Patent Publication Number: US-2012026267-A1

Title: Optical scanning device and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2010-173780 filed in Japan on Aug. 2, 2010, the entire contents of which are herein by incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to optical scanning devices, and relates to image forming apparatuses provided with an optical scanning device. 
     2. Description of the Related Art 
     Conventionally, optical scanning devices are used in image forming apparatuses to expose a photosensitive body. In these optical scanning devices, a beam that is outputted from a light source is converted to a parallel beam by a collimator, and then the beam is shaped by an aperture. The size of the opening of the aperture is determined by the focal point distance of the lens that converges the beam onto the photosensitive body and the beam diameter on the surface of the photosensitive body. Here, when the size of the opening of the aperture is made small, the beam is greatly blocked. Along with this, to maintain the light amount of the attenuated beam, sometimes a high output light source or other measure is used. 
     Furthermore, in regard to the optical scanning device, to correct the beam pitch displacement produced by installation displacement of the light source, techniques have been considered of enabling a cylindrical lens to move (for example, see JP 2009-210760A). 
     Furthermore, techniques have been considered in which control of the light amount is carried out by feeding back the light from the light source (for example, see JP 2006-91157A). 
     In conventional image forming apparatuses, there is a limit to increasing the output of the light source, and therefore there is a problem in that the light amount for exposure cannot be maintained. 
     Further still, for the technique disclosed in JP 2009-210760A, there is no description regarding how to maintain the light amount of an attenuated beam using a diaphragm (aperture), and the above-described problem cannot be addressed. 
     Furthermore, the optical scanning device described in JP 2006-91157A carries out control of the light amount by detecting the light amount of the beam after it has been shaped by the aperture. And the output of the light source is increased to maintain the light amount that has been attenuated by the aperture. With the optical scanning device described in JP 2006-91157A, it is unavoidable that the output of the light source is increased. 
     SUMMARY OF THE INVENTION 
     The present invention has been devised to address the above-described issues, and it is an object thereof to provide an optical scanning device in which the attenuation of a light amount of a beam by an aperture can be reduced. 
     Furthermore, another object of the present invention is to provide an image forming apparatus in which a light amount required for forming an image is secured by providing an optical scanning device in which the attenuation of a light amount can be reduced. 
     An optical scanning device according to the present invention is an optical scanning device in which a beam outputted from a light source is deflected by an optical deflector, and an object to be scanned is scanned by the deflected beam, and is provided with a light source that outputs a beam, an aperture provided with an opening that shapes the beam, and a reducing optical portion that reduces the beam, wherein the aperture and the reducing optical portion are arranged within an interval from the light source to the optical deflector. 
     With this configuration, attenuation of the light amount of the beam can be reduced. 
     In one embodiment according to the present invention, the aperture shapes the beam outputted from the light source, and the reducing optical portion reduces the beam shaped by the aperture. That is, an optical scanning device according to one embodiment of the present invention is directed to an optical scanning device in which a beam outputted from a light source is deflected by an optical deflector, and an object to be scanned is scanned by the deflected beam, and that is provided with a light source that outputs a beam, an aperture provided with an opening that shapes the beam outputted from the light source, and a reducing optical portion that reduces the beam shaped by the aperture, wherein the aperture and the reducing optical portion are arranged within an interval from the light source to the optical deflector. 
     With this configuration, a beam diameter of an optimal size can be obtained by the reducing optical portion. That is, since there is no need to reduce the beam with the aperture, the size of the opening of the aperture can be enlarged to enable a reduction in the attenuation of the light amount of the beam. 
     In the optical scanning device according to the present invention, it is preferable that a size of the opening is determined according to a reduction scaling factor of the reducing optical portion. 
     With this configuration, the opening of the aperture can be set to a size for obtaining a beam diameter of an optimal size. 
     In another embodiment according to the present invention, the reducing optical portion reduces the beam outputted from the light source, and the aperture shapes the beam reduced by the reducing optical portion. That is, an optical scanning device according to one embodiment of the present invention is directed to an optical scanning device in which a beam outputted from a light source is deflected by an optical deflector, and an object to be scanned is scanned by the deflected beam, and that is provided with a light source that outputs a beam, a reducing optical portion that reduces the beam outputted from the light source, and an aperture provided with an opening that shapes the beam reduced by the reducing optical portion, wherein the reducing optical portion and the aperture are arranged within an interval from the light source to the optical deflector. 
