Patent Publication Number: US-2023164275-A1

Title: Diagnostic system for diagnosing image forming apparatus using image formed by image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a diagnostic system for diagnosing an image forming apparatus using an image formed by the image forming apparatus. 
     Description of the Related Art 
     When components of an image forming apparatus reach the end of their useful life, image deficiencies may occur. Japanese Patent No. 5164458 describes identifying components that cause image deficiencies by reading an image on a sheet using an image sensor built into the image forming apparatus. Japanese Patent No. 6350474 describes capturing an image on a sheet using an image capturing apparatus such as a digital camera or a camera-equipped cell phone, and calibrating the image forming apparatus based on a result of the capture. 
     According to the invention of Japanese Patent No. 5164458, an image forming apparatus lacking an image sensor cannot identify the components that cause image deficiencies. In this case, a user must purchase an image forming apparatus equipped with an image sensor. The invention of Japanese Patent No. 6350474 cannot detect image deficiencies in the first place, nor can it identify components that cause image deficiencies. 
     SUMMARY OF THE INVENTION 
     The disclosure provides a diagnostic system comprising: an image forming apparatus that forms a diagnostic image on a sheet; and an image capturing apparatus that captures an image of the sheet on which the diagnostic image is formed and which has been discharged from the image forming apparatus, and outputs the image captured as a sheet image, wherein the image capturing apparatus comprises one or more processor configured to: make a diagnosis for a component of the image forming apparatus based on a position of an image deficiency in the sheet image; and output a diagnosis result for the component of the image forming apparatus. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram illustrating a diagnostic system. 
         FIG.  2    is a diagram illustrating an image capturing apparatus. 
         FIG.  3    is a diagram illustrating an image forming apparatus. 
         FIGS.  4 A and  4 B  are diagrams illustrating a server and a control unit of the image forming apparatus. 
         FIG.  5    is a flowchart illustrating an image diagnosis method. 
         FIGS.  6 A to  6 C  are diagrams illustrating a diagnostic image and a sheet image. 
         FIGS.  7 A and  7 B  are flowcharts illustrating processing by the server and processing by the image forming apparatus. 
         FIG.  8    is a diagram illustrating an image diagnosis method that uses a plurality of sheets. 
         FIGS.  9 A and  9 B  are diagrams illustrating an image diagnosis method related to a fixer. 
         FIGS.  10 A and  10 B  are diagrams illustrating an image diagnosis method using a given image. 
         FIGS.  11 A and  11 B  are flowcharts illustrating an image diagnosis method. 
         FIGS.  12 A to  12 C  are diagrams illustrating user input of a print direction using a direction determination image. 
         FIG.  13    is a diagram illustrating functions of a CPU. 
         FIG.  14    is a diagram illustrating an image capturing apparatus. 
         FIG.  15    is a diagram illustrating an image forming apparatus. 
         FIG.  16    is a diagram illustrating a sheet cassette. 
         FIGS.  17 A and  17 B  are diagrams illustrating a server and a control unit of the image forming apparatus. 
         FIGS.  18 A and  18 B  are flowcharts illustrating an image diagnosis method and the like. 
         FIGS.  19 A to  19 D  are diagrams illustrating a diagnostic image and a sheet image. 
         FIGS.  20 A and  20 B  are diagrams illustrating an image correction method. 
         FIG.  21    is a diagram illustrating an effect of an embodiment. 
         FIG.  22    is a diagram illustrating an image diagnosis method that uses a plurality of sheets. 
         FIGS.  23 A and  23 B  are flowcharts illustrating another image diagnosis method and the like. 
         FIGS.  24 A and  24 B  are flowcharts illustrating yet another image diagnosis method and the like. 
         FIG.  25    is a diagram illustrating functions of a CPU. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, embodiments will be described in detail with reference to the attached drawings. Note, the following embodiments are not intended to limit the scope of the claimed invention. Multiple features are described in the embodiments, but limitation is not made an invention that requires all such features, and multiple such features may be combined as appropriate. Furthermore, in the attached drawings, the same reference numerals are given to the same or similar configurations, and redundant description thereof is omitted. 
     First Embodiment 
     Diagnostic System 
     As illustrated in  FIG.  1   , a diagnostic system  100  includes an image forming apparatus  101  for diagnosis and an image capturing apparatus  102 . A server  103  is optional. It is assumed here that the image forming apparatus  101  does not have an image sensor that reads a diagnostic image formed on a sheet. However, the image forming apparatus  101  may have an image sensor that reads a diagnostic image formed on a sheet. For example, if a diagnostic function provided by the image capturing apparatus  102  is superior to a diagnostic function provided by the image forming apparatus  101 , a user will likely wish to diagnose the image forming apparatus  101  using the diagnostic function of the image capturing apparatus  102 . 
     The image capturing apparatus  102  includes a camera, and is a communication device that can be carried by the user (e.g., a smartphone, a tablet terminal, or a digital camera). Here, the “user” is a human being who can operate the image capturing apparatus  102 , and includes owners, users, and maintenance workers of the image forming apparatus  101 . The image capturing apparatus  102  includes wireless communication circuitry (e.g., wireless LAN, Bluetooth (registered trademark), or cellular wireless) and wired communication circuitry (e.g., a USB interface), and is capable of communicating with the image forming apparatus  101  and the server  103 . 
     In the first embodiment, the image capturing apparatus  102  functions as a diagnostic apparatus for diagnosing the image forming apparatus  101 . Note that the image capturing apparatus  102  may capture a diagnostic image formed on a sheet, generate a sheet image, and transfer the sheet image to the server  103 . The server  103  may diagnose the image forming apparatus  101  based on the sheet image. The server  103  may be a personal computer (PC). 
     Image Capturing Apparatus 
       FIG.  2    illustrates the configuration of the image capturing apparatus  102 . A control unit  201  controls an interface unit  202 , a camera  203 , a communication unit  204 , and a storage unit  205  according to a control program stored in the storage unit  205 . The control unit  201  includes hardware circuitry such as a central processing unit (CPU), an application-specific integrated circuit (ASIC), and a field programmable gate array (FPGA). The interface unit  202  includes an output device (display device) that outputs information to the user and an input device that accepts user inputs (e.g., a touch panel sensor). The camera  203  includes an image sensor (e.g., a CMOS image sensor or a CCD image sensor), a light source that emits illumination light, and the like. The communication unit  204  includes the wireless communication circuitry and the wired communication circuitry mentioned above. The storage unit  205  includes random access memory (RAM), read-only memory (ROM), and the like. The storage unit  205  stores a control program (a diagnostic program  206 ) executed by the control unit  201  and control data (diagnostic image data  207 ) in a ROM region. The diagnostic image data  207  is original image data of a diagnostic image formed on a sheet. The storage unit  205  stores a sheet image  208  obtained by the camera  203 , a diagnostic result  209 , and the like in a RAM region. 
     Image Forming Apparatus 
     Although  FIG.  3    illustrates an electrophotographic image forming apparatus  101 , the technical spirit of the present embodiment can be applied to any image forming apparatus in which components such as a rotating body are involved in the formation of images. For example, the technical spirit of the present embodiment is applicable to any image forming apparatus in which image deficiencies occur in an image formed on a sheet P due to the components having reached the end of their useful life, components malfunctioning, and the like. The technical spirit of the present embodiment is also applicable in any image forming apparatus in which image deficiencies occur when components which require regular maintenance (e.g., cleaning, tuning, and replacement) have not been given such regular maintenance. 
     In  FIG.  3   , the letters Y, M, C, and K appended to the ends of the reference signs indicate toner colors, i.e., yellow, magenta, cyan, and black. For example, components with a Y appended to the end of the reference signs are involved in the formation of a yellow toner image. When there is no need to distinguish among the colors in describing the components, reference signs without the appended letters are used. 
     A control unit  40  is a control circuit (e.g., a CPU, an ASIC, and an FPGA) that controls the various parts of the image forming apparatus  101 . The control unit  40  receives image data and printing instructions from an external device (e.g., the image capturing apparatus  102 ) through a communication unit  41 . The control unit  40  converts the image data to generate an image signal, and supplies the image signal to an exposure device  7 . 
     A photosensitive member  1  is an image carrier that is driven by a drive source such as a motor and rotates clockwise, and carries an electrostatic latent image and a toner image. The photosensitive member  1  is sometimes called a photosensitive drum due to being a cylindrical rotating body. A charging roller  2  is applied with a charging bias voltage by the control unit  40  and charges the surface of the photosensitive member  1  to a uniform potential. The exposure device  7  forms an electrostatic latent image on the surface (circumferential surface) of the photosensitive member  1  by irradiating the surface of the photosensitive member  1  with laser light corresponding to the image signal. A developing roller  3  is applied with a developing bias voltage by the control unit  40 , and forms a toner image on the surface of the photosensitive member  1  by causing toner to adhere to the electrostatic latent image. The primary transfer roller  6  is applied with a primary transfer bias voltage by the control unit  40 , and transfers the toner image from the photosensitive member  1  to an intermediate transfer belt  8 . A cleaner  4  is a cleaning member that removes and collects toner that has not been transferred to the intermediate transfer belt  8  and remains on the photosensitive member  1 . The photosensitive member  1 , the developing roller  3 , the charging roller  2 , and the cleaner  4  may be integrated within a cartridge. Such a cartridge is configured to be removable from the main body of the image forming apparatus  101 . The photosensitive member  1 , the charging roller  2 , the exposure device  7 , the developing roller  3 , and the primary transfer roller  6  function as an image forming unit that forms an image on the intermediate transfer belt  8 . 
     The intermediate transfer belt  8  is an endless belt, and is sometimes referred to as an intermediate transfer body. The intermediate transfer belt  8  is driven by a drive source such as a motor, and rotates counterclockwise. Toner images from each of the four photosensitive members  1  are superimposed and transferred onto the intermediate transfer belt  8 , and a full-color toner image is formed on the intermediate transfer belt  8  as a result. The toner image transferred onto the intermediate transfer belt  8  is conveyed to a secondary transfer section. The secondary transfer section is a nip section formed by the intermediate transfer belt  8  and a secondary transfer roller  11 . 
     A sheet cassette  13  is a holding unit that holds a large number of sheets P. A feed roller  14  feeds the sheet P from the cassette  13  to a conveyance path  15  according to instructions from the control unit  40 . The sheet P is conveyed to the secondary transfer section by conveyance rollers provided along the conveyance path  15 . The secondary transfer roller  11  is applied with a secondary transfer bias voltage by the control unit  40 , and transfers the toner image from the intermediate transfer belt  8  onto the sheet P. The secondary transfer roller  11  conveys the sheet P to a fixer  17 . The fixer  17  includes two rotating bodies (a fixing roller  22  and a pressure roller  21 ), and fixes the toner image onto the sheet P by applying heat and pressure to the sheet P and the toner image. As the fixing roller  22  and the pressure roller  21  rotate, the sheet P is conveyed to a discharge roller  20 . The discharge roller  20  discharges the sheet P to the exterior of the image forming apparatus  101 . 
     Control Unit of Server 
     As illustrated in  FIG.  4 A , the server  103  includes a control unit  401 , a communication unit  404 , and a storage unit  405 . The control unit  401  includes a CPU and the like that execute a control program stored in the storage unit  405 . The communication unit  404  is communication circuitry that communicates with the image capturing apparatus  102  over the network  130 . The storage unit  405  includes ROM, RAM, a solid state drive (SSD), and a hard disk drive (HDD). The storage unit  405  stores, for example, print direction data  406  that holds model information of the image forming apparatus  101  and a standard print direction thereof in association with each other. Upon receiving a query from the image capturing apparatus  102  including the model information of the image forming apparatus  101 , the control unit  401  reads out the print direction corresponding to the model information from the print direction data  406  and responds by sending information indicating the print direction (print direction information) to the image capturing apparatus  102 . The print direction refers to the orientation of the image relative to the sheet P. For example, a forward direction, in which the upper side of the image (the header side) is located on the leading edge side of the sheet P in a conveyance direction of the sheet P, and a reverse direction, in which the lower side of the image (the footer side) is located on the leading edge side of the sheet P in the conveyance direction of the sheet P, can be given as print directions. Even if the standard print direction is the forward direction, reverse direction printing can be achieved by a printer driver rotating the print direction 180 degrees. 
