Abstract:
An optical scanning apparatus includes a housing, light deflector, first optical system and reinforcement member. The housing includes a peripheral wall and a partitioning portion to divide an inner side of the peripheral wall into first and second portion. The partitioning portion includes a first hole to cause the first portion to communicate with the second portion to form an optical path. The light deflector mounted in the first portion has a rotatable polygon mirror to perform deflection scanning of scanning light beams emitted from light sources and a drive unit to drive the rotatable polygon mirror to rotate. The first optical system mounted at least in the first portion causes the scanning light beams reflected from the rotatable polygon mirror to return towards the first hole. The reinforcement member mounted on a surface facing at least the second portion on the partitioning portion reinforces the housing.

Description:
[0001]    This application is based on and claims the benefit of priority from Japanese Patent Application No. 2010-128557, filed on 4 Jun. 2010, the content of which is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an optical scanning apparatus for irradiating a scanning light beam onto a surface of an image carrier, and to an image forming apparatus including the same. 
         [0004]    2. Related Art 
         [0005]    An electrophotographic process is used in such a type of image forming apparatus. A polygon mirror is provided inside a housing body. The polygon mirror is configured to perform deflection scanning in a predetermined direction of scanning light beams emitted from a plurality of light sources. The scanning light beams are irradiated onto image carriers that are charged in advance with electrostatic charges, for example, onto surfaces of photosensitive drums. In this manner, electrostatic latent images are respectively formed on the surfaces of respective photosensitive drums. Toner images corresponding to respective colors are transferred and fixed onto a sheet of paper. 
         [0006]    In this connection, the polygonal mirror is driven by rotation of a motor shaft. When a vibration occurs as a result of driving of the polygonal mirror, a vibration is induced at a scanning (imaging) position of a scanning light beam. Accordingly, it is likely that a writing position of an image by the scanning light beam becomes unstable (an occurrence of a jitter image). As a result, a technique has been developed to suppress the production of such a jitter image. 
       SUMMARY OF THE INVENTION 
       [0007]    In recent years, the demand for downsizing of an image forming apparatus has increased. A light scanning apparatus tends to occupy a large space within the image forming apparatus. As a result, there has been a constantly increasing demand for a reduction in the height or downsizing of the optical forming apparatus. A housing body having an H-shape in sectional view has been proposed as a structure for responding to the above demand. 
         [0008]    More specifically, the housing body having an H-shape in sectional view includes a partitioning plate inside peripheral walls, and for example, divides vertically the inside of the peripheral walls into a first portion and a second portion. The first portion is provided with an optical system such as mirrors and a light deflector composed of a driving unit and a polygon mirror. Another optical system is provided in the second portion. 
         [0009]    A slit is formed in the partitioning plate to cause the first portion to communicate with the second portion. A scanning light beam coming from the polygon mirror is reflected by a mirror in the first portion, and is oriented through the slit towards a mirror in the second portion. 
         [0010]    It should be noted that the housing body reduces its strength if a slit is formed in the partitioning plate. As a result, when the slit is provided in the partitioning plate, the housing body may be susceptible to the effect of vibration produced by driving of the polygon mirror, resulting in a jitter image. 
         [0011]    In this case, it may be considered that a rib made of the same material as the housing body is arranged upright around the polygon mirror to prevent a decrease in the strength of the housing body. However, the housing body having an H-shape in sectional view is in particular susceptible to an effect of heat produced by the driven polygon mirror. 
         [0012]    In other words, since an ambient temperature of the first portion that includes the light deflector is higher than the second portion, a temperature difference occurs between the first portion and the second portion. As a result, the housing body tends to deform such that the light deflector in the first portion deforms to project. 
         [0013]    More specifically, even if a rib to limit vibration is provided in the first portion of the housing body having an H-shape in sectional view, the rib itself expands and an amount of deformation of the first portion increases, accordingly. Furthermore, the first portion deforms in all directions along the shape of the rib. Accordingly, an angle of inclination of the mirror changes to pose a problem that color misregistration of a color image increases (an occurrence of an image out of color registration). Furthermore, this problem conspicuously increases by a difference between amounts of deformation due to a mismatch in coefficients of linear expansion between the housing body and the drive unit. 
         [0014]    Therefore, it is an object of the present invention to provide an optical scanning apparatus that suppresses production of a jitter image or an image out of color registration in a multilayered housing body that includes a hole such as a slit, and to provide an image forming apparatus that includes such an optical scanning apparatus. 
