Patent Publication Number: US-10324407-B2

Title: Image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an image forming apparatus, for example, a copying machine, a laser beam printer, or a multifunction printer. 
     Description of the Related Art 
     In regard to an image forming apparatus, there is a high demand for a direct image printer that eliminates a need for a plate used for offset printing or the like. The direct image printer is capable of handling reduction in time required for printing, service for each individual customer, printing of an enormous number of copies, an environmental issue that paper is discarded due to a failure in printing, or the like. Of the direct image printers, there are particularly often employed: an inkjet printer, which is advantageous in terms of price and suitable for photographic printing; and an electrophotographic printer, which is high in productivity, and exhibits a finish that is close to that of the offset printing. Such an image forming apparatus is required to exhibit stability in colors of a formed image. 
     In order to ensure the stability in colors, there is a technology for conducting the color stabilization control inside the image forming apparatus without a manual operation. For example, an image forming apparatus described in U.S. Pat. No. 6,559,876 is configured to detect a density of a toner-density-detecting image formed on a photosensitive member using a sensor to adjust an exposure amount for exposing the photosensitive member based on a result of detecting the density, and to change a correction amount for tone correction corresponding to a variation of a halftone density. An image forming apparatus described in Japanese Patent Application Laid-open No. 2015-197470 is configured to adjust the exposure amount based on a result of detecting the density of the toner-density-detecting image formed with the highest density, to thereby conduct the tone correction for a period of time shorter than in the case of the related art. 
     In the case of adjusting the exposure amount using the toner-density-detecting image having the highest density, the exposure amount cannot be greatly changed at once due to an influence of density variations in a highlighted portion of the toner-density-detecting image. This is because the influence of density deviation in the highlighted portion becomes larger when the exposure amount is greatly changed at once, which inhibits the correction amount for the tone correction from being changed in time. Therefore, for example, in a case where there is a great change in environment at a time of image formation, the adjustment of the exposure amount and the change of the correction amount for the tone correction may fail to follow the density variations, which may produce a resultant object having a much different density. In view of the foregoing, there is a demand for an image forming apparatus capable of suitably adjusting a density while following even great environmental variations. 
     SUMMARY OF THE INVENTION 
     An image forming apparatus according to the present disclosure includes: a converter configured to convert image data based on a conversion condition; an image forming unit configured to form an image on a sheet based on the image data converted by the converter; a measuring unit configured to measure a measurement image on an image bearing member; and a controller configured to: control the image forming unit to form a first measurement image on the image bearing member; control the measuring unit to measure the first measurement image; adjust, based on a measurement result of the first measurement image, an image forming condition for adjusting a density of the image to be formed on the sheet by the image forming unit; control the image forming unit to form second measurement images on the image bearing member; control the measuring unit to measure the second measurement images; and generate the conversion condition based on measurement results of the second measurement images, control the image forming unit to form a third measurement image on the image bearing member; control the measuring unit to measure the third measurement image; and control whether or not to form the first measurement image and the second measurement images based on a measurement result of the third measurement image. 
     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 diagram of an image forming apparatus. 
         FIG. 2  is an explanatory diagram of an image forming unit. 
         FIG. 3  is a block diagram of a controller. 
         FIG. 4  is an explanatory graph of a table. 
         FIG. 5A  and  FIG. 5B  are diagrams for exemplifying a toner-density-detecting image. 
         FIG. 6  is a flowchart for illustrating tone correction processing. 
         FIG. 7  is an explanatory graph of a predicted density characteristic (tone characteristic). 
         FIG. 8  is an explanatory graph of a tone correction table. 
         FIG. 9  is a diagram for exemplifying a character having jaggies. 
         FIG. 10  is an explanatory graph of a change in density of a halftone portion. 
         FIG. 11  is a flowchart for illustrating processing for increasing or decreasing an exposure amount. 
         FIG. 12  is a flowchart for illustrating processing for resetting the exposure amount and a γ LUT. 
         FIG. 13  is a diagram for exemplifying a Dmax-portion-adjusting image. 
         FIG. 14  is an explanatory graph of a relationship between the exposure amount and a density. 
         FIG. 15  is a flowchart for illustrating processing conducted in a case where a print job is executed. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Now, an embodiment of the present invention is described below in detail with reference to the drawings. 
       FIG. 1  is a configuration diagram of an image forming apparatus according to this embodiment. An image forming apparatus  100  includes an operation unit  20 , a reader A configured to read an image from an original G, and a printer section B configured to conduct image forming processing. The operation unit  20  is a user interface, and includes an input device including various input buttons and a numeric keypad and an output device including a display  218 . The display  218  may be a touch panel display. A user can input a type of the image, the number of sheets to be subjected to image formation, and other such conditions to the image forming apparatus  100  through the operation unit  20 . 
     Reader 
     The reader A includes an original table  102  on which the original G is to be placed. In order to read an image from the original G on the original table  102 , the reader A includes a light source  103 , an optical system  104 , and a reading sensor  105 . The light source  103  is configured to irradiate the original G with light. The applied light is reflected by the original G. The optical system  104  includes a lens and other components, and is configured to image the light reflected by the original G onto a light-receiving surface of the reading sensor  105 . The reading sensor  105  is, for example, a charge-coupled device (CCD) sensor, and is configured to receive the reflected light imaged on the light-receiving surface. The reader A is configured to generate image data representing an image of the original G based on the reflected light received by the reading sensor  105 , and to transmit the generated image data to the printer section B. The light source  103 , the optical system  104 , and the reading sensor  105  are integrally formed, and are configured to move toward a direction indicated by an arrow R 3 . With this configuration, an image on the entire surface of the original G is read. 
