Patent Publication Number: US-7593028-B2

Title: Optical scanning method, optical scanner and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a Rule 1.53(b) Continuation of U.S. Ser. No. 10/452,458, filed Jun. 2, 2003, now U.S. Pat. No. 6,930,700 the entire contents of which are incorporated by reference herein. 

   BACKGROUND 
   1. Technical Field 
   This disclosure generally relates to an optical scanning method, an optical scanner and an image forming apparatus, and more particularly to an optical scanning method and an optical scanner for writing latent images by radiating optical beams on scanned surfaces of a plurality of linearly arranged image carrying members, and an image forming apparatus such as a copier, a printer and a plotter that can form a multi-color image by developing the latent images with distinct color developers and then sequentially transferring the developed color images to a transferred member. 
   2. Description of the Related Art 
   In a conventional tandem type color image forming apparatus, optical beams emitted from a plurality of illuminants are radiated to four linearly arranged image carrying members such as photosensitive drums in order to write latent images thereon. The latent images formed on the image carrying members are developed to visualize the latent images by using distinct color developers, typically, a yellow toner, a magenta toner, a cyan toner and a black toner. Then, a transferred member such as a recorded paper is carried on a transfer belt to each transferring part of the image carrying members, and the individual color images are sequentially superposed on the transferred member. The resulting color image on the transferred member is fixed, and it is possible to produce a multi-color image. 
   In such a conventional tandem type color image forming apparatus, an optical scanner, such as an optical writing apparatus, is prepared for each of the image carrying members, and the optical writing apparatus writes a latent image on the corresponding image carrying member. However, the optical writing apparatus is relatively expensive because the optical writing apparatus contains an optical deflector formed of a polygon mirror and a drive motor for driving the optical deflector. For this reason, components and assembly costs of the conventional tandem type color image forming apparatus can be problematic, as it is necessary to provide a plurality of optical writing apparatuses corresponding to the plurality of image carrying members. In addition, it is necessary to provide an adequate installation space in the image forming apparatus to accommodate the optical writing apparatuses each of which includes an optical deflector. As a result, it is impossible to avoid a size increase in an image forming apparatus in which it is desired to include such optical writing apparatuses. 
   Furthermore, although a tandem type color image forming apparatus is capable of forming a color image, the occasion in offices to produce monochrome manuscripts is greater than that of color manuscripts. As the tandem type color image forming apparatus is required to produce more full-color manuscripts at higher speeds, the tandem type color image forming apparatus has more significant problems, including the following: 
   1. a complicated mechanism for superposing four colors, 
   2. a cost increase of motors and drive parts for driving photosensitive members, 
   3. a short life span of the motors and the drive parts for driving the photosensitive members. 
   In order to meet such office use, conventional color image forming apparatuses are designed to achieve higher productivity in a monochrome mode than in a full-color mode; that is, to operate in the monochrome mode at higher line speed than in the full-color mode. Such color image forming apparatuses can offer monochrome manuscripts at higher speed than full-color manuscripts; that is, the color image forming apparatuses can form more images in the monochrome mode per unit of time than in the full-color mode. 
   On the other hand, there is a color image forming apparatus that allows a user to switch between a quality priority mode and a speed priority mode. For instance, the color image forming apparatus produces an image at a resolution of 1200 dpi in the quality priority mode and at a resolution of 600 dpi in the speed priority mode. In the quality priority mode, the image forming apparatus writes an image at a higher write density under a constraint of lower line speed so that a high-quality manuscript can be obtained, albeit at the cost of a slower operating speed. In contrast, in the speed priority mode, the image forming apparatus writes an image at higher line speed under a constraint of moderate image quality so that high-speed operations can be achieved, albeit at the cost of a lower resolution image quality. 
   In the above-mentioned color image forming apparatus, when a user wants to obtain more monochrome manuscripts in the speed priority mode than in the quality priority mode, a user is allowed to select the operation mode from the quality priority mode and the speed priority mode by switching the pixel density. In the conventional color image forming apparatus, two beams for black (BK) are prepared therein together with a pitch switching mechanism, and one beam for each of yellow (Y), magenta (M) and cyan (C) is provided therein. Then, there are four mode combinations: a monochrome quality priority mode, a monochrome speed priority mode, a color quality priority mode, and a color speed priority mode. In the monochrome quality priority (1200 dpi) mode, the color image forming apparatus operates two BK beams at a pitch of 1200 dpi with respect to the sub-scanning direction at low line speed. In the monochrome speed priority (600 dpi) mode, the color image forming apparatus operates the two BK beams at a pitch of 600 dpi with respect to the sub-scanning direction at high line speed. In the color quality priority (1200 dpi) mode, the color image forming apparatus operates color beams and one of the two BK beams, each of which writes an image at the pitch of 1200 dpi with respect to the sub-scanning direction at low line speed. At this time, only one of the two BK beams is switched ON. In the color speed priority (600 dpi) mode, the color image forming apparatus operates the color beams and one of the two BK beams, each of which writes an image at the pitch of 600 dpi with respect to the sub-scanning direction at high line speed. 