         [0015]    The present invention relates to an optical scanning apparatus, which includes a housing body, a light deflector, a first optical system and a reinforcement member. The housing body includes a peripheral wall and a partitioning portion of a plate shape configured to divide an inner side of the peripheral wall into a first portion and a second portion. The partitioning portion includes a first hole configured to cause the first portion to communicate with the second portion to form an optical path. The light deflector, which is mounted in the first portion, has a rotatable polygon mirror configured to perform deflection scanning of scanning light beams emitted from a plurality of light sources and a drive unit configured to drive the rotatable polygon mirror to rotate through a drive shaft. The first optical system, which is mounted at least in the first portion, is configured to cause the scanning light beams reflected from the rotatable polygon mirror to return towards the first hole. The reinforcement member, which is mounted on a surface that faces at least the second portion on the partitioning portion, is configured to reinforce the housing body. 
         [0016]    In an aspect of the present invention, the reinforcement member is mounted near the second portion that is not provided with the light deflector that acts as a heat source, and more specifically, is mounted on a surface of the partitioning portion that faces the second portion of lower temperature. In this manner, it is possible to prevent an adverse effect on the strength of the housing body as a result of formation of the hole. Therefore, it is possible to suppress vibration of the partitioning portion and to prevent warping due to thermal deformation in comparison to a rib simply provided in an upright orientation to prevent vibration. Therefore, it is possible to prevent an adverse effect on image quality resulting from a jitter image or an image out of color registration. 
         [0017]    Moreover, since it is possible to prevent a jitter image or an image out of color registration by a configuration only using a reinforcement member, a simple structure is satisfactory and the manufacturing cost of the optical scanning apparatus can be reduced, in comparison to the following examples: An example is that a support position is provided between a hole and another hole in the partitioning portion, and an optical system is provided at the support position to ensure the strength of a housing body. Another example is that a housing is cooled with cooling air from an air blower to reduce the temperature difference. 
         [0018]    The present invention provides the optical scanning apparatus that includes the multilayered housing body having the hole and provided with the reinforcement member in the second portion of lower temperature, such that an occurrence of a jitter image or an image out of color registration is prevented. In addition, the present invention provides the image forming apparatus including the optical scanning apparatus. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  illustrates the configuration of a printer according to the present embodiment; 
           [0020]      FIG. 2  illustrates the configuration of the printer according to  FIG. 1  that includes a controller; 
           [0021]      FIG. 3  is a plan view of an exposure device illustrated in  FIG. 1 ; 
           [0022]      FIG. 4  is a sectional view along the line TV-IV in  FIG. 3 ; 
           [0023]      FIG. 5  is a back view of the exposure device illustrated in  FIG. 1 ; 
           [0024]      FIG. 6  is a schematic diagram illustrating an exposure device according to a second embodiment; 
           [0025]      FIG. 7  is a sectional view of an exposure device according to a third embodiment; and 
           [0026]      FIG. 8  is a back view of the exposure device illustrated in  FIG. 7 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    Hereafter, embodiments of the present invention will be described with reference to the figures. 
         [0028]      FIG. 1  illustrates the configuration of a color printer according to the present embodiment. A sectional view illustrated in  FIG. 1  is seen from the left side of a printer  1 . As a result, a front surface of the printer  1  is positioned on the right side of  FIG. 1 , and a back surface of the printer  1  is positioned on the left side of  FIG. 1 . The printer  1  is provided with an apparatus main body  2 . A discharge tray  92  is provided on an upper portion of the apparatus main body  2 . A front cover  81  is provided in proximity to the discharge tray  92 . A plurality of operation keys enabling various operations by a user and a screen for displaying various types of information are disposed on the front cover  81 . 
         [0029]    A paper cassette  3  is disposed at a lower portion of the apparatus main body  2 . Sheets of paper (recording media) before image formation are stacked in a storage unit  82  of the paper cassette  3 . The sheets of paper are separated into individual sheets and fed to inside the apparatus main body  2 . 
         [0030]    More specifically, a feed roller  83  is provided on the right above the storage unit  82  as shown in  FIG. 1 . The sheets of paper are fed right and upward with respect to the paper cassette  3 . A sheet of paper thus fed is conveyed upwardly inside the apparatus main body  2  along the front surface of the printer  1 . The paper cassette  3  is configured to be drawable towards the front surface of the printer  1 ; that is to say, towards the right side of  FIG. 1 . When the paper cassette is pulled out, it is possible that the storage unit  82  is replenished with new sheets of paper, or the existing sheets of paper are substituted with another type of sheets of paper. 
         [0031]    A conveyance roller  84 , registration roller  5 , image forming unit  8 , and transfer unit  12  are disposed in sequence along a direction of paper conveyance inside the apparatus main body  2 . Four drum units  7  are disposed in parallel with each other at the image forming unit  8  according to the present embodiment. Photosensitive drums (image carriers)  10  for supporting visible images (toner images) in respective colors are provided on drum units  7 , respectively (refer to  FIGS. 1 and 2 ). Each photosensitive drum  10  is driven in a clockwise direction as shown in  FIG. 1  by a driving motor (not illustrated). 
