Abstract:
An image forming apparatus having a photosensitive member, an exposure device having a plurality of laser emitting elements that each irradiate a laser beam onto the photosensitive member to form an elactrostatic latent image on the photosensitive member, a developing device that supplies a developing agent to the electrostatic latent image to form a visible image, and a controller that controls the irradiation of the plurality of laser emitting elements. The controller includes a determining device that determines which laser emitting element from the plurality of laser emitting elements is closest to reaching its end of operating life and a restricting device that restricts the irradiation of the laser emitting element determined closest to reaching its end of operating life.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of Invention  
           [0002]    The invention relates to an image forming apparatus that forms an image by exposing a photosensitive member to a plurality of laser beams.  
           [0003]    2. Description of Related Art  
           [0004]    A laser printer is generally known as an electrophotographic image forming apparatus. In the laser printer, a uniformly charged surface of a photosensitive member is exposed to a beam of laser light emitted based on image data, to form an electrostatic latent image on the surface of the photosensitive member. The electrostatic latent image attaches toner thereto. An image is formed onto a sheet as the tone is transferred to the sheet.  
           [0005]    As disclosed in, for example, U.S. Pat. No. 6,476,846, a plurality of laser beam emitting devices that emit laser beams are provided for a laser printer. The resolution of an image is improved by exposing the photosensitive member to the laser beams emitted from each of the plurality of laser beam emitting devices.  
           [0006]    More specifically, as two laser beam emitting devices are controlled in parallel, the photosensitive member is scanned with the laser beams two lines at a time. Thus, line density is increased without reducing the rotating speed of the photosensitive member.  
           [0007]    For example, an image of 1200 dpi resolution is formed with a laser printer having two laser beam emitting devices, by driving the photosensitive member at the same rotating speed as an image of 600 dpi resolution is formed with one laser beam emitting device.  
         SUMMARY OF THE INVENTION  
         [0008]    The invention is directed to an image forming apparatus having a photosensitive member, an exposure device having a plurality of laser emitting elements that each irradiate a laser beam onto the photosensitive member to form an elactrostatic latent image on the photosensitive member, a developing device that supplies a developing agent to the electrostatic latent image to form a visible image, and a controller that controls the irradiation of the plurality of laser emitting elements. The controller includes a determining device that determines which laser emitting element from the plurality of laser emitting elements is closest to reaching its end of operating life and a restricting device that restricts the irradiation of the laser emitting element determined closest to reaching its end of operating life. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    An exemplary embodiment of the invention will be described in detail with reference to the following figures wherein:  
         [0010]    [0010]FIG. 1 is a side cross-sectional view showing a laser printer according to an embodiment of the invention;  
         [0011]    [0011]FIG. 2 is an enlarged side cross-sectional view showing essential parts of an image forming unit of the laser printer;  
         [0012]    [0012]FIG. 3 is a schematic showing a scanner unit of the laser printer;  
         [0013]    [0013]FIG. 4 is a block diagram illustrating structures of a control system for the scanner unit;  
         [0014]    [0014]FIG. 5 is a flowchart illustrating a main routine performed by the control system;  
         [0015]    [0015]FIG. 6 is a flowchart illustrating a laser diode operating life check made during the main routine;  
         [0016]    [0016]FIG. 7 is a flowchart illustrating a laser diode selection process made during the main routine;  
         [0017]    [0017]FIG. 8 is a time chart illustrating changes of various signals in a laser light output adjustment process performed during the main routine;  
         [0018]    [0018]FIG. 9 is a time chart illustrating changes of various signals in a printing operation performed during the main routine; and  
         [0019]    [0019]FIGS. 10A to  10 C are schematics illustrating techniques to reduce a light exposure amount. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0020]    An exemplary embodiment of the invention will be described in detail with reference to the figures.  
         [0021]    [0021]FIG. 1 is a side cross-sectional view showing a laser printer  1  of an image forming apparatus according to an exemplary embodiment of the invention. As shown in FIG. 1, the laser printer  1  is provided in a main casing  2  with a feeder unit  4  that feeds sheets  3  of paper as a recording medium, and an image forming unit  5  that forms an image on the paper sheet  3 .  
         [0022]    The feeder unit  4  includes a paper tray  6 , a paper mounting plate  7 , a pickup roller  8 , a separation pad  9 , paper powder removing rollers  10 , conveying rollers  11  and register rollers  12 . The paper tray  6  is removably set in the bottom of the main casing  2 . The paper mounting plate  7  is provided in the paper tray  6 . The pick-up roller  8  and the separation pad  9  are provided at an upper end of the paper tray  6 . The paper powder removing rollers  10  and the conveying rollers  11  are provided downstream of the pick-up roller  8  in a sheet feeding direction. The register rollers  12  are provided downstream of the conveying rollers  11  in the sheet feeding direction.  
         [0023]    The paper mounting plate  7  supports a stack of the paper sheets  3 . The paper mounting plate  7  pivots on one end far from the pick-up roller  8 , so that the other end of the paper mounting plate  7  near the pick-up roller  8  can move up and down. A spring (not shown) is disposed on the underside of the paper mounting plate  7  to urge the paper mounting plate  7  upwardly. As the amount of the paper sheets  3  stacked on the paper mounting plate  7  increases, the mounting plate  7  pivots downward about the one end far from the pick-up roller  8 , against an urging force of the spring. The pick-up roller  8  and the separation pad  9  are disposed to face each other. A spring  13  disposed on the underside of the separation pad  9  presses the separation pad  9  against the pick-up roller  8 .  
