Abstract:
An image forming apparatus includes an image bearing member having a photosensitive surface and an optical writing apparatus generating a laser beam modulated according to image data. The optical writing apparatus includes a housing, an optical deflector, an f-theta lens and a separator. The optical deflector is configured to rotate to deflect the laser beam. The f-theta lens is configured to correct the laser beam deflected by the optical deflector and to transmit a corrected laser beam towards the image bearing member. The separator is configured to separate an inside space of the housing into at least two sections including a first section in which the optical deflector is mounted and a second section in which the f-theta lens is mounted. The separator includes a heat resistant transparent plate disposed at a position of crossing passage of the laser beam between the optical deflector and the f-theta lens.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   The present application claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2003-154513 filed on May 30, 2003 in the Japanese Patent Office, the entire contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method and apparatus for image forming, and more particularly relates to a method and apparatus for image forming capable of preventing image quality deterioration induced by a heat from an optical deflector by insulating f-theta lenses from the optical deflector. 
   2. Discussion of the Background 
   A conventional color image forming apparatus includes a copying machine, a facsimile machine, a printing machine and other similar image forming apparatus. In a conventional color image forming apparatus having a structure for a tandem operation, an optical writing apparatus includes a plurality of light sources to emit respective laser beams according to image data. The laser beams irradiate respective image bearing members which are arranged in parallel, so that electrostatic latent images are formed on surfaces of the respective image bearing members. 
   The conventional color image forming apparatus also includes developing units for operating with the respective image bearing members. The developing units visualize the respective electrostatic latent images formed on the surfaces of the respective image bearing members as toner images of different colors, such as yellow, magenta, cyan and black toner images. In synchronization with movements in the operations as described above, a transfer member such as a transfer sheet is conveyed on a transfer belt. The toner images of different colors are transferred onto the transfer member as an overlaid toner image. The overlaid toner image is fixed by a fixing unit and then is discharged to a sheet discharging part. 
   The conventional color image forming apparatus generally includes a plurality of optical components separately provided to the optical writing apparatus corresponding to the number of the image bearing members. The plurality of optical components include an optical deflector having a polygon mirror and a motor for driving the polygon mirror. Such optical components are relatively expensive. 
   When the plurality of optical components are separately provided as described above, part cost and production cost increase, and a large space is required in the optical writing system for arranging the plurality of optical components therein. Therefore, the conventional color image forming apparatus may become large in size. 
   Under the above-described circumstances, a technique has been proposed as illustrated in  FIG. 1 . 
     FIG. 1  shows a conventional color laser printer  1 . The conventional color laser printer  1  includes an optical writing apparatus  2 , a sheet feeding system  3 , an intermediate transfer belt  4 , four image bearing members  5 A,  5 B,  5 C and  5 D, four primary transfer rollers  6 A,  6 B,  6 C and  6 D, registration rollers  7 , a secondary transfer roller  8  and a fixing unit  9 . The four image bearing members  5 A,  5 B,  5 C and  5 D are included in respective image forming units (not shown). 
   The optical writing apparatus  2  is located between the sheet feeding system  3  and the image forming units in the background color laser printer  1  of  FIG. 1 . 
   The sheet feeding system  3  is arranged at a bottom location of the background color laser printer  1 . 
   The intermediate transfer belt  4  is arranged above the image forming units. The intermediate transfer belt  4  forms an endless belt extended with pressure by rollers  10  and  11 . The intermediate transfer belt  4  is held in contact between the primary transfer rollers  6 A,  6 B,  6 C and  6 D arranged at a position opposite to the image bearing members  5 A,  5 B,  5 C and  5 D, respectively, such that the toner images formed on the respective image bearing members  5 A,  5 B,  5 C and  5 D are transferred onto the intermediate transfer belt  4  to overlay different color toner images to obtain a recorded image. 
   The image bearing members  5 A,  5 B,  5 C and  5 D are included in the respective image forming units which are arranged in parallel under the intermediate transfer belt  4 . Each of the image forming units further includes a charging unit, a developing unit, a discharging unit and a cleaning unit which are not shown in  FIG. 1 . The image forming units have identical structures with developers of different colors of yellow, magenta, cyan and black. 
