Abstract:
An optical scanning device for scanning a photoreceptor surface with beams, said optical scanning device comprising: a light source that emits a plurality of beams in one direction; a deflector for deflecting the beams; a detector for detecting light intensities of the beams; and a switch for switching travel routes of the beams between a first route leading from the light source to the deflector and a second route leading from the light source to the detector.

Description:
[0001]    This application is based on Japanese Patent Application No. 2010-139060 filed on Jun. 18, 2010, of which content is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an optical scanning device and an image forming apparatus, and more particularly to an optical scanning device for scanning a photoreceptor surface with beams and to an image forming apparatus having the optical scanning device. 
         [0004]    2. Description of Related Art 
         [0005]    A scanning device disclosed by Japanese Patent Laid-Open Publication No. 2002-40350 is well known as a conventional optical scanning device. This scanning device has a surface-emitting laser, a rotative polygon mirror, a half mirror, a light receiving element and a control section, and this device is adapted to scan a photosensitive drum with a plurality of beams concurrently. 
         [0006]    The surface-emitting laser emits a plurality of beams. The rotative polygon mirror deflects the plurality of beams to permit the beams to be scanned on the photosensitive drum. The half mirror is located between the surface-emitting laser and the rotative polygon mirror and reflects part of each of the beams. The light receiving element receives the beams reflected from the half mirror and outputs signals with voltages corresponding to the light intensities of the reflected beams. The control section controls the surface-emitting laser based on the signals outputted from the light receiving element. This permits feedback control of the output of the surface-emitting laser. 
         [0007]    In the optical scanning device disclosed by Japanese Patent Laid-Open Publication No. 2002-40350, each of the plurality of beams is partly reflected from the half mirror to the light receiving element. Therefore, the light intensity of each beam deflected by the rotative polygon mirror is lower than the light intensity immediately after the beam was emitted from the surface-emitting laser by the light intensity reflected from the half mirror. Thus, the light intensity of the beam deflected by the rotative polygon mirror is reduced, and the light intensity that is used as the base of generation of a main-scanning synchronizing signal is reduced. Consequently, the light intensity used for generation of a main-scanning synchronizing signal is insufficient, and it may be impossible to generate a main-scanning synchronizing signal accurately. 
       SUMMARY OF THE INVENTION 
       [0008]    An object of the present invention is to provide an optical scanning device that prevents beams deflected by a deflector from reducing in light intensity and an image forming apparatus having the optical scanning device. 
         [0009]    According to a first aspect of the present invention, an optical scanning device comprises: a light source that emits a plurality of beams in one direction; a deflector for deflecting the beams; a detector for detecting light intensities of the beams; and a switch for switching travel routes of the beams between a first route leading from the light source to the deflector and a second route leading from the light source to the detector. 
         [0010]    According to a second aspect of the present invention, an image forming apparatus comprises the optical scanning device above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    This and other objects and features of the present invention will be apparent from the following description with reference to the accompanying drawings, in which: 
           [0012]      FIG. 1  is an illustration showing the general structure of an image forming apparatus; 
           [0013]      FIG. 2  is a perspective view of an optical scanning device according to a first embodiment; 
           [0014]      FIG. 3  is a time chart showing operation of the optical scanning device according to the first embodiment for a printing process; 
           [0015]      FIG. 4  is a time chart showing a modification of the operation of the optical scanning device according to the first embodiment for a printing process; 
           [0016]      FIG. 5  is an illustration showing the structure of an optical scanning device according to a second embodiment; 
           [0017]      FIG. 6  is a time chart showing operation of the optical scanning device according to the second embodiment for a printing process; 
           [0018]      FIG. 7  is an illustration showing the structure of an optical scanning device according to a third embodiment; and 
           [0019]      FIG. 8  is a time chart showing operation of the optical scanning device according to the third embodiment for a printing process. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    An optical scanning device and an image forming apparatus according to an embodiment of the present invention will be hereinafter described with reference to the accompanying drawings. 
       Structure of the Image Forming Apparatus 
       [0021]    An image forming apparatus provided with an optical scanning device according to an embodiment of the present invention is described with reference to the accompanying drawings.  FIG. 1  shows the general structure of the image forming apparatus. 
         [0022]    The image forming apparatus  1  is an electrophotographic color printer that combines images of four colors, namely, Y (yellow), M (magenta), C (cyan) and K (black) by a tandem method. The image forming apparatus forms an image on a sheet P in accordance with image data read by a scanner. The image forming apparatus  1 , as shown by  FIG. 1 , comprises a printing section  2 , a sheet feeding section  15 , a pair of timing rollers  19 , a fixing device  20 , a printed-sheet tray  21  and a control section  30 . 
         [0023]    The control section  30  controls the operation of the whole apparatus  1 , and the control section  30  is realized by a CPU. The sheet feeding section  15  feeds sheets P one by one, and the sheet feeding section  15  includes a sheet tray  16  and a feed roller  17 . On the sheet tray  16 , sheets to be subjected to printing are stacked. The feed roller  17  picks up sheets from the sheet tray  16  one by one. The pair of timing rollers  19  feeds each sheet P with precise timing so that a toner image can be transferred onto the sheet P in the printing section  2 . 
         [0024]    The printing section  2  forms a toner image on a sheet P fed from the sheet feeding section  15 . The printing section  2  comprises image forming sections  22  ( 22 Y,  22 M,  22 C and  22 K), optical scanning devices  6  ( 6   a  to  6   c ), transferring sections  8  ( 8 Y,  8 M,  8 C and  8 K), an intermediate transfer belt  11 , a driving roller  12 , a driven roller  13 , a secondary transfer roller  14  and a cleaning device  18 . The image forming sections  22  ( 22 Y,  22 M,  22 C or  22 K) each comprise a photosensitive drum  4  ( 4 Y,  4 M,  4 C or  4 K), a charger  5  ( 5 Y,  5 M,  5 C and  5 K), a developing device  7  ( 7 Y,  7 M,  7 C or  7 K), a cleaner  9  ( 9 Y,  9 M,  9 C or  9 K) and an eraser  10  ( 10 Y,  10 M,  10 C or  10 K). 
