Abstract:
An image reading apparatus includes an LED light source, a CCD sensor receiving light reflected from a document irradiated with light from the LED light source, a fan, and a frame housing the LED light source, the CCD sensor and the fan. The fan circulates air in a substantially sealed space formed by the frame. Thus, temperature difference in the image reading apparatus can be reduced while not degrading dust-proof property of the apparatus, and efficient heat radiation to the outside becomes possible. Thus, variation of sensitivity of CCD sensor, deviation of point of focus of image forming system and deviation of reading position caused by temperature increase can be prevented.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This nonprovisional application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2011-240035 filed in Japan on Nov. 1, 2011, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an image reading apparatus capable of preventing temperature increase of a light source for irradiating a document with light and of a reading sensor. 
     Description of the Background Art 
     Generally, in an image reading apparatus, a document surface is irradiated with light emitted from a light source, the light reflected from the document surface is directed to an image pick-up surface of a reading sensor using an optical system including a lens or the like to form an image, and thereby, an image on the document is read. 
     In such an image reading apparatus, the light source is kept on during an image reading period. Therefore, during the image reading period, the light source generates heat and its temperature increases. Typically, the light source and the reading sensor are placed in an enclosure for preventing entrance of dust and, therefore, temperature in the enclosure of image reading apparatus tends to increase because of heat generated by the light source and, naturally, the temperature of reading sensor also increases. 
     As the reading sensor, a CCD (Charge Coupled Device) image sensor is generally used. In the image reading period, when a sensor driving circuit driving the CCD image sensor enters an operating state, clocks are supplied from the sensor driving circuit to the CCD image sensor. Therefore, in the image reading period, the temperature is higher than in a standby state, because of self-heating of the reading sensor. 
     When the temperature of reading sensor increases, sensitivity of the reading sensor may vary, or members forming the reading sensor may deform because of thermal expansion, possibly causing problems such as deviation of a point of focus of the image forming system, or deviation of reading position. Therefore, conventionally, a cooling mechanism has been provided in the enclosure of image reading apparatus, and the light source lamp for the document and the reading sensor are cooled only during the reading period. By way of example, Japanese Patent Laying-Open No. 8-102822 (hereinafter referred to as &#39;822 Reference) discloses a technique of arranging a cooling fan on a wall surface of an enclosure, and by introducing external air from the outside into the enclosure, cooling the light source lamp for the document and the reading sensor. By such an approach, during the image reading period, temperatures of various members including the reading sensor are maintained close to a prescribed temperature. 
     As described above, a reading unit for reading a document is in most cases formed to have a sealed structure to prevent entrance of paper powder or dust, and to prevent any foreign material from adhering to optical components and the like arranged therein. With the reading unit having such a sealed structure, the temperature of light source and its vicinity in the reading unit attains very high. If an opening is provided at the reading unit to prevent temperature increase, however, there arises a problem that inside of the scanner (image reading apparatus) is contaminated by dust or foreign matters. The problem of scanner contamination is similarly experienced if the external air is introduced to the inside of enclosure to cool the light source lamp for the document and the reading sensor, as in &#39;822 Reference. 
     It may be possible to provide a fan on a wall surface of a scanner frame, to discharge air that is heated to a high temperature in the scanner. The scanner unit, however, is substantially sealed and, hence, it is difficult to efficiently discharge high-temperature air in the scanner. Therefore, sufficient cooling effect cannot be attained. If an external air inlet is provided, the efficiency of discharge can be increased, whereas dust-proof property degrades. 
     SUMMARY OF THE INVENTION 
     In view of the problems above, it is desirable to provide an image reading apparatus capable of efficiently preventing temperature increase of the light source for irradiating a document surface and the reading sensor, while maintaining dust-proof property. 
     According to an aspect, the present invention provides an image reading apparatus, including: a light emitting unit emitting light; a light receiving unit receiving light reflected from a document irradiated with light from the light emitting unit; an air blower unit blowing out sucked air to one direction; and a housing unit housing the light emitting unit, the light receiving unit and the air blower unit; and the air blower unit circulates air in a space formed by the housing unit. 
     Preferably, the housing unit forms a substantially sealed space preventing entrance of air from the outside as the air blower unit blows air. 
     More preferably, the air blower unit is arranged close to the light receiving unit. 
     More preferably, the housing unit has a rectangular parallelepiped shape having an upper surface, a lower surface opposite to the upper surface and four side surfaces; the air blower unit is arranged between the light receiving unit and one of the side surfaces of the housing unit; and air blowing direction of the air blower unit is along the side surface opposite to the light receiving unit with the air blower unit positioned in between. 
     Preferably, the image reading apparatus further includes a temperature detecting unit for detecting temperature of at least one of the light receiving unit and the light emitting unit; and the air blower unit blows air if a value detected by the temperature detecting unit is larger than a prescribed value, and stops air blow if the value detected by the temperature detecting unit is equal to or smaller than the prescribed value. 
