Patent Publication Number: US-9851559-B2

Title: Optical scanning unit and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation application of U.S. application Ser. No. 14/717,421, filed May 20, 2015, which is based upon and claims priority to divisional application of U.S. application Ser. No. 13/934,967, filed Jul. 3, 2013, and is based upon and claims priority pursuant to 35 U.S.C. §119 to Japanese Patent Application Nos. 2012-151306, filed on Jul. 5, 2012, and 2013-112429, filed on May 29, 2013 in the Japan Patent Office, the disclosure of which is incorporated by reference herein in their entirety. 
    
    
     BACKGROUND 
     Technical Field 
     The present invention relates to an optical scanning unit and an image forming apparatus, and more particularly to, an optical scanning unit that scans a scanned surface, and an image forming apparatus employing the optical scanning unit. 
     Background Art 
     When recording images, Image forming apparatuses using electrophotography typically include, for example, an optical scanning unit that scans a surface of a photoconductive drum using a laser beam to form a latent image on the surface of the photoconductive drum. 
     The optical scanning unit includes, for example, a light source, a pre-deflector optical system, a rotatable multi-faceted mirror, and a scanning optical system. The laser beam emitted from the light source strikes the rotatable multi-faceted mirror via the pre-deflector optical system, is deflected at the reflection face of the rotatable multi-faceted mirror, and is then guided to the photoconductive drum through the scanning optical system. The reflection face of the rotatable multi-faceted mirror is also referred to as the deflective reflection face. 
     The laser beam can be directed to the multi-faceted mirror using an underfilled (UF) type system or an overfilled (OF) type system. 
     JP-2005-92129-A discloses a UF type system, in which the incident light is narrower than a length of the deflective reflection face extending along a direction corresponding to the main scanning direction, and all the incident light is reflected at the deflective reflection face. 
     By contrast, JP-H10-206778-A discloses a OF type system, in which the incident light is wider than a length of the deflective reflection face extending along a direction corresponding to the main scanning direction, and the periphery of the incident light does not strike the deflective reflection face. 
     Image forming apparatuses are increasingly expected to operate at ever-faster speeds and provide enhanced quality images. However, with conventional image forming apparatuses, it is difficult to enhance image forming speed and image quality without increasing the size and cost of the apparatus. 
     SUMMARY 
     In one aspect of the present invention, an optical scanning unit is devised. The optical scanning unit includes a rotatable multi-faceted mirror having a plurality of reflection faces reflecting light flux emitted from a light source to scan a scanning area on a scanned surface in a main scanning direction. When the light flux is orthographically projected to a plane perpendicular to a rotation axis of the rotatable multi-face mirror, a width of the light flux striking the rotatable multi-faceted mirror is narrower than a length of a reflection face of the rotatable multi-faceted mirror extending along a direction corresponding to the main scanning direction. The entire of light flux striking the rotatable multi-faceted mirror is reflected at a first reflection face when the light flux reflected by the rotatable multi-faceted mirror is directed to the center portion of the scanning area. A part of the light flux striking the rotatable multi-faceted mirror is reflected at the first reflection face while the remaining portion of the light flux striking the rotatable multi-faceted mirror is reflected at a second reflection face when the light flux reflected by the rotatable multi-faceted mirror is directed to a least one of two end portions of the scanning area. 
    
    
     
       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  is a schematic configuration of an image forming apparatuses according to an example embodiment of the present invention; 
         FIGS. 2, 3, 4 and 5  show schematic configurations of an optical scanning unit; 
         FIG. 6  shows a schematic configuration of a pre-deflector optical system and an optical deflector; 
         FIG. 7  shows a schematic view of an inscribed circle of a rotatable multi-faceted mirror; 
         FIG. 8  shows a schematic view of light flux to enter an optical deflector with a light flux width din; 
         FIG. 9  shows an incident light flux and a reflection light flux with respect to an optical deflector when the light flux deflected by the optical deflector is directed to a scanning start position for a scanning area of a corresponding photoconductive drum; 
         FIG. 10  shows an incident light flux and a reflection light flux with respect to an optical deflector when the light flux deflected by the optical deflector is directed to the center position of the scanning area of the corresponding photoconductive drum; 
         FIG. 11  shows an incident light flux and a reflection light flux with respect to an optical deflector when the light flux deflected by the optical deflector is directed to a scanning end position for a scanning area of the corresponding photoconductive drum; 
         FIGS. 12A to 12D  show examples of the aperture plate used in the example embodiments; 
         FIGS. 13A and 13B  show aperture plates used for conventional optical scanning units; 
         FIG. 14  shows a relationship of d 2 /d 1  and an angle θs or θe when the number of faces of the rotatable multi-faceted mirror is six and d 1 =3.0 mm; 
         FIG. 15  shows a relationship of d 2 /d 1  and the angle θs or θe when the number of faces of the rotatable multi-faceted mirror is seven and d 1 =3.0 mm; 
         FIG. 16  shows a relationship of d 2 /d 1  and the angle θs or θe when the number of faces of the rotatable multi-faceted mirror is eight and d 1 =3.0 mm; 
         FIG. 17  shows a relationship of d 2 /d 1  and the angle θs or θe when the number of faces of the rotatable multi-faceted mirror is nine and d 1 =3.0 mm; 
         FIG. 18  shows a relationship of d 2 /d 1  and the angle θs or θe when the number of faces of the rotatable multi-faceted mirror is six and d 1 =3.5 mm; 
         FIG. 19  shows a relationship of d 2 /d 1  and the angle θs or θe when the number of faces of the rotatable multi-faceted mirror is seven and d 1 =3.5 mm; 
         FIG. 20  shows a relationship of d 2 /d 1  and the angle θs or θe when the number of faces of the rotatable multi-faceted mirror is eight and d 1 =3.5 mm; 
         FIG. 21  shows a relationship of d 2 /d 1  and the angle θs or θe when the number of faces of the rotatable multi-faceted mirror is nine and d 1 =3.5 mm; 
         FIG. 22  shows a relationship of d 2 /d 1  and the angle θs or θe when the number of faces of the rotatable multi-faceted mirror is six and d 1 =4.0 mm; 
         FIG. 23  shows a relationship of d 2 /d 1  and the angle θs or θe when the number of faces of the rotatable multi-faceted mirror is seven and d 1 =4.0 mm; 
         FIG. 24  shows a relationship of d 2 /d 1  and the angle θs or θe when the number of faces of the rotatable multi-faceted mirror is eight and d 1 =4.0 mm; 
         FIG. 25  shows a relationship of d 2 /d 1  and the angle θs or θe when the number of faces of the rotatable multi-faceted mirror is nine and d 1 =4.0 mm; 
         FIGS. 26, 27 and 28  show schematic configurations of a first variant example; 
         FIGS. 29, 30, 31, 32 and 33  show schematic configurations of a second variant example; 
         FIGS. 34A to 34D  show examples of aperture plate used for the variant examples. 
         FIG. 35  shows a schematic arrangement of a synchronization detection sensor; 
         FIGS. 36(A) and 36(B)  show examples of fluctuation of light intensity before light intensity correction and after light intensity correction indicating post-correction difference; 
         FIG. 37  shows light intensity ratio at each image height against image height of 0 mm when light flux having only S-polarized light component enters a deflective reflection face of an optical deflector, and when light flux having P-polarized light component stronger than S-polarized light component enters a deflective reflection face of an optical deflector; 
         FIG. 38  shows an example of the light intensity profile when light flux having only S-polarized light component enters a deflective reflection face of an optical deflector; 
         FIG. 39  shows an example of the light intensity profile when light flux having P-polarized light component stronger than S-polarized light component enters a deflective reflection face of an optical deflector; 
         FIG. 40  shows an example of the light intensity profile when setting an intensity ratio of S-polarized light component and P-polarized light component of the light flux exiting from the light source and entering the optical deflector as 27:73; and 
         FIG. 41  shows an effect of shape of an aperture to light intensity profile set by cutting. 
     
    
    
     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 with reference to  FIGS. 1 to 25 . In this disclosure, a scanning area means an area on a scanned surface such as a surface of photoconductive member where an image such as a latent image is formed by an optical scanning process. 
