Patent Publication Number: US-11032443-B2

Title: Image reading apparatus for detecting a dirt substance from a white reference image and a document image

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority of prior Japanese Patent Application No. 2018-044777, filed on Mar. 12, 2018, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to image processing technology. 
     BACKGROUND 
     An image reading apparatus, such as a scanner, typically captures an image of a document while conveying the document using an imaging device, such as a line sensor, where imaging elements are one-dimensionally arrayed. If a dirt substance, such as paper dust, other fine particles, or glue, adheres to a glass surface of the imaging device, noise line extending in a document conveyance direction is generated in an image that captures a document. Thus, an image reading apparatus or an image processing system having an image reading apparatus needs to appropriately detect a dirt substance from an image. 
     There has been disclosed a digital photocopier that reads a shading correction plate for acquiring data for shading correction and detects white level data when a user places a document on a document tray and issues a document reading request by operating an operation panel. The digital photocopier displays a notification on a display device prompting cleaning of a platen glass and a shading correction plate when dirt is detected in a white level detection process, and performs document reading processing when dirt is not detected. The digital photocopier determines that there is no dirt on the platen glass when dirt is not detected in processing of reading a marginal portion of a document (refer to Japanese Unexamined Patent Publication (Kokai) No. 2008-154129). 
     SUMMARY 
     An image reading apparatus or an image processing system having an image reading apparatus is desired to better detect a dirt substance from an image. 
     It is an object to provide an image reading apparatus, an image processing system, a control method, and a computer-readable, non-transitory medium storing a computer program that can better detect a dirt substance from an image. 
     According to an aspect of the apparatus, there is provided an image reading apparatus. The image reading apparatus includes an imaging device for generating a white reference image of a white reference member and a document image of a document and a periphery of the document, and a processor for performing first processing for detecting a dirt substance from the white reference image, generating data for shading correction based on the white reference image, correcting the document image using the data for shading correction to generate a correction image, and performing second processing for detecting a dirt substance from the correction image. One of the first processing or the second processing is performed using a dirt substance detection result of the other one of the first processing or the second processing. According to an aspect of the system, an image processing system is provided. The image processing system includes an image reading apparatus including an imaging device for generating a white reference image of a white reference member and a document image of a document and a periphery of the document, and a first processor for performing first processing for detecting a dirt substance from the white reference image, generating data for shading correction based on the white reference image, and correcting the document image using the data for shading correction to generate a correction image, and an information processing apparatus including a second processor for performing second processing for detecting a dirt substance from the correction image. One of the first processing or the second processing is performed using a dirt substance detection result of the other one of the first processing or the second processing. 
     According to an aspect of the method, there is provided a control method of an image processing system. The method includes acquiring a white reference image of a white reference member and a document image of a document and a periphery of the document, performing first processing for detecting a dirt substance from the white reference image, generating data for shading correction based on the white reference image, correcting the document image using the data for shading correction to generate a correction image, and performing second processing for detecting a dirt substance from the correction image. One of the first processing or the second processing is performed using a dirt substance detection result of the other one of the first processing or the second processing. 
     According to an aspect of the computer-readable, non-transitory medium storing a computer program, there is provided a computer-readable, non-transitory medium storing a computer program, wherein the computer program causes an image reading apparatus to execute a process. The process includes acquiring a white reference image of a white reference member and a document image of a document and a periphery of the document, performing first processing for detecting a dirt substance from the white reference image, generating data for shading correction based on the white reference image, correcting the document image using the data for shading correction to generate a correction image, and performing second processing for detecting a dirt substance from the correction image. One of the first processing or the second processing is performed using a dirt substance detection result of the other one of the first processing or the second processing. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a configuration view of an example of an image processing system according to an embodiment. 
         FIG. 2  is a view for illustrating a conveyance path inside an image reading apparatus. 
         FIG. 3  is a perspective view of a first imaging unit seen from the side of a document conveyance path. 
         FIG. 4  is a view for illustrating the imaging unit and a conveyance mechanism of the upstream and downstream sides of the imaging unit. 
         FIG. 5  is a view for illustrating the arrangement of a first conveyance roller and a first driven roller. 
         FIG. 6  is a view for illustrating how a document is conveyed. 
         FIG. 7  is a view for illustrating the arrangement of a first light source and a first imaging sensor. 
         FIG. 8  is a block diagram depicting schematic components of an image reading apparatus and an information processing apparatus. 
         FIG. 9  is a view depicting schematic components of a first storage device and a first CPU. 
         FIG. 10  is a view depicting schematic components of a second storage device and a second CPU. 
         FIG. 11  is a flowchart depicting an example of the operation of the overall processing of the image reading apparatus. 
         FIG. 12  is a flowchart depicting an example of the operation of first processing. 
         FIG. 13  is a schematic view depicting an example of a white reference image. 
         FIG. 14A  is a schematic view depicting an example of a reception screen. 
         FIG. 14B  is a schematic view depicting an example of a status display screen. 
         FIG. 14C  is a schematic view depicting an example of a status display screen. 
         FIG. 15  is a flowchart depicting an example of the operation of confirmation processing. 
         FIG. 16  is a flowchart depicting an example of the operation of threshold setting processing. 
         FIG. 17  is a flowchart depicting an example of the operation of the overall processing of an information processing apparatus. 
         FIG. 18  is a flowchart depicting an example of the operation of second processing. 
         FIG. 19A  is a schematic view depicting an example of a correction image  1900 . 
         FIG. 19B  is a schematic view for illustrating a plurality of line segments. 
         FIG. 20A  is a schematic view for illustrating a priority range. 
         FIG. 20B  is a graph for illustrating noise pixels. 
         FIG. 21  is a flowchart depicting an example of the operation of correction processing. 
         FIG. 22  is a graph for illustrating a relationship between a noise line and the background of a document. 
         FIG. 23A  is a graph for illustrating a relationship between a noise line region and a content. 
         FIG. 23B  is a graph for illustrating a relationship between a noise line region and a content. 
         FIG. 23C  is a graph for illustrating a relationship between a noise line region and a content. 
         FIG. 24  is a schematic view depicting an example of a correction image in which a noise line region overlaps a character. 
         FIG. 25  is a view depicting schematic components of an imaging unit according to another embodiment. 
         FIG. 26  is a view depicting schematic components of an imaging unit according to still another embodiment. 
         FIG. 27  is a block diagram depicting schematic components of a first processing circuit according to another embodiment. 
         FIG. 28  is a block diagram depicting schematic components of a second processing circuit according to still another embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, a document conveying apparatus, a control method, and computer program according to an embodiment, will be described with reference to the drawings. However, note that the technical scope of the invention is not limited to these embodiments and extends to the inventions described in the claims and their equivalents. 
       FIG. 1  is a configuration view of an example of an image processing system  1  according to an embodiment. 
     The image processing system  1  includes an image reading apparatus  100  and an information processing apparatus  200 . The image reading apparatus  100  is an example of the image processing apparatus, such as an image scanner. The image reading apparatus  100  may be a photocopier, a facsimile, or a multifunction peripheral (MFP). The information processing apparatus  200  is another example of the image processing apparatus, such as a personal computer, a multifunctional mobile terminal, or a mobile phone. The image reading apparatus  100  and the information processing apparatus  200  are mutually connected. 
     The image reading apparatus  100  includes a lower housing  101 , an upper housing  102 , a document tray  103 , side guides  104   a ,  104   b , a discharging tray  105 , a display operation device  106 , etc. 
     The upper housing  102  is arranged at a position for covering the upper surface of the image reading apparatus  100  and engaged with the lower housing  101  by a hinge such that the upper housing  102  can be opened and closed when a document is jammed or for cleaning the inside of the image reading apparatus  100 , especially, the imaging position of the imaging sensor. 
     The document tray  103  is engaged with the lower housing  101  and rotatable in a direction of arrow A 1 . When the image reading apparatus  100  is not in use, the document tray  103  is arranged at a position for covering the upper housing  102  and the lower housing  101  and functions as an exterior cover. Whereas, when the image reading apparatus  100  is in use, the document tray  103  is arranged at a position on which documents can be placed and functions as a document placing tray. 
     The side guides  104   a  and  104   b  are provided on the document tray  103  movably in a direction A 4  perpendicular to a document conveyance direction A 3 . The side guides  104   a  and  104   b  are aligned with the width of a document placed on the document tray  103  to regulate the width direction of the document. 
     The discharging tray  105  is housed inside the lower housing  101  such that the discharging tray  105  can be drawn out in a direction of arrow A 2  where, in a drawn-out state, the discharging tray  105  can retain discharged documents. 
     The display operation device  106  is an example of a display device and an operation device. The display operation device  106  has a display composed of liquid crystal, organic EL (Electro-Luminescence), etc., and an interface circuit that outputs image data on the display so as to display image data on the display. The display operation device  106  further includes a touch-panel style input device and an interface circuit that acquires signals from the input device so as to receive an operation by a user and output signals according to an input by the user. Note that a display device and an operation device may be separately provided. 
       FIG. 2  is a view for illustrating a conveyance path inside the image reading apparatus  100 . 
     The conveyance path inside the image reading apparatus  100  includes: a first sensor  111 , feed rollers  112   a ,  112   b , retard rollers  113   a ,  113   b , an open/close sensor  114 , first conveyance rollers  115   a ,  115   b , first driven rollers  116   a ,  116   b , a second sensor  117 , a first imaging unit  118   a , a second imaging unit  118   b , a third sensor  119 , second conveyance rollers  120   a ,  120   b , and second driven rollers  121   a ,  121   b.    
     Hereinafter, the feed rollers  112   a ,  112   b  may be collectively referred to as the feed roller  112 . Likewise, the retard rollers  113   a ,  113   b  may be collectively referred to as the retard roller  113 . The first conveyance rollers  115   a  and  115   b  may be collectively referred to as the first conveyance roller  115 . The first driven rollers  116   a  and  116   b  may be collectively referred to as the first driven roller  116 . The second conveyance rollers  120   a  and  120   b  may be collectively referred to as the second conveyance roller  120 . The second driven rollers  121   a  and  121   b  may be collectively referred to as the second driven roller  121 . The first imaging unit  118   a  and the second imaging unit  118   b  may be collectively referred to as the imaging unit  118 . 
     The lower surface of the upper housing  102  forms an upper guide  108   a  of the document conveyance path, while the upper surface of the lower housing  101  forms a lower guide  108   b  of the document conveyance path. In  FIG. 2 , an arrow A 3  indicates a document conveyance direction. Hereinafter, upstream refers to upstream of the document conveyance direction A 3 ; downstream refers to downstream of the document conveyance direction A 1 . 
     The first sensor  111  is a contact detecting sensor, is arranged on the upstream side of the feed roller  112  and retard roller  113  and detects whether or not a document is placed on the document tray  103 . 
     The open/close sensor  114  is a contact detecting sensor that detects an open/closed state of the upper housing  102 . The open/close sensor  114  detects whether the upper housing  102  is open or closed in relation to the lower housing  101  by detecting whether or not a projection  114   a  equipped on the upper housing  102  is engaged with a recess  114   b  equipped on the lower housing  101 . 
     The second sensor  117  is a contact detecting sensor and is arranged on the downstream side of the first conveyance roller  115  and first driven roller  116 , as well as, on the upstream side of the imaging unit  118 . The second sensor  117  detects the presence of a document between the first conveyance roller  115  and first driven roller  116 , and the imaging unit  118  in the document conveyance direction A 3 . 
     The third sensor  119  is a contact detecting sensor and is arranged on the downstream side of the imaging unit  118 , as well as, on the upstream side of the second conveyance roller  120  and second driven roller  121 . The third sensor  119  detects the presence of a document between the imaging unit  118 , and the second conveyance roller  120  and second driven roller  121  in the document conveyance direction A 3 . 
