Patent Publication Number: US-7903988-B2

Title: Image forming apparatus capable of detecting ghost image

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2008-312052 filed Dec. 8, 2008. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to an image forming apparatus. 
     2. Related Art 
     Due to environmental considerations, EA (emulsion aggregation) toners using a wet production method are being used as toners in recent image forming apparatuses, enabling a reduction in the fixing temperature thereof. In order to achieve an even greater reduction in the fixing temperature crystalline binder polymers are gradually being introduced. 
     It is known that EA toners have poor cleaning characteristics in comparison to pulverized toners, and that crystalline binder polymers have poor charging characteristics. Consequently, there is a tendency to include a greater proportion of external additives in EA toners, in comparison to pulverized toners, in order to adjust the level of charge and maintain adequate cleaning. 
     Among such additives, in particular external additives of about 100 nm that are comparatively shaped with angular portions removed therefrom are used as transfer agents, in order to improve developability. However, such external additives readily pass beneath cleaning blades, though toner particles do not pass beneath the cleaning blade, so there is a tendency for external additive filming to occur. Therefore, recent toners are in a state susceptible to problems due to external additive filming due to becoming external-additive-rich. 
     A particular example of a problem caused by external additive filming is ghost images (ghosting). Within categories of ghosting there is known ghosting caused by localized changes in sensitivity and known ghosting caused by latent image electrical potential due to changes in Rp (residual potential), and also known ghosting caused by the surface condition of a photoreceptor. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the invention, there is provided an image forming apparatus. The image forming apparatus includes: an image carrier; a charging component that charges the image carrier to a predetermined electrical potential; an exposing component that exposes the image carrier charged by the charging component with light and forms an electrostatic latent image on the image carrier; a developing component that develops the electrostatic latent image formed on the image carrier by the exposing component, using a developer in a state a biasing voltage is applied, the developer comprising a colorant and an external additive; a transfer body that transfers the developer image developed on the image carrier by the developing component onto a transfer body; a detecting component that detects a density of the developer image transferred onto the transfer body by the transfer body; a removing component that removes residual developer on the image carrier after the developer image has been transferred onto the transfer body by the transfer body; a first control component that controls the charging component, the exposing component, and the developing component such that a developer image representing a predetermined image is transferred onto the transfer body; a second control component that, after the first control component has controlled so as to develop the developer image representing the predetermined image formed a plurality of times in the same location on the image carrier by the exposing component, controls such that without performing exposure with the exposing component, the developer image, developed on the image carrier by the developing component applying a biasing voltage to the developer so as to adhere the developer to the image carrier including to the same location on the image carrier, is transferred to the transfer body; and a ghost image generation detecting component that detects whether or not a ghost image is generated by, after control by the second control component, comparing the density detected by the detecting component at a position of the transfer body corresponding to the same location of the image carrier to a reference density. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a schematic cross-section showing an example of an image forming apparatus; 
         FIG. 2  is a drawing showing an electrical configuration of an image forming apparatus; 
         FIG. 3  is a graph showing elemental composition ratios on the surface of a photoreceptor; 
         FIGS. 4A to 4E  are diagrams showing a process that generates a ghost image due to filming; 
         FIG. 5  is a drawing showing actual ghost images; 
         FIG. 6  is a graph showing where a ghost image due to a latent image does not occur; 
         FIG. 7  is a flow chart showing the flow of image forming processing according to an exemplary embodiment; 
         FIG. 8  is a graph showing a relationship between a biasing voltage A and a biasing voltage B; 
         FIG. 9  is a graph showing a relationship between relative peripheral speed and generation of ghosting; 
         FIG. 10  is a schematic cross-section showing an example of an image forming apparatus; 
         FIG. 11  is a schematic cross-section showing an example of an image forming apparatus; 
         FIG. 12  is a graph showing a difference occurring between an electrical potential before developing and an electrical potential after developing; and 
         FIG. 13  is a flow chart showing a flow of image forming processing according to an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Explanation will now be given of details of an exemplary embodiment of the present invention, with reference to the drawings. Note that the developer in the exemplary embodiment of the present invention uses a toner (EA toner etc.) including an external additive that is easy to scrape through a cleaning unit, described later, developer is sometimes referred to below simply as “toner”. 