   The color laser printer  1  produces a full-color image through the following operations. 
   The image bearing members  5 A,  5 B,  5 C and  5 D rotate clockwise, which is a direction A in  FIG. 1 , by a motor (not shown). The charging unit is applied with a charged voltage and then uniformly charges the image bearing members  5 A,  5 B,  5 C and  5 D to a predetermined polarity. 
   The optical writing apparatus  2  optically modulates laser beams L 1 , L 2 , L 3  and L 4  and emits the laser beams L 1 , L 2 , L 3  and L 4  from under the respective image forming units toward the respective image bearing members  5 A,  5 B,  5 C and  5 D, respectively, through gaps between the charging units and the developing units of the respective image forming units. The laser beams L 1 , L 2 , L 3  and L 4  irradiate the surfaces of the respective image bearing members  5 A,  5 B,  5 C and  5 D so that respective electrostatic latent images are formed on the surfaces of the image bearing members  5 A,  5 B,  5 C and  5 D. The electrostatic latent images are generated based on respective single color image data of yellow, magenta, cyan and black which are converted from color image data of a full-color image. The electrostatic latent images are visualized by the respective developing units having developers of different colors corresponding to respective electrostatic latent images as color toner images. 
   As shown in  FIG. 1 , the intermediate transfer belt  4  rotates in the direction A. The color toner images formed on the surfaces of the respective image bearing members  5 A,  5 B,  5 C and  5 D are sequentially overlaid on the surface of the intermediate transfer belt  4  so that an overlaid color toner image is formed on a surface of the intermediate transfer belt  4 . 
   After the color toner images on the surfaces of the respective image bearing members  5 A,  5 B,  5 C and  5 D are transferred onto the intermediate transfer belt  4 , the cleaning units corresponding to the image forming units scrapes the surfaces of the respective image bearing members  5 A,  5 B,  5 C and  5 D to remove residual toner adhering to the surfaces of the respective image bearing members  5 A,  5 B,  5 C and  5 D. 
   After the cleaning units remove the residual toner, the discharging units corresponding to the image forming units discharge the surfaces of the respective image bearing members  5 A,  5 B,  5 C and  5 D so that the image forming units are prepared for the next image forming operations. 
   The sheet feeding system  3  of  FIG. 1  includes a transfer sheet (not shown) in a sheet feeding cassette (not shown). The transfer sheet is fed from the sheet feeding cassette and is conveyed to the registration rollers  7 . The registration rollers  7  stop and feed the transfer sheet in synchronization with a movement of the overlaid color toner image towards a transfer area formed between the intermediate transfer belt  4  and the secondary transfer roller  8 . The secondary transfer roller  8  is applied with an adequate predetermined transfer voltage having a polarity opposite to the overlaid color toner image on the intermediate transfer belt  4 . Thus, the overlaid color toner image is transferred onto the transfer sheet. 
   The transfer sheet that has the overlaid color toner image thereon is conveyed further upward and passes the fixing unit  9 . The fixing unit  9  fixes the overlaid color toner image to the transfer sheet by applying heat and pressure. After the transfer sheet passes the fixing unit  9 , the transfer sheet is discharged to a sheet discharging part  12  provided at the upper location of the background color laser printer  1 . A belt cleaning unit (not shown) scrapes the surface of the intermediate transfer belt  4  and removes residual toner adhering onto the surface of the intermediate transfer belt  4 . 
   Referring to  FIG. 2 , a structure of the optical writing apparatus  2  included in the background color laser printer  1  is described. 
   In  FIG. 2 , the optical writing apparatus  2  includes four light source units (not shown), a polygon mirror wheel  62  and associated optical components. The optical writing apparatus  2  is encased by an optical housing  50 . 
   The four light source units emit the laser beams L 1 , L 2 , L 3  and L 4  towards the polygon mirror wheel  62 . 
   The polygon mirror wheel  62  is an optical deflector having a double-stage structure including mirrors  62 A and  62 B. The polygon mirror wheel  62  distributes the laser beams L 1 , L 2 , L 3  and L 4  emitted from the respective light source units symmetrically in two directions for deflecting and scanning the laser beams L 1 , L 2 , L 3  and L 4 . 
