Patent Publication Number: US-8538294-B2

Title: Charge-removing device

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Applications No. 2010-083668 filed Mar. 31, 2010, and No. 2010-100874 filed Apr. 26, 2010. The entire contents of the priority applications are incorporated herein by reference. 
     TECHNICAL FIELD 
     The invention relates to a charge-removing device. 
     BACKGROUND 
     In an electrophotographic-type image-forming device, first an electrostatic latent image is formed on the surface of a photosensitive drum as the drum rotates. Next, toner is supplied by a developing roller to the surface of the photosensitive drum, for developing the electrostatic latent image into a toner image. Subsequently, the toner image carried on the surface of the photosensitive drum is transferred onto a sheet of paper. Through this process, the image-forming device can form images (toner images) on paper. 
     At this time, any charge remaining on the surface of the photosensitive drum after a toner image has been transferred onto paper can adversely affect the quality of the next electrostatic latent image (toner image) formed on the photosensitive drum. Therefore, some conventional image-forming devices have been provided with a charge-removing device for removing any residual charge from the surface of the photosensitive drum. 
     One charge-removing device known in the art is provided with an LED array. The LED array is disposed in opposition to the surface of the photosensitive drum and is configured of a plurality of light-emitting diodes (LEDs) aligned along the axis of the photosensitive drum. By emitting light from each LED so that the surface of the photosensitive drum is uniformly irradiated across the entire drum in the axial direction thereof, residual charge can be removed from the surface of the drum. 
     Another type of charge-removing device known in the art is configured of an LED lamp, and a light guide for guiding light emitted from the LED lamp onto the surface of the photosensitive drum. The light guide is formed of a transparent resin or glass and is bar-shaped. The bar-shaped light guide is positioned opposite the surface of the photosensitive drum so as to extend in the axial direction of the drum. The LED lamp is positioned opposite one endface of the light guide so that light emitted from the LED lamp enters this endface. V-shaped notches formed of recesses and protrusions are provided in the peripheral surface of the light guide for reflecting light entering the endface from the LED lamp onto the surface of the photosensitive drum. In order to prevent a decrease in the quantity of light irradiated onto the surface of the photosensitive drum as the distance from the LED lamp increases, the width and depth of the recesses and protrusions formed in the light guide are increased monotically in the direction away from the LED lamp. 
     Since the charge-removing device configured of an LED lamp and a light guide requires fewer LEDs than the charge-removing device configured of an LED array, the cost of the charge-removing device can be reduced. 
     SUMMARY 
     However, in thorough research conducted by the inventors of the invention, it was discovered that the conventional charge-removing device configured of an LED lamp and a light guide could not irradiate light onto the surface of the photosensitive drum uniformly across the axial direction thereof, even when the width and depth of the protrusions and recessions formed in the light guide were increased monotically in the direction away from the LED lamp or other light source. 
     Therefore, it is an object of the invention to provide a charge-removing device capable of irradiating light from a light source onto the surface of a photosensitive member so as to be uniform across the photosensitive member in the axial direction. 
     In order to attain the above and other objects, the invention provides a charge-removing device. The charge-removing device includes a light source and a light guide portion. The light source emits light. The light guide portion confronts a surface of an image-bearing member. The light guide portion extends in a first direction. The light guide portion guides light from the light source toward the surface of the image-bearing member. A charge on the surface of the image-bearing member is removed by the light emitted from the light guide portion. The light guide portion includes a first surface, a second surface, a confronting surface, and a rear surface. The first surface opposes the light source. The second surface is an opposite surface of the first surface in the first direction. The confronting surface connects the first surface and the second surface. The confronting surface confronts the surface of the image-bearing member in a second direction orthogonal to the first direction. The rear surface is an opposite surface of the confronting surface. The rear surface has a rough surface part that reflects, toward the confronting surface, the light entered from the first surface. The rough surface part has a length of a third direction orthogonal to the first direction and the second direction. The rough surface part includes an increasing portion and a decreasing portion. The closer a position of the increasing portion is to the second surface, the larger the length of the increasing portion in the third direction becomes. The closer a position of the decreasing portion is to the second surface, the smaller the length of the decreasing portion in the third direction becomes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The particular features and advantages of the invention as well as other objects will become apparent from the following description taken in connection with the accompanying drawings, in which: 
         FIG. 1  is a cross section of an image-forming device having a charge-removing device according to an embodiment; 
         FIG. 2  is a front view of the charge-removing device; 
         FIG. 3  is a right side view of the charge-removing device; 
         FIG. 4  is a table showing an arithmetic average roughness Ra, a root mean square roughness Rq, a maximum height of the roughness profile Rz, and a mean height of roughness profile elements Rc of a rough surface part; 
         FIG. 5  is a front view of a charge-removing device according to a first variation; 
         FIG. 6  is a front view of a charge-removing device according to a second variation; 
         FIG. 7  is a front view of the charge-removing device according to the embodiment; 
         FIG. 8  is a front view of a charge-removing device according to a comparative example; 
         FIG. 9  is a table showing a measurement result of optical powers of the charge-removing devices according to the embodiment and the comparative example; and 
         FIG. 10  is a graph showing the measurement result of optical powers of the charge-removing devices according to the embodiment and the comparative example. 
