Patent Publication Number: US-10310429-B2

Title: Image forming apparatus

Description:
FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to an image forming apparatus such as a copy machine, a printing machine, a facsimile machine, a multifunction machine capable of functioning as two or more of the preceding machines, etc., which employs an electrophotographic method or an electrostatic recording method. 
     There have been known such image forming apparatuses structured so that a toner image is transferred (primary transfer) from its photosensitive drum onto its intermediary transfer belt, and then, the toner image is transferred (secondary transfer) from the intermediary transfer belt onto a sheet of recording medium. There have also been known such image forming apparatuses structured so that a control toner image is formed on its intermediary transfer belt, and various control processes are carried out based on the results of the detection of the control toner image by sensors. 
     In the case of an image forming apparatus structured as described above, it is possible that as foreign particles such as toner particles and paper dust particles adhere to the detection surface of the sensor, the sensor will reduce in detection accuracy. Thus, it has been proposed an image forming apparatuses structured so that the detection surface of its sensor is covered with a protective member having detection holes (Japanese Laid-open Patent Application No. 2002-131997). The image forming apparatus disclosed in Japanese Laid-open Patent Application No. 2002-131997 is provided with a protective member, and is structured so that as the main door with which the main assembly of the apparatus is closed, the protective member is moved to a preset position in which it exposes the detection surface of the sensor through its detection holes, whereas as the door is opened, the protective member is moved to a preset position in which it keeps the detection surface covered. 
     The sensor which detects the toner image formed on the intermediary transfer belt is disposed between the primary transferring portion in which the toner image is transferred from the photosensitive drum onto the intermediary transfer belt, and the secondary transferring portion in which the toner image is transferred from the intermediary transfer belt onto a sheet of recording medium. The sensor is likely to be disposed near the secondary transferring portion, and also, in the adjacencies of the recording medium conveyance passage through which the sheet is conveyed to the secondary transferring portion. Thus, foreign particles such as toner particles and paper duct particles are likely to accumulate on the area which is below the detection hole of the protective member. More specifically, in a case where the detection surface remains covered with the protective member, foreign substances (contaminants) accumulates on the area below the detection hole, which in this case is offset from the detection surface of the sensor. Further, if the protective member is moved to the position in which its keeps the detection surface exposed through its detection hole while the foreign substances (contaminants) having accumulated on the aforementioned area is still on the area, it is likely that the foreign substances (contaminants) are carried by the protective member, and adhere to the detection surface of the sensor. 
     SUMMARY OF THE INVENTION 
     Thus, the primary object of the present invention is to provide a structural design for an image forming apparatus, which makes it unlikely for foreign substance (contaminants) to adhere to the detection surface of a toner image sensor of the apparatus. 
     According to an aspect of the present invention, there is provided an image forming apparatus comprising an image bearing member; a rotatable intermediary transfer member configured to receive a toner image from said image bearing member at a primary transfer portion opposing said image bearing member; a secondary transfer member constituting a secondary transfer portion configured to contact said intermediary transfer member to transfer the toner image from said intermediary transfer member onto the recording material; a feeding path configured to feed the recording material to said secondary transfer portion; a sensor provided opposed to said intermediary transfer member below said intermediary transfer member at a position downstream of said primary transfer portion and upstream of said secondary transfer portion and configured to detect the toner image on said intermediary transfer member; a shutter movably provided between said sensor and said intermediary transfer member and configured to cover at least a part of a window for detection by said sensor, wherein said shutter includes a detection hole configured to uncover said window to permit detection of the toner image on said intermediary transfer member and a cover portion configured to cover said window, and wherein when said shutter is in an opening position, said window and said detection hole are opposed to each other, and when said shutter is in a closing position, said window and said cover portion are opposed to each other; and a holding member configured to hold said sensor, said holding member including a side wall opposed to a side of said sensor remote from said feeding path and extended in a widthwise direction perpendicular to a rotational moving direction of said intermediary transfer member, wherein as viewed in a horizontal direction perpendicular to widthwise direction of said intermediary transfer member, said side wall is provided with a first area overlapping with said detection hole in a vertical direction when said shutter is in the closing position and a second area overlapping with said window in the vertical direction when said shutter is in the closing position, and wherein the first area is below a lower surface of said shutter and is lower than the second area. 
     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 schematic sectional view of the image forming apparatus in one of the preferred embodiments of the present invention; it shows the general structure of the apparatus. 
       Part (a) of  FIG. 2  is a combination of a perspective view of the first sensor of the sensor unit of the apparatus and a schematic side view of the first sensor, and part (b) of  FIG. 2  is a combination of a perspective view of the first sensor of the sensor unit of the apparatus and a schematic side view of the first sensor. 
       Parts (a), (b), (c), (d), (e) and (f) of  FIG. 3  are schematic drawings of the secondary transferring portion and its adjacencies, which show the distances between the second transferring portion and sensor, distance (length of portion of sheet of recording medium) between the secondary transferring portion and a pair of registration rollers, distance between the secondary transferring portion and the pair of registration rollers, distance between the secondary transfer portion inside roller and a secondary transferring portion front roller, distance between the primary transferring portion and sensor, and distance between the secondary transferring portion front roller and sensor, respectively. 
       Part (a) of  FIG. 4  is a perspective view of the sensor unit as seen from the recording medium conveyance passage side, and part (b) of  FIG. 4  is a perspective of the sensor unit as seen from the opposite side of the recording medium conveyance passage from the sensor unit. 
       Part (a) of  FIG. 5  is a schematic side view of two combinations of a solenoid  214 , a shutter moving mechanism  212 , and a protective shutter  211  when the shutter is remaining closed, and is open, respectively, and is for showing the distance the protective shutter  211  is moved, and part (b) of  FIG. 5  is a perspective view of the shutter moving mechanism and its adjacencies. 
         FIG. 6  is a schematic side view of the sensor unit, and its adjacencies, in the image forming apparatus in the preferred embodiment; it shows the general structure of the sensor unit of the image forming apparatus, and its adjacencies. 
       Parts (a) and (b) of  FIG. 7  are schematic side views of the sensor unit when the protective shutter is remaining closed, and open, respectively; they show the positional relationship between the sensor and protective shutter. 
         FIG. 8  is a graph which shows the relationship between the cumulative number by which sheets of recording medium were conveyed through the image forming apparatus, and the amount (%) by which electric power is inputted into the LED of the sensor unit. 
         FIG. 9  is a schematic drawing for showing the positional relationship between the bottom surface of one of the slots of one of the side walls of the frame of the sensor unit, and the detection surface of the sensor. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, referring to  FIGS. 1-9 , one of the preferred embodiments of the present invention is described. First, referring to  FIG. 1 , the image forming apparatus in this embodiment is described about its general structure. 
     [Image Forming Apparatus] 
     The image forming apparatus  100  in this embodiment is a full-color image forming apparatus which uses an electrophotographic image forming method. It is of the so-called tandem type. 
     The image forming apparatus  100  is an electrophotographic full-color printer of the tandem type. It has four image forming portions  2   a ,  2   b ,  2   c  and  2   d , which have photosensitive drums  3   a ,  3   b ,  3   c  and  3   d , as image bearing members, respectively. It forms a toner image on a sheet of recording medium in response to the image formation signals from an original reading apparatus (unshown), which is in connection to the main assembly  100 A of the image forming apparatus  100 , or a host device such as a personal computer, which is in connection to the main assembly  100 A of the image forming apparatus  100  in such a manner that information can be exchanged between the image forming apparatus  100  and host device. As the recording medium, a sheet of ordinary paper, an envelope, a sheet of plastic film, a sheet of cloth, etc., can be listed. The image forming portions  2   a ,  2   b ,  2   c  and  2   d  form yellow, magenta, cyan and black toner images, respectively. 
