Abstract:
An optical scanning device includes: a light source; a first optical element that converts light emitted from the light source to parallel light; a deflection element that deflects the light in a fast scanning direction to scan a surface of an object to be scanned with the light at a constant speed; a second optical element that guides the light to the deflection element; and a third optical element that focuses the light deflected by the deflection element onto the surface of the object to be scanned, at least one surface among surfaces of the third optical element that intersect the light including a surface form which affects only one of fast scanning direction characteristics or slow scanning direction characteristics at an image plane.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2006-223085 filed Aug. 18, 2006. 
       BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present invention relates to an optical scanning device and an image formation apparatus, and more particularly relates to an optical scanning device which deflects light beams emitted from plural light sources with a deflection element to carry out scanning exposure and an image formation apparatus including the optical scanning device. 
         [0004]    2. Related Art 
         [0005]    In recent years, multi-color production of a document has progressed and attempts have been made to improve the productivity of color imaging at an image formation apparatus. A color laser printer which uses plural photoreceptors to improve the productivity of color imaging has come onto the market. 
         [0006]    In an exposure device which is used in the image formation apparatus that utilizes plural photoreceptors, a system is used in which plural scanning devices corresponding to the respective photoreceptors are arranged in a row. However, in order to reduce size, reduce number of components and further reduce cost, a system in which plural beams are deflected by a single deflector to scan the plural photoreceptors has been proposed. 
         [0007]    As a scanning optical system for forming electrostatic latent images on photosensitive surfaces of respective photosensitive drums, there is a system in which polygon mirrors and image focusing optical systems are provided one-to-one for the respective photosensitive drums. However, providing four sets of polygon mirrors and image focusing optical systems is problematic in terms of cost. Therefore, in recent years there has been a scanning optical system in which a single polygon mirror is utilized in common and plural laser beam fluxes are simultaneously scanned therewith, and thereafter the laser beam fluxes are respectively incident at individually corresponding focusing optical systems and are guided to the respective photosensitive drums. 
         [0008]    To respectively separately illuminate the plural light beams onto plural scanned surfaces, it is necessary to separate the plural light beams after deflective reflection by the polygon mirror, and for light sources with the same wavelength, spatial separation is necessary. A required spatial separation can be achieved by, for example, causing the light beam to be incident on a deflection surface (a reflection surface) of the polygon mirror from an oblique angle in a slow scanning plane. However, in a scanning optical device of which the optical structure is compact, because light path length for spatial separation is short, the oblique incidence angle on the reflection surface is large. Consequently, problems arise in that a scanning line on the scanned surface curves and image focusing performance deteriorates. 
       SUMMARY 
       [0009]    In an aspect of the present invention, an optical scanning device includes: a light source; a first optical element that converts light emitted from the light source to parallel light; a deflection element that deflects the light in a fast scanning direction to scan a surface of an object to be scanned with the light at a constant speed; a second optical element that guides the light to the deflection element; and a third optical element that focuses the light deflected by the deflection element onto the surface of the object to be scanned, at least one surface among surfaces of the third optical element that intersect the light including a surface form which affects only one of fast scanning direction characteristics or slow scanning direction characteristics at an image plane. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    An exemplary embodiment of the invention will be described in detail with reference to the following figures, wherein: 
           [0011]      FIG. 1  is a view showing an image formation apparatus which is equipped with an optical scanning device relating to the present invention. 
           [0012]      FIG. 2  is a perspective view showing the interior of the optical scanning device relating to the present invention. 
           [0013]      FIGS. 3A and 3B  are views showing arrangements of components and light paths of the optical scanning device relating to the present invention. 
           [0014]      FIGS. 4A and 4B  are expanded views showing the light paths of the optical scanning device relating to the present invention. 