       FIG. 1  shows a schematic configuration of an image forming apparatus  2000  according to an example embodiment of the present invention. 
     The image forming apparatus  2000 , which can be used as facsimile, printer and copier, includes, for example, a main unit  1001 , a scan unit  1002 , and an automatic document feeder  1003 . As such, the image forming apparatus  2000  may be, for example, a multi-functional apparatus, but not limited hereto. 
     The main unit  1001  is, for example, a multi-color printer employing a tandem system which can form a full-color image by superimposing four colors such as black, cyan, magenta, and yellow. The main unit  1001  includes, for example, an optical scanning unit  2010 , four photoconductors  2030   a ,  2030   b ,  2030   c ,  2030   d , 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 , an intermediate transfer belt  2040 , a transfer roller  2042 , a fusing roller  2050 , a sheet-feed roller  2054 , a sheet-ejection roller  2058 , a sheet tray  2060 , a sheet-ejection tray  2070 , a communication controller  2080 , and an apparatus controller  2090  that controls above each unit as a whole. 
     The scan unit  1002  may be disposed above the main unit  1001  to scan documents. The scan unit  1002  is used as a scanner. Image information scanned by the scan unit  1002  is transmitted to the apparatus controller  2090  of the main unit  1001 . 
     The automatic document feeder  1003  may be disposed above the scan unit  1002  to feed document sheets set on the automatic document feeder  1003 . The automatic document feeder  1003  may be known as an auto document feeder (ADF). 
     The communication controller  2080  controls bi-directional communications with external apparatuses such as personal computers via a network, and data communication via a public line. 
     The apparatus controller  2090  includes, for example, a central processing unit (CPU), a read only memory (ROM), a random access memory (RAM), and an analog/digital (A/D) converter. The ROM stores software programs using coding readable by the CPU, and data to be used with the software programs. The RAM can be used as a working memory. The A/D converter converts analog data to digital data. Further, the apparatus controller  2090  can receive image information from the external apparatuses via the communication controller  2080  or from the scan unit  1002 , and then transmits such image information via to the optical scanning unit  2010 . 
     The photoconductor  2030   a , the charger  2032   a , the development roller  2033   a , and the cleaning unit  2031   a  are assembled as an image forming station to form an image of black (hereinafter, K station). 
     The photoconductor  2030   b , the charger  2032   b , the development roller  2033   b , and the cleaning unit  2031   b  are assembled as an image forming station to form an image of magenta cyan (hereinafter, M station). 
     The photoconductor  2030   c , the charger  2032   c , the development roller  2033   c , and the cleaning unit  2031   c  are assembled as an image forming station to form an image of cyan (hereinafter, C station). 
     The photoconductor  2030   d , the charger  2032   d , the development roller  2033   d , and the cleaning unit  2031   d  are assembled as an image forming station to form an image of yellow (hereinafter, Y station). 
     Each of the photoconductors has a photoconductive layer as a surface layer. Each of the photoconductors can be rotated in a direction shown by an arrow in  FIG. 1  by a driving unit. 
     Each of the chargers charges the surface of corresponding photoconductor uniformly. 
     Based on multi-color image information such as black, cyan, magenta, and yellow image information received from the apparatus controller  2090 , the optical scanning unit  2010  generates a modulated light beam for each color to scan a corresponding charged photoconductor drum using the modulated light beam, by which a latent image corresponding to each color image information can be formed on the corresponding photoconductor drum. Therefore, the surface of each photoconductor drum is used as a scanned surface. Further, photoconductor drum is used as an image bearing member or image carrying member. Therefore, the surface of each photoconductor drum is referred to as the scanned face or the image face, hereinafter. As the photoconductor drum rotates, such latent image is moved to a position facing a corresponding development roller. The optical scanning unit  2010  will be described in detail later. 
     Each development roller is supplied with corresponding color toner, supplied from a corresponding toner cartridge, as the development roller rotates to form a thin layer of toner uniformly on the surface of the development roller. When the toner on the development roller contacts the surface of corresponding photoconductor drum, toner moves only to the light-exposed portion on the surface of photoconductor drum, and adheres on such portion. As such, each development roller supplies toner to the latent image formed on the surface of corresponding photoconductor drum to develop the latent image as a toner image. The toner image is transferred to the intermediate transfer belt  2040  as the photoconductor drum rotates. 
     Yellow, magenta, cyan, and black toner images are sequentially superimposed on the intermediate transfer belt  2040  at a given timing to form a color image. 
     The sheet tray  2060  stores recording media such as recording sheets. The sheet-feed roller  2054  disposed near the sheet tray  2060  can be used to feed out the recording sheets one by one from the sheet tray  2060 . The recording sheet can be fed to a nip between the transfer belt  2040  and the transfer roller  2042  at a given timing to transfer the color image from the intermediate transfer belt  2040  to the recording sheet. Then, the recording sheet transferred with the color image is fed 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. Upon fusing the toner image, the recording sheet is transported to the sheet-ejection tray  2070  via the sheet-ejection roller  2058 , and stacked on the sheet-ejection tray  2070  one by one. 
     Each of the cleaning unit removes remaining toner from the surface of corresponding photoconductor drum. Upon removing the remaining toner, the photoconductor drum can be set at a position facing the corresponding charger again. 
     A description is given of a configuration of the optical scanning unit  2010 . As shown in  FIGS. 2, 3, 4, and 5 , and the optical scanning unit  2010  includes, for example, two light sources  2200 A,  2200 B, four coupling lenses  2201   a ,  2201   b ,  2201   c ,  2201   d , four aperture plates  2202   a ,  2202   b ,  2202   c ,  2202   d , four cylindrical lenses  2204   a ,  2204   b ,  2204   c ,  2204   d , an optical deflector  2104 , four scan lenses  2105   a ,  2105   b ,  2105   c ,  2105   d , eight reflection mirrors  2106 A,  2106 B,  2107   a ,  2107   b ,  2107   c ,  2107   d ,  2108   a ,  2108   d , and a scan controller, and such units and devices are installed in an optical housing. 
     In this description, X, Y, Z three-dimensional orthogonal coordinate system is used, in which the long side direction of each photoconductor drum (i.e., rotation axis direction) is aligned to the Y-axis direction, and the rotation axis direction of the polygon mirror  2104  is aligned to the Z-axis direction. Therefore, the Z-axis direction is parallel to the rotation axis direction of the optical deflector  2104 . 
     Hereinafter, for the simplicity of expression, a direction of each optical part corresponding to the main scanning direction on the photoconductor drum is referred to as “main scanning corresponding direction,” and a direction of each optical part corresponding to the sub-scanning direction on the photoconductor drum is referred to as “sub-scanning corresponding direction”. 
     The light source  2200  A and light source  2200  B are arranged at given locations while maintaining a given distance with each other in the X-axis direction. Each of the light source  2200  A and light source  2200  B includes two light emitting elements, in which the two light emitting elements are arranged at given locations while maintaining a given distance with each other in the Z-axis direction 
     In this description, the light source  2200 A emits two light fluxes such as a light flux La at +Z side and a light flux Lb at −Z side, and the light source  2200  B emits two light fluxes such as a light flux Ld at +Z side and a light flux Lc at −Z side. 
     The coupling lens  2201   a  is disposed on the optical path of the light flux La emitted from the light source  2200 A, and sets the light flux La to a substantially parallel light flux. 
     The coupling lens  2201   b  is disposed on the optical path of the light flux Lb emitted from the light source  2200 A, and sets the light flux Lb to a substantially parallel light flux. 
     The coupling lens  2201   c  is disposed on the optical path of the light flux Lc emitted from the light source  2200  B, and sets the light flux Lc to a substantially parallel light flux. 
     The coupling lens  2201   d  is disposed on the optical path of the light flux Ld emitted from the light source  2200  B, and sets the light flux Ld to a substantially parallel light flux. 
     The cylindrical lens  2204   a  is disposed on the optical path of the light flux La via the coupling lens  2201   a , and focuses the light flux La in the Z-axis. 
     The cylindrical lens  2204   b  is disposed on the optical path of the light flux Lb via the coupling lens  2201   b , and focuses the light flux Lb in the Z-axis. 
