Abstract:
A laser scanner comprises a laser driver which emits two different laser beams by one main-scanning line in correspondence with an input image signal, and two torsion mirrors to reflect the two different laser beams emitted from the laser beam emission means to scan the laser beams on an opposite photosensitive member. The two torsion mirrors conduct a reciprocal torsion movement in mutual opposite phases in synchronization with each other, and the laser beams reflected from the respective torsion mirrors are emitted alternately by one main-scanning line. Thus a laser scanner which enables image formation upon backward movement of the torsion mirror with a simple structure to increase the resolution and realize high-speed image formation, and an image forming apparatus to which the laser scanner is applied can be provided.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a laser scanner and an image forming apparatus to which the laser scanner is applied, and more particularly, to a laser scanner employed in an electrophotographic image forming apparatus using a scan device based on e.g. MEMS (Micro Electro Mechanical System) technique and an image forming apparatus to which the laser scanner is applied. 
   BACKGROUND OF THE INVENTION 
   In a laser scanner used in a conventional electrophotographic image forming apparatus, generally, an electrostatic latent image is formed by emitting a laser beam generated in correspondence with image data on a rotating polygon mirror and scanning the laser beam on an photosensitive member. 
   However, to realize high-speed and/or high-resolution image formation required in recent years, it is necessary to accelerate the rotation of the polygon mirror to determine the number of subscannings. However, as the rotation speed of polygon mirror has already been increased to almost a physical limitation, it is difficult to further increase the speed dramatically. 
   To address the above problem, provided is a laser scanner in which a mirror formed by e.g. the MEMS technique, employed in place of the polygon mirror, is torsionally reciprocated. In the scanner, a laser beam is emitted on this mirror and scanned on an photosensitive member. 
     FIG. 7  shows a schematic structure of the conventional laser scanner. 
   A laser beam  403  emitted from a laser driver  401  in correspondence with an input image signal (not shown) is reflected with a torsion mirror  402  which is torsionally reciprocated, thereby scanned on a rotating electrostatic drum  104 , and forms an electrostatic latent image. 
   In this case, the torsion mirror  402  operates at a phase angle  602  of torsion movement in  FIG. 8 . As the torsion mirror  402  conducts an equiangular velocity movement, image formation by emission of laser beam cannot be performed in the whole period, but laser emission is performed only during an ENB (Enable) period  601 . As a result, a scanning line  603  is drawn on the electrostatic drum. 
     FIG. 9  shows scanning of the laser beam on an electrostatic surface. Reference numeral  503  denotes a main scanning direction by the torsion movement of the torsion mirror; and  504 , a subscanning direction by rotation of the electrostatic drum. As the laser beam is emitted only during the ENB period  601  corresponding to the forward movement of the torsion mirror, an electrostatic latent image is formed only with a scanning line  501  on the electrostatic drum, and laser emission is not performed in a portion indicated with a broken line  502  corresponding to the backward movement of the torsion mirror. If image formation is performed by emitting laser also in the portion  502 , a normal image cannot be obtained since the angle of the scanning line is alternately changed. 
   [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-267995 
   However, in the above conventional structure, as image formation cannot be performed in the portion corresponding to the backward movement of the torsion mirror, the resolution is lowered. To raise the resolution, it is necessary to reduce the speed of the subscanning. Otherwise, complicated control is required so as to attain a high speed in the backward movement of the torsion mirror in comparison with that in the forward movement. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in view of the above conventional problems, and provides a laser scanner and an image forming apparatus, to which the laser scanner is applied, to realize image formation in the backward movement of the torsion mirror with a simple structure and realize high-resolution and high-speed image formation. 
   According to the present invention, the foregoing object is attained by providing a laser scanner comprising: laser beam emission means for emitting two different laser beams by one main-scanning line in correspondence with an input image signal; and two torsion mirrors to reflect the two different laser beams emitted from the laser beam emission means to scan the laser beams on an opposite photosensitive member, wherein the two torsion mirrors conduct a reciprocal torsion movement in mutual opposite phases in synchronization with each other, and the laser beams reflected from the respective torsion mirrors are emitted alternately by one main-scanning line. 
