Patent Publication Number: US-10331068-B2

Title: Image forming apparatus performing density adjustment control

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     Aspects of the present disclosure generally relate to density adjustment control of an image which is formed by an image forming apparatus. 
     Description of the Related Art 
     An electrophotographic-type image forming apparatus includes a charging device, which charges a photosensitive member, an exposure device, which exposes the photosensitive member to laser light to form an electrostatic latent image on the photosensitive member, and a development device, which develops the electrostatic latent image. A visible image is formed by the development device developing the electrostatic latent image with use of a developer. The image forming apparatus further includes a transfer device and a fixing device. The transfer device transfers an image formed on the photosensitive member to a sheet. Then, the fixing device fixes the image on the sheet to the sheet. This enables an image to be formed on the sheet. 
     An image forming apparatus discussed in U.S. Pat. No. 5,566,372 forms a measurement image on an image bearing member for the purpose of adjusting the density of an image to a target density, and generates an image forming condition based on a result of measurement of the measurement image performed by a measurement unit. When a power switch has been turned on, the image forming apparatus discussed in U.S. Pat. No. 5,566,372 forms a measurement image on an image bearing member and generates an image forming condition based on a result of measurement performed by the measurement unit. 
     Moreover, when the image forming apparatus is continuously forming a plurality of images, the density of each image may vary. Therefore, at any time in the process of continuously forming a plurality of images, the image forming apparatus needs to form a measurement image and adjust the image forming condition based on a result of measurement of the measurement image performed by the measurement unit. 
     On the other hand, the image forming apparatus is also able to perform image formation on a sheet longer than a predetermined length (a long sheet). In a case where the image forming apparatus continuously forms images on a plurality of long sheets, a time interval at which to adjust the image forming condition would become longer. Therefore, when such a conventional image forming apparatus continuously forms images on a plurality of long sheets, the density of each image may vary. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present disclosure, an image forming apparatus includes a conveyance roller configured to convey a sheet, an image forming unit configured to form an image on an image bearing member, a transfer portion at which the image is transferred from the image bearing member to the sheet, a sensor configured to measure a measurement image formed on the image bearing member, and a controller configured to, in a case where the image forming unit forms images on a plurality of first sheets, control the image forming unit to form a first measurement image between an image which is to be transferred to a first sheet of the plurality of first sheets and an image which is to be transferred to a different first sheet of the plurality of first sheets, control the sensor to measure the first measurement image, and control an image forming condition based on a result of measurement of the first measurement image performed by the sensor and a first feedback condition, and, in a case where the image forming unit forms images on a plurality of second sheets, control the image forming unit to form a second measurement image between an image which is to be transferred to a second sheet of the plurality of second sheets and an image which is to be transferred to a different second sheet of the plurality of second sheets, control the sensor to measure the second measurement image, and control the image forming condition based on a result of measurement of the second measurement image performed by the sensor and a second feedback condition which is different from the first feedback condition, wherein a length of the second sheet in a conveyance direction is larger than a length of the first sheet in the conveyance direction. 
     Further features of the present disclosure will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic sectional view of an image forming apparatus. 
         FIG. 2  is an essential-portion sectional view of a sensor which measures a measurement image. 
         FIG. 3  is a control block diagram of the image forming apparatus. 
         FIGS. 4A, 4B, 4C, and 4D  are explanatory diagrams of screens which are formed by the image forming apparatus. 
         FIGS. 5A and 5B  are diagrams illustrating measurement images. 
         FIG. 6  is a flowchart of an image forming operation. 
         FIG. 7  is a flowchart concerning laser power (LPW) determination processing. 
         FIG. 8  illustrates a table for a forming condition of a measurement image and scheduling. 
         FIGS. 9A and 9B  are explanatory diagrams illustrating a method for correcting tone characteristics. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     &lt;Configuration of Image Forming Apparatus&gt; 
     An image forming apparatus is described with reference to  FIG. 1 . The image forming apparatus  100  includes a printing unit  101 , a reader unit  400 , and an operation unit  180 . The printing unit  101  includes four stations  120 ,  121 ,  122 , and  123 , which form images for respective colors. The station  120  forms a yellow image, the station  121  forms a magenta image, the station  122  forms a cyan image, and the station  123  forms a black image. 
     Since each station has the same configuration, hereinafter, a configuration of the station  120 , which forms a yellow image, is described. A photosensitive drum  105  is a photosensitive member having a photosensitive layer on the surface thereof. A charging device  111  receives a charging voltage supplied from a high-voltage power source (not illustrated). The charging device  111  charges the surface of the photosensitive drum  105  based on the charging voltage. The photosensitive drum  105  being scanned with laser light emitted from an exposure device  103  controlled based on image data causes an electrostatic latent image to be formed on the photosensitive drum  105 . The intensity of laser light emitted from the exposure device  103  (hereinafter referred to as “LPW”) is controlled based on, for example, a driving current. A development device  112  includes a container portion, in which a developer is contained, and a developing sleeve  12 , which rotates while bearing the developer contained in the container portion. Furthermore, the developer is, for example, a two-component developer containing toner and magnetic carrier. The development device  112  develops an electrostatic latent image with use of the developer contained in the container portion. This causes an image to be borne on the photosensitive drum  105 . The photosensitive drum  105  functions as an image bearing member which bears an image formed by the printing unit  101 . Furthermore, the amount of toner to be supplied from the developing sleeve  12  to the photosensitive drum  105  is adjusted based on a development voltage supplied to the developing sleeve  12 . 
     A primary transfer roller  118  receives a transfer voltage applied from a high-voltage power source (not illustrated). This causes a potential difference between the photosensitive drum  105  and the primary transfer roller  118 . Accordingly, an image on the photosensitive drum  105  is transferred to an intermediate transfer belt  106 . Images of respective colors formed by the respective stations  120 ,  121 ,  122 , and  123  are transferred to the intermediate transfer belt  106  in a superimposed manner, so that a full-color image is borne on the intermediate transfer belt  106 . The intermediate transfer belt  106  functions as an image bearing member which bears an image. Alternatively, the intermediate transfer belt  106  functions as a transfer member to which an image is transferred. An image borne by the intermediate transfer belt  106  is conveyed to a secondary transfer roller  114  according to the intermediate transfer belt  106  rotating. The configuration of the image bearing member (or the transfer member) is not limited to a belt. The configuration of the image bearing member (or the transfer member) can be, for example, an intermediate transfer drum of the drum shape which rotates in a predetermined direction. The surface of the intermediate transfer drum has an elastic layer containing carbon black formed thereon. 
