Patent Publication Number: US-2020292800-A1

Title: Optical device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority from Japanese patent application JP 2019-047407 filed on Mar. 14, 2019, the entire content of which is hereby incorporated by reference into this application. 
     BACKGROUND 
     Technical Field 
     The present disclosure relates to an optical device for assisting in improving the visibility of an image of a blind spot that is obstructed by a shield. 
     Background Art 
     Conventionally, as a technique of such a field, for example, a technique disclosed in JP 2018-127196 A is known. Specifically, JP 2018-127196 A discloses an optical device for allowing one to see an object that is obstructed by a shield, by reflecting or refracting a light beam from the obstructed object so as to avoid the shield using a plurality of reflecting mirrors and lenses. 
     SUMMARY 
     However, since the plurality of reflecting mirrors and lenses are provided on separate members in the aforementioned optical device, it would be necessary to adjust the relative positions of the reflecting mirrors and the lenses when attaching the optical device to the shield, which is problematic in that the attachment operation becomes complex. 
     Accordingly, exemplary embodiments relate to providing an optical device that involves a simple attachment operation without the need for an adjustment process. 
     An optical device according to the present disclosure is an optical device for projecting an image of a blind spot in which a line of sight of a viewer is blocked by a shield, the optical device including a first light guide portion disposed on the shield on the blind spot side of the line of sight, the first light guide portion being adapted to guide an incident light beam coming from the blind spot side in a direction intersecting the line of sight; a reflective portion disposed facing the shield, the reflective portion being adapted to reflect the light beam guided by the first light guiding portion; and a second light guide portion disposed on the shield on an eye point side of the line of sight, the second light guide portion being adapted to guide the light beam reflected by the reflective portion to the eye point side of the line of sight, in which the first light guide portion, the reflective portion, and the second light guide portion are provided on a single transparent member, and the transparent member has formed therein an optical path that is adapted to allow a light beam guided by the first light guide portion to be reflected by the reflective portion and then reach the second light guide portion. 
     In the optical device according to the present disclosure, since the first light guide portion, the reflective portion, and the second light guide portion are provided on a single transparent member, and since the transparent member has formed therein an optical path that is adapted to allow a light beam guided by the first light guide portion to be reflected by the reflective portion and then reach the second light guide portion, the optical path is defined. Therefore, since it is not necessary to adjust the relative positions of reflecting mirrors and lenses as in the conventional optical device, an attachment operation for the optical device can be easily performed without the need for an adjustment process. 
     In the optical device according to the present disclosure, the first light guide portion may be formed of a first curved reflecting mirror that is adapted to reflect an incident light beam coming from the blind spot side to the reflective portion, and the second light guide portion may be formed of a second curved reflecting mirror that is adapted to reflect a light beam reflected by the reflective portion to the eye point side of the line of sight. Accordingly, an image of a blind spot can be projected even when the optical device is tilted in the front-rear direction as seen from the eye point. 
     In the optical device according to the present disclosure, the first light guide portion may be formed of a first lens that is adapted to refract an incident light beam coming from the blind spot side toward the reflective portion, and the second light guide portion may be formed of a second lens that is adapted to refract a light beam reflected by the reflective portion and output the light beam to the eye point side of the line of sight. Accordingly, an image of a blind spot can be projected even when the optical device is tilted in the left-right direction as seen from the eye point. 
     In the optical device according to the present disclosure, each of the first curved reflecting mirror and the second curved reflecting mirror may be formed by vapor-depositing a metal film on the transparent member. Accordingly, the first curved reflecting mirror and the second curved reflecting mirror can be easily formed. 
     According to the present disclosure, an attachment operation for an optical device can be easily performed without the need for an adjustment process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view illustrating an overview of an optical device according to the first embodiment; 
         FIG. 2A  is a plan view illustrating the invisible region of the optical device according to the first embodiment; 
         FIG. 2B  is a plan view illustrating the invisible region of an optical device according to a comparative example; 
         FIG. 3A  illustrates the angle of incidence and the angle of refraction of a light beam when it becomes incident on the body portion from the air; 