     A document placed on the document tray  103  is conveyed in the document conveyance direction A 3  between the upper guide  108   a  and the lower guide  108   b  by the rotation of the feed roller  112  in a direction of arrow A 5 . The retard roller  113  rotates in a direction of arrow A 6  when a document is being conveyed. By the movement of the feed roller  112  and the retard roller  113 , when a plurality of documents is placed on the document tray  103 , only a document being in contact with the feed roller  112  among the documents placed on the document tray  103  is separated. As such, the feed roller  112  and retard roller  113  function as a conveyance member that conveys a document, as well as, as a separating member that separates a document by limiting the conveyance of documents other than the separated document (multi feed prevention). 
     The document is fed between the first conveyance roller  115  and the first driven roller  116  through the guide of the upper guide  108   a  and lower guide  108   b . The document is then fed between the first imaging unit  118   a  and the second imaging unit  118   b  by the rotation of the first conveyance roller  115  in a direction of arrow A 7 . The document read by the imaging unit  118  is discharged on the discharging tray  105  by the rotation of the second conveyance roller  120  in a direction of arrow A 8 . 
       FIG. 3  is a perspective view of the first imaging unit  118   a  seen from the side of the document conveyance path. Note that the second imaging unit  118   b  has the same structure as the first imaging unit  118   a .  FIG. 4  is a view for illustrating the imaging unit  118  and a conveyance mechanism of the upstream and downstream sides of the imaging unit  118 . 
     The first imaging unit  118   a  is arranged above and opposing the second imaging unit  118   b . The first imaging unit  118   a  is equipped with an imaging unit guide  122  for guiding a document between the first imaging unit  118   a  and the second imaging unit  118   b . The first imaging unit  118   a  captures an image of the back surface of a conveyed document and the second imaging unit  118   b  captures an image of the front surface of a conveyed document. 
     While the second imaging unit  118   b  is fixed to the lower housing  101 , the first imaging unit  118   a  is supported by the upper housing  102  such that the first imaging unit  118   a  is movable in a direction A 9  perpendicular to the document conveyance path. An energizing spring  123  is equipped above the imaging unit guide  122  so that the energized spring  123  energizes the first imaging unit  118   a  toward the second imaging unit  118   b.    
     The first imaging unit  118   a  includes a first light source  124   a , a first imaging sensor  125   a , a first white reference member  126   a , a first transparent member  127   a , etc. The second imaging unit  118   b  includes a second light source  124   b , a second imaging sensor  125   b , a second white reference member  126   b , a second transparent member  127   b , etc. 
     The first light source  124   a  is provided on the opposite side of the second white reference member  126   b  across the first transparent member  127   a  and the second transparent member  127   b , as well as, on the upstream side of the first imaging sensor  125   a  in the document conveyance direction A 3 . The first light source  124   a  irradiates light toward the back surface of a document that has been conveyed to the position of the imaging unit  118  (when there is no conveyed document, toward the second white reference member  126   b  of the opposing second imaging unit  118   b ). The first light source  124   a  is equipped with an LED (Light Emitting Diode)  128   a  at an end thereof in a direction A 4  perpendicular to the document conveyance direction A 3 , and further equipped with a light guide member  129   a  that guides light irradiated from the LED along the direction A 4 . The light guide member  129   a  is equipped with a plurality of slits  130   a  along the direction A 4  through which light irradiated from the LED  128   a  passes. The widths of the slits  130   a  are increased as the slits are farther from the LED  128   a  so that the light amount becomes substantially uniform at each position. 
     A shield member  131   a  that has an opening on the side facing the imaging position L 1  is provided around the light guide member  129   a  so that the first light source  124   a  can irradiate light toward the imaging position L 1  of the first imaging sensor  125   a . As such, the first light source  124   a  is provided in a manner in which the direction of the irradiated light is inclined with reference to the imaging direction of the first imaging sensor  125   a.    
     Likewise, the second light source  124   b  is provided on the opposite side of the first white reference member  126   a  across the second transparent member  127   b  and the first transparent member  127   a , as well as, on the downstream side of the second imaging sensor  125   b  in the document conveyance direction A 3 . The second light source  124   b  irradiates light toward the front surface of the document that has been conveyed to the position of the imaging unit  118  (when there is no conveyed document, toward the first white reference member  126   a  of the opposing first imaging unit  118   a ). The second light source  124   b  is equipped with an LED at an end thereof in the direction A 4  and further equipped with a light guide member that guides light irradiated from the LED along the direction A 4 . The second light source  124   b  is provided in a manner in which the direction of the irradiated light is inclined with reference to the imaging direction of the second imaging sensor  125   b.    
     The first imaging sensor  125   a  is an example of the imaging device and is provided on the opposite side of the second white reference member  126   b  across the first transparent member  127   a  and the second transparent member  127   b . The first imaging sensor  125   a  is a Contact Image Sensor (CIS) of a unit magnification optical system type that has imaging elements using complementary metal oxide semiconductor (CMOS) that are linearly arranged in the main scanning direction. Further, the first imaging sensor  125   a  has a lens that forms an image on the imaging device and an A/D (analog to digital) converter that amplifies the electric signals output from the imaging device and converts the analog signals to digital signals (A/D). At the imaging position L 1 , the first imaging sensor  125   a  generates and outputs a document image captured a back surface and periphery of a document that was conveyed between the first imaging unit  118   a  and the second imaging unit  118   b , i.e., between the second transparent member  127   b  and the first imaging sensor  125   a . When there is no conveyed document, the first imaging sensor  125   a  generates and outputs a white reference image captured the second white reference member  126   b.    
     Likewise, the second imaging sensor  125   b , at the imaging position L 2 , is an example of the imaging device and is provided on the opposite side of the first white reference member  126   a  across the first transparent member  127   a  and the second transparent member  127   b . The second imaging sensor  125   b  is a CIS of a unit magnification optical system type that has imaging elements using CMOS that are linearly arranged in the main scanning direction. Further, the second imaging sensor  125   b  has a lens that forms an image on the imaging device and an A/D converter that amplifies the electric signals output from the imaging device and converts the analog signals to digital signals. At the imaging position L 2 , the second imaging sensor  125   b  generates and outputs a document image captured a front surface and periphery of a document that was conveyed between the first imaging unit  118   a  and the second imaging unit  118   b , i.e., between the first transparent member  127   a  and the second imaging sensor  125   b . When there is no conveyed document, the second imaging sensor  125   b  generates and outputs a white reference image captured the first white reference member  126   a.    
     Note that the first imaging sensor  125   a  and the second imaging sensor  125   b  may be an imaging sensor of an optical reduction system type that has imaging elements using charge coupled device (CCD), instead of CMOS. 
     The first white reference member  126   a  is provided at a position above the first transparent member  127   a  and spaced apart from the first transparent member  127   a  and opposing the second light source  124   b  and second imaging sensor  125   b  of the second imaging unit  118   b . The surface of the first white reference member  126   a  facing the second imaging sensor  125   b  is white. Likewise, the second white reference member  126   b  is provided at a position below the second transparent member  127   b  and spaced apart from the second transparent member  127   b  and opposing the first light source  124   a  and first imaging sensor  125   a  of the first imaging unit  118   a . The surface of the second white reference member  126   b  facing the first imaging sensor  125   a  is white. The image reading apparatus  100  can correct an image, such as by shading correction, based on the image signals captured the first white reference member  126   a  and the second white reference member  126   b.    
     The first transparent member  127   a  and the second transparent member  127   b  are formed of transparent glass. Note that the first transparent member  127   a  and the second transparent member  127   b  may instead be formed of transparent plastic etc. 
     Hereinafter, the first light source  124   a  and the second light source  124   b  may be collectively referred to as the light source  124 , and the first imaging sensor  125   a  and the second imaging sensor  125   b  may be collectively referred to as the imaging sensor  125 . The first white reference member  126   a  and the second white reference member  126   b  may be collectively referred to as the white reference member  126 , and the first transparent member  127   a  and the second transparent member  127   b  may be collectively referred to as the transparent member  127 . 
     As depicted in  FIG. 4 , the first driven roller  116  is arranged above and opposing the first conveyance roller  115 . That is, the first driven roller  116  is arranged on the opposite side of the second white reference member  126   b  with reference to the upper surface of the second transparent member  127   b  from the first conveyance roller  115  in the direction A 9  perpendicular to the second transparent member  127   b . The conveyance roller pair including the first conveyance roller  115  and the first driven roller  116  functions as a conveyance member that conveys a document between the first imaging unit  118   a  and the second imaging unit  118   b , i.e., between the second transparent member  127   b  and the first imaging sensor  125   a.    
     A position L 3 , which is the position of the center O 1  as an rotation axis of the first driven roller  116  in the document conveyance direction A 3 , is shifted to the side of the imaging unit  118 , i.e., the side of the first imaging sensor  125   a , than a position L 4  which is the position of the center O 2  as an rotation axis of the first conveyance roller  115  in the document conveyance direction A 3 . A nip position L 5  of the first conveyance roller  115  and the first driven roller  116  is arranged above the upper surface of the second transparent member  127   b , i.e., on the opposite side of the second white reference member  126   b  with reference to the upper surface of the second transparent member  127   b  in the direction A 9  perpendicular to the second transparent member  127   b.    
     In particular, the nip position L 5  is arranged such that a position L 6 , at which a tangent plane P 2  that contacts the first conveyance roller  115  at the nip position L 5  contacts the upper surface of the second transparent member  127   b , is arranged on the upstream side of the imaging positions L 1  and L 2  in the document conveyance direction A 3 . As such, the first conveyance roller  115  and the first driven roller  116  can convey a document such that the document is conveyed along the second transparent member  127   b  at the imaging positions L 1  and L 2 . 
     The second driven roller  121  is arranged above and opposing the second conveyance roller  120 . That is, the second driven roller  121  is arranged on the opposite side of the second white reference member  126   b  with reference to the upper surface of the second transparent member  127   b  from the second conveyance roller  120  in the direction A 9 . 
     A position L 7 , which is the position of the center O 3  as an rotation axis of the second driven roller  121  in the document conveyance direction A 3 , is shifted to the side of the imaging unit  118 , i.e., the side of the first imaging sensor  125   a , than a position L 8  which is the position of the center O 4  as an rotation axis of the second conveyance roller  120  in the document conveyance direction A 3 . A nip position L 9  of the second conveyance roller  120  and the second driven roller  121  is arranged at the same height as the nip position of the first conveyance roller  115  and the first driven roller  116  from the upper surface of the second transparent member  127   b  in the direction A 9  perpendicular to the second transparent member  127   b . In particular, the nip position L 9  is arranged so that a position L 10 , at which a tangent plane P 4  that contacts the second conveyance roller  120  at the nip position L 9  contacts the upper surface of the second transparent member  127   b , is arranged on the downstream side of the imaging positions L 1  and L 2  in the document conveyance direction A 3 . The angle of the tangent plane P 4  with reference to the upper surface of the second transparent member  127   b  is preferably arranged to be the same angle of the tangent plane P 2  with reference to the upper surface of the second transparent member  127   b.    
       FIG. 5  is a view for illustrating the arrangement of the first conveyance roller  115  and the first driven roller  116 . 
     A distance h between the nip position L 5  of the first conveyance roller  115  and the first driven roller  116  and an extension plane P 1  of the upper surface of the second transparent member  127   b  is set to be larger than the height of an embossed bump that may be formed on a card to be conveyed. Likewise, depicted in  FIG. 4 , a distance between the nip position L 9  of the second conveyance roller  120  and the second driven roller  121  and an extension plane P 3  of the upper surface of the second transparent member  127   b  is set to be larger than the height of an embossed bump that may be formed on a card to be conveyed. The height of an embossed bump can be defined according to the specification of the device and may be set at 0.46 mm that is the height of embossing on an identification (ID) card specified by ISO/IEC 7811-1. Alternatively, the height of an embossed bump may be set at 0.48 mm that is the height of embossing on a card specified by Japanese Industrial Standards (JIS). 
     The following formulas can be established with regard to the first conveyance roller  115  and the first driven roller  116 : 
                       sin   ⁢           ⁢     θ   1       =     δ       r   1     +     r   2           ,       tan   ⁢           ⁢     θ   1       =     h   L               [     Math   ⁢           ⁢   1     ]               
θ 1  is an angle of a straight line extending from the imaging position L 2  of the second imaging sensor  125   b  to the nip position L 5  of the first conveyance roller  115  and the first driven roller  116  with reference to the extension plane P 1 . r 1  is the radius of the first conveyance roller  115  and r 2  is the radius of the first driven roller  116 . L is a distance from the nip position L 5  to the imaging position L 2  of the second imaging sensor  125   b  in the document conveyance direction A 3 . δ is a displacement of the center O 1  of the first driven roller  116  with reference to the center O 2  of the first conveyance roller  115  in the document conveyance direction A 3 .
 
