Patent Publication Number: US-8537414-B2

Title: Image forming apparatus and method of adjusting color balance

Description:
BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT 
     The present invention relates to an image forming apparatus capable of forming an image in colors. Further, the present invention relates to a method of adjusting color balance applied to the image forming apparatus. 
     In a conventional image forming apparatus, a toner image as a developer image is formed on a photosensitive drum, and the toner image is transferred to a printing medium or a transfer medium such as a transportation belt. Such a conventional image forming apparatus includes a printer of an electro-photography type, a facsimile, a copier, or a multi function product having three functions of the facsimile, the copier, and the printer. 
     The conventional image forming apparatus may have a capability of forming an image in colors. In the conventional image forming apparatus, it is necessary to accurately adjust color balance for obtaining a color image with high quality. 
     In the conventional image forming apparatus, in order to accurately adjust color balance, it may be configured to control an energy amount of a static latent image forming unit and an energy amount of a developing unit (refer to Patent Reference). 
     Patent Reference: Japanese Patent Publication No. 2004-258281 
     In this case, the energy amount of the static latent image forming unit represents an energy amount of light irradiated on a photosensitive drum. More specifically, the energy amount is represented with an exposure time during which a light irradiation unit such as an LED (Light Emitting Diode) and a laser light source exposes the photosensitive drum, or an amount of drive power applied to the light irradiation unit. In the following description, the energy amount of the static latent image forming unit is referred to as an exposure light energy. 
     Further, the energy amount of the developing unit represents an energy amount defined with one of a developing voltage, a supply voltage, and a charging voltage. In the following description, the energy amount of the developing unit is referred to as a developing energy. 
     In the conventional image forming apparatus described above, when color balance is adjusted, first, different density detection patterns are printed or transferred to a surface of the photosensitive drum or a transfer medium. In the next step, a density detection unit formed of a density sensor and the like detects densities of the density detection patterns thus printed, so that one of the exposure light energy and the developing energy is controlled according to density values thus detected. 
     In this case, a correction value of a light amount is added to a printing condition, thereby controlling the exposure light energy. Alternatively, a correction value of a developing voltage is added to a printing condition, thereby controlling the developing energy. 
     In the next step, in the conventional image forming apparatus, the density detection patterns are printed on the surface of the transfer medium one more time. Then, the density detection unit detects densities of the density detection patterns thus printed, so that the other of the exposure light energy and the developing energy is controlled according to density values thus detected. Through the process described above, in the conventional image forming apparatus, it is possible to obtain proper color balance. 
     As described above, in the conventional image forming apparatus, when color balance is adjusted, first, one of the exposure light energy and the developing energy is controlled according to the density values. After one of the exposure light energy and the developing energy is controlled, the density detection unit detects the densities of the density detection patterns, so that the other of the exposure light energy and the developing energy is controlled according to the density values. 
     In this case, in the conventional image forming apparatus, depending on which one of the exposure light energy and the developing energy is controlled first, a final energy correction amount may be different. When one of the exposure light energy and the developing energy is controlled first, the other of the exposure light energy and the developing energy may be controlled by an amount different from the case in the reversed order. 
     As described above, in the conventional image forming apparatus, when one of the exposure light energy and the developing energy is controlled in a fixed order for adjusting color balance, color balance may be over adjusted. In other words, the light amount or the developing voltage may be adjusted by an excessive correction value. As a result, the conventional image forming apparatus may cause a print quality problem, thereby deteriorating printed image quality. 
     In view of the problems described above, an object of the present invention is to provide an image forming apparatus capable of solving the problems of the conventional image forming apparatus. In the present invention, it is possible to prevent color balance from being overly adjusted, thereby preventing image quality from deteriorating. Further, an object of the present invention is to provide a method of adjusting color balance applied to the image forming apparatus. 
     Further objects and advantages of the invention will be apparent from the following description of the invention. 
     SUMMARY OF THE INVENTION 
     In order to attain the objects described above, according to a first aspect of the present invention, an image forming apparatus is configured to adjust a print density with a plurality of set values including a first set value and a second set value. The image forming apparatus includes a storage unit for storing a first density as a target value of the print density; a density detection unit for detecting a second density as a print density of an image printed on a transfer medium; and a control unit for adjusting the print density. 
     In the first aspect of the present invention, the control unit is arranged to compare the first density stored in the storage unit and the second density detected with the density detection unit. Further, the control unit is arranged to calculate a first correction value as a correction value applied to one of the first set value and the second set value for correcting the second density to the first density according to a density difference generation condition determined in advance. Further, the control unit is arranged to correct one of the first set value and the second set value according to the first correction value, thereby adjusting color balance. 
     According to a second aspect of the present invention, a method of adjusting color balance applied to an image forming apparatus. The image forming apparatus includes a storage unit for storing a first density as a target value of a print density; a density detection unit for detecting a second density as a print density of an image printed on a transfer medium; and a control unit for adjusting the print density. The image forming apparatus is configured to adjust the print density with a plurality of set values including a first set value and a second set value. 
     In the second aspect of the present invention, the control unit is arranged to compare the first density stored in the storage unit and the second density detected with the density detection unit. Further, the control unit is arranged to calculate a first correction value as a correction value applied to one of the first set value and the second set value for correcting the second density to the first density according to a density difference generation condition determined in advance. Further, the control unit is arranged to correct one of the first set value and the second set value according to the first correction value, thereby adjusting color balance. 
     In the first aspect of the present invention, it is possible to provide the image forming apparatus capable of preventing printed image quality from deteriorating due to unnecessary over correction. In the second aspect of the present invention, it is possible to provide the method of adjusting color balance applicable to the image forming apparatus in the first aspect. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing a configuration of an image forming apparatus according to a first embodiment of the present invention; 
         FIG. 2  is a schematic side view showing components of the image forming apparatus according to the first embodiment of the present invention; 
         FIG. 3  is a flow chart showing a method of adjusting color balance applied to the image forming apparatus according to the first embodiment of the present invention; 
         FIGS. 4(   a ) and  4 ( b ) are schematic views showing density detection patterns of the image forming apparatus according to the first embodiment of the present invention; 
         FIG. 5  is a schematic view showing a data flow of the method of adjusting color balance applied to the image forming apparatus according to the first embodiment of the present invention; 
         FIG. 6  is a graph showing a relationship between a density and a duty of the density detection pattern to determine a difference between a target density value and a detected value according to the first embodiment of the present invention; 
         FIG. 7  is a graph showing a relationship between the density and the duty of the density detection pattern to determine a change in a density characteristic of the density detection pattern after color balance is adjusted through controlling an exposure light energy in a conventional image forming apparatus; 
         FIG. 8  is a graph showing a relationship between the density and the duty of the density detection pattern to determine a change in the density characteristic of the density detection pattern after color balance is adjusted through controlling a developing energy in the conventional image forming apparatus; 
         FIG. 9  is a graph showing a relationship between the density and the duty of the density detection pattern to determine a change in the density characteristic of the density detection pattern after color balance is adjusted through controlling the exposure light energy and the developing energy in different orders in the conventional image forming apparatus; 
         FIG. 10  is a block diagram showing a configuration of an image forming apparatus according to a second embodiment of the present invention; 
         FIG. 11  is a flow chart showing a method of adjusting color balance applied to the image forming apparatus according to the second embodiment of the present invention; 
         FIG. 12  is a schematic view showing a data flow of the method of adjusting color balance applied to the image forming apparatus according to the second embodiment of the present invention; 
         FIG. 13  is a block diagram showing a configuration of an image forming apparatus according to a third embodiment of the present invention; 
         FIG. 14  is a flow chart showing a method of adjusting color balance applied to the image forming apparatus according to the third embodiment of the present invention; and 
         FIG. 15  is a schematic view showing a data flow of the method of adjusting color balance applied to the image forming apparatus according to the third embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Hereunder, embodiments of the present invention will be explained with reference to the accompanying drawings. The accompanying drawings are shown for explanation purpose only, and the present invention is not limited to the accompanying drawings. In the accompanying drawings, same components or similar components are designated with the same reference numerals, and explanations thereof are omitted. 
