Abstract:
An image forming apparatus is provided. The image forming apparatus includes: a carrier; a forming unit which forms an image on a carrier; a cleaning unit which cleans the carrier; a detection unit which detects a correction pattern formed on the carrier; and a control unit which performs a correction processing including cleaning a pattern forming region in the carrier by the cleaning unit, forming the correction pattern in the pattern forming region by the forming unit after finishing the cleaning by the cleaning unit, and correcting an image forming characteristic of the forming unit based on a detection result of the correction pattern by the detection unit.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2007-258854, filed on Oct. 2, 2007, the entire subject matter of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     Aspects of the present invention relate to an image forming apparatus. 
     BACKGROUND 
     An image forming apparatus, such as a color laser printer, includes a plurality of image forming units arranged along a sheet conveying belt. In the image forming apparatus, toner images of respective colors are transferred onto the sheet to be conveyed on the belt from the respective image forming units. In such an image forming apparatus, if a transfer position shift (color shift) between the image forming units with respect to the sheet occurs, the quality of an image to be formed is deteriorated. 
     To ensure the quality of the image, there is suggested a technique, called registration, which corrects the shift of a forming position in each color (for example, JP-A-2003-98795). According to this technique, a predetermined pattern is formed on the surface of the belt by each image forming unit, and the position of the pattern is detected by an optical sensor. Then, the forming position in each color is corrected on the basis of the detection result. Similarly, a technique is suggested that a pattern for density correction is formed on the belt, and the pattern is detected by an optical sensor. Then, the density of an image is corrected on the basis of the detection result. 
     During the above-described position shift correction or density correction, if toner is attached to and contaminates the surface of the belt, pattern detection can not be accurately performed. For this reason, an image forming apparatus is provided with a cleaning device for cleaning the belt. Then, after correction processing ends, the cleaning device removes toner attached to the surface of the belt. 
     In a related-art image forming apparatus, however, the belt may not be cleaned immediately before correction is performed. That is, even if the belt is cleaned after correction ends, an image forming unit may be detached or attached later, and as a result, the surface of the belt may be contaminated. In this state, if subsequent correction is performed, pattern detection accuracy may be degraded and the quality of the image to be formed may be deteriorated. 
     SUMMARY 
     Exemplary embodiments of the present invention address the above disadvantages and other disadvantages not described above. However, the present invention is not required to overcome the disadvantages described above, and thus, an exemplary embodiment of the present invention may not overcome any of the problems described above. 
     Accordingly, it is an aspect of the present invention to provide an image forming apparatus which can ensure pattern detection accuracy during correction. 
     According to an exemplary embodiment of the present invention, there is provided an image forming apparatus including: a carrier; a forming unit which forms an image on a carrier; a cleaning unit which cleans the carrier; a detection unit which detects a correction pattern formed on the carrier; and a control unit which performs a correction processing including cleaning a pattern forming region in the carrier by the cleaning unit, forming the correction pattern in the pattern forming region by the forming unit after finishing the cleaning by the cleaning unit, and correcting an image forming characteristic of the forming unit based on a detection result of the correction pattern by the detection unit. 
     According to another exemplary embodiment of the present invention, there is provided a device for determining a contamination degree of a carrier rotatable in a rotating direction. The device includes: a cleaning unit provided around the carrier to clean the carrier; a detection unit which detects an amount of reflected light from the carrier at a upstream of the cleaning unit in the rotating direction; and a determination unit which determines the contamination degree of the carrier at a downstream of the cleaning unit in the rotating direction based on a detection result of the detection unit and a cleaning capability of the cleaning unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects of the present invention will become more apparent and more readily appreciated from the following description of exemplary embodiments of the present invention taken in conjunction with the attached drawings, in which: 
         FIG. 1  is a sectional side view illustrating the schematic configuration of a printer according to an exemplary embodiment of the present invention; 
         FIG. 2  is a block diagram schematically illustrating the electrical configuration of the printer shown in  FIG. 1 ; 
         FIG. 3  is a flowchart illustrating the flow of a contamination degree detection processing; 
         FIG. 4  is a flowchart illustrating the flow of a correction processing; 
         FIG. 5  is a diagram illustrating the position relationship between a belt, and a pattern detection sensor, a cleaning device, and a photosensitive drum provided around the belt; 
         FIG. 6  is a diagram illustrating a pattern for position shift correction; and 
         FIG. 7  is a diagram illustrating a pattern for density correction. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary Embodiment 
     An exemplary embodiment of the invention will now be described with reference to  FIGS. 1 to 7 . 
