Patent Publication Number: US-11656566-B2

Title: Image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present disclosure relates to an image forming apparatus having a calibration function. 
     Description of the Related Art 
     An electrophotographic image forming apparatus is configured to form an image on a sheet by an image forming process described below. First, the image forming apparatus uniformly charges a surface of a photosensitive member. The image forming apparatus irradiates the surface of the photosensitive member whose surface is uniformly charged with a laser beam based on an image signal, to thereby form an electrostatic latent image on the surface of the photosensitive member. The image forming apparatus develops the electrostatic latent image with toner or other developers to form a developer image on the surface of the photosensitive member. The image forming apparatus transfers and fixes this developer image to a sheet, to thereby form an image on the sheet. In a case where a color image is to be formed, the image forming apparatus individually forms developer images of a plurality of colors and transfers the developer images so that the developer images are superimposed onto the sheet, to thereby generate a color image. 
     An image formed by such an image forming apparatus on a sheet may vary in density or hue due to various factors. For example, the density of the image formed by the image forming apparatus changes due to a change of an environment condition such as an air temperature and humidity, and due to a temporal change of a component of the image forming apparatus. Accordingly, the image forming apparatus executes calibration for controlling the density of the image to a target density. In the calibration, there is used a test chart obtained by forming a test pattern for image density detection on a sheet. An image reading apparatus reads the test pattern of the test chart, to thereby obtain the image density of the test pattern. Image forming conditions such as parameters for adjusting the image density are adjusted so that this image density becomes the target density. With the image signal being corrected based on those parameters, even when the change of the environment condition or the temporal change of the component occurs, a stable density and tone characteristic is ensured. In the image forming apparatus described in US 2007/0285743 A1, an auto document feeder (ADF) is used in order to read the test chart and convey the test chart. In this manner, a work load of a user is reduced in a case where the calibration is performed. 
     The image reading apparatus is capable of reading the test pattern from the test chart placed on a platen, in addition to the ADF. Accordingly, the user may be not sure whether to use the ADF or the platen at the time of calibration. The present disclosure has been made in view of the above-mentioned problem, and has a primary object to provide an image forming apparatus with which the user can perform an operation at the time of calibration without confusion. 
     SUMMARY OF THE INVENTION 
     An image forming apparatus according to the present disclosure includes: a platen on which an original is to be placed; a feeder configured to feed an original placed on a tray; a sensor configured to detect a sheet on the tray; a lamp provided to the feeder; a reader configured to read the original placed on the platen and to read the original conveyed by the feeder; an image forming unit configured to form an image on a sheet; and a controller configured to: control the image forming unit to form a first test image on a sheet; control the reader to read the first test image formed on the sheet placed on the platen; control a density of the image to be formed by the image forming unit based on a reading result of the first test image; control the image forming unit to form a second test image on a sheet; control the reader to read the second test image formed on the sheet fed by the feeder; and control the density of the image to be formed by the image forming unit based on a reading result of the second test image, wherein the controller is configured to drive, in a case where the second test image is formed by the image forming unit in order to control the density, the lamp before the sheet on which the second test image is formed is detected by the sensor. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a configuration view of an image forming apparatus according to an embodiment of the present invention. 
         FIG.  2    is an explanatory diagram of a reader image processor. 
         FIG.  3    is an explanatory diagram of a printer controller. 
         FIG.  4    is an explanatory view of a document scanner. 
         FIG.  5    is an exterior appearance perspective view of an ADF. 
         FIG.  6    is an internal configuration diagram of the ADF. 
         FIG.  7    is a four-quadrant chart. 
         FIG.  8    is a flow chart for illustrating tone correction processing. 
         FIG.  9    is an exemplary view of test charts. 
         FIG.  10 A  and  FIG.  10 B  are exemplary views of screens to be displayed on a display. 
         FIG.  11    is a flow chart for illustrating tone correction processing. 
         FIG.  12 A  and  FIG.  12 B  are exemplary views of screens to be displayed on the display. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Now, an embodiment of the present disclosure is described with reference to the drawings. 
     &lt;Image Forming Apparatus&gt; 
       FIG.  1    is a configuration view of an image forming apparatus according to the embodiment of the present disclosure. An image forming apparatus  100  includes a reader  200 , which is an image reading apparatus configured to read an image from an original (sheet), a printer  300  configured to form an image on a sheet, and an operating unit  400 . The reader  200  includes a document scanner  215  and an auto document feeder (hereinafter referred to as “ADF”)  220 . The document scanner  215  is provided on the printer  300 . The ADF  220  is provided on the document scanner  215 . The reader  200  reads an image printed on an original  101 , and transmits an image signal representing the read image to the printer  300 . The printer  300  can perform image formation processing on the sheet based on the image signal acquired from the reader  200 . The operating unit  400  is a user interface, and includes an input device and an output device. Examples of the input device include various key buttons such as an input key, numeric keys, a start key, and a stop key, and a touch panel. The operating unit  400  is used to input user instruction information. Examples of the output device include a display and a speaker. 
