Abstract:
A developing device includes: a plurality of developer retaining bodies that retain developer and rotate, and that respectively convey the developer, which is supplied to each of the developer retaining bodies from a developer supply section, to an image bearing body that rotates and bears a latent image; and a speed changing unit that is adapted to change a rotation speed of at least one of the developer retaining bodies excluding a developer retaining body that is furthest downstream side in a direction of rotation of the image bearing body.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2010-57685 filed on Mar. 15, 2010. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to a developing device and an image forming device. 
     2. Related Art 
     Heretofore, a developing device has been known in which a circumferential speed of a photoreceptor is switchable between two or more levels of circumferential speed within a pre-specified range, and sleeves of a forward developing roller and a backward developing roller of a developing device are turned by respectively separate drive systems and variable speed motors. If the circumferential speed of the sleeve of the forward developing roller is represented by Va (m/s), the circumferential speed of the sleeve of the backward developing roller is represented by Vw (m/s) and the circumferential speed of the photoreceptor is represented by Vp (m/s), the circumferential speeds of the sleeves of the forward developing roller and the backward developing roller are respectively independently changed in accordance with switching of the circumferential speed of the photoreceptor such that Va, Vw and Vp satisfy a pre-specified relationship. 
     SUMMARY 
     A developing device relating to a first aspect of the present invention is configured to include: a plurality of developer retaining bodies that retain developer and rotate, and that respectively convey the developer, which is supplied to each of the developer retaining bodies from a developer supply section, to an image bearing body that rotates and bears a latent image; and a speed changing unit that is adapted to change a rotation speed of at least one of the developer retaining bodies excluding a developer retaining body that is furthest downstream side in a direction of rotation of the image bearing body. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is an overall structural diagram of a printer relating to a first exemplary embodiment of the present invention; 
         FIG. 2  is a sectional diagram of a developing section of the printer relating to the first exemplary embodiment of the present invention; 
         FIG. 3  is a block diagram illustrating structure of a control unit relating to the first exemplary embodiment of the present invention; 
         FIG. 4  is a flowchart illustrating details of a development field control processing routine of the control unit relating to the first exemplary embodiment of the present invention; 
         FIG. 5  is a graph illustrating a relationship between the development field and developed toner amounts; 
         FIG. 6  is a graph illustrating a relationship between the development field and numbers of white spots; 
         FIG. 7  is a graph illustrating a relationship between a ratio of the development field to a cleaning field and fine line density; 
         FIG. 8  is a graph illustrating a relationship between the cleaning field and a fogging grade; 
         FIG. 9  is a graph illustrating a relationship between the development field and a degree of density unevenness; 
         FIG. 10  is a graph illustrating a relationship between circumferential speeds of first and second developing rollers and longitudinal stripes; 
         FIG. 11  is a graph illustrating a relationship between circumferential speeds of first and second developing rollers and density unevenness; 
         FIG. 12  is a block diagram illustrating structure of a control unit relating to a second exemplary embodiment of the present invention; 
         FIG. 13  is a flowchart illustrating details of a development field control processing routine of the control unit relating to the second exemplary embodiment of the present invention; 
         FIG. 14  is a flowchart illustrating details of a development field control processing routine of a control unit relating to a third exemplary embodiment of the present invention; 
         FIG. 15  is a flowchart illustrating details of a development field control processing routine of a control unit relating to a fourth exemplary embodiment of the present invention; 
         FIG. 16  is a flowchart illustrating details of a development field control processing routine of a control unit relating to a fifth exemplary embodiment of the present invention; 
         FIG. 17  is a flowchart illustrating details of a development field control processing routine of a control unit relating to a sixth exemplary embodiment of the present invention; 
         FIG. 18  is a block diagram illustrating structure of a control unit relating to a seventh exemplary embodiment of the present invention; 
         FIG. 19  is a flowchart illustrating details of a development field control processing routine of the control unit relating to the seventh exemplary embodiment of the present invention; and 
         FIG. 20  is a sectional diagram of a developing section of a printer relating to an eighth exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Herebelow, an example of an exemplary embodiment relating to the present invention is described on the basis of the attached drawings. 
     Structure of Printer Relating to the Present Exemplary Embodiment 
     First, the structure of a printer relating to the present exemplary embodiment is described.  FIG. 1  illustrates a printer  10  that serves as an example of an image forming device. 
     The printer  10  is a digital printer that forms a full-color image or a black-and-white image. An image processing device (not illustrated) is provided inside the printer  10 . The image processing device applies image processing to image data that is sent thereto from a personal computer or the like. 
     As illustrated in  FIG. 1 , toner cartridges  11 Y,  11 M,  11 C and  11 K that accommodate respective toners of yellow (Y), magenta (M), cyan (C) and black (K) are replaceably provided in an upper portion of the printer  10 . In the following descriptions, the symbols Y, M, C and K are appended to the reference numerals of members corresponding to the colors yellow, magenta, cyan and black to distinguish therebetween. 
     One ends of toner supply channels  13 Y,  13 M,  13 C and  13 K are connected to the toner cartridges  11 Y,  11 M,  11 C and  11 K, respectively. The toner supply channels  13 Y,  13 M,  13 C and  13 K are structured by piping and are disposed to be oriented downward along a side face of the printer  10 . Intermediate paths thereof are not illustrated. 
     Four image forming units  12  ( 12 Y,  12 M,  12 C and  12 K), corresponding to the developers of Y, M, C and K, are disposed in a central portion of the printer  10 , in an arrangement along a direction diagonally downward to the right in a front view. The developers are two-component developers in which a non-magnetic type of toner and a carrier with magnetism are mixed. The other ends of the toner supply channels  13 Y,  13 M,  13 C and  13 K are connected to the image forming units  12 Y,  12 M,  12 C and  12 K, respectively, so as to supply the toners of the respective colors to the respective image forming units  12 . 
     A transfer section  14  is provided above the image forming units  12 Y,  12 M,  12 C and  12 K. The transfer section  14  includes an intermediate transfer belt  16 , first transfer rollers  18 Y,  18 M,  18 C and  18 K, and a second transfer roller  20 . The intermediate transfer belt  16  serves as an example of an intermediate transfer body. The first transfer rollers  18 Y,  18 M,  18 C and  18 K are disposed inside the intermediate transfer belt  16  and serve as examples of four first transfer members that superposingly transfer respective toner images onto the intermediate transfer belt  16 . The second transfer roller  20  transfers the toner images superposed on the intermediate transfer belt  16  onto recording paper P. 
     The intermediate transfer belt  16  is wound with a certain level of tension around a driving roller  22 , a tensioning roller  24  and a support roller  26 . The driving roller  22  is driven by an unillustrated motor. The tensioning roller  24  regulates the tension of the intermediate transfer belt  16 . The support roller  26  is disposed to oppose the second transfer roller  20 . The intermediate transfer belt  16  is driven to turn in the direction of arrow X (the anticlockwise direction) in  FIG. 1  by the driving roller  22 . 
