Abstract:
The invention comprises a laser beam source that emits a laser beam; a reflecting means that has a plurality of reflecting members and reflects the laser beam emitted from said laser beam source; a deflecting and scanning means that deflects and scans with the laser beam reflected by said reflecting means; and an adjusting means that moves at least part of said reflecting members, and adjusts the size of the laser beam entering the deflecting and scanning means.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    (a) Field of the Invention 
         [0002]    The present invention relates to an image forming apparatus such as an electrophotographic copying machine or printer, and, more particularly, to a method of adjusting a spot diameter with a scanning-type optical device. 
         [0003]    (b) Description of the Related Art 
         [0004]    Referring to  FIGS. 11 and 12 , the related art will be described below. 
         [0005]      FIG. 11  illustrates the entire structure of an image forming apparatus. A photosensitive drum  21  that is an example of an image bearing member has a photosensitive layer applied onto a conductive body, and forms an electrostatic latent image with laser beams emitted from a scanning-type optical device  23 . Reference numeral  22  denotes an image reading unit that reads image information from an original placed onto an original base plate. Reference numeral  23  denotes the scanning-type optical device that irradiates the photosensitive drum  21  with the laser beams based on the image information transmitted from the image reading unit  22  or a personal computer or the like. Reference numeral  24  denotes a charger that charges the photosensitive drum  21 . Reference numeral  25  denotes a developing unit that forms a toner image with triboelectrically charged toner, based on the electrostatic latent image formed on the photosensitive drum  21 . Reference numeral  26  denotes a sheet feeding cassette that stores sheets that serve as recording materials. Reference numeral  27  denotes a fixing unit that fixes the toner image transferred onto a sheet by virtue of heat. Reference numeral  28  denotes a post processing unit on which image-fixed sheets are collectively placed. Reference numeral  29  denotes a cleaner that removes the toner remaining on the photosensitive drum  21  after the image transfer. 
         [0006]    The scanning-type optical device  23  emits laser beams based on image information onto the photosensitive drum  21 , so as to form an electrostatic latent image onto the photosensitive drum  21  charged by the charger  24 . Toner triboelectrically charged in the developing unit  25  is then attached onto the electrostatic latent image, thereby forming a toner image on the photosensitive drum  21 . The toner image is transferred onto an intermediate transfer belt from the photosensitive drum  21 . The toner image is further transferred onto a sheet conveyed from the sheet feeding cassette  26  located at the lower part of the apparatus, so that an image is formed on the sheet. The toner of the image transferred onto the sheet is fixed by the fixing unit  27 , and the sheet is placed onto the sheet output tray of the post processing unit  28 . 
         [0007]    Referring now to  FIG. 12 , the scanning-type optical device  23  is described. Reference numeral  30  denotes a laser beam source that emits beams based on image information. Reference numeral  31  denotes a collimating lens that turns each laser beam emitted from the laser beam source  30  into a collimated light beam. Reference numeral  32  denotes a cylindrical lens that concentrates each laser beam to a belt-like form. Reference numeral  33  denotes a rotary polygon mirror that deflectively scans with a laser beam. Reference numeral  34  denotes a first imaging lens that concentrates a laser beam onto the photosensitive drum  21 . Reference numeral  35  denotes a synchronization sensor that senses synchronization in the main scanning direction. Reference numeral  36  denotes an imaging lens that concentrates a laser beam that enters the synchronization sensor  35 . Reference numeral  37  denotes an optical housing that houses the above described components. The opening of the optical housing  37  is covered with a lid. 
         [0008]    In the image forming apparatus equipped with the scanning-type optical device  23 , the temperature inside the image forming apparatus rises due to the heat generated from the fixing unit  27  and each motor or the like, and the spot diameter on the photosensitive drum  21  might become greater than a predetermined value. This is because there are changes in the refractive indexes of the lenses provided in the scanning-type optical device  23 , due to the heat. 
         [0009]    Also in the laser beam source, there are cases where the diameters of emitted laser beams differ from one another due to a temperature rise caused by the laser beam emission. 
         [0010]    To counter this problem, glass lenses that exhibit smaller refractive index changes than the changes in the refractive indexes of the above described lenses are employed to restrict the variation of the spot diameters on the photosensitive drum  21 . Japanese Unexamined Patent Publication No. 2002-006211 discloses a structure that sets the refractive indexes of lenses so as to cancel a focus deviation caused due to a temperature rise. However, any of the conventional methods cannot sufficiently solve the problem of spot diameter variations due to a temperature rise in the laser beam source. 
