Abstract:
A light scanning device includes a scanning unit and a power consumption unit. The scanning unit faces a scan surface and performs scanning by dividing one scan area into segments by having multiple light-emitting-element groups arranged in a predetermined scanning direction. Each light-emitting-element group writes an image onto the scan surface by causing multiple light-emitting elements arranged in the scanning direction to emit light in a time-division manner based on image information. The power consumption unit operates during a non-writing period occurring between scanning processes repeatedly executed in each light-emitting-element group, so as to cause consumption of electric power corresponding to electric power consumed for light emission in the light-emitting-element group.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2013-213042 filed Oct. 10, 2013. 
       BACKGROUND 
     Technical Field 
       [0002]    The present invention relates to light scanning devices and image forming apparatuses. 
       SUMMARY 
       [0003]    According to an aspect of the invention, there is provided a light scanning device including a scanning unit and a power consumption unit. The scanning unit faces a scan surface and performs scanning by dividing one scan area into segments by having multiple light-emitting-element groups arranged in a predetermined scanning direction. Each light-emitting-element group writes an image onto the scan surface by causing multiple light-emitting elements arranged in the scanning direction to emit light in a time-division manner based on image information. The power consumption unit operates during a non-writing period occurring between scanning processes repeatedly executed in each light-emitting-element group, so as to cause consumption of electric power corresponding to electric power consumed for light emission in the light-emitting-element group. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    An exemplary embodiment of the present invention will be described in detail based on the following figures, wherein: 
           [0005]      FIG. 1  schematically illustrates the overall configuration of an engine section of an image forming apparatus according to an exemplary embodiment; 
           [0006]      FIG. 2  is a block diagram of an image-formation control system in the engine section according to the exemplary embodiment; 
           [0007]      FIG. 3  is an enlarged view illustrating the structure of a light-emitting-diode printer head (LPH) according to the exemplary embodiment; 
           [0008]      FIG. 4  is a plan view illustrating an arrangement configuration of self-scanning light-emitting diodes (SLEDs) according to the exemplary embodiment; 
           [0009]      FIG. 5A  is a front view illustrating a main scanning process based on a relative positional relationship between a photoconductor drum and the LPH, and  FIG. 5B  is an enlarged view of a dotted-chain line area VB in FIG.  52 \; 
           [0010]      FIG. 6  is a control block diagram of a light-emission-time controller-driver; 
           [0011]      FIG. 7  is a light-emission control circuit diagram of an SLED chip according to the exemplary embodiment; 
           [0012]      FIG. 8A  is a characteristic diagram illustrating writing periods and non-writing periods in a main scanning line of each SLED chip shown in  FIG. 5B , and  FIG. 8B  is a timing chart of the writing periods and the non-writing periods in the main scanning line of the SLED chip; 
           [0013]      FIG. 9  is a characteristic diagram illustrating an operating-voltage fluctuation based on main scanning processes (times) between adjacent SLED chips arranged in a main scanning direction; 
           [0014]      FIG. 10  is an operational timing chart (1) of each SLED chip in the LPH according to the exemplary embodiment; 
           [0015]      FIG. 11  is an operational timing chart (2) of each SLED chip in the LPH according to the exemplary embodiment; and 
           [0016]      FIG. 12  is a flowchart illustrating alight-emission-signal switching control routine executed by a signal switching unit of the light-emission-time controller-driver according to the exemplary embodiment. 
       
    
    
     DETAILED DESCRIPTION 
     Overall Configuration 
       [0017]      FIG. 1  schematically illustrates the overall configuration of an engine section  10  of an image forming apparatus according to an exemplary embodiment of the present invention. As shown in  FIG. 1 , the engine section  10  includes a photoconductor drum  12  that rotates at constant speed in a direction indicated by an arrow A in  FIG. 1 . 
         [0018]    The photoconductor drum  12  is surrounded by a charging unit  14 , a light-emitting-diode (LED) printer head (LPH)  16 , a developing unit  18 , a transfer roller  20 , a cleaner  22 , and an erase lamp  24  in this order in the rotational direction (i.e., a clockwise direction indicated by the arrow A in  FIG. 1 ) of the photoconductor drum  12 . 