     Incidentally, when a rotating body such as the photosensitive member  1  reaches a replacement time (lifespan), image deficiencies may occur on the sheet P at intervals corresponding to the rotation cycle of the photosensitive member  1 . Similarly, when the developing roller  3  approaches its replacement time, image deficiencies occur on the sheet P at an interval based on the rotation cycle of the developing roller  3 . The same applies to the charging roller  2 , the intermediate transfer belt  8 , the fixing roller  22 , the pressure roller  21 , and the like. The image capturing apparatus  102  measures a distance from a reference position (e.g., a leading edge of the sheet, a test pattern, the position of another image deficiency) within the sheet image generated by capturing an image of the diagnostic image to the position of an image deficiency. Based on a measurement result, the image capturing apparatus  102  can identify which component requires maintenance. In other words, the image capturing apparatus  102  identifies the component that is the cause of the image deficiency and notifies the user. 
     Here, to accurately make an image diagnosis, it is necessary for the image capturing apparatus  102  to ascertain whether an upper side of the diagnostic image corresponds to a leading edge side or a following edge side of the sheet P in the conveyance direction of the sheet P. As mentioned above, the image forming apparatus  101  has a forward direction and a reverse direction as print directions. Some image forming apparatuses  101  may take the forward direction as the standard print direction, whereas some image forming apparatuses  101  may take the reverse direction as the standard print direction. The image capturing apparatus  102  generates the sheet image by capturing an image of the diagnostic image formed on the sheet P. However, it is difficult for the image capturing apparatus  102  to determine, from the sheet image, whether the diagnostic image was printed in the forward direction with respect to the sheet P or printed in the reverse direction with respect to the sheet P. It is therefore important for the image capturing apparatus  102  that makes the image diagnosis to obtain the print direction of the diagnostic image. For example, the image capturing apparatus  102  obtains the print direction of the image forming apparatus  101  from the server  103  by transmitting model information of the image forming apparatus  101  to the server  103 . 
     Control Unit of Image Forming Apparatus 
       FIG.  4 B  illustrates the control unit  40  of the image forming apparatus  101 . A CPU  411  controls each unit of the image forming apparatus  101  by executing a control program stored in a storage unit  415 . For example, upon receiving a printing instruction for the diagnostic image from the image capturing apparatus  102 , the CPU  411  controls the image forming apparatus  101  to form the diagnostic image on the sheet P. At this time, the CPU  411  controls a power supply circuit  421  to generate the charging bias voltage, the developing bias voltage, and the transfer bias voltages. Additionally, the CPU  411  rotates the various types of rotating bodies, such as the photosensitive member  1 , by driving a motor  422 . The storage unit  415  includes RAM, ROM, an SSD, an HDD, and the like. The storage unit  415  stores model information  416 , which is identification information of the image forming apparatus  101 . The CPU  411  may store information indicating a usage history of the image forming apparatus  101  (e.g., a cumulative number of images formed) or the replacement time of each component in the storage unit  415  as history data  417 . The storage unit  415  may store print direction information  418  indicating the standard print direction of the image forming apparatus  101 . In this case, the image capturing apparatus  102  can obtain the print direction information  418  from the image forming apparatus  101 . Note that the “standard print direction” is the default print direction defined by the design of the image forming apparatus  101 . The user can specify, through a printer driver of the image forming apparatus  101 , that the printing orientation of the image be rotated by  180  degrees with respect to the standard print direction. 
     Flowcharts 
       FIG.  5    illustrates an image diagnosis method executed by a CPU provided in the control unit  201 , according to the diagnostic program  206 . When the control unit  201  is instructed to launch the diagnostic program  206  through the interface unit  202 , the control unit  201  launches the diagnostic program  206  and executes the following processing. 
     In step S 501 , the control unit  201  obtains the model information  416  of the image forming apparatus  101 . For example, the control unit  201  connects to the image forming apparatus  101  through the communication unit  204  and transmits a request to the image forming apparatus  101 . As a result, the control unit  201  receives the model information  416  from the image forming apparatus  101  through the communication unit  204 . The model information  416  includes identification information such as a product number, a model name, and the like of the image forming apparatus  101 . 
     In step S 502 , the control unit  201  obtains the print direction information of the image forming apparatus  101 . For example, the control unit  201  connects to the server  103  through the communication unit  204  and transmits a query for the print direction information. The query includes the model information  416 . The control unit  201  obtains the print direction information  418  corresponding to the model information  416  from the server  103  through the communication unit  204 . 
     In step S 503 , the control unit  201  sets the print direction of the diagnostic image based on the print direction information  418 . 
       FIG.  6 A  illustrates an example of the diagnostic image  600 . The diagnostic image  600  includes a single-color pattern  601  and a halftone pattern  602 . The single-color pattern  601  includes a test image formed from one color among Y, M, C, and K. The halftone pattern  602  includes a gradation image formed by mixing all of the Y, M, C, and K colors. The control unit  201  specifies the print direction of the image forming apparatus  101 , based on the standard print direction of the image forming apparatus  101  indicated by the print direction information  418 , such that the header side of the diagnostic image  600  is formed on the leading edge side of the sheet P in the conveyance direction of the sheet P. For example, if the standard print direction is the forward direction, the control unit  201  sets a rotation angle of the orientation of the image to 0 degrees. If the standard print direction is the reverse direction, the control unit  201  sets the rotation angle of the orientation of the image to 180 degrees. As a result, the single-color pattern  601  of the diagnostic image  600  is formed on the leading edge side of the sheet P. Note that the leading edge of the sheet P passes through the secondary transfer section before the following edge of the sheet P. 
     In step S 504 , the control unit  201  transmits, to the image forming apparatus  101 , a printing instruction to print the diagnostic image  600 . The printing instruction includes the diagnostic image data  207 , which serves as the source of the diagnostic image  600 , and designation information of the print direction (printing orientation) set in step S 503 . 
     In step S 505 , the control unit  201  controls the camera  203  to capture (photograph) the diagnostic image formed on the sheet P by the image forming apparatus  101  and generate a sheet image. For example, the control unit  201  may display a guidance message prompting the user to capture the diagnostic image  600  in the display device of the interface unit  202 . The user then operates the input device of the interface unit  202  in response to the guidance and captures the diagnostic image  600  on the sheet P. As a result, the sheet image  208  is generated and saved in a RAM region of the storage unit  205 . 
       FIG.  6 B  illustrates a sheet image  208   a  showing drum ghosting. From  FIG.  6 B , it can be seen that the leading edge side of the sheet P and the header side of the diagnostic image  600  do not match. Here, the single-color pattern  601  is an image pattern useful for sensing an image deficiency known as “drum ghosting”. “Drum ghosting” is an image deficiency that can occur due to deterioration of the photosensitive member  1 . As illustrated in  FIG.  6 B , the single-color pattern  601  formed first appears as an afterimage  603 . Here, a distance L 1  between the single-color pattern  601  and the afterimage  603  corresponds to the circumferential length of the photosensitive member  1 . The density of the afterimage  603  differs from the density of the regular halftone pattern  602 . The control unit  201  can determine the presence or absence of drum ghosting by comparing the image density at a position distanced from the single-color pattern  601  by the distance L 1  with a reference density (the density of the halftone pattern  602 ). 
     In this manner, it is important for the single-color pattern  601  to be printed on the sheet P before the halftone pattern  602 . If the print direction is incorrect, the single-color pattern  601  will be formed after the halftone pattern  602 , and thus the afterimage  603  of the single-color pattern  601  does not arise in the halftone pattern  602 . In other words, the control unit  201  will not be able to correctly diagnose the deterioration of the photosensitive member  1 . The halftone pattern  602  is an image pattern mainly used to sense the occurrence of defects caused by the transport of the sheet P. Due to wear from years of use, drive gears, conveyance rollers, and the like may deteriorate or break. Image deficiencies  604   a  to  604   c  occur in the halftone pattern  602  of a sheet image  208   b  illustrated in  FIG.  6 C . An interval L 2  between the image deficiencies  604   a  to  604   c  is equivalent to the circumferential length of the drive gear or conveyance roller, and thus the control unit  201  can identify the drive gear or conveyance roller as defective. 
     In step S 506 , the control unit  201  analyzes the sheet image  208  and senses an image deficiency. For example, the control unit  201  determines whether the afterimage  603  occurs at a position distanced from the single-color pattern  601  by the distance L 1 . Alternatively, the control unit  201  may sense the afterimage  603 , identify the position where the afterimage  603  occurs, and determine whether the distance between the single-color pattern  601  and the afterimage  603  is the distance L 1 . Alternatively, the control unit  201  may sense a plurality of the image deficiencies  604   a  to  604   c  that occur periodically and measure the interval L 2  between each of the plurality of the image deficiencies  604   a  to  604   c.  The distance L 1  and the interval L 2  are values that correlate to the rotation cycle (circumferential length) of the rotating body that causes the image deficiency, and the rotating body that causes the image deficiency can therefore be identified. 
     In step S 507 , the control unit  201  generates a diagnosis result. The control unit  201  identifies the component (a causative component) constituting the image forming apparatus  101  based on the position of the image deficiency. The control unit  201  generates the diagnosis result, which indicates the installation location of the causative component within the image forming apparatus  101 , the wear state of the causative component, the replacement time of the causative component, an ordering method of the causative component, and the like. The diagnosis result may include state information for each of the plurality of components, indicating whether the state is a normal state or a state in which maintenance is required. The diagnosis result may include information indicating measures for reducing image deficiencies (e.g., replacement, repair, cleaning, or the like). If no image deficiencies are sensed in step S 506 , the diagnosis result includes information indicating that all components constituting the image forming apparatus  101  are operating normally. 
     In step S 508 , the control unit  201  displays the diagnosis result in the display device of the interface unit  202 . The control unit  201  may notify an administrator or maintenance worker of the diagnosis result by outputting (transmitting) the diagnosis result to the server  103 , the image forming apparatus  101 , or a personal computer through the communication unit  204 . 
       FIG.  7 A  illustrates a model information providing method executed by the CPU  411  of the image forming apparatus  101 . In step S 701 , the CPU  411  receives a request (a transmission request) for the model information  416  from the image capturing apparatus  102 . In step S 702 , the CPU  411  reads out the model information  416  from the storage unit  415  and transmits the model information  416  to the image capturing apparatus  102  through the communication unit  41 . 
     In step S 703 , the CPU  411  receives a printing instruction for the diagnostic image  600  from the image capturing apparatus  102  through the communication unit  41 . In step S 704 , the CPU  411  forms the diagnostic image  600  on the sheet P in the print direction according to the printing instruction. 
       FIG.  7 B  illustrates a print direction information providing method executed by the CPU  411  of the server  103 . In step S 711 , the CPU  411  receives a print direction query from the image capturing apparatus  102  through the communication unit  404 . In step S 712 , the CPU  411  searches and reads out, from the print direction data  406 , the print direction information corresponding to the model information included in the query. In step S 713 , the CPU  411  sends the print direction information to the image capturing apparatus  102  through the communication unit  404 . 
     According to the present embodiment, the diagnostic system  100  can diagnose components of the image forming apparatus  101  using the image capturing apparatus  102 , such as a smartphone, regardless of the model of the image forming apparatus  101 . Accordingly, even if the image forming apparatus  101  is not equipped with a diagnostic image sensor, the user will still be able to obtain a diagnosis result. Note, however, that the image capturing apparatus  102  may perform a similar diagnosis for an image forming apparatus  101  equipped with an image sensor. 
     According to the present embodiment, the leading edge side of the sheet P matches the header side of the diagnostic image  600 , and thus a more accurate diagnosis result can be obtained. In other words, the component that is the cause of the image deficiency can be identified accurately. 
     Because the leading edge side of the sheet P matches the header side of the diagnostic image  600 , the image capturing apparatus  102  can identify the printing orientation of the diagnostic image from the sheet image  208 . In other words, the image capturing apparatus  102  can identify the one of the four sides constituting sheet image  208  that first passed through the secondary transfer section (a diagnostic reference side). For example, the image capturing apparatus  102  may measure the distance from the identified diagnostic reference side to the position of the image deficiency and identify the component that is the cause of the image deficiency based on the measured distance. 
