Patent Publication Number: US-9841699-B2

Title: Optical scanning apparatus and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 13/685,271 filed Nov. 26, 2012, now pending, the contents of which are incorporated by reference as if set forth in full herein; and claims the benefit of Japanese Patent Application Nos. 2011-269394, filed Dec. 8, 2011 and 2012-250587, filed Nov. 14, 2012, which are incorporated by reference herein in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an optical scanning apparatus and an image forming apparatus that uses the optical scanning apparatus. 
     Description of the Related Art 
     An electrophotographic image forming apparatus develops an electrostatic latent image formed on a photosensitive member by a toner and transfers and fixes the developed toner image to a recording material, thereby forming an image on the recording material. To form the electrostatic latent image on the photosensitive member, the image forming apparatus uses an optical scanning apparatus. The optical scanning apparatus includes a laser light source that emits a laser beam, and a deflector such as a rotating polygon mirror that deflects the laser beam emitted by the laser light source so that the laser beam scans the surface of the photosensitive member in a predetermined direction. To control the light power of the laser beam scanning the surface of the photosensitive member to a target light power, the image forming apparatus executes APC (Automatic Power Control). 
     In the APC, the light power of the laser beam emitted by the laser light source is detected using an optical sensor such as a photodiode. A driving current to be supplied to the laser light source is gradually adjusted such that the detected light power of the laser beam reaches the target light power. 
     The APC includes initial APC executed as an initial operation for making preparations for image formation and normal APC executed during image formation. The normal APC is to control the light power of the laser beam during, for example, the period of scanning the surface of the photosensitive member. On the other hand, the initial APC is to perform control to decide the value of the driving current to be supplied to the laser light source in a non-turn-on state as an initial operation when image data is input to the image forming apparatus. 
     Japanese Patent Laid-Open No. 7-171995 describes the initial APC. The light emission amount of a laser light source relative to a supplied driving current changes depending on the temperature of the light-emitting element or the time-rate change of the laser light source. To prevent the laser light source from being damaged by an excessive driving current supplied to it at the time of initial APC, Japanese Patent Laid-Open No. 7-171995 discloses initial APC that increases the driving current to be supplied to the laser light source stepwise from 0, thereby controlling the laser beam to the target light power. 
     However, since the initial APC described in Japanese Patent Laid-Open No. 7-171995 executes the step of increasing the driving current stepwise, a problem is posed that a control time that is relatively long is necessary after the start of the initial APC until the light power of the laser light source stabilizes near the target light power, and image formation can be started. 
     In particular, in a multi-beam system using a plurality of laser light sources, the initial APC is performed first for a specific laser light source to be used to generate a synchronization signal (to be referred to as a BD signal hereinafter) to define the image write position. After the light power has approached the target light power, the APC is started for the remaining laser light sources. For the remaining laser light sources, the APC needs to be performed at a timing so as not to cause the laser beam deflected by a polygon mirror to expose the photosensitive member. To detect such a timing, the light power of the laser beam to be used to generate the BD signal needs to be adjusted to a light power that allows BD signal generation. That is, after the initial APC has been performed for the specific laser light source, the initial APC is performed for the remaining laser light sources. Hence, the time after the light power has been made to approach the target light power by the initial APC until image formation can be started for all laser light sources including the specific laser light source and the remaining laser light sources further prolongs as compared to the case in which a single laser light source is used. Hence, there is deemed necessary a technique of shortening the time after the start of initial APC until the light power of the laser light source approaches the target light power. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in consideration of the above-described problem. The present invention provides a technique of enabling the light power of a laser light source to approach a target light power in a short time after turning on the laser light source when executing APC in an optical scanning apparatus. 
     According to a first aspect of the present invention, there is provided an optical scanning apparatus, for scanning a photosensitive member with a light beam, comprising: a light source configured to output the light beam having a light power dependent on a value of a driving current; a detection unit configured to detect the light power of the light beam output from the light source; a voltage holding unit configured to hold a voltage; a charging unit configured to charge the voltage holding unit; and a control unit configured to control the charging unit so that the voltage holding unit is charged by the charging unit and configured to control the value of the driving current, wherein the control unit controls the charging unit so that the voltage holding unit is charged by the charging unit in a state where the driving current is not supplied to the light source, controls the charging unit based on a detection result of the detection unit so that the voltage held in the voltage holding unit is controlled from the voltage of the voltage holding unit charged in the state where the driving current is not supplied to the light source, and controls the value of the driving current based on the voltage held in the voltage holding unit controlled by the control unit. 
     According to a second aspect of the present invention, there is provided an image forming apparatus, comprising: a photosensitive member; a charger that charges the photosensitive member; an optical scanning apparatus configured to that scan the photosensitive member with a light beam output from a light source when a driving current modulated based on image information is supplied to the light source; a developer configured to develop an electrostatic latent image formed on the photosensitive member by scanning of the light beam by the optical scanning apparatus to form an image on the photosensitive member, and a control unit configured to control the optical scanning apparatus, wherein the optical scanning apparatus comprises: the light source configured to output the light beam having a light power dependent on a value of the driving current; a detection unit configured to detect the light power of the light beam output from the light source; a voltage holding unit configured to hold a voltage; and a charging unit configured to charge the voltage holding unit, wherein the control unit controls the charging unit so that the voltage holding unit is charged by the charging unit and controls the value of the driving current, and wherein the control unit controls the charging unit so that the voltage holding unit is charged by the charging unit in a state where the driving current is not supplied to the light source, controls the charging unit based on a detection result of the detection unit so that the voltage held in the voltage holding unit is controlled from the voltage of the voltage holding unit charged in the state where the driving current is not supplied to the light source, and controls the value of the driving current based on the voltage held in the voltage holding unit controlled by the control unit. 
     According to the present invention, it is possible to provide a technique of enabling the light power of a light source to approach a target light power in a short time after turning on the light source when executing APC in an optical scanning apparatus. 
