Abstract:
A multibeam scanner for scanning an image area with a plurality of laser beams, comprising a plurality of beam emitting points that emit a plurality of laser beams reaching the image area with respective time lags therebetween, a time lag determination unit that determines at least one of the time lags during a predetermined calculation time, a scan controller that controls the plurality of beam emitting points to emit the plurality of laser beams successively in the image area with the at least one of the time lags determined by the time lag determination unit. The multibeam scanner of the invention, comprises a photodetector that detects any one of the laser beams at a predetermined beam detection position. The time lag determination unit determines the time lag between the nth laser beam and the n+1th laser beam. Therefore, to scan the image area, the photodetector detects the first laser beam at the predetermined beam detection position, prior to the image area, and determines a scan start time for the first laser beam. Then, the scan controller starts controlling the first beam emitting point at the determined scan start time to emit the first laser beam in the image area. The scan controller further controls the n+1th beam emitting point to emit the nth laser beam in the image area with the calculated time lag between the nth laser beam and the n+1th laser beam.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of Invention  
           [0002]    The invention relates to a multibeam scanner that scans a photosensitive medium with a plurality of laser beams, thereby exposing an image on the photosensitive medium.  
           [0003]    2. Description of Related Art  
           [0004]    A laser beam scanner (a single beam scanner) deflects a laser beam by a deflector, such as a polygon mirror, to a photosensitive medium and forms an image with scanning lines on the photosensitive medium. (Herein, the operation of forming one scanning line is referred to as “scanning operation”.)  
           [0005]    Conventionally, the laser beam scanner comprises a beam detector that detects the laser beam at a beam detection position, before exposing the image based on image data, and outputs a detection signal (called a BD signal). Specifically, the beam detector receives the laser beam from a photodiode, converts the received laser beam to an output voltage, and amplifies the voltage by an amplifier. The beam detector detects the laser beams and outputs the BD signal, only if the amplified voltage is larger than a reference level. After a predetermined time period has elapsed since the beam detector outputs the BD signal, the laser beam scanner determines that a scan start time sets in, and starts to modulate the laser beam based on the image data. As a result, the laser beam scanner can always start the scanning operation from a predetermined scan start position.  
           [0006]    One particular multibeam scanner comprises a plurality of laser diodes for emitting a plurality of laser beams and forms an image by using the plurality of laser beams. This multibeam scanner also comprises a beam detector for detecting each of the laser beams and outputting a BD signal for each laser beam, and therefore sets a scan start time for each laser beam.  
           [0007]    [0007]FIG. 9 shows how the multibeam scanner, having two laser diodes LD 1 , LD 2 , forms beam spots BS 1 , BS 2  on a photosensitive drum  77  with laser beams LB 1 , LB 2 , respectively. Under optic constraints, the laser diodes LD 1 , LD 2  are arranged with a certain space therebetween. As shown in FIG. 9, this space is distance L 1  along a main-scanning direction. Thus, there is a time lag between the laser beams LB 1  and LB 2  until reaching a certain position along the main-scanning direction. The distance L 1  corresponds to the time lag between the laser beams LB 1  and LB 2  until reaching the certain position along the main-scanning direction. In other words, there is the time lag between the scan start times of the laser beams LB 1  and LB 2 . A distance L 2  along a sub-scanning direction corresponds to a distance in resolution. The photodiode, used in the beam detector, requires some operating time before outputting the BD signal. It is thus difficult to successively output the BD signals against both of the laser beams LB 1 , LB 2  that scans with a very short time lag therebetween.  
           [0008]    In order to overcome such a drawback, the conventional multibeam scanner stores, in a memory, the time lag corresponding to the distance L 1 . This time lag has previously been determined and input in the memory during manufacturing. The multibeam scanner detects only the laser beam LB 1 , so that the beam detector outputs the BD signal for the laser beam LB 1 . Then, the multibeam scanner determines the scan start time of the laser beam LB 2  after the time lag from the scan start time of the laser beam LB 1 .  
           [0009]    However, there are always some errors in the dimensions of the polygon mirror, and the placement of laser diodes in every multibeam scanner that uses a polygon mirror and laser diode. Accordingly, the time lag between scan start times of the laser beams varies with multibeam scanners. The time lag thus has to be determined and stored in the memory on each multibeam scanner during manufacturing. Also, it is time and labor consuming to adjust the time lag for every multibeam scanner.  
