Abstract:
An image forming apparatus including: an optical scanning unit for deflecting and scanning a light on a photosensitive body, the optical scanning unit having: a light source for emitting the light; a deflection section that deflects the emitted light; a driving section that drives the deflection section; and a temperature detecting section that detects the temperature of the driving section or the temperature in vicinity of the driving section; a developing section that develops an electrostatic latent image by depositing a developer on the electrostatic latent image formed on a surface of the photosensitive body by the optical scanning unit; a transfer section that transfers the developer deposited on the surface of the photosensitive body to a recording medium; and a controller for changing a drive speed of the driving section based on the temperature detected by the temperature detecting section.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an image forming apparatus for forming an image on a recording medium, and more particularly to an image forming apparatus of so-called electrophotographic method for forming an image by forming an electrostatic latent image on the surface of a photosensitive body, depositing a developer on the electrostatic latent image and transferring it onto the recording medium. 
   2. Description of the Related Art 
   Conventionally, an image forming apparatus has been provided, including an photosensitive body, an optical scanning unit for deflecting and scanning a light on the photosensitive body to form an electrostatic latent image on the surface of the photosensitive body, a developing section for developing the electrostatic latent image by depositing a developer on the electrostatic latent image formed on the surface of the photosensitive body by the optical scanning unit, and a transfer section for transferring the developer deposited on the surface of the photosensitive body by the developing section to the recording medium. In the image forming apparatus of this type, the electrostatic latent image is formed on the surface of the photosensitive body by deflecting and scanning the light, and developed by the developing section depositing the developer on the electrostatic latent image. The transfer section transfers the developer deposited on the surface of the photosensitive body onto the recording medium, whereby the image according to the deflecting and scanning is formed on the recording medium by the so-called electrophotographic method. 
   The well known optical scanning unit forms an electrostatic latent image according to the image data on the surface of the photosensitive body by emitting and deflecting the light to a polygon mirror driven and rotated by a polygon motor, and by making the emitted light intermittent according to the image data. 
   In recent years, in such image forming apparatus, the rotation speed of the polygon mirror is increased along with the higher image formation speed. Therefore, there is possibility that the temperature within the optical scanning unit rises excessively. It is conceivable to cool the polygon motor by a fan. However, if the fan is provided, the size and cost of the apparatus are increased, with the great noise produced. Therefore, it is desired to make the apparatus silent and reduce the cost by eliminating the fan, if possible. It has been proposed that when a certain number of sheets are printed successively, for example, the sheet feed interval is increased to drive the polygon motor at low speed (e.g., refer to JP-A-2003-66812). 
   SUMMARY OF THE INVENTION 
   However, when the continuous printing is controlled depending on whether or not the number of sheets reaches a prescribed number, the image formation cannot be performed fast in some cases, because the drive speed is reduced even when the temperature of the polygon mirror does not rise too much. Conversely, when the outside air temperature is high and the temperature of the polygon mirror rises quickly, there is possibility that the polygon mirror reaches a high temperature before the certain number of sheets are printed successively. 
   The present invention provides an image forming apparatus in which a driving section for driving a deflection section for deflecting the light is effectively prevented from being superheated without slowing it down unnecessarily. 
   According to an aspect of the present invention, there is provided an image forming apparatus including: a photosensitive body; an optical scanning unit for deflecting and scanning a light on the photosensitive body to form an electrostatic latent image on a surface of the photosensitive body, the optical scanning unit including: a light source for emitting the light; a deflection section that deflects the emitted light; a driving section that drives the deflection section; and a temperature detecting section that detects the temperature of the driving section or the temperature in vicinity of the driving section; a developing section that develops the electrostatic latent image by depositing a developer on the electrostatic latent image formed on the surface of the photosensitive body by the optical scanning unit; a transfer section that transfers the developer deposited on the surface of the photosensitive body by the developing section to a recording medium; and a controller for changing a drive speed of the driving section based on the temperature detected by the temperature detecting section. 