     First Embodiment 
     A configuration of an image forming apparatus  100  according to a first embodiment of the present invention will be explained with reference to  FIGS. 1 and 2 .  FIG. 1  is a block diagram showing the configuration of the image forming apparatus  100  according to the first embodiment of the present invention.  FIG. 2  is a schematic side view showing components of the image forming apparatus  100  according to the first embodiment of the present invention. 
     In the embodiment, the image forming apparatus  100  is a printer of an electro-photography type, a facsimile, a copier, or a multi function product having three functions of the facsimile, the copier, and the printer. Alternatively, the image forming apparatus  100  may be an image forming apparatus of any type. In the following description, the image forming apparatus  100  is a printer of a color electro-photography type. 
     As shown in  FIG. 1 , the image forming apparatus  100  includes an image forming unit  110 , a medium transportation unit  120 , an image fixing unit  130 , a control unit  140 , a storage unit  150 , and a detection unit  160 . 
     In the embodiment, the image forming unit  110  is provided for forming an image on a print medium  200  (refer to  FIG. 2 ) or a transportation belt  123  (refer to  FIG. 2 ). The image forming unit  110  includes an exposure unit  111 , an image forming section  112 , a transfer unit  113 , a photosensitive drum  114 , a developing voltage generation unit  115 , a supply voltage generation unit  116 , a charging voltage generation unit  117 , a transfer voltage generation unit  118 , and a photosensitive drum drive motor  119 . 
     In the embodiment, the exposure unit  111  is provided for forming a static latent image on the photosensitive drum  114  charged with a charging unit  112   c . The exposure unit  111  is formed of an LED (Light Emitting Diode) head. Further, the exposure unit  111  and the charging unit  112   c  constitute a static latent image forming unit  125  for forming a static latent image on the photosensitive drum  114 . 
     In the embodiment, the image forming section  112  is provided for forming a visualized image on the photosensitive drum  114 . The image forming section  112  includes a developing unit  112   a , a toner supply unit  112   b , and the charging unit  112   c.    
     In the embodiment, the developing unit  112   a  is provided for attaching developer (toner) to the static latent image formed on the photosensitive drum  114 , so that the static latent image is visualized. In other words, the developing unit  112   a  is provided for forming a developer image (a toner image). More specifically, the developing voltage generation unit  115  applies a developing voltage (corresponding to a developing energy) to the developing unit  112   a , so that the developing unit  112   a  attaches developer (toner) to the static latent image formed on the photosensitive drum  114 . 
     In the embodiment, the toner supply unit  112   b  is provided for supplying developer to the developing unit  112   a . The toner supply unit  112   b  retains developer in a corresponding color (one of black (B), yellow (Y), magenta (M), and cyan (C)). More specifically, the supply voltage generation unit  116  applies a supply voltage to the toner supply unit  112   b , so that the toner supply unit  112   b  applies electric charges to developer and supplies developer to the developing unit  112   a.    
     In the embodiment, the charging unit  112   c  is provided for charging the photosensitive drum  114 . More specifically, the charging voltage generation unit  117  applies a charging voltage to the charging unit  112   c , so that the charging unit  112   c  charges the photosensitive drum  114 . 
     In the embodiment, the image forming unit  110  is disposed at four locations corresponding to each color of black (B), yellow (Y), magenta (M), and cyan (C). In the following description, when it is necessary to differentiate a component corresponding to each color, the component is designated with a reference numeral with one of letters B, Y, M, and C representing the color. 
     In the embodiment, the transfer unit  113  is provided for transferring the developer image (the toner image) formed on the photosensitive drum  114  to a transfer medium. As shown in  FIG. 2 , the transfer unit  113  is formed of a transportation roller disposed on a backside surface of the transportation belt  123  at a position facing the photosensitive drum  114 . In other words, the transfer unit  113  and the photosensitive drum  114  are arranged to sandwich the transportation belt  123  and the print medium  200  such as a sheet. The transfer voltage generation unit  118  applies a transfer voltage to the transfer unit  113 , so that the transfer unit  113  transfers the developer image (the toner image) formed on the photosensitive drum  114  to the transfer medium. The transfer medium, to which the developer image (the toner image) formed on the photosensitive drum  114  is transferred, includes the transportation belt  123  and the print medium  200 . 
     In the embodiment, the photosensitive drum  114  is an image supporting member for forming the static latent image and the developer image (the toner image) thereon. The developing voltage generation unit  115  is provided for generating the developing voltage (the developing energy) to be applied to the developing unit  112   a  of the image forming section  112 . The supply voltage generation unit  116  is provided for generating the supply voltage to be applied to the toner supply unit  112   b  of the image forming section  112 . The charging voltage generation unit  117  is provided for generating the charging voltage to be applied to the charging unit  112   c . The transfer voltage generation unit  118  is provided for generating the transfer voltage to be applied to the transfer unit  113 . The photosensitive drum drive motor  119  is provided for driving the photosensitive drum  114  to rotate. 
     In the embodiment, the medium transportation unit  120  is provided for transporting the transfer medium (that is, the transportation belt  123  and the print medium  200 ). The medium transportation unit  120  includes medium transportation rollers  121  and a medium transportation roller drive motor  122 . The medium transportation rollers  121  are provided for extending the transportation belt  123 , and are formed of a pair of rollers  121   a  and  121   b  (refer to  FIG. 2 ). The medium transportation roller drive motor  122  is provided for driving one or both of the rollers  121   a  and  121   b  to rotate, so that the transportation belt  123  moves in a direction represented with a hidden line in  FIG. 2  to transport the print medium  200 . 
     In the embodiment, the image fixing unit  130  is provided for fixing the image transferred to the print medium  200 . The image fixing unit  130  is formed of a roller. The image fixing unit  130  includes a heater as an image fixing section  131 . While the image fixing unit  130  is rotating, the image fixing section  131  heats the print medium  200 , so that the image transferred to the print medium  200  is fixed. 
     In the embodiment, the control unit  140  is provided for controlling each component of the image forming apparatus  100 . The control unit  140  is formed of a CPU (Central Processing Unit). The control unit  140  includes a command image processing unit  141 , a mechanic control unit  142 , and a high voltage control unit  143 . 
     In the embodiment, the command image processing unit  141  is provided for processing a command of print data and image data. The command image processing unit  141  is connected to a host device (a computer, not shown) through a host interface unit  171 , so that the command image processing unit  141  receives the print data transmitted from the host device. Further, the command image processing unit  141  is connected to the exposure unit  111  through an exposure unit interface unit  172 , so that the command image processing unit  141  controls the exposure unit  111 . 
     In the embodiment, the mechanic control unit  142  is provided for controlling various mechanical components such as the exposure unit  111 , the high voltage control unit  143 , the photosensitive drum drive motor  119 , the medium transportation roller drive motor  122 , and the image fixing unit  130 . The mechanic control unit  142  is connected to the storage unit  150 , so that the mechanic control unit  142  stores setting data, data transmitted from the host device, and data detected with the detection unit  160  in the storage unit  150 . The mechanic control unit  142  includes a density correction process execution unit  142   a  and a density correction control unit  142   b.    
