Patent Publication Number: US-9835971-B2

Title: Image forming apparatus to emit a plurality of light beams for an exposure of a photosensitive member

Description:
TECHNICAL FIELD 
     The present invention relates to an electrophotographic image forming apparatus including a light source configured to emit a plurality of light beams for an exposure of a photosensitive member. 
     BACKGROUND ART 
     Conventionally, there have been known image forming apparatuses configured to deflect a light beam emitted from a light source by a rotating polygonal mirror, and scan a photosensitive member by the light beam deflected by the rotating polygonal mirror, thereby forming an electrostatic latent image on the photosensitive member. Such image forming apparatuses include an optical sensor configured to detect the light beam deflected by the rotating polygonal mirror. The image forming apparatuses control the light source to emit the light beam therefrom based on a synchronization signal generated by the optical sensor, and match write start positions of electrostatic latent images (images) with each other in a direction in which the light beam scans the photosensitive member (a main scanning direction). 
     There are also known image forming apparatuses including a light source in which a plurality of light emitting elements configured to emit light beams is arranged as illustrated in  FIG. 7A  to increase an image forming speed and a resolution of an image. In  FIG. 7A , an X-axis direction corresponds to the main scanning direction, and a Y-axis direction corresponds to a direction in which the photosensitive member rotates (a sub-scanning direction). In such image forming apparatuses, during an assembling process at a factory, the light source is rotated in a direction indicated by an arrow in  FIG. 7A  to adjust a distance between the light emitting elements in the Y-axis direction. By rotating the light source in this manner, a distance between exposure positions of the light beams emitted from the respective light emitting elements in the sub-scanning direction on the photosensitive member is adjusted to a distance corresponding to a resolution of the image forming apparatus. 
     The rotation of the light source in the direction indicated by the arrow illustrated in  FIG. 7A  changes both the distance between the light emitting elements in the Y-axis direction and the distance between the light emitting elements in the X-axis direction. Therefore, conventional image forming apparatuses cause each of the light emitting elements to emit a light beam at a timing determined for each of the light emitting elements based on the synchronization signal generated by the optical sensor to match the write start positions of electrostatic latent images with each other in the main scanning direction. 
     During the above-described assembling process, an angle by which the light source is rotated (an adjustment amount) varies for each image forming apparatus depending on how the light source is installed at the image forming apparatus and optical characteristics of optical members such as a lens and a mirror. Therefore, the distance between the light emitting elements in the X-axis direction after the rotational adjustment of the light source may not be the same among a plurality of image forming apparatuses. If a same timing is set for all image forming apparatuses as the light beam emission timing set for each light emitting element based on the synchronization signal generated by the optical sensor, this may result in generation of an image forming apparatus in which the write start positions of electrostatic latent images in the main scanning direction are out of alignment in the main scanning direction. 
     To prevent such misalignment among the write start positions of electrostatic latent images in the main scanning direction, which would be caused by rotating the light source during the assembling process, Japanese Patent Application Laid-Open No. 2008-89695 discusses an image forming apparatus that generates a plurality of horizontal synchronization signals by light beams respectively emitted from a first light emitting element and a second light emitting element, and sets a timing at which the second light emitting element emits a light beam relative to a timing at which the first light emitting element emits a light beam based on a difference between timings at which the plurality of horizontal synchronization signals is generated. 
     However, the image forming apparatus discussed in Japanese Patent Application Laid-Open No. 2008-89695 has the following issue. During image formation, heat is generated at a motor that drives a rotating polygonal mirror, and the temperature of a lens disposed near the rotating polygonal mirror increases due to the influence of the heat. The increase in the temperature of the lens causes a change in the optical characteristics of the lens such as a refractive index of a light beam in the main scanning direction. The change in the optical characteristics of the lens causes a change in a relative positional relationship among image forming positions of a plurality of light beams on a photosensitive member, like a change from a state illustrated in  FIG. 7B  to a state illustrated in  FIG. 7C  (or from  FIG. 7C  to  FIG. 7B ). With the change in the optical characteristics and the change in the relative positional relationship among the image forming positions of the plurality of light beams on the photosensitive member during image formation in this manner, a misalignment occurs among write start positions of electrostatic latent images formed by the light beams emitted from the respective light emitting elements. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Patent Application Laid-Open No. 2008-89695 
     SUMMARY OF INVENTION 