     Thus, the displacement  6  of the center O 1  of the first driven roller  116  with reference to the center O 2  of the first conveyance roller  115  in the document conveyance direction can be calculated by the following formula: 
     
       
         
           
             
               
                 
                   δ 
                   = 
                   
                     
                       ( 
                       
                         
                           r 
                           1 
                         
                         + 
                         
                           r 
                           2 
                         
                       
                       ) 
                     
                     × 
                     
                       sin 
                       ⁡ 
                       
                         ( 
                         
                           
                             tan 
                             
                               - 
                               1 
                             
                           
                           ⁡ 
                           
                             ( 
                             
                               h 
                               L 
                             
                             ) 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Math 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                   
                   ] 
                 
               
             
           
         
       
     
     To arrange the position L 6  depicted in  FIG. 4  on the side of the first conveyance roller  115  and first driven roller  116  than the imaging position L 2 , the center O 1  of the first driven roller  116  may be shifted by δ or more from the center O 2  of the first conveyance roller  115  in the document conveyance direction A 3 . For example, when h is 1.5 mm; r 1 , 6.8 mm; r 2 , 6.5 mm; and L, 18.7 mm, the displacement of the center O 1  of the first driven roller  116  with reference to the center O 2  of the first conveyance roller  115  is set at 1.1 mm. 
       FIG. 6  is a view for illustrating how a document is conveyed. 
     In  FIG. 6 , the path R 1  indicates an ideal conveyance path through which the leading end of a document passes. The leading end of a document that is conveyed by the image reading apparatus  100  contacts the first conveyance roller  115  at a position L 11 , and proceeds upward from the extension plane P 1  on the upper surface of the second transparent member  127   b  along the tangent plane P 6  of the first conveyance roller  115 . Then, the leading end of the document contacts the guide member  122   a  of the imaging unit guide  122  at a position L 13 , then, proceeds downward. 
     The leading end of the document that is directed downward by the guide member  122   a  is fed between the first conveyance roller  115  and the first driven roller  116 . The leading end of the document passes through the nip position L 5  of the first conveyance roller  115  and the first driven roller  116 , proceeds along the tangent plane P 2  at the nip position L 5 , and contacts the second transparent member  127   b  of the second imaging unit  118   b  at the position L 6 . 
     The leading end of the document that has contacted the second transparent member  127   b  is conveyed along the second transparent member  127   b . After passing through between the first imaging unit  118   a  and the second imaging unit  118   b , the leading end of the document proceeds along the extension plane P 3  of the upper surface of the second transparent member  127   b  and contacts the second conveyance roller  120  at a position L 14 . The leading end of the document that has contacted the second conveyance roller  120  proceeds along the tangent plane P 7  of the second conveyance roller  120  at the position L 14 , and contacts the second driven roller  121  at a position L 15 . 
     The leading end of the document that has contacted the second driven roller  121  is fed between the second conveyance roller  120  and the second driven roller  121 , and passes through the nip position L 9  of the second conveyance roller  120  and the second driven roller  121 . 
     When the leading end of the document has passed the nip position L 9 , a portion of the document located on the second transparent member  127   b  is pulled along the tangent plane P 4  at the nip position L 9  and separated from the second transparent member  127   b  on the downstream side of the position L 10  in the document conveyance direction A 3 . The document is always maintained at a certain distance from the second transparent member  127   b  at the imaging positions L 1  and L 2 , and the distance from the document to each imaging sensor  125  is constant. As such, even when CIS of a unit magnification optical system type with small depth-of-field is used, occurrence of divergence of focus can be prevented and the imaging sensor  125  can acquire stable images. In particular, as a distance from a document to each imaging sensor  125  in the direction A 4  (main scanning direction) perpendicular to the document conveyance direction A 3  is constant, occurrence of unevenness in the horizontal direction in the document image is prevented. By stabilizing the conveyance path of a document, the image reading apparatus  100  does not need an ample space in the direction A 9  (vertical direction) perpendicular to the second transparent member  127   b , and the device size can be reduced. 
     Since the leading end of a document is conveyed along the second transparent member  127   b , the leading end of the document can clean the second transparent member  127   b , i.e., remove dirt substances from the second transparent member  127   b . Although dirt substances may not only adhere to the second transparent member  127   b  but also possibly adhere to the first transparent member  127   a , dirt substances adhering to the first transparent member  127   a  are more likely to fall by its own weight and adhere to the second transparent member  127   b . In the image reading apparatus  100 , the conveyance of the leading end of a document along the second transparent member  127   b  allows removal of dirt substances that fell from the first transparent member  127   a.    
       FIG. 7  is a view for illustrating the arrangement of the first light source  124   a  and the first imaging sensor  125   a.    
     As depicted in  FIG. 7 , the first light source  124   a  is provided on the upstream side of the first imaging sensor  125   a  in the document conveyance direction A 3  such that the light irradiation direction A 10  is inclined with reference to the imaging direction A 11  of the first imaging sensor  125   a . The second white reference member  126   b  is provided spaced apart from the second transparent member  127   b . As such, when a document D conveyed on the second transparent member  127   b  has reached immediately before the imaging position L 1  of the first imaging sensor  125   a , a shadow of the leading end of the document D is formed on the second white reference member  126   b  at the imaging position L 1 . 
     When a dirt substance (foreign substance), such as paper dust, other fine particles, glue, etc., adheres to the transparent member  127 , noise line (vertical streak noise) extending in a document conveyance direction (sub-scanning direction) is generated in a document image, necessitating removal of such noise line. Dirt substances may in some cases be captured in white and in other cases in black in the document image. When the color of a dirt substance is similar to the color of a background of a document, the dirt substance is less likely to be identified or detected in the document image. In addition, since the white reference member  126  is white, although black dirt substances can be clearly identified and accurately detected in the white reference image, white dirt substances are less likely to be identified and detected in the white reference image. Thus, the image processing system  1  detects noise line in a shadow region formed by the shadow of a document in the correction image that was made based on the document image. Since the shadow formed on the white reference member  126  is gray that is an intermediate color of white and black, both white and black dirt substances can be identified and accurately detected in the shadow region. 
     The correction image preferably has a shadow of 4 pixels or more in the document conveyance direction A 3  so that the noise line can be well detected in the correction image. Accordingly, the width a of a shadow in the document conveyance direction A 3  formed on the second white reference member  126   b  is preferably a length equivalent to 4 pixels (0.3 mm in 300 dpi) or more. 
     The width a is the product of a distance b from the upper surface of the second transparent member  127   b  to the upper surface of the second white reference member  126   b  and the tangent of an angle θ of the light irradiation direction A 10  with reference to the imaging direction A 11 . In order to increase the width a, the angle θ or the distance b should be increased. However, if the angle θ is excessively increased, the dimension of the imaging unit  118  is enlarged. As such, the angle θ is preferably a value between 30° or more and 45° or less. Whereas, if the length b is excessively increased, the luminance of the second white reference member  126   b  becomes lower (darker) in the white reference image captured the second white reference member  126   b , and it becomes difficult to perform satisfactory shading correction using the white reference image. As such, the length b is preferably a value between 0.8 mm or more and 1.8 mm or less. 
     Each imaging unit  118  is constituted by a CIS and irradiates the second white reference member  126   b  only by the directive light emitted from the first light source  124   a . For example, if an image reading apparatus has a reflection member that reflects light emitted from the first light source  124   a  toward the second white reference member  126   b , the dimension of the device increases, as well as, a shadow formed on the second white reference member  126   b  becomes bright and disappears (fades). Whereas, the image reading apparatus  100  does not have a reflection member that reflects the light emitted from the first light source  124   a  toward the side of the second white reference member  126   b . In this way, with the image reading apparatus  100 , the dimension of the device can be reduced, as well as, a shadow can be favorably formed on the second white reference member  126   b.    
     Note that the second light source  124   b  is provided on the downstream side of the second imaging sensor  125   b  in the document conveyance direction A 3  such that the light irradiation direction is inclined with reference to the imaging direction of the second imaging sensor  125   b . The first white reference member  126   a  is provided spaced apart from the first transparent member  127   a . As such, when the rear end of a document D conveyed on the second transparent member  127   b  has passed the imaging position L 2  of the second imaging sensor  125   b , a shadow of the rear end of the document D is formed at the imaging position L 2  on the first white reference member  126   a.    
     When the light source  124  is provided on the upstream side of the imaging sensor  125  in the document conveyance direction A 3 , a shadow is formed by the leading end of the document D, while, when the light source  124  is provided on the downstream side of the imaging sensor  125 , a shadow is formed by the rear end of the document D. A shadow formed by the leading end of the document D allows detection of a shadow area from the document image and detection of a dirt substance before completion of the conveyance of the document D, thus, the light source  124  is preferably provided on the upstream side of the imaging sensor  125  in the document conveyance direction A 3 . 
     As described above, since a sufficiently long shadow is formed on each white reference member  126  in the image reading apparatus  100 , the image processing system  1  can accurately detect both black and white dirt substances at the imaging position of each imaging sensor  125 . 
     In addition, as described above, the first conveyance roller  115  and the first driven roller  116  can convey a document such that the document is conveyed along the second transparent member  127   b  at the imaging positions L 1  and L 2 . In this way, in contrast to a case where a document is conveyed floated in the air without being conveyed along a specific member, the image reading apparatus  100  can stabilize a document conveyance path regardless of the kind of document (thickness, hardness, etc.) and stably generate a shadow on the white reference member  126 . 
     Further, the image reading apparatus  100  has a transparent member  127  between the document conveyance path and each white reference member  126 , protecting the white reference member  126  and preventing the white reference member  126  from dirt or scratches. Since the transparent member  127  has higher rigidity than the white reference member  126 , the transparent member  127  is less likely to sustain scratches when cleaned by users. 
       FIG. 8  is a block diagram depicting schematic components of the image reading apparatus  100  and the information processing apparatus  200 . 
     In addition to the above-described components, the image reading apparatus  100  further includes a driving device  134 , a first interface device  135 , a first storage device  140 , a first Central Processing Unit (CPU)  160 , a first processing circuit  180 , etc. 
     The driving device  134  includes one or a plurality of motors and rotates the feed roller  112 , the retard roller  113 , the first conveyance roller  115 , and the second conveyance roller  120  according to a control signal from the CPU  160  to convey a document. 
     The first interface device  135  has an interface circuit conforming to a serial bus such as Universal Serial Bus (USB). The first interface device  135  transmits and receives various images and information through a communication connection with the information processing apparatus  200 . Instead of the first interface device  135 , a communication device that has an antenna for transmitting and receiving wireless signals and a wireless communication interface circuit for transmitting and receiving signals via a wireless communication channel according to a predetermined communication protocol may be used. The predetermined communication protocol may be, for example, a wireless local area network (LAN). 
     The first storage device  140  includes: a memory device, such as a random access memory (RAM) and a read only memory (ROM); a fixed disk device, such as a hard disk; or a portable storage device, such as a flexible disk and an optical disk. The first storage device  140  stores a computer program, a database, a table, etc., that are used for various processing of the image reading apparatus  100 . The computer program may be installed on the first storage device  140  from a computer-readable, non-transitory medium such as a compact disk read only memory (CD-ROM), a digital versatile disk read only memory (DVD-ROM), etc., by using a well-known setup program, etc. 
     The first storage device  140  further stores a variety of images. The first storage device  140  also stores number of documents which have been scanned as a document scan count, the position of dirt, etc. The scan count is the number of times the image reading apparatus  100  has scanned a document and incremented each time the image reading apparatus  100  scans a document. The position of dirt is a position where a dirt substance is detected in the white reference image. The scan count and the position of dirt are stored in a non-transitory memory to be referred to even after the power of the image reading apparatus  100  is switched off and on again. 
     The first CPU  160  operates according to a program stored in advance in the first storage device  140 . Note that a digital signal processor (DSP), a large scale integration (LSI), etc., may be used instead of the first CPU  160 . Alternatively, an Application Specific Integrated Circuit (ASIC), a field-programming gate array (FPGA) etc., may be used instead of the first CPU  160 . 
     The first CPU  160  is connected to the display operation device  106 , the first sensor  111 , the open/close sensor  114 , the second sensor  117 , the third sensor  119 , the imaging unit  118 , the driving device  134 , the first interface device  135 , the first storage device  140 , the first processing circuit  180 , etc., and controls these components. The first CPU  160  controls driving of the driving device  134 , document reading of the imaging unit  118 , etc., to acquire a document image. 
     The first processing circuit  180  performs predetermined image processing such as correction processing on the document image acquired from the imaging unit  118 . Note that a LSI, a DSP, an ASIC, a FPGA, etc., may be used as the first processing circuit  180 . 
     Whereas, the information processing apparatus  200  further includes a display device  201 , an operation device  202 , a second interface device  203 , a second storage device  220 , a second CPU  240 , a second processing circuit  260 , etc. 
     The display device  201  is an example of a display device, which has a display composed of liquid crystal, organic EL, etc., and an interface circuit for outputting image data on the display and displays image data on the display according to an instruction from the second CPU  240 . 
     The operation device  202  is an example of an operation device, which further includes an input device and an interface circuit that acquires signals from the input device, receives an operation by a user, and outputs signals according to the input by the user to the second CPU  240 . 
     The second interface device  203  includes an interface circuit or a wireless communication interface circuit, similar to the one of the first interface device  135 , and transmits and receives a variety of images and information through a communication connection with the image reading apparatus  100 . 
     The second storage device  220  has: a memory device, such as a RAM and a ROM; a fixed disk device, such as a hard disk; or a portable storage device, such as a flexible disk and an optical disk. Further, the second storage device  220  stores a computer program, a database, a table, etc., that are used for various processing of the information processing apparatus  200 . The computer program may be installed on the second storage device  220  from a computer-readable, non-transitory medium such as a CD-ROM, a DVD-ROM, etc., by using a well-known setup program, etc. 
     The second storage device  220  further stores a variety of images. The second storage device  220  also stores noise line positions etc. The noise line position is a position where a noise line is detected in a correction image. The noise line position is stored in a non-transitory memory, a hard disk, etc., so as to be referred to even after the power of the information processing apparatus  200  is switched off and on again. 
     The second CPU  240  operates according to a program stored in advance in the second storage device  220 . Note that a DSP, a LSI, an ASIC, a FPGA, etc., may be used instead of the second CPU  240 . 
     The second CPU  240  is connected to the display device  201 , the operation device  202 , the second interface device  203 , the second storage device  220 , the second processing circuit  260 , etc., and controls these components. The second CPU  240  controls the components and executes image processing on images acquired from the image reading apparatus  100 . 