     More preferably, the air blower unit blows air if the light emitting unit is emitting light, and stops air blow if the light emitting unit is not emitting light. 
     More preferably, the air blower unit increases air flow rate as resolution of scanning a document by the light receiving unit becomes higher. 
     Preferably, the image reading apparatus further includes a timer detecting passage of time, and the air blower unit starts blowing air if it is detected by the timer that a prescribed time has passed from start of light emission with the light emitting unit fixed in one job. 
     By the present invention, air is circulated in the image reading apparatus, so that temperature difference inside the image reading apparatus can be reduced, and heat can be dissipated efficiently to the outside. As a result, variation in sensitivity of the reading sensor (light receiving unit) can be prevented. Further, deviation of a point of focus of the image forming system, or deviation of reading position, caused by thermal expansion of members forming the reading sensor, can be prevented. 
     Further, the housing unit forms a substantially sealed space and, therefore, dust-proof property of the image reading apparatus is not degraded. 
     Further, since the air blower unit (fan) is arranged near the light receiving unit, temperature increase at the light receiving unit can more efficiently be prevented. 
     Further, the blowing direction of the air blower unit (fan) is set to be along the sidewall of housing unit (frame) and, hence, more efficient heat radiation to the outside can be realized. 
     Further, air is blown when the temperature of at least one of the light receiving unit and the light emitting unit exceeds a prescribed temperature and, hence, temperature increase of the light emitting unit and of the light receiving unit can more efficiently be prevented, and power consumption resulting from unnecessary blowing can be prevented. 
     Further, by increasing the amount of blown air as the resolution of document scanning becomes higher, temperature increase of the light emitting unit and of the light receiving unit can more efficiently be prevented, and power consumption resulting from unnecessary blowing can be prevented. 
     Further, by blowing air only while the light emitting unit is emitting light, temperature increase of the light emitting unit and of the light receiving unit can more efficiently be prevented, and power consumption resulting from unnecessary blowing can be prevented. 
     Further, when a plurality of documents are to be scanned in one job, by starting blowing after a prescribed time from the start of light emission by the light emitting unit, temperature increase of the light emitting unit and of the light receiving unit can more efficiently be prevented, and power consumption resulting from unnecessary blowing can be prevented. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view showing a schematic structure of an image forming apparatus provided with the image reading apparatus in accordance with an embodiment of the present invention. 
         FIG. 2  is a cross-sectional view showing a schematic structure of the image reading apparatus in accordance with an embodiment of the present invention. 
         FIG. 3  is a block diagram showing a schematic configuration of the image forming apparatus shown in  FIG. 1 . 
         FIG. 4  is a plan view showing a schematic configuration of the image reading apparatus shown in  FIG. 2 . 
         FIG. 5  is a flowchart representing a control structure of a program controlling a fan in the image reading apparatus shown in  FIG. 2 . 
         FIG. 6  is a plan view showing a state in which the fan is provided on a sidewall of the image reading apparatus. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following embodiments, the same components are denoted by the same reference characters. Their names and functions are also the same. Therefore, detailed description thereof will not be repeated. 
     Referring to  FIG. 1 , an image forming apparatus  100  provided with image reading apparatus  90  in accordance with the present embodiment forms a multi-colored or monochrome image on a prescribed sheet of recording paper, in accordance with image data formed by reading a document. Image forming apparatus  100  consists of an apparatus main body  110  and an automatic document feeder  120 . The main body  110  includes an image reading apparatus  90 , an optical scanning device  1 , a developer  2 , a photoreceptor drum  3 , a cleaner unit  4 , a charger  5 , an intermediate transfer belt unit  6 , a fixing unit  7 , a paper feed cassette  81 , and a paper discharge tray  91 . In addition to these components, image forming apparatus  100  further includes components necessary for realizing the functions of the image forming apparatus. 
     Image reading apparatus  90  is arranged at an upper portion of apparatus main body  110 . At an upper portion of image reading apparatus  90 , a platen  92  formed of a transparent glass (platen glass) for placing a document is arranged. An automatic document feeder  120  is attached above platen  92 . Automatic document feeder  120  automatically feeds and places a document on platen  92 . Automatic document feeder  120  is formed to be rotatable in the direction of an arrow M, so that when opened, one can manually place the document on platen  92 . 
     The image data handled in the present image forming apparatus  100  are color image data using colors of black (K), cyan (C), magenta (M) and yellow (Y), that is, image data separated to components of these four colors. Therefore, in order to form four different latent images of respective colors, four developers  2 , four photoreceptor drums  3 , four chargers  5  and four cleaner units  4  are provided. These components form four image stations for processing black, cyan, magenta and yellow, respectively. 
     Charger  5  is a device for uniformly charging the surface of photoreceptor drum  3  to a prescribed potential. Other than the charger type device shown in  FIG. 1 , a contact roller type, or a brush type charger may be used. 
     Optical scanning device  1  is a laser scanning unit (LSU) including a laser emitting unit and a reflection mirror. In optical scanning device  1 , a polygon mirror for laser beam scanning and an optical element including a lens and a mirror for guiding the laser beam reflected by the polygon mirror to photoreceptor drum  3  are arranged. Other than such a configuration, a configuration having an EL or LED write head having light emitting elements arranged in an array, may be used as optical scanning device  1 . 