         FIG. 3B  illustrates the angle of incidence and the angle of refraction of a light beam when it enters the air from the body portion; 
         FIG. 4  is a plan view illustrating an overview of an optical device according to the second embodiment; 
         FIG. 5A  is a view illustrating a state in which a light beam in parallel with a line of sight of a viewer and an obliquely incident light beam are allowed to become incident on the optical device according to the second embodiment; 
         FIG. 5B  is a view illustrating a state in which a parallel light beam in parallel with a line of sight of a viewer and an obliquely incident light beam are allowed to become incident on an optical device according to a comparative example; 
         FIG. 6A  is a view for explaining the visible region of the optical device according to the second embodiment; and 
         FIG. 6B  is a view for explaining the visible region of the optical device according to the comparative example. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments of an optical device according to the present disclosure will be described with reference to the drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and repetitive descriptions thereof are omitted. 
     First Embodiment 
       FIG. 1  is a plan view illustrating an overview of an optical device according to the first embodiment. An optical device  10  according to the present embodiment is adapted to be attached to a shield  3 , such as an A-pillar, of an automobile, for example, so as to assist in improving the visibility of an image of a blind spot that is obstructed by the shield  3 . 
     More specifically, a viewer, such as a driver, of an automobile can directly see a scene outside the vehicle through the windshield and the side glass, but the line of sight of the viewer is obstructed by the shield  3 , such as the A-pillar. Thus, a blind spot, which is a region not directly visible to the viewer, is generated. The optical device  10  of the present embodiment is configured to make the shield  3 , such as an A-pillar, appear transparent, and project an image of a blind spot at the same position as when the shield  3  is not present, so as to assist in helping the viewer see an object  2  that is present in the blind spot. Herein, the phrase “to make something appear transparent” means making something, which is present, appear as if it is not present, and means a state in which the background is visible without being hidden by a shield. 
     To help easy understanding of the configuration of the optical device  10 , in the following description, it is assumed that an axis lying along the direction in which the shield  3  is seen from the eye point  1  of the viewer (i.e., a line of sight) is the Y-axis, an axis lying along the direction in which the shield  3 , such as the A-pillar, extends is the Z-axis, and an axis that is orthogonal to the Y-axis and the Z-axis is the X-axis. In addition, a direction in which each of the arrows indicating the X-axis, the Y-axis, and the Z-axis is pointing is defined as the positive direction of each axis, and a direction opposite thereto is defined as the negative direction of each axis. in such a case, the positive direction of the Y-axis indicates a direction toward the eye point  1  from the shield  3 , and the positive direction of the Z-axis indicates a direction toward the vehicle body roof from the vehicle body floor of the automobile along the shield  3 , such as the A-pillar. 
     As illustrated in  FIG. 1 , the optical device  10  includes a transparent body portion (i.e., transparent member)  11  having a plurality of curved faces on its outer peripheral surface; and a blind-spot-side outward facing curved reflecting mirror  12 , a blind-spot-side inward facing curved reflecting mirror  13 , an eye-point-side inward facing curved reflecting mirror  14 , and an eye-point-side outward facing curved reflecting mirror  15 , which are formed integrally with the body portion  11 . 
     The body portion  11  is integrally formed using transparent glass or transparent resin, for example. Examples of the resin material include acrylic, polyethylene terephthalate, polycarbonate, and polyethylene, which are translucent and have low light absorption and low light scattering. 
     The blind-spot-side outward facing curved reflecting mirror  12  is disposed on the shield  3  on the blind spot side of the line of sight (i.e., on the object  2  side), and reflects an incident light beam in the positive direction of the Y-axis coming from the blind spot side, in the positive direction of the X-axis that is a direction intersecting the line of sight and is away from the shield  3 . The blind-spot-side outward facing curved reflecting mirror  12  is formed by vapor-depositing metal on a portion of the outer peripheral surface of the body portion  11  that curves toward the positive direction of the X-axis, for example. The blind-spot-side outward facing curved reflecting mirror  12  corresponds to the “first curved reflecting mirror” recited in the claims, and forms the “first light guide portion” recited in the claims. 