         [0025]    The chargers  5  charge the peripheral surfaces (scanned surfaces) of the photosensitive drums  4 . The optical scanning devices  6  are controlled by the control section  30  to scan the peripheral surfaces of the photosensitive drums  4 Y,  4 M,  4 C and  4 K with beams BY, BM, BC and BK, respectively. Thereby, electrostatic latent images are formed on the photosensitive drums  4 Y,  4 M,  4 C and  4 K. 
         [0026]    As shown in  FIG. 1 , the developing devices  7  ( 7 Y,  7 M,  7 C and  7 K) each comprises a developing roller  72  ( 72 Y,  72 M,  72 C or  72 K), a supplying roller  74  ( 74 Y,  74 M,  74 C or  74 K), a stirring roller  76  ( 76 Y,  76 M,  76 C or  76 K) and a container  78  ( 78 Y,  78 M,  78 C or  78 K). In  FIG. 1 , for simplification of the drawing, only the developing roller  72 Y, the supplying roller  74 Y, the stirring roller  76 Y and the container  78 Y of the developing device  7 Y are provided with reference symbols. 
         [0027]    The container  78  serves as the body of the developing device  7 . The developing roller  72 , the supplying roller  74  and the stirring roller  76  are housed in the container  78 , and also toner is stored therein. The stirring roller  76  stirs the toner stored in the container  78  and charges the toner negatively. The supplying roller  74  supplies the negatively charged toner to the developing roller  72 . The developing roller  72  supplies the toner to the photosensitive drum  4 . More specifically, a negative bias voltage is applied to the developing roller  72  so as to form a development electric field between the photosensitive drum  4  and the developing roller  72 , and by the effect of the electric field, the negatively charged toner is caused to move from the developing roller  72  to the photosensitive drum  4 . In this moment, the toner sticks to the photosensitive drum  4  in accordance with the electrostatic latent image formed on the photosensitive drum  4 . Thus, the electrostatic latent image on the photosensitive drum  4  is developed into a toner image. 
         [0028]    The intermediate transfer belt  11  is stretched out between the driving roller  12  and the driven roller  13 , and the toner image formed on the photosensitive drum  4  is transferred onto the intermediate transfer belt  11  (primary transfer). The transferring section  8  is disposed to face to the inner peripheral surface of the intermediate transfer belt  11 , and a primary transfer voltage is applied to the transferring section  8  so that the toner image on the photosensitive drum  4  can be transferred onto the intermediate transfer belt  11 . The cleaner  9  collects the residual toner from the photosensitive drum  4  after the first transfer. The eraser  10  erases the charge on the photosensitive drum  4 . The driving roller  12  is rotated by an intermediate transfer belt driving section (not shown in  FIG. 1 ) to drive the intermediate transfer belt  11  in a direction of arrow α. Thereby, the intermediate transfer belt  11  carries the toner image to a secondary transfer roller  14 . 
         [0029]    The secondary transfer roller  14 , which is drum-shaped, faces to the intermediate transfer belt  11 . A transfer voltage is applied to the secondary transfer roller  14  so that the toner image carried by the intermediate transfer belt  11  can be transferred to a paper sheet P traveling between the intermediate transfer belt  11  and the second transfer roller  14 . More specifically, the driving roller  12  has the ground potential, and the intermediate transfer belt  11  has a positive potential near the ground potential because the intermediate transfer belt  11  is in contact with the driving roller  12 . Then, a positive voltage that permits the potential of the secondary transfer roller  14  to become higher than those of the driving roller  12  and the intermediate transfer roller  11  is applied to the secondary transfer roller  14  as the transfer voltage. Thereby, an electric field is generated between the driving roller  12  and the secondary transfer roller  14 , and by the effect of the electric field, the negatively charged toner image is transferred from the intermediate transfer belt  11  to the sheet P. 
         [0030]    The cleaning device  18  removes toner remaining on the intermediate transfer belt  11  therefrom after the secondary transfer of the toner image to the sheet P. 
         [0031]    The sheet P with the toner image transferred thereto is fed to the fixing device  20 . The fixing device  20  performs a heating treatment and a pressure treatment toward the sheet P so as to fix the toner image on the sheet P. The sheet P that has been subjected to the printing process is ejected onto the printed-sheet tray  21 . 
       First Embodiment 
     Structure of the Optical Scanning Device 
       [0032]    The structure of an optical scanning device  6   a  according to a first embodiment of the present invention is hereinafter described with reference to the accompanying drawings.  FIG. 2  is a perspective view of the optical scanning device  6   a .  FIG. 2  shows only the structure for irradiating the photosensitive drum  4 K for black with beams BK. Also, the optical scanning device  6   a  actually has optical elements such as mirrors, but the optical elements are omitted from  FIG. 2  to simplify the illustration. In the following paragraphs, scanning in the lengthwise direction of the photosensitive drum  4 K is referred to as main scanning, and scanning in the direction in which the peripheral surface of the photosensitive drum  4 K moves while the photosensitive drum  4 K is rotating is referred to as sub scanning. The main scanning and the sub scanning are performed in orthogonal directions to each other. On a planar view facing to the traveling direction of the beams BK, the leftward direction is referred to as a main-scanning direction, and the upward direction is referred to as a sub-scanning direction. 
         [0033]    The optical scanning device  6   a  comprises a light source  60 K, a collimator lens  61 K, a cylindrical lens  62 K, a deflector  64 , scanning lenses  66 K,  68 K, a mirror  80 K, a sensor  82 K, a reflective liquid crystal element (switching element)  84 K and a sensor  86 K. The optical scanning device  6   a  further comprises a control section  30  although it is not shown in  FIG. 2 . 