     
    
    
     DETAILED DESCRIPTION 
     1. Overall Structure of an Image-Forming Device 
     A charge-removing device  1  according to the embodiment of the invention is provided in an image-forming device  2 , such as that shown in  FIG. 1 . The image-forming device  2  is a tandem-type color laser printer, for example. However, the charge-removing device  1  may be provided in a color laser printer other than a tandem-type printer, or a monochrome laser printer. 
     The image-forming device  2  includes a main casing  3  shaped substantially like a rectangular parallelepiped. A front cover  4  is provided on one side wall of the main casing  3  for covering an access opening providing access to the interior of the main casing  3 . A drum unit  5  is removably accommodated in the interior of the main casing  3  and can be removed from the main casing  3  through the access opening when the front cover  4  is open. 
     In the following description, the side of the main casing  3  on which the front cover  4  is provided will be called the “front side,” and the front, rear, top, bottom, left, and right sides of components in the image-forming device  2 , including the charge-removing device  1 , will be based on the perspective of a user facing the front side of the charge-removing device  1 . 
     The drum unit  5  is provided with four photosensitive drums  6  corresponding to the four colors black, yellow, magenta, and cyan. The photosensitive drums  6  are arranged parallel to each other and are spaced at equal intervals in the front-to-rear direction in the order black, yellow, magenta, and cyan from front to rear. 
     The drum unit  5  also includes a drum subunit  7  and a developer cartridge  8  for each of the photosensitive drums  6 . The drum subunits  7  are fixed relative to the drum unit  5 . The developer cartridge  8  can be mounted in or removed from the drum unit  5  from a position above the drum unit  5  when the drum unit  5  is pulled far enough out of the main casing  3 . 
     Each drum subunit  7  includes a frame  9  formed of a synthetic resin. The frame  9  functions to hold chargers  10 , cleaning rollers  11 , and light guides  12  corresponding to four photosensitive drums  6 . The charger  10 , the cleaning roller  11 , and the light guide  12  are arranged about the circumference of the corresponding photosensitive drum  6  in an order opposite the rotating direction of the photosensitive drum  6  and oppose the circumferential surface of the photosensitive drum  6 . 
     Each developer cartridge  8  includes a case  13  for accommodating toner, and a developing roller  14  held in the lower end of the case  13 . The developing roller  14  is positioned so that a portion of its peripheral surface is exposed from the bottom of the case  13 . This exposed surface of the developing roller  14  contacts the peripheral surface of the corresponding photosensitive drum  6 . 
     An exposure device  15  is disposed above the drum unit  5  for irradiating four laser beams corresponding to the four colors employed in the image-forming device  2 . 
     In an image-forming operation, as the photosensitive drum  6  corresponding to each toner color rotates, the corresponding charger  10  applies a uniform charge to the surface of the photosensitive drum  6  through corona discharge. Subsequently, the exposure device  15  irradiates laser beams for selectively exposing the surfaces of the photosensitive drums  6 . Each laser beam selectively removes charge from the surface of the corresponding photosensitive drum  6 , forming an electrostatic latent image thereon. In the meantime, the thin layer of toner carried on the surface of the developing roller  14  is maintained at a uniform thickness. As the electrostatic latent image formed on the photosensitive drum  6  rotates to a position opposite the developing roller  14 , the developing roller  14  supplies toner to the latent image, developing the latent image into a visible toner image on the surface of the photosensitive drum  6 . 