     In  FIGS.  6 B and  6 C , the diagnosis is made using single sheet images  208   a  and  208   b , but this is only one example. The number of sheets P on which the diagnostic image  600  is printed may be determined according to the size of the constituent members (components) of the image forming apparatus  101  to be diagnosed. For example, a plurality of sheets P are needed to sense image deficiencies caused by scratches on the surface of the intermediate transfer belt  8 . This is because a circumferential length L 3  of the intermediate transfer belt  8  is much longer than a length Lp of the sheet P in the conveyance direction of the sheets P. Therefore, the diagnostic image  600  may be formed continuously for N sheets P corresponding to a length at least one or two times the circumferential length L 3 . 
       FIG.  8    illustrates the relationship between the circumferential length L 3  of the intermediate transfer belt  8  and three sheets Pa, Pb, and Pc on which the diagnostic image  600  is formed. The diagnostic image  600  is formed on the first sheet Pa. Only the halftone pattern  602  of the diagnostic image  600  is formed on the second sheet Pb. Only the halftone pattern  602  of the diagnostic image  600  is formed on the third sheet Pc. In addition, identification patterns  801   a,    801   b,  and  801   c,  which indicate the page number and the printing orientation, are printed on the header side of each of the sheets Pa, Pb, and Pc. The image capturing apparatus  102  senses image deficiencies and measures the distances with respect to the positions of the image deficiencies. In particular, image deficiencies  802   a  and  802   b  caused by the intermediate transfer belt  8  occur periodically at intervals corresponding to the circumferential length L 3 . Therefore, the image capturing apparatus  102  identifies the sequence of the three sheet images based on the identification patterns  801   a,    801   b,  and  801   c,  and measures the distance from the diagnostic reference side to the position of the image deficiency according to the identified sequence. In this manner, the sequence of the three sheet images can be identified, and thus distances can be measured across a plurality of sheet images. For example, assume that the distance from the reference side of the sheet Pa to the position of the image deficiency  802   a  is La, and the distance from the reference side of the sheet Pb to the position of the image deficiency  802   b  is Lb. The length of each sheet P and a sheet spacing d are known. In this case, the interval L 3  is Lp−La+d+Lb. 
     Incidentally, in step S 502 , the image capturing apparatus  102  may obtain the print direction information and the circumferential length L 3  of the intermediate transfer belt  8  from the server  103 . The storage unit  405  of the server  103  stores the circumferential length L 3  associated with the model information of the image forming apparatus  101 . The server  103  may transmit the print direction information and circumferential length L 3  to the image capturing apparatus  102  in response to a query from the image capturing apparatus  102 . The control unit  201  determines the number N of the sheets P based on the circumferential length L 3 . For example, N is determined so that a length Lp×N of the sheets P stored in the cassette  13  is at least L 3  or 2×L 3 . As illustrated in  FIG.  8   , the distance between the preceding sheet P and the following sheet P (the sheet spacing d) may be taken into account as well. In this case, N is determined so that Lp×N+d(N−1) is at least L 3  or 2×L 3 . 
     The storage unit  205  of the image capturing apparatus  102  may store a plurality of instances of the diagnostic image data  207 . Each of the plurality of instances of the diagnostic image data  207  may be prepared according to the usage history of the image forming apparatus  101  or the durability state of various consumables. The control unit  201  may obtain the history data  417  from the image forming apparatus  101  and determine, based on the history data  417 , whether any of the plurality of consumable parts have been used beyond their design lifespan. The control unit  201  selects the diagnostic image data  207  suitable for diagnosing consumable parts that have been used beyond their design lifespan. If there are no consumable parts that have been used beyond their design lifespan, the control unit  201  may select the diagnostic image data  207  suitable for diagnosing consumable parts that are close to their design lifespan. For example, if one or more of the photosensitive members  1 Y,  1 M,  1 C, and  1 K have reached the end of their design lifespan, the diagnostic image  600  illustrated in  FIG.  6 A  is selected. This makes it possible to accurately sense defects caused by deterioration in the durability of the photosensitive member  1 . 
     If the fixer  17  continues to be used beyond its design lifespan, the surface of the fixing roller  22  or pressure roller  21  will become scratched or worn. As a result, the toner adhering to the scratches is transferred onto the sheet P in an offset position. 
       FIG.  9 A  illustrates a diagnostic image  900  suitable for when the fixer  17  has exceeded its design lifespan.  FIG.  9 B  illustrates a sheet image  208   d  obtained by capturing an image of a sheet P on which the diagnostic image  900  has been formed, using the image capturing apparatus  102 . L 4  is the circumferential length of the fixing roller  22  or the pressure roller  21 .  FIG.  9 B  indicates that toner adhering to scratches during the fixing of the single-color pattern  601  results in image deficiencies  901 . Because the interval between the positions where the image deficiencies  901  occur corresponds to the circumferential length L 4 , the image capturing apparatus  102  can identify the fixer  17  as the causative component or causative member of the image deficiencies  901 . 
     There are also cases where a plurality of consumable parts are used beyond their design lifespan. In this case, the control unit  201  may identify the most deteriorated consumable and select the diagnostic images corresponding to the identified consumable. Alternatively, a plurality of diagnostic images, respectively corresponding to a plurality of consumables that have been used beyond their design lifespan, may be selected in sequence. This enables more accurate diagnoses to be made for a plurality of consumables that have been used beyond their design lifespans. 
     In the foregoing descriptions, the image capturing apparatus  102  obtains the print direction information from the server  103 , but this is only one example. The storage unit  205  of the image capturing apparatus  102  may store the print direction data  406 . In this case, the control unit  201  can obtain the print direction information corresponding to the model information of the image forming apparatus  101  from the print direction data  406  in the storage unit  205 . Alternatively, the image capturing apparatus  102  may obtain the print direction information  418  from the image forming apparatus  101 . For example, the control unit  201  transmits, to the image forming apparatus  101 , a transmission request for the print direction information  418  along with the model information  416 . Upon receiving this request, the CPU  411  reads the model information  416  and the print direction information  418  from the storage unit  415  and transmits this information to the image capturing apparatus  102 . In this case, the image capturing apparatus  102  does not need to communicate with the server  103 . In other words, the image capturing apparatus  102  can omit the network environment for communicating with the server  103 . 
     In the foregoing descriptions, the image capturing apparatus  102  obtains the model information  416  from the image forming apparatus  101 , but this is only one example. For example, the image capturing apparatus  102  may obtain the product number, the model name, and the like, which are model information, from user inputs made through the interface unit  202 . In this case, the processing for obtaining the model information becomes unnecessary. Similarly, the control unit  201  may obtain the print direction information from user inputs through the interface unit  202 . In this case, the processing for obtaining the print direction information from the server  103  or the image forming apparatus  101  is also unnecessary. Accordingly, the user can shorten the time required from the start of the diagnosis to the end of the diagnosis. 
     In the foregoing descriptions, dedicated diagnostic images  600  and  900  that include test patterns are used, but this is only one example. The diagnostic image may be any image prepared by a user. Such a diagnostic image may also be marked with an identification pattern that makes it possible to identify the print direction and the page number.  FIG.  10 A  illustrates a given diagnostic image  1000  prepared by the user.  FIG.  10 B  illustrates a sheet image  208   e  obtained by capturing an image of the diagnostic image  1000  formed on the sheet P using the image capturing apparatus  102 . In this case, the control unit  201  compares the diagnostic image  1000  with the sheet image  208   e  and detects a difference. Because the difference corresponds to an image deficiency, the control unit  201  measures a distance L 5  from the diagnostic reference side in the sheet image  208   e  to the position of the difference, and identifies the component corresponding to the distance L 5 . Specific examples of the differences are a difference in image density, irregular toner adhesion to white areas (e.g., horizontal streaking), and the like. 
     Second Embodiment 
     In the first embodiment, the print direction of the image forming apparatus  101  is obtained from the server  103 , the image forming apparatus  101 , or the user. The second embodiment will describe a method of identifying the print direction of the image forming apparatus  101  using a direction determination image for determining the print direction. 
       FIG.  11 A  is a flowchart illustrating an image diagnosis method executed by the control unit  201 . The control unit  201  launches the diagnostic program  206  according to a launch instruction of the diagnostic program  206  input from the interface unit  202 . Note that in the second embodiment, processes that are the same as in the first embodiment will be given the same reference signs, and will not be described. 
     As illustrated in  FIG.  11 A , in step S 1101 , the control unit  201  transmits a printing instruction for the direction determination image to the image forming apparatus  101 .  FIG.  12 A  illustrates a direction determination image  1200 . The direction determination image  1200  is an image that includes an image pattern  1201  formed to be vertically asymmetrical with respect to the conveyance direction of the sheet P in the image forming apparatus  101 . The image data of the direction determination image  1200  is stored in the storage unit  205  and transmitted to the image forming apparatus  101  along with the printing instruction. 
     As illustrated in  FIG.  11 B , the CPU  411  of the image forming apparatus  101  receives the printing instruction and the image data of the direction determination image  1200  from the image capturing apparatus  102  in step S 1111 . In step S 1112 , the CPU  411  controls the image forming apparatus  101  to form the direction determination image  1200  on the sheet P in the standard print direction. As illustrated in  FIG.  12 B , the sheet P on which the direction determination image  1200  is formed is discharged in the discharge direction indicated by arrow  1211 . At this time, a user  1210  observes the discharge direction of the sheet P and the orientation of the direction determination image  1200 . 
     As illustrated in  FIG.  11 A , in step S 1102 , the control unit  201  obtains the print direction information of the image forming apparatus  101  based on a user input made through the interface unit  202 . As illustrated in  FIG.  12 C , the interface unit  202  displays a guidance message  1220  for the user and two operation buttons  1221   a  and  1221   b.  The user  1210  operates one of the operation buttons  1221   a  or  1221   b  according to the guidance message  1220 . In this example, the user  1210  presses the operation button  1221   a  when the discharge direction of the sheet P on which the direction determination image  1200  is formed is the same as the orientation of the direction determination image  1200 . The user  1210  presses the operation button  1221   b  when the discharge direction of the sheet P on which the direction determination image  1200  is formed is the opposite orientation from the orientation of the direction determination image  1200 . The control unit  201  identifies the standard print direction of the image forming apparatus  101  based on which of the operation buttons  1221   a  and  1221   b  is pressed in the interface unit  202 . When the operation button  1221   a  is pressed, the control unit  201  determines that the standard print direction is the forward direction. When the operation button  1221   b  is pressed, the control unit  201  determines that the standard print direction is the reverse direction. 
     Other Embodiments 
       FIG.  13    illustrates the functions realized by a CPU  1300  provided in the control unit  201  executing the diagnostic program  206 . An obtainment unit  1301  acquires the model information  416  or the print direction information  418  of the image forming apparatus  101 . The obtainment unit  1301  may acquire the model information  416  or the print direction information  418  from the image forming apparatus  101 . The obtainment unit  1301  may obtain the print direction information  418  from the server  103  by transmitting the model information  416  to the server  103 . Alternatively, the obtainment unit  1301  may obtain the model information  416  or the print direction information  418  in response to a user input made through the interface unit  202 . 
     An orientation setting unit  1302  sets the printing orientation of the diagnostic image  600  according to the print direction information  418 . As described above, the printing orientation is set so that the leading edge side of the sheet P matches the header side of the diagnostic image  600 . 
     A sheet number setting unit  1303  compares the sizes of a plurality of components and determines the number of sheets P on which the diagnostic image  600  is to be formed according to the maximum size. For example, the circumferential length L 3  of the intermediate transfer belt  8  is longer than the circumferential lengths of other rotating bodies, and the number of sheets P is therefore set based on the circumferential length L 3 . 
     An acquisition unit  1304  acquires the history data  417  from the image forming apparatus  101 . A selection unit  1305  identifies a component that has been used beyond its design lifespan or is about to reach its design lifespan based on the history data  417 . Furthermore, the selection unit  1305  selects, from among a plurality of diagnostic images, a diagnostic image suitable for sensing image deficiencies in which the component in question is the causative component. 
     An instruction unit  1306  transmits a printing instruction based on the orientation set by the orientation setting unit  1302 , the number of sheets P set by the sheet number setting unit  1303 , and the diagnostic image  600  selected by the selection unit  1305  to the image forming apparatus  101 . The instruction unit  1306  may instruct the image forming apparatus  101  to print the direction determination image  1200  before instructing the printing of the diagnostic image  600 . In this case, the obtainment unit  1301  may display the guidance message  1220  and the operation buttons  1221   a  and  1221   b  in the interface unit  202  and accept a user input of the print direction information  418 . 