     A explanation will now be given of details of an image forming apparatus according to the present exemplary embodiment, with reference to the drawings.  FIG. 1  is a schematic cross-section showing an example of an image forming apparatus. The image forming apparatus shown in  FIG. 1  includes: a photoreceptor (image carrier)  10 ; a charging unit (charging component)  11  that charges the photoreceptor  10  to a predetermined electrical potential; a light-exposing unit (exposing component)  13  that exposes the photoreceptor  10  that has been charged by the charging unit  11  with light to form a latent electrostatic image on the photoreceptor  10 ; a developing unit (developing component)  14  that develops the latent electrostatic image formed on the photoreceptor  10  by the light-exposing unit  13  using a developer, containing a colorant and an external additive, in a state applied with a biasing voltage; a transfer unit (transfer component)  15  that transfers the developer image that has been developed by the developing unit  14  onto a transfer body  20 ; a density detection unit (detection component)  22  that detects the density of an image portion of the developer image transferred onto the transfer body  20  by the transfer unit  15 , and detects a non-image portion thereof, and a cleaning unit (removing component)  16  that removes any residual developer on the photoreceptor  10  after the developer image has been transferred onto the transfer body  20  by the transfer unit  15 . 
     Also included in the image forming apparatus are a static eliminator  17  that removes residual electrical potential on the surface of the photoreceptor  10 , and a fixing unit  18  that fixes the toner image transferred onto the surface of the transfer body  20  by application of heat and/or pressure. 
     The non-contact charging unit  11 , such as a charging roller, is disposed above the photoreceptor  10  shown in  FIG. 1 , and the charging unit  11  is operated by a voltage supplied from a power source  12 . A non-contact charging unit is used for the charging unit  11  in the current embodiment, however, both contact charging methods and non-contact charging methods may be used in the present invention. The developing unit  14  is also operated by voltage supplied from the power source  12 , and applies an electrical potential to the toner using the supplied voltage. 
     The cleaning unit  16  is configured with a blade  19 , serving as the cleaning blade of the present invention, disposed at an opening of a box  21 , with the cleaning unit  16  constructed such that residual toner and the like removed from the surface of the photoreceptor  10  is collected within the box  21 . 
     There are no particular limitations in the present invention to other elements of the configuration, the light-exposing unit  13 , developing unit  14 , transfer unit  15 , static eliminator  17  and fixing unit  18 , and these can be configured by application unmodified of known technology in the electrophotographic field. 
     The photoreceptor  10  is also of a circular cylindrical shape, being a rotating body rotating about a central axis connecting the centers of the circles at both ends of the circular cylinder. The developing unit  14  develops by rotating a developing rotating body  25 , to which developer is adhered, about an axis substantially parallel to the axis of the photoreceptor  10 , rotating at a specific rotational speed so as to slide the developing rotating body  25  with friction against the photoreceptor  10 . Control is possible such that the developing rotating body  25  can be rotated at various rotational speeds. 
     Explanation will now be given of an image forming method of the present invention, using the image forming apparatus of  FIG. 1 . The surface of the photoreceptor  10  is uniformly charged by the charging unit  11 , and a latent image is formed by the light-exposing unit  13  (latent image forming process). The latent image formed on the surface of the photoreceptor  10  is developed with toner housed within the developing unit  14 , so as to form a toner image (developing process). The toner image formed on the surface of the photoreceptor  10  is transferred onto the surface of the transfer body  20  passing between the photoreceptor  10  and the facing transfer unit  15  (transfer process), and is then furthermore fixed by application of heat and/or pressure etc. of the fixing unit  18 . 
     Any residual toner on the surface of the photoreceptor  10  after transfer, however, is removed by the cleaning unit  16  equipped with the blade  19  (cleaning process). Before continuing with the next image forming cycle, any residual electrical potential on the surface of the photoreceptor  10  is removed by the static eliminator  17 . 
     Explanation will now be given of an electrical configuration of the image forming apparatus, with reference to  FIG. 2 . The image forming apparatus includes a CPU (Central Processing Unit)  40 , a flash memory  42 , a RAM (Random Access Memory)  44 , a NVM (Non-Volatile Memory)  46 , a UI (User Interface)  48 , a HDD (Hard Disk Drive)  50 , a communications I/F (Interface)  54  and a bus  56 . 
     The CPU  40  controls the overall operation of the image forming apparatus, and the processing shown in a later described flow chart is executed by the CPU  40 . A program expanded in the RAM  44 , and a boot program for operating at start up, etc. are stored on the flash memory  42 . 
     The RAM  44  is a storage device in which an OS (Operating System), program, and image data are expanded. Setting values related to the image forming apparatus and the like are stored on the NVM  46 . 