     A paper cassette  16  for accommodating sheets of a paper P is disposed in the bottom section of the main casing  3 . A conveying belt  17  is disposed between the paper cassette  16  and the drum unit  5  so that the upper portion of the conveying belt  17  is in contact with the four photosensitive drums  6  on their bottom circumferential surfaces. Four transfer rollers  18  are disposed inside the conveying belt  17  at positions confronting each of the photosensitive drums  6  through the upper portion of the conveying belt  17 . 
     The paper P accommodated in the paper cassette  16  is conveyed onto the conveying belt  17  by various rollers. At this time, the conveying belt  17  is circulating so that its upper portion moves in the front-to-rear direction. Thus, when a sheet of paper P is conveyed onto the conveying belt  17 , the conveying belt  17  carries the sheet sequentially through positions between the conveying belt  17  and each of the photosensitive drums  6 . As the sheet passes beneath each photosensitive drum  6 , the corresponding transfer roller  18  functions to transfer the toner image from the surface of the photosensitive drum  6  to the sheet of paper P. 
     A fixing unit  19  is provided on the downstream end of the conveying belt  17  with respect to the direction that the paper P is conveyed. After toner images are transferred onto the paper P, the paper P is conveyed to the fixing unit  19 , where the toner images are fixed to the sheet by heat and pressure. After the toner images are fixed in the fixing unit  19 , various rollers discharge the sheet onto a discharge tray  20  formed on the top surface of the main casing  3 . 
     After a toner image is transferred from the photosensitive drum  6  to the paper P, the region on the surface of the photosensitive drum  6  from which the toner image was transferred rotates to a position opposite the light guide  12 . A light source  21  shown in  FIG. 2  irradiates light into the light guide  12 . As described below in detail, light entering the light guide  12  propagates therethrough and irradiates the surface of the photosensitive drum  6  across the entire width direction (left-to-right direction) thereof. Light irradiated from the light guide  12  removes any residual charge from the surface of the photosensitive drum  6  following the transfer operation. 
     In some cases after a toner image is transferred from the photosensitive drum  6  to the paper P, some toner remains on the portion of the photosensitive drum  6  from which the toner image was transferred. When the photosensitive drum  6  rotates such that this residual toner opposes the cleaning roller  11 , the cleaning roller  11  recovers the toner. 
     2. Charge-Removing Device 
     As shown in  FIG. 2 , the light guide  12  constitutes the charge-removing device  1  together with the light source  21  and a light guide accommodating part  22 . The light source  21  is disposed to the right of the light guide  12 . The light guide accommodating part  22  accommodates the light guide  12 . 
     (1) Light Guide 
     The light guide  12  is formed of a colorless, transparent acrylic resin, for example. The light guide  12  is bar-shaped and elongated in the left-to-right direction, i.e., the width direction (axial direction) of the photosensitive drum  6 . 
     The light guide  12  has a flat right endface  23 . The right endface  23  serves as the light-incident surface upon which light emitted from the light source  21  is irradiated. That is, the right endface  23  opposes the light source  21 . Light incident on the right endface  23  of the light guide  12  travels (passes) through the light guide  12  toward a left endface  24  on the other end of the light guide  12 . 
     The left endface  24  is fabricated to reflect light arriving at the left endface  24  back into the light guide  12 . Specifically, the left endface  24  is fabricated to form a stepped shape including an upper part  24 U, and a lower part  24 L recessed farther rightward than the upper part  24 U. 
     As shown in  FIG. 3 , the circumferential surfaces extending between the right endface  23  and the left endface  24  include a confronting surface  25 , a top surface  26 , a bottom surface  27 , and a rear surface  28 . 
     The confronting surface  25  opposes the surface of the photosensitive drum  6  with a minute gap formed therebetween. The confronting surface  25  is a curved surface of fixed curvature. In a cross section, the confronting surface  25  forms a semicircular arc shape, with the convex side of the arc facing the photosensitive drum  6 . Right and left edges of the confronting surface  25  are connected to the right endface  23  and the left endface  24 , respectively. 
     The top surface  26  is a flat surface that extends rearward from the top edge of the confronting surface  25 . The right and left edges of the top surface  26  are similarly connected to the right endface  23  and the left endface  24 , respectively. 
     The bottom surface  27  extends rearward from the bottom edge of the confronting surface  25  parallel to the top surface  26 . The right and left edges of the bottom surface  27  are similarly connected to the right endface  23  and the left endface  24 , respectively. 