     According to an aspect of the present invention, an image forming apparatus includes a photosensitive member configured to rotate, an optical scanning device including a light source including a plurality of light emitting elements including a first light emitting element configured to emit a first light beam and a second light emitting element configured to emit a second light beam for exposing the photosensitive member, a deflection unit configured to deflect a plurality of light beams emitted from the light source to cause the plurality of light beams to scan the photosensitive member, and a lens configured to guide the plurality of light beams deflected by the deflection unit to the photosensitive member, wherein the first light emitting element and the second light emitting element are disposed at the light source in such a manner that the first light beam and the second light beam expose different positions in a scanning direction in which the first light beam and the second light beam deflected by the deflection unit scan the photosensitive member, a detection unit configured to detect the first light beam and the second light beam deflected by the deflection unit, a storage unit configured to store predetermined data, wherein the predetermined data relates to a detection timing difference between the first light beam and the second light beam detected by the detection unit, and a control unit configured to control a timing at which the second light emitting element emits the second light beam relative to a timing at which the first light emitting element emits the first light beam for forming an electrostatic latent image on the photosensitive member based on a comparison result of a comparison between the detection timing difference between the first light beam and the second light beam detected by the detection unit and the predetermined data. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate exemplary embodiments, features, and aspects of the invention and, together with the description, serve to explain the principles of the invention. 
         FIG. 1  is a schematic cross-sectional view of a color image forming apparatus. 
         FIG. 2A  schematically illustrates an internal configuration of an optical scanning device and a photosensitive drum. 
         FIG. 2B  schematically illustrates an internal configuration of an optical scanning device and a photosensitive drum. 
         FIG. 3A  schematically illustrates a light source. 
         FIG. 3B  illustrates a relative positional relationship among exposure positions of laser light beams on a photosensitive drum. 
         FIG. 3C  schematically illustrates a beam detector (BD). 
         FIG. 4  is a control block diagram of the image forming apparatus according to an exemplary embodiment of the present invention. 
         FIG. 5  is a timing chart indicating timings during one scanning cycle according to the exemplary embodiment of the present invention. 
         FIG. 6  is a flowchart illustrating a control flow executed by a central processing unit (CPU) provided to the image forming apparatus according to the exemplary embodiment of the present invention. 
         FIG. 7A  illustrates an issue with a conventional image forming apparatus. 
         FIG. 7B  illustrates an issue with a conventional image forming apparatus. 
         FIG. 7C  illustrates an issue with a conventional image forming apparatus. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Various exemplary embodiments, features, and aspects of the invention will be described in detail below with reference to the drawings. 
       FIG. 1  is a schematic cross-sectional view illustrating a digital full color printer (a color image forming apparatus) capable of forming an image using toners of a plurality of colors according to a first exemplary embodiment. The present exemplary embodiment will be described based on an example of a color image forming apparatus. However, the present invention does not necessarily have to be embodied by a color image forming apparatus, and may be embodied by an image forming apparatus capable of forming an image using a toner of a single color (for example, black). 
     First, an image forming apparatus  100  according to the present exemplary embodiment will be described with reference to  FIG. 1 . The image forming apparatus  100  includes four image forming units  101 Y,  101 M,  101  C, and  101 Bk, each of which forms an image for each color. The indices Y, M, C, and Bk used herein indicate yellow, magenta, cyan, and black, respectively. That is, the image forming units  101 Y,  101 M,  101 C, and  101 Bk respectively form images using a yellow toner, a magenta toner, a cyan toner, and a black toner. 
     The image forming units  101 Y,  101 M,  101 C, and  101 Bk respectively include photosensitive drums  102 Y,  102 M,  102 C, and  102 Bk which are photosensitive members. Charging devices  103 Y,  103 M,  103 C, and  103 Bk, optical scanning devices  104 Y,  104 M,  104 C, and  104 Bk, and developing devices  105 Y,  105 M,  105 C, and  105 Bk are disposed around the photosensitive drums  102 Y,  102 M,  102 C, and  102 Bk, respectively. Further, drum cleaning devices  106 Y,  106 M,  106 C, and  106 Bk are disposed around the photosensitive drums  102 Y,  102 M,  102 C, and  102 Bk, respectively. 
     An intermediate transfer belt  107  which is an endless belt is disposed below the photosensitive drums  102 Y,  102 M,  102 C, and  102 Bk. The intermediate transfer belt  107  is stretched around a driving roller  108  and driven rollers  109  and  110 , and rotates in a direction indicated by an arrow B illustrated in  FIG. 1  during image formation. Further, primary transfer devices  111 Y,  111 M,  111 C, and  111 Bk are disposed at positions respectively facing to the photosensitive drums  102 Y,  102 M,  102 C, and  102 Bk via the intermediate transfer belt  107  (an intermediate transfer member). 
     The image forming apparatus  100  according to the present exemplary embodiment further includes a secondary transfer device  112  for transferring a toner image on the intermediate transfer belt  107  to a recording medium S, and a fixing device  113  for fixing the toner image on the recording medium S. 
     An image forming process from a charging process to a developing process at the thus-configured image forming apparatus  100  will be described. The respective image forming units  101 Y,  101 M,  101 C, and  101 Bk perform the image forming process in similar manners. Therefore, the image forming process will be described focusing on the image forming unit  101 Y as an example, and the descriptions of the image forming processes at the image forming units  101 M,  101 C, and  101 Bk are omitted herein. 