     The reader  200  reads an original fed from an original tray  501  of the ADF  220 , or the original  101  placed on a platen  102  provided on the ADF  220  side of the document scanner  215 . The platen  102  is a plate-shaped transparent member made of, for example, glass. The document scanner  215  includes therein a reader image processor  108 . The reader image processor  108  converts an electrical signal generated by reading the original  101  into an image signal, and transmits the image signal to the printer  300 . 
     The document scanner  215  includes a reference white plate  106  on the platen  102 . The reader  200  reads the reference white plate  106  before reading the original  101  to perform so-called “shading correction.” The document scanner  215  includes a first mirror unit  104   a , a second mirror unit  104   b , a lens  115 , and an image sensor  105 . The first mirror unit  104   a  includes a light source  103 . The first mirror unit  104   a  and the second mirror unit  104   b  are movable in a direction of an arrow K 1 . When the original  101  placed on the platen  102  is to be read, the first mirror unit  104   a  causes the light source  103  to irradiate the original  101  with light while moving in the direction of the arrow K 1 . Reflected light reflected by the original  101  is received by the image sensor  105 . The image sensor  105  is a reading sensor which includes a plurality of photoelectric conversion elements (light receiving elements) having RGB filters, and is configured to convert the reflected light into an electrical signal for each line. A CCD sensor or a CMOS sensor can be used as the image sensor  105 . The reader image processor  108  acquires the electrical signal from the image sensor  105 , and converts this electrical signal into an image signal (luminance signal). Details of the document scanner  215  are described later. 
     The printer  300  includes therein a printer controller  109 . The printer controller  109  acquires the image signal (luminance signal) from the reader image processor  108  of the document scanner  215 . The printer controller  109  forms an image on the sheet based on the acquired image signal. For image formation, the printer  300  includes image forming units  120 ,  130 ,  140 , and  150 , an exposure device  110 , a transfer belt  111 , and a fixing device  114 . 
     The image forming units  120 ,  130 ,  140 , and  150  are only different in colors of images to be formed, and have similar configurations to perform similar operations. The image forming unit  120  forms a yellow (Y) image. The image forming unit  130  forms a magenta (M) image. The image forming unit  140  forms a cyan (C) image. The image forming unit  150  forms a black (K) image. A description is here given of the configuration of the image forming unit  120 , and a description of the configurations of other image forming units  130 ,  140 , and  150  is omitted. 
     The image forming unit  120  includes a photosensitive drum  121 , a charging device  122 , a developing device  123 , a transfer blade  124 , and a surface electrometer  125 . The photosensitive drum  121  is a drum-shaped photosensitive member having a surface with a photosensitive layer. The photosensitive drum  121  rotates in a clockwise (CW) direction of  FIG.  1   . The charging device  122  uniformly charges the surface of the rotating photosensitive drum  121  at a predetermined potential. On the surface of the photosensitive drum  121 , an electrostatic latent image is formed by scanning the charged surface with a laser beam by the exposure device  110 . The developing device  123  develops the electrostatic latent image with a developer (for example, toner) of a corresponding color (in this example, yellow) to form a toner image on the surface of the photosensitive drum  121 . 
     The exposure device  110  is controlled by the printer controller  109  to irradiate the photosensitive drum  121  with the laser beam. The exposure device  110  scans the photosensitive drum  121  in a rotation axis direction of the photosensitive drum  121 . Accordingly, the rotation axis direction corresponds to a main scanning direction. The printer controller  109  modulates the laser beam, which is emitted from the exposure device  110 , based on a pulse width modulation (PWM) signal that is based on the image signal. 
     The transfer blade  124  is provided so as to sandwich the transfer belt  111  between the transfer blade  124  and the photosensitive drum  121 . The transfer belt  111  conveys a sheet fed from a sheet feeding cassette  152 . The transfer blade  124  discharges electricity to transfer the toner image formed on the photosensitive drum  121  onto the sheet conveyed by the transfer belt  111 . In this manner, a yellow toner image is formed on the sheet. 
     Similarly, a magenta toner image is formed on a photosensitive drum  131  of the image forming unit  130 . A cyan toner image is formed on a photosensitive drum  141  of the image forming unit  140 . A black toner image is formed on a photosensitive drum  151  of the image forming unit  150 . The magenta toner image formed on the photosensitive drum  131  is transferred in superimposition onto the yellow toner image on the sheet. The cyan toner image formed on the photosensitive drum  141  is transferred in superimposition onto the yellow and magenta toner images on the sheet. The black toner image formed on the photosensitive drum  151  is transferred in superimposition onto the yellow, magenta, and cyan toner images on the sheet. The toner images of the four colors are transferred in superimposition, and thus full-color toner images are formed on the sheet. 