     Printer Section 
     The printer section B is configured to acquire the image data from the reader A, and to conduct the image forming processing based on the image data. The printer section B may be configured to acquire the image data to be used for the image forming processing not only from the reader A but also from an external apparatus through a telephone line or a network. 
     The printer section B includes an image forming unit PY configured to form a toner image of yellow, an image forming unit PM configured to form a toner image of magenta, an image forming unit PC configured to form a toner image of cyan, and an image forming unit PK configured to form a toner image of black. The letters Y, M, C, and K at the end of the reference symbols represent yellow, magenta, cyan, and black, respectively. In the following, in a case where there is no need to distinguish the colors, the description is given without adding the letters Y, M, C, and K to the end of the reference symbols. The same also applies to other components provided for each of the colors. In addition, the printer section B includes exposure devices  3 Y,  3 M,  3 C, and  3 K, an intermediate transfer belt  6 , a fixing device  11 , and a conveying mechanism for conveying a recording material S. The exposure devices  3 Y,  3 M,  3 C, and  3 K are provided so as to correspond to the image forming units PY, PM, PC, and PK, respectively. The printer section B is a full-color printer employing a tandem-type intermediate transfer system in which the image forming units PY, PM, PC, and PK are arranged along the intermediate transfer belt  6 . 
     The image forming units PY, PM, PC, and PK have the same configuration. The following description is directed to the configuration of the image forming unit PY, and descriptions of the configurations of the other image forming units PM, PC, and PK are omitted. The image forming unit PY includes a photosensitive drum  1 Y, a charger  2 Y, a developing device  4 Y, a primary transfer roller  7 Y, and a drum cleaner  8 Y. The photosensitive drum  1 Y is configured to be irradiated with laser light by the corresponding exposure device  3 Y after having a surface charged by the charger  2 Y, to thereby have an electrostatic latent image formed thereon. The electrostatic latent image is developed by the developing device  4 Y. With this configuration, the toner image of yellow is formed on the photosensitive drum  1 Y. The primary transfer roller  7 Y is arranged at a position opposed to the photosensitive drum  1 Y across the intermediate transfer belt  6 . The primary transfer roller  7 Y is configured to transfer the toner image formed on the photosensitive drum  1 Y onto the intermediate transfer belt  6 . The toner remaining on the photosensitive drum  1 Y is removed by the drum cleaner  8 Y after the transferring. 
     The exposure device  3 Y includes a rotary mirror. The exposure device  3 Y is configured to scan the photosensitive drum  1 Y by deflecting the laser light modulated based on the image data representing an image of yellow in accordance with the rotation of the rotary mirror. With this configuration, the electrostatic latent image representing the image based on the image data of yellow is formed on the photosensitive drum  1 Y. 
     In the same manner, a toner image of magenta is formed on a photosensitive drum  1 M of the image forming unit PM. The toner image of magenta is transferred from the photosensitive drum  1 M onto the intermediate transfer belt  6  by a primary transfer roller  7 M. A toner image of cyan is formed on a photosensitive drum  1 C of the image forming unit PC. The toner image of cyan is transferred from the photosensitive drum  1 C onto the intermediate transfer belt  6  by a primary transfer roller  7 C. A toner image of black is formed on a photosensitive drum  1 K of the image forming unit PK. The toner image of black is transferred from the photosensitive drum  1 K onto the intermediate transfer belt  6  by a primary transfer roller  7 K. The toner images of the respective colors are transferred onto the intermediate transfer belt  6  in order one over another. 
     The intermediate transfer belt  6  is supported by being stretched around a tension roller  61 , a drive roller  62 , and an opposing roller  63 . In addition, a belt cleaner  68  is provided in the vicinity of the intermediate transfer belt  6 . The intermediate transfer belt  6  is driven by the drive roller  62  to be rotated in a direction indicated by an arrow R 2  at a predetermined process speed. The toner images of the respective colors which have been transferred onto the intermediate transfer belt  6  are conveyed to a secondary transfer portion T 2  by the rotation of the intermediate transfer belt  6 . The secondary transfer portion T 2  is formed between a secondary transfer roller  64  and the opposing roller  63 . At the secondary transfer portion T 2 , the toner images of all the colors are collectively transferred from the intermediate transfer belt  6  onto the recording material S, for example, a sheet, with the intermediate transfer belt  6  and the recording material S being sandwiched between the secondary transfer roller  64  and the opposing roller  63 . When a DC voltage having a positive polarity is applied to the secondary transfer roller  64 , a toner image charged to a negative polarity is transferred from the intermediate transfer belt  6  onto the recording material S. After the transfer, the toner remaining on the intermediate transfer belt  6  is removed by the belt cleaner  68 . 
