Patent Publication Number: US-7903311-B2

Title: Optical beam scanning apparatus and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from: U.S. provisional application 60/971,538, filed on Sep. 11, 2007, the entire contents of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an optical beam scanning apparatus, an optical beam scanning method and an image forming apparatus including the optical beam scanning apparatus. In particular, the invention relates to an optical beam scanning apparatus that can form plural scanning lines by separating one or plural luminous fluxes, which are emitted from one or plural light sources, in a sub-scanning direction for each of color components using deflection surfaces of a deflecting device and then, imaging the luminous fluxes by a post-deflection optical system and an image forming apparatus including the optical beam scanning apparatus. 
     BACKGROUND 
     In an image forming apparatuses of an electrophotographic system such as a laser printer, a digital copying machine, or a laser facsimile includes an optical beam scanning apparatus that forms an electrostatic latent image on a photoconductive drum by irradiating a laser beam (a luminous flux) on the surface of a photoconductive drum and scanning the laser beam. 
     Recently, besides a monochrome machine including a scanning optical system that uses a single light source, a tandem color machine is proposed. For the tandem color machine, a method of increasing the number of laser beams scanned at a time by providing plural light sources (laser diodes) in one laser unit (a multi-beam method) is proposed for the purpose of realizing an increase in speed of scanning on the surface of a photoconductive drum. In the multi-beam method, plural beams for each of color components (e.g., yellow, magenta, cyan, and black) emitted from the respective light sources are subjected to processing in a pre-deflection optical system and are changed to one beam and made incident on a polygon mirror. The beam deflected by the polygon mirror is, after passing through an fθ lens configuring a post-deflection optical system, separated into beams for each of the color components and irradiated on a photoconductive drum for each of the color components. 
     There is also proposed a color image forming apparatus including an optical beam scanning apparatus that forms plural scanning lines by separating one or plural luminous fluxes, which are emitted from one or plural light sources, for each of color components using plural deflection surfaces having different angles with respect to a rotation center axis of a deflecting device (a polygon mirror) and then, imaging the luminous fluxes by a post-deflection optical system. This optical beam scanning apparatus determines timing for drawing an image in a main scanning direction on photoconductive drums by detecting timing of beams irradiated on the respective photoconductive drums with use of a horizontal synchronizing sensor. Several methods are known as a method of guiding luminous fluxes to the horizontal synchronizing sensor. 
     For example, according to a technique proposed in JP-A-11-218991, a horizontal synchronizing sensor provided in an optical path of a beam for black can detect only the beam for black and, without detecting beams for other colors, draw an image on photoconductive drums after predetermined time subsequent to the detection of the beam for black. According to a technique proposed in JP-A-63-274263, it is possible to guide beams of plural scanning lines to one horizontal synchronizing sensor through an optical fiber and then, set a signal time width of a horizontal synchronizing signal for each of the scanning lines. According to a technique proposed in JP-A-63-273814, it is possible to guide beams of plural scanning lines to one horizontal synchronizing sensor through an optical fiber or guide beams of scanning lines to one horizontal synchronizing sensor by changing an angle of a reflection mirror that guides the beams to the horizontal synchronizing sensor. 
     In the technique proposed in JP-A-11-218991, theoretically, it is possible to adjust phases of laser beams of the respective colors by detecting the beam for black. However, in a situation in which there is an error in accuracy of angles of deflection surfaces of a deflecting device and an error can occur in rotating speed of the deflecting device, it is difficult to appropriately adjust phases of laser beams of the respective colors by detecting only the beam for black. In the technique proposed in JP-A-63-274263, it is necessary to make plural beams incident on the optical fiber. In order to make the plural beams sufficiently incident on the optical fiber, it is necessary to provide an adjusting mechanism for adjusting a position of the optical fiber. In the technique proposed in JP-A-63-274263, even if the plural beams are made incident on the optical fiber, it is impossible to discriminate on which of deflecting surfaces the beams are deflected. 
     Moreover, in the technique proposed in JP-A-63-273814, besides using the optical fiber, it is possible to adjust phases of laser beams of the respective colors by changing an angle of the reflection mirror. However, it is difficult to sufficiently adjust the phases of the laser beams of the respective colors. 
