Patent Publication Number: US-9423719-B1

Title: Optical scanning device and image forming apparatus using the same

Description:
INCORPORATION BY REFERENCE 
     This application is based upon and claims the benefit of priority from the corresponding Japanese Patent Application No. 2015-016337 filed on Jan. 30, 2015, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     The present disclosure relates to an optical scanning device that includes a plurality of light sources for emitting light beams and a deflector for reflecting the light beams toward corresponding scanned surfaces, and to an image forming apparatus that uses the optical scanning device. 
     An optical scanning device for use in a color laser printer includes, for example: a plurality of light sources for emitting laser beams respectively for colors of cyan, magenta, yellow and black; a deflector (polygon mirror) for deflecting the laser beams such that the laser beams scan the circumferential surfaces (scanned surfaces) of the photoconductor drums for respective colors; and focus lenses for focusing the deflected laser beams on the circumferential surfaces. The deflector may be used in common by a plurality of laser beams. 
     There has been disclosed a technology in which two optical scanning devices are used to scan four photoconductor drums disposed adjacent to each other. Each of the optical scanning devices includes two light sources and a deflector. In this case, a laser beam emitted from a light source is deflected at a first position on the circumference of the deflector, a laser beam emitted from the other light source is deflected at a second position that is on the opposite side to the first position. 
     SUMMARY 
     An optical scanning device according to an aspect of the present disclosure includes a housing, a light source portion, a deflector, a first focus lens, a second focus lens, a first mirror group, and a second mirror group. The light source portion includes a first light source and a second light source, the first light source emitting a first light beam, the second light source emitting a second light beam. The rotation axis of the deflector is inclined with respect to a first direction and a second direction, the first direction being opposite to the second direction. The deflector is configured to reflect the first light beam diagonally on a side of a third direction that is perpendicular to the first direction and the second direction, and on a side of the first direction, and is configured to reflect the second light beam diagonally on a side of a fourth direction that is opposite to the third direction, and on a side of the second direction, such that the first light beam scans a first scanned surface and the second light beam scans a second scanned surface. The first focus lens is disposed between the deflector and the first scanned surface and configured to focus the first light beam on the first scanned surface. The second focus lens is disposed between the deflector and the second scanned surface and configured to focus the second light beam on the second scanned surface. The first mirror group is disposed between the first focus lens and the first scanned surface and configured to reflect the first light beam to the first scanned surface. The second mirror group is disposed between the second focus lens and the second scanned surface and configured to reflect the second light beam to the second scanned surface. The first mirror group includes a first reflection mirror and a second reflection mirror. The first light beam that has transmitted through the first focus lens is incident on the first reflection mirror. The first reflection mirror is configured to reflect the first light beam in the second direction of separating away from the first scanned surface. The second reflection mirror is configured to reflect the first light beam reflected by the first reflection mirror, toward the first scanned surface. The second mirror group includes a third reflection mirror and a fourth reflection mirror. The second light beam that has transmitted through the second focus lens is incident on the third reflection mirror. The third reflection mirror is configured to reflect the second light beam in the first direction of approaching the second scanned surface. The fourth reflection mirror is configured to reflect the second light beam reflected by the third reflection mirror, toward the second scanned surface. 
     This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description with reference where appropriate to the accompanying drawings. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Furthermore, the claimed subject matter is not limited to implementations that solve any or all disadvantages noted in any part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view showing the configuration of a color printer according to an embodiment of the present disclosure. 
         FIG. 2  is a cross-sectional view showing the internal configuration of an optical scanning device according to an embodiment of the present disclosure. 