     In the embodiment, the density correction process execution unit  142   a  is provided for executing a density correction process according to a print density of an image detected with a density detection unit  161  formed of a density sensor using an optical sensor (described later). More specifically, the density correction process execution unit  142   a  is provided for determining an order of correcting the exposure light energy of the exposure unit  111  and the developing energy of the developing unit  112   a , and for calculating a correction value of the exposure light energy of the exposure unit  111  and the developing energy of the developing unit  112   a . The density correction control unit  142   b  is provided for controlling the density correction process according to a determination result of the density correction process execution unit  142   a.    
     In the embodiment, the high voltage control unit  143  is provided for controlling the developing voltage generation unit  115 , the supply voltage generation unit  116 , the charging voltage generation unit  117 , and the transfer voltage generation unit  118 . When the high voltage control unit  143  receives an instruction from the mechanic control unit  142 , the high voltage control unit  143  controls the developing voltage generation unit  115 , the supply voltage generation unit  116 , the charging voltage generation unit  117 , and the transfer voltage generation unit  118  to apply a high voltage to the corresponding components, respectively. 
     In the embodiment, the storage unit  150  is provided for storing various programs and data. The storage unit  150  is formed of an RAM (Random Access Memory), an ROM (Read Only Memory), an HDD (Hard Disk Drive), and the likes. Further, the storage unit  150  stores a plurality of setting values such as a setting value of the exposure light energy as a first setting value and a setting value of the developing energy as a second setting value. 
     Further, the storage unit  150  stores data received from the host device and detected data. The storage unit  150  includes an exposure light energy correction limit value storage unit  151  and a density difference generation condition storage unit  152 . Further, the storage unit  150  stores in advance a target density  405  (described later) as a target value of a density for adjusting color balance. 
     In the embodiment, the exposure light energy correction limit value storage unit  151  is provided for storing an exposure light energy correction limit value in advance. The exposure light energy correction limit value defines a limit of a correction value of the exposure light energy. In the following description, the exposure light energy correction limit value is referred to as a limit value or a specific value. 
     In the embodiment, in general, the limit value is set to a value of, for example, 15% when the image forming apparatus  100  is delivered. In this case, when the density correction process execution unit  142   a  controls the exposure light energy, the density correction process execution unit  142   a  refers to the limit value stored in advance in the exposure light energy correction limit value storage unit  151 . Then, the density correction process execution unit  142   a  calculates the correction value of the exposure light energy such that the correction value of the exposure light energy has a value within 15% from the target density  405  (refer to  FIG. 6 ). 
     In the embodiment, the density difference generation condition storage unit  152  is provided for storing in advance a density difference generation condition, which is referred to when the order of controlling (correcting) the exposure light energy and the developing energy. The density difference generation condition is determined in advance according to a range of a print density of a density detection pattern printed on the transportation belt  123 . According to the print density, the density difference generation condition defines the order of controlling (correcting) the exposure light energy and the developing energy and a method of calculating the correction value of the exposure light energy and the correction value of the developing energy. 
     In the embodiment, the detection unit  160  is provided for detecting a density of an image, a temperature, and the like. The detection unit  160  includes the density detection unit  161  using an optical sensor, a thermistor  162  as a temperature sensor, and a sensor for detecting a medium. The density detection unit  161  is configured to irradiate light on an image, so that the density detection unit  161  detects an amount of reflected light to detect the density of the image. The density detection unit  161  is formed of a light emitting portion and a light receiving portion, and detects a density of color toner and black toner. The thermistor  162  is provided for measuring an environmental temperature. 
     An arrangement of the components described above will be explained next. As shown in  FIG. 2 , the image forming apparatus  100  is provided with a pair of the medium transportation rollers  121   a  and  121   b , and the transportation belt  123  is extended between the medium transportation rollers  121   a  and  121   b . When the medium transportation roller drive motor  122  (refer to  FIG. 1 ) drives one or both of the medium transportation rollers  121   a  and  121   b  to rotate, the transportation belt  123  moves. 
     In the embodiment, around the transportation belt  123 , the image forming units  110 B,  110 Y,  110 M, and  110 C corresponding to each color of black (B), yellow (Y), magenta (M), and cyan (C) are sequentially disposed from an upstream side along the transportation belt  123 . Each of the image forming units  110 B,  110 Y,  110 M, and  110 C is provided with the photosensitive drum  114 , the static latent image forming unit  125  including the charging unit  112   c  and the exposure unit  111  (refer to  FIG. 1 ), the developing unit  112   a  (refer to  FIG. 1 ), and the transfer unit  113 . 
     In the embodiment, when the exposure unit  111  is formed of the LEDs, a few thousands of the LEDs are arranged in a direction perpendicular to a transportation direction of the print medium  200 . All of the LEDs are configured to emit light according to a drive signal transmitted from the control unit  140  through the exposure unit interface unit  172  (refer to  FIG. 1 ). Note that the mechanic control unit  142  is configured to adjust an exposure time of the exposure unit  111 , so that the control unit  140  controls the exposure light energy, that is, energy of exposing the photosensitive drum  114 . 
     In the embodiment, the image forming apparatus  100  is provided with one or both of a transportation path  124   a  and a transportation path  124   b  as a transportation path  124  for transporting the print medium  200 . The transportation path  124   a  is provided for transporting the print medium  200  from a side of the medium transportation roller  121   a  toward the medium transportation roller  121   b  along the surface of the transportation belt  123 . Accordingly, the transportation path  124   a  transports the print medium  200 , so that the image formed on the photosensitive drum  114  is directly transferred to the print medium  200 . 
     In contrast, the transportation path  124   b  is arranged along a tangential direction relative to a surface of the medium transportation roller  121   b . More specifically, as shown in  FIG. 2 , the transportation path  124   b  is arranged along a direction perpendicular to the transportation path  124   a . The transportation path  124   a  is provided for transporting the print medium  200 , so that the image formed on the photosensitive drum  114  is temporarily transferred to the transportation belt  123 , and the image transferred to the transportation belt  123  is transferred to the print medium  200  through a secondary transfer process. 
     In the embodiment, when the image forming apparatus  100  is provided with the transportation path  124   b , the transfer units  113  are disposed around the transportation path  124   b  for performing the secondary transfer process. Further, when the image forming apparatus  100  is provided with the transportation path  124   b , and the image forming apparatus  100  forms images in multiple colors, the images in the multiple colors are transferred to and overlapped on the transportation belt  123 , and the images are transferred to the print medium  200  through the secondary transfer process. Note that the mechanic control unit  142  of the control unit  140  (refer to  FIG. 1 ) controls timing of transferring the images. 
     In the embodiment, after the image formed with a part or all of the image forming units  110 B,  110 Y,  110 M, and  110 C is transferred to the print medium  200 , the print medium  200  is separated from the transportation belt  123 , and the image fixing unit  130  (refer to  FIG. 1 ) heats the print medium  200 , thereby fixing the image to the print medium  200 . Afterward, the print medium  200  is discharged outside the image forming apparatus  100 . 
     In the embodiment, the density detection unit  161  is arranged below the transportation belt  123  at a position near the medium transportation roller  121   b . The density detection unit  161  is provided for detecting a density of an image formed on the transportation belt  123  as the density detection pattern (referred to as the density detection pattern). 
     In the embodiment, a shutter  181  is arranged above the density detection unit  161  at a position between the density detection unit  161  and the transportation belt  123 . The shutter  181  is provided for preventing a stain such as toner from being adhered to the density detection unit  161 . A shutter drive unit (not shown) formed of a solenoid or a motor is provided for driving the shutter  181 . When color balance is adjusted (more specifically, the density detection unit  161  detects the density), the shutter  181  is opened. In other occasions, the shutter  181  is closed, so that the shutter  181  blocks between the density detection unit  161  and the transportation belt  123 . Accordingly, the shutter  181  prevents a stain such as toner from being adhered to the density detection unit  161 . 
