Abstract:
An image forming apparatus comprising: an image forming section and a residual toner amount detection unit. The image forming section includes a photosensitive member, a charging unit, an exposure unit, a developing unit, and a toner storage unit. The residual toner amount detection unit detects an amount of toner remaining in the developing unit. The residual toner amount detection unit includes a light-emitting element and a light-receiving element. After the toner is deposited on the photosensitive member to form a toner image until electric charge is given onto a photosensitive layer of the photosensitive member by the charging unit again, the light-emitting element removes electric charge remaining on the photosensitive layer.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile machine, etc. for forming an image by electrophotography. Particularly, it relates to an image forming apparatus using toner for forming an image. 
   2. Description of the Related Art 
   In electrophotography, a charging unit such as a charging roller is used for giving electric charge on a photosensitive member evenly. An exposure unit is used for forming an electrostatic latent image corresponding to image data on the photosensitive member to which electric charge is given. Toner which is powder electrostatically charged in accordance with the charge pattern of the electrostatic latent image is deposited on the photosensitive member to thereby form a toner image on the electrostatic latent image. The toner image is transferred onto a recording medium such as a sheet of paper directly or through an intermediate transfer belt as an intermediate transfer member to thereby form an image. 
   Because electric charge remains on the photosensitive member after toner is transferred onto the recording medium, the residual electric charge is erased by an erasing unit before an image is formed again. A method of erasing electric charge by light irradiation or a method of erasing electric charge by an electrically conducting brush etc. brought into contact with the photosensitive member has been proposed as the erasing unit. 
   For formation of a color image, color toners such as yellow (Y), magenta (M), cyan (C), black (K), etc. are superposed on one another to form an image. 
   The color image forming method is roughly classified into two techniques. One is a repetitive development technique for forming a color image by repetitively developing the respective color toners on one photosensitive member. The other is a simultaneous development technique for forming a color image by simultaneously developing the respective color toners on a plurality of photosensitive members. 
   The repetitive development technique uses one photosensitive member for forming a color image. An intermediate transfer technique is an example of the repetitive development technique. The intermediate transfer technique is a technique in which developers that develops different color toners and a medium conveyance member are disposed around a photosensitive member so that toner images formed on the photosensitive member are color by color transferred onto the medium conveyance member successively, for example, as described in JP-A-8-137179. After this operation is repeated color by color so that the toner images of the different colors are superposed on the medium conveyance member, the superposed color toner image formed on the medium conveyance member is transferred onto a sheet of paper to thereby output a color image. 
   In this technique, for example, toner images of the four colors of yellow, magenta, cyan and black are color by color formed on the photosensitive member successively and then superpositively transferred onto the medium conveyance member. After all the toner images are transferred onto the medium conveyance member, the superposed color image formed on the medium conveyance member is transferred onto a medium such as a sheet of paper. Because colors are superposed successively in this manner, a time about four times as much as the printing time in the case of formation of a monochrome image from a color of black is required for forming an image. 
   It is however possible to reduce the number of parts because a charging unit, an exposure unit, a transfer unit, a cleaner unit and an erasing unit necessary for printing and developers corresponding to the four colors can be formed around one photosensitive member. 
   On the other hand, in the simultaneous development technique, photosensitive members corresponding to colors are provided as described in JP-A-5-35097. Toner images are almost simultaneously formed on the photosensitive members. The toner images are transferred in accordance with conveyance of a sheet of paper to thereby form a color image. This technique is also called “tandem electrophotographic”. 
   In the tandem electrophotographic, image forming section each having a photosensitive member, a charging unit, an exposure unit, a developing unit and a cleaner unit are provided independently in accordance with the colors. Accordingly, when a color image is formed from toners of the four colors of yellow, magenta, cyan and black, it is necessary to provide four image forming sections. 
   After toner images are almost simultaneously formed by the independent image forming section corresponding to the four colors, the toner images are transferred onto an intermediate transfer medium or a medium such as a sheet of paper. Because colors are superposed simultaneously in this manner, a color image can be formed in a time approximately as much as the printing time in the case of formation of a monochrome image from a color of black. This technique is suitable for high-speed printing of a color image. 