     Further features of the present invention will become apparent from the following description of embodiments (with reference to the attached drawings). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic sectional view of an image forming apparatus  100  according to the embodiment of the present invention; 
         FIG. 2  is a view showing the arrangement of an exposure controller  10  according to the embodiment of the present invention and the connection relationship between the exposure controller  10  and a sequence controller  47 ; 
         FIG. 3A  is a block diagram showing the arrangement of a laser driving device  31  according to the embodiment of the present invention; 
         FIG. 3B  is a block diagram showing the arrangement of an APC circuit  403  according to the embodiment of the present invention; 
         FIG. 4  is a timing chart showing the light emission sequence of the laser driving device  31  according to the embodiment of the present invention; 
         FIG. 5  is a timing chart showing the relationship between an input voltage and an output voltage Vsh of a hold capacitor  505  according to the embodiment of the present invention; 
         FIG. 6  is a flowchart showing the procedure of an APC operation executed for the laser driving device  31  according to the embodiment of the present invention; and 
         FIG. 7  is a timing chart showing a comparative example of the light emission sequence of the laser driving device  31 . 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. It should be noted that the following embodiments are not intended to limit the scope of the appended claims, and that not all the combinations of features described in the embodiments are necessarily essential to the solving means of the present invention. Each of the embodiments of the present invention described below can be implemented solely or as a combination of a plurality of the embodiments or features thereof where necessary or where the combination of elements or features from individual embodiments in a single embodiment is beneficial. 
     &lt;Arrangement of Image Forming Apparatus  100 &gt; 
     The basic operation of an optical scanning apparatus and an image forming apparatus according to an embodiment will be described first with reference to  FIG. 1 .  FIG. 1  is a schematic sectional view of an image forming apparatus  100  according to this embodiment. 
     In the image forming apparatus  100 , documents stacked on a document feeder  1  are sequentially conveyed onto the surface of a platen glass  2  one by one. When the document is conveyed onto the surface of the platen glass  2 , a lamp unit  3  of a reading unit  4  is turned on, and the reading unit  4  irradiates the document with light while moving in the direction of an arrow  110 . The light reflected by the document passes through a lens  8  via mirrors  5 ,  6 , and  7  and is then input to an image sensor unit  9  and converted into an image signal. The image signal output from the image sensor unit  9  is temporarily stored in an image memory (not shown). After that, the image signal is read out from the image memory and input to an exposure controller  10 . 
     The exposure controller  10  causes a laser light source to be described later to emit a laser beam (light beam) to expose the surface of a photosensitive member  11  (for example, photosensitive drum) based on the input image signal (image information). The photosensitive member  11  is scanned by the laser beam emitted by the laser light source. When the photosensitive member  11  is scanned by the laser beam, an electrostatic latent image is formed on its surface. A potential sensor  30  detects the surface potential of the photosensitive member  11  and simultaneously monitors whether the surface potential has a desired value. A developer  13  develops the electrostatic latent image formed on the surface of the photosensitive member  11  by a toner. A transfer unit  16  transfers the toner image developed by the developer  13  to the surface of a recording material. 
     The recording material to which the toner image is to be transferred by the transfer unit  16  is fed and conveyed from a recording material stacking unit  14  or  15  in synchronization with a timing at which the toner image reaches the transfer unit  16 . The recording material to which the toner image has been transferred by the transfer unit  16  is conveyed to a fixing unit  17 . The fixing unit  17  fixes the toner image on the surface of the recording material. After the fixing processing by the fixing unit  17 , the recording material is discharged from a discharge unit  18  to the outside of the image forming apparatus  100 . 
     After the transfer by the transfer unit  16  has been done, a cleaner  25  collects the toner remaining on the surface of the photosensitive member  11 , thereby cleaning the surface of the photosensitive member  11 . Next, an auxiliary charger  26  removes charges from the surface of the photosensitive member  11  so that the photosensitive member  11  can obtain a satisfactory charge characteristic upon charging by a primary charger  28  at the next time of image formation. In addition, after the residual charges on the surface of the photosensitive member  11  are removed by a pre-exposure lamp  27 , the primary charger  28  charges the surface of the photosensitive member  11 . The image forming apparatus  100  executes image formation for a plurality of recording materials by repeating the above-described processing. 
     &lt;Arrangement of Exposure Controller  10 &gt; 
       FIG. 2  is a view showing the schematic arrangement of the exposure controller  10  according to this embodiment and connection between the exposure controller  10  and a sequence controller  47 . The sequence controller  47  includes a CPU (not shown), and the CPU controls the exposure controller  10  and the photosensitive member  11 . As shown in  FIG. 2 , the exposure controller  10  includes a laser driving device  31 , a collimator lens  35 , a stop  32 , a polygon mirror  33 , an f-θ lens  34 , and a BD (Beam Detect) sensor  36 . The laser driving device  31  includes a semiconductor laser (laser diode (LD))  43  including a plurality of light-emitting points for emitting laser beams, and one photodiode (PD). 
     The operation of the exposure controller  10  based on the control of the sequence controller  47  will be described next. The sequence controller  47  included in the image forming apparatus  100  controls the laser driving device  31  using a control signal S 47  output to the laser driving device  31 . When image formation starts, the sequence controller  47  controls each light-emitting point of the semiconductor laser  43  to a turn-on state or a turn-off state based on the control signal S 47 . Each laser beam emitted by the semiconductor laser  43  is converted into a substantially collimated light beam via the collimator lens  35  and the stop  32 , and then enters the polygon mirror  33  in a predetermined spot diameter. 
     The polygon mirror  33  has a plurality of mirror surfaces and rotates in the direction of an arrow  201  at a uniform angular velocity. Along with the rotation in the direction of the arrow  201 , the polygon mirror  33  reflects each laser beam so that the laser beams that have entered are deflected at continuous angles. Each laser beam deflected by the polygon mirror  33  enters the f-θ lens  34 . The f-θ lens  34  applies a condenser effect to the plurality of laser beams that have entered, and corrects distortion to guarantee temporal linearity when the plurality of laser beams scan the surface of the photosensitive member  11 . The plurality of laser beams scan the surface of the photosensitive member  11  in the direction of an arrow  202  at a uniform velocity. 
     The BD sensor  36  is a sensor used to detect a laser beam reflected by the polygon mirror  33 . The BD sensor  36  detects a laser beam emitted by a specific light-emitting point out of the laser beams reflected by the mirror surfaces of the polygon mirror  33 . That is, the sequence controller  47  controls the specific light-emitting point so that the laser beam emitted by the specific light-emitting point scans the BD sensor  36 . Upon detecting the laser beam, the BD sensor  36  outputs a synchronization signal (BD signal) S 36  indicating the detection of the laser beam to the sequence controller  47 . The sequence controller  47  controls the turn-on timing of each light-emitting point based on image data using the BD signal S 36  as a reference. 
     The sequence controller  47  monitors the period of output of the BD signal S 36  from the BD sensor  36 , thereby monitoring the period of laser beam detection by the BD sensor  36 . In addition, the sequence controller  47  controls to accelerate or decelerate a polygon mirror driver (not shown) for driving the polygon mirror  33  such that the period of one rotation of the polygon mirror  33  is always constant. By this control, the sequence controller  47  sets the polygon mirror  33  in a stable rotation state. 