         [0011]    To further counter this problem, Japanese Unexamined Patent Publication No. 10-142546 discloses a structure that adjusts a focus deviation by inserting a focus lens between an fθ lens and a photosensitive drum. Japanese Unexamined Patent Publication No. 09-105876 discloses a structure that measures each spot diameter with a CCD line sensor, and adjusts the spot diameter by moving the cylindrical lens. Japanese Unexamined Patent Publication No. 07-261103 discloses a structure that measures the magnification in each scanning operation with a sensor provided at either end outside the image region, and moves a mirror so as to reduce the magnification error. 
         [0012]    In any of the above conventional scanning-type optical devices, a spot diameter is adjusted by moving a lens or a mirror along the light path. Therefore, it is necessary to secure a sufficient width for the movement and to employ a motor or the like. As a result, the apparatus becomes large in size. 
       SUMMARY OF THE INVENTION 
       [0013]    A general object of the present invention is to provide a structure that can adjust the spot diameter of a laser beam, without an increase in the size of the apparatus. 
         [0014]    A more specific object of the present invention is to provide an optical device that scans with a laser beam on an image bearing member, comprising: a laser beam source that emits a laser beam; a reflecting means that has a plurality of reflecting members and reflects the laser beam emitted from said laser beam source; a deflective scanning means that deflectively scans with the laser beam reflected by said reflecting means; and an adjusting means that moves at least part of said reflecting members, and adjusts the size of the laser beam entering the deflective scanning means. 
         [0015]    The other specific objects of the present invention will become apparent from the following description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  illustrates a scanning-type optical device according to a first embodiment of the present invention; 
           [0017]      FIG. 2  is a perspective view of a polygon mirror according to the first embodiment of the present invention; 
           [0018]      FIG. 3  illustrates the minute reflecting members of the polygon mirror according to the first embodiment of the present invention; 
           [0019]      FIG. 4  illustrates a change of the reflecting surface formed by the entire polygon mirror according to the first embodiment of the present invention; 
           [0020]      FIG. 5  illustrates the optical effects of the first embodiment of the present invention; 
           [0021]      FIG. 6  illustrates a change of the reflecting surface formed by the entire polygon mirror according to a second embodiment of the present invention; 
           [0022]      FIG. 7  illustrates the optical effects of the second embodiment of the present invention; 
           [0023]      FIG. 8  illustrates a scanning-type optical device according to a third embodiment of the present invention; 
           [0024]      FIG. 9  is a perspective view of the polygon mirror according to the third embodiment of the present invention; 
           [0025]      FIGS. 10A and 10B  illustrate the optical effects of the third embodiment of the present invention; 
           [0026]      FIG. 11  is a schematic cross-sectional view of a conventional image forming apparatus; and 
           [0027]      FIG. 12  illustrates a conventional scanning-type optical device. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0028]    The following is a description of preferred embodiments of the present invention, with reference to the accompanying drawings. It should be noted that the sizes, the materials, the shapes, and the relative locations of components according to the present embodiment are not limited to those of the following embodiments unless specific description. 
       First Embodiment 
       [0029]      FIG. 1  is a schematic view showing the optical paths and principal components of an optical device according to a first embodiment of the present invention. In  FIG. 1 , reference numeral  1  denotes a laser beam source that emits laser beams based on image information. Reference numeral  2  denotes a collimating lens as an optical component that turns each laser beam emitted from the laser beam source  1  into a collimated light beam. Reference numeral  3  denotes a cylindrical lens as an optical component that concentrates each laser beam to a belt-like form. Reference numeral  4  denotes a polygon mirror as a reflecting means that is formed with aligned minute reflecting members  7 . Reference numeral  5  denotes a rotary polygon mirror as a deflective scanning means that deflectively scans with a laser beam. Reference numeral  6  denotes a first imaging lens as an optical component that concentrates a laser beam onto a photosensitive drum as an image bearing member. The above described components are housed in an optical housing, and the opening of the optical housing is covered with a lid. The optical housing and the lid have ribs and walls for reinforcement. 
