Patent Publication Number: US-2005122704-A1

Title: Method for supporting reflector in optical scanner, optical scanner and image formation apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      The disclosure of Japanese Patent Application No. 2003-369479 filed on Oct. 29, 2003 including specification, drawings and claims and the disclosure of Japanese Patent Application No. 2003-383437 filed on Nov. 13, 2003 including specification, drawings and claims are incorporated herein by reference in its entity.  
     BACKGROUND OF THE INVENTION  
      1. Technical Field to which the Invention Belongs  
      The present invention relates to a method for supporting a reflector in an optical scanner, an optical scanner, and an image formation apparatus including the optical scanner.  
      2. Prior Art  
      Conventionally, optical scanners have been used for image formation apparatuses such as laser beam printers, laser facsimile machines and digital copy machines. As an optical scanner of this kind, an apparatus including a semiconductor laser as a light source, a polygon mirror (rotating polygon mirror), a first image formation optical system for making a ray bundle from the semiconductor laser form a line image on the polygon mirror, a second image formation optical system for forming an image of a uniform spot at a uniform velocity on a scanning plane, a scanning start signal detector for detecting the ray bundle scanned by the polygon mirror, and a detection optical system for gathering ray bundles from the semiconductor laser to the scanning start signal detector has been known (see, e.g., Japanese Laid-Open Publication No. 2001-166239, which will be hereinafter referred to as “Patent Reference 1”).  
      As described above, the second image formation optical system exerts the high level function of forming an image of a uniform spot at a uniform velocity on a scanning plane. Therefore, when it is intended to form the second image formation optical system of a single optical element, the optical element has to be formed into a complicated shape. In many cases, a glass lens, i.e., a light transmission type optical element, is used in the second image formation optical system. However, it is difficult to process a glass lens into a complicated shape and thus it is difficult to form the second image formation optical system of a single glass lens. Therefore, when the second image formation optical system is to be formed of glass lenses, a plurality of glass lenses have to be combined. Such a combination of lenses is normally called fθ lens.  
      Glass lenses are, however, of an expensive optical element. When the second image formation optical system is formed of glass lenses, a plurality of expensive glass lenses are needed. Therefore, it has been difficult to reduce the size of and costs for optical scanners.  
      To reduce the size of and costs for optical scanners, then, an apparatus using a reflector in which a reflection plane of a curved plane is provided in the second image formation system has been proposed. That is, use of not a light transmission type optical element but a light reflex type optical element as the second image formation optical system has been proposed.  
      Moreover, the present applicants have proposed that a reflector including a reflection plane of free-form surface is used to form the second image formation optical system of only the reflector (see, e.g., Japanese Laid-open Publication No. 2002-148539). As shown in  FIG. 12 , in a reflector  100  of this kind, a transverse plane has an approximate C shape and, on the other hand, the shape of the transverse plane is not constant along the axis direction but the entire reflector  100  is twisted. With such a shape, the reflector  100  can scan spots in straight line on a scanning plane.  
      In recent years, the size of optical scanners has been reduced more and more and influences of a vibration on an optical element have become a problem which can not be ignored. A reflector as an optical element is more easily influenced by a vibration than a light transmission type optical element (e.g., a lens). Therefore, measures to suppress influences of a vibration on a reflector are desired to be devised.  
      In the scanning optical apparatus of Patent Reference 1, as shown in  FIGS. 13A and 13B , in order to suppress influences of a vibration, support members  102 ,  103 , and  104  are provided under both end and center portions of the reflector  101  in the long side direction, respectively, to support the reflector  101  at three points in the end portions and the center portion. Furthermore, at each of the end potions, the reflector  101  is pressed by an elastic member (not shown) in the orthogonal direction to a reflection plane  105 . On the other hand, at the center portion, the reflector  101  is pressed by an elastic member  106  in the parallel direction to the reflection plane  105 . In this apparatus, the three support members  102 ,  103 , and  104  for supporting the reflector  101  are arranged in an approximately straight line along the long side direction.  
      However, as shown in  FIG. 13B , assume that the center of gravity G and support point S of the reflector  101  do not match each other in the front-rear direction of the reflector  101  (i.e., in the left-right direction in  FIG. 13B ). With an external force applied, the reflector  101  easily vibrates with the support point S as a center.  
      Moreover, in the scanning optical apparatus, the reflection plane  105  of the reflector  101  is not a curved plane but a flat plane. A cross section of the reflector  101  has a rectangular shape such that the reflector  101  is easily supportable. Thus, by the above-described supporting method in which the support points S align approximately linearly, vibration of the reflector  101  can be suppressed. Furthermore, in the reflector  101 , the shape of the cross section is constant along the long side direction. Therefore, the shape of the reflector  101  is relatively simple and the reflector  101  originally has an easily supportable shape.  
      In contrast, the shape of a reflector having a reflection plane of a curved plane is complicated. Therefore, the above-described supporting method can not be used as it is and it has been desirable to develop new supporting methods.  
      Moreover, the reflector of Patent Reference 1 is merely a so-called deflecting mirror and the reflector itself can not form the second image formation optical system. However, especially in a reflector having a reflection plane of a so-called free-form surface, the entire reflector is formed in a twist shape. Therefore, it has been difficult to sufficiently suppress a vibration.  
      In view of the above-described points, the present invention has been devised and it is therefore an object of the present invention to support, in an optical scanner, a reflector including a reflection plane of a curved plane so that influences of a vibration is suppressed. Moreover, it is also an object of the present invention to provide an optical scanner which allows such a supporting method and an image formation apparatus including the optical scanner.  
     SUMMARY OF THE INVENTION  
      A method for supporting a reflector according to the present invention is a method for supporting, in an optical scanner including a reflector for reflecting a ray bundle to be scanned, the reflector, in which the reflector includes a reflection plane formed of a curved plane having a long side extending in the scanning direction in which a ray bundle is scanned and having a positive power at least in the scanning direction, and the reflector is supported at a first support point located in the vicinity of one end of the reflector in the long side direction, a second support point located in the vicinity of the other end of the reflector in the long side direction, and a third support point located in the vicinity of a center of the reflector in the long side direction so as to be more toward a concave side of the reflection plane than an imaginary straight line joining the first support point and the second support point.  
      According to the method, a reflector is stably supported at three points which are not located in a straight line, so that the reflector can be stably supported. Moreover, a third support point is located so as to be more toward a concave side of a reflection plane than an imaginary straight line joining a first support point and a second support point. Thus, the first, second and third support points are arranged along the curve direction so as to correspond to the reflection plane being curved. Accordingly, the reflector is supported in a manner according to the shape of the reflector, so that the reflector is more stably supported. Furthermore, the third support point is located off the imaginary straight line. Thus, even if the reflector receives an external force, the reflector is prevented from rotating with respect to the imaginary straight line. Moreover, a twist of the reflector can be suppressed. As a result, an optical scanner in which influences of a vibration can be suppressed and which is hardly influenced by a vibration can be achieved.  
      It is preferable that a distance between the third support point and the imaginary straight line is larger than a sag of the reflection plane in the scanning direction.  
      Thus, the area of the imaginary triangle joining the first, second and third support points is increased and that the reflector can be stably supported.  