   It is however necessary to increase the number of parts because all printing processes required for forming images corresponding to the colors must be prepared. 
   A method of detecting the amount of toner remaining in a developer has been described in JP-A-5-35097. A residual toner amount detection unit includes a toner sensor, and toner detection windows. The toner sensor is disposed in an image forming apparatus and has a light-emitting element, and a light-receiving element. The toner detection windows are provided in the developer. 
   When there is no toner in the developer, light emitted from the light-emitting element is transmitted through one of the toner detection windows in the developer and further transmitted through the other toner detection window on the opposite side via the inside of the developer so that the light is received by the light-receiving element. 
   When there is toner in the developer, light emitted from the light-emitting element is blocked by the toner in the developer so that the light cannot reach the light-receiving element on the opposite side. On this occasion, the presence of toner can be detected because the light-receiving element outputs a voltage or the like proportional to the quantity of light received by the light-receiving element. 
   A method of erasing residual electric charge has been described in JP-A-11-344909. In this method, a light-emitting element such as an LED lamp or a fluorescent lamp is disposed between a transfer unit and a charging unit so that electric charge remaining on a photosensitive member can be erased by application of light on the photosensitive member. 
   In recent years, color printers have become widespread rapidly and reduction in cost of the printers has advanced because of increasing demands for colorization of documents in offices. In addition, tandem color printers have attracted notice because of demands for increase in printing speed. 
   It is however difficult to reduce the size of the tandem printer because the tandem printer must have four image forming sections as descried above. The tandem printer requires a large number of parts. Accordingly, the size of the tandem printer and the cost of parts in the tandem printer must become larger than those of a printer using the repetitive development technique. Above all, the residual toner amount detection unit needs light-emitting element and light-receiving element pairs, light-emitting element drive circuits and light-receiving element receiving circuits in accordance with the four colors, so that increase in production cost is brought. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in view of above circumstances and provides an image forming apparatus and an image forming unit. According to an aspect of the invention, the number of constituent parts in the image forming apparatus or the image forming unit can be reduced to thereby reduce the cost of production. 
   According to a first aspect of the invention, there is provided an image forming apparatus including: an image forming section and a residual toner amount detection unit. The image forming section includes a photosensitive member having a photosensitive layer in its surface; a charging unit that gives electric charge to the photosensitive member, the charging unit disposed on the photosensitive member; an exposure unit that exposes the photosensitive layer to light based on image data to form an electrostatic latent image; a developing unit that deposits toner on the electrostatic latent image formed on the photosensitive member to form a toner image; and a toner storage unit that stores toner used in the developing unit; wherein the toner image formed on the photosensitive member is transferred onto a recording medium conveyed by a medium conveyance member directly or through an intermediate transfer member to form an image. The residual toner amount detection unit detects an amount of toner remaining in the developing unit. The residual toner amount detection unit includes a light-emitting element and a light-receiving element. After the toner is deposited on the photosensitive member to form the toner image until electric charge is given onto the photosensitive layer by the charging unit again, the light-emitting element removes electric charge remaining on the photosensitive layer. 
   According to a second aspect of the invention, there is provided the image forming apparatus according to the first aspect, wherein at least one of the photosensitive member, the developing unit and the toner storage unit is formed as an detachable image forming unit which is detachable to a main body of the image forming apparatus; and wherein the light-emitting element is provided in the main body of the image forming apparatus. In this configuration, not only can the number of parts be reduced but also the detachable image forming unit can be repaired and exchanged easily. 
   According to a third aspect of the invention, there is provided the image forming apparatus according to the first aspect, wherein at least one of the photosensitive member, the developing unit and the toner storage unit is formed as an detachable unit which is detachable to a main body of the image forming apparatus; and wherein the light-emitting element is provided in the detachable unit. In this configuration, not only can the number of parts be reduced but also the optical element can be repaired and exchanged easily. 