         [0024]    The topmost paper sheet  3  on the paper mounting plate  7  is urged toward the pick-up roller  8  by the spring (not shown) disposed on the underside of the paper mounting plate  7 . As the pick-up roller  8  rotates, a leading end portion of the topmost sheet  3  is nipped between the pick-up roller  8  and the separation pad  9 . In this manner, each topmost sheet  3  is separated from the sheets  3  mounted on the paper mounting plate  7 . The paper sheet  3  is fed to the paper powder removing rollers  10  where paper powders are removed. Thereafter, the paper sheet  3  is fed to a pair of register rollers  12 . The register rollers  12  register the paper sheet  3 , and then feed the paper sheet  3  to an image forming portion where a tone image is transferred onto the sheet  3 . In this exemplary embodiment, the image forming portion is a contact portion where a photosensitive drum  27  and a transfer roller  30  contact each other.  
         [0025]    The feeder unit  4  further includes a multi-purpose tray  14 , as well as a multi-purpose pick-up roller  15  and a multi-purpose separation pad  25  that feed the sheet  3  stacked on the multi-purpose tray  14 . The multi-purpose pick-up roller  15  and the multi-purpose separation pad  25  are disposed so as to face each other. The multi-purpose separation pad  25  is pressed against the multi-purpose pick-up roller  15  by a spring (not shown) provided on the underside of the multi-purpose separation pad  25 . As the multi-purpose pick-up roller  15  rotates, a leading end portion of an uppermost sheet  3  on the multi-purpose tray  14  is nipped between the multi-purpose pick-up roller  15  and the multi-purpose separation pad  25 . In this manner, the sheets  3  stacked on the multi-purpose tray  14  are separated one by one. The sheet  3  separated from the sheet stack on the tray  14  is then delivered to the register rollers  12 .  
         [0026]    The image forming unit  5  includes a scanner unit  16 , a process unit  17 , and a fixing unit  18 .  
         [0027]    The scanner unit  16  is provided in an upper portion of the main casing  2  and below a discharge tray  46 . The scanner unit  16  includes a laser diode array  19  (shown in FIG. 3), a polygon mirror  20  that is driven to spin, lenses  21  and  23 , and a reflecting mirror  22 . The laser diode array  19  includes two laser diodes LD 1 , LD 2 , as shown in FIG. 4, as laser light emitting portions. A beam of laser light emitted from each of the laser diodes LD 1 , LD 2  passes through or reflects off the polygon mirror  20 , the lens  21 , the reflecting mirror  22 , and the lens  23  in this order, as indicated by broken lines in FIG. 1, to scan at high speed across a surface of a photosensitive drum  27 , in the process unit  17 .  
         [0028]    As shown in FIG. 2, the process unit  17  includes a photosensitive member cartridge  26 . The photosensitive member cartridge  26  includes the photosensitive drum  27 , a scorotron charger  29 , a transfer roller  30 , a cleaning roller  51  as a paper powder cleaning device, a secondary roller  52  and a slide member  53 .  
         [0029]    In the laser printer  1 , residual toner remaining on the surface of the photosensitive drum  27  after the toner is transferred by the transfer roller  30  onto the sheet  3 , is collected by a developing roller  31 . This toner collection manner is a so called cleaner-less system in which a blade for wiping out the residual toner and a residual toner container can be eliminated. Therefore, the overall arrangement of the laser printer  1  can be simplified. In addition, the physical size and costs of the laser printer  1  can be reduced.  
         [0030]    The photosensitive drum  27  is provided to a side of the developing roller  31  as a developing device, to face the developing roller  31 . The photosensitive drum  27  is rotatable in a counterclockwise direction as indicated by the arrow in FIG. 2. The photosensitive drum  27  includes a main drum which is grounded. The surface (photosensitive layer) of the photosensitive drum  27  is formed of positively chargeable photosensitive layer including, for example, an amorphous silicon-based material, such as σ-Si:H, a cadmium sulfide-based material, such as Cds, a zinc oxide based-material, such as ZnO, a selenium-based material, such as AsSe3, or an organic photosensitive material, such as polycarbonate. A rotating shaft  27   a , which is a driving shaft of the photosensitive drum  27 , protrudes from the right and left sides of the photosensitive member cartridge  26 . The rotating shaft  27   a  is rotatably driven by a power transmitted from a main motor (not shown).  
         [0031]    The scorotron charger  29  is disposed above the photosensitive drum  27  with a predetermined distance therebetween, to prevent the scorotron charger  29  from contacting the photosensitive drum  27 . The scorotron charger  29  is a positively charging charger that generates corona discharge from a charging wire made from tungsten or other material. The scorotron charger  29  uniformly and positively charges the surface of the photosensitive drum  27 . A charging power source (not shown) turns on and off the scorotron charger  29 .  
         [0032]    The laser light emitted from the scanner unit  16  scans at high speed across the surface of the photosensitive drum  27 , which is uniformly and positively charged by the scorotron charger  29 . The surface of the photosensitive drum  27  is selectively exposed to the laser light. An electrostatic latent image is formed on the photosensitive drum  27  based on image data.  
         [0033]    The transfer roller  30  is disposed below the photosensitive drum  27  to face the drum  27 . The transfer roller  30  is supported in the photosensitive member cartridge  26  so as to rotate in a clockwise direction as indicated by the arrow in FIG. 2. The transfer roller  30  includes a metal roller shaft covered by a roller portion formed of an ionic conductive rubber material. A transfer bias is applied to the transfer roller  30  from a transfer bias application power source (not shown). Accordingly, the visible toner image on the photosensitive drum  27  is transferred onto the paper sheet  3  while the paper sheet  3  passes between the photosensitive drum  27  and the transfer roller  30 .  