     The recording materials S are received in a sheet feeding cassette  65 , and are conveyed to the secondary transfer portion T 2  by the conveying mechanism sheet by sheet. The conveying mechanism includes separation rollers  66  and registration rollers  67 . The separation rollers  66  are configured to convey the recording materials S from the sheet feeding cassette  65  to the registration rollers  67  sheet by sheet. The registration rollers  67  are configured to correct skew feeding or the like of the recording material S, and to convey the recording material S so that the recording material S reaches the secondary transfer portion T 2  at the same timing as a timing at which the toner images formed on the intermediate transfer belt  6  are conveyed to reach the secondary transfer portion T 2 . 
     The recording material S having the toner images transferred thereon at the secondary transfer portion T 2  are conveyed to the fixing device  11 . The fixing device  11  is configured to fix the toner images to the recording material S by heating and pressurizing the recording material S having the toner images transferred thereon. In this manner, the image is formed on the recording material S. The recording material S on which the image based on the image data has been formed is delivered to the outside of the printer section B. 
     Image Forming Unit 
       FIG. 2  is an explanatory diagram of the image forming unit P. 
     The photosensitive drum  1  is an image bearing member formed of, for example, an electrophotographic photosensitive member of a rotary drum type. The photosensitive drum  1  is rotationally driven in a direction indicated by an arrow R 1  at a predetermined process speed. The charger  2  is, for example, a scorotron charger, and is configured to charge the surface of the photosensitive drum  1  to a uniform potential having a negative polarity. The scorotron charger includes a wire to which a high voltage is applied, a shield portion connected to the ground, and a grid portion to which a desired voltage is applied. A predetermined charging bias is applied to the wire from a charging bias power supply (not shown). A predetermined grid bias is applied to the grid portion from a grid bias power supply (not shown). The photosensitive drum  1  is charged to almost the same potential as a potential applied to the grid portion, which depends on the voltage applied to the wire. 
     The photosensitive drum  1  has the electrostatic latent image formed in a part irradiated with the laser light by the exposure device  3 . The developing device  4  is configured to visualize the electrostatic latent image formed on the photosensitive drum  1  as the toner image by supplying a developer thereto. In the vicinity of the photosensitive drum  1 , a potential sensor  5  is provided between a position of exposure by the exposure device  3  and the developing device  4 . The potential sensor  5  is configured to detect a potential of the electrostatic latent image. 
     The primary transfer roller  7  is configured to press an inner surface of the intermediate transfer belt  6  against the photosensitive drum  1  side, and to form a primary transfer portion T 1  between the photosensitive drum  1  and the intermediate transfer belt  6 . When the DC voltage having a positive polarity is applied to the primary transfer roller  7 , the negative-polarity toner image formed on the photosensitive drum  1  is transferred onto the intermediate transfer belt  6  passing through the primary transfer portion T 1 . In the vicinity of the photosensitive drum  1 , an image density sensor  12  is provided between the developing device  4  and the primary transfer portion T 1 . The image density sensor  12  is configured to detect a density of the toner image formed on the photosensitive drum  1 . 
     Control System 
     The image forming apparatus  100  includes, as a control system, a controller  110 , a printer controller  109 , and an image processor  108 . The controller  110  is configured to control an operation of the image forming apparatus  100 . The printer controller  109  is configured to control an operation of the exposure device  3  based on a processing result obtained by the image processor  108 . Such a control system is built into the printer section B. 
     The controller  110  is a computer including a central processing unit (CPU)  111 , a random access memory (RAM)  112 , and a read only memory (ROM)  113 . The CPU  111  is configured to read a computer program from the ROM  113 , and to execute the computer program with the RAM  112  being used as a work area, to thereby control image reading processing conducted by the reader A of the image forming apparatus  100  and the image forming processing conducted by the printer section B of the image forming apparatus  100 . The controller  110  is connected to the operation unit  20 , and is configured to receive various kinds of input from the operation unit  20  and to cause the image reading processing and the image forming processing to be executed. The controller  110  is further configured to cause the display  218  to display a setting screen or the like. 
     The controller  110  can set a plurality of process speeds for the image forming processing. For example, the controller  110  switches the process speed (image forming speed) depending on a basis weight of the recording material S received in the sheet feeding cassette  65 . In this embodiment, two types of process speeds (image forming speeds) can be set between 300 mm/s (constant-speed mode) to be set when the basis weight of the recording material S is smaller than 200 g/m 2  and 150 mm/s (low-speed mode) to be set when the basis weight is equal to or larger than 200 g/m 2 . The process speed (image forming speed) is relatively lower in the low-speed mode than in the constant-speed mode. The driving speeds of the photosensitive drum  1  and the intermediate transfer belt  6 , a charging voltage of the charger  2 , an exposure amount of the laser light applied by the exposure device  3 , a voltage applied at the primary transfer portion T 1 , and other conditions are set based on the process speed. 
     The printer controller  109  includes a laser light amount control circuit  190 , a pattern generator  192 , and a pulse width modulation circuit  191 . The printer controller  109  is connected to the image processor  108  configured to conduct the image processing on the image data representing the image to be formed. The image processor  108  includes a video counter  220  and a γ-correction circuit  209 , and is configured to conduct the image processing, for example, gamma correction, on the image to be formed. 