   The optical components include two f-theta lenses  63  and  64 , imaging lenses  69 ,  70 ,  71  and  72 , which are also referred to as toroidal lenses, first deflecting mirrors  65 ,  66 ,  67  and  68 , second deflecting mirrors  73 ,  74 ,  75  and  76 , and third deflecting mirrors  77 ,  78 ,  79  and  80 . Each of the f-theta lenses  63  and  64  has a vertical double-layer structure having an upper layer and a lower layer. The optical components direct the laser beams L 1 , L 2 , L 3  and L 4  arranged symmetrically in the above-described two directions with respect to the polygon mirror wheel  62  and direct the laser beams L 1 , L 2 , L 3  and L 4  deflected onto the surfaces of the respective image bearing members  5 A,  5 B,  5 C and  5 D so as to form the respective electrostatic latent images thereon. 
   The optical housing  50  has a flat-box-shaped structure which is hermetically closed, and includes a base plate  50 A and a side plate  50 B. The base plate  50 A is located to a bottom portion of the optical housing  50  and mounts the optical components thereon. The side plate  50 B has a shape of a frame surrounding a circumference of the base plate  50 A, which forms a tray-like shape. The polygon mirror wheel  62  is disposed in an approximately central portion of the base plate  50 A of the optical housing  50 , and the optical components as previously described are disposed in the optical housing  50 . A top cover  87  is provided at a top portion of the optical housing  50 . The top cover  87  has four openings provided with dust-proof glasses  81 ,  82 ,  83  and  84  for passing the respective laser beams L 1 , L 2 , L 3  and L 4 . 
   The optical writing apparatus  2  performs image writing through the following operations. 
   An image is input to a document reading apparatus, such as a scanner (not shown), or an image data output system, such as a personal computer, a word processor and a receiving portion of a facsimile machine, is separated into different color image data. The different color image data is converted into respective color image signals for driving the respective light source units. After the conversion of the color image signals, light sources in the respective light source units, such as semiconductor lasers (LD), are driven to emit the laser beams L 1 , L 2 , L 3  and L 4 . The laser beams L 1 , L 2 , L 3  and L 4  emitted from the respective light source units pass through cylindrical lenses (not shown) for correcting an optical face tangle error and reach the polygon mirror  62  directly or after reflected by mirrors (not shown). The laser beams L 1 , L 2 , L 3  and L 4  are deflected in the symmetrical directions by the polygon mirror wheel  62  with the mirrors  62 A and  62 B which are rotated by at a uniform velocity by an electrical motor (not shown). 
   As described above, the polygon mirror wheel  62  of  FIG. 2  includes the mirrors  62 A and  62 B having the double-stage structure in which the mirrors  62 A and  62 B are placed as layers. The mirror  62 A deflects the laser beams L 1  and L 4  and the mirror  62 B deflects the laser beams L 2  and L 3 . As an alternative, a single-stage axially longer polygon mirror may achieve a same performance as the double-stage polygon mirror. 
   After being deflected by the mirrors  62 A and  62 B of the polygon mirror wheel  62  in two directions, the light beams L 1  and L 2  pass through the f-theta lens  63  and the light beams L 3  and L 4  pass through the f-theta lens  64 . The light beam L 1  deflected by the mirror  62 A of the polygon mirror wheel  62  passes through the upper layer of the f-theta lens  63  and the imaging lens  69 , is reflected by the first mirror  62 , the second mirror  73 , the third mirror  77  and then passes through the dust-proof glass  81  to irradiate the image bearing member  5 A. The light beam L 2  deflected by the mirror  62 B passes through the lower layer of the f-theta lens  63 , is reflected by the first mirror  66 , passes through the imaging lens  70 , is reflected by the second mirror  74  and the third mirror  78 , and then passes through the dust-proof glass  82  to irradiate the image bearing member  5 B. The light beam L 3  deflected by the mirror  62 B passes through the lower layer of the f-theta lens  64 , is reflected by the first mirror  67 , passes through the imaging lens  71 , is reflected by the second mirror  75  and the third mirror  79 , and then passes through the dust-proof glass  83  to the image bearing member  5 C. The light beam L 4  deflected by the mirror  62 A of the polygon mirror wheel  62  passes through the upper layer of the f-theta lens  64  and the imaging lens  72 , is reflected by the first mirror  68 , the second mirror  76 , the third mirror  80  and then passes through the dust-proof glass  84  to irradiate the image bearing member  5 D. 
