Patent Publication Number: US-11640120-B2

Title: Image forming apparatus and image forming method

Description:
TECHNICAL FIELD 
     The present disclosure relates to an image forming apparatus and an image forming method. 
     BACKGROUND ART 
     An electrophotographic image forming apparatus collects toner remaining on the circumferential surface of an image bearing member therein using a cleaning member (e.g., a cleaning blade). In order to form high-definition images, it is desirable to use a toner having a small particle diameter and a high roundness. However, such a toner easily passes through a gap between the cleaning member and the circumferential surface of the image bearing member, tending to cause insufficient cleaning. In order to prevent insufficient cleaning, for example, it has been contemplated to tightly press the cleaning member against the image bearing member. However, the cleaning member tightly pressed against the image bearing member rubs hard on the circumferential surface of the image bearing member, and as a result some failure may occur in the image bearing member. 
     In order to reduce friction force between the cleaning member and the circumferential surface of the image bearing member, for example, it has been contemplated to apply a lubricant to the image bearing member. An image forming apparatus for example disclosed in Patent Literature 1 includes a lubricant application mechanism disposed upstream of a cleaning means for the image bearing member. 
     CITATION LIST 
     Patent Literature 
     [Patent Literature 1] Japanese Patent Application Laid-Open Publication No. 2000-075752 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, the image forming apparatus disclosed in Patent Literature 1 includes a lubricant application mechanism. This complicates the configuration of the image forming apparatus to increase manufacturing cost. Furthermore, irregularity in lubricant application on the image bearing member may occur in the image forming apparatus disclosed in Patent Literature 1. The inventors&#39; study revealed that such application irregularity tends to cause a ghost image. 
     The present invention has been made in view of the foregoing and has its object of providing an image forming apparatus and an image forming method capable of inhibiting occurrence of a ghost image and toner charge-up. 
     Solution to Problem 
     An image forming apparatus according to the present invention includes an image bearing member, a charger, a light exposure device, a development device, a transfer belt, a primary transfer device, a secondary transfer device, and a cleaning member. The charger charges a circumferential surface of the image bearing member to a positive polarity. The light exposure device exposes the charged circumferential surface of the image bearing member to light to form an electrostatic latent image on the circumferential surface of the image bearing member. The development device develops the electrostatic latent image into a toner image through supply of a toner to the electrostatic latent image. The transfer belt is in contact with the circumferential surface of the image bearing member. The primary transfer device primarily transfers the toner image from the circumferential surface of the image bearing member to the transfer belt. The secondary transfer device secondarily transfers the toner image from the transfer belt to a recording medium. The cleaning member is pressed against the circumferential surface of the image bearing member and collects residual toner of the toner remaining on the circumferential surface of the image bearing member as a result of the toner being primarily transferred. The transfer belt has a surface resistivity of at least 6 Log Ω and no greater than 11 Log Ω. A linear pressure of the cleaning member on the circumferential surface of the image bearing member is at least 10 N/m and no greater than 40 N/m. The image bearing member includes a conductive substrate and a photosensitive layer of a single layer. The photosensitive layer contains a charge generating material, a hole transport material, an electron transport material, and a binder resin. The image bearing member satisfies formula (1). 
     
       
         
           
             
               
                 
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     In the formula (1), Q represents a charge amount of the image bearing member. S represents a charge area of the image bearing member. d represents a film thickness of the photosensitive layer. ε r  represents a specific permittivity of the binder resin contained in the photosensitive layer. ε 0  represents the vacuum permittivity. V represents a value calculated from an equation V=V 0 −V r . V r  represents a first potential of the circumferential surface of the image bearing member yet to be charged by the charger. V 0  represents a second potential of the circumferential surface of the image bearing member charged by the charger. 
     An image forming method according to the present invention includes charging, exposing to light, developing, performing primary transfer, performing secondary transfer, and performing cleaning. In the charging, a circumferential surface of an image bearing member is charged to a positive polarity. In the exposing to light, the charged circumferential surface of the image bearing member is exposed to light to form an electrostatic latent image on the circumferential surface of the image bearing member. In the developing, the electrostatic latent image is developed into a toner image through supply of a toner to the electrostatic latent image. In the performing primary transfer, the toner image is primarily transferred from the circumferential surface of the image bearing member to a transfer belt that is in contact with the circumferential surface. In the performing secondary transfer, the toner image is secondarily transferred from the transfer belt to a recording medium. In the performing cleaning, cleaning is performed to collect residual toner by pressing a cleaning member against the circumferential surface of the image bearing member. The residual toner is toner of the toner remaining on the circumferential surface of the image bearing member as a result of the primary transfer of the toner image. The transfer belt has a surface resistivity of at least 6 Log Ω and no greater than 11 Log Ω. A linear pressure of the cleaning member on the circumferential surface of the image bearing member is at least 10 N/m and no greater than 40 N/m. The image bearing member includes a conductive substrate and a photosensitive layer of a single layer. The photosensitive layer contains a charge generating material, a hole transport material, an electron transport material, and a binder resin. The image bearing member satisfies formula (1). 
     
       
         
           
             
               
                 
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     In the formula (1), Q represents a charge amount of the image bearing member. S represents a charge area of the image bearing member. d represents a film thickness of the photosensitive layer. ε r  represents a specific permittivity of the binder resin contained in the photosensitive layer. ε 0  represents the vacuum permittivity. V represents a value calculated from an equation V=V 0 −V r . V r  represents a first potential of the circumferential surface of the image bearing member yet to be charged in the charging. V 0  represents a second potential of the circumferential surface of the image bearing member charged in the charging. 
     Advantageous Effects of Invention 
     With the image forming apparatus according to the present invention and the image forming method according to the present invention, occurrence of a ghost image and toner charge-up can be inhibited. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a cross-sectional view of an image forming apparatus according to a first embodiment of the present invention. 
         FIG.  2    is a diagram illustrating a photosensitive member included in the image forming apparatus illustrated in  FIG.  1    and elements around the photosensitive member. 
         FIG.  3    is a graph representation explaining toner charge-up. 
         FIG.  4    is a partial cross-sectional view of an example of the photosensitive member included in the image forming apparatus illustrated in  FIG.  1   . 
         FIG.  5    is a partial cross-sectional view of an example of the photosensitive member included in the image forming apparatus illustrated in  FIG.  1   . 
         FIG.  6    is a partial cross-sectional view of an example of the photosensitive member included in the image forming apparatus illustrated in  FIG.  1   . 
         FIG.  7    is a diagram illustrating a measuring device for measuring a first potential V r  and a second potential V 0 . 
         FIG.  8    is a graph representation illustrating a relationship between surface charge density and charge potential of photosensitive members. 
         FIG.  9    is a diagram illustrating a power supply system for primary transfer rollers included in the image forming apparatus illustrated in  FIG.  1   . 
         FIG.  10    is a diagram illustrating a drive mechanism for implementing a thrust mechanism. 
         FIG.  11    is a graph representation illustrating relationships between number average roundness of toner and linear pressure of a cleaning blade for volume median diameters of toners. 
         FIG.  12    is a graph representation illustrating relationships between transfer current and surface potential drop due to transfer for a photosensitive member according to a comparative example. 
         FIG.  13    is a graph representation illustrating relationships between transfer current and surface potential drop due to transfer for photosensitive members according to an example. 
         FIG.  14    is a graph representation illustrating a relationship between chargeability ratio and surface potential drop due to transfer for photosensitive members. 
         FIG.  15    is a graph representation illustrating a relationship between surface resistivity of a transfer belt and reflection density difference in output images. 
         FIG.  16    is a graph representation illustrating a relationship between surface resistivity of the transfer belt and charge amount of toner on the transfer belt. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First of all, terms used in the present description will be described. The term “-based” may be appended to the name of a chemical compound in order to form a generic name encompassing both the chemical compound itself and derivatives thereof. Also, when the term “-based” is appended to the name of a chemical compound used in the name of a polymer, the term indicates that a repeating unit of the polymer originates from the chemical compound or a derivative thereof. 
     Hereinafter, a halogen atom, an alkyl group having a carbon number of at least 1 and no greater than 8, an alkyl group having a carbon number of at least 1 and no greater than 6, an alkyl group having a carbon number of at least 1 and no greater than 5, an alkyl group having a carbon number of at least 1 and no greater than 4, an alkyl group having a carbon number of at least 1 and no greater than 3, and an alkoxy group having a carbon number of at least 1 and no greater than 4 each refer to the following unless otherwise stated. 
     Examples of the halogen atom (halogen groups) include a fluorine atom (a fluoro group), a chlorine atom (a chloro group), a bromine atom (a bromo group), and an iodine atom (an iodine group). 
     An alkyl group having a carbon number of at least 1 and no greater than 8, an alkyl group having a carbon number of at least 1 and no greater than 6, an alkyl group having a carbon number of at least 1 and no greater than 5, an alkyl group having a carbon number of at least 1 and no greater than 4, and an alkyl group having a carbon number of at least 1 and no greater than 3 as used herein each refer to an unsubstituted straight chain or branched chain alkyl group. Examples of the alkyl group having a carbon number of at least 1 and no greater than 8 include a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, a sec-butyl group, a tert-butyl group, an n-pentyl group, an isopentyl group, a neopentyl group, a 1,1-dimethylpropyl group, a 1,2-dimethylpropyl group, a straight chain or branched chain hexyl group, a straight chain or branched chain heptyl group, and a straight chain or branched chain octyl group. Out of the chemical groups listed as examples of the alkyl group having a carbon number of at least 1 and no greater than 8, the chemical groups having a carbon number of at least 1 and no greater than 6 are examples of the alkyl group having a carbon number of at least 1 and no greater than 6, the chemical groups having a carbon number of at least 1 and no greater than 5 are examples of the alkyl group having a carbon number of at least 1 and no greater than 5, the chemical groups having a carbon number of at least 1 and no greater than 4 are examples of the alkyl group having a carbon number of at least 1 and no greater than 4, and the chemical groups having a carbon number of at least 1 and no greater than 3 are examples of the alkyl group having a carbon number of at least 1 and no greater than 3. 
     An alkoxy group having a carbon number of at least 1 and no greater than 4 as used herein refers to an unsubstituted straight chain or branched chain alkoxy group. Examples of the alkoxy group having a carbon number of at least 1 and no greater than 4 include a methoxy group, an ethoxy group, an n-propoxy group, an isopropoxy group, an n-butoxy group, a sec-butoxy group, and a tert-butoxy group. Through the above, the terms used in the present description have been described. 
     [Image Forming Apparatus According to First Embodiment] 
     The following describes a first embodiment of the present invention with reference to the accompanying drawings. Note that elements in the drawings that are the same or equivalent are marked by the same reference signs and description thereof is not repeated. In the first embodiment, an X-axis, a Y-axis, and a Z-axis are perpendicular to one another. The X axis and the Y axis are parallel with a horizontal plane, and the Z axis is parallel with a vertical line. 
     The following first describes an overview of an image forming apparatus  1  according to the first embodiment with reference to  FIG.  1   . The image forming apparatus  1  according to the first embodiment is a full-color printer. The image forming apparatus  1  includes a feeding section  10 , a conveyance section  20 , an image forming section  30 , a toner supply section  60 , and an ejection section  70 . 
     The feeding section  10  includes a cassette  11  that accommodates a plurality of sheets P. The feeding section  10  feeds the sheets P from the cassette  11  to the conveyance section  20 . The sheets P are paper or made from a synthetic resin, for example. The conveyance section  20  conveys each sheet P to the image forming section  30 . 
     The image forming section  30  includes a light exposure device  31 , a magenta-color unit (also referred to below as an M unit)  32 M, a cyan-color unit (also referred to below as a C unit)  32 C, a yellow-color unit (also referred to below as a Y unit)  32 Y, a black-color unit (also referred to below as a BK unit)  32 BK, a transfer belt  33 , a secondary transfer roller  34 , and a fixing device  35 . Each of the M unit  32 M, the C unit  32 C, the Y unit  32 Y, and the BK unit  32 BK includes a photosensitive member  50 , a charging roller  51 , a development roller  52 , a primary transfer roller  53 , a static elimination lamp  54 , and a cleaner  55 . 
     The light exposure device  31  irradiates each of the M unit  32 M, the C unit  32 C, the Y unit  32 Y, and the BK unit  32 BK with light based on image data to form an electrostatic latent image in each of the M unit  32 M, the C unit  32 C, the Y unit  32 Y, and the BK unit  32 BK. The M unit  32 M forms a magenta toner image based on the electrostatic latent image. The C unit  32 C forms a cyan toner image based on the electrostatic latent image. The Y unit  32 Y forms a yellow toner image based on the electrostatic latent image. The BK unit  32 BK forms a black toner image based on the electrostatic latent image. 
     Each pf the photosensitive members  50  is drum-shaped. Each photosensitive member  50  rotates about a rotational center  50 X (rotation axis, see  FIG.  2   ) thereof. The charging roller  51 , the development roller  52 , the primary transfer roller  53 , the static elimination lamp  54 , and the cleaner  55  are arranged around the photosensitive member  50  in the stated order from upstream in terms of a rotational direction R (see  FIG.  2   ) of the photosensitive member  50 . The charging roller  51  charges a circumferential surface  50   a  of the photosensitive member  50  to a positive polarity. As already described, the light exposure device  31  exposes the charged circumferential surfaces  50   a  of the photosensitive members  50  to light to form electrostatic latent images on the circumferential surfaces  50   a  of the photosensitive members  50 . The development roller  52  carries a carrier CA supporting a toner T thereon by attracting the carrier CA thereto by magnetic force. Application of a developing bias (developing voltage) to the development rollers  52  generates a potential difference between the potential of the development rollers  52  and the potential of the circumferential surfaces  50   a  of the photosensitive members  50  to move and attach the toner T to the electrostatic latent images formed on the circumferential surfaces  50   a  of the photosensitive members  50 . In this manner, the development rollers  52  supply the toner T to the electrostatic latent images to develop the electrostatic latent images into toner images. Through development, the toner images are formed on the circumferential surfaces  50   a  of the photosensitive members  50 . The toner images each include the toner T. The transfer belt  33  is in contact with the circumferential surfaces  50   a  of the photosensitive members  50 . The primary transfer rollers  53  primarily transfer the toner images formed on the circumferential surfaces  50   a  of the photosensitive members  50  to the transfer belt  33  (more specifically, the outer surface of the transfer belt  33 ). The toner images in the four colors are superimposed on and primarily transferred to the outer surface of the transfer belt  33 . The toner images in the four colors include the toner image in the magenta color, the toner image in the cyan color, the toner image in the yellow color, and the toner image in the black color. Through primary transfer, a color toner image is formed on the outer surface of the transfer belt  33 . The secondary transfer roller  34  secondarily transfers the color toner image formed on the outer surface of the transfer belt  33  to the sheet P. The fixing device  35  applies heat and pressure to the sheet to fix the color toner image to the sheet P. The sheet P with the color toner image fixed thereto is ejected onto the ejection section  70 . After primary transfer, the static elimination lamps  54  included in the M unit  32 M, the C unit  32 C, the Y unit  32 Y, and the BK unit  32 BK perform static elimination on the circumferential surfaces  50   a  of the photosensitive members  50 . After primary transfer (more specifically, after primary transfer and static elimination), the cleaners  55  collect toner T remaining on the circumferential surfaces  50   a  of the photosensitive members  50 . 
     The toner supply section  60  includes a cartridge  60 M accommodating a toner T in a magenta color, a cartridge  60 C accommodating a toner T in a cyan color, a cartridge  60 Y accommodating a toner T in a yellow color, and a cartridge  60 BK accommodating a toner T in a black color. The cartridge  60 M, the cartridge  60 C, the cartridge  60 Y, and the cartridge  60 BK respectively supply the toners T to the development rollers  52  of the M unit  32 M, the C unit  32 C, the Y unit  32 Y, and the BK unit  32 BK. 
     Note that the photosensitive members  50  are each equivalent to what may be referred to as an image bearing member. The charging rollers  51  are each equivalent to what may be referred to as a charger. The development rollers  52  are each equivalent to what may be referred to as a development device. The primary transfer rollers  53  are each equivalent to what may be referred to as a primary transfer device. The secondary transfer roller  34  is equivalent to what may be referred to as a secondary transfer device. The static elimination lamps  54  are each equivalent to what may be referred to as a static elimination device. The cleaners  55  are each equivalent to what may be referred to as a cleaning device. The sheets P are each equivalent to what may be referred to as a recording medium. 
     The following further describes the image forming apparatus  1  according to the first embodiment with reference to  FIG.  2   .  FIG.  2    illustrates the photosensitive member  50  and elements around the photosensitive member  50 . The image forming apparatus  1  according to the first embodiment includes photosensitive members  50 , charging rollers  51 , a light exposure device  31 , development rollers  52 , a transfer belt  33 , primary transfer rollers  53 , a secondary transfer roller  34 , and cleaners  55 . Each of the cleaners  55  includes the cleaning blade  81  that is equivalent to what may be referred to as a cleaning member. The cleaning blades  81  are pressed against the circumferential surfaces  50   a  of the photosensitive members  50  and collect residual toner T remaining on the circumferential surfaces  50   a  of the photosensitive members  50  as a result of the toner image being primarily transferred. With the image forming apparatus  1  according to the first embodiment, the following first and second advantages can be obtained. 
     The following describes the first advantage first. In order to form high-definition images, the image forming apparatus  1  is preferably designed so that a slight potential difference in the circumferential surface  50   a  of the photosensitive member  50  is reflected in difference in image density in an output image (image formed on the sheet P). However, such design tends to cause a ghost image on the output image. The ghost image refers to a phenomenon described as appearance of a residual image along with an output image, which in other words is reappearance of an image formed during a previous rotation of the photosensitive member  50 . Non-uniform charging of the circumferential surface  50   a  of the photosensitive member  50  is caused for example due to variation in charge injection to a photosensitive layer  502  of the photosensitive member  50 , presence of residual charge inside the photosensitive layer  502 , or non-uniform current flowing at transfer due to presence or absence of a toner image on the photosensitive layer  502 . Such non-uniform charging causes a ghost image to occur. 
     In order to inhibit occurrence of a ghost image, the transfer belt  33  is preferably set to have a high surface resistivity ρS (e.g., greater than 11 Log Ω). Transfer current flowing in the circumferential surface  50   a  of the photosensitive member  50  from the primary transfer roller  53  through the transfer belt  33  decreases as the surface resistivity ρS of the transfer belt  33  is increased. As such, non-uniform flowing of the transfer current is inhibited that depends on presence or absence of a toner image on the photosensitive layer  502 . However, charge-up of the toner T tends to occur more readily as the surface resistivity ρS of the transfer belt  33  is increased. Charge-up of the toner T refers to a phenomenon in which a toner T on a transfer belt is charged to a charge amount over a desired value. The following describes charge-up of the toner T with reference to  FIG.  3   . The graph representation of  FIG.  3    illustrates a relationship between the number of times of primary transfer of the toner T on the transfer belt  33  and charge amount of the toner T when the toners T in the four colors are primarily transferred onto the transfer belt in a sequential manner using an image forming apparatus of a reference example. As illustrated in  FIG.  3   , the charge amount of the toner T on the transfer belt  33  increases with an increase in the number of times of primary transfer of the toner T on the transfer belt  33 . As further illustrated in  FIG.  3   , the charge amount of the toner T on the transfer belt tends to increase in a case with the transfer belt  33  having a high surface resistivity ρS as compared to a case with a transfer belt  33  having a low surface resistivity ρS (low resistance). 
     In view of the foregoing, in the first embodiment, the transfer belt  33  is set to have a low surface resistivity ρS (e.g., at least 6 Log Ω and no greater than 11 Log Ω) in order to inhibit occurrence of charge-up of the toner T. Furthermore, the present inventors extensively studied upon a photosensitive member  50  that is capable of inhibiting occurrence of a ghost image even if the transfer belt  33  has a low resistivity ρS. As a result of the study, the inventors found that occurrence of a ghost image can be inhibited as long as the photosensitive member  50  satisfies formula (1) described below even if the transfer belt  33  has a low surface resistivity ρS (e.g., at least 6 Log Ω and no greater than 11 Log Ω). 
     The following describes the second advantage. In a case of a toner T having a small particle diameter (e.g., a volume median diameter of at least 4.0 μm and no greater than 7.0 μm) and a high roundness (e.g., a roundness of at least 0.960 and no greater than 0.998), the toner T easily passes through a gap between the cleaning blade  81  and the circumferential surface  50   a  of the photosensitive member  50 , tending to cause insufficient cleaning. In view of the foregoing, in the image forming apparatus  1  according to the first embodiment, the linear pressure of the cleaning blade  81  on the circumferential surface  50   a  of the photosensitive member  50  is set to at least 10 N/m and no greater than 40 N/m. As a result of each cleaning blade  81  being tightly pressed against the corresponding photosensitive member  50  at a linear pressure in the above-specified range, it is possible to eliminate or extremely reduce the gap between the cleaning blade  81  and the circumferential surface  50   a  of the photosensitive member  50 . This can enable favorable cleaning on the circumferential surface  50   a  of the photosensitive member  50  even using a toner T having a small particle diameter and a high roundness. 
     However, the present inventors&#39; study has revealed that a higher linear pressure (e.g., a linear pressure of at least 10 N/m and no greater than 40 N/m) of the cleaning blade  81  on the circumferential surface  50   a  of the photosensitive member  50  is more likely to lead to occurrence of a ghost image. 
     The present inventors&#39; study has also revealed that occurrence of a ghost image is more significant in a case of the photosensitive member  50  having the photosensitive layer  502 , which is a single-layer photosensitive layer, than in a case of a photosensitive member having a multi-layer photosensitive layer. The photosensitive layer  502  of a single-layer is relatively thick. The thicker the photosensitive layer  502  is, the more easily electrons and holes generated from a charge generating material are trapped by residual charge in the photosensitive layer  502 . The trapped electrons and holes prevent the photosensitive member  50  from being uniformly charged, causing a ghost image. 
     The present inventors therefore made intensive study upon a photosensitive member  50  capable of inhibiting occurrence of a ghost image even if the linear pressure of the cleaning blade  81  on the circumferential surface  50   a  of the photosensitive member  50  is high (e.g., a linear pressure of at least 10 N/m and no greater than 40 N/m) and the photosensitive member  50  has the photosensitive layer  502  of a single layer. The present inventors then found that occurrence of a ghost image can be inhibited as long as the photosensitive member  50  satisfies formula (1) described below even if the linear pressure of the cleaning blade  81  is at least 10 N/m and no greater than 40 N/m and the photosensitive member  50  has the photosensitive layer  502  of a single layer. 
     &lt;Photosensitive Member&gt; 
     The following describes the photosensitive member  50  included in the image forming apparatus  1  with reference to  FIGS.  4  to  6   .  FIGS.  4  to  6    are each a partial cross-sectional view of an example of the photosensitive member  50 . The photosensitive member  50  is an organic photoconductor (OPC) drum, for example. 
     As illustrated in  FIG.  4   , the photosensitive member  50  includes a conductive substrate  501  and a photosensitive layer  502 , for example. The photosensitive layer  502  is a single layer (one layer). The photosensitive member  50  is a single-layer electrophotographic photosensitive member including a photosensitive layer  502  of a single layer. The photosensitive layer  502  contains a charge generating material, a hole transport material, an electron transport material, and a binder resin. No particular limitations are placed on film thickness of the photosensitive layer  502 , but the film thickness of the photosensitive layer  502  is preferably at least 5 μm and no greater than 100 μm, more preferably at least 10 μm and no greater than 50 μm, further preferably at least 10 μm and no greater than 35 μm, and yet further preferably at least 15 μm and no greater than 30 μm. 
     As illustrated in  FIG.  5   , the photosensitive member  50  may include the conductive substrate  501 , the photosensitive layer  502 , and an intermediate layer  503  (undercoat layer). The intermediate layer  503  is provided between the conductive substrate  501  and the photosensitive layer  502 . As illustrated in  FIG.  4   , the photosensitive layer  502  may be provided directly on the conductive substrate  501 . Alternatively, the photosensitive layer  502  may be provided on the conductive substrate  501  with the intermediate layer  503  therebetween as illustrated in  FIG.  5   . The intermediate layer  503  may be a single layer or a plurality of layers. 
     As illustrated in  FIG.  6   , the photosensitive member  50  may include the conductive substrate  501 , the photosensitive layer  502 , and a protective layer  504 . The protective layer  504  is provided on the photosensitive layer  502 . The protective layer  504  may be a single layer or a plurality of layers. 
     (Chargeability Ratio) 
     The photosensitive member  50  satisfies formula (1) shown below. 
     