           [0010]    In addition, the multibeam scanner is not always placed at a constant ambient temperature. The operating temperature (especially, the inner temperature) of the multibeam scanner rises under service conditions, for example, due to heat generated by heat-producing components, such as driving motors and a heater, and heat from laser beam emission. The change in such environmental temperatures may alter the properties of the polygon mirror surfaces (such as a shape and a reflectivity), and thermal expansion or shrinkage in various lenses of the optical system. Even if the time lag is adjusted in the above-explained manner, the temperature change may cause the time lag stored in the memory to deviate from the actual time lag between the scan start times of the laser beams.  
           [0011]    For the above reasons, it is impossible for the conventional multibeam scanner to define the scan start time for each laser beam with high precision, and therefore, impossible to provide excellent print quality.  
         SUMMARY OF THE INVENTION  
         [0012]    In various exemplary embodiments, the invention provides a multibeam scanner of good print quality by correcting a time lag between scan start times of laser beams.  
           [0013]    In various exemplary aspects of a multibeam scanner for scanning an image area, the multibeam scanner comprises a plurality of beam emitting points arranged with predetermined spaces, the plurality of beam emitting points emitting a plurality of laser beams reaching the image area with respective time lags corresponding to the predetermined spaces; a time lag determination unit that determines at least one of the time lags based on the predetermined spaces; and a scan controller that controls the plurality of beam emitting points successively to start emitting the laser beams in the image area at respective scan start times, the scan start times having the determined at least one of the time lags therebetween.  
           [0014]    According to the invention, the multibeam scanner further comprises a photodetector that detects any one of the laser beams at a predetermined beam detection position. The plurality of beam emitting points are controlled to not emit the laser beams between the photodetector and the image area. To scan the image area, the photodetector detects a first laser beam first reaching the image area at the predetermined beam detection position, prior to the image area, and determines the scan start time of the first laser beam. Then, the scan controller starts controlling the first beam emitting point at the determined scan start time to emit the first laser beam in the image area. The scan controller further controls the nth beam emitting point to emit the nth laser beam in the image area with the calculated time lag between scan start times of the n−1th laser beam and the nth laser beam.  
           [0015]    Particularly, the multibeam scanner of the invention comprises a deflector that comprises a plurality of deflection surfaces for deflecting the laser beams to the photodetector and to the image area, and rotates in a predetermined direction. The time lag determination unit controls the plurality of beam emitting points to emit the laser beams successively during a predetermined determination time. The photodetector detects each of the laser beams deflected by one deflection surface after the deflector rotates a predetermined number of turns. The time lag determination unit determines the time lag based on the times of detecting the laser beams during the predetermined determination time.  
           [0016]    According to another exemplary aspect of the invention, there is provided a method for scanning an image area, the method comprising the steps of emitting a plurality of laser beams that reach the image area with respective time lags therebetween; determining at least one of the time lags; and controlling the plurality of laser beams to start emitting in the image area with the determined at least one of the time lags therebetween.  
           [0017]    The multibeam scanner of the invention can start scanning the image area with the plurality of laser beams from the same scan start position by correcting any variations between the scan start times of the laser beam. As a result, the multibeam scanner can provide images with no loss of image quality, as described above.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    The above features and advantages of the invention will become more apparent from the following description of exemplary embodiments taken in connection with the accompanying drawings in which:  
         [0019]    [0019]FIG. 1 is a cross-sectional view of a laser printer according to an exemplary embodiment of the invention, when viewed in a direction perpendicular to a sheet transport direction;  
         [0020]    [0020]FIG. 2 shows a main structure of a multibeam scanner according to the exemplary embodiment of the invention;  
         [0021]    [0021]FIG. 3 is a time chart illustrating times of outputting BD signals, and control signals for switching on and off laser diodes LD 1 , LD 2 ;  
         [0022]    [0022]FIG. 4 schematically shows how a time lag Δt is determined between scan start times of laser beams LB 1 , LB 2  emitted by the laser diodes LD 1 , LD 2 ;  
         [0023]    [0023]FIG. 5 is a flowchart for calculating and storing the time lag Δt for the multibeam scanner according to a first exemplary embodiment of the invention;  
         [0024]    [0024]FIG. 6 is a flowchart for calculating and storing the time lag Δt for the multibeam scanner according to a second exemplary embodiment of the invention;  
         [0025]    [0025]FIG. 7 is a flowchart for calculating and storing the time lag Δt for the multibeam scanner according to a third exemplary embodiment of the invention;  
         [0026]    [0026]FIG. 8 is a flowchart for calculating and storing the time lag Δt for the multibeam scanner according to a fourth exemplary embodiment of the invention; and  
         [0027]    [0027]FIG. 9 schematically shows how laser beams LB 1 , LB 2  form laser spots BS 1 , BS 2  on a photosensitive medium in a multibeam scanner.  
