Patent Publication Number: US-2005122547-A1

Title: Image reader and image forming apparatus

Description:
CROSS REFERENCE  
      This Nonprovisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 2003-407450 filed in Japan on Dec. 5, 2003, the entire contents of which are hereby incorporated by reference.  
     BACKGROUND OF THE INVENTION  
      The present invention relates to an image reader for reading the image of a document on a platen and an image forming apparatus incorporating the same.  
      There are various document read systems applicable to image readers to be fitted on image forming apparatus, including: a system adapted to scan a sheet- or book-shaped document stationarily set on the platen (platen glass) by an optical reader; a system adapted to scan a sheet-shaped document automatically fed onto the platen and rendered stationary thereon by an optical reader as disclosed in Japanese Patent Application Laid-Open Nos. S62-49342 and 2001-24856; and a system adapted to read a sheet-shaped document being fed over the platen by an optical reader rendered stationary below the platen.  
      One conventional image reader is capable of selecting one of the two systems of the aforementioned read systems, i.e., the stationary document read system for image reading by causing the optical reader to scan a stationary document and the feed-and-read system for reading a document being fed by the stationary optical reader.  
      Particularly, an image reader which is designed to use a single optical reader in both of the stationary document read system and the feed-and-read system is configured to perform document image reading in a stationary document read mode based on the stationary document read system with the optical reader located differently from the case of a feed-and-read mode based on the feed-and-read system. For this reason, the optical reader is controlled to move from a predetermined standby position to a reading start position established for a selected read mode prior to the starting of document image reading. Further, the standby position of the optical reader is established as the reading start position in one of the stationary document read mode and the feed-and-read mode, whereby the operation of moving the optical reader prior to the starting of document image reading can be eliminated as the case may be.  
      In an image reader, usually, the optical reader rests at the standby position at which the optical reader is ready to read a document image immediately upon the starting of image reading. Generally, the home position, which is the standby position of the optical reader, is established at a location opposite to an end portion of the platen. In the aforementioned image reader using the single optical reader in both of the stationary document read mode and the feed-and-read mode, one of the stationary document read mode and the feed-and-read mode has priority over the other and the reading start position in the priority mode is established at the same location as the standby position of the optical reader in that mode, whereby the operation of moving the optical reader to the reading start position can be eliminated when the priority mode is selected. Usually, the reading start position is established at the same location as the standby position of the optical reader in the feed-and-read mode.  
      In many cases such an image reader of the type using a single optical reader in both of the modes is provided with separate platens to be used in respective of the stationary document read mode and the feed-and-read mode. The platen to be used in the feed-and-read mode has a narrow width in the document feed direction since the optical reader remains stationary during reading. Accordingly, the standby position of the optical reader is close to an end portion of that platen, whichever mode is selected as the priority mode.  
      The end portion of such a platen is fitted with a fixing member fixing the platen on top of the apparatus and a reference member bearing a reference mark indicating a document setting position, which are secured by means of a fastening member (screw) and the like. For this reason, a clearance is likely defined between the platen and the fixing member, the reference member or the like.  
      The fixing member, reference member and the like are removably mounted on support members such as the apparatus frame, cabinet cover and the like so that the platen can be removed for its cleaning and for maintenance of the optical reader.  
      A screw having tapping capability is used as the fastening member for reducing the manufacturing cost. As the screw is screwed into a hole formed in such a support member, the screw fastens the fixing member, reference member or the like to the support member while tapping the bore of the hole. For this reason, it is possible that cuttings or swarf produced from the support member by tapping remains at the end portion of the platen. In addition, powdery paper and dust produced during feeding of documents are likely to collect in the clearance defined between the platen and the fixing member, reference member or the like adjacent the end portion of the platen.  
      Such cuttings, powdery paper and dust (hereinafter referred to as “powdery paper and the like” as the case may be) collecting in the clearance defined between the platen and the fixing member, reference member or the like adjacent the end portion of the platen fall by propagation of vibration generated during the operation of the image forming apparatus or due to removal of the platen for maintenance. This results in a problem that the optical reader resting at the standby position below the end portion of the platen is soiled or damaged by such powdery paper and the like. Particularly where deflection means (mirror), which forms part of the optical reader, for guiding reflected light from the document to photoelectric conversion means is soiled or damaged, a reading failure (missing of information) occurs due to blocking off of the reflected light from the document, thus resulting in a substantial degradation in reading quality. Further, image formation based on data read with a degraded reading quality incurs a substantial degradation in image quality.  