   According to a fourth aspect of the invention, there is provided the image forming apparatus according to the first aspect, wherein light emitted from the light-emitting element is led to the photosensitive member through a light guide unit in order to remove electric charge remaining on the photosensitive layer. In this configuration, the light-emitting element can be arranged in an appropriate position. 
   According to a fifth aspect of the invention, there is provided the image forming apparatus according to the first aspect, further including: an emission amount adjusting unit that changes an amount of light emitted from the light-emitting element. In this configuration, the quantity of light emitted from the light-emitting element is reduced or emission of light is forbidden so that a good image quality can be always provided to a user. 
   According to a sixth aspect of the invention, there is provided an image forming unit including: a light-emitting element. The image forming unit further includes at least one of: a photosensitive member having a photosensitive layer in its surface; a developing unit that deposits toner on an electrostatic latent image formed on the photosensitive member to form a toner image; and a toner storage unit that stores toner used in the developing unit. The light-emitting element detects a residual toner amount in cooperation with a light-receiving unit of a residual toner detection unit and removes electric charge remaining on the photosensitive layer after the toner is deposited on the photosensitive member to form the toner image until electric charge is given onto the photosensitive layer by a charging unit again. In this configuration, it may be possible to provide an image forming unit in which the number of parts can be reduced. 
   According to a seventh aspect of the invention, there is provided the image forming unit according to the sixth aspect, wherein the image forming unit is equipped with the light-receiving element. In this configuration, it may be possible to provide an image forming unit in which not only can the number of parts be reduced but also the light-receiving element can be repaired and exchanged easily. 
   According to above configuration, a light-emitting element serves as the light-emitting element used in the erasing unit that erases electric charge remaining on the photosensitive member and serves also as the light-emitting element used in the residual toner amount detection unit that detects the residual amount of toner. Accordingly, the number of parts such as the light-emitting element and a drive circuit for the light-emitting element can be reduced to attain reduction in cost. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention will be described in detail based on the following figures, wherein: 
       FIG. 1A  is a schematic sectional view of an image forming apparatus according to Embodiment 1 of the invention; and 
       FIG. 1B  is a schematic sectional view of a printing unit according to Embodiment 1; 
       FIG. 2  is a schematic sectional view of an image forming apparatus according to Embodiment 2 of the invention; 
       FIG. 3  is a schematic sectional view of an image forming apparatus according to Embodiment 3 of the invention; 
       FIG. 4  is a schematic sectional view of an image forming section according to Embodiment 4 of the invention; 
       FIG. 5  is a sectional view taken along the line A–A′ on the image forming section depicted in  FIG. 4 ; 
       FIG. 6  is a schematic sectional view of an image forming section according to Embodiment 5 of the invention; 
       FIG. 7  is a sectional view taken along the line B–B′ on the image forming section depicted in  FIG. 6 ; 
       FIG. 8  is a schematic sectional view of an image forming section according to Embodiment 6 of the invention; 
       FIG. 9  is a schematic sectional view of an image forming section according to Embodiment 7 of the invention; 
       FIG. 10  is a schematic sectional view of a developer according to Embodiment 8 of the invention; 
       FIG. 11  is a schematic sectional view of a developer according to Embodiment 9 of the invention; 
       FIG. 12  is a schematic sectional view of an image forming apparatus according to Embodiment 10 of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Embodiment 1 
   An embodiment of an apparatus according to the invention will be described with reference to  FIGS. 1A and 1B .  FIG. 1A  is a schematic sectional view showing the overall configuration of an image forming apparatus using printing units as image forming section.  FIG. 1B  is a schematic sectional view showing one of the printing units. The image forming apparatus and the printing unit will be described below. 
   Sheets of printing paper  19  and a pickup roller  18  for picking up the sheets of printing paper  19  are provided in a lower portion of the image forming apparatus. 