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0028]    An explanation will be given of a multibeam scanner in accordance with the invention based on the following exemplary embodiments. Herein, the exemplary embodiments refer to a multibeam scanner  12  provided in a laser printer  100 .  
         [0029]    Firstly, the overall structure of the laser printer  100  will be described while referring to FIG. 1. FIG. 1 is a cross-sectional view of the laser printer  100  when viewed in a direction perpendicular to a sheet transporting direction. The front surface and the top surface of the laser printer  100  can be seen in directions indicated by arrows X and Y, respectively.  
         [0030]    The laser printer  100  comprises a body frame  11 , a feed unit  19  including a sheet cassette (not shown in figures) in a lower part of the body frame  11 , a transport unit  18 , a developing unit  17  including the photosensitive drum  77 , a transfer roller  87 , a fixing unit  15 , a discharge unit  16 , a sheet stack portion  69  and the laser scanner  12  of the invention, positioned above the developing unit  17 . By the body frame  11 , the overall shape of the laser printer  100  is formed in a generally parallelepiped shape.  
         [0031]    The feed unit  19  holds sheets P in the sheet cassette and feeds the sheet P to the transport unit  18 . The transport unit  18  transports the sheet P from the feed unit  19  to an image transfer position located between the photosensitive drum  77  and the transfer roller  87 . The developing unit  17  has the photosensitive drum  77 , a developing roller  75 , a supply roller  74 , a blade  76  and a charger  78 , and holds therein toner T. Whereupon the charger  78  charges the photosensitive medium  77  uniformly, the laser scanner  12  scans the charged photosensitive drum  77  with two laser beams based on image data, thereby forming a latent image on the photosensitive drum  77 . The supply roller  74  supplies the toner T to the developing toner T, while the blade  76  scrapes excessive toner T off from the developing roller  75  and adjusts the amount of the toner T on the developing roller  75  appropriately. The latent image is developed with the toner T, thereby forming a visible image. The transfer roller  87  transfers the visible image from the photosensitive drum  77  to the sheet P, by sandwiching the sheet P therebetween, at the image transfer position. The transport unit  18  further transports the sheet P with the transferred image to the fixing unit  15 . The fixing unit  15  sandwiches the sheet P between a heat roller  52  and a press roller  54 , and heats and pressurizes the sheet P, so that the toner T is melted and penetrated into fibers of the sheet P. By this, the image is fixed on the sheet P. The discharge unit  16  comprises a discharge roller  55  and subsidiary rollers  56  and  57  that are driven in synchronism with the discharge roller  55 . The discharge unit  16  discharges the sheet P out of the fixing unit  15 , by using the discharge roller  55  and the subsidiary rollers  56  and  57 , into the sheet stack portion  69 .  
         [0032]    Next, the main structure of the multibeam scanner  12  will be explained with reference to FIG. 2. FIG. 2 shows a main structure of the multibeam scanner  12 .  
         [0033]    The multibeam scanner  12  comprises a light emitting device  47 , a polygon mirror  23 , an fθ lens  31 , a beam detector  49  and a controller  9 . The controller  9  has a motor drive circuit  91 , a BD signal detection circuit  92 , a laser drive circuit  93 , a CPU  94 , a RAM  95  and a ROM  96 , for controlling the multibeam scanner  12 . The light emitting device  47  has two laser diodes LD 1 , LD 2  therein. These laser diodes LD 1  and LD 2  emit laser beams LB 1  and LB 2 , respectively, upon receipt of driving signals from the laser drive circuit  93 . The laser beams LB 1 , LB 2  pass through a collimating lens and a cylindrical lens (not shown in figures), and reach the polygon mirror  23 . The polygon mirror  23  rotates in a direction indicated by an arrow R (shown in FIG. 2), driven by a motor  24  upon receipt of driving signals from the motor drive circuit  91 . Thus, the polygon mirror  23  deflects the laser beams LB 1 , LB 2  simultaneously to move at constant angular velocity. The deflected laser beams LB 1  and LB 2  pass through the fθ lens  31 , and move at constant linear velocity in a main-scanning direction (indicated by an arrow MSD in FIG. 2) on the photosensitive drum  77 , thereby forming scanning lines SL 1  and SL 2 , respectively, on the photosensitive drum  77 .  