     The first transfer rollers  18 Y,  18 M,  18 C and  18 K are disposed to oppose below-described photoreceptors  28  ( 28 Y,  28 M,  28 C and  28 K) of the image forming units  12 Y,  12 M,  12 C and  12 K, respectively, sandwiching the intermediate transfer belt  16 . Transfer bias voltages are applied to the first transfer rollers  18 Y,  18 M,  18 C and  18 K by a power supply unit (not illustrated). The transfer bias voltages have the opposite polarity from the toner polarity (for example, positive polarity in the present exemplary embodiment). A transfer bias voltage of the opposite polarity to the toner polarity is also applied by the power supply unit to the second transfer roller  20 . 
     A cleaning device  30  is provided at the outer peripheral face of the intermediate transfer belt  16 , at a position at which the driving roller  22  is disposed. The cleaning device  30  is provided with a cleaning brush  32  and a cleaning blade  34 , and removes residual toner, paper dust and the like on the intermediate transfer belt  16  with the cleaning brush  32  and the cleaning blade  34 . 
     A control unit  36  is provided in the vicinity of a side face of the printer  10  at the opposite side thereof from a conveyance path of the recording paper P. The control unit  36  performs driving control of the respective sections of the printer  10 . An exposure unit  40  is provided at the lower side of the image forming units  12 . The exposure unit  40  illuminates exposure lights L corresponding to the respective colors (LY, LM, LC and LK) at electrostatically charged surfaces of the photoreceptors  28  and forms electrostatic latent images. 
     The exposure unit  40  is structured by a single unit common to the four image forming units  12 Y,  12 M,  12 C and  12 K. The exposure unit  40  is configured to modulate four semiconductor lasers (not illustrated) in accordance with colorant gradation data of the respective colors and emit the exposure lights LY, LM, LC and LK from the semiconductor lasers in accordance with the gradation data. The exposure unit  40  may be provided separately for each of the image forming units  12 . 
     The exposure unit  40  is enclosed in a cuboid frame  38 . Inside the exposure unit  40 , an f−θ lens (not illustrated) and a polygon mirror  42  are provided for scanning the exposure lights L in a main scan direction. Glass windows  44 Y,  44 M,  44 C and  44 K are provided in the top face of the frame  38  for emitting the four exposure lights LY, LM, LC and LK towards the photoreceptors  28  of the image forming units  12 Y,  12 M,  12 C and  12 K. 
     The exposure lights LY, LM, LC and LK emitted from the semiconductor lasers of the exposure unit  40  are irradiated through the f−θ lens at the polygon mirror  42 , and are deflected and scanned by the polygon mirror  42 . The exposure lights LY, LM, LC and LK that have been deflected and scanned by the polygon mirror  42  pass through optical systems (not illustrated) constituted with focusing lenses and plural mirrors, and are scanningly exposed onto exposure points on the photoreceptors  28  from diagonally beneath. 
     A paper supply cassette  46  that accommodates the recording paper P is disposed at the lower side of the exposure unit  40 . A paper supply conveyance path  50  is provided that conveys the recording paper P upward in a vertical direction from an end portion of the paper supply cassette  46 . 
     The paper supply conveyance path  50  is provided with a feed roller  48 , a roller pair  52  for paper separation and conveyance, and paper leading end positioning rollers  54 . The feed roller  48  feeds the recording paper P out from the paper supply cassette  46 . The roller pair  52  supplies the recording paper P one sheet at a time. The paper leading end positioning rollers  54  match a timing of movement of an image on the intermediate transfer belt  16  with a timing of conveyance of the recording paper P. The recording paper P that is sequentially fed out from the paper supply cassette  46  by the feed roller  48  passes along the paper supply conveyance path  50 , is temporarily conveyed to a second transfer position of the intermediate transfer belt  16  by the paper leading end positioning rollers  54  turning intermittently, and is stopped. 
     A fixing device  60  is disposed above the second transfer roller  20 . The fixing device  60  is provided with a heating roller  62 , which is heated, and a pressure roller  64 , which is pressed against the heating roller  62 . The recording paper P to which the toner image of the respective colors has been transferred by the second transfer roller  20  is subjected to fixing by heat and pressure in a contact portion between the heating roller  62  and the pressure roller  64 . The recording paper P is then ejected to an ejection section  68  by ejection rollers  66  provided at the recording paper P conveyance direction downstream side. The ejection rollers  66  serve as an example of an ejection device. The ejection section  68  is provided at a top portion of the printer  10 . Meanwhile, the surface of the intermediate transfer belt  16  for which the second transfer processing of the toner image has been completed is cleared of residual toner, paper dust and the like by the cleaning device  30 . 
     Next, the image forming units  12  are described. As an example, the image forming unit  12 M is described here. The image forming units  12 Y,  12 C and  12 K corresponding to the other colors have the same structure as the image forming unit  12 M, so will not be described. The structural members of the image forming unit  12 M are represented with the reference symbol M being omitted. 
     As illustrated in  FIG. 2 , the image forming unit  12  is provided with the photoreceptor  28 , which is driven to turn in the direction of arrow A (clockwise). Around the photoreceptor  28 , a charging roller (not illustrated), a developing section  70 , an erasure lamp (not illustrated) and a cleaning unit (not illustrated) are provided. The charging roller serves as an example of an electrostatic charging device that touches against a surface of the photoreceptor  28  and uniformly charges the photoreceptor  28 . The developing section  70  is an example of a developing unit that develops the electrostatic latent image formed on the photoreceptor  28  by the aforementioned exposure light L with a developer (toner) of the respective color. The erasure lamp is an example of a de-electrification device that illuminates light at the surface of the photoreceptor  28  after transfer and de-electrifies the surface. The cleaning unit cleans the surface of the photoreceptor  28  after the de-electrification. 
     The charging roller (not illustrated), developing section  70 , erasure lamp (not illustrated) and cleaning unit (not illustrated) are disposed to oppose the surface of the photoreceptor  28  in this order from the turning direction upstream side to the downstream side. 
     Next, an image forming process of the printer  10  is described. 
     As illustrated in  FIG. 1 , image data that has been subjected to image processing by the image processing device (not illustrated) is converted to colorant gradation data of the four colors yellow (Y), magenta (M), cyan (C) and black (K), and sequentially outputted to the exposure unit  40 . From the exposure unit  40 , the respective exposure lights L corresponding to the colorant gradation data of the respective colors are emitted, scanning exposure onto the photoreceptors  28  is implemented, and latent images (electrostatic latent images) are formed. 