     (Overall Configuration of Printer) 
       FIG. 1  is a sectional side view illustrating the schematic configuration of a printer  1  according to an exemplary embodiment of the present invention. In the following description, the right side of  FIG. 1  is taken as a front side and the left side is taken as a rear side. 
     The printer  1  includes a main body casing  2 . In the bottom portion of the main body casing  2 , a sheet feeding tray  4  is provided in which sheets  3  as recording mediums are stacked. Above the front end of the sheet feeding tray  4 , a sheet feed roller  5  is provided. While the sheet feed roller  5  rotates, a sheet  3  at the uppermost position in the sheet feeding tray  4  is fed to a registration roller  6 . The registration roller  6  aligns the sheet  3  and then conveys the sheet  3  onto a belt unit  11  of an image forming unit  10 . 
     The image forming unit  10  includes a belt unit  11 , a scanner unit  19 , a process unit  20 , and a fixing unit  31 . 
     The belt unit  11  includes a belt  13  which is made of polycarbonate or the like and is wound around a pair of front and rear belt support rollers  12 . Then, if the rear belt support roller  12  is rotated, the belt  13  is rotated counterclockwise (in a rotating direction) in  FIG. 1 , and the sheet  3  on the upper surface of the belt  13  is conveyed to the rear side. In the belt  13 , transfer rollers  14  are provided to face respective photosensitive drums  28  of the process unit  20  described below with the belt  13  interposed therebetween. 
     The scanner unit  19  emits laser light of each color from a laser light emitting unit (not shown) onto the surface of the corresponding photosensitive drum  28 . 
     The process unit  20  includes a frame  21  and developing cartridges  22  ( 22 Y,  22 M,  22 C, and  22 K) for four colors (yellow, magenta, cyan, and black), which are detachably mounted on four cartridge mounting portions provided in the frame  21 . The process unit  20  can be pulled out forward when a front cover  2 A provided at the front surface of the main body casing  2  is opened. When the process unit  20  is detached from the main body casing  2 , the belt unit  11  or the cleaning device  17  can be attached to or detached from the main body casing  2 . In the lower portion of the frame  21 , a photosensitive drum  28  and a scorotron type charger  29  are provided to correspond to each developing cartridge  22 . The surface of the photosensitive drum  28  is covered with a positively chargeable photosensitive layer. 
     Each developing cartridge  22  includes a toner containing chamber  23  that contains toner of corresponding color as developer in the upper portion in a boxlike casing, and also includes, below the toner containing chamber  23 , a supply roller  24 , a developing roller  25 , a layer thickness regulating blade  26 , and an agitator  27 . Toner discharged from the toner containing chamber  23  is supplied to the developing roller by rotation of the supply roller  24 , and is positively charged by friction between the supply roller  24  and the developing roller  25 . Toner supplied to the developing roller  25  enters between the layer thickness regulating blade  26  and the developing roller  25  by rotation of the developing roller  25  and is further frictionally charged there. Then, toner is carried on the developing roller  25  as a thin layer having a constant thickness. 
     During image forming, the photosensitive drum  28  is rotated, and accordingly the surface of the photosensitive drum  28  is charged uniformly by the charger  29 . Then, a positively charged portion is exposed by high-speed scanning of laser light from the scanner unit  19 . In this way, an electrostatic latent image corresponding to an image to be formed on the sheet  3  is formed on the surface of the photosensitive drum  28 . 
     Next, by rotation of the developing roller  25 , toner which is carried on the developing roller  25  and positively charged is supplied to the electrostatic latent image formed on the surface of the photosensitive drum  28  when coming into contact with the photosensitive drum  28 . Accordingly, the electrostatic latent image on the photosensitive drum  28  becomes a visible image, that is, a toner image formed by toner adhered thereto is carried on only the exposed portion of the surface of the photosensitive drum  28 . 