      A feature of the present invention is to provide image reader and image forming apparatus which are capable of keeping satisfactory image reading and formation by avoiding powdery paper and the like from falling on the optical reader due to propagation of vibration from the image forming apparatus or the operation of removing the platen for maintenance, by establishing the standby position of the optical reader at a location sufficiently spaced apart from an end portion of the platen thereby preventing the optical reader from being soiled or damaged by such powdery paper and the like.  
     SUMMARY OF THE INVENTION  
      According to the present invention, a standby position at which an optical reader rests during standby for image read processing is established at a location where the optical reader fails to face an end portion of the platen which lies in a direction in which the optical reader moves relative to the image bearing side of the document.  
      The foregoing and other features and attendant advantages of the present invention will become more apparent from the reading of the following detailed description of the invention in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a view illustrating an example of the construction of a digital multifunction machine as an image forming apparatus according to an embodiment of the present invention;  
       FIG. 2  is a sectional view of an image reader forming the image reading section of the digital multifunction machine according to the embodiment;  
       FIG. 3  is a block diagram showing the electrical configuration of the image reader shown in  FIG. 2 ;  
       FIG. 4  is a plan view showing an operating section of the image reader;  
       FIG. 5  is a flowchart of a processing procedure followed by a control section of the image reader;  
       FIG. 6A  is a view illustrating a portion of the image reader around first and second platens for locating a reading start position of a light source unit;  
       FIG. 6B  is a view locating a standby position of the light source unit;  
       FIG. 7  is a view illustrating the light source unit and a mirror unit of the image reader in their fixed conditions; and  
       FIG. 8  illustrates a bundle of reflected light rays extending from an image bearing side of a document to a CCD in the image reader. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The present invention will now be described with reference to the accompanying drawings.  
       FIG. 1  is a view illustrating an example of the construction of a digital multifunction machine as an image forming apparatus according to an embodiment of the present invention. The digital multifunction machine  100  includes an image reading section  110  comprising an image reader according to the present invention, an image forming section  210 , and a paper feed section  250  and is provided with a post-processing device  260  and a multi-tier paper feeder unit  270 . The details of the image reading section  110  will be described later.  
      A document image read by the image reading section  110  is transferred to the image forming section  210 . The document image thus read is inputted as image data to a non-illustrated image data input section in the image forming section  210 . The image data thus inputted is subjected to predetermined image processing and then temporarily stored in memory incorporated in an image processing section. In response to an instruction to output the stored image data, the image data is read out of the memory and transferred to a laser writing unit  227  as an optical writing device included in the image forming section  210 .  
      The image forming section  210  includes a rotatably supported photosensitive drum  222 , and, around the photosensitive drum  222 , an electrostatic charger  223  for charging the surface of the photosensitive drum  222  to a predetermined potential, laser writing unit  227  for forming an electrostatic latent image on the surface of the photosensitive drum  222  by irradiating the surface of the photosensitive drum  222  with laser light modulated according to the image data, a developing device  224  for supplying toner onto the electrostatic latent image formed on the photosensitive drum  222  to make the image tangible, a transfer device  225  for transferring the toner image formed on the surface of the photosensitive drum  222  to a recording sheet, a peeler  229  for releasing the recording sheet from the surface of the photosensitive drum  222 , and a cleaner  226  for recovering excess toner.  
      Instead of the laser writing unit  227 , use may be made of an optical writing head of the solid state scanning type employing a light-emitting device array comprising LEDs, ELs or the like.  
      Below the image forming section  210  is located the paper feed section  250  which comprises a recording sheet tray  251 , a manual feed tray  254  and a reversing unit  255 . Paper feed trays  252  and  253  provided in the multi-tier paper feed unit  270  are included in the paper feed section  250 . The paper feed section  250  defines a sheet feed path for transporting a recording sheet fed from any one of the trays  251  to  254  to an ejected-sheet tray  219  via a transfer position between the photosensitive drum  222  and the transfer device  225  in the image forming section  210 .  