           [0015]      FIG. 5  is a view showing a common f-θ lens of the optical scanning device relating to the present invention. 
           [0016]      FIG. 6  is a view showing an individual f-θ lens of the optical scanning device relating to the present invention. 
           [0017]      FIG. 7  is a graph showing forms of scanning lines of different colors at the optical scanning device relating to the present invention. 
           [0018]      FIG. 8  is a graph showing magnification shifts of the different colors at the optical scanning device relating to the present invention. 
           [0019]      FIG. 9  is a graph showing image plane curvatures separately for sagittal and tangential directions at the optical scanning device relating to the present invention. 
           [0020]      FIGS. 10A ,  10 B and  10 C are graphs showing variations in characteristics in the fast and slow scanning directions when a variable C 0  of an S 2  surface of the individual f-θ lens is varied in the optical scanning device relating to the present invention. 
           [0021]      FIGS. 11A ,  11 B and  11 C are graphs showing variations in characteristics in the fast and slow scanning directions when a variable A 2  of the S 2  surface of the individual f-θ lens is varied in the optical scanning device relating to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    —Basic Structure— 
         [0023]      FIG. 1  shows an image formation apparatus which is equipped with an optical scanning device relating to an exemplary embodiment of the present invention. 
         [0024]    For example,  FIG. 1  shows a full-color laser printer provided with the optical scanning device relating to the exemplary embodiment of the present invention. 
         [0025]    This image formation apparatus  10 , as shown in  FIG. 1 , is structured with main portions thereof being developing devices  30 Y to  30 K for yellow (Y), magenta (M), cyan (C) and black (K), which include respective photoreceptor drums  32 Y to  32 K, charging rollers for primary charging which contact against the photoreceptor drums  32 Y to  32 K, and an ROS (raster output scanner)  20  which emits laser beams  31  Y to  31 K for the colors yellow (Y), magenta (M), cyan (C) and black (K). 
         [0026]    The photoreceptor drums  32 Y,  32 M,  32 C and  32 K are disposed with a fixed spacing therebetween so as to have a common tangential plane, to form, what is called, a tandem-type color printer. Signals corresponding to image information for the respective colors are rasterized at an unillustrated image processing unit and inputted to the ROS  20 . In a laser optical unit, the laser beams for the respective colors yellow (Y), magenta (M), cyan (C) and black (K) are modulated, and are irradiated at the photoreceptor drums  32 Y to  32 K of the corresponding colors. 
         [0027]    At the above-mentioned photoreceptor drums  32 Y to  32 K, image formation processes for the respective colors are carried out with a well-known electrophotography system. Firstly, photoreceptor drums which use, for example, OPC photoreceptors are used as the photoreceptor drums  32 Y to  32 K, and these photoreceptor drums  32 Y to  32 K are driven to rotate. DC voltages are applied to surfaces of the photoreceptor drums  32 Y to  32 K by the charging rollers, and thus the surfaces are charged to around, for example, −300 V. 
         [0028]    The laser beams  31 Y to  31 K corresponding to the colors yellow (Y), magenta (M), cyan (C) and black (K) are irradiated by the ROS  20 , which serves as an exposure device, onto the surfaces of the photoreceptor drums  32 Y to  32 K to which the surface potentials have been applied, and electrostatic latent images corresponding to the inputted image information for the respective colors are formed. The laser beams  31 Y to  31 K are emitted by the ROS  20  and write the images. Thus, the surface potentials of image exposure portions of the photoreceptor drums  32 Y to  32 K are discharged at image line portions, that is, exposed areas, and the electrostatic latent images are formed. 
         [0029]    Then, the electrostatic latent images corresponding to the colors yellow (Y), magenta (M), cyan (C) and black (K) which have been formed at the surfaces of the photoreceptor drums  32 Y to  32 K are developed by the developing devices  30 Y to  30 K of the corresponding colors. Thus, toner images of the colors yellow (Y), magenta (M), cyan (C) and black (K) are developed on the photoreceptor drums  32 Y to  32 K, rendering the images visible. 
         [0030]    Developing agents formed of carriers and toners of the respective different colors yellow (Y), magenta (M), cyan (C) and black (K) are filled in the respective developing devices  30 Y to  30 K. These developing devices  30 Y to  30 K are supplied with toner from unillustrated toner supply devices, and the supplied toners are thoroughly agitated with the carrier by augers inside the developing devices  30 Y to  30 K and charged up by friction. 