     Thereafter, the toner images carried on the surfaces of the respective photosensitive drums  28  are successively transferred onto the sheet  3  by a negative-polarity transfer bias applied to the transfer rollers  14  while the sheet  3  which is conveyed by the belt  13  passes through respective transfer positions between the photosensitive drums  28  and the transfer rollers  14 . Next, the sheet  3  onto which the toner images are transferred in such a manner is conveyed to the fixing unit  31 . 
     The fixing unit  31  includes a heating roller  31 A having a heat source and a pressing roller  31 B that presses the sheet  3  against the heating roller  31 A. In the fixing unit  31 , the toner images transferred onto the sheet  3  are thermally fixed. Then, the sheet  3  onto which the toner images are thermally fixed by the fixing unit  31  is conveyed upward and discharged onto a sheet discharging tray  32  provided at the upper surface of the main body casing  2 . 
     (Cleaning Device) 
     The cleaning device  17  includes a case  40  for containing toner collected from the surface of the belt  13  or sheet dust. In the upper portion of the case  40 , a cleaning roller  41  and a collecting roller  42  are provided to be pressed into contact with each other. The cleaning roller  41  faces a metallic backup roller  43  provided in the belt unit  11  with the belt  13  interposed therebetween. A scraping blade  44  made of rubber is pressed into contact with the collecting roller  42  from below. 
     The entire cleaning device  17  is displaceable up and down by a displacement mechanism (not shown). When power is turned on under the control of a central processing unit (CPU)  50  described below, the cleaning device  17  is displaced to a position in which the cleaning roller  41  comes into contact with the belt  13 . Then, the cleaning roller  41  is driven in a direction opposite to the moving direction of the belt  13  by a driving force from a main motor  57  (see  FIG. 2 ) provided in the main body casing  2 , and a predetermined bias is applied between the cleaning roller  41  and the backup roller  43 . Accordingly, toner attached onto the belt  13  is physically scraped off and electrically attracted toward the cleaning roller  41 . When power is turned off, the cleaning device  17  is descended to a position in which the cleaning roller  41  is not in contact with the belt  13 , and the bias between the cleaning roller  41  and the backup roller  43  is turned off. 
     (Electrical Configuration of Printer) 
       FIG. 2  is a block diagram schematically illustrating the electrical configuration of the printer  1 . As shown in the drawing, the printer  1  includes the CPU  50  (an example of a detection unit, a control unit, a determination unit, and an invalidation unit), a read only memory (ROM)  51 , a random access memory (RAM)  52 , an non-volatile random access memory (NVRAM)  53 , and a network interface  54 . To these, the image forming unit  10 , the pattern detection sensors  15 , and the cleaning device  17  described above, a display unit  55 , an operation unit  56 , a main motor  57 , a cover open/close sensor  58 , and the like are connected. 
     The ROM  51  stores programs for executing various operations of the printer  1 , such as a contamination level detection processing or a correction processing described below. The CPU  50  controls the individual units according to the programs read out from the ROM  51  while storing the processing results in the RAM  52  or the NVRAM  53 . The network interface  54  is connected with an external computer or the like through a communication line (not shown) and enables mutual data communication. 
     The display unit  55  includes a liquid crystal display or a lamp and can display various setup screens and operations states of the printer. The operation unit  56  includes a plurality of buttons. A user can perform various input operations through the operation unit  56 . 
     The main motor  57  rotates the registration roller  6 , the belt support rollers  12 , the transfer rollers  14 , the developing rollers  25 , the photosensitive drums  28 , the heating roller  31 A, the cleaning roller  41 , and the like in synchronization with one another. The cover open/close sensor  58  detects the open/close state of the front cover  2 A. 
     (Contamination Degree Detection Processing) 
     Next, a contamination degree detection processing for detecting a contamination degree of the belt  13  will be described.  FIG. 3  is a flowchart illustrating the flow of a contamination degree detection processing. 