      It is preferable that first, second and third corresponding points located in parts of an opposite side of the reflector to a side in which the first, second and third support points are located and approximately corresponding to the first, second and third support points, respectively, are pressed toward the first, second and third support points, respectively.  
      Note that a point approximately corresponding to a support point may be a point (corresponding point) located in the opposite side to a side in which the support point is located and also may be a point in the vicinity of the corresponding point.  
      Thus, the reflector is sandwiched between each of the support points and its corresponding point. As a result, the reflector can be firmly supported.  
      It is preferable that the reflector includes a thin-plate-shaped reflector body of which one plane serves as a reflection plane and a rib extending from a back side of the reflection plane of the reflector body in the back side direction and having a long side extending in the scanning direction, and the third corresponding point is located on the rib.  
      Thus, the strength of the reflector is improved. Moreover, when the reflector is formed of a resin or like material, it is preferable, in order to suppress the generation of a sink of the material and improve accuracy in processing of the reflector, that the reflector has a smaller thickness. With the reflector, the strength of the reflector can be ensured by the rib. Therefore, the thickness of the reflector body can be reduced.  
      It is preferable that the reflector includes, with the third support point as a boundary, a first portion including the first support point and a second portion including the second support point, the center of gravity of the first portion is located on an imaginary straight line joining between the first support point and the third support point, and the center of gravity of the second portion is located on an imaginary straight line joining the second support point and the third support point.  
      Thus, the center of gravity of each portion is located on each imaginary straight line joining one support point and another, so that the reflector is hardly twisted even when the reflector receives an external force. Therefore, a face tangle error of the reflection plane hardly occurs.  
      It is preferable that the reflector includes, with the third support point as a boundary, a first portion including the first support point and a second portion including the second support point, a shear center of a cross section of the reflector in a center portion of the first portion in the long side direction is located on the imaginary straight line joining the first support point and the third support point, and a shear center of a cross section of the reflector in a center portion of the second portion in the long side direction is located on the imaginary straight line joining the second support point and the third support point.  
      Thus, the reflector is hardly twisted. Therefore, a face tangle error hardly occurs.  
      It is preferable that a center of gravity of the first portion approximately matches the shear center of the cross section of the reflector in the center portion of the first portion in the long side direction, and a center of gravity of the second portion approximately matches the shear center of the cross section of the reflector in the center portion of the second portion in the long side direction.  
      Thus, the reflector is even hardly twisted. Therefore, a face tangle error even hardly occurs.  
      It is preferable that a depth of the center portion of the reflector is larger than a depth of each of end portions of the reflector.  
      Thus, the third support point, i.e., a support point in an approximately center portion can be made to be located in a further back side. Accordingly, the area of a triangle joining the first, second and third support points can be increased, so that the reflector can be more stably supported.  
      It is preferable that a second moment of area in the center portion of the reflector is larger than a second moment of area in each of the end potions of the reflector.  
      Thus, the strength of the reflector is stronger in the center portion than in each of the end portions. Therefore, even when thermal expansion occurs in the reflector, the reflector can easily stretch along the long side direction from the center portion. Accordingly, a twist deformation due to thermal expansion in the thickness direction (i.e., the approximately orthogonal direction with the long side direction) is hardly generated, so that a face tangle error hardly occurs.  
      It is preferable that an area of a cross section of the reflector in the vicinity of each of the support points is larger than an area of a cross section of the reflector in a portion located between one of the support points and another.  
      Thus, compared to the vicinity of each support point, the weight of a portion of the reflector located between one support point and another is reduced. Therefore, even when an external force is applied to the reflector, an inertial force is hardly generated in the portion between one support point and another, compared to the vicinity of each support point. On the other hand, the vicinity of each support point is supported. Thus, a vibration hardly occurs in the first place. Therefore, a vibration of the reflector is suppressed.  
      An optical scanner according to the present invention includes: a light source for outputting a ray bundle; an optical deflector for scanning the ray bundle from the light source; a first image formation optical system, arranged between the light source and the optical deflector, for leading the ray bundle from the light source to a deflecting plane of the optical deflector and for forming a line image on the deflecting plane; and a second image formation optical system, arranged between the optical deflector and a scanning plane to be scanned, for leading the ray bundle from the optical deflector to the scanning plane and forming an image of a uniform spot on the scanning plane at a uniform velocity. In the optical scanner, the second image formation optical system includes a reflector having a reflection plane formed of a curved plane which has a long side extending in the scanning direction in which the ray bundle is scanned and a positive power at least in the scanning direction, and the optical scanner further includes a support member for supporting the reflector at a first support point located in the vicinity of one end of the reflector in the long side direction, a second support point located in the vicinity of the other end of the reflector in the long side direction, and a third support point located in the vicinity of a center of the reflector in the long side direction so as to be more toward a concave side of the reflector plane than an imaginary straight line joining the first support point and the second support point.  
      It is preferable that a distance between the third support point and the imaginary straight line is larger than a sag of the reflection plane in the scanning direction.  
      It is preferable that the optical scanner further includes: a pressure member for pressing first, second and third corresponding points located in parts of an opposite side of the reflector to a side in which the first, second and third support points are located and approximately corresponding to the first, second and third support points, respectively, toward the first, second and third support points, respectively.  
      It is preferable that the reflector includes a thin-plate-shaped reflector body of which one plane serves as a reflection plane and a rib extending from a back side of the reflection plane of the reflector body in the back side direction and having a long side extending in the scanning direction, and the third corresponding point is located on the rib.  
      It is preferable that the reflector includes, with the third support point as a boundary, a first portion including the first support point and a second portion including the second support point, the center of gravity of the first portion is located on an imaginary straight line joining between the first support point and the third support point, and the center of gravity of the second portion is located on an imaginary straight line joining the second support point and the third support point.  
      It is preferable that the reflector includes, with the third support point as a boundary, a first portion including the first support point and a second portion including the second support point, a shear center of a cross section of the reflector in a center portion of the first portion in the long side direction is located on the imaginary straight line joining the first support point and the third support point, and a shear center of a cross section of the reflector in a center portion of the second portion in the long side direction is located on the imaginary straight line joining the second support point and the third support point.  
      It is preferable that a center of gravity of the first portion approximately matches the shear center of the cross section of the reflector in the center portion of the first portion in the long side direction, and a center of gravity of the second portion approximately matches the shear center of the cross section of the reflector in the center portion of the second portion in the long side direction.  
      It is preferable that a depth of the center portion of the reflector is larger than a depth of each of end portions of the reflector.  
      It is preferable that a second moment of area in the center portion of the reflector is larger than a second moment of area in each of the end potions of the reflector.  
      It is preferable that an area of a cross section of the reflector in the vicinity of each of the support points is larger than an area of a cross section of the reflector in a portion located between one of the support points and another.  
      It is preferable that the reflector includes a synthetic resin member having a curved plane and a mirror surface film formed on the curved plane of the synthetic resin member.  
      Thus, the synthetic resin member can be processed in a simple manner and even a curved plane having a complex shape can be formed in a relatively simple manner. Moreover, the synthetic resin member can be formed at low cost, compared to a glass member.  