         [0031]    The toners, which are agitated with the carrier, and electrostatically charged by friction and supplied onto developing rollers  33 , form magnetic brushes structured of the carriers and the toners, due to magnetism of magnetic rollers, and these magnetic brushes touch against the photoreceptor drums  32 Y to  32 K. A developing bias voltage is applied to the developing rollers  33  and the toners on the developing rollers  33  are transferred to the electrostatic latent images formed on the photoreceptor drums  32 Y to  32 K. Thus, the toner images of the colors yellow (Y), magenta (M), cyan (C) and black (K) are formed. 
         [0032]    Then, positioning over paper P of the toner images of the colors yellow (Y), magenta (M), cyan (C) and black (K) that have been formed on the developing devices  30 Y to  30 K is implemented, and the toner images are respectively superposingly transferred. Thus, a final full-color toner image in which the colors cyan (C), magenta (M) and black (K) are respectively superposed on a monochrome Y image is formed as a four-color superposed image. 
         [0033]    Finally, the full-color toner image of yellow (Y), magenta (M), cyan (C) and black (K) that has been formed on the paper P is heated and fused by a fixing device  34  and is fixed onto the paper P, and the image formation processing sequence ends. 
         [0034]    —Optical Scanning Device— 
         [0035]      FIG. 2  shows an optical scanning device relating to the present embodiment, and  FIGS. 3A and 3B  show light paths of the optical scanning device. 
         [0036]    As shown in  FIG. 2 , at the ROS  20 , which is the optical scanning device, the laser beams  31  are emitted from respective light sources  21  for the four colors Y to K. The laser beams  31  are made to be parallel fluxes by collimator lenses  22 , are focused in a slow scanning direction in a line by a cylindrical lens, and are deflected in a fast scanning direction by a polygon mirror  23 . 
         [0037]    As a method for incidence of the beams onto the polygon mirror  23 , tangential offset incidence, in which plural beams are provided with different angles in the fast scanning direction, sagittal offset incidence, in which plural beams are incident at respectively different angles in the slow scanning direction, and the like can be considered. In the case of the present embodiment, the laser beams  31  of the respective colors that are incident at a reflection surface  23 A have respectively predetermined angles in the slow scanning direction (a vertical direction in the drawing), and are incident with offsets from one another in a sagittal direction. Thus, a size of the reflection surface  23 A in the slow scanning direction can be made smaller. 
         [0038]    However, as mentioned earlier, for light sources with the same wavelength, spatial separation is required in order to guide the light beams from the corresponding light sources for the respective colors to the photoreceptor drums  32 Y to  32 K. The required spatial separation can be achieved if, for example, the laser beams  31  are caused to be incident on the reflection surface  23 A from directions oblique in a slow scanning sectional plane. However, if the ROS  20  is reduced in size, light path lengths for spatial separation are shorter, and therefore the oblique incidence angles at the reflection surface  23 A are larger. Consequently, problems arise in that scanning lines on scanned surfaces are curved and imaging performance is adversely affected. In order to counter this, with the present embodiment, surface forms of common and individual f-θ lenses are specified as will be described later. 
         [0039]    The laser beams  31 , that have been deflected by the polygon mirror  23  are incident at a common f-θ lens  24 , are divided in the slow scanning direction into two-color sets, and are incident at first mirrors  25 A and  25 B. That is, the laser beams  31  Y and  31  M for yellow (Y) and magenta (M) are incident at a first mirror  25 A, and the laser beams  31 C and  31 K for cyan (C) and black (K) are incident at a first mirror  25 B. 
         [0040]    The laser beams  31  are further divided into one-color sets in the slow scanning direction after the first mirrors  25 A and  25 B, and are incident at second mirrors  26 Y,  26 M,  26 C,  26 K. That is, as shown in  FIG. 3A , the laser beams  31 Y and  31 M are incident at second mirrors  26 Y and  26 M, respectively, and the laser beams  31 C and  31 K are incident at second mirrors  26 C and  26 K, respectively. The laser beams  31 Y and  31 K, which are closer to two ends in the slow scanning direction, are simply reflected at the second mirrors  26 Y and  26 K. Then, the laser beams  31 Y and  31 K are incident at individual f-θ lenses  28 Y and  28 K and are focused as scanning lines  29 Y and  29 K. 
         [0041]    Meanwhile, the laser beams  31 M and  31 C are incident on and reflected at third mirrors  27 M and  27 C, respectively, are incident at individual f-θ lenses  28 M and  28 C, and are focused as scanning lines  29 M and  29 C. 
         [0042]    Here, for the individual f-θ lenses  28 Y to  28 K mentioned above, rather than four lenses with individual shapes being used, a feature of the present exemplary embodiment being a structure in which light paths are symmetrical in the slow scanning direction is utilized, and sets of two individual f-θ lenses which have the same forms are used. 