     An OCR unit  1310  performs optical character recognition (OCR) on the sheet image, and obtains identification information such as the page number and the like. An order determination unit  1311  determines the print order of the plurality of sheet images based on the identification information of each sheet image. 
     A reference determination unit  1312  determines a reference position that serves as a reference for measuring the distance with respect to the position of the image deficiency. In the example illustrated in  FIG.  6 B , the leading edge of the single-color pattern  601  is sensed by edge detection or pattern matching, and the position of the leading edge that has been sensed is output to a measurement unit  1314  as the reference position. The reference position may be the position of a single image deficiency among a plurality of image deficiencies occurring at regular intervals. 
     A deficiency sensing unit  1313  senses an image deficiency from the sheet image and outputs position information indicating the position of the image deficiency to the measurement unit  1314 . If a plurality of sheet images are present, the deficiency sensing unit  1313  passes the position information of the image deficiency associated with the identification information (e.g., page number) of each sheet to the measurement unit  1314 . The measurement unit  1314  measures the distance between the reference position and the position where the image deficiency occurred and passes the distance to the cause identification unit  1315 . 
     A cause identification unit  1315  identifies the component causing the image deficiency based on the distance measured by the measurement unit  1314 . For example, the cause identification unit  1315  may identify the causative component by comparing the distance to the circumferential length of each component. A result notification unit  1316  generates a diagnosis result and outputs the diagnosis result to the interface unit  202 . The diagnosis result includes information on the component identified by the cause identification unit  1315 . 
     A program that realizes one or more of the functions described in the foregoing embodiments may be supplied to a computer system or a computer device through a network or storage medium. Each of the foregoing embodiments may be realized by one or more processors in a computer system or computer device executing the program. 
     Technical Spirit Derived from Embodiments 
     Aspects A 1 , A 28 , and A 29   
     As illustrated in  FIG.  1   , according to the foregoing embodiments, the diagnostic system  100  is provided. The camera  203  of the image capturing apparatus functions as an imaging unit that captures an image of a sheet on which a diagnostic image is formed and outputs a sheet image. The control unit  201  functions as a diagnosis unit that makes a diagnosis for a component of the image forming apparatus based on a position of an image deficiency in the sheet image. The interface unit  202  functions as an output unit that outputs a diagnosis result from the diagnosis unit. Accordingly, the image diagnosis can also be made for an image forming apparatus that does not include an image sensor. However, the image capturing apparatus  102  may make an image diagnosis for an image forming apparatus including an image sensor in its conveyance path or an image forming apparatus including an image scanner. 
     Aspect A 2   
     The control unit  201 , the communication unit  204 , and the interface unit  202  function as an obtainment unit that obtains direction information pertaining to the direction in which the image forming apparatus forms the diagnostic image with respect to the sheet. The control unit  201  makes the diagnosis for the component of the image forming apparatus using the direction information and the position of the image deficiency in the sheet image. For example, the control unit  201  may set the printing orientation of the diagnostic image with respect to the conveyance direction of the sheet according to the direction information. Ensuring the conveyance direction of the sheet matches the printing orientation of the diagnostic image makes diagnosis results using sheet images more accurate. 
     Aspect A 3   
     The control unit  201  may determine a reference position serving as a reference for the diagnosis based on the direction information. In  FIG.  6 B , the leading edge of the single-color pattern  601  in the conveyance direction of the sheet P is set as the reference position. According to  FIGS.  6 C,  8 , and  9 B , the position of one of a plurality of image deficiencies is set as the reference position. The control unit  201  may identify the component, among the plurality of components constituting the image forming apparatus, that is the cause of the image deficiency, based on the distance from the reference position to the position of the image deficiency. 
     The control unit  201  may determine a reference side, serving as a reference from the diagnosis, from among the four sides constituting the sheet image, based on the direction information. The control unit  201  may identify the component, among the plurality of components constituting the image forming apparatus, that is the cause of the image deficiency, based on the distance from the reference side to the position of the image deficiency. The reference side is the side on the leading edge side in the conveyance direction of the sheet in the image forming apparatus, among the four sides constituting the sheet image. 
     Aspects A 4  and A 5   
     The control unit  201  may set the print direction of the diagnostic image in the image forming apparatus based on the direction information, such that the orientation of the diagnostic image and the conveyance direction of the sheet match. The control unit  201  may set the print direction based on the direction information, such that the orientation of the diagnostic image and the discharge direction of the sheet from the image forming apparatus match. The control unit  201  may cause the image forming apparatus to form the diagnostic image by transmitting, to the image forming apparatus, designation information designating the print direction and image data for forming the diagnostic image. Here, the orientation of the diagnostic image and the conveyance direction of the sheet “matching” means that the header side of the diagnostic image matches the leading edge side of the sheet in the conveyance direction of the sheet. As illustrated in  FIG.  6 A , the afterimage  603  of the single-color pattern  601  arises downstream from the single-color pattern  601  in the conveyance direction of the sheet P. As such, to sense this type of image deficiency, it is, as a rule, necessary for the header side of the diagnostic image to match the leading edge side of the sheet in the conveyance direction of the sheet. 
     Aspects A 6  to All 
     The control unit  201  may obtain the direction information from the image forming apparatus. The communication unit  204  may function as a communication unit that communicates with the server  103 . The control unit  201  may obtain the direction information from the server  103 . In this case, the control unit  201  may obtain the direction information from the server  103  by transmitting identification information of the image forming apparatus to the server  103 . The input device of the interface unit  202  functions as an input unit that accepts user inputs. The control unit  201  may obtain the direction information through the input unit. The control unit  201  functions as an instruction unit that instructs the image forming apparatus to form the direction determination image on the sheet. In this case, the control unit  201  may obtain the direction information based on information input by a user after the direction determination image is formed on the sheet. As illustrated in  FIG.  12 C , a user input may be present pertaining to whether the orientation of the direction determination image matches the discharge direction, from the image forming apparatus, of the sheet on which the direction determination image is formed. The control unit  201  may obtain the direction information through such user input. 
     Aspects A 12  to A 16   
     The control unit  201  functions as a setting unit that sets, in the image forming apparatus, a total number of sheets on which the diagnostic image is formed, according to a size of the component constituting the image forming apparatus. As illustrated in  FIG.  8   , the control unit  201  may make the diagnosis based on a total number of sheet images according to the total number of sheets. Through this, the causative component can be identified from image deficiencies occurring at intervals longer than the length of a single sheet. Accordingly, the total number of the sheets may be set based on a maximum circumferential length among circumferential lengths of a plurality of rotating bodies constituting the image forming apparatus. The total number of sheets may be set based on a length that is at least one times or two times the maximum circumferential length. When the total number of sheets is at least two, the image forming apparatus may form, on each of the sheets, the diagnostic image and identification information (e.g., a page number) of each of the sheets. The control unit  201  may identify the print order and the print direction of the plurality of sheet images based on the position of the identification information of the sheet in the sheet image. The control unit  201  may measure the interval at which an image deficiency occurs across the plurality of sheet images based on the print order and the print direction, and based on the interval, identify a component, among the plurality of components constituting the image forming apparatus, that is a cause of the image deficiency. As illustrated in  FIG.  8   , when the page number is formed on the header side, the control unit  201  can determine the side of the sheet image closest to the page number as the leading edge side of the sheet image. Furthermore, the control unit  201  can obtain the page number through optical character recognition (OCR) processing, and can therefore identify the print order of the plurality of sheet images. 
     Aspects A 17  to A 19   
     The control unit  201  and the communication unit  204  function as an acquisition unit that acquires state information indicating a usage history of the image forming apparatus or a wear state of the component. Furthermore, the control unit  201  functions as a selection unit that selects the diagnostic image corresponding to state information from among the plurality of diagnostic images. The image forming apparatus forms, on the sheet, the diagnostic image selected by the selection unit. The control unit  201  may identify a component, among the plurality of components constituting the image forming apparatus, that has reached a maintenance period specified by design, based on the state information. The control unit  201  may select a diagnostic image corresponding to the component that has reached the maintenance period specified by design. The control unit  201  may identify a component, among the plurality of components constituting the image forming apparatus, that is approaching a maintenance period specified by design, based on the state information. The control unit  201  may select a diagnostic image corresponding to the component that is approaching the maintenance period specified by design. Image deficiencies will be sensed more accurately as a result. 
     Aspects A 20  and A 21   
     The component may be a rotating body, or may be a plate-shaped member such as the cleaner  4 . The control unit  201  may identify the rotating body that is the cause of the image deficiency based on a distance correlated with the rotation cycle of the rotating body and a distance obtained from the position of the image deficiency. Note that when the cleaner  4  wears down, streaks extending parallel to the conveyance direction of the sheet P may appear. 
     Aspects A 22  to A 25   
     The diagnostic image may be any image prepared by a user. The diagnostic image may include a first pattern formed from toner of a single color and a second pattern formed by mixing a plurality of different colors. In the image forming apparatus, the first pattern (e.g., the single-color pattern  601 ) is formed before the second pattern (the halftone pattern  602 ). As illustrated in  FIGS.  6 A and  9 A , the diagnostic image may include a single-color pattern region containing a pattern of a single color, and a white region or a mixed-color pattern region formed nearer to a footer side of the diagnostic image than the single-color pattern region. In this case, the pattern region is formed before the white region or the mixed-color pattern region. In other words, toner from the pattern region adheres to the surface of the component, and the adhering toner is then transferred again from the pattern region to the white region or the mixed-color pattern region. The image deficiency may be made apparent in this manner. 
     Aspect A 26   
     The diagnosis result may include information indicating the component that is the cause of the image deficiency. Through this, the user will be able to understand which component requires maintenance (cleaning, repair, or replacement). 
     Aspect A 27   
     The image capturing apparatus includes a digital camera or a portable communication device (e.g., a smartphone, a tablet terminal) equipped with a camera. These have become commonplace, and will enable the user to make a diagnosis with ease. 
     The image capturing apparatus  102  is an example of an image capturing apparatus that captures an image of the sheet on which the diagnostic image is formed and which has been discharged from the image forming apparatus, and outputs the image captured as a sheet image. The server  103  may function as a diagnostic apparatus that makes a diagnosis for a component of the image forming apparatus based on a position of an image deficiency in the sheet image obtained by the image capturing apparatus, and outputs a diagnosis result. In this case, the functions of the control unit  201  illustrated in  FIG.  13    are provided by the control unit  401  of the server  103 . Furthermore, another information processing apparatus may function as the diagnostic apparatus instead of the image capturing apparatus  102  or the server  103 . 
     Third Embodiment 
     When components of an image forming apparatus reach the end of their useful life, image deficiencies may occur. Japanese Patent No. 5164458 describes identifying components that cause image deficiencies by reading an image on a sheet using an image sensor built into the image forming apparatus. Japanese Patent No. 6350474 describes capturing an image on a sheet using an image capturing apparatus such as a digital camera or a camera-equipped cell phone, and calibrating the image forming apparatus based on a result of the capture. “Calibration” in Japanese Patent No. 6350474 refers to updating a gamma correction table that corrects image tones. 
     According to the invention of Japanese Patent No. 5164458, an image forming apparatus lacking an image sensor cannot identify the components that cause image deficiencies. In this case, a user must purchase an image forming apparatus equipped with an image sensor. The invention of Japanese Patent No. 6350474 cannot detect image deficiencies in the first place, nor can it identify components that cause image deficiencies. 
     Diagnostic System 
     The diagnostic system  100  is as described with reference to  FIG.  1   . 
