Abstract:
An optical scanning apparatus includes a light source, which emits plural light beam groups from plural liminous points which are two-dimensionally arranged in a grid form; a pre-deflection optical system, which transmits the plural light beam groups emitted from the light source; and a deflector, which reflects the plural light beam groups which have been transmitted through the pre-deflection optical system at a deflection surface, for scanningly deflecting the plural light beam groups in a main scanning direction. The pre-deflection optical system includes a first incidence angle-adjuster for adjusting sub-scanning direction incidence angles of the light beam groups which are incident at the deflection surface of the deflector.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority under 35 USC 119 from Japanese Patent Application No. 2005-271062, the disclosure of which is incorporated by reference herein.  
       BACKGROUND  
       [0002]     1. Technical Field  
         [0003]     The present invention relates to an optical scanning apparatus.  
         [0004]     2. Related Art  
         [0005]     Heretofore, an optical scanning apparatus has been known which, in a tandem-system color laser printer or the like, separates plural laser beams, which have been emitted from a single light source and deflected by a single deflector, and scans plural photosensitive bodies therewith.  
         [0006]     An optical scanning apparatus has also been known which, with a view to increasing printing speed and/or raising resolution of color images, separates a total of eight laser beams, which have been emitted from a multi-laser beam array at which four groups of point light sources structured by respective pairs of point light sources are provided, into four laser beam groups of two beams each, and scans four photosensitive bodies therewith.  
         [0007]     As such an optical scanning apparatus, an optical scanning apparatus has been proposed in which a sub-scanning direction spacing of plural scanning beams on a photosensitive body is adjusted by turning a light source about an optical axis to provide the light source with a suitable angle.  
         [0008]     However, in order to separate plural laser beams emitted from a single light source at such a structure, it is necessary to provide a predetermined spacing between the plural laser beams at a location at which the laser beams are to be separated into laser beam groups. Accordingly, a pre-deflection optical system is made to be telecentric, or a predetermined angle is provided in the sub-scanning direction, which intersects a deflection surface. However, because of focusing distance errors of the pre-deflection optical system, errors arise in oblique angles of incidence on the deflection surface of the deflector (see  FIG. 4 ). Hence, because of such errors in the oblique incidence angles, scanning lines on a photosensitive body are curved into arcs, and “bowing” occurs (see  FIG. 8 ). Moreover, in a case in which plural laser beams are scanned in the same scan, curvature amounts thereof differ for each of the laser beams, spacings of the laser beams in the sub-scanning direction vary with scanning positions in a main scanning direction, and a “bow difference” occurs (see  FIG. 10A ).  
       SUMMARY  
       [0009]     According to an aspect of the present invention, an optical scanning apparatus includes: a light source, which emits plural light beam groups from plural light source points which are two-dimensionally arranged in a grid form; a pre-deflection optical system, which transmits the plural light beam groups emitted from the light source; and a deflector, which reflects the plural light beam groups which have been transmitted through the pre-deflection optical system at a deflection surface, for scanningly deflecting the plural light beam groups in a main scanning direction, wherein the pre-deflection optical system includes a first incidence angle-adjuster, for adjusting sub-scanning direction incidence angles of the light beam groups which are incident at the deflection surface of the deflector. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein:  
         [0011]      FIG. 1  is a perspective view showing structure of an optical scanning apparatus of a first exemplary embodiment of the present invention;  
         [0012]      FIG. 2  is a schematic view showing the optical scanning apparatus of the first exemplary embodiment of the present invention;  
         [0013]      FIG. 3A  is a plan view showing a light source of the optical scanning apparatus of the first exemplary embodiment of the present invention;  