         [0034]    Before starting scanning the photosensitive drum  77 , the laser diode LD 1  is switched on to emit the laser beam LB 1 . The beam detector  49  detects the laser beam LB 1  and outputs a BD signal to the BD signal detection circuit  92 . Then, the laser diode LD 1  is switched off. After a predetermined time period has elapsed since the BD signal detection circuit  92  detected the BD signal, the laser diode LD 1  is switched on to emit the laser beam LB 1  and to scan the photosensitive drum  77 . The laser diode LD 2  is switched on to emit the laser beam LB 2  and to scan the photosensitive drum  77 , after a time lag has passed since the laser diode LD 1  was switched on.  
         [0035]    The photosensitive drum  77  moves in a direction indicated by an arrow M (shown in FIG. 2), driven by a stepping motor  88  upon receipt of driving signals from another motor drive circuit  97 , in synchronism with scan start times of laser beams LB 1 , LB 2  along the main-scanning direction. A rotary encoder  79  is provided on the side of the photosensitive drum  77 , and rotates in synchronism with the photosensitive drum  77 . The rotary encoder  79  encodes the rotation of the photosensitive drum  77 , and gives a feedback on the encoding to the controller  9 , whereby the controller  9  controls the rotation of the stepping motor  88 . Being moved in the direction M, the entire photosensitive drum  77  is successively scanned with the laser beams LB 1 , LB 2  in a sub-scanning direction (indicated by an arrow SSD in FIG. 2) opposite to the direction M, thereby forming a series of scanning lines SL 1 , SL 2 . As a result, a latent image is formed with the scanning lines on the photosensitive drum  77 .  
         [0036]    The CPU  94  controls the multibeam scanner  12  according to various programs stored in the ROM  96 . The RAM  95  temporarily stores the programs read out from the ROM  96  and process results from the CPU  94 .  
         [0037]    The explanation will hereinafter be given for determining a scan start time of the laser beam LB 2  based on a scan start time of the laser beam LB 1  with reference to FIGS. 3 and 4, according to a first exemplary embodiment. FIG. 3 is a time chart illustrating times for outputting BD signals and control signals for controlling the laser diodes LD 1 , LD 2 . In FIG. 3, when the control signals are “ON”, the laser diodes LD 1 , LD 2  are switched on to emit the laser beams LB 1 , LB 2  to be deflected by the polygon mirror  23  toward the beam detector  49 , and switched off to not emit the laser beams LB 1 , LB 2  on the photosensitive drum  77 . FIG. 4 shows how a time lag Δt is determined between the scan start times of the laser beams LB 1  and LB 2 . In the exemplary embodiment, the polygon mirror  23  rotates in the direction R, and has six reflective surfaces  1  to  6 .  
         [0038]    When the LD 1  control signal is put into the ON state, the laser beam LB 1  is emitted by the laser diode LD 1 , and deflected by the reflective surface  1  of the polygon mirror  23  to the beam detector  49 . The beam detector  49  outputs the BD signal, when detecting the laser beam LB 1  deflected by the reflective surface  1 .  
         [0039]    The BD signal output for the laser beam LB 1  or LB 2  deflected by the reflective surface  1  is a BD 1  signal. The time of outputting the BD 1  signal is referred to as a time T 1 . As is the same, the BD signals output for the laser beam LB (LB 1  or LB 2 ) deflected by the surfaces  2  to  6  are BD 2  to BD 6  signals, respectively. In FIG. 3, numbers  1  to  6  indicated under a heading of the “BD signal” correspond to the BD  1  to BD 6  signals, respectively.  
         [0040]    Successively, the laser beam LB 1  is deflected by the reflective surfaces  2  to  6  of the polygon mirror  23  to the beam detector  49 . The beam detector  49  detects the laser beam LB 1  deflected by the reflective surfaces  2  to  6  and outputs the BD 2  to BD 6  signals, respectively.  