         [0032]    An exposure apparatus (light scanning apparatus)  6  is provided below the image forming unit  8 , that is to say, between photosensitive drums  10  and the paper cassette  3 . The exposure apparatus  6  irradiates four different colors of laser light, for example, yellow, magenta, cyan, and black, towards the photosensitive drums  10 , respectively. As illustrated in  FIG. 1 , a charging device  85 , development device  86 , intermediate transfer roller  87 , and cleaning unit  88  are provided at a suitable position on a periphery of each photosensitive drum  10 . 
         [0033]    The charging device  85  is positioned at a lower portion of each photosensitive drum  10 , and uniformly charges a surface of each photosensitive drum  10 . As shown in  FIG. 1 , the development device  86  is disposed on a left side of each photosensitive drum  10 . An intermediate transfer belt  9  is disposed above the photosensitive drums  10 . Four toner containers  89  are disposed between the intermediate transfer belt  9  and a discharge tray  92 . The toner containers  89  are disposed in a sequence of yellow, magenta, cyan, and black from the back surface to the front surface of the printer  1 . A toner container for black is configured to have the largest capacity. 
         [0034]    The yellow, magenta, cyan, and black toner are electrostatically attached to respective surfaces of photosensitive drums  10 . In this manner, toner images corresponding to electrostatic latent images created by the exposure device  6  are developed on the respective surfaces of photosensitive drums  10 . 
         [0035]    The toner images formed on the surfaces of the photosensitive drums  10  are transferred in sequence to the intermediate transfer belt  9  and superimposed as a toner image corresponding to one page. A transfer roller  13  is provided at the transfer unit  12 . The transfer roller  13  is configured to be in press contact with the transfer belt  9  obliquely from below. When a sheet of paper passes between the transfer belt  9  and the transfer roller  13 , the toner image that is superimposed on the transfer belt  9  is transferred onto the sheet of paper. 
         [0036]    A fixing unit  14 , discharge branching portion  90  and discharge roller  16  are disposed in sequence downstream of the transfer unit  12  with reference to the direction of paper conveyance. The sheet of paper fed from the fixing unit  14  is discharged to the discharge tray  92 . 
         [0037]    In the present embodiment, a conveyance path  91  for duplex printing is provided between the transfer unit  12  and a manual feed tray  80 . The conveyance path  91  branches from the discharge branching portion  90  on a front side of the apparatus main body  2 , extending downwardly to be connected with an upstream side of the registration roller  5 . 
         [0038]    Various types of optical devices are assembled into the exposure apparatus  6  in the present embodiment. More specifically, as illustrated in  FIGS. 3 to 5 , the exposure apparatus  6  includes a box-shaped housing (housing body)  30  formed of plastic. The housing (housing body)  30  includes a partitioning plate (partitioning portion of a plate shape) that has a substantially quadrilateral shape when viewed in plan. Respective sides of the partitioning plate  36  are enclosed by four peripheral walls including a front surface  31 , back surface  32  and side surfaces  33   a ,  33   b.    
         [0039]    The front surface  31  and the back surface  32  are opposed to each other in a primary scanning direction (direction of a rotational axis of the photosensitive drums  10 ). The side surfaces  33   a ,  33   b  are opposed to each other in a secondary scanning direction (direction of paper conveyance). 
         [0040]    As illustrated in  FIG. 4 , the housing  30  includes a cover  61   u  that includes a slit  62   a  and a slit  62   b  at a predetermined position, and a cover  62   d . One end of the front surface  31 , the back surface  32 , and the side surfaces  33   a ,  33   b  respectively abut with the cover  61   u . The other end of the front surface  31 , the back surface  32 , and the side surfaces  33   a ,  33   b  respectively abut with the cover  62   d .  FIGS. 3 and 5  omit illustration of the above surfaces to facilitate description of the structure inside the housing  30 . 
         [0041]    The exposure apparatus  6  according to the present embodiment is secured to the apparatus main body  2  with screws (not illustrated), in such a configuration that the cover  61   u  is oriented towards the photosensitive drum  10  and the cover  62   d  is oriented towards the paper cassette  3 . The partitioning plate  36  is provided where it partitions the inside of the housing  30 , for example, into upper and lower portions. 
         [0042]    More specifically, the partitioning plate  36  according to the present embodiment is connected to a substantially central position with respect to the front surface  31 , the back surface  32  and the side surfaces  33   a ,  33   b , when seen from a height direction (an H-shaped structure in the sectional view of  FIG. 4 ). The partitioning plate  36  extends substantially parallel to the covers  61   u ,  62   d.    