         FIG. 3  is a plan view showing a part of an optical path of the optical scanning device according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following describes an embodiment of the present disclosure with reference to the drawings.  FIG. 1  is a cross-sectional view showing the configuration of the image forming apparatus  1  according to an embodiment of the present disclosure. The image forming apparatus  1  is a tandem color printer. The image forming apparatus  1  includes a main body housing  10  that is formed approximately in the shape of a rectangular parallelepiped. It is noted that the image forming apparatus may be a full-color copier or multifunction peripheral. 
     The main body housing  10  includes therein a plurality of processing units for performing an image formation process to a sheet. In the present embodiment, the main body housing  10  includes, as the processing units, image forming units  2 Y,  2 C,  2 M and  2 Bk, color containers  25 Y,  25 C,  25 M and  25 Bk for respective colors, an optical scanning device  23 , an intermediate transfer unit  28 , and a fixing device  30 . A sheet discharge tray  11  is provided on the upper surface of the main body housing  10 . A sheet discharge port  12  is opened opposite to the sheet discharge tray  11 . A manual feed tray  13  is attached to a side wall of the main body housing  10  in a freely openable/closable manner. A sheet feed cassette  14  is attached to a lower part of the main body housing  10  in a freely attachable/detachable manner, wherein sheets on which images are to be formed by the image formation process are stored in the sheet feed cassette  14 . 
     The image forming units  2 Y,  2 C,  2 M and  2 Bk are configured to form toner images of yellow, cyan, magenta and black respectively based on image information transmitted from an external apparatus such as a computer, and are aligned at predetermined intervals in tandem in the horizontal direction. Each of the image forming units  2 Y,  2 C,  2 M and  2 Bk includes: a cylindrical photoconductor drum  21  for carrying an electrostatic latent image and a toner image; a charger  22 ; a developing device  24 ; a primary transfer roller  26 ; and a cleaning device  27 . 
     The optical scanning device  23  forms electrostatic latent images on the circumferential surfaces of the photoconductor drums  21  of respective colors. The optical scanning device  23  of the present embodiment includes a plurality of light sources and focusing optical systems, wherein the plurality of light sources are prepared for the respective colors, and the focusing optical systems focus and scan the light beams emitted from the light sources on the circumferential surfaces of the photoconductor drums  21  of the respective colors. The focusing optical systems are not independent optical systems, but a part thereof is used in common. The optical scanning device  23  is described below. 
     The intermediate transfer unit  28  performs a primary transfer of transferring toner images formed on the photoconductor drums  21 . The intermediate transfer unit  28  includes a transfer belt  281 , a driving roller  282  and a driven roller  283 , wherein the transfer belt  281  circumferentially rotates while contacting the circumferential surfaces of the photoconductor drums  21 , and the transfer belt  281  is suspended between the driving roller  282  and the driven roller  283 . The transfer belt  281  is pressed against the circumferential surfaces of the photoconductor drums  21  by the primary transfer rollers  26 . In the primary transfer, the toner images are transferred from the photoconductor drums  21  so as to be overlaid at a same position on the transfer belt  281 . This allows a full-color toner image to be formed on the transfer belt  281 . 
     A secondary transfer roller  29  is disposed opposite to the driving roller  282  across the transfer belt  281  so as to form a secondary transfer nip portion T. In the secondary transfer, the full-color toner image is transferred from the transfer belt  281  to a sheet by the secondary transfer nip portion T. Toner that has remained on the circumferential surface of the transfer belt  281  without being transferred to the sheet, is collected by a belt cleaning device  284  disposed opposite to the driven roller  283 . 
     The fixing device  30  includes a fixing roller  31  and a pressure roller  32 , wherein a heat source is embedded in the fixing roller  31 , and the fixing roller  31  and the pressure roller  32  form a fixing nip portion N. The fixing device  30  performs a fixing process in which the sheet to which the toner image has been transferred by the secondary transfer nip portion T is heated and pressed by the fixing nip portion N so that the toner is fused and fixed to the sheet. The sheet subjected to the fixing process is discharged from the sheet discharge port  12  toward the sheet discharge tray  11 . 