       
         
           
             
               
                 
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     In formula (1), Q represents a charge amount (unit: C) of the photosensitive member  50 . S represents a charge area (unit: m 2 ) of the photosensitive member  50 . d represents a film thickness (unit: m) of the photosensitive layer  502  of the photosensitive member  50 . ε r  represents a specific permittivity of the binder resin contained in the photosensitive layer  502  of the photosensitive member  50 . ε 0  represents the vacuum permittivity (unit: F/m). Note that “d/ε r ·ε 0 ” means “d/(ε r ×ε 0 )”. V represents a value calculated according to equation (2) shown below.
 
 V=V   0   −V   r   (2)
 
     In equation (2), V r  represents a first potential of the circumferential surface  50   a  of the photosensitive member  50  yet to be charged by the charging roller  51 . V 0  in equation (2) represents a second potential of the circumferential surface  50   a  of the photosensitive member  50  charged by the charging roller  51 . 
     In the following, a value represented by the following expression (1′) in formula (1) is also referred to below as a chargeability ratio. The chargeability ratio represented by expression (1′) is a ratio of actual chargeability (a measured value) of the photosensitive member  50  to theoretical chargeability (a theoretical value) of the photosensitive member  50  when the circumferential surface  50   a  of the photosensitive member  50  is charged by the charging roller  51 . Details of the ratio of the actual chargeability of the photosensitive member  50  to the theoretical chargeability of the photosensitive member  50  will be described later with reference to  FIG.  8   . 
     
       
         
           
             
               
                 
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     As a result of the photosensitive member  50  satisfying formula (1), the following third, fourth, and fifth advantages can be obtained. The following describes the third advantage first. As already described, a ghost image is more likely to occur as the linear pressure of the cleaning blade  81  on the circumferential surface  50   a  of the photosensitive member  50  is increased (e.g., a linear pressure of at least 10 N/m and no greater than 40 N/m). However, as a result of the photosensitive member  50  satisfying formula (1), chargeability of the photosensitive member  50  is close to the theoretical value to enable uniform charging of the circumferential surface  50   a  of the photosensitive member  50 . Thus, occurrence of a ghost image can be inhibited even if the linear pressure of the cleaning blade  81  is at least 10 N/m and no greater than 40 N/m. 
     The following describes the fourth advantage. The photosensitive layer  502  of the photosensitive member  50  may abrade away in the course of repeated image formation. One of causes of abrasion of the photosensitive layer  502  is abrasion due to discharge from the charging roller  51  to the photosensitive member  50 , for example. Chargeability of the photosensitive member  50  that satisfies formula (1) is close to the theoretical value. This can achieve favorable charging of the circumferential surface  50   a  of the photosensitive member  50  even if the amount of discharge from the charging roller  51  to the photosensitive member  50  is set low. Setting the discharge amount low can reduce the abrasion amount of the photosensitive layer  502 . Furthermore, reduction in abrasion amount of the photosensitive layer  502  can allow the film thickness of the photosensitive layer  502  to be set thin, thereby enabling reduction in the manufacturing cost. 
     The following describes the fifth advantage. As a result of the photosensitive member  50  satisfying formula (1), chargeability of the photosensitive member  50  is close to the theoretical value to enable favorable charging of the circumferential surface  50   a  of the photosensitive member  50  even if current flowing in the charging roller  51  is set low. As a result of the current flowing in the charging roller  51  being set low, decrease in conductivity of the material (e.g., rubber) of the charging roller  51 , which is caused due to conduction, can be inhibited. As described as the first advantage, it is possible to inhibit occurrence of a ghost image even if the linear pressure of the cleaning blade  81  is high (at least 10 N/m and no greater than 40 N/m) as long as the photosensitive member  50  satisfies formula (1). Because the linear pressure can be high, an external additive of the toner T is prevented from easily passing through the gap between the cleaning blade  81  and the circumferential surface  50   a  of the photosensitive member  50 . As a result of the additive being prevented from easily passing through the gap, the external additive is prevented from easily adhering to the surface of the charging roller  51 . Because conductivity of the material of the charging roller  51  can be prevented from decreasing and the external additive is prevented from easily adhering to the surface of the charging roller  51 , it is possible to prevent elevation of resistance of the charging roller  51 . 
     As to formula (1), the chargeability ratio is preferably at least 0.70 in order to inhibit occurrence of a ghost image, more preferably at least 0.80, and further preferably at least 0.90. The measured value of chargeability of the photosensitive member  50  is equal to the theoretical value thereof when the chargeability ratio is 1.00. That is, the chargeability ratio is no greater than 1.00. 
     A chargeability ratio measuring method will be described next. In formula (1), V represents a value calculated according to the aforementioned equation (2). The following describes a method for measuring the first potential V r  and the second potential V 0  in equation (2) with reference to  FIG.  7   . Note that the first potential V r  and the second potential V 0  are measured under environmental conditions of a temperature of 23° C. and a relative humidity of 50%. 
     The first potential V r  and the second potential V 0  can be measured using a measuring device  100  illustrated in  FIG.  7   . The measuring device  100  can be fabricated by performing first modification and second modification on the image forming apparatus  1 . In the first modification, a first voltage probe  101  is attached to the image forming apparatus  1 . The first voltage probe  101  is placed on the upstream side of the charging roller  51  in terms of the rotational direction R of the photosensitive member  50 . The first voltage probe  101  is connected to a first surface electrometer (not illustrated, “ELECTROSTATIC VOLTMETER Model 344”, product of TREK, INC.). In the second modification, a development roller  52  of the image forming apparatus  1  is replaced by a second voltage probe  102 . The second voltage probe  102  is placed at a location where a rotational center  52 X (rotation axis) of the development roller  52  has been located. The second voltage probe  102  is connected to a second surface electrometer (not illustrated, “ELECTROSTATIC VOLTMETER Model 344”, product of TREK, INC.). 
     The measuring device  100  includes at least a charging roller  51 , the second voltage probe  102 , a static elimination lamp  54 , and the first voltage probe  101 . The photosensitive member  50  that is a measurement target is set in the measuring device  100 . The charging roller  51 , the second voltage probe  102 , the static elimination lamp  54 , and the first voltage probe  101  are arranged around the photosensitive member  50  in the stated order from upstream in terms of the rotational direction R of the photosensitive member  50 . 
     The second voltage probe  102  is placed so that an angle θ 1  between a first line L 1  and a second line L 2  is 120 degrees. Here, the first line L 1  is a line connecting the rotational center  50 X (rotation axis) of the photosensitive member  50  and a rotational center  51 X (rotation axis) of the charging roller  51 , and the second line L 2  is a line connecting the rotational center  50 X (rotation axis) of the photosensitive member  50  and the second voltage probe  102 . The intersection point of the first line L 1  and the circumferential surface  50   a  of the photosensitive member  50  is a charge point P 1 . The intersection point of the second line L 2  and the circumferential surface  50   a  of the photosensitive member  50  is a development point P 2 . 
     The first voltage probe  101  is placed so that an angle θ 2  between a third line L 3  and the first line L 1  is 20 degrees. Here, the third line L 3  is a line connecting the rotational center  50 X (rotation axis) of the photosensitive member  50  and the first voltage probe  101 , and the first line L 1  is the line connecting the rotational center  50 X (rotation axis) of the photosensitive member  50  and the rotational center  51 X (rotation axis) of the charging roller  51 . The intersection point of the third line L 3  and the circumferential surface  50   a  of the photosensitive member  50  is a pre-charge point P 3 . 
     The point of the circumferential surface  50   a  of the photosensitive member  50  where static elimination light of the static elimination lamp  54  is radiated is a static elimination point P 4 . The static elimination lamp  54  is placed so that an angle θ 3  between a fourth line L 1  and the third line L 3  is 90 degrees. Here, the fourth line L 4  is a line connecting the rotational center  50 X (rotation axis) of the photosensitive member  50  and the static elimination point P 4 , and the third line L 3  is the line connecting the rotational center  50 X (rotation axis) of the photosensitive member  50  and the first voltage probe  101 . Note that a modified version of a multifunction peripheral (“TASKalfa356Ci”, product of KYOCERA Document Solutions Inc.) can be used as the measuring device  100 . 
     In measurement of the first potential V r  and the second potential V 0 , a charging voltage applied to the charging roller  51  is set to any of +1000 V, +1100 V, +1200 V, +1300 V, +1400 V, and +1500 V. Alight quantity of the static elimination light emitted from the static elimination lamp  54  when the static elimination light reaches the circumferential surface  50   a  of the photosensitive member  50  (also referred to below as a static elimination light intensity) is set to 5 J/cm 2 . The first potential V r  and the second potential V 0  are measured while the photosensitive member  50  is rotated about the rotational center  50 X (rotation axis). The charging roller  51  charges the circumferential surface  50   a  of the photosensitive member  50  to a positive polarity at the charge point P 1  of the photosensitive member  50 . Next, the static elimination lamp  54  performs static elimination on the circumferential surface  50   a  of the photosensitive member  50  at the static elimination point P 4  of the photosensitive member  50 . The first potential V r  and the second potential V 0  are measured simultaneously at the time when the photosensitive member  50  has been rotated 10 rounds (also referred to below as a timing K) while charging and static elimination as above are performed. Specifically, the potential (first potential V r ) of the circumferential surface  50   a  of the photosensitive member  50  is measured at the pre-charge point P 3  of the photosensitive member  50  at the timing K using the first voltage probe  101 . Also, the potential (second potential V 0 ) of the circumferential surface  50   a  of the photosensitive member  50  is measured at the development point P 2  of the photosensitive member  50  at the timing K using the second voltage probe  102 . In a manner as described above, the first potential V r  and the second potential V 0  are measured under each of conditions of charging voltages applied to the charging roller  51  of +1000 V, +1100 V, +1200 V, +1300 V, +1400 V, and +1500 V. 
     Note that light exposure by a light exposure device  31 , development by a development roller  52 , primary transfer by a primary transfer roller  53 , and cleaning by a cleaning blade  81  are not performed in measurement of the first potential V r  and the second potential V 0 . The cleaning blade  81  is set to have a linear pressure of 0 N/m. The method for measuring the first potential V r  and the second potential V 0  in equation (2) has been described so far. The chargeability ratio measuring method will be described further. 
     The charge amount Q in formula (1) is measured under environmental conditions of a temperature of 23° C. and a relative humidity of 50%. The charge amount Q is measured according to the following method at measurement of the first potential V r  and the second potential V 0 . At the timing K of the simultaneous measurement of the first potential V r  and the second potential V 0 , a current E 1  flowing through the charging roller  51  is measured using an ammeter/voltmeter (“MINIATURE PORTABLE AMMETER AND VOLTMETER 2051”, product of Yokogawa Test &amp; Measurement Corporation). The current E 1  is measured under each of conditions of charging voltages applied to the charging roller  51  of +1000 V, +1100 V, +1200 V, +1300 V, +1400 V, and +1500 V. The charge amount Q under each of the conditions of charging voltages applied to the charging roller  51  of +1000 V, +1100 V, +1200 V, +1300 V, +1400 V, and +1500 V is calculated from the measured currents E t  in accordance with equation (3) shown below.
 
