Patent Publication Number: US-10331064-B2

Title: Image forming apparatus

Description:
This application is a continuation of application Ser. No. 15/461,760, filed Mar. 17, 2017. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an image forming apparatus which forms an image on a recording medium using an electrophotographic technique. 
     Description of the Related Art 
     Conventionally, in image forming apparatuses such as laser beam printers, an in-line color system is known in which a plurality of image forming stations are arranged in parallel in a movement direction of an intermediate transfer belt. With an image forming apparatus adopting the in-line color system, first, an electrostatic latent image is formed on a surface of a photosensitive drum in the plurality of image forming stations. The electrostatic latent image formed on the photosensitive drum is developed by a developing apparatus as a toner image. In addition, toner images of respective colors formed at the plurality of image forming stations are primarily transferred onto the intermediate transfer belt so as to overlap with each other. Furthermore, the toner images of the respective colors primarily transferred onto the intermediate transfer belt are secondarily transferred onto a sheet such as a sheet of paper. Subsequently, as the sheet onto which the toner images have been secondarily transferred is subjected to heat and pressure by a fixing apparatus, the toner images are fixed to the sheet. In this manner, an image is formed on the sheet. In this case, density of the image formed on the sheet is desirably consistent with density desired by a user. In addition, a tinge of the image formed on the sheet is also desirably consistent with a tinge desired by the user. 
     In consideration thereof, in the technique disclosed in Japanese Patent Application Laid-open No. 2013-210489, a color gamut of an image is enlarged and an upper limit value of density of the image is increased by causing a peripheral velocity difference between a photosensitive drum and a developing roller to vary. In addition, the technique disclosed in Japanese Patent Application Laid-open No. 2013-210489 suppresses toner scattering, image thinning, and the like which are caused when the peripheral velocity difference between the photosensitive drum and the developing roller is increased. Specifically, instead of increasing the peripheral velocity difference between the photosensitive drum and the developing roller by increasing a peripheral velocity of the developing roller, the peripheral velocity difference between the photosensitive drum and the developing roller is increased by reducing a peripheral velocity of the photosensitive drum. Accordingly, toner scattering, image thinning, and the like are suppressed. 
     In addition, recently, the widespread use of image forming apparatuses capable of printing images with high image quality in a stable manner has given rise to a new problem in that paper currency, securities, and the like can be easily printed using a full-color image forming apparatus. In order to solve this problem, with a technique disclosed in Japanese Patent Application Laid-open No. H10-076735, a regular dot pattern indicating a machine number of an image forming apparatus is applied to a sheet of paper when printing an image. Accordingly, when, for instance, printed matter resembling paper currency is discovered, a machine number can be deduced from the dot pattern applied to the paper currency and an image forming apparatus used to print the printed matter can be identified. 
     Furthermore, with a technique disclosed in Japanese Patent Application Laid-open No. 2001-103282, density in a vicinity of a portion where a dot pattern indicating information related to an image forming apparatus is added on a sheet is detected and the dot pattern is changed in accordance with the density in the vicinity of the portion. Specifically, when the vicinity of the portion where the dot pattern is added has high density, the dot pattern is made recognizable by reducing spacing among dots constituting the dot pattern. On the other hand, when the vicinity of the portion where the dot pattern is added has low density, the dot pattern is made less visible by increasing the spacing among the dots constituting the dot pattern. 
     SUMMARY OF THE INVENTION 
     However, when the technique disclosed in Japanese Patent Application Laid-open No. 2013-210489 is adopted with respect to the techniques disclosed in Japanese Patent Application Laid-open No. H10-076735 and Japanese Patent Application Laid-open No. 2001-103282, increased density of an image may cause a dot pattern applied to a sheet of paper to become visible. Specifically, in an image forming apparatus which forms a visually-unidentifiable dot pattern for identifying a manufacturer or the like on a sheet of paper, when increasing image density by adjusting a peripheral velocity ratio between a photosensitive drum and a developing roller, density of the dot pattern also increases. In this case, there is a risk that a favorable image can no longer be obtained because the increased density causes the dot pattern applied to the sheet of paper to become visually identifiable by a user. 
     In consideration thereof, an object of the present invention is to provide a technique which enables a favorable image to be obtained even when changing a peripheral velocity ratio between an image bearing member and a developer bearing member during image formation. 
     In order to achieve the object described above, an image forming apparatus according to the present invention is an image forming apparatus, comprising: 
     a rotatable image bearing member on which an electrostatic image is formed; 
     a rotatable developer bearing member supplying a developer to the image bearing member and developing the electrostatic image; and 
     processing portion for processing image information so as to add a specific dot pattern to an image to be formed, wherein 
     the image forming apparatus is capable of executing: 
     a first image formation mode in which an image is formed based on the image information by setting a peripheral velocity ratio Vd/Vdr between a peripheral velocity Vd of the developer bearing member and a peripheral velocity Vdr of the image bearing member to a first peripheral velocity ratio ΔV 1 ; and 
     a second image formation mode in which an image is formed based on the image information by setting the peripheral velocity ratio Vd/Vdr to a second peripheral velocity ratio ΔV 2 , which is higher than the first peripheral velocity ratio ΔV 1 , 
     in the first image formation mode, the image forming apparatus forms an image with a first development contrast C 1  and forms the specific dot pattern with a second development contrast C 2 , which is lower than the first development contrast C 1 , 
     in the second image formation mode, the image forming apparatus forms an image with a third development contrast C 3  and forms the specific dot pattern with a fourth development contrast C 4 , which is lower than the third development contrast C 3 , and 
     when ΔC 1  (=C 2 /C 1 ) denotes a ratio between the second development contrast C 2  and the first development contrast C 1 , and ΔC 2  (=C 4 /C 3 ) denotes a ratio between the fourth development contrast C 4  and the third development contrast C 3 , 
     ΔC 2 &lt;ΔC 1  is satisfied. 
     In order to achieve the object described above, an image forming apparatus according to the present invention is an image forming apparatus, comprising: 
     a rotatable image bearing member on which an electrostatic image is formed; 
     a rotatable developer bearing member supplying a developer to the image bearing member and developing the electrostatic image; and 
     processing portion for processing image information so as to add a specific dot pattern to an image to be formed, wherein 
     the image forming apparatus is capable of executing: 
     a first image formation mode in which an image is formed based on the image information by setting a peripheral velocity ratio Vd/Vdr between a peripheral velocity Vd of the developer bearing member and a peripheral velocity Vdr of the image bearing member to a first peripheral velocity ratio ΔV 1 ; and 
     a second image formation mode in which an image is formed based on the image information by setting the peripheral velocity ratio Vd/Vdr to a second peripheral velocity ratio ΔV 2 , which is higher than the first peripheral velocity ratio ΔV 1 , 
     in the first image formation mode, the image forming apparatus forms an image with a first development contrast C 1  and forms the specific dot pattern with a second development contrast C 2  which is lower than the first development contrast C 1 , 
     in the second image formation mode, the image forming apparatus forms an image with a third development contrast C 3  and forms the specific dot pattern with a fourth development contrast C 4  which is lower than the third development contrast C 3 , and 
     when ΔC 1  (=|C 2 −C 1 |) denotes an absolute value of a difference between the second development contrast C 2  and the first development contrast C 1 , and ΔC 2  (=|C 4 −C 3 |) denotes an absolute value of a difference between the fourth development contrast C 4  and the third development contrast C 3 , 
     ΔC 2 &lt;ΔC 1  is satisfied. 
     In addition, in order to achieve the objects described above, an image forming apparatus according to the present invention is an image forming apparatus, comprising: 
     a rotatable image bearing member on which an electrostatic image is formed; 
     a rotatable developer bearing member supplying a developer to the image bearing member and developing the electrostatic image; and 
     processing portion for processing image information so as to add a specific dot pattern to an image to be formed, wherein 
     the image forming apparatus is capable of executing: 
     a first image formation mode in which an image is formed based on the image information by setting a peripheral velocity ratio Vd/Vdr between a peripheral velocity Vd of the developer bearing member and a peripheral velocity Vdr of the image bearing member to a first peripheral velocity ratio ΔV 1 ; and 
     a second image formation mode in which an image is formed based on the image information by setting the peripheral velocity ratio Vd/Vdr to a second peripheral velocity ratio ΔV 2 , which is higher than the first peripheral velocity ratio ΔV 1 , and 
     the image forming apparatus 
