Patent Publication Number: US-7583408-B2

Title: Image-forming apparatus and control method thereof

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an image-forming apparatus that forms an image by transferring a toner image formed on an image carrier onto a recording medium and a control method for controlling the image-forming apparatus. 
   2. Description of the Related Art 
   In printers, copy machines, etc., using electrophotographic technology, input image data is converted into electrical signals and a laser is driven on the basis of the obtained electrical signals, so that an electrostatic latent image corresponding to the image data is formed on a photosensitive member. The thus formed electrostatic latent image is visualized as a toner image by a developing device and is then transferred onto a recording sheet. 
   In monochrome printers in which images are formed using black developer (toner), the density of the images greatly affects the printing quality. Similarly, in color printers in which color images are formed using yellow (Y), magenta (M), cyan (C), and black (K) toners, the density of the images formed by the toner of each color greatly affects the printing quality. Accordingly, Japanese Patent Laid-Open No. 6-11965 discusses a structure in which a correction pattern used for density correction is formed on a recording sheet every time a predetermined number of recording sheets are subjected to printing. The thus formed correction pattern is optically read and the density of image data is corrected on the basis of a signal obtained by optically reading the correction pattern, thereby maintaining high image quality. 
   According to Japanese Patent Laid-Open No. 6-11965, even when continuous printing is performed, the density correction sequence is executed every time the predetermined number of recording sheets are subjected to printing in order to maintain the printing quality. However, since the timing at which the density correction sequence is performed depends on the number of recording sheets that are subjected to printing, this timing does not always match the timing at which the density correction is required in practice. More specifically, even if the density correction is necessary, the density correction sequence is not executed until the predetermined number of recording sheets are subjected to printing. Therefore, there is a risk that the quality of the printed image is reduced during the printing operation. In addition, if the predetermined number of recording sheets are subjected to printing even though the density correction is not required, the density correction sequence is unnecessarily executed. In such a case, the toner and the recording sheet are wasted and the operating cost is increased as a result. In addition, since printing cannot be performed while the density correction sequence is being executed, the productivity is largely reduced when the density correction sequence is performed unnecessarily. 
   SUMMARY OF THE INVENTION 
   To at least mitigate the above-described problems, some features of the present invention provide an image-forming apparatus that performs density adjustment at a suitable timing by observing the density of images formed in a normal image-forming operation and a control method for controlling the image-forming apparatus. 
   According to a first aspect of the present invention, an image-forming apparatus that forms an image by transferring a toner image formed on an image carrier onto a recording medium includes a density detector that detects a density of a toner image on the image carrier; a density adjuster that forms a correction pattern and performs an image-density adjustment on the basis of the correction pattern and the density of the toner image that is detected the density detector; an image-density calculator that calculates an image density in a predetermined section of an image represented by an image signal which is different from the correction pattern, the image density being calculated on the basis of the image signal; a comparator that compares the image density calculated by the image-density calculator and the result of detection performed by the density detector to obtain a density in the predetermined section of a toner image formed on the basis of the image signal; and a determiner that determines whether or not to cause the density adjuster to perform the image-density adjustment on the basis of the result of comparison performed by the comparator. 
   According to a second aspect of the present invention, a method for controlling an image-forming apparatus that forms an image by transferring a toner image formed on an image carrier onto a recording medium includes a density-detecting step of detecting the density of a toner image on the image carrier; a density-adjusting step of performing an image-density adjustment on the basis of the correction pattern and the density of the toner image that is detected in the density-detecting step; an image-density-calculating step of calculating an image density in a predetermined section of an image represented by an image signal which is different from the correction pattern, the image density being calculated on the basis of the image signal; a comparing step of comparing the image density calculated in the image-density-calculating step and the result of detection performed in the density-detecting step to obtain a density in the predetermined section of a toner image formed on the basis of the image signal; and a determining step of determining whether or not to perform the density-adjusting step of adjusting the image density on the basis of the result of comparison performed in the comparing step. 
   Other features and advantages of the present invention will be apparent from the following description when taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
       FIG. 1  is a schematic diagram illustrating the major part of an image-forming apparatus according to an embodiment of the present invention. 