     In this way, in a color image forming apparatus including an optical beam scanning apparatus that separates one or plural luminous fluxes, which are emitted from one or plural light sources, for each of color components using plural deflection surfaces having different angles with respect to a rotation center axis of a deflecting device (a polygon mirror) and then, images the luminous fluxes by a post-deflection optical system, it is impossible to appropriately adjust phases of laser beams of respective colors. As a result, not only color drift occurs but also distortion occurs in an image of a single color. 
     SUMMARY 
     The present invention was devised in view of such circumstances and it is an object of the present invention to provide an optical beam scanning apparatus that can easily and suitably adjust phases of laser beams of respective colors, an optical beam scanning method and an image forming apparatus including the optical beam scanning apparatus. 
     In order to solve the problems, an optical beam scanning apparatus according to an aspect of the present invention includes a light source configured to emit one or plural luminous fluxes, a pre-deflection optical system configured to shapes the luminous fluxes emitted from the light source to image the luminous fluxes as a line image in a direction corresponding to a main scanning direction, a light deflecting device configured to separate the luminous fluxes imaged by the pre-deflection optical system in a sub-scanning direction for each of color components using plural deflection surfaces having different angles with respect to a rotation center axis and scan the luminous fluxes against a scanning object in the main scanning direction, a post-deflection optical system configured to image the luminous fluxes deflected by the light deflecting device on the scanning object, a first sensor configured to detect a part of the luminous fluxes deflected by the light deflecting device, one or plural first optical elements configured to be provided in the post-deflection optical system and act on the luminous fluxes deflected by all the deflection surfaces of the light deflecting device, and a second optical element having positive power in the sub-scanning direction configured to be provided in an optical path between any one of the first optical elements and the first sensor and act on the luminous fluxes deflected by all the deflection surfaces of the light deflecting device. 
     In order to solves the problems, an optical beam scanning method according to another aspect of the present invention includes the steps of preparing an optical beam scanning apparatus including a light deflecting device, a first sensor, one or plural first optical elements and a second optical element, emitting one or plural luminous fluxes, forming the luminous fluxes emitted from the light source to image the luminous fluxes as a line image in a direction corresponding to a main scanning direction, separating the luminous fluxes in a sub-scanning direction for each of color components using plural deflection surfaces having different angles with respect to a rotation center axis and scanning the luminous fluxes against a scanning object in the main scanning direction, by the light deflecting device, detecting, by the first sensor, a part of the luminous fluxes deflected by the light deflecting device, acting on the luminous fluxes deflected by all the deflection surfaces of the light deflecting device, by the first optical element provided in the post-deflection optical system, and acting on the luminous fluxes deflected by all the deflection surfaces of the light deflecting device, by the second optical element having positive power in the sub-scanning direction provided in an optical path between any one of the first optical elements and the first sensor. 
     In order to solves the problems, an image forming apparatus according to another aspect of the present invention is an image forming apparatus including an optical beam scanning apparatus, the optical beam scanning apparatus including a light source configured to emit one or plural luminous fluxes, a pre-deflection optical system configured to form the luminous fluxes emitted from the light source to image the luminous fluxes as a line image in a direction corresponding to a main scanning direction, a light deflecting device configured to separate the luminous fluxes imaged by the pre-deflection optical system in a sub-scanning direction for each of color components using plural deflection surfaces having different angles with respect to a rotation center axis and scan the luminous fluxes against a scanning object in the main scanning direction, a post-deflection optical system configured to image the luminous fluxes deflected by the light deflecting device on the scanning object, a first sensor configured to detect a part of the luminous fluxes deflected by the light deflecting device, one or plural first optical elements configured to be provided in the post-deflection optical system and act on the luminous fluxes deflected by all the deflection surfaces of the light deflecting device, and a second optical element having positive power in the sub-scanning direction configured to be provided in an optical path between any one of the first optical elements and the first sensor and act on the luminous fluxes deflected by all the deflection surfaces of the light deflecting device. 