     First, the rotatably driven photosensitive drum  102 Y is charged by the charging device  103 Y of the image forming unit  101 Y. The charged photosensitive drum  102 Y (a surface of an image bearing member) is exposed by laser light beams emitted from the optical scanning device  104 Y. Accordingly, an electrostatic latent image is formed on the rotating photosensitive drum  102 Y. Then, the electrostatic latent image is developed as a yellow toner image by the developing device  105 Y. 
     Hereinbelow, the image forming process from a transfer process and subsequent processes will be described based on an example of the image forming units  101 Y,  101 M,  101 C, and  101 Bk. The primary transfer devices  111 Y,  111 M,  111 C, and  111 Bk apply transfer biases to the intermediate transfer belt  107  to transfer yellow, magenta, cyan, and black toner images formed on the photosensitive drums  102 Y,  102 M,  102 C, and  102 Bk of the respective image forming units  101 Y,  101 M,  101 C, and  101 Bk onto the intermediate transfer belt  107 . Accordingly, the toner images of the respective colors are superimposed on one another on the intermediate transfer belt  107 . 
     After the four-color toner image is transferred onto the intermediate transfer belt  107 , the four-color toner image transferred onto the intermediate transfer belt  107  is transferred again (secondary transfer) by the secondary transfer device  112  onto the recording medium S which is conveyed from a manual sheet feeding cassette  114  or a sheet feeding cassette  115  to a secondary transfer portion T2. Then, the toner image on the recording medium S is heated and fixed at the fixing device  113 . The recording medium S is discharged to a sheet discharge portion  116 , and thus, a full color image can be provided on the recording medium S. 
     After the transfer, residual toner is removed from the respective photosensitive drums  102 Y,  102 M,  102 C, and  102 Bk by the drum cleaning devices  106 Y,  106 M,  106 C, and  106 Bk. Then, the above-described image forming process is continuously repeated. 
     Next, the configurations of the optical scanning devices  104 Y,  104 M,  104 C, and  104 Bk which are exposure units will be described with reference to  FIGS. 2A, 2B, 3A, 3B, and 3C . Since the respective optical scanning devices  104 Y,  104 M,  104 C, and  104 Bk are identically configured, the indices Y, M, C, and Bk, which indicate the colors, are omitted in the following descriptions. 
       FIG. 2A  illustrates an exemplary embodiment of the optical scanning device  104 . The optical scanning device  104  includes a light source  201  for emitting a laser light beam (a light beam), a collimator lens  202  for collimating the laser light beam into parallel light, a cylindrical lens  203  for collecting the laser light beam passing through the collimator lens  202  in the sub-scanning direction (a direction corresponding to a rotation direction of the photosensitive drum  102 ), and a polygonal mirror  204  (a rotating polygonal mirror). Further, the optical scanning device  104  includes an f-theta lens A 205  (a scanning lens A, a first lens) and an f-theta lens B 206  (a scanning lens B, a second lens) as a plurality of scanning lenses on which the laser light beam (scanning light) deflected by the polygonal mirror  204  is incident. Furthermore, the optical scanning device  104  includes a beam detector  207  (hereinbelow, referred to as the BD  207 ) which is a signal generation unit configured to detect the laser light beam deflected by the polygonal mirror  204  and output a horizontal synchronization signal according to the detection of the laser light beam. The laser light beam passing through the f-theta lens A 205  and the f-theta lens B 206  is incident on the BD  207 . In a case where the optical performance is satisfied by a single scanning lens, the single scanning lens is provided to the optical scanning device  104 . 
       FIG. 2B  illustrates another exemplary embodiment of the optical scanning device  104 . A difference between the optical scanning device  104  illustrated in  FIG. 2A  and the optical scanning device  104  illustrated in  FIG. 2B  is that, in the optical scanning device  104  illustrated in  FIG. 2B , the laser light beam deflected by the polygonal mirror  204  passes through the f-theta lens A 205 , and the laser light beam reflected by a BD mirror  208  serving as a reflection mirror passes through a BD lens  209  and is incident on the BD  207 . In other words, the laser light beam incident on the BD  207  does not pass through the f-theta lens B 206 . The BD lens  209  has an optical characteristic of collecting laser light beams to the BD  207 , and the optical characteristic of the BD lens  209  is different from that of the f-theta lens B 206 . 
     The light source  201  and the BD  207  will be described with reference to  FIGS. 3A, 3B, and 3C .  FIG. 3A  is an enlarged view of the light source  201 . The light source  201  includes N pieces of light emitting elements (a light emitting element 1 to a light emitting element N) that emit laser light beams. Laser light L1 (a first light beam) is emitted from the light emitting element 1 (a first light emitting element). Laser light L2 is emitted from the light emitting element 2. Laser light Ln is (a second light beam) is emitted from the light emitting element N (a second light emitting element). An X-axis direction illustrated in  FIG. 3A  corresponds to a direction in which a laser light beam deflected by the polygonal mirror  204  scans the surface of the photosensitive drum  102  (the main scanning direction). In addition, a Y-axis direction corresponds to the direction in which the photosensitive drum  102  rotates (the sub-scanning direction). 
     The plurality of light emitting elements 1 to N is arranged so as to form an array as illustrated in  FIG. 3A . Since the light emitting elements 1 to N are arranged as illustrated in  FIG. 3A , the laser light beam L1 to the laser light beam Ln emitted from the respective light emitting elements 1 to N form images at different positions on the photosensitive drum  102  in the main scanning direction. Further, the laser light beam L1 to the laser light beam Ln emitted from the respective light emitting elements 1 to N form images at different positions in the sub-scanning direction. The laser light beam L1 and the laser light beam Ln are laser light beams that expose the positions furthest away from each other in the main scanning direction and the sub-scanning direction. The arrangement of the plurality of light emitting elements 1 to N may be a two-dimensional arrangement. 
