Patent Publication Number: US-2016238849-A1

Title: Light guide prism and head-mounted display apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a Continuing Application based on International Application PCT/JP2013/006440 filed on Oct. 30, 2013, the entire disclosure of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to a light guide prism and a head-mounted display apparatus that guide image light for observation of a virtual image. 
     BACKGROUND 
     In recent years, a light guide prism (eyepiece optical unit) that guides image light from a small display device and enlarges the image light with a lens for observation as a virtual image has been proposed (for example, see JP 2012-203113 A (PTL 1)). Such a compact, lightweight light guide prism can be used as a wearable display by being mounted on a device that can be worn on the head. 
     The light guide prism disclosed in PTL 1 is formed so that, in a state of use in front of the user&#39;s eye, the vertical thickness is uniform at the side of the entrance portion for image light, whereas the exit portion side is a tapered shape such that the vertical thickness becomes thinner from the entrance portion towards the exit portion. This light guide prism is advantageous in that the exit portion side can be made compact, thereby guaranteeing the user&#39;s external field of view. This structure is also advantageous in that the side at the entrance portion for image light can be made thicker, thereby making it easier to guarantee an effective diameter for the image light, to attach a display device to the entrance portion, to guarantee a portion for securing to the housing, and to guarantee a gate space when injection molding the light guide prism. 
     CITATION LIST 
     Patent Literature 
     PTL 1: JP 2012-203113 A 
     SUMMARY 
     As a tapered light guide prism, for example the light guide prism  100  illustrated in the front view and the plan view of  FIGS. 5A and 5B  is envisioned. The light guide prism  100  includes an entrance portion  121 , a reflecting portion  122 , and an exit portion  123 . The exit portion  123  is configured with an eyepiece. In a state of use in front of the user&#39;s eye, the light guide prism  100  guides image light, from a display device  200 , that is incident on the entrance portion  121  to reflect the image light once at the reflecting portion  122  and emit the image light from the exit portion  123  towards the user&#39;s eye, allowing the user to observe the image light as a virtual image. 
     In a state of use in front of the user&#39;s eye, the light guide prism  100  illustrated in  FIGS. 5A and 5B  has a uniform vertical thickness at the entrance portion  121  side and has a tapered shape at the exit portion  123  side. Therefore, the middle portion becomes thicker and correspondingly bulkier as compared to the case illustrated by the phantom lines in  FIG. 5A , in which the entrance portion  121  side and the exit portion  123  side each have a uniform vertical thickness, and a step-like level difference is formed by the thickness at the exit portion  123  side being less than the thickness at the entrance portion  121  side. Accordingly, in order to achieve a thinner, lighter light guide prism, the light guide prism  100  could be formed to have a step-like level difference as illustrated by the phantom lines in  FIG. 5A . 
       FIGS. 6A and 6B  respectively illustrate a front view and a plan view of a light guide prism  101  formed to have a level difference. In  FIGS. 6A and 6B , the light guide prism  101  includes a level difference  124  between the entrance portion  121  and the reflecting portion  122 . The entrance portion  121  side and the reflecting portion  122  and exit portion  123  side each have a uniform vertical thickness, and with the level difference  124  as a boundary, the thickness at the reflecting portion  122  and exit portion  123  side becomes less than the thickness at the entrance portion  121  side. 
     As illustrated in  FIG. 6B , however, when the extending direction of the level difference  124  is nearly parallel to the exit optical axis O e  of the light guide prism in a top view in the state of use, i.e. when the level difference  124  extends in a direction orthogonal to the longitudinal direction of the light guide prism  101 , outside light or the like that is reflected at the level difference  124  enters the user&#39;s eye E. Therefore, the level difference  124  may enter the user&#39;s field of view and cause a sense of discomfort. The same sort of problem also occurs in a head-mounted display apparatus in which a light guide prism is held in a housing. 