     The second processing circuit  260  performs predetermined image processing such as correction processing on an image acquired from the image reading apparatus  100 . Note that a DSP, a LSI, an ASIC, a FPGA, etc., may be used as the second processing circuit  260 . 
       FIG. 9  is a view depicting the schematic components of the first storage device  140  and the first CPU  160  of the image reading apparatus  100 . 
     As depicted in  FIG. 9 , the first storage device  140  stores programs, such as a first image acquiring program  141 , a first dirt substance detection processing program  142 , a correction data generation program  149 , a correction image generation program  150 , etc. The first dirt substance detection processing program  142  includes a first result acquiring program  143 , a dirt degree calculation program  144 , a notifying program  145 , a setting program  146 , a threshold acquiring program  147 , an information acquiring program  148 , etc. Each program is a functional module implemented by software that operates on the processor. The first CPU  160  reads each program stored in the first storage device  140  and operates according to the read program. As such, the first CPU  160  functions as a first image acquiring module  161 , a first dirt substance detection processing module  162 , a first result acquiring module  163 , a dirt degree calculation module  164 , a notifying module  165 , a setting module  166 , a threshold acquiring module  167 , an information acquiring module  168 , a correction data generation module  169 , and a correction image generation module  170 . 
       FIG. 10  is a view depicting the schematic components of the second storage device  220  and second CPU  240  of the information processing apparatus  200 . 
     As depicted in  FIG. 10 , the second storage device  220  stores programs, such as a second image acquiring program  221 , a second dirt substance detection processing program  222 , a determination program  230 , a correction program  231 , etc. The second dirt substance detection processing program  222  includes a second result acquiring program  223 , a device information acquiring program  224 , an edge pixel extracting program  225 , a document region detecting program  226 , a shadow region detecting program  227 , a noise pixel extracting program  228 , a noise line detecting program  229 , etc. Each program is a functional module implemented by software that operates on the processor. The second CPU  240  reads each program stored in the second storage device  220  and operates according to the read program. As such, the second CPU  240  functions as a second image acquiring module  241 , a second dirt substance detection processing module  242 , a second result acquiring module  243 , a device information acquiring module  244 , an edge pixel extracting module  245 , a document region detecting module  246 , a shadow region detecting module  247 , a noise pixel extracting module  248 , a noise line detecting module  249 , a determination module  250  and a correction module  251 . 
       FIG. 11  is a flowchart depicting an example of the operation of the overall processing of the image reading apparatus  100 . The following will describe an example of the operation of the overall processing of the image reading apparatus  100  with reference to the flowchart depicted in  FIG. 11 . Note that the operation flow as will be described below is performed primarily by the first CPU  160  jointly with each component of the image reading apparatus  100  according to programs prestored in the first storage device  140 . This operation flow is performed immediately after start-up of the device or after document reading processing. 
     First, the first image acquiring module  161  causes each imaging sensor  125  to capture an image of each white reference member  126  to generate a white reference image and acquires the generated white reference image (step S 101 ). The white reference image is an image where the number of pixels in a vertical direction (a document conveyance direction A 3 ) is one and a plurality of pixels are arranged in a horizontal direction (a direction A 4  perpendicular to the document conveyance direction A 3 ). Hereinafter, the white reference image captured the second white reference member  126   b  by the first imaging sensor  125   a  may be referred to as the first white reference image, and the white reference image captured the first white reference member  126   a  by the second imaging sensor  125   b  may be referred to as the second white reference image. 
     Next, the first result acquiring module  163  of the first dirt substance detection processing module  162  acquires a second result from the information processing apparatus  200  via the first interface device  135  (step S 102 ). The second dirt substance detection processing module  242  of the information processing apparatus  200  executes second processing for detecting a dirt substance from the correction image. In particular, in the second processing, the second dirt substance detection processing module  242  detects a dirt substance causing noise line from the correction image. The second result is a dirt substance detection result of the second processing by the second dirt substance detection processing module  242  and is a noise line detection result from the correction image. The second result includes information, such as, whether or not a dirt substance is detected from the correction image, the position of the detected dirt substance in the correction image, and whether or not the detected dirt substance has been confirmed by a user. Note that, when the second dirt substance detection processing module  242  has not performed the second processing, the first dirt substance detection processing module  162  does not acquire the second result. 
     Next, the first dirt substance detection processing module  162  executes the first processing (step S 103 ). The first dirt substance detection processing module  162  detects a dirt substance from the white reference image in the first processing. In particular, the first dirt substance detection processing module  162  detects a dirt substance that causes dirt at an imaging position from the white reference image in the first processing. Further, when the first dirt substance detection processing module  162  has detected a dirt substance from the white reference image in the first processing, the first dirt substance detection processing module  162  determines whether the detected dirt substance is on the transparent member on the side of the imaging sensor which captured the white reference image or on the transparent member on the side of the white reference member which was captured in the white reference image. The details of the first processing will be described later. 
     Next, the correction data generation module  169  generates data for shading correction based on the white reference image (step S 104 ). The correction data generation module  169  uses, for example, as the data for shading correction, an image that has a gradient value of each pixel obtained by adding a predetermined offset value to or subtracting a predetermined offset value from the gradient value of a corresponding pixel in the white reference image. The gradient value is, for example, a luminance value. Note that the gradient value may instead be a color value (R value, G value, B value) etc. Note that the correction data generation module  169  may use the white reference image as is as data for shading correction. Hereinafter, the data for shading correction generated from the first white reference image may be referred to as the first data for shading correction and the data for shading correction generated from the second white reference image may be referred to as the second data for shading correction. 
     Next, when, in the first processing, the correction data generation module  169  determines that a dirt substance detected from a white reference image is on the transparent member on the side of the white reference member which was captured in the white reference image, the correction data generation module  169  adjusts the data for shading correction generated from the white reference image (step S 105 ). The correction data generation module  169 , for example, adjusts the data for shading correction by replacing a gradient value of each pixel included in a dirt substance region corresponding to a dirt substance in the data for shading correction with an average value of the gradient values of pixels included in a region of a predetermined width adjacent to the dirt substance region. Whereas, when a dirt substance is not detected from the white reference image in the first processing, or when a dirt substance is determined to be on the transparent member on the side of the imaging sensor which captured the white reference image, the correction data generation module  169  does not adjust the data for shading correction generated from the white reference image. 
     When a position of a dirt substance is on the transparent member on the side of the white reference member, the dirt substance is captured in the white reference image captured the white reference member, but the dirt substance is not captured in the document region of a document image, as there is a document between the imaging sensor and the dirt substance when the document image is captured. Thus, the document image can be appropriately corrected through shading correction by removing elements corresponding to a dirt substance from the data for shading correction generated based on the white reference image captured the dirt substance. 
     Whereas, when the position of a dirt substance is on the transparent member on the side of the imaging sensor, the dirt substance is captured at the corresponding position in both white reference image captured by the imaging sensor and document image subsequently captured by the imaging sensor. Thus, by performing shading correction using data for shading correction generated based on the white reference image captured the dirt substance as it is, the dirt substance captured in the document image can be removed and occurrence of noise line caused by the dirt substance in the correction image can be prevented. 
     Next, the first dirt substance detection processing module  162  executes confirmation processing (step S 106 ). In the confirmation processing, the first dirt substance detection processing module  162  determines whether a confirmation operation by a user has been received. The details of the confirmation processing will be described later. 
     Next, the first image acquiring module  161  determines whether a user has instructed reading of a document using the display operation device  106  and a reading instruction signal that instructs reading of a document has been received via the display operation device  106  (step S 107 ). When a reading instruction signal has not been received yet, the first dirt substance detection processing module  162  returns the processing to step S 106  and re-executes the confirmation processing. 
     Whereas, when a reading instruction signal has been received, the first image acquiring module  161  determines whether a document is placed on the document tray  103  based on a signal received from the first sensor  111  (step S 108 ). When a document is not placed on the document tray  103 , the first dirt substance detection processing module  162  returns the processing to step S 106  and re-executes the confirmation processing. 
     Whereas, when a document is placed on the document tray  103 , the first image acquiring module  161  drives the driving device  134  to rotate the feed roller  112 , retard roller  113 , first conveyance roller  115  and second conveyance roller  120  to convey the document (step S 109 ). 
     Next, the first image acquiring module  161  causes the imaging sensor  125  to capture an image of the document to generate a document image, acquires the generated document image, and increments the scan count stored in the first storage device  140  (step S 110 ). When the image reading apparatus  100  detects dirt, the image reading apparatus  100  generates and provides a warning to a user. However, the image reading apparatus  100  captures the document even if a user has not completed the confirmation. In this way, the image reading apparatus  100  can continue capturing an image of a document when a user does not care about the dirt, thereby improving user convenience. Hereinafter, the document image captured by the first imaging sensor  125   a  may be referred to as the first document image and the document image captured by the second imaging sensor  125   b  may be referred to as the second document image. 
     Next, the correction image generation module  170  performs shading correction on the document image using the data for shading correction based on the white reference image to generate a correction image (step S 111 ). The correction image is an example of an input image. The correction image generation module  170  performs shading correction on the first document image using the first data for shading correction and performs shading correction on the second document image using the second data for shading correction. Hereinafter, the correction image obtained by correcting the first document image may be referred to as the first correction image and the correction image obtained by correcting the second document image may be referred to as the second correction image. 
     Next, the correction image generation module  170  transmits the correction image, first result, and device information to the information processing apparatus  200  via the first interface device  135  (step S 112 ). The first result is a dirt substance detection result of the first processing by the first dirt substance detection processing module  162  and is a dirt detection result at the imaging position in the white reference image. The first result includes information, such as, whether or not a dirt substance is detected from the white reference image, the position of the detected dirt substance in the white reference image, and whether or not the detected dirt substance has been confirmed by a user. 
     The device information indicates the arrangement of the imaging sensor  125  and the light source  124  in the image reading apparatus  100  that generates a correction image. For example, the device information indicates that, with regard to a first correction image, the first light source  124   a  is provided on the upstream side of the first imaging sensor  125   a  in the document conveyance direction A 3  and, with regard to a second correction image, the second light source  124   b  is provided on the downstream side of the second imaging sensor  125   b  in the document conveyance direction A 3 . Note that the correction image generation module  170  may transmit the first result, only when new first result was acquired, and may omit transmission of the first result that has already been transmitted. Further, when the device information has already been transmitted, the correction image generation module  170  may also omit transmission of the device information. 
     Next, the first CPU  160  determines whether there is any document remaining on the document tray  103  based on the signals received from the first sensor  111  (step S 113 ). 
     When there is a document remaining on the document tray  103 , the first CPU  160  returns the processing to step S 109  and repeats the processing of steps S 109  to S 113 . Whereas, when no document is remaining on the document tray  103 , the first CPU  160  ends the set of processing. 
       FIG. 12  is a flowchart depicting an example of the operation of the first processing. The first processing depicted in  FIG. 12  is carried out at step S 103  of the flowchart depicted in  FIG. 11 . 
     First, the dirt degree calculation module  164  calculates a dirt degree at the imaging position for each pixel included in a white reference image (step S 201 ). The dirt degree is the degree of dirt caused by a dirt substance, such as paper dust, on the first transparent member  127   a  and the second transparent member  127   b  at the imaging position of the imaging sensor  125 . The dirt degree calculation module  164  calculates a dirt degree by comparing the gradient value of each pixel with the gradient values of peripheral pixels of the pixel of interest. The peripheral pixels may be, for example, pixels located within a predetermined range (for example, 3 pixels) from the pixel of interest. The dirt degree calculation module  164  calculates, for example, the absolute value of a difference between the gradient value of the pixel of interest and the average value of the gradient values of the peripheral pixels as a dirt degree of the pixel of interest. Note that the dirt degree calculation module  164  may calculate the absolute value of a difference between the gradient value of the pixel of interest and the weighted average value of the gradient values of the peripheral pixels that are weighted such that the weight becomes larger as closer to the pixel of interest, as a dirt degree of the pixel of interest. 
       FIG. 13  is a schematic view depicting an example of a white reference image. 
     The horizontal axis in  FIG. 13  indicates the position of each pixel in a horizontal direction in the white reference image and the vertical axis indicates the gradient value of each pixel. The surface of the white reference member  126  facing the imaging sensor  125  is white, and, as depicted in  FIG. 13 , the gradient values of pixels at horizontal positions in the white reference image are substantially constant. However, when a dirt substance adheres to the transparent member  127 , pixels  1301 - 1303  corresponding to the dirt substance become dark, and the gradient values of the pixels  1301 - 1303  become lower compared with the gradient values of the peripheral pixels  1304 - 1306 . As such, the dirt degree calculation module  164  can accurately detect a dirt substance on the transparent member  127  by comparing the gradient value of each pixel with the gradient values of the peripheral pixels of the pixel of interest. 
     Alternatively, the dirt degree calculation module  164  may calculate a dirt degree by comparing the gradient value of each pixel with a reference value. In such a case, the dirt degree calculation module  164  calculates the absolute value of a difference between the gradient value of the pixel of interest and a preset reference value (for example, 255) as a dirt degree of the pixel of interest. 
     Next, the notifying module  165  determines whether severe dirt exists at the imaging position (step S 202 ). When the dirt degree of any pixel calculated with regard to the white reference image is equal to or more of a first threshold, the notifying module  165  determines that there is severe dirt at the imaging position on the transparent member  127  corresponding to the pixel. Whereas, when the dirt degree of every pixel is less than a first threshold, the notifying module  165  determines that there is no severe dirt at the imaging position on the transparent member  127 . 
     When the notifying module determines that there is no severe dirt at the imaging position, the notifying module  165  determines whether there is moderate dirt at the imaging position (step S 203 ). When the dirt degree of any pixel calculated with regard to the white reference image is less than the first threshold and equal to or more than a third threshold, the notifying module  165  determines that there is moderate dirt at the imaging position on the transparent member  127  corresponding to the pixel. Whereas, when the dirt degree of every pixel is less than the third threshold, the notifying module  165  determines that there is no moderate dirt at the imaging position on the transparent member  127 . The third threshold is set smaller than the first threshold and larger than the second threshold as will be described later. 