         [0014]      FIG. 3B  is a plan view showing a photosensitive body of the first exemplary embodiment of the present invention;  
         [0015]      FIG. 4  is a view of a cylindrical lens viewed in a cross-section along a sub-scanning direction;  
         [0016]      FIG. 5  is a diagram schematically showing a change in sub-scanning direction incidence angles at a deflection surface due to movement of the cylindrical lens in an optical axis direction;  
         [0017]      FIG. 6A  is an exploded perspective view showing a cylindrical lens-moving mechanism;  
         [0018]      FIG. 6B  is a plan view showing the cylindrical lens-moving mechanism;  
         [0019]      FIG. 7  is a view showing a light source-turning mechanism;  
         [0020]      FIG. 8  is an explanatory view for explaining curvature of a scanning line;  
         [0021]      FIG. 9  is a view schematically showing a situation in which curvature amounts of scanning lines are adjusted;  
         [0022]      FIG. 10A  is a view showing a state in which curvature amounts of scanning lines differ and a bow difference arises;  
         [0023]      FIG. 10B  is a view showing a state in which curvature amounts of scanning lines are eliminated and the bow difference is eliminated;  
         [0024]      FIG. 11  is a plan view schematically showing an optical scanning apparatus of a second exemplary embodiment of the present invention;  
         [0025]      FIG. 12  is a diagram schematically showing a situation in which a reflection mirror of the optical scanning apparatus of the second exemplary embodiment of the present invention is turned and an incident angle at a deflection surface is adjusted;  
         [0026]      FIG. 13  is a plan view schematically showing an optical scanning apparatus of a third exemplary embodiment of the present invention; and  
         [0027]      FIG. 14  is a view showing a reflection mirror angle-adjusting mechanism. 
     
    
     DETAILED DESCRIPTION  
       [0028]     In  FIGS. 1 and 2 , a color laser printer is equipped with an optical scanning apparatus  10  of a first exemplary embodiment of the present invention. The optical scanning apparatus  10  irradiates laser beam groups LY, LM, LC and LK, which respectively serve as light flux groups, at photosensitive bodies  12 Y,  12 M,  12 C and  12 K, which each rotate in the direction of arrow V, to form latent images. The latent images formed at the photosensitive bodies  12 Y,  12 M,  12 C and  12 K are developed by unillustrated developing units of the respective colors, to form toner images of yellow (Y), magenta (M), cyan (C) and black (K), respectively. The respective toner images on the photosensitive bodies  12 Y,  12 M,  12 C and  12 K are transferred to be mutually superposed on an unillustrated intermediate transfer body, to form a full-color toner image. Then, the full-color toner image on the intermediate transfer body is transferred by a single operation to a recording medium, such as ordinary paper or the like.  
         [0029]     Hereafter, where Y, M, C and K are to be distinguished, descriptions will be given with ‘Y’, ‘M’, ‘C’ and/or ‘K’ appended to reference numerals, and where Y, M, C and K are not to be distinguished, ‘Y’, ‘M’, ‘C’ and ‘K’ will be omitted.  
         [0030]     The optical scanning apparatus  10  is structured with a light source  14 , a pre-deflection optical system  16 , a rotating polygon mirror  18  which serves as a deflector, and a scanning optical system  20 . The four laser beam groups LY, LM, LC and LK are emitted from the single light source  14  with spacings therebetween in a sub-scanning direction (to be described below), and the laser beam groups LY, LM, LC and LK are separated and are focused and scanned onto the four photosensitive bodies  12 Y,  12 M,  12 C and  12 K.  
         [0031]     At each photosensitive body  12 , the direction of an axis of rotation is a main scanning direction and the direction of rotation is the sub-scanning direction. Further, a direction of deflective scanning due to rotation of the rotating polygon mirror  18  of the optical scanning apparatus  10  is a direction corresponding to the main scanning direction, and a direction intersecting the deflective scanning direction is a direction corresponding to the sub-scanning direction.  
         [0032]     The light source  14  is a surface emission laser beam array in which  32  light emission points P, in eight rows by four columns, are two-dimensionally arranged in a grid form in the main scanning direction and the sub-scanning direction.  