     A light guide prism according to an aspect of this disclosure is used by being placed in front of a user&#39;s eye and guides image light from a display device to the user&#39;s eye for observation as a virtual image, the light guide prism including: 
     a first light-guiding portion, a second light-guiding portion, and a level difference positioned between the first light-guiding portion and the second light-guiding portion, such that 
     in a state of use in front of the user&#39;s eye,
         the second light-guiding portion is positioned at a front side of the user&#39;s eye,   with the level difference as a boundary, a vertical thickness of the second light-guiding portion is less than a vertical thickness of the first light-guiding portion, and   the level difference extends at an inclination with respect to an exit optical axis of the light guide prism so as not to enter the user&#39;s field of view.       

     In the light guide prism, in a top view in the state of use, an extension line in an extending direction of the level difference may intersect with the exit optical axis of the light guide prism between the first light-guiding portion and the eye. 
     In the light guide prism, in a top view in the state of use, an edge of the level difference at an opposite side from an edge of the level difference towards the eye may be positioned closer to entrance of the image light than is an extension line of a line connecting the edge of the level difference towards the eye with the eye. 
     Furthermore, a head-mounted display apparatus according to an aspect of this disclosure includes: 
     an eyepiece optical unit, used by being placed in front of a user&#39;s eye, that guides image light from a display device to the user&#39;s eye for observation as a virtual image; and 
     a housing holding the eyepiece optical unit, such that 
     the housing includes a first housing portion, a second housing portion, and a level difference positioned between the first housing portion and the second housing portion, and 
     in a state of use in front of the user&#39;s eye,
         the second housing portion is positioned at a front side of the user&#39;s eye,   with the level difference as a boundary, a vertical thickness of the second housing portion is less than a vertical thickness of the first housing portion, and   the level difference extends at an inclination with respect to an exit optical axis of the light guide prism so as not to enter the user&#39;s field of view.       

     In the head-mounted display apparatus, in a top view in the state of use, an edge of the level difference at an opposite side from an edge of the level difference towards the eye may be positioned closer to entrance of the image light than is an extension line of a line connecting the edge of the level difference towards the eye with the eye. 
     Furthermore, a head-mounted display apparatus according to an aspect of this disclosure includes: 
     a light guide prism, used by being placed in front of a user&#39;s eye, that guides image light from a display device to the user&#39;s eye for observation as a virtual image; and 
     a housing that holds the light guide prism, wherein 
     the housing holds the light guide prism so that a portion of the light guide prism is exposed, and 
     in a state of use in front of the user&#39;s eye,
         the portion of the light guide prism exposed from the housing is positioned at a front side of the eye, and   an end face of the housing has an opening exposing the light guide prism, and the end face is inclined with respect to an exit optical axis of the light guide prism so as not to enter the user&#39;s field of view.       

     In the head-mounted display apparatus, in a top view in the state of use, an edge of the end face at an opposite side from an edge of the end face towards the eye may be positioned closer to entrance of the image light than is an extension line of a line connecting the edge of the end face towards the eye with the eye. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1A  is a front view illustrating the main structure of a light guide prism according to Embodiment 1 of this disclosure, and  FIG. 1B  is a plan view of the light guide prism in  FIG. 1A ; 
         FIG. 2A  is a front view schematically illustrating the main structure of the head-mounted display apparatus according to Embodiment 2 of this disclosure, and  FIG. 2B  is a plan view of the head-mounted display apparatus in  FIG. 2A ; 
         FIG. 3A  is a front view schematically illustrating the main structure of the head-mounted display apparatus according to Embodiment 3 of this disclosure, and  FIG. 3B  is a plan view of the head-mounted display apparatus in  FIG. 3A ; 
         FIG. 4  is a perspective view illustrating a usage example of the head-mounted display apparatus according to this disclosure; 
         FIG. 5A  is a front view illustrating the main structure of a tapered light guide prism, and  FIG. 5B  is a plan view of the light guide prism in  FIG. 5A ; and 
         FIG. 6A  is a front view illustrating the main structure of a conventionally envisioned light guide prism with a level difference, and  FIG. 6B  is a plan view of the light guide prism in  FIG. 6A . 