     sets spacing between two adjacent dots among a plurality of dots forming the specific dot pattern in the second image formation mode wider than spacing between the two dots in the first image formation mode. 
     In addition, in order to achieve the object described above, an image forming apparatus according to the present invention is an image forming apparatus, comprising: 
     a rotatable image bearing member on which an electrostatic image is formed; 
     a rotatable developer bearing member supplying a developer to the image bearing member and developing the electrostatic image; and 
     processing portion for processing image information so as to add a specific dot pattern to an image to be formed, wherein 
     the image forming apparatus is capable of executing: 
     a first image formation mode in which an image is formed based on the image information by setting a peripheral velocity ratio Vd/Vdr between a peripheral velocity Vd of the developer bearing member and a peripheral velocity Vdr of the image bearing member to a first peripheral velocity ratio ΔV 1 ; and 
     a second image formation mode in which an image is formed based on the image information by setting the peripheral velocity ratio Vd/Vdr to a second peripheral velocity ratio ΔV 2 , which is higher than the first peripheral velocity ratio ΔV 1 , 
     in the first image formation mode, the image forming apparatus forms an image by supplying the developer to the image bearing member so that a region corresponding to the image has a first developer volume level M 1  and forms the specific dot pattern by supplying the developer to the image bearing member so that a region corresponding to the specific dot pattern has a second developer volume level M 2 , which is lower than the first developer volume level M 1 , 
     in the second image formation mode, the image forming apparatus forms an image by supplying the developer to the image bearing member so that a region corresponding to the image has a third developer volume level M 3  and forms the specific dot pattern by supplying the developer to the image bearing member so that a region corresponding to the specific dot pattern has a fourth developer volume level M 4 , which is lower than the third developer volume level M 3 , and 
     when ΔM 1  (=M 2 /M 1 ) denotes a ratio between the second developer volume level M 2  and the first developer volume level M 1 , and ΔM 2  (=M 4 /M 3 ) denotes a ratio between the fourth developer volume level M 4  and the third developer volume level M 3 , 
     ΔM 2 &lt;ΔM 1  is satisfied. 
     In addition, in order to achieve the object described above, an image forming apparatus according to the present invention is an image forming apparatus, comprising: 
     a rotatable image bearing member on which an electrostatic image is formed; 
     a rotatable developer bearing member supplying a developer to the image bearing member and developing the electrostatic image; and 
     processing portion for processing image information so as to add a specific dot pattern to an image to be formed, wherein 
     the image forming apparatus is capable of executing: 
     a first image formation mode in which an image is formed based on the image information by setting a peripheral velocity ratio Vd/Vdr between a peripheral velocity Vd of the developer bearing member and a peripheral velocity Vdr of the image bearing member to a first peripheral velocity ratio ΔV 1 ; and 
     a second image formation mode in which an image is formed based on the image information by setting the peripheral velocity ratio Vd/Vdr to a second peripheral velocity ratio ΔV 2 , which is higher than the first peripheral velocity ratio ΔV 1 , 
     in the first image formation mode, the image forming apparatus forms an image by supplying the developer to the image bearing member so that a region corresponding to the image has a first developer volume level M 1  and forms the specific dot pattern by supplying the developer to the image bearing member so that a region corresponding to the specific dot pattern has a second developer volume level M 2 , which is lower than the first developer volume level M 1 , 
     in the second image formation mode, the image forming apparatus forms an image by supplying the developer to the image bearing member so that a region corresponding to the image has a third developer volume level M 3  and forms the specific dot pattern by supplying the developer to the image bearing member so that a region corresponding to the specific dot pattern has a fourth developer volume level M 4 , which is lower than the third developer volume level M 3 , and 
     when ΔM 1  (=|M 2 −M 1 |) denotes an absolute value of a difference between the second developer volume level M 2  and the first developer volume level M 1 , and ΔM 2  (=|M 4 −M 3 |) denotes an absolute value of a difference between the fourth developer volume level M 4  and the third developer volume level M 3 , 
     ΔM 2 &lt;ΔM 1  is satisfied. 
     In addition, in order to achieve the object described above, an image forming apparatus according to the present invention is an image forming apparatus, comprising: 
     an image bearing member on which an electrostatic image is formed; 
     a developer bearing member for supplying a developer to the image bearing member and developing the electrostatic image; and 
     processing portion for processing image information so as to add a specific dot pattern to an image to be formed, wherein 
     the image forming apparatus is capable of executing: 
     a first image formation mode in which an image is formed based on the image information by setting a peripheral velocity ratio Vd/Vdr between a peripheral velocity Vd of the developer bearing member and a peripheral velocity Vdr of the image bearing member to a first peripheral velocity ratio ΔV 1 ; and 
     a second image formation mode in which an image is formed based on the image information by setting the peripheral velocity ratio Vd/Vdr to a second peripheral velocity ratio ΔV 2 , which is higher than the first peripheral velocity ratio ΔV 1 , 
     wherein, compared to the first image formation mode, in the second image formation mode, density of the specific dot pattern is increased at a rate lower than another rate at which density of a dot pattern of the image other than the specific dot pattern is increased. 
     The present invention enables a favorable image to be obtained even when forming an image by setting a higher peripheral velocity ratio than a prescribed peripheral velocity ratio. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic sectional view of an image forming apparatus according to a first embodiment; 
         FIG. 2  is a schematic sectional view of a process cartridge according to the first embodiment; 
         FIG. 3  is a schematic sectional view of a fixing apparatus according to the first embodiment; 
         FIG. 4  is a diagram showing a flow of processes executed by an image forming apparatus according to the first embodiment; 
         FIG. 5  is a diagram showing a relationship between a toner amount laid on a sheet of paper and density of an image according to the first embodiment; 
         FIGS. 6A and 6B  are diagrams showing a relationship between potential of a photosensitive drum and a toner amount laid on a recording material; 
         FIG. 7  is a diagram showing a traceable pattern in a normal image formation mode; 
         FIG. 8  is a diagram showing a traceable pattern in a wide-color gamut image formation mode; 
         FIG. 9  is a diagram showing a traceable pattern in a normal image formation mode; 
         FIG. 10  is a diagram showing a traceable pattern in a wide-color gamut image formation mode; 
         FIG. 11  is a diagram showing a traceable pattern in a normal image formation mode; and 
         FIG. 12  is a diagram showing a traceable pattern in a wide-color gamut image formation mode. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Modes for carrying out the present invention are illustratively explained in detail below on the basis of embodiments with reference to the drawings. However, dimensions, materials, and shapes of components described in the embodiments, relative arrangement of the components, and the like should be changed as appropriate according to the configuration of an apparatus to which the invention is applied and various conditions. That is, the dimensions, the materials, the shapes, and the relative arrangement are not intended to limit the scope of the present invention to the embodiments. 
     (First Embodiment) 
     &lt;Overall Configuration of Image Forming Apparatus  200 &gt; 
     The present embodiment enables execution of a normal image formation mode in which an image is formed with normal density and a wide-color gamut image formation mode in which a color gamut of an image is enlarged by changing a peripheral velocity ratio Vd/Vdr between a peripheral velocity Vdr of a photosensitive drum  201  and a peripheral velocity Vd of a developing roller  302 . The respective image formation modes differ from each other in the peripheral velocity ratio between the photosensitive drum  201  as an image bearing member and the developing roller  302  as a developer bearing member. In this case, the peripheral velocity ratio between the photosensitive drum  201  and the developing roller  302  is expressed as peripheral velocity ratio=peripheral velocity of developing roller  302 ÷peripheral velocity of photosensitive drum  201 ×100(%). In a portion where the photosensitive drum and the developing roller are in contact with each other, when the photosensitive drum and the developing roller rotate in a same direction as depicted, the peripheral velocity ratio is as follows. When the photosensitive drum is rotating at 200 mm/sec and the developing roller at 200 mm/sec, the peripheral velocity ratio is 100%. On the other hand, when the photosensitive drum and the developing roller rotate in different directions in the contact portion, the peripheral velocity ratio is −100%. 
     In the normal image formation mode, toner as a developer adhered to the developing roller  302  is conveyed to the photosensitive drum  201  by an action of a development contrast between a potential of an electrostatic latent image as an electrostatic image formed on the photosensitive drum  201  and a potential of the developing roller  302 . Accordingly, the electrostatic latent image as an electrostatic image formed on the photosensitive drum  201  as an image bearing member is developed as a toner image. On the other hand, in the wide-color gamut image formation mode, by increasing the peripheral velocity ratio between the photosensitive drum  201  and the developing roller  302 , a toner supply amount per unit area from the developing roller  302  to the photosensitive drum  201  is increased. Accordingly, due to the action of the development contrast between the potential of an electrostatic latent image formed on the photosensitive drum  201  as an image bearing member and the potential of the developing roller, a maximum amount of toner adherable to the developing roller  302  is conveyed to the photosensitive drum  201 . 