     A distance D1 illustrated in  FIG. 3A  is an interval (a distance) between the light emitting element 1 and the light emitting element N which are furthest away from each other in the X-axis direction. Since the light emitting element N among the plurality of light emitting elements is located furthest away from the light emitting element 1 in the X-axis direction, an image forming position Sn of the laser light beam Ln among the plurality of laser light beams is located furthest away from an image forming position S1 of the laser light beam L1 in the main scanning direction on the photosensitive drum  102  as illustrated in  FIG. 3B . According to the present exemplary embodiment, the light emitting element 1 and the light emitting element N are disposed at the light source  201  in such a manner that the laser light beam L1 scans the photosensitive drum  102  before the laser light beam Ln scans the photosensitive drum  102 . Due to this arrangement of the light emitting element 1 and the light emitting element N, the laser light beam L1 is incident on the BD  207 , which will be described below, before the laser light beam Ln is incident on the BD  207 . 
     A distance D2 illustrated in  FIG. 3A  is an interval (a distance) between the light emitting element 1 and the light emitting element N which are furthest away from each other in the Y-axis direction. Since the light emitting element 1 and the light emitting element N are furthest away from each other in the Y-axis direction, the image forming position Sn of the laser light beam Ln among the plurality of laser light beams is located furthest away from the image forming position S1 of the laser light beam L1 in the sub-scanning direction on the photosensitive drum  102  as illustrated in  FIG. 3B . 
     A distance Py=D2/N−1 between the light emitting elements in the Y-axis direction is a distance corresponding to a resolution of the image forming apparatus (for example, the distance would be approximately 21 micrometers if the resolution is 1200 dpi). The distance Py is a value set by rotating and adjusting the light source  201  during an assembling process in such a manner that a distance between image forming positions of laser light beams adjacent to each other in the sub-scanning direction on the photosensitive drum  102  matches a distance corresponding to a predetermined resolution. A distance Px=D1/N−1 between the light emitting elements in the X-axis direction is a value unambiguously determined by adjusting the distance between the light emitting elements in the Y-axis direction to the distance Py. The timing at which a laser light beam is emitted from each light emitting element after a synchronization signal is generated by the BD  207  is set for each light emitting element during the assembling process with use of a predetermined tool, and is stored as an initial value in a memory, which will be described below. The initial value is a value corresponding to the distance Px. 
       FIG. 3C  schematically illustrates the BD  207 . The BD  207  includes a light receiving surface  207   a  on which photoelectric conversion elements are arranged. Laser light is incident on the light receiving surface  207   a , by which a synchronization signal is generated. The BD  207  according to the present exemplary embodiment generates a plurality of BD signals corresponding to the respective laser light beams L1 to Ln according to entries of the laser light beam L1 and the laser light beam Ln into the BD  207 . 
     The width of the light receiving surface  207   a  in the main scanning direction is set to a width D3, and the width of the light receiving surface  207   a  in a direction corresponding to the sub-scanning direction is set to a width D4. As illustrated in  FIG. 3C , the laser light beam L1 emitted from the light emitting element 1 and the laser light beam Ln emitted from the light emitting element N scan the light receiving surface  207   a  of the BD  207 . The width D4 of the light receiving surface  207   a  in the direction corresponding to the sub-scanning direction is set so as to satisfy D4&gt;D2*alpha (alpha: a rate of variation in the distance between the laser light beam L1 and the laser light beam Ln passing through the lenses in the sub-scanning direction). The width D3 of the light receiving surface  207   a  in the main scanning direction is set so as to satisfy D3&lt;D1*beta (beta: a rate of variation in the distance between the laser light beam L1 and the laser light beam Ln passing through the lenses in the main scanning direction), to prevent the laser light beam L1 and the laser light beam Ln from being incident on the light receiving surface  207   a  at the same time even when the light emitting element 1 and the light emitting element N are turned on at the same time. 
       FIG. 4  is a control block diagram of the image forming apparatus  100  according to the present exemplary embodiment. The image forming apparatus  100  according to the present exemplary embodiment includes a CPU  401 , a counter  402 , and a laser driver  403 . The image forming apparatus  100  according to the present exemplary embodiment further includes a clock signal generation unit (a CLK signal generation unit)  404 , an image processing unit  405 , a memory  406 , and a motor  407  for rotationally driving the polygonal mirror  204 . The CPU  401  controls the image forming apparatus  100  according to a control program stored in the memory  406 . The CLK signal generation unit  404  generates a clock signal (a CLK signal) of a predetermined frequency which is higher frequency than an output from the BD  207 , and outputs the clock signal to the CPU  401  and the laser driver  403 . The CPU  401  transmits a control signal to each of the laser driver  403  and the motor  407  in synchronization with the clock signal. 
