The invention is not limited to the above embodiment, but various modifications can be made without departing from the spirit and scope of the invention. For example, the light emitting devices E other than the light emitting device Es that emits light having the wavelengths λ1 and λ2 may emit light having two wavelength components. Further, the light having two wavelengths emitted from such a light emitting device E may be formed into images at different image forming positions in the optical axis direction OA by the image forming optical system of LS 1  and LS 2  other than the image forming optical system of LS 1   s  and LS 2   s.    
     In this case, although there are a plurality of image forming optical systems of LS 1  and LS 2  that form light into images at two different image forming positions in the optical axis direction OA, it is not necessary for all of the plurality of image forming optical systems of LS 1  and LS 2  to satisfy the above relation equation A. Specifically, at least one image forming optical system of LS 1  and LS 2  (LS 1   s  and LS 2   s ) needs to satisfy the above relation equation A. 
     Although the support glass SS is provided in the above embodiment, the line head  29  may be configured without providing the support glass SS. In this case, the front surface SB-h ( FIG. 5 ) of the glass substrate SB of the lens array LA 2  is the “optical surface nearest to the photoconductor drum surface  21   s  in the optical surfaces of the image forming optical system of LS 1   s  and LS 2   s”.    
     Although the plurality of lens arrays LA 1  have the same shape and size in the above embodiment, various modifications can be made to the shape and the size. Further, the same modifications can be made to the plurality of lens arrays LA 2 . 
     Although the plurality of spacers SP 1  have the same shape and size in the above embodiment, various modifications can be made to the shape and the size. Further, the same modifications can be made to the plurality of spacers SP 2 . 
     Although the image forming optical system in the above embodiment forms an upside-down image, the image forming optical system may form a normal image (or not-upside-down image). 
     Although the image forming optical system in the above embodiment forms a reduced-size image, the image forming optical system may form an enlarged image. 
     Although the lenses LS 1  are formed on the back surface (the other side of the thickness direction TKD) of the lens array LA 1  in the above embodiment, the forming position of the lenses LS 1  are not limited to this. This is the same for the lenses LS 2  of the lens array LA 2 . 
     Although lenses are arranged in a three-row zigzag pattern in the lens arrays LA 1  and LA 2  in the above embodiment, the arrangement pattern of the lenses is not limited to this. 
     In the above embodiment, the lens arrays LA 1  and LA 2  are lens arrays in which the lenses LS 1  or LS 2  made of resin are formed on the light transmissive substrate SB made of glass. However, the lens arrays LA 1  and LA 2  can be integrally formed from one material. 
     Although the plurality of light emitting device groups EG are arranged in a three-row zigzag pattern in the above embodiment, the arrangement pattern of the plurality of light emitting device groups EG is not limited to this. 
     In the above embodiment, the light emitting device group EG is constituted by 15 light emitting devices E. However, the number of the light emitting devices E constituting the light emitting device group EG is not limited to this. 
     Although the plurality of light emitting devices E are arranged in a two-row zigzag pattern in the light emitting device group EG in the above embodiment, the arrangement pattern of the plurality of light emitting devices E in the light emitting device group EG is not limited to this. 
     In the above embodiment, bottom emission type organic EL devices are used as the light emitting devices E. However, top emission type organic EL devices may be used as the light emitting devices E, or LEDs (Light Emitting Diodes) or the like other than the organic EL devices may be used as the light emitting devices E. 
     The entire disclosure of Japanese Patent Applications No. 2009-216522, filed on Sep. 18, 2009 is expressly incorporated by reference herein.