Abstract:
A light guide comprising: a main light-guide housing that is elongated shaped, having a first end face, a second end face, and a first lateral face connecting the first end face and the second end face, and the main light-guide housing being elongated in a predetermined direction from the first end face to the second end face; and a protrusion that is connected to the main light-guide housing, having a third end face in an opposite direction from the predetermined direction and a second lateral face connecting the third end face to the first end face, and the protrusion projecting from the first end face in the opposite direction from the predetermined direction, wherein, light emitted toward the third end face by a light source in part exits the protrusion from the second lateral face, then enters the main light-guide housing from the first end face, and thereafter exits the light guide from the first lateral face.

Description:
This application is based on Japanese Patent Application No. 2012-149498 filed on Jul. 3, 2012, the content of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to light guides, more particularly to a light guide for use in an image reading apparatus. 
     2. Description of Related Art 
     For example, a light source device  500  shown in  FIG. 8  is a known light source device including a conventional light guide.  FIG. 8  is a cross-sectional structure diagram of the light source device  500  including a conventional light guide  502 . 
     The light source device  500  includes the light guide  502 , a light source  504 , and a reflective member  506 , as shown in  FIG. 8 . The light guide  502  is made of transparent resin, and extends in a left-right direction in  FIG. 8 . The light guide  502  has a prism that reflects light upwardly inside the light guide  502 , formed on its bottom surface. The light source  504  is directed to the left end of the light guide  502 . Moreover, the reflective member  506  surrounds the gap between the light guide  502  and the light source  504 . 
     In the light source device  500  thus configured, light emitted by the light source  504  enters the light guide  502  from the left end. At this time, light that propagates from the gap between the light guide  502  and the light source  504  toward the outside of the light source device  500  is reflected by the reflective member  506 , and enters the light guide  502  from the left end. The light having entered the light guide  502  travels rightward while repeatedly experiencing total reflection within the light guide  502 , and then the light is reflected upwardly by the prism formed in the light guide  502 . Since the light source device  500  is provided with the reflective member  506 , light is inhibited from leaking out of the light source device  500  through the gap between the light guide  502  and the light source  504 . 
     Incidentally, the light source device  500  uses the reflective member  506  to inhibit leakage of light. When reflecting light, the reflective member  506  absorbs part of the light. Therefore, the light source device  500  cannot efficiently utilize light emitted by the light source  504 . 
     Note that, for example, illumination optics described in Japanese Patent Laid-Open Publication No. 2005-123675 are a known invention related to a conventional light guide. The illumination optics confine light inside a light guiding means through total reflection without using a reflective member. However, in the illumination optics described in Japanese Patent Laid-Open Publication No. 2005-123675, light does not enter the light guiding means from an end in a longitudinal direction but from the bottom surface of the light guiding means. Therefore, Japanese Patent Laid-Open Publication No. 2005-123675 does not describe efficiently utilizing light emitted by a light source in a light guide in which light enters from an end in a longitudinal direction. 
     SUMMARY OF THE INVENTION 
     In an embodiment of the present invention, a light guide comprising: a main light-guide housing that is elongated shaped, having a first end face, a second end face, and a first lateral face connecting the first end face and the second end face, and the main light-guide housing being elongated in a predetermined direction from the first end face to the second end face; and a protrusion that is connected to the main light-guide housing, having a third end face in an opposite direction from the predetermined direction and a second lateral face connecting the third end face to the first end face, and the protrusion projecting from the first end face in the opposite direction from the predetermined direction, wherein, light emitted toward the third end face by a light source in part exits the protrusion from the second lateral face, then enters the main light-guide housing from the first end face, and thereafter exits the light guide from the first lateral face. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a configuration diagram of an image reading apparatus including a light guide according to an embodiment of the present invention; 
         FIG. 2  is an oblique external view of a light guide of a light source device; 
         FIG. 3  is a cross-sectional structure diagram of the light source device taken along line A-A in  FIG. 2 ; 
         FIG. 4  is a cross-sectional structure diagram of a light source device including a light guide according to a first modification; 
         FIG. 5  is a cross-sectional structure diagram of a light source device including a light guide according to a second modification; 
         FIG. 6  is a cross-sectional structure diagram of a light source device including a light guide according to a third modification; 
         FIG. 7  is a cross-sectional structure diagram of a light guide according to a fourth modification; and 
         FIG. 8  is a cross-sectional structure diagram of a light source device including a conventional light guide. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, an image reading apparatus including a light guide according to an embodiment of the present invention will be described. 