     The blind-spot-side inward facing curved reflecting mirror  13  is disposed facing the blind-spot-side outward facing curved reflecting mirror  12 , and reflects a light beam in the positive direction of the X-axis, which has been reflected by the blind-spot-side outward facing curved reflecting minor  12 , in the positive direction of the Y-axis toward the eye point  1 . The blind-spot-side inward facing curved reflecting mirror  13  is formed by vapor-depositing metal on a portion of the outer peripheral surface of the body portion  11  that curves toward the negative direction of the X-axis and faces the blind-spot-side outward facing curved reflecting mirror  12 . for example. 
     The eye-point-side inward facing curved reflecting mirror  14  is disposed on the side closer to the eye point  1  than is the blind-spot-side inward facing curved reflecting mirror  13 , and reflects a light beam in the positive direction of the Y-axis, which has been reflected by the blind-spot-side inward facing curved reflecting mirror  13 , in the negative direction of the X-axis that is a direction intersecting the line of sight and approaching the shield  3 . The eye-point-side inward facing curved reflecting mirror  14  is formed by vapor-depositing metal on a portion of the outer peripheral surface of the body portion  11  that curves toward the negative direction of the X-axis and faces the eye-point-side outward facing curved reflecting mirror  15 . for example. 
     Each of the blind-spot-side inward facing curved reflecting mirror  13  and the eye-point-side inward facing curved reflecting mirror  14  corresponds to the “reflective portion” recited in the claims. 
     The eye-point-side outward facing curved reflecting mirror  15  is disposed on the shield  3  on the eye point  1  side of the line of sight (i.e., the viewer side) and facing the eye-point-side inward facing curved reflecting mirror  14 , and reflects a light beam in the negative direction of the X-axis, which has been reflected by the eye-point-side inward facing curved reflecting mirror  14 , in the positive direction of the Y-axis toward the eye point  1  side of the line of sight. The eye-point-side outward facing curved reflecting mirror  15  is formed by vapor-depositing metal on a portion of the outer peripheral surface of the body portion  11  that curves toward the positive direction of the X-axis and faces the eye-point-side inward facing curved reflecting mirror  14 , for example. The eye-point-side outward facing curved reflecting mirror  15  corresponds to the “second curved reflecting mirror” recited in the claims, and forms the “second light guide portion” recited in the claims. 
     As illustrated in  FIG. 1 , the blind-spot-side outward facing curved reflecting mirror  12  and the eye-point-side outward facing curved reflecting minor  15  are coupled together around the central axis of the optical device  10  so as to form a substantial V-shape. The optical device  10  is fixed to the shield  3  with an adhesive, for example, in a state in which the shield  3  is disposed so as to enter a V-shaped space formed by the blind-spot-side outward facing curved surface reflector  12  and the eye-point-side outward facing curved surface reflector  15 . 
     According to such a configuration, the optical device  10  reflects an incident light beam L 1 , which comes from the blind spot side, in the positive direction of the X-axis using the blind-spot-side outward facing curved reflecting minor  12 , reflects the incident light beam L 1  in the positive direction of the Y-axis using the blind-spot-side inward facing curved reflecting mirror  13 , and further reflects the incident light beam L 1  in the negative direction of the X-axis using the eye-point-side inward facing curved reflecting mirror  14 , and then reflects the incident light beam L 1  in the positive direction of the Y-axis using the eye-point-side outward facing curved reflecting mirror  15 . Therefore, the incident light beam L 1  coming from the blind spot side is output toward the eye point  1  so that an image of the blind spot can be projected at the position of the shield  3  as seen from the eye point  1 . Accordingly, the line of sight of the viewer is not obstructed by the shield  3 , and the shield  3  can thus be made to appear transparent. This allows the viewer to see the object  2  on the other side of the shield  3  and thus can assist in improving the visibility of the image of the blind spot to the viewer. 
     In the present embodiment, the body portion  11  has formed therein an optical path that allows a light beam reflected by the blind-spot-side outward facing curved reflecting mirror  12  to be sequentially reflected by the blind-spot-side inward facing curved reflecting mirror  13  and the eye-point-side inward facing curved reflecting mirror  14  and then reach the eye-point-side outward facing curved reflecting mirror  15 . 
     In the optical device  10  according to the present embodiment, since the blind-spot-side outward facing curved reflecting mirror  12 , the blind-spot-side inward facing curved reflecting mirror  13 , the eye-point-side inward facing curved reflecting mirror  14 , and the eye-point-side outward facing curved reflecting mirror  15  are provided on the body portion  11 , which is a single transparent member, and since the body portion  11  has formed therein the optical path, which allows a light beam reflected by the blind-spot-side outward facing curved reflecting mirror  12  to be sequentially reflected by the blind-spot-side inward facing curved reflecting mirror  13  and the eye-point-side inward facing curved reflecting mirror  14  and then reach the eye-point-side outward facing curved reflecting mirror  15 , the optical path is defined. Therefore, since it is not necessary to adjust the relative positions of reflecting mirrors and lenses as in the conventional optical device, an attachment operation for the optical device  10  can be easily performed without the need for an adjustment process. 