   In the laser scanner, the laser beam emission means has two laser beam emitting units to alternately emit a laser beam by one main-scanning line in correspondence with the input image signal onto one of the two torsion mirrors. 
   Further, the laser beam emission means has: one laser beam emission unit to emit a laser beam in correspondence with the input image signal; and a reversion mirror to change the direction of the laser beam emitted from the laser beam emission unit alternately toward the two torsion mirrors. 
   Further, the present invention provides an image forming apparatus having a laser scanner for scanning a laser beam on a drum in correspondence with an image signal, wherein the laser scanner comprising: laser beam emission means for emitting two different laser beams by one main-scanning line in correspondence with an input image signal; and two torsion mirrors to reflect the two different laser beams emitted from the laser beam emission means to scan the laser beams on an opposite photosensitive member, wherein the two torsion mirrors conduct a reciprocal torsion movement in mutual opposite phases in synchronization with each other, and the laser beams reflected from the respective torsion mirrors are emitted alternately by one main-scanning line. 
   In the image forming apparatus, the laser beam emission means has two laser beam emitting units to alternately emit a laser beam by one main-scanning line in correspondence with the input image signal onto one of the two torsion mirrors. In this case, the image forming apparatus further comprises control means for alternately inputting an image signal into the two laser beam emission units by one main-scanning line. 
   Further, the laser beam emission means has: one laser beam emission unit to emit a laser beam in correspondence with the input image signal; and a reversion mirror to change the direction of the laser beam emitted from the laser beam emission unit alternately toward the two torsion mirrors. 
   Accordingly, the present invention provides a laser scanner and an image forming apparatus, to which the laser scanner is applied, to realize image formation in the backward movement of the torsion mirror with a simple structure and realize high-resolution and high-speed image formation. 
   According to the second and fifth aspects of the invention, by addition of the simple structure having two laser beam emission means and two torsion mirrors corresponding to these laser beam emission means, image formation in the backward movement of the torsion mirror can be realized, and the resolution and the speed of image formation can be increased. 
   According to the third and seventh aspects of the invention, by addition of the simple structure having one reversion mirror to change the direction of the laser beam and two torsion mirrors, image formation in the backward movement of the torsion mirror can be realized without any change in an image processor, and the resolution and the speed of image formation can be increased. 
   Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same name or similar parts throughout the figures thereof. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
       FIG. 1  illustrates an example of the structure of a laser scanner according to an embodiment 1; 
       FIG. 2  is a timing chart showing the operation of the laser scanner according to the embodiment 1; 
       FIG. 3  is a schematic diagram of an electrostatic latent image formed in the embodiments; 
       FIG. 4  illustrates an example of the structure of the laser scanner according to an embodiment 2; 
       FIG. 5  is a timing chart showing the operation of the laser scanner according to the embodiment 2; 
       FIG. 6A  is a schematic cross-sectional view of an image forming apparatus to which the laser scanner of the invention is applied; 
       FIG. 6B  is a block diagram showing a control construction of the laser scanner according to the embodiment 1; 
       FIG. 6C  is a flowchart showing a control procedure in the laser scanner according to the embodiment 1; 
       FIG. 6D  is a block diagram showing the control construction of the laser scanner according to the embodiment 2; 
       FIG. 6E  is a flowchart showing the control procedure in the laser scanner according to the embodiment 2; 
       FIG. 7  illustrates the structure of the conventional MEMS laser scanner; 
       FIG. 8  is a timing chart showing the operation of the conventional MEMS laser scanner; and 
       FIG. 9  is a schematic diagram showing an electrostatic latent image formed by the conventional MEMS laser scanner. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinbelow, a laser scanner and an image forming apparatus to which the laser scanner is applied, according to the embodiments of the present invention, will now be described in detail in accordance with the accompanying drawings. 