     Configuration of Image Reading Apparatus 
       FIG. 1  is a configuration diagram of an image reading apparatus  10  including a light guide according to an embodiment of the present invention. In the following, the vertical direction will be defined as z-axis, and the moving direction of slider units  18  and  20  (i.e., a sub-scanning direction) will be defined as x-axis. Moreover, the direction that is perpendicular to the x- and z-axes (i.e., a main scanning direction) will be defined as y-axis. 
     The image reading apparatus  10  includes a main housing  12 , a document cover  14 , a platen glass  16 , the slider units  18  and  20 , an imaging lens  22 , and an image pickup element  24 , as shown in  FIG. 1 . 
     The main housing  12  is a rectangular casing in which the document cover  14 , the platen glass  16 , the slider units  18  and  20 , the imaging lens  22 , and the image pickup element  24  are provided. The platen glass  16  is a rectangular, transparent plate attached to an opening provided in the main housing  12  in the positive z-axis direction. A document P is placed on the top surface of the platen glass  16 , with the face to be read being directed to the negative z-axis direction. 
     The document cover  14  is placed over the document P, thereby causing the document P to closely contact the platen glass  16 , as shown in  FIG. 1 . 
     When the document P is being read, the slider unit  18  is moved at a velocity V along the document P in the positive x-axis direction by unillustrated transportation means, including a motor, a belt, a pulley, etc., as shown in  FIG. 1 . The slider unit  18  includes a light source device  27 , and mirrors  28  and  29 , as shown in  FIG. 1 . 
     The light source device  27  is configured by, for example, a combination of a light-emitting diode (LED) and a light guide, so as to emit light toward the document P and the mirror  28 . The light source device  27  will be described in more detail later. The mirror  28  reflects the light emitted by the light source device  27 , toward the document P. The mirror  29  redirects light B reflected by the document P, in the negative x-axis direction (toward one side of the sliding direction of the slider unit  18 ), as shown in  FIG. 1 . 
     When the document P is being read, the slider unit  20  is moved at a velocity V/2 along the document P in the positive x-axis direction by unillustrated transportation means, including a motor, a belt, a pulley, etc., as shown in  FIG. 1 . The slider unit  20  includes mirrors  30  and  32 . 
     The mirror  30  reflects the light B redirected by the mirror  29 , in the negative z-axis direction. The mirror  32  redirects the light B reflected by the mirror  30 , in the positive x-axis direction. 
     The imaging lens  22  forms an optical image on the image pickup element  24  on the basis of the light B. The image pickup element  24  is a light receiving element that receives light B reflected by the mirror  32 . Specifically, the image pickup element  24  is a line sensor, such as a CCD camera, which has a linear imaging zone extending in the y-axis direction and reads an image of the document P by scanning the optical image formed by the imaging lens  22 . 
     Configuration of Light Source Device 
     The configuration of the light source device  27  will be described below with reference to the drawings.  FIG. 2  is an oblique external view of the light guide  40  of the light source device  27 .  FIG. 3  is a cross-sectional structure diagram of the light source device  27  taken along line A-A in  FIG. 2 . The x- and z-axes rotated approximately 45 degrees counterclockwise about the y-axis in  FIG. 3  will be referred to below as the X- and Z-axes, respectively. The Z-axis coincides with the direction in which the light source device  27  emits light toward the document P. 
     The light source device  27  includes the light guide  40 , a light source  50 , and a circuit board  52 , as shown in  FIGS. 2 and 3 . 