     An operation of the image forming apparatus  100  for adjusting color balance will be explained next with reference to  FIGS. 3 and 4 .  FIG. 3  is a flow chart showing a method of adjusting color balance applied to the image forming apparatus  100  according to the first embodiment of the present invention.  FIGS. 4(   a ) and  4 ( b ) are schematic views showing the density detection patterns of the image forming apparatus  100  according to the first embodiment of the present invention. 
     As shown in  FIG. 3 , in step S 105 , when the image forming apparatus  100  adjusts color balance, the image forming apparatus  100  prints and detects the density detection pattern. An operation of the image forming apparatus  100  in step S 105  is as follows. 
     In a state that the shutter  181  is closed, the image forming apparatus  100  drives the medium transportation unit  120  to move only the transportation belt  123  without transporting the print medium  200 , and the image forming units  110 B,  110 Y,  110 M, and  110 C are driven. At this moment, the image forming apparatus  100  adjusts the exposure light energy of the exposure unit  111 B,  111 Y,  111 M, and  111 C of the image forming units  110 B,  110 Y,  110 M, and  110 C, respectively. Accordingly, the image forming apparatus  100  sequentially forms the density detection patterns in each color of black (B), yellow (Y), magenta (M), and cyan (C) on the photosensitive drums  114 . Every time the image forming apparatus  100  forms the density detection pattern in each color, the transfer unit  113  transfers (prints) the density detection pattern on the transportation belt  123 . 
     In the embodiment, each of the density detection patterns includes a low duty density detection pattern, a middle duty density detection pattern, and a high duty density detection pattern in each color. A duty represents a toner developing area ratio, that is, a ratio of toner transferred to the transportation belt  123  relative to a specific area. A low duty density represents a range where a print duty is smaller than 50%; a middle duty density represents a range where the print duty is between 30% and 80%; and a high duty density represents a range where the print duty is greater than 60%. 
     Further, the print duty of the low duty density is always smaller than that of the middle duty density, and the print duty of the middle duty density is always smaller than that of the high duty density. In the following description, the low duty density detection pattern, the middle duty density detection pattern, and the high duty density detection pattern are referred to as a low duty portion, a middle duty portion, and a high duty density portion, respectively. 
       FIGS. 4(   a ) and  4 ( b ) are schematic views showing examples of the density detection patterns transferred (printed) on the transportation belt  123  according to the first embodiment of the present invention. 
     As shown in  FIG. 4(   a ), the density detection pattern includes the low density portions in cyan, magenta, yellow, and black; the middle density portions in cyan, magenta, yellow, and black; and the high density portions in cyan, magenta, yellow, and black, in this order. 
     As shown in  FIG. 4(   b ), the density detection pattern includes the low density portion, the middle density portion, and the high density portion in cyan; the low density portion, the middle density portion, and the high density portion in magenta; the low density portion, the middle density portion, and the high density portion in yellow; and the low density portion, the middle density portion, and the high density portion in black, in this order. 
     In the embodiment, the density detection pattern includes the low density portions, the middle density portions, and the high density portions in the orders shown in  FIGS. 4(   a ) and  4 (B). Alternatively, the density detection pattern includes the low density portions, the middle density portions, and the high density portions in a different order. For example, the density detection pattern may include the middle density portion in yellow at first, or the high density portion in black at first. 
     Further, in the embodiment, the image forming apparatus  100  uses toner in the four colors, i.e., black (B), yellow (Y), magenta (M), and cyan (C). Alternatively, the image forming apparatus  100  may use toner in five colors or less than four colors. It is preferred that each of the low duty density detection pattern, the middle duty density detection pattern, and the high duty density detection pattern has a specific density pattern length L (mm) as shown in  FIGS. 4(   a ) and  4 ( b ). 
     After the image forming apparatus  100  transfers (prints) the density detection patterns to the transportation belt  123 , the image forming apparatus  100  opens the shutter  181 , so that the density detection unit  161  detects densities of the density detection patterns. More specifically, when the transportation belt  123  moves, the density detection patterns transferred to the transportation belt  123  pass above the density detection unit  161 . Each time when each of the density detection patterns passes through the density detection unit  161 , the density detection unit  161  detects the density of each of the density detection patterns at an arbitrary timing. Afterwards, the density detection unit  161  transmits the density values of the density detection patterns thus detected to the mechanic control unit  142 . 
     In the next step, when the mechanic control unit  142  receives the density values of the density detection patterns from the density detection unit  161 , the mechanic control unit  142  sequentially stores the density values of the density detection patterns in the storage unit  150  (refer to  FIG. 1 ). After the density values of the density detection patterns in all colors are detected, the image forming apparatus  100  closes the shutter  181 . Accordingly, the image forming apparatus  100  completes the operation of printing and detecting the density detection patterns. 
     When a conventional image forming apparatus includes an exposure unit formed of an LED light source, a few thousands of LEDs are arranged in a direction perpendicular to a moving direction of a transportation belt. When the exposure unit is formed of the LED light source, it is not necessary to dispose a mechanical reflection mirror unit, thereby reducing a size and power consumption of the conventional image forming apparatus. 
     However, in the conventional image forming apparatus using the LED light source, when the LED light source is produced, the LEDs may have variance in an emitting light intensity thereof. When the LEDs have variance in the emitting light intensity, a toner density may have a variance along a transportation direction of a print medium when the print medium is transported during a printing operation. As a result, a lateral streak may be generated in the image printed on the print medium along the transportation direction of the print medium. 
     In the conventional image forming apparatus using the LED light source, in order to prevent such a phenomenon (the generation of the lateral streak), it may be configured to adjust an input current of each of the LEDs, so that the emitting light intensities of the LEDs become uniform. Alternatively, it may be configured to adjust an exposure time of the LEDs for correcting an exposure light energy thereof, or to adjust a developing voltage, a supply voltage, and a charging voltage for correcting a developing energy. 
     However, in the conventional image forming apparatus using the LED light source, when the exposure times of all of the LEDs are uniformly adjusted, a correction balance may be out of a proper balance. Further, in the conventional image forming apparatus using the LED light source, when the developing energy is excessively changed, developer (toner) may be overly charged during the printing operation, or developer may be charged with a polarity opposite to a normal polarity. As a result, in the conventional image forming apparatus using the LED light source, a print quality problem such as a smeared image may occur, thereby deteriorating printed image quality. 
     In the embodiment, in order to prevent such a risk, after step S 105 , the density correction process execution unit  142   a  of the mechanic control unit  142  processes data as shown in  FIG. 5 , so that the density correction process execution unit  142   a  executes the density correction process. More specifically, the density correction process execution unit  142   a  is configured to determine the order of correcting the exposure energy of the exposure unit  111  and the developing energy of the developing unit  112   a , and to calculate the correction values of the exposure light energy of the exposure unit  111  and the developing energy of the developing unit  112   a.    
       FIG. 5  is a schematic view showing a data flow of the method of adjusting color balance applied to the image forming apparatus  100  according to the first embodiment of the present invention. 
     In step S 415 , the density correction process execution unit  142   a  compares a target value of the print density or a target density  405  (refer to  FIG. 5 ) stored in advance in the storage unit  150  for adjusting color balance with a detected value  410  (refer to  FIG. 5 ) of the density of the density detection pattern detected with the density detection unit  161 . Then, the density correction process execution unit  142   a  calculates a difference between the target density  405  and the detected value  410 , thereby identifying a shift value  420  (refer to  FIG. 5 ) of each of the duty portions from the target density  405 . 