      The reversing unit  255  communicates with a switchback path  221  for turning a recording sheet front side back for formation of an image on the reverse side of the sheet. The reversing unit  255  is replaceable with a normal sheet tray. The image forming apparatus  100  can be fitted with a large-capacity paper feed unit capable of holding several thousands of recording sheets.  
      On the ejection side of the image forming section  210  are provided a fixing unit  217  for fixing the toner image to each recording sheet by heating the recording sheet bearing the toner image under pressure, switchback path  221  for turning a recording sheet front side back for the formation of an image on the reverse side of the sheet, and post-processing device  260  which is provided with an up-and-down tray  261  and performs stapling or other processing for recording sheets bearing respective images formed thereon.  
      Each recording sheet bearing a toner image fixed at the fixing unit  217  is guided to the post-processing device  260  by a sheet ejecting roller  219  optionally through the switchback path  221 , subjected to predetermined post-processing, and then ejected to the up-and-down tray  261 .  
       FIG. 2  is a sectional view of the image reader forming the image reading section of the digital multifunction machine according to the embodiment. The image reader  1  includes an optical system equipment section  2 , and an ADF (Automatic Document Feeder)  3  as a feeder of the present invention disposed above the optical system equipment section  2 . The image reader  1  serves not only as the image reading section  110  of the digital multifunction machine  100  but also as the scanner section of the copying machine or facsimile apparatus and is capable of reading images on both sides of a document.  
      The optical system equipment section  2  includes a CCD (Charge Coupled Device) reading unit  11  as a first reader. The optical system equipment section  2  performs image reading by using a light source unit  13  and a mirror unit  14  to form an image of the document, which is placed flat on a first platen  12  as a platen of the present invention, on the CCD reading unit  11 .  
      The CCD reading unit  11  includes an imaging lens  11 A and a CCD image sensor  11 B. The light source unit  13  includes a light source  13 A for emitting document-illuminating light, a reflector  13 B for gathering document-illuminating light emitted from the light source  13 A to a predetermined image reading position on the platen  12 , a slit  13 C allowing only reflected light from the document to pass therethrough, and a mirror  13 D for deflecting the light path of the light having passed through the slit  13 C by 90°. The mirror unit  14  includes a pair of mirrors  14 A and  14 B for deflecting the light path of light from the light source unit  13  by 180°. The light source unit  13  and the mirror unit  14  constitute an optical reader of the present invention.  
      In the stationary document read mode, the light source unit  13  and the mirror unit  14  reciprocate in the secondary scanning direction of arrow  15  between reading start position P 1  and maximum reading finish position P 4  at a velocity of V and a velocity of V/2, respectively, thereby guiding reflected light from the image bearing side of the document on the first platen  12  to the CCD reading unit  11  with the optical path length kept constant.  
      The maximum reading finish position P 4  is a reading finish position used when a document of the maximum readable size is placed on the first platen  12 . When a document of a smaller size than the maximum readable size is placed on the first platen  12 , the reading finish position is located to oppose the right-hand edge of the document placed to abut a document position reference member  12 A at its left-hand edge.  
      The CCD reading unit  11  may be structured to move a unit of an optical reading system for reduced-size reading or actual-size reading comprising CCD image sensor  11 B, imaging lens  11 A and light source  13 A at a velocity V in the stationary document read mode.  
      The optical system equipment section  2  further includes a second platen  16  spaced a predetermined distance apart from the first platen  12  in the secondary scanning direction. In the feed-and-read mode including a traveling document read mode and a both-side read mode, the optical source unit  13  is held stationary at the reading start position P 1  opposed to the lower side of the platen  16 , while document sheets stacked on document tray  22  are fed to the second platen  16  one by one, whereby reflected light from the lower side (hereinafter will be defined as the obverse side) of each document sheet being fed over the second platen  16  is read. Each document sheet thus fed is ejected to an ejected sheet tray  17 .  
