Abstract:
The present invention provides a device for forming an image on a photoconductive image carrier in an image forming apparatus. The device comprises a light source for generating a light beam, an aperture for shaping the light beam generated by the light source, a deflector (polygon mirror) for deflecting the light beam shaped by the aperture, and an optical scanning imaging part for focusing the light beam deflected by the deflector onto the photoconductive image carrier as an optical spot and forming an image thereon. The deflector is provided with at least one blocking member (first blocking plate) for blocking flare beams that do not contribute to image formation. Further blocking members (auxiliary blocking plate and second blocking plate) may be provided to stationary parts of the device so as to block leaked light beams.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention generally relates to an image forming apparatus such as a digital copier, a printer, a facsimile and so on of an electric photographic type, provided with a writing device that performs image exposure on a photoconductive image carrier by deflecting a light beam emitted from a light source by a deflector.  
           [0003]    2. Description of the Related Art  
           [0004]    A brief description of the schematic configuration of such an image forming apparatus is given below by taking a digital copier as an example. The digital copier of an electric photographic type includes an automatic document feeder that feeds document sheets one by one from a pile of document sheets onto a contact glass and sets the document sheet thereon. The digital copier further includes an image reading unit that optically scans the document sheet on the contact glass and forms an image on a CCD sensor, and an optical scan unit that forms an (electrostatic latent) image on a photoconductive image carrier by deflecting a light beam emitted from a light-emitting semiconductor based on a digital image signal. The digital image signal is converted from an analog image signal that is obtained by photoelectrically transforming a reflective light image of the document sheet formed on the CCD sensor by an analog-to-digital converter. The digital copier further includes a transfer unit that transfers an image developed from the electrostatic latent image formed on the photoconductive image carrier onto a transfer paper and a feeder unit that feeds transfer papers to the transfer unit.  
           [0005]    Among these components, the optical scan unit that configures a writing device may be exemplified by an optical scanning system shown in FIG. 1. A laser beam emitted from a semiconductor laser (LD)  1   a,  which is provided in the interior of a semiconductor laser unit (LD unit)  1  as a light source, become a substantially parallel-light beam by passing through a collimator lens  1   b  and become a chief ray L having a cross-section shaped as necessary for image formation by an aperture  2 . The chief ray is condensed in the sub-scanning direction by a cylindrical lens  3  and is reflected by a mirror  4  so as to enter a polygon mirror  5 , which serves as a deflector. The chief ray L entering the polygon mirror  5  is deflected by the reflective side surface of the polygon mirror  5  that rotates with a high velocity in the direction indicated by the arrow, and the deflected chief ray L enters an Fθ lens  6 . By entering the Fθ lens  6 , the constant angular velocity scanning with the chief ray is transformed into constant linear velocity scanning with the same. The chief ray exits to the exterior of the writing device and is focused on a photoconductor drum  7 , which includes the surface to be scanned and thus an electrostatic latent image is formed thereon. In FIG. 1, the numerical references  9   a  and  9   b  represent a synchronism detection mirror provided outside of the image region and a synchronism detection sensor that detects a reflection beam from the synchronism detection mirror, respectively. A synchronization signal as a reference for the beginning of scanning in the main scanning direction can be obtained from the output of the synchronism sensor  9   b.  In addition, the Fθ lens  6  has an optical face tangle correction function.  
           [0006]    [0006]FIG. 2 and FIG. 3 show a configuration of one example of the writing device  10  provided with the above-mentioned optical scanning system. FIG. 2 is a top plan view of such a configuration and FIG. 3 is a sectional side view of such a configuration. As is apparent from FIG. 2 and FIG. 3, in the actual writing device  10 , the polygon mirror  5  is rotationally driven by a polygon motor  8 . A soundproof glass  5   a  is fastened to a polygon cover  8   a  of the polygon motor  8 . The light beam entering the polygon mirror  5  from the mirror  4  and the light beam exiting the polygon mirror  5  to the Fθ lens  6  pass through the soundproof glass  5   a.  The rotational velocity of the polygon mirror  5  is determined based on the rotational velocity of the photoconductor drum  7 , the writing density of the writing device  10 , and the number of side surfaces on the polygon mirror  5 .  