      It is preferable that the second formation optical system is formed of only the reflector.  
      The reflection surface of the reflector is formed of a free-form surface, so that the second image formation optical system can be formed of only the reflector. Therefore, in the optical scanner, even a reflector including a reflection plane formed of a free-form surface can be stably supported.  
      An image formation apparatus according to the present invention includes: an optical scanner; an approximately cylindrical photosensitive body having a rim surface to serve as a scanning plane to be scanned and extending in the scanning direction in which a ray bundle is scanned in the optical scanner; driving mechanism for rotating the photosensitive body; a developer for supplying toner to the photosensitive body; and transferer for transferring a toner image formed on the photosensitive body to a recording medium. In the apparatus, the optical scanner includes a light source for outputting a ray bundle, an optical deflector for scanning a ray bundle from the light source, a first image formation optical system, arranged between the light source and the optical deflector, for leading the ray bundle from the light source to a deflecting plane of the optical deflector and for forming a line image on the deflecting plane, and a second image formation optical system, arranged between the optical deflector and a scanning plane to be scanned, for leading the ray bundle from the optical deflector to the scanning plane and forming an image of a uniform spot on the scanning plane at a uniform velocity, the second image formation optical system includes a reflector having a reflection plane formed of a curved plane which has a long side extending in the scanning direction in which the ray bundle is scanned and a positive power at least in the scanning direction, and the optical scanner further includes a support member for supporting the reflector at a first support point located in the vicinity of one end of the reflector in the long side direction, a second support point located in the vicinity of the other end of the reflector in the long side direction, and a third support point located in the vicinity of a center of the reflector in the long side direction so as to be more toward a concave side of the reflector plane than an imaginary straight line joining the first support point and the second support point.  
      Thus, an image formation apparatus which is hardly influenced by a vibration can be achieved.  
      Another method for supporting a reflector according to the present invention is a method for supporting, in an optical scanner including a reflector for reflecting a ray bundle to be scanned, the reflector, in which the reflector includes a reflection plane formed of a curved plane having a long side extending in the scanning direction in which a ray bundle is scanned and having a positive power at least in the scanning direction, and the reflector is supported at first, second and third support points arranged so as to surround a center of gravity of the reflector when viewed from the top.  
      According to the method, the center of gravity of the reflector is located inside of an imaginary triangle joining the first, second and third support points. Thus, even when an external force is applied, the reflector receives, at any one of the support points, a force in the opposite direction to the direction of a vibration. Therefore, a vibration of the reflector is suppressed.  
      Still another method for supporting a reflector according to the present invention is a method for supporting, in an optical scanner including a reflector for reflecting a ray bundle to be scanned, the reflector, in which the reflector includes a reflection plane formed of a curved plane having a long side extending in the scanning direction in which a ray bundle is scanned and having a positive power at least in the scanning direction, and the reflector is supported at first, second and third support points arranged so as to surround a shear center of a cross section of the reflector in a center portion of the reflector when viewed from the top.  
      According to the method, the shear center of a cross section of the reflector in the center portion is located inside of an imaginary triangle joining the first, second and third support points. Thus, even when an external force is applied, at least a portion of the reflector located in the center portion is hardly twisted. Therefore, a face tangle error of the reflection plane hardly occurs, so that influences of a vibration are suppressed.  
      It is preferable that the first and second support points are located in the vicinity of one end of the reflector in the long side direction, and the third support point is located in the vicinity of the other end of the reflector in the long side direction.  
      According to the method, each of the first, second and third support points is located in an end portion of the reflector. Thus, in part of the reflector other than the end portions, the generation of distortion due to being supported is suppressed. Therefore, in other part of the reflection plane other than the end portions, a face tangle error is effectively suppressed.  
      The first support point may be located in the vicinity of one end portion of the reflector in the long side direction, the second support point may be located in the vicinity of the other end of the reflector in the long side direction, and the third support point may be located in the vicinity of the center portion of the reflector in the long side direction.  
      According to the method, a distance between one support point and another is relatively uniform. Thus, the reflector is more stably supported.  
      It is preferable that the third support point is located so as to be more toward a concave side of the reflection plane of the reflector than an imaginary straight line joining the first support point and the second support point.  
      Thus, the first, second and third support points are arranged along the curve direction so as to correspond to a reflection plane being curved. Therefore, the reflector is supported according to the shape of the reflector plane, so that the reflector can be more stably supported.  
      It is preferable that the center of gravity of the reflector matches a shear center of a cross section of the reflector in a center portion of the reflector in the long side direction.  
      Thus, a vibration and a twist of the reflector are suppressed, so that a face tangle error of the reflection plane is suppressed furthermore.  
      It is preferable that the reflector is formed so that a front portion of the reflector in which the reflection plane is formed and a rear portion of the reflector located in a back side of the reflection plane are symmetrical to each other with respect to a plane including the middle of the reflector in the front-rear direction and having the long side direction of the reflector and the support direction of each of the support points.  
      Thus, the reflector is formed so as to have a form with which the reflector is stably supportable. Therefore, the reflector is more stably supported.  
      Another optical scanner according to the present invention includes: a light source for outputting a ray bundle; an optical deflector for scanning the ray bundle from the light source; a first image formation optical system, arranged between the light source and the optical deflector, for leading the ray bundle from the light source to a deflecting plane of the optical deflector and for forming a line image on the deflecting plane; and a second image formation optical system, arranged between the optical deflector and a scanning plane to be scanned, for leading the ray bundle from the optical deflector to the scanning plane and forming an image of a uniform spot on the scanning plane at a uniform velocity. In the optical scanner, the second image formation optical system includes a reflector having a reflection plane formed of a curved plane which has a long side extending in the scanning direction in which the ray bundle is scanned and a positive power at least in the scanning direction, the optical scanner further includes first, second and third support members for supporting the reflector at first, second and third support points, respectively, and a center of gravity of the reflector is located inside of an imaginary triangle joining the first, second and third support points when viewed from the top.  
      Still another optical scanner according to the present invention includes: a light source for outputting a ray bundle; an optical deflector for scanning the ray bundle from the light source; a first image formation optical system, arranged between the light source and the optical deflector, for leading the ray bundle from the light source to a deflecting plane of the optical deflector and for forming a line image on the deflecting plane; and a second image formation optical system, arranged between the optical deflector and a scanning plane to be scanned, for leading the ray bundle from the optical deflector to the scanning plane and forming an image of a uniform spot on the scanning plane at a uniform velocity. In the optical scanner, the second image formation optical system includes a reflector having a reflection plane formed of a curved plane which has a long side extending in the scanning direction in which the ray bundle is scanned and a positive power at least in the scanning direction, the optical scanner further includes first, second and third support members for supporting the reflector at first, second and third support points, respectively, and a shear center of a cross section of the reflector in a center portion of the reflector in the long side direction is located inside of an imaginary triangle joining the first, second and third support points when viewed from the top.  
      It is preferable that the first and second support points are located in the vicinity of one end of the reflector in the long side direction, and the third support point is located in the vicinity of the other end of the reflector in the long side direction.  