     The sheet having the full-color toner images formed thereon is conveyed to the fixing device  114  by the transfer belt  111 . The fixing device  114  fixes the transferred toner images to the sheet. For example, the fixing device  114  heats and melts the toner images and applies pressure thereto to fix the toner images to the sheet. In this manner, an image is formed on the sheet. The sheet having the image formed thereon is discharged to the outside of the printer  300 . 
     The surface electrometer  125  of the image forming unit  120 , and surface electrometers  135 ,  145 , and  155  of the image forming units  130 ,  140 , and  150  measure surface potentials of the photosensitive drums  121 ,  131 ,  141 , and  151 , respectively. Contrast potentials are adjusted based on results of measurement by the surface electrometers  125 ,  135 ,  145 , and  155 . 
     In this embodiment, the ADF  220  includes a lamp  156 . Further, the document scanner  215  includes a lamp  157 . The ADF  220  has a sensor  504  provided thereto to detect presence or absence of a sheet on the original tray  501 . The lamp  156  of the ADF  220  is turned on when a detection result obtained by the sensor  504  is a detection state representing that a sheet is placed on the original tray  501 . The lamp  156  is turned off in a case where the detection result obtained by the sensor  504  is a non-detection state representing that no sheet is placed on the original tray  501 . At the time of calibration, the lamp  156  or  157  is turned on when the test chart to be used for calibration is to be placed (set). The lamp  156  in this embodiment is turned on regardless of the detection result obtained by the sensor  504  at the time of calibration. 
     &lt;Reader Image Processor&gt; 
       FIG.  2    is an explanatory diagram of the reader image processor  108 . The reader image processor  108  includes an analog front end (AFE) circuit board  201  and a reader controller circuit board  210 . The AFE circuit board  201  includes an analog image processor  202  and an A/D converter  203 . The reader controller circuit board  210  includes a shading processor  212  and a central processing unit (CPU)  211 . The CPU  211  executes a predetermined computer program to control the operation of the reader  200 . 
     The reader image processor  108  causes the AFE circuit board  201  to acquire an electrical signal output from the image sensor  105 . The electrical signal is, for example, an analog signal corresponding to an amount of light received by the image sensor  105 . The AFE circuit board  201  causes the analog image processor  202  to perform analog processing such as gain adjustment. The electrical signal subjected to analog processing is converted into a digital signal by the A/D converter  203 . 
     The shading processor  212  of the reader controller circuit board  210  acquires the digital signal generated by the A/D converter  203 . The shading processor  212  is controlled by the CPU  211  to perform shading correction on the digital signal, to thereby generate an image signal. The image signal is transmitted to the printer controller  109 . The image signal includes pieces of luminance information of red (R), green (G), and blue (B). 
     &lt;Printer Controller&gt; 
       FIG.  3    is an explanatory diagram of the printer controller  109 . The operation of the printer controller  109  is controlled by a CPU  301 . The CPU  301  is a main controller configured to execute a control program stored in a memory  302  to control the operation of the image forming apparatus  100 , to thereby perform processing of forming an image onto a sheet. The memory  302  is a read only memory (ROM) or a random access memory (RAM), and stores control programs and various types of data. The CPU  301  and the memory  302  are provided in the printer  300 . 
     The printer controller  109  acquires the image signal from the reader  200  or a server  500 , for example. The server  500  is an external apparatus which is provided separately from the printer  300 , and is to be connected to the printer  300  via a local area network (LAN) or other networks. In the image signal, the number of tones of R, G, or B is represented by 8 bits. The printer controller  109  includes a color processor  303 , a tone controller  311 , a dither processing portion  307 , a PWM unit  308 , and a laser driver  309 . The printer controller  109  converts respective image signals of R, G, and B into PWM signals, to thereby perform light emission control of a semiconductor laser  310  provided in the exposure device  110 . 
     The image signals of R, G, and B are input to the color processor  303 . The color processor  303  performs image processing and color processing on the input image signals so that a desired output result (image) can be obtained in a case where the printer  300  has an ideal output characteristic. The color processor  303  increases the number of tones of the image signal to 10 bits from 8 bits in order to improve the accuracy. The color processor  303  includes an LUTid  304 , which is a look-up table. The LUTid  304  is a luminance-density conversion table for converting luminance information included in the image signal into density information. The color processor  303  uses the LUTid  304  to convert the luminance information of each of the image signals of R, G, and B into density information of each of yellow (Y), magenta (M), cyan (C), and black (K). The image signals including the density information of Y, M, C, and K are input to the tone controller  311 . 