     Hereinafter, a process cartridge  208  and the image forming apparatus  200  according to the present embodiment will be described.  FIG. 1  is a schematic sectional view of the image forming apparatus  200  according to the first embodiment. The image forming apparatus  200  according to the present embodiment is an in-line system full-color laser printer adopting an intermediate transfer system. The image forming apparatus  200  is capable of forming a full-color image on a recording material P (for example, recording paper) as a recording medium in accordance with image information. The image information is input to a CPU  20  provided inside the image forming apparatus  200  from an image reading apparatus (not shown) which is connected to the image forming apparatus  200  or from a host device (not shown) such as a personal computer which is connected to the image forming apparatus  200  so as to be capable of communication. 
     In addition, as a plurality of image forming portions, the image forming apparatus  200  includes first, second, third, and fourth image forming portions S (SY, SM, SC, and SK) for forming images of the respective colors of yellow (Y), magenta (M), cyan (C), and black (K). In this case, the image forming portion S includes the process cartridge  208  and primary transfer rollers  212  ( 212 Y to  212 K) which are arranged so as to oppose the photosensitive drum  201  via an intermediate transfer belt  205 . In the present embodiment, the first to fourth image forming portions SY to SK are arranged in a single row in a direction intersecting both vertical and horizontal directions. Moreover, in the present embodiment, configurations and operations of the first to fourth image forming portions SY to SK are substantially the same with the exception of differences in colors of images formed. Therefore, unless the image forming portions must be distinguished from one another, the suffixes Y, M, C, and K will be omitted and the image forming portions will be collectively described. However, the present invention is not limited to this configuration and, alternatively, a configuration may be adopted in which black (K) has a larger shape and the image forming portion thereof is also larger than the other image forming portions. In addition, a configuration including a fifth image forming portion using white toner or transparent toner is also valid. 
     As a plurality of image bearing members, the image forming apparatus  200  includes four photosensitive drums  201  which are drum-shaped electrophotographic photoreceptors arranged parallel to each other in a direction intersecting the vertical direction. The photosensitive drum  201  as an image bearing member is rotationally driven in a direction of an arrow A (clockwise) in  FIG. 1  by a driving force of a motor (refer to  FIG. 2 ). In addition, a charging roller  202  as charging means configured to uniformly charge a surface of the photosensitive drum  201  and a scanner unit  203  configured to form an electrostatic latent image on the photosensitive drum  201  by irradiating a laser based on image information are arranged in a periphery of the photosensitive drum  201 . 
     In addition, a developing unit  204  configured to develop an electrostatic image as a toner image and a cleaning blade  206  configured to remove toner as a developer remaining on the surface of the photosensitive drum  201  after the toner image is transferred are arranged in the periphery of the photosensitive drum  201  as an image bearing member. Furthermore, a preliminary exposure LED  216  configured to eliminate a potential on the photosensitive drum  201  is arranged in the periphery of the photosensitive drum  201 . In addition, the intermediate transfer belt  205  for transferring a toner image on the photosensitive drum  201  to a recording material P as a recording medium is arranged so as to oppose the four photosensitive drums  201 . 
     The photosensitive drum  201  as an image bearing member, the charging roller  202 , the developing unit  204 , and the cleaning blade  206  are integrally configured as the process cartridge  208 . The process cartridge  208  is configured to be attachable and detachable to and from an apparatus main body of the image forming apparatus  200 . In addition, in the present embodiment, all of the process cartridges  208  for the respective colors have a same shape, and toners as developers of the respective colors of yellow (Y), magenta (M), cyan (C), and black (K) are housed in the process cartridges  208 . Furthermore, as the toners as developers in the present embodiment, toners having negative-charging characteristics are used. While the present embodiment is described using a process cartridge in which a photosensitive drum and a developing unit are integrally configured, this configuration is not restrictive. A configuration may be adopted in which a photosensitive unit including a photosensitive drum and a developing unit including a developer bearing member are respectively configured to be separately attachable and detachable to and from an apparatus main body of an image forming apparatus. In a similar manner, while the toners according to the present embodiment are one-component developers, two-component developers or magnetic toners may be used depending on the configuration. 
     The intermediate transfer belt  205  formed by an endless belt is in contact with all of the photosensitive drums  201  and moves in a direction of an arrow B (counterclockwise) in  FIG. 1 . In addition, the intermediate transfer belt  205  is stretched over a driver roller  209 , a secondary transfer opposing roller  210 , and a driven roller  211 . Four primary transfer rollers  212  are arranged parallel to each other on a side of an inner peripheral surface of the intermediate transfer belt  205  so as to oppose each photosensitive drum  201 . Furthermore, a bias having an opposite polarity (in the present embodiment, a positive polarity) to a normal charging polarity of the toners is applied to the primary transfer rollers  212  from a primary transfer bias power supply (not shown). Accordingly, a toner image on the photosensitive drum  201  is transferred onto the intermediate transfer belt  205 . 
     In addition, a secondary transfer roller  213  is arranged at a position opposing the secondary transfer opposing roller  210  on a side of an outer peripheral surface of the intermediate transfer belt  205 . Furthermore, a bias having an opposite polarity to the normal charging polarity of the toners is applied to the secondary transfer roller  213  from a secondary transfer bias power supply (not shown). Accordingly, a toner image on the intermediate transfer belt  205  is transferred onto the recording material P as a recording medium. 
     &lt;Configuration of Process Cartridge  208 &gt; 
     Next, an overall configuration of the process cartridge  208  to be attached to and detached from the image forming apparatus  200  according to the present embodiment will be described with reference to  FIG. 2 .  FIG. 2  is a schematic sectional view of the process cartridge  208  according to the first embodiment. Specifically,  FIG. 2  is a schematic sectional view of the process cartridge  208  as viewed from an axial direction of a center of rotation of the photosensitive drum  201 . Moreover, in the present embodiment, configurations and operations of the process cartridges  208  of the respective colors are the same with the exception of types (colors) of toners housed therein. Obviously, a configuration and operations of each process cartridge can be changed as appropriate depending on the configuration. 
     The process cartridge  208  includes a photoreceptor unit  301  including the photosensitive drum  201  as an image bearing member and the like and the developing unit  204  including the developing roller  302  as a developer bearing member and the like. The photoreceptor unit  301  includes a cleaning frame body  303  which supports various elements in the photoreceptor unit  301 . The photosensitive drum  201  is rotatably attached to the cleaning frame body  303  via a bearing member (not shown). In addition, the photosensitive drum  201  as an image bearing member is rotationally driven in the direction of the arrow A (clockwise) in  FIG. 2  in accordance with an image forming operation as a driving force of a motor (refer to  FIG. 2 ) is transferred to the photoreceptor unit  301 . 
     As the photosensitive drum  201  configured to perform a central role of an image forming process, an organic photoreceptor is used in which an outer circumferential surface of an aluminum cylinder is sequentially coated with an undercoat layer, a carrier generation layer, and a carrier transfer layer which are functional membranes. In addition, the cleaning blade  206  and the charging roller  202  are arranged in the photoreceptor unit  301  so as to come into contact with a circumferential surface of the photosensitive drum  201 . Furthermore, untransferred toner removed from the surface of the photosensitive drum  201  by the cleaning blade  206  is housed in the cleaning frame body  303 . 
     The charging roller  202  which is charging means is driven so as to follow the photosensitive drum  201  when a roller portion made of conductive rubber is brought into pressure contact with the photosensitive drum  201 . Prescribed DC voltage is applied to a core of the charging roller  202  and, accordingly, a uniform dark-part potential (Vd) is formed on the surface of the photosensitive drum  201 . In addition, as described earlier, the scanner unit  203  as an exposing apparatus exposes the photosensitive drum  201  with laser light which is emitted in correspondence with image data. 
     Subsequently, as charges on the surface of the photosensitive drum  201  are eliminated by a carrier from the carrier generation layer, the potential of the surface of the exposed photosensitive drum  201  drops. As a result, on the surface of the photosensitive drum  201 , a portion exposed by the laser assumes a prescribed light-part potential (V 1 ) and an unexposed portion not exposed by the laser assumes a prescribed dark-part potential (Vd). Accordingly, an electrostatic latent image as an electrostatic image is formed on the photosensitive drum  201 . 