         [0043]    The partitioning plate  36  partitions the inside of the housing  30  into a first portion  61  and a second portion  62 . As illustrated in  FIG. 4 , the partitioning plate  36  includes a reference surface of higher temperature  36   u  and a reference surface of lower temperature  36   d  (surface facing the second portion). 
         [0044]    More specifically, the first portion  61  is a space enclosed by the cover  61   u , the front surface  31 , the back surface  32 , the side surfaces  33   a ,  33   b  and the reference surface of higher temperature  36   u . On the other hand, the second portion  62  is a space enclosed by the reference surface of lower temperature  36   d , the front surface  31 , the back surface  32 , the side surfaces  33   a ,  33   b  and the cover  62   d.    
         [0045]    The housing  30  according to the present embodiment includes an optical device composed of two light sources  40  (refer to  FIG. 2 ) in addition to a light deflector, an optical system, or the like. 
         [0046]    In other words, an exposure apparatus  6 A corresponding to yellow and magenta images, for example, illustrated on the left side of  FIG. 1  and an exposure apparatus  6 B corresponding to cyan and black images, for example, illustrated on the right side of  FIG. 1  are separately mounted on the printer  1 . 
         [0047]    In the description below, the former exposure apparatus  6 A corresponding to the yellow and magenta images will be used as an example. 
         [0048]    The two light sources  40  of the exposure apparatus  6 A are disposed in parallel at the front surface  31  outside the housing  30 . The two light sources  40  include a yellow laser diode (LD) and a magenta LD. A light beam in a visible region from each independent LD is respectively irradiated towards the light deflector inside the housing  30 . 
         [0049]    The light deflector is disposed in the first portion  61  and includes a polygon mirror (rotatable polygon mirror)  42  and a driving unit  44  (refer to  FIG. 3 ). The polygon mirror  42  has a planar shape of a regular polygon. Each side surface of the polygon mirror  42  is formed as a planar mirror. A central portion of the polygon mirror  42  is fixed to a motor shaft  46  (refer to  FIG. 4 ). 
         [0050]    The drive unit  44  includes a driving motor  45  that is connected to the motor shaft  46 . The motor  45  is mounted on a rectangular circuit board  48  together with electrical components (not shown) such as integrated circuits, electrical resistors, or the like. The circuit board  48  is disposed under the polygon mirror  42 , extending towards the back surface  32  and is fixed to the reference surface of higher temperature  36   u.    
         [0051]    The optical system described above is provided in a region in which laser light reflected by the polygon mirror  42  travels. The above optical system includes fθ lenses  50   a ,  50   b  and  f θ lenses  51   a ,  51   b  for yellow and magenta, and six planar folding mirrors  52   a ,  52   b ,  54   a ,  54   b ,  56   a , and  56   b.    
         [0052]    The yellow and magenta fθ lenses  50   a ,  50   b  are provided in the first portion  61 , and are respectively disposed along the secondary scanning direction to be opposite to each other with respect to the polygon mirror  42 . For example, the yellow fθ lens  50   a  is provided on a left side of the polygon mirror  42  as illustrated in  FIGS. 3 and 4 . The magenta fθ lens  50   b  is provided on a right side of the polygon mirror  42 . 
         [0053]    Next, the yellow fθ lens  51   a  is provided in parallel on a left side of the yellow fθ lens  50   a  as illustrated in  FIGS. 3 and 4 . The magenta fθ lens  51   b  is provided in parallel on a right side of the fθ lens  50   b . The fθ lenses  51   a ,  51   b  are also provided in the first portion  61 . 
         [0054]    The yellow folding mirrors  52   a ,  54   a ,  56   a  are provided on the left with respect to the polygon mirror  42  as illustrated in  FIG. 4 . 
         [0055]    Firstly, the folding mirror  52   a  is disposed in proximity to an intersection of the side surface  33   a  and the reference surface of higher temperature  36   u  in the first portion  61 . Both ends of the folding mirror  52   a  are supported by a plastic mirror support member (not illustrated) formed on the reference surface  36   u , and are disposed inclined at a predetermined angle along a primary scanning direction. 
         [0056]    In contrast, the folding mirrors  54   a ,  56   a  are provided in the second portion  62 . 
         [0057]    More particularly, the folding mirror  54   a  is disposed below opposite to the folding mirror  52   a  with respect to the partitioning plate  36 . The folding mirror  56   a  is disposed below the yellow fθ lens  50   a . Both ends of each of the folding mirrors  54   a ,  56   a  are supported by a plastic mirror supporting member (not illustrated) formed on the reference surface of lower temperature  36   d  and are disposed inclined at a predetermined angle along the primary scanning direction. 