     In order to solve the problems described above, an optical beam scanning apparatus according to still another aspect of the present invention includes a light source configured to emit one or plural luminous fluxes, a pre-deflection optical system configured to form the luminous fluxes emitted from the light source to image the luminous fluxes as a line image in a direction corresponding to a main scanning direction, a light deflecting device configured to separate the luminous fluxes imaged by the pre-deflection optical system in a sub-scanning direction for each of color components using plural deflection surfaces having different angles with respect to a rotation center axis and scan the luminous fluxes against a scanning object in the main scanning direction, a post-deflection optical system configured to image the luminous fluxes deflected by the light deflecting device, a first sensor configured to detect a part of the luminous fluxes deflected by the light deflecting device, and a first optical element having positive power in the sub-scanning direction configured to be provided in an optical path between the light deflecting device and the first sensor and act on the luminous fluxes deflected by all the deflection surfaces provided in the light deflecting device. 
     In order to solves the problems, an optical beam scanning method according to another aspect of the present invention includes steps of preparing an optical beam scanning apparatus including a light deflecting device, a first sensor and a first optical elements, emitting one or plural luminous fluxes, forming the luminous fluxes emitted from the light source to image the luminous fluxes as a line image in a direction corresponding to a main scanning direction, separating the luminous fluxes in a sub-scanning direction for each of color components using plural deflection surfaces having different angles with respect to a rotation center axis and scanning the luminous fluxes against a scanning object in the main scanning direction, by the light deflecting device, detecting, by the first sensor, a part of the luminous fluxes deflected by the light deflecting device, acting on the luminous fluxes deflected by all the deflection surfaces of the light deflecting device, by the first optical element having positive power in the sub-scanning direction provided in an optical path between the light deflecting device and the first sensor. 
     In order to solves the problems, an image forming apparatus according to another aspect of the present invention is an image forming apparatus including an optical beam scanning apparatus, the optical beam scanning apparatus including a light source configured to emit one or plural luminous fluxes, a pre-deflection optical system configured to form the luminous fluxes emitted from the light source to image the luminous fluxes as a line image in a direction corresponding to a main scanning direction, a light deflecting device configured to separate the luminous fluxes imaged by the pre-deflection optical system in a sub-scanning direction for each of color components using plural deflection surfaces having different angles with respect to a rotation center axis and scan the luminous fluxes against a scanning object in the main scanning direction, a post-deflection optical system configured to image the luminous fluxes deflected by the light deflecting device, a first sensor configured to detect a part of the luminous fluxes deflected by the light deflecting device, and a first optical element having positive power in the sub-scanning direction configured to be provided in an optical path between the light deflecting device and the first sensor and act on the luminous fluxes deflected by all the deflection surfaces provided in the light deflecting device. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings, 
         FIG. 1  is a side view showing a configuration of an image forming apparatus having an optical beam scanning apparatus to which the present invention is applied; 
         FIGS. 2A to 2C  are diagrams showing expansion of reflecting by a reflection mirror provided in the optical beam scanning apparatus; 
         FIG. 3  is an explanatory diagram for explaining a shape of a light blocking plate that blocks optical paths of beams LY, LM, and LC to a surface discrimination sensor arranged on a downstream side of the optical path with respect to optical elements; 
         FIGS. 4A to 4F  are a plan view, a sectional view, and side view of a polygon mirror main body of a deflecting device used in a scanning optical system of the optical beam scanning apparatus; 
         FIG. 5  is a diagram showing a state in which a reflection surface (Y surface) of the polygon mirror that reflects beams LY, LM, LC, and LK tilts in a direction for approaching a rotation axis direction; and 
         FIG. 6  is a diagram showing a detailed configuration of the optical beam scanning apparatus shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of the present invention is explained below with reference to the accompanying drawings. 
       FIG. 1  is a diagram showing a configuration of an image forming apparatus  1  having an optical beam scanning apparatus  3  according to an embodiment of the present invention. In the explanation of this embodiment, the image forming apparatus  1  is applied to a color printer. However, the application of the image forming apparatus  1  is not limited to this. The image forming apparatus  1  can also be applied to various image output apparatuses such as a full-color copying apparatus, a facsimile apparatus, and a workstation apparatus. 