   As shown in  FIG. 1B , each of the printing units  11   a ,  11   b ,  11   c  and  11   d  has a photosensitive member  2 , a charging roller  6 , an exposure device  5 , and a developer  4 . The photosensitive member  2  is provided as an endless cylinder having a photosensitive layer in its surface. The charging roller  6 , the exposure device  5  and the developer  4  are disposed around the photosensitive member  2 . The charging roller  6  gives electric charge onto the photosensitive layer of the photosensitive member  2 . The exposure device  5  applies light based on image information on the electric charge given onto the photosensitive layer of the photosensitive member  2  to thereby form an electrostatic latent image. The developer  4  develops toner on the electrostatic latent image. The printing units  11   a ,  11   b ,  11   c  and  11   d , which are units ( 11   a  to  11   d ) corresponding to the four colors of yellow, magenta, cyan and black necessary for forming a color image, are detachably attached to the image forming apparatus body  1  successively. 
   As shown in  FIG. 1B , the developer  4  which is one of image forming section has a developer casing  20 , a developing roller  8 , a supply roller  9 , and a toner vessel  21 . The developing roller  8 , the supply roller  9  and the toner vessel  21  are surrounded by the developer casing  20 . The developing roller  8  is used for developing toner on the electrostatic latent image. The supply roller  9  is used for supplying toner to the developing roller  8 . The toner used for development is stored in the toner vessel  21 . Incidentally, the reference numeral  7  in  FIG. 1B  designates a photosensitive member cleaner. 
   As shown in  FIG. 1A , in this embodiment, a light-emitting element  14  and a light-receiving element  15  for detecting the residual amount of toner are attached to the image forming apparatus body  1  so as to be separate from the printing units ( 11   a  to  11   d ). 
   The photosensitive members  2  of the printing units  11  abut on a medium conveyance member  3 . Each transfer roller  10  is disposed so that the medium conveyance member  3  is put between the transfer roller  10  and the photosensitive member  2 . 
   The medium conveyance member  3  is an endless belt. The medium conveyance member  3  is disposed horizontally in the apparatus body  1  in the condition that the medium conveyance member  3  is laid on a drive roller  24  and a driven roller  25  circularly. Incidentally, the positional relation between the drive roller  24  and the driven roller  25  may be reversed or two or more rollers may be provided. 
   In this embodiment, the image forming apparatus has printing units ( 11   a  to  11   d ) corresponding to the four colors of yellow, magenta, cyan and black necessary for forming a color image. The colors of yellow, magenta, cyan and black are developed in the printing units  11   a  to  11   d  respectively. Toner images thus formed are successively transferred to the medium conveyance member  3  used as an intermediate transfer belt. A second transfer roller  17  transfers the toner images onto a sheet of paper  19  fed by the pickup roller  18 . Thus, an image is formed. 
   The sheet of paper  19  having the toner images transferred thereon is ejected after toners are fixed by a fixing device  16  under heat and pressure. 
   Each residual toner amount detection unit is constituted by a combination of a light-emitting element  14  such as an LED and a light-receiving element  15  such as a photo diode. Light emitted from the light-emitting element  14  passes through the developer casing  20  so as to be incident on the light-receiving element  15 . When the amount of toner remaining in the developer casing  20  is small on this occasion, a large part of light emitted from the light-emitting element  14  becomes incident on the light-receiving element  15 . 
   On the other hand, when the developer casing  20  is filled with toner, the quantity of light incident on the light-receiving element  15  is reduced. The residual amount of toner is detected on the basis of the difference between quantities of light incident on the light-receiving element  15 . 
   Next, the erasing unit will be described. A part of light emitted from the light-emitting element  14  is led to a light guide plate  12  disposed between the transfer roller  10  and the cleaner  7 , via an end portion of the photosensitive member  2  by a light guide path  13 . The light emitted from the light-emitting element  14  and led to the light guide plate  12  is diffused and applied on the photosensitive member  2  to thereby erase electric charge remaining on the photosensitive member  2 . 
   It is preferable that light emitted from the light-emitting element  14  contains the wavelength of sensitivity characteristic of the photosensitive layer on the photosensitive member  2  and the wavelength of sensitivity characteristic of the light-receiving element. When, for example, the photosensitive layer on the photosensitive member  2  has sensitivity at a wavelength of 780 nm and the light-receiving element  15  has sensitivity at a wavelength of 880 nm, light emitted from the light-emitting material  14  contains light in a range of emission wavelengths of 780 nm and 880 nm. 