         [0034]    The light source  60 K is a surface-emitting laser (VCSEL) that emits beams BK 1  to BK 4  in one direction. More specifically, the light source  60 K is a laminate semiconductor element formed of a plurality of semiconductor layers and has emission points  60 K- 1  to  60 K- 4 . The light source  60 K emits beams BK 1  to BK 4  that are diffusion lights only in one direction along the laminate direction of the semiconductor layers from the emission points  60 K- 1  to  60 K- 4 , respectively. The beam BK 1  is used for generation of a main-scanning synchronizing signal (SOS signal), and the beams BK 2  to BK 4  are used for formation of an electrostatic latent image. The emission points  60 K- 1  to  60 K- 4  are aligned in the sub-scanning direction, and also the beams BK 1  to BK 4  are aligned in the sub-scanning direction. In  FIG. 1 , the beam BK collectively means the beams BK 1  to BK 4 . 
         [0035]    The collimator lens  61 K transforms the diffusion light beams BK 1  to BK 4  into parallel light beams. The cylindrical lens  62 K causes the beams BK 1  to BK 4  to converge on reflecting surfaces of the deflector  64  with respect to the sub-scanning direction, so that the forms of the beams BK 1  to BK 4  become linear on the reflecting surfaces of the deflector  64 . 
         [0036]    The deflector  64  comprises a polygon mirror and a motor, and deflects the beams BK 1  to BK 4  in the main-scanning direction at an equiangular velocity. The scanning lenses  66 K and  68 K correct aberrations of the deflected beams BK 1  to BK 4 . Then, the beams BK 1  to BK 4  are imaged on the peripheral surface of the photosensitive drum  4 K. The photosensitive drum  4 K is driven by a motor or any other driving device (not shown) to rotate at a specified constant velocity, so that the beams BK 1  to BK 4  imaged on the photosensitive drum  4 K are scanned in the sub-scanning direction. In this way, by the main scanning and the sub scanning of the beams BK 1  to BK 4 , a two-dimensional image (electrostatic latent image) is formed on the photosensitive drum  4 K. 
         [0037]    The mirror  80 K is located near the main-scanning upstream end of the photosensitive drum  4 K and reflects the beam BK 1 . The sensor  82 K receives the beam BK 1  reflected from the mirror  80 K and generates a main-scanning synchronizing signal (SOS signal). More specifically, the sensor  82 K outputs a signal with a high-level electric potential when the sensor  82 K does not receive the beam BK 1 , and the electric potential of the signal outputted from the sensor  82 K becomes a low level when the sensor  82 K receives the beam BK 1 . Then, the control section  30  detects the SOS signal become a low level, and the light source  60 K is controlled so as to start emitting the beams BK 2  to BK 4  to start writing of an electrostatic latent image a specified time after the detection. 
         [0038]    The reflective liquid crystal element  84 K switches the travel routes of the beams BK 1  to BK 4  emitted from the light source  60 K between a route R 1  to the deflector  64  and a route R 2  to the sensor  86 K. More specifically, the reflective liquid crystal element  84 K is located between the collimator lens  61 K and the cylindrical lens  62 K. While a high-level voltage is applied to the reflective liquid crystal element  84 K, the reflective liquid crystal element  84 K transmits the beams BK 1  to BK 4 , and the beams BK 1  to BK 4  travel to the deflector  64 . On the other hand, while a low-level voltage is applied to the reflective liquid crystal element  84 K, the reflective liquid crystal element  84 K reflects the beams BK 1  to BK 4 , and the beams BK 1  to BK 4  travel to the sensor  86 K. 
         [0039]    The sensor  86 K receives the beams BK 1  to BK 4  reflected from the reflective liquid crystal element  84 K, and generates a detection signal of a voltage depending on the light intensity of the received beams BK 1  to BK 4 . Then, the control section  30  controls the output from the light source  60 K (the light intensities of the beams BK 1  to BK 4 ) based on the detection signal outputted from the sensor  86 K. The structures for irradiation of the photosensitive drums  4 Y,  4 M and  4 C for yellow (Y), magenta (M) and cyan (C) with beams BY, BM and BC, respectively, are the same as the structure for irradiation of the photosensitive drum  4 K for black (K) with the beams BK, and descriptions of the structures are omitted. 
       Operation of the Optical Scanning Device 
       [0040]    The operation of the optical scanning device  6   a  is described with reference to the drawings. In the following, the operation that is executed in the structure for irradiation of the photosensitive drum  4 K for black (K) with the beams BK after the control section  30  receives a print job until the control section  30  starts a printing process is described.  FIG. 3  is a time chart showing actions of the optical scanning device  6   a  for a printing process. 
         [0041]    Upon receiving a print job, the control section  30  controls the output from the light source  60 K such that the light intensities of the beams BK 1  to BK 4  emitted from the emission points  60 K- 1  to  60 K- 4  of the light source  60 K will become predetermined values. Specifically, the control section  30  first turns the reflective liquid crystal element  84 K into a reflecting state and turns a printing enable signal into an inhibiting state. Thereafter, at time t 1 , the control section  30  turns on the emission point  60 K- 1 , and in response, the emission point  60 K- 1  emits the beam BK 1 . The beam BK 1  is reflected by the reflective liquid crystal element  84 K and enters into the sensor  86 K. The light intensity of the beam BK 1  received by the sensor  86 K becomes higher. Then, upon determining, from the detection signal outputted from the sensor  86 K, that the light intensity of the beam BK 1  has reached the predetermined value, the sensor  30  turns off the emission point  60 K- 1  (at time t 2 ). 
         [0042]    At time t 2 , the control section  30  turns on the emission point  60 K- 2 , and in response, the emission point  60 K- 2  emits the beam BK 2 . The beam BK 2  is reflected by the reflective liquid crystal element  84 K and enters into the sensor  86 K. The light intensity of the beam BK 2  received by the sensor  86 K becomes higher. Then, upon determining, from the detection signal outputted from the sensor  86 K, that the light intensity of the beam BK 2  has reached the predetermined value, the sensor  30  turns off the emission point  60 K- 2  (at time t 3 ). 