   According to the above-mentioned color image forming apparatus, when resist positions of four colors (BK, C, M, Y) are adjusted with respect to the main scanning direction and the sub-scanning direction (only one beam is used for BK), it is necessary to properly set a pixel density switching position of BK as either 600 dpi or 1200 dpi. If the pixel density switching position is not properly adjusted, there is a probability that a produced full-color image has a color difference due to misalignment of the BK write position as shown in  FIGS. 1A and 1B . 
     FIGS. 1A and 1B  show dot positions of optical spots for two-beam writing under two pixel densities of 1200 dpi and 600 dpi.  FIG. 1A  shows dot positions of a first beam and a second beam at a resolution of 1200 dpi, and  FIG. 1B  shows dot positions of a first beam and a second beam at a resolution of 600 dpi. 
   As shown in  FIG. 1A , when an image is written at the resolution of 1200 dpi with respect to the sub-scanning direction, a pitch of 21 μm (=25.4 mm/1200) between adjacent optical spots is obtained. As shown in  FIG. 1B , when an image is written at the resolution of 600 dpi with respect to the sub-scanning direction, a pitch of 42 μm (=25.4 mm/600) between adjacent optical spots is obtained. As seen in  FIGS. 1A and 1B , a dot position of an optical spot has a difference L of 10.5 μm between the two resolutions, as computed from the following formula:
 
 L= (42 μm−21 μm)/2.
 
     FIG. 2  shows the difference of dot positions with respect to the sub-scanning direction between the two resolutions. When one of the BK beams is used in the full color modes, there is a probability that a color difference between BK and another color (cyan in  FIG. 2 ) may occur with respect to the sub-scanning direction if beam pitch positions are not properly adjusted for alternation between the two resolutions of 1200 dpi and 600 dpi. This color difference is caused by the narrowed beam pitch between BK and the other color by the difference L. 
   On the other hand,  FIGS. 3A through 3C  show a difference of dot positions with respect to the main scanning direction between the two resolutions. As shown in  FIG. 3A , full-color adjustment for properly producing full-color images is performed for the first beam with respect to the main scanning direction. In fact, however, if the second beam, which is not adjusted, is used to form the full-color images, a color difference arises between the second beam and the other color beams with respect to the main scanning direction, as is shown in  FIG. 3B . As previously mentioned, this color difference is caused by the difference of dot positions of BK beams as shown in  FIG. 3C . As used herein, the phrase “full-color adjustment” means to correct color differences caused at shipment and during use. Japanese Laid-Open Patent Application No. 11-301032 discloses an adjustment technique for correcting such color differences. 
   SUMMARY 
   In an aspect of this disclosure, there is provided an optical scanner that has a write density switching function to correct misalignment of a writing position of a full-color image even if a BK write density is switched. 
   In an aspect of this disclosure, there is provided an image forming apparatus that can form a full-color image without any color difference even if the BK write density is switched. 
   In an exemplary embodiment of this disclosure there is provided an optical scanning method for writing an image in an image formed medium by using a black writing illuminant and a color writing illuminant wherein the black writing illuminant writes the image at a plurality of record densities and the color writing illuminant writes the image at a predetermined record density, the optical scanning method including the steps of: adjusting a resist position for a full-color image with respect to a main scanning direction and a sub-scanning direction by changing a writing position of the black writing illuminant in accordance with a requested one of the record densities; and writing the full-color image at the writing position in the image formed medium. 
   In the above-mentioned embodiment, when the record density or writing speed is changed at formation time of a full-color image, it is possible to write the full-color image at a writing position suitable to the full-color image formation. As a result, there is no probability that a color difference arises due to misalignment of the writing position. 
   In another exemplary embodiment of this disclosure, there is provided an optical scanner for writing an image in an image formed medium, including: a black writing illuminant optically writing the image at a plurality of record densities; a color writing illuminant optically writing the image at a predetermined record density; a storage part storing writing position data of the black writing illuminant corresponding to the record densities; and a writing position switching part switching a writing position of the black writing illuminant based on the writing position data in the storage part so as to properly form a full-color image, wherein the writing position data are used to adjust a resist position for the full-color image with respect to a main scanning position and a sub-scanning position. 
   In the above-mentioned embodiment, when a full-color image is written, it is possible to properly write the full-color image by switching a writing position of the black writing illuminant into a state where color differences due to shipment and use thereof are corrected. As a result, even if the record density and the writing speed have differences from those in the corrected state, there is no probability that a color difference arises due to misalignment of the writing position. 
   In the above-mentioned optical scanner, the black writing illuminant may include at least two semiconductor lasers, a retaining part retaining the semiconductor lasers in a state where the semiconductor lasers are fixed relative to each other, a supporting part supporting the retaining part such that the retaining part can be rotated with respect to a predetermined rotational axis, and a driving part rotating the retaining part with respect to the rotational axis. 
   According to the above-mentioned embodiment, even if the black writing illuminant is constituted as a two-beam illuminant, it is possible to easily adjust writing positions of two beams from the black writing illuminant by simply setting a rotational position thereof. 
   In the above-mentioned optical scanner, the driving part may include a stepping motor. 
   According to the above-mentioned embodiment, since a rotation angle of the black writing illuminant can be determined through the number of steps of the stepping motor, it is possible to easily control the rotation angle. 