     Sensors  117 , each of which measures reflected light from a measurement image, are located in the vicinity of the intermediate transfer belt  106 . Four sensors  117  are arranged side by side in a direction perpendicular to the conveyance direction of the intermediate transfer belt  106 . The sensors  117  detect measurement images formed at respective positions different in the direction perpendicular to the conveyance direction. The image forming apparatus  100  adjusts an image forming condition based on the intensity of reflected light from the measurement images measured by the sensors  117 . Here, the image forming condition refers to, for example, a charging voltage, an LPW, a development voltage, and a transfer voltage. The image forming condition to be adjusted can also be any one of the four parameters, or can also be some of the four parameters. 
     The image forming apparatus  100  includes a plurality of sheet containers  113  and a sheet container  500 . Each of the sheet containers  113  in the present exemplary embodiment is able to contain therein a sheet  110  the length of which in the longitudinal direction thereof is less than 19.2 inches (487.68 mm). Moreover, the sheet container  500  in the present exemplary embodiment is able to contain therein a sheet  501  the length of which in the longitudinal direction thereof is equal to or greater than 19.2 inches (487.68 mm) and less than 35 inches (889 mm). In the following direction, the sheet  501  is referred to as a “long sheet  501 ”. 
     In a case where the image forming apparatus  100  forms an image on the sheet  110 , the sheet  110  is fed from the sheet container  113 . Then, conveyance rollers  140  convey the sheet  110  toward a secondary transfer portion in such a manner that the sheet  110  coincides in timing with an image borne on the intermediate transfer belt  106 . The secondary transfer portion is a nip portion between the secondary transfer roller  114  and the intermediate transfer belt  106 . A transfer voltage (a transfer voltage different from the transfer voltage applied to the primary transfer roller  118 ) is applied to the secondary transfer roller  114 . This causes the secondary transfer roller  114  to transfer the image borne on the intermediate transfer belt  106  to the sheet  110  conveyed to the secondary transfer portion. Then, the sheet  110  having the image transferred thereto is conveyed to fixing devices  150  and  160 . 
     Moreover, in a case where the image forming apparatus  100  forms an image on the sheet  501 , a long sheet  501  contained in the sheet container  500  is fed. Then, the conveyance rollers  140  convey the long sheet  501  toward the secondary transfer roller  114  in such a manner that the long sheet  501  coincides in timing with an image borne on the intermediate transfer belt  106 . Image forming processing which the image forming apparatus  100  performs to form an image on the long sheet  501  is similar to image forming processing which the image forming apparatus  100  performs to form an image on the sheet  110 , and, therefore, hereinafter, the image forming processing which the image forming apparatus  100  performs to form an image on the sheet  110  is described. 
     The fixing devices  150  and  160  fix, to the sheet  110 , an image transferred to the sheet  110  by heating and pressing the image. The fixing device  150  includes a fixing roller  151 , which has a heater that heats the sheet  110 , and a pressure belt  152 , which brings the sheet  110  into press contact with the fixing roller  151 . The fixing device  160  is located more downstream in the conveyance direction of the sheet  110  than the fixing device  150 . The fixing device  160  gives a gloss (sheen) to an image formed on the sheet  110 , which has passed through the fixing device  150 . The fixing device  160  includes a fixing roller  161 , which has a heater that heats the sheet  110 , and a pressure roller  162 . 
     In the case of fixing an image to the sheet  110  in a mode for giving a gloss or in the case of fixing an image to a sheet  110  requiring a large amount of heat for fixing, such as a cardboard, the sheet  110 , which has passed through the fixing device  150 , is conveyed to the fixing device  160 . In the case of fixing an image to a sheet  110  such as plain paper or thin paper, the sheet  110 , which has passed through the fixing device  150 , is conveyed along a conveyance path  130 , which takes a detour around the fixing device  160 . Furthermore, the angle of a diverter (flapper)  131  is controlled so as to control whether to convey the sheet  110  to the fixing device  160  or to convey the sheet  110  around the fixing device  160 . 
     A diverter (flapper)  132  is a guidance member which switches whether to guide the sheet  110  to a conveyance path  135  or to guide the sheet  110  to a conveyance path  139 , which leads to the exterior. The sheet  110 , which has been conveyed along the conveyance path  135 , is conveyed to a reversing portion  136 . When a reversal sensor  137 , which is provided on the conveyance path  135 , detects the trailing edge of the sheet  110 , the conveyance direction of the sheet  110  is reversed. 
     A diverter (flapper)  133  is a guidance member which switches whether to guide the sheet  110  to a conveyance path  138 , which is used for two-sided image formation, or to guide the sheet  110  to the conveyance path  135 . In a case where a face-down sheet discharge mode is performed, the sheet  110  is re-conveyed to the conveyance path  135  and is then discharged from the image forming apparatus  100 . 
     On the other hand, in a case where a two-sided printing mode is performed, the sheet  110  is re-conveyed to the transfer roller  114  along the conveyance path  138 . In the case of the two-sided printing mode, after an image is formed on the first surface of the sheet  110 , the sheet  110  is inverted at the reversing portion  136  and is then conveyed to the transfer roller  114  along the conveyance path  138 , so that an image is formed on the second surface of the sheet  110 . 
     A diverter (flapper)  134  is a guidance member which guides the sheet  110  to the conveyance path  139 , which is used to discharge the sheet  110  from the image forming apparatus  100 . In a case where the sheet  110  is discharged face-down, the diverter  134  guides the sheet  110 , which has been inverted at the reversing portion  136 , to the conveyance path  139  for sheet discharge. The sheet  110 , which has been conveyed along the conveyance path  139  for sheet discharge, is discharged to the exterior of the image forming apparatus  100 . 
     Color sensors  200 , each of which measures reflected light from a measurement image on the sheet  110 , are located on the conveyance path  135 . Four color sensors  200  are arranged side by side in a direction perpendicular to the conveyance direction of the sheet  110 . Each color sensor  200  measures a measurement image on the sheet  110 . The image forming apparatus  100  adjusts an image forming condition including, for example, a charging voltage, an LPW, a development voltage, a transfer voltage which is applied to the primary transfer roller  118 , a transfer voltage which is applied to the secondary transfer roller  114 , and target temperatures of the fixing devices  150  and  160  based on a result of measurement performed by the color sensors  200 . 
     The operation unit  180  includes a liquid crystal display, which serves as a display unit, and a key input unit. The operation unit  180  is an interface used for the user to input the number of printed sheets for an image and a printing mode. The user is allowed to use the operation unit  180  to select between a one-sided printing mode and a two-sided printing mode, to perform a face-down sheet discharge mode, or to give an instruction for image formation with a long sheet. 
     A reader unit  400 , which includes a unit including a light source, an optical system, and a charge-coupled device (CCD) sensor and a platen, reads the image of an original placed on the platen. When an original is placed on the platen and a reading start button of the operation unit  180  is pressed by the user, the reader unit  400  performs a reading operation. When performing the reading operation, the reader unit  400  radiates light from the light source to the original and then causes the CCD sensor to acquire reflected light from the original. The CCD sensor outputs luminance data which represents a result of reading of the original. The reader unit  400  converts the luminance data into density data (image data) with use of a luminance-density conversion table and transfers the density data to a tone correction unit  316  ( FIG. 3 ) of the image forming apparatus  100 . Furthermore, the luminance-density conversion table is previously stored in a read-only memory (ROM)  304  ( FIG. 3 ). 