     A sheet conveyance path for conveying sheets is provided in the main body housing  10 . The sheet conveyance path includes a main conveyance path P 1  that vertically extends from near a lower part of the main body housing  10  to near an upper part via the secondary transfer nip portion T and the fixing device  30 . The downstream end of the main conveyance path P 1  is connected to the sheet discharge port  12 . A reverse conveyance path P 2  for conveying a reversed sheet in the double-side printing is provided to extend from the most downstream end in the main conveyance path P 1  to near the upstream end. In addition, a manually fed sheet conveyance path P 3  extending from the manual feed tray  13  to the main conveyance path P 1  is disposed above the sheet feed cassette  14 . 
     The sheet feed cassette  14  includes a sheet storage portion for storing a stack of sheets. A pick-up roller  151  and a pair of sheet feed rollers  152  are disposed in the vicinity of an upper-right part of the sheet feed cassette  14 , wherein the pick-up roller  151  picks up, one by one, the top sheets of the stack of sheets, and the pair of sheet feed rollers  152  feed the picked-up sheet toward the upstream end of the main conveyance path P 1 . A sheet placed on the manual feed tray  13  is also conveyed to the upstream end of the main conveyance path P 1  via the manually fed sheet conveyance path P 3 . A pair of registration rollers  15  are disposed more on the upstream side than the secondary transfer nip portion T in the main conveyance path P 1 , wherein the pair of registration rollers  15  feed a sheet to the transfer nip portion at a predetermined timing. 
     When a single-side printing process (image formation process) is performed to a sheet, the sheet is fed from the sheet feed cassette  14  or the manual feed tray  13  to the main conveyance path P 1 . A transfer process of transferring a toner image to the sheet is performed in the secondary transfer nip portion T, and the fixing process of fixing the transferred toner to the sheet is performed in the fixing device  30 . Subsequently, the sheet is discharged from the sheet discharge port  12  onto the sheet discharge tray  11 . On the other hand, during a double-side printing process, the transfer process and the fixing process are performed to one surface of the sheet, then the sheet is partially projected outward on the sheet discharge tray  11  from the sheet discharge port  12 . Subsequently, the sheet is switchback-conveyed to be returned to near the upstream end of the main conveyance path P 1  via the reverse conveyance path P 2 . The transfer process and the fixing process are then performed to the other surface of the sheet, then the sheet is discharged on the sheet discharge tray  11  from the sheet discharge port  12 . 
     The optical scanning device  23  of the present embodiment is further described.  FIG. 2  is a cross-sectional view showing the internal configuration of the optical scanning device  23  of the present embodiment.  FIG. 3  is a plan view showing a part of an optical path of the optical scanning device  23 . The optical scanning device  23  includes a first optical scanning unit  23 A and a second optical scanning unit  23 B. The first optical scanning unit  23 A and the second optical scanning unit  23 B are disposed adjacent to each other in the horizontal direction. The first optical scanning unit  23 A is disposed below a yellow photoconductor drum  21 Y (the first scanned surface, the first photoconductor drum) and a magenta photoconductor drum  21 M (the second scanned surface, the second photoconductor drum), and scans the circumferential surfaces of the two photoconductor drums  21 . The second optical scanning unit  23 B is disposed below a cyan photoconductor drum  21 C (the first scanned surface, the third photoconductor drum) and a black photoconductor drum  21 BK (the second scanned surface, the fourth photoconductor drum), and scans the circumferential surfaces of the two photoconductor drums  21 . 
     It is noted that the main scanning direction of the first optical scanning unit  23 A and the second optical scanning unit  23 B is a left-right direction perpendicular to the plane of  FIG. 2 , and the sub scanning direction is a front-rear direction, namely a tangent direction at the lower-end part of each photoconductor drum  21 . In addition, as shown in  FIG. 2 , the photoconductor drums  21  of four colors are disposed such that the rotation shaft centers are disposed adjacent to each other at intervals on a predetermined straight line (a first reference line DL) in a cross section taken along a plane including the sub scanning direction of the optical scanning device  23 , and the photoconductor drums  21  are rotated by a driving portion (not shown). In addition, a second reference line DR is defined as a straight line that is perpendicular to the first reference line DL. In the present embodiment, the second reference line DR extends along the up-down direction. Furthermore, in  FIG. 2 , a pair of conveyance screws  241  and a developing housing lower end portion  242  of the developing device  24  are seen between: the photoconductor drums  21 ; and the first optical scanning unit  23 A and the second optical scanning unit  23 B. The pair of conveyance screws  241  have a function to stir the developer in the developing device  24 . The developing housing lower end portion  242  is an imaginary straight line that extends along the lower end portion of the housing of the developing device  24 . 