     The image forming apparatus  1  includes the optical beam scanning apparatus (an exposing apparatus)  3  that generates image light corresponding to an image signal and an image forming unit that transfers a toner image visualized by a toner as a developer onto sheet-like paper P as a transfer medium used for output, which is called hard copy or printout, on the basis of the image light supplied by the optical beam scanning apparatus  3  and outputs the toner image. Every time the toner image is formed, the paper P is fed to the image forming unit from a paper holding unit  7  that holds an arbitrary number of sheet-like pieces of paper P having a predetermined size and can feed the pieces of paper P one by one according timing when the toner image is formed in the image forming unit. 
     A conveying path  9  that guides the paper P from the paper holding unit  7  to the image forming unit is provided between the paper holding unit  7  and the image forming unit. The conveying path  9  guides the paper P to a fixing device  11  that fixes, on the paper P, the toner image transferred onto the paper P through a transfer device  9 A that transfers the toner image formed in the image forming apparatus. As another function, the conveying path  9  guides the paper P having the toner image fixed thereon by the fixing device  11  to an image-output holding unit  1   a  also serving as a part of a cover that covers the image forming unit. 
     The image forming unit has an intermediate transfer belt  13  obtained by forming an insulative film having predetermined thickness in an endless belt shape. A belt obtained by forming metal in a thin sheet shape and then, protecting the surface thereof with resin may be applied as the intermediate transfer belt  13 . Predetermined tension is applied to the intermediate transfer belt  13  by a driving roller  15 , a first tension roller  17   a  and a second tension roller  17   b , and a transfer roller  19 . An arbitrary position of the intermediate transfer belt  13  parallel to an axis of the driving roller  15  moves in an arrow A direction when the driving roller  15  is rotated. In other words, a belt surface of the intermediate transfer belt  13  turns in one direction at speed of movement of an outer peripheral surface of the driving roller  15 . 
     First to fourth image forming units  21 Y,  21 M,  21 C, and  21 K are arrayed at predetermined intervals in a section in which the belt surface of the intermediate transfer belt  13  moves substantially flat with the predetermined tension applied thereto by the respective rollers (the driving roller  15 , the first tension roller  17   a  and the second tension roller  17   b , and the transfer roller  19 ). 
     The first to fourth image forming units  21 Y,  21 M,  21 C, and  21 K respectively include at least developing devices  22 Y,  22 M,  22 C, and  22 K in which toners of arbitrary colors of Y (yellow), M (magenta), C (cyan), and BK (black) are stored and photoconductive drums  23 Y,  23 M,  23 C, and  23 K that hold electrostatic latent images developed by the respective developing devices  22  (the developing devices  22 Y,  22 M,  22 C, and  22 K). Electrostatic latent images corresponding to images of colors developed by the developing devices  22 Y,  22 M,  22 C, and  22 K provided in the respective image forming units  21  are formed, by image light from the optical light scanning apparatus  3 , on the surfaces (outer peripheral surfaces) of the photoconductive drums  23 Y,  23 M,  23 C, and  23 K included in the respective image forming units  21 . Consequently, the toners are selectively supplied by any one of the developing devices  22 Y,  22 M,  22 C, and  22 K corresponding to the electrostatic latent images. As a result, toner images of predetermined colors are formed on the photoconductive drums  23 Y,  23 M,  23 C, and  23 K, respectively. 
     In the first to fourth image forming units  21 Y,  21 M,  21 C, and  21 K, transfer rollers  31 Y,  31 M,  31 C, and  31 K for transferring the toner images held by the respective photoconductive drums  23  onto the intermediate transfer belt  13  are respectively provided in positions opposed to the photoconductive drums  23 Y,  23 M,  23 C, and  23 K via the intermediate transfer belt  13 . The transfer rollers  31 Y,  31 M,  31 C, and  31 K are provided on a rear side of the intermediate transfer belt  13 . 