     By the way, the four image forming portions  2   a ,  2   b ,  2   c  and  2   d , with which the image forming apparatus  100  is provided, are practically the same in structure, except that they are different in the color of the developer they use. Thus, the image forming portion  2   a  is described as a portion that represents the four image forming portions; the image forming portions  2   b ,  2   c , and  2   d  are not described since their descriptions are the same as that of the image forming apparatus  2   a  except for the suffixes b, c and d which indicates their relationship to the color components, one for one, into which the image to be formed is separated. 
     There is disposed a photosensitive drum  3 , which is a cylindrical photosensitive member as an image bearing member, in the image forming portion  2 . The photosensitive drum  3   a  is rotationally driven in the direction indicated by an arrow mark in the drawing. There are also disposed a charging apparatus  5   a  (charge roller) as a charging means, a developing apparatus  7   a , the primary transfer roller  8   a , and a cleaning apparatus  4   a  as a cleaning means, in the adjacencies of the peripheral surface of the photosensitive drum  3   a . There is also disposed a laser scanner  6   a  (exposing apparatus) as an exposing means, below the photosensitive drum  3   a  in the drawing. 
     Further, there is disposed an intermediary transferring apparatus  120  on the top side of the combination of the image forming portions  2   a ,  2   b ,  2   c  and  2   d  in the drawing. The intermediary transferring apparatus  120  has an intermediary transfer belt  121  as an intermediary transferring member, which is a rotationally movable endless belt. The intermediary transfer belt  121  is suspended and tensioned by multiple rollers, and is rotationally driven in the direction indicated by an arrow mark in the drawing. After being formed on the peripheral surface of the photosensitive drum  2   a , a toner image is transferred (primary transfer) onto the intermediary transfer belt  121  as will be described later. Then, the intermediary transfer belt  121  conveys a toner image by bearing the toner image. The intermediary transferring apparatus  120  is provided with a pair of secondary transfer rollers, more specifically, the secondary transfer inside roller  122  which is disposed on the inward side of the loop which the intermediary transfer belt  121  forms, and the secondary transfer outside roller  124  which is disposed on the outside the belt loop. The two secondary transfer rollers  122  and  124  are disposed so that they sandwich the intermediary transfer belt  121 , forming thereby the secondary transferring portion T 2  in which the toner image on the intermediary transfer belt  121  is transferred onto a sheet of recoding medium. The secondary transfer outside roller  124  is rotatably supported by its lengthwise end portions, by a pair of bearings, in such a manner that it is kept pressed toward the intermediary transfer belt  121  by the force generated in the direction of the intermediary transfer belt  121  by a pair of elastic members. 
     In particular, the intermediary transfer belt  121  in this embodiment, is suspended and tensioned in such a manner that the portion of the intermediary transfer belt  121 , which faces the photosensitive drums  3   a - 3   d  of the image forming portions  2   a - 2   d , respectively, is roughly horizontal, and also, that it is suspended and tension by the secondary transfer front roller  123  as the belt supporting-tensioning second roller, on the immediate upstream side of the secondary transferring portion T 2 . That is, with respect to the direction in which the intermediary transfer belt  121  is rotated, the secondary transfer front roller  123  suspends and keep tensioned the intermediary transfer belt  121  on the downstream side of the primary transferring portion T 1 , and the upstream side of the secondary transfer inside roller  122 . There is disposed a fixing apparatus  15  on the downstream side of the secondary transferring portion T 2  with respect to the recording medium conveyance direction. 
     There is disposed in the bottom portion of the image forming apparatus  100 , a cassette  9 , in which sheets of recording medium are stored. Further, the image forming apparatus  100  is provided with a manual sheet feeding tray  10 , which is attached to the outward side of one of the side walls of the apparatus main assembly  100 A. As a sheet of recording medium is fed into the apparatus main assembly  100 A from the cassette  9  or manual sheet feeder tray  10 , it is conveyed toward the pair of registration rollers  12  by a pair of conveyance rollers  11   a  or  11   b , respectively. With respect to the recording medium conveyance direction, the pair of registration rollers  12  function also as a pair of conveyance rollers which are disposed next to the upstream side of the secondary transferring portion T 2 . As the sheet of recording medium comes into contact with the pair of registration rollers  12  by its leading edge while the registration rollers  12  are remaining stationary, it bends in curvature, being thereby corrected by itself in attitude if it happens to be askew. Then, the rotation of the pair of registration rollers  12  is started with such timing that the sheet will arrive at the secondary transferring portion T 2  at the same time as the toner image on the intermediary transfer belt  121 . Thus, the sheet is conveyed to the secondary transferring portion T 2  by way of a recording medium conveyance passage  14  made up of the pre-transfer guide  13 , etc. In this embodiment, the pair of registration rollers  12  are equivalent to the sheet conveying means which is capable of conveying a sheet of recording medium to the secondary transferring portion T 2 . 
     Next, the process through which a full-color image is formed by the image forming apparatus  100  structured as described above, based on the four primary colors, is described. As an image forming operation is started, first, the photosensitive drum  3   a  begins to be rotated, and the peripheral surface of the rotating photosensitive drum  3   a  is uniformly charged by the charging apparatus  5   a . Then, the photosensitive drum  3   a  is exposed by to the beam of laser light emitted by the exposing apparatus  6   a  while being modulated by image formation signals. As a result, an electrostatic latent image, which reflects the image formation signals, is effected on the peripheral surface of the photosensitive drum  3   a . The electrostatic latent image on the photosensitive drum  3   a  is developed into a visible image, or a toner image, by the toner stored in the developing apparatus  7   a.    
     Then, the toner image on the photosensitive drum  3   a  is transferred (primary transfer) onto the intermediary transfer belt  121 , in the primary transferring portion T 1   a  which is formed between the peripheral surface of the photosensitive drum  3   a  and that of the primary transfer roller  8   a , with the presence of the intermediary transfer belt  121  between the photosensitive drum  3   a  and primary transfer roller  8   a . While the sheet of recording medium is conveyed through the primary transferring portion T 1 , the primary transfer bias is continuously applied to the primary transfer roller  8   a . The toner (transfer residual toner) which is remaining on the peripheral surface of the photosensitive drum  3   a  after the primary transfer is removed by the cleaning apparatus  4   a.    
     The above-described process is sequentially carried in the image forming portions  2   a ,  2   b ,  2   c  and  2   d , which form yellow, magenta, cyan and black toner images, respectively, so that the four toner images, different in color, are transferred in layers onto the intermediary transfer belt  121 . Then, one of the sheets S of recording medium stored in the cassette  9 , or placed on manual feeder tray  10 , is conveyed to the secondary transferring portion T 2  in synchronism with the formation of the toner images. Then, the four toner images, different in color, on the intermediary transfer belt  121  are transferred together (secondary transfer) by the application of a combination of a preset amount of pressure and a present amount of electrostatic load bias (secondary transfer bias). The toner which failed to be transferred in the intermediary transferring portion T 2 , and therefore, is remaining on the intermediary transfer belt  121 , is removed by an intermediary transfer belt cleaner  125 . 