     The contamination degree detection processing is constantly performed as a background operation under the control of the CPU  50  after the printer  1  is powered on. The contamination degree detection processing determines a contamination degree to be stored in the RAM  52 . The contamination degree is a value indicative of a contamination degree of the surface of the belt  13  within a range of 0 to 1. The value “1” indicates a most contaminated state, and the value “0” indicates a cleanest state. 
     If the contamination degree detection processing is started, the CPU  50  first sets the contamination degree to “1” at operation S 101 . Then, it is examined whether the open/close operation of the front cover  2 A is detected by the cover open/close sensor  58  at operation S 102 . When the open/close operation of the front cover  2 A is detected (S 102 : Yes), the contamination degree is set to “1” at operation S 103 . If the contamination degree is “1” before the operation S 103  or if the open/close operation of the front cover  2 A is not detected (S 102 : No), the contamination degree remains unchanged. 
     Subsequently, the CPU  50  examines whether jam (sheet clogging) occurs at operation S 104 . A plurality of sheet sensors (not shown) is provided on the conveying path of the sheet  3 . If the sheet  3  is not detected by the sheet sensors at a predetermined timing while the sheet  3  is conveyed, the CPU  50  determines that a jam occurs. If a jam occurs (S 104 : Yes), the CPU  50  sets the contamination degree to “1” at operation S 105 . Here, if the contamination degree is “1” before the operation S 105  or if a jam does not occur (S 104 : No), the contamination degree remains unchanged. 
     Next, the CPU  50  measures the amount of reflected light from the belt  13  by the pattern detection sensors  15 , and stores the measurement value in the RAM  52  at operation S 106 . The measurement value is accumulated in the RAM  52  each time measurement is performed. However, if the contamination degree is set to “1” in the operations S 103  and S 105 , all the measurement values are cleared from the RAM  52 . Then, the CPU  50  determines whether the measurement values are obtained by the number corresponding to one rotation of the belt  13  at operation S 107 . If the number of obtained measurement values does not correspond to one rotation (S 170 : No), the operation S 102  and later operations are repeated at a predetermined interval. 
     If the measurement values are obtained by the number corresponding to one rotation of the belt  13  (S 107 : Yes), the CPU  50  calculates the contamination degree (that is, a value ranging from 0 to 1) corresponding to the highest value among the measurement values (corresponding to a most contaminated place among the measurement places on the belt  13 ). Then, the calculated value is written into the RAM  52  as a new contamination degree at operation S 108 , and clears the measurement values stored in the RAM  52 . Thereafter, the process returns to the operation S 102 , and the same processing is repeated. 
     As described above, in the contamination degree detection processing, the amount of reflected light from the belt  13  is measured at a predetermined interval, and on the basis of the measurement value, the contamination degree (contamination information) of the belt  13  is determined and stored in the RAM  52 . Then, if an operation which can change the contamination state of the belt  13  is detected, for example, the front cover  2 A is opened or closed, or jam occurs, the contamination degree is overwritten (invalidated). 
     (Correction Processing) 
       FIG. 4  is a flowchart illustrating the flow of a correction processing. 
     When a predetermined condition is satisfied, for example, the front cover  2 A is opened or closed, or the number of printed sheets reaches a predetermined value or the elapsed time reaches a predetermined value from the previous correction processing, the CPU  50  starts the correction processing to perform one of position shift correction and density correction. 
     If the correction processing is started, the CPU  50  refers to the contamination degree stored in the RAM  52  by the above contamination degree detection processing, and determines whether the value is smaller than 0.8 at operation S 201 . Here, the contamination degree of 0.8 is a reference value for determining whether a pattern P 1  for the position shift correction formed on the belt  13  can be accurately detected when the position shift correction is performed as described below. 
     If the contamination degree is smaller than 0.8 (that is, the contamination degree of the belt  13  bears the criteria for performing the position shift correction) (S 201 : Yes), during the correction processing, it is determined whether the position shift correction is to be performed at operation S 202 . Then, if it is determined that the position shift correction is to be performed (S 202 : Yes), the process proceeds to operation S 208  described below, and the pattern P 1  for position shift correction starts to be formed at operation S 208 . 