     The rear surface  28  is a flat surface formed on the opposite side of the confronting surface  25  from the photosensitive drum  6 . The upper and lower edges of the rear surface  28  are connected respectively to the rear edges of the top surface  26  and the bottom surface  27 , and the right and left edges of the rear surface  28  are connected to the right endface  23  and the left endface  24 , respectively. 
     A rough surface part  29  is formed on the rear surface  28  for reflecting light propagating through the light guide  12  toward the confronting surface  25 . More specifically, the region of the rear surface  28  opposite the photosensitive drum  6  in the front-to-rear direction (a direction orthogonal to the width direction of the photosensitive drum  6  and to the top-to-bottom direction) is depressed farther inward toward the confronting surface  25  than the remaining regions of the rear surface  28 , forming steps between the region opposing the photosensitive drum  6  and the regions not opposing the photosensitive drum  6  with respect to the front-to-rear direction. The depressed region in the rear surface  28  is subjected to surface texturing (an etching process) to form micro-protrusions and micro-depressions therein. This processed region serves as the rough surface part  29  and has a textured surface of uniform roughness. 
     As shown in  FIG. 2 , the rough surface part  29  is formed in a region of the rear surface  28  spaced a gap D 1  from the right endface  23  of the light guide  12  (i.e., the right end of the rear surface  28 ). In other words, a gap D 1  is formed between the right end of the rough surface part  29  and the right endface  23 . The gap D 1  is 30 mm. A portion  30  of the rear surface  28  corresponding to the gap D 1  is a non-reflective portion. Accordingly, almost no light incident on the right endface  23  is reflected when traveling through the portion  30  of the rear surface  28  toward the left endface  24 . 
     In  FIG. 2 , the rough surface part  29  has been shaded with hatching lines in order to distinguish the rough surface part  29  from peripheral parts. The rough surface part  29  has a substantially uniform surface roughness. 
     Beginning from its right edge and progressing sequentially toward its left edge, the rough surface part  29  has an increasing width portion  31 , a maximum width portion  32 , a decreasing width portion  33 , and a minimum width portion  34 . That is, the increasing width portion  31  includes the right edge of the rough surface part  29 . 
     The increasing width portion  31  has a length D 2  and extends leftward from the right edge of the rough surface part  29 . The length D 2  is 20 mm. The width of the increasing width portion  31  in a direction orthogonal to the left-to-right direction, i.e., vertically, increases gradually toward the left (toward the left endface  24  of the light guide  12 ). More specifically, the vertical dimension of the increasing width portion  31  is 3 mm on the right end and 4.2 mm on the left end. Moving from the right end toward the left end of the increasing width portion  31 , the vertical dimension of the increasing width portion  31  increases monotonically (gradually) from 3 mm to 4.2 mm. That is, the closer a position of the increasing width portion  31  is to the left endface  24 , the larger the vertical dimension of the increasing width portion  31  becomes. 
     The maximum width portion  32  has a length D 3  that is a maximum vertical length among the vertical lengths of the rough surface part  29  and extends leftward from its border with the increasing width portion  31 . The length D 3  is 50 mm, and the vertical dimension of the maximum width portion  32  is a uniform 4.2 mm. 
     The decreasing width portion  33  has a length D 4  and extends leftward from its border with the maximum width portion  32 . The length D 4  is 40 mm. The vertical dimension of the decreasing width portion  33  decreases monotonically toward the left (toward the left endface  24  of the light guide  12 ). More specifically, the vertical dimension of the decreasing width portion  33  is 4.2 mm on the right end and 2.5 mm on the left end. Moving from the right end toward the left end, the vertical dimension of the decreasing width portion  33  decreases gradually from 4.2 mm to 2.5 mm. That is, the closer a position of the decreasing width portion  33  is to the left endface  24 , the smaller the vertical dimension of the decreasing width portion  33  becomes. 
     The minimum width portion  34  is the portion between its border with the decreasing width portion  33  and the left edge of the rough surface part  29  and has a length D 5  of 110.5 mm in the left-to-right direction. The length D 5  is a minimum vertical length among the vertical lengths of the rough surface part  29 . The vertical dimension of the minimum width portion  34  is a uniform 2.5 mm. 
     A small gap is formed between the left end of the rough surface part  29  and the left end of the light guide  12 . A length D 6  from the border between the decreasing width portion  33  and the minimum width portion  34  to the left endface of the light guide  12  is 112.9 mm. Thus, the gap between the left end of the rough surface part  29  and the left endface of the light guide  12  is 2.4 mm. 