     
    
    
     DETAILED DESCRIPTION 
     The following describes embodiments with reference to the drawings. 
     Embodiment 1 
       FIGS. 1A and 1B  are a front view and a plan view illustrating the main structure of a light guide prism according to Embodiment 1 of this disclosure. The light guide prism  10  according to this embodiment includes an entrance portion  21 , a reflecting portion  22 , an exit portion  23 , and a level difference  24 . The exit portion  23  is configured with an eyepiece. In a state of use in front of the user&#39;s eye, the light guide prism  10  guides image light, from a display device  30 , that is incident on the entrance portion  21  to reflect the image light once at the reflecting portion  22  and emit the image light from the exit portion  23  towards the user&#39;s eye E, allowing the user to observe the image light as a virtual image. In the state of use, the level difference  24  is formed on both the upper surface and the lower surface. 
     In the light guide prism  10  of this embodiment, the vertical thickness in the state of use is uniform on the side of the entrance portion  21 , which constitutes a first light-guiding portion, and on the side of the reflecting portion  22  and the exit portion  23 , which constitute a second light-guiding portion, with the level difference  24  as a boundary therebetween, and the thickness at the side of the reflecting portion  22  and the exit portion  23  is less than the thickness at the side of the entrance portion  21 . In other words, the level difference  24  is formed as a step. In the state of use, the second light-guiding portion that includes the reflecting portion  22  and the exit portion  23  is positioned at the front side of the user&#39;s eye E. In the state of use, the level difference  24  extends at an inclination with respect to the exit optical axis O e  of the light guide prism, so as not to enter the user&#39;s field of view. In other words, the level difference  24  extends at an inclination with respect to the user&#39;s visual axis. 
     Therefore, in this embodiment, the level difference  24  is formed so that in a top view in the state of use, an extension line I 1  in the extending direction of the level difference  24  intersects with the exit optical axis O e  of the light guide prism between the exit portion  23  and the eye E. In other words, in a top view in the state of use, an edge  24   b  of the level difference  24  at the opposite side from an edge  24   a  by the eye E is positioned closer to the entrance portion  21  than is an extension line I 2  of a line connecting the edge  24   a  with the eye E. 
     The distance from the exit portion  23  to the eye E in the state of use of the light guide prism  10  is, for example, approximately 15 mm to 25 mm, and the left-to-right length of the light guide prism  10  is, for example, approximately 24 mm to 30 mm. The level difference  24  is preferably formed in the middle portion. Accordingly, the angle θ between the exit optical axis O e  and the extension line I 1  is, for example, preferably 25° or greater. The level difference  24  may be a right angle with respect to the surface of the entrance portion  21  side and the exit portion  23  side or may be slightly inclined. 
     With the light guide prism  10  of this embodiment, the level difference  24  is formed to be inclined with respect to the exit optical axis O e  of the light guide prism so as not to enter the user&#39;s field of view in the state of use. Hence, the discomfort experienced in the user&#39;s field of view can be reduced. Furthermore, since the level difference  24  is formed as a step, the reflecting portion  22  and exit portion  23  side can be made thinner and lighter. 
     Embodiment 2 
       FIGS. 2A and 2B  are a front view and a plan view schematically illustrating the main structure of a head-mounted display apparatus according to Embodiment 2 of this disclosure. A head-mounted display apparatus  40  according to this embodiment includes a housing  50 . A light guide prism  11 , display device  30 , and the like are held in the housing  50 , and in a state of use in front of the user&#39;s eye, image light from the display device  30  is guided by the light guide prism  11  so as to be emitted towards the user&#39;s eye E, allowing the user to observe the image light as a virtual image. The light guide prism  11  may have a known structure. For the sake of convenience, a structure such as the one illustrated in  FIGS. 1A and 1B  is used here. Accordingly, portions of the light guide prism  11  having the same effects as the constituent elements illustrated in  FIGS. 1A and 1B  are labeled with the same reference signs, and a description thereof is omitted. 