   In the above-mentioned optical scanner, the writing position switching part may drive the stepping motor so as to switch the writing position of the black writing illuminant based on the writing position data in the storage part. 
   According to the above-mentioned embodiment, when a full-color image is formed, it is possible to automatically switch record densities by using the writing position switching part. 
   In the above-mentioned optical scanner, the black writing illuminant may have two semiconductor lasers, and the rotational axis may be located at one of a middle point between writing positions of the two semiconductor lasers and a writing position of one of the two semiconductor lasers. 
   According to the above-mentioned embodiment, it is possible to determine the writing position through the rotation angle. If a relationship between writing positions and rotation angles is prescribed in advance, it is possible to easily set a desired position as the writing position. 
   In another aspect of this disclosure, there is provided an image forming apparatus, including: an optical scanner writing an image in an image formed medium, the optical scanner comprising: a black writing illuminant optically writing the image at a plurality of record densities; a color writing illuminant optically writing the image at a predetermined record density; a storage part storing writing position data of the black writing illuminant corresponding to the record densities; and a writing position switching part switching a writing position of the black writing illuminant based on the writing position data in the storage part so as to properly form a full-color image, wherein the writing position data are used to adjust a resist position for the full-color image with respect to a main scanning position and a sub-scanning position; and an image forming part developing individual color images written by the optical scanner and forming the full-color image on a record medium. 
   According to the above-mentioned apparatus, it is possible to properly form the full-color image written by the optical scanner without any color difference. 
   In the above-mentioned image forming apparatus, the optical scanner optically may write the individual color images on image carrying members, which are linearly arranged, corresponding to the color images. 
   According to the above-mentioned aspect, when the above-mentioned tandem type image forming apparatus is used to write a full-color image in linearly arranged image carrying members corresponding to individual colors, it is possible to suppress color differences. The tandem type image forming apparatus includes a plurality of illuminant units of a multi-beam black writing illuminant having the record density switching part and single-beam color writing illuminants. When the tandem type image forming apparatus writes latent color images by irradiating optical beams on scanned surfaces of the image carrying members therein, the tandem type image forming apparatus changes a writing position of the black writing illuminant by adjusting resist positions of the black image with respect to the main scanning direction and the sub-scanning direction (regardless of record densities of 600 dpi, 1200 dpi and 2400 dpi). Then, the tandem type image forming apparatus writes the black image at that writing position. As a result, there is no probability of writing position differences occurring. Furthermore, users can select a quality priority mode and a speed priority mode by switching the record densities so that the image forming apparatus can produce more monochrome images than full-color images. 
   Other aspects, features and advantages will become more apparent from the following detailed description when read in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A and 1B  are diagrams illustrating dot positions of optical spots for two-beam writing under two pixel densities of 1200 dpi and 600 dpi, respectively; 
       FIG. 2  is a diagram illustrating a difference of dot positions with respect to a sub-scanning direction between the two pixel densities; 
       FIGS. 3A through 3C  are diagrams illustrating a difference of dot positions with respect to the main scanning direction between the two pixel densities; 
       FIG. 4  is a side elevational view roughly illustrating a structure of an image forming apparatus according to the present invention; 
       FIG. 5  is a top plan view of an optical scanner according to the present invention; 
       FIG. 6  is a diagram illustrating arrangement of an optical deflector and optical systems in the optical scanner according to the present invention; 
       FIG. 7  is a cross-sectional view of the optical scanner according to the present invention as viewed from the plane A-A′ in  FIG. 5 ; 
       FIG. 8  is a diagram illustrating arrangement of illuminant units, the optical deflector and the optical systems in the optical scanner according to the present invention; 
       FIG. 9  is an exploded perspective view of a multi-beam illuminant unit of the optical scanner according to the present invention; 
       FIG. 10  is a cross-sectional view of the multi-beam illuminant unit of the optical scanner according to the present invention; 
       FIG. 11  is a diagram for explaining rotation adjustment of the multi-beam illuminant unit of the optical scanner according to the present invention; 
       FIGS. 12A and 12B  are diagrams illustrating shift positions of optical spots on a photosensitive drum corresponding to rotation angles of the multi-beam illuminant unit of the optical scanner according to the present invention; and 
       FIG. 13  is a flowchart of a procedure for adjusting a pitch between optical spots from the multi-beam illuminant unit of the optical scanner according to the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In the following, embodiments of the present invention will be described with reference to the accompanying drawings. 
   A description will now be given, with reference to  FIG. 4 , of an image forming apparatus according to the present invention. 