         [0019]    Specifically, the surface of the photoconductor drum  12  is uniformly charged by the charging unit  14 . Then, the photoconductor drum  12  is irradiated with a light beam from the LPH  16  so that a latent image is formed on the photoconductor drum  12 . The LPH  16  is connected to an LPH driver  26  and is configured to emit a light beam based on image data by being controlled by the LPH driver  26 . 
         [0020]    The latent image formed on the photoconductor drum  12  by the light beam is supplied with toner from the developing unit  18  so that a toner image is formed on the photoconductor drum  12 . 
         [0021]    The transfer roller  20  transfers the toner image on the photoconductor drum  12  onto a sheet  28  transported from a sheet tray (not shown). After the transfer process, residual toner on the photoconductor drum  12  is removed therefrom by the cleaner  22 . Then, the erase lamp  24  diselectrifies the photoconductor drum  12 . Subsequently, the photoconductor drum  12  is electrostatically charged by the charging unit  14  again. The same process described above, is repeated. 
         [0022]    The sheet  28  having the toner image transferred thereon is transported to a fixing unit  30 , which includes a pressing roller  30 A and a heating roller  30 B, where the sheet  28  undergoes a fixing process. Thus, the toner image becomes fixed onto the sheet  28 , whereby a desired image is formed on the sheet  28 . The sheet  28  having the image formed thereon is discharged outside the apparatus. 
         [0023]    Furthermore, a density detection circuit  32  that faces the photoconductor drum  12  is provided on the periphery of the photoconductor drum  12  and between the developing unit  18  and the transfer roller  20 . For example, when a density patch pattern (i.e., a density sample) is formed, the density detection circuit  32  detects the density of the toner image on the photoconductor drum  12 . An output terminal of this density detection circuit  32  is connected to an exposure control unit  162 . The exposure control unit  162  is connected to the LPH driver  26  for driving the LPH  16 . The LPH driver  26  is connected to the LPH  16 . 
         [0024]    As the aforementioned density patch pattern, a patch pattern with an extremely small size of about several hundreds of micrometers by several hundreds of micrometers is used. By using this density patch pattern, the density may be detected by the density detection circuit  32  facing the photoconductor drum  12  without having to print the density patch pattern onto the sheet  28 . 
         [0025]    The density detection circuit  32  is attached to a moving mechanism that is movable in a main scanning direction, and is capable of detecting the density of the density patch pattern in the main scanning direction. 
         [0026]    Engine-Section Control System 
         [0027]      FIG. 2  is a block diagram of an image-formation control system in the engine section  10 . 
         [0028]    A power management unit  150  is connected to a commercial power source (not shown). The power management unit  150  generates a low-voltage power supply (LVPS) and a high-voltage power supply (HVPS) and supplies electric power to each unit via a power supply line. 
         [0029]    A controller  152  is connected to a user interface  154 . The controller  152  receives a command related to, for example, an image forming process from user&#39;s operation and also notifies the user of information about, for example, an image forming process. 
         [0030]    Furthermore, the controller  152  is connected to an external host computer (not shown) via a network line and is configured to receive image data. 
         [0031]    When the controller  152  receives the image data, the controller  152  analyzes, for example, the image data and print command information included in the image data, converts the data into a format (e.g., bitmap data) suitable for the engine section  10 , and then transmits the image data to an image-forming-process controller  156  functioning as a part of an MCU. 
         [0032]    Based on the input image data, the image-forming-process controller  156  synchronously controls the image-forming-process controller  156  as well as a drive-system control unit  158 , a charge control unit  160 , the exposure control unit  162 , a transfer control unit  166 , a fixation control unit  168 , a diselectrification control unit  170 , a cleaner control unit  172 , and a development control unit  164 , which function as the MCU, so as to execute an image forming process. 
         [0033]    The LPH driver  26  is controlled by a light-emission-time controller-driver  162 A provided in the exposure control unit  162 . 
