Abstract:
An image forming apparatus includes a light emission source, a polygon mirror, a plurality of image carrying members, an optical detection mechanism, and an instruction mechanism. The light emission source outputs a plurality of optical beams in accordance with image data. The polygon mirror receives the optical beams at different mirror points, and deflects the beams into a first plurality of scanning optical beams in given directions to scan on predetermined scanning lines in a main scanning direction. The plurality of image carrying members rotating in a sub-scanning direction receive the first plurality of scanning optical beams to form primary separate color images. The optical detection mechanism detects a second plurality of scanning optical beams included in the first plurality of scanning optical beams. The instruction mechanism instructs the light emission source to light on and off at timings based on a plurality of detection signals from the optical detection mechanism.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This patent application is based on Japanese patent application, No. 2005-339871 filed on Nov. 25, 2005 in the Japan Patent Office, the entire contents of which are incorporated by reference herein.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     Exemplary aspects of the present invention relate to a method and apparatus for image forming, and more particularly to a method and an apparatus for toner image forming capable of detecting a reference signal used for a lighting control.  
         [0004]     2. Description of the Related Art  
         [0005]     A related art image forming apparatus such as a laser printer, a digital copying machine, a facsimile, etc., has employed an optical beam scanning method for an image writing. According to the optical beam scanning method, a lighting (light emission) of a laser diode (LD) outputting a plurality of optical beams is controlled by image data, and a rotation polygon mirror deflects the plurality of optical beams to scan periodically in a main scanning direction. Thereby the optical beams irradiate a photoconductor moving towards in a sub-scanning direction so as to write an image on the photoconductor by each line.  
         [0006]     When the image is written on the photoconductor by the optical beam scanning method, the photoconductor has an image writing start position thereon to begin the image writing. Since this image writing position needs to remain constant for each scanning line, a synchronous detection sensor is used to detect that the optical beams are disposed outside an image region of the photoconductor in an image writing start position side. The synchronous detection sensor detects a passage of the optical beams scanned in the main scanning direction (also referred to as a main scanning line) by the rotation polygon mirror, and instructs a lighting timing of the LD for each main scanning line with a synchronous detection signal generated thereby as a reference so as to control the image writing start position to be constant.  
         [0007]     Regarding a color image formed by a related art color image forming apparatus, in general, a number of the photoconductors to be used is substantially equal to a number of color components. The photoconductors are scanned by the optical beams of respective color components, and images of different colors are superimposed by a transfer process so as to form a full color image. This formation of the full color image is referred to as a tandem system, and has been widely used.  
         [0008]     As the tandem system exposes the photoconductors of respective color components to the optical beams, the image writing is controlled for each color component based on the synchronous detection signals generated by the synchronous detection sensors.  
         [0009]     Therefore, the related art color image forming apparatus employing the tandem system generally superimposes the images of four colors, i.e., yellow, magenta, cyan, and black, formed on respective photoconductors so as to form the full color image. Thereby, the synchronous detection signals for the four colors are generated by using four different synchronous detection sensors.  
         [0010]     In addition to using the four synchronous detection sensors with respect to the optical beams of respective colors, one example has attempted to use two synchronous detection sensors in another related art color image forming apparatus. Each synchronous detection sensor is commonly used for two color components.  
         [0011]     According to this example of using the two synchronous detection sensors, one polygon mirror capable of scanning the four color components is employed. The four optical beams for the four color components are divided into two groups, for example, black and cyan, and magenta and yellow. The one polygon mirror has a plurality of mirror faces into which the optical beams are entered with respect to each group. The two optical beams of different color components in each group are detected by one of the two synchronous detection sensors. Each of the two synchronous detection sensors outputs the synchronous detection signals of the two optical beams which can be separated based on a time period by shifting a detection timing of each optical beam.  
       SUMMARY OF THE INVENTION  
       [0012]     According to an aspect of the invention, an image forming apparatus includes a light emission source, a rotary polygon mirror, a plurality of image carrying members, an optical detection mechanism, and an instruction mechanism. The light emission source outputs a plurality of optical beams in accordance with image data. The rotary polygon mirror receives the plurality of optical beams at different mirror points with different mirror positions, and deflects the beams into a first plurality of scanning optical beams in given directions to cyclically scan on a plurality of predetermined scanning lines in a main scanning direction. The plurality of image carrying members rotate in a sub-scanning direction line by line, and respectively receive the first plurality of scanning optical beams to form a plurality of primary separate color images. The optical detection mechanism detects at a specific point a second plurality of scanning optical beams included in the first plurality of scanning optical beams. The instruction mechanism instructs the light emission source to light on and off at timings determined based on a plurality of detection signals from the optical detection mechanism.  