Charge amount  Q =current  E   1 (unit:A)×charging time  t  (unit:second)  (3)
 
     Note that a high-voltage substrate (not illustrated) of the measuring device  100  is connected to the charging roller  51  via the ammeter/voltmeter. The current E t  flowing in the charging roller  51  and the charging voltage mentioned in association with the measurement of the first potential V r  and the second potential V 0  can be constantly monitored using the ammeter/voltmeter while the measuring device  100  is in operation. 
     The charge area S in formula (1) is an area of a charged region of the circumferential surface  50   a  of the photosensitive member  50  charged by the charging roller  51 . The charge area S is calculated in accordance with the following equation (4). A charge width in equation (4) is a length of the charged region of the circumferential surface  50   a  of the photosensitive member  50  charged by the charging roller  51  in a longitudinal direction (a rotational axis direction D in  FIG.  10   ) of the photosensitive member  50 .
 
Charge area  S  (unit:m 2 )=linear velocity of photosensitive member 50 (unit: m/second)×charge width (m)×charging time  t  (unit:second)  (4)
 
     A value “V” in formula (1) is calculated from the first potential V r  and the second potential V 0  each measured according to the above-described method. A value of “Q/S” in formula (1) is calculated from the charge amount Q and the charge area S measured according to the above-described methods. A graph is then produced with “Q/S” value on a horizontal axis and “V” value on a vertical axis. Six points are plotted in the graph, indicating measurement results obtained under the conditions of charging voltages applied to the charging roller  51  of +1000 V, +1100 V, +1200 V, +1300 V, +1400 V, and +1500 V. An approximate straight line on these six points is drawn. A gradient of the approximate straight line is determined from the approximate straight line. The determined gradient is taken to be “V/(Q/S)” in formula (1). 
     A film thickness d of the photosensitive layer  502  in formula (1) is measured under environmental conditions of a temperature of 23° C. and a relative humidity of 50%. The film thickness d of the photosensitive layer  502  is measured using a film thickness measuring device (“FISCHERSCOPE (registered Japanese trademark) MMS (registered Japanese trademark)”, product of Helmut Fischer GmbH). Note that the film thickness of the photosensitive layer  502  is set to 30×10 −6  in the first embodiment. 
     ε 0  in formula (1) represents the vacuum permittivity. The vacuum permittivity ε 0  is constant and is 8.85×10 −12  (unit: F/m). 
     The specific permittivity ε r  of the binder resin in formula (1) is equivalent to a specific permittivity of the photosensitive layer  502  on the assumption that no charge is trapped inside the photosensitive layer  502  and the whole amount of charge supplied from the charging roller  51  is changed to the potential (surface potential) of the circumferential surface  50   a  of the photosensitive member  50 . The specific permittivity εr of the binder resin is measured using a photosensitive member for specific permittivity measurement. The photosensitive member for specific permittivity measurement includes a photosensitive layer only containing the binder resin. Note that the photosensitive member for specific permittivity measurement can be produced according to the same method as in production of photosensitive members described in association with Examples below in all aspects other than that none of a charge generating material, a hole transport material, an electron transport material, and an additive is added thereto. The specific permittivity ε r  of the binder resin is calculated using the photosensitive member for specific permittivity measurement as a measurement target in accordance with equation (5) shown below. The specific permittivity ε r  of the binder resin calculated in accordance with equation (5) is 3.5 in the first embodiment. 
     
       
         
           
             
               
                 
                   
                     V 
                     ε 
                   
                   = 
                   
                     
                       
                         ( 
                         
                           
                             Q 
                             ε 
                           
                           / 
                           
                             S 
                             ε 
                           
                         
                         ) 
                       
                       × 
                       
                         d 
                         ε 
                       
                     
                     
                       
                         ε 
                         r 
                       
                       × 
                       
                         ε 
                         0 
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     In equation (5), Q ε  represents a charge amount (unit: C) of the photosensitive member for specific permittivity measurement. S ε  represents a charge area (unit: m 2 ) of the photosensitive member for specific permittivity measurement. d ε  represents a film thickness (unit: m) of a photosensitive layer of the photosensitive member for specific permittivity measurement. ε r  represents a specific permittivity of the binder resin. ε 0  represent the vacuum permittivity (unit: F/m). V ε  is a value calculated from the following expression “V 0ε −V rε ”. V rε  represents a third potential of the circumferential surface of the photosensitive member for specific permittivity measurement yet to be charged by the charging roller  51 . V 0ε  represents a fourth potential of the circumferential surface of the photosensitive member for specific permittivity measurement charged by the charging roller  51 . 
     The film thickness d ε  in equation (5) is calculated according to the same method as in calculation of the film thickness d of the photosensitive member  50  in the above-described formula (1) in all aspects other than that the photosensitive member for specific permittivity measurement is used instead of the photosensitive member  50 . In the first embodiment, the film thickness d ε  in equation (5) is set to 30×10 −6  m. The vacuum permittivity ε ε  in equation (5) is constant and is 8.85×10 −12  F/m. The theoretical value 0 V is substituted into the third potential V rε  in equation (5). The charge amount Q ε  of the photosensitive member for specific permittivity measurement in equation (5) is measured according to the same method as in measurement of the charge amount Q of the photosensitive member  50  in formula (1) in all aspects other than that the photosensitive member for specific permittivity measurement is used instead of the photosensitive member  50  and the charging voltage is set to +1000 V. The charge area S ε  of the photosensitive member for specific permittivity measurement in equation (5) is calculated according to the same method as in calculation of the charge area S of the photosensitive member  50  in formula (1) in all aspects other than that the photosensitive member for specific permittivity measurement is used instead of the photosensitive member  50 . The fourth potential V 0ε  in equation (5) is measured according to the same method as in measurement of the second potential V 0  of the photosensitive member  50  in equation (2) in all aspects other than that the photosensitive member for specific permittivity measurement is used instead of the photosensitive member  50 . Using the thus obtained values, the specific permittivity εr of the binder resin is calculated in accordance with equation (5). 
     The chargeability ratio measuring method has been described so far. The following further describes the chargeability ratio with reference to  FIG.  8   . As already described, the chargeability ratio is a ratio of actual chargeability (an actual measured value) of the photosensitive member  50  to theoretical chargeability (a theoretical value) of the photosensitive member  50  when the circumferential surface  50   a  of the photosensitive member  50  is charged by the charging roller  51 . The chargeability as used in the present description indicates how much charge potential (unit: V) of the photosensitive member  50  increases for surface charge density (unit: C/m 2 ) of charge supplied from the charging roller  51 . The theoretical chargeability (a theoretical value) of the photosensitive member  50  is a value on the assumption that the whole amount of charge supplied from the charging roller  51  to the photosensitive member  50  is changed to the charge potential of the photosensitive member  50 . The charge potential of the photosensitive member  50  is equivalent to a difference between the potential (first potential V r ) of the circumferential surface  50   a  of the photosensitive member  50  before a portion of the circumferential surface  50   a  of the photosensitive member  50  passes the charging roller  51  and the potential (second potential V 0 ) of the circumferential surface  50   a  of the photosensitive member  50  after the portion of the circumferential surface  50   a  of the photosensitive member  50  has passed the charging roller  51 . 
       FIG.  8    is a graph representation illustrating relationships between surface charge density (unit: C/m 2 ) and charge potential (unit: V) of photosensitive members. The horizontal axis in  FIG.  8    indicates surface charge density. The surface charge density is a value corresponding to “Q/S” in formula (1). The vertical axis in  FIG.  8    indicates charge potential. The charge potential is a value corresponding to “V” in formula (1). The chargeability corresponds to the gradient “V/(Q/S)” of each of graphs shown in  FIG.  8   . 
     Circles on the plot in  FIG.  8    indicate measurement results of a photosensitive member (P-A1) having a chargeability ratio of at least 0.60. Triangles on the plot in  FIG.  8    indicate measurement results of a photosensitive member (P-B1) having a chargeability ratio of less than 0.60. Note that the photosensitive members (P-A1) and (P-B1) are produced according to a method described in association with Examples. The dashed line A in  FIG.  8    indicates the theoretical chargeability (theoretical value) of the photosensitive member  50 . The theoretical chargeability (theoretical value) of the photosensitive member  50  is calculated in accordance with equation (6) shown below. The dashed line A in  FIG.  8    is obtained by plotting values of “Q t /S t ” in equation (6) on the horizontal axis and plotting values “V t ” in equation (6) on the vertical axis. 
     
       
         
           
             
               
                 
                   
                     V 
                     t 
                   
                   = 
                   
                     
                       
                         V 
                         
                           0 
                           ⁢ 
                           t 
                         
                       
                       - 
                       
                         V 
                         
                           rt 
                             
                         
                       
                     
                     = 
                     
                       
                         
                           ( 
                           
                             
                               Q 
                               t 
                             
                             / 
                             
                               S 
                               t 
                             
                           
                           ) 
                         
                         × 
                         
                           d 
                           t 
                         
                       
                       
                         
                           ε 
                           rt 
                         
                         × 
                         
                           ε 
                           o 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     In equation (6), Q t  represents a charge amount (unit: C) of the photosensitive member  50 . S t  represents a charge area (unit: m 2 ) of the photosensitive member  50 . d t  represents a film thickness (unit: m) of the photosensitive layer  502  of the photosensitive member  50 . ε rt  represents a specific permittivity of the binder resin contained in the photosensitive layer  502  of the photosensitive member  50 . ε 0  represents the vacuum permittivity (unit: F/m). V t  is a value calculated in accordance with expression “V 0t −V rt ”. V rt  represents a fifth potential of the circumferential surface  50   a  of the photosensitive member  50  yet to be charged by the charging roller  51 . V 0t  represents a sixth potential of the circumferential surface  50   a  of the photosensitive member  50  charged by the charging roller  51 . 
     The film thickness d t  in equation (6) is calculated according to the same method as in calculation of the film thickness d of the photosensitive member  50  in formula (1). In the first embodiment, the film thickness di in equation (6) is set to 30×10 −6  m. The vacuum permittivity ε 0  in equation (6) is constant and is 8.85×10 −12  F/m. The theoretical value 0 V is substituted into the fifth potential V rt  in equation (6). The charge amount Q t  of the photosensitive member  50  in equation (6) is measured according to the same method as in measurement of the charge amount Q of the photosensitive member  50  in formula (1). The charge area S t  of the photosensitive member  50  in equation (6) is calculated according to the same method as in calculation of the charge area S of the photosensitive member  50  in formula (1). The specific permittivity ε rt  of the binder resin in equation (6) is measured according to the same method as in measurement of the specific permittivity ε r  of the binder resin in formula (1). The specific permittivity ε rt  of the binder resin in equation (6) is 3.5, the same as the specific permittivity ε rt  of the binder resin in formula (1). Using the thus obtained values, the sixth potential V 0t  and V t  are calculated in accordance with equation (6). 
     As shown in  FIG.  8   , the higher and closer to 1.00 the chargeability ratio is, the closer to the dashed line A the chargeability (corresponding to the gradient in  FIG.  8   ) is. Occurrence of a ghost image can be sufficiently inhibited as long as the photosensitive member  50  has a chargeability ratio of at least 0.60. Through the above, the chargeability ratio of the photosensitive member  50  has been described. The following further describes the photosensitive member  50 . 
     The circumferential surface  50   a  of the photosensitive member  50  has a surface friction coefficient of preferably at least 0.20 and no greater than 0.80, more preferably at least 0.20 and no greater than 0.60, and further preferably at least 0.20 and no greater than 0.52. As a result of the surface friction coefficient of the circumferential surface  50   a  of the photosensitive member  50  being no greater than 0.80, adhesion of the toner T to the circumferential surface  50   a  of the photosensitive member  50  can be low enough to further prevent insufficient cleaning. Furthermore, as a result of the surface friction coefficient of the circumferential surface  50   a  of the photosensitive member  50  being no greater than 0.80, friction force of the cleaning blade  81  on the circumferential surface  50   a  of the photosensitive member  50  can be low enough to further reduce abrasion of the photosensitive layer  502  of the photosensitive member  50 . No particular limitations are placed on the lower limit of the surface friction coefficient of the circumferential surface  50   a  of the photosensitive member  50 . The surface friction coefficient of the circumferential surface  50   a  of the photosensitive member  50  may for example be at least 0.20. The surface friction coefficient of the circumferential surface  50   a  of the photosensitive member  50  can be measured according to a method described in association with Examples. 
     In order to obtain output images favorable in image quality, the circumferential surface  50   a  of the photosensitive member  50  has a post-exposure potential of preferably +50 V or higher and +300 V or lower, and more preferably +80 V or higher and +200 V or lower. The post-exposure potential is a potential of a region of the circumferential surface  50   a  of the photosensitive member  50  exposed to light by the light exposure device  31 . The post-exposure potential is measured after light exposure and before development. The post-exposure potential of the photosensitive member  50  can be measured according to a method described in association with Examples. 
     The photosensitive layer  502  has a Martens hardness of preferably at least 150 N/mm 2 , more preferably at least 180 N/mm 2 , further preferably at least 200 N/mm 2 , and yet further preferably at least 220 N/mm 2 . As a result of the photosensitive layer  502  having a Martens hardness of at least 150 N/mm 2 , the abrasion amount of the photosensitive layer  502  is low enough to increase abrasion resistance of the photosensitive member  50 . No particular limitations are placed on the upper limit of the Martens hardness of the photosensitive layer  502 . For example, the Martens hardness of the photosensitive layer  502  may be no greater than 250 N/mm 2 . The Martens hardness of the photosensitive layer  502  can be measured according to a method described in association with Examples. 
     The photosensitive layer  502  contains a charge generating material, a hole transport material, an electron transport material, and a binder resin. The photosensitive layer  502  may further contain an additive according to necessity. The following describes the charge generating material, the hole transport material, the electron transport material, the binder resin, the additive, and preferable material combinations. 
     (Charge Generating Material) 
     No particular limitations are placed on the charge generating material. Examples of the charge generating material include phthalocyanine-based pigments, perylene-based pigments, bisazo pigments, tris-azo pigments, dithioketopyrrolopyrrole pigments, metal-free naphthalocyanine pigments, metal naphthalocyanine pigments, squaraine pigments, indigo pigments, azulenium pigments, cyanine pigments, powders of inorganic photoconductive materials (specific examples include selenium, selenium-tellurium, selenium-arsenic, cadmium sulfide, and amorphous silicon), pyrylium pigments, anthanthrone-based pigments, triphenylmethane-based pigments, threne-based pigments, toluidine-based pigments, pyrazoline-based pigments, and quinacridone-based pigments. The photosensitive layer  502  may contain only one charge generating material or may contain two or more charge generating materials. 
     Examples of phthalocyanine-based pigments that are preferable in terms of inhibiting occurrence of a ghost image include metal-free phthalocyanine, titanyl phthalocyanine, and chloroindium phthalocyanine, among which titanyl phthalocyanine is more preferable. Titanyl phthalocyanine is represented by chemical formula (CGM-1). 
     