   Structure and Operation of Laser Scanner in Embodiment 1 
     FIG. 1  shows an example of the structure of the laser scanner according to an embodiment 1. 
   A laser beam  103 A, emitted from a first laser driver  101 A in correspondence with an input image signal (not shown), is scanned on a rotating electrostatic drum  104  with a torsionally-reciprocating first torsion mirror  102 A, to form an electrostatic latent image. Similarly, a laser beam  103 B, emitted from a second laser driver  101 B in correspondence with an input image signal (not shown), is scanned on the rotating electrostatic drum  104  with a torsionally-reciprocating second torsion mirror  102 B, to form an electrostatic latent image. Note that in the present embodiment, an image signal is alternately inputted into the first laser driver  101 A and the second laser driver  101 B by each main scanning, and the respective laser drivers performs image formation in every alternate lines. 
     FIG. 2  shows an example of the operation of the laser scanner according to the embodiment 1. 
   The phase angles of the torsion movements of the two torsion mirrors  102 A and  102 B are in opposite phases as indicated with mirror A phase angle  302 A and mirror B phase angle  302 B in  FIG. 2 . As the torsion mirrors  102 A and  102 B conduct an /equiangular velocity movement, image formation cannot be performed by emission of laser beam in the whole period. Accordingly, laser emission is made only in periods where an A-laser emission ENB signal  301 A and a B-laser emission ENB signal  301 B are High. As a result, a scanning line  303 , as a synthesis of the A and B scannings, is drawn on the electrostatic drum. 
     FIG. 3  shows the scanning of the laser beam on the electrostatic surface. 
   Numeral  203  denotes a main scanning direction by the torsion movement of the torsion mirror; and  204 , a subscanning direction by rotation of the electrostatic drum. The laser beam is emitted only in the period where the A-laser emission ENB signal  301 A is high, corresponding to the forward movement of the reciprocation of the torsion mirror  102 A, such that only a scanning line  201 A forms an electrostatic latent image on the electrostatic drum, while the laser beam is not emitted in a broken-line portion  202 A corresponding to the backward movement of the torsion mirror  102 A. Similarly, the laser beam is emitted only in the period where the B-laser emission ENB signal  301 B is High, corresponding to the forward movement of the reciprocation of the torsion mirror  102 B, such that only a scanning line  201 B forms an electrostatic latent image on the electrostatic drum, while the laser beam is not emitted in a broken-line portion  202 B corresponding to the backward movement of the torsion mirror  102 B. 
   By this structure and the operation, the resolution in the subscanning direction can be improved without reducing the speed of scanning in the subscanning direction, or without increasing the speed of in the backward movement of the mirror, and as a result, the speed of image formation can be increased. 
   Structure and Operation of Laser Scanner in Embodiment 2 
     FIG. 4  illustrates an example of the structure of the laser scanner according to the embodiment 2. 
   A laser beam  103 , emitted from a laser driver  701  in correspondence with an input image signal (not shown), is reversed with a reversion mirror  705  which alternately moves to two positions (reflection angles) by every main scanning, to be emitted on the two torsion mirrors  102 A and  102 B to be described later. When the reversion mirror is in an A-position (not shown), the laser beam  103 , emitted from a laser driver  701  in correspondence with an input image signal (not shown), is scanned on the rotating electrostatic drum  104  with the torsionally-reciprocating first torsion mirror  102 A, to form an electrostatic latent image. Similarly, when the reversion mirror is in a B-position (not shown), the laser beam  103  is scanned on the rotating electrostatic drum  104  with the torsionally-reciprocating second torsion mirror  102 B, to form an electrostatic latent image. Note that in the present embodiment, the difference from the embodiment 1 is that image formation can be performed by inputting an image signal into the laser driver  701  sequentially in continuous lines in the subscanning direction as in the case of the conventional art. Accordingly, in comparison with the embodiment 1, the image signal can be easily controlled. 
     FIG. 5  shows an example of the operation of the laser scanner according to the embodiment 2. 