         [0030]      FIG. 2  is an enlarged view of the polygon mirror  4 . This polygon mirror  4  is manufactured using a technique generally known as the MEMS (Micro Electro Mechanical Systems) technique. The MEMS technique is to use the exposure process, which is normally used for manufacturing a semiconductor, to integrally form a minute mechanism with an electric circuit on a semiconductor substrate. By this technique, a very small sensor or actuator that cannot be produced by a conventional method can be manufactured at a very low cost. In the past, Japanese Unexamined Patent Publication No. 7-175005 and others disclose techniques relating to galvanometer mirrors that are manufactured using a semiconductor manufacturing process. 
         [0031]    The polygon mirror  4  according to the present invention is now described in greater detail. In this embodiment, the polygon mirror  4  is laminated on a silicon substrate.  FIG. 3  is an enlarged view of one of the minute reflection members  7  that are employed in the polygon mirror  4 . A torsion bar  9  and a movable plate  10  that is a flat plate are integrally formed on a silicon substrate  8  by anisotropic etching. A flat coil  11  formed with a copper thin film that generates a magnetic field by energization is covered with an insulating film and is disposed on the upper peripheral portion of the movable plate  10 . The flat coil  11  of this embodiment is integrally formed on the silicon substrate  8  by performing masking and etching on a silicon wafer, as a peripheral circuit can be formed at the same time as the formation of the above described structure by the MEMS technique. Alternatively, a coil that is produced in another step may be attached onto the substrate, so as to achieve the same effects as above. Reflecting mirrors are formed on the surface of the movable plate  10  through aluminum deposition, and permanent magnets  12  are disposed on the facing sides of the flat coil  11 . The reflecting mirrors of the reflecting members  7  of this embodiment rotate based on the following principle. As current flows through the flat coil  11  (the driving unit) formed on the surface of the movable plate  10 , magnetic force is generated between the magnets disposed on the facing sides, and the reflecting mirrors rotate in the direction of the magnetic force. The polygon mirror  4  of this embodiment has the minute reflecting members  7  that rotate according to this principle. Since the reflecting members  7  of the polygon mirror  4  are integrally formed with the frame of the silicon substrate  8  and the torsion bar  9 , the reflecting members  7  come to a halt where the rotation angle (the rotational quantity) is in proportion to the magnetic force. Therefore, it is possible to control the reflection angle of each reflecting member  7  according to the intensity of the current. 
         [0032]    At the time of energization, the reflecting members  7  of the polygon mirror  4  rotate independently of one another, and form a concave surface, as shown in the section view of the polygon mirror  4  in  FIG. 4 . Each of the reflecting members  7  has a small enough area for the size of the incoming laser beam. Although the size of each of the minute reflecting members  7  is approximately 1/10 of the size of the incoming beam in the figure showing this embodiment, it is more preferable to be smaller. However, in a case where the polygon mirror  4  is formed with two reflecting members  7 , it is possible to perform adjustment. Also, part of the reflecting members  7  may not be movable. 
         [0033]    As described in the description of the related art, the temperature in an image forming apparatus rises with time due to the heat generated from the fixing unit or a heat source such as a motor provided in the apparatus. Because of this, the refractive index of each lens provided in the scanning-type optical device changes, and the imaging point of a laser beam shifts from the photosensitive drum on which the intended focal point is located. As a result, the spot diameter becomes large on the photosensitive drum. In this embodiment, the polygon mirror  4  is interposed between the laser beam source  1  and the rotary polygon mirror  5 , so as to adjust a focus deviation. According to the adjusting method of this embodiment, under the condition that the temperature in the machine has not risen, for example, the condition immediately after the image forming apparatus is activated, the reflecting surfaces of the minute reflecting members  7  exhibit the same reflection angles, as shown in  FIG. 2 . Once the temperature rises, the reflection angles of the reflecting members  7  change so that the polygon mirror  4  forms a concave shape, as shown in  FIG. 4 . In this manner, the laser beam can be concentrated both in the main scanning direction and the sub-scanning direction. 