     Optical scanning device  1  exposes the charged photoreceptor drum  3  in accordance with the input image data, and thereby forms an electrostatic latent image in accordance with the image data on the surface of photoreceptor drum  3 . Developer  2  turns the electrostatic latent images formed on respective photoreceptor drums  3  to visible images with toners of four colors (YMCK), respectively. Cleaner unit  4  removes and recovers the toner left on the surface of each photoreceptor drum  3 , after development and image transfer. 
     Intermediate transfer belt unit  6  arranged above photoreceptor drum  3  includes an intermediate transfer belt  61 , an intermediate transfer belt driving roller  62 , an intermediate transfer belt driven roller  63 , an intermediate transfer roller  64  and an intermediate transfer belt cleaning unit  65 . There are four intermediate transfer rollers  64 , corresponding to respective colors of Y, M, C and K. 
     Around intermediate transfer belt driving roller  62 , intermediate transfer belt driven roller  63 , and intermediate transfer roller  64 , intermediate transfer belt  61  is wound and driven to rotate. Each intermediate transfer roller  64  supplies transfer bias, which will be described later, for transferring the toner image on the corresponding photoreceptor drum  3  to intermediate transfer belt  61 . 
     Intermediate transfer belt  61  is provided to be in contact with each photoreceptor drum  3 . By successively transferring the toner images of respective colors formed on photoreceptor drums  3  onto intermediate transfer belt  61  to be superposed on the last, a color toner image (multi-color toner image) is formed on intermediate transfer belt  61 . Intermediate transfer belt  61  is formed as an endless belt, using a film having the thickness of about 100 μm to 150 μm. 
     Transfer of the toner image from photoreceptor drum  3  to intermediate transfer belt  61  is done by intermediate transfer roller  64  that is in contact with the back side of intermediate transfer belt  61 . A high voltage transfer bias (high voltage of a polarity (+) opposite to the charged polarity (−) of the toner) is applied to intermediate transfer roller  64 , in order to transfer the toner image. Intermediate transfer roller  64  has a metal shaft (for example, of stainless steel) of 8 to 10 mm in diameter as a base, with its surface covered by a conductive elastic member (such as EPDM or urethane foam). Because of this conductive elastic member, uniform high voltage can be applied to intermediate transfer belt  61 . Though a roller-shaped transfer electrode is used in the present embodiment, a brush or the like may be used as an alternative. 
     The electrostatic images turned to visual images in accordance with the hue on respective photoreceptor drums  3  as described above are superposed on intermediate transfer belt  61 . Information of the thus superposed images (density distribution of toner) is transferred to a sheet of recording paper, as intermediate transfer belt  61  is rotated, by a transfer roller  10  arranged at the position of contact between the intermediate transfer belt  61  and the sheet of recording paper. 
     At this time, intermediate transfer belt  61  and transfer roller  10  are brought into pressure-contact with a prescribed nip, and a voltage (high voltage of a polarity (+) opposite to the charged polarity (−) of the toner) for transferring the toner to the sheet of recording paper is applied to transfer roller  10 . Further, in order to constantly secure the nip mentioned above, a hard material (metal or the like) is used for one of the transfer roller  10  and intermediate transfer belt driving roller  62 , and a soft material such as soft roller (elastic rubber roller, foam resin roller or the like) is used for the other one. 
     Further, as described above, the toner adhering to intermediate transfer belt  61  by the contact with photoreceptor drum  3 , or toner not transferred to the sheet of recording paper by transfer roller  10  but left on intermediate transfer belt  61  may cause undesirable mixture of toner colors in subsequent process steps. Therefore, the toner adhering to intermediate transfer belt  61  and the toner left on intermediate transfer belt  61  are removed and recovered by intermediate transfer belt cleaning unit  65 . In intermediate transfer belt cleaning unit  65 , a cleaning blade that is brought into contact with intermediate transfer belt  61  is provided as a cleaning member. Intermediate transfer belt  61  is supported by intermediate transfer belt driven roller  63  at a portion where the cleaning blade contacts. 
     Paper feed cassette  81  is a tray for storing sheets of recording paper to be used for image formation, and provided below optical scanning device  1  in main body  110 . Sheets of recording paper may also be placed on a manual feed cassette  82 . A paper discharge tray  91  provided on main body  110  is for collecting the printed sheets of paper in face-down manner, that is, with the printed surface facing downward. 
     In main body  110 , a paper feeding path S is formed in a substantially vertical direction, for feeding sheets of recording paper in paper feed cassette  81  or manual feed cassette  82  to paper discharge tray  91  through transfer roller  10  and fixing unit  7 . In the vicinity of paper feeding path S from paper feed cassette  81  or manual feed cassette  82  to paper discharge tray  91 , pick-up rollers  11   a  and  11   b , a plurality of conveyor roller pairs  12   a  to  12   d , a registration roller pair  13 , a transfer roller  10 , fixing unit  7  and the like are arranged. 
     Conveyor roller pairs  12   a  to  12   d  are small rollers for promoting and assisting feeding of the recording paper, and a plurality of conveyor roller pairs are provided along paper feeding path S. Pick-up roller  11   a  is arranged near one end of paper feed cassette  81 , picks up sheets of recording paper one by one from paper feed cassette  81  and supplies the sheets to paper feeding path S. Similarly, pick-up roller  11   b  is arranged near one end of manual feed cassette  82 , picks up sheets of recording paper one by one from manual feed cassette  82  and supplies the sheets to paper feeding path S. 
     Registration roller pair  13  temporarily holds the sheet of recording paper fed along paper feeding path S. Then registration roller pair  13  feeds the sheet of recording paper to transfer roller  10  at such timing when the leading edge of toner image on photoreceptor drum  3  is aligned with the leading edge of the sheet of recording paper. 