     &lt;Configuration of Sensor&gt; 
     A configuration of the sensor  117  provided in the image forming apparatus  100  is described with reference to  FIG. 2 . The sensor  117  includes a specular reflection light receiving portion  401 , a diffuse reflection light receiving portion  402 , and a light emitting portion  403 . Furthermore, the sensor  117  can be configured to further include an optical element such as a lens. 
     The light emitting portion  403  is a light emitting element which radiates light to a measurement image formed on the intermediate transfer belt  106 . The wavelength of light to be radiated from the light emitting portion  403  is set to, for example, 800 nanometer (nm) to 850 nm in view of the spectral reflectivity of a developer. Light from the light emitting portion  403  is radiated in a direction that makes an angle of 45 degrees with a direction perpendicular to the surface of the intermediate transfer belt  106 . 
     The specular reflection light receiving portion  401  is provided on an imaginary line that makes an angle of 45 degrees with a direction perpendicular to the surface of the intermediate transfer belt  106 . For example, the light emitting portion  403  and the specular reflection light receiving portion  401  are located at positions symmetrical with respect to a surface perpendicular to the surface of the intermediate transfer belt  106 . The specular reflection light receiving portion  401  receives specular reflection light from a measurement image formed on the intermediate transfer belt  106 . The specular reflection light receiving portion  401  outputs a sensor output value (a voltage value) corresponding to the intensity of reflected light from the measurement image. 
     The diffuse reflection light receiving portion  402  is provided at a position at which no specular reflection light from the intermediate transfer belt  106  is received. The diffuse reflection light receiving portion  402  is provided on an imaginary line that makes angle of, for example, 20 degrees with a direction perpendicular to the surface of the intermediate transfer belt  106 . The diffuse reflection light receiving portion  402  receives diffuse reflection light from the measurement image on the intermediate transfer belt  106 . The diffuse reflection light receiving portion  402  outputs a sensor output value (a voltage value) corresponding to the intensity of reflected light from the measurement image. 
     The image forming apparatus  100  measures the density of the measurement image based on the sensor output value output from the specular reflection light receiving portion  401  and the sensor output value output from the diffuse reflection light receiving portion  402 . For example, the image forming apparatus  100  determines the density of the measurement image by performing calculation based on the sensor output value output from the specular reflection light receiving portion  401  and the sensor output value output from the diffuse reflection light receiving portion  402 . Alternatively, the image forming apparatus  100  determines the density of the measurement image by referring to a table representing a correspondence relationship between a combination of the sensor output value output from the specular reflection light receiving portion  401  and the sensor output value output from the diffuse reflection light receiving portion  402  and a density. 
     &lt;Configuration of Controller&gt; 
     A control block diagram of the image forming apparatus  100  is described with reference to  FIG. 3 . A central processing unit (CPU)  300  is a control circuit (controller) which controls each unit of the image forming apparatus  100 . A control program which is executed by the CPU  300  and is required to perform, for example, various processing operations described in a flowchart to be described below is stored in the ROM  304 . A random access memory (RAM)  309  is a system work memory used for the CPU  300  to operate. 
     The printing unit  101  corresponds to the stations  120 ,  121 ,  122 , and  123 , the primary transfer roller  118 , the intermediate transfer belt  106 , the secondary transfer roller  114 , the fixing device  150 , and the fixing device  160 . Since the operation unit  180  and the reader unit  400  have already been described, the description thereof is omitted here. Moreover, an interface (I/F) unit  302  is an interface to which image data transferred from a personal computer (PC) as an external apparatus is input. 
     The tone correction unit  316  performs various image processing operations on input image data to perform conversion of the image data. The density of an image which is formed by the printing unit  101  may not become an intended density. Therefore, the tone correction unit  316  corrects an input value of image data (an image signal value) in such a manner that the density of an image which is formed by the printing unit  101  becomes an intended density. The tone correction unit  316  performs conversion of image data based on a tone correction table (γ look-up table (LUT)) stored in a memory  310 . Furthermore, the tone correction table is stored for each screen, which is described below, and for each color in the memory  310 . The tone correction table (γ LUT) is equivalent to a conversion condition for conversion of image data. Alternatively, the tone correction table (γ LUT) is an example of an image forming condition for adjusting the density of an output image which is formed by the image forming apparatus  100 . Furthermore, the tone correction unit  316  can be implemented by an integrated circuit such as an application specific integrated circuit (ASIC) or can be implemented by the CPU  300  performing conversion of image data based on a previously-stored program. The tone correction unit  316  is generally referred to as an “image processing unit”. 
     A halftone processing unit  317  performs screening suited to the type of an image on image data subjected to conversion by the tone correction unit  316 . The halftone processing unit  317  performs conversion of image data based on, for example, a 190 Dot screen in such a manner that a photographic image or graphic image becomes an image excellent in tone characteristics. The halftone processing unit  317  performs conversion of image data based on, for example, a 230 Dot screen in such a manner that a character image is printed in a sharp manner. The halftone processing unit  317  performs conversion of image data based on, for example, an error diffusion method in such a manner that a high-resolution image becomes an image with moire prevented or reduced. 
     In a case where input image data is image data for printing generated with use of a page-description language, the halftone processing unit  317  performs conversion of image data based on the 190 Dot screen, the 230 Dot screen, and the error diffusion method. On the other hand, in a case where an image other than a character image of an original read by the reader unit  400  is printed, the halftone processing unit  317  performs conversion of image data transferred from the reader unit  400  based on a copier screen. 
     Moreover,  FIG. 4A  is an enlarged view of an image (halftone) processed based on the 190 Dot screen. Similarly,  FIG. 4B  is an enlarged view of an image (halftone) processed based on the 230 Dot screen,  FIG. 4C  is an enlarged view of an image (halftone) processed based on the copier screen, and  FIG. 4D  is an enlarged view of an image (halftone) processed based on the error diffusion method. Furthermore, the above-mentioned screens are mere examples and the present exemplary embodiment is not limited to these specific screens. 
     Moreover, a tone correction table LUT_SC 1  stored in the memory  310  is a conversion condition for performing conversion of image data about a graphic image. Similarly, a tone correction table LUT_SC 2  is a conversion condition for performing conversion of image data about a character image. A tone correction table LUT_SC 3  is a conversion condition for performing conversion of image data input from the reader unit  400 . A tone correction table LUT_SC 4  is a conversion condition for performing conversion of image data about a photographic image. 
     The image data subjected to screening by the halftone processing unit  317  is input to the printing unit  101 . The printing unit  101  forms, on the sheet  110 , an image that is based on the input image data. 
