Patent Publication Number: US-10325188-B2

Title: Light scanning device and image forming apparatus with the same

Description:
TECHNICAL FIELD 
     The present invention relates to a light scanning device that scans a scan object with light beam and an image forming apparatus with the light scanning device. 
     BACKGROUND ART 
     For example, a color image forming apparatus using an electrophotographic image forming method uniformly charges the surfaces of respective photosensitive bodies (respective scan objects) corresponding to a plurality of colors and then scans the respective photosensitive body surfaces with respective light beams so as to form respective electrostatic latent images on the respective photosensitive body surfaces. The color image forming apparatus develops the electrostatic latent images on the respective photosensitive body surfaces using respective colors of toner to form toner images in the respective colors on the respective photosensitive body surfaces. The color image forming apparatus superimposes and transfers the toner images in the respective colors from the respective photosensitive bodies to an intermediate transfer body so as to form a color toner image on the intermediate transfer body, and then transfers this color toner image from the intermediate transfer body to a recording paper sheet. 
     The respective photosensitive bodies are scanned with the respective light beams by a light scanning device. Typically, four colors, which are black, cyan, magenta, and yellow, of toner are used. Accordingly, it is necessary to scan four photosensitive bodies using at least four light beams, and four light-emitting elements for emitting the four light beams need to be used. 
     Nowadays, there is a need for downsizing and thinning of the image forming apparatus, and a downsized and thinned light scanning device becomes necessary. Accordingly, there is proposed a light scanning device with the following configuration. A polygonal mirror (deflecting section) is arranged approximately in the center of the light scanning device. Two optical systems are arranged symmetrical with respect to the polygonal mirror at the center. Respective light beams emitted from the respective light-emitting elements are reflected by the polygonal mirror so as to be divided into the respective optical systems. The respective optical systems cause the respective light beams to enter the respective photosensitive bodies. 
     On the other hand, a BD sensor is disposed to detect a light beam deflected by the polygonal mirror, and the scanning timing on the photosensitive body using the light beam is set based on the detection timing of the light beam using the BD sensor. In short, the scanning timing on the photosensitive body using the light beam is synchronized with the detection timing of the light beam using the BD sensor. 
     Here, the light beam is reflected by the polygonal mirror to be repeatedly deflected in the range having an approximately fan shape. This range having an approximately fan shape includes the scanning angle range of the light beam that scans a scan object. The BD sensor is often disposed outside the scanning angle range of the light beam. For example, in Patent Literatures 1 and 2, a BD sensor is arranged outside the scanning angle range of the light beam, and the light beam deflected by a polygonal mirror enters the BD sensor. 
     In Patent Literatures 3 and 4, a detecting mirror and a BD sensor are arranged outside the scanning angle range of the light beam, and the light beam deflected by a polygonal mirror is reflected by the detecting mirror such that the light beam enters the BD sensor. 
     CITATION LIST 
     Patent Literature 
     PATENT LITERATURE 1: Japanese Unexamined Patent Application Publication No. 2004-333556 
     PATENT LITERATURE 2: Japanese Unexamined Patent Application Publication No. 2011-242601 
     PATENT LITERATURE 3: Japanese Unexamined Patent Application Publication No. 2011-95559 
     PATENT LITERATURE 4: Japanese Unexamined Patent Application Publication No. 06-59205 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, since the BD sensor is mounted on a substrate, the substrate of the BD sensor provides a negative effect on downsizing and thinning of the light scanning device depending on the arranged position of the BD sensor. For example, in Patent Literatures 1 to 4, since the BD sensor is out of the scanning angle range of the light beam, it is necessary to increase the width and the depth of the light scanning device to cover the scanning angle range of the light beam and the substrate of the BD sensor. In the case where the substrate of the BD sensor is arranged within the scanning angle range of the light beam, the substrate of the BD sensor interferes with the light beam. 
     The present invention has been made to solve the above-described conventional problems, and it is an object of the present invention to provide a light scanning device that ensures further downsizing and an image forming apparatus with that light scanning device. 
     Solutions to the Problems 
     To solve the above-described problems, a light scanning device according to the present invention includes a light-emitting element, a deflecting section, at least one reflective mirror, and an optical sensor. The deflecting section is configured to deflect a light beam emitted from the light-emitting element. The reflective mirror is configured to reflect the light beam and cause the light beam to enter a scan object. The light beam is emitted from the light-emitting element and deflected by the deflecting section. The optical sensor is configured to detect the light beam deflected by the deflecting section. The light scanning device is configured to scan the scan object with the light beam and set scanning timing of the scan object using the light beam based on detection timing of the light beam using the optical sensor. The optical sensor is arranged in a position farther from the deflecting section than a last reflective mirror that reflects the light beam immediately before entering the scan object and arranged inside a scanning angle range of the light beam corresponding to an effective scan area of the scan object. 
     In this light scanning device according to the present invention, the optical sensor is arranged inside the scanning angle range of the light beam corresponding to the effective scan area of the scan object. It is only necessary to set the size of the light scanning device to include the scanning angle range of the light beam. Accordingly, the light scanning device can be downsized. Additionally, the optical sensor is arranged in the position farther from the deflecting section than the last reflective mirror, which reflects the light beam immediately before entering the scan object. Accordingly, the optical sensor and the substrate of the optical sensor do not interfere with the light beam. 
     In the light scanning device according to the present invention, the last reflective mirror may be arranged at one end inside a housing of the light scanning device. 
     In this case, the size of the housing of the light scanning device can be set according to the position of the last reflective mirror so as to set the minimum size of the housing. 
     Further, in the light scanning device according to the present invention, the following configuration is possible. The light scanning device further includes a plurality of the light-emitting elements. The optical sensor detects a light beam of any of the plurality of the light-emitting elements. The last reflective mirror reflects the light beam detected by the optical sensor to the scan object. 
     In the case where a color image is formed, respective light-emitting elements corresponding to a plurality of colors are disposed. The optical sensor detects a light beam of any of the respective light-emitting elements to synchronize the scanning timing of the photosensitive body using the light beam with the detection timing of the light beam using the optical sensor. 