         [0034]      FIG. 5  schematically shows the optical light path in the sub-scanning direction, from the laser beam source  1  to the rotary polygon mirror  5  at the time of a temperature rise. In this embodiment, a control means that controls and adjusts the movement of each reflecting member  7  is employed. The curvature of the polygon mirror  4  cancels the variation of the refractive index of each lens, and exhibits a necessary rate to have laser beams concentrated onto the photosensitive drum. The broken lines in  FIG. 5  indicate the light path of the focused light when there is a change in the refractive index of the cylindrical lens  3 . As can be seen from  FIG. 5 , the light that should be concentrated onto the rotary polygon mirror  5  is concentrated onto another location. To counter this problem, the polygon mirror  4  is disposed in the light path, so that the light can be concentrated onto the rotary polygon mirror  5 . In this manner, even if the focal point of a laser beam is changed due to a temperature rise, the focal point of the laser beam can be adjusted by the polygon mirror  4  and can be irradiated onto the rotary polygon mirror  5 . 
         [0035]    Also in this embodiment, the curvature of the cylindrical lens  3  varies with a temperature rise. Therefore, a temperature sensor is employed in the image forming apparatus (at or in the vicinity of the scanning-type optical device) so as to measure the temperature. According to the detected temperature variation, the curvature of the reflecting surface of the entire polygon mirror  4  is changed. In a case where a temperature rise is +5° C., the movement of each reflecting member  7  is controlled so that the curvature of the entire polygon mirror  4  becomes 1/100. In a case where a temperature rise is +10° C., the movement of each reflecting member  7  is controlled so that the curvature of the entire polygon mirror  4  becomes 1/120. In a case where a temperature rise is +15° C., the movement of each reflecting member  7  is controlled so that the curvature of the entire polygon mirror  4  becomes 1/150. In this embodiment, the temperature sensor is employed to determine the curvature of the entire reflecting surface of the polygon mirror  4 . However, the same effects can be achieved by changing the curvature of the entire reflecting surface of the polygon mirror  4  according to the result of the measurement carried out on the spot diameter on the photosensitive drum. 
         [0036]    Since a spot diameter is inversely proportional to the diameter of a beam entering the imaging lens, each spot diameter can be made smaller by widening the diameter of the beam entering the imaging lens, and can be made greater by reducing the diameter of the beam entering the imaging lens. The curvature of the entire reflecting surface of the polygon mirror  4  can be changed in this manner, so that the optimum spot diameter can be set according to the resolution of each output image. 
         [0037]    Further, the location of the polygon mirror  4  is not necessarily set between the cylindrical lens  3  and the rotary polygon mirror  5 . The same effects as above can also be achieved even if the polygon mirror  4  is placed between the collimating lens  2  and the cylindrical lens  3 . 
         [0038]    In this embodiment, it is possible not only to adjust a spot diameter at the time of a temperature rise, but also to vary the spot diameter according to the resolution of each output image. 
       Second Embodiment 
       [0039]      FIGS. 6 and 7  illustrate a second embodiment of the present invention. Explanation of the same components as those of the first embodiment is not repeated in the following description. 
         [0040]      FIG. 6  is a cross-sectional view illustrating the reflection angle of the polygon mirror  4  of this embodiment. The polygon mirror  4  of this embodiment has a structure in which the reflection angle of the reflecting member  7  at either end differs from the reflection angle of the reflecting member  7  in the center. This is because the light of the unnecessary area does not enter the rotary polygon mirror  5 , and is reflected by the wall or the ribs of the optical housing or the lid to be absorbed. 
         [0041]      FIG. 7  is a schematic view of the optical arrangement in this embodiment. The polygon mirror  4  that is formed by the MEMS technique as in the first embodiment is interposed between the collimating lens  2  and the cylindrical lens  3 . Since the polygon mirror  4  of this embodiment is manufactured through the same procedures as those of the first embodiment, the reflection angle of the reflecting surface of each reflecting member  7  is controlled independently by an adjusting means. A laser beam is emitted onto a wider area than the diameter of a light beam necessary to obtain a predetermined spot diameter on the reflecting surface of the polygon mirror  4 . As can be seen from  FIG. 7 , the laser beam entering outside the predetermined region is prevented from entering the rotary polygon mirror  5  by the reflecting members  7  located at the end portions of the polygon mirror  4  of this embodiment, and is thus shielded by the wall or the ribs of the optical housing or the lid. 