     Image Capturing Apparatus 
       FIG.  14    illustrates the configuration of the image capturing apparatus  102 . A control unit  201  controls an interface unit  202 , a camera  203 , a communication unit  204 , and a storage unit  205  according to a control program stored in the storage unit  205 . The control unit  201  includes hardware circuitry such as a central processing unit (CPU), an application-specific integrated circuit (ASIC), and a field programmable gate array (FPGA). The interface unit  202  includes an output device (e.g., a display device, an audio output device) that outputs information to the user and an input device that accepts user inputs (e.g., a touch panel sensor). The camera  203  includes an image sensor (e.g., a CMOS image sensor or a CCD image sensor), a light source that emits illumination light, and the like. The communication unit  204  includes the wireless communication circuitry and the wired communication circuitry mentioned above. The storage unit  205  includes random access memory (RAM), read-only memory (ROM), and the like. The storage unit  205  stores a control program (e.g., the diagnostic program  206 ) executed by the control unit  201  and control data (e.g., the diagnostic image data  207 ) in a ROM region. The diagnostic image data  207  is original image data of a diagnostic image formed on a sheet. The storage unit  205  stores the sheet image  208  obtained by the camera  203 , the diagnostic result  209 , a case collection  210 , and the like in a RAM region. The case collection  210  is a data group associating the characteristics of image deficiencies with causative components. The control unit  201  identifies the causative component by referring to the case collection  210  based on image deficiency characteristics that have been sensed. 
     Image Forming Apparatus 
     Although  FIG.  15    illustrates an electrophotographic image forming apparatus  101 , the technical spirit of the present embodiment can be applied in the same manner to any image forming apparatus in which components such as a rotating body are used for the formation of images. For example, the technical spirit of the present embodiment is applicable to any image forming apparatus in which image deficiencies occur in an image formed on a sheet P due to the components having reached the end of their useful life, components malfunctioning, and the like. The technical spirit of the present embodiment is also applicable in any image forming apparatus in which image deficiencies occur when components which require regular maintenance (cleaning, tuning, and replacement) have not been given such regular maintenance. 
     In  FIG.  15   , the letters Y, M, C, and K appended to the ends of the reference signs indicate toner colors, i.e., yellow, magenta, cyan, and black. For example, components with a Y appended to the end of the reference signs are involved in the formation of a yellow toner image. When there is no need to distinguish among the colors in describing the components, reference signs without the appended letters are used. 
     The control unit  40  is a control circuit (e.g., a CPU, an ASIC, and an FPGA) that controls the various parts of the image forming apparatus  101 . The control unit  40  receives image data and printing instructions from an external device (e.g., the image capturing apparatus  102 ) through the communication unit  41 . The control unit  40  converts the image data to generate an image signal, and supplies the image signal to the exposure device  7 . 
     The photosensitive member  1  is an image carrier that is driven by a drive source such as a motor and rotates clockwise, and carries an electrostatic latent image and a toner image. The photosensitive member  1  is sometimes called a photosensitive drum due to being a cylindrical rotating body. The charging roller  2  charges the surface of the photosensitive member  1  to a uniform potential by a charging bias voltage being applied by the control unit  40 . The exposure device  7  forms an electrostatic latent image on the surface (circumferential surface) of the photosensitive member  1  by irradiating the surface of the photosensitive member  1  with laser light corresponding to the image signal. The developing roller  3  is applied with a developing bias voltage by the control unit  40 , and forms a toner image on the surface of the photosensitive member  1  by causing toner to adhere to the electrostatic latent image. The primary transfer roller  6  has a primary transfer bias voltage applied by the control unit  40 , and transfers the toner image from the photosensitive member  1  to the intermediate transfer belt  8 . A drum cleaner  4  is a member that removes and collects toner that has not been transferred to the intermediate transfer belt  8  and remains on the photosensitive member  1 . The photosensitive member  1 , the developing roller  3 , the charging roller  2 , and the drum cleaner  4  may be integrated within a cartridge. Such a cartridge is configured to be removable from the main body of the image forming apparatus  101 . The photosensitive member  1 , the charging roller  2 , the exposure device  7 , the developing roller  3 , and the primary transfer roller  6  function as an image forming unit that forms an image on the intermediate transfer belt  8 . 
     The intermediate transfer belt  8  is an endless belt, and is sometimes referred to as an intermediate transfer body. The intermediate transfer belt  8  is driven by a drive source such as a motor, and rotates counterclockwise. Toner images from each of the four photosensitive members  1  are superimposed and transferred onto the intermediate transfer belt  8 , and a full-color toner image is formed on the intermediate transfer belt  8  as a result. The toner image transferred onto the intermediate transfer belt  8  is conveyed to a secondary transfer section. The secondary transfer section is a nip section formed by the intermediate transfer belt  8  and the secondary transfer roller  11 . 
     The image forming apparatus  101  has an upper cassette  13   a  and a lower cassette  13   b,  which are feed trays for feeding sheets. In  FIG.  15   , an “a” at the end of a reference sign indicates that the item relates to the upper cassette  13   a.  A “b” at the end of a reference sign indicates that the item relates to the lower cassette  13   b.  When items common to the upper cassette  13   a  and the lower cassette  13   b  are described, the “a” and “b” at the end of the reference signs are omitted. The cassette  13  is a holding unit that holds a large number of sheets P. A feed roller  14  feeds the sheet P from the cassette  13  to the conveyance path  15  according to instructions from the control unit  40 . The sheet P is conveyed to the secondary transfer section by conveyance rollers  16  and  18  provided along the conveyance path  15 . The conveyance roller  18  is sometimes referred to as a “registration roller”. A sheet sensor  23  may be provided downstream from the conveyance roller  18  in the conveyance direction of the sheet P. The sheet sensor  23  is a sensor that senses the presence or absence of the sheet P. The sheet sensor  23  can sense the arrival of the leading edge (the top edge) of the sheet P, and is therefore sometimes called a “top sensor”. 
     To simplify the descriptions, it is assumed that the upper cassette  13   a  holds A4-size sheets Pa. It is assumed that the lower cassette  13   b  holds B5-size sheets Pb. The long sides of the A4-size sheets Pa and the B5-size sheets Pb are each parallel to the conveyance direction. In other words, the short sides of the A4-size sheets Pa and the short sides of the B5-size sheets Pb are each orthogonal to the conveyance direction. 
     The secondary transfer roller  11  has a secondary transfer bias voltage applied by the control unit  40 , and transfers the toner image from the intermediate transfer belt  8  onto the sheet P. A belt cleaner  9  removes and collects toner that is not transferred to the sheet P and remains on the intermediate transfer belt  8 . The secondary transfer roller  11  conveys the sheet P to the fixer  17 . The fixer  17  includes two rotating bodies (the fixing roller  22  and the pressure roller  21 ), and fixes the toner image onto the sheet P by applying heat and pressure to the sheet P and the toner image. As the fixing roller  22  and the pressure roller  21  rotate, the sheet P is conveyed to a discharge roller  20 . The discharge roller  20  discharges the sheet P to the exterior of the image forming apparatus  101 . 
     Feed Trays 
       FIG.  16    is a plan view of the upper cassette  13   a  and the lower cassette  13   b.  The upper cassette  13   a  and lower cassette  13   b  share the same configuration, and thus the “a” and “b” at the end of the reference signs codes are omitted in  FIG.  16   . 
     Arrow h indicates the conveyance direction of the sheet P by the feed roller  14 . Regulation plates  131  and  132  are members that regulate the position of the sheet P in the width direction. In  FIG.  16   , the width direction of the sheet P is the direction orthogonal to the conveyance direction of the sheet P. In  FIG.  16   , the long side of the sheet P is parallel to the conveyance direction of the sheet P. The short side of the sheet P is orthogonal to the conveyance direction of the sheet P. The regulation plate  131  is movable in the direction indicated by arrow i. The regulation plate  132  is movable in the direction indicated by arrow j. By moving in tandem, the regulation plates  131  and  132  can center the sheet P with respect to the conveyance path  15 . A regulation plate  133  is movable in the direction indicated by arrow k, and regulates the position of an end part of the sheet P in the conveyance direction of the sheet P. By moving the regulation plates  131 ,  132 , and  133 , the upper cassette  13   a  and the lower cassette  13   b  can each accommodate sheets P from A6 size to A4 size. However, all sheets P from A6 size to A4 size are assumed to be placed vertically, as illustrated in  FIG.  16   . “Placed vertically” means that the sheet P is placed so that the long side thereof is parallel to the conveyance direction. 
     Control Unit of Server 
     As illustrated in  FIG.  17 A , the server  103  includes a control unit  1701 , an interface unit  1702 , a communication unit  1704 , and a storage unit  1705 . The control unit  1701  includes a CPU and the like that execute a control program stored in the storage unit  1705 . The control program may be the diagnostic program  206 . This enables the server  103  to function as a diagnostic apparatus instead of the image capturing apparatus  102 . However, even when the server  103  functions as the diagnostic apparatus, the sheet image is generated by the image capturing apparatus  102 . The interface unit  1702  includes an input device and a display device. The interface unit  1702  may display the diagnosis result generated in the image capturing apparatus  102  or the server  103 . The communication unit  1704  is communication circuitry that communicates with the image capturing apparatus  102  and the image forming apparatus  101  over the network. The storage unit  1705  includes ROM, RAM, a solid state drive (SSD), and a hard disk drive (HDD). 
     Control Unit of Image Forming Apparatus 
       FIG.  17 B  illustrates the control unit  40  of the image forming apparatus  101 . A CPU  1711  controls each unit of the image forming apparatus  101  by executing a control program stored in a storage unit  1715 . For example, upon receiving a printing instruction for the diagnostic image from the image capturing apparatus  102 , the CPU  1711  controls the image forming apparatus  101  to form the diagnostic image on the sheet P. At this time, the CPU  1711  controls a power supply circuit  1721  to generate the charging bias voltage, the developing bias voltage, and the transfer bias voltages. Additionally, the CPU  1711  rotates the various types of rotating bodies, such as the photosensitive member  1 , by driving a motor  1722 . An interface unit  1712  includes an input device and a display device. The interface unit  1712  may display the diagnosis result generated by the image capturing apparatus  102  or the server  103 . Alternatively, when CPU  1711  functions as a diagnostic apparatus by executing a diagnostic program, the CPU  1711  acquires the sheet image from the image capturing apparatus  102  through the communication unit  41 , and executes image distortion correction and image diagnosis. The storage unit  1715  includes RAM, ROM, an SSD, an HDD, and the like. The storage unit  1715  stores model information  1716 , which is identification information of the image forming apparatus  101 . The CPU  1711  may store information indicating a usage history of the image forming apparatus  101  (e.g., a cumulative number of images formed) or the replacement time of each component in the storage unit  1715  as history data  1717 . The storage unit  1715  stores size information  1718  indicating a size input or selected by the user through the interface unit  1712  or the size of the sheet P detected by the sheet sensor  23  or size sensors  1723   a  and  1723   b.  The first size sensor  1723   a  and the second size sensor  1723   b  are optional. The first size sensor  1723   a  detects the size of the sheet P (e.g., the length of the long side and the length of the short side) based on the positions of the regulation plates  131 ,  132 , and  133 . The second size sensor  1723   b  may be a line sensor installed in the conveyance path  15 . The line sensor has a plurality of light-receiving elements provided parallel to the width direction of the conveyance path  15  (orthogonal to the conveyance direction), and a light source (a light-emitting element). The light from the light source to the plurality of light-receiving elements is blocked by the sheet P. Accordingly, the size sensor  1723   b  can sense the length of the sheet P in the shorter direction from the number of light-receiving elements that could not receive light among the plurality of light-receiving elements. Furthermore, the size sensor  1723   b  can sense the length of the long side of the sheet P based on the time for which the light is blocked by the sheet P and the conveyance speed of the sheet P (e.g., 180 mm/sec). 
     Flowcharts 
     Processing of Diagnostic Apparatus 
       FIG.  18 A  illustrates an image diagnosis method executed by a CPU provided in the control unit  201  of the image capturing apparatus  102 , according to the diagnostic program  206 . When the control unit  201  is instructed to launch the diagnostic program  206  through the interface unit  202 , the control unit  201  launches the diagnostic program  206  and executes the following processing. 
     In step S 1801 , the control unit  201  obtains the model information  1716  and the size information  1718  of the image forming apparatus  101 . For example, the control unit  201  connects to the image forming apparatus  101  through the communication unit  204  and transmits a request to the image forming apparatus  101 . As a result, the control unit  201  receives the model information  1716  and the size information  1718  through the communication unit  204 . The model information  1716  includes a product number, a model name, and the like of the image forming apparatus  101 . The size information  1718  may include the size of the sheets Pa in the upper cassette  13   a  and the size of sheets Pb in the lower cassette  13   b.    