     The cylindrical lens  2204   c  is disposed on the optical path of the light flux Lc via the coupling lens  2201   c , and focuses the light flux Lc in the Z-axis. 
     The cylindrical lens  2204   d  is disposed on the optical path of the light flux Ld via the coupling lens  2201   d , and focuses the light flux Ld in the Z-axis. 
     The aperture plate  2202   a  has an aperture used for shaping the light flux La coming from the cylindrical lens  2204   a.    
     The aperture plate  2202   b  has an aperture used for shaping the light flux Lb coming from the cylindrical lens  2204   b.    
     The aperture plate  2202   c  has an aperture used for shaping the light flux Lc coming from the cylindrical lens  2204   c.    
     The aperture plate  2202   d  has an aperture used for shaping the light flux Ld coming from the cylindrical lens  2204   d.    
     The light fluxed passing through the aperture of each of the aperture plates enters the optical deflector  2104 . 
     The optical system from each of the light sources to optical elements disposed on the optical path before the optical deflector  2104  is referred to a pre-deflector optical system. 
     The optical deflector  2104  is, for example, a rotatable multi-faceted mirror of two stages having a double deck configuration. The optical deflector  2104  may be a polygon mirror. Each stage of the rotatable multi-faceted mirror has, for example, six mirror faces, and each mirror face is a deflective reflection face. At the first stage or lower stage rotatable multi-faceted mirror, the light flux Lb passing through the aperture of the aperture plate  2202   b  and the light flux Lc passing through the aperture of the aperture plate  2202   c  are deflected. At the second stage or upper stage rotatable multi-faceted mirror, the light flux La passing through the aperture of the aperture plate  2202   a  and the light flux Ld passing through the aperture of the aperture plate  2202   d  are deflected. 
     Further, the light flux La and the light flux Lb are deflected in the +X side of the optical deflector  2104 , and the light flux Lc and the light flux Ld are deflected in −X side of the optical deflector  2104 . 
     The scan lens  2105   a  and the scan lens  2105   b  are disposed on the +X side of the optical deflector  2104 , and the scan lens  2105   c  and the scan lens  2105   d  are disposed on the −X side of the optical deflector  2104 . 
     The scan lens  2105   c  and the scan lens  2105   d  are stacked in the Z-axis direction. The scan lens  2105   a  faces the second stage rotatable multi-faceted mirror, and the scan lens  2105   b  faces the first stage rotatable multi-faceted mirror. The scan lens  2105   c  and the scan lens  2105   d  are stacked in the Z-axis direction. The scan lens  2105   c  faces the first stage rotatable multi-faceted mirror, and the scan lens  2105   d  faces the second stage rotatable multi-faceted mirror. 
     The light flux La deflected by the optical deflector  2104  is irradiated onto the photoconductive drum  2030   a  via the scan lens  2105   a , the reflection mirror  2106 A, the reflection mirror  2107   a , and the reflection mirror  2108   a  to form beam spots. 
     The light flux Lb deflected by the optical deflector  2104  is irradiated onto the photoconductive drum  2030   b  via the scan lens  2105   b , the reflection mirror  2106 A, and the reflection mirror  2107   b  to form beam spots. 
     The light flux Lc deflected by the optical deflector  2104  is irradiated onto the photoconductive drum  2030   c  via the scan lens  2105   c , the reflection mirror  2106 B, and the reflection mirror  2107   c  to form beam spots. 
     The light flux Ld deflected by the optical deflector  2104  is irradiated onto the photoconductive drum  2030   d  via the scan lens  2105   d , the reflection mirror  2106 B, the reflection mirror  2107   d , and the reflection mirror  2108   d  to form beam spots. 
     The plurality of beam spots can be formed on the surface of each photoconductor drum along the axis direction (or long side direction) of each photoconductor drum as the optical deflector  2104 . The forming direction of beam spots corresponds to the main scanning direction (or a first direction), and the rotation direction of the photoconductor drum corresponds to the sub-scanning direction (or a second direction). 
     The optical system disposed on the optical paths between the optical deflector  2104  and the photoconductive drums is referred to a scanning optical system. 
     As shown in  FIG. 6 , when the optical deflector  2104  is viewed from the Z-axis direction, an axis passing the rotation center of the rotatable multi-faceted mirror and extending in parallel to the X-axis is used as a reference axis. 
     When the optical deflector  2104  is viewed from the Z-axis direction, an angle set by the reference axis and the progression direction of the light flux emitted from the light source and entering the deflective reflection face is defined as an angle θin. For example, θin=55.0 degrees is set. 
     Further, as shown in  FIG. 8 , when viewed from the Z-axis direction, when the light flux passes through the aperture of the aperture plate  2202   a , a width of the light flux becomes “din,” and then the light flux enters the optical deflector  2104 . For example, din=3.8 mm is set. 
     A diameter of an inscribed circle of the rotatable multi-faceted mirror ( FIG. 7 ) is, for example, 18 mm, in which the normal line extending from the rotation center of the rotatable multi-faceted mirror to each deflective reflection face has a length of 9 mm. Further, if each of six deflective reflection faces is required to be distinguished with each other, each face is referred to face  1 , face  2 , face  3 , face  4 , face  5 , and face  6  in the counter-clock direction. A description is given of light flux emitted from the light source  2200 A and entering the optical deflector  2104 , and light flux deflected by the optical deflector  2104  with reference to  FIG. 9  to  FIG. 11 . The light flux reflected at the face  1  of the rotatable multi-faceted mirror is directed to a scanning area of the corresponding photoconductive drum. 
       FIG. 9  shows an incident light flux and a reflection light flux with respect to the optical deflector  2104  such as the rotatable multi-faceted mirror when the light flux deflected by the optical deflector  2104  is directed to a scanning start position for a scanning area of the corresponding photoconductive drum. In this case, not all of the light flux entering the optical deflector  2104  enters the face  1  of the rotatable multi-faceted mirror, but it is designed that a part of the light flux entering the optical deflector  2104  strikes the face  6 . Therefore, the width “ds” of the light flux reflected by the face  1  of the rotatable multi-faceted mirror and directed to the scanning start position of the corresponding photoconductive drum becomes smaller than the width “din” of the light flux entering the optical deflector  2104  (ds&lt;din). In this case, a part of the incident light flux is cut at the optical deflector  2104 . For example, ds=3.5 mm is set. 
     The progression direction of the light flux reflected at the face  1  of the rotatable multi-faceted mirror and the reference axis form an angle θs such as 40.0 degrees. Further, the inclined angle θ 1  of the face  1  with respect to the reference axis is, for example, 42.5 degrees. 
       FIG. 10  shows an incident light flux and a reflection light flux with respect to the optical deflector  2104  such as the rotatable multi-faceted mirror when the light flux deflected by the optical deflector  2104  is directed to the center position of the scanning area of the corresponding photoconductive drum. In this case, it is designed that the entire light flux entering the optical deflector  2104  strikes the face  1  of the rotatable multi-faceted mirror. Therefore, the width “dc” of the light flux reflected at the face  1  of the rotatable multi-faceted mirror and directed to the center position of the scanning area of the corresponding photoconductive drum becomes same as the width “din” of the light flux entering the optical deflector  2104  (dc=din). In this case, the incident light flux is not cut at the optical deflector  2104 . Further, the inclined angle θ 1  of the face  1  with respect to the reference axis is, for example, 62.5 degrees. 
       FIG. 11  shows an incident light flux and a reflection light flux with respect to the optical deflector  2104  such as the rotatable multi-faceted mirror when the light flux deflected by the optical deflector  2104  is directed to a scanning end position for a scanning area of the corresponding photoconductive drum. In this case, it is designed that not all of the light flux entering the optical deflector  2104  strikes the face  1  of the rotatable multi-faceted mirror, but a part of the light flux entering the optical deflector  2104  enter the face  2  of the rotatable multi-faceted mirror. Therefore, the width “de” of the light flux reflected at the face  1  of the rotatable multi-faceted mirror and directed to the scanning end position of the corresponding photoconductive drum becomes smaller than the width “din” of the light flux entering the optical deflector  2104  (de&lt;din). In this case, a part of the incident light flux is cut at the optical deflector  2104 . For example, de=3.5 mm is set. 