     &lt;Arrangement of Laser Driving Device  31 &gt; 
     The arrangements of the laser driving device  31  and an APC circuit  403  (APC circuits  403 - 1  to  403 - n ) included in the laser driving device  31  will be described next with reference to  FIGS. 3A and 3B . The arrangement of the laser driving device  31  will be described first with reference to  FIG. 3A . 
     The laser driving device  31  includes the semiconductor laser  43 . The semiconductor laser  43  includes a plurality of (n) light-emitting points (LD 1  to LDn) and one photodiode (PD). The laser driving device  31  is also provided with the plurality of APC circuits  403 - 1  to  403 - n  in correspondence with the plurality of light-emitting points (LD 1  to LDn). 
     The PD in the semiconductor laser  43  detects a laser beam from each of the LD 1  to LDn, and outputs a current Im corresponding to the detected light power to a current/voltage converter  401 . The current/voltage converter  401  converts the received current Im into a voltage and outputs it. An amplifier  402  is used to adjust the gain of the voltage output from the current/voltage converter  401 . That is, the amplifier  402  adjusts the gain of the output from the PD that has detected the laser beam from each of the LD 1  to LDn. The voltage that has undergone the gain adjustment by the amplifier  402  is supplied from the amplifier  402  to the APC circuit  403  as a light power monitor voltage Vpd. Note that the PD, the current/voltage converter  401 , and the amplifier  402  in the semiconductor laser  43  are provided to detect the light power of a laser beam output from each light-emitting point. 
     The laser driving device  31  is controlled by the sequence controller  47  based on various kinds of control signals included in the control signal S 47  output from the sequence controller  47 , as described above. The control signal S 47  includes, for example, a full turn-on signal FULL to be supplied to a logical element  412 , a control signal OFF_LD to be supplied to switches  408 - 1  to  408 - n , and control signals OFF_APC* (OFF_APC*- 1  to OFF_APC*-n) and sample hold signals S/H* (S/H*- 1  to S/H*-n) to be supplied to the APC circuits  403 - 1  to  403 - n . The control signal S 47  also includes a light power control signal to be output to a current controller  506  to be described later. 
     The control signal S 47  (the control signals OFF_APC* and the sample hold signals S/H*) from the sequence controller  47  is input to the APC circuits  403 - 1  to  403 - n . In addition to the control signal S 47 , a reference voltage Vref from the sequence controller  47  is input to the APC circuits  403 - 1  to  403 - n  via digital/analog conversion (D/A) circuits  417 - 1  to  417 - n . The D/A circuits  417 - 1  to  417 - n  convert a digital value representing the reference voltage Vref input from the sequence controller  47  into an analog value and input it to the APC circuits  403 - 1  to  403 - n  as the reference voltage Vref, respectively. Under the control of the sequence controller  47 , each of the APC circuits  403 - 1  to  403 - n  performs control to adjust the light power of a corresponding one of LDs (LD 1  to LDn) so as to cause the plurality of LDs (LD 1  to LDn) to emit light of a predetermined light power. Each of the APC circuits  403 - 1  to  403 - n  executes light power control of a corresponding LD based on the reference voltage Vref in accordance with the control signal S 47  from the sequence controller  47 . 
     A modulator  413  outputs, to the logical element  412 , an image modulation signal to be used to modulate driving currents to be supplied to the LD 1  to LDn using an image signal (image information) input from an image signal generation unit (not shown) or the like. For example, to perform PWM (pulse width modulation) of a driving current, the modulator  413  outputs a pulse signal having a width corresponding to image data to the logical element  412  as an image modulation signal. The logical element  412  outputs, to switches  409 - 1  to  409 - n , a signal representing the OR (logical addition) of the image modulation signal output from the modulator  413  and the full turn-on signal FULL output from the sequence controller  47 . 
     As shown in  FIG. 3A , the laser driving device  31  includes current sources  404 - 1  to  404 - n  and  407 - 1  to  407 - n  for supplying (applying) driving currents to the LD 1  to LDn in the semiconductor laser  43 . The laser driving device  31  also includes the switches  408 - 1  to  408 - n  and  409 - 1  to  409 - n  that switch the current supply states from the current sources to the LD 1  to LDn. For example, the driving current for the LD 1  is supplied from the current sources  404 - 1  and  407 - 1 , and the supply state is switched by the switches  408 - 1  and  409 - 1 . The operations of the current sources  404 - 1  and  407 - 1  and the switches  408 - 1  and  409 - 1  corresponding to the LD 1  out of the LD 1  to LDn will mainly be described below. The description of LD 1  also applies to the remaining lasers LD 2  to LDn. 
     The switching current source  404 - 1  and the bias current source  407 - 1  for supplying a driving current to the LD 1  are connected in parallel between the power supply and the LD 1 . 
     The bias current source  407 - 1  supplies a bias current to the LD 1 . The bias current is a current supplied to the LD 1  to cause it to emit a laser beam of a light power that does not change the potential on the photosensitive member  11 . When the switch  408 - 1  is turned on, the bias current source  407 - 1  supplies the bias current to the LD 1 . In a case in which the bias current is supplied to the LD 1 , the time until the light power reaches the target light power when supplying a switching current to be described below to the LD 1  can be shortened as compared to a case in which no bias current is supplied to the LD 1 . That is, supplying the bias current to the LD 1  enables to improve the light emission responsibility of the LD 1  when the switching current is supplied. In this embodiment, a laser driving device for supplying a bias current having a predetermined value to the LD 1  will be exemplified for the sake of descriptive simplicity. 
     The switching current source  404 - 1  supplies the switching current to the LD 1 . The switching current is a current supplied to the LD 1  to cause it to emit a laser beam of a light power that changes the potential on the photosensitive member, and is supplied to the LD 1  while being superimposed on the above-described bias current. 
     The APC circuit  403 - 1  controls the value of the current to be supplied from the switching current source  404 - 1  to the LD 1  by a current control signal Isw- 1  output to the switching current source  404 - 1 . The switching current source  404 - 1  supplies a switching current corresponding to the current control signal Isw- 1  given by the APC circuit  403 - 1  to the LD 1  as a driving current. The switch  409 - 1  is connected between the LD 1  and the switching current source  404 - 1 . For this reason, driving current supply from the switching current source  404 - 1  to the LD 1  is set to the on/off state in accordance with the on/off state of the switch  409 - 1 . 