     Then, the sheet S of recording medium is conveyed to the fixing apparatus  15  which is a fixing means. The fixing apparatus  15  is equipped with a fixation roller  15   a  and a pressure roller  15   b . It forms a fixation nip between the fixation roller  15   a  and pressure roller  15   b . After the transfer of the toner images onto the sheet S, the sheet S is conveyed through this fixation nip of the fixing apparatus  15 . While the sheet S is conveyed through the fixation nip, the sheet S and the toner images thereon are heated and pressed. Thus, the toner on the sheet S melts and mixes. Then, as the mixture of the toner cools, it becomes fixed to the sheet S. Thereafter, the sheet S is discharged into a delivery tray  17  by a pair of discharge rollers  16 , ending the image formation process. 
     By the way, in a case where an image is formed on both surfaces of a sheet S of recording medium, the sheet S is conveyed to a sheet conveyance passage  18  for the two-sided image formation mode, after the formation of an image on one of two surfaces of the sheet S. Then, the sheet S is conveyed to the secondary transferring portion T 2  for the second time so that an image is formed on the other surface of the sheet S. 
     [Apparatus Adjustment] 
     The image forming apparatus  100  is provided with a sensor unit  200 , which is disposed between the image forming portion  2   d  and secondary transferring portion T 2  in such a manner that it faces the surface of the intermediary transfer belt  121 . The sensor unit  200  has the first and second sensors  201  and  202 , which are enabled to detect the toner image on the intermediary transfer belt  121 , as will be described later. With respect to the direction in which the intermediary transfer belt  121  is rotated, the sensor unit  200  is disposed on the downstream side of the primary transferring portion T 1 , and on the upstream side of the secondary transferring portion T 2 . The positioning of the sensor unit  200  is described later in detail. 
     The sensor unit  200  is provided with three sensors, more specifically, a pair of the first sensors  201 , and one second sensor  202 . With respect to the direction which is perpendicular to the direction in which the intermediary transfer belt  121  is rotated, these sensors  201  and  202  are evenly distributed ( FIG. 4 ). They detect the four monochromatic control toner images, different in color, formed on the intermediary transfer belt  121  for adjusting the image forming apparatus  100  in the color and density of the images it forms (these images may be referred to as “control images”, hereafter). The control portion  300  of the image forming apparatus  100  adjusts (corrects) the image forming apparatus  100 , with regard to the chromatic deviation, density deviation, etc., of each monochromatic control toner image. 
     By the way, the control portion  300  is provided with a CPU (central processing unit) and a memory unit, which has a ROM (Read Only Memory) and a RAM (Random Access Memory). In the ROM, programs and the like, which correspond to control procedures, are stored. The image forming apparatus  100  is structured so that the CPU controls each of its sections while reading the programs stored in the ROM. In the RAM, operational data and input data are stored. The image forming apparatus  100  is structured so that its CPU controls the operation of the image forming apparatus  100  based on the abovementioned programs, with reference to the data stored in the RAM. 
     The process for adjusting (correcting) the image forming apparatus  100  in the color of the images it forms is as follows. First, the control portion  300  forms control toner images, different in color, on the intermediary transfer belt  121  with preset timing. Then, it detects the control toner images with the use of the first and second sensors  201  and  202 . More concretely, first, the exposing apparatuses  6   a - 6   d  form four electrostatic latent images, which correspond to the four control toner images, different in color, on the peripheral surfaces of the photosensitive drums  3   a - 3   d , one for one. The formed electrostatic latent images are developed into control toner images, different in color, by the developing apparatuses  7   a - 7   d , one for one. Then, the toner images on the photosensitive drums  3   a - 3   d  are sequentially transferred (primary transfer) onto the intermediary transfer belt  121  by the combination of the preset amount of pressure and the preset amount of electrostatic load bias applied by the primary transfer rollers  8   a - 8   d . After the transfer (primary transfer) of the control toner images onto the intermediary transfer belt  121 , they are detected by the first and second sensors  201  and  202 . The information regarding the detected control toner images is processed by the control portion  300 . That is, the control portion  300  calculates the amount of color deviation of each control toner image and the amount of density deviation of each control toner image. Then, it adjusts the image forming apparatus  100  in the color and density of the images the apparatus  100  will form (feedback), based on the calculated amount of color deviation and density deviation. 
     That is, the control portion  300  calculates the amount of the positional deviation of each of the control images other than the yellow control image, that is, the control image of the referential color formed in the image forming portion  2   a , or the most upstream image forming portion, relative to the position of the yellow control image. Then, the control portion  300  adjusts the image forming apparatus  100  in the point in each image forming portion  2 , at which the exposure process is started by the exposing apparatus, in order to ensure that the four monochromatic toner images, different in color, will be transferred onto the intermediary transfer belt  120  in layers, in satisfactory alignment. Among the three sensors which are different in position, the second sensor  202  or the central sensor, doubles as a sensor for detecting the density of each control toner image. That is, the control portion  300  calculates the amount by which the image forming apparatus  100  is to be adjusted in the density of the control toner images, different in color, based on the results of the detection by the second sensor  202 . For example, it adjusts the image forming apparatus  100  in the density of each of the four monochromatic toner images, different in color, by adjusting the amount by which each photosensitive drum  3  is to be exposed by the exposing apparatus, the bias to be applied to the developing apparatus during the developing process, etc., or the like factor. 
     [First and Second Sensors] 
     Next, referring to part (a) of  FIG. 2  and part (b) of the Figure, the first and second sensors  201  and  202  of the sensor unit  200  are described. The first sensor  201  is an optical sensor. It has a casing  203 , a light source  204 , a light catching portion  205  (which is for catching nondiffusively reflected light), a substrative plate  206 , and a sensor cover  207 . By the way, part (a) of  FIG. 2  is a perspective view of the first sensor  201 , and shows the internal structure of the portion of the sensor  201  encircled by a dotted line in the drawing. Part (b) of  FIG. 2  is also a schematic perspective view of the first sensor  201 , and shows the internal structure of the portion of the sensor  201  encircled by a dotted line. Part (a) of  FIG. 3  and part (b) thereof are similar to part (a) of  FIG. 2  and part (b) thereof, respectively. 
     The light source  204  (which in this embodiment is LED) is disposed within the casing  203 . The light catching portion  205  (which in this embodiment is photo-diode) catches the light which was emitted by the light source  204  and nondiffusively reflected by the intermediary transfer belt  121 . The substrative plate  206  is where the light source  204  and light catching portion  205  are mounted. The sensor cover  207  is formed of a transparent substance (which in this embodiment is acrylic plate) which is also the material for a condensing lens. By the way, the sensor cover  207  is disposed so that it is perpendicular to the surface of the substrative plate  206 , across which the light source  204  and light catching portion  205  are mounted. That is, the sensor cover  207  is disposed so that after the attachment of the sensor unit  200  to the apparatus main assembly  100 A, the sensor cover  207  is roughly parallel to the surface of the intermediary transfer belt  121 . In this embodiment, the distance between the sensor cover  207  and the surface of the intermediary transfer belt  121  was set to roughly 6 mm. 
     As for the second sensor  202 , it is also an optical sensor. It is similar in structure to the first sensor  201 , except that it is provided with a light catching portion  208 , which is also mounted on the substrative plate  206  in the casing  203 . The light catching portion  208  is such a portion that catches the light which was emitted by the light source  204  and was diffusively reflected by the intermediary transfer belt  121 . Since the second sensor  202  is the same in structure as the first sensor  201  except that it has the light catching portion  208 , the members of the second sensor  203 , which are equivalent to the counterparts of the first sensor  201 , are given the same referential codes, and are not described here. By the way, the distance between the sensor cover  207  of the second sensor  202  and the surface of the intermediary transfer belt  121  was also set to roughly 6 mm. As described above, the first sensors  201  detects the position and density of each control toner image by detecting the light emitted from the light source  204  and nondiffusively reflected by the surface of the intermediary transfer belt  121 , whereas the second sensor  202  detects the position of each control toner image on the intermediary transfer belt  121  by detecting the light emitted from the light source  204  and diffusively reflected by the surface of the intermediary transfer belt  121 . 