     Fixing unit  7  includes a heat roller  71  and a pressure roller  72 . Heat roller  71  and pressure roller  72  rotate, with the sheet of recording paper pinched therebetween. Heat roller  71  is set to a prescribed fixing temperature by a control unit, based on a signal from a temperature detector (not shown). Heat roller  71  presses with heat, together with pressure roller  72 , the toner to the sheet of recording paper, and thus, it has a function of melting, mixing and causing pressure-contact of multi-color toner image that has been transferred to the sheet of recording paper and thereby heat-fixing the image on the sheet of recording paper. Further, an external heating belt  73  for heating heat roller  71  from outside is provided. 
     The path for feeding the sheet of recording paper will be described. In image forming apparatus  100 , paper feed cassette  81  and manual feed cassette  82  for storing sheets of recording paper in advance are provided, as mentioned above. In order to feed sheets of paper from these paper cassettes  81  and  82 , pick-up rollers  11   a  and  11   b  are arranged, respectively, to guide the sheets of recording paper one by one to paper feeding path S. 
     A sheet of recording paper coming from paper feed cassette  81  or  82  is conveyed to registration roller pair  13  by conveyor roller pair  12   a  along paper feeding path S, fed to transfer roller  10  at the timing when the leading edge of the sheet and the leading edge of image information are aligned, and thus, image information is written on the sheet of recording paper. Thereafter, not-yet fixed toner on the sheet of recording paper is melted and fixed as the sheet passes through fixing unit  7 , and through conveyor roller pair  12   b  arranged at the end of paper feeding path S, the sheet is discharged to discharge tray  91 . 
     The feeding path described above corresponds to a request for printing on one side of the sheet of recording paper. If double-sided printing is requested, when the rear end of the sheet that has passed the fixing unit  7  is held by the conveyor roller pair  12   b  at the end of the feeding path, the conveyor roller pair  12   b  rotates in reverse direction. Thus, the sheet of recording paper is fed to conveyor roller pairs  12   c  and  12   d . Thereafter, the sheet of recording paper is fed to registration roller pair  13 , printing is done on the back side of the sheet in the same manner as described above, and then the sheet is discharged to paper discharge tray  91 . 
     Image reading apparatus  90  and automatic document feeder  120  will be described. Referring to  FIG. 2 , automatic document feeder  120  has a deep side pivotally supported by means of a hinge (not shown) on a deep side of image reading apparatus  90 . By moving upward/downward the front portion of automatic document feeder  120 , platen (platen glass)  92  of image reading apparatus  90  is exposed, and a document can be placed on platen  92 . 
     Image reading apparatus  90  includes platen  92 , a first scanning unit  45 , a second scanning unit  46 , an image forming lens  47 , and a CCD  48 . 
     The first scanning unit  45  includes an illumination device  51  and a first reflection mirror  52 . The first scanning unit  45  moves at a constant velocity V to a sub-scanning direction Y by a distance corresponding to the document size, while exposing the document on platen  92  using illumination device  51 . The reflected light is reflected by the first reflection mirror  52  and directed to the second scanning unit  46 . In this manner, an image (including color or black-and-white characters, a figure, a photograph or the like) on the surface of the document is scanned in the sub-scanning direction Y. The second scanning unit  46  includes a second reflection mirror  53  and a third reflection mirror  54 . The second scanning unit  46  moves at a velocity V/ 2  following the first scanning unit  45 , and reflects the light reflected from the document by the second and third reflection mirrors  53  and  54  to image forming lens  47 . Image forming lens  47  collects the light reflected from the document to CCD  48 , and forms an image of the document surface on CCD  48 . CCD  48  scans the document image repeatedly in the main scanning direction (the direction vertical to the surface of  FIG. 2 ), and at every scanning, outputs analog image signals of one main scanning line. 
     The first and second scanning units  45  and  46  have pulleys (not shown), respectively. A wire (not shown) is wound around these pulleys, and as the wire is driven by a stepping motor, the first and second scanning units  45  and  46  move in a synchronized manner. 
     Image reading apparatus  90  can read not only the stationary document on platen  92  but also images on the surface of a document fed by automatic document feeder  120 . In that case, the first scanning unit  45  is moved to a reading area below document reading glass  84  as shown in  FIG. 2 , and the second scanning unit  46  is arranged corresponding to the position of the first scanning unit  45 . In this state, feeding of document by automatic document feeder  120  starts. 
     In automatic document feeder  120 , a pick-up roller  55  is rotated with the roller pressed on a document on document tray  56 , to draw a sheet of document, and the document is conveyed. The leading edge of the document is fed to abut a registration roller pair  85 , so as to be aligned. Thereafter, the document is passed through document reading glass  84  and a reading guide plate  86 , and discharged through a discharge roller pair  58  to a discharge tray  49 . 
     When the document is fed, illumination device  51  of the first scanning unit  45  illuminates the document surface through document reading glass  84 . The light reflected from the document surface is guided by the reflection mirrors of the first and second scanning units  45  and  46  to image forming lens  47 , collected by image forming lens  47  to CCD  48 , and the image on the document surface is formed on CCD  48 . In this manner, the image on the document surface is read. 