     In the embodiment, as described above, the shift value  420  (refer to  FIG. 5 ) of each of the duty portions is identified. Accordingly, it is possible to determine the order of correcting (controlling) the exposure light energy of the exposure unit  111  and the developing energy of the developing unit  112   a , and to calculate the correction values of the exposure light energy of the exposure unit  111  and the developing energy of the developing unit  112   a  (the correction values to be applied to one of the setting values of the exposure light energy of the exposure unit  111  and the developing energy of the developing unit  112   a ). Note that the density difference generation condition is stored in the density difference generation condition storage unit  152  in advance. 
     An example of the target density value  405  and the detected value  410  will be explained next with reference to  FIG. 6 .  FIG. 6  is a graph showing a relationship between the density and the duty of the density detection pattern to determine a difference between the target density value  405  and the detected value  410  according to the first embodiment of the present invention. 
     As shown in  FIG. 6 , there is the difference between the target density value  405  and the detected value  410 . More specifically, a curve A 1  represents the target density value  405 , and a curve A 2  represents the detected value  410  of the density of the density detection pattern. Further, the curve A 2  represents the detected value  410  when the density is detected for the first time, that is, when color balance is not adjusted yet. 
     In step S 425 , the density correction process execution unit  142   a  compares the shift value  420  of each of the duty portions (refer to  FIG. 5 ). Accordingly, the density correction process execution unit  142   a  determines which one of the low duty portion, the middle portion, and the high duty portion has a maximum value of the shift value  420  (referred to as a maximum shift portion). 
     In step S 110  shown in  FIG. 3 , the density correction process execution unit  142   a  determines whether the maximum shift portion is the middle duty portion (the middle portion has the maximum value of the shift value  420 ). In step S 430 , the density correction process execution unit  142   a  determines the order of correcting (controlling) the exposure light energy of the exposure unit  111  and the developing energy of the developing unit  112   a  according to a determination result. 
     In the embodiment, as described above, the density correction process execution unit  142   a  determines the order of correcting (controlling) the exposure light energy of the exposure unit  111  and the developing energy of the developing unit  112   a  according to the determination result. Accordingly, it is possible to prevent the correction from being overly executed upon adjusting color balance when the order of correcting (controlling) the exposure light energy of the exposure unit  111  and the developing energy of the developing unit  112   a  is fixed. In other word, it is possible to prevent the correction value of the light amount of the correction value of the developing voltage from becoming excessively large. 
     In the conventional image forming apparatus using the LED light source, when the exposure energy is controlled, a print dot diameter is increased or decreased, thereby adjusting a distance between dots. In this adjustment, mainly the density of the middle duty portion is corrected, while the densities of the low duty portion and the high duty portion are not significantly corrected. 
     An example of the adjustment described above will be explained with reference to  FIG. 7 .  FIG. 7  is a graph showing a relationship between the density and the duty of the density detection pattern to determine a change in a density characteristic of the density detection pattern after color balance is adjusted through controlling the exposure light energy in the conventional image forming apparatus. 
     More specifically, in  FIG. 7 , a curve B 1  represents the density characteristic of the density detection pattern when the exposure light energy is not corrected; a curve B 2  represents the density characteristic of the density detection pattern when the exposure light energy is increased; and a curve B 3  represents the density characteristic of the density detection pattern when the exposure light energy is decreased. 
     In the conventional image forming apparatus using the LED light source, when the developing energy is controlled, the developing energy is corrected such that the maximum density value is fixed. Through this adjustment, in the conventional image forming apparatus using the LED light source, color balance of an image is adjusted, that is, an amount of developer to be transferred to the print medium is adjusted. In the adjustment, mainly the density of the high duty portion is corrected, while the densities of the low duty portion and the middle duty portion are not significantly corrected. Further, a change amount decreases with the print duty 
     An example of the adjustment described above will be explained with reference to  FIG. 8 .  FIG. 8  is a graph showing a relationship between the density and the duty of the density detection pattern to determine a change in the density characteristic of the density detection pattern after color balance is adjusted through controlling the developing energy in the conventional image forming apparatus. 
     More specifically, in  FIG. 8 , a curve C 1  represents the density characteristic of the density detection pattern when the developing energy is not corrected; a curve C 2  represents the density characteristic of the density detection pattern when the developing energy is increased; and a curve C 3  represents the density characteristic of the density detection pattern when the developing energy is decreased. 
     When the order of controlling the exposure light energy and the developing energy is changed, a final energy correction amount may be different. More specifically, in the conventional image forming apparatus using the LED light source, the final energy correction amount may be different between the case where the exposure light energy is controlled in the first color balance adjustment and the developing energy is controlled in the second color balance adjustment, and the case where the developing energy is controlled in the first color balance adjustment and the exposure light energy is controlled in the second color balance adjustment. In other words, when one of the exposure light energy and the developing energy is controlled first, the other of the exposure light energy and the developing energy may be controlled by an amount different from the case in the reversed order. 
     An example of the adjustment described above will be explained with reference to  FIG. 9 .  FIG. 9  is a graph showing a relationship between the density and the duty of the density detection pattern to determine a change in the density characteristic of the density detection pattern after color balance is adjusted through controlling the exposure light energy and the developing energy in different orders in the conventional image forming apparatus. 
     More specifically, in  FIG. 9 , a curve D 1  represents the density characteristic of the density detection pattern when the developing energy is corrected first; a curve D 2  represents the density characteristic of the density detection pattern when the exposure light energy is corrected first; and a curve D 3  represents the target density (the target value of the density). 
     As described above, in the conventional image forming apparatus using the LED light source, when one of the exposure light energy and the developing energy is controlled in the fixed order for adjusting color balance, color balance may be overly adjusted. In other words, the light amount or the developing voltage may be adjusted by an excessive correction value. As a result, the conventional image forming apparatus may cause a print quality problem, thereby deteriorating printed image quality. 
     In the embodiment, the density correction process execution unit  142   a  determines whether the maximum shift portion is the middle duty portion. Then, the density correction process execution unit  142   a  determines the order of correcting the exposure light energy and the developing energy according to the determination result. Accordingly, in the image forming apparatus  100 , it is possible to prevent the print quality problem, and to mainly correct the duty portion with the density to be corrected. 
     As shown in  FIG. 3 , in step S 110 , the density correction process execution unit  142   a  determines whether the maximum shift portion is the middle duty portion. When the density correction process execution unit  142   a  determines that the maximum shift portion is the middle duty portion in step S 110  (Yes), the density correction process execution unit  142   a  calculates the correction value of the exposure light energy, and stores the correction value in the storage unit  150 . A method of calculating the correction value will be explained later. Alternatively, the density correction control unit  142   b  may calculate the correction value. 
     In step S 115 , the density correction control unit  142   b  adjusts the exposure light energy in the first color balance adjustment before adjusting the developing energy according to the correction value of the exposure light energy stored in the storage unit  150 . 
     In step S 120 , similar to step S 105 , the density correction process execution unit  142   a  prints and detects the density detection pattern. Accordingly, the density correction process execution unit  142   a  detects the density of the density detection pattern after the exposure light energy is adjusted. 
     In the next step, the density correction process execution unit  142   a  (or the density correction control unit  142   b ) calculates the correction value of the developing energy, and stores the correction value in the storage unit  150 . In step S 125 , the density correction control unit  142   b  adjusts the developing energy in the second color balance adjustment according to the correction value of the developing energy stored in the storage unit  150 . 
     When the density correction process execution unit  142   a  determines that the maximum shift portion is not the middle duty portion in step S 110  (No) (that is, the maximum shift portion is the low duty portion or the high density portion), the density correction process execution unit  142   a  (or the density correction control unit  142   b ) calculates the correction value of the developing energy, and stores the correction value in the storage unit  150 . 
     In step S 130 , the density correction control unit  142   b  adjusts the developing energy in the first color balance adjustment before adjusting the exposure light energy according to the correction value of the developing energy stored in the storage unit  150 . 