         [0034]    A developing cartridge  28  used as a developing unit, is detachably mounted on the photosensitive member cartridge  26 . The developing cartridge  28  includes the developing roller  31 , a toner layer thickness regulating blade  32 , a toner supply roller  33 , and a toner box  34 .  
         [0035]    The toner box  34  accommodates a positively chargeable non-magnetic single component toner, as a developing agent. The toner is, for example, a polymerized toner that is obtained by copolymerizing polymerizable monomers using a known polymerization method, such as a suspension polymerization method. The polymerizable monomers may be styrene-based monomers, such as styrene, and acrylic-based monomers, such as acrylic acid, alkyl (C1-C4) acrylate, and alkyl (C1-C4) methacrylate. Polymerized toner particles are spherical in shape, and thus have excellent fluidity. A coloring material, such as carbon black, and wax, as well as silica as an external additive are added to the toner particles to improve the fluidity of the toner. A toner particle size is approximately 6 to 10 μm.  
         [0036]    Disposed in a substantially central portion of the toner box  34  is a rotating shaft  35 . The rotating shaft  35  supports an agitator  36  that agitates the toner in the toner box  34  as the agitator  36  rotates in a counterclockwise direction as indicated by the arrow in FIG.  2 . The toner is discharged from a toner supply opening  37  provided in the toner box  34 . Provided in a side wall of the toner box  34  is a window  38  for detecting the amount of toner remaining in the toner box  34 . The window  38  is cleaned by a cleaner  39  supported by the rotating shaft  35 .  
         [0037]    The toner supply roller  33  is disposed to a side of the toner supply opening  37  and rotates in a clockwise direction as indicated by the arrow in FIG. 2. The developing roller  31  is disposed in contact with the toner supply roller  33  and rotates in a clockwise direction as indicated by the arrow in FIG. 2. The toner supply roller  33  and the developing roller  31  contact each other so as to apply some pressure to each other.  
         [0038]    The toner supply roller  33  includes a metal roller shaft covered by a roller portion formed of a conductive foam material. The developing roller  31  includes a metal roller shaft covered by a roller portion formed of a conductive rubber material. More specifically, the roller portion of the developing roller  31  is formed of conductive urethane rubber or silicone rubber including fine carbon particles. A surface of the roller portion of the developing roller  31  is coated with urethane rubber or silicone rubber including fluorine. A developing bias is applied to the developing roller  31  from a developing bias application power source (not shown).  
         [0039]    Disposed adjacent to the developing roller  31  is the toner layer thickness regulating blade  32  that regulates the thickness of the toner layer on the developing roller  31 . The regulating blade  32  includes a blade portion formed of a metal plate spring and a contact portion  40  attached to one end of the blade portion. The contact portion  40  has a semicircular cross-sectional shape and is formed of insulating silicone rubber. The other end of the blade portion is supported near the developing roller  31  by the developing cartridge  28 . The contact portion  40  presses the developing roller  31  with the elasticity of the plate spring.  
         [0040]    The toner discharged through the toner supply opening  37  is supplied onto the toner supply roller  33  and further onto the developing roller  31  by the rotation of the toner supply roller  33 . The toner is positively charged through friction charging at a contact portion of the toner supply roller  33  and the developing roller  31 . As the developing roller  31  rotates, the toner supplied onto the developing roller  31  enters between the contact portion  40  of the regulating blade  32  and the developing roller  31  where the toner is again charged through friction charging, to a sufficient degree. The toner passing between the contact portion  40  and the developing roller  31  is formed into a uniform-thickness thin toner layer on the developing roller  31 .  
         [0041]    Thereafter, in accordance with the rotation of the developing roller  31 , the toner carried on the developing roller  31  and charged with positive polarity is deposited on the electrostatic latent image formed on the photosensitive drum  27 , thereby making the image visible. In other words, the toner is supplied to parts of the photosensitive drum  27  selectively exposed to the laser light where the potential level is lower than the remaining part of the photosensitive drum surface uniformly positively charged. Thus, a visible toner image is formed on the photosensitive drum  27 .  
         [0042]    As shown in FIG. 1, the fixing unit  18  is disposed downstream of the process unit  17  in the sheet feeding direction. The fixing unit  18  includes a heat roller  41  and a pressure roller  42  that is pressed against the heat roller  41 , as well as a pair of feeding rollers  43  disposed downstream of the heat roller  41  and the pressure roller  42  in the sheet feeding direction. The heat roller  41  is formed of metal and is provided with a halogen lamp as a heat source. When the sheet  3 , which has the toner transferred thereon in the process unit  17 , passes between the heat roller  41  and the pressure roller  42 , the toner is fused by heat to fixedly adhere the toner to the sheet  3 . After the toner is fixedly adhered to the sheet  3 , the sheet  3  is provided by the feeding rollers  43 , to a discharge path  44 . The sheet  3  is fed to discharge rollers  45 , through the discharge path  44 , and is discharged onto the discharge tray  46  by the discharge rollers  45 .  
         [0043]    Exemplary embodiments of the scanner unit  16  will be described below. As shown in FIG. 3, the polygon mirror  20  is driven by a polygon motor (not shown) so as to rotate about a rotating shaft  20   a . In accordance with the rotation of the polygon mirror  20 , the beam of the laser light emitted from the laser diode array  19  scans in an axial direction of the photosensitive drum  27  (in the main scanning direction), across the surface of the photosensitive drum  27 , through the lenses  21 ,  23  and the reflecting mirror  22 . A beam detecting (BD) sensor  70  for determining scanning timing is disposed to a side of a scanning area. The BD sensor  70  receives a beam of the laser light that reaches a region out of a printable area of the sheet  3  used in the laser printer  1 .  
