Patent Publication Number: US-7914096-B2

Title: Image forming apparatus, landing position shift correction method, and landing position shift correction sheet member

Description:
BACKGROUND 
     1. Technical Field 
     This disclosure relates to an image forming apparatus including a recording head for jetting liquid droplets, a method of correcting shifts in landing positions of liquid droplets jetted from the recording head, and a landing position shift correction sheet member used for correcting shifts in the landing positions. 
     2. Description of the Related Art 
     There are image forming apparatuses such as printers, facsimile machines, copiers and multifunction peripherals having the aforementioned functions for performing image formation (recording, printing, etc., are also used synonymously) by the following method. That is, the image forming apparatus uses a liquid jetting device including a recording head with a liquid jetting head (liquid droplet jetting head) for jetting liquid droplets of a recording liquid (liquid). While a medium (hereinafter also referred to as “sheet”, although the material is not limited, and a recording target medium, a recording medium, a transfer material, a recording sheet, etc., are also used synonymously) is being conveyed, the liquid jetting device jets the recording liquid (liquid) so that the recording liquid adheres onto the sheet. 
     An apparatus that forms images by jetting liquid onto a medium such as paper, strings, fiber, fabrics, leather, metal, plastic, glass, wood, ceramics, etc., is referred to as an image forming apparatus. Furthermore, “image formation” not only means to form images that have meaning such as characters or figures on a medium, but also means to form images without meaning such as patterns onto a medium. Therefore, textile printing equipment and devices for forming metal wiring are also included. Moreover, the liquid is not particularly limited as long as it is capable of forming images. Furthermore, a device for jetting liquid from a liquid jetting head is referred to as a liquid jetting device, which is not limited to that for forming images. 
     In such an image forming apparatus employing the liquid jetting method, when printing is performed bidirectionally in forward and backward directions by moving back and forth a carriage provided with a recording head for jetting liquid droplets, the following problem may arise. That is, if the image to be printed includes parallel horizontal lines, the position of a horizontal line printed in a forward direction may be shifted from that of a backward direction (i.e., the lines may not be parallel to each other). 
     For this reason, when using a typical inkjet recording apparatus, a test chart for adjusting positional shifts of horizontal lines is output manually. The user selects and inputs the optimum value. The timing for jetting ink is adjusted based on the input value. However, different individuals may view the test chart in different ways. Furthermore, if the user is not familiar with this operation, there may be errors in the input data. Therefore, adjustment failures may be caused in this method. 
     There are conventional techniques for correcting density inconsistencies in image forming apparatuses employing the liquid jetting method. For example, patent document 1 discloses a technique of printing a test pattern onto a recording medium or a conveying belt, reading color data of the test pattern, and changing the conditions for driving the head based on the read results to correct density inconsistencies. 
     Patent Document 1: Japanese Examined Patent Publication No. H4-39041 
     Furthermore, patent document 2 discloses a technique for detecting nozzle failures of the liquid jetting head. Specifically, a test pattern, which includes colored dots, which are a combination of cyan ink, magenta ink, and yellow ink, is formed in a predetermined region on a member for holding and conveying a printing medium. This combination of dots is read by an RGB sensor. Based on the read results, nozzles having jetting failures are detected. 
     Patent Document 2: Japanese Patent No. 3838251 
     Patent Document 3 discloses a technique of making corrections as follows. A test pattern is recorded onto a part of a conveying belt. The test pattern includes any one of an idling nozzle detection pattern for detecting an idling nozzle, a color shift detection pattern for detecting color shifts of ink, and a head position adjustment pattern for adjusting the position of the recording head, or a combination of these patterns. This test pattern is read by an image pick-up unit such as a CCD, and the correction is made based on this result. 
     Patent Document 3: Japanese Laid-Open Patent Application No. 2005-342899 
     Patent Document 4 discloses a technique of detecting the density of toner images with an image forming apparatus employing the electrophotographic method of using toner. Specifically, a toner image is formed on a photoconductive drum. The image forming apparatus includes light emitting elements and light receiving elements for detecting the density of a toner image. The light receiving elements include one for receiving specular reflection light and one for receiving scattered light. These elements individually detect densities of toner images having different characteristics. 
     Patent Document 4: Japanese Laid-Open Patent Application No. H5-249787 
     Patent document 5 discloses a technique for detecting the amount of adhering toner. Specifically, a sensor simultaneously detects specular reflection light and diffuse reflection light, which are reflected from a toner image that has been formed. The detection results output from this sensor are used for detecting the amount of adhering toner. 
     Patent Document 5: Japanese Laid-Open Patent Application No. 2006-178396 
     However, as described in patent documents 1 through 3, when a test pattern is formed on a conveying belt and the color of the test pattern is detected or the test pattern is read by an image pick-up unit, the following problem may arise. That is, depending on the combination of the color of the conveying belt and the color of the ink, the difference of colors between the conveying belt and test pattern may be small. Thus, it may be difficult to accurately read the test pattern. In such a case, in order to accurately detect the color, it may be necessary to use a light source that changes its wavelength for each color, which increases the cost of the detecting unit. For example, the conveying belt may be an electrostatic belt formed with an insulating layer on its front and a mid-resistance layer on its back, with carbon incorporated to make the mid-resistance layer conductive. The appearance of such an electrostatic belt is black. Therefore, in the process of detecting a test pattern, the electrostatic belt may not be distinguished from black ink, merely with the use of reflections from colors or with an image obtained by an image pick-up unit. For this reason, the test pattern may not be detected with high precision. 
     More specifically, in the device for correcting density inconsistencies disclosed in patent document 1, the colors are read with a sensor. Therefore, if the color of the jetted ink droplets were similar to the color of the holding/conveying member, the detection precision would decrease. Thus, each color needs to be detected through a filter. This increases the number and types of sensors and filters, which leads to increased costs. Furthermore, the device for detecting nozzle failures disclosed in patent document 2 uses an RGB sensor. Therefore, if the color of the jetted ink droplets were similar to the color of the holding/conveying member, the detection precision would decrease. In order to increase the detection precision, it will be necessary to limit the combinations of the ink and the holding/conveying member. Furthermore, if laser light were to be used, extremely small points would be scanned. For this reason, the detection operation would be affected even by small foreign matter particles or scratches on the conveying member, which decreases the detection precision. If an RGB sensor were to be used, it would be necessary to provide at least one unit for reading each of the colors, which leads to increased cost. Moreover, the device disclosed in patent document 3, which uses the image pick-up unit, has the same problem as that of patent document 2. That is, if the color of the jetted ink droplets and the color of the holding/conveying member were similar, the detection precision would decrease. Furthermore, the image is recognized as a two-dimensional image, which requires a processing system with higher performance than the case of recognizing a one-dimensional image, which leads to increased costs. 
     Accordingly, the methods disclosed in patent documents 4 and 5 for detecting the adhering toner amount in the electrophotographic method may be applied. Even when toner particles contact each other, the shape of each particle is maintained. For this reason, it is possible to read the toner density even at portions where the toner is so concentrated that it is piled up along a rectangular line. However, in the case of liquid droplets, the droplets cohere to each other. Therefore, if these methods (of patent documents 4 and 5) were to be directly applied to an image forming apparatus employing the liquid jetting method, it would be possible to perform the detection, but only to the extent of detecting noise. Thus, the test pattern may not be detected with high precision. 
     Furthermore, in a case where the test pattern is formed on plain paper, which is a recording target medium into which ink can permeate, and the test pattern is read by an optical sensor, the following problem arises. That is, the ink will permeate into the paper and form smears, such that the pattern becomes blurred. As a result, the landing positions may not be accurately detected. 
     BRIEF SUMMARY 
     In an aspect of this disclosure, there are provided an image forming apparatus, a landing position shift correction method, and a landing position shift correction sheet member with which a landing position shift correction adjustment pattern formed by liquid droplets can be detected with high precision, and landing position detection and landing position shift correction can be performed with high precision. 
     In another aspect, there is provided an image forming apparatus for forming an image on a recording target medium that is being conveyed, including a recording head configured to jet liquid droplets; a pattern forming unit configured to form, on a water-repellent surface of a water-repellent sheet member, an adjustment pattern used for correcting shifts in landing positions of the liquid droplets, wherein the adjustment pattern includes plural liquid droplets that are separated from each other, and the water-repellent surface having a water-repellent property is formed on at least a part of a surface of the water-repellent sheet member; a reading unit including a light emitting unit configured to irradiate light onto the adjustment pattern and a light receiving unit configured to receive specular reflection light from the adjustment pattern; and a landing position correction unit configured to correct, based on a reading result obtained by the reading unit, the shifts in the landing positions of the liquid droplets that are jetted from the recording head. 
     In another aspect, there is provided a method of correcting landing positions of liquid droplets jetted from a recording head, including a forming step of forming, on a water-repellent surface of a water-repellent sheet member, an adjustment pattern used for correcting shifts in the landing positions of the liquid droplets, wherein the adjustment pattern includes plural liquid droplets that are separated from each other, and the water-repellent surface having a water-repellent property is formed on at least a part of a surface of the water-repellent sheet member; a detecting step of detecting the adjustment pattern by irradiating light onto the adjustment pattern from a light emitting unit and receiving specular reflection light from the adjustment pattern with a light receiving unit; and a correcting step of correcting, based on a detection result obtained by detecting the adjustment pattern at the detecting step, the shifts in the landing positions of the liquid droplets that are jetted from the recording head. 