   In an image forming apparatus including the optical writing apparatus  2  as described above, the polygon mirror wheel  62  is arranged in a vicinity of the f-theta lenses  63  and  64 . In a case where the f-theta lenses  63  and  64  are formed by a resin material, heat generated by rotation of the polygon mirror wheel  62  may deteriorate optical characteristics of the f-theta lenses  63  and  64 . The deterioration of the f-theta lenses  63  and  64  may erroneously vary speeds of writing images on the surfaces of the respective image bearing members. As a result, scales of the respective images may change. This is mainly caused by heat produced by the rotation of the polygon motor which drives the polygon mirror wheel  62 . When the motor of the polygon mirror wheel  62  is controlled or rotated, heat is generated. The heat flows in an air stream generated by rotation of the polygon mirror wheel  62  to hit against the f-theta lenses  63  and  64 . At this time, one of the f-theta lenses  63  and  64  is disposed to a position closer than the other to a heat source located upstream of heated air and the f-theta lenses  63  and  64  symmetrically disposed to two different directions are applied with different temperatures of the air stream. Therefore, the temperature change of the f-theta lenses  63  and  64  are not identical. When the f-theta lenses  63  and  64  are warmed up, their optical characteristics may change and the f-theta lenses  63  and  64  may have different copy scales in the main scanning direction due to thermal expansion. Thus, a writing scale may change and a position of writing an image may also change because of heat generated by the polygon mirror wheel  62  and its electrical motor, resulting in deterioration in image quality. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in view of the above circumstances. It is an object of the present invention to provide a novel image forming apparatus capable of preventing image quality deterioration induced by a heat from an optical deflector by insulating f-theta lenses from the optical deflector. 
   It is another object of the present invention to provide a novel optical writing apparatus included in the novel image forming apparatus. 
   In one exemplary embodiment, a novel image forming apparatus includes an image forming apparatus which includes an image bearing member having a photosensitive surface and an optical writing apparatus generating a laser beam modulated according to image data. The optical writing apparatus includes a housing, an optical deflector configured to rotate to deflect the laser beam, an f-theta lens configured to correct the laser beam deflected by the optical deflector and to transmit a corrected laser beam towards the image bearing member, and a separator configured to separate an inside space of the housing into at least two sections including a first section in which the optical deflector is mounted and a second section in which the f-theta lens is mounted, the separator including a heat resistant transparent plate disposed at a position of crossing passage of the laser beam between the optical deflector and the f-theta lens. 
   Further, in one exemplary embodiment, a novel method of manufacturing a novel image forming apparatus may include the steps of arranging an image bearing member having a photosensitive surface, providing a housing for an optical writing apparatus generating a laser beam modulated according to image data, separating an inside space of the housing into at least two sections including a first section in which the optical deflector is mounted and a second section in which the f-theta lens is mounted, arranging a heat resistant transparent plate at a position of crossing passage of the laser beam between the optical deflector and the f-theta lens, arranging an optical deflector configured to rotate for deflecting the laser beam, and positioning an f-theta lens configured to correct the laser beam deflected by the optical deflector and to transmit a corrected laser beam towards the image bearing member. 