     The printer controller  109  is configured to transmit, to the exposure device  3 , a laser drive signal for controlling a light amount, a light-emitting timing, or the like of the laser light. The laser light amount control circuit  190  is configured to determine the light amount of the laser light to be output from the exposure device  3  so that a suitable image density is obtained from the laser drive signal. The light amount of the laser light is an example of an image forming condition. The pattern generator  192  is configured to hold image data for forming toner-density-detecting images being measurement images described later. The pulse width modulation circuit  191  is configured to generate a binary laser drive signal with a pulse width determined based on the drive signal generated using a correction value (tone correction table) for tone correction which is held by the γ-correction circuit  209 . The γ-correction circuit  209  functions as a converter configured to convert the image data based on the tone correction table. 
     The tone correction table is a gamma look-up table (LUT) for converting the image data so that a density characteristic (tone characteristic) of the image becomes an ideal density characteristic (ideal tone characteristic). The tone correction table provides a conversion condition for converting the image data in order to correct the tone characteristic (density characteristic) of the image formed by the image forming unit P. In another case, the tone correction table provides a tone correction condition for correcting the tone characteristic (density characteristic) of the image formed by the image forming unit P. The pulse width modulation circuit  191  is configured to generate the laser drive signal with the light amount determined by the laser light amount control circuit  190  and using the image data converted based on the tone correction table. The laser drive signal is a pulse width modulation (PWM) signal, and is used to modulate the laser light to be emitted from the exposure device  3 . 
     That is, the printer controller  109  is configured to cause the pulse width modulation circuit  191  to output the laser drive signal being a pulse signal having a pulse width (time width) corresponding to the density for each pixel of the input image data. The laser drive signal has a large pulse width for a pixel having a high density, has a small pulse width for a pixel having a low density, and has a medium pulse width for a pixel having a medium density. 
     The exposure device  3  is configured to form an image having a density tone through area coverage modulation based on the pulse width of the laser drive signal. The exposure device  3  is configured to cause a laser light source, for example, a built-in semiconductor laser, to emit light for a time period corresponding to the pulse width of the laser drive signal. The laser light source is driven for a long time period at a time of forming the pixel having a high density, and is driven for a short time period at a time of forming the pixel having a low density. Therefore, a dot size of the electrostatic latent image formed on the photosensitive drum  1  has a different area depending on a pixel density. The exposure device  3  is configured to expose a range that is long in a main scanning direction at the time of forming the pixel having a high density, and to expose a range that is short in the main scanning direction at the time of forming the pixel having a low density. 
     Image Density Sensor 
     The image density sensor  12  is a photosensor configured to detect the density of the toner image formed on the photosensitive drum  1 . The image density sensor  12  includes a light emitter  12   a  formed of a light emitting diode (LED) or other such light-emitting element and a light receiver  12   b  formed of a photodiode or other such light-receiving element. The light emitter  12   a  is configured to irradiate the surface of the photosensitive drum  1 . The light receiver  12   b  is configured to receive specularly reflected light of the light emitted from the light emitter  12   a , which is specularly reflected by the photosensitive drum  1 . The light receiver  12   b  is configured to measure an amount of the specularly reflected light. The image density sensor  12  is configured to measure the amount of the light reflected by the photosensitive drum  1  at a timing at which the toner-density-detecting image being the toner image formed on the photosensitive drum  1  passes through a detection region. The image density sensor  12  is configured to transmit a measurement result to the CPU  111  of the controller  110 . 
       FIG. 3  is a block diagram of the controller  110  configured to receive the measurement result obtained by the image density sensor  12 . The light receiver  12   b  of the image density sensor  12  is configured to transmit an analog electric signal corresponding to the amount of the received reflected light to the controller  110  as the measurement result. The analog electric signal is expressed by, for example, a voltage value of from 0 V to 5 V. The controller  110  includes an A/D conversion circuit  114  and a density conversion circuit  115  between the image density sensor  12  and the CPU  111 . The density conversion circuit  115  is configured to hold a table  115   a , which is used to convert the measurement result obtained by the image density sensor into a density value, for each color based on a characteristic of the image density sensor  12 . 
     The A/D conversion circuit  114  is configured to convert the analog electric signal acquired from the image density sensor  12  into an 8-bit digital signal. The density conversion circuit  115  is configured to convert the digital signal obtained through the conversion conducted by the A/D conversion circuit  114  into the density value with reference to the table  115   a . The density conversion circuit  115  is configured to input the density value obtained through the conversion to the CPU  111 . 
       FIG. 4  is an explanatory graph of the table  115   a . When the density of the toner-density-detecting image formed on the photosensitive drum  1  is changed stepwise through the area coverage modulation, the measurement result obtained by the image density sensor  12  changes in accordance with the stepwise change. In this case, the measurement result obtained by the image density sensor  12  when a toner does not adhere to the photosensitive drum  1  is 5 V, and the density is expressed by a density value having 255 levels. As the image density becomes higher due to an increase in area coverage ratio of the pixel formed on the photosensitive drum  1  to be covered with the toner, the measurement result (analog electric signal) obtained by the image density sensor  12  becomes smaller. The density conversion circuit  115  can accurately convert the measurement result obtained by the image density sensor  12  into the density value of each color with reference to the table  115   a  indicating such a relationship as shown in  FIG. 4 . 