         [0034]    The image-forming-process controller  156  is connected to a status management unit  176  that determines the operation status of the engine section  10  (e.g., a processing mode, a sleep mode, a start-up from the sleep mode, and an in-progress mode). The operation status determined in the status management unit  176  is transmitted to the controller  152 . 
         [0035]    Furthermore, the power management unit  150  is connected to a power-on monitoring sensor  178 . The power-on monitoring sensor  178  detects that the power is turned on and transmits the power-on information to the controller  152  via the status management unit  176 . 
         [0036]    The controller  152  is also connected to, for example, a temperature sensor  180  and a humidity sensor  182 . The temperature sensor  180  and the humidity sensor  182  respectively detect an ambient temperature and an ambient humidity within the engine section  10 . 
         [0037]    Detailed Configuration of LPH 
         [0038]    Next, the configuration of the LPH  16  will be described in detail. As shown in  FIG. 3 , the LPH  16  includes an LED array  50 , a printed circuit board  52  that supports the LED array  50  and has a circuit for supplying various signals used for controlling the driving of the LED array  50 , and a Selfoc (registered trademark) lens array (SLA)  54 . 
         [0039]    The printed circuit board  52  is disposed within a housing  56  such that an attachment surface of the LED array  50  faces the photoconductor drum  12 , and is supported by a leaf spring  58 . 
         [0040]    As shown in  FIG. 4 , self-scanning LED (SLED) chips  62  each having multiple LEDs  60  arranged in the axial direction of the photoconductor drum  12  are arranged in a so-called zigzag pattern and are capable of radiating light beams with predetermined resolution in the axial direction of the photoconductor drum  12 . 
         [0041]    As shown in  FIG. 5A , with regard to the SLED chips  62  arranged in the zigzag pattern, a scanning process (main scanning process) is repeated by each SLED chip  62 , and the photoconductor drum  12  is rotated about its axis (sub scanning process). 
         [0042]    In other words, as shown in  FIG. 5B , a main scanning line on the photoconductor drum  12  is formed as a single main scanning line constituted of a combination of contemporaneous main scanning lines scanned by the zigzag-arranged SLED chips  62 . Although the combined main scanning line forms a so-called saw-shaped pattern when viewed microscopically, the combined main scanning line may be regarded as a straight line in a condition in which main scanning lines form an image of a single page. 
         [0043]    In  FIG. 5B , thick arrows each correspond to a writing period in which the photoconductor drum  12  is exposed to light, and each dotted arrow in  FIG. 516  denotes an interval between main scanning processes and corresponds to a non-writing period (i.e., an idle period) in which the photoconductor drum  12  is not exposed to light. 
         [0044]    In this exemplary embodiment, in each non-writing period (i.e., a period from the end of a previous scanning process to the start of a subsequent scanning process), the LEDs  60  in each SLED chip  62  emit light with a light quantity that does not cause the photoconductor drum  12  to undergo exposure. Detailed descriptions of light-emission control based on image data in each writing period and forced-light-emission control in each non-writing period will be provided later. 
         [0045]    Light-Emission-Time Controller-Driver 
         [0046]    The light-emission-time controller-driver  162 A provided in the exposure control unit  162  will now be described in detail with reference to  FIG. 6 . 
         [0047]    The light-emission-time controller-driver  162 A corrects a light-emission time for each pixel based on nonuniform-density correction data and generates a control signal for causing the LED  60  of each pixel to emit light. 
         [0048]    As shown in  FIG. 6 , the light-emission-time controller-driver  162 A includes a pre-settable digital one-shot multi-vibrator (PDOMV)  260 , a linearity correction unit  262 , and an AND circuit  270 . The AND circuit  270  receives a trigger signal when the image data is 1 (ON) and does not receive a trigger signal when the image data is 0 (OFF). 
         [0049]    The PDOMV  260  receives nonuniform-density correction data and a reference clock in synchronization with the trigger signal from the AND circuit  270  and generates a light-emission pulse signal. 
         [0050]    The linearity correction unit  262  corrects and outputs the light-emission pulse signal from the PDOMV  260  so as to correct a variation in light-emission start time of each driver output. 