     In the light scanning device according to the present invention, the following configuration is possible. The light scanning device further includes a detecting mirror configured to reflect a light beam deflected by the deflecting section and cause the light beam to enter the optical sensor. The optical sensor is arranged in a position where the light beam enters after the light beam is reflected by the detecting mirror and passes above an upper end or below a lower end of the last reflective mirror. 
     By disposing this detecting mirror, it is possible to arrange the optical sensor inside the scanning angle range of the light beam corresponding to the effective scan area of the scan object. 
     Further, in the light scanning device according to the present invention, the following configuration is possible. The detecting mirror is arranged outside the scanning angle range of the light beam corresponding to the effective scan area of the scan object. The optical sensor and the detecting mirror are arranged in a vicinity of a boundary between an inside and an outside of the scanning angle range of the light beam. 
     As just described, the optical sensor and the detecting mirror are disposed in the vicinity of the boundary between the inside and the outside of the scanning angle range. Accordingly, the light beam reflected by the detecting mirror approximately vertically enters the light receiving surface of the optical sensor such that the light receiving amount of the optical sensor increases. This improves the detection accuracy using the optical sensor. 
     In the light scanning device according to the present invention, the following configuration is possible. The detecting mirror is arranged outside a bottom section of a housing of the light scanning device. A light beam entering and reflected to the detecting mirror passes through a hole formed in the bottom section. 
     This facilitates mounting of the detecting mirror. 
     Further, in the light scanning device according to the present invention, the following configuration is possible. The light scanning device further includes an fθ lens disposed in an optical path of light beam from the deflecting section to the last reflective mirror. The fθ lens includes an optical section configured to transmit the light beam immediately before entering the detecting mirror. The optical section has focusing property. 
     This optical section allows adjustment of the distance from the deflecting section to the detecting mirror and the distance from the detecting mirror to the optical sensor so as to downsize the light scanning device. 
     In the light scanning device according to the present invention, the following configuration is possible. The optical sensor is mounted on a substrate disposed outside a sidewall of a housing of the light scanning device. 
     In this case, when the height of the substrate is set according to the sidewall of the housing of the light scanning device, the substrate does not cause an increase in height of the light scanning device. The substrate is overlapped with the sidewall of the housing of the light scanning device. This downsizes the light scanning device. 
     On the other hand, an image forming apparatus according to the present invention includes the above-described light scanning devices according to the present invention. The image forming apparatus forms a latent image on a scan object by the light scanning device, develops the latent image on the scan object as a visible image, and transfers and forms the visible image from the scan object to a paper. 
     This image forming apparatus also provides operations and effects similar to those of the above-described light scanning devices according to the present invention. 
     Advantageous Effects of Invention 
     According to the present invention, the optical sensor is arranged inside the scanning angle range of the light beam corresponding to the effective scan area of the scan object. It is only necessary to set the size of the light scanning device to include the scanning angle range of the light beam. Accordingly, the light scanning device can be downsized. Additionally, the optical sensor is arranged in the position farther from the deflecting section than the last reflective mirror, which reflects the light beam immediately before entering the scan object. Accordingly, the optical sensor and the substrate of the optical sensor do not interfere with the light beam. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view illustrating an image forming apparatus with one embodiment of a light scanning device according to the present invention. 
         FIG. 2  is a perspective view illustrating an inside of a housing of the light scanning device viewed from obliquely upward and illustrating a state with an upper lid removed. 
         FIG. 3  is a perspective view illustrating a plurality of extracted optical members of the light scanning device and illustrating a state viewed from a back surface side of  FIG. 2 . 
         FIG. 4  is a plan view illustrating the plurality of extracted optical members of the light scanning device. 
         FIG. 5  is a side view illustrating the plurality of extracted optical members of the light scanning device. 
         FIG. 6  is a plan view illustrating the arranged positions of the respective optical members in the housing of the light scanning device. 
         FIG. 7  is a side view illustrating the arranged positions of the respective optical members in the housing of the light scanning device. 
         FIG. 8  is an enlarged perspective view illustrating BD mirrors and BD sensors viewed from the inside of the housing in the obliquely upward direction. 
         FIG. 9  is an enlarged perspective view illustrating the BD mirrors and the BD sensors viewed from the outside of the housing in the obliquely upward direction. 
         FIG. 10  is an enlarged perspective view illustrating the mounting structure for the BD mirrors on the bottom surface in a bottom plate of the housing. 
         FIG. 11  is a perspective view illustrating the mounting structure for the BD mirrors viewed from a direction different from  FIG. 10 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described based on the accompanying drawings. 
       FIG. 1  is a cross-sectional view illustrating an image forming apparatus with one embodiment of a light scanning device according to the present invention. The image data handled by this image forming apparatus  1  corresponds to a color image using respective colors of black (K), cyan (C), magenta (M), and yellow (Y); or corresponds to a monochrome image using a single color (for example, black). In view of this, a development apparatus  12 , a photosensitive drum  13 , a drum cleaning apparatus  14 , a charging unit  15 , and a similar apparatus are disposed for each of four to form four types of toner images according to the respective colors. Each apparatus corresponds to black, cyan, magenta, and yellow. Thus, four image stations Pa, Pb, Pc, and Pd are constituted. 
     The drum cleaning apparatuses  14  remove and recover residual toner at the surfaces of the photosensitive drums  13  of all of the respective image stations Pa, Pb, Pc, and Pd. Then, the charging unit  15  uniformly charges the surfaces of the photosensitive drums  13  at a predetermined electric potential. A light scanning device  11  exposes the surfaces of the photosensitive drums  13  to form electrostatic latent images at the surfaces. Then, the development apparatus  12  develops the electrostatic latent images on the surfaces of the photosensitive drums  13  and form toner images at the surfaces of the photosensitive drums  13 . Thus, a toner image with each color is formed at the surface of the photosensitive drum  13 . 
     Subsequently, while an intermediate transfer belt  21  is moved around the arrow direction C, a belt cleaning apparatus  22  removes and recovers residual toner at the intermediate transfer belt  21 . Then, toner image with each color at the surface of the photosensitive drum  13  is sequentially transferred and superimposed to the intermediate transfer belt  21 , thus a color toner image is formed on the intermediate transfer belt  21 . 