     A not-shown image-signal supplying unit is provided in the image forming apparatus  1  in which the developing devices  22  ( 22 Y,  22 M,  22 C, and  22 K), the photoconductive drums  23  ( 23 Y,  23 M,  23 C, and  23 K), and the transfer roller  31  ( 31 Y,  31 M,  31 C, and  31 K) are arrayed as described above. The image-signal supplying unit supplies an image signal for each of color components to the optical beam scanning apparatus  3 . The optical beam scanning apparatus  3  generates image light corresponding to the image signal supplied from the image-signal supplying unit and irradiates the generated image light on the surfaces of the photoconductive drums  23  ( 23 Y,  23 M,  23 C, and  23 K) integral with the developing devices  22  ( 22 Y,  22 M,  22 C, and  22 K) that holds the toners of the color components corresponding to the image light. At this point, the respective image forming units  21  forms electrostatic latent images at predetermined timing such that the sequentially-transferred toner images are superimposed one on top of another on the intermediate transfer belt  13 . The electrostatic latent images are developed (visualized) by the developing devices  22  corresponding to the image forming units  21 . 
     The toner images formed on the photoconductive drums  23  of the respective image forming units  21  are transferred onto the intermediate transfer belt  13  by the transfer rollers  31  ( 31 Y,  31 M,  31 C, and  31 K) as primary transfer devices corresponding to the respective photoconductive drums  23  ( 23 Y,  23 M,  23 C, and  23 K). At this point, the toner images of Y, M, C, and BK are sequentially stacked on the intermediate transfer belt  13  that moves at predetermined speed. In the case of  FIG. 1 , roller bodies are used as the transfer rollers  31  which are primary transfer devices. However, the transfer rollers  31  are not limited to the roller bodies and may be voltage generating devices such as scorotrons. 
     A secondary transfer roller  71  as a secondary transfer device is provided in the image forming apparatus  1 . The secondary transfer roller  71  comes into contact with the intermediate transfer belt  13  at predetermined pressure in a transfer position  9 A of the conveying path  9 . The secondary transfer roller  71  as the secondary transfer device transfers a full-color toner image formed on the intermediate transfer belt  13  onto the paper P guided to the transfer position  9 A of the conveying path  9 . 
     Registration rollers  61  that temporarily stop the paper P, which is guided from the paper holding unit  7  to the transfer position  9 A, is provided in a predetermined position in the conveying path  9  between the paper holding unit  7  to the transfer position  9 A. The registration rollers  61  include two rollers. At least one roller rotates in a predetermined direction and the other roller is pressed against one roller at predetermined pressure via a not-shown press-contact mechanism. 
     The paper P is guided through the conveying path  9  from the paper holding unit  7  to the transfer position  9 A and temporarily stopped by the registration rollers  61 . This makes it possible to correct a tilt (a tilt of the paper P with respect to a conveying direction) that can occur during conveyance through the conveying path  9  from the paper holding unit  7  to the transfer position  9 A. 
     According to timing when the registration roller  61  is rotated again, timing when a toner image carried to the transfer position  9 A according to the movement of the belt surface of the intermediate transfer belt  13  reaches the transfer position  9 A and timing when the paper P reaches the transfer position  9 A are set. This makes it possible to arbitrarily set a position of the toner image with respect to the paper P and manage the position of the toner image with respect to the paper P. 
       FIGS. 2A to 2C  are diagrams showing expansion of reflecting by the reflection mirror provided in the optical beam scanning apparatus  3 .  FIG. 2A  is a diagram which views  FIG. 2B  from an arrow X direction.  FIG. 2C  is a diagram which views  FIG. 2B  from an arrow Y direction. The optical beam scanning apparatus  3  includes, as shown in  FIGS. 2A to 2C , at least a light source (a semiconductor laser)  33  that outputs image light (exposure light), a deflecting device  35  that scans the image light from the light source  33  in a raster direction for output (hard copy or printout) and guides a beam to the respective photoconductive drums  23  arranged at predetermined intervals in the sub-scanning direction, a post-deflection optical system (an image forming optical system)  37  that focuses the image light, which is raster-deflected (scanned) by the deflecting device  35 , on the photoconductive drums  23  ( 23 Y,  23 M,  23 C, and  23 K) of the first to fourth image forming units  21  under a predetermined condition regardless of a deflection angle, and a pre-deflection optical system (an exposure light shaping optical system)  39  that guides the image light from the light source  33  to the deflecting device  35  under a predetermined condition. 