     Next, the internal configuration of the first optical scanning unit  23 A is described. The first optical scanning unit  23 A includes a first housing  230 A, a light source unit  80  (the light source portion), a polygon motor  51 , a polygon mirror  52  (deflector), and a focusing optical system, wherein the light source unit  80  includes light sources  81  and  82  respectively for yellow and magenta that are stored in the first housing  230 A. It is noted that the second optical scanning unit  23 B has the same internal configuration as the first optical scanning unit  23 A, and includes a second housing  230 B, a light source unit  80  (the light source portion), a polygon motor  51 , a polygon mirror  52  (deflector), and a focusing optical system, wherein the light source unit  80  includes light sources  81  and  82  respectively for cyan and black that are stored in the second housing  230 B. As a result, in the following, detailed description of the components of the second optical scanning unit  23 B is omitted. In addition, in  FIG. 2 , components of the same type included in the first optical scanning unit  23 A and the second optical scanning unit  23 B are assigned a same reference number, for the sake of explanation. 
     The light sources  81  and  82  of the light source unit  80  stored in the first housing  230 A each include a semiconductor laser that emits a laser beam of a single wavelength. A light source  81  for yellow (the first light source) is positioned more on the front side in the plane of the figure than the polygon mirror  52 , and emits a first light beam L 1  that is irradiated on the photoconductor drum  21 Y. On the other hand, a light source  82  for magenta (the second light source) is disposed at a different position from the light source  81  for yellow in the sub scanning direction (front-rear direction), more specifically, is positioned more on the front side than the light source  81  for yellow. In addition, the light source  82  for magenta is positioned more on the rear side in the plane of the figure than the polygon mirror  52 , and emits a second light beam L 2  that is irradiated on the photoconductor drum  21 M. In addition, a collemator lens (not shown), a cylindrical lens (not shown), and a diaphragm (not shown) are disposed respectively between the polygon mirror  52  and the light source  81 , and between the polygon mirror  52  and the light source  82 . The collemator lens converts a laser beam to parallel light beams, wherein the laser beam is emitted from the light source  81 ,  82  and diffused. The cylindrical lens converts the parallel light beams to line-like light beams that are elongated in the main scanning direction and focuses the light beams on the polygon mirror  52 . The diaphragm regulates the laser beam emitted from the light source  81 ,  82 . 
     The polygon mirror  52  deflects laser beams L 1  and L 2  (the first light beam and the second light beam) emitted from the light sources  81  and  82  for respective colors, respectively such that the light beams scan the circumferential surfaces of the photoconductor drums  21 Y and  21 M (the first scanned surface and the second scanned surface), from one end to the other end of a predetermined scanning range. The polygon mirror  52  is a polygon mirror having deflection surfaces formed along the sides of a hexagon. The rotation shaft of the polygon motor  51  is connected to the center of the polygon mirror  52 . That is, the polygon motor  51  is coaxially fixed to the polygon mirror  52  and rotates the polygon mirror  52 . The rotation shaft is inclined at an acute angle with respect to the up direction (the first direction) and the down direction (the second direction that is opposite to the first direction). In this way, in the present embodiment, a single polygon mirror  52  is used in common to scan the photoconductor drums  21  for two colors. As shown in  FIG. 2 , in a cross section taken along a plane including the sub scanning direction, the polygon mirror  52  deflects the first light beam L 1  and the second light beam L 2  in opposite directions. More specifically, the polygon mirror  52  reflects the first light beam L 1  on the side of the rear direction (the third direction) perpendicular to the up-down direction, and in the diagonally upward direction (the rear diagonally upward direction). In addition, the polygon mirror  52  reflects the second light beam L 2  on the side of the front direction (the fourth direction), which is opposite to the rear direction, and in the diagonally downward direction (the front diagonally downward direction). 