     The electrostatic latent images formed on the photoreceptors  28  are manifested and developed as toner images (developer images) of the colors yellow (Y), magenta (M), cyan (C) and black (K) by the developing sections  70 , as illustrated in  FIG. 1  and  FIG. 2 . Then, the toner images of the respective colors formed on the photoreceptors  28  of the image forming units  12 Y,  12 M,  12 C and  12 K are sequentially superposedly transferred onto the intermediate transfer belt  16  by the four first transfer rollers  18 Y,  18 M,  18 C and  18 K. 
     The toner images of the respective colors that have been superposedly transferred onto the intermediate transfer belt  16  are second-transferred by the second transfer roller  20  onto the recording paper P that has been conveyed thereto. Then, the toner image of the respective colors on the recording paper P is fixed by the fixing device  60 , and after fixing the recording paper P is ejected to the ejection section  68 . 
     After the transfer process of the toner image has been completed, the surface of the photoreceptor  28  has residual toner, paper dust and the like removed therefrom by a cleaning unit  76 . Residual toner, paper dust and the like on the intermediate transfer belt  16  is removed by the cleaning device  30 . 
     Structure of the Developing Section  70   
     Next, structure of the developing section  70 , which serves as an example of a developing device, is described. 
     As illustrated in  FIG. 2 , the developing section  70  is provided with a casing  81 , inside which two developer accommodation chambers  80 A and  80 B that accommodate a developer G are formed. 
     As illustrated in  FIG. 2 , agitation and conveyance paths  84 A and  84 B that agitate (mix) and convey the developer G are formed in the developer accommodation chambers  80 A and  80 B. 
     The agitation and conveyance path  84 A is partitioned by a partition wall  93 A provided standing from a bottom face and two agitation paths are provided, a first agitation path  84 C and a second agitation path  84 D. A first aperture and a second aperture (not illustrated) are formed at positions at each of two end portions of the partition wall  93 A. The first agitation path  84 C and the second agitation path  84 D communicate through the first aperture and the second aperture. 
     A toner supply aperture (not illustrated) is formed in the first agitation path  84 C. The other end of the aforementioned toner supply channel  13 M (see  FIG. 1 ) is connected to this toner supply aperture. Accordingly, toner from the toner cartridge  11 M flows down through the toner supply channel  13 M and is supplied to the image forming unit  12 M (the developing section  70 ). 
     A first agitation and conveyance member  91 A is disposed in the first agitation path  84 C. The first agitation and conveyance member  91 A is structured by a first shaft portion (not illustrated), which is turnably supported at the casing  81 , and a helical first vane portion (not illustrated) provided around the first shaft portion. Similarly, a second agitation and conveyance member  92 A is disposed in the second agitation path  84 D. The second agitation and conveyance member  92 A is structured by a second shaft portion (not illustrated), which is turnably supported at the casing  81 , and a helical second vane portion (not illustrated) provided around the second shaft portion. 
     When the first shaft portion and the second shaft portion are respectively turned, the developer G in the agitation and conveyance path  84 A is mixed with the toner that is supplied, is conveyed while being agitated and mixed both in the first agitation path  84 C and in the second agitation path  84 D, and is circulated between the first agitation path  84 C and the second agitation path  84 D. 
     Similarly to the agitation and conveyance path  84 A, the agitation and conveyance path  848  is partitioned by a partition wall  93 B provided standing from a bottom face and two agitation paths are provided, a first agitation path  84 E and a second agitation path  84 F. A first aperture and a second aperture (not illustrated) are formed at positions at each of two end portions of the partition wall  93 B. The first agitation path  84 E and the second agitation path  84 F communicate through the first aperture and the second aperture. 
     A toner supply aperture (not illustrated) is formed in the first agitation path  84 E. The other end of the aforementioned toner supply channel  13 M (see  FIG. 1 ) is connected to this toner supply aperture. Accordingly, toner from the toner cartridge  11 M flows down through the toner supply channel  13 M and is supplied to the image forming unit  12 M (the developing section  70 ). 
     A first agitation and conveyance member  91 B is disposed in the first agitation path  84 E. The first agitation and conveyance member  91 B is structured by a first shaft portion (not illustrated), which is turnably supported at the casing  81 , and a helical first vane portion (not illustrated) provided around the first shaft portion. Similarly, a second agitation and conveyance member  92 B is disposed in the second agitation path  84 F. The second agitation and conveyance member  92 B is structured by a second shaft portion (not illustrated), which is turnably supported at the casing  81 , and a helical second vane portion (not illustrated) provided around the second shaft portion. 
     When the first shaft portion and the second shaft portion are respectively turned, the developer G in the agitation and conveyance path  84 B is mixed with the toner that is supplied, is conveyed while being agitated and mixed both in the first agitation path  84 E and in the second agitation path  84 F, and is circulated between the first agitation path  84 E and the second agitation path  84 F. 
     As illustrated in  FIG. 2 , an aperture portion  98  is formed in a side wall at the photoreceptor  28  side of the casing  81 . A developing roller  70 A is disposed in the developer accommodation chamber  80 A. The developing roller  70 A has its axial direction along the length direction of the photoreceptor  28  and turns in the direction of arrow B (the clockwise direction). 
     A developing roller  70 B is disposed in the developer accommodation chamber  80 B. The developing roller  70 B has its axial direction along the length direction of the photoreceptor  28  and turns in the direction of arrow C (the anticlockwise direction). 
     A regulation roller  97  is disposed between the developing roller  70 A and the developing roller  70 B. 
     The regulation roller  97  is disposed at a spacing from each of an outer peripheral face of the developing roller  70 A and an outer peripheral face of the developing roller  70 B. The regulation roller  97  regulates amounts of developer passing along the surfaces of the developing rollers  70 A and  70 B, and forms developer layers of a pre-decided thickness on the surfaces of the developing rollers  70 A and  70 B. 
     The developing rollers  70 A and  70 B are disposed to oppose the outer peripheral surface of the photoreceptor  28 . The developing rollers  70 A and  70 B are structured by magnetic rolls (not illustrated), which are fixed to the developer accommodation chambers  80 A and  80 B, and hollow circular tube-shaped developing sleeves (not illustrated) that serve as tubular rotating bodies which are provided to be turnable around the outside of the magnetic rolls. A developing bias voltage is applied to the developing rollers  70 A and  70 B from the power supply unit (not illustrated), a developing electric field is formed between the developing rollers  70 A and  70 B, and the photoreceptor  28 , and toner in the developer G transfers to the latent image on the photoreceptor  28  during development. 
     The two developing rollers  70 A and  70 B receive developer from the upper and lower developer accommodation chambers  80 A and  80 B, thereafter retain the developer in the form of thin layers, and implement development. After development, the developing roller  70 A returns developer to the first agitation path  84 C or the second agitation path  84 D, and the developing roller  70 B returns developer to the first agitation path  84 E or the second agitation path  84 F. 