     The progression direction of the light flux reflected at the face  1  of the rotatable multi-faceted mirror and the reference axis form an angle θe such as 40.0 degrees. Further, the inclined angle θ 1  of the face  1  with respect to the reference axis is, for example, 82.5 degrees. 
     A sum of θs and θe (i.e., θs+θe) corresponds to a scanning angle, and the scanning angle is, for example, 80.0 degrees in the above described case. The scanning angle is composed of θs and θe, and each value of θs and θe can be variably set. 
     The scanning start position for the scanning area of the photoconductive drum is at one end of the scanning area in the main scanning direction, and the scanning end position for the scanning area of the photoconductive drum is at another end of the scanning area in the main scanning direction. 
     Further, the light flux emitted from the light source  2200  B, entering the optical deflector  2104  and deflected by the optical deflector  2104  is set same as the light flux emitted from the light source  2200 A. 
       FIGS. 12A to 12D  show examples of aperture plates used in the example embodiments of the present invention, in which an aperture of the aperture plate has following shape. Specifically, a length (or opening width) of the aperture in the sub-scanning corresponding direction is small at the two end portions compared to the center portion in the main scanning corresponding direction. With this configuration, difference of the light intensity of beam spots between the two end portions and the center portion of the scanning area and a difference of diameters of beam spots between the two end portions and the center portion of the scanning area can be smaller. 
     As comparative examples,  FIG. 13A  shows an aperture plate used for an optical scanning unit of conventional UF type, and  FIG. 13B  shows an aperture plate used for an optical scanning unit of conventional OF type. 
     When the optical scanning unit of conventional UF type is used to cope with high speed image forming and higher density of pixels, a length of the deflective reflection face in the main scanning corresponding direction is required to be greater. Therefore, the number of faces of the rotatable multi-faceted mirror is required to be decreased, or a diameter of a circumscribed circle of the rotatable multi-faceted mirror is required to be increased. 
     However, if the number of faces of the rotatable multi-faceted mirror is decreased, the rotation speed of the rotatable multi-faceted mirror is required to be increased, which is not preferable. Further, if the diameter of the circumscribed circle is increased, wind loss of the rotatable multi-faceted mirror increases, and thereby power consumption increases, which is not preferable. 
     Further, a configuration to increase the number of light sources and to increase the number of light beams deflected by one deflective reflection face can be considered. However, as the number of light sources increases, a drive circuit for the light sources becomes greater and a cost of the drive circuit increases. 
     Further, when the optical scanning unit of conventional OF type is used to cope with high speed image forming and higher density of pixels, the rotatable multi-faceted mirror having ten faces or more is required, by which the scan angle becomes small, and the size of the optical scanning unit increases, which is not preferable. Further, because the peripheral of light flux is not used, the light use efficiency becomes low, which is not preferable. The optical scanning unit  2010  according to the example embodiment can design the rotatable multi-faceted mirror smaller than the optical scanning unit of conventional UF type. Therefore, as for the optical scanning unit  2010 , without increasing the power consumption and the number of light sources, the rotatable multi-faceted mirror can be rotated with a high speed, which means without increasing the cost, high speed image forming and higher density of pixels can be conducted. 
     Further, the optical scanning unit  2010  according to the example embodiment can design the scan angle greater than the optical scanning unit of conventional OF type. Therefore, without increasing the size of the optical scanning unit  2010 , high speed image forming and higher density of pixels can be conducted. 
     When a beam diameter on the surface of the photoconductor drum in the main scanning direction is to set, for example, from 60 μm to 100 μm or so, a width of the light flux (light flux width) deflected by the optical deflector in the main scanning corresponding direction is required to be from 3 mm to 4 mm or so. 
     In the following description, a width of light flux in the main scanning direction that irradiates the end portion of the scanning area of the photoconductive drum is defined as the light flux width “d 1 ,” and a width of light flux in the main scanning direction that irradiates the center portion of the scanning area of the photoconductive drum is defined as the light flux width “d 2 .” Further, when viewed from the Z-axis direction, a length of the normal line extending from the rotation center of the rotatable multi-faceted mirror to the deflective reflection face is defined the length A. The length A is same as the radius of the inscribed circle ( FIG. 7 ) of the rotatable multi-faceted mirror. 
       FIGS. 14 to 17  show relationships of d 2 /d 1 , and the angle θs or θe when d 1 =3.0 mm is used. 
       FIG. 14  shows a relationship of d 2 /d 1  and the angle θs or θe when the number of faces of the rotatable multi-faceted mirror is six (6) faces, using cases of A=7 mm, θin=55 degrees; A=8 mm, θin=55 degrees; A=9 mm, θin=55 degrees; A=9 mm, θin=60 degrees; and A=9 mm, θin=65 degrees. 
       FIG. 15  shows a relationship of d 2 /d 1  and the angle θs or θe when the number of faces of the rotatable multi-faceted mirror is seven (7) faces, using cases of A=9 mm, θin=50 degrees; A=10 mm, θin=50 degrees; and A=11 mm, θin=50 degrees. 
       FIG. 16  shows a relationship of d 2 /d 1  and the angle θs or θe when the number of faces of the rotatable multi-faceted mirror is eight (8) faces, using cases of A=11 mm, θin=degrees; A=12 mm, θin=45 degrees; and A=13 mm, θin=45 degrees. 
       FIG. 17  shows a relationship of d 2 /d 1  and the angle θs or θe when the number of faces of the rotatable multi-faceted mirror is nine (9) faces, using cases of A=13 mm, θin=degrees; A=14 mm, θin=45 degrees; and A=15 mm, θin=45 degrees. 
     As indicated in  FIGS. 14 to 17 , when a range of d 2 /d 1  is set from 1 to 1.2, and the angle θs or θe is set 30 degrees or more, for example, when d 1 =3.0 mm is used, the number of faces of the rotatable multi-faceted mirror is preferably eight (8) faces or less. If the number of faces of the rotatable multi-faceted mirror is nine (9) faces, the length A is required to be 15 mm or more, by which the size of the rotatable multi-faceted mirror increases, which is not preferable. 
       FIGS. 18 to 21  show relationships of d 2 /d 1  and the angle θs or θe when d 1 =3.5 mm. 
       FIG. 18  shows a relationship of d 2 /d 1  and the angle θs or θe when the number of faces of the rotatable multi-faceted mirror is six (6) faces, using cases of A=7 mm, θin=55 degrees; A=8 mm, θin=55 degrees; A=9 mm, θin=55 degrees; A=9 mm, θin=60 degrees; and A=9 mm, θin=65 degrees. 
       FIG. 19  shows a relationship of d 2 /d 1  and the angle θs or θe when the number of faces of the rotatable multi-faceted mirror is seven (7) faces, using cases of A=9 mm, θin=50 degrees; A=10 mm, θin=50 degrees; and A=11 mm, θin=50 degrees. 
       FIG. 20  shows a relationship of d 2 /d 1  and the angle θs or θe when the number of faces of the rotatable multi-faceted mirror is eight (8) faces, using cases of A=11 mm, θin=degrees; A=12 mm, θin=45 degrees; and A=13 mm, θin=45 degrees. 
       FIG. 21  shows a relationship of d 2 /d 1  and the angle θs or θe when the number of faces of the rotatable multi-faceted mirror is nine (9) faces, using cases of A=13 mm, θin=degrees; A=14 mm, θin=45 degrees; and A=15 mm, θin=45 degrees. 
     As indicated in  FIGS. 18 to 21 , when a range of d 2 /d 1  is from 1 to 1.2, and the angle θs or θe is 30 degrees or more, for example, when d 1 =3.5 mm is used, the number of faces of the rotatable multi-faceted mirror is preferably eight (8) faces or less. When the number of faces of the rotatable multi-faceted mirror is eight (8) faces, the length A is 11 to 13 mm smaller than the length A of 17 to 18 mm of conventional rotatable multi-faceted mirrors. If the number of faces of the rotatable multi-faceted mirror is nine (9) faces, the length A is required to be 15 mm or more, by which the size of the rotatable multi-faceted mirror increases, which is not preferable. 
       FIGS. 22 to 25  show relationships of d 2 /d 1  and the angle θs or θe when d 1 =4.0 mm. 