         [0041]    When the polygon mirror  23  is turned one turn, the laser beam LB 1  is again deflected by the reflective surface  1  to the beam detector  49 , and then, the beam detector  49  outputs the BD 1  signal. The time of outputting again the BD 1  signal is referred to as a time T 2 . This time T 2  is determined by, for example, counting a number of BD signals generated after the time T 1  and detecting the count value of “7”. The time period from the time T 1  to the time T 2  (namely, a rotation cycle t of the polygon mirror  23 ) is herein referred to as time period t 1 .  
         [0042]    The LD 1  control signal is put into the OFF state immediately after detecting the time T 2 . The LD 2  control signal is put into the ON state to emit the laser beam LB 2  on the reflective surface  1  of the polygon mirror toward the beam detector  49 . The laser beam LB 2  is then deflected by the reflective surfaces  2  to  6  successively to the beam detector  49 . The beam detector  49  detects the laser beam LB 2  deflected by the reflective surfaces  2  to  6  and outputs the BD 2  to BD 6  signals, respectively.  
         [0043]    When the polygon mirror  23  is turned one more turn, the laser beam LB 2  is again deflected by the reflective surface  1 . The beam detector  49  detects the laser beam LB 2  deflected by the reflective surface  1  and outputs the BD 1  signal. This time of still again outputting the BD 1  signal is referred to as a time T 3 . The time period between the times T 2  and T 3  is referred to as time period t 2 .  
         [0044]    Then, both of the laser diodes LD 1  and LD 2  are activated to emit the laser beams LB 1  and LB 2  simultaneously with the beam detector  49  detecting the laser beams LB 1  and LB 2  continuously. The BD 1  to BD 6  signals are then generated in series, as shown in FIG. 4, each forming a pair of signals on the laser beams LB 1 , LD 2 .  
         [0045]    Although the time period t 1  also equals to the rotation cycle of the polygon mirror  23  in this case, the time period t 2  is longer than the rotation cycle t of the polygon mirror  23  by a certain time difference Δt. It is because the time period t 2  is defined as the time period between the times T 2  and T 3  (namely, the time period from the time of outputting the BD 1  signal for the laser beam LB 1  deflected by the reflective surface  1  to the time of outputting the BD 1  signal against the laser beam LB 2  deflected by the reflective surface  1 ), as described above. This time difference corresponds to the time lag Δt between the scan start times of the laser beams LB 1  and LB 2 . That is, the time lag Δt is determined by subtracting the time period t 1  from the time period t 2 .  
         [0046]    Based on the above-explained concept underlying the invention, the time lag between the scan start time of the laser beams LB 1  and LB 2  can be easily determined for every multibeam scanner, without time- and labor-consuming work during manufacturing, in any environmental condition. This allows the multibeam scanner  12  to start scanning the photosensitive drum  77  from the same scan start position with both of the laser beams LB 1 , LB 2 , by determining the scan start time of the laser beam LB 2  as after the time lag Δt from the scan start time of the laser beam LB 1 . That is, it is possible to correct any variations between the scan start positions of the laser beams LB 1  and LB 2 .  
         [0047]    There may be provided n number of laser diodes in the multibeam scanner  12 , wherein n is an integer greater than 3. The time lag Δtn−1 between the scan start times of the laser diode LDn- 1  and the laser diode LDn can be determined as the same as above. Herein, the beam detector  49  detects the laser beam LBn emitted by the laser diode LDn and reflected by the reflective surface  1 , and outputs the BD 1  signal at the time Tn+1. The time period between the time Tn and the time Tn+1 is referred to as the time period tn.  
         [0048]    The time lag Δtn−1 can determined by subtracting the rotation cycle t of the polygon mirror  23  (namely, the time period t 1 ) from the time period tn. The time lag Δtn−1 should substantially equal to the time lag Δt, as long as n number of laser diodes are spaced uniformly in the multibeam scanner  12 . Thus, the time lag Δtn−1 may be set to the time lag Δt, without calculating the time lag Δtn−1 individually in the above-described manners. In this case, the laser diode LDn is switched on, when the time lag Δtn has lapsed after the laser diode LDn- 1  was switched on.  