     The light guide  40  includes a main light-guide housing  42  and a protrusion  44 , and has an elongated shape in the y-axis direction, as shown in  FIGS. 2 and 3 . The main light-guide housing  42  is a transparent columnar member having end faces S 1  and S 2  and a lateral face S 3 . The end face S 1  is a circular surface located at an end of the main light-guide housing  42  in the negative y-axis direction. The end face S 2  is a circular surface located at an end of the main light-guide housing  42  in the positive y-axis direction. The lateral face S 3  is a round surface connecting the end faces S 1  and S 2 . In this manner, the main light-guide housing  42  extends in the direction from the end face S 1  to the end face S 2  (i.e., in the positive y-axis direction). 
     Furthermore, the light guide  40  has a reflective portion  46 , as shown in  FIG. 3 . The reflective portion  46  is provided at a portion of the lateral face S 3  in the negative Z-axis direction, and consists of a plurality of prisms arranged in the y-axis direction. Each prism has a triangular shape in a plan view in the X-axis direction. As a result, the reflective portion  46  reflects light from the inside of the main light-guide housing  42  toward the outside. In the present embodiment, the reflective portion  46  reflects light in the positive Z-axis direction toward the document P, and also in the negative x-axis direction toward the mirror  28 . Note that the reflective portion  46  is not limited to the prisms, and may be a white resin surface for diffuse reflection. 
     The protrusion  44  is connected to the main light-guide housing  42 , and projects in the negative y-axis direction from the center of the end face S 1 . The cross section of the protrusion  44  in the direction perpendicular to the y-axis increases in the positive y-axis direction. In the present embodiment, the protrusion  44  has a truncated cone shape, the diameter of which increases in the positive y-axis direction. Moreover, the central axis of the protrusion  44  coincides with the central axis of the main light-guide housing  42 . 
     Furthermore, the protrusion  44  has an end face S 4  and a lateral face S 5 , as shown in  FIGS. 2 and 3 . The end face S 4  is a surface located at an end of the protrusion  44  in the negative y-axis direction. The lateral face S 5  is a lateral surface connecting the end face S 4  of the protrusion  44  and the end face S 1  of the main light-guide housing  42 . The end face S 4  is a curved surface, which is recessed in the positive y-axis direction, as shown in  FIG. 3 . In the present embodiment, the end face S 4  is a spherical concave. 
     The circuit board  52  is a flat rectangular board provided with a driver circuit for driving the light source  50 . The circuit board  52  is disposed facing the end face S 4  of the protrusion  44 , so as to be perpendicular to the y-axis. Note that, in addition to a board with the light source  50 , another board with the driver circuit may be provided outside the light guide  40  and connected to the board with the light source  50  via a cable. 
     The light source  50  is, for example, an LED, and is mounted on a principal surface of the circuit board  52  in the positive y-axis direction. The light source  50  is accommodated in the recess formed by the end face S 4 , as shown in  FIG. 3 . The light source  50  has a light-emitting surface positioned at the center of the spherical concave of the end face S 4 . Furthermore, the recess formed by the end face S 4  is filled with transparent resin. Note that there may be an air space in the recess in place of the transparent resin. The light source  50  emits light in the positive y-axis direction, the positive and negative Z-axis directions, and the positive and negative X-axis directions. That is, the light source  50  emits light over a 180-degree angular range about the y-axis. The light emitted by the light source  50  enters the protrusion  44  from the end face S 4 , as shown in  FIG. 3 . 
     An optical path of the light source device  27  will be described in detail below with reference to  FIG. 3 . 
     The light source  50  emits light over a 180-degree angular range about the y-axis. Light B 1  enters the lateral face S 5  at an angle equal to or greater than a critical angle between the air space and the light guide  40 , and then enters the main light-guide housing  42  after it is reflected by the lateral face S 5 , as shown in  FIG. 3 . Subsequently, the light B 1  reaches the reflective portion  46  after repeatedly experiencing total reflection by the lateral face S 3  within the main light-guide housing  42 . Then, the light B 1  is reflected by the reflective portion  46  in the positive Z-axis direction or in the negative x-axis direction, and exits the light guide  40 . Note that part of the light B 1  exits the light guide  40  after it is directly reflected by the reflective portion  46  without experiencing total reflection by the lateral face S 3 . 