   In the invention as constituted above, the driving section drives the deflection section to deflect the light emitted from the light source and scan the light on the surface of the photosensitive body. Also, the temperature detecting section detects the temperature of the driving section or the temperature near the driving section, and the controller changes the drive speed of the driving section based on the temperature detected by the temperature detecting section. 
   Therefore, when the temperature of the driving section rises, the controller reduces the drive speed of the driving section to prevent the driving section from being superheated and impaired. Also, since the controller changes the speed of the driving section depending on the actual temperature of the driving section, the speed of the driving section is reduced only when the deceleration is practically required, whereby the image formation is sped up by driving the driving section at high speed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention may be more readily described with reference to the accompanying drawings: 
       FIG. 1  is a side cross-sectional view showing the constitution of a laser printer according to an embodiment of the present invention; 
       FIG. 2  is a perspective view showing the constitution of an optical scanning unit for the laser printer; 
       FIG. 3  is a block diagram showing the configuration of a control system for the laser printer; 
       FIG. 4  is a flowchart showing a printing process that is performed in the control system; and 
       FIG. 5  is a flowchart showing the printing process as continued. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An embodiment of the present invention will be described below with reference to the accompanying drawings.  FIG. 1  is a side cross-sectional view showing the constitution of a laser printer  1  as an image forming apparatus to which the invention is applied. The laser printer  1  has, within a main case  2 , an optical scanning unit  200  having a polygon mirror  220 , a feeder portion  4  for feeding a sheet  3  as the recording medium, a process cartridge  17  making up an image forming section for forming an image on the sheet  3  by developing an electrostatic latent image formed by a light beam scanned by the optical scanning unit  200 , and a fixing unit  18 , as shown in  FIG. 1 . In  FIG. 1 , the right side is a front face of the laser printer  1 . 
   A sheet discharge tray  46  disposed on the top of the main case  2  is formed to be recessed with respect to the outer edge of the main case  2  to receive and stack thereon the printed sheet  3 . The sheet discharge tray  46  is inclined, in which its inclination is smaller on the fore side (right side in  FIG. 1 ) of the laser printer  1  than the rear side (left side in  FIG. 1 ) of the laser printer  1 . Also, a cover  54  is disposed on the front face of the main case  2 , and opened or closed to mount or dismount the process cartridge  17 . 
   A sheet discharge path  44  is provided on the rear side (left side in  FIG. 1 ) within the main case  2  so that the sheet  3  fed from the fixing unit  18  provided on the rear side (left side in  FIG. 1 ) within the main case  2  is led to the sheet discharge tray  46  along the dashed line in  FIG. 1 . A sheet discharge roller  45  for discharging the sheet  3  is provided at an upper end of this sheet discharge path  44 . 
   A feeder portion  4  has a sheet feed roller  8  disposed on the bottom within the main case  2 , a sheet feed cassette  6  that can be mounted or dismounted through the front face of the laser printer  1 , and a sheet pressing plate  7  disposed within the sheet feed cassette  6  to support and press the sheets  3  against the sheet feed roller  8 . Moreover, the feeder portion  4  is provided with a separation pad  9  for separating one sheet  3  in cooperation with the sheet feed roller  8  at the time of sheet feed by being pressed against the sheet feed roller  8 , and a conveying roller  11  disposed on the downstream side of the sheet feed roller  8  in the conveying direction of the sheet  3  to convey the sheet  3 . Further, the feeder portion  8  is provided with a sheet powder cleaning roller  10  for removing the sheet powder of the sheet  3  when the sheet  3  is conveyed in cooperation with the conveying roller  11 , and a registration roller  12  disposed on the downstream side of the conveying roller  11  in the conveying direction of the sheet  3  to adjust the feed timing of the sheet  3  when forming the image. 
   The sheet pressing plate  7  is rotatable about a support shaft  7   a  disposed on the bottom face of the sheet feed cassette  6 , and urged toward the sheet feed roller  8  by a spring  7   b.  The sheet feed roller  8  and the separation pad  9  are opposed to each other, and the separation pad  9  is pressed against the sheet feed roller  8  by a spring  13 . 