     The motor  407  includes a speed sensor (not illustrated). The speed sensor employs a frequency generator (FG) method, according to which, the speed sensor generates a frequency signal proportional to a rotational speed. An FG signal of a frequency corresponding to the rotational speed of the polygonal mirror  204  is output from the motor  407  to the CPU  401 . The counter  402  serving as a counting unit is disposed within the CPU  401 . The counter  402  counts clock signals input to the CPU  401 . The CPU  401  measures a cycle of generation of the FG signal based on the count value of the counter  402 , and determines that the rotational speed of the polygonal mirror  204  reaches a predetermined speed if the cycle of generation of the FG signal is a predetermined cycle. 
     A BD signal output from the BD  207  is input to the CPU  401 . The CPU  401  transmits a control signal for controlling the timing at which the laser light beam is emitted from each of the light emitting elements 1 to N to the laser driver  403  based on the input BD signal. Image data output from the image processing unit  405  is input to the laser driver  403 . The laser driver  403  supplies a driving current based on the image data to each of the light emitting elements 1 to N at the timing based on the control signal transmitted from the CPU  401 . 
     As illustrated in  FIG. 7B , the image forming positions S1 to Sn of the respective laser light beams L1 to Ln are different in the main scanning direction. In the case of conventional image forming apparatuses, a laser light beam is emitted from a certain single light emitting element to generate a single BD signal. Then, a laser light beam is emitted from each of the light emitting elements based on a light beam emission timing (a fixed set value) set for each of the plurality of light emitting elements based on the generated BD signal, so that write start positions of electrostatic latent images (images) are matched in the main scanning direction. 
     If the relative positional relationship among the image forming positions S1 to Sn is constant at all times during image formation, it is possible to match the image write start positions with each other even if the timing at which each of the light emitting elements 1 to N emits a laser light beam is controlled based on the fixed set value set for each of the light emitting elements 1 to N. However, emission of laser light beam causes an increase in the temperature of the light source, and the increase in the temperature of the light source  201  causes a change in the wavelength of the laser light beam emitted from each of the light emitting elements. Further, a rotation of the polygonal mirror  204  causes an increase in the temperature of the motor  407 , and the optical characteristics of the scanning lenses change due to the influence of the heat. As illustrated in  FIGS. 7B and 7C , these changes in the wavelength of the laser light beam and the optical characteristics of the scanning lenses lead to a change in the optical path of the laser light beam emitted from each of the light emitting elements, and therefore a change in the relative positional relationship among the image forming positions S1 to Sn. In other words, a change occurs in the layout of the exposure positions on the photosensitive drum  102 . This results in occurrence of an issue of misalignment among write start positions of electrostatic latent images formed by the respective laser light beams in the main scanning direction. 
     Therefore, the image forming apparatus  100  according to the present exemplary embodiment generates two BD signals by the laser light beam L1 emitted from the light emitting element 1 and the laser light beam Ln emitted from the light emitting element N. The CPU  401  controls relative timings at which the plurality of light emitting elements emit the laser light beams based on a difference between timings at which the two BD signals are generated (a detection timing difference). This control will be described in detail below. The image forming apparatus  100  according to the present exemplary embodiment will be described based on an example that generates the BD signals by the laser light beam L1 and the laser light beam Ln which expose the positions furthest away from each other on the photosensitive drum  102  in the main scanning direction and the sub-scanning direction. However, the present exemplary embodiment is not limited thereto. The BD signals may be generated by a combination of the laser light beam L1 and the laser light beam Ln−1, a combination of the laser light beam L2 and the laser light beam Ln, or a combination of the laser light beam L2 and the laser light beam Ln−1. However, in order to detect a change in the characteristics of the lenses, it is desirable to generate a plurality of BD signals by each of a plurality of laser light beams away from an optical axis of the lenses at opposite sides from each other in the sub-scanning direction. 
       FIG. 5  is a timing chart illustrating timings at which the light emitting elements 1 to N emit the laser light beams L1 to Ln, and timings at which the BD  207  outputs BD signals. The first row indicates CLK signals. The second row indicates timings at which the BD  207  outputs BD signals. The third to sixth rows indicate timings at which the light emitting elements 1, 2, 3, and N output the laser light beams L1, L2, L3, and Ln. 
     During one scanning cycle of the laser light beam, first, the CPU  401  controls the laser driver  403  in such a manner that the light emitting element 1 and the light emitting element N emit the laser light beams L1 and Ln. Accordingly, as illustrated in  FIG. 5 , the BD  207  outputs a BD signal  501  according to detection of the laser light beam L1, and outputs a BD signal  502  according to detection of the laser light beam Ln. The CPU  401  starts counting CLK signals according to an input of the BD signal  501 , and obtains a count value Ca according to an input of the BD signal  502 . The count value Ca is detection data that indicates a difference DT1 between timings at which the BD signal  501  and the BD signal  502  are generated illustrated in  FIG. 5 . 