     The focusing optical system includes a scanning lens  53  (first focus lens), a scanning lens  54  (second focus lens), a reflection mirror  56  (first reflection mirror), a reflection mirror  57  (third reflection mirror), a reflection mirror  58  (second reflection mirror), and a reflection mirror  59  (fourth reflection mirror). 
     The scanning lens  53  is disposed on the optical path of the first light beam L 1  between the polygon mirror  52  and the circumferential surface of the photoconductor drum  21 Y, and focuses the first light beam L 1  on the circumferential surface. In addition, the scanning lens  54  is disposed on the optical path of the second light beam L 2  between the polygon mirror  52  and the circumferential surface of the photoconductor drum  21 M, and focuses the second light beam L 2  on the circumferential surface. In the present embodiment, the scanning lens  53  and the scanning lens  54  are each a single lens. In addition, the scanning lens  53  and the scanning lens  54  have the same shape and are arranged so as to be in point symmetry with respect to the rotation center of the polygon mirror  52 , as shown in the cross section of  FIG. 2 . This makes it possible to commonalize the scanning lens  53  and the scanning lens  54 , thereby the cost of the optical scanning device  23  is reduced. 
     In the present embodiment, the reflection mirror  56  and the reflection mirror  58  constitute the first mirror group of the present disclosure. The first mirror group is disposed on the optical path of the first light beam L 1  between the scanning lens  53  and the circumferential surface of the photoconductor drum  21 Y, and reflects the first light beam L 1  to the circumferential surface. Similarly, the reflection mirror  57  and the reflection mirror  59  constitute the second mirror group of the present disclosure. The second mirror group is disposed on the optical path of the second light beam L 2  between the scanning lens  54  and the circumferential surface of the photoconductor drum  21 M, and reflects the second light beam L 2  to the circumferential surface. 
     The reflection mirror  56  is disposed, among the first mirror group, closest to the scanning lens  53  on the optical path of the first light beam L 1 . The first light beam L 1  having transmitted through the scanning lens  53  is incident on the reflection mirror  56 . As shown in the cross section of  FIG. 2 , the reflection mirror  56  reflects the first light beam L 1  downward away from the circumferential surface of the photoconductor drum  21 Y. The reflection mirror  58  is disposed in the downstream of the reflection mirror  56  on the optical path of the first light beam L 1 . The reflection mirror  58  reflects the first light beam L 1  reflected by the reflection mirror  56 , to the circumferential surface of the photoconductor drum  21 Y. 
     The reflection mirror  57  is disposed, among the second mirror group, closest to the scanning lens  54  on the optical path of the second light beam L 2 . The second light beam L 2  having transmitted through the scanning lens  54  is incident on the reflection mirror  57 . As shown in the cross section of  FIG. 2 , the reflection mirror  57  reflects the second light beam L 2  upward in a direction of approaching the circumferential surface of the photoconductor drum  21 M. The reflection mirror  59  is disposed in the downstream of the reflection mirror  57  on the optical path of the second light beam L 2 . The reflection mirror  59  reflects the second light beam L 2  reflected by the reflection mirror  57 , to the circumferential surface of the photoconductor drum  21 M. 
     It is noted that, in the first housing  230 A, dustproof glasses (not shown) are provided at the portions from which the first light beam L 1  and the second light beam L 2  are emitted. The dustproof glasses prevent foreign materials such as dust from entering the first housing  230 A. 
     The scanning lens  53  and the scanning lens  54  ( FIG. 3 ) of the present embodiment both have a free curved surface that is defined by the equation (1). In the equation (1), “z” represents a surface shape of the scanning lens  53  and the scanning lens  54  in the main scanning direction. Table 1 shows values of coefficients that can be substituted into the equation (1), for each of the incident surface and the emission surface. It is noted that although coefficients for the surface shape in the sub scanning direction are omitted, the sub scanning magnification between the polygon mirror  52  and the photoconductor drum  21 Y (the photoconductor drum  21 M) is set to three times or less. In particular, in the present embodiment, the length of the optical path from the polygon mirror  52  to the scanning lens  53  and the scanning lens  54  is set to be relatively long, and the length of the optical path from the scanning lens  53  and the scanning lens  54  to circumferential surfaces of the photoconductor drums  21 Y and  21 M is set to be relatively short. As a result, due to the scanning lens  53  and the scanning lens  54  each being a single lens, the sub scanning magnification is set to a small value. 
     