     The developing unit  204  includes the developing roller  302  as a developer bearing member (which rotates in a direction of an arrow D), a developing blade  308 , and a toner supplying roller  304  (which rotates in a direction of an arrow E). In addition, the developing unit  204  includes a toner housing chamber  306  which houses the toner as a developer. The toner as a developer is stirred inside the toner housing chamber  306  by an action (a rotation in a direction of an arrow G) of a stirring member  307 . In addition, in the present embodiment, a prescribed DC bias is applied to the developing roller  302  as a developer bearing member. Toner adheres to a light-part potential portion of the photosensitive drum  201  due to a potential difference between the photosensitive drum  201  and the developing roller  302  in a developing portion where the photosensitive drum  201  and the developing roller  302  come into contact with each other. Accordingly, an electrostatic latent image as an electrostatic image on the photosensitive drum  201  is visualized. 
     &lt;Configuration of Fixing Apparatus&gt; 
       FIG. 3  is a schematic sectional view of a fixing apparatus  400  according to the first embodiment. The fixing apparatus  400  according to the present embodiment is a fixing apparatus adopting a pressure roller drive system and includes a heating member  410 , a cylindrical film  430  which comes into sliding contact with the heating member  410 , and a pressure roller  440  which forms a fixing nip portion N with the heating member  410  via the film  430 . In addition, the recording material P as a recording medium is sandwiched and conveyed in the fixing nip portion N and, at the same time, heated by heat from the heating member  410 . Accordingly, an unfixed image formed on the recording material P as a recording medium is fixed by heating to the recording material P. 
     In a state of being held by a heating member supporter  420 , the heating member  410  is in pressure contact with a prescribed pressing force with the pressure roller  440  which is a pressing member via the cylindrical film  430  as a flexible member. In addition, the pressure roller  440  is rotationally driven in a direction of an arrow H in  FIG. 3  by a rotational driving portion  480 . As the pressure roller  440  rotates and slidingly moves against an outer circumferential surface of the film  430 , the film  430  rotates in a direction of an arrow I in  FIG. 3 . Specifically, the film  430  rotates in the direction of the arrow I around the heating member supporter  420  holding the heating member  410 . 
     In addition, the heating member  410  is electrically heated by a heating member driving circuit  470  as power is supplied to the heating member  410  from a commercial power supply. Furthermore, the heating member  410  is controlled to a prescribed temperature adjusted for printing. In this state, the recording material P bearing an unfixed toner image T is sandwiched and conveyed in a direction of an arrow F in the fixing nip portion N. Furthermore, as heat from the heating member  410  is applied to the recording material P via the film  430 , the unfixed toner image T is fixed to the recording material P. Subsequently, the recording material P having passed the fixing nip portion N is separated in a curving manner from a surface of the film  430  and then discharged. Moreover, in the fixing apparatus  400  according to the present embodiment, a reference of paper passage of the recording material P is set to a central portion in a longitudinal direction (a direction perpendicular to the direction of the arrow F of the recording material P) of each member. 
     As the cylindrical film  430 , for example, a thin film cylinder with a thickness of around 30 μm to 100 μm and which uses a polyimide or SUS as a base layer is used. In addition, releasability from the toner is maintained by coating the base layer with PFA, PTFA and the like via a primer layer. Furthermore, a slide grease (not shown) is applied between an inner circumferential surface of the film  430  and the heating member supporter  420  and, accordingly, slidability between the film  430  and the heating member supporter  420  is maintained. 
     The pressure roller  440  is a rotating body in which, for example, an elastic layer such as silicone rubber is formed on a core. In addition, in the present embodiment, a releasing layer with a thickness of around 10 to 100 μm and which is made of FEP, PFA, or the like is provided on the base layer via a primer layer. Accordingly, releasability from the toner is maintained. In addition, the heating member supporter  420  is formed of a highly heat-resistant resin such as PPS, PAI, PI, PEEK, and liquid crystal polymer having a heat insulating property, high heat resistance, and rigidity or a composite material of the resin and ceramics, metal, glass, or the like. In this case, PPS stands for polyphenylene sulfide, PAI stands for polyamide-imide, PI stands for polyimide, and PEEK stands for polyether ether ketone. Furthermore, the rotational driving portion  480  includes a motor  481  which rotationally drives the pressure roller  440 , a control portion (CPU)  482  which controls rotation of the motor  481 , and the like. As the motor  481 , for example, a DC motor or a stepping motor can be used. 
     &lt;Explanation of Traceable Pattern&gt; 
     In the present embodiment, a dot pattern invisible to the naked eye is formed on the recording material P on which an image is formed. Accordingly, a machine number of an image forming apparatus which is a manufacturer can be deduced from the applied dot pattern. Therefore, an image forming apparatus which is a manufacturer having printed paper currency can be identified. This system is generally referred to as a traceable pattern system. A traceable pattern is normally formed using yellow toner which has low visibility. In addition, a traceable pattern indicates various pieces of information on an output device such as a name of a manufacturer, a model name, a machine number, and an output status during output of an image to the recording material P. Moreover, in the present embodiment, a dot pattern formed by a plurality of dots is used as a traceable pattern. Positions where the dot pattern is added onto an image are obtained by means such as converting, based on a certain rule, information to be superimposed. Various rules are conceivable for conversion into positional information. For example, in a conceivable case, a machine number or a model name of a printer main body is expressed by a binary sequence and “1” and “0” are respectively expressed by a presence or an absence of a dot pattern. However, methods are not particularly limited in the present embodiment. In addition, in the present embodiment, an image formed on the recording material P by the image forming apparatus  200  is a digital image formed by a plurality of dots. Furthermore, a traceable pattern is read using a dedicated reading apparatus. An appropriate process of converting a dot pattern into information is performed in reverse to a process of converting information to be superimposed into the dot pattern. Accordingly, the machine number or the model name of the printer main body having output printed matter can be identified. Moreover, in the present embodiment, a toner volume level of a traceable pattern is set to 60% or lower of a toner volume level of portions other than the traceable pattern (portions corresponding to “other than the dot pattern”). In other words, the toner volume level of a traceable pattern is 60% or lower of the toner volume level of portions where a normal image is formed. In addition, in the present embodiment, a size of dots constituting a traceable pattern changes depending on laser output. Specifically, the scanner unit  203  irradiates the photosensitive drum  201  with a laser in accordance with image information converted by a printer driver. Furthermore, the size of dots constituting a traceable pattern is determined in accordance with output of the laser. 
       FIG. 4  is a diagram showing a flow of processes executed by the image forming apparatus  200  according to the first embodiment. As shown in  FIG. 4 , a traceable pattern is generated by an additional information generating process which is independent of a normal image information processing process. The traceable pattern is independent of the normal image information processing process in order to prevent the traceable pattern from being intentionally modified by a user. In addition, information on the traceable pattern is added to normal image information by an additional information superimposing process which is performed after the normal image information processing process is finished. In this manner, the traceable pattern is output for each image forming apparatus  200  without being effected by normal image processing processes. 
     As shown in  FIG. 4 , image information input from an image input terminal is first converted by a color converting process from RGB (red, green, and blue) information into CMYK (cyan, magenta, yellow, and black) information. Next, the CMYK information is corrected by various correcting processes. For example, since a color expressed in RGB and a color expressed in CMYK do not necessarily match each other, the color expressed in CMYK is corrected so as to approach the color expressed in RGB as much as possible. 
     In addition, in a pseudo gradation processing process, dithering is performed so as to smooth shading of an image formed on the recording material P as a recording medium. Furthermore, in the additional information superimposing process, additional information generated by the additional information generating process is added to image information. In the present embodiment, in the additional information generating process, information on a traceable pattern for identifying an image forming apparatus having printed special paper currency and the like is generated. Subsequently, image information to which the additional information has been added is output from an image output terminal. 
     &lt;Normal Image Formation Mode and Wide-Color Gamut Image Formation Mode&gt; 