         [0058]    In the present embodiment as described above, the polygon mirror  42 , the fθ lens  50   a ,  51   a , and the folding mirror  52   a  are provided in the first portion  61 . The folding mirrors  54   a ,  56   a  are provided in the second portion  62 . Moreover, the exposure apparatus  6 A is disposed under each photosensitive drum  10 . For that reason, a slit (hole)  37   a  and a slit (another hole)  38   a  are formed in the partitioning plate  36 . 
         [0059]    More specifically, as illustrated in  FIG. 4 , the slit  37   a  penetrates the reference surface of lower temperature  36   d  and the reference surface of higher temperature  36   u  at a position at which a scanning light beam from the folding mirror  52   a  is guided to the folding mirror  54   a . The slit  38   a  penetrates the reference surface of lower temperature  36   d  and the reference surface of higher temperature  36   u  at a position at which a scanning light beam from the folding mirror  56   a  is guided to the photosensitive drum  10 . 
         [0060]    As illustrated in  FIG. 4 , the slit  38   a  is formed between the fθ lens  50   a  and the fθ lens  51   a  on a left side of the polygon mirror  42  in the partitioning plate  36  according to the present embodiment. The two slits  37   a ,  38   a  for yellow respectively extend along the primary scanning direction similarly with the disposition direction of the folding mirrors  52   a ,  54   a , and  56   a.    
         [0061]    The folding mirrors  52   b ,  54   b , and  56   b  for magenta are disposed on a right side of the polygon mirror  42  in  FIGS. 3 and 4 , and are provided similarly with the folding mirrors  52   a ,  54   a , and  56   a  for yellow. Furthermore, the partitioning plate  36  includes two slits for magenta, a slit (hole)  37   b  and a slit (another hole)  38   b  that are disposed geometrically similar to the slits  37   a ,  38   a  for yellow. 
         [0062]    When each light source  40  for yellow or magenta irradiates a laser light beam in response to a signal from a controller  20  provided in the apparatus main body  2  as illustrated in  FIG. 2 , the laser light beam travels towards the polygon mirror  42  through corresponding collimator lens, prism and cylindrical lens. 
         [0063]    Similarly, when the motor shaft  46  rotates at a high speed with a drive force applied by the driving motor  45  in response to a signal from the controller  20 , the polygon mirror  42  rotates at a high speed. 
         [0064]    Respective yellow and magenta laser light beams enter the polygon mirror  42  with a minute angle of deviation. The rotatable polygon mirror  42  deflects a laser light beam and delivers the deflected laser light beam towards the side surface  33   a  ( 33   b ). 
         [0065]    The laser light beam reflected by the polygon mirror  42  is deflected at an equal speed by the corresponding fθ lenses  50   a  ( 50   b ) and  51   a  ( 51   b ). 
         [0066]    The laser light beam that has passed through the fθ lenses  50   a  ( 50   b ) and  51   a  ( 51   b ) is reflected downwards by the folding mirror  52   a  ( 52   b ), and passes through the slit  37   a  ( 37   b ) in the partitioning plate  36  to reach the second portion  62 . 
         [0067]    Then, the laser light beam is reflected by the folding mirror  54   a  ( 54   b ) towards inside the housing  30 . Thereafter, the laser light beam is reflected again upwardly by the folding mirror  56   a  ( 56   b ). The laser light beam passes through the slit  38   a  ( 38   b ) of the partitioning plate  36  and the slit  62   a  ( 62   b ) of the cover  61   u , and reaches a surface of the photosensitive drum  10  for yellow (the photosensitive drum  10  for magenta). 
         [0068]    Similarly, in the exposure apparatus  6 B corresponding to cyan and black images, a laser light beam of each light source  40  passes through the polygon mirror  42 , and corresponding fθ lenses  50   a  ( 50   b ) and  51   a  ( 51   b ), and the folding mirrors  52   a  ( 52   b ),  54   a  ( 54   b ) and  56   a  ( 56   b ) to reach a surface of the photosensitive drum  10  for cyan (the photosensitive drum  10  for black), in accordance with a signal from the controller  20 . 
         [0069]    A rib (reinforcement member)  70  is provided in the respective second portions  62  of the exposure devices  6 A,  6 B according to the present embodiment. 
         [0070]    More specifically, as illustrated in  FIGS. 4 and 5 , the rib  70  according to the present embodiment includes an inner rib  72  and outer ribs  74   a ,  74   b , and is attached to the reference surface of lower temperature  36   d  of the partitioning plate  36 . 
         [0071]    More specifically, the inner rib  72  and the outer ribs  74   a ,  74   b  are configured substantially in the shape of the letter U when viewed in cross section. The inner rib  72  is provided on the reference surface  36   d ; at a position that straddles the circuit board  48  in the secondary scanning direction (refer to  FIGS. 4 and 5 ). 