     The turning directions of the developing rollers may be any directions as long as each of the upper and lower developing rollers receives developer without the developer having passed along the other developing roller and each returns the developer to an agitation path separately, as described above. 
       FIG. 3  is a diagram illustrating structure of the control unit  36 . 
     The control unit  36  is provided with a CPU  150  that administers overall control of the printer  10 . The CPU  150  is connected to each of a ROM  152 , a RAM  154 , a hard disc storage device  156 , an image data input section  158 , a control and display section  160 , an image formation control section  162 , an image data processing section  164  and an optical sensor  165 , via a bus  166  which is a control bus, a data bus or the like. 
     The ROM  152  stores control programs for controlling the printer  10 . The RAM  154  is used as a workspace for processing various kinds of data and the like. The hard disc storage device  156  stores image data, various kinds of data relating to image formation and the like. 
     The image data input section  158  receives inputs of image data from personal computers and the like. The inputted image data is sent to the hard disc storage device  156 . 
     The control and display section  160  is configured to include a touch panel in which control functions and display functions are integrated, and also control buttons for a user to perform various controls with. The control and display section  160  receives controls for starting image formation on the recording paper P and the like, and reports control states of the printer  10  and the like to the user. 
     The image formation control section  162  controls driving of the image forming units  12 Y,  12 M,  12 C and  12 K and driving of motors of the various rollers and the like (not illustrated) in order to form images on the recording paper P on the basis of image data. 
     The image data processing section  164  performs image processing on the image data stored in the hard disc storage device  156 , such as conversion to colorant gradation data of the respective colors and the like. 
     The optical sensor  165  is disposed over the intermediate transfer belt  16  to downstream relative to the image forming units  12 Y,  12 M,  12 C and  12 K and upstream relative to the second transfer roller  20 , and detects toner densities of the toner images transferred onto the intermediate transfer belt  16 . 
     Next, a development field control processing routine of the first exemplary embodiment is described with reference to  FIG. 4 . For example, when an instruction signal instructing an adjustment of the development field is inputted between one image forming process and another image forming process, a development field adjustment program stored in the ROM  152  is executed by the CPU  150 . Accordingly, the present routine starts for each ink color of cyan, magenta and yellow. In the following description, a case of performing development field adjustment for cyan is described. 
     First, in step  100 , a cyan test pattern for detecting toner density is generated, and a toner image in which the cyan test pattern is developed is formed on the intermediate transfer belt  16  by the image formation control section  162 . In step  102 , when the developed toner image of the test pattern is conveyed to a reading position of the optical sensor  165 , the whole of the toner image of the test pattern is read by the optical sensor  165 , and a cyan toner density is measured from reading data based on the test pattern. 
     Then, in step  104 , the developing bias voltage applied to the developing rollers  70 A and  70 B or an exposure amount onto the photoreceptor  28 , or both, is chosen on the basis of the toner densities measured in step  102 , and a development field to be formed between the developing rollers  70 A and  70 B, and the photoreceptor  28  is chosen. For example, if the measured toner density is not within a pre-specified density range, the developing bias voltage and exposure amount are chosen in order to put the toner density within the pre-specified density range. 
     Then, in step  106 , it is determined whether or not the development field chosen in step  104  is within a pre-specified development field range. If it is determined that the development field is outside the pre-specified range, then, in step  108 , control is performed so as to change a rotation speed of the developing roller  70 A that is disposed at the upstream side of the direction of turning of the photoreceptor  28 , and control returns to step  100 . If the chosen development field is larger than a maximum value of the pre-specified development field range, the rotation speed of the developing roller  70 A is changed so as to increase. For example, if a plural number of levels of rotation speed have been specified beforehand, control is performed to step up the rotation speed. On the other hand, if the chosen development field is smaller than a minimum value of the pre-specified development field range, the rotation speed of the developing roller  70 A is changed so as to decrease. For example, if a plural number of levels of rotation speed have been specified beforehand, control is performed to step down the rotation speed. 
     Alternatively, if the development field is determined to be within the pre-specified electric field range in step  106 , then, in step  110 , the developing bias voltage applied to the developing rollers  70 A and  70 B and/or the exposure amount onto the photoreceptor  28  is adjusted to the developing bias voltage and/or exposure amount chosen in step  104 , and the development field control processing routine ends. Thus, the developing bias voltage or exposure amount is adjusted such that the development field is formed in accordance with the toner density. 
     Now, as illustrated in  FIG. 5 , it can be seen that when development is performed with two developing rollers, an amount of toner that is developed changes and a development capacity changes if the rotation speed of the first developing roller, which is the developing roller disposed at the upstream side of the turning direction of the photoreceptor, is decreased. 
     Further, development fields that prevent reductions in image quality are illustrated. Ordinarily, if a development field is too high, defects in the form of white spots occur, and if a development field is too low, there are defects that mean it is not possible to strike a balance between fine lines and fogging and there are problems with unevennesses in density tending to occur.  FIG. 6  is a diagram illustrating the relationship between numbers of white spots and development fields. It can be seen that white spots are more likely to occur when the development field is large. 
       FIG. 9  is a graph illustrating a relationship between development fields and density mottling (unevenness within images). It can be seen that a degree of density unevenness is larger when the development field is too low.  FIG. 7  is a graph illustrating the relationship between a ratio of the development field (Vdeve) to the cleaning electric field (Vcln) and density reproduction quality of fine lines. It can be seen therefrom that fine lines cannot be reproduced if the ratio Vdeve/Vcln is not at least a certain value. If the development field (Vdeve) is made smaller, the cleaning electric field (Vcln) must also be made smaller in order to maintain the ratio Vdeve/Vcln at least the certain value. However, according to the relationship between the cleaning electric field and fogging illustrated in  FIG. 8 , if the cleaning electric field (Vcln) becomes too small, there is a “fogging” effect in which toner is developed at regions that should not be developed. Thus, it can be seen that it is not possible to strike a balance between reproduction of fine lines and preventing fogging if the development field is too small. 
     Given the above, because problems arise if the development field is either too high or too low, it is desirable to keep the development field within a required development field range in order to prevent a drop in image quality. 
     In the development field control processing routine described above, the rotation speed of the developing roller changes if the development field according to the developing bias voltage or exposure amount chosen in accordance with the toner density is outside a pre-specified range, and the development capacity changes. Thus, the development field is kept within the pre-specified development field range and deteriorations in image quality are prevented. 
     Now a reason for changing the peripheral speed of the first developing roller to keep the development field within the pre-specified range is described. With a structure in which the peripheral speed of the second developing roller changes, a range of control of the development field is wider, but image quality changes when the circumferential speed of the second developing roller is changed, so this is not desirable. For example, if the circumferential speed of the second developing roller were to be made faster, as illustrated in  FIG. 10 , mottling in the form of longitudinal stripes in an image would start to appear. On the other hand, if the circumferential speed of the second developing roller were to be made slower, as illustrated in  FIG. 11 , density mottling would occur. These relationships are illustrated in  FIG. 10  and  FIG. 11 , respectively. 
