Abstract:
A light guide emits lights incident on an end face from an emitting face extending longitudinally of the guide, while having the lights reflected by its internal face. A sectional shape of the guide in a direction orthogonal to the longitudinal direction of this light guide has two opposite parabolas, a line segment connecting the focal points of the two opposite parabolas, and a line segment (bottom face) corresponding to the emitting face. A scattering pattern consisting of white ink is formed on the line segment (bottom face) connecting the focal points. With this guide, the face of a document is to be efficiently illuminated by conversely utilizing the characteristic of the compound parabolic concentrator (CPC) to convert scattered lights extending over a full angle from a limited area into radiant lights confined to a prescribed emission angle and thereby minimizing the expansion of lights.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present invention is the US National Phase of International Application PCT/JP2004/008621, filed 18 Jun. 2004, which claims priority under 35 USC 119 based on Japanese patent application No. 2003-181626, filed 25 Jun. 2003. The entire contents of the International and priority Japanese applications are incorporated herein by reference. 
   TECHNICAL FIELD 
   The present invention relates to a light guide for illuminating documents in a line shape in facsimile machines, copying machines, scanners and the like, and an image reader having this light guide incorporated therein. 
   BACKGROUND ART 
   There is proposed, for the purpose of effectively irradiating the object of irradiation with lights from light sources, an illuminator that uses a light guide whose section is a paraboloid (see Patent Document 1 for instance). 
   There is also known a compound parabolic concentrator (CPC) which is intended to enhance the efficiency of concentration with no regard for image formation performance (Non-Patent Document 1).
     Patent Document 1: Japanese Patent Laid-Open No. 2001-330734   Non-Patent Document 1: Title of book—No. 6, Pencil of Light 5: Non-Image Forming Light Concentrating Optical System; Author—Tadao Tsuruta   