       FIG. 2  is a schematic diagram illustrating the operation principle of the image-forming apparatus according to the embodiment of the present invention. 
       FIGS. 3A to 3C  are diagrams illustrating examples of images in a predetermined section S and video counts, which function as density information, in the predetermined section S. 
       FIGS. 4A and 4B  are diagrams illustrating density distributions in images having the same video count N in the predetermined section S. 
       FIG. 5  is a diagram illustrating a conversion method of the video count, which functions as the density information, based on the characteristic of a density sensor. 
       FIG. 6  is a functional block diagram illustrating the functional structure of an image processor included in a control unit according to the embodiment. 
       FIG. 7  is a flowchart illustrating a process for determining a density correction timing in the image-forming apparatus according to the embodiment. 
   

   DESCRIPTION OF THE EMBODIMENTS 
   An embodiment of the present invention will be described below with reference to the accompanying drawings. The present invention is not limited by the embodiment described below, and not all of the combinations of features described in the following embodiment are essential for carrying out the invention. 
     FIG. 1  is a schematic diagram illustrating the major part of an image-forming apparatus according to an embodiment of the present invention. In the present embodiment, the image-forming apparatus is a multifunction printer (MFP) that includes an electrophotographic printing section and that provides functions of a scanner, a facsimile machine, a copy machine, and a printer that prints data received data from, for example, a personal computer (PC). The printing section provides a color printing function using a photosensitive member and an intermediate transferring member. According to the present embodiment, a color image is formed using a single photosensitive member. However, effects similar to those of the present embodiment can also be obtained by an apparatus in which a plurality of photosensitive members corresponding to different colors are provided or an apparatus in which images are directly transferred onto a recording sheet without using the intermediate transferring member. In addition, the effects of the present embodiment are not limited to color printing, and similar effects can also be obtained by a printing section for monochrome printing. 
   An automatic document feeder (ADF)  41  for automatically feeding a document  44  one sheet at a time and a document reader  42  for reading images from the document  44  fed by the automatic document feeder  41  are disposed in an upper section of a main body  40  of the multifunction printer. In the document reader  42 , the document  44  is placed on a platen glass  43  and is illuminated with light emitted from a light source  45 , and a reflected-light image obtained from the document  44  is guided to a reading device  50 , such as a charge-coupled device (CCD), via a reducing optical system including optical mirrors  46 ,  47 , and  48  and an imaging lens  49 . The image reading element  50  reads the reflected-light image obtained from the color material on the document  44  with a predetermined dot density, converts the image into electrical signals, and outputs the electrical signals. 
   Thus, the reflected-light image of the document  44  is read by the document reader  42  and is transmitted to an image processor  51  as data of three colors, i.e., red (R), green (G), and blue (B). The RGB data of the document  44  is subjected to image processing including shading correction, gamma correction, and color space processing by the image processor  51 , and is output as image data of yellow (Y), magenta (M), cyan (C), and black (K). The multifunction printer also has a function as a printer that prints data received from an external PC (not shown) or the like. The data received from the PC or the like is transmitted to the image processor  51  and is subjected to image processing including image conversion, shading correction, gamma correction, and color space processing. 
   Thus, the image data is subjected to image processing by the image processor  51 , and is transmitted to an exposure device  5  in the form of Y, M, C, and K (black) image data. The exposure device  5  drives a semiconductor laser such that laser light emitted from the semiconductor laser is modulated in accordance with the image data, and the laser light from the semiconductor laser is reflected by a rotating polygon mirror such that a photosensitive member  1  is scanned with the reflected light. The photosensitive member  1  is rotated in the direction shown by the arrow A with a motor (not shown). A primary charging device  4 , the exposure device  5 , a color development unit  7 , a monochrome development unit  8 , a transfer charging device  9 , and a cleaner device  6  are disposed around the photosensitive member  1 . 