     As described hereabove, according to the printer relating to the first exemplary embodiment, the rotation speed of the developing roller disposed at the upstream side of the turning direction of the photoreceptor changes in accordance with detected toner densities and a development capacity according to the plural developing rollers is adjusted, with a development field with which deteriorations of image quality do not occur being maintained rather than the development field being put outside a development field range in which deteriorations of image quality do not occur. 
     In the exemplary embodiment described hereabove, an example is described of a case in which, if the development field chosen on the basis of a detected toner density is outside a pre-specified development field range, the rotation speed of the developing roller disposed at the upstream side of the turning direction of the photoreceptor is changed. However, the present invention is not to be limited thus. For example, the rotation speed of the developing roller disposed at the upstream side of the turning direction of the photoreceptor may be changed when a detected toner density is outside a density range that is specified in advance as a range in which deteriorations in image quality do not occur. For example, if a detected toner density is higher than the maximum value of the pre-specified density range, the rotation speed of the developing roller disposed at the upstream side of the turning direction of the photoreceptor may be changed so as to slow down in order to reduce development capacity, and if a detected toner density is lower than the minimum value of the pre-specified density range, the rotation speed of the developing roller disposed at the upstream side of the turning direction of the photoreceptor may be changed so as to speed up in order to raise development capacity. 
     Next, a second exemplary embodiment is described. Portions with the same structure as in the first exemplary embodiment are assigned the same reference numerals and are not described. 
     The second exemplary embodiment differs from the first exemplary embodiment in that the developing bias voltage applied to the developing rollers is chosen in accordance with humidity. 
     As illustrated in  FIG. 12 , in a control unit  236  of a printer relating to the second exemplary embodiment, the CPU  150  is connected to each of the ROM  152 , the RAM  154 , the hard disc storage device  156 , the image data input section  158 , the control and display section  160 , the image formation control section  162 , the image data processing section  164  and a humidity sensor  265 , via the bus  166 . 
     The humidity sensor  265  is provided inside the printer  10  and detects humidity. 
     Other structures of the printer relating to the second exemplary embodiment are the same as in the first exemplary embodiment, so will not be described. 
     Next, a development field control processing routine relating to the second exemplary embodiment is described with reference to  FIG. 13 . In the following description, a case of performing development field adjustment for cyan is described. 
     First, in step  200 , a humidity is detected by the humidity sensor  265 . Then, in step  202 , the developing bias voltage applied to the developing rollers  70 A and  70 B or the exposure amount onto the photoreceptor  28 , or both, is chosen on the basis of the humidity detected in step  200 , and the development field formed between the developing rollers  70 A and  70 B, and the photoreceptor  28  is chosen. For example, when the detected humidity is higher, a higher developing bias voltage or a larger exposure amount is chosen. 
     Then, in step  106 , it is determined whether or not the development field chosen in step  202  is within a pre-specified development field range. If it is determined that the development field is outside the pre-specified range, then, in step  108 , control is performed so as to change the rotation speed of the developing roller  70 A that is disposed at the upstream side of the direction of turning of the photoreceptor  28 . If the chosen development field is larger than the maximum value of the pre-specified development field range, the rotation speed of the developing roller  70 A is changed so as to increase. On the other hand, if the chosen development field is smaller than the minimum value of the pre-specified development field range, the rotation speed of the developing roller  70 A is changed so as to decrease. 
     In step  204 , the developing bias voltage applied to the developing rollers  70 A and  70 B or the exposure amount onto the photoreceptor  28  is adjusted within a pre-specified voltage range or pre-specified exposure amount range corresponding to the pre-specified development field range, and the development field control processing routine ends. For example, if the developing bias voltage chosen in step  202  is a developing bias voltage corresponding to a development field that is larger than the maximum value of the pre-specified development field, the developing bias voltage to be applied to the developing rollers  70 A and  70 B is adjusted to the maximum value of the pre-specified voltage range. Further, if the developing bias voltage chosen in step  202  is a developing bias voltage corresponding to a development field that is smaller than the minimum value of the pre-specified development field, the developing bias voltage to be applied to the developing rollers  70 A and  70 B is adjusted to the minimum value of the pre-specified voltage range. 
     Alternatively, if the development field is determined to be within the pre-specified electric field range in step  106 , then, in step  110 , the developing bias voltage applied to the developing rollers  70 A and  70 B or the exposure amount onto the photoreceptor  28  is adjusted to the developing bias voltage or exposure amount onto the photoreceptor  28  chosen in step  202 , and the development field control processing routine ends. Thus, the developing bias voltage or exposure amount is adjusted such that the development field is formed in accordance with the humidity. 
     As described hereabove, according to the printer relating to the second exemplary embodiment, the rotation speed of the developing roller disposed at the upstream side of the turning direction of the photoreceptor changes in accordance with detected humidities and a development capacity according to the plural developing rollers is adjusted, with a development field with which deteriorations of image quality do not occur being maintained rather than the development field being put outside the development field range in which deteriorations of image quality do not occur. 
     In the exemplary embodiment described hereabove, an example is described of a case in which, if the development field chosen on the basis of detected humidity is outside a pre-specified development field range, the rotation speed of the developing roller disposed at the upstream side of the turning direction of the photoreceptor changes. However, the present invention is not to be limited thus. For example, the rotation speed of the developing roller disposed at the upstream side of the turning direction of the photoreceptor may be changed when a detected humidity is outside a humidity range that is specified in advance as a range in which deteriorations in image quality do not occur. For example, if a detected humidity is higher than the maximum value of the pre-specified humidity range, because the development field is smaller due to the humidity, the rotation speed of the developing roller disposed at the upstream side of the turning direction of the photoreceptor may be changed so as to speed up in order to raise development capacity, and if a detected humidity is lower than the minimum value of the pre-specified humidity range, because the development field is larger due to the humidity, the rotation speed of the developing roller disposed at the upstream side of the turning direction of the photoreceptor may be changed so as to slow down in order to reduce development capacity. 
     Next, a third exemplary embodiment is described. A printer relating to the third exemplary embodiment has the same structure as in the first exemplary embodiment, so the same reference numerals are assigned and descriptions are not given. 
     The third exemplary embodiment differs from the first exemplary embodiment in that the developing bias voltage applied to the developing rollers is chosen in accordance with a number of white spots that are detected. 
     A development field control processing routine relating to the third exemplary embodiment is described with reference to  FIG. 14 . In the following description, a case of performing development field adjustment for cyan is described. 