     Thus, an image forming apparatus, a landing position shift correction method, and a landing position shift correction sheet member can be provided, with which a landing position shift correction adjustment pattern formed by liquid droplets can be detected with high precision, and landing position detection and landing position shift correction can be performed with high precision. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other aspects, features and advantages will become more apparent from the following detailed description when read in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram illustrating the overall configuration of an image forming apparatus according to an embodiment of the present invention; 
         FIG. 2  is a plan view of an image forming unit and a sub scanning conveying unit of the image forming apparatus shown in  FIG. 1 ; 
         FIG. 3  is a partially transparent side view of the elements shown in  FIG. 2 ; 
         FIG. 4  is a cross-sectional view of an example of a conveying belt; 
         FIG. 5  is a block diagram for describing an overview of a control unit; 
         FIG. 6  is a functional block diagram of parts relevant to detecting and correcting liquid droplet landing positions for describing a first embodiment of the present invention; 
         FIG. 7  is also a functional block diagram of a specific example of parts relevant to detecting and correcting liquid droplet landing positions; 
         FIG. 8  illustrates examples of adjustment patterns; 
         FIG. 9  illustrates a reading sensor; 
         FIG. 10  illustrates how light diffuses from a liquid droplet for describing the principle of pattern detection; 
         FIG. 11  illustrates how light diffuses when the liquid droplet has become flat; 
         FIG. 12  illustrates the relationship between the passage of time after the liquid droplet lands and the sensor output voltage; 
         FIG. 13  is a schematic diagram illustrating an adjustment pattern according to an embodiment of the present invention; 
         FIG. 14  is a schematic diagram illustrating an adjustment pattern of a comparative example; 
         FIG. 15  is a schematic diagram for making a comparison with an adjustment pattern formed with toner; 
         FIGS. 16A and 16B  illustrate a first example of a process for detecting the position of an adjustment pattern; 
         FIGS. 17A and 17B  illustrate a second example of a process for detecting the position of an adjustment pattern; 
         FIG. 18  illustrates a third example of a process for detecting the position of an adjustment pattern; 
         FIG. 19  illustrates a first example of shapes of liquid droplets that have landed to form an adjustment pattern; 
         FIGS. 20A and 20B  illustrate a second example of shapes of liquid droplets that have landed to form an adjustment pattern; 
         FIGS. 21A and 21B  illustrate a third example of shapes of liquid droplets that have landed to form an adjustment pattern; 
         FIGS. 22A through 22C  illustrate different examples of arrangement patterns of liquid droplets forming an adjustment pattern; 
         FIG. 23  is for describing the liquid droplet contact area within a detection range; 
         FIG. 24  illustrates the experimental results of the relationship between the proportion of the diffuse reflection area and the detection output expressed by an approximate line; 
         FIG. 25  is a schematic diagram of a liquid droplet for describing the pattern diffuse reflection rate; 
         FIG. 26  is a diagram for describing the contact angle of a liquid droplet; 
         FIG. 27  is a flowchart for describing a control process for adjusting the light emitting amount of a reading sensor; 
         FIG. 28  is a flowchart of a process for detecting and adjusting liquid droplet landing positions; 
         FIGS. 29A through 29D  illustrate block patterns; 
         FIG. 30  illustrates a horizontal line shift adjustment pattern; 
         FIGS. 31A and 31B  illustrate color shift adjustment patterns; 
         FIG. 32  illustrates an example of adjustment patterns; 
         FIG. 33  illustrates an example of a water-repellent sheet member; 
         FIG. 34  illustrates another example of a water-repellent sheet member; 
         FIGS. 35A and 35B  illustrate the relationship between the luster degree of a water-repellent sheet member and the specular reflection light; 
         FIGS. 36A and 36B  illustrate the relationship between the smoothness degree of a water-repellent sheet member and the specular reflection light; 
         FIG. 37  illustrates yet another example of a water-repellent sheet member; and 
         FIG. 38  is a flowchart of a process for detecting and adjusting liquid droplet landing positions with the use of the water-repellent sheet member shown in  FIG. 37 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A description is given, with reference to the accompanying drawings, of an embodiment of the present invention. 
     An overview of an image forming apparatus according to an embodiment of the present invention is described with reference to  FIGS. 1 through 3 .  FIG. 1  is a schematic diagram illustrating the overall configuration of the image forming apparatus,  FIG. 2  is a plan view of an image forming unit and a sub scanning conveying unit of the image forming apparatus, and  FIG. 3  is a partially transparent side view of the same. 
     The image forming apparatus includes an image forming unit  2  and a sub scanning conveying unit  3  disposed inside an apparatus main body  1  (inside a casing). The image forming unit  2  is for forming images while sheets are being conveyed. The sub scanning conveying unit  3  is for conveying sheets. A sheet feeding unit  4  including a sheet feeding cassette disposed at the bottom of the apparatus main body  1  feeds sheets  5  one by one. The sub scanning conveying unit  3  conveys the sheet  5  to a position facing the image forming unit  2 . While the sheet  5  is being conveyed, the image forming unit  2  jets liquid droplets onto the sheet  5  to form (record) a desired image. Subsequently, the sheet  5  is ejected, through a sheet eject conveying unit  7 , onto a sheet eject tray  8  formed in the upper section of the apparatus main body  1 . 
     Furthermore, the image forming apparatus includes, above the sheet eject tray  8  in the upper section of the apparatus main body  1 , an image scanning unit (scanner unit)  11  for scanning images, which is an input system for image data (printing data) to be used by the image forming unit  2  to form an image. In the image scanning unit  11 , a scanning optical system  15  including an illumination light source  13  and a mirror  14 , and a scanning optical system  18  including mirrors  16  and  17  are moved along for scanning an image of an original placed on a contact glass  12 . The scanned original image is read as image signals by an image reading element  20  disposed behind a lens  19 . The image signals that have been read are converted into digital signals. Image processing is performed on these digital signals. The image-processed printing data can be printed out as an image. 
     As shown in  FIG. 2 , in the image forming unit  2  of this image forming apparatus, a cantilevered carriage  23  is held by a guide rod  21  and a not-shown guide rail in such a manner as to be movable in the main scanning direction. The carriage  23  is moved in the main scanning direction by a main scanning motor  27  via a timing belt  29  that is wound around a driving pulley  28 A and a subordinate pulley  28 B. 
     As shown in  FIG. 2 , in the image forming unit  2  of this image forming apparatus, the carriage  23  is held by the carriage guide (guide rod)  21  and a guide stay  22  (see  FIG. 3 ) in such a manner as to be movable in the main scanning direction. The guide rod  21  is a main guide member bridged across a front side plate  101 F and a rear side plate  101 R. The guide stay  22  is a vertical guide member provided on a rear stay  101 B. The carriage  23  is moved in the main scanning direction by the main scanning motor  27  via the timing belt  29  that is wound around the driving pulley  28 A and the subordinate pulley  28 B. 
     A total of five liquid droplet jetting heads are provided in the carriage  23 . Specifically, there are recording heads  24   k   1 ,  24   k   2 , which are two liquid droplet jetting heads for jetting black (K) ink, and recording heads  24   c ,  24   m , and  24   y , each including one liquid droplet jetting head for jetting cyan (C) ink, magenta (M) ink, and yellow (Y) ink, respectively (hereinafter referred to as “recording head  24 ” when the colors need not be distinguished and when referred to collectively). This carriage  23  is a shuttle type carriage that moves in the main scanning direction to form images by jetting liquid droplets from the recording heads  24 , while the sheet  5  is being conveyed in the sheet conveyance direction (sub scanning direction) by the sub scanning conveying unit  3 . 
     Furthermore, sub tanks  25  are provided in the carriage  23  for supplying recording liquid of necessary colors to the recording heads  24 . Meanwhile, as shown in  FIG. 1 , ink cartridges  26  are removably attached to a cartridge insertion unit  26 A from the front of the apparatus main body  1 . The ink cartridges  26  are recording liquid cartridges for accommodating black (K) ink, cyan (C) ink, magenta (M) ink, and yellow (Y) ink. Ink (recording liquid) is supplied, through tubes, from the ink cartridges  26  each corresponding to one of the colors to the sub tanks  25  each corresponding to one of the colors. The black ink is supplied from one of the ink cartridges  26  to two of the sub tanks  25 . 
     The recording head  24  can be a piezo type head, a thermal type head, or an electrostatic type head. In the piezo type head, a piezoelectric element is used as a pressure generating unit (actuator unit) for pressurizing the ink inside an ink flow path (pressure generating chamber). The walls of the ink flow path are formed with oscillating plates. These oscillating plates are caused to deform by the piezoelectric element, so that the volume inside the ink flow path changes and ink droplets are jetted outside. In the thermal type head, a heating element is used to heat the ink in the ink flow paths so that bubbles are generated. Due to pressure caused by these bubbles, the ink droplets are jetted outside. In the electrostatic type head, an oscillating plate forming a wall of the ink flow path is disposed in such a manner as to face an electrode. An electrostatic force is generated between the oscillating plate and the electrode. This electrostatic force causes the oscillating plate to deform, so that the volume inside the ink flow path changes and ink droplets are jetted outside. 
     Furthermore, a linear scale  128  having slits is stretched across from the front side plate  101 F to the rear side plate  101 R along the main scanning direction of the carriage  23 . The carriage  23  is provided with an encoder sensor  129  that is a transmission photosensor for detecting the slits of the linear scale  128 . The linear scale  128  and the encoder sensor  129  form a linear encoder for detecting movements of the carriage  23 . 
     On one side of the carriage  23 , a pattern reading sensor  401  is provided, which is a reading unit (detecting unit) configured with a reflection photosensor including a light emitting unit and a light receiving unit for reading a landing position detection adjustment pattern according to an embodiment of the present invention. This pattern reading sensor  401  reads a landing position detection adjustment pattern formed on a water-repellent sheet member, as described below. On the other side of the carriage  23 , a sheet member detecting unit (leading edge detecting sensor)  330  is provided, which is a reflection photosensor for detecting the leading edge of a material being conveyed. 
     In a non-printing region on one side of the carriage  23  in the scanning direction, there is provided a maintaining/recovering mechanism (device)  121  for maintaining and recovering the operability of the nozzles of the recording head  24 . This maintaining/recovering mechanism  121  is a cap member for capping a nozzle face  24   a  (see  FIG. 3 ) of the five recording heads  24 . The maintaining/recovering mechanism  121  includes one suction cap  122   a  that also serves as a moisture retention cap, four moisture retention caps  122   b  through  122   e , a wiper blade  124  that is a wiping member for wiping the nozzle face  24   a  of the recording heads  24 , and an idle jetting reception section  125  for performing idle jetting. In a non-printing region on the other side of the carriage  23  in the scanning direction, another idle jetting reception section  126  is provided for performing idle jetting. This idle jetting reception section  126  includes openings  127   a  through  127   e.    