     For connecting to a network the communications I/F  54  is configured with a NIC (Network Interface Card), and the driver thereof, or with a USB device etc. 
     A printer engine  52  is an engine for forming an image on a recording medium, such as paper, and includes the configuration shown in  FIG. 1 . The HDD  50  is a storage device for storing image data and the like. The UI  48  is used when a user operates the image forming apparatus and for inputting data. The bus  56  is used when data is being exchanged. 
     In addition to the configuration described above, for example, a scanner, an interface for connecting to a telephone line, and units relating to image processing may also be included. 
     Explanation will be given below of processing according to the present exemplary embodiment, based on the above configuration. In the present exemplary embodiment, a test image is formed as a latent image on the surface of the photoreceptor, and the portions of the latent image due to the test image are called image portions, and portions thereof other than these are called non-image portions. Also the portions of the transfer body  20  to which the test image is transferred are called the image portions, and portions thereof other than these are called non-image portions. 
     In the present exemplary embodiment an external additive that readily passes under the blade  19  is used, however, the reason for passing under is shown in  FIG. 3 . 
     The horizontal axis of the graph shown in  FIG. 3  shows image portions and non-image portions before developing and after developing, and MOS (the amount of toner per unit surface area of the photoreceptor) is also shown before developing and after developing. The vertical axis shows elemental composition ratios. The elements are Zn and Si. 
     The graph shown in  FIG. 3  therefore shows the elemental composition ratios of image portions and non-image portions before developing and after developing, and is also a graph showing differences in the elemental composition ratio before developing and after developing due to different MOS. 
     As shown in  FIG. 3 , supply of ZnSt (zinc stearate) mainly occurs to the non-image portions. Conditions arise at the image portions in which external additive can readily pass under the blade  19 , due to supply of ZnSt being low, reducing the amount of ZnSt. 
     Consequently, when development is performed plural times in succession at the same position on the photoreceptor, depending on a combination of environmental factors, developing conditions and amount used on the photoreceptor, even at a position after passing the blade  19  the external additive may have passed under the blade  19 , causing filming. 
     A process by which this filming generates ghosting will now be explained, with reference to  FIGS. 4A to 4E . These figures show graphs and states of external additive and toner on the surface of a photoreceptor. The graphs above and diagrams showing the surface of the photoreceptor below correspond to each other. The horizontal axis of each graph indicates corresponding positions on the photoreceptor surface, and the vertical axis indicates the electrical potential. The black circles indicate external additive, and the white circles indicate toner. 
     Note that in the present exemplary embodiment, the photoreceptor surface and the toner are negatively charged. Therefore the vertical axis of the graphs shown in these figures shows the magnitude of the negative charge. 
     First,  FIG. 4A  shows a state of the external additive on the photoreceptor surface after passing under the blade  19 . In this manner, where an image is formed repeatedly on the same portion the supply of ZnSt is low, and in particular under conditions of high sliding friction, due to high MOS and the peripheral speed ratio of the photoreceptor  10  to the developing unit  14 , a layer where external additive has passed under the blade  19  is formed at the image portion where the blade  19  has not been able to scrape off the ZnSt or to stop external additive from passing underneath. 
     In the next  FIG. 4B , a state is shown in which the photoreceptor surface is uniformly charged by the charging unit  11 .  FIG. 4C  shows a state in which a portion corresponding to the image portion has been light-exposed by the light-exposing unit  13 . 
     As shown in  FIG. 4D , the negatively charged external additive layer that has passed beneath the blade is partly removed in the developing unit  14 , and since the electrical potential is lowered at the removed portion, the surface electrical potential is reduced in comparison to where there is no passed external additive. The amount of developer increases, as shown in  FIG. 4E , due to an increase in the difference between the electrical potential applied to the toner and the electrical potential of the image portion. 
     Actual ghost images generated in this manner are shown in  FIG. 5 .  FIG. 5  is a graph showing the relationship between density D and Vdeve, and ghost images. The Vdeve shown in  FIG. 5  shows the difference in electrical potential between the electrical potential charging the image portion and the electrical potential applied to the toner, Vcln shows the difference in electrical potential between the electrical potential applied to the toner and the electrical potential charging the non-image portion. 
     It can be seen from the graph in  FIG. 5  that the density difference is greatest between the image portion and the non-image portion when the Vdeve is in the vicinity of 10V. 