      The ADF  3  includes a CIS (Contact Image Sensor) reading unit  21  as a second reader. In the both-side read mode, the ADF 3  feeds the document sheets stacked on the document tray  22  to the second platen  16  one by one while reading reflected light from the lower side (hereinafter will be defined as the reverse side) of each document sheet being fed over the second platen  16  by means of the CIS reading unit  21 . The ADF  3  defines a document feed path  23  on which rollers R 1  to R 10  and detectors L 1  to L 6  are arranged.  
      The CIS reading unit  21  includes an array of image sensors, an array of optical guide means (for example a lens array of selfoc lenses), and a light source (an LED array light source or a fluorescent lamp).  
      The document tray  22  is lifted up and down about a fulcrum  22 A as the revolution of a lifting motor  61  is transmitted thereto via a lifting mechanism  34  and an up-and-down plate  31 . When an optical document detector L 1  comprising an actuator L 1 A and a sensor body L 1 B detects a stack of document sheets having been set on the document tray  22 , the feed-and-read mode is selected.  
      When a signal to start the feeding of the document sheets is inputted, the document tray  22  is lifted until the detector R 2  detects the pick-up roller R 1  raised by the uppermost surface of the document sheet stack on the document tray  22  and then the pick-up roller R 1  is rotated so that the document sheets are picked up one by one sequentially from the uppermost sheet of the document sheet stack. On the downstream side of the pick-up roller R 1  are disposed separating rollers R 2  and R 2 A. The pick-up roller R 1  is supported by an arm  25 , which is pivotally supported by the rotating shaft of the separating roller R 2 .  
      The roller R 2 A provided with a torque limiter (which may be a friction pad) is arranged to confront the separating roller R 2  so that the document sheets are reliably fed separately one by one. Therefore, even when plural document sheets are collectively picked up by the pick-up roller R 1  at a time, only the uppermost sheet is passed between the separating rollers R 2  and R 2 A and guided toward the document feed path  23 . Each document sheet thus guided to the document feed path  23  is fed to the second platen  16  by feed rollers R 3  to R 7  and register rollers R 8  and R 9  with predetermined timing.  
      Each document sheet having passed between register rollers R 8  and R 9  is guided to the reading start position P 1  for scanning the obverse side of the document sheet by exposure and then passed through reading position P 2  for scanning the reverse side of the document sheet by exposure. The light source unit  13  faces the lower side of the second platen  16  at the reading start position P 1 . The CIS reading unit  21  is exposed on the upper side of the document feed path  23  at the reading position P 2 .  
      Each document sheet having been subjected to image reading on only the obverse side thereof or on both of the obverse and reverse sides thereof is ejected by ejecting rollers R 10  and R 11  onto the ejected-sheet tray  17 . The operation described above is repeated until the document sheets set on the document tray  22  run out, and all the document sheets finished with reading are ejected onto the ejected-sheet tray  17 .  
      As the document sheets on the document tray  22  are fed sequentially, the height level of the uppermost one of the document sheets lowers and, hence, the pick-up roller R 1  also lowers. In order to keep constant the abutting state of the pick-up roller R 1  against the upper side of the uppermost document sheet, the document tray  22  is raised to compensate for a drop in the height level of the pick-up roller R 1 .  
      The position of the light source unit  13  in the secondary scanning direction is controlled by reference to its current position detected by the detector L 10 . During the standby state in which reading is not performed, the light source unit  13  rests at a standby position P 5 . The standby position P 5  is established at a location where the light source unit  13  fails to face a portion adjacent the left-hand end portion of the first platen  12  but faces the lower side of the first platen  12 .  
      The ADF  3  is pivotable about a non-illustrated hinge located between the ADF  3  and the optical system  2  on the rear side of the image reader  1  for a document to be placed on the first platen  12  for reading. This feature allows the upper side of the first platen  12  to be exposed from the front side of the image reader  1 , whereby a document that cannot be fed by the ADF  3 , such as a book-shaped document, can be placed on the first platen  12 . On the bottom of the ADF  3  is provided an elastic document mat  35  facing the first platen  12 . The open/close state of the ADF  3  is monitored by means of a non-illustrated detector included in the optical system  2 .  