     The switch  408 - 1  is connected to the path from the switching current source  404 - 1  and the bias current source  407 - 1  to the LD 1 . The sequence controller  47  controls the switch  408 - 1  between the on and off states using the signal OFF_LD output to the switch  408 - 1 . In this embodiment, if the signal OFF_LD output from the sequence controller  47  is in the high state (“H”), the switch  408 - 1  is turned off, and in the low state (“L”), the switch  408 - 1  is turned on. If the switch  408 - 1  is in the on state, the switching current source  404 - 1  and the bias current source  407 - 1  supply the currents to the LD 1 . On the other hand, if the switch  408 - 1  is in the off state, current supply from the switching current source  404 - 1  and the bias current source  407 - 1  to the LD 1  is cut off. 
     When the switch  408 - 1  is in the on state, and the switch  409 - 1  is in the off state, the switching current is not supplied from the switching current source  404 - 1  to the LD 1 , and the bias current is supplied from the bias current source  407 - 1  to the LD 1 . Note that the switch  409 - 1  is controlled to the on or off state based on a signal supplied from the modulator  413  via the logical element  412 . 
     When the switch  408 - 1  is in the on state, and the switch  409 - 1  is in the on state, the bias current from the bias current source  407 - 1  and the switching current from the switching current source  404 - 1  are supplied to the LD 1  as the driving current. In this case, the LD 1  outputs, to the surface of the photosensitive member  11 , a laser beam of a light power necessary for forming an electrostatic latent image on the surface. 
     &lt;Arrangement of APC Circuit  403  ( 403 - 1  to  403 - n )&gt; 
     The arrangement of the APC circuits  403 - 1  to  403 - n  included in the laser driving device  31  will be described next with reference to  FIG. 3B . Each of the APC circuits  403 - 1  to  403 - n  performs APC for a corresponding one of the LDs (LD 1  to LDn). For the sake of descriptive simplicity, APC by the APC circuit  403 - 1  for the LD 1  will only be explained below. For the remaining lasers (LD 2  to LDn) as well, the APC can be implemented by performing the same control as that of the LD 1 . Since all the APC circuits  403 - 1  to  403 - n  have the same arrangement, the APC circuits  403 - 1  to  403 - n  will be referred to as the APC circuit  403  hereinafter. 
     As described above, the reference voltage Vref corresponding to the target light power of the LD 1  and the light power monitor voltage Vpd output from the amplifier  402  are input to the APC circuit  403 . In addition, out of the control signal S 47  output from the sequence controller  47 , the control signal OFF_APC* and the sample hold signal S/H* are output to the APC circuit  403 . In the APC circuit  403 , the reference voltage Vref is supplied to an analog switch  501  and the current controller  506 . The control signal OFF_APC* is supplied to the analog switch  501  and a logical element  502 . The sample hold signal S/H* is supplied to the logical element  502 . 
     The light power monitor voltage Vpd and the reference voltage Vref are input to the input side of the analog switch  501 . One of the light power monitor voltage Vpd and the reference voltage Vref is output from the output side of the analog switch  501  as an output voltage Vpd 2  based on the control signal OFF_APC* from the sequence controller  47 . More specifically, if the control signal OFF_APC* is “H”, the analog switch  501  outputs the light power monitor voltage Vpd as the output voltage Vpd 2 . If the control signal OFF_APC* is “L”, the analog switch  501  outputs the reference voltage Vref as the output voltage Vpd 2 . 
     The logical element  502  is an element that outputs a signal generated by obtaining a signal representing the AND (logical product) of the received control signal OFF_APC* and sample hold signal S/H* and inverting the logic of the obtained signal (H→L or L→H), and corresponds to a NAND circuit. The signal output from the logical element  502  is supplied to an analog switch  504  as a control signal SEL. 
     The analog switch  504  functions as a sample hold circuit. The output voltage Vpd 2  of the analog switch  501  is applied to the input side of the analog switch  504  via a resistive element  503 . The analog switch  504  switches between a sample state and a hold state by switching based on the control signal SEL supplied from the logical element  502  whether to output, from the output side, the voltage input from the input side. 
     More specifically, if the control signal SEL is “H”, the output-side terminal and the input-side terminal connected to the output-side terminal of the analog switch  501  are connected in the analog switch  504 . The analog switch  504  thus outputs, from the output side, the voltage applied from the analog switch  501  to the input side via the resistive element  503 . On the other hand, if the control signal SEL is “L”, the analog switch  504  opens the input side (the input-side terminal on the unconnected side is connected to the output-side terminal). 
     When the control signal SEL is “H”, the output voltage Vpd 2  of the analog switch  501  is applied to a hold capacitor  505  via the resistive element  503 . The hold capacitor  505  is charged by a predetermined time constant τ when the voltage Vpd 2  is applied to it. The hold capacitor  505  changes the voltage in accordance with the amount of charges accumulated by charging. In the turn-on state in which the LD 1  is on, the hold capacitor  505  outputs a voltage corresponding to the light power monitor voltage Vpd. When the control signal SEL switches to “L”, the input side of the analog switch  504  is opened, and as a result, the voltage of the charged hold capacitor  505  is held. 
     As described above, the analog switch  504  and the hold capacitor  505  are set in the sample state when the control signal SEL is “H”, or in the hold state when “L”. A voltage Vsh of the charged hold capacitor  505  is input to the current controller  506 . Note that the time constant τ when charging the hold capacitor  505  is defined as τ=RC depending on a resistance value R of the resistive element  503  and a capacitance C of the hold capacitor  505 . When executing the APC, the hold capacitor  505  in the sample state is charged to a predetermined voltage Vt in the turn-off state in which the LD 1  is off, or charged to the light power monitor voltage Vpd in the turn-on state in which the LD 1  is on, as will be described later. 
     When the hold capacitor  505  is in the sample state, one of the reference voltage Vref and the light power monitor voltage Vpd corresponding to the light power detected by the PD in the semiconductor laser  43  is applied to the hold capacitor  505  in accordance with switching by the analog switch  501 . That is, in this embodiment, the analog switch  501  functions as a switch for selectively applying one of the reference voltage Vref and the light power monitor voltage Vpd to the hold capacitor  505 . The resistive element  503  functions as a resistive element connected between the switch and the hold capacitor  505 . Additionally, in this embodiment, the analog switch  501 , the resistive element  503 , and the analog switch  504  function as a charging unit. 