     In the present embodiment, the image forming apparatus  200  is capable of executing a wide-color gamut image formation mode in which a color gamut of an image is enlarged in order to form an image with better image quality. In the wide-color gamut image formation mode as a second image formation mode, by varying a peripheral velocity ratio (a ratio of peripheral velocities) between the photosensitive drum  201  and the developing roller  302 , a toner supply amount conveyed from the developing roller  302  to the photosensitive drum  201  can be increased. The peripheral velocity ratio (the ratio of peripheral velocities) between the photosensitive drum  201  and the developing roller  302  can be changed by increasing the peripheral velocity of the developing roller  302  or by reducing the peripheral velocity of the photosensitive drum  201 . 
     In the present embodiment, on the photosensitive drum  201  as an image bearing member, a dark-part potential which refers to a potential of a portion not exposed by a laser is set to −500 [V] and a light-part potential which refers to a potential of a portion exposed by the laser is set to −100 [V]. In addition, in the present embodiment, the light-part potential is acquired by measuring a surface of the photosensitive drum  201  with a potentiometer when forming an image pattern (for example, a solid black image) which causes an image to be formed over the entire recording material P. Furthermore, by setting a developing potential of the developing roller to −350 [V], a difference between the light-part potential of the photosensitive drum  201  and the potential of the developing roller  302  and a difference between the dark-part potential of the photosensitive drum  201  and the potential of the developing roller  302  are respectively set to Δ250 [V]. Hereinafter, the difference between the light-part potential of the photosensitive drum  201  and the potential of the developing roller  302  and the difference between the dark-part potential of the photosensitive drum  201  and the potential of the developing roller  302  will be referred to as a development contrast. 
     In addition, with respect to toner to adhere to the developing roller  302  as a developer bearing member, in the present embodiment, a toner amount per unit area (hereafter, denoted by M/S) is set to 3.0×10 −3  [kg/m 2 ]. Furthermore an electrified charge amount of the toner per unit area (hereafter, denoted by Q/S) is set to −0.15×10 −3  [C/m 2 ]. In the present embodiment, the toner supply amount was confirmed by setting the peripheral velocity of the photosensitive drum  201  to 0.2 [m/s] (constant) and varying the peripheral velocity of the developing roller  302  relative to the photosensitive drum  201 . Moreover, a peripheral velocity ratio of 100% is assumed to represent a case where the peripheral velocities of the photosensitive drum  201  and the developing roller  302  are the same and a peripheral velocity ratio of 140% is assumed to represent a case where the peripheral velocity of the developing roller  302  is 1.4 times the peripheral velocity of the photosensitive drum  201 . In addition, since a tinge of an image and density of the image are strongly related to each other, the present embodiment will be described with a focus on image density. In the present embodiment, the ratio between the peripheral velocity of the photosensitive drum  201  and the peripheral velocity of the developing roller  302  is increased by increasing the peripheral velocity of the developing roller  302  relative to the peripheral velocity of the photosensitive drum  201 . 
     A toner image formed on the photosensitive drum  201  as an image bearing member is eventually fixed onto the recording material P.  FIG. 5  is a diagram showing a relationship between an amount of toner forming an image and density of the image according to the first embodiment. Moreover, since there is no difference among experiment results of the YMC toners, the experiment results will be described using the cyan toner. In the case of a peripheral velocity ratio of 120%, density of 1.45 (Macbeth RD-918) generally required in office documents was obtained and a toner volume level on the recording material P was 3.6×10 −3  kg/m 2 . When the peripheral velocity ratio was subsequently increased to 200%, density of 1.72 was obtained and the toner volume level on the recording material P was 6.0×10 −3  kg/m 2 . 
     In consideration thereof, in the normal image formation mode as a first image formation mode intended for office applications and the like, the peripheral velocity ratio is set to 120% as a first peripheral velocity ratio ΔV 1  so that an image density of 1.45 is attained. In addition, in the present embodiment, in the wide-color gamut image formation mode as a second image formation mode, the peripheral velocity ratio is set to 200% as a second peripheral velocity ratio ΔV 2  so that an image density of 1.7 or higher is attained. As a result, when changing the peripheral velocity ratio from 120% to 200%, a ΔE target enlargement amount of 10 or larger was secured for red. In this case, “a ΔE target enlargement amount of 10 or larger” means that a value of coordinates in the L*a*b* color system increased by 10 or more. Moreover, red is created by mixing Y and M toners at a ratio of 1:1. 
     The colors were measured using i1pro manufactured by X-Rite, Incorporated. Measurements were conducted under conditions of a black backing, a D50 light source, and a 2-degree visual field. In addition, GF-C081 manufactured by Canon Inc. was used as paper for sampling. Furthermore, the fixing apparatus  400  was configured to convey the recording material P to a nip portion of the film  430  and the pressure roller  440  after a lapse of 10 seconds from a temperature of an outlet of the nip portion of the film  430  and the pressure roller  440  reaching 180° C. 
     &lt;Formation of Traceable Pattern&gt; 
     Conventionally, when confirming a state of a traceable pattern, while the traceable pattern is invisible in the normal image formation mode, there is a risk that the traceable pattern may become visible in the wide-color gamut image formation mode due to increased density of the traceable pattern. In this case, a criterion of a traceable pattern being “invisible” or “visible” is whether or not the traceable pattern affects an original image (an image excluding the traceable pattern) as seen by the naked eye. For example, a traceable pattern is “invisible” if the traceable pattern cannot be identified by the naked eye even when the traceable pattern can be identified through a magnifying glass. In the wide-color gamut image formation mode as a second image formation mode, since a larger amount of toner is supplied to dots forming the traceable pattern, density of the traceable pattern is increased. In addition, in the wide-color gamut image formation mode as a second image formation mode, since a larger amount of toner is supplied to dots forming the traceable pattern, dots forming the traceable pattern become larger than those in the normal image formation mode. Accordingly, there is a risk that a traceable pattern may become visible in the wide-color gamut image formation mode as a second image formation mode. 
     In consideration thereof, in the present embodiment, when forming an image on the recording material P in the wide-color gamut image formation mode, an amount of laser light is adjusted so that a light-part potential of a portion corresponding to a dot constituting a traceable pattern on the surface of the photosensitive drum  201  equals −400 [V]. In other words, exposure of a portion corresponding to a traceable pattern on the photosensitive drum  201  causes a potential of the portion to become lower than a potential of portions other than that corresponding to the traceable pattern by 260 V. In this case, a light-part potential refers to a potential of a portion exposed by the scanner unit  203  as an exposing apparatus on the photosensitive drum  201 . In the present embodiment, when toner adheres to the light part, an electrostatic latent image as an electrostatic image formed on the photosensitive drum  201  is developed as a toner image. 
     For example, the image forming apparatus  200  may be provided with a storage portion  500  which stores information on the image forming apparatus  200 . The storage portion  500  is, for example, a storage medium such as a hard disk drive or a memory. In addition, the storage portion  500  may store, in advance, exposure of a traceable pattern in the normal image formation mode as a first image formation mode and exposure of a traceable pattern in the wide-color gamut image formation mode. In this case, a control portion  600  provided in the image forming apparatus  200  executes a program stored in the storage portion  500  to adjust exposure of a traceable pattern to the set exposure in the normal image formation mode and the wide-color gamut image formation mode. The control portion  600  adds, to image information, additional information (corresponding to first information) regarding exposure of a traceable pattern in the normal image formation mode or additional information (corresponding to second information) regarding exposure of a traceable pattern in the wide-color gamut image formation mode. In addition, based on the image information to which the additional information has been added, the control portion  600  controls exposure by the scanner unit  203  as an exposing apparatus and forms an image on the recording material P. 
       FIG. 6A  is a diagram showing potentials on the photosensitive drum  201  of an image portion and a traceable pattern portion in both the normal image formation mode and the wide-color gamut image formation mode according to the first embodiment. The contrast (development contrast) in the drawing refers to a difference between a developing potential and a light-part potential, and an amount in which toner adheres onto the photosensitive drum is determined in proportion to a magnitude of the contrast. Since toner on the photosensitive drum is transferred as-is onto the recording material, a toner volume level on the recording material is also determined in proportion to the contrast.  FIG. 6B  is a diagram showing toner amounts laid onto the recording material under the respective conditions shown in  FIG. 6A . By comparing  FIG. 6B  with  FIG. 6A , it is shown that the toner volume level on the recording material is determined based on a magnitude of the contrast. The following table compiles a potential of each portion and a toner volume level on a recording material under respective conditions. 
     