      The first support point may be located in the vicinity of one end of the reflector in the long side direction, the second support point may be located in the vicinity of the other side of the reflector in the long side direction, and the third support point may be located in the vicinity of a center portion of the reflector in the long side direction.  
      It is preferable that the third support point is located so as to be more toward a concave side of the reflection plane of the reflector than an imaginary straight line joining the first support point and the second support point.  
      It is preferable that the center of gravity of the reflector matches a shear center of a cross section of the reflector in the center portion of the reflector in the long side direction.  
      It is preferable that the reflector is formed so that a front portion of the reflector in which the reflection plane is formed and a rear portion of the reflector which is located in a back side of the reflection plane are symmetrical to each other with respect to a plane including the middle of the reflector in the front-rear direction and having the long side direction of the reflector and the support direction of each of the support points.  
      It is preferable that the reflector includes a synthetic resin member having a curved plane and a mirror surface film formed on the curved plane of the synthetic resin member.  
      Thus, the synthetic resin member can be processed in a simple manner and even a curved plane having a complex shape can be formed in a relatively simple manner. Moreover, the synthetic resin member can be formed at low cost, compared to the case in which a glass member.  
      It is preferable that the second formation optical system is formed of only the reflector.  
      The reflection surface of the reflector is formed of a free-form surface, so that the second image formation optical system can be formed of only the reflector.  
      Another image formation apparatus according to the present invention incluudes: an optical scanner; an approximately cylindrical photosensitive body having a rim surface to serve as a scanning plane and extending in the scanning direction in which a ray bundle is scanned in the optical scanner; driving mechanism for rotating the photosensitive body; a developer for supplying toner to the photosensitive body; and transferer for transferring a toner image formed on the photosensitive body to a recording medium. In the apparatus, the optical scanner includes a light source for outputting a ray bundle, an optical deflector for scanning a ray bundle from the light source, a first image formation optical system, arranged between the light source and the optical deflector, for leading the ray bundle from the light source to a deflecting plane of the optical deflector and for forming a line image on the deflecting plane, and a second image formation optical system, arranged between the optical deflector and a scanning plane to be scanned, for leading the ray bundle from the optical deflector to the scanning plane and forming an image of a uniform spot on the scanning plane at a uniform velocity, the second image formation optical system includes a reflector having a reflection plane formed of a curved plane which has a long side extending in the scanning direction in which the ray bundle is scanned and a positive power at least in the scanning direction, and the optical scanner further includes first, second and third support members for supporting the reflector at first, second and third support points, respectively, and a center of gravity of the reflector is located inside of an imaginary triangle joining the first, second and third support points when viewed from the top.  
      Thus, an image formation apparatus which is hardly influenced by a vibration can be achieved.  
      Still another image formation apparatus includes: an optical scanner; an approximately cylindrical photosensitive body having a rim surface to serve as a scanning plane and extending in the scanning direction in which a ray bundle is scanned in the optical scanner; driving mechanism for rotating the photosensitive body; a developer for supplying toner to the photosensitive body; and transferer for transferring a toner image formed on the photosensitive body to a recording medium. In the apparatus, the optical scanner includes a light source for outputting a ray bundle, an optical deflector for scanning a ray bundle from the light source, a first image formation optical system, arranged between the light source and the optical deflector, for leading the ray bundle from the light source to a deflecting plane of the optical deflector and for forming a line image on the deflecting plane, and a second image formation optical system, arranged between the optical deflector and a scanning plane to be scanned, for leading the ray bundle from the optical deflector to the scanning plane and forming an image of a uniform spot on the scanning plane at a uniform velocity, the second image formation optical system includes a reflector having a reflection plane formed of a curved plane which has a long side extending in the scanning direction in which the ray bundle is scanned and a positive power at least in the scanning direction, and the optical scanner further includes first, second and third support members for supporting the reflector at first, second and third support points, respectively, and a shear center of a cross section of the reflector in a center portion of the reflector in the long side direction is located inside of an imaginary triangle joining the first, second and third support points when viewed from the top.  
      Thus, an image formation apparatus which is hardly influenced by a vibration can be achieved.  
     EFFECTS OF THE INVENTION  
      According to the present invention, a reflector is supported at end portions of the reflector and also at a point which is located in the vicinity of a center portion of the reflector so as to be more toward a concave side of a reflection plane than an imaginary straight line joining respective supporting points in the end portions. Thus, even with the reflection plane curved from the long side direction of the reflector, the reflector is stably supported, so that a vibration can be suppressed.  
      If corresponding portions of the reflector located in the opposite side of the reflector to a side thereof in which the support points are located are pressed, the reflector can be more stably supported.  
      By providing a rib extending in a back side of a reflector body, the strength of the reflector can be improved. Moreover, the strength is ensured by the rib, and thus the reflector body can be formed so as to have a small thickness. Therefore, accuracy in processing the reflection plane can be improved.  
      If the center of gravity of a portion of the reflector located between one of the support points and another is located on an imaginary straight line joining one of the support points and another, a twist of the reflector can be suppressed, so that a face tangle error of the reflection plane can be suppressed. Moreover, if the center of gravity of the reflector and the shear center of a cross section of the reflector can be made to match each other, distortion of the reflector can be effectively suppressed.  
      By setting the depth of the center portion of the reflector to be longer than that of each of the end portions, the area of an imaginary triangle joining the first, second and third support points can be increased. Thus, the reflector can be more stably supported.  
      By setting a second moment of area of a cross section of the reflector in a center portion to be larger than that of each of the end portions, twist deformation of the reflector can be suppressed. Thus, a face tangle error of the reflection plane can be suppressed.  
      By setting the area of a cross section of the reflector in the vicinity of each of the support points to be larger than that of a portion of the reflector located between one of the support points and another, the weight of the portion located between one of the support points and another can be reduced, compared to the vicinity of each of the support points. Thus, a vibration can be effectively suppressed.  
      According to the present invention, the reflector is supported at three points so that the center of gravity of the reflector is located inside of an imaginary triangle joining the first, second and third support points when viewed from the top. Thus, influences of a vibration can be suppressed.  
      Moreover, by supporting the reflector at three points so that the shear center of a cross section of the reflector in the center portion is located inside of an imaginary triangle joining the first, second and third support points when viewed from the top, a twist caused due to a vibration can be suppressed. Thus, influences of a vibration can be suppressed.  
      If the first and second support points are located in the vicinity of one end of the reflector and the third support point is located in the vicinity of the other end of the reflector, a face tangle error in part of the reflector other than the end portions can be effectively suppressed.  
      If the first support point is located in the vicinity of one end of the reflector, the second support point is located in the vicinity of the other end of the reflector, and the third support point is located in the vicinity of a center point of the reflector, a distance between one of the support point and another can be made relatively uniform. Thus, the reflector can be stably supported.  
      If the third support point is located more toward a concave of the reflection plane than an imaginary straight line joining the first support point and the second support point. Thus, the reflector can be supported in a form according to the shape of a curve of the reflection plane.  
      By making the center of gravity of the reflector and the shear center of a cross section in the center portion match each other, a face tangle error of the reflector can be suppressed furthermore.  
      By forming the reflector so as to be symmetric in the front-rear direction, the reflector can be more stably supported.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a plan view of an optical scanner according to an embodiment of the present invention.  