     In addition, since the blind-spot-side outward facing curved reflecting mirror  12 , the blind-spot-side inward facing curved reflecting mirror  13 , the eye-point-side inward facing curved reflecting mirror  14 , and the eye-point-side outward facing curved reflecting mirror  15  are provided on the body portion  11 , which is a single transparent member, the size of the optical device  10  as well as the invisible region of the optical device  10  can be reduced as compared to when such curved reflecting mirrors are provided on separate members. 
     More specifically, for example, an optical device  10 A according to a comparative example illustrated in  FIG. 2B  includes a blind-spot-side outward facing curved reflecting mirror  12 A, a blind-spot-side inward facing curved reflecting mirror  13 A, an eye-point-side inward facing curved reflecting mirror  14 A, and an eye-point-side outward facing curved reflecting mirror  15 A, similarly to the aforementioned optical device  10 . In addition, the blind-spot-side outward facing curved reflecting mirror  12 A and the eye-point-side outward facing curved reflecting mirror  15 A are provided on a single member. 
     Meanwhile, the blind-spot-side inward facing curved reflecting mirror  13 A and the eye-point-side inward facing curved reflecting mirror  14 A are provided on members different from the transparent member on which the blind-spot-side outward facing curved reflecting mirror  12 A and the eye-point-side outward facing curved reflecting mirror  15 A are provided. That is, the optical device  10 A according to the comparative example includes three members. Therefore, attachment of the optical device according to the comparative example to an A-pillar of a vehicle, for example, involves the operations of individually fixing the three members to the A-pillar. 
     In contrast, since the optical device  10  according to the present embodiment is provided on a single transparent member (i.e., body portion  11 ) as described above, the size of the optical device  10  can be reduced as compared to that of the optical device  10 A according to the comparative example. Further, since the attachment operation for the optical device  10  can be completed only by fixing the body portion  11  to the shield  3 , such as an A-pillar of a vehicle, the attachment operation can be simplified as compared to that for the optical device  10 A according to the comparative example. 
     Further, in the comparative example illustrated in  FIG. 2B , each curved reflecting mirror of the optical device  10 A should have a certain thickness to secure a certain strength of each curved reflecting mirror, which results in an increased invisible region. In contrast, in the optical device  10  according to the present embodiment, only the region in which the blind-spot-side inward facing curved reflecting mirror  13  and the eye-point-side inward facing curved reflecting mirror  14  are provided is the invisible region (see  FIG. 24 ). Thus, the invisible region of the optical device  10  can be reduced as compared to that of the optical device  10 A according to the comparative example. 
     Further, since the optical device  10  according to the present embodiment includes the blind-spot-side outward facing curved reflecting mirror  12 , which reflects an incident light beam coming from the blind spot side to the blind-spot-side inward facing curved reflecting mirror  13 , and the eye-point-side outward facing curved reflecting mirror  15 , which reflects a light beam reflected by the eve-point-side inward facing curved reflecting mirror  14  to the eye point side of the line of sight, an image of the blind spot can be projected even when the optical device  10  is tilted in the front-rear direction as seen from the eye point. For example, when an A-pillar is disposed in a tilted state on the vehicle cabin side of the vehicle, an image of a blind spot can be projected even if the optical device  10  is fixed to the A-pillar in a tilted state in the front-rear direction along the tilt of the A-pillar. 
     Further, since each of the blind-spot-side outward facing curved reflecting mirror  12 , the blind-spot-side inward facing curved reflecting mirror  13 , the eye-point-side inward facing curved reflecting mirror  14 , and the eye-point-side outward facing curved reflecting mirror  15  is formed by vapor-depositing a metal film on a transparent member, such curved reflecting mirror can be easily formed. 
     It should be noted that the shape of the body portion  11  of the optical device  10  is not particularly limited as long as it does not totally reflect an incident light beam coming from the blind spot side or a light beam that has been reflected by the eye-point-side outward facing curved reflecting mirror  15  and travels toward the eye point side. 