     With this configuration, due to the reducing optical portion, the beam diameter can be reduced without the light amount of the beam being attenuated. Furthermore, since the beam diameter is reduced, an aperture having a small size can be applied, which is beneficial in making the apparatus more compact. 
     In the optical scanning device according to the present invention, it is preferable that the reducing optical portion is configured provided with a convex lens and a concave lens, and outputs the incoming beam as a parallel beam. 
     With this configuration, a simple configuration can be achieved for outputting the beam as a parallel beam, and greater space-saving is possible. Furthermore, by outputting a beam that has been made a parallel beam, the position of the focal point can be adjusted easily, and there is an increased level of freedom in design. 
     It is preferable that the concave lens is arranged between the convex lens and the optical deflector. Furthermore, it is preferable that the optical scanning device according to the present invention is further provided with a cylindrical lens that is arranged between the concave lens and the optical deflector. 
     It is preferable that the optical scanning device according to the present invention is provided with a collimator, which is arranged within an interval from the light source to the reducing optical portion, and makes the beam parallel. 
     With this configuration, a simple configuration can be achieved for outputting the beam as a parallel beam. 
     In the optical scanning device according to the present invention, it is preferable that a reduction scaling factor of the reducing optical portion is different for a first scanning direction in which a beam scans an object to be scanned and a second scanning direction that is orthogonal to the first scanning direction. 
     With this configuration, a suitable reduction scaling factor can be set in response to the shape of the cross section of the beam irradiated on the object to be scanned. 
     The reduction scaling factor of the reducing optical portion may be one times (same scale as) the reduction scaling factor of the first scanning direction. That is, the reducing optical portion may converge the beam only in the second scanning direction. 
     It is preferable that an image forming apparatus according to the present invention is configured to form an image based on light scanned by the optical scanning device. 
     With this configuration, it is possible provide an image forming apparatus in which a light amount required for forming an image is secured by providing an optical scanning device in which the attenuation of a light amount can be reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic configuration drawing showing an image forming apparatus according to embodiment 1 of the present invention. 
         FIG. 2  is a schematic perspective view showing a configuration of an optical scanning device according to embodiment 2 of the present invention. 
         FIG. 3  is an outline perspective view showing a configuration of a modified example of an optical scanning device according to embodiment 2 of the present invention. 
         FIG. 4A  and  FIG. 4B  are diagrams for describing a relationship between the beam outputted from the aperture and the reducing optical portion.  FIG. 4A  is a schematic top view showing the beam in a case where the reducing optical portion is not provided, and  FIG. 4B  is a schematic top view showing the beam in a case where the reducing optical portion is provided. 
         FIG. 5A  and  FIG. 5B  are diagrams for describing a relationship between the beam diameter and the depth of focus.  FIG. 5A  is a schematic top view showing a case where the beam diameter is large, and  FIG. 5B  is a schematic top view showing a case where the beam diameter is small. 
         FIG. 6  is an outline perspective view showing a configuration of an optical scanning device according to embodiment 3 of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiment 1 
     Hereinafter, description is given with reference to the accompanying drawings regarding an image forming apparatus provided with an optical scanning device according to embodiment 1 of the present invention. 
       FIG. 1  is an outline configuration drawing showing an image forming apparatus according to embodiment 1 of the present invention. 
     An image forming apparatus  100  has a configuration provided with an original paper transport portion  101 , an image reading portion  102 , an image forming portion  103 , a recording paper transport portion  104 , and a paper feeding portion  105 , and is a copier or the like for example. The image forming apparatus  100  forms monochrome images on paper in accordance with image data received externally or from the image reading portion  102 . 
     The original paper transport portion  101  transports originals that have been set to the image reading portion  102 . 
     The image reading portion  102  reads an image of the original and outputs this as image data to the image forming portion  103 . It should be noted that various types of image processing may be executed on the image data by a control circuit such as a microcomputer prior to output. 
     The image forming portion  103  records on paper the original image that is indicated by the image data. The image forming portion  103  has a configuration that is provided with components such as a photosensitive drum  21 , a charging unit  22 , an optical scanning device  23 , a development unit  24 , a transfer unit  25 , a cleaning unit  26 , and a fixing device  27 . 