     The current controller  506  decides the value of the switching current Isw based on the received reference voltage Vref and the voltage Vsh of the hold capacitor  505 . The current controller  506  outputs the current control signal Isw corresponding to the decided value of the switching current Isw to the switching current source  404  ( 404 - 1  to  404 - n ). More specifically, when the LD 1  changes from the turn-off state to the turn-on state, and optical scanning of the photosensitive member  11  by the laser beam output from the LD 1  starts, the APC circuit  403  controls the voltage of the hold capacitor  505  in the following way. That is, the APC circuit  403  controls the driving current to be supplied from the switching current source  404 - 1  to the LD 1  using the predetermined voltage Vt generated in the turn-off state as the initial value, thereby controlling the voltage of the hold capacitor  505 . The current controller  506  designates the driving current to be supplied from the switching current source  404 - 1  to the LD 1  by outputting the decided switching current value Isw (Isw- 1 ) to the switching current source  404 - 1 . 
     As described above, the hold capacitor  505  functions as a charge accumulation unit which causes the laser light source (LD) to output a laser beam of a light power corresponding to the accumulated charge amount. That is, the hold capacitor  505  functions as a voltage holding unit which outputs a voltage corresponding to the accumulated charge amount. The current controller  506  and the switching current source  404 - 1  function as a current supply unit which supplies a driving current corresponding to the voltage of the charge accumulation unit (hold capacitor  505 ) to the laser light source (LD) when optical scanning of the photosensitive member  11  starts. The current controller  506  also functions as a control unit which controls the voltage of the charge accumulation unit (hold capacitor  505 ). 
     &lt;Comparative Example of APC in Laser Driving Device  31 &gt; 
     A comparative example of APC in the laser driving device  31  according to this embodiment will be described next with reference to  FIG. 7 . For the sake of descriptive simplicity, APC by the APC circuit  403  (APC circuit  403 - 1 ) for the LD 1  will only be explained below. For the remaining lasers (LD 2  to LDn) as well, the APC can be implemented by performing the same control as that of the LD 1 . 
     When executing APC for an LD included in the laser driving device  31 , if the light power of the LD is controlled after turning on the LD in the turn-off state, a considerable time may be necessary until the light power sufficiently approaches the target light power.  FIG. 7  shows an example of the light emission sequence of the laser driving device  31  as a comparative example to the embodiment to be described below. In  FIG. 7 , an operation mode including APC to be performed before the image forming apparatus  100  starts image formation will be referred to as an “initial APC mode”, and an operation mode including APC to be performed after image formation will be referred to as a “normal APC mode”.  FIG. 7  shows the light emission sequence for two LDs (LD 1  and LD 2 ) out of the LDs included in the laser driving device  31 . The LD 1  is an LD used to detect a BD signal and is assumed to be an LD for which the APC is executed first out of the plurality of LDs. 
     Referring to  FIG. 7 , first, to start the APC of the initial APC mode, the sequence controller  47  switches the full turn-on signal FULL of the LD 1  from “L” to “H” to turn on the LD 1 . In addition, the sequence controller  47  switches the sample hold signal S/H* (S/H*- 1 ) of the LD 1  from “L” to “H” to shift to a state to sample the light power of the LD 1  detected by the PD. In this state, the detected light power of the LD 1  gradually increases. This is because the sequence controller  47  controls the driving current to be supplied to the LD 1  such that the detected light power of the LD 1  approaches the target light power. 
     More specifically, the light power monitor voltage Vpd corresponding to the light power of the LD 1  detected by the PD in the semiconductor laser  43  is input to the APC circuit  403 . If the APC circuit  403  is in the sample state, the hold capacitor  505  is charged to the light power monitor voltage Vpd. The current controller  506  compares the light power monitor voltage Vpd generated in the hold capacitor  505  with the reference voltage Vref corresponding to the target light power. In addition, the current controller  506  decides the value of the switching current Isw based on the comparison result such that the light power monitor voltage Vpd approaches the reference voltage Vref. The value of the switching current Isw is output from the APC circuit  403  to the switching current source  404 - 1  as a current control signal (Isw- 1 ). The switching current source  404 - 1  supplies the switching current Isw having a value corresponding to the current control signal (Isw- 1 ) to the LD 1 . During the sample state, the APC circuit  403  continuously controls the switching current value Isw based on the light power monitor voltage Vpd and the reference voltage Vref. The sequence controller  47  thus controls the light power of the LD 1  to the target light power using the APC circuit  403 . 
     When the light power of the LD 1  has sufficiently approached the target light power, and it has become possible to stably detect the BD signal, the sequence controller  47  ends the initial APC mode and shifts to the normal APC mode. When APC of the normal APC mode starts, the sequence controller  47  sets the LD 1  in a full turn-on state for a predetermined period Ts and samples the light power every time a BD signal is detected (in every scanning). The sequence controller  47  thus executes the APC by controlling the driving current to the LD 1  such that the light power of the LD 1  approaches the target light power, as in the above-described initial APC mode. The light power of the LD 1  has been made to sufficiently approach the target light power by the APC of the initial APC mode. Hence, in the APC of the normal APC mode executed after the initial APC mode, the light power of the LD 1  can be made to reach the target light power by several times of APC executed every time a BD signal is detected. 
     In the APC of the initial APC mode described above, however, the driving current of, for example, the LD 1  is gradually increased from 0, thereby gradually making the light power of the LD 1  approach the target light power. For this reason, a relatively long time T 1  is necessary until the light power of the LD 1  sufficiently approaches the target light power and it becomes possible to stably detect the BD signal, as shown in  FIG. 7 . 
     In addition, a longer time is necessary for the LD 2  after the driving current is supplied to turn on the LD 2  until its light power sufficiently approaches the target light power. As shown in  FIG. 7 , after the shift from the initial APC mode to the normal APC mode, the sequence controller  47  switches the full turn-on signal FULL of the LD 2  from “L” to “H” to turn on the LD 2 . In addition, the sequence controller  47  switches the sample hold signal S/H* from “L” to “H” to sample the light power of the LD 2 , and performs control to make the light power of the LD 2  approach the target light power, thereby performing the APC of the LD 2 . After that, light power control of the LD 2  is repetitively performed next to the light power control of the LD 1  at a period Tb of BD signal detection. 
     In this manner, after the APC of the initial APC mode for the LD 1  has ended, the APC for the LD 2  is performed in the normal APC mode by performing control to make the light power of the LD 2  gradually approach the target light power from the turn-off state. For this reason, the time until the light power of the LD 2  reaches the target light power is longer than that of the LD 1 . Hence, in the image forming apparatus of the multi-beam system that exposes the photosensitive member by laser beams emitted by a plurality of LDs, the time until the light powers of all of the plurality of LDs are controlled to the target light power by the APC (initial APC mode and normal APC mode) becomes longer as a whole. For example, a time T 2  necessary after the light power control of the LD 2  has started until the light power reaches the target light power is approximately Tb×T 1 /Ts. For example, assume that T 1 =10 [ms], Ts=10 [μs], and Tb=500 [μs]. In this case, T 2 =500 [ms]. In the image forming apparatus of the multi-beam system, when the number of LDs increases, the time until the light powers of all LDs reach the target light power prolongs in proportional to the number of LDs. 