     In step S 135 , similar to step S 105 , the density correction process execution unit  142   a  prints and detects the density detection pattern. Accordingly, the density correction process execution unit  142   a  detects the density of the density detection pattern after the developing energy is adjusted. 
     In the next step, the density correction process execution unit  142   a  (or the density correction control unit  142   b ) calculates the correction value of the developing energy, and stores the correction value in the storage unit  150 . In step S 140 , the density correction control unit  142   b  adjusts the exposure light energy in the second color balance adjustment according to the correction value of the exposure light energy stored in the storage unit  150 . 
     In the embodiment, when the density correction process execution unit  142   a  (or the density correction control unit  142   b ) adjusts the exposure light energy, the density correction process execution unit  142   a  (or the density correction control unit  142   b ) substitutes the density value thus detected into the following correction value calculation equation (1) to calculate the correction value of the light amount of the static latent image forming unit  125  (the exposure light energy), and stores the correction value in the storage unit  150 . 
     Note that the light amount represents an amount corresponding to the exposure light energy. In this case, the light amount represents an exposure time relative to the photosensitive drum  114 , that is, an exposure time of light irradiated from the LED light source to the photosensitive drum  114 .
 
Correction value of light amount=[{high duty detected value×(low duty target value/high duty target value)−low duty detected value}/ K 1+{high duty detected value×(middle duty target value/high duty target value)−middle duty detected value}/ K 2]/2  (1)
 
     In the embodiment, when the density correction process execution unit  142   a  (or the density correction control unit  142   b ) adjusts the developing energy, the density correction process execution unit  142   a  (or the density correction control unit  142   b ) substitutes the density value thus detected into the following correction value calculation equation (2) to calculate the correction value of the developing voltage of the developing unit  112   a  (the developing energy), and stores the correction value in the storage unit  150 .
 
Correction value of developing voltage={(low duty target value−low duty detected value)× W 1 /K 3+(middle duty target value−middle duty detected value)× W 2 /K 4+(high duty target value−high duty detected value)× W 3 /K 5}/( W 1 +W 2 +W 3)  (2)
 
     In the correction value calculation equations (1) and (2), the coefficient K1 is a density change ratio in the low duty portion per change unit of the light amount; the coefficient K2 is a density change ratio in the middle duty portion per change unit of the light amount; the coefficient K3 is a density change ratio in the low duty portion per change unit of the developing voltage; the coefficient K4 is a density change ratio in the middle duty portion per change unit of the developing voltage; and the coefficient K1 is a density change ratio in the middle duty portion per change unit of the developing voltage. 
     Further, in the correction value calculation equation (2), the coefficient W1 is a weighing coefficient of the low duty portion upon correcting the developing voltage; the coefficient W2 is a weighing coefficient of the middle duty portion upon correcting the developing voltage; and the coefficient W3 is a weighing coefficient of the high duty portion upon correcting the developing voltage. Note that the coefficients W1, W2, and W3 may have an arbitrary value. 
     In the embodiment, with the correction value calculation equation (1), it is possible to average the correction values of the light amount for correcting the light amount (the exposure light energy) in good balance regardless of darkness of the printed image. 
     More specifically, in the correction value calculation equation (1), a K1 dividing calculation portion is defined as “{high duty detected value×(low duty target value/high duty target value)−low duty detected value}/K1”, and has a weighing coefficient of one. Similarly, a K2 dividing calculation portion is defined as “{high duty detected value×(middle duty target value/high duty target value)−middle duty detected value}/K2”, and has a weighing coefficient of one. A sum of the K1 dividing calculation portion and the K2 dividing calculation portion is calculated, and is divided by two, i.e., a sum of the weighing coefficients, thereby averaging the correction values of the light amount. 
     In the embodiment, with the correction value calculation equation (2), it is possible to average the correction values of the developing voltage for correcting the developing voltage (the developing energy) in good balance regardless of darkness of the printed image. 
     More specifically, in the correction value calculation equation (2), a K3 dividing calculation portion is defined as “(low duty target value−low duty detected value)×W1/K3”, and has a weighing coefficient of one. Similarly, a K4 dividing calculation portion is defined as “(middle duty target value−middle duty detected value)×W2/K4”, and has a weighing coefficient of one. Similarly, a K5 dividing calculation portion is defined as “(high duty target value−high duty detected value)×W3/K5”, and has a weighing coefficient of one. A sum of the K3 dividing calculation portion, the K4 dividing calculation portion, and the K5 dividing calculation portion is calculated, and is divided by three, i.e., a sum of the weighing coefficients, thereby averaging the correction values of the developing voltage. 
     In the embodiment, the density correction process execution unit  142   a  determines the order of correcting the exposure light energy of the exposure unit  111  and the developing energy of the developing unit  112   a  individually per each color of developer (toner). Further, it is configured such that each of the exposure light energy of the exposure unit  111  and the developing energy of the developing unit  112   a  is corrected once. Alternatively, it may be configured such that the developing energy of the developing unit  112   a  is corrected twice through performing the correction of the developing energy according to the correction of the exposure light energy. 
     In the embodiment, it is configured such that both of the exposure light energy of the exposure unit  111  and the developing energy of the developing unit  112   a  are corrected. Alternatively, it may be configured such that one of the exposure light energy of the exposure unit  111  and the developing energy of the developing unit  112   a  is not corrected when it is determined that it is not necessary to perform the correction of one of the exposure light energy and the developing energy after the other of the exposure light energy and the developing energy is corrected. 
     As described above, when the correction in the high duty portion is mainly performed, even though the correction in the middle duty portion should be performed, the developing energy may be corrected by an excessively large correction value, thereby causing a print quality problem such as a smeared image. In the image forming apparatus  100 , it is possible to prevent such a print quality problem. 
     Further, when the correction in the middle duty portion is mainly performed, even though the correction in the high duty portion should be performed, the exposure light energy may be corrected by an excessively large correction value. As a result, the LED light source may have variance in an emitting light intensity, and a lateral streak may be generated during the printing operation. In the image forming apparatus  100 , it is possible to prevent such a print quality problem. Accordingly, it is possible to prevent printed image quality from deteriorating due to the over correction. 
     Second Embodiment 
     A second embodiment of the present invention will be explained next. In the second embodiment, even when the density correction process execution unit  142   a  determines that the control of the exposure light energy is performed first, if it is considered that a lateral streak may be generated, it is configured such that the control of the developing energy, not the exposure light energy, is performed first. 
     A configuration of an image forming apparatus  100   a  according to the second embodiment of the present invention will be explained with reference to  FIG. 10 .  FIG. 10  is a block diagram showing the configuration of the image forming apparatus  100   a  according to the second embodiment of the present invention. 
     As shown in  FIG. 10 , different from the image forming apparatus  100  in the first embodiment, the image forming apparatus  100   a  includes a density correction order determining unit  142   c  in the mechanic control unit  142 . 
     In the embodiment, the density correction order determining unit  142   c  is provided for changing the order of the control of the exposure light energy and the developing energy determined with the mechanic control unit  142  according to a predetermined condition. 
     More specifically, even when the density correction process execution unit  142   a  determines that the control of the exposure light energy is performed first, the density correction order determining unit  142   c  changes the order of the control of the exposure light energy and the developing energy, so that the control of the developing energy, not the exposure light energy, is performed first when the correction value of the exposure light energy deviates significantly from a specific value, for example, a limit value defined as a 15% value in advance. 
     In the second embodiment, different from the image forming apparatus  100  in the first embodiment, the image forming apparatus  100   a  performs a different operation when the control of the exposure light energy is performed and the correction value of the exposure light energy reaches the limit value, i.e., an exposure light energy correction limit value. 