         [0049]    In other exemplary embodiments, the time lag Δtn′ between the scan start times of the laser beam LB 1  and the laser beam LBn is determined by subtracting n−1 times rotation cycle t (namely, n−1 times time period t 1 ) from a time period between the time T 2  and the time Tn+1. In this case, the laser diode LDn is switched on, when the time lag Δtn′ has lapsed after the laser diode LD 1  was switched on.  
         [0050]    For example, the multibeam scanner  12  comprises the laser diodes LD 1 , LD 2  and LD 3  for emitting the laser beams LB 1 , LB 2  and LB 3 . To determine the scan start time of the laser beam LB 3 , the beam detector  49  detects the laser beam LB 3  and outputs the BD 1  signal at the time T 4 . The time lag Δt′ between the scan start times of the laser beams LB 1  and LB 3  is determined by subtracting twice the time period t 1  from the time period between the times T 2  and T 4 . Thus, the laser diode LD 3  is switched on, when the time lag Δt′ has lapsed after the laser diode LD 1  was switched on. The determined time lag Δt′ would be substantially equal to twice the time lag between the scan start times of the laser beams LB 1  and LB 2 .  
         [0051]    The control procedure for controlling the scan start time of the laser beam LB 2  will be explained, while referring to FIG. 5, according to a first exemplary embodiment of the invention. FIG. 5 is a flowchart for calculating and storing the time lag Δt.  
         [0052]    Firstly, the CPU  94  judges whether the rotation velocity V of the polygon mirror  23  reaches a predetermined velocity V 1  and is stabilized at V 1  in step S 16 . If Yes in step S 16 , the CPU  94  puts the LD 1  control signal into the ON state in step S 18 . As described above, the LD 1  control signal is for switching on the laser diode LD 1  to emit the laser beam LB 1  deflected by any reflective surface of the polygon mirror  23  toward the beam detector  49 , and for switching off the laser diode LD 1  to not emit the laser beams LB 1  on the photosensitive drum  77 . Upon putting the LD 1  control signal into the ON state, the laser diode LD 1  is switched on to emit the laser beam LB 1 .  
         [0053]    When the laser beam LB 1  is deflected by one reflective surface (referred to as a reference reflective surface) to the beam detector  49 , the beam detector  49  detects the laser beam LB 1  and outputs the BD signal. Then, the CPU  94  receives BD signal output by the beam detector  49  in step S 20 , and begins to measure the time period t 1  by using a timer  98  in step S 22 .  
         [0054]    In step S 24 , the CPU  94  judges whether the polygon mirror  23  has been turned one turn. When the polygon mirror  23  has rotated one turn, the laser beam LB 1  emitted by the laser diode LD 1  is deflected by the reference reflective surface to the beam detector  49 . Then, the beam detector  49  detects the laser beam LB 1  and outputs the BD signal.  
         [0055]    If Yes in step S 24 , the CPU  94  receives the BD signal again. Thus, the CPU  94  finishes to measure and determines the time period t 1 , and stores the determined time period t 1  in the RAM  95  in step S 26 . The LD 1  control signal is put into the OFF state to switch off the laser diode LD 1 , at the same time, the LD 2  control signal is put into the ON state to switch on the laser diode LD 2 . As is the same as above, the LD 2  control signal is for switching on the laser diode LD 2  to emit the laser beam LB 2  deflected by any reflective surface toward the beam detector  49 , and for switching off the laser diode LD 2  to not emit the laser diode on the photosensitive drum  77 . The CPU  94  begins to measure the time period t 2  by the timer  98  in step S 26 .  
         [0056]    Then, the CPU  94  judges, in step S 28 , whether the polygon mirror  23  has been turned one turn. When the polygon mirror  23  has rotated one turn, the laser beam LB 2  emitted by the laser diode LD 2  is deflected by the reference reflective surface to the beam detector  49 . Then, the beam detector  49  detects the laser beam LB 2  and outputs the BD signal.  
         [0057]    If Yes in step S 28 , the CPU  94  receives the BD signal. Thus, the CPU  94  finishes to measure and determines the time period t 2 , and stores the determined time period t 2  in the RAM  95  in step S 30 . The LD 2  control signal is put into the OFF state.  
         [0058]    In step S 32 , the CPU  94  reads the time periods t 1  and t 2  out of the RAM  95 , and determines the time lag Δt by subtracting t 1  from t 2 . The CPU  94  stores the determined time lag Δt in the RAM  95  in step S 34 .  