     When the notifying module determines that there is no moderate dirt at the imaging position, the notifying module  165  determines whether there is minor dirt at the imaging position (step S 204 ). When the dirt degree of any pixel calculated with regard to the white reference image is less than the third threshold and equal to or more than a second threshold, the notifying module  165  determines that there is minor dirt at the imaging position on the transparent member  127  corresponding to the pixel. Whereas, when the dirt degree of every pixel is less than a second threshold, the notifying module  165  determines that there is no minor dirt at the imaging position on the transparent member  127 . The second threshold is set at a smaller value than the first threshold and the third threshold. 
     When the notifying module  165  determines that there is no minor dirt at the imaging position, the notifying module  165  ends the set of steps without generating and providing the warning to a user (step S 205 ). 
     Whereas, when the notifying module  165  determines that there is severe or moderate dirt at the imaging position, the notifying module  165  stores the position of a pixel corresponding to the dirt as a dirt position in the first storage device  140  (steps S 206 , S 207 ). In other words, when the notifying module  165  detects a dirt substance causing severe or moderate dirt, the notifying module  165  stores the position where the dirt substance is detected in the white reference image as a dirt position in the first storage device  140 . The notifying module  165  stores the position of a pixel of which calculated dirt degree is equal to or more than the third threshold as a dirt position. 
     Whereas, when the notifying module  165  determines that there is no severe or moderate dirt but there is minor dirt at the imaging position, the notifying module  165  determines whether the position of a pixel corresponding to the minor dirt is stored in the first storage device  140  as a dirt position (step S 208 ). 
     When the position of the pixel corresponding to the minor dirt is not stored as a dirt position, i.e., the dirt has not previously been detected as moderate dirt or severe dirt, the notifying module  165  does not generate and provide the warning to a user (step S 205 ) and ends the set of steps. Whereas, when the position of the pixel corresponding to the minor dirt is stored as a dirt position, i.e., the dirt has previously been detected as moderate dirt or severe dirt, the notifying module  165  transfers the processing to step S 210  in order to processes for the minor dirt in the same way as moderate dirt. 
     Next, the notifying module  165  determines whether the dirt is newly generated dirt (steps S 209 , S 210 ). The notifying module  165  determines that the dirt is newly generated dirt when the position of a specific pixel corresponding to the dirt is not stored in the first storage device  140  as a dirt position, or determines that the dirt is the existing dirt when the position is stored. Note that the notifying module  165  may keep counting the number of detected dirt specks, and, when the number of detected dirt specks that is detected this time is larger than that of previous time, the notifying module  165  may determine that the dirt is newly generated dirt. 
     When the dirt is newly generated dirt, the notifying module  165  sets the scan count stored in the first storage device  140  to a predetermined value (steps S 211 , S 212 ). The notifying module  165  sets a value that is equal to or more than a scan count threshold as a predetermined value. The scan count threshold is a threshold to be compared with the scan count when there is moderate dirt at the imaging position, and the notifying module  165  generates and provides the warning to a user only when the scan count exceeds the scan count threshold. By setting the scan count to a value that is equal to or more than the scan count threshold, the image reading apparatus  100  can immediately generate and provide the warning again when the dirt is detected again without having been confirmed by a user. 
     When there is moderate dirt at the imaging position (or there is minor dirt that has been previously detected as moderate or severe dirt), the notifying module  165  sets a scan count threshold (step S 213 ). The notifying module  165 , for example, changes the scan count threshold according to the number of detected moderate or minor dirt specks, i.e., the number of pixels of which dirt degree is less than the first threshold and equal to or more than the second threshold. The notifying module  165  changes the scan count threshold so that the scan count threshold becomes smaller as the number of pixels of which dirt degree is less than the first threshold and equal to or more than the second threshold is larger. For example, when the number of pixels of which dirt degree is less than the first threshold and equal to or more than the second threshold is larger than a first predetermined value, the notifying module  165  sets the scan count threshold at an average value of scan counts per day. Whereas, when the number of pixels of which dirt degree is less than the first threshold and equal to or more than the second threshold is smaller than a second predetermined value that is smaller than the first predetermined value, the notifying module  165  sets the scan count threshold at an average value of scan counts per week. As the average value of scan counts, the average value with regard to this image reading apparatus  100  may be used or the average value of scan counts by general users that are acquired from a plurality of image reading apparatuses may be used. 
     In this way, the notifying module  165  can increase the frequency of generating and providing the warning to a user to prompt cleaning when the number of moderate dirt and minor dirt is large, or can decrease the frequency of generating and providing the warning to a user to suppress troubling a user when the number of moderate and minor dirt is small. 
     Note that the notifying module  165  may change the scan count threshold according to the number of pixels of which dirt degree is equal to or more than the second threshold, the number of pixels of which dirt degree is equal to or more than the third threshold, or the number of pixels of which dirt degree is less than the first threshold and equal to or more than the second threshold. In such a case, the notifying module  165  changes the scan count threshold so that the scan count threshold becomes smaller as the number of such pixels is larger. Alternatively, the notifying module  165  may omit the processing of step S 213  and use a fixed value that was set as the scan count threshold in advance. 
     Next, the notifying module  165  determines whether the scan count exceeds the scan count threshold (step S 214 ). When the scan count exceeds the scan count threshold, the notifying module  165  transfers the processing to step S 216  in order to process for the moderate dirt (or minor dirt that has been previously detected as moderate or severe dirt) in the same way as severe dirt. 
     Whereas, when the scan count is equal to or less than the scan count threshold, the notifying module  165  determines whether a dirt substance has been detected at a position corresponding to the dirt position in the second processing based on the second result (step S 215 ). 
     When no dirt substance has been detected at a position corresponding to the dirt position in the second processing, the notifying module  165  ends the set of steps without notifying a warning to a user (step S 205 ). Whereas, when a dirt substance has been detected at a position corresponding to the dirt position in the second processing, the dirt is likely to cause noise line in the correction image. In such a case, the notifying module  165  treats the moderate dirt (or minor dirt that has previously been detected as moderate or severe dirt) in the same way as severe dirt and transfers the processing to step S 216 . In this case, a warning is notified to a user in the processing described later. As such, the notifying module  165  changes timing of generating and providing the warning to a user based on the second result when the notifying module  165  has detected a dirt substance causing dirt. The notifying module  165  generates and provides the warning at an earlier stage when there is dirt causing noise line in the correction image, yet, does not generate and provide the warning so as not to trouble a user when the dirt does not affect the correction image. 
     Note that the notifying module  165  may omit the processing of step S 215  and may not generate and provide the warning to a user when the scan count is equal to or less than the scan count threshold, regardless of the second result by the second processing. 
     Next, the notifying module  165  determines whether a user has already confirmed the dirt (step S 216 ). When the display operation device  106  has received a confirmation operation by a user after a warning on the dirt had previously been notified to a user, the notifying module  165  determines that the user has already confirmed the dirt. Further, the notifying module  165  may also determine that the user has already confirmed the dirt, when the second result indicates that a dirt substance has been detected and the position of the detected dirt substance in the correction image corresponds to the present dirt position, as well as, the detected dirt substance has already been confirmed by a user. Note that when a difference between the horizontal position where a dirt substance is detected in the correction image and the horizontal position where dirt is detected in the white reference image is within a predetermined range, the notifying module  165  recognizes that these positions correspond to each other. 
     When a user has already confirmed the dirt, the notifying module  165  ends the set of steps without generating and providing the warning to a user (step S 205 ). As such, when a user has already confirmed a dirt substance in the information processing apparatus  200 , the notifying module  165  does not generate and provide the warning. 
     Whereas, when a user has not confirmed the dirt, the notifying module  165  determines whether the dirt substance causing the dirt is present on the side of the imaging sensor or on the side of the white reference member using the second result (step S 217 ). That is, the notifying module  165  determines whether the dirt substance is on the transparent member on the side of the imaging sensor which captured the white reference image or on the transparent member on the side of the white reference member which was captured in the white reference image. When a noise line has been detected at a position corresponding to the present dirt position within the document region in the correction image, the dirt is likely to exist on front side of the document as seen from the side of the imaging sensor, and, when a noise line has not been detected at the position, the dirt is likely to exist on back side of the document as seen from the side of the imaging sensor. Thus, when the second result indicates that a dirt substance causing a noise line has been detected in the document region of the correction image and the position where the dirt substance has been detected in the correction image and the present dirt position correspond to each other, the notifying module  165  determines that the dirt substance is on the side of the imaging sensor. Whereas, when the second result indicates that no dirt substance causing a noise line has been detected in the document region of the correction image or the position where a dirt substance has been detected in the correction image and the present dirt position do not correspond to each other, the notifying module  165  determines that the dirt substance is on the side of the white reference member. 
     The first dirt substance detection processing module  162 , when it detects a dirt substance, can use the second result to accurately determine whether the position where the dirt substance exists is on the side of the white reference member or on the side of the imaging sensor. 
     Next, the notifying module  165  generates and provides the warning to a user and ends the set of steps (step S 218 ). The notifying module  165  displays an image for providing a warning on the display operation device  106 . Note that the notifying module  165  may provide a warning by turning on an LED (not shown) or outputting voice from a speaker (not shown) etc. 
       FIG. 14A  is a schematic view depicting an example of the reception screen  1400  displayed on the display operation device  106 . 
     The reception screen depicted in  FIG. 14A  displays a read button  1401 , a confirm button  1402 , a set button  1403 , etc. The read button  1401  is a button for instructing reading of a document; when the read button  1401  is pressed, the display operation device  106  outputs a read instruction signal to the first CPU  160 . The confirm button  1402  is a button for receiving a confirmation operation by a user and a request of displaying the status of dirt at the imaging position. When the confirm button  1402  is pressed, the display operation device  106  receives a confirmation operation by a user, transmits a confirmation accept signal that indicates that the confirmation operation has been received, to the notifying module  165 , and displays a status display screen that displays the status of dirt at the imaging position. The set button  1403  is a button for instructing display of a setting screen (not shown) for performing a variety of settings on the image reading apparatus  100 . Note that, when the notifying module  165  detects dirt, a warning image  1404  indicating the presence of dirt is displayed near the confirm button  1402 . The warning image  1404  is an example of an image for providing the warning. 
       FIGS. 14B, 14C  are schematic views depicting examples of status display screens displayed on the display operation device  106 .  FIG. 14B  is an example of a status display image  1410  when a warning is notified to a user and also an example of an image for providing the warning.  FIG. 14C  is an example of a status display screen  1420  when a warning is not notified to a user. 
     As depicted in  FIG. 14B , the status display screen  1410  displays a character  1411  that indicates the presence of dirt and prompts cleaning, an image  1412  that indicates the position of dirt, and an end button  1413 . 
     The image  1412  indicates: whether the dirt is on the transparent member on the side of the imaging sensor that capture a white reference image or on the transparent member on the side of the white reference member that is captured in the white reference image; the dirt position on the transparent member; and if the dirt is severe, moderate, or minor. In the image  1412 , when there is dirt on the transparent member, the position of dirt on the transparent member is indicated distinctly from positions without dirt. For example, to identify a dirt degree by the density of color, a position corresponding to severe dirt is displayed in black, a position corresponding to moderate dirt is displayed in gray, a position corresponding to minor dirt is displayed in lighter gray, and a position without dirt is displayed in white. Alternatively, to identify dirt degree by color, a position corresponding to severe dirt may be displayed in red, a position corresponding to moderate dirt may be displayed in yellow, a position corresponding to minor dirt may be displayed in blue, and a position without dirt may be displayed in white. 
     The notifying module  165  generates and provides, on the status display screen  1410 , the warning, and a notification of the imaging positions of pixels corresponding to severe, moderate, or minor dirt, i.e., pixels of which dirt degree is equal to or more than the first, third, or second threshold, to a user. In this way, a user can accurately recognize the position of a dirt substance causing dirt and clean the position. 
     When the end button  1413  is pressed, the display operation device  106  displays the reception screen  1400  again. 
     Whereas, the status display screen  1420  depicted in  FIG. 14C  displays a character  1421  that indicates there is no dirt, an image  1422 , and an end button  1423 . 
     In this way, the notifying module  165  generates and provides the warning to a user according to the dirt degree at the imaging position in the white reference image. The notifying module  165  generates and provides the warning when there is severe dirt at the imaging position, or does not generate and provide the warning when there is no dirt at the imaging position. Whereas, when there is moderate dirt at the imaging position, the notifying module  165  generates and provides the warning only when the scan count exceeds the scan count threshold. Further, when there is minor dirt at the imaging position and the dirt has not been detected as moderate or severe dirt previously, the notifying module  165  does not generate and provide the warning regardless of whether the scan count exceeds the scan count threshold or not. Whereas, when there is minor dirt at the imaging position and the dirt has been detected as moderate or severe dirt previously, the notifying module  165  generates and provides the warning only when the scan count exceeds the scan count threshold as in a case where there is moderate dirt. 
     In this way, the notifying module  165  can generate and provide the warning at appropriate timing according to a dirt degree at the imaging position. In particular, when there is severe dirt, the notifying module  165  generates and provides the warning until a user confirms the dirt. Thus, the user can recognize the presence of dirt at the imaging position and clean it before scanning a document. This decreases situations where noise line is generated in the document image and re-scanning of the document is required, whereby the image reading apparatus  100  can improve user convenience. Whereas, when there is moderate dirt, the image reading apparatus  100  generates and provides a warning at a certain cycle, thereby preventing troubling a user, while making the user moderately recognize the presence of dirt. 
     The first dirt substance detection processing module  162  performs the first processing using a dirt substance detection result from the second dirt substance detection processing module  242 . As such, the first dirt substance detection processing module  162  can better detect a dirt substance. 
     Note that, at step S 216 , when a user has already confirmed dirt, the notifying module  165  may not go without generating and providing the warning yet may change the timing of generating and providing the warning. In such a case, for example, the notifying module  165  generates and provides the warning at every predetermined count even when a user has already confirmed the dirt. 
     Alternatively, the first dirt substance detection processing module  162  may detect dirt based on other reference images than the white reference image in the first processing. In this case, the first image acquiring module  161  turns on the light source  124  and causes the imaging sensor  125  to capture an image of the white reference member  126  to generate a white reference image, as well as, turns off the light source  124  and causes the imaging sensor  125  to capture an image of the white reference member  126  to generate a black reference image. The first image acquiring module  161  generates a reference image having, as a pixel value of each pixel, a value obtained by subtracting the pixel value of a pixel in the black reference image from the pixel value of a corresponding pixel in the white reference image. 
       FIG. 15  is a flowchart depicting an example of the operation of confirmation processing. The confirmation processing depicted in  FIG. 15  is carried out at step S 106  of the flowchart depicted in  FIG. 11 . 
     First, the notifying module  165  determines whether a warning has been notified to a user (step S 301 ). When no warning has been notified, the notifying module  165  does not perform particular processing and ends the set of steps. 