     Furthermore, light B 2  enters the lateral face S 5  at an angle less than the critical angle between the air space and the light guide  40 , and exits the protrusion  44  from the lateral face S 5 . The protrusion  44  has a truncated cone shape that increases in thickness in the positive y-axis direction. Therefore, the light B 2  is refracted by the lateral face S 5 , so that the propagation direction of the light B 2  leans toward the positive y-axis direction. Then, the light B 2  enters the main light-guide housing  42  from the end face S 1 . Subsequently, the light B 2  reaches the reflective portion  46  after repeatedly experiencing total reflection by the lateral face S 3  within the main light-guide housing  42 . Then, the light B 2  is reflected by the reflective portion  46  in the positive Z-axis direction or in the negative x-axis direction, and exits the light guide  40 . Note that part of the light B 2  exits the light guide  40  after it is directly reflected by the reflective portion  46  without experiencing total reflection by the lateral face S 3 . 
     Furthermore, light B 0 , which propagates in a direction at the largest angle with respect to the positive y-axis direction among the light emitted by the light source  50 , exits the protrusion  44  from the lateral face S 5 . In the present embodiment, the light B 0  is light emitted by the light source  50  in a direction perpendicular to the y-axis (i.e., in the Z-axis direction). Since the protrusion  44  has a truncated cone shape that increases in thickness in the positive y-axis direction, the light B 0  is refracted by the lateral face S 5 , so that the propagation direction of the light B 0  leans toward the positive y-axis direction. As a result, the light B 0  propagates to the end face S 1 , and enters the main light-guide housing  42  from the end face S 1 . Subsequently, the light B 0  reaches the reflective portion  46  after repeatedly experiencing total reflection by the lateral face S 3  within the main light-guide housing  42 . Then, the light B 0  is reflected by the reflective portion  46  in the positive Z-axis direction or in the negative x-axis direction, and exits the light guide  40 . Note that part of the light B 0  exits the light guide  40  after it is directly reflected by the reflective portion  46  without experiencing total reflection by the lateral face S 3 . 
     Here, the light B 0  is incident on the lateral face S 3  in the positive Z-axis direction, preferably at an angle equal to or greater than the critical angle between the air space and the light guide  40 . As a result, the light B 0  experiences total reflection by the lateral face S 3 , so that all of the light emitted by the light source  50  so as to be incident on the lateral face S 3  in the positive Z-axis direction experiences total reflection by the lateral face S 3 . 
     Effects 
     The light guide  40  according to the present embodiment can utilize light emitted by the light source  50  more efficiently than the light source device  500  using a conventional reflective member. More specifically, the light source  50  is attached to the end face S 4  of the protrusion  44 . Therefore, light emitted by the light source  50  enters the protrusion  44 . Then, the light having entered the protrusion  44  is guided into the main light-guide housing  42  through total reflection by the lateral face S 5  of the protrusion  44 , as with the light B 1 , or through refraction by the lateral face S 5  and the end face S 1 , as with the light B 0  and B 2 . In this manner, the light emitted by the light source  50  is guided into the main light-guide housing  42  through total reflection, or through refraction, without being reflected by the reflective member. Optical loss caused by total reflection or refraction is lower than optical loss caused from reflection by the reflective member. Thus, the light guide  40  makes it possible to efficiently utilize light emitted by the light source  50 . 
     Furthermore, also for the following reason, the light guide  40  can efficiently utilize light emitted by the light source  50 . More specifically, the light B 0 , which propagates in a direction at the largest angle with respect to the positive y-axis direction among the light emitted by the light source  50 , exits the protrusion  44  from the lateral face S 5 , and thereafter enters the main light-guide housing  42  from the end face S 1 . As a result, all of the light emitted by the light source  50  enters the main light-guide housing  42 . That is, leakage of light is inhibited between the light source  50  and the main light-guide housing  42 . Thus, the light guide  40  makes it possible to efficiently utilize light emitted by the light source  50 . 