         [0072]    The outer rib  74   a  is disposed at a position that straddles the fθ lens  51   a  in the secondary scanning direction; that is to say, between the slit  37   a  and the slit  38   a  (refer to  FIG. 4 ). The outer rib  74   b  is disposed at a position that straddles the fθ lens  51   b  in the secondary scanning direction; that is to say, between the slit  37   b  and the slit  38   b  (refer to  FIG. 4 ). 
         [0073]    The inner rib  72 , and the outer ribs  74   a ,  74   b  are integrally formed on the reference surface  36   d  using a plastic formed from the same material as the partitioning plate  36 . The inner rib  72  and the outer ribs  74   a ,  74   b  respectively extend along the primary scanning direction similarly with the direction of disposition of the two slits  37   a ,  38   a  for yellow, and the folding mirrors  52   a ,  54   a , and  56   a.    
         [0074]    Returning now to  FIG. 1 , when the printer  1  including the exposure apparatus  6 A,  6 B above executes a printing operation, sheets of paper are separated into individual sheets and conveyed from the paper cassette  3  by the feed roller  83 , and reach the registration roller  5 . The registration roller  5  corrects an inclined feed orientation of a sheet of paper, and feeds the sheet of paper towards the transfer unit  12  while adjusting timing of transferring a toner image formed by the image forming unit  8 . 
         [0075]    An input port  22  illustrated in  FIG. 2  is configured to receive from an external unit an image data based on which a printing operation is performed. The image data include various types of images such as characters and reference numerals, figures, symbols, line diagrams, patterns, and the like, in data format. The controller  20  uses the data to control the irradiation of a laser light beam and the like. 
         [0076]    More specifically, the charging device  85  charges the surface of each photosensitive drum  10 , and the exposure apparatus  6  irradiates a laser light beam onto the surface of each photosensitive drum  10 . In this manner, an electrostatic latent image is formed on the surface of each photosensitive drum  10 , and a toner image in each color is formed using the electrostatic latent image. 
         [0077]    Each toner image is transferred onto the sheet of paper through the transfer belt  9  in the transfer unit  12 . Toner remaining on the surface of each photosensitive drum  10  is removed by the cleaning unit  88 . 
         [0078]    Thereafter, the sheet of paper is fed to the fixing unit  14  with an unfixed toner image thereon. The sheet of paper undergoes heating and pressuring in the fixing unit  14  such that the toner image is fixed. Next, the sheet of paper that has been fed from the fixing unit  14  is discharged to the discharge tray  92  by the discharge roller  16 . 
         [0079]    When duplex printing is executed instead of simplex printing, a direction of conveying the sheet of paper that is discharged from the fixing unit  14  is switched at the discharge branching portion  90 . That is to say, the sheet of paper that is printed on one side is drawn into the apparatus main body  2 , and conveyed to the conveyance path  91  for duplex printing. Next, the sheet of paper is fed upstream of the registration roller  5 , and is fed again to the transfer unit  12 . In this manner, a toner image is transferred onto a surface of the sheet of paper on which printing has not yet been performed. 
         [0080]    The inner rib  72  and outer ribs  74   a ,  74   b  according to the present embodiment that are substantially in the shape of the letter U in sectional view are made of a plastic similar to the partitioning plate  36 . With respect to the outer rib  74   a  ( 74   b ), a width of a portion  75   a  ( 75   b ) closer to the side wall  33   a  ( 33   b ) is substantially equal to a width of a portion  76   a  ( 76   b ) closer to the center of the housing  30  (refer to  FIGS. 4 and 5 ). The outer rib  74   a  ( 74   b ) may be configured such that it is less susceptible to deformation as it approaches more the drive unit  44  that is a heat source. 
         [0081]    More specifically, for example,  FIG. 6  illustrates an enlarged view of a periphery of the outer rib  74   a . In relation to the outer rib  74   a , the width of the portion  76   a  closer to the center of the housing  30  is configured to be greater than the width of the portion  75   a  closer to the side wall  33   a.    
         [0082]    On the other hand, in each of the present embodiments, the inner rib  72  and the outer ribs  74   a ,  74   b  are integrated with the partitioning plate  36 . 
         [0083]    With respect to the outer rib  74   a  ( 74   b ), the portion  76   a  ( 76   b ) closer to the center of the housing  30  may alternatively be made of a material that has a higher stiffness than the portion  75   a  ( 75   b ) closer to the side wall  33   a  ( 33   b ). This configuration also allows the outer rib  74   a  ( 74   b ) to be less susceptible to deformation as it approaches more the drive unit  44 . 
         [0084]    Furthermore, the rib  70  may alternatively be configured to be separate from the partitioning plate  36 , and made of a different material that has a higher stiffness than the partitioning plate  36 . 