       
         
           
             
               
                 
                   Math 
                   . 
                   
                       
                   
                   ⁢ 
                   1 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                       
                   
                   ⁢ 
                   
                     z 
                     = 
                     
                       
                         
                           
                             r 
                             2 
                           
                           / 
                           R 
                         
                         
                           1 
                           + 
                           
                             
                               1 
                               - 
                               
                                 
                                   ( 
                                   
                                     1 
                                     + 
                                     k 
                                   
                                   ) 
                                 
                                 ⁢ 
                                 
                                   
                                     ( 
                                     
                                       r 
                                       / 
                                       R 
                                     
                                     ) 
                                   
                                   2 
                                 
                               
                             
                           
                         
                       
                       + 
                       
                         
                           ∑ 
                           
                             i 
                             = 
                             1 
                           
                           
                               
                           
                         
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     In the equation (1), 
     “R” denotes a main scanning curvature radius, 
     “k” denotes a main scanning conic coefficient, 
     “Ci” denotes a coefficient of the surface shape, and 
     “r” denotes a height in a direction perpendicular to the optical axis direction. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Incident surface 
                 Emission surface 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 R 
                 22.114 
                 21.537 
               
               
                   
                 k 
                 −7.265 
                 −6.818 
               
               
                   
                 C1 
                 0 
                   2.809E−03 
               
               
                   
                 C2 
                 −1.949E−03 
                 −3.945E−03 
               
               
                   
                 C3 
                 0 
                 −2.384E−06 
               
               
                   
                 C4 
                 −2.015E−06 
                 −1.660E−06 
               
               
                   
                 C5 
                 0 
                   4.639E−10 
               
               
                   
                 C6 
                   4.123E−10 
                   9.578E−11 
               
               
                   
                 C7 
                 0 
                 0 
               
               
                   
                 C8 
                 −3.186E−14 
                 −4.537E−14 
               
               
                   
                   
               
            
           
         
       