     Furthermore, also for the following reason, the light guide  40  can efficiently utilize light emitted by the light source  50 . More specifically, the light B 0  is incident on the lateral face S 3  in the positive Z-axis direction, preferably at an angle equal to or greater than the critical angle between the air space and the light guide  40 . Accordingly, the light B 0  experiences total reflection by the lateral face S 3 . As a result, all of the light emitted by the light source  50  so as to be incident on the lateral face S 3  in the positive Z-axis direction experiences total reflection by the lateral face S 3 , so that the light is inhibited from exiting the main light-guide housing  42  without experiencing total reflection by the lateral face S 3 . Thus, the light guide  40  makes it possible to efficiently utilize light emitted by the light source  50 . 
     Furthermore, also for the following reason, the light guide  40  can efficiently utilize light emitted by the light source  50 . More specifically, light emitted by the light source  50  enters the protrusion  44 , and is guided into the main light-guide housing  42  through refraction or total reflection. Accordingly, more light can reach the reflective portion  46 . As a result, there is an increase in the amount of light that is reflected toward the document P and the mirror  28  by the reflective portion  46 . On the other hand, there is a decrease in the amount of light that leaves the light guide  40  from the end face S 2  without being reflected by the reflective portion  46 . Thus, the light guide  40  makes it possible to efficiently utilize light emitted by the light source  50 . 
     Furthermore, the main light-guide housing  42  of the light guide  40  can be reduced in dimension in the Z-axis direction. More specifically, the protrusion  44  has a truncated cone shape, the diameter of which increases in the positive y-axis direction. Accordingly, the lateral face S 5  is inclined with respect to the y-axis. As a result, the light B 0  and the light B 2 , which are to exit the protrusion  44 , are refracted by the lateral face S 5  so as to lean in the positive y-axis direction. Therefore, light that exits the protrusion  44  is inhibited from diffusing in the Z-axis direction. Thus, it is rendered possible to use the main light-guide housing  42  with a reduced dimension in the Z-axis direction. 
     First Modification 
     A light guide  40   a  according to a first modification will be described below with reference to the drawings.  FIG. 4  is a cross-sectional structure diagram of a light source device  27  including the light guide  40   a  according to the first modification. 
     The light guide  40   a  differs from the light guide  40  in shape of the end face S 1 . More specifically, the end face S 1  of the light guide  40  is a flat surface perpendicular to the y-axis, as shown in  FIG. 3 . On the other hand, the end face S 1  of the light guide  40   a  is inclined toward the protrusion  44 , as shown in  FIG. 4 . More specifically, the Z-axis component VZ of the normal vector V to the end face S 1  is entirely directed toward the protrusion  44  (i.e., the center of the end face S 1 ). Moreover, the end face S 1  is linear in y-Z cross section. As a result, the light B 0  and the light B 2  enter the end face S 1  more readily in the light guide  40   a  than in the light guide  40 . Thus, the light guide  40   a  can utilize light emitted by the light source  50  more efficiently than the light guide  40 . Moreover, the light B 0  and the light B 2  enter the end face S 1  at closer positions from the center of the end face S 1  in the light guide  40   a  than in the light guide  40 . Thus, the dimension of the main light-guide housing  42  in the Z-axis direction can be reduced in the light guide  40   a  more than in the light guide  40 . 
     Furthermore, the end face S 1  of the light guide  40   a  is inclined toward the protrusion  44 , as shown in  FIG. 4 . As a result, the light B 0  and the light B 2  are incident on the lateral face S 3  in the positive Z-axis direction at smaller angles of incident in the light guide  40   a  than in the light guide  40 . Therefore, the light B 0  and the light B 2  can be reflected toward the reflective portion  46  more readily in the light guide  40   a  than in the light guide  40 . Thus, there is an increase in the amount of light that is reflected toward the document P and the mirror  28  by the reflective portion  46 . 