   The sheet powder produced due to friction between the sheet  3  and the separation pad  9  in feeding the sheet is electrostatically adsorbed to the sheet powder cleaning roller  14  disposed on the downstream side of the separation pad  9  to cooperate with the sheet feed roller  8 , and then collected and removed by a sponge  14   a.  The sheet powder not removed by the sheet powder cleaning roller  14  is removed by a sheet powder cleaning roller  10 . 
   A double-side printing unit  26  is disposed above the sheet feed cassette  6 . The double-side printing unit  26  has the reversal conveying rollers  50   a,    50   b  and  50   c  disposed almost horizontally, and the reversal conveying paths  47   a,    47   b  are disposed at both ends of the double-side printing unit  26 . At the time of double-side printing, the sheet  3  is firstly printed on the surface, discharged from the fixing unit  18 , and switched back by the sheet discharge roller  45 . Then, the switched back sheet  3  is branched from the path of the dashed line in  FIG. 1  and conveyed via a reversal conveying path  47   a  to the double-side printing unit  26 . Then, the sheet  3  conveyed to the double-side printing unit  26  is conveyed via a reversal conveying path  47   b  to the registration roller  12 , and is printed on the back face by the image forming section. 
   A low voltage power supply board  90 , a high voltage power supply board  95  and an engine board  85  are disposed above the double-side printing unit  26 . To segregate these boards from the fixing unit  18  and the process cartridge  17 , a chute  80  is provided above these boards. A guide plate  81  formed on the top of the chute  80  makes up a part of the conveying path for the sheet  3 . The chute  80  bridges the main frames (not shown) at the left and right sides of the laser printer  1  while extending in a direction perpendicular to the sheet of  FIG. 1  to increase the rigidity of the laser printer  1 . 
   The low voltage power supply board  90  drops a single phase voltage of 100V supplied from the outside of the laser printer  1  to a voltage of 24V which is supplied to each part inside the laser printer  1 . Also, the high voltage power supply board  95  generates a high voltage bias applied to each part of the process cartridge  17 . The engine board  85  drives various DC motors including a main motor  100  (see  FIG. 3 ) for rotating the photosensitive drum  27  and each roller of the laser printer  1 , and drives a solenoid (not shown) for switching the rotation and stop of parts such as the registration roller  12 . 
   The process cartridge  17  making up a part of the image forming section has a drum cartridge  23  and a development cartridge  24  that can be mounted or dismounted on or from the drum cartridge  23 . The drum cartridge  23  has a photosensitive drum  27 , a Scolotron type charger  29  and a transfer roller  30 . The development cartridge  24  has a development roller  31 , a supply roller  33  and a toner hopper  34 . 
   The photosensitive drum  27  of the drum cartridge  23  is rotatable in a direction as indicated by the arrow in  FIG. 1  in a state contact with the development roller  31 . This photosensitive drum  27  has a positively charged organic photosensitive material coated on a conductive substrate, in which a charge generating material is dispersed into a charge transportation layer. When a light beam is applied to the photosensitive drum  27 , electric charges arise on the charge generating material due to optical absorption, and are transported through the charge transportation layer to the surface of the photosensitive drum  27  and the conductive substrate When the electric potential on the surface of the photosensitive drum  27  charged by the Scolotron type charger  29  is negated, a potential difference occurs between the potential of irradiated portion and the potential of non-irradiated portion. That is, the light beam is exposed and scanned according to the image data to be written, so that an electrostatic latent image is formed on the photosensitive drum  27 . 
   The Scolotron type charger  29  is disposed a certain distance above the photosensitive drum  27  to be out of contact with the photosensitive drum  27 . The Scolotron type charger  29  generates a corona discharge from a discharging wire of tungsten or the like, and is driven by a charging bias circuit (not shown) of the high voltage power supply board  95  to positively charge the surface of the photosensitive drum  27  uniformly. 