         [0044]    As shown in FIG. 4, the laser diode array  19  includes two laser diodes LD 1 , LD 2 , and a photodiode PD for detecting an amount of the laser light emitted from the laser diodes LD 1 , LD 2 . The beam of the laser light emitted from each of the laser diodes LD 1 , LD 2  is slightly shifted from each other in a sub scanning direction, reaching the surface of the photosensitive drum  27 . Accordingly, the surface of the photosensitive drum  27  is scanned with the laser light, two lines at a time.  
         [0045]    The laser diodes LD 1 , LD 2  are driven by laser diode drivers (LDDs)  71 ,  72 , respectively. The current that passes through the photodiode PD in response to the intensity of the light beam that the photodiode PD receives, is detected by a photodiode light amount detector  73 . The LDDs  71 ,  72 , the light amount detector  73 , and the BD sensor  70  communicate with a light amount controlling circuit  75 .  
         [0046]    An anode of the laser diodes LD 1 , LD 2  is connected to a power source  76 . When an output level of the LDD  71 , LDD  72  is low, the current is supplied to the laser diode LD 1 , LD 2 . Accordingly, the laser diodes LD 1 , LD 2  emit the laser light. An amount of the light emitted from the laser diodes LD 1 , LD 2  is associated with the current that passes therethrough. The LDDs  71  and  72  input thereto from the light amount controlling circuit  75 , Data 1 and Data 2 pulse signals, respectively, that makes a transition to LOW or HIGH in association with print data, as well as light amount data indicating the value of current that passes through the laser diodes LD 1 , LD 2  when the laser diodes LD 1 , LD 2  emit the light.  
         [0047]    The light amount controlling circuit  75  communicates with an image processing device  80  that communicates with a control panel  77  provided on the main casing  2  and an interface  78  through which the print data from an external device, such as a personal computer (PC) is input. The image processing device  80  includes a central processing unit (CPU)  81 , a read-only memory (ROM)  82 , and a random-access memory (RAM)  83 . The image processing device  80  generates and outputs rectangular Data 1 and Data 2 pulses, based on the print data input from the PC through the interface  78 . The image processing device  80  also outputs Cal 1 and Cal 2 pulse signals for providing timing for the photodiode PD to detect the laser light emitted from the laser diodes LD 1 , LD 2 , and a Half signal for reducing the amount of the laser light emitted from the laser diodes LD 1 , LD 2  to half of a normal light emitting amount. The Data 1, Data 2, Cal 1, Cal 2, and Half signals are input to the light amount controlling circuit  75 .  
         [0048]    The RAM  83  in the image processing device  80  includes an operating life register  83   a  for the laser diode LD 1  that stores data on an operating life of the laser diode LD 1 , and an operating life register  83   b  for the laser diode LD 2  that stores data on an operating life of the laser diode LD 2 . The image processing device  80  performs following operations based on the data stored in the operating life registers  83   a ,  83   b  for the laser diodes LD 1 , LD 2 , respectively.  
         [0049]    FIGS.  5  to  7  show flowcharts that the image processing device  80  performs. When the laser printer  1  is turned on, the image processing device  80  starts the main routine shown in FIG. 5. In step S 1 , it is determined whether a print command is entered as print data is input through the interface  78 . When the print command is not entered (S 1 : NO), flow waits in step S 1  for the print command to be entered. When the print command is entered (S 1 : YES), flow proceeds to step S 2  where a laser light output adjustment process is performed. The laser light output adjustment process will be described in detail below with reference to FIG. 8.  
         [0050]    [0050]FIG. 8 is a time chart showing changes of various signals during the laser light output adjustment process. As shown in FIG. 8, the image processing device  80  outputs low Data 2 and Data 1 rectangular pulses in this order at an interval that the polygon mirror  20 , which starts to rotate as the power is turned on, is rotated 60 degrees.  
         [0051]    As shown in FIG. 3, the polygon mirror  20  according to the embodiment is substantially regular hexagonal when viewed from the top. Each of the six sides of the polygon mirror  20  has a mirror finished surface. One side of the polygon mirror  20  is associated with one scanning line on the photosensitive drum  27 .  
         [0052]    While the polygon mirror  20  is rotated 60 degrees, a surface of the photosensitive drum  27  is scanned with the laser light beam in one way along a longitudinal direction of the photosensitive drum  27 .  
         [0053]    When the Data 2 or Data 1 pulse signal is turned into LOW, the laser diode LD 2  or LD 1  emits the laser light, as described above. The low rectangular Data 2 and Data 1 pulses are sequentially output at the timing that the laser light emitted from the laser diodes LD 2 , LD 1  are reflected off the polygon mirror  20  and directed to a region outside the printable area toward the BD sensor  70 . The low Data 1 pulse is output longer to ensure the BD sensor  70  to detect the laser light emitted from the laser diode LD 1 . A signal output by the BD sensor  70  is shown as BDN in FIG. 8. Based on the BDN signals, output timings of the Data 1 and Data 2 pulses are determined.  
         [0054]    The image processing device  80  outputs low Cal 1 and Cal 2 rectangular pulses, in synchronization with the output of the low Data 1 and Data 2 pulses, respectively. When the low Cal 1 and Cal 2 pulses are output, an adjustment is made to the light amount data based on the intensity of the light detected by the light amount detector  73  in order for the laser diodes LD 1 , LD 2  to emit the laser light at a predetermined intensity. For example, at a timing indicated by “A” in FIG. 8, a value of the current supplied to the laser diode LD 1  is adjusted such that the laser diode LD 1  emits the beam of the laser light at the predetermined intensity. At a timing indicated by “B” in FIG. 8, a value of the current supplied to the laser diode LD 2  is adjusted such that the laser diode LD 2  emits the beam of the laser light at the predetermined intensity.  
