Abstract:
An exposure device to expose an exposure element includes a light source; an optical system to guide light emitted from the light source to the exposure element; and an optical housing, configured with a plurality of plates, to support the light source and the optical system. At least one of the plurality of plates configuring the optical housing is formed with a plurality of grooves on each of a first face and a second face with a given pitch on the one of the plurality of plates, the first face and the second face being opposite faces with each other. The plurality of grooves are arranged by shifting the center of each of grooves formed on the first face and the center of each of grooves formed on the second face.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority pursuant to 35 U.S.C. §119 to Japanese Patent Application No. 2012-068657, filed on Mar. 26, 2012 in the Japan Patent Office, the disclosure of which is incorporated by reference herein in its entirety. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to an exposure device and an image forming apparatus, and more particularly to an exposure device having an optical housing, and an image forming apparatus having the exposure device. 
     2. Background Art 
     In the field of image forming technologies using electrophotography, typical image forming apparatuses include an exposure device, a photoconductor drum, and an optical deflector such as a polygon mirror. In such image forming apparatuses, the exposure device scans the photoconductor drum along the axial direction using a laser beam while rotating the photoconductor drum to form a latent image on the photoconductor drum. 
     With advances in technology, image forming apparatuses having color printing and high-speed printing capabilities have been introduced into the market. In line with such market trends, image forming apparatuses equipped with a plurality of photoconductor drums (typically four) have been introduced. Such tandem-type image forming apparatuses are larger in size due to the increase in the number of photoconductor drums. However, increasingly the market also demands more compact image forming apparatuses, including a concomitant demand for smaller, thinner exposure devices. 
     For example, JP-S60-32019-A, JP-H07-144434-A, and JP-2010-160295-A disclose configurations to reduce the size and thickness of the exposure device by partially overlapping optical paths of a plurality of laser beams directed from the optical deflector onto each one of the photoconductor drums. 
     In such image forming apparatuses, when a latent image is formed on a surface of the photoconductor drum, rotation of the optical deflector causes vibrations that result in deformation of the optical housing of the exposure device. Such deformation causes stripes or banding to appear on the output images. 
     Various methods have been proposed to suppress the vibrations at the exposure device. For example, JP-4299103-B (JP-2005-138442-A) discloses an optical scanner having a housing made of sheet metal that encases optical parts. Attachments are disposed on the bottom face of the housing at at least two places along the long side of the optical parts at positions corresponding to nodes of vibration. An optical part supporting member that supports the optical parts is attached at the attachments to isolate the optical part supporting member from the bottom face of the housing while attached to the nodes of vibration. 
     Moreover, there is an additional source of vibration. When forming a latent image on the surface of a photoconductor drum, vibrations from mechanical parts disposed in the image forming apparatus are transmitted to the exposure device. By reducing the size of the exposure device, the size of optical elements included in the exposure device also becomes smaller, and thereby the optical elements are more vulnerable to external vibrations. Further, by reducing the thickness of the exposure device, the rigidity and natural frequency of the optical housing are decreased, making the optical housing more likely to resonate with external vibrations. As a result, the vibrations of the optical elements and vibrations of the optical housing are more likely to be superimposed, resulting in marked deterioration of image quality. 
     The configuration of the optical scanner disclosed in JP-4299103-B (JP-2005-138442-A) can suppress the effects of vibration on the exposure device. However, it is difficult to design the optical housing to locate the nodes at a given position, and also difficult to dispose the optical elements exactly at the nodes. 
     SUMMARY 
     As one aspect of the present invention, an exposure device to expose an exposure element is devised. The exposure device includes a light source; an optical system to guide light emitted from the light source to the exposure element; and an optical housing, configured with a plurality of plates, to support the light source and the optical system. At least one of the plurality of plates configuring the optical housing is formed with a plurality of grooves on each of a first face and a second face with a given pitch on the one of the plurality of plates, the first face and the second face being opposite faces with each other. The plurality of grooves is arranged by shifting the center of each of the grooves formed on the first face and the center of each of the grooves formed on the second face. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the disclosure and many of the attendant advantages and features thereof can be readily obtained and understood from the following detailed description with reference to the accompanying drawings, wherein: 
         FIG. 1  shows a schematic configuration of an image forming apparatus according to an example embodiment; 
         FIG. 2  shows an optical scanner used for the image forming apparatus of  FIG. 1 ; 
         FIG. 3  shows a pre-deflector optical system shown in  FIG. 2 ; 
         FIG. 4  shows two scanning optical systems shown in  FIG. 2 ; 
         FIG. 5  shows a polarized light separation face of a polarization splitter; 
         FIG. 6  shows an anti-reflection film of the polarization splitter; 
         FIG. 7  shows an effect of a polarization splitter; 
         FIG. 8  shows another effect of a polarization splitter; 
         FIG. 9  shows a first holder holding a polarization splitter and a reflection mirror; 
         FIG. 10  shows a cross-sectional view of the first holder of  FIG. 9 ; 
         FIG. 11  shows another first holder holding a polarization splitter and a reflection mirror; 
         FIG. 12  shows a second holder holding two reflection mirrors; 
         FIG. 13  shows a cross-sectional view of the second holder of  FIG. 12 . 