     In this embodiment, the housing  50  includes a first housing portion  51 , a second housing portion  52 , and a level difference  53 . In the state of use, the second housing portion  52  is positioned at the front side of the user&#39;s eye E. In the state of use, the vertical thickness of the first housing portion  51  and of the second housing portion  52  is uniform with the level difference  53  as a boundary, and the thickness of the second housing portion  52  is less than the thickness of the first housing portion  51 . In other words, the level difference  53  is formed as a step. In the state of use, the level difference  53  is formed on both the upper surface and the lower surface. In the state of use, the level difference  53  extends at an inclination with respect to the exit optical axis O e  of the light guide prism, so that outside light reflected at the level difference  53  does not enter the user&#39;s eye E, i.e. so that the level difference  53  does not enter the user&#39;s field of view. 
     So that the level difference  24  of the light guide prism  11  is positioned at the level difference  53  in the housing  50 , the entrance portion  21  side of the light guide prism  11 , the display device  30 , and the like are held in the first housing portion  51 , and the reflecting portion  22  and exit portion  23  side of the light guide prism  11  is held in the second housing portion  52 . An opening  52   a  for emitting image light is formed in the second housing portion  52  in correspondence with the exit portion  23  of the light guide prism  11 . In this embodiment, the level difference  53  of the housing  50  and the level difference  24  of the light guide prism  11  are formed at an inclination with respect to the adjacent surface but may be perpendicular instead. 
     Like the light guide prism  10  of Embodiment 1, the level difference  53  of the housing  50  is formed so that in a top view in the state of use, an extension line I 1  in the extending direction of the level difference  53  intersects with the exit optical axis O e  of the light guide prism between the opening  52   a  and the eye E. In other words, in a top view in the state of use, an edge  53   b  of the level difference  53  at the opposite side from an edge  53   a  by the eye E is positioned closer to the entrance portion  21  of the light guide prism  11  than is an extension line I 2  of a line connecting the edge  53   a  with the eye E. The angle θ between the exit optical axis O e  and the extension line I 1  is, for example, preferably 25° or greater, as in the case of Embodiment 1. 
     Accordingly, with this embodiment, outside light reflected at the level difference  53  does not enter the user&#39;s eye E in the state of use. Hence, the discomfort experienced in the user&#39;s field of view can be reduced. Furthermore, since the level difference  53  is formed as a step, the second housing portion  52  can be made thinner and lighter than when the second housing portion  52  is formed in a tapered shape. 
     Embodiment 3 
       FIGS. 3A and 3B  are a front view and a plan view schematically illustrating the main structure of a head-mounted display apparatus according to Embodiment 3 of this disclosure. A head-mounted display apparatus  41  according to this embodiment includes a housing  60 . A light guide prism  12 , display device  30 , and the like are held in the housing  60 , and in a state of use in front of the user&#39;s eye, image light from the display device  30  is guided by the light guide prism  12  so as to be emitted towards the user&#39;s eye E, allowing the user to observe the image light as a virtual image. The light guide prism  12  may have a known structure. For the sake of convenience, a structure such as the one illustrated in  FIGS. 1A and 1B  is used here. Accordingly, portions of the light guide prism  12  having the same effects as the constituent elements illustrated in  FIGS. 1A and 1B  are labeled with the same reference signs, and a description thereof is omitted. In  FIGS. 3A and 3B , the level difference  24  of the light guide prism  12  is formed at an inclination with respect to the adjacent surface but may be perpendicular instead. 