     When the back side of the document is to be read, an intermediate tray  67  is rotated about its shaft  69  as represented by a chain-dotted line. In this state, while the document is discharged from discharge roller pair  58  to discharge tray  49 , discharge roller pair  58  is stopped, and the document is received by intermediate tray  67 . In this state, discharge roller pair  58  is rotated in reverse direction, so that the document is guided through a reverse feeding path  68  to registration roller pair  85 , and the document is turned over. In the similar manner as reading the image on the front side, the image on the back side of the document is read. Thereafter, the intermediate tray  67  is returned to the original position represented by the solid line, and the document is discharged through discharge roller pair  58  to discharge tray  49 . 
     The image on the document surface read by CCD  48  in this manner is output as an analog image signal from CCD  48 . The analog image signal is subjected to A/D conversion, and a digital image signal is generated. The digital image signal is subjected to various image processing operations and thereafter input to optical scanning device (laser exposure device)  1  of image forming apparatus  100 , in which the image is recorded on a sheet of recording paper and the sheet is output as a copied document. 
     When a document on platen  92  or document reading glass  84  is to be irradiated by illumination device  51  of first scanning unit  45 , it is desired to reduce loss of emitted light so that substantially the light emitted from LED array  77  is fully incident on the document. For this purpose, illumination device  51  shown in  FIG. 2  has a light guiding member  78  and a reflector  79 . Light guiding member  78  directly guides the light emitted from LED array  77  toward the document, and guides light, reflected in a direction that cannot directly irradiate the document, to reflector  79 . Reflector  79  reflects the light that has been guided by light guiding member  78  toward the document. Thus, the loss of light emitted from LED array  77  can be reduced, and the emitted light can almost fully be directed to the document. 
     Referring to  FIG. 3 , image forming apparatus  100  includes: a control unit (hereinafter referred to as a CPU (Central Processing Unit))  130  for overall control of image forming apparatus  100 ; an ROM (Read Only Memory)  132 ; an RAM (Random Access Memory)  134 ; an HDD (Hard Disk Drive)  136 ; and a bus  142 . ROM  132  is a non-volatile storage device that retains data even when power is turned off. ROM  132  stores programs and data necessary for controlling operations of image forming apparatus  100 . RAM  134  is a volatile storage device. HDD  136  is a non-volatile storage device. 
     CPU  130 , ROM  132 , RAM  134  and HDD  136  are connected to bus  142 . Data (including control information) is exchanged between each of these components through bus  142 . CPU  130  reads a program from ROM  132  to RAM  134  through bus  142 , and executes the program, using a part of RAM  134  as a work area. Specifically, CPU  130  controls various units and components forming image forming apparatus  100  in accordance with the program stored in ROM  132 , and realizes various functions of image forming apparatus  100 . 
     Image forming apparatus  100  further includes a power supply unit  172  for supplying electric power to a fan  170 , a temperature sensor  174 , automatic document feeder  120 , image reading apparatus  90 , image forming unit  150 , image processing unit  152 , image memory  154 , paper feed unit  156 , and an operation unit  160 . These are also connected to bus  142 . 
     Operation unit  160  receives inputs such as an instruction by the user to first image forming apparatus  100 . Operation unit  160  includes an operation panel and an operation key portion (both not shown). The operation panel includes a display panel formed, for example, by a liquid crystal panel, and a touch-panel arranged on the display panel, allowing detection of a touched position. In order to operate image forming apparatus  100 , soft keys are displayed on the display panel, and on the operation key portion, hard keys are arranged. CPU  130  monitors user operation of these keys. The user can input an instruction of image formation, setting of conditions for image formation and the like to image forming apparatus, by pressing or touching these keys. Selection of a key displayed on the display panel is done by touching the corresponding portion of the touch-panel superposed on the display panel. 
     When the user operates operation unit  160  and instructs image formation, document is read by image reading apparatus  90  and image data is generated, as described above, and the image data is temporarily stored in image memory  154 . Image processing unit  152  executes various image processing operations on the image data stored in image memory  154 . The image data is stored in HDD  136  as needed. 
     Paper feed unit  156  includes paper feed cassettes  81  and  82  mentioned above, and holds sheets of recording paper for image formation. Image forming unit  150  includes photoreceptor drum  3 , charger  5 , optical scanning device  1 , developer  2 , transfer roller  10  and fixing unit  7 , as mentioned above. Image forming unit  150  forms image data read from image memory  154  or from HDD  136  on a sheet of recording paper fed from paper feed unit  156 , as described above. 
     Fan  170  is arranged near a CCD substrate  176  on which CCD  48  is mounted, as shown in  FIG. 4 . In  FIG. 4 , fan  170  is arranged between an end portion in the longitudinal direction of CCD substrate  176  and a frame sidewall  94  of image reading apparatus, and blows air from the right side to the left side. 
     Temperature sensor  174  is arranged on CCD substrate  176 , and detects the temperature of CCD substrate  176 . 
     Assuming that fan  170  and temperature sensor  174  are arranged in image reading apparatus  90  as shown in  FIG. 4 , the function of controlling the operation of fan  170  in image reading apparatus  90  at the time of operation of image forming apparatus will be described. 
     Referring to  FIG. 5 , at a step  300  of the program controlling the operation of fan  170 , image forming apparatus  100  is powered on and warmed-up. At this time, CPU  130  reads a prescribed program from ROM  132  and displays a screen image including a prescribed key or keys on the operation panel of operation unit  160 , and waits for an operation by the user. 
     At step  302 , CPU  130  detects temperature of CCD substrate  176  using temperature sensor  174 , and determines whether or not the detected temperature is at a prescribed temperature or lower. If the detected temperature is equal to or lower than the prescribed temperature, the control proceeds to step  306 . Otherwise, the control proceeds to step  304 . The prescribed temperature is, for example, 60° C. In the present specification, the temperature is not the value actually measured by temperature sensor  174  but “35° C. equivalent temperature.” The 35° C. equivalent temperature is calculated from the actually measured value and the room temperature (temperature of the site where image reading apparatus  90  is installed), as 35° C. equivalent temperature=35° C.−room temperature+actually measured temperature. 