     The image forming apparatus according to this embodiment, when executing APC of the initial APC mode for the laser driving device  31 , enables light power control to starts from a light power close to the target light power in order to make the light power of the LD approach the target light power in a short time after turning on the LD. More specifically, the hold capacitor that holds the voltage used to cause the LD to output a laser beam is charged in advance to a predetermined voltage close to the reference voltage for the target light power during the turn-off state (before turning on) of the LD before the start of optical scanning of the photosensitive member  11 . That is, charges in a predetermined amount corresponding to the predetermined voltage close to the reference voltage for the target light power are accumulated in the hold capacitor during the turn-off state of the LD before the start of optical scanning. The voltage of the hold capacitor is used to decide the driving current to be supplied to the LD based on the result of comparison with the reference voltage. In this embodiment, since the hold capacitor has been charged in advance to the voltage close to the reference voltage when turning on the LD and starting the light power control of the LD, the LD can be turned on in a light power close to the target light power at the start of APC of the initial APC mode. This allows the light power of the LD to reach the target light power in a short time by the APC of the initial APC mode and the normal APC mode. 
     This embodiment assumes an image forming apparatus of the multi-beam system. In the image forming apparatus of the multi-beam system, for each of the LDs, charges in a predetermined amount are accumulated in a corresponding hold capacitor during the turn-off state before the start of optical scanning, thereby charging the hold capacitor to a predetermined voltage. This allows all LDs to make the light power reach the target light power in a short time by the APC after turn on. Processing executed for the laser driving device  31  in this embodiment will be described below in more detail. 
     &lt;APC in Laser Driving Device  31 &gt; 
     APC in the laser driving device  31  according to this embodiment will be described next with reference to  FIG. 4 . For the sake of descriptive simplicity, APC by the APC circuit  403  (APC circuit  403 - 1 ) for the LD 1  will only be explained below. For the remaining lasers (LD 2  to LDn) as well, the APC can be implemented by performing the same control as that of the LD 1 . 
     In the image forming apparatus  100 , the APC executed for light power control of each of the LD 1  to LDn is divided into APC of the initial APC mode and APC of the normal APC mode, as described above. The initial APC mode is an operation mode including APC to be performed as a preparation operation before the image forming apparatus  100  starts image formation. In the APC of the initial APC mode, control is performed from a complete turn-off state of each LD such that the light power of the laser beam emitted by each LD approaches the target light power. The normal APC mode is an operation mode including APC to be performed after the start of image formation. In the APC of the normal APC mode, the light power of the laser beam emitted by each LD to expose the photosensitive member  11  is controlled to the target light power. 
     The initial APC mode of this embodiment includes an initial charging operation of charging the hold capacitor  505  to the predetermined voltage Vt in the turn-off state in which each LD is off before the start of driving current supply to each LD. The initial charging operation need only be executed, for example, at the time of activation of the image forming apparatus  100  or at the time of a preparation operation before the start of formation of an image to be transferred to a recording material. Assume here that the initial charging operation is executed at the time of a preparation operation of the image forming apparatus  100 . 
     In the initial APC mode of this embodiment, the APC to control the light power of each LD to a light power near a predetermined target light power is executed in the turn-on state in which each LD is on, after the initial charging operation has ended and driving current supply to each LD has started. In this APC, when supply of the driving current (switching current) to each LD starts, the hold capacitor  505  is charged from the voltage Vt to the light power monitor voltage Vpd corresponding to the light power detected by the PD. In addition, the switching current is controlled based on the result of comparison between the reference voltage Vref and the light power monitor voltage Vpd generated in the hold capacitor  505 . In this APC, the light power monitor voltage Vpd is controlled to approach the reference voltage Vref from not voltage=0 but the voltage Vt close to the reference voltage Vref corresponding to the target light power, as will be described later. That is, control of the driving current (light power) based on the light power monitor voltage Vpd (corresponding to the light power of each LD) is started from the voltage Vt close to the reference voltage Vref, thereby controlling the light power of each LD to the target light power in a shorter time. After that, when the image forming apparatus  100  has started image formation, the initial APC mode changes to the normal APC mode, and APC of the normal APC mode is executed at a predetermined timing. The initial charging operation in the initial APC mode and the APC of the initial APC mode and the normal APC mode will be described below in detail in accordance with the light emission sequence shown in  FIG. 4 . 
     (Initial Charging Operation in Initial APC Mode) 
     In the initial state before the start of image formation in the image forming apparatus  100  (before a time  421  in  FIG. 4 ), the sequence controller  47  outputs the signal OFF_LD of “H”. In this state, the switch  408 - 1  is off, and the bias current and the switching current to the LD 1  are not supplied. Hence, since the LD 1  is in the turn-off state, the light power monitor voltage Vpd input to the APC circuit  403  is 0. Additionally, in the initial state, the sequence controller  47  outputs the sample hold signal S/H* of “H” and the control signal OFF_APC* of “H” to the APC circuit  403 . 
     At the time  421 , the sequence controller  47  changes the control signal OFF_APC* from “H” to “L”. Accordingly, the analog switch  501  outputs not the light power monitor voltage Vpd but the reference voltage Vref as the output voltage Vpd 2 . In addition, since the control signal OFF_APC* is “H”, and the sample hold signal S/H* is “L”, the control signal SEL is set to “H”. For this reason, the analog switch  504  sets the hold capacitor  505  in the sample state. Hence, at the time  421 , the reference voltage Vref (=voltage Vpd 2 ) starts being applied to the hold capacitor  505  via the resistive element  503 . 
     The reference voltage Vref is applied to the hold capacitor  505  for a predetermined period Tc (the period from the time  421  to a time  422  in  FIG. 4 ). The period Tc is defined as a period after the charging of the hold capacitor  505  by the reference voltage Vref has started until the hold capacitor  505  is charged to the predetermined voltage Vt. At the time  422 , the sequence controller  47  changes the control signal OFF_APC* from “L” to “H”. Accordingly, the control signal SEL changes from “H” to “L”, and the analog switch  504  changes the hold capacitor  505  to the hold state. As a result, at the time  422 , the hold capacitor  505  is charged to the predetermined voltage Vt by the time constant τ and held at the voltage. After that, at a time  423 , the initial charging operation ends, and the processing switches to execution of APC of the initial APC mode. 