     The tone controller  311  includes an under color removal (UCR) unit  305  and an LUTa  306 , which is a lookup table. The tone controller  311  corrects the tone of each of the image signals of Y, M, C, and K so that a desired output result (image) can be obtained in accordance with the actual output characteristic of the printer  300 . The UCR unit  305  regulates the integrated value of the image signal in each pixel to limit the total sum of the image signal levels. In a case where the total sum exceeds a specified value, the UCR unit  305  performs under color removal (UCR) processing of replacing a predetermined amount of C, M, and Y image signals into K image signals, to thereby reduce the total sum of the image signal levels. The regulation of the total sum of the image signal levels is performed in order to regulate a toner laid-on level at the time of forming an image by the printer  300 , to thereby optimize the operation of the printer  300 . The optimization of the operation of the printer  300  in this embodiment refers to prevention of image defects and the like caused in a case where the toner laid-on level exceeds a specified value. The LUTa  306  is a 10-bit conversion table for correcting the density characteristic, and is used to change the y characteristic of the printer  300 , for example. In this embodiment, as an example, the LUTa is described as an image forming condition to be adjusted by the calibration. The image signals of Y, M, C, and K subjected to tone correction are input to the dither processing portion  307 . 
     The dither processing portion  307  performs dither processing on the 10-bit image signals of Y, M, C, and K subjected to tone correction, to thereby perform halftone processing (dither processing) of converting the respective 10-bit image signals of Y, M, C, and K into 4-bit signals. The PWM unit  308  performs pulse width modulation on the signals subjected to dither processing to generate a PWM signal corresponding to a control signal for the exposure device  110 . The PWM signal is input to the laser driver  309 . The laser driver  309  controls the light emission of the semiconductor laser  310  in accordance with the PWM signal. 
     &lt;Document Scanner&gt; 
       FIG.  4    is an explanatory view of the document scanner  215 . As described above, the document scanner  215  includes, in a housing, the first mirror unit  104   a , the second mirror unit  104   b , the lens  115 , and the image sensor  105 . The document scanner  215  further includes a motor  116  and a home position sensor  412 . The first mirror unit  104   a  includes the light source  103  and a first mirror  107   a . The second mirror unit  104   b  includes a second mirror  107   b  and a third mirror  107   c . The first mirror unit  104   a  and the second mirror unit  104   b  are movable in the direction of the arrow K 1  of  FIG.  1    by being driven by the motor  116 . The document scanner  215  having such a configuration receives an instruction to read an image from the operating unit  400  to start its operation. 
     The document scanner  215  can perform image reading in accordance with a first reading mode of reading the original  101  conveyed by the ADF  220  and a second reading mode of reading the original  101  placed on the platen  102 . The first reading mode is sometimes called “flow reading” and “ADF reading.” The second reading mode is sometimes called “fixed reading” and “platen reading.” 
     In either of the first reading mode and the second reading mode, the operation itself performed by the document scanner  215  to read the image is the same. When the image reading is started, the document scanner  215  causes the motor  116  to move the first mirror unit  104   a  and the second mirror unit  104   b  temporarily to a home position corresponding to a detection position of the home position sensor  412 . After that, the document scanner  215  turns on the light source  103 , and irradiates a reading surface (surface on which an image is printed) of the original  101  with light. The first mirror  107   a , the second mirror  107   b , and the third mirror  107   c  polarize reflected light (image light) of the light applied to the original  101  and guide the image light to the lens  115 . The lens  115  forms an image from the image light onto a light receiving surface of the image sensor  105 . The image sensor  105  photoelectrically converts the image light into an electrical signal. 
     As described above, the first mirror unit  104   a  and the second mirror unit  104   b  are driven by the same motor  116  to be moved in the direction of the arrow K 1 . With use of a movable pulley, a speed at which the second mirror unit  104   b  is moved becomes half (V/2) of a speed V at which the first mirror unit  104   a  is moved. Light is applied to the original  101  while the first mirror unit  104   a  and the second mirror unit  104   b  are moved so that the image on the entire surface of the original  101  is read. 
     &lt;ADF&gt; 
       FIG.  5    is an exterior appearance perspective view of the ADF  220 .  FIG.  6    is an internal configuration diagram of the ADF  220 . The ADF  220  includes an original stacker  601 , an original feeder  614 , an original conveyor  615 , and an original reversing unit  608 . 
     The original stacker  601  includes the original tray  501 . On the original tray  501 , one or more originals  101  can be stacked on a stacking surface thereof. The original tray  501  functions as a feeder. The original stacker  601  is provided with an original indicator  503  configured to turn on when the originals  101  are stacked on the original tray  501 . Accordingly, the sensor  504  configured to detect the original placed on the original tray  501  is arranged between a pickup roller  602  and a feed roller  603  to be described later. The originals  101  stacked on the original tray  501  are conveyed one by one onto the platen  102  by the original feeder  614 , pass on the platen  102 , and are discharged to a discharge tray  617  by the original reversing unit  608 . 