         [0043]    At time t 3 , the control section turns on the emission point  60 K- 3 , and in response, the emission point  60 K- 3  emits the beam BK 3 . The beam BK 3  is reflected by the reflective liquid crystal element  84 K and enters into the sensor  86 K. The light intensity of the beam BK 3  received by the sensor  86 K becomes higher. Then, upon determining, from the detection signal outputted from the sensor  86 K, that the light intensity of the beam BK 3  has reached the predetermined value, the sensor  30  turns off the emission point  60 K- 3  (at time t 4 ). 
         [0044]    At time t 4 , the control section  30  turns on the emission point  60 K- 4 , and in response, the emission point  60 K- 4  emits the beam BK 4 . The beam BK 4  is reflected by the reflective liquid crystal element  84 K and enters into the sensor  86 K. The light intensity of the beam BK 4  received by the sensor  86 K becomes higher. Then, upon determining, from the detection signal outputted from the sensor  86 K, that the light intensity of the beam BK 4  has reached the predetermined value, the sensor  30  turns off the emission point  60 K- 4  (at time t 5 ). By the above-described actions, the light intensities of the beams BK 1  to BK 4  emitted from the emission points  60 K- 1  to  60 K- 4  of the light source  60 K are adjusted to the predetermined values. 
         [0045]    At time t 5 , the control section  30  turns the reflective liquid crystal element  84 K into a transmitting state and turns on the emission point  60 K- 1 . Because the reflective liquid crystal element  84 K has become the transmitting state, the beam BK 1  emitted from the emission point  60 K- 1  passes through the reflective liquid crystal element  84 K and is deflected by the deflector  64 . Thereafter, when the beam BK 1  enters into the sensor  82 K, a low-level pulse p 1  is generated in the SOS signal. Then, the emission point  60 K- 1  is turned off. 
         [0046]    At time  6  that is a specified time after the generation of the pulse p 1 , the reflective liquid crystal element  84 K is turned from the transmitting state to the reflecting state by the control section  30  and is kept in the reflecting state until time t 7 . During this time, the control section  30  turns on the emission points  60 K- 4 ,  60 K- 3 ,  60 K- 2  and  60 K- 1  to emit the beams BK 4 , BK 3 , BK 2  and BK 1  in this order. The beams BK 4 , BK 3 , BK 2  and BK 1  are reflected by the reflective liquid crystal element  84 K and enter into the sensor  86 K individually in order. The control section  30  adjusts the outputs from the emission points  60 K- 4 ,  60 K- 3 ,  60 K- 2  and  60 K- 1  (the light intensities of the beams BK 4 , BK 3 , BK 2  and BK 1 ), based on the detection signal outputted from the sensor  86 K. 
         [0047]    At time t 7 , the control section  30  turns the reflective liquid crystal element  84 K from the reflecting state to the transmitting state. In this moment, the emission point  60 K- 1  is kept on by the control section  30  to keep emitting the beam BK 1 . Accordingly, the beam BK 1  is deflected by the deflector  64 . Then, when the beam BK 1  enters into the sensor  82 K, a low-level pulse p 2  is generated in the SOS signal. Then, the emission point  60 K- 1  is turned off. 
         [0048]    At time t 8  that is a specified time after the generation of the pulse p 2 , the reflective liquid crystal element  84 K is turned from the transmitting state to the reflecting state by the control section  30  and is kept in the reflecting state until time t 9 . During this time, the control section  30  turns on the emission points  60 K- 4 ,  60 K- 3 ,  60 K- 2  and  60 K- 1  to emit the beams BK 4 , BK 3 , BK 2  and BK 1  in this order. The beams BK 4 , BK 3 , BK 2  and BK 1  are reflected by the reflective liquid crystal element  84 K and enter into the sensor  86 K individually in order. The control section  30  adjusts the outputs from the emission points  60 K- 4 ,  60 K- 3 ,  60 K- 2  and  60 K- 1  (the light intensities of the beams BK 4 , BK 3 , BK 2  and BK 1 ), based on the detection signal outputted from the sensor  86 K. 
         [0049]    At time t 9 , the control section  30  turns the reflective liquid crystal element  84 K into the transmitting state and turns the printing enable signal into an enabling state. In this moment, the emission point  60 K- 1  is kept on by the control section  30  to keep emitting the beam BK 1 . Accordingly, the beam BK 1  is deflected by the deflector  64 . Then, when the beam BK 1  enters into the sensor  82 K, a low-level pulse p 3  is generated in the SOS signal (at time t 10 ). The adjustment of the outputs from the emission points  60 K- 1  to  60 K- 4  executed between the generation of the pulse p 1  and the generation of the pulse p 2  and adjustments of the outputs from the emission points  60 K- 1  to  60 K- 4  executed between the generation of the pulse p 2  and the generation of the pulse p 3  include substantially the same actions. Thus, the optical scanning device  6   a  gets ready for a printing process through the actions from time t 1  until time t 10 . 
         [0050]    From time t 11  (a specified time after t 10 ) until time t 12 , the control section  30  makes the emission points  60 K- 1  to  60 K- 4  emit the beams BK 1  to BK 4  in accordance with image data. During this time, the reflective liquid crystal element  84 K is kept in the transmitting state, and the beams BK 1  to BK 4  are deflected by the deflector  64  and scanned on the peripheral surface of the photosensitive drum  4 K. Thereby, from time t 11  until time t 12 , three lines of an electrostatic latent image are written on the photosensitive drum  4 K. Thereafter, at time t 13 , the control section  30  turns the reflective liquid crystal element  84 K from the transmitting state to the reflecting state. From time t 13  until time t 15 , the same actions executed from time t 8  until time t 10  are executed, and descriptions of the actions are omitted. On and after time t 15 , the same actions executed from time t 10  until time t 15  are repeated, and thereby, an electrostatic latent image is formed three lines at a time. 