     Second Modification 
     A light guide  40   b  according to a second modification will be described below with reference to the drawings.  FIG. 5  is a cross-sectional structure diagram of a light source device  27  including the light guide  40   b  according to the second modification. 
     The light guide  40   b  differs from the light guide  40  in shape of the recess formed by the end face S 4 . More specifically, the end face S 4  of the light guide  40  forms a spherically concave recess. On the other hand, the end face S 4  of the light guide  40   b  forms a rectangular parallele piped recess. Accordingly, the end face S 4  has a flat surface. As a result, the light B 2  is refracted by passing through the end face S 4 , as shown in  FIG. 5 . Thus, by adjusting the refractive index of the resin to fill the recess formed by the end face S 4  or the shape of the recess, the propagation direction of the light B 2  can be changed. For example, the refractive index of the resin to fill the recess is set higher than the refractive index of the light guide  40 , so that the light B 2  is refracted so as to lean in the positive y-axis direction. As a result, the light B 2  enters the end face S 1  more readily in the light guide  40   b  than in the light guide  40 . Thus, the light guide  40   b  makes it possible to utilize light emitted by the light source  50  more efficiently than the light guide  40 . Moreover, the light B 2  enters the end face S 1  at a closer position from the center of the end face S 1  in the light guide  40   b  than in the light guide  40 . Thus, the dimension of the main light-guide housing  42  in the Z-axis direction can be reduced in the light guide  40   b  more than in the light guide  40 . 
     Third Modification 
     A light guide  40   c  according to a third modification will be described below with reference to the drawings.  FIG. 6  is a cross-sectional structure diagram of a light source device  27  including the light guide  40   c  according to the third modification. 
     The light guide  40   c  differs from the light guide  40   a  in shape of the end face S 1 . More specifically, the end face S 1  of the light guide  40   a  is linear in y-Z cross section. On the other hand, the end face S 1  of the light guide  40   c  is curvilinear in y-Z cross section. Such a light guide  40   c  also makes it possible to efficiently utilize light emitted by the light source  50 . 
     Fourth Modification 
     A light guide  40   d  according to a fourth modification will be described below with reference to the drawings.  FIG. 7  is a cross-sectional structure diagram of the light guide  40   d  according to the fourth modification. 
     The light guide  40   d  differs from the light guide  40  in that it further includes a protrusion  44 ′. The protrusion  44 ′ is provided to the end face S 2 , as shown in  FIG. 7 . Thus, light can enter the light guide  40   d  from either side in the y-axis direction. Such a light guide  40   d  also makes it possible to efficiently utilize light emitted by the light source  50 . 
     Other Embodiments 
     The present invention is not limited to the light guides  40  and  40   a  to  40   d , and variations can be made within the spirit of the invention. 
     Note that, for the light guides  40  and  40   a  to  40   d , the light B 0 , which propagates in a direction at the largest angle with respect to the positive y-axis direction among the light emitted by the light source  50 , exits the protrusion  44  from the lateral face S 5 , and enters the main light-guide housing  42  from the end face S 1 . However, this is not restrictive. The light emitted by the light source  50  at least in part exits the protrusion  44  from the lateral face S 5 , and enters the main light-guide housing  42  from the end face S 1 . That is, for the light guides  40  and  40   a  to  40   d , the light that has exited the protrusion  44  from the lateral face S 5  enters the main light-guide housing  42  from the end face S 1 . This makes it possible to efficiently utilize light having exited the lateral face S 5 . 
     Note that the light source  50  emits light over a 180-degree angular range about the y-axis, but the light source  50  may emit light over an angular range of less than 180 degrees. 
     Although the present invention has been described in connection with the preferred embodiment above, it is to be noted that various changes and modifications are possible to those who are skilled in the art. Such changes and modifications are to be understood as being within the scope of the invention.