         [0055]    The thus adjusted values of the current are stored in a predetermined area of the RAM  83 , as well as in the operating life registers  83   a ,  83   b  for the laser diodes LD 1 , LD 2 . As a total light emitting time of a laser diode increases, a current required for the laser diode to emit the light at a predetermined intensity also increases. In this embodiment, the values of the current after being adjusted are used as parameters for determining the end of the operating life of the laser diodes LD 1 , LD 2 .  
         [0056]    When the Cal 1 and Cal 2 pulses are output, voltage or power applied to the laser diodes LD 1 , LD 2  may be adjusted instead of current. The values of the voltage or power after being adjusted may be stored in the operating life registers  83   a ,  83   b  for the laser diodes LD 1 , LD 2 . The values of current, voltage, or power properly reflect the substantial operating life of each of the laser diodes LD 1 , LD 2 , which generally have some quality variances between the laser diodes LD 1 , LD 2 .  
         [0057]    Referring back to FIG. 5, after the laser light output adjustment process is performed in step S 2 , the process continues to step S 3  whether it is determined whether a low resolution mode is designated using the control panel  77  of the laser printer  1  or the PC. When the low resolution mode is not designated (S 3 : NO), the number of the laser diodes to be used for scanning is set to two in step S 4  to use both laser diodes LD 1 , LD 2 . Thereafter, a laser diode operating life check is made in step S 5 .  
         [0058]    The laser diode operating life check will now be described in detail with reference to FIG. 6. In step S 51 , it is determined whether the current value stored in the operating life register  83   a  for the laser diode LD 1  is greater than a maximum allowable current value (hereinafter referred to as the maximum value). More specifically, as the total light emitting time of the laser diode LD 1  becomes longer, that is, as the laser diode LD 1  approaches the end of its operating life, a current value to be stored in the operating life register  83   a  becomes greater. Accordingly, it is determined in step S 51  whether the value stored in the operating life register  83   a  exceeds the maximum value and the life of the laser diode LD 1  has expired.  
         [0059]    When the value stored in the operating life register  83   a  for the laser diode LD 1  is equal to or less than the maximum value (S 51 : NO), it is determined in step S 52  whether the value stored in the operating life register  83   b  for the laser diode LD 2  is greater than the maximum value. When the value stored in the operating life register  83   b  is equal to or less than the maximum value (S 52 : NO), flow proceeds to step S 6  shown in FIG. 5. In other words, the operating life of each laser diode LD 1 , LD 2  has not yet expired, so that the laser diodes LD 1 , LD 2  can be both usable. Thus, the setting made in step S 4  to use both laser diodes LD 1 , LD 2  can be maintained.  
         [0060]    In step S 52 , when it is determined that the value stored in the operating life register  83   b  for the laser diode LD 2  exceeds the maximum value and the operating life of the laser diode LD 2  has expired (S 52 : YES), flow proceeds to step S 53  where non-use of the laser diode LD 2  and use of the laser diode LD 1  are set. Thereafter, flow proceeds to step S 6  in FIG. 5. Accordingly, the laser diode LD 2  whose life has expired is not used. Using only the laser diode LD 1 , a printing operation is carried out at substantially a lower resolution, while one line is being thinned out or skipped, even when the low resolution mode is not designated.  
         [0061]    In step S 51 , when it is determined that the value stored in the operating life register  83   a  for the laser diode LD 1  exceeds the maximum value and the life of the laser diode LD 1  has expired (S 51 : YES), flow proceeds to step S 54  where it is determined whether the value stored in the operating life register  83   b  for the laser diode LD 2  exceeds the maximum value. When the value stored in the operating life register  83   b  is equal to or less than the maximum value (S 54 : NO), non-use of the laser diode LD 1  and use of the laser diode LD 2  are set in step S 55 . Thereafter, flow proceeds to step S 6  in FIG. 5. Accordingly, the laser diode LD 1  whose life has expired is not used. Using only the laser diode LD 2 , the printing operation is carried out at substantially a lower resolution, while one line is being thinned out or skipped, even when the low resolution mode is not designated.  
         [0062]    When it is determined that the operating life of the laser diode LD 1  has expired (S 51 : YES) and then determined that the value stored in the operating life register  83   b  for the laser diode LD 2  exceeds the maximum value (S 54 : YES), neither laser diodes LD 1  nor LD 2  can be used, because the operating lives of both of the laser diodes LD 1 , LD 2  have expired. In this case, an error occurrence is stored in the RAM  83  in step S 56  and then flow proceeds to step S 6 .  
         [0063]    Referring back to FIG. 5, when the low resolution mode is designated (S 3 : YES), flow proceeds to step S 7  where a laser diode selection process is performed to select which laser diode LD 1  or LD 2  is used for scanning. Then, flow proceeds to step S 6 .  
         [0064]    The laser diode selection process performed in step S 7  will be described in detail with reference to a flowchart shown in FIG. 7. In step S 71 , it is determined whether the value stored in the operating life register  83   a  for the laser diode LD 1  is equal to the value stored in the operating life register  83   b  for the laser diode LD 2 . When the values stored in the operating life registers  83   a ,  83   b  are not equal (S 71 : NO), flow proceeds to step S 72  where it is determined whether the value stored in the operating life register  83   a  for the laser diode LD 1  is greater than the value stored in the operating life register  83   b  for the laser diode LD 2 .  