     A nip region is formed between the intermediate transfer belt  21  and a transfer roller  23   a  of a secondary transfer apparatus  23 . The recording paper sheet conveyed through an S-shaped paper sheet transport path R 1  is conveyed while being sandwiched by the nip region. The color toner image on the surface of the intermediate transfer belt  21  is transferred on the recording paper sheet. Then, the recording paper sheet is sandwiched between a heating roller  24  and a pressing roller  25  of a fixing apparatus  17 , and heated and pressurized for fixing the color toner image on the recording paper sheet. 
     On the other hand, a pickup roller  33  extracts the recording paper sheets from a sheet feed cassette  18 . The recording paper sheets are conveyed through the paper sheet transport path R 1 , pass through the secondary transfer apparatus  23  and the fixing apparatus  17 , and then are carried out to a discharge tray  39  via a discharge roller  36 . This paper sheet transport path R 1  includes a registration roller  34 , a conveyance roller  35 , or the discharge roller  36 , and a similar part. The registration roller  34  starts conveying the recording paper sheets matching transfer timing of the toner image at the nip region between the intermediate transfer belt  21  and the transfer roller  23   a  after the recording paper sheets are once stopped and the top of the recording paper sheets are aligned. The conveyance roller  35  promotes conveyance of the recording paper sheets. 
     Next, the constitution of the light scanning device  11  according to this embodiment will be described in detail using  FIG. 2  to  FIG. 5 .  FIG. 2  is a perspective view illustrating an inside of a housing  41  of the light scanning device  11  of  FIG. 1  viewed from obliquely upward and illustrating a state with an upper lid removed.  FIG. 3  is a perspective view illustrating a plurality of extracted optical members of the light scanning device  11  and illustrating a state viewed from a back surface side of  FIG. 2 . Further,  FIG. 4  and  FIG. 5  are a plan view and a side view illustrating the plurality of extracted optical members of the light scanning device  11 . 
     The housing  41  includes a rectangular bottom plate  41   a  and four side plates  41   b  and  41   c  that surround the bottom plate  41   a . A polygonal mirror  42 , which has a square shape in plan view, is disposed at approximately center of the bottom plate  41   a . A polygonal motor  43  is secured at approximately center of the bottom plate  41   a . The center of the polygonal mirror  42  is coupled to and secured to a rotation axis of the polygonal motor  43 , and the polygonal motor  43  rotates the polygonal mirror  42 . 
     A drive substrate  46  is secured to the outside of one side plate  41   b  of the housing  41 . The drive substrate  46  includes two first semiconductor lasers  44   a  and  44   b  and two second semiconductor lasers  45   a  and  45   b  (total of four semiconductor lasers). The respective first semiconductor lasers  44   a  and  44   b  and the respective second semiconductor lasers  45   a  and  45   b  go into the inside of the housing  41  through respective holes formed at the side plate  41   b.    
     Assuming that an imaginary straight line M extends in a main-scanning direction X passing through the center of the polygonal mirror  42 , each of the first semiconductor lasers  44   a  and  44   b  is disposed symmetry to the respective second semiconductor lasers  45   a  and  45   b  placing the imaginary straight line M as the center. A direction perpendicular to the main-scanning direction X is set as a sub-scanning direction Y. A direction perpendicular to the main-scanning direction X and the sub-scanning direction Y (the longitudinal direction of the rotation axis of the polygonal motor  43 ) is set as a height direction Z. 
     The drive substrate  46  is a plane plate-shaped printed circuit board and includes circuits for driving the respective first semiconductor lasers  44   a  and  44   b  and the respective second semiconductor lasers  45   a  and  45   b . The respective first semiconductor lasers  44   a  and  44   b  and the respective second semiconductor lasers  45   a  and  45   b  are disposed on an approximately the same plane (YZ plane) by being mounted on the plane plate-shaped printed circuit board. The first semiconductor lasers  44   a  and  44   b  and the second semiconductor lasers  45   a  and  45   b  emit light beams L 1  to L 4 , respectively. The respective light beams L 1  to L 4  are emitted in the vertical direction (the main-scanning direction X) with respect to the plane and to the inside of the housing  41 . 
     On the drive substrate  46  (YZ plane), the respective first semiconductor lasers  44   a  and  44   b  are disposed at different positions from one another in the sub-scanning direction Y and the height direction Z. Similarly, the respective second semiconductor lasers  45   a  and  45   b  are also disposed different positions from one another in the sub-scanning direction Y and the height direction Z. 
     The light scanning device  11  includes a first incident optical system  51  and a second incident optical system  52 . The first incident optical system  51  guides the light beams L 1  and L 2  of the respective first semiconductor lasers  44   a  and  44   b  to the polygonal mirror  42 . The second incident optical system  52  guides the light beams L 3  and L 4  of the respective second semiconductor lasers  45   a  and  45   b  to the polygonal mirror  42 . The first incident optical system  51  includes two collimator lenses  53   a  and  53   b , two apertures  54 , two mirrors  55   a  and  55   b , a cylindrical lens  56 , and a similar component. Similarly, the second incident optical system  52  includes two collimator lenses  57   a  and  57   b , two apertures  58 , two mirrors  59   a  and  59   b , the cylindrical lens  56 , and a similar component. The respective collimator lens  53   a  and  53   b , the respective apertures  54 , and the respective mirrors  55   a  and  55   b  of the first incident optical system  51  are disposed symmetrical to the respective collimator lens  57   a  and  57   b , the respective apertures  58 , and the respective mirrors  59   a  and  59   b  of the second incident optical system  52  placing the imaginary straight line M as the center. The imaginary straight line M passes through the center of the cylindrical lens  56 . One half side of the cylindrical lens  56  divided by the imaginary straight line M is disposed at the first incident optical system  51  while the other half side of the cylindrical lens  56  is disposed at the second incident optical system  52 . 