     Toner-Density-Detecting Image 
       FIG. 5A  and  FIG. 5B  are diagrams for exemplifying the toner-density-detecting image.  FIG. 5A  is a diagram for exemplifying the toner-density-detecting image including a halftone portion within a predetermined number of tone levels, for example, three tone levels, and a Dmax portion (highest-density portion) having the highest density.  FIG. 5B  is a diagram for exemplifying the toner-density-detecting image within a larger number of tone levels (ten tone levels) than in  FIG. 5A . The image forming unit P is configured to form the toner-density-detecting image being such a toner image on the photosensitive drum  1  under the control of the controller  110  and the printer controller  109 . The controller  110  is configured to execute image density control processing described later so that the density of the toner-density-detecting image converges within a range of a reference density based on the measurement result of the density of the toner-density-detecting image obtained by the image density sensor  12 . 
     The printer controller  109  is configured to acquire image data representing the toner-density-detecting image from the pattern generator  192 , and to control the operation of the exposure device  3 . The image data is data to be used for forming the toner-density-detecting image with a predetermined image density. The pulse width modulation circuit  191  is configured to generate the laser drive signal having a pulse width corresponding to the predetermined image density based on the image data representing the toner-density-detecting image which is acquired from the pattern generator  192 . The pulse width modulation circuit  191  is configured to supply the generated laser drive signal to the exposure device  3 . The exposure device  3  is configured to emit the light of the semiconductor laser for a time period corresponding to the pulse width of the laser drive signal to scan the photosensitive drum  1 . With this configuration, the electrostatic latent image of the toner-density-detecting image corresponding to a predetermined density is formed on the photosensitive drum  1 . When the electrostatic latent image is developed by the developing device  4 , the toner image of the toner-density-detecting image is formed on the photosensitive drum  1 . 
     Image Density Control Processing 
     The image density control processing includes tone correction processing, processing for increasing or decreasing the exposure amount, and processing for resetting the exposure amount and the tone correction table. The image density control processing is conducted for each of the colors of yellow, magenta, cyan, and black. 
     Tone Correction Processing 
       FIG. 6  is a flowchart for illustrating the tone correction processing. 
     The printer controller  109  causes the laser light amount control circuit  190  to adjust the exposure amount of the laser light output from the exposure device  3  based on an increase/decrease amount of the exposure amount obtained in the processing for increasing or decreasing the exposure amount, which is described later (Step S 3001 ). The controller  110  and the printer controller  109  form the toner-density-detecting image illustrated in  FIG. 5A  on the photosensitive drum  1  (Step S 3002 ). The toner-density-detecting image of  FIG. 5A  is formed, and thus a processing time period is shorter compared with the case of forming the toner-density-detecting image of  FIG. 5B . 
     The image density sensor  12  detects the density of the toner-density-detecting image being the toner image formed on the photosensitive drum  1 . The controller  110  acquires the measurement result of the density of the toner-density-detecting image from the image density sensor  12  (Step S 3003 ). The controller  110  acquires the density value from the measurement result obtained by the image density sensor  12 , and plots the density value with respect to a density target being a target density value set in advance, to thereby predict the density characteristic (tone characteristic).  FIG. 7  is an explanatory graph of the predicted density characteristic (tone characteristic). The density target is indicated by the solid line. The density target is set so that a relationship between the laser drive signal and the density exhibits a linear function. The density characteristic (tone characteristic) predicted by plotting the density values is indicated by the broken line. 
     The controller  110  calculates a density difference Δd (=(Dmax portion density)−(highest density of density target)) between the measured density value of the Dmax portion being the highest density of the toner-density-detecting image of  FIG. 5A  and the highest density of the density target (Step S 3004 ). In this case, the Dmax portion of the toner-density-detecting image corresponds to a third measurement image. The controller  110  determines whether or not the absolute value of the calculated density difference Δd is equal to or larger than “60” (Step S 3005 ). When the absolute value is not equal to or larger than “60” (N in Step S 3005 ), the controller  110  conducts inverse conversion processing so as to match the predicted density characteristic (tone characteristic) with the density target, and generates the tone correction table (Step S 3015 ). The controller  110  stores the generated tone correction table in the γ-correction circuit  209 . With this, the image data is subjected to tone correction, and normal image forming processing is conducted. After generating the tone correction table, the controller  110  conducts the processing for increasing or decreasing the exposure amount based on the density difference Δd calculated in the processing of Step S 3004  (Step S 3016 ), and brings the tone correction processing to an end. That is, the controller  110  generates the tone correction table based on the measurement result of the density of the toner-density-detecting image acquired in the processing of Step S 3003  when the density difference Δd falls within a predetermined range (in this case, within a range of from −60 to +60). The controller  110  further sets the increase/decrease amount of the exposure amount in the laser light amount control circuit  190  based on the density difference Δd. 
     When the absolute value of the density difference Δd is equal to or larger than “60” (Y in Step S 3005 ), the controller  110  conducts the processing for resetting the exposure amount and the tone correction table, which is described later, (Step S 3006 ), and brings the tone correction processing to an end. That is, the controller  110  conducts the processing for resetting the exposure amount and the tone correction table when the density difference Δd falls out of the predetermined range (in this case, out of the range of from −60 to +60). 