         [0043]    That is, the individual f-θ lenses  28 Y and  28 K are lenses which have the same forms but which differ in arrangement position and orientation, and the individual f-θ lenses  28 M and  28 C are lenses which have the same forms but which differ in arrangement position and orientation. Thus, for the individual f-θ lenses  28 , it is sufficient to provide two kinds of lenses in sets of two for the whole apparatus, and therefore component numbers can be reduced and cost can be lowered. 
         [0044]    Furthermore, molded products from plastic mold are used for the common f-θ lens  24 , the individual f-θ lenses  28  and suchlike. Thus, there are advantages in a reduction of a number of parts (f-θ lenses and cylinder mirrors) in the scanning optical system, a reduction in thickness of the polygon mirror  23 , a reduction in component costs, an improvement in a degree of freedom of layout of the optical elements, and so forth. 
         [0045]    —Light Path and Surface Form— 
         [0046]      FIGS. 4A and 4B  show expanded views of light paths of the optical scanning device relating to the present embodiment. 
         [0047]    As shown in  FIG. 4A , in the present embodiment, the light paths of the four colors coincide in the slow scanning direction at the polygon mirror, and the individual f-θ lenses are provided for the respective colors as a final f-θ lens. 
         [0048]    As a consequence, the occurrence of curvature of the scanning lines (bowing) is unavoidable. In a “tandem-type” color printer as for the present embodiment, it is necessary to reduce the number of lenses in order to reduce size and lower cost of the optical scanning device. Therefore, functionality that is required from each lens is greater. 
         [0049]    However, as mentioned earlier, if a complex surface form in order to produce desirable characteristics in the fast scanning direction and a complex surface form in order to produce desirable characteristics in the slow scanning direction are applied to the same surface, independently modifying of the fast scanning direction characteristics and the slow scanning direction characteristics at a time of f-θ lens mold modification or the like in order to push initial performance, is difficult. 
         [0050]    If a surface with a form such that fast-scanning characteristics (beam diameter, magnification and the like) and slow scanning characteristics (beam diameter, bow correction and the like) are corrected at the same surface is used at an f-θ lens, the characteristics cannot be modified independently for fast scanning and slow scanning by modifications during making of the lens (molding conditions, mold modification and the like), and establishing lens performance is difficult. For example, in a case of unsatisfactory slow scanning characteristics and satisfactory fast scanning characteristics, if the slow scanning characteristics are modified, fast scanning characteristics will deteriorate. 
         [0051]    That is, if fast scanning direction characteristics are adjusted, slow scanning direction characteristics will also be affected thereby, and if slow scanning direction characteristics are adjusted, fast scanning direction characteristics will also be affected thereby. Therefore, it is difficult to satisfactorily adjust capabilities for both. 
         [0052]    Accordingly, with the present embodiment, a surface form which does not feature characteristics that will affect fast scanning direction performance is applied to a surface for satisfying slow scanning direction characteristics (for example, as mentioned earlier, slow scanning direction image plane curvature correction and scanning line curvature correction in a case of sagittal offsetting), and a surface form which does not feature characteristics that will affect slow scanning direction performance is applied to a surface for satisfying fast scanning direction characteristics (linearity correction and fast scanning direction image plane curvature correction), which will be mentioned later. Thus, it is possible to implement pushing of performance of the lenses independently for the fast scanning direction and the slow scanning direction. 
         [0053]    —Common f-θ Lens— 
         [0054]      FIG. 5  shows a common f-θ lens of an optical scanning device relating to the present embodiment. 
         [0055]    As shown in  FIG. 5 , in the present embodiment, in the common f-θ lens  24 , if a surface of at which the laser beam  31  is incident is S 1  and a surface of from which the laser beam  31  is emitted is S 2 , the incidence surface S 1  is an anamorphic aspherical surface, and the emission surface S 2  is a y toric surface. 
         [0056]    Now, in the emission surface S 2  which is a y toric surface, curvature in an x direction, that is, the slow scanning direction, is always constant, and the emission surface S 2  has a surface form which is made by rotating a form represented by z(y) mentioned below about a y axis. 
         [0057]    That is, if 
         [0058]    CUY is a fast scanning direction curvature at an optical axis origin, 
         [0059]    K is a conic constant, and 
         [0060]    A, B, C and D are higher-order coefficients in the y-axis direction, 
         [0061]    then the emission surface S 2  of the common f-θ lens  24  is represented by the equation: 
         [0000]    
       