     [Positioning of Sensor Unit] 
     Next, referring to  FIG. 1  and parts (a)-(f) of  FIG. 3 , the positioning of the sensor unit  200  is described. Referring to  FIG. 1 , the image forming apparatus  100  in this embodiment is of the so-called vertical conveyance type. That is, it is structured so that sheets S of recording medium are conveyed upward from the cassette  9  which is in the bottom portion of the apparatus main assembly  100 A. Thus, the secondary transferring portion T 2  is above the pair of registration rollers  21 , and the sensor unit  200  is positioned next to (as seen from horizontal direction) the recording medium conveyance passage  14  through which the sheets S of recording medium are conveyed to the secondary transferring portion T 2  from the pair of registration rollers  12 . In other words, the sensor unit  200  is disposed so that as the image forming apparatus  100  is seen from the direction which is horizontal and perpendicular to the recording medium conveyance direction, the sensor unit  200  overlaps with the recording medium conveyance passage  14 . 
     In particular, referring to part (a) of  FIG. 3 , the sensor unit  200  is disposed so that the length L 1  of the portion of the intermediary transfer belt  121  between the secondary transferring portion T 2  and the sensor unit  200  becomes no more than the dimension of the largest sheet of recording medium, with respect to the recording medium conveyance direction, on which an image can be formed by the image forming apparatus  100 . For example, in a case where a sheet of recording medium of size A 3 , that is, the largest sheet of recording medium on which an image can be formed by the image forming apparatus  100 , is conveyed in the portrait mode, the dimension of the sheet in terms of the recording medium conveyance direction is 420 mm. Thus, the sensor unit  200  is positioned so that the dimension L 1  become no more than 420 mm. 
     Preferably, the sensor unit  200  is disposed so that the length L 1  becomes no more than the measurement of the smallest sheet of recording medium, in terms of the recording medium conveyance direction, on which an image can be formed by the image forming apparatus  100 . For example, in a case where an envelop, which is the smallest recording medium on which an image can be formed by the image forming apparatus  100 , is conveyed in the landscape mode, the measurement of the medium is 98.4 mm. Thus, it is desired that the sensor unit  200  is disposed so that the length L 1  becomes no more than 98.4 mm. In this embodiment, the sensor unit  200  was disposed so that the length L 1  becomes roughly 32.4 mm. 
     By the way, the position described in this specification as the position in which the sensor unit  200  is disposed is an area which corresponds in position to the portion of the intermediary transfer belt  121 , which is illuminated by the light source  204  of the sensor unit  200 . Further, the distance, in terms of the recording medium conveyance direction, from the sensor unit  200  to the secondary transferring portion T 2  means the distance from the sensor unit  200  to the center of the secondary transferring portion T 2 , or the recording medium sandwiching nip between the intermediary transfer belt  121  and secondary transfer outside roller  124 . Moreover, the lengthwise measurement of a sheet of recording medium means the measurement of the sheet P in terms of the recording medium conveyance direction. Further, the widthwise measurement of a sheet of recording medium means the measurement of the sheet in terms of the direction which is perpendicular to the recording medium conveyance direction. 
     To describe the positioning of the sensor unit  200  in greater detail, with respect to the distance, measured along the surface of the intermediary transfer belt  121 , between the first transferring unit T 1   d , and the secondary transferring section T 2 , the sensor unit  200  is disposed closer to the secondary transferring portion T 2  than to the first transferring portion T 1   d . More concretely, the distances of the sensor unit  200  from the points of the image forming apparatus  100 , which are related to the present invention, are as follows. 
     To begin with, referring to part (b) of  FIG. 3 , with respect to the recording medium conveyance direction, L 2  stands for the measurement of the portion of a sheet S of recording medium, which is between the nip of the secondary transferring portion T 2  and that of the pair of registration roller  12 , and which remains bent in curvature by an amount large enough to come into contact with the secondary transferring portion front guide  13 . In this case, the image forming apparatus  100  is structured so that “L 1 ≤≤L 2 ” is satisfied. That is, the image forming apparatus  100  is structured so that L 1  is no more than L 2 , that is, the measurement of the portion of a sheet S of recording medium, which is between the center of the nip of the secondary transferring portion T 2  and that of the pair of registration rollers  12  when the amount by which the sheet S bends in curvature is largest. In this embodiment, the image forming apparatus  100  is structured so that L 3  becomes roughly 49.4 mm. 
     Next, referring to part (c) of  FIG. 3 , L 3  stands for the rectilinear distance between the center of the nip (which nips sheet of recording medium) of the secondary transferring portion T 2 , in terms of the recording medium direction, and the center of the nip (which nips sheet of recording medium) of the pair of registration roller  12 . In this case, the image forming apparatus  100  is structured so that “L 1 ≤L 3 ” is satisfied. That is, L 1  is no more than the rectilinear distance between the pair of registration roller  12  and the secondary transferring portion T 2 . In this embodiment, the image forming apparatus  100  was structured so that L 3  becomes rough 45.4 mm. 
     Next, referring to part (d) of  FIG. 3 , L 4  stands for the rectilinear distance between the center of the secondary transfer inside roller  122  and that of the secondary transfer front roller  123 . Further, referring to part (e) of  FIG. 3 , L 5  stands for the distance from the sensor unit  200  to the primary transferring portion T 1   d . By the way, “to the primary transferring portion T 1   d ” means to the intersection between the surface of the intermediary transfer belt  121  and the straight line which coincides with the center of the photosensitive drum  3   d  and is perpendicular to the surface of the intermediary transfer belt  121 . Moreover, referring to part (f) of  FIG. 4 , L 6  stands for the distance from the sensor unit  200  to the secondary transfer front roller  123 . By the way, “to the secondary transfer front roller  123 ” means to the intersection between the surface of the intermediary transfer belt  121 , and the straight line which coincides with the center of the second transfer front roller  123  and is perpendicular to the surface of the intermediary transfer belt  121 . 
     In this case, the image forming apparatus  100  is structured so that “L 6 &lt;L 5 ” is satisfied. Further, it is preferred that the image forming apparatus  100  is structured so that “L 6 &lt;L 4 ” is satisfied. That is, it is desired that the image forming apparatus  100  is structured so that L 6 , or the distance from the sensor unit  200  to the center of the secondary transfer front roller  123 , is shorter than L 4 , or the distance between the center of the secondary transfer inside roller  122  and that of the secondary transfer front roller  123 . In this embodiment, the image forming apparatus  100  is structured so that L 6  becomes roughly 19.2 mm; L 5 , roughly 43.8 mm; and L 6  becomes roughly 3.8 mm. 
     That is, in this embodiment, the sensor unit  200  is disposed as close as possible to the secondary transfer front roller  123 , for the following reason. That is, if the surface of the intermediary transfer belt  121  is detected by the sensor unit  200 , in an area which is not in the adjacencies of the area of contact between the intermediary transfer belt  121  and second transfer front roller  123 , it is possible that the fluttering of the intermediary transfer belt  121  will reduce the sensor unit  200  in accuracy. In this embodiment, therefore, the sensor unit  200  is disposed so that it opposes the portion of the intermediary transfer belt  121 , which is in contact with the secondary transfer front roller  123 , or as close as possible to this portion of the intermediary transfer belt  121 , in order to minimize the amount by which the sensor unit  200  is reduced in accuracy by the fluttering of the intermediary transfer belt  121 . 