         [0033]     As shown in  FIG. 3A , at the light source  14 , four light emission point groups PK, PC, PM and PY, each of which is structured by eight light emission points P, are arranged in the sub-scanning direction. Each light emission point group is structured by the eight light emission points P, which are arranged in a straight line which is angled with respect to the main scanning direction and the sub-scanning direction. The light emission point groups PK, PC, PM and PY emit the laser beam groups LK, LC, LM and LY, respectively.  
         [0034]     As shown in  FIG. 3B , each of the laser groups LY, LM, LC and LK is constituted by eight laser beams, and the eight laser beams simultaneously scan the photosensitive body  12  corresponding to the respective color.  
         [0035]     Hereafter, the laser beam groups may be referred to as ‘laser beam groups LY-K’, and may be referred to as ‘laser beam(s) L’.  
         [0036]     As shown in  FIGS. 1 and 2 , the pre-deflection optical system  16  is structured with a coupling lens  22 , an aperture  24  and a cylindrical lens  26 , which are used in common for the four laser beam groups LY-K. The coupling lens  22  is disposed facing the light source  14 . The aperture  24  is provided at a back side focusing position of the coupling lens  22 . The cylindrical lens  26  is disposed with a front side focusing position thereof coinciding with an opening  24 A of the aperture  24 . Here, the cylindrical lens  26  has no power in the main scanning direction and positive power in the sub-scanning direction.  
         [0037]     The laser beam groups LY-K emitted from the light source  14  are condensed by the coupling lens  22 , pass through the opening  24 A of the aperture  24  while being truncated, are condensed by the cylindrical lens  26  only in a direction corresponding to the sub-scanning direction, as shown in  FIG. 4 , and are incident at a deflection surface  18 A of the rotating polygon mirror  18 .  
         [0038]     As shown in  FIG. 1 , the rotating polygon mirror  18  features six of the deflection surface  18 A, and rotates at a speed of 30,000 rpm. Thus, scanning lines are formed at the photosensitive bodies  12 Y,  12 M,  12 C and  12 K. Here, as described earlier, the direction of deflective scanning due to the rotation of the rotating polygon mirror  18  is a direction corresponding to the main scanning direction, and a direction intersecting the deflective scanning direction is a direction corresponding to the sub-scanning direction.  
         [0039]     The scanning optical system  20  is structured by an anamorphic aspherical lens  28 , through which the laser beam groups LY-K pass, a plane mirror group  30 , which serves as a separating section, and toroidal lenses  32 Y,  32 M,  32 C and  32 K, which are provided one for each of the laser beam groups LY-K. The aspherical lens  28  and each toroidal lens  32  together have positive power.  
         [0040]     The aspherical lens  28  is disposed on an optical path of the laser beam groups LY-K which have been deflected by the rotating polygon mirror  18 . A sub-scanning direction focusing length of the aspherical lens  28  is 60 mm, and a distance of the aspherical lens  28  from the deflection surface  18 A is also 60 mm. Hence, the respective laser beam groups LY-K intersect at a back side focusing position of the aspherical lens  28  and are incident at the plane mirror group  30 . Each laser beam L is formed of substantially parallel light.  
         [0041]     The aspherical lens  28  is structured so as to co-operate with the toroidal lenses  32 Y,  32 M,  32 C and  32 K to provide f-θ characteristics with respect to the main scanning direction.  
         [0042]     The plane mirror group  30  is structured by first plane mirrors  34 Y,  34 M,  34 C and  34 K and second plane mirrors  36 Y,  36 M,  36 C and  36 K, which are provided for the laser beam groups LY-K, respectively.  
         [0043]     The first plane mirrors  34 Y,  34 M,  34 C and  34 K reflect the laser beam groups LY-K which are incident at the plane mirror group  30  in negative directions. The second plane mirrors  36 Y,  36 M,  36 C and  36 K reflect the laser beam groups LY-K that have been reflected by the first plane mirrors  34 Y,  34 M,  34 C and  34 K towards the respective photosensitive bodies  12 .  