     The surface of the photosensitive drum  21  is an organic photosensitive body. The surface of the photosensitive drum  21  is cleaned by the cleaning unit  26  then uniformly charged by the charging unit  22 . 
     The charging unit  22  may be a charger type or may be a roller type or brush type that makes contact with the photosensitive drum  21 . 
     The optical scanning device  23  is a laser scanning unit (LSU). The optical scanning device  23  emits laser beams corresponding to the inputted image data onto the photosensitive drum  21  to expose the uniformly charged surface of the photosensitive drum  21  such that an electrostatic latent image is formed on the surface of the photosensitive drum  21 . That is, the image forming apparatus  100  is configured to form an image based on laser beams that are scanned by the optical scanning device  23 . With this configuration, an image forming apparatus  100  can be provided that ensures the amount of light necessary for forming an image. It should be noted that configurations of the optical scanning device  23  are described in detail in embodiment 2 and embodiment 3. 
     The development unit  24  supplies toner to the surface of the photosensitive drum  21  to develop the electrostatic latent image and form a toner image (visible image) on the surface of the photosensitive drum  21 . 
     The transfer unit  25  transfers the toner image on the surface of the photosensitive drum  21  to a recording paper that has been transported in by the recording paper transport portion  104 . The transfer unit  25  is provided with such components as a transfer belt  31 , a drive roller  32 , an idler roller  33 , and an elastic conductive roller  34 , and the transfer belt  31  is caused to rotate while spanning the rollers  32  to  34  and other rollers in a tensioned state. 
     The transfer belt  31  has a predetermined volume resistivity value (for example, 1×10 9  to 1×10 13  Ω·cm). Furthermore, the elastic conductive roller  34 , which is for applying a transfer electric field, is arranged near a region (an image transfer portion  57 ) where the photosensitive drum  21  and the transfer belt  31  contact each other. 
     The elastic conductive roller  34  applies pressure to the transfer belt  31  and the photosensitive drum  21  so that the transfer belt  31  presses against the photosensitive drum  21 . Due to this, the image transfer portion  57  is not a line shape, but rather a surface shape having a predetermined width. Thus, the transfer efficiency onto the transported recording paper can be improved. 
     A transfer electric field of a polarity opposite to the charge of the toner image that has been formed on the surface of the photosensitive drum  21  is applied to the elastic conductive roller  34 , and the toner image on the surface of the photosensitive drum  21  is transferred to the recording paper due to the opposite polarity transfer electric field. For example, in a case where the toner image takes on a charge of a negative polarity, the polarity of the transfer electric field applied to the elastic conductive roller  34  is a positive polarity. 
     Further still, a charge removal roller  54  is arranged on a downstream side in the paper transport direction from the image transfer portion  57 . The charge removal roller  54  carries out a charge removal process on the paper that has been charged when passing through the image transfer portion  57 . Due to the charge removal process, the transport of the recording paper to the fixing device  27  can be performed smoothly. In the present embodiment, the charge removal roller  54  is arranged at the rear surface of the transfer belt  31 . 
     Furthermore, the transfer unit  25  is provided is provided with a belt cleaning unit  56 , which removes toner smearing on the transfer belt  31 , and a charge removal unit  55 , which executes a charge removal process on the transfer belt  31 . 
     Various charge removal methods are available for the charge removal unit  55 , including for example a method in which the transfer belt  31  is grounded via the apparatus, and a method in which an electric field of the opposite polarity to the polarity of the transfer electric field is applied to the transfer belt  31 . 
     The cleaning unit  26  removes and collects toner that is residual on the surface of the photosensitive drum  21  after development and transfer. 
     The fixing device  27  is provided with a heating roller  35  and a pressure roller  36 , and applies heat and pressure to the recording paper to cause the toner image to fix onto the recording paper. 
     A heat source is arranged inside the heating roller  35  to heat the outer peripheral surface thereof to a predetermined temperature (for example, 160° C. to 200° C.). 
     The pressure roller  36  is provided with a mechanism such as a load spring at its axial direction end portions and due to this mechanism, a configuration is achieved in which the pressure roller  36  presses against the heating roller  35  with a predetermined load. Furthermore, a paper separation claw and a roller surface cleaning member are arranged on an outer periphery of the pressure roller  36 . 