     FIG. 4  roughly shows a structure of the image forming apparatus according to the present invention. The image forming apparatus comprises a plurality of drum-shaped photoconductive photosensitive members (which are referred to as photosensitive drums hereinafter)  1 ,  2 ,  3  and  4 , electrifying parts  6 ,  7 ,  8  and  9 , an optical scanner  5  serving as an exposing part, developing parts  10 ,  11 ,  12  and  13 , a transferring-carrying apparatus  22 , cleaning parts  18 ,  19 ,  20 , and  21 . As shown in  FIG. 4 , the image forming apparatus is a full-color image forming apparatus in that the photosensitive drums  1 ,  2 ,  3  and  4 , which are linearly arranged therein, are used to form color images corresponding to individual colors such as black (BK), cyan (C), magenta (M) and yellow (Y), respectively. The photosensitive drums  1 ,  2 ,  3  and  4  are not limited to the as-shown arrangement and may be arranged in any suitable manner. As shown in  FIG. 4 , the other above-mentioned parts (the electrifying parts  6 ,  7 ,  8  and  9 , the developing parts  10 ,  11 ,  12  and  13 , and the cleaning parts  18 ,  19 ,  20  and  21 ) for forming images through electrophotographic processing are provided around the respective photosensitive drums  1 ,  2 ,  3  and  4 . The electrifying parts  6 ,  7 ,  8  and  9  are formed of charge rollers, charge brushes, electrifying charger, for example. The optical scanner  5 , which is to be described in greater detail below, uses optical beams L 1 , L 2 , L 3  and L 4  to expose scanned surfaces of the photosensitive drums  1 ,  2 ,  3  and  4 . The developing parts  10 ,  11 ,  12  and  13  serve as developing apparatuses, each of which corresponds to individual colors of BK, C, M and Y. The transferring-carrying apparatus  22  includes a transferring-carrying belt  22   a  and transferring parts  14 ,  15 ,  16  and  17 , which are formed of transferring rollers and transferring brushes, for example, in the inward-facing side of the transferring-carrying belt  22   a.  The cleaning parts  18 ,  19 ,  20  and  21  are formed of cleaning blades and cleaning brushes, for example. In this configuration, the image forming apparatus according to the present invention can form individual color images on the photosensitive drums  1 ,  2 ,  3  and  4 . 
   In  FIG. 4 , the X and Y directions represent horizontal directions of a space where the image forming apparatus is located, and the Z direction represents a vertical direction thereof. As shown in  FIG. 4 , the four photosensitive drums  1 ,  2 ,  3  and  4  are linearly arranged to have a slope with respect to the X-Y plane. In  FIG. 4 , the photosensitive drums  1 ,  2 ,  3  and  4  are arranged to have a negative slope in the Z-X coordinate directions. The transferring-carrying apparatus  22  is provided slantingly relatively to the X-Y plane in nearly parallel relation to the arrangement of the four photosensitive drums  1 ,  2 ,  3  and  4 . A transferred member such as a record paper is fed from the lower end of the sloped arrangement of parts and is carried upwards to transferring parts  14 ,  15 ,  16  and  17  of the photosensitive drums  1 ,  2 ,  3  and  4  sequentially on the transferring-carrying belt  22   a.  A fixing apparatus  26  is provided at upper end of the sloped arrangement of parts, that is, the lower stream of the carrying direction of the transferred member. Also, the optical scanner  5  is mounted around an upper corner of the linearly arranged photosensitive drums  1 ,  2 ,  3  and  4 , which serve as image forming parts. A housing  50  of the optical scanner  5  is mounted slantingly relative to the X-Y plane such that the housing  50  is nearly parallel to the arrangement of the photosensitive drums  1 ,  2 ,  3  and  4 . The housing  50  is fixed to sloped frame members  29  and  30  of the image forming apparatus. 
     FIG. 5  is a top plan view of the optical scanner  5  from the upper portion thereof.  FIG. 6  shows an arrangement of an optical deflector and optical systems in the optical scanner  5 . 
   Referring to  FIG. 5  and  FIG. 6 , the optical scanner  5  comprises four illuminant units  52 ,  53 ,  54  and  55 , an optical deflector  62 , beam focusing lenses  63 ,  64 ,  69 ,  70 ,  71  and  72 , optical path folding mirrors  65 ,  66 ,  67 ,  68 ,  73 ,  74 ,  75 ,  76 ,  77 ,  78 ,  79  and  80 . These components are accommodated in the housing  50 . The four illuminant units  52 ,  53 ,  54  and  55  emit optical beams L 1 , L 2 , L 3  and L 4 , respectively. The optical deflector  62  deflects the optical beams L 1 , L 2 , L 3  and L 4  such that two pairs of the four beams L 1 , L 2 , L 3  and L 4  propagate in two directions symmetric to each other. Image forming optical systems, which include the beam focusing lenses  63 ,  64 ,  69 ,  70 ,  71  and  72 , and the optical path folding mirrors  65 ,  66 ,  67 ,  68 ,  73 ,  74 ,  75 ,  76 ,  77 ,  78 ,  79  and  80 , lead the deflected optical beams L 1 , L 2 , L 3  and L 4  on scanned surfaces of the corresponding photosensitive drums  1 ,  2 ,  3  and  4 , as illustrated in  FIG. 6 . 
     FIG. 7  is a cross-sectional view of the optical scanner  5  as viewed from the A-A′ plane in  FIG. 5 . 