   In this embodiment, the number of parts such as a drive circuit belonging to the light-emitting element can be reduced because the light-emitting element  14  serves as the light-emitting element of the residual toner amount detection unit and also as the light-emitting element of the erasing unit. Incidentally, the reference numeral  22  in  FIG. 1A  designates a belt cleaner. 
   Embodiment 2 
   Another embodiment of the invention will be described below with reference to  FIG. 2 .  FIG. 2  is a sectional view showing the overall configuration of an image forming apparatus in which one  11   a  of printing units ( 11   a  to  11   d ) corresponding to the four colors of yellow, magenta, cyan and black necessary for forming a color image is separated. 
   The arrangement and functions of various kinds of parts constituting the image forming apparatus are the same as those in the image forming apparatus described above in Embodiment 1. Although Embodiment 1 shows the case where the light-emitting element  14  used in common to the residual toner amount detection unit and the erasing unit is attached to the image forming apparatus body  1 , this embodiment shows the case where the light-emitting element  14  is attached to the developer  4 . 
   For this reason, the light-emitting element  14  together with the developer  4  can be exchanged for a new one. In this configuration, not only can the number of parts shown in Embodiment 1 be reduced but also electric charge erasing failure in the erasing unit can be prevented from being caused by reduction in quantity of light due to deterioration of the light-emitting element  14  with the passage of time. 
   Embodiment 3 
   A further embodiment of the invention will be described below with reference to  FIG. 3 .  FIG. 3  is a sectional view showing the overall configuration of an image forming apparatus in which one  11   a  of printing units ( 11   a  to  11   d ) corresponding to the four colors of yellow, magenta, cyan and black necessary for forming a color image is separated. 
   The arrangement and functions of various kinds of parts constituting the image forming apparatus are the same as those in the image forming apparatus described above in  FIGS. 1A and 1B  or  FIG. 2 . Although Embodiment 2 shows the case where the light-receiving element  15  of the residual toner amount detection unit is attached to the image forming apparatus body  1 , this embodiment shows the case where the light-receiving element  15  together with the light-emitting element  14  is attached to the developer  4 . 
   For this reason, the light-emitting element  14  and the light-receiving element  15  together with the developer  4  can be exchanged for new ones. In this configuration, not only can the same improvement effect as in Embodiment 2 be obtained but also residual toner amount detection failure etc. can be prevented from being caused by deterioration of the light-receiving element  15  with the passage of time etc. 
   Embodiment 4 
   A further embodiment of the invention will be described below with reference to  FIGS. 4 and 5 . 
     FIG. 4  is a view showing a state in which only the printing unit  11  described in  FIG. 2  or  3  is removed.  FIG. 5  is a sectional view taken along the line A–A′ in  FIG. 4 , as seen from the upper surface. 
   The printing unit  11  shown in  FIGS. 4 and 5  operates in the same manner as in the image forming apparatus described in Embodiments 1 to 3. In this embodiment, light emitted from the light-emitting element  14  braches into an optical path for the erasing unit and an optical path for the residual toner amount detection unit, so that the light-emitting element  14  can be incorporated in the developer  4  easily. 
   The reference numerals  30  and  31  in  FIG. 5  designate through-holes which are formed in the developer casing  20  so that light emitted from the light-emitting element  14  toward the light-receiving element  15  can be transmitted. Transparent plastic sheets  32  are stuck to inner surfaces of the though-holes  30  and  31  respectively in order to prevent toner from scattering from the casing  20 . 
   Although this embodiment shows the case where the transparent plastic sheets  32  are used, the plastic sheets need not be provided and lenses or the like may be joined to the through-holes so that light emitted from the light-emitting element  14  can converge or diverge. 
   Embodiment 5 
   A further embodiment of the invention will be described below with reference to  FIGS. 6 and 7 . 