      The image reader  1  thus constructed is capable of reading a document image in each of the three modes: stationary document read mode, traveling document read mode, and both-side read mode. In the stationary document read mode, the CCD reading unit  11  reads an image of a document, such as a book-shaped document, manually placed on the first platen  12 , while the light source unit  13  and the mirror unit  14  are moved to scan the document. The traveling document read mode and the both-side read mode are included in the feed-and-read mode for reading each of document sheets set on the document tray  22  while feeding them one by one automatically. In the traveling document read mode the image on the obverse side of each document sheet is read by the CCD reading unit  11  only, whereas in the both-side read mode the images on respective of the obverse and reverse sides of each document sheet are read by both of the CCD reading unit  11  and the CIS reading unit  21 .  
       FIG. 3  is a block diagram showing the electrical configuration of the image reader  1 . A control section  41  realized by a microcomputer or the like performs switching controls associated with recording sheets to be used in the image forming section  210  and with the operation timing of each section, and like controls based on the results of detections performed by document size detectors L 0  and L 7  adapted to detect the size of a document set on the document tray  22  and document size detector L 9  disposed in the optical system  2  and adapted to detect the size of a document placed on the first platen  12 .  
      In the stationary document read mode in which only the CCD reading unit  11  performs reading, the control section  41  drives a scanning motor  42 , such as a stepping motor, to move the light source unit  13  and the mirror unit  14  while driving exposure lamp  13 A and CCD  11 B depending on the position of the light source unit  13  detected by light source detector L 10 , thereby reading the document image.  
      In each of the traveling document read mode and both-side read mode which use the ADF  3 , the control section  41  drives the lifting motor  33  in accordance with the output of the pick-up roller position detector L 2  to keep constant the height level of the uppermost one of document sheets set on the document tray  22  while controlling feed motor  43 , clutch  44  of the pick-up roller R 1  and clutch  45  of registration rollers R 8  and R 9  based on the results of detections by the detectors L 3  to L 6  until the document detector L 1  detects the absence of any document sheet on the document tray  22 . Under these conditions, the CCD  11 B and CIS  21  are driven appropriately to read the image of each document sheet.  
      The control section  41  is connected to the operating section  46  shown in  FIG. 4 . The operating section  46  is provided on top of the image reader  1  and includes various types of keys K 1  to K 8  and an operation panel D realized by a liquid crystal touch panel or the like. On the right-hand side of the operation panel D are arranged numeric keypad K 1 , interrupt key K 2 , clear key K 3 , clear-all key K 4  and start key K 5 . On the left-hand side of the operation panel D are arranged fax key K 6 , printer key K 7  and copy key K 8 , which are mode keys for changing the operation mode.  
      The operation panel D is capable of displaying various screens by switching to one from another. Keys assigned various read conditions are displayed on these screens. By touching the display position of each key on the operation panel D, the read condition assigned to that key is established. The operation panel D also displays operation guidance, warning and the like. The control section  41  creates and outputs a screen to be displayed on the operation panel D of the operating section  46  while receiving operation signals from the keys K 1  to K 8  and operation panel D.  
      In the case where the digital multifunction machine  100  includes the image reader  1  as the image read section  110 , the control section  41  may form part of the control section of the digital multifunction machine  100 .  
       FIG. 5  is a flowchart of a processing procedure followed by the control section of the image reader. When the image reader  1  is powered on, the control section  41  starts processing and causes the light source unit  13  to move leftwards in  FIG. 2  until the light source unit  13  is detected by the light source unit detector L 10  (S 12  and S 13 ). Subsequently, the control section  41  causes the light source unit  13  to move a predetermined distance rightwards in  FIG. 2  to the reading start position P 3  using the position detected by the detector L 10  as a reference, whereby reference data for shading correction is read (S 14 ).  
      As shown in  FIG. 6A , a white reference plate  202  is affixed to a left-hand end portion of the upper side of the first platen  12 . At the reading start position P 3  the light source unit  13  faces the white reference plate  202 . The control section  41  reads the image of the white reference plate  202  as reference data on a white image to be used in shading correction for subsequent document image reading by means of the light source unit  13  stopped at the reading start position P 3 .  