     More specifically, as illustrated in  FIG. 3A , when an incident light beam coming from the blind spot side travels from the air (having an refractive index of n 1 ) to the body portion  11  (i.e., the transparent member, having a refractive index of n 2 ) of the optical device  10 , a relational expression n 1 sinθ 1 =n 2 sinθ 2  is established between the angle of incidence θ 1  and the angle of refraction θ 2  according to the Snell&#39;s law. If the refractive index of the air is 1 (n 1 =1) and the angle of refraction θ 2  is 90° (θ 2 =90° ), an incident light beam will be totally reflected at the boundary between the air and the transparent member. Therefore, when the condition (sinθ 1 /n 2 )≥sin90° is satisfied, an incident light beam will be totally reflected at the boundary between the air and the transparent member. That is, as long as the condition (sinθ 1 /n 2 ) &lt;1 is satisfied, an incident light beam will not be totally reflected and will pass through the boundary between the air and the transparent member. 
     Meanwhile, as illustrated in the  FIG. 3B , when a light beam, which has been reflected by the eye-point-side outward facing curved reflecting mirror  15  and travels toward the eye point side (hereinafter simply referred to as an “outgoing light beam”), travels from the body portion(i.e., the transparent member, having a refractive index of n 3 ) to the air (having a refractive index of n 4 ), a relational expression n 3 sinθ 3 =n 4 sinθ 4  is established between the angle of incidence θ 3  and the angle of refraction θ 4  according to Snell&#39;s law. If the refractive index of the air is 1 (n 4 =1) and the angle of refraction θ 4  is 90° (θ 4 =90° ), an outgoing light beam will be totally reflected at the boundary between the transparent member and the air. Therefore, when the condition (n 3 sinθ 3 /n 4 )≥sin90° is satisfied, an outgoing light beam will be totally reflected at the boundary between the transparent member and the air. That is, as long as the condition (n 3 sinθ 3 ) &lt;1 is satisfied, an outgoing light beam will not be totally reflected and will pass through the boundary between the transparent member and the air. 
     Therefore, various modifications may be made to the shape of the body portion  11  of the optical devices  10  as long as the condition (sinθ 1 /n 2 ) &lt;1 and (n 3 sin θ 3 ) &lt;1 is satisfied because total reflections of an incident light beam and an outgoing light beam will not occur under such condition. 
     Second Embodiment 
       FIG. 4  is a plan view illustrating an overview of an optical device according to the second embodiment. An optical device  20  according to the present embodiment differs from that of the aforementioned first embodiment in its structure. 
     As illustrated in  FIG. 4 , the optical device  20  according to the present embodiment includes a transparent body portion  21 ; and a first lens  22 , a reflective portion  23 , and a second lens  24 , which are integrally formed with the body portion  21 . 
     The body portion  21  has a cross-section with a V-shaped groove that is recessed toward the center of a semicircle from its outer circumference. The body portion  21  is integrally formed using transparent glass or transparent resin, for example. Examples of the resin material include acrylic, polyethylene terephthalate, polycarbonate, and polyethylene, which are translucent and have low light absorption and low light scattering. 
     The first lens  22  has a convex surface and is formed of a part of the body portion  21 . That is, the first lens  22  is a part of the body portion  21  The first lens  22  is disposed on the shield  3  on the blind spot side of the line of sight, and refracts an incident light beam coining from the blind spot side toward the reflective portion  23 . The first lens  22  forms the “first light guide portion” recited in the claims. 
     The reflective portion  23  is formed of a plane reflecting mirror, and is disposed facing the shield  3  so as to reflect a light beam from the first lens  22 . The reflective portion  23  is formed by vapor-depositing metal on a portion of the flat outer peripheral surface of body portion  21  that faces the shield  3 , for example. 
     The second lens  24  has a convex surface and is formed of a part of the body portion  21 . That is, the second lens  24  is a part of the body portion  21 . The second lens  24  is disposed on the shield  3  on the eye point  1  side of the line of sight, and refracts a light beam, which has been reflected by the reflective portion  23 , toward the eye point  1  side. The second lens  24  forms the “second light guide portion” recited in the claims. 
     As illustrated in  FIG. 4 , the optical device  20  is fixed to the shield  3  with an adhesive, for example, in a state in which the shield  3  is disposed so as to enter a V-shaped groove formed in the body portion  21 . 
     According to such a configuration, the optical device  20  refracts a parallel light beam L 2 , which is in parallel with the line of sight, out of incident light beams coming from the blind spot side, toward the reflective portion  23  using the first lens  22 , reflects the parallel light beam L 2  onto the second lens  24  using the reflective portion  23 , and further refracts the parallel light beam L 2  using the second lens  24 , thereby outputting the parallel light beam L 2  to the eve point  1  side. Therefore, the parallel light beam L 2  coming from the blind spot side can be output to the eye point  1  side, and an image of the blind spot can be projected at the position of the shield  3  as seen from the eye point  1 . Accordingly, the line of sight of the viewer is not obstructed by the shield  3 , and the shield  3  can thus be made to appear transparent. This allows the viewer to see the object  2  on the other side of the shield  3  and thus can assist in improving the visibility of the image of the blind spot to the viewer. 