     FIG. 6  is a view showing a state in which only the printing unit  11  described in  FIG. 2  or  3  is removed.  FIG. 7  is a sectional view taken along the line B–B′ in  FIG. 6 , as seen from the upper surface. 
   The printing unit  11  shown in  FIGS. 6 and 7  operates in the same manner as in the image forming apparatus described in Embodiments 1 to 3. In this embodiment, light emitted from the light-emitting element  14  is guided by the light guide plate  12  used as an erasing unit so that a part of the emitted light is applied on the photosensitive member  2 . A part of the emitted light is transmitted through the light guide path  13  as light for the residual toner amount detection unit so that the light is guided to the light-receiving element  15 . 
   In this configuration, loss in quantity of light at the time of splitting light emitted from the light-emitting element  14  can be reduced. 
   The reference numeral  33  in  FIG. 7  designates a through-hole which permits insertion of a front end of the light guide path  13  and through which light emitted from the front end of the light guide path  13  is applied on the light-receiving element  15 . 
   Embodiment 6 
   A further embodiment of the invention will be described below with reference to  FIG. 8 .  FIG. 8  is a view showing a state in which only the printing unit  11  described in  FIG. 2  or  3  is removed. 
   The printing unit  11  shown in  FIG. 8  operates in the same manner as in the image forming apparatus described in Embodiments 1 to 3. In this embodiment, the light-emitting element  14  and the light guide plate  12  are disposed in a lower portion of the developer  4  so that the light-emitting element  14  and the light guide plate  12  can be incorporated in the developer  4  easily. 
   Embodiment 7 
   A further embodiment of the invention will be described below with reference to  FIG. 9 .  FIG. 9  typically shows paths of light emitted from the light-emitting element  14 . 
   Light emitted from the light-emitting element  14  splits into two optical paths. One is an optical path  34  which leads to the light-receiving element  15  via the developer casing  20  and which is used for the toner detection unit. The other is an optical path  35  which leads to the photosensitive member  2  after diffused by the light guide plate  12  via the light guide path  13  and which is used for the erasing unit. 
   The light guide path  13  is made of a transparent material such as polycarbonate, polyester, acrylic resin, glass, etc. The light guide path  13  leads light emitted from the light-emitting element  14  to the light guide plate  12 . The light guide plate  12  is made of a transparent light-diffusing material such as polycarbonate, polyester, acrylic resin, glass, etc. The light guide plate  12  plays a role of reflecting/diffusing the emitted light to apply the light on the photosensitive member  2 . 
   Embodiment 8 
   A further embodiment of the invention will be described below with reference to  FIG. 10 .  FIG. 10  is a typical view of a controller for controlling the quantity of light emitted from the light-emitting element  14 . 
   The developer  4  shown in  FIG. 10  operates in the same manner as in the image forming apparatus described in Embodiments 1 through 3. In this embodiment, a current control circuit  23  for controlling the quantity of light emitted from the light-emitting element  14  is attached to the image forming apparatus body  1 . When, for example, the developing ability of the developer  4  is lowered because of the expiration of its life, the current control circuit  23  applies an overcurrent to the light-emitting element  14  to disable the light-emitting element  14  from emitting light. 
   By this measure, the image forming apparatus is informed of the time of exchange of the developer  4  having the developing ability lowered, so that a good image quality can be always provided to the user. 
   Embodiment 9 
   A further embodiment of the invention will be described below with reference to  FIG. 11 .  FIG. 11  is a typical view of a controller for controlling the quantity of light emitted from the light-emitting element  14 . The developer  4  shown in  FIG. 11  operates in the same manner as in the image forming apparatus described in Embodiments 1 through 3. 
   In this embodiment, a current control circuit  23  for controlling the quantity of light emitted from the light-emitting element  14  is attached to the developer  4 . When, for example, the developing ability of the developer  4  is lowered because of the expiration of its life, the current control circuit  23  controls the current applied to the light-emitting element  14  so that the quantity of light emitted from the light-emitting element  14  can be reduced or light emission can be forbidden. 