   In a state where the development cartridge  24  is attached on the drum cartridge  23 , the development roller  31  is placed on the downstream side of the Scolotron type charge  29  in the rotational direction of the photosensitive drum  27 , and rotatable in a direction as indicated by the arrow in  FIG. 1 . The development roller  31  is made by covering a conductive rubber material around a metallic shaft. A development bias is applied to the development roller  31  by a development bias circuit (not shown) of the high voltage power supply board  95 . 
   The supply roller  33  is disposed on the opposite side of the photosensitive drum  27  across the development roller  31 , and rotatable in a direction as indicated by the arrow in  FIG. 1 . The supply roller  33  is contacted with the development roller  31 . The supply roller  33  is made by covering a conductive foaming material around a metallic shaft, in which the toner supplied to the development roller  31  is frictionally charged. That is, the supply roller  33  is rotated in the same direction (clockwise in  FIG. 1 ) as the development roller  31 . 
   The toner hopper  34  is disposed beside the supply roller  33 , in which a developer filled inside the toner hopper  34  is supplied via the supply roller  33  to the development roller  31 . In this embodiment, the positively charged toner of nonmagnetic one-component is employed as the developer. This toner may be polymer toner that is produced by copolymerizing more than one species of polymeric monomer, for example, styrene based monomer such as styrene, or acryl based monomer such as acrylic acid, alkyl (C1 to C4) acrylate or alkyl (C1 to C4) methacrylate, by a well known polymerization method such as suspension polymerization. Such polymer toner is mixed with a coloring agent such as carbon black or a wax, and an additive agent such as silica is added to improve fluidity. The particle diameter is about 6 to 10 μm. 
   An agitator  36  is a coarse net plate extending axially (in the direction perpendicular to the sheet of  FIG. 1 ), and has a cross-sectional shape of a “dogleg character” as shown in  FIG. 1 . The agitator  36  is rotatable about a rotation axis  35  in a direction as indicated by the arrow in  FIG. 1 . A film member  36   a  for rubbing against an inner wall of the toner hopper  34  is provided at a radially outer end of the agitator  36  and a middle abdomen of the “dogleg character”. When the agitator  36  is rotated, the toner contained with the toner hopper  34  is agitated. 
   The transfer roller  30  rotatable in a direction as indicated by the arrow in  FIG. 1  is disposed on the downstream side of the development roller  31  in the rotation direction of the photosensitive drum  27 . The transfer roller  30  is made by covering an ion conductive rubber material around a metallic shaft. When a toner image on the photosensitive drum  27  is transferred onto the sheet  3 , a transfer bias is applied to the transfer roller  30  by a transfer bias circuit (not shown) of the high voltage power supply board  95 . The transfer bias means the bias applied to the transfer roller  30  to produce a potential difference between the surface of the photosensitive drum  27  and the surface of the transfer roller  30  so that the toner electrostatically deposited on the surface of the photosensitive drum  27  may be electrically sucked toward the surface of the transfer roller  30 . 
   In this laser printer  1 , a so-called cleanerless development system is employed in which after the toner is transferred from the photosensitive drum  27  onto the sheet  3  by the transfer roller  30 , the residual toner remaining on the surface of the photosensitive drum  27  is withdrawn by the development roller  31 . 
   The fixing unit  18  making up a part of the image forming section is disposed on the downstream side of the process cartridge  17  in a conveying direction of the sheet  3 . The fixing unit  18  has a fixing roller  41 , a pressure roller  42  for pressing the fixing roller  41 , and a pair of conveying rollers  43  disposed on the downstream side of the fixing roller  41  and the pressure roller  42  in the conveying direction of the sheet  3 . The fixing roller  41  is coated with a fluororesin around a hollow aluminum shaft and sintered. A heating halogen lamp  41   a  is disposed inside the fixing roller  41 . The pressure roller  42  is covered with a fluororesin tube around a shaft made of low hardness silicone rubber, and pressed against the fixing roller  41  by a spring (not shown). The toner image transferred on the sheet  3  in the process cartridge  17  is pressurized and heated when the sheet  3  passes between the fixing roller  41  and the pressure roller  42 , and fixed on the sheet  3 . Then, the sheet  3  is conveyed to the sheet discharge path  44  by the conveying roller  43 . 