   As shown in  FIG. 5  and  FIG. 7 , the housing  50  includes a substrate  50 A to which the optical deflector  62  and the optical systems are mounted, and a sidewall  50 B for surrounding the substrate  50 A. The substrate  50 A is located near the center of the housing  50  with respect to the vertical direction thereof (top to bottom in  FIG. 7 ) and partitions an inner space in the housing  50  into upper and lower portions. The four illuminant units  52 ,  53 ,  54  and  55  are mounted to the sidewall  50 B ( FIG. 5 ) in almost parallel relation to the arrangement of the photosensitive drums  1 ,  2 ,  3  and  4  ( FIG. 4 ). As shown in  FIG. 7 , the optical deflector  62  is located in the center of the substrate  50 A. The above-mentioned optical systems such as the beam focusing lenses  63 ,  64 ,  69 ,  70 ,  71  and  72  and the optical path folding mirrors  65 ,  66 ,  67 ,  68 ,  73 ,  74 ,  75 ,  76 ,  77 ,  78 ,  79  and  80  are provided in both portions (the upper portion and the lower portion) of the inner space partitioned by the substrate  50 A. Also, covers  87  and  88  are provided in the lower side and the upper side of the housing  50 , respectively. The lower cover  87  has apertures for passage of optical beams, and dustproof glasses  83 ,  84 ,  85  and  86  are provided to each of the apertures. 
     FIG. 8  shows an arrangement of the illuminant units  52 ,  53 ,  54  and  55 , the optical deflector  62  and the optical systems in the optical scanner  5 . As shown in  FIG. 8 , when image data of individual colors are provided to the optical scanner  5  from a manuscript reading apparatus such as a scanner or an image data output apparatus such as a personal computer, a word processor, or a facsimile receiver, which are not illustrated, the optical scanner  5  converts the individual color image data into signals for driving the illuminant units  52 ,  53 ,  54  and  55 . The illuminant units  52 ,  53 ,  54  and  55  emit optical beams in accordance with the signals. The optical beams propagate through cylindrical lenses  56 ,  67 ,  58  and  59  for correcting optical face tangle errors and then arrive at the optical deflector  62  directly or via mirrors  60  and  61 . The optical beams are deflected in the two symmetric directions by two-tiered polygon mirrors  62   a  and  62   b  rotated by a polygon motor  62   c  at constant speed as shown in  FIGS. 7 and 8 . Here, the two-tiered polygon mirrors  62   a  and  62   b  deflect a pair of the optical beams L 2  and L 3  and a pair of the optical beams L 1  and L 4 , respectively. Although the optical deflector  62 , as illustrated, uses two polygon mirrors  62   a  and  62   b,  the optical deflector  62  may use, for example, one large polygon mirror to deflect the four optical beams L 1 , L 2 , L 3  and L 4 . 
   As shown in  FIG. 7 , the deflected optical beams are transmitted through image forming lenses  63  and  64 . For instance, two-tiered fθ lenses can be used as the image forming lenses  63  and  64 . The optical beams L 1 , L 2 , L 3  and L 4  are folded by the first folding mirrors  65 ,  66 ,  67  and  68  and then travel through the apertures of the substrate  50 A. After passage through the apertures of the substrate  50 A, the optical beams L 1 , L 2 , L 3  and L 4  travel through the second image forming lenses  69 ,  70 ,  71  and  72 , and then arrive on the scanned surfaces of the photosensitive drums  1 ,  2 ,  3  and  4  via the second folding mirrors  73 ,  75 ,  77  and  79 , the third folding mirrors  74 ,  76 ,  78  and  80 , and the dustproof glasses  83 ,  84 ,  85  and  86 . When the optical beams L 1 , L 2 , L 3  and L 4  are irradiated on the scanned surfaces of the photosensitive drums  1 ,  2 ,  3  and  4 , it is possible to write latent images on the scanned surfaces. 
   In the optical scanner  5 , each of the illuminant units  52 ,  53 ,  54  and  55  comprises a semiconductor laser (LD) working as an illuminant and a collimate lens for collimating a luminous flux emitted by the semiconductor laser. The semiconductor laser and the collimate lens are integrally accommodated in a retaining member such as a holder. In the illustrated embodiment, the illuminant unit  52  is a BK illuminant unit. Since the BK illuminant unit  52  is more frequently used than any other color illuminant units to form monochrome images, it is preferable that the illuminant unit  52  be constituted as a multi-beam illuminant unit wherein at least two pairs of illuminants and collimate lenses are integrally accommodated in the retaining member thereof. As a result, when the monochrome images are formed, it is possible to optically write the monochrome images at a high speed and, therefore, to improve productivity of the image forming apparatus with respect to monochrome image formation. 
   A description will now be given, with reference to  FIG. 9  and  FIG. 10 , of a multi-beam illuminant unit serving as a BK illuminant unit.  FIG. 9  is an exploded perspective view of the multi-beam illuminant unit.  FIG. 10  is a cross-sectional view of the multi-beam illuminant unit. 
   Referring to  FIG. 9  and  FIG. 10 , semiconductor lasers  111  and  112 , which serve as illuminants of the multi-beam illuminant unit  52 , are fixed to supporting members  113  and  114 , respectively. The semiconductor lasers  111  and  112  are connected to a collimate lens holder  115  with fasteners  118  and  119  via the supporting members  113  and  114  such that optical beams from the semiconductor lasers  111  and  112  coincide with optical axes of collimate lenses  116  and  117 , respectively. The collimate lenses  116  and  117  are accommodated in cylindrical mirror holders and are connected to holes  115   a  and  115   b  in the collimate lens holder  115  by a suitable adhesive such that the collimate lenses  116  and  117  are positioned, as illustrated, for example, relative to the respective semiconductor lasers  111  and  112 . The collimate lenses  116  and  117  convert the optical beams from the semiconductor lasers  111  and  112  into parallel luminous fluxes. An iris plate  120  is provided at the exit end of the collimate lenses  116  and  117  so that each of the outgoing optical beams can have a predetermined beam diameter. A beam synthesizing part  121  such as a prism is provided behind or downstream of the iris plate  120 . 