   By this measure, the light-emitting element  14  can be recycled while the same effect as in Embodiment 8 can be obtained. 
   Embodiment 10 
   A further embodiment of the invention will be described below with reference to  FIG. 12 .  FIG. 12  is a sectional view showing the overall configuration of a printing unit and an image forming apparatus using the printing units. 
   The arrangement and functions of various parts constituting the image forming apparatus are the same as those of the image forming apparatus described in Embodiment 1. Although Embodiment 1 shows the case where the light-emitting element  14  used in common to the residual toner amount detection unit and the erasing unit is provided for each of the printing units ( 11   a  to  11   d ) of the four colors of yellow, magenta, cyan and black, this embodiment shows the case where the light guide path  13  is used so that one light-emitting element  14  can be used for all the printing units. 
   For this reason, the number of parts belonging to the light-emitting element  14  can be reduced compared with the image forming apparatus described in Embodiment 1. 
   Although each of the embodiments of the invention exemplifies the image forming apparatus using the medium conveyance member  3  as an intermediate transfer medium, the medium conveyance member  3  may be constituted by a paper conveyance member or the like. 
   Although each of the embodiments of the invention exemplifies the image forming apparatus provided with the printing units  11  disposed horizontally, the printing units  11  may be disposed vertically or obliquely. 
   Although each of the printing units  11  has been described on the case where a roller coated with an elastic material is used as the charging roller  6 , a brush roller coated with a brush or a non-contact charger may be used as the charging roller  6 . 
   For example, a device which is an array of LEDs arranged in one row in the widthwise direction of the photosensitive member and which has 600 to 1200 light-emitting diodes arranged per 1 inch (25.4 mm) so that each diode can be turned on and off at predetermined timing may be used as the exposure device  5 . As another example, a device constituted by a combination of a laser and a rotary mirror or an exposure device constituted by a combination of a light-emitting device and an optical switch such as a micro-mirror may be used. 
   Although each of the embodiments of the invention shows the case where the light-emitting element  14  as the erasing unit applies light on the photosensitive member  2  through the light guide plate  12 , light may be applied on the photosensitive member  2  directly without interposition of the light guide plate  12  or the light-emitting element  14  may apply light on the light-receiving element  15  without use of the light guide path  13 . 
   The case where the light-emitting element  14  is formed to apply light on the photosensitive member  2  or the light-receiving element  15  through the light guide unit such as the light guide plate  12 , the light guide path, etc. as shown in each of the embodiments of the invention is however preferred because not only a small light-emitting element  14  can be used but also there is room for arrangement of other units. 
   Although each of the embodiments of the invention shows the case where a sheet of paper which is a paper member is used as the printing medium used in the image forming apparatus, a plastic material such as a plastic sheet may be used as the printing medium. 
   Although each of the embodiments of the invention shows the case where printing units ( 11   a  to  11   d ) corresponding to the four colors of yellow, magenta, cyan and black necessary for forming a color image are used as the printing units, one printing unit corresponding to only a color of black may be used or a repetitive development technique using four rotations may be used. 
   Although each of the embodiments of the invention shows the case where printing units ( 11   a  to  11   d ) each having all of the photosensitive member  2 , the developing unit  4  and the toner storage unit  21  (Embodiments 1 and 10) or printing units ( 11   a  to  11   d ) each having the developing unit  4  and the toner storage unit  21  (Embodiments 2 to 9) are used as the printing units ( 11   a  to  11   d ) which are image forming sections, the invention is not limited thereto. For example, at least one of the photosensitive member  2 , the developing unit  4  and the toner storage unit may be used as a unit which can be separated from the apparatus body  1 . 
   The image forming apparatus according to the invention can be applied not only to the color printer shown in the aforementioned embodiments but also to a copying machine, a printer, a facsimile machine, etc. for forming an image by electrophotography. 
   The entire disclosure of Japanese Patent Application No. 2005-062431 filed on Mar. 7, 2005 including specification, claims, drawings and abstract is incorporated herein be reference in its entirety.