   Referring to  FIGS. 1 and 2 , the optical scanning unit  200  will be described below.  FIG. 2  is a perspective view showing the optical scanning unit  200  in a state where an upper lid member  201  is removed, as seen from the right upper in  FIG. 1 . As shown in  FIG. 1 , a housing of the optical scanning unit  200  is composed of a scanner frame  202  made of resin mixed with a reinforcing agent such as glass fiber, an upper lid member  201  made of iron to cover its upper part, and a tray  203  made of steel secured by screws to the main frame (not shown) at both the left and right sides of the laser printer  1  to support the scanner frame  202 . 
   The tray  203  is formed like a rectangular shallow box, as shown in  FIG. 2 . An opening portion  203   a  (see  FIG. 1 ) for applying a light beam L to the photosensitive drum  27  is provided at an almost central position of the tray  203 . The scanner frame  202  is provided with an outer wall  202   a  extending vertically from the bottom face of the tray  203 , and a partition wall  202   b  extending from near the middle of the outer wall  202   a  to partition the scanner frame  202  into two upper and lower layers. 
   A laser unit  300  for emitting a light beam L and a cylindrical lens  210  for refracting the light beam L from the laser unit  300  vertically and forming an image on the polygon mirror  220  are disposed on an upper layer of the scanner frame  202  located on the partition wall  202   b.  Moreover, the polygon mirror  220  having six reflecting surfaces for reflecting the light beam L, an fθ lens  230  for converting the light beam L reflected from the polygon mirror  220  and scanned at equal angular velocity into equal velocity of scanning, and a mirror  240  for reflecting the light beam L passing through the fθ lens  230  to a lower layer of the scanner frame  202  are disposed on the upper layer of the scanner frame  202 . 
   A mirror  250  for further reflecting the light beam L reflected from the mirror  240 , a cylindrical lens  260 , and a mirror  270  for reflecting the light beam L passing through the cylindrical lens  260  toward the surface of the photosensitive drum  27  are disposed on the lower layer of the scanner frame  202 , as shown in  FIG. 1 . 
   Also, a polygon motor  221  for driving and rotating the polygon mirror  220 , a circuit board  204  for adjusting the output of light beam emitted from the laser unit  300 , and a thermistor  207  for detecting the atmospheric temperature near the polygon mirror  221  are provided within the scanner frame  202  in which the laser unit  300  is placed, and held in the scanner frame  202 , as shown in  FIG. 2 . An adjusting hole  205  is punched in the circuit board  204  to insert a driver when adjusting the optical axis of the laser unit  300 . Also, an inspection hole  202   c  for inspecting the optical axis is provided on a wall face of the scanner frame  202  on the extension line of the optical axis (dashed line in  FIG. 2 ) of the light beam emitted from the laser unit  300  to the polygon mirror  220 . 
   A control system for the main motor  100  and the polygon mirror  221  will be described below. A main board (not shown) is provided with a controller  510  as shown in  FIG. 3 . This controller  510  is a well known microcomputer having, as the main parts, a CPU  511 , ROM  512  and RAM  513 . 
   This controller  510  is connected to an air temperature sensor  520  for detecting the atmospheric temperature (hereinafter referred to as air temperature) of the laser printer  1 , in addition to the thermistor  207 , and a connector  530  for reading the image data to be printed from a network or a personal computer. The controller  510  performs the following process, based on the data inputted from these sensors, to drive the main motor  100  via a motor drive circuit  101  and the polygon mirror  221  via a motor drive circuit  222 . Though the components related with the following process are only illustrated in  FIG. 3 , various other sensors are also connected to the controller  510  in practice. Also, the air temperature sensor  520  is provided at a position far away from the fixing unit  18  and near the polygon motor  221 , as shown in  FIG. 1 . 