     A pattern generator  305  outputs measurement image data. The printing unit  101  forms a measurement image in a region between an image borne on the intermediate transfer belt  106  and an image adjacent to the first-mentioned image based on the measurement image data output from the pattern generator  305 . The CPU  300  acquires sensor output values at timing at which the measurement image on the intermediate transfer belt  106  passes through the measurement position of the sensors  117 . Then, the CPU  300  can determine the density of the measurement image based on the sensor outputs. Furthermore, the CPU  300  can determine the amount of adhesion of a developer based on the sensor outputs. 
     A γ LUT generation unit  307  generates a tone correction table (γ LUT) based on the density of the measurement image measured by the CPU  300  and the sensors  117  and a previously-determined target density. Furthermore, the measurement image is formed for each color and for each screen. The γ LUT generation unit  307  generates a tone correction table corresponding to the measurement image based on a result obtained by measuring each measurement image. 
     &lt;Density Adjustment Control&gt; 
     Next, density adjustment control is described. The image forming apparatus  100  is able to perform two types of density adjustment control, i.e., a first calibration and a second calibration. 
     The first calibration is control in which the image forming apparatus  100  causes the printing unit  101  to form a pattern image on the sheet  110  and generates an image forming condition based on the density of the pattern image measured by the color sensors  200  or the reader unit  400 . A pattern image having 22 tone levels is formed on the sheet  110  for each screen and for each color. 
     On the other hand, in the case of the first calibration, since a pattern image is formed on the sheet  110 , sheets  110  may be consumed. Therefore, the image forming apparatus  100  is able to perform the second calibration to correct the tone correction table without consuming sheets  110 . 
     The second calibration is control in which the image forming apparatus  100  causes the printing unit  101  to form a measurement image P on the intermediate transfer belt  106  and generates an image forming condition based on a result of measurement of the measurement image P performed by the sensors  117 . The image forming apparatus  100  in the present exemplary embodiment forms a measurement image Pa processed based on the 190 Dot screen, a measurement image Pb processed based on the 230 Dot screen, and a measurement image Pc processed based on the copier screen. 
     The measurement image Pa in the present exemplary embodiment includes five measurement images having respective different tones. The measurement image Pb in the present exemplary embodiment also includes five measurement images having respective different tones. Image signal values of measurement image data used for forming the measurement image Pa are assumed to be, for example, 20%, 40%, 60%, 80%, and 100% (maximum density). The same also applies to image signal values of measurement image data used for forming the measurement image Pb. Furthermore, the image signal value of measurement image data used for forming the measurement image Pc is assumed to be, for example, 40%. 
     Moreover, the image forming apparatus  100  in the present exemplary embodiment updates an image forming condition by performing the second calibration even during a period in which the printing unit  101  is continuously forming a plurality of images. In a case where the image forming apparatus  100  is continuously forming a plurality of images, each measurement image P is formed for each color in a space between an image and a subsequent image (an inter-sheet region) in the conveyance direction of the intermediate transfer belt  106 . This aims at improving the productivity of the image forming apparatus  100  by narrowing an interval between an image and a subsequent image. 
     Furthermore, the image forming apparatus  100  in the present exemplary embodiment does not update the γ LUT for the 190 Dot screen before the measurement results (sensor outputs) corresponding to measurement images Pa for five tones are acquired. Similarly, the image forming apparatus  100  in the present exemplary embodiment does not update the γ LUT for the 230 Dot screen before the measurement results (sensor outputs) corresponding to measurement images Pb for five tones are acquired. 
     Here, if the image forming condition is significantly changed during a period in which images are continuously being formed, the density of an image formed after the image forming condition is changed may become different from the density of an image formed before the image forming condition is changed. Therefore, the image forming apparatus  100  in the present exemplary embodiment restricts the amount of correction of the image forming condition in such a manner that the density of an output image gradually converges on a target density. For example, the image forming apparatus  100  determines an image forming condition by multiplying a difference between the density of the measurement image P and the target density by a coefficient. However, even if the image forming condition is updated based on one coefficient, in a case where a plurality of images continues being formed on a plurality of long sheets, the density of an output image may not converge on the target density. 
       FIG. 5A  is a schematic diagram of measurement images P which are formed on the intermediate transfer belt  106  in a case where the image forming apparatus  100  continuously forms images on sheets of paper the length of which in the longitudinal direction thereof is 420 mm (non-long sheets). The measurement image P_C is a cyan measurement image, the measurement image P_M is a magenta measurement image, the measurement image P_Y is a yellow measurement image, and the measurement image P_K is a black measurement image. As illustrated in  FIG. 5A , while the printing unit  101  is continuously forming images on five sheets (the N-th sheet to the (N+4)-th sheet), an image and measurement images P are alternately formed on the intermediate transfer belt  106 . 
       FIG. 5B  is a schematic diagram of measurement images P which are formed on the intermediate transfer belt  106  in a case where the image forming apparatus  100  continuously forms images on sheets of paper the length of which in the longitudinal direction thereof is 1000 mm (long sheets). The measurement image P_C is a cyan measurement image, the measurement image P_M is a magenta measurement image, the measurement image P_Y is a yellow measurement image, and the measurement image P_K is a black measurement image. As illustrated in  FIG. 5B , while the printing unit  101  is continuously forming images on five sheets (the M-th sheet to the (M+4)-th sheet), an image and measurement images P are alternately formed on the intermediate transfer belt  106 . 
     As illustrated in  FIGS. 5A and 5B , the image forming apparatus  100  does not update the tone correction table before completing acquiring results of measurement of measurement images for five tones. Therefore, in a case where images continue being formed on long-sheets, the amount of variation in the density of output images may exceed the capability of converging the density of an output image on a target density. In other words, in a case where images continue being formed on long sheets, even if the image forming apparatus  100  updates the image forming condition, the density of an output image may gradually deviate from the target density. 
     Thus, when continuously forming images on a plurality of long sheets, the image forming apparatus  100  increases the amount of correction of the image forming condition in such a manner that the density of an output image converges on a target density. The image forming apparatus  100  in the present exemplary embodiment switches between a plurality of adjustment conditions according to the size of a sheet on which to form an image. More specifically, when continuously forming images on a plurality of non-long sheets, the image forming apparatus  100  generates an image forming condition using a first coefficient. On the other hand, when continuously forming images on a plurality of long sheets, the image forming apparatus  100  generates an image forming condition using a second coefficient. At this time, the absolute value of the coefficient for the long sheet is greater than the absolute value of the coefficient for the non-long sheet. Coefficients (feedback rates) and other parameters in the present exemplary embodiment are shown in Table 1. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 190 Dot, 230 Dot 
                 Non-long sheet 
                 Long sheet 
               