     Reference count value data Cref and count values C1 to Cn corresponding to the data Cref are stored in the memory  406 . The reference count value data Cref is reference data (predetermined data) corresponding to a difference Tref between generation timings at which a plurality of BD signals is generated in a certain arbitrary condition. In the present example, the reference count value data Cref is defined to correspond to a difference between generation timings at which a plurality of BD signals is generated in the above-described initial condition. Each of the count values C1 to Cn is a count value (write start timing data) for matching the write start positions of the respective light emitting elements 1 to N in the main scanning direction, in a case where a difference between generation timings at which the plurality of BD signals is generated is the difference Tref. The count values C1 to Cn correspond to times T1 to Tn illustrated in  FIG. 5 , respectively. 
     The CPU  401  compares the count value Ca corresponding to the difference DT1 between the timings at which the BD signals  501  and  502  are generated, with the reference count value data Cref. If the comparison result is Ca=Cref, the CPU  401  turns on the light emitting element 1 in response to that the count value of the CLK signals from the generation of the BD signal  501  reaches the count value C1 (the time T1 has elapsed). In other words, as illustrated in  FIG. 5 , a period during which the light emitting element 1 forms an electrostatic latent image starts in response to that the count value of the CLK signals from the generation of the BD signal  501  reaches the count value C1 (the time T1 has elapsed). Further, the CPU  401  turns on the light emitting element N in response to that the count value of the CLK signals from the generation of the BD signal  501  reaches the count value Cn (the time Tn has elapsed). In other words, as illustrated in  FIG. 5 , a period during which the light emitting element N forms an electrostatic latent image starts in response to that the count value of the CLK signals from the generation of the BD signal  501  reaches the count value Cn (the time Tn has elapsed). Accordingly, the write start position of the electrostatic latent image (the image) formed by the light emitting element 1 can be matched with the write start position of the electrostatic latent image (the image) formed by the light emitting element N in the main scanning direction. 
     According to the present exemplary embodiment, the laser light emission timing of each of the light emitting elements 1 to N is controlled based on the BD signal generated by the laser light beam L1. However, the laser light emission timing of each of the light emitting elements 1 to N may be controlled based on the BD signal generated by the laser light beam Ln. Further, the laser light emission timing of each of the light emitting elements 1 to N may be controlled based on an arbitrary timing determined based on a plurality of BD signals generated by the laser light beam L1 and the laser light beam Ln. 
     Next, a method for determining the reference count value data Cref will be described. First, at the time of adjustment at a factory, the laser light beam L1 and the laser light beam Ln deflected by the rotating polygonal mirror  204  are incident on the BD  207  at the respective timings when the polygonal mirror  204  continues rotating in such a state that the temperature of the light source  201  is a reference temperature (for example, 25 degrees Celsius). Then, a difference DTref between timings at which a BD signal generated by the laser light beam L1 and a BD signal generated by the laser light beam Ln are detected is input into a measurement device. CLK signals are input from the CLK signal generation unit  404  to the measurement device, and the measurement device converts the detection timing difference DTref to a count value. The measurement device determines this count value as the reference count value data Cref, and stores it into the memory  406 . 
     Further, at the time of the adjustment, a light receiving device is disposed at a position corresponding to a write start position of an electrostatic latent image on the surface of the photosensitive drum  102 . The light receiving device receives the laser light beam L1 and the laser light beam Ln deflected by the polygonal mirror  204 . The light receiving device transmits light reception signals indicating a timing at which the laser light beam L1 is received and a timing at which the laser light beam Ln is received, to the measurement device. 
     The measurement device converts a difference between the timing at which the BD signal is generated by the laser light beam L1 and the timing at which the light reception signal is generated in response to that the light receiving device receives the laser light beam L1, into a count value. This count value is set as the count value C1, and the measurement device stores the count value C1 into the memory  406  by associating with the reference count value data Cref. On the other hand, the measurement device converts a difference between the timing at which the BD signal is generated by the laser light beam L1 and the timing at which the light reception signal is generated in response to that the light receiving device receives the laser light beam Ln, into a count value. This count value is set as the count value Cn, and the measurement device stores the count value Cn into the memory  406  by associating with the reference count value data Cref. The measurement device stores the count values C1 to Cn into the memory  406  by performing the above-described processing on the respective light emitting elements 1 to N at the time of the adjustment. 
     The present exemplary embodiment may be configured in such a manner that the count values C1 and Cn are stored in the memory  406 , but the write start timing data for a light emitting element M (the light emitting element 2 to the light emitting element N−1) which is located between the light emitting element 1 and the light emitting element N in the X-axis direction in  FIG. 3  is not stored in the memory  406 . In this case, the CPU  401  calculates the write start timing data for the light emitting element M based on the count values C1 and Cn, and an arranged position of the light emitting element M relative to the light emitting elements 1 and N in the X-axis direction. In other words, the CPU  401  calculates the write start timing data Cm (a count value) for the light emitting element M located between the light emitting element 1 and the light emitting element N based on the following equation 1. 
     