     In the original feeder  614 , the pickup roller  602 , the feed roller  603 , and a registration roller pair  604  are provided along a conveying path of the originals  101 . The pickup roller  602  is a roller that is rotatable and vertically movable. At the time of feeding the originals  101 , the pickup roller  602  is lowered on an uppermost original of an original bundle stacked on the original tray  501  to be brought into contact with this original. At this time, a middle plate of the original tray  501  on which the original bundle is placed is raised to press the original bundle toward the feed roller  603 . After the pickup roller  602  is brought into contact with the uppermost original, the pickup roller  602  and the feed roller  603  rotate in the clockwise (CW) direction of  FIG.  6    to start the conveyance of the originals. 
     The pickup roller  602  and the feed roller  603  feed the originals  101  one by one by a frictional separation method. For example, the second and subsequent originals which are about to be fed by the pickup roller  602  together with the uppermost original are restricted by a friction piece so as to stay on the original stacker  601 . The originals conveyed one by one are detected by a separation sensor (not shown) provided on the downstream of the feed roller  603  in the conveying direction of the originals. The feed roller  603  conveys the originals  101  that have been conveyed by the pickup roller  602  to the registration roller pair  604 . 
     The registration roller pair  604  is stopped at the time when a tip end of the original  101  reaches the registration roller pair  604 . Even after the tip end of the original  101  collides with the registration roller pair  604 , the feed roller  603  continues the conveyance of the originals  101 . In this manner, the original  101  forms a loop. With the formation of the loop, skew feeding in the conveying direction of the original  101  is corrected. The registration roller pair  604  starts to rotate after the skew feeding is corrected, and conveys the originals  101  to the original conveyor  615 . 
     The original conveyor  615  includes a conveyor belt  605 , a drive roller  606 , a driven roller  607 , and a plurality of pressing rollers  616 . The original conveyor  615  conveys the original  101  with the use of the conveyor belt  605 . The conveyor belt  605  is tensioned around the drive roller  606  and the driven roller  607 . Moreover, the conveyor belt  605  is pressed against the platen  102  by the pressing rollers  616 . The conveyor belt  605  conveys, by frictional force, the original  101  that enters between the conveyor belt  605  and the platen  102 . Thus, the original  101  is conveyed on the platen  102 . 
     When the original  101  reaches a predetermined position on the platen  102 , the conveyor belt  605  is stopped. The image of the original  101  is read by the document scanner  215  under a stopped state. After the image is read, the conveyor belt  605  conveys the original  101  to the original reversing unit  608 . When there is a subsequent original, the subsequent original is conveyed to the predetermined position by the conveyor belt  605  and stopped thereat similarly to the preceding original, and an image thereof is read. While the subsequent original is read, the original reversing unit  608  reverses the front and the back of the preceding original and discharges the preceding original to the discharge tray  617 . 
     The original reversing unit  608  includes a reverse roller  609 , a conveyor roller pair  610 , a reverse flapper  611 , a discharge flapper  613 , and a reverse roller  612 . The reverse roller  609  and the conveyor roller pair  610  are driven by a drive motor (not shown). This drive motor can perform forward and reverse rotation. With the use of a drive motor different from that of the original conveyor  615 , the original reversing unit  608  can operate independently of the original conveyor  615 . 
     The original  101  conveyed by the conveyor belt  605  of the original conveyor  615  is lifted up by the reverse flapper  611  when entering the original reversing unit  608 , and is conveyed to the reverse roller  609 . The reverse flapper  611  regulates the entry of the original in the vicinity of an original entrance of the original reversing unit  608 , and is controlled by a solenoid (not shown) to take a posture illustrated in  FIG.  6   , to thereby lift up the original. The original  101  is sandwiched between the reverse roller  609  that rotates in a counterclockwise (CCW) direction, and the reverse roller  612  that faces the reverse roller  609 , and is conveyed to the conveyor roller pair  610 . When a rear end of the original  101  passes through the discharge flapper  613 , the discharge flapper  613  rotates in the CW direction. Moreover, the reverse roller  609  also rotates in the CW direction. Thus, the original  101  is conveyed in a switchback manner, and is discharged to the discharge tray  617  of a discharged sheet stacking portion. 
     &lt;Calibration Operation&gt; 
     In this embodiment, a description is given of a case in which, in the image forming apparatus  100  capable of operating in the first reading mode (ADF reading) and the second reading mode (platen reading) at the time of calibration, the ADF reading is set as a default setting. After outputting a test chart obtained by forming a test pattern on a sheet when executing the calibration, the image forming apparatus  100  turns on the lamp  156  of the ADF  220 . In this manner, the user can recognize the place to set the test chart, and is less confused at the time of processing, thereby reducing the load. 