       
         
         
             
             
         
       
     
     Titanyl phthalocyanine may have a crystal structure. Examples of titanyl phthalocyanine having a crystal structure include titanyl phthalocyanine having an α-form crystal structure, titanyl phthalocyanine having a β-form crystal structure, and titanyl phthalocyanine having a Y-form crystal structure (also referred to below as α-form titanyl phthalocyanine, β-form titanyl phthalocyanine, and Y-form titanyl phthalocyanine, respectively). Y-form titanyl phthalocyanine is preferable as the titanyl phthalocyanine. 
     Y-form titanyl phthalocyanine for example exhibits a main peak at a Bragg angle (2θ±0.2°) of 27.2° in a CuKα characteristic X-ray diffraction spectrum. The main peak in the CuKα characteristic X-ray diffraction spectrum refers to a peak having a highest or second highest intensity in a range of Bragg angles (2θ±0.2°) from 30 to 40°. 
     The following describes an example of a method for measuring the CuKα characteristic X-ray diffraction spectrum. A sample (titanyl phthalocyanine) is loaded into a sample holder of an X-ray diffraction spectrometer (e.g., “RINT (registered Japanese trademark)  1100 ”, product of Rigaku Corporation), and an X-ray diffraction spectrum is measured using a Cu X-ray tube, a tube voltage of 40 k, a tube current of mA, and CuKα characteristic X-rays having a wavelength of 1.542 Å. The measurement range (2θ) is for example from 3° to 40° (start angle: 3°, stop angle: 40°), and the scanning rate is for example 10°/minute. 
     Y-form titanyl phthalocyanine is for example classified into the following three types (A) to (C) based on thermal characteristics in differential scanning calorimetry (DSC) spectra. 
     (A) Y-form titanyl phthalocyanine that exhibits a peak in a range of from 50° C. to 270° C. in a differential scanning calorimetry spectrum thereof, other than a peak resulting from vaporization of adsorbed water. 
     (B) Y-form titanyl phthalocyanine that does not exhibit a peak in a range of from 50° C. to 400° C. in a differential scanning calorimetry spectrum thereof, other than a peak resulting from vaporization of adsorbed water. 
     (C) Y-form titanyl phthalocyanine that does not exhibit a peak in a range of from 50° C. to 270° C. and exhibits a peak in a range of higher than 270° C. and no higher than 400° C. in a differential scanning calorimetry spectrum thereof, other than a peak resulting from vaporization of adsorbed water. 
     Y-form titanyl phthalocyanine is preferable that does not exhibit a peak in a range of from 50° C. to 270° C. and exhibits a peak in a range of higher than 270° C. and no greater than 400° C. in a differential scanning calorimetry spectrum thereof, other than a peak resulting from vaporization of adsorbed water. Y-form titanyl phthalocyanine exhibiting such a peak is preferably that exhibiting a single peak in a range of higher than 270° C. and no greater than 400° C., and more preferably that exhibiting a single peak at 296° C. 
     The following describes an example of a differential scanning calorimetry spectrum measuring method. A sample (titanyl phthalocyanine) is loaded on a sample pan, and a differential scanning calorimetry spectrum is measured using a differential scanning calorimeter (e.g., “TAS-200 DSC8230D”, product of Rigaku Corporation). The measurement range is for example from 40° C. to 400° C. The heating rate is for example 20° C./minute. 
     The charge generating material has a content ratio to mass of the photosensitive layer  502  of preferably greater than 0.0% by mass and no greater than 1.0% by mass, and more preferably greater than 0.0% by mass and no greater than 0.5% by mass. As a result of the content ratio of the charge generating material to the mass of the photosensitive layer  502  being no greater than 1.0% by mass, an increased chargeability ratio can be attained. The mass of the photosensitive layer  502  is total mass of the materials contained in the photosensitive layer  502 . Where the photosensitive layer  502  contains a charge generating material, a hole transport material, an electron transport material, and a binder resin, the mass of the photosensitive layer  502  is a total of mass of the charge generating material, mass of the hole transport material, mass of the electron transport material, and mass of the binder resin. Where the photosensitive layer  502  contains a charge generating material, a hole transport material, an electron transport material, a binder resin, and an additive, the mass of the photosensitive layer  502  is a total of mass of the charge generating material, mass of the hole transport material, mass of the electron transport material, mass of the binder resin, and mass of the additive. 
     (Hole Transport Material) 
     No particular limitations are placed on the hole transport material. Examples of the hole transport material includes nitrogen-containing cyclic compounds and condensed polycyclic compounds. Examples of the nitrogen-containing cyclic compounds and condensed polycyclic compounds include triphenylamine derivatives; diamine derivatives (specific examples include N,N,N′,N′-tetraphenylbenzidine derivatives, N,N,N′,N′-tetraphenylphenylenediamine derivatives, N,N,N′,N′-tetraphenylnaphtylenediamine derivatives, di(aminophenylethenyl)benzene derivatives, and N,N,N′,N′-tetraphenylphenanthrylenediamine derivatives); oxadiazole-based compounds (specific examples include 2,5-di(4-methylaminophenyl)-1,3,4-oxadiazole); styryl-based compounds (specific examples include 9-(4-diethylaminostyryl)anthracene); carbazole-based compounds (specific examples include polyvinyl carbazole); organic polysilane compounds; pyrazoline-based compounds (specific examples include 1-phenyl-3-(p-dimethylaminophenyl)pyrazoline); hydrazone-based compounds; indole-based compounds; oxazole-based compounds; isoxazole-based compounds; thiazole-based compounds; thiadiazole-based compounds; imidazole-based compounds; pyrazole-based compounds; and triazole-based compounds. The photosensitive layer  502  may contain only one hole transport material or may contain two or more hole transport materials. 
     Examples of hole transport materials that are preferable in terms of inhibiting occurrence of a ghost image include a compound represented by general formula (10) (also referred to below as a hole transport material (10)). 
     
       
         
         
             
             
         
       
     
     In general formula (10), R 13  to R 15  each represent, independently of each other, an alkyl group having a carbon number of at least 1 and no greater than 4 or an alkoxy group having a carbon number of at least 1 and no greater than 4. m and n each represent, independently of each other, an integer of at least 1 and no greater than 3. p and r each represent, independently of each other, 0 or 1. q represents an integer of at least 0 and no greater than 2. Where q represents 2, two chemical groups R 14  may be the same as or different from each other. 
     R 14  in general formula (10) is preferably an alkyl group having a carbon number of at least 1 and no greater than 4, more preferably a methyl group, an ethyl group, or an n-butyl group, and particularly preferably an n-butyl group. q Preferably represents 1 or 2, and more preferably represents 1. Each of p and r preferably represents 0. Each of m and n preferably represents 1 or 2, and more preferably represents 2. 
     A preferable example of the hole transport material (10) is a compound represented by chemical formula (HTM-1) (also referred to below as a hole transport material (HTM-1)). 
     
       
         
         
             
             
         
       
     
     The hole transport material has a content ratio to the mass of the photosensitive layer  502  of preferably greater than 0.0% by mass and no greater than 35.0% by mass, and more preferably at least 10.0% by mass and no greater than 30.0% by mass. 
     (Binder Resin) 
     Examples of the binder resin include thermoplastic resins, thermosetting resin, and photocurable resins. Examples of the thermoplastic resins include polycarbonate resins, polyarylate resins, styrene-butadiene copolymers, styrene-acrylonitrile copolymers, styrene-maleic acid copolymers, acrylic acid polymers, styrene-acrylic acid copolymers, polyethylene resins, ethylene-vinyl acetate copolymers, chlorinated polyethylene resins, polyvinyl chloride resins, polypropylene resins, ionomer resins, vinyl chloride-vinyl acetate copolymers, alkyd resins, polyamide resins, urethane resins, polysulfone resins, diallyl phthalate resins, ketone resins, polyvinyl butyral resins, polyester resins, and polyether resins. Examples of the thermosetting resins include silicone resins, epoxy resins, phenolic resins, urea resins, and melamine resins. Examples of the photocurable resins include acrylic acid adducts of epoxy compounds and acrylic acid adducts of urethane compounds. The photosensitive layer  502  may contain only one binder resin or may contain two or more binder resins. 
     In order to inhibit occurrence of a ghost image, preferably, the binder resin includes a polyarylate resin including a repeating unit represented by general formula (20) (also referred to below as a polyarylate resin (20)). 
     
       
         
         
             
             
         
       
     
     In general formula (20), R 20  and R 21  each represent, independently of each other, a hydrogen atom or an alkyl group having a carbon number of at least 1 and no greater than 4. R 22  and R 23  each represent, independently of each other, a hydrogen atom, a phenyl group, or an alkyl group having a carbon number of at least 1 and no greater than 4. R 22  and R 23  may be bonded to each other to form a divalent group represented by general formula (W). Y represents a divalent group represented by chemical formula (Y1), (Y2), (Y3), (Y4), (Y5), or (Y6). 
     
       
         
         
             
             
         
       
     
     In general formula (W), t represents an integer of at least 1 and no greater than 3. The asterisks each represent a bond. Specifically, each of the asterisks in general formula (W) represents a bond to a carbon atom to which Y in general formula (20) is bonded. 
     
       
         
         
             
             
         
       
     
     In general formula (20), each of R 20  and R 21  is preferably an alkyl group having a carbon number of at least 1 and no greater than 4, and more preferably a methyl group. R 22  and R 23  are preferably bonded to each other to form a divalent group represented by general formula (W). Y is preferably a divalent group represented by chemical formula (Y1) or (Y3). Preferably, t in general formula (W) is 2. 
     Preferably, the polyarylate resin (20) only includes a repeating unit represented by general formula (20). However, the polyarylate resin (20) may further include another repeating unit. A ratio (mole fraction) of the number of the repeating units represented by general formula (20) to a total number of repeating units in the polyarylate resin (20) is preferably at least 0.80, more preferably at least 0.90, and further preferably 1.00. The polyarylate resin (20) may include only one type of the repeating unit represented by general formula (20) or include two or more types (e.g., two types) of the repeating unit represented by general formula (20). 
     Note that in the present description, the ratio (mole fraction) of the number of repeating units represented by general formula (20) to the total number of repeating units in the polyarylate resin (20) is not a value obtained from one resin chain but a number average obtained from the entirety (a plurality of resin chains) of the polyarylate resin (20) contained in the photosensitive layer  502 . The mole fraction can for example be calculated from a  1 H-NMR spectrum of the polyarylate resin (20) measured using a proton nuclear magnetic resonance spectrometer. 
     Examples of preferable repeating units represented by general formula (20) include repeating units represented by chemical formula (20-a) and chemical formula (20-b) (also referred to below as repeating units (20-a) and (20-b), respectively). The polyarylate resin (20) preferably includes at least one of the repeating units (20-a) and (20-b), and more preferably includes both the repeating units (20-a) and (20-b). 
     
       
         
         
             
             
         
       
     
     In a case of the polyarylate resin (20) including both the repeating units (20-a) and (20-b), no particular limitations are placed on the sequence of the repeating units (20-a) and (20-b). The polyarylate resin (20) including the repeating units (20-a) and (20-b) may be any of a random copolymer, a block copolymer, a periodic copolymer, and an alternating copolymer. 
     Examples of preferable polyarylate resins (20) including both the repeating units (20-a) and (20-b) include a polyarylate resin having a main chain represented by general formula (20-1). 
     
       
         
         
             
             
         
       
     
     In general formula (20-1), a sum of u and v is 100. u is a number greater than or equal to 30 and less than or equal to 70. 
     u is preferably a number of at least 40 and no greater than 60, further preferably a number of at least 45 and no greater than 55, yet further preferably a number of at least 49 and no greater than 51, and particularly preferably a number of 50. Note that u represents a percentage of the number of the repeating units (20-a) relative to a sum of the number of the repeating units (20-a) and the number of the repeating units (20-b) in the polyarylate resin (20). v represents a percentage of the number of the repeating units (20-b) relative to the sum of the number of the repeating units (20-a) and the number of the repeating units (20-b) in the polyarylate resin (20). Examples of preferable polyarylate resins having a main chain represented by general formula (20-1) include a polyarylate resin having a main chain represented by general formula (20-1a). 
     
       
         
         
             
             
         
       
     
     The polyarylate resin (20) may have a terminal group represented by chemical formula (Z). In chemical formula (Z), the asterisk represents a bond. Specifically, the asterisk in chemical formula (Z) represents a bond to a main chain of the polyarylate resin. In a case of the polyarylate resin (20) including the repeating unit (20-a), the repeating unit (20-b), and the terminal group represented by chemical formula (Z), the terminal group may be bonded to the repeating unit (20-a) or may be bonded to the repeating unit (20-b). 
     
       
         
         
             
             
         
       
     
     In order to inhibit occurrence of a ghost image, preferably, the polyarylate resin (20) includes a polyarylate resin having a main chain represented by general formula (20-1) and a terminal group represented by chemical formula (Z). More preferably, the polyarylate resin (20) includes a polyarylate resin having a main chain represented by general formula (20-1a) and a terminal group represented by chemical formula (Z). The polyarylate resin having a main chain represented by general formula (20-1a) and a terminal group represented by chemical formula (Z) is also referred to below as a polyarylate resin (R-1). 
     The binder resin has a viscosity average molecular weight of preferably at least 10,000, more preferably at least 20,000, still more preferably at least 30,000, further preferably at least 50,000, and particularly preferably at least 55,000. As a result of the viscosity average molecular weight of the binder resin being at least 10,000, the photosensitive member  50  tends to have improved abrasion resistance. The viscosity average molecular weight of the binder resin is preferably no greater than 80,000 by contrast, and more preferably no greater than 70,000. As a result of the viscosity average molecular weight of the binder resin being no greater than 80,000, the binder resin tends to readily dissolve in a solvent for photosensitive layer formation, facilitating formation of the photosensitive layer  502 . 
     The binder resin has a content ratio to the mass of the photosensitive layer  502  of preferably at least 30.0% by mass and no greater than 70.0% by mass, and more preferably at least 40.0% by mass and no greater than 60.0% by mass. 
     (Electron Transport Material) 
     Examples of the electron transport materials include quinone-based compounds, diimide-based compounds, hydrazone-based compounds, malononitrile-based compounds, thiopyran-based compounds, trinitrothioxanthone-based compounds, 3,4,5,7-tetranitro-9-fluorenone-based compounds, dinitroanthracene-based compounds, dinitroacridine-based compounds, tetracyanoethylene, 2,4,8-trinitrothioxanthone, dinitrobenzene, dinitroacridine, succinic anhydride, maleic anhydride, and dibromomaleic anhydride. Examples of the quinone-based compounds include diphenoquinone-based compounds, azoquinone-based compounds, anthraquinone-based compounds, naphthoquinone-based compounds, nitroanthraquinone-based compounds, and dinitroanthraquinone-based compounds. The photosensitive layer  502  may contain only one electron transport material or may contain two or more electron transport materials. 
     Examples of electron transport materials that are preferable in terms of inhibiting occurrence of a ghost image include compounds represented by general formula (31), general formula (32), and general formula (33) (also referred to below as electron transport materials (31), (32), and (33), respectively). 
     
       
         
         
             
             
         
       
     
     In general formulas (31) to (33), R 1  to R 4  and R 9  to R 12  each represent, independently of one another, an alkyl group having a carbon number of at least 1 and no greater than 8. R 5  to R 8  each represent, independently of one another, a hydrogen atom, a halogen atom, or an alkyl group having a carbon number of at least 1 and no greater than 4. 
     In general formulas (31) to (33), the alkyl group having a carbon number of at least 1 and no greater than 8 that may be represented by any of R 1  to R 4  and R 9  to R 12  is preferably an alkyl group having a carbon number of at least 1 and no greater than 5, and further preferably a methyl group, a tert-butyl group, or a 1,1-dimethylpropyl group. Preferably, R 5  to R 8  each represent a hydrogen atom. 
     Preferably, the electron transport material (31) is a compound represented by chemical formula (ETM-1) (also referred to below as an electron transport material (ETM-1)). Preferably, the electron transport material (32) is a compound represented by chemical formula (ETM-3) (also referred to below as an electron transport material (ETM-3)). Preferably, the electron transport material (33) is a compound represented by chemical formula (ETM-2) (also referred to below as an electron transport material (ETM-2)). 
     
       
         
         
             
             
         
       
     
     In order to inhibit occurrence of a ghost image, the photosensitive layer  502  preferably contains at least one of the electron transport materials (31) and (32) as the electron transport material, and more preferably contains both (two of) the electron transport material (31) and the electron transport material (32). 
     In order to inhibit occurrence of a ghost image, the photosensitive layer  502  preferably contains at least one of the electron transport materials (ETM-1) and (ETM-3) as the electron transport material, and more preferably contains both (two of) the electron transport material (ETM-1) and the electron transport material (ETM-3). 
     The electron transport material has a content ratio to the mass of the photosensitive layer  502  of preferably at least 5.0% by mass and no greater than 50.0% by mass, and more preferably at least 20.0% by mass and no greater than 30.0% by mass. Where the photosensitive layer  502  contains two or more electron transport materials, the content ratio of the electron transport material is a total content ratio of the two or more electron transport materials. 
     (Additive) 
     The photosensitive layer  502  may further contain a compound represented by general formula (40) (also referred to below as an additive (40)) according to necessity. However, in order to increase the chargeability ratio, preferably, the photosensitive layer  502  contains no additive (40). Where the additive is used as necessary, the content ratio of the additive (40) is set to be greater than 0.0% by mass and no greater than 1.0% by mass to the mass of the photosensitive layer  502 , for example. The additive (40) can for example be used to adjust the chargeability ratio.
 
R 40 -A-R 41   (40)
 
     In general formula (40), R 40  and R 41  each represent, independently of each other, a hydrogen atom or a monovalent group represented by general formula (40a) shown below. 
     
       
         
         
             
             
         
       
     
     In general formula (40a), X represents a halogen atom. Examples of the halogen atom represented by X include a fluorine atom, a chlorine atom, a bromine atom, and an iodine atom. A chlorine atom is preferable as the halogen atom represented by X. 
     In general formula (40), A represents a divalent group represented by chemical formula (A1), (A2), (A3), (A4), (A5), or (A6) shown below. Preferably, the divalent group represented by A is the divalent group represented by chemical formula (A4). 
     
       
         
         
             
             
         
       
     
     A specific example of the additive (40) is a compound represented by chemical formula (40-1) (also referred to below as an additive (40-1)). 
     