       
         
           
             
               
                 
                   Cm 
                   = 
                   
                     
                       
                         
                           ( 
                           
                             Cn 
                             - 
                             
                               C 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               1 
                             
                           
                           ) 
                         
                         * 
                         
                           
                             ( 
                             
                               m 
                               - 
                               1 
                             
                             ) 
                           
                           / 
                           
                             ( 
                             
                               n 
                               - 
                               1 
                             
                             ) 
                           
                         
                       
                       + 
                       
                         C 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         1 
                       
                     
                     = 
                     
                       
                         C 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         1 
                         * 
                         
                           
                             ( 
                             
                               n 
                               - 
                               m 
                             
                             ) 
                           
                           / 
                           
                             ( 
                             
                               n 
                               - 
                               1 
                             
                             ) 
                           
                         
                       
                       + 
                       
                         Cn 
                         * 
                         
                           
                             ( 
                             
                               m 
                               - 
                               1 
                             
                             ) 
                           
                           / 
                           
                             ( 
                             
                               n 
                               - 
                               1 
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     EQUATION 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ) 
                 
               
             
           
         
       
     
     For example, in a case where the light source  201  includes four light emitting elements 1 to 4, the CPU  401  calculates write start timing data C2 for the light emitting element 2 and write start timing data C3 for the light emitting element 3 based on the following equation. 
     
       
         
           
             
               
                 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                   
                   = 
                   
                     
                       
                         C 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         1 
                       
                       + 
                       
                         
                           ( 
                           
                             
                               C 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               4 
                             
                             - 
                             
                               C 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               1 
                             
                           
                           ) 
                         
                         * 
                         
                           1 
                           / 
                           3 
                         
                       
                     
                     = 
                     
                       
                         C 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         1 
                         * 
                         
                           2 
                           / 
                           3 
                         
                       
                       + 
                       
                         C 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         4 
                         * 
                         
                           1 
                           / 
                           3 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     EQUATION 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                   
                   ) 
                 
               
             
             
               
                 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     3 
                   
                   = 
                   
                     
                       
                         C 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         1 
                       
                       + 
                       
                         
                           ( 
                           
                             
                               C 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               4 
                             
                             - 
                             
                               C 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               1 
                             
                           
                           ) 
                         
                         * 
                         
                           2 
                           / 
                           3 
                         
                       
                     
                     = 
                     
                       
                         C 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         1 
                         * 
                         
                           1 
                           / 
                           3 
                         
                       
                       + 
                       
                         C 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         4 
                         * 
                         
                           2 
                           / 
                           3 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     EQUATION 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     3 
                   
                   ) 
                 
               
             
           
         
       
     
     Next, when a difference between timings at which a BD signal  503  and a BD signal  504  are generated is a difference DT2, how the CPU  401  performs control will be described. As illustrated in  FIG. 5 , the BD  207  outputs the BD signal  503  according to detection of the laser light beam L1, and outputs the BD signal  504  according to detection of the laser light beam Ln. The CPU  401  detects a difference DT′1 between the timings at which the BD signal  503  and the BD signal  504  are generated as illustrated in  FIG. 5 , as a count value C′a. The CPU  401  compares the count value C′a and the reference count value data Cref. At this time, an example in which the count value C′a is equal to the reference count value data Cref (C′a=Cref) will be described. The CPU  401  corrects the write start timing data Cn based on a difference between the count value C′a and the reference count value data Cref to calculate C′n.
 
 C′n=Cn*K ( C ref− C′a )  (EQUATION 4)
 
(K is an arbitrary coefficient including 1)
 