      Thereafter, the control section  41  causes the light source unit  13  to move a predetermined distance rightwards in  FIG. 2  to the standby position P 5  (S 15 ) and waits for the occurrence of a preparatory event such as the operation of opening/closing the ADF  3 , setting of a document on the document tray  22 , or the operation of selecting the both-side mode in the operating section  46  (S 16  to S 18 ). Open/close detector L 8  for detecting the operation of opening/closing the ADF  3 , document detector L 1  for detecting the setting of a document on the document tray  22  and mode keys K 6  to K 8  constitute a detector of the present invention.  
      During waiting for such a preparatory event, when the control section  41  detects a document having been set on the document tray  22  via the document detector L 1 , the control section  41  judges that reading in either of the traveling document read mode and the both-side mode is to be performed and then causes the light-source unit  13  to move from the standby position P 5  to the reading start position P 1  (S 31 ). At that time, the control section  41  causes the light source unit  13  to pass through the reading start position P 1  and move to a position where the light source unit  13  is detected by the light source unit detector L 10  and then move a predetermined distance in the opposite direction by reference to that detected position, thereby stopping the light source unit  13  at the reading start position P 1  accurately.  
      With the light source unit  13  stopped at the reading start position P 1 , the control section  41  judges the condition of the document set on the document tray  22  and whether or not the start key K 5  of the operating section  46  has been operated (S 32  and S 33 ). When the start key K 5  is operated with the document set on the document tray  22 , the control section  41  performs reading in the traveling document read mode or the both-side read mode according to a selection made at the operating section  46  (S 34 ). Upon completion of reading of all the document sheets set on the document tray  22 , the control section  41  causes the light source unit  13  to move to the standby position P 5  and returns into the standby state (S 35 ).  
      In the case where the document sheets on the document tray  22  run out without the start key K 5  being operated, the control section  41  waits for the next setting of a document on the document tray  22  (S 36 ) in a predetermined time period. If the predetermined time period has elapsed without the next document setting, the control section  41  performs a restoring operation to cause the light source unit  13  to return to the standby position P 5  (S 36 ?S 35 ).  
      In the case where the document on the document tray  22  is removed without the start key K 5  being operated, the control section  41  can judge that document reading is aborted and cause the light source unit  13  to move to the standby position P 5  immediately. However, it is possible that chattering occurs at the document detector L 1  during the setting of the document on the document tray  22  or that the document once set on the document tray  22  is reset for correction of the condition of the document set. For this reason the control section  41  causes the light source unit  13  to move to the standby position P 5  after lapse of a predetermined time period, for example, about several tens of seconds.  
      When the control section  41  detects the opening of the ADF  3  by means of the open/close detector L 8  during waiting for the occurrence of the preparatory event, the control section  41  judges that reading in the stationary document read mode is to be performed and then causes the light source unit  13  to move from the standby position P 5  to the reading start position P 3  (S 16 ?S 21 ). At that time, the control section  41  causes the light source unit  13  to pass through the reading start position P 3  and move to a position where the light source unit  13  is detected by the light source unit detector L 10  and then move a predetermined distance in the opposite direction by reference to that detected position, thereby stopping the light source unit  13  at the reading start position P 13  accurately.  
      With the light source unit  13  stopped at the reading start position P 1 , the control section  41  waits for a document to be placed on the first platen  12  in a predetermined time period (S 22  and S 23 ). If the document is placed on the first platen  12  in the predetermined time period of about several ten seconds, the control section  41  waits for the start key K 5  to be operated and then performs reading in the stationary document mode in response to an operation on the start key K 5  (S 34 ). Upon completion of reading of the document placed on the first platen  12 , the control section  41  causes the light source unit  13  to move to the standby position P 5  and returns into the standby state (S 35 ).  
      Though not illustrated in  FIG. 5 , if a predetermined time period (about several ten seconds to about several minutes for example) has elapsed from the placement of the document on the first platen  12  without the start key K 5  being operated, the control section  13  may return the light source unit  13  to the standby position P 5 . In this case, when the start key K 5  is operated with the light source unit  13  at the standby position P 5 , reading is performed following the detection of the position of the light source unit  13  by the detector L 10 .  