         [0044]     The first plane mirrors  34 Y,  34 M,  34 C and  34 K are disposed at positions which are separated by 300 mm from the aspherical lens  28 . Sub-scanning direction spacings of the laser beam groups LY-K at these positions are 2.8 mm. Thus, space in which the first plane mirrors  34 Y,  34 M,  34 C and  34 K are to be disposed can be thoroughly assured.  
         [0045]     The toroidal lenses  32 Y,  32 M,  32 C and  32 K focus the respective laser beam groups LY-K that have been reflected by the second plane mirrors  36 Y,  36 M,  36 C and  36 K onto the respective photosensitive bodies  12  with predetermined spacings in the sub-scanning direction. Further, as mentioned above, the toroidal lenses  32 Y,  32 M,  32 C and  32 K feature f-θ characteristics in combination with the aspherical lens  28 .  
         [0046]     As shown in  FIG. 5 , the cylindrical lens  26  is movable in parallel with the direction of an optical axis G. Hence, as is shown in  FIG. 5 , by moving the cylindrical lens  26  in the direction of the optical axis Q it is possible to adjust a magnification ratio of the pre-deflection optical system  16 . In addition, the state shown by solid lines, in which sub-scanning direction incidence angles of the laser beam groups LY-K onto the deflection surface  18 A of the rotating polygon mirror  18  diverge, changes such that main optical axes of the respective laser beam groups LY-K become parallel, as shown by the broken lines. Note that  FIG. 5  is schematically, exaggeratedly illustrated.  
         [0047]     Further, as shown in  FIGS. 1 and 2 , a pitch of the laser beams L can be adjusted by adjusting an angle of the light source  14  about the optical axis G.  
         [0048]     Next, a cylindrical lens-moving mechanism  100 , which moves the cylindrical lens  26  in parallel with the optical axis G direction, will be described.  
         [0049]     As shown in  FIG. 6A , the cylindrical lens  26  is mounted at a lens holder  102 . The lens holder  102  is fitted in, to be movable in the optical axis G direction, at a cylinder-form insertion portion  104 A of a lens holder-mounting member  104 . An arm portion  106  is formed at the lens holder  102 . As shown in  FIG. 6B , the arm portion  106  is pulled on by tension coil springs  108  and  110 , and abuts against a distal end of a screw  112 . The screw  112  is threadingly engaged with an attachment portion  116  of the lens holder-mounting member  104 . Hence, when the screw  112  turns, the distal end of the screw  112  moves along the optical axis G direction, and the arm portion  106  moves accordingly in the optical axis G direction as shown by the arrow Y. With this movement, the mechanism moves the lens holder  102 , and thus the cylindrical lens  26 , in the direction of the optical axis G.  
         [0050]     Next, a light source-turning mechanism  150 , which turns the light source  14  about the optical axis G, will be described.  
         [0051]     As shown in  FIG. 7 , the light source  14  is mounted at a bracket  152 , amd the bracket  152  is mounted, to be rotatable about the optical axis G, at a bracket-mounting member  154 . An arm portion  155  is formed at the bracket  152 . The arm portion  155  is pulled on by a tension coil spring  156 , and abuts against a distal end of a screw  158 . The screw  158  is threadingly engaged with an attachment portion  160 . Hence, when the screw  158  turns, the distal end moves up/down, and the arm portion  155  accordingly moves up/down as shown by the arrow Z. Because of this vertical movement, the mechanism turns (swivels) the bracket  152 , and thus the light source  14 , about the optical axis G.  
         [0052]     Note that the cylindrical lens-moving mechanism  100  and the light source-turning mechanism  150  may have, rather than structures in which the screws  112  and  158  are manually turned, structures in which stepper motors and the like are employed, and may have further different structures. That is, it is sufficient that there be structures which are capable of moving the cylindrical lens  26  in the optical axis G direction and capable of turning the light source  14  about the optical axis G.  