     First, in step  300 , a cyan test pattern for detecting white spots is generated, and a toner image in which the cyan test pattern is developed is formed on the intermediate transfer belt  16  by the image formation control section  162 . In step  302 , when the developed toner image of the test pattern is conveyed to the reading position of the optical sensor  165 , the whole of the toner image of the test pattern is read by the optical sensor  165 , and white spots are detected from reading data based on the test pattern. 
     Then, in step  304 , the developing bias voltage applied to the developing rollers  70 A and  70 B or the exposure amount onto the photoreceptor  28 , or both, is chosen on the basis of the number of white spots detected in step  302 , and a development field to be formed between the developing rollers  70 A and  70 B, and the photoreceptor  28  is chosen. For example, if a detected number of white spots is greater than a pre-specified fixed value (e.g., four on an A3 size area), the developing bias voltage and exposure amount are chosen in order to reduce the number of white spots. 
     Then, in step  106 , it is determined whether or not the development field chosen in step  304  is within a pre-specified development field range. If it is determined that the development field is outside the pre-specified development field range, then, in step  108 , control is performed so as to change the rotation speed of the developing roller  70 A, and control returns to step  300 . If the chosen development field is larger than the maximum value of the pre-specified development field range, the rotation speed of the developing roller  70 A is changed so as to increase. On the other hand, if the chosen development field is smaller than the minimum value of the pre-specified development field range, the rotation speed of the developing roller  70 A is changed so as to decrease. 
     Alternatively, if the development field is determined to be within the pre-specified development field range in step  106 , then, in step  110 , the developing bias voltage applied to the developing rollers  70 A and  70 B or the exposure amount onto the photoreceptor  28  is adjusted to the developing bias voltage or exposure amount onto the photoreceptor  28  chosen in step  304 , and the development field control processing routine ends. Thus, the developing bias voltage or exposure amount is adjusted such that the development field is formed in accordance with results of detection of white spots. 
     As described hereabove, according to the printer relating to the third exemplary embodiment, the rotation speed of the developing roller disposed at the upstream side of the turning direction of the photoreceptor changes in accordance with results of detection of white spots and a development capacity according to the plural developing rollers is adjusted, with a development field with which deteriorations of image quality do not occur being maintained rather than the development field being put outside the development field range in which deteriorations of image quality do not occur. 
     In the exemplary embodiment described hereabove, an example is described of a case in which, if the development field chosen on the basis of white spot detection results is outside a pre-specified development field range, the rotation speed of the developing roller disposed at the upstream side of the turning direction of the photoreceptor is changed. However, the present invention is not to be limited thus. For example, the rotation speed of the developing roller disposed at the upstream side of the turning direction of the photoreceptor may be changed when a detected number of white spots is outside a range that is specified in advance as a range in which deteriorations in image quality do not occur. For example, if a detected number of white spots is larger than the maximum value of the pre-specified range, because it is determined that the development field is large, the rotation speed of the developing roller disposed at the upstream side of the turning direction of the photoreceptor may be changed so as to speed up. 
     Next, a fourth exemplary embodiment is described. A printer relating to the fourth exemplary embodiment has the same structure as in the first exemplary embodiment, so the same reference numerals are assigned and descriptions are not given. 
     The fourth exemplary embodiment differs from the first exemplary embodiment in that the developing bias voltage applied to the developing rollers is chosen in accordance with a measured density of fine lines. 
     A development field control processing routine relating to the fourth exemplary embodiment is described with reference to  FIG. 15 . In the following description, a case of performing development field adjustment for cyan is described. 
     First, in step  400 , a cyan test pattern for measuring one-bit fine line densities is generated, and a toner image in which the cyan test pattern is developed is formed on the intermediate transfer belt  16  by the image formation control section  162 . In step  402 , when the developed toner image of the test pattern is conveyed to the reading position of the optical sensor  165 , the whole of the test pattern toner image is read by the optical sensor  165 , and fine line density is measured from reading data based on the test pattern. 
     Then, in step  404 , the developing bias voltage applied to the developing rollers  70 A and  70 B or the exposure amount onto the photoreceptor  28 , or both, is chosen on the basis of the fine line density detected in step  402 , and a development field to be formed between the developing rollers  70 A and  70 B, and the photoreceptor  28  is chosen. For example, if a detected fine line density is not within a pre-specified density range, the developing bias voltage and exposure amount are chosen in order to put the fine line density into the pre-specified density range. 
     Then, in step  106 , it is determined whether or not the development field chosen in step  404  is within a pre-specified development field range. If it is determined that the development field is outside the pre-specified development field range, then, in step  108 , control is performed so as to change the rotation speed of the developing roller  70 A, and control returns to step  400 . If the chosen development field is larger than the maximum value of the pre-specified development field range, the rotation speed of the developing roller  70 A is changed so as to increase. On the other hand, if the chosen development field is smaller than the minimum value of the pre-specified development field range, the rotation speed of the developing roller  70 A is changed so as to decrease. 
     Alternatively, if the development field is determined to be within the pre-specified development field range in step  106 , then, in step  110 , the developing bias voltage applied to the developing rollers  70 A and  70 B or the exposure amount onto the photoreceptor  28  is adjusted to the developing bias voltage or exposure amount onto the photoreceptor  28  chosen in step  404 , and the development field control processing routine ends. Thus, the developing bias voltage or exposure amount is adjusted such that the development field is formed in accordance with results of detection of fine line density. 
     As described hereabove, according to the printer relating to the fourth exemplary embodiment, the rotation speed of the developing roller disposed at the upstream side of the turning direction of the photoreceptor changes in accordance with results of detection of fine line densities and a development capacity according to the plural developing rollers is adjusted, with a development field with which deteriorations of image quality do not occur being maintained rather than the development field being put outside the development field range in which deteriorations of image quality do not occur. 
     In the exemplary embodiment described hereabove, an example is described of a case in which, if the development field chosen on the basis of detected fine line density is outside a pre-specified development field range, the rotation speed of the developing roller disposed at the upstream side of the turning direction of the photoreceptor changes. However, the present invention is not to be limited thus. For example, the rotation speed of the developing roller disposed at the upstream side of the turning direction of the photoreceptor may be changed when a detected fine line density is outside a density range that is specified in advance as a range in which deteriorations in image quality do not occur. For example, if a detected fine line density is higher than the maximum value of the pre-specified density range, the rotation speed of the developing roller disposed at the upstream side of the turning direction of the photoreceptor may be changed so as to speed up, and if a detected fine line density is lower than the minimum value of the pre-specified density range, the rotation speed of the developing roller disposed at the upstream side of the turning direction of the photoreceptor may be changed so as to slow down. 
     Next, a fifth exemplary embodiment is described. A printer relating to the fifth exemplary embodiment has the same structure as in the first exemplary embodiment, so the same reference numerals are assigned and descriptions are not given. 