     
     The scanning lens  53  and the scanning lens  54  allow an excellent focus performance to be obtained when the distances from the emission surfaces of the lenses to the circumferential surfaces (scanned surfaces) of the photoconductor drums  21 Y and  21 M are each less than 110 mm. In addition, as shown in  FIG. 3 , the lens thickness (center thickness) of the scanning lens  53  and the scanning lens  54  is 9.0 mm. Furthermore, the distance from the deflection surface of the polygon mirror  52  to the incident surface of each lens is set to 23.7 mm. 
     As shown in  FIG. 2 , in the present embodiment, the polygon mirror  52  is arranged such that the rotation shaft thereof is inclined at a predetermined angle with respect to the first reference line DL that connects the rotation shafts centers of a pair of photoconductor drums  21  (photoconductor drums  21 Y and  21 M). The trajectory of the light beam deflected by the polygon mirror  52  is inclined at an acute angle θX with respect to the horizontal line (a straight line parallel to the first reference line DL). An optical path of the first light beam L 1  is formed in rear of the polygon mirror  52 , and an optical path of the second light beam L 2  is formed in front of the polygon mirror  52 . In this way, the first mirror group and the second mirror group are arranged such that the optical path of the first light beam L 1  and the optical path of the second light beam L 2  do not intersect with each other. 
     The rotation shaft of the polygon mirror  52  is inclined with respect to the up-down direction such that the front of the rotation shaft is located above, and the rear of the rotation shaft is located below. As a result, the first light beam L 1  emitted from the light source  81  for yellow is deflected by the polygon mirror  52  in the rear diagonally upward direction toward the scanning lens  53  in a direction of approaching the first reference line DL. The first light beam L 1  having transmitted through the scanning lens  53  is reflected by the reflection mirror  56  downward opposite to the first reference line DL. The reflection mirror  58 , in the cross section shown in  FIG. 2 , reflects the first light beam L 1  upward such that it passes through between the polygon mirror  52  and the scanning lens  53 . Subsequently, the first light beam L 1  is irradiated on the circumferential surface of the photoconductor drum  21 Y. With such a configuration, even if a part of the first light beam L 1  is reflected in a direction of separating away from the circumferential surface of the photoconductor drum  21 Y, the optical path length of the first light beam L 1  can easily be set to be the same as the optical path length of the second light beam L 2 . It is noted that an optical path of the first light beam L 1  that travels from the polygon motor  51  to the scanning lens  53  is arranged to be shifted, in the left-right direction (a direction perpendicular to the plane of  FIG. 2 ), from an optical path of the first light beam L 1  that travels from the reflection mirror  58  to the circumferential surface of the photoconductor drum  21 Y. 
     Similarly, the rotation shaft of the polygon mirror  52  is inclined with respect to the up-down direction such that the front of the rotation shaft is located above, and the rear of the rotation shaft is located below. As a result, the second light beam L 2  emitted from the light source  82  for magenta is deflected by the polygon mirror  52  in the front diagonally downward direction toward the scanning lens  54 , away from the first reference line DL. The second light beam L 2  having transmitted through the scanning lens  54  is reflected by the reflection mirror  57  upward in a direction of approaching the first reference line DL. Subsequently, the second light beam L 2  is reflected upward by the reflection mirror  59  and irradiated on the circumferential surface of the photoconductor drum  21 M. 
     In order to obtain good focusing on the photoconductor drums  21 Y and  21 M, the optical path lengths of the first light beam L 1  and the second light beam L 2 , in particular, the distance from the scanning lens  53  to the photoconductor drum  21 Y and the distance from the scanning lens  54  to the photoconductor drum  21 M, need to set to the same distance (length). 
     In conventional technologies, two laser beams deflected by the polygon mirror  52  are reflected by different mirrors and irradiated on the photoconductor drums. In order to ensure that each of the laser beams has a predetermined optical path length or more, the trajectories of the two laser beams reflected by the mirrors are arranged to intersect with each other above the polygon mirror  52  when viewed in a cross section taken along a plane including the sub scanning direction. This causes a problem that the height of the optical scanning device  23  is increased. 
     In the present embodiment, the light beams that have transmitted through the scanning lens  53  and the scanning lens  54  are reflected to be opposite to each other in the up-down direction. In addition, due to the configuration where the rotation shaft center of the polygon mirror  52  is inclined, even if the height of the first housing  230 A is set to be low in the up-down direction, the optical path lengths of the first light beam L 1  and the second light beam L 2  can easily be set to the same length. In other words, it is possible to ensure that each of the first light beam L 1  and the second light beam L 2  has a predetermined optical path length or more, while restricting the height of the first housing  230 A in the up-down direction to be low. In particular, compared to the other configuration where the optical paths of the first light beam L 1  and the second light beam L 2  are arranged to intersect with each other above the polygon mirror  52  in the cross section shown in  FIG. 2 , it is possible to reduce the height of the first optical scanning unit  23 A in a direction (up-down direction) in which the light beam is irradiated on the scanned surfaces (the photoconductor drums  21 Y and  21 M), while setting the optical path lengths of the first light beam L 1  and the second light beam L 2  to be the same. 
     Suppose that the first light beam L 1  reflected by the reflection mirror  56  passes through between the scanning lens  53  and the reflection mirror  56  and is irradiated on the photoconductor drum  21 Y in  FIG. 2 , then the optical path length of the first light beam L 1  is short. In the present embodiment, as described above, the optical path of the first light beam L 1  reflected by the reflection mirror  58  is arranged by using the space between the polygon mirror  52  and the scanning lens  53 . This makes it possible for the first light beam L 1  to proceed to below the scanning lens  53 , thereby the optical path length of the first light beam L 1  is ensured. As a result, it is possible to set the optical path length of the first light beam L 1  to be the same as that of the second light beam L 2  that is deflected by the polygon mirror  52  in a direction of separating away from the first reference line DL. It is noted that the second optical scanning unit  23 B, which has the same configuration as the first optical scanning unit  23 A, produces the same act and effect as those described above. 