     Calibration for obtaining a desired density and tone characteristic is performed by controlling the LUTa  306  corresponding to a correcting circuit configured to perform y correction.  FIG.  7    is a four-quadrant chart for illustrating how the image signal is converted in order to correct the tone characteristic. 
     Quadrant I represents a reading characteristic of the reader  200 . The reading characteristic of the reader  200  is a characteristic of converting, by the reader  200 , an original density representing the density of the original image formed on the original into a density signal. The characteristic of converting the original density into the density signal may vary depending on the reading mode (ADF reading or platen reading). Quadrant II represents a conversion characteristic of the tone controller  311  (LUTa  306 ). The conversion characteristic of the tone controller  311  is a characteristic of converting, by the LUTa  306 , the density signal into a laser output signal representing the amount of light of the laser beam to be output from the semiconductor laser  310 . Quadrant III represents a recording characteristic of the printer  300 . The recording characteristic of the printer  300  is a characteristic of converting, by the printer  300 , the laser output signal into an output density representing the density of the image to be formed on the sheet. Quadrant IV represents a relationship between the original density and a recorded density of the image formed on the sheet. This relationship represents a tone reproducing characteristic of the entire image forming apparatus  100 . 
     The printer  300  in this embodiment corrects a non-linear part of the recording characteristic of the printer  300  in Quadrant III by the conversion characteristic of the tone controller  311  in Quadrant II in order to obtain a linear tone characteristic in Quadrant IV. The LUTa  306  is created by exchanging the input and the output of the characteristic of Quadrant III obtained in a case where the test chart is created without performing the processing by the tone controller  311 . In this embodiment, the output number of tones is 256 (8 bits), but the number of tones in the tone controller  311  is 1,024 because the tone controller  311  processes 10-bit digital signals. 
     &lt;Tone Correction&gt; 
     Tone correction is executed when reproducibility of the density or hue of an image formed by the printer  300  drops. To execute the tone correction, a test chart for tone correction, which is formed by the printer  300 , is read with the reader  200  and an LUTa for correcting the density characteristic (y characteristic) is created based on the result of the reading. 
       FIG.  8    is a flow chart for illustrating the tone correction processing.  FIG.  9    is an exemplary view of test charts to be used in the tone correction.  FIG.  10 A  and  FIG.  10 B  are exemplary views of screens to be displayed on a display of the operating unit  400  during the tone correction processing. 
     The CPU  301  causes the printer  300  to create a test chart for tone correction exemplified in  FIG.  9    (Step S 1 ). Sheets having a predetermined size are stored in advance in a sheet feeding cassette of the printer  300 . The CPU  301  displays a guide screen exemplified in  FIG.  10 A  on the display of the operating unit  400 . On the guide screen, a message of “PRINT FOR EXECUTING CALIBRATION” and a “PRINT” button for giving an instruction to create the test chart are displayed. When the “PRINT” button is pressed through the operating unit  400 , the CPU  301  transmits the density signals of the image signals (test pattern) for creating the test chart to the color processor  303 . The density signals processed by the color processor  303  are transmitted to the dither processing portion  307  via the tone controller  311 . At this time, the LUTa  306  is not used. That is, the density signals of YMCK output from the UCR unit  305  bypass the LUTa  306  to be input to the dither processing portion  307 . In this manner, a test pattern corresponding to the density signals of YMCK bypassing the LUTa  306  is printed on a sheet, and thus the test chart is created. 
     As illustrated in  FIG.  9   , each of test charts  801   a  and  801   b  includes test patterns having 10 tones for each color of Y, M, C, and K. For each color, for example, test patterns having 10 tones are formed of density signals of 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, and 100%. The dither processing portion  307  can apply a plurality of procedures of halftone processing. For example, the dither processing portion  307  includes a small number-of-lines screen (160 lines per inch (lpi) to 180 lpi) and a large number-of-lines screen (250 lpi to 300 lpi). The test chart  801   a  is a test chart to which the small number-of-lines screen is applied. The test chart  801   b  is a test chart to which the large number-of-lines screen is applied. Tone images may be formed by the small number-of-lines screen, and letters or other line images may be formed by the large number-of-lines screen. When the tone characteristic greatly varies due to the difference in number of screen lines, it is more preferred to set the tone levels depending on the number of screen lines. In a case where the printer  300  has ability to form an image with three types or more of the number of lines, the number of types of test charts may be three or more. In this case, the number of test charts created at the time of tone correction is defined to be one for the sake of convenience. 
     After creating the test chart, the CPU  301  turns on the lamp of the apparatus on the side on which the test chart is to be set (Step S 2 ). In this embodiment, the ADF reading is set as a default setting, and hence the CPU  301  turns on the lamp  156  of the ADF  220 . In the case of the platen reading, the CPU  301  turns on the lamp  157  of the document scanner  215 . The CPU  301  turns on the lamp to instruct the user on the apparatus to set the test chart. The user sets the test chart to the specified apparatus (place). 