         [0051]    Specifically, the linearity correction unit  262  has multiple (eight in this exemplary embodiment) delay circuits  264  (the numbers 0 to 7 provided as suffixes to the reference numeral  264  are for differentiating between the individual delay circuits  264 ), a delay selection register  266 , a delay-signal selecting unit  265 , an AND circuit  267 , an OR circuit  268 , and a light-emission-signal selecting unit  269 . 
         [0052]    The delay circuits  264  (i.e., the delay circuits  264 - 0  to  264 - 7 ) are connected to the PDOMV  260  and delay the light-emission pulse signal from the PDOMV  260  by different times. 
         [0053]    The delay selection register  266  is connected to the delay-signal selecting unit  265  and the light-emission-signal selecting unit  269 . The delay selection register  266  stores therein delay selection data for each driver and light-emission-signal selection data. 
         [0054]    The delay selection data for each driver and the light-emission-signal selection data are measured in advance and are stored in a nonvolatile memory (not shown), such as an electrically erasable and programmable read-only memory (EEPROM) or a flash read-only memory (ROM). In a case where the delay selection data for each driver and the light-emission-signal selection data are stored in the EEPROM, the delay selection data is downloaded into the delay selection register  266  when the apparatus is turned on. In a case where the delay selection data for each driver and the light-emission-signal selection data are stored in the flash ROM, the flash ROM functions as the delay selection register  266 . 
         [0055]    The delay-signal selecting unit  265  is connected to the AND circuit  267  and the OR circuit  68  and selects any one of outputs from the delay circuits  264 - 0  to  264 - 7  based on the delay selection data stored in the delay selection register  266 . 
         [0056]    The AND circuit  267  outputs a light-emission pulse if a logical product of the light-emission pulse signal from the PDOMV  260  and a delay light-emission pulse signal selected by the delay-signal selecting unit  265  is in a light-emission state, that is, if both the pre-delayed light-emission pulse signal and the delayed light-emission pulse signal are in a light-emission state. 
         [0057]    The OR circuit  268  outputs a light-emission pulse if a logical sum of the light-emission pulse signal from the PDOMV  260  and the delay light-emission pulse signal selected by the delay-signal selecting unit  265  is in a light-emission state, that is, if at least one of the pre-delayed light-emission pulse signal and the delayed light-emission pulse signal is in a light-emission state. 
         [0058]    The light-emission-signal selecting unit  269  selects one of outputs from the AND circuit  267  and the OR circuit  268  based on the light-emission-signal selection data stored in the delay selection register  266 . 
         [0059]    The light-emission-signal selecting unit  269  is connected to an image-data light-emission-signal output unit  272 . A metal-oxide semiconductor field-effect transistor (MOSFET)  272 A may be used as the image-data light-emission-signal output unit  272 . 
         [0060]    In the image-data light-emission-signal output unit  272 , a light-emission time according to the image data is generated based on a predetermined light quantity and is transmitted to drive circuits of the SLED chips  62  via a signal switching unit  273  so as to be used as a light-emission control signal (I). 
         [0061]    The signal switching unit  273  is connected to a forced-light-emission-signal output unit  275 . The forced-light emission-signal output unit  275  constantly outputs a light emission signal toward the signal switching unit  273 . 
         [0062]    Furthermore, the signal switching unit  273  receives a horizontal synchronization signal. Based on this horizontal synchronization signal, the signal switching unit  273  switches an output source for the light-emission control signal (I) to the image-data light-emission-signal output unit  272  or the forced-light-emission-signal output unit  275 . The light-emission signal to be input to the forced-light-emission-signal output unit  275  is preliminarily limited to an exposure light quantity that does not lead to exposure. 
         [0063]    SLED Drive Circuit 
         [0064]    Next, an internal circuit configuration provided in each SLED chip  62  for driving the LEDs  60  in the SLED chip  62  will be described with reference to  FIG. 7 . 
         [0065]    With regard to each SLED chip  62 , the multiple (e.g., 128) LEDs  60  arranged within the SLED chip  62  are individually provided with thyristors  90 . The anodes of the thyristors  90  are connected to a SUB terminal  80 . 