         [0042]    A laser beam is emitted onto a wider area than the predetermined area in this embodiment, so as to vary each spot diameter according to the resolution. Here, the diameter of the light beam is approximately the same as the minimum spot diameter. As described in the first embodiment, where it is necessary to increase the output resolution and to reduce the spot diameter, the reflection angles of the reflecting members  7  on the end portions of the polygon mirror  4  are changed, and the laser beam entering region on the polygon mirror  5  is widened so as to also widen the diameter of the light beam to enter the imaging lens. As a result, the spot diameter can be reduced. By this method, each spot diameter can be changed according to the output resolution. 
         [0043]    As in the first embodiment, the polygon mirror  4  is not necessarily located between the cylindrical lens  3  and the rotary polygon mirror  5 . The same effects as above can be achieved even if the polygon mirror  4  is placed between the collimating lens  2  and the cylindrical lens  3 . 
         [0044]    In this embodiment, the optimum spot diameter can be set with a compact structure according to the resolution of each image to be output. 
       Third Embodiment 
       [0045]      FIG. 8  illustrates a third embodiment of the present invention. Explanation of the same components as those of the first and second embodiments is not repeated in the following description. 
         [0046]    In this embodiment, a polygon mirror  13  shown in  FIG. 9  is disposed between the cylindrical lens  3  and the rotary polygon mirror  5 . This embodiment further includes a second torsion bar (not shown) to rotate the same mirror as the polygon mirror  4  of the first and second embodiments, so that the entire polygon mirror  13  can be rotated in the direction indicated by the arrow in  FIGS. 8 ,  9 . Like the minute reflecting members  7  of the polygon mirror  4  of the first and second embodiments, the polygon mirror  13  is manufactured by the MEMS technique. The torsion bars integrally formed by anisotropic etching are disposed to extend in a direction perpendicular to the optical housing to be attached. 
         [0047]    Accordingly, when current is applied, the polygon mirror  13  can be rotated in the direction indicated by the arrow in  FIGS. 8 ,  9 , and the applied current is varied as alternating current, so as to vibrate the polygon mirror  13 . In this embodiment, each spot diameter is adjusted with this structure, and the magnification of each image is also adjusted. 
         [0048]      FIGS. 10A and 10B  illustrate the principle of the adjusting operation. As shown in  FIG. 10A , the light beam that enters the rotary polygon mirror  5  at an angle indicated by the broken line when the polygon mirror  13  is in a stationary state is changed through a small angle, as indicated by the solid line, by minutely vibrating the polygon mirror  13  with alternating current. The deflecting angle formed by the rotary polygon mirror  5  is then slightly changed. As a result, the irradiation position after the deflection is changed in the main scanning direction from the broken line to the solid line, as shown in  FIG. 10B . 
         [0049]    When the magnification of an image changes with a temperature rise or the like, the polygon mirror  13  is vibrated at the same frequency as the cycle of scanning the photosensitive drum with the rotary polygon mirror  5 . The vibration amplitude (the rotation angle) of the polygon mirror  13  can be changed according to the variation of the magnification. With the structure of this embodiment, the irradiation position can be adjusted to the main scanning direction on the photosensitive drum, and thus, the magnification of each image can be adjusted. 
         [0050]    Also, as described in the first and second embodiments, the reflection angle of each of the reflecting members  7  of the polygon mirror  13  is adjusted so as to adjust the magnification as well as the spot diameter. 
         [0051]    The adjusting value for the magnification of each image may be calculated by directly measuring the magnification with a spot diameter measuring sensor or the like provided in the image forming apparatus. 
         [0052]    However, it is also possible to use a magnification adjusting value that can be estimated from a temperature rise measured with a temperature sensor provided in the image forming apparatus. In general, the magnification adjusting value varies with the degree of each temperature rise. If the temperature rise is small or the variation of the entire magnification is narrow, the rotation angle should be small. If the temperature rise is great or the variation of the entire magnification is wide, the rotation angle should be great. Further, the magnification may be adjusted every time it changes with respect to a value that is preset in the image forming apparatus, or may be adjusted only when the temperature becomes higher than a predetermined temperature. 
         [0053]    In this embodiment, the magnification of each image can be maintained uniform, regardless of an environmental change such as a temperature rise. Thus, stable image quality can be provided. 
         [0054]    Although the present invention has been described by way of examples, it is not limited to the above described embodiments, and various modifications may be made to it without departing from the scope of the present invention. 
         [0055]    This application claims priority from Japanese Patent Application No. 2004-168043 filed Jun. 7, 2004, which hereby incorporated by reference herein.