   The phase angles of the torsion movements of the two torsion mirrors  102 A and  102 B are in opposite phases as a mirror A phase angle  802 A and mirror B phase angle  802 B in FIG  5 . As the torsion mirrors  102 A and  102 B conduct an equiangular velocity movement, image formation cannot be performed by emission of laser beam in the whole period. Accordingly, laser emission is made only in a period where a laser emission ENB signal  801 B is High. As a result, a scanning line  803 , as a synthesis of the A and B scannings, is drawn on the electrostatic drum. 
   The scanning of the laser beam on the electrostatic surface is as shown in  FIG. 3  as in the case of the embodiment 1. In the embodiment 2, the laser beam is emitted only in a period corresponding to the forward movement of the reciprocation of the torsion mirror  102 A, where the reversion mirror  705  is in the A-position, such that only a scanning line  201 A forms an electrostatic latent image on the electrostatic drum, while the laser beam is not emitted in a broken-line portion  202 A corresponding to the backward movement of the torsion mirror  102 A. Similarly, the laser beam is emitted only in the period corresponding to the forward movement of the reciprocation of the torsion mirror  102 B, where the reversion mirror  705  is in the B-position, such that only a scanning line  201 B forms an electrostatic latent image on the electrostatic drum, while the laser beam is not emitted in a broken-line portion  202 B corresponding to the backward movement of the torsion mirror  102 B. 
   By this structure and the operation, the resolution in the subscanning direction can be improved without reducing the speed of scanning in the subscanning direction or without increasing the speed of the backward movement of the mirror, and as a result, the speed of image formation can be increased. Further, the input control of image signal can be simplified. 
   &lt;Configuration of Image Forming Apparatus Using Laser Scanner&gt; 
     FIG. 6A  shows an example of the configuration of the image forming apparatus to which the laser scanner of the present invention is applied.  FIG. 6A  shows a color complex machine as an example of the image forming apparatus, however, apparently the image forming apparatus may be another machine such as a copier or a printer, otherwise, a part including the laser scanner as a part of the apparatus. The image forming apparatus of the present invention includes the above devices as long as the laser scanner of the present invention is applicable. 
   An image forming apparatus  50  has a color image reader  1  (hereinbelow, “reader  1 ”) in an upper part, and a color image printer  2  (hereinbelow, “printer  2 ”) in a lower part. 
   First, the structure of the reader  1  will be described. 
   Numeral  1101  denotes a glass plate (platen); and  1102 , an automatic document feeder (ADF). The automatic document feeder  1102  may be replaced with a mirror-finished platen or white platen (not shown). As light sources  1103  and  1104  to illuminate an original, halogen lamps, fluorescent lamps, xenon lamps or the like are used. Numerals  1105  and  1106  denote reflectors to gather light from the light sources  1103  and  1104  on the original;  1107  to  1109 , mirrors; and  1110 , a lens to gather reflected light from the original or projection light on a CCD (Charge Coupled Device) image sensor (hereinbelow, “CCD”)  1111 . 
   Numeral  1112  denotes a base plate on which the CCD  1111  is mounted;  1100 , a controller to control the entire image forming apparatus; and  1113 , a digital image processor. 
   Numeral  1114  denotes a carriage accommodating the light sources  1103  and  1104 , the reflectors  1105  and  1106  and the mirror  1107 ; and  1115 , a carriage accommodating the mirrors  1108  and  1109 . Note that the carriage  1114  and the carriage  1115  mechanically move respectively at speeds V and V/2 in a subscanning direction Y orthogonal to an electric scanning direction (main scanning direction X) of the CCD  1111 , thereby scan the whole surface of the original. Numeral  1116  denotes an external interface (I/F) for communication with another device. 
   Next, the structure of the color printer  2  will be described. 
   A control signal from the controller  1100  is received with a printer control I/F  1218 , and the printer  2  operates based on the control signal from the printer control I/F  1218 . 