         FIG. 14  shows another second holder holding two reflection mirrors two reflection mirrors; 
         FIG. 15  shows an perspective view of an optical housing; 
         FIG. 16  shows a upper face of a bottom plate of the optical housing of  FIG. 15 ; 
         FIG. 17  shows a lower face of a bottom plate of the optical housing of  FIG. 15 ; 
         FIG. 18  shows an expanded view of a groove-formed portion; 
         FIG. 19  shows a cross-sectional view of the optical housing of  FIG. 16  cut at A-A line; 
         FIG. 20  an expanded view of the optical housing of  FIG. 19 ; 
         FIG. 21  shows a random vibrational analysis result for the optical housing according to an example embodiment; 
         FIG. 22  shows an optical housing having formed of a plurality of grooves on the upper face of the bottom plate and the lower face of the bottom plate, and the plurality of grooves on the upper face and the lower are set at corresponding positions; 
         FIG. 23  shows a random vibrational analysis result for the optical housing of FIG.  22 ; 
         FIG. 24  shows a conventional optical housing; 
         FIG. 25  shows a random vibrational analysis result for the optical housing of  FIG. 24 ; 
         FIG. 26  shows an optical housing having thin portions at four corners; 
         FIG. 27  shows a random vibrational analysis result for the optical housing of  FIG. 26 ; 
         FIG. 28  shows an optical housing having concave and convex portions for reducing vibration transmission; 
         FIG. 29  shows a random vibrational analysis result for the optical housing of  FIG. 28 ; 
         FIG. 30  shows an optical housing formed of a plurality of grooves on a bottom plate and a side plates; and 
         FIG. 31  shows an example of a polarized light separation device. 
     
    
    
     The accompanying drawings are intended to depict exemplary embodiments of the present invention and should not be interpreted to limit the scope thereof. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted, and identical or similar reference numerals designate identical or similar components throughout the several views. 
     DETAILED DESCRIPTION 
     A description is now given of exemplary embodiments of the present invention. It should be noted that although such terms as first, second, etc. may be used herein to describe various elements, components, regions, layers and/or sections, it should be understood that such elements, components, regions, layers and/or sections are not limited thereby because such terms are relative, that is, used only to distinguish one element, component, region, layer or section from another region, layer or section. Thus, for example, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the present invention. 
     In addition, it should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present invention. Thus, for example, as used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. Moreover, the terms “includes” and/or “including”, when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     Furthermore, although in describing views shown in the drawings, specific terminology is employed for the sake of clarity, the present disclosure is not limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that, have a similar function, operate in a similar manner, and achieve a similar result. Referring now to the drawings, apparatuses or systems according to example embodiments are described hereinafter. 
     A description is given of an image forming apparatus according to an example embodiment with reference to  FIGS. 1 to 29 .  FIG. 1  shows a schematic configuration of an image forming apparatus  2000  such as a color printer according to an example embodiment. 
     The image forming apparatus  2000  is, for example, a tandem type color printer to form color images by superimposing four colors of black, cyan, magenta, and yellow. The image forming apparatus  2000  includes, for example, an optical scan unit  2010 , four photoconductor drums  2030   a ,  2030   b ,  2030   c ,  2030   d  used as image carriers or image bearing members, four cleaning units  2031   a ,  2031   b ,  2031   c ,  2031   d , four chargers  2032   a ,  2032   b ,  2032   c ,  2032   d , four development rollers  2033   a ,  2033   b ,  2033   c ,  2033   d , a transfer belt  2040 , a transfer roller  2042 , a fusing roller  2050 , a sheet feed roller  2054 , a sheet ejection roller  2058 , a sheet feed tray  2060 , a sheet ejection tray  2070 , a communication controller  2080 , and a main controller  2090  that controls such unit as a whole. Such units are encased in a housing of the image forming apparatus  2000 . 
     The communication controller  2080  controls bi-directional communications with an external apparatus such as a personal computer via a network. 
     The main controller  2090  includes, for example, a central processing unit (CPU), a read only memory (ROM), a random access memory (RAM), an amplification circuit, and an analog/digital (A/D) converter. The ROM stores programs codes decode-able by the CPU and various data for executing the programs. The RAM is used as a working memory. The A/D converter converts analog data to digital data. The main controller  2090  controls each unit in response to demands or requests from the external apparatus, and transmits multi-color image data received from the external apparatus to the optical scan unit  2010 . 
     The photoconductor drum  2030   a , the charger  2032   a , the development roller  2033   a , and the cleaning unit  2031  are used an image forming station to form black images (hereinafter, K station). 
     The photoconductor drum  2030   b , the charger  2032   b , the development roller  2033   b , and the cleaning unit  2031   b  are used an image forming station to form cyan images (hereinafter, C station). 
     The photoconductor drum  2030   c , the charger  2032   c , the development roller  2033   c , and the cleaning unit  2031   c  are used an image forming station to form magenta images (hereinafter, M station). 
     The photoconductor drum  2030   d , the charger  2032   d , the development roller  2033   d , and the cleaning unit  2031   d  are used an image forming station to form yellow images (hereinafter, Y station). 