     (2) Light Source 
     The light source  21  is an LED light provided with an LED, for example. The light source  21  is fixed inside the main casing  3  at a position opposing the right endface  23  of the light guide  12  over a prescribed distance (22.9 mm, for example) in the left-to-right direction. 
     (3) Light Guide Accommodating Unit 
     The light guide accommodating part  22  is formed in the frame  9  of the drum subunit  7 . The light guide accommodating part  22  has a square C-shaped cross section and is open on the front and right sides. The light guide accommodating part  22  has interior space for accommodating the light guide  12 . More specifically, the light guide accommodating part  22  has a left wall  35 , a top wall  36 , a bottom wall  37 , and a rear wall  38  respectively corresponding to (and covering) the left endface  24 , the top surface  26 , the bottom surface  27 , and the rear surface  28  of the light guide  12 . The right and front sides of the light guide accommodating part  22  are open and expose the right endface  23  and the confronting surface  25  of the light guide  12 . The left wall  35  is formed with a step corresponding to the shape of the left endface  24  formed on the light guide  12 . The rear wall  38  is shaped to form contact with the entire rear surface  28  formed on the light guide  12 . The light guide accommodating part  22  is formed of a non-transparent resin in a color other than black, such as a white resin. 
     (4) Measurement of Roughness of the Rough Surface Part 
     The roughness of the rough surface part  29  was measured at three points spaced in the left-to-right direction using a contact-type instrument for measuring surface roughness. Using a position on the rough surface part  29  corresponding to the right edge of the photosensitive drum  6  as a reference point, the three measurement points were set at left positions separated 33.5, 122, and 213 mm from the reference position. 
     The maximum width portion  32  is formed at a biased position in the left-to-right direction of the rough surface part  29  to the right of the center region. 
     Parameters, including the arithmetic average roughness Ra, the root mean square roughness Rq, the maximum height of the roughness profile Rz, and the mean height of roughness profile elements Re were found for each point from the above measurements. The calculated values are shown in  FIG. 4 . 
     Based on the results shown in  FIG. 4 , it is readily apparent that the rough surface part  29  has a substantially uniform surface roughness. 
     3. Operations and Effects 
     As described above, the light guide  12  is disposed in opposition to the surface of the photosensitive drum  6 , extending along the photosensitive drum  6  in the width direction. Light emitted from the light source  21  is incident on the right endface  23  of the light guide  12 . The rough surface part  29  is formed in the rear surface  28  constituting the side of the light guide  12  opposite the confronting surface  25 , which opposes the surface of the photosensitive drum  6 . The rough surface part  29  functions to reflect light incident on the right endface  23  toward the confronting surface  25 . 
     The vertical dimension of the rough surface part  29  does not increase monotonically from the right endface  23  side to the left endface  24  side; rather, the rough surface part  29  has the increasing width portion  31  whose vertical dimension increases in the direction from the right endface  23  side toward the left endface  24  side, and the decreasing width portion  33  whose vertical dimension decreases in the direction from the right endface  23  side toward the left endface  24  side. With this configuration, the rough surface part  29  does not have its maximum vertical dimension at the end portion on the left endface  24  side but rather in a midway portion between the right endface  23  and the left endface  24 . 
     Light incident on the right endface  23  of the light guide  12  travels through the light guide  12  toward the left endface  24  and decays along the path to the left endface  24 . Some of the light that reaches the left endface  24  is emitted from the light guide  12  through the left endface  24 , while some is reflected back into the light guide  12  by the left endface  24 . Therefore, both light traveling from the right endface  23  toward the left endface  24  and light reflected back into the light guide  12  by the left endface  24  are reflected by the rough surface part  29  in the end portion of the rough surface part  29  on the left endface  24  side, and this reflected light is emitted from the confronting surface  25 . For this reason, in a conceivable case where a rough surface part  29  has maximum vertical dimension on the end portion near the left endface  24  side, a quantity of reflected light (a larger quantity of light emitted from the confronting surface  25 ) in the end portion on the left endface  24  side is larger than in portions of the rough surface part  29  closer to the light-incident surface side. 