     If it is determined that the density correction is to be performed (S 202 : No), it is determined whether the contamination degree of the belt  13  is smaller than 0.5 at operation S 203 . Here, the contamination degree of 0.5 is a reference value for determining whether a density can be accurately measured with a pattern P 2  for density correction formed on the belt  13  if the density correction described below is performed. That is, in case of the density correction, the reference of the contamination degree needs to be lower than that for position shift correction, and the belt  13  needs to be cleaner. If the contamination degree is smaller than 0.5 (that is, the contamination degree of the belt  13  bears the criteria for the density correction) (S 203 : Yes), the process proceeds to operation S 215  described below, and the pattern for density correction starts to be formed at operation S 215 . 
     If the contamination degree of the belt  13  is not less than 0.8 (S 201 : No), or if the density correction is to be performed and the contamination degree is not less than 0.5 (S 203 : No), the CPU  50  turns on and starts to operate the cleaning device  17  at operation S 204 . Accordingly, the cleaning roller  41  comes into contact with the belt  13 , and according to the movement of the belt  13 , a portion facing the cleaning roller  41  on the surface of the belt  13  is cleaned. 
     Subsequently, the CPU  50  determines whether the position shift correction is to be performed at operation S 205 . If the position shift correction is to be performed (S 205 : Yes), the CPU  50  waits until a time period in which an output level V of each pattern detection sensor  15  satisfies Equation 1 continues a time period corresponding to a length La of the belt  13  (described below) while cleaning the belt  13  by using the cleaning device  17  at operation S 206 .
 
 V 0/2 &gt;V *( K 0 −K )/ K 0  [Equation 1]
         K 0 : expected maximum thickness of a toner layer on the belt  13     K: cleaning capability of the cleaning device  17 , that is, the thickness of a toner layer to be removed when the belt  13  passes through the cleaning device  17  once   V: output level of the pattern detection sensor  15     V 0 : expected maximum value of an output level of the pattern detection sensor  15  (that is, an output level when a portion of a toner layer having a maximum thickness is measured)       

     In Equation 1, “K 0 −K” corresponds to the maximum thickness of the toner layer on the surface of the belt  13  on a downstream side from the cleaning device  17 , which is after being cleaned by the cleaning device  17 . On the right side of Equation 1, “V*(K 0 −K)/K 0 ” corresponds to an output level when it is supposed that reflected light is measured by the pattern detection sensor  15  on the downstream side from the cleaning device  17 , which is an output level obtained by subtracting the amount to be cleaned by the cleaning device  17 . 
     The left side of Equation 1 is a threshold value for determining the contamination degree of the belt  13 . This value is half of the maximum output level of the pattern detection sensor  15 , and is an intermediate value between an output level V 0  when the amount of reflected light from the pattern surface is measured and an output level when the amount of reflected light from the surface of the belt  13  is measured. Then, if the value of the right side is less than the value of the left side, Equation 1 is satisfied. If Equation 1 is satisfied, the contamination degree at a portion on the belt  13  measured by the pattern detection sensor  15  becomes lower than a reference value. 
       FIG. 5  is a diagram illustrating the position relationship between the belt  13 , and the pattern detection sensors  15 , the cleaning device  17  and the photosensitive drums  28  provided around the belt  13 .  FIG. 6  illustrates an example of a pattern for position shift correction.  FIG. 7  illustrates an example of a pattern for density correction. 
     As shown in  FIG. 6 , the pattern P 1  for position shift correction has a plurality of marks  60  which are arranged in two lines on left and right sides of the surface of the belt  13  at predetermined intervals. A pair of pattern detection sensors  15  are arranged to face the marks  60  of the left and right columns, respectively. The marks  60  correspond to the colors of toner used in the process unit  20 , and a plurality of sets of marks  60 , each set having four marks of yellow ( 60 Y), magenta ( 60 M), cyan ( 60 C), and black ( 60 K), are arranged in a predetermined order along the sheet conveying direction. A length on the belt  13  in which the pattern P 1  for position shift correction is formed is La. As shown in  FIG. 5 , the length La is smaller than a length L 0  on the belt  13  from the pattern detection sensor  15  to the initial image forming position (a position facing the photosensitive drum  28  of yellow). 