         [0065]    When the value in the operating life register  83   a  is greater than the value in the operating life register  83   b  (S 72 : YES), flow proceeds to step S 73  where the use of the laser diode LD 2  is set. In the subsequent step S 74 , it is determined whether the value in the operating life register  83   b  exceeds the maximum value. When it is determined that the value in the register  83   b  does not exceed the maximum value (S 74 : NO), flow proceeds to step S 6  in FIG. 5. Thus, use of the laser diode LD 2  associated with the operating life register  83   b  storing the smaller value, is selected for scanning.  
         [0066]    In step S 72 , when the value in the operating life register  83   a  for the laser diode LD 1  is not greater than the value in the operating life register  83   b  for the laser diode LD 2  (S 72 : NO), flow proceeds to step S 75  where the use of the laser diode LD 1  is set. Thereafter, in the step S 76 , it is determined whether the value in the operating life register  83   a  for the laser diode LD 1  exceeds the maximum value. When it is determined that the value in the register  83   a  does not exceed the maximum value (S 76 : NO), flow proceeds to step S 6  in FIG. 5. Thus, use of the laser diode LD 1  associated with the operating life register  83   a  storing the smaller value, is selected for scanning.  
         [0067]    When the values stored in the operating life registers  83   a ,  83   b  for the laser diodes LD 1 , LD 2  are equal (S 71 : YES), flow proceeds to step S 77  where it is determined whether the laser diode used for a previous printing operation performed using either laser diode LD 1  or LD 2  is the laser diode LD 2 . When it is determined that the laser diode LD 2  was used for the previous printing operation (S 77 : YES), flow proceeds to step S 75  where the use of the laser diode LD 1  is set. When it is determined that the laser diode LD 2  was not used for the previous printing operation (S 77 : NO), flow proceeds to step S 73  where the use of the laser diode LD 2  is set. In other words, when the values stored in the operating life registers  83   a ,  83   b  are equal (S 71 : YES), the laser diodes LD 1 , LD 2  are alternatively selected.  
         [0068]    In step S 74 , when it is determined that the value in the operating life register  83   b  for the laser diode LD 2  exceeds the maximum value (S 74 : YES), an error occurrence is stored in the RAM  83  in step S 78 , because the value stored in the operating life register  83   a  for the laser diode LD 1  also exceeds the maximum value, that is, the operating live of both of the laser diodes LD 1 , LD 2  have expired. Thereafter, flow proceeds to step S 6  in FIG. 5.  
         [0069]    Similarly, in step S 76 , when it is determined that the value in the operating life register  83   a  for the laser diode LD 1  exceeds the maximum value (S 76 : YES), an error occurrence is stored in the RAM  83  in step S 78 , because the value stored in the operating life register  83   b  for the laser diode LD 2  also exceeds the maximum value, that is, the operating lives of both of the laser diodes LD 1 , LD 2  have expired. Thereafter, flow proceeds to step S 6  in FIG. 5.  
         [0070]    Referring back to FIG. 5, when flow proceeds to step S 6  after step S 5  or S 7 , it is determined whether the error occurrence is stored in the RAM  83  in step S 56  in the laser diode operating life check in step S 5 , or in step S 78  in the laser diode selection process in step S 7 . When the error occurrence is stored in step S 56  or S 78  (S 6 : YES), flow proceeds to step S 14  where an error message is output to the control panel  77  and the laser printer  1  is stopped, whereupon the process flow ends.  
         [0071]    When the error occurrence is not stored in the RAM  83  (S 6 : NO), flow proceeds to step S 11  where a print start is instructed. Thereafter, the printing operation is started in accordance with another routine (not shown), using both or one of the laser diodes LD 1 , LD 2 .  
         [0072]    [0072]FIG. 9 is a time chart showing changes of various signals when the printing operation is performed using both laser diodes LD 1 , LD 2 . The low Data 2 pulse is output at the time when the beam of laser light emitted from the laser diode LD 2  reaches an area of the photosensitive drum  27  associated with a region outside of the printable area. The low Data 1 pulse is output at the time when the Data 2 pulse signal makes a LOW to HIGH transition.  
         [0073]    Based on the Data 1 and Data 2 pulses, the laser diodes LD 1 , LD 2  emit the laser light. At the time the laser diodes LD 1 , LD 2  emit the laser light, the low Cal 1 and Cal 2 pulses are output in synchronization with the Data 1 and Data 2 pulses, respectively. At this time, the amounts of laser light emitted from the laser diodes LD 1 , LD 2  are detected by the light amount detector  73 . Based on the detected light emitting amounts, the light amount controlling circuit  75  determines the current to be supplied to the laser diodes LD 1 , LD 2  by the driving of the LDD 1 , LDD 2 . The determined current values are stored in the operating life registers  83   a ,  83   b  for the laser diodes LD 1 , LD 2 .  
         [0074]    The beam of the laser light emitted from the laser diode LD 1  in accordance with the low Data 1 rectangular pulses, is detected by the BD sensor  70 . Based on the detection the laser beam by the BD sensor  70 , a BDN rectangular pulse is output. Detection by the light amount controlling circuit  75  of a falling edge of the BDN pulse output from the BD sensor  70  triggers the output of the Data 1, Data 2 pulse signals for one line to the LDD 1 , LDD 2 . More specifically, the light amount controlling circuit  75  outputs the Data 1, Data 2 pulse signals for one line to the LDD 1 , LDD 2 , in accordance with the Data 1, Data 2 pulse signals that are output from the image processing device  80  based on image data. In FIG. 9, the Data 1, Data 2 pulse signals for one line is indicated by “X”, and the transitions of the pulse signals are not specified for the simplicity of the illustration. In practice, the Data 1, Data 2 pulse signals make fine transitions from LOW to HIGH and vice versa, in accordance with the image data.  