       FIG. 22  shows a relationship of d 2 /d 1  and the angle θs or θe when the number of faces of the rotatable multi-faceted mirror is six (6) faces, using cases of A=7 mm, θin=55 degrees; A=8 mm, θin=55 degrees; A=9 mm, θin=55 degrees; A=9 mm, θin=60 degrees; and A=9 mm, θin=65 degrees. 
       FIG. 23  shows a relationship of d 2 /d 1  and the angle θs or θe when the number of faces of the rotatable multi-faceted mirror is seven (7) faces, using cases of A=9 mm, θin=50 degrees; A=10 mm, θin=50 degrees; and A=11 mm, θin=50 degrees. 
       FIG. 24  shows a relationship of d 2 /d 1  and the angle θs or θe when the number of faces of the rotatable multi-faceted mirror is eight (8) faces, using cases of A=11 mm, θin=degrees; A=12 mm, θin=45 degrees; and A=13 mm, θin=45 degrees. 
       FIG. 25  shows a relationship of d 2 /d 1  and the angle θs or θe when the number of faces of the rotatable multi-faceted mirror is nine (9) faces, using cases of A=13 mm, θin=degrees; A=14 mm, θin=45 degrees; and A=15 mm, θin=45 degrees. 
     As indicated in  FIGS. 22 to 25 , when a range of d 2 /d 1  is set from 1 to 1.2, and the angle θs or θe is 30 degrees or more, for example, when d 1 =4.0 mm is used, the number of faces of the rotatable multi-faceted mirror is preferably eight (8) faces or less. When the number of faces of the rotatable multi-faceted mirror is seven (7) faces, the length A is 9 to 11 mm, by which the rotatable multi-faceted mirror is relatively small. When the number of faces of the rotatable multi-faceted mirror is eight (8) faces, the length A is 13 to 14 mm, by which the rotatable multi-faceted mirror can be smaller than conventional rotatable multi-faceted mirrors having the length A of 17 to 18 mm. If the number of faces of the rotatable multi-faceted mirror is nine (9) faces, the length A is required to be 15 mm or more, by which the size of the rotatable multi-faceted mirror increases, which is not preferable. 
     When manufacturing the optical scanning unit  2010 , the optical scanning unit  2010  can be provided with a beam diameter detector disposed near a position optically equal to the face of the photoconductive drum such as both ends of the scanning area to detect a beam diameter of incident light flux. At both ends of the scanning area, the incident position of incident light flux to the rotatable multi-faceted mirror is adjusted to set the equal diameter for beam spots of the incident light flux at both ends of the scanning area. 
     Further, when manufacturing the optical scanning unit  2010 , the optical scanning unit  2010  can be provided with a light intensity detector such as light power meter disposed near a position optically equal to the surface of the photoconductive drum such as both ends of the scanning area to detect light intensity of incident light flux. At both ends of the scanning area, the incident position of incident light flux to the rotatable multi-faceted mirror is adjusted to set the equal light intensity for the incident light flux at both ends of the scanning area. 
     As described above, the optical scanning unit  2010  includes, for example, the light sources  2200 A,  2200 B, the pre-deflector optical system, the optical deflector  2104 , and the scanning optical system. 
     The pre-deflector optical system includes, for example, the four aperture plates  2202   a ,  2202   b ,  2202   c ,  2202   d  having the corresponding aperture to shape the light flux emitted from the four light sources. Each of the aperture plates has the aperture having a given shape. Specifically, the opening width of the aperture in the sub-scanning corresponding direction at the two end portions in the main scanning corresponding direction is small compared to the center portion in the main scanning corresponding direction. 
     Further, it is designed, when the light flux reflected by the rotatable multi-faceted mirror is directed to the center portion of the scanning area on the corresponding photoconductive drum, the entire light flux entering the rotatable multi-faceted mirror is reflected on one reflection face, and it is designed, when the light flux reflected by the rotatable multi-faceted mirror is directed to the end portion of the scanning area on the corresponding photoconductive drum, a part of the light flux entering the rotatable multi-faceted mirror is reflected on one reflection face while other remaining part of the light flux is reflected on other reflection face 
     With this configuration, the rotatable multi-faceted mirror can be compact in size, and the scan angle can be set greater, and thereby the scanned surface can be optically scanned with a high speed and high precision without increasing of size and cost. 
     Therefore, the image forming apparatus  2000  including the optical scanning unit  2010  can form high quality image with a high speed without the increase of size and cost. 
     In the above described example embodiment, cases having din=3.8 mm are disclosed but not limited hereto. For example,  FIGS. 26 to 28  show cases using din=3.5 mm. 
       FIG. 26  shows the incident light flux and the reflection light flux with respect to the rotatable multi-faceted mirror when the light flux deflected at the optical deflector  2104  is directed to the scanning start position for the scanning area of the photoconductive drum, in which ds=3.2 mm and a relationship of “ds&lt;din” is used. 
     Further, the progression direction of light flux reflected at the face  1  of the rotatable multi-faceted mirror and the reference axis form an angle θs of, for example, 40.0 degrees. Further, the inclined angle θ 1  of the face  1  with respect to the reference axis is, for example, 42.5 degrees. 
       FIG. 27  shows the incident light flux and the reflection light flux with respect to the rotatable multi-faceted mirror when the light flux deflected at the optical deflector  2104  is directed to the center position of the scanning area of the photoconductive drum, in which dc=3.5 mm and a relationship of “dc=din” is used. Further, the inclined angle θ 1  of the face  1  with respect to the reference axis is, for example, 62.5 degrees. 
       FIG. 28  shows the incident light flux and the reflection light flux with respect to the rotatable multi-faceted mirror when the light flux deflected at the optical deflector  2104  is directed to the scanning end position for the scanning area of the photoconductive drum, in which de=3.5 mm and a relationship of “de=din” is used. 
     Further, the progression direction light flux reflected at the face  1  of the rotatable multi-faceted mirror and the reference axis form an angle θe of, for example, 40.0 degrees. Further, the inclined angle θ 1  of the face  1  with respect to the reference axis is, for example, 82.5 degrees. 
     In this case, at the timing directed to the scanning start position for the scanning area of the photoconductive drum, the incident light flux is cut at the optical deflector  2104 , but at the timing directed to the scanning end position for the scanning area of the photoconductive drum, the incident light flux is not cut at the optical deflector  2104 . Further, in this case, the scan angle is, for example, 80.0 degrees. 
     Further, in the above described example embodiment, the diameter of the inscribed circle of the rotatable multi-faceted mirror is set 18 mm but not limited such value. Depending on a required value for d 2 /d 1  and the angle θs or θe, the diameter of the inscribed circle of the rotatable multi-faceted mirror can be variably set. 
     Further, in the above described example embodiment, the rotatable multi-faceted mirror has six mirror faces but not limited hereto. For example, the rotatable multi-faceted mirror can have seven mirror faces as shown in  FIG. 29 . 
     When  FIG. 29  is viewed from the Z-axis direction, it is designed that the progression direction of the light flux emitted from the light source and entering the deflective reflection face and the reference axis form an angle θin such as 60.0 degrees. 
     A diameter of an inscribed circle of the rotatable multi-faceted mirror ( FIG. 30 ) may be, for example, 26 mm. Further, if the seven deflective reflection faces are required to be distinguished, the seven deflective reflection faces may referred to the face  1 , the face  2 , the face  3 , the face  4 , the face  5 , the face  6 , and the face  7  in the counter clockwise direction. 
     Further,  FIGS. 31 to 33  show example configurations when din=3.5 mm.  FIG. 31  shows the incident light flux and the reflection light flux with respect to the rotatable multi-faceted mirror when the light flux deflected at the optical deflector  2104  is directed to the scanning start position for the scanning area of the photoconductive drum, in which ds=3.5 mm and a relationship of “ds=din” is used. 
     Further, the progression direction of light flux reflected at the face  1  of the rotatable multi-faceted mirror and the reference axis form an angle θs of, for example, 36.0 degrees. Further, the inclined angle θ 1  of the face  1  with respect to the reference axis is, for example, 42.0 degrees. 