               
                   
                   
               
             
            
               
                   
                 Measurement image  
                 20%, 40%, 60%, 
                 20%, 40%, 60%, 
               
               
                   
                 data signal value 
                 80%, 100% 
                 80%, 100% 
               
               
                   
                 Feedback rate 
                 40% 
                 60% 
               
               
                   
                 LPW adjustment  
                 −1, 0, +1 
                 −2, −1, 0, +1, 
               
               
                   
                 amount 
                   
                 +2 
               
               
                   
                 LPW adjustment  
                 D5 ≤ D5tgt − 30 
                 D5 ≤ D5tgt − 60 
               
               
                   
                 threshold value 
                 D5 ≥ D5tgt + 30  
                 D5 ≥ D5tgt + 60 
               
               
                   
                   
                   
                 5 ≤ D5tgt − 30 
               
               
                   
                   
                   
                 D5 ≥ D5tgt + 30 
               
               
                   
                   
               
            
           
         
       
     
     Here, the feedback rate (hereinafter also referred to as an “FB rate”) represents the rate of having correction for a difference ΔD between the density of a measurement image and a target density. The feedback rate (coefficient) corresponds to a feedback condition. In the case of non-long sheets, the tone correction table is generated in such a manner that 40% of the difference ΔD between the density of a measurement image and a target density is corrected. Moreover, in the case of long sheets, the tone correction table is generated in such a manner that 60% of the difference ΔD between the density of a measurement image and a target density is corrected. 
     In a case where the image forming apparatus  100  continuously performs image formation on a plurality of long sheets, the execution timing of update processing of the tone correction table would become delayed. Therefore, in a case where the image forming apparatus  100  continuously performs image formation on a plurality of long sheets, the variation in the density of an output image may be unable to be restricted to within an allowable range. Thus, to converge a variation in density caused while the image forming apparatus  100  is continuously performing image formation on a plurality of long sheets, the image forming apparatus  100  in the present exemplary embodiment increases the feedback rate when continuously forming images on a plurality of long sheets. 
     More specifically, the feedback rate which is used in a period during which images are continuously formed on a plurality of long sheets is higher than the feedback rate which is used in a period during which images are continuously formed on a plurality of non-long sheets. This converges a variation in density caused when the image forming apparatus  100  is continuously performing image formation on a plurality of long sheets, thus preventing the variation in density from exceeding the allowable range. 
     Here,  FIGS. 9A and 9B  are diagrams used to explain concepts in which the γ LUT generation unit  307  generates a tone correction table. The horizontal axis indicates an image signal value, and the vertical axis indicates the density. A solid line indicates an ideal tone characteristic representing a correspondence relationship between an image signal and a target density. A dashed line indicates a tone characteristic of the printing unit  101  calculated by linear interpolation from the densities of measurement images P measured by the sensors  117 . In a case where the feedback rate is 100%, to convert the density Di of an image signal value i into a target density Ditgt, the image signal value i only needs to be converted into an image signal value i′100% corresponding to the target density Ditgt of the image signal value i. 
       FIG. 9A  is a diagram used to explain a concept in which the γ LUT generation unit  307  generates a tone correction table in a case where the feedback rate is 40%. A dashed-dotted line is equivalent to a target density displaced with the feedback rate set to 40%. The γ LUT generation unit  307  displaces a target density by 40% of the difference ΔD between the predicted density and the target density and generates such a tone correction table that the image signal value i is converted into an image signal value i′40% corresponding to the target density Ditgt. In other words, the γ LUT generation unit  307  displaces the target density based on a value obtained by multiplying the density difference ΔD by 0.4 (a first coefficient) and generates a tone correction table based on the displaced target density and the predicted density. 
       FIG. 9B  is a diagram used to explain a concept in which the γ LUT generation unit  307  generates a tone correction table in a case where the feedback rate is 60%. A dashed-dotted line is equivalent to a target density displaced with the feedback rate set to 60%. The γ LUT generation unit  307  displaces a target density by 60% of the difference ΔD between the predicted density and the target density and generates such a tone correction table that the image signal value i is converted into an image signal value i′60% corresponding to the target density Ditgt. In other words, the γ LUT generation unit  307  displaces the target density based on a value obtained by multiplying the density difference ΔD by 0.6 (a second coefficient) and generates a tone correction table based on the displaced target density and the predicted density. 
     The difference in feedback rate is a difference in a coefficient used to generate a tone correction table. The first coefficient corresponding to the feedback rate 40% is 0.4, and the second coefficient corresponding to the feedback rate 60% is 0.6. If the feedback rate differs, the amount of correction in a case where the difference between the target density and the predicted density is a predetermined value differs. The amount of correction corresponding to the feedback rate 60% is larger than the amount of correction corresponding to the feedback rate 40%. 