         [0013]     According to another aspect of the invention, a method of image forming includes providing, rotating, diving, detecting, and instructing. The proving step provides a light emission source to output a plurality of optical beams in accordance with image data. The rotating step rotates a rotary polygon mirror to receive the plurality of optical beams at different mirror points with different mirror positions so as to deflect the beams into a first plurality of scanning optical beams in given directions to cyclically scan on a plurality of predetermined scanning lines in a main scanning direction. The driving step drives a plurality of image carrying members to rotate in a sub-scanning direction line by line so as to respectively receive the first plurality of scanning optical beams to form a plurality of primary separate color images. The detecting step detects at a specific point a second plurality of scanning optical beams included in the first plurality of scanning optical beams with an optical detection mechanism. The instructing step instructs the light emission source to light on and off at timings determined based on the plurality of detection signals from the optical detection mechanism.  
         [0014]     According to another aspect of the invention, an optical writing apparatus includes a light emission source, a rotary polygon mirror, an optical detection mechanism, and an instruction mechanism. The light emission source outputs a plurality of optical beams in accordance with image data. The rotary polygon mirror receives the plurality of optical beams at different mirror points with different mirror positions, and deflects the beams into a first plurality of scanning optical beams in given directions so as to cyclically scan along a plurality of predetermined scanning lines in a main scanning direction on a plurality of image carrying members provided to rotate in a sub-scanning direction line by line and to respectively receive the first plurality of scanning optical beams to form a plurality of primary separate color images. The optical detection mechanism detects at a specific point a second plurality of scanning optical beams included in the first plurality of scanning optical beams. The instruction mechanism instructs the light emission source to light on and off at timings determined based on a plurality of detection signals from the optical detection mechanism.  
         [0015]     According to still another aspect of the invention, an image forming apparatus includes plural light emission sources each configured to output an optical beam, a light reflecting unit configured to receive the optical beam from each light emission source of the plural light emission sources and to reflect the optical beam, plural image carrying members each configured to receive a corresponding optical beam reflected by the light reflecting unit, a single optical detection mechanism configured to receive each optical beam of at least half of the plural light emission sources prior to arriving at the plural image carrying members, and to generate detection signals, each corresponding to one of the at least half of the plural light emission sources, and an instruction mechanism configured to instruct each light emission source when to light on and off, at different timings, based on the detection signals corresponding to the at least half of the plural light emission sources. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     A more complete appreciation of the exemplary aspects of the invention and many of the attendant advantage thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:  
         [0017]      FIG. 1  is a schematic diagram illustrating a color image forming apparatus according to an exemplary embodiment of the present invention;  
         [0018]      FIG. 2  is a schematic diagram illustrating an optical beam scanning device and an image forming unit included in the image forming apparatus of  FIG. 1 ;  
         [0019]      FIG. 3  is a schematic diagram illustrating a synchronous detection of an optical beam;  
         [0020]      FIG. 4  is a schematic diagram illustrating an example of synchronous detection signals generated by a synchronous detection sensor in the image forming apparatus of  FIG. 1 ;  
         [0021]      FIG. 5  is a schematic diagram illustrating a separation of the synchronous detection signals by a time base;  
         [0022]      FIG. 6  is a schematic diagram illustrating another example of the synchronous detection signals generated by the single synchronous detection sensor according to another exemplary embodiment of the present invention; and  
         [0023]      FIG. 7  is a schematic diagram illustrating a state of the optical beam entering into the synchronous detection sensor. 
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0024]     In describing exemplary embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner. Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, a color image forming apparatus according to an exemplary embodiment of the present invention is described.  