     Each of the photoconductor drums includes a photoconductive layer as its top face. As such, the surface of the photoconductor drum is used as a scan face. Each of the photoconductor drums can be rotated in a direction shown by an arrow in FIG. l using a rotation mechanism for the photoconductor drum. 
     The charger charges the surface of the photoconductor drum uniformly. 
     The optical scan unit  2010  is used as an exposure device. The optical scan unit  2010  scans the surface of the each of the photoconductor drums, charged by the charger, using light modulated based on image data such as black, cyan, magenta, and yellow image data received from the main controller  2090 . Then, a latent image corresponding to each image data is formed on each of the photoconductor drums. Such latent image moves to the development roller with a rotation of the photoconductor drum. The optical scan unit  2010  will be described later in detail. 
     As the development roller rotates, corresponding color toner is applied on the surface of the development roller as a uniform thin layer from a corresponding toner cartridge. When the toner on the development roller contacts the surface of the photoconductor drum, the toner is transferred and adhered onto the light exposed face of the photoconductor drum. As such, the latent image formed on the photoconductor drum is developed by toner supplied from the development roller. The toner image moves toward the transfer belt  2040  as the photoconductor drum rotates. 
     The toner images of yellow, magenta, cyan, and black are sequentially transferred onto the transfer belt  2040  at a given timing, and superimposed to form a color image. 
     The sheet feed tray  2060  stores recording sheets. The sheet feed roller  2054  disposed near the sheet feed tray  2060  feeds out the recording sheets from the sheet feed tray  2060  one by one. The recording sheet is then fed to a space between the transfer belt  2040  and the transfer roller  2042  at a given timing to transfer the color image from the transfer belt  2040  to the recording sheet. The recording sheet transferred with the color image is transported to the fusing roller  2050 . 
     The fusing roller  2050  applies heat and pressure to the recording sheet to fuse the toner on the recording sheet. The recording sheet fused with the toner is transported to the sheet ejection tray  2070  via the sheet ejection roller  2058 , and stacked on the sheet ejection tray  2070 . 
     Each of the cleaning units removes toner remaining on the photoconductor drum. Upon removing the remaining toner, the surface of the photoconductor drum is faced to the charger again. 
     A description is given of the optical scan unit  2010 . The optical scan unit  2010  includes, for example, four light sources  2200   a ,  2200   b ,  2200   c ,  2200   d , a pre-deflector optical system, which is disposed before a deflector, a polygon mirror  2104  used as a deflector that deflects light, a scanning optical system A , a scanning optical system B, and a scan controller as shown in  FIG. 2 . Such units are encased in an optical housing  2300  as shown in  FIG. 15 . 
     In the three dimensional orthogonal coordinate system of X, Y, Z, the long side direction or rotation axis direction of each of the photoconductor drums is set as Y axis direction, and the direction along a rotation axis of the polygon mirror  2104  is set as Z axis direction. Further, when the direction is required to be referred for each optical part, the main scanning direction and the sub-scanning direction are used. 
     Each of the light sources includes a semiconductor laser such as a laser diode (LD). The light source  2200   b  and the light source  2200   c  are separated in the X axis direction, and emit lights in the −Y direction. The light source  2200   a  and the light source  2200   d  are opposed with each other in the X axis direction, and the light source  2200   a  emits light in the +X direction, and the light source  2200   d  emits light in the −X direction. 
     As shown in  FIG. 3 , the pre-deflector optical system includes, for example, four coupling lenses  2201   a ,  2201   b ,  2201   c ,  2201   d , four half-wave plates  2202   a ,  2202   b ,  2202   c ,  2202   d , two polarization beam splitters  2205   1 ,  2205   2 , two aperture plates  2203   1 ,  2203   2  , and two cylindrical lenses  2204   1 ,  2204   2 . 
     The coupling lens  2201   a  is disposed on the optical path of light emitted from the light source  2200   a , and such light is used as substantially parallel light (hereinafter, “light LBa”). 
     The coupling lens  2201   b  is disposed on the optical path of light emitted from the light source  2200   b , and such light is used as substantially parallel light (hereinafter, “light LBb”). 
     The coupling lens  2201   c  is disposed on the optical path of light emitted from the light source  2200   c , and such light is used as substantially parallel light (hereinafter, “light LBc”). 
     The coupling lens  2201   d  is disposed on the optical path of light emitted from the light source  2200   d , and such light is used as substantially parallel light (hereinafter, “light LBd”). 
     The half-wave plate  2202   a  is disposed on the optical path of the light LBa via the coupling lens  2201   a , and such light is used as s-polarized light with respect to an incidence plane of the polarization beam splitters  2205   1 . 
     The half-wave plate  2202   b  is disposed on the optical path of the light LBb via the coupling lens  2201   b , and such light is used as p-polarized light with respect to the incidence plane of the polarization beam splitters  2205   1 . 
     The half-wave plate  2202   c  is disposed on the optical path of the light LBc via the coupling lens  2201   c , and such light is used as p-polarized light with respect to an incidence plane of the polarization beam splitters  2205   2 . 