     In the fixing device  27 , the unfixed toner image on the recording paper is subjected to thermal melting and pressure by a fixing process portion, which is the pressing portion between the heating roller  35  and the pressure roller  36 , thereby fixing the toner image onto the recording paper. 
     The recording paper transport portion  104  is provided with transport paths  43  for transporting the recording papers, registration rollers  42 , and discharge rollers  46 . 
     In the transport paths  43 , the recording paper is taken in from the paper feeding portion  105 , then the recording paper is transported until the leading edge of the recording paper reaches the registration rollers  42 . 
     The registration rollers  42  transport the recording paper to the transfer unit  25 . 
     The discharge rollers  46  transport to the discharge tray  47  the recording paper on which a toner image has been fixed by the fixing device  27 . 
     The paper feeding portion  105  is provided with a plurality of paper feed trays  51 . 
     The paper feed trays  51  are trays for storing recording paper and are provided in the lower portion of the image forming apparatus  100 . Furthermore, the paper feed trays  51  are provided with a pickup roller or the like for withdrawing the recording paper sheet by sheet, and recording paper that has been withdrawn is fed to the transport paths  43  of the recording paper transport portion  104 . It should be noted that the image forming apparatus  100  according to the present embodiment is provided with multiple paper feed trays  51  capable of accommodating from 500 to 1,500 sheets of standard size papers to enable high-speed print processing. 
     Furthermore, a manual feeding tray  53  is provided at a lateral surface of the image forming apparatus  100  primarily for supplying nonstandard size recording papers, and moreover a large capacity cassette (LCC)  52  capable of accommodating large volumes of multiple types of recording papers may also be provided. 
     The discharge tray  47  is arranged at a lateral surface of an opposite side to the manual feeding tray  53 . Instead of the discharge tray  47 , configurations in which post processing devices of the recording paper (stapling, punching and the like) or a plurality of levels of discharge trays are arranged as options are also possible. 
     Embodiment 2 
       FIG. 2  is an outline perspective view showing a configuration of an optical scanning device according to embodiment 2 of the present invention. 
     In the optical scanning device  23  according to this embodiment of the present invention, a beam LB outputted from a light source  61  is deflected by an optical deflector  68 , and an object to be scanned (the photosensitive drum  21 ) is scanned by the deflected beam LB. The optical scanning device  23  is provided with a light source  61  that outputs the beam LB, an aperture  63  provided with an opening  63   a  that shapes the beam LB outputted from the light source  61 , and a reducing optical portion  64  that reduces the beam LB shaped by the aperture  63 . The aperture  63  and the reducing optical portion  64  are arranged within an interval from the light source  61  to the optical deflector  68 . 
     With this configuration, a beam diameter of an optimal size can be obtained by the reducing optical portion  64 . That is, since there is no need to reduce the beam LB with the aperture  63 , the size of the opening  63   a  of the aperture  63  can be enlarged to enable a reduction in the attenuation of the light amount of the beam LB. 
     In the optical scanning device  23 , the light source  61 , a collimator  62 , the aperture  63 , the reducing optical portion  64 , a first cylindrical lens  66 , a mirror  67 , the optical deflector  68 , scanning lenses  69  and  70 , a second cylindrical lens  71 , and a turning mirror  72  are arranged in order from upstream to downstream along an advancement direction of the beam LB. 
     The beam LB outputted from the optical scanning device  23  is irradiated onto the surface of the photosensitive drum  21 . It should be noted that hereinafter the direction in which the beam LB irradiated onto the surface of the photosensitive drum  21  scans is referred to as a first scanning direction H, and the direction orthogonal to the optical axis of the beam LB and orthogonal to the first scanning direction H is referred to as a second scanning direction V. 
     The optical scanning device  23  is provided with the collimator  62 , which is arranged within an interval from the light source  61  to the reducing optical portion  64 , and makes the beam LB parallel. With this configuration, a simple configuration can be achieved for outputting the beam LB as a parallel beam. It should be noted that the collimator  62  is arranged on the upstream side from the aperture  63 . 
     The light source  61  is a laser diode for example. A cross section (beam cross section) that is vertical to the optical axis of the beam LB outputted from the light source  61  is a circular shape. 