   Lights emitted from a light guide expand. As a result, where the distance between the document and the emitting face is great, the illumination of the face of the document may decline, which is not desirable. 
   Incidentally, as stated in Non-Patent Document 1, a compound parabolic concentrator (CPC) is an optical system devised for efficient concentration of lights, and has a characteristic that all the lights incident on the light receiving face at an angle of incidence not greater than θ concentrate on the light concentration face. 
   Then, an object of the present invention is to provide a light guide capable of minimizing the expansion of light by conversely utilizing the characteristic of the compound parabolic concentrator (CPC) to convert scattered lights extending over a full angle from a limited area into radiant lights confined to a prescribed emission angle and using them as an optical system efficiently illuminating the face of the document, and a line illuminator using that light guide. 
   SUMMARY OF THE INVENTION 
   A light guide according to an embodiment of the invention for solving the problem noted above emits lights incident on an end face from an emitting face disposed along the longitudinal direction while having the lights reflected by the internal face thereof, wherein the sectional shape of the light guide in a direction orthogonal to the longitudinal direction of this light guide has two opposite parabolas, a line segment connecting the focal points of the two opposite parabolas and a line segment corresponding to the emitting face. The face containing the line segment connecting the focal points of the two opposite parabolas constitutes the reflective face and the face opposite to this reflective face constitutes the emitting face. 
   The configuration described above makes it possible to restrain the expansion of lights emitted from the emitting face. In order to emit lights most efficiently, it is preferable to arrange the side faces of the light guide on the emitting face side substantially in parallel with the optical axis. 
   An image reader according to an embodiment of the invention has two, for instance, of illuminating units each including the light guide with a light source on one end or both ends of the light guide, and the illuminating units are so arranged as to cause lights emitted from the emitting faces of the guides to irradiate the same area of the face of the document to be illuminated. 
   Advantages of the Invention 
   The light guide according to the invention and the line illuminator using that light guide, since they conversely utilize the characteristic of the compound parabolic concentrator (CPC) to convert diffused lights extending over a full angle from a limited area into radiant lights confined to a prescribed emission angle and use them as an optical system efficiently illuminating the face of the document, can minimize the expansion of lights. This makes it possible to efficiently illuminate the face of the document. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram showing the sectional shape of the light guide according to the present invention having a compound parabolic shape; 
       FIG. 2  is a diagram showing the emitted lights of the light guide according to the invention shown in  FIG. 1 ; 
       FIG. 3  is a sectional view of a contact image sensor (CIS) provided with a line illuminator in which the light guide according to the invention is incorporated; 
       FIG. 4  is a diagram showing the fitting positions of light emitting diodes as light sources disposed on an end face of the light guide according to the invention; 
       FIG. 5  is a diagram showing the sectional shape of another light guide according to the invention; 
       FIG. 6  is a diagram showing the sectional shape of another light guide according to the invention; and 
       FIG. 7  is a diagram showing the sectional shape of a line illuminator provided with a compound parabolic reflector. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The best modes for carrying out the present invention will be described below with reference to the accompanying drawings.  FIG. 1  shows the sectional shape of a light guide according to the invention having a compound parabolic shape.  FIG. 2  is a diagram showing the emitted lights of the light guide according to the invention shown in  FIG. 1 . 
   A light guide  10  is formed of a transparent resin, such as acryl for instance, and its sectional shape is constant over the full length (e.g. 320 mm) of the light guide  10 . 
   The emitting face  4  of the light guide  10  has a larger width than that of the bottom face  1  of the light guide  10 . The bottom face  1  of the light guide  10  measures 0.52 mm in width, and a scattering pattern consisting of white ink or fine convexes and concaves is formed on this bottom face  1 . The scattering pattern is formed in dots for instance. 
   This scattering pattern may be formed all over the bottom face  1 , but the area in which the scattering pattern is formed may also become greater with an increase in distance from the end face on which lights from light sources, not shown, come incident. 
   A side face  2  is part of a curve resulting from the rotation of a quadratic curve (y=0.81927x 2 −0.30515) represented by a virtual line around the origin (x, y)=(0, 0) as the center of rotation by θ=10 degrees and displaced in parallel by −W1/2=−0.26 mm in the direction of the x axis (the range of y: 0≦y≦9.97). By inclining the quadratic curve represented by the virtual line, the side face  2  on the emitting face  4  side is made substantially parallel to the y axis (the optical axis). A side face  3  is a curve symmetric to the side face  2  with respect to the y axis. 
   In this case, the coordinate (x, y)=(−0.26, 0) is the focal point a of the side face  2 , which is a paraboloid. As shown in  FIG. 2 , out of the scattered lights from this focal point a, lights having directly reached the side face  2  satisfy every condition of reflection, and reach the emitting face  4  as parallel lights inclined at 10 degrees to the y axis within the light guide  10 . 
   Where the light guide  10  is made of acryl whose refractive index n=1.49: 1.49·sin 10°=sin θd (Snell&#39;s law) 
   Hence θd=15°. 
   Therefore, like the light ray represented by reference numeral  5  in  FIG. 2 , parallel lights inclined at 15 degrees to the y axis are emitted from the emitting face  4 . 
   On the other hand, out of the scattered lights from the focal point a, lights having directly reached the emitting face  4  are inclined by −15 degrees to the y axis when they pass the coordinate (x, y)=(1.50, 9.97) like the light ray represented by reference numeral  6  in  FIG. 2 . 
   Similarly, the coordinate (x, y)=(0.26, 0) is the focal point b of the side face  3 , which is a paraboloid. Out of the scattered lights from this focal point b, lights having directly reached the side face  3  satisfy every condition of reflection, and reach the emitting face  4  as parallel lights inclined at −10 degrees to the y axis within the light guide  10 . 
   Therefore, scattered lights from the section (−0.26≦x≦0.26, y=0) (namely the scattered lights from the bottom face  1 ), including the reflection of the scattered lights directly reaching the side face  3 , are confined to a range of ±15 degrees to the y axis. 
   This makes it possible to prevent the expansion of lights emitted from the emitting face  4  from becoming too wide, with the result that the face of the document can be efficiently illuminated. 
     FIG. 3  is a sectional view of a contact image sensor (CIS) provided with a line illuminator in which the light guide according to the invention is incorporated, and  FIG. 4 , a diagram showing the fitting positions of light emitting diodes as light sources disposed on an end face of the light guide according to the invention. 
   A contact image sensor (CIS)  30  shown in  FIG. 3  is provided with a box  31 ; two pairs of line illuminators  20 L and  20 R are built into this box  31 ; a lens array  32  of an erecting unit magnification system is arranged in the box  31 ; and further a substrate  34  provided with a line image sensor  33  is fitted into the lower part of the box  31 . Reference numeral  35  denotes a cover glass constituting a document mount. 
   Each of the line illuminators  20 L and  20 R comprises the light guide  10  shown in  FIG. 1  and  FIG. 2 , a light guide case  11 , and a light source board (not shown) provided with light emitting diodes  12 R,  12 G and  12 B shown in  FIG. 4 . The light emitting diodes  12 R,  12 G and  12 B respectively emit red, green and blue lights, and these light emitting diodes  12 R,  12 G and  12 B are chip type diodes (LED chips). 
   In this mode for implementing the invention, as shown in  FIG. 4 , the light emitting diodes  12 R,  12 G and  12 B are arranged in a row along the y axis (the optical axis). This makes the normal of the scattered dot pattern formed on the bottom face  1  of the light guide  10  coincide with the optical axis of the light emitting diodes  12 R,  12 G and  12 B. 
   Lights from the light emitting diodes  12 R,  12 G and  12 B propagate within the light guide  10 , and cause scattered lights to be generated on the bottom face  1 . As shown in  FIG. 3 , these scattered lights are reflected by the side faces  2  and  3 , or directly emitted from the emitting face  4  and constitute illuminating lights  7  to illuminate the face to be read of the document, not shown, mounted on a cover glass  35 . 
   The illuminating lights  7  reflected by the face to be read of the document, not shown, are detected by the line image sensor  33  via the cover glass  35  and the lens array  32 . This causes the document to be read. 
   Since the illuminating lights emitted from the light guide  10  are confined to the range of ±15 degrees to the y axis (the optical axis), the expansion of the illuminating lights is prevented from becoming too wide even where the distance to the document is long. Therefore, the face of the document can be efficiently illuminated. 
     FIG. 5  is a diagram showing the sectional shape of another light guide according to the invention. A light guide  10 A shown in  FIG. 5  has an emitting face  8  made convex to reduce the expanding angle of emitted lights. 
     FIG. 6  is a diagram showing the sectional shape of another light guide according to the invention. The sectional shape in a direction orthogonal to the longitudinal direction of this light guide has side faces  2 ′ and  3 ′ each of an oval curve, a bottom face  1 ′ and an emitting face  4 ′, and the two ends of the bottom face  1 ′ coincide with the respective focal points a′ and b′ of the side faces  2 ′ and  3 ′. 
   Thus, to figure out the coordinate of a focal point from an oval curve (x/1.6) 2 +(y/6) 2 =1 having an aspect ratio of 1.6:6:
 