     The half-wave plate  2202   d  is disposed on the optical path of the light LBd via the coupling lens  2201   d , and such light is used as s-polarized light with respect to the incidence plane of the polarization beam splitters  2205   2 . 
     The polarization beam splitters  2205   1  is disposed at +X side of the half-wave plate  2202   a  and −Y side of the half-wave plate  2202   b . The polarization beam splitter  2205   1  has a property that passes through p-polarized light, and reflects s-polarized light. Therefore, the polarization beam splitters  2205   1  reflects the light LBa that has passed through the half-wave plate  2202   a  in the −Y direction, and passes through the light LBb that has passed through the half-wave plate  2202   b . Further, the optical path of the light LBa emitted from the polarization beam splitters  2205   1  and the optical path of the light LBb emitted from the polarization beam splitters  2205   1  become almost the same optical path, in which the polarization beam splitter  2205   1  synthesizes two lights. 
     The polarization beam splitters  2205   2  is disposed at −X side of the half-wave plate  2202   d  and −Y side of the half-wave plate  2202   c . The polarization beam splitters  2205   2  has a property that passes through p-polarized light and reflects s-polarized light. Therefore, the polarization beam splitters  2205   2  reflects the light LBd that has passed through the half-wave plate  2202   d  in the −Y direction, and passes through the light LBc that has passed through the half-wave plate  2202   c . Further, the optical path of the light LBc emitted from the polarization beam splitters  2205   2  and the optical path of the light LBd emitted from the polarization beam splitters  2205   2  become almost the same optical path, in which the polarization beam splitter  2205   2  synthesizes two lights. 
     The aperture plate  2203   1  includes an aperture to adjust the beam shape of the light LBa and the light LBb coming from the polarization beam splitters  2205   1 . 
     The aperture plate  2203   2  includes an aperture to adjust the beam shape of the light LBc and the light LBd coming from the polarization beam splitters  2205   2 . 
     The cylindrical lens  2204   1  focuses the light LBa and the light LBb that have passed through the aperture of the aperture plate  22031  near a reflection face of the polygon mirror  2104  in the Z axis direction. As such, the cylindrical lens  2204   1  forms a line image on the reflection face of the polygon mirror  2104 . 
     The cylindrical lens  2204   2  focuses the light LBc and the light LBd that have passed through the aperture of the aperture plate  2203   2  near a reflection face of the polygon mirror  2104  in the Z axis direction. As such, the cylindrical lens  2204   2  forms a line image on the reflection face of the polygon mirror  2104 . 
     The polygon mirror  2104  has, for example, four minor faces, and each mirror face is used as a reflection face. The polygon minor  2104  rotates with a uniform speed about a mirror rotation axis parallel to the Z axis direction, and deflects lights coming from each of the cylindrical lens. 
     The light LBa and the light LBb coming from the cylindrical lens  2204   1  are deflected to the −X side of the polygon mirror  2104 , and the light LBc and the light LBd coming from the cylindrical lens  2204   2  are deflected to the +X side of the polygon minor  2104 . Further, a light flux plane generated by light deflected at the reflection face of the polygon minor  2104  along the time line is referred to as “plane of deflection” as described in JP-H11-202252-A. In this disclosure, the plane of deflection is a plane parallel to the X-Y plane. 
     As shown in  FIG. 4 , the scanning optical system A includes, for example, a first scan lens  2105   1 , a second scan lens  2107   1 , a polarization splitter  2110   1 , and five reflection mirrors  2106   a ,  2106   b ,  2108   a ,  2108   b ,  2109   a . The first scan lens  2105   1  is disposed near to the optical deflector such as the polygon mirror, and the second scan lens  2107   1  is disposed near to the image bearing member such as the photoconductor in the optical path of the light. 
     The first scan lens  2105   1  is disposed at the −X side of the polygon minor  2104 , and is disposed on the optical paths of the light LBa and the light LBb coming from the cylindrical lens  2204   1  deflected by the polygon mirror  2104 . 
     The second scan lens  2107   1  is disposed at the −X side of the first scan lens  2105   1 , and is disposed on the optical paths of the light LBa and the light LBb via the first scan lens  2105   1 . 
     The polarization splitter  2110   1  is disposed at the −X side of the second scan lens  2107   1 , and is disposed on the optical paths of the light LBa and the light LBb via the second scan lens  2107   1 . 
     The polarization splitter  2110   1  has a polarized light separation face. The polarized light separation face is, for example, a wire grid, a multi-layered dielectric film or the like. The multi-layered dielectric film is preferably used to suppress the increase of wavefront aberration. 
     Further, the polarization splitter  2110   1  may be a quadrangular prism composed of two triangular prisms made of glass or resin material having a cross-sectional face of isosceles right triangle having interposing the polarized light separation face between the triangular prisms. The polarization splitter can be prepared with less processing using a plate member made of glass or transparent resin material as a base member, and forming the polarized light separation face on one side of the plate member. 
     As shown in  FIG. 5 , the polarization splitter  2110   1  includes, for example, a polarized light separation face which is angled 45° with respect to the plane of deflection. 
     As shown in  FIG. 6 , a anti-reflection film is formed on a face of the polarization splitter  2110   1  opposite to the polarized light separation face. By providing the anti-reflection film, the separated light that has passed through the base member does not reflect on a rear face of the base member, by which the generation of ghost light can be suppressed. 