         [0066]    A point P (the numbers 1 to 128 added as suffixes to points P denote the order of multiple arranged LEDs  60 ) connected to the gate of the thyristor  90  in the first stage is connected to a φS input terminal  88 . As a trigger for causing the LEDs  60  in the SLED chip  62  to emit light, a start signal φS (voltage) is applied to the points P (P1 to P128). 
         [0067]    The points P (P1 to P128) connected to the gates of the thyristors  90  in the respective stages are connected to each other in series via diodes  92 . Furthermore, the points P (P1 to P128) in the respective stages are connected, via resistors  94 , to a base line  96  that is connected to a video-graphics-array (VGA) terminal  78 . The base line  96  maintains a predetermined voltage in the first stage and decrements the voltage by a predetermined potential (Vf) with increasing stages. 
         [0068]    The points P (P1 to P128) are connected to the anodes of the LEDs  60 . The cathodes of the LEDs  60  are connected to a φI input terminal  82  via a light-emission control signal line  98  that outputs a pulse wave acting as the light-emission control signal (I) in each stage. When this light-emission control signal is at a low level (L), the LEDs  60  emit light if the thyristors  90  with the points P (P1 to P128) acting as gates are turned on. 
         [0069]    The cathodes of the thyristors  90  in the odd-numbered stages are connected to a first transmission line  100 , and the cathodes of the thyristors  90  in the even-numbered stages are connected to a second transmission line  102 , such that transmission signals CK 1  and CK 2  are supplied. In accordance with these transmission signals CK 1  and CK 2 , the potential at each of the points P (P1 to P128) is incremented by a predetermined potential (Vf). Specifically, the potentials at the points P reach predetermined potentials, which may cause the LEDs  60  to emit light, sequentially from the point P1 in the first stage to the points P in the subsequent stages, thereby allowing for self-scanning of the SLED chip  62 . 
         [0070]    Forced-Light-Emission Control 
         [0071]    As shown in  FIG. 8A , due to the photoconductor drum  12  rotating at constant speed, the main scanning lines by the SLED chips  62  are sub-scanned in the following order: n-th line, (n+1)-th line, (n+2)-th line, . . . , (n+i)-th line. 
         [0072]    In this case, as shown in  FIG. 8E , each main scanning line has non-writing periods as intervals between writing periods. The LEDs  60  emit light in each writing period, whereas the LEDs  60  do not emit light in each non-writing period, thus causing a voltage fluctuation to occur between the writing period and the non-writing period. As indicated by a period A in  FIG. 8 , a lack of light quantity caused by the voltage fluctuation occurs during a start-up of a writing period, leading to the occurrence of streakiness (see a dotted line (comparative example) in  FIG. 9 ). 
         [0073]    In this exemplary embodiment, the LEDs  60  are forced made to emit light with an exposure light quantity that does not lead to exposure even during a non-writing period (i.e., an idle period), so that the voltage fluctuation may be suppressed (see a solid line (exemplary embodiment) in  FIG. 9 ) as compared with a case where the LEDs  60  do not emit light, thereby preventing a lack of light quantity during a start-up of each SLED chip  62 . 
         [0074]    In this exemplary embodiment, the signal switching unit  273  is provided at a terminal of the light-emission-time controller-driver  162 A as a unit for forcedly making the LEDs  60  emit light during a non-writing period in the above-described manner. Based on a horizontal synchronization signal, the signal switching unit  273  switches the output source for the light-emission control signal (I) to the image-data light-emission-signal output unit  272  or the forced-light-emission-signal output unit  275 . 
         [0075]    More specifically, based on a horizontal synchronization signal, the signal switching unit  273  switches the output source to the image-data light-emission-signal output unit  272  during each writing period (see  FIGS. 8A and 8B ), and switches the output source to the forced-light-emission-signal output unit  275  during each non-writing period (see  FIGS. 8A and 8B ). A light-emission signal to be input to the forced-light-emission-signal output unit  275  is preliminarily limited to an exposure light quantity that does not lead to exposure. As a result, the light-emission control signal (I) is changed from the comparative example indicated by the dotted line in  FIG. 9  to this exemplary embodiment indicated by the sold line in  FIG. 9 , so that electric power is continuously consumed even during a non-writing period (i.e., an idle period), whereby a voltage fluctuation may be suppressed. 