         [0025]     Referring to  FIG. 1 , the color image forming apparatus includes an optical beam scanning device  20 , image forming units  200 BK,  200 C,  200 M, and  200 Y, and a transfer belt  36 . The optical beam scanning device  20  includes a plurality of optical elements such as a polygon mirror  22 , a polygon motor  22   m , fθ lenses  23 BKC and  23 MY, first mirrors  251 BK,  251 C,  251 M, and  251 Y, second mirrors  252 BK,  252 C,  252 M, and  252 Y, barrel toroidal lenses (BTL)  24 BK,  24 C,  24 M, and  24 Y, laser diodes  10 BK,  10 C,  10 M, and  10 Y (shown in  FIG. 4 ), a synchronous detection sensor  7  (shown in  FIG. 3 ) and third mirrors  253 BK,  253 C,  253 M, and  253 Y. The image forming units  200 BK,  200 C,  200 M, and  200 Y respectively includes photoconductors  29 BK,  29 C,  29 M, and  29 Y, charging devices  30 BK,  30 C,  30 M, and  30 Y, discharge devices  34 BK,  34 C,  34 M, and  34 Y, cleaning units  33 BK,  33 C,  33 M, and  33 Y, transfer devices  32 BK,  32 C,  32 M, and  321 Y, and development units  31 BK,  31 C,  31 M, and  31 Y as image forming elements. This color image forming apparatus of the exemplary embodiment employs a tandem system having the four image forming units  200 BK,  200 C,  200 M, and  200 Y for respective four color components, black, cyan, magenta, and yellow which are respectively abbreviated as BK, C, M, and Y. These abbreviations may be omitted as necessary.  
         [0026]     The optical beam scanning device  20  emits optical beams of the four color components BK, C, M, and Y to irradiate respective photoconductors  29 BK,  29 C,  29 M, and  29 Y. The image forming unit  200 BK,  200 C,  200 M, and  200 Y form and develop electrostatic latent images to form the toner images so as to transfer the toner images onto a transfer sheet. The transfer belt  36  conveys the transfer sheet (not shown).  
         [0027]     As stated above, the optical beam scanning device  20  includes the plurality of optical elements as follows. The polygon mirror  22  deflects the optical beams of four color components. The polygon motor  22   m  drives the polygon mirror  22 . The fθ lenses  23 BKC and  23 MY correct scanning speeds of the optical beams. The first mirrors  251 BK,  251 C,  251 M, and  251 Y, the second mirrors  252 BK,  252 C,  252 M, and  252 Y, and the third mirrors  253 BK,  253 C,  253 M, and  253 Y reflect the optical beams. The laser diodes  10 BK,  10 C,  10 M, and  10 Y are light emission sources. The synchronous detection sensor  7  detects the optical beams and generates synchronous detection signals. The barrel toroidal lenses (BTL)  24 BK,  24 C,  24 M, and  24 Y correct, for example, a focusing function and an optical face tangle error in a sub-scanning direction.  
         [0028]     Like the optical beam scanning device  20  including the plurality of optical elements, the image forming image forming units  200 BK,  200 C,  200 M, and  200 Y include the plurality of image forming elements as follows. The photoconductors  29 BK,  29 C,  29 M, and  29 Y form electrostatic latent images thereon by the optical beams emitted from the optical device  20 . The charging devices  30 BK,  30 C,  30 M, and  30 Y uniformly charge surfaces of the photoconductor  29 BK,  29 C,  29 M, and  29 Y. The discharge devices  34 BK,  34 C,  34 M, and  34 Y discharge residual charges of the photoconductors. The cleaning units  33 BK,  33 C,  33 M, and  33 Y remove remaining toners from the surfaces of the photoconductors. The transfer devices  32 BK,  32 C,  32 M, and  32 Y transfer the toner images onto the transfer sheet. The development units  31 BK,  31 C,  31 M, and  31 Y develop the electrostatic latent images on the photoconductors to form the toner images.  
         [0029]     This color image forming apparatus employs an electrophotographic method with the tandem system to form the full color image. The color image forming apparatus controls a lighting of each laser diode  10  by image data of respective color component, writes images on photoconductors  29 BK,  29 C,  29 M, and  29 Y by a plurality of optical beams emitted from the optical beam scanning device  20 , develops the images on the photoconductors with toners by development units  31 BK,  31 C,  31 M, and  31 Y, and superimposes the toner images of four colors so as to form the full color image on the transfer sheet.  