     In a case where the ADF  220  does not include the lamp  156 , the CPU  301  may turn on the original indicator  503  to instruct the user on the apparatus to set the test chart. Further, in addition to the lighting of the lamp  156  or  157 , the CPU  301  may instruct the user on the apparatus to set the test chart by sound. As a matter of course, the lighting of the lamp  156  or  157  and the sound may be used in combination. 
     The lamp  156  or  157  may be set to have a specific blinking pattern at the time of calibration, such as continuous lighting, blinking, or lighting in a pattern different from that in a normal case. The color of the lamp  156  or  157  may be different between the case of calibration and the normal case. For example, in a case where the original is placed on the original tray  501  in a copy mode of copying the original, the lamp  156  is turned on when the sensor  504  detects that the original is placed on the original tray  501 . Meanwhile, at the time of calibration, the lamp  156  is turned on before the sensor  504  detects that the test chart is placed on the original tray  501 . Further, the light intensity of the lamp  156  or  157  may be increased to raise the awareness of the user. 
     After instructing the user on the place to set the test chart, the CPU  301  reads the test chart by the reader  200  (Step S 3 ). The CPU  301  displays an input screen for inputting an instruction to read the test chart on the display of the operating unit  400  after instructing the user on the place to set the test chart.  FIG.  10 B  exemplifies such an input screen. On the input screen, a message of “PLEASE SET TEST CHART ON ADF TO READ TEST CHART” and a “READ” button for giving a reading instruction are displayed. In a case where the “READ” button is pressed through the operating unit  400 , the CPU  301  starts the conveyance of the test chart by the ADF  220 , to thereby read the test chart by the document scanner  215 . 
     The CPU  301  acquires the density signals of the test pattern based on the reading result (luminance signals) (Step S 4 ). The CPU  301  converts the luminance signals into the density signals with the use of the LUTid  304  of the color processor  303 . Thus, a density signal for each of the images of 10 tones is obtained. 
     The CPU  301  creates the LUTa based on the density signals used to create the test pattern and the density signals obtained from the reading result of the test chart (Step S 5 ). The CPU  301  stores the created LUTa in the memory  302 . At this stage, the CPU  301  can obtain the recording characteristic of the printer  300  represented in Quadrant III of  FIG.  7   . The CPU  301  exchanges the input and the output of the recording characteristic to determine the LUTa of the printer  300 , and sets the LUTa to the tone controller  311 . Data is insufficient to obtain the LUTa through calculation. The reason is because the test pattern is only provided for 10 tones although 256 tones are usually required. Accordingly, the CPU  301  interpolates the insufficient data to create required data. With such calibration, a tone characteristic that is linear with respect to the target density can be achieved. In the manner described above, the tone correction processing is performed. 
     In this embodiment described above, in the image forming apparatus  100  capable of performing ADF reading and platen reading, the user is instructed on the place to set the test chart. In this manner, the user is less confused at the time of setting the test chart on the instructed place, and can perform the operation at the time of calibration without confusion. In this manner, the image forming apparatus  100  is capable of executing the calibration while the work load of the user is reduced. 
     Modification Example 
     A description is given of a case in which the user can select the reading mode at the time of tone correction between the ADF reading and the platen reading.  FIG.  11    is a flow chart for illustrating the tone correction processing in this case.  FIG.  12 A  and  FIG.  12 B  are exemplary views of screens to be displayed on the display of the operating unit  400  during the tone correction processing. In this example, the test chart is created by different test patterns between the ADF reading and the platen reading. 
     In a case where the tone correction is executed based on the user instruction information, the CPU  301  displays a guide screen illustrated in  FIG.  12 A  on the display of the operating unit  400  in order to allow the user to select the reading method (Step S 11 ). In the guide screen illustrated in  FIG.  12 A , a message for urging the user to select the reading method, and buttons that allow selection of the ADF reading or the platen reading are displayed. The user selects either one of the ADF reading and the platen reading through this guide screen. The CPU  301  functions as a selector configured to select the reading mode based on the user instruction information input from the operating unit  400 . 
     In a case where the ADF reading is selected (Step S 11 : ADF), the CPU  301  starts the lighting of the lamp  156  of the ADF  220  (Step S 12 ). Further, the CPU  301  displays a guide screen exemplified in  FIG.  12 B  on the display of the operating unit  400 . The guide screen exemplified in  FIG.  12 B  is the same as the guide screen illustrated in  FIG.  10 A . 