     Whereas, when the warning has been generated and provided, i.e., after generating and providing the warning, the notifying module  165  determines whether the display operation device  106  has received a confirmation operation by a user based on whether a confirmation acceptance signal has been received from the display operation device  106  (step S 302 ). 
     When the display operation device  106  has received a confirmation operation by a user, the notifying module  165  stores the confirmation operation information that indicates that the confirmation operation has been received, in the first storage device  140 , in association with the dirt position displayed on the image  1412  of the status display screen  1410  (step S 303 ). Note that the notifying module  165  thereafter periodically monitors the confirmation operation information, and when dirt is not detected at a position associated with the confirmation operation information, the notifying module  165  deletes the confirmation operation information from the first storage device  140 . 
     Next, the notifying module  165  initializes (resets) the scan count stored in the first storage device  140  to zero (step S 304 ). In this way, as the scan count becomes equal to or less than the scan count threshold, the notifying module  165  does not generate and provide the warning to a user even when moderate dirt on which the warning has already been notified to a user is redetected. Thus, the notifying module  165  keeps generating and providing the warning to a user who has not confirmed the warning, and prevents repeating providing the same warning to a user who has confirmed the warning, thereby preventing troubling the user. 
     Next, the notifying module  165  changes the scan count threshold according to the confirmation time from providing the warning until the display operation device  106  receives a confirmation operation by a user (step S 305 ). For example, the notifying module  165  changes the scan count threshold larger as the confirmation time is longer. In this way, when a user does not care about the dirt, the notifying module  165  can prevent troubling the user by reducing the frequency of providing the warning. 
     Next, the information acquiring module  168  determines whether or not the cover of the image reading apparatus  100  has been opened and closed based on an open/close signal, which indicates whether the upper housing  102  is open or closed with respect to the lower housing  101 , output from the open/close sensor  114  (step S 306 ). When the state of the upper housing  102  has changed from a closed state to an open state with respect to the lower housing  101 , then, further changed to a closed state, the information acquiring module  168  determines that the cover of the image reading apparatus  100  has been opened and closed. In such a case, the information acquiring module  168  acquires open/close information that indicates that the cover of the image reading apparatus  100  has been opened and closed. Note that the open/close sensor  114  may determine whether or not the cover of the image reading apparatus  100  has been opened and closed and the information acquiring module  168  may acquire the open/close information from the open/close sensor  114 . 
     When the information acquiring module  168  has acquired the open/close information, the first CPU  160  re-executes the overall processing depicted in  FIG. 11  from step S 101 . In such a case, the first image acquiring module  161  acquires newly captured a white reference image, the dirt degree calculation module  164  newly calculates a dirt degree based on the newly captured white reference image, and the notifying module  165  generates and provides the warning to a user according to the newly calculated dirt degree. As such, as the notifying module  165  automatically determines whether there is remaining dirt after the image position was cleaned and generates and provides the warning to a user if there is remaining dirt, the user can immediately recognize that there is remaining dirt. 
     Whereas, when the information acquiring module  168  has not acquired open/close information, the notifying module  165  determines whether a certain time period (for example, one minute) has passed after providing the warning (step S 307 ). When a certain time period has not passed after providing the warning, the notifying module  165  does not perform particular processing and ends the set of steps. 
     When a certain time period has passed after providing the warning, i.e., when the information acquiring module  168  has not acquired open/close information within the certain time period after providing the warning, the notifying module  165  changes the scan count threshold (step S 308 ) and ends the set of steps. For example, the notifying module  165  increases the scan count threshold. Note that when the information acquiring module  168  has not acquired open/close information even though the display operation device  106  had received a confirmation operation by a user, the notifying module  165  may further increase the scan count threshold. Further, the notifying module  165  may change the scan count threshold so that the scan count threshold becomes larger as the time period from providing the warning to acquisition of open/close information is longer. In this way, when a user does not care about the dirt, the notifying module  165  can prevent troubling the user by reducing the frequency of providing the warning. 
     Note that, even when a warning has been notified to a user in the second processing instead of the first processing, the information acquiring module  168  may determine whether or not the cover of the image reading apparatus  100  has been opened and closed, and, when the cover has been opened and closed, the first CPU  160  may re-execute the overall processing as depicted in  FIG. 11 . In this way, even when a warning was notified to a user on the side of the information processing apparatus  200  and a user cleaned the dirt, the image reading apparatus  100  can automatically determine whether there is remaining dirt and, if there is remaining dirt, generate and provide the warning to a user. 
     Note that each processing of steps S 304 , S 305 , S 306 , S 307  to S 308  may be omitted. 
       FIG. 16  is a flowchart depicting an example of the operation of threshold setting processing of the image reading apparatus  100 . The following will describe an example of the operation of the threshold setting processing of the image reading apparatus  100  with reference to the flowchart depicted in  FIG. 16 . The operation flow as will be described below is performed primarily by the first CPU  160  jointly with each component of the image reading apparatus  100  according to a program prestored in the first storage device  140 . This operation flow is performed immediately after start-up of the device. 
     First, the setting module  166  sets a first threshold, a second threshold, and a third threshold at predefined values (step S 401 ). The setting module  166  sets the first threshold based on a binarization threshold for binarizing a correction image to detect characters from the correction image in the optical character recognition (OCR) processing performed by the information processing apparatus  200 . The setting module  166 , for example, sets the first threshold as a value obtained by subtracting the binarization threshold (or plus or minus a predetermined margin value) from the gradient value representing white (for example, 255). In this way, the image reading apparatus  100  can detect dirt that has density that can be recognized as a part of black characters in contrast to a white background in the character recognition processing as severe dirt and immediately a user of a warning. 
     Whereas, the setting module  166  sets the third threshold as a value that can distinguish visually recognizable dirt for human eyes from visually unrecognizable dirt in a multi-level image. In prior evaluation, the setting module  166  accepts, from an administrator, gradient values corresponding to visually recognizable dirt and gradient values corresponding to visually unrecognizable dirt in a white reference image (multi-level image) captured dirt of a variety of densities. The setting module  166  calculates an average value of gradient values corresponding to visually recognizable dirt and gradient values corresponding to visually unrecognizable dirt, and sets the third threshold as a value obtained by subtracting the average value (or plus or minus a predetermined margin value) from a gradient value representing white. In this way, the image reading apparatus  100  can detect visually recognizable dirt in a multi-level image as moderate dirt and generate and provide the warning at a certain cycle. 
     Further, the setting module  166  sets the second threshold as a value obtained by subtracting an allowable error value that is set in the image reading apparatus  100  with regard to the reading value of the imaging sensor  125  from the third threshold. In this way, the image reading apparatus  100  can prevent failing to detect visually recognizable dirt as moderate dirt due to the precision error of the image reading apparatus  100 . 
     Next, the setting module  166  determines whether the threshold acquiring module  167  has newly acquired a binarization threshold from the information processing apparatus  200  via the first interface device  135  (step S 402 ). When the threshold acquiring module  167  has newly acquired a binarization threshold, the setting module  166  resets the first threshold based on the newly acquired binarization threshold (step S 403 ). 
     Next, the setting module  166  determines whether the first result acquiring module  163  has newly acquired a second result from the information processing apparatus  200  via the first interface device  135  (step S 404 ). When the first result acquiring module  163  has newly acquired a second result, the setting module  166  changes the first threshold, the second threshold, or the third threshold based on the newly acquired second result (step S 405 ), and returns the processing to step S 402 . The setting module  166  changes the first threshold, the second threshold or the third threshold to a smaller value than the present value, for example, when a dirt substance causing noise line is detected in the second processing, and changes the first threshold, the second threshold or the third threshold to a larger value than the present value, when a dirt substance is not detected in the second processing. In this way, the image reading apparatus  100  can more accurately generate and provide the warning when there is dirt that can cause noise line in the correction image. 
     Note that each processing of steps S 402  to S 403 , S 404  to S 405  may be omitted. 
       FIG. 17  is a flowchart depicting an example of the operation of the overall processing of an information processing apparatus  200 . The following will illustrate an example of the operation of the overall processing of the information processing apparatus  200  with reference to the flowchart depicted in  FIG. 17 . Note that the operation flow as will be described below is performed primarily by the second CPU  240  jointly with each component of the information processing apparatus  200  according to a program prestored in the second storage device  220 . This operation flow is periodically performed. 
     First, the second image acquiring module  241  acquires a correction image from the image reading apparatus  100  via the second interface device  203 . Further, the second result acquiring module  243  acquires a first result from the image reading apparatus  100  via the second interface device  203 . The second result acquiring module  243  acquires a second result relating to the previously conveyed document from the noise line detecting module  249 . The device information acquiring module  244  acquires device information from the image reading apparatus  100  via the second interface device  203  (step S 501 ). Note that the second result acquiring module  243  may acquire the first result only when a new first result is transmitted from the image reading apparatus  100 . Further, the device information acquiring module  244  may omit acquisition of the device information when the device information has already been acquired. 
     Next, the second dirt substance detection processing module  242  executes the second processing (step S 502 ) and ends the set of steps. The second dirt substance detection processing module  242  detects a dirt substance from the correction image in the second processing. The details of the second processing will be described later. 
       FIG. 18  is a flowchart depicting an example of the operation of the second processing. The second processing depicted in  FIG. 18  is carried out at step S 502  of the flowchart depicted in  FIG. 17 . 
     First, the edge pixel extracting module  245  extracts first edge pixels in horizontal and vertical directions from the correction image (step S 601 ). The edge pixel extracting module  245  generates a first edge image constituted by the first edge pixels in horizontal and vertical directions of the correction image. 
     The edge pixel extracting module  245  calculates the absolute value of a difference of gradient values between both side pixels of a pixel of interest in a horizontal direction in the correction image or the absolute value of a difference of gradient values of pixels that are apart by a predetermined distance from the pixel of interest (hereinafter, referred to as a peripheral difference value) and, when the peripheral difference value exceeds a fourth threshold, the edge pixel extracting module  245  extracts the pixel of interest as a first vertical edge pixel. This peripheral difference value indicates the intensity of the edge in the edge pixels. The fourth threshold may be set, for example, at a difference of gradient values in an image that can be visually distinguished by human eyes (for example, 20). The edge pixel extracting module  245  performs the same processing in a vertical direction and extracts first horizontal edge pixels. Then, the edge pixel extracting module  245  generates a first horizontal edge image and a first vertical edge image for a horizontal direction and a vertical direction respectively. 
     Note that the edge pixel extracting module  245  may instead calculate, as a peripheral difference value, the absolute value of a difference between a gradient value of the pixel of interest in the correction image and the average value of gradient values of peripheral pixels located both sides of the pixel of interest or located within a predetermined range from the pixel of interest in a horizontal or vertical direction. Alternatively, the edge pixel extracting module  245  may extract the first edge pixel by comparing the gradient value of each pixel with a threshold. For example, when the gradient value of the pixel of interest is less than a threshold and the gradient values of pixels on both sides of the pixel of interest or pixels that are apart from the pixel of interest by a predetermined distance in a horizontal or vertical direction are equal to or more than the threshold, the edge pixel extracting module  245  determines the pixel of interest as the first edge pixel. 
       FIG. 19A  is a schematic view depicting an example of a correction image  1900 . 
     In the correction image  1900  depicted in  FIG. 19A , a document  1901  and the periphery thereof, as well as, a shadow  1903  formed on the white reference member  126  by the leading end  1902  of the document  1901  are captured. In the correction image  1900 , pixels corresponding to the ends  1904  to  1907  of the document  1901  are extracted as the first edge pixels. 
     Next, the document region detecting module  246  detects a document region based on the first edge pixels (step S 602 ). 
     The document region detecting module  246  first extracts a plurality of straight lines from the first edge pixels extracted by the edge pixel extracting module  245 . The document region detecting module  246  extracts straight lines from the first horizontal edge image and the first vertical edge image. The document region detecting module  246  extracts straight lines using Hough transform. Note that the document region detecting module  246  may extract straight lines using a least squares method. Alternatively, the document region detecting module  246  may group the first edge pixels adjacent to one another into one by labeling them and extract straight lines by connecting the first edge pixels at both ends in a horizontal or vertical direction from among the first edge pixels included in each group. 
     Next, the document region detecting module  246  detects a rectangle from the extracted plurality of straight lines. The document region detecting module  246  extracts a plurality of rectangle candidates, each constituted by four straight lines where each two of the extracted plurality of straight lines are substantially perpendicular to each other. The document region detecting module  246  first selects one straight line in a horizontal direction (hereinafter, referred to as the first horizontal line) and extracts another straight line in a horizontal direction that is substantially parallel (for example, ±3° or less) to the selected straight line and apart from the selected straight line by threshold Th1 or more (hereinafter, referred to as the second horizontal line). Next, the document region detecting module  246  extracts a straight line in a vertical direction substantially perpendicular to the first horizontal line (for example, ±3° or less with respect to 90°) (hereinafter, referred to as the first vertical line). Next, the document region detecting module  246  extracts a straight line in a vertical direction that is substantially perpendicular to the first horizontal line and apart from the first vertical line by a threshold Th2 or more (hereinafter, referred to as the second vertical line). Note that the thresholds Th1 and Th2 may be predefined according to the size of a document to be read by the image reading apparatus  100  and may be the same values. 
     The document region detecting module  246  extracts all combinations of a first horizontal line, a second horizontal line, a first vertical line, and a second vertical line that satisfy the above conditions from among all the extracted straight lines and extracts rectangles constituted by the extracted combinations as rectangle candidates. The document region detecting module  246  calculates the areas of the extracted rectangle candidates and eliminates the rectangle candidates of which area is less than a predetermined value. The document region detecting module  246  detects a rectangle candidate with the largest area from the remaining rectangle candidates as a document region. Whereas, when there is no remaining rectangle candidate, the document region detecting module  246  detects no document region. 
     In the correction image  1900  depicted in  FIG. 19A , straight lines corresponding to the ends  1904  to  1907  of the document  1901  are extracted and the document region  1908  surrounded by the straight lines is extracted. 
     Next, the shadow region detecting module  247  determines a predetermined range for detecting a shadow region in a correction image based on the device information acquired by the device information acquiring module  244  (step S 603 ). The shadow region is a region that captures a shadow formed on the white reference member  126  by the leading end or rear end of the document in the correction image and the predetermined range is determined by an adjacent region outside of the document region detected in the correction image. For example, when the device information indicates that the first light source  124   a  is provided on the upstream side of the first imaging sensor  125   a  in the document conveyance direction A 3 , as depicted in  FIG. 7 , a shadow is formed by the leading end of the document on the second white reference member  126   b . In such a case, the shadow region detecting module  247  determines a range within a predetermined distance from the upper end of the document region corresponding to the leading end of the document (for example, a distance equivalent to 5 mm) as a predetermined range in a first correction image. Likewise, when the device information indicates that the second light source  124   b  is provided on the downstream side of the second imaging sensor  125   b  in the document conveyance direction A 3 , a shadow is formed by the rear end of the document on the first white reference member  126   a . In such a case, the shadow region detecting module  247  determines a range within a predetermined distance from the lower end of the document region corresponding to the rear end of the document as a predetermined range in a second correction image. 