         [0076]    The operation of this exemplary embodiment will be described below. 
         [0077]    Image Forming Process 
         [0078]    A known electrophotographic image forming (printing) process is performed for each color around the periphery of the corresponding photoconductor drum  12  in the following manner. 
         [0079]    First, the photoconductor drum  12  is rotationally driven at a predetermined rotation speed. 
         [0080]    Then, as shown in  FIG. 1 , the charging unit  14  applies a direct-current voltage at a predetermined charge level (or a voltage in which alternating-current voltage is superimposed on direct-current voltage) onto the surface of the photoconductor drum  12  so as to uniformly charge the surface of the photoconductor drum  12  to a predetermined level. 
         [0081]    Subsequently, the PPM  16  causes the LEDs  60  to radiate a light beam onto the uniformly charged surface of the photoconductor drum  12 , so that an electrostatic latent image according to image information is formed on the surface. The light-emission control of the LEDs  60  will be described later. 
         [0082]    With the light emission from the LEDs  60 , the surface potential of the area in the photoconductor drum  12  exposed to the light beam changes to a predetermined level. 
         [0083]    The electrostatic latent image formed on the surface of the photoconductor drum  12  is developed into a visible toner image on the photoconductor drum  12  by the corresponding developing unit  18 . 
         [0084]    Specifically, the developing unit  18  takes out a two component developer from a development cartridge and spreads toner over the electrostatic latent image from a developing roller so that the toner is adhered onto the surface of the photoconductor drum  12 . 
         [0085]    With regard to the developer in this case, a carrier having a function for transporting the toner remains on the developing roller, and only the toner is transferred to the photoconductor drum  12 . 
         [0086]    Subsequently, the color toner images formed on the respective photoconductor drums  12  are transferred, by the transfer rollers  20 , onto a sheet  28  traveling through the sheet transport path. After the sheet  28  undergoes the transfer process, the toner images formed on the sheet  28  are heated, pressed, and transported by the fixing unit  30 , so that the toner becomes fused and solidified, whereby the toner becomes fixed onto the sheet  28 . After the fixing process, the sheet  28  is output by an output roller, and the image forming process ends. 
         [0087]    Light-Emission Control 
         [0088]    Signal Generation 
         [0089]    The AND circuit  170  in the light-emission-time controller-driver  162 A receives a trigger signal and image data. The AND circuit  270  outputs the trigger signal to the PDOMV  260  only when the image data is ON. The PDOMV  260  receives nonuniform-density correction data, a reference clock, and the trigger signal. When the image data is ON, the PDOMV  260  generates light-emission pulses for the number of reference clocks corresponding to the nonuniform-density correction data. 
         [0090]    A light-emission pulse is output to the AND circuit  267  and the OR circuit  268  and is also split and output to the delay circuit  264 - 0 . The light-emission pulse is delayed by a predetermined time at the delay circuit  264 - 0  and is output to the delay-signal selecting unit  265 . A light-emission pulse CKi delayed at the delay circuit  264 - 0  is also output to the delay circuit  264 - 1 . Each of the delay circuits  264 - 1  to  264 - 7  receives a light-emission pulse CKi from the preceding delay circuit  264 , delays the light-emission pulse by a predetermined time, and outputs the delayed light-emission pulse to the delay-signal selecting unit  265  and the subsequent delay circuit  264 . However, the delay circuit  264 - 7  does not output the light-emission pulse to the subsequent delay circuit  264 . 
         [0091]    The delay-signal selecting unit  265  selects any one of the light-emission pulses CKi output from the delay circuits  264 - 0  to  264 - 7  based on the delay selection data stored in advance in the delay selection register  266 . The selected light-emission pulse is output to the AND circuit  267  and the OR circuit  268 . 