     In the four images surrounding the ghost image shown as a rectangle, the image portion thereof is in the vicinity of the central bottom portion, with the remaining portions being a non-image portion, as shown in the diagram above. As shown in  FIG. 5 , an image starts to be visible when Vcln of the image portion is about 50V, and a dense-feint contrast can be clearly ascertained when the above described Vdeve is in the vicinity of 10V. When the Vdeve reaches 50V, the density difference between the image portion and the non-image portion decreases and all over becomes dense. 
     By use of  FIG. 6  it can be shown that the ghost image generated in the above described manner is not a latent image-caused ghost image, caused by a factor other than the external additive. 
       FIG. 6  is a graph showing the passage of time on the horizontal axis and the electrical potential of the photoreceptor surface on the vertical axis. The graphs show that, after 30 times of forming a latent image of the test image, the photoreceptor surface electrical potential is set before developing such that a latent image is not formed. However, as shown by the arrow, after setting the photoreceptor surface electrical potential so as not to form a latent image there is no reduction in the surface electrical potential before developing, and so it is clear that the ghost image is not one caused by a latent image. 
     A process for detecting the generation of an above described ghost image caused by external additive will now be explained. This process is a process executed by control of the CPU  40 . 
     First, in step  101 , the charging unit  11  uniformly charges the surface of the photoreceptor  10 . Next, in step  102 , the light-exposing unit  13  light-exposes a latent image of the test image. Next, in step  103 , the developing unit  14  develops by applying biasing voltage A normally used when image forming. Then, at step  104 , the transfer unit  15  transfers the image onto the surface of the transfer body  20 , the above described external additive passing occurs with residual toner not being removed by the blade  19 , however the static eliminator  17  removes the residual electrical potential on the surface of the photoreceptor  10 . 
     Determination is made at step  105  as to whether or not the above described process has been performed a predetermined number of times (a minimum of two times), and if this determination is negative then the process returns to step  101 . The processing of above described step  101  to step  105  is processing to develop a plural number of times a developer image representing the test image formed by the light-exposing unit  13  in the same location on the photoreceptor  10 . 
     However, when the determination at step  105  is affirmative, then at step  106  the charging unit  11  uniformly charges the surface of the photoreceptor  10 . Then at step  107 , without forming a latent image, developing with toner is performed by application of a biasing voltage B, different from the biasing voltage A, and at step  108  the image is transferred onto the transfer body  20  by the transfer unit  15 . The relationship between the biasing voltage A and the biasing voltage B will be explained later. 
     The processing of step  106  to step  108  is processing to control such that, with no light-exposure being performed by the light-exposing unit  13 , a developer image is developed on the photoreceptor  10  by the developing unit  14  applying a biasing voltage to adhere developer to the photoreceptor  10 , including the same location on the photoreceptor  10  as that of the test image, and the developed image on the photoreceptor  10  is transferred by the transfer unit  15 . 
     At the next step  109 , the density detection unit  22  detects a density X of toner at a position corresponding to an image portion on the surface of the transfer body  20  (a location at which the test image was formed (“the same location”)), and detects a density Y of toner at a position corresponding to a non-image portion on the surface of the transfer body  20  (a location different for the same location: more particularly a location not formed with the test image in the vicinity of where the test image was formed). 
     Then at step  110 , the densities detected by the density detection unit  22  for each of the image portion and the non-image portion are compared with each other. More specifically, a state in which ghost images are generated due to external additive is detected is made at the next step  111  when determination that the difference between the density X and the density Y is greater than a predetermined value Z. At step  112  a state in which a ghost image due to external additive is not generated is detected when the determination is negative at step  110 , and the processing is ended. Z is a value predetermined based on test results etc. 
     The relationship between the above described biasing voltage A and biasing voltage B will now be explained, with reference to  FIG. 8 .  FIG. 8  is a graph showing electrical potential on the vertical axis and position on the surface of the photoreceptor  10  on the horizontal axis. The biasing voltage A normally used during image forming is, as shown in  FIG. 8 , an electrical potential between the electrical potential of the surface of the photoreceptor  10  of the non-image portion and the electrical potential of the surface of the photoreceptor  10  of the image portion. The biasing voltage B is an electrical potential that is about the electrical potential of the surface of the photoreceptor  10  of the non-image portion, or an electrical potential with a greater absolute value. Namely, a biasing voltage is applied to the toner that adheres the toner to the photoreceptor  10 . 
     By using the biasing voltage B as the electrical potential applied to the toner, the toner is adhered to the image portion and to the non-image portion. The reason for such a biasing voltage B is that when biasing voltage A is used it is not possible to discriminate between a ghost image due to external additive and a ghost image due to a factor other than due to external additive. 