           [0007]    However, in such a writing device of the image forming apparatus according to the related art, some undesired light beam may occur from the LD unit  1 , which light beam is different from the chief ray L and does not contribute to the proper image formation, due to the side lobes occurring from the LD  1   a  and being reflected internally of the LD unit  1 , etc. The undesired light beam is represented as flare beams FL  1 , FL  2  in FIG. 2. These flare beams appear on both sides of the traveling direction of the chief ray L. FIG. 4 shows a magnified view of a part of the optical path of these flare beams. It is often the case that the energy of the first flare beam FL  1  and the energy of the second flare beam FL  2  are smaller than that of the chief ray L. However, if they travel at the same height as the chief ray L, the first flare beam FL  1  passes through the gap between one end of the mirror  4  and the cover  8   a  of the polygon motor  8  and the second flare beam FL  2  travels on the side of the chief ray L, which side is outer with respect to the other end of the mirror  4  as shown in FIG. 2 and FIG. 4. Both flare beams pass through the Fθ lens  6  shown in FIG. 2 and become flare beams FL  1   a,  FL  2   a  that reach the photoconductor drum  7 . The flare beams FL  1   a,  FL  2   a  always expose themselves to the photoconductor drum  7  as a certain image height and these flare beams form on finally obtained image black lines or black bands in the sub-scanning direction, therefore the image quality is significantly degraded. These flare beams may also be generated at both the upper side and the under side of the chief ray; however, such flare beams are blocked by an optical housing or a cover member that accommodates most of the optical devices and therefore they do not reach the photoconductor drum  7 .  
         SUMMARY OF THE INVENTION  
         [0008]    Accordingly, it is necessary to prevent the first flare beam and the second flare beam from reaching the photoconductor drum by blocking the relevant flare beams. However, when the first flare beam FL  1  and the second flare beam FL  2  are traveling on the same scanning plane as the chief ray L and the parts FL 1   a,  FL  2   a  of the first flare beam FL  1  and the second flare beam FL  2  enter the scan region of the chief ray L as shown in FIG. 2, it is difficult to separate the flare beams from the chief ray L. Accordingly, it is necessary to block the first flare beam FL  1  and the second flare beam FL  2  in the optical path between the LD unit  1  and the polygon mirror  5 .  
           [0009]    In particular, for blocking the first flare beam FL  1 , it is necessary to arrange a blocking member between the polygon motor  8  and the mirror  4  avoiding obstructing the traveling of the chief ray L and this is spatially difficult. Further, the polygon motor  8  is removably mounted to the housing of the writing device  10  by screws threaded in the direction perpendicular to the plane of the figure so that the polygon motor can be replaced in case of a failure, etc. However, under a certain design condition, the blocking member may obstruct the removal of the polygon motor and thus may impede a smooth removal of the polygon motor. Therefore, it is extremely difficult to completely block the first flare beam FL  1  by providing a blocking member between the motor  8  and the mirror  4  leaving no space between them, while avoiding obstructing the removal of the motor  8  and blocking the chief ray L.  
           [0010]    The present invention is proposed in view of the mentioned problems, and therefore, it is a general object of the present invention to provide an image forming apparatus provided with a writing device with a simple configuration, by means of which writing device the flare beams emitted from the LD unit can be effectively blocked.  
           [0011]    According to a first aspect of the present invention, there is provided a device for forming an image on a photoconductive image carrier in an image forming apparatus. The device according to the present invention comprises a light source for generating a light beam, an aperture for shaping the light beam suitable for proper image formation, which light beam is generated by the light source, a deflector assembly for deflecting the light beam shaped by the aperture, an optical scanning imaging part for focusing the light beam deflected by the deflector assembly onto the photoconductive image carrier as an optical spot and forming an image thereon, and a blocking member provided on an optical path between the aperture and the deflector assembly for blocking a light beam that does not contribute to the proper image formation.  