     The polarization splitter  2110   1  passes through the light LBa ( FIG. 7 ), and reflects the light LBb in the −Z direction ( FIG. 8 ). 
     Referring back to  FIG. 4 , the light LBa that has passed through the polarization splitter  2110   1  is guided to the surface of the photoconductor drum  2030   a  via three reflection mirrors  2106   a ,  2108   a ,  2109   a . Further, the light LBb reflected by the polarization splitter  2110   1  is guided to the surface of the photoconductor drum  2030   b  via two reflection mirrors  2106   b ,  2108   b. The first scan lens  2105     1  and the second scan lens  2107   1  are used by two image forming stations. 
     The scanning optical system B includes, for example, the first scan lens  2105   2 , the second scan lens  2107   2 , the polarization splitter  2110   2 , and five reflection mirrors  2106   c ,  2106   d ,  2108   c ,  2108   d ,  2109   d.    
     The first scan lens  2105   2  is disposed at the +X side of the polygon mirror  2104 , and is disposed on the optical paths of the light LBc and the light LBd coming from the cylindrical lens  2204   2  and deflected by the polygon mirror  2104 . 
     The second scan lens  2107   2  is disposed at the +X side of the first scan lens  2105   2 , and is disposed on the optical paths of the light LBc and the light LBd via the first scan lens  2105   2 . 
     The polarization splitter  2110   2  is disposed at the +X side of the second scan lens  2107   2 , and is disposed on the optical paths of the light LBc and the light LBd via the second scan lens  2107   2 . The polarization splitter  2110   2  and the polarization splitter  2110   1  are the same type of polarization splitter. 
     The polarization splitter  2110   2  passes through the light LBd, and reflects the light LBc in the −Z direction. 
     The light LBc reflected by the polarization splitter  2110   2  is guided to the surface of the photoconductor drum  2030   c  via two reflection mirrors  2106   c ,  2108   c.    
     The light LBd that has passed the polarization splitter  2110   2  is guided to the surface of the photoconductor drum  2030   d  via three reflection mirrors  2106   d ,  2108   d ,  2109   d.    
     The first scan lens  2105   2  and the second scan lens  2107   2  are used by two image forming station. 
     The light spot on each of the photoconductor drums moves along the long side direction of the photoconductor drum as the polygon mirror  2104  rotates. The moving direction of the light spot corresponds to the main scanning direction, and the rotation direction of the photoconductor drum corresponds to the sub-scanning direction. 
     As shown in  FIG. 9 , the polarization splitter  2110   1  and a reflection mirror  2106   b  are retained integrally, for example, in a first holder  10 . 
     As shown in  FIG. 10 , which is a cross-sectional view of  FIG. 9 , the first holder  10  integrally retains the polarization splitter  2110   1  and the reflection mirror  2106   b  while the polarization splitter  2110   1  and the reflection mirror  2106   b  are disposed within the first holder  10  in the Z axis direction. 
     The first holder  10  is, for example, a die-cast aluminum having two faces perpendicular with each other and extending along the long side direction (or Y axis direction), by which forming a right-angled face as described in JP-H06-50739-A. The polarization splitter  2110   1  is retained on the +Z side face in the right-angled face, and the reflection mirror  2106   b  is retained on the −Z side face in the right-angled face. Further, the first holder  10  is formed with a through-hole having a rectangular shape to pass the light LBa through the polarization splitter  2110   1 . 
     The polarization splitter  2110   1  and the reflection minor  2106   b  are pressed against the right-angled face at a plurality of portions along the long side direction (or Y axis direction) using plate springs, or the polarization splitter  2110   1  and the reflection minor  2106   b  are adhered on the right-angled face using adhesive agent. With such a configuration, the rigidity of the polarization splitter  2110   1  and the reflection mirror  2106   b  can be made more rigid than setting the polarization splitter  2110   1  or the reflection mirror  2106   b  alone, and the natural frequency of the polarization splitter  2110   1  and the reflection mirror  2106   b  shifts to the high frequency side. Therefore, resonance by external vibrations can be suppressed, and an anti-vibration performance can be enhanced. 
     Further, because the polarization splitter  2110   1  and the reflection mirror  2106   b  are retained on the right-angled face, even if the dihedral angle is deviated from 90° due to manufacturing error or the like, the light progressing direction reflected by the reflection mirror  2106   b  (+X direction) does not change. If a reflection mirror alone is disposed conventionally, when the angle of a mirror face changes from the designed angel for θ, the light progressing direction reflected by the mirror face changes from the designed direction for 2θ. 
     Further, as shown in  FIG. 11 , the polarization splitter  2110   2  and a reflection mirror  2106   c  are retained integrally, for example, in the first holder  10 . With such a configuration, the polarization splitter  2110   2  and the reflection mirror  2106   c  can be made more rigid than setting the polarization splitter  2110   2  or the reflection mirror  2106   c  are alone, and the natural frequency of the polarization splitter  2110   2  and the reflection mirror  2106   c  shifts to the high frequency side. Therefore, resonance by external vibrations can be suppressed, and an anti-vibration performance can be enhanced. 