     A direction in which respective laser beams are deflected (scanned) by the deflecting device  35  (a rotation axis direction of the photoconductive drums  23 ) is defined as “main scanning direction” and a direction perpendicular to the optical axis of the optical system and the main scanning direction is defined as “sub-scanning direction”. Therefore, the sub-scanning direction is a drum rotating direction on the photoconductive drum  23 . 
     The deflecting device  35  includes a polygon mirror main body (a so-called polygon mirror) in which, for example, eight plane reflection surfaces (plane reflection mirrors) are arranged in a regular polygonal shape and a motor that rotates the polygon mirror main body in the main scanning direction at predetermined speed. The polygon mirror main body is a rotatable reflection element and fixed to a shaft of the motor. The number of reflection surfaces provided in the polygon mirror main body as the reflection element and the number of revolutions are set according to requirements of output (i.e., resolution and output speed required of the image forming apparatus  1  and other requirements). The reflection surfaces (polygon mirror surfaces) of the deflecting device  35  have required angles with respect to a rotation center axis of the polygon mirror main body such that a beam can be guided to a scanning line position where electrostatic latent images are formed on the respective photoconductive drums  23 . 
     The post-deflection optical system  37  includes at least a shared lens  37 - 1  used for all scanning lines for forming electrostatic latent images of the respective colors guided to the respective photoconductive drums  23  and an individual lens  37  corresponding to each of the scanning lines for forming electrostatic images of the respective colors guided to the respective photoconductive drums  23 . Different light focusing properties are given to the shared lens  37 - 1  according to positions in a longitudinal direction of the respective photoconductive drums  23 Y,  23 M,  23 C, and  23 K (i.e., positions on the photoconductive drums  23  that depend on swing angles (deflection angles) of image light caused by raster-scanning of the deflecting device  35  in the main scanning direction orthogonal to a direction in which the paper P is conveyed (a direction in which the photoconductive drums  23  are rotated). The shared lens  37 - 1  has a slender shape extending in the longitudinal direction of the photoconductive drums  23 . 
     The post-deflection optical system  37  includes, besides the shared lens  37 - 1  and the individual lens  37 - 2 , various optical elements (e.g., a mirror and a filter) for guiding the image light raster-scanned by the deflecting device  35  to the respective photoconductive drums  23 Y,  23 M,  23 C, and  23 K of the first to fourth image forming units  21 . The shared lens  37 - 1  and the individual lens  37 - 2  may be replaced with mirrors having curved surfaces similar to those of these lenses by optimizing types and shapes of optical elements and combining arrays. The replacement with mirrors may be applied to both the shared lens  37 - 1  and the individual lens  37 - 2  or may be applied to only one of the lenses. 
     A focus position (a focus position on a front side in the sub-scanning direction) of the shared lens  37 - 1  is set further on an upstream side (a side where the rotation center axis of the polygon mirror main body is present; the upstream side may extend beyond the rotation center axis) than the reflection surface (the polygon mirror surface) of the deflecting device  35  such that an inter-beam distance of beams emitted from the shared lens  37 - 1  for generating electrostatic latent images of the respective colors increases toward downstream of the optical paths. 
     The pre-deflection optical system  39  forms the image light from the light source  33  such that the image light is formed in (condensed in) a sectional beam shape that satisfies a predetermined condition when the image light is raster-scanned by the deflecting device  35  and focused in predetermined positions in the longitudinal direction of the respective photoconductive drums  23 Y,  23 M,  23 C, and  23 K in the post-deflection optical system  37 . The pre-deflection optical system  39  includes optical elements such as a condenser lens, a mirror, and an aperture. 
     Predetermined intervals corresponding to positions where the respective image forming units  21  are arrayed (substantially equal intervals on the belt surface of the intermediate transfer belt  13 ) are given to the image light emitted from the optical beam scanning apparatus  3 . Intervals of the image light emitted from the optical beam scanning apparatus  3  are defined to integer times as large as a circumference (a rotation pitch of the driving roller  15 ) obtained by adding up the diameter of the driving roller  15  and the thickness of the intermediate transfer belt  13 . Therefore, even if there is eccentricity or the like in the driving roller  15 , since the same period is given when images are formed in the first to fourth image forming units  21 , it is possible to reduce the influence of the eccentricity such as color drift. 