     The CPU  401  turns on the light emitting element N in response to that the count value of the counter  402  from the generation of the BD signal  503  reaches the corrected write start timing data C′n. Even if a change occurs in the difference between the timings at which the BD signals are generated, it is possible to match the write start position of an image formed by the light emitting element 1 and the write start position of an image formed by the light emitting element N in the main scanning direction. 
     The coefficient K is a coefficient multiplied to a change amount (Cref−C′a) in the time interval on the BD  207  (on the light receiving surface  207   a ), and is determined by measuring the optical characteristics of the lenses provided to the optical scanning device  104  at the time of the above-described adjustment at the factory. In the optical scanning device  104  illustrated in  FIG. 2A , the laser light beam L1 and the laser light beam Ln incident on the BD  207 , and the laser light beams L1 to Ln reaching the photosensitive drum  102  pass through the same lenses. Therefore, the detection timing difference DTref measured by the measurement device at the time of the adjustment, and the light reception timing difference between the laser light beam L1 and the laser light beam Ln received by the light receiving device are substantially the same. Therefore, the coefficient K is set to one (K=1) for the optical scanning device  104  illustrated in  FIG. 2A . 
     On the other hand, in the optical scanning device  104  illustrated in  FIG. 2B , while the laser light beam L1 and the laser light beam Ln incident on the BD  207  pass through the scanning lens A 205  and the BD lens  209 , the laser light beams L1 to Ln reaching the photosensitive drum  102  pass through the scanning lens A 205  and the scanning lens B 206 . In other words, the laser light beam L1 and the laser light beam Ln incident on the BD  207 , and the laser light beams L1 to Ln reaching the photosensitive drum  102  pass through different lenses. Therefore, the speed at which the laser light beam L1 and the laser light beam Ln scan the BD  207  is different from the speed at which the laser light beams L1 to Ln scan the photosensitive drum  102 . In such an optical scanning device, the coefficient K is set to a positive value other than 1, based on the detection timing difference DTref measured by the measurement device at the time of the adjustment, and the light reception timing difference between the laser light beam L1 and the laser light beam Ln received by the light receiving device. In a case where the optical scanning device  104  includes a single scanning lens, the BD  207  may be configured so as to receive laser light passing through the single scanning lens, or may be configured so as to receive laser light that does not pass through the scanning lens. 
     Next, a flow of control executed by the CPU  401  will be described with reference to  FIG. 6 . This control starts according to an input of image data into the image forming apparatus  100 . First, in step S 601 , the CPU  401  causes the polygonal mirror  204  to rotate by driving the motor  407  according to the input of the image data. Subsequently, in step S 602 , the CPU  401  determines whether the rotational speed of the polygonal mirror  204  reaches a predetermined rotational speed. If the CPU  401  determines in step S 602  that the rotational speed of the polygonal mirror  204  does not reach the predetermined rotational speed (NO in step S 602 ), in step S 603 , the CPU  401  increases the rotational speed of the polygonal mirror  204 , and returns the control to step S 602 . 
     If the CPU  401  determines in step S 602  that the rotational speed of the polygonal mirror  204  reaches the predetermined rotational speed (YES in step S 602 ), in step S 604 , the CPU  401  turns on the light emitting element 1. Subsequently, in step S 605 , the CPU  401  determines whether a BD signal is generated by the laser light beam L1 emitted from the light emitting element 1. If the CPU  401  determines in step S 605  that a BD signal is not generated by the laser light beam L1 (NO in step S 605 ), the CPU  401  repeats the control in step S 605  until the CPU  401  confirms that a BD signal is generated. On the other hand, if the CPU  401  determines in step S 605  that a BD signal is generated by the laser light beam L1 (YES in step S 605 ), in step S 606 , the CPU  401  causes the counter  402  to start counting CLK signals according to the generation of the BD signal. 
     After step S 606 , in step S 607 , the CPU  401  turns off the light emitting element 1. Then, in step S 608 , the CPU  401  turns on the light emitting element N. In step S 609 , the CPU  401  determines whether a BD signal is generated by the laser light beam Ln emitted from the light emitting element N. If the CPU  401  determines in step S 609  that a BD signal is not generated by the laser light beam Ln (NO in step S 609 ), the CPU  401  repeats the control in step S 609  until the CPU  401  confirms that a BD signal is generated. On the other hand, if the CPU  401  determines in step S 609  that a BD signal is generated by the laser light beam Ln (YES in step S 609 ), in step S 610 , the CPU  401  samples the count value of CLK signals by the counter  402  according to the generation of the BD signal. Then, in step S 611 , the CPU  401  turns off the light emitting element N. 
     After step S 611 , in step S 612 , the CPU  401  compares a sampled count value C with the reference count value data Cref to determine whether the count value C is equal to the reference count value data Cref (C=Cref). If the CPU  401  determines that the count value C is equal to the reference count value data Cref (C=Cref) (YES in step S 612 ), in step S 613 , the CPU  401  sets the laser light emission timing corresponding to the respective light emitting elements based on the BD signal generated by the laser light beam L1 from the count value C1 to the count value Cn. On the other hand, if the CPU  401  determines in step S 612  that the count value C is not equal to the reference count value data Cref (C not equal Cref) (NO in step S 612 ), in step S 614 , the CPU  401  calculates Ccor=C−Cref. Then, in step S 615 , the CPU  401  sets the laser light emission timing corresponding to the respective light emitting elements based on the BD signal generated by the laser light beam L1 from the count value C′a to the count value C′n based on the difference Ccor. 
     After step S 613  or step S 615 , in step S 616 , the CPU  401  exposes the photosensitive drum  102  by causing the light source  201  to emit laser light beams based on the image data according to the laser light emission timings set in the respective steps. After step S 616 , in step S 617 , the CPU  401  determines whether the image formation is completed. If the CPU  401  determines that the image formation is not completed (NO in step S 617 ), the CPU  401  returns the control to step S 614 . On the other hand, if the CPU  401  determines in step S 617  that the image formation is completed (YES in step S 617 ), the CPU  401  ends the control. 
     As described above, the image forming apparatus according to the present exemplary embodiment generates a plurality of BD signals by causing the light beams emitted from the different light emitting elements to enter to the BD during image formation, and controls the relative timings at which images start to be written by the respective light emitting elements in the main scanning direction based on the difference between the timings at which the plurality of BD signals is generated. Therefore, it is possible to prevent occurrence of a variation in the image write start positions during the image formation. 
     According to the present invention, it is possible to prevent occurrence of a variation in positions at which a plurality of light beams start writing electrostatic latent images during image formation. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all modifications, equivalent structures, and functions. 
     This application claims the benefit of Japanese Patent Application No. 2012-098682, filed Apr. 24, 2012, which is hereby incorporated by reference herein in its entirety. 
     REFERENCE SIGNS LIST 
     
         
         
           
               201  light source 
               207  BD 
               401  CPU 
               402  counter 
               403  laser driver 
               404  clock signal generation unit 
               406  memory