     In short, the controller  110  controls whether or not to conduct the processing for resetting the exposure amount and the tone correction table based on the measurement result (density difference Δd) for the Dmax portion. When the density difference Δd falls out of the predetermined range, the controller  110  forms a Dmax-portion-adjusting image described later and the toner-density-detecting image illustrated in  FIG. 5B . Meanwhile, when the density difference Δd falls within the predetermined range, the controller  110  skips the formation of the Dmax-portion-adjusting image and the toner-density-detecting image illustrated in  FIG. 5B . Then, the controller  110  generates the tone correction table based on the measurement result of the toner-density-detecting image formed in Step S 3002 . 
     The density target is described. The density target is generated from a density value acquired by automatic tone correction control using an image formed on the recording material S, and is stored in the RAM  112 . The automatic tone correction control is executed in response to an instruction issued through the operation unit  20  by the user. 
     When the execution of the automatic tone correction control is instructed, the image forming apparatus  100  causes the printer section B to form an image pattern having a large number of tone levels (in this case, 64 tone levels) on the recording material S for each color. The recording material S on which the image pattern has been formed is placed on the original table  102  of the reader A by the user. The reader A reads the image pattern from the placed recording material S. With this processing, the reader A detects the density value of the image pattern. A result of the detection is transmitted from the reader A to the controller  110  of the printer section B. 
     The controller  110  conducts storing processing and smoothing processing on the density value detected from the image pattern to acquire the density characteristic (tone characteristic) for an entire density region. The controller  110  generates the tone correction table for the image data based on the obtained density characteristic (tone characteristic) and a tone target set in advance.  FIG. 8  is an explanatory graph of the tone correction table. The controller  110  conducts the inverse conversion processing on the density characteristic (tone characteristic) so that the density characteristic is matched with the tone target, to thereby create the tone correction table. The image data is corrected based on the tone correction table and subjected to the image forming processing, to thereby match the density of the image formed on the recording material S with the tone target over the entire density region. 
     The image forming apparatus  100  uses such a tone correction table to form the toner image having a plurality of image patterns on the photosensitive drum  1 . The image density sensor  12  detects the density of the toner image having the image pattern formed on the photosensitive drum  1 . The controller  110  can acquire a target density for the image data on the photosensitive drum  1  based on the density value representing the detected density of the toner image. In this embodiment, after creating the tone correction table, the controller  110  forms the toner-density-detecting image having ten tone levels illustrated in  FIG. 5B  on the photosensitive drum  1  to acquire the density target. The controller  110  stores the acquired density target in the RAM  112  to use the density target for the processing. 
     Processing for Increasing or Decreasing Exposure Amount 
     When the density correction is conducted by only the tone correction, a portion of the image having the highest density may be excessively subjected to halftoning depending on the density characteristic (tone characteristic) of the image forming apparatus  100 . In this case, jaggies occur in a character as exemplified in, for example,  FIG. 9 . Therefore, not only the tone correction table but also the adjustment of the exposure amount conducted by the exposure device  3  is important in order to ensure image quality. In this embodiment, the exposure amount is adjusted based on a result of the tone correction processing. Specifically, the controller  110  conducts the processing for increasing or decreasing the exposure amount based on the density difference Δd calculated in the processing of Step S 3004  illustrated in  FIG. 6  of the tone correction processing. 
     However, it is necessary to suppress an influence on the density of the image of the halftone portion due to the increase or decrease of the exposure amount. To that end, the increase/decrease amount of the exposure amount is determined based on the result obtained when the exposure amount is changed using a common correction table for each image forming apparatus. 
       FIG. 10  is an explanatory graph of a change in density of the halftone portion at the time of the increase or decrease of the exposure amount. When an exposure increase/decrease amount is large, the image forming apparatus  100  cannot correct the density of a halftone by the tone correction. Therefore, density deviation occurs in the halftone. In order to inhibit the density deviation from occurring in the halftone, the image forming apparatus  100  needs to maintain the exposure increase/decrease amount within such a range as to allow the halftone portion to be corrected by the tone correction. In this embodiment, as shown in  FIG. 10 , when the exposure amount level exhibits three levels within the 255 levels, it is possible to correct a halftone density by the tone correction. Therefore, in this embodiment, the maximum value of the increase/decrease amount of the exposure amount is set to ±3 levels. 
       FIG. 11  is a flowchart for illustrating the processing for increasing or decreasing the exposure amount. When the absolute value of the density difference Δd calculated in the processing of Step S 3004  of the tone correction processing is not equal to or larger than “60”, the controller  110  starts the processing for increasing or decreasing the exposure amount after generating the tone correction table (N in Step S 3005  and Step S 3015 , which are illustrated in  FIG. 6 ). 
     The controller  110  initializes the exposure increase/decrease amount being the increase/decrease amount of the exposure amount to “0” (Step S 2001 ). The controller  110  determines whether or not the density difference Δd is equal to or smaller than “−30” (Step S 2002 ). When the density difference Δd is equal to or smaller than “−30” (Y in Step S 2002 ), the controller  110  determines that the measured density value of the toner-density-detecting image is extremely low with respect to the density target. In this case, the controller  110  sets the exposure increase/decrease amount to “+3” so that the density value becomes closer to the density target (Step S 2003 ). When the density difference Δd falls within a range of from “−30” to “−20” (Y in Step S 2004 ), the controller  110  sets the exposure increase/decrease amount to “+2” (Step S 2005 ). When the density difference Δd falls within a range of from “−20” to “−10” (Y in Step S 2006 ), the controller  110  sets the exposure increase/decrease amount to “+1” (Step S 2007 ). 