       FIG. 32  shows the incident light flux and the reflection light flux with respect to the rotatable multi-faceted mirror when the light flux deflected at the optical deflector  2104  is directed to the center position of the scanning area of the photoconductive drum, in which dc=3.5 mm and a relationship of “dc=din” is used. Further, the inclined angle θ 1  of the face  1  with respect to the reference axis is, for example, 60.0 degrees. 
       FIG. 33  shows the incident light flux and the reflection light flux with respect to the rotatable multi-faceted mirror when the light flux deflected at the optical deflector  2104  is directed to the scanning end position for the scanning area of the photoconductive drum, in which de=3.35 mm and a relationship of “de&lt;din” is used. 
     Further, the progression direction of light flux reflected at the face  1  of the rotatable multi-faceted mirror and the reference axis form an angle θe of, for example, 36.0 degrees. Further, the inclined angle θ 1  of the face  1  with respect to the reference axis is, for example, 78.0 degrees. 
     In this case, at the timing directed to the scanning end position for the scanning area of the photoconductive drum, the incident light flux is cut at the optical deflector  2104 , but at the timing directed to the scanning start position for the scanning area of the photoconductive drum, the incident light flux is not cut at the optical deflector  2104 . Further, in this case, the scan angle is, for example, 72.0 degrees. 
     When the incident light flux is cut at the optical deflector  2104  at the timing directed to the scanning start position for the scanning area of the photoconductive drum or at the timing directed to the scanning end position for the scanning area of the photoconductive drum, the aperture of the aperture plate can be formed into a shape shown in  FIGS. 34A to 34D , in which the length (opening width) of one end of the aperture in the sub-scanning corresponding direction is smaller compared to the center portion of the aperture in the sub-scanning corresponding direction, wherein the aperture extends in the main scanning corresponding direction. The smaller length (opening width) of one end of the aperture is corresponded to the side that the incident light flux is cut in the main scanning corresponding direction. 
     A position in the main scanning area in the main scanning direction on the scanned surface is referred to as an image height. Hereinafter, the center position in the main scanning area with respect to the main scanning direction is referred to as a center image height, and each of the both end portions in the main scanning area with respect to the main scanning direction is referred to as a peripheral image height. Typically, the image height is expressed by coordinates setting the coordinates of the center image height as zero. For example, when an entire length of the main scanning area with respect to the main scanning direction is 300 mm, the center image height is set zero (0) mm, the peripheral image height at one side is set +150 mm, and the peripheral image height at other side is set −150 mm. Further, an area in the main scanning area that an image is formed is referred to as an image area. Further, hereinafter, for the simplicity of expression, as for the light flux deflected by the optical deflector, when a part of the incidence light flux is cut at the optical deflector, such light flux is referred to as a light flux cut at the optical deflector. 
     In the above described example embodiment, the light flux directed to the end portion of the main scanning area on the photoconductor drum is the light flux cut at the optical deflector  2104 . Therefore, for example, if a synchronization detection sensor  2115  is disposed at a position shown in  FIG. 35 , the light flux deflected by the optical deflector  2104  enters the synchronization detection sensor  2115  before the writing starts, and the light flux cut at the optical deflector  2104  can be received by the synchronization detection sensor  2115 . Hereinafter, the light flux received by the synchronization detection sensor  2115  is referred to as a synchronization detection light flux. In this case, an increase of beam spot diameter and a decrease of light intensity may occur to the synchronization detection light flux. 
     When the light flux is projected onto a plane perpendicular to a rotation axis of the rotatable multi-faceted mirror, an angle defined by the light flux directed to the synchronization detection sensor  2115  and the light flux entering or striking the optical deflector  2104  is smaller than an angle defined by the light flux directed to the main scanning area and the light flux entering or striking the optical deflector  2104 . Further, hereinafter, a position where the synchronization detection sensor  2115  is disposed is referred to as a synchronization detection position. 
     An effect of the increase of beam spot diameter to a finally-formed image is small because the changed amount of beam spot diameter is small such as a several μm or so. In contrast, the decrease of light intensity may cause a synchronization detection error due to lack of enough light intensity. 
     Further, the light flux directed to the end portion of the main scanning area on the photoconductor drum and used within the image area may be the light flux cut at the optical deflector  2104 . In this case, the density unevenness may occur at the end portion of image, thereby degrading the image quality. 
     Typically, the fluctuation of light intensity can be corrected by changing the output of the light source with respect to the main scanning direction. For example, theoretically, the emission light intensity of the light source is increased for the amount equal to the decrease of light intensity, by which the light intensity on the surface of the photoconductor drum can become even or uniform. Further, the decrease of light intensity of the synchronization detection light flux can be compensated by increasing the sensitivity of the synchronization detection sensor  2115 . 
     The fluctuation of light intensity on the surface of the photoconductor drum with respect to the main scanning direction does not change abruptly such as unpredictable change but may change gradually such as a mild curve line. Further, the fluctuation of light intensity caused by the cutting at the optical deflector can be the decrease of light intensity expressed by a substantially linear function. 
     The light intensity correction may be conducted in view of adjustment time. Specifically, the light intensity may not be corrected based on the measurement result of the light intensity using a small adjustment pitch such as 1 mm to 5 mm with respect to the main scanning direction, but the light intensity may be corrected based on the measurement result of the light intensity using an adjustment pitch of 5 mm (minimum pitch) or more, or rough adjustment pitch such as 30 mm to 50 mm with respect to the main scanning direction, by which an optical scanning apparatus that can reduce an adjustment time can be provided with a lower cost. 
       FIGS. 36(A) and 36(B)  show examples of the fluctuation of light intensity before the light intensity correction and the fluctuation of light intensity after the light intensity correction (post-correction difference). The vertical axis of  FIGS. 36(A) and 36(B)  represents the light intensity on the surface of the photoconductor drum, and the horizontal axis represents positions (i.e., image height) with respect to the main scanning direction.  FIG. 36(A)  shows a case that the fluctuation of light intensity before the light intensity correction is great, and  FIG. 36(B)  shows a case that that the fluctuation of light intensity before the light intensity correction is small. As indicated by  FIGS. 36(A) and 36(B) , if the fluctuation of light intensity before the light intensity correction is great and a rough adjustment pitch is applied, the post-correction difference may become great. 
     The post-correction difference shown in  FIGS. 36(A) and 36(B)  is the light intensity difference between two adjacent areas adjacent with each other via one adjustment position. If the difference of light intensity at the two adjacent areas is about 2% to 3%, the difference appears as a density change such as a thick-thin-portion-included image, and thereby the image quality deteriorates, which is confirmed by experiments. Therefore, to set the adjustment pitch as a rough pitch, it is preferable that the fluctuation of light intensity is small as much as possible. The post-correction difference can be reduced by making the adjustment pitch smaller, but the smaller adjustment pitch increases the adjustment time and cost as a whole. 
     For example, if the post-correction difference of  FIG. 36(B)  is used as the limit that no thick-thin-portion occurs on the image, the post-correction difference of  FIG. 36(A)  causes density change and thereby unnecessary thick-thin-portion-included image occurs. 
     An output range of the light source is not infinite but has a given specific value such as 1 mW to 10 mW. Because the output range of the light source is used to adjust the fluctuation of transmittance and reflectance of optical parts disposed in the optical scanning apparatus and to adjust conditions of photoconductor drum which may be changed due to aging, it is preferable that the decrease of light intensity caused by the cutting at the optical deflector is small as much as possible. 
     Therefore, in the above described example embodiment, the light source may be pivoted or about an exit direction of the light flux (i.e., about the axis of the light source) so that the light flux that enters or strikes the optical deflector becomes a light flux having P-polarized light component stronger than the S-polarized light component with respect to the deflective reflection face of the optical deflector. 
     Typically, the light source includes a single laser diode (LD) having one light emitter, and the light source is disposed with a given arrangement so that the light flux exiting from the light source becomes the S-polarized light component with respect to the deflective reflection face of the optical deflector. Further, the angle of divergence is wide in the main scanning corresponding direction and narrow in the sub-scanning corresponding direction. In this case, the light intensity of the light flux, reflected at the deflective reflection face of the optical deflector and irradiated onto the photoconductor drum via a scan lens and a reflection mirror, becomes smaller at the peripheral image height compared to the center image height with respect to the main scanning direction. 