         [0059]    Afterwards, scanning operations are repeatedly performed based on image data, whereby the laser beams LB 1 , LB 2  form the scanning lines SL 1 , SL 2 . In the scanning operations, the CPU  94  controls the photosensitive drum  77  to start scanning with the laser beam LB 1  after the predetermined time period has passed since the beam detector  49  detected the laser beam LB 1 . Then, the CPU  94  controls the photosensitive drum  77  to start scanning with the laser beam LB 2  after the time lag Δt has elapsed since CPU  94  started scanning the photosensitive medium  77  with the laser beam LB 1 .  
         [0060]    According to the first exemplary embodiment, as described above, the multibeam scanner  12  detects the time periods t 1  and t 2  by switching on the laser diodes LD 1  and LD 2  alternately, and determines the time lag Δt by subtracting t 1  from t 2 . Thus, it is possible to determine the time lag Δt between the scan start times of the laser beams LB 1  and LB 2  with high precision and to define the scan start time of the laser beam LB 2  relative to the scan start time of the laser beam LB 1 . Namely, the multibeam scanner  12  of the invention is capable of correcting any variations between the scan start times of the laser beams LB 1 , LB 2 , and starting scanning the photosensitive drum  77  from the same scan start position with the laser beams LB 1 , LB 2 . Therefore, the multibeam scanner  12  can provide images without a loss of image quality, as described above.  
         [0061]    Next, the control procedure for controlling the scan start time of the laser beam LB 2  will be explained, while referring to FIG. 6, according to a second exemplary embodiment of the invention. According to the second exemplary embodiment, the multibeam scanner  12  determines the time lag Δt every time the laser printer  1  is turned on. Herein, like parts and components are designated by the same reference numerals to avoid duplicating the description.  
         [0062]    [0062]FIG. 6 is a flowchart for calculating and storing the time lag Δt according to the second exemplary embodiment. The description will only be made of the differences between the first and the second exemplary embodiments, as most operation steps of FIG. 6 are the same as those of FIG. 5  
         [0063]    In step S 10 , the CPU  94  detects that a power source of the laser printer  1  is turned on. If Yes in step S 10 , the CPU  94  starts to count time T by using the timer  98  in step S 12 . Then, the CPU  94  judges whether the count value T of the timer reaches a predetermined time Tα in step S 14 . If Yes in step S 14 , the CPU  94  executes steps S 18  to S 34 . These steps S 18  to S 34  are the same as the above-explained steps S 18  to S 34  of the first exemplary embodiment.  
         [0064]    According to the second exemplary embodiment, it is possible for the multibeam scanner  12  of the invention to determine the time lag Δt between the scan start times of the laser beams LB 1  and LB 2  with high precision, every time the laser printer  100  is switched on. This allows correcting variations between the scan start positions of the laser beams LB 1  and LB 2 , appropriate to the temperature condition of its surroundings, and at the same time, avoiding such variations lingering for the next power-up.  
         [0065]    The control procedure for controlling the scan start time of the laser beam LB 2  will be explained, while referring to FIG. 7, according to a third exemplary embodiment of the invention. According to the third exemplary embodiment, the multibeam scanner  12  detects the time lag Δt repeatedly, calculates an average value Δta between the repeatedly detected time lags, and uses the average value Δta for controlling the scan start time of the laser beam LB 2 . Like reference numerals are used throughout to designate like parts and elements.  
         [0066]    [0066]FIG. 7 is a flowchart for calculating and storing the average value Δta between the time lags Δt according to the third exemplary embodiment. The description will be made only of main points of a difference between the first and the third exemplary embodiments, as most operation steps of FIG. 7 are the same to those of FIG. 5.  
         [0067]    The CPU  94  firstly performs steps S 16  to S 34 , thereby determining the time lag Δt and storing the determined time lag Δt in the RAM  95 . These steps S 16  to S 34  are the same as the above-explained steps S 16  to S 34  of the first exemplary embodiment. Then, in step S 36 , the CPU  94  increments a calculation number n, indicating a number for determining the time lag Δt, by 1. In step S 38 , the CPU  94  judges whether the calculation number n reaches a predetermined calculation number n 1 . This calculation number n 1  may be a fixed value that has previously been set, or an input value input by a user. If No in step S 38 , the CPU  94  starts to counts time T by using the timer  98  in step S 40 . Then, the CPU  94  judges whether the count value T of the timer reaches a predetermined time Tβ in step S 42 . If Yes in step S 42 , the CPU  94  executes again steps S 16  to S 34 . If Yes in step S 38 , the CPU  94  calculates the average value Δta among n number of the determined time lag Δt in step S 44 . In step S 46 , the CPU  94  stores the average value Δta in the RAM  95 .  