       
         
         
             
             
         
       
     
     The photosensitive layer  502  may further contain an additive other than the additive (40) (also referred to below as an additional additive) according to necessity. Examples of the additional additive include antidegradants (specific examples include an antioxidant, a radical scavenger, a quencher, and an ultraviolet absorbing agent), softeners, surface modifiers, extenders, thickeners, dispersion stabilizers, waxes, donors, surfactants, and leveling agents. Where an additional additive is contained in the photosensitive layer  502 , the photosensitive layer  502  may contain only one additional additive or may contain two or more additional additives. 
     (Material Combinations) 
     In order to inhibit occurrence of a ghost image, the photosensitive layer  502  preferably contains materials of types and at content ratios shown in combination example Nos. 1 to 3 in Table 1, materials of types and at content ratios shown in combination example Nos. 4 to 6 in Table 2, or materials of types and at content ratios shown in combination example Nos. 7 to 9 in Table 3. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Combination 
                 CGM 
                 ETM 
                 Additive 
               
            
           
           
               
               
               
               
               
            
               
                 example 
                 Content ratio 
                 Type 
                 Type 
                 Content ratio 
               
               
                   
               
               
                 No. 1 
                 0.5 wt % &lt; CGM ≤ 1.0 wt % 
                 ETM-1/ETM-3 
                 40-1 
                 0.0 wt % &lt; Additive ≤ 1.0 wt % 
               
               
                 No. 2 
                 0.5 wt % &lt; CGM ≤ 1.0 wt % 
                 ETM-1/ETM-3 
                 — 
                 — 
               
               
                 No .3 
                 0.0 wt % &lt; CGM ≤ 0.5 wt % 
                 ETM-1/ETM-3 
                 — 
                 — 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Combination 
                 CGM 
                 HIM 
                 EIM 
                 Additive 
               
            
           
           
               
               
               
               
               
               
            
               
                 example 
                 Content ratio 
                 Type 
                 Type 
                 Type 
                 Content ratio 
               
               
                   
               
               
                 No. 4 
                 0.5 wt % &lt; CGM ≤ 1.0 wt % 
                 HTM-1 
                 ETM-1/ETM-3 
                 40-1 
                 0.0 wt % &lt; Additive ≤ 1.0 wt % 
               
               
                 No. 5 
                 0.5 wt % &lt; CGM ≤ 1.0 wt % 
                 HTM-1 
                 ETM-1/ETM-3 
                 — 
                 — 
               
               
                 No. 6 
                 0.0 wt % &lt; CGM ≤ 0.5 wt % 
                 HTM-1 
                 ETM-1/ETM-3 
                 — 
                 — 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Combination 
                 CGM 
                 HTM 
                 ETM 
                 Resin 
                 Additive 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 example 
                 Type 
                 Content ratio 
                 type 
                 Type 
                 Type 
                 Type 
                 Content ratio 
               
               
                   
               
               
                 No. 7 
                 CGM-1 
                 0.5 wt % &lt; CGM ≤ 1.0 wt % 
                 HTM-1 
                 ETM-1/ETM-3 
                 R-1 
                 40-1 
                 0.0 wt % &lt; Additive ≤ 1.0 wt % 
               
               
                 No. 8 
                 CGM-1 
                 0.5 wt % &lt; CGM ≤ 1.0 wt % 
                 HTM-1 
                 ETM-1/ETM-3 
                 R-1 
                 — 
                 — 
               
               
                 No. 9 
                 CGM-1 
                 0.0 wt % &lt; CGM ≤ 0.5 wt % 
                 HTM-1 
                 ETM-1/ETM-3 
                 R-1 
                 — 
                 — 
               
               
                   
               
            
           
         
       
     