   In an image-forming operation, first, the surface of the photosensitive member  1  is uniformly charged to a predetermined negative potential by the charging device  4 . Then, the exposure device  5  including a laser scanner scans the charged surface of the photosensitive member  1  with the laser light emitted from the semiconductor laser that is driven by a signal that is pulse-width modulated on the basis of the image data. Accordingly, an electrostatic latent image corresponding to the image data is formed on the photosensitive member  1 . The color development unit  7  includes three development devices  7 Y (yellow toner),  7 M (magenta toner), and  7 C (cyan toner) to perform full-color development, and each development device is supplied with toner of the corresponding color. The color development devices  7 Y,  7 M, and  7 C and the monochrome development unit  8  can develop the latent image on the photosensitive member  1  with Y, M, C, and K toners, respectively. When the latent image is developed with color toner, the color development unit  7  is rotated in the direction shown by the arrow R with a motor (not shown) until the development device for that color comes into contact with the photosensitive member  1 . Each time a toner image is developed with color toner, the toner image is transferred onto a transfer belt  2 , which functions as the intermediate transferring member. Accordingly, a full-color image can be formed by superimposing images of the three colors on the transfer belt  2 . 
   The density of the toner image of each color that is developed on the photosensitive member  1  is detected by, for example, a density sensor  21  including a light-emitting element and a light-receiving element for detecting the density of the developed image. The density sensor  21  is disposed between a development position of the development device  7  and a transfer position of the transfer device  9  along the periphery of the photosensitive member  1 . The density sensor  21  detects the amount reflection of light that is emitted from the light-emitting element and reflected by the surface of the photosensitive member  1  using a light-receiving element, and determines the density of a single-color toner image formed on the photosensitive member  1  on the basis of the result of detection. The density sensor  21  transmits a detection signal to the image processor  51 . In the present embodiment, the density sensor  21  detects the density of the single-color toner image on the photosensitive member  1 . However, the density may also be detected at other positions, such as a position on the intermediate transferring belt or a photosensitive belt, where the single-color toner image is formed. 
   The toner images of different colors are successively developed on the photosensitive member  1  and are transferred onto the transfer belt  2 , which functions as the intermediate transferring member, by the transfer device  9 . Accordingly, the toner images of four colors are superimposed on the transfer belt  2 . The toner images that are thus transferred onto the transfer belt  2  are transferred onto a recording sheet by a secondary transfer device  15 . In full-color printing, the toner images of four colors are superimposed on the transfer belt  2 , and are then transferred onto the recording sheet. The recording sheet is fed from a paper cassette  16  into a conveying path due to rotation of a pickup roller  17 , and is conveyed to a nip portion, that is, a contact portion between the secondary transfer device  15  and the belt  2 , by conveying rollers  18  and  19 . A belt cleaner  14  id disposed at a position where the belt cleaner  14  faces a transfer-belt driving roller  10  with the transfer belt  2  disposed therebetween. The belt cleaner  14  removes the toner that remains on the transfer belt  2  without being transferred onto the recording sheet with a blade. 
   The toner that remains on the surface of the photosensitive member  1  is removed and collected by a cleaner device  6  after the amount of charge of the toner is reduced by a preliminary cleaner to facilitate the cleaning process. Then, the photosensitive member  1  is uniformly discharged to about 0V by a discharging device (not shown) to prepare for the next cycle of the image-forming operation. 
   The recording sheet on which the toner image is transferred is conveyed to a fixing device  3 , where the toner image is fixed, and is ejected from the apparatus. The fixing device  3  includes a pair of rollers having halogen heaters, which function as heating elements, contained therein. The rollers can rotate while being pressed against each other by a pressing mechanism (not shown). 
   In this multifunction printer, an image-forming timing is controlled on the basis of a reference position on the transfer belt  2 . The transfer belt  2  is wound around rollers  10 ,  11 ,  12 , and  13 . Among these rollers, the transfer-belt-driving roller  10  is connected to a drive source (not shown) and functions as a drive roller for driving the transfer belt  2 . In addition, transfer-belt tension rollers  11  and  12  function as tension rollers for adjusting the tension applied to the transfer belt  2 . A back-up roller  13  functions as a back-up roller for the transfer roller  15 , which functions as a secondary transfer device. In addition, a reflective sensor  20  that detects the passage of the reference position on the transfer belt  2  is disposed near the tension roller  12 . The reflective sensor  20  detects a mark, such as a reflection tape, provided at an edge of the outer peripheral surface of the transfer belt  2  and outputs an I top  signal. 