     Meanwhile, as shown in  FIG. 7 , the pattern P 2  for density correction has a plurality of marks  61  which are arranged in a line on one side of the surface of the belt  13 . The pattern P 2  for density correction has a plurality of marks  61  which have different densities for the respective colors (yellow ( 61 Y), magenta ( 61 M), cyan ( 61 C), and black ( 61 K)) of toner used in the process unit  20 , for example, 10%, 50%, and 100%. A length on the belt  13  in which the pattern P 2  for density correction is formed is Lb. As shown in  FIG. 5 , the length Lb is smaller than the length L 0  on the belt  13  from the pattern detection sensor  15  to the initial image forming position. 
     In operation S 206 , if the state satisfying Equation 1 is kept until the belt  13  passes through the position of the pattern detection sensor  15  by the length La (S 206 : Yes), the CPU  50  sets the region on the belt  13  corresponding to the length La as a pattern forming region where the pattern P 1  for position shift correction is to be formed. Then, after the rear end of the pattern forming region passes through the cleaning device  17 , the cleaning device  17  is turned off at operation S 207 . Accordingly, if the contamination degree of the belt  13  is large (does not bear the criteria of Equation 1), the operation amount of the cleaning device  17  (the number of times of cleaning the same place on the belt  13 , a cleaning range, an operation time, and the like) is increased. 
     Subsequently, if the front end of the pattern forming region on the belt  13  is moved from the position of the pattern detection sensor  15  by the length L 0 , that is, reaches the initial image forming position, the CPU  50  starts to form the pattern P 1  for position shift correction at a timing at which an initial yellow mark  60 Y is transferred from the photosensitive drum  28  at operation S 208 . As described above, when the position shift correction is to be performed, if the contamination degree of the belt  13  is smaller than the reference of 0.8 (S 202 : Yes), the pattern P 1  for position shift correction immediately starts to be formed in S 208  without operating the cleaning device  17 . 
     Subsequently, the CPU  50  sets a threshold value Vt which is used to measure the position of the pattern P 1  for position shift correction at operation S 209 . This threshold value Vt is determined, for example, by using Equation 2.
 
 Vt =( Vm−V 0/2)*1.2, if ( Vm−V 0/2)/ V 0≧0.3
 
 Vt=V 0*0.3, if ( Vm−V 0/2)/ V 0&lt;0.3  [Equation 2]
 
     Vm: maximum value of a measured output value V of the pattern detection sensor  15   
     On the belt  13 , a place where the output level Vm of the pattern detection sensor  15  is slightly large, and the contamination degree is comparatively large is cleaned while passing through the cleaning device  17 . Then, if it is supposed that reflected light was measured by using the pattern detection sensor  15 , it is considered that the output is decreased by at least V 0 / 2 . Accordingly, “Vm−V 0 / 2 ” of Equation 2 corresponds to the maximum output level on the downstream side from the cleaning device  17  if the amount of reflected light from the surface of the belt  13  is measured by using the pattern detection sensor  15 . In Equation 2, when a value obtained by dividing “Vm−V 0 / 2 ” by V 0  is not less than 0.3, the value is multiplied by 1.2 and then the calculated value is set as the threshold value Vt. Accordingly, the threshold value Vt is set as the intermediate value between the output level when the amount of reflected light from the surface of the belt is measured and the output level V 0  when the amount of reflected light from the mark  60  is measured. The larger the amount of reflected light from the surface of the belt is, the larger the threshold value is. When the value obtained by dividing “Vm−V 0 / 2 ” by V 0  is less than 0.3, the threshold value Vt is set to be 0.3 times V 0  (lower value). 
     Subsequently, when the pattern forming region where the pattern P 1  for position shift correction is formed reaches the position of the pattern detection sensor  15 , the CPU  50  starts to measure the position of the pattern P 1  for position shift correction (S 210 ). The CPU  50  compares the output V from the pattern detection sensor  15  with the threshold value Vt. Then, if the output V is larger than the threshold value Vt, that is, if the amount of reflected light from the belt  13  is close to the amount V 0  of reflected light from the surface of the pattern, the CPU  50  determines that a mark  60  exists at a position facing the pattern detection sensor  15 . On the other hand, if the output V is smaller than the threshold value Vt, that is, if it is close to the amount of reflected light from the surface of the belt  13 , the CPU  50  determines that no mark  60  exists on the belt  13 . 