     (Setting of Period Tc) 
     The period Tc will be explained here with reference to  FIG. 5 . Referring to  FIG. 5 , a waveform  511  represents the output voltage Vpd 2  of the analog switch  501 , and has a step at time t=0 corresponding to the time  421  at which the voltage switches from 0 to the reference voltage Vref. A waveform  512  represents the voltage Vsh of the hold capacitor  505  when the reference voltage Vref is applied to the hold capacitor  505  via the resistive element  503 . The hold capacitor  505  accumulates charges as the reference voltage Vref is applied to the hold capacitor  505  via the resistive element  503 . As a result, the voltage Vsh of the hold capacitor  505  moderately increases with the time constant τ defined by the capacitance C of the hold capacitor  505  and the resistance value R of the resistive element  503 . 
     The voltage Vsh (waveform  512 ) of the hold capacitor  505  shown in  FIG. 5  is the step response to the waveform  511  and is generally given by
 
 Vsh=V ref(1−exp(− t /τ))  (1)
 
When the time t at which the voltage Vsh reaches the predetermined voltage Vt is defined as Tc, Tc is determined depending on the reference voltage Vref, the voltage Vt, and the time constant τ, as is apparent.
 
     The voltage Vt may be designated in advance at a ratio to the reference voltage Vref. That is, the voltage Vt may be designated as a ratio x (%) of the light power to the voltage Vt based on the target light power. In this case, using the ratio x, the period Tc during which the hold capacitor  505  is charged is obtained by
 
 Tc =−τ×ln(1− x/ 100)  (2)
 
     The period Tc can be calculated by equation (2) using the ratio x and the time constant τ. Note that Tc may be calculated by the sequence controller  47 . The sequence controller  47  switches the control signal OFF_APC* such that the reference voltage Vref is applied to the hold capacitor  505  during the calculated period Tc. 
       FIG. 5  shows a case in which the ratio x is set to 80, 90, and 95(%) as an example. Using equation (2),
 
 x= 80(%), Tc ( T 80)=1.61τ
 
 x= 90(%), Tc ( T 90)=2.30τ
 
 x= 95(%), Tc ( T 95)=2.97τ
 
are obtained. As can be seen from  FIG. 5 , when the ratio x is increased, the period Tc until the voltage Vsh reaches the voltage (0.80 Vref, 0.90 Vref, 0.95 Vref) corresponding to the ratio x becomes long. Hence, the closer the light power from which light power control by APC executed after the initial charging operation starts is to the target light power, the longer the period Tc necessary for charging the hold capacitor  505  in the initial charging operation is. It is therefore necessary to set the period Tc within a period assignable to the initial charging operation. Note that the period Tc designated by the ratio x is constant independently of the target light power even when the target light power is changed, as indicated by equation (2).
 
     (APC of Initial APC Mode and Normal APC Mode) 
     When the above-described initial charging operation is completed in the image forming apparatus  100 , the processing shifts to execution of APC of the initial APC mode at the time  423 . At the time  423 , the sequence controller  47  switches the signal OFF_LD from “H” to “L” to start supplying the driving current to each LD, thereby setting each LD in the turn-on state. At this time, the current controller  506  in the APC circuit  403  decides the driving current (switching current value Isw) to be supplied to each LD in accordance with the voltage Vsh (=Vt) of the hold capacitor  505  charged in the turn-off state of the LD. 
     The hold capacitor  505  has been charged up to the voltage Vt close to the reference voltage Vref corresponding to the target light power by the initial charging operation in the initial APC mode, as shown in  FIG. 4 . Hence, the decided driving current has a current value close to the driving current corresponding to the target light power. As a consequence, the light power of each LD is controlled to the target light power in a short time by several times of APC executed later in response to detection of a BD signal. Referring to  FIG. 4 , after the time  423 , the sequence controller  47  sets each LD in the full turn-on state and detects the BD signal. In addition, the sequence controller  47  switches the sample hold signal S/H* (H→L) to switch the hold capacitor  505  from the hold state to the sample state at the timing the BD signal has stably been detected twice. The first APC of the initial APC mode is thus executed during a period  424 , and the voltage of the hold capacitor  505  approaches the reference voltage Vref corresponding to the target light power from the voltage Vt (initial value). 
     After that, when the APC during the period  424  is completed, and image formation starts, the image forming apparatus  100  shifts from the initial APC mode to the normal APC mode. In every scanning of the photosensitive member  11  by a laser beam output from each LD (every time a BD signal is detected), the APC operation is repetitively performed during a predetermined period (periods  425  and  426 ). In  FIG. 4 , the voltage Vsh of the hold capacitor  505  is set to a value sufficiently closer to the reference voltage Vref during the periods  425  and  426 . That is, the light power of each LD is controlled to a light power sufficiently close to the target light power, and the light power is considered to have reached the target light power. 
       FIG. 4  illustrates only the light emission sequence of one LD. In this embodiment, the same light emission sequence is executed for n LDs (LD 1  to LDn). As described above, the APC circuits  403  ( 403 - 1  to  403 - n ) are provided for the n LDs, respectively. Hence, the light emission sequence shown in  FIG. 4  is executed for each LD. 
     &lt;Procedure of APC in Laser Driving Device  31 &gt; 
     The procedure of the series of APC operations (initial APC mode and normal APC mode) in the laser driving device  31  described with reference to  FIGS. 4 and 5  will be explained next with reference to the flowchart of  FIG. 6 . Note that the processing of each step shown in  FIG. 6  is implemented on the image forming apparatus  100  by causing the CPU (not shown) of the sequence controller  47  to read out a control program stored in advance in a memory or the like to a RAM (not shown) and execute the program. The sequence controller  47  is assumed to start the processing shown in  FIG. 6  upon power-on of the image forming apparatus  100  and end the processing upon power-off. 
     In step S 601 , the CPU of the sequence controller  47  (to be simply referred to as a “CPU” hereinafter) sets the period Tc based on, for example, an instruction input by the user via the operation unit (not shown) of the image forming apparatus  100  before the start of image formation. The period Tc can be set based on equation (1) or (2), as described above. That is, the CPU controls the operation unit such that the user can set the ratio x (%). After that, the CPU advances the process to step S 602 . 
     In step S 602 , the CPU determines whether to start image formation. In accordance with input of an image formation command, or the like, the CPU determines whether to start image formation. Upon determining in step S 602  not to start image formation, the CPU repeats the determination of step S 602 . Upon determining in step S 602  to start image formation, the process advances to step S 603 . 