         [0030]     Since this color image forming apparatus with the tandem system superimposes the toner images of four color components to form the full color image, the four image forming units  200 BK,  200 C,  200 M, and  200 Y for respective four color components are disposed therein. As stated above, the photoconductors  29 BK,  29 C,  29 M, and  29 Y form the electrostatic latent images thereon by irradiation of the optical beams. The irradiation of the optical beams is provided by one polygon mirror  22  instead of four polygon mirrors as deflecting devices for the four color components. Thereby, a configuration of the color image forming apparatus may be simplified, and a cost thereof may be reduced.  
         [0031]     As shown in  FIG. 1 , the optical beam scanning device  20  deflects the optical beams of different colors by using the polygon mirror  22  that is driven by the polygon motor  22   m . The polygon mirror  22  has a plurality of mirror faces. One of the plurality of mirror faces deflects the optical beams of two colors at upper and lower portions thereof. In other words, each one of the optical beams of two colors is deflected by either upper portion or lower portion of the mirror face. Another mirror face deflects the optical beams of other two colors at upper and lower portions thereof. These two mirror faces are opposed each other. Thereby, the optical beams deflected by the polygon mirror  22  are spread and centered opposite to each other around the polygon mirror  22 . Consequently, each color of the optical beams is deflected by the polygon mirror  22 , passes through the fθ lens  23 , is reflected off the first mirrors  251  and second mirror  252 , passes through the BTL  24 , is reflected off the third mirror  253 , and scans the photoconductor  29 . Thereby, the optical beams of four color components scan respective photoconductors  29 BK,  29 C,  29 M, and  29 Y.  
         [0032]     When the electrostatic latent images on the photoconductors  29  are developed and transferred onto the transfer sheet by respective image forming units  200 , the transfer belt  36  conveys the transfer sheet in a direction shown with an arrow in  FIG. 1  so that a first color image is transferred onto the transfer sheet. As the transfer sheet is further conveyed in the arrow direction, second, third, and fourth color images are sequentially transferred onto the transfer sheet. Thereby, the full color image is formed on the transfer sheet by superimposing one image on another. The full color image on the transfer sheet is fixed by a fixing device (not shown).  
         [0033]     In the exemplary embodiment shown in  FIG. 1 , the images are directly transferred from the photoconductors  29  to the transfer sheet to form the full color image. However, an image forming apparatus having an intermediate transfer member can be applied to this exemplary embodiment. When the intermediate transfer member is used, the images are transferred from the photoconductors to the intermediate transfer member, and the images transferred on the intermediate transfer member are secondarily transferred onto the transfer sheet.  
         [0034]     Referring to  FIG. 2 , the optical beam scanning device  20  and one of the four image forming units  200 BK,  200 C,  200 M, and  200 Y included in  FIG. 1  are explained in detail. Since each of the four image forming units  200 BK,  200 C,  200 M, and  200 Y is similar to other, except for the color components, one of the image forming units is shown as an example without the color abbreviation.  
         [0035]     In the optical beam scanning device  20 , the laser diode  10  (shown in  FIG. 3 ), the lighting of which is controlled by the image data, outputs the optical beams, and a collimate lens (not shown) collimates the optical beams output from the laser diode  10 . As shown in  FIG. 2 , after the optical beams pass through a cylinder lens (not shown), the optical beams enter into the mirror faces of the polygon mirror  22 . The optical beams are deflected by the polygon mirror  22 , pass through the fθ lens  23  and the barrel toroidal lens  24 , are reflected off the third mirror  253 , and irradiate the photoconductor  29 .  
         [0036]     As also shown in  FIG. 2 , the image forming unit  200  includes the image forming elements such as the photoconductor  29 , the charging device  30 , the development unit  31 , the transfer device  32 , the cleaning unit  33 , and the discharge device  34 . The photoconductor  29  includes other image forming elements in a vicinity thereof.  
         [0037]     The image forming unit  200  using the electrophotographic method forms the full color image on the transfer sheet by carrying out an image forming process such as charging the photoconductor  29  by the charging device  30 , irradiating on the photoconductor  29  by the optical beam scanning device  20  so as to form the electrostatic latent image, developing the electrostatic latent image on the photoconductor  29  with toner by the development unit  31 , transferring the toner image onto the transfer sheet by the transfer device  32 , fixing the toner image on the transfer sheet by the fixing device (not shown), removing a residual toner from the photoconductor  29  by the cleaning unit  33 , and discharging the photoconductor  29  by the discharge device  34  to prepare for a next image forming.  