     [Structure of Sensor Unit] 
     Next, referring to parts (a) and (b) of  FIG. 4 , the structure of the sensor unit  200  is described. The sensor unit  200  is made up primarily of a frame  209 , a sensor holder  210 , a protective shutter  211 , and a shutter moving mechanism  212 , in addition to the above-described first and second sensors  201  and  202 . 
     The frame  209  is a holding member. It is the base of the sensor unit  200 . It is roughly rectangular, and is disposed so that its longer edges are perpendicular to the rotational direction of the intermediary transfer belt  121 . It has a side wall  209   a , and a pair of positioning portions  213   a  and  213   b , which are the lengthwise end portions of the side wall  209   a , one for one. The side wall  209   a  holds the first and second sensors  201  and  202 , with the placement of a sensor holder  210  between itself and the sensors  201  and  202 . It is disposed so that it is on the opposite side of the first and second sensors  201  and  202  from the recording medium conveyance passage  14 . That is, the sensor holder  210  is where the first and second sensors  201  and  202  are fixed. That is, the first and second sensors  201  and  202  are attached to the sensor holder  210 , and the sensor holder  210  is attached to the side wall  209   a , whereby the first and second sensors  201  and  202  are held by the frame  209 . The positioning portions  213   a  and  213   b  are attached to parts of the frame of the apparatus main assembly  100 A ( FIG. 1 ), whereby the sensor unit  200  is disposed in a preset location in the apparatus main assembly  100 A. 
     The sensor unit  200  is disposed in such an attitude that it squarely faces the surface of the intermediary transfer belt  121 , that is, the detection surface  201   a  and  202   a  of the first and second sensors  201  and  202 , respectively, are roughly parallel to the surface of the intermediary transfer belt  121 . Further, the sensor unit  200  is disposed so that its detection surfaces  201   a  and  202   a  face upward. Further, it is disposed so that the distance between the surface of the intermediary transfer belt  121  and the detection surface  201   a  of the first sensor  201 , and the distance between the surface of the intermediary transfer belt  121  and the detection surface of the sensor  202   a , remain stable. By the way, “detection surfaces  201   a  and  202   a ” means two parts, one for one, of surface of the sensor cover  207  (through which control toner images are detected) which covers the first and second sensors  201  and  202 . 
     Further, the sensor unit  200  is disposed so that its lengthwise direction is parallel to the widthwise direction, which is perpendicular to the rotational direction of the intermediary transfer belt  121 . Further, it is disposed so that, with respect to the widthwise direction of the intermediary transfer belt  121 , the second sensor  202  faces the center portion of the intermediary transfer belt  121 , and the pair of first sensors  201  face the edge portions of the intermediary transfer belt  121 , one for one. By the way, this embodiment is not intended to limit the present invention in terms of the number and positioning of the first and second sensors  201  and  202 . That is, the numbers and positioning of the first and second sensors  201  and  202  are optional. 
     The protective shutter  211  is a covering member. It is disposed so that it is between the first sensor  201  and intermediary transfer belt  121 , and also, between the second sensor  202  and intermediary transfer belt  121 , and also, so that it can be placed in the first or second position. It protects the sensor cover  207  for the first and second sensor  201  and  202 . Since the detection surfaces  201   a  and  202   a  of the first and second sensor  201  and  202 , respectively, face upward, the protective shutter  211  is disposed above both the first and second sensors  201  and  202 . The direction in which the protective shutter  211  is movable is parallel to the lengthwise direction of the sensor unit  200 . 
     The protective shutter  211  is provided with a pair of detection holes  216   a , which correspond to the pair of first sensors  201 , one for one, and one detection hole  216   b  which corresponds to the second sensor  202 . It is when the protective shutter  211  is in the first position that the protective shutter  211  exposes the detection surfaces  201   a  and  202   a  of the first and second sensors  201  and  202  through its detection holes  216   a  and  216   b , respectively. The image forming apparatus  100  is structured so that when the protective shutter  211  is in the first position, the toner images on the intermediary transfer belt  121  (intermediary transferring member) can be detected by the first and second sensors  201  and  202 , whereas when the protective shutter  211  is in the second position, its covering portion  211   a , that is, the portion which does not have the detection holes  216   a  and  216   b , covers the detection surfaces  201   a  and  202   a  of the first and second sensors  201  and  202 , respectively. 
     The shutter moving mechanism  212  can move the protective shutter  211  to the first or second position. That is, it can move the protective shutter  211  to the position where the detection surfaces  201   a  and  202   a  are exposed through the detection holes  216   a  and  216   b , or the position where the detection surfaces  201   a  and  202   a  are covered with the covering portion  211   a  of the protective shutter  211 . 
     [Shutter Moving Mechanism] 
     Next, referring to parts (a) and (b) of  FIG. 5 , the abovementioned shutter moving mechanism  212  is described. Part (a) of  FIG. 5  shows the combination of the solenoid  214 , shutter moving mechanism  212 , and protective shutter  211  when the protective shutter  211  is open (top), and that when the protective shutter  211  is closed (bottom). It is for showing the distance the protective shutter  211  is moved. The top side of the drawing shows the protective shutter  211  when the shutter  211  is open, and the bottom side shows the protective shutter  211  when the shutter  11  is closed. The shutter moving mechanism  212  has the solenoid  214  as a driving force source, and a linkage  215 . As the solenoid  214  is driven by the control portion  300 , the protective shutter  211  is moved in the lengthwise direction of the solenoid  214  by the solenoid by way of the linkage  215 . For example, as the solenoid  214  is turned on (supplied with electric power), the protective shutter  211  moves to its first position in which it keeps the protective shutter  211  open as the bottom side of part (a) of  FIG. 5  shows. On the other hand, as the solenoid  214  is turned off (not supplied with electric power), the protective shutter  211  moves to its second position in which it keeps the protective shutter  211  closed as shown in the bottom side of part (a) of  FIG. 5 . By the way, the relationship between the state (on or off) of the solenoid  214  and the position of the protective shutter  211  may be opposite from the above-described one. 
     As described above, the protective shutter  211  is movable by the shutter moving mechanism  212  to its first position in which it remains open, or its second position in which it remains closed. The process to be carried out with preset timing to adjust (correct) the image forming apparatus  100  in image properties is as follows. First, the control portion  300  drives the shutter moving mechanism  212  to move the protective shutter  211  to the first position to open the protective shutter  211  so that the detection holes  216   a  and  216   b  move to positions in which they align with (expose) the detection surfaces  201   a  and  202   a  of the first and second sensors  201  and  202 , that is, parts of the top surface of the sensor cover  207 . Thus, the light emitted from the light sources  204  of the first and second sensors  201  and  202  is focused on the surface of the intermediary transfer belt  121  through the sensor cover  207  and detection holes  216   a  and  216   b , and reflected by the surface the intermediary transfer belt  121 . As the light is reflected by the surface of the intermediary transfer belt  121 , it travels through the detection holes  216   a  and  216   b  and the sensor cover  207 , and is caught by the light catching portion  205  for catching (sensing) the portion the light, which was diffusively reflected by the surface of the intermediary transfer belt  121 , and the light catching portion  208  for catching (sensing) the portion of the light, which was diffusively reflected by the surface of the intermediary transfer belt  121 . That is, the first and second sensors  201  and  202  detect the control toner images on the intermediary transfer belt  121  so that the control portion  300  can adjust (correct) the image forming apparatus  100  in image properties (color, density, etc.). 