     In step S 1802 , the control unit  201  selects a diagnostic image and a cassette. For example, the control unit  201  selects the diagnostic image data  207  associated with the model information  1716  from the plurality of diagnostic image data  207  stored in the storage unit  205 . This association may be held in the case collection  210 . The control unit  201  may also select the diagnostic image data  207  based on the history data  1717  obtained from the image forming apparatus  101 . For example, the control unit  201  identifies a component, among the components of the image forming apparatus  101 , that is nearing or has exceeded its use-by date by design, based on the history data  1717 . In addition, the control unit  201  selects diagnostic image data  207  for a diagnostic image that makes it easy to sense image deficiencies that may occur in the identified component. The control unit  201  selects the upper cassette  13   a  or the lower cassette  13   b  based on the size information  1718 . In general, a larger sheet P makes it possible to sense image deficiencies over a broader range in the width direction of the sheet P than a smaller sheet P. Accordingly, the control unit  201  may select the upper cassette  13   a  based on the size information  1718 . 
       FIG.  19 A  illustrates an example of a diagnostic image  1900  generated based on the diagnostic image data  207 . The diagnostic image  1900  includes a single-color pattern  1901  and a halftone pattern  1902 . The single-color pattern  1901  includes a test image formed from one color among Y, M, C, and K. The halftone pattern  1902  includes a gradation image formed by mixing all of the Y, M, C, and K colors. 
     In step S 1803 , the control unit  201  transmits, to the image forming apparatus  101 , a printing instruction to print the diagnostic image  1900 . The printing instruction includes the diagnostic image data  207  that is the source of the diagnostic image  1900 , and the designation information of the cassette  13 . 
     In step S 1804 , the control unit  201  controls the camera  203  to capture the diagnostic image  1900  formed on the sheet P by the image forming apparatus  101  and generate a sheet image. For example, the control unit  201  may display a guidance message prompting the user to capture the diagnostic image  1900  in the display device of the interface unit  202 . The user then operates the input device of the interface unit  202  in response to the guidance and captures the diagnostic image  1900  on the sheet P. As a result, the sheet image  208  is generated and saved in a RAM region of the storage unit  205 . 
       FIG.  19 B  illustrates a sheet image  208   a  showing drum ghosting. The single-color pattern  1901  is an image pattern useful for sensing an image deficiency known as “drum ghosting”. “Drum ghosting” is an image deficiency that can occur due to deterioration of the photosensitive member  1 . As illustrated in  FIG.  19 B , the single-color pattern  1901  formed first appears as an afterimage  1903 . Here, a distance L 1  between the single-color pattern  1901  and the afterimage  1903  corresponds to the circumferential length of the photosensitive member  1 . The density of the afterimage  1903  differs from the density of the regular halftone pattern  1902 . The control unit  201  can determine the presence or absence of drum ghosting by comparing the image density at a position distanced from the single-color pattern  1901  by the distance L 1  with a reference density (the density of the halftone pattern  1902 ). Determination rules for such drum ghosting are included in the case collection  210 . 
     The halftone pattern  1902  is an image pattern mainly used to sense the occurrence of defects caused by the transport of the sheet P. Due to wear from years of use, drive gears, conveyance rollers, and the like may deteriorate or break. Image deficiencies  1904   a  to  1904   c  occur in the halftone pattern  1902  of a sheet image  208   b  illustrated in  FIG.  19 C . An interval L 2  between the image deficiencies  1904   a  to  1904   c  is equivalent to the circumferential length of the drive gear or conveyance roller, and thus the control unit  201  can identify the drive gear or conveyance roller as defective. Such determination rules are also included in the case collection  210 . 
       FIG.  19 D  illustrates an image deficiency  1905  (a line in the vertical direction) that can occur when foreign objects accumulate in the conveyance path  15 . In this example, the foreign objects causing the image deficiency accumulate at the position of a distance L 3  away from the center of the conveyance path  15  in the width direction. 
     In step S 1805 , the control unit  201  corrects the sheet image  208  based on the size information  1718 . Because the user holds the image capturing apparatus  102  by hand to capture the diagnostic image  1900  on the sheet P, the sheet P and the diagnostic image  1900  appearing in the generated sheet image  208  may be rotated, distorted, or the like. Therefore, it is necessary to reduce the distortion of the sheet P and the diagnostic image  1900  in the sheet image  208 . 
       FIG.  20 A  illustrates the sheet image  208 . A shooting frame  2000  is a frame that shows the shooting field of view of the camera  203 . An image region  2001  indicates the region in the sheet image  208  where the sheet P on which the diagnostic image  1900  is formed appears. In this example, the sheet P is rotated with respect to the shooting frame  2000 . Furthermore, if the imaging plane (image sensor) of the camera  203  and the sheet P are not parallel, the image of the sheet P will be distorted. In this manner, measuring the position of the image deficiency in a distorted sheet image  208  results in a large error in the measurement result. Accordingly, the control unit  201  corrects distortion in the sheet image  208 . 
     First, the control unit  201  measures the lengths of the four sides of the image region  2001  in which the sheet P appears in the sheet image  208 . The control unit  201  measures a length U 0  of one long side, a length V 0  of the other long side, a length W 0  of one short side, and a length X 0  of the other short side by counting the number of pixels corresponding to the four sides of the image region  2001 . The control unit  201  may use a measurement function provided in the camera  203  to measure the lengths of the four sides. As an example, it is assumed that U 0 =260 mm, V 0 =320 mm, W 0 =200 mm, and X 0 =220 mm. 
       FIG.  20 B  illustrates an image region  2002  obtained by correcting the image region  2001 . The image region  2002  is rotationally corrected after the dimensions of each side are corrected. The rotation correction may be performed prior to correcting the dimensions of each side. As  FIG.  20 A  shows, the original image region  2001  is tilted at tilt  60  with respect to the shooting frame  2000 . Internal angles α 0 , β 0 , γ 0 , and δ 0  of the image region  2001  are also not 90 degrees. Accordingly, image correction is performed such that the tilt ε 0 =0° and the internal angles α 0 , β 0 , γ 0 , and δ 0  are α 1 , β 1 , γ 1 , and δ 1 =90°. 
     Furthermore, the control unit  201  recognizes that the original size of the image region  2001  is the A 4  size, based on the size information  1718 . Accordingly, the control unit  201  corrects the image region  2001 , in which the long sides U 0 =260 mm and V 0 =320 mm, to the image region  2002 , in which U 1  and V 1 =297 mm. Furthermore, the control unit  201  corrects the image region  2001 , in which W 0 =200 mm and V 0 =220 mm, to the image region  2002 , in which W 1  and V 1 =210 mm. As a result, the sheet image  208 , in which image deformation, rotation, and this like have been corrected, is generated. 
     In step S 1806 , the control unit  201  analyzes the sheet image  208  and senses an image deficiency. For example, the control unit  201  determines whether the afterimage  1903  occurs at a position distanced from the single-color pattern  1901  by the distance L 1 . Alternatively, the control unit  201  may sense the afterimage  1903 , identify the position where the afterimage  1903  occurs, and determine whether the distance between the single-color pattern  1901  and the afterimage  1903  is the distance L 1 . Alternatively, the control unit  201  may sense a plurality of the image deficiencies  1904   a  to  1904   c  that occur periodically and measure the interval L 2  between each of the plurality of the image deficiencies  1904   a  to  1904   c.  The distance L 1  and the interval L 2  are values that correlate to the rotation cycle (circumferential length) of the rotating body that causes the image deficiency, and the rotating body that causes the image deficiency can therefore be identified. 
     Incidentally, the distance L 1 , the interval L 2 , and the like may be measured based on the lengths U 1  and V 1  of the long sides. For example, the control unit  201  may calculate the actual lengths of the distance L 1  and the interval L 2  by measuring the ratio of the distance L 1  and the interval L 2  to the length U 1  (=V 1 ) and multiplying the ratio by the length U 1 . 
     Similarly, the distance L 3  for the vertical stripe-type image deficiency  1905  illustrated in  FIG.  19 D  is measured with respect to the short side lengths W 1  and X 1 . For example, the control unit  201  may calculate the actual length of the distance L 3  by measuring the ratio of the distance L 3  to the length W 1  (=X 1 ) and multiplying the ratio by the length W 1 . From the size information  1718 , the control unit  201  has ascertained that U 1  and V 1 =297 mm, and that W 1  and X 1 =210 mm. Accordingly, the control unit  201  can accurately measure L 1 , L 2 , and L 3 . 
     In step S 1807 , the control unit  201  generates a diagnosis result. The control unit  201  identifies the component (a causative component) constituting the image forming apparatus  101  based on the position of the image deficiency. The control unit  201  generates the diagnosis result, which indicates the installation location of the causative component within the image forming apparatus  101 , the wear state of the causative component, the replacement time of the causative component, an ordering method of the causative component, and the like. The diagnosis result may include state information for each of the plurality of components, indicating whether the state is a normal state or a state in which maintenance is required. The diagnosis result may include information indicating measures for reducing image deficiencies (e.g., replacement, repair, cleaning, or the like). If no image deficiencies are sensed in step S 1806 , the diagnosis result includes information indicating that all components constituting the image forming apparatus  101  are operating normally. 
     In step S 1808 , the control unit  201  displays the diagnosis result in the display device of the interface unit  202 . The control unit  201  may notify an administrator or maintenance worker of the diagnosis result by outputting (transmitting) the diagnosis result to the server  103 , the image forming apparatus  101 , or a personal computer through the communication unit  204 . 
     Processing by Image Forming Apparatus 
       FIG.  18 B  illustrates a method for providing the model information  1716  and the size information  1718 , executed by the CPU  1711  of the image forming apparatus  101 . In step S 1811 , the CPU  1711  receives a request (a transmission request) for the model information  1716  ad the size information  1718  from the image capturing apparatus  102 . In step S 1812 , the CPU  1711  reads out the model information  1716  and the size information  1718  from the storage unit  1715 , and transmits the model information  1716  and the size information  1718  to the image capturing apparatus  102  through the communication unit  41 . 
     In step S 1813 , the CPU  1711  receives a printing instruction for the diagnostic image  1900  from the image capturing apparatus  102  through the communication unit  41 . The printing instruction includes designation information for the cassette  13 . 
     In step S 1814 , the CPU  1711  forms the diagnostic image  1900  on the sheet P according to the printing instruction. For example, the CPU  1711  drives the feed roller  14   a  to feed the sheet Pa from the upper cassette  13   a  designated by the printing instruction. Furthermore, the CPU  1711  controls the exposure device  7  based on the diagnostic image data  207  received from the image capturing apparatus  102 . The diagnostic image  1900  is formed on the sheet Pa as a result. 
     The storage unit  205  stores the case collection  210  (cause identifying information) of image deficiencies that are expected to occur for each combination of the model information  1716  and diagnostic images. For example, if the afterimage  1903  occurs with the combination of the image forming apparatus  101  and the diagnostic image  1900 , and the distance L 1  is equal to the circumferential length of the photosensitive member  1 , the case collection  210  indicating that deterioration of the photosensitive member  1  is the cause may be stored. In this manner, the case collection  210  may include a combination of the identification information of the image forming apparatus  101  and the identification information of the diagnostic image, characteristic information indicating the characteristics of the image deficiency, and identification information of the component that is the cause of the image deficiency. The control unit  201  selects the diagnostic image data  207  by referring to the case collection  210  stored in the storage unit  205 , senses an image deficiency based on the characteristics of the image deficiency, and identifies the causative component based on the position of the image deficiency. The case collection  210  may include countermeasure information for reducing image deficiencies. By referring to the case collection  210 , the control unit  201  identifies a method for reducing image deficiencies, and includes the method in the diagnostic result  209 . 
       FIG.  21    is a table illustrating effects of the third embodiment. Here, the interval L 2  has been measured for a horizontal stripe-type image deficiency  704 , illustrated in  FIG.  19 C . Comparative Example  1  measures the interval L 2  based on the length U 0  of the long side in the image region  2001  illustrated in  FIG.  20 A , in which the correction processing of the third embodiment is not applied. Comparative Example 2 measures the interval L 2  based on the length V 0  of the long side in the image region  2001  illustrated in  FIG.  20 A , in which the correction processing of the third embodiment is not applied. Here, the circumferential length of the photosensitive member  1  is assumed to be 75.4 mm. The circumferential length of the pressure roller  21  is assumed to be 72.2 mm. The circumferential length of the fixing roller  22  is assumed to be 78.5 mm. 