     The CPU  50  calculates a shift amount of the image forming position of each color with respect to black on the basis of the measurement result of each mark  60 , and registers a position correction amount corresponding to the shift amount in the NVRAM  53  at operation S 211 . During image forming, when exposure is performed by the scanner unit  19 , the write position of each photosensitive drum  28  is corrected on the basis of the position correction amount. 
     After the position shift correction ends through operations S 208  to S 211 , the CPU  50  turns on the cleaning device  17  to perform a cleaning processing of the belt  13  at operation S 212 . During the cleaning processing, the amount of reflected light from the belt  13  is measured by using the pattern detection sensor  15 . Then, cleaning of the belt  13  is continued until the output level of the pattern detection sensor  15  is less than the threshold value over the entire belt  13 , and the pattern P 1  for position shift correction is removed. 
     In operation S 205 , when density correction is to be performed (S 205 : No), the CPU  50  waits until a time period in which the output of the pattern detection sensor  15  satisfies Equation 3 continues by a time period corresponding to the length Lb of the belt  13  while cleaning the belt  13  by using the cleaning device  17  at operation S 213 .
 
 V 0/3 &gt;V *( K 0 −K )/ K 0  [Equation 3]
 
     Equation 3 is different from Equation 1 in that the value of the left side is V 0 / 3 . That is, on the downstream side from the cleaning device  17 , if the maximum output level when it is supposed that reflected light is measured by using the pattern detection sensor  15  is less than ⅓ of the maximum output level of the pattern detection sensor  15 , Equation 3 is satisfied. In Equation 3, the value of the left side for determining the contamination degree of the belt  13  becomes smaller than that in Equation 1. That is, in case of density correction, the belt  13  needs to be cleaner than in case of position shift correction. 
     If the state satisfying Equation 3 continues until the belt  13  passes through the position of the pattern detection sensor  15  by the length Lb (S 213 : Yes), the CPU  50  sets the region on the belt  13  corresponding to the length Lb as a pattern forming region in which the pattern P 2  for density correction is to be formed. Then, after the rear end of the pattern forming region passes through the cleaning device  17 , the cleaning device  17  is turned off at operation S 214 . When density correction is to be performed, the condition represented by Equation 3 is stricter than in case of position shift correction. Therefore, the operation amount of the cleaning device  17  is increased according to the contamination degree of the belt  13 , as compared with position shift correction. 
     Subsequently, when the front end of the pattern forming region on the belt  13  is moved from the position of the pattern detection sensor  15  by the length L 0 , that is, reaches the initial image forming position, the CPU  50  starts to form the pattern P 2  for density correction at a timing at which an initial mark  61  is transferred from the photosensitive drum  28  at operation S 215 . As described above, when density correction is to be performed, if the contamination degree of the belt  13  is less than the reference of 0.5 (S 203 : Yes), the CPU  50  immediately starts to form the pattern P 2  for density correction at operation S 215  without operating the cleaning device  17 . 
     Subsequently, if the pattern forming region reaches the position of the pattern detection sensor  15 , the CPU  50  starts to measure the pattern P 2  for density correction at operation S 216 . Here, the CPU  50  measures the densities of the respective marks  61 , and registers a density correction value based on the measurement results in the NVRAM  53  at operation S 217 . During image forming, the densities of the respective colors when exposure is performed by the scanner unit  19  are corrected on the basis of the density correction amount. 
     After the density correction through operations S 213  to S 217  ends, the CPU  50  turns on the cleaning device  17  to clean the belt  13  at operation S 212 . During the cleaning processing, the amount of reflected light from the belt  13  is measured by using the pattern detection sensor  15 . The belt  13  is continuously cleaned until the output level of the pattern detection sensor  15  is not more than a predetermined threshold value over the entire belt  13 , and the pattern P 2  for density correction is removed. In this way, the correction processing ends. 