       FIG. 2  is a perspective view illustrating a main portion of the optical scanner of the embodiment.  
       FIG. 3  is a perspective view illustrating an optical scanner and a photosensitive drum.  
       FIGS. 4A, 4B  and  4 C are explanatory illustrations of a reflector according to an embodiment:  FIG. 4A  is a plan view of the reflector;  FIG. 4B  is a front view thereof; and  FIG. 4C  is a side view thereof.  
       FIGS. 5A, 5B  and  5 C are explanatory illustrations of a reflector according to a modified example of the embodiment:  FIG. 5A  is a plan view of the reflector;  FIG. 5B  is a front view thereof; and  FIG. 5C  is a cross-sectional view thereof taken along the line V-V of  FIG. 5B .  
       FIGS. 6A, 6B  and  6 C are explanatory illustrations of a reflector according to a modified example of the embodiment:  FIG. 6A  is a plan view of the reflector;  FIG. 6B  is a front view thereof; and  FIG. 6C  is a side view thereof.  
       FIGS. 7A, 7B  and  7 C are explanatory illustrations of a reflector according to a modified example of the embodiment:  FIG. 7A  is a plan view of the reflector;  FIG. 7B  is a front view thereof; and  FIG. 7C  is a cross-sectional view thereof taken along the line VII-VII of  FIG. 7B .  
       FIGS. 8A, 8B  and  8 C are explanatory illustrations of a reflector according to an embodiment:  FIG. 8A  is a plan view of the reflector;  FIG. 8B  is a front view thereof; and  FIG. 8C  is a side view thereof.  
       FIGS. 9A, 9B  and  9 C are explanatory illustrations of a reflector according to a modified example of the embodiment:  FIG. 9A  is a plan view of the reflector;  FIG. 9B  is a front view thereof; and  FIG. 9C  is a side view thereof.  
       FIGS. 10A, 10B  and  10 C are explanatory illustrations of a reflector according to a modified example of the embodiment:  FIG. 10A  is a plan view of the reflector;  FIG. 10B  is a front view thereof; and  FIG. 10C  is a side view thereof.  
       FIG. 11  is a cross-sectional view schematically illustrating an image formation apparatus according to an embodiment of the present invention.  
       FIG. 12  is an explanatory illustration of a reflector having a reflection plane formed of a free-form plane.  
       FIGS. 13A, 13B  and  13 C are explanatory illustrations of a known reflector according to a modified example of the embodiment:  FIG. 13A  is a plan view of the reflector;  FIG. 13B  is a front view thereof; and  FIG. 13C  is a cross-sectional view thereof taken along the line XIII-XIII of  FIG. 13B . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.  
     Embodiment 1  
      As shown in  FIGS. 1 and 2 , an optical scanner  1  according to this embodiment includes a light source unit  2 , a polygon mirror  9 , a reflector  10  and a synchronization sensor  13 . These members are provided in a case  15 . Note that the right hand side of  FIG. 1  is referred to as a “rear side” and the left hand side of  FIG. 1  is referred to as a “front side” for convenience.  
      The light source unit  2  is formed of an assembly of a laser driving substrate (which will be hereinafter referred to as a semiconductor laser)  3  in which a semiconductor laser circuit is provided, a collimator lens  4 , a main concave cylinder lens  5  and a sub convex cylinder lens  6 . In the direction in which laser light of the light source unit  2  is irradiated, i.e., in the front of the light source unit  2 , a deflecting mirror  7  is provided. A main convex cylinder lens  8  is provided between the deflecting mirror  7  and the polygon mirror  9 .  
      The collimator lens  4 , the main concave cylinder lens  5 , the sub convex cylinder lens  6 , and the main concave cylinder lens  8  lead beam (a ray bundle) from the semiconductor laser  3  to a deflecting plane of the polygon mirror  9  and also together form a first image formation optical system for forming a line image on the deflecting plane. Note that in this application, an element such as the deflecting mirror  7  which merely reflects light by a flat plane thereof is not included in the image formation optical system.  
      The polygon mirror  9  is a rotating polygon mirror including a plurality of reflection planes (deflecting planes) and is rotary-driven by a motor (not shown). Due to a rotation of the polygon mirror  9 , light reflected by the polygon mirror  9  is scanned in the following order: a beam  60   a , a beam  60   b  and a beam  60   c . Note that the three beams  60   a ,  60   b  and  60   c  are illustrated at the same time for convenience, but in an actual situation, a single beam is scanned in the long side direction of a reflector  10  at a time. The reflector  10  for reflecting a beam from the polygon mirror  9  is provided in the front of the deflecting mirror  7 . Details of the reflector  10  will be described later.  
      As shown in  FIG. 2 , deflecting mirrors  11  and  12  are provided in the rear of the reflector  10 . Each of the deflecting mirrors  11  and  12  is formed so as to have a long length. The deflecting mirror  12  is provided under the deflecting mirror  11 . Then, a beam reflected by the reflector  10  is reflected by the deflecting mirrors  11  and  12  in this order and irradiated in the frontward direction.  
      In the rear of a start point in the scanning direction (the position of an end potion in the left hand side of  FIG. 2 ) in the reflector  10 , a deflecting mirror  14  for reflecting light only when a beam is located in the point is provided. Light reflected by the deflecting mirror  14  is entered into a synchronization sensor  13 . That is, at a start time of scanning, light is entered into the synchronization sensor  13  and a start of scanning is detected.  
      As shown in  FIG. 3 , a beam irradiated from the optical scanner  1  is lead onto a photosensitive drum  16  having a cylindrical shape. A rim surface of the photosensitive drum  16  forms a scanning plane on which a beam from the optical scanner is scanned and is covered with a photosensitive body in which charges vary when light is irradiated thereto. A beam from the optical scanner  1  is scanned, so that a beam spot is scanned on the photosensitive drum  16  in the parallel direction to the axis direction of the photosensitive drum  16  (i.e., a main scanning direction). The photosensitive drum  16  is rotary-driven by a motor, i.e., a driving mechanism (not shown). Thus, by a combination of scanning of a beam and rotation of the photosensitive drum  16 , a two-dimensional latent image is formed on a surface of the photosensitive drum  16 .  
      Next, the reflector  10  and a method for supporting the same will be described.  
      The reflector  10  forms a second image formation optical system for leading a beam from the polygon mirror  9  to a scanning plane of the photosensitive drum  16  and forming an image of a uniform spot at a uniform velocity on the scanning plane. As described above, the reflector  10  is formed so as to have a long side along the direction in which light is scanned. The reflector  10  includes a thin-plate-shaped reflector body  23  (see  FIG. 1 ) having a reflection plane  20 , upper and lower ribs  21   a  and  21   b  (see  FIG. 4B ) each extending from an upper or lower end of the reflector body  23  in the back side direction (i.e., the left hand side direction in  FIG. 1  or, in other words, the front of the optical scanner  1 ), and end portion ribs  22  each extending to from a right or left end of the reflector body  23  in the back side direction and is formed as a unit by plastic resin. Note that each of the ribs  21   a  and  21   b  is formed so as to have a long side in the scanning direction as the reflector body  23 .  