     An operation of the image forming apparatus  100   a  for adjusting color balance will be explained next with reference to  FIGS. 11 and 12 .  FIG. 11  is a flow chart showing a method of adjusting color balance applied to the image forming apparatus  100   a  according to the second embodiment of the present invention.  FIG. 12  is a schematic view showing a data flow of the method of adjusting color balance applied to the image forming apparatus  100   a  according to the second embodiment of the present invention. 
     In the following description, features of the operation of the image forming apparatus  100   a  different from those of the operation of the image forming apparatus  100  in the first embodiment are mainly explained. Other features of the operation of the image forming apparatus  100   a  similar to those of the operation of the image forming apparatus  100  in the first embodiment are considered to be the same, and explanations thereof are omitted. 
     As shown in  FIG. 11 , the image forming apparatus  100   a  performs step S 116  and step S 117  between step S 115  (the control of the exposure light energy) and step S 120  (the printing and detection of the density detection pattern). In this case, in the image forming apparatus  100   a , the density correction process execution unit  142   a  (or the density correction control unit  142   b ) performs the correction value calculation (refer to step S 416  in  FIG. 12 ) to calculate a correction value  421  of the exposure light energy (refer to  FIG. 12 ), and stores the correction value  421  in the storage unit  150 . 
     More specifically, after the image forming apparatus  100   a  performs step S 115 , the density correction order determining unit  142   c  refers to the specific value stored in the exposure light energy correction limit value storage unit  151  in advance to compare the specific value with the correction value  421  of the exposure light energy stored in the storage unit  150  and calculated in step S 115 . Accordingly, in step S 116 , the density correction order determining unit  142   c  determines whether the correction value  421  of the exposure light energy is less than the specific value, for example, 15% in the second embodiment. 
     In step S 117 , when the density correction order determining unit  142   c  determines that the correction value  421  of the exposure light energy is less than the specific value in step S 116  (Yes), the density correction order determining unit  142   c  adopts the correction value  421  for the control of the exposure light energy, that is, the control of the exposure light energy is performed first. 
     When the density correction order determining unit  142   c  determines that the correction value  421  of the exposure light energy is not less than the specific value in step S 116  (No), the density correction order determining unit  142   c  determines that the control of the developing energy is performed first. In this case, the process proceeds to step  5130 . In step S 130 , the density correction control unit  142   b  performs the control of the developing energy. 
     In the embodiment, the specific value is not limited to 15%, and may be set arbitrarily. When the amount of the exposure light energy is changed, the LED light source tends to have a variance in the emitting light intensity, thereby causing a lateral streak during the printing operation. 
     In the second embodiment, when the exposure time of the LED light source is changed (corrected) to a large extent, a lateral streak tends to be generated more easily. Accordingly, the specific value, i.e., a threshold value relative to the correction value of the exposure light energy, is set to 15%, and may be adjusted according to an operational environment of the image forming apparatus  100   a , a type of print data, and the like. 
     In the second embodiment, the exposure unit  111  stores a reference condition corresponding to each of LED elements for correcting an emitted light amount to compensate a variance in the LED elements due to manufacturing variances. The reference condition corresponding to each of the LED elements is sent to the mechanic control unit  142  through the exposure unit interface unit  172 . Accordingly, the mechanic control unit  142  determines a light emitting time of each of the LED elements, a voltage to be applied to each of the LED elements, and a current to be applied to each of the LED elements according to the reference condition corresponding to each of the LED elements, so that a toner image with a specific toner density (for example, a 100% duty image) is formed under an ideal operational environment without correction. 
     In this case, when the exposure unit  111  emits light through the exposure unit interface unit  172 , the emitted light energy of each of the LED elements is defined as 100% of the emitted light energy as the specific value. Accordingly, the mechanic control unit  142  determines whether the correction less than 15% (the correction within 85% to 115% of the emitted light energy) is applied to the specific value, i.e., 100% of the emitted light energy of each of the LED elements, as the correction of the current operational environment in order to form the toner image with the specific toner density (for example, a 100% duty image). 
     In the image forming apparatus  100   a  in the second embodiment, similar to the image forming apparatus  100  in the first embodiment, the density correction process execution unit  142   a  determines the order of correcting the exposure light energy of the exposure unit  111  and the developing energy of the developing unit  112   a  individually per each color of developer (toner). Further, similar to the image forming apparatus  100  in the first embodiment, it may be configured such that the developing energy of the developing unit  112   a  is corrected twice through performing the correction of the developing energy according to the correction of the exposure light energy. 
     As described above, when the correction in the high duty portion is mainly performed, even though the correction in the middle duty portion should be performed, the developing energy may be corrected by an excessively large correction value, thereby causing a print quality problem such as a smeared image. In the image forming apparatus  100   a  in the second embodiment, similar to the image forming apparatus  100  in the first embodiment, it is possible to prevent such a print quality problem. 
     Further, when the correction in the middle duty portion is mainly performed, even though the correction in the high duty portion should be performed, the exposure light energy may be corrected by an excessively large correction value. As a result, the LED light source may have variance in an emitting light intensity, and a lateral streak may be generated during the printing operation. In the image forming apparatus  100   a  in the second embodiment, similar to the image forming apparatus  100  in the first embodiment, it is possible to prevent such a print quality problem. Accordingly, it is possible to prevent printed image quality from deteriorating due to the over correction. 
     Further, in the second embodiment, even when the density correction process execution unit  142   a  determines that the control of the exposure light energy is performed first, if it is considered that a lateral streak may be generated (that is, the correction value of the exposure light energy deviates significantly from the specific value set in advance), it is configured such that the control of the developing energy, not the exposure light energy, is performed first. Accordingly, as compared with the image forming apparatus  100  in the first embodiment, it is possible to more accurately prevent print quality problem. 
     Third Embodiment 
     A third embodiment of the present invention will be explained next. 
     In a conventional image forming apparatus using a LED light source, a limit value of a correction value of the exposure energy light is determined in advance for controlling the exposure light energy. Accordingly, when the exposure light energy is controlled, the correction value of the exposure light energy is calculated such that the correction value of the exposure light energy is within a range from a target value by the limit value, thereby controlling the exposure light energy according to the correction value. 
     As described above, in the conventional image forming apparatus using the LED light source, the exposure light energy is controlled within the range of the limit value. Accordingly, when the developing energy is controlled after the exposure light energy is controlled, the correction value of the developing energy may be excessively increased. As a result, in the conventional image forming apparatus using the LED light source, developer (toner) may be overly charged during the printing operation, or developer may be charged with a polarity opposite to a normal polarity. As a result, a print quality problem such as a smeared image may happen, thereby deteriorating printed image quality. Further, the correction value of the exposure light energy reaches the limit value, so that a lateral streak tends to be generated in the printing operation. 
     In the third embodiment, the developing energy is corrected twice through performing the correction of the developing energy according to the correction of the exposure light energy. Accordingly, it is possible to prevent the risks described above. 
     In an image forming apparatus  100   b  in the third embodiment, different from the image forming apparatus  100  in the first embodiment and the image forming apparatus  100   a  in the second embodiment, a different operation is performed when the exposure light energy is controlled and the correction value of the exposure light energy reaches the exposure light energy correction limit value as the limit value. 
     A configuration of the image forming apparatus  100   b  according to the third embodiment of the present invention will be explained with reference to  FIG. 13 .  FIG. 13  is a block diagram showing the configuration of the image forming apparatus  100   b  according to the third embodiment of the present invention. 
     As shown in  FIG. 13 , different from the image forming apparatus  100   a  in the second embodiment, the image forming apparatus  100   b  includes an exposure light energy correction value changing unit  142   d  in the mechanic control unit  142 . Further, the image forming apparatus  100   b  includes the exposure light energy correction limit value storage unit  151  in the storage unit  150 . 