     Further, the light scanning device  11  includes a first image-forming optical system  61  and a second image-forming optical system  62 . The first image-forming optical system  61  guides the light beams L 1  and L 2  of the respective first semiconductor lasers  44   a  and  44   b  reflected by the polygonal mirror  42  to the two photosensitive drums  13  (not illustrated). The second image-forming optical system  62  guides the light beams L 3  and L 4  of the respective second semiconductor lasers  45   a  and  45   b  reflected by the polygonal mirror  42  to the other two photosensitive drums  13  (not illustrated). The first image-forming optical system  61  is formed of an fθ lens  63 , respective four reflective mirrors  64   a ,  64   b ,  64   c , and  64   d , and a similar lens. Similarly, the second image-forming optical system  62  is formed of an fθ lens  65 , respective four reflective mirrors  66   a ,  66   b ,  66   c , and  66   d , and a similar lens. The fθ lens  63  and the respective reflective mirrors  64   a ,  64   b ,  64   c , and  64   d  of the first image-forming optical system  61  are disposed symmetrical to the fθ lens  65  and the respective reflective mirrors  66   a ,  66   b ,  66   c , and  66   d  of the second image-forming optical system  62  placing the imaginary straight line M as the center. 
     A BD substrate  73  is disposed at the first image-forming optical system  61  side while a BD substrate  76  is also disposed at the second image-forming optical system  62  side. The BD substrate  73  includes a BD mirror  71  and a BD sensor  72 . The BD substrate  76  includes a BD mirror  74  and a BD sensor  75 . The BD mirror  71  and the BD sensor  72  at the first image-forming optical system  61  side are disposed symmetrical to the BD mirror  74  and the BD sensor  75  at the second image-forming optical system  62  side placing the rotation axis of the polygonal mirror  42  as the center. 
     Next, optical paths for the light beams L 1  and L 2  of the respective first semiconductor lasers  44   a  and  44   b  to enter the respective photosensitive drums  13 , and optical paths for the light beams L 3  and L 4  of the respective second semiconductor lasers  45   a  and  45   b  to enter the respective photosensitive drums  13  will be described. 
     The light beam L 1  of the first semiconductor laser  44   a  transmits the collimator lens  53   a  and is made to parallel light. The light beam L 1  enters the reflecting surface  42   a  of the polygonal mirror  42  via the aperture  54 , the mirror (semi-transparent mirror)  55   a , and the cylindrical lens  56 . The light beam L 2  of the first semiconductor laser  44   b  transmits the collimator lens  53   b  and is made to parallel light. The light beam L 2  enters and is reflected by the respective mirrors  55   a  and  55   b  via the aperture  54 , and enters the reflecting surface  42   a  of the polygonal mirror  42 , via the cylindrical lens  56 . The cylindrical lens  56  condenses the respective light beams L 1  and L 2  so as to almost converge the respective light beams L 1  and L 2  at the reflecting surface  42   a  of the polygonal mirror  42  only in the height direction Z. 
     Here, on the drive substrate  46  (YZ plane), the respective first semiconductor lasers  44   a  and  44   b  are disposed at different positions from one another in the sub-scanning direction Y. However, the light beam L 2  of the first semiconductor laser  44   b  is reflected by the respective mirrors  55   a  and  55   b  to be shifted to a first optical path J 1  in common with the light beam L 1  of the first semiconductor laser  44   a  in the sub-scanning direction Y. The first optical path J 1  is the optical path from the mirror  55   a  to the reflecting surface  42   a  of the polygonal mirror  42  via the cylindrical lens  56 . In this first optical path J 1 , as illustrated in  FIG. 4 , the respective light beams L 1  and L 2  overlap with each other in plan view. 
     On the drive substrate  46  (the YZ plane), the respective first semiconductor lasers  44   a  and  44   b  are disposed at different positions from one another in the height direction Z. However, setting of the emission directions of the light beams L 1  and L 2  of the respective first semiconductor lasers  44   a  and  44   b  or the orientations of the respective mirrors  55   a  and  55   b  almost superimposes incident spots (first incident spots) of the respective light beams L 1  and L 2  on the reflecting surface  42   a  of the polygonal mirror  42 . In view of this, in the first optical path J 1 , the light beams L 1  and L 2  of the respective first semiconductor lasers  44   a  and  44   b  enter from obliquely upward and obliquely downward to the reflecting surface  42   a  of the polygonal mirror  42 . Then, the respective light beams L 1  and L 2  reflected by the reflecting surface  42   a  of the polygonal mirror  42  are away from one another in the obliquely downward direction and the obliquely upward direction. The light beam L 1  at one side is reflected by the reflecting surface  42   a  of the polygonal mirror  42  to obliquely downward, transmits the fθ lens  63 , is reflected by the one mirror  64   a , and enters the photosensitive drum  13  (not illustrated) where yellow toner image is to be formed. The light beam L 2  at the other side is reflected by the reflecting surface  42   a  of the polygonal mirror  42  to obliquely upward, transmits the fθ lens  63 , is sequentially reflected by the three mirrors  64   b ,  64   c , and  64   d , and enters the photosensitive drum  13  (not illustrated) where a magenta toner image is to be formed. 
     The polygonal motor  43  rotates the polygonal mirror  42  at equal angular velocity. Then, the polygonal mirror  42  sequentially reflects the respective light beams L 1  and L 2  at the respective reflecting surfaces  42   a , and causes the respective light beams L 1  and L 2  to be repeatedly deflected at the equal angular velocity in the main-scanning direction X. The fθ lens  63  condenses and emits the respective light beams L 1  and L 2  in both the main-scanning direction X and the sub-scanning direction Y such that the respective light beams L 1  and L 2  may have a predetermined beam diameter at the surface of the respective photosensitive drums  13 . Moreover, the fθ lens  63  transforms the respective light beams L 1  and L 2  deflected at the equal angular velocity in the main-scanning direction X by the polygonal mirror  42  such that the respective light beams L 1  and L 2  may move at the equal linear velocity along the main-scanning line on respective photosensitive drums  13 . Thus, the respective light beams L 1  and L 2  are repeatedly scanned on the surface of respective photosensitive drums  13  in the main-scanning direction X. 
     Immediately before start of main scanning of the respective photosensitive drums  13  with the respective light beams L 1  and L 2 , the light beam L 1  at one side transmits a convex lens portion  63   a  formed in the end portion of the fθ lens  63  to enter the BD mirror  71  and is reflected by the BD mirror  71  to enter the BD sensor  72 . The BD sensor  72  receives the light beam L 1  at timing immediately before the start of main scanning of the respective photosensitive drums  13 , and outputs a BD signal indicating timing immediately before the start of the main scanning According to this BD signal, the timing of starting main scanning of the respective photosensitive drums  13  on which yellow and magenta toner images are formed is determined. Then, modulation of the respective light beams L 1  and L 2  according to the respective image data with yellow and magenta is started. 