         
           
             
               z 
                
               
                 ( 
                 y 
                 ) 
               
             
             = 
             
               
                 
                   CUY 
                   · 
                   
                     y 
                     2 
                   
                 
                 
                   1 
                   + 
                   
                     
                       1 
                       - 
                       
                         
                           ( 
                           
                             1 
                             + 
                             k 
                           
                           ) 
                         
                          
                         
                           c 
                           2 
                         
                          
                         
                           y 
                           2 
                         
                       
                     
                   
                 
               
               + 
               
                 Ay 
                 4 
               
               + 
               
                 By 
                 6 
               
               + 
               
                 Cy 
                 8 
               
               + 
               
                 Dy 
                 10 
               
             
           
         
       
     
         [0062]    Furthermore, if 
         [0063]    CUX is a slow scanning direction curvature at the optical axis origin, 
         [0064]    CUY is the fast scanning direction curvature at the optical axis origin, 
         [0065]    Kx is a conic constant in the slow scanning direction 
         [0066]    Ky is a conic constant in the fast scanning direction 
         [0067]    AR, BR, CR and DR are even-order coefficients of rotational symmetry, 
         [0068]    AP, BP, CP and DP are odd-order coefficients of rotational symmetry, and 
         [0069]    C 0  is a slow scanning direction radius of curvature at the optical axis origin, 
         [0070]    then the incidence surface S 1  of the common f-θ lens  24  is represented by the equation: 
         [0000]    
       
         
           
             z 
             = 
             
               
                 
                   CUX 
                   · 
                   
                     x 
                     2 
                   
                 
                 + 
                 
                   CUY 
                   · 
                   
                     y 
                     2 
                   
                 
               
               
                 
                   
                     
                       1 
                       + 
                       
                         
                           1 
                           - 
                           
                             
                               ( 
                               
                                 1 
                                 + 
                                 kx 
                               
                               ) 
                             
                             · 
                             
                               CUX 
                               2 
                             
                             · 
                             
                               x 
                               2 
                             
                           
                           - 
                           
                             
                               ( 
                               
                                 1 
                                 + 
                                 ky 
                               
                               ) 
                             
                             · 
                             
                               CUY 
                               2 
                             
                             · 
                             
                               y 
                               2 
                             
                           
                         
                       
                       + 
                     
                   
                 
                 
                   
                     
                       
                         AR 
                          
                         
                           
                             { 
                             
                               
                                 
                                   ( 
                                   
                                     1 
                                     - 
                                     AP 
                                   
                                   ) 
                                 
                                  
                                 
                                   x 
                                   2 
                                 
                               
                               + 
                               
                                 
                                   ( 
                                   
                                     1 
                                     + 
                                     AP 
                                   
                                   ) 
                                 
                                  
                                 
                                   y 
                                   2 
                                 
                               
                             
                             } 
                           
                           2 
                         
                       
                       + 
                       
                         BR 
                          
                         
                           
                             { 
                             
                               
                                 
                                   ( 
                                   
                                     1 
                                     - 
                                     BP 
                                   
                                   ) 
                                 
                                  
                                 
                                   x 
                                   2 
                                 
                               
                               + 
                               
                                 
                                   ( 
                                   
                                     1 
                                     + 
                                     BP 
                                   
                                   ) 
                                 
                                  
                                 
                                   y 
                                   2 
                                 
                               
                             
                             } 
                           
                           3 
                         
                       
                       + 
                     
                   
                 
                 
                   
                     
                       
                         CR 
                          
                         
                           
                             { 
                             
                               
                                 
                                   ( 
                                   
                                     1 
                                     - 
                                     CP 
                                   
                                   ) 
                                 
                                  
                                 
                                   x 
                                   2 
                                 
                               
                               + 
                               
                                 
                                   ( 
                                   
                                     1 
                                     + 
                                     CP 
                                   
                                   ) 
                                 
                                  
                                 
                                   y 
                                   2 
                                 
                               
                             
                             } 
                           
                           4 
                         
                       
                       + 
                       
                         DR 
                          
                         
                           
                             { 
                             
                               
                                 
                                   ( 
                                   
                                     1 
                                     - 
                                     DP 
                                   
                                   ) 
                                 
                                  
                                 
                                   x 
                                   2 
                                 
                               
                               + 
                               
                                 
                                   ( 
                                   
                                     1 
                                     + 
                                     DP 
                                   
                                   ) 
                                 
                                  
                                 
                                   y 
                                   2 
                                 
                               
                             