     In step S 603 , the CPU starts the above-described initial APC mode and also starts the initial charging operation. That is, the CPU starts the operation of charging the hold capacitor  505  to the voltage Vt based on the ratio x in the turn-off state without turning on the lasers. More specifically, the CPU switches the control signal OFF_APC* to be output to the APC circuit  403  from “H” to “L”, and starts time count from time t=0. In step S 604 , the CPU determines whether the period Tc has elapsed after the switching of the control signal OFF_APC* in step S 603  (t≧Tc is satisfied). Upon determining that the period Tc has elapsed, the CPU advances the process to step S 605  to return the control signal OFF_APC* from “L” to “H”. The hold capacitor  505  is thus charged from the voltage 0 to the voltage Vt (the voltage corresponding to x % of the reference voltage Vref corresponding to the target light power). 
     In step S 606 , the CPU starts driving the polygon mirror  33  and also starts supplying the driving current to each of the lasers (LD 1  to LDn), thereby turning on the lasers and setting them in the full turn-on state. The image forming apparatus  100  thus starts the APC (of the initial APC mode). When a BD signal is detected as the BD sensor  36  receives the laser beam from a representative laser, the CPU starts the APC of each LD in step S 607  (period  424  in  FIG. 4 ). 
     In step S 608 , the CPU starts supplying a driving current (switching current) based on the image information to each laser, thereby stating image formation. The image forming apparatus  100  thus shifts from the initial APC mode to the normal APC mode. After the start of image formation, the CPU may execute the APC (of the normal APC mode) in response to BD signal detection using a laser beam. In step S 609 , the CPU determines whether processing designated by the image formation command is completed, thereby determining whether to end the image formation. As long as determining not to end the image formation processing, the CPU repeats the determination of step S 609 . Upon determining to end, the process advances to step S 610 . In step S 610 , the CPU turns off the lasers, and returns the process to step S 602 . The image forming apparatus  100  stands by until image formation starts again. 
     As described above, when performing APC for an LD that outputs a laser beam corresponding to the driving current controlled based on the voltage of the hold capacitor, the optical scanning apparatus according to this embodiment controls the driving current to be supplied to the LD such that the light power monitor voltage generated in the charged hold capacitor approaches the reference voltage from the initial value that is a voltage corresponding to the amount of charges accumulated in the hold capacitor in advance at the time of turning on the LD. The hold capacitor accumulates charges in advance in a state in which the LD is off before the start of optical scanning of the photosensitive member. When the LD is turned on, the hold capacitor outputs a voltage corresponding to the amount of charges accumulated in advance at the time of turning on the LD, and then outputs a voltage corresponding to the light power of the LD. The optical scanning apparatus thus controls the driving current (that is, the voltage of the hold capacitor) to be supplied to the LD such that the voltage corresponding to the light power of the LD approaches the reference voltage from the initial value that is the voltage corresponding to the amount of charges accumulated in the hold capacitor in advance before turning on the LD. According to this embodiment, the voltage of the hold capacitor can approach the reference voltage from a voltage closer to the reference voltage corresponding to the target light power as compared to a case in which no charges are accumulated in the hold capacitor in advance. That is, when executing the APC, the light power of the LD can be made to approach the target light power in a shorter time after turning on the LD. 
     More specifically, the optical scanning apparatus may charge the hold capacitor to a predetermined voltage close to the reference voltage corresponding to the target light power before turning on the LD. When performing the APC, the voltage of the hold capacitor approaches the reference voltage from the predetermined voltage set as the initial value. That is, since the light power control of the LD can be started from the level close to the target light power after turning on the LD, it is possible to control the light power to the target light power in a short time. 
     Note that in the image forming apparatus according to this embodiment, the target light power is the light power of the laser beam input to the BD sensor  36 . The light power that enters the BD sensor  36  is desired to be constant. The rising speed and falling speed of the signal output from the BD sensor  36  depend on the light power of the laser beam that enters the BD sensor  36 . That is, when the light power that enters the BD sensor  36  changes, the rising speed and falling speed of the signal output from the BD sensor  36  change depending on the light power of the laser beam. For this reason, to always attain the same image write position, the light power of the laser beam that enters the BD sensor  36  is desired to be made constant. 
     On the other hand, the light power of the laser beam to expose the surface of the photosensitive member  11  to form an electrostatic latent image on the photosensitive member  11  is controlled in the following way. The image forming apparatus according to this embodiment is provided with the potential sensor  30  to measure the charges on the surface of the photosensitive member  11 . The sequence controller  47  performs control to expose, by a plurality of light powers of laser beams, the photosensitive member  11  charged by the primary charger  28  at a predetermined timing, thereby forming a plurality of latent image patterns on the photosensitive member  11 . The potential of each of the plurality of latent image patterns is detected by the potential sensor  30 . The sequence controller  47  selects a latent image pattern formed with a predetermined potential out of the plurality of latent image patterns, and sets the light power of the laser beam corresponding to the latent image pattern to the light power of the laser beam to expose the surface of the photosensitive member  11 . Note that a density sensor may be attached to the image forming apparatus, and the light power of the laser beam to expose the surface of the photosensitive member  11  may be set based on not the latent image patterns but toner patterns of a plurality of densities. 
     The light power control signal included in the control signal S 47  is a signal (control coefficient) representing the degree of control of the light power of the laser beam to scan the surface of the photosensitive member  11  with respect to the target light power. The sequence controller  47  outputs the light power control signal to the current controller  506  of the APC circuit  403 . The current controller  506  controls the switching current Isw such that the light power of the laser beam to scan the surface of the photosensitive member  11  is controlled to a light power obtained by multiplying the target light power (a light power corresponding to Vref) by the control coefficient. 
     That is, the image forming apparatus according to this embodiment controls the light power of the laser beam that enters the BD sensor  36  to the target light power (first light power). On the other hand, the image forming apparatus according to this embodiment controls the light power of the laser beam to scan the surface of the photosensitive member  11  to form a latent image pattern on the photosensitive member  11  to a second light power based on the target light power and the detection result of the potential sensor. 
     In this embodiment, for each of the plurality of LDs of the image forming apparatus of the multi-beam system, the corresponding hold capacitor is charged to a predetermined voltage in advance before turning on the LDs. This allows to the light power control to start from the level close to the target light power for all of the plurality of LDs. Hence, according to this embodiment, it is possible to shorten the time necessary until the light power reaches the target light power by the APC, which is particularly problematic in the image forming apparatus of the multi-beam system. 
     While the present invention has been described with reference to embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. 
     This application claims the benefit of Japanese Patent Application Nos. 2011-269394, filed Dec. 8, 2011 and 2012-250587, Nov. 14, 2012, which are hereby incorporated by reference herein in their entirety.