         [0053]     Next, operations of this exemplary embodiment will be described.  
         [0054]     As shown in  FIG. 8 , when a laser beam L is incident on the deflection surface  18 A of the rotating polygon mirror  18  at an incline in the sub-scanning direction, a curved scanning line S will be formed at the respective photosensitive body  12 . Further, a curvature amount varies with this incidence angle.  
         [0055]     Now, with the optical scanning apparatus  10  as in this exemplary embodiment, in which the plural laser beams L emitted from the single light source  14 , at which the light emission points P are two-dimensionally arranged in the grid form, are separated and plural scanning lines S are simultaneously formed at each of the plural photosensitive bodies  12 , as shown in  FIG. 1 , etc., if the sub-scanning direction incidence angles of the laser beam groups LY-K on the deflection surface  18 A of the rotating polygon mirror  18  differ, degrees of curvature of the scanning lines S at each photosensitive body  12  differ and form color offsets. Further, because sub-scanning direction incidence angles on the deflection surface  18 A of the rotating polygon mirror  18  also differ within the plural laser beams L that scan one photosensitive body  12 , degrees of curvature of the scanning lines S that are formed by this plurality of laser beams L differ, as shown in  FIG. 10A , and a pitch between the laser beams L on the photosensitive body  12 , that is, a spacing of scanning lines S 1  to S 4 , varies between central portions and respective end portions. In other words, sub-scanning direction spacings of the scanning lines vary with main scanning direction scanning positions, and a bow difference occurs.  
         [0056]     Note that although there are actually eight of the scanning lines S in this exemplary embodiment, in order to facilitate comprehension, only four scanning lines, scanning line S 1  to scanning line S 4 , are illustrated in  FIGS. 10A and 10B , and descriptions are given in accordance with the drawings. Further, other drawings may also be illustrated with suitable omissions, with descriptions being given in accordance with the drawings.  
         [0057]     Anyway, conventionally, a “tangle error correction optical system” featuring power in a sub-scanning direction has been provided subsequent to scanning deflection (that is, subsequent to the rotating polygon mirror  18 ), and curvature amounts of scanning lines have been made substantially equal by curving a generating line of this correction optical system to moderate a color offset.  
         [0058]     However, such a method cannot correct within plural laser beams that are scanning the same photosensitive body, and portions at which sub-scanning direction spacings of the scanning lines are different would arise (i.e., a bow difference occurs). With a tangle error correction optical system, although a degree of bow difference is moderated, because the deflection surface of the rotating polygon mirror and the photosensitive body which is a scanning-object surface have a conjugative relationship, the bow difference is not reduced to zero. When a magnification ratio (a coupling magnification) of the conjugative relationship is of an enlarging type (i.e., a lateral magnification ratio is greater than 1), curvature amounts of the respective scanning lines differ remarkably, and sub-scanning direction spacings of the scanning lines differ. In such a case, even when a predetermined angle is provided to the light source to adjust a scanning line pitch, as in Japanese Patent Application (JP-A) No. 2004-276532, scanning line spacings will not be equal over the whole surface of the photosensitive body  12  (i.e., the bow difference cannot be eliminated). Consequently, density differences due to pitch variations of the scanning lines occur.  
         [0059]     The main factors behind such occurrences are due to focusing distance errors of the respective optical systems, mechanical mounting accuracies and the like. For example, in this exemplary embodiment, if a curvature of the cylindrical lens  26  differs by 1%, incidence angles on the rotating polygon mirror  18  change by 0.01° and inter-beam spacings change by about 1 μm.  
         [0060]     Accordingly, in this exemplary embodiment, as shown in  FIGS. 5 and 9 , by moving the cylindrical lens  26  in the optical axis G direction to adjust the magnification and adjusting such that the sub-scanning direction incidence angles of the laser beam groups LY-K at the deflection surface  18 A of the rotating polygon mirror  18  are substantially perpendicular with respect to the deflection surface (i.e., such that the laser beam groups LY-K are substantially parallel to one another), the curvature amounts of the scanning lines S are reduced and are made substantially equal. Further, by rotating the light source  14  about the optical axis G, pitch of the scanning lines S is adjusted.  