     Further, as shown in  FIG. 12 , a reflection mirror  2106   a  and a reflection mirror  2108   a  are retained integrally, for example, in a second holder  20 . 
     As shown in  FIG. 13 , which is a cross-sectional view of  FIG. 12 , the second holder  20  integrally retains the reflection mirror  2106   a  and the reflection mirror  2108  while the reflection mirror  2106   a  and the reflection mirror  2108   a  are disposed within the second holder  20  in the Z axis direction. 
     The second holder  20  is, for example, a die-cast aluminum having two faces perpendicular with each other and extending along the long side direction (or Y axis direction), by which forming a right-angled face. The reflection mirror  2106   a  is retained on the +Z side face in the right-angled face, and the reflection mirror  2108   a  is retained on the −Z side face in the right-angled face. 
     The reflection mirror  2106   a  and the reflection mirror  2108   a  are pressed against the right-angled face at a plurality of portions along the long side direction (or Y axis direction) using plate springs, or the reflection mirror  2106   a  and the reflection mirror  2108   a  are adhered on the right-angled face using an adhesive agent. With such a configuration, the reflection mirror  2106   a  and the reflection mirror  2108   a  can be made more rigid than setting the reflection minor  2106   a  or the reflection mirror  2108   a  alone, and the natural frequency of the reflection mirror  2106   a  and the reflection mirror  2108   a  shifts to the high frequency side. Therefore, resonance by external vibrations can be suppressed, and an anti-vibration performance can be enhanced. 
     Further, as shown in  FIG. 14 , a reflection minor  2106   d  and a reflection minor  2108   d  are retained integrally, for example, in the second holder  20 . With such a configuration, the reflection mirror  2106   d  and the reflection minor  2108   d  can be made more rigid than setting the reflection mirror  2106   d  or the reflection mirror or  2108   d  alone, and the natural frequency of the reflection mirror  2106   d  and the reflection mirror  2108   d  shifts to the high frequency side. Therefore, resonance by external vibrations can be suppressed, and an anti-vibration performance can be enhanced. 
       FIG. 15  shows the optical housing  2300  attached with four light sources, the pre-deflector optical system, the polygon minor  2104 , the scanning optical system A, and the scanning optical system B. The optical housing  2300  is made of, for example, a resin material having Young&#39;s modulus, for example, 1.25×10 10  (Pa). The optical housing  2300  is shaped, for example, as a box-shape having a top plate, a bottom plate, and four side plates, and the top plate is used as a cover.  FIG. 15  shows the optical housing  2300  when the top plate is removed. 
     Further, the polygon minor  2104  is disposed at the center portion of the optical housing  2300 . The bottom plate and the four side plates can be formed, for example, as one integral part. 
     Each end side of the optical housing  2300  in the Y axis direction can be fixed to a casing of the image forming apparatus  2000  via a stay  2401  ( FIG. 15 ). The stay  2401  is made of, for example, a metal sheet, and has a long side along the X axis direction. The stay  2401  has a screw hole at its each end in the X axis direction so that the stay  2401  can be fixed to the casing of the image forming apparatus  2000  by screwing a screw through the screw hole. 
     Each of side plates of the optical housing  2300  disposed at each end in the Y axis direction is provided with a coupling member at each end in the X axis direction of the optical housing  2300 , wherein the coupling member can be fixed with the stay  2401 . 
     In the above-described example embodiment, the length of scanning optical system in the Z axis direction is set shorter using a polarization splitter, by which reducing the apparatus thickness of the optical scanner. Therefore, the height of side plates of the optical housing  2300  can be set smaller than the height of side plates of a conventional optical housing. If the height of the side plates is set smaller, the natural frequency of the optical housing decreases, by which the optical housing is more likely to resonate by external vibrations, wherein the external vibrations are transmitted to the optical housing  2300  via the coupling member fixed with the stay  2401 . 
     In light of such vibration issues, as for the optical housing  2300  according to an example embodiment, a groove-formed portion having formed with a plurality of grooves is provided to the +Z side face of the bottom plate and a groove-formed portion having formed with a plurality of grooves is provided to the −Z side face of the bottom plate. Hereinafter, the +Z side face of the bottom plate may be referred to as the upper face or front face of the bottom plate, and the −Z side face of the bottom plate may be referred to as the lower face or rear face of the bottom plate. Further, the upper face of the bottom plate may be referred to as a first face, and the lower face or rear face of the bottom plate may be referred to as a second face, which are the opposite faces of one plate such as the bottom plate. 
       FIG. 16  shows the groove-formed portion provided to the upper face of the bottom plate, and  FIG. 17  shows the groove-formed portion provided to the lower face of the bottom plate. For example, the groove-formed portion is provided at each of four corners of the upper face of the bottom plate, and each of four corners of the lower face of the bottom plate. 
     The plurality of grooves at each of the groove-formed portions can be formed as arcs of concentric circles using the coupling member, fixable with the stay  2401  as their center or base point as shown in  FIG. 18 . 