   In one exemplary embodiment, another novel image forming apparatus includes a plurality of image bearing members each having a photosensitive surface and an optical writing apparatus generating a plurality of laser beams modulated according to image data. The optical writing apparatus includes a housing, an optical deflector configured to rotate for deflecting a part of the plurality of laser beams in a first direction and a rest of the plurality of laser beams in a second direction symmetrically opposite to the first direction, at least two f-theta lenses configured to correct the plurality of laser beams deflected by the optical deflector and to transmit a plurality of corresponding corrected laser beams towards the plurality of image bearing members, the at least two f-theta lenses including first and second f-theta lenses disposed at respective places oppositely located in the first and second directions, respectively, relative to the optical deflector, and a separator configured to separate an inside space of the housing into at least two sections including a first section in which the optical deflector is mounted and a second section in which the first and second f-theta lenses are mounted, the separator including a heat resistant transparent plate disposed at a position of crossing passage of one part of the plurality of laser beams between the optical deflector and the first f-theta lens and another heat resistant transparent plate disposed at a position of crossing passage of the rest of the plurality of laser beams between the optical deflector and the second f-theta lens. 
   The housing may be a flat box, and the first section may be located at an upper side of the housing and the second section may be located at a lower side of the housing. 
   The first section may include a concave portion at an approximately central portion thereof protruding downward to the second section for placing the optical deflector. 
   The concave portion may be insulated from the second section of the housing with the heat resistant transparent plates. 
   The heat resistant transparent plates may be arranged tilted away from the optical deflector, expanding the concave portion upward. 
   The housing may include a resin material. 
   The first and second f-theta lenses may include a resin material. 
   The above-described image forming apparatus may further include a top cover member configured to seal the first section. 
   The top cover member may include a material having heat conductivity which is higher than a material of the housing. 
   A part of the first section which is located in a vicinity of the concave portion above the second section may include side walls separated from the concave portion. 
   The above-described image forming apparatus may further include a concave cover member configured to cover the concave portion. 
   The concave cover member may include a material having a heat conductance which is higher than a material of the housing. 
   The above-described image forming apparatus may further include a controller configured to control the rotation of the optical deflector. 
   The controller may be disposed outside of the housing. 
   Further, in one exemplary embodiment, a novel method of manufacturing an image forming apparatus includes the steps of arranging a plurality of image bearing members each having a photosensitive surface, providing a housing for an optical writing apparatus generating a plurality of laser beams modulated according to image data, separating an inside space of the housing into at least two sections including a first section in which the optical deflector is mounted and a second section in which the first and second f-theta lenses are mounted, arranging heat resistant transparent plates, one plate at a position of crossing passage of one part of the plurality of laser beams between the optical deflector and the first f-theta lens and another plate at a position of crossing passage of the rest of the plurality of laser beams between the optical deflector and the second f-theta lens, arranging an optical deflector configured to deflect one part of the plurality of laser beams in a first direction and a rest of the plurality of laser beams in a second direction, symmetrically opposite to the first direction. The first and second f-theta lenses are then positioned at respective places oppositely located in the first and second directions, respectively, relative to the optical deflector, the first and second f-theta lenses being configured to correct the plurality of laser beams deflected by the optical deflector and to transmit a plurality of corresponding corrected laser beams towards the plurality of image bearing members. 
   The above-described method may further include the step of sealing the first section by providing a top cover member. 
   The above-described method may further include the step of covering the concave portion by providing a concave cover member. 
   The above-described method may further include controlled rotation of the optical deflector by a controller. 
   In one exemplary embodiment, a novel optical writing apparatus generating a plurality of laser beams modulated according to image data includes a housing, an optical deflector configured to rotate to deflect part of the plurality of laser beams in a first direction and the rest of the plurality of laser beams in a second direction, symmetrically opposite to the first direction, at least two f-theta lenses configured to correct the plurality of laser beams deflected by the optical deflector and to transmit a plurality of corresponding corrected laser beams towards the plurality of image bearing members, the at least two f-theta lenses including first and second f-theta lenses disposed at respective places oppositely located in the first and second directions, respectively, relative to the optical deflector, and a separator configured to separate an inside space of the housing into at least two sections including a first section in which the optical deflector is mounted and a second section in which the first and second f-theta lenses are mounted, the separator including a heat resistant transparent plate disposed at a position of crossing passage of one part of the plurality of laser beams between the optical deflector and the first f-theta lens and another heat resistant transparent plate disposed at a position of crossing passage of the rest of the plurality of laser beams between the optical deflector and the second f-theta lens. 