         [0092]    The AND circuit  267  generates a light-emission pulse CK 1 , which is a logical product of a pre-delayed light-emission pulse and a delayed light-emission pulse, and outputs the light-emission pulse CK 1  to the light-emission-signal selecting unit  269 . 
         [0093]    The OR circuit  268  generates a light-emission pulse CK 2 , which is a logical sum of a pre-delayed light-emission pulse and a delayed light-emission pulse, and outputs the light-emission pulse CK 2  to the light-emission-signal selecting unit  269 . 
         [0094]    The light-emission-signal selecting unit  269  selects one of the output from the AND circuit  267  and the output from the OR circuit  268  based on the light-emission-signal selection data stored in advance in the delay selection register  266 . The selected light-emission pulse (i.e., light-emission control signal (I)) is output to the LPH  16  via the MOSFET  272 A if the signal switching unit  273  has switched toward the image-data light-emission-signal output unit  272 . 
         [0095]    SLED-Chip Operation Control 
         [0096]    Next, the operation of the SLED chips  62  of the LPH  16  will be described with reference to timing charts shown in  FIGS. 10 and 11 . 
         [0097]    As shown in  FIGS. 10 and 11 , a start signal φS (CKS) is set to a high (H) level so that the potential at the point P1 becomes H level and the potential at the point P2 connected to the point P1 via a diode  92  becomes P2=φS−Vf (due to a voltage decrease in LED). Likewise, the potential at the point P3 becomes P3=P2−Vf, the potential at the point P4 becomes P4=P3−Vf, the potential at the point PN becomes PN=P(N−1)−Vf, and so on. However, the potential does not decrease to φga or lower since saturation occurs at a potential of φga. 
         [0098]    When CK 1  becomes a low (L) level, the thyristor  90  corresponding to the point P1 is turned on. In this case, the potential φS at the point P1 becomes 0 V, and the potential φ 1  of CK 1  becomes −Vf. With regard to a point P equivalent to the point P1, that is, an odd-numbered point P, the thyristor  90  corresponding thereto is not turned on since the potential is decremented by 2Vf. 
         [0099]    By changing OT from H to L in this state, the LED  60  in the first stage emits light. By changing φI from L to H, the LED  60  in the first stage is turned off. In this case, the potential of φI becomes −Vf. 
         [0100]    Subsequently, by setting CK 2  to L, the thyristor  90  corresponding to the point P2 is turned on so that P2=0 V, P3=−Vf, and P4=−2Vf. In this case, since the potential φ 2  of CK 2  becomes −Vf, the thyristors  90  corresponding to the points P4 and onward in the even-numbered stages are not turned on. 
         [0101]    In a state where the thyristor  90  corresponding to the point P2 is turned on, CK 1  is set to H so that the thyristor  90  corresponding to the point P1 is turned off, whereby the LED  60  in the first stage does not emit light in response to a subsequent data signal. 
         [0102]    In this state, φI is changed from H to L so that the LED  60  in the second stage emits light. In this case, the potential of φI becomes −Vf. The φI is changed from L to H so that the LED  60  in the second stage is turned off (the potential of φI becomes 0 V). 
         [0103]    The on state (and the light emission) of the thyristor (and the LED  60 ) in each odd-numbered stage is controlled by CK 1 , the on state (and the light emission) of the thyristor  90  (and the LED  60 ) in each even-numbered stage is controlled by CK 2 , and the exposure light quantity by each LED  60  is controlled by the light-emission, control signal φI. 
         [0104]    Forced-Light-Emission Control 
         [0105]    As shown in  FIG. 5A , when sub scanning is performed in the following order: n-th line, (n+1)-th line, (n+2)-th line, . . . , (n+i)-th line, each main scanning line of each SLED chip  62  has non-writing periods as intervals between writing periods. 
         [0106]    The LEDs  60  emit light in each writing period, whereas the LEDs  60  do not emit light in each non-writing period, thus causing a voltage fluctuation to occur between the writing period and the non-writing period. This may sometimes lead to the occurrence of streakiness in the sub scanning direction at a juncture of each SLED chip  62  (see the dotted line (comparative example) in  FIG. 9 ). 