     As shown in  FIG. 3 , the sub scanning conveying unit  3  includes an endless conveying belt  31 , a charging roller  34 , a guide member  35 , pressurizing rollers  36  and  37 , a guide plate  38 , and a separating claw  39 . The conveying belt  31  is for changing the conveyance direction of the sheet  5 , which has been fed from below, by substantially 90 degrees, and conveying the sheet  5  in such a manner as to face the image forming unit  2 . The conveying belt  31  is stretched around a conveying roller  32  that is a driving roller and a subordinate roller  33  that is a tension roller. The charging roller  34  is a charging unit to which a high voltage alternating current is applied from a high voltage power source for charging the surface of the conveying belt  31 . The guide member  35  is for guiding the conveying belt  31  in a region facing the image forming unit  2 . The pressurizing rollers  36  and  37  are rotatably held by a holding member  136 . The pressurizing rollers  36  and  37  are for pressing the sheet  5  against the conveying belt  31  at a position facing the conveying roller  32 . The guide plate  38  is for guiding the top face of the sheet  5  with an image formed by the image forming unit  2 . The separating claw  39  is for separating, from the conveying belt  31 , the sheet  5  with an image. 
     The conveying belt  31  is configured to revolve in the sheet conveyance direction (sub scanning direction) as the conveying roller  32  is rotated by a sub scanning motor  131  using a DC brushless motor via a timing belt  132  and a timing roller  133 . As shown in  FIG. 4 , the conveying belt  31  has, for example, a two layer structure including a front layer  31 A to which the sheet adheres and a back layer (mid-resistance layer, earth layer)  31 B. The front layer  31 A is made of a pure resin material such as an ETFE pure material that has not been subjected to resistance control. The back layer  31 B is made of the same material as the front layer  31 A except that carbon has been added to control the resistance. However, the structure is not limited thereto; the conveying belt  31  can have a single layer structure or a structure with three or more layers. 
     Furthermore, a Mylar (paper powder removing unit)  191 , a cleaning brush  192 , and a discharging brush  193  are provided between the subordinate roller  33  and the charging roller  34 , arranged in this order from the upstream side of the movement direction of the conveying belt  31 . The Mylar  191  is a cleaning unit for removing paper powder, etc., adhering to the surface of the conveying belt  31 . The Mylar  191  is an abutment member made of a PET film, which abuts the surface of the conveying belt  31 . The cleaning brush  192  is a brush that also abuts the surface of the conveying belt  31 . The discharging brush  193  is for removing electric charges from the surface of the conveying belt  31 . 
     Moreover, a high-resolution code wheel  137  is attached to a shaft  32   a  of the conveying roller  32 . An encoder sensor  138  is provided, which is a transmission photosensor for detecting slits  137   a  formed on this code wheel  137 . The code wheel  137  and the encoder sensor  138  form a rotary encoder. 
     The sheet feeding unit  4  includes a sheet feeding cassette  41 , a sheet feeding roller  42 , a friction pad  43 , and a pair of resist rollers  44 . The sheet feeding cassette  41  is an accommodation unit for accommodating multiple stacked sheets  5 , which sheet feeding cassette  41  can be inserted in/removed from the apparatus main body  1 . The sheet feeding roller  42  and the friction pad  43  are for separating the sheets  5  in the sheet feeding cassette  41  from each other and sending them out one by one. The resist rollers  44  are for resisting the sheet  5  being fed. 
     Furthermore, the sheet feeding unit  4  includes a manual feed tray  46 , a manual feed roller  47 , and a vertical conveying roller  48 . The manual feed tray  46  is for accommodating multiple stacked sheets  5 . The manual feed roller  47  is for feeding the sheets  5  one by one from the manual feed tray  46 . The vertical conveying roller  48  is for conveying the sheet  5  that is fed from a sheet feeding cassette that is optionally installed at the bottom of the apparatus main body  1  or from a double-side unit. Members for feeding the sheet  5  to the sub scanning conveying unit  3 , such as the sheet feeding roller  42 , the resist rollers  44 , the manual feed roller  47 , and the vertical conveying roller  48 , are rotated by a sheet feeding motor (driving unit)  49  that is an HB type stepping motor, via a not-shown electromagnetic clutch. 
     The sheet eject conveying unit  7  includes an openable/closable sheet eject guide plate  73  provided with three conveying rollers  71   a ,  71   b , and  71   c  (referred to as “conveying rollers  71 ” when not distinguished) and spurs  72   a ,  72   b , and  72   c  (referred to as “spurs  72 ” when not distinguished) that face the conveying rollers  71 , a pair of reverse rollers  77 , and a pair of reverse sheet eject rollers  78 . The conveying rollers  71  are for conveying the sheet  5  which has been separated from the conveying belt  31  by the separating claw  39  of the sub scanning conveying unit  3 . The reverse rollers  77  and the reverse sheet eject rollers  78  are for reversing the sheet  5  and sending the sheet  5  face-down to the sheet eject tray  8 . Moreover, the sheet eject conveying unit  7  is provided with a separating claw  60  for conveying the sheet to the double-side unit in a case where the double-side unit is installed in the apparatus main body  1 . 
     Furthermore, in order to manually feed a single sheet, as shown in  FIG. 1 , on one side of the apparatus main body  1  there is provided a single sheet manual feed tray  141  that can be opened and closed (in such a manner as to be unfolded) with respect to the apparatus main body  1 . When a single sheet is to be fed manually, the single sheet manual feed tray  141  is opened (unfolded) to the position indicated by a horizontal virtual line. By opening this single sheet manual feed tray  141 , a single sheet manual feed shutter  144  opens, so that the sheet  5  can enter the image forming unit  2 . The sheet  5  that is fed manually from the single sheet manual feed tray  141  is guided along the top surface of a guide plate  110  and is then linearly inserted in between the conveying roller  32  and the pressurizing roller  36  of the sub scanning conveying unit  3 . 
     Meanwhile, in order to eject the sheet  5  on which an image has been formed face-up and in a straight manner, a straight sheet eject tray  181  that can be opened and closed (unfolded) is provided on the other side of the apparatus main body  1 . By opening (unfolding) this straight sheet eject tray  181 , the sheet  5  that is sent out from the sheet eject conveying unit  7  can be linearly ejected to a stack part  82  of the straight sheet eject tray  181 . 
     Next, an overview of a control unit of this image forming apparatus is described with reference to a block diagram shown in  FIG. 5 . 
     A control unit  300  includes a main control unit  310  for controlling the entire apparatus as well as specific operations according to embodiments of the present invention such as forming adjustment patterns, detecting adjustment patterns, and adjusting (correcting) landing positions. The main control unit  310  includes a CPU  301 , a program to be executed by the CPU  301 , a ROM  302  for storing other fixed data, a RAM  303  for temporarily storing image data, etc., a nonvolatile memory (NVRAM)  304  for holding data even while the power of the apparatus is shut off, and an ASIC  305  for performing various signal processings on the image data, image processings such as sorting, and other processings on input/output signals to control the entire apparatus. 
     Furthermore, the control unit  300  includes an external I/F  311 , a head driving control unit  312 , a main scanning driving unit (motor driver)  313 , a sub scanning driving unit (motor driver)  314 , a sheet feed driving unit  315 , a sheet eject driving unit  316 , and an AC bias supplying unit  319 . The external I/F  311  is provided between the host side and the main control unit  310  for transmitting/receiving data and signals. The head driving control unit  312  includes a head driver (actually provided in the recording head  24 ) configured with a head data generating rearranging ASIC for driving/controlling the recording head  24 . The main scanning driving unit  313  is for driving the main scanning motor  27  to move the carriage  23 . The sub scanning driving unit  314  is for driving the sub scanning motor  131 . The sheet feed driving unit  315  is for driving the sheet feeding motor  49 . The sheet eject driving unit  316  is for driving a sheet eject motor  79  which drives the rollers of the sheet eject conveying unit  7 . The AC bias supplying unit  319  is for supplying an AC bias to the charging roller  34 . Although not shown, the control unit  300  also includes a recovering system driving unit for driving a maintaining/recovering motor which drives the maintaining/recovering mechanism  121 , a double side driving unit for driving a double side unit if the double side unit is installed, a solenoid driving unit (driver) for driving various solenoids (SOL), a clutch driving unit for driving electromagnetic clutches, and a scanner control unit  325  for controlling the image scanning unit  11 . 
     Various detection signals of an environment sensor  234  for detecting, for example, the temperature and the humidity around the conveying belt  31  (environment conditions) are input to the main control unit  310 . Detection signals of other not-shown sensors are also input to the main control unit  310 . Furthermore, the main control unit  310  acquires necessary key input from various keys provided in the apparatus main body  1  such as a numeric keypad and a print start key, and outputs display information to an operations/display unit  327  including various display devices. 
     Moreover, output signals from the photosensor (encoder sensor)  129 , which is a part of the linear encoder for detecting the above-described carriage position, are input to the main control unit  310 . Based on these output signals, the main control unit  310  moves the carriage  23  back and forth in the main scanning direction by driving/controlling the main scanning motor  27  via the main scanning driving unit  313 . Furthermore, output signals (pulses) from the photosensor (encoder sensor)  138 , which is a part of the rotary encoder for detecting the movement amount of the above-described conveying belt  31 , are input to the main control unit  310 . Based on these output signals, the main control unit  310  moves the conveying belt  31  via the conveying roller  32  by driving/controlling the sub scanning motor  131  via the sub scanning driving unit  314 . 
     The main control unit  310  moves a water-repellent sheet member to a position for forming an adjustment pattern based on detection signals from the sheet member detecting unit  330 . The main control unit  310  performs a process for forming an adjustment pattern on the water-repellent sheet member. The main control unit  310  performs a light emitting driving control operation for emitting light onto the formed adjustment pattern from the pattern reading sensor  401  installed in the carriage  23 . Output signals from the light receiving unit are input to the main control unit  310  to detect the adjustment pattern and to detect the landing position shift amount. Based on these detection results, the main control unit  310  performs a control operation to correct the timings at which liquid droplets are jetted from the recording heads  24  so as to eliminate shifts of landing positions. 