     In Tables 1 to 3, “wt %”, “CGM”, “HTM”, “ETM”, and “Resin” respectively refer to “% by mass”, “charge generating material”, “hole transport material”, “electron transport material”, and “binder resin”. In Tables 1 to 3, “Content ratio” refers to each content ratio of a corresponding material to the mass of the photosensitive layer  502 . In Table 1 to 3, “ETM-1/ETM-3” means each of the electron transport material (ETM-1) and the electron transport material (ETM-3) being contained as the electron transport material. In Table 1 to 3, “-” refers to no corresponding materials being contained. In Table 3, “CGM-1” refers to Y-form titanyl phthalocyanine represented by chemical formula (CGM-1). Y-form titanyl phthalocyanine shown in Table 3 is preferably Y-form titanyl phthalocyanine that does not exhibit a peak in a range of from 50° C. to 270° C. and exhibits a peak in a range of higher than 270° C. and no greater than 400° C. (specifically one peak at 296° C.) in a differential scanning calorimetry spectrum thereof, other than a peak resulting from vaporization of adsorbed water. 
     (Intermediate Layer) 
     The intermediate layer  503  contains inorganic particles and a resin used in the intermediate layer  503  (intermediate layer resin), for example. Provision of the intermediate layer  503  can facilitate flow of current generated when the photosensitive member  50  is exposed to light and inhibit increasing resistance while also maintaining insulation to a sufficient degree so as to inhibit occurrence of leakage current. 
     Examples of the inorganic particles include particles of metals (specific examples include aluminum, iron, and copper), particles of metal oxides (specific examples include titanium oxide, alumina, zirconium oxide, tin oxide, and zinc oxide), and particles of non-metal oxides (specific examples include silica). Any one type of the inorganic particles listed above may be used independently, or any two or more types of the inorganic particles listed above may be used in combination. Note that the inorganic particles may be surface-treated. No particular limitations are placed on the intermediate layer resin other than being a resin that can be used for forming the intermediate layer  503 . 
     (Photosensitive Member Production Method) 
     In an example of production methods of the photosensitive member  50 , an application liquid for forming the photosensitive layer  502  (also referred to below as an application liquid for photosensitive layer formation) is applied onto the conductive substrate  501  and dried. Through the above, the photosensitive layer  502  is formed, thereby producing the photosensitive member  50 . The application liquid for photosensitive layer formation is produced by dissolving or dispersing in a solvent a charge generating material, a hole transport material, an electron transport material, a binder resin, and an optional component added as necessary. 
     No particular limitations are placed on the solvent contained in the application liquid for photosensitive layer formation so long as each component contained in the application liquid can be dissolved or dispersed therein. Examples of the solvent include alcohols (specific examples include methanol, ethanol, isopropanol, and butanol), aliphatic hydrocarbons (specific examples include n-hexane, octane, and cyclohexane), aromatic hydrocarbons (specific examples include benzene, toluene, and xylene), halogenated hydrocarbons (specific examples include dichloromethane, dichloroethane, carbon tetrachloride, and chlorobenzene), ethers (specific examples include dimethyl ether, diethyl ether, tetrahydrofuran, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, and propylene glycol monomethyl ether), ketones (specific examples include acetone, methyl ethyl ketone, and cyclohexanone), esters (specific examples include ethyl acetate and methyl acetate), dimethyl formaldehyde, dimethyl formamide, and dimethyl sulfoxide. Any one of the solvents listed above may be used independently, or any two or more of the solvents listed above may be used in combination. In order to improve workability in production of the photosensitive member  50 , a non-halogenated solvent (a solvent other than a halogenated hydrocarbon) is preferably used. 
     The application liquid for photosensitive layer formation is prepared by dispersing the components in the solvent by mixing. Mixing or dispersion can for example be performed using a bead mill, a roll mill, a ball mill, an attritor, a paint shaker, or an ultrasonic disperser. 
     The application liquid for photosensitive layer formation may for example contain a surfactant in order to improve dispersibility of the components. 
     No particular limitations are placed on the method by which the application liquid for photosensitive layer formation is applied other than being a method that enables uniform application of the application liquid for photosensitive layer formation on the conductive substrate  501 . Examples of application methods that can be used include blade coating, dip coating, spray coating, spin coating, and bar coating. 
     No particular limitations are placed on the method by which the application liquid for photosensitive layer formation is dried other than being a method that enables evaporation of the solvent in the application liquid for photosensitive layer formation. An example of the method involves heat treatment (hot-air drying) using a high-temperature dryer or a reduced pressure dryer. The heat treatment temperature is for example from 40° C. to 150° C. The heat treatment time is for example from 3 minutes to 120 minutes. 
     Note that the production method of the photosensitive member  50  may further include either or both a process of forming the intermediate layer  503  and a process of forming the protective layer  504  as necessary. The process of forming the intermediate layer  503  and the process of forming the protective layer  504  are each performed according to a method appropriately selected from known methods. 
     Through the above, the photosensitive member  50  has been described. Referring again to  FIG.  2   , the following describes the toners T, the charging rollers  51 , the primary transfer rollers  53 , the static elimination lamps  54 , and the cleaners  55  included in the image forming apparatus  1 . 
     &lt;Toner&gt; 
     The following describes the toners T that are loaded in the cartridge  60 M, the cartridge  60 C, the cartridge  60 Y, and the cartridge  60 BK illustrated in  FIG.  1    and that are to be supplied to the circumferential surfaces of the photosensitive members  50 . Each toner T includes toner particles. The toner T is a collection (a powder) of the toner particles. The toner particles each include a toner mother particle and an external additive. The toner mother particle includes at least one of a binder resin, a releasing agent, a colorant, a charge control agent, and a magnetic powder. The external additive is attached to the surface of the toner mother particle. The toner particles do not need to contain any external additive if unnecessary. In a situation in which the toner particles do not contain any external additive, the toner mother particles are equivalent to the toner particles. The toner T may be a capsule toner or a non-capsule toner. The capsule toner T can be produced by forming a shell layer on the surface of each toner mother particle. 
     The toner T preferably has a number average roundness of at least 0.960 and no greater than 0.998. As a result of the toner T having a number average roundness of at least 0.960, development and transfer can be favorably performed, so that a truer image can be output. As a result of the number average roundness of the toner T being no greater than 0.998, the toner T is prevented from easily passing through the gap between the cleaning blade  81  and the circumferential surface  50   a  of the photosensitive member  50 . The toner T preferably has a number average roundness of at least 0.960 and no greater than 0.980, more preferably at least 0.965 and no greater than 0.980, further preferably at least 0.970 and no greater than 0.980, and particularly preferably at least 0.975 and no greater than 0.980. The number average roundness of the toner T can be measured according to a method described in association with Examples. 
     The toner T preferably has a volume median diameter (also referred to below as D 50 ) of at least 4.0 m and no greater than 7.0 μm. As a result of the D 50  of the toner T being no greater than 7.0 μm, non-grainy high-definition output images can be obtained. The amount of the toner T necessary to obtain a desired image density decreases with a decrease in D 50  of the toner T. It is therefore possible to reduce the amount of the toner T to be used as long as the D 50  of the toner T is no greater than 7.0 m. As a result of the D 50  of the toner T being at least 4.0 μm, the toner T does not easily pass through the gap between the cleaning blade  81  and the circumferential surface  50   a  of the photosensitive member  50 . The D 50  of the toner T is preferably at least 4.0 μm and no greater than 6.0 μm, and more preferably at least 4.0 μm and no greater than 5.0 μm. The D 50  of the toner T can be measured according to a method described in association with Examples. Note that the Do of the toner T is a value of particle diameter at 50% of cumulative distribution of a volume distribution of the toner T measured using a particle size distribution analyzer. 
     According to the first embodiment, occurrence of a ghost image can be inhibited even if the toner T having a small particle diameter and a high roundness as above is employed and the cleaning blades  81  are tightly pressed against the photosensitive members  50 . 
     &lt;Charging Roller&gt; 
     Each charging roller  51  is located to be in contact with or close to the circumferential surface  50   a  of the corresponding photosensitive member  50 . The image forming apparatus  1  adopts a direct discharge process or a proximity discharge process. The charging time is shorter and the amount of charge to the photosensitive member  50  is smaller in a configuration including the charging roller  51  located to be in contact with or close to the circumferential surface  50   a  of the photosensitive member  50  than in a configuration including a scorotron charger. In image formation using the image forming apparatus  1  including the charging roller  51  located to be in contact with or close to the circumferential surface  50   a  of the photosensitive member  50 , therefore, it is difficult to uniformly charge the circumferential surface  50   a  of the photosensitive member  50  and a ghost image can easily occur. However, as already described, the image forming apparatus  1  according to the first embodiment can inhibit occurrence of a ghost image. Therefore, it is possible to sufficiently inhibit occurrence of a ghost image even in a configuration in which the charging roller  51  is located to be in contact with or close to the circumferential surface  50   a  of the photosensitive member  50 . 
     The distance between the charging roller  51  and the circumferential surface  50   a  of the photosensitive member  50  is preferably no greater than 50 μm, and more preferably no greater than 30 μm. Even in a configuration in which the distance between the charging roller  51  and the circumferential surface  50   a  of the photosensitive member  50  is in such a range, the image forming apparatus  1  according to the first embodiment can satisfactorily inhibit occurrence of a ghost image. 
     The charging voltage (charging bias) applied to the charging roller  51  is a direct current voltage. Where the charging voltage is a direct current voltage, an amount of discharge from the charging roller  51  to the photosensitive member  50  is smaller than that in a case of the charging voltage being a composite voltage. Thus, an abrasion amount of the photosensitive layer  502  of the photosensitive member  50  can be reduced. 
     A ghost image tends to occur particularly when the charging roller  51  is located in contact with or close to the circumferential surface  50   a  of the photosensitive member  50  and the charging voltage is a direct current voltage. However, as a result of the photosensitive member  50  satisfying formula (1), the image forming apparatus  1  according to the first embodiment can inhibit occurrence of a ghost image even in a configuration in which the charging roller  51  is located in contact with or close to the circumferential surface  50   a  of the photosensitive member  50  and the charging voltage is a direct current voltage. 
     The charging roller  51  has a resistance of preferably at least 5.0 log Ω and no greater than 7.0 log Ω, and more preferably at least 5.0 log Ω and no greater than 6.0 log Ω. As a result of the charging roller  51  having a resistance of at least 5.0 log Ω, leakage hardly occurs in the photosensitive layer  502  of the photosensitive member  50 . As a result of the charging roller  51  having a resistance of no greater than 7.0 log Ω, the resistance of the charging roller  51  hardly increases. The resistance of the charging roller  51  can be measured according to a method described in association with Examples. 
     &lt;Transfer Belt&gt; 
     The transfer belt has a surface resistivity ρS of at least 6 Log Ω and no greater than 11 Log Ω. Note that 6 Log Ω is equivalent to 1.0×10 6 Ω and 11 Log Ω is equivalent to 1.0×10 11 Ω. Also, Ω, which is a unit of the surface resistivity ρS, is also called a/square. As a result of the transfer belt  33  having a surface resistivity ρS of at least 6 Log Ω, occurrence of a ghost image can be inhibited. As a result of the transfer belt  33  having a surface resistivity ρS of no greater than 11 Log Ω, occurrence of charge-up of the toner T on the transfer belt  33  can be inhibited. The lower the surface resistivity ρS of the transfer belt  33  is (e.g., no greater than 11 Log Ω), the more likely a ghost image tends to occur. However, the photosensitive member  50  of the image forming apparatus  1  according to the first embodiment satisfies formula (1). This can inhibit occurrence of a ghost image and charge-up of the toner T even if the transfer belt  33  has a surface resistivity ρS of no greater than 11 Log Ω. 
     In order to inhibit occurrence of a ghost image, the transfer belt  33  has a surface resistivity ρS of preferably at least 7 Log Ω, more preferably at least 8 Log Ω, further preferably at least 9 Log Ω, and yet further preferably at least 10 Log Ω. In order to inhibit occurrence of charge-up of the toner T, the transfer belt  33  has a surface resistivity ρS of preferably no greater than 10 Log Ω, more preferably no greater than 9 Log Ω, further preferably no greater than 8 Log Ω, and yet further preferably no greater than 7 Log Ω. In order to inhibit occurrence of a ghost image while inhibiting occurrence of charge-up of the toner T, preferably, the transfer belt  33  has a surface resistivity ρS of at least 8 Log Ω and no greater than 11 Log Ω. In order to inhibit occurrence of a ghost image while inhibiting occurrence of charge-up of the toner T, the transfer belt  33  may have a surface resistivity ρS in a range between two values selected from 6 Log Ω, 7 Log Ω, 8 Log Ω, 9 Log Ω, 10 Log Ω, and 11 Log Ω. The surface resistivity ρS of the transfer belt  33  can be measured according to a method described in association with Examples. 
     &lt;Primary Transfer Roller&gt; 
     Each of the primary transfer rollers  53  primarily transfers the toner image from the circumferential surface  50   a  of the corresponding photosensitive member  50  to the transfer belt  33  in a state in which static elimination is not performed on the circumferential surface  50   a  of the photosensitive member  50 . The static elimination lamps  54  perform static elimination after transfer but do not perform static elimination before transfer. The image forming apparatus  1  adopts what is called a pre-transfer erasure-less process. Typically, static elimination is performed on the circumferential surface  50   a  of the photosensitive member  50  preferably before primary transfer by the primary transfer roller  53  in order to inhibit occurrence of a ghost image. This is because transfer current uniformly flows into the photosensitive member  50 . However, the photosensitive member  50  satisfies formula (1) in the first embodiment. This can enable sufficient inhibition of occurrence of a ghost image even in a configuration in which static elimination is not performed on the circumferential surface  50   a  of the photosensitive member  50  before primary transfer by the primary transfer roller  53 . Furthermore, when static elimination is performed before transfer, a tendency to cause toner scattering on an output image is observed which is due to production of an artifact of an electrostatic latent image formed on the circumferential surface  50   a  of the photosensitive member  50 . In the first embodiment, toner scattering on an output image can be inhibited because static elimination is not performed before transfer. 
     The following describes the primary transfer rollers  53 , which are under constant-voltage control, with reference to  FIG.  9   .  FIG.  9    is a diagram illustrating a power supply system for the four primary transfer rollers  53 . As illustrated in  FIG.  9   , the image forming section  30  further includes a power source  56  connected to the four primary transfer rollers  53 . The power source  56  is capable of charging each of the primary transfer rollers  53 . The power source  56  includes a constant voltage source  57  connected to the four primary transfer rollers  53 . The constant voltage source  57  applies a transfer voltage (transfer bias) to the primary transfer rollers  53  to charge the primary transfer rollers  53  in primary transfer. The constant voltage source  57  generates a constant transfer voltage (e.g., a constant negative transfer voltage). That is, the primary transfer rollers  53  are under constant-voltage control. A potential difference (transfer fields) between the surface potential of the circumferential surfaces  50   a  of the photosensitive members  50  and the surface potential of the primary transfer rollers  53  causes primary transfer of the toner images carried on the circumferential surfaces  50   a  of the respective photosensitive members  50  to the outer surface of the circulating transfer belt  33 . 
     In primary transfer, current (e.g., negative current) flows from the primary transfer rollers  53  into the respective photosensitive members  50  through the transfer belt  33 . In a configuration in which the primary transfer rollers  53  are disposed directly above the respective photosensitive members  50 , the current flows from the primary transfer rollers  53  into the photosensitive members  50  in a thickness direction of the transfer belt  33 . The current flowing into the photosensitive members  50  (flow-in current) changes as the surface resistivity ρS and the volume resistivity of the transfer belt  33  change provided that a constant transfer voltage is applied to the primary transfer rollers  53 . The tendency of a ghost image to occur increases with an increase in the flow-in current. That is, a ghost image is more likely to occur in an image formed by the image forming apparatus  1  including the primary transfer rollers  53 , which are under constant-voltage control, than in an image formed by an image forming apparatus that adopts constant-current control. However, the image forming apparatus  1  according to the first embodiment includes the photosensitive members  50  capable of inhibiting occurrence of a ghost image. It is therefore possible to inhibit occurrence of a ghost image even if an image is formed using the image forming apparatus  1  including the primary transfer rollers  53  under constant-voltage control. Furthermore, in the image forming apparatus  1  including the primary transfer rollers  53  under constant-voltage control, the number of constant voltage sources  57  can be smaller than the number of primary transfer rollers  53 . Thus, the image forming apparatus  1  can be simplified and miniaturized. 
     In order to perform stable primary transfer of the toners T from the primary transfer rollers  53  to the transfer belt  33 , current (transfer current) flowing in the primary transfer rollers  53  in transfer voltage application is preferably at least −20 μA and no greater than −10 μA. 
     &lt;Static Elimination Lamp&gt; 
     The static elimination lamps  54  are arranged downstream of the primary transfer rollers  53  in terms of the rotational direction R of the photosensitive members  50 . The cleaners  55  are arranged downstream of the static elimination lamps  54  in terms of the rotational direction R of the photosensitive members  50 . The charging rollers  51  are arranged downstream of the cleaners  55  in terms of the rotational direction R of the photosensitive members  50 . As a result of each static elimination lamp  54  being arranged between the corresponding primary transfer roller  53  and the corresponding cleaner  55 , it is ensured that a time from static elimination of the circumferential surface  50   a  of the photosensitive member  50  by the static elimination lamp  54  to charging of the circumferential surface  50   a  of the photosensitive member  50  by the charging roller  51  (also referred to below as a static elimination-charging time) is sufficiently long. Thus, a time for eliminating excited carriers generated inside the photosensitive layer  502  can be ensured. The static elimination-charging time is preferably 20 milliseconds or longer, and more preferably 50 milliseconds or longer. 
     The static elimination light intensity of each static elimination lamp  54  is preferably at least 0 μJ/cm 2  and no greater than 10 μJ/cm 2 , and more preferably at least 0 μJ/cm 2  and no greater than 5 μJ/cm 2 . As a result of the static elimination light intensity of the static elimination lamp  54  being no greater than 10 μJ/cm 2 , the amount of charge trapped inside the photosensitive layer  502  of the photosensitive member  50  decreases to enable chargeability of the photosensitive member  50  to increase. A smaller static elimination light intensity of the static elimination lamp  54  is more preferable. Note that the static elimination light intensity of the static elimination lamps  54  being 0 μJ/cm 2  means a static elimination-less system, which is a system without static elimination of the photosensitive members  50  by the static elimination lamps  54 . The static elimination light intensity of the static elimination lamp  54  can be measured according to a method described in association with Examples. 
     &lt;Cleaner&gt; 
     The cleaners  55  each include a cleaning blade  81  and a toner seal  82 . The cleaning blade  81  is located downstream of the primary transfer roller  53  in term of the rotational direction R of the photosensitive member  50 . The cleaning blade  81  is pressed against the circumferential surface  50   a  of the photosensitive member  50  and collects residual toner T on the circumferential surface  50   a  of the photosensitive member  50 . The residual toner T refers to toner of the toner T remaining on the circumferential surface  50   a  of the photosensitive member  50  as a result of primary transfer. Specifically, a distal end of the cleaning blade  81  is pressed against the circumferential surface  50   a  of the photosensitive member  50 , and a direction from a proximal end to the distal end of the cleaning blade  81  is opposite to the rotational direction R at a point of contact between the distal end of the cleaning blade  81  and the circumferential surface  50   a  of the photosensitive member  50 . The cleaning blade  81  is in what is called counter-contact with the circumferential surface  50   a  of the photosensitive member  50 . Thus, the cleaning blade  81  is tightly pressed against the circumferential surface  50   a  of the photosensitive member  50  such that the cleaning blade  81  digs into the photosensitive member  50  as the photosensitive member  50  rotates. Insufficient cleaning can be further prevented through the cleaning blade  81  being tightly pressed against the circumferential surface  50   a  of the photosensitive member  50 . The cleaning blade  81  is for example a plate-shaped elastic member. More specifically, the cleaning blade  81  is made from rubber with a plate shape. The cleaning blade  81  is in line-contact with the circumferential surface  50   a  of the photosensitive member  50 . 
     The linear pressure of the cleaning blade  81  on the circumferential surface  50   a  of the photosensitive member  50  is at least 10 N/m and no greater than 40 N/m. As a result of the linear pressure of the cleaning blade  81  on the circumferential surface  50   a  of the photosensitive member  50  being at least 10 N/m, insufficient cleaning can be prevented. As a result of the linear pressure of the cleaning blade  81  on the circumferential surface  50   a  of the photosensitive member  50  being no greater than 40 N/m, occurrence of a ghost image can be inhibited. In order to particularly prevent insufficient cleaning while inhibiting occurrence of a ghost image, the linear pressure of the cleaning blade  81  on the circumferential surface  50   a  of the photosensitive member  50  is preferably at least 15 N/m and no greater than 40 N/m, more preferably at least 20 N/m and no greater than 40 N/m, still more preferably at least 25 N/m and no greater than 40 N/m, further preferably at least 30 N/m and no greater than 40 N/m, and particularly preferably at least 35 N/m and no greater than 40 N/m. The linear pressure of the cleaning blade  81  on the circumferential surface  50   a  of the photosensitive member  50  may be in a range between two values selected from 10 N/m, 15 N/m, 20 N/m, 25 N/m, 30 N/m, 35 N/m, and 40 N/m. 
     The cleaning blade  81  preferably has a hardness of at least 60 and no greater than 80, and more preferably at least 70 and no greater than 78. As a result of the hardness of the cleaning blade  81  being at least 60, the cleaning blade  81  is not too soft, favorably preventing insufficient cleaning. As a result of the hardness of the cleaning blade  81  being no greater than 80, the cleaning blade  81  is not too hard, reducing the abrasion amount of the photosensitive layer  502  of the photosensitive member  50 . The hardness of the cleaning blade  81  can be measured according to a method described in association with Examples. 
     The cleaning blade  81  preferably has a rebound resilience of at least 20% and no greater than 40%, and more preferably at least 25% and no greater than 35%. The rebound resilience of the cleaning blade  81  can be measured according to a method described in association with Examples. 
     The toner seal  82  is located in contact with the circumferential surface  50   a  of the photosensitive member  50  between the corresponding primary transfer roller  53  and the cleaning blade  81 , and prevents the toner T collected by the cleaning blade  81  from scattering. 
     &lt;Thrust Mechanism&gt; 
     The following describes a drive mechanism  90  for implementing a thrust mechanism with reference to  FIG.  10   .  FIG.  10    is a plan view explaining the photosensitive members  50 , the cleaning blades  81 , and the drive mechanism  90 . Each of the photosensitive members  50  has a circular tubular shape elongated in a rotational axis direction D of the photosensitive member  50 . Each of the cleaning blades  81  has a plate-like shape elongated in the rotational axis direction D. 
     The image forming apparatus  1  further includes the drive mechanism  90 . The drive mechanism  90  causes either the photosensitive members  50  or the cleaning blades  81  to reciprocate in the rotational axis direction D. In the first embodiment, the drive mechanism  90  causes the photosensitive members  50  to reciprocate in the rotational axis direction D. The drive mechanism  90  for example includes a drive source such as a motor, a gear train, a plurality of cams, and a plurality of elastic members. The cleaning blades  81  are secured to a housing of the image forming apparatus  1 . 
     As described with reference to  FIG.  10   , the photosensitive members  50  are moved reciprocally in the rotational axis direction D relative to the cleaning blades  81  according to the first embodiment. Accordingly, local accumulation on and around the edge of each cleaning blade  81  can be moved in the rotational axis direction D, preventing a scratch in a circumferential direction (referred to below as “a circumferential scratch”) from being made on the circumferential surface  50   a  of the corresponding photosensitive member  50 . As a result, streaks that may occur in output images due to the toner T stuck in such a circumferential scratch are prevented from being made. Thus, good quality of resulting output images can be maintained over a long period of time. 
     Furthermore, according to the first embodiment, in which the photosensitive members  50  are caused to reciprocate, it is easy to obtain driving force required for the reciprocation and restrict occurrence of toner leakage over opposite ends of each of the cleaning blades  81  as compared to a configuration in which the cleaning blades  81  are caused to reciprocate. 
     The thrust amount of each photosensitive member  50  refers to a distance by which the photosensitive member  50  travels in one way of one back-and-forth motion. Note that in the first embodiment, an outward thrust amount and a return thrust amount are the same. The thrust amount of the photosensitive members  50  is preferably at least 0.1 mm and no greater than 2.0 mm, and more preferably at least 0.5 mm and no greater than 1.0 mm. As a result of the thrust amount of each photosensitive member  50  being within the above-specified range, circumferential scratches on the photosensitive member  50  can be favorably prevented from being made. 
     The thrust period of each photosensitive member  50  refers to a time taken by the photosensitive member  50  to make one back-and-forth motion. In the present description, the thrust period of the photosensitive member  50  is indicated in terms of the number of rotations of the photosensitive member  50  per back-and-forth motion of the photosensitive member  50 . The rotation speed of the photosensitive member  50  is constant. Accordingly, a longer thrust period of the photosensitive member  50  (i.e., a larger number of rotations of the photosensitive member  50  per back-and-forth motion of the photosensitive member  50 ) means that the photosensitive member  50  reciprocates more slowly. A shorter thrust period of the photosensitive member  50  (i.e., a smaller number of rotations of the photosensitive member  50  per back-and-forth motion of the photosensitive member  50 ) by contrast means that the photosensitive member  50  reciprocates more quickly. 
     The thrust period of each photosensitive member  50  is preferably at least 10 rotations and no greater than 200 rotations, and more preferably at least 50 rotations and no greater than 100 rotations. As a result of the thrust period of the photosensitive member  50  being at least 10 rotations, it is easy to clean the circumferential surface  50   a  of the photosensitive member  50 . Furthermore, as a result of the thrust period of the photosensitive member  50  being at least 10 rotations, the color image forming apparatus  1  tends not to undergo unintended coloristic shift. As a result of the thrust period of the photosensitive member  50  being no greater than 200 rotations by contrast, circumferential scratches on the photosensitive member  50  can be prevented from being made. 
     Through the above, the image forming apparatus  1  according to the first embodiment has been described. Although a configuration has been described in which the charging rollers  51  are employed as chargers, the image forming apparatus  1  may have a configuration in which the chargers are charging brushes located to be in contact with or close to the circumferential surfaces  50   a  of the respective photosensitive members  50 . Although the chargers adopting a direct discharge process or a proximity discharge process (specifically, the charging rollers  51 ) have been described, the present invention is also applicable to chargers adopting a discharge process other than the direct discharge process and the proximity discharge process. Although a configuration in which the charging voltage is a direct current voltage has been described, the present disclosure is also applicable to a configuration in which the charging voltage is an alternating current voltage or a composite voltage. The composite voltage refers to a voltage of an alternating current voltage superimposed on a direct current voltage. Although the development rollers  52  each using a two-component developer containing the carrier CA and the toner T have been described, the present invention is also applicable to development devices each using a one-component developer. Furthermore, although the image forming apparatus  1  has been described that adopts an intermediate transfer process using the primary transfer rollers  53 , the secondary transfer roller  34 , and the transfer belt  33 , the present invention is also applicable to an image forming apparatus that adopts a direct transfer process. 
     [Image Forming Method Implemented by Image Forming Apparatus According to First Embodiment] 
     The following describes an image forming method that is implemented by the image forming apparatus  1  according to the first embodiment. This image forming method includes charging, exposing to light, developing, performing primary transfer, performing secondary transfer, and cleaning. In the charging, the charging rollers  51  charge the circumferential surfaces  50   a  of the photosensitive members  50  to a positive polarity. In the exposing to light, the charged circumferential surfaces  50   a  of the photosensitive members  50  are exposed to light to form electrostatic latent images on the circumferential surfaces  50   a  of the photosensitive members  50 . In the developing, the electrostatic latent images are developed into toner images through supply of the toner T to the electrostatic latent images. In the performing primary transfer, the toner images are primarily transferred from the circumferential surfaces  50   a  of the photosensitive members  50  to the transfer belt  33  that is in contact with the circumferential surfaces  50   a . In the performing secondary transfer, the toner images are secondarily transferred from the transfer belt  33  to a sheet P. In the cleaning, residual toner T remaining on the circumferential surfaces  50   a  of the photosensitive members  50  as a result of the primary transfer of the toner images is collected by pressing the cleaning blades  81  against the circumferential surfaces  50   a  of the photosensitive members  50 . The transfer belt  33  has a surface resistivity ρS of at least 6 Log Ω and no greater than 11 Log Ω. The linear pressure of the cleaning blades  81  on the circumferential surfaces  50   a  of the photosensitive members  50  is at least 10 N/m and no greater than 40 N/m. The photosensitive members  50  each include the conductive substrate  501  and the photosensitive layer  502  of a single layer. The photosensitive layer  502  contains a charge generating material, a hole transport material, an electron transport material, and a binder resin. The photosensitive member  50  satisfies formula (1) described above. With the image forming method that is implemented by the image forming apparatus  1  according to the first embodiment, occurrence of a ghost image and charge-up of the toner T can be inhibited. 
     [Image Forming Apparatus According to Second Embodiment and Image Forming Method] 
     The following describes an image forming apparatus according to a second embodiment. The image forming apparatus according to the second embodiment includes an image bearing member, a charger, a light exposure device, a development device, a transfer belt, a primary transfer device, a secondary transfer device, and a cleaning member. The charger charges a circumferential surface of the image bearing member to a positive polarity. The light exposure device exposes the charged circumferential surface of the image bearing member to light to form an electrostatic latent image on the circumferential surface of the image bearing member. The development device develops the electrostatic latent image into a toner image through supply of a toner to the electrostatic latent image. The transfer belt is in contact with the circumferential surface of the image bearing member. The primary transfer device primarily transfers the toner image from the circumferential surface of the image bearing member to the transfer belt. The secondary transfer device secondarily transfers the toner image from the transfer belt to a recording medium. The cleaning member is pressed against the circumferential surface of the image bearing member and collects residual toner of the toner remaining on the circumferential surface of the image bearing member as a result of the toner image being primarily transferred. The transfer belt has a surface resistivity of at least 6 Log Ω and no greater than 11 Log Ω. A linear pressure of the cleaning member on the circumferential surface of the image bearing member is at least 10 N/m and no greater than 40 N/m. The image bearing member includes a conductive substrate and a photosensitive layer of a single layer. The photosensitive layer contains a charge generating material, a hole transport material, an electron transport material, and a binder resin. The charge generating material has a content ratio to mass of the photosensitive layer of greater than 0.0% by mass and no greater than 0.5% by mass. No particular limitations are placed on values related to formula (1) for the image bearing member in the image forming apparatus according to the second embodiment. The same description and preferred examples given with respect to the image forming apparatus according to the first embodiment apply to the image forming apparatus according to the second embodiment except values related to formula (1) for the image bearing member. With the image forming apparatus according to the second embodiment, occurrence of a ghost image and toner charge-up can be inhibited. 
     The following describes an image forming method that is implemented by the image forming apparatus according to the second embodiment. This image forming method includes charging, exposing to light, developing, performing primary transfer, performing secondary transfer, and performing cleaning. In the charging, a circumferential surface of an image bearing member is charged to a positive polarity. In the exposing to light, the charged circumferential surface of the image bearing member is exposed to light to form an electrostatic latent image on the circumferential surface of the image bearing member. In the developing, the electrostatic latent image is developed into a toner image through supply of a toner to the electrostatic latent image. In the performing primary transfer, the toner image is primarily transferred from the circumferential surface of the image bearing member to a transfer belt that is in contact with the circumferential surface of the image bearing member. In the performing secondary transfer, the toner image is secondarily transferred from the transfer belt to a recording medium. In the performing cleaning, cleaning is performed to collect residual toner by pressing a cleaning member against the circumferential surface of the image bearing member. The residual toner is toner of the toner remaining on the circumferential surface of the image bearing member as a result of the primary transfer of the toner. The transfer belt has a surface resistivity of at least 6 Log Ω and no greater than 11 Log Ω. A linear pressure of the cleaning member on the circumferential surface of the image bearing member is at least 10 N/m and no greater than 40 N/m. The image bearing member includes a conductive substrate and a photosensitive layer of a single layer. The photosensitive layer contains a charge generating material, a hole transport material, an electron transport material, and a binder resin. The charge generating material has a content ratio to mass of the photosensitive layer of greater than 0.0% by mass and no greater than 0.5% by mass. No particular limitations are placed on values related to formula (1) for the image bearing member in the image forming method implemented by the image forming apparatus according to the second embodiment. With the image forming method that is implemented by the image forming apparatus according to the second embodiment, occurrence of a ghost image and toner charge-up can be inhibited. 
     [Image Forming Apparatus According to Third Embodiment and Image Forming Method] 
     The following describes an image forming apparatus according to a third embodiment. The image forming apparatus according to the third embodiment includes an image bearing member, a charger, a light exposure device, a development device, a transfer belt, a primary transfer device, a secondary transfer device, and a cleaning member. The charger charges a circumferential surface of the image bearing member to a positive polarity. The light exposure device exposes the charged circumferential surface of the image bearing member to light to form an electrostatic latent image on the circumferential surface of the image bearing member. The development device develops the electrostatic latent image into a toner image through supply of a toner to the electrostatic latent image. The transfer belt is in contact with the circumferential surface of the image bearing member. The primary transfer device primarily transfers the toner image from the circumferential surface of the image bearing member to the transfer belt. The secondary transfer device secondarily transfers the toner image from the transfer belt to a recording medium. The cleaning member is pressed against the circumferential surface of the image bearing and collects residual toner of the toner remaining on the circumferential surface of the image bearing member as a result of the toner image being primarily transferred. The transfer belt has a surface resistivity of at least 6 Log Ω and no greater than 11 Log Ω. A linear pressure of the cleaning member on the circumferential surface of the image bearing member is at least 10 N/m and no greater than 40 N/m. The image bearing member includes a conductive substrate and a photosensitive layer of a single layer. The photosensitive layer contains a charge generating material, a hole transport material, an electron transport material, and a binder resin. The charge generating material has a content ratio to mass of the photosensitive layer of greater than 0.0% by mass and no greater than 1.0% by mass. The photosensitive layer contains no additive (40) or further contains an additive (40) at a content ratio to the mass of the photosensitive layer of greater than 0.0% by mass and no greater than 1.0% by mass. No particular limitations are placed on values related to formula (1) for the image bearing member in the image forming apparatus according to the third embodiment. The same description and preferred examples given with respect to the image forming apparatus according to the first embodiment apply to the image forming apparatus according to the third embodiment except values related to formula (1) for the image bearing member. With the image forming method that is implemented by the image forming apparatus according to the third embodiment, occurrence of a ghost image and toner charge-up can be inhibited. 
     The following describes an image forming method implemented by the image forming apparatus according to the third embodiment. This image forming method includes charging, exposing to light, developing, performing primary transfer, performing secondary transfer, and performing cleaning. In the charging, a circumferential surface of an image bearing member is charged to a positive polarity. In the exposing to light, the charged circumferential surface of the image bearing member is exposed to light to form an electrostatic latent image on the circumferential surface of the image bearing member. In the developing, the electrostatic latent image is developed into a toner image through supply of a toner to the electrostatic latent image. In the performing primary transfer, the toner image is primarily transferred from the circumferential surface of the image bearing member to a transfer belt that is in contact the circumferential surface of the image bearing member. In the performing secondary transfer, the toner image is secondarily transferred from the transfer belt to a recording medium. In the performing cleaning, cleaning is performed to collect residual toner by pressing a cleaning member against the circumferential surface of the image bearing member. The residual toner is toner of the toner remaining on the circumferential surface of the image bearing member as a result of the primary transfer of the toner image. The transfer belt has a surface resistivity of at least 6 Log Ω and no greater than 11 Log Ω. A linear pressure of the cleaning member on the circumferential surface of the image bearing member is at least 10 N/m and no greater than 40 N/m. The image bearing member includes a conductive substrate and a photosensitive layer of a single layer. The photosensitive layer contains a charge generating material, a hole transport material, an electron transport material, and a binder resin. The charge generating material has a content ratio to mass of the photosensitive layer of greater than 0.0% by mass and no greater than 1.0% by mass. The photosensitive layer contains no additive (40) or further contains an additive (40) at a content ratio to the mass of the photosensitive layer of greater than 0.0% by mass and no greater than 1.0% by mass. No particular limitations are placed on values related to formula (1) for the image bearing member in the image forming method implemented by the image forming apparatus according to the third embodiment. With the image forming method that is implemented by the image forming apparatus according to the third embodiment, occurrence of a ghost image and toner charge-up can be inhibited. 
     EXAMPLES 
     The following provides further specific description of the present invention through use of Examples. Note that the present invention is not limited to the scope of Examples. 
     &lt;Measuring Method&gt; 
     The following first describes methods for measuring physical properties in tests of examples and comparative examples. 
     (D 50  of Toner) 
     The D 50  of a target toner was measured using a particle size distribution analyzer (“COULTER COUNTER MULTISIZER 3”, product of Beckman Caulter, Inc.). 
     (Number Average Roundness of Toner) 
     The number average roundness of the target toner was measured using a flow particle imaging analyzer (“FPIA (registered Japanese trademark) 3000”, product of Sysmex Corporation). 
     (Static Elimination Light Intensity) 
     An optical power meter (“OPTICAL POWER METER 3664”, product of HIOKI E.E. CORPORATION) was embedded in a position of the circumferential surface of a target photosensitive member opposite to a static elimination lamp. Static elimination light having a wavelength of 660 nm was radiated onto the photosensitive member using the static elimination lamp, and the intensity of the static elimination light at the circumferential surface of the photosensitive member was measured using the optical power meter. 
     (Linear Pressure of Cleaning Blade) 
     The linear pressure of a target cleaning blade was measured using a load cell (“LMA-A SMALL-SIZED COMPRESSION LOAD CELL”, product of Kyowa Electronic Instruments Co., Ltd.). Specifically, the load cell was replaced with a photosensitive member in an evaluation apparatus such that the load cell was disposed in a position of contact between the cleaning blade and the circumferential surface of the photosensitive member. The angle of contact between the cleaning blade and the load cell was set to 23 degrees. The cleaning blade was pressed against the load cell. The linear pressure of the cleaning blade was measured using the load cell ten seconds after the start of the pressing. The thus measured linear pressure was taken to be the linear pressure of the cleaning blade. 
     (Hardness of Cleaning Blade) 
     The hardness of the cleaning blade was measured using a rubber hardness tester (“ASKER RUBBER HARDNESS TESTER Type JA”, product of KOBUNSHI KEIKI CO., LTD.) by a method in accordance with JIS K 6301. 
     (Rebound Resilience of Cleaning Blade) 
     The rebound resilience of the cleaning blade was measured using a rebound resilience tester (“RT-90”, product of KOBUNSHI KEIKI CO., LTD) by a method in accordance with JIS K 6255 (corresponding to ISO 4662). The rebound resilience was measured under environmental conditions of a temperature of 25° C. and a relative humidity of 50%. 
     (Surface Resistivity ρS of Transfer Belt) 
     The surface resistivity ρS of a target transfer belt was measured using a resistivity meter (“HIRESTA-UX MCP-HT800”, product of Mitsubishi Chemical Analytech Co., Ltd.) by a method in accordance with JIS K 6911. Measurement conditions included an application voltage of 250 V and a load of 2 kgf. The surface resistivity ρS was measured ten seconds after voltage application. 
     &lt;Evaluation Apparatus&gt; 
     The following describes an evaluation apparatus used for the tests of the examples and the comparative examples. The evaluation apparatus was a modified version of a multifunction peripheral (“TASKalfa 356Ci”, product of KYOCERA Document Solutions Inc.). The configuration and settings of the evaluation apparatus were mostly as follows. 
     Photosensitive member: positively-chargeable single-layer OPC drum 
     Diameter of photosensitive member: 30 mm 
     Film thickness of photosensitive layer of photosensitive member: 30 μm 
     Linear velocity of photosensitive member: 250 mm/second 
     Thrust amount of photosensitive member: 0.8 mm 
     Thrust period of photosensitive member: 70 rotations/back-and-forth motion 
     Charger: charging roller 
     Charging voltage: direct current voltage of positive polarity 
     Material of charging roller: epichlorohydrin rubber with an ion conductor dispersed therein 
     Diameter of charging roller: 12 mm 
     Thickness of rubber-containing layer of charging roller: 3 mm 
     Resistance of charging roller: 5.8 log Ω upon application of a charging voltage of +500 V 
     Distance between charging roller and circumferential surface of photosensitive member: 0 μm (contact) 
     Effective charge length: 226 mm 
     Transfer process: intermediate transfer process 
     Transfer voltage: direct current voltage of negative polarity 
     Material of transfer belt: polyimide 
     Transfer width: 232 mm 
     Pre-transfer static elimination: not done 
     Post-transfer static elimination: done 
     Static elimination light intensity: 5 μJ/cm 2    
     Static elimination-charging time: 125 millisecond 
     Cleaner: counter-contact cleaning blade 
     Contact angle of cleaning blade: 23 degrees 
     Material of cleaning blade: polyurethane rubber 
     Hardness of cleaning blade: 73 
     Rebound resilience of cleaning blade: 30% 
     Thickness of cleaning blade: 1.8 mm 
     Pressing method of cleaning blade: by fixing digging amount of cleaning blade in photosensitive member (fixed deflection) 
     Digging amount of cleaning blade in photosensitive member: value in range of from 0.8 mm to 1.5 mm (value varying depending on linear pressure of cleaning blade) 
     &lt;Photosensitive Member Production&gt; 
     Photosensitive members of the examples and the comparative examples to be mounted in an image forming apparatus were produced next. Materials for forming photosensitive layers used in the production of the photosensitive members and methods for producing the photosensitive member are as follows. 
     As the materials for forming the photosensitive layers of the photosensitive members, a charge generating material, a hole transport material, electron transport materials, a binder resin, and an additive described below were prepared. 
     (Charge Generating Material) 
     Y-form titanyl phthalocyanine represented by chemical formula (CGM-1) described in association with the first embodiment was prepared as the charge generating material. This Y-form titanyl phthalocyanine did not exhibit a peak in a range of from 50° C. to 270° C. and exhibited a peak in a range of higher than 270° C. and no greater than 400° C. (specifically, a single peak at 296° C.) in a differential scanning calorimetry spectrum thereof, other than a peak resulting from vaporization of adsorbed water. 
     (Hole Transport Material) 
     The hole transport material (HTM-1) described in association with the first embodiment was prepared as the hole transport material. 
     (Electron Transport Material) 
     The electron transport materials (ETM-1) and (ETM-3) described in association with the first embodiment were prepared as the hole transport material. 
     (Binder Resin) 
     The polyarylate resin (R-1) described in association with the first embodiment was prepared as the binder resin. The polyarylate resin (R-1) had a viscosity average molecular weight of 60,000. 
     (Additive) 
     The additive (40-1) described in association with the first embodiment was prepared as the additive. 
     (Production of Photosensitive Member (P-A1)) 
     A vessel of a ball mill was charged with 1.0 part by mass of the Y-form titanyl phthalocyanine as the charge generating material, 20.0 parts by mass of the hole transport material (HTM-1), 12.0 parts by mass of the electron transport material (ETM-1), 12.0 parts by mass of the electron transport material (ETM-3), 55.0 parts by mass of the polyarylate resin (R-1) as the binder resin, and tetrahydrofuran as a solvent. The vessel contents were mixed for 50 hours using the ball mill to disperse the materials (the charge generating material, the hole transport material, the electron transport materials, and the binder resin) in the solvent. Thus, an application liquid for photosensitive layer formation was obtained. The application liquid for photosensitive layer formation was applied onto a conductive substrate—an aluminum drum-shaped support—by dip coating to form a liquid film. The liquid film was hot-air dried at 100° C. for 40 minutes. Through the above, a single-layer photosensitive layer (film thickness 30 μm) was formed on the conductive substrate. As a result, a photosensitive member (P-A1) was obtained. 
     (Production of Photosensitive Members (P-A2) and (P-B1)) 
     Photosensitive members (P-A2) and (P-B1) each were produced according to the same method as in the production of the photosensitive member (P-A1) in all aspects other than that the charge generating material in an amount specified in Table 4 was used, the hole transport material in an amount specified in Table 4 was used, the electron transport material(s) of type and in an amount specified in Table 4 was used, and the binder resin in an amount specified in Table 4 was used. 
     (Production of Photosensitive Members (P-A3) and (P-B2)) 
     Photosensitive members (P-A3) and (P-B2) each were produced according to the same method as in the production of the photosensitive member (P-A1) in all aspects other than that the additive of type and in an amount specified in Table 4 was added. Note that the additive (40-1) was added in order to adjust chargeability of the photosensitive members. 
     &lt;Measurement of Chargeability Ratio&gt; 
     The chargeability ratio of each of the photosensitive members (P-A1) to (P-A3), (P-B1), and (P-B2) was measured according to the chargeability ratio measuring method described in association with the first embodiment. Table 4 shows results of chargeability ratio measurement. 
     In Table 4, “wt %”, “CGM”, “HTM”, “ETM”, and “Resin” respectively refer to “% by mass”, “charge generating material”, “hole transport material”, “electron transport material”, and “binder resin”. In Table 4, “ETM-1/ETM-3” and “12.0/12.0” refer to addition of both 12.0 parts by mass of the electron transport material (ETM-1) and 12.0 parts by mass of the electron transport material (ETM-3). In Table 4, “-” refers to no addition of a corresponding material. The amount of each material in Table 4 indicates a percentage (unit: % by mass) of the mass of the material relative to the mass of the photosensitive layer. The mass of the photosensitive layer is equivalent to the total mass of solids (more specifically, the charge generating material, the hole transport material, the electron transport material(s), the binder resin, and the additive) added to the application liquid for photosensitive layer formation. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                   
                 CGM 
                 HTM 
                 ETM 
                 Resin 
                 Additive 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Photosensitive 
                   