       Advantages 
       [0051]    In the optical scanning device  6   a  of the above-described structure, the beams deflected by the deflector  64  can be prevented from reducing in light intensity. In the optical scanning device disclosed by 2002-40350, on the other hand, each of a plurality of beams emitted from a surface-emitting laser is partly reflected by a half mirror and directed to a light receiving element, and therefore, the beams deflected by a polygon mirror reduces in light intensity, compared with the beams immediately after emitted from the surface-emitting laser, by the light intensity reflected by the half mirror. 
         [0052]    More specifically, in the optical scanning device  6   a , the reflective liquid crystal element  84 K switches the travel routes of the beams BK 1  to BK 4  emitted from the light source  60 K between the route R 1  to the deflector  64  and the route R 2  to the sensor  86 K. The control section  30  keeps the reflective liquid crystal element  84 K in a transmitting state for generations of pulses in the SOS signal and formation of an electrostatic latent image, and keeps the reflective liquid crystal element  84 K in a reflecting state for adjustments of the outputs from the emission points  60 K- 1  to  60 K- 4 . With this control, it does not occur that the light intensities of the beams BK 1  to BK 4  for generations of pulses in the SOS signal and formation of an electrostatic latent image decrease due to reflection of the beams BK 1  to BK 4  by the reflective liquid crystal element  84 K. Thus, in the optical scanning device  6   a , the beams deflected by the deflector  64  can be prevented from reducing in light intensity. 
         [0053]    When a surface-emitting laser is used in the optical scanning device  60   a  as the light source  60 K, the advantage is remarkable. Ordinary lasers emit two beams traveling forward and backward, respectively. When such an ordinary laser is used in an optical scanning device, one of the beams is used for formation of an electrostatic latent image and generations of pulses in an SOS signal, and the other is used for adjustment of the output from the light source. Therefore, in such an optical scanning device using an ordinary laser as a light source, it is not necessary to adjust the output from the light source by splitting the beams and/or by switching the travel routes of the beams. 
         [0054]    On the other hand, surface-emitting lasers emit beams in one direction. When such a surface-emitting laser is used in an optical scanning device as a light source, the beams emitted in one direction must be used both for formation of an electrostatic latent image and generations of pulses in an SOS signal and for adjustment of the output from the light source. Therefore, in such an optical scanning device, it is necessary to adjust the output from the light source by splitting the beams and/or by switching the travel routes of the beams. In the optical scanning device  6   a , the travel routes of the beams BK 1  to BK 4  are switched by the reflective liquid crystal element  84 K. Thereby, in the optical scanning device  6   a , although a surface-emitting laser is used as the light source  60 K, it does not occur that the beams deflected by the deflector  64  reduce in light intensity. 
         [0055]    Moreover, for the following reason, the advantage is remarkable when a surface-emitting laser is used as the light source  60 K in the optical scanning device  60   a . Surface-emitting lasers emit beams with less light intensities compared with ordinary lasers. Therefore, when a surface-emitting laser is used as the light source in an optical scanning device, the problem of reductions in light intensity of beams deflected by a deflector is more significant. In such a case, by using the reflective liquid crystal element  84 K as a switching element for switching the travel routes of the beams BK 1  to BK 4 , the problem of reduction of the beams in light intensity can be effectively prevented. 
       Modification 
       [0056]    A modification of the operation of the optical scanning device  6   a  is described with reference to the drawings.  FIG. 4  is a time chart showing modified actions of the optical scanning device  6   a  for a printing process. 
         [0057]    Upon receiving a print job, the control section  30  controls the output from the light source  60 K such that the light intensities of the beams BK 1  to BK 4  emitted from the emission points  60 K- 1  to  60 K- 4  of the light source  60 K will become predetermined values. Specifically, the control section  30  first turns the reflective liquid crystal element  84 K into a reflecting state and turns a printing enable signal into an inhibiting state. Thereafter, at time t 1 , the control section  30  turns on the emission point  60 K- 1 , and in response, the emission point  60 K- 1  emits the beam BK 1 . The BK 1  is reflected by the reflective liquid crystal element  84 K and enters into the sensor  86 K. The light intensity of the beam BK 1  received by the sensor  86 K becomes higher. Then, upon determining, from the detection signal outputted from the sensor  86 K, that the light intensity of the beam BK 1  has reached a predetermined value, the sensor  30  turns the reflective liquid crystal element  84 K to a transmitting state (at time t 2 ). 
         [0058]    At time t 2 , the emission point  60 K- 1  is on, and therefore, the beam BK 1  is deflected by the deflector  64 . Then, when the beam BK 1  enters into the sensor  82 K, a low-level pulse p 1  is generated in the SOS signal. Then, the emission point  60 K- 1  is turned off. 
         [0059]    At time t 3  that is a specified time after the generation of the pulse p 1 , the reflective liquid crystal element  84 K is turned from the transmitting state to the reflecting state by the control section  30  and is kept in the reflecting state until time t 4 . During this time, the control section  30  turns on the emission points  60 K- 4 ,  60 K- 3 ,  60 K- 2  and  60 K- 1  in this order, and the beams BK 4 , BK 3 , BK 2  and BK 1  emitted from the emission points  60 K- 4 ,  60 K- 3 ,  60 K- 2  and  60 K- 1  are reflected by the reflective liquid crystal element  84 K and enter into the sensor  86 K individually in order. In this stage, the emission points  60 K- 1  to  60 K- 4  emit the beams BK 1  to BK 4  for a very short time, and therefore, the light intensities of the beams BK 2  to BK 4  cannot reach the predetermined values. With respect to the beam BK 1 , however, because the adjustment was completed at time t 2 , the light intensity reaches the predetermined value. 
         [0060]    At time t 4 , the control section  30  turns the reflective liquid crystal element  84 K to the transmitting state and keeps the emission point  60 K- 1  on, and therefore, the beam BK 1  is deflected by the deflector  64 . Then, when the beam BK 1  enters into the sensor  82 K, a low-level pulse p 2  is generated in the SOS signal. Thereafter, the actions executed between the generations of pulses p 1  and p 2  are repeated between pulses p 2  and p 3 , between pulses p 3  and p 4  and between pulses p 4  and p 5 . In this way, the light intensities of the beams BK 2  to BK 4  are adjusted to the predetermined values. 