         [0038]     The optical beams output from the laser diode  10 , which are deflected by the polygon mirror  22 , periodically scan a surface of the photoconductor  29  in a main scanning direction by a line scanning, and irradiate a receiving surface of the photoconductor  29  moving in a sub-scanning direction by each line so as to write a two-dimensional image on the receiving surface.  
         [0039]     When the photoconductor  29  is irradiated, the image writing position on the photoconductor  29  needs to be constant (the same) for each scanning line so that an occurrence of an image misregistration may be reduced. The synchronous detection sensor  7  (shown in  FIG. 3 ) is disposed to detect the optical beams at a certain position on the scanning line of an image writing start side and generate a synchronous detection signal.  
         [0040]     Referring to  FIG. 3 , the synchronous detection sensor  7  detects the optical beams. The color abbreviations BK, C, M, and Y for the optical elements may be omitted as necessary.  
         [0041]     As shown in  FIG. 3 , the plurality of optical beams emitted from the laser diode  10  in the optical beam scanning device  20  are deflected by the polygon mirror  22 , pass through the fθ lens  23 , are reflected off the first mirror  251  so as to scan the photoconductor  29  of  FIGS. 1 and 2  in the main scanning direction. One of the optical beams, for example, is directed towards the synchronous detection sensor  7  by reflecting off a mirror  71  which is disposed at a location before the optical beams enter into the first mirror  251 . In other words, the mirror  71  is disposed outside the photoconductor  29  in the image writing start side. The synchronous detection sensor  7  detects the optical beam entering at the certain position on the scanning line, and generates the synchronous detection signal.  
         [0042]     The synchronous detection sensor  7  detects the optical beams, and instructs a lighting timing of the laser diode  10  for each main scanning line with the synchronous detection signal as a reference in such a manner that the image writing is controlled to begin at the certain position. The lighting timing of the laser diode  10  is based on the image data written in an effective image area. The control of the image writing, for example, may be provided with using the synchronous detection signal as a trigger to begin the image writing after a given time. The given time may be determined by a predetermined clock period.  
         [0043]     When the image forming apparatus with the tandem system forms the full color image, a number of the photoconductors to be used is equal to a number of color components. Consequently, a synchronous detection signal may be needed for each color component.  
         [0044]     When the images are written on the photoconductors  29 BK,  29 C,  29 M, and  29 Y by the optical beams, the synchronous detection sensor  7  detects the optical beams and controls writing timings based on synchronous detection signals for respective color components so that image writing positions on the photoconductors  29 BK,  29 C,  29 M, and  29 Y may be constant for respective scanning lines.  
         [0045]     According to this exemplary embodiment, a single synchronous detection sensor  7  is disposed to detect all the optical beams and generate all the synchronous detection signals for all the colors. Thereby, the configuration of the color image forming apparatus may be simplified, and the cost thereof may be reduced.  
         [0046]     Referring to  FIG. 4 , the color image forming apparatus shown in  FIG. 1  of the exemplary embodiment of the present invention is provided with synchronous detection signals generated by the synchronous detection sensor  7  for respective color components. As stated above in  FIG. 1 , the optical beam scanning device  20  uses the opposite mirror faces of the polygon mirror  22  to scan and spread the deflected optical beams in opposite directions. Each mirror face deflects the optical beams of two different color components at the upper and lower portions thereof. This exemplary embodiment, for example, the optical beams of cyan and black are respectively entered into the upper and lower portions of the same mirror face while the optical beams of magenta and yellow are respectively entered into the upper and lower portions of another mirror face. The another mirror face may be directly opposite to the same mirror face.  