   The above-described optical writing apparatus may further include a top cover member configured to seal the first section. 
   The above-described optical writing apparatus may further include a concave cover member configured to cover the concave portion. 
   The above-described optical writing apparatus may further include a controller configured to control the rotation of the optical deflector. 
   Further, in one exemplary embodiment, a novel method of manufacturing a novel optical writing apparatus includes the steps of providing a housing for the optical writing apparatus generating a plurality of laser beams modulated according to image data, separating an inside space of the housing into at least two sections including a first section in which the optical deflector is mounted and a second section in which the first and second f-theta lenses are mounted, arranging heat resistant transparent plates, one plate at a position of crossing passage of one part of the plurality of laser beams between the optical deflector and the first f-theta lens and another plate at a position of crossing passage of the rest of the plurality of laser beams between the optical deflector and the second f-theta lens, arranging an optical deflector configured to deflect part of the plurality of laser beams in a first direction and a rest of the plurality of laser beams in a second direction symmetrically opposite to the first direction, and positioning first and second f-theta lenses at respective places oppositely located in the first and second directions, respectively, relative to the optical deflector, the first and second f-theta lenses being configured to correct the plurality of laser beams deflected by the optical deflector and to transmit a plurality of corresponding corrected laser beams towards the plurality of image bearing members. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the disclosure and many of the resulting advantages thereof will be better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
       FIG. 1  is a schematic view of a structure of a conventional color laser printer; 
       FIG. 2  is a schematic cross-sectional view of a structure of an optical writing apparatus in the color laser printer of  FIG. 1 ; 
       FIG. 3  is a schematic view of a structure of a color laser printer according to an exemplary embodiment of the present invention; 
       FIG. 4  is a schematic cross-sectional view of an exemplary optical writing apparatus in the color laser printer of  FIG. 3 ; 
       FIG. 5  is a schematic cross-sectional view of an exemplary hollow space associated with a heated-air flow around an optical deflector in the optical writing apparatus of  FIG. 4 ; 
       FIG. 6  is a schematic cross-sectional view of another exemplary hollow space associated with a heated-air flow around an optical deflector in the optical writing apparatus of  FIG. 4 ; and 
       FIG. 7  is a schematic cross-sectional view of another exemplary hollow space associated with a heated-air flow around an optical deflector in the optical writing apparatus of  FIG. 6 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In describing preferred embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology selected therefore and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner. 
   Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, particularly to  FIG. 3 , a color laser printer  100  as shown as one example of an electrophotographic image forming apparatus including an optical writing apparatus  102  according to an exemplary embodiment of the present invention is described. 
   The color laser printer  100  of  FIG. 3  has a similar structure to the color laser printer  1  of  FIG. 1 , except for a structure of the optical writing apparatus  102 . 
   Referring to  FIG. 4 , the detailed structure of the optical writing apparatus  102  is described. The optical writing apparatus  102  is provided to a color image forming apparatus having a structure for a tandem operation, such as the color laser printer  1  of  FIG. 1  and the color laser printer  100  of  FIG. 3 , so that laser beams can be emitted from below to upward towards respective image bearing members of respective image forming units. 
   The optical writing apparatus  102  of  FIG. 4  has a structure similar to the optical writing apparatus  2  of  FIG. 2 , except for a separation plate  150 A of an optical housing  150 . 
   In the optical housing  150 , the separation plate  150 A possesses greater rigidity to function as a main frame of the optical housing  150  as the base plate  50 A does and is arranged at an approximately central portion of a side plate  150 B to vertically separate the optical housing  150  into two sections such that a cross-sectional view of the optical housing  150  is framed as an approximately H-shaped structure. That is, the separation plate  150 A of the optical housing  150  is placed between an upper section  151  and a lower section  152  of the optical writing apparatus  102 . The optical writing apparatus  102  further includes a top cover  187  and a bottom cover  155  of the optical housing  150 . 