         [0075]    When the Data 1, Data 2 pulse signals output from the LDD 1 , LDD 2  make a transition to LOW, the laser beams are emitted from the respective laser diodes LD 1 , LD 2 . The laser beams emitted from the laser diodes LD 1 , LD 2  are reflected off the polygon mirror  20  and scan across the surface of the photosensitive drum  27 .  
         [0076]    When the Data 1, Data 2 pulse signals for one line are completely output based on the image data, the light amount controlling circuit  75  again outputs the low Data 2 rectangular pulse, and then low Data 1 rectangular pulse at the timing when the Data 2 pulse makes a LOW to HIGH transition, as described above. The pulse signals are thus output repeatedly during the printing operation.  
         [0077]    Referring back to FIG. 5 , in step S 12 , when printing for one page is finished (S 12 : YES), it is determined in step S 13  whether there is another page to be printed. When there is another page to be printed (S 13 : YES), flow returns to step S 3  and the abovedescribed process steps are repeated. When there is no page to be printed (S 13 : NO), flow ends.  
         [0078]    [0078]FIG. 9 shows the time chart when both laser diodes LD 1 , LD 2  are used. In the case where only a single laser diode LD 1  or LD 2  is used, the laser diode LD 1  or LD 2 , whichever is used, may be controlled in accordance with the Data 1 and Cal 1 pulse signals. In this case, an output timing of a falling edge of the low Data 1 pulse, may be the same as the output timing of that of the low Data 2 pulse shown in FIG. 9, or slightly thereafter.  
         [0079]    In the laser printer  1  according to the embodiment, for the printing operation using only one of the laser diodes LD 1 , LD 2 , the values stored in the operating life registers  83   a ,  83   b  are compared. Based on the comparison result, driving of the laser diode LD 1 , LD 2  associated with the operating life register  83   a ,  83   b  storing the greater value, that is, the driving of the laser diode LD 1 , LD 2  having the shortest operating life, is stopped. Accordingly, both laser diodes LD 1 , LD 2  become unusable at timings close to each other.  
         [0080]    The laser diode array  19  is normally soldered onto a printed circuit board. When the laser diode LD 1 , LD 2  becomes unusable, replacement of the printed circuit board, the scanner unit  16 , or the laser printer  1  is required. In the case where a laser printer includes a plurality of laser diodes, like the laser printer  1  according to the embodiment, if one of the laser diodes becomes unusable, other usable laser diodes also have to be replaced with new ones together with the unusable laser diode. However, in the laser printer  1  according to the embodiment, the usable laser diode LD 1  or LD 2  is not wasted, because the laser printer  1  is controlled such that a plurality of the laser diodes LD 1 , LD 2  become unusable at the timings close to each other. Thus, the best possible use of the laser diodes LD 1 , LD 2  during their usable periods, may be made without wasting any of the usable laser diode LD 1  or LD 2 .  
         [0081]    When only one laser diode LD 1  or LD 2  is used for the printing with the laser printer  1 , the values stored in the operating life registers  83   a ,  83   b  are compared. Based on the comparison result, driving of the laser diode LD 1 , LD 2  associated with the operating life register  83   a ,  83   b  storing the greater value is stopped, as described above. Therefore, a specific laser diode is not continuously used or does not become unusable earlier. Thus, the best possible use of the laser diodes LD 1 , LD 2  may be made during their usable periods.  
         [0082]    The current values that reflect quality variances among the laser diodes are used for the comparison of an operating life of the laser diodes LD 1 , LD 2 . By the use of the current values, the time when the laser diodes LD 1 , LD 2  reach the end of the operating life thereof can be brought close to each other with great accuracy.  
         [0083]    While the invention has been described in detail and with reference to the specific embodiment thereof, it would be apparent to those skilled in the art that various changes, arrangements and modifications may be applied therein without departing from the spirit and scope of the invention.  
         [0084]    For example, although the operating life registers  83   a ,  83   b  write thereinto the current values in the above-described embodiment, the operating life registers  83   a ,  83   b  may write thereinto data on the light emitting amount when a predetermined current is supplied to the laser diodes LD 1 , LD 2 . In this case, the light emitting amount becomes smaller as the total driving time of the laser diode LD 1 , LD 2  becomes longer. Thus, determination as to which laser diode LD 1  or LD 2  is closer to the end of the operating life thereof, can be made.  
         [0085]    Instead, the operating life registers  83   a ,  83   b  may write thereinto the number of image dots formed with the laser diodes LD 1 , LD 2 . In this case, the number of dots becomes greater as the total driving time of the laser diode LD 1 , LD 2  becomes longer. Thus, it may be determined which the laser diode LD 1 , or LD 2  is closer to the end of its operating life.  
         [0086]    Thus, a comparison process using the number of dots may be simplified as compared with that using the current values.  
         [0087]    A driving amount of the laser diode LD 1  during a period associated with the low Cal 1 pulse is slightly greater than that of the laser diode LD 2  during a period associated with the low Cal 2 pulse, because the Cal 1 pulse is longer than the Cal 2 pulses. However, the difference in the driving amounts of the laser diodes LD 1 , LD 2  is such a negligible amount for the calculation of the number of dots to determine the operating life of the laser diode LD 1 , LD 2 .  
         [0088]    In the above-described embodiment, when the low resolution mode is designated, the laser diode LD 1  or LD 2  which is closer to the end of its operating life is prevented from being driven. In the low resolution mode, the density of lines forming an image is reduced. Accordingly, the amount of the toner attached to the photosensitive drum  27  is reduced. Therefore, the low resolution mode refers to a toner saving mode. To save the consumption amount of the toner, the same processes according to the above-described embodiment may be performed.  