     Moreover, the image forming apparatus  100  adjusts laser power (LPW) serving as an image forming condition based on a difference between a result of measurement of any one measurement image included in a plurality of measurement images P and a target measurement result. The image forming apparatus  100  in the present exemplary embodiment controls LPW based on a difference between the density D 5  of a measurement image Pb 100  with an image signal value 100% formed with use of the 230 Dot screen and the target density. The image forming apparatus  100  decreases LPW if the density D 5  of the specific measurement image Pb 100  is higher than the target density and increases LPW if the density D 5  of the specific measurement image Pb 100  is lower than the target density. Furthermore, the timing at which the image forming apparatus  100  changes LPW is assumed to be after the amount of LPW adjustment is determined and timing at which any leading measurement image of the measurement images Pa, Pb, and Pc is formed. 
     Moreover, the image forming apparatus  100  in the present exemplary embodiment increases the amount of adjustment of LPW in the case of continuously forming images on a plurality of long sheets. Regardless of a long sheet or a non-long sheet, the image forming apparatus  100  changes LPW by one level if the difference between the density D 5  of the specific measurement image Pb 100  and the target density is greater than a first predetermined value. Moreover, in a case where the image forming apparatus  100  continuously forms images on a plurality of long sheets, the image forming apparatus  100  changes LPW by two levels if the difference between the density D 5  of the specific measurement image Pb 100  and the target density is greater than a second predetermined value. Here, the absolute value of the second predetermined value is greater than the absolute value of the first predetermined value. 
     &lt;Image Forming Processing&gt; 
     Next, the second calibration, which the CPU  300  performs while the image forming apparatus  100  is continuously forming images, is described with reference to the flowchart of  FIG. 6  and the table of  FIG. 8 . The CPU  300  starts an image forming operation when image data for copying corresponding to an original has been input from the reader unit  400  or when image data for printing transferred via the I/F unit  302  has been input. 
     First, in step S 001 , the CPU  300  determines whether the sheet on which an image is to be formed is a long sheet  501 . For example, the CPU  300  acquires information about the size of a sheet designated by the user via the operation unit  180 , and determines whether the long sheet  501  has been designated based on the acquired information. If an image is to be formed on the long sheet  501  (YES in step S 001 ), then in step S 002 , the CPU  300  sets the feedback rate to 60% and then advances the processing to step S 004 . On the other hand, if an image is to be formed on the non-long sheet (sheet  110 ) (NO in step S 001 ), then in step S 003 , the CPU  300  sets the feedback rate to 40% and then advances the processing to step S 004 . 
     In step S 004 , the CPU  300  determines whether it is necessary to change LPW. The CPU  300  determines whether it is necessary to change LPW based on a result of LPW determination processing last performed. In other words, in a case where the amount of adjustment of LPW has been set, the CPU  300  determines that it is necessary to change LPW. 
     Moreover, the image forming apparatus  100  in the present exemplary embodiment determines that LPW is changeable at timing when a measurement image Pa with an image signal value 20% is formed or at timing when a measurement image Pb with an image signal value 20% is formed. This is because, if LPW is changed, the density of a measurement image would also change. Therefore, at timing when an image corresponding to the L-th sheet is formed or at timing when an image corresponding to the (L+5)-th sheet is formed, the CPU  300  changes LPW in a case where the amount of adjustment of LPW has been determined. 
     If it is necessary to change LPW (YES in step S 004 ) and at timing when LPW is changeable, then in step S 005 , the CPU  300  changes LPW based on the amount of adjustment. Then, in step S 006 , the CPU  300  controls the printing unit  101  to form an image based on image data, and, in step S 007 , the CPU  300  controls the printing unit  101  to form a measurement image P based on measurement image data. In step S 008 , the CPU  300  controls the sensors  117  to measure the measurement image P at timing when the measurement image P arrives at the measurement position of the sensors  117 . 
     The image forming apparatus  100  in the present exemplary embodiment forms one measurement image P for each color each time an image for one page is formed. Therefore, the CPU  300  selects one image signal value for forming the measurement image P in step S 007  from among image signal values (20%, 40%, 60%, 80%, and 100%) illustrated in  FIG. 8 . Moreover, the CPU  300  selects one screen for forming the measurement image P in step S 007  from among the 190 Dot screen, the 230 Dot screen, and the copier screen illustrated in  FIGS. 4A to 4C . 
     Then, in step S 007 , the CPU  300  causes the pattern generator  305  to output measurement image data with the selected image signal value and causes the tone correction unit  316  to correct the measurement image data based on a tone correction table corresponding to the selected screen. Then, the CPU  300  causes the halftone processing unit  317  to perform conversion of image data based on the selected screen and controls the printing unit  101  to form a measurement image P on the intermediate transfer belt  106 . 
     Next, in step S 009 , the CPU  300  determines whether the printing unit  101  has completed forming all of the images that are based on the image data. If the printing unit  101  has completed forming all of the images (YES in step S 009 ), the CPU  300  ends processing for the second calibration. 
     On the other hand, if the printing unit  101  has not yet completed forming all of the images (NO in step S 009 ), the CPU  300  advances the processing to step S 010 . Then, in step S 010 , the CPU  300  determines whether the CPU  300  has completed acquiring results of measurement of a number of measurement images P previously determined for each screen. If the CPU  300  has not yet completed acquiring results of measurement of a number of measurement images P previously determined for each screen (NO in step S 010 ), the CPU  300  returns the processing to step S 006 . With this, the CPU  300  does not update an image forming condition before the CPU  300  completes acquiring results of measurement of a number of measurement images P previously determined for each screen. Furthermore, measured data (density) about the measurement image P is stored in the RAM  309 . 