       
         
           
               
               
               
            
               
                   
                   
               
               
                   
                   
                 Wide-color gamut image formation 
               
               
                   
                 Normal image formation mode 
                 mode 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 Traceable pattern 
                   
                 Traceable pattern 
               
               
                   
                 Image portion 
                 portion 
                 Image portion 
                 portion 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                 D roller peripheral 
                 120% 
                 200% 
               
               
                 velocity ratio 
               
            
           
           
               
               
               
               
               
            
               
                 Dark-part potential 
                 −500 V 
                 −500 V 
                 −750 V 
                 −750 V 
               
               
                 Developing 
                 −350 V 
                 −350 V 
                 −580 V 
                 −580 V 
               
               
                 potential 
               
               
                 Light-part potential 
                 −100 V 
                 −250 V 
                 −180 V 
                 −440 V 
               
               
                 Contrast 
                  250 V 
                  100 V 
                  400 V 
                  140 V 
               
               
                 Toner volume 
                  0.36 mg/m 2   
                  0.15 mg/m 2   
                  0.60 mg/m 2   
                  0.22 mg/m 2   
               
               
                 level (on recording 
               
               
                 material) 
               
               
                   
               
            
           
         
       
     
     Let C 1  (a first development contrast C 1 ) denote a contrast of the image portion in the normal image formation mode and C 2  (a second development contrast C 2 ) denote a contrast of the traceable pattern portion in the normal image formation mode. In a similar manner, let C 3  (a third development contrast C 3 ) denote a contrast of the image portion in the wide-color gamut image formation mode and C 4  (a fourth development contrast C 4 ) denote a contrast of the traceable pattern portion in the wide-color gamut image formation mode. By reducing the contrast of the traceable pattern portion in comparison to the image portion in the normal image formation mode, a toner volume level of the traceable pattern portion is lower than that of the image portion to make the traceable pattern less visible to the naked eye. In a similar manner, in the wide-color gamut image formation mode, the contrast of the traceable pattern portion is smaller than that in the image portion and a toner volume level of the traceable pattern portion is lower. As a result, even in the wide-color gamut image formation mode, the traceable pattern cannot be identified by the naked eye. 
     Furthermore, when contrast ratios between the image portion and the traceable pattern portion in the respective modes are denoted by ΔC 1 =C 2 /C 1  and ΔC 2 =C 4 /C 3 , 
     ΔC 1 =C 2 /C 1 =100V/250V=2/5 and 
     ΔC 2 =C 4 /C 3 =140V/400V=7/20 are satisfied, 
     which in turn satisfy a relationship expressed as
 
ΔC2&lt;ΔC1.
 
Since a toner volume level increases in the wide-color gamut image formation mode and a volume level in a traceable pattern portion increases when a contrast ratio remains similar to that in the normal image formation mode, the settings described above are adopted in order to prevent an occurrence of a state where the traceable pattern becomes identifiable to the naked eye. By reducing the contrast ratio in the wide-color gamut image formation mode in comparison to the normal image formation mode, a toner volume level of the traceable pattern portion is suppressed to maintain a state where the traceable pattern is invisible to the naked eye.
 