     In this embodiment, the thin reflecting portion  22  and exit portion  23  side, positioned by the user&#39;s eye E in the state of use, of the light guide prism  12  is exposed from the housing  60 , and the thick entrance portion  21  side that includes the level difference  24  is held in the housing  60 . In other words, the portion of the light guide prism  12  that is exposed from the housing  60  is positioned at the front side of the user&#39;s eye E in the state of use. Accordingly, the vertical thickness of the housing  60  in the state of use is greater than the vertical thickness at the entrance portion  21  side of the light guide prism  12 . An end face  61  of the housing  60  has an opening  60   a  exposing the light guide prism  12 , and in the state of use, the end face  61  is inclined with respect to the exit optical axis O e  of the light guide prism, so that outside light reflected at the end face  61  does not enter the user&#39;s eye E, i.e. so that the end face  61  does not enter the user&#39;s field of view. 
     Like the light guide prism  10  of Embodiment 1, the end face  61  of the housing  60  is formed so that in a top view in the state of use, an extension line I 1  of the plane of the end face  61  intersects with the exit optical axis O e  of the light guide prism between the exit portion  23  of the light guide prism  12  and the eye E. In other words, in a top view in the state of use, an edge  61   b  of the end face  61  at the opposite side from an edge  61   a  by the eye E is positioned closer to the entrance portion  21  of the light guide prism  12  than is an extension line I 2  of a line connecting the edge  61   a  with the eye E. The angle θ between the visual axis O e  and the extension line I 1  is, for example, preferably 25° or greater, as in the case of Embodiment 1. 
     Accordingly, with this embodiment, outside light reflected at the end face  61  does not enter the user&#39;s eye E in the state of use. Hence, the discomfort experienced in the user&#39;s field of view can be reduced. Furthermore, by holding the light guide prism  12  that has a similar structure to Embodiment 1, the portion of the light guide prism  12  exposed from the housing  60  can be made thinner and lighter. 
       FIG. 4  is a perspective view illustrating a usage example of the head-mounted display apparatus according to this disclosure.  FIG. 4  illustrates an example of the head-mounted display apparatus  41  illustrated in  FIGS. 3A and 3B  being held by eyeglasses  70  and used. The head-mounted display apparatus  41  is configured so that when the user is wearing the eyeglasses  70 , the housing  60  is supported by a temple  72 , an end piece  73 , and the like of the eyeglasses  70  in order to position the light guide prism  12  exposed from the housing  60  in front of one lens  71  of the eyeglasses  70 , i.e. in front of one of the user&#39;s eyes. Accordingly, in this case, the image light emitted from the light guide prism  12  is guided through the lens  71  to the user&#39;s eye, thereby allowing the user to observe a virtual image produced by the image light. The head-mounted display apparatus  40  illustrated in  FIGS. 2A and 2B  can also be supported similarly by glasses and used. 
     This disclosure is not limited to the above embodiments, and a variety of changes and modifications may be made. For example, the level difference  24  of the light guide prism  10  illustrated in Embodiment 1 is not limited to being formed on both vertical sides in the state of use and may instead be formed on only the top side or the bottom side. Similarly, the level difference  53  of the housing  50  in the head-mounted display apparatus  40  illustrated in Embodiment 2 is also not limited to being formed on both vertical sides in the state of use and may instead be formed on only the top side or the bottom side. The light guide prism is not limited to a structure such as the one illustrated in the above embodiments that emits image light after reflecting the image light once inside the light guide prism. This disclosure may also be applied effectively to a light guide prism that emits image light after reflecting the light multiple times as in PTL 1. Furthermore, the head-mounted display apparatus of this disclosure is not limited to the case of being supported by the eyeglasses  70  and used as illustrated in  FIG. 4  and may instead be supported by a support member in the shape of eyeglasses without lenses, a headset, or the like and used. 
     REFERENCE SIGNS LIST 
       10 ,  11 ,  12  Light guide prism 
       21  Entrance portion 
       22  Reflecting portion 
       23  Exit portion 
       24  Level difference 
       24   a,    24   b  Edge 
       30  Display device 
       40 ,  41  Head-mounted display apparatus 
       50  Housing 
       51  First housing portion 
       52  Second housing portion 
       52   a  Opening 
       53  Level difference 
       53   a,    53   b  Edge 
       60  Housing 
       60   a  Opening 
       61  End face 
       70  Eyeglasses