   An electrostatic drum  1202  rotates in a counterclockwise direction. An electrostatic latent image is formed by a laser scanner  1201  according to the present embodiment on the electrostatic drum  1202 . developers  1221 ,  1222 ,  1223  and  1224  corresponding to black, yellow, magenta and cyan colors are provided around a rotation shaft  1200  of the electrostatic drum. Upon formation of toner image on the electrostatic drum  1202 , in the case of color image formation, a rotating color developer  1203  is rotated so as to selectively move one of the developers  1221  to  1224  to a developing position adjacent to (or in contact with) the electrostatic drum  1202  with the rotation shaft  1200  as the center, in correspondence with separated color in the development. The electrostatic latent image is developed with toner supplied from one of the developers  1221  to  1224  by an amount corresponding to the charge on the electrostatic drum  1202 . 
   Note that in the present embodiment, the developers  1221  to  1224  can be easily attached/removed to/from the rotating color developer  1203 . In the rotating color developer  1203 , setting positions corresponding to black, yellow, magenta and cyan colors are designated in a clockwise direction, and the respective color developers  1221  to  1224  are set in the designated positions. When a monochrome image is developed, only the black developer  1221  is used. In this case, the rotating color developer  1203  is rotated to move the sleeve (not shown) of the black developer  1221  to a visualizing position  1226  opposite to the electrostatic drum  1202 , and toner is supplied. When a full color image is developed, all the developers  1221  to  1224  are used. In this case, the rotating color developer  1203  is rotated to move the sleeves of the black, yellow, magenta and cyan developers  1221  to  1224  sequentially to the visualizing position  1226  opposite to the electrostatic drum  1202 . A toner image formed on the electrostatic drum  1202  is transferred onto an intermediate transferring medium  1205  rotating in the clockwise direction by rotation of the electrostatic drum  1202  in the counterclockwise direction. In the case of monochrome image, the transfer onto the intermediate transferring medium  1205  is completed by 1 rotation of the intermediate transferring medium  1205 ; and in the case of full color image, the transfer is completed by 4 rotations of the intermediate transferring medium  1205 . When image formation is performed within a particular sheet size such as A4 size, two images can be formed on the intermediate transferring medium  1205 . 
   On the other hand, a sheet (print sheet), picked up from an upper cassette  1208  or a lower cassette  1209  with a pickup roller  1211  or  1212  and conveyed with a paper feed roller  1213  or  1214 , is conveyed with a conveyance roller  1215  to a registration roller  1219 . At timing of completion of the transfer onto the intermediate transferring medium  1205 , the sheet is conveyed to a position between the intermediate transferring medium  1205  and a transferring belt  1206 . Thereafter, the sheet is conveyed with the transferring belt  1206  and press-attached to the intermediate transferring medium  1205 , and the toner image on the intermediate transferring medium  1205  is transferred onto the sheet. The toner transferred onto the sheet is heated and pressed with a fixing roller and a pressure roller  1207 , thereby fixed to the sheet. The sheet to which the image has been fixed is discharged to a face-up paper discharge opening  1217 . 
   Note that residual toner on the intermediate transferring medium  1205  which has not been transferred onto the sheet is cleaned by postprocessing control in the last half of image forming sequence. In the postprocessing control, the residual toner on the intermediate transferring medium  1205  after the completion of the transfer, as waste toner, is charged with a cleaning roller  1230  to an opposite polarity to the original toner polarity. Then the residual toner with the opposite polarity is re-transferred onto the electrostatic drum  1202 . In the electrostatic drum unit, the toner with the opposite polarity is scraped with a blade (not shown) from the drum surface and conveyed to a waste toner box  1231  integrated in the electrostatic drum unit. Thus, the residual toner on the intermediate transferring medium  1205  is completely cleared, and the postprocessing control ends. 
   Construction of Laser Scanner Controller in Embodiment 1 
     FIG. 6B  is a block diagram showing an example of the construction of a laser scanner controller according to the embodiment 1. 
   Input image data is converted with a digital image processor  1113  to an image data signal for image formation, and is transferred to a laser scanner controller  1201 . 