     On the other hand, as the process for adjusting (correcting) the image forming apparatus  100  in image properties is ended, the shutter moving mechanism  212  is driven by the control portion  300  to move the protective shutter  211  to the second position to close the shutter  211 . As the protective shutter  211  is moved to its second position, the covering portion  211   a  of the protective shutter  211 , or the portions which are not the detection holes  216   a  and  216   b , covers the portions of the top surface of the sensor cover  207 , which correspond in position to the first and second sensors  201  and  202 . Thus, it is unlikely for foreign particles such as toner particles and paper duct to adhere to the surface of the sensor cover  207 . 
     [Adhesion of Foreign Substances (Contaminants) to Sensor Unit] 
     When the protective shutter  211  is closed, the detection holes  216   a  and  216   b  are not in alignment with the portions of the sensor cover  207 , which correspond in position to the first and second sensors  201  and  202 , respectively. Thus, it sometimes occurs that foreign substances (contaminants) accumulate on the areas in the sensor unit  200 , which are below the detection holes  216   a  and  216   b , but, are not directly below the portions of the sensor cover  207 , which correspond in position to the first and second sensors  201  and  202 . 
     In particular, in this embodiment, the sensor unit  200  is disposed close to the secondary transferring portion T 2 , and also, as close as possible to the secondary transfer front roller  123  as described above. Therefore, the sensor unit  200  is disposed close to the recording medium conveyance passage  14  for conveying a sheet S of recording medium to the secondary transferring portion T 2 , as shown in  FIG. 6 . 
     Thus, as a sheet S of recording medium reaches the secondary transferring portion T 2 , the recording medium conveyance passage side of the sensor unit  200  is blocked by the sheet S and recording medium conveyance passage  14 , which is made up of the transferring portion front guide  13 , etc. Here, the airflow A which is generated by the rotational movement of the intermediary transfer belt  121  moves rightward of  FIG. 6 , as seen from the front side of the apparatus main assembly  100 A, and collides with the sheet S and/or the conveyance passage  14 . Therefore, the airflow A is directed backward toward the sensor unit  200  as indicated by an arrow mark in  FIG. 6 . 
     Thus, the foreign substances (contaminants) such as paper dust and stray toner particles which are generated as a sheet S of recording medium rubs against rollers and recording medium conveyance passages  14  while the sheet S is conveyed, are likely to flow back toward the sensor unit  200  and adhere to the sensor unit  200 . In particular, in recent years, image forming apparatuses have been substantially reduced in size, and therefore, in internal space. Consequently, they have been reduced in the distance from the sensor unit  200  to the secondary transferring portion T 2 . As an image forming apparatus is reduced in this distance, it is also reduced in the distance from the sensor unit  200  to the recording medium conveyance passage  14 , and the distance from the sensor unit  200  to the sheet S which is being conveyed through the conveyance passage  14 . Therefore, it has become more likely for the foreign substances (contaminants) to adhere to the sensor unit  200  as they are carried by the airflow A than in the past. That is, in the case of the current crop of image forming apparatuses, it is more likely for the foreign substances (contaminants) to adhere to the detection surface  201   a  of the first sensor  201  and the detection surface  202   a  of the second sensor  202  than in the past. 
     Next, referring to parts (a) and (b) of  FIG. 7 , the above-described concern is addressed in detail, assuming, first, that the image forming apparatus does not have slots  217  and a blocking portion  218 . Incidentally, although parts (a) and (b) of  FIG. 7  show only the relationship in position between the detection surface  202   a  of the second sensor  202 , and the detection hole  216   b , the relationship in position between the detection surface  201   a  of the first sensor  201 , and the detection hole  216   a , is the same as that of the detection surface  202   a  and detection hole  216   a.    
     Referring to part (a) of  FIG. 7 , when the protective shutter  211  is remaining closed, the detection surfaces  201   a  and detection surface  202   a  of the first and second sensors  201  and  202 , respectively, remain covered by the covering portion  211   a . Therefore, it is unlikely for the foreign substances (contaminants) to adhere to the detection surfaces  201   a  and detection surface  202   a . However, it sometimes occurs that as foreign substances (contaminants) are borne by the airflow A, they go through the detection holes  216   a  and  216   b , and accumulate on the areas below the detection holes  216   a  and  216   b.    
     Referring to part (b) of  FIG. 7 , if the protective shutter  211  is moved to its first position to be closed, while the foreign substances (contaminants) having accumulated on the areas below the detection holes  216   a  and  216   b  are still there, the foreign substances (contaminants) are carried by the protective shutter  211 . When the protective shutter  211  is in the first position, the detection holes  216   a  and  216   b  align with the detection surfaces  201   a  and detection surface  202   a  of the first and second sensors  201  and  202 , respectively. Therefore, it is possible that the foreign substances (contaminants) will be carried by the protective shutter  211 , and be positioned in a manner to face the detection surfaces  201   a  and  202   a  . In this embodiment, therefore, the image forming apparatus  100  is structured as follows to minimize the amount by which the detection surfaces  201   a  and  202   a  are soiled by these foreign substances (contaminants). 
     [Slots of Frame] 
     The first and second sensors  201  and  202  are disposed between the side wall  209   a  of the sensor unit  200 , and the recording medium conveyance passage  14  through which a sheet of recording medium is conveyed from the pair of registration rollers  12  to the secondary transferring portion T 2 . In other words, they are disposed on the bottom side of the airflow A which was bounced back from the recording medium passage  14 , in the sensor unit  200 . In this embodiment, therefore, the side wall  209   a  is provided with three slots  217 , which are located so that when the protective shutter  211  is in its second position, they align with the detection holes  216   a  and  216   b . The frame  209  is structured so that the bottom edge of each slots  217  is positioned lower than the bottom surface protective shutter  211 , and also, so that the bottom surface (edge)  217   a  of the slot is positioned lower than the top edge of the portions of the side wall  209   a , which correspond in position to the detection surfaces  201   a  and detection surface  202   a . That is, with respect to the direction of the movement of the protective shutter  211 , the bottom surface (edge)  217   a  of each slot  217  is positioned lower than the portions of the side wall  209   a , which correspond in position to the detection surfaces  201   a  and  202 . In this embodiment, the frame  209  is structured so that when the protective shutter  211  is in its second position (closed), the slots  217  align with the detection holes  216   a  and  216   b . That is, the frame  209  is structured as if the portions of the side wall  209   a , which correspond in position to the detection holes  216   a  and  216   b  when the protective shutter  211  is remaining closed, were cut out downward from their top side. By the way, this embodiment is not intended to limit the present invention, regarding how the slots are made. That is, the method for providing the frame  209  with the slots  207  does not need to be limited to the one that cuts parts of the frame  209  away. For example, instead of subjecting the precursor of the frame  209  to the secondary process of cutting, the material for the frame  209  may be punched out so that the precursor will be provided with the slots through the first process of punching. 