      If the predetermined time has elapsed without any document being placed on the first platen  12 , the control section  41  performs a restoring operation to cause the light source unit  13  to return to the standby position P 5  (S 23 ?S 35 ).  
      When the both-side mode is selected by an operation on an appropriate mode key of the operating section  46  during waiting for the occurrence of the preparatory event, the control section  41  causes the light-source unit  13  to move from the standby position P 5  to the reading start position P 3  (S 18 ?S 41 ). At that time, the control section  41  causes the light source unit  13  to pass through the reading start position P 3  and move to a position where the light source unit  13  is detected by the light source unit detector L 10  and then move a predetermined distance in the opposite direction by reference to that detected position, thereby stopping the light source unit  13  at the reading start position P 1  accurately.  
      The control section  41  waits for a document to be set on the document tray  22  in a predetermined time period (S 36 ). If the predetermined time period has elapsed without the start key K 5  being operated, the control section  13  performs a restoring operation to cause the light source unit  13  to return to the standby position P 5  (S 36 ?S 35 ). If the document is set on the document tray  22  before lapse of the predetermined time period, the control section  41  waits for the start key K 5  to be operated and then performs reading in the both-side mode in response to an operation on the start key K 5  (S 34 ). Upon completion of reading of all the document sheets set on the document tray  22 , the control section  41  causes the light source unit  13  to move to the standby position P 5  and returns into the standby state (S 35 ).  
      In the case where the restoring operation is performed to cause the light source unit  13  to return to the standby position P 5  in the step S 35 , the control section  41  may cause the light source unit  13  to move from the reading start position P 1  or P 3  to the standby position P 5  directly because the standby position P 5  does not necessarily require strict positional precision. Alternatively, the control section  41  may cause the light source unit  13  to move to the reference position once and then return to the standby position P 5  as in the case where the light source unit  13  is caused to move from the standby position P 5  to the reading start position P 1  or P 3 .  
      The control by the control section  41  can be simplified by establishing a common reading start position for the reading start positions P 1  and P 3  in both of the stationary document read mode and the traveling document read mode. In this case, however, the light source unit  13  and the mirror unit  14  have to move a longer distance in the stationary document read mode and, hence, reading in this mode takes a longer time.  
      As shown in  FIG. 6B , when the image forming apparatus  100  is powered off with the light source unit  13  resting at the standby position P 5 , the light source unit  13  is not moved. Accordingly, the light source unit  13  can be visually recognized from above through the platen  12  when the apparatus  100  is normally powered off.  
      If an operation for fixing the light source unit  13  and the mirror unit  14  is performed before the image forming apparatus  100  is powered off, the light source unit  13  is stopped at a position on the left-hand side of the reading start position P 1  in  FIG. 2 . For example, after the light source unit detector L 10  has detected the light source unit  13 , the light source unit  13  is caused to further move a predetermined distance leftwards in  FIG. 2  and stop at a position adjacent the cabinet panel of the image reader  1 . Alternatively, the light source unit  13  may be caused to stop at the position where the light source unit  13  has been detected by the light source unit detector L 10 . Thus, the light source unit  13  can be fixed at the position where the light source unit  13  is to be detected by the light source unit detector L 10 . Alternatively, the light source unit  13  may be fixed at the reading start position P 1 .  
      As shown in  FIG. 7 , the image reader  1  has an arrangement which allows at least one of the light source unit  13  and the mirror unit  14  to be fixed to cabinet panel  1 A or frame  1 B of the image reader  1  from the exterior of the cabinet panel  1 A by means of a fixing screw  203  or the like after powering off, thereby preventing damage to the image reader  1  due to the light source unit  13  or mirror unit  14  moving during transportation or the like. Operations needed in fixing the light source unit  13  and/or the mirror unit  14  include, for example, inputting of a code from the numeric keypad of the operating section  46 , and touching an optical system fixing key displayed on the operation panel D.  