     A brief description is given of an image forming operation of the image forming apparatus having the above configuration. The rotation amount of the conveying roller  32  for driving the conveying belt  31  is detected. According to the detected rotation amount, the sub scanning motor  131  is driven/controlled, and high voltage alternating current rectangular waves of positive and negative polarities are applied from the AC bias supplying unit  319  to the charging roller  34 . Accordingly, positive and negative charges are alternately applied onto the conveying belt  31  in a striped manner with respect to the conveyance direction of the conveying belt  31 . Thus, the conveying belt  31  is charged with predetermined charge widths so that a non-uniform electric field is generated. 
     The sheet  5  is fed from the sheet feeding unit  4 , and is sent in between the conveying roller  32  and the first pressurizing roller  36 . When the sheet  5  is sent onto the conveying belt  31 , on which charges of positive and negative polarities are formed so that a non-uniform electric field is generated, the sheet  5  immediately becomes polarized according to the direction of the electric field. Then, the sheet  5  adheres onto the conveying belt  31  due to an electrostatic adhering force, so that it is conveyed along with the movement of the conveying belt  31 . 
     The sheet  5  is intermittently conveyed by the conveying belt  31 . The carriage  23  is moved in the main scanning direction to jet liquid droplets of recording liquid from the recording heads  24  onto the halting sheet  5  to record (print) an image. The leading edge of the sheet  5  which has undergone the printing operation is separated from the conveying belt  31  with the separating claw  39 . The sheet  5  is then sent out to the sheet eject conveying unit  7  and is ejected onto the sheet eject tray  8 . 
     Furthermore, during standby periods between printing (recording) operations, the carriage  23  is moved to the maintaining/recovering mechanism  121 . The nozzle faces of the recording heads  24  are capped by caps  122  so that the nozzles are maintained in a moist state. This prevents jetting failures that may be caused when the ink becomes dry. Furthermore, a recovery operation is performed by suctioning the recording liquid through the nozzles and discharging viscous recording liquid and bubbles, where the recording heads  24  are capped by suction and moisture retention caps  122 . By performing this recovery operation, ink adheres to the nozzle faces of the recording heads  24 . In order to clean/remove this ink, the wiper blade  124  wipes off the ink. Furthermore, before starting the recording operation or during the recording operation, the recording heads  24  perform idle jetting operations by jetting ink into the idle jetting reception section  125 , which ink is unrelated to the recording operation. Accordingly, the jetting performance of the recording heads  24  can be maintained at a stable level. 
     Next, the units relevant to liquid droplet landing position shift correction control in the image forming apparatus are described with reference to  FIGS. 6 and 7 .  FIG. 6  is a block diagram illustrating the functions of the liquid droplet landing position shift correction unit.  FIG. 7  is a block diagram of the functional flow of the liquid droplet landing position shift correction operation. 
     As shown in  FIGS. 7 and 9 , the carriage  23  is provided with the pattern reading sensor  401  for reading an adjustment pattern (also referred to as DRESS pattern, test pattern, detection pattern, etc.)  400 , which is formed on a water-repellent sheet member  700 . The pattern reading sensor  401  includes a light emitting element  402  and a light receiving element  403 , which are arranged in a direction perpendicular to the main scanning direction, and are held and packaged in a holder  404 . The light emitting element  402  is a light emitting unit for emitting light onto the adjustment pattern  400  on the water-repellent sheet member  700 . The light receiving element  403  is a light receiving unit for receiving specular reflection light from the adjustment pattern  400 . A lens  405  is provided at the light beam outgoing part and the light beam incoming part of the holder  404 . 
     Inside the pattern reading sensor  401 , the light emitting element  402  and the light receiving element  403  are arranged in a direction perpendicular to the main scanning direction of the carriage  23 , which main scanning direction is indicated in  FIG. 2 . Accordingly, the detection results (reading results) are less affected by fluctuations in the movement speed of the carriage  23 . Furthermore, a relatively simple and inexpensive light source can be used as the light emitting element  402 , for example, light of an infrared region such as an LED or visible light. Furthermore, the spot diameter (detection range, detection region) of the light source is detected in units of millimeters because an inexpensive lens is used instead of a high-precision lens. 
     An adjustment pattern forming/reading control unit  501  causes the conveying belt  31  to convey the water-repellent sheet member  700 . When the sheet member detecting unit  330  detects the leading edge of the water-repellent sheet member  700 , the water-repellent sheet member  700  is further conveyed to the position for forming the adjustment pattern  400 . Subsequently, the carriage  23  is moved back and forth in the main scanning direction while a liquid droplet jetting control unit  502  causes the recording heads  24 , which are liquid droplet jetting units, to jet liquid droplets. Accordingly, the line-shaped adjustment patterns  400  ( 400 B 1 ,  400 B 2 ,  400 C 1 ,  400 C 1 , and so forth) as shown in  FIG. 8  are formed with plural liquid droplets (ink drops)  500  on the water-repellent sheet member  700 . The adjustment pattern forming/reading control unit  501  is a part of the CPU  301  of the main control unit  310 . 
     The adjustment pattern forming/reading control unit  501  reads, with the pattern reading sensor  401 , the adjustment pattern  400  formed on the water-repellent sheet member  700 . This adjustment pattern reading control operation is performed by emitting light from the light emitting element  402  of the pattern reading sensor  401  while moving the carriage in the main scanning direction. Specifically, as shown in  FIG. 7 , the CPU  301  of the main control unit  310  sets, in a light eminence control unit  511 , a PWM value for driving the light emitting element  402  of the pattern reading sensor  401 . Output from the light eminence control unit  511  is smoothed by a smoothing circuit  512 , and the smoothed output is supplied to a driving circuit  513 . Accordingly, the driving circuit  513  causes the light emitting element  402  to emit light, so that light output from the light emitting element  402  is irradiated onto the adjustment pattern  400  on the water-repellent sheet member  700 . 
     In the pattern reading sensor  401 , as light output from the light emitting element  402  is irradiated onto the adjustment pattern  400  on the water-repellent sheet member  700 , the specular reflection light reflected from the adjustment pattern  400  is irradiated into the light receiving element  403 . The light receiving element  403  outputs detection signals according to the amount of the specular reflection light received from the adjustment pattern  400 . These detection signals are input to a landing position shift amount computing unit  503  of a landing position correction unit  505 . Specifically, as shown in  FIG. 7 , the output signals from the light receiving element  403  of the pattern reading sensor  401  are subjected to photoelectric conversion with a photoelectric conversion circuit  521  (not shown in  FIG. 5 ) included in the main control unit  310 . Noise components are removed from these photoelectric-converted signals (sensor output voltage) with a lowpass filter  522 . Then, the signals are subjected to A/D conversion with an A/D conversion circuit  523 . The A/D converted sensor output voltage data are loaded in a common memory  525  by a signal processing circuit (DSP)  524 . 
     The landing position shift amount computing unit  503  of the landing position correction unit  505  detects the position of the adjustment pattern  400  based on output results from the light receiving element  403  of the pattern reading sensor  401 , and calculates the shift amount with respect to a standard position (liquid droplet landing position shift amount). The landing position shift amount calculated by the landing position shift amount computing unit  503  is given to a jetting timing correction amount computing unit  504 . The jetting timing correction amount computing unit  504  calculates the correction amount of the jetting timing so that there are no shifts in the landing position when the liquid droplet jetting control unit  502  drives the recording heads  24 . The jetting timing correction amount computing unit  504  sets the calculated jetting timing correction amount in the liquid droplet jetting control unit  502 . Accordingly, the liquid droplet jetting control unit  502  can drive the recording heads  24  at jetting timings that have been corrected based on the correction amount. Thus, the shifts in the liquid droplet landing positions can be reduced. 
     Specifically, as shown in  FIG. 7 , with the use of a processing algorithm  526  that is executed by the CPU  301 , the center position (point A) of each adjustment pattern  400  (one of the line patterns is denoted by “ 400   a ”) is detected from a sensor output voltage So as indicated by (a) in  FIG. 7  loaded in the common memory  525 . Then, the jetting timing correction amount computing unit  504  calculates the shift amount of the actual landing position of liquid jetted from the corresponding head with respect to a standard position (standard head). Based on the shift amount, the jetting timing correction amount computing unit  504  calculates the correction amount of the printing jetting timings. The jetting timing correction amount computing unit  504  sets this correction amount in the liquid droplet jetting control unit  502 . 
     The adjustment pattern  400  according to an embodiment of the present invention is further described with reference to  FIG. 10  onward. 
     First, a description is given of the principle of landing position detection (pattern detection) according to an embodiment of the present invention. With reference to  FIG. 10 , a description is given of how light diffuses from a liquid droplet (hereinafter, “ink drop”) when the light is irradiated onto the ink drop. 
     As shown in  FIG. 10 , incident light  601  hits an ink drop  500  that has landed on a landing target member  600  (the landed ink drop becomes a hemisphere). Because the liquid droplet  500  has a round, lustrous surface, most of the incident light  601  turns into diffuse reflection light  602 . Hence, only a small amount of the light can be detected as specular reflection light  603 . However, as shown in  FIG. 11 , the liquid droplet  500  dries with the passage of time, and therefore the surfaces looses luster, and the shape gradually changes into a flat shape from the hemispheric shape. As a result, the range and proportion of the specular reflection light  603  becomes relatively larger than those of the diffuse reflection light  602 . Thus, when the specular reflection light  603  is received by the light receiving element  403 , as shown in  FIG. 12 , the sensor output voltage decreases with the passage of time, and therefore the detection precision also decreases with the passage of time. 
     Next, with reference to  FIG. 13 , a description is given of the operation of detecting the position of the ink drop  500  included in the adjustment pattern  400  (more specifically, one pattern  400   a ). 