     On the other hand, the respective photosensitive drums  13  where yellow and magenta toner images are to be formed are rotatably driven. The respective light beams L 1  and L 2  scan a two-dimensional surface (a circumference surface) of the respective photosensitive drums  13 . Thus, respective electrostatic latent images are formed at the surfaces of the respective photosensitive drums  13 . 
     Next, the light beam L 3  of the second semiconductor laser  45   a  transmits the collimator lens  57   a  and is made to parallel light. The light beam L 3  enters and is reflected by the respective mirrors  59   a  and  59   b  via the aperture  58 , and transmits the cylindrical lens  56  to enter the reflecting surface  42   a  of the polygonal mirror  42 . The light beam L 4  of the second semiconductor laser  45   b  transmits the collimator lens  57   b  and is made to parallel light. The light beam L 4  enters the reflecting surface  42   a  of the polygonal mirror  42  via the aperture  58 , the mirror (semi-transparent mirror)  59   b , and the cylindrical lens  56 . 
     On the drive substrate  46  (YZ plane), the respective second semiconductor lasers  45   a  and  45   b  are disposed at different positions from one another in the sub-scanning direction Y. However, the light beam L 3  of the second semiconductor laser  45   a  is reflected by the respective mirrors  59   a  and  59   b  to be shifted to a second optical path J 2  in common with the light beam L 4  of the second semiconductor laser  44   b  in the sub-scanning direction Y. The second optical path J 2  is the optical path from the mirror  59   b  to the reflecting surface  42   a  of the polygonal mirror  42  via the cylindrical lens  56 . In the second optical path J 2 , as illustrated in  FIG. 4 , the respective light beams L 3  and L 4  overlap with each other in plan view. 
     On the drive substrate  46  (the YZ plane), the respective second semiconductor lasers  45   a  and  45   b  are disposed at different positions from one another in the height direction Z. However, setting of the emission directions of the light beams L 3  and L 4  of the respective second semiconductor lasers  45   a  and  45   b  or the orientations of respective mirrors  59   a  and  59   b  almost superimposes incident spots (second incident spots) of the respective light beams L 3  and L 4  on the reflecting surface  42   a  of the polygonal mirror  42 . In view of this, in the second optical path J 2 , the light beams L 3  and L 4  of the respective second semiconductor lasers  45   a  and  45   b  enter from obliquely downward and obliquely upward to the reflecting surface  42   a  of the polygonal mirror  42 . Then, when being reflected by the reflecting surface  42   a  of the polygonal mirror  42 , the respective light beams L 3  and L 4  are away from one another in the obliquely upward direction and the obliquely downward direction. The light beam L 3  at one side is reflected by the reflecting surface  42   a  of the polygonal mirror  42  to obliquely upward, transmits the fθ lens  65 , is sequentially reflected by the three mirrors  66   b ,  66   c , and  66   d , and enters the photosensitive drum  13  (not illustrated) where cyan toner image is to be formed. The light beam L 4  at the other side is reflected by the reflecting surface  42   a  of the polygonal mirror  42  to obliquely downward, transmits the fθ lens  65 , is reflected by the one mirror  66   a , and enters the photosensitive drum  13  (not illustrated) where black toner image is to be formed. 
     Immediately before start of main scanning of the respective photosensitive drums  13  with the respective light beams L 3  and L 4 , the other light beam L 4  transmits a convex lens portion  65   a  formed in the end portion of the fθ lens  65  to enter the BD mirror  74  and is reflected by the BD mirror  74  to enter the BD sensor  75 . The BD sensor  75  outputs a BD signal indicating timing immediately before the start of the main scanning of the respective photosensitive drums  13  with the respective light beams L 3  and L 4 . According to this BD signal, the timing of starting main scanning of the respective photosensitive drums  13  where cyan and black toner images are to be formed is determined. Then, modulation of the respective light beams L 3  and L 4  according to respective cyan and black image data is started. 
     On the other hand, the respective photosensitive drums  13  where cyan and black toner images are to be formed are rotatably driven. The respective light beams L 3  and L 4  scan a two-dimensional surface (a circumference surface) of the respective photosensitive drums  13 . Thus, respective electrostatic latent images are formed at the surfaces of the respective photosensitive drums  13 . 
     The light scanning device  11  with this constitution includes the polygonal mirror  42  at the approximately center of the bottom plate  41   a  of the housing  41 . The light scanning device  11  includes the respective first semiconductor lasers  44   a  and  44   b  and the respective second semiconductor lasers  45   a  and  45   b  disposed symmetrically to one another placing the imaginary straight line M passing through the center of the polygonal mirror  42  as the center. Here, the first incident optical system  51  is disposed symmetrically to the second incident optical system  52 , and the first image-forming optical system  61  is disposed symmetrically to the second image-forming optical system  62 . This allows approximately downsizing the light scanning device  11  viewed from the side by aggregating the polygonal mirror  42 , the respective first semiconductor lasers  44   a  and  44   b , the respective second semiconductor lasers  45   a  and  45   b , the first incident optical system  51 , the second incident optical system  52 , or a similar component in a small space. 
     The light beams L 1  and L 2  of the respective first semiconductor lasers  44   a  and  44   b  enter the approximately identical first incident spots on the reflecting surface  42   a  of the polygonal mirror  42 . Additionally, the light beams L 3  and L 4  of the respective second semiconductor lasers  45   a  and  45   b  enter the approximately identical second incident spots on the reflecting surface  42   a  of the polygonal mirror  42 . This thins the thickness of the polygonal mirror  42 , and the polygonal mirror  42  does not cause an increase in height of the light scanning device  11 . 