     When the density difference Δd is equal to or larger than “+30” (Y in Step S 2008 ), the controller  110  determines that the measured density value of the toner-density-detecting image is extremely high with respect to the density target. In this case, the controller  110  sets the exposure increase/decrease amount to “−3” so that the density value becomes closer to the density target (Step S 2009 ). When the density difference Δd falls within a range of from “+20” to “+30” (Y in Step S 2010 ), the controller  110  sets the exposure increase/decrease amount to “−2” (Step S 2011 ). When the density difference Δd falls within a range of from “+10” to “+20” (Y in Step S 2012 ), the controller  110  sets the exposure increase/decrease amount to “−1” (Step S 2013 ). 
     The controller  110  sets the exposure increase/decrease amount in the laser light amount control circuit  190 . The laser light amount control circuit  190  adjusts the exposure amount based on the set exposure increase/decrease amount before the toner-density-detecting image is formed next time the tone correction processing is executed (Step S 3001  of  FIG. 6 ). Therefore, the tone correction table is generated after the exposure amount is adjusted, and the density deviation is inhibited from occurring in the halftone portion. There may be employed a configuration for executing the tone correction processing illustrated in  FIG. 6 , for example, at a time of initial adjustment immediately after a main power source of the image forming apparatus  100  is turned on, and executing other tone correction processing while the image is being formed based on the image data. The other tone correction processing is, for example, processing for causing, after the measurement result of the density of the toner-density-detecting image is acquired in the processing of Step S 3003  of  FIG. 6 , the controller  110  to execute the processing of Step S 3015  and then execute the processing of Step S 3016 . According to this configuration, when the other tone correction processing is executed while the image formation is being conducted continuously, the image forming apparatus  100  can inhibit downtime. In addition, the exposure amount is not greatly changed, and hence a difference between the density of the image before the other tone correction processing is executed and the density of the image after the other tone correction processing is executed can be inhibited from increasing. The toner-density-detecting image formed in Step S 3002  of the tone correction processing illustrated in  FIG. 6  corresponds to a plurality of measurement images including the third measurement image. Meanwhile, the toner-density-detecting image formed in the other tone correction processing corresponds to other second measurement images. The types of densities of the plurality of measurement images of this embodiment include four measurement images having different densities. The toner-density-detecting image of this embodiment includes ten measurement images having different densities. While the image forming unit P is continuously forming a plurality of images, the controller  110  executes the other tone correction processing, and generates the tone correction table. In addition, the Dmax portion of the toner-density-detecting image formed in the other tone correction processing, which is illustrated in  FIG. 5A , corresponds to a predetermined measurement image to be used for determining the image forming condition. 
     Processing for Resetting Exposure Amount and Tone Correction Table 
     In the above-mentioned processing for increasing or decreasing the exposure amount, adjustment can be conducted only within the range of ±3 levels at maximum due to the influence of the density of the halftone. However, when there is a great change in installation environment of the image forming apparatus  100 , it may no longer be possible to form a desired image because the image density being the highest density cannot be optimized through the correction with ±3 levels at maximum irrespective of the large density difference Δd being the highest density. Therefore, when the density difference Δd falls out of the predetermined range, the image forming apparatus  100  resets the exposure amount and regenerates the tone correction table. In this embodiment, when the absolute value of the density difference Δd exceeds “60”, the exposure amount is reset, and the tone correction table is regenerated. 
       FIG. 12  is a flowchart for illustrating processing for resetting the exposure amount and the tone correction table. When the absolute value of the density difference Δd calculated in the processing of Step S 3004  of the tone correction processing is equal to or larger than “60”, the controller  110  starts the processing for resetting the exposure amount and the tone correction table (Y in Step S 3005  and Step S 3006 , which are illustrated in  FIG. 6 ). 
     The controller  110  first resets the exposure amount. The controller  110  forms the Dmax-portion-adjusting image (first measurement image) being the measurement image exemplified in  FIG. 13  on the photosensitive drum  1  (Step S 3007 ). The Dmax-portion-adjusting image is an image to be used for adjusting the density of the Dmax portion being the highest density of the image. The Dmax-portion-adjusting image is formed of a measurement image exhibiting a plurality of exposure amounts including ±10%, ±20%, and ±30% with respect to the exposure amount (LPW_Ref) being a reference set at that point in time. The controller  110  acquires the measurement result of the density of the Dmax-portion-adjusting image detected by the image density sensor  12  (Step S 3008 ). The controller  110  calculates the exposure amount corresponding to the density target by linear interpolation from a relationship between the exposure amount of the Dmax-portion-adjusting image and the measured density based on the acquired measurement result, and sets the calculated exposure amount (Step S 3009 ).  FIG. 14  is an explanatory graph of a relationship between the exposure amount of each measurement image of the Dmax-portion-adjusting image and the detected density. The controller  110  calculates a new exposure amount from such a relationship to set the new exposure amount. The controller  110 , which has set the new exposure amount, initializes the exposure increase/decrease amount set in the processing for increasing or decreasing the exposure amount to “0” (Step S 3010 ). The controller  110  thus brings the resetting of the exposure amount to an end. The exposure amount is an example of the image forming condition for adjusting the density of an output image to be formed on the recording material S by the image forming unit P. The image forming condition includes not only the exposure amount but also a charging voltage of the charger  2 , a developing bias applied to the developing device  4 , and a voltage applied at the primary transfer portion T 1 . 