     On the other hand, if the CPU  300  has completed acquiring results of measurement of a number of measurement images P previously determined for each screen (YES in step S 010 ), then in step S 011 , the CPU  300  performs LPW determination processing ( FIG. 7 ), and then advances the processing to step S 012 . In step S 012 , the γ LUT generation unit  307  generates a tone correction table (γ LUT) corresponding to a screen and a color of the measurement image P based on the density of the measurement image P, the target density, and the feedback rate (coefficient). Then, the CPU  300  returns the processing to step S 001 . 
     Here, in a case where the measured data has been acquired while images are being formed on five long sheets  501 , the feedback rate (coefficient) which is used to generate a tone correction table in step S 012  is set to 60% ( 0 . 6 ). For example, in a case where the image forming apparatus  100  forms images on the L-th sheet to the (L+4)-th sheet all of which are long sheets  501 , the feedback rate (coefficient) which the γ LUT generation unit  307  uses is 60% ( 0 . 6 ). For example, in a case where the image forming apparatus  100  forms images on the (L+5)-th sheet to the (L+8)-th sheet which are long sheets  501  and forms an image on the (L+9)-th sheet which is a sheet  110 , the feedback rate (coefficient) which the γ LUT generation unit  307  uses is 40% (0.4). The image forming apparatus  100  in the present exemplary embodiment changes the feedback rate (coefficient) from 40% (0.4) to 60% (0.6) in the case of continuously forming images on a predetermined number of long sheets  501 . 
     Next, the LPW determination processing performed in step S 011  illustrated in  FIG. 6  is described with reference to the flowchart of  FIG. 7 . The image forming apparatus  100  determines whether it is necessary to change LPW based on a difference between the density D 5  of the specific measurement image Pb 100  and the target density D 5   tgt . Moreover, the image forming apparatus  100  determines the amount of adjustment of LPW based on the difference between the density D 5  and the target density D 5   tgt . Furthermore, in the following description, it is supposed that the smaller the numerical value of the density D 5 , the lower the density of the measurement image is, and, the larger the numerical value of the density D 5 , the higher the density of the measurement image is. 
     First, in step S 101 , the CPU  300  determines whether images continuously formed in a period in which a previously-determined number of measurement images P have been formed (hereinafter referred to as a “test period”) are images all of which are formed on long sheets  501 . In the image forming apparatus  100  in the present exemplary embodiment, in a case where images continuously formed in the test period are images which are formed on five long sheets  501 , a variation in density may increase during a period in which images are being formed on a plurality of long sheets  501 . Therefore, when updating a tone correction table (γ LUT) during a period in which images are continuously formed on five long sheets  501 , the image forming apparatus  100  in the present exemplary embodiment changes the amount of adjustment of LPW as well as the feedback rate (coefficient). 
     If images continuously formed in the test period are images all of which are formed on long sheets  501  (YES in step S 101 ), the CPU  300  advances the processing to step S 102 . Then, in step S 102 , the CPU  300  determines whether the density D 5  of the measurement image Pb 100  is equal to or lower than a lower limit value (=D 5   tgt −60). If the density D 5  is equal to or lower than the lower limit value (=D 5   tgt −60) (YES in step S 102 ), then in step S 104 , the CPU  300  increases LPW by two levels. 
     Then, the CPU  300  determines that it is necessary to change LPW and ends the LPW determination processing. Here, in a case where LPW increases by two levels, since the intensity of laser light which the exposure device  103  emits increases, the density of an image formed on the photosensitive drum  105  increases. 
     On the other hand, if the density D 5  of the measurement image Pb 100  is higher than the lower limit value (=D 5   tgt −60) (NO in step S 102 ), the CPU  300  advances the processing to step S 103 . In step S 103 , the CPU  300  determines whether the density D 5  of the measurement image Pb 100  is equal to or higher than an upper limit value (=D 5   tgt +60). If the density D 5  is equal to or higher than the upper limit value (=D 5   tgt +60) (YES in step S 103 ), then in step S 105 , the CPU  300  decreases LPW by two levels. 
     Then, the CPU  300  determines that it is necessary to change LPW and ends the LPW determination processing. Here, in a case where LPW decreases by two levels, since the intensity of laser light which the exposure device  103  emits decreases, the density of an image formed on the photosensitive drum  105  decreases. 
     Moreover, if the density D 5  of the measurement image Pb 100  is lower than the upper limit value (=D 5   tgt +60) (NO in step S 103 ), the CPU  300  advances the processing to step S 106 . In step S 106 , the CPU  300  determines whether the density D 5  of the measurement image Pb 100  is equal to or lower than a low density threshold value (=D 5   tgt −30). If the density D 5  is equal to or lower than the low density threshold value (=D 5   tgt −30) (YES in step S 106 ), then in step S 108 , the CPU  300  increases LPW by one level. 
     Then, the CPU  300  determines that it is necessary to change LPW and ends the LPW determination processing. Here, in a case where LPW increases by one level, since the intensity of laser light which the exposure device  103  emits increases, the density of an image formed on the photosensitive drum  105  increases. Furthermore, as the level of LPW is larger, the amount of change also increases. 
     On the other hand, if the density D 5  of the measurement image Pb 100  is higher than the low density threshold value (=D 5   tgt −30) (NO in step S 106 ), the CPU  300  advances the processing to step S 107 . In step S 107 , the CPU  300  determines whether the density D 5  of the measurement image Pb 100  is equal to or higher than a high density threshold value (=D 5   tgt +30). If the density D 5  is equal to or higher than the high density threshold value (=D 5   tgt +30) (YES in step S 107 ), then in step S 109 , the CPU  300  decreases LPW by one level. 
     Then, the CPU  300  determines that it is necessary to change LPW and ends the LPW determination processing. In a case where LPW decreases by one levels since the intensity of laser light which the exposure device  103  emits decreases, the density of an image formed on the photosensitive drum  105  decreases. 
     Moreover, if the density D 5  of the measurement image Pb 100  is lower than the high density threshold value (=D 5   tgt +30) (NO in step S 107 ), the CPU  300  determines that it is unnecessary to change LPW and ends the LPW determination processing. In this case, in step S 004  ( FIG. 6 ) of the second calibration, the CPU  300  determines that it is not necessary to change LPW. Thus, LPW is not changed. 