      As shown in  FIG. 2 , a condensation-preventive heater  201  as a heater of the present invention is disposed on the inside bottom of the image reader  1  at a location intermediate the light source unit  13  resting at the standby position and the mirror unit  14 . The condensation-preventive heater  201  is turned on to heat ambient air around the light source unit  13  and mirror unit  14  when the image reader  1  is off. Particularly where the image reader  1  is installed under a low ambient temperature condition such as in a cold district, the condensation-preventive heater  201  heats the light source unit  13  and the mirror unit  14  to a sufficiently high temperature before the inside of the image reader  1  is warmed following powering on of the image forming apparatus  100  having been off for a relatively long time, thereby preventing condensation on mirrors and the like.  
      As shown in  FIG. 8 , reflected light from document G is deflected by mirrors  13 D,  14 A and  14 B and then forms an image on the light-receiving portion of the CCD  11 B through the imaging lens  11 A. The CCD  11 B reads reflected light corresponding to only one scanning line of the image of the document G and hence can read a region of the image having a mere width of about 42 μm even at a resolution of 600 dpi. That is, the light-receiving portion of the CCD  11   b  receives reflected light from a region of the document G having a dimension of about 42 μm in the secondary scanning direction and a dimension equal to the width of the document G in the primary scanning direction.  
      In the case where the light-receiving portion of the CCD  11 B is capable of reading reflected light corresponding to the longitudinal dimension of a A4-size document in the primary scanning direction, a bundle of rays having a diameter of about 0.1 mm is deflected at the mirror  13 D for each of about 7,000 photoelectric converters arranged in the primary scanning direction, though depending on the diameter W of the aperture of the imaging lens  11 A. Accordingly, deflection of light by a mirror is impeded more seriously by very small particles of powdery paper and the like adhering thereto as the position of the mirror becomes closer to the document G. Therefore, deflection-impeded portions of reflected light do not reach the light-receiving portion of the CCD  11 B, causing missing of image data. This phenomenon is most conspicuous at the mirror  13 D which is closest of the three mirrors  13 D,  14 A and  14 B to the document G. Accordingly, it is necessary to prevent powdery paper and the like from falling onto the light source unit  13  including the mirror  13 D from above.  
      As shown in  FIG. 6 , the document position reference member  12 A defining the position in which a document is to be placed on the first platen  12 , together with support member  16 A, is fixed at a location adjacent the left-hand end portion of the first platen  12  by means of fixing screw  204 . Powdery paper, dust and the like are likely to collect in clearances between the first platen  12  and the document position reference member  12 A and between the document position reference member  12 A and the support member  16 A.  
      While the standby position P 5  is established at a location spaced about 70 mm apart from the reading start position P 1  in the image reader  1  according to the present embodiment, the standby position P 5  may be established at any location at which dust, waste, powdery paper or cuttings fail to fall onto the light source unit  13  from above, without limitation. Therefore, it is possible to establish the standby position P 5  at a location on the opposite side of the first platen  12  away from the reading start position P 1 . However, this arrangement can increase the size of the image reader  1  and hence is not preferable.  
      In the present embodiment the standby position P 5  of the light source unit  13  is established at a location where the light source unit  13  faces the lower side of the first platen  12 , so that the light source unit  13  can be visually recognized through the first platen  12  from above when power to the image reader  1  is off. This arrangement can avoid cuttings, which are produced on such an occasion when the screw fixing the document position reference member  12 A and the like is screwed out for maintenance of the first platen  12 , second platen  16 , light source unit  13  and the like, from falling onto the light-source unit  13  and prevent powdery paper and dust collecting in clearances from falling onto the light source unit  13  due to propagation of vibration during the operation of the digital multifunction machine  100  or on the occasion of removal of the first platen  12  or the second platen  16 .  
      Further, as compared with the case where the light source unit  13  is positioned under the frame supporting the first platen  12  or the second platen  16 , the arrangement of the present embodiment in which the light source unit  13  is positioned so as to be visually recognized from outside can eliminate the operation of drawing out the light source unit  13  by hand after the removal of the first platen during maintenance and hence can prevent the operator from inadvertently soiling or damaging the light source  13 A, reflector  13 B or mirror  13 D included in the light source unit  13  by touching with the same by his or her hand or finger.  
      The foregoing embodiment is illustrative in all points and should not be construed to limit the present invention. The scope of the present invention is defined not by the foregoing embodiment but by the following claims. Further, the scope of the present invention is intended to include all modifications within the meanings and scopes of claims and equivalents.