     In the present embodiment, the body portion  21  has formed therein an optical path, which allows a light beam having passed through the first lens  22  to be reflected by the reflective portion  23  and then reach the second lens  24 . 
     In the optical device  20  according to the present embodiment, since the first lens  22 , the reflective portion  23 , and the second lens  24  are provided on the body portion  21 , which is a. single transparent member, and since the body portion  21  has formed therein the optical path that allows a light beam having passed through the first lens  22  to be reflected by the reflective portion  23  and then reach the second lens  24 , the optical path is defined. Therefore, since it is not necessary to adjust the relative positions of reflecting mirrors and lenses as in the conventional optical device, an attachment operation for the optical device  10  can be easily performed without the need for an adjustment process. 
     In addition, since the first lens  22 , the reflective portion  23 , and the second lens  24  are provided on the body portion  21 , which is a single transparent member, the attachment operation for the optical device can be further simplified and the compatibility with an obliquely incident light beam can be increased as compared to when lenses and reflecting mirrors are provided on separate members, so that the range of the field of view of the viewer can be increased. 
     More specifically, for example, an optical device  20 A according to a comparative example illustrated in  FIG. 5B  includes a first lens  22 A, a reflective portion  23 A, and a second lens  24 A similarly to the aforementioned optical device  20 , but the first lens  22 A, the reflective portion  23 A, and the second lens  24 A are provided on separate members. The first lens  22 A, the reflective portion  23 A, and the second lens  24 A are fixed to a single attachment member  21 A, and are attached to an A-pillar of a vehicle, for example, via the attachment member  21 A, Therefore, fixing the first lens  22 A, the reflective portion  23 A, and the second lens  24 A to the attachment member  21 A involves the operations of adjusting the positions of the respective components. 
     In contrast, since the optical device  20  according to the present embodiment is provided on a single transparent member as described above, the attachment operation for the optical device  20  can be completed only by fixing the transparent member to a target. Thus, the attachment operation can be simplified as compared to that for the optical device  20 A according to the comparative example. In addition, since the attachment member  21 A of the optical device  20 A according to the comparative example can be omitted, the number of components can be reduced. 
     In the comparative example illustrated in  FIG. 5B , when an obliquely incident light beam L 3  intersecting the line of sight comes from the blind spot side, the obliquely incident light beam L 3  may not reach the second lens  24 A if it is refracted by the first lens  22 A toward the reflective portion  23 A, passes through the boundary between the first lens  22 A and the air, and is further reflected by the reflective portion  23 A. In such a case, since the obliquely incident light beam L 3  does not reach the eye point  1  side, the range of the field of view of the viewer becomes narrow. 
     In contrast, in the optical device  20  according to the present embodiment, as illustrated in  FIG. 5A , since an optical path, which allows a light beam having passed through the first lens  22  to be reflected by the reflective portion  23  and then reach the second lens  24 , is formed inside the body portion  21 , the boundary between the air and the transparent member can be reduced as compared to that of the optical device  20 A according to the comparative example. Thus, the compatibility with an obliquely incident light beam can be increased. Consequently, the range of the field of view of the viewer can be increased as compared to that of the optical device  20 A according to the comparative example. Therefore, an image of the blind spot can be projected even when the optical device  20  is tilted in the left-right direction as seen from the eye point. 
     This will he described in detail with reference to  FIGS. 6A and 6B .  FIG. 6A  illustrates the lens of the present disclosure, and  FIG. 6B  illustrates the lens of the comparative example. Each lens is approximated to a triangular prism. In  FIGS. 6A and 6B , it is assumed that each lens is made of glass (having a refractive index of n 6 =1.5), the angle θ a  of incidence of an incident light beam in parallel with the line of sight is 30°, and the angle θ a  of incidence of an obliquely incident light beam is 25°. In addition, symbols θ b , θ c , and θ d  indicate the angles of refraction, incidence, and emergence, respectively. Herein, the difference between the angle θ d  of an obliquely incident light beam and the angle θ d  of an incident light beam in parallel with the line of sight is determined on the basis of the Snell&#39;s Law, for example. 
     Table 1 shows the process of calculating the angle θ d  of an incident light beam in parallel with the line of sight for each of the lens of the present disclosure and the lens of the comparative example, and the angle θ d  of an obliquely incident light beam for each of the lens of the present disclosure and the lens the comparative example. 
     