     Moreover, ΔC 1  and ΔC 2  can be used as “absolute difference values” instead of “ratios”. For example, it can be expressed that ΔC 1 =|C 2 −C 1 | and ΔC 2 =|C 4 −C 3 |. In this case, the relationship expressed as ΔC 2 &lt;ΔC 1  is also satisfied. 
     Due to this effect, in the present embodiment, while a toner volume level in the image portion in the wide-color gamut image formation mode is 0.60 mg/m 2  which represents an increase of 0.24 mg/m 2  from the normal image formation mode, a toner volume level in the traceable pattern portion is 0.22 mg/m 2  which represents an increase of only 0.07 mg/m 2  from the normal image formation mode. Therefore, compared to the normal image formation mode, in the wide-color gamut image formation mode, the toner volume level in the traceable pattern portion (specific dot pattern), that is density of the traceable pattern portion is increased at a rate (0.15 mg/m 2  to 0.22 mg/m 2 ) lower than another rate (0.36 mg/m 2  to 0.60 mg/m 2 ) at which the toner volume level of the image portion (a dot pattern of the image other than the specific dot pattern), that is, density of the image portion is increased. Compared to the toner volume level of the image portion, that is the density of the image portion to be developed, the toner volume level in the traceable pattern portion, that is, the density of the traceable pattern portion to be developed is lowered by adjusting exposure of an electrostatic image corresponding to the traceable pattern portion by the exposing apparatus. According to adjustment of the exposure amount of the present embodiment in the wide-color gamut image formation mode, a potential of a portion, corresponding to the traceable pattern portion on the photosensitive drum  201  in a case where the toner volume level (density) of the traceable pattern portion is increased at 0.07 mg/m 2  (the rate) and the toner volume level (density) of the image portion is increased at 0.24 mg/m 2  (the another rate), becomes lower than that in a case where the toner volume level (density) of both the traceable pattern portion and the image portion are increased at 0.24 mg/m 2  (the another rate). Thus, in the wide-color gamut image formation mode, a difference between (A) a potential of a portion corresponding to the traceable pattern portion in a case where the toner volume level (density) of the traceable pattern portion is increased at 0.07 mg/m 2  (the rate) and the toner volume level (density) of the image portion is increased at 0.24 mg/m 2  (the another rate) and (B) a potential of the developing roller  302  is smaller than a difference between (i) a potential of a portion corresponding to the traceable pattern portion in a case where the toner volume level (density) of both the traceable pattern portion and the image portion are increased at 0.24 mg/m 2  (the another rate) and (ii) the potential of the developing roller 302 . 
     Let M 1  (a first developer volume level M 1 ) denote a toner volume level in an image portion (a region corresponding to an image) in the normal image formation mode and M 2  (a second developer volume level M 2 ) denote a toner volume level in a traceable pattern portion (a region corresponding to a dot pattern) in the normal image formation mode. In a similar manner, let M 3  (a third developer volume level M 3 ) denote a toner volume level in an image portion in the wide-color gamut image formation mode and M 4  (a fourth developer volume level M 4 ) denote a toner volume level in a traceable pattern portion in the wide-color gamut image formation mode. When ratios of toner volume levels between the image portion and the traceable pattern portion in the respective modes are denoted by ΔM 1 =M 2 /M 1  and ΔM 2 =M 4 /M 3 , 
     ΔM 1 =M 2 /M 1 =0.15 mg/m 2 /0.36 mg/m 2 =5/12 and 
     ΔM 2 =M 4 /M 3 =0.22 mg/m 2 /0.60 mg/m 2 =11/30 are satisfied, which in turn satisfy a relationship expressed as
 
ΔM2&lt;ΔM1.
 