     In contrast, if the single LD is pivoted about the exit direction of the light flux (i.e., axis) so that the light flux having P-polarized light component stronger than S-polarized light component with respect to the deflective reflection face of the optical deflector enters or strikes the optical deflector, the light intensity of the light flux irradiated to the photoconductor drum becomes greater at the peripheral image height compared to the center image height with respect to the main scanning direction. In this case, even if the light intensity at the peripheral image height decreases due the cutting at the optical deflector, the decrease of the light intensity at the peripheral image height can be reduced. 
     A description is given with reference to  FIGS. 37, 38 and 39 .  FIG. 37  shows the light intensity ratio at each image height against the image height of 0 mm when the light flux having only S-polarized light component enters the deflective reflection face of the optical deflector, and when the light flux having P-polarized light component stronger than S-polarized light component enters the deflective reflection face of the optical deflector. When the light flux having only S-polarized light component enters the deflective reflection face of the optical deflector, the light intensity is greater at the center image height, and when the light flux having P-polarized light component stronger than S-polarized light component enters the deflective reflection face of the optical deflector, the light intensity is greater at the peripheral image height. 
     Because the decrease of light intensity due to the cutting at the optical deflector is the same for both light fluxes, as schematically shown in  FIGS. 38 and 39 , when the light flux irradiates the scanned face of the photoconductor drum, the decrease of light intensity becomes smaller when the light flux having P-polarized light component stronger than S-polarized light component enters the deflective reflection face of the optical deflector. 
       FIG. 40  shows an example of the light intensity profile when a scanning optical system including one single scan lens having an angle of θs or θe of about 40 degrees, a reflection mirror at the optical deflector side having a deflection angle of about 36 degrees, and a mirror at the scanned surface side having a deflection angle of about 104 degrees is used while setting the intensity ratio of S-polarized light component and P-polarized light component of the light flux exiting from the light source and entering the optical deflector as 27:73. 
     With this configuration, the post-correction difference can be reduced without setting a smaller adjustment pitch and without increasing the cost. 
     If the synchronization detection sensor  2115  is disposed at the position shown in  FIG. 35 , to which the light flux deflected by the optical deflector and before starting the writing enters. Further, if the light source is disposed in a given arrangement so that the light flux having only S-polarized light component enters the deflective reflection face of the optical deflector, the light intensity decreases at the peripheral image height. In this case, the light intensity received at the synchronization detection sensor  2115  may be decreased to about 50% of the light intensity at the center image height, wherein the light intensity decrease may be influenced by an optical configuration between the optical deflector  2104  and the synchronization detection sensor  2115  such as an arrangement of a synchronization lens and a reflection mirror. 
     If the decrease of light intensity due to the cutting at the optical deflector  2104  further occurs under this condition, the light intensity correction may not be conducted just by increasing the sensitivity of the synchronization detection sensor  2115 , but the emission light intensity of the light source may need to be increased. However, because the output range of the light source is not infinite but has a specific value, increasing the output range of the emission light intensity may not be preferable. 
     In this case, by pivoting the light source, the light flux having P-polarized light component stronger than S-polarized light component can enter the deflective reflection face of the optical deflector  2104 , by which the light intensity at the peripheral image height can be increased, and thereby the light intensity at the peripheral image height can be enhanced. 
     Further, in the above described example embodiment, the cutting amount of the light flux cut at the optical deflector and directed to one end portion of the main scanning area and the cutting amount of the light flux cut at the optical deflector and directed to other end portion of the main scanning area may be different values. 
     When the polarized-light direction of the light flux entering the optical deflector is angled from the S-polarized light component for the deflective reflection face of the optical deflector, depending on the deflection angle and coating condition of a reflection mirror that guides the light to the photoconductor drum, the reflectance near one end portion of the reflection mirror in the main scanning corresponding direction and the reflectance near other end portion of the reflection mirror in the main scanning corresponding direction become different, and thereby the light intensity on the face of the photoconductor drum also becomes different at the one end portion and the other end portion of the main scanning area, in which the deflection angle of the reflection mirror is determined based on a layout of the optical scanning apparatus, and the light intensity may differ at the one end portion and the other end portion in the main scanning area. 
     In the above described example embodiment, by preferably setting an incidence position and an incidence angle of the light flux, emitted from the light source, at the optical deflector, the cutting amount of the light flux, cut at the optical deflector and directed to the one end portion of the main scanning area, and the cutting amount of the light flux cut, at the optical deflector and directed to the other end portion of the main scanning area, can be different. 
     Specifically, if the incidence position of the light flux to the optical deflector is shifted with respect to the main scanning corresponding direction so that the light flux directed to the one end portion of the main scanning area is cut greater at the optical deflector, the light flux directed to the other end portion the of the main scanning area is cut smaller at the optical deflector. With this configuration, the light intensity at the one end portion and the other end portion of the main scanning area can become different. 
     With this configuration, the light intensity difference at the one end portion and the other end portion with respect to the main scanning corresponding direction, which occurs in the scanning optical system, can be canceled, and the light intensity can be set substantially evenly with respect to the main scanning corresponding direction. With this configuration, the fluctuation of light intensity profile for the entire area with respect to the main scanning corresponding direction can be set smaller, and the post-correction difference can be reduced. 
     As above described, the scanning optical system is disposed between the rotatable multi-faceted mirror and the scanned surface, in which the light flux reflected at the rotatable multi-faceted mirror is directed to the main scanning area on the scanned surface. When the light flux reflected at the rotatable multi-faceted mirror is directed to one end and the other end of the main scanning area, a part of one light flux entering the rotatable multi-faceted mirror may be reflected at one reflection face (first reflection face), and the remaining part of the one light flux may be reflected at another reflection face (second reflection face), in which the decrease of light intensity at one end of the main scanning area may become great due to the reflection at the another reflection face (second reflection face) compared to the decrease of light intensity at the other end of the main scanning area. In an example embodiment, in the light intensity profile in the main scanning direction by the scanning optical system when the light flux entering the rotatable multi-faceted mirror is assumed to entirely reflect on the one reflection face (first reflection face), the light intensity by the scanning optical system is greater at least one end in the main scanning direction as shown in  FIG. 39 . In this configuration, the above mentioned one end of the main scanning area having the greater decrease of light intensity is corresponded to the one end in the main scanning direction having greater light intensity. 
     For example, when the optical system sets the light intensity profile having greater intensity at the end portion as shown in the top view of  FIG. 39 , and when this light intensity profile by the optical system is superimposed with a light intensity profile by cutting shown in the middle view of  FIG. 39 , the light intensity profile on the scanned surface becomes substantially M-shape as shown in the bottom view of  FIG. 39 . Therefore, the fluctuation of light intensity ratio on the scanned surface can be reduced compared to a comparison example shown in  FIG. 38 . In this example case shown in  FIG. 39 , the horizontal axis representing the main scanning direction corresponds to the image height on the scanned surface, and the decrease of light intensity by cutting occurs at both ends of the image height but not limited hereto. For example, the above described configuration of example embodiment can be applied when the decrease of light intensity by cutting occurs at least one end of the image height on the scanned surface. Although the light intensity profile by the optical system schematically shown in  FIG. 39  is symmetrical with respect to the center image height, an actual light intensity profile may become unsymmetrical with respect to the center image height as shown in  FIG. 40 . 
     Further, the synchronization detection sensor  2115  may be disposed at the position shown in  FIG. 35 , to which the light flux deflected by the optical deflector and before starting the writing enters. In this case, how to secure the light intensity of the synchronization detection light flux becomes an issue. Because the synchronization detection sensor  2115  is disposed at a position that the light flux before starting the writing enters, the decrease of light intensity at the synchronization detection sensor  2115  becomes greater than an area extending from a writing start position to a writing end position. 
     The decrease of light intensity near the end portion of the main scanning area on the photoconductor drum can be reduced by entering the light flux having P-polarized light component stronger than S-polarized light component with respect to the deflective reflection face of the optical deflector (see  FIG. 39 ). In this case, the decrease of light intensity of the synchronization detection light flux caused by the cutting at the optical deflector can be smaller. Therefore, without increasing the emission light intensity of the light source but by adjusting the sensitivity of the synchronization detection sensor  2115 , an effect of the decrease of light intensity of the synchronization detection light flux to synchronization detection precision can be reduced. 