   The ratio of the circumference of the photosensitive member  1  to the circumference of the transfer belt  2  is set to a ratio of 1:n (n is an integer). Accordingly, the photosensitive member  1  rotates n turns (n is an integer) while the transfer belt  2  rotates one turn. After the transfer belt  2  rotes one turn, the positions of the surfaces of the transfer belt  2  and the photosensitive member  1  return to the initial positions. Therefore, when the toner images of four colors are superimposed on the intermediate transferring belt  2  (which means that the transfer belt  2  rotates four turns), the color misalignment due to variation in rotation of the photosensitive member  1  can be prevented. 
   In the above-described image-forming apparatus using the intermediate transferring method, when a predetermined time elapses after the detection of the above-described I top  signal, the exposure device  5  (including the laser scanner) starts the exposure process. As described above, while the transfer belt  2  rotates one turn, the photosensitive member  1  rotates n turns (n is an integer) and return to exactly the same position as the position before the rotation of the transfer belt  2  and the photosensitive member  1 . Therefore, the toner image is always formed at the same position on the transfer belt  2 . Although the size of the toner image varies depending on the size of the recording sheet, the transfer belt  2  has a region where the toner image is never formed. 
   Next, the operation principle of the present embodiment will be described below with reference to  FIG. 2 . 
     FIG. 2  is a schematic diagram illustrating the operation principle of the image-forming apparatus according to the embodiment of the present invention. In  FIG. 2 , components similar to those shown in  FIG. 1  are denoted by the same reference numerals. 
   When image data S 201  is input to a control unit  30  from the document reader  42  or the PC (not shown), the input image data S 201  is subjected to image processing by the image processor  51 . Then, the control unit  30 , which will be described in detail below, determines whether or not the density of the image can be measured in a predetermined section S thereof. The control unit  30  includes a CPU, a ROM that stores programs executed by the CPU, and a RAM used as a work area when the CPU executes the programs. If it is determined that the density measurement can be performed, the control unit  30 , which functions as an image-density calculator, calculates an image density D ref  in the predetermined section S of the image to be printed for each color, that is, for each of yellow (Y), magenta (M), cyan (C), and black (K), on the basis of a video count and the characteristic of the density sensor  21 . Then, the obtained result is stored in the memory (RAM). Next, in order to detect the density in the predetermined section S of the toner image formed on the photosensitive member  1  using the density detection sensor  21 , which functions as a density detector, the control unit  30  calculates a delay time between the output of the I top  signal for each color and the start of measurement and the measurement time. Accordingly, the density sensor  21  detects the density in the predetermined section S for each color and outputs a density signal S 202  is transmitted to the image processor  51 . The image processor  51 , which functions as a density detector, converts the density signal S 202  into digital data using an A/D converter  31  ( FIG. 6 ) and calculates a print density D of the actually formed image using a density conversion table  32  that is prepared in advance. Then, the control unit  30 , which also functions as a comparator, compares the image density D ref  and the print density D and calculates a density difference AD. 
     FIGS. 3A to 3C  are diagrams illustrating examples of images in the predetermined section S and video counts, which function as density information, in the predetermined section S. 
   As shown in  FIGS. 3A and 3B , when the video count is equal to or more than N (e.g., N=1000) in the predetermined section S, it is determined that the density can be detected by the density sensor  21 . 
   However, when the video count is less than N, as shown in  FIG. 3C , the S/N ratio is reduced and it is difficult to reliably measure the density. Therefore, it is determined that the density cannot be detected by the density sensor  21 . 
   Here, the term “video count” refers to the sum of the pixel data in the predetermined section when the pixel data is expressed as multilevel data. 
     FIGS. 4A and 4B  are diagrams illustrating density distributions of images having the same video count N in the predetermined section S. 