     The controller  110 , which has brought the resetting of the exposure amount to an end, conducts tone correction control in order to match the density of each tone level with the density target based on the new exposure amount. In this embodiment, the toner-density-detecting image exemplified in  FIG. 5B  is used to conduct the tone correction control. 
     The controller  110  forms the toner-density-detecting image of  FIG. 5B  (second measurement images) on the photosensitive drum  1  (Step S 3011 ). The controller  110  acquires the measurement result of the density of the toner-density-detecting image detected by the image density sensor (Step S 3012 ). The controller  110  detects the density characteristic (tone characteristic) by conducting the linear interpolation on discrete density values of the toner-density-detecting image corresponding to ten tone levels. The controller  110  regenerates the tone correction table by conducting the inverse conversion processing on the detected density characteristic (tone characteristic) so as to match the density characteristic (tone characteristic) with the density target (Step S 3013 ). The newly regenerated tone correction table is stored in the γ-correction circuit  209 . The controller  110 , which has regenerated the tone correction table, sets a Dmax-portion-adjustment flag for the low-speed mode to “True” (Step S 3014 ). 
     The Dmax-portion-adjustment flag for the low-speed mode is a flag to be used for adjusting the Dmax portion being the image having the highest density when a print job is started with the process speed being set to the low-speed mode. In a case where the process speed is set to the low-speed mode when the Dmax-portion-adjustment flag is “True”, the controller  110  conducts the adjustment of the Dmax portion. When the absolute value of the density difference Δd is equal to or larger than “60” in the processing of Step S 3005  illustrated in  FIG. 6 , the controller  110  determines that the Dmax portion being the image having the highest density needs to be adjusted. In the low-speed mode, the image density being the highest density is highly liable to greatly differ from an ideal density. Therefore, it is necessary to reset the exposure amount even in the low-speed mode. 
       FIG. 15  is a flowchart for illustrating processing conducted when the print job is executed with the process speed being set to the low-speed mode. 
     The controller  110  examines the process speed of the print job to be started (Step S 4001 ). When the process speed is set to the constant-speed mode (N in Step S 4001 ), the controller  110  conducts the image forming processing without resetting the exposure amount and the tone correction table (Step S 4011 ). When the process speed is set to the low-speed mode (Y in Step S 4001 ), the controller  110  examines whether or not the Dmax-portion-adjustment flag for the low-speed mode is “True” (Step S 4002 ). When the Dmax-portion-adjustment flag for the low-speed mode is “False” (N in Step S 4002 ), the controller  110  conducts the image forming processing without resetting the exposure amount and the tone correction table (Step S 4011 ). 
     When the Dmax-portion-adjustment flag for the low-speed mode is “True” (Y in Step S 4002 ), the controller  110  adjusts the Dmax portion in the low-speed mode. The controller  110  first forms the Dmax-portion-adjusting image being the toner-density-detecting image exemplified in  FIG. 13  on the photosensitive drum  1  (Step S 4003 ). The controller  110  acquires the measurement result of the density of the Dmax-portion-adjusting image detected by the image density sensor  12  (Step S 4004 ). The controller  110  calculates the exposure amount corresponding to the density target by linear interpolation from a relationship between the exposure amount of the Dmax-portion-adjusting image and the measured density based on the acquired measurement result. The controller  110  sets the calculated exposure amount in the laser light amount control circuit  190  as the exposure amount in the low-speed mode (Step S 4005 ). The controller  110 , which has set the exposure amount in the low-speed mode, initializes the exposure increase/decrease amount set in the processing for increasing or decreasing the exposure amount to “0” (Step S 4006 ). 
     Subsequently, the controller  110  forms the toner-density-detecting image of  FIG. 5B  on the photosensitive drum  1  (Step S 4007 ). The controller  110  acquires the measurement result of the density of the toner-density-detecting image detected by the image density sensor  12  (Step S 4008 ). The controller  110  detects the density characteristic (tone characteristic) by conducting the linear interpolation on the discrete density values of the toner-density-detecting image corresponding to ten tone levels. The controller  110  regenerates the tone correction table by conducting the inverse conversion processing on the detected density characteristic (tone characteristic) so as to match the density characteristic (tone characteristic) with the density target (Step S 4009 ). The newly regenerated tone correction table is stored in the γ-correction circuit  209 . The controller  110 , which has regenerated the tone correction table, sets the Dmax-portion-adjustment flag for the low-speed mode to “False” (Step S 4010 ). The controller  110  uses the regenerated tone correction table to conduct the image forming processing (Step S 4011 ). 
     The image forming apparatus  100  described above can optimally adjust the density of the image having the highest density by appropriately conducting the adjustment, that is, the increase or decrease, of the exposure amount and the tone correction. Therefore, the image forming apparatus  100  can form an image with a satisfactory density even when the density deviation becomes larger due to a great change in installation environment or the like. That is, the image forming apparatus  100  sets the conversion condition and the image forming condition based on different toner images depending on the density difference from the toner image having the highest density, to thereby be able to suitably adjust a density while following even great environmental variations. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2016-137227, filed Jul. 12, 2016 which is hereby incorporated by reference herein in its entirety.