         [0061]     As a result of such adjustments, as shown in  FIGS. 10A and 10B , bowing of the scanning lines S is substantially eliminated (curvature amounts of the scanning lines S become substantially equal), and sub-scanning direction spacings become substantially equal regardless of main scanning direction scanning positions. That is, the spacings of the scanning lines are substantially equal in the sub-scanning direction over the whole surface of the photosensitive body  12 .  
         [0062]     In other words, “bowing” and “bow differences” are eliminated, and as a result, color shifts and density variations are eliminated.  
         [0063]     Next, an optical scanning apparatus  200  of a second exemplary embodiment of the present invention will be described.  
         [0064]     As shown in  FIG. 11 , the optical scanning apparatus  200  is provided with two light sources  214  and  215 , at which point light sources P are two-dimensionally arranged in grid forms similarly to the first exemplary embodiment (i.e., two light sources are provided). Note that  FIG. 11  is schematically drawn and does not accurately show actual arrangements of the various members.  
         [0065]     Laser beam groups LK and LC, which are constituted by pluralities of laser beams L emitted from the light source  214 , are condensed by a coupling lens  222 , pass through an aperture  224  while being truncated, and are then reflected by a reflection mirror  250 . After being reflected, the laser beam groups LK and LC are condensed by a lens  260  and a cylindrical lens  226  only in a direction corresponding to the sub-scanning direction, are reflected by reflection mirrors  252  and  254 , and are then incident at a deflection surface  218 A of a rotating polygon mirror  218 .  
         [0066]     Similarly, laser beam groups LY and LM, which are constituted by pluralities of laser beams L emitted from the light source  215 , pass through a coupling lens  223  and an aperture  225 , and are then reflected by a reflection mirror  251 . The laser beam groups LY and LM are condensed by a lens  261  and a cylindrical lens  227  only in a direction corresponding to the sub-scanning direction, are reflected by reflection mirrors  253  and  255 , and are then incident at the deflection surface  218 A of the rotating polygon mirror  218 .  
         [0067]     Here, the laser beam groups LK and LC emitted from the light source  214  and the laser beam groups LY and LM emitted from the light source  215  are incident at the same deflection surface  218 A of the same rotating polygon mirror  218 . Hence, after being scanningly deflected by the rotating polygon mirror  218 , the laser beam groups LK, LC, LY and LM pass through an f-θ lens  228 , etc., and are then focused at the respective photosensitive bodies  12 .  
         [0068]     Similarly to the first exemplary embodiment, the light source  214  and light source  215  turn about optical axes thereof, and the cylindrical lenses  226  and  227  move in parallel with the optical axis directions.  
         [0069]     Further, as shown in  FIG. 12 , the reflection mirror  254  can turn about an axis which intersects the sub-scanning direction, and sub-scanning direction incidence angles at which the laser beam groups LK and LC emitted from the light source  214  are incident at the deflection surface  218 A of the rotating polygon mirror  218  can be adjusted. Note that only the laser beam groups LY and LC are illustrated in  FIG. 12 .  
         [0070]     An incidence angle-adjusting mechanism of the reflection mirror  254  is shown in  FIG. 14 . A mounting reference surface  271  is provided at an unillustrated casing body. The reflection mirror  254  is pushed, from a rear face of the reflection mirror  254 , against the mounting reference surface  271  by a spring  272 . An adjustment screw  273  is provided at a lower portion of one side of the mounting reference surface  271 . When the adjustment screw  273  is turned, the adjustment screw  273  pushes a lower portion of the mirror, and the reflection mirror  254  can be altered to a downward-facing angle.  