   When the density dispersion is low, as shown in  FIG. 4B , the density can be determined with high accuracy. However, when the density dispersion is high, as shown in  FIG. 4A , the response speed of the density sensor  21  and the S/N ratio are reduced, and it is difficult to detect the density with high accuracy. 
     FIG. 5  is a diagram illustrating a conversion method of the video count, which functions as the density information, based on the characteristic of the density sensor. 
   In order to simplify the sensitivity characteristic of the density sensor  21 , it is assumed that the sensitivity is linearly reduced from the center. When the sensitivity of the density sensor  21  has a characteristic as denoted by  500  in  FIG. 5 , the density signal (a) at each spot can be converted into signal (b), which is used for determining the image density, in accordance with the relationship between the position and sensitivity of the density sensor  21 . 
   The image density is determined by the steps described below. When the predetermined section S satisfies both the condition that the video count is N or more as described with reference to  FIGS. 3A to 3C  and the condition that the density dispersion is within a predetermined value as described with reference to  FIGS. 4A and 4B , the video count of the predetermined section S is converted as described with reference to  FIG. 5  on the basis of the installation position and the characteristic of the density sensor  21 . Then, the control unit  30  calculates the integrated value or the average value to determine the image density D ref  at the predetermined section S, and stores the image density D ref  in the memory (RAM). 
   In this embodiment, the area of the predetermined section S corresponds to the detectable range of the density sensor  21 , and can cover the overall length of the photosensitive member  1  in the longitudinal direction thereof. In such a case, a line-shaped sensor can be used as the density sensor. In addition, the sensitivity distribution is not limited to that shown in  FIG. 5 , and the sensor elements may have substantially the same sensitivity. 
   The predetermined section may also extend along the circumference of the photosensitive member  1 . 
     FIG. 6  is a functional block diagram illustrating the functional structure of the image processor  51  included in the control unit  30  according to the embodiment. 
   The density in the predetermined section S of the toner image for each color is measured by the density sensor  21  at the above-described timing. The thus measured value is output from the density sensor  21  as an analog signal, and the A/D converter  31  performs the A/D conversion of the obtained analog signal in or out of the control unit  30  at a sampling interval Δt. The value obtained by the A/D conversion is converted into the density data by referring to the density conversion table (ROM)  32  prepared in advance. Then, the print density D in the predetermined section S is calculated using the density data obtained by conversion. 
     FIG. 7  is a flowchart illustrating a process for determining the density correction timing in the image-forming apparatus according to the present embodiment. The program for this process is stored in the ROM included in the control unit  30 , which functions as a determiner, and is executed under the control of the CPU. 
   First, in step S 701 , to start an operation of printing on a recording sheet (not shown) commanded by a user, the data of the document  44  read by the document reader  42  or the data transmitted from the PC or the like is input and transmitted to the image processor  51 . Then, in step S 702 , the image data transmitted to the image processor  51  is subjected to image processing such as shading correction, gamma correction, color space processing, etc. Then, after the image processing is performed by the image processor  51 , the image data is divided into image signals for, for example, Y (yellow), M (magenta), C (cyan), K (black), etc., used for printing. Next, in step S 703 , it is determined whether or not the video count, which is the density information, in the predetermined section S set arbitrarily in a range that can be read by the density sensor  21  is equal to or more than a predetermined number (N) on the basis of the image data subjected to image processing or the image signals. Thus, it is determined whether or not the density can be reliably detected by the density sensor  21 . When the video count in the predetermined section S is less than the predetermined number (N), the process proceeds to step S 706  and the image density D ref  is not set since it is determined that the print density in the predetermined section S cannot be monitored by the density sensor  21 . 
   If it is determined that the video count in the predetermined section S is equal to or more than the predetermined number (N), the process proceeds to step S 704 , where the density dispersion of the image information in the predetermined section S is calculated. Then, it is determined whether or not the determined density dispersion is within a given range. If the density dispersion is within the given range, it is determined that the density of the image in the predetermined section S can be reliably detected and the process proceeds to step S 705 , where the image density D ref  in the predetermined section S is determined on the basis of the characteristic of the density sensor  21  and the video count. If it is determined that the density dispersion in the predetermined section S is out of the given range in step S 704 , the print density in the predetermined section S cannot be monitored by the density sensor  21 . Therefore, the process proceeds to step S 706  and the image density D ref  is not set. 