     As described above, in this embodiment, the frame  209  is structured so that the side wall  209   a , or the downstream side wall in terms of the direction in which the airflow A is bounced back, is provided with the slots  217 . Therefore, when the protective shutter  211  is remaining closed, as the foreign substances (contaminants) enter the sensor unit  200  through the detection holes  216   a  and  216   b  by being carried by the airflow A, they fall out of the sensor unit  200  through the slots  217 . Therefore, the image forming apparatus  100  in this embodiment is significantly smaller in the amount by which foreign substances (contaminants) accumulate on the area below the detection holes  216   a  and  216   b.    
     Further, the image forming apparatus  100  in this embodiment is structured so that the distance between the bottom surface (edge)  217   a  of each slot  217  and the protective shutter  211  is no less than the distance between the protective shutter  211  and detection surfaces  201   a  and  202   a . That is, it is structured so that the height of the slot  217  is no less than the distance from the protective shutter  211  to the detection surfaces  201   a  and detection surface  202   a . Incidentally, it is preferred that, in terms of the moving direction of the protective shutter  211 , the length of the slot  217  is no less than the length of the detection holes  216   a  and  216   b . By structuring the image forming apparatus  100  so that the height and length of the slot  217  becomes as described above, it is possible to ensure that the foreign substances (contaminants) having passed through the detection holes  216   a  and  216   b  are easily discharged through the slot  217 . 
     However, the image forming apparatus  100  is to be structured so that, in terms of the moving direction of the protective shutter  211 , the slots  217  do not overlap with the detection surfaces  201   a  and  202   a  of the first and second sensors  201  and  202 , respectively, in order to prevent the problem that the foreign substances (contaminants) having passed through the detection holes  216   a  and  216   b  adhere to the detection surfaces  201   a  and  202   a  when they are discharged through the slots  217 . 
     By the way, the image forming apparatus  100  is structured so that the side wall  209   a  is on the opposite side of the first and second sensors  201  and  202  from the recording medium conveyance passage  14 . However, there is no requirement regarding the recording medium conveyance passage side of the side wall  209   a . It depends of the strength or the like of the side wall  209   a . In this embodiment, the image forming apparatus  100  is structured so that in terms of the moving direction of the protective shutter  211 , each slot  207  is longer than the detection holes  216   a  and  216   b , and both edges of the slot  207  are on the outward side of the edges of the corresponding detection hole  201   a  and detection surface  202   a , respectively. With respect to the moving direction of the protective shutter  211 , each slot  217  may overlap with the substrative plate  206  for the first and second sensors  201  and  202 , as long as they do not overlap with the detection surfaces  201   a  and detection surface  202   a . In the case of the example shown in  FIG. 7 , the image forming apparatus  100  is structured so that the slots  217  do not overlap with the substrative plate  206 . 
     Since the side wall  209   a  is provided with the slots  217 , foreign substances (contaminants) are discharged through the slots  217  as they enter the sensor unit side of the sensor unit  200  through the slots  217 . Therefore, it is unlikely for the foreign substances (contaminants) to accumulate on the areas below the detection holes  216   a  and  216   b  when the protective shutter  211  is in the second position. Therefore, it is possible to reduce the image forming apparatus  100  in the amount by which the foreign substances (contaminants) are carried to the sensor unit  200  by the airflow A when the protective shutter  211  is moved to its first position to be opened to enable the first and second sensors  201  and  202  to detect the control toner images. That is, this embodiment can make it unlikely for foreign substances (contaminants) to adhere to the detection surfaces  201   a  and detection surface  202   a.    
     [Blocking Portion] 
     Next, referring to part (a) of  FIG. 7 , there are provided blocking portions  218  which will be between the detection holes  216   a  and  216   b  and the first and second sensors  201  and  202 , respectively, when the protective shutter  211  is in its second position. The blocking portion  218  is disposed in a space  219  which is between the protective shutter  211  and the first sensor  201  and between the protective shutter  211  and second sensor  202 . With respect to the moving direction of the protective shutter  211 , the blocking portion  218  at least partially blocks the portion of the space  219 , which is between the detection hole  216   a  and detection surface  201   a , and the portion of the space  219 , which is between the detection hole  216   a  and the detection surface detection surface  202   a . In this embodiment, the blocking portion  218  is a protrusion which protrudes into (downward) the space  19  from the protective shutter  211 . By the way, the blocking portion  218  does not need to a part of the protective shutter  211 . For example, it may be formed as a part of the frame  209  or sensor holder  210  so that it protrudes into (upward) the space  219  from the frame  209  or sensor holder  210 , respectively. 
     In this embodiment, the sensor unit  200  is provided with the blocking portions  218  as described above. Therefore, it is unlikely for foreign substances (contaminants) to enter the sensor unit  200  through the detection holes  216   a  and  216   b , and then, enter where the detection surfaces  201   a  and detection surface  202   a  of the first and second sensors  201  and  202 , respectively, are present. That is, this embodiment makes it unlikely for foreign substances (contaminants) to adhere to the detection surfaces  201   a  and detection surface  202   a.    
     [Confirmation of Effects of this Embodiment] 
     Next, referring to  FIGS. 8 and 9 , the experiments conducted to confirm the effectiveness of this embodiment are described.  FIG. 8  is a graph which shows the relationship among multiple conditions under which the image forming apparatus  100  was operated, cumulative number of sheets of recording medium conveyed through the secondary transferring portion T 2  (cumulative sheet count), and amount (%) of power inputted into the LEDs as a light sources  204  of the first and second sensors  201  and  202 . That is, the horizontal axis of  FIG. 8  represents the cumulative number of the sheets of recording medium conveyed through the secondary transferring portion T 2 , and the vertical axis of  FIG. 8  represents the amount (% relative to initial amount) by which electrical current was flowed to keep the output of the light catching portion  205  for the nondiffusively reflected light, stable at a preset level as the light catching portion  205  catches the light emitted by the light source (LED) and reflected by the surface of the intermediary transfer belt  121 .  FIG. 8  shows the changes in the amount (%) of power inputted into the LED, relative to the initial amount. 
     That is, as foreign substances (contaminants) adhere to the detection surfaces  201   a  and  202   a  of the first and second sensors  201  and  202 , respectively, the amount by which light can be caught by the light catching portions  205  for catching the nondiffusively reflected light. Thus, in order to keep the output voltage of the light catching portion  205  stable at a preset level, the light source  204  is increased in the amount of the light it emits. That is, the LED is increased in the amount of power input. Thus, the smaller the LED in input value, the smaller the amount of the foreign substances (contaminants) on the detection surfaces  201   a  and detection surface  202   a . In this embodiment, the initial value of the amount by which light source was provided with electric current was roughly 9 mA, and the maximum amount by which the light source was provided with electric current was 18.7 mA. 
     In these experiments, the light source  204  (LED) was adjusted in the amount by which it was supplied with power in such a manner that the output (background light amount) of the light catching portion  205  remains at 2.3 V as the light nondiffusively reflected by the surface of the intermediary transfer belt  121  is caught by the light catching portion  205 . If the amount of the background light is no more than 2.3 V, it is impossible to detect the difference between the output of the light catching portion  205  and the amount of the background light. That is, the sensors becomes lower in accuracy. 
       FIG. 9  is a drawing for describing the abovementioned multiple conditions. It shows the positional relationship between the bottom surface (edge)  217   a  of each slot  217 , and the detection surfaces  201   a  and  202   a  of the first and second sensors  201  and  202 , respectively. The conditions ( 1 )-( 5 ) presented in  FIGS. 8 and 9  are related to presence or absence of the slots  217 , size (position of bottom surface in terms of vertical direction) of the slot  217 , and presence or absence of the blocking portions  218 . Referring to  FIG. 9 , z 1  stands for the distance from the protective shutter  211  to the bottom surface (edge)  217   a  of the slot  217 , and z 2  stands for the distance from the protective shutter  211  to the detection surfaces  201   a  and  202   a.    