     Compared to conventional optical scanning apparatuses using the OF type system, the above described optical scanning apparatus  2010  can set a shorter length for the optical path and thereby the optical scanning apparatus  2010  can be compact in size. The optical scanning apparatus can be compact in size by setting the length of optical path in the scanning optical system (i.e., distance from the deflective reflection face of the optical deflector to the scanned surface) shorter, in which a focal length of a scan lens is required to be set shorter. 
     In this case, because the field angle of the scan lens becomes greater, a position that the light flux emitted from the light source is started to be cut at the optical deflector is shifted toward a center portion of the image area with respect to the main scanning direction, and the cutting amount of the light flux, directed to the peripheral image height, at the optical deflector becomes greater. With this configuration, at the peripheral image height such as at the synchronization detection position, the decrease of light intensity due to the cutting at the optical deflector becomes greater. 
     If the light intensity for the synchronization detection light flux is not enough, the incidence position and incidence angle of the light flux emitted from the light source to the optical deflector are adjusted to change the cutting amount at the optical deflector, by which the decrease of light intensity for the synchronization detection light flux can be reduced. 
     Specifically, the light intensity at the write-start position and the light intensity at the write-end position is not set at the same level, but the incidence position and incidence angle of the light flux is adjusted so that a difference between the light intensity at the synchronization detection position and the light intensity at the write-end position becomes smaller. Because the light intensity decrease of the synchronization detection light flux can be compensated by increasing the sensitivity of the synchronization detection sensor  2115 , the light intensity at the synchronization detection position and the light intensity at the write-end position are not required to set at the same level as long as desired synchronization detection precision is obtained. In this case, the light intensity at the write-start position may be set greater than the light intensity at the write-end position. 
     The above description describes a case that the light flux cut at the optical deflector is directed to within the image area, but not limited this case. For example, the light flux directed to the synchronization detection position may be the only light flux that is cut at the optical deflector. In this case, the incidence position and the incidence angle of the light flux emitted from the light source to the optical deflector are set with preferable values to secure the light intensity of the synchronization detection light flux, in which both of the light flux directed to the write-start position and the light flux directed to the write-end position may not be the light flux cut at the optical deflector. In this case, even if the synchronization detection sensor  2115  is disposed, the light intensity at the write-start position is not required to be set greater than the light intensity at the write-end position. 
     Further, as the cutting amount at the optical deflector becomes greater as the field angle of the scan lens becomes greater, the post-correction difference becomes greater unless a smaller adjustment pitch is used for the light intensity correction. To set a rough adjustment pitch, smaller fluctuation of light intensity is preferable. The rough adjustment pitch can be set using a rectangle shaped aperture for the aperture plate disposed between the light source and the optical deflector. 
     If the rectangle aperture is used and the same beam spot diameter is to be set on the surface of the photoconductor drum, the aperture length with respect to the main scanning corresponding direction becomes the shortest compared other shapes. Therefore, if the rectangle aperture is used to set the same beam spot diameter, the cutting at the optical deflector is started for the light directed to near the peripheral image height, by which the light intensity fluctuation in the image area can be reduced (see  FIG. 41 ). 
     Further, if a size of aperture becomes smaller in the sub-scanning corresponding direction at near the both end portions with respect to the main scanning corresponding direction, it is preferable that the light passing near the both end portions of the aperture are cut at the optical deflector. 
     The aperture plates shown in  FIGS. 12A to 12D  can be preferably used. By using the shapes of aperture shown in  FIGS. 12A to 12D , the decrease of light intensity that occurs when the end portion of the light flux with respect to the main scanning corresponding direction is cut at the optical deflector can be reduced, in particular minimized. Further, compared to the rectangle aperture, the cut area can be set smaller, and thereby the fluctuation of light intensity can be reduced. Further, compared to the rectangle aperture, the increase of beam spot diameter can be reduced. In this case, the aperture size with respect to the main scanning corresponding direction becomes greater, and a cut-starting position shifts toward the center image height side (i.e., an area in the main scanning area that is not cut becomes small), but the effect of the cutting can be reduced. 
     For example, as shown in  FIG. 41 , if the shape of aperture is ellipse (see  FIG. 12B ), compared to the shape of aperture is rectangle  1 , the image height that is started to be cut (i.e., start of decrease of light intensity) shifts toward the center image height simply because the aperture size with respect to the main scanning corresponding direction is greater. Further, compared to the shape of aperture is rectangle  2 , the cutting starts at the same image height but the decrease of the light intensity is small if the shape of aperture is ellipse. 
     The area that can be cut at the optical deflector is determined based on physical conditions, thereby the decrease of light intensity and the increase of beam spot diameter are required to be reduced within the physical conditions. By setting a cut area of the light flux cut at the optical deflector within the light flux area corresponding to a width-reduced portion in the sub-scanning corresponding direction of the aperture plate, the decrease of light intensity and the increase of beam spot diameter can be reduced. 
     As above described, by selectively using positions that are started to be cut, the decrease of light intensity and the increase of beam spot diameter can be reduced in view of the field angle of the scanning optical system. Specifically, the cutting-start position closer to the center portion of the main scanning area, and the cutting-start position only at the peripheral (e.g., only synchronization detection position) can be selectively used. By selectively using these cutting-start positions, the decrease of light intensity and the increase of beam spot diameter can be reduced. 
     Further, in the above described example embodiment, the light source can employ a monolithic edge emitting laser array, a surface emitting laser array, or the like. 
     Further, in the above described example embodiment, each of the two light sources includes two light emitting elements but not limited such configuration. For example, four light sources can be used while each light source has a single light emitting element. Further, two light sources can be used while each light source has a single light emitting element, in which light flux emitted from each light source is split into two light fluxes. 
     Further, in the above described example embodiment, the image forming apparatus is described as a multi-functional apparatus but not limited hereto. For example, the image forming apparatus may be a single function apparatus such as a copier, a printer, or a facsimile machine. 
     Further, the image forming apparatus can use a configuration that directly irradiates laser beams to a medium (e.g., sheet) that can generate colors by the irradiation of laser beams. 
     Further, the image forming apparatus can use a configuration that silver halide film is used as the image carrying member, in which a latent image is formed on the silver halide film by the optical scanning process, and the latent image can be developed using the same development process of silver halide process, and the developed image can be transferred to a photographic paper using the same baking finish of silver halide process. Further, the image forming apparatus can be used as a photoengraving apparatus, and a light beam lithography apparatus such as an apparatus that generates a computed tomography (CT) scan image or the like. 
     Further, in the above described example embodiment of the optical scanning unit, the light flux striking the rotatable multi-faceted mirror has P-polarized light component stronger than the S-polarized light component with respect to the deflective reflection face of the optical deflector. 
     Further, in the above described example embodiment of the optical scanning unit, when the light flux striking the rotatable multi-faceted mirror is entirely reflected on one reflection face, a light intensity profile in the main scanning direction, settable by a scanning optical system composed from the rotatable multi-faceted mirror until the scanned surface, becomes a light intensity profile that light intensity at an image height at least at one end in the main scanning direction is greater than light intensity at other image heights at other portions in the main scanning direction. The image height corresponds to a position in the main scanning direction. The image height at the one end in the main scanning direction having the greater light intensity is matched to an image height at one end in the main scanning direction having a greater decrease of light intensity due to the reflection of the remaining of the light flux striking the rotatable multi-face mirror at the second reflection face. 
     Further, in the above described example embodiment of the optical scanning unit, a synchronization detection sensor can be disposed to which the light flux reflected at the rotatable multi-faceted mirror and before starting writing enters, and the light intensity at the write-start position is greater than the light intensity at the write-end position. 
     Further, in the above described example embodiment of the optical scanning unit, a plate can be disposed in an optical path between the light source and the rotatable multi-face mirror. The plate has an aperture to shape light flux emitted from the light source, and the aperture is rectangle. 
     As described above, the optical scanning unit according to the example embodiment can optically scan a scanned surface with high speed and high precision without increasing size and cost. 
     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.