     The surface of the water-repellent sheet member  700  is lustrous, and therefore tends to reflect specular reflection light when light is received from the light emitting element  402 . For this reason, as indicated by (b) in  FIG. 13 , the amount of specular reflection light  603  is large in the region on the water-repellent sheet member  700  without any ink drops  500 , because almost all of incident light  601  from the pattern reading sensor  401  becomes specular reflection light when reflected from the water-repellent sheet member  700 . Accordingly, as indicated by (a) in  FIG. 13 , the output (sensor output voltage) from the light receiving element  403  for receiving the specular reflection light  603  is relatively large. 
     Meanwhile, as indicated by (b) in  FIG. 13 , in the region where the landed ink drops  500  are separated from each other but close to each other, the amount of the specular reflection light  603  is reduced because the light is diffused on the surfaces of the lustrous, hemispheric ink drops  500 . Therefore, as indicated by (a) in  FIG. 13 , the output (sensor output voltage) from the light receiving element  403  for receiving the specular reflection light  603  is relatively small. Incidentally, when ink drops that are “close to each other” refers to where the area between the ink drops  500  is smaller than the area on which the ink drops  500  have landed (adhering area), within a predetermined detection region. 
     In a comparative example, as indicated by (b) in  FIG. 14 , when the adjacent ink drops have contacted each other and have become connected to each other on the water-repellent sheet member  700 , the top surface of the connected ink drops  500  becomes flat. As a result, the amount of specular reflection light  603  will increase. Therefore, as indicated by (a) in  FIG. 14 , the output value of the sensor output voltage will become substantially the same for the region on the water-repellent sheet member  700  without the ink drops  500  and the region with the ink drops  500 , which makes it difficult to detect the positions of the ink drops  500 . Even when the ink drops  500  have become connected to each other, diffuse light is generated at the edges of the connected ink drop  500 . Nevertheless, detection is still difficult because the diffuse light is generated from extremely small portions. If an attempt were made to detect the ink drops, the area to be examined with the light receiving element  403  (region to be detected) would need to be narrowed down. Accordingly, the detection may be affected by noise elements such as slight scratches or dust on the surface of the water-repellent sheet member  700 , which may decrease the detection precision and/or degrade the reliability of detection results. 
     The landing positions of ink drops can be detected by identifying the portions where specular reflection light is attenuated, from the output from the light receiving unit for receiving specular reflection light from the ink drops. In order to detect the landing positions of ink drops with high precision, the adjustment pattern  400  needs to include plural liquid droplets that are separated from each other but close to each other (in the detection region, the area between the ink drops is smaller than the adhering area where the ink drops are adhering to the member). By forming such an adjustment pattern, the adjustment pattern (liquid droplet landing position) can be detected with high precision with a simple structure including a light emitting unit and a light receiving unit. 
     The difference between toner used in an electrophotographic method and liquid droplets used in the liquid jetting method is described with reference to  FIG. 15 . 
     The shape of each toner particle used in an electrophotographic method is maintained even when the toner is adhering to an adhering target member. Therefore, even when toner particles  611  included in the adjustment pattern on a adhering target member  610  are stacked on top of each other as shown in  FIG. 15 , the amount of specular reflection light from an area on the adhering target member  610  on which toner is adhering (toner adhering surface) is smaller than that from an area on the adhering target member  610  without any toner. Accordingly, the adjustment pattern can be detected with the output from the light receiving unit for receiving specular reflection light. 
     Conversely, liquid droplets have a peculiar property. As described above, when liquid droplets that have landed on the landing target member become connected with adjacent liquid droplets, the top part becomes flat. As a result, the amount of specular reflection light from the connected liquid droplets will become substantially the same as that from the surface of the landing target member without any liquid droplets. If the adjustment pattern is simply detected according to the difference in the amount of specular reflection light received from the adjustment pattern, without considering such a peculiar property of liquid droplets, the detection precision will be significantly degraded. Particularly, when the adjustment pattern is formed by making ink drops land onto a media into which ink can permeate, such as a recording target medium, the pattern may not be accurately detected. 
     An embodiment of the present invention takes into consideration this peculiar property of liquid droplets. That is, an adjustment pattern is formed on a water-repellant sheet member. On this water-repellant sheet member, an adjustment pattern is formed with plural liquid droplets that are separated from each other. The area between the liquid droplets is smaller than the area where the liquid droplets are adhering to the member, within the detection region. Accordingly, the adjustment pattern can be detected with high precision based on the difference in the amount of specular reflection light from the adjustment pattern. As a result, shifts in liquid droplet landing positions can be adjusted (corrected) with high precision. 
     Next, different examples of a position detection process (reading process) of the adjustment pattern  400  formed on the water-repellent sheet member  700  is described with reference to  FIGS. 16A through 18 . 
       FIGS. 16A and 16B  illustrate a first example. For example, as shown in  FIG. 16A , a line-shaped pattern  400   k   1  is formed with the recording head  24   k   1 , and a line-shaped pattern  400   k   2  is formed with the recording head  24   k   2 . These are scanned with the pattern reading sensor  401  in the sensor scanning direction (carriage main scanning direction). Based on the output results from the light receiving element  403  of the pattern reading sensor  401 , as shown in  FIG. 16B , a sensor output voltage So is obtained, which falls at the pattern  400   k   1  and the pattern  400   k   2 . 
     By comparing this sensor output voltage So with a predetermined threshold Vr, the positions at which the sensor output voltage So becomes lower than the threshold Vr can detected as edges of the pattern  400   k   1  and the pattern  400   k   2 . The area centroid of the region surrounded by the lines representing the threshold Vr and the sensor output voltage So (the hatched parts in the figure) is calculated. This area centroid can be set to be the center of the adjustment pattern  400 . By using a centroid, it is possible to reduce errors caused by microscopic variations of the sensor output voltage. 
       FIGS. 17A and 17B  illustrate a second example. By scanning the same patterns  401   k   1 ,  400   k   2  as those of the first example with the pattern reading sensor  401 , a sensor output voltage So as shown in  FIG. 17A  can be obtained.  FIG. 17B  is an enlarged view of the portion where the sensor output voltage So falls. 
     This portion where the sensor output voltage So falls is searched in a direction indicated by an arrow Q 1  shown in  FIG. 17B , and the point where the sensor output voltage So falls below (becomes less than or equal to) a lower limit threshold Vrd is stored as a point P 2 . Next, from the point P 2 , the sensor output voltage So is searched in a direction indicated by an arrow Q 2 , and the point where the sensor output voltage So exceeds an upper limit threshold Vru is stored as a point P 1 . Then, a regression line L 1  is calculated from the output voltage So between the point P 1  and the point P 2 . An obtained regression line formula is used to calculate an intersection point C 1  of the regression line L 1  and an intermediate value Vrc of the upper and lower thresholds. In the same manner, a regression line L 2  is calculated for the stand up portion of the sensor output voltage So. An intersection point C 2  of the regression line L 2  and the intermediate value Vrc of the upper and lower thresholds is calculated. Based on the intermediate point of the intersection point C 1  and the intersection point C 2 , a line center C 12  is obtained by (intersection point C 1 +intersection point C 2 )/2. 
       FIG. 18  illustrates a third example. As shown in  FIG. 18(   a ), similar to the first example, on the water-repellent sheet member  700 , the line-shaped pattern  400   k   1  is formed with the recording head  24   k   1 , and the line-shaped pattern  400   k   2  is formed with the recording head  24   k   2 . These are scanned with the pattern reading sensor  401  in the main scanning direction. Accordingly, a sensor output voltage So (photoelectric conversion output voltage) is obtained, as shown in  FIG. 18(   b ). 
     With the above-described processing algorithm  526 , a process is performed to remove harmonic noise with an IIR filter, and then the quality of the detected signals is evaluated (whether there are missing signals, unstable signals, or excessive signals), tilted portions near the threshold Vr are detected, and a regression curve is calculated. Furthermore, intersection points a 1 , a 2 , b 1 , and b 2  of the regression curve and the threshold Vr are calculated (in a practical situation, the calculation is performed by a position counter, which position is configured with an ASIC: application-specific integrated circuit). Moreover, an intermediate point A of the intersection points a 1  and a 2 , and an intermediate point B of the intersection points b 1  and b 2  are calculated, and a length L between the intermediate point A and the intermediate point B is calculated. Accordingly, an intermediate position between the pattern  400   k   1  and the pattern  400   k   2  is determined. 
     Then, the difference between the calculated length L and an ideal length between the recording head  24   k   1  and the recording head  24   k   2  is calculated by (ideal length between heads-L). This difference corresponds to the shift amount of an actually printed image. Therefore, based on this obtained shift amount, a correction value is calculated for correcting the timing of jetting liquid droplets from the recording heads  24   k   1 ,  24   k   2  (liquid droplet jetting timing). The correction value is set in the liquid droplet jetting control unit  502 . Accordingly, the liquid droplet jetting control unit  502  drives the heads at the corrected liquid droplet jetting timings, and therefore positional shifts are reduced. 
     Next, examples of different shapes of landed ink drops forming the adjustment pattern  400  are described with reference to  FIGS. 19 through 21B . 
       FIG. 19  illustrates a first example in which plural liquid droplets  500  are arranged separately from each other in a lattice. 
       FIG. 20A  and  FIG. 20B  illustrate two variations of a second example. In the variation shown in  FIG. 20A , a large drop (for example, a main drop) and a small drop (for example, a satellite drop or a small drop) are combined together to form a single liquid droplet  500 A having a lageniform shape. These liquid droplets  500 A are arranged separately from each other in a lattice. In the variation shown in  FIG. 20B , two liquid droplets having substantially the same size are combined together to form a single liquid droplet  500 B. These liquid droplets  500 B are arranged separately from each other in a lattice. 
       FIG. 21A  and  FIG. 21B  illustrate two variations of a third example. In the modification shown in  FIG. 21A , the liquid droplets are consecutively combined together in the direction perpendicular to a scanning direction of the pattern reading sensor  401 , thus forming a single liquid droplet  500 C shaped as a line. Plural line-shaped liquid droplets  500 C are arranged in the sensor scanning direction. In the variation shown in  FIG. 21B , liquid droplets  500 D are shaped as irregularly broken lines. That is, there are partially missing portions in the lines of the variation shown in  FIG. 21A  (the lengths can be the same or different). Plural line-shaped liquid droplets  500 D are arranged in the sensor scanning direction. 