                             } 
                           
                           5 
                         
                       
                     
                   
                 
               
             
           
         
       
     
         [0071]    —Individual f-θ Lens— 
         [0072]      FIG. 6  shows an individual f-θ lens of an optical scanning device relating to the present embodiment. 
         [0073]    As shown in  FIG. 6 , in the present embodiment, in the individual f-θ lens  28 , if a surface at which the laser beam  31  is incident is S 1  and a surface from which the laser beam  31  is emitted is S 2 , the incidence surface S 1  is a y toric surface, and the emission surface S 2  is a surface which is made by linking circular arcs which have curvature radii R(y) determined for positions y in the fast scanning direction with a generatrix produced by x1(y) in the x-y plane serving as peak points. This emission surface S 2  is a surface at which the generatrix curves and the curvature in the slow scanning direction varies along the fast scanning direction. 
         [0074]    The incidence surface S 1  of the individual f-θ lens  28  is defined by an equation the same as for the above-described emission surface S 2  of the common f-θ lens  24 . 
         [0075]    On the other hand, the emission surface S 2  of the individual f-θ lens  28  is described by the equations: 
         [0000]    
       
         
           
             
               
                 
                   ( 
                   
                     x 
                     - 
                     
                       
                         x 
                         1 
                       
                        
                       
                         ( 
                         y 
                         ) 
                       
                     
                   
                   ) 
                 
                 2 
               
               + 
               
                 
                   ( 
                   
                     z 
                     - 
                     
                       R 
                        
                       
                         ( 
                         y 
                         ) 
                       
                     
                   
                   ) 
                 
                 2 
               
             
             = 
             
               
                 R 
                  
                 
                   ( 
                   y 
                   ) 
                 
               
               2 
             
           
         
       
       
         
           
             
               
                 x 
                 1 
               
                
               
                 ( 
                 y 
                 ) 
               
             
             = 
             
               
                 x 
                 0 
               
               + 
               
                 
                   ∑ 
                   
                     n 
                     = 
                     1 
                   
                   
                     2 
                      
                     n 
                   
                 
                  
                 
                   
                     A 
                     
                       2 
                        
                       n 
                     
                   
                    
                   
                     y 
                     
                       2 
                        
                       n 
                     
                   
                 
               
             
           
         
       
       
         
           
             
               R 
                
               
                 ( 
                 y 
                 ) 
               
             
             = 
             
               
                 C 
                 0 
               
               + 
               
                 
                   ∑ 
                   
                     n 
                     = 
                     1 
                   
                   
                     2 
                      
                     n 
                   
                 
                  
                 
                   
                     B 
                     
                       2 
                        
                       n 
                     
                   
                    
                   
                     y 
                     
                       2 
                        
                       n 
                     
                   
                 
               
             
           
         
       
     