     Further, the respective light beams L 1  and L 2  reflected by the reflecting surface  42   a  of the polygonal mirror  42  move apart from each other in the obliquely downward direction and the obliquely upward direction. On the other hand, the arranged position of the fθ lens  63  with respect to the polygonal mirror  42  is set such that the respective light beams L 1  and L 2  enter the fθ lens  63  before the separation distance between the respective light beams L 1  and L 2  in the up-down direction becomes long. Similarly, the respective light beams L 3  and L 4  reflected by the reflecting surface  42   a  of the polygonal mirror  42  move apart from each other in the obliquely downward direction and the obliquely upward direction. On the other hand, the arranged position of the fθ lens  65  with respect to the polygonal mirror  42  is set such that the respective light beams L 3  and L 4  enter the fθ lens  65  before the separation distance between the respective light beams L 3  and L 4  in the up-down direction becomes long. This thins the respective thicknesses of the fθ lenses  63  and  65 , and the respective fθ lenses  63  and  65  do not cause an increase in height of the light scanning device  11 . 
     The respective mirrors  55   a  and  55   b  shift the light beam L 2  of the first semiconductor laser  44   b  to the first optical path J 1  near the imaginary straight line M (the center of the device) in the sub-scanning direction Y and then causes the light beam L 2  to enter the polygonal mirror  42 . Additionally, the respective mirrors  59   a  and  59   b  shift the light beam L 3  of the second semiconductor laser  45   a  to the second optical path J 2  near the imaginary straight line M (the center of the device) in the sub-scanning direction Y and then causes the light beam L 3  to enter the polygonal mirror  42 . Accordingly, the diameter of the polygonal mirror  42  can be reduced such that the first image-forming optical system  61  and the second image-forming optical system  62  becomes closer to each other. This reduces the depth and the lateral width of the light scanning device  11  so as to downsize the light scanning device  11 . 
     Further, the respective first semiconductor lasers  44   a  and  44   b  and the respective second semiconductor lasers  45   a  and  45   b  are mounted on the identical drive substrate  46 . This ensures a small parts count and simplifies the wiring for the respective semiconductor lasers  44   a ,  44   b ,  45   a , and  45   b.    
     Now, the arranged positions or similar parameter of a plurality of optical members such as mirrors and lenses are appropriately set so as to downsize the light scanning device  11 . On the other hand, it is also necessary to appropriately set the arranged positions of the respective BD mirrors  71  and  74 , the respective BD sensors  72  and  75 , and the respective BD substrates  73  and  76  so as to downsize the light scanning device  11 . In particular, since the respective BD substrates  73  and  76  have large sizes, the BD substrates  73  and  76  might block the respective light beams L 1  to L 4  or hinder downsizing of the light scanning device  11  depending on the arranged positions of the BD substrates  73  and  76 . 
     Here,  FIG. 6  and  FIG. 7  are a plan view and a side view illustrating the arranged positions of the respective optical members in the housing  41  of the light scanning device  11 . As illustrated in  FIG. 6 , the respective light beams L 1  to L 4  are reflected by the polygonal mirror  42  so as to be repeatedly deflected in an approximately fan-shaped range (not illustrated). This approximately fan-shaped range includes a scanning angle range α for the respective light beams L 1  to L 4 . The scanning angle range α is needed for scanning an effective scan area H for the respective photosensitive drums  13 . 
     The effective scan area H is an area on each photosensitive drums  13  scanned by each of the light beams L 1  to L 4 , and is a region including a formation region of an electrostatic latent image. In practice, the effective scan areas H of the respective photosensitive drums  13  are positioned upward of the respective reflective mirrors  64   a ,  64   d ,  66   a , and  66   d . However,  FIG. 6  illustrates the effective scan area H expanded in a two-dimensional plane. 
     If the respective BD substrates  73  and  76  on which the respective BD sensors  72  and  75  are mounted are arranged at positions W 1  within the scanning angle range α as illustrated in  FIG. 6 , the respective BD substrates  73  and  76  interfere with the respective light beams L 1  to L 4  so as to hinder formation of electrostatic latent images. Alternatively, if the respective BD substrates  73  and  76  are arranged at positions W 2  outside the scanning angle range α, it is necessary to increase the depth of the housing  41  to form arrangement spaces of the respective BD substrates  73  and  76 . 
     Accordingly, in the light scanning device  11  according to this embodiment, the arranged positions of the respective BD mirrors  71  and  74 , the respective BD sensors  72  and  75 , and the respective BD substrates  73  and  76  are set as illustrated in  FIG. 6  and  FIG. 7 . 
     In detail, the respective BD substrates  73  and  76  are overlapped with the outside of the respective side plates  41   c  in the housing  41 , and the respective BD sensors  72  and  75  face the inside of the housing  41  through the holes of the respective side plates  41   c . The respective BD mirrors  71  and  74  are arranged inside the housing  41  and outside the scanning angle range α. 
     The respective light beams L 1  to L 4  are reflected by the polygonal mirror  42  and repeatedly deflected in an approximately fan-shaped range. The respective BD mirrors  71  and  74  reflect the respective light beams L 1  and L 4  immediately before entering the scanning angle range α and cause the light beams L 1  and L 4  to enter the respective BD sensors  72  and  75 . The respective BD sensors  72  and  75  detect the respective light beams L 1  and L 4  and output respective BD signals. 
     The respective light beams L 1  and L 4  are reflected by the polygonal mirror  42  in the obliquely downward direction and enter the respective BD mirrors  71  and  74 . The directions of the reflecting surfaces of the respective BD mirrors  71  and  74  are set to the obliquely upward directions so as to reflect the respective light beams L 1  and L 4  in the obliquely upward directions. When reflected in the obliquely upward directions at the BD mirrors  71  and  74 , the respective beams L 1  and L 4  pass through the upper side of the respective mirrors  64   a  and  66   a  and enter the respective BD sensors  72  and  75 . Then, the respective BD sensors  72  and  75  output the respective BD signals. 
     Based on the respective BD signals, when the respective light beams L 1  to L 4  enter the scanning angle range α, modulation of the respective light beams L 1  to L 4  according to the respective image data is simultaneously started so as to form respective electrostatic latent images on the surfaces of the respective photosensitive drums  13 . Accordingly, the timing when scanning on the respective photosensitive drums  13  using the respective light beams L 1  to L 4  is started is synchronized with the detection timing of the respective light beams L 1  and L 4  using the respective BD sensors  72  and  75 . 
     The following describes the mounting structures, the positions, and similar configuration of the respective BD mirrors  71  and  74  and the respective BD sensors  72  and  75  in detail. 