     The collimator  62  is an optical component that shapes the conical beam LB, which is outputted from the light source  61  in a diffused manner, into the parallel beam LB. 
     The aperture  63  is a plate member in which the rectangular opening  63   a  is centrally formed, and is an optical component that, when the beam LB passes there-through, shapes the beam cross section from an elliptical shape to a rectangular shape. 
     The reducing optical portion  64  is configured provided with a convex lens  64   a  and a concave lens  64   b,  and outputs the incoming beam LB as a parallel beam. With this configuration, a simple configuration can be achieved for outputting the beam LB as a parallel beam, and greater space-saving is possible. Furthermore, by outputting a beam that has been made a parallel beam, the position of the focal point can be adjusted easily, and there is an increased level of freedom in design. 
     In the present embodiment, the convex lens  64   a  is configured as a component that converges the beam LB only in the second scanning direction V. The concave lens  64   b  is a component that makes parallel the beam LB that has been converged in the second scanning direction V by the convex lens  64   a.  For example, the reduction scaling factor of the reducing optical portion  64  is one times (same scale) with respect to the first scanning direction H and ⅕ times with respect to the second scanning direction V. 
     As described above, the reduction scaling factor of the reducing optical portion  64  may be configured differently for the first scanning direction H and the second scanning direction V. With this configuration, a suitable reduction scaling factor can be set in response to the shape of the cross section of the beam LB irradiated on the object to be scanned (the photosensitive drum  21 ). 
     The size of the opening  63   a  is determined according to the reduction scaling factor of the reducing optical portion  64 . With this configuration, the opening  63   a  of the aperture  63  can be set to a size for obtaining a beam diameter of an optimal size. Furthermore, the processing for forming the opening  63   a  becomes easier by increasing the size of the opening  63   a  of the aperture  63 . It should be noted that the size of the opening  63   a  refers to a width of the opening with respect to the first scanning direction H or the second scanning direction V, and the beam diameter refers to a width of the beam LB with respect to the first scanning direction H or the second scanning direction V. 
     Furthermore, it is preferable that the size of the opening  63   a  is smaller in the first scanning direction H and the second scanning direction V than the diameter of the beam that is incident on the aperture  63 . With this configuration, the shape of the beam cross section can be shaped reliably by the aperture  63 . 
     The first cylindrical lens  66  and the mirror  67  are optical components for converging the beam LB onto the reflective surfaces of the optical deflector  68 . 
     The optical deflector  68  is a polygon mirror on which multiple reflective surfaces are formed, and is rotationally driven by an unshown driver. The optical deflector  68  is rotationally driven so that the reflected beam LB scans along the first scanning direction H. Hereinafter, the range in which the beam LB scans in the first scanning direction H is referred to as a scanning range. Furthermore, the first scanning direction H is a direction parallel to the rotational axis of the photosensitive drum  21 . 
     As described above, the optical scanning device  23  is provided with the optical deflector  68 , which deflects the beam LB outputted from the light source  61  to scan the object to be scanned (the photosensitive drum  21 ) in the first scanning direction H. With this configuration, the optical scanning device  23  can be achieved that scans the beam LB onto the object to be scanned (the photosensitive drum  21 ) to form an electrostatic latent image. 
     The scanning lenses  69  and  70  are optical components for correcting the image distortion that is produced due to the disparity between the optical path length of the beam LB irradiated at the end portions of the scanning range and the optical path length of the beam LB irradiated at the center of the scanning range That is, the scanning lenses  69  and  70  are optical components that cause the beam LB scanned by the optical deflector  68  to scan on the photosensitive drum  21  with a constant velocity, and are also referred to as f-theta lenses. 
     The second cylindrical lens  71  is an optical component for correcting an optical face tangle error of the optical deflector  68  through a reciprocal action with the first cylindrical lens  66 . 
     The turning mirror  72  is a light reflecting member that reflects the irradiated beam LB and guides it to the surface of the photosensitive drum  21 . 
     Furthermore, the optical scanning device  23  is further provided with a reflective mirror  73  and a BD (beam detector) sensor  74 . 
     The reflective mirror  73  reflects the beam LB that is irradiated from the optical deflector  68  to an end portion of the scanning range and guides it to the BD sensor  74 . 
     The BD sensor  74  receives the beam LB to detect timings of a scanning commencement and scanning completion for each line on the photosensitive drum  21 , and outputs the results thereof as a signal. 