     Next, in a case where the button for starting printing is pressed by the user through the guide screen of  FIG.  12 B , the CPU  301  controls the printer  300  in order to print a test pattern for ADF reading on a sheet (Step S 13 ). In a case where a test chart created with the use of the test pattern for ADF reading is output from the printer  300 , the CPU  301  displays a button for starting the reading of the test chart on the display of the operating unit  400 . At this time, the lamp  156  is turned on, and hence the user can recognize that the test chart is required to be placed on the original tray  501  of the ADF  220 . Further, a guidance for urging the user to place the test chart on the original tray  501  of the ADF  220  may be displayed on the display of the operating unit  400 . 
     In a case where the button for starting the reading is pressed by the user, the CPU  301  controls the ADF  220  to convey the test chart placed on the original tray  501  to the reading position, to thereby read the test chart (Step S 14 ). In this case, after the button for starting the reading is pressed by the user, the CPU  301  turns off the lamp  156  of the ADF  220 . Then, the CPU  301  acquires the density signals of the test pattern based on the reading result (luminance signals) (Step S 15 ). The CPU  301  converts the acquired luminance signals into density signals based on the LUTid  304  of the color processor  303 . In this manner, the density signals can be obtained for the respective 10-tone images. The CPU  301  creates the LUTa based on the density signals used to generate the test pattern and on the density signals obtained from the reading result of the test chart (Step S 16 ), and ends the tone correction processing. 
     In a case where the platen reading is selected in the processing of Step S 11  (Step S 11 : platen), the CPU  301  starts the lighting of the lamp  157  of the document scanner  215  (Step S 17 ). Further, the CPU  301  displays the guide screen exemplified in  FIG.  12 B  on the display of the operating unit  400 . The guide screen exemplified in  FIG.  12 B  is the same as the guide screen illustrated in  FIG.  10 A . 
     Next, in a case where the button for starting printing is pressed by the user through the guide screen of  FIG.  12 B , the CPU  301  controls the printer  300  in order to print a test pattern for platen reading on a sheet (Step S 18 ). In a case where a test chart created with the use of the test pattern for platen reading is output from the printer  300 , the CPU  301  displays a button for starting the reading of the test chart on the display of the operating unit  400 . At this time, the lamp  157  is turned on, and hence the user can recognize that the test chart is to be placed on the platen  102 . Further, a guidance for urging the user to place the test chart on the platen  102  may be displayed on the display of the operating unit  400 . 
     In a case where the button for starting the reading is pressed by the user, the CPU  301  reads the test chart on the platen  102  (Step S 19 ). In this case, after the button for starting the reading is pressed by the user, the CPU  301  turns off the lamp  157  of the document scanner  215 . Then, the CPU  301  acquires the density signals of the test pattern based on the reading result (luminance signals) (Step S 20 ). The CPU  301  converts the acquired luminance signals into density signals based on the LUTid  304  of the color processor  303 . In this manner, the density signals can be obtained for the respective 10-tone images. The CPU  301  creates the LUTa based on the density signals used to generate the test pattern and on the density signals obtained from the reading result of the test chart (Step S 21 ), and ends the tone correction processing. 
     The timing to start the lighting of the lamp  156  is not limited to when the ADF reading is selected by the user, and may be, for example, before the test chart is placed on the original tray  501 . Similarly, the timing to start the lighting of the lamp  157  is not limited to when the platen reading is selected by the user, and may be, for example, before the test chart is placed on the platen  102 . Further, the lighting of the lamp  156  and the lamp  157  may be continued until, for example, the LUTa is created. 
     In a case where the ADF  220  does not include the lamp  156 , the CPU  301  may turn on the original indicator  503  to instruct the user on the apparatus to set the test chart. In a case where the document scanner  215  does not include the lamp  157 , the CPU  301  may turn on the light source  103  to instruct the user on the apparatus to set the test chart. 
     Further, in addition to the lighting of the lamp  156  or  157 , the CPU  301  may instruct the user on the apparatus to set the test chart by sound. In this case, a name of the set apparatus and an instruction, such as “Please set to platen” or “Please set to ADF,” are output by voice. As a matter of course, the lighting of the lamp  156  or  157  and the sound may be used in combination. 
     The lamp  156  or  157  may be set to have a specific blinking pattern at the time of calibration, such as continuous lighting, blinking, or lighting in a pattern different from that in a normal case. The color of the lamp  156  or  157  may be different between the case of calibration and the normal case. Further, the light intensity of the lamp  156  or  157  may be increased to raise the awareness of the user. 
     In this embodiment described above, in the image forming apparatus  100  capable of performing ADF reading and platen reading, the user is allowed to select the reading mode between the ADF reading and the platen reading. The image forming apparatus  100  instructs the user on the place to set the test chart in accordance with the selection by the user so that the user is less confused at the time of setting the test chart on the instructed place, and can perform the operation at the time of calibration without confusion. In this manner, the image forming apparatus  100  is capable of executing the calibration while the work load of the user is reduced. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2020-049103, filed Mar. 19, 2020, which is hereby incorporated by reference herein in its entirety.