     In the embodiment, the exposure light energy correction value changing unit  142   d  is provided for changing the correction value of the exposure light energy calculated with the density correction process execution unit  142   a  (or the density correction order determining unit  142   c ). More specifically, the exposure light energy correction value changing unit  142   d  determines whether the correction value of the exposure light energy is greater than a specific value, for example, a limit value set as 15% in advance in the third embodiment. 
     When the exposure light energy correction value changing unit  142   d  determines that the correction value of the exposure light energy is greater than the specific value, the exposure light energy correction value changing unit  142   d  changes the correction value of the exposure light energy. 
     In the embodiment, the exposure light energy correction value changing unit  142   d  changes the correction value of the exposure light energy to 50% of the correction value of the exposure light energy calculated. 
     An operation of the image forming apparatus  100   b  for adjusting color balance will be explained next with reference to  FIGS. 14 and 15 .  FIG. 14  is a flow chart showing a method of adjusting color balance applied to the image forming apparatus  100   b  according to the third embodiment of the present invention.  FIG. 15  is a schematic view showing a data flow of the method of adjusting color balance applied to the image forming apparatus  100   b  according to the third embodiment of the present invention. 
     In the following description, features of the operation of the image forming apparatus  100   b  different from those of the operation of the image forming apparatus  100  in the first embodiment and the image forming apparatus  100   a  in the second embodiment are mainly explained. Other features of the operation of the image forming apparatus  100   b  similar to those of the operation of the image forming apparatus  100  in the first embodiment and the image forming apparatus  100   a  in the second embodiment are considered to be the same, and explanations thereof are omitted. Further, in  FIG. 14 , steps similar to those in  FIG. 11  are designated with the same reference numerals with characters a or b, and explanations thereof are omitted. 
     As shown in  FIG. 14 , the image forming apparatus  100   b  performs step S 118  (the control of the developing energy) between step S 117  (the adoption of the correction value  421  for the control of the exposure light energy) and step S 120  (the printing and detection of the density detection pattern). More specifically, after the density correction order determining unit  142   c  adopts the correction value  421  for the control of the exposure light energy in step S 117 , in step S 118 , the image forming apparatus  100   b  performs the control of the developing energy for the first time. Further, in step S 125   a , the image forming apparatus  100   b  performs the control of the developing energy for the second time. 
     It is supposed that, at this moment, in the image forming apparatus  100   b  in the embodiment, similar to the image forming apparatus  100   a  in the second embodiment, in step S 115 , the density correction process execution unit  142   a  (or the density correction control unit  142   b ) performs the correction value calculation (refer to step S 416  in  FIG. 15 ) to calculate the correction value  421  of the exposure light energy (refer to  FIG. 15 ), and stores the correction value  421  in the storage unit  150 . 
     When the density correction process execution unit  142   a  determines that the maximum shift portion is not the middle duty portion in step S 110  (No), similar to step S 130 , the image forming unit  110   b  performs the control of the developing energy before performing the control of the exposure light energy in step S 130   a . Further, similar to step S 135  (refer to  FIG. 3 ), the density correction process execution unit  142   a  prints and detects the density detection pattern. Accordingly, the density correction process execution unit  142   a  detects the density of the density detection pattern after the developing energy is adjusted. 
     Afterward, the image forming apparatus  100   b  concurrently performs the control of the exposure light energy and the control of the developing energy. More specifically, in step S 136   a , similar to step S 130  (refer to  FIG. 3 ), the image forming apparatus  100   b  performs the control of the developing energy for the second time. Further, in step S 140   a , similar to step S 140  (refer to  FIG. 3 ), the image forming apparatus  100   b  performs the control of the exposure light energy. 
     When the density correction order determining unit  142   c  determines that the correction of the exposure light energy is not less than the specific value in step S 116  (No), similar to step S 130   a , the image forming unit  110   b  performs the control of the developing energy before performing the control of the exposure light energy in step S 130   b . Further, similar to step S 135   a , the density correction process execution unit  142   a  prints and detects the density detection pattern in step S 135   b . Accordingly, the density correction process execution unit  142   a  detects the density of the density detection pattern after the developing energy is adjusted. 
     Afterward, the image forming apparatus  100   b  concurrently performs the control of the exposure light energy and the control of the developing energy. More specifically, in step S 136   b , similar to step S 136   a , the image forming apparatus  100   b  performs the control of the developing energy for the second time. Further, in step S 140   b , similar to step S 140   a , the image forming apparatus  100   b  performs the control of the exposure light energy. 
     In the embodiment, in step S 140   b , the exposure light energy correction value changing unit  142   d  of the image forming apparatus  100   b  performs the control of the exposure light energy according to a new correction value different from the correction value used in step S 115  (referred to as the correction value ( 1 )  421  of the exposure light energy shown in  FIG. 15 ). 
     More specifically, in step S 140   b , the exposure light energy correction value changing unit  142   d  compares a specific value or the limit value set in advance, for example, 15% in the third embodiment, with the correction value ( 1 )  421  of the exposure light energy used in step S 115 . Accordingly, the exposure light energy correction value changing unit  142   d  determines whether the correction value ( 1 )  421  of the exposure light energy is greater than the specific value (refer to step S 426  in  FIG. 15 ). 
     When the exposure light energy correction value changing unit  142   d  determines whether the correction value ( 1 )  421  of the exposure light energy is greater than the specific value, the exposure light energy correction value changing unit  142   d  determines to change from the correction value ( 1 )  421  to the new correction value (refer to step S 427  in  FIG. 15 ). 
     When the exposure light energy correction value changing unit  142   d  determines to change to the new correction value (referred to as a correction value ( 2 )  431  shown in  FIG. 15 ), the exposure light energy correction value changing unit  142   d  calculates the correction value ( 2 )  431  less than the correction value ( 1 )  421  (more specifically, 50% of the correction value ( 1 )  421 ). Afterward, the image forming apparatus  100   b  performs the control of the exposure light energy according to the new correction value, i.e., the correction value ( 2 )  431  of the exposure light energy. 
     In the embodiment, the correction value ( 2 )  431  of the exposure light energy is not limited to 50% of the correction value ( 1 )  421 , and may be set arbitrarily according to the specific value, i.e., the limit value set in advance, for example, 15% in the third embodiment, and a control unit, i.e., a ratio defining an amount of the exposure light energy to be change per which %. 
     For example, when the specific value is set to a relatively large value such as when it is determined to be acceptable through an experiment and the like even when the amount of the exposure light energy is changed to a relatively large extent, the correction value ( 2 )  431  of the exposure light energy may exceed 50% of the correction value ( 1 )  421 . 
     On the other hand, when the specific value is set to a relatively small value such as when it is determined to cause a risk of an image quality problem when the amount of the exposure light energy is changed, the correction value ( 2 )  431  of the exposure light energy should be less than 50% of the correction value ( 1 )  421 . 
     In the image forming apparatus  100   b  in the third embodiment, similar to the image forming apparatus  100   a  in the second embodiment, the density correction process execution unit  142   a  determines the order of correcting the exposure light energy of the exposure unit  111  and the developing energy of the developing unit  112   a  individually per each color of developer (toner). 
     As described above, in the image forming apparatus  100   b  in the third embodiment, in addition to the effects of the image forming apparatus  100   a  in the second embodiment, even when developer (toner) is overly charged during the printing operation, or developer is charged with a polarity opposite to a normal polarity, it is possible to prevent printed image quality from deteriorating and a lateral streak. 
     The disclosure of Japanese Patent Application No. 2009-156514, filed on Jul. 1, 2009, is incorporated in the application. 
     While the invention has been explained with reference to the specific embodiments of the invention, the explanation is illustrative and the invention is limited only by the appended claims.