         [0076]    C 0  is a slow scanning direction radius of curvature at the optical axis origin, B 2n , is a higher-order coefficient, with respect to a fast scanning direction, of a slow scanning direction radius of curvature, X 0 , A 2n  each is a form of generatrix in a slow scanning direction. 
         [0077]    C 0 , B 2n , X 0 , A 2n  are handled as variables in designing. After the desired characteristics are obtained, these become constants which express surface form. 
         [0078]    If the variables C 0 , B 2n , X 0 , A 2n , etc. used in these equations are altered, they will have no effect at all on fast scanning direction characteristics, as described below. 
         [0079]    Because the surface form of the emission surface S 2  is prescribed as described above, situations in which slow scanning direction performance is affected when fast scanning direction characteristics are adjusted, or fast scanning direction performance is affected when slow scanning direction characteristics are adjusted can be prevented. 
         [0080]    That is, a surface form which does not provide characteristics that will affect fast scanning direction performance is applied to a surface for satisfying slow scanning direction characteristics (for example, as mentioned earlier, slow scanning direction image plane curvature correction and scanning line curvature correction at a time of sagittal offsetting), which is to say the emission surface S 2  of the individual f-θ lens. Conversely, a surface form which does not provide characteristics that will affect slow scanning direction performance is applied to a surface for satisfying fast scanning direction characteristics (linearity correction and fast scanning direction image plane curvature correction). Thus, it is possible to implement pushing of capabilities of the lenses independently for the fast scanning direction and the slow scanning direction. 
         [0081]    —Lens Characteristics— 
         [0082]      FIGS. 7 to 11C  show design performances of color registration characteristics and imaging characteristics of the optical scanning device relating to the present embodiment. 
         [0083]    As shown in  FIG. 3A , in the optical scanning device relating to the present embodiment, pairs of the individual f-θ lenses  28  with respectively different forms are used for the outer side two colors (the colors Y and K) and the inner side two colors (the colors M and C), and various characteristics principally differ between these two systems. 
         [0084]    With a center in the fast scanning direction being 0, shapes of scanning lines of the laser beams  31  are shown in  FIG. 7  and linearities (magnification shifts) are shown in  FIG. 8 . 
         [0085]    As shown in  FIG. 7 , the shapes of the laser beams  31  are substantially flat for both the outer side two colors (the colors Y and K) and the inner side two colors (the colors M and C), and offset between scanning lines of those colors over the whole of a scanning region is kept to below a few μm. With regard to inclinations of the scanning lines, the scanning lines of the respective colors can be made to respectively coincide by rotation adjustment of the individual lenses in a plane which is perpendicular to the optical axes. 
         [0086]    As shown in  FIG. 8 , there are substantially no differences in magnification variation characteristics between the outer side two colors (the colors Y and K) and the inner side two colors (the colors M and C), and these can similarly be kept to below a few microns over the whole of the scanning region. 
         [0087]      FIG. 9  shows an image plane curvature characteristic with the center in the fast scanning direction being 0. 
         [0088]    As shown in  FIG. 9 , defocus values for all the colors are kept to within 1.0 mm peak-to-peak, and the image plane curvature characteristic is excellently corrected. 
         [0089]    Accordingly, in order to adjust characteristics in a sagittal direction while maintaining characteristics in a meridional direction, a structure of the present embodiment is used, that is, a surface form which does not feature characteristics that will affect fast scanning direction performance is applied to a surface for satisfying slow scanning direction characteristics (for example, correction of beam diameter in the sagittal direction). Thus, it is possible to correct an image plane curvature characteristic without unpreferably affecting characteristics in the meridional direction. 
         [0090]    For example, as shown in  FIGS. 10A to 10C , the variable C 0  of the S 2  surface of the individual f-θ lens  28  relating to the present embodiment, that is, the slow scanning direction radius of curvature at the optical axis origin, is altered by −5% to +5%, and fast scanning direction characteristics (linearities and fast scanning image planes) and slow scanning direction image plane positions are compared for these cases. Here, even though slow scanning direction image plane position is shifted as shown in  FIG. 10C , the fast scanning direction characteristics hardly change at all, as shown in  FIGS. 10A and 10B . Thus, it is possible to move slow scanning direction image plane position without affecting fast scanning characteristics. 
         [0091]    Alternatively, as shown in  FIGS. 11A to 11C , the variable A 2  of the S 2  surface of the individual f-θ lens  28  relating to the present embodiment, that is, a coefficient which determines the form of the generatrix, is altered by −5% to +5%, and fast scanning direction characteristics (linearities and fast scanning image planes) and scanning line forms are compared for these cases. Here, even though the scanning line form is changed as shown in  FIG. 11C , the fast scanning direction characteristics hardly change at all, as shown in  FIGS. 11A and 11B . Thus, it is possible to adjust the scanning line form without affecting fast scanning characteristics. 
         [0092]    —Concluding Remarks— 
         [0093]    In the present embodiment as described above, a surface form which does not feature characteristics that will affect fast scanning direction performance is applied to a surface for satisfying slow scanning direction characteristics (for example, slow scanning direction image plane curvature correction and scanning line curvature correction in a case of sagittal offsetting), and a surface form which does not feature characteristics that will affect slow scanning direction performance is applied to a surface for satisfying fast scanning direction characteristics (linearity correction and fast scanning direction image plane curvature correction). Therefore, it is possible to adjust characteristics of the lenses independently for the fast scanning direction and the slow scanning direction, and to pursue optical performance. 
         [0094]    —Other Remarks— 
         [0095]    An exemplary embodiment of the present invention has been described hereabove, but the present invention is not in any way limited to the example described above, and obviously various embodiments are possible within a scope not departing from the spirit of the present invention. 
         [0096]    That is, although the present exemplary embodiment is applied to a tandem-type full-color image formation apparatus, this is not a limitation. Obviously, for example, single-color monochrome image formation apparatus, image formation apparatus of three colors or less, and the like may also be used.