     In the present embodiment, the convex lens  64   a  is configured as a component that converges the beam LB only in the second scanning direction V, but it is also possible for the convex lens  64   a  to converge the beam LB in the first scanning direction H and the second scanning direction V. 
       FIG. 3  is an outline perspective view showing a configuration of a modified example of an optical scanning device according to embodiment 1 of the present invention. It should be noted that same reference symbols are assigned to constituent elements whose function and structure are essentially the same as in  FIG. 2  and description thereof is omitted. 
     In the modified example, a convex lens  64   c  is configured as a component that converges the beam LB in the first scanning direction H and the second scanning direction V. Furthermore, a concave lens  64   d  is a component that makes parallel the beam LB that has been converged in the first scanning direction H and the second scanning direction V by the convex lens  64   c.  It should be noted that the reduction scaling factor of the reducing optical portion  64  may be configured differently for the first scanning direction H and the second scanning direction V. With this configuration, a suitable reduction scaling factor can be set in response to the shape of the cross section of the beam irradiated on the object to be scanned. 
       FIG. 4A  and  FIG. 4B  are diagrams for describing a relationship between the beam outputted from the aperture and the reducing optical portion.  FIG. 4A  is a schematic top view showing the beam in a case where the reducing optical portion is not provided, and  FIG. 4B  is a schematic top view showing the beam in a case where the reducing optical portion is provided. 
     In a case where the reducing optical portion is not provided as in  FIG. 4A , an opening  163   a  of an aperture  163  has a narrow opening width AW 1 . The beam LB outputted from a light source  161  becomes a parallel beam having an irradiated beam width BW due to a collimator  162 . In passing through the aperture  163 , the beam LB becomes a parallel beam of a beam width D equivalent to the opening width AW 1 . Here, due to the beam LB being blocked by the aperture  163 , the light amount of the beam LB attenuates. Along with the difference becoming greater between the irradiated beam width BW and the opening width AW 1 , the light amount is greatly attenuated. 
     In a case where the reducing optical portion is provided as in  FIG. 4B , the opening  63   a  of the aperture  63  has an opening width AW 2  that is wider than the opening width AW 1  of  FIG. 4A . That is, by reducing the difference between the opening width AW 2  and the irradiated beam width BW, attenuation of the light amount of the beam LB is reduced. 
     The beam LB outputted from the light source  61  becomes a parallel beam having the irradiated beam width BW due to the collimator  62 . In passing through the aperture  63  having the opening width AW 2 , the beam LB becomes a parallel beam of a beam width equivalent to the opening width AW 2 . The beam LB that has passed through the aperture  63  becomes a parallel beam having the beam width D due to the reducing optical portion  64 . 
     In the case shown in  FIG. 4A , the opening width AW 1  is narrow compared to the irradiated beam width BW, and therefore the beam LB is greatly blocked and the light amount is greatly attenuated. In the present embodiment, the opening width AW 2  is widened as shown in  FIG. 4B  so that attenuation of the light amount of the beam LB is reduced. Furthermore, due to the reducing optical portion  64 , an optimal beam width D required on the downstream side is achieved. 
       FIG. 5A  and  FIG. 5B  are diagrams for describing a relationship between the beam diameter and the depth of focus.  FIG. 5A  is a schematic top view showing a case where the beam diameter is large, and  FIG. 5B  is a schematic top view showing a case where the beam diameter is small. 
     As described above, the beam LB outputted from the light source  61  is converged onto the reflective surfaces of the optical deflector  68  by the first cylindrical lens  66  and the mirror  67 , and the surface of the photosensitive drum  21  is exposed by the converged beam LB. At this time, if the beam LB is not sufficiently converged, the light amount required for exposing the photosensitive drum  21  cannot be obtained. 
     Generally, the depth of focus varies according to the width of the beam that is incident on the lens. Here, the depth of focus refers to the range on the optical axis where a certain level of resolving power can be maintained. That is, if the image surface (the surface of the photosensitive drum  21 ) is contained in the depth of focus, then the light amount required for exposure can be secured. The relationship between the beam width and the depth of focus can be expressed by the following equations. 
         d= 2.44×(λ× f )/ D  
 
         A= 2×(λ× f   2 )/ D   2  
 
     Here, λ is the beam wavelength, f is the focal point distance (the distance from the lens to the focal point), D is the incoming beam width, d is the spot diameter (beam width at the focal point), and A is the depth of focus. 