                 Amount 
                   
                 Amount 
                   
                 Amount 
                   
                 Amount 
                   
                 Amount 
                 Chargeability 
               
               
                 member 
                 Type 
                 [wt %] 
                 Type 
                 [wt %] 
                 Type 
                 [wt %] 
                 Type 
                 [wt %] 
                 Type 
                 [wt %] 
                 ratio 
               
               
                   
               
               
                 P-B1 
                 CGM-1 
                 1.7 
                 HTM-1 
                 36.0 
                 ETM-1 
                 23.0 
                 R-1 
                 39.3 
                 — 
                 — 
                 0.32 
               
               
                 P-B2 
                 CGM-1 
                 1.0 
                 HTM-1 
                 20.0 
                 ETM-1/ETM-3 
                 12.0/12.0 
                 R-1 
                 53.6 
                 40-1 
                 1.4 
                 0.48 
               
               
                 P-A3 
                 CGM-1 
                 1.0 
                 HTM-1 
                 20.0 
                 ETM-1/ETM-3 
                 12.0/12.0 
                 R-1 
                 54.2 
                 40-1 
                 0.8 
                 0.61 
               
               
                 P-A1 
                 CGM-1 
                 1.0 
                 HTM-1 
                 20.0 
                 ETM-1/ETM-3 
                 12.0/12.0 
                 R-1 
                 55.0 
                 — 
                 — 
                 0.71 
               
               
                 P-A2 
                 CGM-1 
                 0.5 
                 HTM-1 
                 20.0 
                 ETM-1/ETM-3 
                 12.0/12.0 
                 R-1 
                 55.5 
                 — 
                 — 
                 0.95 
               
               
                   
               
            
           
         
       
     