     Furthermore, in the cross section of  FIG. 2 , the reflection mirror  58  and the reflection mirror  59  cause the first light beam L 1  and the second light beam L 2  to be irradiated on the photoconductor drum  21 Y and the photoconductor drum  21 M along directions that intersect with each other at a predetermined angle with respect to the second reference line DR, respectively. With reference to  FIG. 2 , the incident angle θY of the first light beam L 1  with respect to the second reference line DR is set to 10 degrees. As a result, compared to the case where the first light beam L 1  is incident along the second reference line DR, the reflection light from the photoconductor drum  21 Y is restricted. It is noted that this also applies to the incident angle of the second light beam L 2 . 
     In addition, as described above, when the scanning lens  53  and the scanning lens  54  are each a single lens, the sub scanning magnification between them tends to be small. Even in such a case, with the above-described configuration where a part of the first light beam L 1  is reflected in a direction opposite to the photoconductor drum  21 Y, the optical path length of the first light beam L 1  is ensured. As a result, it is possible to focus the first light beam L 1  on the circumferential surface of the photoconductor drum  21 Y with a high accuracy. 
     Furthermore, in the present embodiment, as shown in  FIG. 2 , the outer appearances of the first housing  230 A of the first optical scanning unit  23 A and the second housing  230 B of the second optical scanning unit  23 B are each a parallelogram in a cross section. As a result, the housings of the first optical scanning unit  23 A and the second optical scanning unit  23 B can be commonalized. As shown in  FIG. 2 , the first housing  230 A and the second housing  230 B are disposed so as to overlap with each other in a direction perpendicular to the first reference line DL (a direction in which the second reference line DR extends, the up-down direction) when viewed in a cross section taken along a plane including the sub scanning direction. In  FIG. 2 , a second housing rear wall  230 B 1  of the second housing  230 B is disposed above a first housing front wall  230 A 1  of the first housing  230 A with a gap therebetween. In particular, in the present embodiment, the first mirror group and the second mirror group are disposed such that the reflection mirror  57  of the first optical scanning unit  23 A and the reflection mirror  56  of the second optical scanning unit  23 B overlap with each other in the direction perpendicular to the first reference line DL. With such a configuration where parts of adjacent housings are set to overlap with each other, the width of the optical scanning device  23  in the horizontal direction (the front-rear direction) can be reduced. As a result, the inter-axial distance between a plurality of photoconductor drums  21  that are disposed in alignment at predetermined intervals can be set to be small. In the present embodiment, by using the arrangement of the optical path of the second light beam L 2  in the first optical scanning unit  23 A and the optical path of the first light beam L 1  in the second optical scanning unit  23 B, the two housings can be arranged to overlap with each other as described above. 
     Up to now, an embodiment of the present disclosure has been described. With such a configuration, spaces in front and rear of the polygon mirror  52  can be used to form the optical paths of the first light beam L 1  and the second light beam L 2 . The optical path of the first light beam L 1  is arranged such that it once approaches, then separates away from, and then approaches again the circumferential surface of the photoconductor drum  21 Y. This makes it possible to ensure the optical path length of the first light beam L 1 . On the other hand, the optical path of the second light beam L 2  is arranged such that it once separates away from, and then approaches the circumferential surface of the photoconductor drum  21 M. This makes it possible to reduce the height of the optical scanning device  23  in a direction in which the light beams are irradiated on the photoconductor drums  21 , while setting the optical path lengths of the first light beam L 1  and the second light beam L 2  to be the same. It is noted that the present disclosure is not limited to the above-described configuration, but can be modified, for example, as follows. 
     (1) In the above-described embodiment, two housings are arranged to overlap with each other partially such that the reflection mirror  56  of the second optical scanning unit  23 B is disposed above the reflection mirror  57  of the first optical scanning unit  23 A. However, the present disclosure is not limited to this configuration. In a modified embodiment, the first mirror group and the second mirror group may be arranged such that, when viewed in a cross section taken along a plane including the sub scanning direction, the optical path of the second light beam L 2  in the first optical scanning unit  23 A overlaps with the optical path of the first light beam L 1  in the second optical scanning unit  23 B in a direction perpendicular to the first reference line DL. With this configuration, too, the two optical scanning units  23 A and  23 B installed in the image forming apparatus  1  are arranged to overlap with each other partially, thereby making it possible to reduce the height and the width in the sub scanning direction, of the image forming apparatus  1 . As a result, the inter-axial distance between a plurality of photoconductor drums  21  can be set to be small. 
     (2) In the above-described embodiment, a plurality of photoconductor drums  21  are arranged above the optical scanning device  23 . However, the present disclosure is not limited to this configuration. In another modified embodiment, the members shown in  FIG. 2  may be reversed in the up-down direction such that the optical scanning device  23  is disposed above the plurality of photoconductor drums  21 , and the exposure light is irradiated downward. 
     It is to be understood that the embodiments herein are illustrative and not restrictive, since the scope of the disclosure is defined by the appended claims rather than by the description preceding them, and all changes that fall within metes and bounds of the claims, or equivalence of such metes and bounds thereof are therefore intended to be embraced by the claims.