       FIG. 19  shows a shows a cross-sectional view of the optical housing  2300  along a line A-A in  FIG. 16 . Further,  FIG. 20  shows a partially expanded view of the bottom plate of the optical housing  2300  of  FIG. 19 . As shown in  FIG. 20 , the shape of the groove along the YZ plane is, for example, rectangular. In the YZ cross-sectional plane, the plurality of grooves is formed with a pitch “T,” a width of “L,” and a depth of “d.” Further, in the YZ cross-sectional plane, the center of the plurality of grooves formed on the upper face of the bottom plate and the center of the plurality of grooves formed on the lower face of the bottom plate are shifted for the length of half of pitch T (T/2). As shown in  FIG. 20 , the bottom plate has the thickness of “D.” 
     For example, each of the groove-formed portions includes twenty grooves formed such that L=3 mm, d=0.75, T=6 mm for the bottom plate having thickness of D=2.5 mm. 
     To check the vibration reducing effect of the optical housing  2300 , a random vibrational analysis using an analysis software such as ANSYS (a registered trademark of ANSYS, Inc.) is conducted. In such random vibrational analysis, acceleration is applied to the coupling member fixed with the stay  2401  along the Z axis direction for each of frequency range for causing vibrations, and the maximum deformation amount in the Z axis direction is computed. 
     As for the optical housing  2300 , the greatest deformation occurs at substantially the center portion, and the maximum deformation amount was, for example, 0.075 mm ( FIG. 21 ). 
     As shown in  FIG. 22 , the plurality of grooves formed on the upper face of the bottom plate and the plurality of grooves formed on the lower face of the bottom plate can be corresponded one by one, in which the center of the plurality of grooves formed on the upper face of the bottom plate and the center of the plurality of grooves formed on the lower face of the bottom plate are matched. In such a configuration, the maximum deformation amount was 0.083 mm ( FIG. 23 ). 
     Further, as for a conventional optical housing without grooves shown in  FIG. 24 , the maximum deformation amount was 0.087 mm ( FIG. 25 ). 
     Further, to determine the effect of grooves, an optical housing having no grooves is prepared as shown in  FIG. 26 , in which the optical housing has a thin portion having 1 mm thickness at a portion corresponding to the groove-formed portion of the optical housing  2300 . In such optical housing having no grooves, the maximum deformation amount was 0.132 mm ( FIG. 27 ). 
     Further, instead of the groove-formed portion of the optical housing  2300 , an optical housing can be formed with concave/convex portions used for reducing vibration transmission as shown in  FIG. 28 , which is disclosed in JP-4223175-B (JP-2002-023095-A). In such a configuration, the maximum deformation amount was 0.084 mm ( FIG. 29 ). 
     Based on the comparison of maximum deformation amounts, which are the results of above-described vibrational analysis, the maximum deformation amount of the optical housing  2300  becomes the smallest one, and it is confirmed that such optical housing  2300  can reduce an effect of vibrations. 
     The anti-vibration performance of an optical housing can be enhanced by adding ribs. However, such optical housing will increase its weight, and its material cost. In an Example embodiment, the plurality of grooves are formed on the upper face of the bottom plate and the lower face of the bottom plate while shifting the positions of grooves formed on the upper face and the positions of grooves formed on the lower face for the half of pitch of grooves as shown in  FIG. 20 . For example, the groove formed on the upper face and the groove formed on the lower face of the bottom plate are shifted with each for a half of pitch T. With such a configuration, the anti-vibration performance of the optical housing can be enhanced without increasing weight and material cost. As such, even if the size of the scanning optical system in the Z axis direction becomes smaller, the optical housing can reduce its apparatus thickness while preventing or suppressing the decrease of the anti-vibration performance. 
     As above-described, the optical scan unit  2010  includes the four light sources  2200   a ,  2200   b ,  2200   c ,  2200   d , the pre-deflector optical system, the polygon mirror  2104 , the scanning optical system A , the scanning optical system B, and the optical housing  2300 , wherein the optical housing  2300  is attached with such units or devices. 
     Each of scanning optical systems includes a polarization splitter that separates two lights having different polarization directions with each other. In such a configuration, the optical paths of such two lights deflected by the polygon mirror  2104  can be partially overlapped, by which the apparatus thickness of optical scanner can be reduced. 
     As for the optical housing  2300 , the plurality of grooves is formed on the upper face of the bottom plate with an equal pitch, and the plurality of grooves is formed on the lower face of the bottom plate with an equal pitch. Further, the center of each of the grooves formed on the upper face of the bottom plate and the center of each of the grooves formed on the lower face of the bottom plate is shifted for the half of pitch of grooves. With such a configuration, even if the optical housing reduces its apparatus thickness, the anti-vibration performance of the optical housing can be enhanced without increasing weight and material cost. Therefore, the rigidity of the optical scan unit  2010  against mechanical disturbance can be enhanced. 
     Resultantly, the image forming apparatus  2000  can reduce its apparatus size without degrading image quality. 
     Further, in the above-described example embodiment, the plurality of grooves is formed on the bottom plate of the optical housing  2300 , but the plurality of grooves can be formed on the side plate instead of the bottom plate, or the plurality of grooves can be formed on both of the bottom plate and the side plate. 