   The two semiconductor lasers  111  and  112  are arranged in the same plane such that a pn junction surface of the semiconductor laser  111  coincides with that of the semiconductor laser  112 . A ½ wavelength plate  122  is provided at the entrance end of the beam synthesizing part  121  so as to rotate by 90° a polarization surface of one of the two optical beams from the semiconductor lasers  111  and  112 , for example, the optical beam from the semiconductor laser  111  in the illustrated embodiment. The resulting optical beam whose polarization surface is rotated by 90° travels to a polarization beam splitter surface  121   b  ( FIG. 9 ) of the beam synthesizing part  121 . The optical beam from the semiconductor laser  112 , on the other hand, is inner-reflected on a sloped surface  121   a  of the beam synthesizing part  121  and also is reflected on the polarization beam splitter surface  121   b.  The resulting optical beam from the semiconductor laser  112  is synthesized with the optical beam from the semiconductor laser  111  in the vicinity of the optical axis of the optical beam from the semiconductor laser  111 , which is considered as a reference optical beam. At this time, the optical axes of the semiconductor lasers  111  and  112  are directed in slightly different directions from each other with respect to the main scanning direction. Here, an angle between the optical axes is set as θ in the exit side of the beam synthesizing part  121  as shown in  FIG. 9 . 
   The beam synthesizing part  121  and the iris plate  120  are mounted at predetermined positions on an entrance or upstream surface of a flange member  123 , and the flange member  123  is fixed to the collimate lens holder  115  with fasteners  124  and  125 . The flange member  123  and/or the collimate lens holder  115  is fixed on (not illustrated) a substrate  126  on which a drive circuit for driving the semiconductor lasers  111  and  112  is provided. In this configuration, the members along the optical paths between the semiconductor lasers  111  and  112  and the flange member  123  are fixed on the substrate  126 , and these members constitute the illuminant unit  52 . 
   As shown in  FIG. 9 , a cylinder part  123   a  is mounted to the exit or downstream end of the flange member  123 . The cylinder part  123   a  is inserted into a hole  132   a  of a frame  132  provided on the sidewall  50 B of the housing  50  ( FIG. 7 ). The cylinder part  123   a  is inserted through the interior of a helical spring  130  and further through a hole  131   a  of a spring pressure plate  131 . In this configuration, if the BK illuminant unit  52 , which has the members between the semiconductor lasers  111  and  112  and the flange  123  on the substrate  126 , is pulled in the direction of the arrow α in  FIG. 9  and then the spring pressure plate  131  is rotated 90°, it is possible to hook a projection  131   b  of the spring pressure plate  131  on a projection  123   b  of the cylinder part  123   a.  As a result, the BK illuminant unit  52  is mounted to the frame  132  in a state where the BK illuminant unit  52  can be freely rotated with respect to the center (optical axis) of the cylinder part  123   a  of the flange member  123 . 
   Since the illuminant unit  52  can be rotated with respect to the optical axis, it is possible to adjust a pitch between optical spots on the photosensitive drum. A pitch changing part, which is described hereinafter, is used to adjust the optical spot pitch. 
   In  FIG. 9 , a male screw whose nominal diameter is M 3  is shaped on a feed screw  128 , and a female screw is shaped in the interior of a moving member  127 . The moving member  127  has a somewhat D-shaped outer body. The male screw of the feed screw  128  is inserted into the female screw of the moving member  127 . The moving member  127  is inserted into a D-shaped hole of a cylinder  132   b  that is provided in the frame  132  in the housing  50 , as illustrated. The moving member  127  is slidably movable in the cylinder  132   b.  Here, a rotation shaft  129   a  of a pitch change stepping motor  129  is inserted through the hole of the cylinder  132   b  of the frame  132 . The lower end of the feed screw  128  is fixed to the top end of the rotation shaft  129   a , for example, by means of a pressure fit. The pitch change stepping motor  129  is connected to the frame  132  so that the feed screw  128 , which is pressed to the rotation shaft  129   a , can be rotated through rotation of the pitch change stepping motor  129 . Since the cylinder  132   b  has the corresponding D-shaped hole, the moving member  127  is able to make up-and-down motions ( FIG. 9 ). 
   The flange member  123  has an arm  123   c . The arm  123   c  extends toward the moving member  127 , and the end of the arm  123   c  is in contact with the top end of the moving member  127 . A tension spring  135  is provided between the arm  123   c  and the frame  132 . The tension spring  135  pulls down the arm  123   c  so that the end of the arm  123   c  can be depressed on the moving member  127 . As a result, when the moving member  127  moves in the vertical direction through rotational motions by the pitch change stepping motor  129 , the arm  123   c  moves up-and-down in the vertical direction. According to such an arrangement, the illuminant unit  52  may be rotated wherein the center of the cylinder  123   a  of the flange member  123  is the rotational axis. 