     At step  304 , CPU  130  controls power supply unit  172  such that a prescribed voltage is supplied to fan  170  to rotate fan  170 . If fan  170  is already rotating, the state is maintained. The supplied voltage is, for example, rated voltage of fan  170 . Then, the control proceeds to step  308 . 
     At step  306 , CPU  130  controls power supply unit  172  such that the power supply to fan  170  is stopped, to stop fan  170 . If fan  170  is not rotating, the state is maintained. 
     At step  308 , CPU  130  determines whether or not the user has operated operation unit  160  and any instruction has been input. If it is determined that an operation has been done, the control proceeds to step  310 . Otherwise, the control returns to step  302 . 
     At step  310 , determination is made as to whether the instruction at step  308  is an instruction to operate image reading apparatus  90 , that is, an instruction to execute a job involving scanning. An instruction to execute a job involving scanning is, for example, an instruction to copy a document. If it is an instruction to execute a job involving scanning, the control proceeds to step  314 . Otherwise, the control proceeds to step  312 . 
     At step  312 , CPU  130  executes the instructed process. By way of example, if an operation to set copy conditions (for example, selection of black-and-white/color, number of copies, copy density, scanning resolution and the like) is received before executing the copying operation, corresponding process or processes are executed. 
     At step  314 , CPU  130  determines whether or not the scanning resolution is low. Here, it is assumed that in image forming apparatus  100 , a plurality of scanning resolutions can be set and specifically, the resolution is classified to three types, that is, low resolution (for example, 300 DPI or lower), middle resolution (for example, 300 DPI&lt;middle resolution&lt;600 DPI) and high resolution (for example, 600 DPI or higher). If it is determined to be the low resolution, the control proceeds to step  316 . Otherwise, the control proceeds to step  318 . 
     At step  316 , CPU  130  controls power supply unit  172  such that fan  170  is rotated at a low speed. Here, the number of rotations of fan  170  depends on the supplied voltage. A low voltage is supplied from power supply unit  172  to fan  170 . Thereafter, the control proceeds to step  324 . 
     At step  318 , CPU  130  determines whether or not the scanning resolution is the high resolution. If it is determined to be the high resolution, the control proceeds to step  320 . Otherwise, the control proceeds to step  322 . 
     At step  320 , CPU  130  controls power supply unit  172  such that fan  170  is rotated at high speed. A high voltage is supplied from power supply unit  172  to fan  170 . Thereafter, the control proceeds to step  324 . 
     At step  322 , CPU  130  controls power supply unit  172  such that fan  170  is rotated at a middle speed. A middle level voltage is supplied from power supply unit  172  to fan  170 . Thereafter, the control proceeds to step  324 . 
     At step  324 , CPU  130  executes the designated job. Specifically, CPU  130  controls image reading apparatus  90  such that the document on platen  92  is scanned and the image data is temporarily stored in image memory  154 . If a document is set on automatic document feeder  120 , CPU  130  controls automatic document feeder  120  such that the document is fed to platen  92 , and then it controls image reading apparatus  90  such that the document is scanned. At this time, fan  170  rotates at the speed in accordance with the voltage supplied at step  316 ,  318  or  320 . The temporarily stored image data is processed in accordance with the designated job. 
     At step  326 , CPU  130  determines whether or not the job has been completed. If it is determined that the job is completed, the control proceeds to step  328 . Otherwise, the control returns to step  324 , and scanning is repeated. Thus, fan  170  is kept rotating. 
     At step  328 , CPU  130  stops voltage supply from power supply unit  172 , and stops rotation of fan  170 . 
     At step  330 , CPU  130  determines whether or not an end instruction has been received. If it is determined that an end instruction has been received, the program ends. Otherwise, the control returns to step  302 . The end instruction is, for example, power off of image forming apparatus  100 . 
     In the manner as described above, when image reading apparatus  90  operates and the temperature of CCD substrate  176  becomes higher than the prescribed value because of the heat generated by the light source (LED array  77 ), fan  170  can be rotated. Thus, as represented by arrows in  FIG. 4 , the air in image reading apparatus  90  can be circulated. Specifically, fan  170  sucks air in the space behind CCD substrate  176  (back side of the surface on which CCD  48  is mounted), and discharges the air to the side of first scanning unit  45 . The air discharged from fan  170  is reversed mainly at the front surface (the surface opposite to CCD  48 ) of first scanning unit  45 , and flows to image forming lens  47  and the back side of CCD substrate  176 . Therefore, efficient heat radiation through the sidewall of a frame of image reading apparatus  90  becomes possible, and the temperature inside image reading apparatus  90 , especially the temperature of CCD  48  can be prevented from attaining too high. Any special opening communicated to the outside is not formed in the frame of image reading apparatus  90  and, therefore, dust-proof property is not degraded. 
     Particularly, as the speed of rotation of fan  170  is controlled in accordance with the resolution of scanning at steps  314  to  322 , power consumption by fan  170  can be properly adjusted. For scanning at high resolution, the light source is kept on for a long period and much heat is generated. Therefore, in order to prevent CCD  48  from being heated to a high temperature, it is desirable to rotate fan  170  at a high speed. On the other hand, for scanning at low resolution, the light source is kept on only for a short period and not much heat is generated. Therefore, by rotating fan  170  at a low speed, unnecessary power consumption can be avoided. 