     &lt;Relationship Between Linear Pressure of Cleaning Blade and Number Average Roundness of Toner for D 50  of Toner&gt; 
     The relationship was studied first between linear pressure of a cleaning blade necessary for cleaning and number average roundness of toners for D 50  of the toners. Specifically, the photosensitive member (P-B1) was mounted in the evaluation apparatus. A toner was loaded into a toner container of the evaluation apparatus, and a developer containing the toner and a carrier was loaded into a development device of the evaluation apparatus. The surface resistivity ρS of the transfer belt was 10.5 Log Ω. An image I (a black longitudinal band-shaped image having a length of 100 mm parallel with the rotation direction of the photosensitive member) was printed on 100,000 successive sheets of paper using the evaluation apparatus under low-temperature and low-humidity environmental conditions (temperature: 10° C., relative humidity: 10%). The 100,000-sheet printing was a condition for the surface roughness of the cleaning blade and the surface roughness of the circumferential surface of the photosensitive member to increase. The low-temperature and low-humidity environmental conditions were for the hardness of the cleaning blade to increase and for the cleaning blade to easily decrease in performance. The evaluation apparatus was set so as not to perform toner transfer, specifically, so as not to perform transfer voltage application during printing of the image I. Due to non-performance of toner transfer, all toner developed on the circumferential surface of the photosensitive member was collected by the cleaning blade. After the 100,000-sheet printing, the circumferential surface of the photosensitive member was visually observed to confirm presence or absence of toner that had escaped capture by the cleaning blade on the circumferential surface of the photosensitive member. The above-described test was repeated by gradually increasing the linear pressure of the cleaning blade to determine the lowest linear pressure at which the cleaning blade was able to completely prevent the toner from escaping its capture (a minimum linear pressure necessary for preventing insufficient cleaning). 
     The minimum linear pressure for preventing insufficient cleaning was measured with respect to each of 15 toners having a D 50  of any of 4.0 μm, 6.0 μm, and 8.0 μm and a number average roundness of any of 0.960, 0.965, 0.970, 0.975, and 0.980.  FIG.  11    shows measurement results. In  FIG.  11   , the vertical axis indicates minimum linear pressure for preventing insufficient cleaning (unit: N/m), and the horizontal axis indicates number average roundness of the toners. In  FIG.  11   , circles on the plot indicate measurement results of the toners having a D 50  of 4.0 μm, diamonds on the plot indicate measurement results of the toners having a D 50  of 6.0 μm, and crosses on the plot indicate measurement results of the toners having a D 50  of 8.0 μm. 
       FIG.  11    demonstrates that the smaller the D 50  of toner is, the higher the minimum linear pressure necessary for preventing insufficient cleaning is.  FIG.  11    also demonstrates that the higher the number average roundness of toner is, the higher the minimum linear pressure necessary for preventing insufficient cleaning is. It can be understood from  FIG.  11    that a linear pressure of at least 10 N/m is necessary for the use of the toner having a D 50  of 6.0 μm and a number average roundness of 0.960. It can be also understood from  FIG.  11    that it is preferable to set the linear pressure to approximately 40 N/m for the use of the toner having a D 50  of 4.0 μm and a number average roundness of 0.980. The above-described tendency of the photosensitive member (P-B1), which had a chargeability ratio of lower than 0.60, indicated in  FIG.  11    is expected to be true for photosensitive members having a chargeability ratio of at least 0.60. Therefore, study was made as follows on photosensitive members that can inhibit occurrence of a ghost image even if the linear pressure of the cleaning blade is set to at least 10 N/m and no greater than 40 N/m. 
     &lt;Ghost Image Evaluation&gt; 
     (Ghost Image Evaluation on Photosensitive Member (P-B1)) 
     The photosensitive member (P-B1) was mounted in the evaluation apparatus. The transfer belt of the evaluation apparatus had a surface resistivity ρS of 10.5 Log Ω. The transfer current of a primary transfer roller of the evaluation apparatus was set to −10 μA. The linear pressure of a cleaning blade of the evaluation apparatus was set to 20 N/m. A charging roller of the evaluation apparatus was used to charge the circumferential surface of the photosensitive member to a potential of +500 V. The potential (+500V) of the charged circumferential surface of the photosensitive member was taken to be a surface potential V A  (Unit: +V). Next, the primary transfer roller of the evaluation apparatus was used to apply a transfer voltage to the charged circumferential surface of the photosensitive member. The potential of the circumferential surface of the photosensitive member after the transfer voltage application was measured using a surface electrometer (not illustrated, “ELECTROSTATIC VOLTMETER Model 344”, product of TREK, INC.), and taken to be a surface potential V B  (unit: +V). The surface potential drop ΔV B−A  (unit: V) due to transfer was calculated from the thus measured surface potential V B  in accordance with the following equation: “ΔV B−A =surface potential V B − surface potential V A =surface potential V B-500 ”. 
     Next, the transfer current of the primary transfer roller of the evaluation apparatus was set to 0 μA, −5 μA, −15 μA, −20 μA, −25 μA, and −30 μA, and the surface potential drop ΔV B−A  (unit: V) due to transfer at each of these values of the transfer current was measured according to the same method as described above. Next, the linear pressure of the cleaning blade of the evaluation apparatus was set to 0 N/m, 5 N/m, and 10 N/m, and the surface potential drop ΔV B−A  (unit: V) due to transfer at each of these values of the linear pressure was measured according to the same method as described above. No transfer voltage was applied for a transfer current of 0 μA. The cleaning blade was removed from the evaluation apparatus for a linear pressure of the cleaning blade of 0 N/m.  FIG.  12    shows measurement results of the surface potential drop ΔV B−A  due to transfer for the photosensitive members (P-B1). 
     (Ghost Image Evaluation on Photosensitive Member (P-A1)) 
     The photosensitive member (P-A1) was mounted in the evaluation apparatus. The surface potential drop ΔV B−A  (unit: V) due to transfer was measured according to the same method as in the ghost image evaluation on the photosensitive member (P-B1). The transfer current of the primary transfer roller of the evaluation apparatus was set to 0 μA, −5 μA, −10 μA, −15 μA, −20 μA, −25 μA, and −30 μA, and the surface potential drop ΔV B−A  (unit: V) due to transfer at each of these values of the transfer current was measured. Furthermore, the linear pressure of the cleaning blade of the evaluation apparatus was set to 25 N/m, 30 N/m, 35 N/m, 40 N/m, and 45 N/m, and the surface potential drop ΔV B−A  (unit: V) due to transfer at each of these values of the linear pressure was measured.  FIG.  13    shows measurement results of the surface potential drop ΔV B−A  due to transfer for the photosensitive member (P-A1). 
     (Criteria for Ghost Image Evaluation) 
     When the absolute value of the surface potential drop ΔV B−A  due to transfer is 10 V or higher, a ghost image tends to occur on an output image. Further, a range of the set transfer current (transfer current setting range) is preferably at least −20 μA and no greater than −10 μA in order to perform stable primary transfer of a toner to a transfer belt. From the above consideration, the photosensitive members were evaluated as being capable of inhibiting occurrence of a ghost image (denoted by “Ghost OK”) if the absolute value of the surface potential drop ΔV B−A  due to transfer was lower than 10 V under any of conditions of set transfer currents of −20 μA, —15 μA, and −10 μA. The photosensitive members were evaluated as being incapable of inhibiting occurrence of a ghost image (denoted by “Ghost NG”) if the absolute value of the surface potential drop ΔV B−A  due to transfer was 10 V or higher under at least one of the conditions of set transfer current values of −20 μA, −15 μA, and −10 μA. 
     (Result of Ghost Image Evaluation) 
     As shown in  FIGS.  12  and  13   , the absolute value of the surface potential drop ΔV B−A  due to transfer increased with an increase in the linear pressure of the cleaning blade. As also shown in  FIGS.  12  and  13   , the absolute value of the surface potential drop ΔV B−A  due to transfer increased with a decrease (to be closer to −30 μA) in the set transfer current. 
       FIG.  12    indicates the following about the photosensitive member (P-B1) having a chargeability ratio of lower than 0.60. As indicated in  FIG.  12   , when the linear pressure of the cleaning blade was set to 10 N/m or 20 N/m, the absolute value of the surface potential drop ΔV B−A  due to transfer for the photosensitive member (P-B1) was 10 V or higher under at least one of the conditions of set transfer currents of −20 μA, −15 μA, and −10 μA. The absolute value of the surface potential drop ΔV B−A  due to transfer increases with an increase in the linear pressure of the cleaning blade. Accordingly, as for the photosensitive member (P-B1), the absolute value of the surface potential drop ΔV B−A  due to transfer is expected to be 10 V or higher under at least one of the conditions of set transfer currents of −20 μA, −15 μA, and −10 μA also when the linear pressure of the cleaning blade is set to 30 N/m or 40 N/m. It is therefore decided that the photosensitive member (P-B1) having a chargeability ratio of lower than 0.60 is incapable of inhibiting occurrence of a ghost image when the linear pressure of the cleaning blade is at least 10 N/m and no greater than 40 N/m and the transfer current of the primary transfer roller is at least −20 μA and no greater than −10 μA. 
       FIG.  13    indicates the following about the photosensitive member (P-A1) having a chargeability ratio of at least 0.60. As for the photosensitive member (P-A1), as shown in  FIG.  13   , the absolute value of the surface potential drop ΔV B−A  due to transfer was lower than 10 V under any of the conditions of set transfer currents of −20 μA, −15 μA, and −10 μA when the linear pressure of the cleaning blade was set to any of 25 N/m, 30 N/m, 35 N/m, and 40 N/m. The absolute value of the surface potential drop ΔV B−A  due to transfer decreases with a decrease in the linear pressure of the cleaning blade. Accordingly, as for the photosensitive member (P-A1), the absolute value of the surface potential drop ΔV B−A  due to transfer is expected to be lower than 10 V under any of the conditions of set transfer currents of −20 μA, −15 μA, and −10 μA also when the linear pressure of the cleaning blade is set to any of 10 N/m, 15 N/m, and 20 N/m. It is therefore decided that the photosensitive member (P-A1) having a chargeability ratio of at least 0.60 is capable of inhibiting occurrence of a ghost image when the linear pressure of the cleaning blade is at least 10 N/m and no greater than 40 N/m and the transfer current of the primary transfer roller is at least −20 μA and no greater than −10 μA. 
     &lt;Relationship Between Chargeability Ratio of Photosensitive Member and Ghost Image Evaluation&gt; 
     The photosensitive member (P-B1) was mounted in the evaluation apparatus. The surface resistivity ρS of the transfer belt of the evaluation apparatus was 10.5 Log Ω. The transfer current of the primary transfer roller of the evaluation apparatus was set to −20 μA. The linear pressure of the cleaning blade of the evaluation apparatus was set to 40 N/m. The charging roller of the evaluation apparatus was used to charge the circumferential surface of the photosensitive member to a potential of +500 V. The potential (+500 V) of the charged circumferential surface of the photosensitive member was taken to be a surface potential V A  (Unit: +V). Next, the primary transfer roller of the evaluation apparatus was used to apply a transfer voltage to the charged circumferential surface of the photosensitive member. The potential of the circumferential surface of the photosensitive member after the transfer voltage application was measured using a surface electrometer (not illustrated, “SURFACE ELECTROMETER MODEL 344”, product of TREK, INC.), and the measured value was taken to be a surface potential V B  (Unit: +V). The surface potential drop ΔV B−A  (unit: V) due to transfer was calculated from the thus measured surface potential V B  in accordance with an equation “ΔV B−A =surface potential V B −surface potential V A =surface potential V B −500”. The photosensitive member (P-B1) was changed to the photosensitive members (P-A1), (P-A2), (P-A3), and (P-B2), and the surface potential drop ΔV B−A  due to transfer for each of the photosensitive members was measured according to the same method as described above. 
       FIG.  14    shows measurement results of the surface potential drop ΔV B−A  due to transfer for the photosensitive members. The photosensitive members were evaluated as being capable of inhibiting occurrence of a ghost image (denoted by “Ghost OK”) if the absolute value of the surface potential drop ΔV B−A  due to transfer was lower than 10 V in  FIG.  14   . The photosensitive members were evaluated as being incapable of inhibiting occurrence of a ghost image (denoted by “Ghost NG”) if the absolute value of the surface potential drop ΔV B−A  due to transfer was 10V or higher in  FIG.  14   . 
     The photosensitive members (P-B1) and (P-B2), which had a chargeability ratio of less than 0.60, each had an absolute value of the surface potential drop ΔV B−A  due to transfer of 10 V or higher as shown in  FIG.  14   . It is therefore decided that the photosensitive members (P-B1) and (P-B2) were incapable of inhibiting occurrence of a ghost image when used to form images. By contrast, the photosensitive members (P-A1) to (P-A3), which had a chargeability ratio of at least 0.60, each had an absolute value of the surface potential drop ΔV B−A  due to transfer of lower than 10 V as shown in  FIG.  14   . It is therefore decided that the photosensitive members (P-A1) to (P-A3) were capable of inhibiting occurrence of a ghost image when used to form images. 
     &lt;Relationship between Surface Resistivity ρS of Transfer Belt and Ghost Image Evaluation or Toner Charge-Up Evaluation&gt; 
     The photosensitive member (P-A1) was mounted in the evaluation apparatus. The transfer current of the primary transfer roller of the evaluation apparatus was set to −10 μA. The linear pressure of the cleaning blade of the evaluation apparatus was set to 20 N/m. A toner (number average roundness: 0.968, D50: 6.8 μm) was loaded into the toner container of the evaluation apparatus, and a developer containing the toner and a carrier was loaded into the development device of the evaluation apparatus. The surface resistivity ρS of the transfer belt of the evaluation apparatus was set to 5 Log Ω, 6 Log Ω, 8 Log Ω, 10 Log Ω, 11 Log Ω, 12 Log Ω, and 13 Log Ω, and the following printing was performed for each of the values of the surface resistivity ρS. An image I was printed on one sheet of paper using the evaluation apparatus under environmental conditions of a temperature of 23° C. and a relative humidity of 50%. The image I included an image region IA on a leading edge side of the paper and an image region IB on a trailing edge side of the paper in terms of a paper conveyance direction. The image region IA included a circular solid image portion and a background blank image portion. The image region IA corresponded to an image region formed through the first rotation of the photosensitive member in formation of the image I. The image region IB included a halftone image portion. The image region IB corresponded to an image region formed through the second rotation of the photosensitive member in formation of the image I. 
     (Ghost Image Evaluation) 
     A spectrophotometer (“SPECTROEYE (registered Japanese trademark) available at SAKATA INX ENG CO., LTD.) was used to measure the reflection density (reflection density A) of an area of the halftone image portion of the image I corresponding to the solid image portion of the image I and the reflection density (reflection density B) of an area of the halftone image portion of the image I corresponding to the background blank image portion of the image I. Then, a reflection density difference ΔE was calculated in accordance with an equation “ΔE=|reflection density A−reflection density B|”. According to the reflection density difference ΔE, whether or not occurrence of a ghost image was inhibited was evaluated based on the following criteria. 
     Good: ΔE was no greater than 3.0 and occurrence of ghost image was inhibited. 
     Poor: ΔE was greater than 3.0 and occurrence of ghost image was not inhibited. 
     (Evaluation of Toner Charge-Up) 
     Directly after the printing of the image I, a compact toner draw-off charge measurement system (“MODEL 212HS”, product of TREK, INC.) was used to suck toner on the transfer belt after the toner had passed through the primary transfer roller of the BK unit (after fourth primary transfer) and before the toner had passed through the secondary transfer roller. The charge amount (unit: μC/g) of the sucked toner was then measured using the compact toner draw-off charge measurement system. Whether or not occurrence of toner charge-up was inhibited was evaluated from the measured charge amount based on the following criteria. 
     Good: charge amount was no greater than 70 μC/g and occurrence of toner charge-up was inhibited. 
     Poor: charge amount was greater than 70 ρC/g and occurrence of toner charge-up was not inhibited. 
     Table 5 shows measurement results of reflection density differences ΔE and charge amounts when transfer belts having the respective surface resistivities ρS were used. Also,  FIG.  15    shows measurement results of reflection density differences ΔE when the transfer belts having the respective surface resistivities ρS were used.  FIG.  16    also shows measurement results of charge amounts when the transfer belts having the respective surface resistivities ρS were used. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 5 
               
               
                   
               
               
                 Photosensitive 
                 ρS 
                 Ghost image 
                 Toner charge-up 
               
               
                 member 
                 [LogΩ] 
                 ΔE 
                 Charge amount [μC/g] 
               
               
                   
               
             
            
               
                 P-A1 
                  5 
                 3.5 
                 38 
               
               
                   
                  6 
                 2.9 
                 40 
               
               
                   
                  8 
                 2.2 
                 48 
               
               
                   
                 10 
                 1.5 
                 58 
               
               
                   
                 11 
                 1.0 
                 69 
               
               
                   
                 12 
                 0.8 
                 82 
               
               
                   
                 13 
                 1.0 
                 88 
               
               
                   
               
            
           
         
       
     
     As shown in Table 5 and  FIGS.  15  and  16   , the image forming apparatus including the photosensitive member (P-A1) having a chargeability ratio of at least 0.60 achieved inhibition of occurrence of both a ghost image and toner charge-up when the transfer belt had a surface resistivity ρS of at least 6 Log Ω and no greater than 11 Log Ω. 
     &lt;Other Characteristics of Photosensitive Member&gt; 
     With respect to each of the photosensitive members, surface friction coefficient, Martens hardness of the photosensitive layer, and sensitivity were measured. 
     (Surface Friction Coefficient of Circumferential Surface of Photosensitive Member) 
     With respect to each of the photosensitive members, a non-woven fabric (“KIMWIPES S-200”, product of NIPPON PAPER CRECIA CO., LTD.) was placed on the photosensitive member and a weight (load: 200 gf) was placed on the circumferential surface of the non-woven fabric. An area of contact between the weight and the circumferential surface of the photosensitive member with the non-woven fabric therebetween was 1 cm 2 . The photosensitive member was caused to laterally slide at a rate of 50 mm/second while the weight was fixed. Lateral friction force in the lateral sliding was measured using a load cell (“LMA-A, small-sized compression load cell”, product of Kyowa Electronic Instruments Co., Ltd.). The surface friction coefficient of the circumferential surface of the photosensitive member was calculated in accordance with the following equation “surface friction coefficient=measured lateral friction force/200”. The circumferential surfaces of the photosensitive members (P-A1) to (P-A3) had surface friction coefficients of 0.45, 0.52, and 0.50, respectively. By contrast, the circumferential surfaces of the photosensitive members (P-B1) and (P-B2) had surface friction coefficients of 0.55 and 0.53, respectively. 
     (Martens Hardness of Photosensitive Layer) 
     The Martens hardness was measured using a hardness tester (“FISCHERSCOPE (registered Japanese trademark) HM2000XYp”, product of Fischer Instruments K.K.) by a nanoindentation method in accordance with ISO 14577. The measurement was carried out as described below under environmental conditions of a temperature of 23° C. and a relative humidity of 50%. That is, a square pyramidal diamond indenter (opposite sides angled at 135 degrees) was brought into contact with the circumferential surface of the photosensitive layer, a load was gradually applied to the indenter at a rate of 10 mN/5 seconds, the load was retained for one second once the load reached 10 mN, and the load was removed five seconds after the retention. The thus measured Martens hardness of the photosensitive layer of the photosensitive member (P-A1) was 220 N/mm 2 . 
     (Sensitivity of Photosensitive Member) 
     With respect to each of the photosensitive members (P-A1) to (P-A3), sensitivity was evaluated. Sensitivity was evaluated under environmental conditions of a temperature of 23° C. and a relative humidity of 50%. First, the circumferential surface of the photosensitive member was charged to +500 V using a drum sensitivity test device (product of Gen-Tech, Inc.). Next, monochromatic light (wavelength: 780 nm, half-width: 20 nm, light intensity: 1.0 μJ/cm 2 ) was obtained from white light of a halogen lamp using a band-pass filter. The thus obtained monochromatic light was radiated onto the circumferential surface of the photosensitive member. A surface potential of the circumferential surface of the photosensitive member was measured when 50 milliseconds elapsed from termination of the radiation. The thus measured surface potential was taken to be a post-exposure potential (unit: +V). The photosensitive members (P-A1), (P-A2), and (P-A3) resulted in a post-exposure potential of +110 V a post-exposure potential of +108 V, and a post-exposure potential of +98 V respectively. 
     These results demonstrated that the photosensitive members (P-A1) to (P-A3) each have a surface friction coefficient of the circumferential surface, a Martens hardness of the photosensitive layer, and sensitivity that are suitable for image formation. 
     The above demonstrated that the image forming apparatus according to the present invention, which encompasses image forming apparatuses including any of the photosensitive members (P-A1) to (P-A3), can achieve inhibition of occurrence of both a ghost image and toner charge-up. 
     INDUSTRIAL APPLICABILITY 
     The image forming apparatus according to the present invention is applicable for image formation on recording media.