     The fifth exemplary embodiment differs from the first exemplary embodiment in that the developing bias voltage applied to the developing rollers is chosen in accordance with detected density unevenness. 
     A development field control processing routine relating to the fifth exemplary embodiment is described with reference to  FIG. 16 . In the following description, a case of performing development field adjustment for cyan is described. 
     First, in step  500 , a cyan test pattern for detecting density unevenness is generated, and a toner image in which the cyan test pattern is developed is formed on the intermediate transfer belt  16  by the image formation control section  162 . In step  502 , when the developed toner image of the test pattern is conveyed to the reading position of the optical sensor  165 , the whole of the test pattern toner image is read by the optical sensor  165 , and density unevennesses are detected from reading data based on the test pattern. For example, CIELAB L* values of the test pattern image are calculated for respective pixels, and a maximum value in L* value differences between neighboring pixels is detected to serve as a degree of density unevenness. 
     Then, in step  504 , the developing bias voltage applied to the developing rollers  70 A and  70 B or the exposure amount onto the photoreceptor  28 , or both, is chosen on the basis of the degree of density unevenness detected in step  502 , and a development field to be formed between the developing rollers  70 A and  70 B, and the photoreceptor  28  is chosen. For example, if a detected degree of density unevenness is not within a pre-specified density range, the developing bias voltage and exposure amount are chosen in order to suppress density unevenness. 
     Then, in step  106 , it is determined whether or not the development field chosen in step  504  is within a pre-specified development field range. If it is determined that the development field is outside the pre-specified development field range, then, in step  108 , control is performed so as to change the rotation speed of the developing roller  70 A, and control returns to step  500 . If the chosen development field is larger than the maximum value of the pre-specified development field range, the rotation speed of the developing roller  70 A is changed so as to increase. On the other hand, if the chosen development field is smaller than the minimum value of the pre-specified development field range, the rotation speed of the developing roller  70 A is changed so as to decrease. 
     Alternatively, if the development field is determined to be within the pre-specified development field range in step  106 , then, in step  110 , the developing bias voltage applied to the developing rollers  70 A and  70 B or the exposure amount onto the photoreceptor  28  is adjusted to the developing bias voltage or exposure amount onto the photoreceptor  28  chosen in step  504 , and the development field control processing routine ends. Thus, the developing bias voltage or exposure amount is adjusted such that the development field is formed in accordance with results of detection of density unevenness. 
     As described hereabove, according to the printer relating to the fifth exemplary embodiment, the rotation speed of the developing roller disposed at the upstream side of the turning direction of the photoreceptor changes in accordance with detected degrees of density unevenness and a development capacity according to the plural developing rollers is adjusted, with a development field with which deteriorations of image quality do not occur being maintained rather than the development field being put outside the development field range in which deteriorations of image quality do not occur. 
     In the exemplary embodiment described hereabove, an example is described of a case in which, if the development field chosen on the basis of detected density unevenness is outside a pre-specified development field range, the rotation speed of the developing roller disposed at the upstream side of the turning direction of the photoreceptor changes. However, the present invention is not to be limited thus. For example, the rotation speed of the developing roller disposed at the upstream side of the turning direction of the photoreceptor may be changed when a detected degree of density unevenness is outside a range that is specified in advance as a range in which deteriorations in image quality do not occur. For example, if a detected degree of density unevenness is higher than the maximum value of the pre-specified range, the rotation speed of the developing roller disposed at the upstream side of the turning direction of the photoreceptor may be changed so as to slow down, and if a detected degree of density unevenness is lower than the minimum value of the pre-specified range, the rotation speed of the developing roller disposed at the upstream side of the turning direction of the photoreceptor may be changed so as to speed up. 
     Next, a sixth exemplary embodiment is described. A printer relating to the sixth exemplary embodiment has the same structure as in the first exemplary embodiment, so the same reference numerals are assigned and descriptions are not given. 
     The sixth exemplary embodiment differs from the first exemplary embodiment in that the developing bias voltage applied to the developing rollers or the exposure amount is chosen in accordance with an amount of toner that is developed over a certain duration. 
     A development field control processing routine relating to the sixth exemplary embodiment is described with reference to  FIG. 17 . In the following description, a case of performing development field adjustment for cyan is described. 
     First, in step  600 , it is determined whether or not an image forming process has been carried out, and if it is determined that no image forming process has been carried out, control passes to step  604 . On the other hand, if it is determined that an image forming process has been carried out, a development amount of toner is calculated on the basis of the image data of the image forming process, and is counted into a total value of toner development amounts. 
     In step  604 , it is determined whether or not a pre-specified measurement duration has passed since the start of execution of the development field control processing routine. If the pre-specified measurement duration has not passed, control returns to step  600 . On the other hand, if the pre-specified measurement duration has passed, then, in step  606 , the developing bias voltage applied to the developing rollers  70 A and  70 B or the exposure amount onto the photoreceptor  28 , or both, is chosen on the basis of the counted-up total value of toner development amounts, and a development field to be formed between the developing rollers  70 A and  70 B, and the photoreceptor  28  is chosen. For example, when the total value of toner development amounts is larger, because charge amounts of the toner are lower, a lower developing bias voltage and smaller exposure amount are chosen. 
     Then, in step  106 , it is determined whether or not the development field chosen in step  606  is within a pre-specified development field range. If it is determined that the development field is outside the pre-specified development field range, then, in step  108 , control is performed so as to change the rotation speed of the developing roller  70 A. If the chosen development field is larger than the maximum value of the pre-specified development field range, the rotation speed of the developing roller  70 A is changed so as to increase. On the other hand, if the chosen development field is smaller than the minimum value of the pre-specified development field range, the rotation speed of the developing roller  70 A is changed so as to decrease. 
     In step  204 , the developing bias voltage applied to the developing rollers  70 A and  70 B or the exposure amount onto the photoreceptor  28  is adjusted within a pre-specified voltage range or pre-specified exposure amount range corresponding to the pre-specified development field range, and the development field control processing routine ends. 
     Alternatively, if the development field is determined to be within the pre-specified development field range in step  106 , then, in step  110 , the developing bias voltage applied to the developing rollers  70 A and  70 B or the exposure amount onto the photoreceptor  28  is adjusted to the developing bias voltage or exposure amount onto the photoreceptor  28  chosen in step  606 , and the development field control processing routine ends. Thus, the developing bias voltage or exposure amount is adjusted such that the development field is formed in accordance with the total value of toner development amounts over the pre-specified duration. 
     As described hereabove, according to the printer relating to the sixth exemplary embodiment, the rotation speed of the developing roller disposed at the upstream side of the turning direction of the photoreceptor changes in accordance with total values of toner development amounts over the pre-specified duration and a development capacity according to the plural developing rollers is adjusted, with a development field with which deteriorations of image quality do not occur being maintained rather than the development field being put outside the development field range in which deteriorations of image quality do not occur. 