   An optical home position sensor  133  for controlling a rotation angle of the illuminant unit  52  is fixed with fasteners that are not illustrated. The optical home position sensor  133  has an illuminant part  133   a  and a receiver part  133   b  in the side of the frame  132 . A filler  123   d  is provided in the side opposite to the arm  123   c  of the flange member  123 . The filler  123   d  has an edge part for screening between the illuminant part  133   a  and the receiver part  133   b  of the home position sensor  133 . A home position (HP) of the optical home position sensor is determined as the position at the time when the edge part  123   e  screens between the illuminant part  133   a  and the receiver part  133   b.  The home position is used as a reference position for adjusting rotation of the illuminant unit  52 . 
     FIG. 11  is a diagram for explaining the rotation adjustment of the illuminant unit  52  for the purpose of changing a pitch of optical spots with respect to the sub-scanning direction. In  FIG. 11 , dotted lines indicate positions of the illuminant part  133   a  and the receiver part  133   b.  As mentioned above, the home position is set as the position of the home position sensor  133  at the screening time of the edge part  123   e.  The position B in  FIG. 11  is a position of the home position sensor  133  where the illuminant unit  52  is rotated by a rotation angle of θ 1  from the home position with respect to a rotational axis as the optical axis thereof. In order to rotate the illuminant unit  52  by the rotation angle of θ 1 , it is necessary to shift the moving member  127  at a predetermined distance in the upper direction by rotating the pitch change stepping motor  129  by a predetermined number of pulses. Similarly, the position A in  FIG. 11  indicates a position of the home position sensor  133  where the illuminant unit  52  is rotated by a rotation angle of θ 2  from the home position with respect to the rotational axis. 
     FIGS. 12A and 12B  show positions of optical spots on a photosensitive drum when the illuminant unit  52  is rotated such that the home position sensor  133  is located at the home position and the position A and B.  FIG. 12A  is a diagram illustrating a case where a position of one of optical beams from the two semiconductor lasers  111  and  112  is set as the rotational axis, and  FIG. 12B  is a diagram illustrating a case where a middle position between the two optical beams is set as the rotational axis. In  FIGS. 12A and 12B , the lengths P 1  and P 2  represent pitches of the optical spots on the photosensitive drum with respect to the sub-scanning direction corresponding to the rotation angles of θ 1  and θ 2 . As seen in  FIGS. 12A and 12B , if the illuminant unit  52  is rotated from the home position, it is possible to change the pitches of the optical spots on the photosensitive drum and easily control the rotation by adjusting the driving number of pulses of the pitch change stepping motor  129 . 
   Normally, the pitches of optical spots on the photosensitive drum with respect to the sub-scanning direction are changed in accordance with a record density. For instance, it may be assumed that the driving pulse number Pa corresponding to the record density of 600 dpi is set as 42 μm and the driving pulse number Pb corresponding to the record density of 1200 dpi is set as 21 μm. If the pulse numbers Pa and Pb are stored in a memory in a control part of an image forming apparatus, it is possible to easily switch the pitches of optical spots on the photosensitive drum with respect to the sub-scanning direction by rotating the pitch change stepping motor  129  based on the stored data regarding the driving pulse numbers Pa and Pb in accordance with a requested record density. 
   Once the image forming apparatus is switched ON, the image forming apparatus locates the illuminant unit  52  at a predetermined position, for instance, by rotating the illuminant unit  52  by the rotation angle (to position B) corresponding to the record density of 600 dpi. In order to locate the illuminant apparatus  52  at that position, when the image forming apparatus is switched ON, the image forming apparatus returns the illuminant unit  52  to the home position. Thereafter, the pitch change stepping motor  129  is driven by the pulse number Pa in a predetermined direction so as to locate the pitch change stepping motor  129  at the position B. As a result, it is possible to rotate the illuminant unit  52  by the rotation angle of θ 1  so that optical spots on the photosensitive drum can have the pitch P 1  corresponding to the position B with respect to the sub-scanning direction. Here, the image forming apparatus has information regarding the predetermined rotation angles in the memory of the control part such as a CPU (Central Processing Unit). Accordingly, when the record density of 1200 dpi is requested, the image forming apparatus drives the pitch change stepping motor  129  by the pulse number of (Pb-Pa) so that the illuminant unit  52  can be rotated from the position B, which is the position corresponding to the record density of 600 dpi. As a result, it is possible to properly change the pitch of the optical spots by rotating the illuminant unit  52  to the position A, which is the position corresponding to the record density of 1200 dpi. 
     FIG. 13  is a flowchart of the above-mentioned procedure. As shown in  FIG. 13 , a user selects the record density of 1200 dpi in a black mode (BK 1200 dpi mode) at step S 1 . At step S 2 , it is determined whether or not a pitch position of a BK beam is located at the position A. At step S 3 , if the pitch position is currently located at the position A, the image forming apparatus receives print data and then performs a normal printing process by rotating the polygon motor  62   c.  On the other hand, if the pitch position is not located at the position A, the pitch position is shifted to the position A at step S 4 . After the pitch position has been shifted to the position A by rotating the pitch change stepping motor  129  by the pulse number of (Pb-Pa) at step S 6 , the image forming apparatus is ready to write the sent print data at the position A corresponding to the record density of 1200 dpi. Then, the image forming apparatus proceeds to the step S 3  to perform the writing procedure. Here, it is noted that the pitch position is located at the position A just after step S 3 . 