   After steps S 705  and S 706 , the process proceeds to step S 707 , where the semiconductor laser is driven on the basis of the image information input in step S 701  and a toner image is formed on the photosensitive member  1  by the above-described method. Then, in step S 708 , whether or not the image density D ref  is set for the formed toner image is checked. If the image density D ref  is set, it is determined that the density detection can be performed and the process proceeds to step S 709 , where the density in the predetermined section S is detected at the above-described timing. Then, the thus obtained density signal is subjected to A/D conversion and the print density D for each color is calculated using the density conversion table  32 . Then, in step S 710 , the control unit  30  compares the image density D ref  in the predetermined section S calculated in step S 705  and the print density D calculated in step S 709  and calculates the difference ΔD therebetween. Then, if the difference ΔD is out of a given range A, the process proceeds to step S 713 , where the density correction is performed immediately. 
   When the difference ΔD is within the given range between A and B (A&gt;ΔD&gt;B), the process proceeds to step S 714 . After images of all of the colors required by the image data are formed and the toner images of all colors are transferred onto the transfer belt  2  in step S 715 , the process proceeds to step S 713 . In step S 713 , the control unit  30 , which functions as a density adjuster, forms a correction pattern and performs density correction for image data on the basis of signals obtained by optically reading the correction pattern with the density sensor  21 . If the difference ΔD is within the given range B in step S 711 , it is determined that the density correction is not necessary and the process proceeds to step S 716 , where it is determined whether or not the images of all of the colors required by the image data are formed. If it is determined that the formation of images of all colors is not yet finished, the process returns to step S 707 . After the toner images of all colors are formed, the toner images are transferred onto the transfer belt  2  and then onto the recording sheet in step S 717 . 
   If the difference ΔD is more than the given range B (for example, 5%) and less than A (20%), the quality of a resulting image on the recording sheet would not be particularly low. Therefore, it is determined that the image can be used as a normal image and is formed on the recording medium so as not to waste the toner images formed on the photosensitive member  1  and the transfer belt  2 . 
   However, when the calculated difference is more than the given range A in step S 710 , it is decided that the density correction must be performed immediately and the process proceeds to step S 713  without transferring the toner images onto the recording sheet. 
   If it is determined that the image density D ref  is not set in step S 708 , it is determined that the density detection cannot be performed and the process proceeds to step S 712 . In step S 712 , it is determined whether or not factors including the number of recording sheets on which images are printed without density correction satisfy predetermined conditions for ensuring the print density. If it is determined that the conditions are satisfied, the process returns to step S 707  and the image-forming operation is continued. 
   When it is determined that density correction is necessary in step S 710  or S 712 , the image-forming operation is temporarily stopped and the process proceeds to step S 713 , where a density correction sequence similar to that disclosed in Japanese Patent Laid-Open No. 6-11965 is performed as soon as the state in which the correction can be performed is obtained. Then, when the density correction sequence is finished, the process returns to step S 707  and the image-forming operation is restarted. 
   The given ranges A and B used in steps S 710  and S 711  can be determined as described below. When the spot diameter of the sensor is about 1 mm, the size of the predetermined section S can be set to about 3 mm×3 mm. In this case, the number of sensor spots included in the section S is about 5,000. When each pixel data is expressed with 8 bits and the pixel average of the density difference ΔD in the predetermined section S (5,000 pixels) is about 5%, the difference in each pixel is about 10. Therefore, the density correction is necessary when the difference is 5,000 pixels×(10/pixel)=50,000 in the predetermined section S. If, for example, the density difference ΔD largely exceeds 5% and is 20% of more (e.g., 30%) in step S 710 , the density of the printed image cannot be ensured. Accordingly, the image is not transferred onto the recording sheet and the density correction is performed even though the toner is wasted. In step S 711 , it is determined whether or not the density of the resulting image can be somewhat ensured. If the difference is within the predetermined range, the image transfer process is performed to the end and the image is printed, so that the toner can be prevented from being wasted. Then, after the image is transferred onto the recording sheet, the density correction is performed. 