      A metal layer is formed on one plane (an obverse side plane) of the reflector body  23  and the metal layer forms the reflection plane  20  as a mirror surface. The reflection plane  20  is a curved plane having a long side in the direction in which a ray bundle is scanned and a positive power at least in the scanning direction. In other words, the reflector  20  is a curved plane which is curved in a concave arc at least so that a center portion of the plane in the long side direction is located more toward a back side with respect to the front-rear direction of the reflector  10  than each of end portions of the reflector  20 . Also, the reflection plane  20  is a three-dimensional curved plane having an approximate C shaped lateral cross section and also an approximate C shaped longitudinal cross section. Furthermore, the reflection plane  20  is formed of a so-called free-form surface whose lateral cross section does not have a constant shape in the long side direction. This is because the second image formation optical system is formed of only the reflector  10 . A specific shape of the reflection plane  20  can be appropriately set based on a distance between the optical scanner  1  and the photosensitive drum  16 , a specification of each optical system and the like.  
      As shown in  FIG. 4 , the reflector  10  is supported by the first, second and third protrusions  31 ,  32  and  33  provided in the case  15  at three points in the end portions and center portion thereof. Specifically, the reflector  10  is supported at a first support point S 1  located in the vicinity of one end of the reflector  10  in the long side direction, a second support point S 2  located in the vicinity of the other end thereof, and a third support point S 3  located around the center of the reflector  10 . As shown in  FIG. 4A , the third support point S 3  is located more toward a concave side (the back side with respect to front-rear direction of the reflector  10 ) of the reflection plane  20  than an imaginary straight line V 1  joining the first support point S 1  and the second support point S 2 . In other words, the third support point S 3  is located more toward a back side with respect to the front-rear direction of the optical scanner  1  than the imaginary straight line V 1 . Accordingly, the first support point S 1 , the third support point S 3  and the second support point S 2  are not arranged in line but along a curve shape of the reflector body  23 .  
      To stably support the reflector  10 , it is preferable that a distance L 1  between the third support point S 3  and the imaginary straight line V 1  is set to be as long as possible. In this case, the distance L 1  between the third support point S 3  and the imaginary straight line V 1  is larger than a sag L 2  of the reflector body  23  in the scanning direction.  
      On parts of the reflector  10  corresponding to the first, second and third support points S 1 , S 2  and S 3 , provided are first, second and third pressure springs  41 ,  42  and  43 , respectively. Each of the corresponding parts may be directly on each of the first, second and third support points S 1 , S 2  and S 3  and also in the vicinity of each of the first, second and third support points S 1 , S 2  and S 3 . Each of the pressure springs  41 ,  42  and  43  presses the reflector  10  in the downward direction. Therefore, the reflector  10  is sandwiched between each of the first, second and third pressure springs  41 ,  42  and  43  and each of the first, second and third protrusions  31 ,  32  and  33 .  
      At the first, second and third support points S 1 , S 2  and S 3 , the protrusions  31 ,  32  and  33  contact against the lower rib  21   b . Moreover, the first, second and third pressure springs  41 ,  42  and  43  contact against the upper rib  21   a . However, at the first and second support points S 1  and S 2 , the protrusions  31  and  32  may contact against the reflector body  23 . Also, the first and second pressure springs  41  and  42  may press against the reflector body  23 . With the reflector body  23  pressed or supported at both of the end portions thereof, distortion might be caused in the vicinity of the pressed or supported portions of the reflector body  23 . In this embodiment, however, both of the end portions of the reflector  10  are not used as reflection planes (i.e., light is not reflected at both of the end portions) and, therefore, actual problems hardly arise. By pressing or supporting the reflector body  23  at both of the end portions, the area of an imaginary triangle V 3  joining the first, second and third support points S 1 , S 2  and S 3  or the area of an imaginary triangle joining the first, second and third pressure points can be increased. Thus, the reflector  10  can be stably supported.  
      In the optical scanner  1  of this embodiment, the reflector  10  is supported at three points, i.e., the first, second and third support points S 1 , S 2  and S 3  which are not arranged in a straight line and thus the reflector  10  is stably supported. Therefore, even when an external force (e.g., a vibration of the motor of the polygon mirror  9  and external impact) is applied to the reflector  10 , the reflector  10  hardly vibrates and a face tangle error of the reflection plane  20  can be effectively prevented.  
      Specifically, in this embodiment, the distance L 1  between the imaginary straight line V 1  joining the first support point S 1  and the second support point S 2  and the third support point S 3  is larger than the sag L 2  of the reflection plane  20  in the scanning direction. Therefore, the reflector  10  can be stably supported.  
      Furthermore, the reflector  10  is pressed at the respective corresponding points to the first, second and third support points S 1 , S 2  and S 3 . Thus, the reflector  10  can be more firmly held.  
      The reflector  10  is formed of a synthetic resin. Thus, even with the reflection plane  20  having a complicated shape, the reflection plane  20  can be achieved at low cost. Moreover, each of the ribs  21   a ,  21   b  and  22  is provided so as to extend from the reflector body  23  in the back side of the reflection plane  20 . Thus, even if the reflector body  23  is formed so as to have a very small thickness, the strength of the entire reflector  10  can be maintained at a high level. By forming the reflector body  23  so as to have a small thickness, the generation of sinks of materials caused during processing can be suppressed. Therefore, processing accuracy for the reflector plane  20  can be improved.  
      At the third support point S 3  and the pressure point corresponding to the third support point S 3 , the ribs  21   a  and  21   b  are supported and pressed. Thus, support and pressure forces do not directly affect the reflector body  23 . Therefore, distortion in the reflection plane  20  to be generated as the center potion of the reflector  10  are supported and pressed can be suppressed.  
      As has been described, with the optical scanner  1 , the reflector  10  including the reflection plane  20  formed of a free-form surface can be stably supported. The second image formation optical system is formed of only a reflection type optical element, originally, and thus the optical scanner  1  is easily influenced by a vibration in the first place, compared to an apparatus using a light transmission type optical element. However, in the optical scanner  1  of this embodiment, the reflector  10  can be stably supported and, therefore, a vibration of the reflector  10  can be suppressed at a high level. Accordingly, performance of the optical scanner  1  can be improved. Moreover, this can facilitate reduction in the size of optical scanners.  
      The shape of the reflector  10  and a method for supporting the reflector  10  are not limited to the above-described shape and supporting method. Next, modified examples for the shape of the reflector  10  and the method for supporting the reflector  10  will be described.  
      A modified example shown in  FIGS. 5A, 5B  and  5 C is obtained by changing the pressure point at which pressure is applied by the third pressure spring  43 . As shown in  FIG. 5C , in this example, the third pressure spring  43  presses the lower rib  21   b  located in the center portion of the reflector  10 . Thus, the pressure spring  43  presses the rib  21   b  itself supported by the protrusion  33 . Accordingly, distortion in the reflector  10  due to a pressure force of the third pressure spring  43  can be suppressed.  