           [0012]    In the device according to the present invention, the deflector assembly may comprise a polygon mirror, a polygon motor that rotationally drives the polygon mirror, and a polygon cover that accommodates the polygon mirror and the polygon motor therein, and the blocking member may be integrally formed on the polygon cover.  
           [0013]    Therefore, according to the first-aspect of the present invention, by providing the blocking member on the optical path between the aperture and the deflector assembly, it is possible to effectively and efficiently block the undesired light beam that dose not contribute to the proper image formation and therefore it is possible to improve the image quality of the finally obtained image.  
           [0014]    According to a second aspect of the present invention, there is provided another device for forming an image on a photoconductive image carrier in an image forming apparatus. The device according to the present invention comprises a light source for generating a light beam, a deflector assembly for deflecting the light beam generated by the light source, an optical scanning imaging part for focusing the light beam deflected by the deflector onto the photoconductive image carrier as an optical spot and forming an image thereon, and a blocking member integrally formed on the deflector assembly for blocking a light beam that does not contribute to the proper image formation.  
           [0015]    In the device according to the present invention, the deflector assembly may comprise a polygon mirror, a polygon motor that rotationally drives the polygon mirror, and a polygon cover that accommodates the polygon mirror and the polygon motor therein, and the blocking member may be integrally formed on the polygon cover.  
           [0016]    In the device according to the present invention, the blocking member may be formed with an opening that allows only a light beam that contributes to the proper image formation to pass therethrough, and the opening may function as an aperture for shaping the light beam suitable for the proper image formation, which light beam is generated by the light source.  
           [0017]    Therefore, according to the present invention, an independent aperture may be dispensed with, which aperture is present in the related art, and a fewer number of components may be necessary for the device for forming an image on the photoconductive image carrier according to the present invention.  
           [0018]    In the device according to the present invention, the opening formed in the blocking member may have a dimension that can be determined correspondingly to a rotational velocity of the polygon motor.  
           [0019]    In the device according to the present invention, the blocking member may be provided on an optical path between the light source and the deflector assembly.  
           [0020]    Therefore, according to the second aspect of the present invention, by integrally forming the blocking member on the polygon cover that configures a part of the deflector assembly, it is possible to readily remove the deflector assembly for replacement, etc. without the blocking member obstructing the removal of the deflector assembly.  
           [0021]    When the first aspect and the second aspect of the present invention are considered together, it is possible to provide a device for forming an image on a photoconductive image carrier in an image forming apparatus, which device can effectively and efficiently block the undesired light beam that does not contribute to the proper image formation and, at the same time, the deflector assembly that configures a part of the device according to the present invention can be readily removed from the device when necessary without being obstructed by the blocking member.  
           [0022]    As a third aspect of the present invention, there is provided an image forming apparatus that comprises a document feeder that feeds document sheets one by one from a pile of document sheets onto a contact glass and sets the document sheet thereon, an image reading unit that optically scans the document sheet and forms an image corresponding to the document sheet on a sensor, an optical scan unit that forms a latent image on a photoconductive image carrier based on a digital image signal provided from the sensor via an analog-to-digital converter, a transfer unit that transfers an image developed from the latent image formed on the photoconductive image carrier onto a transfer paper, and a feeder unit that feeds the transfer paper to the transfer unit. The optical scan unit comprises the above-mentioned device for forming an image on the photoconductive image carrier according to the first and the second aspects of the present invention.  