         [0047]     As shown in  FIG. 4 , the optical beams of cyan, black, magenta and yellow are respectively output from the laser diodes  10 C,  10 BK,  10 M, and  10 Y. The laser diode  10 C is disposed above the laser diode  10 BK while the laser diode  10 M is disposed above the laser diode  10 Y so that the optical beams are deflected by respective portions of the opposite mirror faces. The deflected optical beams pass the fθ lenses  23 BKC and  23  MY and are reflected off by the first mirrors  251 BK,  251 C,  251 M, and  251 Y towards respective photoconductors  29 BK,  29 C,  29 M and  29 Y. Here, a pair of deflected optical beams of cyan and black and another pair of deflected optical beams of magenta and yellow are spread symmetrically around the polygon mirror  22 , in opposite directions, so that the optical beams scan respective photoconductors  29 BK,  29 C,  29 M and  29 Y.  
         [0048]     The optical beams of the four color components to scan periodically are deflected by the polygon mirror  22 , and are directed towards the synchronous detection sensor  7  by the mirrors  71 BK,  71 C,  71 M, and  71 Y. As stated above, the mirrors  71  are disposed at locations before the optical beams are entered into the first mirrors  251 . In other words, the mirrors  71  are disposed outside the photoconductors  29  in the image writing start sides. The synchronous detection sensor  7  detects the optical beams of the four color components entering at the certain positions on the scanning lines, and generates the synchronous detection signals for respective color components.  
         [0049]     Since the synchronous detection sensor  7  generates the synchronous detection signals, the optical beams may need to be entered into the synchronous detection sensor  7  at different timings in such a manner that the optical beams of four color components are separated by the time base.  
         [0050]     Referring to  FIG. 5 , the synchronous detection signals generated by the synchronous detection sensor  7  from the respective optical beams are separated by the time base. The optical elements including mirrors to reflect the optical beams may be omitted as necessary in  FIG. 5 .  
         [0051]     The optical beams of yellow and black, for example, are reflected off the opposite mirror faces of the polygon mirror  22 , and incident beams to the synchronous detection sensor  7  are fluctuated as shown in situations  1  and  2  of  FIG. 5 .  
         [0052]     The situation  1  of  FIG. 5  shows the optical beams at a certain timing at which, for example, the polygon mirror  22  is in a position rotated by θ from a reference position. The optical beam emitted from the laser diode  10 Y is deflected by the polygon mirror  22  and is entered into the synchronous detection sensor  7  at the timing at which the polygon mirror  22  is in the position of the θ rotation. At the same time, the optical beam BK emitted by the laser diode  10 BK is not deflected towards the synchronous detection sensor  7 .  
         [0053]     The situation  2  of  FIG. 5  shows the optical beams at another timing at which the polygon mirror  22  is in a position rotated by θ+Δθ from the reference position. The optical beam emitted from the laser diode  11 BK is deflected by the polygon mirror  22  and is entered to the synchronous detection sensor  7  at this timing and the optical beam Y is not deflected towards the synchronous detection sensor  7 . The optical beams emitted from the laser diodes  10 Y and  10 BK are stated as examples. However, the optical beams of cyan and magenta are similar to those of black and yellow. For example, the optical beams emitted from the laser diodes  10 C and  10 M are entered to the synchronous detection sensor  7  at timings at which the polygon mirror  22  is in a position rotated by θ+2Δθ and θ+3Δθ respectively from the reference position.  
         [0054]     As the polygon mirror  22  is rotated in different positions by different angles, the optical beams of the four color components are detected at different timings. Thereby, the synchronous detection signals are separated one from another by the time base.  
         [0055]     The synchronous detection signals separated by the time base are used as reference signals for respective color components so as to instruct lighting timings of laser diodes  10 Y,  10 M,  10 C, and  10 BK and control the beginning of image writing at the certain positions. The lighting timings of laser diodes  10 Y,  10 M,  10 C, and  10 BK are based on the image data written in the effective image areas for respective main scanning lines.  
         [0056]     A related art image forming apparatus applies a synchronous detection signal of only a single color to commonly use for all four color components so that optical beams are spread and opposed. In other words, only the optical beams of the single color are deflected by one mirror face of a polygon mirror, and are detected by a detection sensor so as to be commonly used for all the four color components. However, the related art image forming apparatus has generated a jitter that disturbs the image.  
         [0057]     The image forming apparatus of this exemplary embodiment shown in  FIG. 1  though  FIG. 5 , on the other hand, employs the polygon mirror  22  that generates the synchronous detection signals of each of the four color components from the optical beams of respective color components. Therefore, an occurrence of the jitter may be reduced and a quality of the writing image may remain high.  