     FIGS. 4 and 5  are flowcharts showing a printing process that is performed by the controller  510 . The controller  510  repeatedly performs this process after turning on the power. When the process is started, the controller  510  firstly waits for a rotation request to be made at S 1  (S denotes a step, same below). That is, when the controller  510  receives the image data via the connector  530 , the image data is converted into bit map data through a well known process of another routine, and after the completion of the process, a rotation request of rotating the sheet feed roller  8  is issued. Thus, the controller waits for the rotation request to be made at S 1 . 
   When the rotation request is issued (S 1 : YES), it is determined at S 2  whether or not the air temperature is sufficiently low, based on a detection signal from the air temperature sensor  520 . When the air temperature is sufficiently low (e.g., lower than 30° C. (corresponding to the first atmospheric temperature)), no process for preventing the polygon mirror  221  from being superheated is required. Thus, if the air temperature is sufficiently low (S 2 : YES), the operation goes to S 16 , or if not (S 2 : NO), the operation continues to S 3 . 
   At S 3 , it is determined whether or not the atmospheric temperature (hereinafter referred to as the temperature of the polygon motor  221 ) near the polygon mirror  221  exceeds 60° C. (corresponding to the second temperature), based on a detection signal from the thermistor  207 . Since the temperature is usually 60° C. or less immediately after the start of printing, a negative determination is made here, and the operation goes to S 4 . At S 4 , it is determined whether or not the temperature of the polygon motor  221  exceeds 55° C. (corresponding to the third temperature). Since the temperature is usually 55° C. or less immediately after the start of printing, a negative determination is made here, and the operation goes to S 5 . At S 5 , it is determined whether or not the temperature of the polygon motor  221  exceeds 50° C. (corresponding to the first temperature). Since the temperature is usually 50° C. or less immediately after the start of printing, a negative determination is made here, and the operation goes to S 6 . 
   At S 6 , the drive speed of the polygon mirror  221  is set to a normal speed (e.g., 28 ppm), the drive speed of the main motor  100  is set to a corresponding normal speed, and the operation goes to S 7 . At S 7 , the controller waits for one page of printing to be ended while driving the polygon motor  221  and the main motor  100  at the normal speeds. When one page of printing is ended (S 7 : YES), it is determined at S 8  whether or not the image data received via the connector  530  contains the next page of data. 
   When the image data contains the next page of data (S 8 : YES), the operation transits to S 3 . This transition timing is almost the same as the timing when the transfer roller  30  finishes conveying the sheet  3  at the previous page. On the other hand, if all the image data has been completely printed and there is no next page of data (S 8 : NO), the polygon mirror  221  and the main motor  100  are stopped successively at S 9 , and the process is once ended. When the amount of image data is small, the printing is usually continued at the normal speed while repeating the processing of S 3 , S 4 , S 5 , S 6 , S 7  and S 8 . When the printing is ended, each motor is stopped (S 9 ). 
   However, when the amount of image data is large, the temperature of the polygon mirror  221  rises during the printing. Particularly, nowadays when the high speed printing at high pixel density is required, this problem is significant. Thus, the following process is performed in this embodiment. 
   That is, if the temperature of the polygon mirror  221  exceeds 50° C. during the printing (S 5 : YES), the operation goes to S 11 , where the drive speed of the polygon motor  221  is set to the second speed (e.g., 24 ppm), and the drive speed of the main motor  100  is set to the corresponding second speed. Then, the printing is performed at the second speed during the process following S 7 , whereby the temperature elevation of the polygon motor  221  is relieved. 
   Also, if the temperature of the polygon mirror  221  exceeds  55 C during the printing (S 4 : YES), the operation goes to S 12 , where the drive speed of the polygon motor  221  is set to the third speed (e.g., 20 ppm), and the drive speed of the main motor  100  is set to the corresponding third speed. Then, the printing is performed at the third speed during the process following S 7 , whereby the temperature elevation of the polygon motor  221  is further relieved. 