         [0061]    Then, between pulses p 5  and p 6 , the control section  30  controls the optical scanning device  6   a  such that three lines of an electrostatic latent image are written on the photosensitive drum  4 K. The actions of the optical scanning device  6   a  between the pulses p 5  and p 6  in  FIG. 4  are the same as the actions of the optical scanning device  6   a  between the pulses p 3  and p 4  in  FIG. 3 , and descriptions of the actions between the pulses p 5  and p 6  in  FIG. 4  are omitted. 
         [0062]    Also in the optical scanning device  6   a  operating in this way, the beams deflected by the deflector  64  can be prevented from reducing in light intensity. 
       Second Embodiment 
     Structure of the Optical Scanning Device 
       [0063]    Next, an optical scanning device  6   b  according to a second embodiment of the present invention will be described with reference to the accompanying drawings.  FIG. 5  shows the structure of the optical scanning device  6   b .  FIG. 5  shows only the structure for irradiating the photosensitive drum  4 K for black with beams BK. Also, the optical scanning device  6   b  actually has optical elements such as mirrors, but the optical elements are omitted from  FIG. 5  to simplify the illustration. 
         [0064]    The optical scanning device  6   b  is different from the optical scanning device  6   a  in that the device  6   b  has reflective liquid crystal elements  84 Ka and  84 Kb. The reflective liquid crystal element  84 Ka switches the travel route of the beam BK 1  used for generations of pulses in the SOS signal. The reflective liquid crystal element  84 Kb switches the travel routes of the other beams BK 2  to BK 4 . Thereby, in the optical scanning device  6   b , the travel route of the beam BK 1  can be switched independently of the other beams BK 2  to BK 4 . The other components of the optical scanning device  6   b  are the same as those of the optical scanning device  6   a , and descriptions of these components are omitted. 
       Operation of the Optical Scanning Device 
       [0065]    The operation of the optical scanning device  6   a  is described with reference to the drawings. In the following, the operation that is executed in the structure for irradiation of the photosensitive drum  4 K for black (K) with the beams BK after the control section  30  receives a print job until the control section  30  starts a printing process is described.  FIG. 6  is a time chart showing actions of the optical scanning device  6   b  for a printing process. 
         [0066]    Upon receiving a print job, the control section  30  controls the output from the light source  60 K such that the light intensities of the beams BK 1  to BK 4  emitted from the emission points  60 K- 1  to  60 K- 4  of the light source  60 K will become predetermined values. Specifically, the control section  30  first turns the reflective liquid crystal elements  84 Ka and  84 Kb into a reflecting state and turns a printing enable signal into an inhibiting state. Thereafter, at time t 1 , the control section  30  turns on the emission point  60 K- 1 , and in response, the emission point  60 K- 1  emits the beam BK 1 . The BK 1  is reflected by the reflective liquid crystal element  84 K and enters into the sensor  86 K. The light intensity of the beam BK 1  received by the sensor  86 K becomes higher. Then, upon determining, from the detection signal outputted from the sensor  86 K, that the light intensity of the beam BK 1  has reached the predetermined value, the sensor  30  turns the reflective liquid crystal element  84 Ka into a transmitting state (at time t 2 ). 
         [0067]    At time t 2 , the emission point  60 K- 1  is kept on by the control section  30 , and therefore, the beam BK 1  passes through the reflective liquid crystal element  84 Ka and is deflected by the deflector  64 . Then, when the beam BK 1  enters into the sensor  82 K, a low-level pulse p 1  is generated in the SOS signal. Then, the emission point  60 K- 1  is turned off. 
         [0068]    At time t 3 , the control section  30  turns on the emission point  60 K- 2 , and in response, the emission point  60 K- 2  emits the beam BK 2 . In this moment, the reflective liquid crystal element  84 Kb is in the reflecting state, and the beam BK 2  is reflected by the reflective liquid crystal element  84 Kb and enters into the sensor  86 K. Thereafter, the light intensity of the beam BK 2  received by the sensor  86 K becomes higher. 
         [0069]    At time t 4 , the control section  30  turns the reflective liquid crystal element  84 Ka into the reflecting state. Thereafter, upon determining, from the detection signal outputted from the sensor  86 K, that the light intensity of the beam BK 2  has reached the predetermined value, the sensor  30  turns off the emission point  60 K- 2  (at time t 5 ). At time t 5 , the emission point  60 K- 1  is turned on. 
         [0070]    At time t 6 , the control section turns the reflective liquid crystal element  84 Ka into the transmitting state. At this time, the emission point  60 K- 1  is on, and the beam BK 1  passes through the reflective liquid crystal element  84 Ka and is deflected by the deflector  64 . Then, when the beam BK 1  enters into the sensor  82 K, a low-level pulse p 2  is generated in the SOS signal. Then, the emission point  60 K- 1  is turned off. 
         [0071]    At time t 7 , the control section  30  turns on the emission point  60 K- 3 , and in response, the emission point  60 K- 3  emits the beam BK 3 . In this moment, the reflective liquid crystal element  84 Kb is in the reflecting state, and the beam BK 3  is reflected by the reflective liquid crystal element  84 Kb and enters into the sensor  86 K. Thereafter, the light intensity of the beam BK 3  received by the sensor  86 K becomes higher. 
         [0072]    At time t 8 , the control section  40  turns the reflective liquid crystal element  84 Ka into the reflecting state. Thereafter, upon determining, from the detection signal outputted from the sensor  86 K, that the light intensity of the beam BK 3  has reached the predetermined value, the sensor  30  turns off the emission point  60 K- 3  (at time t 9 ). At time t 9 , the emission point  60 K- 1  is turned on. 