     Explanation will now be given of the fact that ghost images are readily generated with conditions of a large relative peripheral speed, with reference to the graph shown in  FIG. 9 . The graph of  FIG. 9  shows the difference in output of a provided ESV (surface electrical potential meter) between before and after developing on the vertical axis, a characteristic value roughly equivalent to how readily ghost images are generated, shown as ΔVh (the difference in photoreceptor surface charge electrical potential before and after passing development: the higher the value thereof the more readily ghost images are generated), and with relative peripheral speed shown on the horizontal axis. The relative peripheral speed shows the ratio of the rotational speed of the developing rotating body  25  to that of the photoreceptor  10 . 
     This graph is a graph made by forming a latent image of a test image 30 times at a relative peripheral speed of 1.75 so as to form an adhered layer of external additive on the photoreceptor  10 , then without forming a latent test image changing the relative peripheral speed while leaving Vcln=120, passing the surface of the photoreceptor  10  past the developing unit  14 , and shows the ΔVh after passing the developing unit  14 . 
     As shown in  FIG. 9 , as the relative peripheral speed increases conditions become more susceptible to ghost image generation. Consequently, the detection sensitivity may be raised by forming latent test images so as to rotate with various high sliding friction relative peripheral speeds, so as to produce on the photoreceptor  10  a surface that readily generates a ghost image by passage of external additive. 
     Since there is a high dependency on the amount of toner component adhered to the photoreceptor surface and to the surface roughness, conditions which do not readily generate ghost images may be found by trials using plural relative peripheral speeds. 
     When operating with various relative peripheral speeds in this manner, a step may be provided just after starting the routine in the flow chart explained for  FIG. 7  for setting the relative peripheral speed. 
     In the exemplary embodiment explained above, the density detection unit  22  detects the density of toner transferred to the transfer body  20 , however, as shown in  FIG. 10 , the density detection unit  22  may be provided below the developing unit  14 , so as to detect the density of developed toner that has been developed on the photoreceptor  10 . 
     In such cases the image forming apparatus includes: a photoreceptor (image carrier)  10 ; a charging unit (charging component)  11  that charges the photoreceptor  10  to a predetermined electrical potential; a light-exposing unit (exposing component)  13  that exposes the photoreceptor  10  that has been charged by the charging unit  11  with light to form a latent electrostatic image on the photoreceptor  10 ; a developing unit (developing component)  14  that develops the latent electrostatic image formed on the photoreceptor  10  by the light-exposing unit  13  using a developer, containing a colorant and an external additive, in a state applied with a biasing voltage; and a density detection unit (detection component)  22  that detects the density of a developer image transferred onto the photoreceptor  10  by the developing unit  14 . 
     Also included in the image forming apparatus are: a transfer unit  15  that transfers the developer image that has been developed on the photoreceptor  10  by the developing unit  14  onto a transfer body  20 ; a static eliminator  17  that removes residual electrical potential on the surface of the photoreceptor  10 ; and a fixing unit  18  that fixes the toner image transferred onto the surface of the transfer body  20  by application of heat and/or pressure. 
     When the image forming apparatus is configured in such a manner, after controlling so as to develop a plural number of times developer images representing the same test image, control is performed such that light-exposure is not carried out with the light-exposing unit  13  and a biasing voltage is applied to developer by the developing unit  14  so as to adhere developer to the photoreceptor  10 , thereby forming a developer image on the photoreceptor  10 . Then detection is made as to whether or not a ghost image due to external additive is generated by detecting with the density detection unit  22  the density of the same location of the image carrier as that of the test image and the density at a location of the image carrier that is different to the same location, then comparing these two densities. Namely, at step  108  in the flow chart explained for  FIG. 7 , the density detection unit  22  detects the density X of toner at the position corresponding to an image portion on the surface of the photoreceptor  10 , and the density Y of toner at a position corresponding to a non-image portion on the surface of the transfer body  20 , so as to detect whether or not a ghost image due to external additive is generated. 
     Explanation will now be given of a configuration for detecting ghost image generation by electrical potential, using  FIG. 11 , this being different to the configuration for detecting ghost image generation by density explained in the above exemplary embodiment. 