   Moreover, if the temperature of the polygon mirror  221  during the printing exceeds 60° C. that is its critical operation temperature (S 3 : YES), the polygon motor  221  and the main motor  100  are stopped at S 13 , and the operation goes to S 7 . In this case, each motor is stopped and the printing does not proceed, whereby a standby state at S 7  is continued. In this case, after the polygon motor  221  is cooled, the user performs a well known reset operation, whereby the process of this routine is performed again from S 1 , and if the temperature of the polygon motor  221  is 60° C. or less, the printing is performed. 
   Also, if the temperature of the polygon mirror  221  during the printing at the third speed (S 12 ) drops to 55° C. or below (S 4 : NO), the printing is performed by switching to the normal speed or the second speed (S 6  or S 11 ). Moreover, if the temperature of the polygon mirror  221  during the printing at the second speed (S 11 ) drops to 50° C. or below (S 5 : NO), the printing is performed by switching to the normal speed (S 6 ). 
   Through the above process, the polygon motor  221  is effectively prevented from being superheated. And since the drive speed is reduced stepwise according to the actual temperature of the polygon motor  221 , the printing is performed at as high speed as possible. Moreover, when the temperature of the polygon mirror  221  falls, the drive speed is increased from the third speed to the second speed and from the second speed to the normal speed, whereby the printing is further sped up. Also, the switching of the drive speed occurs after the printing of one page and before the printing of the next page (i.e., when the sheet  3  is not opposed to the transfer roller  30 ) (see S 7 ), and the speed of the polygon mirror  221  and the speed of the main motor  100  are changed at the same time, whereby the image formation (printing) is made more excellently. 
   On the other hand, when the air temperature is sufficiently low, and there is no fear that the polygon motor  221  is superheated (S 2 : YES), the same processing (S 16  to S 19 ) of S 6  to S 9  is performed without referring to the detection signal of the thermistor  207 , as shown in  FIG. 5 . That is, at S 16  the drive speed of the polygon motor  221  is set to the normal speed, and the drive speed of the main motor  100  is set to the corresponding normal speed. At S 17 , the controller waits for one page of printing to be ended while driving the polygon motor  221  and the main motor  100  at the normal speeds. When one page of printing is ended (S 17 : YES), it is determined whether or not the image data contains the next page of data (S 18 ). When the image data contains the next page of data (S 18 : YES), the operation goes to S 16  to continue printing at the normal speed. When all the pages have been completely printed (S 18 : NO), each motor is stopped (S 19 ), and the process is once ended. In this way, in this embodiment, when the air temperature is sufficiently low and there is no fear that the polygon motor  221  is superheated, the excess process is omitted. 
   In this embodiment, the photosensitive drum  27  functions as the photosensitive body, the development roller  31  functions as the development section, the transfer roller  30  functions as the transfer section, the laser unit  300  functions as the light source, the polygon mirror  220  functions as the deflection section, the polygon motor  221  functions as the driving section, the thermistor  207  functions as the temperature detecting section, the controller  510  functions as the controller, the main motor  100  functions as the second driving section, the air temperature sensor  520  functions as the atmospheric temperature detecting section, and the scanner frame  202  functions as the housing. 
   This invention is not limited to the above embodiment, but may be implemented in various other forms without departing from the spirit or scope of the invention. For example, when the temperature of the polygon mirror  221  exceeds 60° C., or immediately before 60° C., the drive speed of the polygon mirror  221  may be further reduced. If the speed is reduced excessively, the control of the polygon mirror  221  becomes difficult. Thus, the speed is changed in a range up to 30% in the embodiment. Accordingly, the control of the polygon motor  221  becomes easier in this embodiment. 
   Also, when the detected temperature of the thermistor  207  at the time of power on is regarded as the air temperature at that time, the air temperature sensor  520  may be eliminated and the detected temperature of the thermistor  207  may be employed as the air temperature. Further, the temperature of the polygon mirror  221  may be detected directly from the impedance of a coil for the polygon mirror  221  or the housing temperature of the polygon motor  221 , and the drive speed may be switched based on its temperature, as previously described. Furthermore, the deflection section is not limited to the polygon mirror but may be implemented in various forms.