   The laser scanner controller  1201  has a laser emission/mirror rotation controller  601  which controls laser emission and mirror rotation in synchronization with the digital image processor  1113  by an image formation synchronizing signal (including a horizontal synchronizing signal, a vertical synchronizing signal and the like). The laser emission/mirror rotation controller  601  may be constructed with hardware or software executed by a CPU or firmware as a combination of hardware and software. Note that in the following description, the respective controllers may also be constructed with any of hardware, software and firmware, and further, the controllers may be respectively constructed as an independent unit, otherwise plural controllers may be controlled by e.g. a common CPU. 
   First, in the laser emission control, an A-laser emission controller  201 A causes light emission from an A-laser  101 A based on the image data signal from the digital image processor  1113  and the A-laser emission ENB signal  301 A from the laser emission/mirror rotation controller  601 . On the other hand, a B-laser emission controller  201 B causes light emission from a B-laser  101 B based on the image data signal from the digital image processor  1113  and the B-laser emission ENB signal  301 B from the laser emission/mirror rotation controller  601 . As shown in  FIG. 2 , when the respective laser emission ENB signals are in the “enable” status, the laser emission controllers  201 A and  201 B ON/OFF control the lasers in correspondence with the image signal. 
   Next, in the mirror rotation control, the motor of the mirror A  102 A is rotate-driven with a rotation driving signal  402 A by a mirror A rotation controller  202 A which receives the mirror A control signal  302 A as shown in  FIG. 2 . The current phase angle of the mirror A  102 A is detected with a sensor, then fed back as a mirror A angle signal  502 A to the laser emission/mirror rotation controller  601 , and the phase angle of the mirror A  102 A is controlled. On the other hand, the motor of the mirror B  102 B is rotate-driven with a rotation driving signal  402 B by a mirror B rotation controller  202 B which receives the mirror B control signal  302 B as shown in  FIG. 2 . The current phase angle of the mirror B  102 B is detected with a sensor, then fed back as a mirror B angle signal  502 B to the laser emission/mirror rotation controller  601 , and the phase angle of the mirror B  102 B is controlled. 
   Example of Operation Procedure in Laser Scanner Controller in Embodiment 1 
     FIG. 6C  is a flowchart showing an example of the operation procedure in the laser scanner controller in  FIG. 6B . Note that as described in the above  FIG. 6B , the flowchart does not mean control only by software but also means hardware control and firmware control. 
   First, at step S 11 , the rotational positions of the A-laser and the B-laser and the mirror A and the mirror B are initialized. At the same time, the value of a counter for selection of A-laser emission/B-laser emission is initialized to 1. Next, at step S 12 , the horizontal synchronizing signal is waited, and when the horizontal synchronizing signal is detected, the process proceeds to step S 13 , at which driving of the mirror A and the mirror B is started. 
   At step S 14 , laser emission branching is performed based on whether the counter value is an even number or an odd number. If the counter value is an odd number, light emission is caused from the A-laser  101 A at step S 15 . If the counter value is an even number, light emission is caused from the B-laser  101 B at step S 16 . At step S 17 , the counter value is incremented, and while the vertical synchronizing signal is not detected, the process returns from step S 18  to step S 12 , at which image formation for the next line is performed by alternately causing light emission from the A-laser  101 A and the B-laser  101 B. 
   When the vertical synchronizing signal is detected, the process proceeds to step S 19 , at which it is determined whether or not the image formation has been completed. If it is determined that the image formation has not been completed, the process returns to step S 11 , at which initialization for image formation for the next page is performed. 
   Construction of Laser Scanner Controller in Embodiment 2 
     FIG. 6D  is a block diagram showing an example of the construction of the laser scanner controller according to an embodiment 2. 
   Input image data is converted with a digital image processor  1113  to an image data signal for image formation, and is transferred to a laser scanner controller  1201 . 