     The scanning optical system of the optical beam scanning apparatus  3  includes a surface discrimination sensor  43  that outputs a signal only when the beam LK of BK (black) is scanned on the polygon mirror surface and a horizontal synchronizing sensor  44  for determining timing for drawing an image in the main scanning direction. A beam made incident on the horizontal synchronizing sensor  44  passes through the shared lens  37 - 1  and, then, passes through an optical element  51 . The optical element  51  focuses beams passing through the different optical paths on the horizontal synchronizing sensor  44  in the sub-scanning direction while setting heights in the sub-scanning direction of all the optical paths substantially identical on the surface of the horizontal synchronizing sensor  44 . The optical element  51  is a convex cylindrical lens on a surface on one side (a surface on the downstream side of the optical paths) thereof in this embodiment. A light blocking plate  52  is arranged on the upstream side of the optical path of the optical element  51 . As shown in  FIG. 3 , the light blocking plate  52  has a shape for blocking the optical path of the beams LY, LM, and LC to the surface discrimination sensor  43  arranged on the downstream side of the optical paths with respect to the optical element  51 . The light blocking plate  52  causes only the beam LK to pass through the surface discrimination sensor  43  via the optical element  51 . In this embodiment, as shown in  FIG. 3 , the four beams LY, LM, LC, and LK emitted from the shared lens  37 - 1  after being deflected by the deflecting device  35  are arrayed in order of the beams LC, LK, LM, and LY from the upstream side in the sub-scanning direction to the downstream side in the sub-scanning direction. However, the array of the beams LC, LK, LM, and LY is not limited to this. The beams LC, LK, LM, and LY may be arrayed in order of the beams LY, LM, LK, and LC from the upstream side in the sub-scanning direction to the downstream side in the sub-scanning direction. 
     On the other hand, the light blocking plate  52  causes all the beams LY, LM, LC, and LK to pass through, via the optical element  51 , the horizontal synchronizing sensor  44  arranged on the downstream side of the optical paths with respect to the optical element  51 . This makes it possible to suitably adjust, while discriminating the black laser beam among the laser beams of the respective colors guided from the deflecting device  35  via the optical element  51 , phases of the laser beams of the respective colors for each of the laser beams. Further, it is possible to prevent occurrence of color drift even in a situation in which there is an error in accuracy of an angle of the deflection surface of the deflecting device  35  and in which an error is likely to occur in rotating speed of the deflecting device  35 . Moreover, it is possible to prevent occurrence of distortion in an image of a single color. 
     It goes without saying that the light blocking plate  52  may cause any one of the beams LY, LM, and LC to pass through the surface discrimination sensor  43  rather than causing only the beam LK to pass through the surface discrimination sensor  43 . In this embodiment, the optical element  51  is provided in the optical paths between the optical element  37 - 1  that acts on the luminous fluxes deflected by all the deflection surfaces of the deflecting device  35  and the horizontal synchronizing sensor  44 . However, the optical element  51  may be provided in the optical paths between the deflecting device  35  and the horizontal synchronizing sensor  44 . 
       FIGS. 4A to 4F  are a plan view, a sectional view, and side view of the polygon mirror main body of the deflecting device  35  used in the scanning optical system of the optical beam scanning apparatus  3 .  FIG. 4A  is a plan view of the polygon mirror main body of the deflecting device  35 .  FIG. 4B  is a sectional view of the polygon mirror main body of the deflecting device  35 .  FIGS. 4C to 4F  are sides views of the polygon mirror main body of the deflecting device  35  viewed from a predetermined direction. 