         [0089]    In the above-described embodiment, when the low resolution mode is designated, the driving of the one of the laser diodes LD 1 , LD 2  is stopped. However, when the low resolution mode or toner saving mode is designated, the photosensitive drum  27  may be exposed to the laser beams emitted from both of the laser diodes LD 1 , LD 2 , while an amount of the laser light emitting from one of the laser diodes LD 1 , LD 2  is reduced. Thus, a light exposure amount for the photosensitive drum  27  may be reduced without stopping the driving of one of the laser diodes LD 1 , LD 2 . With this structure, deterioration of image quality may be restricted as compared with the case where the driving of one of the laser diodes LD 1 , LD 2  is stopped.  
         [0090]    Shortening degree of the operating life of the laser diode LD 1 , LD 2  is lower when one dot image is formed with the light exposure amount being reduced, than when one dot image is formed without reducing the light exposure amount. When the number of dots are used for the determination of the operating life of the laser diodes LD 1 , LD 2 , weight needs to be given for the calculation of the number of dots.  
         [0091]    The light exposure amount may be reduced, for example, by reducing the current supplied to the laser diodes LD 1 , LD 2 , or by reducing the width of a pulse associated with one dot.  
         [0092]    In the former case where the current supplied to the laser diodes LD 1 , LD 2  is reduced, the light amount controlling circuit  75  outputs to the LDD 1   71  or LDD 2   72 , the light amount data indicating such current value that reduces the light emitting amount of the laser diode LD 1  or LD 2  to half, based on the Half signals output from the image processing device  80 , as shown in FIG. 4.  
         [0093]    [0093]FIG. 10A shows a dot formed by exposing the photosensitive drum  27  to the laser beam emitted from the laser diode LD 1 , LD 2  by the application of normal current A during a period corresponding to a normal pulse width B. FIG. 10B shows a dot formed by exposing the photosensitive drum  27  to the laser beam emitted from the laser diode LD 1 , LD 2  by the application of half of the normal current A during a period corresponding to the normal pulse width B.  
         [0094]    When the photosensitive drum  27  is exposed to the laser beam by the application of half of the normal current (A/2), as shown in FIG. 10B, the dot is shaped substantially the same as that formed by the application of the normal current A. However, the amount of the toner attached to the photosensitive drum  27  is reduced and the density of an image becomes lower. In this case, no changes are required for the Data 1, Data 2 rectangular pulses, which are output based on the image data. Only the output from the laser diode is adjusted, and the relatively good image quality is obtained.  
         [0095]    In the latter case where the width of a pulse associated with one dot is reduced, the image processing device  80  reduces the width of the Data 1, Data 2 rectangular pulse per dot. No changes in the light amount data indicating the current value are made.  
         [0096]    [0096]FIG. 10C shows a dot formed by exposing the photosensitive drum  27  to the laser beam emitted from the laser diode LD 1 , LD 2  by the application of the normal current A during a period corresponding to half of the normal pulse width B. As compared with the dot shown in FIG. 10A, the width of the dot shown in FIG. 10C is reduced approximately to half, though the density of the dot is not changed. As the width of the dots is reduced, the density of an image becomes lower and relatively good image quality is not obtained.  
         [0097]    In the above-described embodiment, even when the low resolution mode is not designated, the printing operation is performed at low resolution, if one of the laser diodes LD 1 , LD 2  has reached the end of its operating life (S 52 : YES or S 54 : NO), by stopping the driving of the laser diode LD 1  or LD 2  whose operating life has expired. In such case, however, the printing operation may be carried out at higher resolution using the other laser diode LD 1  or LD 2 , whose operating life has not yet expired, by reducing the rotating speed of the photosensitive drum  27  to half.  
         [0098]    In steps S 53  and S 55  shown in FIG. 6, the printing operation is performed by thinning out or skipping one line. Instead, the rotating speed of the photosensitive drum  27  may be reduced to half in steps S 53  and S 55 . In this case, the printing speed becomes slower, though an image can be formed using only one laser diode LD 1  or LD 2 , at the substantially same resolution as that of the image formed using two laser diodes LD 1 , LD 2 .  
         [0099]    If one of the laser diodes LD 1 , LD 2  has reached the end of its operating life (S 52 : YES or S 54 : NO) at the time of printing at the higher resolution, an message may be displayed on the control panel  77  to allow a user to select by the operation or input on the control panel  77 , either a low-resolution printing by thinning out or skipping one line, or a high-resolution printing by reducing the rotating speed of the photosensitive drum  27  to half. With this structure, it can be determined according to the user&#39;s desire, which of the printing speed or resolution takes precedence over the other.  
         [0100]    The invention may be applied to various electrophotographic image forming devices, such as facsimile machines, that include a plurality of laser diodes, other than laser printers. Although the number of the laser diodes used in the above-described embodiment is two, the invention may be applied to image forming devices including three or more laser diodes. In this case, the above-described processes may be performed by focusing on at least two specific laser diodes, so that the times when the at least two specific laser diodes reach the end of the operating life thereof can be brought close to each other.  
         [0101]    When the image forming device includes three or more laser diodes, the number of the laser diodes used for the low resolution mode or the toner saving mode may be predetermined. The predetermined number of the laser diodes may be selected from the laser diode having the longer operating life. For example, when an image forming device includes four laser diodes and the number of the laser diodes used for the low resolution mode is set to two, two laser diodes closer to the end of the operating life thereof may not be used, or may be used with the light emitting amount being reduced. If such setting is made that only one laser diode is used for the toner saving mode, the laser diode having the longest operating life may be used, and the other three laser diodes may not be used, or may be used with the light emitting amount being reduced.