     Further, in the above-described example embodiment, the shape of grooves in the plane parallel to the Z axis is rectangular, but the shape of grooves is not limited to the rectangular. 
     Further, in the above-described example embodiment, the grooves formed on the upper face of the bottom plate and the grooves formed on the lower face of the bottom plate are shifted for T/2. However, the shifting distance is not limited to T/2. Specifically, if the center of each of the the grooves formed on the upper face of the bottom plate and the center of each of the the grooves formed on the lower face of the bottom plate are shifted for a given distance, such optical housing has the above-described effect. 
     Further, in the above-described example embodiment, the plurality of grooves is formed as arc of a concentric circle setting the coupling member as the center of the concentric circle (see  FIG. 18 ), but the center of the concentric circle is not limited to the coupling member fixed with the stay  2401 , which means the center of the concentric circle can be deviated from the coupling member. Further, the plurality of grooves is not limited to the arc pattern. For example, the plurality of grooves can be formed with straight lines. 
     Further, in the above-described example embodiment, the plurality of grooves is formed with an equal pitch, but the pitch is not limited to the equal pitch. 
     Further, in the above-described example embodiment, the plurality of grooves is formed on the bottom plate of the optical housing  2300 , but the plurality of grooves can be formed differently. For example, the plurality of grooves can be formed on the side plate of the optical housing  2300  instead of the bottom plate, or the plurality of grooves can be formed on both of the bottom plate and the side plate as shown in  FIG. 30 . Further, the plurality of grooves can be formed on the top plate of the optical housing  2300  instead of the bottom plate, or the plurality of grooves can be formed on both of the bottom plate and the top plate. 
     Further, in the above-described example embodiment, the number of grooves, the groove width L, the groove depth d, and the groove pitch T can be set any values. 
     Further, in the above-described example embodiment, the first holder  10  and the second holder  20  are die-cast aluminum, but the first holder  10  and the second holder  20  can be made of other materials. For example, the first holder  10  and the second holder  20  can be formed by machining, by metal sheet processing, or can be formed using metal material other than aluminum, or can be formed using resin material. For example, the first holder  10  and the second holder  20  can be formed by the injection molding using resin material. 
     Further, the shape of the first holder  10  and the shape of the second holder  20  perpendicular to the Y axis direction are not limited to the above described shape. Further, the size of the first holder  10  and the second holder  20  is not limited to the above described size. As long as the first holder  10  integrally retains a polarization splitter and a reflection mirror while the polarization splitter and the reflection mirror are disposed within the first holder  10  in the Z axis direction, and further, as long as the second holder  20  integrally retains two reflection mirrors in the Z axis direction while the two reflection mirrors are disposed within the second holder  20  in the Z axis direction, the optical system can be used effectively. 
     Further, in the above-described example embodiment, the polarization splitter  2110   1  is used. Instead of the polarization splitter  2110   1 , as shown in  FIG. 31 , a polarized light separation device  16   1  can be used. The polarized light separation device  16   1  includes a beam splitter  16   10 , and two polarizers  16   11 ,  16   12 . 
     The beam splitter  16   10  is disposed at the −X side of the second scan lens  2107   1  and is disposed on the optical paths of the light LBa and the light LBb via the second scan lens  2107   1 . The beam splitter  16   10  passes through one polarized light in the incidence light and reflects other polarized light in the incidence light while maintaining the polarization direction of the incidence lights. 
     The polarizer  16   11  is disposed at the −X side of the beam splitter  16   10  and is disposed on the optical path of the light that has passed the beam splitter  16   10 . The polarizer  16   12  is disposed at the −Z side of the beam splitter  16   10  and is disposed on the optical path of the light reflected by the beam splitter  16   10 . Each of the polarizers may be a polarization film, which can be prepared by dyeing the film with iodine or dichroic dye and extending such film in one direction. 
     Only the light LBa passes through the polarizer  16   11 , and only the light LBb passes through the polarizer  16   12 . 
     Further, another polarized light separation device can be used instead of the polarization splitter  2110   2 . 
     Further, in the above-described example embodiment, each light source includes one light emitting element, but the number of light emitting elements is not limited one. For example, each light source can include a plurality of semiconductor lasers. Further, each light source can include a semiconductor laser array having a plurality of light emitting elements. 
     Further, in the above-described example embodiment, the image forming apparatus of color printer having the four photoconductor drums is described, but the image forming apparatus is not limited such color printer. 
     Further, in the above-described example embodiment, the optical scanner is used for printers, but the optical scanner can be used for other image forming apparatuses such as copiers, facsimile machines, or multi-functional apparatuses combining such machines. 
     Further, in the above-described example embodiment, in the image forming apparatus, the surface of an image carrier is optically scanned to form a latent image on the surface of the image carrier, but the image forming apparatus that can form a latent image on the surface of an image carrier without the optical scanning can be used. 
     As for the above described exposure device, the rigidity of the exposure device against mechanical disturbance can be enhanced. 
     Numerous additional modifications and variations are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the disclosure of the present invention may be practiced otherwise than as specifically described herein. For example, elements and/or features of different examples and illustrative embodiments may be combined each other and/or substituted for each other within the scope of this disclosure and appended claims.