     Though not shown in  FIG. 4 , there may be a cover arranged to partially cover CCD substrate  176 . In that case, the shape of cover should preferably be designed not to block air flow from fan  170  and to form air flow near CCD substrate  176 . 
     Though an example in which temperature sensor  174  is provided on CCD substrate  176  has been described above, arrangement of the temperature sensor is not limited to the above. The temperature sensor may be arranged at an arbitrary position, provided that the temperature of CCD  48  can be evaluated. It may be arranged to enable direct measurement of CCD  48 . It is also possible to arrange the temperature sensor near the light source for scanning (LED array  77 ), to determine whether or not its temperature is equal to or lower than a prescribed temperature, and the fan may be rotated if the temperature exceeds the prescribed temperature. It is also possible to arrange temperature sensors near CCD substrate  176  and near the light source (for example, first scanning unit  45 ), respectively, and whether or not the detected temperatures are equal to or lower than a prescribed temperature may be determined. The sensor may be arranged to enable direct measurement of the light source. 
     The arrangement of fan  170  is not limited to the position shown in  FIG. 4 . Fan  170  may be arranged at any position, provided that it is out of the movable range of first scanning unit  45  and that circulation of air in image reading apparatus  90  can be attained (more preferably, air circulation along the sidewall of the frame of image reading apparatus  90  can be attained). For instance, in  FIG. 4 , the fan may be arranged on the side where temperature sensor  174  is arranged. The fan may be arranged between the back surface of CCD substrate  176  and sidewall  96  of the frame. In that case, the direction of blowing may be determined such that the air flows along the back surface of CCD substrate  176 . If prescribed amount of air flow cannot be attained by one fan, two or more fans may be provided. 
     When controlling the number of rotations of the fan in accordance with the scanning resolution, the classification to three types mentioned above is not limiting. What is necessary is classification to at least two types, and classification to four or more types to control the number of rotations of the fan is also possible. 
     Though an example in which the speed of rotation of the fan is controlled in accordance with scanning resolution has been described in the foregoing, the fan can be rotated in intermittent manner, linked to on/off of the light source. By way of example, the fan may be rotated while the light source is on (light emitting state), and the rotation of the fan may be stopped if the light source is off (light-off state). This can further reduce wasteful power consumption. 
     If a plurality of documents are set at automatic document feeder  120  and scanning is to be executed as one job, scanning is executed while the first scanning unit  45  is fixed at the position shown in  FIG. 2  (reading area below document reading glass  84 ). If the number of documents is small and scanning ends in a short time period, it is unnecessary to rotate the fun. If a large number of documents are to be scanned, however, the light source is heated to a high temperature. Therefore, it is desirable to rotate the fan. Therefore, it is preferred to rotate the fun if a prescribed time period has passed from when the documents are set to automatic document feeder and scanning is started (emission of light from the light source started). By appropriately setting the prescribed time period, it is possible to prevent CCD  48  from attaining high temperature, even without using any temperature sensor. 
     In order to prevent any influence of vibration caused by the fan on the optical system (image forming lens  47  and the like) and on CCD  48 , it is preferred to arrange an elastic member (for example, rubber) to absorb vibration of fan  170 , between fan  170  and the bottom surface of image reading apparatus  90 . 
     In the foregoing, copy function (printing on a sheet of recording paper) of the image forming apparatus has been mainly described. The image forming apparatus, however, may be a multifunctional peripheral having functions other than the copying function. Specifically, the present invention is also applicable to an image reading apparatus mounted on a multifunctional peripheral having the functions of transmitting image data obtained by scanning by FAX, transmitting the image data by attaching it to an e-mail, transferring the image data to a computer such as a server and the like. 
     Results of experiments shown below indicate effectiveness of the present invention. A fan was arranged as shown in  FIG. 4 , and the temperatures of CCD and LED light source were measured while circulating air in the image reading apparatus. As a comparative example, a fan  178  was attached to a side wall of image reading apparatus  90  as shown in  FIG. 6 , and the temperatures of CCD and LED light source were measured while discharging the air of image reading apparatus  90 . 
     In both examples, an image forming apparatus having a DSPF (Duplex Single Pass Feeder) was used as automatic document feeder  120 , and the experiment was done in duplex copying mode, in which much heat is generated from the LED light source and the CCD. Specifically, copying operation was executed for about 2 hours, with scanning speed of 372 mm/sec, which corresponds to scanning resolution of 400 DPI, and paper feeding rate through DSPF of 75 sheets/min. 
     As the fan, a small DC fan manufactured by NMB Technologies Corporation (part number: 1608KL-05W-B59-L00) was used. Specification of the fan was as follows: rated voltage 24V; power 1.92 W; size 40 mm (length)×40 mm (height) (air blowing surface), 20 mm (depth) (blowing direction); air flow rate 9.5 cubic feet/min (0.27 m 3 /min). When the fan was arranged as shown in  FIG. 4 , a voltage of 12V was continuously supplied to the fan, from the start of copying. When the fan was arranged as shown in  FIG. 6 , a voltage of 24V was continuously supplied to the fan, from the start of copying. 
     The results are as shown in Table 1. In Table 1, Measurement Condition A corresponds to the fan arrangement of  FIG. 4 , and Measurement Condition B corresponds to the fan arrangement of  FIG. 6 . The temperatures are all 35° C. equivalent temperature. Ambient temperature was both 25° C. (actually measured value). 
     