     From the above equations, it is evident that along with the incoming beam width D becoming smaller, the spot diameter d and the depth of focus A become larger. 
     In  FIG. 5A , the beam LB having a large incoming beam width Da due to an aperture  81  is converged on a lens  82 . When the incoming beam width Da is large, a spot diameter da can be narrowed down and made small, but a depth of focus Aa becomes narrow. Furthermore, when the image surface is out of the focal point, the variation in the beam diameter becomes larger. 
     In  FIG. 5B , the beam LB having a small incoming beam width Db due to the aperture  81  is converged on the lens  82 . Compared to the case of  FIG. 5A , when the incoming beam width Db is small, a spot diameter db becomes larger and a depth of focus Ab becomes wider. Furthermore, even if the image surface is out of the focal point, the variation in the beam diameter is small. 
     As described above, if the incoming beam width D is made small, the depth of focus A becomes wide, and therefore image surface displacement or the like can be easily addressed. As shown in  FIG. 5A , in a case where the beam LB having the incoming beam width Da is converged without being reduced by the reducing optical portion  64 , it becomes difficult to address image surface displacement. In the present embodiment, a beam diameter having an optimal size is obtained by reducing the beam LB using the reducing optical portion  64 . 
     Embodiment 3 
       FIG. 6  is an outline perspective view showing a configuration of an optical scanning device according to embodiment 3 of the present invention. It should be noted that same reference symbols are assigned to constituent elements whose function and structure are essentially the same as in embodiment 2 and description thereof is omitted. 
     In an optical scanning device  23   a  according to this embodiment of the present invention, the beam LB outputted from the light source  61  is deflected by the optical deflector  68 , and an object to be scanned (the photosensitive drum  21 ) is scanned by the deflected beam LB. The optical scanning device  23   a  is provided with the light source  61  that outputs the beam LB, the reducing optical portion  64  that reduces the beam LB outputted from the light source  61 , and the aperture  65  provided with an opening  65   a  that shapes the beam LB reduced by the reducing optical portion  64 . The reducing optical portion  64  and the aperture  65  are arranged within an interval from the light source  61  to the optical deflector  68 . 
     With this configuration, due to the reducing optical portion  64 , the beam diameter can be reduced without the light amount of the beam LB being attenuated. Furthermore, since the beam diameter is reduced, an aperture  65  having a small size can be applied, which is beneficial in making the apparatus more compact. 
     In the optical scanning device  23   a,  the light source  61 , the collimator  62 , the reducing optical portion  64 , the aperture  65 , the first cylindrical lens  66 , the mirror  67 , the optical deflector  68 , scanning lenses  69  and  70 , the second cylindrical lens  71 , and the turning mirror  72  are arranged in order from upstream to downstream along the advancement direction of the beam. The beam LB outputted from the optical scanning device  23   a  is irradiated onto the surface of the photosensitive drum  21 . That is, embodiment 3 is different from embodiment 2 in that the reducing optical portion  64  is arranged upstream from the aperture  65 . 
     The reducing optical portion  64  is configured provided with the convex lens  64   a  and the concave lens  64   b,  and outputs the incoming beam LB as a parallel beam. With this configuration, a simple configuration can be achieved for outputting the beam LB as a parallel beam, and greater space-saving is possible. Furthermore, by outputting a beam that has been made a parallel beam, the position of the focal point can be adjusted easily, and there is an increased level of freedom in design. 
     The optical scanning device  23   a  is provided with the collimator  62 , which is arranged within an interval from the light source  61  to the reducing optical portion  64 , and makes the beam LB parallel. With this configuration, a simple configuration can be achieved for outputting the beam LB as a parallel beam. It should be noted that the collimator  62  is arranged on the upstream side from the aperture  65 . 
     The present invention can be embodied and practiced in other different forms without departing from the spirit and essential characteristics thereof. Therefore, the above-described working examples are considered in all respects as illustrative and not restrictive. The scope of the invention is indicated by the appended claims rather than by the foregoing description. All variations and modifications falling within the equivalency range of the appended claims are intended to be embraced therein.