     After conducting thorough research, the inventors of the invention discovered that these irregularities in reflected light quantities are one factor preventing the uniform irradiation of light over the surface of the photosensitive drum  6  in the width direction. In order to eliminate these irregularities in reflected light quantities, the inventors formed the rough surface part  29  with its maximum width at a midway point between the right endface  23  and the left endface  24  rather than at the end region near the left endface  24 . Compared to the conceivable example having the maximum width at the end region of the rough surface part  29  near the left endface  24 , the construction of the rough surface part  29  according to the embodiment can suppress the quantity of light reflected by the light guide  12  in the end portion of the rough surface part  29  near the left endface  24  while increasing the quantity of reflected light in the portions of the rough surface part  29  closer to the light-incident surface side than the end region near the left endface  24 . 
     More specifically, the inventors formed the increasing width portion  31  as the right end portion of the rough surface part  29  and formed the decreasing width portion  33  as a region of the rough surface part  29  near the left side of the increasing width portion  31  and separated from the increasing width portion  31  in the width direction. Further, the maximum width portion  32 , having a uniform vertical dimension set at the maximum width, was formed to connect the increasing width portion  31  and decreasing width portion  33 . Further, the minimum width portion  34 , having a uniform vertical dimension set at the minimum width, was formed to connect to the left side of the decreasing width portion  33 . The maximum width portion  32  is formed at a biased position in the left-to-right direction of the rough surface part  29  to the right of the center position of the rough surface part  29 . 
     With this construction, the rough surface part  29  ensures that light emitted from the confronting surface  25  of the light guide  12  is uniform across the longitudinal dimension thereof (the width direction of the photosensitive drum  6 ). Thus, light emitted from the light source  21  can be irradiated uniformly over the surface of the photosensitive drum  6  across the width direction thereof. 
     Since the rough surface part  29  has a uniform roughness, the amount of light reflected by the rough surface part  29  is dependent on the width thereof. Accordingly, by varying the width of the rough surface part  29 , it is possible to adjust the quantity of light reflected thereby. Thus, use of the rough surface part  29  is a simple method for ensuring that light irradiated over the surface of the photosensitive drum  6  (light emitted from the confronting surface  25 ) is uniform in the width direction. 
     The confronting surface  25  is a curved surface having a fixed curvature with its convex side facing the photosensitive drum  6 . Thus, light exiting the light guide  12  through the confronting surface  25  is bent uniformly by the confronting surface  25 . As a result, the light can be irradiated uniformly in the width direction onto the surface of the photosensitive drum  6 . 
     The rear surface  28 , on the other hand, is flat and, hence, can contact the rear wall  38  of the light guide accommodating part  22  across its entire surface. As a result, it is possible to prevent light from leaking out of the rear surface  28 , thereby improving the efficiency in which light is emitted from the confronting surface  25 . 
     The rear surface  28  has stepped parts formed between the rough surface part  29  and the region outside the rough surface part  29 . With this construction, the rough surface part  29  is formed a step closer toward the confronting surface  25  than the region outside the rough surface part  29  and, therefore, is also positioned closer to the surface of the photosensitive drum  6  than the region outside the rough surface part  29 . Accordingly, light reflected off the rough surface part  29  can be irradiated onto the surface of the photosensitive drum  6  more efficiently. 
     The gap D 1  is formed between the end of the rough surface part  29  on the right endface  23  side and the right endface  23 . That is, the light guide  12  has the portion  30  on which the rough surface part  29  is not formed positioned between the right endface  23  and the right endface  23  side end of the rough surface part  29 . It would be possible to eliminate the portion  30  and align the edge of the rough surface part  29  on the right endface  23  side with the light-incident surface on which light from the light source  21  is incident. However, providing the light guide  12  with the portion  30  makes it possible to shorten the distance from the light source  21  to the light-incident surface so that light emitted from the light source  21  strikes the light-incident surface more efficiently. Consequently, light emitted from the light source  21  can be irradiated more efficiently onto the surface of the photosensitive drum  6 . 
     The left endface  24  has been processed for reflecting light back into the light guide  12  after the light has passed through the light guide  12  and reached the left endface  24 . Specifically, the lower part  24 L is formed by cutting a notch out of the left endface  24 . With this configuration, light traveling from the right endface  23  toward the left endface  24  can be actively reflected back into the light guide  12  by the left endface  24 , thereby improving light emission efficiency from the confronting surface  25 . 