      A modified example shown in  FIG. 6  is obtained mainly by changing the shapes of the ribs  21   a  and  21   b . The reflector  10  can be considered as a combination of two separate parts, i.e., right and left parts into which the reflector  10  is divided with the third support point S 3  assumed to be a boundary. That is, the reflector  10  can be divided, with the third support point S 3  as a boundary, into a first portion  10   a  located in the first support point S 1  and a second portion  10   b  located in the second support point S 2  side. In this example, the center of gravity G 1  of the first portion  10   a  is located on an imaginary straight line Q 1  joining the first support point S 1  and the third support point S 3 . Moreover, the center of gravity G 2  of the second portion  10   b  is located on an imaginary straight line Q 2  joining the second support point S 2  and the third support point S 3 . Note that “being located in a straight line” not only means to be located on a straight line in a strict sense but also being slightly shifted from a straight line. That is, the case where the center of gravity G 1  or the center of gravity G 2  can be considered to be substantially located on a straight line is included.  
      The reflector  10  is fixed by the protrusions  31 ,  32  and  33  and the pressure springs  41 ,  42  and  43 . Thus, when a disturbance is applied to the reflector  10 , with the reflector  10  fixedly supported at each of the support points S 1 , S 2  and S 3  and the pressure points corresponding to the support points S 1 , S 2  and S 3 , a minute vibration of the reflector  10  is caused. In this case, an inertial force which acts in each member between the support points, i.e., the first potion  10   a  and the second portion  10   b  is considered to act in each of the centers of gravity G 1  and G 2 . Therefore, if the centers of gravity G 1  and G 2  are located off the imaginary straight lines Q 1  and Q 2 , respectively, a twist moment M (see  FIG. 6C ) which might cause a face tangle error of the reflection plane  20  is generated in the reflector  10 . In this example, however, the centers of gravity G 1  and G 2  are located on the imaginary straight lines Q 1  and Q 2 , respectively, the generation of such a twist moment M can be suppressed.  
      Note that in the modified examples, a shear center of a cross section at the center portion of the first portion  10   a  (i.e., a lateral cross section located at a middle point between a lateral cross section including the first support point S 1  and a lateral cross section including the third support point S 3 ) corresponds to the center of gravity G 1  of the first portion  10   a . Moreover, a shear center of a cross section at the center portion of the second potion  10   b  (i.e., a lateral cross section located at a middle point between a lateral cross section including the second support point S 2  and a lateral cross section including the third support point S 3 ) corresponds to the center of gravity G 2  of the second portion  10   b . Accordingly, the shear center of the cross section at the center portion of the first portion  10   a  is located on the imaginary straight line Q 1  joining the first support point S 1  and the third support point S 3  and the shear center of the cross section at the center portion of the second portion  10   b  is located on the imaginary straight line Q 2  joining the second support point S 2  and the third support point S 3 . With the shear center of the cross section at the center portion of each of the first and second portions  10   a  and  10   b  located on the imaginary straight lines Q 1  and Q 2 , respectively, distortion of the reflector  10  can be suppressed furthermore. Therefore, a face tangle error of the reflection plane  20  can be more effectively suppressed.  
      Moreover, in the modified example of  FIG. 6 , the area of a cross section in the vicinity of each of the support points S 1 , S 2  and S 3  is larger than the area of a cross section located between the first and third support points S 1  and S 3  and the area of a cross section located between the second and third support points S 2  and S 3 . Accordingly, compared to the vicinity of each of the support points S 1 , S 2  and S 3 , the weight of each portion between one of the support points and another is reduced. Thus, even when an external force is applied to the reflector  10 , an inertial force is hardly generated in each portion between one of the support points and another, compared to the vicinity of each of the support points S 1 , S 2  and S 3 . Therefore, each portion between one of the support points and another hardly vibrates, compared to the vicinity of each of the support portions. On the other hand, the vicinity of each of the support points S 1 , S 2  and S 3  is supported and therefore no vibration occurs in the vicinity of each of the support points S 1 , S 2  and S 3  in the first place. Therefore, according to this example, a vibration of the reflector  10  can be suppressed and a face tangle error of the reflection plane  20  can be effectively suppressed.  
      A modified example shown in  FIG. 7  is obtained by changing the ribs  21   a  and  21   b  so that a depth of the center portion of the reflector  10  (i.e., a length in the up-down direction of an optical scanner of  FIG. 7A ) is larger than a depth of each of the end portions thereof. Moreover, in this example, the third pressure spring  43  presses the lower side rib  21   b . In this example, the depth of each of the ribs  21   a  and  21   b  gradually decreases in the direction from the center potion of the reflector  10  to each of the end potions thereof. Therefore, the area of a lateral cross section of the reflector  10  is larger in the center portion than in each of the end portions. Moreover, a second moment of area in the lateral cross section of the reflector  10  is larger in the center portion than in each of the end portions.  
      According to this example, the center portion of the reflector  10  can be more firmly supported, so that a face tangle error of the reflection plane  20  in the center portion can be effectively prevented. Moreover, even when thermal expansion occurs in the reflector  10 , the reflector  10  can easily stretch in the direction from the center portion to each of the end portions. Thus, a thermal stress in the reflector  10  is hardly generated. Accordingly, distortion due to a thermal stress is hardly generated and thus a twist of the reflector  10  can be suppressed. As a result, a face tangle error of the reflection plane  20  can be suppressed.  
     Embodiment 2  
      As shown in  FIG. 8 , in an optical scanner  1  according to this embodiment, the first, second and third support points S 1 , S 2  and S 3  are provided so that the center of gravity G of the reflector  10  is located inside of the imaginary triangle V 3  joining the support points S 1 , S 2  and S 3  when viewed from the top. Moreover, although illustration is omitted, a shear center of a cross section of reflector  10  in the center potion in the long side direction is located inside of the imaginary triangle V 3 . In other points, the optical scanner of this embodiment is substantially the same as the optical scanner of EMBODIMENT 1.  
      As described above, in the optical scanner  1 , the reflector  10  is supported so that the center of gravity G is located inside of the imaginary triangle V 3  joining the support points S 1 , S 2  and S 3 . Thus, even when an external force (e.g., a vibration of the motor of the polygon mirror  9  and external impact) is applied to the reflector  10 , the reflector  10  receives a force in the reverse direction to the direction in which the reflector  10  vibrates. For example, when the reflector  10  receives an external force and is likely to rotate backward with respect to the imaginary straight line V 1  joining the first support point S 1  and the second support point S 2 , the reflector  10  receives from the support point  33  a force in the reverse direction to the direction in which the reflector  10  is likely to rotate. Thus, rotation of the reflector  10  is prevented. Therefore, with the optical scanner  1 , even when an external force is applied to the reflector  10 , the reflector  10  hardly vibrates and a face tangle error of the reflection plane  20  can be effectively suppressed.  
      Moreover, the shear center of a cross section of the reflector  10  in the center portion in the long side direction is also located inside of the imaginary triangle V 3  joining the support points S 1 , S 2  and S 3 . Thus, even when a force is applied to the reflector  10 , a twist can be suppressed.  
      Specifically, in this embodiment, the reflector  10  is supported at three points, i.e., at the vicinity of each of the end portions and the vicinity of the center portion. Thus, a distance between one support point and another can be made uniform. Therefore, the reflector  10  can be stably supported.  