     Moreover, ΔM 1  and ΔM 2  can also be used as “absolute difference values” instead of “ratios”. For example, it can be expressed that ΔM 1 =|M 2 −M 1 | and ΔM 2 =|M 4 −M 3 |. In this case, the relationship expressed as ΔM 2 &lt;ΔM 1  is also satisfied. 
     Moreover, since it is difficult to actually measure a toner volume level in a traceable pattern portion, a measurement of the toner volume levels described herein are substituted by measuring a toner amount when forming a solid image on a recording material under same contrast conditions as a traceable pattern portion. 
     Normally, when measuring a toner volume level on a recording material, toner is laid onto the recording material under measurement conditions (type of recording material, conditions of respective potentials, and the like) and, without fixing the toner, the toner is collected in an unfixed state and a mass thereof is measured. At the same time, an area of a range onto which the toner had been laid is measured and the “toner volume level” in the table described earlier is calculated based on the measured mass and area of the toner. 
     For example, when measuring a toner volume level of an image portion in the normal image formation mode, first, toner is laid onto an appropriate recording material (for example, a sheet of Image Coat Gloss 158 manufactured by Canon Inc.) under same conditions as the image portion (D roller peripheral velocity ratio: 120%, dark-part potential: −500 V, developing potential: −350 V, light-part potential: −100 V, and contrast: 250V). An area on which toner is to be laid is set in advance to, for example, a rectangle of 20 mm×50==1000 mm 2 . By collecting the toner laid onto this range and measuring a mass thereof, the toner volume level (in this case, 0.36 mg/m 2 ) in the table described earlier is calculated. A similar measurement method is used for a toner volume level in the wide-color gamut image formation mode with the exception of changing the D roller peripheral velocity ratio, conditions of respective potentials, and the like to those corresponding to the wide-color gamut image formation mode. 
     Incidentally, since a traceable pattern portion is usually surrounded by an image portion and finely divided on a recording material, a toner volume level of only the toner volume level cannot be measured by a normal print operation. In consideration thereof, when measuring a toner volume level of a traceable pattern (in this case, the normal image formation mode will continue to be described as an example), the toner volume level of the traceable pattern will be substituted by laying toner onto an appropriate recording material (for example, a sheet of Image Coat Gloss 158 manufactured by Canon Inc.) under same conditions as the traceable pattern portion (D roller peripheral velocity ratio: 120%, dark-part potential: −500 V, developing potential: −350 V, light-part potential: −250 V, and contrast: 100 V) and then measuring a mass of the toner. A measurement of a toner volume level of a traceable pattern portion in the wide-color gamut image formation mode is performed using a similar method by changing the D roller peripheral velocity ratio, conditions of respective potentials, and the like to those corresponding to the wide-color gamut image formation mode. 
     As described above, in the present embodiment, even in the wide-color gamut image formation mode, a toner volume level on a recording material can be suppressed and a traceable pattern can be prevented from becoming visible by adjusting a contrast on a photosensitive drum. 
     In the present embodiment, a case has been described in which the two modes of a normal image formation mode as a first image formation mode and a wide-color gamut image formation mode as a second image formation mode are executed. However, modes are not necessarily limited to the above. For example, the image forming apparatus  200  may have two wide-color gamut image formation modes, namely, a first wide-color gamut image formation mode (density: 1.7 or higher, peripheral velocity ratio: 200%) and a second wide-color gamut image formation mode (density: 1.9 or higher, peripheral velocity ratio: 300%). In this case, an amount of laser light which irradiates a portion corresponding to a traceable pattern on the photosensitive drum  201  may be respectively set in the first wide-color gamut image formation mode and the second wide-color gamut image formation mode. Accordingly, in both the first wide-color gamut image formation mode and the second wide-color gamut image formation mode, by adjusting exposure of the portion corresponding to the traceable pattern (by setting a density of the traceable pattern to or below a prescribed density), the traceable pattern can be prevented from becoming visible. 
     As described above, according to the present embodiment, in the wide-color gamut image formation mode as a second image formation mode, density of a traceable pattern is set lower than the density of the traceable pattern in a case where density of an entire image including the traceable pattern is increased in the wide-color gamut image formation mode. Accordingly, a traceable pattern is prevented from becoming visible to a user in the wide-color gamut image formation mode. 
     (Second Embodiment) 
     Parts of the present embodiment which have same functions as those of the first embodiment will be denoted by the same reference characters and a description thereof will be omitted. In the present embodiment, unlike in the first embodiment, in the wide-color gamut image formation mode, visualization of a traceable pattern is suppressed by increasing spacing between two adjacent dots among a plurality of dots forming the traceable pattern. In this case, a traceable pattern is normally formed by a plurality of dots. In addition, in the present embodiment, separate traceable patterns are respectively set in the normal image formation mode and the wide-color gamut image formation mode. In the present embodiment, in the wide-color gamut image formation mode in which density of an image formed on the recording material P is high (a toner volume level is high), spacing between dots forming a traceable pattern is set wider than in the normal image formation mode. In other words, in the present embodiment, shapes of the traceable pattern are changed between the wide-color gamut image formation mode and the normal image formation mode. 
     On the other hand, in the normal image formation mode in which density of an image formed on the recording material P is not high, spacing between dots forming a traceable pattern is not widened. In the present embodiment, a traceable pattern formed on the recording material P can be read using a dedicated reading apparatus. In addition, in order to make the traceable pattern easily readable by the reading apparatus, in the normal image formation mode, spacing between the dots forming the traceable pattern is set to an appropriate spacing. In the present embodiment, in the normal image formation mode, spacing between dots forming a traceable pattern is set narrower than in the wide-color gamut image formation mode. 
     Specifically, for example, the storage portion  500  stores, in advance, a traceable pattern in the normal image formation and a traceable pattern in the wide-color gamut image formation mode. As described above, it is assumed that, in the wide-color gamut image formation mode, spacing between dots forming a traceable pattern is set wider than in the normal image formation mode. In this case, the control portion  600  provided in the image forming apparatus  200  executes a program stored in the storage portion  500  to form a traceable pattern set in advance on the recording material P in the normal image formation mode and in the wide-color gamut image formation mode. The control portion  600  adds, to image information, additional information on a traceable pattern in the normal image formation mode as a first image formation mode, or additional information on a traceable pattern in the wide-color gamut image formation mode. In addition, based on image information to which the additional information has been added, the control portion  600  controls operations of the image forming apparatus  200  so that the traceable pattern is formed and an image is formed on the recording material P. Moreover, the control portion  600  is, for example, a processing unit such as a CPU and is capable of controlling operations of devices inside the image forming apparatus  200  by executing a program stored in the storage portion  500 . 
       FIG. 7  is a diagram showing a traceable pattern A formed in the normal image formation mode as a first image formation mode. In addition,  FIG. 8  is a diagram showing a traceable pattern B formed in the wide-color gamut image formation mode. For example, let us assume that the traceable pattern formed on the recording material P in the normal image formation mode is the traceable pattern A (refer to  FIG. 7 ). In the normal image formation mode as a first image formation mode, the traceable pattern A is formed on the recording material P on which an image is formed. In this case, the traceable pattern A formed on the recording material P is invisible. 
     However, when forming the traceable pattern A in the wide-color gamut image formation mode as a second image formation mode, there is a risk that a part of the traceable pattern A may become visible due to an increase of a toner amount per unit area of the traceable pattern A. In the wide-color gamut image formation mode, since a larger amount of toner is laid on the traceable pattern A than in the normal image formation mode, there is a risk that the toner may even spread to blank portions between dots forming the traceable pattern A. In this case, as the blanks between the dots forming the traceable pattern A become filled, as shown in  FIG. 8 , a plurality of separate dots may merge into one large dot. As a result, there is a risk that the traceable pattern A may become visible. 
       FIG. 9  is a diagram showing the traceable pattern B in the normal image formation mode as a first image formation mode. In addition,  FIG. 10  is a diagram showing the traceable pattern B in the wide-color gamut image formation mode. In the present embodiment, in the first wide-color gamut image formation mode (density: 1.7 or higher, peripheral velocity ratio: 200%), spacing between dots forming the traceable pattern B is widened as shown in  FIG. 9 . In the present embodiment, by widening the spacing between dots forming the traceable pattern B, a plurality of dots forming the traceable pattern B are prevented from merging into one large dot. As a result, in the first wide-color gamut image formation mode, even when toner is laid in an amount larger than usual on the dots forming the traceable pattern B, since the dots are sufficiently separated from each other, situations where the traceable pattern B becomes visible can be reduced. For the dots to fulfill their roles as a traceable pattern, independency of the dots constituting the traceable pattern must be secured. When the dots constituting a traceable pattern are identifiable, the dots have “independency”. On the other hand, in a case where adjacent dots are connected to each other and the dots are not identifiable, the dots do not have “independency”. Therefore, in the present embodiment, a dot on which toner is laid is at least not formed adjacent to a dot constituting a traceable pattern. Generally, a traceable pattern is formed in a size of around 3×3 dots (a square with 3 dots on a side) to 5×5 dots (a square with 5 dots on aside). Since authentication performance declines when using a large traceable pattern, a size of a traceable pattern is set to a certain size. In consideration thereof, in the present embodiment, when enlarging the spacing between dots in the wide-color gamut image formation mode, an appropriate size of a traceable pattern is around 10×10 dots (a square with 10 dots on a side). 
     In addition,  FIG. 11  is a diagram showing a traceable pattern C in the normal image formation mode. Furthermore,  FIG. 12  is a diagram showing the traceable pattern C in the wide-color gamut image formation mode. In the present embodiment, in the second wide-color gamut image formation mode (density: 1.9 or higher, peripheral velocity ratio: 300%), the traceable pattern C is formed on the recording material P. Spacing between dots forming the traceable pattern C is set wider than the dots forming the traceable pattern B. Therefore, although a toner amount per unit area in the second wide-color gamut image formation mode is larger than a toner amount per unit area in the first wide-color gamut image formation mode, even in the second wide-color gamut image formation mode, the traceable pattern C can be prevented from becoming visible. 
     As described above, in the present embodiment, in the wide-color gamut image formation mode, spacing between dots forming a traceable pattern is set wider than in a case where density of an entire image including the traceable pattern is increased by the wide-color gamut image formation mode. Accordingly, a traceable pattern is prevented from becoming visible to a user in the wide-color gamut image formation mode. 
     Moreover, in the respective embodiments, a color gamut and/or density of an image need not necessarily be changed in accordance with a peripheral velocity ratio between the photosensitive drum  201  and the developing roller  302 . For example, a color gamut and/or density of an image may be changed by changing exposure by the scanner unit  203  with respect to the photosensitive drum  201 . 
     In addition, in the first embodiment, the exposure by the scanner unit  203  with respect to the photosensitive drum  201  need not necessarily be stored in advance in a storage medium. For example, density of a traceable pattern may be adjusted by correcting the exposure by the scanner unit  203  with respect to the photosensitive drum  201  as an image bearing member. 
     Furthermore, in the second embodiment, spacing between dots forming a traceable pattern need not necessarily be stored in advance in a storage medium. For example, the spacing between dots forming a traceable pattern may be widened by correcting the spacing between the dots forming the traceable pattern. 
     In addition, while the traceable pattern A, the traceable pattern B, and the traceable pattern C are used as traceable patterns in the second embodiment, traceable patterns are not limited thereto. Instead of being limited to the traceable patterns A to C, traceable patterns may be used without particular limitation as long as a similar effect can be obtained. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefits of Japanese Patent Application No. 2016-057714, filed on Mar. 22, 2016, and Japanese Patent Application No. 2017-019608, filed on Feb. 6, 2017, which are hereby incorporated by reference herein in their entirety.