         [0058]     This exemplary embodiment of the present invention stated above uses the polygon mirror  22  to deflect the optical beams. The deflected optical beams are spread and (symmetrically) centered opposite to each other around the polygon mirror  22 . For example, when the images are written on the four photoconductors, the sensor outputs of the optical beams of the four colors are used as the synchronous detection signals of respective colors. As shown in  FIG. 1  and  FIG. 5 , the optical beams of the four color components are spread and opposed to scan, and each of the optical beams of two color components are entered into one of the mirror faces of the polygon mirror  22 . Each mirror face has upper and lower portions to deflect the optical beams of the two colors. When one mirror face deflects a plurality of optical beams in this exemplary embodiment shown  FIG. 1  through  FIG. 5 , a likelihood of an error occurrence between the plurality of optical beams may be significantly small. Therefore, the color image forming apparatus of this embodiment may be simplified so as to, for example, reduce costs thereof by another exemplary embodiment shown in  FIG. 6 .  
         [0059]     In the exemplary embodiment shown in  FIG. 6 , the synchronous detection sensor  7 , detecting the optical beam of one color component deflected by one mirror face of the polygon mirror  22 , generates the synchronous detection sensor that may be commonly used for the optical beam of another color component deflected by the same mirror face. Thereby, a number of the optical beams to be detected may be reduced, and the color image forming apparatus may be configured to be simplified, for example, without having the mirrors  71 C and  71 Y. In other words, only one control signal is generated for those optical beams reflected on faces provided on a same side of the polygon mirror  22 .  
         [0060]     Referring to  FIG. 6 , another exemplary embodiment of the present invention employing another method to generate the synchronous detection signal by the synchronous detection sensor  7  in the color image forming apparatus with the tandem system of  FIG. 1  is described. As the optical elements of  FIG. 6  are similar to those of  FIG. 4 , except for the mirrors  71 C and  71 Y, reference numbers used in  FIG. 4  and  FIG. 6  may be similar.  
         [0061]     Similar to  FIG. 4 , the optical beam scanning device  20  of this exemplary embodiment shown in  FIG. 6  uses the polygon mirror  22  having the mirror faces. Thereby, the optical beams of two color components are entered into one of the mirror faces while the optical beams of other two color components are entered into another mirror face so that the optical beams emitted from the laser diodes  10 BK,  10 C,  10 Y, and  10 M are spread into two, for example, black and cyan, and magenta and yellow. The optical beams of black and cyan, and magenta and yellow scan respective photoconductors in opposite directions. As these optical elements are similar to those of  FIG. 4 , a detailed description of each optical element may be omitted.  
         [0062]     As shown in  FIG. 6 , the synchronous detection sensor  7  detects the optical beams of black and magenta. The optical beams of four color components are deflected by the polygon mirror  22 . However, the optical beams of black and magenta out of the four colors are respectively reflected off the mirrors  71 BK and  71 M so as to be directed towards the synchronous detection sensor  7 . The mirrors  71 BK and  71 K are disposed at the locations before the optical beams are entered into the first mirrors  251 BK and  251 M. In other words, the mirrors  71 BK and  71 M are disposed outside the photoconductors  29  in the image writing start sides. The synchronous detection sensor  7  detects the optical beams of black and magenta entering at the certain positions on the scanning lines, and generates the synchronous detection signals for respective color components.  
         [0063]     Since one piece of the synchronous detection sensor  7  generates the synchronous detection signals of different color components, the optical beams may need to be entered into the synchronous detection sensor  7  at different timings in such a manner that the optical beams of different color components are separated by the time base. That is similar to the exemplary embodiment previously stated in  FIG. 5 .  
         [0064]     The synchronous detection signals of the optical beams of black and magenta detected by the synchronous detection sensor  7  are separated by the time base and are used as the reference signals for respective color components so as to instruct the lighting timings of the laser diodes  10 BK and  10 M and control the beginning of the image writing at the certain positions. The lighting timings of laser diodes  10 BK and  10 M are based on the image data written in the effective image areas for respective main scanning lines.  
         [0065]     The optical beams of other color components such as cyan and yellow are described as follows. The optical beam of cyan is deflected by the mirror face of the polygon mirror  22  by which the optical beam of black is deflected. The optical beams of cyan and black use the same mirror face so as to be deflected. Similarly, the optical beams of yellow and magenta are deflected by another mirror face of the polygon mirror  22 . The optical beams of cyan and yellow respectively use the synchronous detection signals of the optical beams of black and magenta.  