     Next, configurations for improving the precision of landing position detection are described with reference to  FIGS. 22A through 23 . 
     First, it is assumed that the proportion of diffuse reflection light included in the reflection light from the adjustment pattern  400  is fixed. That is, the liquid droplets  500  are made to land in such a manner that the reflection light from the adjustment pattern  400  diffuses in a uniform manner, as in the ink droplets that have landed in the center part of  FIG. 13 . Accordingly, the sensor output voltage (detection potential) applied to the process algorithm will have high reproducibility, and therefore a highly precise adjustment pattern  400  (liquid droplet landing position) can be detected with high precision, and shifts in the liquid droplet landing position can be adjusted with high precision. 
     In order to make the reflection light from the adjustment pattern  400  diffuse in a uniform manner, the area that emits diffuse reflection light on the ink droplet surfaces is fixed. For example, as shown in  FIG. 22A , the plural ink drops  500  included in the adjustment pattern are arranged separately from each other, provided in every other dot. In this case, adjacent ink drops will not be combined with each other, the ink drops  500  will adhere onto the water-repellent sheet member  700  in a regular manner, and the area that emits diffuse reflection light is fixed. As long as the ink drops are arranged separately from each other without being combined, the ink drops  500  can be arranged in a staggered manner as shown in  FIG. 22B  or the ink drops  500  can be arranged in all of the dots as shown in  FIG. 22C . 
     Furthermore, as shown in  FIG. 12 , with the passage of time after liquid droplets land, the ink drops will dry and the extent to which the reflection light diffuses will change. Accordingly, by fixing the time from when the ink drop lands to when the pattern reading sensor  401  receives the specular reflection light, the detection potential can be made to have high reproducibility. 
     Moreover, as long as the reflection light diffuses in a uniform manner, each of the regularly arranged ink drops  500  can be formed with two combined liquid droplets (for example, a main drop and a satellite drop) as illustrated in  FIGS. 20A and 20B . 
     Furthermore, in order to make the reflection light diffuse in a uniform manner, for example, as shown in  FIG. 23 , the contact area of the ink drops  500  and the water-repellent sheet member  700  within a detection range (detection region)  450  is fixed. For example, the plural ink drops  500  included in the aforementioned adjustment pattern  400  are arranged separately from each other, in every other dot. By jetting the same amount ink to form each ink drop  500  and arranging the ink drops  500  separately from each other, it is possible to fix the contact area where the ink drops are adhering to the surface of the water-repellent sheet member  700 . Also in this case, as long as the ink drops are arranged separately from each other without being combined, the ink drops  500  can be arranged in a staggered manner. It will help to fix the contact area by using pigment ink having a water-repellent relationship with the water-repellent sheet member  700 . 
     Furthermore, a synergistic effect will be attained by fixing both the area that emits diffuse reflection light among the total area of the ink drops, and also the contact area of the ink drops on the water-repellent sheet member. As a result of this synergistic effect, the reflection light from the adjustment pattern will diffuse in an even more uniform manner, and the detection potential will have even higher reproducibility. 
     If the ink drops are not arranged close enough to each other, the detection output indicating whether the adjustment pattern  400  exists will not be large enough. This point needs to be taken into consideration. Specifically, an experiment was conducted to confirm the correlation between the area on the ink droplets where reflection light diffuses and the detection output amount. As shown in  FIG. 24 , it was found that this relationship can be expressed by an approximate line, and that a desired level of detection output can be attained if the reflection light diffuses on 10% or more of the area of the adjustment pattern. 
     Next, a description is given of the liquid droplets forming the adjustment pattern, from the perspective of diffuse reflection of the pattern. 
     A pattern diffuse reflection rate refers to the proportion of a part in which diffuse reflection occurs (part where diffuse light is generated) within a detection range (detection region) detected by the pattern reading sensor  401 . An example of a detection range is shown in  FIG. 23  as described above. Specifically, the pattern diffuse reflection rate can be calculated as follows:
 
pattern diffuse reflection rate=total area of diffuse reflection/area of detection range
 
     If the detection range is fixed, the pattern diffuse reflection rate can be increased by increasing the area of diffuse reflection. The area of diffuse reflection is described as follows. As shown in  FIG. 25 , when the ink drop  500  adheres to the surface of the water-repellent sheet member  700 , the ink drop  500  will have a hemispheric shape if the wettability is low (if the contact angle θ shown in  FIG. 26  is large). In such a case, above the outer periphery of the ink drop  500 , a specular reflection part  500   a  and a diffuse reflection part  500   b  will be formed with respect to light from a certain direction. By controlling the operation of jetting ink drops, this diffuse reflection part  500   b  (drop diffuse reflection rate) can be increased for each ink drop  500 . 
     The drop diffuse reflection rate refers to the proportion of the diffuse reflection part with respect to the contact area of the ink drop with the water-repellent sheet member. This can be calculated as follows:
 
drop diffuse reflection rate=area of diffuse reflection part in one drop/contact area of one drop with water-repellent sheet member
 
     Specifically, among liquid droplets that can be used to form images, the adjustment pattern  400  is preferably formed with liquid droplets having maximum jetting amounts (drop volume). That is, the adjustment pattern  400  is to be formed by jetting liquid droplets in a print mode for jetting maximum drops. Accordingly, the height of the liquid droplet  500  shown in  FIG. 25  can be increased, so that the drop diffuse reflection rate is increased. 
     Furthermore, the composition of ink may be different for each color (cyan, magenta, yellow, and black). Therefore, the shape of the liquid droplet  500  may be different for each color. Accordingly, the jetting amount (drop volume) of each liquid droplet is to be controlled according to the color of the liquid droplet to be jetted to increase the drop reflection rate. 
     As described above, the image forming apparatus is provided with the liquid droplet jetting unit (recording head) for jetting liquid droplets; water-repellent sheet members for receiving liquid droplets; a unit for forming an adjustment pattern including plural liquid droplets arranged separately from each other, which pattern is used for detecting landing positions of the liquid droplets; a reading unit including a light emitting unit for emitting light to be irradiated onto the adjustment pattern and a light receiving unit for receiving specular reflection light of the light irradiated onto the adjustment pattern; and a unit for correcting landing positions of the liquid droplets by calculating the landing position shift amount based on attenuation signals of the specular reflection light output from the reading unit. Therefore, the following effects can be achieved. That is, by controlling the liquid droplet jetting operation in such a manner as to maximize the pattern diffuse reflection rate of the liquid droplets included in the adjustment pattern, it is possible to increase the output sensitivity of the light receiving unit (sensor), and to improve the reading performance such as the shift amount detection performance and repetition precision. 
     In this case, by controlling the liquid droplet jetting unit in such a manner as to maximize the diffuse reflection area for each individual liquid droplet (drop diffuse reflection rate), the detection sensitivity and performance can be further improved. In order to maximize the diffuse reflection area, the liquid droplet jetting unit is preferably controlled as follows.
     (1) Control the jetting amount of liquid droplets.   (2) Control the jetting amount of liquid droplets according to the color of the liquid droplets.   (3) Control the jetting operation in such a manner as to minimize the time difference between when the liquid droplets for forming the pattern are jetted and when the light emission/light reception is performed to read the pattern. Further, control the jetting operation so as to simultaneously jet the liquid droplets and perform the light emission/light reception with a single operation.   (4) Use a combination of materials with which the contact angle can be increased between the surface of the water-repellent sheet member and the liquid droplet.   (5) Make the shape of the liquid droplet in contact with the surface of the water-repellent sheet member circular or lageniform.   (6) Control the liquid droplet jetting operation in such a manner as to maximize the area of the liquid droplets, which are substantially separated from each other, within a range detectable by the light emitting unit and the light receiving unit. For example, the liquid droplets are preferably arranged so that the intervals between the liquid droplets are minimized.   

     Next, the operation of forming and detecting the adjustment pattern  400  is described. As described above, the shape of the ink drop changes with the passage of time after the ink drop adheres onto the water-repellent sheet member because the moisture of the liquid droplet evaporates. Accordingly, the amount of specular reflection light increases with the passage of time, starting immediately after the liquid droplet is formed. Thus, the output voltage of the pattern reading sensor  401  decreases with the passage of time. 
     For this reason, the adjustment pattern  400  is preferably detected by the pattern reading sensor  401  immediately after being formed, in order to accurately detect the landing positions of ink drops. 
     In one example, the printing speed of forming the adjustment pattern  400  is made to be the same as the speed of reading the adjustment pattern  400 . As the adjustment pattern  400  is being printed, the position of the pattern is immediately detected. To perform such an operation, the pattern reading sensor  401  is to be provided on the upstream side of the carriage  23  in the scanning direction of printing the adjustment patter  400 . However, such a configuration is only applicable to either one of the forward direction or the backward direction. 
     Accordingly, in another example, the printing speed of forming the adjustment pattern  400  is made to be different from the speed of reading the adjustment pattern  400 . In both the forward direction and the backward direction, the adjustment pattern  400  is printed on the water-repellent sheet member  700  and is immediately detected without rotating the conveying belt  31 . In this case, the pattern reading sensor  401  is located above the region where the adjustment pattern  400  is formed. 
     Next, with reference to  FIG. 27 , a description is given of a light emitting amount adjustment process according to an embodiment of the present invention, which is performed before the main control unit  310  corrects the shifts in the liquid droplet landing position. 
     First, the water-repellent sheet member  700  is fed. The sheet member detecting unit  330  (leading edge position detecting sensor) is moved to a position above the water-repellent sheet member  700  (step S 271 ). The light emitting element  402  of the pattern reading sensor  401  is driven to emit light at a predetermined PWM value (for example, 50% duty) (step S 272 ). The light receiving element  403  of the pattern reading sensor  401  receives light reflected from the water-repellent sheet member  700  (step S 273 ). The main control unit  310  determines whether the light receiving level is greater than or equal to a predetermined reference value (a predetermined light receiving level) (step S 274 ). If the light receiving level is not greater than or equal to the reference value (No in step S 274 ), the PWM value is increased to increase the amount of light to be emitted from the light emitting element  402  of the pattern reading sensor  401  (step S 275 ). Under such a condition, light is emitted once again from the light emitting element  402  (step S 273 ) and the light receiving level is compared once again with the standard value (step S 274 ). This operation is repeated. 