     Moreover, if images continuously formed in the test period are not images all of which are formed on long sheets  501  (NO in step S 101 ), the CPU  300  advances the processing to step S 106 . Processing in step S 106  and subsequent steps has been described above, and is, therefore, omitted from description here. With this, in a case where images continuously formed in the test period are not images all of which are formed on long sheets  501 , the CPU  300  determines that the adjustment range of LPW is ±1 level. This is smaller than the adjustment range of LPW (±2 levels) determined in a case where images continuously formed in the test period are images all of which are formed on long sheets  501 . 
     The image forming apparatus  100  according to the present exemplary embodiment is able to adjust LPW in such a manner that the density of an output image converges on a target density even when images are continuously formed on long sheets  501 . 
     Moreover, the image forming apparatus  100  in the present exemplary embodiment changes both the feedback rate (coefficient) and the adjustment range of LPW, but can be configured to change any one of these. If any one of the feedback rate (coefficient) and the adjustment range of LPW can be changed, the density of an output image can be prevented from greatly deviating from the target density. 
     &lt;Formation Sequence of Measurement Image P&gt; 
     The explanation and use application of various screens and the error diffusion method are described as follows. The 190 Dot screen is used when a photographic image or graphic image is printed. The 190 Dot screen is frequently used, and is also used for a portrait photograph, in which color reproduction is significant. 
     The 230 Dot screen is used when a character image is printed. The 230 Dot screen is greater in the number of lines per inch than the 190 Dot screen, and makes jaggies of a halftone character inconspicuous. The density of a character image is often consciously designated by the user. 
     The copier screen is used when an image other than character images among images for copying is printed. The images for copying are lower in tone characteristic, resolution, and granularity than images for printing. 
     The error diffusion method is used for a map mode or when a character image among images for copying is printed. Moreover, the error diffusion method is used in a case where it is designated by the user when an image for printing is printed. The error diffusion method is suited for printing a high-resolution image. The error diffusion method makes moire unlikely to occur in an image. 
     The image forming apparatus  100  forms measurement images Pa and Pb corresponding to the 190 Dot screen and the 230 Dot screen, a variation of the hue of which the user intends to prevent or reduce, each for five tones. This enables correcting the tone characteristics of graphic images or photographic images over a wide range and with a high degree of accuracy. Moreover, the image forming apparatus  100  forms a measurement image P corresponding to the copier screen for only one tone. 
     Since a measurement image P corresponding to the copier screen is formed for only one tone, the γ LUT generation unit  307  calculates tone characteristics from the density of a measurement image P for one tone based on a previously stored model formula, thus generating a tone correction table for the copier screen. Furthermore, since the number of tones of the 190 Dot screen or the 230 Dot screen used for print images is less than the number of tones of a measurement image P corresponding to the copier screen, the tone characteristics can be corrected more accurately in print images than in copy images. 
       FIG. 8  is a table showing screens of measurement images P which are formed in tone correction and image signal values. During a period in which images for twelve pages are being formed, five measurement images Pa corresponding to the 190 Dot screen, five measurement images Pb corresponding to the 190 Dot screen, and one measurement image Pc corresponding to the copier screen, i.e., eleven measurement images P in total, are formed. The γ LUT generation unit  307  repeatedly forms eleven measurement images P. The image forming apparatus  100  updates a tone correction table (γ LUT) corresponding to each screen each time images for eleven pages are formed. 
     The number of tones of the copier screen, which is used to form copy images, is set less than that of the 190 Dot screen or 230 Dot screen, which is used to form print images. This enables correcting a tone correction table used for forming print images in a wide range from a low density to a high density with a high degree of accuracy. Moreover, since the tone correction table used for forming print images can be updated with a high degree of accuracy, even in a case where the density of an image changes rapidly, the density of a print image can be stabilized. 
     Furthermore, while, in the above-described configuration, four sensors  117  are provided at a position at which to measure measurement images formed on the intermediate transfer belt  106 , the sensor  117  can be configured to be provided at a position at which to measure a measurement image formed on each photosensitive drum  105 . For example, in a case where the color of the intermediate transfer belt  106  is black so that the accuracy of measurement of a measurement image for black is low, a configuration in which the measurement image for black is measured by the sensor  117  provided for the photosensitive drum  105  for black can be employed. 
     Moreover, the image forming apparatus  100  can be configured such that a measurement image for black is measured by the sensor  117  provided for the photosensitive drum  105  for black and measurement images for cyan, magenta, and yellow are measured by the sensors  117  provided for the intermediate transfer belt  106 . The sensor  117  provided for the photosensitive drum  105  for black functions as a first measurement unit configured to measure a first measurement image on a first photosensitive member, and the sensors  117  provided for the intermediate transfer belt  106  function as a second measurement unit configured to measure a second measurement image on an image bearing member. 
     According to an exemplary embodiment of the disclosure, even in a case where images are continuously formed on a plurality of long sheets, since adjustment processing for an image forming condition suited for the long sheet  501  is performed, a variation in the density of an output image can be prevented or reduced. 
     While the present disclosure has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2017-109253 filed Jun. 1, 2017, which is hereby incorporated by reference herein in its entirety.