         [0085]    More specifically, in an example illustrated in  FIGS. 7 and 8 , an opening  39  is formed in a partitioning plate  36 , extending from the proximity of the center of the partitioning plate  36  towards the back surface  32 . Except for the differences described above, the example illustrated in  FIGS. 7 and 8  is similar to the above embodiments. Accordingly, the same reference numerals are used for components of the example illustrated in  FIGS. 7 and 8 , which have functions similar to those of the above embodiments. Descriptions about such components will not be repeated. A rib  70  in the example illustrated in  FIGS. 7 and 8  includes inner ribs  72   a ,  72   b  and outer ribs  74   a ,  74   b , and is attached to a reference surface of lower temperature  36   d.    
         [0086]    The inner ribs  72   a ,  72   b  and the outer ribs  74   a ,  74   b  are formed from a plate having a higher stiffness than the material used in the partitioning plate  36 , for example, a metal plate or a glass plate. An inner rib  72   a  ( 72   b ) is provided on the reference surface  36   d  opposite to the fθ lens  50   a  ( 50   b ); at a position that straddles the circuit board  48  in the secondary scanning direction. 
         [0000]    An outer rib  74   a  is disposed between the slit  37   a  and the slit  38   a . An outer rib  74   b  is provided between the slit  37   b  and the slit  38   b , extending along the primary scanning direction similarly with the disposition direction of the inner rib  72  and the outer ribs  74   a ,  74   b  in the embodiment described above (refer to  FIG. 8 ). 
         [0087]    As described above, according to each of the above embodiments, since the housing  30  is configured with an H-shaped structure when viewed in cross section, it is possible to achieve a reduction in the height and downsizing of the exposure apparatus  6 . 
         [0088]    The inside of the housing  30  is partitioned into the first portion  61  and the second portion  62  by the partitioning plate  36 . The polygon mirror  42  and the drive unit  44  are provided in the first portion  61 . A scanning light beam from the polygon mirror  42  is reflected by the folding mirror  52   a  ( 52   b ) through the slit  37   a  ( 37   b ) formed in the partitioning plate  36  of the first portion  61 , and is oriented towards the second portion  62 . 
         [0089]    When the partitioning plate  36  of the housing  30  includes the slits  37   a ,  37   b , the strength of the housing  30  reduces in comparison to a configuration in which the partitioning plate  36  does not include the slits  37   a ,  37   b.    
         [0090]    Furthermore, as illustrated in the present embodiment, the slits  38   a ,  38   b  are required of the partitioning plate  36  to enable the scanning light beam from the polygon mirror  42  to pass from the first portion  61  through the second portion  62  to reach the first portion  61  again. Consequently, the strength of the housing  30  further reduces. 
         [0091]    When the polygon mirror  42  is driven by the rotation of the motor shaft  46 , a vibration is produced due to the rotation of the motor shaft  46 . This vibration is easily transmitted from the partitioning plate  36  provided with the slits  37   a ,  37   b ,  38   a ,  38   b  to the mirror support member. As a result, there is a risk that a writing position of an image started by the scanning light beam will not be stable (occurrence of a jitter image). 
         [0092]    Furthermore, heat is also produced by the rotation of the motor shaft  46 . This heat increases an ambient temperature of the first portion  61 . Accordingly, it applies an expansion force to the fθ lenses  50   a ,  50   b ,  51   a ,  51   b  and the folding mirrors  52   a ,  52   b  of the first portion  61 . Since the drive unit  44  that acts as a heat source is not provided in the second portion  62 , an ambient temperature of the second portion  62  will be lower than the first portion  61 . As a result, an expansion force as much as that of the first portion  61  does not act on the folding mirrors  54   a ,  54   b ,  56   a , and  56   b  of the second portion  62 . 
         [0093]    That is to say, a temperature difference is present between the first portion  61  and the second portion  62 . The expansion produced in the reference surface of higher temperature  36   u  of the partitioning plate  36  is restricted by the reference surface of lower temperature  36   d . As a result, a tensional force acts on the first portion  61  of higher temperature and a compression force acts on the second portion  62  of lower temperature. The polygon mirror  42  of the first portion  61  rises upwardly in the housing  30 . A central portion of the partitioning plate  36  in an orientation illustrated in  FIG. 4 , for example, is deformed to project upwardly. Accordingly, inclination angles of the folding mirrors  52   a ,  52   b  will change. Consequently, it is concerned that the color misregistration of a color image will increase (occurrence of an image out of color registration). 
         [0094]    In particular, in the partitioning plate  36  that includes the slits  37   a ,  37   b ,  38   a ,  38   b , the slits  37   a ,  37   b ,  38   a ,  38   b  extend in the primary scanning direction similarly with the folding mirrors  52   a ,  52   b . Therefore, the partitioning plate  36  tends to warp in a direction that is substantially orthogonal to the primary scanning direction (that is to say, the secondary scanning direction). Accordingly, the color misregistration of a color image in the secondary scanning direction will further increase. 