           [0023]    By configuring the image forming apparatus as mentioned above, it is possible to efficiently and effectively block undesired light beams that impede image formation by way of an extremely simple configuration, and thus the image quality of the finally obtained image can be significantly improved. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    [0024]FIG. 1 is an optical path diagram illustrating one example of a writing device according to the related art;  
         [0025]    [0025]FIG. 2 is a top plan view of one example of the writing device according to the related art;  
         [0026]    [0026]FIG. 3 is a sectional side view of one example of the writing device according to the related art;  
         [0027]    [0027]FIG. 4 is a magnified view of a part of one example of the writing device shown in FIG. 2;  
         [0028]    [0028]FIG. 5 is a plan view of a writing device according to a first embodiment of the present invention;  
         [0029]    [0029]FIG. 6 shows a sectional side view of the writing device according to the first embodiment of the present invention;  
         [0030]    [0030]FIG. 7 is a magnified view of a part of the writing device according to the first embodiment of the present invention;  
         [0031]    [0031]FIG. 8 is a perspective view of a polygon motor in a writing device according to a second embodiment of the present invention;  
         [0032]    [0032]FIG. 9 is a magnified view of a part of the writing device according to the second embodiment of the present invention;  
         [0033]    [0033]FIG. 10 is a plan view of an overall configuration of the writing device according to the second embodiment of the present invention; and  
         [0034]    [0034]FIG. 11 is a schematic configuration illustrating a digital copier provided with the writing device according to the second embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]    In the following, preferred embodiments of the present invention are more specifically described with reference to the accompanying figures.  
         [0036]    [0036]FIG. 5 is a plan view of a writing device according to a first embodiment of the present invention and FIG. 6 is a sectional side view of the writing device according to the first embodiment. Parts corresponding to those shown in FIG. 2 and FIG. 4 are denoted with the same numerical references and the descriptions of which are omitted.  
         [0037]    In the writing device  10  according to the first embodiment of the present invention, a first blocking plate  11  is integrally formed on the polygon cover  8   a  of the polygon motor  8  so as to block the first flare beam FL  1 . This eliminates the possibility of forming a gap between the first blocking plate  11  and the polygon cover  8   a  of the polygon motor  8 , and at the same time, the removal of the polygon motor  8  is not obstructed since the first blocking plate  11  is simultaneously removed with the polygon motor  8 . Since the polygon mirror  5  is normally arranged with high precision with respect to the LD unit  1  and other optical components, it is possible to arrange the first blocking plate  11  at the ultimate position where the first blocking plate  11  can block the first flare beam FL  1  but does not block the chief ray L.  
         [0038]    However, since the distance between the first blocking plate  11  and the aperture  2  is shorter than the distance between the first blocking plate  11  and the mirror  4 , there is a possibility that a part of the first flare beam FL  1  may pass through due to a slight error. Therefore, it is preferable that an auxiliary blocking plate  12  may be provided in parallel with the first blocking plate  11  on the optical housing  10   a  of the writing device  10  between the first blocking plate  11  and the mirror  4  in order to completely block the first flare beam FL  1 . The auxiliary blocking plate  12  may be also integrally formed on the polygon cover  8   a  of the polygon motor  8  as is the first blocking plate  11 .  
         [0039]    As for the second flare beam FL  2 , as shown in FIG. 7, a second blocking plate  13  is provided on the optical housing  10   a  of the writing device  10  as is the auxiliary blocking plate  12 . The second blocking plate  13  can be arranged comparatively easily since there are no particular intricate movable/stationary members in its surroundings.  
         [0040]    According to the first embodiment of the present invention, it is possible to achieve an efficient and effective blocking of the flare beams emitted from the LD unit, which is conventionally said to be very difficult, and to prevent the degradation in image quality by simply modifying a part of the shape of the polygon cover  8   a  of the polygon motor  8  configuring the deflector.  
         [0041]    [0041]FIG. 8 through FIG. 10 describe a writing device according to a second embodiment of the present invention. FIG. 8 is a perspective view illustrating the exterior of the cover of the polygon motor in the writing device according to the second embodiment. FIG. 9 shows a magnified view of a part of the optical path of the flare beams in the surroundings of the polygon motor. FIG. 10 is a plan view illustrating an overall configuration of the writing device according to the second embodiment.  