         [0107]    In this exemplary embodiment, control is performed such that the LEDs  60  are forcedly made to emit light even during a non-writing period (i.e., an idle period), so that the voltage fluctuation may be suppressed (see the solid line (exemplary embodiment) in  FIG. 9 ) as compared with a case where the LEDs  60  do not emit light, thereby preventing a lack of light quantity during a start-up of each SLED chip  62 . 
         [0108]      FIG. 12  is a flowchart illustrating alight-emission-signal switching control routine executed by the signal switching unit  273  shown in  FIG. 6 . Although the flow of processing will be described with reference to the flowchart, the processing is not limited to light-emission-signal switching control based on so-called software. In view of the processing speed, a logical circuit may be established by using an electronic component that includes a switching circuit, such that the light-emission-signal switching control may be executed based on hardware. 
         [0109]    The flowchart shown in  FIG. 12  commences in synchronization with a writing process. In step  300 , it is determined whether or not a writing process for one line has been completed. This determination process is looped until a positive determination result is obtained. This looping period corresponds to a writing period shown in  FIGS. 8A and 8B  in which the SLED chips  62  execute main scanning. 
         [0110]    When a positive determination result is obtained in step  300 , the processing proceeds to step  302  where the light-emission-signal output source is switched to the forced-light-emission-signal output unit  275 . The processing then proceeds to step  304 . Due to this switching, the LEDs  60  are forcedly made to emit light during a non-writing period. Since the light forcedly emitted from the LEDs  60  is limited to an exposure light quantity that does not lead to exposure, the light does not affect the image quality. 
         [0111]    In step  304 , it is determined whether or not (a start-up of) a horizontal synchronization signal is detected. If a positive determination result is obtained, the processing proceeds to step  306  where the light-emission-signal output source is switched to the image-data light-emission-signal output unit  272 . The processing then proceeds to step  308 . 
         [0112]    In step  308 , it is determined whether or not the scanning has been completed for a predetermined number of lines, for example, lines equivalent to a single page. If a negative determination result is obtained, the processing returns to step  300  so as to repeat the above-described process. On the other hand, if a positive determination result is obtained in step  308 , the routine ends. 
         [0113]    With the switching control described above, the signal switching unit  273  switches the output source to the image-data light-emission-signal output unit  272  during a writing period (see  FIGS. 5A and 5B ), and switches the output source to the forced-light-emission-signal output unit  275  during a non-writing period (see  FIGS. 5A and 5B ). 
         [0114]    As a result, the light-emission control signal (I) is changed from the comparative example indicated by the dotted line in  FIG. 9  to the exemplary embodiment indicated by the solid line in  FIG. 9 , so that electric power is continuously consumed even during a non-writing period (i.e., an idle period), whereby a voltage fluctuation may be suppressed. 
         [0115]    Modifications 
         [0116]    In this exemplary embodiment, in order to suppress a voltage fluctuation during a non-writing period, the LEDs  60  are forcedly made to emit light that does not lead to exposure. As a solution for suppressing a voltage fluctuation other than forcedly making the LEDs  60  emit light, the following solutions may be applied. 
         [0117]    First Modification 
         [0118]    in the drive circuit of each SLED chip  62 , transfer thyristors (thyristors  90 ) that do not affect other components and reset thyristors (not shown) for turning off the LEDs  60  in the light emitting state may be driven (continuously turned on or repeatedly turned on and off) during a non-writing period. 
         [0119]    Second Modification 
         [0120]    The electric power (electric current) consumed by the light-emission-time controller-driver  162 A or an application specific integrated circuit (ASIC) used in the drive circuit of each SLED chip  62  is increased. For example, in the case of the light-emission-time controller-driver  162 A, a clock generated at the PDOMV  260  may be quickened, or a wasteful calculation process may be intentionally performed in a calculation process at the delay-signal selecting unit  265 . 
         [0121]    The foregoing description of the exemplary embodiment of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiment was chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.