     The image forming apparatus shown in  FIG. 11  includes: a photoreceptor  10 ; a charging unit  11  that charges the photoreceptor  10  to a predetermined electrical potential; a light-exposing unit  13  that exposes the photoreceptor  10  that has been charged by the charging unit  11  with light to form a latent electrostatic image on the photoreceptor  10 ; a first electrical potential detection unit  23  that detects the electrical potential of the photoreceptor  10  that has had an electrostatic latent image formed thereon by the light-exposing unit  13 ; a developing unit  14  that develops the latent electrostatic image formed on the photoreceptor  10  by the light-exposing unit  13  using a developer, containing a colorant and an external additive, in a state applied with a biasing voltage; a second electrical potential detection unit  24  that detects the electrical potential of the photoreceptor  10  after passing past the developing unit  14 ; a transfer unit  15  that transfers the developer image that has been developed on the photoreceptor  10  by the developing unit  14  onto a transfer body  20 ; and a cleaning unit  16  that removes any residual developer after the developer image has been transferred onto the transfer body  20  by the transfer unit  15 . 
     Namely, the image forming apparatus shown in  FIG. 11 , is one that detects the electrical potential of the photoreceptor before developing and detects the electrical potential of the photoreceptor after developing. Generation of a ghost image can be detected in this configuration of image forming apparatus too. 
     Specific explanation will be given with reference to  FIG. 12 .  FIG. 12  is a graph showing the passage of time on the horizontal axis and the electrical potential of the photoreceptor surface on the vertical axis. The graphs shows that, after 30 repetitions of forming a latent image of the test image, the photoreceptor surface electrical potential is set before developing such that a latent image is not formed. However, as shown by the arrow, after setting the photoreceptor surface electrical potential so as not to form a latent image, a difference occurs between the electrical potential of the photoreceptor  10  before developing and the electrical potential of the photoreceptor  10  after developing. 
     The processing using electrical potentials to detect the generation of the above ghost image due to external additive will be explained with reference to the flow chart of  FIG. 13 . This processing is processing executed under control of the CPU  40 . 
     First, at step  201 , the charging unit  11  uniformly charges the surface of the photoreceptor  10 . Next, at step  202 , the light-exposing unit  13  performs light-exposure to form a latent test image. Next, at step  203 , the developing unit  14  develops the test image with toner using the above described biasing voltage A. Then, at step  204 , the transfer unit  15  transfers the toner image onto the surface of the transfer body  20 . As described above, the blade  19  is used to remove residual toner however the passage thereunder of the external additive occurs, and the static eliminator  17  removes the residual electrical potential on the surface of the photoreceptor  10 . 
     Determination is made at step  205  as to whether or not the above described process has been performed a predetermined number of times (a minimum of two times), and if this determination is negative then the process returns to step  201 . The processing of above described step  201  to step  205  is processing to develop a developer a plural number of times an image representing the test image formed by the light-exposing unit  13  in the same location on the photoreceptor  10 . 
     However, when the determination at step  205  is affirmative, then at step  206  the charging unit  11  uniformly charges the surface of the photoreceptor  10 . Then at step  207 , without forming a latent image, the electrical potential X of an image portion of the photoreceptor  10  is detected by the first electrical potential detection unit  23 . At step  208  the developing unit  14  applies a biasing voltage B with absolute value smaller than the photoreceptor charged electrical potential (this may be the same as biasing voltage A). 
     The processing of step  206  to step  208  is processing to control such that while no light-exposure is performed by the light-exposing unit  13 , a developer image is developed on the photoreceptor  10  by the developing unit  14  applying a biasing voltage to adhere developer to the photoreceptor  10 , including the same location on the photoreceptor  10 , and the photoreceptor  10  is developed with developer. 
     At the next step  209 , the second electrical potential detection unit  24  detects the electrical potential Y of an image portion on the photoreceptor  10 . A state of ghost image generation is detected at the next step  211  when determination is made at step  210  that the difference between the electrical potential X and the electrical potential Y is greater than a predetermined value Z. At step  212  a state in which a ghost image is not generated is detected when the determination is negative at step  210 , and the processing is ended. Note that the above described Z is a value pre-determined on the basis of test results or the like. 
     In the configuration shown in  FIG. 11  as well, as the relative peripheral speed increases conditions become more susceptible to ghost image generation. Consequently, the detection sensitivity may be raised by forming latent test images so as to rotate with various high sliding friction relative peripheral speeds, so as to produce the photoreceptor  10  a surface that readily generates a ghost image by passage of external additive. 
     Since there is a high dependency on the amount of toner component adhered to the photoreceptor surface and to the surface roughness, conditions which do not readily generate ghost images may be found by trials using plural relative peripheral speeds. 
     The processing of each of the flow charts explained above are only examples, and obviously the processing sequence may be changed, new steps added, and redundant steps removed, without departing from the scope of the present invention.