       FIG. 8  is an enlarged perspective view illustrating the respective BD mirrors  71  and  74  and the respective BD sensors  72  and  75  viewed from the inside of the housing  41  in the obliquely upward direction.  FIG. 9  is an enlarged perspective view illustrating the respective BD mirrors  71  and  74  and the respective BD substrates  73  and  76  viewed from the outside of the housing  41  in the obliquely upward direction. As illustrated in  FIG. 8  and  FIG. 9 , a depressed portion  41   d  is formed outside the side plate  41   c  of the housing  41 , and the BD substrate  73  (or  76 ) is secured to this depressed portion  41   d . Inside the depressed portion  41   d , a rectangular hole  41   e  is formed. The BD sensor  72  (or  75 ) faces the inside of the housing  41  through the rectangular hole  41   e.    
       FIG. 10  is an enlarged perspective view illustrating the mounting structure for the respective BD mirrors  71  and  74  on the bottom surface in the bottom plate  41   a  in the housing  41 .  FIG. 11  is a perspective view illustrating the mounting structure for the respective BD mirrors  71  and  74  viewed from a direction different from  FIG. 10 . As illustrated in  FIG. 10  and  FIG. 11 , a depressed portion  41   f  is formed on the bottom surface in the bottom plate  41   a , and the BD mirror  71  (or  74 ) is arranged in the depressed portion  41   f . Inside this depressed portion  41   f , a supporting piece  41   g  protrudes. The BD mirror  71  (or  74 ) overlaps with the supporting piece  41   g , and the supporting piece  41   g  and the BD mirror  71  (or  74 ) are sandwiched by a spring member  81  having an approximately U-shaped cross-sectional shape so as to hold the BD mirror  71  (or  74 ). 
     In the depressed portion  41   f , an entrance hole  41   h  and an emission hole  41   i  are formed. The entrance hole  41   h  causes the light beam L 1  (or L 4 ) reflected by the polygonal mirror  42  to pass through and enter the BD mirror  71  (or  74 ). The emission hole  41   i  emits the light beam L 1  (or L 4 ) reflected by the BD mirror  71  (or  74 ) to the BD sensor  72  (or  75 ). 
     As apparent from  FIG. 6 , in plan view of the respective BD mirrors  71  and  74 , the respective BD mirrors  71  and  74  are arranged at the lower side of the respective mirrors  64   b ,  64   c ,  66   b , and  66   c  while the respective BD mirrors  71  and  74  are disposed in the respective depressed portions  41   f  on the bottom surface in the bottom plate  41   a . Accordingly, the respective BD mirrors  71  and  74  can be mounted and removed from the bottom surface side in the bottom plate  41   a  regardless of the existence or absence of the respective mirrors  64   b ,  64   c ,  66   b , and  66   c.    
     In this configuration, the respective mirrors  64   a  and  66   a  reflect the respective light beams L 1  and L 4  immediately before entering the respective photosensitive drums  13 , and are the mirrors separated from the polygonal mirror  42  the most among the respective mirrors of the first and second image-forming optical systems  61  and  62 . Further, the respective mirrors  64   a  and  66   a  are the mirrors that reflect the respective light beams L 1  and L 4  detected by the respective BD sensors  72  and  75 . Accordingly, the respective mirrors  64   a  and  66   a  are referred to as respective last mirrors  64   a  and  66   a.    
     Here, as apparent from  FIG. 2  and  FIG. 6 , the respective BD substrates  73  and  76  are overlapped with the outside of the respective side plates  41   c  in the housing  41 . Accordingly, the respective BD substrates  73  and  76  are arranged outside the respective last mirrors  64   a  and  66   a  arranged inside the respective side plate  41   c  (at both ends inside the housing  41 ), that is, in the positions farther from the polygonal mirror  42  than the respective last mirrors  64   a  and  66   a . The respective BD sensors  72  and  75  are also arranged in the identical position. Accordingly, the respective BD sensors  72  and  75  and the respective BD substrates  73  and  76  do not interfere with the respective light beams L 1  to L 4 . 
     Since the respective BD substrates  73  and  76  are overlapped with the outside of the respective side plates  41   c  in the housing  41 , it is not necessary to peculiarly form arrangement spaces of the respective BD sensors  72  and  75  and the respective BD substrates  73  and  76  in the sub-scanning direction Y. Accordingly, the lateral width of the housing  41  can be set according to the positions of the respective last mirrors  64   a  and  66   a  so as to set the minimum lateral width of the light scanning device  11 . Further, since the heights of the respective BD substrates  73  and  76  are set to be equal to or less than the heights of the respective side plates  41   c  in the housing  41 , the respective BD substrates  73  and  76  do not cause an increase in height of the light scanning device  11 . 
     As apparent from  FIG. 6 , the respective BD sensors  72  and  75  and the respective BD substrates  73  and  76  are arranged within the scanning angle range α. 
     Accordingly, regarding the depth of the light scanning device  11 , it is not necessary to peculiarly form arrangement spaces of the respective BD sensors  72  and  75  and the respective BD substrates  73  and  76  and it is only necessary to fit the scanning angle range α to the inside of the housing  41  between the respective mirrors  64   a  and  66   a . Thus, it is not necessary to increase the depth of the light scanning device  11 . 
     As illustrated in  FIG. 6 , the respective BD mirrors  71  and  74  are arranged outside the scanning angle range α, and the respective BD sensors  72  and  75  and the respective BD mirrors  71  and  74  are arranged in the vicinity of a boundary line β between the inside and the outside of the scanning angle range α. This increases bending angles γ of the respective light beams L 1  and L 4  when being reflected and bent by the respective BD mirrors  71  and  74 . The respective light beams L 1  and L 4  approximately vertically enter the light receiving surfaces of the respective BD sensors  72  and  75  such that the light receiving amounts of the respective BD sensors  72  and  75  increase. This improves the accuracy of the detection timing of the respective light beams L 1  and L 4  using the respective BD sensors  72  and  75 . 