     Advantage of the Exemplary Embodiment 
     As described above, according to this exemplary embodiment, if the correction processing is to be performed, first, the belt  13  is cleaned, then the pattern is formed in a cleaned portion on the belt  13 , and subsequently pattern detection and correction are performed. With this configuration, the belt  13  is cleaned before the pattern is formed. Therefore, it is possible to ensure pattern detection accuracy and to increase correction accuracy, thereby ensuring the quality of an image to be formed. 
     The operation amount during cleaning by the cleaning device  17  can be changed. Therefore, the operation amount can be changed as occasion demands. For example, when the belt  13  is not contaminated so much, the operation amount is set to be small, and as a result, a waiting time of the user can be reduced. 
     During density correction, pattern detection tends to be affected by contamination of the belt  13 , compared with position shift correction. For this reason, during density correction, the operation mount of the cleaning device  17  is set to be large, compared with position shift correction, thereby ensuring the pattern detection accuracy. During position shift correction, the operation amount of the cleaning device  17  is set to be small, compared with density correction, thereby reducing a processing time. 
     The contamination degree of the belt  13  is determined, and the operation amount during cleaning is changed according to the contamination degree. Therefore, appropriate cleaning can be performed. 
     The amount of reflected light from the belt  13  is measured, and the contamination degree is determined on the basis of the measurement result. Therefore, the contamination degree can be accurately determined, as compared with a case where the contamination degree is estimated according to the number of printed sheets. 
     Comparison is performed to determine whether the light to be measured is close to the amount of reflected light from the surface of the belt  13  or the amount of reflected light from the surface of the pattern. Therefore, the contamination degree can be appropriately determined. 
     When a place on the belt  13  measured by the pattern detection sensor  15  is arranged to reach the image forming position of the image forming unit while passing through the cleaning device  17 , the contamination degree is determined by subtracting the amount to be cleaned by the cleaning device  17  from the measurement result. For this reason, as compared with a case where the contamination degree is determined on the basis of the measurement result, an actual contamination degree can be promptly determined, and a pattern can start to be formed. 
     The amount of reflected light from the belt  13  is measured at a predetermined interval, and the contamination information based on the measurement result is stored, and when correction is performed, the contamination degree is determined on the basis of the contamination information. Therefore, the contamination degree can be determined in a short time, as compared with a case where measurement is not started until correction is performed. 
     Further, for example, when a contamination state of the belt  13  is changed, the contamination information is invalidated. Therefore, appropriate determination can be performed. 
     When it is determined that the contamination degree of the belt  13  is less than the reference, the operation amount of the cleaning device  17  is set to 0. Therefore, an unnecessary cleaning processing is not performed, and as a result, a processing time can be reduced. 
     Since the amount of reflected light from the surface of the belt  13  is changed due to a wear pattern on the surface of the belt  13 , by changing the threshold value for position detection according to the amount of light, pattern detection accuracy can be increased. 
     Other Exemplary Embodiments 
     While the present invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. 
     (1) During the correction processing, it may be configured such that, by a selection unit, such as the operation unit or the like, the user selects whether to perform cleaning before a pattern is formed. Therefore, on urgent business, cleaning can be omitted, and correction may be promptly completed. 
     (2) In the foregoing exemplary embodiment, the amount of light from the belt is optically measured, and the contamination degree is directly determined Alternatively, according to an exemplary embodiment of the present invention, the contamination degree may be supposed from the number of rotations of the carrier. In addition, the belt may be divided into a plurality of sections, and a contamination degree about each section may be stored. 
     (3) In the foregoing exemplary embodiment, the cleaning device is switched on or off. However, the inventive concept of the present invention may be applied to a case where a front end of a fixed blade is in contact with the surface of the carrier, such as a belt or the like, and the carrier is constantly cleaned when being moved (not switched on or off). 
     (4) In the foregoing exemplary embodiment, the belt is used as the carrier on which the pattern is formed. Alternatively, according to an exemplary embodiment of the present invention, in an image forming apparatus using a transfer drum or an intermediate transfer belt, a pattern may be formed on the transfer drum or the intermediate transfer belt.