     In this case, Condition ( 1 ) is “z 1 ≤0” (if z 1  is negative, button surface (edge)  217   a  of slot  217  is positioned higher than bottom surface of protective shutter  211 ). That is, Condition ( 1 ) is that a sensor having no slot  217  was used. Condition ( 2 ) is “0&lt;z 1 ≤z 2 ”. That is, it is such a condition that the sensor unit  200  which has the slots  217  was used, but, the bottom surface (edge)  217   a  of each slot  217  was positioned higher than the detection surfaces  201   a  and  202   a . Condition ( 3 ) is “z 1 =z 2 ”. That is, it is such a condition that in terms of the vertical direction, the bottom surface  217   a  of the slot  217  is the same in position as the detection surfaces  201   a  and  202   a . Condition ( 4 ) is “z 1 &gt;z 2 &gt;0”. That is, it is such a condition that the bottom surface  217   a  of the slot  217  is positioned lower than the detection surfaces  201   a  and  202   a . Condition ( 5 ) is a combination of Condition ( 4 ) and employment of a sensor unit, the protective shutter  211  of which is provided with the blocking portion  218 . 
     In these experiments, images were formed, with the sensor units, which satisfy these conditions, being positioned as shown in  FIG. 6 , in order to obtain the relationship between the cumulative number of the sheets S of recording medium conveyed, and the amount by which the LED was supplied with electric power. The results of the experiments are shown in  FIG. 8 . 
     As will be evident from  FIG. 8 , in the case of Condition ( 1 ) in which the sensor unit  200  was provided with no slit  217 , the detection surfaces  201   a  and  202   a  began to be soiled by foreign substances (contaminants) as the cumulative number of the conveyed sheets S of recording medium reached approximately 15,000, making it impossible for the sensors to detect the control toner images. In comparison, in the cases of Conditions ( 2 )-( 4 ), in which the sensors were provided with the slots  217 , the lower was the bottom surface  217   a  of the slot  217  positioned, the smaller the amount by which the detection surfaces  201   a  and  202   a  were soiled by the foreign substances (contaminants). By the way, Conditions ( 3 ) and ( 4 ) were roughly the same in effectiveness, proving that all that is necessary is that the sensor unit  200  is structured so that the bottom surface  217   a  of the slot  217  is positioned no higher than the detection surfaces  201   a  and  202   a , that is, “z 1 ≥z 2 ” is satisfied. To summarize, in this embodiment, it was preferred that “z 1 ≥z 2 ” is satisfied as in Conditions ( 3 ) and ( 4 ). However, as long as “z 1 &lt;z 2 ” is satisfied as in Condition ( 2 ), it is possible to reduce the amount by which the detection surfaces  201   a  and  202   a  are soiled by foreign substances (contaminants). 
     In this embodiment, the size of each slot  217  was set so that “z 1 ≈z 2 ” is satisfied. Further, it became evident that providing the protective shutter  211  with the blocking portions  218  further reduces the amount by which the detection surfaces  201   a  and  202   a  are soiled. In this embodiment, the protective shutter  211  is provided with the blocking portions  218  as in the case of Condition ( 5 ). Therefore, even if an image forming operation is continued for a substantial length of time, the image forming apparatus  100  in this embodiment is substantially smaller in the amount by which the detection surfaces  201   a  and  202   a  are soiled by the foreign substances (contaminants). By the way, in this embodiment, the image forming apparatus  100  is structured so that the portion of the frame  209  which is not provided with the slot  217  is positioned higher than the bottom surface of the protective shutter  211 . However, the portion of the frame  209 , which is not provided with the slot  217 , may be positioned lower than the bottom surface of the protective shutter  211 . It is preferred that, in terms of the vertical direction, the portion of the frame  209 , which is not provided with the slot  217 , is positioned at roughly the same level as, or higher than, the protective shutter  211 . 
     As is evident from the detailed description of the structure of the image forming apparatus  100  and its sensor unit  200 , this embodiment can make it unlikely for foreign substances (contaminants) to adhere to the detection surfaces  201   a  and  202   a , and therefore, can keep the first and second sensors  201  and  202  accurate for a long time. Thus, it can prevent, for a long time, an image forming apparatus from forming such unsatisfactory images that suffer from color deviation and density deviation. That is, the present invention makes it possible to provide an image forming apparatus which can output high quality images for a long period time. 
     [Miscellanies] 
     The intermediary transferring member does not need to be an endless belt. For example, it may be a drum. The sensor for detecting the toner images on the intermediary transferring member does not need to be an optical sensor like those described above. For example, it may be a means for detecting the toner images with the use of an image sensor. That is, this embodiment is not intended to limit the sensor unit in structure. All that is necessary is that the sensors are structured so that they detect toner images through the detection surface. 
     In the embodiment described above, the secondary transfer front roller  123  was positioned on the upstream side of the secondary transfer inside roller  122 . However, the secondary transfer front roller  123  may be eliminated. In a case where the secondary transfer front roller  123  is not provided, the sensor unit  200  is to be positioned as close as possible to the secondary transfer inside roller  122 . 
     Further, the openings with which the side walls of the sensor unit are provided do not need to be slots. For example, they may be ordinary holes. For example, in a case where the focal distance of the sensors is large, and therefore, the distance between the protective shutter  211  and detection surfaces  201   a  and  202   a  has to be large, the side walls may be provided with ordinary hole instead of the slots. In such a case, it is desired that the position of the bottom portion, or the bottom portion of the hole, that is, the opposite portion of the hole from the protective shutter  211 , satisfies the same conditions as the bottom surface  217   a  of the slot  217 . That is, the image forming apparatus  100  is to be structured so that in terms of the moving direction of the protective shutter  211 , the openings align with the detection holes  216   a  and  216   b  when the protective shutter  211  is in its second position, and also, that in terms of the vertical direction, the openings are at least between the protective shutter  211 , and the detection surfaces  201   a  and  202   a  of the first and second sensors  201  and  202 . In a case where the openings are ordinary holes, the image forming apparatus  100  is to be structured so that the highest point of the holes is higher than the detection surfaces  201   a  and  202   a.    
     Further, in the embodiment described above, the image forming apparatus  100  was structured so that the positional relationship between the bottom surface  217   a  of the slot  217  and the detection surfaces  201   a  and  202   a  satisfies “z 1 ≥z 2 .” However, the image forming apparatus  100  may be structured so that “z 1 &lt;z 2 ” is satisfied. Even Condition ( 2 ), in which “z 1 &lt;z 2 ”, is effective compared to Condition ( 1 ) in which the side wall  209   a  is not provided with slots. Thus, the image forming apparatus  100  may be structured so that the slots  217  satisfy Condition ( 2 ). 
     By the way, in this embodiment, the detection surfaces  201   a  and  202   a  of the sensors  201  and  202  were parts of a transparent member (which in this embodiment was acrylic plate). However, the transparent member may be replaced with a piece of nontransparent plate provided with openings as light passages. In this embodiment, the openings through which light is allowed to reach the sensors  201  and  202  are also considered as detection windows. 
     The present invention makes it possible to provide an image forming apparatus structured so that foreign substances (contaminants) are unlikely to adhere to the detection surfaces of the sensors of the sensor unit of the apparatus. 
     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. 2016-245735 filed on Dec. 19, 2016, which is hereby incorporated by reference herein in its entirety.