      Moreover, in this embodiment, the reflector  10  is pressed at points corresponding to the first, second and third support points S 1 , S 2  and S 3 . Thus, the reflector  10  can be firmly held.  
      Moreover, the reflector  10  is formed of a synthetic resin. Thus, even when the reflection plane  20  has a complicated shape, the reflector plane  20  can be achieved at low cost. Moreover, the ribs  21   a ,  21   b  and  22  are provided so as to extend from the reflector body  23  in the back side of the reflection plane  20 . Thus, even if the reflector body  23  is formed so as to have a very small thickness, the strength of the entire reflector  10  can be maintained at a high level. By forming the reflector body  23  so as to have a small thickness, the generation of sinks of materials caused during processing can be suppressed. Therefore, accuracy in processing the reflector plane  20  can be improved.  
      At the third support point S 3  and the pressure point corresponding to the third support point S 3 , the ribs  21   a  and  21   b  are supported and pressed. Thus, support and pressure forces do not directly affect the reflector body  23 . Therefore, distortion in the reflection plane  20  caused by supporting and pressing the center potion of the reflector  10  can be suppressed.  
      As has been described, in the optical scanner  1 , the reflector  10  including the reflection plane  20  formed of a free-form surface can be stably supported. The second image formation optical system is formed of only a reflection type optical element, and thus the optical scanner  1  is easily influenced by a vibration, originally, compared to an apparatus using a light transmission type optical element. However, in the optical scanner  1  of this embodiment, the reflector  10  can be stably supported and, therefore, a vibration of the reflector  10  can be suppressed. Accordingly, performance of the optical scanner  1  can be improved. Moreover, this can facilitate reduction in the size of optical scanners.  
      The shape of the reflector  10  and a method for supporting the reflector  10  are not limited to the above-described shape and supporting method. Next, modified examples for the shape of the reflector  10  and the method for supporting the reflector  10  will be described.  
      A modified example shown in  FIG. 9  is obtained by changing the numbers and positions of support points and pressure points. Specifically, each of the first support point S 1  and the second support point S 2  is located in the vicinity of one end of the reflector  10  while the third support point S 3  is located in the vicinity of the other end of the reflector  10 . The first support point S 1  and the second support point S 2  are arranged in the front-rear direction (i.e., the up-down direction in  FIG. 9A ). In this example, the center of gravity G of the reflector  10  and the shear center (not shown) of a cross section of the reflector  10  in the center portion in the long side direction is located inside of the imaginary triangle V 3  joining the support points S 1 , S 2  and S 3 .  
      Accordingly, in this example, even when an external force is applied to the reflector  10 , the reflector  10  hardly vibrates and also hardly twists at least in the center portion. Therefore, a face tangle error of the reflection plane  20  can be effectively suppressed.  
      In addition, according to this example, each of the first, second and third support points S 1 , S 2  and S 3  is located in the vicinity of an end potion of the reflector  10 . Thus, in other part of the reflector  10  than the vicinity of each of the end portions, the generation of minute distortion to be locally generated due to a support force can be prevented. Note that in this example, part of the reflection plane  20  other than the end portions is used for reflection of light. Therefore, even if minute distortion due to a support force is generated in each of the end portions, no particular problem actually arises.  
      A modified example shown in  FIG. 10  is obtained by further changing the shape of the reflector  10 . The reflector  10  of this example is formed so as to be symmetric in the front-rear direction. Specifically, the reflector  10  is formed so that a front portion of the reflector  10  in which the reflection plane  20  is formed and a rear portion of the reflector  10  located in the back side of the reflection plane  20  are symmetrical to each other with respect to an imaginary plane L 3  having the long side direction (i.e., the left-right direction of  FIG. 10B ) and the support direction of each of the support points S 1 , S 2  and S 3  (i.e., the up-down direction of  FIG. 10B ) at the middle of the reflector  10  in the front-rear direction. Note that in this example, the reflector  10  is formed of a solid rod-shaped body.  
      In this example, the center of gravity G is also located inside of the imaginary triangle joining the first, second and third support points S 1 , S 2  and S 3 . Moreover, in the reflector  10  of this embodiment, the center of gravity G matches the shear center of a cross section of the reflector  10  at the center in the long side direction. Therefore, the shear center is also located inside of the imaginary triangle V 3 .  
      According to this example, in addition to the above-described effects, the reflector  10  itself is formed so as to be stably supportable. Thus, the reflector  10  can be more stably supported and influences of a vibration can be suppressed furthermore. Moreover, the center of gravity G and the shear center match each other, so that a vibration and a twist of the reflector  10  can be effectively suppressed.  
     Embodiment 3  
      An image formation apparatus according to this embodiment includes the optical scanner  1 . Next, embodiments of an image formation apparatus in which the optical scanner  1  is provided. The image formation apparatus including the optical scanner  1  can be used for various types of image formation apparatuses such as a laser beam printer, a laser facsimile machine and a digital copy machine. As the optical scanner  1 , the reflector  10  according to any one of the above-described embodiments and the reflector  10  according to any one of the above-described modified examples can be used.  
      As shown in  FIG. 11 , the optical scanner  1  (illustration of the case  15  and other elements is omitted) including the light source unit  2 , the polygon mirror  9  and the reflector  10  is stored in a casing  51  an image formation apparatus  50 . Moreover, in the casing  51 , a photosensitive drum  16 , a primary charger  52  for attaching electrostatic ions to a rim surface of the photosensitive drum  16  to charge, a developer  53  for attaching charged toner to a printing section, a transfer charger  54  for transferring attached toner to a print paper, a cleaner  55  for removing remaining toner, a printer fuser  56  for fusing transferred toner into the print paper, and a paper feed cassette  57  are provided.  
      In the image formation apparatus  50  of this embodiment, the above-described optical scanner  1  is used. Thus, reduction in the size of and costs for the apparatus and improvement of performance of the apparatus can be achieved.  
      Note that in the optical scanner  1 , the reflector  10  is supported by the protrusions  31 ,  32  and  33  formed in the case  15 . However, the protrusions  31 ,  32  and  33  do not have to be united as one but each of them may be formed of a separate member from the case  15 . Moreover, the protrusions  31 ,  32  and  33  can be provided in the reflector  10 .  
      In each of the embodiments of  FIGS. 4 and 8 , the third support point S 3  does not have to be located at the middle of the reflector  10  in the long side direction but may be located at a point shifted from the middle thereof. That is, the third support point S 3  can be located substantially in the vicinity of the center potion.  
      The pressure springs  41 ,  42  and  43  are provided in points corresponding to the support points S 1 , S 2  and S 3 , respectively, but may be provided at different points. Moreover, the number of pressure springs does not have to match the number of supporting points. Furthermore, the pressure springs  41 ,  42  and  43  are useful for firmly supporting the reflector  10  but are not always necessary.  
      A material for the reflector  10  is not limited to a synthetic resin but some other material can be used. If the second image formation optical system is not formed of only the reflector  10 , the reflection plane  20  of the reflector  10  does not have to be a free-form surface.  
      As has been described, the present invention is useful for an image formation apparatus such as a laser beam printer, a laser facsimile machine and digital copy machine, and an optical scanner used in the image formation apparatus.