     The sectional view of the polygon mirror main body of the deflecting device  35  shown in  FIG. 4B  shows a reference surface in setting a tilt of the reflection surfaces of the polygon mirror main body. A motor is provided on an A side of the reference surface via a not-shown shaft. As shown in  FIGS. 4C to 4F , the reflection surfaces of the polygon mirror main body (the polygon mirror) have required tilts with respect to the rotation center axis (a rotation center axis of the motor, in other words, a hole center axis of the polygon mirror main body). Absolute values of the tilts of the reflection surfaces are maximum and equal at θ 1  and θ 3  and signs of the tilts are set opposite. The tilts have a relation of θ 1 =−θ 3  and have a relation of θ 1 &gt;θ 2 &gt;θ 4 &gt;θ 3  or θ 1 &lt;θ 2 &lt;θ 4 &lt;θ 3 . For example, when a value of θ is a minus numerical value, this means that the reflection surface tilts in a direction closer to a rotation axis direction as the reflection surface is further away from the reference surface A. When a value of θ is a plus numerical value, this means that the reflection surface tilts in the direction closer to the rotation axis direction as the reflection surface is further away from a surface on the opposite side of the reference surface A. Specifically, for example, as shown in  FIG. 5 , the reflection surface (a C surface) that reflects the beam LC of the color component C and the reflection surface (K surface) that reflects the beam LK of the color component K tilt in the direction closer to the rotation axis direction as the reflection surfaces are further away from the surface on the reference surface A. The reflection surface (M surface) that reflects the beam LM of the color component M and the reflection surface (Y surface) that reflects the beam LY of the color component Y tilt in the direction away from the rotation axis direction as the reflection surfaces are further away from the reference surface A. 
     By arranging the reflection surfaces in this way, it is possible to control a maximum value of a tilt angle of the reflection surfaces of the polygon mirror main body to be as small as possible compared with other those in other arrangements. Since deterioration in a focusing characteristic increases as the tilts of the reflection surfaces of the polygon mirror main body increase, it is possible to suitably control the deterioration in the focusing characteristic. 
     In  FIG. 6 , optical paths for guiding beams to the respective photoconductive drums  23 Y,  23 M,  23 C, and  23 K using two reflection mirrors  40  and  41  for the three colors of Y, M, and C and using one reflection mirror  42  for one color of BK are shown. As described above, the focus position (the focus position on the front side in the sub-scanning direction) of the shared lens  37 - 1  is set further on the upstream side (the side where the rotation center axis of the polygon mirror main body is present; the upstream side may extend beyond the rotation center axis) than the reflection surface (the polygon mirror surface) of the deflecting device  35  such that an inter-beam distance of beams emitted from the shared lens  37 - 1  for generating electrostatic latent images of the respective colors increases toward downstream of the optical paths. Consequently, beams further on the upstream side of the optical paths in reflection mirrors  40 Y,  40 M,  40 C, and  42 K for separating, for each of the color components, the beams raster-deflected by the deflecting device  35  have wider intervals in the same position in a beam traveling direction. The four reflection mirrors are arranged in order of the reflection mirrors  40 Y,  40 M,  40 C, and  42 K in order from the upstream side. The intervals in the same position in the beam traveling direction are in a relation of LY−LM&gt;LM−LK&gt;LK−LC. 
     In optical paths of beams reflected by the sets of the two reflection mirrors  40  and  41 , individual lenses  37 - 2  ( 37 - 2 Y,  37 - 2 M, and  37 - 2 C) are arranged between the sets of the two reflection mirrors  40  and  41 , respectively. On the other hand, in an optical path of a beam reflected by the one reflection mirror  42 K, an individual lens  37 - 2 K is arranged after the reflection mirror  42 K is arranged. In this embodiment, the beams LC and LY are beams at both the ends in the sub-scanning direction. The beam LY at one end in the sub-scanning direction is reflected by the reflection mirror  40 Y on the most upstream side. The beam LC at the other end in the sub-scanning direction is reflected by the reflection mirror  40 C second from the most downstream side. The reflection mirror  40 C is chamfered in advance not to block the optical path of the beam L. 
     In this embodiment, in the optical beam scanning apparatus  3  explicitly described above, the individual lens  37 - 2  ( 37 - 2 Y,  37 - 2 M,  37 - 2 C, or  37 - 2 K) as an individual imaging lens is provided in the post-deflection optical system  37  for each of the color components. However, the present invention can also be applied to the optical beam scanning apparatus  3  in which all imaging lenses provided in the post-deflection optical system  37  are shared lenses.