 x= 0 , y=f =−(6×6−1.6×1.6) 0.5 
 
is given. Wherein y is a variation in the section:
 
( f≦y≦ 0),  f =−(6×6−1.6×1.6) 0.5 
 
and the oval curve in that section is:
 
 x= 1.6(1−( y/ 6) 2 ) 0.5 
 
Now,
 
   x where y=f being supposed to be x 0 (=0.42667) 
   by making the side faces of the light guide curves of which
 
 x= 1.6(1−( y/ 6) 2 ) 0.5   −x   0 /2,
 
the two ends of the bottom face  1 ′ are made coincident with the focal points a′ and b′ of the side faces (oval curves)  2 ′ and  3 ′.
 
   When light guides of the type shown in  FIG. 6  were used as  20 L and  20 R shown in  FIG. 3 , substantially equal outputs to those where light guides of the type shown in  FIG. 1  were used were detected by the sensor  33 . 
   Besides the illustrated examples described above, the paraboloids  2  and  3  or the oval curves  2 ′ and  3 ′ may be made asymmetric between right and left to differentiate the right and left angles of expansion. 
     FIG. 7  is a diagram showing the sectional shape of a line illuminator provided with a compound parabolic reflector. A line illuminator  60  shown in  FIG. 7  is such that lights emitted from the emitting face  62  of a light guide  61 , whose sectional shape is rectangular for instance, are caused to be reflected by reflectors  63  and  64  having paraboloidal side faces to be emitted as illuminating lights from an opening  65 . Reference numeral  66  denotes a plastic case. This line illuminator  60 , as it has no reflective face in the refractive index medium, serves to restrain the angle of expansion. Thus, since the contents of the compound parabolic reflectors  63  and  64  are air, lights do not expand when they are emitted from the opening  65 , which is the outlet of illuminating lights. 
   Although there have been described what are the present exemplary embodiments of the invention, it will be understood that variations and modifications may be made thereto within the spirit and scope of the appended claims.