     The left endface  24  and the rear surface  28  of the light guide  12  are covered with the light guide accommodating part  22 , which is formed of a non-transparent resin in a color other than black, and specifically a white resin. Accordingly, light emitted from the left endface  24  and the rear surface  28  of the light guide  12  can be reflected back into the light guide  12  by the light guide accommodating part  22 , thereby further improving light emission efficiency from the confronting surface  25 . 
     4. Variations of the Embodiment 
     While the invention has been described in detail with reference to the embodiment thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention. 
     (1) First Variation 
     In the example shown in  FIG. 2 , the left endface  24  of the light guide  12  has been processed so that the lower part  24 L is recessed a step rightward from the upper part  24 U, as an example of processing the left endface  24  to reflect light back into the light guide  12 . However, the left endface  24  may be formed as a hemispherical surface with its convex side on the left, as shown in  FIG. 5 , for achieving the same objective, 
     (2) Second Variation 
     Alternatively, as shown in  FIG. 6 , the left endface  24  of the light guide  12  may be formed flat, and a reflective film  61 , such as a silver thin film may be formed on the left endface  24  through a plating process. 
     Alternatively, the left endface  24  may be formed as a flat surface, and micro-protrusions and micro-depressions may be formed in the left endface  24  through surface texturing. 
     Next, the charge-removing device according to the embodiment of the invention will be compared to a comparative device (hereinafter referred to as the “comparative example”). However, the invention is not limited to the charge-removing device of the embodiment. 
     As shown in  FIG. 7 , the charge-removing device according to the embodiment is manufactured with the same structure as the charge-removing device  1  shown in  FIG. 2  (i.e., the structure according to the embodiment of the invention). 
     The charge-removing device in the comparative example, on the other hand, has the structure shown in  FIG. 8 . The charge-removing device shown in  FIG. 8  has the maximum width at the left end region of the rough surface part. Excluding the shape of the rough surface part formed on the rear surface of the light guide, the structure of the charge-removing device shown in  FIG. 8  has the same structure as the charge-removing device shown in  FIG. 7 . The rough surface part shown in  FIG. 8  has a length of 220.5 mm along the longitudinal direction of the light guide and is spaced a gap of 30 mm from the longitudinal endface of the light guide (the light-incident surface on which light emitted from the light source is incident). The width of the rough surface part (that is, the dimension in a direction orthogonal to the longitudinal direction of the light guide) on the endface nearest the light source is 1.5 mm, while the width on the opposite end is 4.2 mm. 
     Using the charge-removing devices according to the embodiment and the comparative example, the quantity of light emitted from their corresponding light guides was measured. In these measurements, an LED light emitting infrared light at a wavelength of 780 nm was provided as the light source and positioned a distance of 22.9 mm from the endface of each light guide. Optical power meters for measuring light intensity were positioned at distances of 55, 100, 145, 190, and 235 mm from the light source. 
     The light source used in the experiment is the SLI-580UT manufactured by ROHM (a high-brightness circular-type, with a diameter of 5.0 mm). The measuring instrument used in the experiment is the Optical Power Meter 3664 manufactured by Hioki E.E. Corporation. A gap of 2 mm was set between each light guide and the optical power meter. The table in  FIG. 9  and the graph in  FIG. 10  show the measurement results. In the graph of  FIG. 10 , the horizontal axis represents the distance from the light source (mm), while the vertical axis represents the optical power (μW). 
     As shown in the table of  FIG. 9 , optical power produced by the charge-removing device according to the comparative example was 2.8, 3.8, 4.5, 5.3, and 7.2 μW at the respective distances from the light source of 55, 100, 145, 190, and 235 mm. Thus, the optical power reaches a maximum of 7.2 μW at a position 235 mm from the light source and a minimum of 2.8 μW at a position 55 mm from the light source. The difference between the maximum and minimum values is 4.4 
     On the other hand, optical power produced by the charge-removing device according to the embodiment was 5.2, 5.8, 6.1, 5.2, and 5.4 μW at the respective distances from the light source of 55, 100, 145, 190, and 235 mm. Thus, the optical power reaches a maximum of 6.1 μW at a position 145 mm from the light source and a minimum of 5.2 μW at positions 55 mm and 190 mm from the light source. The difference between the maximum and minimum values is 0.9 μW. 
     These results confirm that the charge-removing device according to the embodiment has superior balance in the quantity of light emitted from the light guide (greater uniformity of optical power in the longitudinal direction of the light guide) to the charge-removing device of the comparative example.