         [0073]    At time t 10 , the control section  30  turns the reflective liquid crystal element  84 Ka into the transmitting state. In this moment, the emission point  60 K- 1  is on, and the beams BK 1  passes through the reflective liquid crystal element  84 Ka and is deflected by the deflector  64 . Then, when the beam BK 1  enters into the sensor  82 K, a low-level pulse p 3  is generated in the SOS signal. Then, the emission point  60 K- 1  is turned off. 
         [0074]    At time t 11 , the control section  30  turns on the emission point  60 K- 4 , and in response, the emission point  60 K- 4  emits the beam BK 4 . The reflective liquid crystal element  84 Kb is in the reflecting state, and therefore, the beam BK 4  is reflected by the reflective liquid crystal element  84 Kb and enters into the sensor  86 K. Thereafter, the light intensity of the beam BK 4  received by the sensor  86 K becomes higher. 
         [0075]    At time t 12 , the control section  30  turns the reflective liquid crystal element  84 Ka into the reflecting state. Thereafter, upon determining, from the detection signal outputted from the sensor  86 K, that the light intensity of the beam BK 4  has reached the predetermined value, the sensor  30  turns off the emission point  60 K- 4  (at time t 13 ). At time t 13 , the emission point  60 K- 1  is turned on. 
         [0076]    At time t 14 , the control section  30  turns the reflective liquid crystal elements  84 Ka and  84 Kb into the transmitting state and turns the printing enable signal into the enabling state. In this moment, the emission point  60 K- 1  is on, and the beam BK 1  passes through the reflective liquid crystal element  84 Ka and is deflected by the deflector  64 . Then, when the beam BK 1  enters into the sensor  84 K, a low-level pulse p 4  is generated in the SOS signal. Through the actions above, the light intensities of the beams BK 1  to BK 4  emitted from the emission points  60 K- 1  to  60 K- 4  of the light source  60 K are adjusted to the predetermined values. 
         [0077]    Thereafter, between pulses p 4  and p 5 , the control section  30  controls the optical scanning device  6   b  such that three lines of an electrostatic latent image are written on the photosensitive drum  4 K. The actions of the optical scanning device  6   b  between the pulses p 4  and p 5  in  FIG. 6  are the same as the actions of the optical scanning device  6   b  between the pulses p 3  and p 4  in  FIG. 3 , and descriptions of the actions between the pulses p 5  and p 6  in  FIG. 4  are omitted. 
       Advantages 
       [0078]    Like in the optical scanning device  6   a , in the optical scanning device  6   b  that operates in the above-described way, the beams deflected by the deflector  64  can be prevented from reducing in light intensity. 
         [0079]    In the optical scanning device  6   b , further, the preparation period (from time t 1  to time t 14  in  FIG. 6 ) required for the adjustments of the light intensities of the beams BK 1  to BK 4  emitted from the emission points  60 K- 1  to  60 K- 4  of the light source  60 K and for preliminary generations of pulses in the SOS signal can be shortened. In the optical scanning devices  6   a  and  6   b , from the view point of shortening of the preparation period, it is undesired that the beams BK 1  to BK 4  are scanned on the peripheral surface of the photosensitive drum  4 K during the preparation period. In the optical scanning device  6   a  having only one reflective liquid crystal element  84 K, in order to shorten the preparation period, the light intensities of the beams BK 1  to BK 4  are adjusted to the predetermined values first, as shown in  FIG. 3 . 
         [0080]    In the operation shown by  FIG. 3 , the control section  30  does not start actions for generations of pulses in the SOS signal until the adjustments of the light intensities of the beams BK 1  to BK 4  are completed. Due to the delay of the generations of pulses, the synchronization timing in the optical scanning device  6   a  is delayed. Accordingly, the optical scanning device  6   a  needs a long preparation period. 
         [0081]    As shown in  FIG. 4 , it is possible to control the optical scanning device  6   a  such that the periods wherein the beams BK 1  to BK 4  are not scanned on the peripheral surface  4 K are used for the adjustments of the light intensities of the beams BK 1  to BK 4 . 
         [0082]    In the case of  FIG. 4 , the emission points  60 K- 1  to  60 K- 4  are turned on and immediately turned off by the control section  30 , and the turn-ons and turn-offs are repeated. Accordingly, in this case also, the optical scanning device  6   a  needs a long preparation period. 
         [0083]    In the optical scanning device  6   b , on the other hand, because two reflective liquid crystal elements  84 Ka and  84 Kb are provided, it is possible to switch the travel route of the beam BK 1  independently of the other beams BK 2  to BK 4 . Thereby, for example, the following actions become possible: the light intensity of the beam BK 2  is adjusted with the reflective liquid crystal element  84 Kb kept in the reflecting state from time t 3  to time t 5 ; and immediately after that (at time t 6 ), the reflective liquid crystal element  84 Ka is turned from the reflecting state to the transmitting state so as to cause the beam BK 1  to enter into the sensor  82 K, thereby resulting in the generation of pulse p 2  in the SOS signal. Thus, in the optical scanning device  6   b , it is possible to generate a pulse immediately after the adjustment of the light intensity of each of the beams BK 1  to BK 4  is completed. Accordingly, the optical scanning device  6   b  needs a shorter preparation period, compared with the optical scanning device  6   a.    
       Third Embodiment 
     Structure of the Optical Scanning Device 
       [0084]    Next, an optical scanning device  6   c  according to a third embodiment of the present invention will be described with reference to the drawings.  FIG. 7  shows the structure of the optical scanning device  6   c .  FIG. 7  shows only the structure for irradiating the photosensitive drum  4 K for black with beams BK. Also, the optical scanning device  6   b  actually has optical elements such as mirrors, but the optical elements are omitted from  FIG. 7  to simplify the illustration. 