   The laser scanner controller  1201  has a laser emission/mirror rotation controller  801  which controls laser emission and mirror rotation in synchronization with the digital image processor  1113  by an image formation synchronizing signal (including a horizontal synchronizing signal, a vertical synchronizing signal and the like). The laser emission/mirror rotation controller  801  may be constructed with hardware or software executed by a CPU or firmware as a combination of hardware and software. Note that in the following description, the respective controllers may also be constructed with any of hardware, software and firmware, and further, the controllers may be respectively constructed as an independent unit, otherwise plural controllers may be controlled by e.g. a common CPU. 
   First, in the laser emission control, a laser emission controller  701 A causes light emission from a laser  701  based on the image data signal from the digital image processor  1113  and the laser emission ENB signal  801 B from the laser emission/mirror rotation controller  801 . As shown in  FIG. 5 , when the laser emission ENB signal is in the “enable” status, the laser emission controller  701 A ON/OFF controls the laser in correspondence with the image signal. 
   Next, in the mirror rotation control, the motor of the reversion mirror  705  is rotate-driven with a rotation driving signal  705 C from a reversion mirror rotation controller  705 A which receives a reversion mirror control signal  705 B. The current phase angle of the reversion mirror  705  is detected with a sensor, then fed back as a reversion mirror angle signal  805  to the laser emission/mirror rotation controller  801 , and the angle of the reversion mirror  705  is controlled. 
   Further, the motor of the mirror A  102 A is rotate-driven with the rotation driving signal  402 A from the mirror A rotation controller  202 A which receives the mirror A control signal  802 A as shown in  FIG. 5 . The current phase angle of the mirror A  102 A is detected with a sensor, then fed back as a mirror A angle signal  502 A to the laser emission/mirror rotation controller  801 , and the phase angle of the mirror A  102 A is controlled. On the other hand, the motor of the mirror B  102 B is rotate-driven with a rotation driving signal  402 B from a mirror B rotation controller  202 B which receives the mirror B control signal  802 B as shown in  FIG. 5 . The phase angle of the mirror B  102 B is detected with a sensor, then fed back as a mirror B angle signal  502 B to the laser emission/mirror rotation controller  801 , and the phase angle of the mirror B  102 B is controlled. 
   Example of Operation Procedure in Laser Scanner Controller in Embodiment 2 
     FIG. 6E  is a flowchart showing an example of the operation procedure in the laser scanner controller in  FIG. 6D . Note that as described in the above  FIG. 6D , the flowchart does not mean control only by software but also means hardware control and firmware control. 
   First, at step S 21 , the rotational positions of the laser and the mirror A and the mirror B are initialized. At the same time, the value of a counter for selection of the angle of the reversion mirror  705  is initialized to 1. Next, at step S 22 , the horizontal synchronizing signal is waited, and when the horizontal synchronizing signal is detected, the process proceeds to step S 23 . 
   At step S 23 , branching of reversion mirror angle is performed based on whether the counter value is an even number or an odd number. If the counter value is an odd number, the reversion mirror  705  is rotated to the A-position (an angle where the laser is turned toward the mirror A  102 A) at step S 24 . If the counter value is an even number, the reversion mirror  705  is rotated to the B-position (an angle where the laser is turned toward the mirror B  102 B) at step S 25 . At step S 26 , rotation driving of the mirror A and the mirror B is started, and at step S 27 , light emission from the laser  701  is started. 
   At step S 28 , the counter value is incremented, and while the vertical synchronizing signal is not detected, the process returns from step S 29  to step S 22 , at which image formation for the next line is performed by rotating the reversion mirror  705  so as to reverse the laser beam to the mirror A/mirror B by line. 
   When the vertical synchronizing signal is detected, the process proceeds to step S 30 , at which it is determined whether or not the image formation has been completed. If it is determined that the image formation has not been completed, the process returns to step S 21 , at which initialization for image formation for the next page is performed. 
   As many apparently widely different embodiments of the present invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims. 
   CLAIM OF PRIORITY 
   This application claims priority from Japanese Patent Application No. 2004-355889 filed on Dec. 8, 2004, which is hereby incorporated by reference herein.