     According to the third embodiment, the interval L 2  is measured as 75.4 mm based on the lengths U 1  and V 1  of the long sides being 297 mm, based on the size information  1718 . Accordingly, the control unit  201  correctly determines that the photosensitive member  1  is the causative component of the image deficiency  704 . On the other hand, in Comparative Example 1, the interval L 2  is measured as 72.4 mm, based on the length U 0  before correction. As such, it is erroneously determined that the causative component is the pressure roller  21 . In Comparative Example 2, the interval L 2  is measured as 77.4 mm, based on the length V 0  before correction. As such, it is erroneously determined that the causative component is the fixing roller  22 . In this manner, it is important to obtain a reference length when diagnosing a plurality of rotating bodies, each of which has a similar outer diameter. 
     In the third embodiment, it is assumed that a plurality of diagnostic images are stored in the storage unit  205  of the image capturing apparatus  102 , but this is only one example. The diagnostic image data  207  of the diagnostic image may be stored in the storage unit  1705  of the server  103 . In this case, the control unit  201  may transmit the model information of the image forming apparatus  101  to the server  103 , and the server  103  may read out the diagnostic image data  207  corresponding to the model information from the storage unit  1705  and transfer that data to the image capturing apparatus  102 . This increases the free space in storage unit  205 . Similarly, the case collection  210  for the image deficiencies may also be stored in the storage unit  1705  of the server  103 . The control unit  201  of the image capturing apparatus  102  transmits the model information and the diagnostic image identification information to the server  103 , and the control unit  1701  of the server  103  transmits the corresponding case collection  210  to the image capturing apparatus  102 . The control unit  201  of the image capturing apparatus  102  may use the received case collection  210  to measure the characteristics of the image deficiency (the distance L 1 , the interval L 2 , and the like) and identify the causative component. 
     The server  103  may function as the diagnostic apparatus. In this case, the server  103  obtains the size information  1718  and the sheet image  208  through the image capturing apparatus  102 . The control unit  1701  then identifies the causative component and generates a diagnosis result by executing processing similar to that executed by the control unit  201  described above. In this case, the diagnostic program  206 , the diagnostic image data  207 , the sheet image  208 , and the diagnostic result  209  are stored in the storage unit  1705 . Note that the control unit  1701  of the server  103  may obtain the size information  1718  directly from the image forming apparatus  101 . This is because the control unit  1701  can communicate with the image forming apparatus  101  through the communication unit  1704 . 
       FIG.  19 A  illustrates a one-page diagnostic image  1900 , but this is only one example. As illustrated in  FIG.  22   , a diagnostic image  1900  constituted by a plurality of pages may be employed. This example indicates a relationship between the circumferential length L 4  of the intermediate transfer belt  8  and three sheets Pa 1 , Pa 2 , and Pa 3  on which the diagnostic image  1900  is formed. The diagnostic image  1900  is formed on the first sheet Pa 1 . Only the halftone pattern  1902  of the diagnostic image  1900  is formed on the second sheet Pa 2 . Only the halftone pattern  1902  of the diagnostic image  1900  is formed on the third sheet Pa 3 . Image deficiencies  2200   a  and  2200   b  caused by the intermediate transfer belt  8  occur periodically at intervals corresponding to the circumferential length L 4 . d represents the sheet interval between the preceding and following sheets as the sheets pass through the secondary transfer section. The control unit  201  generates the sheet image  208  from each of the three sheets Pa 1 , Pa 2 , and Pa 3 , applies the corrections described above, and measures an interval L 4 ′ of the image deficiencies  2200   a  and  2200   b.  The interval L 4 ′ is almost equal to the circumferential length L 4 , and thus the control unit  201  determines that the causative component is the intermediate transfer belt  8 . 
     According to the third embodiment, the diagnostic system  100  can diagnose components of the image forming apparatus  101  using the image capturing apparatus  102 , such as a smartphone. This enables the user to diagnose the image forming apparatus  101  more simply than before. Furthermore, a reading result of the diagnostic image  1900  formed on the sheet P (the sheet image  208 ) is corrected based on the size information  1718  of the sheet P. Accordingly, the measurement accuracy of the characteristics of image deficiencies is improved, and the accuracy of the diagnosis result is also improved. 
     Fourth Embodiment 
     In the third embodiment, the size sensor  1723   a  senses the size of the sheet P mainly by monitoring the positions of the regulation plates  131 ,  132 , and  133  in the upper cassette  13   a  and the lower cassette  13   b,  respectively. The size sensor  1723   a  may incorrectly sense the size of the sheet P if the regulation plates  131 ,  132 , and  133  are not positioned correctly. 
     Therefore, in the fourth embodiment, size information of the sheet P is obtained by the sheet sensor  23  provided in the conveyance path  15 , in addition to the size information obtained by the size sensor  1723   a.    
       FIG.  23 A  illustrates an image diagnosis method executed by a CPU provided in the control unit  201  of the image capturing apparatus  102 , according to the diagnostic program  206 . In  FIG.  23 A , processes identical or similar to those in  FIG.  18 A  are given the same reference signs, and the descriptions thereof will apply here as well. In the fourth embodiment, the size information  1718  obtained by the size sensor  1723   a  is denoted as “first size information”. The size information  1718  obtained by the sheet sensor  23  is denoted as “second size information”. In the fourth embodiment, the first size information is primarily used to select the cassette  13 , while the second size information is used to correct the sheet image. 
     When step S 1803  ends, the control unit  201  proceeds to step S 2301 . In step S 2301 , the control unit  201  obtains the second size information from the image forming apparatus  101  through the communication unit  204 . For example, a request signal for obtaining the second size information may be transmitted. The control unit  201  then moves to step S 1804 , and generates the sheet image  208 . In step S 2302 , the control unit  201  corrects the sheet image  208  based on the second size information. Step S 2302  is similar to step S 1805 . The first size information is used in step S 1805 , while the second size information is used in step S 2302 . Compared to the first size information, the second size information is more accurate. As such, the sheet image  208  is more accurately corrected, the measurement of the characteristics of the image deficiency is more accurate, and the diagnosis result is therefore more accurate as well. 
       FIG.  23 B  illustrates a method for providing the model information  1716 , the first size information, and the second size information, executed by the CPU  1711  of the image forming apparatus  101 . In  FIG.  23 A , processes identical or similar to those in  FIG.  18 A  are given the same reference signs, and the descriptions thereof will apply here as well. 
     As illustrated in  FIG.  23 B , step S 2311  is added as a new step between steps S 1813  and S 1814 . In addition, step S 2312  is added after step S 1814 . Step S 2312  may be added before step S 1814 . 
     In step S 2311 , the CPU  1711  obtains the size information  1718  (the second size information) of the sheet P using the sheet sensor  23 . When the sheet sensor  23  is a sensor that senses the presence or absence of sheet P, the CPU  1711  uses a timer or counter to measure the time over which the sheet sensor  23  senses the following edge of the sheet P from the leading edge (a transit time T). The CPU  1711  may calculate the length of the long side of the sheet P by multiplying the transit time T by the conveyance speed (e.g., 180 mm/s). This calculation may be executed by the control unit  201 . In this case, the control unit  201  may identify the conveyance speed based on the model information  1716 , or may be notified of the conveyance speed by the CPU  1711 . In this manner, the CPU  1711  obtains the second size information. The CPU  1711  then executes step S 1814 , and moves to step S 2312 . Note that if the second size information only indicates the length of the long side (the length in the conveyance direction) of the sheet P, the length of the short side (the length in the width direction) of the sheet P may be obtained from the first size information. 
     In step S 2312 , the CPU  1711  transmits the second size information through the communication unit  41 . The second size information may be any information making it possible to identify the size of sheet P, and can be (i) the length of the long side of sheet P or (ii) the transit time T and conveyance speed. 
     The foregoing descriptions assume that the sheet sensor  23  can measure the length of the sheet P in the conveyance direction. However, instead of the sheet sensor  23 , the second size sensor  1723   b  installed in the conveyance path  15  may sense the length of sheet P in the conveyance direction and the length of the sheet P in the width direction. The size sensor  1723   b  may be a sensor that utilizes a line sensor. In this case, the line sensor extends in a direction orthogonal to the conveyance direction in the conveyance path  15 . As a result, the second size information may include both the length of the long side and the length of the short side of the sheet P. In this case, the length of the long side and the length of the short side of the sheet P, obtained from the second size information, are used in the image correction of step S 2302 . The first size information obtained by the first size sensor  1723   a  provided in the cassette need not be used for image correction. 
     According to the fourth embodiment, the second size information, which is more accurate than the first size information, is used. As such, the sheet image  208  is more accurately corrected, the measurement of the characteristics of the image deficiency is more accurate, and the diagnosis result is therefore more accurate as well. 
     Fifth Embodiment 
     If the diagnostic image  1900  is an image that occupies a broad range of the sheet P, many unfixed toner images are transferred to the sheet P at the secondary transfer section. An unfixed toner image has a lubricating effect between the intermediate transfer belt  8  and the sheet P. As a result, the sheet P may slip, reducing the conveyance speed of sheet P and resulting in an inaccurate measurement result of the sheet P by the sheet sensor  23 . Accordingly, in the fifth embodiment, the sheet P for which the size is measured by the sheet sensor  23  or the second size sensor  1723   b  and the sheet P on which the diagnostic image  1900  is printed are different sheets P. In particular, the sheet P for which the size is measured has less toner transferred than the sheet P on which the diagnostic image  1900  is printed. For example, the sheet P for which the size is to be measured may have no toner transferred at all. 
       FIG.  24 A  illustrates an image diagnosis method executed by a CPU provided in the control unit  201  of the image capturing apparatus  102 , according to the diagnostic program  206 . In  FIG.  24 A , processes identical or similar to those in  FIG.  18 A  or  FIG.  23 A  are given the same reference signs, and the descriptions thereof will apply here as well. Note that the user stores a plurality of the sheets P in the upper cassette  13   a  in advance. This may be achieved by the control unit  201  displaying a message in the interface unit  202  prompting the user to perform such storage. In the fifth embodiment, steps S 2401  and S 2402  are inserted between steps S 1802  and S 1803 . The sheet P on which the measurement image is formed and the sheet P on which the diagnostic image  1900  is formed are each the same brand and from the same production lot. Generally, the sheets P are contained in a single package containing a predetermined number of sheets (e.g., 1,000 sheets). Therefore, a sheet P on which a measurement image is formed and a sheet P on which the diagnostic image  1900  is formed may be sheets P sold in the same package. Doing so is likely to result in a very small error between the size of the sheet P on which the measurement image is formed and the size of the sheet P on which the diagnostic image  1900  is formed. 
     In step S 2401 , the control unit  201  transmits, to the image forming apparatus  101 , a printing instruction for a measurement image suited to measuring the size of the sheet P. The “measurement image” is an image that is less likely to cause a drop in the conveyance speed due to unfixed toner images compared to the diagnostic image  1900 . For example, the measurement image is a solid white image to which no toner is transferred, or an image to which a small amount of toner is transferred. The cassette specified by the printing instruction for the measurement image is basically the same as the cassette specified by the printing instruction for the diagnostic image  1900 . This improves the accuracy of the correction of the sheet image. 
     In step S 2402 , the control unit  201  obtains the second size information from the image forming apparatus  101  through the communication unit  204 . Here, the second size information includes size information obtained based on the sheet P on which the measurement image is formed. The second size information may be any information making it possible to identify the size of sheet P, and can be (i) the length of the long side of sheet P or (ii) the transit time T and conveyance speed. 
     The second size information is then used in step S 2302  to correct the sheet image  208 . As described with reference to  FIGS.  20 A and  20 B , the image region  2002  is corrected such that the lengths U 1 , V 1 , W 1 , and X 1  of the four sides match the second size information. Incidentally, the control unit  201  may tentatively correct the image region  2002  such that the lengths U 1 , V 1 , W 1  and X 1  of the four sides match the first size information, as described in the third embodiment. The control unit  201  may then correct the image region  2002  again such that the lengths U 1 , V 1 , W 1 , and X 1  of the four sides of the corrected image region  2002  match the second size information. 