     Since the respective convex lens portions  63   a  and  65   a  are disposed in the end portions of the respective fθ lenses  63  and  65 , the respective light beams L 1  and L 4  can be condensed by the respective convex lens portions  63   a  and  65   a  and then reflected by the respective BD mirrors  71  and  74  to enter the respective BD sensors  72  and  75 . This reduces the spots of the respective light beams L 1  and L 4  on the light receiving surfaces of the respective BD sensors  72  and  75 . This also improves the accuracy of the detection timing of the respective light beams L 1  and L 4  using the respective BD sensors  72  and  75 . Adjustment of the focal lengths of the respective convex lens portions  63   a  and  65   a  allows freely setting the distances from the polygonal mirror  42  to the respective BD mirrors  71  and  74  and the distances from the respective BD mirrors  71  and  74  to the respective BD sensors  72  and  75  while maintaining the small spots of the respective light beams L 1  and L 4  on the light receiving surfaces of the respective BD sensors  72  and  75 . Thus, the light scanning device  11  can be downsized. 
     As just described, in the light scanning device  11  according to this embodiment, the respective BD substrates  73  and  76  are overlapped with the outside of the respective side plates  41   c  in the housing  41 . The respective BD substrates  73  and  76  are arranged in the positions farther from the polygonal mirror  42  than the respective last mirrors  64   a  and  66   a . Accordingly, the respective BD sensors  72  and  75  and the respective BD substrates  73  and  76  do not interfere with the respective light beams L 1  to L 4 . 
     Since the respective BD substrates  73  and  76  are overlapped with the outside of the respective side plates  41   c  in the housing  41 , it is not necessary to peculiarly form arrangement spaces of the respective BD sensors  72  and  75  and the respective BD substrates  73  and  76  in the sub-scanning direction Y. Accordingly, the lateral width of the housing  41  can be set according to the positions of the respective last mirrors  64   a  and  66   a  so as to set the minimum lateral width of the light scanning device  11 . Further, since the heights of the respective BD substrates  73  and  76  are set to be equal to or less than the heights of the respective side plates  41   c  in the housing  41 , the respective BD substrates  73  and  76  do not cause an increase in height of the light scanning device  11 . 
     Since the respective BD sensors  72  and  75  and the respective BD substrates  73  and  76  are arranged within the scanning angle range α, regarding the depth of the light scanning device  11 , it is not necessary to peculiarly form arrangement spaces of the respective BD sensors  72  and  75  and the respective BD substrates  73  and  76 . Thus, it is not necessary to increase the depth of the light scanning device  11 . 
     The respective BD mirrors  71  and  74  are arranged outside the scanning angle range α, and the respective BD sensors  72  and  75  and the respective BD mirrors  71  and  74  are arranged in the vicinity of a boundary line β between the inside and the outside of the scanning angle range α. The respective light beams L 1  and L 4  approximately vertically enter the light receiving surfaces of the respective BD sensors  72  and  75  such that the light receiving amounts of the respective BD sensors  72  and  75  increase. This improves the accuracy of the detection timing of the respective light beams L 1  and L 4  using the respective BD sensors  72  and  75 . 
     Further, the light beams L 1  and L 4  are be condensed by the respective convex lens portions  63   a  and  65   a  of the respective fθ lenses  63  and  65  and then reflected by the respective BD mirrors  71  and  74  to enter the respective BD sensors  72  and  75 . This improves the accuracy of the detection timing of the respective light beams L 1  and L 4  using the respective BD sensors  72  and  75 . This allows freely setting the distances from the polygonal mirror  42  to the respective BD mirrors  71  and  74  and the distances from the respective BD mirrors  71  and  74  to the respective BD sensors  72  and  75  while maintaining the small spots of the respective light beams L 1  and L 4  on the light receiving surfaces of the respective BD sensors  72  and  75 . Thus, the light scanning device  11  can be downsized. 
     Here, in the above-described embodiment, as illustrated in  FIG. 7 , the respective beams L 1  and L 4  are reflected by the respective BD mirrors  71  and  74  in the obliquely upward directions and passes above the respective mirrors  64   a  and  66   a . In the case where the light scanning device  11  is inverted in the up-down direction, the respective beams L 1  and L 4  are reflected by the respective BD mirrors  71  and  74  in the obliquely downward directions and enter the respective BD sensors  72  and  75  while passing below the respective mirrors  64   a  and  66   a.    
     The preferred embodiment according to the present invention is described above with reference to the attached drawings; however, it is needless to say that the present invention is not limited to the above examples. It would be obvious that an ordinary skilled person conceives various modifications and corrections within scopes defined in the claims, and it should be understood that those modified examples fall within the technical scope of the present invention. 
     INDUSTRIAL APPLICABILITY 
     The present invention is appropriate for a light scanning device that includes: a light-emitting element that emits a light beam; an optical sensor that detects a light beam; a deflecting section that deflects a light beam; and a reflective mirror that reflects a light beam and that scan a scan object with a light beam, and is appropriate for an image forming apparatus with the light scanning device. 
     This application is based on and claims priority to Japanese Patent Application 2012-230098, filed in Japan on Oct. 17, 2012, the entire contents of which are incorporated herein by reference. 
     DESCRIPTION OF REFERENCE SIGNS 
       1  image forming apparatus 
       11  light scanning device 
       12  development apparatus 
       13  photosensitive drum (scan object) 
       14  drum cleaning apparatus 
       15  charging unit 
       17  fixing apparatus 
       21  intermediate transfer belt 
       22  belt cleaning apparatus 
       23  secondary transfer apparatus 
       33  pickup roller 
       34  registration roller 
       35  conveyance roller 
       36  discharge roller 
       41  housing 
       42  polygonal mirror (deflecting section) 
       43  polygonal motor 
       44   a ,  44   b  first semiconductor laser (light-emitting element) 
       45   a ,  45   b  second semiconductor laser (light-emitting element) 
       46  drive substrate 
       51  first incident optical system 
       52  second incident optical system 
       53   a ,  53   b ,  57   a ,  57   b  collimator lens 
       55   a ,  55   b ,  59   a ,  59   b  mirror 
       56  cylindrical lens 
       61  first image-forming optical system 
       62  second image-forming optical system 
       63 ,  65  fθ lens 
       63   a ,  65   a  convex lens portion 
       64   a  to  64   d ,  66   a  to  66   d  mirror (reflective mirror) 
       71 ,  74  BD mirror (detecting mirror) 
       72 ,  75  BD sensor (optical sensor) 
       73 ,  76  BD substrate