         [0066]     In other words, the synchronous detection signal of black is commonly used for the optical beams of cyan and black while the synchronous detection signal of magenta is commonly used for the optical beams of yellow and magenta. Thereby, the optical beams of cyan and yellow respectively use the synchronous detection signals of black and magenta to instruct the lighting timings of the laser diodes  10 C and  10 Y, and control the beginning of the image writing at the certain positions.  
         [0067]     According to this exemplary embodiment, the synchronous detection sensor  7  detecting the optical beam of one color component deflected by one mirror face of the polygon mirror  22  generates the synchronous detection signal that may be commonly used for the optical beam of another color component deflected by the same mirror face. Therefore, an occurrence of the jitter stated above in the related art image forming apparatus may be reduced, and the quality of the writing image may remain high.  
         [0068]     Still another exemplary embodiment of the present invention will be described in  FIG. 7 . This exemplary embodiment of  FIG. 7  includes an adjustment process to reduce an error occurrence when the synchronous detection sensor  7  shown in the exemplary embodiments of  FIG. 4  and  FIG. 6  is used to detect the synchronous detection signals of a plurality of the optical beams.  
         [0069]     These exemplary embodiments shown in  FIG. 4  and  FIG. 6  respectively detect the optical beams of four and two color components by the synchronous detection sensor  7 . Regardless of  FIG. 4  and  FIG. 6 , the optical beams entering into the synchronous detection sensor  7  are inclined against a detection face of the synchronous detection sensor  7  because of an arrangement of the optical elements.  
         [0070]     Referring to  FIG. 7 , the optical beams are entered into the synchronous detection sensor  7  in a state that the optical beams are inclined relative to a normal to the surface of the synchronous detection sensor  7 . When the inclined optical beams are entered into the synchronous detection sensor  7 , a detection light intensity detected by the synchronous detection sensor  7  may be reduced compared to a largest light intensity which may be detected in a case where the optical beams are vertically entered. Consequently, the synchronous detection signals from the synchronous detection sensor  7  may have an error. When the detection amount fluctuates with a variation in an incident angle that is an angle of the optical beam to enter into the synchronous detection sensor  7 , timings of the synchronous detection signals may be slightly fluctuated.  
         [0071]     When the inclined optical beams are entered into the synchronous detection sensor  7 , the light intensity of the inclined optical beams are adjusted in such a manner that a suitable light intensity is provided. For example, when the optical beams are spread and opposed to scan as shown in  FIG. 7 , one side of the optical beams is called an optical beam  1 , and another side of the optical beams is called an optical beam  2 . The optical beams  1  and  2  are assumed to have incident angles α and β degrees respectively. When the optical beam  1  is entered at the α degree of incident angle, the light intensity may be reduced by α% from a case where the optical beam is vertically entered. When the optical beam  2  is entered at the β degree, the light intensity may be reduced by β% from a case where optical beam is vertically entered. The reduction of the light intensity may be controlled by a lighting control unit  12  of the laser diode as shown in  FIG. 3 . Therefore, the lighting control unit  12  controlling the lighting of the laser diode  10  transmits an intensity adjustment signal to a driving unit  14  of the laser diode so as to adjust the light intensity of the laser diode and obtain the suitable light intensity. The intensity adjustment signals are transmitted to, for example, increase the optical beams  1  and  2  by α% and β% respectively in this exemplary embodiment. Therefore, an occurrence of shifting the writing timing between the optical beams  1  and  2  may be reduced.  
         [0072]     Each exemplary embodiment of the present invention above is illustrated by applying to the color image forming apparatus with the electrophotographic method that the images are written by the optical beams having the image data of the four color components. However, the stated disclosure and description of the exemplary embodiments are illustrative only and are not to be considered limiting. The present invention may be applied to an area employing an optical writing method by using a plurality of optical beams, for example, an area in which data is written and/or recorded to an optical recording medium utilizing a photo-magnetic effect.  
         [0073]     Numerous additional modifications and variation are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the disclosure of this patent specification may be practiced otherwise than as specifically described herein.