     In the correction image  1900  depicted in  FIG. 19A , a range  1910  within a predetermined distance, in an outward direction of the document region  1908 , from the upper end  1904  of the document region  1908  is determined as a predetermined range. 
     Next, the shadow region detecting module  247  extracts second edge pixels within the determined predetermined range and extracts a plurality of straight lines based on the extracted second edge pixels (step S 604 ). 
     The shadow region detecting module  247  extracts the second edge pixels in the same way in which the edge pixel extracting module  245  extracts the first edge pixels. However, the shadow region detecting module  247  extracts a plurality of levels of second edge pixels with different degrees of peripheral difference values (edge intensities). The shadow region detecting module  247  first sets an initial value at a fifth threshold and extracts the second edge pixels of which peripheral difference value exceeds the fifth threshold. The initial value is set, for example, at the same or a smaller value than the fourth threshold. Next, the shadow region detecting module  247  extracts straight lines from the extracted second edge pixels in the same way in which the document region detecting module  246  extracts straight lines, and extracts line segments of a region corresponding to the extracted second edge pixels in a horizontal direction from among the extracted straight lines. 
     Next, the shadow region detecting module  247  changes the fifth threshold to a smaller value than the present value, extracts the second edge pixels of which peripheral difference value exceeds the changed fifth threshold from a range where line segments have not extracted in a horizontal direction, and extracts new line segments from among the extracted second edge pixels. The shadow region detecting module  247  extracts a plurality of levels of second edge pixels with different degrees of peripheral difference values while changing the fifth threshold to smaller values until line segments are extracted from the entire range in a horizontal direction and extracts a line segment from the second edge pixels at each level. 
       FIG. 19B  is a schematic view for illustrating a plurality of line segments extracted based on the second edge pixels.  FIG. 19B  is an enlarged view of the predetermined range  1910  depicted in  FIG. 19A . 
     As depicted in  FIG. 19B , when an end of the document  1901  is curved, the shadow region  1911  does not become a rectangular region. In the example depicted in  FIG. 19B , first, the second edge pixels  1921  of a first level are extracted and a line segment  1922  is extracted from the second edge pixels  1921 . Next, the second edge pixels  1923  of a second level that is lower than the first level are extracted and a line segment  1924  is extracted from the second edge pixels  1923 . Then, the second edge pixels  1925  of a third level that is lower than the second level are extracted and a line segment  1926  is extracted from the second edge pixels  1925 . 
     Next, the shadow region detecting module  247  detects a region in a second predetermined range (for example, a distance equivalent to 1 min) from each of a plurality of extracted line segments as a shadow region (step S 605 ). The shadow region detecting module  247  detects a shadow region obtained by connecting regions within the second predetermined range in a vertical direction from the line segments. 
     In the example depicted in  FIG. 19B , a shadow region is detected as a region  1930  obtained by connecting a region  1927  within the second predetermined range from the line segment  1922 , a region  1928  within the second predetermined range from the line segment  1924 , and a region  1929  within the second predetermined range in a vertical direction from the line segment  1926 . In this way, even when an end of the document  1901  is curved, the shadow region detecting module  247  can accurately detect a shadow region by detecting a shadow region using a plurality of line segments. 
     Next, the noise pixel extracting module  248  specifies a priority range for preferentially detecting a dirt substance in the shadow region using a position within a white reference image where a dirt substance is detected, i.e., a position of the dirt substance detected by the first dirt substance detection processing module  162  as indicated in the first result (step S 606 ). 
     To reduce a load of noise pixel determination processing, the noise pixel extracting module  248  sets pixels apart from one another by a first distance in a horizontal direction as target pixels for determining whether the pixels are noise pixels or not, instead of determining whether they are noise pixels or not for all pixels in the shadow region. Whereas, the noise pixel extracting module  248  sets a range within a predetermined distance from a position corresponding to the position where a dirt substance is detected in the white reference image as a priority range in the shadow region. As for pixels in the priority range, the noise pixel extracting module  248  sets pixels adjacent to one another in a horizontal direction or pixels apart from one another by a second distance that is shorter than the first distance as target pixels. 
       FIG. 20A  is a schematic view for illustrating a priority range.  FIG. 20A  is an enlarged view of the shadow region  1930  depicted in  FIG. 19B . 
     In the example depicted in  FIG. 20A , pixels spaced apart from one another by a first distance (4 pixels) in a horizontal direction are set as target pixels  2001 . Further, a dirt substance is detected at a position in a white reference image corresponding to a position  2002  in the shadow region  1930 , and a range within a predetermined distance (4 pixels) from the position  2002  is set as a priority range  2003 . In the priority range  2003 , pixels spaced apart from one another by a second distance (2 pixels) are set as the target pixels  2004 . In this way, the noise pixel extracting module  248  can efficiently extract noise pixels since the noise pixel extracting module  248  can selectively scan a region where a dirt substance is likely to exist while reducing the load of noise pixel determination processing. 
     Further, the noise pixel extracting module  248  specifies a priority range for preferentially detecting a dirt substance in a shadow region using a position within a correction image where a dirt substance has been previously detected, indicated by a previous second result by the second dirt substance detection processing module  242 . The noise pixel extracting module  248  specifies the priority range in the same way as the case where a priority range is specified using a position of a dirt substance detected by the first dirt substance detection processing module  162 . 
     Next, the noise pixel extracting module  248  extracts noise pixels in the shadow region based on the specified priority range (step S 607 ). In this way, the noise pixel extracting module  248  specifies a priority range before executing the second processing. The noise pixel extracting module  248  extracts, as a noise pixel, a pixel where a difference between the gradient value of the shadow region and the gradient value of the noise pixel is equal to or more than a sixth threshold. 
     As depicted in  FIG. 19A , the shadow region has gradation along a vertical direction; the shadow region is the darkest (lower luminance) around the upper end of the document and gradually becomes brighter (higher luminance) as it goes upward. The noise pixel extracting module  248  calculates an average value of the gradient values of pixels for each horizontal line in the shadow region, and extracts, as a noise pixel, a pixel where the absolute value of a difference between the gradient value of the pixel of interest and the average value calculated for a horizontal line to which the pixel of interest belongs is equal to or more than the sixth threshold. As described above, as the shadow formed on the white reference member  126  is gray that is an intermediate color between white and black, not only a black dirt substance but also a white dirt substance is extracted as noise pixels from the shadow region in the white reference image. 
       FIG. 20B  is a graph for illustrating noise pixels. 
     The horizontal axis of  FIG. 20B  indicates a gradient value and the vertical axis indicates a position in a vertical direction in a correction image. The straight line  2010  of  FIG. 20B  indicates an average value of the gradient values of pixels for each horizontal line in the shadow region. As described above, the straight line  2010  is inclined as the shadow region has gradation along the vertical direction. The pixels  2013 ,  2014  plotted within a range  2012  where a distance  2011  in a horizontal axis direction from this straight line  2010  is equal to or less than the sixth threshold are not extracted as noise pixels, rather, pixels  2015 ,  2016  plotted outside the range  2012  are extracted as noise pixels. 
     Next, the noise line detecting module  249  detects noise pixels, the number of which connected with one another is equal to or more than a predetermined number (for example, 4 pixels), from noise pixels extracted by the noise pixel extracting module  248  as a noise line (step S 608 ). The noise line detecting module  249  determines whether to connect noise pixels extracted by the noise pixel extracting module  248  in a vertical direction (whether adjacent noise pixels exist). The noise line detecting module  249  groups mutually connected noise pixels into one and, when the length of each group in a vertical direction is equal to or more than a predetermined value (for example, a distance equivalent to 0.5 mm), detects the group as a noise line. In this way, the noise line detecting module  249  detects a dirt substance causing a noise line based on a difference between the gradient value of a shadow region and the gradient value of the dirt substance. By detecting a noise line from a shadow region outside of a document region, the noise line detecting module  249  can accurately detect a noise line without being affected by the content, such as ruled lines, in the document. 
     Next, the noise line detecting module  249  determines whether a noise line is detected from the correction image (step S 609 ). When no noise line is detected, the noise line detecting module  249  transfers the processing to step S 616 . 
     Whereas, when a noise line is detected, the noise line detecting module  249  stores a position in a horizontal direction where the noise line is detected in the correction image as a noise line position in the second storage device  220  (step S 610 ). 
     Next, the noise line detecting module  249  determines whether a user has confirmed the noise line (step S 611 ). When the operation device  202  has received a confirmation operation by a user after a warning on the noise line had previously been notified to a user, the noise line detecting module  249  determines that the user has confirmed the noise line. When the first result indicates that dirt is detected and the dirt position of the detected dirt corresponds to the noise line position of the present noise line, and the detected dirt has been confirmed by a user, the noise line detecting module  249  may also determine that the user has already confirmed the noise line. 
     When a user has already confirmed the noise line, the noise line detecting module  249  transfers the processing to step S 615 . As such, when a user has already confirmed a dirt substance in the image reading apparatus  100 , the noise line detecting module  249  does not generate and provide the warning. 
     Whereas, when a user has not confirmed the noise line, the noise line detecting module  249  generates and provides the warning to a user (step S 612 ). The noise line detecting module  249  generates and provides the warning to a user by displaying the similar image as the reception screen depicted in  FIG. 14A  and the status display screen depicted in  FIGS. 14B and 14C  on the display device  201 . 
     In this way, the noise line detecting module  249  keeps generating and providing the warning to a user who has not confirmed the warning and prevents repeating providing the same warning to a user who has confirmed the warning, thereby preventing troubling the user. 
     Next, the noise line detecting module  249  determines whether the display device  201  has received a confirmation operation by a user based on whether the operation device  202  has received a confirmation acceptance signal (step S 613 ). 
     When the operation device  202  has received a confirmation operation from a user, the noise line detecting module  249  stores the confirmation operation information that indicates that the confirmation operation has been received, in the second storage device  220 , in association with the noise line position of the noise line (step S 614 ). Note that the noise line detecting module  249  thereafter periodically monitors the confirmation operation information, and, when the noise line is not detected at a position associated with the confirmation operation information, the noise line detecting module  249  deletes the confirmation operation information from the second storage device  220 . 
     Next, the noise line detecting module  249 , the determination module  250 , and the correction module  251  perform correction processing (step S 615 ). In the correction processing, the correction module  251  corrects the document region of the correction image based on the noise line detection result by the noise line detecting module  249 . The details of the correction processing will be described later. 
     Next, the noise line detecting module  249  stores the second result in the second storage device  220 , as well as, transmits the second result to the image reading apparatus  100  via the second interface device  203  (step S 616 ). 
     Next, the correction module  251  stores the correction image in the second storage device  220 , as well as, displays the correction image on the display device  201  (step S 617 ), and ends the set of steps. 
     As such, the second dirt substance detection processing module  242  executes the second processing using the dirt substance detection result by the first dirt substance detection processing module  162  to detect a dirt substance from the correction image. As such, the second dirt substance detection processing module  242  can better detect a dirt substance. 
     Note that, at step S 611 , when a user has already confirmed the noise line, the noise line detecting module  249  may not go without generating and providing the warning yet may change the timing of generating and providing the warning. In such a case, for example, the noise line detecting module  249  generates and provides the warning at every predetermined count even when a user has already confirmed the noise line. 
       FIG. 21  is a flowchart depicting an example of the operation of the correction processing. The correction processing depicted in  FIG. 21  is carried out at step S 615  of the flowchart depicted in  FIG. 18 . 
     First, the noise line detecting module  249  specifies the noise line range, corresponding to a noise line detected in the shadow region, in the document region detected by the document region detecting module  246  (step S 701 ). 
     As depicted in  FIG. 19A , when there are noise lines  1912  to  1914  in the shadow region, the noise lines are likely to extend in a vertical direction and present in the document region  1908 . Thus, the noise line detecting module  249  estimates that a noise line is present in the document region at a horizontal position corresponding to the noise line position of the noise line detected in the shadow region, and specifies the horizontal position in the document region corresponding to the noise line position of the noise line detected in the shadow region as a noise line region. 
     The noise line detecting module  249  further determines whether or not a noise line is present in the identified noise line region. In the same way as the edge pixel extracting module  245 , the noise line detecting module  249  extracts third edge pixels in the noise line region, and determines that there are noise lines in the specified noise line region, i.e., in the document region, when the number or ratio of the extracted third edge pixels is equal to or more than a predetermined number or ratio. The noise line detecting module  249  includes the determination result in the second result. This determination result is used by the notifying module  165  to determine whether a dirt substance is on the side of the imaging sensor or on the side of the white reference member. 
     Next, the determination module  250  determines whether the noise line in the document region overlaps the content based on a difference between the gradient value of the noise line region identified by the noise line detecting module  249  and the gradient values of the peripheral pixels in the noise line region (step S 702 ). The peripheral pixels may be, for example, pixels located within a predetermined range (for example, 20 pixels) from the noise line region. 
       FIG. 22  is a graph for illustrating a relationship between a noise line and a background of a document. 
     The horizontal axis of  FIG. 22  indicates the gradient value of the background of a document and the vertical axis indicates the gradient value of noise lines. Each dot  2201  of  FIG. 22  corresponds to each document image of a document of a different single color such that a noise line is generated in the document image, and is plotted at a coordinate corresponding to the gradient value of the background of the document and the gradient value of the noise line in each document image. A noise line occurs when light from the light source  124  is reflected against a dirt substance, such as paper dust, adhered to the transparent member  127  between the imaging sensor  125  and the document. As such, as depicted in  FIG. 22 , when a noise line overlaps the background of a document in the document image, the gradient value of the noise line pixels corresponding to the noise line becomes slightly higher than the gradient value of the background pixels capturing the background of the document around the noise line. Further, as the background is darker, the influence of reflection, against the background, of the light from the light source  124  becomes larger, while, as the background is brighter, the influence of reflection, against the background, of the light from the light source  124  becomes smaller. As such, as the gradient value of the background pixel is lower, the difference between the gradient value of the noise line pixels and the gradient value of the background pixels becomes larger; as the gradient value of the background pixel is higher, the difference between the gradient value of the noise line pixels and the gradient value of the background pixels becomes smaller. 
     As the result of measurement using a variety of documents, it is found that the relationship of the following formulas (1) and (2) can be established between the gradient value of pixels corresponding to a noise line and the gradient value of peripheral background pixels:
 