     Accordingly, it is possible to improve the precision in repetitive detection operations. 
     Next, with reference to  FIG. 28 , a description is given of a liquid droplet landing position shift correction process according to an embodiment of the present invention, which is performed by the main control unit  310 . 
     First, when a liquid droplet landing position shift correction process operation is instructed with a not shown operations panel, the main control unit  310  determines whether the sheet feeding tray (or manual feeding tray) contains the water-repellent sheet member  700  (step S 281 ). If there are no water-repellent sheet members  700  (No in step S 281 ), the main control unit  310  requests the user to set the water-repellent sheet member  700  in the tray (step S 282 ). 
     When the water-repellent sheet member  700  is set (Yes in step S 281 ), the main control unit  310  moves the carriage  23  to the center of the image forming region in the main scanning direction (step S 283 ), conveys the water-repellent sheet member  700  (step S 284 ), detects the water-repellent sheet member  700  with the sheet member detecting unit  330  (leading edge position detecting sensor) (step S 285 ), and then conveys the water-repellent sheet member  700  to the adjustment pattern forming region (step S 286 ). Subsequently, as described with reference to  FIG. 27 , the light emitting element  402  of the pattern reading sensor  401  is driven to emit light at a predetermined PWM value (for example, 50% duty) (step S 287 ). The light receiving element  403  of the pattern reading sensor  401  receives light reflected from the water-repellent sheet member  700  (step S 288 ). The main control unit  310  determines whether the light receiving level is greater than or equal to a predetermined reference value (a predetermined light receiving level) (step S 289 ). If the light receiving level is not greater than or equal to the reference value (No in step S 289 ), the main control unit  310  determines whether it is possible to increase the amount of light to be emitted (step S 2810 ). If the light emission amount can be increased (Yes in step S 2810 ), the PWM value is increased to increase the amount of light to be emitted from the light emitting element  402  of the pattern reading sensor  401 , light is emitted once again from the light emitting element  402  (step S 2811 ), and the light receiving level is once again compared with the standard value (steps S 288  and S 289 ). If the light emission amount cannot be increased (No in step S 2810 ), the main control unit  310  sends a notice to this effect the user to change (exchange) the water-repellent sheet member  700  (step S 2812 ). 
     Subsequently, if a predetermined light receiving level can be attained (Yes in step S 289 ), a predetermined adjustment pattern (test pattern) is formed on the water-repellent sheet member  700  (step S 2813 ), the test pattern is read with the pattern reading sensor  401  (step S 2814 ), the positional shift amount is calculated based on the read results, and the landing position correction is performed by changing the liquid droplet jetting timing according to the calculated positional shift amount (step S 2815 ). 
     With reference to  FIGS. 29A through 29D , a description is given of a block pattern (also referred to as basic pattern) for each minimum unit for detecting landing position shifts included in the adjustment pattern  400  according to an embodiment of the present invention. 
     In the above-described landing position shift correction method for this image forming apparatus, with the use of a recording head (color) that is to be a reference head, a line-shaped pattern is formed on the conveying belt, which pattern extends in a direction perpendicular to the movement direction of the conveying belt. Then, other recording heads (of other colors) are used to form similar line-shaped patterns with fixed intervals along the movement direction of the conveying belt. The distance between the reference head and another head is calculated (measured). 
     There are four types of block patterns (basic patterns) for each minimum unit, as follows. In the basic pattern shown in  FIG. 29A , when the image formation is performed in the forward direction (first scan), a pattern FK 1  formed by the recording head  24   k   1  is used as a reference for detecting the landing position shift of a pattern FK 2  formed by the recording head  24   k   2 . In the basic pattern shown in  FIG. 29B , when the image formation is performed in the backward direction (second scan), a pattern BK 1  formed by the recording head  24   k   1  is used as a reference for detecting the landing position shift of a pattern BK 2  formed by the recording head  24   k   2 . In the basic pattern shown in  FIG. 29C , when the image formation is performed in the forward direction (third scan), each of the patterns FK 1  formed by the recording head  24   k   1  is used as a reference for detecting the landing position shifts of patterns FC, FM, and FY of colors C, M, and Y formed by the recording heads  24   c ,  24   m , and  24   y , respectively. In the basic pattern shown in  FIG. 29D , when the image formation is performed in the backward direction (fourth scan), each of the patterns FK 1  formed by the recording head  24   k   1  is used as a reference for detecting the landing position shifts of patterns FC, FM, and FY of colors C, M, and Y formed by the recording heads  24   c ,  24   m , and  24   y , respectively. These block patterns can be combined to form an adjustment pattern for obtaining various detection results. 
     Next, with reference to  FIGS. 30 ,  31 A, and  31 B, adjustment patterns including the above block patterns are described. One adjustment pattern is for detecting shifts in monochrome horizontal lines and another is for detecting color shifts. 
     In a horizontal line shift adjustment pattern  400 B shown in  FIG. 30 , the position of the pattern FK 1  in the reference direction (assumed to be forward direction) is used as a reference (the pattern FK 1  is used as a reference pattern) for printing, at predetermined intervals, the pattern BK 1  in the backward direction, the pattern FK 2  in the forward direction, and the pattern BK 2  in the backward direction (these are the patterns to be measured). Thus, based on the position information of each of the patterns FK 1 , BK 1 , FK 2 , and BK 2 , it is possible to detect the landing position shifts with respect to the pattern FK 1  which is the reference pattern. The sensor scanning direction (reading direction) indicates a case where only one direction is read. 
       FIGS. 31A and 31B  illustrate color shift adjustment patterns  400 C 1  and  400 C 2 , respectively. In these patterns, the reference color is used as a reference (the patterns FK 1  recorded by the recording head  24   k   1  are used as reference patterns) for printing patterns FY, FM, and FC of the respective colors at predetermined intervals (these are the patterns to be measured). The landing positions of patterns FK 1  and FY, FK 1  and FM, and FK 1  and FC are detected to detect the landing positions of each color pattern with respect to the corresponding reference pattern FK 1 . The sensor scanning direction (reading direction) indicates a case where only one direction is read. 
     Next, a specific example of an adjustment pattern is described with reference to  FIG. 32 . 
     As for the scanning direction of the carriage  23 , as shown in  FIG. 2 , the forward direction is the direction from the back side of the apparatus toward the front side of the apparatus, and the backward direction is the direction from the front side of the apparatus toward the back side of the apparatus. Furthermore, the carriage  23  is provided with the recording heads  24   c ,  24   k   1 ,  24   k   2 ,  24   m , and  24   y  in the stated order from the downstream side in the forward direction (front side of apparatus). 
     In this example, horizontal line positional shift adjustment patterns  400 B 1  and  400 B 2  are formed at edges of the water-repellent sheet member  700 . The color shift adjustment patterns  400 C 1  and  400 C 2  are formed in the middle of the water-repellent sheet member  700 . That is, in this example, plural block patterns are arranged within the width of the printing region in the direction perpendicular to the movement direction of the water-repellent sheet member  700 . 
     Each time the horizontal line positional shift adjustment patterns  400 B 1 ,  400 B 2  and the color shift adjustment patterns  400 C 1 ,  400 C 2  are printed, they are read plural times by the pattern reading sensor  401 . The reading can be performed plural times in one direction (same direction) or the reading can be performed plural times in both directions. 
     For example, the carriage  23  is moved in the forward direction to sequentially read and detect the positions of the patterns  400 B 1 ,  400 C 1 ,  400 C 2 , and  400 B 2  with the pattern reading sensor  401 . For example, in the horizontal line positional shift adjustment pattern  400 B 1 , the pattern FK 1  is assumed to be the reference pattern and the pattern BK 1  is assumed to be the pattern to be measured, and the distance between these are calculated. Accordingly, it is possible to obtain the landing position shift amount between the forward direction and the backward direction for the recording head  24   k   1 . 
     For example, in the horizontal line positional shift adjustment pattern  400 B 1 , the pattern FK 1  is assumed to be the reference pattern and the pattern BK 2  is assumed to be the pattern to be measured, and the distance between these are calculated. Accordingly, it is possible to obtain the landing position shift amount with respect to the actual distance between the recording head  24   k   1  and the recording head  24   k   2 . 
     Furthermore, in the color shift adjustment pattern  400 C 1 , each of the patterns FK 1  is assumed to be the reference pattern. The distance between the pattern FK 1  and the pattern FY, the distance between the pattern FK 1  and the pattern FM, and the distance between the pattern FK 1  and the pattern FC are calculated. Accordingly, it is possible to obtain the landing position shift amounts with respect to the actual distances between the recording heads  24   k   1 ,  24   y ,  24   m , and  24   c.    
     In order to improve detection precision, plural patterns  400 B 1 ,  400 B 2 ,  400 C 1 , and  400 C 2  are read/scanned, and average values are calculated. 
     Based on the calculated shift amount, the main control unit  310  controls (changes) the timings at which liquid droplets are jetted from the recording heads  24   k   1 ,  24   k   2 ,  24   y ,  24   m , and  24   c . Accordingly, the landing positions of jetted liquid droplets the can be made consistent for the recording heads  24   k   1 ,  24   k   2 ,  24   y ,  24   m , and  24   c.    
     As described above, a water-repellent sheet member has a water-repellent surface formed on at least one side. On this water-repellent surface, a landing position shift adjustment pattern is formed with plural liquid droplets that are separated from each other. Light is irradiated from a light emitting unit onto this adjustment pattern. Specular reflection light reflected from the adjustment pattern is received by the light receiving unit to detect the adjustment pattern. Based on results obtained by detecting the adjustment pattern, the landing position shifts of liquid droplets jetted from recording heads are corrected. Accordingly, with a simple structure, the landing positions of the liquid droplets can be detected with high precision and liquid droplet landing position shifts can be corrected with high precision. 