         [0071]     The incidence angles of the laser beam groups LK and LC on the deflection surface  218 A are altered by this mechanism. Note that this incidence angle-adjusting mechanism is not necessarily limited to the present mode, and could be, for example, a structure for turning a mirror holder in a state in which the reflection mirror  254  is retained at the mirror holder.  
         [0072]     Next, operations of this second exemplary embodiment will be described.  
         [0073]     In this exemplary embodiment, when the reflection mirror  254  is turned, as shown in  FIG. 12 , sub-scanning direction incidence angles at which the laser beam groups LK and LC emitted from the light source  214  are incident on the deflection surface  218 A of the rotating polygon mirror  218  are adjusted, and are made substantially equal to incidence angles of the laser beam groups LY and LM emitted from the light source  215 . As a result, bow differences between the laser beam groups emitted from the light source  214  and the light source  215  can be substantially eliminated.  
         [0074]     Next, an optical scanning apparatus  300  of a third exemplary embodiment will be described.  
         [0075]     As shown in  FIG. 13 , the optical scanning apparatus  300  is provided with two light sources  314  and  315 , at which point light sources P are two-dimensionally arranged in grid forms similarly to the first exemplary embodiment (i.e., two light sources are provided). Note that  FIG. 13  is schematically drawn and does not accurately show actual arrangements of the various members.  
         [0076]     Laser beam groups LK and LC, which are constituted by pluralities of laser beams L emitted from the light source  315 , are condensed by a coupling lens  323  and pass through an aperture  325  while being truncated. The laser beam groups LK and LC are then condensed by a lens  361  and a cylindrical lens  327  only in a direction corresponding to the sub-scanning direction, are reflected by a reflection mirror  355 , and are then incident at deflection surfaces  318 A of a rotating polygon mirror  318 . After being scanningly deflected by the rotating polygon mirror  318 , the laser beam groups LC and LK pass through an f-θ lens  428  or the like, and are then separated between LK and LC by a separation mirror  330 . Thereafter, the laser beam groups LK and LKC are focused at photosensitive bodies  312 K and  312 C, respectively.  
         [0077]     Similarly, laser beam groups LY and LM, which are constituted by pluralities of laser beams L emitted from the light source  314 , are condensed by a coupling lens  322  and pass through an aperture  324  while being truncated. The laser beam groups LY and LM are then condensed by a lens  360  and a cylindrical lens  326  only in a direction corresponding to the sub-scanning direction, are reflected by a reflection mirror  354 , and are then incident at the deflection surfaces  318 A of the rotating polygon mirror  318 . After being scanningly deflected by the rotating polygon mirror  318 , the laser beam groups LY and LM pass through an f-θ lens  328  or the like, and are then separated between LY and LM by another of the separation mirror  330 . Thereafter, the laser beam groups LY and LM are focused at photosensitive bodies  312 Y and  312 M, respectively.  
         [0078]     Now, as can be seen from  FIG. 13 , the laser beam groups LY and LM emitted from the light source  314  are incident at one of the deflection surfaces  318 A, which differs in facing from another of the deflection surfaces  318 A at which the previously described laser beam groups LK and LC emitted from the light source  315  are incident.  
         [0079]     Further, the light source  314  and the light source  315  turn about optical axes thereof. By moving the cylindrical lenses  326  and  327  along the optical axis directions, it is possible to adjust relative differences between sub-scanning direction incidence angles at the deflection surfaces  318 A of the laser beam groups LK and LC and the laser beam groups LY and LM. Furthermore, similarly to the second exemplary embodiment, the reflection mirror  354  can turn about an axis which intersects the sub-scanning direction, and sub-scanning direction incidence angles of the laser beam groups LY and LM emitted from the light source  314  can be adjusted relative to sub-scanning direction incidence angles at which the laser beam groups LK and LC are incident at the deflection surfaces  318 A of the rotating polygon mirror  318  (see  FIG. 12 ).  
         [0080]     The present embodiment implements similar operations to the first exemplary embodiment and the second exemplary embodiment, so descriptions thereof will not be given.