       
         
           
               
               
               
             
               
                 TABLE 11 
               
               
                   
               
               
                   
                   
                 Lens of 
               
               
                   
                 Lens of Present  
                 Comparative 
               
               
                   
                 Disclosure 
                 Example 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Angle of Incident Light 
                 n 5 sinθ a  = n 6 sinθ b   
                 n 5 sinθ a  = n 6 sinθ b   
               
               
                 Beam in Parallel 
                 1sin30° = 1.5sinθ b   
                 1sin30° = 1.5sinθ b   
               
               
                 with Line of Sight 
                 θ b  = 19.4° 
                 θ b  = 19.4° 
               
               
                 θ a  = 30° 
                 θ c  = θ a  − θ b  = 10.5° 
                 θ c  = θ a  − θ b  = 10.5° 
               
               
                   
                 θ d  = θ c  = 10.5° 
                 n 6 sinθ c  = n 5 sinθ d   
               
               
                   
                 (because there 
                 1.5sin10.5° = 1sinθ d   
               
               
                   
                 is no boundary 
                 θ d  = 15.8° 
               
               
                 Angle of Obliquely 
                 n 5 sinθ a  = n 6 sinθ b   
                 n 5 sinθ a  = n 6 sinθ b   
               
               
                 Incident Light Beam 
                 1sin25° = 1.5sinθ b   
                 1sin25° = 1.5sinθ b   
               
               
                 θ a  = 25° 
                 θ b  = 16.3° 
                 θ b  = 16.3° 
               
               
                   
                 θ c  = θ a  − θ b  = 13.6° 
                 θ c  = θ a  − θ b  = 13.6° 
               
               
                   
                 θ d  = θ c  = 13.6° 
                 n 6 sinθ c  = n 5 sinθ d   
               
               
                   
                 (because there 
                 1.5sin13.6° = 1sinθ d   
               
               
                   
                 is no boundary 
                 θ d  = 20.7° 
               
               
                 Difference between θ d  of 
                 3.1° 
                 4.9° 
               
               
                 Obliquely Incident Light 
                   
                   
               
               
                 Beam and θ d  of Incident 
                   
                   
               
               
                 Light Beam in parallel 
                   
                   
               
               
                 with Line of Sight 
               
               
                   
               
            
           
         
       
     
     As shown in Table 1, regarding the lens of the comparative example, the difference between the angle θ d  of an obliquely incident light beam and the angle θ d  of an incident light beam in parallel with the line of sight is 4.9°. Meanwhile, regarding the lens of the present disclosure, the difference between the angle θ d  of an obliquely incident light beam and the angle θ d  of an incident light beam in parallel with the line of sight is 3.1°. This can confirm that the lens of the present disclosure is less likely to be influenced by an obliquely incident light, that is, the lens of the present disclosure has higher compatibility with an obliquely incident light beam. Therefore, the optical device  20  of the present embodiment can increase the range of the field of view of the viewer. 
     Although the embodiments of the present disclosure have been described in detail above, the present disclosure is not limited thereto, and various design modifications can be made without departing from the spirit or scope of the present disclosure recited in the claims. 
     For example, although the aforementioned first embodiment illustrates an example in which the blind-spot-side inward facing curved reflecting mirror  13  and the eye-point-side inward facing curved reflecting mirror  14  forming the reflective portion are formed separately, such mirrors may also be integrally formed such that they are coupled together. 
     DESCRIPTION OF SYMBOLS 
     
         
           1  Eye point 
           2  Object 
           3  Shield 
           10 ,  20  Optical device 
           11 ,  21  Body portion (transparent member) 
           12  Blind-spot-side outward facing curved reflecting mirror (first curved reflecting mirror) 
           13  Blind-spot-side inward facing curved reflecting mirror (reflective portion) 
           14  Eye-point-side inward facing curved reflecting mirror (reflective portion) 
           15  Eye-point-side outward facing curved reflecting mirror (second curved reflecting mirror) 
         
           22 
         
       
    
     First lens (first light guide portion)
       23  Reflective portion     24  Second lens (second light guide portion