     In the exemplary embodiment described hereabove, an example is described of a case in which, if the development field chosen on the basis of a counted-up total value of toner development amounts is outside a pre-specified development field range, the rotation speed of the developing roller disposed at the upstream side of the turning direction of the photoreceptor changes. However, the present invention is not to be limited thus. For example, the rotation speed of the developing roller disposed at the upstream side of the turning direction of the photoreceptor may be changed when a counted-up total value of toner development amounts is outside a range that is specified in advance as a range in which deteriorations in image quality do not occur. For example, if a counted-up total value of toner development amounts is higher than the maximum value of the pre-specified range, the rotation speed of the developing roller disposed at the upstream side of the turning direction of the photoreceptor may be changed so as to speed up, and if a counted-up total value of toner development amounts is lower than the minimum value of the pre-specified range, the rotation speed of the developing roller disposed at the upstream side of the turning direction of the photoreceptor may be changed so as to slow down. 
     Next, a seventh exemplary embodiment is described. Portions with the same structure as in the first exemplary embodiment are assigned the same reference numerals and are not described. 
     The seventh exemplary embodiment differs from the first exemplary embodiment in that the developing bias voltage applied to the developing rollers is chosen in accordance with electrostatic charge amounts of the toner. 
     As illustrated in  FIG. 18 , in a control unit  736  of a printer relating to the seventh exemplary embodiment, the CPU  150  is connected with each of the ROM  152 , the RAM  154 , the hard disc storage device  156 , the image data input section  158 , the control and display section  160 , the image formation control section  162 , the image data processing section  164  and a charge amount measurement section  765 , via the bus  166 . 
     The charge amount measurement section  765  is constituted using a sensor provided at the agitation and conveyance path  84 A or the agitation and conveyance path  84 B of the developing section  70  of the image forming unit  12 , and measures charge amounts of the developer. 
     Other structures of the printer relating to the seventh exemplary embodiment are the same as in the first exemplary embodiment, so will not be described. 
     Next, a development field control processing routine relating to the seventh exemplary embodiment is described with reference to  FIG. 19 . In the following description, a case of performing development field adjustment for cyan is described. 
     First, in step  700 , a toner charge amount is measured by the corresponding charge amount measurement section  765 . Then, in step  702 , the developing bias voltage applied to the developing rollers  70 A and  70 B or the exposure amount onto the photoreceptor  28 , or both, is chosen on the basis of the charge amount measured in step  700 , and the development field formed between the developing rollers  70 A and  70 B, and the photoreceptor  28  is chosen. For example, when the measured toner charge amount is larger, a higher developing bias voltage and a larger exposure amount are chosen. 
     Then, in step  106 , it is determined whether or not the development field chosen in step  702  is within a pre-specified development field range. If it is determined that the development field is outside the pre-specified electric field range, then, in step  108 , control is performed so as to change the rotation speed of the developing roller  70 A. 
     In step  204 , the developing bias voltage applied to the developing rollers  70 A and  70 B or the exposure amount onto the photoreceptor  28  is adjusted within a pre-specified voltage range or pre-specified exposure amount range corresponding to the pre-specified development field range, and the development field control processing routine ends. 
     Alternatively, if the development field is determined to be within the pre-specified electric field range in step  106 , then, in step  110 , the developing bias voltage applied to the developing rollers  70 A and  70 B or the exposure amount onto the photoreceptor  28  is adjusted to the developing bias voltage or exposure amount onto the photoreceptor  28  chosen in step  702 , and the development field control processing routine ends. Thus, the developing bias voltage or exposure amount is adjusted such that the development field is formed in accordance with the toner charge amount. 
     As described hereabove, according to the printer relating to the seventh exemplary embodiment, the rotation speed of the developing roller disposed at the upstream side of the turning direction of the photoreceptor changes in accordance with measured toner charge amounts and a development capacity according to the plural developing rollers is adjusted, with a development field with which deteriorations of image quality do not occur being maintained rather than the development field being put outside the development field range in which deteriorations of image quality do not occur. 
     In the exemplary embodiment described hereabove, an example is described of a case in which, if the development field chosen on the basis of measured toner charge amounts is outside a pre-specified electric field range, the rotation speed of the developing roller disposed at the upstream side of the turning direction of the photoreceptor is changed. However, the present invention is not to be limited thus. For example, the rotation speed of the developing roller disposed at the upstream side of the turning direction of the photoreceptor may be changed when a measured toner charge amount is outside a range that is specified in advance as a range in which deteriorations in image quality do not occur. For example, if a measured toner charge amount is higher than the maximum value of the pre-specified range, the rotation speed of the developing roller disposed at the upstream side of the turning direction of the photoreceptor may be changed so as to speed up, and if a measured toner charge amount is lower than the minimum value of the pre-specified range, the rotation speed of the developing roller disposed at the upstream side of the turning direction of the photoreceptor may be changed so as to slow down. 
     Rather than toner charge amounts being directly measured using a sensor, toner charge amounts may be measured from other information, such as image data or the like. 
     Next, an eighth exemplary embodiment is described. Portions with the same structure as in the first exemplary embodiment are assigned the same reference numerals and are not described. 
     The eighth exemplary embodiment differs from the first exemplary embodiment in that the developing section has a configuration in which developer is transferred from one developing roller to the other developing roller. 
     As illustrated in  FIG. 20 , the developing section  70  of a printer relating to the eighth exemplary embodiment is provided with the casing  81  inside which the developer accommodation chamber  80 A that accommodates the developer G is formed. As illustrated in  FIG. 20 , the agitation and conveyance path  84 A that agitates (mixes) and conveys the developer G is formed in the developer accommodation chamber  80 A. 
     The agitation and conveyance path  84 A is partitioned by the partition wall  93 A provided standing from a bottom face and two agitation paths are formed, the first agitation path  84 C and the second agitation path  84 D. 
     The regulation roller  97  is disposed at the periphery of the developing roller  70 B. 
     The regulation roller  97  is disposed at a spacing from the outer peripheral face of the developing roller  70 B. The regulation roller  97  regulates amounts of developer passing along the surface of the developing roller  70 B, and forms a developer layer of a pre-decided thickness on the surface of the developing roller  70 B. 
     The developing roller  70 B transfers a portion of the developer in the developer layer to the developing roller  70 A, retains developer remaining in the developer layer, and performs development. The developing roller  70 A forms the transferred developer into a thin layer, retains the developer, and performs development. After development, both of the developing rollers  70 A and  70 B return the developer to the first agitation path  84 C and the second agitation path  84 D. 
     The above-described development field control processing routines of the first exemplary embodiment to the seventh exemplary embodiment may be executed in combination. 
     The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The exemplary embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.