(Gradient value of noise line pixels)&gt;(Gradient values of background pixels)  (1)
 
(Gradient value of noise line pixels)&lt;0.8×(Gradient values of background pixels)+80  (2)
 
     Thus, when the gradient value of a noise line region and the average value of the gradient values of peripheral pixels of the noise line region satisfy the relationship of the following formulas (3) and (4), the determination module  250  determines that the noise line region overlaps the background yet does not overlap the content. Whereas, when the gradient value of the noise line region and the average value of the gradient values of peripheral pixels of the noise line region do not satisfy the relationship of the following formulas (3) and (4), the determination module  250  determines that the noise line region does not overlap the content.
 
(Gradient value of noise line pixels)&gt;(Average value of gradient values of peripheral pixels)  (3)
 
(Gradient value of noise line pixels)&lt;α×(Average value of gradient values of peripheral pixels)+β  (4)
 
where a is a value larger than 0.6 and smaller than 1.0, and preferably 0.8; β is a value larger than 0 and smaller than 160, and preferably 80.
 
       FIGS. 23A to 23C  are graphs for illustrating a relationship between a noise line region and a content. The horizontal axis of  FIGS. 23A to 23C  indicates the horizontal position in a correction image and the vertical axis indicates the gradient value. 
       FIG. 23A  depicts the gradient value of a horizontal line in a correction image with a noise line overlapping a background. In this correction image, a noise line is in a region  2301  and a background is in peripheral regions  2302 ,  2303 . In the example depicted in  FIG. 23A , the gradient value  2304  of the noise line region  2301  is larger than the average value  2305  of the gradient values of the peripheral regions  2302 ,  2303  and a difference therebetween is sufficiently small, thus, formulas (3) and (4) are satisfied and the noise line region is determined not to overlap the content. 
       FIG. 23B  depicts the gradient value of a horizontal line in a correction image with a noise line overlapping a content that has a higher gradient value than the background. In this correction image, a noise line is in a region  2311 , a content with a higher gradient value is in peripheral regions  2312 ,  2313 , and a background is in further peripheral regions  2314 ,  2315 . The content regions  2312 ,  2313  have sufficiently higher gradient values compared with the background regions  2314 ,  2315  so that a user can easily identify the content. Thus, the noise line region  2311  is buried in the content regions  2312 ,  2313 , and the gradient value  2316  of the noise line region  2311  becomes a substantially similar level as the gradient values of the content regions  2312 ,  2313 . Whereas, as the peripheral regions  2312  to  2315  include the background in addition to the content, the average value  2317  of the gradient values of the peripheral regions  2312  to  2315  becomes a value close to the gradient value of the background. Thus, a difference between the gradient value  2316  of the noise line region  2311  and the average value  2317  of the gradient values of the peripheral regions  2312  to  2315  becomes large, and the formula (4) is not satisfied, whereby the noise line region is determined to overlap the content. 
       FIG. 23C  depicts the gradient value of a horizontal line in a correction image with a noise line overlapping a content that has a lower gradient value than the background. In this correction image, a noise line is in a region  2321  and a content with a lower gradient value is in peripheral regions  2322 ,  2323 , and a background is in further peripheral regions  2324 ,  2325 . The content regions  2322 ,  2323  have sufficiently low gradient values compared with the background regions  2324 ,  2325  so that a user can easily identify the content. Thus, the gradient value  2326  of the noise line region  2321  becomes a higher value than the gradient values of the content regions  2322 ,  2323 , but is a sufficiently lower value than the gradient values of the background regions  2324 ,  2325 . As the peripheral regions  2322  to  2325  include the background in addition to the content, the average value  2327  of the gradient values of the peripheral regions  2322  to  2325  becomes a value close to the gradient value of the background. Thus, the gradient value  2326  of the noise line region  2321  becomes smaller than the average value  2327  of the gradient values of the peripheral regions  2322  to  2325 , and the formula (3) is not satisfied, whereby the noise line region is determined to overlap the content. 
     When the determination module  250  determines that the noise line region does not overlap the content, the correction module  251  corrects the vertical region based on the peripheral pixels of the noise line region (step S 703 ) and ends the set of steps. The correction module  251  uses, for example, a known linear interpolation technique to correct the noise line region using the gradient values of the peripheral pixels of the noise line region. In this way, when the periphery of the noise line region is a monotonous background, the correction module  251  can bury the noise line region in the background. 
     Whereas, when the noise line region is determined to overlap the content, the determination module  250  determines whether the content is a character or not (step S 704 ). The determination module  250  detects a character from the correction image, for example, using a known character recognition (OCR) technique. The determination module  250  calculates the likelihood (coincidence) of including each preregistered character in each region in the correction image and, when the likelihood of the character with the highest likelihood is equal to or more than a predetermined threshold, the determination module  250  determines that the region includes the character. When a character is detected, the determination module  250  determines whether the overlapping region that is determined to overlap the content in the noise line region overlaps a character region where the character is detected. When the overlapping region overlaps the character region, the determination module  250  determines that the content overlapping the overlapping region is a character, or when the overlapping region does not overlap the character region, the determination module  250  determines that the content overlapping the overlapping region is not a character. 
     When the determination module  250  determines that the content overlapping the overlapping region is not a character, the correction module  251  does not correct the vertical region (step S 705 ) and ends the set of steps. When the content is not a character, the content is likely a photograph, a pattern, etc., and such a content is likely bright (high luminance value). Noise lines are conspicuous when they are brighter than the document and are inconspicuous when they are darker than the document, thus, when a noise line overlaps a content other than characters, the noise line may preferably be left as is without correction. Whereas, when a noise line overlaps a content such as ruled lines, elimination of the noise line may also eliminate ruled lines. The correction module  251  does not correct a noise line when the noise line overlaps other contents than characters, thereby preventing inappropriately correcting a noise line and exacerbating the correction image. 
     When the determination module  250  determines that the content overlapping the overlapping region is a character, the correction module  251  estimates the character overlapping the overlapping region based on the context of the content (step S 706 ). The context is words, sentences, a text, etc., constituted by a plurality of characters. 
       FIG. 24  is a schematic view depicting an example of a correction image in which a noise line region overlaps a character. 
     In the correction image  2400  depicted in  FIG. 24 , a character “r”  2402  overlaps a noise line region  2401 . When character recognition processing is performed on this correction image  2400 , the character  2402  is unlikely to be recognized correctly (for example, mistakenly recognized as “1”), while characters  2403  “L,” “i,” “b,” “r,” “a,” “y” that do not overlap the noise line region  2401  are correctly recognized. Thus, the correction module  251  estimates the character  2402  overlapping the noise line region  2401  based on the characters  2403  that do not overlap the noise line region  2401 . 
     For example, the information processing apparatus  200  stores a context table recording a variety of contexts and image patterns of characters in the second storage device  220  in advance. The correction module  251  retrieves and refers to the context table in the second storage device  220  and identifies a context where characters that do not overlap the noise line region match characters included in each context from among the contexts stored in the context table. Note that the correction module  251  identifies a context that includes characters that do not overlap the noise line region and are lined up in sequence except for the character that overlaps the noise line region. Then, the correction module  251  estimates the character that overlaps the noise line region from the identified context. 
     Next, the correction module  251  corrects the noise line region using the estimated characters (step S 707 ), and ends the set of steps. The correction module  251  identifies the character portion of characters that do not overlap the noise line region and the background portion in the periphery of the characters, and calculates the average value of the color values of the identified character portion and the average value of the color values of the background portion for each RGB color. The correction module  251  retrieves an image pattern of the estimated character from the second storage device  220 , and generates an image by defining each color value of pixels corresponding to the image pattern as the calculated average value of the color values of the character portion and defining each color value of other pixels as the calculated average value of the color values of the background portion. Then, the correction module  251  corrects the noise line region by replacing the character portion overlapping the noise line region with the generated image. 
     If a noise line is corrected by linear interpolation etc., when the noise line overlaps a character, the character portion may possibly be blurred, erased, or destroyed. The correction module  251  can accurately correct a character by replacing the whole character portion that overlaps the noise line region with a preset image pattern. 
     Alternatively, the correction module  251  may estimate a character that overlaps a noise line using a discriminator that has learned beforehand to output character information when an image in which a character and a noise line overlap each other is input. This discriminator learns in advance using a plurality of images in which a character and a noise line overlap each other by deep learning etc., and is stored in the second storage device  220  in advance. The correction module  251  inputs an image including a character portion that overlaps a noise line region to the discriminator and acquires character information output from the discriminator to estimate the character that overlaps the noise line. In such a case, the correction module  251  can accurately estimate a character that overlaps a noise line and appropriately correct the character portion that overlaps the noise line region. 
     Alternatively, the context table may not be stored in the second storage device  220 , yet instead, stored in a server (not shown) that connects and communicates with the information processing apparatus  200 . In such a case, the correction module  251  transmits characters that do not overlap the noise line region to the server via a communication circuit (not shown), receives a context that matches the characters from the server, and estimates the character overlapping the noise line region from the received context. Likewise, the discriminator may not be stored in the second storage device  220 , and instead, stored in a server (not shown) that connects and communicates with the information processing apparatus  200 . In such a case, the correction module  251  transmits an image including the character portion that overlaps the noise line region to the server via a communication circuit (not shown) and receives information of characters that are in the character portion from the server. 
     In this way, the correction module  251  changes the method of correcting a noise line region according to the determination result of the determination module  250 . Thus, the correction module  251  can appropriately correct a noise line region for each target overlapping the noise line region. 
     Note that, in the image processing system  1 , the image reading apparatus  100  may have a second dirt substance detection processing module  242  and execute the second processing, instead of the information processing apparatus  200 . In such a case, the image reading apparatus  100  executes the second processing after generating the correction image at step S 111  of  FIG. 11  and detects a dirt substance from the correction image. 
     Alternatively, in the image processing system  1 , the information processing apparatus  200  may have a first dirt substance detection processing module  162  and execute the first processing and confirmation processing, instead of the image reading apparatus  100 . In such a case, the image reading apparatus  100  transmits the white reference image acquired at step S 101  of  FIG. 11  to the information processing apparatus  200 . When having received a white reference image from the image reading apparatus  100 , the information processing apparatus  200  executes the first processing, detects a dirt substance from the white reference image, and generates the warning and provides the warning to a user via the display device  201 . The information processing apparatus  200  further executes the confirmation processing and receives a confirmation operation by a user via the operation device  202 . 
     Further, the second dirt substance detection processing module  242  may detect a dirt substance from the document image instead of the correction image in the second processing. In such a case, at step S 112  of  FIG. 11 , the image reading apparatus  100  transmits the document image, instead of or in addition to the correction image, to the information processing apparatus  200  and the information processing apparatus  200  acquires the document image as an input image. 
     Further, in the same way as the second dirt substance detection processing module  242 , the first dirt substance detection processing module  162  may specify a priority range for preferentially detecting dirt in the white reference image using a position in the correction image where a dirt substance is detected as indicated in the second result. In such a case, the first dirt substance detection processing module  162  calculates dirt degrees of pixels that are spaced apart from one another by a first distance in a horizontal direction, without calculating dirt degrees of all pixels in the white reference image. Whereas, the first dirt substance detection processing module  162  sets a range, in the white reference image, that is within a predetermined distance from a position corresponding to the position where a dirt substance is detected in the correction image as a priority range. With regard to the pixels within the priority range, the first dirt substance detection processing module  162  calculates dirt degrees of pixels that are adjacent to one another or spaced apart from one another in a horizontal direction by a second distance that is shorter than the first distance. In this way, the first dirt substance detection processing module  162  can reduce the load of dirt detection processing while selectively scanning a region where dirt is highly likely to be present, which allows efficient detection of dirt. 
     Alternatively, the image processing system  1  may include a plurality of information processing apparatuses  200 , instead of one, which may jointly operate to share each processing in the overall processing and second processing. In such a case, the plurality of information processing apparatuses  200  may be distributed over a network so that the image processing service can be provided in the form of cloud computing. 
     As detailed above, the image reading apparatus  100  generates the warning when a dirt degree at the imaging position is severe, or generates the warning when a dirt degree at the imaging position is moderate (or minor) only when the document scan count exceeds a threshold, yet, does not generate the warning in other cases. Thus, the image reading apparatus  100  can generate the warning at more appropriate timing when the imaging position is dirty. 
     Further, the image processing system  1  is provided with a light source  124  in the image reading apparatus  100  so that the leading end or rear end of a document conveyed onto the transparent member  127  forms a shadow on the white reference member  126 , whereby the image reading apparatus  100  detects a shadow region from a correction image and detects a dirt substance from the detected shadow region. Thus, the image processing system  1  can detect both white dirt substance and black dirt substance from a shadow region and can better detect a dirt substance from a correction image. 
     Further, the image processing system  1  detects a shadow region from a region outside the document region in the correction image and detects noise pixels, the number of which connected with one another in the shadow region is equal to or more than a predetermined number, as a noise line. Thus, the image processing system  1  can detect both white noise line and black noise line from a shadow region and can more accurately detect a noise line from an image. 
     Further, in the image processing system  1 , a dirt substance detection result is fed back to and used by each other between the first processing for detecting a dirt substance from a white reference image before generation of data for shading correction and the second processing for detecting a dirt substance from the correction image obtained by performing shading correction on a document image. Thus, the image processing system  1  can better detect a dirt substance from an image. 
       FIG. 25  is a view depicting schematic components of an imaging unit  318  according to another embodiment. 
     A first imaging unit  318   a  and a second imaging unit  318   b  depicted in  FIG. 25  are used instead of the first imaging unit  118   a  and the second imaging unit  118   b  in the image reading apparatus  100 . In the first imaging unit  318   a , a first light source  324   a  is provided on the downstream side of the first imaging sensor  125   a  in a document conveyance direction A 3 . In the second imaging unit  318   b , a second light source  324   b  is provided on the upstream side of the second imaging sensor  125   b  in the document conveyance direction A 3 . Note that the first light source  324   a  and the second light source  324   b  may be provided on the upstream side of the first imaging sensor  125   a . Alternatively, the first light source  324   a  and the second light source  324   b  may be provided on the downstream side of the first imaging sensor  125   a . The arrangement of each imaging sensor  125  and light source  324  is indicated in the device information, and the information processing apparatus  200  can acquire the arrangement of each imaging sensor  125  and light source  324  from the device information. 
     The image processing system according to this embodiment can also provide the same effects as those described above. 
       FIG. 26  depicts schematic components of the imaging unit  418  and a conveyance mechanism of the upstream and downstream sides of the imaging unit  418  according to still another embodiment. 
     In this embodiment, an imaging unit  418 , a first conveyance roller  415 , a first driven roller  416 , a second conveyance roller  420 , and a second driven roller  421  are arranged upside down in a direction A 4  perpendicular to the document conveyance path in contrast to the arrangement state depicted in  FIG. 4 . 
     That is, the first imaging unit  418   a  is arranged below the second imaging unit  418   b . The first imaging unit  418   a  is provided with an imaging unit guide  422 . The second imaging unit  418   b  is fixed to the upper housing  102  and the first imaging unit  418   a  is supported by the lower housing  101  so that the first imaging unit  418   a  can move in a direction perpendicular to the document conveyance path and is energized in a direction toward the side of the second imaging unit  418   b  by an energizing spring  423 . The first white reference member  426   a  is provided below the first transparent member  427   a , and the second light source  424   b  and the second imaging sensor  425   b  are provided on the opposite side of the first white reference member  426   a  across the first transparent member  427   a  and the second transparent member  427   b . Likewise, the second white reference member  426   b  is provided above the second transparent member  427   b , and the first light source  424   a  and the first imaging sensor  425   a  are provided on the opposite side of the second white reference member  426   b  across the first transparent member  427   a  and the second transparent member  427   b.    
     Further, the first driven roller  416  and the second driven roller  421  are arranged below the first conveyance roller  415  and the second conveyance roller  420  respectively. The first conveyance roller  415  and the first driven roller  416  convey a document such that the document is conveyed along the second transparent member  427   b  at the imaging positions L 1  and L 2 . 
     The image processing system according to this embodiment can also provide the same effects as those described above. 
       FIG. 27  is a block diagram depicting schematic components of a first processing circuit  180  according to another embodiment. 
     In place of the first CPU  160 , the first processing circuit  180  performs the overall processing, first processing, confirmation processing, threshold setting processing etc. The first processing circuit  180  includes a first image acquiring circuit  181 , a first dirt substance detection processing circuit  182 , a correction data generation circuit  189 , a correction image generation circuit  190  etc. The first dirt substance detection processing circuit  182  includes a first result acquiring circuit  183 , a dirt degree calculation circuit  184 , a notifying circuit  185 , a setting circuit  186 , a threshold acquiring circuit  187 , an information acquiring circuit  188 , etc. 
     The first image acquiring circuit  181  is an example of the first image acquiring module and has the same function as the first image acquiring module  161 . The first image acquiring circuit  181  acquires a white reference image and a document image from the imaging unit  118 , outputs the acquired white reference image to the dirt degree calculation circuit  184  and the correction data generation circuit  189  and outputs the acquired document image to the correction image generation circuit  190 . 
     The first dirt substance detection processing circuit  182  is an example of the first dirt substance detection processing module and has the same function as the first dirt substance detection processing module  162 . The first dirt substance detection processing circuit  182  performs the first processing for detecting a dirt substance from a white reference image. 
     The first result acquiring circuit  183  is an example of the first result acquiring module and has the same function as the first result acquiring module  163 . The first result acquiring circuit  183  acquires a second result from the information processing apparatus  200  via the first interface device  135 , and outputs the second result to the notifying circuit  185 , the setting circuit  186 , and the correction data generation circuit  189 . 
     The dirt degree calculation circuit  184  is an example of the dirt degree calculation module and has the same function as the dirt degree calculation module  164 . The dirt degree calculation circuit  184  acquires a white reference image from the first image acquiring circuit  181  and each threshold from the first storage device  140 , calculates a dirt degree at the imaging position from the white reference image, and outputs the dirt degree to the notifying circuit  185 . 
     The notifying circuit  185  is an example of the notifying module and has the same function as the notifying module  165 . The notifying circuit  185  acquires a dirt degree from the dirt degree calculation circuit  184 , a second result from the first result acquiring circuit  183 , and open/close information from the information acquiring circuit  188 , and generates and provides the warning on the display operation device  106  according to the dirt degree. 
     The setting circuit  186  is an example of the setting module and has the same function as the setting module  166 . The setting circuit  186  acquires a binarization threshold from the threshold acquiring circuit  187  and a second result from the first result acquiring circuit  183  and sets each threshold in the first storage device  140 . 
     The threshold acquiring circuit  187  is an example of the threshold acquiring module and has the same function as the threshold acquiring module  167 . The threshold acquiring circuit  187  acquires a binarization threshold from the information processing apparatus  200  via the first interface device  135  and outputs the binarization threshold to the setting circuit  186 . 
     The information acquiring circuit  188  is an example of the information acquiring module and has the same function as the information acquiring module  168 . The information acquiring circuit  188  acquires an open/close signal from the open/close sensor  114  and outputs the open/close information to the notifying circuit  185 . 
     The correction data generation circuit  189  is an example of the correction data generation module and has the same function as the correction data generation module  169 . The correction data generation circuit  189  acquires a white reference image from the first image acquiring circuit  181  and a second result from the first result acquiring circuit  183 , generates data for shading correction from the white reference image, and outputs the data for shading correction to the correction image generation circuit  190 . 
     The correction image generation circuit  190  is an example of the correction image generation module and has the same function as the correction image generation module  170 . The correction image generation circuit  190  acquires a document image from the first image acquiring circuit  181  and data for shading correction from the correction data generation circuit  189 , and generates a correction image by correcting the document image using the data for shading correction. The correction image generation circuit  190  outputs the correction image to the information processing apparatus  200  via the first interface device  135 . 
     The image processing system according to this embodiment can also provide the same effects as those described above. 
       FIG. 28  is a block diagram depicting schematic components of a second processing circuit  260  according to another embodiment. 
     In place of the second CPU  240 , the second processing circuit  260  preforms the overall processing, second processing, correction processing etc. The second processing circuit  260  includes a second image acquiring circuit  261 , a second dirt substance detection processing circuit  262 , a determination circuit  270 , a correction circuit  271  etc. The second dirt substance detection processing circuit  262  includes a second result acquiring circuit  263 , a device information acquiring circuit  264 , an edge pixel extracting circuit  265 , a document region detecting circuit  266 , a shadow region detecting circuit  267 , a noise pixel extracting circuit  268 , a noise line detecting circuit  269  etc. 
     The second image acquiring circuit  261  is an example of the second image acquiring module and has the same function as the second image acquiring module  241 . The second image acquiring circuit  261  acquires a correction image from the image reading apparatus  100  via the second interface device  203 , and outputs the acquired correction image to the edge pixel extracting circuit  265  and the correction circuit  271 . 
     The second dirt substance detection processing circuit  262  is an example of the second dirt substance detection processing module and has the same function as the second dirt substance detection processing module  242 . The second dirt substance detection processing circuit  262  performs the second processing for detecting a dirt substance from a correction image. 
     The second result acquiring circuit  263  is an example of the second result acquiring module and has the same function as the second result acquiring module  243 . The second result acquiring circuit  263  acquires the first result from the image reading apparatus  100  via the second interface device  203  and the second result from the noise line detecting circuit  269 , and outputs the acquired first result and second result to the noise pixel extracting circuit  268  and the noise line detecting circuit  269 . 
     The device information acquiring circuit  264  is an example of the device information acquiring module and has the same function as the device information acquiring module  244 . The device information acquiring circuit  264  acquires device information from the image reading apparatus  100  via the second interface device  203  and outputs the acquired device information to the shadow region detecting circuit  267 . 
     The edge pixel extracting circuit  265  is an example of the edge pixel extracting module and has the same function as the edge pixel extracting module  245 . The edge pixel extracting circuit  265  acquires a correction image from the second image acquiring circuit  261 , extracts first edge pixels from the correction image, and outputs the extracted first edge pixel information to the document region detecting circuit  266 . 
     The document region detecting circuit  266  is an example of the document region detecting module and has the same function as the document region detecting module  246 . The document region detecting circuit  266  acquires the first edge pixel information from the edge pixel extracting circuit  265 , detects a document region based on the first edge pixels, and outputs the detected document region information to the shadow region detecting circuit  267 . 
     The shadow region detecting circuit  267  is an example of the shadow region detecting module and has the same function as the shadow region detecting module  247 . The shadow region detecting circuit  267  acquires document region information from the document region detecting circuit  266 , detects a shadow region from within a predetermined range outside the document region, and outputs the detected shadow region information to the noise pixel extracting circuit  268 . 
     The noise pixel extracting circuit  268  is an example of the noise pixel extracting module and has the same function as the noise pixel extracting module  248 . The noise pixel extracting circuit  268  acquires the shadow region information from the shadow region detecting circuit  267  and the first result and the second result from the second result acquiring circuit  263 , extracts noise pixels from the shadow region, and outputs the extracted noise pixel information to the noise line detecting circuit  269 . 
     The noise line detecting circuit  269  is an example of the noise line detecting module and has the same function as the noise line detecting module  249 . The noise line detecting circuit  269  acquires the noise pixel information from the noise pixel extracting circuit  268  and the first result and the second result from the second result acquiring circuit  263 , detects a noise line based on the noise pixels, and outputs the noise line region information to the determination circuit  270 . 
     The determination circuit  270  is an example of the determination module and has the same function as the determination module  250 . The determination circuit  270  acquires the noise line region information from the noise line detecting circuit  269 , determines whether the noise line region overlaps a content, and outputs the determination result to the correction circuit  271 . 
     The correction circuit  271  is an example of the correction module and has the same function as the correction module  251 . The correction circuit  271  acquires a correction image from the second image acquiring circuit  261  and a determination result from the determination circuit  270 , corrects the correction image based on the determination result, stores the corrected correction image in the second storage device  220 , and displays it on the display device  201 . 
     The image processing system according to this embodiment can also provide the same effects as those described above. 
     According to the image processing system, the control method, and the computer-readable, non-transitory medium storing a computer program, it is possible to better detect a dirt substance from an image. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiment(s) of the present inventions have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.