   Although a single-color toner image is monitored by the density sensor  21  in the present embodiment, a density sensor may be disposed so as to face the roller  11  of the transfer belt  2  and toner images of a plurality of colors superimposed on the transfer belt  2  can be measured using the density sensor. In such a case, a similar control operation can be performed by subjecting the obtained signal to color separation. 
   As described above, according to the present embodiment, when the user performs the image-forming operation, the density of images formed by the image-forming apparatus is monitored in real time and the timing for adjusting the density is determined accordingly. Therefore, the density correction can be performed at an adequate timing. As a result, the density correction can be prevented from being performed unnecessarily. In addition, the down time of the image-forming apparatus can be reduced, so that the productivity can be increased. 
   In addition, since the pattern for density correction is not formed when the density correction is not necessary, the toner can be prevented from being wasted. 
   The process for determining the density correction timing is performed every time a predetermined number of recording sheets are subjected to printing. In the present embodiment, the predetermined number of recording sheets is set to one, and it is determined whether or not to perform the density adjustment each time the image-forming operation is performed. However, if it can be assumed that sudden variation does not occur, the predetermined number of recording sheets may also be set to, for example, five, and the process for determining the density correction timing may be performed every time five recording sheets are subjected to printing. In addition, the predetermined number of recording sheets can be set to more than five as long as the expected variation is allowable. In such a case, the processing load on the control unit  30  can be reduced. 
   According to the present invention, a software program for carrying out the functions of the above-described embodiment can be directly or remotely supplied to a system or an apparatus. Thus, the present invention includes a case in which the thus supplied program code is read out and executed by a computer included in the system or the apparatus. The form of the program code is not limited to the program as long as the functions of the program can be provided. Thus, the present invention can also be achieved by the program code itself that is installed in the computer for allowing the computer to carry out the functions of the present invention. In other words, the present invention includes the computer program for achieving the functions of the present invention. In this case, the form of the program is not limited as long as the functions of the program can be obtained, and may be, for example, an object code, a program executed by an interpreter, script data supplied to an OS, etc. 
   A storage medium for supplying the program may be, for example, a floppy disk (registered trademark), a hard disk, an optical disc, a magneto-optical disk, an MO, a CD-ROM, a CD-R, a CD-RW, a magnetic tape, a nonvolatile memory card, a ROM, a DVD (DVD-ROM and DVD-R), etc. In addition, the program can also be obtained by accessing a Web page on the Internet using a browser on a client computer. The computer program according to the present invention or a file including the program in a compressed form and having an automatic installation function can be downloaded from the Web page to a storage medium, such as a hard disk. Alternatively, the program code that functions as the program according to the present invention may be divided into a plurality of files, and these files may be downloaded from different Web pages. Thus, a WWW server from which a program file for allowing the computer to carry out the functions of the present invention is downloaded by a plurality of users is also included in the present invention. 
   The program according to the present invention may be stored in storage media, such as CD-ROMs, in an encrypted form, and be distributed to users. The users can download key information for decoding the encrypted program from a Web page via the Internet. Thus, the encrypted program can be executed using the key information and installed into the computer. 
   The computer can carry out the functions of the above-described embodiment by reading out and executing the program. In addition, the functions of the above-described embodiment can also be carried out by causing the OS or the like running on the computer to perform all or part of the actual processes on the basis of the instructions of the program. 
   The program read out from the recording medium may be written in a memory provided in a function expansion board included in the computer or in a function expansion unit connected to the computer. In such a case, the functions of the above-described embodiments can also be carried out by causing the CPU or the like included in the function expansion board or the function expansion unit to perform all or part of the actual processes on the basis of the instructions of the program. 
   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 modifications, equivalent structures and functions. 
   This application claims the priority of Japanese Application No. 2005-252467 filed Aug. 31, 2005 and Japanese Application No. 2006-200873 filed Jul. 24, 2006, both of which are hereby incorporated by reference herein in their entirety.