   Subsequently, if the user selects a color mode print under the record density of 600 dpi, the image forming apparatus proceeds to step S 8 . At step S 9 , it is determined whether or not the pitch position is located at the position B. However, since the pitch position is located at the position A after step S 3 , the branch condition at step S 9  is normally determined as NO. Then, the pitch change stepping motor  129  is driven by the pulse number of (Pb-Pa) in the direction opposite to the rotational direction at step S 5  so as to move the pitch position to the position B at step S 11  and step S 12 . After shifting the pitch position to the position B at step S 13 , the image forming apparatus receives write data and then performs the normal printing process by rotating the polygon motor  62   c  at step S 10 . In this fashion, the whole procedure is completed at step S 14 . 
   Here, the above-mentioned procedure is automatically performed by a printer driver of the image forming apparatus in accordance with user&#39;s requests, received data or received instructions. Then, an image is printed out at a requested record density. 
   The optical scanner  5  shown in  FIG. 5  and  FIG. 6  has synchronization detecting mirrors, which are not illustrated, for retrieving luminous fluxes of start scanning positions in the main scanning direction on optical paths of the optical beams L 1 , L 2 , L 3  and L 4 . Synchronization detectors  81  and  82  receive the luminous fluxes reflected on the synchronization detecting mirrors and supply synchronization signals for start timings of scanning. Furthermore, stepping motors  92 ,  93  and  94  for adjusting skew are provided in the third folding mirrors  74 ,  75  and  76  on the optical paths of the optical beams L 1 , L 2  and L 3 , respectively, as shown in  FIG. 6 . The stepping motors  92 ,  93  and  94  are used to correct misalignment of scanning lines of the optical beams L 1 , L 2  and L 3  with reference to the scanning line of the optical beam L 1 . Here, the main scanning direction is defined as a direction where the optical beams deflected by the optical deflector  62  scan the photosensitive drums, that is, the axis directions of the photosensitive drums. Also, the sub-scanning direction is a direction perpendicular to the main scanning direction, that is, the rotation direction of the photosensitive drums (moving directions of the surfaces of the photosensitive drums). Also, the sub-scanning direction is a carrying direction of a transferring-carrying belt  22   a  to be mentioned later. For this reason, it is concluded that the width direction of the transferring-carrying belt  22   a  is the main scanning direction, and the carrying direction thereof is the sub-scanning direction. 
   As shown in  FIG. 4 , the transferring-carrying belt  22   a  is disposed under the four photosensitive drums  1 ,  2 ,  3  and  4 . The transferring-carrying belt  22   a  is supported by drive rollers and dependent rollers and is carried in the arrow direction in  FIG. 4  by the drive rollers. Furthermore, paper feeding parts  23  and  24  for accommodating transferred members such as record papers are provided in the lower part of the image forming apparatus. The transferred members in the paper feeding parts  23  and  24  are fed to the transferring-carrying belt  22   a  via paper feeding rollers, carrying rollers and a resist roller  25  and then are carried by the transferring-carrying belt  22   a.    
   After the optical scanner  5  forms latent images for the individual photosensitive drums  1 ,  2 ,  3  and  4 , the latent images are developed with individual color toners of BK, C, M and Y by the developing parts  10 ,  11 ,  12  and  13 . The developed toner images of individual colors are sequentially superposed on a transferred member on the transferring-carrying belt  22   a  by the transferring parts  14 ,  15 ,  16  and  17  of the transferring-carrying apparatus  22 . Then, the transferred member on which the four color images are transferred is delivered to the fixing apparatus  26  and then is fixed therein. Thereafter, the resulting transferred member is delivered to the output tray  28  by the paper output roller  27 . Here, if the image forming apparatus is in the monochrome image forming mode, the above-mentioned process is performed for only the BK photosensitive drum  1 . 
   According to the above-mentioned image forming apparatus, when resist adjustment is performed for a full-color image with respect to the main scanning direction and the sub-scanning direction regardless of the resolutions of 600 dpi and 1200 dpi, the image forming apparatus can adjust the BK pixel density position at a predetermined position and write the full-color image at the adjusted pixel density position. As a result, it is possible to provide the tandem type color image forming apparatus that can overcome misalignment of writing positions of the color image. 
   Here, the above-mentioned embodiments concentrate on the optical scanner and the image forming apparatus that can switch the write density into the two resolutions of 600 dpi and 1200 dpi. However, the optical scanner and the image forming apparatus according to the present invention can also deal with a resolution of 2400 dpi in addition to the resolutions of 600 dpi and 1200 dpi in a similar configuration. 
   The present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the present invention. 
   The present application is based on Japanese priority application No. 2002-169989 filed Jun. 11, 2002, the entire contents of which are hereby incorporated by reference.