         [0042]    According to the second embodiment of the present invention, a first blocking plate  14 , which is integrally formed on the polygon cover  8   a  of the polygon motor  8 , is extended to a position across the chief ray L. An opening  15  with a predetermined shape for allowing the chief ray L to pass therethrough is formed in the first blocking plate  14 . The opening  15  substitutes for the aperture  2  for forming the chief ray L and thus the independent aperture  2  such as shown in FIG. 5 and FIG. 6 is eliminated. As previously mentioned, since the arrangement accuracy of the polygon motor  8  is essentially very high, the position accuracy the first blocking plate  14  that is integrally formed on the polygon motor  8  and the opening  15  formed in the blocking plate  14  is also high. It is possible to make the opening  15  sufficiently fulfill the function of an aperture. Since the first flare beam FL  1  emitted from the LD unit  1  is reliably blocked by the portions of the first blocking plate  14  other than the opening  15 , the auxiliary blocking plate  12  provided in the first embodiment of the present invention is not necessary any more. As for the second flare beam FL  2 , the second blocking plate  13  as shown in FIG. 5 and FIG. 7 is provided. According to the second embodiment of the present invention, since the function of the aperture, which is conventionally provided in the LD unit  1  or on the optical housing  10   a  as an independent member, can be combined with the opening  15  in the first blocking plate  14 , it is possible to reduce the production cost in addition to the blocking of the flare beams.  
         [0043]    Accordingly, by integrally forming an opening having the function of the aperture on the polygon cover of the polygon motor, further effects can be obtained as described below.  
         [0044]    In general, when the writing densities of the plurality of image forming apparatus differ, it is necessary to change the diameter of the beam incident on the photoconductor drum as required by the image formation process. For example, an image forming apparatus with a writing density of 600 dpi needs a smaller beam diameter than an image forming apparatus with a writing density of 400 dpi. In order to obtain the required beam diameter, an appropriate LD unit and superior performance of the lens system that focuses the laser beam emitted from the LD unit onto the photoconductor and forms images thereon are essential. However, as a method for changing the beam diameter without modifying the lens system, changing the shape or the dimension of the aperture may be effectively implemented.  
         [0045]    For example, in order to configure a plurality of device types having different writing densities as series devices of the image forming apparatus, it may be only necessary to change the number of rotations of the polygon motor and the shape of the opening from an optical point of view. According to the second embodiment of the present invention shown in FIG. 8 through FIG. 10, since the polygon cover  8   a  of the polygon motor  8  and the opening  15  configuring the aperture of the first blocking plate  14  are integrally formed on the same component, it is possible to readily achieve the configurations of the series devices by combining a plurality of polygon motors having different numbers of rotations and a plurality of openings having different dimensions during production. In addition, it is possible to provide an image forming apparatus with a plurality of grades by simply replacing the above-mentioned integral component. Therefore, it is possible to achieve good productivity.  
         [0046]    [0046]FIG. 11 shows a schematic configuration of a digital copier provided with such a writing device as mentioned above. It is noted that, as a matter of convenience, the writing device  10  has an optical path that travels in the opposite direction as compared to that shown in FIG. 6. The writing device  10  is configured as shown in FIG. 8 through FIG. 10.  