     Next, the water-repellent sheet member  700  according to an embodiment of the present invention is described with reference to  FIGS. 33 and 34 . 
     One side of a water-repellent sheet member  700 A shown in  FIG. 33  is partially coated with a water-repellent agent. These parts are referred to as water-repellent regions  701 . In this example, a total of nine water-repellent regions  701  are formed. Specifically, the water-repellent sheet member  700 A is divided into a top part  700   a , a middle part  700   b , and a bottom part  700   c , in the stated order in the conveyance direction of the water-repellent sheet member  700 A, and each of these parts includes three water-repellent regions  701 . 
     Therefore, the adjustment patterns  400  are formed on this water-repellent sheet member  700 A in the following manner. In a first landing position shift correction operation, the adjustment patterns  400  are formed in the water-repellent regions  701  included in the top part  700   a  of the water-repellent sheet member  700 A. In a second landing position shift correction operation, the adjustment patterns  400  are formed in the water-repellent regions  701  included in the middle part  700   b  of the water-repellent sheet member  700 A. In a third landing position shift correction operation, the adjustment patterns  400  are formed in the water-repellent regions  701  included in the bottom part  700   c  of the water-repellent sheet member  700 A. 
     In this case, when the water-repellent sheet member  700 A is set, a scanning operation is performed at a predetermined position to determine whether adjustment patterns  400  have already been formed, so that the adjustment patterns  400  can be formed in new regions (without patterns). 
     By printing the test pattern at separate positions in the above manner, i.e., the leading edge part, the middle part, and the trailing edge part of the water-repellent sheet member, the landing position shift correction can be performed in consideration of a floating portion in the water-repellent sheet member that is formed due to the resilience of the water-repellent sheet member. Furthermore, by providing plural patterns across the entire water-repellent sheet member and detecting them plural times, it is possible to improve reading performance such as repetition precision. 
     As a matter of course, as shown in  FIG. 34 , a water-repellent sheet member  700 B is also applicable, which is entirely coated with a water-repellent agent so that the entire surface corresponds to the water-repellent region  701 . 
     The water-repellent region  701  of the water-repellent sheet member  700  preferably provides a high specular reflection rate, a high degree of luster, and a high degree of smoothness on a recording target medium used for a typical image forming operation performed by the image forming apparatus, such as a plain paper sheet into which ink permeates. 
     The specular reflection rate refers to the proportion of the parts where specular reflection occurs with respect to a surface where liquid droplets have landed within the detection range  450  of the pattern reading sensor  401 . This value is calculated as follows:
 
specular reflection rate=(total area of specular reflection parts/area of detection range)
 
By using a water-repellent sheet member having a high specular reflection rate, the diffuse reflection rate on the surface of the water-repellent sheet member decreases, and therefore the sensitivity increases.
 
     Furthermore, the specular reflection rate will increase as the degree of luster and the degree of smoothness increase on the surface of the water-repellent sheet member. Specifically, as shown in  FIG. 35A , if the luster degree is low, part of the incident light is absorbed or transmitted in the sheet member, and therefore the specular reflection light will decrease. Conversely, as shown in  FIG. 35B , if the luster degree is high the entire incident light is reflected from the sheet member, and therefore the specular reflection light will increase. Furthermore, as shown in  FIG. 36A , if the smoothness degree is low the incident light is diffused, and therefore the specular reflection light will decrease. Conversely, as shown in  FIG. 36B , if the smoothness degree is high the entire incident light is reflected, and therefore the specular reflection light will increase. 
     Next, another example of the water-repellent sheet member  700  according to an embodiment of the present invention is described with reference to  FIG. 37 . 
     A water-repellent sheet member  700 C is formed by providing marks  703  onto the water-repellent sheet member  700 A described with reference to  FIG. 35 . These marks  703  are identification units for indicating that this is a water-repellent sheet member. The marks  703  are provided on the leading edge and the trailing edge of the water-repellent sheet member  700 A in the conveyance direction, thereby forming the water-repellent sheet member  700 C. The marks  703  can be two-dimensional patterns such as bar codes, three dimensional patterns with recessions/projections, or magnetic patterns. 
     Next, a description is given of a landing position correction process performed with the use of the water-repellent sheet member  700 C, with reference to  FIG. 38 . 
     First, to detect the leading edge of a conveying target member that is being conveyed, the carriage  23  is moved to the center of the image forming region in the main scanning direction (step S 381 ). The conveying target member is conveyed (step S 382 ). The sheet member detecting unit  330  detects the leading edge of the conveying target member (step S 383 ). It is determined whether the mark  703  is detected (step S 384 ). If the mark  703  is not detected (No in step S 384 ), the conveying target member is determined to be a regular recording target medium and a regular printing operation will be performed (step S 385 ). 
     Conversely, if the mark  703  is detected at the time of detecting the leading edge (Yes in step S 384 ), the conveying target member is determined to be the water-repellent sheet member  700 C. Therefore, the water-repellent sheet member  700 C is conveyed to the test pattern forming region (step S 386 ), and then the test pattern  400  is formed on the water-repellent sheet member  700 C (step S 387 ). The pattern reading sensor  401  reads the test pattern  400  (step S 388 ) and the landing position shift amount is calculated. The landing positional correction is performed (step S 389 ) by correcting the liquid droplet jetting timing based on the calculated landing position shift amount. 
     As described above, at the time of detecting the leading edge of a conveying target member being conveyed, it is determined whether the conveying target member is a water-repellent sheet member. If it is a water-repellent sheet member, the landing position correction operation of forming a test pattern is started automatically. Accordingly, the operation for performing the landing position correction can be simplified. 
     According to one embodiment of the present invention, an image forming apparatus for forming an image on a recording target medium that is being conveyed includes a recording head configured to jet liquid droplets, a pattern forming unit configured to form, on a water-repellent surface of a water-repellent sheet member, an adjustment pattern used for correcting shifts in landing positions of the liquid droplets, wherein the adjustment pattern includes plural liquid droplets that are separated from each other, and the water-repellent surface having a water-repellent property is formed on at least a part of a surface of the water-repellent sheet member, a reading unit including a light emitting unit configured to irradiate light onto the adjustment pattern and a light receiving unit configured to receive specular reflection light from the adjustment pattern, and a landing position correction unit configured to correct, based on a reading result obtained by the reading unit, the shifts in the landing positions of the liquid droplets that are jetted from the recording head. 
     Additionally, the image forming apparatus is preferably configured to determine whether a conveying target medium that is being conveyed is the water-repellent sheet member. In the event that the conveying target medium is determined to be the water-repellent sheet member, operations for correcting the shifts in the landing positions are started. In this case, the image forming apparatus is preferably configured to detect an identification unit provided on a leading edge of the water-repellent sheet member, which identification unit is configured to indicate that a member to which it is provided is the water-repellent sheet member. 
     Additionally, the image forming apparatus preferably has a configuration in which the water-repellent sheet member includes an upstream region, a midstream region, and a downstream region in the stated order in a conveyance direction of the water-repellent sheet member, and the adjustment pattern is formed on at least one of the upstream region, the midstream region, and the downstream region. In this case, the image forming apparatus preferably has a configuration in which before the adjustment pattern is formed, the light emitting unit irradiates the light onto the water-repellent sheet member and the light receiving unit receives the specular reflection light to obtain a light reception result, and in the event that the light reception result is less than or equal to a predetermined reference value, a light emission amount of the light emitting unit is increased. 
     According to one embodiment of the present invention, a method of correcting landing positions of liquid droplets jetted from a recording head includes a forming step of forming, on a water-repellent surface of a water-repellent sheet member, an adjustment pattern used for correcting shifts in the landing positions of the liquid droplets, wherein the adjustment pattern includes plural liquid droplets that are separated from each other, and the water-repellent surface having a water-repellent property is formed on at least a part of a surface of the water-repellent sheet member, a detecting step of detecting the adjustment pattern by irradiating light onto the adjustment pattern from a light emitting unit and receiving specular reflection light from the adjustment pattern with a light receiving unit, and a correcting step of correcting, based on a detection result obtained by detecting the adjustment pattern at the detecting step, the shifts in the landing positions of the liquid droplets that are jetted from the recording head. 
     According to one embodiment of the present invention, a water-repellent sheet member used for correcting landing positions of liquid droplets includes, at its leading edge, an identification unit configured to indicate that a member to which it is provided is the water-repellent sheet member. At least one of a specular reflection rate, a degree of luster, and a degree of smoothness of the water-repellent sheet member according to one embodiment of the present invention is higher than that of a plain paper sheet into which liquid permeates. 
     In the image forming apparatus and the landing position shift correction method according to an embodiment of the present invention, on a water-repellent surface of a water-repellent sheet member, an adjustment pattern used for correcting shifts in the landing positions of the liquid droplets is formed. The adjustment pattern includes plural liquid droplets that are separated from each other. The water-repellent surface having a water-repellent property is formed on at least a part of a surface of the water-repellent sheet member. The adjustment pattern is detected by irradiating light onto the adjustment pattern from a light emitting unit and receiving specular reflection light from the adjustment pattern with a light receiving unit. Therefore, with a simple structure, liquid droplet landing positions can be detected with high precision, and liquid droplet landing position shifts can be corrected with high precision 
     The sheet member according to an embodiment of the present invention is provided with an identification unit configured to indicate that a member to which it is provided is the water-repellent sheet member. Therefore, operations for correcting the shifts in the landing positions can be automatically started. Furthermore, at least one of a specular reflection rate, a degree of luster, and a degree of smoothness of the water-repellent sheet member is higher than that of a recording target medium. Therefore, the precision in detecting the adjustment pattern can be improved. 
     The present invention is not limited to the specifically disclosed embodiment, and variations and modifications may be made without departing from the scope of the present invention. 
     The present application is based on Japanese Priority Patent Application No. 2007-069676, filed on Mar. 17, 2007, the entire contents of which are hereby incorporated by reference.