     According to the fifth embodiment, the second size information is obtained using a different sheet P from the sheet P on which the diagnostic image  1900  is formed. In particular, the sheet P on which the measurement image is formed is less likely to experience a drop and variations in the conveyance speed than the sheet P on which the diagnostic image  1900  is formed. Accordingly, the length of the sheet P in the conveyance direction can be obtained more accurately. As a result, the fifth embodiment can provide even more accurate diagnosis results compared to the fourth embodiment. 
     Sixth Embodiment 
       FIG.  25    illustrates an example of the functions realized by a CPU  2500 , which can be provided in the control unit  201  or  1701 , executing the diagnostic program  206 . The plurality of functions illustrated in  FIG.  25    may be distributed throughout the control units  201  and  1701  and the control unit  40 , or may be centralized in one of the control units  201  and  1701  and the control unit  40 . However, it is sufficient for only the camera  203 , which captures images photographically, to be provided independently of the image forming apparatus  101 . 
     An obtainment unit  2501  obtains the size information  1718  from the image forming apparatus  101  or the interface unit  202 . The obtainment unit  2501  obtains the model information  1716  of the image forming apparatus  101 . The obtainment unit  2501  obtains the sheet image  208  generated by the camera  203 . In other words, the obtainment unit  2501  functions as an acquisition unit that acquires the sheet image  208  obtained by capturing an image of the sheet P on which the diagnostic image  1900  has been formed by the image forming apparatus  101 . 
     A selection unit  2505  selects the diagnostic image  1900  corresponding to the model information  1716  by referring to the case collection  210 . The selection unit  2505  may select the diagnostic image  1900  corresponding to the model information  1716  and the history data  1717  by referring to the case collection  210 . Furthermore, the selection unit  2505  may select the measurement image and pass the measurement image to an instruction unit  2506  before the diagnostic image. The instruction unit  2506  selects the cassette  13   a  based on the size information  1718 . For example, the size information  1718  may indicate that cassette  13   a  contains A4-size sheets Pa and cassette  13   b  contains B5-size sheets Pb. In this case, the instruction unit  2506  selects the cassette  13   a  and reflect the selection in the printing instruction. The instruction unit  2506  transmits the printing instruction for the image selected by the selection unit  2505  to the image forming apparatus  101 . 
     A correction unit  2512  corrects the image region  2001  in the sheet image  208  to the image region  2002  based on the size information  1718  obtained by the sheet sensor  23 , the size sensors  1723 , or the like. As described above, the correction unit  2512  rotates the image region  2001  such that the tilt  60  of the image region  2001  is zero. The correction unit  2512  deforms the image region  2001  such that the angles α 0 , β 0 , γ 0  and δ 0  of the four corners of the image region  2001  are 90 degrees each. Furthermore, the correction unit  2512  changes the lengths U 0 , V 0 , W 0 , and X 0  of the four sides so as to match the size information  1718 . As a result, a sheet image  208  that includes the corrected image region  2002  is generated. 
     A deficiency sensing unit  2513  refers to the case collection  210  and senses an image deficiency (e.g., the afterimage  1903 ) and the like from the image region  2002 . The case collection  210  includes characteristics of image deficiencies (the size of the afterimage  1903  and a density difference from the single-color pattern  1901 ) and the like. A measurement unit  2514  refers to the case collection  210  and measures the characteristics (e.g., L 1 , L 2 , L 3 , L 4 ) of the image deficiency. A cause identification unit  2515  refers to the case collection  210 , identifies a causative component corresponding to the characteristics obtained by the measurement unit  2514  and a countermeasure method (e.g., replacement, repair, cleaning) corresponding to the causative component, and generates a diagnosis result. A result notification unit  2516  generates a diagnosis result and outputs the diagnosis result to the interface units  202 ,  1702 , and  1712 . The interface units  202 ,  1702 , and  1712  display the diagnosis result. 
     Technical Spirit Derived from Embodiments 
     As described above, the diagnostic system  100  has the camera  203 , a reception function for receiving an image from the camera  203 , a diagnostic function, and a notification function. Here, the reception function, the diagnostic function, and the notification function may be provided in the image capturing apparatus  102 , or may be provided in another device. For example, the reception function, the diagnostic function, and the notification function may be distributed among the image forming apparatus  101 , the image capturing apparatus  102 , and the server  103  (information processing apparatus) including a personal computer. For example, the image capturing apparatus  102  may only obtain the sheet image  208 , and the remaining functions may be provided in an information processing apparatus such as a personal computer. 
     The diagnostic apparatus may be implemented by a single application program by having the image capturing apparatus  102  also function as the diagnostic apparatus, as described in the foregoing embodiments. This makes it possible to improve the convenience for a worker. 
     Aspects B 1  and B 19  to B 22   
     The image forming apparatus  101  is an example of an image forming apparatus that forms the diagnostic image  1900  on the sheet P. The image capturing apparatus  102  and the camera  203  are examples of an image capturing apparatus that captures an image of the sheet P on which the diagnostic image  1900  has been formed by the image forming apparatus  101  and outputs the sheet image  208 . The control units  201  and  1701  and the communication unit  1704  may function as a reception unit that receives the sheet image from the image capturing apparatus that outputs the sheet image (the camera  203 ). The image capturing apparatus  102  and the server  103  are examples of the diagnostic apparatus that diagnoses the image forming apparatus  101  based on the sheet image  208 . The control unit  201 , the interface unit  202 , and the communication unit  204  are examples of an obtainment unit that obtains the size information  1718  indicating the size of the sheet P. The control units  201  and  1701  function as a diagnosis unit that makes a diagnosis for a component of the image forming apparatus  101  based on the size information  1718  and a position of an image deficiency in the sheet image  208 . The interface units  202  and  1702  and the communication units  204  and  1704  function as an output unit that outputs a diagnosis result from the diagnosis unit. In this manner, according to the third embodiment, using the image capturing apparatus  102  makes it possible to diagnose the image forming apparatus  101  more easily than before. Furthermore, the size information  1718  of the sheet P on which the diagnostic image  1900  is formed is used in the diagnosis, and thus more accurate diagnosis results can be expected. 
     Aspect B 2   
     The control units  201  and  1701  may resize the image region  2001  on the sheet P appearing in the sheet image  208  based on the size information  1718 . Furthermore, the control units  201  and  1701  may make the diagnosis of the component of the image forming apparatus  101  based on the position of the image deficiency in the image region  2002  resulting from the resizing. The image capturing apparatus  102  captures a photograph while being held in the user&#39;s hand, and it is thus easy for the image region  2001  in the sheet image  208  to become distorted. Accordingly, correcting the image region  2001  based on the size information  1718  results in a more accurate diagnosis result. 
     Aspect B 3   
     The size information  1718  may include at least one of a length of a long side of the sheet P and a length of a short side of the sheet P. One of the length of the long side and the length of the short side may be used to estimate the length of the other. This is because the length of the long side and the length of the short side are already known for standard sizes such as A 4 , B 5 , and the like. 
     Aspect B 4   
     The size information  1718  may include both the length of a long side of the sheet P and the length of a short side of the sheet P. The control units  201  and  1701  may resize the image region  2001  in the sheet P such that the length of the long side and the length of the short side of the image region  2001  of the sheet P appearing in the sheet image  208  approach the lengths of the long side and the short side of the sheet P included in the size information  1718 . As a result, the image region  2001  is corrected more accurately, which will make the diagnosis result more accurate as well. 
     Aspect B 5   
     The server  103  (this may be a personal computer) is an example of an information processing apparatus capable of communicating with the image forming apparatus  101 . The control unit  201  may obtain the size information  1718  through the information processing apparatus. 
     Aspect B 6   
     The sheet sensor  23  and the size sensors  1723   a  and  1723   b  are examples of a sensing unit that senses the size of the sheet P. The control units  201  and  1701  may obtain the size information  1718  indicating the size of the sheet P sensed by the sensing unit. 
     Aspect B 7   
     The cassettes  13   a  and  13   b  are examples of holding units capable of holding a plurality of sheets. The size sensor  1723   a  may be configured to sense the size of the sheets P held in the holding unit. 
     Aspects B 8  and B 9   
     The conveyance path  15  is an example of a conveyance path along which the sheet P is conveyed. The sheet sensor  23  and the size sensor  1723   b  may be configured to sense the size of the sheet Pin the conveyance path  15 . This will make it possible to obtain even more accurate size information  1718 . The sheet sensor  23  and the size sensor  1723   b  may sense the size of the sheet P conveyed along the conveyance path  15  based on the transit time T of the sheet P and the conveyance speed of that sheet. 
     Aspect B 10   
     The size sensor  1723   b  may include a plurality of light-receiving elements disposed in a direction orthogonal to the conveyance direction of the sheet P in the conveyance path  15 . Furthermore, the size sensor  1723   b  may be configured to sense the size of the sheet P in the direction orthogonal to the conveyance direction of the sheet P based on light reception results from the plurality of light-receiving elements. This makes it possible to obtain not only the length of the sheet P in the conveyance direction, but also the length of the sheet P in the width direction. 
     Aspect B 11   
     As described in the fifth embodiment, the sheet P for which the size is sensed by the sensing unit and the sheet P on which the diagnostic image is formed may be different sheets. This may further increase the accuracy of the size information of the sheet P. 
     Aspect B 12   
     The information processing apparatus (e.g., the server  103  and the personal computer) may include a notification unit (e.g., the interface unit  1702 ) that makes a notification of the diagnosis result. This makes it possible to notify the user of the information processing apparatus of the diagnosis result as well. 
     Aspects B 13  and B 14   
     The diagnosis result may include information indicating the component estimated to be a cause of the image deficiency (e.g., the photosensitive member  1 ). Through this, the user will easily be able to understand which component requires maintenance. The diagnosis result may include information indicating a response method (countermeasure) for reducing the image deficiency. Through this, the user will easily be able to understand which type of maintenance is required. 
     Aspects B 15 , B 16 , and B 21   
     The diagnostic apparatus (e.g., the control unit  201 ) and the image capturing apparatus  102  may be contained within a single housing. In other words, the image capturing apparatus  102  may function as the diagnostic apparatus. The diagnostic apparatus and the image capturing apparatus  102  may be realized by a camera-equipped mobile communication device (e.g., a smartphone or a tablet terminal), or by a digital camera. Camera-equipped mobile communication device have become extremely commonplace. Accordingly, the user can download the diagnostic program  206  from a server device and install the program in the camera-equipped mobile communication device. In other words, the user will be able to introduce the diagnostic apparatus with ease. 
     Aspect B 17   
     The diagnostic apparatus may be provided in the image forming apparatus  101 . In this case, some of the processing described above as being executed by the control unit  201  (e.g., steps S 1805  to S 1808 ) is instead executed by the control unit  40 . Even in such a case, the image capturing apparatus  102  is provided independent from the image forming apparatus  101 . In other words, the image capturing apparatus  102  still obtains the sheet image  208 . This will make it possible for even an image forming apparatus  101  that does not have an image sensor to execute diagnostic processing. 
     Aspect B 18   
     The diagnostic apparatus may be installed in a server computer (e.g., the server  103 ) capable of communicating with the image capturing apparatus  102 . Accordingly, diagnosis results can be obtained even when the information processing capabilities of the image capturing apparatus  102  are low or there is little free space in the storage unit  205 . 
     Other Embodiments 
     Embodiment(s) of the present invention can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions (e.g., one or more programs) recorded on a storage medium (which may also be referred to more fully as a ‘non-transitory computer-readable storage medium’) to perform the functions of one or more of the above-described embodiment(s) and/or that includes one or more circuits (e.g., application specific integrated circuit (ASIC)) for performing the functions of one or more of the above-described embodiment(s), and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described embodiment(s) and/or controlling the one or more circuits to perform the functions of one or more of the above-described embodiment(s). The computer may comprise one or more processors (e.g., central processing unit (CPU), micro processing unit (MPU)) and may include a network of separate computers or separate processors to read out and execute the computer executable instructions. The computer executable instructions may be provided to the computer, for example, from a network or the storage medium. The storage medium may include, for example, one or more of a hard disk, a random-access memory (RAM), a read only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)TM), a flash memory device, a memory card, and the like. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2021-142697, filed Sep. 1, 2021 and Japanese Patent Application No. 2021-142698, filed Sep. 1, 2021 are hereby incorporated by reference herein in their entirety.