         [0047]    The digital copier comprises, from the top to the bottom, the automatic document feeder  20 , the document reading unit  30 , the writing device  10 , the transfer unit  40 , and the feeder unit  50 . The automatic document feeder  20  feeds document sheets one by one from a pile of document sheets placed on a table for the document sheets (not shown) to the contact glass  21  and discharges the document sheet when it is finished copying. The document reading unit  30  comprises a first carriage  30 A provided with a light source configured from an illumination lamp  31  and a reflection mirror  32 , and a first mirror  33  and a second carriage  30 B provided with a second mirror  34  and a third mirror  35 . At the time of reading out the document sheet, the first carriage  30 A moves in the direction indicated by an arrow X with predetermined velocity and the second carriage  30 B follows the first carriage  30 A in the direction indicated by the arrow X at half of the predetermined velocity of the first carriage  30 A. This enables the document sheet on the contact glass  21 , which is illuminated by the illumination lamp  31  provided with the reflection mirror  32 , to be optically scanned, and a corresponding image is formed on the CCD sensor  38  through a color filter  36  and a lens  37 .  
         [0048]    The document sheet is read by the CCD sensor  38 , which performs the photoelectric transfer on the reflective light image of the document sheet so as to provide an analog signal. After reading of the document sheet is completed, the first carriage  30 A and the second carriage  30 B return to their original positions. When a 3-line CCD provided with a red (R) filter, a green (G) filter, and a blue (B) filter is used as the CCD sensor  38 , color images can be read. The analog image signals provided from the CCD sensor  38  are converted to digital image signals by an A/D converter and various image processes including digitization, a gradation process, a magnification process, and a compilation process are applied on the digital image signals at an image processing plate  39 . The digital image signals having been processed are sent to the writing device  10  via a semiconductor drive plate (not shown) and a laser beam is emitted from the LD  1   a  in the LD unit  1  in accordance with the provided digital signals.  
         [0049]    As previously mentioned, the laser beam emitted from the LD  1   a  becomes a substantially parallel light beam by passing through the collimator lens  1   b  and become a chief ray L having a cross-section shaped as necessary for image formation by the opening  15  (FIG. 9) of the first blocking plate  14 . The first flare beam FL  1  and the second flare beam FL  2  (FIG. 9) other than the chief ray L are blocked by the first blocking plate  14  and the second blocking plate  13 . The chief-ray that passes through the opening  15  is condensed in the sub-scanning direction by the cylindrical lens  3  and is reflected by the mirror  4  so as to enter the polygon mirror  5  via the soundproof glass  5   a.  The chief ray L entering the polygon mirror  5  is deflected by the polygon mirror  5  and enters the Fθ lens  6 . The direction of the chief ray L is changed by a turning mirror  16  and the chief ray L exits to the interior of the transfer unit  40  through a window portion  10   b  of the optical housing  10   a  and a dustproof glass  17 .  
         [0050]    The chief ray L emitted to the interior of the transfer unit  40  is focused on the photoconductive image carrier, which is the photoconductor drum  7 , and forms an image thereon. The photoconductor drum  7  is rotationally driven by a drive (not shown) during copying and uniformly charged by a charging device  41 . Afterward, electrostatic latent images are formed by being exposed to the image light beams from the writing device  10  and the latent images are developed by a developing device  42 .  
         [0051]    Meanwhile, the feeder unit  50  comprises a plurality of feeding cassettes  50   a,    50   b,  and  50   c  corresponding to different dimensions. Transfer papers are fed to resist rollers  43  from the required feeding cassette in precise timing with the images on the photoconductor drum  7 . The latent images formed on the photoconductor drum  7  are transferred to the transfer paper by a transfer device  44  and the transfer paper is separated from the photoconductor drum  7  by a separation device  45 . After being conveyed by a conveying device  46 , the transfer paper is discharged into a tray  48  as a copy. Remaining toner is removed from the photoconductor drum  7  by a cleaning device  49  after the transfer paper is separated therefrom.  
         [0052]    In the digital copier shown in FIG. 11, a double-side copier, etc. may be provided inside, however, the descriptions of which are omitted since they are not directly related to the present invention.  
         [0053]    Further, the present invention is not limited to these embodiments, and variations and modifications may be made without departing from the scope of the present invention.  
         [0054]    The present application is based on Japanese priority application No. 2002-053929 filed on Feb. 28, 2002, the entire contents of which are-hereby incorporated by reference.