       
         
               
               
             
               
               
               
             
               
               
               
             
           
               
                   
               
               
                   
                 Measured Temperature 
               
             
          
           
               
                 Place of 
                 Measurement  
                 Measurement  
               
               
                 Measurement 
                 Condition A 
                 Condition B 
               
               
                   
               
             
          
           
               
                 Center of LED substrate 
                 84.5 
                 85.7 
               
               
                 Surface of CCD substrate 
                 60.3 
                 65 
               
               
                 Center of frame bottom 
                 51.9 
                 51.3 
               
               
                 surface 
                   
                   
               
               
                   
               
             
          
         
       
     
     As can be seen from Table 1, though the temperature of frame bottom surface representing the atmosphere in the image reading apparatus are substantially the same under Measurement Conditions A and B, the temperatures of CCD substrate and of LED substrate are both lower under Measurement Condition A. Therefore, it is understood that, in order to prevent temperature increase of CCD substrate and LED substrate, it is better to arrange the fan as shown in  FIG. 4  to circulate air in the image forming apparatus, than to arrange the fan on the frame sidewall to discharge air to the outside. Further, considering that the speed of rotation of fan is lower (supplied voltage is lower) under Measurement Condition A than Measurement Condition B, it can be understood that the arrangement of  FIG. 4  is very advantageous. 
     The embodiments as have been described here are mere examples and should not be interpreted as restrictive. The scope of the present invention is determined by each of the claims with appropriate consideration of the written description of the embodiments and embraces modifications within the meaning of, and equivalent to, the languages in the claims.