   In the upper section  151 , the separation plate  150 A has a hollow  153  at an approximately central position thereof. The hollow  153  downwardly extends to the bottom cover  155  of the lower section  152  to place a polygon mirror  162  having a double-stage structure therein. The hollow  153  includes side walls  154 A and  154 B for separating the hollow  153  from the lower section  152 . The side walls  154 A and  154 B have respective portions made of transparent dust-proof and/or sound-proof glasses so that the laser beams L 1 , L 2 , L 3  and L 4  deflected by the polygon mirror wheel  62  pass through the side walls  154 A and  154 B. The upper section  151  further includes imaging lenses  170  and  171 , second deflecting lenses  173 ,  174 ,  175  and  176 , and third deflecting lenses  177 ,  178 ,  179  and  180 . The top cover  187  includes dust-proof glasses  181 ,  182 ,  183  and  184  to pass through the respective laser beams L 1 , L 2 , L 3  and L 4  to irradiate each surface of the respective image bearing members  5 A,  5 B,  5 C and  5 D. 
   In the lower section  152 , f-theta lenses  163  and  164  are mounted on a bottom side of the separation plate  150 A so that the f-theta lenses  163  and  164  are oppositely placed to each other having the polygon mirror wheel  162  there between. The lower section  152  further includes imaging lenses  169  and  172 , first deflecting mirrors  165 ,  166 ,  167  and  168 . The optical housing  150  and the f-theta lenses  163  and  164  are formed by resin molding. 
   In the optical writing apparatus  102 , the polygon mirror wheel  162  and the f-theta lenses  163  and  164  are provided to different sections separated by the separation plate  150 A of the optical housing  150 . That is, the polygon mirror wheel  162  is disposed in the upper section  151  and the f-theta lenses  163  and  164  are disposed in the lower section  152 . In a case where a stream of heated air is generated in the upper section  151  by rotation of the polygon mirror wheel  162 , the heated air flow does not directly hit against the f-theta lenses  163  and  164  disposed in the lower section  152 . Further, in a case where the heated air flow generated by the rotation of the polygon mirror wheel  162  has a temperature difference, optical characteristics are not changed significantly by heat. Therefore, temperature change of the f-theta lenses  163  and  164  is restrained and deterioration in image quality is reduced or prevented. 
   An entire portion of the bottom cover  155  is preferably made of a metal plate having a heat conductance which is higher than the optical housing  150 . With the above-described bottom cover  155 , a heat in the lower section  152  of the optical housing  150  is uniformly distributed so that the f-theta lenses  163  and  164  are substantially equally warmed. As a result, this reduces or prevents color deviation which is caused when the f-theta lenses  163  and  164  are distorted by heat generated by the polygon mirror wheel  162 . 
   The heat generated by the polygon mirror wheel  162  is distributed in the upper section  151  of the optical housing  150 . To release the heat, the top cover  187  may have an entire portion or at least a portion just above the polygon mirror wheel  162  made of a metal cover  156  having a heat conductance which is higher than the optical housing  150 . With the above-described top cover  187  having the metal cover  156  in part, a temperature in the upper section  151  will not be drastically increased. 
   Referring to  FIG. 5 , a structure of an optical writing apparatus  202  is described according to another exemplary embodiment of the present invention. The optical writing apparatus  202  of  FIG. 5  has a similar structure to the optical writing apparatus  102  of  FIG. 4 , except for a shape of a space or hollow  253 . 
   In  FIG. 5 , the optical writing apparatus  202  includes the hollow  253  formed on a separation plate  250 A so that the polygon mirror wheel  162  is disposed therein. The hollow  253  of the separation plate  250 A includes side walls  254 A and  254 B which are arranged oppositely on both sides of the polygon mirror wheel  162 . The side walls  254 A and  254 B are tilted away from the polygon mirror wheel  162  so that a distance between the side walls  254 A and  254 B becomes greater as the side walls  254 A and  254 B are extending upward. 
   A top cover  287  of the optical writing apparatus  202  also has a hollow  288  corresponding to the hollow  253  of the separation plate  250 A. The hollow  288  of the top cover  287  includes side walls  289 A and  289 B which are arranged substantially in parallel with the tilted side walls  254 A and  254 B of the separation plate  250 A. The top cover  287  may be made of a heat-conductive metal plate for over an entire portion or at least a portion just above the polygon mirror wheel  162 . 