         [0085]    The optical scanning device  6   c  is different from the optical scanning device  6   a  in that the optical scanning device  6   c  has reflective liquid crystal elements  84 Ka to  84   kd  and sensors  86 Ka to  86 Kd. The reflective liquid crystal elements  84 Ka to  84   kd  are located in the paths of the beams BK 1  to BK 4 , respectively, and switch the respective travel routes of the beams BK 1  to BK 4 . Thus, in the optical scanning device  6   c , the travel routes of the beams BK 1  to BK 4  can be switched independently of one another. The sensors  86 Ka to  86 Kd receive the beams BK 1  to BK 4 , respectively. The other components of the optical scanning device  6   c  are the same as those of the optical scanning device  6   a , and descriptions thereof are omitted. 
       Operation of the Optical Scanning Device 
       [0086]    The operation of the optical scanning device  6   a  is described with reference to the drawings. In the following, the operation that is executed in the structure for irradiation of the photosensitive drum  4 K for black (K) with the beams BK after the control section  30  receives a print job until the control section  30  starts a printing process is described.  FIG. 8  is a time chart showing actions of the optical scanning device  6   c  for a printing process. 
         [0087]    Upon receiving a print job, the control section  30  controls the output from the light source  60 K such that the light intensities of the beams BK 1  to BK 4  emitted from the emission points  60 K- 1  to  60 K- 4  of the light source  60 K will become predetermined values. First, the control section  30  turns the reflective liquid crystal elements  84 Ka to  84 Kd into a reflecting state and turns the printing enable signal into an inhibiting state. Then, at time t 1 , the control section  30  turns on the emission points  60 K- 1  to  60 K- 4 , and in response, the emission points  60 K- 1  to  60 K- 4  emit beams BK 1  to BK 4 . Since the reflective liquid crystal elements  84 Ka to  84 Kd are in the reflecting state, the beams BK 1  to BK 4  are reflected by the reflective liquid crystal elements  84 Ka to  84 Kd, respectively, and enter into the sensors  86 Ka to  86 Kd, respectively. The light intensities of the beams BK 1  to BK 4  received by the sensors  86 Ka to  86 Kd become higher. Thereafter, upon determining from the detection signals outputted from the sensors  86 Ka to  86 Kd that the light intensities of the beams BK 2  to BK 4  have reached the predetermined values, the control section  30  turns off the emission points  60 K- 2  to  60 K- 4  (at time t 2 ). At time t 2 , further, the control section  30  turns the reflective liquid crystal elements  84 Ka to  84 Kd into a transmitting state. At time t 2 , the emission point  60 K- 1  is kept on. 
         [0088]    The reflective liquid crystal elements  84 Ka to  84 Kd are in the transmitting state, and the beam BK 1  is deflected by the deflector  64 . Then, when the beam BK 1  enters into the sensor  82 K, a low-level pulse p 1  is generated in the SOS signal. Through these actions, the light intensities of the beams BK 1  to BK 4  emitted from the emission points  60 K- 1  to  60 K- 4  are adjusted to the predetermined values. 
         [0089]    At time t 3 , the control section  30  turns the reflective liquid crystal elements  84 Ka to  84 Kd into the reflecting state. Thereafter, at time t 4 , the control section  30  makes the emission points  60 K- 1  to  60 K- 4  emit the beams BK 1  to BK 4  concurrently. The beams BK 1  to BK 4  are reflected by the reflective liquid crystal elements  84 Ka to  84 Kd and enter into the sensors  86 Ka to  86 Kd. The control section  30  adjusts the outputs from the emission points  60 K- 1  to  60 K- 4  (the light intensities of the beams BK 1  to BK 4 ) on the basis of the detection signals outputted from the sensors  86 Ka to  86 Kd. Thereafter, at time t 5 , the control section  30  turns off the emission points  60 K- 2  to  60 K- 4  and turns the reflective liquid crystal elements  84 Ka to  84 Kd into the transmitting state. 
         [0090]    The emission point  60 K- 1  is not turned off at time t 5  and kept on. Therefore, the beam BK 1  emitted from the emission point  60 K- 1  passes through the reflective liquid crystal element  84 Ka and is deflected by the deflector  64 . Then, when the beam BK 1  enters into the sensor  82 K, a low-level pulse p 2  is generated in the SOS signal. 
         [0091]    Thereafter, between pulses p 2  and p 3 , the optical scanning device  6   c  takes the same actions as taken between the pulses p 1  and p 2 , and descriptions of the actions between the pulses p 2  and p 3  are omitted. Then, between pulses p 3  and p 4 , the control section  30  controls the optical scanning device  6   c  such that three lines of an electrostatic latent image are written on the photosensitive drum  4 K. The process of forming an electrostatic latent image has been already described, and the description thereof is omitted here. 
       Advantages 
       [0092]    Like in the optical scanning device  6   a , in the optical scanning device  6   c  of the above-described structure, the beams deflected by the deflector  64  can be prevented from reducing in light intensity. 
         [0093]    In the optical scanning device  6   c , further since the sensors  86 Ka to  86 Kd are disposed to receive the respective beams BK 1  to BK 4 , it is possible that the beams BK 1  to BK 4  are entered into the sensors  86 Ka to  86 Kd concurrently. Therefore, as shown in  FIG. 8 , it is possible to adjust the light intensities of the beams BK 1  to BK 4  concurrently. Accordingly, the optical scanning device  6   c  needs a shorter preparation period. 
       Other Embodiments 
       [0094]    In the optical scanning devices  6   a  to  6   c , the reflective liquid crystal elements  84 K and  84 Ka to  84 Kd are used as the switching elements for switching the travel routes of the beams BK 1  to BK 4 . However, the switching elements are not limited to the reflective liquid crystal elements and may be combinations of motors and mirrors. When switching elements of this type are used, the mirrors reflect the beams BK 1  to BK 4 , and the mirrors are moved by motors to cause the beams BK 1  to BK 4  to travel forward. The switching of the travel routes of the beams BK 1  to BK 4  may be realized by other mechanisms. 
         [0095]    Although the present invention has been described in connection with the preferred embodiments above, it is to be noted that various changes and modifications are possible to those who are skilled in the art. Such changes and modifications are to be understood as being within the scope of the present invention.