         [0068]    According to the third exemplary embodiment, it is possible for the multibeam scanner  12  of the invention to determine the time lag Δt between the scan start times of the laser beams LB 1  and LB 2  with high precision, by using the average value Δta of a plurality of time lags. This allows correcting variations between the scan start positions of the laser beams LB 1  and LB 2  with high precision  
         [0069]    The control procedure for controlling the scan start time of the laser beam LB 2  will be explained, while referring to FIG. 8, according to a fourth exemplary embodiment of the invention. According to the fourth exemplary embodiment, the multibeam scanner  12  gives an alarm, when a difference between the determined time lag Δt and a standard time lag Δtb is more than or equal to a predetermined dot scan time period td. Herein, like parts and components are designated by the same reference numerals to avoid duplicating the description.  
         [0070]    [0070]FIG. 8 is a flowchart for calculating and storing the time lags Δt and giving an alarm according to the fourth exemplary embodiment. The description will be made of the differences between the first and the fourth exemplary embodiments, as most operation steps of FIG. 8 are the same to those of FIG. 5.  
         [0071]    The CPU  94  performs steps S 16  and S 34 , thereby determining the time lag Δt and storing the determined time lag Δt in the RAM  95 . These steps S 16  to S 34  are the same to the above-explained steps S 16  to S 34  of the first exemplary embodiment. Then, in step S 48 , the CPU  94  calculates a difference between the time lag Δt and the standard time lag Δtb, and compares the difference (Δt−Δtb) with the dot scan time period td. In other words, the CPU  49  judges whether a variation in the time lag Δt is larger than the dot scan time period td. The standard time lag Δtb and the dot scan time period td have been previously stored in the ROM  96 . The dot scan time period is defined as, for example, a half of a dot clock. One dot clock corresponds to a dot pitch between two dots. If Yes in step S 48 , the CPU  49  gives an alarm in step S 50 , by lighting up an alarm LED (not shown in figures) provided on the laser printer  1 , or indicating an error message on an operation panel (not shown in figures).  
         [0072]    According to the fourth exemplary embodiment, as described above, the multibeam scanner  12  of the invention gives an alarm, when the difference (Δt−Δtb) between the determined time lag Δt and the standard time lag Δtb is more than or equal to the predetermined dot scan time period td. This allows users to confirm the operation status of the multibeam scanner  12  in case a malfunction occurs.  
         [0073]    Further, the multibeam scanner  12  according to any of the above-mentioned exemplary embodiments could determine the time lag Δt between the scan start times of the laser beams LB 1  and LB 2 , while rotating the polygon mirror  23  two or more turns during the time periods t 1  and t 2 . In such a case, the time lag Δt is determined by dividing a time difference (t 2 −t 1 ) by a number of rotations for the polygon mirror  23 .  
         [0074]    Still further, the time lag Δt may be regularly determined at established time intervals after the rotation velocity V of the polygon mirror  23  is stabilized at the predetermined velocity V 1 , or the power source is turned on. The time intervals could be managed on a year, a monthly, a dairy, an hour, a minute, a second or their combination basis. Also, the time intervals could be managed only while switching on the laser printer  100 , or regardless of switching on and off the laser printer  100 , or while performing printing operations.  
         [0075]    Yet further, the time lag Δt may be determined prior to every printing operation, or after performing a plurality of printing operations.  
         [0076]    The laser printer  100  according any of the above-described exemplary embodiments may have a temperature sensor at a predetermined location therein. In this case, the multibeam scanner  12  determines the time lag Δt between the scan start times of the laser beams LB 1  and LB 2 , when the temperature sensor detects a higher temperature than a fixed temperature value.  
         [0077]    Although the invention has been described in detail through the specific exemplary embodiments, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention.  
         [0078]    The multibeam scanner of the invention is not limited for use in the laser printer. The multibeam scanner of the invention will be widely used for, for instance, an image projector.