   In the above-described optical writing apparatus  202 , rotation of the polygon mirror wheel  162  generates a stream of heated air. The stream of heated air is dispersed in an upward direction, as indicated by arrows, along the side walls  254 A and  254 B of the hollow  253  into an upper section of the optical writing apparatus  202 . Since the side walls  254 A and  254 B are tilted, the stream of heated air is dispersed in the upward direction more smoothly, compared to the optical writing apparatus  102  having the side walls  154 A and  154 B which are arranged in a vertical direction perpendicular to the separation plate  150 A. The side walls  254 A and  254 B can reduce or prevent a heat conductance to the f-theta lenses of  FIG. 4  and keep a temperature of heat around the f-theta lenses  163  and  164  stable. Therefore, deterioration in image quality caused by the heat may be reduced or prevented. 
   Referring now to  FIG. 6 , a structure of an optical writing system  302  is described according to another exemplary embodiment of the present invention. The optical writing apparatus  302  of  FIG. 6  has an identical structure to the optical writing apparatus  102  of  FIG. 4 , except for a shape of a hollow  353 . 
   In  FIG. 6 , the optical writing apparatus  302  includes the hollow  353  formed on a separation plate  350 A so that the polygon mirror wheel  162  is disposed therein. The hollow  353  of the separation plate  350 A includes side walls  354 A and  354 B which are arranged oppositely on both sides of the polygon mirror wheel  162 . The side walls  354 A and  354 B are tilted so that a distance between the side walls  354 A and  354 B becomes greater as the side walls  354 A and  354 B are extending upward. The side walls  354 A and  354 B of  FIG. 6  are made to have a length longer than the side walls  254 A and  254 B of  FIG. 5 . A top cover  387  includes side walls  389 A and  389 B which are tilted by same angles as the side walls  354 A and  354 B of the separation plate  350 A. In this embodiment, the top cover  387  is integrally connected with the separation plate  350 A such that the side walls  354 A and  354 B are arranged in alignment with  389 A and  389 B, respectively. Two opposite ends of the metal cover  356  are conjoined either with  354 A and  354 B, and/or with  389 A and  389 B respectively, at a predetermined height of the hollow  353 . 
   The metal cover  356  covers the hollow  353  over the polygon mirror wheel  162  so that the hollow  353  is hermetically closed and a stream of heated air is not distributed into an upper section  351  as well as a lower section  352  of the optical writing apparatus  302 . The metal cover  356  is made of a metal plate having a high heat conductance. With the above-described structure of the optical writing apparatus  302 , the stream of heated air is dispersed through the metal cover  356  to an outside of the optical writing apparatus  302 . Therefore, the amount of the heated air flow affecting the optical writing section is reduced, more particularly affected on the f-theta lenses  163  and  164  of  FIG. 4  is reduced, and as a result, damage to the f-theta lenses  163  and  164  caused by heat can be reduced or prevented. 
   Referring to  FIG. 7 , another structure of the optical writing apparatus  302  is described. 
   The optical writing apparatus  302  of  FIG. 7  further includes a controller  358  for controlling the polygon mirror  362 . The controller  358  which is generally disposed in an optical housing of the optical writing apparatus  302  is arranged outside of the optical housing, so as to reduce the heat generated inside the hollow  353 . 
   With the above-described structure, a degree of a temperature in the hollow  353  is not drastically increased and damage to the f-theta lenses  163  and  163  of  FIG. 4  are securely reduced. 
   In  FIG. 7 , the controller  358  is arranged to a bottom surface of the separation plate  350 A. As an alternative, the controller  358  may be arranged to any other place outside the optical housing by connecting a polygon motor (not shown) via a harness or a cable (not shown). 
   The image forming apparatus according to the present invention has a structure that an optical writing apparatus emits laser beams to irradiate respective image bearing members from under respective image forming units including the respective image bearing members. As an alternative, the optical writing apparatus of the present invention may be applied to an image forming apparatus in which the optical writing apparatus emits laser beams from a side of or from above the respective image bearing members. 
   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 this patent specification may be practiced otherwise than as specifically described herein.