         [0095]    However, in the present embodiment, the rib  70  is attached to the second portion  62  in which the circuit board  48  and the driving motor  45  of heat sources are not disposed (more specifically, the reference surface of lower temperature  36   d  of the partitioning plate  36 ). In this manner, a reduction in the strength of the housing  30  resulting from the formation of the slits  37   a ,  37   b ,  38   a ,  38   b  is prevented. In comparison to a case in which a rib simply provided in an upright configuration for the purpose of suppressing vibration, it is possible to realize a compact exposure apparatus  6 , in addition to not only suppressing vibration of the partitioning plate  36 , but also avoiding warping of the partitioning plate  36  due to thermal deformation. Therefore, it is possible to prevent deterioration in image quality due to a jitter image or color misregistration of the color image. 
         [0096]    In addition, the prevention of a jitter image or color misregistration of the color image is implemented by the rib  70 . Consequently, it is possible to allow the exposure apparatus  6  to be simply configured and suppress the manufacturing cost in comparison to the following cases: A support position is provided between the slit  37   a  and the slit  38   a  of the partitioning plate  36 , and folding mirrors are provided at this support position to maintain the strength of the housing. Alternatively, an air blower is provided to supply a cooling air to reduce the temperature difference. 
         [0097]    The rib  70  is provided between the slit  37   a  and the slit  38   a  or at a position straddling the drive unit  44 , respectively. As a result, even when the strength of the housing  30  reduces further, it is possible to prevent warping of the polygon mirror  42  of the first portion  61  into a projection. 
         [0098]    As illustrated in the example in  FIG. 4 , when the rib  70  is formed of the same material as the housing  30  to be integral with the housing  30 , it is possible to eliminate the trouble associated with providing the rib  70  as a separate component on the reference surface of lower temperature  36   d.    
         [0099]    With the example illustrated in  FIG. 7 , when the rib  70  is configured separate from the housing  30  using a material having a higher stiffness than the housing  30 , the transmission of the vibration of the motor shaft  46  to the partitioning plate  36  is suppressed on the whole. In addition, it is possible to suppress the resonance due to the natural frequency of the housing  30  being close to the rotation frequency of the motor shaft  46 . 
         [0100]    As illustrated in the example in  FIG. 6 , a locational temperature difference between the first portion  61  and the second portion  62  increases as a location approaches more to the heat source. However, since the outer rib  74   a  is configured to be more resistant to deformation as it approaches more the heat source, it is possible to prevent more efficiently warping of the polygon mirror  42  of the first portion  61  into a projecting configuration. 
         [0101]    Furthermore, since deterioration of image quality due to a jitter image or color misregistration of the color image is prevented, and superior image quality is obtained, the reliability of the printer  1  is increased. 
         [0102]    The present invention is not limited to the above embodiments, and various modifications thereof may be performed within a scope that does not depart from the scope of the appended claims. 
         [0103]    For example, two light sources  40  are provided in the exposure apparatus  6  according to the above embodiments. However there is no particular limitation to the above embodiments. Four light sources  40  may be provided in one exposure apparatus  6 . 
         [0104]    The partitioning plate  36  according to the above embodiments partitions the housing  30  into an upper and a lower portion. However, partitioning of the housing  30  in various directions such as left and right directions is possible according to the orientation of the exposure apparatus  6 . 
         [0105]    In the above embodiments, the folding mirrors  54   a ,  54   b ,  56   a , and  56   b  that act as optical systems are also provided in the second portion  62 . However, it may be alternatively possible that an optical system is not arranged in the second portion  62 . 
         [0106]    This is due to the fact that when the photosensitive drum  10  is disposed below the exposure apparatus  6 , the scanning light beam from the first portion  61  passes through the slits  37   a ,  37   b  and reaches the second portion  62 , and then arrives at the photosensitive drum without further modification. 
         [0107]    The present invention is not limited to a configuration in which the reinforcement member is disposed only in the second portion  62 . In other words, the reinforcement member may also be provided in the first portion  61 . More specifically, when the reinforcement member of the second portion  62  of lower temperature is configured to be stronger than the reinforcement member of the first portion  61  of the higher temperature, it is possible to suppress thermal deformation of the housing  30  due to the temperature difference between the first portion  61  and the second portion  62 . 
         [0108]    The examples in the above embodiments are applied to a printer as an example of an image forming apparatus. However, the image forming apparatus according to the present invention can also be applied to a copying machine, a facsimile or the like. 
         [0109]    In any of the above examples, in accordance with the description above, the effect of preventing the production of a jitter image or color misregistration of the color image is obtained in a housing having a multilayered structure including a hole.